Index of content:
Volume 43, Issue 11, 01 December 1965
43(1965); http://dx.doi.org/10.1063/1.1696562View Description Hide Description
The vibrational spectrum of the ``perturbed'' molecules H11BBr2, H10BBr2, D11BBr2, and D10BBr2 were generated from the Green's function of the ``unperturbed'' boron tribromides 11BBr3 and 10BBr3. Effect of geometry changes imposed by replacing a halogen by hydrogen on the B 1 modes has been calculated and the results indicate a strong dependence on the BH and BD bond length. Using information provided by the calculations, we have observed and identified five of the six HBBr2 and DBBr2 fundamentals. The bands are reported in the following order (cm−1): H11BBr2, D11BBr2, and D10BBr2: ν1 (BBr symmetric stretch), 595, ___, 585, ___; ν2 (BH and BD stretch), 2622, 2622, 1960, ___; ν3 (BBr symmetric deformation), ≈180–190 cm−1, calculated values only; ν4 (BBr asymmetric stretch), 1036, 1048, 870, 901; ν5 (BH and BD in‐plane deformation), 772, ___, 656, ___; ν6 (out‐of‐plane deformation), 731, ___, 625?, ___. Blanks refer to unobserved bands. The calculated in‐plane frequencies have been brought into alignment with the observed frequencies by considering mass and geometry changes without altering the force field.
43(1965); http://dx.doi.org/10.1063/1.1696563View Description Hide Description
A theory is developed for exciton—phonon interactions in ionic molecular crystals showing triplet‐exciton magnetic resonance. By using a simple model for the phonon Hamiltonian it is shown that the triplet excitons tend to be localized, or ``self‐trapped,'' and to move in a diffusional manner.
The activation energy for diffusion is found to be essentially ½Λ, where Λ is the ``self‐energy'' of the triplet exciton due to exciton—phonon interaction. It is suggested that the anomalous excess activation energy for exciton—exciton spin‐exchange line broadening is ½Λ rather than the phonon‐coupled exciton—exciton repulsion suggested previously. It is also shown that exciton creation and annihilation with the absorption and emission of phonons has no significant effect on the excitonmagnetic resonancelinewidths at low temperatures.
Motions of Molecules in Condensed Systems. XIV. Polarized Infrared Spectra of Single Crystals of Diborane43(1965); http://dx.doi.org/10.1063/1.1696564View Description Hide Description
The polarized infrared spectra of single crystals of diborane as well as the spectra of the vapor and liquid are presented and analyzed. Evidence for the existence of three solid phases is discussed and the structure of the low‐temperature form partially elucidated. This phase is found to be monoclinic with two or more molecules per primitive unit cell located on sites of symmetry no higher than Ci . Confirmatory evidence for Lord and Nielsen's assignment of ν9≈950 cm−1 and Smith and Mills' assignment of ν7≈850 cm−1 is presented. It is suggested that the strong electronic absorptions in the not‐too‐far ultraviolet are of B 1u symmetry. Some indication of a special interaction in the solid state is found.
Thermodynamics of the Dimerization and Trimerization of Gaseous Tungsten Trioxide and Molybdenum Trioxide43(1965); http://dx.doi.org/10.1063/1.1696565View Description Hide Description
A mass‐spectrometric investigation of gaseous tungsten oxides has yielded the thermodynamic values ΔH°1800=−127 kcal/mole and ΔS°1800=−40±1 eu for 2 WO3(g)=W2O5(g), and ΔH°1800=−240 kcal/mole and ΔS°1800=−77±3 eu for 3 WO3(g)=W3O9(g). The experiments were conducted at reduced activity of WO3 to increase the relative proportion of monomer in the gas phase and permit simultaneous measurement of partial pressures of WO3, W2O6, and W3O9. The activity was reduced by dissolution of tungsten oxide in a molten silicate. Use of similar techniques in studies of gaseous molybdenum oxides yielded ΔH°1800=−112 kcal/mole for 2 MoO3(g)=Mo2O6(g) and ΔH°1800=−209 kcal/mole for 3 MoO3(g)=Mo3O9(g).
