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15 Jun 1971

Volume 54, Issue 12, pp. 4969-5446

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Interactions between Chemisorbed Species: H2 and CO on (100) Tungsten

John T. Yates and Theodore E. Madey

J. Chem. Phys. 54, 4969 (1971); http://dx.doi.org/10.1063/1.1674786 (10 pages) | Cited 37 times

Online Publication Date: 10 September 2003

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The interaction of CO with chemisorbed β‐hydrogen on the (100) plane of tungsten has been investigated using flash desorption and work function methods. CO efficiently displaces chemisorbed hydrogen from tungsten at temperatures near 300°K. This is a result of the CO‐induced lowering of the activation energy for H2 desorption, with the CO chemisorbing initially on sites other than those occupied by hydrogen. At ∼ 100°K, little CO‐induced displacement of chemisorbed hydrogen occurs, and the production of three weakly bound hydrogen states (designated ν‐hydrogen) is observed. Each ν state exhibits a range of desorption energies. During the CO‐induced surface conversion of β‐hydrogen to ν‐hydrogen, a decrease in work function is observed. All of this behavior is indicative of a major interaction between adsorbed CO and adsorbed hydrogen. A weakly‐bound molecular H2 state (δ‐hydrogen) may be populated by adsorption of H2 on top of partial CO layers on W(100) at ∼ 100°K. The sites adsorbing this molecular state of hydrogen are related to the presence of β‐CO species, and are probably the same sites which could adsorb α‐CO at higher CO coverages. In thermal desorption, the liberation of two discrete hydrogen binding states (β1andβ2) from pure hydrogen layers on W(100) is probably a result of coverage‐dependent interactions between identical adsorbed H‐atom species. No chemical differences are observed in this work between hydrogen species responsible for the two β‐hydrogen binding states. No catalytic production of H2CO, HCO, CH4, or C2H6 was observed in coadsorption of hydrogen and CO on W(100).

Microwave Spectrum, Vibration–Rotation Interaction, and Ring Puckering Vibration in Silacyclobutane and Silacyclobutane‐1,1‐d2

Wallace C. Pringle

J. Chem. Phys. 54, 4979 (1971); http://dx.doi.org/10.1063/1.1674787 (10 pages) | Cited 20 times

Online Publication Date: 10 September 2003

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The microwave spectrum of the molecules silacyclobutane and silacyclobutane‐1,1‐d2 has been observed between 8 and 55 GHz and the first four vibrational states assigned. The ground state rotational constants of the former molecule are 8815.75, 6289.00, and 4245.32 MHz. The first two inversionlike intervals of the ring puckering mode were determined to be 75.75 and 7790 MHz in the normal isotopic species; the corresponding intervals in the deuterated species are 43.06 and 4430 MHz. The effect of these frequencies on the determination of the potential energy for the ring puckering vibration was investigated. The ring configuration was shown to be significantly puckered in the ground state. The a component of the dipole moment was found to be nearly constant in the four vibrational states of the normal molecule, μa  =  0.4396D, while the out‐of‐plane transition moment between inversion doublets dropped rapidly as the inversion levels approached the top of the barrier.

Dielectric and Pressure Virial Coefficients of Imperfect Gases. III. CClF2H

H. Sutter and R. H. Cole

J. Chem. Phys. 54, 4988 (1971); http://dx.doi.org/10.1063/1.1674788 (2 pages) | Cited 3 times

Online Publication Date: 10 September 2003

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Second dielectric and pressure virial coefficients at 50°, 100°, and 150° are reported, together with low‐density dielectric results which give a dipole moment μ  =  1.48D and polarizability 5.3 × 10−24 cm3. The second virial data can be reasonably well fitted by a pair potential including a Lennard‐Jones radial function, polarizable dipole interactions, and an anisotropic shape factor proposed by Buckingham and Pople, but difficulties in consistently accounting for these and other results for polar molecules are pointed out.

