Index of content:
Volume 46, Issue 1, 01 January 1967
46(1967); http://dx.doi.org/10.1063/1.1840356View Description Hide Description
Polarized crystal spectra are reported for the trans‐[Coen2Cl2]+ ion and the trans‐[Co(1‐pn2Cl2]+ ion in various crystals at temperatures down to 25°K. The spectra are analyzed in terms of several vibronic intensity gaining models. The optical activity of the latter compound has been used to establish the most appropriate analysis for that spectrum and this is then used in order to discuss the circular dichroism of that ion.
46(1967); http://dx.doi.org/10.1063/1.1840432View Description Hide Description
Rotational distributions of CH (A 2Δ, v=0) were measured at pressures from 0.1 to 8.5 torr in the O+C2H2reaction in the presence of large excess of N2, Ar, or He. The reaction N+NO→N2+O was used to produce O without O2. Under all conditions the distributions are found to be a superposition of two Boltzmann distributions, one at 1200°—1400°K characteristic of the process leading to the formation of excited CH (A 2Δ), and one which stays close to the temperature of the reactor. Assuming that the observed behavior of the rotational distributions is caused by collisions of excited CH with heat‐bath molecules and interpreting the fraction of molecules in this low‐temperature distribution as a measure of the extent of relaxation of the initial rotational distribution of excited CH, average relaxation rate constants are derived. With the collision partners mentioned above, 10 to 30 collisions are required for relaxation. A 14‐level phenomenological model including only transitions between neighboring rotational levels (K→K±1) fits the experimental distributions well at all pressures studied if the downward rate constantsk K−1,K are approximately proportional to exp(−2BK/kT), where B=14.6 cm−1 is the rotational constant for CH(A 2Δ, v=0) and T=360°K. Other models are discussed. Rotational and vibrational distributions for CD(A 2Δ) are also described. Quenching of the CH(A→X) emission by added oxygen was studied quantitatively and the results are consistent with a mechanism in which not CH(A 2Δ) but a precursor to its formation is removed by O2. In the absence of O2 the lifetime of the precursor is determined primarily by the atomic‐oxygen concentration.
When an electrical discharge through O2 is used as the source of oxygen atoms for reaction with acetylene, the rotational distribution of CH(A 2Δ) and its pressure dependence is quite different from that described above. When acetylene reacts with the products of a discharge through CO2 with excess CO2 as heat bath, the rotational distribution of CH(A 2Δ) has a Boltzmann form at all pressures, with only the rotational temperature depending on pressure and approaching the gas kinetic temperature at high pressure.
46(1967); http://dx.doi.org/10.1063/1.1840378View Description Hide Description
The EPR and optical spectra of vanadium in the Na2O–B2O3 glass system have been analyzed. It is found that the data can best be interpreted in terms of a vanadyl‐type structure in a ligand field of fourfold symmetry. Good correlation of the optical and EPR data is achieved through molecular orbital calculations, which provide a qualitative description of the behavior of the coordination sphere as the Na2O content of the glass is varied. The model proposed is one in which there is a dilation of the ligand shell corresponding to the boric oxide anomaly superposed on an over‐all tendency of the ligand—metal distance to decrease as [Na2O] increases. The hyperfine coupling terms as well as the g tensor and optical data support this model.
46(1967); http://dx.doi.org/10.1063/1.1840392View Description Hide Description
Recent experiments have shown that the electronic ground state of ScF is 1Σ+, contrary to the 3Δ r ground state known for the isoelectronic TiO molecule. Previous theoretical calculations have shown that the 1Σ+ state of TiO lies above 3Δ r , in agreement with experiment. Truncated matrix Hartree—Fock wavefunctions for ScF have been calculated here to study the apparent ground‐state term inversion of the 1Σ+ and 3Δ r levels of this molecule. The ab initio calculations are based on the LCAO expansion method of Roothaan and employ an extended basis that contains the essential functions necessary to represent uncorrelated ground states reliably when there are close‐lying valence states. The basis is larger than those used in previous calculations on related monoxides to ensure that the computed ground state and qualitative ordering of ther low‐lying states are stationary to further improvements in the basis. For reference purposes, similar extended calculations have been carried out on the TiO molecule. It is found that 3Δ r of configuration δσ is the symmetry restricted Hartree—Fock ground state of both ScF and TiO. A semiempirical estimate of the correlation energy contributions shows that correlation depresses the 1Σ+ state relatively more than 3Δ r in both cases and suggests that 1Σ+ is the ground state of ScF, in agreement with experiment. This study provides additional information on the electronic structures of molecules of the transition metals and demonstrates the possibilities of term inversions in other monoxides of the titanium family.
