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Volume 71, Issue 3, 01 August 1979

Microwave, infrared, and Raman spectra and structure of germyl isothiocyanate
View Description Hide DescriptionThe microwave spectra of H_{3} ^{70}Ge ^{14}NCS, H_{3} ^{72}Ge ^{14}NCS, H_{3} ^{74}Ge ^{14}NCS, the corresponding ‐d _{3} compounds, H_{3} ^{70}Ge ^{15}NCS, H_{3} ^{72}Ge ^{15}NCS, H_{3} ^{74}Ge ^{15}NCS, and H_{3} ^{74}Ge ^{14}N ^{13}CS have been recorded and analyzed from 12.4 to 40.0 GHz. The spectra are consistent with those for a symmetric top with a very low‐frequency bending mode. Relative intensity measurements of the excited states indicate that the GeNC bending mode is about 50 cm^{−1}. The r _{ s } distance for the Ge–N and N=C bonds were determined to be 1.817±0.015 Å and 1.144±0.015 Å, respectively. Other structural parameters were estimated which were consistent with the rotational constants of the 10 isotopic species. Infrared spectra of gaseous and solid GeH_{3}NCS and GeD_{3}NCS and Raman spectra of the corresponding liquids and solids were recorded from 30 to 3000 cm^{−1}. The vibrational spectra of the gases are only consistent with a molecule with C _{3v } symmetry. A complete vibrational analysis is presented and a normal coordinate calculation has been carried out.

Exciton transitions in quasi‐one‐dimensional crystals. 9‐Cyanoanthracene
View Description Hide DescriptionThe polarized reflection spectra of 9‐cyanoanthracene crystal at 2 °K taken at near normal angles of incidence are reported for a wavelength range corresponding to the first singlet electronic transition. The dielectric funtion is calculated by means of the Kramers–Kronig transformation and the positions and oscillator strengths of the main exciton transitions are located. The factor group splitting in reflection is nearly 80 cm^{−1}, but in the derived absorptionspectrum ε_{2}(ω) the splitting is much larger, being approximately 200 cm^{−1}. One component of the dielectric function is negative in the region of the 0–0 transition and the calculation of the theoretical attenuated total reflectionspectrum suggests that at 2 °K the (100) crystal surface should support an exciton surface polariton. The structure of the exciton band has been calculated for three orthogonal directions in the first Brillouin zone using the point dipole approximation. All four branches show strong negative dispersion in the k _{ c } direction, implying that the bottom of the exciton band is not a k=0 level. This is one reason why the crystal shows no ’’monomer’’ fluorescence. The qualitative validity of the point dipole model has been established by calculating the nondipolar part of the exciton transfer interaction for near neighbors. It is found that the nondipolar parts are 25% or less of the individual intermolecular exciton transfer interactions.

Tunable diode laser measurements of line strengths in the ν_{4} band of ^{12}CH_{4}
View Description Hide DescriptionAbsolute intensities have been measured, using a diode laser spectrometer, for several vibration–rotation lines in the ν_{4} fundamental of methane. The data have been analyzed, with the inclusion of Coriolis interactions, to obtain a transition moment of (0.0508±0.0015) ×10^{−18} esu cm. This work appears to resolve some earlier difficulties in interpreting measurements and calculations of intensities in ν_{4} of CH_{4}.

Electroneutrality effects on aluminum order in sodium feldspar: A two‐dimensional model
View Description Hide DescriptionA two‐dimensional analog model for the mineral albite is investigated in order to evaluate the effect of local charge neutrality on the aluminum–silicon order–disorder in the crystal. The configurational entropy and order parameter, as functions of the temperature, are calculated within the quasichemical approximation. Model calculations using our definition of local charge balance show a significant change in the temperature dependence of the entropy and order parameter over what would be expected if local charge balance were not rigorously obeyed.

Relations between SCF and Slater’s orbital energies in closed‐shell configurations of isoelectronic atoms and ions
View Description Hide DescriptionSimple relations between SCF and Slater’s orbital energies are established in closed‐shell atomic electronic configurations. Using elementary screening constants taken from the literature, ionization potentials of various isoelectronic ions are evaluated and compared with experimental values.

An ENDOR study of the geometry of the lowest triplet state of benzil
View Description Hide DescriptionWe present the results of our optically detected ENDOR investigation of the lowest triplet state of benzil. The interaction between the total electron spin and nuclear spins placed at strategic positions throughout the molecule was used to probe the molecular geometry. ^{13}C substitution at the carbonyl carbons led to the dihedral angle, while proton A tensors were used to deduce the ring twist. The geometric information resulted from a point–dipole calculation which reproduced the ^{13}C and proton A tensors by optimizing geometry and spin density distribution. A dihedral angle of 157° was found, substantially larger than the ground state value of 111°, though significantly different from a t r a n s‐planar configuration. The ring twist angle was found to be 24°.

