Chem. 67, 2227 (1989).Ab initio calculations have been used to study the conformational potential surface of sulfamide, by considering the S-N bond rotations and the nitrogen inversion processes. The lowest energy conformation (b) is found for a cis-trans arrangement of the amino groups, although conformations with cis-cis (a), trans-trans (c), and near staggered (c') arrangements lie close in energy. Nitrogen inversion barriers are very low, and consequently one may expect forms b and c' to be the only ones present in the gas phase. Conformer a is very polar, its dipole moment being twice that of b, so it may be favored in condensed media or in polar-solvent solutions. The relative stability of the different isomers is governed by interactions between the amino protons and between the nitrogen lone pairs. Our results show that d-n backbonding, involving the d orbitals on sulfur, is responsible for the multiple bond character of the S-0 linkage, but is very small in the S-N interactions. The role of the sulfur d-orbital exponent, when a 6-31G* basis is used, is analyzed on a series of model compounds containing S", STV, and SV1. Although the inclusion of d functions on sulfur is crucial to describing correctly the bonding in sulfamide, the results obtained do not change appreciably if a second set of d functions is centered on sulfur. Nevertheless, only when polarization functions are also included for first-row atoms is the description of the system reliable.Key