Theoretical and spectroscopic studies on molecular structure and hydrogen bonding of 1,2-bis (monochloroacetyl) cyclopentadiene

1,2-Bis (monochloroacetyl) cyclopentadiene (MCACP) was synthesized and its molecular structure, intramolecular hydrogen bonding, and vibrational frequencies were investigated by means of density functional theory (DFT) calculations, NMR, and IR spectroscopies. It was found that the most stable conformers are those stabilized by intramolecular hydrogen bridges. Calculations at the B3LYP level, using 6-31G(d,p), 6-311G(d,p), and 6-311++G(d,p) basis sets, have been carried out for understanding the strength of hydrogen bond. In addition, the energies of the stable chelated conformers and their corresponding open structures were obtained at the MP2/6-31G(d,p) level of theory. The harmonic vibrational wavenumbers of MCACP and its deuterated analogue were obtained at the B3LYP/6-311++G(d,p) level. 1H and 13CNMR spectra were recorded and 1H and 13C nuclear magnetic resonance chemical shifts of the molecule were calculated using the gauge independent atomic orbital (GIAO) method. The calculated geometrical parameters and relative energies show formation of a very strong intramolecular hydrogen bond that is consistent with the frequency shifts for OH/OD stretching, OH/OD out-of-plane bending, and O⋯O stretching modes and proton chemical shift. The rotation of terminal CH2Cl groups indicates existence of two stable conformers that their hydrogen bond energy was estimated to be, on average, about 75.6 kJ/mol. The nucleus-independent chemical shift (NICS) data in the keto and enol forms indicated that in addition to differences in stability due to hydrogen bonding, differing stability also arises from the aromaticity increase of cyclopentadene (CP) ring in the chelated forms.