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22 Jun 1997

Volume 106, Issue 24, pp. 9979-10386

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Hyperfine decoupling in electron spin resonance

Gunnar Jeschke and Arthur Schweiger

J. Chem. Phys. 106, 9979 (1997); http://dx.doi.org/10.1063/1.474073 (13 pages) | Cited 12 times

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A 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 S and an arbitrary number of coupled nuclear spins and is used to describe spin decoupling in pulse ESR and ENDOR spectroscopy. Analytical expressions are given for the hyperfine-decoupled nuclear frequencies and the residual hyperfine splittings of spin-1/2 nuclei during strong decoupling. Pulse sequences are proposed for hyperfine-decoupled electron spin echo envelope modulation (ESEEM) and electron nuclear double resonance (ENDOR) experiments as well as for the correlation of the hyperfine-decoupled ESEEM spectrum with two-pulse and three-pulse ESEEM spectra and of hyperfine-decoupled ENDOR with the hyperfine splittings. It is shown that hyperfine-decoupled ESEEM and ENDOR spectra can reveal information on the magnetic quantum numbers involved in an ESR observer transition, and that choosing a transition mS↔mS+1 with mS ≠ −1/2 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 two-dimensional DECENT (decoupled ESEEM correlated to nuclear transition frequencies) experiment is demonstrated on weakly coupled 14N nuclei in both an ordered and a disordered system and on the hexaquo manganese (II) complex (S = 5/2) in a single crystal. It is also shown that for the ESR observer transition mS = (−3/2↔−5/2) the S = 5/2 system yields highly resolved hyperfine-decoupled ENDOR spectra which allow for a complete assignment of the ENDOR lines. © 1997 American Institute of Physics.
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33.35.+r Electron resonance and relaxation
76.30.-v Electron paramagnetic resonance and relaxation
33.15.Pw Fine and hyperfine structure
76.70.Dx Electron-nuclear double resonance (ENDOR), electron double resonance (ELDOR)

Resonance-enhanced multiphoton electron detachment spectra of C7−, C9−, and C11−

M. Ohara, H. Shiromaru, and Y. Achiba

J. Chem. Phys. 106, 9992 (1997); http://dx.doi.org/10.1063/1.474074 (4 pages) | Cited 16 times

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Electronic spectra of C7−, C9−, and C11− were measured in the gas phase for the first time using resonance-enhanced detachment. The band origin of the transition to the lowest excited state for each of these three anions was determined. For C7− and C9−, several other higher excited states were also detected. The 000 transitions for a series of linear chain anions from C5− to C11− were compared with those previously reported for even-numbered series. Slight differences were noted between the even- and odd-numbered series. © 1997 American Institute of Physics.
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36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Wa Charged clusters
33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
31.50.Df Potential energy surfaces for excited electronic states
33.20.Tp Vibrational analysis

A 250 GHz ESR study of o-terphenyl: Dynamic cage effects above Tc

Keith A. Earle, Jozef K. Moscicki, Antonino Polimeno, and Jack H. Freed

J. Chem. Phys. 106, 9996 (1997); http://dx.doi.org/10.1063/1.474114 (20 pages) | Cited 38 times

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Three 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 ortho-terphenyl (OTP) over a wide temperature range from 380 to 180 K. Comparative studies at 9.5 GHz have also been performed. Perdeuterated 2,2′,6,6′-tetramethyl-4-methyl aminopiperidinyl-N-oxide (MOTA), and 3,3-dimethyloxazolidinyl-N-oxy-2′,3-5α-cholestane (CSL) are, respectively, comparable in size to and larger than the OTP host molecule, whereas Perdeuterated 2,2′,6,6′-tetramethyl-4-piperidine-N-oxide (PDT) is substantially smaller. The sensitivity of 250 GHz ESR to the details of the rotational tumbling for T≳Tc (where Tc 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 least-squares analysis shows that below the melting point, Tm, CSL, and MOTA dynamics are well-described 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 Tm. Spectra of PDT do not show any significant non-Brownian 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 Tm is discussed in terms of a model of frustration limited domain sizes proposed by Kivelson and co-workers. Evidence for the existence of substantial voids in OTP below Tm, especially from the spectra of the small PDT probe, is discussed in terms of the structure and packing of the OTP solvent. © 1997 American Institute of Physics.
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33.35.+r Electron resonance and relaxation
76.30.-v Electron paramagnetic resonance and relaxation
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis

