Volume 119, Issue 17, 01 November 2003
 COMMUNICATIONS


Circular dichroism in the photoelectron angular distribution from randomly oriented enantiomers of camphor
View Description Hide DescriptionCircular dichroism in the angular distribution of valence photoelectrons emitted from randomly oriented chiral molecules has been observed in experiments that use circularly polarized VUVsynchrotron radiation.Photoionization of the outermost carbonyl oxygen lone pair electrons from pure enantiomers of the prototype chiral molecule camphor is shown to have an asymmetry in the forward–backward scattering of photoelectrons (relative to the propagation direction of the light beam) of magnitude approaching 3% at 9.2 eV photon energy. The asymmetry reverses on exchange of either the helicity of the radiation or of the molecular enantiomer, confirming theoretical predictions of an effect that arises in the pure electricdipole approximation.

Influence of vibrational excitation on the reaction
View Description Hide DescriptionThe reaction is studied to understand the effect of vibrational excitation on the reaction pathways. The hydrogen molecules in the levels are populated by using stimulated Raman pumping (SRP). A pump–probe technique is employed simultaneously to initiate the reaction and monitor the products. The pump–SRP and SRP–probe delay time are short enough to allow for the products to be in a nascent state. The population fraction in the level can be estimated to be 7.5% by using coherent antiStokes Raman spectroscopy. As characterized by Boltzmann rotational temperature of 730 K, the rotational state distributions of obtained with appear to be unimodal, similar to those obtained with but the product yield is enhanced by a factor of 7–8 times. According to the potential energy surfaces calculations, the insertion mechanism in (near) collision configuration is favored. The collision is initially along the surface in the entrance channel and then transits to the ground surface, from which the products are formed. When stretches to its outer turning point (∼0.9 Å), the surface may diabatically couple to the surface in the attractive region. An energy barrier of 4300 cm^{−1} will otherwise obscure the reaction if the bond distance is fixed at 0.75 Å. The energy deposited in the level simply enlarges the bond distance to help facilitate the reaction and increase the subsequent product yield. The lack of detection of the product implies that the exoergic energy by 2530 cm^{−1} should not be distributed statistically among different freedom of motions. The vibrational excitation does not seem to open up an additional pathway for the reaction.

Classical and quasiclassical spectral analysis of using an ab initio potential energy surface
View Description Hide DescriptionWe report a potential energy surface and calculations of power spectra for The potential surface is obtained by precise fitting of MP2/ccpVTZ electronic energies and gradients, which are obtained in classical directdynamics calculations. The power spectra are obtained using standard microcanonical classical and novel quasiclassical calculations of the velocity autocorrelation function, from which the power spectrum is obtained in the usual way. Both calculations agree qualitatively that the overall spectrum is quite complex; however, the latter calculations indicate that some spectral features may be assignable.
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 ARTICLES

 Theoretical Methods and Algorithms

Localizeddensitymatrix implementation of timedependent densityfunctional theory
View Description Hide DescriptionA linearscaling firstprinciples quantum mechanical method is developed to evaluate the optical responses of large molecular systems. Instead of a manybody wave function, the equation of motion is solved for the reduced singleelectron density matrix in the time domain. The locality of the reduced singleelectron density matrix is utilized to ensure that computational time scales linearly with system size. The twoelectron Coulomb integrals are evaluated with the fast multipole method, and the calculation of exchangecorrelation quadratures utilizes the locality of an exchangecorrelation functional and the integral prescreening technique. As an illustration, the resulting timedependent densityfunctional theory is used to calculate the absorption spectra of polyacetylene oligomers and linear alkanes. The linearscaling of computational time versus the system size is clearly demonstrated.

Using force fields methods for locating transition structures
View Description Hide DescriptionA previously proposed strategy of using force field methods for generating approximations to the geometry of transition structures is extended to also estimating an approximate Hessian matrix. These two components allow an automated method for locating first order saddle points, which is an essential requisite for studying chemical reactions of systems with many degrees of freedom. The efficiency of using an approximate force field Hessian matrix for initiating the geometry optimization is compared with the use of an exact Hessian. The force field Hessian in general requires more geometry steps to converge, but the additional computational cost is offset by the savings from not calculating the exact Hessian at the initial geometry.

