Theoretical studies of the reaction of hydroxyl radical with methyl acetate

The mechanisms and the kinetics of the OH (OD) radicals with methyl acetate CH3C(O)OCH3 are investigated theoretically. The dual-level direct dynamics method is employed in the calculation of the rate constants. The optimized geometries and frequencies and the gradients of the stationary points are calculated at the MP2/6-311G(d,p) level. The energetic information of potential energy surfaces is further refined by the multicoefficient correlation method based on QCISD (MC-QCISD) using the MP2/6-311G(d,p) geometries. Four channels are found for the title reaction. The calculated results reveal that there exists an attractive well (reactant complex) in each entrance H-abstraction channel, that is, the H-abstraction reaction makes a stepwise mechanism. The rate constants are calculated by the canonical variational transition-state theory (CVT) with the interpolated single-point energies (ISPE) approach in the temperature range of 200-1200 K. The small-curvature tunneling effect (SCT) approximation is used to evaluate the transmission coefficient. The calculated rate constants are in good agreement with the experimental ones in the measured temperature range. It is shown that the "out-of-plane hydrogen abstraction" from the methoxy end is the dominant channel at the lower temperatures, and the other two H-abstraction channels should be taken into account with the temperatures increasing. The kinetic isotope effects (KIEs) for the three H-abstraction channels and the total reaction are "inverse", and these theoretically calculated KIEs as a function of temperature are expected to be useful for the future laboratory investigation.     

Conformational dependence of the n→π* rotatory strength in dissymmetric carboxylic acid compounds

The conformational dependence of the chiroptical properties associated with electronic transitions in α-hydroxy, α-fluoro, α-chloro, and α-mercapto carboxylic acids is examined with a theoretical model in which rotatory strengths are calculated directly from molecular orbitals calculated using INDO and CNDO semi-empirical molecular orbital models, and excited state wave functions constructed in the virtual orbital-configuration interaction approximation. Binding energies, ground state dipole moments, and vertical ionization potentials (calculated according to Koopman's theorem) are calculated, as well as transition energies, dipole strengths, oscillator strengths, rotatory strengths and dissymmetry factors. Special emphasis is placed on correlating the signs of the rotatory strengths calculated for the lowest energy singlet-singlet transitions with conformational isomerism about the C(α)-COOH band of acyclic α-substituted carboxylic acids. The calculated results for S-lactic acid are in agreement with and strongly support the empirically based spectra-structure relationships previously proposed for S-lactic acid and its derivatives. The calculated results for 2(S)-chloropropionic acid do not agree with previously proposed spectra-structure relationships and this finding is discussed. The results obtained for 2(S)-mercaptopropionic acid are in partial agreement with experiment and spectra-structure relationships in this case involve some uncertainty.     

Radiative lifetimes for the B1piu state of the Na2 molecule

A study of the radiative lifetimes calculation of the Na2 B1piu state is presented. RKR electronic potentials are considered. The studied vibrational levels are for v' = 0-33 (B1piu) and v" = 0-65 (chi1sigmag+). The rotation is considered for values of J' = 1-225 (B1piu). The Einstein emission coefficients are calculated for the specified B1piu rovibrational levels (for Q line and R, P lines, for all ground state vibrational levels). With the inverse of Einstein emission coefficients sum, the radiative lifetimes are calculated. These calculated lifetimes are in good agreement with the experimental and previously calculated (with RKR potentials) lifetimes, but now great extension of considered rovibrational levels is considered. The bound-free contribution is irrelevant for Na2 lifetimes of the B1piu state. The perturbation between Na2 B1piu and alpha1sigmau+ states is considered.     