The equilibrium structure of silene H2C�SiH2 from millimeter wave spectra and from ab initio calculations

Stephane Bailleux, Marcel Bogey, Jean Demaison, Hans Bürger, Michael Senzlober, Jürgen Breidung, Walter Thiel, Radek Fajgar, and Josef Pola

J. Chem. Phys. 106, 10016 (1997); http://dx.doi.org/10.1063/1.473642 (11 pages) | Cited 15 times

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Silene, H2CSiH2, has been efficiently produced by pyrolysis of 5,6-bis(trifluoromethyl)- 2-silabicyclo[2.2.2]octa-5,7-diene (SBO). Seven isotopomers have been observed by millimeter- and submillimeter-wave 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: re(Si�C) = 1.7039(18) Å, re(C–H) = 1.0819(12) Å, re(Si–H) = 1.4671(9) Å, ∠HCSi = 122.00(4)°, and ∠HSiC = 122.39(3)°. This experimental structure is in excellent agreement with the equilibrium geometry calculated at the CCSD(T) level of theory with a cc-pV(Q,T)Z basis set. This is the first experimental determination without any constraint of the Si�C 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. © 1997 American Institute of Physics.
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33.15.Bh General molecular conformation and symmetry; stereochemistry
33.20.Bx Radio-frequency and microwave spectra
33.20.Vq Vibration-rotation analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.15.A- Ab initio calculations
33.20.Sn Rotational analysis
33.15.Dj Interatomic distances and angles
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.70.Jg Line and band widths, shapes, and shifts
02.60.-x Numerical approximation and analysis

Vibrational anharmonicity and multilevel vibrational dephasing from vibrational echo beats

K. D. Rector, A. S. Kwok, C. Ferrante, A. Tokmakoff, C. W. Rella, and M. D. Fayer

J. Chem. Phys. 106, 10027 (1997); http://dx.doi.org/10.1063/1.474060 (10 pages) | Cited 46 times

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Vibrational echo experiments were performed on the IR active CO stretching modes ( ∼ 2000 cm−1) of rhodium dicarbonylacetylacetonate [Rh(CO)2acac] and tungsten hexacarbonyl [W(CO)6] in dibutylphthalate and a mutant of myoglobin-CO (H64V-CO) 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 25.4 cm−1, for W(CO)6, Rh(CO)2acac, and H64V-CO, respectively. From the components of the nonexponential decays, the vibrational dephasing at very low temperature of both the v = 0–1 and v = 1–2 transitions are determined. At the lowest temperatures, T2 ≈ 2T1, so the v = 2 lifetimes are obtained for the three molecules. These are found to be significantly shorter than the v = 1 lifetimes. Although the v = 1 lifetimes are similar for the three molecules, there is a wide variation in the v = 2 lifetimes. © 1997 American Institute of Physics.
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33.20.Ea Infrared spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis

Fourier-transform microwave spectrum, structure, harmonic force field, and hyperfine constants of sulfur chloride fluoride, ClSF

