Conformational preference of N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium cation ([DEME]+), which is one of the quaternary ammonium-based ionic liquid cation, in the gas phase has been investigated using a density functional theory (DFT) calculation. Eight candidates for the stable conformers of [DEME]+ exist in the gas phase, and can it energetically classify into two groups. One is a five conformers group, which has the intramolecular attractive interaction form (the folded form). The other is a three conformers group, which is the noninteraction form (the extended form). The transformation from the folded form to the extended form induces large changes in the dipole moment and partial charges of N and O atoms. Here we show that the difference in the dipole moment and partial charges of N and O atoms associated with the conformational change of [DEME]+ are closely related to the molecular orientation of [DEME]-based ionic liquids in the liquid state. 1. Introduction The conformational analysis of room temperature ionic liquids (RTILs) using a vibrational spectroscopy combined with the quantum chemical calculation has been studied because the investigation of the conformational equilibria in RTILs is a good indicator of the molecular orientation for the phase transition and solution structure of RTILs [1–3]. Recently, the phase behaviors and solution structures of N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate ([DEME][BF4]) and iodide ([DEME][I]), which are typical quaternary ammonium-based RTILs, in the pure solution and also in the aqueous solutions have been investigated using various experimental techniques such as Raman, deferential thermal analysis (DTA), and X-ray diffraction methods [4–12]. Imai et al. [4] and Abe et al. [8] reported the phase diagram of [DEME][BF4]-water mixed solutions as functions of temperature and water concentration. They found that [DEME][BF4]-water mixed solutions show complicated phase behavior. Moreover, Yoshimura et al. [7] found the existence of the nearly free hydrogen bonds of water in the [DEME][BF4]-water mixed solutions at 298 and 77？K using a Raman spectroscopy. In this relation, Imai et al. [9, 12] reported that [DEME][ ] ( ？= BF4 and I)-water mixed solutions have double glass transition temperatures ( ) due to the liquid-liquid immiscibility using a DTA method. It is, however, still unclear the relationship between the molecular orientation such as conformation and the complicated phase behavior in the aqueous [DEME][ ]( = BF4 and I) solutions. Although the conformations of
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