Volume 48, Issue 11, 01 June 1968

Matrix‐Isolation Study of the Reaction of F_{2} and of F Atoms with NCN. The Infrared Spectra of the Species NF_{2}CN and FNCN
View Description Hide DescriptionNCN, produced by the photolysis of cyanogen azide isolated in an Ar matrix at 14°K, has been found to react readily with F_{2} also present in the system, leading to the stabilization of NF_{2}CN. The absorption patterns observed for the most prominent infrared absorptions of NF_{2} ^{13}CN and of the two singly ^{15}N‐substituted difluorocyanamides are consistent with the previous vibrational assignment. When both NCN and F atoms are present in appreciable concentration, absorptions appear at 873 and at 2068 cm^{−1} which may be tentatively assigned to the FNCN free radical. Upon prolonged photolysis, incompletely characterized products of more extensive fluorination reactions are observed.

Crystal Spectra of Tetracene and Rubrene under Pressure
View Description Hide DescriptionPressure‐induced spectral shifts (0–20 kbar) are compared for the absorption and fluorescence of tetracene and rubrene as crystals or embedded in polystyrene matrices. At 20 kbar the red shift of the tetracene crystal absorption is two to three times larger than either the red shift of tetracene in polystyrene or rubrene in both crystal and solid‐solution environments. The crystal shifts and increased Davydov splitting at high pressure are related to calculations on compressed anthracene by Rice and Jortner. The spectral shifts indicate that crystal‐field effects are negligible for rubrene due to steric factors. Strong specific interactions in the excited state between the phenyl substituents of rubrene and the phenyl groups of polystyrene are indicated from the fluorescence spectra at atmospheric pressure. The influence of pressure on tetracene crystal luminescence can be described in terms of excitation‐energy transfer to tetracene dimers at defects. High pressures enhance the transformation of excitation energy into heat, probably by increasing non‐radiative excition trapping at imperfections.

Resolution Enhancement of ESR Spectra from Irradiated Single Crystals of Glycine
View Description Hide DescriptionThe electron spin resonancespectrum of ^{+}H_{3}NĊHCOO^{−} in single crystals of glycine irradiated at room temperature has been re‐examined at X band and evaluated by resolution‐enhancement technique. The resolution enhancement is obtained by reducing the linewidth by a factor of maximum 0.55 at the expense of the signal‐to‐noise ratio. The calculations, which require a high‐speed digital computer, consist of three steps: (1) transformation of the experimental spectrum to a complex Fourier plane, (2) multiplication of this Fourier spectrum with a suitable linewidth‐reduction function, and (3) inverse transformation back to the real plane. From the obtained constants, the hyperfine coupling tensors have been calculated, including contributions from second‐order effects . One of the principal values of one of the tensors was appreciably affected by these second‐order effects. The results fully confirm the structure of the radical.

Anomalous Emission from Substituted Hafner's Hydrocarbons
View Description Hide DescriptionThe emissive behavior of substituted, pericondensed, nonalternant Hafner's hydrocarbons has been investigated in fluid solution at room temperature. The compounds investigated showed fluorescence from electronic states other than the first excited singlet.

Charge Distribution of the NO_{2}‐ Radical in Ferroelectric Sodium Nitrite
View Description Hide DescriptionThe ^{14}N nuclear quadrupole resonance spectrum of ferroelectric sodium nitrite is interpreted to yield the charge distribution of the nitrite radical. Contributions to the electric field gradient due to lattice changes are estimated by performing a lattice sum, while the local electrons are treated using the approximations of the Townes and Dailey theory. Occupation numbers for the N–O sigma‐ and pi‐bond wavefunctions are obtained, and the effective charge on the nitrogen is deduced. The latter quantity is found to be very close to zero.

