Conformational Space and Photochemistry of Tyramine ... › ~reva › FULL › 102.pdfspectra of seven diﬀerent conformers were observed. All the gas phase experiments agree that

Conformational Space and Photochemistry of Tyramine Isolatedin Argon and Xenon CryomatrixesBarbara M. Giuliano,† Sonia Melandri,*,†,‡ Igor Reva,† and Rui Fausto*,†

†Department of Chemistry, University of Coimbra, P-3004-535 Coimbra, Portugal‡Department of Chemistry, University of Bologna, I-40126 Bologna, Italy

*S Supporting Information

ABSTRACT: The infrared spectra of tyramine monomers trapped in low-temperature argonand xenon matrixes were recorded. The presence of the flexible ethylamino side chain givesrise to a complex conformational surface that contains several minima of relatively lowenergies, some of them stabilized by a weak N−H···π hydrogen bond interaction between theamino group and the phenyl ring. The experimental infrared spectra confirm the presence of atleast two stable conformers isolated in the matrixes. Annealing experiments performed on thexenon matrix revealed a change in the relative population of the experimentally relevantconformers upon isolation in this polarizable matrix, compared to the gas phase. The general interpretation of the spectra wasbased on harmonic and anharmonic quantum chemical calculations, undertaken at the DFT/B3LYP and MP2 levels of theorywith the 6-311++G(d,p) basis set. The photochemical behavior of the matrix-isolated compound upon narrow-band UVirradiation was also investigated. Identification of ketene species in the spectra of the irradiated matrixes suggests the occurrenceof a ring-opening reaction, which in the xenon matrix occurs concomitantly with the conformational isomerization of tyramine.

1. INTRODUCTION

Tyramine (4-hydroxyphenethylamine; TYR) is a biogenicamine derived from the decarboxylation of the amino acid tyro-sine. It can be found in aged foods as a degradation product1,2

and can act as a releasing agent.3,4 The presence of the flexibleethylamino group as a side chain of its phenolic aromatic ringis an essential structural feature of tyramine, leading to a com-plex conformational surface, which contains several minima ofrelatively low energy.This substance has already been investigated by microwave

rotational spectroscopy and laser spectroscopy in the super-sonic jet environment.5−9 Such experiments have evidencedthe presence of a minimum of four, up to seven, different con-formers in the gas phase, depending on the detection techniqueemployed. In particular, the dispersed fluorescence and IRspectra of tyramine are reported in ref 7, where the vibrationalspectra of seven different conformers were observed. All the gasphase experiments agree that the most stable conformers arestabilized by weak intramolecular hydrogen bond interactionsand exhibit a folded geometry in which the amino hydrogenatoms interact with the π cloud of the aromatic ring.To the best of our knowledge, the infrared spectra of tyra-

mine isolated in cryogenic matrixes have never been reportedbefore, nor has its photochemistry been investigated hitherto.In order to fill these gaps, we present in this article a study onthe structure and infrared spectra of tyramine isolated in argonand xenon matrixes, exploring its conformational landscapewith the help of harmonic and anharmonic quantum chemicalcalculations, and analyzing its reactivity following narrow-bandUV irradiation.

2. EXPERIMENTAL AND CALCULATION METHODS

A sample of tyramine (99% purity) was purchased from Sigma-Aldrich and used without further purification. Matrixes wereprepared by codeposition of tyramine vapors from a heatedglass furnace containing a sample of the compound, and a largeexcess of the matrix gas (argon N60 from Air Liquide; scientificxenon 5.0 from Linde), onto the CsI optical substrate of thecryostat kept at 15 and 30 K, respectively, for argon and xenonmatrixes. An APD Cryogenics closed-cycle helium refrigerationsystem with a DE-202A expander was used. The temperaturewas measured directly at the sample holder by a silicon diodetemperature sensor, connected to a digital controller (ScientificInstruments, Model 9650-1), with an accuracy of 0.1 K.The IR spectra were recorded using a Nicolet 6700 Fourier

transform infrared spectrometer, equipped with a deuteratedtriglycine sulfate (DTGS) detector and a Ge/KBr beam splitter,with 0.5 cm−1 resolution.In the photochemical experiments, narrow-band tunable UV

radiation was provided by the frequency doubled signal beam ofthe Quanta-Ray MOPO-SL optical parametric oscillator (fullwidth at half-maximum (fwhm) ∼0.2 cm−1, repetition rate 10 Hz,pulse energy ∼3 mJ) pumped with a pulsed Nd:YAG laser.The quantum chemical calculations were performed with the

Gaussian 03 software package.10 Equilibrium geometries for allstudied species were fully optimized at the DFT level of theory,using the standard 6-311++G(d,p) basis set and the three-parameter density functional B3LYP, which includes the Beckegradient exchange correction and the Lee, Yang, and Parr

Received: May 13, 2013Revised: September 2, 2013Published: September 8, 2013

Article

pubs.acs.org/JPCA

© 2013 American Chemical Society 10248 dx.doi.org/10.1021/jp404707e | J. Phys. Chem. A 2013, 117, 10248−10259

pubs.acs.org/JPCA

correlation functional. The geometries and relative energies ofthe TYR minimum energy conformations were also computedusing the MP2 method, with the 6-311++G(d,p) basis set.Both harmonic and anharmonic vibrational calculations were

undertaken. The first were obtained at both the B3LYP andMP2 levels of theory. Anharmonic frequencies were obtainedjust at the B3LYP level of approximation. The theoretical nor-mal modes were analyzed by carrying out potential energydistribution (PED) calculations. Transformations of the forceconstants with respect to the Cartesian coordinates to the forceconstants with respect to the molecule fixed internal coor-dinates allowed the PED analysis to be carried out as describedby Schachtschneider and Mortimer.11 The internal coordinatesused in this analysis were defined as recommended by Pulayet al.12 and are listed in Table S1 (Supporting Information).

3. RESULTS AND DISCUSSION

The molecule of tyramine (Figure 1) has four internal rota-tional degrees of freedom. Two of them are associated with thespatial orientation of the ethylamino side chain and originatefrom rotations around the CH2−CH2 and CH2−N bonds. Theremaining two define the relative positions of the ethylaminomoiety and hydroxyl group in relation to the aromatic ring,and correspond to internal rotations about the Caryl−CH2 andCaryl−O bonds, respectively.The systematic inspection of the PES of the molecule indi-

cated that the optimized orientations of both the ethylaminoand hydroxyl substituents remained essentially perpendicularand parallel to the ring plane, respectively, for all minimumenergy conformations, independently of the conformationwithin the ethylamino moiety. Illustrative relaxed potentialenergy scans around the Caryl−CH2 and Caryl−OH bonds areshown in Figure 1. We choose, as example, the case where theconformations about the CH2−CH2 and CH2−N bonds of theethylamino group are anti (Caryl−C−C−N dihedral angle is∼180°) and gauche (C−C−N−Lp is ∼60°; Lp = lone electronpair of N). As can be seen in Figure 1, one full rotation aroundthe Caryl−CH2 or Caryl−OH bond results in two minima, desig-nated as Ag and Ag′ (first letter indicates the conformationabout the CH2−CH2 bond; the second indicates that about the

