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15 Jul 1997

Volume 107, Issue 3, pp. 675-1034

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Nonadiabatic coupling of the 3p Rydberg and ππ∗ valence states of acetone

Ruth McDiarmid and Xing Xing

J. Chem. Phys. 107, 675 (1997); http://dx.doi.org/10.1063/1.475152 (5 pages) | Cited 6 times

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The 3p Rydberg region of the spectra of acetone and acetone-d6 has been studied by reson-antly enhanced multiphoton ionization and photoacoustic spectroscopies. Differences between the spectra of cold and room temperature samples, between REMPI and PA spectral measure-ments, and between the laser power dependences of the different3p←math origins have been deduced to arise from a nonadiabatic coupling of the A1 3p Rydberg and A1ππ∗ states of acetone. The coupling is proposed to arise from a seam of conical intersection between these two A1 states on the CO stretch–C2CO pyramidalization plane. An analogous seam is proposed to exist between theA1 3d Rydberg and A1 ππ∗ states. © 1997 American Institute of Physics.
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31.50.Df Potential energy surfaces for excited electronic states
33.80.Wz Other multiphoton processes
33.80.Eh Autoionization, photoionization, and photodetachment

Fluorescence excitation spectroscopy of the Ar–HCO(math 2A′,math 2A′) van der Waals complex

Scott A. Wright and Paul J. Dagdigian

J. Chem. Phys. 107, 680 (1997); http://dx.doi.org/10.1063/1.474469 (11 pages) | Cited 3 times

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The fluorescence excitation spectrum of the Ar–HCO van der Waals complex in the vicinity of the math 2A′–math 2A′ 000 band of free HCO is reported. At least eight bands associated with the complex have been detected. From the spectral shift of the lowest energy Ar–HCO band from the origin of the free HCO transition, we estimate the Ar–HCO binding energy in the excited electronic state to be at least 13 cm−1 greater than that in the ground state. Rotational analysis of some of the bands has been carried out, and average Ar–HCO separations ( ∼ 3.7 Å) in both electronic states determined. Several of the bands were assigned as hot bands from the first excited bend–stretch level (K″ = 1) in the ground electronic state. From the derived A rotational constants, we conclude that the ArCO framework has an approximately T-shaped geometry in both electronic states. The decay lifetime of the upper state of the strongest Ar–HCO band was measured and was found to be somewhat smaller than those previously measured for low rotational levels of free HCO. With the high signal-to-noise ratio in this study, it was also possible to observe transitions in the free H13CO isotopomer. A rotational analysis of the math 2A′–math 2A′ 000 band of the H13CO isotopomer was carried out. The isotopic shifts of the origins of the 301 and 201 bands were also measured, and a normal mode analysis of HCO(math) was carried out. © 1997 American Institute of Physics.
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33.50.Dq Fluorescence and phosphorescence spectra
33.70.Jg Line and band widths, shapes, and shifts
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.15.Bh General molecular conformation and symmetry; stereochemistry

CH-stretching overtone spectra and internal methyl rotation in 2,6-difluorotoluene

Chenxi Zhu, Henrik G. Kjaergaard, and Bryan R. Henry

J. Chem. Phys. 107, 691 (1997); http://dx.doi.org/10.1063/1.474434 (11 pages) | Cited 13 times

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Vapor phase overtone spectra of 2,6-difluorotoluene are recorded in the ΔvCH = 2 and 3 regions by conventional near-infrared spectroscopy and in the ΔvCH = 4–6 regions by intracavity dye/titanium: sapphire laser photoacoustic spectroscopy. The spectra are interpreted on the basis of ab initio calculations at the HF/6-31G∗ level. The methyl regions of the spectra are complex due to coupling between the nearly freely rotating methyl rotor and CH stretching. A model has been developed to predict the methyl spectral profiles, which uses the harmonically coupled anharmonic oscillator local mode model and the rigid rotor model for stretching and torsion, respectively. A dipole moment function is formulated which combines a Taylor series for CH stretching and a Fourier series for torsion. The dipole moment function includes both angular dependence and higher order expansion terms in the CH-stretching coordinate. The model is successful in predicting the methyl overtone spectral profiles and attributes these profiles to a very large number of transitions that arise from terms involving torsion–stretching coupling, both in the Hamiltonian and in the dipole moment function. © 1997 American Institute of Physics.
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33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.15.A- Ab initio calculations
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.20.Sn Rotational analysis
33.20.Ea Infrared spectra
33.70.Jg Line and band widths, shapes, and shifts

Measuring Patterson functions of inhomogeneous liquids using the nuclear dipolar field