43(1965); http://dx.doi.org/10.1063/1.1696566View Description Hide Description
Sodium chloride single crystals with edge dimensions as large as 2.5 cm were grown from aqueous solutions.Solutions were purified by chemical precipitation of trace contaminants and filtration through membrane filters. Crystal defects were apparent in the initial stages of growth, but the outer portions of crystals were free of dislocations.F‐band coloration by x rays at room temperature was not linear and the ``first stage'' was either absent or nearly so. Transition from extrinsic to intrinsic dc conductivity occurred at 350°C with the extrinsic conductivity two orders of magnitude lower than for ``pure'' melt‐grown crystals.
43(1965); http://dx.doi.org/10.1063/1.1696567View Description Hide Description
The magnetic behavior of CeIn3, CePb3, and the series of compounds represented by the formula LnSn3 with Ln=Ce, Pr, Nd, Sm, and Gd has been investigated over the temperature range 2° to 300°K. Curie—Weiss behavior was observed in the upper portion of the temperature range for all compounds except SmSn3, and effective moments were in close agreement with those expected for the free tripositive ions. The behavior for SmSn3 is attributed to contribution of the first excited multiplet (J=7/2). Néel points were observed for CeIn3, PrSn3, NdSn3, and SmSn3. Deviations from Curie—Weiss behavior at low temperatures were observed in all cases and were ascribed to antiferromagnetic ordering, crystal‐field effects, and ionization of Ce in the Ce compounds into the quadripositive state. GdSn3 is complex in that it shows deviation from Curie—Weiss below 40°K but it has no susceptibility peak.
43(1965); http://dx.doi.org/10.1063/1.1696568View Description Hide Description
The photolysis of cyclo‐C4H8 and of cyclo‐C4H8—cyclo‐C4D8 mixtures has been investigated at 1470 and 1236 Å in the absence and presence of NO. In addition, a series of experiments carried out in which H2S was used as a free‐radical interceptor in the photolysis of cyclo‐C4D8 at 1236 Å. Approximately 90% of the observed products can be accounted for by the primary process: C4H8+hv→2C2H4. The extent to which the internally excited ethylene formed in this process will decompose further increases with diminishing pressure and wavelength.
The photolysis of cyclobutane has been investigated at energies above the ionization energy (I.E.=10.3 eV), using the argon resonance lines at 11.54 and 11.72 eV. It is shown that when NO is added to the system, cis‐2‐butene, trans‐2‐butene, and 1‐butene are major products. These butenes are, however, not produced when NO (I.E.=9.25 eV) is replaced by O2 (I.E.=12.1 eV) and are only formed in trace amounts when no additives are present. It is suggested that the parent ion acquires the olefinic structure and undergoes charge exchange with NO.
43(1965); http://dx.doi.org/10.1063/1.1696569View Description Hide Description
An attempt has been made to measure the zero‐field splitting parameter of the manganous ion in glassy and polycrystalline media from the relative intensities of the allowed transitions of the EPR hyperfine spectrum. A value of this parameter D=96±5 G was determined for manganous ion in glassy 12N HCl for T≤140°K, and of D=145 G in glassy methnol for T≤118°K. In the crystalline phase, the D term in both cases approximately doubled. A value of D intermediate between that of the glassy phase and the final crystalline phase was found for methanol between temperatures of 120° and 135°K.
43(1965); http://dx.doi.org/10.1063/1.1696570View Description Hide Description
The fluorescence lifetime and self‐quenching of anthracene vapor has been examined as a function of temperature. The fluorescence lifetime in the absence of quenching is found to be 5.7±0.2 nsec and is independent of temperature over the range 200° to 300°C. This value, together with published Stern—Volmer constants is used to determine the quenching probability for quenchers such as O2 and NO. Probabilities considerably less than unity are obtained.
The second‐order rate constant for self‐quenching of anthracene vapor is found to have a negative temperature coefficient. The previously reported negative coefficient found in the Stern—Volmer constant for self‐quenching can be explained by a temperature effect on the actual quenching process without resort to excimer decomposition.
Vapor‐phase absorption and fluorescence spectra are reported. The oscillator strength and the quantum yield calculated from the actual and natural lifetimes are compared with corresponding values for solution.