CH2−N bond). If the conformation around the C−C−N−Lpdihedral angle was chosen as anti (∼180°), the two minimawould be mirror reflections of each other (Aa), correspondingto reflection either through the plane of the phenyl ring (flip ofthe ethylamine group) or in the plane bisecting the phenyl ring(flip of the OH group).According to the B3LYP calculations, the transformation

between Ag and Ag′ via rotation about the Caryl−CH2 bond isassociated with a barrier lower than 9 kJ mol−1, while thatobtained via rotation of the OH group involves a larger barrier(about 15 kJ mol−1).It is important to note that transformations between the

symmetry-equivalent minima can be accomplished via internalrotation either of the ethylamino fragment or, with the sameeffect, of the hydroxyl group. Then, to characterize a set ofunique conformations of tyramine, one of these internal rota-tions does not need an explicit consideration. Since the OHgroup internal rotation is related to a higher energy barrier, inthe further analysis we have oriented the OH group to havethe cis value of the C2−C1−O−H dihedral angle. We kept thisOH group orientation as a reference (or, equivalent to say, asan “asymmetry marker” of the phenyl ring) while analyzing allpossible orientations of the ethylamino side chain.For a more detailed conformational characterization of the

ethylamino fragment, we have performed a two-dimensionalpotential energy scan where the driving coordinates correspondto the dihedral angles related to the internal rotations aroundthe CH2−CH2 and CH2−N bonds. These angles wereincrementally fixed with a step of 20°, in all possible 324 com-binations, while all other geometric parameters were fully op-timized. The resulting potential energy surface is represented inFigure 2.The complete potential energy surface contains nine different

minima. According to the adopted naming scheme, these min-ima are designated using two letters, appearing on the top andright sides of the surface representation in Figure 2. The first(capital) letter refers to the position of the amino group withrespect to the phenyl ring, defined by the Caryl−C−C−Ndihedral angle. The second (lower case) letter specifies theorientation of the amino group with respect to the ethylaminochain, defined by the C−C−N−Lp dihedral angle. This naming

Figure 1. Relaxed B3LYP/6-311++G(d,p) potential energy scans around the C8−C7−C4−C3 (left) and C2−C1−O−H21 (right) dihedral anglesin tyramine. Along the Caryl−CH2 scan (left), the optimized C2−C1−O−H, N−C8−C7−C4, and Lp−N−C8−C7 dihedral angles stayed around cis,anti, and gauche orientations, respectively. Along the Caryl−OH scan (right), the C8−C7−C4−C3, N−C8−C7−C4, and Lp−N−C8−C7 dihedralangles stayed around perpendicular, anti, and gauche orientations, respectively. “Lp” stands for lone pair of electrons of the nitrogen atom, and itsorientation is defined to be in the plane bisecting the C8−N9−H19 and C8−N9−H20 planes. Center: optimized structure of Gg conformer withatom numbering.

The Journal of Physical Chemistry A Article

dx.doi.org/10.1021/jp404707e | J. Phys. Chem. A 2013, 117, 10248−1025910249

system is very similar to that implemented previously in thestudy of tryptamine,13 a molecule that also bears an ethylaminogroup connected to an asymmetric aromatic ring.The CH2−CH2 bond rotation gives rise to two types of

conformers: the gauche conformers (denoted with the capitalletters G and G′), adopting a “folded” position of the NH2group with respect to the aromatic ring, and the anti forms(denoted with A) which assume an unfolded geometry. Theinternal rotation of the NH2 group produces three moreorientations (denoted with g, a, and g′) for each gauche or antiarrangement around the CH2−CH2 bond. Depending on theorientation of the amino group, some of the folded conformersmay involve the presence of a weak N−H···π hydrogen bondinteraction, while in the case of the unfolded conformationssuch an interaction is impossible.The structures of the nine different conformers of tyramine

are presented in Figure 3. Their calculated energies are listed inTable 1. According to both the B3LYP and MP2 calculations,the lowest energy conformations are those stabilized by theintramolecular hydrogen bonding between one of the aminohydrogen atoms and the π electron cloud of the aromatic ring(Gg, G′g′, Ga, and G′a; see also Figure 3). In the two foldedconformations, where the nitrogen lone pair is pointing towardthe phenyl ring (Gg′ and G′g), this interaction is not possible,and their energies relative to the most stable conformers arepredicted to be much higher. The energies of the anti formsabout the CH2−CH2 bond were predicted by the MP2 methodto be only slightly lower than those of the Gg′ and G′g formsand considerably higher than those of the folded conformationsbearing the stabilizing N−H···π interaction. At the B3LYP level

of theory the relative energies of the anti forms about theCH2−CH2 bond were estimated to be smaller, with the Aaconformer being predicted to correspond to the second moststable form of tyramine. We can expect the relative energies ofthe conformers to be better predicted by the MP2 calculations,since this method is well-known to describe more consistentlythe intramolecular correlation energy than DFT basedmethods.5,14,15

From the MP2 calculated Gibbs energies at 298 K, therelative abundances of the different tyramine conformers inthe gas phase equilibrium at that temperature were estimated(Table 1). The abundance of the conformers in a low-temperaturematrix may, however, differ substantially from those, because ofconformer interconversion either during deposition of the matrixor in the matrix itself. The probability of the occurrence of theconformational relaxation is related to the energy barriers betweenthe conformers. In matrixes, conformational cooling has beendemonstrated to occur in compounds possessing barriers tointramolecular rotation of a few kilocalories per mole.16−20

The B3LYP and MP2 predicted barriers for interconversionsinvolving the nine conformers of tyramine can be seen in

Figure 2. Relaxed potential energy surface of tyramine for rotationaround CH2−CH2 and CH2−NH2 bonds calculated at the B3LYP/6-311++G(d,p) level. The names of conformers correspond to thecombinations of letters (G/A/G′) and (g/a/g′) shown at the edges ofthe surface. The global energy minimum is Gg, and its energy waschosen as the relative zero level. The isoenergy levels are presented bycontinuous thin lines and are spaced by 3 kJ mol−1. Two additionalisoenergy levels are shown at 2 (bold line) and 16 (dotted line)kJ mol−1. “Lp” stands for “lone pair” of the nitrogen atom, and itsorientation was chosen to be in the plane bisecting the two CNHplanes.

Figure 3. The nine conformers of tyramine and their optimizedgeometries. Note that positions of the conformers in this figurecorrespond to their positions on the two-dimensional potential energysurface shown in Figure 2. The dashed lines specify the amino grouphydrogen atoms involved in the stabilizing interaction with thearomatic ring.



