Volume 102, Issue 18, 08 May 1995
Index of content:

Microwave spectrum of alkali metal tetrahydroborate. III. Rotational transitions of LiBD_{4}, NaBD_{4}, and KBH_{4} in the ground vibrational states and molecular structures of MBH_{4} (M=Li, Na, and K)
View Description Hide DescriptionIn order to determine the molecular structure of alkali metal tetrahydroborates in a systematic way, we have extended the measurements of rotational transitions to LiBD_{4}, NaBD_{4}, and KBH_{4} in the ground vibrational states. The observed spectra, which all conformed well to the pattern expected for a symmetric top molecule, yielded rotational and centrifugal distortion constants for ^{7}Li^{11}BD_{4}, ^{7}Li^{10}BD_{4}, ^{6}Li^{11}BD_{4}, ^{6}Li^{10}BD_{4}, Na^{11}BD_{4}, Na^{10}BD_{4}, ^{39}K^{11}BH_{4}, ^{39}K^{10}BH_{4}, and ^{41}K^{11}BH_{4}. The four observed rotational constants of LiBD_{4} gave r _{ s }(Li–B) to be 1.931 09(14) Å, which, when combined with an assumption that θ(D_{ b }–B–D_{ t })=113.0°, led to r(B–D_{ b }) and r(B–D_{ t }) to be 1.250±0.025 Å and 1.212∓0.032 Å, respectively, where the uncertainties of the B–D distances are primarily due to that of θ(D_{ b }–B–D_{ t }) estimated to be ∓1.0°. [The suffixes attached to the deuterium atom, b and t, denote bridge and terminal, respectively.] The r _{ s }(Li–B) distance in LiBD_{4} is significantly shorter than that in LiBH_{4}, 1.939 38(10) Å.
This large secondary isotope effect is ascribed partly to the large amplitude rocking (or internal rotation) motion of the BH_{4} group. By fixing the Li–B distance to the respective r _{ s } values, all of the eight rotational constants of LiBH_{4} and LiBD_{4} were simultaneously analyzed to determine the structure of the BH_{4} group, where the isotope effect was taken into account only for the B–H distances in a form δ=r(B–H)–r(B–D). Again θ(H_{ b }–B–H_{ t })=θ(D_{ b }–B–D_{ t }) had to be fixed to 113.0∓1.0°. The isotope shift δ was found to be not very dependent on the value of θ and was determined to be 0.006 26(6) Å. The two B–H distances were obtained to be r(B–H_{ b })=1.257±0.025 Å and r(B–H_{ t })=1.218∓0.032 Å. The data on NaBD_{4} were analyzed by assuming θ(D_{ b }–B–D_{ t })=111.0∓1.0° and r(B–D_{ b })−r(B–D_{ t })=0.04(I) or 0.03(II) Å, yielding r(Na–B)=2.2978(I) or 2.2987(II)±0.0055 Å and r(B–D_{ b })=1.269(I) or 1.258(II)∓0.040 Å. An alternative way of the analysis is to combine the data on the H and D isotopic species; the assumptions on θ(H_{ b }–B–H_{ t })=θ(D_{ b }–B–D_{ t })=111.0∓1.0° and on the difference r(B–H_{ b } or D_{ b })–r(B–H_{ t } or D_{ t })=0.04(I) or 0.03(II) Å lead to the following results: r(Na–B)=2.3075(I) or 2.3080(II)±0.0028 Å, r(Na–B) (in NaBH_{4})–r(Na–B) (in NaBD_{4})=0.0097(I) or 0.0092(II)∓0.0028 Å, r(B–H_{ b })=1.278(I) or 1.267(II)∓0.040 Å, and δ=0.0086∓0.0001 Å for both I and II. The r _{ s }(K–B) distance was obtained from the three observed rotational constants to be 2.656 41(20) Å, which, combined with the assumption of θ(H_{ b }–B–H_{ t })=110.8∓1.0°, led to r(B–H_{ b })=1.272±0.030 Å and r(B–H_{ t })=1.233∓0.030 Å.

