Volume 103, Issue 16, 22 October 1995
Index of content:

Analysis of static distributions in hydrogen hyperfine interactions in randomly oriented radicals in the solid state by using ^{2}H electron spin echo envelope modulation spectroscopy: Conformational dispersion of β ‐^{2}H coupling in the model tyrosyl radical
View Description Hide DescriptionThe experimental analysis of static distributions in hydrogen hyperfineinteractions in randomly‐oriented organic radicals in the solid state by using ^{2}H electron spin echo envelope modulation spectroscopic techniques has been examined systematically. The hyperfineinteraction between the two β‐methylene‐^{2}H nuclei and coupling π‐spin density (ρ_{π}) at ring carbon atom C_{1} in the tyrosine neutral radical trapped in a low temperature aqueous glass was addressed specifically. Stimulated echo envelope modulation generated by the microwave pulse‐swapping sequence was collected for τ values of 176–1295 ns at external magnetic field strengths of 0.3258 and 0.3983 T. The spectra reveal weak (β‐^{2}H_{ w }) and strong (β‐^{2}H_{ s }) sets of hyperfine couplings. The envelope modulation depths and line shape responses to changes in τ and magnetic field strength could not be reproduced by simulations that incorporated discrete principal hyperfinetensors. Successful simulations were achieved by using two sets of distributed principal hyperfinetensors. The principal tensor distribution is caused by a variation in the isotropic coupling for β‐^{2}H_{ w } and β‐^{2}H_{ s } of 0–2.0 MHz and 2.0–6.8 MHz, respectively. The range of isotropic couplings corresponds to a distribution in the dihedral angles, θ, between the phenol ring normal and the C_{β}–H_{β} bonds of ‖θ_{ w }‖=60°–90° and ‖θ_{ s }‖=60°–30°. A common, θ‐independent dipolar coupling constant for each rotamer yields a value for ρ_{π} at C_{1} of 0.35±0.03. Relative rotamer populations in the distribution are given by the conformational weighting function required for exact reproduction of the line shapes. The quantitative relation between modulation amplitudes and the details of the electron–nuclear interactions that determine the hyperfine frequencies gives the ^{2}H electron spin echo envelope modulation method unique merit for the detection and accurate description of static distributions of principal hyperfinetensors in randomly‐oriented paramagnetic systems in the solid state.

Fourier transform infrared observation of the ν_{7} stretching mode of linear C_{9} in Ar at 10 K
View Description Hide DescriptionA second stretching fundamental ν_{7}(σ_{ u }), of the linear C_{9}carboncluster has been observed for the first time at 1601.0 cm^{−1} by means of a Fourier transform infrared investigation of the products of the evaporation of graphite trapped in solid Ar at ∼10 K. Comparison of the measured isotopic shifts of the single ^{13}C and single ^{12}C isotopomers with the predictions of new density functional theory and coupled clusterab initio calculations carried out as part of this study, conclusively confirms the assignment of the new mode.

Resonance Raman spectroscopy of the S _{1} and S _{2} states of pyrazine: Experiment and first principles calculation of spectra
View Description Hide DescriptionNew experimental and theoretical data on the resonance Raman (RR) spectroscopy of the S _{1} and S _{2} states of pyrazine are presented. Based on recent ab initio CASSCF (complete‐ active‐space‐self‐consistent‐field) and MRCI (multireference configuration interaction) calculations of Woywod et al. [J. Chem. Phys. 100, 1400 (1994)], we construct a vibronic coupling model of the conically intersecting S _{1} and S _{2} states of pyrazine, which includes the seven most relevant vibrational degrees of freedom of the molecule. Employing a time‐dependent approach that treats the intramolecular couplings in a nonperturbative manner, we calculate RR cross sections for this model, taking explicitly into account the nonseparability of all vibrational modes. The combination of high‐level ab initio calculations and multimode propagation techniques makes it possible, for the first time, to make first‐principles predictions of RR spectra for vibronically coupled electronic states of an aromatic molecule. The theoretical data are compared to experimental gas‐phase RR spectra which have been obtained for five different excitation wavelengths. The comparison reveals that the ab initio predictions match the experimental results in almost every detail.