Figure 2 and in Scheme 1, respectively. As a general feature, thebarriers for the NH2 group torsion are lower than those for the

torsion of the ethylamino backbone. The conversion of Gg′into the global minimum Gg shows the smallest calculatedbarrier, less than 3 kJ mol−1. This value is small enough toassume that the Gg′ form will be converted into the Gg formduring matrix deposition. The same assumption can be madefor the conversion of G′g into G′g′. Likewise, the barriers forthe interconversions between the anti CH2−CH2 conformers(Ag, Ag′, and Aa), which are between 6 and 10 kJ mol−1, canalso be considered low enough to lead us to expect that only

one of these conformers (either the most stable Ag′ form or theAg one which possesses the highest value of the dipolemoment, according to the MP2 calculations; see Scheme 1) willbe present in the low-temperature matrixes after deposition.Similarly, the barriers for the amino group rotation con-

verting the Ga form into the most stable Gg, and accordinglythe G′a into the G′g′, have been calculated to be slightly higher(∼12 kJ mol−1), which is in agreement with the presence of theweak N−H···π interaction in these conformers. Hence, thepresence of Ga and G′a conformers in the matrixes is notexpected. Also, we will present below the experimental featuresfor postulating the absence of these two conformers in thedeposited matrixes. Note that in alanine, as in tyramine, manyconformers differing by the NH2 group orientation arepossible. The corresponding barriers for the NH2 groupinternal rotation in alanine are of the order of ∼10 kJ mol−1.The less stable forms of alanine readily collapse to the moststable conformer around this coordinate during deposition ofthe matrixes, and only two forms of alanine with different heavyatom backbone orientations could be trapped upon the matrixdeposition.21

Conformational isomerization about the CH2−CH2 bondinvolves rotation of heavy atoms. The associated barriers werecalculated to be no less than 16 kJ mol−1 (Scheme 1), beingthe highest barriers for conformational isomerization in the stud-ied molecule. This result can be used in favor of the presence ofboth gauche and anti type conformers in the deposited matrixes.Finally, the barrier for the rotation of the OH substituent was

calculated to be around 15 kJ mol−1, independently of theconformation of the ethylamino substituent. Our recent stud-ies on cytosine22 showed that two hydroxy conformers couldconvert to each other in the matrix even over the barrier of30 kJ mol−1.Rotation of the OH group interconverts the Gg, Ga, Ag′, and

Gg′ conformers into the G′g′, G′a, Ag, and G′g forms, respec-tively. It is interesting to note that the conformers that differonly in the orientation of the OH group are almost isoenergetic(as can be seen from Table 1 and Scheme 1) but the conformerwith the highest dipole moment should be favored in thematrix. Nevertheless, since their calculated spectra are practi-cally coincident, at our level of experimental resolution it isimpossible to discriminate between these conformers. For thisreason in the spectral analysis presented below, only one mem-ber of each of these classes of conformers will be consideredand it will be the one with the highest dipole moment. We willthen use G′g′ for the couple Gg/G′g′, Ag for Ag/Ag′, Ga forGa/G′a, and G′g for G′g/Gg′.After all these considerations, taking into account all the

calculated values for the isomerization barriers, and the dipolemoments, the species that are expected to be predominant inthe matrix after deposition are the Ag and G′g′ conformers.With the assumptions resulting from the analysis of the

potential energy landscape of tyramine and taking into accountthe expected conformational conversions between conformersduring matrix deposition, we can now roughly estimate therelative abundance (%) of the conformers in the as-depositedmatrixes, using the theoretically predicted (MP2) populationsfor the conformers in the gas phase before deposition presentedin Table 1. The obtained value, G′g′:Ag = 71:29 (i.e., ∼2.4), is,as could be anticipated (see Figure 4), somewhat smaller thanthat found in ref 6 for jet cooled tyramine using R2PI laserspectroscopy (in the spectrum shown in Figure 3 of ref 6,the relative intensity of the bands assigned to the gauche

Table 1. Zero Point Corrected Relative Energies (ΔEZPE/kJ mol−1),

Dipole Moments (μ/D), Relative Gibbs Energies at 298 K(ΔG298/kJ mol−1), and Relative Populations (P298, %; at T =298 K) of the Nine Different Conformers of TYR, Calculatedat the B3LYP and MP2 Levels of Approximation with the6-311++G(d,p) Basis Set

ΔEZPE μ ΔG298 P298b

conformera B3LYP MP2 B3LYP MP2 MP2 MP2

Gg 0.00c 0.00d 0.65 0.67 0.00e 23.9G′g′ 0.26 0.53 2.47 2.65 0.34 20.8Ga 0.52 1.81 2.09 2.23 1.88 11.2G′a 0.42 1.95 1.57 1.67 1.85 11.3Aa 0.17 5.25 1.86 2.02 2.40 9.1Ag′ 0.63 4.77 0.87 0.90 1.52 12.9Ag 0.73 5.16 2.46 2.57 3.14 6.7Gg′ 5.39 7.60 2.37 2.70 6.26 1.9G′g 5.60 7.73 2.37 2.72 5.87 2.2

aConformer structures are shown in Figure 3. bEstimated using thecalculated ΔG298 values.

cAbsolute energy: −441.397 289 Eh.dAbsolute

energy: −440.109 036 Eh.eAbsolute Gibbs energy: −440.144 844 Eh.

Scheme 1. Relative Energies (ΔE/kJ mol−1) of TransitionStates between the Nine Conformers of TYR Calculated atthe MP2/6-311++G(d,p) Level of Theorya

aData enclosed in squares represent names, relative energies (kJ mol−1),and net dipole moments (debye, in brackets) of all the minima,positioned in the same way as on the map shown in Figure 2. Databetween squares represent the energies of the respective transitionstates. Values on the perimeter show the transition state energiesbetween the minima on the opposite edges, and for emphasis, they arerepeated on the right/left and top/bottom. bAbsolute energy:−440.287 276 Eh.



conformers is ca. 3 times larger than the intensity of the bandsassigned to the anti forms).3.1. Band Assignments. Due to the characteristics of the

molecular system under study (high flexibility of the side chain,relatively large molecule, similar spectra for the variousconformers), the assignment of the main spectral features toindividual species is difficult to undertake unequivocally. Themost intense bands are split into various components that canbe assigned to the presence of different conformers as well as tomatrix site effects, which are particularly common for argonmatrixes. A further complication arises from the fact that themost conformation-sensitive vibrational modes have a highlyanharmonic character. Therefore, anharmonic predictions ofthe spectra have been computed for the seven most stable con-formers of tyramine. In other cases,23,24 where the assignmentsare clearer, the B3LYP predicted frequencies have been foundto reproduce better the experimental spectra than the MP2ones, and therefore those calculations have been used to makethe assignments also in the case of tyramine.In the case of ref 7 the IR spectra of different conformers are

obtained by IR dip spectroscopy or laser induced fluorescenceand the conformers are collected in groups based on thespectral similarities. The conformers belonging to the G or Agroups show similarities in the C−H out of plane bending inthe benzene ring (around 550−1000 cm−1), while groupings aand g are based on the C−H methylene stretching region(2800−3000 cm−1). The main features of the observed spectraare well reproduced by B3LYP/aug-cc-pVDZ calculations. Inour case the overlap of bands does not allow a straightforwardassignment to a single conformation; nevertheless, informationon populations and population changes can be extracted fromthe spectra. In this case the most informative region of thespectra is the fingerprint region.A detailed comparison of the B3LYP harmonic and

anharmonic predictions will be provided in the next paragraphs;briefly, the predicted harmonic frequencies, scaled by acommon factor of 0.978, have been found to reproduce betterthe experimental spectra in the fingerprint region, and havebeen used to make the detailed assignment in this region ofthe spectra, while the anharmonic predictions have been veryeffective in reproducing the position of the OH stretchingvibration for the various conformers.25

