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Volume 66, Issue 10, 15 May 1977

Light scattering studies of rotational and vibrational relaxations of acetonitrile in carbon tetrachloride
View Description Hide DescriptionThe rotational and vibrational relaxation times of acetonitrile–carbon tetrachloride solutions were investigated as a function of concentration, viscosity, and temperature using depolarized Rayleigh and Raman scattering. Using a Fabry‐Perot interferometer and single frequency laser source, we have shown that reliable results for the single particle orientational correlation times (τ_{s}) for CH_{3}CN can be obtained by carrying out a concentration dependent depolarized Rayleigh scattering study. Raman scattering was shown to yield inconsistent results for τ_{s} in CH_{3}CN. At constant viscosity, it was found that the Rayleigh scatteringrelaxation time (τ_{Ray}) of CH_{3}CN in CCl_{4} does not change with CH_{3}CN concentration, indicating that the orientational pair correlation factor of liquid CH_{3}CN is close to unity. This result suggests that the dynamic pair correlation in CH_{3}CN is just as important as the static pair correlation. The experimental data were also compared with the predictions of the hydrodynamic stick and slip models for a rotational diffusion. The CH_{3}CN data were found to be close to the prediction of the slip model. The isotropic relaxation time (τ_{iso}) of the C≡N stretching mode was also studied as a function of concentration and viscosity using Raman spectroscopy. This viscosity dependence of τ_{iso} also decreases with decreasing number density of CH_{3}CN, suggesting that pair correlations are also important in the Raman scattering of CH_{3}CN.

On the glass transition in mixtures between the normal alcohols and various Lewis bases
View Description Hide DescriptionThe glass transition temperatures (T _{ g }) of binary mixtures of the lower normal alkanols with several aprotic Lewis bases have been measured as a function of composition. The initial molar slope (IMS) at the pure alcohol end of the curve of T _{ g } against mole fraction of base was accurately determined for each base. A related quantity, the reduced IMS, is shown to be proportional to the excess configurational entropy per bead of base introduced into dilute solution with the base. The reduced IMS is found to be a linear function of the length of the alcohol backbone for the heavier alkanols and to be an increasing function of the base T _{ g }. The results are interpreted in terms of alcohol and base self‐association. Some form of network association is suggested for the lighter alkanols in the supercooled liquid, while the bases most likely associate under the action of dipole–dipole or other specific nonhydrogen bonding forces. The reduced IMS values suggest association between alcohol and base, but this association usually does not seem to be by means of hydrogen bonding in solutions dilute in the base. The T _{ g } values of concentrated base solutions in the alcohols are discussed as well.

Vibrational spectra and intermolecular forces of mercuric halide complexes of 1,4‐dioxane and tetrahydrofuran
View Description Hide DescriptionInfrared and Raman spectra of mercuric chloride complexes of 1,4‐dioxane and 1,4‐dioxane‐d _{8} and mercuric bromide complexes of tetrahydrofuran and tetrahydrofuran‐d _{8} were measured in a solid state at various temperatures. Optically active Raman and far‐infrared bands due to the lattice vibrations and to the stretching and bending modes of the mercuric halide molecules were detected. On the basis of the observed frequency data the intra‐ and intermolecular force constants were evaluated by the least‐squares fitting of the normal frequencies. A strong interacting force was estimated for the nearest Hg⋅⋅⋅O pairs compared with the ordinary van der Waals‐type intermolecular interactions. The frequencies of the Hg–X (X is Cl or Br) stretching vibrations in the complexes are lower than those of the corresponding crystalline mercuric halides, and they increase with rising temperature. These facts suggest a coordination character of the nearest Hg⋅⋅⋅O bonding in the complexes.

Electronic relaxation of acridine as studied by picosecond spectroscopy
View Description Hide DescriptionInternal conversion (IC) and intersystem crossing (ISC) rates of the lowest excited singlet state (S _{1}) of acridine have been measured in n‐hexane at room temperature by picosecond spectroscopy. The rates deduced from the buildup of triplet–triplet absorption and the quantum yield of intersystem crossing were found to be 3×10^{10} sec^{−1} for the internal conversion and 3×10^{10} sec^{−1} for the intersystem crossing. Corresponding rates for perdeuterated acridine are found to be similar. These rates, when combined with the upper limit of the unobserved fluorescence, lead to the nπ* assignment of the lowest excited singlet state. The kinetics of repopulation of the ground state following a pulsed excitation of the sample give strong indication of the occurrence of S _{1}→S _{0}internal conversion.

