Conformational Equilibrium and Internal Dynamics of E-Anethole: A

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Conformational Equilibrium and Internal Dynamics of E-Anethole: A Rotational Study Camilla Calabrese, Qian Gou, Assimo Maris, Sonia Melandri, and Walther Caminati J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b04883 • Publication Date (Web): 24 Jun 2016 Downloaded from http://pubs.acs.org on June 26, 2016

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The Journal of Physical Chemistry

Conformational Equilibrium and Internal Dynamics of E-Anethole: A Rotational Study Camilla Calabrese,a Qian Gou,a,b Assimo Maris,a Sonia Melandri,a Walther Caminatia,*

a

Dipartimento di Chimica “G. Ciamician” dell’Università, Via Selmi 2, I-40126 Bologna, Italy.

b

Permanent address: School of Chemistry and Chemical Engineering, Chongqing University, No.55

Daxuecheng South Rd., Shapingba, Chongqing, P. R. China, 401331

Corresponding Author *Walther Caminati. Email: [email protected]; Ph.: +39-0512099480

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Abstract The rotational spectra of the two conformers of E-anethole have been investigated using the free jet broadband millimeter-wave spectroscopic technique combined with theoretical calculations. Anti and syn conformers differ for the relative orientation of the propenyl and methoxy chains, with all heavy atoms co-planar to the benzene ring. Relative intensity measurements prove that the anti form is the global minimum, about 2.0(5) kJ mol-1 lower in energy with respect to the syn conformer, solving the contrasting results supplied by different theoretical methods. For both conformers the barriers to internal rotation of the propenyl –CH3 group are low enough to generate fully resolved A-E splittings of the rotational transitions. The corresponding V3 barriers have been determined to be 7.080(5) and 6.978(4) kJ mol-1, respectively.


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1-Introduction There is a considerable interest in flavor and fragrances, due to their irreplaceable roles in food and perfume industries. The introduction and development of new molecules that carry new scents continues to be the primary target, which is now still an exceedingly random and difficult task. The understanding of the relationship between structures and odors, which would guide the chemical synthesis of new odorants, is not straightforward, since the odor varies much with respect to small structural changes.1 Nevertheless, precise structural information of aroma compounds is believed to be a crucial step in the development of our knowledge on the structure-odor relationship. Gas phase studies, free from environmental effects due to the solvent, are very useful to describe the properties of the isolated molecules. Combined with supersonic expansion technique, rotational spectroscopy has been proved to be especially suitable to investigate the conformational behavior, structures and internal dynamics at the molecular level. Indeed, a number of fruit esters were investigated with rotational spectroscopy.2-11 Results show that molecular geometries and barriers to internal rotation of acyl –CH3 tops change slightly with the length of the hydrocarbon chain, which might shed a light on the shifting odor of the fruit esters with the growing hydrocarbon chain length. The rotational spectra of several kinds of odorants, such as camphor,12 ethylvanillin,13 ovanillin,14 S-(+)-carvone and R-(+)-limonene,15 estragole,16 linalool,17 thymol and carvacrol,18 1,4cineole and 1,8-cineole,19 menthol, menthone and isomenthone20 and trans-cinnamaldehyde21 have been previously studied, providing precise information on their structures, conformational equilibria and internal dynamics. Particularly, rotational studies on some isomers are helpful to understand how small structural differences lead to diversities of the biological activities. A comparative rotational study of two bicyclic ethers, 1,4-cineole and 1,8-cineole (which differ only in the connectivity of an ether functional group) show that 1,4-cineole has a significantly more strained geometry than 1,8-cineole: 1,4-cineole has a smaller ether bond angle (98°) with respect to 1,8cineole (115°), which might explain why 1,8-cineole has much longer atmospheric lifetime.19 Anethole (anise camphor; IUPAC: 1-methoxy-4-(1-propenyl)benzene)) is an aromatic compound, unsaturated ether, naturally occuring in essential oils of many plants such as the spice star anise (Illicium verum Hook. f., Illiciacea). It has a characteristic flavor and is widely used in alcoholic drinks, seasoning, confectionery applications, toilet soap, oral hygiene product, etc. It was also found to have antioxidant, antibacterial, antifungal, and anesthetic properties,22, 23 whilst nongenotoxic and noncarcinogenic.24 Differing in the relative orientation of methyl group with respect to the C=C bond in the propenyl substituent, anethole exists in two forms: E and Z isomers. In nature, plants synthesize the E isomer almost exclusively, which holds the desired odor and taste