The experimental spectra of TYR isolated in argon andxenon matrixes are shown in Figure 5. The simulated spectrumobtained by adding the frequency scaled calculated harmonicspectra of conformers G′g′ and Ag in a 2.4:1 ratio is also shownin Figure 5. The sum of the predicted spectra of these two

conformers is the minimum set of data necessary to satis-factorily reproduce the recorded spectra. This result is com-patible with the expected presence in the matrixes of both G′g′and Ag forms (vide supra). The small differences between thespectra obtained in Ar and Xe matrixes are due to the differenttrapping sites which result in a slightly different splittingpattern. In addition, the different polarizabilities of the twomatrixes may induce different stabilization orders of the trappedconformers.The experimentally measured frequencies and their compar-

ison with the B3LYP predictions, together with the PED anal-ysis, for the G′g′ and Ag conformers are reported in Table 2.The calculated frequencies and PEDs for the remainingseven conformers are listed in Tables S2 and S3 (SupportingInformation). A more detailed discussion about the vibrationalassignments proposed for the different regions of the spectrumis given below.

3.1.1. 3700−2800 cm−1 Region (ν(OH), ν(NH2), and ν(CH)Stretching Modes). The most intense band in this region is dueto ν(OH), which appears as an intense sharp band at 3631 and3617 cm−1 in Ar and Xe matrixes, respectively (see Figure 8).Since the OH group of tyramine is not involved in anyintramolecular interaction, the ν(OH) band appears at almostthe same position as in matrix-isolated phenol.26 In argon, theband shows different components, possibly due to matrix sitesplitting effects (similarly as it is for phenol), in addition to thepresence of different conformers of tyramine in the matrix. Ascan be seen in Table 2, the anharmonic predictions for theν(OH) mode are much closer to the observed frequencies and,

Figure 4. Calculated Boltzmann relative populations of G- and A-typeconformers as a function of temperature, according to the MP2calculated Gibbs energies.

Figure 5. Experimental infrared spectra of TYR isolated in freshlydeposited (a) Ar and (b) Xe matrixes, at 15 and 30 K, respectively. (c)Simulated theoretical spectrum which includes population-weightedcontributions of the G′g′ (71%) and Ag (29%) conformers. Beforesimulation, all frequencies were calculated at the B3LYP/6-311++G(d,p) level of theory and scaled by a factor of 0.978. Then, theabsorption bands were simulated using Lorentzian functions centeredat the calculated (scaled) frequencies and with the full width at half-maximum (fwhm) equal to 2 cm−1.



Table

2.Experim

entalWavenum

bers

(ν/cm

−1 )

andRelativeIntegral

Intensities(I)of

theAbsorptionBands

intheInfrared

Spectrum

ofTyram

ineIsolated

inArandXe

Matrixes,Com

paredwithWavenum

bers

(ν/cm

−1 ),A

bsoluteIntensities(A

th/km

mol

−1 ),and

PotentialEnergyDistribution(PED,%

)CalculatedforTyram

ineat

theB3L

YP/

6-311+

+G(d,p}Levelof

Theory

experim

entalνa(Ib)

calculated

νc(A

th)

PEDd(%

)

Ar

Xe

G′g′h

arm.scaled

G′g′a

nhAgharm

.scaled

Aganh

G′g′

Ag

3633.8;3631.4;3627.7

(195.5)

3622.2;3616.6(170.5)

3752.3

(66.1)

3650.2

37519(66.0)

3655.6

ν(OH)(100)

ν(OH)(100)

n.o.e

3458.3;3443.7

(10.2)

3499.5

(2.7)

3412.3

3498.0

(1.2)

3416.0

ν a(N

H2)

(99)

ν a(N

H2)

(99)

n.o.

3416.8

(5.3)

3422.5

(0.7)

3353.0

3420.2

(0.5)

3354.0

ν x(N

H2)

(98)

ν x(N

H2)

(98)

3036.6;3020.9;2964.9;

2964.9;2958.3;

2922.9;2

861.0(271.0)

3023.4;3009.8;2952.7;

2942.3;2928.7;2912.7;

2905.6;2840.8

(250.9)

3121.2

(5.1)

3071.4

3121.5

(5.5)

3050.4

ν(C2−

H13)(87)

+ν(C3−

H14)(12)

ν(C2−

H13)(93)

3098.5

(5.1)

3017.5

3094.7

(12.5)

3027.6

ν(C3−

H14)(86)

+ν(C2−

H13)(12)

ν(C5−

H10)(72)

+ν(C6−

H11)(25)

3093.3

(15.6)

3038.6

3089.4

(13.0)

3003.9

ν(C5−

H10)(71)

+ν(C6−

H11)(27)

ν(C3−

H14)(92)

3075.0(17.6)

2999.7

3075.7

(18.9)

2998.9

ν(C6−

H11)(72)

+ν(C5−

H10)(27)

ν(C6H

11)(75)

+ν(C5−

H10)(25)

3006.1

(21.6)

2926.4

3004.1

(28.0)

2928.0

ν a(C

7H2)

(93)

ν a(C

7H2)

(76)

+ν(C8−

H18)(14)

2980.8

(68.8)

2926.9

2981.6

(23.1)

2937.9

ν(C8−

H18)(89)

ν(C8−

H18)(80)

+ν a(C

7H2)

(10)

2959.7

(19.0)

2918.0

2949.5

(27.7)

2896.4

ν a(C

7H2)

(87)

ν a(C

7H2)

(81)

+ν a(C

7H2)

(15)

2877.6

(67.5)

2801.0

2888.0

(61.8)

2806.5

ν(C8−

H17)(98)

ν(C8−

H17)(98)

1620.1;1615.8(49.5)

1618.7;1616.2

(67.8)

1619.8

(47.4)

1567.4

1622.1

(33.0)

1638.6

scis(N

H2)

(76)

+ν b(C

C)(16)

scis(N

H2)

(101)

1615.8

(36.5)

1597.0

1617.8

(40.2)

1603.9

scis(N

H2)

(25)

+ν b(C

C)(47)

+δ b(C

H)

(13)

ν b(C

C)(63)

+δ b(C

H)(18)

+δ(R2)

(10)

1602.8;

1597.1

(12.8)

1593.4

(11.4)

1590.1

1594.6

(14.1)

1590.3

ν c(C

C)(65)

ν c(C

C)(66)

1599.1(21.3)

1517.7

(198.7)

1513.8

(181.8)

1508.1

(104.7)

1508.7

1509.3

(107.8)

1499.7

δ c(C

H)(50)

ν c(C

C)(36)

δ c(C

H)(51)

ν c(C

C)(37)

n.o.

n.o.

