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Volume 37, Issue 10, 15 November 1962
37(1962); http://dx.doi.org/10.1063/1.1732981View Description Hide Description
The quenching by 0‐terphenyl of the gamma‐ray induced fluorescence of 0‐xylene/9,10‐diphenylanthracene solutions does not give the usual linear dependence of I 0/I on quencher concentration. The effect is ascribed to a quencher concentration‐dependent energy transfer from the solvent to 0‐terphenyl which has a finite transfer probability to the fluorescent solute. This interpretation is confirmed by the enhancement of energy transfer caused by the addition of 0‐terphenyl to a solution of a fluorescent solute in tri‐n‐butylphosphate, a solvent having a very low transfer efficiency.
37(1962); http://dx.doi.org/10.1063/1.1732982View Description Hide Description
High‐resolution spectra of H2O, HDO, and D2O suspended in solid Ar have been observed in the range 4°—27°K. Evidence has been obtained for the rotation of these species in Ar matrices. The majority of the observed lines are found to be in very good agreement with low‐J transitions of the gas spectra, allowing for a small matrix shift. Transitions arising from both spin modifications of H2O and D2O have been observed and the observed intensities suggest that a large degree of spin equilibration has taken place. A small contamination of O2 may be responsible for this. A perturbation of one of the rotational levels of ν2—H2O is discussed.
37(1962); http://dx.doi.org/10.1063/1.1732983View Description Hide Description
The decay of metastable ions through transitions giving an ionized fragment and a neutral radical has been investigated with a 180° mass spectrometer. Four aspects of the main problem have been studied. These are: (a) the decay of metastable ions within the ion source to produce a continuous distribution of ``metastable ions''; (b) an examination of the mathematical conditions relating dissociation beyond the ion‐source exit slit to the formation of a ``metastable'' peak, to the shape of such a peak, to the cutoff imposed by the analyzer walls and to the length of travel during which detectable dissociation may occur; (c) an examination of the conditions for determining lifetimes with a 180° instrument; and (d) a consideration of the elements leading to the broadness of metastable peaks.
The distribution function resulting from dissociation within the slit drops very rapidly and is ordinarily hidden within the peak due to the daughter ion. The formation of a metastable peak in a 180° instrument results from a low‐order dependence of where the ion fragment resulting from dissociation hits the focal plane on the position of dissociation beyond the exit slit. The cutoff mass due to ions striking the walls of the analyzer tube may be readily calculated. In a CEC model 21–103 instrument there is adequate distance of free travel beyond the exit slit to allow the instrument to be used for lifetime measurements. Using repeller voltages up to 120 V, shorter lifetimes than those previously reported have been observed. For the m *=31.9 metastable peak from n‐butane, for example, there is evidence that the metastable ions are created in at least three classes, each with its own lifetime. The shortest lifetime observed was of the order of 9×10—8 sec. Evidence has been obtained to indicate that the broadness of the observed metastable peaks is probably the result of the perturbation of focusing conditions by the increased angular spread in the ion beam.
Statistical Thermodynamics of Polymer Solutions. III. The Second Virial Coefficient for Branched Star Molecules37(1962); http://dx.doi.org/10.1063/1.1732984View Description Hide Description
The effect of branching on the osmotic second virial coefficient is calculated for a homogeneous solute of ``star'' molecules, each composed of a number of identical, flexible, linear, polymeric, chain elements joined together at one end through a common junction. The perturbation method of Zimm, familiar for linear chains, is generalized slightly to include the star molecule of arbitrary functionality. This calculation, applicable to thermodynamic interactions in poor solvents, is then combined with a method developed in the first paper of this series, to obtain an approximate theory for the virial coefficient in good solvents where thermodynamic interactions between solute molecules are large.
37(1962); http://dx.doi.org/10.1063/1.1732985View Description Hide Description
The size of the critical nucleus in the formation of metal deposits from the vapor is very small, probably containing 10 atoms or less. For nuclei of this magnitude the uncertainties concerned with the liquid‐drop model and the concept of surface energy in the Volmer and Weber and Becker and Doring treatment of nucleation become serious. An expression for the density of clusters of atoms is developed using statistical mechanics. From this an expression for the nucleation rate is derived on a kinetic basis. It is also observed that certain orientation effects in the deposit can be explained if the small size of the critical nucleus is taken into account.
