Synthesis, structural, spectral and optical studies of (4-methylphenyl)methanaminium bromide hemihydrate: a new semiorganic crystal for laser applications

Abstract

Semiorganic single crystals of (4-methylphenyl)methanaminium bromide hemihydrate (4MMBH) have been grown by reacting 4-methylbenzylamine (4MLBA) and hydrobromic acid in deionized water (1:1 molar ratio). Hydrobromic acid donates H⁺ cation to NH₂ group of 4MLBA and protonates it. Single crystal X-ray diffraction analysis reveals that 4MMBH is isostructural with its chloride congener and crystallizes in monoclinic crystal system with centrosymmetric space group C₂/c. Intramolecular (C–H⋅⋅⋅Cg) and intermolecular (C–H⋅⋅⋅Br, O–H⋅⋅⋅Br, N–H⋅⋅⋅Br, and N–H⋅⋅⋅O) interactions were observed in the structure of the title crystal. These interactions shifts the NH, OH and CH stretching vibrations to lower frequency and their bending vibrations (–NH, –OH, –CH) to higher frequency when compared with pure 4MLBA vibrations. CH stretching vibration frequency is explained by difference in the bond length observed in the crystal structure. Also, the Br⁻ vibration predicted through Raman spectral analysis ensures the association of bromide anion in the molecular structure. The molecular structural framework was established through ¹³C and ¹H NMR spectral analysis. In ¹H NMR, the interaction of Br⁻ with protonated NH₃⁺ group causes the deshielding of NH₃⁺ protons results in downfield shift and was observed at 8.211 ppm and the same for pure 4MLBA is observed at 1.42 ppm. C–H⋅⋅⋅Br interaction shifts the carbon signal of CH₂ towards downfield due to electronegative Br⁻ which deshields CH₂ group and the hydrogen bond associated with NH₃⁺ group splits the signals as quartet. The lower cut off wavelength of 4MMBH was found to be 260 nm and the crystal is transparent from 260 to 1100 nm. The third order nonlinear susceptibility χ⁽³⁾ value is found to be 3.15758 × 10⁻⁵ esu and this is due to the inter and intramolecular interactions of hydrogen bond present in the crystal.