1476.4

(2.2)

1463.0

1483.2

(3.4)

1474.8

scis(C

8H2)

(99)

scis(C

8H2)

(88)

+scis(C

7H2)

(13)

n.o.

n.o.

1449.9

(7.5)

1447.8

1460.0

(2.6)

1452.9

scis(C

7H2)

(93)

scis(C

7H2)

(86)

+scis(C

8H2)

(13)

1444.7;1434.0(41.1)

14385(26.7)

1432.4

(17.3)

1432.0

1433.0

(16.2)

1433.2

ν d(C

C)(39)

+δ d(C

H)(27)

ν d(C

C)(41)

+δ d(C

H)(28)

1384.0;1382.2(12.7)

1382.6

(14.4)

1384.1

(14.6)

1380.6

1386

(15.9)

1373.4

wag(C

8H2)

(74)

+rock(N

H2)

(16)

wag(C

8H2)

(74)

+rock(N

H2)

(13)

n.o.

1354.0

(6.1)

1329.1

(26.5)

1326.8

δ a(C

H)(44)

+ν f(C

C)(26)

+δ(OH)(19)

n.o.

1335.9

(3.4)

1331.1

(10.3)

1333.4

wag(C

7H2)

(47)

+δ a(C

H)(15)

+twist(C8H

2)(17)

1330.4;1327.4(25.2)

1329.3;1326.9

(13.3)

1329.0

(21.4)

1326.6

1321.6

(1.5)

1312.6

ν f(C

C)(31)

+δ a(C

H)(34)

+δ(OH)(18)

wag(C

7H2)

(30)

+ν f(C

C)(21)

+δ a(C

H)

(14)

+twist(C7H

2)(13)

1317

(4.9)

1319.9:1315.4

(3.9)

1317.4

(4.6)

1314.3

1315.7

(3.0)

1303.3

ν f(C

C)(28)

+wag(C

7H2)

(17)

+δ a(C

H)

(16)

+twist(C7H

2)(18)

twist(C8H

2)(24)

+ν r(C

C)(21)

+wag

(C7H

2)(18)

+twist(C7H

2)(13)

n.o.

1303.4:1297.8

(3.1)

1299.6

(4.0)

1296.5

1289.6

(2.6)

1277.2

twist(C8H

2)(37)

+rock(N

H2)

(14)

+ν f(C

C)(13)

+δ a(C

H)(11)

twist(C8H

2)(46)

+wag(C

7H2)

(15)

n.o.

n.o.

1247.4

(107.7)

1244.9

ν(C1−

O12)(49)

+ν a(C

C)(15)

+δ c(C

H)

(15)

1262.2;1259.0;1257.6

(101.9)

1258.5;1254.2(84.9)

1247.1

(111.0)

1245.9

1234.7

(3.6)

1222.8

ν(C1−

O12)(50)

+ν a(C

C)(15)

+δ c(C

H)

(15)

twist(C7H

2)(21)

+rock(C

8H2)(18)

+rock

(NH

2)(15)

+wag(C

8H2)

(13)

n.o.

n.o.

1198.5

(23)

1198.7

ν(C4−

C7)

(33)

+ν a(C

C)(16)

+wag

(C7H

2)(13)

+δ c(C

H)(13)

1199.7;1192.6(19.8)

1197.2

(2.9)

1187.8

(10.9)

1185.9

1199.2

(1.3)

1198.3

twist(C7H

2)(47)

ν(C4−

C7)

(34)

+ν a(C

C)(18)

+δ c(C

H)

(14)

+wag(C

7H2)

(13)

n.o.

n.o.

1168.1

(5.0)

1179.7

1168.2

(5.2)

1176.2

δ b(C

H)(76)

+ν b(C

C)(21)

δ b(C

H)(76)

+ν b(C

C)(21)

1173.8;1170.5;1167.3

(136.4)

1171.2:1169.2;1167.1:

1163.8

(189.3)

1160.9

(155.7)

1163.2

1162.1

(169.4)

1164.0

δ(OH)(52)

+ν f(C

C)(17)

δ(OH)(56)

+ν f(C

C)(11)



Table

2.continued

experim

entalνa(Ib)

calculated

νc(A

th)

PEDd(%

)

Ar

Xe

G′g′h

arm.scaled

G′g′a

nhAgharm

.scaled

Aganh

G′g′

Ag

n.o

n.o.

1134.2

(1.1)

1131.8

1128.6

(2.6)

1124.2

rock(N

H2)

(36)

+twist(C8H

2)(25)

rock(N

H2(21)

+twist(C7H

2)(20)

+δ d(C

H)(16)

+twist(C8H

2)(14)

1106.7;1103.4(21.6)

1100.6;1097.9

(24.9)

1102.3

(18.5)

1109.1

1098.5

(16.3)

1099.9

δ d(C

H)(49)

+ν d(C

C)(27)

δ d(C

H)(39)

+ν d(C

C)(25)

1084.2

(3.6)

1071.8

(12.2)

1071.7

(12.7)

1063.0

1053.2

(32.8)

1046.7

ν(C8−

N9)

(47)

+ν(C7−

C8)

(23)

ν(C8−

N9)

+(61)

+ν(C7−

C8)

(18)

1016.1

(4.2)

n.o.

1018.9

(8.1)

1012.8

1018.8

(3.5)

1012.4

rock(C

8H2)

(25)

+rock(C

7H2)

(23)

+ν(C8−

N9)

(22)

ν(C8−

N9)

(22)

+rock(C

8H2)

(22)

+rock

(C7H

2)(18)

+twist(C7H

2)(12)

n.o.

n.o.

1006.3

(0.9)

1014.0

1006.8

(0.05)

1012.2

δ(R1)

(45)

+ν c(C

C)(37)

+δ c(C

H)(16)

δ(R1)

(45)

+ν c(C

C)(38)

+δ c(C

H)(16)

n.o.

n.o.

951.1(0.1)

956.8

950.5(0.0)

960.9

γ h(C

H)(102)+γ d(C

H)(11)

γ d(C

H)(70)

+ν(C7−

C8)

(14)

n.o.

n.o.

919.6(0.3)

946.8

942.5(0.4)

949.8

γ d(C

H)(98)

+γ b(C

H)(10)

γ d(C

H)(29)

+γ b(C

H)(25)

+ν(C7−

C8)

(22)

+rock(N

H2)

(11)

n.o.

878.7;

871.4(3.6)

896.1(0.6)

892.3

913.1(0.3)

934.2

ν(C7−

C8)

(37)

+rock(C

8H2)

(15)

+rock

(NH

2)(12)

+ν a(C

C)(10)

γ b(C

H)(85)

+γ d(C

H)(16)

n.o.

