Volume 42, Issue 4, 15 February 1965
Index of content:
42(1965); http://dx.doi.org/10.1063/1.1696092View Description Hide Description
The molecular structures of gaseous B2H6 and B2D6 were determined by the sector‐microphotometer method of electron diffraction. Structural parameters and standard errors for B2H6 were rg (B–B) = 1.7750±0.0035 Å; rg (B–H)term = 1.1960+0.008, −0.006 Å; rg (B–H)bridge = 1.3393+0.002, −0.006 Å; and <BBH t = 120.5±0.9°. Corresponding parameters for B2D6 were 1.7712±0.0035 Å; 1.1980+0.006, −0.005 Å; 1.3335+0.002, −0.004 Å; and 119.3±0.9°. Root‐mean‐square amplitudes of vibration were also determined. Bridge hydrogen amplitudes exceeded terminal hydrogen amplitudes in conformity with an extension of Badger's rule. Hydrogen amplitudes exceeded deuterium amplitudes in accordance with the difference in reduced mass. Isotope effects on bond lengths were obscured by an unanticipated experimental flaw which influenced results by 1 or 2 parts per thousand.
42(1965); http://dx.doi.org/10.1063/1.1696093View Description Hide Description
Approximate expressions for the viscosity and thermal conductivity of monatomic gas mixtures have been derived from the rigorous kinetic theory formulas for binary mixtures. The approximations, which are of the Sutherland—Wassiljewa form, are compared with rigorous calculations for binary and ternary mixtures of the noble gases. These approximations are very accurate: maximum errors are three parts in ten thousand for viscosity (He–Ne–Ar at 20°C) and seven parts in ten thousand for thermal conductivity (He–Kr–Xe at 29°C); the corresponding root‐mean‐square errors for all the calculations are 0.015% for viscosity and 0.036% for thermal conductivity. Thus, these approximations seem adequate for all practical applications; indeed, if greater accuracy is required, the higher Chapman—Enskog approximations should be considered as well.
42(1965); http://dx.doi.org/10.1063/1.1696094View Description Hide Description
The structures of gaseous H2O and D2O were investigated using the sector‐microphotometer method of electron diffraction. Structural parameters and estimated standard errors for H2O included the mean bond length rg (OH) = 0.976±0.003 Å, rms amplitude lg (OH) = 0.0666±0.003 Å, and mean HOH angle α g = 107.2°±3°. Corresponding values for D2O were 0.970±0.0025 Å, 0.564±0.002 Å, and 104.2°±3°. Results agreed with spectroscopic values to within the larger limits of error of the diffraction results. Analyses were based on the radial distribution method and on a new automatic ``smooth‐background'' method. Observed atomic intensities deviated significantly at small scattering angles from intensities calculated using the conventional approximations. The resulting discrepancy led to a greater uncertainty in structural parameters than did the random experimental intensity errors. This almost universally neglected source of error is briefly discussed.
42(1965); http://dx.doi.org/10.1063/1.1696095View Description Hide Description
The variational theory of chemical reactions has been extended to provide estimates for reaction rates across surfaces parallel to the tangent of the rotational barrier and for the case of three‐body recombination in the presence of a catalyst whose dominant interaction with the recombining atoms is attractive.
42(1965); http://dx.doi.org/10.1063/1.1696096View Description Hide Description
High‐resolution microwave spectroscopy has been used to study the spin—rotation interaction of the two 19F nuclei in OF2. Several transitions were resolved from J=9 to J=26 where half‐widths at half‐heights of 12–15 kc/sec were obtained. The chemical shift of the 19F nucleus was also measured. The value of the magnetic shielding relative to F in CFCl3 is −249±1 ppm. The paramagnetic component of the shielding in OF2(σOF2 p ) is then related to the paramagnetic shielding in F2(σF2 p ) yielding σOF2 p —σF2 p =205×10−6. σF2 p can be computed from the known spin—rotation constant in F2 yielding σOF2 p =−569×10−6. This data is then combined with the microwave data on the spin—rotation interaction in OF2 to yield Maa =−42kc/sec, Mbb =−22 kc/sec, and Mcc =−49 kc/sec.
