Ray Engelke

Diatomic molecule vibrational potentials: Accuracy of representations

A method is presented for increasing the radius of convergence of certain representations of diatomic molecule vibrational potentials. The method relies on using knowledge of the analytic structure of such potentials to the maximum when attempting to approximate them. The known singular point (due to the centrifugal and/or Coulomb potentials) at zero internuclear separation should be included in its exact form in an approximate representation. The efficacy of this idea is tested [using Peek’s ’’exact’’ numerical Born–Oppenheimer potential for the (1sσg)2Σ+g state of H+2 as a test problem] when the representational form is the series of (1) Dunham, (2) Simons, Parr, and Finlan, (3) Thakkar, and (4) Ogilvie–Tipping, and also (5) when the form is a [2, 2] or a [3, 3] Pade approximant. Significant improvements in accuracy are obtained in some of these cases, particularly on the inner wall of the potential. A comparison of the effectiveness of the five methods is made both with and without the origin behavior ...

Microscopic evidence that the nitromethane aci ion is a rate controlling species in the detonation of liquid nitromethane

We present microscopic evidence that the aci ion (H2CNO−2) of nitromethane (H3CNO2) plays an important role in the detonation kinetics of liquid‐phase nitromethane. It is known from previous detonation experiments that very minute additions of organic bases (e.g., amines) have a profound effect on the detonation properties of nitromethane; i.e., the explosive is strongly sensitized. Here we show that, under conditions similar to the detonation experiments, the only new chemical species generated in nitromethane by the bases sodium hydroxide (NaOH), diethylenetriamine (NH2CH2CH2NHCH2CH2NH2), and pyridine (C5H5N) is the aci ion, within the sensitivity of the experiments. The primary tool used to demonstrate this is 13C NMR spectroscopy. Ab initio quantum‐mechanical calculations of the chemical shifts are used to support the experimental interpretation. Qualitative arguments concerning the increased reactivity of the aci ion, relative to normal nitromethane, are given. We review earlier work and relate it to...

A study of the steady‐state reaction‐zone structure of a homogeneous and a heterogeneous explosive

The two‐dimensional steady‐state reaction‐zone structure of a homogeneous and a heterogeneous explosive is studied. To do this theoretical results obtained from the Euler equations of compressible flow are combined with experimental data on steady‐state detonation shock‐wave speed and shape as a function of the explosive charge size. The theoretical results, constrained by the experiments, define an inverse problem for the chemical heat‐release function in the reaction zone which follows the shock wave; this problem is solved. The heterogeneous explosive is made from the homogeneous one by adding small quantities of other materials. Because of this, the two explosives were closely related in many respects. In spite of this, quite large differences in the detonation characteristics are observed between the two explosives, both in the wave speed as a function of charge size and in the shape of shock‐wave loci near the explosive edge. It is found that a single forward rate exponentially dependent on the inve...

Effect of a chemical inhomogeneity on steady‐state detonation velocity

The detonation behavior of the liquid explosive, nitromethane, sensitized by the liquid organic base, diethylene triamine, has been studied experimentally. The work to be described complements earlier work concerning the effect of physical impurities on steady detonation behavior. By interpretation of detonation‐velocity measurements, the following three goals are reached: (1) The evidence that physical inhomogeneities (usually voids) produce the downward concave region of the diameter‐effect curves of condensed high explosives is enhanced; (2) the hypothesis that homogeneous materials have linear diameter‐effect curves is placed on a firmer basis; and (3) the experimental evidence indicates that the sensitizing material has altered the effective chemical kinetics and consequently, has produced an explosive with a shorter one‐dimensional reaction zone. A remarkably small amount of the chemical impurity produces a significant reduction of the nitromethane failure diameter; e.g., addition of one molecule of...

Diatomic‐molecule vibrational potentials. II. New representationsa)

Two new representations of diatomic‐molecule vibrational potentials are presented. One of these includes most of the previously employed series approximations as special cases. The new representations are tested against older ones by using Peek’s ’’exact’’ numerical Born–Oppenheimer potentials for the 1sσg, 2pπu, and 3dσg states of H2+ as test problems. Accuracy comparisons are made with Dunham, Thakkar, Ogilvie–Tipping, Coulomb‐subtracted Ogilvie–Tipping, and Pade representations. A central idea of the new treatment is that it is not necessary to use a global representation of the potential over the region 0?R/Re<∞. Rather one can choose separate representations suitable for the subregions R/Re?1 and R/Re?1 and then match them smoothly to each other and to the Dunham expansion at R/Re=1. Attention is focused on finding improved approximations for R/Re≳1, since this region exerts strong control on the vibrational eigenvalue spectrum and it is here, perhaps, where the older techniques are at their weakest....

