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Volume 42, Issue 5, 01 March 1965

Paramagnetic Absorption of Single Crystals of Succinic Acid Irradiated at Low Temperature
View Description Hide DescriptionElectron spin resonance absorption was observed in single crystals of (–CH_{2}–COOH)_{2}, (–CH_{2}–;COOD)_{2}, and (–CH_{2}–^{13}COOH)_{2}irradiated at 77°K. The principal paramagnetic species observed at low temperature has an unpaired electron in a 2pπ orbital on the carbon atom of the carboxyl group. It is probably ionized succinic acid with the oxygen of the carboxyl group carrying a negative charge.

Ultrasonic Relaxation and Cis—Trans Isomerization in Isopropyl Formate
View Description Hide DescriptionThe ultrasonic absorption of dilute solutions of isopropyl formate has been measured over the frequency range 0.6 to 45 Mc/sec at temperatures from 30° to 80°C. The frequency dependence of the absorption is that of a single relaxation process. The concentration dependence of the relaxation parameters is consistent with the mechanism being the interconversion of trans and cis isomers by rotation about the carboxyl C–O bond, while the temperature dependence yields a difference in enthalpy and entropy between the isomers of 3.7±0.5 kcal/mole and 1.5±0.6 cal/mole/deg, respectively, and an activation enthalpy of 5.8±0.4 kcal/mole. It is suggested that a large effect in addition to dipolar interaction and steric repulsion is necessary to explain the differences in enthalpy between formate isomers. The ultrasonic absorption in pure isopropyl formate was also measured at 45 Mc/sec over the temperature range −60° to +60°C, and the results have been interpreted in terms of a second relaxation caused by internal rotation about the methoxyl C–O bond.

Use of Computer Programs in the Interpretation of Electron Paramagnetic Resonance Spectra of Dilute Radicals in Amorphous Solid Samples. I. High‐Field Treatment. X‐Band Spectra of π‐Electron Unconjugated Hydrocarbon Radicals
View Description Hide DescriptionThe successive stages of the calculation of a high‐field electron paramagnetic resonanceline shape for a radical dilute in a polycrystalline or glassy sample from the parameters of its spin Hamiltonian have been fitted into a computer program. We have considered the general case where there may be a tensor, anisotropic hyperfine couplings, and satellite lines arising from nuclei of any spin up to and approximated the component line shape by a Gaussian function of constant width. Possible uses of such a program are: (1) determining the line shapes associated with some model radicals and establishing approximate rules to facilitate the interpretation of experimental spectra; (2) checking the validity of an assignment of tensors made on the basis of some plausible structure for a species under investigation; and (3) finding the parameters which make the best fit to an observed spectrum. Applications to some π‐electron hydrocarbon radicals are presented, and the possibilities offered by such calculations are evaluated.

Use of Computer Programs in the Interpretation of Electron Paramagnetic Resonance Spectra of Dilute Radicals in Amorphous Solid Samples. II. Zero‐Field Treatment for Several Nuclei of Spin 1/2
View Description Hide DescriptionThe successive stages of the calculation of a zero‐field magnetic resonance spectrum for a radical with nuclei of spin ½ from its hyperfine coupling tensors have been fitted into a computer program. An application is made to two three‐proton radicals studied by Cole, Kushida, and Heller.

Kink Poisons and the Reduction of Dissolution Rate at Crystal—Liquid Interfaces
View Description Hide DescriptionThe effect of contaminants on solubility and dissolution rate was studied in a system of a (100)‐faced lithium fluoride single crystal and a very dilute solution containing ferric ions. The solubility of the crystal was determined in contaminant solutions of varying concentrations and at different temperatures. The dissolution‐rate experiments were carried out in a rotating‐disk apparatus. It was found that the reduction of the dissolution rate produced by ferric ions could be explained by the changed solubility of the (100)‐faced crystals.

On the Difference between the Thermodynamic Behavior of Argon in D_{2}O and H_{2}O
View Description Hide DescriptionThe solubility of argon was measured in D_{2}O (99.7%) at five temperatures: 5°, 10°, 15°, 20°, 25°C. The standard entropies,enthalpies, and free energies of solution were calculated, and compared with the corresponding values in H_{2}O. The differences in the thermodynamic functions of solution in D_{2}O and in H_{2}O are interpreted in terms of the ``two‐structure'' model for liquid water, and are attributed to the difference in the ``degree of crystallinity'' of the two solvents.