The 3 ^{1}Π_{ g } and 3 ^{1}Δ_{ g } states of ^{39}K_{2} studied by optical–optical double resonance spectroscopy
View Description Hide DescriptionThe 3 ^{1}Π_{ g } and 3 ^{1}Δ_{ g } states of K_{2} have been observed for the first time via the intermediate levels of the A ^{1}Σ^{+} _{ u } and B ^{1}Π_{ u } states using optical–optical double resonance (OODR) excitation spectroscopy. The absolute vibrational numberings are obtained by comparing the calculated Franck–Condon (FC) factors with the excitation intensities and resolved fluorescence spectra. The Rydberg–Klein–Rees (RKR) potential curves are determined with the fitted molecular constants. The dependence of the molecular constants on the principal quantum numbers for the bonding and antibonding orbitals and the quantum defects are discussed. The adiabatic and diabatic dissociation limits for those states are given based on comparison with theoretical calculations.

Depolarized Rayleigh scattering in water up to supercritical conditions
View Description Hide DescriptionDepolarized Rayleigh spectra of water have been measured as a function of density along several isotherms in both liquid and supercritical phases. At all the thermodynamical states the spectral line shape can be fitted by a Lorentzian line superimposed to an exponential background. The behavior of the fitting parameters shows that at temperatures lower than ∼470 K the temperature is the most relevant parameter in determining the dynamics of the system, whereas above this temperature both temperature and density play a relevant role. Both the line shape and intensity of the spectra can be consistently interpreted in the framework of the first‐order dipole induced dipole approximation to the collision‐induced light scattering. The data show a clear evidence for cancellation effects.

Study of the low‐lying states of Ge_{2} and Ge^{−} _{2} using negative ion zero electron kinetic energy spectroscopy
View Description Hide DescriptionThe low‐lying states of Ge_{2} and Ge^{−} _{2} are probed using negative ion zero electron kinetic energy (ZEKE) spectroscopy. The ZEKE spectrum of Ge^{−} _{2} yields an electron affinity of 2.035±0.001 eV for Ge_{2}, as well as term energies and vibrational frequencies for the low‐lying states of Ge^{−} _{2} and Ge_{2}. Specifically, we observe transitions originating from the anion ^{2}Π_{ u }(3/2) ground state and ^{3}Σ^{+} _{ g }excited state (T _{ e }=279±10 cm^{−1}) to several triplet and singlet states of Ge_{2}. Term values and vibrational frequencies are determined for the Ge_{2} ^{3}Σ^{+} _{ g }ground state, the low‐lying ^{3}Π_{ u }excited state (T _{ e }=337 cm^{−1} for the 2_{ u } spin–orbit component), and the somewhat higher lying ^{1}Δ_{ g }, ^{3}Σ^{+} _{ g }, and ^{1}Π_{ u } states. We also determine the zero‐field splitting for the X0^{+} _{ g } and 1_{ g } components of the ^{3}Σ^{+} _{ g } state and the splittings between the 2_{ u }, 1_{ u }, and 0^{±} _{ u } spin–orbit components of the ^{3}Π_{ u } state. Detailed comparisons are made with Si_{2} and Si^{−} _{2}.

Electric field effect observed on the infrared spectra of the N_{2}O molecule adsorbed in NaA zeolite
View Description Hide DescriptionThe spectrum of the N_{2}O molecule adsorbed in the cavity of NaA zeolite presents two main components for each of the stretching vibrational modes. It is assumed that they correspond to molecules parallel and antiparallel to the electric field of the inner surface of the zeolite cavity. In order to verify this assumption, the frequency shifts and the intensities of these components have been calculated for the two orientations of the molecule with respect to the field, by applying the model of Bishop for the vibrational Stark effect. These calculations require the knowledge of molecular quantities such as derivatives of permanent electric moments and polarizability.

