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28 Jun 2007

Volume 126, Issue 24, Articles (24xxxx)

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Announcement: Farewell to Editor Donald H. Levy

H. Frederick Dylla

J. Chem. Phys. 126, 240201 (2007); http://dx.doi.org/10.1063/1.2759503 (1 page)

Online Publication Date: 29 June 2007

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Abstract Unavailable
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01.10.Cr Announcements, news, and awards
01.30.-y Physics literature and publications
01.60.+q Biographies, tributes, personal notes, and obituaries
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Editorial

Donald H. Levy

J. Chem. Phys. 126, 240401 (2007); http://dx.doi.org/10.1063/1.2759435 (1 page) | Cited 1 time

Online Publication Date: 29 June 2007

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Abstract Unavailable
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01.10.Cr Announcements, news, and awards
01.30.-y Physics literature and publications
01.60.+q Biographies, tributes, personal notes, and obituaries
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Approaching the full set of energy levels of water

Pavlo Maksyutenko, John S. Muenter, Nikolai F. Zobov, Sergei V. Shirin, Oleg L. Polyansky, Thomas R. Rizzo, and Oleg V. Boyarkin

J. Chem. Phys. 126, 241101 (2007); http://dx.doi.org/10.1063/1.2748751 (4 pages) | Cited 9 times

Online Publication Date: 22 June 2007

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We report here the measurements of rovibrational levels in the electronic ground state of water molecule at the previously inaccessible energies above 26 000 cm−1. The use of laser double-resonance overtone excitation extends this limit to 34 200 cm−1, which corresponds to 83% of the water dissociation energy. We use experimental data to generate a semiempirical potential energy surface that now allows prediction of water levels with sub-cm−1 accuracy at any energy up to the new limit.
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33.20.Vq Vibration-rotation analysis
33.15.Fm Bond strengths, dissociation energies
31.50.Bc Potential energy surfaces for ground electronic states
61.25.Em Molecular liquids

High-energy conformer of formic acid in solid neon: Giant difference between the proton tunneling rates of cis monomer and trans-cis dimer

Kseniya Marushkevich, Leonid Khriachtchev, and Markku Räsänen

J. Chem. Phys. 126, 241102 (2007); http://dx.doi.org/10.1063/1.2752152 (4 pages) | Cited 17 times

Online Publication Date: 22 June 2007

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We study the conformational reorganization of formic acid (FA) in solid neon and report the higher-energy cis-FA monomer and one form of the trans-cis FA dimers. They were prepared by selective vibrational excitation of the trans-FA monomer and trans-trans dimer. The proton tunneling decay of cis-FA monomer is surprisingly very fast in solid neon, two orders of magnitude faster than in solid argon. It was also found that the stability of the trans-cis dimer against proton tunneling is enormously enhanced in solid neon compared to the monomer (by a factor of ∼ 300). These results are discussed in terms of matrix solvation and hydrogen bonding.
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34.50.Ez Rotational and vibrational energy transfer
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Fm Bond strengths, dissociation energies

Structure of coexisting liquid phases of supercooled water: Analogy with ice polymorphs

Pál Jedlovszky, Lívia B. Pártay, Albert P. Bartók, Giovanni Garberoglio, and Renzo Vallauri

J. Chem. Phys. 126, 241103 (2007); http://dx.doi.org/10.1063/1.2753145 (4 pages) | Cited 2 times

Online Publication Date: 29 June 2007

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The structural changes occurring in supercooled liquid water upon moving from one coexisting liquid phase to the other have been investigated by computer simulation using a polarizable interaction potential model. The obtained results favorably compare with recent neutron scattering data of high and low density water. In order to assess the physical origin of the observed structural changes, computer simulation of several ice polymorphs has also been carried out. Our results show that there is a strict analogy between the structure of various disordered (supercooled) and ordered (ice) phases of water, suggesting that the occurrence of several different phases of supercooled water is rooted in the same physical origin that is responsible for ice polymorphism.
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61.20.Ja Computer simulation of liquid structure
64.60.Cn Order-disorder transformations
61.25.Em Molecular liquids

