Sulfur-Containing Flavors: Gas Phase Structures of Dihydro-2-methyl-3-thiophenone

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-3-thiophenone, 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 (32)S-isotopologue of the main conformer, also the (34)S-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.

The heavy atom structures and 33S quadrupole coupling constants of 2-thiophenecarboxaldehyde: insights from microwave spectroscopy

“…This angle was kept at a fixed value while the remaining degrees of freedom were allowed to relax during optimizations at the MP2/6-311++G(d,p) level of theory [28]. We chose this level of theory since previous microwave spectroscopic studies on related sulfur-containing rings such as 2methyltetrahydrothiophene [29], 2-methyl-1,3-dithiolane [30], and tetrahydrothiophen-3-one [8] have shown that it provides reliable rotational constants that can be used directly for the assignment of the experimental spectra. All calculations were carried out using the Gaussian 16 program package [31].…”

“…These types of small, volatile organic compounds possess numerous applications, e.g., as precursors in the drug and cosmetic industry [1], as agricultural agents [2], and in energetics as fluorescents and organic light-emitting diodes, as well as components in fossil fuels [3][4][5]. A large number of their alkyl derivatives, e.g., 2-methyltetrahydrofuran-3-one and 2-acetyl-5-methylfural, also contribute to the scent and flavor composition of food, such as coffee beans [6][7][8] and smoked salmon [9]. In an early investigation, 2TPC has been measured by Mönnig et al in the microwave region from 6 to 20 GHz using a Stark modulated spectrometer, and a small number of low-J transitions were reported.…”

Conformational Transformations of Sulfur‐Containing Rings: 2‐Methyltetrahydrothiophene Gas‐Phase Structures

“…Under molecular beam conditions, we expect to observe several twist conformations of MTTP, which are linked by envelope transition states. The twist conformations often appear as the most stable structures in many other five‐membered rings 6–8…”

What are the spectroscopic properties of HFC-32? Answers from DFT

“…[ ), and conformational energetic and structures of biomolecules (e.g., Refs. [ ), by means of infrared (IR) and microwave (MW) spectroscopy. Therefore, it is not surprising that several investigations have appeared, aimed at assessing the performances of DFT for the solution of the vibrational problem, and properly tailored basis sets, as well as composite CC/DFT approaches have been proposed .…”