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Volume 80, Issue 3, 01 February 1984

Integrated infrared intensities and the atomic polar tensors in SO_{2}
View Description Hide DescriptionThe integrated infrared intensities of the fundamental modes in SO_{2} have been measured by the Wilson–Wells–Penner–Weber method. The results are 20.60±0.07 km/mol for ν_{1} at 1151 cm^{−} ^{1}, 25.33±0.04 km/mol for ν_{2} at 518 cm^{−} ^{1}, and 192.13±0.14 km/mol for ν_{3} at 1362 cm^{−} ^{1}. The polar tensors derived from the observed intensities are in essential agreement with those obtained from an ab initio calculation. The effective atomic charge for the oxygen atoms in SO_{2} is shown to have a magnitude close to that found in other oxygen‐containing molecules.

Integrated infrared intensities in cyanogen
View Description Hide DescriptionThe integrated intensities of the fundamental modes in cyanogen have been measured by the Wilson–Wells–Penner–Weber method. The results are 4.55±0.04 km mol^{−} ^{1} for the fundamental at 2149 cm^{−} ^{1} and 36.28±0.32 km mol^{−} ^{1} for the fundamental at 226 cm^{−} ^{1}. The dipole moment derivative for the Π_{ u } mode obtained from these measurements and that obtained from ab initio calculations using a STO‐NG basis set are found to be in essentially exact agreement with each other, although there is some disagreement in magnitude for the Σ^{−} _{ u } mode.

Infrared intensities in chloroform^{a)}
View Description Hide DescriptionThe integrated intensities of the ν_{2}(A _{1}), ν_{4}(E), and ν_{5}(E) modes in HCC1_{3} and DCC1_{3} have been measured by the Wilson–Wells–Penner–Weber method and the atomic polar tensors derived. The effective atomic charge for hydrogen was found to be χ_{H}/e=0.048±0.004, a value that is close to that in HCF_{3}, χ_{H}/e=0.049±0.004, although there is a sign discrepancy in the H atom polar tensor between chloroform and fluoroform.

Atom anisotropies and effective charges in vibrational intensity analysis^{a)}
View Description Hide DescriptionIn the analysis of infrared intensity data, it is shown that the value of the atom anisotropy relative to the effective atomic charge is a very useful parameter in characterizing chemical bonds in terms of atomic properties. The application of this parameter reveals that the anomalously large effective charges of hydrogen in HCCH and HCN in comparison with those in other hydrocarbons can be adequately explained by virtue of their rather ionically bonded protons.

Theoretical study of competing photoionization and photodissociation processes in the NO molecule
View Description Hide DescriptionThe multichannel quantum defect theory of dissociative recombination is reviewed and adapted to treat simultaneous vibrational preionization and electronic predissociation in NO. The photoionization,photodissociation, and photoabsorption cross sections of NO corresponding to ^{2}Π excited states are computed in the range from the N(^{2} D)+O(^{3} P) dissociation limit up beyond the second vibrational level of NO^{+}(X ^{1}Σ^{+}). The calculations are based on the ^{2}Π Rydberg and valence state potential curves and Rydberg‐valence state interaction energies extracted by Gallusser and Dressler from the high‐resolution spectroscopic data of Miescher and co‐workers. These data pertain to the discrete range and furnish precise information on numerous spectral perturbations involving levels of the npπ Rydberg series and the B and L valence states. The calculated cross section profiles reproduce with reasonable accuracy the observed absorption and photoionization curves, and in particular the widths of the Rydberg resonances. The coexistence of Δv=1 and Δv≳1 preionization mechanisms is demonstrated. While the Δv=1 process is related to the R dependence of the quantum defect, the Δv≳1 preionization is shown to be i n d u c e d by the strong electronic Rydberg‐valence state coupling which primarily (and simultaneously) is responsible for predissociation. This result is generalized to the competition between any different decay mechanisms affecting a discrete excited level, and several limiting cases of competition are characterized. The relative contributions of different partial waves lλ to the photoionizationspectrum of NO are estimated on the basis of the medium‐resolution Rydbergabsorptionspectrum.

Limitations of the rotating wave approximation in the theory of molecular collisions in a laser field
View Description Hide DescriptionThe radiative transition in a molecular collision system is usually described as a motion on so‐called electronic‐field potential surfaces, obtained by application of the rotating wave approximation (RWA). The RWA is shown to be valid if the time of interaction in the resonance region is larger than a third of the light period T/3. The interaction time is estimated for a radiationX _{3/2}→X _{1/2} transition in the F+H_{2} and Br+H_{2} systems.

