Index of content:
Volume 106, Issue 24, 22 June 1997

Hyperfine decoupling in electron spin resonance
View Description Hide DescriptionA new class of experiments is introduced to electron spin resonance(ESR)spectroscopy that utilizes hyperfine decoupling for resolution enhancement and spectrum simplification, and that provides a basis for correlation techniques. A general framework is provided for the discussion of pulse ESR experiments on systems with arbitrary effective electron spin and an arbitrary number of coupled nuclear spins and is used to describe spin decoupling in pulse ESR and ENDORspectroscopy. Analytical expressions are given for the hyperfinedecoupled nuclear frequencies and the residual hyperfine splittings of spin nuclei during strong decoupling. Pulse sequences are proposed for hyperfinedecoupled electron spin echo envelope modulation (ESEEM) and electron nuclear double resonance(ENDOR) experiments as well as for the correlation of the hyperfinedecoupled ESEEM spectrum with twopulse and threepulse ESEEM spectra and of hyperfinedecoupled ENDOR with the hyperfine splittings. It is shown that hyperfinedecoupled ESEEM and ENDOR spectra can reveal information on the magnetic quantum numbers involved in an ESR observer transition, and that choosing a transition with can improve the resolution of a nuclear frequency spectrum. In addition, such experiments can be used to determine the relative signs of hyperfine couplings. The potential of the twodimensional DECENT (decoupled ESEEM correlated to nuclear transition frequencies) experiment is demonstrated on weakly coupled nuclei in both an ordered and a disordered system and on the hexaquo manganese (II) complex in a single crystal. It is also shown that for the ESR observer transition the system yields highly resolved hyperfinedecoupled ENDOR spectra which allow for a complete assignment of the ENDOR lines.

Resonanceenhanced multiphoton electron detachment spectra of and
View Description Hide DescriptionElectronic spectra of and were measured in the gas phase for the first time using resonanceenhanced detachment. The band origin of the transition to the lowest excited state for each of these three anions was determined. For and several other higher excited states were also detected. The transitions for a series of linear chain anions from to were compared with those previously reported for evennumbered series. Slight differences were noted between the even and oddnumbered series.

A 250 GHz ESR study of terphenyl: Dynamic cage effects above
View Description Hide DescriptionThree nitroxide spin probes of different sizes and geometrical shape were used in a 250 GHz ESR study of the probe rotational dynamics in the fragile glass former orthoterphenyl (OTP) over a wide temperature range from 380 to 180 K. Comparative studies at 9.5 GHz have also been performed. Perdeuterated tetramethyl4methyl aminopiperidinyloxide (MOTA), and (CSL) are, respectively, comparable in size to and larger than the OTP host molecule, whereas Perdeuterated (PDT) is substantially smaller. The sensitivity of 250 GHz ESR to the details of the rotational tumbling for (where is the crossover temperature) was exploited to show that the relaxation is fit by a model that is characteristic of a homogeneous liquid. A nonlinear leastsquares analysis shows that below the melting point, CSL, and MOTA dynamics are welldescribed by a model of dynamic cage relaxation proposed by Polimeno and Freed wherein the probe relaxation is significantly influenced by a fluctuating potential well created by the neighboring OTP molecules. A model of simple Brownian reorientation does not fit the experimental spectra of CSL or MOTA as well as the dynamic cage model below Spectra of PDT do not show any significant nonBrownian dynamics for this probe. It was found that the characteristic rates of the cage model, viz., the reorientation of the probe and the cage relaxation, were describable by activated processes; however, the “average” rotational diffusion rates (defined in the usual manner as the time integral of the correlation function) derived from the dynamic cage parameters follow the Stokes–Einstein–Debye (SED) relation rather well, in agreement with previous studies by other physical techniques. It is then shown that the usual stretched exponential fit to the motional correlation function, interpreted in terms of an inhomogeneous distribution of simple reorientational rates, is clearly inconsistent with the observed ESR spectrum. The absence of a significant cage potential above is discussed in terms of a model of frustration limited domain sizes proposed by Kivelson and coworkers. Evidence for the existence of substantial voids in OTP below especially from the spectra of the small PDT probe, is discussed in terms of the structure and packing of the OTP solvent.

