Chemical Physics Letters 580 (2013) 37–42

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Chemical Physics Letters

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The millimetre-wave spectrum of estragole

0009-2614/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cplett.2013.06.067

⇑ Corresponding author.E-mail address: cje8@le.ac.uk (C.J. Evans).

Peter D. Godfrey a, Don McNaughton a, Corey J. Evans b,⇑a School of Chemistry, Monash University, Box 23, Victoria 3800, Australiab Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom

a r t i c l e i n f o

Article history:Received 20 March 2013In final form 17 June 2013Available online 5 July 2013

a b s t r a c t

The rotational spectrum of estragole has been investigated using a free-jet millimetre-wave spectrome-ter. Computational chemistry calculations at different levels of theory predict three possible conformersof estragole that are within 200 cm�1 of each other. Analysis of the millimetre-wave spectrum clearlyshows lines from two of the three possible conformers of estragole. The observed lines have beenassigned to conformers I and III and their corresponding spectroscopic constants have been evaluated.Examination into why no lines from conformer II were observed was carried out by evaluating the poten-tial energy surfaces linking the three conformers.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Estragole (1-allyl-4-methoxybenzene) has been found to be amajor component of biogenic volatile organic compound (BVOC)emissions from a number of plants including the ponderosa pine(Pinus ponderosa), oil palm trees, straggerly baecka (Ochrospermalineare), tarragon (Artemisa dracunculus), sweet basil (Ocimumbasilicum), sweet fennel (Foeniculum vulgare), and anise hyssop(Agastache foeniculum) [1–6].

Interest in estragole emissions from biogenic sources stemsfrom the late 1960s study of ponderosa pine trees that were af-fected by photochemical air pollution in southern California. Thesestudies showed that injured ponderosa pine trees were infestedwith bark beetles, while the more healthy trees were less infested.-Work by Cobb et al. showed there was a significant difference inthe amount of estragole (also known as methyl chavicol and4-allylanisole) in the tree resin between the injured and healthytrees [5]. Further work has shown that estragole is an efficient in-sect aggregation inhibitor and can be used effectively to protectplants, including the ponderosa pine, from a wide range of barkbeetles [7–11]. Estragole is present in the oleoresin of plants andit is synthesized by plants from phenylalanine via the Shikimatepathway [12,13]. As well as being an insect inhibitor, estragole isalso a significant attractant for a number of agricultural pestsincluding the American western corn rootworm (Diabroticavirgifera virgifera LeConte) and the weevil (Elaeidobius kamerunicusFaust). Studies on the western corn rootworm have found that theattractive nature of estragole is controlled by its structure and theconfiguration of receptor cells inside the insects antenna segments[14].

In more recent years there has been growing interest in the roleestragole plays in atmospheric chemistry. In 2006, laboratory workby Lee et al. showed that the secondary organic aerosol yield ofestragole from full photochemical oxidation was the highest ofall oxygenated terpenes (40%) [15]. Real-time emission measure-ments by Bouvier-Brown et al. in 2009 found that the average day-time ecosystem flux of estragole from ponderosa pine trees wasestimated to be 1.37 lmol m�2 h�1 [16]. A comprehensive studyin 2010 by Misztal et al. showed that the average total flux ofestragole emitted by oil palms was around 0.44 mg m�2 h�1 andthat 500 Gg of estragole is emitted from oil palm plantations annu-ally [1]. Although emissions of estragole were three orders of mag-nitude lower than the global emission of isoprene, themeasurements of Misztal et al. suggest that estragole may have asignificant impact on regional atmospheric chemistry.

There has been a limited amount of spectroscopic work carriedout on estragole with most studies focussing on the classificationof essential oils in different plants using hybrid techniques suchas GC–FTIR [17]. The only attempt at assigning the infrared spec-trum of estragole was carried out by Sirichote et al. in 1998 [18].In this study, the condensed phase infrared spectrum of estragolewas recorded between 3500 and 400 cm�1. To aid in the assign-ment of the infrared spectrum a number of computational chemis-try calculations were carried out at different levels of theoryranging from the semi-empirical methods CNDO, INDO, MNDO,AM1 and PM3 to the HF level of theory using the 3-21G, 3-21G(d) and 6-31G(d) basis sets. The results from the computa-tional chemistry calculations indicated two possible conformersof estragole; however, the results were rather ambiguous.

