Sulfur-Containing Flavors: Gas Phase Structures of Dihydro-2-methyl

Article pubs.acs.org/JPCA

Sulfur-Containing Flavors: Gas Phase Structures of Dihydro-2-methyl-3-thiophenone Halima Mouhib,* Vinh Van, and Wolfgang Stahl Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany S Supporting Information *

ABSTRACT: Dihydro-2-methyl-3-thiophenone was investigated using a combination of quantum chemical calculations and molecular beam Fourier transform microwave spectroscopy. The substance is present in coffee, roasted peanuts, and whiskey. The microwave spectrum was recorded under molecular beam conditions in the frequency range from 9 to 14 GHz. We report on the two main conformers of dihydro-2-methyl-3thiophenone, for which highly accurate rotational constants and centrifugal distortion constants were obtained. No splittings due to internal rotation of the methyl group could be observed in the microwave spectrum. This is in agreement with the theoretical predictions of the barrier heights, which have been determined to be more than 1000 cm−1 at the MP2/6-311++G(d,p) level of theory. In addition to the most abundant 32S-isotopologue of the main conformer, also the 34S-isotopologue was assigned, which occurs with a natural abundance of about 4%. Using the experimental rotational constants, different quantum chemical calculations were validated for the two observed conformers. To complete the theoretical investigation of dihydro-2-methyl-3-thiophenone, different transition states were optimized to understand the intramolecular conversion between the two conformers at the MP2/6-311++G(d,p) level. The transition states were optimized using the Berny algorithm.

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INTRODUCTION Sulfur containing compounds find widespread applications in the flavor industry, especially when meaty, garlic, onion, and vegetable scents are needed. The scents of sulfurous odorants are highly dependent on the concentration applied. In higher concentrations most sulfurous compounds are rather repelling, especially when sulfur is present in the form of a thiol group. The endless structural variety and odor composition of these compounds make them fascinating, especially as their oxygen analogues are usually free of any malodors. Here, we report on the gas phase structures of dihydro-2methyl-3-thiophenone (see Scheme 1), investigated using a

discovered and shows the general interest in this small molecule.8 While dihydro-2-methyl-3-thiophenone lacks the strong sulfurous malodor of thiols, its oxygen analogue, 2-methyldihydrofuran-3(2H)-one, still possesses a different bread-like scent.9 Although, the structures of such small odorants might not be as relevant for structure-odor correlations as in the case of Cassyrane, an artificial blackcurrant odorant, we were still interested in its structure and dynamics.10 There is still significant interest in characterizing the various conformers of organic molecules in terms of relative energies, structures, and dipole moments. Until now, only little physical data is available on small volatile molecules such as dihydro-2-methyl-3thiophenone.

Scheme 1. Dihydro-2-methyl-3-thiophenone

EXPERIMENTAL AND COMPUTATIONAL SECTION Spectral Assignment. To record the spectrum, a molecular beam Fourier transform microwave (MB-FTMW) spectrometer was used operating in the frequency range from 3.0 to 26.5 GHz.11 Dihydro-2-methyl-3-thiophenone was purchased from Alfa-Aesar, Karlsruhe, Germany, with a stated purity of approximately 97%. It was used without further purification. Its vapor pressure of 1.2 hPa12 is sufficiently high to record an intense spectrum. For this purpose, a 5 cm piece of a pipe cleaner, serving as a carrier for the sample, was inserted into a stainless steel tube with an inner diameter of 4 mm and

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combination of microwave spectroscopy and quantum chemical calculations. It has a sulfurous meat-like odor and is rather repellent at higher concentrations.1,2 Dihydro-2-methyl-3thiophenone can be found as a natural aroma in several beverages like coffee, whiskey, and wine.3 It is formed during food preparation such as roasting peanuts4 and contributes to the savory flavor composition. Furthermore, it is produced as an important metabolite by microorganisms such as yeasts and bacteria.5−7 The biosynthetic pathway of dihydro-2-methyl-3thiophenone in the bacterium Chitinophaga Fx7914 was recently © XXXX American Chemical Society

Received: April 27, 2013 Revised: June 28, 2013

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dx.doi.org/10.1021/jp4041748 | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Table 1. Spectroscopic Data of the Two Main Isotopologues and the 34S Isotopic Species of the Most Abundant Conformer of Dihydro-2-methyl-3-thiophenone As Obtained with the Xiam Code: Watson’s A Reduction and Ir Representation Were Used unit b

