Ab initio and density functional study of the resonance Raman spectra of Methyl Red, Ethyl Red and their protonated derivatives

JOURNAL OF RAMAN SPECTROSCOPY, VOL. 29, 447È462 (1998)

Ab Initio and Density Functional Study of theResonance Spectra ofRaman Methyl Red, Ethyl

and Their Protonated DerivativesRed

Stephen Bell, Alison Bisset and Trevor J. Dines*Department of Chemistry, The University, Dundee DD1 4NH, UK

Resonance Raman (RR) spectra of the acid–base indicators Methyl Red and Ethyl Red and their monoprotonatedderivatives were recorded using excitation in the region of their lowest energy electronic absorption bands. Themolecular geometries and vibrational spectra of Methyl Red and its protonated derivative were calculated using abinitio (HF/3–21G) and density functional (B3-LYP/3–21G) methods. Force constants and potential energy dis-tributions were determined from the calculated spectra, after appropriate scaling. Comparison of the RR spectrawith the calculated Raman band positions enabled precise vibrational band assignments to be made. The RRspectra provide clear evidence for the structural and electronic changes that accompany protonation. 1998 John(

Wiley & Sons, Ltd.

J. Raman Spectrosc. 29, 447È462 (1998)

INTRODUCTION

In the last 20 years there has developed considerableinterest in the ab initio calculation of vibrational wave-numbers and IR and Raman intensities.1h4 Much ofthis work has been centred on relatively small mol-ecules, but the availability of more powerful computa-tional resources has now made it possible to carry outcalculations on much larger molecules, although this isusually with smaller basis sets and less rigorousmethods. The ability to compute force constants, dipolederivatives and polarizability derivatives is crucial tothe interpretation of the vibrational spectra of largermolecules. For molecules of up to about 10È15 atoms acomplete normal coordinate analysis is generally pos-sible, based on experimental data alone. This is animpossible task for larger molecules owing to theextremely large number of force constants in relation tothe number of observable IR and Raman bands. Fur-thermore, it is likely that the complete set of 3N [ 6vibrational modes may not be observed, since it is morelikely that some will have wavenumbers that are suffi-ciently close for them not to be resolved, and some mayyield extremely weak bands in both the IR and Ramanspectra. The interpretation of resonance Raman (RR)spectra of larger molecules, especially those with little orno symmetry, can be especially difficult owing to theselective nature of resonance enhancement.

Ab initio calculations provide a complete set of forceconstants, from which a potential energy distributionmay be determined for every normal coordinate.However, calculations at the HartreeÈFock level yieldforce constants that are consistently high for two

* Correspondence to : T. Dines, Department of Chemistry, TheUniversity, Dundee DD1 4NH, UK.

reasons. First, such calculations tend to give excessivelyshort bond lengths, even with large basis sets, andsecond, the harmonic oscillator approximation isassumed. The latter leads to abnormally large force con-stants for vibrations which are known to be appreciablyanharmonic, e.g. for the stretching of bonds involvinghydrogen atoms. Typically, force constants computed atthe HF-SCF level require scaling by factors of the orderof 0.80È0.85, and are not generally improved by invok-ing larger basis sets than 3È21G. More realistic calcu-lations of both molecular geometries and force Ðeldsrequire calculation at the MP2 level with at least a6È31G basis set. MP2/6È31G calculations generallyyield vibrational wavenumbers which are within 2È3%of the experimental values, whence most of the devi-ation is accounted for by anharmonicity and possiblyalso Fermi resonance.

In the last few years, hybrid density functionalmethods, particularly B3-LYP, have become popular5h7since they provide vibrational wavenumbers that arecomparable to or better than those obtained at theMP2 level, but require signiÐcantly less computationtime. For the large molecules considered in studies suchas this one, when only small basis sets can be usedbecause of limits on computer resources, although agood Ðt is obtained to vibrational spectra using DFTmethods this is obtained at the expense of poor agree-ment with experimental geometrical structures. Forsmall molecules, when large bases can be employed withthe B3-LYP method, excellent agreement is obtainedwith experimental bond lengths.8 It has been noted9 fora number of compounds with CxO and CyC bondsthat DFT calculations yield bond lengths much nearerto experimental data than those obtained by HF calcu-lations, which are very short, or MP2 methods, whichare very long. DFT methods are now being applied tolarger and larger molecules.10h13 In the present study,the molecules had up to 36 atoms and 212 basis func-tions. The good Ðt to experimental vibrational spectra

CCC 0377È0486/98/060447È16 $17.50 Received 2 December 1997( 1998 John Wiley & Sons, Ltd. Accepted 26 January 1998

448 S. BELL, A. BISSET AND T. J. DINES

of large molecules with small basis sets a†orded byDFT methods may be fortuitous in view of the poorbond lengths calculated and the success of these calcu-lations in modelling vibrational spectra of large mol-ecules may come about by a kind of cancellation oferrors.

The calculation of vibrational spectra of moleculescontaining more than 20 atoms becomes prohibitivelyexpensive at the MP2 level and lower levels of theorymust suffice. It follows, therefore, that studies of largerpolyatomic molecules must be restricted to the HF orDFT methods with a modest basis set, although suchcalculations can still be of tremendous beneÐt in theinterpretation of vibrational spectra. In this paper wereport the results of HF/3È21G and B3-LYP/3È21Gcalculations and normal coordinate analyses of 4-(N,N-dimethylamino)azobenzene-2@-carboxylic acid (MethylRed, MR) and its 4-diethylamino homologue (EthylRed, ER) in their unprotonated and protonated states,and an assignment of the RR spectrum.

Derivatives of 4-amino-trans-azobenzene are animportant class of dyes for both natural and syntheticÐbres. They are also widely used as acidÈbaseindicators14 and as spectrophotometric probes inprotein chemistry.15 Such dyes undergo a colour changeon protonation, due to changes in their molecular andelectronic structure. Intense electronic absorption in thevisible region allows their investigation by resonanceRaman (RR) spectroscopy, and RR studies of severalazo dyes have been reported.16h33 As part of an RRinvestigation of adsorption on acidic sites on oxidesurfaces34h37 we have examined in detail the spectra of

a wide range of azobenzene derivatives in order toassess their suitability as probes of surface acidity.

MR is a weakly basic azo dye with andpKin\ 4.8,29is a useful pH indicator in the range 4.4È6.0, in which itchanges colour from yellow in basic or neutral solutionto red in acidic solution. In recent years its use as anindicator has declined since it is readily reduced, withconsequent loss of color. ER is used in the(pKin\ 5.0)pH range 4.5È6.5 and exhibits an identical colourchange. The structures of MR and ER in their depro-tonated (I), monoprotonated (II) and diprotonated (III)forms are shown in Fig. 1. Protonation may initiallyoccur at either of two sites, resulting in a tautomericmixture of the azo (IIa and b) and hydrazo (IIc) forms.The azo tautomer exists in only very small amounts andthe diprotonated species (III) is formed at much higheracid strengths. The absorption maxima for these speciesare at much shorter wavelength and they therefore donot contribute signiÐcantly to RR spectra recorded withvisible excitation. We have investigated the RR spectraof MR and ER and their monoprotonated derivatives(designated MRH` and ERH`) in the solid state and invarious solvents, using laser excitation close to thelowest energy electronic absorption maxima.

EXPERIMENTAL

MR and ER (Aldrich) were recrystallized from meth-anol and their protonated derivatives were prepared byrecrystallization from 0.1 mol dm~3 HCl in methanol.

Figure 1. Protonation equilibria for Methyl Red and Ethyl Red.

