Volume 107, Issue 2, 08 July 1997
Index of content:

Structure and dynamics of the silacyclobutane radical cation, studied by ab initio and density functional theory and electron spin resonance spectroscopy
View Description Hide DescriptionThe geometricstructure, the proton isotropic hyperfine coupling constants and temperature dependence of the isotropic hyperfine coupling constants have been investigated for the silacyclobutane radical cation using Mo/llerPlesset perturbation theory to second order, the coupled cluster approximation and density functional theory. The temperature dependence of the hyperfine coupling constants is explained employing a onedimensional model for the description of the ring puckering motion. For the geometricalstructure the study supports conclusions made in earlier work based on the measurements of the temperature dependence of the electron spin resonance(ESR) spectra, e.g. the silacyclobutane radical cation possesses an asymmetrically distorted structure in which one of the Si–C bonds is considerably elongated, and the ring is puckered. A reassignment of part of the ESR spectrum is obtained on the basis of the theoretical results in combination with experimental measurements of the low temperature dynamical behaviour.

Laser spectroscopy and density functional calculations on niobium monocarbide
View Description Hide DescriptionA survey of the jetcooled and radicals has been carried out between 13 500 and using laserinduced fluorescence and resonant twophotonionization spectroscopy. Several vibronic bands belonging to at least six band systems have been identified. Three of these systems appear to belong to transitions in which the lower state is the ground electronic state of the molecule. The other three systems also terminate to the same three upper states, but originate from a state lying above the state. This state is assigned as the state. The ionization potential has been determined to be or using twocolor photoionization efficiency spectroscopy. This value, combined with the ionization potential of Nb and the bond energy of yields an improved bond energy of for NbC. The (4,0) band of the system has been studied at a resolution of approximately using laserinduced fluorescencespectroscopy. The nuclear magnetic hyperfine structure has been resolved in both states, and an analysis confirms that the ground state arises from the electron configuration in which the unpaired δ electron is a pure Nb electron associated with the term arising from the excited electron configuration. Density functional calculations have been carried out on the lowest and states of the neutral and the and states of the cations. These calculations fully support the experimental evidence for the ground state.

Transient infrared spectrum of vibrationally excited
View Description Hide DescriptionThe transient IR spectrum of in with one quanta of excitation in the COstretch modes has been measured by picosecond timeresolved IR saturation spectroscopy as a function of pump–probe time delay and polarization. At short time delays the spectra can be consistently deconvoluted into a bleach of the fundamental at and two hotband transitions at and which we tentatively assign to the and symmetry overtones of the fundamental. The broader hotband feature observed at long time delay is assigned to an inhomogeneous superposition of transitions to overtone and combination modes from an equilibrated population of CO stretch modes. A mean fieldmodel of coupled anharmonic oscillators substantially overestimates the magnitude of average frequency shift of the hot band in this intermediate size molecular system. The rapid decay of the pumpinduced dichroism in the sample suggests that vibrational coupling of the degenerate modes proceeds on a time scale similar to or faster than physical rotation of the molecule.

Coherent antiStokes Raman spectroscopy of shockcompressed liquid carbon monoxide–oxygen and nitrogen–oxygen mixtures
View Description Hide DescriptionA twostage light gas gun and singlepulse multiplex coherent antiStokes Raman scattering(CARS) have been used to obtain carbon monoxide, nitrogen, and oxygen vibrational spectra for several highpressure/hightemperature, dense fluid, carbon monoxide–oxygen, and nitrogen–oxygen mixtures. The experimental spectra were compared to synthetic spectra calculated with a semiclassical model for CARS intensities and using best fit vibrational frequencies, peak Raman susceptibilities, and Raman linewidths for each mixture component. Up to a maximum shock pressure of 6.75 GPa for carbon monoxide–oxygen mixtures, the CO and vibrational frequencies were found to increase monotonically with pressure and depended on the carbon monoxide–oxygen mixture ratio. For the nitrogen–oxygen mixtures, the vibrational frequency increased monotonically with pressure to a maximum experimental pressure of 12.9 GPa, however the vibrational frequency increased with pressure to about 11 GPa and then appeared to decrease slightly as the pressure increased to the experiment maximum of 12.9 GPa. Empirical fits of the measured Raman frequencies incorporating previously published neat nitrogen, carbon monoxide, and oxygen data and using a functional form dependent on pressure, temperature, and mixture ratio, accurately describe the CO, and vibrational frequency shifts for both the carbon monoxide–oxygen and the nitrogen–oxygen mixtures. The transition intensity and linewidth data suggest that thermal equilibrium of the vibrational levels is attained in less than 10 ns at these shock pressures. The vibrational temperatures obtained for the nitrogen–oxygen mixtures were used to improve the oxygen potential function used to calculate equationofstate pressures and temperatures. The measuredlinewidths for CO, and were different for the different mixtures and did not appear to depended significantly on mixture ratios. The broadening of all spectral lines suggested that the vibrational dephasing time for each species decreased to a few ps at the highest pressure shock states.

