Infrared and Raman Spectroscopy of Eugenol, Isoeugenol, and Methyl

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Infrared and Raman Spectroscopy of Eugenol, Isoeugenol and Methyl Eugenol: Conformational Analysis and Vibrational Assignments from DFT Calculations of the Anharmonic Fundamentals Babur Zahurriddin Chowdhry, John P Ryall, Trevor John Dines, and Andrew P Mendham J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 23 Oct 2015 Downloaded from http://pubs.acs.org on October 23, 2015

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The Journal of Physical Chemistry

Infrared and Raman Spectroscopy of Eugenol, Isoeugenol and Methyl Eugenol: Conformational Analysis and Vibrational Assignments from DFT Calculations of the Anharmonic Fundamentals Babur Z. Chowdhrya, John P. Ryalla, Trevor J. Dinesb* and Andrew P. Mendhama a. Faculty of Engineering & Science, University of Greenwich (Medway Campus), Chatham Maritime, Kent ME4 4TB, UK. b. Carnegie Laboratory of Physics, University of Dundee, Dundee, DD1 4HN, UK.

* Author for correspondence (email [email protected] )

Abstract IR and Raman spectra of eugenol, isoeugenol and methyl eugenol have been obtained in the liquid phase.

Vibrational spectroscopic results are discussed in relation to computed

structures and spectra of the low energy conformations of these molecules obtained from DFT calculations at the B3LYP/cc-pVTZ level.

Although computed differences in

vibrational spectra for the different conformers were generally small, close examination, in conjunction with the experimental spectra, enabled conformational analysis of all three molecules. Anharmonic contributions to computed vibrational spectra were obtained from calculations of cubic and quartic force constants at the B3LYP/DZ level. This permitted the determination of the anharmonic fundamentals for comparison with the experimental IR and Raman band positions, leading to an excellent fit between calculated and experimental spectra. Band assignments were obtained in terms of potential energy distributions (p.e.d.s).

Keywords: Raman spectra; IR spectra; eugenol; isoeugenol; methyl eugenol, conformational analysis

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Introduction Eugenol (2-methoxy-4-[prop-2-enyl]phenol, Figure 1(a)) and its derivatives and analogues are used both in fundamental scientific investigations and also commercially for a variety of purposes.1,2 Uses in the former category include e.g. as a reagent in molecular biology studies,3 physiological studies of vertebrates (vaso-relaxant properties4) and invertebrates (e.g. for its insecticidal5 and anti-helminthic properties6) and as a xenobiotic (in vitro tests show that eugenol appears to display both pro-cytotoxic7,8 and anti-oxidant activity9). Eugenol, an “essential oil” and the chief component of clove oil (derived from e.g. the stem, leaves and buds of the Eugenia caryohypllata tree), is commonly used in the food industry10, and it has been shown that spray-dried zein/casein nanocapsules containing eugenol may have the potential to be used as antimicrobial preservatives in food products.11 It was, until recently, also used extensively in aromatherapy12 and as a therapeutic agent in dentistry.13 In the latter it was used in many forms one of which included zinc oxide-eugenol based endodontic sealers/fillers (dental cements); more recently eugenol-functionalised polymers have been developed as dental cements.14 However its use in aromatherapy is contentious and although eugenol is still occasionally used in dentistry other reagents have largely superseded it.

Eugenol-based anaesthetics (e.g. clove oil) have significant potential as

anaesthetics for some aquatic animals, particularly fish.

This is because eugenol is

inexpensive, has good efficacy at low dosages, and is used for the purposes of handling, grading, vaccination, transport and disease or parasitic treatment of fish.15

Isoeugenol (2-methoxy-4-(prop-1-enyl)-phenol), which can exist as either the cis (Figure 1(b)) or trans isomer (Figure 1(c)), is another constituent of clove oil and it is also used in perfumes, flavourings and essential oils. It is commonly used as an antioxidant and flavoring agent in food products, and in medicine as a local antiseptic and analgesic16 and has been


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shown to demonstrate anti-inflammatory17 and anti-arthritic18 activity.

Additionally,

isoeugenol is used in the manufacture of vanillin.19 Sensitive analytical techniques have been developed for the detection of eugenol and isoeugenol in cosmetics20, and for the detection of isoeuegnol in seawater and fish.21 Methyl eugenol (1,2-dimethoxy-4-prop-2-en-1-ylbenzene, Figure 1(d)) is the methyl ether of eugenol; it also occurs naturally in several essential oils and is used in aromatherapy.22

Many of the useful properties of eugenol and its derivatives are a consequence of the presence of both hydrophilic and hydrophobic functional groups within the molecule. An understanding of the conformational properties of eugenol is therefore of key importance, and such information can be provided by vibrational spectroscopic studies in conjunction with high-level quantum-chemical calculations. Until now, vibrational studies of eugenol have been very limited. The only detailed vibrational spectroscopic study reported for eugenol and isoeugenol involved IR matrix isolation investigation supported by DFT calculations of several conformers.23,24 However, nine of the twelve conformers of eugenol do not contain an intramolecular hydrogen bond and are therefore at much higher energy. Only three of the conformers were therefore of plausible existence and conformational analysis of a molecule in a matrix at low temperature is not relevant to the conformational behaviour in the liquid phase at room temperature.

