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15 Jul 1998

Volume 109, Issue 3, pp. 869-1205

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Low temperature CO oxidation triggered by the gas-phase D atom incident on Pt(111) covered with O2 and CO

Jae-Young Kim and Jihwa Lee

J. Chem. Phys. 109, 869 (1998); http://dx.doi.org/10.1063/1.476626 (4 pages) | Cited 6 times

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Exposing an O2-saturated Pt(111) surface at 85 K to a beam of D atom leads to desorption of O2 and D2O. A series of post D-exposure thermal desorption spectra shows that D2O is produced by consecutive D-addition reactions via adsorbed OD intermediate, i.e., O2mathO(ad)+mathD2O(ad)+D2O(g). When CO is coadsorbed with O2 on Pt(111) at 85 K, the incident D atom also induces prompt desorption of CO2 but not CO. We propose that CO is oxidized by the nascent hot O∗ and OD∗ formed in a highly exothermic initiation reaction D(g)+O2(ad)→DO2 ≠ →O(ad)+OD(ad) with an energy release of ∼−4.6 eV before they become accommodated to the surface. Possible mechanisms for O2 desorption are also briefly discussed. © 1998 American Institute of Physics.
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82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
68.43.-h Chemisorption/physisorption: adsorbates on surfaces
68.03.Fg Evaporation and condensation of liquids
68.43.Mn Adsorption kinetics
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
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Variational resonance valence bond study on the ground state of C60 using the Heisenberg model

N. Flocke, T. G. Schmalz, and D. J. Klein

J. Chem. Phys. 109, 873 (1998); http://dx.doi.org/10.1063/1.476627 (8 pages) | Cited 11 times

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A detailed variational resonance valence bond (RVB) study is performed for the S = 0 ground state of the C60 molecule in the framework of the Heisenberg model. It is shown that the 12 500-dimensional Kekulé space can be divided into two subspaces of respective dimensions 5828 and 6672, of which the first one recovers 99.82% of the energy of the full Kekulé space. This 5828-dimensional subspace is derived from the main Kekulé function, which is formed from spin pairs on hexagon–hexagon bonds only, by simple rotations of the three spin pairs in disjoint sets of hexagonal rings of C60 in all possible ways. This indicates that the concept of the stability of the aromatic sextet still plays an important role even in this nonalternant system. Further, the inclusion of some longer range RVB functions like Dewar-type functions and functions involving Claus structures is investigated, and the effect on the ground-state energy as well as on the nearest neighbor correlation functions is examined. © 1998 American Institute of Physics.
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36.40.Cg Electronic and magnetic properties of clusters
31.15.xt Variational techniques
31.15.xw Valence bond calculations

Rigorous formula for the fast calculation of the electron repulsion integral over the solid harmonic Gaussian-type orbitals

Kazuhiro Ishida

J. Chem. Phys. 109, 881 (1998); http://dx.doi.org/10.1063/1.476628 (10 pages) | Cited 23 times

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A rigorous general formula for calculating the electron repulsion integral (ERI) over the solid harmonic (SH) Gaussian-type orbitals (GTOs) can be derived. A general algorithm can be obtained from this formula named as accompanying coordinate expansion (ACE) b3k3. This algorithm is capable of computing very fast SH-ERIs, especially for SH contracted GTOs. Numerical assessment is shown for the (LL∣LL) class of SH-ERIs (L = 2–5). It is found that the present ACE-b3k3 algorithm is the fastest among all algorithms in the literature in the floating-point-opration (FLOP) count assessment when the degree of contraction is large. © 1998 American Institute of Physics.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics

Derivation of the phase factor and geometrical phase for an N-state degenerate system

Michael Baer

J. Chem. Phys. 109, 891 (1998); http://dx.doi.org/10.1063/1.476629 (4 pages) | Cited 5 times

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This work considers the case of degenerate vibronic coupling of N states all at one single point. For this case we first derived the extended single-state Born–Oppenheimer equation [M. Baer and R. Englman, Chem. Phys. Lett. 265, 105 (1997)] and then showed that such a system, like the two-state system, can be described in terms of a single phase factor and is characterized by a well-defined geometrical phase. © 1998 American Institute of Physics.
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33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions

Calculation of DFT-GIAO NMR shifts with the inclusion of spin-orbit coupling

S. K. Wolff and T. Ziegler

J. Chem. Phys. 109, 895 (1998); http://dx.doi.org/10.1063/1.476630 (11 pages) | Cited 91 times

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A formulation for the calculation of nuclear magnetic resonance (NMR) shielding tensors, based on density functional theory (DFT), is presented. Scalar-relativistic and spin-orbit coupling effects are taken into account, and a Fermi-contact term is included in the NMR shielding tensor expression. Gauge-including atomic orbitals (GIAO) and a frozen-core approximation are used. This formulation has been implemented, and 1H and 13C NMR shifts of hydrogen and methyl halides have been calculated and show good agreement with experiment. 13C NMR shifts of 5d transition metal carbonyls have been calculated and show improved agreement with experiment over previous scalar-relativistic calculations. For the metal carbonyls it is shown explicitly that the combination of spin-orbit coupling and magnetic field mixes spin triplet states into the ground state, inducing a spin density that then interacts with the nuclei of the metal carbonyl via the Fermi-contact term. Results indicate that the Fermi-contact contribution to the 13C NMR of the metal carbonyl ions increases with increasing oxidation state of the ion. It is reasoned that as the oxidation state increases, π back bonding decreases and σ bonding increases, within the metal–carbon bond, thus facilitating a greater transfer of spin density from the metal to the carbon nucleus, and thus increasing the Fermi-contact contribution to the NMR shielding of the carbon. © 1998 American Institute of Physics.
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33.25.+k Nuclear resonance and relaxation
31.15.E- Density-functional theory
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Laser spectroscopy of free pentacene molecules (I): The rotational structure of the vibrationless S1←S0 transition

E. Heinecke, D. Hartmann, R. Müller, and A. Hese

J. Chem. Phys. 109, 906 (1998); http://dx.doi.org/10.1063/1.476631 (6 pages) | Cited 14 times

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The rotational structure of the vibrationless S1←S0 transition of pentacene has been investigated using a strongly collimated seeded supersonic argon beam. Because single rotational lines could not be completely resolved, a band contour analysis was performed. The rotational constants of the electronic ground state X 1A1g were found to be under the asymmetric rotor approximation A″ = 1320.6(9), B″ = 117.97(9), C″ = 108.28(15) MHz, whereas the differences to the first electronic excited state A 1B2u are ΔA = A′−A″ = 13.2(3), ΔB = −0.764(45), ΔC = −0.54(6) MHz. A new value of the band origin was determined to be ν00 = 18 648.996(4) cm−1 and the band type was confirmed to be of type b as proposed by symmetry arguments. Good agreement between observed and calculated spectra was obtained assuming planarity in both ground and excited state. From the fit procedure a rotational temperature of about 7 K was deduced. The nuclear statistical weights of the electronic ground state are reported. © 1998 American Institute of Physics.
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33.20.Kf Visible spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis
33.70.Jg Line and band widths, shapes, and shifts

The G 1Πg state of 7Li2 revisited: Observation and analysis of high vibrational levels

K. Urbanski, S. Antonova, A. M. Lyyra, Li Li, and B. Ji

J. Chem. Phys. 109, 912 (1998); http://dx.doi.org/10.1063/1.476632 (7 pages) | Cited 5 times

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CW optical–optical double resonance has been employed to study the upper portion of the G 1Πg state of 7Li2. The use of fluorescence and ion detection enabled the observation of vibrational levels ranging from v = 20 to 48 and rotational levels ranging from J = 1 to 25. For optical–optical double resonance experiments the Franck–Condon factors between these levels and useful A 1Σu+ state levels (levels that can be reached from the thermally populated levels of the ground state) were exceedingly small. The sensitive ionization detection made possible observation of this upper, anharmonic region of the potential. From the data a Rydberg–Klein–Rees potential curve was constructed which represents over 99% of the potential well depth. A C5 coefficient has been calculated and represents the first experimental determination of a long range coefficient for a doubly excited state in diatomic lithium.© 1998 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
33.40.+f Multiple resonances (including double and higher-order resonance processes, such as double nuclear magnetic resonance, electron double resonance, and microwave optical double resonance)
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
31.90.+s Other topics in the theory of the electronic structure of atoms and molecules (restricted to new topics in section 31)

The effect of spin-orbit coupling on the magnetic properties of H2Ti(μ–H)2TiH2

Simon P. Webb and Mark S. Gordon

J. Chem. Phys. 109, 919 (1998); http://dx.doi.org/10.1063/1.476633 (9 pages) | Cited 2 times

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Excited states of singlet and triplet H2Ti(μ−H)2TiH2 have been calculated using multiconfigurational wave functions. The effects of orbital relaxation are determined by optimizing orbitals for all states separately and comparing to state-averaged calculations, and are found to be small. Dynamic electron correlation included through second-order perturbation theory is found to have a considerable effect on excited state relative energies, but not on the ordering of states. Spin-orbit coupling effects are introduced by a one-electron operator which uses an effective nuclear charge to replace two-electron effects. The resulting splittings of the lowest energy triplet state components are 0.027 and 0.199 cm−1, respectively. The former is due to the angular momentum operator which acts along the Ti–Ti axis; the latter is due almost entirely to the angular momentum operator which acts in the direction perpendicular to the plane of the Ti–H–Ti bridge. An overall ferromagnetic effect of 0.660 cm−1 on the ground state singlet-first excited triplet energy gap is predicted. Orbital interactions responsible for spin-orbit coupling effects are identified. © 1998 American Institute of Physics.
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31.50.Df Potential energy surfaces for excited electronic states