Photostability of amino acids: Internal conversion versus dissociation

Ming-Fu Lin, Cheng-Ming Tzeng, Yuri A. Dyakov, and Chi-Kung Ni

J. Chem. Phys. 126, 241104 (2007); http://dx.doi.org/10.1063/1.2751150 (5 pages) | Cited 6 times

Online Publication Date: 29 June 2007

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Photodissociation dynamics for various tryptophan chromophores was studied at 193 or 248 nm using multimass ion imaging techniques. The competition between internal conversion to the ground electronic state and dissociation from the repulsive excited state reveals size-dependent photostability for these amino acid chromophores. As the size of chromophore increases, internal conversion to the ground state becomes the major nonradiative process. For tryptophan and larger chromophores, dissociation directly from the repulsive state is completely quenched.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.50.Hv Radiationless transitions, quenching
31.15.vj Electron correlation calculations for atoms and ions: excited states
31.15.ve Electron correlation calculations for atoms and ions: ground state
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back to top Theoretical Methods and Algorithms

Calculation of the distribution of eigenvalues and eigenvectors in Markovian state models for molecular dynamics

Nina Singhal Hinrichs and Vijay S. Pande

J. Chem. Phys. 126, 244101 (2007); http://dx.doi.org/10.1063/1.2740261 (11 pages) | Cited 31 times

Online Publication Date: 22 June 2007

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Markovian state models (MSMs) are a convenient and efficient means to compactly describe the kinetics of a molecular system as well as a formalism for using many short simulations to predict long time scale behavior. Building a MSM consists of grouping the conformations into states and estimating the transition probabilities between these states. In a previous paper, we described an efficient method for calculating the uncertainty due to finite sampling in the mean first passage time between two states. In this paper, we extend the uncertainty analysis to derive similar closed-form solutions for the distributions of the eigenvalues and eigenvectors of the transition matrix, quantities that have numerous applications when using the model. We demonstrate the accuracy of the distributions on a six-state model of the terminally blocked alanine peptide. We also show how to significantly reduce the total number of simulations necessary to build a model with a given precision using these uncertainty estimates for the blocked alanine system and for a 2454-state MSM for the dynamics of the villin headpiece.
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87.15.A- Theory, modeling, and computer simulation
87.14.E- Proteins
87.15.H- Dynamics of biomolecules

Time-dependent density-functional theory/localized density matrix method for dynamic hyperpolarizability

Fan Wang, Chi Yung Yam, and GuanHua Chen

J. Chem. Phys. 126, 244102 (2007); http://dx.doi.org/10.1063/1.2746034 (10 pages) | Cited 7 times

Online Publication Date: 22 June 2007

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Time-dependent density-functional theory/localized density matrix method (TDDFT/LDM) was developed to calculate the excited state energy, absorption spectrum and dynamic polarizability. In the present work we generalize it to calculate the dynamic hyperpolarizabilities in both time and frequency domains. We show that in the frequency domain the 2n+1 rule can be derived readily and the dynamic hyperpolarizabilities are thus calculated efficiently. Although the time-domain TDDFT/LDM is time consuming, its implementation is straightforward because the evaluation of the derivatives of exchange-correlation potential with respect to electron density is avoided. Moreover, the time-domain method can be used to simulate higher order response which is very difficult to be calculated with the frequency-domain method.
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33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
31.15.E- Density-functional theory

Steric effect: A quantitative description from density functional theory

Shubin Liu

J. Chem. Phys. 126, 244103 (2007); http://dx.doi.org/10.1063/1.2747247 (5 pages) | Cited 25 times

Online Publication Date: 22 June 2007

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The concepts of steric energy, steric potential, and steric charge are introduced within the density functional theory framework. The steric energy, representing a hypothetical state with all electrons packed into the lowest orbital and other effects entirely excluded, is a measure of the intrinsic space occupied by an electronic system. It is exclusive, repulsive, and extensive, and it vanishes for homogeneous electron gas. When Bader’s zero-flux boundary condition is adopted, atoms in molecules are found to achieve balanced steric repulsion among one another with vanished steric energy density interfaces. A few molecular systems involving conformation changes and chemical reactions have been investigated to examine the relative contribution of the steric and other effects, providing insights for a few controversial topics from a different perspective.
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31.15.E- Density-functional theory
33.15.Bh General molecular conformation and symmetry; stereochemistry