Anharmonicity of weakly bound M(+)-H2 complexes

The anharmonicity of weakly bound complexes is studied using the vibrational self-consistent field (VSCF) approach for a series of metal cation dihydrogen (M(+)-H(2)) complexes. The H-H stretching frequency shifts of M(+)-H(2) (M(+) = Li(+), Na(+), B(+), and Al(+)) complexes are calculated with the coupled-cluster method including all single and double excitations with perturbative triples (CCSD(T)) level of theory with the cc-pVTZ basis set. The calculated H-H stretching frequency of Li(+)-H(2), B(+)-H(2), Na(+)-H(2), and Al(+)-H(2) is red-shifted by 121, 202, 74, and 62 cm(-1), respectively, relative to that of unbound H(2). The calculated red shifts and their trends are in good agreement with the available experimental and previously calculated data. Insight into the observed trends is provided by symmetry adapted perturbation theory (SAPT).     

The accuracy of diffusion quantum Monte Carlo simulations in the determination of molecular equilibrium structures

For a test set of 17 first-row small molecules, the equilibrium structures are calculated with Ornstein-Uhlenbeck diffusion quantum Monte Carlo simulations guiding by trial wave functions constructed from floating spherical Gaussian orbitals and spherical Gaussian geminals. To measure performance of the Monte Carlo calculations, the mean deviation, the mean absolute deviation, the maximum absolute deviation, and the standard deviation of Monte Carlo calculated equilibrium structures with respect to empirical equilibrium structures are given. This approach is found to yield results having a uniformly high quality, being consistent with empirical equilibrium structures and surpassing calculated values from the coupled cluster model with single, double, and noniterative triple excitations [CCSD(T)] with the basis sets of cc-pCVQZ and cc-pVQZ. The nonrelativistic equilibrium atomization energies are also presented to assess performance of the calculated methods. The mean absolute deviations regarding experimental atomization energy are 0.16 and 0.21 kcal/mol for the Monte Carlo and CCSD(T)/cc-pCV(56)Z calculations, respectively.     

Effect of temperature and pressure on the structure, dynamics, and hydrogen bond properties of liquid N-methylacetamide: a molecular dynamics study

The structure and dynamical properties of liquid N-methylacetamides (NMA) are calculated at five different temperatures and at four different pressures using classical molecular dynamics simulations. Our results are analyzed in terms of pressure-induced changes in structural properties by investigating the radial distribution functions of different atoms in NMA molecule. It is found that the first peak and also the second peak of C-O and N-H are well defined even at higher temperature and pressure. It is also observed that the number of hydrogen bonds increase with application of pressure at a given temperature. On the other hand, the calculated hydrogen bond energy (E(HB)) shows that the stability of hydrogen bond decreases with increasing of pressure and temperature. Various dynamical properties associated with translational and rotational motion of neat NMA are calculated and the self-diffusion coefficient of NMA is found to be in excellent agreement with the experiment and the behavior is non-Arrhenius at low temperatures with application of pressures. The single particle orientational relaxation time for dipole vector and N-C vector are also calculated and it is found that the orientational relaxation time follows Arrhenius behavior with a variation of temperature and pressure.     

Quantum-chemical calculations of the relative stability of the keto-enol tautomers of 5-chlorouracyl

Relative energies (by MP4(SQTQ)/def2-TZVPP) for 13 tautomeric conformers of 5-chlorouracyl in the gas phase and in solution with consideration of nonspecific solvation in water are calculated. The geometrical parameters of the calculated conformers are analyzed. A stability series of tautomers with respect to the diketo form is obtained. It is shown that nonspecific solvation leads to changes in the stability series.     

