Index of content:
Volume 68, Issue 12, 15 June 1978

Geminal nuclear spin–spin couplings in AB_{ n }‐type molecular systems
View Description Hide DescriptionAn approximate formula for the second‐order perturbation energy previously obtained by the author is applied to the calculation of geminal nuclear spin–spin couplings in AB_{ n }‐type molecular systems. It is shown why the electron correlation is unusually important in the geminal couplings. The signs of c i s and t r a n s geminal couplings are discussed for the AB_{6}‐type molecular systems.

Energy transfer in A ^{2}Σ^{+} OH. II. Vibrational
View Description Hide DescriptionVibrational energy transfer within the A ^{2}Σ^{+} state of OH and OD has been studied. A frequency doubled tunable dye laser excites individual N′, J′ levels in v′=1 or 2, and the intensities of rotationally resolved fluorescence emitted in the presence of collision partners (He, Ar, H_{2}, D_{2}, and N_{2}) furnished state‐to‐state transfer rates. It is found that the transfer rates are strongly dependent on initial rotational level (decreasing as N _{ i } increases), that the final rotational state distributions are near thermal but hot, that isoenergetic transfer is small, and that the magnitudes for transfer 1→0, 2→1, and 2→0 are all similar. The rates themselves are large; for example, with N_{2}, k _{1→0} (N _{ i }=3) =1.7×10^{−10} cm^{3} sec^{−1}. The results, taken together, are supportive of a long‐lived collision in which anisotropic attractive forces are of importance in the dynamics of the entrance channel.

Temperature dependence of electron spin‐lattice relaxation of methyl radicals in glassy organic matrices:Relation to environmental dynamics and radical decay
View Description Hide DescriptionThe electron spin‐lattice relaxation of trapped methyl radicals in methanol , 2−methyltetrahydrofuran, and 3−methylhexane glasses has been directly studied as a function of temperature from 5‐100 K by the saturation recovery method. The relaxation rate remains fast at low temperatures and shows a large component linear in temperature. This is interpreted in terms of the recently developed model of tunneling relaxation involving modulation of the electron nuclear dipolar interaction by tunneling of nuclei in the radical’s environment [J.Phys.Chem.81,456 (1977)]. Deuteration experiments show that CH_{3} and [J.Phys.Chem.81,456 (1977)]. Deuteration experiments show that CH_{3} and CD_{3} have the same relaxation rate while it is decreased by about four−fold by deuterating the matrix. There also appears to be a correlation between the spin‐lattice relaxation rate and the decay of methyl radicals in these matrices.

An ESR study of the CH_{2}ClCȮ radical at 77 K in x‐irradiated chloroacetamide crystals
View Description Hide DescriptionX‐irradiation of chloroacetamide crystals at 77 K yields at least two radicals which are not stable near 300 K. One of these give rise to a four line ESR spectrum due to two nonequivalent protons with the principal hyperfinetensor components for H (1) equal to 268.4, 260.3, and 256.1 MHz and for H (2) equal to 129.9, 109.7, and 109.9 MHz. The principal g values are g _{max}=2.0674, g _{int}=2.0144, and g _{min}=1.9909. Comparing the corresponding direction cosines to selected molecular directions obtained from an x‐ray crystal structure as well as deuteration studies suggests the species to be the π‐radical CH_{2}ClCȮ. The unpaired electron is largely distributed between the oxygen and carbon 2p orbitals whose symmetry axis lies perpendicular to the CCO plane. Crystallographically, the chlorine atom lies nearly in the CCO plane and does not give rise to any resolvable ESRhyperfine couplings. The second radical gives rise to a complex ESR spectrum which has been tentatively assigned as due to ⋅CH_{2}Cl. This radical decays upon warming above 100 K with the resultant formation of ĊHClCONH_{2}.

Vibrational normal modes of SiO_{2}. III. α‐Berlinite (AlPO_{4}) and its relations to Γ–A phonon dispersion curves in α‐quartz
View Description Hide DescriptionThe valence force field, previously obtained to describe the vibrational normal modes of quartz, cristobalite, and tridymite forms of SiO_{2} (parts I and II), has been used to calculate the phonon dispersion curves and elastic constants of α‐quartz. The reasonably good fit to the experimental data suggested that the same valence force field may be valid in α‐berlinite, an isomorph of α‐quartz. The following results confirm the analogy between SiO_{2} and AlPO_{4}.