Generalized Formula for Light Scattering of Optically Anisotropic Macromolecules

Yukiko Tagami

J. Chem. Phys. 54, 4990 (1971); http://dx.doi.org/10.1063/1.1674789 (21 pages) | Cited 9 times

Online Publication Date: 10 September 2003

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A general expression for the spectrum of scattered light from monodispersed, infinitely dilute solutions of optically anisotropic molecules is presented. Application is made to the particular cases of rigid‐rod macromolecules and long, flexible coils. The calculations involve extensive use of the theory of the rotation group. New results are obtained for the anisotropic part of the Fourier transform of the frequency spectrum of light scattered from optically anisotropic, rigid‐rod macromolecules. In the case of the random coil, Pecora's results are extended to more general scattering angles and the full range of molecular weight. Horn's formulas for the total scattering of rods and coils are reobtained.

Dipole Moments of Dimethylsiloxane Oligomers and Poly(dimethylsiloxane)

C. Sutton and J. E. Mark

J. Chem. Phys. 54, 5011 (1971); http://dx.doi.org/10.1063/1.1674790 (4 pages) | Cited 18 times

Online Publication Date: 10 September 2003

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Dielectric constants at 20, 30, 40, and 50°C have been determined for pure liquid dimethylsiloxane oligomers (CH3)3Si–[OSi(CH3)2]xOSi(CH3)3 having chain lengths corresponding to x  =  3,5,7,and9. Mean‐square dipole moments 〈μ2〉, calculated from the Onsager equation, are in good agreement with predicted values based on a rotational isomeric state model with neighbor dependence and chain conformational energies obtained in an independent analysis of the random‐coil dimensions of such chains. In addition, the observed temperature coefficients of 〈μ2〉 are in qualitative agreement with calculated results for the stated range of x. An experimental value of the ratio 〈μ2〉 / nm2 (where n is the number of bond dipoles, each of magnitude m) in the limit of large x is estimated from published data on the pure liquid polymer at 25°C. This tentative result is in fair agreement with theory; agreement is improved if the large specific solvent effect previously observed for this polymer is interpreted as being primarily due to the effect of the dielectric constant of the medium on the Coulombic contribution to the conformational energy.

Energy of Excess Electrons in Nonpolar Liquids by Photoelectric Work Function Measurements

R. A. Holroyd and Mark Allen

J. Chem. Phys. 54, 5014 (1971); http://dx.doi.org/10.1063/1.1674791 (8 pages) | Cited 44 times

Online Publication Date: 10 September 2003

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The energy of the excess electron state has been determined for six nonpolar liquids by a photoelectric injection method. The functional dependence of the photocurrents on wavelength is the same in the liquid as in the vacuum and work functions can be evaluated from Fowler plots. For several liquids the work functions are less than the vacuum value and these lowerings are characteristic for each liquid. For tetramethylsilane, neopentane, cyclopentane, and 2,2,4‐trimethylpentane the work functions are lowered by 0.62, 0.43, 0.28, and 0.18 eV, respectively. For n‐pentane and n‐hexane the work functions are close to the vacuum value. The magnitude of the lowering is correlated with the energy of the first electronic absorption band. A variation of electron mobility with the work function changes is noted and discussed relative to the trap theory. The photocurrents measured in the liquids increase with voltage, and at the voltage employed the currents in neopentane and tetramethylsilane are comparable to the vacuum photocurrent. The magnitude of the observed photocurrent depends on the distance the electrons penetrate.

Cascade Measurements of Thermal Transpiration for Ar, N2, CO, and the Helium Isotopes

W. L. Taylor and G. T. McConville

J. Chem. Phys. 54, 5022 (1971); http://dx.doi.org/10.1063/1.1674792 (5 pages)

Online Publication Date: 10 September 2003

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A new experimental technique was employed to measure the thermal transpiration effect (thermomolecular pressure difference) for Ar, N2, and CO between 77 and 97°K and for helium‐3 and helium‐4 between 4.2 and 7°K. A cascade device consisting of seven large tubes connected top to bottom with capillaries was partially immersed in either liquid nitrogen or liquid helium while a suitable temperature difference was applied by heating the upper ends of the tubes and capillaries. Small pressure differences were established in each of the capillaries and the cumulative effect was large enough to be accurately measured with a differential capacitance manometer. The modified Weber–Schmidt theory was used to analyze the data and represented the argon data quite well. Some difficulty, which was traced to the coefficient of the temperature gradient, was encountered in obtaining a satisfactory theoretical explanation of the results for the other gases. We concluded that either an incorrect or oversimplified treatment of the temperature jump distance probably caused the discrepancy. Until further elucidation of this term, which also contains the translational Eucken factor, is forthcoming, use of this model for obtaining rotational relaxation times from thermal transpiration measurements is probably of limited value.