For isoeugenol these DFT calculations identified six

conformations of the trans-isomer and eight of the cis-isomer, although only two of each were judged to be significant. Wu et al.25 obtained the IR spectrum of eugenol as part of a study of the components of Honghua Oil (a Chinese medicine preparation) but did not report any band assignments. Longarte et al. obtained IR-UV double resonance spectra of eugenolwater clusters26 and eugenol dimers27 but carried out only partial analysis of the vibrational spectrum, supported by MP2 and DFT calculations. The only other quantum-chemical study,



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using semi-empirical methods, examined the structure-genotoxicity relationships of a series of allylbenzene derivatives, including eugenol and methyl eugenol.28

There have been no previous studies of the Raman spectra of any of these three molecules, other than some measurements obtained about eighty years ago.29

In the present study we

report the IR and Raman spectra of eugenol, isoeugenol and methyl eugenol in the liquid phase. DFT calculations have been carried out for various conformations of each molecule, including those of both the cis- and trans-isomers of isoeugenol. Experimental IR and Raman spectra are interpreted by comparison of spectra computed from DFT calculations of anharmonic fundamentals for the lowest energy conformers.

The significance of this

vibrational spectroscopic investigation is related to the importance of gaining knowledge of the conformational properties of eugenol and its derivatives, which is crucial to the understanding of their biological activity.

Experimental Eugenol (C10H12O2, molar mass 164.20 g mol-1, 99%), isoeugenol (C10H12O2, molar mass 164.20 g mol-1, 98%) and methyl eugenol (C11H14O2, molar mass 178.22 g mol-1, analytical standard, >98%) were purchased from Aldrich Ltd (Dorset, UK) and used as received.

Raman spectra were obtained using a Labram Raman spectrometer (Instruments S.A., Ltd.) equipped with an 1800 line mm-1 holographic grating, a holographic super-notch filter (Kaiser), an Olympus BX40 microscope, and a Peltier-cooled CCD (MPP1 chip) detector. A helium-neon laser provided exciting radiation at 632.8 nm, which was attenuated by a 10% neutral density filter, resulting in a laser power of ca. 0.8 mW at the sample in all experiments. Measurements consisted of acquiring a spectral window centred at 1150 cm-1;


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the spectrometer has a linear reciprocal dispersion of 2 nm mm-1 in the first order resulting in a spectral window of 1070 cm-1 centered on a Stokes shift of 1150 cm-1 from the 632.8 nm line. Raman spectra of liquid sample were collected using a quartz cuvette, 40 mm macro lens and measurement of scattered radiation at 90º to the direction of the incident laser radiation. Raman measurements consisted of acquiring multiple spectral windows in the range 50–4000 cm-1 (Stokes shifts). The Raman instrument was calibrated using the silicon band at 520.7 cm-1, whose position was checked by using the frequencies of the emission lines of a neon lamp.

FTIR measurements, from samples contained in cells with NaCl windows, were conducted using a Perkin Elmer 1710 FTIR spectrometer. Spectra were obtained within the range 4000400 cm-1 at a resolution of 1 cm-1. A dry nitrogen gas purge was maintained in the sample compartment to facilitate a simpler background subtraction.

Computational methods DFT calculations of various conformations of eugenol, cis- and trans-isoeugenol and methyl eugenol were performed with the Gaussian 09 program,30 using the hybrid SCF-DFT method B3LYP, incorporating Becke’s three parameter hybrid functional31 and the Lee, Yang and Parr correlation functional.32 All calculations were performed with the cc-pVTZ basis set.33 The vibrational spectrum was calculated at the optimised geometry for each conformer and normal coordinate analyses were carried out using a program derived from those of Schachtschneider.34

Cartesian quadratic force constants obtained from the Gaussian 09

output were converted to force constants expressed in terms of a full set of internal coordinates, which were then reduced to a non-redundant set of 3N‒6 symmetry adapted internal coordinates. Local symmetry coordinates were used where appropriate, e.g. for



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methyl groups, and Wilson symmetry coordinates were used for benzene ring stretches and deformations.35

Band assignments were then determined in terms of potential energy

distributions.

In order to facilitate comparison of computed vibrational spectra, containing harmonic fundamentals, with experimental IR and Raman data it has been customary to scale force constants using arbitrary scale factors chosen to give the best fit to experimental data. For the B3LYP and other hybrid SCF-DFT methods scale factors in the range 0.9-1.0 have typically been employed. Nevertheless this approach is not entirely satisfactory and it is preferable to calculate anharmonic fundamentals for comparison with experimental data, which can be undertaken using Barone’s second-order perturbative treatment based on quadratic, cubic, and semi-diagonal quartic force constants.36 The computation of anharmonicity constants using the cc-pVTZ basis set would present a formidable computational challenge for molecules such as eugenol and its derivatives. However, Barone37 has shown that the combination of harmonic frequencies computed at high level with anharmonic corrections computed at a lower level provided an effective method for larger molecules. We calculated anharmonicity corrections using the much smaller Dunning double-zeta (DZ) basis set,38 which is referred to as “D95” in the Gaussian 09 software, having initially optimised the molecular geometry at this lower level. Anharmonic fundamentals were thus obtained using the B3LYP/cc-pVTZ harmonic fundamentals in conjunction with the B3LYP/DZ anharmonicity constants.

Any serious discrepancies between experimental and calculated band wavenumbers can be attributed to either deficiencies in the second order perturbative treatment of anharmonicity, the random errors inherent in DFT calculations (as opposed to the systematic error found in


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HF calculations), or to intermolecular interactions. Anharmonic corrections were mostly negative, as expected, although for a small number of lower wavenumber vibrations, mainly (although not exclusively) corresponding to out-of-plane motions, there were positive anharmonic corrections.

This represents a breakdown of the second-order perturbative

model.

IR and Raman intensities were determined from the dipole and polarizability derivatives from the B3LYP/cc-pVTZ calculations, obtained within the harmonic approximation. Thus the band wavenumbers, but not the IR and Raman intensities were corrected for anharmoncity. Simulated IR and Raman spectra were calculated for each conformer by convolution with a Lorentzian lineshape function (full-width-half-maximum = 10 cm-1). For comparison with experimental spectra, population-weighted average spectra were computed.

