Electronic Structure Investigation and Nuclear Quadrupole Interactions in β-HMX

Abstract

The nuclear quadrupole coupling constants (e 2 qQ) and asymmetry parameters (η) for the 14 N nuclei in the β-HMX molecule are investigated by the Hartree-Fock-Roothaan variational procedure. For the two pairs of equivalent nuclei in the ring positions, the theoretical values are found to be -5.936 and -6.069 MHz for e 2 qQ and 0.432 and 0.490 for η, which are in good agreement with the corresponding experimentally observed magnitudes of 5.791 and 6.025 MHz for e 2 qQ and the values 0.4977 and 0.5180 for η obtained from singlecrystal measurements by the nuclear quadrupole resonance technique. Possible factors that could further improve the already close quantitative agreement between theory and experiment are also discussed. For the NO 2 groups in the peripheral positions the calculated values for e 2 qQ were found to be 1.442 and 1.369 for the two pairs of equivalent 14 N nuclei, substantially smaller than for the ring nuclei, while for η the corresponding values of 0.553 and 0.571 are close to those for the ring nuclei. Experimental values obtained for the magnitudes of e 2 qQ for 14 N in the NO 2 groups are found to be 0.840 and 0.806 MHz, the corresponding η being 0.42 and 0.48. Reasons for the weaker agreement with experiment, in contrast to the case of ring 14 N nuclei, are suggested, the principal one being the possibility of strong intermolecular bonding between the oxygen ligands of the NO 2 groups and hydrogens in neighboring molecules. Calculated values of e 2 qQ and η for 17 O and 2 H are presented for the β-HMX molecule with the hope that experimental values will become available for them in future for comparison with theory, allowing one to test whether the intermolecular interactions between oxygen and hydrogen atoms also influence their nuclear quadrupole interaction parameters.

Theory of Electronic Structure and Nuclear Quadrupole Interactions in Heroin

“…It is important to emphasize that while the need for the improvements in theoretical and experimental investigations discussed here is important to attempt near-exact agreement between theory and experiment, our results and currently available experimental data are in better than 10% agreement for e 2 qQ and the sizes of η from experiment and theory are both small, about 6% or less. In this respect, the good agreement between experiment and theory, using the selfconsistent-field Hartree-Fock cluster approach for heroin, is in keeping with the similar type of agreement found in our recent investigations [8][9][10] on other physiologically and energetically important molecules. One could therefore utilize the energy levels, electronic wave functions, and electron densities obtained from the Hartree-Fock cluster approach we have used, quite effectively for interpretation of other properties for heroin that may become available in the future, for instance, chemical shifts and indirect spin-spin interactions from nuclear magnetic resonance (NMR) measurements.…”

“…Additionally, it would be helpful to have a revised X-ray structure determination to test the available C, N, and O atom positions as well as information on the H-atom positions by Patterson synthesis, as in cocaine, to compare with our results for the H-atom positions (Table 1). A second source that could bridge the small differences between theoretical and experimental NQI parameters is the possibility of intermolecular interactions, especially between the nitrogen atom (and other atoms bonded to it) and atoms in neighboring molecules, which has been suggested from our earlier work 9 on β-HMX to influence the anisotropy of the electron density distribution around the 14 N nuclei in the latter system. The influence of intermolecular interactions would be somewhat time-consuming to test but would be desirable to carry out in the future.…”

“…We have not made use of even more extensive basis functions like 6-311G because of the large amount of time that would be needed for this large molecule. However, from our experience 9,10 in RDX and β-HMX we do not expect the results to change significantly from D95U to 6-311G. To give an idea about the sizes of the basis sets utilized in this work on heroin involving 196 electrons, STO-3G involves 158 basis functions and 474 primitive Gaussians, whereas D95 involves 316 basis functions, twice as big as STO-3G, and 740 primitive Gaussians.…”

“…Next, turning to the choice of basis functions employed to obtain the molecular orbital wave functions that were used to evaluate the efg tensor components, we have carried out our investigations using STO-3G as well as two much more flexible basis sets D95 and D95U, which were utilized in our earlier investigations [8][9][10] on RDX, β-HMX, and cocaine. The last named basis set (D95U) involves uncontraction 8 of the outermost p-orbitals for C, N, O in the D95 basis set.…”

DFT Investigation of Nuclear 14N Quadrupole Coupling Constants for Four HMX Polymorphs

“…Although many spectrum properties of HMX have been found in literature, [9][10][11][12] to our knowledge, the NQR spectrum of 14 N in HMX polymorph is still not available except the NQCC of 14 N in β-HMX. [13][14][15] Its theoretical value of η for 14 N in NO 2 group on the basis of single molecule model deviates greatly from experiment. To look for a better scheme to calculate the parameters of NQCC and η of HMX polymorph, single molecule model combined with higher basis set in this work is used to investigate the dependence of the accuracy of these parameters on basis set.…”