Volume 40, Issue 9, 01 May 1964
Index of content:
40(1964); http://dx.doi.org/10.1063/1.1725544View Description Hide Description
A study was made of the initiation of a deflagration wave at the surface of ammonium perchlorate—copper chromite—carbon pellets in nitrogen at 25 atm. The relationship between energy flux density I and exposure time τ I , describing the boundary conditions necessary for initiation, was measured. Integration of the governing partial differential equation using experimentally determined chemical and physical parameters and the boundary conditions gave the temperature T(0, τ I ) of the initiating surface at the moment τ I of cutoff of the energy pulse, and a lower bound for the minimum thermal ignition time τ im .
T(0, τ I ) for ammonium perchlorate with 5% copper chromite was 380±31°C. It was drastically lowered when carbon was added, the amount of lowering being proportional to carbon concentration. It ranged from 300° to 215°C for carbon percentages ranging from 0.5 to 4. For given composition T(0, τ I ) was independent of flux density over the range 9 to 63 cal/cm2·sec. Initiation occurred substantially before the calculated minimum thermal ignition time τ im . The results provide support for the view that thermal ignition of the solid material is not a prerequisite for initiation of a deflagration wave.
Calculation of Electronic Spectra of Nonalternant Conjugated Hydrocarbons by the Semiempirical Method of Limited Configuration Interaction40(1964); http://dx.doi.org/10.1063/1.1725545View Description Hide Description
The semiempirical method of configuration interaction (the Pariser—Parr approximation) assuming equal bond lengths has been used to calculate the excitation energies of π‐electron transitions for 45 nonalternant hydrocarbons (fluoranthene, azulene, the tropylium cation, the cyclopentadienide anion, fulvene, heptafulvene, their benzo derivatives, acenaphthylene, pleiadiene, and acepleiadylene). Oscillator strength has also been calculated for the most interesting of the compounds. A critical comparison with experimental data in presented and the agreement is found satisfactory. The extent to which the present results resemble those of other methods is discussed and some explanation of the successes of the simple Hückel method is obtained.
40(1964); http://dx.doi.org/10.1063/1.1725546View Description Hide Description
HNCO and DNCO suspended in Ar and in N2 matrices at 4° and at 20°K are shown to yield upon photolysis the corresponding tautomers, HOCN and DOCN, for which vibrational assignments are proposed. HOCN may be produced by recombination at the O atom of H+NCO produced in a primary photolytic rupture of the N–H bond, as well as by the attack of NH (3Σ—) on the carbon—oxygen bond of CO, giving an unstable cyclic intermediate.
40(1964); http://dx.doi.org/10.1063/1.1725547View Description Hide Description
The infrared spectra of FN3, ClN3, and BrN3 suspended in Ar and N2 matrices have been observed at 4° and at 20°K. Ultraviolet photolysis of these systems results in the appearance of absorptions assigned to the free radicals NF, NCl, and NBr. Evidence is presented for the matrix deactivation of NBr to its ground 3Σ— state. Slow photolytic decomposition of NCl and NBr has been observed. Possible mechanisms are discussed for the origin of a green phosphorescence identified as resulting from N atoms trapped in the matrix.
Semiempirical SCF—LCAO—MO Calculation of the Electronic Structure of the Guanine—Cytosine Base Pair: Possible Interpretation of the Mutagenic Effect of Radiation40(1964); http://dx.doi.org/10.1063/1.1725548View Description Hide Description
The π‐electronic structure of the guanine—cytosine nucleotide base pair has been investigated in the semiempirical SCF—LCAO—MO approximation.
An examination of the energy levels with respect to the single bases showed that the transition between the highest filled and lowest unfilled π orbitals (n *→n *+1) takes place between the levels originating from two different single bases. This statement follows from considering the n * and n *+1 levels of the base pair as perturbed ones of the guanine, cytosine single bases, respectively. This view is substantiated by the structure of the eigenvectors of the respective states. The transition between these states is associated with a transfer of charge, which in the investigated case involves the transfer of a unit charge from guanine to cytosine.
Since in the charge‐transfer state the bases are essentially in an ionic form, a considerable increase in the probability of a single‐proton tunnelling, proposed by Löwdin, may be expected. The interpretation of radiation‐induced mutations in terms of the charge‐transfer state and single‐proton tunneling is further discussed.
