Volume 110, Issue 6, 08 February 1999
 GAS PHASE DYNAMICS AND STRUCTURE: SPECTROSCOPY, MOLECULAR INTERACTIONS, SCATTERING, AND PHOTOCHEMISTRY


The microwave spectrum of the rubidium monoxide RbO radical
View Description Hide DescriptionThe rotational spectra of in the ground and the first excited vibrational states and of in the ground vibrational state were observed in the millimeter and submillimeterwave region. The RbO radical was generated in a hightemperature absorption cell by the reaction of with Rb vapor, which was produced in the cell by heating a mixture of RbCl and lithium metal to 500–550 °C. Analysis of the observed spectra yielded the rotational and centrifugal distortion constants, spin–rotation interaction constants, and hyperfineinteraction constants. The bond length and the vibrational frequency were calculated from the rotational and centrifugal distortion constants to be 2.254 193 1 (15) Å and 387.22 (20) cm^{−1}, respectively, for The ground electronic state of RbO was confirmed to be and the lowlying state was estimated to be located at 650–700 cm^{−1} above the ground state by using a secondorder perturbation expression with the vibrational energy in the denominator for the spin–rotation interaction constant. The observed hyperfineinteraction constants indicated that the RbO radical is essentially an ionic molecule.

Torsional potential of biphenyl: Ab initio calculations with the Dunning correlation consisted basis sets
View Description Hide DescriptionThe internal rotational barrier heights of biphenyl were calculated with the Dunning correlation consisted basis sets (up to ccpVQZ, 960 basis functions) and the electron correlation correction by the second order Mo/llerPlesset method (MP2). Although previous Hartree–Fock (HF) and MP2 calculations showed that the internal rotational barrier height at 0° was substantially larger than that at 90° our MP2/ccpVQZ//MP2/631G^{*} calculations showed that (2.28 kcal/mol) was close to (2.13 kcal/mol), which agreed with the estimation from experimental measurements. The calculations of benzene dimers suggested that the dispersion interaction increased the relative stability of the coplanar conformer. The basis sets employed in the previous calculations were not large enough to evaluate the attractive dispersion interaction. The underestimation of the stabilization of the coplanar conformer by the dispersion interaction would be one of the reasons for the overestimation of in the previous calculations.

The unimolecular dissociation of vinylcyanide: A theoretical investigation of a complex multichannel reaction
View Description Hide DescriptionAb initio molecular orbital calculations were performed toward the determination of the potential energy surface for the unimolecular groundstatedissociation of vinylcyanide. Reaction pathways for the three and fourcenter elimination reactions of HCN and as well as migration and radical elimination channels of H and CN, were examined. MP2 gradient geometry optimizations and QCISD(T) single point energy calculations were performed for all the relevant product species and transition states. The results are compared to the analogous unimolecular dissociation of vinylchloride which has been theoretically investigated by Morokuma and coworkers [J. Chem. Phys. 100, 8976 (1994)]. The unimolecular rates for all reaction channels have been calculated using Rice–Ramsperger–Kassel–Marcus (RRKM) theory employing ab initio transition state energies and MP2 vibrational frequencies. Our calculations indicate that the elimination of and HCN preferentially proceed via threecenter transition states. We also find that Hmigration reactions are rapid, suggesting that substantial Hatom scrambling precedes dissociation. This result is important for the interpretation of several isotopic substitution measurements which are all consistent with exclusive threecenter elimination of and HCN.

Ab initio calculation of the frequencydependent interaction induced hyperpolarizability of
View Description Hide DescriptionThe frequencydependent interaction induced polarizability and second hyperpolarizability of the argon dimer are computed for a range of internuclear distances employing the coupled cluster singles and doubles response approach. The frequency dependence of the interactioninduced properties is treated through second order in the frequency arguments using expansions in Cauchy moments and hyperpolarizability dispersion coefficients. The dielectric, the refractivity, the Kerr and the hyperpolarizability second virial coefficients are computed for a range of temperatures employing a recent accurate ab initio potential for the ground state of the argon dimer. For most of the computed virial coefficients good agreement is obtained between the present ab initio results and the available experimental data.