Jürgen Preusser and Michael C. L. Gerry

J. Chem. Phys. 106, 10037 (1997); http://dx.doi.org/10.1063/1.474061 (11 pages) | Cited 3 times

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The 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 Fourier-transform microwave spectrometer. The unstable sample molecule has been prepared using a pulsed electrical discharge in jets containing a mixture of SF6 and SCl2 in Ne. Besides the parent species 35Cl32S19F, the isotopomers 37Cl32S19F and 35Cl34S19F could be observed in natural abundance. Rotational constants and quartic centrifugal distortion constants as well as nuclear quadrupole coupling constants due to 35Cl and 37Cl and spin–rotation constants due to 35Cl, 37Cl, and 19F are given. The data were used for the determination of r0, rΔP, rs 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 ground-state average structure, rz, and the estimation of the equilibrium structure, re. The rs structure is r(SF)=160.653(162) pm, r(SCl)=199.437(65) pm, and ∠(ClSF)=100.732(81)°. The diagonal elements of the 35Cl/37Cl quadrupole coupling tensors have been obtained, and are interpreted in terms of the bonding at 35Cl/37Cl. Negative 19F spin–rotation constants suggest a close analogy of the electronic structures of ClSF and SF2. © 1997 American Institute of Physics.
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33.20.Bx Radio-frequency and microwave spectra
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
31.30.Gs Hyperfine interactions and isotope effects
33.15.Pw Fine and hyperfine structure
33.20.Sn Rotational analysis

Spectroscopic studies of the math 2A′′-math 2A′′ system of the jet-cooled vinoxy radical

L. R. Brock and Eric A. Rohlfing

J. Chem. Phys. 106, 10048 (1997); http://dx.doi.org/10.1063/1.474091 (18 pages) | Cited 22 times

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We apply several techniques to the study of the math 2A′′-math 2A′′ band system of the jet-cooled vinoxy radical, CH2CHO. Vibronically resolved excitation spectra are obtained using both laser-induced fluorescence (LIF) and a two-color resonant four-wave mixing (TC-RFWM) scheme that provides the nonlinear equivalent of hole-burning spectra. Rotationally resolved LIF spectra recorded at low temperatures ( ⩽ 3 K) provide rotational constants for 9 math-state levels. We also measure the fluorescence lifetimes of 19 math-state levels and obtain high-quality dispersed fluorescence (DF) spectra from seven of the most strongly fluorescing levels in the math 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 3650 cm−1 of the math state and for 57 levels in the first 3100 cm−1 of the math state. These assignments include the identification of the a′ fundamentals for ν4 through ν9 and all three a′′ overtones, 2ν10 through 2ν12, in both states. The differences between the TC-RFWM and LIF spectra and the measured lifetimes indicate a dramatic increase in the predissociation rate of the math state beginning at 1190 cm−1 above the origin. The predissociation rate is markedly mode-specific and is enhanced by out-of-plane excitation, possibly due to vibronic coupling with either the  2A′ or math 2A′ electronic states. The congestion and complexity of the DF spectra at high energies provides direct evidence of extensive intramolecular vibrational redistribution on the ground-state potential surface. © 1997 American Institute of Physics.
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33.50.Dq Fluorescence and phosphorescence spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
33.20.Sn Rotational analysis

Ab initio study of NO2. VI. Vibrational and vibronic coupling in the math2A1/2B2 conical intersection up to 16 000 cm−1

Erminia Leonardi and Carlo Petrongolo

J. Chem. Phys. 106, 10066 (1997); http://dx.doi.org/10.1063/1.474062 (6 pages) | Cited 24 times

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Following our previous work [J. Chem. Phys. 105, 9051 (1996)], we have investigated the vibrational and vibronic mixings in the math2A1/2B2 conical intersection spectrum of NO2. By analyzing the expansion coefficients of the nonadiabatic states, we have discussed the math2A1 vibrational resonances above 4600 cm−1 and the interplay among the math2A1 vibrational and math2A1/2B2 vibronic couplings in the energy range 8900–12 000 cm−1. The assignment and the vibrational mixing of the nonadiabatic bands have been studied by comparing two-dimensional contour plots of Born–Oppenheimer and of generalized vibrational functions. A Dunham fit to the 2B2 vibrational bands has extended our previous assignment and has shown that Fermi, Darling–Dennison, and two new 2B2 vibrational resonances contribute to several nonadiabatic states above 12 000 cm−1. We have finally reported the 2B2 vibrational bands up to 16 000 cm−1 and the corresponding clusters of nonadiabatic levels, which have been compared with some experimental results. © 1997 American Institute of Physics.
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31.15.A- Ab initio calculations
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
31.50.Df Potential energy surfaces for excited electronic states