Electron transfer at sensitized TiO_{2} electrodes
View Description Hide DescriptionElectron transfer from the excited state of tetra‐iodo, tetra‐chloro fluorescein (rose bengal) to the conduction band of TiO_{2} has been studied through photoelectrochemical techniques. The measured transfer rate was correlated with information from absorbance and adsorption measurements of this dye molecule on the (001) surface of the single crystals used as electrodes. The quantum efficiency for the photoinjection of electrons was determined to be 4.0×10^{−3} independent of the pH of the electrolyte and the dye surface concentration. With these data, an argument is given supporting the existence of excitation transfer among the dye molecules on the surface. A temperature study of the electron transfer efficiency yielded an activation enthalpy of 3.2±0.3 kcal/mole with a pre‐exponential factor of 1.0. A heat of adsorption of 7.6±0.5 kcal/mole for this dye on TiO_{2}single crystals was derived from analysis of adsorption isotherms measured at 21.5 and 36.5°C. In addition to the photo‐oxidation process, a photoreduction was also observed upon cathodic polarization of the semiconductor. Action spectra revealed participation of solution dye molecules in this reaction; the reduction rate is dependent upon oxygen concentration in solution. It was concluded that the oxidized dye in solution accepts an electron from the TiO_{2} to generate this current. This photoreduction was found with triphenylmethane and thiazine dyes as well as with other fluorescein derivatives.

Application of complementary variational principles to the Kirkwood equation for hard spheres at very high densities
View Description Hide DescriptionThe theory of complementary variational principles for a nonlinear integral equation with symmetric positive‐definite kernel is applied to the Kirkwood equation (under the superposition approximation) for g ^{(2)}(x) for a system of hard spheres at very high densities. At all λ values beyond the lower bound on the limit of stability of the dense fluid regime, variational analysis of the approximate analytical g ^{(2)}(x) functions suggests infinite periodicity in g ^{(2)}(x). This behavior is in agreement with an earlier study of high density solutions to the Kirkwood equation for hard spheres, where the solutions were obtained by iterative techniques.

Theoretical study of molecular dipole moment functions. I. The X ^{1}Σ^{+} state of CO
View Description Hide DescriptionApproximate dipole momentfunctions for the X ^{1}Σ^{+} state of CO have been calculated using several configuration interactionwavefunctions. The best theoretical results obtained for the dipole derivatives at R _{ e } and vibrational transition matrix elements are found to be in quantitative agreement with available experimental data. The theoreticaldipole momentfunctions, in contrast to empirical functions, possess correct long range behavior. Through a detailed comparison of different approximate wavefunctions, useful information has been obtained on the relative importance of different types of correlation effects on the accurate determination of molecular dipole momentfunctions.

Infrared laser‐induced photochemistry of cyclopropane
View Description Hide DescriptionThe ir laser photolysis of cyclopropane (CP), with the focused gigawatt power of the 9.552 μm(P20) line of a CO_{2} laser, results in two chemical processes. One, a non‐Boltzmann high energy decomposition, gives rise to the major products acetylene, propylene, methane, and ethylene and the minor product methylacetylene. The other is a typical flamereaction characterized by the luminescence of the C_{2} ^{+}(d ^{3}Π_{ g }) Swan band. The latter process plays a minor role in terms of the precent consumption of CP. Both temporal and wavelength resolved spectroscopy were used in the identification of the emitting species as well as in the characterization of the elementary reactions producing them. In mixed CP–NO photolysis, CN^{+}(B ^{2}Σ), C_{2} ^{+}(d ^{3}Π_{ g }), CH^{+}(A ^{2}Δ), and NH^{+}(A ^{3}Π) were observed emitters, whereas only C_{2} ^{+}(d ^{3}Π_{ g }) and CH(A ^{2}Δ, B ^{2}Σ^{−}, and C ^{2}Σ^{+}) emissions were observed in the CP–O_{2}photolysis.

Negative ion properties of tetracyanoquinodimethan: Electron affinity and compound states
View Description Hide DescriptionBound and excited negative ion states of gaseous 7,7,8,8‐tetracyanoquinodimethan (TCNQ) are studied from experiments involving collisions of electrons and fast cesium beams with TCNQ. The electron affinity of TCNQ is measured to be 2.8^{+0.05} _{−0.3} eV by the collisional ionization technique. TCNQ attaches electrons with energies of ∼0, 0.7, and 1.3 eV (values represent peaks in cross section) to form long‐lived compound negative ions which are metastable with respect to autodetachment. The lifetime for the decay of TCNQ^{−}* decreases from ∼2×10^{−3} sec at ∼0 eV to ∼10^{−4} sec at ∼4 eV. Evidence for a third compound negative ion state is seen as a peak in the TCNQ^{−} signal at ∼3.2 eV. This state is metastable with respect to autodetachment a n ddissociation into the products C_{11}N_{3}H^{−} _{3}+HCN^{0}.