Wave Propagation in Nematic Liquid Crystals

James D. Lee and A. Cemal Eringen

J. Chem. Phys. 54, 5027 (1971); http://dx.doi.org/10.1063/1.1674793 (8 pages) | Cited 26 times

Online Publication Date: 10 September 2003

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Basic laws of motion of micropolar continuum are presented, and the adequacy of applying micropolar theory to liquid crystals is indicated. A set of constitutive equations is derived for nematic liquid crystals. Wave propagation problems are solved, and it is shown that the theoretical analysis is in good agreement with the experimental data, which indicates the isotropy of the phase velocity of the longitudinal wave and the anisotropy of the damping coefficient. The coupling, although small, is shown to exist between longitudinal and rotational waves.

Coexistence Curves of CO2, N2O, and CClF3 in the Critical Region

J. M. H. Levelt Sengers, J. Straub, and M. Vicentini‐Missoni

J. Chem. Phys. 54, 5034 (1971); http://dx.doi.org/10.1063/1.1674794 (17 pages) | Cited 58 times

Online Publication Date: 10 September 2003

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The coexistence curves of CO2, N2O, and CClF3 are analyzed in the critical region. The curves were obtained by refractive index measurements which are virtually free of gravity effects and contain much detail near Tc. After proper weight assignment, it is established that the top of the coexistence curve is asymptotically symmetric: ρ±  =  ρc ± Btβ; that the exponent β is independent of the range, varies little from substance to substance, and is insensitive to impurities; and that the data are in agreement with the law of the rectilinear diameter. “Best” values for β,B, and for the slope of the diameter are presented. An analysis of earlier coexistence curves for CO2 and N2O, including a weight assignment, is presented; there is agreement between the older and newer data.

Study of Self‐Avoiding Walks with Excluded First Nearest Neighbors

Mahadevappa Kumbar and Stanley Windwer

J. Chem. Phys. 54, 5051 (1971); http://dx.doi.org/10.1063/1.1674795 (7 pages) | Cited 10 times

Online Publication Date: 10 September 2003

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A study of the direct enumeration of self‐avoiding walks on the tetrahedral and four choice cubic lattices with first nearest neighbors excluded is presented. Data is given for the mean square end‐to‐end distance, the mean square radius of gyration, the number of returns to the origin, the mean square radius of gyration of the rings, and probability calculations. The results are interpreted in light of the theories of excluded volume.

Ultraviolet Spectra of Alkali Halides in Inert Matrices

Michael Oppenheimer and R. Stephen Berry

J. Chem. Phys. 54, 5058 (1971); http://dx.doi.org/10.1063/1.1674796 (16 pages) | Cited 24 times

Online Publication Date: 10 September 2003

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Ultraviolet absorption spectra have been observed for some of the alkali halides in several inert matrices at wavelengths between 2000 and 4500 Å. The matrix‐isolated molecules exhibit simpler spectra than those in the gas phase, permitting vibrational analysis. The LiI spectrum contains a strong absorption showing at least nine peaks with a mean spacing of 363 cm−1 in argon and 367 cm−1 in krypton, at wavelengths below 2900 Å. In addition, LiI exhibits one broad structureless peak near 3500 Å in both matrices. The NaI spectrum shows a weak absorption with as many as five peaks just below 4000 Å. The mean spacings are 166 cm−1 in argon, 129 cm−1 in kyrpton, and 141 cm−1 in nitrogen. NaBr exhibits a strong absorption at wavelengths below 3100 Å showing as many as 16 peaks with mean spacings of 453 cm−1 in argon, 529 cm−1 in krypton, and 472 cm−1 in nitrogen. LiBr, KI, NaCl, and RbBr show no absorption in this region at similar concentrations. The observed absorption peaks are interpreted as arising from transitions between the ground states (1Σ+) and different vibronic levels of the first excited 0+ bound states of these molecules. The structureless peak near 3500 Å in LiI probably represents a transition to a purely repulsive state. The appearance of absorption in LiI in a region where none existed in the gas phase is attributed to a change in Franck–Condon factors induced by the matrix. The results are interpreted in terms of a charge transfer model of molecular absorption and support the interpretation of adiabatic behavior when the ionic and atomic potential curves cross at a relatively short internuclear separation; the smaller the internuclear distance at the crossing, the more likely the noncrossing rule is obeyed. The unexpected appearance of discrete structure in the NaBr spectrum is discussed in terms of matrix‐induced charge transfer.

