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Volume 75, Issue 7, 01 October 1981

A charge flow model for vibrational rotational strengths
View Description Hide DescriptionThe valence‐optical approach is used to construct a model applicable for the calculation of rotational strengths associated with fundamentals of vibrational transitions. It is shown that the model has the essential property of giving rotational strengths that are independent of the choice of origin.

Intramolecular and intermolecular interactions in methyl substituted pentanes as revealed by high energy CH‐stretching overtone spectra
View Description Hide DescriptionThe overtone spectra of pentane and 12 methyl substituted pentanes are measured in the liquid phase in the region of the pure local mode overtones corresponding to Δv _{CH} = 3, 4, 5, and 6. The local mode structure of the spectra is discussed. The local mode frequencies ω_{CH} and diagonal local mode anharmonicities X _{CH} for the methyl oscillators are obtained from an analysis of the spectra. The change in ω_{CH} and X _{CH} with increased methyl substitution is related to a barrier to large amplitude vibrational motion arising from steric crowding. The height of this barrier is obtained from dissociation energies calculated from ω_{CH} and X _{CH} by taking the local methyl CH stretching potential to be a Lippincott–Schroeder potential. The methyl steric interactions are described in terms of nonbonded Lennard‐Jones and Morse potentials. The parameters for these nonbonded potentials are calculated from the barrier heights and structural information obtained from molecular models for their least strained conformations. These parameters are used to identify a positional dependence in methyl steric hindrance. Force constants evaluated from the nonbonded potentials are correlated with bulk liquid properties.

Molecular mechanics for phase transition and melting of n‐alkanes: A spectroscopic study of molecular mobility of solid n‐nonadecane
View Description Hide Descriptionn‐Nonadecane shows a phase transition from orthorhombic to so called α phase near 22.8 °C and melts at 32 °C. Temperature dependence studies of infrared and Raman spectra of n‐nonadecane and of selectively deuterated nonadecanes (2D_{2} and 10D_{2}) allow one to obtain a detailed description of the structural changes which take place at the phase transition and melting. Experiments are interpreted on the basis of normal mode calculations. Comparison with the information available in the literature from other experiments or calculations shows that additional information are derived in this work. A model for the mechanism of phase transition is proposed.

Theory of photon echoes from a pair of coupled two level systems: Impurity dimers and energy transfer in molecular crystals
View Description Hide DescriptionWe present the theory of the photon echo and stimulated photon echo from a four level system that results from a pair of weakly coupled two level systems. This model describes an impurity dimer in a molecular crystal. We include the effect of relaxation processes, focusing on energy transfer between delocalized states. We find that both the photon and stimulated echoes contain information about static level splittings and dynamic energy transfer processes. Moreover, these observables show a dramatic dependence on the excitation power. We discuss implications of our results for recent experiments on concentration dependent dephasing of impurities in molecular crystals. Our analysis casts considerable doubt on the intermolecular interaction interpretation of these experiments.

High pressure luminescence of indole and substituted indoles in liquid solution
View Description Hide DescriptionHigh pressureluminescencemeasurements have been made on indole, 5‐methoxyindole, and tryptophan in a number of solvents. Radiative and nonradiative rates have been calculated as a function of pressure. It has been shown that the nonradiative rates are determined by viscosity in all solvents except water and, to some extent, methanol for which the peak shift as well as possibly viscosity are the controlling factors. The radiative rates and the peak position and shift have been scaled with the dielectric constant in a continuous and consistent way. Some peculiarities have been observed when water and methanol are used as solvents.

Single‐mode versus multimode calculations of Raman intensities of cytochrome c
View Description Hide DescriptionThe simple single‐mode model for Raman scattering is examined with regard to the relative Raman intensity ratios of cytochrome c at the 0–0 and 0–1 transition frequencies of S _{1} and S _{2}. Interference between A‐ and B‐term amplitudes as well as nonadiabatic effects are considered. The model is found to generate the experimentally observed intensity ratios only under the condition that the damping factors in S _{2} (Soret band) and S _{1} (Q band) are nearly equal. This condition is at variance with observation unless multimode effects are included to account for the large bandwidth of S _{2} in cytochrome c. Use of the multimode model generates the observed Raman intensity ratios in S _{2} without the need for A‐ and B‐term interference effects.

