Experimental and computational studies of the structure and vibrational spectra of 4-dimethylamino pyridinium-betaine of squaric acid

The FTIR spectra of 4-dimethylamino pyridinium-betaine of squaric acid in 4000–100 cm21 frequency region in solid state were measured. In addition, the structure and harmonic vibrational frequencies of this molecule were theoretically evaluated using restricted Hartree–Fock and B3LYP density functional methods. The computed vibrational frequencies are used to determine the types of molecular motions associated with each of the experimental bands observed. The results of the optimized molecular structure obtained on the basis of restricted Hartree–Fock and density functional theory calculations are presented and compared with the experimental X-ray diffraction data for the 4-dimethylamino pyridinium-betaine of squaric acid single crystal. Comparison with the experimental spectra provides important information about the ability of these computational methods to describe the vibrational modes in these highly polar strained ring compounds. q 2003 Elsevier B.V. All rights reserved.