P. Robyr and R. Bowtell

J. Chem. Phys. 107, 702 (1997); http://dx.doi.org/10.1063/1.474435 (5 pages) | Cited 13 times

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The long-range nuclear dipolar interactions in liquids are not averaged out by molecular motion and give rise to the dipolar field. In nuclear magnetic resonance experiments, this field can be used to probe the structure of heterogeneous samples. In this contribution, we demonstrate theoretically and experimentally how the signal generated by the dipolar field can provide structural information without the use of any model structure. In the limit where the dipolar field weakly perturbs the evolution of the magnetization, and where the molecular motion is not significantly restricted by the structure, the autocorrelation function, or Patterson function, of the spin density can be obtained. The signal generated by the dipolar field is measured as a function of the spatial modulation imposed on the magnetization and an integral transform of the signal amplitude yields the Patterson function. If the structure is anisotropic, a three-dimensional data set has to be acquired and Fourier transformed. If the sample is isotropic, modulation of the magnetization along a single direction is sufficient and the Patterson function can be calculated from a Hankel transform of the signal amplitude. © 1997 American Institute of Physics.
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76.60.-k Nuclear magnetic resonance and relaxation
02.30.Nw Fourier analysis
02.30.Uu Integral transforms
02.30.Vv Operational calculus

Nonlinear optical response of cofacial phthalocyanine dimers and trimers

Eric S. Manas, Frank C. Spano, and Lin X. Chen

J. Chem. Phys. 107, 707 (1997); http://dx.doi.org/10.1063/1.474436 (13 pages) | Cited 17 times

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The effects of intermacrocycle interactions on the second hyperpolarizabilities 〈γ(−ω;ω,−ω,ω)〉 of cofacial phthalocyanine dimers and trimers are studied. A theoretical analysis is presented based on the Frenkel exciton model for a chain of three level molecules. Using a simplified analysis in the static and near-resonant regimes we identify two mechanisms which lead to enhancements in the dimer or trimer value of 〈γ(−ω;ω,−ω,ω)〉 over that of the monomer. The first mechanism is a disruption of the balance between type I and type II terms in the sum over states expression for the second hyperpolarizability tensor γkjih(−ω;ω,−ω,ω), caused by weak intermacrocycle interactions. The second is a near-resonance enhancement of the type II terms due to an intermacrocycle interaction induced shift in the monomer derived two-photon allowed states towards twice the laser photon energy. This analysis is in good agreement with recent degenerate four wave mixing experiments [SPIE Proc. 2527, 61 (1995)] which showed a strong enhancement of 〈γ(−ω;ω,−ω,ω)〉 for SiPcO oligomers as a function of the number of macrocycles. Our calculations suggest that the first mechanism is responsible for the 25-fold monomer to dimer enhancement measured in this system, and that the additional 4-fold enhancement found in going from the dimer to the trimer is primarily the result of the second mechanism. © 1997 American Institute of Physics.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.65.-k Nonlinear optics
42.70.Mp Nonlinear optical crystals
71.35.Aa Frenkel excitons and self-trapped excitons