Ground state relaxation measurements by laser‐induced depopulation
View Description Hide DescriptionWe have directly measured the repopulation rate for an individual rotational level in the electronic ground state of a homonuclear molecule. A pulsed dye laser was used to depopulate (v ^{″}, J ^{″}) level of Na_{2} by pumping a transition of the form (v′, J′) ← (v ^{″}, J ^{″}) in the B ^{1}Π_{ u }‐X ^{1}Σ^{+} _{ g } blue–green band system. The (v ^{″}, J ^{″}) state was repopulated by collisions with other sodium species and with an argon buffer gas. The transmission of a second, delayed, pulsed dye laser was monitored, serving as a probe of the (v ^{″}, J ^{″}) state’s population at various delay times. The argon pressure was varied and the procedure repeated. The Na_{2}–argon collision cross section may also be determined by this method.

Matrix isolation spectroscopic study of the free radical HCCN
View Description Hide DescriptionThe free radical HCCN was produced in an Ar matrix via photolysis of its precursor diazoacetonitrile, NNCHCN, with radiation of wavelengths longer than 350 nm. The ir spectra of HCCN and three isotopic modifications (DCCN, HC^{13}CN, and HCC^{15}N) were obtained, and normal coordinate analyses based on two different geometries, bent and linear, were carried out. The results provide a determination of the general valence force field, and favor the linear, allenic configuration H–Ċ=C=Ṅ. The electronic spectrum of HCCN in an argon matrix was also obtained. A tentative assignment of the band system observed in the uv part of the spectrum has been made.

Intermolecular vibrations of a crystalline molecular complex
View Description Hide DescriptionThe nature of intermolecular vibrations in a crystalline molecular complex is examined using a simple model of a crystal with one molecule of a 1:1 complex per unit cell. It is shown that two limiting descriptions can result depending on the coupling strength between molecular components in the complex: (i) The g i a n t m o l e c u l e l i m i t describes the case of a strong coupling where the intermolecular motions are the motions of the complex as one entity. (ii) The s u b l a t t i c e l i m i t is applicable in the weak coupling case. Here, the motion can be characterized as an intermolecular motion belonging to the sublattice of the individual components. The phonon bands of naphthalene:octafluoronaphthalene crystals are examined within the framework of the simple model. The results predict phonons to be close to the sublattice limit. Our experimental study of Raman phononspectra using isotopic mixed crystals of the complex is in agreement with the theoretical predictions and show that the low frequency phonons belong predominantly to the C_{10}F_{8} sublattice, while the high frequency phonons characterize the C_{10}H_{8} sublattice motions.

Beam maser double resonance measurements on HDO
View Description Hide DescriptionHigh resolution measurements of hyperfine structure on the 2_{20}–2_{21} transition of HDO were made with a two‐cavity beam maser using the Ramsey method and using a new double resonance technique. Resonances for magnetic dipole transitions between hyperfine sublevels of the rotational states were detected by observing intensity changes in the two‐cavity interference pattern for microwave rotational transitions. The linewidth (fwhm) for the magnetic dipole transitions was 1 kHz. This linewidth allows better effective resolution than the two‐cavity Ramsey resonance when nozzle beams are used. Deuterium quadrupole coupling and spin–rotation interaction strengths were obtained. This double resonance technique should be useful for other small molecules, particularly for those cases where the data cannot be obtained from the microwave transitions.

A theoretical investigation of symmetry‐forbidden transitions in magnetic circular dichroism and electronic spectra of benzene
View Description Hide DescriptionIn this paper, the vibronic coupling calculation in the π‐electron framework has been carried out for 〈B _{2u }‖?′_{ v c }‖E _{1u }〉, 〈A _{1g }‖?′_{ v c }‖B _{2u }〉, and 〈B _{1u }‖?′_{ v c }‖E _{1u }〉 of benzene by using the so‐called linear displacement method. Our method differs from that of Liehr’s by the fact that in our calculation all multicenter integrals have been evaluated exactly. The vibronic coupling matrix elements obtained have then been applied to calculate the magnetic rotational strengths and dipole strengths of symmetry‐forbidden transitions^{1} A _{1g }→^{1} B _{1u } and ^{1} A _{1g }→^{1} B _{2u } and the electronic matrix elements involved in radiationless transitions. It is well known that the ^{1} A _{1g }→^{1} B _{2u }transition is symmetry‐forbidden in MCD and uv spectra and becomes allowed through the mixing of both B _{1u } and B _{2u } states with the E _{1u } state through the E _{2g } modes of vibration. We have developed a method for determining the dipole strengths and magnetic rotational strengths associated with each individual E _{2g } promoting mode from the experimental MCD and uv spectra. Theoretical and experimental results are compared.