Pulse Radiolysis Studies of Acetone Solutions of Biphenyl and Anthracene: Formation of Ions and Excited Singlet State

Shigeyoshi Arai, Akira Kira, and Masashi Imamura

J. Chem. Phys. 54, 5073 (1971); http://dx.doi.org/10.1063/1.1674797 (9 pages) | Cited 2 times

Online Publication Date: 10 September 2003

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The radiolysis of acetone solutions of biphenyl and anthracene has been studied by means of the microsecond‐pulse technique. Transient spectra, as well as the effects of ion scavengers, show the presence of both positive and negative ions of biphenyl and anthracene in irradiated solutions. Assuming that molar extinction coefficients of positive and negative ions are identical, one can determine the yields of ions to be G(biphenyl++biphenyl−)  =  1.1at0.69M, and G(anthracene++anthracene−)  =  1.5 at concentrations above 5 × 10−3M. Decay kinetics of aromatic ions fit the second‐order rate law, which can be ascribed to the charge neutralization reaction between the positive and negative ions; rate constants are found to be k(biphenyl++biphenyl−)  =  3.9 × 1011M−1⋅sec−1 and k(anthracene++anthracene−)  =  2.6 × 1011M−1⋅sec−1. The light emitted upon irradiation of acetone solutions of anthracene is anthracene fluorescence; a part of fluorescence is delayed. The relationship between the intensity of fluorescence and the concentration, as well as the comparison of the energy level of anthracene with that of acetone, suggests the occurrence of energy transfer from the first excited singlet state of acetone to anthracene. The rate constant of energy transfer is estimated to be 6 × 1010M−1⋅sec−1. The yield of excited singlet anthracene is determined to be G(1anthracene)  =  0.31 at the concentration of 1.0 × 10−2M. Addition of various ion scavengers to acetone solutions of anthracene reduces the yields of both excited singlet and triplet anthracene. These results may be explained by the reactions of ion scavengers with the primary ions in spurs. Thus these reactions result in the decreases in the yields of excited acetone which may be produced by geminate recombination.

Positron Annihilation in Rare‐Earth Hydrides

M. P. Chouinard and D. R. Gustafson

J. Chem. Phys. 54, 5082 (1971); http://dx.doi.org/10.1063/1.1674798 (3 pages) | Cited 6 times

Online Publication Date: 10 September 2003

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Analysis of the angular correlations of positron‐annihilation radiation from erbium, gadolinium, holmium, and ytterbium hydrides indicates that the electronic structure of these materials is basically metallic in nature. As was previously found in the case of cerium hydride, the angular correlations were found to be broader than those from their parent metals by amounts which were consistent with the assumption that the rare‐earth metal valence electrons and the electrons of the hydrogen atoms are all contributing to metallic bonding.

ESR Measurement of the Reaction between H Atoms and N2H4

P. D. Francis and A. R. Jones

J. Chem. Phys. 54, 5085 (1971); http://dx.doi.org/10.1063/1.1674799 (4 pages) | Cited 7 times

Online Publication Date: 10 September 2003

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An ESR atom detection method has been used to measure the rate of the reaction of H atoms with hydrazine, over the temperature interval 300–540°K. From the Arrhenius plot the specific rate constant for the reaction H+N2H4☒N2H3+H2 was found to be (1.5 ± 0.3) × 1012exp[(− 1300 ± 200) / RT] in units of cubic centimeters mole−1⋅second−1. Although the stoichiometry of the reaction was not determined a possible reaction sequence is deduced from a mass spectrometric analysis of the products. The value of this rate constant is in good agreement with the rate constant of 1.7 × 1011cm3 mole−1⋅sec−1 measured by Gehring et al. over the temperature interval 251–315°K and the activation energy of 1300 ± 200 cal agrees fairly closely with the 2000 cal measured by Schiavello and Volpi.

