Polarizability of acetanilide and RDX in the crystal: effect of molecular geometry

Reassessment of large dipole moment enhancements in crystals: a detailed experimental and theoretical charge density analysis of 2-methyl-4-nitroaniline

The molecular dipole moment of MNA in the crystal has been critically reexamined, to test the conclusion from an earlier experimental charge density analysis that it was substantially enhanced due to a combination of strong intermolecular interactions and crystal field effects. X-ray and neutron diffraction data have been carefully measured at 100 K and supplemented with ab initio crystal Hartree-Fock calculations. Considerable care taken in the measurement and reduction of the experimental data excluded most systematic errors, and sources of error and their effects on the experimental electron density have been carefully investigated. The electron density derived from a fit to theoretical structure factors assisted in the determination of the scale and thermal motion model. The dipole moment enhancement for MNA in the crystal is much less than that reported previously and only on the order of 30-40% (approximately 2.5 D). In addition to the dipole moment, experimental deformation electron density maps, bond critical point data, electric field gradients at hydrogen nuclei, and atomic and group charges all agree well with theoretical results and trends. Anisotropic modeling of the motion of hydrogen atoms, integral use of periodic ab initio calculations, and improved data quality are all aspects of this study that represent a considerable advance over previous work.

Equilibrium Partitioning Model Applied to RDX−Halide Adduct Formation in Electrospray Ionization Mass Spectrometry

An equilibrium partitioning model is applied for the first time to the sequential formation of 1:1 and then 2:1 adducts between the high explosive cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX) and halide anions fluoride, chloride, bromide, and iodide in electrospray ionization interface (ESI) mass spectrometry. The equilibrium partitioning model is developed and model calculations are presented to demonstrate the generic behavior of the system, which is in qualitative agreement with the observed changes in 1:1 (RDX-halide) and 2:1 (RDX-halide) responses in ESI-MS. The model is successfully applied to the experimental data with the use of octanol-water partitioning coefficients to predict interior-to-surface partitioning behavior of the complexes in droplets formed in the ESI. The data and model suggest that the significantly more hydrophobic 2:1 complexes are readily observed in ESI-MS, even though their formation constants may be several orders of magnitude less than that of the 1:1 complex. Structures for RDX-halide 1:1 and 2:1 complexes are proposed based on ion-dipole attractions and destabilizing dipole-dipole interactions.

Energy of charged states in the RDX crystal: Trapping of charge-transfer pairs as a possible mechanism for initiating detonation

Calculations for the crystalline energetic material RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) yield the effective polarizability (17.2 angstroms3), local electric field tensor, effective dipole moment (9.40 D), and dipole-dipole energy (-27.2 kJ/mol). Fourier-transform techniques give the polarization energy P for a single charge in the perfect crystal as -1.14 eV; the charge-dipole energy W(D) is zero if the crystal carries no bulk dipole moment. Polarization energies for charge-transfer (CT) pairs combine with the Coulomb energy E(C) to give the screened Coulomb energy E(scr); screening is nearly isotropic with E(scr) approximately = E(C)2.6. For CT pairs W(D) reduces to a term deltaW(D) arising from the interaction of the charge on each ion with the change in dipole moment on the other ion relative to the neutral molecule. The dipole moments are calculated as 7.40 D for the neutral molecule and 6.84 D and 7.44 D for the anion and cation, giving the lowest two CT pairs at -1.34 eV and -0.94 eV. The changes in P and W(D) near a molecular vacancy yield traps with depths that reach 400 meV for single charges and 185 meV for the nearest-neighbor CT pair. Divacancies yield traps with depths nearly equal to the sum of those produced by the separate vacancies. These results are consistent with a mechanism in which detonation of RDX is initiated by mechanical generation of CT pairs that localize at vacancies, recombine, and release energy sufficient to break bonds; crystals of molecules with lower dipole moments should be less sensitive.