Five‐dimensional local mode‐Fermi resonance model for overtone spectra of ammonia
View Description Hide DescriptionA five‐dimensional local mode‐Fermi resonance model for overtone spectra of the ammonia molecule has been constructed. The model Hamiltonian is expressed in terms of curvilinear internal valence coordinates and it includes the three stretching vibrations and the doubly degenerate bending vibration. The symmetric bending vibration associated with the inversion motion has been excluded. Thus the model is useful for energy levels with the totally symmetric bending vibration on its ground state. Vibrational energy levels have been calculated using van Vleck perturbation theory. Nonlinear least‐squares method has been used to optimize potential energy parameters. Observed vibrational band origins for ^{14}NH_{3}, ^{14}ND_{3}, and ^{14}NT_{3} have been employed as data. A fit with the standard deviation of 5.5 cm^{−1} has been obtained using one set of isotope invariant potential energy parameters. The optimised potential energy surface compare well with results of ab initio electronic structure calculations and with results of customary anharmonic force field calculations.

Relation of vibrational hyper‐Raman intensities to γ‐hyperpolarizability densities
View Description Hide DescriptionBand intensities for nonresonant vibrational hyper‐Raman scattering depend on the derivatives of the β hyperpolarizability, a nonlinear electronic response tensor, with respect to normal mode coordinates. In this work, we derive a new result for the change in β(−ω_{σ}; ω_{1},ω_{2}) due to small shifts in nuclear positions within a molecule. We prove that the derivative of β(−ω_{σ}; ω_{1},ω_{2}), taken with respect to the position R ^{ K } of nucleus K, depends on the nonlocal hyperpolarizability density γ(r,r′,r″,r‴; −ω_{σ}; ω_{1},ω_{2},0) of second order, the charge on nucleus K, and the dipole propagator from R ^{ K } to r‴. Thus γ(r,r′,r″,r‴; −ω_{σ}; ω_{1},ω_{2},0) determines the origins of vibrational hyper‐Raman intensities on the intramolecular scale. Two observations provide the physical basis for this result: The effective value of β for a molecule in a static applied field is governed by the γ hyperpolarizability density. When a nucleus shifts infinitesimally, the electrons respond to the resulting change in the nuclear Coulomb field via the same nonlocal susceptibilities that characterize their response to an applied electric field.

Rotational excitations of a symmetric top in cubic orientational potentials: CH_{3}D matrix‐isolated in argon and krypton
View Description Hide DescriptionMeasurements with inelastic neutron scattering of the rotational spectra of the symmetric top molecule CH_{3}D as substitutional impurities in crystalline argon and krypton are presented. The energy of the J=1 doublet is shifted by 33% and 27% respectively, compared to the completely free rotor. Spin‐rotational wave functions for the free CH_{3}D rotor are constructed and used for the calculation of the dependence of the scattered intensities on momentum transfer. In contrast to the CH_{4} molecule, the center of mass does not coincide with the position of the carbon atom in the case of the CH_{3}D molecule. However, energies and intensities can only be explained if this fact is disregarded. The spin conversion behavior has been studied on a CH_{3}D/argon sample within the temperature range 1.5 K≤T≤5 K. Two different mechanisms for the conversion process (libron‐phonon process and Raman process) are discussed.

Conformational equilibrium and orientational ordering: ^{1}H‐nuclear magnetic resonance of butane in a nematic liquid crystal
View Description Hide DescriptionIn this study we use multiple‐quantum ^{1}H‐NMR spectroscopy to study butane, the simplest flexible alkane, dissolved in a nematic solvent. An analysis of the highly accurate ^{1}H dipolar coupling constants gives important information about conformational and orientational behavior, including the trans–gauche energy difference, E _{ tg }, and the conformer probabilities and order parameters. An essential component of the analysis involves the use of mean‐field models to describe the orientational ordering of solutes in a nematic solvent. Several models were found to adequately describe the molecular ordering, including the chord model of Photinos et al. [D. J. Photinos, E. T. Samulski, and H. Toriumi, J. Phys. Chem. 94, 4688 (1990)] and recent versions of a model proposed by Burnell and co‐workers [D. S. Zimmerman and E. E. Burnell, Mol. Phys. 78, 687 (1993)]. It was found that E _{ tg } lies in the range 2.1–3.0 kJ/mol, which is significantly below most experimental estimates of the gas–phase value. An attempt to describe more realistically the conformational states by including torsional fluctuations about the rotational isomeric states did not significantly improve the quality of the fits or alter the results. Finally, the anisotropic component of the solute–solvent interaction was found to perturb only marginally the conformational probabilities from the isotropic values.