Theoretical studies of the effects of matrix composition, lattice temperature, and isotopic substitution on isomerization reactions of matrix‐isolated HONO/Ar
View Description Hide DescriptionTheoreticalmolecular dynamics studies of matrix composition, lattice temperature, and isotopic substitution effects upon cis–transisomerization rates and the vibrational relaxation rates to lattice phonon modes of matrix‐isolated HONO, DONO, and H^{18}ON^{18}O systems are reported. The results show that isomerization is usually slower in an argon matrix than in xenon. The calculated ratios of the rates for different initial vibrational energy distributions correlate well with the ratio of the well‐depth parameters for the lattice/HONO interactions. In all cases examined, the matrix‐isolated isomerization rate is enhanced relative to the gas‐phase rate. This behavior is attributed to a vibration → lattice phonon modes → rotation → torsional vibration) isomerization mechanism. Isomerization in both Xe and Ar matrices is nonstatistical with pronounced mode specificity present in both environments. In the gas phase, deuterium and ^{18}O substitution produce small, positive enhancements of the isomerization rate by 13% and 26%, respectively, due to an increased kinetic coupling to the torsional modes. In the matrix, however, the isotope effects are negative and larger in magnitude. This reversal is attributed to a reduced rate of energy transfer from the lattice to rotation of DONO and H^{18}ON^{18}O due to the increased moment of inertia. In general, all of the present results support a matrix HONO isomerization mechanism via a (vibration→lattice phonon modes →rotation→torsional vibration) energy transfer pathway.

A lumped model for H_{2}/O_{2} oxidation in the oscillatory regime
View Description Hide DescriptionA lumped model for H_{2}/O_{2}oxidation in the oscillatory regime is constructed by using the approach of approximate constrained nonlinear lumping based on an algebraic method within nonlinear perturbation theory. The fast variables approach zero rapidly such that the lumped model is constructed within a slow manifold by setting the identified fast variables to zero. The model dimension is reduced from 7 to 4. The variables representing the concentrations of H_{2}, O_{2}, H_{2}O, and H are explicitly kept unlumped. The lumped model accurately reproduces the main features of the original system, such as oscillatory periods and concentration profiles.

A crossed‐beam scattering study of CH^{+} _{4} and CH^{+} _{3} formation in charge transfer collisions of Kr^{+} with CH_{4} at about 1 eV
View Description Hide DescriptionThe dynamics of CH^{+} _{4} and CH^{+} _{3} ion formation in collisions of Kr^{+} (^{2} P _{3/2} ^{’1/2}) with thermal CH_{4} has been investigated in a crossed beam experiment at a hyperthermal collision energy of 1.18 eV. The scattering data show that the CH^{+} _{4} product is formed in a near‐resonant exoergic process in which the most probable energy transferred to the target is practically equal to the recombination energy of the Kr^{+} projectile (resonantenergy transfer); in addition a wide band of internal states of CH^{+} _{4} up to ±0.6 eV is populated in inelastic and superelastic collisions. In contrast, the CH^{+} _{3} product is formed in dissociative charge transfer, with about one‐half of the yield due to nonresonant, endoergic collisions of Kr^{+} (^{2} P _{3/2}). The other half of the CH^{+} _{3} product is found to originate in near‐resonant exoergic collisions of Kr^{+} (^{2} P _{1/2}). An estimate is given of the distribution of the total energy deposited in methane by the above processes.

