The LAM of the Rings: Large Amplitude Motions in Aromatic Molecules Studied by Microwave Spectroscopy

Abstract

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.

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

“…The only exception of thiophene derivatives where the anti -conformer is more stable is the isomer 2A3MT 23 of 2A5MT due to steric interactions between the ring methyl group and the acetyl group. 36…”

“…The addition of a methyl group at the 4-and the 5-position of the thiophene ring makes the molecule more prolate compared to 2-acetylthiophene or 2-acetylfurane and decreases the barrier, while the addition of a methyl group at the 3-position makes the molecule more globular and increases the barrier. 36 Overall, it is noticeable that the barriers of the acetyl methyl group fall into a much wider range for furan derivatives (212.7 cm À1 to 319.8 cm À1 ) than that for thiophene derivatives (281.2 cm À1 to 330.2 cm À1 ). In ref.…”

“…The only exception of thiophene derivatives where the anti-conformer is more stable is the isomer 2A3MT 23 of 2A5MT due to steric interactions between the ring methyl group and the acetyl group. 36 In our previous investigations on thiophene derivatives, 23,24,45,64 we often observed discrepancies while calculating the molecular structures at different levels of theory concerning the planarity. A number of MP2 calculations predicted the molecules to be nonplanar due to an out-of-plane tilt of the acetyl group.…”

“…Aromatic rings are of special interest while coupled with multiple methyl internal rotors as the conjugated double bond system is able to transfer electrostatic effects over a longer distance through the p-electrons, making the barrier to methyl torsion sometimes unpredictable. 36 Heterocycles such as thiophene and furan additionally have the influence of the heteroatom on the torsional barrier. It was shown in the literature that changing from sulphur to oxygen increases the barrier, i.e., dimethyl sulfide (736 cm À1 ) 66 vs. dimethyl ether (944 cm À1 ), 67 trans-ethyl methyl sulfide (693 cm À1 ) 68 vs. trans-ethyl methyl ether (893 cm À1 ), 69 conformer III of n-propyl sulfide (699 cm À1 ) 70 vs. trans-trans-methyl-n-propyl ether (1154 cm À1 ), 71 and S-phenyl thioacetate (48 cm À1 ) 72 vs. phenyl acetate (137 cm À1 ).…”

“…35 We were interested in exploring the torsional barriers in 2A5MT, and comparing them with the values found for 2A3MT 23 and 2A4MT 24 as well as related molecules to better understand the steric and electrostatic effects in thiophene containing molecules with methyl internal rotation(s). 36 For aliphatic ketones containing an acetyl group CH 3 -CO, Andresen et al proposed a classification to link the barrier height of the acetyl methyl group CH 3 -to the molecular structure. 37,38 Linear aliphatic ketones were categorised into two classes: the C 1 class with a barrier of about 240 cm À1 and the C s class with a barrier of approximately 180 cm À1 .…”