Effect of the Prolactin Inhibitor Cabergoline and the Gonadotropin Releasing-Hormone Agonist Deslorelin in the Suppression of Plasma Prolactin Concentrations and Egg Laying in Quail (Coturnix japonica).

Egg binding and excessive laying frequently affect avian patients, and in many cases the treatment includes suppression of egg production. Currently, for the suppression of egg production in avian patients, a gonadotropin-releasing hormone agonist, in the form of a deslorelin implant, is often used. However, the commercially available deslorelin implants have an undesired delayed onset, as well as a potential brief increase in gonadotropin secretion after administration ("flare-up" effect) that can lead to oviposition before the actual suppression of gonadotropins. The objective of this study was to investigate whether the prolactin inhibitor cabergoline suppresses ovulation and whether it could be used to bridge the time until the onset of effect by the deslorelin implant. We measured the effect of cabergoline (30 µg/kg PO q24h × 14 days), deslorelin implants (4.7 mg SC), and a combination of both on egg laying and plasma prolactin concentrations in 37 quail (Coturnix japonica) over 6 weeks. Quail were divided into 4 groups: group DesCab (deslorelin implant and cabergoline oral; n = 9); group DesPlac (deslorelin implant and placebo oral; n = 9); group PlacCab (placebo implant and cabergoline oral; n = 9); and group PlacPlac (placebo implant and placebo oral; n = 10). Regular egg laying stopped in 100% (9/9) of birds in group DesCab and 78% (7/ 9) of birds in group DesPlac within 5 days of placing the deslorelin implant. No bird ceased egg production in group PlacCab (0/9), and 10% of birds ceased egg production intermittently in group PlacPlac (1/10). Treatment with the deslorelin implant (P < .001) and with cabergoline (P = .04) had a significant (negative) influence on plasma prolactin concentrations compared with the baseline. The interaction of deslorelin and cabergoline treatment, as well as time after initiation of treatment, did not have a significant effect on plasma prolactin concentrations. These results show that daily oral cabergoline has no significant influence on egg laying and only a minor biologically nonsignificant effect on lowering the relative plasma prolactin concentrations in quail.

Effects of Epinephrine, Detomidine, and Butorphanol on Assessments of Insulin Sensitivity in Mares.

Sympathoadrenal stimulation may perturb results of endocrine tests performed on fractious horses. Sedation may be beneficial; however, perturbation of results may preclude useful information. Four experiments were designed to 1) determine the effects of epinephrine on insulin response to glucose (IR2G), 2) assess the effects of detomidine (DET), alone or combined with butorphanol (DET/BUT), on IR2G and glucose response to insulin (GR2I), and 3) assess the effects of BUT alone on IR2G. In Experiment 1, mares were administered saline or epinephrine (5 µg/kg BW) immediately before infusion of glucose (100 mg/kg BW). Glucose stimulated (P < .05) insulin release in controls at 5 minutes that persisted through 30 minutes; insulin was suppressed (P < .05) by epinephrine from 5 to 15 minutes, rising gradually through 30 minutes. Experiments 2 (IR2G) and 3 (GR2I) were conducted as triplicated 3 × 3 Latin squares with the following treatments: saline (SAL), DET, and DET/BUT (all administered at .01 mg/kg BW). Glucose stimulated (P < .05) insulin release that persisted through 30 minutes in SAL mares; DET and DET/BUT severely suppressed (P < .0001) the IR2G. Sedation did not affect resting glucose and had inconsistent effects on the GR2I when mares were treated with 50 mIU/kg BW recombinant human insulin. Butorphanol had no effect on IR2G. In conclusion, adrenergic agonists severely suppress the IR2G and cannot be used for sedation for this test. The use of DET did not alter the GR2I, and therefore may be useful for conducting this test in fractious horses.

Melanocyte-Stimulating Hormone Response to Exercise, Twitching, Epinephrine Injection, Substance P Injection, and Prostaglandin-F2α Administration in Mares.