Results and Discussion Structures The calculated structures of the stable conformers of eugenol, cis-isoeugenol and methyl eugenol are all non-planar, while those of trans-isoeugenol are planar, with Cs symmetry. Atom numbering schemes for eugenol, isoeugenol and methyl eugenol are shown in Figures 1(e), 1(f) and 1(g), respectively, and the computed bond distances, inter-bond angles and torsional angles for all conformations are listed in Tables S1, S2 and S3 (Supplementary Information).

Note that the designation of the second methyl group (CH3′) differs for

isoeugenol (where it is part of the propenyl side-chain) and methyl eugenol. The structure of eugenol is considered to exhibit conformational flexibility only in the propenyl side-chain because the juxtaposition of the OH and OCH3 groups is fixed by a weak intramolecular hydrogen bond between the hydroxyl H atom and the methoxy O atom. Conformations



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which do not have this arrangement were found to have much higher energy, in agreement with previous work,23,24 and were not considered further. Similar conformational behaviour is found for isoeugenol but in methyl eugenol the intramolecular hydrogen bond does not exist, but in all the low energy conformations the two methoxy groups are necessarily trans to each other in order to minimise steric hindrance.

In order to verify the lowest energy conformations of the propenyl side-chain of eugenol we carried out a molecular mechanics simulation using the Moldraw program,39 scanning the potential energy surface obtained by variation of the torsional angles τ(C5C4C10C11) and τ(C4C10C11C12). The potential energy surface, shown in Figure 2, reveals that there are four “islands” of low energy, in the regions of τ(C5C4C10C11) = ± 100º and τ(C4C10C11C12) = ± 100º, corresponding to four possible conformations. However, two of these conformations are mirror images of each other and therefore identical; these are the ones in which the two torsional angles are either both positive or both negative. DFT calculations were carried out with initial geometries corresponding to each of the three distinct combinations of torsional angles leading to the optimised geometries shown in Figure 3, together with their computed energies, populations at 298.15 K, and τ(C5C4C10C11) and τ(C4C10C11C12) angles. These optimised geometries correspond to energy minima, as evidenced by the lack of imaginary frequencies and each of these conformations is significantly populated at 298.15 K.

In the case of isoeugenol C4C10 is a double bond so the τ(C4C10C11C12) dihedral angle can only be either 0 (cis) or 180º (trans) and in fact there is less conformational flexibility because the exocyclic C=C bond is conjugated with the benzene ring.

A molecular

mechanics potential energy scan of the τ(C5C4C10C11) dihedral angle for both cis- and


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trans-isoeugenol reveals that there are minima close to ±90° and maxima close to 0° and 180°, although the curve is much shallower for the trans isomer (Figure S1, Supplementary Information). The DFT calculations revealed that the trans isomer has an energy minimum for planar geometry, possessing Cs symmetry, whence the τ(C5C4C10C11) angle may be either 0 or 180º. By contrast, computed vibrational spectra for two conformers of the cis isomer with Cs symmetry both yielded one imaginary frequency corresponding to the normal mode involving torsion about the C4C10 bond. It appears therefore that for the cis isomer the exocylic C=C double bond is not coplanar with the benzene ring, presumably on account of steric hindrance. There are two distinct conformers of the cis-isomer i.e. that are not mirror images. DFT calculations were carried out for each of the two conformers of cis-isoeugenol and the two conformers of trans-isoeugenol, resulting in the optimised geometries shown in Figure 4, together with their computed energies and populations at 298.15 K.

These

optimised geometries correspond to energy minima, as evidenced by the lack of imaginary frequencies and both conformations of trans-isoeugenol are significantly populated at 298.15 K. However, the energies of the cis-isoeugenol conformers are predicted to be ca. 12 kJ mol-1 higher than those of trans-isoeugenol, indicating that at room temperature isoeugenol is predominantly in the trans form.

The potential energy surface for methyl eugenol is very similar to that of eugenol, showing the same four “islands” of low energy; it is shown in Figure S2 (Supplementary Information). DFT calculations reveal that three conformations of methyl eugenol are significantly populated at 298.15 K. Their optimised geometries are shown in Figure 5, together with the computed energies and populations at 298.15 K.



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Vibrational spectra Eugenol and isoeugenol each consist of 24 atoms and therefore exhibit 66 fundamental vibrations; methyl eugenol contains 27 atoms and exhibits 76 fundamentals. All vibrations are active in both IR and Raman spectra. The sets of 3N‒6 independent internal coordinates used in the normal coordinate analyses are listed in Tables S4, S5 and S6 (Supplementary Information).

The Raman and IR spectra of eugenol are shown in Figures 6 and 7,

respectively, together with the computed spectra for each conformation and the populationweighted average spectrum of the three conformers.

Experimental and calculated band

positions for eugenol are listed in Table 1, with band assignments in terms of potential energy distributions (p.e.d.s) for the lowest energy conformer. Raman and IR spectra for isoeugenol are shown in Figures 8 and 9, respectively, together with the computed spectra for each conformation of trans-isoeugenol, and the population-weighted average spectrum of the two conformers of trans-isoeugenol.

Experimental and calculated band positions for trans-

isoeugenol are listed in Table 2, with band assignments in terms of p.e.d.s for the lowest energy conformer of trans-isoeugenol. The Raman and IR spectra of methyl eugenol are shown in Figures 10 and 11, respectively, together with the computed spectra for each conformation and the population-weighted average spectrum of the three conformers. Experimental and calculated band positions for methyl eugenol are listed in Table 3, with band assignments in terms of potential energy distributions (p.e.d.s) for the lowest energy conformer. For all three molecules the band positions, IR and Raman intensities and p.e.d.s for each of the low energy conformers are slightly different, although we consider that the differences in p.e.d.s are not sufficient to warrant further discussion.