A Bb Cb ΔJc ΔJKc ΔKc δJc δKc κd Ne σf

GHz GHz GHz kHz kHz kHz kHz kHz

kHz

32

S-conformer I obsda

34

2.95574439(27) 2.36570849(23) 1.40084533(25) 0.1701(96) 0.4559(42) −0.0509(30) 0.05898(63) 0.3651(24) 0.24106 73 2.0

S-conformer I obsda 2.94001045(31) 2.32443805(17) 1.38307777(16) 0.1770(36) 0.4135(80) −0.0509(fixed)g 0.0665(14) 0.3587(39) 0.20925 36 1.6

32

S-conformer II obsda 3.0237787(13) 2.21279981(66) 1.51465720(58) 0.665(17) −1.134(48) −2.013(82) 0.0906(42) 1.553(26) −0.07477 33 5.2

a

Fitted constant, single standard errors in units of the least significant digit in parentheses. bRotational constants. cQuartic centrifugal distortion constants. dRay’s asymmetry parameter. eNumber of lines included in the fit. fStandard deviation of the fit. gParameter fixed to the experimental value of the 32S-conformer I.

Figure 1. Broadband scan of dihydro-2-methyl-3-thiophenone (upper trace) and the simulated spectrum (lower trace). For the simulation, an intensity ratio of 1(32S-conformer I):0.33(32S-conformer II):0.04(34S-conformer I) and a rotational temperature of 0.6 K were assumed. The calculations were carried out using the Xiam code. The simulated spectra of the 32S-isotopologue of conformer I is shown in green, of the 32 S-isotopologue of conformer II in blue, and of the 34S-isotopologue of conformer I in orange.

moment in a direction with 31,2 ← 21,1, 30,3 ← 20,2, and 31,3 ← 21,2 transitions of high intensity. Therefore, these transitions were searched and identified as a-type R-branches. After assigning the a-type lines using the Xiam code, more b- and c-type transitions were found and added to the fit via trial and error. This procedure was performed for both conformers. Because of the intense spectrum of conformer I, which is also the most abundant in the molecular beam, we expected to observe also the 34S-isotopologue, which occurs with a natural abundance of about 4%. In total, 73 lines were included in the fit of the 32S-isotopologue of conformer I, 36 lines in the fit of the 34S-isotopologue of conformer I, and 33 lines in the fit of the 32S-isotopologue of conformer II. All strong lines were included in the fit. Unassigned lines were rather weak and may be attributed to numerous 13C-isotopologues. No splitting was observed arising from the internal rotation of the methyl group. Most lines were narrow and only in some cases broadened, probably due to magnetic coupling effects of the protons. The microwave spectroscopic data of dihydro-2-methyl-3-thiophenone is shown in Table 1. The standard deviations of the fits are close to our experimental accuracy of 2 kHz.

mounted upstream the nozzle. The carrier gas helium was flown over the sample at a pressure of approximately 150− 200 kPa. Using the low resolution mode of the spectrometer a strong spectrum was recorded in the frequency range from 9 to 14 GHz. Using the program Xiam,13 three rotational constants and five quartic centrifugal distortion constants could be determined with high accuracy for two different conformers. The constants were fitted using Watson’s A reduction and Ir representation.14 The initial rotational constants used to predict the theoretical spectrum were obtained from geometry optimizations at the MP2/6-311++G(d,p) level (see the section below). Using the theoretical geometries, a strong a-type spectrum was expected for both conformers. In the case of dihydro-2methyl-3-thiophenone, both assigned conformers are highly asymmetric with Ray’s asymmetry parameters of 0.241 and −0.075 for conformer I and conformer II (see Figure 2), respectively.15 Because of this, the characteristic structures of a-type R-branches, which usually form groups of lines in intervals of B + C in the spectrum, were missing. This initially made the assignment difficult. However, we expected a strong dipole B



