Index of content:
Volume 44, Issue 5, 01 March 1966
Electron Spin Resonance Study of Radical Pairs Trapped in X‐Irradiated Single Crystals of Dimethylglyoxime‐O,O‐d 2, Glyoxime, and Methylglyoxime at Liquid‐Nitrogen Temperature44(1966); http://dx.doi.org/10.1063/1.1726929View Description Hide Description
The ESR spectra of x‐irradiated single crystals of dimethylglyoxime‐O,O‐d 2, glyoxime, and methylglyoxime have been measured at 77°K. An analysis of the spectra reveals the presence of radical pairs R 1—CR2NȮ···R1—CR2NȮ , where R1=C(CH3)NOD and R2=CH3 for dimethylglyoxime‐d 2 (the average distance between the unpaired electrons R=5.4 Å), R1=CHNOH and R2=H for glyoxime ( Å), and R1=CHNOH and R2=CH3 for methylglyoxime ( Å). Radical pairs decay in dimethylglyoxime faster than in dimethylglyoxime‐d 2. The fact shows that the iminoxy radicals CR1NOH–CR2NȮ diffuse by hydrogen‐atom abstraction from nearby hydroxyl groups.
44(1966); http://dx.doi.org/10.1063/1.1726930View Description Hide Description
Motional narrowing of the hyperfine interaction for a localized, diffusional model of one‐dimensional triplet excitons is shown to lead, at low temperatures, to Lorentzian lines, with half‐width ωobs ω p is the half‐width of the Gaussian envelope of the hyperfine pattern of an exciton in the absence of motion, ω i is the exciton jumping rate along the chain, and ω r represents all other, slower randomizing processes. The Lorentzian line shape is in agreement with experiment, and the jumping rate ω j agrees with previous calculations. These results give further evidence that a localized, diffusional description of triplet excitons is appropriate at low temperatures.
44(1966); http://dx.doi.org/10.1063/1.1726931View Description Hide Description
The Zeeman patterns for 14N pure quadrupole resonances in powders have been deduced, calculating the relative intensities around the resonance frequencies. The shape of the resonance curve to be recorded can be found by the difference between the unperturbed and perturbed lines. When the asymmetry parameter η has a finite value, the recorded peak shifts from the unperturbed position. This effect is more evident for the weaker modulation field or the broader unperturbed line, and also for larger values of η. Furthermore, it is shown that the higher resonance line is much stronger than the lower, when η is large.
44(1966); http://dx.doi.org/10.1063/1.1726932View Description Hide Description
Several different paramagnetic centers have been identified and studied in specimens of natural and synthetic α quartz. The dominant feature of the EPR spectrum of natural amethyst and citrine is a center S 1, which was previously identified as substitutional Fe3+ with a charge‐compensating alkali‐metal ion on a neighboring interstitial site. Synthetic brown and green quartz also contain a small proportion of S 1 centers, but the dominant feature of their EPR spectra is a center I which is identified as an interstitial Fe3+ lying in one of two possible interstitial sites in the structure, with a charge neutralizing ion in a neighboring substitutional site. This spectrum is fitted to a spin Hamiltonian with D=2.333 Gc/sec, E=0.63 Gc/sec, F=0.60 Gc/sec, the z axis of the center coinciding with the c axis of the crystal. Interconversion of S 1 and I centers by heat treatment indicates that the latter are more stable. In a crystal of synthetic amethyst, a new EPR center, denoted S 2, has been characterized, which is converted to S 1 by annealing at 350° and obtained from S 1 by irradiation with x rays. Citrine and heat‐treated amethyst display a broad isotropic EPR absorption at g=2, believed to be caused by nuclei of precipitated Fe2O3. The brown color is partly due to these inclusions and partly to a shift of the charge‐transfer band [Fe3++O2—→Fe2++O−] into the visible as S 1 centers are converted to less constricted I centers.
44(1966); http://dx.doi.org/10.1063/1.1726933View Description Hide Description
A recent observation that apparent vibronic fine structure in the R 2absorption band of the R center in alkali halide crystals corresponds energetically to certain lattice phonons at the Brillioun zone boundary is considered in the light of possible selection rules for optical absorption. A perturbation calculation is employed. The selection rules found are not inconsistent with the lattice‐phonon interpretation, but they do not characterize or allow identification of the fine structure.