IR and Raman band assignments for eugenol and its derivatives are generally straightforward and there is a good agreement between experimental and calculated band wavenumbers. For


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this reason we focus discussion on a few important details. Our calculations did not take account of intermolecular hydrogen bonding, thus the ν(OH) band, observed for eugenol and isoeugenol, is lower (by ca. 30-40 cm-1) than the DFT prediction. For both compounds an additional band appears in the IR spectra on the low wavenumber side at around 3450 cm-1 and in the case of methyl eugenol some very weak broad bands are seen at 3430, 3545 and 3635 cm-1. These are not observed in the Raman spectra and may be due to a hydrogenbonded impurity.

The strongest band in the Raman spectra of all three compounds is assigned to the stretching vibration of the exocyclic C=C bond, which appears at 1640 cm-1 in eugenol and methyl eugenol, and at 1658 cm-1 in isoeugenol. The shift to higher wavenumber for isoeugenol is attributed to conjugation with the benzene ring, although this observation is surprising in that usually a reduction in wavenumber would be expected to accompany conjugation. However, this band has recently been observed at 1656 cm-1 in the Raman spectrum of the related compound 1-methoxy-4-(1-propenyl)benzene (anethole).40 By contrast, stretching of the two exocyclic CC single bonds contributes to a large number of normal modes, spanning a wide range (300-1500 cm-1), where they are extensively mixed with several other types of motion. In fact the extensive mixing of vibrational motion is most noticeable for skeletal deformations of the propenyl side-chain, which occur below 500 cm-1. The positions of these bands are similar for eugenol and methyl eugenol but are substantially different for isoeugenol, which is not surprising given that the plane of symmetry in trans-isoeugenol prohibits mixing of in-plane and out-of-plane coordinates.

For all three compounds the symmetric ring breathing coordinate, S1, contributes to several normal modes. In eugenol these give rise to bands at 1268, 795 and 747 cm-1 in the IR



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spectrum, which also appear in the Raman spectrum. For isoeugenol there are bands in the IR spectrum at 1291, 804 and 735 cm-1, and in the Raman spectrum at 1280, 808 and 737 cm-1. An additional band is predicted at 965 cm-1 but not observed – this is not predicted for either of the other species. The S1 coordinate is also distributed over several normal modes for methyl eugenol, showing behaviour which is similar to that of eugenol, but with bands shifted to lower wavenumber. It is significant to note that for all molecules there is not a single vibration which could accurately be described as ring breathing since S1 is extensively mixed with other CC stretching coordinates, and also ν(CO). This is in stark contrast with the Raman spectrum of benzene, where the ring breathing mode appears as a very strong band at 980 cm-1, and a wide range of substituted benzene derivatives where it gives rise to a characteristic strong Raman band.35

CH3 and CH2 deformations, and also rocking, wagging and twisting of the CH2 group, as well as the in-plane and out-of-plane rocking of CH3 groups, mostly occur within the expected ranges. Although the majority of the predicted bands are observed there are some exceptions. Notably the methoxy CH3 symmetric deformation is not observed for eugenol and although its harmonic wavenumber is predicted at 1489 cm-1, rather surprisingly the anharmonic fundamental is predicted at 1533 cm-1. This indicates a breakdown of the second order perturbative treatment of anharmonicity, which was also found for some CH3 deformation modes in the other two species. For this reason we suggest that band assignments in this region can only be regarded as tentative and the predicted band positions are all outside the usual range for a CH3 group bonded to oxygen, where the δs(CH3) mode is expected at 14381450 cm-1 for methoxy-substituted benzene.41


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Conformational differences in the computed IR and Raman spectra of eugenol and methyl eugenol The biological activity of eugenol and its derivatives in the liquid state is expected to be closely related to their conformational behavior, which can ideally be studied using vibrational spectroscopy.

It is surprising that very little attention has been given to

conformational analysis of eugenol and is limited to the matrix-isolation IR studies of OlbertMajkut et al.23,24 and the IR-UV double resonance studies of Longarte et al.26,27 In our examination of eugenol in the liquid state it appears that differences between the computed IR and Rama spectra of the conformers of both eugenol and methyl eugenol are subtle and in many cases, not readily apparent. The most significant differences in the Raman spectra are in the 1100-1500 cm-1 region, and in the region below 500 cm-1. However, although the computed Raman spectra of conformers 2 and 3 differ significantly from those of conformer 1, the IR spectra are remarkably similar for all three conformers. Thus Raman spectroscopy provides a better tool for conformational analysis of eugenol and methyl eugenol. Comparison of the computed Raman spectra of the three conformers reveals that those of conformers 2 and 3 are markedly different from each other, and from the experimental spectrum around 1300 cm-1. These can therefore be discounted in accord with their higher energies and consequent low populations at room temperature. On the other hand, the computed Raman spectra of conformer 1 for eugenol and methyl eugenol give the closest match to the experimental Raman spectra and we therefore suggest that in the liquid phase at room temperature this conformer is dominant for both molecules. Conformer 1 of eugenol was established by Olbert-Majkut and Wierzejewska23 to be the conformer of lowest energy in the matrix-isolated state (designated SAA+ in their study).



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For eugenol the first important part of the Raman spectrum, with regard to conformational differences, consists of the three bands observed at 1311, 1294 and 1272 cm-1, which are predicted at 1315, 1303 and 1264 cm-1. For conformer 1 the predicted positions and relative intensities of these three bands are in close agreement with the experimental Raman spectrum. The strongest of these bands (1294 cm-1) is attributed to a vibration localized on the propenyl side-chain, with contributions from ν(C=C), ρ(=CH2) and δip(CH) coordinates. On the other hand, the band at 1272 cm-1 is assigned to a vibration which does not involve the propenyl side-chain and the dominant contributions are from benzene ring and CO stretching motions. Very similar behavior is seen in the Raman spectrum of methyl eugenol, where we observe a trio of bands at 1338, 1296 and 1262 cm-1, in excellent agreement with the predicted positions for conformer 1 (1342, 1304 and 1258 cm-1), which also involve motion of both the benzene ring atoms, the propenyl side-chain and the other substituents. Thus, the orientation of the propenyl side-chain clearly influences the electron density within the benzene ring and also the other two substituents. The second important region, with regard to conformational differences, is below 500 cm-1, where the Raman spectrum is dominated by vibrations which involve out-of-plane motions. In this region it is clear the calculated Raman spectra for conformer 1 of both eugenol and methyl eugenol give a much better fit to the observed spectra than conformers 2 or 3.