Modeling the adiabatic connection in H2

Michael J. G. Peach, Andrew M. Teale, and David J. Tozer

J. Chem. Phys. 126, 244104 (2007); http://dx.doi.org/10.1063/1.2747248 (9 pages) | Cited 10 times

Online Publication Date: 22 June 2007

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Full configuration interaction (FCI) data are used to quantify the accuracy of approximate adiabatic connection (AC) forms in describing the ground state potential energy curve of H2, within spin-restricted density functional theory (DFT). For each internuclear separation R, accurate properties of the AC are determined from large basis set FCI calculations. The parameters in the approximate AC form are then determined so as to reproduce these FCI values exactly, yielding an exchange-correlation energy expressed entirely in terms of FCI-derived quantities. This is combined with other FCI-derived energy components to give the total electronic energy; comparison with the FCI energy quantifies the accuracy of the AC form. Initial calculations focus on a [1/1]-Padé-based form. The potential energy curve determined using the procedure is a notable improvement over those from existing DFT functionals. The accuracy near equilibrium is quantified by calculating the bond length and vibrational wave numbers; errors in the latter are below 0.5%. The molecule dissociates correctly, which can be traced to the use of virtual orbital eigenvalues in the slope in the noninteracting limit, capturing static correlation. At intermediate R, the potential energy curve exhibits an unphysical barrier, similar to that noted previously using the random phase approximation. Alternative forms of the AC are also considered, paying attention to size extensivity and the behavior in the strong-interaction limit; none provide an accurate potential energy curve for all R, although good accuracy can be achieved near equilibrium. The study demonstrates how data from correlated ab initio calculations can provide valuable information about AC forms and highlight areas where further theoretical progress is required.
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31.15.ve Electron correlation calculations for atoms and ions: ground state
31.15.E- Density-functional theory
33.15.Dj Interatomic distances and angles
33.20.Tp Vibrational analysis
31.15.A- Ab initio calculations

Extrapolating to the one-electron basis-set limit in electronic structure calculations

A. J. C. Varandas

J. Chem. Phys. 126, 244105 (2007); http://dx.doi.org/10.1063/1.2741259 (15 pages) | Cited 45 times

Online Publication Date: 25 June 2007

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A simple, yet reliable, scheme based on treating uniformly singlet-pair and triplet-pair interactions is suggested to extrapolate atomic and molecular electron correlation energies calculated at two basis-set levels of ab initio theory to the infinite one-electron basis-set limit. The novel dual-level method is first tested on extrapolating the full correlation in single-reference coupled-cluster singles and doubles energies for the closed-shell systems CH2(1A1), H2O, HF, N2, CO, Ne, and F2 with correlation-consistent basis sets of the type cc-pVXZ (X = D,T,Q,5,6) reported by Klopper [Mol. Phys. 6, 481 (2001) ] against his own benchmark calculations with large uncontracted basis sets obtained from explicit correlated singles and doubles coupled-cluster theory. Comparisons are also reported for the same data set but using both single-reference Møller-Plesset and coupled-cluster doubles methods. The results show a similar, often better, accordance with the target results than Klopper’s extrapolations where singlet-pair and triplet-pair energies are extrapolated separately using the popular X−3 and X−5 dual-level laws, respectively. Applications to the extrapolation of the dynamical correlation in multireference configuration interaction calculations carried out anew for He, H2, HeH+, He2++, H3+(1 1A′), H3+(1 3A′), BH, CH, NH, OH, FH, B2, C2, N2, O2, F2, BO, CO, NO, BN, CN, SH, H2O, and NH3 with standard augmented correlation-consistent basis sets of the type aug-cc-pVXZ (X = D,T,Q,5,6) are also reported. Despite lacking accurate theoretical or experimental data for comparison in the case of most diatomic systems, the new method also shows in this case a good performance when judged from the results obtained with the traditional schemes which extrapolate using the two largest affordable basis sets. For the Hartree-Fock and complete-active space self-consistent field energies, a simple pragmatic extrapolation rule is examined whose results are shown to compare well with the ones obtained from the best reported schemes.
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31.15.V- Electron correlation calculations for atoms, ions and molecules
31.15.A- Ab initio calculations
31.15.bw Coupled-cluster theory
31.15.xr Self-consistent-field methods

Second- and third-order triples and quadruples corrections to coupled-cluster singles and doubles in the ground and excited states

