bluuebunny/crossref_metadata_2025_split huggingface dataset:
Theory of the Phase Transition between Helix and Random Coil in Polypeptide Chains
Authors: Zimm B. H. , Bragg J. K. 
Published: January 2005

The transition between the helical and randomly coiled forms of a polypeptide chain is discussed by reference to a simple model that allows bonding only between each group and the third preceding. Two principal parameters are introduced, a statistical parameter that is essentially an equilibrium constant for the bonding of segments to a portion of the chain that is already in helical form, and a special correction factor for the initiation of a helix. A third parameter which specifies the minimum number of segments in a random section between two helical portions has only a minor effect on the results. The partition function for this model is handled in two alternative ways, either as a summation suitable for short chains, or in terms of the eigenvalues and eigenvectors of a characteristic matrix; the latter is more suitable for long chains. A transition from the random to the helical form is encountered as either the bonding parameter or the chain length is increased. The critical value of the bonding parameter is unity for long chains, while the sharpness of the transition depends on the initiation parameter.
Depending on the values of the bonding parameter and the chain length, one of the following configurations dominates: random coils, single helices with occasional disorder at the ends, and for longer chains, helices occasionally broken by random sections. In rather narrow transition regions, mixtures of these forms may be found. A diagram is given that displays the relationships of these forms.
The theory is compared with published data on polybenzyl-glutamate with fair agreement.
Microwave Spectrum and Structure of Perrhenyl Fluoride
Authors: Lotspeich J. F. , Javan A. , Engelbrecht A. 
Published: January 2005

Rotational absorption lines of the symmetric-top molecule ReO3F have been observed in the K-band regions of 21 400 and 28 500 Mc, with a Stark modulation spectrometer designed specially to accommodate this and other chemically reactive molecules The spectrum exhibits a hyperfine structure arising from interaction of the Re nuclear electric quadrupole moment with the surrounding charge. The hfs pattern establishes the Re nuclear spin as 5/2 and yields a quadrupole coupling constant eqQ (Re187) = (—48.4±1.3) Mc for the ground vibration state. Additional groups of lines associated with molecular states of vibrational excitation were also present. Of the six normal vibration frequencies, three were sufficiently low to contribute an observable fraction of molecules to their excited states: one non-degenerate, ν3 (a1), and two doubly degenerate, ν5 (e) and ν6 (e). The corresponding rotation-vibration and l type doubling constants were measured to be α3 = (+12.30±0.06) Mc, α5 = (—10.91±0.06) Mc, ql5 = (16.31±0.25) Mc, α6 = (+2.52±0.06) Mc, ql6 = (5.00±0.24) Mc. Strong dependence of quadrupole coupling on molecular vibration was observed, the values of eqQ (Re187) being —27, —35, and —58 Mc respectively in the singly-excited states ν3, ν5, and ν6. Comparative measurements of line intensities relative to those of the ground vibration state yield the following approximate values for the three lowest normal vibration frequencies: ν3 = (325±40) cm—1, ν5 = (410±25) cm—1, ν6 = (345±40) cm—1. Measurements of the Stark effect in ReO3F give as the dipole moment of this molecule a value μ = (0.85±0.05) debye.
Precise determination of molecular structure was effected from frequency measurements of lines belonging to the slightly asymmetric top ReO216O18F. The results give Re–O = (1.692±0.003) A, Re–F = (1.859±0.008) A, F–Re–O = 109° 31′±16′. Comparison is made between these and the known values of two other structurally similar molecules, ReO3Cl and MnO3F.
Quadrupole Coupling and Bond Structure in ReO3F, ReO3Cl, and MnO3F
Authors: Lotspeich J. F. 
Published: January 2005

A theoretical explanation is offered to account for the magnitude and sign of the quadrupole coupling constants in ReO3F, ReO3Cl, and MnO3F. Subject to the requirement that the valence electron bonding orbitals of Re and Mn have a tetrahedral configuration, a combination of s, p, and d atomic orbitals is chosen which tends to minimize the energies of bond formation. Results of the analysis reveal that, in order to account for the observed quadrupole couplings, the effect of atom core polarization must be considered. It is shown in particular that the contribution to the electric field gradient at the Re nucleus from the polarized electron core is more than 25 times that of the external polarizing ions themselves.
Comparison of the quadrupole coupling in ReO3F with that of atomic Re shows that the amount of unbalanced p electron along the molecular symmetry axis is nearly zero and thus provides a reasonable explanation for the pronounced dependence of eqQ(Re) on the state of molecular vibration.
Dynamical Theory of Electrosedimentation
Authors: Peterlin A. , Ribarič M. 
Published: January 2005

