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Volume 63, Issue 10, 15 November 1975

Short‐range order in liquid Li–Pb alloys
View Description Hide DescriptionThe neutron diffraction patterns of liquidlithium lead alloys containing 0, 17, 34, 48, 61, 80, and 100 at.% ^{7}Li were measured. The long‐wavelength limit of the partial structure factors a _{ i j }(k) and of the concentration–concentration correlation functionS _{ C C }(k) is calculated. These values agree well with the values calculated from activity data. For the 80 at.% alloy the total S _{ C C }(k) function was measured. It shows a pronounced first peak at the k value of a prepeak observed with x rays. The Fourier tranform of S _{ C C }(k) is related to the short‐range order‐size coefficients which have been introduced by Warren e t a l. (Ref. 19) to describe the local order of solid solutions. The data for the liquid Li–Pb alloys indicate a preference for unlike nearest neighbors and a reduction of the distance between unlike nearest neighbors as compared with the mean distance of the pure components. This short‐range order is probably due to a partially saltlike bonding between lithium and lead ions. The over‐all structure of the liquid alloys is similar to the structure of a hard sphere system.

Contraction effects in ideal networks of flexible chains
View Description Hide DescriptionThe mean‐square distances between various points of regular Gaussian networks are calculated using a new electrical analog and solving exactly a set of difference equations both in two and in three variables through an appropriate Fourier lattice transformation. The results show that the mean‐square radius of gyration of the network is of the same order of magnitude as that of a single chain for networks having a three‐dimensional connectivity, while it increases logarithmically with the total number of chains for networks having a two‐dimensional connectivity.

Raman spectra of gases. XVIII. Internal rotational motions in ethylamine and ethylamine‐d _{2}
View Description Hide DescriptionThe Raman spectra of gaseous CH_{3}CH_{2}NH_{2} and CH_{3}CH_{2}ND_{2} have been recorded from 100 to 600 cm^{−1}. Both ground and excited states of the methyl and amino torsional modes for the g a u c h e conformer have been observed. The overtone of the methyl torsion has also been observed for the g a u c h e conformer. The methyl barriers in the g a u c h e conformers of the CH_{3}CH_{2}NH_{2} and CH_{3}CH_{2}ND_{2} molecules were found to be 3.71±0.05 and 3.59±0.05 kcal/mole, respectively. The apparent difference in the barrier heights for these two isotopic species is attributed to a difference in the coupling between the methyl and amino torsions in these two molecules. The potential function for internal rotation around the C–N bond in CH_{3}CH_{2}ND_{2} was determined and the following potential constants found: V _{1}=279±16 cm^{−1}, V _{3}=707±4 cm^{−1}, V _{6}=−12±3 cm^{−1}. The energy difference between the potential energy minima of the g a u c h e and t r a n s conformations is 207 cm^{−1} (0.592 kcal/mole) with the t r a n s being the more stable. The calculated g a u c h e–g a u c h e barrier is 779 cm^{−1} (2.23 kcal/mole) and the t r a n s–g a u c h e barrier is 572 cm^{−1} (1.64 kcal/mole). A similar potential function was determined for the CH_{3}CH_{2}NH_{2} isomer but the fit to this one‐dimensional model was not as good as that obtained for the deuterium compound probably because of the greater coupling in the light molecule.

A kinetic model for clustering of water on hydrated protons in a supersonic free jet expansion
View Description Hide DescriptionA simple model with three variable parameters is developed for calculating observed clustering data from a free jet expansion. One parameter, σ_{ R } _{ 1 }, can be associated with a reactive collision diameter for a monomer associating with a cluster to form a larger, excited cluster. A second parameter, σ_{ R } _{ 3 }, can be associated with a reactive collision diameter for energy transfer from the excited cluster to the surrounding gas. The third parameter, K′, is a constant in a unimolecular decay rate expression for the excited cluster. The reactive cross sections that give the best fit of experimental data are given by (π/4) σ^{2} _{ R } _{ 1 } =9.90×10^{−16} cm^{2} and (π/4) σ^{2} _{ R } _{ 3 }=5.22 ×10^{−17} cm^{2}. No value could be assigned to K′. Apparently the excited clusters are stabilized by energy exchange collisions in this particular flow field before there is appreciable unimolecular decay. Good fits of clustering data are obtained even though these same values are used for clusters varying in size from 6 to 26 water molecules. Since the clustering environment is radically different than in near‐equilibrium experiments, it is not clear that these numbers can be used with other types of experimental data.

