Cooperative Effects in Aminothiols: Acid-Base Equilibria and the Molecular Structure of 2-(N, N-Dimethylaminomethyl)thiophenol

Abstract

Abstract 2-(N,N-Dimethylaminomethyl)thiophenol, (1, HL), is present as a non-zwitterionic aminothiol in the solid state, exhibiting an intramolecular S-H···N hydrogen bond. The S···N distances of the two independent molecules in the asymmetric unit are 2.929(10) and 3.050(10) Å. This structural feature is also present in an ab initio (MP2/6-31G*) optimized molecular structure. The investigation of the hydrogen bond by ab initio methods supports an n(N)-σ *(S-H) interaction as the reason for this bond type. On the basis of data from potentiometric acid-base titrations of a 0.01 M aqueous solution of [H2L]Cl with a 0.1 M aqueous solution of sodium hydroxide, values of 4.09±0.01 and 11.50±0.01 were obtained for pKa1 and pKa2 of [H2L]+. pKa1 is much smaller than pKa of thiophenol while pKa2 is bigger than pKa of benzyldimethylamine. The increased difference between pKa1 and pKa2 is attributed to the stabilization of HL by the intramolecular S-H···N hydrogen bond.