Five experiments were performed to test the hypothesis that α-melanocyte-stimulating hormone (MSH) is secreted in response to various stressors in horses similar to prolactin, growth hormone, and adrenocorticotropin (ACTH). There was considerable variation in resting concentrations of MSH and in the degree of stimulation in responders; thus all data sets were tested for heterogeneity of variance and corrected for as needed before analysis. In experiment 1, 12 mares were used in a switchback design to test the effect of a 2-minute exercise bout on MSH secretion. Plasma MSH concentrations were constant when mares were not exercised but increased (P < .05) immediately (2 minutes) after exercise and were still elevated 5 minutes later. In experiment 2, six mares were twitched for 2 minutes and six mares were not twitched. Twitching stimulated (P < .05) both MSH and ACTH relative to controls. Experiments 3, 4, and 5 tested the acute effects of intravenous injection of epinephrine at 5 µg/kg of body weight, intravenous injection of 100 µg substance P, and intramuscular injection of 10 mg prostaglandin-F2α in mares compared to controls (6, 5, and 6 mares per treatment group, respectively). Concentrations of MSH increased (P ≤ .05) after treatment in all three experiments. Plasma concentrations of ACTH also increased (P < .01) after administration of epinephrine and prostaglandin-F2α in experiments 3 and 5; plasma ACTH was not measured in experiment 1 or 4 because we have previously reported that exercise and substance P stimulate plasma ACTH concentrations. As hypothesized, MSH is secreted in response to various stimuli similar to that observed previously for prolactin, growth hormone, and ACTH.

Identifying Collisions of Various Molecularities in Molecular Dynamics Simulations.

We present a method based on kinetic molecular theory that identifies reactions of various molecularities in molecular dynamics (MD) simulations of bulk gases. The method allows characterization of the thermodynamic conditions at which higher than bimolecular reactions are a factor in the mechanisms of complex gas-phase chemistry. Starting with Bodenstein's definition of termolecular collisions we derive analytical expressions for the frequency of higher molecularity collisions. We have developed a relationship for the ratio of the frequencies of termolecular to bimolecular collisions in terms of the temperature, density, and collision times. To demonstrate the method, we used ReaxFF in LAMMPS to carry out MD simulations for NVT ensembles of mixtures of H2-O2 over the density range 120.2-332.7 kg m-3 and temperature range 3000-5000 K. The simulations yield ReaxFF-based predictions of the relative importance of termolecular collisions O2···H2···O2 and bimolecular collisions O2···H2 in the early chemistry of hydrogen combustion.

Endocrine and metabolic profile of peripubertal Standardbred colts.

The objectives of this study were to determine the concentrations of reproductive and metabolic hormones during the peripubertal period and to assess their relationship with testicular development and body fat deposition. Blood samples were collected from 23 healthy Standardbred colts every four weeks for twelve months. Colts were weighed monthly, and percent of body fat and testicular volume estimated by ultrasound. Onset of puberty was determined as the month when testosterone was two standard deviations above the previous mean. Plasma FSH, LH, leptin, estradiol-17ß, androstenedione, IGF-1, insulin, inhibin-A, and inhibin-B were analyzed for a seven month peripubertal period. Spring born Standardbred colts underwent puberty at 13 months of age; onset of puberty coincided with exponential testicular growth but did not coincide with an increase in cutaneous body fat deposition or leptin (p > 0.05). Plasma inhibin-B concentrations were significantly increased in the postpubertal period (p < 0.05), but no increase was seen in inhibin-A, androstenedione, FSH, LH, or estradiol-17ß. In conclusion, the rise in testosterone and subsequent onset of puberty coincides with rapid testicular growth but is not correlated with an increase in gonadotropins, IGF-1, cutaneous body fat or leptin in the horse.

Effects of Various Methods of Sulpiride Administration on Prolactin Release in Horses.