Coupled cluster response calculation of natural chiroptical spectra
View Description Hide DescriptionWe present the first coupled cluster singles and doubles calculations of the scalar rotatory strength and the rotatory strength tensor, which determine the intensity of isotropic and oriented electronic circular dichroismspectra, respectively, for the ethylene chromophore of (−)transcyclooctene. Results for the oscillator strength of the ordinary electronic absorption spectra are also reported. The results are presented in pictorial form for seven different basis sets and two formally equivalent formulations of the intensity quantities. The theoretical results are compared with the experimental absorption and isotropic circular dichroismspectra of (+)transcyclooctene.

Infrared transition intensities in acetylene: An algebraic approach
View Description Hide DescriptionThe twodimensional algebraic model for degenerate bending modes has been extended to include interaction terms with stretching modes and to provide a detailed description of the dipole operator for linear molecules. An algebraic mixed model is used to investigate, for the first time, the absolute infrared intensities in up to Intensities of hitherto unmeasured vibrational bands are also given.

Formation of He^{*} by ion–ion neutralization reactions of with (X=F, Cl, Br, ) in a helium flowing afterglow
View Description Hide DescriptionThe ion–ion neutralization reactions of with (X=F, Cl, Br, ) leading to He^{*} have been spectroscopically studied in a helium flowing afterglow. Although both singlet and triplet He^{*} states were formed when X=F and Cl, only triplet He^{*} states were produced when X=Br and More than 99% of the product He^{*} atoms were formed in the lowlying He( or ) states for all the reactions, and their electronicstate distributions were similar. The electronicstate populations decreased rapidly with increasing excitation energy of He^{*}. They were represented by effective electronic temperatures of 0.070–0.19 eV. The observed electronicstate distributions were compared with those predicted from a simple statistical theory.

Unimolecular reaction dynamics from kinetic energy release distributions. III. A comparative study of the halogenobenzene cations
View Description Hide DescriptionThe translational kinetic energy release distribution (KERD) in the halogen loss reaction of the chloro, bromo, and iodobenzene cations has been experimentally determined in the microsecond time scale and theoretically analyzed by the maximum entropy method. The KERD is constrained by the square root of the translational energy, i.e., by the momentum gap law. This can be understood in terms of quantummechanical resonancescontrolled by a matrix element involving a localized bound state and a rapidly oscillating continuum wave function, as in the case of a vibrational predissociation process. The energy partitioning between the reaction coordinate and the set of the remaining coordinates is nearly statistical, but not quite: less translational energy is channeled into the reaction coordinate than the statistical estimate. The measured entropy deficiency leads to values of the order of 80% for the fraction of phase space sampled by the pair of fragments with respect to the statistical value. In the case of the dissociation of the chlorobenzene ion, it is necessary to take into account a second process which corresponds to the formation of the chlorine atom in the excited electronic state in addition to the ground state The observations are compatible with the presence of a small barrier (of the order of 0.12 eV) along the reaction path connecting the state of to the asymptote.

Photodissociation of bromobenzene at 266 nm
View Description Hide DescriptionThe photodissociation of at 266 nm has been investigated on the universal crossed molecular beam machine, and timeofflightspectra as well as the angular distribution of Br atom have been measured. Photofragment translational energy distribution reveals that about 47% of the available energy is partitioned into translational energy. The anisotropy parameter β at this wavelength is −0.7±0.2. From and β, we deduce that photodissociation is a fast process and the transition dipole moment is almost perpendicular to the C–Br bond. Ab initio calculations have been performed, and the calculated results show that the geometry of the first excited state of bromobenzene has changed apparently compared with that of the ground state. Two kinds of possible fast dissociation mechanism have also been proposed.

Structure and stability of the AlX and species
View Description Hide DescriptionThe electronic and geometrical structures of the ground and lowlying excited states of the diatomic AlX and series (X=H, Li, Be, B, C, N, O, and F) are calculated by the coupledcluster method with all singles and doubles and noniterative inclusion of triples using a large atomic natural orbital basis. All the groundstate AlX molecules except for AlF can attach an additional electron and form groundstate anions. The groundstate and anions possess excited states that are stable toward autodetachment of an extra electron; also has a second excited state. Lowlying excited states of all AlX but AlN can attach an extra electron and form anionic states that are stable with respect to their neutral (excited) parent states. The groundstate and anions are found to be thermodynamically more stable than their neutral parents. The most stable is whose dissociation energy to is 6.4 eV. Correspondingly, AlO possesses the largest electron affinity (2.65 eV) in the series.