Time‐delayed two‐color photoelectron spectra of aniline, 2‐aminopyridine, and 3‐aminopyridine: Snapshots of the nonadiabatic curve crossings
View Description Hide DescriptionWe present time‐delayed two‐color photoionizationphotoelectron spectra of aniline, 2‐aminopyridine, and 3‐aminopyridine seeded in a cold molecular beam. The molecules are prepared in their S _{1} electronic states by a picosecond UV laser pulse and ionized by a time‐delayed 200 nm probe pulse. The photoelectron spectrum is observed with a time‐of‐flight spectrometer. All time‐delayed spectra reveal only one product of the nonradiative relaxation process. Careful considerations of electronic and vibrational overlap propensity rules for the ionization step lead to the conclusion that the dominant nonradiative decay mechanism in these molecules is the intersystem crossing to a bath of vibrationally excited levels of the T _{1} electronic state. Our observations reveal no admixtures of T _{2} or higher triplet levels. The pathway of the nonradiative electronic relaxation in 2‐aminopyridine is found to be independent of the initially prepared vibrational states up to 1000 cm^{−1} of vibrational energy. We find no evidence of intramolecular vibrational relaxation preceding the electronic curve crossing.

Expected significance of weakly chaotic vibrational motions in single molecule spectroscopy
View Description Hide DescriptionIt is known that a new kind of large amplitude motion gives rise to as a very characteristic mode of weak chaos. This is essentially an unpredictable and intermittent motion taking place in a thin quasiseparatrix which wanders among several very clear vibrational modes. In this paper, we study the spectroscopic characterization of the quantum version of this large amplitude motion in terms of the dynamics of a wave packet, which is prepared in a narrow energy‐range so that it is localized along a thin quasiseparatrix. In particular, we discuss possible significance of the weak chaos in single molecule spectroscopy, for which the spectra are supposed to be averaged neither in ensemble of molecules nor in time. That this wavepacket state is unusual originates from the extremely long‐time behavior and the strong sensitivity to the initial condition at which the wave packet is prepared. The weak chaos combined with the statistical nature of quantum mechanics brings about a notion of unreproducibility in the spectrum. Conversely, it is anticipated that one of the distinguished features inherent to a single molecule spectroscopy manifests itself when weak chaos is observed.

Fourier transform infrared study and ab initio calculation of ClNO complex with HCl
View Description Hide DescriptionThe molecular complex in a 1:1 ratio of nitrosyl chloride ClNO with hydrogen chloride has been characterized in argon matrices by infrared spectroscopy.Ab initio calculations using the self‐consistent‐field Hartree–Fock, the second order Mo/ller–Plesset, and the Becke–Lee–Yang–Paar density functional(DFT methods) were undertaken on all possible structures. Only one stable structure was found. It showed simultaneously interaction of the Cl, N, and O atoms of ClNO with the H atom of HCl. The complexation induces a mean variation of about −0.005 Å of the NO bond and about +0.005 Å of the ClN bond. The HCl bond length is calculated to lengthen by about +0.008 Å.

Experimental and theoretical determination of the temperature dependence of deuteron and oxygen quadrupole coupling constants of liquid water
View Description Hide DescriptionQuadrupole coupling constants, χ_{ Q }, for the deuteron and the oxygen nuclei in neat, liquid water were determined by both theoretical and experimental methods. The theoretical values of χ_{D}=264 kHz and χ_{O}=8.4 MHz obtained from ab initio calculations at the MP2/6‐31+G* level in combination with a quantum cluster equilibrium model for liquids are in good agreement with results from NMRrelaxation timeexperiments. Both theory and experiment show no observable temperature dependence of the quadrupole coupling constants. The theoretical values reported here for the oxygen quadrupole coupling constant and both the oxygen and deuterium asymmetry parameters are different from values obtained from ab initio calculations of clusters using molecular dynamics methods. This may be due to the use of pairwise additive potentials in the molecular dynamics simulations which do not take into account many‐body or polarizability effects.

Heteronuclear decoupling in rotating solids
View Description Hide DescriptionA simple two pulse phase modulation (TPPM) scheme greatly reduces the residual linewidths arising from insufficient proton decoupling power in double resonance magic angle spinning (MAS) experiments. Optimization of pulse lengths and phases in the sequence produces substantial improvements in both the resolution and sensitivity of dilute spins (e.g., ^{13}C) over a broad range of spinning speeds at high magnetic field. The theoretical complications introduced by large homo‐ and heteronuclear interactions among the spins, as well as the amplitude modulation imposed by MAS, are explored analytically and numerically. To our knowledge, this method is the first phase‐switched sequence to exhibit improvement over continuous‐wave (cw) decoupling in a strongly coupled homogeneous spin system undergoing sample spinning.