( 1998 John Wiley & Sons, Ltd. J. Raman Spectrosc. 29, 447È462 (1998)

RESONANCE RAMAN SPECTRA OF METHYL RED AND ETHYL RED 449

Solid samples were prepared as discs andKBr/K2SO4contained within a spinning cell to minimize laser-induced decomposition and the Raman scattering col-lected at 90¡. Solutions in water, methanol and toluene(1.0] 10~4È1.0] 10~3 mol dm~3) were contained inglass cells and the Raman scattering was collected at180¡.

Raman spectra were excited by a Coherent RadiationInnova 90-6 argon ion laser and recorded on a Spex1403 spectrometer interfaced to a DM1B computer. Inall cases the laser power was maintained at \100 mWat the sample. Detection was by standard photoncounting techniques with a Hamamatsu R928 photo-multiplier. Data were collected at 2 cm~1 intervals with1 s integration time and spectral slit width 2È4 cm~1,and averaged over Ðve scans. Wavenumber calibrationof the spectrometer was established by reference to theemission spectrum of neon and all spectra were cor-rected for the spectral sensitivity of the spectrometer.

Electronic absorption spectra were recorded on aCary 219 spectrophotometer.

RESULTS AND DISCUSSION

Ab initio calculations

Ab initio calculations were performed using the Gauss-ian 94 program,38 initially at the HartreeÈFock levelwith the 3È21G basis set. Subsequently, density func-tional calculations were carried out using the B3-LYPmethod and the same basis set. The optimized geome-tries of MR and MRH` are displayed in Fig. 2 and thebond lengths and interbond angles obtained with bothmethods are compared in Table 1. The calculatedgeometry of MR is generally in good agreement withthat obtained from the crystal structure,39 althoughthere are some di†erences between the HF and B3-LYPgeometries, the latter mostly yielding signiÐcantlylonger bonds than in the experimental geometry. In itsunprotonated state MR has a non-planar azobenzenechromophore, the dihedral angle between the twobenzene rings being 11.8¡ ; calculations yielded 31¡ (HF)

Figure 2. Optimized geometries (HF/3–21G) for (a) MR and (b) MRH½.



Table 1. Experimental and calculated bond lengths and inter-(A� )bond angles (Ä) for Methyl Red and the calculatedgeometries for protonated Methyl Red

MR MRH½

Calc. Calc.

Bond Expt.39 HF B3-LYP HF B3-LYP

r(C1C2) 1.371 1.384 1.399 1.384 1.400

r(C2C3) 1.381 1.381 1.391 1.380 1.390

r(C3C4) 1.400 1.385 1.403 1.384 1.400

r(C4C5) 1.406 1.390 1.410 1.396 1.415

r(C5C6) 1.394 1.385 1.399 1.389 1.400

r(C1C6) 1.379 1.381 1.392 1.377 1.390

r(C1H17) 1.071 1.083 1.070 1.082

r(C2H16) 1.072 1.084 1.071 1.083

r(C3H15) 1.070 1.082 1.067 1.080

r(C4N7) 1.421 1.422 1.419 1.413 1.408

r(C5C23) 1.512 1.481 1.485 1.478 1.480

r(C6H18) 1.071 1.083 1.070 1.082

r(N7N8) 1.268 1.243 1.298 1.280 1.313

r(N8C9) 1.385 1.412 1.408 1.324 1.353

r(C9C10) 1.401 1.383 1.403 1.420 1.427

r(C10C11) 1.369 1.376 1.383 1.349 1.366

r(C11C12) 1.415 1.403 1.419 1.430 1.434

r(C12C13) 1.426 1.407 1.424 1.440 1.441

r(C13C14) 1.359 1.372 1.379 1.345 1.365

r(C9C14) 1.411 1.388 1.407 1.426 1.429

r(C10H19) 1.071 1.083 1.071 1.082

r(C11H20) 1.068 1.080 1.066 1.079

r(C12N27) 1.361 1.372 1.376 1.321 1.345

r(C13H21) 1.068 1.081 1.066 1.080

r(C14H22) 1.069 1.082 1.071 1.084

r(C23O24) 1.319 1.352 1.379 1.377 1.410

r(C23O26) 1.199 1.206 1.230 1.198 1.220

r(O24H25) 0.968 0.997 0.971 0.997

r(N27C28) 1.470 1.461 1.471 1.481 1.487

r(N27C29) 1.460 1.461 1.471 1.481 1.487

r(C28H30) 1.085 1.098 1.082 1.095

r(C28H31) 1.085 1.098 1.082 1.095

r(C28H32) 1.079 1.091 1.076 1.089

r(C29H33) 1.085 1.098 1.082 1.095

r(C29H34) 1.085 1.098 1.082 1.095

r(C29H35) 1.079 1.091 1.076 1.089

r(N7H36) — — 1.011 1.041

h(C1C2C3) 120.7 119.9 119.8 120.6 120.4

h(C2C3C4) 120.2 120.9 121.2 120.2 120.3

h(C3C4C5) 119.6 119.2 118.7 119.7 119.8

h(C4C5C6) 118.4 119.6 119.5 119.2 119.0

h(C5C6C1) 120.8 121.1 121.1 121.0

h(C6C1C2) 119.7 119.4 119.4 119.2 119.5

h(C6C1H17) 120.1 120.1 120.2 120.1

h(C1C2H16) 120.3 120.3 120.2 120.1

h(C2C3H15) 121.2 121.4 120.6 121.3

h(C5C6H18) 118.4 117.7 117.9 117.6

h(C3C4N7) 122.7 115.7 115.5 120.3 120.5

h(C4C5C23) 124.2 124.7 125.4 125.9 126.1

h(C5C4N7) 117.7 125.1 125.8 120.0 119.7

h(C4N7N8) 114.5 115.0 113.4 121.0 121.0

h(C6C5C23) 117.4 115.6 115.0 114.9 114.9

h(C5C23O24) 117.7 113.0 112.7 114.3 113.2

h(C5C23O26) 120.6 124.8 124.8 125.5 126.3

h(N7N8C9) 116.8 117.1 115.0 123.7 120.4

h(N8C9C10) 115.6 116.9 116.7 116.0 115.3

h(N8C9C14) 126.3 124.5 125.1 126.7 127.3

h(C9C10C11) 121.6 121.3 121.4 122.1 121.9

h(C10C11C12) 120.4 120.8 120.8 120.8 120.9

h(C11C12C13) 122.7 117.2 117.3 117.1 117.1

h(C12C13C14) 120.9 121.2 121.2 121.4 121.5

h(C13C14C9) 120.9 121.1 121.3 121.2

h(C14C9C10) 118.2 118.6 118.2 117.3 117.4



Table 1.ÈContinued

MR MRH½

Calc. Calc.

Bond Expt.39 HF B3-LYP HF B3-LYP

h(C9C10H19) 118.4 117.8 117.4 117.2

h(C10C11H20) 118.8 119.1 119.4 119.2

h(C12C13H21) 120.1 119.7 119.4 119.4

h(C13C14H22) 120.4 121.1 118.2 118.3

h(C11C12N27) 121.7 121.5 121.4 121.7 121.7

h(C13C12N27) 120.4 121.3 121.3 121.2 121.2

h(C12N27C28) 119.8 120.4 120.4 121.4 121.1

h(C12N27C29) 121.5 120.2 120.2 120.9 120.7

h(C23O24H25) 111.5 107.6 111.1 108.6

h(O24C23O26) 121.7 121.9 122.1 120.2 120.5

h(N27C28H30) 111.5 111.5 110.7 110.7

h(N27C28H31) 111.5 111.4 110.7 110.7

h(N27C28H32) 109.0 108.7 108.6 108.5

h(C28N27C29) 118.6 119.4 119.4 117.8 118.2

h(N27C29H33) 111.5 111.5 110.6 110.6

h(N27C29H34) 111.5 111.4 110.6 110.6

h(N27C29H35) 109.0 108.7 108.7 108.6

h(H30C28H31) 108.5 108.4 109.7 109.6

h(H30C28H32) 108.1 108.4 108.5 108.6

h(H31C28H32) 108.1 108.4 108.5 108.6

h(H33C29H34) 108.4 108.4 109.7 109.5

h(H33C29H35) 108.1 108.4 108.6 108.8

h(H34C29H35) 108.2 108.4 108.6 108.8

h(N7N8H36) — — 123.7 125.2

or 37¡ (B3-LYP). The planarity of the parent moleculetrans-azobenzene is open to question. Although x-raydi†raction results40 indicate a planar structure, electrondi†raction measurements41 show that the phenyl ring istwisted by 30¡ around the NxNÈC plane. The non-planar structure is also supported by RamanC

