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Volume 62, Issue 10, 15 May 1975

Molecular reorientation in liquids by Rayleigh scattering: Pressure dependence of rotational correlation functions
View Description Hide DescriptionResults of high pressure study of the depolarized Rayleigh scattering in acetone, benzene, and methyl iodide are presented. The density dependences of the angular velocity and angular position correlation functions are determined. The angular velocity correlation function of each liquid shows a negative overshoot. In addition, benzene at all pressures and methyl iodide at high pressures exhibit additional oscillations in the angular velocity correlation function. The details of the correlation functions are discussed in terms of ''collision interrupted'' models such as the well known J‐diffusion model. It is concluded that a more general description than Jdiffusion capable of allowing incomplete loss of momentum upon collision is needed. The results are compared to studies of reorientation by means of NMR. Finally, the relation between shear viscosity and reorientation in these liquids is discussed.

Determination of the torsional potential function of 1,3‐butadiene
View Description Hide DescriptionA series of 10 lines in the Raman spectrum of 1,3‐butadiene has been observed and assigned as torsional overtones. An analysis of the data leads to a potential of the form V (φ) = J^{4} _{ i=1}(V i/2) (1 − cosiφ). The less stable species is found to be in the c i s configuration with a c i s–t r a n s energy difference of 873 cm^{−1}. The barrier for t r a n s → c i s interconversion was found to be 2504 cm^{−1}.

Charge exchange of rare gas ions: An ion kinetic energy study
View Description Hide DescriptionThe charge exchangereactionm ^{++} + N → m ^{+‐} + N ^{+‐} has been studied at 8 keV impact energy in a double‐focusing mass spectrometer for each combination of the rare gas ions and atoms. The kinetic energy gained or lost by the fast species m has been measured and from this the states of m ^{+‐} and N ^{+‐} have been assigned. Generally all the observed peaks correlate with known states of the product ions and the relative intensities of the peaks are in approximate agreement with previously determined cross sections. The Massey ''near‐resonance'' criterion is qualitatively applicable. Extension of the work to the study of polyatomic species is reported.

Microwave spectrum of ethyl mercaptan
View Description Hide DescriptionThe microwave spectrum of the t r a n s and g a u c h e isomers of the normal and two isotopic species (CH_{3}CH_{2}SD and CH_{2}DCH_{2}SH) of ethyl mercaptan has been studied. Identification of the spectrum of the t r a n s isomer of the normal and s y m‐CH_{2}DCH_{2}SH molecules has been extended up to J=19, enabling preliminary centrifugal distortion constants to be determined. From a and c dipole transitions full sets of rotational constants of the g a u c h e isomer of all species have been determined. For the normal molecule they are A=28 746.37 MHz, B=5294.85 MHz, and C=4846.96 MHz. The tunneling frequency of the thiol top between its two equivalent g a u c h e configurations has been measured directly for the normal species (1754.09 MHz), s y m‐CH_{2}DCH_{2}SH (1790.05 MHz), and CH_{3}CH_{2}SD (70.40 MHz). Identification has been made of excited thiol torsion satellites in both isomers. Dipole moments of all configurations for each species have been determined using second‐order Stark effects. For the t r a n s isomer of the normal molecule they are μ_{ a }=1.06±0.03 D, μ_{ b }=1.17±0.03 D,μ_{ c }=0, μ=1.58±0.04 D; for the g a u c h e isomer of the normal molecule they are μ_{ a }=1.48±0.02 D, μ_{ b }=0.19±0.10 D, μ_{ c }=0.59±0.02 D, μ=1.61±0.05 D. An approximate structure, which has been fit to the data, indicates that the thiol top rotates about an axis with the CS bond tilted away from the axis by 3.16 °±0.05 °, while & CSH=95.23 °±0.50 ° and the angle of the axis with the CC bond is 111.71 °±0.10 °. The barrier to methyl internal rotation in the t r a n s isomer was found to be 1156±30 cm^{−1}. Splittings observed in the g a u c h eground state of the normal and s y m‐CH_{2}DCH_{2}SH molecules have been effectively reproduced on the basis of structural effects and tunneling. Relative intensity measurements show the g a u c h e isomer to be lower in energy with the difference between the lowest states (t r a n s − g a u c h e) equal to 142±15 cm^{−1} for the normal species and 138±30 cm^{−1} for CH_{3}CH_{2}SD. Using the measured energy differences of the ground and excited states, the first three Fourier coefficients of the thiol torsional potential were found to be V _{1} = −65 cm^{−1}, V _{2} = −135 cm^{−1}, and V _{3} = 440 cm^{−1}. Evidence of coupling between the methyl and thiol internal rotors was also investigated.

