Barrier to Methyl Internal Rotation and Equilibrium Structure of 2-Methylthiophene Determined by Microwave Spectroscopy

Abstract

The microwave spectrum of 2-methylthiophene was recorded in a frequency range from 2 to 26.5 GHz using a molecular-jet Fourier transform microwave spectrometer with a Fabry–Pérot type resonator chamber and coaxial arrangement of the resonator and the molecular beam. Measuring and assigning spectra of the 34S and 13C isotopologues allowed the determination of the semiexperimental equilibrium structure (reSE). Comparing the structure to that of thiophene revealed a decrease in the ∠(S−C2−C3) angle from 111.595(6)° to 111.37(1)° by addition of the methyl group to the C(2) position, as well as an increase in the S−C2 bond length from 1.7102(1) Å to 1.7219(2) Å. A–E splittings from internal rotation of the methyl group were observed, and the V3 potential in the vibrational ground state was determined to be 197.7324(18) cm−1. The V3 value and the rotational constants A, B, C were calculated with a large number of different methods and basis sets for benchmarking purposes by comparing them to the fitted parameters. The V3 value was also compared to those of other thiophene and furan derivatives to gain a better understanding of the steric and electrostatic effects in these classes of compounds.

Noncovalent Interactions in the Molecular Geometries of 4-Methylthiazole···H2O and 5-Methylthiazole···H2O Revealed by Microwave Spectroscopy

“…The magnitude of this barrier is denoted as V 3 and has been reported to be 34.1 cm –1 for 2-methylthiazole, 357.6 cm –1 for 4-methylthiazole, and 332.0 cm –1 for 5-methylthiazole. Similar studies have been performed for isomers of methylimidazole, methylpyrrole, − methyloxazole, , methylisoxazole, , methylfuran, − methylisothiazole, and methylthiophene. − Generally, for five-membered heteroaromatic rings containing two heteroatoms, the lowest barrier is observed where CH 3 is substituted onto the 2-position (the convention for atom numbering is shown in Figure ). Higher barriers are observed where CH 3 is substituted onto the 4- or 5- positions because of differences in electronic structure.…”

“…O and 5-MT••• H 2 16 O are calculated to be −3.6540 (23) and −3.2681(22) u Å 2 , respectively. Evidently, the attachment of H 2 O to each of 4-methylthiazole and 5methylthiazole adds a very small amount of out-of-plane mass such that Δ 0 becomes more negative.…”