bluuebunny/crossref_metadata_2025 huggingface dataset:
Interdependence of Spectral Line Breadth, Sound Velocity Dispersion, and Viscosity of Small Molecules in the Gas Phase
Authors: Bird George R.
Published: January 2005

An examination of the data on bimolecular collisions of small molecules has produced several new insights and predictions. The data on viscosity, sound velocity dispersion, and spectral line breadths in the microwave, infrared, and Raman regions have been assembled for H2, O2, N2, CH3Cl, and HCN. These data have been put on an equivalent basis of calculated or observed spectral line breadth constants. When this is done, the relative importance of elastic and inelastic rotational collisions may be assessed.

The data on H2 are here used to predict a collisional narrowing of the ordinary Doppler linewidth; this narrowing effect can be found in the literature, where the significance of the extremely narrow lines has not been fully appreciated.

The line breadths observed in the microwave spectrum of oxygen are seen to be peculiar to some phenomenon which reorients electron spins but does not alter rotational states. Spin exchange is here identified as the most probably mechanism for microwave line broadening in O2.

The differences between isotropic and anisotropic linewidths in the Raman spectra of H2, O2, and N2 indicate that a thorough theoretical treatment of Raman linewidths is required. Some additional experiments on line breadths in the Raman spectra of H2, O2, and N2 are suggested. The basic approximation in Anderson's impact theory for broadening of spectral absorption and emission lines is seen to be quantitatively invalid for self-broadening of these homonuclear diatomic molecules. This is the approximation of classical (straight) paths.

Two new phenomena are predicted for the propagation of rotational energy in small polar molecules. The first of these is a separation of the rotational sound velocity dispersion into two regions for symmetrictop molecules. The region of lower frequency dispersion should account for R/2 of heat capacity, and be associated with changes in rotation about the unique axis of the molecule. Small polar molecules with large dipoles (particularly the alkali halide vapors) should also exhibit a peculiar form of energy propagation in the rotational degrees of freedom only. This ``roton propagation'' may also influence the behavior of shock waves in polar gases and the heat conductivity at very low pressures.
Molecular Interaction and Linewidth of Asymmetric Molecule SO2. III. SO2–CH3Br and SO2–SO2 Collisions
Authors: Krishnaji  , Chandra Suresh
Published: January 2005

The linewidth of the 91,9→82,6 transition of sulfur dioxide broadened by methyl bromide and sulfur dioxide itself has been measured at different pressures of the broadener. The measured linewidth for SO2–CH3Br collisions is 13.64 Mc/mm. The hard-sphere collision diameter corresponding to this is 12.1 Å as compared to the kinetic collision diameter of 4.0 Å. The calculated width on the basis of Anderson's theory, assuming dipole—dipole interaction, comes out to be 22.4 Mc/mm, which is in disagreement with the experimental results. The measured linewidth for SO2–SO2 collisions comes out to be 11.23 Mc/mm. The effective collision diameter for this is 10.5 Å.
Microwave Spectra and Structure of Dimethyl Ether
Authors: Blukis Uldis , Kasai Paul H. , Myers Rollie J.
Published: January 2005

The microwave spectra of six isotopic species of dimethyl ether were investigated from 8–48 Gc. From the changes in moments of inertia with C13 and O18 substitutions the CO distance and <COC were found to be 1.410±0.003 Å and 111°43′±20′, respectively.

The conformation of the methyl groups was determined from the (CH2D) (CH3)O species. Dimethyl ether has C2v symmetry and the methyl groups are staggered with respect to the opposite CO bonds.

The data from deuterium substitutions could only be satisfactorily fit by a structure containing asymmetric methyl groups. The two CH distances in this structure differ by 0.009 Å, the < HCH by 49′, and the angle between the CO internuclear line and a perpendicular to its respective hydrogen plane is 2°31′±40′. If an average CH distance of 1.0956 Å is assumed the average <HCH is 109°29′. The structure of the asymmetric methyl groups is shown to be consistent with s and p hybridization of the carbon orbitals. In this scheme the carbons form ``bent bonds'' with the oxygen.

From the Stark effect the dipole moment of dimethyl ether was determined to be 1.31±0.01×10—18 esu.
Infrared Spectrum and Vibrational Potential Function of Ketene and the Deuterated Ketenes
Authors: Moore C. Bradley , Pimentel George C.
Published: January 2005

Infrared spectra of solid ketene and ketene in argon reveal a new fundamental at lower frequency (CH2CO, 438 cm—1; CD2CO, 371 cm—1; CHDCO, 398 cm—1) than any previously reported. These spectra and additional gas-phase spectra provide a basis for a reassignment of the vibrational spectrum. The vibrational potential function, centrifugal distortion constants, Coriolis coupling constants, and thermodynamic properties of ketene have been calculated. The out-of-plane hydrogen bending force constant is found to be surprisingly low, about one-third of that for ethylene. In addition, the analysis of the rotational structure of several perpendicular bands yields improved estimates of the A moment of inertia and hence of the HCH angle (122.3°) and the C–H bond length (1.079 Å) of ketene.
Spin—Orbit Coupling and the Radiationless Processes in Nitrogen Heterocyclics
Authors: El-Sayed M. A.
Published: January 2005