High‐temperature thermodynamic properties of MnS by the dynamic Knudsen–torsion effusion method
View Description Hide DescriptionThe vaporization and thermodynamics of MnS were investigated from 1536–1693 K by Knudsen effusion and dynamic‐torsion effusion. The principal vaporizationreaction was confirmed to be MnS(s) = Mn(g)+(1/2)S_{2}(g). Equations representing vapor pressures as functions of temperature were derived. Third‐law ΔH °_{ m } (298 K) of the principal vaporizationreaction was 563.7±0.3 kJ from torsion effusion and 562.8±0.2 kJ from Knudsen effusion. A vapor analysis from the combined Knudsen and torsion effusion results revealed the average molecular weight of effusing vapor was 62.5±8.3 at 1600 K. Results were compared with those from previous investigations and with the enthalpy calculated from a thermodynamic cycle. Efficacy of the dynamic torsion effusion method was tested by observing the torsion effusion pendulum in three ways, at different parts of its oscillation, and comparing the results. The method gave results in excellent agreement with Knudsen effusion results and was effective in detecting and analyzing extraneous torques on the torsion pendulum.

Excitation of nitrogen and carbon monoxide ionic emissions by He(2 ^{3} S), He^{+}, and He^{+} _{2}
View Description Hide DescriptionThe N^{+} _{2}(B ^{2}Σ^{+} _{ u }–X ^{2}Σ^{+} _{ g }), CO^{+}(B ^{2}Σ^{+}–X ^{2}Σ^{+}), and CO^{+}(A ^{2}Π–X ^{2}Σ) emission spectra excited by reaction of He^{+} and He^{+}+He^{+} _{2} mixtures with N_{2} and CO are compared with the spectra from the reaction with He(2 ^{3} S) metastable atoms. The experiments were done in a room temperature flowing afterglow apparatus. Charge exchange between He^{+} and N_{2} gave N^{+} _{2}(B) and a small amount of N^{+} _{2}(D) in addition to N^{+} _{2}(C); the yield of N^{+} _{2}(A) was insignificant. The vibrational population distribution of N^{+} _{2}(B) excited by He^{+} differed greatly from that given by the reaction with He(2 ^{3} S). No CO^{+} emission could be identified from the He^{+}+CO reaction. The reaction of He^{+} _{2} with N_{2} and CO gave strong emissions from N^{+} _{2}(B), CO^{+}(B), and CO^{+}(A) with vibrational populations resembling the distributions expected for Franck–Condon transitions from neutral N_{2} and CO. Although the N^{+} _{2}(A–X) emission could not be studied in detail, N^{+} _{2}(A) appeared to be a product from reaction of He^{+} _{2} and N_{2}.

Experimental investigation of the thermomagnetic force in N_{2}, CO, and NH_{3} gases at 300°K
View Description Hide DescriptionThe thermomagnetic force is observed as a magnetic field induced change in the thermal force exerted on an object suspended in a heat‐conducting rarefied polyatomic gas. A recently developed extremely sensitive stress measuring technique and higher magnetic fields have permitted the experimental investigation of the thermomagnetic force to saturation levels in the gases N_{2}, CO, and NH_{3} at 300°K. We have studied the force effects for three well‐defined temperature fields produced by isothermal boundaries with different geometry. The increased accuracy of the experimental results has permitted the fit of the data to a typical saturation curve. From this curve saturation values and the position on the H/p axis at which the effect reaches half‐magnitude (H/p)_{1/2} values have been determined for the various gases under different conditions of temperature field and pressure. A comparison is made with the Senftleben–Beenakker thermal conductivityeffects.