Anomalous enrichment of C2+ ions by laser ablation of graphite in Ar jet

Seung Min Park, Heebyung Chae, Sangwook Wee, and Inhyung Lee

J. Chem. Phys. 109, 928 (1998); http://dx.doi.org/10.1063/1.476634 (4 pages) | Cited 15 times

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The formation mechanisms of C1+, C2+, and C3+ ions by laser ablation of graphite are investigated using time-of-flight (TOF) quadrupole mass spectroscopy. The laser ablation is performed in conjunction with a pulsed Ar expansion to elucidate the third-body effect on the formation of the small carbon cluster ions. Drastic enhancement of the C2+ ion signal is observed by an increase of the local pressure near the target, indicating that C2+ ions are efficiently formed by recombination of carbon atoms and subsequent ionization. The branching ratio of C1+, C2+, and C3+ ions and their mean translational energies are different from those of neutrals. Also, the TOF spectra for Cn+ ions show multiple peak structures, which implies that different mechanisms are involved in the formation of Cn+ ions. © 1998 American Institute of Physics.
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61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
79.20.Ds Laser-beam impact phenomena
34.50.Gb Electronic excitation and ionization of molecules
33.15.Ta Mass spectra
07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment
07.60.-j Optical instruments and equipment

Electron impact ionization of the hydroxyl radical

V. Tarnovsky, H. Deutsch, and K. Becker

J. Chem. Phys. 109, 932 (1998); http://dx.doi.org/10.1063/1.476635 (5 pages) | Cited 16 times

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We employed the fast-neutral-beam technique in a measurement of absolute partial cross sections for the electron-impact ionization and dissociative ionization of the hydroxyl free radical from threshold to 200 eV. The deuterated OD radical rather than the protonated OH radical was used as a target in our studies in order to allow a better separation of the various product ions in our apparatus. The total (single) OD ionization cross section was found to have a value of slightly less than 2.0×10−16 cm2 at 70 eV. The ionization of OD is dominated by the formation of parent ions with a parent ionization cross section of 1.85×10−16 cm2 at this energy. A comparison of the experimentally determined total single OD ionization cross section with a calculated OH cross section using a modified additivity rule showed good agreement in terms of the absolute value and the cross section shape (at least above 50 eV). In the course of this work, we also measured the partial ionization cross sections for the D2O molecule and found good agreement between our cross sections and the most recent measurements of Straub et al. [J. Chem. Phys. 108, 109 (1998)] as well as with recent calculations. © 1998 American Institute of Physics.
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34.80.Gs Molecular excitation and ionization

All the adiabatic bound states of NO2

R. F. Salzgeber, V. Mandelshtam, Ch. Schlier, and H. S. Taylor

J. Chem. Phys. 109, 937 (1998); http://dx.doi.org/10.1063/1.476636 (5 pages) | Cited 28 times

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We calculated all 2967 even and odd bound states of the adiabatic ground state of NO2, using a modification of the ab initio potential energy surface of Leonardi et al. [J. Chem. Phys. 105, 9051 (1996)]. The calculation was performed by harmonic inversion of the Chebyshev correlation function generated by a DVR Hamiltonian in Radau coordinates. The relative error for the computed eigenenergies (measured from the potential minimum), is 10−4 or better, corresponding to an absolute error of less than about 2.5 cm−1. Near the dissociation threshold the average density of states is about 0.2/cm−1 for each symmetry. Statistical analysis of the states shows some interesting structure of the rigidity parameter Δ3 as a function of energy. © 1998 American Institute of Physics.
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31.15.A- Ab initio calculations
03.65.Ge Solutions of wave equations: bound states

Theory for magnetic linear dichroism of electronic transitions between twofold-degenerate molecular spin levels