Infrared laser absorption spectroscopy of rotational and vibration rotational transitions of up to the dissociation threshold
View Description Hide DescriptionMany high pure rotational transitions of up to the dissociation threshold have been measured by infrared diode laserabsorption spectroscopy between 590 and in an air cooled discharge. Additional centrifugal distortion terms up to have been included in the rotational Hamiltonian to fit these transitions. Ten new vibration rotational transitions have also been detected in the same spectral region, arising from both low and high vibrational states. Some of the transitions involving high are observed as stimulated emission signals. The measured rotational transitions are in excellent agreement with ab initio calculations based on the form of the adiabatic potential model developed by Fournier and Richard.

Ab initio configuration interaction determination of the overtone vibrations of methyleneimine in the region
View Description Hide DescriptionAb initioconfiguration interaction (CI) potential function calculated from a basis set at a MP2 level of theory is used to compute the vibrational energy levels of methyleneimine between 2800 and The most important configurations selected by an iterative process by means of a variational perturbational method are diagonalized. Results show a perfect agreement with the most reliable experimental values and predict the and unobserved combinations.

Solidstate nuclear magnetic resonance techniques for studying slow molecular motions
View Description Hide DescriptionA variety of transitionselective solidstate NMR techniques are demonstrated for the first time to be useful for quantitatively describing slow molecular motions in the solid state. These techniques are validated by quantitative measurements of molecular reorientation by tetrahedral jumps in hexamethylenetetramine (HMT). A new foursite magnetizationexchange model, capable of being generalized to sites, which includes the effects of spinlattice relaxation is developed. This model provides the limiting conditions under which the orientation dependence of spinlattice relaxation values can be safely neglected. The model is used to analyze results from a frequencyselective DANTE train used to burn a hole in the spectrum, that provide a direct indication of the existence of 4site exchange. The measuredcorrelation time for the motion in HMT of 103±6 ms at room temperature agrees well with previous studies by other techniques. In a novel application to molecular dynamics, the repeated holeburning pulse trains of the SINK experiment are used to measure a magnetization recovery time constant due to spinlattice relaxation in HMT of 0.99±0.12 s. Both experiments employ frequencyselective as well as transitionselective radio frequency pulses on a relatively small frequency region (<100 kHz) of the entire quadrupolar powder pattern of HMT (NQCC=4.414 MHz, η=0). The Hahn spinecho used for detection can be understood in terms of the fictitious spin1/2 formalism. Quantitative dynamical information is obtained from measurements at only one frequency position of a wide inhomogeneously broadened powder pattern. Because we are operating in this unusual regime, the sensitivity can be significantly improved by replacing the DANTE holeburning train with a series of π/2 pulses that saturate all observable magnetization. Results from such an experiment compare well with those obtained using DANTE trains.

