Formation of an Ion-Pair Molecule with a Single NH+...Cl- Hydrogen Bond: Raman spectra of 1,1,3,3-Tetramethylguanidinium chloride in the solid state, in solution and in the vapor phase

Some ionic compounds (salts) form liquids when heated to temperatures in the range of 200-300 °C. They may be referred to as moderate temperature ionic liquids. An example of such a compound is the 1,1,3,3- tetramethylguanidinium chloride, [TMGH]Cl, melting at ∼212 °C. The chemistry of this compoundscontaining a dimeric ion-pair “molecule”swas investigated in the solid state, in solutions in water and ethanol, and in the vapor phase, based on ab initio molecular orbital density functional theory (DFT)-type calculations with 6-311+G(d,p) basis sets. Calculations on the monomeric [TMGH]+ ion and the dimeric chloride ion-pair salt converged to give geometries near the established crystal structure of [TMGH]Cl. The structures and their binding energies are given as well as calculated vibrational harmonic normal modes (IR and Raman band wavenumbers and intensities). Experimentally obtained Raman scattering spectra are presented and assigned, by comparing to the quantum mechanical calculations. It is concluded that dimeric molecular ion pairs with four N-H+ · · · Cl- hydrogen bonds probably exist in the solutions and are responsible for the relatively high solubility of the “salt” in ethanol. It was discovered that the compound can be easily sublimed by heating to about 200-230 °C. In the Raman spectrum of the vapor at 225 °C, a characteristic strong band at 2229 cm-1 was found and interpreted to show that the gas phase consists of monomeric ion-pair “molecules” held together by a single N-H+ · · · Cl- hydrogen bond, the stretching band of which is causing the band.