Search

This paper, the first of a series of papers, examines equilibrium properties of N[sub 2]O[sub 4]⇋NO[sub 2] in liquid state by classical molecular dynamics simulations of liquid NO[sub 2]. An ab initio MO calculation has been carried out to elucidate NO[sub 2]–NO[sub 2] potential, and an orientation-sensitive pairwise potential (OSPP), which can reproduce highly anisotropic character of covalent bonding between N–N, has been formulated. The OSPP potential is parameterized by the well depth D[sub e] and by two anisotropy factors: A[sub θ] (0≤A[sub θ]≤1) the anisotropy factor for the rocking angle between NN bond and ONO direction, and A[sub τ] (0≤A[sub τ]≤1) for torsional angle of the two NO[sub 2] about NN bond. The reactive liquid N[sub 2]O[sub 4] is modeled as liquid NO[sub 2] which interacts with the OSPP potential between N–N atoms and Lennard-Jones potentials between N–O and O–O atoms. Equilibrium properties were found to be very sensitive to the well depth D[sub e] and anisotropy factors of OSPP. The population of more than the NO[sub 2] dimer (3-mer, 4-mer,...) is considerable when anisotropy factors of the NN bond are small. On the other hand, the equilibrium liquid N[sub 2]O[sub 4]⇋2NO[sub 2] is formed, that is, most NO[sub 2] form monomer or dimer and the population of more than 3-mer is very small when A[sub θ]+A[sub τ]>=0.4–0.5. In simulated liquid NO[sub 2]/N[sub 2]O[sub 4], concentration of N[sub 2]O[sub 4] is found to increase as D[sub e] increases, A[sub θ] increases, and A[sub τ] decreases. The equilibrium constant for the dissociation reaction has been derived by computing the potential of mean force as a function of the N–N distance r[sub c] (the reaction coordinate). The OSPP potential for D[sub e]=0.12×10[sup -18] J, A[sub θ]=0.5 and A[sub τ]=0.1 is found to reproduce the observed liquid phase equilibrium properties fairly wel... [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations were performed for the dissociation and association (D/A) reactions N2O4[larger_closed_square]2 NO2 in the gas phase and in liquid N2O4. The trajectory was initialized from an equilibrium distribution of all variables in liquid N2O4, except the reactive mode, the NN distance of a reactant NO2 pair, was excited above the dissociation limit of the Morse-like potential between NO2 fragments, and the dynamics were calculated for 500 fs both forward and backward in time. Characteristics of the translational and vibrational energy relaxations of the reactant were studied in detail. Energy ERT, which is defined to be the sum of the potential and kinetic energies of interfragment motion, is found to play a key role in the D/A dynamics; a reactant pair is associated when ERT<0 and the pair is dissociated when ERT>=0. The transition state to the D/A reactions is hence defined by the last associated phase curve ERT=0 in the phase space. Energy transfer between intrafragment vibrational modes and the interfragment translational mode, which occurs at the inner turning point of the interfragment potential, is found to be the dominant prompter of the D/A reactions. The vibration–translation (V–T) energy transfer is found to excite the relative translational motion between fragments or gives rise to dissociation, and T–V energy transfer often causes deactivation of the relative translational motion or association in both the gas and liquid phases. In minor cases, the D/A reaction is found to occur by an energy transfer between reactant relative translational mode and solvent modes. The reaction rates are determined essentially by the rates of energy transfers among relative translational mode, intrafragment vibrational modes, and solvent modes. [ABSTRACT FROM AUTHOR]