42(1965); http://dx.doi.org/10.1063/1.1696097View Description Hide Description
A study has been made of alkane‐type chains of Nbonds on a diamond lattice with atom overlap (both carbon and hydrogen) excluded. An exact enumeration of all allowed chains and their end‐to‐end distances and numbers of transbonds has been made for chains of up to 18 bonds. The allowed polymethylene rings have also been determined through 18 bonds. The results have been compared with those for the simple model in which only gauche—gauche prime sequences are disallowed. The number of configurations and the fraction of transbonds do not appear to differ significantly for the two models. Definite conclusions regarding the mean‐square end‐to‐end distance cannot be drawn from the enumerations reported here, but the two models probably are not in serious disagreement if N is not more than a few thousand.
42(1965); http://dx.doi.org/10.1063/1.1696098View Description Hide Description
The mobility of substitutional boron in single crystals of graphite has been measured by an etch‐decoration method, which makes it possible to locate individual boron atoms by electron microscopy. The mobility of boron is not very sensitive to the boron concentration, for B/C values between 10−2 and 10−8. The diffusion constants measured between 1700° and 2400°C are 6320 exp(−157 000/RT) for motion parallel to the layer planes and 7.1 exp(−153 000/RT) for motion perpendicular to the layers.
42(1965); http://dx.doi.org/10.1063/1.1696099View Description Hide Description
The infrared‐absorption spectra of 18 haloalkanes have been studied and the carbon—halogen bending vibration assigned for each molecule. The effect of rotational isomerism on this vibration was determined. The low‐frequency torsional modes have also been assigned. Solid‐state studies contributed to the support of the assignments and made it possible to determine rotational effects.
42(1965); http://dx.doi.org/10.1063/1.1696100View Description Hide Description
The absorptionspectrum of the pink afterglow of nitrogen in the 1080–1900‐Å region has been studied in an attempt to determine the active substances responsible for the production of the afterglow. The Lewis—Rayleigh afterglow, which appears in conjunction with the pink afterglow was also studied, and various features of the two afterglows are compared. The afterglows were studied at room temperature and at liquid‐nitrogen temperature. Comparison was made between the emission spectra of an ordinary discharge of nitrogen and the absorptionspectrum of the pink afterglow.
Most of the absorption bands observed in the pink afterglow were found to originate in the higher vibration levels of the ground state of N2, which indicates the presence of highly vibrating ground‐state molecules in activated nitrogen. The upper states of the absorption bands were determined to be the b, b′, g, j, m, o, q, r, and possibly other states. Bass' results are discussed and compared with those of the present work.
Nuclear Magnetic Double Resonance. Transmission of Modulation Information through the Nuclear Spin—Spin Coupling42(1965); http://dx.doi.org/10.1063/1.1696101View Description Hide Description
Nuclear magnetic double‐resonance experiments are described in which the perturbing radio‐frequency field is amplitude or frequency modulated, causing the precessing moment vector of a group of nuclear spins (S) to mutate at a low audio frequency. Through the action of the nuclear spin—spin coupling this modulation information is transmitted to a second group of nuclear spins (I) in the form of a selective modulation of the local magnetic field, which may be used to excite a magnetic resonance ``sideband'' signal. A general theory is given, covering both the present modulation transfer experiments and also the removal of residual splitting in spin decoupling experiments by modulation of the irradiating frequency. Numerical calculations to predict the frequencies and relative intensities of the observed signals have been carried out on a high‐speed computer and presented in graphical form. In all the experimental measurements the S spins are 13C nuclei in their natural abundance, and the I spins are coupled protons, examined at 60 Mc/sec. Good agreement with the theoretical predictions has been obtained in a detailed study of the AX spin system of chloroform. The application of modulation transfer experiments to the indirect measurement of 13C chemical shifts has been illustrated for chloroform, acetic acid, and diethyl ether. The technique has also been used to discriminate between signals from molecules with different isotopic constitution; for example, in acetic acid all proton signals from molecules not containing 13C may be suppressed, clearly revealing the small long‐range spin coupling between carboxyl 13C and the methyl protons. In more complex molecules the theoretical frequency and intensity diagrams can be built up by superposition of the diagrams for simpler systems. Often the relative signs of certain coupling constants may be inferred by inspection of the modulated double resonance spectrum—the relative signs of J(13CH), J(13CCH), and J(HH) in diethyl ether have been determined in this way.
