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Volume 104, Issue 14, 08 April 1996

High pressure investigation of absorption spectra of J‐aggregates
View Description Hide DescriptionThe spectral shift of J‐bands under high pressure up to 60 kbar has been investigated for J‐aggregates of different cyanine dyes. Under conditions where no J‐band can be observed at normal pressure,J‐aggregates are formed at higher pressure. A red shift of absorption upon increasing pressure was found with a linear dependence of line position on pressure. The results can be explained by a change of dipolar coupling of monomer molecules in the aggregate due to decreasing center‐to‐center distances. The different pressure slopes of various aggregates are interpreted from their different aggregate structures. The monomer absorption is also red‐shifted under pressure, but the pressure dependence is quite different and can be described by a solvent shift. From the linear dependence, the dipole–dipole coupling energy J can be determined with a simple theory. The general applicability and limitations of this method are discussed.

Quantum beat study of the nuclear hyperfine structure of OD and Ar⋅OD in their A ^{2}Σ^{+} electronic states
View Description Hide DescriptionThe nuclear hyperfine structure of OD and Ar⋅OD in their A ^{2}Σ^{+} electronic states has been studied by quantum beatspectroscopy. The very cold transient species were produced in a supersonic expansion using a pulsed discharge nozzle. Coherent excitation of hyperfine (hf) states, arising from one fine structure (OD) or rotational (Ar⋅OD) level, created quantum beats on the fluorescence decay. The beat frequencies, which correspond to energy separations between hf levels, could be measured to ±75 kHz. The splitting of the hf levels into their Zeeman components was investigated in a weak magnetic field. A fit of the zero field and Zeeman data yielded the relevant constants for the nuclear magnetic and electric quadrupolehyperfine interactions as well as the pertinent g‐factors in each species. In the case of OD, the hf parameters agree well with those reported previously but are more accurately defined. For Ar⋅OD the previously unknown hyperfine and spin‐rotation parameters of the A ^{2}Σ^{+} state were determined. A comparison of the hf parameters in the two systems allowed assessment of the effect of van der Waals complex formation on the electron distribution. Thus complexation is found to reduce the unpaired electron density on the deuteron by 7% which is indicative of significant chemical bonding between the Ar atom and the OD moiety in the A ^{2}Σ^{+} state of Ar⋅OD. For both systems, the g‐factors g _{ S } and g _{ l } obtained suggest an admixture of other, possibly quartet, electronic states into the A ^{2}Σ^{+} state.

Multi‐rank nuclear magnetic resonance studies of half‐integer quadrupolar nuclei in solids by three‐dimensional dynamic‐angle correlation spectroscopy
View Description Hide DescriptionThe present work introduces a new three‐dimensional nuclear magnetic resonance (3D NMR) experiment for the analysis of half‐integer quadrupolar nuclei in solids. The method is based on the multi‐rank expansion of the high‐field NMR Hamiltonian governing the central transition of these spins in terms of irreducible spherical tensor elements. This approach leads to a temporal spin evolution given by an isotropic term characteristic of each chemical site, as well as by second‐ and fourth‐rank anisotropies depending on the principal values and relative orientations of the shielding and quadrupolar interactions. A method for extracting the 3D spectral distribution correlating these three frequency components is presented, based on the acquisition of dynamic‐angle spinning NMR signals collected as a function of different initial and final spinning axes. Computational and instrumental details involved in the acquisition of these 3D dynamic‐angle correlation spectroscopy (DACSY) data are discussed, and applications of the DACSY methodology to the analysis of different rubidium salts are illustrated. The new type of chemical information that this experiment can provide and its relation to other NMR techniques that have been recently developed for the analysis of quadrupolar nuclei in solids are also discussed.

Diaelastic pressure‐induced effects on spectral holes in crystals
View Description Hide DescriptionA statistical theory is developed in order to describe the pressure‐induced shift and broadening of spectral holes in pressure‐tuning experiments in crystals. The theory accounts for the defect‐related diaelastic effect (induction of internal inhomogeneous strain fields by the applied hydrostatic pressure due to the host‐defect compressibility and/or size mismatch). General results are specified and analyzed in the case of similar defects and for two different types of point defects. The former case yields no hole broadening, while the latter one does. A similar consideration applies to electric‐ and magnetic‐field‐induced effects on spectral holes in crystals as well.

