Conformational comparison of urea and thiourea near the <scp>CCSD</scp>(<scp>T</scp>) complete basis set limit

AbstractThe global minima of urea and thiourea were characterized along with other low‐lying stationary points. Each structure was optimized with the CCSD(T) method and triple‐ζ correlation consistent basis sets followed by harmonic vibrational frequency computations. Relative energies evaluated near the complete basis set limit with both canonical and explicitly correlated CCSD(T) techniques reveal several subtle but important details about both systems. These computations resolve a discrepancy by demonstrating that the electronic energy of the C2v second‐order saddle point of urea lies at least 1.5 kcal mol−1 above the C2 global minimum regardless of whether the structures were optimized with MP2, CCSD, or CCSD(T). Additionally, urea effectively has one minimum instead of two because the electronic barrier for inversion at one amino group in the Cs local minimum vanishes at the CCSD(T) CBS limit. Characterization of both systems with the same ab initio methods and large basis sets conclusively establishes that the electronic barriers to inversion at one or both NH2 groups in thiourea are appreciably smaller than in urea. CCSDT(Q)/cc‐pVTZ computations show higher‐order electron correlation effects have little impact on the relative energies and are consistently offset by core correlation effects of opposite sign and comparable magnitude.