Influence of Paramagnetic Resonance on the Static Susceptibility. The Lattice—Bath Relaxation Time of Neodymium Ethyl Sulfate46(1967); http://dx.doi.org/10.1063/1.1840428View Description Hide Description
The spin—lattice—bath relaxation process of neodymium ethyl sulfate was investigated by measuring simultaneously the change in the static susceptibility and the microwave power absorbed at electron spin resonance. This technique can be used at constant temperature to distinguish the spin—lattice process from the lattice—bath process. The relaxation time of neodymium ethyl sulfate was studied at a microwave frequency of 14.5 GHz as a function of temperature, helium‐exchange gas pressure, microwave power absorbed, two crystal orientations, and crystal size. At this microwave frequency the energy transfer from the lattice to the bath appears to be the rate‐determining process. The dominant lattice—bath relaxation time, τ p , is inversely proportional to the square of the bath temperature τ p T 2=0.20 sec·°K2 but is apparently independent of the crystal size, the helium‐exchange gas pressure, and crystal orientation. The experimental data are in essential agreement with the data obtained by other researchers using different methods.
46(1967); http://dx.doi.org/10.1063/1.1840429View Description Hide Description
The influence of a liquid‐crystal solvent on paramagnetic relaxation processes involving Zeeman and hyperfineanisotropies is examined. Secular and pseudosecular relaxation mechanisms are reduced by molecular ordering. Secular contributions are most effectively quenched, and, for high degrees of order in viscous media, pseudosecular processes are most important.
ESR studies were made of solutions of vanadyl complexes with six β‐diketones in p‐azoxyanisole. The linewidths in the spectra depend upon the 51V nuclear spin quantum number M, and vary as A+BM+CM 2. As the degree of molecular ordering increases, one predicts and observes that first C and then B change in sign.
It was also found that almost perfect ordering of a solute in an imperfectly ordered liquid crystal can be achieved, provided the solute molecule is sufficiently large and rigidly planar. Implications of ESR observations concerning the structure of liquid‐crystal phases are discussed.
46(1967); http://dx.doi.org/10.1063/1.1840430View Description Hide Description
Through the use of Wang Chang and Uhlenbeck's theory of polyatomic gases, rotational relaxation times for homonuclear diatomic molecules have been calculated. The model adopted is that of two molecules rotating in the same plane which also serves as the collision plane. The intermolecular potential used is Parker's modification of the Morse potential, where the molecular orientations are taken into account only in the repulsive part. This leads to a relaxation time, for which the ``collision number'' Z rot is essentially independent of temperature. Owing to the lack of any experimental information concerning the temperature dependence of rotational relaxation the discussion has been limited to a few comments about earlier calculations.
Heat Capacity and Entropy of NiSiF6·6H2O from 0.35° to 4.2°K with Magnetic Fields 0–90 kG Perpendicular to the c axis. The Use of 3He Gas Conduction in Calorimetry46(1967); http://dx.doi.org/10.1063/1.1840431View Description Hide Description
The heat capacity of nickel fluosilicate hexahydrate has been measured over the range 0.35–4.2°K in stabilized magnetic fields of 0, 1, 5, 10, 20, 40, 60 and 90 kG. The field was perpendicular to the crystallographic c axis of the 3.5‐cm‐diam single crystal. The entropy changes in NiSiF6·6H2O have been accurately tabulated over the ranges 0.35°—4.2°K and 0–90 kG, by combining the heat‐capacity data and the change in temperature with field on isentropics. The temperature was measured by means of a carbon thermometer which was also used to introduce calorimetric heat. The total electronic entropy removed to magnetic saturation was Rln3. The substance gave no indication of hysteresis at any field or temperature. At the higher fields and lower temperatures it was necessary to consider a small entropy change due to nuclear magnetization of the hydrogen and fluorine atoms. As a practical rule it has been shown that a 2 mtorr pressure of 3He, which is sufficient to cause liquefaction near 0.3°K, is acceptable in a calorimeter over the range 0.4°—4°K, provided that the ratio of sample surface to volume is small, as, e.g., with a very large spherical single crystal.