Resonant nonlinear polarizabilities in the timedependent density functional theory
View Description Hide DescriptionThe response of the density matrix to an external field is calculated in the adiabatic timedependent density functional (TDDFT) theory by mapping the equation of motion for the driven singleelectron density matrix into the dynamics of coupled harmonic oscillators. The resulting nonlinear response functions and the closed expressions for nonlinear frequencydependent polarizabilities are derived. These expressions include transition densities and frequencies calculated in the linear response TDDFT, and higher order functional derivatives of the exchangecorrelation functional. Limitations of the applicability of the traditional sum over states approach for computing the nonlinear response to the TDDFT are discussed.

Canard phenomenon and localization of oscillations in the Belousov–Zhabotinsky reaction with global feedback
View Description Hide DescriptionThe occurrence of spatial domains of large amplitude oscillation on a background of small amplitude oscillation in a reaction–diffusion system is called localization. We study, analytically and numerically, the mechanism of localization in a model of the Belousov–Zhabotinsky reaction subject to global feedback. This behavior is found to arise from the canard phenomenon, in which a limit cycle suddenly undergoes a significant change in amplitude as a bifurcation parameter, in this case the feedback strength, is varied. In the system studied here, the oscillations arise via a supercritical Hopf bifurcation, but our analysis suggests that the same mechanism is relevant for systems undergoing a subcritical Hopf bifurcation.

Exponential variational expansion in relative coordinates for highly accurate bound state calculations in fourbody systems
View Description Hide DescriptionExponential variational expansions in relative coordinates are considered for fourbody systems. All matrix elements needed for boundstate calculations are expressed as linear combinations of fifth and sixthorder derivatives of a basic fourbody integral. Computation of the basic fourbody integral and its derivatives is performed directly, i.e., without any use of the branch tracking in the complex plane that is required in the Fromm/Hill approach, and by methods that take into account the termwise singularities of the formulas. The final computational procedure is relatively simple, physically transparent, and numerically stable. The methods are illustrated with sample data that show the importance of a singularitycanceling approach and that the increased precision thereby made possible permits more accurate wave functionoptimization than heretofore.

Preconditioned iterative minimization for linearscaling electronic structure calculations
View Description Hide DescriptionLinearscaling electronic structure methods are essential for calculations on large systems. Some of these approaches use a systematic basis set, the completeness of which may be tuned with an adjustable parameter similar to the energy cutoff of planewave techniques. The search for the electronic ground state in such methods suffers from an illconditioning which is related to the kinetic contribution to the total energy and which results in unacceptably slow convergence. We present a general preconditioning scheme to overcome this illconditioning and implement it within our own firstprinciples linearscaling density functional theory method. The scheme may be applied in either real space or reciprocal space with equal success. The rate of convergence is improved by an order of magnitude and is found to be almost independent of the size of the basis.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

The carbon pentamer dication Toward thermochemical stability
View Description Hide DescriptionVarious computational abinitio methods are employed to study the doublycharged carbon system For its electronic ground state, equilibrium geometries are identified, and their stability against dissociation investigated. Multireference configurationinteraction results indicate a weakly bound but thermochemically stable dication.

Theoretical study of photoinduced electron transfer from tetramethylethylene to tetracyanoethylene
View Description Hide DescriptionUsing ab initio and density functional calculations, we studied photoexcitation of a chargebalanced electron donor–acceptor (DA) complex comprised of tetracyanoethylene (TCE) and tetramethylethylene (TME). We considered both the TCETME stacked conformer and a possible conformer with a solvent molecule (dichloromethane) inserted between TCE and TME. The photoexcitation of the DA complex can directly form a charge transfer(CT) state. Our theoretical investigations show that the CT state can also be produced from the decay of higher excited states. Using the continuum model, we investigated the solvent effects on CT absorption, local excitation, and CT emission in the polar solvent. The equilibrium solvation energies of the ground and excited states of the DA complex were calculated using the selfconsistent reaction field method, and then the correction of nonequilibrium solvation energies for the vertical transitions was made. The transition energies (i.e., CT absorption for the DA complexes and CT emission for the contact ion pair complexes) in the polar solvent show redshifts relative to those in the gas phase.