An EPR–ENDOR study of phosphorus in phenacite
View Description Hide DescriptionAn EPR and ENDOR study on a phosphorus impurity in natural single crystals of phenacite (Be_{2}SiO_{4}) is reported. The EPR studies on this strongly coupled electron–nuclear spin system include the forbidden or ΔM _{ F }=0 transitions. The nuclear g factor of the impurity ion which was calculated from an analysis of the forbidden transitions shows the impurity to be ^{31}P. The principal values and directions of the electronic gtensor and the hyperfine coupling tensor were determined through a least‐squares fit to the data employing direct diagonalization of the Hamiltonian matrix. ENDOR studies show a superhyperfine (shf) coupling of this center to several near neighbor ^{9}Be nuclei. The shf coupling parameters for three ^{9}Be nuclei were determined and are presented along with a detailed model of the center and a discussion of the crystallographic positions of the Be nuclei. Our analyses indicate a P^{5+} ion substitutes for a Si ion at a normal site and is converted to P^{4+} by x rays.

Light scattering from spheroids in shear flows. I. The orientation correlation
View Description Hide DescriptionThe time dependent orientation distribution function for ellipsoids of revolution (a,b,b) in shear flow is obtained as an expansion in terms of the parameter r=[1−(b/a)^{2}]/[1+(b/a)^{2}]. Our results reduce to Peterlin’s steady state results at infinite time (t=∞). Using this distribution, the orientation correlation function and its contribution to the light spectrum of polarized light are calculated. The results for the correlation function as an expansion in r lead also simultaneously to an expansion in the ratio σ of the shear rate to the Brownian rotation diffusion coefficient, σ=γ/D. The expansion procedure is thus valid for reasonably high shear rates in the case of nearly spherical spheroids (r≪1) or for small shear rates but large values of r (‖r‖?1, rods and disks).

Exchangeable proton couplings in free radicals: Radiation products of hydroxyproline HCl
View Description Hide DescriptionFree radicals were generated by X irradiation in single crystals of hydroxyproline HCl irradiated at low temperature. Proton hyperfine couplings were deduced from ENDOR measurements on two species, namely, a primary reduction product and a secondary free radical formed in a hydrogen abstraction process. These two radicals are representative of two general types of radicals, one nearly planar and the other markedly pyramidal, in which the unpaired electron interacts with nonexchangeable β protons attached to carbon and exchangeable β protons attached to oxygen. The characteristics of the hyperfine coupling tensors of these protons are discussed.

Theory of elasticity of polymer networks. II. The effect of geometric constraints on junctions
View Description Hide DescriptionThe effects of junction–chain interactions in a real network are analyzed in terms of a model in which the entanglements restricting fluctuations of the junctions are represented by vertical‐wall potential domains. Within these domains the junctions are allowed to fluctuate freely. The domains are assumed to transform linearly with macroscopic strain. The additional stress that arises due to the restrictions on the fluctuations of junction points accounts for the principal departures of experimental stress–strain curves from phantom‐network theory. The size of the entanglement domain, which is the only arbitrary parameter introduced, measures the deviations of the real network from phantom network theory. Calculations based on this model show the reduced force to be approximately independent of strain in the compression region (α<1) and to decrease approximately linearly with the reciprocal of the elongation α in tension (α≳1) over the range accessible to experiment.

Spectroscopic characterization of ytterbium monohalide emission in a high pressure electrodeless microwave arc discharge
View Description Hide DescriptionMolecular emission has been observed in high‐pressure electrodeless microwave arc discharges from the monochloride, monobromide, and monoiodide of ytterbium. The vibrational analyses of the electronic transitions in YbBr and YbI represent the first spectroscopic data for these molecules. Emission from the A (^{2}Σ_{1/2}) →X (^{2}Π^{+}) and A (^{2}Π^{+}) and R (^{2}Π_{3/2}) →X (^{2}Σ^{+}) transitions in YbCl have been observed. By analogy with YbF and YbCl, the blue degraded emission in YbBr and YbI has been assigned to the A (^{2}Π_{1/2}) →X (^{2}Σ^{+}) and A (^{2}Π_{3/2}) →X (^{2}Σ^{+}) transitions. The term values for YbBr and YbI are shifted to the red with respect to YbCl by <100 cm^{−1} and <300 cm^{−1}, respectively, for both transitions. The spin–orbit coupling constants of the A (2Π) state in YbBr and TbI have been determined from the difference in the band origins of the two A→X transitions. The effect of the lanthanide contraction in the ytterbium monohalides has been examined by comparing the ground state vibrational frequencies of the ytterbium monohalides and the Group IIa monohalides. Finally, the spectra of YbCl observed under high‐pressure arc conditions have been compared to the YbCl emission previously observed in a low‐pressure chemiluminescent beam experiment.