The equilibrium structure of silene from millimeter wave spectra and from ab initio calculations
View Description Hide DescriptionSilene, has been efficiently produced by pyrolysis of 5,6bis(trifluoromethyl) 2silabicyclo[2.2.2]octa5,7diene (SBO). Seven isotopomers have been observed by millimeter and submillimeterwave spectroscopy. From the different sets of experimental molecular parameters and from ab initio calculations of the rovibrational interaction parameters, the equilibrium structure has been obtained by a least squares analysis of the rotational constants. The results are: and This experimental structure is in excellent agreement with the equilibrium geometry calculated at the CCSD(T) level of theory with a ccpV(Q,T)Z basis set. This is the first experimental determination without any constraint of the double bond length in the parent compound of the silaalkene family. A lifetime of 30 ms has been observed for this molecule in the gas phase at low pressure.

Vibrational anharmonicity and multilevel vibrational dephasing from vibrational echo beats
View Description Hide DescriptionVibrational echo experiments were performed on the IR active CO stretching modes of rhodium dicarbonylacetylacetonate and tungsten hexacarbonyl in dibutylphthalate and a mutant of myoglobinCO (H64VCO) in glycerol–water using ps IR pulses from a free electron laser. The echo decays display pronounced beats and are nonexponential. The beats and nonexponential decays arise because the bandwidths of the laser pulses exceed the vibrational anharmonicities, leading to the excitation and dephasing of a multilevel coherence. From the beat frequencies, the anharmonicities are determined to be 14.7, 13.5, and for and H64VCO, respectively. From the components of the nonexponential decays, the vibrational dephasing at very low temperature of both the and transitions are determined. At the lowest temperatures, so the lifetimes are obtained for the three molecules. These are found to be significantly shorter than the lifetimes. Although the lifetimes are similar for the three molecules, there is a wide variation in the lifetimes.

Fouriertransform microwave spectrum, structure, harmonic force field, and hyperfine constants of sulfur chloride fluoride, ClSF
View Description Hide DescriptionThe rotational spectrum of sulfur choride fluoride, ClSF, has been observed for the first time in the frequency range 8–26 GHz by means of a pulsed molecular jet Fouriertransform microwave spectrometer. The unstable sample molecule has been prepared using a pulsed electrical discharge in jets containing a mixture of and in Ne. Besides the parent species the isotopomers and could be observed in natural abundance.Rotational constants and quartic centrifugal distortion constants as well as nuclear quadrupole coupling constants due to and and spin–rotation constants due to and are given. The data were used for the determination of structural parameters. Additionally, the new data were used for the refinement of the molecular harmonic force field. Results from those harmonic force field calculations were applied in the evaluation of the groundstate average structure, and the estimation of the equilibrium structure, The structure is (SF)=160.653(162) pm, (SCl)=199.437(65) pm, and ∠(ClSF)=100.732(81)°. The diagonal elements of the quadrupole coupling tensors have been obtained, and are interpreted in terms of the bonding at Negative spin–rotation constants suggest a close analogy of the electronic structures of ClSF and

Spectroscopic studies of the system of the jetcooled vinoxy radical
View Description Hide DescriptionWe apply several techniques to the study of the band system of the jetcooled vinoxy radical, Vibronically resolved excitation spectra are obtained using both laserinduced fluorescence(LIF) and a twocolor resonant fourwave mixing (TCRFWM) scheme that provides the nonlinear equivalent of holeburning spectra. Rotationally resolved LIFspectra recorded at low temperatures provide rotational constants for 9 state levels. We also measure the fluorescence lifetimes of 19 state levels and obtain highquality dispersed fluorescence (DF) spectra from seven of the most strongly fluorescing levels in the state. The excitation and DF spectra reveal far more vibrational levels in the two electronic states than have been previously observed. In total, we provide assignments for 54 levels observed in the first of the state and for 57 levels in the first of the state. These assignments include the identification of the fundamentals for through and all three overtones, through in both states. The differences between the TCRFWM and LIFspectra and the measured lifetimes indicate a dramatic increase in the predissociation rate of the state beginning at above the origin. The predissociation rate is markedly modespecific and is enhanced by outofplane excitation, possibly due to vibronic coupling with either the or electronic states. The congestion and complexity of the DF spectra at high energies provides direct evidence of extensive intramolecular vibrational redistribution on the groundstatepotential surface.