Calculation of Raman intensities for the ring‐puckering vibrations of cyclopentene and 2,5‐dihydrofuran
View Description Hide DescriptionRaman intensities are calculated for the ring‐puckering vibrations of cyclopentene and 2,5‐dihydrofuran using an anisotropic atom–point dipole interaction model to calculate the elements of the molecular polarizabilitytensor. Large second‐order terms are calculated for both molecules in the expansions of the molecular polarizabilitytensor elements in the ring‐puckering coordinate [‖(∂^{2}α_{μν}/∂Z ^{2})_{0}‖≫0]. These terms result in intense calculated Δv = 2 overtone transitions, in agreement with experimental observations. The calculations predict that the Δv = 2 transitions of cyclopentene are one to two orders of magnitude more intense than the Δv = 1 fundamentals, while the two sets of transitions are comparable in intensity in 2,5‐dihydrofuran. Although the shape of the band contours precludes observation of the Δv = 1 transitions for either molecule, the calculations suggest that the fundamental transitions of cyclopentene would not be observable even if the band shape were amenable to observation.

Study of mode Grüneisen parameters in various quasi‐one‐dimensional crystals
View Description Hide DescriptionThis work reports the pressure dependence of the Raman spectra of TCNQ^{0}, TTF^{0}, K(TCNQ), Cs_{2}(TCNQ)_{3}, perylene–TCNQ, and phenazine–TCNQ in the range from 0 to 70 kbar. By using the average compressibility of ∼0.01 kbar^{−1} of TTF–TCNQ, approximate mode Grüneisen parameters are derived spanning a range of several orders of magnitude. In the gas phase spectrum of TCNQ, low lying internal modes are observed at 40, 76, and 145 cm^{−1}, lowering the so far assumed demarcation between internal and external modes from about 200 cm^{−1} down to less than 50 cm^{−1}. From the pressure and gas phase data it is concluded that all of the observed low frequency modes in this class of compounds actually are modes of mixed internal and external character. In TTF^{0} and TCNQ^{0} a continuous change in color from yellow to dark red is observed when pressure is applied until the samples become opaque at about 80 kbar. Furthermore, TCNQ shows a phase transition at 12 kbar.

Raman spectroscopic investigation of the structure of silicate glasses. III. Raman intensities and structural units in sodium silicate glasses
View Description Hide DescriptionThe Raman scattering intensity of the 1100 cm^{−1} polarized band, which appears on the addition of Na_{2}O to SiO_{2}glass, reaches a maximum at the disilicate composition. The intensity of the polarized band at 950 cm^{−1} increases sharply as the Na_{2}O concentration increases above 30 mole %. These data were interpreted by normal mode calculations and by IR and Raman intensity calculations for the silicate anion structural units: SiO_{4} isolated tetrahedra, Si_{2}O_{7} dimers, Si_{2}O_{6} chain links, Si_{2}O_{5} sheet units, and Si_{2}O_{4} framework units. According to these simplified models, the polarized high frequency band is due to symmetric stretching of Si–O^{−} nonbridging bonds and the frequency increases with degree of polymerization of the tetrahedra. The previous assignments of the 1100 cm^{−1} band to the symmetric stretch of tetrahedra containing one nonbridging oxygen and of the 950 cm^{−1} band to the symmetric stretch of tetrahedra containing two nonbridging oxygens were confirmed. The other main feature of the alkali silicate glasses, an intense polarized band in the range of 400–600 cm^{−1}, was shown to be a mixed stretching bending mode of the Si–O–Si bridging bond. The model also accounts for the loss of intensity of the high frequency band with increasing degree of silicapolymerization.

Characteristics of low resolution Q‐branch microwave bands: Applications to 2,2,2‐trifluoroethyl vinyl ether and 7‐methylbicyclo[2.2.1]‐hept‐2‐ene‐5‐one
View Description Hide DescriptionA rigid rotor analysis of μ_{ b }‐ and μ_{ c }‐type Q‐branch series indicates overlapping low J transitions giving rise to the low resolution microwave bands which follow a half‐integer frequency rule. At higher J values certain Q‐branch series form bandheads shifted to a lower frequency from the series origin. Under favorable circumstances low resolution bands are expected and the frequency of these bands may be used to estimate the rotational constant parameter (A−C). The microwave spectrum of 2,2,2‐trifluoroethyl vinyl ether exhibits bands which are assigned to the bandhead phenomenon. This provides a value of (A−C) and in conjunction with previous data yields the three rotational constantsA, B, and C uniquely from low resolution data. The results are consistent with the s‐c i s‐planar as well as g a u c h e conformations. Previously reported low resolution bands of 7‐methylbicyclo[2.2.1]‐hept‐2‐ene‐5‐one are interpreted as two half‐integer band series arising from low‐lying vibrational states with slightly different values of (2A−B−C).