Vacuum-UV fluorescence spectroscopy of SiF4 in the range 10–30 eV

H. Biehl, K. J. Boyle, D. P. Seccombe, D. M. Smith, R. P. Tuckett, K. R. Yoxall, H. Baumgärtel, and H. W. Jochims

J. Chem. Phys. 107, 720 (1997); http://dx.doi.org/10.1063/1.474437 (10 pages) | Cited 8 times

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The vacuum-UV and visible spectroscopy of SiF4 using fluorescence excitation and dispersed emission techniques is reported. The fluorescence excitation spectrum has been recorded following excitation with synchrotron radiation from the BESSY 1, Berlin source in the energy range 10–30 eV with an average resolution of ∼ 0.05 eV. By comparison with vacuum-UV absorption and electron energy loss spectra, all the peaks in the Rydberg spectra that photodissociate to a fluorescing state of a fragment have been assigned. Dispersed emission spectra have been recorded at the energies of all the peaks in the excitation spectra. Four different decay channels are observed: (a) SiF3 fluorescence in the range 380–650 nm for photon energies around 13.0 eV, (b) SiF2  3B1–math 1A1 phosphorescence in the range 360–440 nm for photon energies in the range 15.2–18.0 eV, (c) SiF2  1B1–math 1A1 fluorescence in the range 210–270 nm for photon energies in the range 17.0–20.0 eV, and (d) emission from the math 2A1 state of SiF4+ predominantly in the range 280–350 nm for photon energies greater than 21.5 eV. These assignments are confirmed by action spectra in which the excitation energy of the vacuum-UV radiation is scanned with detection at a specific (dispersed) wavelength. Using the single-bunch mode of the synchrotron, lifetimes of all the emitting states have been measured. The lifetimes of the unassigned emitting state in SiF3, the  1B1 state of SiF2, and the math 2A1 state of SiF4+ are 3.9±0.7, 11.2±1.5, and 9.16±0.02 ns, respectively. This is the first measurement of the lifetimes of these excited states in SiF3 and SiF2. The decay from the  3B1 state of SiF2 has a fast component of 2.6±0.4 ns. We conclude that the lifetime of the  3B1 state of SiF2 is either as low as 2.6 ns or too high (τ> ∼ 200 ns) to measure with the timing profile of the single-bunch mode of BESSY 1. If the latter interpretation is correct, as seems likely for a spin-forbidden phosphorescence to the 1A1 ground state, the 2.6 ns component could be the lifetime of intersystem crossing from higher vibrational levels of the  3B1 state of SiF2 into its ground state. © 1997 American Institute of Physics.
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33.50.Dq Fluorescence and phosphorescence spectra
34.80.-i Electron and positron scattering
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
31.50.Df Potential energy surfaces for excited electronic states
33.50.Hv Radiationless transitions, quenching
33.15.Mt Rotation, vibration, and vibration-rotation constants

Spin–lattice relaxation study of the photoexcited triplet states in solid C60

Yu. I. Prilutski

J. Chem. Phys. 107, 730 (1997); http://dx.doi.org/10.1063/1.474438 (3 pages)

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The spin–lattice relaxation (SLR) of the photoexcited triplet states in fullerene C60 crystal is investigated at low temperatures and high magnetic fields. The mixing of the translational and rotational motions of the molecules (TRM relaxation mechanism) is chosen as the major phenomenon, leading to the coupling of the spin system with acoustic vibrations. The general expressions of the SLR rates for the direct one-phonon processes are obtained. The results of calculation are in accordance with available experimental data. © 1997 American Institute of Physics.
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76.30.-v Electron paramagnetic resonance and relaxation
76.60.Es Relaxation effects
71.35.-y Excitons and related phenomena

Polarization of emission from asymmetric rotors. II. Vector reorientation through intramolecular coupling and inelastic collisions

Kaspars Truhins, Anthony J. McCaffery, Zeyad T. Alwahabi, and Zaid Rawi

J. Chem. Phys. 107, 733 (1997); http://dx.doi.org/10.1063/1.474439 (11 pages) | Cited 5 times

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We report measurements of the linear and circular polarization ratios from fully resolved rotational levels of the asymmetric rotor NH2 populated by collisions with H atoms. The results compare well with a theoretical model that includes the depolarizing effects of intramolecular coupling of rotational angular momentum N to nuclear and to electron spin. These have a very significant influence on fluorescence polarization. The model also incorporates the tilting of the N vector in the molecule frame that occurs when inter-k stack transitions take place. Changes in N vector orientation are described with the aid of the angular momentum sphere, a classical representation of the motion of the N vector in a molecule fixed frame. The theoretical treatment assumes the classically impulsive limit for the collisional interaction with conservation of the m quantum number along the kinematic apse. This description of the fate of the N vector under the influence of intra- and intermolecular interactions allows stereodynamical conclusions to be drawn from experimental observations of fluorescence polarization. © 1997 American Institute of Physics.
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34.50.-s Scattering of atoms and molecules
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Proton and hydrogen atom transfer in hydrogen bonded clusters: Ammonia as a paradigm

E. M. Snyder and A. W. Castleman

J. Chem. Phys. 107, 744 (1997); http://dx.doi.org/10.1063/1.474440 (7 pages) | Cited 8 times

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The competition between proton and hydrogen atom transfer in ammonia clusters is studied in the excited math′ state using femtosecond pump-probe spectroscopy. Little effect of solvation is seen for the case of proton transfer, while the hydrogen transfer processes display significant dependence on the degree of clustering. The former have lifetimes from 85 to 135 fs for (NH3)3–40 while the latter display values ranging from 300 to 1500 fs. Similar effects are found for deuterated systems, with a relatively large isotope effect for the atom transfer process. The results are consistent with the formation of ion pairs in the excited state and published findings for related phenomena in isolated ion-molecule reactions [Conaway et al., J. Chem. Phys. 87, 3453 (1987)]. © 1997 American Institute of Physics.
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36.40.-c Atomic and molecular clusters
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.30.Nr Association, addition, insertion, cluster formation
07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment
07.60.-j Optical instruments and equipment
33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Wz Other multiphoton processes