istudies.42Bond lengths calculated at the HF level are usually

shorter than those determined experimentally, but forMR there is an equal distribution between shorter andlonger bonds in the calculations and in most cases thedeviation is small (1È2%). The largest deviation is forthe CxO bond, which the calculated distance at theHF level (1.352 is 2.5% larger than the experimentalÓ)value (1.319 This discrepancy may, in part, be a con-Ó).sequence of a reduction in the bond distance in the solidstate, due to crystal packing forces, which could beresponsible for the peculiarities observed in the solid-state RR spectrum (see below). Calculated bond anglesare mostly within 1È2¡ of the experimental values,although there are some exceptions. In particular, theangle between the ÈNxNÈ group and thecarboxylate-substituted benzene ring di†ers from theexperimental geometry by 7¡. The B3-LYP geometrygives bond lengths which are generally longer thanthose calculated at the HF level, although bond anglesare comparable for the two methods.

No experimental geometry is available for MRH`,but the calculations reveal signiÐcant geometry changesupon protonation. The calculated geometry of the

ring is in agreement with the pos-ÈNMe2-substitutedtulated quinonoid structure, i.e. shortening of theC10ÈC11 and C13ÈC14 bonds and lengthening of thefour other bonds in the ring, a substantial contractionof the N8ÈC9 and C12ÈN27 bonds (by 0.09 and 0.05

respectively) and extension of the NxN bond (byÓ,0.04 However, what is particularly striking is that onÓ).protonation the two benzene rings become coplanar, asalso is the group, giving rise to symmetry. WeNC2 C

stherefore propose that the changes in the electronicstructure attendant upon protonation, responsible forthe red shift of the lowest energy absorption band andits increased intensity, are a consequence of the greaterextent of conjugation and n-electron delocalizationacross the whole molecule in the protonated species.

The vibrational wavenumbers for MR and MRH`were calculated at their optimized geometries usingboth HF and B3-LYP methods. Calculations were alsoperformed using the 6È31G basis set (HF method only),but these gave less satisfactory results, yielding generallylarger vibrational wavenumbers, especially for the CH,NH and OH stretching modes. A B3-LYP/6È31G cal-culation was therefore not attempted. It is commonpractice to scale force constants calculated at the HFlevel using factors such as 0.9 for stretching force con-stants, 0.8 for bending force constants and 1.0 for tor-sions.43 CH stretching wavenumbers are often stillslightly high with a scale factor of 0.9 and for CHstretching force constants alone a scale factor of 0.83 issuitable,44 although for MR and MRH` a more severescaling of 0.8 was found necessary. These were chosento give the best Ðt to the experimental data. The necessi-ty for scaling of the calculated force constants arisesÐrst because the calculation is performed assuming har-monic oscillator behaviour, and second because theHF-SCF and B3-LYP methods do not give an exactmatch to the experimental geometry. Since the devi-ation from experimental geometry di†ers for the varioustypes of bonds and bond angles, it is not surprising thatseveral di†erent scaling factors may be required. For



example, a better Ðt to the experimental geometry isobtained for CH and CC bond lengths than for NxNand CxO bond lengths. Furthermore, the neglect ofanharmonicity leads to wide di†erences in scalingfactors. In particular, vibrations involving motion ofhydrogen atoms are known to be signiÐcantly anhar-monic.

Scaling was done using normal coordinate analysisprograms derived from those of Schachtschneider.45,46Density functional methods gives force constants whichare generally lower than those determined by HF-SCFcalculations, i.e. closer to the experimental values, andless severe scaling is therefore required. Scaling factorsfor both methods are given in Table 2. The unusuallylarge scale factor required for NxN stretching is a con-sequence of the abnormally long bond length, withrespect to the experimental geometry, that is obtainedusing this method. Raman band intensities were alsoobtained from the HF/3È21G calculations, althoughthese are determined on the assumption of ground-statepolarizability derivatives and are therefore inapprop-riate for laser excitation under resonance conditions.

For convenience the benzene ring skeletal coordi-nates have been grouped together as symmetry coordi-nates, assuming microsymmetry, in accordance withD6hthe Wilson notation. Likewise, the methyl group coordi-nates are also grouped as symmetry coordinatesassuming microsymmetry. The use of these sym-C3vmetry coordinates results in simpler and more realisticpotential energy distributions, and facilitates compari-son with other benzene derivatives, including the parentmolecule azobeneze for which detailed vibrationalanalyses have recently been reported.47,48 The scaledvibrational wavenumbers for MR and MRH` are givenin Tables 3 and 4, together with the potential energydistributions, determined using the scaled force Ðeld.The data in these tables are from the HF/3È21G calcu-lations, the scaled B3-LYP/3È21G results (not shown)are within 10È20 cm~1 of these, and there are minordi†erences in the potential energy distributions. Atomnumbers used in the potential energy distributions arethose shown in Fig. 2 ; only coordinates contributingP10% to the potential energy distribution have beenincluded in the list. Scaled force constants, expressed ininternal coordinates, were also obtained from thenormal coordinate analysis ; the principal force con-stants only are given in Table 5.

Examination of the scaled wavenumbers and theirassignments reveals that the CH stretching vibrations ofthe methyl groups and benzene ring chromophores arenot substantially mixed. It is also found most of the CCand CN stretches and deformations and the CH defor-

Table 2. Force constant scaling factors used for thecalculation of vibrational wavenumbers andpotential energy distributions for MR andMRH‘

Vibrational coordinate HF/3–21G B3-LYP/3–21G

l(OH) 0.80 0.99

l(CH), l(NH) 0.80 0.88

l(CO) 0.75 0.93

l(NxN) 0.71 1.15

l(CC), l(CN), deformations 0.82 0.96

mations can be clearly identiÐed with just one of thetwo benzene rings of the molecule. Many of the vibra-tions associated with the carbon atoms of the twobenzene rings are within the ranges expected for ortho-and para-substituted benzene.49,50 The match betweencalculated and experimental band vibrational wave-numbers is remarkably good after appropriate scaling,although the scaling is more severe than that requiredfor smaller molecules. Ab initio HF calculations werealso attempted using a larger basis set (6È31G), but thisyielded results which deviated even further from theexperimental data, requiring more sever scaling than thevibrations determined with the smaller 3È21G basisset. The size of these molecules ruled out the use ofimproved methods such as MP2, which has been usedfor the parent molecule trans-azobenene.47