37(1962); http://dx.doi.org/10.1063/1.1732986View Description Hide Description
The secondary processes occurring in organic sulfur compounds, irradiated with ionizing radiation in the dry state, have been followed by ESRspectroscopy. Samples were irradiated in vacuum at 77°K and the ESR spectra recorded at this temperature before and after stepwise heat treatment at successively increasing temperatures.
The resonance spectra seem to indicate that after irradiation at 77°K, several kinds of ESR centers are formed. No or very few sulfur radicals seem to be formed at this temperature.
The results show that few secondary processes proceed efficiently in solids in the temperature range from 77° to about 110°K. Depending upon the compound, decay processes seem to dominate in the temperature range varying from about 110°K and up to about 250°K. Upon further heat treatment, secondary reactions take place which lead to the formation of sulfur radicals. The threshold temperature for these reactions is about 170°K for simple thiols and disulfides, 240°K for peptides like glutathione, and about 280°K for proteins. The formation of sulfur radicals is found to coincide with an increase in the total number of observable ESR centers. Thus, in the case of penicillamine, the number of unpaired spins observed after annealing to 295°K exceeds the initial value at 77°K by about 70%.
Evidence is presented that diffusion‐controlled intermolecular reactions take place in irradiated solids, and it is suggested that the observed formation of sulfur radicals may in part be explained by such a mechanism.
The resonance pattern due to polyoriented radicals of the type R—CH2–S· (e.g., cysteine, cysteamine, glutathione, proteins) was found to exhibit a temperature dependent hyperfine splitting which is not found for radicals of the type R—CX2—S· (e.g., penicillamine). This observation supports the model for organic sulfur radicals proposed by Kurita and Gordy.
37(1962); http://dx.doi.org/10.1063/1.1732987View Description Hide Description
An algebraic analysis is given of the relative effects of molecular asymmetry and differences in conformational populations in causing nuclei to be magnetically nonequivalent. Primary importance is attached to the nature of the molecular asymmetry. The temperature dependence of the chemical shift observed between F a and F b , in CF a F b BrCFBrCl, is compatable with a small splitting, an ``asymmetry effect,'' which persists when the three rotamers have the same population.
37(1962); http://dx.doi.org/10.1063/1.1732988View Description Hide Description
An interpretation is presented for the additivity of substituent effects on the C13–H coupling constant, which has been observed previously in the high‐resolution NMR spectra of substituted methanes. Each atom or group X is assigned a characteristic ``affinity'' for s character in the carbon hybrid orbital of the C—X bond. Distribution of s character among the carbon orbitals in accord with the relative s affinities of the four substituents leads to the observed additivity relation provided that the total s character is conserved. The valence‐bond approach used with this model gives a linear relation between the s character of the carbon hybrid orbital involved in the C–H bond (αH 2) and the observed C13–H coupling constant (J CH=500 αH 2). Also, it allows the determination of the s character of the other carbon orbitals. The dependence of the s character of the C—X bond on the electronegativity of X is discussed in terms of electron spin and charge correlation. It is noted that the hybridization changes should affect not only J CH but also J HH, which is consistent with the observed proportionality between J CH(CH3X) and the cis and trans H–H coupling constants in CH2=C13HX.
The treatment developed for methanes has been extended to J CH in substituted ethylenes and to the Si29–H coupling in silanes. For the former, introduction of the s‐electron affinities, obtained from the values of J CH(CH3X) observed for the substituted methanes, leads to the result that J CH(CH2=C13HX) = J CH(CH2=CH2)+ [J CH(CH3 X) – J CH(CH4)]. Values predicted in this manner are systematically larger than those observed, which implies that there is a small, negative π‐electron contribution of 5 to 10 cps to J CH(CH2=C13HX). Such a contribution is compatible with current theories for proton and C13hyperfine splittings in ESR spectra of free radicals. The Si29–H coupling constants observed in substituted silanes exhibit large, systematic deviations from the simple additivity found in the methanes. These deviations are explicable qualitatively in terms of changes in the Si–H‐bond polarity.