Competition between photodetachment and photodissociation in
View Description Hide DescriptionUsing fast beam photofragment spectroscopy, we have studied the competition between photodissociation and photodetachment in The photodissociation fraction is ∼10% and increases somewhat between 285 and 266 nm. High resolution photofragment spectroscopy shows that the beam is composed of ions only. The photofragment angular distribution has an anisotropy parameter β=1.9, indicating that the process of dissociation has a parallel character, attributed to the transition. A detailed analysis of the kinetic energy release of the photofragments shows the structure related to the distribution over the six finestructure dissociation limits The results are compared with an analytical diabatic dissociation model, taking into account the long range charge–quadrupole interactions; the is described in a Born–Oppenheimer electrostatic aproximation and projected onto the different finestructure states at large internuclear separation using analytical expressions derived from the long range charge–quadrupole interactions between and O fragments.

Potential energy surfaces and bound states for the openshell van der Waals cluster Br–HF
View Description Hide DescriptionSemiempiricalpotential energy surfaces for the lowest three electronic states of the openshell complex Br–HF are constructed, based on existing empirical potentials for Kr–HF and Kr–Ne and coupledclusterelectronic structure calculations for Br–Ne. Coupled cluster calculations are also described for He–F, Ne–F and Ar–F. Electrostaticinteractions that arise from the quadrupole of the Br atom and the permanent multipoles of HF are also included in the Br–HF surfaces. The well depth of the lowest adiabatic surface is found to be 670 cm^{−1} at a linear equilibrium geometry. The results of helicity decoupled and full closecoupling calculations of the bound states of the complex are also described. The ground state, with total angular momentum projection quantum number is found 435 cm^{−1} below dissociation to The lowestfrequency intermolecular bending and stretching vibrations are predicted around 145 and 211 cm^{−1}, respectively. Parity splittings are found to be extremely small for bound states with projection quantum number The relevance of the results to recently recorded spectra of Br–HF is discussed.

Zero electron kinetic energy photoelectron spectroscopy and density functional theory calculations of gallium–methylamine complexes
View Description Hide DescriptionGallium–dimethylamine and –trimethylamine were produced in pulsed laservaporization molecular beams and studied by threshold photoionization and pulsedfield ionization zero electron kinetic energy (ZEKE) photoelectron spectroscopies and density functional theory calculations. Analyses of the ZEKE spectra yield molecular adiabatic ionization potentials and metal–ligand and ligandbased vibrational frequencies. Comparisons of the experimental and theoretical results establish the ground electronic states of the neutral and ionic complexes. The ionization potentials, stretching, and bending frequencies are 38 790, 206/177, and 132/128 cm^{−1} for the dimethylamine complex and 38 081 cm^{−1}, 188/133, and 111/92 cm^{−1} for the trimethylamine species. The electronic ground states are for and for The calculated metal–ligand binding energies of and are 36.2/9.7 and 37.1/8.5 kcal mol^{−1}, respectively.

Hyperpolarization transfer from parahydrogen to deuterium via carbon13
View Description Hide DescriptionHyperpolarization arising from parahydrogen can be transferred via carbon13 to deuterium after hydrogenation of a perdeuterated substrate. The model compound is acetylene hydrogenated to yield ethylene Transfer to deuterium occurs in ALTADENA experiments (the hydrogenation reaction being performed outside the magnet of the NMR spectrometer prior to the insertion of the sample tube into the NMR probe). The proposed theory, limited to the case where the two protons remain isochronous (same chemical shift), is based on the concept of a steadystate density operator which prevails subsequently to the hydrogenation reaction. The outcome quantity is the carbon–deuterium longitudinal spin order, denoted as Calculations simply involve commutators of all relevant spin quantities with the Jcoupling Hamiltonian (denoted as In particular, it is shown that the necessary condition for polarization transfer toward deuterium via carbon13 is that does not commute with The structure of is thus of prime importance and it appears that transfer to carbon13 occurs for both types of experiments, ALTADENA and PASADENA (hydrogenation reaction in the presence of the NMR spectrometer magnetic field). Conversely, transfer toward deuterium via carbon13 is possible only with ALTADENA experiments.

