Index of content:
Volume 35, Issue 6, 01 December 1961
Absolute Infrared Intensities of the CS2 Fundamentals in CCl4 Solution and of the 768–797 cm—1 Fermi Doublet of CCl4 in Cyclohexane Solution35(1961); http://dx.doi.org/10.1063/1.1732188View Description Hide Description
The absolute infrared intensities of two nonpolar solute molecules were determined in nonpolar solvents by extrapolating the apparent intensities to zero concentration. For CS2 the intensities were found to be 5.8×107 cm mole—1 in CCl4 in 5.2×107 cm mole—1 in C2Cl4 for the v 3 (1523 cm—1) mode, and 9.5×105 cm mole—1 in CCl4 for the v 2 (395 cm—1) mode. For CCl4 the intensity of the 768–797 cm—1 Fermi doublet was found to be 5.4×107 cm mole—1 in cyclohexane. The results are discussed in relation to the Polo‐Wilson, Person, and Buckingham expressions for the gas to solution intensity ratio. Modifications of the dielectric expressions were investigated.
s‐Tetrazine. I. High‐Resolution Vapor‐Phase Study of the Visible n→π Vibronic Absorption Band Systems35(1961); http://dx.doi.org/10.1063/1.1732189View Description Hide Description
The low‐resolution electronic spectrum of s‐tetrazine vapor has been measured down to about 190 mμ, and the complex systems of vibronic bands that comprise the n→π* visible absorptionspectrum have been studied under higher resolution. These latter spectra were photoelectrically recorded in the third order of a 21‐ft grating spectrograph, using path lengths from 0.05 to 48 m and temperatures from —70° to 70°C. In the absorption region, 4500 to 6100 A, the spectrum consists of over 500 sharp, vibronic band peaks. Their collected Q‐branch maxima have been measured to the nearest cm—1, but no discrete rovibronic structure was resolved.
The outstanding feature of the visible spectrum is a set of four intense progressions, each of which is propagated by the consecutive excitation of the same vibration (believed to be the totally symmetric mode 6a). In the ground state, the frequency of this vibration is 737 cm—1 for s‐tetrazine‐d 0 and 720 cm—1 for s‐tetrazine‐d 2; these frequencies drop to about 700 and 690 cm—1, respectively, in the excited n→π* state(s). From simple molecular orbital theory and the Herzberg‐Sponer‐Teller selection rules, a plausible, preliminary, singlet absorption model is developed and is qualitatively applied to the spectral data. Because there may be an ``almost accidental degeneracy'' of some of the n→π* states of the azines (excluding pyridine), the need for a more elaborate model that will properly treat the electronic‐vibrational interaction problem is emphasized. Although a complete vibronic analysis has not yet been obtained, the presence of three electronic transitions forbidden on the basis of D 2h excited statemolecular symmetry, as well as the electronically allowed n→π* transition, is suspected.
35(1961); http://dx.doi.org/10.1063/1.1732190View Description Hide Description
To aid in the vibronic analysis of the n→π* visible absorption band systems of s‐tetrazine vapor, infrared spectra of vapor, solution, and cold, solid film phases of natural and deuterated mixtures of s‐tetrazine have been obtained. By use of the product rule and by correlation of the fundamental modes of s‐tetrazine with those of benzene and the other azines, it is possible to make tentative vibrational assignments for all seven of the fundamental bands that would be expected to be infrared active in the wavelength region from 2 to 15 μ. However, the low‐frequency modes 16a and 16b, Au and B 1u , respectively, have not yet been observed, and the lack of knowledge of the frequencies of eight of the nine gerade modes (Raman active only) has prevented any definite assignment of combination bands. The observation of an interesting and peculiar phenomenon—what is believed to be the totally symmetrical mode 6a near 730 cm—1 in the solid film spectra of the various deuterated mixtures—is also reported.
Effects of Anharmonicity of Molecular Vibrations on the Diffraction of Electrons. II. Interpretation of Experimental Structural Parameters35(1961); http://dx.doi.org/10.1063/1.1732191View Description Hide Description
The effects of anharmonic vibrations on the molecular intensity function M (s) of electron diffraction and on the radial distribution functionf(r) considered in paper I, are treated more rigorously to meet current needs. More definitive general relations for interpreting experimental diffraction data in terms of molecular structure parameters of known absolute significance are presented.
35(1961); http://dx.doi.org/10.1063/1.1732192View Description Hide Description
The complete‐matrix least‐squares analysis on the neutron diffraction intensities of CaC2, YC2, LaC2, CeC2, TbC2, YbC2, LuC2, and UC2 yielded the C–C distances in the C2 groups, 1.191±0.009, 1.275±0.002, 1.303±0.012, 1.283±0.004, 1.293±0.009, 1.287±0.010, 1.276±0.012, and 1.340±0.007 A, respectively. The statistical averages of the neutron scattering amplitudes obtained from the dicarbide data and those reported previously are 0.488±0.007, 0.796±0.009, 0.832±0.014, 0.482±0.006, 0.756±0.020, 1.262±0.012, 0.73±0.02, and 0.851±0.022, all in 10—12 cm with positive sign, for Ca, Y, La, Ce, Tb, Yb, Lu, and U, respectively.