Line shapes for attachment of threshold electrons to SF_{6} and CFCl_{3}: Threshold photoelectron (TPSA) studies of Xe, CO, and C_{2}H_{2}
View Description Hide DescriptionUsing the technique of threshold photoelectron spectroscopy by electron attachment (TPSA), the shapes of the threshold electron attachment cross sections for SF_{6} and CFCl_{3} are determined by direct photoionization measurements of the ^{2} P _{1/2} level of Xe^{+}. The observed TPSA line shape is deconvoluted using a Lorentizian attachment profile with full‐width at half‐maximum of 30 meV for SF_{6} and 25 meV for CFCl_{3}. In addition, TPSA spectra of the ground electronic states of CO^{+} and C_{2}H_{2} ^{+} are reported. In CO^{+} the intensity of the threshold spectral features are dominated by autoionization, while for the vibrational features of C_{2}H_{2} ^{+} direct photoionization is more important. The two processes (autoionization and direct ionization) are characterized by different TPSA spectral line profiles.

Laser induced fluorescence spectra of symmetrically substituted trihalobenzene radical cations
View Description Hide DescriptionLaser induced fluorescence excitation and resolved emission spectra are obtained in a Ne matrix for the radicals cations of C_{6}H_{3}F_{3}, C_{6}H_{3}Cl_{3} and all their deuterated analogs. The excitation spectra show that the excited state in each of the ions is characterized by extremely sharp, extremely regular vibrational structure. The majority of the a′_{1} and e′ species vibrational modes are assigned. The resolved emission spectra of the ions, however, show irregular structure for the e′ modes, which is entirely consistent with a Jahn–Teller distortion of the ground state. Gas phase hot band spectra confirm that the Ne matrix does not perturb the vibrational structure. Although spectral assignments are more difficult in the ground state, it is shown that ν^{″} _{13}, the C–C–C bond angle deformation mode, is principally affected by this distortion. Jahn–Teller distortion parameters are derived for the four species: C_{6}H_{3}F_{3} ^{+}, C_{6}D_{3}F_{3} ^{+}, C_{6}H_{3}Cl_{3} ^{+} and C_{6}D_{3}Cl_{3} ^{+}.

Application of a dynamical S matrix method to the three‐dimensional H+H_{2} exchange reaction
View Description Hide DescriptionA quantum dynamical S matrix formalism which enables population distributions to be computed as a function of a reaction coordinate u is described and applied to the three‐dimensional H+H_{2}exchange reaction. Quantum dynamical R matrix methods and programs of Stechel, Walker, and Light have been adapted for this purpose. The method has been applied to examine the suitability of the simple surprisal formula [p _{ j }∝p _{ j } ^{0}exp(λk _{ j })] describing rotational product state population distributions {p _{ j }}. Previous results on the semiempirical Porter–Kaplus (PK) potential energy surface for total angular momentumJ=0 showed that the computed quantum dynamical population distributions can be fitted accurately by the surprisal formula for all values of u. The microcanonical prior distribution function,p ^{0} _{ j }∝k _{ j }(2j+1), for final product states was found to be appropriate, even though the statistical justification of this function fails because of angular momentumconservation. In the present work this surprisal theory study has been extended and a comparison has been made between the population distributions computed using the PK potential and the potential of Truhlar and Horowitz derived from the a b i n i t i o CI study of Siegbahn and Liu (THSL). Although the surprisal formula is accurate for all values of u for the PK potential and for small values of u for the THSL potential, it does not give such a good fit for larger values of u for the THSL potential. This result is linked directly with the fact that for the THSL potential the H_{3}conformation of minimum potential energy is nonlinear for relatively large values of u. These results strongly suggest that a justification for the successes of surprisal theory must result from a detailed examination of the collision dynamics, rather than from statistical considerations. The dynamical S matrix method is also used to explain an approximate reactive flux rule for the H+H_{2}reaction: the sum of unnormalized reaction probabilities into product states with even rotational quantum numbers is approximately equal to the sum of reaction probabilities into odd product states.