On the Singlet‐Exciton States of Crystalline Anthracene
View Description Hide DescriptionIn this paper we present a detailed analysis of the lower excited states of crystalline anthracene. Starting with zero‐order product wavefunctions, the treatment differs from standard formulations in that interactions between molecules are computed directly by the use of π‐electron theory, by the inclusion of the effects of extensive configuration mixing, and by the inclusion of long‐range interactions out to the convergence limit. It is found that:
(1) The computation of interaction energies cannot be reduced to dipole—dipole terms alone. By the use of π‐electron theory it is shown that short‐range high‐order multipole (greater than dipole) interactions make important contributions to both the diagonal and off‐diagonal elements of the energy matrix.
(2) Long‐range interactions of the dipole—dipole type are of importance for distances of the order of the wavelength of light. By application of momentum‐conservation conditions, it is shown that the long‐range dipole—dipole interactions, including the effects of retardation of the potential, are absolutely convergent. Major contributions to the Davydov splitting arise from molecular separations ranging from 50 Å to the convergence limit.
(3) For the case of allowed singlet—singlet transitions, electron‐exchange interactions are small relative to other contributions to the interaction energy.
(4) Under the experimental conditions used to date, the Davydov splitting should be independent of crystal thickness.
(5) In anthracene, crystal‐field mixing of the p and β molecular states has a large effect on the Davydov splitting. Inclusion of mixing with higher excited π states has little effect on the Davydov splitting, but is required in the calculation of the polarization ratios in the vibronic components of the p band.
(6) Charge‐transferexciton states play only a minor role in altering the properties of singlet exciton states arising from allowed transitions.
(7) The detailed calculations reported herein yield good agreement with the observed Davydov splitting (ΔE) and polarization (P) ratios in anthracene, e.g., for the ^{1} A _{1g }→^{1} B _{2u } band:

Search for a Charge‐Transfer State in Crystalline Anthracene
View Description Hide DescriptionIn this paper we examine the calculation of the location of the charge‐transfer state in crystalline anthracene. Also reported are the results of spectroscopicexperiments designed to find the transition from the ground state to the charge‐transfer state. No experimental evidence could be found for this transition, and it is thereby concluded that: (a) The CT state lies above the first exciton state in anthracene. (b) The polarization energy in the CT state is much less than that calculated on the basis of classical considerations. There is a brief discussion of the nature of the polarization in aromatic crystals, especially with respect to Conclusion (b).

Threshold Shapes and Resonances in the Photodetachment Cross Sections of Chloride, Bromide, and Iodide
View Description Hide DescriptionThe photodetachment curves of Cl^{—}, Br^{—}, and I^{—} have been examined in their threshold regions by absorption spectroscopy of shock‐heated alkali halide vapors. The chloride curve appears smooth and normal in terms of threshold laws, but the bromide and iodide curves show maxima about 500 cm^{−1} above the thresholds, with widths of about 200 cm^{−1} at half‐height. The threshold positions are not affected by changes in conditions. The anomalously large absorption or resonances seem to be due to the halide ions, but to be influenced to some degree by the alkali atoms or ions. Three explanations, none entirely satisfactory, are put forth for the maxima; these are (1) near‐resonant transfer of energy from X+e ^{—} to M, giving X^{—}+M^{*}; (2) metastable autodetaching states of the halides; and (3) mixing of vacant alkali ion orbitals with continua of the halides. The third is considered most plausible.

Molecular Collision Cross Section Due to Quadrupole‐Induced Dipole and Dipole‐Induced Quadrupole Interactions
View Description Hide DescriptionA theory for molecular collision cross section due to quadrupole‐induced dipole and dipole‐induced quadrupole interactions has been developed following the Anderson—Tsao—Curnutte approach. The OCS–He, OCS–Ar, and HCl–Ar collisions are discussed. For HCl–Ar collisions, μ_{1} q _{1}α_{2} interactions have been found to be important, whereas in OCS–He and OCS–Ar collisions these are negligibly small.

Calculation of Zero Field Splitting in NH. I. One‐Center Minimal Basis and Atomic Orbital Representations of the Ground State
View Description Hide DescriptionThe zero field splitting parameter D is calculated for the ground state of NH (^{3}Σ_{ g } ^{—}) using a variety of single Slater‐type atomic orbitals and atomic SCF functions for nitrogen. A minimal basis (five basis functions) one‐center expansion SCF calculation has been performed for ^{3}Σ_{ g } ^{—} NH. The resulting molecular orbitals produce D=1.6279 cm^{−1}. Spin—orbit contributions to D are estimated and found to be small.

