Volume 33, Issue 6, 01 December 1960

Electron Spin Resonance of an Irradiated Single Crystal of N‐Acetylglycine
View Description Hide DescriptionElectron spin resonance absorption of an irradiatedsingle crystal of N‐acetylglycine was observed at room temperature at 9 kMc and 12 kMc. From the analysis of the anisotropy in the spectroscopic splitting factor and in the nuclear hyperfineinteraction constant, a chemical structure[Complex chemical formula]is deduced for the free radical. The C–H bond is in the NCC plane and approximately along the bisector of the NCC angle. The unpaired electron spin density is essentially in a π orbital, about 72% of which is the p orbital of the CH carbon directed perpendicular to the NCC plane.

Forces in Molecules. I. Application of the Virial Theorem
View Description Hide DescriptionIt is shown that the virial theorem can be used to derive a force constant expression that sheds new light on the process of molecular binding. In connection with the latter the quantum‐mechanical virial theorem is derived in such a way as to be useful in discussing molecular applications in which the Born‐Oppenheimer approximation is applied.

Molecular Size Distribution and Gelation of Irradiated Copolymers
View Description Hide DescriptionA theory is developed about the molecular size distribution in a system of copolymers which are composed of two different kinds of monomer units and undergo random crosslinking when exposed to high‐energy radiations. The radiation‐induced change in the molecular size distribution is described by an integro‐differential equation, which we solve exactly by means of Laplace transform for an arbitrary initial distribution to obtain the number‐average as well as the weight‐average degree of polymerization as a function of radiation dose. Further, the gel point, or the dose required for incipient gelation, is given as a function of concentrations of the monomer units constituting the copolymeric system. The present theory, which also involves the problem of crosslinking in a mixture of two kinds of polymers as a special case, leads to a condition for gelation of such a mixture due to irradiation when crosslinking competes with main‐chain scission. This condition proves to be completely independent of the initial distribution.

Infrared Absorption Spectrum of NdCl_{3}
View Description Hide DescriptionThe infrared absorptionspectrum of NdCl_{3} is obtained under high resolution which show the transitions from the ground state to ^{4} I _{13/2} and ^{4} I _{15/2} the latter of which was hitherto unknown. The crystals were 5% and 50% NdCl_{3} in LaCl_{3} and the higher Nd concentration shows a decided frequency shift of the lines.

Physical Properties of Isotopically Substituted Classical Fluids
View Description Hide DescriptionThe observed differences in physical properties between isotopically substituted classical fluids are treated in terms of the theorem of corresponding states. It is shown that these differences are at least partially due to changes in the intermolecular potential energies of interaction. When the potential function of an isotopically substituted molecule is conformal with that of the unsubstituted molecule, the differences in the physical properties of the fluids can be quantitatively related to the differences in the depth and spatial extension of the potential functions, and to the changes in the internal degrees of freedom of the molecules. It is shown that the measured properties of perdeutero benzene‐benzene and perdeutero cyclohexane‐cyclohexane are consistent with these equations, and that the values of the parameters chosen to fit the theory to the data are reasonable. The limits of applicability of this treatment and its possible extension to less simple systems are discussed.

Pressure Effects in Nickel Dimethylglyoxime and Related Chelates
View Description Hide DescriptionThe effect of pressure on the absorption peak characteristic of the solid phase of certain metal chelates has been measured for Ni(DMG)_{2}, Pd(DMG)_{2}, Pt(DMG)_{2}, Ni nioxime, Pd nioxime, Ni benzoxime, and Ni heptoxime. The peak shifts strongly red with pressure. At high pressure the shift levels off for nickel, tends to reverse for palladium and reverses strongly for plantinum. There is considerable pressure broadening. These and other related observations are consistent with the assignment of the transition as from the A _{1g } (or A _{1g } and E_{g} ) ground state to the A _{2u } (p_{z} ) excited state.

Mass Spectrometric Study of Heats of Dimerization of Alkali Chlorides
View Description Hide DescriptionThe heats of sublimation of the important species in equilibrium with the five alkali chlorides have been determined using the Bendix time‐of‐flight mass spectrometer. The mass spectrometrically determined differences between the heats of sublimation of monomer and dimer have been combined with the best available value for the monomer heat of sublimation to calculate the dimerization energies for all five salts. For LiCl a trimerization energy was also determined. These results are compared with the results of previous studies.

Potential Energy Curves for CO
View Description Hide DescriptionPotential energy curves for the X ^{1}Σ^{+}, a ^{3}π_{ r }, a′ ^{3}Σ^{+}, d ^{3}Δ, e ^{3}Σ^{—}, A ^{1}π, and B ^{1}Σ^{+} electronic states of the CO molecule have been calculated by the Rydberg‐Klein‐Rees method. The curve for the A ^{1}π state will have to bend sharply in the range between 1.9 and 2.1 A or it will have to pass through a maximum to reach the proper dissociation limit.