The matrix elements of the spin—orbit operator between the zero-order (spin-free) (n, π*) states of nitrogen heterocyclics are examined. It is found that generally, to the first order, there is no spin—orbit coupling between singlet and triplet states of the same configuration. The coupling between states of different configurations due to differences in the occupancy of the π* orbitals, but having the same nonbonding orbital, is found to have matrix elements similar to those resulting from the coupling between (π, π*) states, which McClure has shown to be unimportant. For the diazines (or polyazines), the coupling between different configurations due to a different occupancy of the nonbonding orbitals is found to have matrix elements which cancel one another. The general conclusion is thus reached that spin—orbit (s.o.) coupling between singlet and triplet (n, π*) states is unimportant to the first order. Using the observed polarization and lifetime data, it is estimated that the ratio |〈S(n,π*)|Hs.o.|T(n,π*)〉〈S(π,π*)|Hs.o.|T(n,π*)〉|2has an upper value of 10—3. These results as well as vibrational overlap considerations suggest that the enhancement of the efficiency of the intersystem crossing in nitrogen heterocyclics with the lowest singlet state of the (n, π*) type might be due to an S(n, π*)→T(π, π*) rather than an S(n, π*)→T(n, π*) radiationless process. An alternative explanation to the one previously given for the observation of fluorescence of 9, 10-diazaphenanthrene and s-tetrazine is thus suggested. In these molecules, the strong interaction between the (n, π*) singlet levels might cause the lowest S(n, π*) excited level to have a lower energy value than the (π, π*) triplet level. This would therefore lead to a retardation of the intersystem crossing process and allow the fluorescence to be observed. The characteristics of the observed weak phosphorescence of 9,10-diazaphenanthrene give spectroscopic support for this proposal.

For nitrogen heterocyclic molecules with the lowest singlet and lowest triplet states of the (π, π*) types, an examination of the quinoline total emission in comparison with that of the parent hydrocarbon, nath-thalene, suggests an increase in the efficiency of the intersystem crossing process in quinoline. This is suggested to be due to the efficient S1(π, π*)→T2(n, π*)→T1(π, π*) radiationless process. A probable enhancement of the internal conversion processes in these molecules is suggested to be due to an increase in the density of states and the efficiency of the S(π, π*)↔S(n, π*) radiationless process due to a large vibrational overlap integral.
Electron Spin Exchange in Aqueous Solutions of K2(SO3)2NO
Authors: Jones M. T.
Published: January 2005

The electron spin exchange frequency of potassium peroxylamine disulfonate as a function of concentration has been studied by electron spin resonance techniques. The exchange frequencies have been calculated from measurements of the linewidths and the line separations using the modified Bloch equations. The frequencies thus obtained are in agreement with each other. The functional dependence of exchange frequency upon concentration can be expressed as ω=bCm, where m is of the order of unity. The effect of molecular oxygen upon the spin exchange measurements is described. The resonance line shape has been found to deviate from Lorentzian at low concentrations.
Zero-Field Electron Magnetic Resonance in Some Inorganic and Organic Radicals
Authors: Cole Terry , Kushida Toshimoto , Heller Hanan C.
Published: January 2005

Zero-field electron magnetic resonance (EMR) spectra have been observed for several dilute paramagnetic systems. Among these are peroxylamine disulfonate ions in aqueous solutions, Cr3+ in single crystals of MgO, free radicals in x-irradiated powders of malonic acid, potassium hydrogen malonate, dipotassium malonate, succinic acid, and glutaconic acid. Zero-field spectra were observed by means of a marginal oscillator—detector similar to that of Benedek and Kushida. The zero-field splittings in the systems studied were due exclusively to the hyperfine couplings between electronic and nuclear spins. Results obtained were in good agreement with high-field EMR measurements on these systems. In several cases the hyperfine couplings were obtained with greater precision than at high field. In addition, the radical —OOC–ĊH–COO— was identified, for the first time, in irradiated dipotassium malonate by means of its zero-field spectrum. No zero-field lines could be detected in electron-irradiated polyethylene. Advantages and disadvantages of the zero-field technique are discussed.
Spin—Lattice Relaxation due to Translational Diffusion in LiBr
Authors: Allen R. R. , Weber M. J.
Published: January 2005

Nuclear spin—lattice relaxation in LiBr due to translational diffusion of Li ion vacancies has been established from an investigation of the temperature dependence of the relaxation time T1. The Li and Br T1's exhibit minima at different temperatures, and are found to arise from magnetic dipole—dipole and quadrupolar interactions, respectively. Since both relaxation mechanisms are observed in the same crystal environment, the results have been used to compare and correlate the relaxation theories of Torrey and Reif and their extensions to LiBr. The magnitude and temperature dependence of the relaxation are in agreement with theoretical predictions. The effects of vacancies caused by (1) the presence of divalent impurity ions, and (2) intrinsic thermal generation are observed and the activation energies for their formation and diffusion are determined. Additional measurements of line narrowing and ionic conductivity are reported which confirm the properties of the diffusive motion.
Nature of s-Tetrazine Emission Spectra
Authors: Chowdhury Mihir , Goodman Lionel
Published: January 2005

Fluorescence is established as the unique emission spectrum of s-tetrazine and dimethyl s-tetrazine in several ways—notably by polarization measurements (out of plane) and life-time measurements of the emitting state (4×10—9 sec for dimethyl s-tetrazine at 300°K). The vibrational features (which indicate an allowed transition) are not fully understood on the basis of emission solely from the lowest singlet state. Experiments involving H-bonding solvents suggest that the hydrogen bond exists in the (n, π*) singlet state in s-tetrazine as contrasted to the situation in the (n, π*) triplet state in pyrazine and pyrimidine, where the H bond is broken. Polarization measurements on the emission as a function of the exciting wavelength confirms Mason's observation of a second n→π* band system at about 3200 Å. The polarization of the 3200-Å band system is out of plane, thereby supporting Mason's assignment of this band as an allowed n→π* transition. Different viewpoints on the mechanism of the fluorescence are summarized. The localized exciton splitting prediction of a forbidden lowest singlet state is contradicted by the experimental evidence. It seems probable that the lowest excited state is a triplet.
Theory of Pressure-Induced Shifts of Infrared Lines
Authors: Ben-Reuven A. , Friedmann H. , Jaffe J. H.
Published: January 2005