Ionization in the track of a high‐energy electron in hydrocarbon liquids; study of positive and negative ion scavenging using nanosecond pulse radiolysis
View Description Hide DescriptionThe lifetime distribution of the charged particles in the track of a high‐energy electron in cyclohexane and isooctane has been considered. Results on the decay of biphenyl ions in solutions of biphenyl obtained by means of pulse radiolysis with nanosecond time resolution have been compared with lifetime distributions calculated from the concentration dependence of the steady state scavenging, using recently determined absolute rate constants for reaction of excess electrons and mobilities of the charged species. The discrepancy is relatively small for cyclohexane but large for isooctane. It is concluded that the scavengers considerably affect the initial spatial distribution of the charged species in the track. The kinetics of the reaction of the positive species in cyclohexane and isooctane with biphenyl, TMPD, and pyrene has been considered. It has been concluded that the mobile positive species in cyclohexane reacts with pyrene with a rate constant of 5×10^{11} mol^{−1}⋅s^{−1}, and has a lifetime of 50 ns. In isooctane no evidence for the existence of a mobile positive species has been obtained.

Magnetism down to 0.89 °K and the ligand field in a planar spin triplet compound of cobalt(III)
View Description Hide DescriptionThe magnetism down to 0.89 °K and the optical spectra of solid KCo(3‐prbi)_{2}⋅2H_{2}O have been studied where 3‐prbi = 3‐propylbiuret. The data are uniquely reproduced on the basis of a complete 3d electron calculation where B=811.5 cm^{−1}, C=4B, ζ=580 cm^{−1}, D q=2223 cm^{−1}, D t=1002 cm^{−1}, and D s=4587 cm^{−1}. The results confirm the unusual occurence of a ^{3} B _{2}(e ^{4} b _{2} a _{1}) ground state in this compound.

Configuration interaction studies of O_{3} and O^{+} _{3}. Ground and excited states
View Description Hide DescriptionWe report the results of extensive configuration interaction calculations (double ζ basis) on the ground and 15 excited states of ozone and the eight lowest states of the ozone positive ion. The studies on ozone are carried out as a function of both bond length and bond angle. We find that three excited states of ozone may be bound with respect to the ground states of O_{2} and O. Additional information is provided on the ring state (60 ° bond angle) previously reported. Two of the excited states of ozone are known spectroscopically and the ionization potentials of the first three states of O^{+} _{3} are now known experimentally. These experimental results are in good agreement with the theoretical calculations.

Continuum hydrodynamic interactions and diffusion
View Description Hide DescriptionThe Oseen tensor is not an adequate characterization for the contribution of hydrodynamicinteractions to the homodyne (’’self‐beat’’) spectrum of light scattered from a dilute macromoleculesolution. Physical constraints on the possible form of the true cross diffusiontensor D^{ T }(R), which describes hydrodynamicinteractions in macromoleculesolutions, indicate that hydrodynamicinteractions do not by themselves contribute to the homodynespectrum. However, if the macromolecules in a solutioninteract directly as well as hydrodynamically, the direct and hydrodynamicinteractions couple to contribute jointly to the mutual diffusion coefficient of the solute molecules, and thence to the spectrum of the quasielastically scattered light. Tracer diffusion techniques may be used to study the cross‐diffusion tensor. Comparison with the available experimental data indicates that the joint contribution of direct and coherent hydrodynamicinteractions to diffusion need not be large, even in extremely concentrated solutions.

The Raman spectrum of (TMA)_{2}UCl_{6} and (TMA)_{2}SnCl_{6} crystals and of TMACl in solution
View Description Hide DescriptionThe Raman spectrum of (TMA)_{2}UCl_{6}, not reported previously, is presented and the assignments of the observed bands to normal modes of vibration are discussed based on polarized room temperature scans of single crystals. Seventeen of the eighteen fundamental vibrational bands and seven nonfundamental ones are identified. The missing fundamental vibrational band, containing mainly methyl internal symmetric stretching motions, is at least three orders of magnitude weaker than another band composed of the same methyl internal motions. Five totally symmetric nonfundamental bands are resolved near 3000 cm^{−1}. It is suggested that they interact by Fermi resonance with the highly intense totally symmetric methyl stretching vibration. Their proposed assignments imply that one of the nonfundamental bands is an overtone of a combination level, thus involving four vibrational quanta. Another one of these bands is assigned to a combination level involving three different vibrational quanta. Light scattering by three and four phononexcitations has not been reported before, except in resonanceRaman scattering. An unpolarized Raman spectrum of (TMA)_{2}SnCl_{6} is shown and the bands are assigned based on the close correlation with the (TMA)_{2}UCl_{6}spectrum. The Raman spectrum of TMACl in solution is reexamined. Four of the frequencies reported previously could not be verified, and five bands of this spectrum, including a fundamental one, are reported here for the first time. The polarization information of the latter spectrum is completed. The crystal structures of (TMA)_{2}UCl_{6} and (TMA)_{2}SnCl_{6} are discussed. Four variations of the structure compatible with O ^{5} _{ h }, the space group suggested for both crystals by x‐ray investigations, are described.