Halogen Bond Character in the Alkyl Halides
View Description Hide DescriptionThe quantity (I+s ^{2}), the sum of the ionicity and s hybridization, has been calculated from nuclear quadrupole coupling data for 24 alkyl halides. The ionicity has been estimated independently from calculated values of ε_{ x }, the effective charge on the halogen. The average derived values of s ^{2}, the amount of s character in the hybrid halogen bonding orbital, are 13.6% for carbon‐chlorine bonds, 8.6% for carbon‐bromine bonds, and 1.8% for carbon‐iodine bonds.

Trimethylene Oxide. I. Microwave Spectrum, Dipole Moment, and Double Minimum Vibration
View Description Hide DescriptionThe microwave spectra of four isotopic species of trimethylene oxide have been investigated. Analysis of the spectra indicates that the four‐membered ring is essentially planar. From the intensity measurements of the rotational transitions and their vibrational satellites, it was concluded that the energy levels in the ring‐puckering vibration are single levels. The Stark effect also provided independent evidence that the ring is not highly puckered. The dipole moment of the molecule was found to lie solely on the C–O molecular axis. A value of 1.93±0.01 Debye was obtained. The existence of a small barrier in the potential function, however, was established by a detailed analysis of the observed vibration‐rotation interaction. Quantitative considerations have led to an accurate determination of the barrier height and the general shape of the potential function. The barrier restricting the ring‐puckering motion has been found to be 35±5 cm^{—1}. The ground vibrational level is 8±4 cm^{—1} above the top of the barrier.

Small Angle X‐ray Scattering Study of Sodium Montmorillonite Clay Suspensions
View Description Hide DescriptionSuspensions of montmorillonite clay with concentrations of ½ and 2% have been investigated by small angle x‐ray scattering, with two different centrifuge fractions being studied for each concentration. At a given concentration the scattering was observed to be the same for both centrifuge fractions. The scattered intensity was always proportional to the concentration. These results are what would be expected from dilute suspensions of thin platelets of identical thickness but different surface areas. When corrections for the finite height of the collimating slits are applied to the scattering curves, good agreement with the theoretical curves for thin platelets is obtained. From the deviation between the experimental and the theoretical curves at the largest observed scattering angles, the thickness of the platelets was calculated to be 9 A. An independent determination of the thickness can be made from a measurement of the zero angle scattered intensity from some material for which the ratio of the scattered intensity to the scattering from a single electron can be calculated. Two heavy gases, SF_{6} and C_{2}Cl_{2}F_{4} (Freon 114), were used for this purpose. The feasibility of using Ludox colloidalsilica as a third standard was investigated. Within experimental accuracy, the thickness values calculated from these data agree with the values obtained from the shape of the scattering curves. Both values agree with the thickness determined from the light scattering experiments of M. B. M'Ewen and M. I. Pratt [Trans. Faraday Soc. 53, 535 (1957)]. Since the scattering curve for the clay is proportional to the inverse square of the angle at the smallest observed angles, the large dimensions of the particle must be at least 500 A.

Excitation and Negative Ions in H_{2}O
View Description Hide DescriptionBy combining the trapped‐electron method with the study of negative ions and positive ions, it is possible to obtain data which can be used for construction of an approximate potential energy diagram of molecules. The experimental methods used are described and the H_{2}O molecule is discussed. The negative ion current peaks at 6.5±0.1 and 8.8±0.1 ev. Kinetic energy measurements on the negative ions show that the latter peak is associated with the same state at infinite separation as the first peak. The inelastic loss processes are essentially in agreement with optical absorption experiments. An inelastic process with a threshold at about 3.4 ev is observed.

Ultraviolet and Structural Studies of Polythionates
View Description Hide DescriptionThe ultraviolet spectral results of the polythionates are discussed in light of various available theoretical approaches in evaluating inorganic complex spectral intensities and band positions. A semiempirical approach shows that the oscillator strengths of the polythionate electronic spectra are related to the π orbitals of the sulfur chain.

Infrared Spectra of Complexes of NO_{2}F with BF_{3}, PF_{5} and SbF_{5}
View Description Hide DescriptionThe infrared spectra of complexes of NO_{2}F with BF_{3}, PF_{5}, and SbF_{5} have been studied. In each case bands at about 2360 cm^{—1} were recorded and have been attributed to the nitronium ion NO_{2} ^{+}. Bands consistent with a complex anion of the form MF_{ n+1} ^{—} have been observed, though with less accuracy. It is considered correct to designate these complexes as NO_{2} ^{+}BF_{4} ^{—}, NO_{2} ^{+}PF_{6} ^{—}, and NO_{2} ^{+}SbF_{6} ^{—}.