Four experiments assessed factors affecting prolactin responses to sulpiride administration in horses. Experiment 1 compared the efficacy of the (-) enantiomer of sulpiride to that of the commonly used (+/-) racemic mixture. Mares were used in an 8 × 8 Latin square to compare the prolactin responses to four doses of levosulpiride to four corresponding doses of the racemic mixture at twice the dose. Responses at each dose indicated equal and similar (P > .1) responses. Experiment 2 compared the efficacy of 1 gram of orally administered racemic sulpiride to 100 mg of intramuscularly injected sulpiride in oil in mares primed with 50 mg of estradiol cypionate (ECP). Prolactin responses in groups receiving sulpiride were robust but similar in magnitude with minor differences in timing. In experiment 3, ECP-primed geldings received subcutaneous injections of 1.8 grams racemic sulpiride in vegetable shortening in one of three sites: the neck, the back below the withers, or the lower girth region; control geldings received no sulpiride. Prolactin responses to sulpiride lasted a minimum of 96 hours. In experiment 4, prolactin responses to 3 g of racemic sulpiride in vegetable shortening were compared to similar injections (3 g) in 5 mL of sucrose acetate isobutyrate (SAIB; SucroMate) or just SAIB (control) in ECP-primed geldings. Controls had no prolactin response to SucroMate, whereas both treatment groups had extended prolactin responses lasting at least 10 days. It is concluded that prolactin responses to sulpiride in horses can be greatly extended by using hydrophobic vehicles like vegetable shortening or SAIB.

Effects of Combined Estradiol-Sulpiride Treatment and Follicle Ablation on Vernal Transition in Mares: Evaluation of Plasma and Follicular Fluid Hormones and Luteinizing Hormone Receptor Gene Expression.

This experiment assessed the hormonal production, secretory aspects, and changes in luteinizing hormone (LH) receptor gene expression of early induced ovulatory-sized follicles relative to the first ovulatory-sized follicles occurring naturally in the spring. Anovulatory mares were treated on January 21 with (1) 50 mg of estradiol cypionate (ECP, n = 8) alone or (2) with ECP followed by two 3-g sulpiride injections (n = 8), 5 and 12 days later. Half of each group also received complete follicle ablation via transvaginal aspiration before ECP treatment. Ovaries were scanned regularly until detection of a 32-35 mm follicle; follicular fluid was recovered via aspiration for analysis of hormonal concentrations. Blood was collected regularly to characterize plasma prolactin, LH, follicle stimulating hormone, progesterone, and estradiol concentrations. Mean date to first 35-mm follicle was earlier (P < .05) in sulpiride-treated mares: five of eight (63%) responded within 28 days of the first sulpiride injection. Ablation did not affect ovarian response. Plasma prolactin was stimulated (P < .0001) in ECP-sulpiride-treated mares for 16 days but did not dictate ovarian response. Estradiol stimulated plasma LH (P < .05), which was higher (P < .05) in treated mares that responded. There was no effect of treatment or ablation on follicular fluid concentrations of estradiol, progesterone, leptin, or insulin-like growth factor 1 or on LH receptor gene expression. These latter similarities indicate that ECP-sulpiride early induced follicles have apparently reached a degree of maturity equivalent to naturally occurring ovulatory-sized follicles later in the spring.

General application of Tolman's concept of activation energy.

We present a generalization of Tolman's concept of activation energy applicable to thermal and non-thermal reactions in molecular dynamics simulations of reactions in bulk gases. To illustrate the applicability of the method, molecular dynamics calculations were carried out for the NVT ensemble to determine the activation energies of O2 + H2 → H + HO2 and 2O2 + H2 → 2HO2 from MD simulation results for [H2]/[O2] = 1 at 3000 K using the reactive force field, ReaxFF. Assuming local thermodynamic equilibrium, we define the reaction cluster local energy, the energy of the atoms participating in an individual reaction, which is conserved. The generalized Tolman activation energy (GTEa) approach is applicable to reactions of any molecularity. Although we have applied GTEa for thermal conditions, it is applicable to chemistry occurring under non-thermal conditions because it rests upon local rather than global equilibrium. We have defined the transition configurations, unique points that define a seam separating reactants and products at which the local energies of the reactants and products become equal.

Short-Term Effects of Changing Precipitation Patterns on Shrub-Steppe Grasslands: Seasonal Watering Is More Important than Frequency of Watering Events.