Reactant-product decoupling approach to half-scattering problems: Photodissociation of H2O in three dimensions

Dunyou Wang, Wei Zhu, John Z. H. Zhang, and Donald J. Kouri

J. Chem. Phys. 107, 751 (1997); http://dx.doi.org/10.1063/1.474373 (6 pages) | Cited 3 times

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In this paper, we present the RPD (reactant-product decoupling) approach to the calculation of final-state distribution in photodissociation of H2O in three-dimensional space. Although the RPD approach was recently developed for bimolecular state-to-state reactive scattering calculations, its application to photodissociation dynamics is very attractive. Specifically in photodissociation, the interaction (reactant) component wavefunction ψr (which in the present case of photodissociation is replaced by the interaction component ψint) is nonzero only in the strong interaction region, which greatly simplifies the numerical calculation for ψint in comparison to that for ψr in a full bimolecular reactive scattering calculation. In the following report, the time-dependent implementation of the RPD approach to the photodissociation of H2O in three dimensions is given and the calculated rovibrational state distributions of the OH fragment are presented. © 1997 American Institute of Physics.
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82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.20.Vq Vibration-rotation analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants

Unimolecular dissociation of trivalent metal cluster ions: The size evolution of metallic bonding

E. Cottancin, M. Pellarin, J. Lermé, B. Baguenard, B. Palpant, J. L. Vialle, and M. Broyer

J. Chem. Phys. 107, 757 (1997); http://dx.doi.org/10.1063/1.474374 (15 pages) | Cited 5 times

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The unimolecular decomposition of size selected cluster cations of trivalent metals (Aln+, Gan+, and Inn+), induced by high fluence laser ionization, has been investigated in the n = 7 to n = 85, 55, and 75 size ranges, respectively. This method is applied for the first time to photoexcited trivalent clusters generated in an evaporative ensemble and the experimental data cover a size range that was not explored in previous pioneering experiments on their dynamics. Small clusters dissociate through the loss of a neutral or a charged atom whereas clusters larger than a well defined critical size merely dissociate through the first channel. In the framework of the RRK statistical theory, the measured evaporation rates provide some information about the size evolution of the cluster dissociation energies and their ionization potentials in the low size range. The competition between the ion and the atom evaporation is found to be consistent with the size evolution of the ionization potentials independently measured by direct photoionization. The agreement between theory and experiment is discussed in relation to cluster structure, especially in the case of gallium. © 1997 American Institute of Physics.
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36.40.Qv Stability and fragmentation of clusters
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
33.15.Fm Bond strengths, dissociation energies
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

Measurement of the rotational distribution for the OD product from the reaction ND3++D2O→ND4++OD under translationally thermal conditions

Richard J. Green and Richard N. Zare

J. Chem. Phys. 107, 772 (1997); http://dx.doi.org/10.1063/1.474375 (7 pages)

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The state-to-state ion-molecule reaction ND3+(ν2=1)+D2O→ND4++OD(v = 0,N) is investigated. A slowly flowing, 2:1 mixture of ND3 and D2O at a total pressure of 50 mTorr is irradiated with a two-color sequence of laser pulses that prepares ND3+ in either the ν2 = 1 umbrella bending mode or the ground vibrational state by 1+1+1 resonance-enhanced multiphoton ionization via the ND3 and math states. After a delay of 200 ns to allow product buildup, the rotational distribution of the OD(v = 0) product is measured by recording the OD A2Σ+−X2Π laser-induced fluorescence spectrum on the (1,1) band following excitation of the (1,0) band. Rotational distributions are presented for the 2Π3/2 and 2Π1/2 fine-structure states of the OD product for the reaction of the vibrationally excited reactant ion; for the experimentally difficult case of the reactant ion in the ground state, a rotational distribution is presented for the 2Π3/2 fine-structure state of the OD product. For the case of the reaction with excited ND3+, the relative rotational populations are fit to a Boltzmann distribution to yield temperatures of 990±30 K and 890±70 K for the OD 2Π3/2 and 2Π1/2 fine-structure components, respectively. For the ground state ion, such a fit yields a temperature of 700±100 K for the OD 2Π3/2 fine-structure component. The results are compared to an RRKM-type model that predicts a rotational distribution of 800 K, and 940 K for the reaction of ion with ν2 = 0 and ν2 = 1, respectively. The excellent agreement is evidence for reaction through a long-lived complex. © 1997 American Institute of Physics.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
33.50.Dq Fluorescence and phosphorescence spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis
33.20.Tp Vibrational analysis

