Investigation of the molecular conformations of ethanol using electron momentum spectroscopy

Abstract

The valence electronic structure and momentum-space electron density distributions of ethanol have been investigated with our newly constructed high-resolution electron momentum spectrometer. The measurements are compared to thermally averaged simulations based on Kohn-Sham (B3LYP) orbital densities as well as one-particle Green's function calculations of ionization spectra and Dyson orbital densities, assuming Boltzmann's statistical distribution of the molecular structure over the two energy minima defining the anti and gauche conformers. One-electron ionization energies and momentum distributions in the outer-valence region were found to be highly dependent upon the molecular conformation. Calculated momentum distributions indeed very sensitively reflect the distortions and topological changes that molecular orbitals undergo due to the internal rotation of the hydroxyl group, and thereby exhibit variations which can be traced experimentally. The B3LYP model Kohn-Sham orbital densities are overall in good agreement with the experimental distributions, and closely resemble benchmark ADC(3) Dyson orbital densities. Both approaches fail to quantitatively reproduce the experimental momentum distributions characterizing the highest occupied molecular orbital. Since electron momentum spectroscopy measurements at various electron impact energies indicate that the plane wave impulse approximation is valid, this discrepancy between theory and experiment is tentatively ascribed to thermal disorder, i.e. large-amplitude and thermally induced dynamical distortions of the molecular structure in the gas phase.

A comparison of force fields for ethanol–water mixtures

“…[39] One of the first models for liquid ethanol is the rigid united-atom model from Jorgensen, published in 1981. [40] In 1986, Jorgensen et al developed optimised potentials for liquid simulations (OPLS) and, based on liquid hydrocarbon models, [41] they developed OPLS models for liquid alcohols.…”

Theoretical study of molecular vibrations in electron momentum spectroscopy experiments on furan: An analytical versus a molecular dynamical approach

“…[40][41][42] Further physical phenomena that can strongly influence the recorded momentum distributions pertain to nuclear dynamics. Following Hajgató et al 43 or Deleuze et al, 44,45 it is useful to distinguish nuclear dynamics in the initial and neutral electronic ground state, comprising thermally induced conformational rearrangements [46][47][48][49][50][51][52][53][54][55][56][57][58][59][60] or pseudo-rotational motions, 61,62 from nuclear dynamics in the final ionized state, more specifically, geometrical relaxation, vibronic coupling interactions, and ultra-fast bond cleavages [43][44][45][63][64][65][66][67] induced by the ionization processes of interest.…”

Time-resolved electron(e,2e)momentum spectroscopy: Application to laser-driven electron population transfer in atoms

“…The validity of this planewave approximation can also be verified experimentally by comparing (e, 2e) measurements for different energies of the incident electrons (see, e.g., Refs. [28,35,37,38]). Since we are interested in the motion of valence electrons, whose momentum distributions are typically concentrated within the range of 0.0-2.0 a.u., this approximation is adequate for our purposes.…”

“…As a result, EMS measurements have been employed as benchmarks for evaluating the accuracy of theoretical calculations of molecular momentum distributions [35,36]. Moreover, owing to the sensitivity of valence electrons to the structure of a molecule, EMS can detect and study modifications of electron momentum densities due to molecular conformation changes [37], molecular pseudorotations [38], multicenter effects [39,40], or vibrations [41,42]. Recently, (e,2e) measurements have been carried out for excited molecular states [43,44].…”