46(1967); http://dx.doi.org/10.1063/1.1840433View Description Hide Description
The rate of photochemical production of hydrogen iodide in a hydrogen—iodine system irradiated with 5780‐Å light was measured and found to be proportional to the square of the iodine atom concentration. Absolute light intensities were determined and rate constants were calculated for the termolecular reaction H2+2I→2HI at 417.9°, 480.7°, 520.1°K. The previously obtained thermal data in the range 633°—738°K were treated considering one of the elementary reactions to be the termolecular reaction and not H2+I2→2HI; the rate constants obtained have the same Arrhenius parameters as those from the photochemical measurements. The bimolecular reaction, H2+I2→2HI, does not occur. Two mechanisms consistent with the photochemical and the thermal data are H2+2I→2HI; H2+I+M=H2I+M, H2I+I→2HI. The specific rate calculated for the termolecular reaction is .
46(1967); http://dx.doi.org/10.1063/1.1840434View Description Hide Description
The circular‐dichroism (CD) spectra of several dinucleoside phosphates containing guanine and cytosine in aqueous solution at −17°C are reported in the range 210–300 mμ. The results differ from previously reported dinucleoside phosphate curves in that the CD bands do not appear in pairs of nearby positive and negative bands. This is in disagreement with the exciton treatment which has been successfully used to interpret other dinucleoside phosphate CD and ORD.
Tinoco's general theory of polymeroptical activity is examined in an attempt to explain the nonconservative CD curves we observe. Both the cases of base planes perpendicular to the helix axis and that of the base planes tilted with respect to the helix axis are considered. We conclude that tilting cannot be expected to give large nonconservative rotational strengths. In addition we show by means of a polarizability approximation, that interaction of the far‐uv transitions can give rise to rotational strengths such as those we observe, even for the case of base planes perpendicular to the helix axis. This nonconservative contribution is composed of the Kirkwood polarizability term and an analogous term containing the magnetic transition moment and the polarizability perpendicular to the base plane. The contribution also depends on the direction of the electronic transition dipole moment of the transition whose rotational strength is being calculated. We propose that it is through this dependence that the nonconservative effect is not present in the CD of dinucleoside phosphates previously measured but is prominent in the curves reported for dinucleoside phosphates containing guanine or cytosine. The study suggests that the adenosyl—adenosine CD curve, which is conservative, is a special case due to its electronic structure and that the geometry of several dinucleoside phosphates is the same as that of single‐strand RNA whose CD curve is nonconservative.
46(1967); http://dx.doi.org/10.1063/1.1840435View Description Hide Description
Reaction of monosilane with hot tritium atoms was investigated thoroughly, determining the extent of formation of SiH3T, HT, and higher tritiated silanes. A special study was made of the effect of I2 as radical scavenger and the effect of 4He as moderator in order to verify, experimentally, the applicability of the kinetic theory, elaborated by Wolfgang for the reaction of hot atoms, to the examined system. The bond energy of Si–H was also correlated with the variation of the found ratio HT/RT, as compared with the same ratio in the system H+CH4.
46(1967); http://dx.doi.org/10.1063/1.1840436View Description Hide Description
The drift mobilities of electrons and holes in metal‐free and lead phthalocyanine were measured using a transient space‐charge‐limited‐current method and a time‐of‐flight method. The mobilities were found to be in the range 0.05–0.1 cm2/V·sec for metal‐free phthalocyanine and 0.6–4.0 cm2/V·sec for lead phthalocyanine. Fast trapping of carriers in the bulk and emptying of traps at the surface under illumination were observed.