Dynamics of molecular inversion: An instanton approach
View Description Hide DescriptionTo describe tunneling of light atoms, a method is developed that takes account of the multidimensional nature of the process but remains tractable without becoming inaccurate. It combines the instanton formalism with ab initio potentials and force fields and makes effective use of a number of practical approximations suggested by the nature of the calculations. The tunneling potential is constructed from ab initio calculations that are fully optimized at stationary points. All other vibrations are represented by their harmonic force fields. Changes in the harmonic force fields between stationary points are expressed as couplings with the tunneling mode. The transfer rate is calculated for the instanton path, i.e., the path of least resistance, modulated by adjacent paths which define the damping required for nonoscillatory transfer. The multidimensional transfer integrals, involving all modes that change between the initial state and the transition state, are reduced to quasi‐one‐dimensional integrals by a number of approximation schemes. Modes with frequencies much higher than the tunneling mode are included in the adiabatic potential. Modes with frequencies much lower than the tunneling mode are treated classically. Modes that are linearly coupled to the tunneling mode are handled by separation of the variables followed by analytical integration. These approaches permit the calculation of most transfer rates without the explicit evaluation of the multidimensional instanton path.
They also specify the parts played by the various modes coupled to the tunneling mode. Totally symmetric modes generally promote tunneling by allowing a more favorable trajectory. Hence low‐frequency symmetric modes tend to govern the temperature dependence of the transfer. Modes of the same symmetry as the transfer mode will generally contribute to the barrier and thus cause friction, represented by a Franck–Condon factor in the transfer integral. The method is applied to three molecular inversions whose rate constants have been deduced from magnetic resonance measurements. Inversion rate constants for aziridine and the oxiranyl radical together with their relevant deuterium analogs are calculated as a function of temperature and are found to be in a good agreement with the observed rates. For the dioxolanyl radical such agreement is obtained only after the introduction of an anharmonic correction. While inversions are generally low‐frequency modes, the method is equally applicable to transfer governed by high‐frequency XH stretch vibrations.

Study of N_{2}O_{2} by photoelectron spectroscopy of N_{2}O_{2} ^{−}
View Description Hide DescriptionPhotoelectron spectra of the N_{2}O^{−} _{2} anion, collected at hν=4.657 and 5.822 eV, are presented. The spectra originate from the C _{2v } isomer of the anion. Vibrationally resolved progressions corresponding to transitions to several electronic states of the previously unobserved N_{2}O_{2} molecule are observed. All of the observed transitions lie above the dissociation asymptotes for N_{2}+O_{2}, NO+NO, and O+N_{2}O, and several lie above the N+NO_{2} and N_{2}+O+O asymptotes. Ab initio calculations have been carried out for the anion ground state and several singlet and triplet states of neutral N_{2}O_{2}. By comparing the observed spectra with Franck–Condon simulations based on these calculations, the lowest bands observed in our spectra were assigned to transitions to the ^{3} A _{2} and ^{3} A _{1} states (C _{2v } symmetry) of N_{2}O_{2}. These spectra thus represent the first experimental characterization of metastable, high energy forms of N_{2}O_{2}. Both the N_{2}O^{−} _{2} and the N_{2}O_{2} species are considered in terms of their roles as reactive intermediates in the O^{−}+N_{2}O and N+NO_{2}chemical reactions.

State‐resolved rotational energy transfer in open shell collisions: Cl(^{2} P _{3/2})+HCl
View Description Hide DescriptionTime‐ and frequency‐resolved infrared (IR) laser absorption methods are used to probe hot atom energy transfer in open shell interactions of Cl(^{2} P _{3/2})+HCl(J) in the single collision regime. The Cl(^{2} P _{3/2}) atoms are prepared by 308 nm laser photolysis of Cl_{2}, and suffer collisions at E _{rel}̄∼3500 cm^{−1} with a room temperature HCl distribution in a fast flow cell. Selective collisional excitation of final HCl(J _{ f }) states is monitored by transient IR absorption on R(J≥4) branch lines in the HCl(v=1←0) band, while depletion of the initial HCl(J _{ i }) states is monitored by transient bleaching of the room temperature Doppler profiles. Analysis of the J dependent Doppler profiles permits extraction of rotational loss [σ_{loss}(J _{ i })=∑_{ fP }(J _{ i })⋅σ_{ f←i }] and gain [σ_{gain}(J _{ f })=∑_{ iP }(J _{ i })⋅σ_{ f←i }] cross sections, as a function of initial and final J states, respectively. Absolute transient concentrations of the HCl(J _{ i }) and HCl(J _{ f }) are measured directly from absorbances via Beer’s Law, and used to extract absolutecollisional cross sections. The results are compared with quasiclassical trajectory(QCT) calculations on a hybrid ab initio/LEPS surface of Schatz and Gordon, which prove remarkably successful in reproducing both the J dependent trends and absolute values of the state‐resolved gain and loss collision cross sections.

