title: The 'partial resonance' of the ring in the NLO crystal melaminium formate: study using vibrational spectra, DFT, HOMO-LUMO and MESP mapping.
abstract: 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 N(3)C(1)N(1) moiety leading to special type resonance of the ring and the resonance structure of CO(2) 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.
authors.affiliation: Department of Physics, Government College, Nedumangadu, Thiruvananthapuram 695 541, Kerala, India. binjohndas@yahoo.co.in
authors.forename: J
authors.initials: J
authors.lastname: Binoy
authors.forename: M K
authors.initials: MK
authors.lastname: Marchewka
authors.forename: V S
authors.initials: VS
authors.lastname: Jayakumar
mesh_terms: D003460:Crystallization; D005561:Formates / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D017711:Nonlinear Dynamics; D011522:Protons; D011789:Quantum Theory; D017550:Spectroscopy, Fourier Transform Infrared; D013059:Spectrum Analysis, Raman; D055672:Static Electricity; D013816:Thermodynamics; D014227:Triazines / Q000737:chemistry*; D014732:Vibration; D014961:X-Ray Diffraction
chemical_list: D005561:Formates; D011522:Protons; D014227:Triazines; C582188:melaminium formate