Volume 94, Issue 11, 01 June 1991
Index of content:

Microwave spectrum, structure, dipole moment, and deuterium nuclear quadrupole coupling constants of the acetylene–sulfur dioxide van der Waals complex
View Description Hide DescriptionThirty‐three a‐ and c‐dipole transitions of the acetylene–SO_{2} van der Waals complex have been observed by Fourier transformmicrowave spectroscopy and fit to rotational constantsA=7176.804(2) MHz, B=2234.962(1) MHz, C=1796.160(1) MHz. The complex has C _{ s } symmetry with the C_{2}H_{2} and SO_{2} moieties both straddling an a–c symmetry plane (i.e., only the S atom lies in the plane). The two subunits are separated by a distance R _{cm}=3.430(1) Å and the C _{2} axis of the SO_{2} is tilted 14.1(1)° from perpendicular to the R _{cm} vector, with the S atom closer to the C_{2}H_{2}. The dipole moment of the complex is 1.683(5) D. The deuterium nuclear quadrupolehyperfine structure was resolved for several transitions in both C_{2}HD⋅SO_{2} and C_{2}D_{2}⋅SO_{2}. A lower limit for the barrier to internal rotation of the C_{2}H_{2} was estimated to be 150 cm^{−1} from the absence of tunneling splittings. The binding energy was estimated by the pseudo‐diatomic model as 2.1 kcal/mol. A distributed multipole analysis was investigated to rationalize the structure and binding of the complex.

The microwave spectrum, structure, and tunneling motion of the sulfur dioxide dimer
View Description Hide DescriptionThe microwave spectrum of (SO_{2})_{2} has been reinvestigated using a pulsed beam Fourier‐transform microwave spectrometer. Several new a‐type transitions for the normal species and the a‐type spectra of eight isotopically substituted species were measured. The spectra indicate that the SO_{2} dimer undergoes a high‐barrier tunneling motion. Based on the analysis used for (H_{2}O)_{2} by Coudert and Hougen [J. Mol. Spectrosc. 1 3 0, 86 (1988)], the internal motion is identified as a geared interconversion motion similar to that displayed by (H_{2}O)_{2}. From the analysis of the moments of inertia of the various isotopic species, an a c plane of symmetry is established for the dimer and the tilt angles of the C _{2} axes of each subunit relative to the line joining their centers of mass were determined. From Stark effect measurements, μ_{ a } was redetermined and μ_{ c } was shown to be nearly zero. Electrostatic calculations using distributed multipoles were carried out to explore the structure of this dimer.

Electronic absorption spectroscopy of matrix‐isolated polycyclic aromatic hydrocarbon cations. I. The naphthalene cation (C_{1} _{0}H^{+} _{8})
View Description Hide DescriptionThe ultraviolet, visible, and near infrared absorption spectra of naphthalene (C_{1} _{0}H_{8} ) and its radical ion (C_{1} _{0}H^{+} _{8} ), formed by vacuum ultraviolet irradiation, were measured in argon and neon matrices at 4.2 K. The associated vibronic band systems and their spectroscopic assignments are discussed together with the physical and chemical conditions governing ion production in the solid phase. The absorption coefficients were calculated for the ion and found lower than previous values, presumably due to the low polarizability of the neon matrix. This study presents the first spectroscopic data for naphthalene trapped in a neon matrix, where the perturbation of the isolated species by its environment is minimum; a condition crucial to astrophysical applications.

Observation of an electronic state of C_{2}H near 9 eV by resonance ionization spectroscopy
View Description Hide DescriptionA new electronic state of C_{2}H, tentatively assigned as the 3pσ ^{2}Π Rydberg state, has been observed by resonance‐enhanced multiphoton spectroscopy (REMPI). The observed absorptions originate from vibrationally excited C_{2}H and C_{2}D formed in the photodissociation of C_{2}H_{2}, C_{2}D_{2}, and C_{2}HD at 193 nm in a molecular beam. Two‐laser experiments and REMPI spectroscopy of photofragments of the dissociation of C_{2}HD were used to assign the carriers of the observed spectra to C_{2}H and C_{2}D. Two‐photon resonant, three‐photon ionization (2+1 REMPI) of C_{2}H and C_{2}D is accompanied by substantial fragmentation to C^{+} _{2} ions caused by multiple photon absorption by resonantly excited C_{2}H and C_{2}D. The identity of the lower C_{2}H electronic state(s) (Ã ^{2}Π or X̃ ^{2}Σ^{+} ) was not established.