Semiclassical Theory of Inelastic Scattering: An Infinite‐Order Distorted‐Wave Approximation
View Description Hide DescriptionA previously derived semiclassical theory of inelastic scattering is generalized to include collisions where the trajectory is nonlinear. Schrödinger's equation is expressed as an infinite‐order perturbation series using the radial wavefunctions corresponding to the spherically symmetric part of the potential as the unperturbed solution. In the classical limit this series simplifies, and the scattering matrix is obtained by exponentiating a phase shift matrix which is obtained as a simple integral over the classical trajectory corresponding to the spherically symmetric part of the potential. A second‐order term is obtained to correct for the deviation from this trajectory.

Absorption Spectrum and Zeeman Effect of the Transitions in Holmium Ethylsulfate
View Description Hide DescriptionThe absorptionspectrum and Zeeman effect of the ground term and two excited terms of Ho^{3 +} were measured in the point symmetry of hexagonal single crystals of holmium ethylsulfate. Magnetic‐field strengths from 0 to 27 900 G were employed for studying the behavior of the energy levels in both the HsC (magnetic field perpendicular to axis) and HpC(magnetic field parallel to axis) crystal orientations. When the single crystals were rotated about the axis in the HsC case, a 60° periodicity was observed in the energies of some of the Zeeman components and in the intensities of some of the lines. The angle defined by Murao, Spedding, and Good (MSG), which describes the angle between the and axes in the basal plane of the crystal was found to be 5° ± 2° for both holmium ethylsulfate and 10 mole % holmium in yttrium ethylsulfate. At high magnetic‐field strengths, since the magnetic splitting could no longer be considered small compared to the crystal‐field splitting, the second‐order perturbation treatment of MSG was not sufficient to account for the periodic energy behavior of the levels or the intensity behavior of the lines. The energy behavior was in good agreement with a more recent theoretical treatment for the case where the crystal and magnetic splittings are comparable and with calculations performed in this work in which the observed crystal‐field levels were perturbed by approximate matrix elements of the Zeeman interaction. These matrix elements were calculated using the approximate wavefunctions obtained by diagonalizing the parameterized crystal‐field matrix which gave the best agreement with the crystal‐field splittings of the as observed in this paper. Due to the large nuclear moment and high values of the holmium ion, we were able to resolve the nuclear hyperfine structure in a number of absorption lines. When the magnetic field was along the axis of the crystal, it was found that an apparent change in the selection rules, and a distinct change in the hyperfine patterns occurred in the neighborhood of field strengths where levels in the ground term with different crystal quantum numbers were expected to cross. This behavior was explained theoretically by including the effects of matrix elements of hyperfine interaction which are not diagonal in . These elements prevent the crossing, and patterns calculated in this way were, within the limits of our resolution, in good agreement with the experimental results.

Rydberg Absorption Series and Ionization Energies of the Oxygen Molecule. I
View Description Hide DescriptionRydberg absorption series of O_{2} have been investigated with a 6.6‐m normal‐incidence vacuum spectrograph in the first and second orders in the 600–750‐Å region. Previous observations have been improved upon, and it has been possible to obtain much more accurate ionization energies for the and states of O_{2} ^{+}. The data are compared with recent results obtained in the investigation of the electron energy spectrum of photoionized O_{2} ^{+}. Another Rydberg series converging to the state has been observed and is now reported for the first time.

Separation of Gas Mixtures in Chemical Equilibrium in a Temperature Gradient. II
View Description Hide DescriptionThe theory of separation of gas mixtures in chemical equilibrium has been verified experimentally for the systems: (a) Ar/He/NO_{2}/N_{2}O_{4} and (b) 2NO_{2}⇄2NO + O_{2} with a large excess of O_{2}. The separation in the second system was smaller than calculated, which was attributed to the slowness of the chemical reaction. This was tested by experiments with two different concentrations of NO_{2}. The results agree well with a simple model.