In this letter, we have recorded the rotational spectrum(48–72 GHz) of estragole in its ground vibrational state using amillimetre-wave free-jet spectrometer. Computational chemistrycalculations have been carried out on the possible conformers ofestragole and the results have been used to aid in the assignment

38 P.D. Godfrey et al. / Chemical Physics Letters 580 (2013) 37–42

of the observed spectrum. Investigations on the potential energysurfaces linking the conformers of estragole have also been carriedout and will be discussed.

2. Computational chemistry calculations and experimentalmethod

2.1. Computational chemistry calculations

Rotational constants, dipole moments and harmonic vibrationalfrequencies of the conformers of estragole were determined at theB3LYP, MP2(FC) and MP3(FC) levels of theory using the 6-31+G(d,p) and 6–311++G(d,p) basis sets [19–21]. In all cases, thegeometry was optimized using a tight convergence criterion. Forthe DFT calculations, an ultrafine pruning grid was used. Furthercalculations were carried out on the optimized geometries to esti-mate the centrifugal distortion constants for each conformer at theB3LYP/6-31+G(d,p) and MP2/6-31+G(d,p) levels of theory. To ex-plore the effects of diffuse functions additional calculations werecarried out using the 6-31G(d,p) and 6-311G(d,p) basis sets, whilein an attempt to get more accurate energies a G2MP2 calculationwas carried out on each conformer [22]. To determine the barriersof isomerisation linking the conformers of estragole together, re-laxed potential energy surface (PES) scans were carried out byrotating the appropriate functional group with a step size of 10�.The relaxed PES scans were evaluated at the B3LYP/6-31+G(d)and MP2(FC)/6-31+G(d) levels of theory. All calculations were car-ried out using the GAUSSIAN09 suite of programs and visualised usingGaussView 5 [23,24].

2.2. Experimental method

The millimetre-wave (48–72 GHz) spectrum of estragole wasrecorded using a Stark modulated free-jet spectrometer wherethe basic design has been described previously [25]. The originalinstrument was modified with solid-state sweep oscillators ratherthan klystron sources to provide improved frequency agility. Starkmodulation was provided at 33 kHz between parallel-plate elec-trodes separated by �3.5 cm. Estragole was purchased from Sigmato Aldrich (98%) and used without further purification. The estrag-ole was vaporized at a temperature of 100 �C in a stream of argonat a pressure of 30 kPa. The sample, entrained in hot argon, wasintroduced into the spectrometer via a 350 lm diameter pinhole

Figure 1. The structures of the th

nozzle held 10 �C above the vaporization temperature. Under theseconditions, the post-expansion rotational temperature was �10 Kand no evidence of thermal decomposition was observed. Electricfields of up to 1600 V cm�1 were used to maximise the degree ofStark modulation. A quick, low resolution spectrum was first re-corded over a wide frequency range (48–52 GHz) using a stripchart with observed lines subsequently digitally averaged byrepetitive narrow band scans. A Lorentzian line-shape functionwas fitted to each averaged line profile giving a typical full widthhalf height (FWHH) for each line of 200–400 kHz, thus leading toexperimental line center-frequency uncertainties of between 20–40 kHz. Lines were fitted using Watson’s A-reduced Hamiltonianwith an Ir representation within the PGOPHER simulation and fit-ting program written by C. M. Western [26,27].

3. Result and discussion

3.1. Computational chemistry calculations

A conformer search was carried out on estragole at the B3LYP/6-31+G(d,p) level of theory and it was found that only three conform-ers exist which gave positive eigenvalues of the Hessian matrix(i.e., no imaginary vibrational frequencies). Calculations were alsocarried out using the MP2 and MP3 levels of theory with the 6-31+G(d,p) and 6-311++G(d,p) basis sets. The structures of the threeconformers of estragole can be seen in Figure 1. Table 1 lists someof the important structural characteristics of each conformer. Con-former II is the cis form of conformer I, with the methoxy groupand the vinyl part of the allyl group having a dihedral angle of 0�while, conformer III has the vinyl part of the allyl group coming di-rectly out of the plane of the phenyl ring and is in line with the Oatom on the methoxy group. In comparing the work by Sirichoteet al., the structure from their HF/6-31G(d) calculation correlateswell with conformer II while, the structure from their PM3 calcula-tion is in better agreement with conformer I(18). In their study,Sirichote et al. did not find a structure equivalent to conformer III.