Besides the steady-state solution the time dependence of particle distribution in the cylindrical cell in both limiting cases, t→0 and t→∞, is given. The initial rise of electric capacity and the asymptotic approach to the steady state yield information concerning the diffusion constant of the suspended particles (weight average and number average). The dependence of the capacity change on the applied field permits, at least in principle, the determination of particle size averages of increasing order.
The experimental arrangement can be greatly improved by using suspension liquids having higher dielectric constant than the particles, e.g., polar liquids and nonpolar particles. The field then drives the particles away from the central wire into areas of lower field strength. In such a case the concentration in no part of the cell noticeably surpasses the initial concentration and it is therefore much easier to remain in the linear concentration range. A rough estimate indicates that the method may be promising for suspensions of compact particles of M∼106 where a rather simple measurement yields the average size. With true solutions of linear macromolecules the situation is much less favorable because the large extension of the coils requires so much lower concentrations.
Molecular Constants of Carbonyl Sulfide and a Coriolis Perturbation
Authors: Saksena G. D. , Wiggins T. A. , Rank D. H. 
Published: January 2005

The band origins and the changes in B and D for five bands of carbonyl sulfide have been measured. The highly precise values obtained are the result of precise infrared measurements used in conjunction with microwave results. The changes in B for the 002 and 003 bands agree with the reported value of α3 from microwave measurements. The other bands, members of resonating polyads, have anomalous values of ΔB. The ΔD values also reflect this resonance.
A Coriolis rotational perturbation in the 003 band has been observed and analyzed.
Spectroscopic Evidence for the Enhancement by Dissolved Salts of the Mole Ratio of the Gauche to the Trans Form of Ethylene Chloride
Authors: Inami Y. Harry , Ramsey J. B. 
Published: January 2005

The infrared absorption spectra of two solutions of tetra-n-butylammonium perchlorate in ethylene chloride, 0.77 and 1.23 v.f., respectively, have been determined at ∼25°C. By comparison of these spectra with the spectrum of pure ethylene chloride it is shown that the mole ratio of the gauche to the trans form of ethylene chloride in each of these salt solutions (1.69 and 1.92, respectively) is significantly greater than it is in pure ethylene chloride (1.3).
The degree of dissociation of the salt in each of the two solutions is estimated from the measured values of their equivalent conductance-viscosity products. On this basis it is shown that in each of the solutions an appreciable fraction of ethylene chloride exists near enough to an ion to be appreciably influenced by its electric field.
Molecular Partition Functions in Terms of Local Properties
Authors: Herschbach Dudley R. , Johnston Harold S. , Rapp Donald 
Published: January 2005

The classical partition function for a polyatomic molecule is obtained in a form which has, for each atom, a factor that corresponds to free translational motion of the atom in an effective volume defined by the average vibrational amplitudes of the atom and the geometrical configuration of the neighboring atoms. A method is given for calculating the appropriate volume elements for any choice of internal coordinates. The treatment includes harmonic vibrations, internal rotations, and ``reaction coordinates.'' Quantum mechanical corrections introduce additional factors which shrink the volume elements for high-frequency vibrations.
Since the principal factors in this form of the partition function are determined by local properties largely characteristic of the bonds in the vicinity of each atom, many of these factors remain practically constant and can be omitted in calculations which compare the reactants in a chemical reaction with the products or with the ``activated complex.''
Studies of the Evaporation of Condensates Containing Nitrogen Atoms
Authors: Herron John T. , Dibeler Vernon H. 
Published: January 2005