H atom formation dynamics in the dissociation of CH3–CF2Cl (HCFC-142b) after UV and VUV laser photoexcitation

R. A. Brownsword, M. Hillenkamp, T. Laurent, H.-R. Volpp, J. Wolfrum, R. K. Vatsa, and H.-S. Yoo

J. Chem. Phys. 107, 779 (1997); http://dx.doi.org/10.1063/1.474376 (7 pages) | Cited 9 times

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Using the laser photolysis/laser-induced fluorescence (LIF) “pump-and-probe” technique, the dynamics of H atom formation in the photodissociation of CH3–CF2Cl (HCFC-142b) after excitation at 193 nm and the Lyman-α wavelength were studied under collision-free conditions in the gas-phase at room temperature. The H atoms produced were detected by (2p2P←1s2S)-LIF using tunable narrow-band Lyman-α laser radiation (λLα ≈ 121.6 nm) generated by resonant third-order sum-difference frequency conversion of pulsed dye laser radiation. In the VUV photodissociation experiments the Lyman-α laser radiation was used both to photodissociate the parent molecules and to detect the produced nascent H atoms via laser induced fluorescence. The following quantum yields ΦH for H atom formation were determined by a photolytic calibration method: ΦH(193 nm) = (0.06±0.02) and ΦH(Lα) = (0.53±0.12). From the measured H atom Doppler profiles the average H atom kinetic energy was determined to be ET(193 nm) = (51±10) kJ/mol and ET(Lα) = (72±4) kJ/mol, respectively. © 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
82.20.Hf Product distribution
33.50.Dq Fluorescence and phosphorescence spectra

On the appearance of resonances in reactive scattering: An experimental study of the H+D2→HD+D reaction at collision energies near 1.29 eV

E. Wrede and L. Schnieder

J. Chem. Phys. 107, 786 (1997); http://dx.doi.org/10.1063/1.474378 (5 pages) | Cited 45 times

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The hydrogen exchange reaction H+D2(v = 0,j = 0)→HD(v′,j′)+D was investigated at collision energies between 1.27 and 1.30 eV in a high resolution crossed beam experiment. The angle-velocity distribution of nascent D-atoms was measured using the technique of Rydberg atom time-of-flight spectroscopy. The resolution of this technique allows the identification of individual ro-vibrational states of the associated HD product molecule. Calculations done on the Liu–Siegbahn–Truhlar–Horowitz (LSTH) potential energy surface (PES) explicitly including the Geometric Phase effect predict a resonance in reactive scattering for collision energies close to 1.29 eV. The experimental data do not show signatures of this resonance in the energy range investigated. Instead of this a general good agreement between experiment and theory even on the basis of state-to-state differential cross sections is already found for calculations on the LSTH PES at a collision energy of 1.30 eV not including the Geometric Phase indicating that this effect does not play an important role at these collision energies. © 1997 American Institute of Physics.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
33.15.Mt Rotation, vibration, and vibration-rotation constants
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies

Semiclassical initial value theory for dissociation dynamics

G. Campolieti and Paul Brumer

J. Chem. Phys. 107, 791 (1997); http://dx.doi.org/10.1063/1.474379 (13 pages) | Cited 33 times

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A time-dependent initial value semiclassical propagator is developed and applied to dissociation dynamics. Numerically implementable formulas are given for computing detailed dissociation dynamics and photofragmentation matrix elements. The method is applied to the study of two- and three-dimensional HOH/HOD photodissociation in the state. In the two-dimensional case, results obtained by a grid-based numerical integration method using relatively few classical trajectories show very good agreement with known quantum results. The three-dimensional study uses a stationary-phase Monte Carlo approach to computing dissociation cross sections. In this case a comparison with exact quantum calculations shows only qualitative agreement. © 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
82.20.-w Chemical kinetics and dynamics
02.60.-x Numerical approximation and analysis

Distributed approximating functional fit of the H3 ab initio potential-energy data of Liu and Siegbahn

