Dynamical disorder in 2-methyl-4-nitroaniline and its deuterated analogue crystals studied by Fourier transform infrared and nuclear magnetic resonance

Abstract

The Fourier transform infrared spectra of the thin layers of 2-methyl-4-nitroaniline (MNA) and its deuterated analog were recorded in the 500-4000 cm(-1) region in the 10-300 K temperature range. Activation energies of the -CH(3), -NH(2), and -NO(2) groups reorientations were estimated. The (1)H-NMR spin-lattice relaxation time, T(1), and the second moment of (1)H-NMR resonance line, M(2), measured in the 80-298 K temperature range, were used to determine the parameters of the -CH(3) group motion. The experimental potential barriers for the amine, nitro, and methyl group reorientations are considered in the context of strengths of the N-H([ellipsis (horizontal)])O, C-H([ellipsis (horizontal)])O intermolecular hydrogen bonds, and other short contacts, recognized recently [U. Okwieka et al., J. Raman Spectrosc. 39, 849 (2008)], and they agree with the barriers calculated by quantum chemical methods. The dynamical disorder found in the MNA crystal in the large temperature range seems to be important from the point of view of its nonlinear optical and other properties.

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 Raman excitation profiles of lattice Table 2 The calculated and experimentally obtained values of second moment of 1 H NMR line (M 2 ) in BNA solid for rigid structure, methyl, phenyl group rotations and trans-gauche motion of methylene group. The M 2 values in the m-NA, MNA and 4-IP crystals are given for comparison [8,15,16]. Note that M 2 value for m-NA was given for -NH 2 180°p roton jumps.…”

“…The activation energy estimated from the Carabatos-Nédelec and Becker model [22] amounts to 11.09 ± 0.12 and 10.68 ± 0.08 kJ/mol for the -CH 2 and -NH stretching vibration bands (Table 1) while for the -NO 2 symmetric and antisymmetric stretching vibrations is equal to 9.07 ± 0.24 and 9.61 ± 0.18 kJ/mol, respectively. These activation energy values are typical though usually depend on compound and the type of motion and/or libration [15,[36][37][38].…”

“…The temperature dependence of C is used to determine the rotational correlation time, s c , and the activation energy, E a . To fit the experimental points alike in the MNA case [15] we used the formula (1) given by Carabatos-Nédelec and Becker which is composed of the vibrational relaxation term (linear dependence on temperature, T) and the rotational relaxation of diffuse nature (exponential) term [22]:…”

“…This work is a continuation of studies on optically nonlinear (NLO) benzene derivatives crystals such as meta-nitroaniline (m-NA) [3,4,8], 2-methyl-4-nitroaniline (MNA) [6,14,15], 4-isopropylphenol (4-IP) [16] and N-benzyl-2-methyl-4-nitroaniline (BNA) [7,17,18]. The earlier studies showed that the charged polarons (radical anions), dynamic disorder of phenyl ring and both (three) nitro and amine (methyl) substituents leads to the instantaneous helical superstructure in the m-NA [3,4] (MNA [6,15]) and contribute to the SHG in these crystals.…”

“…The earlier studies showed that the charged polarons (radical anions), dynamic disorder of phenyl ring and both (three) nitro and amine (methyl) substituents leads to the instantaneous helical superstructure in the m-NA [3,4] (MNA [6,15]) and contribute to the SHG in these crystals. It was found by the 1 H NMR method that methyl group rotations are dominant motions in the solid MNA and 4-IP at about 90 K and 100 K, respectively [15,16]. It is worth to add that the charged polarons were also detected in the 4-IP crystals [16].…”