The products of an electrodeless discharge in nitrogen or mixtures of nitrogen in neon or argon were condensed at 4.2°K.
The products evolved from this condensate when it was allowed to warm up slowly were continuously analyzed by means of a mass spectrometer having a lower detection limit of 2×10—8 mm of nitrogen atoms. Experimental evidence is presented for the presence of nitrogen atoms in the warmup products. It was estimated that less than 10—3% of the nitrogen atoms originally condensed could be recovered.
Recombination and Disproportionation Reactions of Iodine Atoms and Ethyl Radicals
Authors: Guercione J. A. , Wijnen M. H. J. 
Published: January 2005

In order to obtain information regarding disproportionation and recombination reactions of ethyl radicals and iodine atoms we have investigated the photolysis of various mixtures of diethyl ketone and methyl iodide.
Ethyl radicals, produced by photolysis of diethyl ketone, react with iodine atoms, produced by photolysis of methyl iodide, according to reactions (13) and (14) C2H5+I→C2H4+HI,C2H5+I→C2H5I.The results indicate k13/k14=0.33±0.03. The influence of added gas and the effect of temperature are discussed.
Kinetic Isotope Effects in the Pyrolysis of Cyclopropane and Cyclobutane
Authors: Benson Sidney W. , Nangia Prakash S. 
Published: January 2005

An analysis of the recent kinetic isotope effects in the pyrolysis of cyclopropane and cyclopropane-d6 is shown to be in excellent agreement with the diradical path for the reaction. This is true at the high-pressure limit and possibly near the low-pressure limit for the reaction.
Using the same transition state for the isomerization with a structure about half-way between trimethylene and propylene and containing a bridged H atom, leads to a frequency factor for the high-pressure rate constant in excellent agreement with the experimental data. It is also shown, from the examples of the C2H6 and H2O2 pyrolyses that ``secondary'' isotope effects may have temperature coefficients as large as or larger than those associated with ``primary'' isotope effects.
Extending the same type of analysis to cyclobutane pyrolysis leads to an excellent a priori calculation of the frequency factor for the high-pressure rate constant and of the kinetic isotope effects in cyclobutane-d8. It is further shown that the apparently discordant kinetic isotope effects reported for[Complex chemical formula]are actually in good agreement with those for cyclo-C4D8. Finally, the use of group additivity relations in kinetics is explored and shown to lead to excellent predictions in the reactions of isotopically labeled species with different isotopic content. Their application to other types of reactions cannot be evaluated at present because of insufficient data.
Structural Interpretation of Asymmetrically Broadened NMR Fine-Structure Lines
Authors: Holcomb D. F. , Pedersen B. 
Published: January 2005

The determination of precise structural information about solids by means of NMR fine-structure measurements is investigated theoretically. Attention is directed to the interpretation of broadening effects resulting from subsidiary dipolar interactions. Consideration is limited primarily to systems in which the fine structure is that produced by a strongly interacting pair of spin ½ nuclei, of which the water molecule in hydrate crystals is the prototype. It is shown that the fine-structure lines are expected to broaden asymmetrically and that the asymmetry is a measure of the anisotropy in the pair distribution. The nature of the asymmetry can often be interpreted in terms of the relative angular position of nearby interacting pairs in the crystal. It is further proved that, within the validity of a first-order perturbation theory approach, the center of gravity of a broadened fine-structure line is the correct measure of the interaction responsible for the main splitting. This result is independent of the number or relative positions of the interacting pairs.
The breakdown of the first-order theory with increasing interpair interaction strength is discussed. Using a simple two-pair model, calculations are made which show typical interpair distances at which one may expect a significant shift of the center-of-gravity position from that determined only by the intrapair interaction. Extension of the center-of-gravity theorem to other systems in which fine structure produced by a dominant interaction is asymmetrically broadened by subsidiary nuclear magnetic dipolar interactions is discussed briefly.
Application of Space-Group Theory to the Vibrational Problem of di-Tetramethyl Ammonium Uranium Hexachloride
Authors: Pollack S. A. 
Published: January 2005