Theory for BCl_{3}+H_{2} laser induced chemistry
View Description Hide DescriptionThe experimental data of Rockwood and Hudson are analyzed in terms of a binary collision mechanism for H_{2}+BCl_{3}→BHCl_{2} +HCl. The model assumes the atom interchange occurs during the collision and uses the known rate for H_{2}+Cl→HCl+H to explain the experimental results. A qualitative account is given for the dependence of yield of BHCl_{2} on flux and gas pressure.

A one‐dimensional model for exciton motional correlation effects in dense triplet exciton systems
View Description Hide DescriptionA model for motional correlation effects of random‐walking particles with spin in one dimension is developed. For a system of particle site density ρ, the hard sphere interactions between particles are modeled by considering a single particle moving on a line segment of length ρ^{−1} undergoing perfectly elastic reflective collisions at the end points of the lattice; the collision rate is used to determine the spin exchange rate. The model is applied to proton spin–lattice relaxation in the MTPA(TCNQ)_{2} triplet exciton ion radical salt. Using mean theoretical estimates for the anisotropy parameters, a jump rate ν_{ j } is calculated indicative of an activated process with an activation energy of approximately 0.12 eV ith a preexponential factor of the order of 1.5×10^{+14} Hz. Limitations and extensions of the model are discussed.

Ring puckering in the ^{1} B _{2}(n, 3s) Rydberg electronic state of cyclobutanone
View Description Hide DescriptionThe first vacuum ultraviolet absorptionspectrum of cyclobutanone has been photographed under conditions of moderately high resolution. This system, which is given the assignment ?^{1} B _{2}(n, 3s) ←?^{1} A _{1}, has been observed to have unusually discrete rotational and vibrational fine structure. To higher frequencies from a weak (0–0) band are a pair of weak vibronic bands at (+105) and (+196) cm^{−1} which are assigned to the ring puckering sequence transitions (1–1) and (2–2). From a fit of the energy levels derived for the upper state to a quartic–quadratic potential function it was determined that the C_{1}C_{2}C_{3}C_{4}carbon atoms are coplanar in this state. The increase in puckering frequency from 35 to 140 cm^{−1} is interpreted in terms of a large increase in ring strain on excitation.

The kinetics of solvent reorientation in hydroxylated solvents from the exciting‐wavelength dependence of chromophore emission spectra
View Description Hide DescriptionThe disappearance of the exciting‐wavelength dependence of the phosphorescencespectra of polar, aromatic chromophores in supercooled glycol–water mixtures is utilized to monitor the kinetics of solvent reorientation. Reorientation times in the nanosecond to second range are obtained for (3:2 v/v) glycerol–water and (1:1 v/v) ethylene glycol–water at 140–240 °K. The results suggest that the process is one involving a cluster of solvent molecules and in which the chromophore plays a relatively passive role. Steady‐state data and direct measurements of phosphorescence shifts as a function of time indicate that the solvent reorientation process is nonexponential in nature. The decay function derived from the temperature dependence of the steady state data is consistent with the decays observed directly as a function of time. Interpretation of this nonexponential decay in terms of a summation of rate processes leads to a distribution dominated by two reorientation rate constants. The relative contributions of the slow and faster reorientation rate constants in addition to their activation parameters differ for the glycerol–water and ethylene glycol–water mixtures.

