Density functional calculations at the B3LYP/6-31G* level are used to generate electron-diffraction intensity curves for six conformers of hexahydro-1,3,5-trinitro-s-triazine (RDX). These are compared with an experimental curve (Shishkov, I. F.; Vilkov, L. V.; Lolonits, M.; Rozsondai, B. Struct. Chem. 1991, 2, 57) for which a structural model of gas-phase RDX was proposed. The calculations were done to investigate possible causes of the discrepancies between the theoretical structures and the molecular model proposed by Shishkov et al. The results show that alternative structures to that proposed from experiment can reproduce the measured intensities. Also, barriers to interconversion between the conformers were calculated to investigate the possibility of rapid interconversion at the temperature of the experiment (433 K). Barriers range from 1.5 to 5 kcal/mol, suggesting that measurements of RDX in the gas phase might reflect a dynamically averaged structure that represents contributions from the individual conformers.
The equation of state for hard spheres in a spherical cavity is calculated and compared with the results previously found when the containing boundary was a channel. It was found that the spherical surface provides a focusing effect on both the pressure and the compressibility factor. An examination of the results suggested that the density and compressibility factors could be expanded in a series of Maxwellian functions. It was found that a six-term series which was modified in the immediate vicinity of the wall was sufficient. This has an advantage over the usual Fourier series expansion in that less terms are required. In addition, an argument can be made that such a series has a foundation in the physics of the problem.
McWeeny group functions, in conjunction with the popular Ramsey formulation, are used to theoretically account for the additive influence of substituents observed in NMR spectra. In this treatment the group wavefunctions are assumed to perturb one another only to first order. This approach is applied to chemical shifts and coupling constants involving various types of nuclei. In general, the substituent effect is found to be pairwise additive. If the substituent is far removed from the nuclei in question then the pairwise rule reduces to direct additivity. For a local operator, such as the Fermi contact term in spin coupling, the effects of substituents are directly additive even though the substituents are bonded to one of the nuclei in question. If the group wavefunctions act as a large perturbation on one another then the second-order terms cannot be neglected, and the simple direct and pairwise additivity rules fail. Existing data and new data are discussed in terms of the above theoretical treatment.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.