Molecular motions contributions to optical nonlinearity of N-benzyl-2-methyl-4-nitroaniline studied by temperature-dependent FT-IR, 1H NMR spectroscopy and DFT calculations

The fourier transform infrared (FT-IR) spectra of polycrystalline orthorhombic N-benzyl-2-methyl-4-nitroaniline (BNA) were recorded in the 380–4000 cm−1 region within the 11–297 K temperature range while the 1H NMR spin–lattice relaxation time and the second moment of 1H NMR resonance line were measured in the 145–373 K and 80–400 K temperature range, respectively. Above 80 K the methyl group rotation is found to be dominant among other motions and leading to a significant reduction of the proton second moment which agrees with proposed models for the rigid structure and for the internal rotations in the BNA crystal such as methyl group C3 rotation, phenyl ring rotation and trans-gauche motion of methylene group. Activation energies for the nitro and methyl group rotations were experimentally estimated by FT-IR and 1H NMR, appropriately. For comparison the potential barriers for these two group motions in BNA molecule were calculated by DFT quantum chemical methods. The role of molecular motions in the BNA crystal in the large temperature range is discussed from the point of view of its nonlinear optical properties.