Conformational differences in the computed IR and Raman spectra of trans-isoeugenol Isoeugenol can exist as cis- and trans-isomers and the DFT calculations indicate that there are two stable conformers of the trans isomer, both possessing Cs symmetry. By contrast the two lowest energy conformers of cis-isoeugenol lack a plane of symmetry and their populations are predicted to be very low at room temperature. Accordingly, only the two conformers of trans-isoeugenol need be considered. As for eugenol and methyl eugenol the most striking


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differences between the computed vibrational spectra for the conformers are in the 1100-1500 cm-1 region. However, there are also significant differences in the IR spectra of the two conformers as well as in their Raman spectra. Comparison of the computed spectra with the experimental Raman and IR spectra in this region clearly indicates that that the experimental spectra display features associated with both conformers. Furthermore, comparison with the population-averaged computed IR and Raman spectra demonstrates that the predicted population ratio between conformers 1 and 2 is very close to that apparent from the experimental spectra. The results obtained here are in agreement with Krupa et al.,24 whose B3LYP/6-311++G(2d,2p) calculations predicted an energy difference of 1.62 kJ mol-1 between the trans-1 and trans-2 conformations (labelled E1 and E2 in their terminology).

Conclusions Investigation of the room temperature liquid-phase Raman and IR spectra of eugenol, isoeugenol and methyl eugenol has facilitated a detailed conformational analysis of all three molecules. DFT calculations at the B3LYP/cc-pVTZ level for the conformers of each of these molecules have enabled the structures and energies of each of these conformers to be determined and the observed IR and Raman spectra to be attributed to the lowest energy conformer in each case.

These calculations have established that the existence of cis-

isoeugenol at room temperature can be ruled out on the grounds that its conformers have much higher energies than those of trans-isoeugenol and therefore very low populations. The results are in accordance with the IR studies of eugenol and isoeugenol in the matrix-isolated state reported by Olbert-Majkut et al.23,24

The vibrational spectra of the stable conformers of each molecule were computed at the B3LYP/cc-pVTZ level, providing the harmonic band wavenumbers and detailed band



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assignments in terms of p.e.d.s. However, the computed differences in vibrational spectra for the different conformers were generally small apart from the 1100-1500 cm-1 region. In order to properly compare the computed spectra with experimental Raman and IR spectra we calculated anharmonic fundamentals by combining anharmonicity constants computed at a lower level (B3LYP/DZ) with harmonic band wavenumbers computed at the much higher B3LYP/cc-pVTZ level. This is a far superior approach to the ad hoc scaling of quadratic force constants, which is commonly used to fit the computed vibrations to the experimental observations.

Acknowledgements J.P.R. would like to acknowledge the University of Greenwich for the award of a Research Studentship.

Supporting Information Available: Computed bond distances, bond angles and torsional angles for conformations of eugenol, isoeugenol and methyl eugenol, definitions of the sets of 3N − 6 independent internal coordinates used in the normal coordinate analyses, and PE diagrams for isoeugenol and methyl eugenol. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Table 1 Experimental IR and Raman spectra of eugenol, computed harmonic and anharmonic fundamentals for the lowest energy conformer (model 1) and band assignments in terms of potential energy distributions (p.e.d.s). Exp. ν% / cm-1 Calc. ν% / cm-1 Raman IR harm. anharm. 3530 3516 3769 3554 3450 3076 3209 3049 3068 3195 3045 3059 3192 3040 3026 sh 3168 3026 3007 3003 3130 2982 2980 3131 2975 2975 3124 2965 2963 sh 3012 2956 2941 2938 3011 2940 2907 2906 3068 2915 2846 2842 3045 2887 1640 1638 1703 1663 1615 1613 1652 1609 1605 sh 1605 sh 1644 1603 1489 1533 1515 1518 1548 1514 1513 1493 1485 1465 sh 1465 1478 1462 1452 1451 1509 1461 1434 1431 1465 1431

Assignments (% p.e.d.) ν(OH) (100) νas(=CH2) (96) ν(C3H13) (84), ν(C6H15) (14) ν(C3H13) (15), ν(C6H15) (76) ν(C5H14) (89) ν(C6H15) (10) νas(CH3) (91) νs(=CH2) (31), ν(C11H22) (64) νs(=CH2) (66), ν(C11H22) (33) νs(CH2) (99) νs(CH3) (91) νas(CH3) (100) νas(CH2) (98) ν(C=C) (70), δip(=CH2) (14) S8a (40), S8b (24) S8a (27), S8b (41) δs(CH3) (66) S19a (34), δip(C3H13) (17), δip(C6H15) (14) δas(CH3) (92) δ(CH2) (78), δs(CH3) (11) δas(CH3) (89) S19b (30), δs(CH3) (12)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


- 21 1414 1367 1311 sh 1294

1413 sh 1367

1272 1233 1210 1185

1268 1233 1208

1154 1123 1097 1037 1001

1182 1149 1122 1097 sh 1035 995 946

938 sh 914 sh 901 854 819 798 749 722 651 597

915 851 818 795 747 720 648 595

1455 1408 1344 1328 1306 1296 1264 1238 1230 1208 1176 1172 1147 1123 1065 1038 948 952 962 942 913 878 839 813 762 732 659 615