The structures of c i s‐polyacetylene and highly conducting derivatives
View Description Hide DescriptionThe structures of c i s‐polyacetylene and polyacetylene–iodine derivatives have been investigated by x‐ray diffraction and crystal packing analysis. Results obtained for c i s‐polyacetylene are consistent with the space group P n m a with one CH per asymmetric unit and a chain symmetry (m1̄)_{ t }. Hence, the structure is analogous to that of orthorhombic polyethylene. For a structure with such a concentration of π electrons there is minimal overlap between parallel π orbitals. The π orbitals are almost orthogonal for four of the six closest neighbors of a given chain. Based on x‐ray diffraction and spectroscopic results, a model is proposed for the structure of the highly conducting polyacetylene–iodine complexes. According to this model, linear arrays of polyhalide ions (principally in the form of I^{−} _{3}) substitute in positions of displaced polyacetylene chains. At high dopant concentrations, alternate close‐packed layers of polyacetylene are separated by a layer consisting of such polyhalide arrays. This model predicts a limiting charge transfer of 0.11 electron on the average per carbon atom in the polyacetylene chain.

The temperature dependence of the rate constant for Cl+NO+N_{2} → NOCl+N_{2}
View Description Hide DescriptionThe rate constant for the reaction Cl+NO+N_{2}→k _{1} ClNO+N_{2}, of interest in stratospheric chemistry, has been measured by the flash photolysis–resonance fluorescence technique over the temperature range 200–400 K. The measured rate constants obey the Arrhenius equation k _{1,N} _{2}= (1.18±0.10) ×10^{−32} exp[(532±20)/T] cm^{6} molecule^{−2} sec^{−1}. At room temperature with the two additional third bodies, He and Ar, the measured termolecular rate constants are k _{1,He}= (4.11±0.20) ×10^{−32} and k _{1,Ar}= (4.40±0.30) ×10^{−32} cm^{6} molecule^{−2} sec^{−1}. These results are theoretically discussed and are compared with previous work.

Rate constants for the reactions of O^{−}, O_{2} ^{−}, NO_{2} ^{−}, CO_{3} ^{−}, and CO_{4} ^{−} with HCl and ClO^{−} with NO, NO_{2}, SO_{2}, and CO_{2} at 300 K
View Description Hide DescriptionThe rate constants for several ion–molecule reactions involving chlorine‐containing ions or molecules have been measured at 300 K with a flowing afterglow apparatus. These include the reaction of O^{−} with CCl_{4}, several negative ions with HCl, ClO^{−} with several neutrals, and Cl^{−} with O_{3}. These reactions are used to place limits on the electron affinity of ClO: E.A.(ClO) =1.95±0.25 eV. The implications of these results to the negative ion chemistry of the atmosphere are discussed.

Semiempirical theory of chemisorption on narrow d‐band metals
View Description Hide DescriptionA previous developed model is applied to chemisorption of various adsorbates on Ni,Cu, Pd, and Agsurfaces. The aim of the calculations was mainly to predict the trends of various physical properties over a series of different adsorption systems. Adsorption energies, ionization energies, and energy profiles are evaluated and compared with experiment. The difference photoelectron spectra are derived to a first approximation by including the optical matrix elements for excitation into plane wave final states. Chemisorption of atomic hydrogen and oxygen is treated in detail. In the case of hydrogen chemisorption the results agree qualitatively well with conclusions drawn from more elaborate numerical calculations published recently. For oxygen chemisorption the importance of the multiplet structure of the O atom is stressed. In this model the large exchange splitting of the O 2p level is preserved in the adsorbed state; the main effect caused by coupling to the metal is a screening shift by about 8 eV towards lower ionization energies. The earlier treated adsorbates CO, NO, and N_{2} are included in the concluding discussion which attempts to extract the dominating physical aspects. Comparison with experimental data is generally encouraging.

A Monte Carlo study of the classical octopolar solid
View Description Hide DescriptionA Monte Carlo simulation of the effect of temperature on the structure of the classical octopolar solid predicts the existence of three phases. The structures are identical with those predicted by the molecular field method but the transition temperatures are significantly different. Results are also given for the energy, specific, and lattice correlation functions as a function of temperature.

Wien effect in mixed strong electrolytes
View Description Hide DescriptionA method of Fourier transformations and matrix representations is formulated for the computation of Wien effect in diluted solutions of mixed strong electrolytes. The method permits the extension of the computation to systems containing any number of species of ions of any valence type. Thus, for the first time Wien effect of mixed electrolytes is computed from the fundamental equations of ion–ion interactions. At very low field strengths, where Ohm’s law is obeyed, the integral representation of Onsager and Kim is reproduced. At extremely high field strengths, the relaxation effect and the electrophoreticeffect are computed to the second order term. At intermediate field strengths, the equations for the computation of the relaxation effect and the electrophoresis by triple integrations are derived.