Nitrogen nuclear quadrupole interactions in RDX, β-HMX and cocaine systems

Using the Hartree-Fock-Roothaan procedure, the nuclear quadrupole interactions (NQI) of the ¹⁴N (I = 1) nucleus in the energetically important molecules RDX, β-HMX and the physiologically important molecule Cocaine, are studied. The coupling constants (e² q Q) and asymmetry parameters M for the three ring nitrogens in RDX are found to be -5.671, -5.808 and -5.838 MHz and 0.542, 0.556 and 0.562, respectively, in good quantitative agreement with the experimental results of 5.735, 5.799 and 5.604 MHz for the magnitudes of e2qQ and 0.6215, 0.6146 and 0.6024 for η obtained in the single crystal. For β-HMX, where two sets of e² q Q and η are expected from symmetry considerations, our calculated values are -5.936 and - 6.069 MHz for e2 q Q and 0.432 and 0.490 for η, compared to experimentally measured magnitudes of 5.791 and 6.025 MHz. and η-values of 0.4977 and 0.5180, respectively, obtained in the single crystal. For Cocaine free base which contains only one ¹⁴N nucleus, our calculated values of e² q Q and η are -5.038 MHz and 0.067, in very good agreement with the experimental results of 5.0229 MHz for the magnitude of e² q Q and 0.0395 for η. Possible reasons for the small remaining differences between theory and experiment in e² q Q and η for all three systems and the significant differences in trends over the three nitrogens in RDX between theory and experiment are discussed. Also, the calculated quadrupole interaction parameters for the ¹⁴N nuclei in the NO₂ groups outside the ring for both RDX and β-HMX are presented with the hope that they will be measured in the future to provide a more complete understanding of the electron distributions in these systems.