Binary diffusion coefficients of helium/hydrogen isotope mixtures
View Description Hide DescriptionMass diffusion coefficients have been measured for the six binary mixtures formed from ^{3}He and ^{4}He with hydrogen, deuterium, and tritium. An intermolecular potential of the Hartree–Fock‐dispersion form is presented which, when employed in the quantum mechanical theoretical evaluation of the diffusion coefficient, correlates experimental results for all six systems within the experimental uncertainty. No isotopic effect is displayed by the correlating potential.

Photodissociation of CBrCl_{3} at 193 nm by translational spectroscopy
View Description Hide DescriptionThe photodissociation of CBrCl_{3} at 193 nm has been studied by translational spectroscopy. Two major dissociation channels, (1) CBrCl_{3}→CCl_{3}+Br and (2) CBrCl_{3}→CBrCl_{2}+Cl, are detected with product translational energies of 17 and 22 kcal/mol, respectively. The relative yield of (1):(2) is calculated to be 7:3. The primary product, CBrCl_{2}, which is internally excited, undergoes unimolecular decay to form the CCl_{2}+Br products. From the derived values of the anisotropy parameter β, we conclude that these processes take place rapidly after excitation via an A _{1}←A _{1} transition, with the transition moment aligned parallel to the threefold axis. Results obtained for these two primary pathways and other minor processes are discussed in terms of a simple direct dissociation mechanism.

Ab initio potential energy surfaces and quantum scattering studies of NO(X ^{2}Π) with He: Λ‐doublet resolved rotational and electronic fine‐structure transitions
View Description Hide DescriptionNew ab initiopotential energy surfaces (PES’s) are presented for the interaction of He with the NO radical in its ground (X ^{2}Π) electronic state, determined within the coupled electron pair approximation (CEPA) with a large atomic orbital basis set. The dynamics of the collisions of NO with He are then investigated, in particular the coupling between nuclear motion (rotation and translation) and the internal electronic motion of the open‐shell partner. State‐to‐state integral and differential cross sections are calculated using full close coupling and coupled states methods. These cross sections are compared with the results of the two separate measurements at different initial collision energies, 508 and 1186 cm^{−1} (63 and 147 meV). Excellent agreement is obtained in both cases. Also comparisons with previous calculations, based on an earlier local density potential energy surface, are made at 508 and 2420 cm^{−1} (63 and 300 meV).

A fast Fourier transform method for the quasiclassical selection of initial rovibrational states of triatomic molecules
View Description Hide DescriptionThis paper describes the use of an exact fast Fourier transform method to prepare specified vibrational–rotational states of triatomic molecules. The method determines the Fourier coefficients needed to describe the coordinates and momenta of a vibrating–rotating triatomic molecule. Once the Fourier coefficients of a particular state are determined, it is possible to easily generate as many random sets of initial Cartesian coordinates and momenta as desired. All the members of each set will correspond to the particular vibrational–rotational state selected. For example, in the case of the ground vibrational state of a nonrotating water molecule, the calculated actions of 100 sets of initial conditions produced actions within 0.001ℏ of the specified quantization values and energies within 5 cm^{−1} of the semiclassical eigenvalue. The numerical procedure is straightforward for states in which all the fundamental frequencies are independent. However, for states for which the fundamental frequencies become commensurate (resonance states), there are additional complications. In these cases it is necessary to determine a new set of ‘‘fundamental’’ frequencies and to modify the quantization conditions. Once these adjustments are made, good results are obtained for resonance states. The major problems are in labeling the large number of Fourier coefficients and the presence of regions of chaotic motion. Results are presented for the vibrational states of H_{2}O and HCN and the rovibrational states of H_{2}O.

Theoretical study on the excitation spectrum and the photofragmentation reaction of Ni(CO)_{4}
View Description Hide DescriptionThe ground and excited states of Ni(CO)_{4} are studied using the symmetry adapted cluster (SAC)/SAC‐configuration interaction (SAC‐CI) method. The experimental absorptionspectrum is well reproduced by the present calculations. All the peaks observed in the range of 200∼350 nm are assigned to the electronic allowed ^{1} T _{2}excited states. The third peak is assigned to the 3 ^{1} T _{2} and 4 ^{1} T _{2} states. Next, the potential energy curves of the ground and the low‐lying excited states are calculated by the same method and utilized to clarify the mechanism of the photofragmentation reaction of Ni(CO)_{4} by a XeCl laser (308 nm). A reaction pathway involving several excited states is proposed for the photofragmentation reaction into the excited Ni(CO)_{3} and CO. The calculated emission energy from the former agrees well with the observed luminescencespectrum.