Rotational spectra, structure, and intramolecular force field of the Hg–OCS van der Waals complex
View Description Hide DescriptionPure rotational spectra of the mercury–carbonyl sulfide complex have been observed by a pulsed‐nozzle Fourier‐transform microwave spectrometer, which has been recently constructed. The structure of the complex is found to be T‐shaped, which is the same as the rare‐gas–carbonyl sulfide complexes analogous to this system. The mercury–carbon distance and the Hg–C–O angle have been determined to be 3.7075(2) Å and 86.98(1) deg, respectively, for ^{202}Hg–OCS from the rotational constants. Harmonic force constants of the van der Waals modes have been derived by the centrifugal distortion constants. The electric quadrupole coupling constants for ^{201}Hg–OCS have also been determined and the values indicate small perturbation on the electric charge distribution of the mercury atom by complex formation.

Vibrational circular dichroism and electric‐field shielding tensors: A new physical interpretation based on nonlocal susceptibility densities
View Description Hide DescriptionMotion of nuclei within a molecule induces a magnetic momentm ^{ e } in the electronic charge distribution, giving a nonzero electronic contribution to the magnetic transition dipole that produces vibrational circular dichroism. In this paper, we develop a new susceptibility density theory for the induced magnetic moment. The theory is based on the response of the electrons to changes in the nuclear Coulomb field, due to shifts in nuclear positions. The electronic response to these changes depends on the s a m esusceptibility densities that determine response to external fields. Our analysis suggests a new physical picture of vibrational circular dichroism. It yields an equation for the density of the induced electronic magnetic moment within a molecule; it also yields a new relation connecting the electric‐field shielding at nucleus I of a molecule in an applied magnetic field of frequency ω to the derivative of m ^{ e } with respect to the velocity of nucleus I, regarded as a parameter in the electronic wave function. Within our theory, the derivative of m ^{ e } with respect to nuclear velocity separates into quantum‐mechanical and classical components in close analogy with the Hellmann–Feynman theorem for forces on nuclei. In matrix‐element form, results from our theory are identical to those obtained with nonadiabaticperturbation theory, to leading order. In general, the leading nonadiabatic corrections to electronic properties are determined directly by the electrons’ response to the changes in the nuclear Coulomb field, when the nuclei move.

Infrared spectroscopy of HX (X=Br,Cl) adsorbed on LiF(001): Alignment and orientation
View Description Hide DescriptionThe infrared spectra of HBr and HCl on LiF(001) single crystal surfaces were measured as a function of coverage at temperatures ≤83 K using Fourier‐transform infrared (FTIR)spectroscopy. For each hydrogen halide three different spectral features could be distinguished. At low coverages broad absorptions centered at 2265±20 cm^{−} ^{1} (HBr) and at 2515±20 cm^{−} ^{1} (HCl) were observed. These absorptions were attributed to molecules hydrogen‐bonded to F^{−} anions of the surface, the angle between the molecular axis and the surface being 21±5° for HBr and 19±5° for HCl as determined from experiments employing polarized infrared radiation.Hydrogen bonding was evidenced by: (i) redshifts with respect to the gas phase (∼300 cm^{−} ^{1}), (ii) broad infrared absorption (FWHM: 265±25 cm^{−} ^{1} for HBr, 295±15 cm^{−} ^{1} for HCl), and (iii) enhancement of the infrared absorption intensity compared to the gas phase by more than one order of magnitude for both HBr and HCl. With increasing coverage a second absorption was observed before the first one saturated (HBr:ν=2461±5 cm^{−} ^{1}, FWHM=75±10 cm^{−1}; HCl: ν=2763±5 cm^{−} ^{1}, FWHM=80±10 cm^{−} ^{1}). This absorption was attributed to molecules adsorbed in a second layer. The smaller redshift and spectral width for the second layer were consistent with weaker hydrogen bonding, probably to the halogen of molecules adsorbed in the first layer. Further increase in coverage resulted in the appearance of the well‐known doublet absorptions due to formation of solid. Coadsorption of HBr and HCl, as well as experiments under adsorption–desorption equilibrium conditions, confirmed that the first and second layers could coexist. The isotherms could best be understood on the assumption of a repulsive interaction within the first layer.

