Volume 52, Issue 8, 15 April 1970
Index of content:

Molecular Structure of Cyclobutane from Its Proton NMR in a Nematic Solvent
View Description Hide DescriptionThe molecular structure of cyclobutane has been deduced from its protonnuclear magnetic resonancespectra in the nematic phase of , p′‐di‐n‐hexyloxyazoxybenzene. The observed spectra include satellites due to molecules containing carbon‐13 in natural abundance. Cyclobutane is found to be bent with a symmetry of , but the molecule oscillates between the two equivalent bent conformers to give a spin Hamiltonian of symmetry . From the observed spectra, it is not possible to determine both coordinates required to locate the carbon atoms, and an extra assumption must be made to determine the geometry. If the two C–H bond lengths to a carbon are assumed equal, then we find a bent geometry with a dihedral angle of 27°, with , with the methylene group tilted 4° in a way which brings axial protons on the same side of the ring together. If the size of cyclobutane is scaled by taking the C–C bond length to have the electron diffraction value of 1.548 Å, then we find a C–H bond length of 1.133 Å. Other assumptions to fix the undetermined coordinate are explored.

Infrared Spectrum of the NO_{2} ^{−} Ion Isolated in an Argon Matrix
View Description Hide DescriptionThe molecular ion NO_{2} ^{−} has been stabilized in an argon matrix in sufficient concentration for detection of its antisymmetric stretching fundamental, , at 1244 cm^{−1} by electron bombardment or photoionization of matrix‐isolated NO_{2} and by the interaction of an alkali‐metal atomic beam with NO_{2} in an argon matrix. In contrast to the position of this fundamental in an inert, nonionic environment, a value of approximately 1275 cm^{−1} is characteristic of the crystalline material. Isotopic data are consistent with a 115° valence angle for NO_{2} ^{−}, independent of environment. Irradiation of the sample with light of wavelength near 3150 Å leads to the destruction of the NO_{2} ^{−} absorption in the studies of the electron bombardment and photoionization of NO_{2}, but not in the experiments in which the alkali metal atoms provide a reservoir of photoelectrons.

Dissociation Energy and Long‐Range Potential of Diatomic Molecules from Vibrational Spacings of Higher Levels
View Description Hide DescriptionAn expression is derived which relates the distribution of vibrational levels near the dissociation limit of a given diatomic species to the nature of the long‐range interatomic potential, in the region where the latter may be approximated by . Fitting experimental energies directly to this relationship yields values of , , and . This procedure requires a knowledge of the relative energies and relative vibrational numbering for at least four rotationless levels lying near the dissociation limit. However, it requires no information on the rotational constants or on the number and energies of the deeply bound levels. can be evaluated with a much smaller uncertainty than heretofore obtainable from Birge–Sponer extrapolations. The formula predicts the energies of all vibrational levels lying above the highest one measured, with uncertainties no larger than that of the binding energy of the highest level. The validity of the method is tested with model potentials, and its usefulness is demonstrated by application to the precise data of Douglas, Mo/ller, and Stoicheff for the state of Cl_{2}.

Dynamic Model for Locally Stiff Ring and Straight Chain Polymers
View Description Hide DescriptionThe Rouse–Bueche bead–spring model for high polymers is extended to account for stiffness arising from local torsional and bending interactions. A mechanical model including second‐ and third‐nearest‐neighbor bead–spring interactions is developed. The effect of local stiffness is introduced into the normal‐mode eigenvalue spectrum via second‐ and third‐nearest‐neighbor Hooke's law spring constants. A calculation is made of the effect of stiffness on both the free‐draining and hydrodynamic interaction properties of linear and circular high polymers. Experiments for supercoiled circular DNA where local stiffness is expected to be large and circular DNA are compared with theory. The effect of local stiffness is sufficient to account for the differences in intrinsic viscosity between the two species.