40(1964); http://dx.doi.org/10.1063/1.1725549View Description Hide Description
Taking as model systems the polynucleotides poly A, poly T, poly G, poly C, and poly U in a stereo arrangement according to the Watson—Crick model of DNA, we have calculated the energy bands of these macromolecules in the Hückel approximation. Simple polynomial expressions were found for the energy values as functions of cosk, and with the aid of these the limits of all the occurring energy bands were determined. Further in the paper we discuss some features of the obtained band structures.
40(1964); http://dx.doi.org/10.1063/1.1725550View Description Hide Description
The extended Hückel theory is applied to compounds of boron and nitrogen, with emphasis placed upon similarities and differences with isoelectronic and isosteric carbon analogs. The barrier to internal rotation in borazane is predicted to be ∼1.5 kcal/mole and the torsional barrier in aminoborane∼10 kcal/mole. The internal charge‐transfer nature of the expected electronic transitions is stressed. The highest occupied orbitals in aminoborane and borazine are computed to be σ type, with important consequences for the reactions of these molecules. It is proposed that the accepted valence picture of borazine is incorrect and that in all B–N molecules N is more negative. Stabilization due to nonbonded electrostatic interactions is studied. Predictions are made regarding the geometry of B–N analogs of cyclobutadiene and cyclooctatetraene. Borazine is calculated to be inefficient in transmitting electronic effects. Computations of the relative stabilities of heteroaromatic B–N compounds are made.
40(1964); http://dx.doi.org/10.1063/1.1725551View Description Hide Description
The conformations, relative stabilities, and electronic distribution of a sample of the more important carbonium ions and positively charged hypothetical transition states are examined. The species studied include the methyl, ethyl, isopropyl, tertiary butyl, and higher alkyl carbonium ions; protonated ethylene, acetylene, benzene, cyclopropane; the cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl carbinyl, allyl, and benzyl cations; the carbonium ions based on norbornane, norbornene, norbornadiene. Significant charge delocalization for a classical carbonium ion geometry is observed—the extent of this phenomenon is wider than anticipated. For the alkyl carbonium ions it is shown that the order of stabilities may be obtained from a calculation in which the polarity of the C–H bond is C—–H+. Protonated ethylene and acetylene show local minima for a symmetrical complex, but with rearrangement to an unsymmetrical cation favored. Protonated cyclopropane prefers an unsymmetrical three‐center bonded structure, protonated benzene stabilizes in the familiar benzenium. The orientation of the empty carbonium p orbital with respect to other π‐type orbitals determines the conformation in cyclopropyl carbinyl, benzyl, and allyl. The peculiar nature of the cyclopropane electron distribution is studied. The carbonium ions based on the bicyclo[2.2.1]‐heptane structure show some nonclassical features; confirming experimental conclusions, the unusual 7‐norbornadienyl cation is calculated to prefer an unsymmetrical geometry. Difficulties in applying the extended Hückel theory to charged species make some of the conclusions from the calculations less certain.
40(1964); http://dx.doi.org/10.1063/1.1725552View Description Hide Description
An examination is made of some new data obtained by photon and electron impact, in an attempt to determine more accurately the threshold laws for processes of autoionization. In the cases of Kr and Xe, it is found that marked interference can occur between the two competing processes of direct and autoionization. The results, and also some of the apparently anomalous ionization efficiency data obtained by electron impact for several of the metals, can be interpreted in terms of configuration‐interaction phenomena involving excited states of very short lifetimes.
40(1964); http://dx.doi.org/10.1063/1.1725553View Description Hide Description
Use of the 6:12 potential for Cl2, and presumably for other halogen molecules, shows that the observed orthorhombic Cmca space group in which they crystallize is not that predicted; both the cubic Pa3 and the hexagonal would be more stable crystal structures. A molecular model is discussed in which, compared to the free halogen atoms, the molecule has, in addition to concentration of charge at its center, a charge concentration at its outer ends. This model correctly predicts the Cmca space group and the orientation of the molecules in the unit cell. The calculations also show why certain other simple molecules crystallize in Pa3.
40(1964); http://dx.doi.org/10.1063/1.1725554View Description Hide Description
Absorption spectra of the pentacene mono and dinegative ions in the 5–42 kK region were measured. To determine the relative directions of polarization of the electronic transitions a glassy solution of the dinegative ion was photo‐oxidized with plane‐polarized ultraviolet light and extinctions of this dichroic solution were measured for plane‐polarized light parallel and perpendicular to the vector of the photo‐oxidizing light. The experimental spectra are compared with those of the mono and dinegative ions of some other polyacenes. A qualitative interpretation of the spectra is given.