RR spectrum of Methyl Red

The electronic absorption spectrum of MR in methanoldisplays an intense, broad band at 460 nm, attributed tothe lowest energy electric-dipole-allowed nÈn* tran-sition. On protonation this band is shifted to 500 nmowing to the dominance of the quinonoid structure(IIb). Excitation within the contours of these bandsresults in intense RR spectra, examples of which areshown for methanol solutions of MR and MRH` inFig. 3. The band wavenumbers, relative intensities andassignments for MR and MRH` in various solutionsare listed in Table 6 and 7. For MR the strongest bandis at ca. 1400 cm~1, assigned to the l(NxN) vibration,and other strong bands occur at 1472 and 1603 cm~1,attributed to ring stretching vibrations. The assign-ments were deduced by comparison of the experimentaldata with the calculated band positions given in Table3. The scale factors used for the HF/3È21G force con-stants were chosen to give the best Ðt to the experimen-tal data, with the constraint that internal coordinates ofthe same kind were scaled by the same factor. In propo-sing the RR band assignments, it has been assumed thatthe vibrations of the methyl groups, the benzene ringCÈH stretches and vibrations involving predominantlyout-of-plane motion of the hydrogen atoms are notsubject to resonance enhancement and do not thereforeappear in the RR spectra. It is clear from the assignedRR spectra that MR exists in the azo form in bothpolar and non-polar solvents. The l(NxN) wavenum-ber in MR is signiÐcantly reduced with respect to thatin trans-azobenzene (1443 cm~1), attributed to thee†ects of substituents. For this reason, l(NxN) is notsigniÐcantly mixed with other vibrations. By contrast, intrans-azobenzene l(NxN) contributes to other Ramanbands, although the nature of the mixing di†ers accord-ing to the MP247 and DFT48 interpretations. In theformer it is claimed that the 1473 cm~1 vibrationinvolves CC and NxN stretching, whereas the lattersuggests that is the case for the 1493 cm~1 band. Onlythe 1473 cm~1 band has an analogue in MR (1473cm~1), which is attributed to CC stretching and in-plane CH deformation.

The RR spectra of solutions of MRH` [Fig. 3(b),Table 7] are markedly di†erent from those of theunprotonated species. The strongest band at 1620 cm~1is assigned to the 8a vibration of the N-ring, conÐrming



Table 3. Ab initio (HF/3–21G) scaled vibrational wavenumbers and potential energy distribution for MR

No. l6 /cmÉ1 Potential energy distribution (%)

1 3473 l(OH) (100)

2 3060 l(C11H20) (40) ½l(C13H21) (39) ½l(C14H22) (18)

3 3058 l(C11H20) (54) ½l(C13H21) (31) ½l(C14H22) (12)

4 3043 l(C13H21) (29) ½l(C14H22) (67)

5 3042 l(C1H17) (12) ½l(C2H16) (14) ½l(C3H15) (54) ½l(C6H18) (16)

6 3036 l(C1H17) (15) ½l(C3H15) (30) ½l(C6H18) (54)

7 3023 l(C10H19) (89)

8 3023 l(C1H17) (35) ½l(C2H16) (27) ½l(C6H18) (25)

9 3009 l(C1H17) (35) ½l(C2H16) (56)

10 2955 las

(CH3) (44) ½l

as(CH

3@) (43)

11 2938 las

(CH3) (44) ½l

as(CH

3@) (45)

12 2883 las

(CH3) (54) ½l

as(CH

3@) (45)

13 2880 las

(CH3) (45) ½l

as(CH

3@ ) (54)

14 2847 ls(CH

3) (43) ½l

as(CH

3@ ) (43)

15 2841 ls(CH

3) (45) ½l

as(CH

3@ ) (45)

16 1689 l(CxO) (77)

17 1611 S8b

ÍCË (52)

18 1606 S8a

ÍNË (50)

19 1574 S8a

ÍCË (61) ½d(C3C2H16) (13)

20 1560 S8a

ÍNË (57) ½d(C10C11H20) (12)

21 1550 das

(CH3) (44) ½d

as(CH

3@ ) (44)

22 1537 das

(CH3) (43) ½d

as(CH

3@ ) (44)

23 1532 S19a

ÍNË (14) ½d(C12C13H21) (19) ½d(C13C14H22) (16)

24 1520 das

(CH3) (49) ½d

as(CH

3@) (44)

25 1511 das

(CH3) (44) ½d

as(CH

3@) (49)

26 1496 S19b

ÍNË (18) ½d(C4C3H15) (22) ½d(C5C6H18) (25)

27 1493 ds(CH

3) (41) ½d

s(CH

3@ ) (41)

28 1468 S19a

ÍCË (18) ½d(C2C1H17) (29) ½d(C3C2H16) (30)

29 1462 ds(CH

3) (47) ½d

s(CH

3@) (47)

30 1449 S19b

ÍNË (19) ½S21b

ÍNË (10) ½d(C12C13H21) (14)

31 1403 l(NxN) (68) ½d(C10C11H20) (10)

32 1355 d(C9C10H19) (20) ½d(C12C13H21) (11) ½d(C13C14H22) (17)

33 1353 l(C5C23) (11) ½l(CwO) (14) ½d(COH) (30) ½d(C5C23O26) (11)

34 1339 l(C12N27) (35) ½oip

(CH3@) (11)

35 1298 d(C4C3H15) (27) ½d(C5C6H18) (27)

36 1277 S14

ÍNË (12) ½las

(NC) (32) ½oip

(CH3) (14) ½o

ip(CH

3@ ) (12)

37 1240 S8a

ÍNË (16) ½l(C9N8) (16) ½d(C10C11H20) (12) ½d(C12C13H21) (15)

38 1227 S14

ÍCË (16) ½d(C2C1H17) (27) ½d(C3C2H16) (22) ½d(C4C3H15) (11)

39 1201 l(C4N7) (17) ½d(C13C14H22) (12)

40 1187 oip

(CH3) (20) ½o

ip(CH

3@ ) (26)

41 1178 S12

ÍCË (11) ½l(C5C23) (17) ½d(COH) (28)

42 1175 S14

ÍNË (66)

43 1152 S14

ÍCË (34) ½l(C9N8) (10)

44 1139 oop

(CH3) (41) ½o

op(CH

3@ ) (53)

45 1138 oop

(CH3) (50) ½o

op(CH

3@ ) (39)

46 1136 S19b

ÍNË (23) ½S21b

ÍNË (12) ½d(C9C10H19) (14) ½d(C10C11H20) (10) ½d(C13C14H22) (13)

47 1127 S14

ÍCË (33) ½S1ÍNË (11) ½l(C9N8) (12)

48 1110 S12

ÍCË (30) ½S19a

ÍCË (18) ½S21a

ÍCË (10)

49 1091 u(C13H21) (26) ½u(C14H22) (76)

50 1087 u(C1H17) (33) ½u(C2H16) (42) ½u(C3H15) (22) ½u(C6H18) (15)

51 1070 u(C10H19) (75) ½u(C11H20) (29)

52 1064 l(C–O) (27)

53 1061 las

(NC) (34) ½oip

(CH3) (24) ½o

ip(CH

3@) (24)

54 1049 u(C1H17) (17) ½u(C3H15) (29) ½u(C6H18) (43)

55 1023 S1ÍCË (41) ½S

19bÍCË (17) ½l(C–O) (12)

56 1011 S12

ÍNË (51) ½S19a

ÍNË (24) ½S21a

ÍNË (12)

57 971 u(C1H17) (13) ½u(C2H16) (17) ½u(C3H15) (23) ½u(C6H18) (20) ½u(C4N7) (10)

58 928 ls(NC) (39)

59 919 u(C11H20) (11) ½u(C13H21) (19) ½u(C12N27) (14)

60 915 u(C11H20) (14) ½u(C13H21) (12) ½u(C12N27) (11)

61 877 u(C10H19) (14) ½u(C11H20) (39) ½u(C13H21) (34) ½u(C14H22) (11)

62 844 S4ÍCË (24) ½u(C3H15) (11) ½u(C5C23) (13) ½q(C5C23) (25)

63 832 S1ÍNË (14) ½S

12ÍCË (10) ½l(C4N7) (10)

64 814 S1ÍNË (10) ½u(C1H17) (21) ½u(C2H16) (15) ½u(C6H18) (10)

65 798 S4ÍNË (64) ½u(C9N8) (10) ½u(C12N27) (13)

66 742 S6a

ÍNË (16) ½S6b

ÍCË (12) ½ls(NC) (10)