44(1966); http://dx.doi.org/10.1063/1.1726934View Description Hide Description
The structure of O2PtF6 has been refined using neutron‐diffraction data from a polycrystalline sample. The O–O bond length has not been determined with high precision owing to the probability of dynamic or static disorder of the O2 groups. O–O distances in the range 0.9 to 1.4 Å are compatible with the data if a suitable model is chosen for the disorder. Thus, although the data are compatible with the probable formulation as O2 +PtF6 −, other formulations cannot be eliminated. All models display interionic distances O···F of 2.5 Å or less. These distances are indicative of strong interionic attractions and may be responsible for the stability of the compound. The PtF6 − ion is a regular octahedron with a Pt–F bond length of 1.82±0.03 Å.
44(1966); http://dx.doi.org/10.1063/1.1726935View Description Hide Description
Additively colored KBr and KCl crystals were deformed at room temperature, illuminated with F light at −140° and −120°C, respectively, to produce F′ centers and then warmed from −186°C to room temperature. Absorption spectra were obtained at a series of temperatures during warming. Curves of absorption in the region of the F′ band vs temperature showed two temperature regions in which the absorbance decreased rapidly, corresponding to the decomposition of two sets of centers of differing thermal stability. Undeformed crystals displayed only one such temperature region, in which ordinary F′ centers were destroyed. The centers in deformed crystals may be F′ centers whose stability is either increased or decreased by the influence of neighboring dislocations. In deformed KCl a new center, absorbing near 1500 mμ and more stable thermally than F′ centers, was found.
Atomic Structure and Correlation in Liquid Binaries by X‐Ray and Neutron Diffraction with Application to NaK44(1966); http://dx.doi.org/10.1063/1.1726936View Description Hide Description
For a liquid or amorphous binary three independent diffraction patterns are required to completely characterize the atomic structure, including the correlation of the two atom types. However if the distribution of atomic density is the same about either atom type only two patterns are required. This two‐pattern method, as developed by Keating, was applied to 50 at. % liquid sodium—potassium using x‐ray and neutron diffraction. The results indicated a very nearly random mixing of atom types, but further analysis of the data showed that the assumption of identical distributions about the Na and K atoms is invalid. Indeed, consideration of the factors which may determine the liquid structure makes this assumption highly questionable for most binaries. A new method of combining the x‐ray and neutron‐diffraction patterns was developed which can yield qualitative information on the correlation and serve as a test for theoretical structural models. The new approach utilizes the ratio of the two weighted atomic distributions obtained separately from the x‐ray and neutron patterns. For the 50% NaK system the neutron pattern alone gives the true atomic distribution with nearest‐neighbor concentrations at 4.4, 7.7, and 11.2 Å, and the comparison with the x‐ray distribution indicates that at the first three nearest‐neighbor distances a given atom tends to be paired with atoms of the opposite type, with some concentration of Na–Na pairs at 5.0 Å and K–K pairs at 6 to 7 Å.
Glow‐Discharge Shock Tube for Studying Chemiluminescent, Surface‐Catalytic, and Gas‐Phase Reaction Rates; Temperature Dependence of NO–O and CO–O Chemiluminescence44(1966); http://dx.doi.org/10.1063/1.1726937View Description Hide Description
The ordinary glow‐discharge tube has been used extensively to study both surface and gas‐phase recombination rates, as well as chemiluminescent reactions, typically at room temperature. The combination of a glow‐discharge flow tube with a shock‐tube driver to provide a considerable extension of atom fluxes, temperatures, and densities for the study of these kinetic processes is described. In operation, the ordinary glow‐discharge‐tube situation appropriate to the particular process under study is first established and, then, a shock is propagated into the predissociated gas by a driver upstream of the rf region. Typically, the shock strength is less than that which causes further dissociation. Under these conditions, the effect of the shock is to compress, heat, and accelerate to high speed those species already in the glow tube. The high temperature and compression provide a means for studying the temperature dependence of gas‐phase recombination processes and chemiluminescent reactions, while the high‐speed flow, in increasing the atom flux, provides a means for studying surface‐catalysis phenomena occurring on a short time scale (<1 msec). Application of the present technique to determine the temperature dependences of the NO–O and CO–O chemiluminescent reactions is described in considerable detail.