Table 2 lists the different conformers and their relative energiesat various levels of theory. It can be seen that there are some majorinconsistencies between the DFT method and the Møller-Plesset(MP) methods. The B3LYP calculations using the 6-31+G(d,p) and6-311++G(d,p) basis sets predict that conformer I is the lowest en-ergy conformer, while conformer III is much higher in energy (over100 cm�1). However, the MP2 and MP3 calculations give an

ree conformers of estragole.

Table 1The optimized structures of conformers I, II and III of estragole at the B3LYP/6-311++G(d,p) and MP3/6-311++G (d,p) levels of theory. Bond lengths in picometres and bond anglesin degrees.

Parameter Conformer I Conformer II Conformer III

B3LYP MP3 B3LYP MP3 Ref [18] B3LYP MP3

O(9)–C(4) 136.7 136.3 136.7 136.3 135.0 136.7 136.3O(9)–C(20) 142.0 141.2 142.0 141.2 142.0 141.2C(1)–C(12) 152.0 152.0 152.0 152.1 151.2 151.3C(12)–C(13) 150.7 151.1 150.7 151.0 151.3 151.7C(14)–C(13) 133.1 133.8 133.1 133.7 131.9 133.1 133.8C(14)–H(15) 108.4 108.5 108.4 108.5 108.4 108.5C(14)–H(16) 108.6 108.7 108.6 108.7 108.4 108.6\C(4)–O(9)–C(20) 118.5� 117.0� 118.5� 117.1� 118.5� 117.1�\C(3)–C(4)–O(9) 115.8� 115.6� 116.0� 115.8� 115.9� 115.7�\C(5)–C(4)–O(9) 124.7� 124.8� 124.6� 124.7� 124.7� 124.7�\C(2)-C(1)–C(12) 120.8� 120.5� 121.3� 121.3� 121.4� 121.1� 120.9�\C(6)–C(1)–C(12) 121.5� 121.6� 121.0� 120.8� 121.1� 121.3� 121.2�\C(1)–C(12)–C(13) 113.4� 112.4� 113.5� 112.6� 116.0� 114.6�\C(12)–C(13)–C(14) 125.1� 124.3� 125.1� 124.3� 126.7� 125.7�\C(13)–C(14)–H(16) 121.6� 121.2� 121.6� 121.2� 121.9� 121.8�\C(5)–C(4)–O(9)–C(20) 0.1� 0.9� 0.1� �0.2� �0.2� 0.4�\C(3)–C(4)–O(9)–C(20) �180.0� �179.0� �180.0� 179.7� �180.0� �180.0� �179.2�\C(2)–C(1)–C(12)–C(13) �61.8� �59.4� �121.7� �124.9� �120.8� �87.4� �86.3�\C(6)–C(1)–C(12)–C(13) 118.4� 120.1� 58.7� 55.3� 91.9� 92.3�\C(1)–C(12)–C(13)–C(14) 122.7� 120.3� �122.7� �120.0� �122.6� �0.8� �1.0�

Table 2Predicted spectroscopic constants in (MHz) of the three conformers of estragole. Relative energies in cm�1, Rotational Constants in MHz and Dipole moments in Debye.

