Infrared Spectra of H<sub>2</sub>ThS and H<sub>2</sub>US in Noble Gas Matrixes: Enhanced H‑An‑S Covalent Bonding

Laser-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 H<sub>2</sub>ThS (<sup>1</sup>A′) and H<sub>2</sub>US (<sup>3</sup>A″) molecules. The lowest three spin states of triplet H<sub>2</sub>US are calculated to be within 3 kcal/mol using all three methods, just as in H<sub>2</sub>UO. The major products are compared with the oxygen analogues H<sub>2</sub>ThO and H<sub>2</sub>UO, and the sulfides have 71–85 cm<sup>–1</sup> <i>higher</i> 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 H<sub>2</sub>ThS and H<sub>2</sub>US.