Paramagnetic defects in irradiated ferroelectric (NH_{4})_{2}SO_{4}
View Description Hide DescriptionEPR studies of x‐ray damaged crystals of (NH_{4})_{2}SO_{4} show that long lived defects, NH_{3} ^{+} ions, are produced in two inequivalent sites with each site having two magnetically inequivalent orientations. All sites have approximately the same isotropic hyperfine splittings which are found to be 19.0 G for nitrogen and −24.6 G for hydrogen. The anisotropic components are different for the two inequivalent sites indicating different amounts of restricted rotation for the two sites. A second transient defect is identified as SO_{3} ^{−}.

A b i n i t i o valence bond calculations of the potential energy surface for H+H_{2}
View Description Hide DescriptionValence bond calculations of the potential energy surface of H+H_{2} have been done. In all the calculations the hydrogen molecule bond length was kept at 1.4 a _{0}. The isotropic potential V_{0}(r) and the leading anisotropic potential V_{2}(R) were obtained. Our caclulated V_{0} is in good agreement with the experimentally determined V_{0} reported by Gengenbach, Hahn, and Toennies. Our calculations predict that the triangular arrangement of H_{2}+H is favored over the linear configuration at long distances of R with alignment to the linear geometry at approximately 3.1 a _{0}. A populaion analysis of our wavefunction is also presented.

Inelastic light scattering from isotropic MBBA
View Description Hide DescriptionThe spectrum of light scattered from the isotropic phase of p‐methoxybenzylidene p‐ (n‐butylaniline) (MBBA) has been studied over a 50 °C temperature range above the nematic–isotropic transition temperature (46.2 °C). Light scattered through a wave vector K?2.7×10^{5} cm^{−1} with frequency shifts f in the range 1 GHz<f<15 GHz shows two major contributions: a polarized shifted (Brillouin) peak of f∼6 GHz and a depolarized 1/(f)^{2} tail of an unshifted line. The adiabatic sound speed and the sound attenuation coefficient in the vicinity of 6 GHz have been determined from the Brillouin splitting and width, respectively, and compared to values measured at 23, 150, and 400 MHz. It is found that at 6 GHz substantial parts of the bulk and shear viscosities have relaxed out. Dispersion in the sound speed is also observed and used along with the linewidth measurements to estimate the relaxation frequency f _{ r }, which is found to be ∼2.5 GHz. The total dispersion and relaxation frequency are similar to those found in benzene. The ratio of peak Brillouin intensity to 1/f ^{2} intensity is analyzed and shows the local dielectric anisotropy to be weakly temperature independent for 46<T<97 °C.

Piezoelectricity and long‐range order in NH_{4}Cl
View Description Hide DescriptionThe temperature dependence of the piezoelectric constantd _{14} of NH_{4}Cl has been measured in the vicinity of the order–disorder phase transition. An analysis of data obtained in the ordered phase yields β=0.134 for the critical exponent associated with the order parameter. Above the transition temperature, there is an anomalous ’’tail’’ of nonzero d _{14} values that indicates a persistence of long‐range order in the ’’disordered’’ phase. The values of d _{14} in this tail are well described by the empirical expression d _{14}=const×exp(−σT/T _{ c }) with σ=335. This anomalous behavior is essentially identical to that observed previously for the second‐harmonic scattering intensity.