Low resolution microwave spectroscopy. I. The structural significance of the band spacing
View Description Hide DescriptionCompounds which are nearly symmetric tops, have ∼20 mtorr vapor pressure and have a sizeable dipole moment along the a axis display band microwave rotational spectra under low resolution conditions. The frequencies of the band maxima are adequately represented by a symmetric rotor type equation, ν = (B + C)(J + 1). From calculated spectra it is shown that the B + C value defined above exceeds the ground state effective rotational constantB _{0} + C _{0}. Low resolution B + C values and high resolution B _{0} + C _{0} values are compared for nineteen species. B + C differs from B _{0} + C _{0} by less than 2% in all cases and less than 0.8% for molecules with κ < −0.8. Empirically it is found that (B + C)/(B _{0} + C _{0}) − 1 = ν/ν_{0} − 1 = 0.025(±0.005)(1 + κ), where k is the asymmetry parameter and ν_{0} is the hypothetical symmetric rotor frequency, (B _{0} + C _{0})(J + 1). This empirical relation allows B _{0} + C _{0} to be estimated from B + C with an uncertainty of only a few tenths of a percent.

Low resolution microwave spectroscopy. II. The conformation of 2,2,2‐trifluoroethyl vinyl ether
View Description Hide DescriptionThe low resolution microwave spectrum of gaseous 2,2,2‐trifluoroethyl vinyl ether displays an intense series of a‐type bands and a weak b‐type series which identify the species to have an s‐c i s‐planar configuration, τ (C=C–O–C) = 0° and τ (C–O–C–CF_{3}) = 180° [Fig. 1(b)]. This confirms the determination by Charles, Cullen, and Owen based on infrared evidence and analogy to unsubstituted ethyl vinyl ether. No other species is observed which excludes any significant population of an s‐t r a n s‐planar species, τ (C=C–O–C) = 180° and τ (C–O–C–CF_{3}) = 180° [Fig. 1(d)]. Any other species significantly present in the vapor would have to be relatively nonpolar, i.e., have τ (C–O–C–CF_{3}) < 90°.

Theoretical model for fluorescence in gases subject to continuous infrared excitation
View Description Hide DescriptionThe emission from gaseous molecular systems subject to continuous excitation in the infrared has been modeled on a computer. The effects of collisionally induced transitions are specifically included through adaptation of an existing analysis by Rapp. In its present form, the computer program is capable of determining the fluorescence of a single polyatomic gas as well as simulating quenching by a noble gas. Potentially, the program is capable of handling as many constituents, energy levels, and laser ’’pumps’’ as desired. Computations for a 20 level system require about one minute of computer time on a Univac 1108. Specific results are given for excitation at 10 W/cm^{2} by a CO_{2} laser for ozone, ammonia, and ethylene, each at 10 Torr and also for ethylene quenched by argon. We conclude that (1) the occurrence of fluorescence at frequencies a b o v e the laser frequency is a natural consequence of the collisional energy transfer mechanism, (2) states separated by small differences in energy or by small quantum number intervals are stongly coupled by collisions, (3) fluorescence from certain levels associated with strong absorption peaks is predicted to be weak, and (4) t r u e laser‐induced fluorescence is weak compared to simple thermal emission from the laser‐heated gas.

