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Vibrational dephasing of the N-N dissociation mode in equilibrium liquid N[SUB2]O[SUB4] ⇋ 2NO[SUB2] in the extreme limit of pure N[SUB2]O[SUB4] is studied by classical molecular dynamics simulations of liquid NO[SUB2]. An OSPP+LJ potential between NO[SUB2] molecules, which is a sum of an orientation-sensitive pairwise potential (OSPP) between N-N atoms proposed in Paper I [J. Chem. Phys. 115, 10852 (2001)] and Lennard-Jones potentials between N-O and O-O atoms, has been used in the simulation. The vibrational correlation function and Raman spectrum of the N-N stretching ν[SUB3] mode of N[SUB2]O[SUB4] are calculated, and are found to be very sensitive to the well depth D[SUBe] and anisotropy factors of OSPP: A[SUBθ2] (0 ⩽A[SUBθ] ⩽ 1) for rocking angle between the N-N bond and ONO direction, and A[SUBτ] (0 ⩽A[SUBτ] ⩽ 1 ) for the torsional angle. As A[SUBτ] increases in equilibrium liquid N[SUB2]O[SUB4]&lahar2;2NO[SUB2], the equilibrium shifts to dissociation, and the isotropic Raman band shifts to low frequency and broadens. The OSPP potential for D[SUBe]=0.12×10[SUP-18] J, A[SUBθ]0=0.5, and A[SUBτ]=0.1 is found to reproduce both the observed liquid phase equilibrium properties and Raman band shapes (peak frequency, bandwidth, and band asymmetry) of the dissociation mode very well. The long persistent positive correlation in the vibrational frequency fluctuation correlation function G[SUBω](t)= 〈Δω(0)Δω(t)〉/〈Δω(0)[SU P2]〉 is mainly ascribed to the slow energy fluctuation in the dissociation mode. It is concluded that the asymmetry of the ν[SUB3] Raman band is due to the scattering by molecular pairs excited in the dissociation mode. [ABSTRACT FROM AUTHOR]

An expression of the microcanonical unimolecular rate for an arbitrary transition state surface in phase space is derived and applied to fragmentation reactions into polyatomic molecules without potential barrier. The transition state which has a ‘‘point of no return’’ property in unimolecular dissociation is defined as an interfragment bottleneck in phase space. The fragmentation rate based on the interfragment bottleneck in phase space is compared with the rate based on the transition state defined in configuration space. The rate derived from the flux which crosses the interfragment bottleneck by intermode energy transfer is found to be smaller than the rate derived from the Rice–Ramsperger–Kassel–Marcus or phase space theory by an approximate factor (s+r/2)|W|/E, where E is the total energy and |W| is the magnitude of the coupling energy between the reaction coordinate and the s-dimensional vibrational and r-dimensional rotational modes of the fragments. Phase space theory grossly overestimates the rate of fragmentation of small molecules with small |W| in the high energy range, because the theory does not take into account the slow process of intramolecular energy redistribution. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Thermal dissociation reaction into polyatomic molecules on the ground state no-barrier potential energy surface is studied by classical molecular dynamics simulations of N2O4⇋2NO2. A phase space surface ET=Veff(Rl)>=0 is identified as the transition state (TS), where ET is the sum of the potential and kinetic energies of interfragment motion and Veff(Rl) is the orbital angular momentum-dependent effective barrier. By dividing the motion of the system into fragments’ vibrational (V), rotational (R), and interfragment (T) modes, where the T mode is composed of translational (TT) and orbital (TL) modes, a scheme of reactive energy transfer for fragmentation is presented. The present energy condition for the TS is in accord with the one of phase space theory (PST). The observed photofragmentation rates of NCNO and CH2CO which increase with energy slower than predicted by PST suggest that intrareactant energy redistribution may influence the rate. Dissociation is found to occur by energy redistribution among T-R-V modes followed by the one among TT-TL-R modes, which determine the product vibrational and rotational distributions, respectively. This scheme supports separate statistical ensemble method in reproducing the nascent distributions from unimolecular photofragmentation at excess energies above the vibrational threshold. © 1996 American Institute of Physics. [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]

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 isotropic Raman spectra of the ν1(A’1) mode of NO-3 are measured in molten NaNO3–TlNO3 mixtures at various concentrations (the mole fraction of NaNO3, 1≥CA≥0) at 600 K. Vibrational correlation functions in mixtures at CA[bar_over_tilde:_approx._equal_to]0.8–0.5 show a considerable contribution of inhomogeneous vibrational dephasing. A model is presented which accounts simultaneously for the coordination number fluctuation and the concentration fluctuation at the reference anion. Concentration dependence of the isotropic Raman band shape (peak frequency, bandwidth, and asymmetry) is fully explained by the static model for the molten NaNO3–TlNO3 and LiNO3–RbNO3 mixtures. The bandwidths at mole fractions of about 0.5, slightly smaller than those calculated with the static model, suggest a motional narrowing due to diffusion dynamics. The model also makes predictions on the concentration dependence of the spectral line profile in terms of the interaction with the nearest neighbors and diffusion dynamics in liquid mixtures. [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]