A treatment is presented that accounts to a large extent for the j dependence of pressure-induced shifts due to noble gases of lines in HCl bands. After a brief critical review of the theoretical work done on this subject to date, an improved phase-shift approximation for the intermolecular collision process is worked out with a coordinate system fixed in space during the collision. The main features of the observed phenomena are accounted for using only parameters calculated directly from known molecular properties. A discussion of the limitations of this theory is given. It appears that in order to achieve further improvement, the finite probability of j transitions and the influence of short-range forces should be taken into account.
Self-Diffusion and Impurity-Controlled Proton Relaxation in Liquid Methane
Authors: Gaven J. V. , Waugh J. S. , Stockmayer W. H.
Published: January 2005

Self-diffusion in liquid methane, under its own vapor pressure, has been studied between the triple point and the normal boiling point by means of the spin-echo technique. The activation energy for self-diffusion is approximately 0.67 kcal/mole. These results are compared with the constant-pressure tracer results of Naghizadeh and Rice [J. Chem. Phys. 36, 2710 (1962)], which show somewhat higher (∼15%) self-diffusion coefficients and an activation energy of approximately 0.83 kcal/mole. The temperature dependence of self-diffusion in liquid methane appears to be comparable on the basis of a classical reduction to corresponding states, by means of critical constants, to that in liquid Ar, Kr, and Xe.

Measurements of the proton spin-lattice relaxation time in our sample indicate that the relaxation is predominantly controlled by the mutual diffusion of the hydrocarbon and small amounts of dissolved oxygen.
Ohmic Contact to Zinc Sulfide Single Crystals
Authors: Cooke I.
Published: January 2005

Gallium-metal electrodes have been used with ZnS and it is established that these are Ohmic. With such electrodes it has been possible to make use of theoretical work on currents in insulators to estimate that the carrier density in these crystals is in the order of 106 per cm3 and that there is an effective trap depth of 0.9 eV. Experiments with crystals other than ZnS indicate that the mechanism of Ohmic contact formation is understood in principle.
Hexagonal Close-Packed Structures in Bi–Pb Alloys and the Polymorphism of Lead at High Pressure
Authors: Klement William
Published: January 2005

Lattice spacings have been obtained for the hexagonal close-packed structures in lead alloys containing 27.0 to 32.0 at.% bismuth, which were rapidly cooled from the melt. Axial ratios for these structures vary from 1.653 to 1.654. By extrapolation to pure lead of these volume/atom determinations and of some activity data, the free energy of hexagonal close-packed lead relative to the face-centered cubic form is estimated. It is suggested that the structure of the lead polymorph stable at pressures greater than 0.16 Mbar is hexagonal close-packed.
Calculation of Molecular Properties with Limited-Basis Hartree—Fock Functions
Authors: Goodisman Jerry
Published: January 2005

The use of the limited-basis (Roothaan) approximation to Hartree—Fock functions and its implications for the accuracy of calculations on one-electron operators are discussed. Only for the true Hartree—Fock function does Brillouin's theorem guarantee second-order errors for expectation values of these operators. The mean square deviation of one Roothaan function from a second with an incremented basis set provides a measure of how closely the true Hartree—Fock function is being approached. Expectation values of one-electron operators reflect this as well, but it is shown that the improvement or lowering of energy on addition of functions to the basis is much less suitable.
Kinetics of Desorption. II. Cs+ and Ba+ from Rhenium
Authors: Scheer Milton D. , Fine Joseph
Published: January 2005

The mean desorption lifetimes (τ3) of Cs+ and Ba+ on an atomically clean rhenium surface have been measured. Under conditions of low surface coverage τ3=(1.9±0.9)×10−13exp(23 300±500T)secwas obtained for Cs+ in the temperature range 950<T<1080°K. This is in excellent agreement with the data obtained previously for the Cs+–W system. For Ba+ on rheniumτ3=(0.6±0.4)×10−13exp(54 700±2400T)secwas obtained in the temperature range 2100<T<2310°K. The Cs+–Re, like the Cs+–W bonding, can be attributed to the electrostatic image energy of a Cs+ ion on an electrically conducting surface. The Ba+–Re interaction, on the other hand, must include a large exchange force in addition to the electrostatic image force because of the unpaired valence electron remaining in the univalent barium positive ion. The presence of an adsorbed layer of residual vacuum gases decreased the Cs+–Re binding energy by 0.4 eV and increased the pre-exponential factor by two orders of magnitude. These results are almost identical with that reported previously for Cs+ desorbing from a contaminated tungsten surface.
Vibronic—Spin-Orbit Perturbations and the Assignment of the Lowest Triplet State of Benzene
Authors: Albrecht A. C.
Published: January 2005

