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Volume 74, Issue 12, 15 June 1981

The rotational and hyperfine spectrum of Ar–HF
View Description Hide DescriptionThe radio frequency and microwave spectrum of Ar–HF has been remeasured by molecular beam electric resonance spectroscopy. Analysis of the data yields a revised value for the HF nulear spin–spin constant S, which in turn provides information about the vibrationally averaged geometry of the complex. Rotational and hyperfine constants and the dipole moment have also been measured for Ar–DF: The average structures and estimated harmonic force constants are shown to be consistent with those observed for other Ar–HX complexes. Comparison of the two hyperfine constants for Ar–DF indicates that the average charge distribution around the deuterium is only weakly perturbed by the argon atom.

CO_{2}–HF: A linear molecule
View Description Hide DescriptionThe results of microwave and radio frequency spectroscopy of the weakly bound complexes of hydrogen fluoride with carbon dioxide and carbonyl sulfide are presented. The following spectroscopic constants are determined: A nearly linear hydrogen bonded equilibrium structure with an O–H bond length of ∼1.9 Å is consistent with all the experimental data. The stretching force constant, estimated from the distortion constant to be k _{ s } = 0.021 mdyne/Å for CO_{2}–HF, appears anomalously small for such a short bond.

The structure of N_{2}O–HF
View Description Hide DescriptionThe rotational spectra of four isotopes of the weakly bound complex N_{2}O–HF have been measured by molecular beam electric resonance spectroscopy. The following spectroscopic constants and structural parameters were derived from the observations: In the average structure, the N_{2}O axis makes an angle of ∼47° with a line r _{c.m.} drawn between the centers of mass of the two submolecules. HF is hydrogen bonded toward the oxygen on N_{2}O to form an angle of ∼31° with the center of mass line.

Laser‐excited emission spectra of In atoms trapped in Kr and Xe matrices
View Description Hide DescriptionThe first measurements of the fluorescence spectra of In atoms trapped in Kr and Xe matrices are reported. The In atoms were trapped at 4 °K and the fluorescence was excited by a pulsed dye laser tuned to the matrix‐isolated In absorption bands. The results are compared with the previously reported emission spectra of matrix‐isolated Tl atoms.

Far infrared laser magnetic resonance spectrum of NF(a ^{1}Δ)
View Description Hide DescriptionThe NF radical in its a ^{1}Δ metastable state has been detected by far infrared lasermagnetic resonancespectroscopy at 513 μm. The spectrum arises from the J = 7→8 rotational transition, and yields an accurate measurement for this spacing of 19.5517_{2} cm^{−1}.

Theory of resonance Raman scattering beyond the Condon approximation for a system with two modes of different symmetry
View Description Hide DescriptionA theoretical study is presented of resonance Raman excitation profiles and depolarization dispersion curves for a molecule with two Raman‐active normal modes of vibration and three electronic states. The modes are taken to be of different symmetry, one totally and the other non‐totally symmetric. The three electronic states are the ground state and a pair of excited states coupled through the non‐totally symmetric mode. The energy gap separating the pair is allowed to vary from zero (Jahn–Teller limit, strong coupling) to large values (Herzberg–Teller case, weak coupling). The totally symmetric coordinate is allowed to assume different equilibrium values not only between the ground state and the resonant state, but also between the two coupled excited states. As a result, the effective coupling depends indirectly on the totally symmetric coordinate, causing breakdown of the Condon approximation. The effect of this breakdown is demonstrated by calculating excitation profiles for all one and two quantum transitions involving the two modes. The profiles show progression of the totally symmetric mode with or without superimposed non‐totally symmetric quanta. If the Condon approximation breaks down, the intensity distribution in these progressions generally depends on the presence of such quanta. For weak coupling, the effect should be most readily observable in combination bands. For strong coupling, it should also be observable in totally symmetric fundamentals and overtones. Special attention is given to systems with resonance–preresonance interference and to intermediate‐coupling systems.