Bonded and nonbonded interactions in saturated hydrocarbons
View Description Hide DescriptionThe enthalpy of formation (ΔH°_{ f }) of an alkane is expressed as a linear function of eight structure parameters (N) that are essential in characterizing stable conformations of the molecule. Using the least squares criterion, energy parameters (X) which correspond to the linear coefficients of the structure parameters are calculated from experimental ΔH°_{ f }. For acyclic alkanes and polycyclic alkanes with six‐membered rings, 80 liquids and 63 gases are used. The mean and root‐mean‐square deviations between the calculated and experimental ΔH°_{ f } are, respectively, ±0.18 and ±0.24 kcal mole^{−1} for liquids and ±0.20 and ±0.28 kcal mole^{−1} for gases. The resulting energy parameters may be used for predicting the unknown ΔH°_{ f } of higher alkanes and for estimating enthalpy differences between certain conformers. For the latter application, those involving the 1,4 and 1,5 interactions represented by the g a u c h e CCCC (X ^{ G }), g a u c h e–g a u c h e prime CCCCC (X ^{ V }), and anti–anti CCCCC (X ^{ W }) interactions are particularly useful. The energy parameter for ring strain (X _{ R }) appears necessary for bringing the predicted ΔH°_{ f } of gaseous adamantane and diamantane into reasonable agreement with available experimental data.

Bonded and nonbonded interactions in saturated alcohols, ethers, thiols, thioethers, and amines
View Description Hide DescriptionThe enthalpies of formation (ΔH°_{ f }) of alcohols, ethers, thiols, thioethers, and amines are expressed as linear combinations of structure parameters. The linear coefficients are determined from experimental ΔH°_{ f } using the least squares criterion. For these calculations eight structure parameters are used for thiols and thioethers, nine for amines, and ten for alcohols and ethers. The mean and root‐mean‐square deviations between calculated and experimental values are, respectively, ±0.17 and ±0.25 kcal mole^{−1} for 30 gaseous alcohols and ethers, ±0.17 and ±0.25 kcal mole^{−1} for 32 liquid and ±0.19 and ±0.27 kcal mole^{−1} for 31 gaseous thiols and thioethers, and ±0.12 and ±0.23 kcal mole^{−1} for 18 liquid amines. Liquid phase data of 28 alcohols and ethers and gas phase data of 13 amines are also treated with this model but the results are not satisfactory. The linear coefficients may be used for predicting unknown ΔH°_{ f } and for estimating enthalpy differences between certain conformers. Those corresponding to the various 1,4 nonbonded interactions indicate that the g a u c h e CCCC, CCOC, CCNC, CCOH, and CCNH interactions are repulsive, but the g a u c h e CCCO and CCCN, interactions are attractive. The only 1,5 interactions considered are the g a u c h e–g a u c h e prime CCOCC and CCSCC interactions and they are found highly repulsive. The relative stability of conformers predicted on the basis of these nonbonded interactions agrees quite well with results from experiments and molecular orbital calculations.

Dynamic effects of phase transition of K_{3}Fe(CN)_{6} on EPR Of Cr^{3+}
View Description Hide DescriptionA marked broadening of the resonances and a coalescing of the fine structure splitting of the EPR of the 3d ^{3} state of Cr^{3+} is observed in the temperature range where previous data indicate the possibility of a phase change. The effects are attributed to motional reorientaion of the [Cr(CN)_{6}]^{−3} complexes.

Effect of reagent electronic excitation on the chemical reaction Br(^{2} P _{1/2,3/2})+HI
View Description Hide DescriptionBr(^{2} P _{3/2})+HI are found to react at room temperature to give HBr+I(^{2} P _{3/2}) with a rate constant of (1.0±0.3) ×10^{−11} cm^{3} molecule^{−1}⋅sec^{−1}. For Br+HI, electronic excitation of Br inhibits the reaction to HBr+I. Collisions with HI remove excited Br(^{2} P _{1/2}) with a rate of about 0.25×10^{−11} cm^{3} molecule^{−1}⋅sec^{−1}. This rate includes electronic quenching and electronic‐to‐vibration energy transfer as well as reaction. From correlation arguments the reaction of excited Br(^{2} P _{1/2}) with HI is expected to be much slower than that of ground state. The observed rate is comparable to that for quenching plus electronic‐to‐vibration transfer for Br(^{3} P _{1/2}) by HCl and HBr.

