Index of content:
Volume 35, Issue 4, 01 October 1961
35(1961); http://dx.doi.org/10.1063/1.1732016View Description Hide Description
The interactions of the surface hydroxyl groups on amorphous silica with two series of adsorbates, the methylbenzenes and the chloromethanes, have been studied. A correlation is demonstrated between the frequency shifts of the surface hydroxyl stretching vibration and the ionization potential of the adsorbate. A similar correlation is found for the interaction of the hydroxyl group of a carbinol with the methylbenzene series. The importance of charge‐transfer interaction in hydrogen bonding, which is indicated by these correlations, is discussed.
Long‐Range Nuclear Magnetic Shielding in Molecules and the Anisotropy in the Magnetic Susceptibility of a Carbon‐Carbon Single Bond35(1961); http://dx.doi.org/10.1063/1.1732017View Description Hide Description
A method is discussed for obtaining quantitative measurements of long‐range NMR shielding effects in molecules based on the theory of Ramsey as extended by McConnell. These effects are shown to arise principally from the diamagnetic part of the magnetic susceptibility of the individual bonds when the origin of coordinates is taken at the geometric center of the bond. Application is made to saturated hydrocarbon alcohols for which the only long‐range shielding contributions come from tetrahedral carbon‐carbon bonds. From high‐resolution NMRmeasurements of tertiary protons on carbocyclic molecules the existence of these effects is verified and the magnitude of the magnetic anisotropy on the individual bonds causing such effects is determined.
35(1961); http://dx.doi.org/10.1063/1.1732018View Description Hide Description
Analysis of the electron magnetic resonance of x‐ray damaged crystals of NH4ClO4 shows that the long‐lived defects are NH3 + ions. Isotropic hyperfine splittings are found to be 54.6 Mc for nitrogen and 72.5 Mc for hydrogen. These radicals appear to be planar and execute restricted rotation at room temperature. A second transient defect is identified as ClO3.
35(1961); http://dx.doi.org/10.1063/1.1732019View Description Hide Description
It is found that the chemical shifts of various gaseous compounds, using gaseous methane as a standard, vary with temperature. The slopes of chemical shift versus temperature at 50°C are tabulated for several compounds. This effect is ascribed to excitation of vibrational modes of the molecules, the protons in the excited molecules being differently shielded than the protons in the ground vibrational states. The data are interpreted to yield approximate chemical shifts associated with the excitation of various types of modes. NMRisotopic shifts are discussed on the basis of these data.
35(1961); http://dx.doi.org/10.1063/1.1732020View Description Hide Description
Dielectric dispersion measurements at 400 Mc and 40.0°C are reported for CH2=CHCl and CH2=CF2 in the gas phase. The results for these two molecules and earlier measurements on C2H5Cl are compared with theoretical expectations for nonresonant dispersion resulting from pressure‐broadened transitions between asymmetry doublet levels.
35(1961); http://dx.doi.org/10.1063/1.1732021View Description Hide Description
A nonadiabatic theory of flames is formulated which describes the features of flame propagation in free radical containing cryogenic solids. This ``cryogenicflame'' theory is an extension of nonadiabaticflame theory associated with gaseous flames. The general features of cryogenicflames prescribed by the theory include: the characteristic propagation velocity of the reaction wave, the spatial structure of the composition and temperature fields, and the extinction limits associated with flame existence‐nonexistence. Application of the formulation is illustrated for the case of flame propagation in N‐atom‐containing solid N2.
35(1961); http://dx.doi.org/10.1063/1.1732022View Description Hide Description
Mercuric oxide exists in two forms, the red and the yellow oxides. Magnetic susceptibility determinations showed no appreciable difference between the two oxides; the red one has a mass susceptibility of (—0.2217±0.0002)·10—6 emu/g, while the yellow one has a slightly lower mass susceptibility of (—0.2165±0.0002)·10—6 emu/g at room temperature (25°C).
The temperature variation of their mass susceptibility between room temperature (25°C) and about 300°C was also investigated. The general shape of the curve for the two oxides is essentially the same, namely, an increase in diamagnetism with rise in temperature followed by a mild drop and subsequently a sharp rise. The position of the peak occurs at about 50°C for the red oxide and at about 114°C for the yellow oxide. A tentative explanation of their behavior was given, based on impurity ionization.
35(1961); http://dx.doi.org/10.1063/1.1732023View Description Hide Description
A variation calculation on the first excited state of H2 has been carried out using an expansion in elliptical coordinates. All results are reported in Hartree atomic units.
The potential function resulting from this calculation is characterized by having minima at R=1.9 and R=4.3 and a maximum at R=3.3.