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of anethole, while Z form is toxic and possesses an unpleasant scent and flavor.25 The biological nature of E-anethole (E-An hereinafter) has motivated its investigation using different techniques: jet-cooled fluorescence spectroscopy,26 matrix-isolation IR,27 FTIR and FT-Raman spectroscopy.28 The authors discussed structures and energies of the two most stable conformers mainly basing on theoretical calculations. As shown in Figure 1, E-An presents two stable conformers with all the heavy atoms co-planar: anti and syn, which correspond to the relative orientation between the methoxyl and propenyl groups; syn if they stay on the same side with respect to a plane perpendicular to the benzene ring and anti if on opposite sides. We believe that precise experimental information on the structure and internal dynamics can be supplied by rotational spectroscopy. In the present investigation, we report the rotational study of E-An in its ground vibrational state using a millimeter-wave free-jet spectrometer. The conformational equilibrium and the internal dynamics in this molecule are discussed in detail.

anti

syn

Figure 1. The two most stable conformers of E-An in the principal axes system. 2-Experimental The millimeter-wave (59.6-74.4 GHz) spectrum of E-An was recorded using the free jet absorption millimeter wave spectrometer, the basic design of which has been described previously.29 E-An was purchased from Sigma Aldrich (99%) and used without further purification. Carrier gas argon, at a pressure of 0.2 MPa, passed over the sample which was heated to about 353 K, and expanded through a 0.35 mm diameter nozzle into the chamber (about 0.05 kPa) reaching an estimate rotational temperature of about 10 K. No evidence of thermal decomposition was observed. Electric fields of 750 V/cm were used to maximize the degree of Stark modulation. 3-Theoretical Calculations ACS Paragon Plus Environment

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Since no spectroscopic constants (eg. rotational constants) useful for the rotational spectroscopic investigation have been reported, we performed our own calculations. Geometry optimizations were carried out at MP2/6-311++G(d,p), B3LYP/6-311++G(d,p) and B3LYP/aug-cc-pVTZ levels, respectively, by using the Gaussian 09 program package, rev. D.01.30 Computed relative energies, rotational constants and dipole moment components of the two conformers are summarized in Table 1. Harmonic vibrational frequency calculations at the same levels proved both conformers to be real minima and provided the zero-point correction energies and quartic centrifugal distortion (CD) constants. Here only DK is reported since the others CD parameters are nearly zero. It is worth noting that the use of different theoretical methods (MP2 and B3LYP) leads to different global minima: DFT calculations suggest the anti conformer to be slightly lower in energy than the syn one, while the situation for MP2 calculations is reversed. The relative Gibbs free energies calculated using DFT methods at the work temperature of 353 K, led to two almost isoenergetic conformers, instead with the MP2 method the energetic gap doubled. Moreover, in the MP2 structures the propenyl chain is not coplanar (about 30°) to the benzene ring. Table 1. Calculated spectroscopic parameters of the two stable conformations of E-An. anti MP2

B3LYP

syn B3LYP

MP2

B3LYP

B3LYP

6-311++G(d,p) 6-311++G(d,p) -cc-pVTZ 6-311++G(d,p) 6-311++G(d,p) -cc-pVTZ -1

∆E/kJ mol 0.23 ∆E0/kJ·mol-1 0.25 ∆G0353K/kJ mol-1 g 0.46 A/MHz 4359 B/MHz 459 C/MHz 420 DK/kHz 3.4 ∆c/uÅ2 -14.61 µa/D 0.4 µb/D -1.1 µc/D -0.2 V3propenyl/kJ mol-1 7.94