1418 1370 1315 1303 1278 1264 1236 1209 1188 1172 1146 1145 1125 1100 1030 1011 927 927 925 921 892 857 822 800 744 717 645 602

δip(=CH2) (67), δip(C11H22) (11), δ(CH2) (10) S14 (32), δ(COH) (26), δip(C5H14) (11) δip(C6H15) (10), ω(CH2) (35) ν(C=C) (15), ρ(=CH2) (10), δip(C11H22) (56) δip(C3H13) (12), ω(CH2) (48) S1 (18), S14 (19), ν(C1O7) (24), ν(C2O8) (11), δip(C3H13) (11) S12 (22), ν(C1O7) (20), ν(C2O8) (12), δip(C5H14) (12) τ(CH2) (42) δ(COH) (17), δip(C6H15) (10), τ(CH2) (12), ρip(CH3) (26) δ(COH) (11), ρip(CH3) (48) ρop(CH3) (93) ν(C2O8) (12), ν(C4C10) (17), ν(O8C9) (14), δip(C3H13) (14) S19b (18), δip(C5H14) (25), δip(C6H15) (21) ν(C10C11) (17), ρ(=CH2) (21), δip(C11H22) (10), τ(CH2) (19) S12 (20), S19a (16), ν(O8C9) (56) δop(C11H22) (41), τ(C=C) (55) δop(C5H14) (48), δop(C6H15) (44) , ω(=CH2) (16) ν(C10C11) (15), ρ(=CH2) (10), ω(=CH2) (54) ν(C2O8) (10), ν(C4C10) (11), ρ(CH2) (12) ν(C10C11) (32), ρ(=CH2) (10), ω(=CH2) (23) ρ(=CH2) (17), ρ(CH2) (36), δop(C3H13) (11) δop(C3H13) (86) δop(C5H14) (37), δop(C6H15) (40), δip(C1O7) (10) S1 (18), S12 (15), ν(C1O7) (20) S1 (15), S4 (11), S6b (14) S4 (47), δip(C1O7) (20) δip(C2O8) (20) S6b (15), δop(C11H22) (13), τ(C=C) (11) S4 (16), S16b (11), δop(C11H22) (12), δip(C2O8) (18), δop(C4C10) (14)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

- 22 559 484 456 425 397 372 307 233 186 163 146 132 119

563 547 495 445 471 402 376 360 297 278 230 179 198

552 535 481 431 423 398 368 353 283 252 196 166 161

S6b (12), ν(C2O8) (10), δip(C1O7) (33), δip(C2O8) (11) S6b (23), δip(C2O8) (14), δ(COC) (19) S6a (42), δ(C10C11C12) (13) S16a (22), τ(OH) (70) S16a (30), S16b (10), δop(C2O8) (18), τ(CO) (31) S4 (15), δ(C10C11C12) (29), δop(C1O7) (17) S6a (10), ν(C4C10) (10), δop(C4C10) (13), δ(C10C11C12) (19) S6b (13), δip(C1O7) (16), δip(C4C10) (10), δ(COC) (31) S16a (11), δ(C4C10C11) (12), δip(C1O7) (19), δip(C4C10) (12), τ(CH3) (11) S16a (12), δip(C1O7) (13), δip(C4C10) (17), τ(CH3) (25) S16b (12), δip(C4C10) (14), τ(CH3) (46) S4 (13), S16a (15), S16b (28) δip(C2O8) (29), δ(COC) (18), τ(CH3) (10)

117 80 72 27

109 64 64 16

S16b (38), δop(C4C10) (25), τ(C10C11) (21) τ(OCH3) (97) δ(C4C10C11) (17), δip(C4C10) (11), τ(C10C11) (51) τ(C4C10) (90)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


- 23 -

Table 2 Experimental IR and Raman spectra of isoeugenol, computed harmonic and anharmonic fundamentals for the lowest energy conformer (model trans-1) and band assignments in terms of potential energy distributions (p.e.d.s). Calc. ν% / cm-1 Exp. ν% / cm-1 Raman IR harm. anharm. a′ symmetry 3530 3512 3766 3549 3446 3072 3207 3081 3058 3194 3058 3017 sh 3017 3171 3028 3006 3130 3001 2960 3131 2980 2940 2937 3117 2950 2917 2913 3089 2938 2854 2850 3010 2895 2734 3007 2770 1658 1657 1713 1671 1612 1608 sh 1650 1609 1597 sh 1597 1637 1599 1516 1513 1549 1516 1465 1489 1479 1454 1451 1509 1471 1426 1465 1432 1495 1407 1418 1404 1378 1377 1409 1377 1369 1368 1360 1334 1304 1316 sh 1332 1326

Assignments (% p.e.d.)

ν(OH) (100) ν(C3H13) (99) ν(C5H14) (12), ν(C6H15) (87) ν(C5H14) (87), ν(C6H15) (13) νas(CH3) (90) ν(C10H20) (11), ν(C11H21) (83) ν(C10H20) (88), ν(C11H21) (11) νas(CH3′) (91) νs(CH3) (91) νs(CH3′) (95) ν(C=C) (64) S8a (49), S8b (14) S8a (18), S8b (48) S19a (34), δip(C3H13) (17), δip(C6H15) (13) δs(CH3) (67) δas(CH3) (87) S19b (28), δs(CH3) (23) δas(CH3′) (81) δs(CH3′) (78) S14 (25), δ(COH) (24), δs(CH3′) (18) δip(C10H20) (52) δip(C11H21) (62)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

