Hydrogen bonded [formula omitted] graph set in inducing charge transfer mechanism in guanidinium-3,5-dinitrobenzoate: A combined experimental, theoretical and Hirshfeld surface study

Abstract

A supramolecular crystal, guanidinium-3,5-dinitrobenzoate (GDNB), was synthesised and its structure was confirmed by single crystal X-ray diffraction at 25 °C. Distinct NH⋯O hydrogen bonding interactions as well as π-π interactions in GDNB were ascertained from the crystal structure. An exhaustive Hirshfeld surface analysis spells out variety of intermolecular contacts in GDNB proving the existence of a supramolecular isomer at 25 °C. The NH⋯O hydrogen bond led to the formation of R2,2(8) graph set which functions as a bridge for the intermolecular charge transfer occur in GDNB. Theoretical calculations were carried out at B3LYP level of theory at 6-311 G(d,p) and cc-pvtz basis sets using Gaussian 09 software package. FT-IR and FT-Raman spectral analyses were also carried out to confirm various functional groups in GDNB. The 1H, 13C and DEPT-135 NMR spectra of GDNB confirmed the various types and number of protons as well as carbons in the molecule. A systematic and complete DFT calculation covering optimisation, HOMO-LUMO analysis, CHELPG charge analysis, hyperpolarizability and Mulliken charge analysis was carried out. There is no absorption in the entire visible region and the maximum cut off wavelength of GDNB crystal is 267 nm. The charge transfer was confirmed by both HOMO-LUMO and NBO analyses where electrons are promoted from guanidinium moiety to the 3,5-dinitrobenzoate through NH⋯O hydrogen bonds. NBO analysis supports the interactions of chemical bonds and structural properties of GDNB. Wiberg bond indices expose the nature of donor and acceptor moieties. The thermal stability of GDNB was confirmed from TG/DTA and DSC analyses. The compound is stable up to 250 °C. DFT calculations of static as well as frequency dependant electro optic properties were also undertaken. The formation of strong hydrogen bonded dimer as ring graph set favours the charge transfer which influences the optical properties of the compound.