67 739 S4ÍCË (25) ½q(C5C23) (47)



Table 3.ÈContinued

No. n6 /cmÉ1 Potential energy distribution (%)

68 713 S6a

ÍCË (18)

69 661 S8b

ÍNË (14) ½S6b

ÍNË (72)

70 640 S6a

ÍCË (46Ë

71 624 S6b

ÍCË (24) ½d(C5C23O26) (20) ½d(C5C23O24) (13)

72 594 S4ÍCË (29) ½S

16aÍCË (11) ½u(C4N7) (12)

73 571 S4ÍCË (11) ½S

16aÍCË (10) ½S

16bÍNË (11) ½u(C12N27) (16)

74 563 d(C3C4N7) (11) ½d(C5C23O26) (10)

75 537 q(CO) (56)

76 524 S6a

ÍNË (22) ½d(CNC) (32)

77 491 S16b

ÍCË (10) ½d(C11C12N27) (14) ½o(CNC) (16) ½q(CO) (12)

78 474 S4ÍNË (12) ½S

16aÍNË (16) ½o(CNC) (11)

79 460 S16a

ÍNË (71)

80 432 S4ÍNË (18) ½S

16aÍNË (20) ½S

16bÍCË (17) ½u(C9N8) )(18)

81 413 d(CNC) (26)

82 399 S16a

ÍCË (12) ½S16b

ÍCË (31) ½d(C5C23O24) (16)

83 365 S6b

ÍCË (11) ½S16b

ÍCË (11) ½l(C5C23) (12) ½d(CNC) (15) ½d(C5C23O24) (10)

84 319 S16a

ÍCË (12) ½d(C3C4N7) (17)

85 294 S16b

ÍNË (28) ½q(C4N7) (18) ½q(CH3) (13) ½q(CH

3@) (17)

86 268 S16a

ÍCË (10) ½d(C11C12N27) (11) ½o(CNC) (17)

87 221 d(C6C5C23) (25) ½q(C5C23) (10)

88 216 q(CH3) (38) ½q(CH

3@) (24)

89 212 d(C11C12N27) (18) ½o(CNC) (11) ½d(C6C5C23) (20)

90 196 u(C9N8) (10) ½q(C4N7) (27) ½q(CH3) (11) ½q(CH

3@) (22)

91 145 u(C5C23) (33)

92 138 q(C12N27) (38) ½q(C5C23)(10) ½q(CH3@) (10)

93 125 S16a

ÍCË (19) ½u(C5C23) (11)

94 94 S16b

ÍNË (17) ½q(C12N27) (23) ½q(C5C23) (14)

95 75 q(C9N8) (20) ½q(C12N27) (19) ½q(C5C23) (10)

96 65 q(C12N27) (55)

97 43 S16b

ÍNË (10) ½d(C4N7N8) (14) ½u(C4N7) (10) ½q(C9N8) (13)

98 33 S16b

ÍNË (13) ½d(C3C4N7) (14) ½d(C4N7N8) (10) ½u(C9N8) (16) ½q(NxN) (29)

99 25 q(C4N7) (48) ½q(C9N8) (28)

the existence of the quinonoid structure of this ring inthe protonated species. Although the positions of somebands are essentially unaltered on protonation theirpotential energy distributions are signiÐcantly di†erent,as also are their relative Raman intensities. In accord-ance with the quinonoid structure, the NxN bondorder is markedly reduced, as evidenced by the absenceof the l(NxN) band at 1400 cm~1, which is replaced bybands at 1180 and 1144 cm~1, both of which have acontribution from l(NÈN).

RR spectra were also recorded for MR and MRH` inthe solid state, in discs (Fig. 4). AlthoughKBr/K2SO4the band positions are very similar to those of the solu-tion species, it is apparent that the spectra of bothcontain bands due to the azo and the quinonoid struc-tures. It appears that in the solid state the MR andMRH` samples both contain a mixture of theunprotonated and protonated species. It is believed thatthis could be the results of intramolecular proton trans-fer in the solid state, although the crystal structure ofMR39 gives no indication of this. The relative intensityof the band at 981 cm~1 serves to illus-l1(a1) K2SO4trate the variation in Raman intensity with wavelengthand upon protonation.

RR spectrum of Ethyl Red

The electronic absorption spectra of ER in acidic andbasic solution are very similar to those of MR andMRH`. RR spectra of ER solutions in methanol and0.1 mol dm~3 methanolic HCl are shown in Fig. 5. Theband wavenumbers, relative intensities and assignmentsfor ER and ERH` in various solutions are listed inTables 8 and 9. Similar behaviour to MR was observed :the strongest band of ER at 1392 cm~1 is attributed tothe l(NxN) vibration while the strongest band ofERH` at 1620 cm~1 is assigned to the 8a ring stretch-ing vibration.

CONCLUSIONS

Although there are gross changes in the electronicabsorption spectra of MR and ER upon protonation,much more detailed information is revealed by their RRspectra. The most signiÐcant di†erences in the RRspectra of the unprotonated species are (a) the domin-ance of the l(NxN) band for the former and the 8a ring



Table 4. Ab initio (HF/3–21G) scaled vibrational wavenumbers and potential energy distribution forMRH‘

Modes No. l6 /cmÉ1 Potential energy distribution (%)

A¾ 1 3452 l(OH) (100)

2 3200 l(NH) (99)

3 3079 l(C11H20) (95)

4 3075 l(C13H21) (96)

5 3069 l(C3H15) (95)

6 3051 l(C1H17) (29) ½l(C6H18) (66)

7 3038 l(C1H17) (46) ½l(C2H16) (21) ½l(C6H18) (31)

8 3037 l(C10H19) (96)

9 3024 l(C1H17) (24) ½l(C2H16) (72)

10 3021 l(C14H22) (97)

11 2994 las

(CH3) (45) ½l

as(CH

3@) (46)

12 2978 las

(CH3) (46) ½l

as(CH

3@) (46)

13 2881 ls(CH

3) (37) ½l

s(CH

3@) (54)

14 2876 ls(CH

3) (55) ½l

s(CH

3@ ) (38)

15 1717 l(CxO) (79)

16 1641 S8a

ÍCË (14) ½S8a

ÍNË (11) ½d(NNH) (45)

17 1610 S8b

ÍCË (17) ½S8a

ÍNË (38)

18 1595 S8a

ÍCË (10) ½S8b

ÍCË (35)

19 1568 l(C12N27) (27)

20 1552 S8a

ÍCË (31) ½d(NNH) (23)

21 1530 das

(CH3) (24) ½d

as(CH

3@) (33)

22 1529 S14

ÍNË (15) ½S19b

ÍNË (12) ½das

(CH3) (24) ½d

as(CH

3@) (10)

23 1509 S14

ÍNË (10) ½das

(CH3) (24) ½d

as(CH

3@) (32)

24 1498 S19b

ÍCË (11) ½d(C4C3H15) (18) ½d(C5C6H18) (16) ½ds(CH

3) (10)

25 1487 ds(CH

3) (24) ½d

s(CH

3@) (25)

26 1476 S8b

ÍNË (15) ½d(C2C1H17) (11) ½d(C10C11H20) (14)

27 1467 d(C2C1H17) (19) ½d(C3C2H16) (14) ½ds(CH

3) (12) ½d

s(CH

3@) (15)

28 1459 ds(CH

3) (34) ½d

s(CH

3@) (29)

29 1410 l(C12N27) (29) ½d(C13C14H22) (14)

30 1349 S8b

ÍNË (21) ½d(C9C10H19) (16) ½d(C10C11H20) (11) ½d(C12C13H21) (10)

31 1334 l(C9N8) (38) ½d(C12C13H21) (15)

32 1328 S14

ÍNË (36)

33 1319 l(C5C23) (11) ½d(COH) (35) ½d(C5C23O26) (11)