In order to study the cation dependence of the ionic dynamics in molten nitrates, molecular dynamics simulations including vibrational degrees of freedom were carried out for molten LiNO3, NaNO3, and RbNO3. Coulomb pair potential with Born-type repulsion was adopted for the interionic interaction. The simulated diffusion coefficient was smaller for a larger cation, and that of nitrate ions did not change with changing cation species. The mean squared charge displacements showed that the static conductivity decreased considerably as the cation size increased from Li+ to Rb+. The simulated orientational correlation function of nitrate ions decayed more quickly as the cation size increased. Far infrared absorption spectrum simulated from the time evolution of the dipole moment (or the current) of the system showed that the peak shifted to the low energy side and the intensity decreased as the cation size increased. Results of the simulation were compared with the experimental diffusion constants, static and dynamic conductivities, and rotational behavior revealed by Raman spectroscopy. The simulated vibrational correlation functions and the power spectra of NO3- could reproduce the observed cation dependence of the peak frequencies of the ν1(A’1) and ν2(A‘2) modes. However, the assumed interionic potentials in the present simulation were found to result in too slow vibrational dephasing of the ν1 mode and too fast dephasing of the ν2 mode as compared with the infrared and isotropic Raman spectra. Strong correlation between radial and angular distributions of cations was found in the first coordination spheres of nitrate ions in the simulated molten nitrates. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulation has been used to investigate the infrared spectra of ordered and disordered benzoic acid crystals consisting of various proportions of the A and B configurations, whose C–O and C=O bonds are approximately parallel to the a axis, respectively. The simulation reproduced well the splitting and the temperature dependence in intensity for the C=O stretching bands observed. The absorption coefficient of the lower frequency band was simulated as several times larger than that of the higher frequency band in accord with the observed. To interpret this peculiar result of the molecular dynamics simulation, a simple calculation of the band splitting has been made in terms of the transition dipole–transition dipole interaction. It has been shown that the interaction is an essential factor to produce the large difference in intensity and the splitting for the C=O stretching bands. The higher and lower bands are interpreted as due to the A configuration reduced in intensity by the B form, and due to the B form enhanced in intensity by the A form, respectively. Compression effect in the O···O distance of the hydrogen bond is also studied in comparison with the observed value. [ABSTRACT FROM AUTHOR]

The vibrational dephasing of nitrate ions was studied in a molecular dynamics simulation of molten LiNO3, which included all degrees of freedom of vibrating nitrate ions. For the interionic interaction, a Coulomb pair potential with a Born-type repulsion was adopted as a standard potential, and the effect on vibrational dephasing of a potential well of varying depth between Li+ and O of NO-3 was studied. Vibrational correlation functions 〈Qi(0)Qi(t)> for the ν1 and ν2 modes of NO-3 were calculated and the vibrational spectra were obtained from their Fourier transforms. It was found that the vibrational correlation functions for the two modes decayed rapidly and the vibrational linewidths increased considerably as the well depth increased. Two simulations for the harmonic and the anharmonic intraionic potentials for NO stretching suggested that pure interionic interaction induced broadening dominated the band width of the ν1 mode in this melt, while vibrational anharmonicity coupled to the forces due to the environment did not play any important role. Results of the simulation were compared with the infrared and isotropic Raman band shapes in molten LiNO3. The assumed interionic potentials in the present simulation were found to result in two slow vibrational dephasing of the ν1 mode and too fast dephasing of the ν2 mode as compared with the spectroscopic results. The effect of vibration–rotation coupling on the vibrational spectra was found to be small in this system. [ABSTRACT FROM AUTHOR]

In order to study the effect of the potential well on the single ion dynamics and the charge transport in molten LiNO3, a molecular dynamics simulation, which included all degrees of freedom of vibrating nitrate ions, was carried out. Simple Coulomb pair potential with Born-type repulsion was adopted as a standard, and a potential well of varying depth between Li+ and O of NO-3 was added to the standard potential. As the well depth increased, both the translational and orientational diffusion were found to slow down, leading to solid like dynamics. The potential dependence of the ionic dynamics was studied from the translational, orientational, and current correlation functions. Results of the simulation were compared with the experimental diffusion constants, static conductivity, and rotational behavior revealed by Raman spectroscopy. Using the relation between the infrared absorption spectrum and the autocorrelation function of the dipole moment (or the current) of the system, we carried out the first simulation of translational band shapes in an ionic melt, and demonstrated how the absorption spectrum was related to the velocity correlation functions of cations and anions. The infrared and Raman spectroscopy were found to give sensitive criteria for finding a good potential of an ionic liquid. The lifetime of local structures surrounding nitrate ions, which were characterized by the coordination number, was found to increase remarkably as the well depth increased. The coupling between the environmental structure and the translational and rotational behavior of a nitrate ion was examined through selectively sampled correlation functions. [ABSTRACT FROM AUTHOR]