Ab initio studies of reactions. IX. Combination and disproportionation reactions of ClO and radicals
View Description Hide DescriptionThe mechanism for the reaction on both singlet and triplet state potential surfaces has been investigated with the modified Gaussian2 method based on the optimized stationarypoint geometries. The result shows that the barrierless association reaction producing and two lower barrier Oatom abstraction reactions take place primarily on the singlet state potential surface; they are energetically more favorable than those occurring on the triplet state surface.Rate constants calculated by variational transition state and Rice–Ramsperger–Kassel–Marcus theories suggest that the major products are at low temperatures (<700 K) and at high temperatures. The following rate constants are recommended for atmospheric chemistry and combustion applications: (200–3000 K) and (200–800 K) for He as the thirdbody. In addition, a potential hightemperature reverse reaction involving Cl and has been predicted to yield exclusively the ClO and products; its rate constant is predicted to be: (200–3000 K). The heats of formation at 0 K for and have been predicted to be 46.7, 59.4, and 38.3 kcal/mol with about 1 kcal/mol uncertainty using the new heat of formation of OClO, based on the most recent bond dissociation energy of O–ClO reported in the literature.

Separation of a benzene and nitric oxide mixture by a molecule prism
View Description Hide DescriptionIn molecule optics, a matter wave of molecules is manipulated by a moleculeoptical component made out of external, typically radiative, fields. The moleculeoptical index of refraction, for a nonresonant IR laser pulse focused onto a molecular beam can be obtained from the energy conservation and wave properties of molecules. Experimentally measured values of for benzene and nitric oxide agreed well with the calculated values. Since depends on the properties of molecules as well as those of the laser field, a molecule prism composed of the focused nonresonant laser field can separate a multicomponent molecular beam into several components according to their moleculeoptical refractive indices We obtained a chromatographic resolution of 0.62 for the spatial separation of a mixture beam of benzene and nitric oxide using a focused Nd:YAG laser pulse as a molecule prism.

Fluorescence detected microwave Stark effect measurements in excited vibrational states of
View Description Hide DescriptionWe describe a microwaveoptical double resonanceexperiment designed for Stark effectmeasurements on molecules in excited vibrational energy levels of ground electronic states. This technique, which employs pulsed laser excitation and laser induced fluorescence detection, has been used to measure electric dipole moments in the and vibrational states of formaldehyde. In addition, a Stark induced quantum beat experiment is briefly described.

Heteronuclear coherence transfer in solidstate nuclear magnetic resonance using a γencoded transferred echo experiment
View Description Hide DescriptionA novel type of solidstate nuclear magnetic resonance experiment for efficient transfer of coherence between different nuclear spin species under magicangle spinning conditions is introduced. The method combines the attractive features of γencoded dipolar recoupling [Nielsen et al., J. Chem. Phys. 101, 1805 (1995)] with coherence transfer mediated by a longitudinal spinorder operator in a transferred echo experiment. Using twochannel rotary resonance recoupling with different phase and amplitude modulation schemes, the transferred echo sequence can be tuned to achieve dipolar recoupling and coherence transfer over a welldefined range of chemical shifts while keeping the ratio between the rf field strength and the sample spinning frequency relatively low. The method, referred to as gammaencoded transfer echo, is described analytically, by numerical simulations for various different spin systems, and experimentally by to coherence transfers in a powder sample of labeled glycine.

Water dimer hydrogen bond stretch, donor torsion overtone, and “inplane bend” vibrations
View Description Hide DescriptionWe report the measurement and analysis of 64 new and transitions of and 16 new transitions of by terahertz laser vibration–rotation–tunneling spectroscopy of a planar supersonic expansion between 140.5 and 145.5 cm^{−1}. The transitions in both isotopomers correspond to vibrations assigned to the hydrogen bond stretch (translational) and donor torsion overtone vibrations. The interchange splitting is 56.3 GHz in of the excited state of nearly 3 times the value of the ground state, and the bifurcation tunneling splitting is 1.8 GHz, over 2 times the value of the ground state. We compare the existing experimental spectra with calculations on stateoftheart intermolecular potential energy surfaces and critically review the vibrational assignments reported in the literature. We show that the discrepancy between theory and experiment regarding the assignment of the feature near 103 cm^{−1} can be resolved by considering transitions, which had not been considered previously.