Ab initio and localized molecular orbital studies of the integrated intensities of infrared absorption bands of polyatomic molecules. Part 4.—Complexes between chloroform and three aliphatic nitriles (CH3CN, CCl3CN and HCCCN); influence of hydrogen bonding on the CH and CN infrared characteristics in proton donor and acceptor molecules

Abstract

STO-3G ab initio calculations are performed on the hydrogen-bond complexes between chloroform and three aliphatic nitriles (CH₃CN, CCl₃CN and HCCCN). The molecular electrostatic potential (m.e.p.) around the nitriles indicates the N lone-pair as the preferential site for H-bond formation. A partial inter- and intra-molecular geometry optimization and force constant evaluation is performed. Stabilization energies, inter-and intra-molecular geometry parameters and force constants are correlated with various properties of the isolated molecules such as the m.e.p. characteristics and the N lone-pair orbital energy. Vibration frequencies are obtained via the Wilson GF matrix procedure. The decrease in the CH stretching frequency in chloroform and the increase in the CN stretching frequency of the nitriles parallel the stabilization energies of the complexes and are in agreement with experimental infrared results. The form of the CN and CH stretching normal coordinates changes very little upon complexation. The equilibrium charge distribution is compared with experimental n.m.r. data and is related to the charge-transfer character of hydrogen bonding. The calculated CH intensity evolution upon complexation parallels the experimental results in solution, where a strong intensity increase is observed. The variation in intensity is seen to be linearly related to the frequency variation. The CN intensity results do not directly correspond to the experimental solution data. This is ascribed to a smaller contribution of the specific intermolecular interactions as compared with the CH bond.