Magnetic resonance study of Mo^{5+} in Ca_{1−x }Y_{ x }MoO_{4}
View Description Hide DescriptionThe ESR of MoO^{3−} _{4} has been studied at 4.2 K in single crystals of Ca_{1−x }Y_{ x }MoO_{4} (x<0.01). The g values and Mo hyperfine constants are g _{ x } = 1.8925, g _{ y } = 1.8753, g _{ z } = 1.9780, A _{ x } = 38.6×10^{−4} cm^{−1}, A _{ y } = 42.8×10^{−4} cm^{−1}, and A _{ z } = 22.2×10^{−4} cm^{−1}. The resonances show an isotropic superhyperfine splitting due to a nearby charge compensating yttrium ion. The ground state of the d ^{1} electron is d _{ z 2 } as expected for the MoO_{4} tetrahedron which is distorted to D _{2d } symmetry by a c axis compression in this host. Crystal field parameters are calculated by fitting the ESR data to equations assuming x zmirror symmetry at the Mo^{5+} site.

Halogenated benzene radical cations and ground state degeneracy splitting by asymmetric substitution
View Description Hide DescriptionThe absorption and laser induced fluorescence of several halogenated benzene radical cations were studied in solid Ne matrices. The spectra of 1,2,4‐trifluorobenzene, 1,3‐dichloro‐5‐fluorobenzene, and 1‐chloro‐3,5‐difluorobenzene radical cations are observed and analyzed. Studies of fluorescencepolarization and a photoselection technique were used to examine the splitting of the degeneracy of the benzene cation ground state by asymmetric subsitution.

Fluctuations in optical activity: A probe of fast reactions using light scattering
View Description Hide DescriptionTheory is presented for light scattering from a liquid where there is a change in optical activity of the molecules upon chemical reaction. The results indicate that the difference spectrum for 90° scattering between right and left circularly polarized input is dominated by the reactive component through the cross term which couples fluctuations in the dielectric permittivity and the optical activity. The feasibility of the technique is illustrated by calculations for a unimolecular reaction.

Coriolis interactions and infrared intensities in fluoroform
View Description Hide DescriptionInfrared intensities of the fundamental bands of the CHF_{3} molecule were remeasured. The Coriolis interaction between ν_{3} and ν_{5} bands as well as that between ν_{6} and ν_{5} bands was analyzed to determine the sign of the perturbation. As a result, the relative signs of all the (∂p/∂Q _{ i })’s in this molecule have been determined without using the CNDO/2 method or an a b i n i t i o calculation. An empirical rule on the sign of the CF bond intensity parameters is proposed by the name of ’’CF‐bond criteria’’, according to which a negative dipole moment is enhanced in the direction of the displacements of the F atoms in both the CF stretching and CF deformation modes. Further, a possibility is suggested to extend this rule to all halogen atoms.

Intramolecular vibrational relaxation in jet‐cooled phthalocyanine
View Description Hide DescriptionWe have observed the dispersed fluorescencespectrum of free base phthalocyanine cooled in supersonic free jet. The spectrum consists of a single strong feature and some weaker red shifted structure. The frequencies of the single strong feature and the red shifted structure change only slightly as a function of the exciting frequency. This indicates that the potential surface does not change greatly upon electronic excitation and that the strong fluorescence transitions are Δv = 0. For excitation at frequencies greater than 720 cm^{−1} above the origin, the features in the fluorescencespectrum are broadened although the absorption features remain narrow. This is taken to indicate a mixing of background vibrational levels leading to intramolecular vibrational relaxation that is fast compared to the fluorescence lifetime. The intramolecular vibrational relaxation in phthalocyanine is compared to that observed in a recent series of experiments on alkylbenzenes.