Light scattering properties of paramagnetic particles
View Description Hide DescriptionWe present an experimental study of light scatteringproperties of paramagnetic particles. To account for the magnetic dipole radiation and the Brownian motion of the particles in a thermal equilibrium solution, we calculate the scattering intensity and its autocorrelation function Experimentally, we examine the scatteringproperties of the paramagnetic particles and compare the results with those from isotropic and anisotropicdielectric particles. The experiment verifies the calculation and reveals that the magnetic dipole radiation of the paramagnetic particles is unusually large and equals to approximately of the electric dipole radiation of the particles. Dynamic light scatteringmeasurements show that the measured for the depolarized scattering is strongly influenced by the size distribution of the particles. This is because the large paramagnetic particles tend to have more magnetite content and hence weigh more in the depolarized scattering. With a simple sedimentation method, we are able to separate the particles of different sizes and obtain relatively monodispersed scattering samples. These samples give the expected translational and rotational diffusion constants of the particles.

An analytical derivation of MCSCF vibrational wave functions for the quantum dynamical simulation of multiple proton transfer reactions: Initial application to protonated water chains
View Description Hide DescriptionThis paper presents an analytical derivation of a multiconfigurational selfconsistentfield (MCSCF) solution of the timeindependent Schrödinger equation for nuclear motion (i.e. vibrational modes). This variational MCSCF method is designed for the mixed quantum/classical molecular dynamics simulation of multiple proton transferreactions, where the transferring protons are treated quantum mechanically while the remaining degrees of freedom are treated classically. This paper presents a proof that the Hellmann–Feynman forces on the classical degrees of freedom are identical to the exact forces (i.e. the Pulay corrections vanish) when this MCSCF method is used with an appropriate choice of basis functions. This new MCSCF method is applied to multiple proton transfer in a protonated chain of three hydrogenbonded water molecules. The ground state and the first three excited state energies and the ground state forces agree well with full configuration interaction calculations. Sample trajectories are obtained using adiabatic molecular dynamics methods, and nonadiabatic effects are found to be insignificant for these sample trajectories. The accuracy of the excited states will enable this MCSCF method to be used in conjunction with nonadiabaticmolecular dynamics methods. This application differs from previous work in that it is a realtime quantum dynamical nonequilibrium simulation of multiple proton transfer in a chain of water molecules.

A combined method for determining reaction paths, minima, and transition state geometries
View Description Hide DescriptionMapping out a reaction mechanism involves optimizing the reactants and products, finding the transition state and following the reaction path connecting them. Transition states can be difficult to locate and reaction paths can be expensive to follow. We describe an efficient algorithm for determining the transition state, minima and reaction path in a single procedure. Starting with an approximate path represented by points, the path is iteratively relaxed until one of the points reached the transition state, the end points optimize to minima and the remaining points converged to a second order approximation of the steepest descent path. The method appears to be more reliable than conventional transition state optimization algorithms, and requires only energies and gradients, but not second derivative calculations. The procedure is illustrated by application to a number of modelreactions. In most cases, the reaction mechanism can be described well using 5 to 7 points to represent the transition state, the minima and the path. The computational cost of relaxing the path is less than or comparable to the cost of standard techniques for finding the transition state and the minima, determining the transition vector and following the reaction path on both sides of the transition state.

Photodissociation of rovibrationally excited Observation of two pathways
View Description Hide Descriptionis prepared in the (five quanta of C–H stretch) vibrational state and photodissociated by 243.135 nm photons that also probe the H photofragments via (2+1) resonanceenhancedmultiphoton ionization (REMPI) in a timeofflightmass spectrometer. The production of H atoms is greatly enhanced upon rovibrational excitation. The REMPI action spectrum shows the characteristic features of a band and mimics the absorptionspectrum, except that the line intensity is an order of magnitude higher than that expected for a Boltzmann distribution. The maximum translational energy of the H atoms obtained from dissociation of the regularly distributed rotational states is 0.67±0.10, whereas for it is 1.34±0.10 eV. The observed intensities and linewidths indicate the existence of two photodissociation pathways following the preparation of where the fragment is produced in two different states. In the pathway an additional bent state is prepared, or an accidental coincidence resonance is involved.