The van der Waals vibrations of aniline–(argon)_{2} in the S _{1} electronic state
View Description Hide DescriptionVibrational structure associated with van der Waals modes of the aniline–(argon)_{2} complex has been observed in the region near the origin of the S _{1}←S _{0} electronic transition of the complex using resonance enhanced, multiphoton ionization (REMPI) spectroscopy. The aniline–Ar_{2}spectrum in this region displays only a few discrete bands built on an intense electronic origin. The dominant vibrational band, associated principally with the symmetric van der Waals stretching motion of the two argon atoms against the aromatic frame, occurs at 38.5 cm^{−1} displacement relative to the 0^{0} _{0} band, with weaker transitions at 15 and 30 cm^{−1} displacement. A simple model for the van der Waals vibrations of aromatics bound to one and two rare gas atoms is developed and allows allow us to explain the aniline–Ar_{2}spectrum, using van der Waals bond parameters determined from the previously measured An–Ar_{1}spectrum. The agreement between the predicted and observed aniline–Ar_{2}spectrum confirms the view that van der Waals stretching vibrations are coupled anharmonically to near resonant bending vibrations in aniline–Ar_{1} and aniline–Ar_{2}.

Mass selected resonance enhanced multiphoton ionization spectroscopy of aniline–Ar_{ n } (n=3,4,5, ...) van der Waals complexes
View Description Hide DescriptionThe origin region of the S _{1}←S _{0} transitions of the aniline–Ar_{3}, aniline–Ar_{4}4, and aniline–Ar_{5} molecules have been measured using mass selected resonance enhanced, multiphoton ionization (REMPI) spectroscopy. The aniline–Ar_{3} spectrum exhibits two distinct groups of peaks. The more prominent group displays a regular vibrational progression, with five obvious members and a spacing of ∼16 cm^{−1}. Vibrational structure in the other group is less distinctive. On the basis of cluster potential calculations described in this paper, we believe that two stable aniline–(argon)_{3} isomers exist in the supersonic expansion and that the two groups of peaks correspond to absorption by these two isomers. Spectra recorded at masses corresponding to aniline–(argon)_{4} and aniline–(argon)_{5} display broadened structure that probably reflects contributions from larger aniline–(argon)_{ n } clusters which fragment upon ionization. There is, however, some evidence for a progression with a spacing of ∼16 cm^{−1} in the aniline–(argon)_{4} spectrum. Dispersed fluorescence spectra from relatively small aniline–Ar_{ n } clusters (4<n<10) indicate that vibrational redistribution from Franck–Condon active van der Waals modes occurs with rates of at least 5×10^{9} s^{−1}.

J coupling in chemically equivalent spin pairs as studied by solid‐state nuclear magnetic resonance
View Description Hide DescriptionThe presence of J coupling was observed in the solid‐state nuclear magnetic resonance(NMR) spectrum of the ^{31}P spin pair in polycrystalline 1,2‐bis (2,4,6‐tri‐t e r t‐butylphenyl)‐diphosphene (TBPDP) using the magic‐angle spinning (MAS) technique. This represents the first example of J coupling in a spin‐pair system with nuclei having identical isotropic chemical shifts. Average Hamiltonian theory is used to derive the time‐independent eigenstates for the system. It is shown that a second‐order perturbation term, which is dependent on the difference between the chemical‐shift tensor orientations in addition to the strength of the dipolar coupling, allows the recoupling of the J interaction for the two chemically equivalent ^{31}P nuclei. In the absence of the perturbation term, the system is reduced to an A _{2} case in solution‐state NMR for which, of course, no J coupling may be observed. The magnitude of the J coupling, namely, 580±20 Hz obtained from two‐dimensional (2D) J‐resolved experiments was found to be in excellent agreement with the value of 577±15 Hz estimated using the abovementioned theory and experimental data from the one‐dimensional (1D) spectra measured as a function of the rotor spinning frequency.

n f Rydberg complexes of NO in a magnetic field, probed by double resonance multiphoton ionization
View Description Hide Descriptionn f (v=1) Rydberg states of NO have been probed by double resonancemultiphoton ionization in a 1 T external magnetic field. Due to the nonpenetrating character of the f orbitals, these Rydberg states are very sensitive probes of any external perturbation. As n increases, a decoupling of the angular momentuml of the Rydberg electron from the molecular frame occurs gradually, as the magnetic interaction becomes more and more important with respect to intramolecular forces. Up to n≂15, only the linear Zeeman perturbation has been taken into account. The rotational–electronic structure of the 7f and 15f states has been interpreted theoretically by considering the linear Zeeman perturbation in addition to the Coulombic interaction and the long range interaction due to the quadrupole moment and the polarizability of the ion core. The intensities and line positions of the transitions from the intermediate A ^{2}Σ^{+},v=1 level to the 7f and 15f levels have been calculated. The alignment of the N, M _{ S }, M _{ N }Zeeman sublevels of the A state by the two‐photon pump excitation from the ground state as well as the polarization of both lasers have been taken into account in the calculations. A good agreement between the observed and the calculated transitions has been obtained. For the 7f levels, the electronic–rotational structure is well described in a coupled case (d) representation. For the 15f levels, the strong coupling of l to the field axis led to the first observation of the Paschen–Back effect in a molecule, within each rotational N ^{+} series, with an accompanying drastic simplification of the spectra. This level is better described in a decoupled case (d) representation corresponding to a moderately strong field regime.