The absorption spectrum of [(CH3)4N]2UCl6 (abbr. TMA) is discussed. The spectrum is vibronic in nature, i.e., the electronic transitions between levels belonging to the 5f2 configuration of the U4+ ion are coupled with the vibrations of the TMA lattice. The number and symmetries of the normal modes of vibration of a unit cell are calculated in the factor group approximation. The Winston—Halford decomposition formula is used to reduce the representation of motions among the irreducible representations of the symmetry group of TMA.
Calculations are compared with the experimental results. The gross features of the spectrum are found to be in agreement with the theoretical predictions.
The method of obtaining the character tables for finite symmorphic space groups is presented in summary form. A general vibrational problem in crystals is discussed.
Accurate and Facile Approximation for Vibrational Energy-Level Sums
Authors: Whitten G. Z. , Rabinovitch B. S. 
Published: January 2005

Some exact sums of harmonic vibrational-energy-level densities for 45 molecules were obtained at various energies with a digital computer. An improved ``corresponding vibrational states'' type of approximation to these sums at low and moderate energies is described. The approximation has the great advantage or being very rapid, of requiring scarcely any calculational detail and of using the actual molecular frequencies. The agreement of the approximation with a smoothed true count is usually within a few percent at quite low energies, ε≳0.1 εz. It has the further advantage of being good also for the difficult case of a complex polyatomic molecule with noncommensurable frequencies.
The behavior of the sums for various vibrational patterns is considered. The importance of noncommensurability and number of frequency groups is displayed.
Interaction of Low-Energy Electrons with Water Vapor and with Other Polar Molecules
Authors: Hurst G. S. , Stockdale J. A. , O'Kelly L. B. 
Published: January 2005

Electron attachment coefficients and drift velocities have been measured in mixtures of H2O vapor with N2, CH4, C2H4, and with CO2. Electron capture takes place only in the H2O–CO2 mixture, and even in this case the attachment coefficient is quite small. Thus, low-energy (less than 1 eV) electrons do not, in general, form stable negative ions with H2O.
Electron drift velocities are very sensitive to the addition of small quantities of H2O vapor in all the mixtures. When certain other polar molecules (acetone, heavy water, methyl alcohol, dimethyl ether, hydrogen sulfide, toluene and nitrous oxide) were mixed with C2H4, it was found that the decreases in drift velocity correlated well with the magnitude of the electrical dipole moments. The drift velocity data were analyzed to obtain the cross sections for momentum transfer. These cross sections are in fair agreement with the theory of Altshuler, except for H2O, D2O, and H2S, where the experimental values are a factor of 2 larger than theory. This discrepancy may be due to molecular rotational enhancement of the momentum transfer cross section, or it may be due to the temporary formation of negative ions, an effect which would decrease the observed drift velocity.
Infrared Spectra of Methylgermane, Methyl-d3-Germane, and Methylgermane-d3
Authors: Griffiths James E. 
Published: January 2005

The infrared spectra of gaseous CH3GeH3, CD3GeH3, and CH3GeD3 were observed in the range 4000–400 cm—1. The 12 fundamental frequencies for each molecule are assigned to normal modes and the spectra are discussed in some detail. Teller—Redlich product-rule ratios support the assignments. The height of the barrier hindering internal rotation was found from measurements on twelve combination bands involving the torsional modes to have an average value of 1270 cal/mole. The torsional force constant was estimated to be 0.31×10—12 erg/rad. The corresponding barrier height and torsional force constant for methylsilane were estimated to be 1715 cal/mole and 0.44×10—12 erg/rad, respectively, from published data.
Analyses of the rotational structures of ν7–10 and of ν12 and 2ν12 in the spectrum of CH3GeH3 and of ν7–10 and ν12 in the spectrum of CD3GeH3 afforded values for the band origins, certain rotational constants and ζ7–12 for both molecules.
On the Diffusion of Ions in Membranes. II. Generalized Planck—Teorell Equations
Authors: Harris Joseph David 
Published: January 2005

The equations given by Planck and Teorell for the fluxes of univalent ions across a uniformly charged membrane are generalized from the case of parallel plane membranes considered by the original authors to the case of membranes of arbitrary shape. It is shown that a change in membrane geometry results solely in the reciprocal membrane thickness being replaced by the vacuum membrane capacitance per unit area when the fluxes are averaged over the outer membrane surface. The generalized equations are in all other respects, in particular in their dependence on the boundary concentrations and potentials, identical with the ones for parallel plane membranes.
Generalized equations for the potential and ionic concentrations within the membrane are also given. Again the functional dependence of these quantities on their boundary values is independent of the geometry.
Analysis of Molecular Weight Changes and Gel Data of Some Irradiated Vinyl Polymers
Authors: Inokuti Mitio , Dole Malcolm 
Published: January 2005

