Imaging Momentum Orbital Densities of Conformationally Versatile Molecules:  A Benchmark Theoretical Study of the Molecular and Electronic Structures of Dimethoxymethane

Abstract

The main purpose of the present work is to predict from benchmark many-body quantum mechanical calculations the results of experimental studies of the valence electronic structure of dimethoxymethane employing electron momentum spectroscopy, and to establish once and for all the guidelines that should systematically be followed in order to reliably interpret the results of such experiments on conformationally versatile molecules. In a first step, accurate calculations of the energy differences between stationary points on the potential energy surface of this molecule are performed using Hartree-Fock (HF) theory and post-HF treatments of improving quality (MP2, MP3, CCSD, CCSD(T), along with basis sets of increasing size. This study focuses on the four conformers of this molecule, namely the trans-trans (TT), trans-gauche (TG), gauche-gauche (G+G+), and gauche-gauche (G+G-) structures, belonging to the C2v, C1, C2, and Cs symmetry point groups, respectively. A focal point analysis supplemented by suited extrapolations to the limit of asymptotically complete basis sets is carried out to determine how the conformational energy differences at 0 K approach the full CI limit. In a second step, statistical thermodynamics accounting for hindered rotations is used to calculate Gibbs free energy corrections to the above energy differences, and to evaluate the abundance of each conformer in the gas phase. It is found that, at room temperature, the G+G+ species accounts for 96% of the conformational mixture characterizing dimethoxymethane. In a third step, the valence one-electron and shake-up ionization spectrum of dimethoxymethane is analyzed according to calculations on the G+G+ conformer alone by means of one-particle Green's function [1p-GF] theory along with the benchmark third-order algebraic diagrammatic construction [ADC(3)] scheme. A complete breakdown of the orbital picture of ionization is noted at electron binding energies above 22 eV. A comparison with available (e,2e) ionization spectra enables us to identify specific fingerprints of through-space orbital interactions associated with the anomeric effect. At last, based on our 1p-GF/ADC(3) assignment of spectral bands, accurate and spherically averaged (e,2e) electron momentum distributions at an electron impact energy of 1200 eV are computed from the related Dyson orbitals. Very significant discrepancies are observed with momentum distributions obtained for several outer-valence levels using standard Kohn-Sham orbitals.

High resolution electron momentum spectroscopy of the valence orbitals of water

“…The overlap of the ion and neutral wavefunctions in Eq. (2) is referred to as a Dyson orbital [5][6][7][8][9][10][11][12]. Dyson orbitals are straightforwardly obtained from CI [31] or Green's Function (GF) calculations [9][10][11]54].…”

“…However, the available EMS spectrometers still badly need improvements with regards to energy and momentum resolution for probing on more quantitative grounds the outcome of the dynamical correlation and relaxation effects induced by (e, 2e) ionization processes on the effective shape and spread of the orbitals involved in these processes. In an exact theory of ionization at the limit of high kinetic energies for the impinging electrons, the measured electron momentum distributions relate to Dyson orbitals [5][6][7][8][9][10][11][12], measuring partial overlaps between the neutral ground state and the corresponding cationic states of the target.…”

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.…”

Experimental and Theoretical Electron Momentum Spectroscopic Study of the Valence Electronic Structure of Tetrahydrofuran under Pseudorotation

“…[19][20][21][22][23][24][25][26][27] Previous studies have established that EMS is an attractive technique to investigate conformations of organic molecules and small biomolecules. [17][18][19][20][21][28][29][30][31][32][33] It has been demonstrated that the symmetries of the HOMOs with respect to the minimal structures of C s and C 2 on the potential-energy surface (PES) produced by pseudorotation differentiate the conformers of THF. 17 This finding is further supported by a detailed theoretical analysis of the individual valence orbitals of the three important structures, C s , C 2 , and C 1 produced by pseudorotation of THF.…”