Volume 37, Issue 2, 15 July 1962
Index of content:
37(1962); http://dx.doi.org/10.1063/1.1701304View Description Hide Description
The photolysis of CH4—CD4 mixtures, CH2D2, and CHD3 with 1236 Å light shows that molecular elimination of hydrogen from methane is an important but not an exclusive primary photochemical process. The photolysis products are principally those expected from the insertion reaction of the methylene radical.
37(1962); http://dx.doi.org/10.1063/1.1701305View Description Hide Description
Sum rules are developed for the vibrational‐rotational energy levels of an asymmetric rotor. These sum rules contain the corrections for centrifugal distortion correct for terms of the order P 4 in the Hamiltonian. If the P 4 terms of the Hamiltonian are neglected these sum rules reduce to those derived for a rigid asymmetric rotor by Mecke.
37(1962); http://dx.doi.org/10.1063/1.1701306View Description Hide Description
The rotational magnetic moments μ J / J have been measured for Li6F and Li7F in the same magnetic field. These measurements, when combined with the experimental value of μ e 2 A for Li6F, yield a value for the internuclear distance which disagrees with the value obtained from vibrational spectra. It is proposed that the discrepancy is due to an error in assuming that the high‐frequency or paramagnetic term χHF in the magnetic susceptibility is the same for each isotopic species. If the internuclear distances calculated from infrared spectra are assumed to be correct the high‐frequency term is found to beThe isotopic shift Δχ HF is calculated separately and found to beThis represents a decrease of 0.4% from Li6F to Li7F. A calculation of the electric dipole moment of Li6F from the same data yields,
37(1962); http://dx.doi.org/10.1063/1.1701307View Description Hide Description
The low‐energy structure of the x‐ray K‐absorption edge of nickel in nickel dimethylglyoxime, nickel phthalocyanine, and potassiumnickel cyanide is re‐examined for evidence of the second‐order transition, the excitation of a 1s electron to a bound orbital, plus the simultaneous excitation of a plasmon. A perturbation calculation indicates that such a process may be observable. The absorption data are consistent with this interpretation if the electron transition is to a perturbed 3d orbital of the K‐excited nickel atom or a molecular orbital which in the LCAO—MO approximation contains some d orbital wave function. The appearance of two strong absorption maxima (2 and 6.5 eV) in nickel dimethylglyoxime is explained by the formation of two distinct molecular species when nickel dimethylglyoxime is precipitated from a basic solution of dimethylglyoxime by a nickel salt.
37(1962); http://dx.doi.org/10.1063/1.1701308View Description Hide Description
The nuclear magnetic resonance of Li7 in Li x V2O5 shows no Knight shift, but there is a small diamagnetic shift relative to LiCl solution, amounting to 0.0023 and 0.0058% at 296° and 77°K, respectively. The spin—lattice relaxation timeT 1 as measured by pulse experiments, decreases from 0.16 sec at 296°K to a minimum of 0.060 sec at 163°K and then rises to 0.13 sec at 77°K. The free‐induction decay half‐width decreases from 850 μsec at 296°K to 100 μsec at 77°K. The minimum in T 1 is attributed to back‐and‐forth motion of the lithium ions within but normal to the axis of channels in the oxygen framework; the line narrowing, to diffusion along the channel from one unit cell to an adjacent one. EPR studies give Lorentzian‐shaped lines, centered at g=1.96, of intensity proportional to 1/T. Results are consistent with a model in which lithium atoms in a host V2O5 structure are completely ionized to give Li+ ions and electrons which are almost, but not completely, localized as V4+ centers.
Effects of Electronic Exchange on the Efficiency of Vibrational Excitation by Molecular Collisions. Part I. Interaction Potential37(1962); http://dx.doi.org/10.1063/1.1701309View Description Hide Description
A theoretical study has been made of the mechanism for the transfer of molecule translational energy into the internal energy of vibration by molecular collisions. In particular, the research considered the interaction between molecules which can combine chemically to form a covalent bond. Previous theoretical treatments using a Lennard‐Jones type of interaction potential resulted in calculated efficiencies which were low by several orders of magnitude for the vibrational excitation process. A stable covalent bond is formed by the sharing of electrons, i.e., electronic exchange, between the various atoms in a molecule. The concept of electronic exchange was used in determining a more realistic interaction potential then the Lennard‐Jones for a typical case, namely, the collision of a proton and a diatomic hydrogen molecule which can combine to form a stable molecule ion. The resulting interaction potential was entirely of negative energy as opposed to the Lennard—Jones which is of positive energy and strongly repulsive for small separation distances between the colliding particles. Furthermore, the depth of the potential well with electronic exchange was about 2 eV compared to Lennard‐Jones well depths of about 0.01 eV. In addition, the equilibrium separation between the nuclei of the hydrogen molecule increased to 2½ times the normal value as the proton approached, and the spring constant of the molecule as a vibrator decreased to less than ⅕ the normal value. The above results all tend to increase the ease of vibrational excitation and should result in higher excitation efficiencies.