Anatoli Frishman, David K. Hoffman, and Donald J. Kouri

J. Chem. Phys. 107, 804 (1997); http://dx.doi.org/10.1063/1.474380 (8 pages) | Cited 23 times

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We report a distributed approximating functional (DAF) fit of the ab initio potential-energy data of Liu [J. Chem. Phys. 58, 1925 (1973)] and Siegbahn and Liu [ibid. 68, 2457 (1978)]. The DAF-fit procedure is based on a variational principle, and is systematic and general. Only two adjustable parameters occur in the DAF leading to a fit which is both accurate (to the level inherent in the input data; RMS error of 0.2765 kcal/mol) and smooth (“well-tempered,” in DAF terminology). In addition, the LSTH surface of Truhlar and Horowitz based on this same data [J. Chem. Phys. 68, 2466 (1978)] is itself approximated using only the values of the LSTH surface on the same grid coordinate points as the ab initio data, and the same DAF parameters. The purpose of this exercise is to demonstrate that the DAF delivers a well-tempered approximation to a known function that closely mimics the true potential-energy surface. As is to be expected, since there is only roundoff error present in the LSTH input data, even more significant figures of fitting accuracy are obtained. The RMS error of the DAF fit, of the LSTH surface at the input points, is 0.0274 kcal/mol, and a smooth fit, accurate to better than 1 cm−1, can be obtained using more than 287 input data points. © 1997 American Institute of Physics.
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31.15.A- Ab initio calculations
31.90.+s Other topics in the theory of the electronic structure of atoms and molecules (restricted to new topics in section 31)

Unimolecular reactions in the gas and liquid phases: A possible resolution to the puzzles of the trans-stilbene isomerization

Gidon Gershinsky and Eli Pollak

J. Chem. Phys. 107, 812 (1997); http://dx.doi.org/10.1063/1.474381 (13 pages) | Cited 20 times

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Previous theoretical and experimental investigations of the trans-stilbene isomerization reaction in the excited S1 state indicated that the gas phase thermal rate at room temperature is much smaller than the thermal rate in the liquid phase. This was based on the observations that: (a) A combination of measured energy-dependent rates and RRKM calculations led to an isolated molecule thermal rate at T = 300 K of 2×109 s−1; (b) An experiment of Balk and Fleming [J. Phys. Chem. 90, 3975 (1986)] in which stilbene vapor at 300 K excited at the S0 to S1 zero point to zero point electronic transition energy (000), gave a lifetime in the excited state of ∼ 780 ps. The liquid state lifetime in ethane is ∼ 30 ps. In this paper we present theoretical computations of the rate in the gas and liquid phases, based on a new potential model of Vachev et al. [J. Phys. Chem. 99, 5247 (1995)]. We find that: (a) RRKM rates are in agreement with measured energy-dependent rates; (b) The thermal rate derived from the new RRKM rates is the same as the thermal rate in liquid ethane; (c) The laser excitation experiment of Balk and Fleming leads to laser cooling of the excited state suggesting that their measured lifetime is longer than the lifetime in the liquid. The surrounding liquid heats up the molecule on a time scale which is faster than the isomerization lifetime. Experiments are suggested to verify this interpretation. © 1997 American Institute of Physics.
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82.30.Qt Isomerization and rearrangement
82.20.Pm Rate constants, reaction cross sections, and activation energies
37.10.Mn Slowing and cooling of molecules
37.10.Pq Trapping of molecules
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
37.10.Vz Mechanical effects of light on atoms, molecules, and ions
31.50.Df Potential energy surfaces for excited electronic states
33.80.-b Photon interactions with molecules

Mean-field molecular dynamics with surface hopping

Oleg V. Prezhdo and Peter J. Rossky

J. Chem. Phys. 107, 825 (1997); http://dx.doi.org/10.1063/1.474382 (10 pages) | Cited 111 times

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Molecular dynamics simulations of many degree of freedom systems are often comprised of classical evolutions on quantum adiabatic energy surfaces with intermittent instantaneous hops from one surface to another. However, since quantum transitions are inherently nonadiabatic processes, the adiabatic approximation underlying the classical equations of motion does not hold in the regions where quantum transitions take place, and the restriction to classical trajectories for adiabatic quantum states is an approximation. Alternatives which employ classical paths that account more fully for nonadiabaticity can be computationally expensive and algorithmically complicated. Here, we propose a new method, which combines the surface hopping idea with the mean-field approximation for classical paths. Applied to three test systems, the method is shown to outperform the methods based on an adiabatic force without significant extra effort. This makes it an appealing alternative for modeling complex quantum–classical processes. © 1997 American Institute of Physics.
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61.20.Ja Computer simulation of liquid structure

Photofragmentation of chlorotoluenes and dichlorobenzenes: Substituent effects on the dissociation mechanism, and angular distribution of the Cl fragment