Radio‐Frequency and Microwave Spectra of ^{6}Li^{127}I by the Molecular‐Beam Electric‐Resonance Method
View Description Hide DescriptionThe radio‐frequency and microwave spectra of ^{6}Li^{127}I have been observed by the molecular‐beam electric‐resonance method of molecular spectroscopy.Spectra of ^{6}Li^{127}I in the first two vibrational states were obtained. From the J=1 radio frequency, Stark spectra values were obtained for the electric dipole moment μ, the iodine nuclear quadrupoleinteractioneqQ _{I}, and the iodine spin‐rotation constant c _{I}. The values of these constants are
The observation of J=1→J=0 microwave transitions yielded the following rotational constants:

Splitting of ^{19}F NMR Line in a Co^{2+}‐Doped NaF Crystal
View Description Hide DescriptionSuperimposed on the primary ^{19}F nuclear magnetic resonance line from a room‐temperature Co^{2+}‐doped NaF single crystal are 10 intense, evenly spaced, field‐independent, strongly anisotropic secondary ^{19}F lines of unknown origin. Neither these lines nor several additional anomalies of the NaF:Co^{2+} crystal are observable in an undoped but otherwise identical NaF crystal.

Molecular Orbital Theory of Diamagnetism. V. Anisotropies of Some Aromatic Hydrocarbon Molecules
View Description Hide DescriptionMolecular orbital theory is used to calculate the diamagnetic susceptibility contributions of local intraatomic currents in a series of polycyclic aromatic hydrocarbons. It is found that a considerable part of the excess diamagnetism perpendicular to the molecular plane is due to the anisotropy of these local terms so that the contribution of interatomic ring currents is less than has been believed previously. The theory also reproduces the trend for the in‐plane susceptibility to be (numerically) greatest along the long axis of the molecule.

Molecular Magnetic Moments and Susceptibility in Formaldehyde
View Description Hide DescriptionHigh‐resolution microwave spectroscopy has been used to measure the molecular Zeeman effect in formaldehyde where both ΔM = 0 and ΔM = ±1 transitions were observed. The magnitudes and signs of the diagonal elements of the gtensor were determined.
The results for OCH_{2} are  g_{bb}—g_{cc}  = 0.08±0.03, g_{aa} = −2.935±0.03, g_{bb} = −0.17±0.05, and g_{cc} = −0.11±0.05. These results combined with the value of the bulk susceptibility yield the following value depending on the ground‐state wavefunction for OCH_{2}: 〈ψ^{0}  Σ_{ i } r_{i} ^{2} ψ^{0}〉 = 14.61×10^{−16} cm^{2}, where the sum is over all electrons.
The average value of the diamagneticsusceptibility in OCH_{2} is χ^{ d } = −41.3×10^{−6}. The average value of the paramagneticsusceptibility is χ^{ p } = 40.8×10^{−6}. The anisotropy in the paramagneticsusceptibility along the O–C bond is χ⊥^{ p }—χ∥^{ p } = 5.6×10^{−6} where χ⊥^{ p } is perpendicular to the CH_{2} plane.

Scattering of Slow Neutrons from Ammonia and Hydrogen Sulfide
View Description Hide DescriptionMeasurements of the scattering of slow neutrons from samples of gaseous ammonia and hydrogen sulfide have been made using the MTR phased‐chopper velocity selector. The energies of incident neutrons were 0.025, 0.07, and 0.1 eV and the scatteredneutrons were detected in counter banks in an angular range of 16° to 145°. The data obtained have been converted to reduced partial differential cross sections and have been compared with theoretical calculations based on scattering from a symmetric top molecule, from a spherical‐top molecule and from a Krieger—Nelkin gas. The symmetric top gives the best fit to the data for both samples. This theory fits the ammonia data better than the hydrogen sulfide data; the deviation is at large energy transfers and is larger for hydrogen sulfide than for ammonia. The zeroth and first moments of energy transfer have been obtained from the data for the experimental range of momentum transfer. The zeroth moments for ammonia agree with the predicted values based on classical theory, but they are about 10% high for hydrogen sulfide. The experimental first moments for both molecules suggest an effective mass roughly 20% lower than the Sachs—Teller mass.