851.2(3.8);834.2(3.4)

859.9(7.5)

866.2

842.8(12.6)

846.4

rock(C

7H2)

(22)

+γ c(C

H)(13)

+ν(C7−

C8)

(12)

+ν(C8−

N9)

(12)

ν a(C

C)(45)

+δ(R2)

(11)

+ν(C4−

C7)

(10)

826.7(8.1)

826.1;

822.9;

820.9

(44.3)

825.6(24.6)

821.1

822.1(28.0)

831.0

ν a(C

C)(41)

+ν(C1−

O12)(17)

+δ(R2)

(15)

γ c(C

H)(69)

+γ(C1−

O12)(11)

822.0(17.2)

808.0;

802.0(119.9)

815.5(5.4);808.5;807.0;

805.7;

802.7(122.2)

820.3(41.1)

819.6

810.9(91.0)

778.1

810.3(131.4)

776.7

wag(N

H2)

(63)

+γ c(C

H)(20)

wag(N

H2)

(72)

+rock(C

7H2)

(13)

794.3(8.8)

789.2;

787.3(14.4);

780.4(5.5)

795.1(18.5)

807.7

795.6(14.6)

807.5

γ a(C

H)(83)

γ a(C

H)(74)

+γ c(C

H)(11)

767.9;

765.9(35.8)

766.6(15.9)

771.4(7.7)

775.1

rock(C

8H2)

(30)

+rock(C

7H2)

(29)

n.o.

n.o.

761.8(24.6)

763.7

δ(R1)

(15)

+wag(N

H2)

(13)

+ν(C4−

C7)

(12)

+rock(C

8H2)

(10)

n.o.

723.3(1.1);776.6(2.5)

721.0(4.4)

735.9

τ(R1)

(47)

n.o.

698.3(1.0)

701.0(5.5)

714.4

706.5(0.6)

739.3

τ(R1)

(36)

+ν(C4−

C7)

(19)

+δ(R1)

(15)

τ(R1)

(79)

+γ(Cl−O12)(11)

n.o.

n.o.

641.5(0.6)

649.4

641.9(0.4)

649.1

δ(R3)

(75)

+ν c(C

C)(12)

δ(R3)

(75)

+ν c(C

C)(12)

n.o.

552.2;

551.0(14.2)

547.1(24.8)

557.5

546.5(20.3)

557.8

scis(C

4−C7−

C8)

(17)

+γ(C1−

O12)(15)

+τ(R2)

(13)

+γ(C4−

C7)

(13)

δ(R2)

(25)

+γ(Cl−O12)(12)

+scis(C

4−C7−

C8)

(12)

+τ(R2)

(10)

n.o.

503.5(9.4)

479.3(6.1)

488.9

501.5(21.5)

510.7

γ(Cl−O12)(23)

+δ(R2)

(23)

+τ(R2)

(16)

+scis(C

7−C8−

N9)

(10)

δ(R2)

(25)

+γ(Cl−O12)(22)

+τ(R2)

(18)

+scis(C

7−C8−

N9)

(10)

n.o.

n.o.

425.4(14.0)

432.3

418.3(7.7)

427.1

scis(C

7−C8−

N9)

(27)

+δ(R2)

+(25)

δ(Cl−O12)(51)

+δ(C4−

C7)

(16)

+τ(R3)

(14)

+δ(R3)

(14)

n.o.

n.o.

417.7(2.8)

422.0

412.2(2.3)

418.5

δ(Cl−O12)(43)

+τ(R3)

(18)

+δ(C4−

C7)

(11)

+δ(R3)

(11)

τ(R3)

(101)

n.o.

n.o.

411.8(2.7)

418.9

374.0(3.2)

380.2

τ(R3)

(96)

+δ(C1−

O12)(11)

τ (R1)

(32)

+γ(C1−

O12)(23)

+γ(C

4−C7)

(18)

+scis(C

4−C7−

C8)

(15)

n.i.

n.i.

354.9(1.1)

356.4

320.8(12.5)

324.8

τ(R1)

(16)

+γ(C8−

N9)

(13)

+γ(C1−

O12)

(12)

+scis(C

7−C8−

N9)

(12)

scis(C

7−C8−

N9)

(46)

+δ(R2)

(17)

+ν(C4−

C7)

(15)

n.i.

n.i.

296.6(12.9)

310.6

3061(3.3)

310.1

δ(C4−

C7)

(33)

+scis(C

7−C8−

N9)

(14)

+δ(C1−

O12)(11)

δ(C4−

C7)

(55)

+δ(C

1−O12)(20)

+ν d(C

C)(11)

n.i.

n.i.

287.2(106.2)

373.2

706.5(0.6)

412.4

γ(OH)(90)

γ(OH)(99)

n.i.

n.i.

265.4(62.9)

212.6

641.9(0.4)

183.0

τ(NH

2)(84)

τ(NH

2)(87)

n.i.

n.i.

195.3(6.9)

191.2

546.5(20.3)

202.9

τ(R2)

(27)

+scis(C

4−C7−

C8)

(23)

+γ(C8−

N9)

(15)

τ(R2)

(48)

+scis(C

4−C7−

C8)

(29)



in particular, the predicted vibrational shifts of the G′g′ and Agconformers are in good agreement with the separation of thetwo observed components in the xenon matrix spectra.The intensity of the ν(NH2) bands is predicted to be very

low, and these bands are hardly discernible in the measuredspectra. The calculations do not predict a strong effect on theν(NH2) frequencies caused by the intramolecular N−H···πhydrogen bond. The shifts in the calculated frequencies for theconformers bearing this interaction in relation to the Gg′ andG′g conformers, in which this interaction is absent, are within afew wavenumbers. The proposed assignments for these modesare then tentative, due to their low intensity.The ν(CH) modes can be roughly separated into two

groups, i.e., the aromatic CH stretching, giving rise to bandsabove 3000 cm−1, and the aliphatic CH2 stretching vibrations,expected to absorb below 3000 cm−1. Although the theoreticalpredictions agree fairly well with the experimental findings,these modes cannot distinguish between the different confor-mations because this region of the spectrum is very congested.

3.1.2. 1850−1000 cm−1 Region. The main vibrationalmodes observed in this region, though giving rise to very in-tense bands, are not specific for distinguishing the variousconformers. Nevertheless, the general agreement between thecalculated (particularly in the case of the scaled harmonic calcu-lations) and experimental data is rather good. As for ν(OH),these bands appear with several components (especially in Armatrix), which can be ascribed to matrix site splitting and thepresence of different conformers in the matrix.The band exhibiting the largest peak intensity in the argon

matrix spectrum, observed at 1518 cm−1 (1514 cm−1 in Xe),originates from the all-in-phase in-plane bending CH vibrationof the aromatic ring and is predicted at very close values for allconformers. The ν(CO) mode is observed at ca. 1260 (Ar) and1254 (Xe) cm−1, also in good agreement with the theoreticalpredictions. The intense band at 1170 cm−1 (1169 cm−1 in Xe)is assigned to the δ(OH) in-plane bending mode. Compared tothese three bands, all the remaining features observed in thisspectral region are, in agreement with the theoretical pre-dictions, very much less intense (see Table 2 for detailedassignments).

