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Volume 106, Issue 13, 01 April 1997

Rotational spectra, structure, and internal dynamics of isotopomers
View Description Hide DescriptionSeveral groups have studied the rotational spectra of various isotopomers of the pseudolinear dimer. We have found two additional progressions, for and , which show that the symmetrical isotopomers (with or ) exist in lower and upper states giving progressions with differing by about 50 MHz. When assigned and analyzed on this basis, the overall spectra are quite similar to those of the analogue where the doubling is caused by and internal rotor states of the or . Such states of the most likely cause the doubling found in the isotopomers. However, some of the details differ in interesting ways, indicating substantial differences in their potential energy surfaces. The lower states of and have ’s about 28 MHz larger than those for , while the isotopomers of the upper states exhibit a more normal decrease of about 12 MHz. Somewhat related to this is the effect of deuteration on the difference in between lower and upper states. Perdeuteration of decreases from 76 to 68 MHz. The corresponding change for is much larger, from 44 to 5 MHz. For the internal rotor model, analysis of the hfs for assigns its upper and lower states to and rotor states of the water, respectively. But this assignment is reversed in . Also, we have observed and fitted the quadrupolar hfs for the lower and upper states of , finding values for of −7.89 and −17.36 MHz.

The weak interaction potential of
View Description Hide DescriptionAb initio calculations have been carried out to generate a potential energy surface for the weakly bonded cluster. The surface displays a low energy trough with small barriers for the “orbit” of Ar about . Basis set and correlation effects have been analyzed through a series of calculations at different levels, and the feature of a low energy trough seems assured. The nature of this surface has only little to do with electrostaticinteraction, and so, a fairly good fit of the nonelectrical part of the potential surface is obtained with atom–atom LennardJones potential terms. With different surface fits and a fully anharmonic analysis of the ground vibrational state, onaverage rotational constants were calculated via rigidbody diffusion quantum Monte Carlo. These calculations show that certain intriguing spectroscopic manifestations of deuterium substitution are associated primarily with the trough feature of the surface.

Degenerate fourwave mixing spectroscopy and spectral simulation of in an atmospheric pressure oxyacetylene flame
View Description Hide DescriptionThe Swan bands of have been recorded with high resolution using DFWM in the nearly Doppler free, phase conjugate geometry. was probed in a standard oxyacetylene weldingflame with excellent signaltonoise ratio and spectral resolution. Theoretical spectra were simulated and fitted directly to the complex overlapping spectra. The good agreement obtained shows that DFWM holds promise to become a robust and reliable tool for flame thermometry. Current theories of DFWM are reviewed in context of the present work and advantages and disadvantages of the technique are discussed.

Unimolecular dissociation dynamics of highly vibrationally excited DCO (X̃ ^{2}A). I. Investigation of dissociative resonance states by stimulated emission pumping spectroscopy
View Description Hide DescriptionThe vibrational level structure and unimolecular dissociation dynamics of highly vibrationally excited X̃ ^{2}A DCO were investigated using the method of stimulated emission pumping spectroscopy (SEP). Single vibrationrotation states were probed with excitation energies up to E(X̃)=18 200 cm^{−1}, ≈12 700 cm^{−1} above the asymptotic DCO dissociation limit. The vibrational level structure of the molecule was found to be determined by distinctive polyads arising from a 1:1:2 resonance between the CD stretching, CO stretching, and DCO bending vibrations. Anharmonic coupling mechanisms give rise to considerable level mixings, especially regarding the CD and CO stretching motion. Thus, only a minority of vibrational states can be unambiguously assigned. The spectral line shape profiles of ≈100 highly excited “resonance states” in the continuum above the DCO dissociation limit were measured at high resolution. The profiles are homogeneously broadened. The unimolecular decay rates, obtained from the observed line widths, were observed to fluctuate by more than two orders of magnitude in a strikingly state specific manner. The decay rates on average increase with increasing vibrational excitation energy. The state resolved experimental data are compared to predictions of the microcanonical specific unimolecular rate coefficients calculated from different statistical models. Serious problems were encountered considering the calculation of the density of states of the molecules in the continuum region of the potential energy surface regarding the contribution of the disappearing oscillator. Despite tentative corrections, the calculated rate coefficients were to too high by one to two orders of magnitude. Overall, the unimolecular dynamics of DCO appears to conform to an intermediate case between the strictly vibrationally “mode specific” and the “statistical” limits.