Method DEa Ae Be Ce lx ly lz

Conformer IB3LYP/6-31+G(d,p) 0.0 3669.3 494.3 452.9 �0.89 1.23 0.11B3LYP/6-311++G(d,p) 0.0 3671.8 496.6 455.3 �0.88 1.25 0.12MP2/6-31+G(d,p) 0.0 3565.2 506.5 462.5 �0.65 1.43 0.13MP2/6-311++G(d,p) 0.0 3551.9 508.1 463.4 �0.66 1.41 0.12MP3/6-31+G(d,p) 0.0 3629.7 502.0 458.8 �0.64 1.41 0.11MP3/6-311G(d,p) 0.0 3613.9 504.3 460.0 �0.65 1.36 0.14MP3/6-311++G(d,p) 0.0 3620.6 502.7 458.9 �0.64 1.38 0.11G2MP2 0.0

Conformer IIB3LYP/6-31+G(d,p) 24.2 3116.3 514.8 456.6 0.48 �1.14 �0.20B3LYP/6-311++G(d,p) 28.3 3124.1 517.0 458.9 0.47 �1.13 �0.20MP2/6-31+G(d,p) 25.9 3018.0 529.3 466.8 0.17 �1.29 �0.24MP2/6-311++G(d,p) 31.8 2999.6 531.3 467.9 0.17 �1.25 �0.22MP3/6-31+G(d,p) 33.2 3068.7 524.3 462.6 0.15 �1.27 �0.22MP3/6-311++G(d,p) 40.7 3055.3 525.2 462.8 0.16 �1.23 �0.20G2MP2 30.9

Conformer IIIB3LYP/6-31+G(d,p) 199.4 2898.0 547.7 528.3 0.54 �1.18 �0.42B3LYP/6-311++G(d,p) 147.9 2909.3 550.3 530.8 0.53 �1.18 �0.41MP2/6-31+G(d,p) 0.1 2852.4 562.2 542.4 0.25 �1.37 �0.43MP2/6-311++G(d,p) �72.2 2836.2 566.8 547.2 0.26 �1.33 �0.45MP3/6-31+G(d,p) �8.3 2883.0 556.7 527.3 0.26 �1.35 �0.40MP3/6-311++G(d,p) �34.8 2871.5 558.8 539.7 0.28 �1.31 �0.41G2MP2 �117.6

a Relative energy. No ZPE correction included except for the G2MP2 calculation.

P.D. Godfrey et al. / Chemical Physics Letters 580 (2013) 37–42 39

alternative assignment. The MP2/6-31+G(d,p) calculation indicatesconformer III is only 0.1 cm�1 higher in energy than conformer I,while the MP2/6-311++G(d,p) calculation shows that conformerIII is the lowest lying conformer by nearly 72 cm�1. Likewise, theMP3/6-31+G(d,p) and MP3/6-311++G(d,p) calculations both giveconformer III as the lowest lying conformer by 8 and 35 cm�1

wavenumbers, respectively. All of the different levels of theory em-ployed indicate that conformer II is between 20 and 40 cm�1 high-er in energy than conformer I.

Similar inconsistencies between conformers and levels of the-ory have been observed in allylbenzene. Panja and Chakrabortycarried out a series of calculations ranging from HF/6-31G(d,p) toMP2/aug-cc-pVTZ and found that the order of stability betweenthe eclipsed conformer (similar to conformer III of estragole) andthe 120� conformer (similar to conformer I of estragole) reversed

in going from the HF level of theory to the MP2 level of theory[28]. This reversal in stability was explained through CH���p hydro-gen bonding between the aromatic p electrons and the H atom ofthe allyl chain that is pointing towards the phenyl ring. Obviouslyelectron correlation will be important in taking account any inter-action between H16 (in terms of Figure 1) and the phenyl ring, andso the MP2 and MP3 calculations should be more reliable than theDFT calculations. The interaction is attractive in nature as theC14–H16 bond length is slightly longer than the C14–H15 bondlength at each level of theory (see Table 1). It has been observedthat the inclusion of diffuse functions can have a dramatic effecton the relative energies of conformers of different molecules[29]. To explore this, calculations were also carried out using theMP2 and MP3 levels of theory using the 6-31G(d,p) and 6-311G(d,p) basis sets. The results (see the Supplementary data)

40 P.D. Godfrey et al. / Chemical Physics Letters 580 (2013) 37–42

showed that without the diffuse functions the relative energy ofconformer III is either similar to or significantly higher than con-former II with the most extreme case being the MP2/6-31G(d,p)calculation which estimates conformer III to be over 90 cm�1

higher in energy than conformer I, which when compared to the6-31+G(d,p) result shows there is a 90 cm�1 difference in energybetween the different basis sets. The large changes observed withand without the inclusion of diffuse functions for these basis setswould indicate that larger basis sets are required to correctlydescribe the interactions occurring in conformer III.