Second-order perturbation theory in electron configuration and spin space using both vibronic and spin-orbit perturbations reveals a number of routes for bringing dipole allowed character into a spin-forbidden transition. Apart from direct spin-orbit interaction, there are three additional mechanisms: (1) direct spin-vibronic coupling (a very weak, though first-order term), (2) spin-orbit coupling with vibronic coupling in the singlet manifold, and (3) vibronic coupling in the triplet manifold with spin-orbit coupling. In the case of benzene, vibrational analysis alone of the phosphorescence spectrum can neither discern the route for bringing allowed singlet character into the triplet state nor can it lead to an assignment of that state. It happens that polarized luminescence data are able to show that route (3) is dominant, and, given this, vibrational analysis leads to a 3B1u assignment. The vibronic characteristics of the various routes are discussed. Estimates are made of the relative importance of the various routes based on energy considerations within the framework of second-order perturbation theory. Approximate oscillator strengths are obtained and compared with the experimental value showing, independently, that route (3) is dominant and that the assignment must be 3B1u. Other possible examples where mixed vibronic—spin-orbit terms are important to phosphoresence are mentioned suggesting the fruitfulness of detailed polarized phosphorescence experiments, both to aid in assigning excited states and to test vibronic theories.
Electron Spin Resonance of Titanium (III) Acetylacetonate
Authors: McGarvey B. R.
Published: January 2005

The ESR spectra of titanium (III) acetylacetonate present in small amounts in single crystals of aluminum (III) acetylacetonate has been investigated both at room temperature and at liquid-nitrogen temperature. The spectra arising from the isotopes with no nuclear spin gave the following: g‖ = 2.000±0.002; g‖ = 1.921±0.001. The linewidths at room temperature were about 70 G, but narrowed down to less than 10 G at liquid-nitrogen temperatures. At liquid-nitrogen temperatures, the hyperfine spectrum due to isotopes Ti47 and Ti49 was observed and is shown to confirm previous assignments of nuclear spins. The hyperfine components of the spin Hamiltonian were found to be: A = 6.3±3.0×10—4 cm—1; B = 17.5±0.5×10—4 cm—1. It is shown that these results are consistent with an A1 ground state and with the Ea state, being from 2000 to 4000 cm—1 above the ground state. Further, it is shown that the results suggest a considerable degree of covalent interaction involving π electrons of the ligands.
Exchange of Methanol between Solvated Cations and Solvent. I
Authors: Swinehart J. H. , Rogers T. E. , Taube H.
Published: January 2005

The exchange of methanol between solvated Li+, Cd++, and Mg++, and the solvent has been studied by the isotope dilution technique at —82.5° and —96.9°C. Even at —96.9°C the exchange is complete for Li+ and Cd++ within the time of sampling, 0.5 min. However, the exchange for solvated Mg++ is observable. At —82.5° and —96.9°C the half-lives for the isotopic exchange are 3.7 and 64 min, respectively. The activation energy ΔH‡ is 12.5±2.0 kcal/mole and the entropy of activation ΔS‡ is +(4±4) eu. The ``kinetic solvation number'' of Mg++ with respect to methanol as determined by the isotope dilution technique is less than six, where six is the coordination number of Mg++ in these solutions. The defect is caused by residual water which both displaces methanol from the coordination sphere and labilizes it for exchange.
Electron Transfer Among the Transition Elements; the Controlling Role of the Franck—Condon Principle on Rates
Authors: Libby W. F.
Published: January 2005

The principle [W. F. Libby, J. Phys. Chem. 56, 863 (1952)] that electrons cannot be readily exchanged between aqueous ions of different valence, unless they possess sufficient geometrical similarity to reduce to a minimum the energy transfer required by the simultaneous and instantaneous conversion of an ion of one valence to another while a second is changed in the exact opposite way, has now been firmly established. The Franck—Condon principle as applied to electron movement as well as to optical transitions thus has been found to be completely commanding in the case of electron exchange reactions between transition element ions in aqueous solution; e.g., Fe2+ and Fe3+ exchange electrons slowly while Fe(CN)64+ and Fe(CN)63— exchange very rapidly. Rapid electron exchange occurs also between MnO4—— and MnO4— even though the oxygen atoms do not exchange. Thus a type of tunneling of electrons is firmly proved to occur when the source and sink (reducing and oxidizing) ions (or molecules) satisfy the requirement of the Franck—Condon principle of being sufficiently similar geometrically to leave little need for change in hydration energy after the electron transfer.

It seems clear that this principle should apply also to oxidation and reduction reactions among the transition elements, and the present work examines the available kinetic data (particularly those of Taube and co-workers) to establish the extent of this applicability which appears to be substantial.
Electron Requirements of Bonds in Metal Borides
Authors: Johnson Robert W. , Daane A. H.
Published: January 2005

Electrical measurements have been made on CaB6, SrB6, BaB6, YB2, YB4, YB6, and YB12 for the purpose of testing models of the electronic structure of the boron atoms in the four boride types represented. The compounds CaB6, SrB6, and BaB6 were found to be semiconductors. The Hall coefficients of YB2, YB6, and YB12 were measured and found to correspond to one free electron per yttrium atom in each compound. The Hall coefficient of YB4 was also measured, but could not be interpreted because the free-electron theory does not provide a theoretical value for comparison with experiment. The results are in agreement with the models which have been proposed.
Diffusion of Hydrogen in Natural and Synthetic Fused Quartz
Authors: Lee R. W.
Published: January 2005