Optothermal‐detected microwave‐sideband CO_{2}‐laser spectroscopy of Ar–NH_{3}
View Description Hide DescriptionA microwave‐sideband CO_{2}‐laser optothermal spectrometer with a resolution better than 1 MHz has been used to record the infrared spectrum of Ar–NH_{3} in the vicinity of the a R(0,0) line of the ν_{2} vibration of free NH_{3}. A Π←∑ type band is observed, giving a positive l‐type doubling constant q, of 90.9 MHz for the upper state. The positive q indicates that the j=1, k=0, ∑ state is above the j=1, k=0, Π state in the v _{2}=1 excited state, where j and k specify the correlation of the internal‐rotor state of the Ar–NH_{3} complex to the NH_{3}monomer rotational state j,k. The ν_{2} vibrationally excited complex is found to predissociate in less than the 0.9 ms transit time between the bolometer detector and laser‐excitation region. A lower limit to the upper‐state lifetime can be obtained from the observed linewidths, which range from 1.5 to 3 MHz (FWHM). The present results agree with and extend the previous free‐jet diode‐laser absorption measurements on this band.

Theoretical characterization of the potential energy surface for H+O_{2} = HO^{*} _{2} = OH+O. III. Computed points to define a global potential energy surface
View Description Hide DescriptionRecent a b i n i t i o calculations have focused on the minimum energy path region of this surface [J. Chem. Phys. 8 8, 6273 (1988), Paper I] and on the saddle point region for H atom exchange via a T‐shaped HO_{2} complex (J. Chem. Phys. 9 1, 2373 (1989), Paper II). In this paper, the results of additional calculations are discussed that, combined with the previously reported results, provide a global representation of the potential energy surface for this reaction. Complete active space SCF/externally contracted configuration interaction calculations (CASSCF/CCI) were carried out using the same wave function employed in II. The new calculations presented here characterize the potential energy surface for a variety of H atom approach angles, ranging from perpendicular to collinear (measured with respect to the O_{2}bond) and for a variety of H atom to O_{2} center‐of‐mass distances. Additionally, a new collinear exchange saddle point is reported. Using the a b i n i t i o results from these new calculations, optimal geometries, and harmonic frequencies based on local polynomial representations of the potential energy surface are reported along the constrained energy minimum path, while anharmonic frequencies are calculated at both the O_{2} and OH asymptotes and at the HO_{2} intermediate.

O(^{3} P) attack on boranes. II. B_{5}H_{9}
View Description Hide DescriptionWhen B_{5}H_{9} is injected into a stream of He that is carrying O(^{3} P) atoms (approximately 100/1), at a total pressure of 5–15 Torr, a blue‐green flame develops. The major chemiluminescent species is BO(A ^{2}Π). While its translational and rotational temperatures are ≊350 K, the vibrational temperature in the A state is high, ≊3800 K. From among the many products of this reaction, the OH radical can be most easily quantitated by measuring the intensity of its laser‐induced fluorescence. The central streamline from a flow‐tube reactor was extracted into an evacuated plenum via a pinhole. The time‐intensity profile was calibrated using C_{2}H_{6} for the fuel. Check runs were made with B_{2}H_{6}. A multistep mechanism was developed for B_{5}H_{9}+O(^{3} P) that simulates the shape as well as the magnitude of the OH concentration over a reactor residence time 0.5–10 ms. Less than a dozen crucial reactions were identified by means of an extended sensitivity analysis. Breakdown schemes for the oxidation of B_{2}H_{6} and B_{5}H_{9} have been developed.

Symmetry‐invariant reaction‐path potentials
View Description Hide DescriptionA general method is presented for constructing a molecular potential‐energy surface, based on a reaction path, which is invariant to operations of the complete nuclear permutation inversion group. The method is illustrated with a number of examples, including an approximate potential for NH^{+} _{3}+D_{2} which allows both abstraction and exchange reactions.