Binding energies of Li_{2}H and Li_{2}H^{+} and the ionization potential of Li_{2}H
View Description Hide DescriptionThe existence of the stable molecule Li_{2}H was proved experimentally for the first time by mass spectrometric measurements over dilute solutions of hydrogen in liquid lithium. From measurements of the gaseous equilibria Li_{2}H(g)+Li(g) ?LiH(g)+Li_{2}(g) and Li_{2}H(g)+Li_{2}(g) ?LiH(g) +Li_{3}(g), a dissociation energyD ^{0} _{0}(Li_{2}H) =89.7±5.0 kcal/mole was obtained. The ionization potential of Li_{2}H was determined as I.P.(Li_{2}H) =4.5±0.2 eV, from which the binding energy D ^{0} _{0}(Li_{2}H^{+}) =299.0±6.0 kcal/mole results.

Experimental evidence on the source of negative peaks in coherent anti‐Stokes Raman spectra
View Description Hide DescriptionUnder certain conditions, when the laser pump frequency approaches an electronic transition frequency, negative peaks have been observed in coherent anti‐Stokes Raman spectra. Two conflicting explanations of this phenomenon have been offered. We have noted that these two explanations lead to a different dependence of the intensity of the negative peaks on the power in the pump beam. We have experimentally determined this power dependence in the case of a 5×10^{−3} M solution of N, N‐diethyl‐p‐nitrosoaniline in benzene and found that the intensity of the negative peaks varies with the square of the power in the pump beam. This result is consistent with the explanation that the negative peaks arise from a cross term between the background susceptibility and the imaginary part of the Raman susceptibility and not from the inverse Raman effect.

Correlation functions of classical fluids. V. The perturbation theory for soft spheres
View Description Hide DescriptionFrom the inhomogeneous functional derivation formalism of Percus, we derive here a perturbationequation on correlation functions which represents a generalization of the hard sphere equation of Barker, Henderson, and Smith. It is particularly suited to the expansion from the hard spheres to the soft spheres due to fast numerical convergence. To render our development independent of machine simulation input, we use the Verlet–Weis hard sphere construction. Comparison of our calculations with the Monte Carlo (MC) results on two types of soft spheres, the inverse‐12 model and the repulsive part of the Lennard‐Jones (LJ) potential due to the Weeks–Chandler–Andersen (WCA) division, shows that this theory improves over the widely used WCA approximation and the Percus–Yevick (PY) theory for the radial distribution functions. For the shorter‐ranged LJ repulsion, superior agreement with MC data on equilibrium properties and radial distribution functions is obtained for temperatures T<2.8 and all densities. A mixed theory combining the perturbationequation and the PY approximation is found to work well for the inverse‐12 potential for x=ρT ^{−1/4}<0.63.

Gaussian basis sets for molecular calculations. The representation of 3d orbitals in transition‐metal atoms
View Description Hide DescriptionAugmented (4d) and (5d) Gaussian basis sets are presented which provide a balanced description of the 4s ^{2}3d ^{ n−2}, 4s3d ^{ n−1}, and 3d ^{ n } configurations of transition−metal atoms. When compared to accurate Hartree–Fock calculations, the (4d) and (5d) expansions in the literature, which have been optimized for the 4s ^{2}3d ^{ n−2} configuration, can lead to errors of several eV for states of the latter configurations where the 3d orbitals become more diffuse. The augmented sets reported here consist of a single diffuse basis function added to the original sets, where the orbital exponent of the added function was optimized for the 3d ^{ n } configuration. Basis sets and contraction schemes are presented for the atoms Sc through Cu.

Frequency‐dependent rate coefficient in diffusion‐controlled reactions
View Description Hide DescriptionDiffusion‐controlled reactions between particles and a nondilute system of sinks are studied. It is shown that in time‐dependent situations it is necessary to take account of retardation effects in the building‐up of correlations between competing sinks. The retardation leads to a frequency dependence of the rate coefficient. We derive corrections to the dilute limit value of the rate coefficient in the low concentration, low frequency regime. As an application we consider the effect on the time dependence of a quenching process.

The arbitrary dynamic behavior of open chemical reaction systems
View Description Hide DescriptionWe show that if the kinetic equations describing a set of chemical reactions occurring in a one‐phase closed system are only required to obey a very general set of postulates (mass conserved, concentrations nonnegative, vector fields which are continuous with continuous derivatives, existence of equilibrium, stability of equilibrium, existence of potential functions which are extremal at equilibrium, detailed balance, law of definite proportions, and a dissipation inequality equivalent to the second law of thermodynamics), then when coupled to input and output flows the kinetic equations are compatible with any local dynamical behavior consistent with the dimension of the chemical state space. From this one can draw one of two conclusions: either open chemical systems are capable of exhibiting any behavior or that there exist additional restrictions on the types of equations that can be used to describe chemical reaction systems.