Climate change is expected to alter precipitation patterns. Droughts may become longer and more frequent, and the timing and intensity of precipitation may change. We tested how shifting precipitation patterns, both seasonally and by frequency of events, affects soil nitrogen availability, plant biomass and diversity in a shrub-steppe temperate grassland along a natural productivity gradient in Lac du Bois Grasslands Protected Area near Kamloops, British Columbia, Canada. We manipulated seasonal watering patterns by either exclusively watering in the spring or the fall. To simulate spring precipitation we restricted precipitation inputs in the fall, then added 50% more water than the long term average in the spring, and vice-versa for the fall precipitation treatment. Overall, the amount of precipitation remained roughly the same. We manipulated the frequency of rainfall events by either applying water weekly (frequent) or monthly (intensive). After 2 years, changes in the seasonality of watering had greater effects on plant biomass and diversity than changes in the frequency of watering. Fall watering reduced biomass and increased species diversity, while spring watering had little effect. The reduction in biomass in fall watered treatments was due to a decline in grasses, but not forbs. Plant available N, measured by Plant Root Simulator (PRS)-probes, increased from spring to summer to fall, and was higher in fall watered treatments compared to spring watered treatments when measured in the fall. The only effect observed due to frequency of watering events was greater extractable soil N in monthly applied treatments compared to weekly watering treatments. Understanding the effects of changing precipitation patterns on grasslands will allow improved grassland conservation and management in the face of global climatic change, and here we show that if precipitation is more abundant in the fall, compared to the spring, grassland primary productivity will likely be negatively affected.

Anisotropy in surface-initiated melting of the triclinic molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene: A molecular dynamics study.

Surface-initiated melting of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), a triclinic molecular crystal, was investigated using molecular dynamics simulations. Simulations were performed for the three principal crystallographic planes exposed to vacuum, with the normal vectors to the planes given by b × c, c × a, and a × b (where a, b, and c define the edge vectors of the unit cell), denoted as (100), (010), and (001), respectively. The best estimate of the normal melting temperature for TATB is 851 ± 5 K. The nature and extent of disordering of the crystal-vacuum interface depend on the exposed crystallographic face, with the (001) face exhibiting incomplete melting and superheating. This is attributed to the anisotropy of the inter-molecular hydrogen bonding and the propensity of the crystal to form stacking faults in directions approximately perpendicular to the (100) and (010) faces. For all three crystal orientations, formation of molecular vacancies in the lattice at the crystal-vacuum (or crystal-quasi-liquid layer) interface precedes the complete loss of order at the interface.

The effectiveness of clinician feedback in the treatment of depression in the community mental health system.

OBJECTIVE: We describe the development and evaluation of a clinician feedback intervention for use in community mental health settings. The Community Clinician Feedback System (CCFS) was developed in collaboration with a community partner to meet the needs of providers working in such community settings. METHOD: The CCFS consists of weekly performance feedback to clinicians, as well as a clinical feedback report that assists clinicians with patients who are not progressing as expected. Patients in the randomized sample (N = 100) were predominantly female African Americans, with a mean age of 39 years. RESULTS: Satisfaction ratings of the CCFS indicate that the system was widely accepted by clinicians and patients. A hierarchical linear models (HLM) analysis comparing rates of change across conditions controlling for baseline gender, age, and racial group indicated a moderate effect in favor of the feedback condition for symptom improvement, t(94) = 2.41, p = .017, d = .50. Thirty-six percent of feedback patients compared with only 13% of patients in the no-feedback condition demonstrated clinically significant change across treatment, χ2(1) = 6.13, p = .013. CONCLUSIONS: These results indicate that our CCFS is acceptable to providers and patients of mental health services and has the potential to improve the effectiveness of services for clinically meaningful depression in the community mental health setting.

Pressure effects on the relaxation of an excited nitromethane molecule in an argon bath.

Classical molecular dynamics simulations were performed to study the relaxation of nitromethane in an Ar bath (of 1000 atoms) at 300 K and pressures 10, 50, 75, 100, 125, 150, 300, and 400 atm. The molecule was instantaneously excited by statistically distributing 50 kcal/mol among the internal degrees of freedom. At each pressure, 1000 trajectories were integrated for 1000 ps, except for 10 atm, for which the integration time was 5000 ps. The computed ensemble-averaged rotational energy decay is ∼100 times faster than the vibrational energy decay. Both rotational and vibrational decay curves can be satisfactorily fit with the Lendvay-Schatz function, which involves two parameters: one for the initial rate and one for the curvature of the decay curve. The decay curves for all pressures exhibit positive curvature implying the rate slows as the molecule loses energy. The initial rotational relaxation rate is directly proportional to density over the interval of simulated densities, but the initial vibrational relaxation rate decreases with increasing density relative to the extrapolation of the limiting low-pressure proportionality to density. The initial vibrational relaxation rate and curvature are fit as functions of density. For the initial vibrational relaxation rate, the functional form of the fit arises from a combinatorial model for the frequency of nitromethane "simultaneously" colliding with multiple Ar atoms. Roll-off of the initial rate from its low-density extrapolation occurs because the cross section for collision events with L Ar atoms increases with L more slowly than L times the cross section for collision events with one Ar atom. The resulting density-dependent functions of the initial rate and curvature represent, reasonably well, all the vibrational decay curves except at the lowest density for which the functions overestimate the rate of decay. The decay over all gas phase densities is predicted by extrapolating the fits to condensed-phase densities.

