Understanding (coupled) large amplitude motions: the interplay of microwave spectroscopy, spectral modeling, and quantum chemistry

Abstract

A large variety of molecules contain large amplitude motions (LAMs), inter alia internal rotation and inversion tunneling, resulting in tunneling splittings in their rotational spectrum. We will present the modern strategy to study LAMs using a combination of molecular jet Fourier transform microwave spectroscopy, spectral modeling, and quantum chemical calculations to characterize such systems by the analysis of their rotational spectra. This interplay is particularly successful in decoding complex spectra revealing LAMs and providing reference data for fundamental physics, astrochemistry, atmospheric/environmental chemistry and analytics, or fundamental researches in physical chemistry. Addressing experimental key aspects, a brief presentation on the two most popular types of state-of-the-art Fourier transform microwave spectrometer technology, i.e., pulsed supersonic jet expansion–based spectrometers employing narrow-band pulse or broad-band chirp excitation, will be given first. Secondly, the use of quantum chemistry as a supporting tool for rotational spectroscopy will be discussed with emphasis on conformational analysis. Several computer codes for fitting rotational spectra exhibiting fine structure arising from LAMs are discussed with their advantages and drawbacks. Furthermore, a number of examples will provide an overview on the wealth of information that can be drawn from the rotational spectra, leading to new insights into the molecular structure and dynamics. The focus will be on the interpretation of potential barriers and how LAMs can act as sensors within molecules to help us understand the molecular behavior in the laboratory and nature.

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

“…Microwave spectroscopy is classically connected with the topic of structural chemistry, because the primary parameters deduced from a microwave spectrum, the rotational constants, directly reflect the mass distribution of the atoms in the molecule [1]. However, along the history of almost one century since the first microwave spectrum of ammonia was measured in 1934 [2], large amplitude motions (LAMs) have been involved, either in the form of inversion motion [3], ring puckering [4], or internal rotation [5]. If the molecule features a methyl group undergoing internal rotation, a torsional fine structure consisting of A-E doublets can be observed.…”

A simplistic computational procedure for tunneling splittings caused by proton transfer

“…One of the most prominent LAM manifestation is the tunneling splitting of the ground vibrational state, whenever a motion between equivalent minima is involved. Such splittings can be observed, for instance, using microwave (MW) or millimeter wave (MMW) rotational spectroscopy [6][7][8][9][10][11][12][13][14][15]. A knowledge of the LAM and of its characteristics can be helpful for the interpretation of the experimental data [11,14,16,17].…”

Local vs global approaches to treat two equivalent methyl internal rotations and 14N nuclear quadrupole coupling of 2,5-dimethylpyrrole

“…Due to challenges in both spectral assignment and fit, only a limited number of two-top molecules have been investigated, as reviewed by Nguyen and Kleiner. 2 Unlike inequivalent two-top cases where the point group of the frame symmetry can be C 1 or C s , in the cases of two equivalent methyl groups, due to the higher molecular symmetry requirement, e.g., C 2 , C 2h , or C 2v , the number of studies reported in the literature are even smaller. Some examples of equivalent two-top molecules are dimethylgermane, 5 dimethylamine, 6 dimethylketene, 7 isobutylene, 8 propane, 9 2,6-lutidine, 10,11 difluorodimethylsilane, 12 2-bromopropane, 13 dimethyl sulfate, 14 acetone, 15,16 dimethyl ether, 17 diethyl ketone, 18 dimethyl sulfide, 19 2,5-dimethylthiophene, 20 2,5-dimethylfuran, 21 and 2,6dimethylfluorobenzene.…”