Volume 70, Issue 4, 15 February 1979
Index of content:

Infrared spectra of liquid and solid DT and T_{2} a)
View Description Hide DescriptionThe fundamental vibration–rotation spectra of condensed DT and T_{2} were measured. The 24 separate spectral lines identified consist mainly of Q _{ Q }, Q _{ R }, Q _{ Q }+S _{ o }(0), and Q _{ Q }+S _{ o }(1) transitions. The last two appear as triad sets in D_{2}–DT–T_{2} solutions because of double transitions between different isotopes. The Q _{ Q } frequencies are 2458 cm^{−1} for T_{2} and 2736 cm^{−1} for DT. Some approximate kinetic half‐times have been measured. In the 21 K liquid, these include: 18 h for the reaction of D_{2}+T_{2}→2DT and 25 h for the T_{2} rotational J=1 to J=0 conversion. In solid T_{2} at 20 K, the J=1 to J=0 conversion time is also about one day, but it is only 1 h at 4 K. Catalysis by atomic tritium is considered to be the cause of the fast 4 K‐conversion time.

Non‐Franck–Condon transitions in resonant autoionization of N_{2}O
View Description Hide DescriptionAutoionization of N_{2}O between 12.89 and 16.4 eV was investigated by photoionization using the pulsed synchrotron radiation from ACO, Orsay’s storage ring. Measurements were performed of threshold photoelectron spectra,photoionizationspectra, and of photoelectron energy spectra. The latter were obtained from photoelectron time of flight distributions at selected wavelengths. The results suggest that autoionization in the Franck–Condon gap between the ? ^{2}Π and the ? ^{2}Σ^{+} states of N_{2}O^{+} proceeds via two distinct mechanisms. The major autoionization process (?90%) produces the ? ^{2}Π state in its low vibrational levels, while a resonantautoionization path (?10%) produces vibrationally excited ? ^{2}Π ions. The latter process is associated with the production of low energy electrons with a distribution peaking sharply at zero energy. This resonantautoionization process appears to be a general phenomena for polyatomic molecules.

Criterion of minimum state density in the transition state theory of bimolecular reactions
View Description Hide DescriptionWe discuss two minimum‐density‐of‐states criteria for the location of generalized transition states for chemical reactions. One is due to Bunker and Pattengill; the other is due to Wong and Marcus. We prove that both provide upper bounds on the exact classical equilibrium rate constant. In addition, we show that for several‐dimensional systems both methods are exact at threshold, and in the limit of an infinite number of dimensions they agree with the variational theory of reactions of Wigner, Horiuti, and Keck. However, it is also shown that for a finite number of degrees of freedom both methods yield rate constants which are only as accurate as or less accurate than rate constants given by the variational theory of reactions. We note that, where tested by others for actual systems, the differences of the results obtained with the variational and Bunker–Pattengill criteria have been minor.

Application of the RISM theory to Lennard‐Jones interaction site molecular fluids
View Description Hide DescriptionIt seems that reference interaction site model (RISM) theory atom–atom distribution functions have been obtained directly from the RISM equations only for fused hard sphere molecular fluids. RISM distribution functions for Lennard‐Jones interaction site fluids are presented. Results presented suggest that these distribution functions are as accurate as RISM distribution functions for fused hard sphere molecular fluids.

Classical theory of light scattering by an adsorbed molecule. I. Theory
View Description Hide DescriptionWe develop a classical theory for the intensity and the depolarization ratio of the light scattered (Raman or Rayleigh) by an absorbed molecule. It is assumed that the optical properties of the system can be described by the polarizability of the molecule and the dielectric constant of the two media. The presence of the surface modifies the field incident upon the molecule as well as the field emitted by the induced dipole. We compute these effects, exactly, by using a dyadic Green’s function method, and approximately, by using a perfect mirror model. The theory provides the angular distribution of the scattered radiation, and its polarization as a function of the polarization, the frequency and the direction of incidence of the incoming radiation, as well as of the dielectric properties of the metal and the position of the molecule with respect to the surface. We use these equations to analyze the possible sources for the experimentally observed enhancement of the scattering caused by the presence of the metallic surface.