Calculation of anharmonic couplings and THz linewidths in crystalline PETN.

We have developed a method for calculating the cubic anharmonic couplings in molecular crystals for normal modes with the zero wave vector in the framework of classical mechanics, and have applied it, combined with perturbation theory, to obtain the linewidths of all infrared absorption lines of crystalline pentaerythritol tetranitrate in the terahertz region (<100 cm(-1)). Contributions of the up- and down-conversion processes to the total linewidth were calculated. The computed linewidths are in qualitative agreement with experimental data and the results of molecular dynamics simulations. Quantum corrections to the linewidths in the terahertz region are shown to be negligible.

Molecular dynamics simulations of shock waves in hydroxyl-terminated polybutadiene melts: mechanical and structural responses.

The mechanical and structural responses of hydroxyl-terminated cis-1,4-polybutadiene melts to shock waves were investigated by means of all-atom non-reactive molecular dynamics simulations. The simulations were performed using the OPLS-AA force field but with the standard 12-6 Lennard-Jones potential replaced by the Buckingham exponential-6 potential to better represent the interactions at high compression. Monodisperse systems containing 64, 128, and 256 backbone carbon atoms were studied. Supported shock waves were generated by impacting the samples onto stationary pistons at impact velocities of 1.0, 1.5, 2.0, and 2.5 km s(-1), yielding shock pressures between approximately 2.8 GPa and 12.5 GPa. Single-molecule structural properties (squared radii of gyration, asphericity parameters, and orientational order parameters) and mechanical properties (density, shock pressure, shock temperature, and shear stress) were analyzed using a geometric binning scheme to obtain spatio-temporal resolution in the reference frame centered on the shock front. Our results indicate that while shear stress behind the shock front is relieved on a ∼0.5 ps time scale, a shock-induced transition to a glass-like state occurs with a concomitant increase of structural relaxation times by several orders of magnitude.

A classical trajectory study of the intramolecular dynamics, isomerization, and unimolecular dissociation of HO2.

The classical dynamics and rates of isomerization and dissociation of HO2 have been studied using two potential energy surfaces (PESs) based on interpolative fittings of ab initio data: An interpolative moving least-squares (IMLS) surface [A. Li, D. Xie, R. Dawes, A. W. Jasper, J. Ma, and H. Guo, J. Chem. Phys. 133, 144306 (2010)] and the cubic-spline-fitted PES reported by Xu, Xie, Zhang, Lin, and Guo (XXZLG) [J. Chem. Phys. 127, 024304 (2007)]. Both PESs are based on similar, though not identical, internally contracted multi-reference configuration interaction with Davidson correction (icMRCI+Q) electronic structure calculations; the IMLS PES includes complete basis set (CBS) extrapolation. The coordinate range of the IMLS PES is limited to non-reactive processes. Surfaces-of-section show similar generally regular phase space structures for the IMLS and XXZLG PESs with increasing energy. The intramolecular vibrational energy redistribution (IVR) at energies above and below the threshold of isomerization is slow, especially for O-O stretch excitations, consistent with the regularity in the surfaces-of-section. The slow IVR rates lead to mode-specific effects that are prominent for isomerization (on both the IMLS and XXZLG) and modest for unimolecular dissociation to H + O2 (accessible only on the XXZLG PES). Even with statistical distributions of initial energy, slow IVR rates result in double exponential decay for isomerization, with the slower rate correlated with slow IVR rates for O-O vibrational excitation. The IVR and isomerization rates computed for the IMLS and XXZLG PESs are quantitatively, but not qualitatively, different from one another with the largest differences ascribed to the ~2 kcal/mol difference in the isomerization barrier heights. The IMLS and XXZLG results are compared with those obtained using the global, semi-empirical double-many-body expansion DMBE-IV PES [M. R. Pastrana, L. A. M. Quintales, J. Brandão, and A. J. C. Varandas, J. Chem. Phys. 94, 8073 (1990)], for which the surfaces-of-section display more irregular phase space structure, much faster IVR rates, and significantly less mode-specific effects in isomerization and unimolecular dissociation. The calculated IVR results for all three PESs are reasonably well represented by an analytic, coupled three-mode energy transfer model.