Unimolecular dissociation dynamics of highly vibrationally excited DCO. II. Calculation of resonance energies and widths and comparison with highresolution spectroscopic data
View Description Hide DescriptionWe present a theoretical study of the unimolecular dissociation of DCO in the electronic ground state, using a new ab initiopotential energy surface. Altogether we have analyzed about 140 resonances up to an energy of 1.4 eV above the D+CO threshold, corresponding to the ninth overtone in the CO stretching mode The agreement of the resonance positions and widths with recent stimulated emission pumping measurements of Stöck et al. [J. Chem. Phys. 106, 5333 (1997), the preceding article] is pleasing. The rootmeansquare deviation from the experimental energies is only 16 cm over a range of about 16 500 cm and all trends of the resonance widths observed in the experiment are satisfactorily reproduced by the calculations. A strong 1:1:2 stretch–stretch–bend resonance prohibits a unique assignment for the majority of vibrational states.

Vibrational spectroscopy of CH/NH stretches in pyrrole: An algebraic approach
View Description Hide DescriptionThe complete vibrational spectroscopy of pyrrole is addressed within the onedimensional algebraic framework. Particular attention is devoted to the study of the CH/NH stretching levels, including their possible anharmonic resonance mechanisms with other ring modes.

Twodimensional deuterium magicanglespinning nuclear magnetic resonance of paramagnetic compounds: Separation of paramagnetic and quadrupole interactions
View Description Hide DescriptionA new twodimensional magicanglespinning NMR experiment to study paramagnetic compounds is presented. It can be carried out by the twopulse sequence –(, acquisition) or the threepulse sequence –(, acquisition). The latter has the advantages over the former in that pureabsorption spectra can be obtained, and also finite pulsewidth effects are less apparent. The intensities of the twodimensionally spread sidebands are calculated as a function of the paramagnetic and the quadrupole coupling parameters, the timeevolution of the spin system during the rf pulse being involved. The method is applied to paramagnetic coordination compounds with selectively deuterated acetate groups: and . The principal values of the two interaction tensors can be separately estimated from the projections of the twodimensional spectrum onto the corresponding diagonals. These values are further refined by simulations of the twodimensional spinning sideband pattern, which also allows us to determine the mutual orientation of the two tensors. The paramagnetic shift tensors are calculated on the basis of the crystal structures and are compared with those obtained by the experiments. The effect of the bulk magnetic susceptibility on the observed paramagnetic shift tensor is discussed. Some methods to remove this effect are suggested.

The dissociative recombination of hydrocarbon ions. I. Light alkanes
View Description Hide DescriptionRate coefficients for the dissociative recombination of a series of hydrocarbon ions derived from normal alkanes have been measured using the FALPMS technique. Despite the increasing complexity of the ions along the series, the measured rate coefficients are found to be remarkably similar having values between 5.5 and .

Laser probing of velocitysubgroup dependent rotational alignment of drifted in He
View Description Hide DescriptionResults are presented for Dopplerresolved laserinduced fluorescencemeasurements of collisioninduced rotational alignment of N_{2} ^{+}(v″=0) drifted in He in a drifttube apparatus. A singlefrequency ring dye laser is used to probe the rotational line of the system both parallel and perpendicular to the drift field at three different field strengths and at several different Dopplerselected velocities. A strong correlation is found between the degree of rotational alignment and the velocity subgroup probed along the field direction. For field strengths of 8 and 16 Td and laser probe parallel to the drift field, there is a monotonic increase in the quadrupole alignment parameter with higher velocity subgroup, up to a maximum value of (6) for 16 Td at the highvelocity tail. There is evidence that the correlation between alignment and velocity increases with increasing field strength. The mechanisms of the alignment are discussed and these results are attributed primarily to the change in anisotropy of the relative velocity vector distribution of the N_{2} ^{+}–He pair with field strength.