Rotational Freedom of Ammonium Ions and Methyl Groups by Cross‐Section Measurements with Slow Neutrons37(1962); http://dx.doi.org/10.1063/1.1701310View Description Hide Description
The slope Δσ s /Δλ n of a plot of the scattering cross section per proton vs the wavelength of subthermal neutrons is shown to increase with increasing freedom of rotation of ammonium ions. For ammonium iodide, the slope increases from 6.2 b/Å—H in phase II to 11.2 b/Å—H in phase I, where the ammonium ions are rotating relatively freely. Measurements on five liquid methylbenzenes indicate greater hindrance to rotation of methyl groups in o‐xylene (slope=9.5±0.3 b/Å—H), than in toluene, m‐xylene, p‐xylene, and mesitylene (slopes=11.4±0.3 b/Å—H).
From an empirical plot of slopes vs barriers to rotation of ammonium ions in several ammonium salts, slopes of 5.5 and 5.8 b/Å—H for (NH4)2CrO4 and NH4CNS indicate a barrier of about 4 kcal mole−1 for both these salts, while a high slope of 13 b/Å—H for NH4ClO4 predicts a barrier of only about 0.1 to 0.2 kcal mole−1. Assuming a similar type curve for methyl groups, the barrier to rotation of the methyl groups in o‐xylene is estimated to be about 1 kcal mole−1, and a value of 0.1 to 0.4 kcal mole−1 is predicted for the other methylbenzenes.
37(1962); http://dx.doi.org/10.1063/1.1701311View Description Hide Description
The frequency dependence of the complex dielectric constant of glycerol—water mixtures has been measured in the megacycle frequency range at −7.5°, −15.3°, and −19.5°C. for various concentrations between nearly pure glycerol and an equal molar mixture. Cole—Cole plots of the data indicate a single main relaxation with an asymmetric distribution of relaxation times of the Davidson—Cole form. The limiting dielectric relaxation time of the Davidson—Cole distribution was found to be strongly dependent on the water content and to vary exponentially with the molar concentration of water. The width of the distribution of relaxation times is independent of the concentration over the range studied and only slightly temperature dependent. The average dielectric relaxation times of the mixtures were extrapolated to 100% water. The dielectric relaxation time of water obtained by this extrapolation agrees satisfactorily with the measured value, indicating a smooth transistion between the average relaxation time of pure glycerol and pure water, as well as indicating a change in the distribution of times from that of the Davidson—Cole form to a single time. These experimental findings can be explained in terms of the ``group'' concept of Schallamach in which it is assumed that in mixtures of associated—associated liquids, such as glycerol and water, dielectric orientation occurs through a cooperative effect in groups of molecules, these groups containing molecules of both kinds.
The concentration dependence of the dielectric relaxation time is also compared to that of viscosity. The comparison indicates that the variation of dielectric times with water content is similar to, but not exactly the same as, the variation of viscosity.
37(1962); http://dx.doi.org/10.1063/1.1701312View Description Hide Description
It has been found that x‐irradiation of a single crystal of glutaconic acid (HOOC)CH=CH–CH2(COOH) produces the oriented allyl‐type free radical (HOOC)C(1)H=C(2)H–C(3)H(COOH). A detailed analysis of the electron magnetic resonance spectra of this radical confirms that the spin density on carbon 2 is opposite in sign to the spin densities on carbons 1 and 3. Additional structural features determined are: (a) The unpaired electron occupies a delocalized π molecular orbital extending over carbon atoms 1, 2, and 3; (b) spin densities ρ ii for carbonp orbitals on carbon atoms 1, 2, and 3 are ρ11=ρ33=+0.57, ρ22=−0.19; (c) principal values of the hyperfine tensors for protons 1 and 3 are −53, −36, and −18 Mc, and for proton 2 are +17, +12, +7 Mc; (d) all C–H bonds in the radical are nearly parallel. The parallelism of all C–H bonds indicates that the undamaged parent molecule is the trans isomer of the acid.