Statistical model for nonadiabatic decay of an exciplex strongly coupled to a dissociative continuum
View Description Hide DescriptionWe present a statistical model for the decay of an exciplex that is strongly coupled to repulsive groundstatepotential energy surface. The motion of the exciplex is assumed to be governed by the excited electronic state adiabatic potential surface, and transitions to the ground state are caused by nonadiabatic transitions at a seam of avoided crossing of two strongly coupled diabatic states underlying the adiabatic representation. The model assumes that motion in the exciplex is ergodic. The predictions of the model are tested against accurate quantum dynamics and trajectory surface hopping calculations for .

Comparison of the Smoluchowski approach with modern alternative approaches to diffusioninfluenced fluorescence quenching: The effect of intense excitation pulses
View Description Hide DescriptionFor low intensity excitation pulses, the fluorescence intensity in the presence of quenchers is obtained, within the framework of the Smoluchowski approach, by convoluting the pulse profile with the survival probability of an excited fluorophore initially surrounded by an equilibrium distribution of quenchers. This conventional approach is generalized to handle excitation pulses of arbitrary intensity. The resulting expression is exact in the limit that the fluorophore is static and the quenchers diffuse independently. Modern alternative approaches to this problem are based either explicitely or implicitely on truncating the reduced manyparticle distribution function hierarchy by means of a superposition approximation. For a deltafunction excitation pulse all approaches yield the identical result. For arbitrary pulses, the modern approaches predict different, albeit numerically similar, results. This difference, however, does not constitute an improvement over the Smoluchowski approach. Rather, it is a reflexion of an additional approximation that is made in the modern approaches.

Theoretical study of the interaction of benzene with and cations
View Description Hide DescriptionExtensive ab initio calculations have been carried out on the benzene– complex, the sandwich complex and the benzene interaction products, viz., and The “physisorbed” benzene– was found to be in a state with symmetry. The ground state of the sandwich complex is with symmetry. Although the optimization of the ground state of the sandwich complex was carried out in symmetry, no symmetry breaking distortion in the structure of the benzene moiety was found in the final optimized geometry. The relative strength of binding of benzene with the cation in these two complexes has also been compared. The interaction of and benzene has been found to favor the formation of two different products consistent with the recent experiment of Bondybey and coworkers. The probability of the formation of the chemisorbed species with respect to the other cleaved product resulting from the physisorbed species, benzene– has been discussed in the light of energy calculations.

Theoretical study of photodissociation on the Pt(111) surface
View Description Hide DescriptionThe photodissociation of is studied by density functional theory and the stateaveraged complete active space selfconsistent field (SACASSCF) method using a cluster model With the small clusters the equilibrium molecule–surface distances are less than 2.3 Å and the binding energies are 4–14 kcal/mol, the order of the chemisorption. With larger clusters, the molecule–surface distance and the binding energy are calculated to be 3.00 Å and 0.67 kcal/mol, respectively, of the order of the physisorption, which coincides with the experiments. The SACASSCF calculations verify that, in spite of the weak interaction between and in the ground state, the first excited state of (Rydberg type) interacts with unoccupied states strongly, resulting in the chargetransfer state and finally leading to the dissociation to on the Pt(111) surface, the excitation energy to the Rydberg state of decreases by compared to that in an isolated molecule. These results support the experimental results that the direct excitation of is invoked on the Pt(111) surface by irradiation of the 193 nm photon, leading to the dissociation to and H.

Semiempirical methods with conjugate gradient density matrix search to replace diagonalization for molecular systems containing thousands of atoms
View Description Hide DescriptionConventional semiempirical methods using diagonalization are not practical for calculations on molecular systems containing more than a few hundred atoms because of time and memory requirements, where is the number of atoms. Currently, the time dominating step is diagonalization of the Fock matrix. This paper demonstrates how diagonalization and memory requirements are eliminated by using a conjugate gradient search for the density matrix with sparse matrix techniques. Our method makes high accuracy energy calculations on molecules containing thousands of atoms possible on the typical workstation. Benchmark examples are presented on polyglycine chains (20000 atoms), water clusters (up to 1800 atoms), and nucleic acids (up to 6304 atoms).