Organic metals. Molecular coordination along the stacking direction of a conducting organic compound: TTF–TCNQ (tetrathiafulvalene–tetracyanoquinodimethane)
View Description Hide DescriptionCNDO/2 molecular orbital calculations have been performed on ionized TCNQ and TTF dimers for different values of relative displacement of the monomers. The intermolecular separation is held fixed. The total energy of the TCNQ dimer goes through two minima, one when the monomers are exactly over each other (eclipsed geometry) and one for which the monomers are slipped with respect to each other (slipped geometry) as observed in TTF–TCNQ. The total energy of the TTF dimer exhibits a minimum for the eclipsed geometry only. The qualitative agreement with previous extended Huckel results highlights the close screening of the core–core interaction by the electrons. The implications of the different geometries with respect to electronic banding of the affinity and ionization levels in segregated stack structures is discussed.

Laser induced fluorescence spectrum of matrix isolated CS^{+} _{2}
View Description Hide DescriptionThe laser induced fluorescencespectrum of the CS_{2} cation has been obtained in solid Ne and Ar matrices. The transitions observed originate in the X ^{2}Π state and terminate in the A ^{2}Π state. In the Ne matrix, sharp lines with well resolved vibrational structure are observed. There is only a very small difference between gas‐phase and Ne matrix values for the electronic origin and vibrational frequencies, although the latter analysis can be extended somewhat in the matrix. In the Ar matrix only a poorly resolved and strongly shifted CS^{+} _{2}spectrum is observed. A qualitative explanation for this different behavior is advanced.

A theoretical study of the predissociation of the c ^{4}Σ^{−} _{ u } state of O^{+} _{2}
View Description Hide DescriptionExtensive configuration‐interaction calculations have been carried out to determine the potential curve for the c ^{4}Σ^{−} _{ u } state of O^{+} _{2}. The results show that this state is quasibound with a low potential barrier that could accomodate only few vibrational levels. The lifetimes with respect to the tunneling through the barrier are found to be too short, but the radiative lifetime of 2×10^{−8} sec for the v=O level is shown to be reliable. Based on these results with available experimental data, a semiempirical curve has been constructed and the mean lifetimes have been determined to be 10^{−8} and 5×10^{−11} sec for the levels v=O and 1, respectively. This result clearly explains absence of the v=1 level in emission, and leads to a conclusion that the tunneling should be the dominant mechanism for the predissociation of the c ^{4}Σ^{−} _{ u } state of O^{+} _{2}.

Infrared measurements of sulfur dioxide thermal decomposition rate in shock waves
View Description Hide DescriptionThe rate of thermal decomposition of sulfur dioxide dilute in argon has been measured in a shock tube, using IR emission from the ν_{3} band to monitor SO_{2}. These results yielded a rate coefficient k=8(±2) ×10^{15} exp(−108 000/R T) cm^{3} mole^{−1} sec^{−1} for the reaction SO_{2}+M→SO+O+M in the temperature range 2800 to 3880°K and the total concentration range 8×10^{−6} to 3.4×10^{−5} mole/cc. The measured rate coefficient agrees well with the results of Just and Kiefer, determined respectively from O‐atom formation rate observations and densitometric measurements, and the combined data yield a common rate coefficient between 2500 and 5200°K.

Second harmonic generation in collagen
View Description Hide DescriptionOrdered collagen in native, intact rat tail tendon has been studied by means of optical second harmonic generation(SHG). The two independent hyperpolarizability tensor elements of the collagen molecule, β_{3} and β_{1}, have been found to be of opposite sign and roughly equal in absolute magnitude. Evidence is presented for a systematic variation in the ratio β_{3}/β_{1} with age which may reflect the effects of progressive cross linking. Our experimental findings are in reasonably good agreement with a simple theory constructed on the basis of current models of the t e n d o n s t r u c t u r e, and thereby confirm this structure in the native, intact tissue. The many advantages of SHG for the structural study of intact partially ordered biological tissues are outlined.

A statistical model for rotationally and vibrationally inelastic collisions
View Description Hide DescriptionA statistical model is described which is able to treat the collisional excitation of internal degrees of freedom in collisions of large molecules in a simple way. In contrast to most previous statistical theories, this theory is able to treat direct collisions as well as collisions involving a long lived complex. A wide variety of collisional interaction strengths can be handled by the model. This is accomplished by the use of a coupling strength parameter, which limits the energy in any internal degree of freedom. This energy limit is the only adjustable parameter in the model and, when held small, allows a good description of glancing collisions and threshold effects, which are not normally considered to be amenable to statistical treatment. The results of this model are compared both with an exact theoretical treatment of the direct collinear atom–diatom collision and also with experimental ion beam data on average energy transfer in direct collisions with polyatomic molecules. In both cases good agreement is found. A simple semiempirical formula is obtained which can be used to predict the energy and angular dependence of the average relative energy transfer under conditions encountered in hot atom and in radiation chemistry and plasmas.