A theoretical study of possible benzene dimerizations under high‐pressure conditions

We offer a theoretical explanation of the rate processes observed macroscopically in materials composed of aromatic ring structures subjected to high pressure. Earlier workers have made qualitative suggestions that the origin of these processes may be due to interring pi bonding. By making quantum‐mechanical calculations on a simple special case of such systems (i.e., two interacting benzene rings), we attempt to produce a quantitative microscopic foundation for the suggestions. We briefly review earlier experimental and theoretical work on the subject and thereby motivate the working hypotheses used in the calculations. The principal hypothesis is that by studying restricted parts of the two benzene‐ring energy hypersurface, we can learn something about the pressure‐induced rate process for all the arene structures. By use of the modified‐neglect‐of‐diatomic‐differential‐overlap (MNDO) method and the generalized valence bond ‘‘perfect‐pairing’’ (GVP–PP) method supplemented by configuration interaction, w...

Kinetics study of a condensed detonating explosive

We discuss a method for determining chemical–kinetic parameters for condensed explosive materials under the extreme pressure and temperature conditions of detonation. It utilizes theoretical results obtained from the Euler equations of fluid mechanics and experimental detonation data, which consist of steady‐state shock shapes and detonation velocities measured as a function of charge size. We determine Arrhenius parameters for detonating commercial‐grade nitromethane. The sensitivity of the method to various uncertainties in the input is tested. Results for the Arrhenius activation energy (EA = 92±23 kcal/mole) and vibrational factor [log10 k(sec−1) = 18±2] are larger than the gas phase values. These results are compared with those of other workers.

Effect of the number density of heterogeneities on the critical diameter of condensed explosives

Experiments are reported which examine the dependence of the critical diameter of a condensed explosive on the number density of heterogeneities present within the material. The form and quantity of heterogeneities present were closely controlled by constructing the heterogeneous materials from a homogeneous one. Reductions in critical diameter of over 40% were produced in some cases, even though a chemically inert material was being introduced into the explosive. The experiments indicate that the distance between heterogeneities is an important parameter in the production of a critical‐diameter reduction.

Chemical dimerization of crystalline anthracene produced by transient high pressure

We report the production of a chemically bound dimer of anthracene produced by the shock compression of crystalline anthracene. The experimental probe used to detect the dimer structure was time‐of‐flight (TOF) mass spectrometry. The principal method used to produce the shock compression was the impact of electrically accelerated flyers (‘‘slappers’’) with the surface of the anthracene crystals. Our work correlates well with earlier experimental and theoretical work concerning chemical processes that can occur in pressurized anthracene and other aromatic materials. We briefly review the earlier work. The anthracene crystals were shocked to various pressures in the interval 9 to 22 GPa. Also various crystal thicknesses were employed; this allowed us to control the time interval over which various anthracene ‘‘particles’’ in the crystal were held at high pressure and temperature. At a pressure of ∼22.1 GPa we observe dimer formation in 20 ns or less. For a shock pressure of 9 GPa no dimer is produced, where...

Reduction of detonating liquid nitromethane’s chemical reaction-zone length by chemical sensitization

We examine the effect of the addition of small amounts of the organic base diethylenetriamine [NH2(CH2CH2)NH(CH2CH2)NH2] on the chemical reaction-zone length (CRZ) of detonating liquid nitromethane (CH3NO2). This is done by making accurate measurements of the detonating materials’ diameter-effect curves (i.e., detonation speed versus lateral charge size) as a function of the amount of chemical sensitizer added. Detonation speed experiments were performed with additions of the organic base in amounts between 0.00 and 0.25wt%. Reductions in the CRZ of as much as 25% were produced by base addition. Most of the reduction in length is produced by very small amounts of base addition—i.e., ca. 0.05wt% of base or less (i.e., 1 molecule of the base per 3300 nitromethane molecules or less). Measured detonation speeds are given for five compositions of nitromethane and base as a function of charge internal diameter. Absolute CRZs are estimated using a value of liquid nitromethane’s CRZ obtained by other means. Earli...