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

The global minima of urea and thiourea were characterized along with other lowlying stationary points. Each structure was optimized with the CCSD(T) method and triple-zeta 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 C(2v )second-order saddle point of urea lies at least 1.5 kcal mol(-1) above the C-2 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 C-s 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.

Automated summary

The global minima of urea and thiourea were characterized using the CCSD(T) method and triple-zeta correlation consistent basis sets. The electronic energy discrepancy of urea's C(2v) second-order saddle point was resolved, showing that it is at least 1.5 kcal mol(-1) above the C-2 global minimum regardless of the optimization method. The electronic barriers to inversion in thiourea are considerably smaller than in urea.