The paramagneticscattering analyses revealed that all metal atoms in these dicarbides are in their trivalent Hund ground state, except Ca (divalent), Yb (possible valency of 2.8), and U (possible tetravalent). The scattering data also deduced the screening constants for the 4felectrons in Ce, Tb, and Yb as 39, 43, and 43, respectively, which are compared with those obtained by other methods. The bond numbers for the rare‐earth dicarbides are briefly discussed.
35(1961); http://dx.doi.org/10.1063/1.1732193View Description Hide Description
The neutron diffraction data of the rare‐earth sesquicarbides with the Pu2C3‐type structure (D5 c type), La2C3, Ce2C3, Pr2C3, and Tb2C3, were analyzed using the complete‐matrix least‐squares method. These sesquicarbides contain the C2 groups with the C–C distances, 1.236±0.009, 1.276±0.005, 1.239±0.003, and 1.240±0.005 A, respectively, for La2C3, Ce2C3, Pr2C3, and Tb2C3. These values are, except for Ce2C3, significantly longer than the C–C distance in CaC2, 1.191 A, but are shorter than the average C–C distance in the rare‐earth dicarbides, 1.278 A.
The paramagneticscattering analyses show that all metal atoms in these sesquicarbides are in their trivalent Hund ground state, except Ce in Ce2C3, whose possible valency is about 3.4. The screening constants for the 4f hydrogenic radial wave functions of Pr and Tb as determined from the diffuse scattering analyses of their sesquicarbides are 40 and 43, respectively. The valency and metallic bonds in these sesquicarbides are briefly discussed.
Studies in Molecular Structure. VI. Potential Curve for the Interaction of Two Hydrogen Atoms in the LCAO MO SCF Approximation35(1961); http://dx.doi.org/10.1063/1.1732194View Description Hide Description
The LCAO MO SCF method, both in the single‐configuration and a limited configuration interaction approximation, has been utilized to evaluate the potential curve of the ground state of H2; wave functions and total molecular energies are presented for a wide range of the internuclear distance (1.0 a.u. ≤R≤18.0 a.u.). These solutions were then used to construct potential curves for the ground states of the molecular ions H2 + and H2 — and for the lower‐lying excited states of the neutral molecule. Results of vibration‐rotation analyses for the stable states are presented. The results for a population analysis of the single‐configuration ground‐state function over the whole range of R are given, and compared to similar results for He2.
35(1961); http://dx.doi.org/10.1063/1.1732195View Description Hide Description
Calcium sulfide activated by nickel and copper or nickel and silver gives an emission band at 680 and 625 mμ, respectively. In strontium sulfide the emission bands appear at 775 and 710 mμ, respectively. The presence of both activators is necessary for the production of these emissions.
35(1961); http://dx.doi.org/10.1063/1.1732196View Description Hide Description
A theory is proposed to explain the relationship between the structure and the scintillation efficiency of organic liquid scintillators.
The requirements for good scintillation properties in given solvent‐solute systems are found to be: (a) the existence of pairs of energy levels in the solute which are identical with those involved in the excitation of the solvent and (b) a nonrigid structure in the solute, capable of undergoing conformational changes in the excited state.
It was found that the behavior of liquid scintillators satisfied these requirements. Good scintillation properties were predicted for some hitherto unknown scintillators.
35(1961); http://dx.doi.org/10.1063/1.1732198View Description Hide Description
The NMR absorption of protons in hydroxyl and hydronium ions is displaced about 10 ppm to low field from the proton‐resonance absorption in water. This shift is roughly explained by a model locating all the ionic charge on the oxygen atom for both ions.
Energy Distribution and Fragmentation Processes Resulting from Electron Impact on Propane and n‐Butane35(1961); http://dx.doi.org/10.1063/1.1732199View Description Hide Description
The internal energy distribution has been determined for propane and n‐butane ions produced by electron impact. The method involves the use of differential ionization efficiency curves and depends on the validity of the linear threshold law for simple ionization. The experimental distributions are in fair agreement with theoretical calculations of Lennard‐Jones and Hall and indicate that the major fraction of the parent ions are formed by simple removal of a valence electron. Only a minor fraction of the parent ions are apparently formed by removal of one valence electron together with excitation of one or more remaining electrons. Breakdown curves for propane and n‐butane have been determined experimentally and, except for the energy scales, are in fair qualitative agreement with the predictions of the statistical theory of mass spectra. The discrepancy between theoretical and experimental energy scales is ascribed at least in part to inadequacy of the theoretical rate equations used in the statistical theory. There is no evidence for gross failure of the assumptions of the theory regarding energy randomization although the number of crossings of potential surfaces of the molecular ion is shown to be very much smaller than originally assumed by Rosenstock et al.