A mass spectrometer study was made to compare the transport characteristics of hydrogen and deuterium through hollow, thin walled cylinders of three types of fused quartz. The experiments in the temperature range (300°—1000°C) resulted in the following observations: (1) The permeability of hydrogen (and deuterium) in fused quartz is virtually independent of purity level or hydroxyl content. (2) The corresponding diffusivity is unaffected by purity or hydroxyl content; however, the ``anomalously low evolution rates'' which are observed at elevated temperatures in ``dry'' (low OH) quartz are much less predominant in ``wet'' quartz. (3) The total hydrogen content is higher in ``wet'' quartz. (4) The chemical exchange of diffusing (interstitial) hydrogen with residual hydrogen does not obey the same rate laws in ``wet'' and ``dry'' fused quartz. Various hydrogen trapping sites are considered in the light of infrared data and a model is proposed which accounts for the diffusion anomaly (2) as well as the exchange rate differences (4).
Microwave Spectrum and Structure of Difluoramine
Authors: Lide David R.
Published: January 2005

The microwave spectra of NF2H and NF2D have been analyzed. Rotational constants are 53 017.12, 10 895.43, 9307.22 Mc for NF2H, and 44 884.92, 10 749.38, 9 126.26 Mc for NF2D. The structure obtained from these constants is: rNF=1.400±0.002 Å, rNH=1.026±0.002 Å, ∠FNF=102.9°±0.2°, and ∠HNF=99.8°±0.2°. Quadrupole coupling constants have been measured for both isotopic species and the complete field-gradient tensor has been determined. The dipole moment has a magnitude of 1.93±0.02 D and makes an angle of 19° with the NH bond. An upper limit of 0.2 Mc may be set on the inversion splitting in the ground state.
Infrared Spectrum of Difluoramine
Authors: Comeford J. J. , Mann D. E. , Schoen L. J. , Lide David R.
Published: January 2005

The infrared spectrum of gaseous difluoramine, NF2H, has been investigated over the range 250–4000 cm—1. Four bands of NF2D have also been observed. The fundamental frequencies (in cm—1) and their species are as follows: NF2H:ν1(a′):3193;ν2(a′):1307;ν3(a′):972;ν4(a′):500;ν5(a′′):1424;ν6(a′′):888.NF2D:ν1(a′):[2333];ν2(a′):1008;ν3(a′):972;ν4(a′):[500];ν5(a′′):1042;ν6(a′′):888.The bracketed values were not observed directly. The fine structure resolved in several of the bands led to a value for A — (B+C)/2 in agreement with the microwave result. No inversion splitting was detected.
Perturbed Hartree—Fock Calculations. I. Magnetic Susceptibility and Shielding in the LiH Molecule
Authors: Stevens R. M. , Pitzer R. M. , Lipscomb W. N.
Published: January 2005

The limited basis set Hartree—Fock problem is solved in the presence of a perturbation term in the Hamiltonian to obtain the first-order perturbed wavefunction. The formulation is applied to the calculation of electric polarizability, magnetic susceptibility, and magnetic shielding. The effect of the limited basis set on the gauge invariance of the magnetic quantities is discussed. The magnetic susceptibility and shielding, the rotational magnetic moment, and the spin rotational constants are calculated for lithium hydride using two different wavefunctions. The agreement with experiment is reasonably good for the spin rotational constants and rotational magnetic moment. It is felt that further refinement of the calculation would not change the results by more than a few percent.
Problem of the Lengths and Strengths of Carbon—Fluorine Bonds
Authors: Peters David
Published: January 2005

A simple explanation of the problem of the lengths and strengths of carbon—fluorine bonds is proposed. Qualitative molecular orbital (MO) theory is used and it is suggested that the charge transfer from carbon to fluorine is the primary cause of the observed progressive shortening of all bonds from a given carbon atom as the number of fluorines on this atom increases. A reasonable table of carbon—fluorine bond strengths is constructed.
Simple Model for the Helix—Random Coil Transition in Polypeptides
Authors: Baur M. E. , Nosanow L. H.
Published: January 2005

A simple model for the helix—random coil transition in polypeptides is constructed. Only the limiting case of a sharp transition is considered; however, the model could easily be extended to treat the actual case of a diffuse transition. The formulation of the problem given by Lifson and Roig is used; the main innovation is the introduction of a model interaction with a small number of parameters. For transitions occurring near room temperature, the transition temperature and thermodynamic functions are given in terms of only two parameters, each of which has a simple physical interpretation. The parameters are treated as temperature independent. The model describes in a natural way systems in which both normal and inverted transitions occur. In a normal transition, the helix is the phase stable below the transition temperature; in an inverted transition, it is the phase stable above the transition temperature. In the latter case, the model also describes a reversion of the helix to coil form at elevated temperatures. On the basis of experimental data, the parameters of the theory and the thermodynamic functions are calculated for two systems, one having a normal, and one an inverted transition. The interpretation of the theory in terms of the nature of the polymer—solvent interaction is discussed. The relationship of our work to that of Gibbs and DiMarzio is discussed.
Ultrasonic Investigation of the Rate of Hydrolysis of Potassium Cyanide
Authors: Stuehr John , Yeager Ernest , Sachs Thomas , Hovorka Frank
Published: January 2005

The kinetics of the hydrolysis of the cyanide ion have been studied by ultrasonic absorption measurements over the frequency range 30 kc to 25 Mc. A single relaxation frequency has been observed which depends on concentration in the manner expected theoretically for the hydrolysis reaction. The relaxation frequency at 25°C is 450 kc for a 0.01M solution and 950 kc for 0.05M. The corresponding forward and reverse rate constants calculated for the reaction CN—+H2O⇌HCN+OH— are k1=5.2×104 sec—1 and k—1=3.7×109 M—1 sec—1 at 25°C. From the magnitude of the maximum excess acoustic absorption per wavelength (90×10—6 at 25°C for the 0.05M solution), the partial molal volume change ΔV° for the process is calculated to be — 12.4 cm3/mole. The temperature dependence of the relaxation frequency indicates an energy of activation for the process HCN+OH—→CN—+H2O of 6±2 kcal/mole.
On the Robinson and Price (Kramers—Kronig) Method of Interpreting Reflection Data Taken through a Transparent Window
Authors: Plaskett J. S. , Schatz P. N.
Published: January 2005

