Volume 111, Issue 13, 01 October 1999
 GAS PHASE DYNAMICS AND STRUCTURE: SPECTROSCOPY, MOLECULAR INTERACTIONS, SCATTERING, AND PHOTOCHEMISTRY


Direct observation of rotational transitions of the CO–CO dimer
View Description Hide DescriptionMeasurements of five pure rotational transitions of a mixed isotopomer of the CO–CO van der Waals dimer with a Fourier transformmicrowave spectrometer in the frequency range from 3 to 19 GHz are reported. For symmetry reasons, pure rotational transitions in the vibrational ground state are not accessible in the symmetric species, and the mixed isotopomer was studied instead. The observed lines were identified as belonging to the dimer by isotopomeric variation of the sample composition, monitoring of the microwave excitation pulse conditions, and comparison of the measured frequencies with those predicted in a recent infrared study. [M. D. Brookes and A. R. W. McKellar, J. Chem. Phys. (submitted).]

Collisional dynamics of I. Quantumresolved vibrational energy transfer for
View Description Hide DescriptionVibrationaltotranslational energy transfer between the lowest vibrational levels of the state of has been investigated using spectrally resolved, laserinduced fluorescence techniques. The small vibrational spacing leads to highly nonadiabatic conditions, particularly for the collision pair. However, the transition probabilities for collisions with the rare gases range from 0.75% to 1.75% per collision, considerably lower than would be anticipated from standard vibrational energy transfertheory. Multiquantum transfer rates are low, consistent with the low anharmonicity of the state. The rates for transitions scale linearly with vibrational quantum number as expected near the bottom of this nearly harmonic potential.

The structure and ground state dynamics of Ar–IH
View Description Hide DescriptionThe structure and ground state dynamics of the atom–diatom dimer interaction between Ar and HI has been investigated by microwave and near infrared supersonic jet spectroscopy.Ab initio molecular orbital calculations were used to provide greater insight into the nature of the interaction. The ground state is shown to be in the isomeric form Ar–IH with for the normal isotopomer and for Ar–ID. The potential surface from an ab initio molecular orbital calculation was scaled and shifted to yield a nonlinear leastsquares fit of the rovibrational state energies to the experimental data. The ground statepotential energy surface obtained in this manner has a barrier between the Ar–IH and Ar–HI isomers of 88.5 cm^{−1} with respect to the global minimum. Such calculations are also used to predict the presence of localized states in the secondary minimum associated with isomers Ar–HI and Ar–DI. Attempts to experimentally identify transitions associated with the latter were unsuccessful. The ground state, Ar–IH isomeric structure, contrasts with the corresponding ground state of the other members of the homologous series Ar–HX ( Cl, and Br) in which the Ar is bound to the proton.

Photodissociation dynamics of studied using resonance enhanced multiphoton ionization (REMPI) with timeofflight mass spectrometry
View Description Hide DescriptionThe photodissociation dynamics of have been studied using resonanceenhanced multiphoton ionization with timeofflight mass spectrometry.Polarization dependent timeofflight profiles were collected for a range of wavelengths from 248 to 268 nm, corresponding to the red wing of the absorptionspectrum. Forward convolution fits to the data have provided translational energy distributions and anisotropy parameters over the entire wavelength range for both and The average translational energies for the Br and channels are 20 and 23 kcal/mol, respectively. The measured anisotropy parameters indicate that both channels arise preferentially from a parallel transition and that the relative contribution of this transition increases with decreasing wavelength. Nonadiabatic transitions appear to play a smaller role in dissociation than in its monohalogenated analogues, specifically We suggest that this difference is the result of the intrinsic symmetry and lower radial velocity of and it is discussed in terms of a onedimensional Landau–Zener model. A C–Br bond dissociation energy of 67.5 kcal/mol in was also calculated using ab initio methods at the MP2/ccpVtz//MP2/ccpVdz level.