Toru Shiozaki, Kimihiko Hirao, and So Hirata

J. Chem. Phys. 126, 244106 (2007); http://dx.doi.org/10.1063/1.2741262 (11 pages) | Cited 27 times

Online Publication Date: 26 June 2007

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Second- and third-order perturbation corrections to equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) incorporating excited configurations in the space of triples [EOM-CCSD(2)T and (3)T] or in the space of triples and quadruples [EOM-CCSD(2)TQ] have been implemented. Their ground-state counterparts—third-order corrections to coupled-cluster singles and doubles (CCSD) in the space of triples [CCSD(3)T] or in the space of triples and quadruples [CCSD(3)TQ]—have also been implemented and assessed. It has been shown that a straightforward application of the Rayleigh-Schrödinger perturbation theory leads to perturbation corrections to total energies of excited states that lack the correct size dependence. Approximations have been introduced to the perturbation corrections to arrive at EOM-CCSD(2)T, (3)T, and (2)TQ that provide size-intensive excitation energies at a noniterative O(n7), O(n8), and O(n9) cost (n is the number of orbitals) and CCSD(3)T and (3)TQ size-extensive total energies at a noniterative O(n8) and O(n10) cost. All the implementations are parallel executable, applicable to open and closed shells, and take into account spin and real Abelian point-group symmetries. For excited states, they form a systematically more accurate series, CCSD<CCSD(2)T<CCSD(2)TQ<CCSD(3)T<CCSDT, with the second- and third-order corrections capturing typically ∼ 80% and 100% of such effects, when those effects are large (>1 eV) and the ground-state wave function has single-determinant character. In other cases, however, the corrections tend to overestimate the triples and quadruples effects, the origin of which is discussed. For ground states, the third-order corrections lead to a rather small improvement over the highly effective second-order corrections [CCSD(2)T and (2)TQ], which is a manifestation of the staircase convergence of perturbation series.
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31.15.bw Coupled-cluster theory
31.15.vj Electron correlation calculations for atoms and ions: excited states

Diminished gradient dependence of density functionals: Constraint satisfaction and self-interaction correction

Gábor I. Csonka, Oleg A. Vydrov, Gustavo E. Scuseria, Adrienn Ruzsinszky, and John P. Perdew

J. Chem. Phys. 126, 244107 (2007); http://dx.doi.org/10.1063/1.2743985 (7 pages) | Cited 12 times

Online Publication Date: 26 June 2007

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The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation for the exchange-correlation energy functional has two nonempirical constructions, based on satisfaction of universal exact constraints on the hole density or on the energy. We show here that, by identifying one possible free parameter in exchange and a second in correlation, we can continue to satisfy these constraints while diminishing the gradient dependence almost to zero (i.e., almost recovering the local spin density approximation or LSDA). This points out the important role played by the Perdew-Wang 1991 nonempirical hole construction in shaping PBE and later constructions. Only the undiminished PBE is good for atoms and molecules, for reasons we present, but a somewhat diminished PBE could be useful for solids; in particular, the surface energies of solids could be improved. Even for atoms and molecules, a strongly diminished PBE works well when combined with a scaled-down self-interaction correction (although perhaps not significantly better than LSDA). This shows that the undiminished gradient dependence of PBE and related functionals works somewhat like a scaled-down self-interaction correction to LSDA.
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31.15.E- Density-functional theory

Treatment of geometric singularities in implicit solvent models

Sining Yu, Weihua Geng, and G. W. Wei

J. Chem. Phys. 126, 244108 (2007); http://dx.doi.org/10.1063/1.2743020 (13 pages) | Cited 28 times

Online Publication Date: 28 June 2007

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Geometric singularities, such as cusps and self-intersecting surfaces, are major obstacles to the accuracy, convergence, and stability of the numerical solution of the Poisson-Boltzmann (PB) equation. In earlier work, an interface technique based PB solver was developed using the matched interface and boundary (MIB) method, which explicitly enforces the flux jump condition at the solvent-solute interfaces and leads to highly accurate biomolecular electrostatics in continuum electric environments. However, such a PB solver, denoted as MIBPB-I, cannot maintain the designed second order convergence whenever there are geometric singularities, such as cusps and self-intersecting surfaces. Moreover, the matrix of the MIBPB-I is not optimally symmetrical, resulting in the convergence difficulty. The present work presents a new interface method based PB solver, denoted as MIBPB-II, to address the aforementioned problems. The present MIBPB-II solver is systematical and robust in treating geometric singularities and delivers second order convergence for arbitrarily complex molecular surfaces of proteins. A new procedure is introduced to make the MIBPB-II matrix optimally symmetrical and diagonally dominant. The MIBPB-II solver is extensively validated by the molecular surfaces of few-atom systems and a set of 24 proteins. Converged electrostatic potentials and solvation free energies are obtained at a coarse grid spacing of 0.5 Å and are considerably more accurate than those obtained by the PBEQ and the APBS at finer grid spacings.
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87.15.A- Theory, modeling, and computer simulation
87.14.E- Proteins
87.15.N- Properties of solutions of macromolecules