Millimeter and Submillimeter Wave Spectrum and Molecular Constants of Aluminum Monofluoride
View Description Hide DescriptionRotational transitions of ^{27}Al^{19}F have been measured in the millimeter and submillimeter wave regions to frequencies of 461 Gc/sec. Vibrational states up to were observed. From analysis of the data the following Dunham rotational constants were obtained: , , , , , and . The Dunham potential constants found are: , , , and . The nuclear quadrupole coupling constants obtained are: for and for the vibrational state. Vibrational constants derived are and . The equilibrium internuclear distance was obtained.

Spectroscopic Behavior and Coordination of Nickel(II) in Liquid Mixtures of Zinc and Cesium Chlorides
View Description Hide DescriptionOptical absorptionspectra of dilute solutions of NiCl_{2} in molten ZnCl_{2}–CsCl mixtures were measured over the complete range of solvent compositions. The temperature extremes investigated were 260°C for melts containing 38 mole % CsCl and 1000°C for 75 mole % CsCl. At temperatures within 100–300°C of the liquidus (depending on melt composition) the spectra were highly composition and temperature dependent and consisted of relatively narrow absorption bands. Analysis of these spectra showed the presence under different conditions of six kinds of nickel(II) centers with well‐defined coordination geometries. An equilibrium between octahedral and tetrahedral centers occurred in 0–8 mole % CsCl such that the octahedral form was favored with increasing CsCl content. A different octahedral–tetrahedral pair occurred in 20–50 mole % CsCl with the tetrahedral form favored by increasing CsCl content and temperature. In CsCl‐rich melts tetrahedrally coordinated Cl_{3}NiClZnCl_{3} ^{3−} and NiCl_{4} ^{2−} were found. In ZnCl_{2}‐rich melts at high temperatures the absorptions became broad and only mildly dependent on solvent composition. Suggestions are made regarding the relationships between these coordination effects and structure in the solvent.

Theory of Inelastic Collisions. II. The Proof of Conjectured Rules for the WKB‐Type General Solutions
View Description Hide DescriptionA set of the rules which were conjectured in the first paper of this series for obtaining the WKB‐type general solutions of N‐coupled radial Schrödinger equations are either derived or proved under an assumption of distinct eigenvalues of a matrix which is directly related to the coupled equations. These coupled radial Schrödinger equations occur in the multichannel approach in the theory of inelastic collisions of atoms and molecules.

Kinetics of Chemically Activated Isobutane and Neopentane from the 4358‐ and 3660‐Å Photolyses of Diazomethane with Propane and Isobutane
View Description Hide DescriptionThe results of a study of chemically activated isobutane and neopentane produced by excited singlet‐state methylene radical insertion into the secondary and tertiary C–H bonds of propane and isobutane, respectively, are reported. The methylene radicals were produced at two energies by the 4358‐ and 3660‐Å photolyses of diazomethane. The measured decomposition rates for the energized isobutane are 1.9 × 10^{7} and 3.6 × 10^{7} sec^{−1} in the 4358‐ and 3660‐Å photolysis systems, respectively, and for the energized neopentane are 4.4 × 10^{6} and 6.8 × 10^{6} sec^{−1} in the 4358‐ and 3660‐Å photolysis systems, respectively. These rates correlate well via RRKM theoretical calculations with thermal factors in the range of 10^{16.3}–10^{16.9} sec^{−1} for isobutane and 10^{16.5}–10^{16.9} sec^{−1} for neopentane.

Powder Line Shapes in the Electron Paramagnetic Resonance Spectra of High‐Spin Ferric Complexes
View Description Hide DescriptionThe positions of powder lines in the electron paramagnetic spectra of high‐spin ferric systems have been calculated by solving the spin Hamiltonian for a broad range of parameters. Powder lines are obtained for every transition when the magnetic field points along the principal axes of the fine structure tensor. However, it was found that for most transitions extra powder lines are often found when the field lies in any of the principal planes but not along the axes. Particular attention is directed to the transition responsible for the absorption in nearly rhombic ferric complexes. The calculations show that, depending on the value of the ratio between the microwave quantum and the parameter , this transition may consist of 3–6 powder lines near . The values for all these powder lines were also obtained from a third‐order perturbation calculation which assumes nearly rhombic symmetry and . The 9.2‐ and 34‐GHz spectra of Fe(III)–EDTA diluted in the corresponding diamagnetic Co(III) compound and the 2.7‐, 9.2‐, and 34‐GHz spectra of native human serum transferrin have been analyzed by the aid of the calculations. It was determined that for and , while for transferrin and .