Emile L. Bominaar, Catalina Achim, and Jim Peterson

J. Chem. Phys. 109, 942 (1998); http://dx.doi.org/10.1063/1.476637 (9 pages) | Cited 1 time

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Magnetic linear dichroism (MLD) spectroscopy is a relatively new technique which previously has been almost exclusively applied to atoms. These investigations have revealed that the study of MLD, in conjunction with electronic absorption and magnetic circular dichroism (MCD) spectroscopies, provides significant additional information concerning the electronic structure of atoms. More recent measurements have indicated that MLD is also observable from transition ions in inorganic compounds and metalloproteins. While the theory for atomic MLD has been worked out in considerable detail during the last two decades, an MLD theory of practical utility for the analysis of the spectra derived from the majority of paramagnetic molecules is not available. In the present contribution, the MLD of an electric-dipole-allowed transition between twofold-degenerate molecular spin levels is analyzed, assuming nonsaturating conditions. As for atomic systems, it is found that the MLD of a single molecule is dominated by the term G0. However, this term vanishes in the powder average evaluated for a randomly oriented ensemble of molecules, leading to a drastic reduction of the MLD differential absorption for systems with spin S = 1/2 compared to that observed for systems with higher ground-state spin. It is found that MLD and MCD spectroscopies on solution samples have complementary spin-state specific sensitivities which suggest that the two methods can be used to selectively probe the individual metal sites in multicenter metalloprotein assemblies. © 1998 American Institute of Physics.
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33.55.+b Optical activity and dichroism
33.57.+c Magneto-optical and electro-optical spectra and effects

Investigations of pure rotational transitions of H2 self-perturbed and perturbed by He. I. Measurement, modeling, and quantum calculations

X. Michaut, R. Saint-Loup, H. Berger, M. L. Dubernet, P. Joubert, and J. Bonamy

J. Chem. Phys. 109, 951 (1998); http://dx.doi.org/10.1063/1.476638 (11 pages) | Cited 12 times

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High resolution stimulated gain Raman spectroscopy is used to investigate the collisional parameters of pure rotational S0(j = 0–4) lines of H2 in pure H2 and H2–He mixture. Measurements are performed between 300 and 1000 K in a density regime where the lines are essentially collisionally broadened (typically 10 amagat). For the first time, these highly accurate measurements of the frequencies of pure rotational lines allow one to correct previously measured values that did not take into account the collisional frequency shift. For both collisional systems, the shifting coefficients exhibit a linear behavior with the square root of temperature, similar to the behavior already observed in the Q branch. The broadening coefficients of the S0 branch increase nonlinearly with temperature contrary to the Q branch. For the H2–He system, both these new S0(j) data and previously measured Q(j) data are analyzed using a modeling of the broadening coefficients in terms of elastic and inelastic contributions. These different contributions are analyzed as a function of temperature and of the rotational quantum number j. Preliminary quantum calculations are used to assess the validity of the model. Further calculations will be presented in paper II. © 1998 American Institute of Physics.
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34.50.Ez Rotational and vibrational energy transfer
42.65.Dr Stimulated Raman scattering; CARS
42.65.Es Stimulated Brillouin and Rayleigh scattering
33.70.Jg Line and band widths, shapes, and shifts
33.20.Fb Raman and Rayleigh spectra (including optical scattering)

A study of Gen− and Gen (n = 2–6) using B3LYP-DFT and CCSD(T) methods: The structures and electron affinities of small germanium clusters

Edet F. Archibong and Alain St-Amant

J. Chem. Phys. 109, 962 (1998); http://dx.doi.org/10.1063/1.476639 (11 pages) | Cited 40 times

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The structures of the anionic germanium Gen− clusters and the corresponding neutral Gen clusters (n = 2–6) have been investigated using B3LYP-DFT and CCSD(T) methods. The 6-311+G(3df ) basis set is employed for the dimers and trimers, while the smaller 6-311+G(d) basis set is used for clusters with n>3. The most stable structures for the germanium cluster anions Ge3−, Ge4−, Ge5−, and Ge6− are found to be C2v(2A1), D2h(2B2g), D3h(2A2″), and D4h(2A2u), respectively. In the case of Ge2−, our calculations show that the low lying 2Πu and 2Σg+ states are within 1 kcal/mol of each other and both states are candidates for the ground state of the anion. The adiabatic electron affinities calculated for the Gen clusters with n = 2,3,4,6 are within 0.1 eV of the corresponding experimental values. Furthermore, the adiabatic excitation energies computed at the CCSD(T) level for the low lying states of Ge3 and Ge4 compare quite well with the assignments of the bands observed in the photoelectron spectra of Ge3− and Ge4− by Burton, Xu, Arnold, and Neumark [J. Chem. Phys. 104, 2757 (1996)]. © 1998 American Institute of Physics.
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36.40.Cg Electronic and magnetic properties of clusters
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
31.15.E- Density-functional theory

Structures and stability of hydrated clusters of hydrogen chloride, HCl(H2O)n, n = 1–5