40(1964); http://dx.doi.org/10.1063/1.1725555View Description Hide Description
It has been possible to assign the 10 lowest energy levels of Am IV. By using these assignments and the complete spin—orbit matrices, a fit was obtained with 5f hydrogenic wavefunctions. This fit was then improved by using a fitting program and varying all four parameters. Comparison calculations using hydrogenic wavefunctions are made for Pu I. For Am IV the hydrogenic values are F 2=268.6 cm—1 and ζ=2605 cm—1; the corresponding values for Pu I are F 2=230.6 cm—1 and ζ=2174.6 cm—1.
40(1964); http://dx.doi.org/10.1063/1.1725556View Description Hide Description
Nuclear hyperfine structure has been observed in the electron spin resonance(ESR)absorption spectra of potassium and lithium dissolved in some alkylated amines. The four‐line spectrum displayed by the potassiumsolutions is attributed to 39K‐electron contact interaction. This interaction increases with increasing temperature. The spin density at the potassium nucleus in ethylamine solution is 3.37±0.05×1023 cm—3 at —40°C and 1.21±0.05×1024 cm—3 at 50°C. The contact interaction also decreases when methylamine is added to an ethylamine solution. This behavior is interpreted in terms of a modified view of the monomer of the theory of Becker, Lindquist, and Alder. The monomer changes its structure as its environment changes. A statistical model which relates the experimental observations to these changes is constructed.
The differences in the ESRabsorption spectra observed for the three systems Na–NH3, K—EtNH2, and Li—EtNH2 are discussed, and in part, attributed to differences among the three systems in the rates of spin exchange, electron exchange, and solvent exchange. Order‐of‐magnitude calculations relating measured line breadths to the rates of these processes are carried out in certain limiting cases. The results of these calculations lend support to the idea that the monomer includes remote solvent molecules within its structure. The rate constant for spin exchange is shown to increase with decreasing temperature in some potassium—amine solutions.
No evidence for the solvated electron is found in the ESR spectra of K dissolved in ethylamine, or in a range of mixtures of ethylamine with methylamine. An upper limit of 10—4 is placed on the dissociation constant of the monomer. For Li in ethylamine a composite spectrum is observed under some conditions. It consists of a nine‐line hyperfine pattern superimposed on a single relatively sharp line with the same g value. This sharp line is attributed to the solvated electron. The nine‐line pattern is attributed to hyperfine interaction between the unpaired electron and four equivalent 14N nuclei in the Li monomer. The spin density at each 14N is 1.4±0.1×1023 cm—3, independent of temperature. Similar spectra are observed for Li dissolved in two mixed amine solvents.
The size of the monomer is discussed in terms of an expanded orbital approach. The K monomer is shown to vary between two and three times the size of atomic K. The trend toward lower percent atomic character with decreasing atomic number of the alkali metal is also discussed. Part of this trend may come about because the small atoms undergo a larger expansion in forming a monomer.
40(1964); http://dx.doi.org/10.1063/1.1725557View Description Hide Description
The results of recent experimental studies of the scattering of beams of Kr, Ar, Ne, He, and D2 from carbonized polycrystallinenickel targets are presented and compared with the findings of other investigations of the phenomenology of heavy particle scattering at a solid surface. In the present experiments, as in certain earlier experiments utilizing alkali—halide cleavage planes, large deviations from diffuse (cosine law) scattering are observed, which have not yet been satisfactorily interpreted. The present series of experiments tends to support a model for the scattering mechanism in which a fraction of the incident molecules are scattered diffusely and the remainder are scattered preferentially. From experimental observations, it is inferred that the slower particles in the incident beam contribute to the diffuse component while the faster particles are scattered preferentially. In addition, an angular variation in the degree of thermal accommodation of the incident particles is observed.
40(1964); http://dx.doi.org/10.1063/1.1725558View Description Hide Description
This article presents an elementary derivation of time‐correlation formulas for transport coefficients. The derivation, based on classical statistical mechanics, is modeled on one by Kubo, Yokota, and Nakajima. In agreement with theirs, it makes use of the assumption of regression of fluctuations. It differs, however, in its avoidance of Markoffian assumptions. The decisive point of the derivation is the introduction of a parameter ξ to measure the rate of the transport process. In the limit of a very slow process, i.e., in the limit ξ→0 and time t→∞ with ξt held constant, the results coincide with those obtained by Markoffian assumptions. The relation to other work, and in particular to Kirkwood's friction constant theory, is touched on briefly.
