³⁵Cl NQR Study of Thermoactivated Motions of Nitro Groups in Picryl Chloride

Abstract

The temperature dependences of the 35Cl NQR frequency (ν), spin-lattice relaxation time (T1), and spin-spin relaxation time (T2) have been studied in 2,4,6-trinitrochlorobenzene (picryl chloride) from 77 K up to the melting point (354 K). The T1(T) curve exhibits a pronounced composite mimimum near 300 K which gives evidence for the reorientations of the two ortho-NO2 groups around their two-fold symmetry axes with the activation energies of 27.4 kJ mol - 1 and 31.2 kJ mol - 1. These values can be related to the ortho-NO2 groups having the twist angles of 33° and 81°, respectively (the crystal structure of picryl chloride is known). The T2(T) dependence exhibits interesting features, too: a deep minimum about 140 K and a new rapid decrease above 270 K.

The revision of intermolecular interactions in 1,3‐dinitrobenzene crystal—the role of nitro groups in optical nonlinearity

“…The theoretical calculation for a free nitrobenzene molecule yielded 13 ± 4 kJ·mol −1 . [50,51] The values obtained in this work were lower than the theoretical value and much lower than in other crystals containing nitro groups, like in 2chloro-nitrobenzene where Perez et al [50] estimated the energy values to be 21.9 and 23.6 kJ·mol −1 , whereas Kyuntsel [52] found 27.4 and 31.2 kJ·mol −1 in 2,4,6-trinitrochlorobenzene crystal. The increasing values of reorientation energy were connected with intermolecular forces and steric effects.…”

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 180°rotation barrier energy for nitro group is very high and amounts to 28.07 kJ/mol in BNA while in the MNA (optimized molecule) amounts to 20.6 kJ/mol [15]. These results may be compared with the 21.9 and 23.6 kJ/mol in 2-chloronitrobenzene crystal and 27.4 and 31.2 kJ/mol in 2,4,6-trinitrochlorobenzene crystal where the higher values of rotation energy were connected with strong intermolecular forces and steric effects [41,42]. However, the discrepancies between experimental and theoretically predicted activation energies values result from the comparison between the experimental data for BNA polycrystalline sample and the calculated for BNA free molecule where interactions and steric effects were not considered.…”