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The performance of an ab initio computational scheme for molecular crystals based on density functional theory DFT was investigated by computing several structural and energetic properties of hydrogen bonded infinite chains and of two-dimensional infinite periodic networks of formamide. The applied DFT potentials covered a wide range in quality, starting with a simple local exchange X without correlation C , and then gradually introducing C and gradient corrections for both X and C.
At the same time, five atomic basis sets of systematically increasing size, in the range of DZ to TZ 2df,2pd were used to construct the Bloch-type crystal orbitals, to optimize the structures, and to extrapolate different physical quantities to the limit of a hypothetical infinite basis set. Infinite lattice sums were computed by the multipole expansion technique, and basis set superposition errors were partly eliminated by the counterpoise method. To be able to assess the accuracy of the theoretical models, the formamide monomer and two different dimers were also investigated using the same methods.
Detailed comparisons were made for all models also with recent results obtained by using different orders of many-body perturbation theory. Structural optimizations for the dimers and the infinite crystal demonstrated the importance of gradient terms both for exchange and correlation. The hydrogen bond structure of liquid methanol was investigated as a function of pressure and temperature up to 2.
Chemical shifts of the CH3 and OH groups were monitored throughout this pressure and temperature regime, and the chemical shift difference between these two groups was used to describe changes of the hydrogen bond network in methanol. The hydrogen bond equilibrium was investigated using molecular dynamics simulations and a phenomenological model describing clustering in liquid methanol.
Results are presented concerning the size and distribution of hydrogen-bonded clusters in methanol as a function of pressure and temperature. The results indicate that the extent of hydrogen bonding decreases upon an increase in temperature. The results for pressure are equivocal, the phenomenological model suggests that hydrogen bonding decreases with increasing pressure, which supports earlier interpretations regarding the measured self-diffusion coefficients in deuterated methanol as a function of pressure. The molecular dynamics simulations, however, show an increase in hydrogen bonding with increasing pressure.
Calculations are reported with a variety of basis sets and a variety of correlation levels based on molecular orbital MO theory as well as with a variety of density functional theory DFT gradient-corrected nonlocal functionals. The convergence of the hydrolysis energies with respect to the computational method is discussed. The highest level calculations yield 0. Ab initio calculations are reported on 7-azaindole and the complexes between 7-azaindole and water and methanol.
Energetics were predicted using second-order perturbation theory. Vertical excitation of the complex with water, qualitatively estimated using singly excited configuration interaction, is predicted to reverse the order of stability of the two tautomers. A harmonic force field for difluorodioxirane was derived from fundamental wavenumbers, isotopic shifts, centrifugal distortion constants, and inertial defect differences. The results of the aqueous ionic solution simulations provide a reasonable description of many structural and thermodynamic properties of the solvated ion such as the solvation enthalpy, the radial distribution function, and the hydration number.
Several energetically favorable interconversions were identified; however, it is clear that even though some of these reactions are exothermic, geometrical constraints will inhibit some from taking place. Bonding occurred in five cases, but the anticipated cyclic dimer s were not found. In the gas phase the gauche conformers are of lower energy than the anti conformers by 0. These dimeric radical cations are expected to arise in gas phase experiments mass spectrometry and matrix isolation studies.
This paper deals with the high-temperature decomposition of reactive intermediates with low reaction thresholds. If these intermediates are created in situ, for example, through radical chain processes, their initial molecular distribution functions may be characteristic of the bath temperature and, under certain circumstances, peak at energies above the reaction threshold. Such an ordering of reaction thresholds and distribution functions has some similarities to that found during chemical activation. This leads to consequences that are essenially the inverse larger rate constants than those deduced from steady-state distributions of the situation for stable compounds under shock-heated conditions and hence reduces falloff effects.
To study this behavior, rate constants for the unimolecular decomposition of allyl, ethyl, n-propyl, and n-hexyl radicals have been determined on the basis of the solution of the time-dependent master equation with specific rate constants from RRKM calculations. The time required for the molecules to attain steady-state distribution functions has been determined as a function of the energy-transfer parameter the step size down molecular size heat capacity , high-pressure rate parameters, temperature, and pressure.
At kPa 1 atm pressure, unimolecular rate constants near s-1 represent a lower boundary, above which steady-state assumptions become increasingly questionable.
