Isotope effects in X–H···π type hydrogen-bonded complexes (X = F, Cl or Br)

Abstract

Ab initio calculations were performed at the B3LYP, MP2 and QCISD levels of theory using the 6-311++G(3df,2pd) basis set to obtain optimized geometries, dipole moments, binding energies and harmonic vibrational frequencies for the symmetrical T-shaped structures of XH···π bonded complexes involving acetylene (as the proton acceptor) and the halides HF, HCl and HBr (as the proton donors). The relative stabilities of the D-containing isotopomers C₂H₂···DX and C₂HD···HX were also determined from their zero-point energies; vibrational frequency shifts of HX and acetylene on bonding were also calculated and used to gain insight into these relative stabilities. It was found that the D-bonded isotopomer (C₂H₂···DX) was more thermodynamically stable than the H-bonded isotopomer (C₂HD···HX) for HF-, HCl- and HBr-containing complexes; previous studies on the weakly bound linear complexes CO–acetylene and N₂–acetylene also found that the D-bonded species was more stable than the H-bonded species.