37(1962); http://dx.doi.org/10.1063/1.1732989View Description Hide Description
The theory of the resonancefluorescence of degenerate or near‐degenerate states is derived and it is applied to a description of magnetic rotation spectra of triplet—singlet transitions. The transition probability is obtained as a quotient of two determinants, each containing the matrix of the damping constants between the different states. The mathematical formalism becomes reasonably simple only if the energy differences between the states are large with respect to the absolute values of the off‐diagonal damping constants. Therefore our results for the magnetic rotation spectra of triplet—singlet transitions are restricted to the case of large magnetic fields so that the splittings of the triplet levels are large compared to the natural line breadths of their absorption lines. It is found that the relative intensities of singlet—triplet transitions in magnetic rotation spectra are compatible with or even larger than the intensities of singlet—singlet transitions. The possible importance of spin—spin interactions is mentioned.
37(1962); http://dx.doi.org/10.1063/1.1732990View Description Hide Description
The compound (n‐C4H9)4N·C6H5COO·39½H2O forms tetragonal crystals with four molecules in the unit cell. The water structure is a hydrogen‐bonded clathrate framework, in the cavities of which are located the tetra alkyl ammonium cations. The oxygen atoms of the benzoate anions are hydrogen‐bonded to the water framework and form part of the polyhedral structure. This structure contains 10 pentagonal dodecahedra, 16 tetrakaidecahedra, and four pentakaidecahedra per unit cell. The 20 larger polyhedra are occupied by the 16 alkyl and four benzyl groups with a statistical disorder over two sets of equivalent cation and anion positions. There is evidence of additional hydrogen‐bonded water inside some of the dodecahedra which are distorted due to the presence of the ions.
The clathrate framework is iso‐structural with that of other salts in this series, which have a very similar tetragonal lattice but differ in containing five tetra n‐butyl ammonium salt molecules per unit cell.
37(1962); http://dx.doi.org/10.1063/1.1732991View Description Hide Description
The preparation and crystal structure of (n‐C4H9)3S+F—·20H2O is reported. This hydrate is isostructural with the chlorine gas hydrate, 6Cl2·46H2O, having a pseudo body‐centered arrangement of H40O20 pentagonal dodecahedra in a cubic unit cell of approximately 12 Å edge. The cations occupy large voids formed by the tetrakaidecahedra in the structure and are statistically disordered. The water structure is distorted as compared with the idealized polyhedral framework and there is evidence that the pentagonal dodecahedra may be occupied by a water molecule or a fluoride ion.
37(1962); http://dx.doi.org/10.1063/1.1732992View Description Hide Description
The classical adiabatic calculation of the rate of dissociation of diatomic molecules is extended to Morse potential oscillators in one‐dimensional colinear collisions. The limits of error of the method are formulated and numerical calculations of the rates of dissociation of H2 in Xe and I2 in Ar are made assuming both the Morse and harmonic potentials. The harmonic results are in good agreement with experiment. Estimates of the errors involved show that the anharmonic results are not as reliable as the harmonic results. The distribution functions for a strongly interacting harmonic oscillator are also calculated.
37(1962); http://dx.doi.org/10.1063/1.1732993View Description Hide Description
The Schwartz—Slawsky—Herzfeld (SSH) theory of vibrational relaxation times assumes that the most effective relative kinetic energy for causing a vibrational transition is much greater than the quantum of energy transferred. For many actual cases, this assumption is not a good one. However, it appears that this approximation can be eliminated with little change in the resulting equations. The force law for N2, determined with the modified equations from the experimental data on relaxation times, does not differ appreciably from previous results.
