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

Abstract

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-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 C 2v second-order saddle point of urea lies at least 1.5 kcal mol À1 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 NH 2 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. | INTRODUCTIONThe first synthesis of urea (O C(NH 2 ) 2 ) from inorganic starting materials was reported in 1828 [1]. Since then, urea and its derivatives have been produced commercially for a wide range of industrial purposes. For example, due to the high nitrogen content of urea, it has become a crucial component utilized by the agricultural industry when manufacturing fertilizers [2] as well as animal feed [3]. In addition, urea is a common synthetic precursor used in both the chemical and pharmaceutical industries to produce various resins [4] and barbiturates [5]. Urea derivatives have also been found to play an instrumental role in drug design by acting as a biological modulator [6]. Thiourea (S C(NH 2 ) 2 ) is the sulfur analog of urea. Although urea and thiourea are isovalent and structurally similar, they can exhibit significantly different properties. An example of such can be seen in their corresponding crystal structures, where the hydrogen-bonding framework experimentally observed for the crystal structure of urea forms linear hydrogen bonds in a head-to-tail manner, while thiourea preferentially adopts a unique zig-zag or ribbon hydrogen-bonding motif [7]. These different hydrogen-bonding preferences have also been observed in the crystal structures of many of their derivatives and are thought to be responsible for why certain thiourea derivatives were effective components of healable materials whereas their urea analogs were