A microwave study of hydrogen-transfer-triggered methyl-group rotation in 5-methyltropolone

Abstract

We present here the first experimental and theoretical study of the microwave spectrum of 5-methyltropolone, which can be visualized as a seven-membered "aromatic" carbon ring with a five-membered hydrogen-bonded cyclic structure at the top and a methyl group at the bottom. The molecule is known from earlier studies in the literature to exhibit two large-amplitude motions, an intramolecular hydrogen transfer and a methyl torsion. The former motion is particularly interesting because transfer of the hydrogen atom from the hydroxyl to the carbonyl group induces a tautomerization in the molecule, which then triggers a 60° internal rotation of the methyl group. Measurements were carried out by Fourier-transform microwave spectroscopy in the 8-24 GHz frequency range. Theoretical analysis was carried out using a tunneling-rotational Hamiltonian based on a G(12)(m) extended-group-theory formalism. Our global fit of 1015 transitions to 20 molecular parameters gave a root-mean-square deviation of 1.5 kHz. The tunneling splitting of the two J=0 levels arising from a hypothetical pure hydrogen-transfer motion is calculated to be 1310 MHz. The tunneling splitting of the two J=0 levels arising from a hypothetical pure methyl top internal-rotation motion is calculated to be 885 MHz. We have also carried out ab initio calculations, which support the structural parameters determined from our spectroscopic analysis and give estimates of the barriers to the two large-amplitude motions.

Nuclear Spin Conversion to Probe the Methyl Rotation Effect on Hydrogen‐Bond and Vibrational Dynamics

“…These two latter processes involve large‐amplitude motions, and their coupling can induce complex behaviors. For example, it was predicted theoretically3 and confirmed experimentally4 that the transfer of the H‐bonded H‐atom in 5‐methyltropolone can drive the rotation of the remote methyl group through long range chemical interactions. The present study is focused on acetylacetone, a prototypical β‐diketone with two methyl groups.…”

SERS investigation of neocuproine adsorption on silver: Influence of electrode potential on methyl groups

“…The conformational transformations of neocuproine consist in the rotation of methyl groups, characteristic for the aromatic systems with alkyl substituents. The rotational conformers are usually studied with a wide variety of experimental techniques, including nuclear magnetic resonance, microwave spectroscopy, and inelastic neutron scattering . SERS spectroscopy should be also considered as promising method because of its unsurpassed sensitivity to the conformational equilibria occurring at an interface …”

Tautomerism, dynamic properties and level crossing in 2‐(pyridin‐2‐yl) furan‐3‐ol by density functional theory and natural bond orbital analysis

“…Studies of molecular dynamics have been of interest in both computational and experimental research areas . Recently, dynamic phenomena such as rotations and tautomerism have been the subject of theoretical researches. In this line, potential energy surfaces (PESs) for dynamic motion have been studied by both computational and experimental methods .…”