It is shown that the usual integral formula used to obtain the phase shift from the reflectivity (at all frequencies) must be modified when the reflectivity is measured off a surface covered by a transparent window. The discussion is first restricted to reflection at normal incidence but is later generalized to include all possible angles of incidence. There are found to be three distinct cases defined by the conditions (1)sin2α<1/εw,(2) 1/εw<sin2α<ε0/εw,(3)sin2α>ε0/εw, where ε0 is the static dielectric constant of the sample, εw is the dielectric constant of the window, and α is the angle of incidence. Case (1) involves the addition of an extra factor to the usual expression for the phase shift. This extra factor involves either zero, one, or two unknown parameters depending upon the particular values of α, εw and ε0. In case (2) the expression for the phase shift is quite complicated and of doubtful practical value. In case (3), it was found possible to write a simple expression for the phase shift in terms of a different integral than the one ordinarily used.
Thermoelectric Effects in Fused Ionic Materials
Authors: Detig R. H. , Archer D. H.
Published: January 2005

Measured values of the thermoelectric power of fused PbCl2, PbBr2, AgNO3, NaCl, and KCl are reported for temperatures in the range 200°—1000°C. Theoretical correlations of the thermoelectric power derived by Holtan from the principles of irreversible thermodynamics do not check closely with the empirical results if it is assumed that the entropies of transport for the materials investigated are negligible. Such an assumption, in general therefore, seems unjustified.
Mechanism of the Conversion of CdF2 from an Insulator to a Semiconductor
Authors: Prener J. S. , Kingsley J. D.
Published: January 2005

In a previous publication, the authors proposed a mechanism for the conversion of CdF2 containing small amounts of trivalent ion impurities from an insulator to an n-type semiconductor when heated in Cd vapor at 500°C. Additional experimental evidence is presented in support of this mechanism in which the interstitial F— ions, which normally compensate the extra positive charge of the trivalent ion impurity, diffuse to the surface of the crystal and combine with Cd atoms from the gas phase to form a surface layer of CdF2. Simultaneously electrons from the Cd atoms diffuse into the crystal and give rise to observed conductivity and optical absorption. It is demonstrated using Cd109 that no appreciable diffusion of Cd into the crystals takes place during heating in Cd vapor. The dependence of the amount of added Cd upon the Cd pressure is in good agreement with the proposed mechanism. Finally the observed changes in the fluorescence spectrum of Sm3+ after heating in Cd vapor are used to show that the symmetry of the crystal field at the Sm site is changed from noncubic symmetry to a higher symmetry, most probably cubic.
Kinetics of Reactant Isolation. I. One-Dimensional Problems
Authors: Cohen E. Richard , Reiss H.
Published: January 2005

This paper treats the time-dependent statistics of bond formation (both irreversible and reversible) between fixed sites. This model corresponds to a number of real physical processes. Among these are reaction of adjacent functional groups on a linear polymer, matrix isolation of free radicals, and chemisorption. Because the sites are fixed, certain of them become isolated from others, not yet reacted, and must survive to infinite time. A certain amount of survival occurs even when bonds can be undone.

Exact descriptions of the kinetics of these processes are given for one-dimensional systems. The mean probability of site survival as a function of time is calculated for linear arrays and for rings. The probability of site survival as a function of position on the chain (end effects) is treated in Sec. IV. A generating function is introduced in order to calculate higher moments of the survival probability distribution function and to treat the effect of a random diluent. An approximate method of solution is developed for the time dependence of the number of reacted particles in the reversible case.
Primary Reactions in the Mercury Photosensitized Decomposition of Butene-2 at Low Pressures
Authors: Kebarle P. , Avrahami M.
Published: January 2005

The reaction was studied in a flow system attached to a mass spectrometer. The pressure of the butene was in the micron range while He at a pressure of 10 mm was used as a carrier gas. Under these conditions it was found that the greater part of the excited butene-2 is deactivated.

The decomposition proceeds by two primary reactions: 2−C4H8*→CH3CHCHCH2+H,2−C4H8*→ lim rearrangementallyl+methyl.The two reactions occur roughly in the ratio 2.5:1. A rearrangement to methylcyclopropane occurs to a very minor extent if at all.
WKB Analysis of Molecular Beam Scattering and the Lennard-Jones Potential
Authors: Mueller C. R. , Marchi R. P.
Published: January 2005

The WKB method was used in the interpretation of molecular beam scattering. Particular attention was given to small angle scattering where the sensitivity to the intermolecular potential is greatest. It was found that the Lennard-Jones potential does not fit the data and that, for the case investigated, the minimum in the potential probably needs to be broadened. Empirical relationships between the phase shifts and the intermolecular potential were found which should be of value in the inversion problem. A calculation of rainbow scattering was made.
Analytical Self-Consistent Field Functions for Positive Ions. II. Isoelectronic Series with 11 to 18 Electrons
Authors: Clementi Enrico
Published: January 2005