Spectroscopy and dynamics of rare gas–spherical top complexes. II. The infrared spectrum of the ν_{3} band of Ne–SiH_{4} ( j=1←0 and j=0←1 transitions)
View Description Hide DescriptionThe infrared spectrum of the rare gas–spherical top complex Ne–SiH_{4} has been recorded in a supersonic jet in the region of the SiH_{4} ν_{3} triply degenerate stretching vibration at ∼2189 cm^{−1}. In contrast to the previously measured Ar–SiH_{4}spectrum which showed almost equal rotational spacings within each band (corresponding to transitions between different internal rotor states of SiH_{4} within the complex), the Ne–SiH_{4}spectrum is complex with no obvious regular band structure. However, by analogy with the Ar–SiH_{4}spectrum, four bands of the Ne–SiH_{4} have been assigned and analyzed in terms of Hamiltonians incorporating Coriolis interaction between the angular momentum of the SiH_{4}monomer unit and the overall end over end rotation of the complex. These bands correlate with the SiH_{4} R(0) (K=0←0, K=1←0) and P(1) (K=0←0, K=0←1) transitions. Derived rotational constants demonstrate that the neon–silane separation (∼4.13 Å in the ground vibrational state) is larger than expected by analogy with Ar–SiH_{4}, indicative of nearly free internal rotation by the silane monomer unit in Ne–SiH_{4}. The smaller anisotropy of Ne–SiH_{4} compared with Ar–SiH_{4} results in a new angular momentum coupling scheme. Transitions arising from ^{22}Ne–SiH_{4} correlating to SiH_{4} R(0) have also been observed and fitted; the higher than anticipated intensities demonstrate a novel isotope enrichment effect in the supersonic jet which is discussed.

The coupled electronic oscillators vs the sum‐over‐states pictures for the optical response of octatetraene
View Description Hide DescriptionA coupled electronic oscillator (CEO) analysis of the third harmonic generation (THG) spectrum for octatetraene is presented. The dominant oscillators and their couplings are identified using tree diagrams. The correspondence between the dominant oscillators in the CEO picture and the relevant excited states in the sum‐over‐states (SOS) description is demonstrated. The important channels in the SOS are related to the dominant oscillator pathways in the CEO picture.

Optical Stark spectroscopy of molecular aggregates
View Description Hide DescriptionEffects of static disorder and interaction with phonons on the dynamics of Frenkel excitons in molecular aggregates are studied by calculating the absorption of a weak probe in the presence of a strong resonant and off‐resonant pump field. To second order in the pump amplitude, the self‐energy which determines the Stark shift and dynamical broadening of the probe absorption is expressed in terms of the single exciton Green function and the two‐exciton scattering matrix. For stronger pump intensities the self‐energy is calculated using higher‐order optical response functions of the system.

The thermal diffusivity tensor and lattice dynamics of β‐oxygen at high pressure
View Description Hide DescriptionAn experimental approach to the study of thermal transport in the diamond‐anvil high pressure cell is described and the elements of the thermal diffusivitytensor of β‐oxygen are reported to a pressure of 9 GPa. The anisotropy in the thermal conductivity is found to be opposite in sense to that in the velocity of acoustic waves. Results are interpreted in terms of a lattice dynamical model in which the strong interactions between an atom and its first‐, second‐, and third‐nearest neighbors are treated in tensorial form with parameters evaluated directly from experiment.

Impulsive stimulated thermal scattering study of α relaxation dynamics and the Debye–Waller factor anomaly in Ca_{0.4}K_{0.6}(NO_{3})_{1.4}
View Description Hide DescriptionImpulsive stimulated light scattering experiments were carried out on an ionic glass former Ca_{0.4}K_{0.6}(NO_{3})_{1.4} to determine the temperature dependence of the Debye–Waller factor f _{ q }(T) in the q→0 limit and to investigate the structural relaxation dynamics. A square‐root cusp anomaly was observed in f _{ q→0}(T) at a crossover temperature T _{ c } of 378 K. This and the relaxation dynamics observed were consistent with predictions of the mode‐coupling theory of the liquid–glass transition.

