B. Sztaray et Pg. Szalay, STRUCTURE AND PHOTOELECTRON-SPECTRUM OF TETRAMETHYLDIARSANE, Journal of the American Chemical Society, 119(49), 1997, pp. 11926-11932
A systematic theoretical investigation of tetramethyldiarsane is prese
nted to help understand its structure and photoelectron spectrum. Full
potential energy curve along the C-As-As-C torsion was calculated at
the Hartree-Fock (HF) level, and complete geometry optimizations were
performed at HF, second-order Many-Body Perturbation Theory (MBPT(2)),
and Coupled-Cluster Singles and Doubles (CCSD) levels of theory. Two
conformers, anti and gauche, have been found in accordance with the ex
perimental observations. The calculated geometries are in good agreeme
nt with the electron diffraction results. Ionization energies were com
puted by the Equation-of-Motion Coupled-Cluster (EOM-CC) method. The c
alculations predict a substantial lone-pair splitting for both conform
ers which contradicts the original assignment of the photoelectron spe
ctrum by Cowley et al, According to the new assignment, the first and
third bands belong to the lone pairs of the anti rotamer, the second b
and is attributed to the n(+) lone-pair combination in the gauche conf
ormer, while the peak of the n(-) combination is merged with the inten
se first band previously ascribed exclusively to the anti rotamer. The
conformer ratios calculated from the present assignment of the photoe
lectron spectrum are in good agreement with the quantum-chemical resul
ts. Since our assignment contradicts the theoretical reasoning, which
the original assignment was based on, a new explanation is presented.
We find that the s-character of the lone-pair orbitals increases in th
e order of nitrogen, phosphorus, arsenic, and antimony which explains
not only the increasing pyramidalization of the Me2E moiety but also t
he increasing splitting of the energy of the lone-pair MOs.