34 1304 S14

ÍNË (11) ½d(C4C3H15) (20) ½d(C5C6H18) (15)

35 1237 S14

ÍCË (13) ½d(C2C1H17) (23) ½d(C3C2H16) (29)

36 1230 las

(NC) (13) ½d(C12C13H21) (13) ½d(C13C14H22) (13) ½rip

(CH3@) (15)

37 1223 l(C4N7) (17) ½d(C9C10H19) (12)

38 1201 rip

(CH3) (33) ½o

ip(CH

3@) (26)

39 1189 S8b

ÍCË (13) ½d(COH) (10)

40 1173 S1ÍCË (11) ½l(NN) (24) ½l(C5C23) (10) ½d(COH) (15)

41 1160 S14

ÍCË (45)

42 1137 S14

ÍCË (29) ½l(NN) (16) ½l(C9N8) (10)

43 1126 S19b

ÍNË (19) ½S21b

ÍNË (10) ½d(C9C10H19) (10)

44 1100 S19a

ÍCË (15) ½S12

ÍCË (40)

45 1058 S19b

ÍCË (23) ½S21b

ÍCË (12) ½l(C–O) (15)

46 1042 las

(NC) (56) ½oip

(CH3) (19) ½o

ip(CH

3@) (19)

47 1010 S1ÍCË (27) ½l(C–O) (35)

48 977 S12

ÍNË (59) ½S19a

ÍNË (17) ½S21a

ÍNË (10)

49 903 ls(NC) (39) ½S

19aÍNË (20) ½S

21aÍNË (10)

50 883 S1ÍCË (13) ½S

1ÍNË (11) ½l(C4N7N8) (18) ½l(C9N8N7) (15)

51 817 S1ÍNË (23) ½S

6bÍNË (11) ½S

12ÍCË (14) ½l(C4N7) (14)

52 738 S1ÍNË (18) ½S

6bÍNË (12) ½l(C5C23) (11)

53 696 S1ÍNË (10) ½S

6aÍCË (10) ½S

6aÍNË (14) ½l

s(NC) (24)

54 644 S6b

ÍNË (47) ½S8b

ÍNË (47)

55 635 S6a

ÍCË (51)

56 628 S6b

ÍCË (17) ½S6b

ÍNË (15) ½d(C5C23O26) (23) ½d(C5C23O24) (13)

57 568 S6b

ÍNË (11) ½d(C3C4N7) (18) ½d(C5C23O26) (11)

58 526 S6a

ÍNË (21) ½d(CNC) (28)

59 497 d(C11C12N27) (12) ½d(CNC) (11) ½o(CNC) (18)

60 454 o(CNC) (20) ½d(C5C23O24) (10)

61 405 l(C5C23) (18) ½d(CNC) (24) ½d(C5C23O26) (11)

62 381 S6b

ÍCË (22) ½l(C5C23) (18) ½d(CNC) (15) ½d(C5C23O24) (24)

63 300 d(C3C4N7) (14) ½d(C10C9N8) (10) ½d(C6C5C23) (18) ½d(C5C23O26) (17)

½d(C5C23O24) (12)

64 265 d(C11C12N27) (23) ½o(CNC) (21) ½d(C6C5C23) (29)

65 170 d(C3C4N7) (21) ½d(C10C9N8) (20) ½d(C11C12N27) (21)



Table 4.ÈContinued

Modes No. l6 /cmÉ1 Potential energy distribution (%)

66 184 S6a

ÍNË (17) ½l(C4N7) (12) ½d(C4N7N8) (14) ½d(C6C5C23) (14)

67 47 d(C4N7N8) (27) ½d(C9N8N7) (40) ½d(C3C4N7) (11)

½d(C10C9N8) (16)

AÂ 68 2936 las

(CH3) (31) ½l

as(CH

3@) (69)

69 2933 las

(CH3) (69) ½l

as(CH

3@) (31)

70 1524 das

(CH3) (50) ½d

as(CH

3@) (42)

71 1510 das

(CH3) (42) ½d

as(CH

3@) (51)

72 1154 oop

(CH3) (44) ½o

op(CH

3@) (46)

73 1121 oop

(CH3) (47) ½o

op(CH

3@ ) (45)

74 1105 u(C1H17) (39) ½u(C2H16) (35) ½o(C3H15) (14) ½u(C6H18) (27)

75 1094 u(C10H19) (44) ½u(C11H20) (18) ½u(C13H21) (17)

½u(C14H22) (33)

76 1085 u(C10H19) (34) ½u(C11H20) (18) ½u(C13H21) (26) ½u(C14H22) (37)

77 1072 u(C2H16) (25) ½u(C3H15) (34) ½u(C6H18) (44)

78 995 u(C1H17) (12) ½u(C2H16) (11) ½u(C3H15) (17) ½u(C4N7) (11)

½u(NH) (36)

79 958 u(C1H17) (10) ½u(C3H15) (18) ½u(NH) (61)

80 920 u(C10H19) (10) ½u(C11H20) (28) ½u(C13H21) (21) ½u(C14H22) (10)

½u(C12N27) (27)

81 853 S4ÍCË (20) ½u(C3H15) (14) ½u(C5C23) (19) ½q(C5C23) (30)

82 848 u(C10H19) (16) ½u(C11H20) (29) ½u(C13H21) (33) ½u(C14H22) (18)

83 830 u(C1H17) (30) ½u(C2H16) (19) ½u(C6H18) (14) ½u(C4N7) (13)

84 803 S4ÍNË (50) ½u(C9N8) (11) ½u(C12N27) (19)

85 734 S4ÍCË (32) ½q(C5C23) (22) ½q(C5C23) (30)

86 580 S4ÍCË (17) ½S

16aÍNË (12) ½u(C4N7) (15) ½q(CO) (23)

87 562 u(C12N27) (10) ½q(CO) (49)

88 536 u(C4N7) (16) ½u(C12N27) (13) ½q(CO) (14)

89 481 S4ÍNË (11) ½S

16aÍNË (56) ½q(C9N8) (18)

90 447 S16a

ÍCË (18) ½S16b

ÍCË (60) ½u(C5C23) (10) ½q(C5C23) (12)

91 416 S4ÍNË (20) ½S

16aÍNË (43) ½u(C9N8) (16) ½q(C12N27) (11)

92 333 S16b

ÍNË (15) ½q(C4N7) (14) ½q(C9N8) (20) ½q(C12N27) (19)

93 295 S16a

ÍNË (16) ½S16b

ÍNË (25) ½q(NxN) (14)

94 202 S4ÍNË (11) ½u(C12N27) (10) ½u(C5C23) (12) ½q(C9N8) (17)

½q(C12N27) (10)

95 183 q(C12N27) (15) ½q(CH3) (29) ½q(CH

3@) (36)

96 159 q(C12N27) (18) ½q(C12N27) (12) ½q(CH3) (35) ½q(CH

3@) (31)

97 147 S16a

ÍCË (36) ½q(C5C23) (30)

98 106 q(C5C23) (31) ½q(C5C23) (21)

99 80 q(C12N27) (30) ½q(C12N27) (21) ½q(CH3) (17) ½q(CH

3@) (11)

100 68 S16b

ÍNË (16) ½q(C5C23) (25) ½q(C5C23) (11)

101 35 S16b

ÍNË (36) ½u(C9N8) (18) ½q(NxN) (35)

102 30 q(C4N7) (58) ½q(C9N8) (27)

Figure 3. RR spectra of 10É4 mol dmÉ3 Methyl Red in (a) methanol and (b) 0.1 mol dmÉ3 HCl in methanol. nm; integrationl0¼457.9

time, 1 s ; signal averaged over (a) five and (b) four scans.