Experimental and computational studies of the structure and vibrational spectra of aminomethyl-dimethyl-phosphine oxide and its 15N labeled isomer

Infrared (4000-100 cm(-1)) spectra of aminomethyl-dimethyl-phosphine oxide and 15N-aminomethyl-dimethyl-phosphine oxide have been measured. Geometric parameters (bond distances and angles), net electronic charges and vibrational spectroscopic data of both compounds calculated at various levels of theory (B3LYP/6-31G* and Moeller-Plesset perturbational theory (MP2)/6-31G*) are reported. The theoretical spectral results are discussed mainly in terms of comparison with infrared (4000-100 cm(-1)) spectral data. Better coincidence was achieved with the frequencies calculated at the MP2/6-31G* level: the standard deviation is 16 cm(-1). The calculated isotopic frequency shifts, induced by the 15N labeling, are in good accordance with the measured ones. Complete vibrational assignment is made with the help of MP2 force field calculations. Data obtained here are used to reassign some of the vibrational frequencies.     

Theoretical prediction of emulsion color

The perceived quality of many commercial products that are based on emulsions is determined by their color. In this article, a theory is presented to relate the color of emulsions to their composition and microstructure. First, the scattering characteristics (Qs and g) of individual droplets are calculated using Mie Theory. Second, the scattering (S) and absorption (K) coefficients of a concentrated emulsion are calculated using radiative transfer theory. Third, the reflectance spectrum (R) of the emulsion is calculated using Kubelka-Munk Theory. Finally, the tristimulus coordinates (XYZ, or L*a*b*) of the emulsion are calculated using color theory. There is excellent agreement between theoretical predictions and experimental measurements of the influence of droplet and chromophore characteristics on the tristimulus coordinates of concentrated oil-in-water emulsions.     

Effect of removing the no-virtual-pair approximation on the correlation energy of the He isoelectronic sequence

The correlation energies (CEs) for the He-like ions are studied with the virtual-pair approximation (VPA) and with the no-virtual-pair approximation (NVPA). In contrast to the nonrelativistic CEs, the CEs calculated with relativity fell sharply as the nuclear charge Z increased, although the CE calculated with the NVPA was considerably lower than with the VPA for the heavier atoms. It is shown that CE calculated with a Hylleraas-type function implicitly includes the effects of the excitations into negative-energy states, which corresponds to the VPA. The present results verify that the strong dependence on Z of the CE of He-like ions is an essential effect of the relativity.     

Ab initio study on far-infrared spectra of dihalodiammine complexes of palladium(II) and platinum(II).

The far-infrared spectra of dihalodiammine complexes of Pd(II) and Pt(II) are calculated using ab initio method at RHF/LANL2DZ level. The calculated vibrational frequencies are in good agreement with the experimental ones except for M-N stretching frequencies in cis-M(NH3)2X2 complexes, and the reason for the deviation is discussed.     

Calculation of entropy and heat capacity of organic compounds in the gas phase. Evaluation of a consistent method without adjustable parameters. Applications to hydrocarbons

The purpose of this study has been to determine how well a consistent ab initio thermostatistical method reproduces experimental values of heat capacity and entropy. The method has been applied to calculation of heat capacity and entropy of a representative set of hydrocarbons that includes compounds consisting of multiple conformers. All Cp and S values are for the gaseous state at 1 atm; units are cal K-1 mol-1. A detailed sensitivity (error) analysis has been performed to determine the root mean square (rms) values of errors expected of the calculated values: these are 0.27 cal for Cp and 0.36 cal for entropy. In comparing calculated values with experimental values, it is necessary to consider also the uncertainties of the experimental data. When these are included, the expected rms values of Cp(experimental) - Cp(calculated) values at 298.15 K range from 0.21 to 0.73. For S(experimental) - S(calculated), they range from 0.36 to 0.72. Calculations with frequencies derived with the 6-31G(d,p) basis set and scaled by 0.91 yielded rms values for Cp(experimental) - Cp(calculated) of individual compounds from 0.14 to 0.84 cal and rms values for S(experimental) - S(calculated) of individual compounds from 0.07 to 1.11 cal. Calculated Cp values for 7 out of 16 compounds agree with experimental values within the rms uncertainty estimated for the compound, and 11 fall within twice that estimate. For entropy, the calculated values for 13 of 18 compounds agree with the very limited available experimental data within the rms estimated uncertainty for the compound, and 16 of 18 fall within twice the uncertainty.     

