Normal coordinate analyses of trimethylarsine sulfide and trimethylarsine selenide

Abstract

Trimethylarsine sulfide and trimethylarsine selenide and their perdeuterated analogues were synthesized for the purpose of carrying out vibrational analyses. The mid-infrared (4000 - 200 cm⁻¹) and far-infrared (400 - 30 cm⁻¹) spectra of the solid compounds as well as the Raman spectra of saturated aqueous solutions were measured for each compound. The Raman spectra of the solid trimethylarsine sulfides-d₀ and -d₉ are reported. The 33 fundamental vibrational frequencies (omitting the methyl torsions) are assigned. The assignments are made on the basis of symmetry selection rules and comparison with structurally related molecules such as trimethylarsine oxide and trialkylphosphine chalcogenides. Detailed normal coordinate calculations were carried out to reinforce these assignments. A valence force field and C[subscript 3v] symmetry were assumed for each molecule. A set of 24 significant force constants were refined to give a least squares fit of the calculated frequencies to the observed frequencies. The values of the force constants for trimethylarsine sulfide and trimethylarsine selenide were found to be essentially the same except for the As-S and As-Se bond stretches, the CAsS and CAsSe angle bends, and the interaction constants involving any of these bond stretches or angle bends. Calculations of the potential energy distributions for each molecule show that there exists little coupling between the different symmetry coordinates for each of the compounds.