0a 0d 0 4516 455 415 0.6 -6.33 0.6 -1.1 0.0 8.02

0b 0c 0e 0f 0 0 4539 3812 457 471 417 424 0.6 3.5 -6.30 -14.77 0.7 -0.1 -0.9 -1.4 0.0 0.0 8.05 8.17

0.92 0.79 0.12 3962 465 418 1.9 -6.34 -0.3 -1.4 0.0 7.76

0.75 0.67 0.02 3985 467 420 1.9 -6.30 -0.4 -1.3 0.0 8.02

a

Absolute energy value: -463.616631 Eh; bAbsolute energy value: -463.659571 Eh; cAbsolute energy value: -

462.223083 Eh; dAbsolute energy value: -463.423821 Eh; eAbsolute energy value: -463.466261 Eh; fAbsolute energy value: -462.029632 Eh; gCalculated using the output from the Gaussian calculations and the tools supplied by NIST [http://www.nist.gov/mml/csd/informatics_research/thermochemistry_cgi.cfm] (see Ref. 31).

Conformational relaxation upon supersonic expansion in our conditions is not expected, since it takes place easily in Ar carrier when the interconversion barrier is smaller than 2kT,32 where T is the ACS Paragon Plus Environment


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temperature before supersonic expansion (2kT is about 5.9 kJ mol-1 at 353 K, the pre-expansion temperature in our case). However, as already mentioned in Ref. 27, barriers to the conformational interconversion along two plausible pathways are relatively high: the one related to the torsion of the methoxy group is about 10.5 kJ mol-1; whilst the other one, related to the re-orientation of the propenyl moiety, is about 16 kJ mol-1. 4-Rotational Spectra According to the computational predictions, which suggest µb to be the largest component of the electric dipole moment for both conformers, the µb-R-branch lines with J ranging from 9 to 20 and Ka = 7 ← 6 of the anti conformer were identified first. Each transition displayed a triplet as shown in Figure 2 (left) for the transition 117←106, due to the effects of internal rotation of a methyl group on rotational transitions connecting degenerate levels (see the following section on the Internal Dynamics). Later on, many more lines with Jupper and Ka values up to 29 and 8, respectively, were measured. At much lower frequency range, we found another set of transition lines which could be assigned to the second conformer (syn). These lines have a similar pattern (see Figure 2 (right), for the transition 118←107) as the ones of anti but are weaker in intensity.

Figure 2. The 117←106 rotational transition of the anti-(left) and the 118←107 rotational transition of the syn-(right) conformers. The triplet pattern is due to the internal rotation of the propenyl –CH3 group. All the transitions (available as Supporting Information) were fitted with the XIAM program based on the Combined Axis Method (CAM).33 This code supplies, apart from the rotational and centrifugal distortion constants, parameters with a clear physical meaning, such as the V3 barrier to internal rotation, the angles (∠(i,g), g = a, b, c) between the internal rotation axis and the principal axes, and the moment of inertia of the internal top (Iα). In the fits, Watson’s S reduced semirigidACS Paragon Plus Environment

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rotor Hamiltonian (Ir representation) has been used.34 The results have been summarized in Table 2. Only the ∠(i,a) angle is reported, because ∠(i,b) is the complement to 90° of ∠(i,a), and ∠(i,c) is 90° by symmetry. ∠(i,a) was not satisfactorily determined from the fits, so that it was fixed to the B3LYP/cc-pVTZ value. Also Iα was poorly determined in the fit, so also its value has been fixed at the value derived from the theoretical geometry. Table 2 Experimental spectroscopic constants of conformers anti and syn. A/MHz B/MHz C/MHz ∆c/uÅ2 DK/kHz V3/kJ mol-1 Iα/uÅ2 ∠(i, a)/° σc/MHz Nd a

anti 4478.059(5)a 457.667(5) 417.702(4) -7.20(2) 0.80(4) 7.080(5) 3.142b 15.2b 0.04 78

syn 3915.566(4) 468.833(4) 421.288(3) -7.42(1) 1.84(2) 6.978(4) 3.142b 7.9b 0.04 120

Error in parentheses in units of the last digit; bFixed at the value derived from the B3LYP/cc-pVTZ

geometry. cRoot-mean-square deviation of the fit; dNumber of lines in the fit.