Electron–electron–nuclear three-spin mixing in spin-correlated radical pairs

Gunnar Jeschke

J. Chem. Phys. 106, 10072 (1997); http://dx.doi.org/10.1063/1.474063 (15 pages) | Cited 20 times

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Electron–electron–nuclear three-spin mixing occurs in radical pairs in solid-state matrices if the radicals feature a significant electron–electron spin coupling and an anisotropic hyperfine coupling. The perturbation of nuclear frequencies by the electron–electron spin coupling and the three-spin mixing have to be generally considered in the calculation of transition frequencies and probabilities in such radical pairs. Analytical descriptions of three-spin 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 three-spin 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 three-spin mixing is significant, spin-correlated radical pairs are born in a state that features nuclear coherence in addition to the electron spin zero-quantum 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 microwave irradiation. 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 three-spin mixing is significant. The novel CINC, CIDNC, and CIDNP effects in the solid state might be used in the structure determination of spin-correlated radical pairs with applications to photosynthesis research. © 1997 American Institute of Physics.
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33.35.+r Electron resonance and relaxation
33.40.+f Multiple resonances (including double and higher-order resonance processes, such as double nuclear magnetic resonance, electron double resonance, and microwave optical double resonance)
33.25.+k Nuclear resonance and relaxation
33.57.+c Magneto-optical and electro-optical spectra and effects

Photodissociation spectroscopy and dynamics of the HCCO free radical

David L. Osborn, David H. Mordaunt, Hyeon Choi, Ryan T. Bise, Daniel M. Neumark, and Celeste McMichael Rohlfing

J. Chem. Phys. 106, 10087 (1997); http://dx.doi.org/10.1063/1.474064 (12 pages) | Cited 24 times

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The photodissociation spectroscopy and dynamics of the HCCO radical have been investigated using fast radical beam photofragment translational spectroscopy. An electronic band with origin at 33 424 cm−1 has been identified. This band exhibits rotational resolution near the band origin, but the well-defined rovibronic structure is homogeneously broadened at higher photon energies. Based on the rotational structure this band is assigned to the math 2Π←math 2A′′ transition. Photofragment translational energy and angular distributions were obtained at several excitation energies. At excitation energies close to the origin, the excited, spin-forbidden CH(a 4Σ−)+CO channel dominates, while the ground state CH(X 2Π)+CO channel is the major channel at higher photon energies. The translational energy distributions provide evidence of competition between intersystem crossing and internal conversion dissociation mechanisms, with some evidence for nonstatistical dynamics in the CH(X 2Π)+CO channel. This work yields an improved heat of formation for HCCO, ΔHf,2980 = 1.83±0.03 eV. © 1997 American Institute of Physics.
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33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.50.Hv Radiationless transitions, quenching
33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis
33.70.Jg Line and band widths, shapes, and shifts
31.90.+s Other topics in the theory of the electronic structure of atoms and molecules (restricted to new topics in section 31)
82.20.Kh Potential energy surfaces for chemical reactions
82.60.Cx Enthalpies of combustion, reaction, and formation

Finding transition states using contangency curves

Alex Ulitsky and David Shalloway

J. Chem. Phys. 106, 10099 (1997); http://dx.doi.org/10.1063/1.474043 (6 pages) | Cited 9 times

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We present a new method for approximating reaction paths and transition states of conformational transitions in polyatomic systems. The method uses the quasi-harmonic properties of the metastable state to construct a bi-Gaussian 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 twisted-boat isomers of cyclohexane. © 1997 American Institute of Physics.
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82.20.Db Transition state theory and statistical theories of rate constants
82.30.Qt Isomerization and rearrangement