High-pressure study of lithium azide from density-functional calculations

The structural, electronic, optical, and vibrational properties of LiN(3) under high pressure have been studied using plane wave pseudopotentials within the generalized gradient approximation for the exchange and correlation functional. The calculated lattice parameters agree quite well with experiments. The calculated bulk modulus value is found to be 23.23 GPa, which is in good agreement with the experimental value of 20.5 GPa. Our calculations reproduce well the trends in high-pressure behavior of the structural parameters. The present results show that the compressibility of LiN(3) crystal is anisotropic and the crystallographic b-axis is more compressible when compared to a- and c-axes, which is also consistent with experiment. Elastic constants are predicted, which still awaits experimental confirmation. The computed elastic constants clearly show that LiN(3) is a mechanically stable system and the calculated elastic constants follow the order C(33) > C(11) > C(22), implying that the LiN(3) lattice is stiffer along the c-axis and relatively weaker along the b-axis. Under the application of pressure the magnitude of the electronic band gap value decreases, indicating that the system has the tendency to become semiconductor at high pressures. The optical properties such as refractive index, absorption spectra, and photoconductivity along the three crystallographic directions have been calculated at ambient as well as at high pressures. The calculated refractive index shows that the system is optically anisotropic and the anisotropy increases with an increase in pressure. The observed peaks in the absorption and photoconductivity spectra are found to shift toward the higher energy region as pressure increases, which implies that in LiN(3) decomposition is favored under pressure with the action of light. The vibrational frequencies for the internal and lattice modes of LiN(3) at ambient conditions as well as at high pressures are calculated from which we predict that the response of the lattice modes toward pressure is relatively high when compared to the internal modes of the azide ion.     

Investigation of fundamental transport properties and thermodynamics in diglyme-salt solutions

Ionic mobility, the thermodynamics of ionic association, and the structure of associated species are studied in solutions of diglyme containing either lithium triflate or tetrabutylammonium triflate. Infrared spectroscopic, PFG NMR, thermodynamic, and crystallographic data suggest that the solute species existing in diglyme-lithium triflate are "free" ions, contact ion pairs, and dimers. Equilibrium constants, S(o), deltaH(o), and deltaG(o) are calculated for processes occurring between these species. In particular, the equilibrium constant, corrected for nonideality using a modified Debye-Hückel expression, is calculated for the dissociation of contact ion pairs into "free" cations and anions. A second equilibrium constant for the formation of dimers from contact ion pairs is also calculated; these constants do not significantly vary with salt concentration up to about 1.3 x 10(-3) mol cm(-3). The measured temperature dependence of equilibrium constants was used to calculate deltaH(o) and deltaS(o) for the two processes. The value of deltaS(o) = -102 J mol(-1) K(-1) for the dissociation of contact ion pairs shows that the large entropy decrease due to cation solvation outweighs the entropy increase due to dissociation of a contact ion pair. Ionic mobilities are calculated in lithium triflate-diglyme solutions using conductivity data in conjunction with information about the nature and concentrations of solute species obtained from IR spectroscopy. Mobilities in tetrabutlyammonium triflate-diglyme solutions are calculated directly from conductivity data. It was concluded that the concentration dependence of the molar conductivity is due in large part to the variation of the ion mobilities with concentration.     

Comparison of Dorris-Gray and Schultz methods for the calculation of surface dispersive free energy by inverse gas chromatography

Inverse gas chromatography (IGC) is an important technique for the characterization of surface properties of solid materials. A standard method of surface characterization is that the surface dispersive free energy of the solid stationary phase is firstly determined by using a series of linear alkane liquids as molecular probes, and then the acid-base parameters are calculated from the dispersive parameters. However, for the calculation of surface dispersive free energy, generally, two different methods are used, which are Dorris-Gray method and Schultz method. In this paper, the results calculated from Dorris-Gray method and Schultz method are compared through calculating their ratio with their basic equations and parameters. It can be concluded that the dispersive parameters calculated with Dorris-Gray method will always be larger than the data calculated with Schultz method. When the measuring temperature increases, the ratio increases large. Compared with the parameters in solvents handbook, it seems that the traditional surface free energy parameters of n-alkanes listed in the papers using Schultz method are not enough accurate, which can be proved with a published IGC experimental result.     

