The solid state VCD of a novel N-acylhydrazone trifluoroacetate

A novel N-acylhydrazone with pharmaceutical importance was subject of structural and IR/VCD investigations in the solid state. In the crystal structure, dimers of anion-cation pairs are stabilized by H-bonding and ionic interactions. Some less common interaction types, like C=N···C-NH₃⁺ (σ-hole) interactions, hydrazone-aromatic interactions and dispersive contacts of the CF₃ groups are also present in the crystal. Satisfactory reproduction of the solid state IR and VCD spectra required that quantum-chemical calculations be done on a tetramer (four cation-anion pairs) cut out from the crystal structure, exhibiting key intermolecular interactions. Ten DFT functionals were assessed as to the agreement between the calculated and experimental spectra. Various approaches to scaling of the calculated frequencies were applied. The best results were yielded with individual (optimized) frequency scaling factors (FSFs) and band half-widths at half maximum-(HWHM) for four separate spectral subregions. The best matching between the experimental and theoretical spectra (according to SimIR, SimVCD and SimVDF indices) was found for the B3PW91 functional, however, a few other functionals follow closely in the ranking. Based on the quantum chemical calculations, spectral assignments have been made.