An adiabatic model for the photodissociation of CH_{3}SH in the first ultraviolet absorption band
View Description Hide DescriptionThe photodissociation of CH_{3}SH in the first absorption band is studied via ab initio computation of the relevant potential energy surfaces and exact quantum scattering calculations. The effective valence shell Hamiltonian (H ^{ν}) ab initio many‐body perturbation technique is used to calculate the global ground X ^{1} A’ and 1 ^{1} A‘surfaces as functions of the C–S and S–H internuclear distances. The finite range scatteringwave function (FRSW) time‐independent quantum scattering method is used to compute the adiabatic dynamics of S–H and C–S bond fission on the 1 ^{1} A‘surface following excitation. Two calculations are performed, one in which the ground state is represented by a cubic spline function fitted to the ab initio data and another in which it is represented as the sum of two uncoupled Morse oscillators. Absorption spectra as well as the branching ratios and photofragment translational energy distributions corresponding to various excitation energies are presented and compared to recent experimental results. A final calculation examines how the branching ratio and product vibrational state distribution changes for the photodissociation of a CH_{3}SH molecule with one quantum of vibrational excitation in the C–S stretch.

The photodissociation of FNO in the S _{1} state: Three‐dimensional calculation on a new potential energy surface
View Description Hide DescriptionWe present three‐dimensional wave packet calculations for the photodissociation of FNO in the first excited singlet state S _{1} using a new ab initiopotential surface. While the calculated absorptionspectrum agrees satisfactorily with the measured spectrum, the energy dependence of the partial cross sections for particular NO product states is only in fair agreement with experiment. The same is true for the vibrational and rotational state distributions of NO for selected energies. Because of the interference between direct and indirect dissociation, details of the cross sections are highly sensitive to subtleties of the potential surface. Altogether, the new calculation reproduces the available experimental data more satisfactorily than a previous one.

Ab initio studies of critical conformations in ethane, methylamine, methanol, hydrazine, hydroxylamine, and hydrogen peroxide
View Description Hide DescriptionAb initio calculations on conformations corresponding to the torsional energy minima and maxima of ethane, methylamine, methanol, hydrazine, hydroxylamine, and hydrogen peroxide were carried out with geometry optimizations using second‐order Mo/ller–Plesset perturbation theory (MP2) in the 6–311+G(3df,2p) basis. Compared with experiments the MP2/6–311+G(3df,2p) calculations yield absolute average deviations of 0.128 kcal/mol for 6 rotational barriers, 0.009 Å for 18 bond lengths, 0.7° for 16 bond angles, 0.5° for 2 dihedral angles, and 0.17 D for 5 dipole moments. Three smaller basis sets, 6–31G(d), 6–31+G(d,p), and 6–311G(d,p), were also used to study variations in the total energies and barrier heights as a result of basis set expansions. Several single‐point calculations were performed to estimate effects of electron correlation enhancement from MP2 to quadratic configuration interaction [QCISD(T)]. Simulating a high level calculation with lower level calculations in a procedure similar to the recent G2(MP2) theory was found very successful.

An accurate computational model for the study of intermolecular interactions
View Description Hide DescriptionA new computational method within the framework of extended group function models is introduced for describing intermolecular interactions between closed shell systems. The model is size extensive, applicable for any intersystem distance, and has a conceptual structure which facilitates interpretation. The basis set superposition error can be eliminated at the correlation level. Test calculations have been performed on the helium dimer and the helium trimer.