Possible Sources of Large Error in Determinations of Ion–Molecule Reaction Rates with Drift Tube–Mass Spectrometers
View Description Hide DescriptionDrift tube–mass spectrometers are now used extensively to measure the rate coefficients of ion–molecule reactions. The accuracy which can be obtained in favorable cases is about 10%, but if careful attention is not paid to a number of factors, very large errors can result. The purpose of this paper is to call attention to this fact, which seems not to be widely known, and to enumerate the factors which can lead to these large errors.

Magnetothermodynamics of α‐MnCl_{2}·4H_{2}O. III. Heat Capacity, Entropy, Magnetic Moment, from 0.3 to 4.2°K with Fields to 90 kG along the [100] Crystal Axis
View Description Hide DescriptionThe heat capacity and magnetic moment of a 3.934 cm diameter spherical single crystal of α‐MnCl_{2}·4H_{2}O have been measured with stabilized solenoid fields of 0, 1, 5, 10, 14, 22, 25, 40, 65, and 90 kG, along the [100] crystal axis. The magnetic saturation value, 27 999 G·cm^{3} mole^{−1}, obtained at both 65 and 90 kG, and 0.46°K, indicates that the temperature independent susceptibility is zero and the saturation moment of the temperature‐dependent system is 27 999 G·cm^{3} mole^{−1}, equivalent to . At 90 kG the nuclear spins contributed a heat capacity term of . The amount of enthalpy required to remove quanta of angular momentum from the saturated condition at 90 kG was found to be 20.2 cal mole^{−1}. This is less than , the magnetic work, by 3.9 cal mole^{−1}. This difference is the amount of stored internal energy contributed by the saturated condition to this limiting process, and is consistent with antiferromagnetic interactions. Extrapolation of the magnetic moment–temperature curves to the absolute zero yields the expression M = 0.9946H + 2.50 × 10^{−6} H ^{2}G·cm^{3} mole^{−1} at 0°K. On this slightly curved line the saturation value, 27 999 G·cm^{3} mole^{−1}, is reached at 26 400 G, and the work of magnetization to saturation,, is 9.02 cal mole^{−1}. Temperature field observations on 26 isentropes were used to correlate the entropies along 10 isoerstedic heat capacity series. The zero of electronic and lattice entropy was located from the low temperature heat capacities at 90 kG. The upper limit of the electronic entropy was found to be 3.561 gibbs mole^{−1} in complete agreement with the Rln6 expected for an state. Smoothed correlated values of the heat capacity,entropy,enthalpy, internal energy, magnetic moment and its isoerstedic temperature coefficient, and the work of magnetization have been tabulated.

Vibration Spectra of the Orthorhombic Alkaline‐Earth Hydrides by the Inelastic Scattering of Cold Neutrons and by Infrared Transmission Measurements
View Description Hide DescriptionThe vibration spectra of the orthorhombic alkaline‐earth hydrides have been investigated by measuring the energy distribution of cold neutrons which have been scattered inelastically from polycrystalline samples. In each case the measuredspectrum consists of a band or bands primarily due to the metal atom vibrations (below ∼300 cm^{−1}) and two broad optical hydrogen vibration bands (above ∼300 cm^{−1}) presumably caused by hydrogen occupying two different sites in the metal lattice. An approximate frequency distribution has been derived for each hydride sample. The optical bands are peaked at 1050 and 720 cm^{−1} for YbH_{2}, at 1010 and 690 cm^{−1} for CaH_{2}, at 940 and 600 cm^{−1} for SrH_{2}, and at 810 and 560 cm^{−1} for BaH_{2}. Splitting and fine structure of the optical peaks have been observed and have been attributed to the asymmetry of the hydrogen sites in the metal lattice. Infrared transmission measurements on the same compounds are in good agreement with the neutron results. Additional splitting of the optical peaks have been observed in the infrared.