Optical third‐harmonic generation in the fluorinated methanes and sulfur hexafluoride
View Description Hide DescriptionOptical third‐harmonic generation experiments are used to determine nonlinear polarizabilities for the fluorinated methanes and sulfur hexafluoride. An ambiguity present in previous measurements of third‐harmonic generation in gases is resolved.

Ab initio configuration interaction calculations of the hyperfine structure in small radicals
View Description Hide DescriptionExtended basis set natural orbital CI wave functions have been used to compute the isotropic and anisotropichyperfine coupling of ten small radicals for which experimental measurements exist. The configuration spaces employed in the CI’s were of two kinds: One involved all single and double excitations from the Hartree–Fock configuration plus limited triples and quadrupoles, while the other consisted of singles and doubles from a multireference set coupled with various configuration selection techniques. At the highest level of theory used in the present study, agreement with experiment was generally within 10%, but differences for the isoelectronic series H_{2}CN, H_{2}CO^{+}, and H_{2}BO as well as the related acetaldehyde radical cation H_{3}C–CHO^{+} are larger than would be expected based on the data from the other six radicals.

An ESR investigation of the formaldehyde cation radicals (H_{2} ^{12}CO^{+} and H_{2} ^{13}CO^{+}) in neon matrices at 4 K
View Description Hide DescriptionThe H_{2} ^{12}CO^{+} and H_{2} ^{13}CO^{+} radicals have been trapped in neon matrices at 4 K for detailed ESR(electron spin resonance) study. Both photoionization (17 eV) from an open tube neon discharge source and electron bombardment, recently developed techniques for generating matrix isolated cations, were employed in this investigation. The ESR spectra were sufficiently resolved to allow a complete characterization of the nuclear A and gtensors. The magnetic parameters are: g _{ x }=2.0069(2), g _{ y }=2.0015(2), g _{ z }=2.0025(2), A _{ x }(H)=129.4(2), A _{ y }=(H)=134.5(2), A _{ z }(H)=134.3(2) g, ‖A _{ x }(^{13}C)‖ =43.9(2), ‖A _{ y }(^{13}C)‖=37.3, and ‖A _{ z }(^{13}C)‖ =35.1(2) g. Of particular interest is the conclusion that the direction of the largest g component (x) lies perpendicular to the radical plane rather than along the in‐plane C _{2} symmetry axis. These H_{2}CO^{+} results were compared with MINDO and ab initio CI type theoretical calculations and to previous experimental results for the isoelectronic radicals H_{2}CN, H_{2}BO, H_{2}C_{2} ^{−}, and the acetaldehyde cation CH_{3}HCO^{+}. Theoretical values for the isotropic nuclear hyperfine interaction for this series of H_{2}XY radicals are consistently below the experimental results, although good agreement is obtained for the H and ^{13}C dipolar interactions in H_{2} ^{13}CO^{+}.

Photoconductivity measurements of x‐ray absorption of liquids: Fe K‐edge spectrum of ferrocene in 2, 2, 4, 2, 4‐trimethylpentane
View Description Hide DescriptionUsing a conductivity technique, we have obtained EXAFSspectra of ferrocene Fe(C_{5}H_{5})_{2} in 2,2,4‐trimethylpentane under various conditions. The behavior of the collection efficiency of the conductivity cell affects the appearance of the photoconductivityspectrum. In thin cells, the photocurrent increases at the K edge and normal EXAFS structure is observed. In the black cell limit, i.e., for total absorption the photocurrent yield actually drops at the Fe K edge but still exhibits x‐ray absorption fine structure (EXAFS) behavior. Both conductivity and transmission measurements give identical Fe(C_{5}H_{5})_{2} bond lengths.

