Volume 114, Issue 13, 01 April 2001
 ARTICLES

 Theoretical Methods and Algorithms

Calculation of heat capacities and entropies of metal halides with quantum chemical methods
View Description Hide DescriptionStatistical thermodynamics and quantum chemistry are combined to evaluate the temperature dependence of heat capacities and entropiesS for metal halides. Density functional calculations are used to supply the structural data and vibrational frequencies. The accuracy of these data is established by comparison with experimental data. It is shown that they provide a reliable basis for the calculation of and S. Discrepancies between calculated values and experimental literature values can mainly be attributed to vibrational anharmonicities and contributions from excited electronic states. It is demonstrated that quantum chemical calculations can provide a powerful tool for systems where experimental data are not available.

Effect of correlating core orbitals in calculations of nuclear spin–spin couplings
View Description Hide DescriptionElectron correlationeffects on nuclear spin–spin coupling tensors arising from corevalence interaction were investigated with ab initio calculations based on multiconfigurational selfconsistent field (MCSCF) reference states, an adequate basis set, and large restricted active spaces. Calculations of first and secondrow hydrides were performed to examine the effect of including the core and semicore orbitals in the active molecular orbital space. The effects of using a multireference wave function instead of a singlereference one, as well as different numbers of electrons in the virtual orbitals, were considered. In secondrow hydrides, the inclusion of semicore orbitals is found to be necessary for accurate calculations of spin–spin couplings involving the heavy nucleus. A smaller but nonnegligible effect is observed also for the hydrogen–hydrogen coupling. Comparison is made with nuclear shielding and electric field gradient, for which the effect is considerably smaller. Recommendations for wave functions for calculations of spin–spin couplings are given.

Scaled higherorder correlation energies: In pursuit of the complete basis set full configuration interaction limit
View Description Hide DescriptionA simple multiplicative approach is presented for approximating the full configuration interaction (FCI) limit at the complete basis set limit from the small basis set FCI and coupled cluster [most notably CCSD(T) and CCSDT] calculations. The proposed scaled higherorder correlation (SHOC) correction scheme is demonstrated to extrapolate CCSD(T) and CCSDT correlation energies for BH and AlH to the FCI limit with remarkable accuracy, and to correct the dissociation energies of computed at the CCSD(T) and CCSDT levels by [+1.4, +2.3, +1.5] kcal/mol, respectively, bringing them in much closer agreement with the best experimental values. The approach is also well suited for the generation of accurate potential energy hypersurfaces.

A new parametrization of exchange–correlation generalized gradient approximation functionals
View Description Hide DescriptionA new “HCTH” generalized gradient approximation (GGA) functional is presented. Its 15 parameters have been refined against data from a training set containing 407 atomic and molecular systems. We believe that the much enhanced training set means that the new functional HCTH/407 has a much greater universality than previous GGA functionals. Statistical data is presented for the 407 set for the new functional, as well as other functionals.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

New theoretical insight into the thermal cis–trans isomerization of azo compounds: Protonation lowers the activation barrier
View Description Hide DescriptionThe detailed mechanism of thermal cis–trans inversion of protonated diazenes and azobenzenes has been investigated for the first time using ab initio Hartree–Fock (HF), density functional theory(DFT), second order Mo/ller–Plesset (MP2), and complete active space self consistent field (CASSCF) approaches. Protonation of one of the nitrogens at the bond leads to a considerable (5–20 kcal/mol) reduction of the activation barrier. The magnitude of this effect depends on the electron withdrawing nature of the substituents and the differential stabilization of the transition state by positively charged functional groups. In the case of 4phenylazopirydine protonated at pyridine nitrogen, the barrier is dramatically reduced to just 7 kcal/mol. This result establishes the lowest (close to an experimental estimate) ab initio value for the cis–transisomerization for a substituted azobenzene. The structures of neutral and protonated transition states are similar, and the correlation effects for molecular parameters as well as the energy are negligible for extended basis set calculations.

Excess energies of n and ioctane molecular clusters
View Description Hide DescriptionCloud chamber data for the nucleation of droplets from supersaturated n and ioctane vapors are analyzed using nucleation theorems. We obtain the excess energies of pure and mixed component molecular clusters with sizes ranging from 19 to 58 molecules. We plot this information in the form of an excess energy surface for a range of cluster compositions. Since the two species are similar we also combine the data into a plot of excess energy against the total number of molecules in the cluster. We show that the capillarity approximation fails to predict the critical cluster composition, though it does provide a rough estimate of the excess energy of a specified cluster.

