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

Semi-empirical (AMI 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 (AMI 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 B3LY-P/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. (C) 2002 Elsevier Science B.V. All rights reserved.