37(1962); http://dx.doi.org/10.1063/1.1701313View Description Hide Description
The fundamental infrared vibration bands of hydrogen sulfide exhibit an interesting intensity anomaly characterized by P branches which are much weaker than the corresponding R branches. This work shows that this anomaly is caused by centrifugal distortion of the nonrigid molecule. A formalism based on a semi‐classical approach is used to calculate the vibration—rotational line positions and their intensities. The computed theoretical spectra are in good agreement with the experimental spectra. This formalism also gives a method of determining the relative sign of (∂μ/∂Q) with respect to μ0. The values of (∂μ/∂Q) needed in the calculations were obtained from the experimental integrated band intensities and were found to be (∂μ/∂Q 1) = −4.4 esu g−½ and (∂μ/∂Q 2) = +8.7 esu g−½.
37(1962); http://dx.doi.org/10.1063/1.1701314View Description Hide Description
Magnetic shielding constants calculated for all nuclei in LiH, BH, NH, BF, HF, LiF, CO, H2, Li2, N2, and F2 have yielded good results for lighter nuclei bonded to heavier nuclei. The difficult paramagnetic contribution of Ramsey's theory is reduced to tractable terms involving only the ground‐state wave function by a choice of the gauge of the vector potential, and by an approximation involving cancellation of average excitation energies of matrix elements depending upon angular momentum and linear momentum operators. The limitations of this approximation are explored.
37(1962); http://dx.doi.org/10.1063/1.1701315View Description Hide Description
The use of expansions of solid spherical harmonics, which are the functionsis discussed with respect to molecular integrals, and it is found that terms of the delta function type must be included in the expansion for the r −n−1 case if n—m≥2. These expansions are especially useful for operators involving inverse powers of r such as electromagnetic interaction operators. General formulas are given for integrals in which two atomic orbitals, e.g., Slater orbitals, are on one center, and one of these operators is on the other center. Formulas are also given for the corresponding one‐center integrals, and methods of evaluating three‐center integrals and other two‐center integrals are discussed briefly.
37(1962); http://dx.doi.org/10.1063/1.1701316View Description Hide Description
The chemical shift of the bridge proton relative to the terminal proton in diborane is discussed in several approximations based on Pople's and on Ramsey's atomic and molecular theories of magnetic shielding, respectively. The analysis, which depends on Yamazaki's 12‐electron SCF LC—STO MO wave function, gives the result, generally observed in the H1NMR spectra of the higher hydrides, that the bridge protons show resonance at higher fields than do the BH2 terminal ones.
37(1962); http://dx.doi.org/10.1063/1.1701317View Description Hide Description
The anomalous high‐field chemical shift observed in general for H bonded to a heavy metal has been calculated in the localized diatomic approximation for the CoH bond in HCo(CO)4 with the use of the theory of Kern and Lipscomb, valid for a light atom bonded to a heavy atom. It is shown for the first time that reasonable agreement with experiment is obtainable at a bond distance corresponding to a normal covalent bond, and that, aside from the electron around H, the major contribution to the high‐field shift comes from the electron populations in the large and diffuse 4s and 4p orbitals of Co.
Rotational Isomerism and Microwave Spectroscopy. I. The Microwave Spectrum of Normal Propyl Fluoride37(1962); http://dx.doi.org/10.1063/1.1701318View Description Hide Description
The normal propyl fluoride molecule has been confirmed by microwave spectroscopy to exist in two rotational isomers, trans and gauche.Rotational constants in the ground vibrational state are A = 26 986.73, B = 3748.32, C = 3509.88 Mc/sec for the trans form and A = 14 503.69, B = 5085.71, C = 4295.28 Mc/sec for the gauche form. The dihedral angle of the gauche form is about 63° from the cis position.
The barrier height hindering methyl rotation has been determined, using lines in the first excited state of methyl rotation, to be 2.69 kcal/mole and 2.87 kcal/mole for the trans and the gauche forms, respectively. The dipole moment and its components along the principal axes have been determined by the Stark effect: μ a = 1.970±0.026, μ b = 0.566±0.064, μtotal = 2.050±0.043D for the trans form and μ a = 1.137±0.008, μ b = 1.450±0.033, μ c = 0.472±0.137, μtotal = 1.902±0.064 D for the gauche form. The total dipole moment is nearly parallel to the C–F bond for both forms; the angle between them is 10.6° in the trans form and 7.4° in the gauche form.