Photodissociation of in the ultraviolet and nearinfrared: Wavelength dependence of KrF yield
View Description Hide DescriptionThe photoabsorptionspectrum of the excited state of has been measured in the 280–850 nm region by fluorescence suppression spectroscopy. Both the and bands, peaking at 320 and ∼710 nm, respectively, have been observed—the latter for the first time. Although the position of the ultraviolet band is consistent with both theory and previous experiments, its spectral width is ∼40% of that measured by Greene and McCown [Appl. Phys. Lett. 54, 1965 (1989)] in absorption experiments in a pulsed discharge. The relative yield of produced in the photodissociation of was found to be independent of wavelength over the 280–360 nm and 590–840 nm spectral intervals and the mechanism responsible for photodissociation appears to be predissociation of the and states by potentials correlated with the and limits, respectively.

Laserinduced fluorescence spectroscopy of the band system of jetcooled CCN radical
View Description Hide DescriptionLaserinducedfluorescence spectra of the system of the CCN radical were observed under jetcooled conditions. The radical was generated in a supersonic expansion by a pulsed electric discharge of methyl cyanide diluted in Ar. Sixteen vibronic bands with , , and were observed, including some very weak vibronically induced paralleltype transitions with . Rotational analyses were performed to give the band origins and the rotational constants for vibrationally excited states of all the three normal modes and their combinations in the state. From the determined band origins, the term value and the vibrational parameters in the state have been determined: , , , , and . A combination of the present frequency in the state with the data on the hot bands reported so far in literature has yielded the vibronic structures in the and states, which are subject to a prominent Renner–Teller effect.

Classical and approximate quantum investigations of vibrational energy transfer in difluorobenzene
View Description Hide DescriptionA simple modelpotential energy surface is constructed and used in both quasiclassical trajectory calculations and quantum vibrational closecoupling, infinite order sudden approximation calculations of collisioninduced vibrational energy transfer from four vibrational states of difluorobenzene. Classical and quantum statetostate cross sections are compared for excitation of the two lowest energyvibrational states and collision with He or Ar. Qualitatively, the same trends are seen in both sets of results. Classical cross sections, however, are significantly larger at very low collision energies as a consequence of the binning procedures used to determine classical final states and, in the case of the Ar collider, as a result of the possible breakdown of the sudden approximation. Rotational excitation of the difluorobenzene molecule is also investigated and found to have only small effects on the dominant energy transfer channels. The theoretical results are compared with recent experimental results of Mudjijono and Lawrance [J. Chem. Phys. 104, 7444 (1996)]. The classical results, for the He, Ne, Ar, and Kr collision partners, show good agreement with experiment, reproducing the major energy transfer channels and the experimental collision partner dependence. Quantum results agree well with experiment for the He collider and are also used to assign experimentally ambiguous product states and to investigate vibrational energy transfer channels that are not experimentally observable. The propensity toward the transfer of multiple quanta of vibrational energy is analyzed and, in general, found to increase with the intermolecular well depth and with the mass of the collision partner. The He collision partner, however, behaves anomalously.

Fragment rotational distributions from the dissociation of : Experimental and classical trajectory studies
View Description Hide DescriptionThe fragment rotational distributions that result from the vibrational predissociation of in the electronic state have been measured for several initial vibrational levels. In each case, the rotational distributions extend to the effective energetic limit determined by the amount of energy available for disposal into the fragment rotational and translational degrees of freedom. Analysis of the data allows refinement of the dissociation energy; we find that for the electronic state, . Both and −2 dissociation events have been examined. For dissociation pathways with approximately the same value of the pathways are observed to have a higher fraction of the fragment energy in rotational excitation. The overall shape of the distributions are insensitive to the value of suggesting that a Franck–Condon model for the dissociation may have some validity, though quantitative quantum mechanical calculations demonstrate that this model does not reproduce the large degree of fragment rotational excitation. Two classical models for the dissociation also fail to reproduce the extent of fragment rotational distribution. This result is discussed in light of previous experimental and theoretical investigations, focusing on the apparent agreement of classical models with the IBr fragment rotational distributions that result from the dissociation of NeIBr.