The photodissociation of SiO

G. Jolicard, J.-M. Zucconi, I. Drira, A. Spielfieldel, and N. Feautrier

J. Chem. Phys. 106, 10105 (1997); http://dx.doi.org/10.1063/1.474044 (8 pages) | Cited 4 times

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The photodissociation of the SiO molecule, which is of considerable astrophysical interest, is studied within the framework of the Floquet theory. Only the one-photon transition process between the two lowest 1Σ+ 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 E 1Σ+ surface. © 1997 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
31.50.Df Potential energy surfaces for excited electronic states

Improved treatment of inertia and non-Markovian effects on short-time dynamics of diffusion-controlled reaction based on generalized diffusion equation

Kazuyasu Ibuki and Masakatsu Ueno

J. Chem. Phys. 106, 10113 (1997); http://dx.doi.org/10.1063/1.474045 (10 pages) | Cited 13 times

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Starting from a generalized diffusion equation and the Collins–Kimball boundary condition, we investigated the inertia and the non-Markovian effects on the time-dependent rate constant of a diffusion-controlled reaction at short times. In the short-time 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 non-Markovian 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 non-Markovian 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. © 1997 American Institute of Physics.
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05.60.-k Transport processes
66.10.C- Diffusion and thermal diffusion
66.30.-h Diffusion in solids
82.30.-b Specific chemical reactions; reaction mechanisms
82.20.-w Chemical kinetics and dynamics

Translational energy distributions of the products of the 193 and 157 nm photodissociation of chloroethylenes

Kei Sato, Shigeru Tsunashima, Toshiyuki Takayanagi, Ginji Fujisawa, and Atsushi Yokoyama

J. Chem. Phys. 106, 10123 (1997); http://dx.doi.org/10.1063/1.473643 (11 pages) | Cited 19 times

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The 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 time-of-flight spectra of C2H2+, Cl+, HCl+, C2HCl+, and C2Cl2+ 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 cis-DCE, 1,1-DCE, and TCE, while only the C–Cl bond rupture occurred for trans-DCE. This result suggests that the 157 nm photodissociation of trans-DCE proceeds via the direct photodissociation following the photoexcitation to the repulsive 1nσ∗ state. A minor C–H bond rupture was also found in the 157 nm photodissociations of cis-DCE and TCE. On the basis of the present mechanisms, the translational energy distributions and the branching ratios were estimated for all the possible processes. © 1997 American Institute of Physics.
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82.20.Rp State to state energy transfer
33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light

First-order intermolecular diatomics-in-molecule potentials. Potential energy surfaces, spectra, and fragmentation dynamics of the Ne⋯Cl2 complex

A. A. Buchachenko and N. F. Stepanov

J. Chem. Phys. 106, 10134 (1997); http://dx.doi.org/10.1063/1.474059 (11 pages) | Cited 15 times

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First-order perturbative approximations to the diatomics-in-molecule (DIM) approach are implemented for studying interactions between the neon atom and chlorine molecule in the X 1Σg+(0+) and B 3Πu(0+) states. Intermolecular DIM perturbation theory (IDIM PT1) [J. Chem. Phys. 104, 9913 (1996)], which accounts for the atomic component of spin-orbit interaction, is compared to the anisotropic model by Naumkin and Knowles [J. Chem. Phys. 103, 3392 (1995)] which is proven to be a first-order approximation to the nonrelativistic DIM approach. An importance of the spin-orbit effects for the ground-state potential energy surface (PES) is demonstrated. Semiempirical PESs are used in the accurate quantum calculations on the vibrationally averaged geometry, B←X vibronic spectra, and vibrational predissociation dynamics of the Ne⋯Cl2 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 DIM-based PESs to the results of ab initio calculations are discussed. © 1997 American Institute of Physics.
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31.90.+s Other topics in the theory of the electronic structure of atoms and molecules (restricted to new topics in section 31)
34.20.Gj Intermolecular and atom-molecule potentials and forces
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Rate coefficients for the endothermic reactions C+(2P)+H2(D2)→CH+(CD+)+H(D) as functions of temperature from 400–1300 K

