Molecular structure of thiourea.

The molecular structure of thiourea has been investigated under C(s), C(2), and C(2v) symmetry constraints. At the coupled-cluster level in conjunction with a triple-ζ basis set, only the C(2) conformer has been found to be a real minimum on the potential energy surface. Its equilibrium structure has therefore been accurately evaluated using both theoretical and experimental data. With respect to the former, high-level quantum-chemical calculations at the coupled-cluster level in conjunction with correlation-consistent basis sets ranging in size from triple- to quintuple-zeta have been carried out. Extrapolation to the complete basis-set limit as well as core-correlation effects and inclusion of full treatment of triple excitations in the cluster operator have been considered. On the basis of the vibrational ground-state rotational constants available for five isotopic species and the corresponding computed vibrational corrections, the semiexperimental equilibrium geometry of thiourea has also been determined for the first time.