An NQR study of the crystalline structure of TNT

Abstract

A comparison of the NQR parameters of the monoclinic and orthorhombic phases of TNT and their relation to the twist or dihedral angle between the plane of the NO 2 substituents and that of the benzene ring as determined in the X-Ray crystal structure analysis enables an assignment of different frequencies to specific sites in the two independent molecules in the unit cell of both forms to be made. The slow transformation of the metastable orthorhombic phase to monoclinic can then be followed by monitoring the NQR spectrum in which specific lines can be assigned to molecular sites in the two phases. NQR spectra of TNT referred to in the literature often differ; this could be due partly to the TNT often being a mixture of monoclinic and orthorhombic phases and partly to changes in the spectral line width, factors which must be taken into account when NQR is used to detect landmines.

14N NQR, relaxation and molecular dynamics of the explosive TNT

“…The 14 N NQR ν + and ν − line frequencies as a function of temperature have already been published for both the monoclinic and orthorhombic form of TNT [1,2,3,13,14]. Over the temperature range −30°C to +50°C (243 K to 323 K) used for our measurements here the variation of line frequency with temperature is close to linear.…”

“…We have measured the various 14 N NQR parameters for a number of the ν + and ν − lines of monoclinic and orthorhombic TNT over a range of temperatures and discussed the temperature variation of some of these parameters in terms of the molecular dynamics. The temperature dependence of T 1 has been explained as due to hindered rotation of the NO 2 group about the C-NO 2 bond with an activation energy of 89 kJ mol -1 for the ortho and para groups of monoclinic TNT and 70 kJ mol -1 for the para group of orthorhombic TNT.…”

“…A theoretical analysis [24] suggests that these features are "cog-wheel" torsional modes involving the 2,6 NO 2 groups with the adjacent methyl group and predicts frequencies close to 70 and 91 cm -1 for the two molecules (I and II) of the crystallographic repeat unit. Table 1 14 N NQR ν + and ν − line frequencies for monoclinic and orthorhombic TNT have previously been published over a range of temperatures [1,2,3,13,14].This table presents the linear fit parameters, ν(0) and temperature coefficients b, of Eq. 2 for the ν + and ν − NQR lines of both polymorphs of TNT, obtained from our line frequency measurements in the 243 -323 K temperature range.…”

“…Table 1 also shows the twist angle of the plane of the nitro groups with respect to that of the benzene ring and the site assignments [14] to the two ortho (oA and oB) and para (p) nitro groups of the two molecules (I and II) of the crystallographic repeat unit. In both polymorphs, the lowest value of b and the twist angle are found for the two para nitro groups which from Eq.…”

“…1) have 14 N nuclear quadrupole resonance (NQR) frequencies [1,2] below 1 MHz, where inductive methods of signal detection have poor sensitivity. In detection applications of NQR [3] various multiple pulse sequences are often used to improve sensitivity by rapidly regenerating signals.…”