The intensities of satellites due to the first and the second excited states of the C–C torsion have been measured relative to the ground‐state lines. The energies of the first and the second excited states are 98.0 cm−1 and 211.3 cm−1 for the trans form and 160.8 cm−1 and 322.6 cm−1 for the gauche form. The same technique of the relative‐intensity measurement has been applied to the temperature dependence of the intensity ratio of trans and gauche lines, giving the result that the gauche form is more stable than the trans form by 0.47±0.31 kcal/mole. Using these data the first seven constants were determined in the Fourier expansion of the potential function around the central C–C bond. The barrier at the cis position seems to be higher than that separating the trans form from two equivalent gauche forms.
37(1962); http://dx.doi.org/10.1063/1.1701319View Description Hide Description
The matrix isolation technique has been extended to allow molecules which are in equilibrium with solids at high temperatures to be trapped and studied at low temperatures. A beam of the hot vapor issuing from a Knudsen cell or a heated surface is premixed with a large excess of argon or xenon just prior to condensation at 20°K. The method was applied to boric oxide vapor which was vaporized from the liquid at 1400°K. The infrared absorptionspectrum of the B2O3 molecule in a solid inert gas matrix was measured between 280 and 3600 cm−1 and compared with the known vapor emission spectrum. Several new bands were found near 500 cm−1 which led to a considerable alteration in the vibrational assignment and the thermodynamic properties of the gaseous molecule. The infrared spectrum of B2O2 (produced by heating boron plus boric oxide) isolated in a matrix yielded one absorption band which agreed with the emission spectrum. An extensive normal coordinate treatment of B2O3 was made utilizing the observed spectra of the B10 and B11 substituted molecules.
Contribution of Octopole—Octopole Interactions to the Excess Properties of Mixtures of Tetrahedral Molecules37(1962); http://dx.doi.org/10.1063/1.1701320View Description Hide Description
The contribution to thermodynamic properties of the interaction between bond dipoles of adjacent tetrahedral molecules (such as CF4) has been estimated by calculating the octopole—octopole effect (a) between molecules in an isolated pair and (b) in a rigid lattice. The effect is small and cannot account, at best, for more than a small part of the anomalous excess free energies of systems such as CF4+CH4.
NMR‐Relaxation Mechanisms of O17 in Aqueous Solutions of Paramagnetic Cations and the Lifetime of Water Molecules in the First Coordination Sphere37(1962); http://dx.doi.org/10.1063/1.1701321View Description Hide Description
An investigation was made of the temperature and frequency dependence of T 2 for O17 in aqueous solutions containing Mn2+, Fe2+, Co2+, Ni2+, and Cu2+. This represented an extension of the studies previously performed in this laboratory on these ions. Virtually all of the temperature effects predicted by the modified Bloch equations for a two‐species system were verified experimentally. Rates of exchange of water molecules between the bulk of the solution and the first coordination sphere of the paramagnetic cations were determined for all the ions studied. Activation energies for exchange were measured and electronic T 1's and coupling constants were determined in some cases. Evidence was found for a tetrahedral Co2+(H2O)4 species in aqueous solutions near 100°C. The data for cupric ion were interpreted in terms of six coordinated water molecules in a distorted octahedron, with a ratio of ∼105 existing for the axial‐water‐exchange rate over that of the equatorial waters. The rates of exchange were compared with other physical measurements and the nature of the bonding was considered.
37(1962); http://dx.doi.org/10.1063/1.1701322View Description Hide Description
The vaporization behavior of niobiumcarbide was studied over the temperature range from 2260° to 2940°K utilizing the Langmuir method. NbC was observed to lose carbon preferentially down to a composition of NbC0.75, at which composition the vaporization proceeds congruently at a temperature of 2940°K. The vapor pressure of carbon above nearly stoichiometric NbC could be represented by the equation log P (atm) = 5.296–3.276×104/T with a corresponding second law heat of vaporization of 150 kcal/mole of C vaporized. However, third law calculations and heat of formation data indicate a ΔH 0 298 of from 180 to 189 kcal/mole. Several possible explanations of this disagreement are discussed. The total rate of evaporation from NbC x was found to increase by a factor of two as the value of x decreased from 0.97 to 0.80. Finally, a semi‐quantitative evaluation of the diffusion constant of C in NbC was made, resulting in a value of D≅8×10−7 cm2/sec at 2940°K.
37(1962); http://dx.doi.org/10.1063/1.1701323View Description Hide Description
A new mechanism for singlet—triplet transitions is developed by considering the direct interactions between the radiation field and the electron spins. It is concluded that this effect may in general be neglected with respect to contributions due to the perturbation of the wave functions by spin‐orbit interactions, but that it might become important in some cases where the spin‐orbit coupling is small.