- 24 -

1280 1232 sh 1210 1191 1158 1125 1099

910 808 762 737 614 591 562 411 337

1291 1273 1263 1234 1206 1185 1155 1122 1095

907 804 758 735 613 588 556

227 150 a″ symmetry 2916 2914 2887 2883 1429

1426

1301 1303

1280 S1 (12), S14 (41), ν(C1O7) (10), ν(C4C10) (11) 1279 ν(C1O7) (15), δip(C3H13) (36)

1267 1231 1210 1179 1148 1117 1064 984 934 822

1246 1209 1186 1157 1130 1094 1041 965 918 810

750 600 564 533 410 332 325 224 113

743 593 558 527 405 327 321 219 113

3067 3043 1492 1479 1177

2920 2898 1500 1441 1151

S12 (19), ν(C1O7) (19), δip(C5H14) (16), δ(COH) (12) δ(COH) (18), δip(C6H15) (14), ρip(CH3) (33) S14 (11), δ(COH) (14), ρip(CH3) (37) ν(C2O8) (15), ν(C4C10) (14), ν(O8C9) (14), δip(C3H13) (10), ρip(CH3) (12) S19b (18), δip(C5H14) (26), δip(C6H15) (20) ν(C11C12) (35), ρip(CH3′) (32) S12 (19), S19a (18), ν(O8C9) (54) S1 (12), S19a (10), ν(C11C12) (24) ν(C11C12) (16), ρip(CH3′) (30) S1 (22), S12 (21), ν(C1O7) (25) S1 (17), S6b (24), S12 (13), ν(C2O8) (12) S6a (16), S6b (24) δip(C1O7) (25), δip(C2O8) (25) S6a (23), S6b (23), δ(COC) (10) δ(C4C10C11) (11), δip(C4C10) (23), δ(COC) (25) S6a (17), ν(C4C10) (17), δ(C10C11C12) (21) δ(C10C11C12) (17), δip(C1O7) (38) δip(C2O8) (40), δ(COC) (28) δ(C4C10C11) (32), δ(C10C11C12) (10), δip(C4C10) (39) νas(CH3) (100) νas(CH3′) (100) δas(CH3) (95) δas(CH3′) (93) ρop(CH3) (94)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


- 25 1038 969 923 859 829 sh 797 724 657 454 426 sh 392 285 191 167 140

1034 962 920 856 824 792 786 720

1067 997 949 881 850

1045 970 924 854 831

809

δop(C11H21) (17), ρop(CH3′) (75) δop(C10H20) (38), δop(C11H21) (15), τ(C=C) (40) δop(C5H14) (56), δop(C6H15) (57) δop(C3H13) (76), δop(C4C10) (13) δop(C3H13) (22), δop(C5H14) (18), δop(C6H15) (32) 792 δop(C5H14) (22), δop(C6H15) (12), δop(C10H20) (25), δop(C11H21) (21)

728 625 468 446 398 283 224 193 178 102 84 22

700 610 491 435 390 280 240 221 196 123 74 83

S4 (54), δop(C1O7) (23), δop(C2O8) (19) S4 (15), S16b (12), δop(C1O7) (10), δop(C2O8) (26), δop(C4C10) (23) S16a (20), δop(C2O8) (20), τ(OH) (54) S16a (36), τ(OH) (47) S4 (22), δop(C1O7) (36), δop(C4C10) (15) S16a (28), δop(C2O8) (11), τ(CH3) (38) S16b (17), τ(CH3) (44), τ(CH3′) (16) τ(CH3′) (71) S4 (14), S16a (15), S16b (34) S16b (27), δop(C4C10) (32), τ(C=C) (20) τ(OC) (95) τ(C4C10) (83)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

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Table 3 Experimental IR and Raman spectra of methyl eugenol, computed harmonic and anharmonic fundamentals for the lowest energy conformer (model 1) and band assignments in terms of potential energy distributions (p.e.d.s). Exp. ν% / cm-1 Raman IR 3635br 3545br 3430br 3077 3075 3052 sh 3059 sh 3004

3000

2978

2975 sh

2957 sh 2936 2909 2901 sh

2954 sh 2934 2906 2888 sh

2835 1640 1607 1592

2834 1638 1608 1591

1514

1515

Calc. ν% / cm-1 harm. anharm.

3207 3209 3167 3199 3131 3126 3127 3124 3012 3055 3052 3045 3001 2999 1703 1645 1626 1488 1552

3068 3049 3038 3033 2980 2975 2974 2964 2934 2907 2907 2888 2855 2832 1660 1604 1588 1526 1511

Assignments (% p.e.d.)

ν(C6H15) (94) νas(=CH2) (96) ν(C5H14) (96) ν(C3H13) (98) νs(=CH2) (35), ν(C11H22) (60) νs(CH3′) (11), νas(CH3′) (87) νs(CH3) (10), νas(CH3) (87) νs(=CH2) (62), ν(C11H22) (37) νs(CH2) (100) νas(CH3) (100) νas(CH3′) (100) νas(CH2) (99) νs(CH3) (86), νas(CH3) (10) νs(CH3′) (86), νas(CH3′) (11) ν(C=C) (70), δip(=CH2) (14) S8a (63), S8b (10) S8b (67) δs(CH3) (63) S19a (29), δip(C3H13) (15), δip(C6H15) (17)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


- 27 -

1464 sh

1466

1452 1441 sh 1434 sh 1418 sh 1412 1406 sh 1338 1321 sh 1296 1269 sh 1262 1234 1209 1185

1452 1441 1432 1419 1416 sh 1406 sh 1337

1153

1154

1141 1099 1041 sh 1030 996 950 930h

1140 1098

1261 1235 1189

1030 995 953 929 sh 913

1479 1477 1494 1508 1495 1456 1508 1448 1376 1334 1326 1307 1289 1268 1236 1215 1210 1181 1176 1175 1167 1124 1060 1073 1038 970 952 943

1472 1472 1464 1464 1462 1424 1421 1419 1342 1304 1304 1284 1258 1244 1206 1193 1188 1154 1153 1149 1145 1099 1035 1032 1010 930 923 923