The energy obtained at R=1.9 using a 20‐term, open‐shell, covalent‐type wave function was —0.7162 as compared with the ``experimental'' value of —0.7181. Due to convergence difficulties, many more configurations are required to improve this result significantly. At R=4.3, on the other hand, the best result was obtained with an open‐shell wave function including nine ionic terms and nine atomic terms. This result of —0.7007 is below the reported ``experimental'' value of —0.6935 because the two lowest vibrational levels have not been found experimentally. The energy at the top of the barrier was computed to be —0.6844 and probably is as low as —0.6884.
The vibrational levels for this potential function have been computed by a numerical integration technique. The fact that these are in one‐to‐one correspondence with a combination of the experimental 1s2s and (2pσ)2 levels indicates that both of these sets of levels belong to the first excited state.
35(1961); http://dx.doi.org/10.1063/1.1732024View Description Hide Description
The spectrum of NBr near 6000 A, excited by the action of active nitrogen on bromine, has been photographed at high dispersion and the rotational structure of the bands has been analyzed. The bands have been attributed to a transition in which the 3Σ— state is the ground state of the molecule. Only two of the five possible branches were observed, both of them belonging to the F 1(J=N+1) component of the 3Σ— state. The unusual structure of the bands is attributed to the fact that the coupling in the 3Σ— state is intermediate between Hund's case b and case c. The equilibrium internuclear distances are r e ′=1.731 A and r e ″=1.79±0.02 A and the dissociation energy is estimated to be 67±5 kcal. The emission spectrum is believed to be caused by the combination of N(4 S) and atoms into some unknown excited state of NBr which is induced, by wall collisions, into the 1Σ+ state.
Mean and Equilibrium Molecular Structures of Methane and Deuteromethane as Determined by Electron Diffraction35(1961); http://dx.doi.org/10.1063/1.1732025View Description Hide Description
The structural parameters of methane and methane‐d 4 were determined with the new rotating‐sector electron diffraction apparatus of Iowa State University. The results demonstrated, for the first time by electron diffraction, the existence of isotope effects on structure. Mean internuclear distances were C–H=1.1068±0.001 A, C—D=1.1027±0.001 A, H···H=1.811±0.007 A and D···D=1.805±0.008 A. Mean amplitudes, reckoned from mean positions, were C–H=0.0751±0.002 A, C—D=0.0663±0.002 A, H···H=0.120±0.006 A, and D···D=0.105±0.006 A.
The isotope effects were just those expected for atoms of different mass vibrating in identical anharmonic force fields. Vibrational anharmonicity led to measurable phase shifts in the molecular diffraction patterns from which the asymmetry of the internuclear distribution functions could be determined. This allowed a direct experimental determination of the approximate equilibrium C–H and C—D bond lengths, giving 1.082 A. Somewhat more rigorous calculations of the correction from mean to equilibrium distances based on an anharmonic Urey‐Bradley field led to equilibrium C–H and C—D bond lengths of 1.0847 A and 1.0863 A, respectively. Corresponding equilibrium bond lengths computed from the spectroscopicr 0 values of 1.0940 A and 1.0923 A, respectively, were 1.0850 A and 1.0856 A. The comparison clearly illustrates the appreciable difference between electron diffraction and spectroscopic methods in the manner of averaging over molecular motions, but also confirms the essential equivalence of molecular information derived, if suitable corrections are applied to each.
35(1961); http://dx.doi.org/10.1063/1.1732026View Description Hide Description
The 3000‐A absorption band of s‐triazine has been examined under low resolution. The long wavelength portion of this band is assigned to the allowed n→π* promotion with 0–0 band at 31 574±15 cm—1. Evidence is presented for the onset of a second n→π* transition at ∼32 500 cm—1. The observed sequence of states contradicts previous predictions based on approximate sp 2 hybridization, and indicates that in the excited states the n orbital has very little s character (∼0.1).
35(1961); http://dx.doi.org/10.1063/1.1732027View Description Hide Description
The results of further studies of the mercury photosensitized reactions at room temperature of n‐butane and of its mixtures with nitrous oxide are reported. The importance of a self‐scavenging process responsible for low quantum yields of hydrogen at finite conversions has been demonstrated by carrying out experiments at low conversions closely approaching initial conditions. The initial quantum yield of hydrogen has been determined relative to that of nitrogen from the mercury photosensitized decomposition of nitrous oxide. The mechanism of the self‐scavenging process is discussed.
35(1961); http://dx.doi.org/10.1063/1.1732028View Description Hide Description
Hydrogen atoms were produced by the mercury‐photosensitized decomposition of n‐butane at 435±5 mm pressure and at 23.5±1°C. By introducing known amounts of olefine, the rate of addition of hydrogen atoms to the olefine could be measured relative to the rate of abstraction from the n‐butane by observing the fall in the rate of production of hydrogen. In this way, relative rates of addition to eight olefines were obtained, and the logarithms of the relative rate constants were found to correlate reasonably well with the atom localization energies of the olefines, but not with their maximum free valence.