Study of Some Electronic Transition Probabilities in CO and CN
View Description Hide DescriptionThe transition probabilities of the CO third positive , the CO Ångstrom , and the CN violet systems have been obtained from measurements of radiative lifetimes by the phase‐shift method. In the CO Ångstrom system the probability of was distinguished from the (greater) probability for by taking advantage of reabsorption (or resonance trapping) of the shorter wavelength radiation. In the CN transition there is found to be no measurable dependence of the electronic transition moment on internuclear distance.

Chapman–Enskog Method in Chemical Kinetics
View Description Hide DescriptionNonequilibrium effects in gas‐phase reactions associated with the perturbation of the Maxwell speed distribution by the reaction have been investigated by Prigogine and others using the Chapman–Enskog method in the lowest‐order approximation. We have extended these calculations to the second order in the Burnett expansion of the perturbation in a series of Sonine polynomials. In the case of the reaction in which the products B and C are removed and no inert gas is present, the appropriate elastic collision integrals have been evaluated for the model of rigid elastic spheres and the reactive collision integrals have been evaluated for two reaction cross sections, one corresponding to the simple collision theory of bimolecular reactions with activation energy and the other to the “centrifugal barrier” theory of free‐radical recombinations. For the activated reactions the detailed results show that the Chapman–Enskog method is useless when the reaction is fast because the perturbation method fails and is also useless when the reaction is slow because the Sonine polynomial expansion then diverges, as anticipated by Takayanagi. In free‐radical reactions the Chapman–Enskog procedure is satisfactory and the nonequilibrium corrections to the reaction rate are negligibly small (<0.1%) in first and second orders.

Magnetic Shielding of a Hydrogen Molecule in an Electric Field
View Description Hide DescriptionWe evaluate the effect of an electric field along the molecular axis on the magnetic shielding constants and of a hydrogen molecule. It was found that the two shielding constants can be represented as if the axis is taken as the molecular axis. We found that and if is expressed in atomic units It is concluded that the splitting is observable if is of the order of 10^{6} to 10^{7} V/cm.

Valence Structure from Coherent X‐Ray Scattering: Fourier Difference Synthesis
View Description Hide DescriptionThe one‐electron density function for a molecule of first‐row atoms is partitioned into core‐electron and valence‐electron density pieces. The SCF 1 AO's are used to define the core‐electron density pieces. The residual density, after removal of the core density, constitutes the valence density, which is assumed to be chemically interpretable. The partitioning is explored in reciprocal space to provide insight into x‐ray diffraction experiments. The contribution from valence scattering within the sphere is 10%–30% of the total scattering. Valence‐electron density maps have been Fourier synthesized from x‐ray diffraction data of uracil. The density maps reveal trigonal bonding for the nitrogen atoms and bridge densities in the middle of the C(5)–C(6) and C(4)–C(5) bonds. The densities near the time‐average nuclear positions are unreliable.

Solid Fluorine and Solid Chlorine: Crystal Structures and Intermolecular Forces
View Description Hide DescriptionExtended Hückel molecular‐orbital (EHMO) calculations and molecular multipole sums are compared for various assemblies of Cl_{2} molecules. It is shown that the multipole sums does not predict the correct space group for Cl_{2} but that the EHMO calculation does, if there is d‐orbital involvement. Some comments are made on the crystal structures of α‐ and of β‐F_{2}, both different from those of the other halogens.

Computational Procedure for the Close‐Coupled Rotational Excitation Problem: Scattering of Diatomic Molecules by Atoms
View Description Hide DescriptionA computational procedure is described for the integration of the coupled differential equations and determination of the probability matrices required for the accurate evaluation of cross sections for rotational excitation of diatomic molecules. The Arthurs–Dalgarno theory of scattering of an atom by a rigid rotator is employed. The approach developed takes advantage of several of the computational schemes used in the field of electron–atom scattering, particularly those of Smith. The principal virtue of the present computational method is its capability of generating “exact” results to serve as standards against which to compare various approximations such as sudden, dominant coupling, and distorted wave.

