Multiple-H-bonded-zwitterionic tetramer structure for L-(+)-2-chlorophenylglycine, as investigated by UV, IR and Raman spectroscopy and electronic structure calculations

Abstract A DFT tetramer model constructed of multi-H-bonds at B3LYP/6-311G(d,p) level has been proposed for L-(+)-2-Chlorophenylglycine. Motivation in part for the tetramer model comes from the observed IR spectra near 3400−2500 cm−1 showing a very broad sub-band structure spread over ∼ 900 cm−1 without distinct sharp bands; secondly, XRD structures of similar molecular solids show multiple H-bonds among monomers. Therefore, the proposed tetramer consists of fourteen H-bonds arising from a network of inter-/intra-molecularly X−H···Y bonds: N−H···O, C−H···O and Cα−H···Cl bonds, all from four monomer species. The strengths and degree of these H-bonds being covalent, ionic or partial have been determined using topological parameters from AIM calculations. Other weaker van der Waals interactions and steric clashes in relation to the N−H···O, C−H···O and Cα−H···Cl bonds have also been evaluated by the method of ‘non-covalent interactions (NCI)’. Further, the charge transfer from the Y lone pair orbital (donor) to the X−H anti-bonding orbital (acceptor) have been explained on the basis of overlapping of orbitals using NBO analysis. Computed stabilization energies for the X−H···Y bonds have shown the inter-molecular N−H···O bonds to be the strongest of the H-bonds. Concentration-dependent UV spectral analysis aided by TD-DFT-based calculations at B3LYP/6-311G(d,p) with SMD model in water has shown that a strong band at 221 nm, among other bands, blue-shifts to 194 nm with a concomitant arrival of a shoulder band at 219 nm. The bands are assigned to the π→π* transition. This observation has been interpreted to be the result of dissociation of the tetrameric species into monomeric species and is supported by the stabilization of computed ground and excited electronic levels. By the same interpretation, three vibronic bands seen at 260−280 nm also have been explained. Computed vibrational modes of the tetramer fit very well with the experimental IR and Raman band features including low frequency Raman modes below 350 cm−1. It has been shown the consistency among the H-bonding descriptors - H···Y bond distance, change in X−H bond length, X−H frequency shifts, electron density, H-bond energy and stabilization energy by graphical correlations.