Generation of Free Carriers in Photoconducting Anthracene. I
View Description Hide DescriptionThe source of free carriers in photoconducting anthracene was determined from an investigation of the spatial distribution of trapped electrons. Free carriers are generated in the bulk in addition to electrons being injected into the anthracene at the negative electrode. The bulk generated carriers cannot come from an intrinsic process which simultaneously yields a free electron and a free hole. A tentative extrinsic model for the generation of free carriers is proposed in which electrons are injected at the negative electrode, free holes and trapped electrons are generated by the incident radiation in the bulk at impurities or other defects, and free holes are generated by the incident radiation at the positive illuminated electrode. On the basis of these results, one concludes that anthracene is an extrinsic rather than an intrinsic photoconductor.

Methyl Affinities Determined by Photolysis of Azomethane
View Description Hide DescriptionThe thermal decomposition of acetyl peroxide forms acetate radicals which decarboxylate into methyl radicals. The reaction products may result, therefore, either from methyl radicals or from acetate radicals which decarboxylate simultaneously as they interact with the substrate. To identify the reacting species, reactions previously investigated in an acetyl peroxide system were reinvestigated using the photolysis of azomethane as the source of radicals. The results prove that at least in hydrocarbon solvents methyl radicals, and not acetate radicals, are the reacting species in the acetyl peroxide system. Furthermore, it was shown that photolysis of azomethane in solution provides us with a clean and simple system to study reactions of methyl radicals.

Transferability of Urey‐Bradley Force Constants. V. Bromoethylenes
View Description Hide DescriptionUrey‐Bradley force constants (UBFC's), calculated by a least‐squares method, were obtained for ethylene, vinyl bromide, vinylidene bromide, trans‐dibromoethylene, cis‐dibromoethylene, tribromoethylene, and tetra bromoethylene. Even with complete isotopic data it was found impossible to determine all the force constants for an individual bromoethylene; moreover, from the viewpoint of giving stability to the calculation, values have been supplied by transfer from other molecules within this series and these force constants have been constrained in the least‐squares refinement. By inspection of the correlation matrix, the uncertainties in the various force constants were, in many cases, found to be intimately related, and, it was usually found unnecessary to constrain both force constants of an indeterminate pair; in this manner we have kept the number of transferred force constants to a minimum. A comparison of the remaining UBFC's shows that they are moderately transferable. Although the flexible bond parameter S _{HH} was found to be significant (but not transferable) in those molecules containing hydrogen atoms trans to one another, i.e., ethylene, vinyl bromide, and trans‐dibromoethylene, its value in cis‐dibromoethylene was small and made no significant contribution in explaining the observed frequencies. A potential function which includes a quadratic cross term between trans‐hydrogen bending coordinates was found to be adequate throughout the series and moderately transferable.

Dynamical Jahn‐Teller Effect in Hydrocarbon Radicals
View Description Hide DescriptionThe first part of this paper shows how the ordinary Born‐Oppenheimer approximation for separating nuclear and electronic motion can be adapted to a degenerate electronic state. To set up equations of motion for the dynamical Jahn‐Teller effect in their simplest form we use vibrational amplitudes associated with special linear combinations of the degenerate electronic wave functions, chosen to vary as slowly as possible with nuclear displacements. We also discuss briefly the symmetry‐forbidden electronic transitions allowed by a Jahn‐Teller distortion. In the second we make molecular orbital calculations of the energies and distorted shapes of some aromatic hydrocarbon molecules. The differences in energy between the distorted and symmetrical shapes (in kcal/mole) are cyclobutadiene 11.43; cyclopentadienyl 1.414, cycloheptatrienyl 0.859; benzene negative ion 1.077; triphenylene and coronene negative ions 0.385 and 0.299. In the last three each shape of minimum energy is separated from two equivalent ones by a small potential barrier, respectively, 0.000, 0.001, and 0.002. The ground state of each radical is doubly degenerate, and it can oscillate about a continuous series of distorted shapes. In the excited ^{1} E _{1u } ^{+} and ^{3} E _{1u } ^{+} states of benzene the distortions are much smaller, and the CC bonds probably bend rather than stretch.

Frequency Distribution and the Specific Heat of NaCl at Low Temperatures
View Description Hide DescriptionUsing the Kellerman model of the NaCl crystal, the eigenfrequencies at 0°K are found from those at room temperature by a first‐order perturbation theory, which is developed here. From this the frequency distribution at 0°K is obtained and is compared with the room‐temperature frequency spectrum. The specific heat and apparent Debye temperatures below 100°K based on both distributions as well as a linearly weighted average are given.

Statistical Mechanics of Molecular Ions
View Description Hide DescriptionThe N_{4} ^{+} ion is shown to be able to be in equilibrium against dissociation into N_{2} ^{+} and N_{2} under specified experimental conditions only if it possesses a considerable degree of vibrational excitation. The A_{2} ^{+} ion, lacking the numbers of modes of vibrational freedom possessed by the N_{4} ^{+}, is therefore unstable under the same conditions. It would become stable in much weaker electric fields and simultaneously higher pressures. Experimentally, the A_{2} ^{+} ion is known to exist in contradiction to this prediction, but theoretical and experimental evidence is advanced that it is metastable and subject to dissociation with sufficient numbers of collisions in the gas.