The ‘partial resonance’ of the ring in the NLO crystal melaminium formate: Study using vibrational spectra, DFT, HOMO–LUMO and MESP mapping

The molecular geometry and vibrational spectral investigations of melaminium formate, a potential material known for toxicity and NLO activity, has been performed. The FT IR and FT Raman spectral investigations of melaminium formate is performed aided by the computed spectra of melaminium formate, triazine, melamine, melaminium and formate ion, along with bond orders and PED, computed using the density functional method (B3LYP) with 6-31G(d) basis set and XRD data, to reveal intermolecular interactions of amino groups with neighbor formula units in the crystal, intramolecular H⋯H repulsion of amino group hydrogen with protonating hydrogen, consequent loss of resonance in the melaminium ring, restriction of resonance to N3C1N1 moiety leading to special type resonance of the ring and the resonance structure of CO2 group of formate ion. The 3D matrix of hyperpolarizability tensor components has been computed to quantify NLO activity of melamine, melaminium and melaminium formate and the hyperpolarizability enhancement is analyzed using computed plots of HOMO and LUMO orbitals. A new mechanism of proton transfer responsible for NLO activity has been suggested, based on anomalous IR spectral bands in the high wavenumber region. The computed MEP contour maps have been used to analyze the interaction of melaminium and formate ions in the crystal.

Graphical abstractThe crystalline network of melaminium formate and the ‘partial resonance’ of melaminium cation.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► DFT calculations have been performed on the NLO crystal melaminium formate. ► The IR and Raman spectra of the compound were analyzed. ► The ‘partial’ resonance of the ring of melaminium cation has been investigated. ► A new mechanism of proton transfer responsible for NLO activity has been suggested. ► The HOMO LUMO plots and MEP contour maps have been analyzed.