Suyong Re, Yoshihiro Osamura, Youhei Suzuki, and Henry F. Schaefer

J. Chem. Phys. 109, 973 (1998); http://dx.doi.org/10.1063/1.476640 (5 pages) | Cited 74 times

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The molecular structures of the hydrated clusters of the HCl molecule, HCl(H2O)n, n = 1–5, are examined by employing density functional molecular orbital methods. The most stable structures of the n = 1–3 clusters are found to be of the proton nontransferred type. In the case of the n = 4 cluster, the proton nontransferred and proton transferred structures have nearly similar energies. There are several stable isomers for the n = 5 case and the structures of these isomers are found to be all proton transferred. The relative stabilities of the direct ion-pair H+Cl−(H2O)n and the indirect ion-pair H3O+(H2O)n−1Cl− are discussed in conjunction with their structures. The prediction of the IR spectra of the stable HCl(H2O)n clusters clearly indicate the large red-shifts of the H–Cl stretching and hydrogen-bonded O–H stretching frequencies. © 1998 American Institute of Physics.
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31.15.E- Density-functional theory
36.40.Mr Spectroscopy and geometrical structure of clusters
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.20.Ea Infrared spectra
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Absorption and fluorescence of OClO  2A2–math 2B1 in solid Ne, Ar, and Kr. I. Vibrationally unrelaxed →math emission

Chin-Ping Liu, Lih-Huey Lai, Ying-Yu Lee, Shao-Ching Hung, and Yuan-Pern Lee

J. Chem. Phys. 109, 978 (1998); http://dx.doi.org/10.1063/1.476668 (10 pages) | Cited 8 times

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Dispersed laser-induced fluorescence of the  2A2→math 2B1 transition of OClO in solid Ne in the spectral range 500–770 nm was recorded when the origin at 20 991 cm−1 was excited. Progressions with spacings near 939 and 446 cm−1 are associated with vibrational modes ν1 and ν2 of the math state. A simultaneous fit of both modes yields ω1″ = 957.1±1.4, ω2″ = 452.6±0.4, x11″ = 4.47±0.04, x22″ = 0.54±0.05, and x12″ = 4.00±0.05 cm−1. When the 101 line of the –math system at 21 699 cm−1 was excited, vibrationally unrelaxed emission was observed in the spectral region 480–600 nm. Excitation of the 201 line at 21 284 cm−1 generated weak vibrationally unrelaxed progressions. The visible absorption spectrum of OClO in solid Ne in the region 415–488 nm was recorded with a Fourier-transform spectrometer, yielding ν00 = 20 991.3, ν1′ = 707.9, ν2′ = 292.5, and 2ν3′ = 887.6 cm−1 for the state. Simultaneous fits considering either only ν1 and ν2 modes or all three modes yield corresponding spectral parameters. Similar experiments were performed with OClO in solid Ar and Kr. Pronounced increases in ν1′ (716.0 cm−1 in Ar and 712.5 cm−1 in Kr) and ν2′ (302.3 cm−1 in Ar and 303.0 cm−1 in Kr) and a decrease in ν00 (188 cm−1 and 331 cm−1 red-shifted, respectively) from that in the gas phase indicate substantial perturbation of the state in solid Ar and Kr. An absorption continuum underlying the –math system is attributed to absorption to the math 2A1 state above the predissociation barrier. The phonon interaction increases and the threshold of the continuum decreases as the matrix host is altered from Ne to Ar to Kr. © 1998 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
33.20.Kf Visible spectra

Laser photolysis of OClO in solid Ne, Ar, and Kr. II. Site selectivity, mode specificity, and effects of matrix hosts

Lih-Huey Lai, Chin-Ping Liu, and Yuan-Pern Lee

J. Chem. Phys. 109, 988 (1998); http://dx.doi.org/10.1063/1.476641 (9 pages) | Cited 6 times

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Irradiation of low-lying vibronic levels of  2A2 of OClO isolated in solid Ne, Ar, or Kr produces ClOO. Destruction of OClO and production of ClOO in various matrix sites were monitored with IR absorption lines near 1100 and 1440 cm−1, respectively. For OClO in solid Ar at 5 K, site selectivity is observed after irradiation at the vibronic line associated with a specific site; decreased intensity of the IR line associated with a specific site of OClO correlates well with increased intensity of the corresponding line of ClOO. Similar behavior is observed for OClO in solid Kr except that formation of ClOO in one site (1412.1 cm−1) is invariably dominant. The nature of the carriers of two major sites of ClOO (corresponding to 1416.7 and 1442.5 cm−1 in solid Ar, 1412.1 and 1440.7 cm−1 in solid Kr) is discussed. Temporal profiles of absorbance AOClO and AClOO were probed after various periods of irradiation until approximately 20%–60% of OClO was destroyed. Excitation of the ν3 vibrational mode of OClO  2A2 enhances photodissociation of OClO appreciably. Rates of destruction of OClO in solid Ar or Kr are much smaller than that in solid Ne. The apparent conversion factor, ΔAClOO/(−ΔAOClO) is greater for OClO in solid Ne than that in solid Ar or Kr. Effects of matrix host on photolysis are discussed. © 1998 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
33.70.Fd Absolute and relative line and band intensities
33.20.Ea Infrared spectra
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions

The ground state spectroscopic parameters and molecular geometry of SbH3

L. Fusina, G. Di Lonardo, and P. De Natale

J. Chem. Phys. 109, 997 (1998); http://dx.doi.org/10.1063/1.476642 (7 pages) | Cited 6 times

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The rotation spectrum of stibine, SbH3, in the ground vibrational state was recorded in the region between 30 and 100 cm−1 at an unapodized resolution of about 0.0015 cm−1. ΔJ = +1, ΔK = 0 transitions were measured and assigned up to J″ = 15 and K = 12 for both 121Sb and 123Sb isotopomers. The data of each molecule were analyzed together with the J = 0, 1 rotational transitions reported in the literature, a few A1–A2, K = 3 splitting transitions and K = 1←2, ΔK = 3 “perturbation allowed” transitions measured recently with Fourier transform microwave spectroscopy, and ground state combination differences from the analysis of the high resolution spectra of the stretching and bending fundamental bands. The theoretical models adopted for the analysis included Δk = ±3 or Δk = ±6 interaction terms and correspond to two different reductions of the rotational Hamiltonian. From the obtained parameters, the r0 and the equilibrium structure were also determined for this molecule. © 1998 American Institute of Physics.  
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33.20.Ea Infrared spectra
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
31.30.Gs Hyperfine interactions and isotope effects

Mode-specific tunneling splittings in 9-hydroxyphenalenone: Comparison of two methods for direct tunneling dynamics

Antonio Fernández-Ramos, Zorka Smedarchina, Marek Z. Zgierski, and Willem Siebrand

J. Chem. Phys. 109, 1004 (1998); http://dx.doi.org/10.1063/1.476643 (10 pages) | Cited 23 times

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A benchmark comparison is presented of two direct dynamics methods for proton tunneling, namely variational transition-state theory with semiclassical tunneling corrections (VTST/ST) and the instanton method. The molecules chosen for the comparison are 9-hydroxyphenalenone-d0 and -d1, which have 64 vibrational degrees of freedom and show large tunneling splittings for the zero-point level and several vibrationally excited levels of the electronic ground state. Some of the excited-level splittings are larger and some smaller than the zero-level splitting, illustrating the multidimensional nature of the tunneling. Ab initio structure and force field calculations at the Hartree–Fock/6-31G** level are carried out for the two stationary points of the tunneling potential, viz. the equilibrium configuration and the transition state. The VTST/ST calculations are based on both the small- and the large-curvature approximation; the additional quantum-chemical calculations required at intermediate points of the potential are performed at the semiempirical modified neglect of differential overlap (MNDO)/H2 level. The VTST/ST computations use the MORATE 6.5 code developed by Truhlar and co-workers. The instanton dynamics calculations are based on the method we previously developed and applied to tropolone, among others. It uses the transition state rather than the equilibrium configuration as reference structure and approximates the least action analytically. The computations use our “dynamics of instanton tunneling” (DOIT) code. It is found that the large-curvature approximation and the instanton method both reproduce the observed zero-level splitting of the d0 isotopomer if the calculated barrier is reduced by a factor 0.87. With this adjusted barrier, the instanton method also reproduces the zero-level and excited-level splittings of the d1 isotopomer. However, both the small- and the large-curvature approximations severely underestimate all these splittings. These methods, which use relatively inflexible trajectories, do not handle the isotope effect well and also are not developed to the point where they can deal satisfactorily with vibronic level splittings. In addition, there is a striking difference in efficiency between the two methods: the MORATE 6.5 code took 40 h on an R8000 workstation to perform the dynamics calculations, whereas the DOIT code took less than 1 min and produced superior results. The main reason for this superior performance is ascribed to the effective use made of the least-action principle by the instanton method and to the avoidance of the adiabatic approximation, which is not valid for modes with a frequency equal to or lower than the tunneling-mode frequency. © 1998 American Institute of Physics.
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31.15.A- Ab initio calculations
31.15.xr Self-consistent-field methods
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.30.Gs Hyperfine interactions and isotope effects