Effect of electron correlation on the H_{2}CO‐He interaction potential
View Description Hide DescriptionA previously reported Hartree‐Fock (HF) interaction potential between H_{2}CO(^{1} A _{1}) and He(^{1} s) is modified through a series of configuration interaction (CI) calculations. The CI contribution is described by a three‐term (l=0,1,2) Legendre polynomial expansion in the angle τ formed by the direction of incidence of He and the CO bond of formaldehyde. No significant azimuthal angle dependence is obtained. Correlation is found to have little effect in the strongly anisotropic repulsive region of the interaction potential but dominates the well and long‐range regions. The maximum well depth is attained for in‐plane approaches of He and lies in the range 35–40 °K for arbitrary τ at center of mass separation of 7.5 a.u. The CI contribution in the region of the minimum is beleived accurate to ∼20%.

Vibrational relaxation in H_{2}–CO and D_{2}–CO mixtures, measured via stimulated Raman‐ir fluorescence
View Description Hide DescriptionRate constants for vibration–vibration and vibration–translation energy transfers were measured by exciting the hydrogenic species in a stimulated Raman cavity and recording the subsequent rise and decay of ir fluorescence from the admixed CO at 4.7 μm. Under the justifiable assumption that the intraspecies redistribution of vibrational energy is very rapid, the time variation of fluorescence intensities and their dependence on the sample composition can be accounted for by the following processes: CO^{(v)}+H_{2}→CO+H_{2}, k _{C,H}=14.3±0.7 sec^{−1}⋅torr^{−1}; H_{2} ^{(v)}+H_{2}→H_{2}+H_{2}, k _{H,H}=4.3±0.1; D_{2} ^{(v)}+D_{2}→D_{2}+D_{2}, k _{D,D}=0.9±0.3; H_{2} ^{(v)}+CO→H_{2}+CO^{(v)}, k ^{ v } _{H,C}=4.5{^{+2.1} _{−1.5}; D_{2} ^{(v)}+CO→D_{2}+CO^{(v)}, k ^{ v } _{D,C}=138±25. The ratio (k ^{ v } _{H,C}/k ^{ v } _{D,C}) is surprisingly small in view of the near resonant condition for Δv (H_{2}) =−1 coupled with Δv (CO) =+2.

Nuclear magnetic relaxation of ^{31}P in phosphorous halides
View Description Hide Description^{31}P NMR spin–lattice relaxation timemeasurements in the laboratory frame, T _{1}, and in the rotating coordinate frame, T _{1ρ}, are reported for an equimolar mixture of PBr_{3} and PCl_{3}. Relaxation times were measured as a function of temperature. T _{1ρ} was measured also as a function of offset and lock‐field strength. The lock‐field dependent T _{1ρ} technique is shown to be inadequate for exact determinations of scalar spin–spin coupling constants. We report for all four molecular species PBr_{3}, PBr_{2}Cl, PBrCl_{2}, and PCl_{3} the following quantities: activation energies and frequency factors for chemical exchange and molecular reorientation and spin–lattice relaxation times for the bromines and chlorines. These quantities are obtained from the temperature dependent T _{1ρ}measurements, which are shown to be suited for the study of chemical exchange processes when the exchange rates are in the order of 10^{3}–10^{5} s^{−1}. Using known values for quadrupolar coupling constants, we derive the temperature dependence of the reorientation correlation time. The known temperature dependence of the correlation time is in turn used for a complete analysis of the temperature dependent T _{1}measurements. All possible relaxation mechanisms contributing to T _{1} are identified and experimental values for the spin–rotation interaction constant and the chemical shiftanisotropy are determined. The accuracy of the various parameters varies from a few percent to mere estimates.