Normal order and extended Wick theorem for a multiconfiguration reference wave function
View Description Hide DescriptionA generalization of normal ordering and of Wick’s theorem with respect to an arbitrary reference function as some generalized “physical vacuum” is formulated in a different (but essentially equivalent) way than that suggested previously by one of the present authors. Guiding principles are that normal order operators with respect to any reference state must be expressible as linear combinations of those with respect to the genuine vacuum, that the vacuum expectation value of a normal order operator must vanish (with respect to the vacuum to which it is in normal order), and that the wellknown formalism for a single Slater determinant as physical vacuum must be contained as a special case. The derivation is largely based on the concepts of “Quantum Chemistry in Fock space,” which means that particlenumberconserving operators (excitation operators) play a central role. Nevertheless, the contraction rules in the frame of a generalized Wick theorem are derived, that hold for nonparticlenumberconserving operators as well. The contraction rules are formulated and illustrated in terms of diagrams. The contractions involve the “residual particle density matrices” , which are the irreducible (nonfactorizable) parts of the conventional particle density matrices , in the sense of a cumulant expansion for the density. A spinfree formulation is presented as well. The expression of the Hamiltonian in normal order with respect to a multiconfiguration reference function leads to a natural definition of a generalized Fock operator. MCSCFtheory is easily worked out in this context. The paper concludes with a discussion of the excited configurations and the firstorder interacting space, that underlies a perturbative coupled cluster type correction to the MCSCF function for an arbitrary reference function, and with general implications of the new formalism, that is related to “internally contracted multireference configuration interaction.” The present generalization of normal ordering is not only valid for arbitrary reference functions, but also if the reference state is an ensemble state.

Ab initio quantum chemical calculation of electron transfer matrix elements for large molecules
View Description Hide DescriptionUsing a diabatic state formalism and pseudospectral numerical methods, we have developed an efficient ab initio quantum chemical approach to the calculation of electron transfer matrix elements for large molecules. The theory is developed at the Hartree–Fock level and validated by comparison with results in the literature for small systems. As an example of the power of the method, we calculate the electronic coupling between two bacteriochlorophyll molecules in various intermolecular geometries. Only a single selfconsistent field (SCF) calculation on each of the monomers is needed to generate coupling matrix elements for all of the molecular pairs. The largest calculations performed, utilizing 1778 basis functions, required on an IBM 390 workstation. This is considerably less cpu time than would be necessitated with a supermolecule adiabatic state calculation and a conventional electronic structure code.

Theory of the kinetic critical nucleus in binary systems
View Description Hide DescriptionWe extend the theory of kinetic critical nucleus [K. Nishioka and I. L. Maksimov, J. Cryst. Growth 163, 1 (1996)] to binary systems for vapor–liquid transition and derive the equations to determine its size and composition. The kinetic critical nucleus corresponds to the extreme point (or saddle point) on the surface of the kinetic potential. By employing an available formula for the reversible work of forming a cluster, it is shown that the composition of the kinetic critical nucleus is the same as that of the thermodynamic one and the equation to determine the size of the kinetic critical nucleus has the same features as those for singlecomponent systems. Similar to singlecomponent systems, there are two values for the size of the kinetic critical nucleus.

Attraction of electrostatic origin between colloids
View Description Hide DescriptionThe interaction between two spherical colloids immersed in an electrolyte of screening constant κ is studied using the recent charge regulation primitive model and the hypernetted chain integral equation. The charge of the particles is not fixed a priori but results from the adsorption of positive and/or negative ions onto the colloidalsurfaces. In the case of symmetrical adsorption, the model exhibits a longrange attraction between the globally neutral colloids. At large distance, the effective colloid–colloid potential behaves as for small colloids where is the center to center interparticles distance and as for large colloids where is the surface to surface distance. For nonsymmetrical cases, such an attraction adds to the usual screened Coulombic repulsion between the globally charged colloids. The numerical results and the physical origin of the attraction are explained in terms of elementary diagrams. The attraction arises from the ion exchange between the adsorbed layers and the bulk and from the non meanfield ion–ion correlations.