A coupledcluster study of the HOBr→HBrO transition state
View Description Hide DescriptionThe structural and energetic properties of the HOBr→HBrO transition state are examined using the single and doubles coupledcluster method that includes a perturbational estimate of the effect of connected triple excitations [CCSD(T)]. The energy change for the isomerizationreaction is best estimated to be 56.5 endothermic, and the activation energy for the process is 75.0

Magnetic and microwave field effects for single rotational levels of the band of oxalylfluoride in cooled jet conditions
View Description Hide DescriptionFluorescence intensity and decay in oxalylfluoride vapors excited to single rotational levels (SRLs) of the state of the transition, were measured as a function of an external magnetic field. On excitation to these levels, dynamics in zero field may be described in the smallmolecule limit, with fluorescence exhibiting an almost exponential decay. However, at increased field strength B the initial fluorescence decay becomes faster, the decay profile becoming biexponential at higher fields. Thus, a magnetic fieldinduced change of dynamics occurs in the state, from that of a small molecule, to the intermediate case. The decay rate constant of the fast component was measured for different SRLs, being independent on the magnetic field strength, while the slow component lifetime is field dependent, increasing at higher fields. Both the fast and slow decay lifetimes depend on the studied SRL. At higher fields, the slow component amplitude decreases, while that of the fast component increases with subsequent saturation at high fields. Halfwidth value of the field dependence of the slow component amplitude increases linearly with Structure of the OD EPR spectrum of excited to the level was resolved. Experimental data are interpreted using the indirect mechanism theory in the low level density limit.

Nuclear spin relaxation in paramagnetic complexes of Electron spin relaxation effects
View Description Hide DescriptionElectron spin relaxation for an system and its field dependence in the presence of static zerofield splitting (ZFS) has been described and incorporated in a model for nuclear spinlattice relaxation in paramagnetic complexes in solution, proposed earlier by the group in Florence. Slow reorientation is assumed and the electron spin energy level structure (at any orientation of the molecule with respect to the laboratory frame) is described in terms of the Zeemaninteraction and of the static ZFS. The electron spin relaxation is assumed to be caused by a transient ZFS modulated by the deformation of the complex described as a distortional (or pseudorotational) motion and the Redfield theory is used to derive the electron spin relaxation matrices. In the description of the electron spin relaxation we neglect any contribution from mechanisms involving modulation by reorientation, such as those of the static ZFS and the less important Zeemaninteraction, as we limit ourselves to the slowrotation limit (i.e., This in general covers the behavior of proteins and macromolecules. The decomposition (DC) approximation is used, which means that the reorientational motion and electron spin dynamics are assumed to be uncorrelated. This is not a serious problem, due to the slowrotation condition, since reorientational and distortional motions are timescale separated. The resulting nuclear magnetic relaxation dispersion (NMRD) profiles obtained using the Florence model are calculated and compared with the calculations of the Swedish approach, which can be considered essentially exact within the given set of assumed interactions and dynamic processes. That theory is not restricted by the Redfield limit and can thus handle electron spin relaxation in the slowmotion regime, which is a consequence of not explicitly defining any electron spin relaxation times. Furthermore, the DC approximation is not invoked, and in addition, the electron spin relaxation is described by reorientationally modulated static ZFS and Zeemaninteraction besides the distortionally modulated transient ZFS. The curves computed with the Florence model show a satisfactory agreement with these more accurate calculations of the Swedish approach, in particular for the axially symmetric static ZFS tensor, providing confidence in the adequacy of the electron spin relaxationmodel under the condition of slow rotation. The comparison is also quite instructive as far as the physical meaning of the electron spin relaxation and of its interplay with the nuclear spin system are concerned.

Modified BornOppenheimer basis for nonadiabatic coupling: Application to the vibronic spectrum of
View Description Hide DescriptionNonadiabatic matrix elements, when computed using a BornOppenheimer (BO) basis, do not vanish asymptotically because the motion of the electrons with the nuclei at large internuclear separations is not taken into account. We apply a method suggested by Delos [Rev. Mod. Phys. 53, 287 (1981)] to include the effect of electron translation factors in a quantummechanical framework, thus correcting the BO basis to incorporate proper boundary conditions. We calculate the nonadiabatic matrix elements for and its isotopic variants. We focus our calculations on for which experimental results exist, and calculate its vibronic spectrum. This is the first application of this method to calculate high precision spectroscopic information for molecular systems.

A failing of coupledstates calculations for inelastic and pressurebroadening cross sections: Calculations on –Ar
View Description Hide DescriptionFully quantal benchmark calculations of pressurebroadening cross sections for infrared and Raman lines of perturbed by Ar are carried out using both closecoupling (CC) and coupledstates (CS) calculations. CS calculations are found to underestimate the cross sections by up to 15%. The effect occurs even for isotropic Raman cross sections, which are not affected by reorientation contributions. The discrepancy arises mostly for collisions with large orbital angular momenta occurring on the longrange part of the potential. It may be attributed to collisions that are adiabatic rather than sudden in nature. A hybrid computational method, employing CS calculations for low and decoupled dominant (DLD) calculations for high offers a promising solution.