Investigations of Soret excited resonance Raman excitation profiles in cytochrome c
View Description Hide DescriptionResonance Raman excitation profiles of the ’’oxidation state marker bands’’ in ferric‐ and ferrocytochrome c are obtained using tunable laser excitation in the Soret region (350–450 nm). The excitation profiles are asymmetric and peak distinctively to the blue of the Soret absorption maximum. The data are explained using a simple model that analytically includes the c o m p l e t esubspace of Franck–Condon active vibrations associated with each intermediate electronic state. In this simple multimode model the damping factor is a ’’soft’’ fitting parameter taken to be wavelength independent. Nevertheless, an excellent fit of the Soret absorption band and Raman excitation profile is achieved. Our theoretical approach places upper limits on the Soret damping factors that are on the order of 50 cm^{−1} (HWHM) for ferrocytochrome c and on the order of 300 cm^{−1} (HWHM) for ferricytochrome c.

Potential energy curves for diatomic zinc and cadmium
View Description Hide DescriptionMolecular electronic structure theory has been applied to the low‐lying electronic states of Zn_{2} and Cd_{2}. Gaussian basis sets of size Zn (13s 9p 5d) and Cd (15s 11p 7d) have been optimized in atomic calculations on the ground ^{1} S and excited ^{3} P electronic states. The general contraction scheme of Raffenetti has been used to reduce these primitive Gaussian bases to size Zn (5s 4p 1d) and Cd (6s 4p 2d) without any degradation in the atomic SCF energies. Following X ^{1}Σ^{+} _{ g }ground state SCF calculations, full configuration interaction was performed for the four valence electrons. The resulting potential energy curves for Zn_{2} and Cd_{2} are, with some notable exceptions, qualitatively similar. In the case of Cd_{2}, we have obtained potential curves which include spin–orbit coupling and have carried out a detailed analysis of the fluorescence intensity from the first 1_{ u } (^{3}Σ^{+} _{ u }) excited state.

Energy dependence of the differential cross sections for simple models of ion‐induced dipole capture collisions
View Description Hide DescriptionAt low relative kinetic energies, the product angular distributionscharacteristic of the short range part of a reactive scattering event may become distorted by a long range attractive force. We examine this behavior for several different simple models of direct, short range encounters, each of which is coupled at long range to an attractive potential term proportional to r ^{−4}. The long range attraction does n o t generally lead to symmetric angular distributions for low‐energy capture collisions. The anisotropy of the differential cross section which is observed at high energies may be preserved at extremely low kinetic energies, or it may be substantially altered toward either more symmetric or less symmetric distributions. It is even possible for a distribution which is forward‐peaked at high energy to become backward‐peaked at low energy, and v i c e v e r s a. These results suggest that caution is warranted when one wishes to interpret a change in the product angular distribution, in going from high kinetic energies to low kinetic energies, as due to a change in the short range reaction mechanism.

Picosecond relaxation processes in excited state quinoxaline
View Description Hide DescriptionExcited stateabsorption spectra of quinoxaline in benzene, isooctane, and c i s‐1,3‐pentadiene at 7 and 130 ps after excitation are reported. Picosecond absorption kinetics at 422 and 533 nm are also reported. In the nonaromatic solvents, bands at ∼425 nm (S _{ n }←S _{1} a n d T _{ n }←T _{1}) and ∼ 530 nm (mostly S _{ n }←S _{1}) are observed in the transient spectra. Decay of the ∼530‐nm band gives k ^{−1} _{isc}?23 ps in the nonaromatic solvents. In benzene solvent, the excited state spectra are more complex, the decay kinetics are longer, and quinoxaline–benzene exciplex formation is suggested.

Laser magnetic resonance spectrum of the ν_{2} band of PH_{2}
View Description Hide DescriptionThe ν_{2} bending fundamental band (ν_{0}=1101.908 cm^{−1}) of PH_{2} in the ? ^{2} B _{1}ground electronic state has been studied using CO_{2} laser magnetic resonance in the 9.0 to 9.6 μm region. The short‐lived PH_{2} radicals were produced in a flow system by the reaction of H atoms with PH_{3} molecules. A total of 25 molecular parameters (3 rotational, 5 distortion, and 4 spin‐rotation constants in each of the (000) and (010) vibrational states, plus the ν_{2} band origin) have been determined from a least‐squares fit of 122 observed resonances. The (000) state parameters are more accurate than, and in satisfactory agreement with, previous spectroscopic determinations, and the changes in the parameters upon excitation of the ν_{2} bending mode are discussed.

A generalized phase shift approach to the semiclassical S matrix
View Description Hide DescriptionThe relation between the generalized phase shift and semiclassical S matrix formulations of molecular scatteringtheory is discussed. It is shown that the classical limit of the generalized phase shift is the modified action. The set of coupled first order or ’’trajectory’’ equations provides an exact quantum formulation of scattering theory which leads smoothly to the semiclassical formulation.