Analytical self-consistent field (SCF) functions for the neutral atoms and positive ions isoelectronic series of 11 to 18 electrons with Z=11 to Z=36 are presented. These computations include the ground state and the excited states for the lowest configurations. As in previous reported work, the basis set of Slater-type orbitals is fully optimized in the orbital exponents, and considerable effort is spent in ensuring accurate and short expansions. The computational results are discussed only in the most important aspects. The total energies for each isoelectronic series are condensed in a power series of Z. These empirical power expansions show a regular dependency on the number of electrons and give an accurate fitting of the several hundreds of states computed. The relation between the ionization potentials and orbital energy is discussed. The experimental excitation energies are compared with the computed. The problem of the ``goodness'' of the SCF functions is briefly examined in relation to the one-electron operator expectation values.
Oxygen-17 NMR Shifts Caused by Cr++ in Aqueous Solutions
Authors: Jackson Jasper A. , Lemons Joe F. , Taube Henry
Published: January 2005

Cr++ in solution produces a paramagnetic shift in the NMR absorption of O17 in ClO4—, as well as the expected paramagnetic shift for O17 in H2O. As the concentration of ClO4— increases, the shift in the H2O17 absorption is diminished, and eventually changes sign. The effects are ascribed to preferential replacement by ClO4— of water molecules from the axial positions in the first coordination sphere about Cr++.
Transformations of Ice II, Ice III, and Ice V at Atmospheric Pressure
Authors: Bertie J. E. , Calvert L. D. , Whalley E.
Published: January 2005

The transformations that occur in ices II, III, and V at atmospheric pressure when they are heated from liquid-nitrogen temperature have been examined by simple thermal analysis and by x-ray diffraction. They transform first to cubic ice I (ice Ic). The rate of transformation of ices II and III depends upon the temperature, and that of ice II probably depends upon its thermal history. The temperature dependence of the rate for ice V, and the history dependence of the rate for ices III and V were not examined. Ice Ic has previously been made only in small quantities; it can now be made in as large quantities as required by transformation of the high-pressure ices. The transformation of ice Ic to ordinary hexagonal ice (ice Ih) has also been examined. The rate depends markedly on the thermal history of the sample and this may help to explain the apparently inconsistent rates that have been reported by various workers.
Nuclear Magnetic Resonance in Transition-Metal Diborides
Authors: Silver Arnold H. , Kushida Toshimoto
Published: January 2005

Nuclear magnetic-resonance studies of the groups IV, V, and VI transition-metal diborides show that the boron nuclei have a small quadrupole coupling constant, and negative frequency shift of the order of 10—4. The V51 resonance in VB2 provides a measure of the quadrupole coupling constant for V51 as 2.35 Mc/sec and a frequency shift of —3.37×10—3. The frequency shifts are enhanced at low temperatures. These data are explained within the framework of a suggested metal—boron donor—acceptor bonds by proposing s—d exchange polarization and diamagnetic ``ring current'' contributions to the field at the nucleus.
Optical Absorption Spectrum of Metal-Free Phthalocyanine Single Crystals
Authors: Heilmeier George H. , Warfield George
Published: January 2005

The optical absorption spectrum of the molecular crystal metal-free phthalocyanine has been investigated in the region 25 000 to 3500 Å. The spectrum of the phthalocyanine molecule retains much of its identity in the crystal, as evidenced by the similarity in the regions of main absorption. This is a consequence of the weak intermolecular binding forces. The crystal absorptions in the region 8000 to 5000 Å, and below 4000 Å can be attributed to excitations from the highest filled molecular orbital to the vibrationally broadened and shifted excited states of the molecule which occur in the crystal field. Vibrational structure was observed which was attributed to a symmetrical breathing vibration of the peripheral benzene rings and, quite possibly, a similar vibration of the entire phthalocyanine molecule.
Photoconductivity in Metal-Free Phthalocyanine Single Crystals
Authors: Heilmeier George H. , Warfield George
Published: January 2005

The spectral response of photoconductivity in metal-free phthalocyanine crystals was obtained in the region 2500–25 000 Å, and was found to have the same response edges as the optical absorption spectrum. The correspondence between the valley-to-valley separation of the structure found in the strong response regions of the spectral response of photoconductivity and the peak-to-peak separation of the infrared optical absorption spectrum has been used to formulate a model for photoconductivity in metal-free phthalocyanine crystals which involves the direct creation of carriers by suitable radiation.

An expression for the effective lifetime of a photoexcited carrier in a crystal with a finite surface recombination velocity was derived. This was found experimentally to be approximately 0.9×10—8 sec. Using this lifetime and the mobility determined by Hall-effect measurements, the diffusion length for electrons was calculated to be 5×10—6 cm. A comparison of this diffusion length with the reciprocal of the extinction coefficient indicates that the free carriers excited by 7300-Å radiation are produced further from the surface than a diffusion length. Thus, the lifetime of the free carriers should be more nearly characteristic of the bulk. The experimentally determined lifetime is in rough agreement with the theoretical decay time for fluorescence. This emphasizes the importance of the first excited singlet state in the conduction process. The structure of the spectral response of photoconductivity was used in conjunction with previous discussions of the applicability of the band model to infer the nature of the conduction band in metal-free phthalocyanine crystals.
Vibrational Spectra of Molten Halides of Mercury. I. Mercuric Chloride, Mercuric Bromide, and Mercury Chlorobromide
Authors: Janz George J. , James David W.
Published: January 2005