Magic numbers in copperdoped aluminum cluster anions
View Description Hide DescriptionCopperdoped aluminum cluster anions, were generated in a laser vaporization source and examined via mass spectrometry and anion photoelectron spectroscopy The mass spectrum of the series is dominated by with other magic numbers also appearing at 19, and 23. The electron affinity versus cluster size trend shows a peak at and a dip at These results are discussed in terms of the reordering of shell model energy levels and the enhanced stability of neutral Reordering, which is a consequence of the copper atom residing in the central region of these clusters, provides an anionoriented electronic rationale for the observed magic numbers.

The structure of a weakly bound ionic trimer: Calculations for the complex
View Description Hide DescriptionThe weakly bound diatomic systems and have been found able to support only one bound state for although the latter also supports an additional bound state for In the present paper we, therefore, study the structure of the bound states which might exist for the weakly bound triatomic in its state, by describing the full potential as a simple addition of twobody (2B) interactions. We carry out bound state calculations using both Jacobi coordinates within a discrete variable representation (DVR) and pair coordinates with a distributed Gaussian function (DGF) expansion. The system is shown to possess two bound states with respect to its lower dissociation threshold and two further “ghost” states before the complete breakup threshold. The spatial structures of such states and of the floppy complex are analyzed in detail, as is the possibility of detecting Efimovtype states in such a weakly bound aggregate. Finally, the inclusion of threebody (3B) forces in the description of the full interaction and its effect on the number of possible bound states is also discussed.

Computation of large systems with an economic basis set: Ab initio calculations of silicon oxide clusters
View Description Hide DescriptionSilicon oxide plays an important role in many technological areas. Extensive ab initio calculations for related systems are thus desired but are limited due to the requirement of a huge computation resource. We show here that an economic basis set can be used to determine reliable atomic and electronic structures of the silicon oxide system. Binding energies in a singlepoint energy calculation using a standard higherlevel basis set on the geometric structures optimized with the economic basis set found reasonable agreement with reported ones of highlevel calculations. The approach has enabled the computation of silicon oxide clusters as large as 16 atoms being performed with considerably high accuracy using a medium level of computation resources.

Quantum interference and asymptotic interactions in the photodissociation of SH: Total cross section and branching ratios
View Description Hide DescriptionA theoretical analysis is presented for the photodissociation processes of SH to and Transition dipole moments from the ground state to the states are computed by the effective valence shell Hamiltonian method. Two frame transformation matrices are constructed and used to describe the correlations between the two sulfur atomic terms and and the adiabatic Born–Oppenheimer molecular states. Very interesting dynamics of quantum interferenceeffects and asymptotic interactions are found. At energies between the thresholds to the and limits, the resonances are mostly Lorentzian with more or less constant branching ratios. The effects of the intrastate interactions between the repulsive states are predicted to be very strong. At energies above the threshold to limit, quantum interferences between the dissociative pathways through the optically bright repulsive states and states) are predicted to give asymmetric resonances of multichannel character. Partial cross sections to the triplet sulfur fine structure states exhibit different degrees of asymmetry and, consequently, the branching ratios display strong variations across the asymmetric resonances, suggesting the possibility of controlling the product distributions by scanning the excitation wavelengths across a single asymmetric resonance in a onephoton excitation process. At higher energies, the interference between the two direct dissociation routes (by and states) is shown to produce highly oscillatory variations of the total cross section for dissociation to and of the branching ratios of

An interatomic potential for mercury dimer
View Description Hide DescriptionThe potential energy curve of the ground electronic state of the Hg dimer has been calculated using the CCSD(T) procedure and relativistic effective core potentials. The calculated binding energy (0.047 eV) and equilibrium separation (3.72 Å) are in excellent agreement with experiment. A variety of properties, including the second virial coefficient, rotational and vibrational spectroscopic constants, and vibrational energy levels, have been calculated using this interatomic potential and agreement with experiment is good overall.