Dynamic NMR in liquid crystalline solvents—ring inversion of s‐trioxane
View Description Hide DescriptionThe protonmagnetic resonance spectrum of s‐trioxane in nematic liquid crystalline solvents was studied over a wide range of temperatures. The spectra exhibit striking line shape variations with temperature which are attributed to the ring inversion of s‐trioxane. At high temperatures (≳100 °C) the spectra correspond to an average Hamiltonian due to the two fast interconverting chair forms of the molecule. As the temperature is lowered the resonance lines undergo selective broadening due to incomplete averaging of the two conformers. In order to obtain quantitative kinetic results the line shapetheory for dynamic NMR in anisotropicliquids is formulated and is used to calculate theoretical spectra for s‐trioxane undergoing ring inversion. Comparison of the experimental and theoretical spectra yields kinetic parameters for the reaction in the temperature range at which the liquid crystalline solutions were studied, i.e., −20 to +110 °C. The protonNMR of s‐trioxane was also studied in the isotropic solvent freon in the temperature range −80 to −40 °C. Analysis of these spectra yield essentially the same kinetic parameters as in the anisotropicsolvents. The final overall results are ΔH ^{≠} = 11.2 kcal/mole, ΔS ^{≠} = +3.1 e.u., and 1/τ (300 °K) = 2.0×10^{5} sec^{−1}. The advantages and limitations of dynamic NMR in liquid crystalline solvent are discussed.

Infrared emission spectroscopy of the products of UV‐dissociated chloroethylenes
View Description Hide DescriptionThe chloroethylenes are known to undergo dissociation under ultraviolet light to form chloroacetylenes. We report here the infrared emission spectra of the products of 193 nm dissociation of vinyl chloride, t r a n s‐ and c i s‐dichloroethylene, vinylidene chloride, and trichlorothylene. The spectra reported here represent molecules that have undergone from four to 40 collisions. The most prominent features of these spectra are that the products are very highly vibrationally excited. The energy partitioning possibilities that are consistent with these results are discussed.

The high temperature generation and ESR investigation of ScH_{2}, YH_{2}, and YD_{2} radicals in rare gas matrices: A nonequilibrium vaporization technique for solid samples
View Description Hide DescriptionThe ScH_{2}, YH_{2}, and YD_{2} radicals have been generated from metal hydride powders at high temperatures by a nonequilibrium vaporization technique. The MH_{2} radicals were isolated in rare gas matrices at 12 K and investigated by electron spin resonance(ESR). The vaporization procedure has potential applications for the production of other vapor species which are difficult to generate by conventional means. The electronic structure, geometry, and ground state of these dihydride radicals were estimated from the ESR results and compared with recent theoretical conclusions. The metal ’’d’’ electron was found not to participate significantly in the M–H bond. These findings for MH_{2} are compared with recent results for ScF_{2} and YF_{2}. The magnetic parameters for ScH_{2} are g _{∥} = 1.980 (1), g _{⊥} = 1.987 (1), A _{∥}(^{45}Sc) = −212 (2) MHz, A _{⊥}(^{45}Sc) = −128, ‖A _{iso}‖(H) = 12 (2). For YH_{2}, g _{⊥} = 1.991 (1), A _{⊥}(^{89}Y) = −40 (10) MHz, ‖A _{⊥}‖ (H) = 13 (2) MHz.

A simple interpretation of Raman excitation spectra of metalloporphyrins
View Description Hide DescriptionA simple treatment of vibronic coupling in metalloporphyrins is described. Electronic degeneracy in the excited states is treated to arrive at simple expressions for the Raman excitation spectra and the absorptionspectrum. The expressions are simple analogs of the usual A and B term expressions for the scattering tensor and are easily programmed for a small computer. They are only applicable in the weak vibronic coupling limit, but are nevertheless suitable for interpretation of the spectra of most biologically relevant porphyrinsystems, at least as a starting point. They allow a simple discussion of a number of interference phenomena that influence the Raman intensity distribution in the excitation profiles of metalloporphyrins and other molecular systems. Some of these phenomena are consequences of electronic degeneracy in the resonant excited state.Nonadiabatic coupling contributions are also included in the expressions. Experimental Raman excitation spectra are discussed in light of this simple model.