Normal‐Coordinate Analysis and Molecular Properties of Four CH2XY Halomethanes (X = F or I; Y = Cl or Br)44(1966); http://dx.doi.org/10.1063/1.1726938View Description Hide Description
A normal‐coordinate treatment for the series CH2XY halosubstituted methanes (X = F or I; Y = Cl or Br) was carried out with a 709 digital computer and a perturbation program supplied by Schachtschneider of the Shell Development Company. A reasonable set of potential constants was obtained for each molecule that reproduced exactly the observed fundamentals. Trends that were noticed among the potential constants of the obtained sets were demonstrated and discussed. The potential‐energy distributions for each molecule were calculated. The effect of the redundancy on the quadratic potential constants as well as an estimate of the first‐order potential energy constants were obtained according to the method of Crawford and Overend. In the present work, the assignments of the ν3 and ν8 fundamentals of the two iodinated compounds, as given by Pitzer and Gelles, were interchanged and the argument for this change was given. A new value for the unperturbed ν5 of the fluorobromomethane, which is undergoing a Fermi resonance with 2ν6, was adopted. This new value, higher than that adopted by Pitzer and Gelles, is backed by the results of the normal‐coordinate treatment. Mean‐square amplitudes of vibration for bonded and nonbonded atoms for the four molecules, as well as the Coriolis coupling coefficients were calculated. With the adopted values of the fundamentals, the usual ideal‐gas thermodynamic properties were calculated for the four compounds based on the latest values of the physical constants and the carbon−12 atomic mass scale.
44(1966); http://dx.doi.org/10.1063/1.1726939View Description Hide Description
The photolysis of O3 dissolved in liquid Ar at 87°K has been studied at 2537 Å and at wavelengths near 3130 Å. Quantum yields of O3photodecomposition were determined for O3 concentrations ranging from 5×10−4 to 0.3 mole/liter at 2537 Å and 0.03 to 0.67 mole/liter at 3130 Å. Disappearance of O3 is attributed to the reactions where Φ is the quantum yield of O(1 D) formation at a given wavelength. The limiting photodecomposition quantum yield at high O3 concentration is about 2 at 2537 Å and about ½ at 3130 Å.
Quantum yields were also determined at several wavelengths in the interval 2970 to 3340 Å for the photo‐exchange reaction
The foregoing studies, together with previous work, show that the O(1 D) quantum yield in O3photolysis is unity below 3000 Å but falls to a value of 0.4±0.15 in the 3130‐Å region. The exchange experiments show that the total quantum yield of atomic oxygen, including O(1 D) and O(3 P), retains a constant value of unity throughout the wavelength range studied, with onset of O(3 P) formation occurring at 3000 Å.
From these results the rate of O(1 D) production in the lower atmosphere is calculated to be 1.2±0.5 pphm/h (pphm is parts per hundred million), based on an assumed O3 concentration of 10 pphm and a solar zenith angle of 45°.
44(1966); http://dx.doi.org/10.1063/1.1726940View Description Hide Description
The temperature and frequency dependence of the 1H and 19F spin—lattice relaxation times in TGS, TGSe, and TGBe have been measured both in the ferroelectric and nonferroelectric phases, using a pulse technique. The results show that though a considerable slowing down of the proton motion occurs at the Curie points, the characteristic frequencies of internal motion are, in this range, still greater than the nuclear Larmor frequencies. The changes in the linewidth observed at certain orientations in the 1H and the additional splittings in the deuteron spectra on going to the ferroelectric phase are thus not due to a freezing in of internal motion but rather to a static rearrangement of the glycine units.
44(1966); http://dx.doi.org/10.1063/1.1726941View Description Hide Description
The fluorescence and absorption spectra of anthracene‐d 10 in a fluorene matrix at 4.2°K have been measured and analyzed. In the ground electronic state seven ag and two fundamentals have been assigned, while in the first‐excited electronic state (1 B 1u ) six ag and four fundamentals were located. Other possible fundamentals are indicated. Earlier data concerning ag and fundamentals of anthracene‐h 10 are critically reviewed and compared with the present results.
Observations on the Yellow Nitrogen Afterglow Bands at Low Pressure and the Radiative Lifetimes of the B 3Π g and A 3Σ u + States of Nitrogen44(1966); http://dx.doi.org/10.1063/1.1726942View Description Hide Description
The rotational profile of the (11, 7) first positive band in active nitrogen is shown to be pressure dependent in the range 0.05 to 1.0 torr. The effect is ascribed to rotational relaxation of nitrogen molecules in the B 3Π(v=11) level, and this interpretation leads to an estimation of tentative lower limits to the radiative lifetimes of B 3Π(v=11) and A 3Σ(v=0) of (2.4±0.5)×10−6 sec and 12±2.4 sec, respectively.
