Ab initio molecular orbital calculations on ion-molecule and ion-pair-molecule complexes of formamide with LiF and LiCl

Abstract

Ab initio molecular orbital (MO) calculations are reported on a series of ion-molecule and ion-pair-molecule complexes of (HCONH₂)_n+Li⁺X^- (n=1, 2; X=F, Cl) systems using 3-21G, 6-31G and 6-31G^∗∗ basis sets. Stabilization energies, optimized geometrical parameters, internal force constants and harmonic vibrational frequencies are evaluated for 28 structures of interest. Interaction of Li⁺, F^- and Cl^- ions with formamide (FA) leads to highly stabilized complexes. The FA---F^- complex reveals a proton transfer. The calculated properties of the solvated contact ion-pair complexes follow patterns similar to those noted for the corresponding ion-molecule complexes. However, the presence of the counterion reduces the interaction energies significantly. The interaction of FA with Li⁺ ions as well as with F^- and Cl^- ions results in a red shift in the CO and NH stretching frequencies and a blue shift in the CN stretching frequency. The CH stretching frequency is found to be influenced by the ions in a different manner, cations causing a substantial blue shift and anions a red shift. The variations in the harmonic frequencies are compared with the trends observed in the experimental results for the corresponding systems as reported in the literature.