Photochemistry and dynamics of clusters at 226 nm
View Description Hide DescriptionThe photochemistry and dynamics of small clusters were studied in a supersonic expansion using 226 nm laser excitation and multiphoton ionization probes. We were able to detect a strong signal due to when mixed clusters were present in the expansion but no O atom fragments could be observed in the absence of benzene in the expansion mixture. Photofragmentation of in the unique environment of the cluster is enhanced by at least three orders of magnitude compared to the isolated oxygen molecule. The kinetic energy release of the was determined with a timeofflight method and found to be relatively small and characterized by a completely isotropic spatial distribution. The fine structure population of the was also examined and the resultant branching fractions, , 0.26±0.06, 0.06±0.01, are similar to those obtained for photodissociation of isolated by other workers. We also find that photochemical production of oxygen containing products, such as , becomes feasible in larger cluster species due to solvent cage effects which trap the recoiling O atom fragments. The observed dynamics can be attributed to either excitation of the supramolecular chargetransfer state, or localized excitation of a perturbed transition in . The net effect of cluster absorption is to greatly enhance a chemical pathway that is only weakly observed in the separated molecules, similar to the behavior that has recently been described for the complex.

Kinematic mass model of activated bimolecular reactions: Molecular shape effects and zeropoint energy corrections
View Description Hide DescriptionThe recently proposed simple collision model of activated bimolecular reactions which takes into account the nonspherical shape of molecules and includes the effects of reagent rotation has been studied in considerable detail, in order to determine its range of applicability. By taking the limit of hard nonspherical molecules in which the formulation becomes rigorous it is possible to show that the model is likely to work well if the shape of the potential surface is prolate in the region of the reaction barrier, as long as a direct reaction mechanism is involved. The model cannot be expected to work for diatomic reactants if the shape of the potential surface is pronouncedly oblate because of the complicated reorientation effects which such a surface exerts on the trajectories and which do not appear amenable to simple modeling. The original model is improved by including the zeropoint energy corrections on the assumption of vibrational adiabaticity en route to the barrier. With the improved model, crosssections are calculated for the reactions:, , , , , , and the results are compared with those of quasiclassical trajectory calculations and, in the last two cases, also with those of quantum mechanical calculations. The possible origins of the significant discrepancies which occur in some cases are also discussed.

The reactive flux method applied to complex isomerization reactions: Using the unstable normal mode as a reaction coordinate
View Description Hide DescriptionA basic problem when calculating reaction rates using the reactive flux method is the introduction of a reaction coordinate. In this paper we show that it is advantageous to define a reaction coordinate by means of the unstable normal mode of the saddle point of the potential energy surface. This particular choice is made since it yields a high transmission function. Moreover, the reaction coordinate is calculated via a rapidly converging algorithm, and its derivative, which is needed in constrained runs, is calculated analytically. Calculations on the transmission coefficient of the isomerization of butane are in good agreement with results published by others. Runs with an isomerizing calix[4]arene in vacuo produce a very high transmission coefficient, as is the purpose of the reaction coordinate. The same molecule is also studied in chloroform.

Excited state kinetics of neutral transition metal atoms by stimulated emission pumping:
View Description Hide DescriptionStimulated emissionpumping (SEP) by two tunable, ns dye lasers selectively creates metastable excited states near 8500 in a fast flow reactor with 1.2 Torr He buffer gas at 300 K. Subsequent collisions with He equilibrate the spinorbit levels on a time scale of 1 μs but do not quench the , multiplet on a time scale of at least 50 μs. A third probe laser placed 5 mm downstream of the SEP lasers monitors the population at fixed reaction time μs. The exponential decay of vs calibrated hydrocarbon flow yields total removal rate constants that include both chemical reaction and electronic quenching of the excited state. Coupling of diffusion with reaction/quenching evidently does not cause the slight positive curvature observed in the kinetics plots. We demonstrate this by solving two simplified models of the reacting, diffusing, flowing gas for our experimental geometry, with SEP and probe laser beams transverse to the flow tube axis. Instead, the curvature is probably caused by the delayed, collisional cascade of a small population of higher excited states into the state. Ethylene removes the state on essentially every hardspheres collision. The alkanes , , and remove on one in 75, one in 25, and one in 12 collisions. This is remarkably inefficient reaction/quenching, given that the lowspin state is ideally configured for CH or CC bond insertion in alkanes. We interpret these new results and earlier work of Honma and coworkers on the sextet excited state in terms of avoided intersections between repulsive and attractive diabatic potential surfaces (conserving electron spin and configuration).