Phase Equilibria in Fluid Mixtures at High Pressures: The Neon–Argon System

William B. Streett and James L. E. Hill

J. Chem. Phys. 54, 5088 (1971); http://dx.doi.org/10.1063/1.1674800 (7 pages) | Cited 14 times

Online Publication Date: 10 September 2003

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Results of phase equilibria studies are reported for the neon–argon system at temperatures from 87.34 to 123.83°K and pressures to 3880 atm. These results, combined with those of earlier studies, provide a complete description, partly quantitative and partly qualitative, of the lines and surfaces comprising an important segment of the three‐dimensional, pressure–temperature–composition, phase diagram for neon–argon. A three‐dimensional sketch of the diagram is used to show the important phase boundary lines and the qualitative features in the region of transition from fluid to solid at high pressures. The relevance of these studies to problems of deep atmosphere structures in the outer planets is briefly discussed.

Dipole Moment of the First Excited π* ← π State of Styrene

Harriet Parker and John R. Lombardi

J. Chem. Phys. 54, 5095 (1971); http://dx.doi.org/10.1063/1.1674801 (2 pages) | Cited 4 times

Online Publication Date: 10 September 2003

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The Stark effect is observed in the rotational fine structure of the 0–0 band of the lowest lying π* ← π transition of styrene. This resulted in a ∣ Δμ ∣  =  0.13D from the ground state. It is shown that the smallness of the change in dipole moment is consistent with the π* ← π transitions in similar substituted benzenes studied in this laboratory.

Intermolecular Coupling of Water Molecules in Copper Chloride Dihydrate

R. A. Fifer and J. Schiffer

J. Chem. Phys. 54, 5097 (1971); http://dx.doi.org/10.1063/1.1674802 (6 pages) | Cited 9 times

Online Publication Date: 10 September 2003

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Polarized and unpolarized infrared spectra have been obtained for copper chloride dihydrate samples, containing low mole ratios of H2O and D2O. The positions of the absorption bands of these samples are shown to differ from those of samples, containing high percentages of these same molecules. The spectral differences noted are shown to be related to the interactions between water molecules in the solid. Furthermore, a criterion is provided for sensing a variation of hydrogen bond strength with deuteration, and the origin of bands in the ν2 overtone regions of H2O, D2O, and HOD is also considered.

Collision Mechanism Leading to the Formation of NO+ in O+☒N2 Collisions

J. J. Leventhal

J. Chem. Phys. 54, 5102 (1971); http://dx.doi.org/10.1063/1.1674803 (2 pages) | Cited 13 times

Online Publication Date: 10 September 2003

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The energetics of the reaction O++N2→NO++N have been investigated in the laboratory for the kinetic‐energy range 3–12 eV. The results indicate that the product NO+ ion is formed with excess internal energy. The heats of reaction obtained are consistent with formation of NO+ in a stripping‐type collision process above about 4‐eV incident ion energy.

Reflection and Absorption Spectra of the Higher π → π* Transitions of Solid Benzene

M. Brith, R. Lubart, and I. T. Steinberger

J. Chem. Phys. 54, 5104 (1971); http://dx.doi.org/10.1063/1.1674804 (10 pages) | Cited 18 times

Online Publication Date: 10 September 2003

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Reflection and absorption spectra of C6H6 and C6D6 were obtained at temperatures near 4.2°K, in the wavelength region of 1700Å < λ < 2700Å. The samples were prepared as films on a cold window. Wellresolved spectra were obtained with both methods. The reflectance data (obtained at near normal incidence) were converted, using a combination of Kramers–Kronig and curve‐fitting techniques, to yield the real and imaginary parts of the dielectric constant as well as the refractive index and the absorption coefficient. The results were in good correspondence with those obtained by direct absorption measurements. The absorption bands observed below 2200 Å belong to the electronic transitions 1A1g → 1B1u(math < 50000cm−1,weaksystem)and1A1g → 1E1u(math > 50000cm−1,strongsystem), respectively. The possible assignments of the individual bands within each system are discussed on the basis of the present results, taking into account conclusions derived from spectra of benzene embedded in rare‐gas matrices. Semiempirical calculations are carried out on the shift of the different progressions within the 1A1g → 1B1u transition in the solid.