Photoionization of N_{2} X ^{1}Σ^{+} _{ g }, v″=0 and 1 near threshold. Preionization of the Worley–Jenkins Rydberg series
View Description Hide DescriptionThe high resolution relative photoionization cross section for N_{2} is reported in the wavelength region from the ionization threshold to 650 Å, with particular attention given to the region between the N^{+} _{2} X ^{2}Σ^{+} _{ g }, v′=0 and 1 ionization limits. Cross sections from both the X ^{1}Σ^{+} _{ g }, v″=0 and 1 vibrational levels were determined in this region, and preionized members of the n pπ_{ u } ^{1}Π_{ u } Worley–Jenkins Rydberg series converging to N ^{+} _{2} X ^{2}Σ^{+} _{ g }, v′=1 are observed in both spectra. In the spectrum excited from v″=1, the Worley–Jenkins series appears prominently as a result of good Franck–Condon overlap between the N_{2} X ^{1}Σ^{+} _{ g }, v″=1 and the N^{+} _{2} X ^{2}Σ^{+} _{ g }, v′=1 levels; the intensities of the series members decrease approximately as 1/n*^{3}, in accord with simple theoretical predictions. However, in the spectrum excited from v″=0, the Worley–Jenkins series converging to N^{+} _{2} X ^{2}Σ^{+} _{ g }, v′=1 is weak as a result of a poor Franck–Condon overlap with the ground vibrational level; the intensities of the series members show large deviations from the simple theory as a result of channel interactions with Rydberg states converging to N^{+} _{2} A ^{2}Π_{ u }. These perturbing Rydberg states have low photoabsorptionoscillator strengths for excitation from v″=1 (in contrast to excitation from v″=0) and hence have only a small effect on the cross section from the excited vibrational level. Just as in the case of photoionization of H_{2}, the results demonstrate that channel interaction can redistribute the oscillator strength of a perturbing Rydberg state of low principal quantum number over a number of members of an interacting Rydberg series and that the spectral range affected by the perturber can be much greater than the width of the perturbing level.

Rotational spectrum and structure of the complex HCNCO_{2}
View Description Hide DescriptionRadiofrequency and microwave rotational spectra of the complexes HCN CO_{2} and DCN CO_{2} have been obtained using molecular beam electric resonance spectroscopy. The spectra are characteristic of prolate asymmetric rotors. The spectroscopic constants obtained are These are shown to be consistent with a C _{2v } structure in which the nitrogen of the HCN bonds to the carbon of the CO_{2} with a weak bond length of 3.00 Å. The HCN subunit subtends an average angle of 17.4 (2)° with the line joining the centers of mass of the two submolecules, and the average bending vibrational amplitude of the CO_{2} is 11(1)°. The difference between the average amplitudes of the in‐plane and out‐of‐plane bending vibrations of the HCN is less than 4°. The stretching force constant for the weak bond is 0.049 mdyne/Å in HCNCO_{2}, and the induced dipole moment is 0.361 D.

Iron K x‐ray absorption‐edge structures of FeS and FeS_{2}
View Description Hide DescriptionThe Fe K absorption‐edge structures of the NiAs‐type FeS and the pyrite‐type FeS_{2} have been measured with a high‐resolution two‐crystal spectrometer. The Kabsorption edges consist of a step‐like structure and are interpreted in terms of the energy‐band structures for these sulfides. The presence of the absorption structure is much clearer in Fes_{2} than in FeS. This difference is attributed to the difference of the iron 3d states in FeS and FeS_{2} and the distortion of the octahedron [FeS_{6}]^{1}0− in these sulfides. The Fe Kabsorptionspectrum of FeS_{2} is compared with the sulfur Kabsorptionspectrum reported previously and a good agreement is obtained between them.

Fluorescence excitation spectra of indole, 3‐methyl indole, and 3‐indole acetic acid in supersonic jets
View Description Hide DescriptionIn this paper we report on the laser‐induced fluorescence spectra of indole, 3‐methyl indole, and 3‐indole acetic acid in pulsed supersonic expansions of He. The electronic origin of the first excited (^{1} L _{ b }) state is 35 233 cm^{−} ^{1} for indole, 34 887 cm^{−} ^{1} for 3‐methyl indole, and 34 044 cm^{−} ^{1} for 3‐indole acetic acid. On the basis of the energetics and fluorescence lifetimes, the spectral features at excess vibrational energies E_{ V }=0–2200 cm^{−} ^{1} for indole and E _{ V }=0–1600 cm^{−} ^{1} for 3‐methyl indole were assigned to the vibrational level structure within a single electronic manifold. The second electronically excited (^{1} L _{ a }) state is not located in the above energy ranges. The energetics of selective solvation of 3‐methyl indole was inferred from the spectra of the van der Waals complexes with Ar, H_{2}O, and D_{2}O, which provide information on microscopic solvent shifts of the electronic origin and of some prominent vibrational excitations of the ^{1} L _{ b } states.

