Gas phase model of an ionic liquid: semi-empirical and ab initio bonding and molecular structure

Abstract

Semi-empirical (AM1 and PM3) and ab initio structures of the gas phase ion pair, bmim+(1-butyl-3-methyl imidazolium)–PF6−, have been calculated. The ab initio calculations include fully optimized structures at the RHF/3-21G(∗), RHF/6-31G∗, RHF/6-31G∗∗, MP2/6-31G∗, B3LYP/6-31G∗ and B3LYP/6-31G∗∗ levels. Semi-empirical structures (AM1 and PM3) are similar to those obtained from ab initio methods. The non-bonded interactions in these ion pairs are of the C–H⋯X type (C–H⋯F hydrogen bonds) and exist between F's on PF6− and (a) the imidazolium ring N–H, (b) the methyl group H adjacent to the ring and (c) H's on the butyl (bmim+) side chain. The ab initio ion pair structures contain hydrogen bonds that shorten with increasing basis set complexity. The movement of the bmim+ imidazolium ring C2–H along the proton transfer coordinate towards PF6− produces similar anharmonic functions for the RHF/6-31G∗//6-31G∗∗, MP2/6-31G∗//6-31G∗∗, RHF/6-31G∗∗/6-31+G(d,p)//6-31G∗∗, MP2/6-31G∗∗/6-31+G(d,p)//6-31G∗∗, B3LYP/6-31G∗//6-31G∗∗ and B3LYP/6-31G∗∗/6-31+G(d,p)//6-31G∗∗ levels. The existence of anharmonic functions that describe the proton transfer coordinate is consistent with the Del Bene model of hydrogen bonding.