3.1.3. 1000−400 cm−1 Region. Unlike the other regions ofthe spectrum, the differences of the calculated frequencies forthe various conformers in this spectral region are adequate tobe resolved and allow tentative assignment of bands to differentconformers. Figure 6 shows a closer view of the 900−750 cm−1

region of the experimental spectra and compares it with thepredicted spectra of the G′g′ and Ag conformers. To aid in thevisual comparison of the calculated and measured spectra, therelative intensity of the calculated bands has been scaledaccordingly with the theoretical predictions.The most intense absorption in this region is due to the

NH2 wagging mode, predicted at ca. 800 cm−1. The band at767 cm−1 is clearly distinguishable from the other componentsand shall be ascribed to the Ag′/Ag conformers exclusively.In turn, the bands at ca. 825 cm−1 are also clearly separatedfrom the others, and are distinctive of the Gg/G′g′ conformers.The bands between 775 and ca. 815 cm−1, on the other hand,are more difficult to doubtlessly assign based only on thecomparison of the experimental spectra under discussion andthe calculated data. However, as shown in sections 3.2 and 3.3,annealing and UV-irradiation experiments in xenon matrixallowed us also to make additional safe detailed band assign-ments to individual conformers in this range (see Table 2).T

able

2.continued

experim

entalνa(Ib)

calculated

νc(A

th)

PEDd(%

)

Ar

Xe

G′g′h

arm.scaled

G′g′a

nhAgharm

.scaled

Aganh

G′g′

Ag

n.i.

n.i.

128.3(2.8)

122.0

501.5(21.5)

86.6

γ(C8−

N9)

(58)

+τ(R2)

(25)

γ(C8−

N9)

(86)

n.i.

n.i.

72.6(0.1)

74.5

418.3(7.7)

73.3

γ(C4−

C7)

(34)

+scis(C

4−C7−

C8)

(28)

+τ(R2)

(24)

+γ(C8−

N9)

(11)

γ(C4−

C7)

(40)

+τ(R2)

(33)

+scis(C

4−C7−

C8)

(21)

n.i.

n.i.

36.6(0.2)

56.9

412.2(2.3)

30.4

τ(C7−

C8)

(99)

τ(C7−

C8)

(92)

aWavenum

bers

ofthestrongestcomponentsof

split

bandsaregivenin

italics.bRelativeintegrated

intensities.c Theoretical

positio

nsof

absorptio

nbandswerescaled

usingtheem

pirical

form

ula

ν scaled=0.978ν

unscaled.dPE

Dslower

than

10%

arenotincluded.D

efinitio

nof

internalcoordinatesisgivenin

TableS1

intheSupportin

gInform

ation.

e n.o.,notobserved;n.i.,notinvestigated.



It is interesting to compare tyramine amino group vibrationswith those of the model compound n-propylamine. The lattercompound, like tyramine, may also form two types of con-formers around the CCCN backbone, gauche and anti. As intyramine, the wagging vibrations of the gauche forms of matrix-isolated n-propylamine were found at higher wavenumbers thanthose due to anti forms.27 Unlike tyramine, n-propylaminecannot establish intramolecular H-bond interactions. Interest-ingly, the gauche forms of n-propylamine have their NH2wagging modes at lower frequencies than in gauche-tyramine.The increase of frequency of NH2 wagging modes usuallyoccurs when the corresponding groups are involved in H-bondinteractions.16 The observed increase of NH2 wagging fre-quency in tyramine, compared to n-propylamine, is consistentwith the presence of a weak stabilizing interaction in gauche-tyramine.CH out-of-plane vibrations in phenol produce a very strong

IR band around 750 cm−1. Due to proximity with NH2 wag-ging, CH out-of-plane vibrations could also contribute around800 cm−1 in tyramine, along with NH2 wag. This may contrib-ute to the complexity of observed spectrum around 800 cm−1.We refer also to spectra of a similar molecule, propylbenzene,isolated in argon matrix. Instead of an NH2 group, propyl-benzene has a CH3 group at the same position as in tyramine. Itshows a strong infrared band around 748 cm−1, and nothingparticularly strong around 800 cm−1. This can be proof that inour case the bands around 800 cm−1 are due to the aminogroup wagging modes.Another interesting analogy with tyramine is that both

gauche and anti conformers of propylbenzene were trapped inargon matrixes at 12 K, without any conformational cooling.This should reinforce our interpretation, confirming that itshould be possible to trap also gauche (Gx) and anti (Ax) con-formers of tyramine.3.2. Annealing of the Matrixes. Annealing experiments

were performed in both argon and xenon matrixes to aid inthe interpretation of the spectra, in particular to confirm thepresence of different conformers in the matrixes. Due to itsphysical characteristics, annealing of the argon matrix (depos-ited at 15 K) was carried out only up to 30 K, to avoid

destruction of the matrix upon further heating. The xenonmatrix was deposited at 30 K and annealed up to 60 K withoutremarkable alteration of the baseline, which is an indication ofthe physical state of the matrix, and without signals of tyramineaggregation in the recorded spectrum.In the argon matrix, annealing did not produce any band

intensity changes, while in the experiments performed withxenon, changes in the relative intensity of the components ofthe most intense bands started to occur between 40 and 42 K(Figures 7−9). A similar annealing behavior was observed for

rearrangements of conformers of trimethyl phosphate (TMP).28

The rotations of methoxy groups in TMP (similar in size to theethylamino fragment in tyramine) have calculated barriers of ca.

Figure 6. Part of the experimental infrared spectra of TYR isolated infreshly deposited (a) Ar and (b) Xe matrixes, at 15 and 30 K,respectively. (c) Simulated infrared spectra of the G′g′ (black line) andAg (red line) conformers. For the details of the simulation, see thecaption of Figure 5.

Figure 7. (a) Experimental infrared spectra of TYR in a freshlydeposited Xe matrix at 30 K (black line) and after annealing at 42 K(red line). (b) simulated spectra for the G′g′ (black line) and Ag (redline) conformers. For the details of the simulation, see the caption ofFigure 5.

Figure 8. (a) OH stretching region of the experimental infraredspectra of TYR in a freshly deposited Xe matrix at 30 K (black line)and after annealing at 42 K (red line). (b) Anharmonic prediction ofν(OH) for the G′g′ (black line) and Ag (red line) conformers.Absorption bands were simulated using Lorentzian profiles centered atthe B3LYP calculated (nonscaled) anharmonic frequency and havingfwhm = 2 cm−1. Infrared intensities were taken from the harmonicB3LYP calculation and then scaled by 0.71 and 0.29, respectively,according to the calculated relative population of the conformers.