Molecular dynamics study of the pressure-dependent terahertz infrared absorption spectrum of α- and γ-RDX.

Terahertz infrared absorption spectra of the α and γ polymorphs of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) were predicted using two different theoretical approaches based on molecular dynamics simulations. The thermodynamic conditions studied were T = 298 K and hydrostatic pressures P = 0.0, 1.0, and 2.0 GPa for α-RDX and P = 3.0, 5.2, and 7.0 GPa for γ-RDX. The spectra obtained using the two methods are similar but not identical. In the case of α-RDX for pressure P = 0.0 GPa both spectra agree reasonably well with experimental data. The predicted spectra for α-RDX exhibit red-shifting (mode softening) of the main absorption peak with increasing pressure while for γ-RDX the spectra exhibit overall blue-shifting with increasing pressure.

A classical trajectory study of the dissociation and isomerization of C2H5.

Motivated by photodissociation experiments in which non-RRKM nanosecond lifetimes of the ethyl radical were reported, we have performed a classical trajectory study of the dissociation and isomerization of C2H5 over the energy range 100-150 kcal/mol. We used a customized version of the AIREBO semiempirical potential (Stuart, S. J.; et al. J. Chem. Phys. 2000, 112, 6472-6486) to more accurately describe the gas-phase decomposition of C2H5. This study constitutes one of the first gas-phase applications of this potential form. At each energy, 10,000 trajectories were run and all underwent dissociation in less than 100 ps. The calculated dissociation rate constants are consistent with RRKM models; no evidence was found for nanosecond lifetimes. An analytic kinetics model of isomerization/dissociation competition was developed that incorporated incomplete mode mixing through a postulated divided phase space. The fits of the model to the trajectory data are good and represent the trajectory results in detail through repeated isomerizations at all energies. The model correctly displays single exponential decay at lower energies, but at higher energies, multiexponential decay due to incomplete mode mixing becomes more apparent. At both ends of the energy range, we carried out similar trajectory studies on CD2CH3 to examine isotopic scrambling. The results largely support the assumption that a H or a D atom is equally likely to dissociate from the mixed-isotope methyl end of the molecule. The calculated fraction of products that have the D atom dissociation is ∼20%, twice the experimental value available at one energy within our range. The calculated degree of isotopic scrambling is non-monotonic with respect to energy due to a non-monotonic ratio of the isomerization to dissociation rate constants.

Post-shock relaxation in crystalline nitromethane.

Molecular dynamics simulations of shocked (100)-oriented crystalline nitromethane were carried out to determine the rates of relaxation behind the shock wave. The forces were described by the fully flexible non-reactive Sorescu-Rice-Thompson force field [D. C. Sorescu, B. M. Rice, and D. L. Thompson, J. Phys. Chem. B 104, 8406 (2000)]. The time scales for local and overall thermal equilibration in the shocked crystal were determined. The molecular center-of-mass and atomic kinetic energy distributions rapidly reach substantially different local temperatures. Several picoseconds are required for the two distributions to converge, corresponding to establishment of thermal equilibrium in the shocked crystal. The decrease of the molecular center-of-mass temperature and the increase of the atomic temperature behind the shock front exhibit essentially exponential dependence on time. Analysis of covalent bond distance distributions ahead of, immediately behind, and well behind the shock front showed that the effective bond stretching potentials are essentially harmonic. Effective force constants for the C-N, C-H, and N-O bonds immediately behind the shock front are larger by factors of 1.6, 2.5, and 2.0, respectively, than in the unshocked crystal; and by factors of 1.2, 2.2, and 1.7, respectively, compared to material sufficiently far behind the shock front to be essentially at thermal equilibrium.

Molecular dynamics simulations of shock waves in oriented nitromethane single crystals: plane-specific effects.