Time of flight measurements of the vibrational excitation of CH_{4} by collision with 3−6 eV Li^{+} ions
View Description Hide DescriptionAngular distributions and time of flightspectra for the scattering of Li^{+} ions from ground state CH_{4} have been measured over a range of center of mass scattering angles (7° to 19°) and energies (2.7 to 6.5 eV). The spectra reveal transitions to groups of CH_{4} vibrational states which could be clearly resolved. As in previous studies of CO_{2} and N_{2}O, very little rotational excitation is observed. The vibrational transition probabilities show an extremely strong dependence on the scattering angle, not seen in similar experiments using simpler target molecules. Certain state to state differential cross sections as well as the average energy transfer and the second and third central moments of the energy transfer are compared for Li^{+} + CH_{4}, CO_{2}, and CO. The different behavior of the average energy trnasfer for these targets is well explained by a statistical model recently developed by two of the authors.

Rate constant for the reaction HO_{2}+NO→OH+NO_{2}
View Description Hide DescriptionThe rate constant for the reaction HO_{2}+NO→OH+NO_{2} has been determined over the temperature range 270 to 425 K in a discharge flow/mass spectrometer/resonance fluorescence apparatus. Results, expressed in the Arrhenius form k _{1}= (5.7^{+5.6} _{−4.0}) ×10^{−12} exp[(130±270)/T] cm^{3} sec^{−1}, are compared with previous measurements.

ESR of phosphite radicals trapped in x‐irradiated single crystals of o‐phosphorylethanolamine
View Description Hide DescriptionTwo different species of phosphite radicals are trapped at 300 K in x‐irradiated single crystals of o‐phosphorylethanolamine, H_{3}N^{+}CH_{2}CH_{2}OPO_{3}H^{−}. One radical, resulting from cleavage of the P–OH bond, is characterized by eigenvalues of 2173±16 MHz, 1733±19 MHz, and 1713±19 MHz, and 2.0065±0.0004, 2.0045±0.0004, and 2.0038±0.0004, for the hyperfine coupling tensor and gtensor, respectively. The other radical, produced by cleavage of the P–OCH_{2}CH_{2}NH^{+} _{3}bond, has corresponding eigenvalues of 2131±15 MHz, 1710±15 MHz, and 1689±13 MHz, and 2.0054±0.0004, 2.0052±0.0004, and 2.0044±0.0004. Both radical species are present in approximately equal concentrations in the crystal, suggesting that both the P–OH and P–OCH_{2}CH_{2}NH^{+} _{3}bonds are equally susceptable to electron dissociative capture, the assumed mechanism for radical formation.

Extended London theory of diamagnetism. I. General theory and saturated hydrocarbons
View Description Hide DescriptionIn this paper a method is proposed to calculate diamagneticsusceptibilities of organic compounds. This method uses the extended Hückel formalism generalized to include the perturbation due to a magnetic fieldH. Parameters are varied to obtain agreement with experimental results. The diamagnetic properties of the alkanes are examined and all the terms contributing to the susceptibility are analyzed. The importance of a coupling term between matrix elements of first order in H and London terms is shown. Some results are discussed, particular to methane and the cycloalkanes.

The far‐ultraviolet spectra of CH_{3}SCH_{2}SCH_{3} and CH_{3}SCH_{2}COCH_{3}
View Description Hide DescriptionThe far ultraviolet spectra of CH_{3}SCH_{2}SCH_{3} and of CH_{3}SCH_{2}COCH_{3} have been measured in the vapor state. The spectra are compared to those of related thioethers and ketones, and are interpreted in terms of valence‐shell and Rydberg transitions. In the 41 000–46 000 cm^{−1} part of the spectrum the σ*←? (lone pair) and 4s←? transitions give clearly separate bands for both compounds. In the 47 000–55 000 cm^{−1} part of the spectrum of CH_{3}SCH_{2}SCH_{3} new bands appear which might be related to a split in the 4p levels.