δ(CH2) (13), δs(CH3') (76) δ(CH2) (71), δs(CH3) (12) δas(CH3) (92) δas(CH3) (88) δas(CH3') (92) δip(=CH2) (68), δip(C11H22) (11) δas(CH3') (89) S19b (27), δip(C5H14) (18), δs(CH3) (16) S14 (53) ν(C=C) (10), δip(C11H22) (24), ω(CH2) (29) δip(C11H22) (34), δip(C6H15) (10) δip(C3H13) (22), ω(CH2) (40) S1 (13), S14 (21), ν(C1O7) (11), ν(C2O8) (20), δip(C6H15) (10) S12 (24), ν(C1O7) (24) ρ(=CH2) (10), τ(CH2) (54) ρip(CH3) (63) ρip(CH3) (11), ρip(CH3′) (65) S19a (10), ν(C4C10) (16), δip(C3H13) (16) ρop(CH3′) (93) ρop(CH3) (93) S8a (10), S19b (17), δip(C5H14) (29), δip(C6H15) (20) ν(C10C11) (17), ρ(=CH2) (22), δip(C11H22) (10), τ(CH2) (20) S12 (28), ν(O7C16) (45), ν(O8C9) (15) S19a (11), ν(O7C16) (27), ν(O8C9) (48) δop(C11H22) (41), τ(C=C) (55) ν(C2O8) (11), ν(C4C10) (15), ν(O8C9) (10) ν(C10C11) (15), ω(=CH2) (63) ν(C10C11) (29), ρ(=CH2) (13), ω(=CH2) (34)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

- 28 903 853 807 766 748 725 646 597 559 540 474 sh 463 402 sh 364 332 280 240 sh 220 sh 206

898 sh 859 806 767 747 w 725 645 598 565

931 913 879 823 784 764 736 658 618 607 546 485 474 409 393 363 354 288 259 209 232 190 170 128 96 82 67 27

908 894 857 801 760 745 720 644 603 602 535 477 465 402 389 354 346 277 248 212 203 191 160 118 88 73 62 55

δop(C5H14) (56), δop(C6H15) (43) ρ(=CH2) (18), ρ(CH2) (35), δop(C3H13) (13) δop(C3H13) (85) δop(C5H14) (35), δop(C6H15) (47), δop(C1O7) (10) S1 (16), S12 (11), ν(C1O7) (22) S1 (17), S4 (12), S6b (13) S4 (42), δop(C1O7) (19), δop(C2O8) (22) S6b (16) S4 (16), δop(C11H22) (16), δop(C2O8) (14), δop(C4C10) (12), τ(C=C) (12) δ(C1O7C16) (15), δip(C1O7) (22), δip(C2O8) (16) S6b (35) S6a (12), δ(C10C11C12) (32) S16a (40), S16b (11), δip(C1O7) (18), δip(C2O8) (20) S4 (16), δ(C10C11C12) (13), δ(C4C10C11) (10), δop(C1O7) (22) δ(C1O7C16) (20), δ(C10C11C12) (21), δ(C2O8C9) (17) S6a (14), δ(C1O7C16) (16), δ(C10C11C12) (10) δ(C10C11C12) (13), δip(C4C10) (24), δ(C2O8C9) (22) S16a (17), δ(C4C10C11) (11), τ(O8C9) (40) τ(O8C9) (14), τ(O7C16) (71) δ(C1O7C16) (14), δip(C1O7) (21), δip(C4C10) (21) δ(C4C10C11) (10), τ(O8C9) (33), τ(O7C16) (17) δip(C1O7) (26), δip(C2O8) (37), δ(C2O8C9) (12) S4 (16), S16a (15), S16b (22) S16b (19), δop(C4C10) (20), τ(C1O7) (32), τ(C10C11) (14) S16b (19), τ(C2O8) (62) τ(C1O7) (43), τ(C2O8) (23), τ(C10C11) (19) δ(C4C10C11) (13), τ(C1O7) (12), τ(C10C11) (34) τ(C4C10) (84)


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Figure 1 Structures of (a) eugenol, (b) cis-isoeugenol, (c) trans-isoeugenol and (d) methyl eugenol, and atom numbering schemes for (e) eugenol, (f) isoeugenol and (g) methyl eugenol.



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Figure 2

Potential energy surface for eugenol as a function of the torsional angles τ(C4C5C10C11) and τ(C4C10C11C12).


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Figure 3 Structures and energies for conformations of eugenol.



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Figure 4 Structures and energies for conformations of trans- and cis-isoeugenol.


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Figure 5 Structures and energies for conformations of methyl eugenol.



- 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6 Calculated Raman spectra of eugenol: model 1 (a), model 2 (b), model 3 (c), population-weighted average of models 1-3 (d), and the experimental Raman spectrum (e).

Figure 7 Calculated IR spectra of eugenol: model 1 (a), model 2 (b), model 3 (c), Boltzmann-average of models 1-3 (d), and the experimental IR spectrum (e).


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Figure 8 Calculated Raman spectra of trans-isoeugenol: model trans-1(a), model trans-2 (b), population-weighted average of models trans-1 and trans-2 (c), and the experimental Raman spectrum of isoeugenol (d).

Figure 9 Calculated IR spectra of trans-isoeugenol: model trans-1 (a), model trans-2 (b), population-weighted average of models trans-1 and trans-2 (c), and the experimental IR spectrum of isoeugenol (d).



- 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 10 Calculated Raman spectra of methyl eugenol: model 1 (a), model 2 (b), model 3 (c), population-weighted average of models 1-3 (d), and the experimental Raman spectrum (e).

Figure 11 Calculated IR spectra of methyl eugenol: model 1 (a), model 2 (b), model 3 (c), population-weighted -average of models 1-3 (d), and the experimental IR spectrum (e).


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Table of Contents Image


Infrared and Raman Spectroscopy of Eugenol, Isoeugenol, and Methyl

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