Exciton–charge carrier interactions in the electroluminescence of crystalline anthracene
View Description Hide DescriptionThe interactions of triplet excitons with trapped charge carriers are studied in anthracenesingle crystals. The triplet excitons are generated by the recombination of charge carriers through double injection. The lifetime of the triplet excitons is studied from the decay of the phosphorescence intensity in a transient measurement. The interaction rate constant for the trapped charge carriers is of the same order of magnitude as previous published data measured with single injection of electrons. It is further shown that the rate constant varies with different conditions of crystal growth. The simultaneous investigation of double injection delayed fluorescence reveals also a strong quenching of the delayed fluorescence from the excited singlet state by trapped charge carriers. Magnetic field measurements show that this quenching is partly due to an increased dissociation of the triplet pair state through the presence of charge carriers. The possibility of enhanced spin relaxation of the triplets in the pair state is also discussed.

Long‐lived K _{ a }=0, ^{2} B _{1} states of NO_{2}: A direct measurement using a tunable dye laser
View Description Hide DescriptionNarrow‐band (1 GHz) excitation coupled with selection of specific emission bands has been used to measure the lifetimes of some K _{ a }=0, ^{2} B _{1} states of NO_{2}. In contrast with previous experiments and expectations of short lifetimes based on analysis of possible perturbations in the electronic manifold, these states exhibit long collisionless lifetimes of 33±4 μsec. Their quenching cross sections are found to be larger than gas kinetic, and much larger than that for the unresolved fluorescence.

Validity of approximate methods in molecular scattering: Thermal HCl–He collisions
View Description Hide DescriptionAccurate close coupling scattering calculations are presented for thermal energy HCl–He collisions. The interaction potential is obtained from the Gordon–Kim electron gas model, adjusted to have the correct long‐range multipole form. A variety of phenomenological cross sections are computed from the close coupling S matrix, and these are compared with results from several commonly employed approximate methods. In particular, it is found that the total integral, total differential, and gas kinetic cross sections are accurately predicted by the central field approximation which retains just the spherical average of the interaction. Integral inelastic cross sections are represented quite accurately by the coupled states approximation of McGuire and Kouri, but only qualitatively by the effective potential method of Rabitz. Pressure broadening cross sections from the close coupling calculation are in much better agreement with experiment than either Anderson theory calculations or the classical trajectory study of Gordon. NMR spin–lattice relaxation cross sections are also presented.

Vibrational energy exchange in CO–CO collisions at low temperature
View Description Hide DescriptionThe V–V exchange rates for CO in the ground electronic state have been measured at a temperature of 75 °K. The rates determined were for the processes CO(v=n)+CO(v=0) →CO(v=n−1)+CO(v=1)−ΔE, with 2?n⩽8. The values obtained are close to published 100 °K data, thus indicating an insensitivity to temperature. It appears that multirotational quantum jumps may be important in explaining the measured rates.

The microwave spectrum of HO_{2} near 65 GHz
View Description Hide DescriptionUsing a Zeeman‐modulated cavityspectrometer with a 10 sec time constant and a phase locked klystron, we have observed in the products of a discharge‐flow system, Zeeman components of the six allowed zero‐field lines at 65 070±2, 65 082±2, 65 098±2, 65 373±2, 65 397±2, and 65 401±2 MHz, and of one forbidden zero‐field line at 65 369±4 MHz. The Q of the Fabry–Perot cavity is about 10 000, and the magnetic field was swept from 0–30 G. Chemical tests indicate that the observed lines are due to HO_{2}. They have been assigned and least‐squares fitted using a simple theoretical model to yield a value of 65 185±2 MHz for the 1_{01}–0_{00} asymmetric rotor transition frequency of HO_{2}, a value of −208±2 MHz for the linear combination (ε_{ b b }+ε_{ c c })/2 of elements of the electron spin–molecular rotation interactiontensor, a value of −28±2 MHz for the nuclear spin–electron spin Fermi contact interaction parameter σ, and a value of +4±2 MHz for the spin–spin tensorinteraction parameter λ. These constants are in excellent agreement with three less precise constants obtained from an earlier laser magnetic resonance study and have been confirmed by recent more accurate measurements of Saito.