46(1967); http://dx.doi.org/10.1063/1.1840437View Description Hide Description
The cross section for resonant vibration—vibration energy exchange between infrared‐active molecules due to the long‐range dipole potential is formulated. It is found that despite certain restrictions imposed by rotational selection rules, energy‐exchange cross sections may be as much as 0.01 to 0.1 times as large as the molecular gas‐kinetic cross section and are greater than those estimated using only an exponential repulsive interaction between molecules.
46(1967); http://dx.doi.org/10.1063/1.1840357View Description Hide Description
Previous reports from this laboratory described the characteristic luminescence of γ‐irradiated, pure 3‐methylpentane glass and the influence of preirradiation standing time at 77°K on the luminescence intensity (annealingeffect: decrease in intensity). In this paper it is shown: (a) that cracks in the annealed glasses are not necessarily required for the annealingeffect to be observed, although they apparently accelerate the glass relaxation markedly, and (b) that the luminescence spectra of isopentane are different for the glassy and polycrystalline states. It is also shown, using a simple model, that the lowest charge‐separated state can be stationary and lower in energy than the first excited state of the molecule; consequently, the charge‐separated state can be the luminescent state for pure alkanes.
46(1967); http://dx.doi.org/10.1063/1.1840358View Description Hide Description
A simple, low‐energy (100–200 J), short‐duration (3‐μsec) flash‐photolysis apparatus for the vacuum ultraviolet down to the transparency limit of lithium fluoride (1100 Å) is described.
The photolysis of CO2 has been studied. Recombination rate of CO+O(1 D) is 2×109 liters mole−1·sec−1 in the presence of a large excess of nonphotolyzed CO2 which may act as a third body. From kinetic considerations, the average lifetime of O(1 D) was estimated to be of the order of 50 μsec under these experimental conditions. The fact that O(1 D) plays a role is supported by the observation at 1152±0.5 Å of an absorption which could be attributed to the (2p 4 1 D−3s 1 1 D°) transition known in emission.
46(1967); http://dx.doi.org/10.1063/1.1840359View Description Hide Description
Classical single‐component fluids are considered in order to investigate the asymptotic behavior of the joint distribution function of l+m particles, ρ l+m , for large separation between the set of l and m particles. Near the critical point the authors find ρ l+m to be the product of ρ l and ρ m plus the long‐range corrections. The critical singularities, such as the heat capacity at constant volume, are explained in terms of the authors' result on the asymptotic distribution function.
46(1967); http://dx.doi.org/10.1063/1.1840360View Description Hide Description
The rate coefficient for the gas‐phase recombination of atoms or free radicals in the presence of third‐body molecules is calculated, based upon the radical—molecule complex mechanism. The rate coefficient, kr, is given by , z=ε L /kT, where A is a constant factor depending on the molecular constants, ε L is the energy depth of the Lennard‐Jones (12–6) potential between the reactive atom and the third body, k is the Boltzmann constant, T is the absolute temperature, and 2 F 2(z) is a generalized hypergeometric function. The experimental temperature dependence of iodine atom recombination in the presence of argon atoms is accurately described by the theoretical expression with ε L /k=700, A = 5.4×108 liters2 mole−2·sec−1.
46(1967); http://dx.doi.org/10.1063/1.1840361View Description Hide Description
From experimental determinations of the rate constants for the reaction, an activation energy,Ea =0.067 eV, was calculated. At 77°K, no contribution from this reaction was found in the afterglow of a pulsed discharge in helium; rather at 77°K the decay was found to be second order with a reaction constant about 2×10−9 cm3 atom−1·sec−1.
46(1967); http://dx.doi.org/10.1063/1.1840362View Description Hide Description
Accurate spin Hamiltonians for one paramagnetic center in irradiated KClO4 are derived from ESR spectra at five temperatures in the range from −170° to 0°C. The quadrupole interaction gives rise to ``forbidden'' transitions of high intensity. An iterative procedure is used for computation of the principal values of the gtensor as well as the tensors for the magnetic and electric hyperfine interaction. Comparison of the low‐temperature spin Hamiltonian for the center with that derived from microwave spectra for the free ClO2 molecule demonstrates that the center is adequately described in terms of a trapped ClO2 molecule as previously suggested by Cole. A pronounced temperature dependence of the spin Hamiltonian indicates a considerable influence of the host lattice.