In an attempt to overcome some of these contradictions, aG2MP2 calculation was carried out on each conformer. TheG2MP2 method is designed to compute very accurate energies byusing a number of calculation steps at different levels of theory.The results from these calculations are shown in Table 2. At theG2MP2 level of theory conformer III is the most stable conformerby 117 cm�1 from conformer I, while conformer II is 30 cm�1 highin energy than conformer I. It is obvious that the only way areliable estimate of the energies of the conformers (in particularconformer III) can be obtained is by doing additional calculationswith larger basis sets. This conclusion is supported by the workdone on 3-fluoropropene which showed that larger basis sets wererequired to get reliable results as energies calculated using smallerbasis sets were greatly affected by the inclusion of diffusefunctions [29].

3.2. Experimental results

Initially a quick 48–52 GHz scan was carried out using the free-jet millimetre-wave spectrometer. Simulations based on the rota-tional constants and dipole moments calculated at the B3LYP/6-31+G(d,p) level of theory showed a number of strong lines in thisregion from each conformer. The lines observed in the quick scanwere then re-recorded at higher resolution. Initial fits showed thatlines from only two conformers of estragole could be assigned withany certainty. Based on the rotational constants obtained from thecalculations the two conformers that were observed were tenta-tively assigned as conformer I and III. Using the results from the

Figure 2. (a) A portion of the Ka = 11 ? Ka = 12 Q-branch for conformer III of estrag

initial fits subsequent lines from conformer I and III were predictedand then measured. Figure 2(a) and (b) shows Q-branches ob-served for conformers III and I, respectively. The presence of theseQ-branches in the spectrum was extremely important in assigningthe observed lines to a given conformer as their spectral profilesare significantly different from each other.

The program PGOPHER utilizing Watson’s A-reduced Hamilto-nian was employed to fit the lines observed for conformer I andIII. In total, 167 b-type transitions from conformer I were fitted,while for conformer III 181 b-type transitions were fitted. The fit-ted parameters are given in Table 3. Additional computationalchemistry calculations were also carried out at the B3LYP/6-31+G(d,p) and MP2/6-31+G(d,p) levels of theory in order to evalu-ate the centrifugal distortion constants of conformers I and III, andthese are listed in Table 3 for comparison. The rotational constantsfrom the MP3/6-31+G(d,p) and MP3/6-311++G(d,p) calculationsgive the best agreement (between 0.1–0.8%) with the experimentalresults and they clearly indicate that the observed lines do in factbelong to conformers I and III. The rotational constants calculatedusing the B3LYP method are around 1–2% out from the experimen-tal values; while the MP2 calculations usually give good estimatesof the B- and C- rotational constants, but are up to 3% out on the A-rotational constant. The predicted centrifugal distortion constantsare in good agreement with those fitted, with the MP2 resultsbeing more in line with the experimental values.

Of the lines observed, 10 could not be assigned to either con-former I or conformer III. Several attempts were made to assignthese lines to conformer II; however, these attempts failed whenpredicted lines based on initial fits were not experimentally ob-served. It was concluded these observed lines were from anunidentified contaminant either in the sampling line or in the ac-tual sample. To explore the reason why conformer II was not ob-served a series of potential energy surface scans were carried out.

4. Potential energy surface scans

The aim of these calculations were, firstly to calculate thebarrier height in going from conformer I to II by rotation of the

ole (b) A portion of the Ka = 10 ? Ka = 11 Q-branch for conformer I of estragole.

Table 3Fitted Spectroscopic constants in (MHz) of conformers I and III of Estragole.