44(1966); http://dx.doi.org/10.1063/1.1726943View Description Hide Description
Measurements of the NMR spin—lattice relaxation timeT 1 have been carried out at various temperatures and pressures for eight solids that demonstrate rotational degrees of freedom. Apparatus is described which permits NMR pulse experiments to be performed over the temperature interval between 169° and 400°K under applied hydrostatic pressures between 1 and 680 atm. The intramolecular rotation of groups within molecules is insensitive to pressures within this range, and the rotation of entire molecules in the solid state is pressure dependent in varying degrees. The observed pressure coefficients of T 1 in these solids are related to the thermal expansion coefficient of the lattice by a simple model that postulates a localized lattice expansion as an important step in the reorientation process.
44(1966); http://dx.doi.org/10.1063/1.1726944View Description Hide Description
The weak mystery band found in the mono‐olefins at 5–6 eV is here reassigned as a symmetry‐allowed π→CH*(1 Ag →1 B 2u ) transition in which the CH* sigma upper orbital has considerable nsRydberg character. Evidence supporting this assignment and militating against the recent CH→π*(1 Ag →1 B 3g ) assignment of Berry [J. Chem. Phys. 38, 1934 (1963)], the only other plausible assignment, is drawn from (a) ab initio calculations of the electronic spectrum and orbital energies of ethylene in large Gaussian bases, (b) a study of the intensities and vibronic band shapes of the mystery bands of cis—trans pairs of dialkylethylenes, (c) the observed effect of ring strain on the mystery‐band intensity, (d) a measurement of the Cotton effect and rotational strength of the mystery‐band transition, and (e) the spectra of olefins in condensed phases which demonstrate the Rydberg nature of the mystery‐band upper state. Evidence is also presented that shows that the first electron affinity of ethylene places an electron in the CH* orbital, rather than in the π* orbital, as previously assumed. The implication of this low‐lying π→σ* transition for the common assumption of π—σ separability is mentioned, and the importance of the almost certain presence of relatively low‐lying π→σ* transitions with out‐of‐plane polarization in hydrocarbons of all sizes is stressed.
44(1966); http://dx.doi.org/10.1063/1.1726945View Description Hide Description
A version of the statistical theory of electronic energies, arising from a consideration of alternative partitionings of the Hamiltonian, has been applied to the H2 + ion, yielding electronic energies ∼6% greater in magnitude than the exact theoretical values at several values of the internuclear separation. The binding energy at the theoretical equilibrium separation was calculated to be 0.1024 a.u. compared to the experimental value of 0.1026 a.u.
44(1966); http://dx.doi.org/10.1063/1.1726946View Description Hide Description
With an 18‐term wavefunction, using 16 generalized Gaussian orbitals as a basis, we find a minimum energy of −1.3185 a.u. for H3 + in an equilateral triangular configuration having sides 1.66 bohr The A 1′ normal mode of vibration has ω0=3610 cm−1. Several energy contours are presented and their relation to H2+D+→HD+H+reactions is sketched.
44(1966); http://dx.doi.org/10.1063/1.1726947View Description Hide Description
Numerical results are presented for the lithium hydride molecule, obtained by different techniques of the constrained‐variation method. The constrained wavefunction, the degree to which the constraint is satisfied, and the sacrifice in energy are compared. The constraint operator used was the electronic dipole moment in one case and the total Hellmann—Feynman force in another. In both cases, the parametrization procedure in the direct solution approach (Method I B) was found to be superior.
44(1966); http://dx.doi.org/10.1063/1.1726948View Description Hide Description
SCF—LCAO wavefunction calculations with a minimal basis set were performed for H2O2 at a series of seven different dihedral angles, as well as for a modified form, H2–O–O. The multicenter integrals were computed to an accuracy of about 0.00002 a.u. The internal rotation barriers obtained were 2.2 kcal/mole through the trans position and 11.8 through cis, compared with the best experimental values of 1.1 and 7.0 kcal/mole, respectively. Dipole moments,ionization energies, and electron‐distribution data were also obtained.