Field‐swept and frequency‐swept ELDOR and the manifestation of rapid‐passage effects
View Description Hide DescriptionField‐swept and frequency‐swept electron‐electron double resonance(ELDOR) have been quantitatively compared as a function of temperature and of modulation frequency (ω_{ m }). At ω_{ m } ? 1 kHz the ELDOR reduction factor for field‐swept and frequency‐swept experiments, R (H) and R (ν)_{ o }, respectively, show the normal inverse dependence on temperature.R (H) is about 2 to 4 times greater than R (ν)_{ o } for either the ΔM _{ I } = 1 or 2 transition of inhomogeneously broadened lines of a nitroxide radical. At 100 MHz modulation, ELDOR shows rapid‐passage effects. The maximum frequency‐swept signal lags the modulation field by 90° and tends to decrease with the decrease of temperature at low temperature. The results are interpreted in terms of present theories of ELDOR.

NMR study of CuAl_{2}: ^{27}Al, ^{63}Cu, and ^{65}Cu quadrupole and anisotropic shift interactions
View Description Hide DescriptionThe nuclear magnetic resonance spectra of ^{27}Al, ^{63}Cu, and ^{65}Cu have been studied in a polycrystalline specimen of the intermetallic compound CuAl_{2} of the C16 structure type at room temperature between 3 and 40 MHz. The ^{27}Al spectra yield to interpretation in terms of a completely anisotropicKnight shift and an asymmetric nuclear electric quadrupole interaction. The observed field separations of the several ^{27}Al quadrupolesatellite spectral features at fixed frequencies yield values of the lowest quadrupole frequency ν_{ Q } = 420.2(8) kHz and asymmetry parameter η = 0.413(9). The ^{27}Al satellite features also yield the three Knight shift parameters K _{ x } = 0.159(1) %, K _{ y } = 0.153(2) %, and K _{ z } = 0.148(10) %. Excellent agreement has also been obtained between observed ^{27}Al central line shape and the theoretical shapes of J. F. Baugher, H. M. Kriz, P. C. Taylor, and P. J. Bray, J. Magn. Reson. 3, 415 (l970). The ^{63}Cu and ^{65}Cu central lines exhibit a strong second and a fourth order quadrupole splitting with ν_{ Q }(^{63}Cu) = 7.54(17) MHz and ν_{ Q }(^{65}Cu) = 7.05(18) MHz. An analysis of the copperKnight shift is also presented.

Vibrational spectrum and force field of molybdenum hexafluoride
View Description Hide DescriptionInfrared and Raman spectra are reported for ^{92}MoF_{6} and ^{100}MoF_{6} vapor (isotopic purities ≳ 97%). The infrared‐active stretching fundamental ν_{3} exhibits partially resolved PQR structure with an asymmetric Q branch, from which the band origin ν_{0} can be estimated; the isotopic frequency shift for the band origin is Δν_{3} = 8.1±0.3 cm^{−1}. The shift of the Q branch maximum of the other F _{1u } fundamental is Δν_{4} = 3.0±0.4 cm^{−1}. With the aid of these two isotope shifts, the general quadratic symmetry and valence force constants of MoF_{6} are calculated. The primary valence force constants are f _{ r } = 4.94 and f _{α} ? 0.22 mdyn/Å. The force fields of MoF_{6}, WF_{6}, and UF_{6} are compared and discussed.

A quantum kinetic theory of moderately dense gases. II. Coupled integral equations
View Description Hide DescriptionThe triplet Wigner distribution function developed in a previous paper leads to a closed set of coupled integro–differential equations for the pair and singlet Wigner distribution functions. These equations are simplified by a perturbation technique which assumes that the distribution functions differ only slightly from their equilibrium forms. The equations are expanded and only terms at most linear in the gradients of the physical properties are retained. Separating the equations on the basis of a density expansion, a set of equations is obtained, the solution of which is sufficient to determine the first density corrections to the transport coefficients.

The partial‐wave self‐consistent‐field method for diatomic molecules: Computational formalism and results for small molecules
View Description Hide DescriptionA method for performing Hartree–Fock calculations on diatomic molecules using partial‐wave expansions in spheroidal coordinates is presented. This method makes no use of conventional basis set techniques but, instead, relies heavily on numerical integration. The computational formalism is described, and results of partial‐wave calculations are compared with results from the standard matrix method. The partial‐wave method is expected to be particularly useful for those expectation values which are difficult to compute with the matrix method. Although it is limited to diatomic molecules, the partial wave method is not limited to the Hartree–Fock level of approximation.