Theoretical study of urea and thiourea. 2. Chains and ribbons

One-dimensional hydrogen-bonding aggregates of urea and thiourea correspond ing to the two patterns, chains and ribbons, which are found in crystal str uctures of these molecules have been studied using ab initio and semiempiri cal molecular orbital theory. In accord with experimental evidence, long ch ains are found to be more stable than comparably sized ribbons for urea, al though ribbons are favored for dimers and small aggregates. Thiourea ribbon s are favored over chains for all aggregates, large and small, again in agr eement with the experimental crystal structure. Thus, cooperative interacti ons dictate the hydrogen-bonding structure for urea, but not thiourea, crys tals. Various ab initio (HF, MP2, and B3PW91 all the with the D95** basis s et) and semiempirical (AM1 and SAM1) molecular orbital calculations were us ed for this study.