The isotropic Raman spectra of the ν1 (A’1) mode of NO-3 ions are measured in molten LiNO3–RbNO3 mixtures at various concentrations and at temperatures from 478 to 750 K. The isotropic Raman bandwidth Γiso is found to increase almost linearly with the mole fraction c(Li+) of LiNO3, from 9–13 cm-1 for molten RbNO3 to 26–27 cm-1 for molten LiNO3. Vibrational correlation functions are analyzed on the basis of a model of simultaneous homogeneous and inhomogeneous vibrational dephasing. The contribution of inhomogeneous dephasing was found to be negligibly small in mixtures of c(Li+)≾0.33. While in mixtures of concentrations c(Li+)>=0.5, homogeneous and inhomogeneous processes are found to give comparable contributions to the total vibrational widths, where the latter contribution increases as the temperature decreases. The observed isotropic Raman bands are asymmetric. The asymmetry parameter α=Γ1/Γh , where Γ1 and Γh are the half-widths at half-maximum height of the low and high frequency sides of the band, is larger than one in molten LiNO3 and RbNO3, while α is smaller than one in molten mixtures of concentrations c(Li+)=0.33–0.67. This concentration dependent asymmetry is found to arise predominantly from coordination number fluctuation, i.e., fluctuation of the environmental cation number of a reference NO-3 ion, while concentration fluctuation in the coordination sphere plays a minor role. A model of local environmental states in a molten mixture, which assumes a linear change of vibrational frequency with neighboring cation numbers, is presented. The observed spectral asymmetry can be interpreted by assuming that the equilibrium distribution has a peak at the small coordination number side in pure molten LiNO3 and RbNO3, while the distribution peak shifts to the high coordination number side in molten mixtures. [ABSTRACT FROM AUTHOR]

The Raman spectra of the ν1(A’1) mode of NO-3 ions in molten LiNO3–RbNO3 mixtures are measured at various concentrations and at temperatures from 478 to 750 K. Rotational correlation functions are calculated from the Raman spectra. Reorientation of the trigonal axis of an NO-3 ion is found to be represented by the inertial rotation of an associated ion cluster for short times and by the rotational diffusion at longer times. The speeds of inertial rotation and rotational diffusion are found to increase as the temperature rises and as the mole fraction c(Li+) of LiNO3 decreases. [ABSTRACT FROM AUTHOR]

Transition state dynamics of dissociation and association reactions N2O4 <==> 2NO2 in liquid state are studied by classical molecular dynamics simulations of reactive liquid NO(2) at 298 K. An OSPP+LJ potential between NO(2) molecules proposed in paper I [J. Chem. Phys. 115, 10852 (2001)], which takes into account the orientational sensitivity of the chemical bond, has been used in the simulation. The trajectory and energy evolution of various reactions are studied in the OSPP+LJ liquid, which reproduces both the observed liquid phase equilibrium constant and Raman band shape of the dissociation mode. It is found that a NO(2) pair in reactive liquid NO(2) is bound when E(T)<0 and dissociates when E(T)>0, and the dissociation of a reactant pair occurs when the transition state (TS) surface of E(T)=0 is crossed from negative to positive, where E(T) is the sum of the potential and kinetic energies of intermolecular motion of the pair. Two types of dissociation are found depending on the source of energy for dissociation; the first type D is the dissociation via collisional activation of the reactive mode by solvent molecules, and the second type T is the dissociation via bond transfer from a dimer to a monomer NO(2) through the TS of NO(2) trimer. It is concluded that the type T dissociation is found to be much more probable than the type D dissociation because of easy energy conservation. The reactant experiences the TS of NO(2) trimer for a long time (1-10 ps) in NO(2) mediated bond transfer reactions, and crossing and recrossing trajectories and dynamics in the TS neighborhood are studied., ((c) 2004 American Institute of Physics.)

The dissociation and association dynamics of N2O4 [see text] 2NO2 in liquid state are studied by classical molecular dynamics simulations of reactive liquid NO2. An OSPP+LJ potential between NO2 molecules, which is a sum of an orientation-sensitive pairwise potential (OSPP) between N-N atoms proposed in Paper I [J. Chem. Phys. 115, 10852 (2001)] and Lennard-Jones potentials between N-O and O-O atoms, has been used in the simulation. The reaction dynamics is studied as a function of well depth De and anisotropy factors of the OSPP potential: Atheta (0< or =Atheta< or =1) for the rocking angle and Atau (0< or =Atau< or =0.5) for the torsional angle of relative NO2-NO2 orientation. The lifetime tauD of initially prepared NO2 dimers is found to increase as De increases, Atheta increases, and Atau decreases. Dissociation and association dynamics are studied in detail around the extreme limit of pure NO2-dimer liquid: De=0.12 x 10(-18) J, Atheta=0.5, and Atau=0.1, which has been found to reproduce both the observed liquid phase equilibrium properties and Raman band shapes of the dissociation mode very well. The dissociation dynamics from microscopic reaction trajectories is compared with the potential of the mean force (PMF) as a function of the N-N distance R. The PMF of reactive liquid NO2 shows a transition state barrier at R=2.3-2.5 A, and NO2-trimer structure is found to be formed at the barrier. Two types of dissociation of the NO2 dimer-the dissociation by collisional activation of the reactive mode to cross the dissociation limit and the NO2-mediated dissociation via bond transfer-are studied. The latter needs less free energy and is found to be much more probable. The dissociation trajectories and PMF in reactive liquid NO2 are compared with those of a reactive NO2 pair in inert solvent N2O4., ((c) 2004 American Institute of Physics)