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the (R)-enantiomer was studied by means of quantum chemical calculations. For this purpose, several start geometries were created and optimized at the MP2/6-311++G(d,p) level. Additionally, harmonic frequency calculations were performed to verify the nature of the stationary points. This method and basis set was chosen empirically, as we know from experience that it yields rotational constants that usually agree within 1% of the experimental constants.16,17 It is therefore widely used in the microwave spectroscopic community.18,19 The geometry of conformers I and II observed in the molecular beam was also optimized again at different levels of theory including HF, MP2, and DFT methods in order to check for convergence. Finally, different transition states of dihydro-2-methyl-3-thiophenone were optimized to understand the intramolecular conversion between the two conformers at the MP2/6-311++G(d,p) level of theory. The transition states were optimized using the Berny algorithm.20 All geometry and transition state optimizations were performed using the Gaussian09 program package.21

Figure 1 shows the recorded spectrum of dihydro-2-methyl3-thiophenone together with the simulated spectra of the assigned conformers and the 34S-isotopologue of conformer I. For the simulation, an intensity ratio of 1(32S-conformer I): 0.33(32S-conformer II):0.04(34S-conformer I) and a rotational temperature of 0.6 K were assumed. It should be noted that the simulation temperature is not very accurate. Also, simulations at 1.5 K yielded reasonable agreement with the experimental spectrum. The calculations were carried out using the Xiam code. Quantum Chemical Calculations. Dihydro-2-methyl-3thiophenone is a small heterocyclic molecule of medium ring size, which possesses a stereo center at the attached methyl group in proximity to the carboxylic group. Because of the considerable torsional strain through the ring structure, only two conformations arise for the molecule. Since enantiomers cannot be distinguished using our experimental method, only

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RESULTS AND DISCUSSION In total, two conformers were obtained from the quantum chemical calculations at the MP2/6-311++G(d,p) level. The geometries of both conformers are shown in Figure 2. The barriers to internal rotation of the methyl group were theoretically determined to be 1266 and 1068 cm−1 at the MP2/6-311++G(d,p) level, for conformer I and conformer II, respectively. Sample calculations using Xiam have shown that these barriers will not lead to splittings, which can be observed at our resolution. Table 2 shows an overview of the quantum chemical results for both conformers of dihydro-2methyl-3-thiophenone at the MP2/6-311++G(d,p) level. The observed intensities of the spectrum (see Figure 1) are in good agreement with the strong dipole moment components of both conformers predicted in a direction from the ab initio calculations. The validation of the quantum chemical results at different levels of theory is given in Tables 3 and 4. The rotational constants obtained at the MP2/6-311++G(d,p) level agree in both cases within less than 1% deviation with the experimental

Figure 2. Structures of dihydro-2-methyl-3-thiophenone calculated at the MP2/6-311++G(d,p) level and observed in the molecular beam. TS1 and TS2 are the transition state geometries. The colors red, gray, white, and yellow correspond to oxygen, carbon, hydrogen, and sulfur, respectively.

Table 2. Quantum Chemical Data of Dihydro-2-methyl-3-thiophenone at the MP2/6-311++G(d,p) Level of Theory isotopologue 32

S-conf. S-conf. 34 S-conf. 32 S-conf. 33

I Ie Ie II

Aa

dev.b [%]

Ba

dev.b [%]

Ca

dev.b [%]

μac

μbc

μcc

μtotal

ΔE [kJ/mol]d

2.974 2.967 2.960 3.044

−0.62 f −0.70 −0.68

2.350 2.328 2.307 2.199

0.68 f 0.75 0.60

1.401 1.392 1.383 1.521

−0.02 f 0.01 −0.44

2.03 2.01 2.00 1.76

0.72 0.76 0.80 0.69

0.60 0.60 0.60 1.27

2.23 2.23 2.23 2.28

0.00

0.54

Rotational constants. b[(obsd − calcd)/obsd] × 100 with the constants calculated at the MP2/6-311++G(d,p) level of theory. cDipole moment components with respect to the Cartesian coordinates in the principal axes of inertia as defined in the Supporting Information. dRelative energy with respect to the lowest energy conformer. eValues derived from the geometry of the 32S-conformer I at the MP2/6-311++G(d,p) level. f33 S-isotopologue was not observed experimentally. a

Table 3. Quantum Chemical Data Obtained for 32S-Conformer I of Dihydro-2-methyl-3-thiophenone at Different Levels of Theory; for Additional Results See Table S9 in the Supporting Information method/basis set

Aa

dev.b [%]

Ba

dev.b [%]