Reactive scattering using efficient time‐dependent quantum mechanical wave packet methods on an L‐shaped grid
View Description Hide DescriptionMost quantum mechanical time‐dependent wave packet methods represent the wave packet on Cartesian grids. For reactive systems these grids can contain many points at which the wave function amplitude is negligible throughout the calculation. A significant fraction of the computational resources in a calculation may be used in calculating and storing values of the wave function at these grid points. This work describes a method which uses the Fourier transform method to calculate the kinetic energy of a wave function on a grid which covers only reactant and product regions of the potential. For a two‐dimensional system this corresponds to the use of an L‐shaped grid. The time propagation is performed using the Chebyshev propagation method. The method is demonstrated for a model problem treating dissociative adsorption and associative desorption of H_{2} from a flat surface. The amount of computational resources required for these calculations is much less than would be needed if the full rectangular grid was used.

On the geometry of transient relaxation
View Description Hide DescriptionCoupled chemical reactions are often described by (stiff) systems of ordinary differential equations (ODEs) with widely separated relaxation times. In the phase space Γ of species concentration variables, relaxation can be represented as a cascade through a nested hierarchy of smooth hypersurfaces (inertial manifolds) {Σ}: If d is the number of independent concentration variables, then Γ≡Γ_{ d }⊇Σ_{ d−1}⊇Σ_{ d−2}⋅⋅⋅. The last three sets in this hierarchy have special chemical importance: Σ_{0} is the stagnation point of the ODEs, i.e., chemical equilibrium; M(≡Σ_{1}) is the linelike slow manifold describing the dynamical steady state in closed systems; Σ(≡Σ_{2}) is the two‐dimensional surface containing the s l o w e s t transient flow that reaches M. Thus M and Σ are the structures underlying most steady‐state and transient kinetics experiments. The ODEs describe the velocity field in Γ, which may be used to define f u n c t i o n a l e q u a t i o n s for M, Σ, and other members in the hierarchy {Σ}. These functional equations can be solved to give explicit formulas for M, Σ, etc. In a model three‐step mechanism, M is described by a v e c t o rfunctional equation involving ordinary derivatives, whereas Σ is described by a s c a l a rfunctional equation involving partial derivatives. We show how Σ may be found by iterative solution of this functional equation if decay is monotonic, and comment on the complications introduced by oscillatory transients. The functional equation approach may be generally applied in higher dimensional systems.

The rotation–vibration potential of He–H_{2} and its connection with physical phenomena
View Description Hide DescriptionThis paper examines the effect of infinitesimal functional variations in a three‐dimensional vibration–rotation He–H_{2}potential surface on several different levels of physical observables: inelastic cross sections, rate constants, and energy level populations. Earlier equations for a rigid‐rotor system are extended and a comparison of the current results with earlier rigid‐rotor results is made. A significant difference in the sensitivity of observables to the potential components has been observed between those observables which are purely rotationally inelastic and those which are vibrationally inelastic. The region of highest sensitivity is dependent upon the energy or temperature as well as the states related by the individual observable. Significant information loss has been observed in the transition from the microscopic observables to the macroscopic ones for those observables which are vibrationally inelastic.

The role of the potential surface in transport and relaxation phenomena in the He–H_{2} system
View Description Hide DescriptionThis paper examines the role of the potential surface of the rigid rotor He–H_{2} system upon a variety of transport and relaxation cross sections, including some involving field effects. The technique of functional sensitivity analysis is used to explore these issues. Three different levels of cross sections were studied: microscopic, thermally averaged, and effective cross sections. The cross sections studied were found to be sensitive to differing components of the potential energy surface, with some cross sections being more sensitive to either the slope or the magnitude of the potential components. The degree of information loss in the progression from microscopic to bulk observables is highly dependent on the individual phenomenon.

Use of scaled external correlation, a double many‐body expansion, and variational transition state theory to calibrate a potential energy surface for FH_{2}
View Description Hide DescriptionA new potential energy surface is presented for the reaction F+H_{2}→HF+H. The regions of the surface corresponding to collinear and bent geometries in the F–H–H and H–F–H barrier regions are based on scaled external correlation (SEC) electronic structure calculations, and the F–H⋅⋅⋅H exit channel region is based on the previously developed surface No. 5. The functional form of the new surface includes dispersion forces by a double many‐body expansion (DMBE), and the surface was adjusted so that the van der Waals well in the F⋅⋅⋅H–H region agrees with available experimental predictions. We have calculated stationary point properties for the new surface as well as product–valley barrier maxima of vibrationally adiabatic potential curves for F+H_{2}→HF(v’=3)+H,F+HD→HF(v’=3)+D, and F+D_{2}→DF(v’=4)+D. The new surface should prove useful for studying the effect on dynamics of a low, early barrier with a wide, flat bend potential, as indicated by the best available electronic structure calculations.