35(1961); http://dx.doi.org/10.1063/1.1732029View Description Hide Description
The analysis of broad bands in the polarized electronic spectra of transition metal ions in crystals is discussed. The band of cobalt(III) acetylacetonate due to the transition 1 T 1←1 A 1 is found to be split by the trigonal field into components at 16 200 cm—1 (1 A 2) and at 17 000 cm—1 (1 E). The spectra indicate that the trigonal field of the free molecule is not appreciably distorted in the monoclinic crystal.
35(1961); http://dx.doi.org/10.1063/1.1732030View Description Hide Description
The spectra of several fluorine‐substituted, saturated, organic compounds have been investigated. It has been found that the coupling constants between 1,2 fluorine atoms are usually near zero. The coupling constants between 1,3 fluorine atoms in a free chain are usually between 7 and 10 cps if all of the intermediate skeletal atoms are carbon atoms. If one of the intermediate skeletal atoms is a nitrogen atom, these coupling constants go up to between 10 and 17 cps. The coupling constants between 1,4 fluorine atoms are usually in the range 2–7 cps when a nitrogen atom is in the intermediate skeletal chain. Rings usually reduce all coupling constants below the above stated values.
Several exceptions are found to these generalizations. These generalizations with their exceptions lead the authors to believe that in the case of coupling between fluorine atoms, the main effect is not through‐the‐bonds, but rather a direct‐through‐space interaction. It is demonstrated that the restricted rotation about the carbon‐carbon bonds has little to do with the near‐zero coupling constants, but that these are readily explained on the basis of through‐space coupling.
35(1961); http://dx.doi.org/10.1063/1.1732031View Description Hide Description
Within the π‐electron approximation, formulas are displayed for some of the properties of the benzene molecule. The integrals which enter the formulas are treated as empirical parameters. The one‐electron integrals are interpreted as being over‐orthogonalized atomic orbitals consistent with the use of zero differential overlap for the two‐electron integrals. An attempt is made to find one set of parameters which will simultaneously account for several properties of the benzene molecule. The resolution of benzene integrals into empirically evaluated components in a quantum mechanically well‐defined way firstly provides a more critical test of the level of the π‐electron approximation and secondly renders more sure the use of these parameter values for other molecules.
35(1961); http://dx.doi.org/10.1063/1.1732032View Description Hide Description
The in‐line vibrations of a linear, symmetric, triatomic molecule having Morse bond‐stretching potentials are studied numerically. The significance of the results for unimolecular rate theory is discussed. Harmonic and weakly anharmonic approximations appear to be inadequate.
35(1961); http://dx.doi.org/10.1063/1.1732033View Description Hide Description
The heat capacity of terbium metal has been measured between 0.25° and 1.0°K. The high temperature data can be expressed as cal/mole‐deg which is in excellent agreement with the paramagnetic resonance experiments of Baker and Bleaney on terbium ethyl sulfate. The data are compared with a Schottky formula for the heat capacity, assuming equal spacing between the nuclear hyperfine levels with an over‐all spacing of Δ=0.45°K, corresponding to the high temperature expression above. The agreement is within experimental error except at the very lowest temperatures obtained. The effective field at the nucleus of the terbium atom can be calculated and is found to be gauss.
35(1961); http://dx.doi.org/10.1063/1.1732034View Description Hide Description
It is shown that previous methods for calculating the mean‐square end‐to‐end distance 〈r 0 2〉 of polyethylene are invalid, since no account is taken of interactions between rotations around chain bonds. With the aid of recently developed matrix methods the value of is calculated to be 6.75 at 160°C in excellent agreement with the results deduced from intrinsic viscosity measurements.
35(1961); http://dx.doi.org/10.1063/1.1732035View Description Hide Description
Electron density distributions have been determined for two ZnO crystals that had been heated in Zn vapor and for an undoped crystal by very accurate measurements of their x‐ray diffraction intensities. An examination of the difference between the electron densities of doped and undoped crystals shows that the doped crystals contain 15–60×1019 atoms/cm3 in the octahedral interstices. The incorporation of this large number of interstitialZn atoms increases the axial ratio of the hexagonal unit cell from 1.6019 to 1.6025. The density of interstitials is approximately 1000 times greater than that calculated from the electrical conductivity of these crystals so that it must be concluded that most of the interstitialZn atoms are electrically neutral. The presence of these interstitials explains why the time required to reach saturation for electrical conductivity is much greater in virgin crystals than in previously doped ones.