10 kJ mol−1, and internal conversion around these barriersin TMP in xenon matrixes was observed at temperaturesabove 40 K.Figure 7 shows the observed changes in the region of the

NH2 wagging vibration, compared with the frequency scaledharmonic predicted spectra for the G′g′ and Ag conformers.Relative intensities are scaled according to the calculated rel-ative population of the conformers. The OH stretching and theOH bending regions are shown in Figures 8 and 9, respectively.In these cases comparison with anharmonic calculations is alsopresented.The anharmonic prediction in the NH2 wagging region is still

very shifted from the experimental values and does not aid theinterpretation. Indeed, the amino group wagging vibrationsare known to be problematic for anharmonic simulation.29,30

The large deviation from the experimental value for this modeis in agreement with anharmonic calculations performed onglycine.31 The components that increase upon annealing fit wellthe calculated spectrum of the anti (Ax) conformers. Theseconformers are predicted by the calculations to be less stablethan the gauche forms. Hence, based on the calculated relativeenergies, the reverse conversion could be expected, sinceannealing of the matrix must lead the conformational system toapproach the low-temperature equilibrium, i.e., as a generalrule, to conversion of the less stable conformers into the morestable ones. These results suggest that, at least in the highlypolarizable xenon matrix, the anti conformers become morestable than the gauche forms. Nevertheless, the putative conflictbetween the experimental and calculated data may also be aresult of insufficient accuracy of calculations to predict eitherrelative conformational energies or vibrational spectra.Conclusions about the relative stability of the two con-

formers in the argon matrix could not be extracted due to thelack of observation of any spectral changes in the annealingexperiments performed in this matrix. However, we can assumethat the barrier is large enough to prevent the conformationalinterconversion, for those forms that are trapped in the matrix,in the temperature range accessible for experiments with argon.3.3. Narrow-Band UV Irradiation Experiments. Freshly

deposited argon and xenon matrixes of tyramine were irradiatedusing narrow-band tunable UV laser light, through a quartz

window. In the gas phase, tyramine exhibits an absorption bandin the UV spectrum with origin at about 282 nm.7 In theperformed experiments, irradiation of the deposited sampleswas then undertaken starting at 290 nm and ending at 270 nm,with steps of 1 nm. The irradiation time for each step was2 min.Taking into account previous results in related com-

pounds,32−35 for matrix-isolated tyramine the expected photo-chemical processes should be mainly photoisomerizations(Scheme 2). Depending on the reaction site of the molecule,

isomerization processes in tyramine may lead to three classesof products: isomerization of the ethylamine side chain candeplete one of the gauche or anti conformers in favor of theother one (path A in Scheme 2); isomerization of the phenolring can lead either to production of a Dewar isomer (path B)or an open ring conjugated ketene (path C).The formation of a Dewar isomer could be easily excluded,

since no new bands were observed in the OH stretching regionof the spectra of the irradiated matrixes (the OH stretchingvibrations for the Dewar forms are calculated at least 16 cm−1

away from the ones of the other conformers of TYR).On the other hand, new bands in the 2100−2150 cm−1

region started to appear at the early stages of irradiation at290 nm, in the spectra of both argon and xenon irradiated tyra-mine matrixes. The presence of these bands is an unequivocalindication of the formation of a ketene species as can be de-duced from the infrared spectra of ketenes reported in the lit-erature.36,37 For ketenes, the shapes of absorption bands of thesame species are different in different environments, and evenin the same environment; the discussion of this effect is nottrivial and requires a dedicated study which is beyond the scopeof this paper. Ketenes have been found to be main photo-products resulting from the opening of aromatic six-memberedrings bearing oxygen-containing substituents.32−35 The ketenemarker bands in the 2100−2150 cm−1 region continuously in-creased along the performed irradiation experiments. Figure 10shows this region of the spectra obtained during a series ofirradiations of the argon and xenon matrixes. As can be seenfrom the total intensity of the new bands, the rate of the ring-opening reaction is quite low.Irradiation of the xenon matrix at 287 nm has shown, in

addition to the bands associated with the ketene species, resultssimilar to those obtained during the annealing experiments.Some components of the most intense bands, assigned to the

Figure 9. (a) Experimental infrared spectra of TYR in a freshlydeposited Xe matrix at 30 K (black line) and after annealing at 42 K(red line). (b) Simulated anharmonic spectra for the G′g′ (black line)and Ag (red line) conformers. For details of the simulation, see thecaption of Figure 8.

Scheme 2. Possible Reaction Pathways after UV Irradiationof Matrix-Isolated TYR



Ag conformer, increased, while bands ascribed to the G′g′ formdecreased. It can then be concluded that in the xenon matrixthe photoisomerization of gauche form into anti form occurssimultaneously with the ring-opening reaction. On the otherhand, irradiation of the argon matrix did not lead to a clearobservation of the gauche→ anti isomerization process, with allthe components of the recorded bands being observed todecrease simultaneously. In argon matrix, the single reliablyobserved photoprocess was then the formation of the isomericopen ring conjugated ketene.

4. CONCLUSIONSThe infrared spectra of tyramine isolated in low-temperatureargon and xenon matrixes were described for the first time. Thespectra of the as-deposited matrixes are compatible with thepresence of at least two types of conformers: one has beenassigned to the most stable gauche forms about the CH2−CH2bond (G′g′ + Gg) and the other one has been assigned to themost stable anti type conformers (Ag′ + Ag). The most stableconformer in the gas phase (Gg) has a folded structurestabilized by the presence of a weak N−H···π hydrogen bondinteraction. Observation of both gauche and anti forms in theas-deposited matrixes is consistent with the calculated relativevalues for the different conformational isomerization barriers,with that associated with the rotation around the CH2−CH2bond, involving rotation of heavy atoms, being predicted to bethe highest among all interconversion barriers. Annealingexperiments performed in the xenon matrix, compared withharmonic and anharmonic predictions for the most stable con-formers, seem to confirm the presence in the deposited matrixof the gauche and anti conformers and the occurrence of thegauche → anti isomerization reaction upon temperature in-crease. This last result indicates a change in the relative energiesof the two experimentally relevant conformers upon isolationin the polarizable xenon matrix, compared to the gas phase,with the more polar anti form being stabilized in the matrixenvironment.

New bands formed after UV irradiation of the matrix-isolatedtyramine (both in argon and xenon matrixes) are consistentwith the formation of a conjugated ketene species, generated viaring-opening reaction, which has already been shown to occurfor similar six-membered-ring compounds bearing oxygen-containing substituents.32−35 Concomitant occurrence of thegauche → anti isomerization reaction was observed in the xe-non matrix. For both matrixes, no evidence of photoinducedformation of Dewar isomers of tyramine has been found.

■ ASSOCIATED CONTENT*S Supporting InformationInternal coordinates used in normal mode analyses for tyramine(Table S1) as well as calculated harmonic and anharmonicwavenumbers, absolute intensities, and potential energydistribution for seven tyramine conformers at the B3LYP/6-311++G(d,p) level of theory (Tables S2 and S3), and completeref 10. This material is available free of charge via the Internetat http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: [email protected]. Tel.: +351 239 854483.*E-mail: [email protected]. Tel.: +39 051 2099502NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe research leading to these results has received funding fromthe European Community’s Seventh Frame Work Programmeunder Grant Agreement No. 228334, and the Portuguese“Fundacao para a Ciencia e a Tecnologia” (FCT) ProjectsPTDC/QUI-QUI/111879/2009 and PTDC/QUI-QUI/118078/2010, FCOMP-01-0124-FEDER-021082, cofunded byQREN-COMPETE-UE. B.M.G. acknowledges FCT for thepostdoctoral grant SFRH/BPD/44689/2008.

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Figure 10. Region of the CCO antisymmetric stretchingvibration in the infrared spectra of TYR isolated in (a) argon and(b) xenon matrixes, in the freshly deposited matrix (black line) andafter UV irradiations.



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mailto:[email protected]

mailto:[email protected]

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