On the origin and nature of internal methyl rotation barriers: an information-theoretic approach study

The microwave spectrum of 2,3-dimethylfluorobezene, one of the six isomers of dimethylfluorobenzene, was recorded using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range from 2 to 26.5 GHz. The internal rotations of two inequivalent methyl groups, causing splittings of up to several hundred MHz of all rotational energy levels into quintets, were analyzed and modeled. The torsional barriers of the methyl groups at the ortho and the meta positions were determined to be 215.5740(56) cm-1 and 488.53(11) cm-1. A comparison with the barrier heights observed for the two isomers 2,6-dimethylfluorobenzene and 3,4-dimethylfluorobenzene has shown that the methyl group at the meta position seems to be invisible to its neighboring ortho-methyl group, while the meta-methyl group clearly senses the ortho one. Steric effects are not able to explain this observation, and electrostatic effects are most probably the reason. Highly accurate molecular parameters determined experimentally were compared with those obtained from quantum chemical calculations at different levels of theory.

Large Amplitude Motions in 2,3-Dimethylfluorobenzene: Steric Effects Failing to Interpret Hindered Methyl Torsion.

Two methyl internal rotations of 2-acetyl-4-methylthiophene explored by microwave spectroscopy and quantum chemistry.

The microwave spectrum of 2-acetyl-5-methylthiophene (2A5MT) was recorded using a molecular jet Fourier transform microwave spectrometer working in the frequency range of 2 to 26.5 GHz. The spectrum was assigned to the syn-conformer of the molecule while that of anti-2A5MT was not observable. For the assignment of the spectrum of 2A5MT, adequate spectral analysis skill and quantum chemical benchmarking helped to significantly reduce the time required for recording survey scans. The rotational and centrifugal distortion constants were determined with high accuracy. The experimental values of the rotational constants are compared to those derived from quantum chemical calculations in the course of ongoing benchmarking effort. Splitting of each rotational transition into quintets due to internal rotations of the acetyl methyl and ring methyl groups could be resolved and analysed to yield barriers to internal rotations of 301.811(41) cm-1 and 157.2612(13) cm-1, respectively. These values are compared to those found in other thiophene and furan derivatives in order to understand the electronic effects transmitted through aromatic rings, as well as how different heteroatoms affect torsional barriers. The acetyl methyl group features torsional barriers of around 300 cm-1 if a thiophene derivative is attached at the other side of the carbonyl group. This finding allows the establishment of the so-called "thiophene class" for the acetyl group containing ketones.

Benchmarking acetylthiophene derivatives: methyl internal rotations in the microwave spectrum of 2-acetyl-5-methylthiophene.

Large amplitude motions (LAMs) form a fundamental phenomenon that demands the development of specific theoretical and Hamiltonian models. In recent years, along with the strong progress in instrumental techniques on high-resolution microwave spectroscopy and computational capacity in quantum chemistry, studies on LAMs have become very diverse. Larger and more complex molecular systems have been taken under investigation, ranging from series of heteroaromatic molecules from five- and six-membered rings to polycyclic-aromatic-hydrocarbon derivatives. Such systems are ideally suited to create families of molecules in which the positions and the number of LAMs can be varied, while the heteroatoms often provide a sufficient dipole moment to the systems to warrant the observation of their rotational spectra. This review will summarize three types of LAMs: internal rotation, inversion tunneling, and ring puckering, which are frequently observed in aromatic five-membered rings such as furan, thiophene, pyrrole, thiazole, and oxazole derivatives, in aromatic six-membered rings such as benzene, pyridine, and pyrimidine derivatives, and larger combined rings such as naphthalene, indole, and indan derivatives. For each molecular class, we will present the representatives and summarize the recent insights on the molecular structure and internal dynamics and how they help to advance the field of quantum mechanics.

https://www.mdpi.com/1420-3049/27/12/3948/pdf?version=1655795333

Ha Vinh Lam Nguyen

Papers

Neighborhood matters: Steric effects on methyl internal rotation and chlorine nuclear quadrupole coupling in 2-fluoro-4-chlorotoluene

The two-top molecule 3-penten-2-one: Acetyl methyl torsion in α,β-unsaturated ketones