Billiard‐Ball Collisions in the Reactions of Hot Hydrogen Atoms. Isotope Effect on the Hydrogen Displacement in Methane35(1961); http://dx.doi.org/10.1063/1.1732201View Description Hide Description
The nature of collisions leading to reactions of hydrogen atoms of high kinetic energy (in the form of recoil tritium) is investigated. Two extreme models, both originally due to Libby, are examined. One assumes very weak coupling by valence bonds in the molecule attacked, so that the reaction can be represented by a quasi‐elastic atom‐atom collision (billiard‐ball model) and the other assumes strong valence bond coupling (epithermal model). Calculations on the billiard‐ball (hard sphere) model of the reaction T*+CH4→H+CH3T leads to an estimate of its relative cross section as a function of energy. An isotope effect for reaction with CH4 and with CD4 is calculated and measured experimentally. Consideration of both the calculations and the experimental results indicate that the billiard‐ball model is unimportant in the reactions of gas phase hot hydrogen (as recoil tritium), and that reactions occur nearly exclusively at less than 10–20 ev of kinetic energy, by mechanisms involving strong bond coupling. The possible importance of the billiard‐ball model in the reactions of other hot atoms is briefly discussed.
35(1961); http://dx.doi.org/10.1063/1.1732202View Description Hide Description
The absolute reaction‐rate theory for the unimolecular decomposition of isolated excited molecules is applied to two models. The first considers the molecule as a set of harmonic oscillators in which only a finite amount of energy may be stored in a particular degree of freedom. In the second model the spacing between adjacent levels is assumed to decrease exponentially. The first leads to a decrease in the lifetime above that calculated for the conventional harmonic oscillator model for short‐lived molecules. The second produces an increase depending directly on the rate of change of the level spacing.
Organic Semiconductors. V. Comparison of Measurements on Single‐Crystal and Compressed Microcrystalline Molecular Complexes35(1961); http://dx.doi.org/10.1063/1.1732203View Description Hide Description
Utilizing specialized micromanipulative techniques, measurements of conductivity, activation energy of conduction, and Seebeck coefficient have been performed on charge‐transfer complex single crystals. The results are compared with previous measurements on compressed microcrystalline materials.
35(1961); http://dx.doi.org/10.1063/1.1732204View Description Hide Description
Three previously proposed mechanisms of nuclear relaxation in fluids are qualitatively discussed: (1) modulation of intramolecular fields by collisions, (2) transient, and (3) diffusively modulated dipolar interactions with paramagnetic impurities. It is shown that the dependence of T 1 for the protons in methaneoxygen mixtures on composition, density, and temperature is adequately described by a superposition of these contributions to 1/T 1, and that the magnitudes involved are consistent with a priori estimates. It is suggested that the spin‐rotation interaction provides an important relaxation mechanism in both gaseous and liquidmethane.
35(1961); http://dx.doi.org/10.1063/1.1732205View Description Hide Description
The ultraviolet spectrum of dibenzene chromium vapor has been observed under medium resolution conditions. At least three separate band systems are found, at least two of which display progressions of the totally symmetric vibration frequencies. Possible assignments are considered and compared with alternative level structures. It is concluded that the 3de 2u level probably lies below the metal a 1g orbital, and that term splitting may play a considerable role in determining the order of the excited states.
Molecular Friction in Dilute Gases. II. Thermal Relaxation of Translational and Rotational Degrees of Freedom35(1961); http://dx.doi.org/10.1063/1.1732206View Description Hide Description
Expressions for the rates of equilibration of the translational energies in multicomponent gas mixtures of structureless molecules are derived in terms of characteristicrelaxation times. The approach to equilibrium is found to be completely dominated by an exponential decay factor for molecules interacting with inverse power potentials; the decay should be essentially exponential for more realistic potential functions. The calculations of rotational relaxation times are presented for several rigid molecular models: rough and partially rough spheres, spherocylinders, and loaded spheres. The relaxation times obtained are in good agreement with those observed experimentally and with those predicted by previous theories. It is found that, in general, on the order of 10 collisions are required to reduce an initial temperature difference by a factor of 1/e for translational‐rotational equilibration.
35(1961); http://dx.doi.org/10.1063/1.1732207View Description Hide Description
Protonmagnetic resonance spectra of vinyltrimethylsilane and divinyldimethylsilane in dilute cyclohexane solution, at 40 Mc/sec, have been analyzed and the parameters compared with those obtained for alkylethylenes under similar conditions. The results indicate the presence of d π—p π structures in these silanes. Additional anisotropy effects due to the presence of Si may also be involved but are believed not to be solely responsible for the effects observed. All the vinyl coupling constants are considerably greater than those observed in the alkenes, but are comparable to values obtained for molecules containing vinyl‐metal bonds.