Peter M. Hierl, Robert A. Morris, and A. A. Viggiano

J. Chem. Phys. 106, 10145 (1997); http://dx.doi.org/10.1063/1.474093 (8 pages) | Cited 10 times

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We have measured the bimolecular rate coefficients for the reactions of C+(2P) with H2 and D2 as functions of temperature from 400 to 1300 K using a high temperature flowing 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 beam measurements. 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 v = 1 state enhances the rate coefficients by a factor of ∼1000 for the reaction with H2 and by ∼6000 for the reaction with D2 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 three-body association reaction of C+(2P) with H2 in a helium buffer over the temperature range 300–600 K. © 1997 American Institute of Physics.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.20.-w Chemical kinetics and dynamics

Binding energies of Ti+(H2)1–6 clusters: Theory and experiment

John E. Bushnell, Philippe Maître, Paul R. Kemper, and Michael T. Bowers

J. Chem. Phys. 106, 10153 (1997); http://dx.doi.org/10.1063/1.474046 (15 pages) | Cited 21 times

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Formation of Ti+(H2)n clusters (n = 1–6) has been studied by both temperature-dependent 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 n = 1–6, respectively. The relatively low value of the n = 1 BDE is due to a curve crossing from the Ti+[a4F(sd2)] ground state to the Ti+[b4F(d3)] 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 M+(H2)n series. The present DFT calculations show these relatively constant BDE values for the Ti+(H2)n clusters are due to an electronic occupation which allows the Ti+ ion to interact equally with up to six H2 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 Ti+(H2)4 and found to be endoergic. © 1997 American Institute of Physics.  
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36.40.-c Atomic and molecular clusters
31.15.E- Density-functional theory
31.90.+s Other topics in the theory of the electronic structure of atoms and molecules (restricted to new topics in section 31)
33.15.Dj Interatomic distances and angles

Quantum simulations of energy transfer and state-to-state transitions in collision of an atom with a large anharmonic cluster: He+Ar13

A. Y. Rom and R. B. Gerber

J. Chem. Phys. 106, 10168 (1997); http://dx.doi.org/10.1063/1.474100 (7 pages) | Cited 5 times

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A time-dependent self-consistent field approach is used to simulate a He atom colliding with an Ar13 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 state-to-state 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 post-collision energy redistribution in Ar13 , 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 post-collision intracluster energy distribution. (2) The long-time final state distribution of Ar13 is less state selective than the distribution immediately after the impact, but it is also not completely statistical. (3) There are state-to-state 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 collision-induced dynamics in scattering of large anharmonic systems. © 1997 American Institute of Physics.
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34.50.Ez Rotational and vibrational energy transfer
34.50.-s Scattering of atoms and molecules
31.15.xr Self-consistent-field methods
36.40.-c Atomic and molecular clusters

Improving harmonic vibrational frequencies calculations in density functional theory

R. Eric Stratmann, John C. Burant, Gustavo E. Scuseria, and Michael J. Frisch

J. Chem. Phys. 106, 10175 (1997); http://dx.doi.org/10.1063/1.474047 (9 pages) | Cited 19 times

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Using 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 exchange-correlation contribution to the Coupled-Perturbed 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 exchange-correlation portion of a coronene (C24H12) frequency calculation with a 3-21G 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, gradient-corrected functionals, and on even larger systems (C54H18 and C96H24), to illustrate the various effects and improvements that we have accomplished. © 1997 American Institute of Physics.
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31.15.E- Density-functional theory
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.15.vq Electron correlation calculations for polyatomic molecules
31.15.xr Self-consistent-field methods
31.15.V- Electron correlation calculations for atoms, ions and molecules

Distributions and averages of electron density parameters: Explaining the effects of gradient corrections