A combined quantum chemical and transition state theory study of the C_{2}H^{+} _{2}+CH_{4} reaction dynamics
View Description Hide DescriptionThe stationary points of importance in the initial branching between complex formation and H transfer in the reaction of acetylene cation with methane are studied at the Gaussian‐2 level of theory. Three separate C_{2}H^{+} _{2}...CH_{4} ion‐neutral complexes are found. One complex leads smoothly to the bridged form of C_{2}H^{+} _{3} plus CH_{3}. A slightly exothermic saddlepoint also connects this ‘‘bridged’’ complex to a stable C_{3}H^{+} _{6} species. A ‘‘classical’’ complex leads to the more endothermic classical form of C_{2}H^{+} _{3}. These classical and bridged ion‐neutral complexes are nearly degenerate and are directly connected via a low energy saddlepoint which is substantially exothermic relative to reactants. The third, more weakly bound complex, is of a more electrostatic nature and appears to be of no importance in the subsequent branching between the C_{3}H^{+} _{6} and C_{2}H^{+} _{3}+CH_{3} products. Transition state theory calculations employing the ab initio determined energetics and quadratic force fields provide estimated reactive cross sections which are in qualitative accord with recent experimental data. The quantitative discrepancies may be explained on the basis of the neglect of anharmonicites and/or errors in the estimated energetics. Consideration of the vibrational frequencies for the various stationary points provides a qualitative explanation for the observed variations in the reaction cross sections with initial vibrational excitations in the CC stretching and bending modes of C_{2}H^{+} _{2}.

Investigating conformation dependence and nonadiabatic effects in the photodissociation of allyl chloride at 193 nm
View Description Hide DescriptionThe experiments presented here investigate the competing photodissociation pathways for allyl chloride upon excitation of the nominally ππ*(C=C) transition at 193 nm. The measured photofragment velocity distributions evidence C–Cl bond fission and HCl elimination. The recoil kinetic energy distribution for the HCl products is bimodal, indicating two primary processes for HCl elimination. The experimental measurements show C–Cl bond fission dominates, giving an absolute branching ratio of HCl:C–Cl=0.12±0.03 when the parent molecule is expanded through a nozzle at 200 °C. The branching ratio depends on the nozzle temperature; at 475 °C, the absolute branching ratio measured is HCl:C–Cl=0.24±0.03. We analyze the experimental results along with supporting ab initio calculations and earlier photodissociation studies of vinyl chloride in order to examine the potential influence of nonadiabaticity along the C–Cl fission reaction coordinate and its dependence on molecular conformation.

A generalized approach to the control of the evolution of a molecular system
View Description Hide DescriptionThe theory of active control of molecular motion by use of shaped laser pulses is developed emphasizing the role of interference and using thermodynamic analogies. Attention is focused on the control of the dynamics in a system with n states coupled by radiation, and the phase relations which generate particular control schemes are derived. Among the new results reported is an optimal control scheme which constrains the value of the phase. The n‐state model can be considered to represent a molecule with n electronic potential energy surfaces and an arbitrary number of degrees of freedom or as the skeleton spectrum of system where each level in the spectrum can be associated with a specific set of quantum numbers for all of the degrees of freedom. We show how the control of the dynamics of an n‐state molecule can be represented in terms of the control of the dynamics of a precisely defined surrogate fewer state system. This reduction is illustrated by use of a surrogate two state system to describe the dynamics of population transfer in a three state system.

Partitioning of the nonfixed excess energy and the reverse critical energy in CH_{2}OH^{+}→CHO^{+}+H_{2}: A classical trajectory study
View Description Hide DescriptionDynamics of the four‐centered elimination reaction CH_{2}OH^{+}→CHO^{+}+H_{2} has been investigated over the internal energy range 4.6–5.9 eV using the classical trajectory method. A realistic semiempirical potential reported previously [J. Chem. Phys. (in press, 1996)] has been used for the calculation. It has been found that the disposal of the nonfixed excess energy at the transition state and of the reverse critical energy can be considered independently as manifest in the sum rule analysis. The former is determined statistically while the latter dynamically. Based on the above idea, a method to determine the kinetic energy release distribution originating only from the reverse critical energy has been developed.