Conformer Ia Conformer IIIa

A0 3643.1386 (13) 2877.6060 (10)B0 503.59742 (36) 560.82999 (42)C0 459.61635 (40) 541.42060 (41)DJ 2.859 (24) � 10�5 4.538 (28) � 10�5

DJK �6.2814 (193) � 10�4 �1.123 (10) � 10�4

DK 1.05381 (86) � 10�2 3.04254 (506) � 10�3

UKJ -9.202 (994) � 10�8 �2.086 (367) � 10�8

Fitting detailsRMS error = 0.87 RMS error = 1.02167 lines fitted 181 lines fitted

Predicted centrifugal distortion constants: B3LYP/6-31+G(d,p) and MP2/6-31+G(d,p)Conformer I Conformer IIIB3LYP MP2 B3LYP MP2

Ae 3669.31 3565.28 2897.99 2852.44Be 494.28 506.47 547.68 562.15Ce 452.89 462.51 528.27 542.39DJ 2.36 � 10�5 2.68 � 10�5 3.26 � 10�5 4.28 � 10�5

DJK �5.12 � 10�4 �5.96 � 10�4 �3.32 � 10�5 �9.97 � 10�5

DK 1.06 � 10�2 9.61 � 10�3 3.17 � 10�3 3.66 � 10�3

dJ �5.16 � 10�7 �1.57 � 10�7 5.50 � 10�6 4.22 � 10�7

dK 1.95 � 10�4 2.07 � 10�4 �3.84 � 10�3 �4.18 � 10�3

a The numbers in parentheses are the uncertainties given to one standard deviation.

Figure 3. (a) Relaxed potential energy surface scan of rotation about the C4–O9bond. Conformer II is at 0� and 360� while conformer I is at 180�. (b) Relax potentialenergy surface scan of rotation about the C13–C12 bond. Conformer III is at 0�,conformer II at 120� and conformer I at 240�. (c) Relax potential energy surface scanof rotation about the C1–C12 bond. Conformer I is at 0� and 360� and conformer II isat 180�.

P.D. Godfrey et al. / Chemical Physics Letters 580 (2013) 37–42 41

methoxy group, secondly to calculate the barrier height of rotationof the vinyl group which links conformers I, II and III and lastly tocalculate the barrier height from the rotation about the allyl groupwhich links conformers I and II. In all cases, the relaxed PES scanswere calculated at the B3LYP/6-31+G(d) and MP2/6-31+G(d) levelsof theory.

Firstly, the rotation of the methoxy group around the O9–C4bond. This results in a one-dimensional PES plot as seen in Figure3(a). There are two minima, corresponding to conformers I (0�)and II (180�). The points from the relaxed PES scan were fittedusing the general form of the potential function for a torsionalmotion:

VðhÞ ¼ 12

X

n

Vnð1� cos nhÞ ð1Þ

The fitted MP2 values are V2 = 629.00 ± 6.48 cm�1 and V4 =138.63 ± 6.48 cm�1 and the B3LYP values are V2 = 1003.23 ±6.52 cm�1 and V4 = 186.99 ± 6.52 cm�1.

In terms of rotation of the vinyl group about the C12–C13 bond,the results from the one-dimensional relaxed PES scan can be seenin Figure 3(b). It has three minima corresponding to conformer III(0�), conformer II (120�) and conformer I(240�). The MP2 values areV0 = �138.5 ± 37.5, V1 = �62.3 ± 18.6 cm�1, V2 = 160.7 ± 18.6 cm�1

and V3 = 921.7 ± 18.6 cm�1 while the B3LYP values areV0 = 319.6 ± 26.6, V1 = �324.0 ± 15.1 cm�1,V2 = 131.22 ± 15.1 cm�1 and V3 = 883.1 ± 15.1 cm�1.

Another possible link between conformer I and conformer II isrotation of allyl group about the C1–C12 bond. The one-dimensionalPES plot for rotation about the C1–C12 bond is shown in Figure 3(c).The points from the relaxed PES scans were fitted using three termsin equation 1 and the results are MP2: V2 = 677.4 ± 27.4 cm�1,V3 = 45.2 ± 27.4 cm�1, and V4 = �80.4 ± 27.4 cm�1. The B3LYPresults are V2 = 627.7 ± 17.7 cm�1, V3 = 44.2 ± 17.7 cm�1, andV4 = �81.8 ± 17.7 cm�1.