43(1965); http://dx.doi.org/10.1063/1.1696571View Description Hide Description
Studies on electron behavior in finite model systems provide information relevant to electrons in lattices. Approximate schemes for testing electron correlation may be tested by such model calculations. We report here the results of a study on the 1Γ1ground state of the H6 hexagonal ring, using a novel type of wavefunction containing both valence bond and molecular orbital components. The method has a number of significant advantages and gives results as good as those of the simple alternant molecular orbital (AMO) method. Possible implications of these results for molecular and solid‐state electron behavior are discussed briefly.
43(1965); http://dx.doi.org/10.1063/1.1696572View Description Hide Description
Vibrational excitation cross sections of diatomic molecules have been calculated using the Born approximation by including the polarization interaction between the incident electron and the molecule. Because of the divergence of the polarization potential at the origin, a cutoff procedure must be used. Four different cutoff methods have been examined, and the excitation cross sections are not very sensitive to the choice of the cutoff parameters. For the nonpolar molecules, inclusion of the polarization term increases the cross sections by an order of magnitude or more over the previous calculations, while in the case of the CO molecule, the contribution from the polarization term is comparable to that from the monopole—dipole interaction.
43(1965); http://dx.doi.org/10.1063/1.1696573View Description Hide Description
The optical density of the vapors at 860°C that are in equilibrium with Hg‐saturated, Se‐saturated, and congruently subliming HgSe(c) in the range 450° to 816°C has been measured between 2000 and 7000 Å. No indication of the species HgSe(g) is found. Assuming the optical density is the sum of those obtained with pure Se and pure Hg, the partial pressure of Hg(g), p Hg, and the total selenium pressure,P Σ, are determined along a substantial portion of the HgSe(c) three‐phase lines. Hg‐saturated HgSe(c) is in equilibrium with essentially pure Hg(l) up to 659°C and is at least 48.5 at. % Se. The total selenium pressure attains a maximum value of 0.62 atm for Se‐saturated HgSe near 722°C. The value of p Hg for congruently subliming HgSe(c) is close to that required by the value of P Σ and the published thermodynamics of selenium vapor. Between about 450° and 600°C the standard Gibbs free energies of formation for both Se‐saturated and congruently subliming HgSe(c) are obtained and are the same within experimental error. Taking an average, ΔGf °[Hg(g)+½Se2(g) = HgSe(c)] = −41.92+42.40(10−3)T kcal/mole. Extrapolating to 300°K, we obtain ΔHf ° = −10.8 kcal/mole and ΔSf ° = −4.99 eu/mole for the formation of HgSe(c) from Hg(l) and Se(c).
43(1965); http://dx.doi.org/10.1063/1.1696574View Description Hide Description
NMRrelaxation times have been determined as a function of temperature and composition for the protons of benzene adsorbed on charcoal. These relaxation times have been interpreted to give the following conclusions. The benzene molecule rotates more rapidly in a plane perpendicular to its sixfold axis than it does about other axes. The average enthalpy of activation for surfacediffusion lies in the range 5–7 kcal/mole. At any composition, surfacediffusion is characterized by a distribution of activation enthalpies with a standard deviation of about 1 kcal/mole. There are close to 1020 unpaired electronic spin/cm3. Some evidence for high‐energy adsorption sites is presented.
43(1965); http://dx.doi.org/10.1063/1.1696581View Description Hide Description
The terms of the general molecular vibration—rotation Hamiltonian that are of the sixth power in the angular‐momentum components are studied. Those terms which are expected to contribute to the allowed energies in fourth order of approximation in the Nielsen—Amat—Goldsmith formulation are compiled. Nonlinear triatomic molecules are discussed as a special case.
43(1965); http://dx.doi.org/10.1063/1.1696599View Description Hide Description
All coefficients of centrifugal distortion that are expected to contribute to the molecular vibration—rotation energies to fourth or lower orders of approximation are calculated for the nonlinear XYX molecule as functions of the fundamental molecular constants. The Nielsen—Amat—Goldsmith expansion of the vibration—rotation Hamiltonian forms the basis of this calculation.
Refractive Indices and Piezo‐optic Coefficients of Deuterium Oxide, Methanol, and Other Pure Liquids43(1965); http://dx.doi.org/10.1063/1.1696614View Description Hide Description
The piezo‐optic coefficients of deuterium oxide and of methanol and the refractive index of methanol have been determined over a range of temperatures up to 35°C. These results, and data available in the literature for benzene and a number of other pure liquids, have been examined in terms of an equation recently proposed for the dependence of refractive index of pure liquids on temperature and volume.