Dynamics of tautomerism in porphine: An instanton approach

Zorka Smedarchina, Marek Z. Zgierski, Willem Siebrand, and Pawel M. Kozlowski

J. Chem. Phys. 109, 1014 (1998); http://dx.doi.org/10.1063/1.476644 (11 pages) | Cited 32 times

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Ab initio calculations are reported of the rate of tautomerization by double-hydrogen transfer of porphine and three of its isotopomers. Both synchronous (one-step) and asynchronous (two-step) hydrogen tunneling mechanisms are considered. Geometries and force fields are calculated at the stationary points by means of a nonlocal density functional method that yields accurate equilibrium structures and vibrational spectra. Potential-energy surfaces are constructed in terms of all 73 in-plane normal-mode coordinates at the transition state, the mode with imaginary frequency being taken as the reaction coordinate. Hydrogen tunneling calculations are performed by means of a simplified instanton method that has proved reliable in calculations on smaller systems. The full multidimensional potential is used, and adiabatic separation of the normal modes from the reaction coordinate is avoided. The coordinates of the transverse modes are coupled linearly to the reaction coordinate and all modes are allowed to mix freely with each other along the reaction path. Direct evaluation of the instanton path is not necessary. To calculate the tunneling rate constant, it is sufficient to evaluate the one-dimensional instanton action along the reaction coordinate and to correct it for coupling with transverse vibrations. This makes the method computationally very efficient compared to other multidimensional approaches. For the synchronous mechanism, the calculations closely follow the previously established procedure, but for the asynchronous mechanism, generalization to an asymmetric barrier is required. This is achieved by dividing the normal-mode displacements that determine the couplings into symmetric and antisymmetric components which enhance and suppress the tunneling rate, respectively. The relative energies at the stationary points of the density-functional potential are calculated both by density functional theory (DFT) and by the Hartree–Fock method at the DFT geometry. The two methods yield results that are quite different. Comparison with a large set of experimental data comprising four isotopomers and a wide range of temperatures, indicates that neither method yields accurate energies but that some adjustment of the barrier height and the cis–trans energy difference is necessary to obtain satisfactory rate constants for the asynchronous mechanism. The other calculated parameters are used without adjustment. All parameters are combined to construct the potential required for the instanton calculations. A good fit to all available kinetic data is obtained, indicating that the method accounts accurately both for the isotope and the temperature dependence of the rate of tautomerization. It is shown that, in order to achieve this result, it is essential to include all linear couplings, since the balance between symmetric couplings, which enhance the tunneling rate, and antisymmetric couplings, which suppress it, varies between isotopomers. All dynamics calculations are performed with a newly developed code, which is designed to use the output of standard quantum-chemical codes and requires only minutes of CPU time on a standard workstation. © 1998 American Institute of Physics.
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31.15.A- Ab initio calculations
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
82.30.Qt Isomerization and rearrangement
31.15.E- Density-functional theory

Vibronic structure of the valence π-photoelectron bands in furan, pyrrole, and thiophene

A. B. Trofimov, H. Köppel, and J. Schirmer

J. Chem. Phys. 109, 1025 (1998); http://dx.doi.org/10.1063/1.476645 (16 pages) | Cited 37 times

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The 2A2 and 2B1 states formed in the ionization of the outermost π orbitals in furan, pyrrole and thiophene are shown to interact vibronically via nontotally symmetric b2 vibrational modes. The interaction is strongest in pyrrole and thiophene, where the conical intersection between the two adiabatic surfaces occurs near the minimum of the upper (2B1) state. The resulting nonadiabatic effects manifest themselves in the 2B1 bands by a lack of resolved structure in case of pyrrole and thiophene, and by a line broadening in case of furan. The spectra are investigated using a linear vibronic coupling model. All totally symmetric a1 (tuning) modes and nontotally symmetric b2 (coupling) modes describing the ring motion are taken into account. The parameters of the model are obtained with the aid of ab initio calculations. The ground state optimized geometries and vibrational frequencies are computed at the level of the second-order Møller–Plesset perturbation theory, while the dependence of the ionization energies on the nuclear configuration is evaluated using the outer valence Green’s function method. Where appropriate, assignments of the observed structure are given. © 1998 American Institute of Physics.
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33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
31.15.A- Ab initio calculations
33.70.Jg Line and band widths, shapes, and shifts

Vibrational structure in the carbon 1s ionization of hydrocarbons: Calculation using electronic structure theory and the equivalent-cores approximation

