Structural, Thermal, and Vibrational Properties of N,N-Dimethylglycine–Chloranilic Acid—A New Co-Crystal Based on an Aliphatic Amino Acid

Abstract

The new complex of N,N-Dimethylglycine (DMG) with chloranilic acid (CLA) was synthesized and examined for thermal, structural, and dynamical properties. The structure of the reaction product between DMG and CLA was investigated in a deuterated dimethyl sulfoxide (DMSO-d6) solution and in the solid state by Nuclear Magnetic Resonance (NMR) (Cross Polarization Magic Angle Spinning-CPMAS NMR). The formation of the 1:1 complex of CLA and DMG in the DMSO solution was also confirmed by diffusion measurement. X-ray single crystal diffraction results revealed that the N,N-dimethylglycine–chloranilic acid (DMG+–CLA−) complex crystallizes in the centrosymmetric triclinic P-1 space group. The X-ray diffraction and NMR spectroscopy show the presence of the protonated form of N,N-dimethylglycine and the deprotonated form of chloranilic acid molecules. The vibrational properties of the co-crystal were investigated by the use of neutron (INS), infrared (IR), and Raman (RS) spectroscopies, as well as the density functional theory (DFT) with periodic boundary conditions. From the band shape analysis of the N–CH3 bending vibration, we can conclude that the CH3 groups perform fast (τR ≈ 10−11 to 10‒13 s) reorientational motions down to a temperature of 140 K, with activation energy at ca. 6.7 kJ mol−1. X-ray diffraction and IR investigations confirm the presence of a strong N+–H···O− hydrogen bond in the studied co-crystal.

Keywords

• N,N-dimethylglycine–chloranilic acid (DMG⁺–CLA⁻) co-crystal
• X-ray diffraction
• inelastic neutron scattering (INS)
• temperature dependent IR spectroscopy
• DFT calculation