Density dependence of the light scattering spectrum of fluid nitrogen
View Description Hide DescriptionThe light scatteringspectrum of fluid nitrogen at 157 K and densities between 120 and 462 amagat was fitted to a third order virial expansion in the density. The one‐body intensity contains the unresolved rotational spectrum of the isolated molecules. The two‐body intensity contains a rotation–collision cross contribution with a zero integrated intensity, as expected, and a purely collision‐induced contribution with the exponential form and decay constant predicted by the model of Bucaro and Litovitz. This latter contribution, unlike all systems previously studied, was found to be negative. The three‐body intensity is positive with a shape similar to that of the two‐body intensity.

Tensor patterns and polarization ratios for three‐photon transitions in fluid media
View Description Hide DescriptionExcitation of polyatomic molecules by the simultaneous absorption of three photons provides a method for observing and unambiguously assigning molecular electronic and vibronic transitions which may be inaccessible to one‐ or two‐photon spectroscopy. Such three‐photon allowed states may be forbidden to one‐photon processes by reason of symmetry, even though the states may be parity (g→u) allowed. Also, the states may be one‐photon allowed but lie at inaccessibly high energies, such as in the vacuum ultraviolet. Calculation of the polarization dependence of the three‐photon absorption intensity requires a knowledge of the third rank tensor pattern for the three‐photon transition. These patterns are presented here for the symmetry species of several molecular point groups, and their applications are discussed. Tensor patterns for the special case of three‐photon excitation by a single laser are also presented. In several symmetry species the polarization ratio (circular/linear) for excitation of fluid media becomes 5/2. The intensities vanish for a few species possessing antisymmetric tensors. This phenomenon of ’’identity forbiddenness’’ is discussed.

Location and movement on dehydration of Cu^{2+} in Cs_{7}Na_{5}A zeolite from electron spin echo modulation analysis
View Description Hide DescriptionIt is shown that paramagnetic ions like Cu^{2+} can be located and that their motion upon dehydration in zeolites can be monitored by electron spin echo modulation analysis. In Cs_{7}Na_{5}A zeolite,Cu^{2+} is located inside the β cage about 0.1 nm below a hexagonal window between the β and α cages. The Cu^{2+} is coordinated to three lattice oxygens and one water molecule. On dehydration Cu^{2+} moves out of the β cage along the threefold axis of the hexagonal window to about 0.1 nm above the hexagonal window into the α cage.

Microwave, infrared, and Raman spectra, structures, quadrupole coupling constants, barrier to internal rotation, and vibrational assignment of 1‐chloro‐2‐methylpropene
View Description Hide DescriptionThe microwave spectra of (CH_{3})_{2}CCH^{35}Cl and (CH_{3})_{2}CCH^{37}Cl have been recorded from 12.5 and 39.0 GHz. a‐type transitions were observed and R‐branch assignments have been made for the ground vibrational states for both isotopes. The rotational constants were found to have the following values: for (CH_{3})_{2}CCH^{35}Cl, A = 8400.77±1.18, B = 2258.18±0.02, C = 1818.72±0.02 MHz; for (CH_{3})_{2}CCH^{37}Cl, A = 8399.31±0.66, B = 2199.93±0.02, C = 1780.73±0.02 MHz. From a diagnostic least‐squares adjustment to fit the six rotational constants and with reasonable assumed carbon–hydrogen distances and angles, the following heavy atom structural parameters were obtained: r(C–C) = 1.507±0.013 Å, r(C = C) = 1.355±0.019 Å, r(C–C1) = 1.750±0.019 Å, ∢C–C = C(c i s) = 122.20°±4.50°, ∢C–C = C(t r a n s) = 119.39°±1.84°, and ∢CCCl = 124.24°±2.79°. The quadrupole coupling constants for the (CH_{3})_{2}CCH^{35}Cl molecule were found to have the following values: χ_{ a a } = −49.2, χ_{ b b } = 19.5, and χ_{ c c } = 29.7 MHz. From an analysis of the internal rotational splittings, the threefold barrier for the c i s methyl group was found to be 288±5 cm^{−1} (0.824 kcal/mole). The infrared (3500–50 cm^{−1}) and Raman spectra (3500–10 cm^{−1}) have been recorded for both the gas and solid states. Additionally, the Raman spectrum of the liquid was recorded and qualitative depolarization values were obtained. All of the normal modes have been assigned based on band contours, depolarization values, and group frequencies. Several torsional transitions for the t r a n s methyl group have been observed in the far infrared spectrum of the vapor, and from these transitions a barrier of 745±60 cm^{−1} (2.14 kcal/mole) has been calculated for the internal rotation of the t r a n s methyl group. These results are compared to the corresponding quantities in similar molecules.