Ab initio study of VI. Vibrational and vibronic coupling in the conical intersection up to
View Description Hide DescriptionFollowing our previous work [J. Chem. Phys. 105, 9051 (1996)], we have investigated the vibrational and vibronic mixings in the conical intersection spectrum of By analyzing the expansion coefficients of the nonadiabatic states, we have discussed the vibrational resonances above and the interplay among the vibrational and vibronic couplings in the energy range The assignment and the vibrational mixing of the nonadiabatic bands have been studied by comparing twodimensional contour plots of Born–Oppenheimer and of generalized vibrational functions. A Dunham fit to the vibrational bands has extended our previous assignment and has shown that Fermi, Darling–Dennison, and two new vibrational resonances contribute to several nonadiabatic states above We have finally reported the vibrational bands up to and the corresponding clusters of nonadiabatic levels, which have been compared with some experimental results.

Electron–electron–nuclear threespin mixing in spincorrelated radical pairs
View Description Hide DescriptionElectron–electron–nuclear threespin mixing occurs in radical pairs in solidstate matrices if the radicals feature a significant electron–electron spin coupling and an anisotropichyperfine coupling. The perturbation of nuclear frequencies by the electron–electron spin coupling and the threespin mixing have to be generally considered in the calculation of transition frequencies and probabilities in such radical pairs. Analytical descriptions of threespin mixing for different ratios of the spin Hamiltonian parameters are introduced. It is found that nuclear frequencies are strongly perturbed if the difference of the Zeeman energies of the two electron spins is matched to half the hyperfine coupling and that threespin mixing is maximum, if also the nuclear Zeeman frequency matches the former two interactions. Such double matching situations may be encountered for pairs of organic radicals under the conditions where transient electron spin resonance(ESR) experiments are usually performed. If threespin mixing is significant, spincorrelated radical pairs are born in a state that features nuclear coherence in addition to the electron spin zeroquantum coherence that is created irrespective of this mixing. The possibility is discussed to detect such chemically induced nuclear coherence (CINC) in transient electron spin resonance experiments by selective microwaveirradiation. It is shown that subsequent electron transfer reactions can yield chemically induced dynamic nuclear coherence (CIDNC) in isolated radicals and chemically induced dynamic nuclear polarization (CIDNP) in diamagnetic products if threespin mixing is significant. The novel CINC, CIDNC, and CIDNP effects in the solid state might be used in the structure determination of spincorrelated radical pairs with applications to photosynthesis research.

Photodissociation spectroscopy and dynamics of the HCCO free radical
View Description Hide DescriptionThe photodissociationspectroscopy and dynamics of the HCCO radical have been investigated using fast radical beam photofragment translational spectroscopy. An electronic band with origin at has been identified. This band exhibits rotational resolution near the band origin, but the welldefined rovibronic structure is homogeneously broadened at higher photonenergies. Based on the rotational structure this band is assigned to the transition. Photofragment translational energy and angular distributions were obtained at several excitation energies. At excitation energies close to the origin, the excited, spinforbidden channel dominates, while the ground state channel is the major channel at higher photonenergies. The translational energy distributions provide evidence of competition between intersystem crossing and internal conversiondissociation mechanisms, with some evidence for nonstatistical dynamics in the channel. This work yields an improved heat of formation for HCCO,

Finding transition states using contangency curves
View Description Hide DescriptionWe present a new method for approximating reaction paths and transition states of conformational transitions in polyatomic systems. The method uses the quasiharmonic properties of the metastable state to construct a biGaussian model for the energy landscape. Its reaction path is approximated by the equipotential contour cotangency (contangency) curve which connects the two equilibrium states with the saddle point lying between them. Unlike the reaction path, the contangency curve has an explicit analytic definition which makes it a useful starting point for conventional reaction path calculations. The method is illustrated for the conformational transition between the chair and twistedboat isomers of cyclohexane.

The photodissociation of SiO
View Description Hide DescriptionThe photodissociation of the SiO molecule, which is of considerable astrophysical interest, is studied within the framework of the Floquet theory. Only the onephoton transition process between the two lowest states is considered. The computation of the electronic energy surfaces is performed with the MOLPRO code. The wave operator theory is used to select the active space related to this process, and to build the Floquet resonance states which constitute this active space. Particular attention is paid to the role of the shape resonance states induced by the presence of a barrier on the surface.