Chemical Applications of Metastable Argon Atoms. III. Production of Krypton and Xenon Metastable Atoms
View Description Hide DescriptionAn inexpensive experimental technique has been developed for the production of argon, krypton, and xenon metastable atoms using a discharge‐flow system. The reactions of these metastable atoms with N_{2}, CO, and N_{2}O were investigated in the pressure range 0.3–10 torr, and the emission spectra resulting from the reactions were identified. These spectra gave information about the collision event and also about some of the relaxation processes. Franck–Condon factors apparently do not control the molecular excitation processes in collisions of excited heavy atoms with light diatomic molecules. This discharge flow source of metastable atoms provides emission spectra of sufficient intensity from added CO and N_{2}O for study of emission bands which currently are not well understood. Molecular emission from N_{2}O has been observed, apparently for the first time.

Reactions of Ar, Kr, and Xe Metastables with Simple NH‐Containing Compounds
View Description Hide DescriptionThe reactions of Ar*, Kr*, and Xe* with HN_{3}, NH_{3}, and HNCO were studied. Emission spectra from , , , and were observed. The data are consistent with the recent value of 91 kcal mole^{−1} for , but not with standard values in some tables, and support a lower value for the separation than is currently accepted. Ar* and Xe* were used in further work on the bond energies of CN‐containing compounds leading to . The validity of spin conservation in metastable atom reactions and comparison of metastable impact excitation with vacuum uvphotolysis are discussed.

Kinetics of Electron Scavenging and Ion Recombination in the Radiolysis of Hydrocarbon Solutions
View Description Hide DescriptionA functional description of the lifetimes of the ion pairs produced in the radiolysis of pure hydrocarbons is derived from the concentration dependence observed for ion scavenging. A number of consequences which follow from this description are explored. In particular it is shown that in the absence of scavenger the decay of the geminate ions must be very closely described by , where is a constant and is the fraction of ions present at time . Appropriate descriptions for the growth and decay of secondary ions are given, and the results are applied to real systems of finite pulses. Comparison is made between the predictions given here for the time dependence of the population of secondary negative ions and the experimental observations of Thomas and co‐workers on the decay of diphenylide negative ion in pulse irradiated cyclohexane solutions of diphenyl. From this comparison it is concluded that the rate constant for electron scavenging by the diphenyl is and that the preponderant fraction of ion recombination occurs in the time region of 10^{−11}–10^{−9} sec.

Vibrational Energy Transfer in CO–He Lasers
View Description Hide DescriptionTime‐resolved fluorescence arising from transitions in CO has been observed in a flowing Q‐switched CO laser containing only He and CO. The time for equilibration of the vibrationally excited states of CO at translational temperatures near 120°K has been found to be given by , where is the partial pressure of CO. The relaxation time for loss of vibrational energy due to processes other than stimulated emission is typically 6–15 msec. The net rate of pumping energy into the vibrations of CO is estimated to be 2 × 10^{19} quanta sec^{−1}·cm^{−3}. This compares favorably with calculations based on direct vibrational excitation by electron impact. The observations of vibrational relaxation are consistent with vibrational energy exchange in binary CO–CO collisions subject to harmonic‐oscillator selection rules.