Decomposition of density matrix renormalization group states into a Slater determinant basis

Gerrit Moritz and Markus Reiher

J. Chem. Phys. 126, 244109 (2007); http://dx.doi.org/10.1063/1.2741527 (16 pages) | Cited 17 times

Online Publication Date: 28 June 2007

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The quantum chemical density matrix renormalization group (DMRG) algorithm is difficult to analyze because of the many numerical transformation steps involved. In particular, a decomposition of the intermediate and the converged DMRG states in terms of Slater determinants has not been accomplished yet. This, however, would allow one to better understand the convergence of the algorithm in terms of a configuration interaction expansion of the states. In this work, the authors fill this gap and provide a determinantal analysis of DMRG states upon convergence to the final states. The authors show that upon convergence, DMRG provides the same complete-active-space expansion for a given set of active orbitals as obtained from a corresponding configuration interaction calculation. Additional insight into DMRG convergence is provided, which cannot be obtained from the inspection of the total electronic energy alone. Indeed, we will show that the total energy can be misleading as a decrease of this observable during DMRG microiteration steps may not necessarily be taken as an indication for the pickup of essential configurations in the configuration interaction expansion. One result of this work is that a fine balance can be shown to exist between the chosen orbital ordering, the guess for the environment operators, and the choice of the number of renormalized states. This balance can be well understood in terms of the decomposition of total and system states in terms of Slater determinants.
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31.15.V- Electron correlation calculations for atoms, ions and molecules

Electronic optical response of molecules in intense fields: Comparison of TD-HF, TD-CIS, and TD-CIS(D) approaches

H. Bernhard Schlegel, Stanley M. Smith, and Xiaosong Li

J. Chem. Phys. 126, 244110 (2007); http://dx.doi.org/10.1063/1.2743982 (13 pages) | Cited 15 times

Online Publication Date: 28 June 2007

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Time-dependent Hartree-Fock (TD-HF) and time-dependent configuration interaction (TD-CI) methods with Gaussian basis sets have been compared in modeling the response of hydrogen molecule, butadiene, and hexatriene exposed to very short, intense laser pulses (760 nm, 3 cycles). After the electric field of the pulse returns to zero, the molecular dipole continues to oscillate due to the coherent superposition of excited states resulting from the nonadiabatic excitation caused by the pulse. The Fourier transform of this residual dipole gives a measure of the nonadiabatic excitation. For low fields, only the lowest excited states are populated, and TD-CI simulations using singly excited states with and without perturbative corrections for double excitations [TD-CIS(D) and TD-CIS, respectively] are generally in good agreement with the TD-HF simulations. At higher field strengths, higher states are populated and the methods begin to differ significantly if the coefficients of the excited states become larger than ∼ 0.1. The response of individual excited states does not grow linearly with intensity because of excited state to excited state transitions. Beyond a threshold in the field strength, there is a rapid increase in the population of many higher excited states, possibly signaling an approach to ionization. However, without continuum functions, the present TD-HF and TD-CI calculations cannot model ionization directly. The TD-HF and TD-CIS simulations are in good accord because the excitation energies obtained by linear response TD-HF [also known as random phase approximation (RPA)] agree very well with those obtained from singly excited configuration interaction (CIS) calculations. Because CIS excitation energies with the perturbative doubles corrections [CIS(D)] are on average lower than the CIS excitation energies, the TD-CIS(D) response is generally stronger than TD-CIS.
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33.80.-b Photon interactions with molecules
31.15.xr Self-consistent-field methods
31.15.vn Electron correlation calculations for diatomic molecules
31.15.vq Electron correlation calculations for polyatomic molecules
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Dihedral angle principal component analysis of molecular dynamics simulations

