Hydrogen Bonding Abilities of Thioamide

Abstract

The strengths of hydrogen bonding interaction between formamide (FA) and thioformamide (TFA) were investigated at the B3LYP level with the 6-311G(d,p), 6-31+G(d,p), and 6-311++G(2d,2p) basis sets. The 18 minimum energy structures of FA-FA, TFA-TFA, and TFA-FA dimers were examined. The average strength of the OCN−H- - -OC, SCN−H- - -SC, OCN−H- - - SC, and SCN−H- - -OC hydrogen bonds at the B3LYP/6-311++G(2d,2p) level was −6.1 ± 0.3, −5.0 ± 0.1, −4.8 ± 0.3, and −7.3 ± 0.4 kcal/mol, respectively, when the basis set superposition error (BSSE) was corrected. The results show that TFA is a good hydrogen bond donor but a poor hydrogen bond acceptor as compared to FA. For the OC−H- - -OC, SC−H- - -SC, OC−H- - - SC, and SC−H- - -OC hydrogen bonds, the average strength has been predicted to be −2.2 ± 0.3, −2.2 ± 0.2, −1.0 ± 0.3, and −3.1 ± 0.3 kcal/mol, respectively. It is remarkable that the thioformyl hydrogen atom of TFA has a strong hydrogen bonding ability as compared to that of FA. The abilities of the hydrogen bond donor have a good correlation with the proton affinities of the deprotonated anion.