Using stretching and bending vibrations to direct the reaction of Cl atoms with isocyanic acid (HNCO)
View Description Hide DescriptionReaction of wellcharacterized vibrational states prepared in the region of three quanta of N–H stretching excitation explores how vibrations with different components along the reaction coordinate influence the bimolecular reaction of Cl atoms with isocyanic acid (HNCO) to form HCl and NCO. Near prolate symmetric top states corresponding to different amounts of aaxis rotation are well separated in energy, and perturbations by background states make each of the eigenstates a different mixture of zeroorder states. Molecules in the essentially unperturbed and 4 states, which are nearly pure N–H stretching excitation, react efficiently, but those in the perturbed states, 2, and 3, which are a mixture of N–H stretching and lower frequency vibrations react only half as well. Detailed analysis of resolved, perturbed eigenstates for and 7 of reveals the relative reactivity of the two interacting zeroorder states. The less reactive zeroorder state, which most likely contains only two quanta of N–H stretch and several quanta of other vibrations, reacts only 10% as well as the pure N–H stretch zeroorder state. Ab initio calculations suggest that bending excitation alters the interaction potential to reduce the fraction of reactive collisions.

A theoretical investigation of the nature of the πH interaction in ethene– benzene– and benzene–
View Description Hide DescriptionWe have carried out a detailed investigation of the nature of the πH interaction in the ethene– benzene– and benzene– complexes using large basis sets (ranging from 631+G^{*} to TZ2P++) and high levels of theory. The minimum geometries, and hence the vibrational frequencies, of all the complexes have been obtained at the second order Mo/ller–Plesset (MP2) level of theory. The binding energy of the ethene– complex is only about 1 kcal/mol lower than that of the benzene– complex. In the benzene– complex, the interaction of benzene with the πbonded water to that with the second water is nearly equivalent. In order to explain the above interesting facets of the interaction of water with benzene and ethene, the interactionenergies were decomposed into the individual interactionenergy components using the recently developed symmetry adapted perturbation theory (SAPT) program. The SAPT results indicate that the repulsive exchange energies play a crucial role in governing the energies and geometric preferences of these complexes. A detailed analysis of the vibrational frequencies of these complexes has also been done to examine the changes in the frequencies of the monomers upon complexation. It is found that changes in the outofplane bending frequencies of benzene and ethene can be correlated to the interactionenergies of these complexes, in particular the exchange energy.

Valence oneelectron and shakeup ionization bands of carbon clusters. I. The chains
View Description Hide DescriptionThe (onehole) and (twohole; oneparticle) shakeup bands in the valence ionization spectrum of small carbon chains are investigated up to 40 eV, using the oneparticle Green’s function approach. Calculations have been performed at the second and thirdorders of an algebraic diagrammatic construction (ADC) scheme based on partial renormalization series, which incorporate static and dynamic electronic correlation consistently through those orders. The results obtained indicate a major or complete breakdown of the orbital picture of ionization extending down into the outervalence energies for the largest chains (12.4 eV for Cumulenic carbon chains represent the only case reported so far where outervalence ionization lines of π character can be affected by severe fragmentation in shakeup sets. The onehole states associated with the terminal carbon lone pairs are also very strongly affected by electronic relaxation.

Photoswitch and nonlinear optical switch: Theoretical studies on 1,2bis(3thienyl)ethene derivatives
View Description Hide DescriptionThe 1,2bis(3thienyl)ethene derivatives are known to be good photoswitches. A large number of experiments have been carried out on different classes of these molecules to find out the most effective photoswitch. We have selected several highly efficient representative model photoswitch molecules of this class and studied their structures, photophysics, and different molecular properties at the ground and vertically excited states using density functional technique together with its timedependent analog. These analyses are motivated toward the understanding of the effective molecular criteria, which are to be satisfied by a molecule in order to be good photoswitch. The theoretical investigations indicate that the capped ethene derivatives of this class of molecules are more effective photoswitches than the uncapped ones. Our contention has been verified by carrying out similar calculations on a wellknown thermally irreversible photoswitch molecule of this class. Since the transition of the open to the closed form in photoswitching devices is the key factor for the molecules to exhibit such properties, and, moreover, since the molecules are thermally stable, they could be used for designing nonlinear optical (NLO) switches. One such possibility has been explored theoretically using a model molecular system.

