Vibrational analysis and Y-aromaticity in bis (N,N′-diphenyl guanidinium) oxalate crystal: A DFT study

The compound of therapeutic interest, Bis (N,N′-diphenyl guanidinium) oxalate, have been crystallized and is subjected to FT IR and Raman spectral studies, along with quantum chemical computations using density functional theory. In the crystal, diphenyl guanidinium ion is found to possess anti anti conformation although syn syn conformer is energetically favored based on the single point energy calculations at B3LYP/6-31G(d) level, which shows the counter ion influence on conformation. The optimized geometry at the same level of theory indicates that guanidinium moiety of diphenyl guanidinium ion is planar, possibly induced by the sp2 hybridized central carbon atom and the twisting of phenyl ring with respect to guanidinium moiety has been observed. The charge delocalization of the planar oxalate ion predicted by XRD is confirmed by the DFT values of bond lengths. The presence of N–H⋯O intermolecular interactions can be observed in IR spectrum by the broadness of the band in the stretching region and its vibrational spectral consequences can be found as the shifting of band positions for amino group and CO group vibrations. The geometry, vibrational spectra, covalent bond orders and atomic charges of guanidinium ion, diphenyl guanidinium ion and of their corresponding neutral species have been used to investigate the effects of charge delocalization leading to Y-aromaticity. The FT IR and Raman spectra of diphenyl guanidine indicates that phenyl ring modes 8b, 18a, 9b and 6b, active in diphenyl guanidine molecule, are found to be dormant in their ionic species.