T. Darrah Thomas, Leif J. Saethre, Stacey L. Sorensen, and Svante Svensson

J. Chem. Phys. 109, 1041 (1998); http://dx.doi.org/10.1063/1.476646 (11 pages) | Cited 58 times

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A simple ab initio procedure is used to calculate the vibrational structure observed in the carbon 1s ionization of seven hydrocarbons (methane, deuteromethane, ethane, ethene, deuteroethene, ethyne, and deuteroethyne), with good agreement between experiment and theory. The method involves use of the equivalent-cores approximation, localized holes in molecules with equivalent carbons, and the harmonic oscillator approximation. The approach provides insight into the vibrational modes of the core-ionized molecules. It is potentially useful in extracting carbon 1s ionization energies from spectra from molecules having inequivalent carbons or in finding information on inner-hole lifetimes from inner-shell spectra. © 1998 American Institute of Physics.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.15.A- Ab initio calculations
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Generalized thermodynamic perturbation theory for polyatomic fluid mixtures. I. Formulation and results for chemical potentials

William R. Smith, Ivo Nezbeda, Martin Strnad, Bohumil Tříska, Stanislav Labík, and Anatol Malijevský

J. Chem. Phys. 109, 1052 (1998); http://dx.doi.org/10.1063/1.476647 (10 pages) | Cited 9 times

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For general mixtures of polyatomic molecules and their constituent atoms, we first rigorously derive an exact statistical mechanical result relating the background pair correlation function y(1,2,…,m) to a certain excess chemical potential difference involving its components, βΔμe, extending and generalizing our previous results. Second, using only thermodynamic methods, we develop a perturbation theory for the equation of state (EOS) which involves βΔμe; we then express this EOS in an alternative form involving y(1,2,…,m). The latter form coincides with results recently obtained by Zhou and Stell using a different approach and with the EOS of the Wertheim first-order perturbation theory (TPT1); our approach explicitly exposes the underlying thermodynamic approximations involved. Third, we show for the case of tangent fused-hard sphere (FHS) systems, under the approximation that βΔμe is independent of composition, that implementation of the former form of the theory yields results analytically equivalent to those obtained from the Boublik–Nezbeda (BN) EOS; and that the alternative implementation is only slightly less accurate, due to a (numerically small) internal inconsistency in this EOS. This sheds light on the remarkable accuracy obtained for several previous implementations of TPT1 for such systems. We present new computer simulation results for a particular ternary tangent FHS heteronuclear diatomic mixture, which support the approximation that βΔμe for mixtures of such molecules is nearly composition independent. Finally, for several FHS mixture model systems, we test the Lewis–Randall rule and several other approximations for calculation of the mixture chemical potentials. The Lewis–Randall rule is generally superior for the individual chemical potentials, and is competitive for βΔμe. © 1998 American Institute of Physics.
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61.20.-p Structure of liquids
65.20.-w Thermal properties of liquids
65.40.gd Entropy
05.70.Ce Thermodynamic functions and equations of state
64.10.+h General theory of equations of state and phase equilibria

Crystallization of dipolar hard spheres: Density functional results

Sabine Klapp and Frank Forstmann

J. Chem. Phys. 109, 1062 (1998); http://dx.doi.org/10.1063/1.476648 (8 pages) | Cited 15 times

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We investigate the thermodynamic conditions under which an isotropic dipolar hard sphere fluid freezes into an unpolarized or ferroelectric crystalline state by minimizing a free-energy functional. This functional is based on the direct correlation function of the isotropic fluid, calculated by reference hypernetted chain integral equations. In the framework of our approach, the ferroelectric fluid phase, found in simulations, is surpassed by a ferroelectric body-centered tetragonal crystal. © 1998 American Institute of Physics.
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64.70.D- Solid-liquid transitions
61.20.Ja Computer simulation of liquid structure

Thermodynamics of water-cubic ice and other liquid-solid coexistence in nanometer-size particles

G. P. Johari

J. Chem. Phys. 109, 1070 (1998); http://dx.doi.org/10.1063/1.476649 (4 pages) | Cited 8 times

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When contributions from the interfacial energy become significant and comparable to the bulk energy, liquid and crystalline phases can coexist at a temperature much lower than the usual melting point. A formalism for this coexistence is given, and thermodynamic conditions for the melting of nanometer-size cubic ice crystals are derived when both the ice and water are at an equilibrium vapor pressure. By using the approximate values of surface energy and the enthalpy and entropy of melting, it is shown that nanometer-size water droplets can coexist with cubic ice particles of about the same size at temperatures in the 150–180 K range. The unusually large decrease in the temperature of a liquid-solid phase equilibrium is expected to be a general phenomenon in the nanometer-size films, clusters, and particles of materials.© 1998 American Institute of Physics.
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64.70.D- Solid-liquid transitions
65.20.-w Thermal properties of liquids
65.40.gd Entropy
61.46.-w Structure of nanoscale materials
81.07.-b Nanoscale materials and structures: fabrication and characterization
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