The Raman spectra for HgCl2, HgBr2, and HgBrCl in the pure molten state are: HgCl2: 313 cm—1 (10, p), 376 (0, dp); HgBr2: 195 cm—1 (10, p), 271 (0, dp); HgBrCl: 111 (v.w.), 203 (v.w.), 236 (m), 319 (v.w.), and 335 (m). The vibrational assignment and force constants are reported in accord with the view that the linear X—Hg—X molecular species are retained in these melts as in the gaseous and crystalline states. The broad-band shape for the intense Raman line in these melts and the appearance of a Raman forbidden frequency as a very weak line in the spectra of molten HgCl2 and HgBr2 are examined from the viewpoint of the nature of the Hg—X chemical bond and cooperative rotational interactions in these molecular molten salts. The appearance of the two Raman lines of HgCl2 and HgBr2 (319 and 203 cm—1) in the spectrum of HgBrCl is understood if a disproportion reaction is recognized in this mixed halide salt in the molten state.
Infrared Absorption Spectra of Carbon Suboxide and Malononitrile in Solid Argon Matrices
Authors: Ames L. L. , White David , Mann D. E.
Published: January 2005

The infrared spectra of carbon suboxide and malononitrile isolated in solid argon matrices at low temperatures have been measured in the spectral region 440–4400 cm—1. A detailed comparison with prior spectroscopic data is made. It is shown that the matrix spectra of C3O2 support the linear symmetric model. A comparison of the matrix spectra and vibrational assignments of the three isoelectronic molecules C3O2, CH2(CN)2, and B2O3 is shown to favor a large apex angle in B2O3.
Analysis of the Spectra of Tripositive Praseodymium in Ethylsulfate Crystals
Authors: Gruber John B.
Published: January 2005

In the first portion of this paper a theoretical calculation is presented for the ``free-ion'' spectrum of Pr3+ (4f2). A least-squares calculation is made to obtain a set of Slater and spin-orbit parameters that gives a theoretical spectrum in closest possible agreement with the observed absorption spectrum of Pr3+ (4f2) in ethylsulfate crystals. The disagreement between the calculated and observed spectrum may be explained as due primarily to the neglect of the configuration interaction in the ``free-ion'' calculation, and to a much smaller extent as due to the effect of the lattice environment surrounding the Pr3+ (4f2) ion.

In comparing the spectrum of Pr3+ (4f2) in ethylsulfate crystals with the spectrum of the same ion in other crystals, the shift in energy of the J levels may be explained as due to the difference in chemical bonding in the various crystals. This ``chemical bonding'' shift may be accounted for by a simple change in the Slater and spin-orbit parameters.

Assuming that the local symmetry about the Pr3+ (4f2) ion is D3h, a first-order perturbation calculation is carried out for the crystal-field split components of the Pr3+ (4f2) ion J-levels, 3P2, 1I6, 3P1, 3P0, 1D2, 1G4, 3F4, 3F3, and 3H4.

The following set of lattice parameters, Anm 〈rn〉, obtained by a least-squares calculation, gives the best over-all agreement between the theoretical and observed spectrum: A20 〈r2〉=16.32 cm—1, A40 〈r4〉=—83.46 cm—1, A60 〈r6〉=—45.76 cm—1, and A66 〈r6〉=530.21 cm—1. A more complete calculation is then made using an IBM 7090 program that diagonalizes the entire intermediate-coupling and D3h crystal-field-splitting matrix. Again, from a least-squares calculation, the following ``best'' set of parameters is found: F2=307.4 cm—1, F4=49.44 cm—1, F6=5.138 cm—1, ξ=727.9 cm—1, A20 〈r2〉=15.31 cm—1, A40 〈r4〉=—88.32 cm—1, A60 〈r6〉=—48.76 cm—1, and A66 〈r6〉=548.48 cm—1. When one compares the theoretical energy spectrum based on these parameters with the observed spectrum, one finds an average deviation of 3.0 cm—1, and a standard deviation of only 1.0 cm—1.
Effective Diffusion Constant in a Polyelectrolyte Solution
Authors: Jackson Julius L. , Coriell Sam R.
Published: January 2005

A general theory of the effective diffusion constant of ions in a periodic electric field is presented. The predictions of the theory are compared with the results of experiments on the diffusion of labeled sodium in a sodium hydroxide—polyacrylic acid solution.
Operator Techniques in the Phenomenological Treatment of Magnetic Resonance
Authors: Corio P. L.
Published: January 2005

Operator methods are used to obtain solutions of the Bloch equations for circularly polarized fields and pulsed fields. These methods allow the solutions to be obtained in vector form. When T1 = T2, explicit analytical results are obtained. A perturbation method is given for T1 ≠ T2. An explicit formal solution is given for the magnetization after a sequence of n rf pulses. This treatment does not neglect relaxation effects during the pulses and the pulse sequence is of the form {θ0/2, θ0, θ0.···}, where θ0 is arbitrary. In the special case where θ0 = π, one obtains the usual 90°—180° pulse sequence. Formulas are derived which enable one to obtain explicit results in some important special cases.
Reinvestigation of Some Absolute Rate Measurements of O-Atom Reactions with Olefins
Authors: Elias Lorne
Published: January 2005

The rate constants, activation energies, and pre-exponential factors for the primary reactions of atomic oxygen with ethylene, butene-1, iso-butene, and cis-butene-2 have been determined. The measurements were carried out in a fast-flow system in which O atoms were produced by electrical discharge. Possible sources of error such as the presence of O2 and the effect of surface reactions are shown to be unimportant. The results show a systematic deviation from those of previous studies in which a photolysis technique was used to obtain relative rate data. The possibility of secondary attack on the olefins is discussed and considered an unlikely cause for the observed discrepancies.