Shape Resonances and Rotationally Predissociating Levels: The Atomic Collision Time‐Delay Functions and Quasibound Level Properties of H2(X1Σg+)

Robert J. Le Roy and Richard B. Bernstein

J. Chem. Phys. 54, 5114 (1971); http://dx.doi.org/10.1063/1.1674805 (13 pages) | Cited 108 times

Online Publication Date: 10 September 2003

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The energy dependence of the collisional time‐delay function has been computed for H(1S) atoms interacting via the ab initio H2(X1Σg+) potential. Peaks in this function determine the scattering resonance energies Er and widths Γ, and the lifetimes for each of the corresponding quasibound vibrational–rotational levels. Small differences are found between these Er and Γ, and the values obtained by a “maximum internal amplitude” approach (intended to characterize the spectroscopically observable predissociating levels). Approximate procedures for rapid, accurate numerical evaluation of Er are appraised; a new outer‐boundary‐condition criterion for resonances leads to the best agreement with the exact results. Also, a primitive WKB procedure yields Γ's of usable accuracy. For ground‐state H2, HD, and D2 the onset of line broadening due to centrifugal barrier penetration is found to occur at energies some hundreds of cm−1 below the locus of barrier maxima. The predissociation method of estimating long‐range interatomic forces therefore cannot be expected to yield valid results for hydridic diatomics.

Kinetic Energy of Ions Produced by Photoionization of HF and F2

W. A. Chupka and J. Berkowitz

J. Chem. Phys. 54, 5126 (1971); http://dx.doi.org/10.1063/1.1674806 (7 pages) | Cited 16 times

Online Publication Date: 10 September 2003

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Kinetic energies were measured for the positive ions produced in the ion‐pair formation processes
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at a number of wavelengths in the region of the thresholds for these processes. The measurements prove that D0(HF) is either 5.91 ± 0.02 or 5.87 ± 0.02 eV, depending on the value of the electron affinity of the F atom. Since the latter value for D0(HF) is in good agreement with other more precise determinations, these results support the value E.A.(F) = 3.400 eV. The kinetic energy measurements also prove that D0(F2)  =  1.58 ± 0.03eV. The measurements strongly suggest that the F+ ions from F2 are produced not only in the 3P2 ground state but also in 3P1 and/or 3P0 states as well. At higher photon energies, the F+ ions from F2 are also produced in the 1D state. Dissociation and predissociation processes of the molecular ions are discussed.

Detection of Iodine Atoms by an Atomic Fluorescence Technique: Application to Study of Diffusion and Wall Recombination

Leo Brewer and Joel B. Tellinghuisen

J. Chem. Phys. 54, 5133 (1971); http://dx.doi.org/10.1063/1.1674807 (6 pages) | Cited 12 times

Online Publication Date: 10 September 2003

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An atomic fluorescence method has been used to detect iodine atoms in concentrations lower than 1010/cm3. The heart of the apparatus is a “solar‐blind” photomultiplier having practically no sensitivity to radiation of wavelengths longer than 2500 Å, but with good response in the region of the two strong iodine atomic resonance lines near 1800 Å. In combination with a small photolysis flashlamp, the system is used to study recombination of ground‐state (2P3/2) iodine atoms in pure I2 vapor. At pressures below 0.2 torr the removal of atoms is seen to be predominantly a first‐order diffusion‐controlled process, characterized by a wall sticking probability of 0.040–0.065 and a binary diffusion cross section of 100 Å2. The reaction I+I2→I3 is found to be of minor significance in this study; a rough estimate indicates that fewer than one in 104 I☒I2 collisions lead to the formation of I3.