Improved treatment of inertia and nonMarkovian effects on shorttime dynamics of diffusioncontrolled reaction based on generalized diffusion equation
View Description Hide DescriptionStarting from a generalized diffusion equation and the Collins–Kimball boundary condition, we investigated the inertia and the nonMarkovian effects on the timedependent rate constant of a diffusioncontrolled reaction at short times. In the shorttime limit, we obtained the rate constant analytically, and found that the rate constant was independent of the friction coefficient, and was always smaller than the result of the classical Smoluchowski–Collins–Kimball (SCK) theory in which both of the inertia and the nonMarkovian effects were neglected. At finite times, we obtained the rate constant numerically, and found that the decay of the rate constant was slower than that of the SCK result. When the nonMarkovian effect became larger, the decay became much slower. Our results were consistent with a relevant theory based on a generalized Fokker–Planck equation. The results were compared with computer simulations, and a good agreement was obtained for the case of the maximum reactivity.

Translational energy distributions of the products of the 193 and 157 nm photodissociation of chloroethylenes
View Description Hide DescriptionThe 193 and 157 nm photodissociations of three isomers of dichloroethylene (DCE) and trichloroethylene (TCE) were investigated using a technique of photofragmentation translational spectroscopy. The photofragmentation mechanisms were constructed by analyzing the timeofflight spectra of and produced by electron impact of neutral photofragments. In the 193 nm photodissociation, both the HCl elimination and the C–Cl bond rupture were important for all the compounds examined. It was concluded that secondary dissociation of the vibrationally excited chlorinated vinyl radical produced by the C–Cl bond rupture was important even at 193 nm. In the 157 nm photodissociation, the mechanisms were similar to those at 193 nm for cisDCE, 1,1DCE, and TCE, while only the C–Cl bond rupture occurred for transDCE. This result suggests that the 157 nm photodissociation of transDCE proceeds via the direct photodissociation following the photoexcitation to the repulsive state. A minor C–H bond rupture was also found in the 157 nm photodissociations of cisDCE and TCE. On the basis of the present mechanisms, the translational energy distributions and the branching ratios were estimated for all the possible processes.

Firstorder intermolecular diatomicsinmolecule potentials. Potential energy surfaces, spectra, and fragmentation dynamics of the complex
View Description Hide DescriptionFirstorder perturbative approximations to the diatomicsinmolecule (DIM) approach are implemented for studying interactions between the neon atom and chlorine molecule in the and states. Intermolecular DIM perturbation theory (IDIM PT1) [J. Chem. Phys. 104, 9913 (1996)], which accounts for the atomic component of spinorbit interaction, is compared to the anisotropic model by Naumkin and Knowles [J. Chem. Phys. 103, 3392 (1995)] which is proven to be a firstorder approximation to the nonrelativistic DIM approach. An importance of the spinorbit effects for the groundstatepotential energy surface (PES) is demonstrated. Semiempirical PESs are used in the accurate quantum calculations on the vibrationally averaged geometry, vibronic spectra, and vibrational predissociation dynamics of the van der Waals complex. The IDIM PT1 model is shown to provide good agreement with available experimental data. The effects of interaction potential topology on the spectroscopic and dynamic properties of the complex and the relation of DIMbased PESs to the results of ab initio calculations are discussed.

Rate coefficients for the endothermic reactions as functions of temperature from 400–1300 K
View Description Hide DescriptionWe have measured the bimolecular rate coefficients for the reactions of with and as functions of temperature from 400 to 1300 K using a high temperatureflowing afterglow apparatus. The temperature dependences of these rate coefficients are accurately fit by the Arrhenius equation, with activation energies equal within experimental uncertainty to the reaction endothermicities. Internal energy dependences have been deduced by combining the present data with previous drift tube and ion beammeasurements. We found that reactant rotational energy and translational energy are equally effective in surmounting the energy barrier to reaction, and that vibrational excitation of the neutral reactant to the state enhances the rate coefficients by a factor of ∼1000 for the reaction with and by ∼6000 for the reaction with at temperatures of 800 and 500 K, respectively. This vibrational enhancement is larger than the enhancement that would be produced if the same amount of energy were put into translational and/or rotational modes of the reactants. In addition, rate coefficients have been derived for the threebody association reaction of with in a helium buffer over the temperature range 300–600 K.