Teijiro Ichimura, Yuji Mori, Hisanori Shinohara, and Nobuyuki Nishi

J. Chem. Phys. 107, 835 (1997); http://dx.doi.org/10.1063/1.474383 (8 pages) | Cited 30 times

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Time-of-flight spectra of the Cl photofragments were measured for molecular beams of o-, m-, and p-chlorotoluene (ClC6H4CH3) and o-, m-, and p-dichlorobenzene (ClC6H4Cl) irradiated by a 193 nm excimer laser pulse. The observed translational energy distributions of photofragments revealed that these chlorinated benzene derivatives dissociate via three different channels: (1) very fast predissociation and/or a direct dissociation, (2) predissociation through the triplet state, and (3) predissociation via highly excited vibrational levels of the ground electronic state (hot molecules). The three dissociation channels for dichlorobenzene have similar probabilities ( ∼ 0.3) in accord with those for chlorobenzene, indicating no significant change caused by the additional chlorine atom. The methyl substituent on chlorobenzene (chlorotoluene), however, remarkably induces dissociation through triplet states, probably due to the enhanced intersystem crossing by the methyl group. The angular distribution of the photofragment was also measured for p-chlorotoluene and p-dichlorobenzene excited by linearly polarized laser light. Angular distributions of the Cl fragments via the 2nd and 3rd channels were isotropic, while the fastest fragment via the 1st channel has an anisotropic distribution, confirming that the dissociation rate of the 1st channel is shorter than a molecular rotation. © 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
82.20.Hf Product distribution
33.50.Hv Radiationless transitions, quenching

Spatial organization in the A+B→0 reaction under confined-scale mixing

R. Reigada, F Sagués, I. M. Sokolov, J. M. Sancho, and A. Blumen

J. Chem. Phys. 107, 843 (1997); http://dx.doi.org/10.1063/1.474470 (6 pages) | Cited 10 times

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We consider the kinetics of the two-dimensional, stoichiometric A+B→0 reaction under confined-scale turbulent mixing and concentrate on the interplay between the kinetic patterns and the spatial organization of the system. We study the properties of the arising clusters and of the reaction zones, both in the presence and in the absence of mixing. We show that the two- point correlation function CAB(r) = 〈cA(r′+r)cB(r′)〉/〈cA(r)〉2 is closely related to the effective reaction rate, while the functional form of the quartic correlation function Q(r,t) = 〈cA(r′,t)cB(r′,t)cA(r′+r,t)cB(r′+r,t)〉/〈cA2(r,t)cB2(r,t)〉 is connected to the geometry of the reaction zones. We pay special attention to the occurrence of time windows of fast (classical) concentration decay even when the reactants show strong segregation. © 1997 American Institute of Physics.
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82.30.-b Specific chemical reactions; reaction mechanisms
82.20.-w Chemical kinetics and dynamics

First-order one-electron properties in the integral-direct coupled cluster singles and doubles model

Asger Halkier, Henrik Koch, Ove Christiansen, Poul Jørgensen, and Trygve Helgaker

J. Chem. Phys. 107, 849 (1997); http://dx.doi.org/10.1063/1.474384 (18 pages) | Cited 74 times

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An integral-direct implementation of first-order one-electron properties in the coupled cluster singles and doubles (CCSD) model is presented. The implementation increases the range of applicability of CCSD first-order one-electron property calculations significantly compared to nondirect approaches. As an application a thorough basis set investigation is performed on five diatomic molecules at the Hartree–Fock and CCSD levels for the molecular electric dipole moment, the molecular electric quadrupole moment, and the electric field gradient at the nuclei. In general, basis sets of polarized triple-zeta quality are the smallest to be recommended, and the convergence towards the basis set limit is faster at the Hartree–Fock level than at the CCSD level. Among the properties considered, the electric dipole moment is the easiest to converge. The electric dipole and especially the electric quadrupole moment require diffuse functions for high accuracy. With standard basis sets, it is not possible to calculate electric field gradients consistently within three thousandths of an atomic unit of the basis set limit—for this purpose, elaborate nonstandard basis sets are required. The electric field gradients at the nuclei in HCN and the electric dipole moment of the furan molecule are calculated at the CCSD level employing up to 417 basis functions, further demonstrating the large-scale applicability of the implementation. © 1997 American Institute of Physics.
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33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
31.15.bw Coupled-cluster theory
31.15.xr Self-consistent-field methods

Energy switching approach to potential surfaces. II. Two-valued function for the water molecule