Asymmetry in Magnetic Second‐Rank Tensor Quantities
View Description Hide DescriptionThe possibility of asymmetry in a variety of second‐rank tensor quantities of importance in magnetic resonance phenomena is discussed. A classical model demonstrating this effect in the case of shielding is described. The present methods for the measurement of anisotropy of tensor quantities in crystals are discussed, and the contributions of the antisymmetric part of such a tensor are shown to occur in terms quadratic in the shielding (or ) tensor components.

Atomic Wavefunctions and Diamagnetic Susceptibilities
View Description Hide DescriptionThe predictions of early and modern wavefunctions concerning the diamagnetic susceptibilities of the elements up to are compared with experiment. It is concluded that self‐consistent‐field wavefunctions with optimized orbital exponents give the best over‐all description.

Studies of Phase Transitions in Ammonium Salts and Barriers to Rotation of Ammonium Ions by Neutron‐Scattering Cross Sections as a Function of Temperature
View Description Hide DescriptionThe total scattering cross sections per hydrogen atom for NH_{4}F, NH_{4}Cl, NH_{4}Br, NH_{4}I, (NH_{4})_{2}SO_{4}, (NH_{4})_{2}Cr_{2}O_{7}, and NH_{4}ClO_{4} have been measured as a function of temperature using subthermal neutrons. No significant change in the slope is observed in the region of the ^{λ}‐point transitions of the ammonium halides. At the first‐order transitions of NH_{4}Cl, NH_{4}Br, and NH_{4}I where the motion of the ammonium ions changes from torsional oscillation to relatively free rotation, the cross sections increase sharply by 18 to 20 b/H atom. The slope of the linear sections of the plot of atom vs increases with decreasing torsional frequency or barrier to rotation of the ammonium ions. A plot of vs the barriers for the ammonium halides and (NH_{4})_{2}Cr_{2}O_{7} gave an empirical curve, . This curve was used to obtain an average barrier for ammonium ions in (NH_{4})_{2}SO_{4} , and in NH_{4}ClO_{4} , . Calculations of the scattering cross sections using harmonicoscillator approximations for torsional and translation–vibration motions are in reasonable agreement with the experimental results. A comparison of calculations based on a model of free rotation above the first‐order transitions of the ammonium halides indicates that there is significant hindrance to free rotation in the order NH_{4}Cl > NH_{4}Br > NH_{4}I.

Microwave Studies of Collision‐Induced Transitions between Rotational Levels. IV. Steady‐State Measurements in NH_{3}
View Description Hide DescriptionThe technique of high‐power microwave double resonance has been applied to the study of collision‐induced transitions between rotational levels of ammonia. The inversion doublets for selected values have been “pumped” by high microwave power, and the increase of absorption in other inversion doublets have been observed. Because of the abundance and intensities of ammonia lines, a systematic choice of 18 four‐level systems was possible. The observed results were analyzed by steady‐state equations, and relative values of rate constants have been determined. The experiments and analysis have led to the following qualitative conclusions:
(i) Collision‐induced transitions with dipole selection rules are “preferred.”
(ii) The dipole‐type transitions have much greater probability than the dipole‐type transitions for levels with , but they have probabilities of equal order of magnitude for levels with .
(iii) The transitions have much smaller probabilities than the transitions.
(iv) The transitions have much smaller probabilities than the transitions.
A comparison of the observed results with values calculated by using Anderson's theory suggests that the probabilities of the quadrupole‐type transition (parity +↔ +) are the same order of magnitude as those of the corresponding dipole‐type transitions.

Laser Raman Spectrum and Force‐Constant Calculation of Co(CO)_{3}NO
View Description Hide DescriptionThe laser‐excited Raman spectrum of liquid Co(CO)_{3}NO is presented and, in addition, the infrared spectrum of the liquid. A reassignment of fundamentals is made, and symmetry force constants are calculated using reported vapor‐phase frequencies.