43(1965); http://dx.doi.org/10.1063/1.1696615View Description Hide Description
The Rayleigh ratio and the Cabannes factor for water, deuterium oxide, and a number of other pure liquids were determined at 25°C, at angles of observation ranging from 30° to 150°, at 546 and 436 mμ. It was found that the Rayleigh ratio of water is substantially lower than the values reported in the literature; the values found by us Ru 546(90)=0.865 10−6 cm−1, Ru 436(90)=2.32 10−6 cm−1 for H2O, and Ru 436(90)=0.843×10−6 cm−1, Ru 436(90)=2.30 10−6 cm−1 for D2O are in excellent agreement with the predictions of the Einstein density‐fluctuation theory, modified by the Cabannes anisotropy factor. The temperature dependence of the Rayleigh ratios for H2O and D2O is also satisfactorily predicted by the variation of the bulk parameters appearing in the Einstein—Cabannes equations. In contradistinction to such liquids as benzene, the Rayleigh ratios of H2O and D2O have a minimum, at about 22° and 28°C, respectively. No significant new information on the structure of water as a function of temperature can be derived from light‐scattering experiments.
43(1965); http://dx.doi.org/10.1063/1.1696616View Description Hide Description
The observations that β−1 (∂ lnf/∂P)T=B is a constant (of order unity, independent of temperature), and —γ−1 (∂ lnf/∂T)P=B+Cγ−1, lead to an equationwhich describes the refractive indexn of water between 0° and 60°C (at any given wavelength in the visible spectrum) to within a few digits in the seventh decimal. Here ρ is the density, β=(∂ lnρ/∂P)T and γ=—(∂ lnρ/∂T)P. The constant C=—(d lnf/∂T)V reflects the change in the structure of water at different temperatures. The temperature of maximum index [(∂n/∂T)P=0] corresponds to γ=—C/B, and the minimum in the Lorenz—Lorentz specific refraction (n 2−1)/(n 2+2)ρ corresponds to γ=C/(1—B). The values of the parameters A, B, and C are independent of pressure up to 1100 bar in the temperature range examined. It is suggested that refractive index, or dielectric,measurements at higher pressures and temperatures may yield further information on the structure of water in terms of specific models related to the variation of B and C with the variables of state.
43(1965); http://dx.doi.org/10.1063/1.1696617View Description Hide Description
The van der Waals, Cahn—Hilliard theory of interfacial tension is reformulated for a fluid in the neighborhood of its critical point. The reformulated theory becomes equivalent to the Ornstein—Zernike, Debye theory of molecular correlations when the interface thickness is identified with the correlation length. When account is taken of the nonclassical behavior of the compressibility and coexistence curve, the theory is found to be in good agreement with independently known facts in three‐dimensional systems, yet slightly but unambiguously wrong in two dimensions. When one of the hypotheses of the original theory is replaced by an alternative hypothesis, the resulting theory is found to be correct in both two and three dimensions.
43(1965); http://dx.doi.org/10.1063/1.1696618View Description Hide Description
A specific form is proposed for the equation of state of a fluid near its critical point. A function Φ(x, y) is introduced, with x a measure of the temperature and y of the density. Fluids obeying an equation of state of van der Waals type (``classical'' fluids) are characterized by Φ being a constant. It is suggested that in a real fluid Φ(x, y) is a homogeneous function of x and y, with a positive degree of homogeneity (Sec. 2). This leads to a nonclassical compressibility, the behavior of which is determined by the degree of homogeneity of Φ (Sec. 3). A previously derived relation connecting the degree of the critical isotherm, the degree of the coexistence curve, and the compressibility index, again follows, this time without the restrictive assumption of effective isochore linearity (Sec. 4). The locus in the temperature—density plane of the points of inflection in the pressure—density isotherms, as determined experimentally by Habgood and Schneider, is accounted for (Sec. 5). It is shown that if a certain combination of the compressibility and coexistence curve indices is an integer, then the constant‐volume specific heat on the critical isochore has a logarithmic singularity at the critical temperature with, in general, a superimposed finite discontinuity (Sec. 6).