Ales Zupan, Kieron Burke, Matthias Ernzerhof, and John P. Perdew

J. Chem. Phys. 106, 10184 (1997); http://dx.doi.org/10.1063/1.474101 (10 pages) | Cited 46 times

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We analyze the electron densities n(r) of atoms, molecules, solids, and surfaces. The distributions of values of the Seitz radius rs=(3/4πn)1/3 and the reduced density gradient s=∣∇n∣/(2(3π2)1/3n4/3) in an electron density indicate which ranges of these variables are significant for physical processes. We also define energy-weighted averages of these variables, 〈rs〉 and 〈s〉, from which local spin density (LSD) and generalized gradient approximation (GGA) exchange-correlation 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 thermodynamic-like inequality (essentially d〈s〉/〈s〉>d〈rs〉/2〈rs〉) 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. © 1997 American Institute of Physics.
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31.15.E- Density-functional theory
33.15.Dj Interatomic distances and angles
61.50.Lt Crystal binding; cohesive energy
68.03.Cd Surface tension and related phenomena
68.35.Md Surface thermodynamics, surface energies

Charge penetration in dielectric models of solvation

Daniel M. Chipman

J. Chem. Phys. 106, 10194 (1997); http://dx.doi.org/10.1063/1.474048 (13 pages) | Cited 38 times

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Dielectric continuum models are widely used for treating solvent effects in quantum chemical calculations of solute electronic structure. These invoke a reaction field wherein solute-solvent electrostatic interactions are explicitly or implicitly described by means of certain apparent polarization charges. Most implementations represent this polarization through an apparent surface charge distribution spread on the boundary of the cavity that nominally encloses the solute. However, quantum chemical calculations usually lead to a tail of the wave function penetrating outside the cavity, thereby causing an additional volume polarization contribution to the reaction field that is rarely recognized or treated. In principle the volume polarization should be represented by a certain apparent volume charge distribution spread throughout the entire dielectric medium. It is shown here that this effect can be closely simulated by means of a certain additional apparent surface charge distribution. This provides a convenient and efficient route to treat volume polarization in practice. A very simple approximation to this correction can be obtained from knowledge only of the amount of penetrating solute charge. This supplies a theoretical context as well as justification for the concept of surface charge renormalization that some workers have advocated. The analysis also points to a new prescription for properly making this renormalization in practice, improving on various ad hoc procedures that have been previously suggested for this purpose. © 1997 American Institute of Physics.
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82.30.Nr Association, addition, insertion, cluster formation
31.70.Dk Environmental and solvent effects
77.22.-d Dielectric properties of solids and liquids

Structure, vertical electron-detachment energy, and O–H stretching frequencies of e+(H2O)12

Jongseob Kim, Jung Mee Park, Kyung Seok Oh, Jin Yong Lee, Sik Lee, and Kwang S. Kim

J. Chem. Phys. 106, 10207 (1997); http://dx.doi.org/10.1063/1.474106 (8 pages) | Cited 44 times

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The first comprehensive ab initio study is performed on an excess electron bound to the water dodecamer to find out if this wet electron can be regarded as a precursor of the fully solvated electron. Various structures of the wet electron are explored using ab initio calculations. Among a number of possible geometries categorized as unbounded, surface, internal, and partially internal excess-electron states, the lowest-energy conformer is predicted to be a structure of a partially internal state. The predicted vertical electron-detachment energy of this structure is in good agreement with the experimental value of Coe et al. [J. Chem. Phys. 92, 3980 (1990)]. This indicates that in the experiment the partially internal excess-electron state structure would have been detected. The electronic structure, interactions between the excess electron and dangling H atoms (e⋯ H interaction), and red-shifts of the O–H stretching frequencies with strong IR intensities are discussed. © 1997 American Institute of Physics.
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82.30.Nr Association, addition, insertion, cluster formation
31.15.A- Ab initio calculations
33.15.Bh General molecular conformation and symmetry; stereochemistry

The effect of two- and three-body interactions in ArnCO2 (n = 1,2) on the asymmetric stretching CO2 coordinate: An ab initio study