The comparison of the experimental rotational constants in Table 2 with the theoretical ones in Table 1 gives a straightforward conformational assignment. The experimental values of the inertial defects are ∆c = -7.20 and -7.42 uÅ2 for the anti and syn species, respectively. These values are in agreement with the planarity of the mainframe, once considered the out of plane contributions of the hydrogens of the two methylenic groups and the torsional contributions of the vinyl group. We can then draw some conclusion about the performance of the used computational methods: our experimental evidences indicate that the B3LYP methods are more effective in determining the right configurations for this kind of molecules. Accordingly, the calculated rotational constants obtained using the B3LYP method are in better agreement with experimental values, with the largest discrepancy of ~1%. The B3LYP/cc-pVTZ level seems to provide the most precise values of the rotational constants, and the corresponding results will be used throughout the following sections. The full B3LYP/cc-pVTZ geometries of both conformers are also given as Supporting Information. 5-Internal Dynamics By inspection of Figure 1, one can see that in E-An there are two different methyl groups: the propenyl and the methoxy one. The V3 barriers to internal rotation of the methoxy CH3 group is ACS Paragon Plus Environment


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expected to be quite high for both conformers, in such a way that we did not observe measurable splitting, concordantly with the results found for the prototype molecule anisole35 and for the similar compound estragole which presents the same group.16 On the other hand, V3 for the propenyl CH3 group is much lower for both conformers, and, as a consequence, the measured transitions are split into triplets. This feature is typical of transitions connecting asymmetry degenerate levels.36 In this case internal rotation of the methyl group splits the torsional levels into three sublevels, labelled as A, E+ and E-, and the rotational transitions take place within each sublevel, forming triplets with intensity ratios 2/1/1 for the A, E+ and E- sublevels, respectively.36 The ∠(i, a) angles (see Table 2) have small values for both conformers, in agreement with what expected for the propenyl CH3 groups (which internal rotation axes are nearly parallel to the a principle axes). The experimental values of the V3 barriers to internal rotation of the propenyl CH3 top) (V3anti = 7.080(5) kJ mol-1 and V3syn = 6.978(4) kJ mol-1) are in agreement with the theoretical values (see Table 1) and have a considerably smaller uncertainty with respect to the value (7.45 kJ mol-1), common to both conformers, reported in the fluorescence spectroscopy study.26 The obtained V3 values are ca. 10% and 20% lower, respectively, than the corresponding parameters of the isopropenyl methyl groups in propene37 and isopropropenyl acetate.38 Probably such V3 lowering is related to the conjugation of the ethene group with the abenzene ring. The similar values of the V3 barriers imply that these quantities are not affected in any appreciable way by the orientation of methoxyl group. 6-Conformational Equilibria Based on the assumption that no conformational relaxation takes place during the supersonic expansion, an estimation of the relative energy differences ∆E0.0(=E0.0(syn)- E0.0(anti)) between the two conformers can be obtained, according to the following equation:39 ∆E0,0 = kT ln(IA ∆νA µb,S2 γS νS2/IS ∆νS µb,A2 γA νA2)

(1)

Here I, ∆ν, γ and ν are the peak intensity, the full width of the line, the line strength and the frequency, respectively, of the considered transition. In the same equation µb is the calculated dipole moment component at B3LYP/aug-cc-pVTZ level, while T is the temperature before the supersonic expansion (that in this case is 353 K). The subscript A and S denote the anti and syn conformers, respectively. Four pairs of µb-type transitions close in frequency, were chosen for this purpose (the data used for this analysis are given as Supporting Information). For each rotational transitions split into a triplet, the A-E+ doublet (see Figure 2) was considered giving the result of ∆E0.0= 2.0(5) kJ mol-1,


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which is in the reverse trend suggested by ab initio calculations and also disagree with isoenergetical results of DFT methods. 7-Organoleptic Properties of E-An: a Comparison with Estragole It is worth noting that an isomer of E-An, estragole (1-allyl-4-methoxybenzene) which shows a different location of the C=C bond in the propenyl substituent, has a scent similar to that of E-An but it is suspected to be carcinogenic and genotoxic.40 Schemes of molecular structures of Eanethole and estragole are shown in Figure 3.

