Rotational Spectroscopy of the Acetone-Water Complex: Large Amplitude Motions.

Acetone (CH3COCH3), the simplest ketone, has recently attracted considerable attention for its important role in atmospheric chemistry and in the formation of ices in extraterrestrial sources that contain complex organic molecules. In this study, we employed a combination of experimental rotational spectroscopy and quantum chemistry calculations to investigate the structure and dynamics of the acetone-water complex. Our aim was to understand how non-covalent interactions with water affect the methyl internal rotation dynamics of acetone, and how water-centered large amplitude motions alter the observed physical properties compared to those predicted at the equilibrium position. Detailed rotation-tunneling analyses of acetone-H2O and -D2O reveal that the interactions with water disrupt the equivalence of the two methyl rotors, resulting in a noticeably lower methyl rotor barrier for the top with the close-by water compared to that of free acetone. The barrier for the methyl group further from water is also lower, although to a lesser degree. To gain further insights, extensive theoretical modelling was conducted, focusing on the associated large amplitude motions. Furthermore, quantum theory of atoms in molecules and non-covalent interactions analyses were utilized to visualize the underlying causes of the observed trends.

The Heavy Atom Structure, "cis effect" on Methyl Internal Rotation, and 14N Nuclear Quadrupole Coupling of 1-Cyanopropene from Quantum Chemical and Microwave Spectroscopic Analysis.

The microwave spectrum of 1-cyanopropene (crotonitrile) was remeasured using two pulsed molecular jet Fourier transform microwave spectrometers operating from 2.0 to 40.0 GHz. The molecule exists in two isomer forms, E and Z, with respect to the orientation between the methyl and the cyano groups. The spectrum of the Z isomer is more intense. Due to internal rotation of the methyl group, doublets containing A and E torsional species were found for all rotational transitions. Hyperfine splittings arising from the 14N nuclear quadrupole coupling were resolved. The heavy atom structure of the Z isomer was determined by observation of 13C and 15N isotopologue spectra in natural abundances. The experimental results were supported by quantum chemistry. The complex spectral patterns were analyzed and fitted globally, and the barriers to methyl internal rotation are determined to be 478.325(28) cm-1 and 674.632(76) cm-1 for the Z and E isomers, respectively. The non-bonded intramolecular electrostatic attraction between the methyl group and the 1-cyano substituent overcomes steric hindrance, leading to higher stability of the Z isomer. The consequence is a slight opening of 3.2° of the C(1)-C(2)-C(3) angle and a radical decrease of the methyl torsional barrier in the Z isomer due to steric repulsion.

Ab initio and NBO studies of methyl internal rotation in 1-methyl-2(1H)-quinolinone: effect of aromatic substitution to 1-methyl-2(1H)-pyridone.

1-methyl-2(1H)-quinolinone (MeQone) forms the framework of several hundred alkaloid molecules both natural and synthetic being used for various biological applications. From chemical structure point of view, the molecules can also be seen as an aromatic ring fused to 1-methyl-2(1H)-pyridone (1-MPY). In this work, we present theoretical investigations on internal rotation of methyl group in MeQone in light of 1-MPY. We looked into the change in the three-fold (V3) methyl internal rotation barrier resulted from the aromatic ring substitution to 1-MPY. The V3 term in two molecules were calculated using density functional theory and Hartree-Fock theory with different basis sets. MeQone has calculated V3 term (in S0 state) three times higher in magnitude compared with that of 1-MPY. The role of aromatic substitution in increase of V3 term is investigated using natural bond orbital (NBO) analyses. In the NBO analysis, it is found that the aromatic ring as highly delocalized π-system lowers the magnitude of hyperconjugation energy in MeQone compared with 1-MPY. This is due to the extension of delocalization of π-electrons to pyridone ring which lowers the orbital overlap. However, the Lewis energy increases substantially and make the overall barrier energy higher in MeQone compared with 1-MPY. From our study, we conclude that in the molecules such as 1-MPY and MeQone where the methyl group has two single bonds vicinal to it, the overall hyperconjugation energy is always barrier forming with nonlocal interactions playing significant role. Also, the Lewis energy plays the decisive role in barrier formation, and its magnitude can be tuned by tuning the π-electron delocalization. We have also looked into the change in methyl group conformation upon electronic excitation to S1 state. In 1-MPY, the methyl group rotated by 60° upon excitation whereas in MeQone, there was no conformational change. Strong π*-σ* interaction in LUMO in top-of-barrier conformation is responsible for the change in the methyl group conformation in 1-MPY, whereas same π*-σ* interaction in LUMO of minimum energy conformation results in unchanged excited state conformation in MeQone. Graphical abstract.