Alexandros Altis, Phuong H. Nguyen, Rainer Hegger, and Gerhard Stock

J. Chem. Phys. 126, 244111 (2007); http://dx.doi.org/10.1063/1.2746330 (10 pages) | Cited 23 times

Online Publication Date: 29 June 2007

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It has recently been suggested by Mu et al. [Proteins 58, 45 (2005)] to use backbone dihedral angles instead of Cartesian coordinates in a principal component analysis of molecular dynamics simulations. Dihedral angles may be advantageous because internal coordinates naturally provide a correct separation of internal and overall motion, which was found to be essential for the construction and interpretation of the free energy landscape of a biomolecule undergoing large structural rearrangements. To account for the circular statistics of angular variables, a transformation from the space of dihedral angles {φn} to the metric coordinate space {xn = cos φn,yn = sin φn} was employed. To study the validity and the applicability of the approach, in this work the theoretical foundations underlying the dihedral angle principal component analysis (dPCA) are discussed. It is shown that the dPCA amounts to a one-to-one representation of the original angle distribution and that its principal components can readily be characterized by the corresponding conformational changes of the peptide. Furthermore, a complex version of the dPCA is introduced, in which N angular variables naturally lead to N eigenvalues and eigenvectors. Applying the methodology to the construction of the free energy landscape of decaalanine from a 300 ns molecular dynamics simulation, a critical comparison of the various methods is given.
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87.15.B- Structure of biomolecules
87.15.H- Dynamics of biomolecules
36.20.Ey Conformation (statistics and dynamics)
36.20.Hb Configuration (bonds, dimensions)

Zero-variance zero-bias quantum Monte Carlo estimators of the spherically and system-averaged pair density

Julien Toulouse, Roland Assaraf, and C. J. Umrigar

J. Chem. Phys. 126, 244112 (2007); http://dx.doi.org/10.1063/1.2746029 (11 pages) | Cited 13 times

Online Publication Date: 29 June 2007

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We construct improved quantum Monte Carlo estimators for the spherically and system-averaged electron pair density (i.e., the probability density of finding two electrons separated by a relative distance u), also known as the spherically averaged electron position intracule density I(u), using the general zero-variance zero-bias principle for observables, introduced by Assaraf and Caffarel. The calculation of I(u) is made vastly more efficient by replacing the average of the local delta-function operator by the average of a smooth nonlocal operator that has several orders of magnitude smaller variance. These new estimators also reduce the systematic error (or bias) of the intracule density due to the approximate trial wave function. Used in combination with the optimization of an increasing number of parameters in trial Jastrow-Slater wave functions, they allow one to obtain well converged correlated intracule densities for atoms and molecules. These ideas can be applied to calculating any pair-correlation function in classical or quantum Monte Carlo calculations.
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31.10.+z Theory of electronic structure, electronic transitions, and chemical binding
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Single photon ionization of hydrogen bonded clusters with a soft x-ray laser: (HCOOH)x and (HCOOH)y(H2O)z

S. Heinbuch, F. Dong, J. J. Rocca, and E. R. Bernstein

J. Chem. Phys. 126, 244301 (2007); http://dx.doi.org/10.1063/1.2746036 (11 pages) | Cited 12 times

Online Publication Date: 22 June 2007

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Pure, neutral formic acid (HCOOH)n+1 clusters and mixed (HCOOH)/(H2O) clusters are investigated employing time of flight mass spectroscopy and single photon ionization at 26.5 eV using a very compact, capillary discharge, soft x-ray laser. During the ionization process, neutral clusters suffer little fragmentation because almost all excess energy above the vertical ionization energy is taken away by the photoelectron, leaving only a small part of the photon energy deposited into the (HCOOH)n+1+ cluster. The vertical ionization energy minus the adiabatic ionization energy is enough excess energy in the clusters to surmount the proton transfer energy barrier and induce the reaction (HCOOH)n+1+→(HCOOH)nH++HCOO making the protonated (HCOOH)nH+ series dominant in all data obtained. The distribution of pure (HCOOH)nH+ clusters is dependent on experimental conditions. Under certain conditions, a magic number is found at n = 5. Metastable dissociation rate constants of (HCOOH)nH+ are measured in the range (0.1–0.8)×104 s−1 for cluster sizes 4<n<9. The rate constants display an odd/even alternating behavior between monomer and dimer loss that can be attributed to the structure of the cluster. When small amounts of water are added to the formic acid, the predominant signals in the mass spectrum are still (HCOOH)nH+ cluster ions. Also observed are the protonated mixed cluster series (HCOOH)n(H2O)mH+ for n = 1–8 and m = 0–4. A magic number in the cluster series n = 5, m = 1 is observed. The mechanisms and dynamics of formation of these neutral and ionic clusters are discussed.
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36.40.Jn Reactivity of clusters
36.40.Qv Stability and fragmentation of clusters
82.50.-m Photochemistry
33.80.Gj Diffuse spectra; predissociation, photodissociation
82.20.Pm Rate constants, reaction cross sections, and activation energies
33.80.Eh Autoionization, photoionization, and photodetachment