Linear scaling computation of the Fock matrix
View Description Hide DescriptionComputation of the Fock matrix is currently the limiting factor in the application of HartreeFock and hybrid HartreeFock/density functional theories to larger systems. Computation of the Fock matrix is dominated by calculation of the Coulomb and exchange matrices. With conventional Gaussianbased methods, computation of the Fock matrix typically scales as where is the number of basis functions. A hierarchical multipole method is developed for fast computation of the Coulomb matrix. This method, together with a recently described approach to computing the HartreeFock exchange matrix of insulators [J. Chem. Phys. 105, 2726 (1900)], leads to a linear scaling algorithm for calculation of the Fock matrix. Linear scaling computation the Fock matrix is demonstrated for a sequence of water clusters at the restricted HartreeFock/321G level of theory, and corresponding accuracies in converged total energies are shown to be comparable with those obtained from standard quantum chemistry programs. Restricted HartreeFock/321G calculations on several proteins of current interest are documented, including endothelin, charybdotoxin, and the tetramerization monomer of P53. The P53 calculation, involving 698 atoms and 3836 basis functions, may be the largest HartreeFock calculation to date. The electrostatic potentials of charybdotoxin and the tetramerization monomer of P53 are visualized and the results are related to molecular function.

Theoretical studies of large water clusters: (H_{2}O)_{28}, (H_{2}O)_{29}, (H_{2}O)_{30}, and (H_{2}O)_{31} hexakaidecahedral structures
View Description Hide DescriptionThe 28, 29, 30, and 31 mer hexakaidecahedral water clusters were studied by applying the intermediate neglect of differential overlap selfconsistent field restricted Hartree–Fock method (INDO SCF RHF) after parametrization for H and O atoms. The most stable 29 and 30 mer clusters have one and two water molecules, respectively within the cavity of the distorted hexakaidecahedral cage with stabilization energy/monomer values of around 10.9 and 11.0 kcal, respectively. The 31 mer cluster with three water molecules within the cavity is less stable than the isomer with two molecules within the cavity and the third one bonded outside of the cage by around 3 kcal/mol.

Evidence of quinonoid structures in the vibrational spectra of thiophene based conducting polymers: Poly(thiophene), , and
View Description Hide DescriptionBy combining vibrational spectra and ab initio calculations, we obtained a consistent description of the IR and nonresonant Raman spectra, including intensities, of four thiophene based polymers—undoped and heavily doped poly(thiophene) (PTh), undoped (PITN), and (PThP) for the first time. Predicted spectra for poly(thiophene) agree with experiment very well. Based on the calculated force constants and Badger’s rule, we also estimated the average interring bond lengths of undoped and doped PTh to be 1.47 and 1.42 Å, respectively. The latter leads to an estimated 33% quinonoid character on average for heavily doped PTh. The average interring bond lengths of undoped PITN and PThP, that are consistent with their vibrational spectra, are estimated to be 1.41, and 1.42 Å, respectively. These values showed that undoped PITN and PThP have quinonoid character close to that of heavily doped PTh. Further, we also estimated that, upon doping the average bond lengths of PTh changed by −0.01, 0.11, and −0.05 Å for intraring , , and interring bonds, respectively. These bond length changes are significantly different from those of Hartree–Focktype calculations, reflecting significant correlation contributions and are also in conflict with earlier empirical fits of the vibrational spectrum of the highly doped phase of PTh. However, our results are more in line with the generally accepted picture of an aromatic to quinonoid “transition” of the doping process. Furthermore, the counterintuitive downward frequency shifts in the vibrational spectra of PTh upon doping can be explained by the structural change from an essentially aromatic to a partially quinonoid form.