Ca

dev.b [%]

μac

μbc

μcc

μtotald

MP2/6-311++G(3df,2dp) MP2/6-311++G(d,p) B3LYP/6-311++G(3df,2dp) B3LYP/6-311++G(d,p) HF/6-311++G(3df,2dp) HF/6-311++G(d,p)

2.988 2.974 2.941 2.925 2.970 2.959

−1.10 −0.62 0.52 1.04 −0.50 −0.10

2.376 2.350 2.348 2.329 2.387 2.374

−0.45 0.68 0.77 1.55 −0.91 −0.36

1.412 1.401 1.387 1.378 1.408 1.401

−0.80 −0.02 1.01 1.65 −0.50 0.00

2.05 2.03 1.63 1.61 1.79 1.80

0.66 0.72 0.67 0.71 0.67 0.72

0.82 0.60 0.59 0.58 0.60 0.60

2.31 2.23 1.85 1.85 2.00 2.03

a

Rotational constants in GHz. bRelative deviation in percent with respect to the experimental value. cDipole moment components with respect to the Cartesian coordinates in the principal axes of inertia as defined in the Supporting Information. dTotal dipole moment. C



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Table 4. Quantum Chemical Data Obtained for 32S-Conformer II of Dihydro-2-methyl-3-thiophenone at Different Levels of Theory; for Additional Results See Table S10 in the Supporting Information method/basis set

Aa

dev.b [%]

Ba

dev.b [%]

Ca

dev.b [%]

μac

μbc

μcc

μtotald

MP2/6-311++G(3df,2dp) MP2/6-311++G(d,p) B3LYP/6-311++G(3df,2dp) B3LYP/6-311++G(d,p) HF/6-311++G(3df,2dp) HF/6-311++G(d,p)

3.075 3.044 2.985 2.967 3.016 3.003

−1.71 −0.68 1.28 1.89 0.25 0.69

2.201 2.199 2.232 2.218 2.275 2.260

0.54 0.60 −0.85 −0.22 −2.81 −2.13

1.550 1.521 1.471 1.459 1.494 1.489

−2.35 −0.44 2.89 3.68 1.35 1.70

1.77 1.76 1.34 1.32 1.50 1.49

−0.62 −0.69 −0.69 −0.73 −0.71 −0.74

1.39 1.27 0.84 0.84 0.90 0.96

2.33 2.28 1.72 1.73 1.88 1.93

a

Rotational constants in GHz. bRelative deviation in percent with respect to the experimental value. cDipole moment components with respect to the Cartesian coordinates in the principal axes of inertia as defined in the Supporting Information. dTotal dipole moment.

Figure 3. Energy diagram for the intramolecular conversion of dihydro-2-methyl-3-thiophenone and the corresponding transition states connecting the two energy minima as calculated at the MP2/6-311++G(d,p) level.

MP2/6-311++G(d,p) level and the list of observed frequencies included in the fits. This material is available free of charge via the Internet at http://pubs.acs.org.

constants. This may also be considered as an a posteriori justification for using this level of theory to calculate the starting structures and facilitate the spectrum assignment. The calculations for the intramolecular conversion of dihydro2-methyl-3-thiophenone yielded two different transition states (TS) at the MP2/6-311++G(d,p) level. The TS have a nonplanar geometry and correspond to two different envelope conformations of the ring. Both TS represent a different pathway for the conversion from one energy minimum to the other. The intramolecular conversion pathway of dihydro-2-methyl-3-thiophenone optimized at the MP2/6-311++G(d,p) level is shown in Figure 3. The optimized structures of the TS are shown in Figure 2. The activation energy for the lowest TS was determined to be 8.49 kJ/mol with respect to conformer I. The envelope TS1 is about 1.00 kJ/mol lower in energy than the envelope TS2.

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AUTHOR INFORMATION

Corresponding Author

*(H.M.) Phone: +49/241/80/94759. Fax: +49/241/80/92365. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Center for Computing and Communication of the RWTH Aachen University for free computer time and the Land Nordrhein-Westfalen for funds.

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ASSOCIATED CONTENT

S Supporting Information *

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Cartesian coordinates in the principal axis system of inertia for all conformers and transition states optimized at the D



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Sulfur-Containing Flavors: Gas Phase Structures of Dihydro-2-methyl

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