Theoretical investigation of excited and Rydberg states of imidogen radical NH: Potential energy curves, spectroscopic constants, and dipole moment functions

L. C. Owono Owono, N. Jaidane, M. G. Kwato Njock, and Z. Ben Lakhdar

J. Chem. Phys. 126, 244302 (2007); http://dx.doi.org/10.1063/1.2741260 (13 pages) | Cited 7 times

Online Publication Date: 22 June 2007

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A search is conducted for the calculation of potential energy curves (PECs), spectroscopic constants, and dipole moment functions for excited and Rydberg states of imidogen radical NH, with a particular emphasis on the Rydberg states arising from 3s configuration of nitrogen and 2s and 2p configurations of hydrogen. A range of about 11 eV above the electronic ground state X 3Σ− atomic separation limit which corresponds to the first eight asymptotes of dissociation is spanned. Computations are carried out at the internally contracted multireference singles plus doubles configuration interaction level of theory, including the Davidson correction to account for quadruple excitations. The Gaussian basis set used has been modified from a standard basis to give a balanced description of valence-Rydberg interactions. States of 1Σ−, 1Π, 1Δ, 3Σ−, 3Π, 3Δ, and 5Σ− symmetries are computed accurately in the range of energy investigated. PECs of the three lowest 5Π states are obtained for the first time. Our spectroscopic constants show good agreement with experimental data in comparison with other theoretical studies reported in the literature. A discussion on the variations of dipole moment functions helps to understand the strong interactions between excited and Rydberg states as well as the avoided crossings. The present study may be of great practical interest for investigations in astrophysical research as well as in laboratory experiments.
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31.50.Df Potential energy surfaces for excited electronic states
31.15.vj Electron correlation calculations for atoms and ions: excited states
31.50.Gh Surface crossings, non-adiabatic couplings

Frontier electronic structures of Ru(tcterpy)(NCS)3 and Ru(dcbpy)2(NCS)2: A photoelectron spectroscopy study

E. M. J. Johansson, M. Hedlund, M. Odelius, H. Siegbahn, and H. Rensmo

J. Chem. Phys. 126, 244303 (2007); http://dx.doi.org/10.1063/1.2738066 (9 pages) | Cited 10 times

Online Publication Date: 25 June 2007

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The frontier electronic structures of Ru(tcterpy)(NCS)3 [black dye (BD)] and Ru(dcbpy)2(NCS)2 (N719) have been investigated by photoelectron spectroscopy (PES), X-ray absorption spectroscopy (XAS) and resonant photoelectron spectroscopy (RPES). N1s XAS has been used to probe the nitrogen contribution in the unoccupied density of states, and PES, together with RPES over the N1s edge, has been used to delineate the character of the occupied density of states. The experimental findings of the frontier electron structure are compared to calculations of the partial density of states for the nitrogens in the different ligands (NCS and terpyridine/bipyridine) and for Ru4d. The result indicates large similarities between the two complexes. Specifically, the valence level spectra show two well separated structures at low binding energy. The experimental results indicate that the outermost structure in the valence region largely has a Ru4d character but with a substantial character also from the NCS ligand. Interestingly, the second lowest structure also has a significant Ru4d character mixed into the structure otherwise dominated by NCS. Comparing the two complexes the BD valence structures lowest in binding energy contains a large contribution from the NCS ligands but almost no contribution from the terpyridine ligands, while for N719 also some contribution from the bipyridine ligands is mixed into the energy levels.
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71.20.Rv Polymers and organic compounds
79.60.Fr Polymers; organic compounds
78.70.Dm X-ray absorption spectra

Zero electron kinetic energy spectroscopy of the para-fluorotoluene cation

Victoria L. Ayles, Chris J. Hammond, Denis E. Bergeron, Owen J. Richards, and Timothy G. Wright