SCF treatment of charge polarization effects in intermediate‐energy electron scattering calculations with applications to N_{2}
View Description Hide DescriptionWe report converged rotational close coupling calculations of the differential, integral, and momentum‐transfer cross sections for seven model potentials for electron–N_{2}scattering at an impact energy of 30 eV. The model potentials involve a static potential calculated by the INDO/1s or INDOXI/1s method, an exchange potential calculated by the semiclassical exchange approximation from the INDO/1s or INDOXI/1s unperturbed electronic density, and a polarization potential. The polarization potentials used include the Buckley–Burke semiempirical one and various modifications of the INDOXI and INDO self‐consistent‐field adiabatic polarization potentials. We are able, without adjusting parameters, to obtain good agreement with the angle dependence of the experimentally measured sum of the elastic and rotational excitation differential cross sections although the absolute value of our calculated cross sections is about 20%–30% larger than the measured values in the best case, perhaps indicating that the model potentials are too strong or should have a nonzero imaginary part. We discuss the sensitivity of the computed results to details of both the static and polarization parts of the model potentials, and we present some predictions of the rotationally resolved state‐to‐state cross sections.

Simulation of chain molecules in solutions at finite concentrations under several solvent conditions
View Description Hide DescriptionComputer experiments simulating chain molecule solutions on a tetrahedral lattice have been performed at various concentrations and under several solvent conditions. In good and athermal solvents, mean squares of end to end distance 〈R ^{2}〉/b ^{2} and of radius of gyration 〈S ^{2}〉/b ^{2} decrease monotonically with the increase in the concentration of chain molecules. In poorer solvents, the monotonic decrease was sometimes not found and in the solutions under very poor solvent conditions, they seemed to have a slight positive dependence on the concentration. The principal moments of inertia of chain molecules were also computed in this connection. The element density distributions around the mass center of a chain were also evaluated and discussed. The numbers of chain element to chain element and chain element to solvent contact pairs, which are closely related to the heat of mixing, were evaluated and compared with the prediction of the quasichemical theory. The agreement was good in good or athermal solvent solutions at moderate concentrations. The disagreement, however, was found in dilute solutions and in poorer solvent solutions.

Measurements of the branching ratios for the reaction of O(^{1} D _{2}) with N_{2}O
View Description Hide DescriptionThe relative rate constants for the reaction channels were determined using chemical‐ionization mass spectrometry to measure the products of Reaction (1). O(^{1} D _{2}) atoms were produced by the photodissociation of N_{2}O at wavelengths greater than 1950 Å. The measurements were carried out in mixtures of N_{2}O with helium added to remove the excess kinetic energy carried by the O(^{1} D _{2}). We find the ratio k _{1a }/k _{1b } is sensitive to the presence of helium increasing from a value of 0.68 in pure N_{2}O to a value of 0.80 with [He]/[N_{2}O]?10. In addition, at 300 K we find that (1a) and (1b) consititute the only significant channels for the reaction with (k _{1c }+k _{1d }+k _{1e })/(k _{1a }+k _{1b } ) ?0.038. The ratio k _{1a }/k _{1b } is found to decrease slowly with increasing temperature over the range 170–434 K. These data can be fitted by the expression k _{1a }/k _{1b }= (0.72±0.11) +(21.6±7.0)/T.

Temperature dependence of HF(v _{1}=1)+HF(v _{2}=0) vibrational relaxation
View Description Hide DescriptionA kinetics model of infrared laser‐induced fluorescenceexperiments has been used to simulate the experimental quenching rate coefficients reported between 300 and 4000 K for the vibrational relaxation of HF(v _{1}=1) by HF. This rotational nonequilibrium model is based on the predicted energy‐transfer mechanisms in hydrogen fluoride systems reported in a trajectory study by Wilkins. This model includes v→R, R→v, R→ (R′, T′), and (R′, T′) →R energy‐transfer processes. A key process is vibrational‐to‐rotational intramolecular energy transfer in which HF(v _{1}=1,J_{1}) terminates on high J′ _{1} states ofv′_{1}=0. The calculated temperature‐ dependent quenching rate coefficient for self relaxation of HF(v _{1}=1) at temperatures between 300 and 2000 K is dependent on v→R andR→v energy‐transfer processes, and beyond 2000 K only on v→R processes. The temperature dependence observed for HF(v _{1}=1) vibrational relaxation by HF(v _{2}=0) is explained by this model. For the high roational states in the v′ _{1}=0 manifold, this model predicts incomplete rotational therma lization at high temperatures. No mechanisms involving dimerization appear to be necessary in understanding the inverse temperature dependence of the re ported quenching rate coefficients.