Hydrogen bond cooperativity and electron delocalization in hydrogen fluoride clusters
View Description Hide DescriptionWe investigate the energetic, structural, electronic and thermodynamics properties of hydrogen fluoride cluster, in the range by ab initio methods and density functional theory(DFT). The ab initio methods chosen were Hartree–Fock (RHF) and secondorder Mo/ller–Plesset perturbation theory (MP2). The DFT calculations were based on Becke’s hybrid functional and the Lee–Yang–Parr correlation functional (B3LYP). We found that symmetric cyclic clusters are the most stable structure, and that large cooperative effects, particularly from trimer to tetramer are present, in binding energy, and hydrogen bond distance. An analysis of the topology of the electron density reveals a linear correlation between the binding energy per hydrogen bond and the density at the hydrogen bond critical point and the Cioslowski covalent bond order. Based on these correlations,hydrogen bond cooperativity is associated with the electronic delocalization between monomers units. Analysis of the thermodynamics properties shows that the enthalpy changes are determined by the electronic cooperative effects, while the entropic statistical factors are fundamental in the relative stability of these clusters. Finally, for the trimer and tetramer, nonstable linear zigzag chains where found in a detailed analysis of the potential energy surfaces.

Unexpected stability of and their larger clusters
View Description Hide DescriptionExperimental observations following the ionization of neutral group 11 metal/argon complexes have revealed the presence of doubly charged ions of the form for n in the range 1–6. Of particular interest are two features of the results. First, the unexpected stability of the dimer ions, since similar species involving a molecule rather than a rare gas atom are often unstable with respect to charge transfer.Ab initio calculations show the dimers owe their stability to a combination of a strong electrostaticinteraction and the high ionization energy of argon. A second feature to the results is the high relative intensities of the and ions. Calculations show these complexes to consist of squareplanar structures, with the additional two atoms in occupying axial sites, which are Jahn–Teller distorted. The calculated relative binding energies support the preferential stability of these two structures.

Onephoton massanalyzed threshold ionization spectroscopy of 1 and 2iodopropanes in vacuum ultraviolet
View Description Hide DescriptionOnephoton massanalyzed threshold ionization (MATI) spectroscopy of 1 and 2iodopropanes has been studied using coherent vacuum ultraviolet (VUV)radiation generated by fourwave mixing in Kr gas. Accurate ionization energies to the lower and upper spin–orbit states of the molecular ions have been determined. These are 9.1755±0.0005 and 9.6903±0.0017 eV for the lower and upper spin–orbit states, respectively, of 2iodopropane ion. For 1iodopropane ion, gauche and trans peaks were resolved in the MATI spectra. Ionization energies to the lower spin–orbit states are 9.2567±0.0005 and 9.2718±0.0005, respectively, for the gauche and trans conformers. The corresponding values are 9.8332±0.0017 and 9.8466±0.0017 for the upper spin–orbit states. The pure ion beam of the gaucheonly or that of transonly could be selectively generated by tuning the VUV wavelength. Dissociation of 1 and 2iodopropane ions, occurring in the ion core of highly excited Rydberg neutrals has been observed. Fragmentation thresholds for these reactions have been determined. This has led to an accurate potential energy diagram for the dissociation of the system in the threshold region. The heat of formation at 0 K of has been determined, 821.7±3.8 kJ mol^{−1}, together with the proton affinity at 0 K of 741.6±3.9 kJ mol^{−1}. Plausible mechanisms for the production of from have been proposed.

The complex. I. IRREMPI double resonance spectroscopy
View Description Hide DescriptionThe rotationally resolved infrared spectrum of the NO–Ne complex associated with the first overtone transition in is measured in an IRUV double resonance experiment. resonance enhanced multiphoton ionization (REMPI) involving the Rydberg states and is used to detect the infrared absorption. In the complex, the NO overtone transition is located at i.e., it is blueshifted by from the transition in the monomer. Three other bands detected at frequencies 3727.85, 3732.56, and are assigned to combination bands involving different intermolecular bendstretch vibrations. The rotational structure of the observed bands is consistent with a near Tshaped complex. Details of the rotational structure resemble the spectrum of a symmetric top subject to weak electronic interactions causing a Starktype and Ptype doubling of the rotational levels.

