Probing Molecular Conformations with Electron Momentum Spectroscopy: The Case of n-Butane

Abstract

High-resolution (e,2e) measurements of the valence electronic structure and momentum-space electron density distributions of n-butane have been exhaustively reanalyzed in order to cope with the presence of two stable structures in the gas phase, namely the all-staggered and gauche conformers. The measurements are compared to a series of Boltzmann-weighted simulations based on the momentum-space form of Kohn-Sham (B3LYP) orbital densities, and to ionization spectra obtained from high-level [ADC(3)] one-particle Green's Function calculations. Indubitable improvements in the quality of the simulated (e,2e) ionization spectra and electron momentum profiles are seen when the contributions of the gauche form of n-butane are included. Both the one-electron binding energies and momentum distributions consistently image the distortions and topological changes that molecular orbitals undergo due to torsion of the carbon backbone, and thereby exhibit variations which can be traced experimentally. With regard to the intimate relation of (e,2e) cross sections with orbital densities, electron momentum spectroscopy can therefore be viewed as a very powerful, but up to now largely unexploited, conformational probe. The study also emphasizes the influence of thermal agitation in photoionization experiments of all kind.

Norbornane: An investigation into its valence electronic structure using electron momentum spectroscopy, and density functional and Green’s function theories

“…For these orbitals, the fractions of intensity recovered under the form of lines with a spectroscopic strength larger than 0.005 amount to 0.765, 0.697, 0.725, and 0.481, respectively. This observation is entirely consistent with previous oneparticle Green's function [50][51][52][53] or MR-SDCI ͑Ref. 54͒ studies of the ionization spectra of saturated hydrocarbons larger than ethane.…”

“…1͑a͔͒. This, the fact that the 1p-GF/ ADC͑3͒ and density functional theories of ionization and (e,2e) cross sections provide very consistent insights into the shape, energy location, and into the momentum distributions characterizing the neighboring peaks 11 and 13, and the vast experience accumulated over the last 25 years with 1p-GF calculations of the shake-up transitions of saturated hydrocarbons [51][52][53] and many other molecules ͑see, for instance, Refs. 40, 42, 55-57 and references therein͒, lead us to believe that band 12 does not belong to the vertical oneelectron and 2h-1p shake-up ionization spectrum of norbornane in its ground electronic state, as described by the ADC͑3͒ model of ionization.…”

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.…”

High-level theoretical study of the conformational equilibrium of n-pentane

“…An accurate determination of the conformational energies of n-pentane has become a mandatory step for, among others, 6-17 a sound interpretation of its ionization, 15 electron momentum, 18 infrared or Raman 19 spectra, due to the impact of the conformation on orbital energies, 15,18 electron density distributions, 18 and molecular vibrations. 19 Other properties of interest are, quite naturally, thermodynamic, structural, and dynamic functions of all kind 7,9,12,16,17 ͑heat capacities, specific volumes, electric polarizability, intermolecular interactions, gyration radius, rotational relaxation time, heat flux-, velocity-, stress-, or pressure-autocorrelation functions, diffusion self-coefficients, viscosities, ...͒.…”