Table 5. Methyl Red principal force constants (mdyn from the scaledA� —1)HF/3–21G calculations

Type Group Bond/Angle MR MRH½

Stretches OH 6.75 6.67

NH — 5.64

CH (C-ring) C1H17 5.00 5.06

C2H16 4.99 5.03

C3H15 5.06 5.16

C6H18 5.04 5.09

CH (N-ring) C10H19 5.01 5.06

C11H20 5.12 5.20

C13H21 5.11 5.18

C14H22 5.09 5.01

CH (methyl) C28H30 4.54 4.68

C28H31 4.52 4.68

C28H32 4.77 4.88

C29H33 4.54 4.68

C29H34 4.52 4.68

C29H35 4.77 4.88

NxN 8.29 6.16

CN C4N7 4.43 5.05

C9N8 4.66 6.74

C12N27 6.00 7.65

N27C28 4.74 4.36

N27C29 4.75 4.36

CC (C-ring) C1C2 5.00 5.04

C2C3 5.07 5.13

C3C4 4.90 4.89

C4C5 5.18 5.71

C5C6 4.95 4.89

C6C1 5.15 5.25

CC (N-ring) C9C10 5.01 4.22

C10C11 5.25 6.14

C11C12 4.56 3.99

C12C13 4.54 3.90

C13C14 5.35 6.33

C14C9 4.93 4.15

CxO 10.9 11.5

CCOOH

4.36 4.61

C–O 5.14 4.62

In-plane deformations COH 0.68 0.74

NNH — 1.37

CCH (C-ring) C2C1H17 1.07 1.07

C3C2H16 1.07 1.08

C4C3H15 1.04 1.08

C5C6H18 1.09 1.09

CCH (N-ring) C9C10H19 1.03 1.05

C10C11H20 1.13 1.14

C12C13H21 1.13 1.15

C13C14H22 1.04 1.13

NCH (methyl) N27C28H30 0.64 0.64

N27C28H31 0.65 0.64

N27C28H32 0.67 0.69

N27C29H33 0.64 0.64

N27C29H34 0.65 0.64

N27C29H35 0.67 0.69

HCH (methyl) H30C27H31 0.53 0.50

H30C27H32 0.51 0.51

H31C27H32 0.52 0.51

H33C27H34 0.53 0.50

H33C27H35 0.51 0.50

H34C27H35 0.52 0.50

CNN C4N7N8 1.66 2.04

C9N8N7 1.70 1.39

CCN C3C4N7 2.34 3.51

C10C9N8 2.07 2.12

C11C12N27 2.92 2.99

CNC C12N27C28 2.35 2.47

C12N27C29 2.35 2.47



Table 5.ÈContinued

Type Group Bond/Angle MR MRH½

CCC (C-ring) C1C2C3 1.32 1.32

C2C3C4 1.39 1.49

C3C4C5 1.49 1.61

C4C5C6 1.53 1.89

C5C6C1 1.41 1.56

C6C1C2 1.35 1.35

CCC (N-ring) C9C10C11 1.35 1.32

C10C11C12 1.40 1.37

C11C12C13 1.64 1.54

C12C13C14 1.36 1.33

C13C14C9 1.34 1.30

C14C9C10 1.48 1.39

CCCOOH

2.41 4.30

CCxO 2.28 2.35

CC–O 2.22 3.45

Out-of-plane deformations NH — 0.45

CH (C-ring) C1H17 0.50 0.51

C2H16 0.51 0.52

C3H15 0.52 0.51

C6H18 0.52 0.55

CH (N-ring) C10H19 0.55 0.54

C11H20 0.46 0.46

C13H21 0.47 0.46

C14H22 0.57 0.50

CN C4N7 1.14 0.76

C9N8 0.79 0.58

C12N27 0.91 0.85

CCOOH

0.80 0.61

Torsions NxN 0.52 0.29

CN C4N7 0.42 0.21

C9N8 0.23 0.25

C12N27 0.16 0.24

CC (C-ring) C1C2 0.20 0.17

C2C3 0.19 0.18

C3C4 0.25 0.19

C4C5 0.32 0.21

C5C6 0.26 0.18

C6C1 0.19 0.17

CC (N-ring) C9C10 0.20 0.14

C10C11 0.20 0.10

C11C12 0.16 0.10

C12C13 0.16 0.17

C13C14 0.20 0.14

C14C9 0.20 0.14

CCOOH

0.83 0.52

CO 0.13 0.14

Methyl 0.01 0.01

stretching vibration for the latter, and (b) the alterna-tions of the relative intensities of most other bands.

In this study, we have shown that ab initio calcu-lations, even with a small basis set, can provide a satis-factory model for the Raman spectra of large molecules.The scale ab initio force constants for MR and MRH`permit a detailed vibrational analysis, including poten-

tial energy distributions for all 3N [ 6 normal modes.This would otherwise be an impossible task and haspermitted detailed RR band assignments to be made forboth MR and MRH`, with a clear indication of thechanges that occur upon protonation. The data havealso enabled assignments to be made of the RR spectraof the homologue ER and ERH`.



Table 6. Raman band positions (m6 /cm—1), relative intensities and assignments for Methyl Red in toluene, methanol,water and 0.1 mol dm—3 KOH

C7H

8CH

3OH H

2O KOH Calc.

l6 I l6 I l6 I l6 I l6 Assignment

1602 14 1603 19 1602 99 1602 15 1611, 8b (C-ring)

1606 8a (N-ring)

1574 12 1576 2 1580 3 1574 8a (C-ring)

1470 22 1472 16 1472 5 1474 sh 1496 19b (N-ring), dip

(CH) (C-ring)

1460 sh 1454 7 1456 14 1449 19b, 21b, dip

(CH) (N-ring)

1420 sh

1408 sh

1402 100 1402 100 1402 100 1410 100 1402 l(NxN)

1366 21 1368 71 1372 sh 1355 dip

(CH) (N-ring)

1312 4 1314 6 1316 5 1339 l(C12N27)

1276 9 1282 10 1276 7 1277 14 (N-ring), las

(NC)

1260 6 1266 14 1240 8a, dip

(CH) (N-ring), l(C9N8)

1234 3 1246 12

1228 3 1227 14, dip

(CH) (C-ring)

1208 8 1201 l(C4N7), dip

(CH) (N-ring)

1194 7 1198 14 1194 10 1196 18 1178, 12 (C-ring), l(C5C23)

1175 14 (N-ring)

1148 11 1148 17 1152 6 1150 41 1152 14 (C-ring), l(C9N8)

1144 6 1136 19b, 21b, dip

(CH) (N-ring)

1112 9 1118 18 1110 12, 19a, 21a (C-ring)

1086 16 1090 16 1061 las

(NC)

1040 2 1023 1, 19b (C-ring)

922 13 922 17 922 18 924 18 928 ls(NC)

834 7 836 7 832 1 (N-ring), 12 (C-ring), l(C4N7)

784 16 790 2 798 4 (N-ring)

764 Ä1 742 6a (N-ring), 6b (C-ring), ls(NC)

732 sh 713 6a (C-ring)

666 sh 661 6b, 8b (N-ring)

622 624 6b (C-ring), d(CCO)

564 1 563 d(CCN), d(CCO)

538 3 550 542 3 537 q(CO)

520 4 524 6a (N-ring), d(CNC)

492 sh 491 16b (C-ring), d(CCN), o(CNC)

462 4 468 5 474, 4, 16a (N-ring), o(CNC)

460 16a (N-ring)

434 3 434 3 432 4, 16a (N-ring), 16b (C-ring), u(C9N8)

396 1 399 16a, 16b (C-ring), d(CCO)

322 3 319 16a (C-ring), d(CCN)

298 3 294 16b (N-ring), q(C4N7)

Figure 4. RR spectra of MR and MRH½ in the solid state in discs. MR: 457.9 and (b) 514.5 nm. MRH½ : 457.9KBr/K2SO

4l

0¼(a) l

0¼(c)

and (d) 514.5 nm. Integration time, 1 s ; signal averaged over four scans in each case. The vertical dashed line indicates the mode ofl(a1)

K2SO

4.