Theory of Diffusion in Simple Liquid Mixtures
View Description Hide DescriptionA method is presented for obtaining the diffusion coefficient in terms of functions of molecular properties for a liquid system of monatomic molecules near equilibrium. The approach considers the reduced probability density functions for molecules () in the limit of a steady state. By operating on the complete probability density function with the conventional Liouville operator and by integrating over the phase space of molecules, we write the time dependence of in a form not explicitly involving for . The reduced probability densities and the average vector forces are developed as a power series in a small parameter λ. Starting with singlet functions corresponding to a gradient in the density of a binary isothermal solution at constant pressure and the corresponding flux of molecules, we determine a singlet momentum‐dependent force which maintains the flux and gradient stationary in time. The force can only be consistent with a certain class of pair probability functions, and we write the required general forms. Repeating the operation, we compute the average pair force which maintains the fluid in a steady state. The operation may, in principle, be carried to for any . From symmetry requirements we obtain a correction to the Kirkwood closure for the nonequilibrium factors in the triplet probability density. The closure fixes the ratio between the assumed density gradient and flux, which leads to the diffusion coefficient in terms of functions of pairs and triples of molecules and the intermolecular potential energy.

Chemically Activated Tetramethylsilane from the Reaction of Singlet Methylene Radicals with Trimethylsilane
View Description Hide DescriptionAn experimental study of the photolysis of trimethylsilane–diazomethane–oxygen mixtures and tetramethylsilane–trimethylsilane–diazomethane–oxygen mixtures at 4358 and 3660 Å is reported. Experimental rate constants for the decomposition of chemically activated ethyltrimethylsilane, ethyldimethylsilane, and tetramethylsilane are determined. These specific rates range from 3 × 10^{4} sec^{−1} to 6.5 × 10^{5} sec^{−1}. A comparison of the 4358‐ and 3660‐Å rate constants for tetramethylsilane decomposition with RRKM theory calculations suggest that a thermal factor of 10^{15.0±0.5} sec^{−1} is correct for primary decomposition by Si–C bond rupture. The uncertainty in this factor reflects the uncertainties in the and values for chemically activated tetramethylsilane. A discussion of this factor relative to that found previously for neopentane decomposition is given.

ESR of TMPD–TCNQ: Spin Excitations of the Heisenberg Regular Linear Chain
View Description Hide DescriptionThe detailed electron spin resonance of single crystals of the charge‐transfer complex tetramethylphenylenediamine–tetracyanoquinodimethan (TMPD–TCNQ) at X band and Q band are reported. The ESR spectra are interpreted as resulting from the thermally excited (activation energy, 0.068 eV) spin excitations of the Heisenberg regular antiferromagnetically coupled chain. Although the exact solution to the excited states of the Heisenberg chain with antiferromagnetic coupling is not available, an approximate solution treating the quasiparticles as Wannier spin excitons is very successful at predicting the spin resonanceproperties of the excitations; fundamental to the Wannier spin exciton model is the absence of spin correlation in the excitations. The spin–spin and spin–lattice relaxation rates are observed as functions of temperature and are determined mainly by spin exchange between excitons. Exchange is proportional to exciton concentration, and the low activation energy in TMPD–TCNQ allows exchange frequencies on the order of the Larmor frequency to be achieved without decomposition. Nonsecular broadening of the linewidth is observed and interpreted in terms of the Kubo–Tomita formalism, modified for the uncorrelated spins of the excitations. Similar equations describe the spin–lattice relaxation. The form of the correlation function for spin fluctuations in one dimension can be deduced from the data and is shown to be closely Lorentzian, rather than Gaussian. The crystal structure is known and both the tensor and the principal parts of the dipolar secular moment for the excitations are shown to have the molecular rather than crystal symmetry.

Positron Annihilation in Helium and the System of He Bound State
View Description Hide DescriptionWithin the framework of the valence bond method, the system of He bound state was treated with a two‐center problem approximation. The He bound state was found to have a binding energy of 5.03 eV at its equilibrium‐positron–helium‐nucleus distance, . The ratio of the effective nuclear charges for the positron and for the helium nucleus ε was found to have a value of 0.63 at . An attempt was made to compare the results of the present work with those of others.