Theoretical study of the mechanism of electron transfer at photosynthetic reaction centers. I. Singlet excited states of free base porphin
View Description Hide DescriptionFree base porphin is a key unit in the electron transferreaction at photosynthetic reaction centers. For the electron transferreaction, the transfer integral of the rate constant depends strongly on the quality of the wave functions of porphinbased chromophores. Therefore, we need a stable method for calculating the wave functions of optically allowed excited states of the porphin. We developed such a method and verified its stability by calculating the wave functions for an ethylene molecule. We confirmed that the optically allowed excited states required the entire amount of valence molecular orbitals for the active space to adequately describe the wave function with molecular in–out polarization. We applied our calculation method to investigate the wave function of free base porphin. Our ab initio calculation used 431 G plus dpolarization functions for the carbon and nitrogen atoms, and Rydberg basis functions on the center of each pyrrol ring simultaneously. We also proposed an assignment for the controversial B and N bands. This assignment is reasonably consistent with experimental data within 0.36 eV for Q and B bands, and 0.75 eV for N bands. The reproduction accuracy of the lower excitation spectra up to B band confirms the reliability of the transfer integral part to calculate the rate constants of the electron transferreaction at photosynthetic reaction centers.

Excitation and decay of states in a pulsed discharge: Kinetics of electrons and longlived species
View Description Hide DescriptionThe vibrational excitation and decay of state has been investigated in a pulsed rf discharge. The effect of the pulsing frequency and the duty cycle on the vibrational distribution, obtained from the 1PG spectra taken at different times in discharge and afterglow, has been examined in the pressure range 2.4 mTorr–1.5 Torr. The measured distributions have been analyzed by a steadystate kinetic model taking into account the main excitation processes, like the electron impact from and states, the associative excitation of with the pooling by molecules, the atomic recombination of as well as the quenching processes. For the various processes, experimental statetostate rate coefficients from the literature and/or calculated data sets have been used. Measured and electron energy distribution functions, and estimated distributions have been used as input data for the model. The model satisfactorily reproduces almost all the measured distributions. The kinetic analysis evidences that the shape of the distribution depends on the competition between the different processes and in turn on the time variation of both density and the degree of internal excitation of the reaction species with discharge repetition rate, duty cycle, and pressure. In general, in the discharge the electron impact is always the predominant excitation mechanism, while the processes involving longlived species: and are important in the postdischarge regime. Under conditions of high metastable density, the associative collision processes contribute to excitation in the discharge, and the electron impact process from the A and X states gives a nonnegligible contribution to the low levels in the millisecond time scale afterglow. An analysis of the B quenching is carried out.

Reactive and inelastic collisions of H atoms with vibrationally excited water molecules
View Description Hide DescriptionThe dynamics of the collisions of H atoms with vibrationally excited were studied using classical mechanical reactive and quantum mechanical nonreactive scattering calculations. The classical trajectory calculations were performed with the I5 potential surface of Isaacson. These results show the expected behavior for an endoergic reaction with a late barrier, with the cross section exhibiting a high threshold when the water is unexcited, and a much lower threshold if the asymmetric stretch of water is highly excited. Qualitatively this matches experimental results, although the threshold energy for reaction of water in the ground vibrational state is too low to reproduce the measured rate coefficients. The rate coefficient is higher than for ground state water by six orders of magnitude when the asymmetric stretch mode is excited by four quanta. However the rate for reaction from this excited state is still two orders of magnitude smaller than the total reactive+inelastic rate coefficient obtained in recent measurements by Smith and coworkers. Quantum scattering calculations of the vibrational energy transfer rate coefficients show that the pure stretch excited states can have very high deactivation rate coefficients, resulting from transitions to states that are separated by a small energy gap (<50 cm^{−1}) from the initial state. The calculated rate coefficients for reactive+inelastic transitions are therefore dominated by inelastic scattering, and the results we obtain are in good agreement with the Smith data.
