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Infrared Spectra of H2ThS and H2US in Noble Gas Matrixes: Enhanced H‑An‑S Covalent Bonding
journal contribution
posted on 2016-02-18, 21:45 authored by Xuefeng Wang, Lester Andrews, K. Sahan Thanthiriwatte, David A. DixonLaser-ablated thorium and uranium
atoms have been co-deposited
at 4 K with hydrogen sulfide in excess noble gas matrixes. The major
dihydride sulfide reaction products were observed for each actinide
and identified on the basis of S-34 and D isotopic substitution. These
assignments were confirmed by frequency and structure calculations
using density functional theory with the B3LYP and PW91 exchange-correlation
functionals and the CCSD(T) method for the pyramidal H2ThS (1A′) and H2US (3A″)
molecules. The lowest three spin states of triplet H2US
are calculated to be within 3 kcal/mol using all three methods, just
as in H2UO. The major products are compared with the oxygen
analogues H2ThO and H2UO, and the sulfides have
71–85 cm–1 higher hydrogen-actinide
stretching frequencies. The actinide-hydrogen bonding appears to be
enhanced in the actinide sulfides through back-bonding of a S 3p electron
pair to a vacant 6d orbital, which is delocalized over the H atoms.
This unique covalent bond is favored by the inductive effect of the
hydride substituents, the pyramidal structures, and the lower electronegativity
of sulfur. Sulfur back-bonding gives polarized triple bond character
to the US and ThS bonds and enhanced metal hydride bonding in H2ThS and H2US.