42(1965); http://dx.doi.org/10.1063/1.1696102View Description Hide Description
The infrared and Raman spectra of KrF2 vapor have been obtained. They clearly indicate a linear and symmetric molecule with the symmetric stretching frequency at 449 cm−1, the asymmetric stretching frequency at 588 cm−1, and the bending frequency at 232.6 cm−1. The force constants are fr =2.46, frr =−0.20, and f α=0.21 mdyn/Å. The negative bond—bond interaction constant can be explained neither by valence bond models nor by a simple molecular orbital model.
Luminescence of Solids Excited by Surface Recombination of Atoms. IV. Mechanisms of Excitation and Luminescence42(1965); http://dx.doi.org/10.1063/1.1696103View Description Hide Description
This experimental study examines processes of excitation and luminescence of solids subjected to surface‐catalyzed atom recombination and to photoexcitation. Fourteen lumophors were tested for their luminescence response to nitrogen atoms, to oxygen atoms, and to photoexcitation. The results indicate that photoexcitation processes provide inadequate criteria for atom‐excited luminescence. For three of these lumophors, measurements were also made of the atom recombination coefficient and the heat flux due to atom recombination.
The greater response of CaO lumophors to atom excitation than to photoexcitation is attributed to luminescence centers at the surface which permit close coupling with surface‐recombining atoms. The decrease in luminescence response of CaO caused by exposure to oxygen atoms is attributed to electron transfer from the luminescence centers to the adsorbed oxygen species, and to reaction of adsorbed oxygen species with oxygen‐ion vacancies. Nitrogen atoms, however, cause no decrease in luminescence and are even able to restore the luminescence response of a lumophor previously exposed to oxygen atoms.
Mechanisms of excitation of CaO lumophors and of reduced luminescence response to oxygen atoms are discussed in terms of an energy‐band model.
Luminescence of Solids Excited by Surface Recombination of Atoms. V. Quantitative Dependence of Luminescence Response on Oxygen‐ and Nitrogen‐Atom Densities42(1965); http://dx.doi.org/10.1063/1.1696104View Description Hide Description
The quantitative relationship between luminescence response and atom density was investigated for the lumophor CaO:Sb:Cl excited by surface recombination of nitrogen or oxygen atoms. Atom densities were determined by electron spin resonance employing molecular oxygen as a standard of spin density.
Luminescence response is essentially linearly proportional to nitrogen‐atom density for total nitrogen pressures of 0.007 to 0.29 Torr. Luminescence response to oxygen atoms, determined at a total oxygen pressure of 0.016 Torr, is linearly proportional to oxygen‐atom density when account is taken of the decreased response caused by oxygen‐atom exposure. However, for certain operating conditions of the discharge, enhanced luminescence response occurred at oxygen‐atom densities less than 3×1013 atom/cc; also, no significant decrease of response occurred with exposure to oxygen atoms at densities less than 0.4×1013 atom/cc. The enhanced response and the absence of a decreased response upon continued exposure are attributed to electronically excited oxygen species, probably O(1 D) or O(1 S). Equal densities of nitrogen and oxygen atoms produced about the same luminescence response.
42(1965); http://dx.doi.org/10.1063/1.1696105View Description Hide Description
The absolute infrared absorption intensities of the fundamental vibrations of GeH4 and GeD4 have been measured using nitrogen pressures of 900 psi to broaden the individual rotational lines. Alternative sets of the derivatives of the dipole moment with respect to designated stretching and bending symmetry coordinates are obtained. A least‐squares analysis of the isotopic data indicates that the positive ratio of the two dipole‐moment derivatives is the preferred solution.
