2013. 54, 4 UDC 541.6:548.737 STRUCTURE AND INTERACTION BETWEEN THE [BMIM][Ala] ALANINE ANION AND THE 1-BUTYL-3-METHYLIMIDAZOLIUM CATION IN ION PAIRS Z.W. Li1, W.S. Wu1, Z.Y. Du2, X.Y. Hao1 1College of Chemistry and Chemical Engineering, Zhaoqing University, Zhaoqing 526061, China E-mail:lzwgzzd@163.com Received March, 12, 2012 The equilibrium geometries and vibrational frequencies of the ionic liquid 1-butyl-3-methylimidazolium cation and the alanine anion [BMIM][Ala] are studied using density functional theory (DFT) at the B3PW91/6-311+G(d,p) leve1. <...> The most stable structures of the anion, the cation, and the ion pairs are obtained and characterized, and the geometry parameters of the ion pairs confirm the presence of a hydrogen bonding interaction between the anion and the cation. <...> Natural bond orbital (NBO) analysis is also performed to analyze the atomic charge distribution and charge transfer in the [BMIM]+ cation and [BMIM][Ala] ionic liquids. <...> The results show that there are the electrostatic interaction and multiple hydrogen bond interactions between the cation and the anion of the ionic liquids, and the stability of the ground state of the ion pairs mostly results from the hydrogen bonding between the lone pairs of O atoms in the anion and H in the imidazole cycle of the cation. <...> There are some changes in microstructures and the charge distribution during the formation of the ion pairs. <...> Keywords: amino acid ionic liquids (AAILs), density functional theory (DFT), natural bond orbital (NBO), second order interaction energy. <...> They included traditional molecular dynamics simulations, first-principles molecular dynamics simulations, and electronic structure methods. <...> For example, AAIL of [emim][Gly] formed by the glycine anion demonstrated that the intermolecular H bonds were the evident characterristics of it [ 20 ]. <...> Gao et al. [ 22 ] carried out a theoretical study on the structure and the cation-anion interaction of [Pro]+[NO3]– AAIL, and Chiappe [ 23 ] studied the novel glycerol borate-based ILs by combining the experimental and theoretical studies. <...> Zhang et al. [ 24 ] characterized the structural properties of N,N-dimethylformamide based IL using a DFT calculation. <...> Zhang et al. completed the DFT study on the structure and the cation-anion interaction of [C3mim]+[Glu]– AAIL [27 ]. <...> These theoretical studies mentioned above redound to understand the molecular level interaction <...>