The complex. II. Investigation of the lower bound states based on new potential energy surfaces
View Description Hide DescriptionHighquality ab initiopotential energy surfaces were calculated and subsequently used to predict the positions of the lowest bendstretch vibrational states of the complex. The vibrational wavefunctions and basis set expansion coefficients, determined within the adiabatic bender model, were then used to simulate the observed spectrum for excitation of the complex. The overall position and rotational substructure matches well the experimental results for this system, which are presented in the preceding article [Y. Kim, J. Fleniken and H. Meyer, J. Chem. Phys. 114, 5577 (2001)]. A heuristic Hamiltonian, which includes the most important couplings and splittings, is used to improve the fit to experiment.

Ionization and fragmentation dynamics of benzene in intense laser fields by tandem mass spectroscopy
View Description Hide DescriptionUsing a tandem timeofflightmass spectrometer, benzene cations produced by the resonantly enhanced multiphoton ionization are mass separated and are exposed to intense laser fields at λ∼790 and 395 nm with the pulse duration of ∼50 fs. Comparing the yields of the product ions with those obtained from neutral benzene molecules, the ionization and dissociation dynamics of benzene in intense laser fields is investigated. At λ∼790 nm, the formation of parent benzene ions is a dominant process irrespective of the initial charge states, i.e., major products obtained when starting from neutral benzene are benzene cations and dications and those obtained when starting from benzene cation are benzene dications. On the other hand, at λ∼395 nm, the fragmentation processes to produce and dominate over further ionization to the benzene dication for both cases starting from neutral benzene and benzene cation, indicating the population trapping occurs by the efficient confinement in the lightdressed mixture of the and states of benzene cations assisted by the ultrafast intramolecular decay process.
 Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Microscopic theory of hyperRayleigh scattering for molecular crystals
View Description Hide DescriptionMolecular crystals produce hyperRayleigh scattering (HRS) at the second harmonic frequency via nonzero meansquare dynamical fluctuations in the quadratic optical susceptibility. The intensity of HRS depends on the lattice normalcoordinate derivatives of the quadratic susceptibility. Expressions are derived for these quantities, including polarizability changes induced by polar molecules and the first hyperpolarizability induced by quadrupolar molecules, drawing upon previous results from microscopic theories of nonlinear optics, Raman scattering, and optical activity. A simple illustrative calculation for the hexamine crystal indicates that its HRS should be governed by a susceptibility that is about 7% of the usual quadratic susceptibility for secondharmonic generation (SHG). HRS should be detectable for a centrosymmetric crystal where SHG is zero by symmetry.

Electronic coupling between Watson–Crick pairs for hole transfer and transport in desoxyribonucleic acid
View Description Hide DescriptionElectronic matrix elements for hole transfer between Watson–Crick pairs in desoxyribonucleic acid(DNA) of regular structure, calculated at the Hartree–Fock level, are compared with the corresponding intrastrand and interstrand matrix elements estimated for models comprised of just two nucleobases. The hole transfer matrix element of the GAG trimer duplex is calculated to be larger than that of the GTG duplex. “Throughspace” interaction between two guanines in the trimer duplexes is comparable with the coupling through an intervening Watson–Crick pair. The gross features of bridge specificity and directional asymmetry of the electronic matrix elements for hole transfer between purine nucleobases in superstructures of dimer and trimer duplexes have been discussed on the basis of the quantum chemical calculations. These results have also been analyzed with a semiempiricalsuperexchange model for the electronic coupling in DNA duplexes of donor (nuclobases)–acceptor, which incorporates adjacent base–base electronic couplings and empirical energy gaps corrected for solvation effects; this perturbationtheorybased model interpretation allows a theoretical evaluation of experimental observables, i.e., the absolute values of donor–acceptor electronic couplings, their distance dependence, and the reduction factors for the intrastrand hole hopping or trapping rates upon increasing the size of the nucleobases bridge. The quantum chemical results point towards some limitations of the perturbationtheorybased modeling.

New insights into the fragility dilemma in liquids
View Description Hide DescriptionA compilation of data for small molecule organic, polymeric, and inorganic glassforming liquids shows that the original expectation, that there be a positive correlation between the thermodynamic measure of fragility (or and the dynamic fragility index m, is not generally true. The results are consistent with three classes of behavior: (1) a decrease in m with increasing for the polymeric glass formers; (2) a nearly constant value of m independent of for small molecule organics and hydrogen bonding small molecules; (3) an increasing value of m with increasing for inorganic glass formers as originally considered by Angell.