Chirality of and gear motion in isopropyl methyl sulfide: A Fourier transform microwave study

Abstract

Isopropyl methyl sulfide (CH(3))(2)CHSCH(3) was investigated by Fourier transform microwave spectroscopy. Two rotational isomers gauche and trans were detected. The rotational spectra of gauche were found fit to an asymmetric rotor pattern, except for being split by the internal rotation of CH(3) attached to S with the potential barrier V(3) of 601.642 (65) cm(-1) and for exhibiting the effect of tunneling between the two equivalent gauche forms in a few high-K transitions. The tunneling was discussed from a viewpoint of chirality. The trans spectra appeared generally similar to those of gauche, with V(3) to the S-CH(3) internal rotation of 559.00 (11) cm(-1), but satellite lines accompanied the ground torsional state lines in some high-K transitions. These satellites were ascribed to the excited state of the C(isop)-S torsion. In fact, the potential function for this torsion was shown by an ab initio calculation to be flat or even of double minima around the trans position, which was presumably caused by a gear coupling between the two methyl groups of the isopropyl group and the one in the S-CH(3).

Laboratory rotational spectroscopy of methyl n-propyl sulfide: Conformational analysis and methyl internal rotation

“…An interesting case is methyl isopropyl ether, for which Nakagawa et al reported a torsional barrier of 603 cm 1 for the methoxy methyl group O-CH3 determined by fitting the doublet structure observed for some transitions [57], which is unusually low for such methyl group and quite similar to the value of 601.642(65) cm 1 obtained for the gauche conformer of methyl isopropyl sulfide with comparable structure [58]. Currently, no conclusive statement could be made to explain the origin of different values of methyl torsional barrier in thioethers and ethers with the small number of available studies.…”

Unusual Dynamics of Alanine Residues in Polyalanine Regions with Staggered Packing Structure of Samia cynthia ricini Silk Fiber in Dry and Hydrated States Studied by 13C Solid-State NMR and Molecular Dynamics Simulation

“…In the case of closely located two methyl groups in the same molecule, a specially correlated methyl rotation which is called geared methyl rotation has been reported. − In a 9-methyltriptycene derivative, a raising barrier for the isolated rotation of the two methyl groups obtained by 1 H NMR line shape indicated that the two methyl groups in space could be considered as a pair of gear . Methyl groups in hexamethyl benzene have been reported to undergo a geared disrotatory motion in the solid state. , It is important to note that the geared motions in methyl groups have been reported in the densely located methyl groups in the molecules and tightly packed in the solid state.…”

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

“…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. , It is therefore widely used in the microwave spectroscopic community. , 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.…”

“…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.…”