O

O

E-Anethole

Estragole

Figure 3. Schemes of molecular structures of E-An and estragole According to the results revealed by rotational studies,16 estragole has two stable conformers in the supersonic jet, both with the vinyl tail not coplanar to the aromatic ring. As already mentioned above, no splitting has been observed due to the internal rotation of methoxy –CH3, which is in agreement with the case of E-An. One can note how a small change in the electronic structure of the molecule (the shift of one carbon position of the C=C double bond) greatly affects its organoleptic properties.

8-Conclusions The rotational spectra of the two conformers, syn and anti of E-An have been assigned using the free jet broadband millimeter-wave spectroscopic technique. The two conformers differ in the



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orientation of the methoxyl group with respect to the propenyl moiety, with all heavy atoms lying in the benzene plane. The –CH3 group of both conformers in the propenyl chain undergoes a hindered internal rotation with more or less the same V3 barrier (~ 7 kJ mol-1), which gives rise to the observable splittings of the rotational transitions. Despite the fact that the conformational energy surface is rather simple, only two stable conformers are predicted and observed in the gas phase, the results show that theoretical calculations are still faced with challenges in describing accurately the conformational landscapes of flexible molecules. Different theoretical methods give the opposite order of relative energies of these two conformers, whilst from intensity measurements it is possible to confirm that the global minimum is the anti form. This rotational study sheds light on a complete picture of E-An, and can be further used as benchmark to the theoretical calculations. In this case the experimental estimation is more consistent with B3LYP calculations rather than with MP2 ones. Acknowledgment. We thank the Italian MIUR (PRIN project 2010ERFKXL_001) and the University of Bologna (RFO) for financial support. Supporting Information Available: Full reference [30], B3LYP/aug-cc-pVTZ starting geometries and tables of transition frequencies of both anti and syn conformers of E-anethole. This information is available free of charge via the Internet at http://pubs.acs.org. References (1) Sell, C. On the Unpredictability of Odor. Angew. Chem. Int. Ed. 2006, 45, 6254-6261. (2) Jelisavac, D.; Cortés Gómez, D. C.; Nguyen, H. V. L.; Sutikdja, L. W.; Stahl, W.; Kleiner, I. The Microwave Spectrum of the trans Conformer of Ethyl Acetate. J. Mol. Spectrosc. 2009, 257, 111-115. (3) Mouhib, H.; Zhao, Y.; Stahl, W. Two Conformers of Ethyl Pivalate Studied by Microwave Spectroscopy. J. Mol. Spectrosc. 2010, 261, 59-62. (4) Mouhib, H.; Jelisavac, D.; Sutikdja, L. W.; Isaak, E.; Stahl, W. Structural Studies on Ethyl Isovalerate by Microwave Spectroscopy and Quantum Chemical Calculations. J. Phys. Chem. A 2011, 115, 118-122. (5) Tudorie, M.; Kleiner, I.; Hougen, J. T.; Melandri, S.; Sutikdja, L. W.; Stahl, W. A Fitting Program for Molecules with Two Inequivalent Methyl Tops and a Plane of Symmetry at


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EMEA/HMPC/137212/2005, Committee on Herbal Medicinal Products. Final Public

Statement

on

the

Use

of

Herbal

Medicinal

Products

http://www.ema.europa.eu/docs/en_GB/document_library


Containing

Estragole


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

Table of Contents artwork

Graphical abstract:


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Conformational Equilibrium and Internal Dynamics of E-Anethole: A

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