Felder and Günthard, Ruoff et al. and Godfrey et al. have lookedat the relaxation of higher energy conformers of molecules withsimple hindered rotations and molecules of biological interest[30–32]. In the study by Ruoff et al., when using a Fourier Trans-form microwave (FTMW) spectrometer, they found that whenthe barrier to internal rotation was less than �400 cm�1 relaxationto a lower energy conformer occurred. For those systems with

higher barriers it was found the populations of the conformerswere basically ‘frozen’ at the temperature prior to expansion.Felder and Günthard indicate that this relaxation phenomena is aresult of a thermally activated rate process, with the critical barrierheight for relaxation being dependent on the initial temperature,the stagnation pressure of the gas and the size of the molecule.

42 P.D. Godfrey et al. / Chemical Physics Letters 580 (2013) 37–42

Work by Godfrey et al., using a similar instrument to that used inthis Letter found that for molecules like glycine and alanine, thecritical barrier height for relaxation is much higher, �800 cm�1.

The reason for the difference in the critical barrier height is inthe temperature of the rare gas stream prior to expansion. To getthese compounds into the gas phase the gas stream (argon in thiscase) is heated, in this Letter it was heated to �373 K. If we assumethe relative energies predicted at the MP2/6-31+G(d,p) level of the-ory are correct then the Boltzmann factors for the estragole con-formers (I:II:III) indicate that the proportions in the argon streamat �373 K before expansion are: 34.4:31.2:34.4. Based on thisinformation conformer II should be observable if its population isfrozen in the expansion or if there is a low barrier linking con-former II with other conformers of higher energy that are able torelax to conformer II. The critical barrier height for estragole canbe roughly estimated as �575 cm�1 based on the glycine and ala-nine study of Godfrey et al. which, using a pre-expansion temper-ature of �520 K, resulted in a critical barrier height of �800 cm�1.The lowest barrier connecting conformer I and II is�600 cm�1 (Fig-ure 3(a)) and since conformer I is lower in energy than conformer IIrelaxation from II to I is likely to occur, which would result in theproportions after expansion to become 65.6:34.4 ({I:II}:III). A sim-ilar analysis using the G2MP2 results gives the proportions afterexpansion as 54.6:45.4 ({I:II}:III). The other PES scans give barrierheights linking conformers I and II to be 700–800 cm�1 (Figure3(b) and (c)), which are on the high side of the critical barrierheight and probably wouldn’t result in additional relaxation fromII to I. The barrier linking conformer I to conformer III is�1000 cm�1 (Figure 3(b)) which would mean no relaxation is pos-sible from III to I (or I to III), resulting in only these two conformersbeing observed, which supports what is found experimentally.

5. Conclusion

The millimetre-wave spectrum of estragole has been recordedbetween 48–72 GHz and lines from two different conformers havebeen identified and fitted and their spectroscopic parameters eval-uated. There are some disagreements in the computational chem-istry calculations with the B3LYP calculations showing thatconformer III is significantly higher in energy than conformer I,while the MP2 and MP3 calculations show that conformer III isthe lowest lying conformer or has similar energy to conformer I.The larger and more accurate G2MP2 calculations indicate con-former III is 117 cm�1 more stable than conformer I. The difficultyin obtaining reliable energies for conformer III are due in part toCH���p hydrogen bonding between the aromatic p electrons andthe H atom of the allyl chain that is pointing towards the phenylring. To take better account of this interaction, larger basis sets willbe needed as the inclusion of diffuse functions with smaller basis

sets (as used in this Letter) significantly affects the relative ener-gies of the three conformers.

The fitted parameters from the observed millimetre-wave linesof conformer I and conformer III are in good agreement with thespectroscopic parameters predicted at the MP3/6-31+G(d,p) andMP3/6-311++G(d,p) levels of theory. Lines from conformer II werenot observed and an analysis of the potential energy surfaces link-ing the three conformers together indicates that during the expan-sion process conformer II undergoes relaxation to conformer I,resulting in only two conformers being observed.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cplett.2013.06.067.

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