Bounded error analysis of experimental distributions of relaxation times
View Description Hide DescriptionA method is presented for analyzing experimental data from dielectric relaxation, acoustic relaxation, or other measurements related to a distribution of relaxation times. A consistency test is derived showing whether or not there is a n y distribution of relaxation times consistent with the data. For consistent data, the analysis then provides bounds on possible interpolations at frequencies between the measured points. Bounds are placed on the cumulative distribution of relaxation times and these bounds are shown to be the most precise attainable on the basis of any given set of experimental data. The analysis is illustrated by application to ’’data’’ generated theoretically from some simple distributions of relaxation times and to some experimental data on dielectric relaxation.

Ratio analysis of electron spin echo modulation envelopes in disordered matrices and application to the structure of solvated electrons in 2‐methyltetrahydrofuran glass
View Description Hide DescriptionA new analysis for two pulse and three pulse electron spin echo modulation data is presented which is independent of the echo decay function. The experimental ratios of maxima to minima of the echo modulation curve are compared with simulated ones to obtain the number of interacting nuclei, their distances, and their isotropic coupling by a least squares fit. From the best simulation, the empirical echo decay function can be determined and the original experimental data can be simulated. This new method is applied to new three pulse and old two pulse [L. Kevan e t a l., J. Chem. Phys. 63, 409 (1975)] electron spin echo data on solvated electrons in 2‐methyltetrahydrofuran (MTHF) glass at 77 K. This analysis together with previous second moment data gives a revised model for electron solvation in this matrix based on a statistical orientation of the MTHF ring carbon planes toward the electron. In this statistical configuration model three equivalent MTHF molecules form the first solvation shell with their ring carbon atom planes perpendicular to the electron and oriented statistically. The closest protons are 3.4 Å from the electron with a protonhyperfine constant of 0.4 MHz which is equivalent to a deuteron hyperfine constant of 0.06 MHz.

Low frequency vibrational spectra and internal rotation of 2‐chlorobuta‐1,3‐diene, propenoyl fluoride, and propenoyl chloride
View Description Hide DescriptionThe infrared and Raman spectra of gaseous 2‐chlorobuta‐1,3‐diene, propenoyl fluoride, and propenoyl chloride have been recorded below 300 cm^{−1}. It was observed that the low frequency spectra of these three molecules were quite similar. The infrared spectrum of 2‐chlorobuta‐1,3‐diene exhibited a series of sharp torsional transitions whereas in the Raman spectrum a B‐type band was observed. These torsional frequencies have been assigned to the s‐t r a n s conformer and no evidence for a higher energy conformer was observed. In the infrared and Raman spectra of the propenoyl halides, bands assigned to the torsional series of both conformers have been observed. The analyses of these spectra were simplified by comparison with that of the 2‐chlorobuta‐1,3‐diene. Asymmetric potential functions and values for ΔH for propenoyl fluoride and propenoyl chloride were calculated. The enthalpy differences of 36 and 215 cm^{−1} between the lowest energy levels of the s‐t r a n s and s‐c i s wells for the propenoyl fluoride and chloride, respectively, were obtained.

Barrier to internal rotation in ethane from the microwave spectrum of CH_{3}CHD_{2}
View Description Hide DescriptionRotational spectra of CH_{3}CHD_{2} were observed by using a source‐modulation microwave spectrometer. Of 26 observed transitions, 16 showed splittings due to internal rotation. An analysis of the observed spectra, which utilized an internal axis method modified to include centrifugal distortion effects, gave the following results: A=60 809.592 (59) MHz, B=17 770.87 (16) MHz, C=17 084.84 (16) MHz, F=257.70 (35) GHz, and V _{3}=2.8783 (28) kcal/mole, with 2.5 times standard errors in parentheses. In the analysisV _{6}=0 was assumed, and D, a constant associated with a cross product of inertia, was fixed to a calculated value. When the uncertainty in D is taken into account, the error of V _{3} increases to 0.0029 kcal/mole. Effects of the molecular vibrations on the barrier are discussed briefly.

Infinite order sudden approximation for reactive scattering. I. Basic l‐labeled formulation
View Description Hide DescriptionAn infinite order sudden (IOS) treatment of reactive scattering is developed taking into account recent results of nonreactive collision studies on the importance of l‐labeling, nonconservation of helicity, and transformation properties of sudden approximation wave functions. The present IOS method should be sufficiently simple to apply to a number of chemically interesting atom–diatom reactions. Such applications are currently in progress.