The phonon-assisted proton transfer between hydrazinium cations in (N 2H 5) 2HMF 6·2H 2O (M∈{Ga, Al, Fe}) type compounds. FT IR and quantum theoretical study

Abstract

FT IR spectra of a series of compounds with a general formula (N 2H 5) 2HMF 6·2H 2O (where M∈{Ga, Al, Fe}) were recorded at variable temperatures (from ∼100 to 300 K, at 10 K intervals). The appearance of the spectral region of ν(N–N) modes due to hydrazinium cations further supports the conclusions regarding the N 2H 5 +⋯H +⋯N 2H 5 + hydrogen bond potential well based on Raman spectroscopic data [J. Raman Spectrosc. 28 (1997) 315]. The appearance of two bands corresponding to the ν(N–N) modes in the low temperature FT IR spectra that merge into one upon heating is a clear evidence of a symmetric potential well through which a phonon-assisted proton transfer (PAPT) occurs at higher temperatures. Ab initio MP2/6-311++G(2d,p) quantum chemical study of the proton transfer potential within the N 2H 5 +⋯H +⋯N 2H 5 + cluster confirmed its double-minimum character. The first-order saddle point found on the MP2/6-311++G(2d,p) potential energy hypersurface corresponds to a centrosymmetric structure ( C 2 h symmetry), with the proton placed at the inversion center. The potential energy curve along the tunnelling coordinate was calculated by the intrinsic reaction coordinate (IRC) methodology, leading to an adiabatic PT barrier height of 3.94 kcal mol −1 and a tunneling rate of 1.98 s −1. The corresponding MP4(SDTQ)/6-311++G(2d,p)//MP2/6-311++G(2d,p) value of the adiabatic PT barrier height is 4.26 kcal mol −1.