Binding energies of clusters: Theory and experiment
View Description Hide DescriptionFormation of clusters has been studied by both temperaturedependent equilibrium measurements and density functional theory(DFT). The successive binding energies (BDEs) were measured to be 7.5±0.5, 9.7±0.6, 9.3±0.7, 8.5±0.4, 8.2±0.4, and 8.7±0.4 kcal/mol for respectively. The relatively low value of the BDE is due to a curve crossing from the ground state to the first excited asymptote with the addition of the first ligand. The first BDE is 10 kcal/mol when measured with respect to the excited state asymptote. This series of almost constant BDEs is unlike any other series. The present DFT calculations show these relatively constant BDE values for the clusters are due to an electronic occupation which allows the ion to interact equally with up to six ligands. Bond lengths, geometries, and vibrational frequencies from the DFT calculations are reported here for all clusters. The influence of basis set size and computational method on the first two clusters was also examined. It was determined that a multireference wave function was required to describe these first two clusters accurately. A possible crossing to the lowest doublet potential energy surface was examined for and found to be endoergic.

Quantum simulations of energy transfer and statetostate transitions in collision of an atom with a large anharmonic cluster:
View Description Hide DescriptionA timedependent selfconsistent field approach is used to simulate a He atom colliding with an cluster. Direct energy transfer during the collision, and energy redistribution among the vibrational degrees of freedom of the anharmonic cluster following the collision, are studied. An important advantage of the method used is that quantum statetostate transition cross sections can be computed for large systems. The following main results are obtained: (1) The process can be interpreted in terms of a direct collision, followed by postcollision energy redistribution in a description that appears only when the cluster vibrations are not described by the eigenstates of this system. A time scale of one picosecond is found for the postcollision intracluster energy distribution. (2) The longtime final state distribution of is less state selective than the distribution immediately after the impact, but it is also not completely statistical. (3) There are statetostate transitions having cross sections of observable magnitude. (4) The dominant transitions are those involving zero, one, and two “phonon” excitations. Some of the “two phonon” excitations have cross sections comparable to strong “single phonon” transitions. (5) Different types of modes show different propensities for excitations in the collision, in close relation to the geometric character of the modes. The results show that the TDSCF approximation is a powerful tool for treating both direct collision dynamics and collisioninduced dynamics in scattering of large anharmonic systems.

Improving harmonic vibrational frequencies calculations in density functional theory
View Description Hide DescriptionUsing a previously introduced weight scheme, microbatching, and grid compression [R. E. Stratmann, G. E. Scuseria and M. J. Frisch, Chem. Phys. Lett. 257, 213 (1996)], we significantly speed up the numerical integration of the exchangecorrelation contribution to the CoupledPerturbed Kohn–Sham equations. In addition, we find that the nature of the integrand is such that it is possible to employ substantially fewer grid points in the quadrature and to use the Gaussian very Fast Multipole Method (GvFMM) with very short multipole expansions for the Coulomb contribution, with negligible loss in accuracy. As a representative example, the computational demand for the exchangecorrelation portion of a coronene (CH frequency calculation with a 321G basis is reduced by more than one order of magnitude. The overall speed up achieved in this calculation is between a factor of 4 to 6, depending on the specific functional. We also present sample calculations using polarized bases, gradientcorrected functionals, and on even larger systems (CH and CH, to illustrate the various effects and improvements that we have accomplished.

Distributions and averages of electron density parameters: Explaining the effects of gradient corrections
View Description Hide DescriptionWe analyze the electron densities of atoms, molecules, solids, and surfaces. The distributions of values of the Seitz radius = and the reduced density gradient =/ in an electron density indicate which ranges of these variables are significant for physical processes. We also define energyweighted averages of these variables, and , from which local spin density (LSD) and generalized gradient approximation (GGA) exchangecorrelation energies may be estimated. The changes in these averages upon rearrangement of the nuclei (atomization of molecules or solids, stretching of bond lengths or lattice parameters, change of crystal structure, etc.) are used to explain why GGA corrects LSD in the way it does. A thermodynamiclike inequality (essentially determines whether the gradient corrections drive a process forward. We use this analysis to explain why gradient corrections usually stretch bonds (but not for example H–H bonds), reduce atomization and surface energies, and raise energy barriers to formation at transition states.