42(1965); http://dx.doi.org/10.1063/1.1696106View Description Hide Description
A single‐shutter drift tube, together with a hot‐filament‐type ion source, was used to investigate the conversion and drift properties of ions in nitrogen. Three different species of ions, namely N+, N2 +, and N3 +, were detected, and their drift velocities in nitrogen were measured over a range of E/p 0 (ratio of electric field strength to pressure) from 10 to 200 V/(cm·mm Hg). N2 + was found to undergo time‐rate‐controlled conversion to N3 +. Theoretical solutions of such a drift transient have been obtained and were compared with experiment. The attachment frequency α was deduced from the transient form, and its dependence on pressure and E/p 0measured. It was found that .
42(1965); http://dx.doi.org/10.1063/1.1696107View Description Hide Description
A description is given of the mercury‐sensitized photolysis of propylene and mixtures of propylene with H2, D2, and propylene‐d 6 at 26°C using 253.7 and 184.9‐nm light. There are at least five primary steps:The ratios of the primary reaction rates depend upon wavelength of the light and deuteration of the propylene. Addition of hydrogen atoms to the propylene double bond gives 5% n‐propyl radicals.
We find no evidence of vinyl—vinyl radical combination to form butadiene. The cross‐combination ratio of isopropyl and allyl radicals is 6½.
Internal Energy of Reaction Products by Velocity Analysis. I. Scattered KBr* from the Crossed Molecular Beam Reaction K+HBr42(1965); http://dx.doi.org/10.1063/1.1696108View Description Hide Description
The velocity distribution of excited KBr* formed in reactive collision between crossed beams of velocity‐selected K and thermal HBr has been measured as a function of laboratory scattering angle for two different initial relative kinetic energies,E. The most probable value of the recoil velocity of the KBr* yields the final relative kinetic energyE′ and thus the value of the most probable ``collision‐exothermicity'' Q ≡ E′ − E = ΔD 0° − ΔE int, where ΔE int is the most probable value of the internal energy change. Results of a series of experiments at E = 1.9 kcal/mole gave Q = 0.5±0.5 kcal/mole (out of an a priori range from − 1.9 to +5.1 kcal/mole) and thus the most probable internal (excitation) energy for the KBr: E * KBr = 4.6±0.7 kcal/mole. The observed angular and velocity distributions of KBr imply that the reactive scattering in the center‐of‐mass (c.m.) system is predominantly confined to low angles (i.e., ``forward hemisphere,'' with the KBr* product preferring the direction of the incident K in the c.m.). Less extensive experiments at E = 3.4 kcal/mole served to confirm these conclusions.
42(1965); http://dx.doi.org/10.1063/1.1696109View Description Hide Description
Comparison of the Infrared Spectra (4000–70 cm−1) of Several Hydrated and Anhydrous Salts of Transition Metals42(1965); http://dx.doi.org/10.1063/1.1696110View Description Hide Description
The infrared spectra from 4000 to 70 cm−1 of several hydrated and anhydrous salts of transition metals have been obtained. A comparison of the spectra of a hydrate with a full coordination shell of water, of a lower hydrate, and of the anhydrous salt is made. Differences in the spectra are discussed.
42(1965); http://dx.doi.org/10.1063/1.1696111View Description Hide Description
Measurements of the static dielectric constant and the density of o‐methylanisole, m‐methylanisole, p‐methylanisole, 2,6‐dimethylanisole, and 3,5‐dimethylanisole from 20° to 60°C, are reported. These data are combined with estimates of the limiting dielectric constant at high frequency in order to obtain information about the interaction of a dipolar molecule with its neighbors. The dielectric constant and loss at 2‐mm wavelength are also reported. These data are used to provide a partial characterization of the dielectric relaxation. The dielectric constants permit more accurate estimates of the high‐frequency dielectric constants than possible previously.