Discharge flow‐tube studies of O(^{3} P)+N_{2}H_{4} reaction: The rate coefficient values over the temperature range 252–423 K and the OH(X ^{2}Π) product yield at 298 K
View Description Hide DescriptionThe absolute second‐order reaction rate coefficient, k _{1}, for the gas phase reaction, O(^{3} P)+N_{2}H_{4}→products, was studied in a discharge flow‐tube apparatus. The reaction was studied under pseudo‐first‐order conditions in O(^{3} P) concentration (i.e., [N_{2}H_{4}]≫[O(^{3} P)]). The O atoms were generated by a microwavedischarge of a suitable precursor gas in He in a fixed side‐arm reactor upstream of the flow tube, or in the sliding inner injector of the flow tube. The hydrazine concentration was photometrically measured and introduced into the apparatus in a flow of He via the sliding injector or the fixed side‐arm port, respectively. The kinetics of the O‐atoms in the reaction was directly followed by 130.2–130.6 nm cw‐resonance fluorescence detection of O(^{3} P) at the fixed detector situated downstream of the flow tube. The Arrhenius expression, k _{1}=(7.35±2.16)×10^{−13} exp[(640±60)/T] cm^{3} molec^{−1} s^{−1}, in the temperature range 252–423 K, was fit to the data points. The rate coefficient at room temperature was, within experimental errors, independent of the He buffer gas pressure in the range 1.74 to 8.30 Torr, or the O‐atom source reactor. The formation of OH(X ^{2}Π) in the reaction, which can be vibrationally excited (v″≤2), was directly detected by pulsed laser‐induced fluorescence. The total yield of OH in the reaction was determined to be (0.15±0.05) at 298 K, of which ∼50% is thought to be produced vibrationally hot. These results suggest that the single‐H‐atom removal channel is a minor process, in agreement with earlier molecular beam studies in which a direct two‐H‐atom removal channel was proposed to be the principal reaction mechanism by which O(^{3} P) reacts with N_{2}H_{4}.

Two‐photon dissociation/ionization beyond the adiabatic approximation
View Description Hide DescriptionAn accurate theory of two‐photon dissociation with strong laser pulses, which goes beyond the adiabatic approximation, is developed. Criteria for adiabatic behavior in two photon dissociation (enabling via adiabatic passage the complete population transfer to the continuum), are derived. We obtain the minimal pulse duration needed to ensure adiabaticity as a function of the field intensities and detuning. In addition, we develop a simple, rapidly converging, iterative scheme, built on the adiabatic approximation, for the exact solution of the two‐photon dissociation problem. Each iteration step requires a computational effort that scales as N ^{2} (N being the system dimension) for full matrices, or N log N for sparse matrices. The iterative scheme is tested by comparing it to the Runge–Kutta–Merson direct integration algorithm. It is found to work well even when the adiabatic approximation fails completely.

A comparison of models for calculating nuclear magnetic resonance shielding tensors
View Description Hide DescriptionThe direct (recomputation of two‐electron integrals) implementation of the gauge‐including atomic orbital (GIAO) and the CSGT (continuous set of gauge transformations) methods for calculating nuclear magnetic shieldingtensors at both the Hartree‐Fock and density functional levels of theory are presented. Isotropic ^{13}C, ^{15}N, and ^{17}O magnetic shielding constants for several molecules, including taxol (C_{47}H_{51}NO_{14} using 1032 basis functions) are reported. Shielding tensor components determined using the GIAO and CSGT methods are found to converge to the same value at sufficiently large basis sets; however, GIAO shielding tensor components for atoms other than carbon are found to converge faster with respect to basis set size than those determined using the CSGT method for both Hartree‐Fock and DFT. For molecules where electron correlation effects are significant, shielding constants determined using (gradient‐corrected) pure DFT or hybrid methods (including a mixture of Hartree‐Fock exchange and DFT exchange‐correlation) are closer to experiment than those determined at the Hartree‐Fock level of theory. For the series of molecules studied here, the RMS error for ^{13}C chemical shifts relative to TMS determined using the B3LYP hybrid functional with the 6‐311+G(2d,p) basis is nearly three times smaller than the RMS error for shifts determined using Hartree‐Fock at this same basis. Hartree‐Fock ^{13}C chemical shifts calculated using the 6‐31G* basis set give nearly the same RMS error as compared to experiment as chemical shifts obtained using Hartree‐Fock with the bigger 6‐311+G(2d,p) basis set for the range of molecules studied here. The RMS error for chemical shifts relative to TMS calculated at the Hartree‐Fock 6‐31G* level of theory for taxol (C_{47}H_{51}NO_{14}) is 6.4 ppm, indicating that for large systems, this level of theory is sufficient to determine accurate ^{13}C chemical shifts.