Table 7. Raman band positions (m6 /cm—1), relative intensities and assignments for proto-nated Methyl Red in aqueous and methanolic 0.1 mol dm—3 HCl

H2O CH

3OH Calc.

l6 I l6 I l6 Assignment

1620 100 1620 100 1641 8a (C,N-rings) ; d(NNH)

1606 36 1606 sh 1610 8a (N-ring), 8b (C-ring)

1588 33 1588 40 1595 8a, 8b (C-ring)

1496 21 1496 37 1498 19b, dip

(CH) (C-ring)

1456 5 1452 41 1476, 8b, dip

(CH) (N-ring), dip

(CH) (C-ring)

1467 dip

(CH) (C-ring)

1406 46 1402 26 1410 l(C12N27)

1376 50 1374 23 1349 8b, dip

(CH) (N-ring)

1366 sh 1364 23 1334 l(C9N8)

1284 9 1282 3 1304 14 (N-ring), dip

(CH) (C-ring)

1250 16 1256 25 1237, 14, dip

(CH) (C-ring)

1230 las

(NC), dip

(CH) (N-ring)

1220 9 1220 7 1223 l(C4N7), dip

(CH) (N-ring)

1192 34 1190 25 1189 8b (C-ring), d(COH)

1180 33 1173 1, dip

(CH) (C-ring), l(NN), l(C5C23), d(COH)

1144 6 1144 5 1160, 14 (C-ring)

1137 14 (C-ring), l(C9N8), l(NN)

1084 3 1100, 12, 19a (C-ring)

1058 19a, 21b (C-ring), l(C–O)

972 6 974 3 977 12, 19a, 21a (N-ring)

926 2 930 9 920 dop

(CH) (N-ring), dop

(C12N27)

916 18 903 19a, 21a (N-ring), ls(NC)

840 2 838 4 817 1, 6b (N-ring), 12 (C-ring), l(C4N7)

726 2 724 3 738, 1, 6b (N-ring), l(C5C23)

734 4 (C-ring), q(C5C23)

524 16 526 14 526 6a (N-ring), d(CNC)

472 9 470 7 481 4, 16a (N-ring), q(C9N8)

438 Ä1 447 6b, 16a (C-ring)

392 3 405, l(C5C23), d(CNC), d(CCO)

381 6b (C-ring), l(C5C23), d(CNC), d(CCO)

332 4 300 d(CCN), d(CCO), d(C6C5C23)

Figure 5. RR spectra of 10É3 mol dmÉ3 Methyl Red in (a) methanol and (b) 0.1 mol dmÉ3 HCl in methanol. nm; integrationl0¼488.0

time, 1 s ; signal averaged over (a) four and (b) five scans.



Table 8. Raman band positions (m6 /cm—1), relative intensities and assignments for EthylRed in toluene and ethanolic 0.1 mol dm—3 KOH

C7H

8C

2H

5OH/KOH Calc.

l6 I l6 I l6 Assignment

1616 sh 1611 8b (C-ring)

1602 14 1600 24 1606 8a (N-ring)

1576 14 1576 3.5 1574 8a (C-ring)

1470 22 1472 8 1496 19b (N-ring), dip

(CH) (C-ring)

1462 sh 1449 19b, 21b, dip

(CH) (N-ring)

1392 100 1388 100 1402 l(NxN)

1354 9.2 1354 13 1355 dip

(CH) (N-ring)

1312 7 1312 9.5 1339 l(C12N27)

1282 6

1276 15 1278 6 1277 14 (N-ring), las

(NC)

1236 18 1230 sh

1224 sh 1227 14, dip

(CH) (C-ring)

1208 11 1212 17 1201 l(C4N7), dip

(CH) (N-ring)

1196 12 1198 26 1178, 12 (C-ring), l(C5C23)

1175 14 (N-ring)

1166 sh

1154 11 1156 7.6 1152 14 (C-ring), l(C9N8)

1144 11 1146 7.9 1136 19b, 21b, dip

(CH) (N-ring)

1112 11 1116 8.9 1110 12, 19a, 21a (C-ring)

1096 2.8 1091 dop

(CH) (N-ring)

1086 3.1 1087 dop

(CH) (C-ring)

1074 7 1076 3.0 1061 las

(NC)

1046 2.9 1023 1, 19b (C-ring)

924 7 922 11 928 ls(NC)

836 13 836 13 832 1 (N-ring), 12 (C-ring), l(C4N7)

628 11 624 6b (C-ring), d(CCO)

620 11

600 5.2 594 4, 16a (C-ring), dop

(C4N7)

588 3.8 563 d(CCN), d(CCO)

544 1.8 537 q(CO)

518 4.8 524 6a (N-ring), d(CNC)

482 1.1 484 5.1 491 16b (C-ring), d(CCN), o(CNC)

438 5.5 432 4, 16a (N-ring), 16b (C-ring), u(C9N8)

414 1.4 420 5.5 399 16a, 16b (C-ring), d(CCO)

334 1

300 18 319 16a (C-ring), d(CCN)

292 2.7 296 5 294 16b (N-ring), q(C4N7)

220 sh 221 dip

(C5C23)

214 4.5 212 dip

(C12N27), dip

(C5C23), o(CNC)

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Table 9. Raman band positions (m6 /cm—1), relative intensities and assign-ments for protonated Ethyl Red in ethanolic 0.1 mol dm—3 HCl

expt. Calc.

l6 I l6 Assignment

1620 100 1641 8a (C,N-rings), d(NNH)

1606 sh 1610 8a (N-ring), 8b (C-ring)

1588 sh 1595 8a, 8b (C-ring)

1498 1498 19b, dip

(CH) (C-ring)

1456 8 1476, 8b, dip

(CH) (N-ring), dip

(CH) (C-ring)

1467 dip

(CH) (C-ring)

1432 8

1414 7 1410 l(C12N27)

1386 1

1364 5 1349 8b, dip

(CH) (N-ring)

1354 15 1334 l(C9N8)

1282 19 1304 14 (N-ring), dip

(CH) (C-ring)

1252 16 1237 14, dip

(CH) (C-ring)

1246 16 1230 las

(NC), dip

(CH) (N-ring)

1194 23 1189 8b (C-ring), d(COH)

1182 sh 1173 1, dip

(CH) (C-ring), l(NN), l(C5C23), d(COH)

1162 4.7 1160 14 (C-ring)

1144 4 1137 14 (C-ring), l(C9N8), l(NN)

1134 sh 1126 19b, 21b (N-ring)

1090 1.5 1100 12, 19a (C-ring)

1082 1.5 1058 19a, 21b (C-ring), l(C–O)

972 Ä1 977 12, 19a, 21a (N-ring)

924 6.2 920 dop

(CH) (N-ring), dop

(C12N27)

904 7 903 19a, 21a (N-ring), ls(NC)

836 3.1 817 1, 6b (N-ring), 12 (C-ring), l(C4N7)

706 1.1 696 1, 6a (N-ring), 6b (C-ring), ls(NC)

618 1 628 6b (C,N-rings), d(CCO)

522 6 526 6a (N-ring), d(CNC)

482 1.2 481 4, 16a (N-ring), q(C9N8)

450 1.9 454 o(CNC), d(CCO)

420 1.9 416 4, 16a (N-ring)

392 1.2 381 6b (C-ring), d(CCO), l(C5C23), d(CNC)

338 1.6 333 16b (N-ring), q(CN)

320 1.6 300 d(CCO), d(CCN), dip

(C5C23)

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Documents

Ab initio and density functional study of the resonance Raman spectra of Methyl Red, Ethyl Red and their protonated derivatives