37(1962); http://dx.doi.org/10.1063/1.1732994View Description Hide Description
A low‐temperature cell suitable for the study of the infrared spectra of pure liquids in the fundamental region is described. Selection rules are derived based on the assumption that diatomic molecules in the liquid state can undergo hindered rotation. The infrared absorption band shape is predicted to resemble the P‐ and R‐branch envelope of the gas phase molecule in the wings, but to have an intense Q branch at the band center. The observed spectra of carbon monoxide as a liquid is in reasonable agreement with these selection rules. It is concluded that a considerable fraction of molecules in the liquid state can undergo nearly free rotation. The spectrum of carbon monoxide just below its freezing point suggests, however, that the rotatory motion of the molecules in the solid can best be described as librational.
37(1962); http://dx.doi.org/10.1063/1.1732995View Description Hide Description
Deuterium labeled isobutane (CH3)3CD was pyrolyzed to generate (CH3)3C and CH2CD(CH3)2 radicals. The competition between reactions (5) and (6) was studied by isotopic analysis of the hydrogen, methane, and isobutene fractions.
Reaction (5) was found to be of negligible importance relative to reaction (6) up to 540°C and it is concluded that previous estimates of k 5/k 6 (in the nondeuterated species) are too high by at least an order of magnitude. The expulsion of molecular hydrogen by isobutyl or t‐butyl does not occur in competition with other unimolecular reactions of these radicals.
37(1962); http://dx.doi.org/10.1063/1.1732996View Description Hide Description
Fluorescence‐polarization data on porphyrins are reported. The results can be readily interpreted in terms of the theory of polarizationspectroscopy and the theory of the electronic and vibrational structure of these molecules. It is shown how these two theories combine to interpret most of the data, but some anomalies remain. The results hint at the richness of data that may be expected from fluorescence‐polarization experiments when techniques are developed further.
37(1962); http://dx.doi.org/10.1063/1.1732997View Description Hide Description
Absorption spectra of porphyrins in solution with trinitrobenzene indicate formation of a fairly strong one‐to‐one charge‐transfer complex. The ΔF and ΔH values are given for etioporphyrin and tetraphenylporphin; a rough ΔF value was obtained for Zn tetraphenylporphin. It was also found that trinitrobenzene strongly quenches the porphyrinfluorescence.
37(1962); http://dx.doi.org/10.1063/1.1732998View Description Hide Description
The absorptionspectrum of 0.5 mole % Nd3+ in a LaBr3single crystal was obtained and most of the crystalline field Stark levels identified. The spectrum is very similar to that obtained from Nd3+ in LaCl3 and, in fact, the crystalline field parameters that fit LaCl3 to 5 cm—1 fit LaBr3 to 7 cm—1. The major difference in the two spectra is a general shift of levels to the red in the LaBr3. This is similar to the spectra of Pr3+ in the two crystals discussed previously [E. Y. Wong and I. Richman, J. Chem. Phys. 36, 1889 (1962)]. The explanation for this shift in both Pr3+ and Nd3+ spectra is probably the same.
37(1962); http://dx.doi.org/10.1063/1.1732999View Description Hide Description
The relative intensities of the electron spin resonance(ESR)spectra of H and D atoms were determined for mixtures of H2O and D2O which were irradiated and observed at 4.2°K. A careful analysis of the results indicates that the yield of atoms per unit irradiation dosage exhibits no isotope effect, except for the system containing trace amounts of H2O in D2O. The presence of a large isotope effect for a system of D2O containing approximately 0.1% H2O is discussed in terms of energy transfer in the solid state.
Absorption Spectra of Gaseous Halide Ions and Halogen Electron Affinities : Chlorine, Bromine, and Iodine37(1962); http://dx.doi.org/10.1063/1.1733000View Description Hide Description
Absorption spectra have been taken of partially dissociated vapors of chloride, bromide, and iodide salts of rubidium and cesium. The salts have been heated by shock waves. Continuous absorption spectra characterized by sharp thresholds are attributed to the ions Cl—, Br—, and I—. The low‐energy thresholds for the three ions fall at approximately 3424, 3685, and 4040 Å, respectively. From the threshold wavelengths and appropriate small environmental corrections, the electron affinities of chlorine, bromine, and iodine are determined to be 3.628±0.005, 3.378±0.005, and 3.078±0.005 eV, respectively.