The effects on rate expressions and branching ratios for decomposition reactions during the pre-steady-state period are described. The turbulent flow tube technique has been used to determine the reaction rate constants of chlorine atoms with nitrogen dioxide, methane, and ozone. The room temperature rate constant was determined to be 7. The stated uncertainties for the values of rate constants and Arrhenius parameters are reported at the one standard deviation level and represent only the precision of the data.
In each case the experimentally determined rate constant is in good agreement with earlier results obtained by using flash-photolysis and conventional low-pressure discharge flow systems. This study demonstrates that the turbulent flow tube method is a viable technique for studying gas phase reaction kinetics over a wide range of temperatures and pressures.
At room temperature, the rate constant was found to be independent of pressure between 70 and Torr of N2 and given by 8. The temperature dependence of the rate constant was investigated between and K; the Arrhenius expression over this temperature range is 3. The uncertainties of the room temperature rate constant and the Arrhenius parameters represent the precision of the data and are reported at the 1 standard deviation level. In each system the association product channel occurs in competition with exothermic bimolecular channels.
A semiempirical fitting approach was used in which A and B were obtained from a transition state theory TST calculation and C was determined from a nonlinear least squares fit to the experimental data.
This semiempirical fit is shown to be superior to a purely empirical fit to the data. The expressions for k1 and k2 are in good agreement with the previous studies between temperatures of and K. Above K, our extended temperature measurements verify previous TST predictions. The experimental results indicate that k1 is about 5 times as fast as k2 at K.
The molecular structure and photophysical behavior of several secondary and tertiary N- aminoalkyl phenanthrenecarboxamides have been investigated.
Secondary aminoalkyl amides exist predominantly in the Z conformation, whereas tertiary amides exist as mixtures of Z and E conformers and semirigid piperazines as mixtures of chair conformers. Rate constants for endergonic intramolecular electron transfer are found to be highly dependent upon molecular structure. The aromatic and amide groups of the tertiary amides are essentially orthogonal, and thus, an E aminoalkyl group can adopt low-energy conformations in which there is spatial overlap between the aromatic and amine groups, whereas such overlap is not possible for either a Z aminoalkyl group or the piperazines.
The observation of more rapid intramolecular electron transfer quenching of the phenanthrene singlet by an appended trialkylamine in the E vs Z conformation is attributed to this difference in overlap. In the case of appended tertiary anilines, efficient electron transfer quenching occurs for both Z and E conformers. Exciplexes formed by the E conformers are nonfluorescent and apparently undergo rapid intersystem crossing.
The strong exciplex emission observed at low temperatures both in solution and in frozen glasses is attributed to ground state dimers or aggregates. The parameter values were then changed to include the more commonly used solvent, acetonitrile, and a wide range of electron transfer rates.
These results cannot be obtained simply by referring to the distance dependence of the rate constant of the reaction. The distribution considered here has experimental significance, since it is directly related to the yield of free ions. The comparison with the quantum mechanical QM calculations and experimental results shows that, in general, QCT thermal rate constants are smaller than their QM and experimental counterparts, and this can be traced back to a decrease in the classical reactivity in the threshold region with rotational excitation of the reagents.
In addition, the analysis of the QCT results provides an explanation for the differences found in thermal rate constants calculated on the three PESs in terms of specific features of each of these potentials. Both O 1D and O 3Pj atoms produced in the photodissociation of O3 were directly detected, using a vacuum ultraviolet laser-induced fluorescence technique.
The photofragment yield spectrum for O 3Pj has the vibrational structure as appearing in the absorption spectrum in the Huggins band, while the spectrum for O 1D is smooth and has no structure. The wavelength dependence of the quantum yield for O 1D from the O3 photolysis exhibits a dip at every peak of the vibrational structure in the absorption spectrum in the Huggins band. The partial cross sections for the O 1D production process in the photoabsorption of O3 were determined.
The existence of the tail around nm in the wavelength dependence of the quantum yield of O 1D produced from O3 photolysis was verified. Our results indicate that the tail arises predominantly from the hot-band excitation of O3. Electron-reduced intermediates display g anisotropy within the 2.
Both oxidized and reduced intermediates add oxygen on annealing forming peroxyl radicals which are identified by their characteristic oxygen hyperfine couplings.