Photofragment translational spectroscopy of IBr at 304 nm: Polarization dependence and dissociation dynamics
View Description Hide DescriptionThe photodissociation dynamics of IBr has been studied at 304 nm by state‐selective photofragment translational spectroscopy. Velocity distributions, anisotropy parameters, and relative quantum yields are obtained for the ground I(^{2} P _{3/2}) and spin–orbit excited state I*(^{2} P _{1/2}) iodine atoms, which are produced from photodissociation of IBr at this wavelength. Two sharp velocity distributions observed for the I channel suggest the two dissociation pathways that correlate with ground‐state iodine formation. Based on the expected translational energy release and the energy separation between the peaks, the two distributions have been assigned to dissociation of IBr to form I(^{2} P _{3/2})+Br(^{2} P _{3/2}) and I(^{2} P _{3/2})+Br*(^{2} P _{1/2}) with the former channel appearing at higher translational energy. The I* distribution shows one strong peak indicating that there is one dominant channel for formation of I* atoms at this wavelength which has been assigned to dissociation of IBr to form I*(^{2} P _{1/2})+Br(^{2} P _{3/2}) with a quantum yield of 0.1. The I* signal formed from the I*(^{2} P _{1/2})+Br*(^{2} P _{1/2}) dissociation channel is observed very weakly.
The observed anisotropy parameter indicates that the I+Br* product (β=−0.7) is formed mainly from the perpendicular ^{1}Π_{1}(2341)←X transition while the I*+Br channel (β=1.8) is formed predominantly from the parallel ^{3}Π_{0+}(2341)←X transition followed by curve crossing to the ^{3}Σ_{0+} ^{−}(2422) state. The recoil energy dependence of the anisotropy parameter in the I atom produced in the I+Br channel shows a positive β value above maximum of the peak recoil energy and a negative value below the peak maximum of the recoil energy distribution. These results are interpreted in terms of the presence of more than one path for the formation of I+Br photoproduct with opposite polarization for their absorbing transitions, most likely the ^{3}Π_{0+}(2341)←X and the ^{3}Π_{1}(2341)←X transitions. The possible excited state dynamics which give the observed results are discussed in terms of the previously proposed potential energy diagrams for IBr and ICl.

Photodissociation of the HCO^{+} ion. I. Two‐dimensional calculations through the I ^{1}Π state
View Description Hide DescriptionThe HCO^{+} ion plays an important role in the chemistry of interstellar space and in combustionflames. The 1 ^{1}Π state is the only dissociative state below the hydrogen ionization potential, to which transitions are dipole allowed. Ab initio MRD‐CI calculations have been performed for the two‐dimensional potential energy surfaces of the ground state and the 1 ^{1}Π state of HCO^{+} as functions of the C–H and C–O bond distances, keeping the ion in the linear configuration. The 1 ^{1}Π state is interesting because of an avoided crossing with the 2 ^{1}Π state. The potential energy surfaces and geometry dependent dipole transition moments have been employed in two‐dimensional photodissociationdynamics calculations. Total and vibrationally resolved partial cross sections are calculated as functions of the excitation energy. The potential barrier arising from the avoided crossing leads to vibrational resonances in the cross sections, which correspond to levels of the v _{3} (C–O) stretching motion in the excited state. The structure of the cross sections reveals the competition between resonant and direct photodissociation. The difference between the total cross section derived from the Fourier transform of the autocorrelation function and from the sum of the partial cross sections of the CO^{+}+H dissociation channel indicates the existence of a second photodissociation channel leading to CH^{+}+O. The total and partial cross sections of this channel have been computed in an independent calculation; its contribution to the integrated total cross section amounts to about 4%.

The investigation of the (CO)^{+} _{2} ion by dissociative ionization of argon/carbon monoxide clusters
View Description Hide DescriptionThe formation of (CO)^{+} _{2} ion has been investigated by the dissociativeionization of Ar(CO)_{2} clusters in a threshold photoelectron photoion coincidence experiment. The kinetic energy released (KER) in the reaction Ar(CO)^{+} _{2}→(CO)^{+} _{2}+Ar has been measured as a function of the internal energy of the Ar(CO)^{+} _{2} ion. The comparison between the experimental KER and the statistically expected KER allows one to extrapolate back to zero KER which corresponds to the thermochemical threshold for the formation of (CO)^{+} _{2}. The ionization potential (IP) of (CO)_{2} is determined to be 12.24±0.15 eV. This leads to a binding energy of the (CO)^{+} _{2} of 1.80 eV one of the strongest known for any ionized van der Waals dimer. Our data are in line with a trans planar structure of the CO dimer ion as suggested by our ab initio calculations at the CI level.