Accurate nonadiabatic couplings for Application to predissociation
View Description Hide DescriptionWe report the results of an ab initio calculation, using analytic techniques, of nonadiabatic couplings for between the degenerate ground state and the and Rydberg states. The calculations employed extensive correlated wave functions that accurately reproduced the energy levels of these states. We have used these couplings as input to a twodimensional wave packet calculation of dissociation dynamics in order to predict the predissociation lifetimes and final state vibrational distributions produced by the predissociation. A significant isotope effect is found in the final vibrational distributions produced from and predissociation.

The photodissociation of HNCO in the band: A fivedimensional classical trajectory study
View Description Hide DescriptionWe present a classical trajectory study of the photodissociation of HNCO in the electronic state using a fivedimensional potential energy surface with all atoms confined to a plane. The potential energy surface has been determined by ab initio calculations (multireference configuration–interaction method, triplezeta basis set). Because any coupling to other electronic states is neglected in our calculations, direct comparison with experimental data is limited and basically restricted to energies well above the channel. Nevertheless, some aspects are described in a realistic way and help to interpret experimental results. In particular, the calculated vibrational–rotational state distributions of the diatomic fragments are in good agreement with the measured ones. Moreover, our calculations support the most recently published data on the Hatom quantum yield for photolysis with 193nm photons. It is found that preexcitation of the HN stretching mode by several quanta significantly enhances the lifetime with respect to breaking the NC bond, which may partly explain the drastic increase of the Hatom quantum yield in the vibrationally mediated photodissociation.

Investigation of an Hbonded dimer: Calculations of bonding structures and temperature dependence of the librational substructure of the OHstretching band
View Description Hide DescriptionWe demonstrate data from timeresolved spectroscopy and quantum statistical thermodynamics of ab initio calculated molecular clusters for 2,2dimethyl3ethyl3pentanol (DMEP). The association of the molecules to dimers, only, is supported by the numerical investigation indicating a weak Hbond. This is in agreement with NMR experiments and data from conventional IR spectroscopy. From timeresolvedIR spectroscopy of a 2 M DMEP sample diluted in in the temperature range from 260 K to 343 K a librational substructure of the proton donor OHband is suggested from spectral holes and satellite holes in the sample bleaching. With these spectral components we are able to fit the temperature dependent conventional IRabsorption spectrum of the sample in the investigated range from 260 K up to 343 K.

Infrared laser velocity modulation spectrum of the fundamental band of
View Description Hide DescriptionThe infrared absorptionspectrum of the (B–Br stretching) mode of has been observed using a tunable diode laser and velocity modulation spectroscopy. The ion was formed in an ac glow discharge through a flowing mixture of and Thirtythree transitions of and the same number of lines of the isotopomer have been assigned. The band origins and rotational constants are and and Ab initio calculations of the band origins and the internuclear distances have been performed using the GAUSSIAN 94 package. The calculated values are in highly satisfactory agreement with the experimental results.

The averaged density matrix in the coordinate representation: Application to the calculation of the farwing line shapes for
View Description Hide DescriptionThe farwing line shapetheory within the binary collision and quasistatic framework developed previously for linear molecules using the coordinate representation has been generalized to symmetric and asymmetrictop molecular systems. However, due to more variables needed to specify the orientation of these complicated molecules, one has to evaluate multidimensional integrals with higher dimensionality and this would be intractable for practical calculations. In cases where the anisotropic interaction contains cyclic coordinates, one can carry out the integration of the density matrix over these coordinates analytically and obtain the “averaged” density matrix. This reduces the dimensionality of the multidimensional integrals and thus dramatically reduces the computational time necessary to obtain converged results. In addition, a new interpolation method that enables one to treat more realistic potential models has been formulated. Using these results, calculations for the bandaverage farwing line shapes and corresponding absorption coefficients in the spectral range 300–1100 cm^{−1} have been carried out for and pairs for a few temperatures. These results improve the agreement with experimental data over previous calculations that were limited in the number of states that could be included and in the sophistication of the anisotropic interaction potential model that was used.

Benzeneargon intermolecular potential energy surface
View Description Hide DescriptionThe benzeneargon intermolecular potential energy surface is evaluated using coupledcluster linear response methods and an augmented correlationconsistent polarized valence doublezeta basis set extended with midbond functions. As a result of the excitation, the well depth of the potential increases and the benzeneargon equilibrium distance is decreased by 0.065 Å. Full threedimensional vibrational calculations of the van der Waals vibrational energy levels, using the ab initio potential, are in good agreement with available experimental data.