Magnetic Circular Dichroism of Al2O3:V(III); the 3A2 → 3T2 No‐Phonon Band and Possible 3A2 → 1A1 Ion Pair Transitions

J. A. Spencer

J. Chem. Phys. 54, 5139 (1971); http://dx.doi.org/10.1063/1.1674808 (5 pages) | Cited 4 times

Online Publication Date: 10 September 2003

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The magnetic circular dichroism and axial absorption spectra of the 6295‐Å 3A2 → 3T2 no‐phonon band in V(III)‐doped corundum have been measured as a function of temperature and field under high resolution. The relatively concentrated crystal [∼ 1.5 mole% V(III)] used gave only a broad, featureless absorption. The MCD, however, clearly revealed the expected Ham effect fine structure first resolved in absorption studies of dilute samples by Scott and Sturge. Use of the nonuniform dynamic Jahn–Teller quenching model permitted direct prediction of the MCD band shape. Good agreement with both absorption and MCD data was found when component line half‐widths were assumed to be ∼ 3 cm−1. In addition, sharp MCD satellite structure has been observed in the 3A2 → 1A1 transition. This is attributed to V(III) ion pair interactions in the corundum lattice.

Linear Viscoelasticity in Entangling Polymer Systems

William W. Graessley

J. Chem. Phys. 54, 5143 (1971); http://dx.doi.org/10.1063/1.1674809 (15 pages) | Cited 39 times

Online Publication Date: 10 September 2003

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Molecular models for polymers, such as the Rouse model, yield steady state shear compliance Je and terminal relaxation time τmax of the forms Je,∝M / cRT and τmax∝ηM / cRT, in which c is the polymer concentration, M is the molecular weight, and η is the zero shear viscosity. Recent experiments have shown that in entangling systems these forms change, becoming Je∝ / c2RT and τmax∝η / c2RT. It is proposed here that the changes are a consequence of the highly uncorrelated nature of entanglement drag interactions, as opposed to the smoothly varying interactions inherent in the Rouse analysis. A new model is proposed to account for this difference. The resulting predictions are consistent with the experimental forms of G′(ω),G″(ω), and G(t) as well as those of θmax and Je. The molecular weight between entanglement points, Me, was estimated from several different viscoelastic properties of undiluted polystyrene. With the exception of that from Je, similar values of Me were obtained in all cases. Calculations of Je for mixtures of two molecular weights MA and MB were made with and without the assumption of an uniformly effective drage coefficient. The former predicted an unrealistically weak dependence on polydispersity; the latter agreed with experimental results for cases in which MB / MA was in the range of 2, but overestimated the effects of polydispersity when the difference in molecular weights was greater.

Measurement of the Rotational Diffusion Coefficient of Lysozyme by Depolarized Light Scattering: Configuration of Lysozyme in Solution

Stuart B. Dubin, Noel A. Clark, and George B. Benedek

J. Chem. Phys. 54, 5158 (1971); http://dx.doi.org/10.1063/1.1674810 (7 pages) | Cited 80 times

Online Publication Date: 10 September 2003

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We have determined the rotational diffusion coefficient (DR) of lysozyme from the spectrum of the depolarized light scattered by this protein. The sample was illuminated by a single‐frequency He☒Ne laser and the spectrum of the depolarized scattered light was analyzed using a high‐resolution spherical Fabry–Perot interferometer. We have determined D20,wR to be (16.7 ± 0.8) × 106/sec at 15% protein concentration. We have, in addition, measured the translational diffusion coefficient (DT) of lysozyme by employing the now‐familiar techniques of optical mixing spectroscopy to observe the spectrum of the polarized light scattered by the protein. We find D20,wT to be (10.6 ± 0.1) × 10−7 cm2/sec, independent of protein concentration between 1% and 15%. Our results for DR and DT, combined with Perrin's expressions for the rotational and translational diffusion coefficients of ellipsoids, indicate that lysozyme in solution is hydrodynamically equivalent to a prolate ellipsoid of revolution with major and minor axes of (55 ± 1) Å and (33 ± 1) Å, respectively. These dimensions are shown to be consistent with an unsolvated molecule of dimensions (48 ± 1) Å by (26 ± 0.8) Å covered with a shell of solvent about 3.5 Å thick. The hydrodynamically equivalent oblate ellipsoid of revolution is shown to be inconsistent with other physical properties of the protein. We have, in addition, determined from our diffusion coefficient results the degree of solvation of lysozyme and find this to be (0.60 ± 0.03) g of solvent per gram of (dry) protein. Finally, we present a novel and unambiguous method for the determination of the depolarization ratio (ρυ) of small molecules and find ρυ  =  (1.4 ± 0.1) × 10−3 for lysozyme. This value of the depolarization ratio is shown to imply an intrinsic anisotropy in the polarizability of lysozyme.
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