An experimental and theoretical study of the HOMO of W(CO)6: Vibrational effects on the electron momentum density distribution

Electron momentum spectroscopy of metal carbonyls: a reinvestigation of the role of nuclear dynamics

“…However, the more recent EMS experiments by K. Liu et al [45] on W(CO) 6 at electron impact energies of 1.2 and 2.4 keV gave at first glance quite similar momentum profiles for the HOMO and led thus on the contrary to the conclusion that distorted wave and post-collision effects are too weak to explain the experimentally observed turnups at low electron momenta. Since the target compounds contain relatively heavy metal atoms, this discrepancy between theory and experiment was then also thought to be the outcome of the limitations inherent to a non-relativistic depiction.…”

“…Further investigations of scalar relativistic and spin-orbit coupling effects indicated, however, a very marginal influence of these effects upon the computed momentum distributions [45]. In their ultimate attempt to reconcile theory with experiment, Liu et al [45] invoked large-amplitude structural distortions in the form of wagging or bending motions associated with the three lowest and triply degenerate vibrational frequencies and the corresponding 1T 2u , 1T 1u , and 1T 2g eigenmodes (Fig. 1), regardless of anharmonic effects and of excessive energy demands resulting from exacerbated electrostatic repulsions between strongly polarized C=O substituents.…”

“…In their study, it was simply assumed that thermally induced nuclear dynamics in the electronic ground state can be conveniently described by displacing nuclei away from their equilibrium position, according to so-called normalized vibrational eigenvectors (i.e., to eigenvectors the norm of which amounts to 1 Å 2 , according to the ADF output for atomic displacements), and averaging thereby the obtained momentum distributions for the first three 1T 2u , 1T 1u , and 1T 2g eigenmodes. The main argument by Liu et al [45] in support to their approach was that the population of the vibrational ground state accounts for less than 10 % of the total vibrational populations of the three lowest eigenmodes, at an estimated experimental temperature of 310 K, according to (Maxwell-) Boltzmann statistics. With this simple depiction of nuclear dynamics in the electronic ground state, Liu et al [45] managed to obtain a much better match between theory and experiment and claimed therefore the case of the frontier orbital momentum distributions of W(CO) 6 to be understandable in terms of lowfrequency vibrations.…”

“…Since the target compounds contain relatively heavy metal atoms, this discrepancy between theory and experiment was then also thought to be the outcome of the limitations inherent to a non-relativistic depiction. Further investigations of scalar relativistic and spin-orbit coupling effects indicated, however, a very marginal influence of these effects upon the computed momentum distributions [45]. In their ultimate attempt to reconcile theory with experiment, Liu et al [45] invoked large-amplitude structural distortions in the form of wagging or bending motions associated with the three lowest and triply degenerate vibrational frequencies and the corresponding 1T 2u , 1T 1u , and 1T 2g eigenmodes (Fig.…”

“…Because of their high volatility and octahedral symmetry, they are also most useful molecular models for studies of bond formation between carbonyl groups and metal surfaces. Investigations of the outermost valence orbitals of these compounds comprise the ultra-violet [He I, He II] and X-ray photoelectron experiments by Higginson et al [42] and various studies [43][44][45] employing Electron Momentum Spectroscopy (EMS). In the EMS studies of Cr(CO) 6 , Mo(CO) 6 , and W(CO) 6 by Chornay et al [43], and by Rolke et al [44], the measured (e, 2e) ionization cross-sections for the HOMO were found to be much larger than expected in the low momentum region, according to standard theoretical models of (e, 2e) ionization processes in the high energy limit, and to be largely inconsistent with the nodal characteristics of an uncorrelated (Hartree-Fock) 2t 2g orbital at p = 0.…”