40(1964); http://dx.doi.org/10.1063/1.1725559View Description Hide Description
Photocurrents in anthracenesingle crystals excited by the 6943‐Å ruby laser line have been studied. At room temperature (and at higher temperatures) the currents are proportional to the square root of the intensity of exciting light, as reported previously. As the temperature is lowered this changes progressively to a linear dependence at —25°C. In the vicinity of 0°C the photocurrent pulses exhibit long decay times following termination of the laser pulse. These are interpreted in terms of carrier‐trapping effects. The observed photocurrents are attributed mainly to single photon excitation of localized anthracene ions or trapped carriers. The enhanced currents observed in experiments using preillumination with uv light support this interpretation.
40(1964); http://dx.doi.org/10.1063/1.1725560View Description Hide Description
The attenuation of ultrasonicwaves has been measured in single crystals of naphthalene and p‐dichlorobenzene at frequencies from 5 to 95 MHz for the former crystal, and 5 to 65 MHz for the latter. The absorption coefficients of the crystals were proportional to the square of the acoustic frequency. The change in absorption coefficient for the temperature range from 30°C down to —70°C was very small to about —40°C; a slight decrease was observed below this point. There was no appreciable change in the absorption coefficient within the limit of experimental error, up to a (uniaxial) pressure of 4.25×106 dyn/cm2 applied in a direction perpendicular to the direction of propagation of the acoustic wave. From these results, it is concluded that any contributions of dislocation damping and thermal conduction in the crystals to the total acoustic absorption were negligibly small.
The sound velocity was determined for these same crystals at room temperature. This velocity was to be found to be a linear function of the temperature, with negative slope, and shows approximately the same temperature coefficient in both cases.
40(1964); http://dx.doi.org/10.1063/1.1725561View Description Hide Description
The absorption and fluorescenceemission spectra of solutions of the chelate europium benzoylacetonate (EuB), the fluorescence decay times of these solutions, and the fluorescenceemission spectrum of the solid chelate are described in detail. From this information an energy‐level structure for the lower states of the ground multiplet is obtained for the solid and for the solutions and the room temperature radiative lifetime of the europiumfluorescence in solutions of the chelate has been determined. The temperature dependence of the intensity and lifetimes of the fluorescence reveals that the efficiency of the intramolecular energy transfer must decrease as the temperature increases.
40(1964); http://dx.doi.org/10.1063/1.1725562View Description Hide Description
The laser application of europium chelates is considered in some detail by means of an analysis of the steady‐state kinetics of the fluorescence and of the optical pumping. An energy‐level model of the chelate molecule is proposed and the kinetics analyzed in terms of the various transition probabilities. The solutions for the steady state indicate the effect of the various transition probabilities on the threshold for laser action. It is also pointed out that the triplet state associated with the phosphorescence can function as a trap state.
A consideration of the pumping problem reveals that the intense absorption leads to extremely non‐uniform excitation which reduces the width of the effective pump band. The rate of pumping becomes relatively insensitive to concentration but is extremely dependent on the diameter of the cylindrical optical cavity. These considerations are then applied to a comparison of the thresholds for laser action in ruby and EuB.
40(1964); http://dx.doi.org/10.1063/1.1725563View Description Hide Description
Measurements have been made of the absolute intensity of the lowest‐energy electronic state of perylene. The intensities of the Au and Bu factor group states have been found to deviate from an oriented gas model to such an extent that the model proves valueless in this case. The perturbation treatment of crystal‐induced intensity stealing has been very briefly reviewed to examine the effect of the orientation of molecules on the relative importance of second‐order terms. It has been shown that these terms can be responsible for most of the intensity when the active molecular axis is very nearly 90° to the crystal symmetry axis. Numerical values have been obtained for the influence of five higher‐energy states on the intensity of the lowest state. The oriented gas polarization ratio is Ia/Ib =344: 1; the observed value is 1:1.76; the calculated value is 1: 2.05. The calculated and experimental values of the absolute a and b polarized intensity are in fair agreement.
The observed splitting (5250 cm—1) between the origin of the b spectrum and the peak of the a spectrum has been considered as a strong coupling Davydov splitting which is calculated to be 5013 cm—1 using a dipole approximation.
Vibrational frequencies of 350 and 1270 cm—1 have been obtained from absorption spectra data with perylene at 4.2°K. The possible role of vibronic coupling and charge resonance on the spectrum, and the nature of the diffuse fluorescence are also discussed.