Molecular dynamics simulations of supported shock waves (shock pressure P(s) ∼ 15 GPa) propagating along the [110], [011], [101], and [111] directions in crystalline nitromethane initially at T = 200 K were performed using the nonreactive Sorescu-Rice-Thompson force field [D. C. Sorescu, B. M. Rice, and D. L. Thompson, J. Phys. Chem. B 104, 8406 (2000)]. These simulations, combined with those from a preceding study of shocks propagating along [100], [010], and [001] directions in nitromethane for similar conditions of temperature and shock pressure [L. He, T. D. Sewell, and D. L. Thompson, J. Chem. Phys. 134, 124506 (2011)], have been used to study the post-shock relaxation phenomena. Shocks along [010] and [101] lead to a crystal-crystal structure transformation. Shocks propagating along [011], [110], [111], [100], and [001] exhibit plane-specific disordering, which was characterized by calculating as functions of time the 1D mean square displacement (MSD), 2D radial distribution function (RDF), and 2D orientation order parameter P(2)(θ) in orthogonal planes mutually perpendicular to the shock plane; and by calculating as functions of distance behind the shock front the Cartesian components of intermolecular, intramolecular, and total kinetic energies. The 2D RDF results show that the structural disordering for shocks along [100], [110], and [111] is strongly plane-specific; whereas for shocks along [001] and [011], the loss of crystal structural order is almost equivalent in the orthogonal planes perpendicular to the shock plane. Based on the entire set of simulations, there is a trend for the most extensive disordering to occur in the (010) and (110) planes, less extensive disordering to occur in the (100) plane, and essentially no disordering to occur in the (001) plane. The 2D P(2)(θ) and 1D MSD profiles show, respectively, that the orientational and translational disordering is plane-specific, which results in the plane-specific structural disordering observed in the 2D RDF. By contrast, the kinetic energy partitioning and redistribution do not exhibit plane specificity, as shown by the similarity of spatial profiles of the Cartesian components of the intermolecular, intramolecular, and total kinetic energies in orthogonal planes perpendicular to the shock plane.

Molecular dynamics simulations of shock waves in oriented nitromethane single crystals.

The structural relaxation of crystalline nitromethane initially at T = 200 K subjected to moderate (~15 GPa) supported shocks on the (100), (010), and (001) crystal planes has been studied using microcanonical molecular dynamics with the nonreactive Sorescu-Rice-Thompson force field [D. C. Sorescu, B. M. Rice, and D. L. Thompson, J. Phys. Chem. B 104, 8406 (2000)]. The responses to the shocks were determined by monitoring the mass density, the intermolecular, intramolecular, and total temperatures (average kinetic energies), the partitioning of total kinetic energy among Cartesian directions, the radial distribution functions for directions perpendicular to those of shock propagation, the mean-square displacements in directions perpendicular to those of shock propagation, and the time dependence of molecular rotational relaxation as a function of time. The results show that the mechanical response of crystalline nitromethane strongly depends on the orientation of the shock wave. Shocks propagating along [100] and [001] result in translational disordering in some crystal planes but not in others, a phenomenon that we refer to as plane-specific disordering; whereas for [010] the shock-induced stresses are relieved by a complicated structural rearrangement that leads to a paracrystalline structure. The plane-specific translational disordering is more complete by the end of the simulations (~6 ps) for shock propagation along [001] than along [100]. Transient excitation of the intermolecular degrees of freedom occurs in the immediate vicinity of the shock front for all three orientations; the effect is most pronounced for the [010] shock. In all three cases excitation of molecular vibrations occurs more slowly than the intermolecular excitation. The intermolecular and intramolecular temperatures are nearly equal by the end of the simulations, with 400-500 K of net shock heating. Results for two-dimensional mean-square molecular center-of-mass displacements, calculated as a function of time since shock wave passage in planes perpendicular to the direction of shock propagation, show that the molecular translational mobility in the picoseconds following shock wave passage is greatest for [001] and least for the [010] case. In all cases the root-mean-square center-of-mass displacement is small compared to the molecular diameter of nitromethane on the time scale of the simulations. The calculated time scales for the approach to thermal equilibrium are generally consistent with the predictions of a recent theoretical analysis due to Hooper [J. Chem. Phys. 132, 014507 (2010)].