Behavior of Thirteen Charge‐Transfer Complexes at High Pressures
View Description Hide DescriptionThe effect of pressure to several hundred kilobars has been measured on the properties of 13 complexes containing aromatic molecules. These complexes are classified in four groups: those containing tetracyanoethylene (TCNE), those containing iodine, those involving p‐phenylenediamine, and halogenated p‐benzoquinones, and those involving the halogenated p‐benzoquinones and other materials.
In all cases the resistance decreased rapidly with increasing pressure at low pressures. A number of complexes exhibited minima in resistance at pressures from 60 to 200 kbar and an irreversible rise at higher pressures. The nature of the irreversible process could in several cases be postulated from difference in the visible and infrared spectra between the transformed and untransformed complexes.

Spin‐Echo Studies of Chemical Exchange. II. Closed Formulas for Two Sites
View Description Hide DescriptionClosed expressions are derived for the decay in amplitude of successive echoes in nuclear magnetic resonance experiments for systems with two exchanging sites having different resonance frequencies but equal transverse relaxation times ``in the absence of exchange.'' The decay for a Carr—Purcell train of echoes is found to be the sum of two exponential terms and two ``pseudoexponential'' terms; the absolute values of the latter decay exponentially but the sign alternates from one echo to the next. For the case of two equally populated sites the decay simplifies to the sum of just one exponential and one ``pseudoexponential'' term. In most cases of experimental importance, a single exponential term is much larger than the rest and adequately describes the decay in echo amplitude.
An iterative computer procedure is described which uses the expressions to extract information about the system from measurements of the apparent decay time constant (1/T _{2}) as a function of 180° pulse separation (t_{cp} ). In favorable cases the method yields values not only of the lifetime between exchanges (1/2τ) but also of the chemical shift (δω) and the relaxation time in the absence of exchange (1/T _{2} ^{0}). Moreover, it is applicable to both slow and fast exchange, whereas use of the approximate equation derived by Luz and Meiboom leads to inaccurate values for relatively slow rates with large chemical shifts between the sites. Our experience shows that the spin‐echo technique can be very useful for the study of fast exchange processes, but even when closed expressions of appropriate accuracy are used the nature of these expressions is such that a high‐speed computer is required for fitting the experimental data, unless some ``outside'' information is available about δω and 1/T _{2} ^{0}.

Luminescence and Energy Transfer in Solutions of Rare‐Earth Complexes. I. Enhancement of Fluorescence by Deuterium Substitution
View Description Hide DescriptionFluorescence yields and lifetimes of solutions of europium and terbium salts in solution are greatly enhanced by using deuterated solvents. The magnitude of increase varies from an 18‐fold enhancement in yield and lifetime for an aqueous solution of europium nitrate to a twofold enhancement for europium ethylenediaminetetra‐acetate dissolved in methanol. The enhancement is proportional to the fraction of deuterated solvent present. Results lead to conclusions that the dominant mode of radiationless deactivation of a rare‐earth ion in solution is via loss of energy to hydrogen vibrations. Further, such loss is proportional to the number of hydrogen vibrations in the local rare‐earth environment. Values for the rate constant for radiationless loss of energy to hydrogen vibrations are determined for the salts studied. The influence upon radiationless processes of complexing between the rare earth and anion is discussed. Likewise the dependence of the enhancement factor upon the rare‐earth ion is considered. There is no evidence that the lifetime from the ^{5} D _{1} level of europium nitrate is enhanced by deuterium substitution. This lifetime is ≤5 μsec in both D_{2}O and H_{2}O solutions.

Theoretical Investigations on the Light Scattering of Spheres. XVI. Range of Practical Validity of the Rayleigh Theory
View Description Hide DescriptionThe errors committed on using the Rayleigh equations of light scattering at finite α values were evaluated numerically for spherical nonabsorbing particles using exact Mie data as the reference. The errors considered are Δ_{α}, Δ_{α}′, Δ_{τ}, and Δ_{ I }. Considering an error of 2% as the tolerable maximum, the upper limiting particle diameters which may be evaluated by means of the Rayleigh equations are, expressed in units relative to the wavelength (in the medium) used, 0.124 and 0.207 at m=1.00 and 1.30, respectively. These data which apply to the use of lateral scattering differ little from the limiting data obtained for diameters derived from turbidity. The errors committed on evaluating turbidity or lateral scattering from the Rayleigh equations are distinctly larger at identical true α values, and at identical m values. The interesting fact that the Rayleigh equations perform better the larger m—as long as m<∼1.8—opens up the possibility of using these equations for the evaluation of relative large particle diameters provided m is sufficiently large. Thus at m=1.63±0.2, particle diameters, dimensionally one‐third the wavelength in the medium, are obtainable from the Rayleigh equations with 100% theoretical accuracy. This `^{.}second'' range of validity of the Rayleigh theory appears to be of major meteorological and astrophysical interest.