Linear-cyclic isomer competition in protonated ethanol-methanol clusters probed by infrared spectroscopy and deep-learning structural and dynamical simulations.

The competition between linear and cyclic isomer structures in protonated ethanol tetramers has been investigated using spectroscopic and theoretical approaches. Infrared spectroscopy of protonated ethanol-methanol mixed tetramers, cooled by the inert gas tagging technique, revealed a significant dependence of the isomer structure competition on the mixing ratio and the tag species. To investigate isomer competition, structure searches were performed using the parallelized Basin-Hopping algorithm with neural network potentials that approximate the accuracy of density functional theory. Spectral simulations were conducted via harmonic vibrational analysis of key stable isomers using density functional theory and power spectral density calculations from molecular dynamics trajectories based on the neural network potentials. Comparison with experimental data reveals that the global minimum of the protonated ethanol tetramer is a linear structure. Additionally, the tag species significantly influence the relative stability of linear and cyclic isomers, as well as the isomerization barrier between these two structures.