Ab initio study of the electronic spectrum of peroxyacetyl nitrate

A theoretical study of the ground and excited states of peroxyacetyl nitrate (PAN), CH3C(O)OONO2, has been carried out using high level ab initio molecular orbital methods. The ground state geometry and vibrational frequencies are calculated using the coupled-cluster method. The vertical excitation energies for the lowest three excited states are calculated using the complete active space self-consistent field method along with the multireference internally contracted configuration interaction method. These results are compared with vertical excitation energies calculated with the coupled cluster equation of motion method. The calculation provides relevant insight into the origin of PAN absorption in the UV wavelength region from 200 to 300 nm. The nature of the electron transitions for these excited states is discussed.     

Theoretical determination of the excited states and of g-factors of the Creutz-Taube ion, [(NH3)5-Ru-pyrazine-Ru-(NH3)5]5+

The Creutz-Taube complex [(NH3)5-Ru-pyrazine-Ru-(NH3)5]5+ is studied using wave function-based methods, namely the CASSCF/MS-CASPT2 method. Spin-orbit effects have been calculated with the SO-RASSI program. The nature of the ground state is analyzed, and all the excited states up to 50,000 cm(-1) are calculated. They form a quasi continuum from 25,000 cm(-1), theoretical bands are assigned to UV-visible spectra, and MCD bands are assigned by calculating transition moments from first principles for both spectroscopies. g-Factors are calculated from first principles and modeled by a model Hamiltonian: they compare well to experimental values and are shown to be the same as in the monomeric species.     

HCN和氯反应动态学及产物振动态分布的计算

采用自洽场分子轨道UHF／6－311G**从头计算法,获得了HCN＋Cl→HCl＋CN反应的内禀反应坐标（IRC）。沿着IRC,计算了反应的动态学性质,根据传统过渡态、变分过渡态理论,得到了反应的理论速率常数,可供实验工作者参考,利用SCP－IOS近似下的产物振动跃迁几率公式,自编程序计算了HCN（004,302）＋Cl→HCl＋CN反应中产物CN的振动态分布,获得了和实验相一致的结果。     

Influence of the Screening Function on Theoretical Calculations of the Structure Factor of Liquid Aluminum

Abstract

The structure factor S(k) of liquid aluminum is calculated using the Metropolis Monte Carlo method. The effective two-body ion-ion interaction used in the calculations are the Shaw optimized model and the local approximation suggested by Harrison calculated using two screening function in each case, the screening function of Vashishta and Singwi and that of Utsumi and Ichimaru. The calculated structure factors from each case are compared with experiment.     

High level ab initio study of the structure and vibrational spectra of HO(2)NO(2)

A high-level ab initio study has been performed on the conformational structure and vibrational spectra of HO(2)NO(2). Calculations carried out with coupled-cluster methods using a series of Pople and Dunning basis sets reveal that there is a significant basis set dependence on the predicted ab initio structure. Higher angular momentum basis sets are shown to be necessary in order to bring the calculated structure into agreement with experimental rotational constants. Harmonic vibrational frequencies of HO(2)NO(2) are computed at the CCSD(T)/aug-cc-pVTZ level of theory while the corresponding vibrational anharmonicities are calculated at the MP2/cc-pVTZ level. In addition, the absorption cross sections of OH stretching overtones in HO(2)NO(2) are calculated using a dipole function computed at the QCISD level of theory and found to be in good agreement with the available experimental data.