The microdynamics of orientational motion (the tumbling of the symmetry axis and the spinning about the axis) of nitrate ions has been investigated in a molecular dynamics simulation of molten LiNO3. Simple Coulomb potential with Born type repulsion has been adopted and all degrees of freedom of vibrating nitrate ions have been included in the simulation. The relaxation of the orientational correlation functions is studied in the cumulant expansion scheme, and a correlation function which reflects only the spinning motion is separated by the method of Lynden-Bell et al. It is found that the angular momentum correlation function for the perpendicular component C⊥J(t) has a pronounced negative portion, which is clear evidence of librations in a temporary solvent cage. C||J(t) has a shallow negative lobe. The spinning motion is less hindered and the reorientation is faster. Both the tumbling and the spinning motions are found to be predominantly Markovian. The difference between the tumbling and the spinning motions is examined from the magnitude of the torque impulse as compared to the average angular momentum. The coupling between the environmental structure and the rotational behavior of a nitrate ion has been examined through selectively sampled correlation functions. It is found that the rotational diffusion caused by the tumbling motion is slow and that by the spinning motion is fast if the coordination number remains low. [ABSTRACT FROM AUTHOR]

In order to study the microdynamics of translational and orientational motion of ions in liquid electrolytes, a molecular dynamics simulation of molten LiNO3, which includes all degrees of freedom of vibrating nitrate ions, has been performed. Simple Coulomb pair potentials with Born-type repulsions have been adopted in the simulation. The calculated properties include: translational velocity and force correlation functions, rotational correlation functions, and angular momentum and torque correlation functions. The nature of the ionic motion is analyzed in terms of quasioscillation and quasilibration. Local structures surrounding nitrate ions, which can be characterized by, e.g., the coordination number, are found to persist for a long time (∼1 ps). The coupling between the environmental structure and the translational and orientational behaviors of a given nitrate ion has been examined through selectively sampled correlation functions. We found that the change of coordination number enhances the translational diffusion and the rotational diffusion is slow if the coordination number remains low. This shows that the dynamics are very sensitive to the exact shape of the potential and the dynamics itself. Results of the simulation are compared with the experimental diffusion constants and dynamical behaviors revealed by Raman spectroscopy. While the interionic interaction assumed has been found to describe quite well the static structure of molten LiNO3, simulated translational diffusion and rotational relaxation seem to be somewhat too fast, suggesting too shallow an attractive well in the potential we have assumed. [ABSTRACT FROM AUTHOR]

Studies vibrational effects on structural parameters obtained by solid-state nuclear magnetic resonance (NMR). Theoretical calculations and molecular-dynamics simulations; Reduction of dipolar interactions by molecular vibration and intramolecular vibrations; Vibrational effects on bond and dihedral angles; Molecular dynamics simulations.

The intra- and intermolecular potentials of potassium palmitate B-form crystal were estimated by a normal mode analysis and a molecular dynamics simulation. Based on these potentials, we calculated the time history of the dipolemoment in the nine unit cells (3a×3b) containing 18 molecules and obtained the polarized infrared spectra by a Fourier transformation. The frequencies and the intensities of the intense bands agreed well with the observed ones, and were consistent with the crystal structure. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In order to explain the large permittivity of liquid cyanogen consisting of centrosymmetric molecules, a molecular dynamics simulation which includes all degrees of freedom of vibration, rotation, and translation has been performed. The intramolecular potential energy is expressed in terms of the internal coordinates with quadratic, cubic, and quartic force constants. The intermolecular potential consists of an atom–atom pair potential with Coulombic and Buckingham terms. The simulation has given an adequate value of self-diffusion coefficient and a typical form of radial distribution function for the liquid state. The simulation has reproduced well the large value of the permittivity observed in experiments, suggesting that the value results from the extraordinarily large atomic polarization brought by the intramolecular bending motion in an antisymmetric mode. The simulation has also suggested that the nitrogen atoms collide with each other more frequently than with carbon atoms and the collision shifts the normal frequencies related to the N≡C bond to higher frequency. The infrared and Raman spectra together with the real and imaginary parts of the refractive index have also been calculated. [ABSTRACT FROM AUTHOR]