Angle‐resolved photoelectron spectroscopy of CS_{2} and COS measured with synchrotron radiation
View Description Hide DescriptionAngle‐resolved photoelectron spectra of CS_{2} and COS have been taken as a function of photon energy. From these data the angular asymmetry parameter β has been obtained for the five valence orbitals of CS_{2} and the three least tightly bound valence orbitals of COS. Monochromatized polarized photons produced from synchrotron radiation have permitted studies from near ionization threshold up to 29 eV. The experimental data were used to evaluate previous theoretical calculations which employ a multiple scattering method. The agreement for the least tightly bound orbitals of the nonpolar CS_{2} molecule were good. There were some problem areas, particularly with the more tightly bound orbitals of CS_{2} and the polar molecule COS. However, when previous results for CO_{2} are taken together with the results on CS_{2} and COS, the calculations in general give a reasonable account of the experimental β values and suggest that the multiple‐scattering method will be useful for the study of differential cross sections of polyatomic molecules.

Line shapes and transport for excitons with stochastic coupling
View Description Hide DescriptionThe effect of nonlocal fluctuations on the line shape and dynamics of exciton states is considered, using a stochastic perturbation approach. The cumulant expansion is truncated at second order. The usual Gaussian and motional narrowing limits are treated and the limitations of the model are discussed.

NMR relaxation dispersion in an aqueous nitroxide system
View Description Hide DescriptionNMR relaxation rate measurements are reported for protons in aqueous nitroxide solutions over the frequency range from 0.01 to 20 MHz. A complete relaxation dispersion curve is observed that is analyzed in terms of dynamical models for the interaction between the solvent and solute. Both translational and rotational motions contribute to the relaxation behavior, although the translational contribution is dominant. Dipole–dipole interactions are sufficient to account for the data over this frequency range. At 298 K translational motions account for 75% for the relaxation, and are characterized in the present analysis by correlation time of 73 ps and 2.5 Å as the distance of closest approach. The small rotational term requires a 22 ps correlation time and an interdipole separation of 2.1 Å.

Quasiclassical dynamics of light+heavy–heavy and heavy+heavy–light atom reactions: The reaction X+F_{2}→XF+F(X = Mu, H)
View Description Hide DescriptionThe effect of mass variation on the dynamics of exoergic chemical reactions has been investigated using one and three dimensional quasiclassical trajectories. The reactions studied are the light+heavy–heavy atom reactions X+F_{2}→XF+F(X = Mu, H) and their heavy+heavy–light atom counterparts. For all four reactions, the best extended LEPS potential energy surface No. 2 of Jonathan e t a l. has been used. Calculations have been performed for the total reaction cross sections,reaction probabilities as a function of impact parameter, product energy disposal, angular distributions, and, for the Mu+F_{2} and H+F_{2}reactions, rate coefficients and activation energies. It is found that many features of the reactivity of the three dimensional reactions can be understood in terms of the corresponding one dimensional reactions. The reactivity of all four one dimensional reactions in the threshold region is found to be determined by the heights of their vibrationally adiabatic barriers, which occur in the entrance valley, and not by exit valley properties. Comparison with thermal experimental rate coefficients and activation energies for the Mu+F_{2} and H+F_{2}reactions suggests that tunneling is very important for the Mu reaction, and that a potential surface with a different barrier region topology is probably required in order to account for the experimental results.

A unitary approximation to the inelastic binary collision problem
View Description Hide DescriptionThe memory function formulation of the time‐dependent Schrödinger equation is used to analyze the internal state problem associated with the decoupled motions or impact parameter approximation to binary collisional events. The exact, time‐orderd memory functions are then replaced with their time‐disordered counterparts. This generates a unitary approximation to the collision problem as well as simple, easily interpretable formulas for the transition probabilities. Two specific examples are considered in detail, namely, (i) the three‐state resonant transfer model and (ii) a ’’paired‐states’’ model for the dynamics of a many‐state problem.

Excitation of the O_{2}(a ^{1}Δ_{ g }) state by low energy electrons
View Description Hide DescriptionCoefficients for the excitation of the a ^{1}Δ_{ g } metastable state of O_{2} by low energy electrons in O_{2}–Ar mixtures have been measured using a drift tube technique. The time dependence of the absolute intensity of the 1.27 μm band emission was measured for 1% and 5% O_{2} in Ar at total gas densities of (1–20)×10^{24} m^{−3}. The electric field to gas density ratio E/N was varied from (0.3–10)×10^{−21} V m^{2}, corresponding to calculated mean electron energies between about 0.8 and 4 eV. The measured decay constants yield a normalized diffusion coefficient for O_{2}(a ^{1}Δ_{ g }) in Ar of (2.8±0.2)×10^{20} m^{−1} s^{−1} and a rate coefficient for quenching by O_{2} of (1.57±0.1)×10^{−24} m^{3} s^{−1}. Our experimental electron excitation coefficients agree satisfactorily with values calculated using previously derived sets of electron collision cross sections for O_{2} and Ar.