Absorption and dispersion in the O_{2} microwave spectrum at atmospheric pressures
View Description Hide DescriptionCalculations are performed for absorption and phase dispersion at various frequencies within the 60 GHz band of O_{2} from low pressures where the spectral lines are isolated, to atmospheric pressures where they merge to form a continuum band. A perturbation theory proposed by Rosenkranz was tested and found to be valid for pressures up to 100 kPa (1 atm). The ’’line coupling coefficients’’, which describe the transfer of excitation from one radiating state to another, are also studied and various methods for evaluating these coefficients are analyzed and compared with experimental data. It is found that dispersion measurements are extremely sensitive to these coefficients and an experimental procedure for systematically measuring them is outlined; it is shown that such measurements can provide a very sensitive test for theoretical calculations of inelastic transition amplitudes.

Time resolved study of rotational energy transfer in A ^{2}Σ^{+}(ν′ = 0) state of OH in a flame by laser induced fluorescence
View Description Hide DescriptionTime resolved study of rotational transfer in the A ^{2}Σ^{+}(ν′ = 0) state of OH in a flame is performed by analyzing the fluorescence spectra scanned at different delays after the short exciting laser pulse. The conditions under which a three level model apply are discussed. Then comparison between this kinetic model and experiment provides direct total transfer rate out of the J′ = 7 1/2 excited level: The specific transfer cross sections derived are 70 Å^{2} for N_{2} and 18 Å^{2} for Ar. The fluorescence spectra show a strong propensity for spin conservation upon rotational transfer. As the delay increases, the rotational population distribution among the upper state tends to a Boltzmann distribution.

Vibrational dephasing in liquid ethane‐d _{6}
View Description Hide DescriptionThe spontaneous Raman spectrum of liquid C_{2}D_{6} has been obtained. The ν_{2} and ν_{3} isotropic bands have been analyzed by a memory‐functions procedure. The analysis suggests that the vibrational dephasing arises from rotation about the symmetry axis. The tumbling of the symmetry axis is much slower, and in the case of the ν_{3} band leads to ’’homogeneous narrowing.’’ The 2ν_{2} overtone bandwidth was found to have a first‐power dependence on the vibrational quantum number. The depolarization ratio of the ν_{3} band is 0.14; the ν_{2} band has a depolarization ratio less than 0.02.

Theoretical study of shape resonances in the partial photoionization cross sections for the b ^{4}Σ_{ g } ^{−} and B ^{2}Σ_{ g } ^{−} states of O_{2} ^{+}
View Description Hide DescriptionUsing the single‐center static‐exchange approximation, the 3σ_{ g }photoionization cross section of O_{2} is calculated. The R‐averaged results are obtained for the production of O^{+} _{2} in the b ^{4}Σ^{−} _{ g } and B ^{2}Σ^{−} _{ g } states. Shape resonances, a few electron volts wide, are found at 3 and 2.5 eV from the threshold for these two states, respectively. We find reasonable agreement with the experimental cross sections for the B ^{2}Σ^{−} _{ g } state and with the β values for the b ^{4}Σ^{−} _{ g } state. For this last state, comparison between theory and experiment leads us to a possible interpretation of the data, but a definite answer can not be given without taking into account the autoionized Rydberg states.

Multiphoton ionization mass spectroscopy of deuterated analogs of acetaldehyde: Evidence for deuterium scrambling
View Description Hide DescriptionFragmentation patterns as a function of laser flux are obtained for CD_{3}CHO using a quadrupolemass spectrometer for excitation at the origin of the two‐photon resonant n→3sRydberg transition at 55 138 cm^{−1}. Extensive fragmentation is found with product ion branching ratios showing a strong dependence on laser flux. Fragments such as CDO^{+} appear, indicating that bond rearrangement occurs. In the case of the CD^{+} _{3}, CD_{2}H^{+} ion pair, the CD_{2}H^{+} scrambled product dominates, although in no case does the scrambling reach the statistical limit obtained by assuming that all hydrogen species are equivalent. The individual ion signal data can be fit with a rate equation model. Isotopic selectivity for mixtures of CD_{3}CDO/CH_{3}CHO is demonstrated; no partially deuterated products are found, indicating that the scrambling in CD_{3}CHO is not collisionally induced.