A. J. C. Varandas

J. Chem. Phys. 107, 867 (1997); http://dx.doi.org/10.1063/1.474385 (12 pages) | Cited 27 times

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A recently proposed energy switching scheme is used to improve the two-valued many-body expansion potential energy surface of Murrell, Carter, Mills, and Guest [Mol. Phys. 42, 605 (1981)] for H2O by merging it with the spectroscopically accurate polynomial-type form of Polyanski, Jensen, and Tennyson [J. Chem. Phys. 105, 6490 (1996)]. An attempt is also made to improve its long range forces, and Coulombic behavior at the collapsed molecular limits. The resulting ES two-valued surface has almost spectroscopic accuracy up to 13 650 cm−1, and like the original many-body expansion form may be used for studies of reaction dynamics. A brief analysis of the Σ–Π locus of conical intersection is also presented. © 1997 American Institute of Physics.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
02.10.De Algebraic structures and number theory
82.20.Kh Potential energy surfaces for chemical reactions

Fast, accurate semiempirical molecular orbital calculations for macromolecules

Steven L. Dixon and Kenneth M. Merz

J. Chem. Phys. 107, 879 (1997); http://dx.doi.org/10.1063/1.474386 (15 pages) | Cited 69 times

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A detailed review of the semiempirical divide-and-conquer (D&C) method is given, including a new approach to subsetting, which involves dual buffer regions. Comparisons are drawn between this method and other semiempirical macromolecular schemes. D&C calculations are carried out using a basic 32 Mbyte memory workstation on a variety of peptide systems, including proteins containing up to 1960 atoms. Aspects of storage and SCF convergence are addressed, and parallelization of the D&C algorithm is discussed. © 1997 American Institute of Physics.
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36.20.-r Macromolecules and polymer molecules
31.15.xr Self-consistent-field methods
87.15.-v Biomolecules: structure and physical properties

The Cotton-Mouton effect of liquid water. Part I: The dielectric continuum model

Kenneth Ruud, Trygve Helgaker, Antonio Rizzo, Sonia Coriani, and Kurt V. Mikkelsen

J. Chem. Phys. 107, 894 (1997); http://dx.doi.org/10.1063/1.474387 (8 pages) | Cited 11 times

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We present a gauge-origin independent method for calculating the electric-field dependence of the molecular magnetizability—that is, the hypermagnetizability, related to the Cotton–Mouton Effect (CME)—of solvated molecules. In our approach, the solvated molecule is placed in a spherical cavity surrounded by a linear, homogeneous, and polarizable dielectric medium. We apply the model to investigate the dielectric-medium effects on the CME of liquid water. The effects of electron correlation, molecular geometry, and the surrounding dielectric continuum on the hypermagnetizability and the CME are investigated. The change induced in the hypermagnetizability anisotropy by the dielectric medium is the dominating effect, being almost twice as large as the correlation contribution. The combined effect of electron correlation and the dielectric continuum leads to a doubling of the hypermagnetizability anisotropy when going from the SCF gas phase value (Δη = 17.89 a.u.) to the value obtained for the MCSCF wave function in the dielectric medium (Δη = 39.74 a.u.). The effects of change in geometry are shown to be small. Our result for the static Cotton–Mouton constant averaged in the temperature range 283.15 K to 293.15 K, mC = 15.2×10−20 G−2 cm3 mol−1, differs from experiment still by the sign and by a factor of almost 8. The major reason for this discrepancy is the neglect of short-range interactions such as hydrogen bonding and van der Waals interactions not accounted for by the continuum model. © 1997 American Institute of Physics.
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78.20.Ls Magneto-optical effects
77.22.-d Dielectric properties of solids and liquids

Ab initio potential-energy surface and rotationally inelastic integral cross sections of the Ar–CH4 complex

Tino G. A. Heijmen, Tatiana Korona, Robert Moszynski, Paul E. S. Wormer, and Ad van der Avoird

J. Chem. Phys. 107, 902 (1997); http://dx.doi.org/10.1063/1.474388 (12 pages) | Cited 29 times

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Symmetry-adapted perturbation theory has been applied to compute the intermolecular potential-energy surface of the Ar–CH4 complex. The interaction energy, including high-level intramonomer correlation effects, is found to be dominated by the first-order exchange contribution and the dispersion energy. The ab initio potential has four equivalent minima of ϵm = −144.30 cm−1 at Rm = 7.00 bohr, for structures in which the argon atom approaches the face of the CH4 tetrahedron. The computed potential-energy surface has been analytically fitted and used in converged close-coupling calculations to generate state-to-state integral cross sections for rotational excitation of CH4 in collisions with argon. The computed cross sections are generally in good agreement with the experimental data [W. B. Chapman et al., J. Chem. Phys. 105, 3497 (1996)]. Some discrepancies for the smallest cross sections can be explained by the influence of sequential collision channels, with the use of a master equation approach. © 1997 American Institute of Physics.
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34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
34.50.Ez Rotational and vibrational energy transfer
02.30.-f Function theory, analysis
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