Theory of spin relaxation by translational diffusion in two‐dimensional systems
View Description Hide DescriptionSpin relaxation rates T ^{−1} _{1} and T ^{−1} _{2} are calculated for nuclear (or electron) spins diffusing on finite two‐dimensional, planar, or spherical surfaces. The spin relaxation is assumed to be due to modulation of the intermolecular dipole–dipole interactions. It is shown that the mathematical divergences encountered in a number of previous theoretical treatments of this problem for infinite planar surfaces have no physical significance; these divergences are avoided by limiting the calculations to two‐dimensional systems that are finite, but that are otherwise of arbitrarily large size. The theoretical relaxation rates T ^{−1} _{1} and T ^{−1} _{2} for finite, planar two‐dimensional systems are found to have a number of unique properties that should facilitate the interpretation of magnetic resonancespectra of molecules physically adsorbed on solid surfaces. For example, the reduction in dimensionality of rapid diffusive motion yields relaxation rates typical of slow motion in three‐dimensional systems. Under certain conditions the relaxation rate T ^{−1} _{1} is strongly dependent on the size of the two‐dimensional surface on which atoms or molecules diffuse. Moreover the shape of the surface (planar or spherical), which is of particular importance in the description of the two‐dimensional dynamics, can profoundly alter the frequency and temperature dependences of the spin‐relaxation rates. The theory appears to be directly applicable to recent experiments by J. Tabony [Prog. Nucl. Magn. Reson. Spectrosc. 14, 1 (1980)].

Vibrational circular dichroism and vibrational optical rotatory dispersion in molecular crystals. I. Fundamental tones
View Description Hide DescriptionA theory is advanced of the vibrational circular dichroism (VCD) and of the vibrational optical rotatory dispersion (VORD) in molecular crystals for the frequency region of the intramolecular vibrations. The substantial role of the symmetry of the crystal lattice is shown. The contributions of the fundamental tones of nondegenerate and degenerate vibrations in VORD and VCD are discussed. The effect of the separate molecule and of the intermolecular interaction on the gyrotropy of the crystal is estimated. The most favorable conditions for experimental examination of VCD are commented upon, as well as the information obtained from the VCD spectra.

Vibrational circular dichroism and vibrational optical rotatory dispersion in molecular crystals. II. Selection rules
View Description Hide DescriptionThe selection rules for the contribution of the electric and magnetic dipole and electric quadrupole moments of the optical active vibrations to the gyrotropy of molecular crystals are derived by group‐theoretical analysis. The peculiarities of nondegenerate and degenerate vibration modes are discussed. The selection rules for k → 0 case are tabulated for all 32 crystal classes, taking into account the time‐reversal symmetry and Wigner co‐representation theory.

Vibrational circular dichroism and vibrational optical rotatory dispersion in molecular crystals. III. Overtones of nondegenerate vibrations
View Description Hide DescriptionThe contribution of overtones of nondegenerate intramolecular vibrations to the vibrational circular dichroism of molecular crystals with one and two molecules in an elementary cell is calculated. The dielectricpermittivity and the gyration tensor are found in the region of the bound states of the two phonons and in the band of many particle states.

^{1}H and ^{13}C NMR study on rotation of congested methyl groups in methyl substituted phenanthrenes, fluorenes, and fluorenones
View Description Hide DescriptionWe have investigated methyl rotation in 4‐methyl and 4,5‐dimethyl substituted phenanthrenes, fluorenes, and fluorenones by the temperature dependence of the ^{1}H spin‐lattice relaxation times and high‐resolution solid‐state^{13}C NMR spectra. Distinctly unsymmetric curves of the relaxation times with temperature are ascribed to the existence of inequivalent methyl groups in the unit cell. The inequivalence is considered to be brought about by distortion about the central C_{4a }–C_{4b }bonds and anisotropicintermolecular forces in the crystalline state. The steric effect of the protruding 5‐hydrogen atom in a planar molecular structure gives rise to a remarkably high barrier (5.05±0.13 kcal mol^{−1}) to rotation of the methyl group in 4‐methylphenanthrene (1).

Scaling behavior in collinear atom–diatom collisions: Energy transfer from high vibrational states
View Description Hide DescriptionClassical energy transfer in a simple collinear collisionsystem is studied within the framework of a recently derived classical scaling formalism. In the present system which corresponds to collinearly colliding I_{2} (Morse oscillator) + rare gas (He, Ar), a simple polynomial function in the initial vibrational quantum number of I_{2} scales the first moment of the energy transfer(ET). Using the computed ET moment from only five states as input, it is possible to predict that from any other bound state in the vibrational manifold 0≤n≤110. There is correspondence—in a limiting case—between the classical scaling law and the (quantum) energy corrected sudden scaling theory given earlier; this allows for the interpretation of the classical scaling coefficients and indicates that the higher order terms are necessary in order to account for multiquantum vibrational transitions in the V‐T process.