J. Chem. Phys. 126, 244304 (2007); http://dx.doi.org/10.1063/1.2741542 (9 pages) | Cited 5 times

Online Publication Date: 25 June 2007

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Zero electron kinetic energy (ZEKE) spectroscopy is employed to gain information on the vibrational energy levels of the para-fluorotoluene (pFT) cation. Vibrationally resolved spectra are obtained following excitation through a range of intermediate vibrational energy levels in the S1 state. These spectra allow the observation of different cationic vibrational modes, whose assignment is achieved both from a knowledge of the S1 vibrational states and also by comparison with density functional calculations. In one notable case, clean ZEKE spectra were obtained from two overlapped S1 features. From the authors' data, the adiabatic ionization energy of pFT was derived as 70 946±4 cm−1. The information on the cationic energy levels obtained will be useful in untangling the intramolecular vibrational redistribution dynamics of pFT in the S1 state.
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33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.60.+q Photoelectron spectra

Optimal internal coordinates, vibrational spectrum, and effective Hamiltonian for ozone

José Zúñiga, José Antonio G. Picón, Adolfo Bastida, and Alberto Requena

J. Chem. Phys. 126, 244305 (2007); http://dx.doi.org/10.1063/1.2743441 (19 pages) | Cited 4 times

Online Publication Date: 27 June 2007

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In this paper the authors use the optimal internal vibrational coordinates previously determined for the electronic ground state of the ozone molecule to study the vibrational spectrum of the molecule employing the second empirical potential energy surface calculated by Tyuterev et al. [Chem. Phys. Lett. 316, 271 (2000)] . First, the authors compute variationally all the bound vibrational energy levels of the molecule up to the dissociation limit and state the usefulness of the optimal coordinates in this respect, which allows us to converge all the bound levels using relatively small anharmonic basis sets. By analyzing the expansion coefficients of the wave functions, they show then that a large portion of the vibrational spectrum of O3 can be structured in nearly separable polyadic groups characterized by the polyad quantum number N = n1+n2+nθ corresponding to the optimal internal coordinates. Accordingly, they determine an internal effective vibrational Hamiltonian for O3 by fitting the effective Hamiltonian parameters to the experimental vibrational frequencies, using as input parameters in the fit those extracted from an analytical second-order Van Vleck perturbation theory calculation. It is finally shown that the internal effective Hamiltonian thus obtained accurately describes the vibrational spectrum of ozone in the low and medium energy regimes.
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33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
31.15.xp Perturbation theory

Time-resolved photoelectron imaging of large anionic methanol clusters: (Methanol)n−(n ∼ 145–535)

Aster Kammrath, Graham B. Griffin, Jan R. R. Verlet, Ryan M. Young, and Daniel M. Neumark

J. Chem. Phys. 126, 244306 (2007); http://dx.doi.org/10.1063/1.2747618 (6 pages) | Cited 14 times

Online Publication Date: 27 June 2007

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The dynamics of an excess electron in size-selected methanol clusters is studied via pump-probe spectroscopy with resolution of ∼ 120 fs. Following excitation, the excess electron undergoes internal conversion back to the ground state with lifetimes of 260–175 fs in (CH3OH)n−(n = 145–535) and 280–230 fs in (CD3OD)n−(n = 210–390), decreasing with increasing cluster size. The clusters then undergo vibrational relaxation on the ground state on a time scale of 760±250 fs. The excited state lifetimes for (CH3OH)n− clusters extrapolate to a value of 157±25 fs in the limit of infinite cluster size.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.20.Tp Vibrational analysis
33.60.+q Photoelectron spectra
07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment
07.60.-j Optical instruments and equipment

Observation of the math 1A″ state of isocyanogen

W. Bryan Lynch, Hans A. Bechtel, Adam H. Steeves, John J. Curley, and Robert W. Field

J. Chem. Phys. 126, 244307 (2007); http://dx.doi.org/10.1063/1.2745295 (4 pages) | Cited 2 times

Online Publication Date: 28 June 2007

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The math 1A″ state of isocyanogen, CNCN, is observed using photofragment fluorescence excitation spectroscopy in a room temperature cell and in a molecular beam. The spectra are highly congested, but progressions that correspond to the Franck-Condon active C–N–C bending vibration in the excited state are evident. Linewidth measurements indicate that the excited state lifetime is <10 ps. These measurements are consistent with previous ab initio calculations, which predicted a bent excited state with a short lifetime due to predissociation. Although we do not believe that we have observed the origin band of the electronic transition, we place an upper limit of 42 523 cm−1 on the energy of the excited state zero point level.
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33.50.Dq Fluorescence and phosphorescence spectra
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.70.Jg Line and band widths, shapes, and shifts
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