An accurate calculation of the three‐body potential for the ground state of the helium trimer
View Description Hide DescriptionAn extended group function model has been applied to determine the three‐body potential for the ground state of the helium trimer. The basis set for the calculations is an uncontracted (17s,6p,4d,3f) set of Gaussian type functions contracted to [8s,6p,4d,3f]. Three different types of configurations were considered: (i) equilateral triangles of side R, R∈{3.5,4.0,...,8.5,9.0} a.u., (ii) linear configurations with R _{12}=R _{23}, R _{12}∈{3.5,4.0,...,8.5,9.0} a.u., and (iii) a set of pseudo‐random configurations. For the equilateral triangular configurations and the linear configurations, and with equal distances larger than 5.5 a.u., the magnitude of the ratio between the three‐body potential and the sum of the corresponding two‐body potentials, is less than 0.003. The value of the three‐body potential for the triangular and the linear configuration with equal distance R=5.669 289 a.u.=3 Å, is respectively 0.040 μH and −0.045 μH. In the long range region the calculated three‐body potential is very close to the Axilrod–Teller triple‐dipole energy.

Conductivity in polyacetylene. I. Ab initio calculation of charge localization, bond distances, and reorganization energy in model molecules
View Description Hide DescriptionThe electronic structure and geometry of neutral or charged molecules of the type trans‐H(CH)_{ n }H is calculated using ab initio methods. The reorganization energy λ for adding or subtracting electrons is obtained for the cases with n=10, 11, and 12 using (U)MP2 with a 6‐31G basis set. We find contributions to λ not only from C–C bond distance changes but also from C–C–C bond angle changes, whereas the contribution from C–H bond length and C–C–H bond angle changes are unimportant. The change of bond length when electrons are added to or subtracted from a neutral molecule with an even number of carbon atoms is typically 0.05–0.08 Å and occurs over a large part of the molecule, even if the donated charge is concentrated near the positive alkali ion. λ for one‐electron reduction or oxidation is large in an even C chain but small in an odd C chain. The introduction of electronic correlation at (at least) the level of second‐order Mo/ller–Plesset perturbation theory is important in the calculation of bond lengths and reorganization energies.

On the singlet–triplet separation in methylene: A critical comparison of single‐ versus two‐determinant (generalized valence bond) coupled cluster theory
View Description Hide DescriptionWe present a critical comparison of the performance of the single‐ vs two‐determinant‐reference coupled cluster method, including its generalized valence bond version, for the classic multireference problem of the singlet–triplet separation in methylene. After demonstrating excellent agreement between the two‐determinant coupled cluster method with single and double excitations and the full configuration interaction method for a double zeta polarization (DZP) basis, we adopt an extended atomic natural orbital basis and obtain harmonic frequencies for the two states to give T _{0}=10.30 kcal mol^{−1} and 8.86 kcal mol^{−1}, respectively, for the single‐ and two‐ determinant‐reference coupled cluster results, compared to the experimental value, 8.998±0.014 kcal mol^{−1}. Adding triples, the corresponding single‐reference value is 9.35 kcal mol^{−1}. We also consider stretched geometries of CH_{2} as a stringent test of our approach. Comparisons are made between the two‐determinant coupled cluster values, including the single and double excitations and their triples excitation corrected coupled cluster counterparts, obtained using various choices of orbitals, including the generalized valence bond ones.

Shape dependences in pulsed laser–matter interaction
View Description Hide DescriptionIt is shown that the shape dependence of final state probability distributions in pulsed laser–atom or molecule interactions has a simple explanation in the theory of quantum transport of Floquet states. More precisely, the predictions of a quasienergy resonance model—complex poles in the quasienergy plane produced by quantum transport and populated by state‐mixing at near‐degeneracies—are successfully compared with numerical solutions of the initial value problem of the Schrödinger equation for a Morse‐potential model.

Gaussian‐type orbitals basis sets for the calculation of continuum properties in molecules: The differential photoionization cross section of Li_{2} and LiH
View Description Hide DescriptionThe differential cross section of the one‐photon ionization of the valence shell of Li_{2} and LiH is computed ab initio adopting the static‐exchange approximation (SEA) and the random phase approximation (RPA). Using large L ^{2}basis sets of polynomial spherical Gaussian‐type orbitals (PSGTO) the matrix elements involving the final states in the electronic continuum are computed by a K‐matrix based technique which also allows to obtain the phase shifts of the partial wave channels. The results reported here suggest that, with the methods proposed, the generally employed GTO’s could advantageously be exploited to evaluate reliable values of the properties of the continuum.