Many‐body potentials and dynamics based on diatomics‐in‐molecules: Vibrational frequency shifts in Ar_{ n }HF (n=1–12,62) clusters
View Description Hide DescriptionThe conjecture that limited basis diatomics‐in‐molecules type potentials may serve as an accurate representation of many‐body interactions is explored through molecular dynamics simulations of Ar_{ n }HF (n=1–12,62). The important ingredient in the constructed potentials is the inclusion of ionic configurations of HF. Once the admixture between ionic and covalent configurations is calibrated by reference to an ab initiosurface of the ArHF dimer, a single three‐body potential energy surface is defined, and used in subsequent simulations of larger clusters. The vibrational frequencies of HF, which are computed from velocity–velocity autocorrelation functions, quantitatively reproduce the cluster size dependent redshifts.

Quantum mechanical and semiclassical dynamics at a conical intersection
View Description Hide DescriptionWe present simulations of wave‐packet dynamics for a model of a conical intersection in two dimensions. The potential energy surfaces and couplings are functions of a total symmetrical coordinate and of a symmetry breaking one. The wave packet crosses the coupling region once, moving essentially in the direction of the symmetrical coordinate. The dynamics are determined by two methods, one quantum mechanical and the other semiclassical, based on trajectories and surface hopping. The semiclassical approximation is quite adequate for low coupling strengths in the diabatic representation, less so for larger couplings. Approximate analytic solutions for the two‐dimensional problem and for one‐dimensional analogs are provided, in order to generalize the numerical results and to analyze the reasons of the discrepancies between semiclassical and quantum mechanical results.

Electronic properties of polymers based on thienothiadiazole and thiophene
View Description Hide DescriptionAb initio crystal orbital (CO) studies on the geometric and the electronic structures of poly(thienothiadiazole) (Poly‐1a) and periodic copolymers of thienothiadiazole and thiophene with the ratio of 1:1 (Poly‐2b) and 1:2 (Poly‐3b) are presented. In Poly‐1a and Poly‐2b, considerable change in the geometries is found to occur as one moves from their oligomers to the polymers. Although thienothiadiazole oligomers have a very small highest occupied molecular orbital (HOMO)‐lowest unoccupied molecular orbital (LUMO) separation, the corresponding polymer (Poly‐1a) has a considerably large band gap. On the contrary, the geometric and electronic structures of Poly‐3b are almost identical to those of the oligomer, and Poly‐3b possesses a direct band gap of 1.3 eV estimated by simple scaling. The reasons for these differences are discussed in terms of orbital interactions and nonbonding molecular orbitals in the oligomers, and the reduced π‐conjugation and enhanced intercell interaction in the polymers.

Solvent effects on the potential energy surface of the 1:1 complex of water and formamide: Application of the polarizable continuum model to the study of nonadditive effects
View Description Hide DescriptionA study of the solvent effect on the potential energy surface of the 1:1 complex of water and formamide have been performed. In the description of the solvent we have employed the polarizable continuum model. The calculations were done at Hartree–Fock ab initio and Mo/ller–Plesset (MP) levels. We found that the geometry of the system is appreciably modified by the solvent. The most important changes are the inversion of the water molecule orientation and the increase of the O(formamide)–H(water) distance by about 0.2 Å. In the gas phase binding to the carbonyl is energetically equivalent to binding to the amino group. However, in solution, water binds better to the carbonyl oxygen that to the NH group. The nonadditive contributions are, in general, important and can be related to the change in the monomer energies when one passes from the monomeric to the dimeric reaction potential.