Janusz Rak, M. M. Szczȩśniak, Grzegorz Chałasiński, and Sławomir M. Cybulski

J. Chem. Phys. 106, 10215 (1997); http://dx.doi.org/10.1063/1.474105 (7 pages) | Cited 11 times

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The dependence of the two-body and three-body interactions in the ArnCO2 cluster upon the intramolecular asymmetric stretching coordinate of CO2 is studied by the ab initio method. In the T-shaped binary complex Ar–CO2, the influence of the components of the interaction energy on the shift of the asymmetric stretching frequency of CO2 (ν3) is estimated within a one-dimensional vibrational model and compared with the experimental data of Sperhac, Weida, and Nesbitt [J. Chem. Phys. 104, 2202 (1996)]. The interaction energy is dissected into Heitler–London, induction, and dispersion energies and their respective intrasystem correlation corrections. The redshift represents a delicate balance of these effects on the v = 0 and v = 1 levels. The highly correlated treatment is required to describe the dependence of two-body potential upon the stretching coordinate. The supermolecular coupled cluster calculations with the single, double, and noniterative triple excitations reproduce the shift observed by Sperhac et al. with excellent accuracy. In the Ar2CO2 trimer with the two Ar atoms in equatorial positions, the influence of the three-body interaction components on the v = 0 and v = 1 levels is analyzed. A model of the three-body potential, including three nonadditive components, exchange, induction, and dispersion is applied. It describes the departure from additivity of the two-body shifts observed by Sperhac et al. with excellent accuracy. The analytical models of the energy components are also discussed. © 1997 American Institute of Physics.
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36.40.Sx Diffusion and dynamics of clusters
33.20.Tp Vibrational analysis
34.20.Gj Intermolecular and atom-molecule potentials and forces
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
31.15.A- Ab initio calculations
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.70.Jg Line and band widths, shapes, and shifts
31.15.bw Coupled-cluster theory

Ab initio calculations and dynamical tests of a potential energy surface for the Na+FH reaction

Antonio Laganà, José M. Alvariño, M. Luz Hernandez, Paolo Palmieri, Ernesto Garcia, and Teresa Martinez

J. Chem. Phys. 106, 10222 (1997); http://dx.doi.org/10.1063/1.474049 (8 pages) | Cited 15 times

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Ab initio calculations of the potential energy surface for the ground state Na+FH reaction were performed. Calculated potential energy values were fitted using a polynomial in bond order coordinates. Quasiclassical trajectories integrated on the fitted surface were used to calculate reactive properties of the system. Calculated quasiclassical properties agree with available experimental information. Quasiclassical trajectories allowed also a rationalization of the reactive dynamics of the system. © 1997 American Institute of Physics.
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82.20.Kh Potential energy surfaces for chemical reactions
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Fd Collision theories; trajectory models
02.10.De Algebraic structures and number theory

Symmetrized density matrix renormalization group studies of the properties of low-lying states of the poly-para-phenylene system

Y. Anusooya, Swapan K. Pati, and S. Ramasesha

J. Chem. Phys. 106, 10230 (1997); http://dx.doi.org/10.1063/1.474076 (8 pages) | Cited 9 times

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We report the symmetrized density matrix renormalization group (DMRG) study of neutral and doped oligomers of poly-para-phenylene (PPP) system within an extended Hubbard model. Model parameters are determined by comparing the existing results for an interacting small system. We compute a number of properties in the ground state as well as in the one-photon, two-photon and triplet states to completely characterize these states. Bond-order studies show that the lowest two-photon state corresponds to a localized excitation while one-photon and triplet excitations are extended in nature. The bipolaronic state shows clear evidence for charge separation and disproportionation into two polarons. We find that the extended nature of one-photon and triplet states of the neutral system are very similar to those of the bipolaronic ground states. © 1997 American Institute of Physics.
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36.20.Kd Electronic structure and spectra
31.50.Df Potential energy surfaces for excited electronic states
71.38.-k Polarons and electron-phonon interactions
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