The theoretical equations of Inokuti of the preceding paper are applied to the interpretation of data obtained during the irradiation of isotactic and atactic polypropylene, and poly-(1-pentene) and poly-(1-hexene). Inokuti's modified Charlesby—Pinner function calculated for initial values of the Mw/Mn ratio greater than 2 can be made to agree with observation within experimental uncertainties in the case of polypropylene, but in the case of the other two vinyl polymers, the observed discrepancies are probably too great to be explained by experimental error. A G(S) value decreasing at high doses would account for the observed gel data.
It is shown that in the pregelation dose range irrespective of the initial molecular-weight distribution and scission to crosslinking ratio, both Pw,0/Pw and Pn,0/Pn change very nearly linearly with the dimensionless parameter y/yg where yg is the number of crosslinks per initial number-average molecule at the gel point.
Decomposition of Chemically Activated Ethyl-d3 Radicals. Primary Intramolecular Kinetic Isotope Effect in a Nonequilibrium System
Authors: Current J. H. , Rabinovitch B. S. 
Published: January 2005

The rate of decomposition of ethyl-d3 radicals formed by chemical activation was studied at 195° and 300°K over the pressure range from 0.05 to 3.0 mm. The excited radicals decomposed by hydrogen or deuterium atom rupture or were collisionally stabilized. The rate of decomposition was determined relative to the rate of stabilization by complete product analysis. Detailed description is given. The limiting high-pressure rate for hydrogen rupture from energized ethyl-d3 radicals was determined to be 6.5×107 sec—1 at 300°K and 2.5×107 sec—1 at 195°K. The limiting low-pressure rate for hydrogen rupture was found to be 2.0×107 sec—1 at 300°K and 1.0×107 sec—1 at 195°K. The intramolecular hydrogen to deuterium rupture ratio was determined to be ∼2.0 at 300°K and ∼3.0 at 195°K for this nonequilibrium reaction system, and appreciably less than similar ratios determined earlier for the ethyl-d2 system. The disproportionation to recombination ratio for ethyl radicals was found to be 0.14 at 300°K and to be 0.17 at 195°K. The apparent isotopic H/D disproportionation ratio in ethyl-d3, measured at both temperatures, was 1.6 in reasonable agreement with the value of 1.4 of Boddy and Steacie.
Theoretical rates were calculated from a quantum statistical harmonic oscillator formulation of the rate constant. Good qualitative and semiquantitative agreement has been found between the various experimental quantities of this study (as well as a previous one dealing with ethyl-d2 radicals) and magnitudes calculated from a model which has quite a loose activated complex (150 cm—1, C–C–H bends) with the figure axis rotation active, and in which the energized radical has both an active free internal rotation and figure axis rotation.
Kinetics of the Gas-Phase Reaction of CH3I with HI
Authors: Flowers Michael C. , Benson Sidney W. 
Published: January 2005

By modification of a Beckmann DB spectrophotometer for use with high-temperature gas systems it has been possible to follow the kinetics of reaction of CH3I+HI which goes quantitatively to CH4+I2 in the temperature range, 260° to 316°C. The kinetics fit the mechanism proposed earlier by Benson and O'Neal very well. I2⇋2 I,(K),I+CH3I⇋ lim 21CH3+I2,CH3+HI⇋ lim 43CH4+I.The rate law for this system is (neglecting 4) d(I2)/dt=k1k3K12(CH3I)(HI)(I2)12/[k2(I2)+k3(HI)].Although this equation can be integrated exactly, the integrated forms are not sensitive to the data, and various expansions must be used. Values for k1 and k3/k2 are found to be in excellent agreement with earlier estimates. From the activation energy E1 and the assumption that E2≤0.5 kcal, the bond energy of CH3I is found to be about 55±1 kcal/mole, in excellent agreement with other estimates. Values are given, in addition, for the individual rate constants, k2, k3 and k4, as well as their Arrhenius parameters.
The A factors for reactions 1 and 4 are shown to be in excess of collision frequencies, indicating steric factors slightly in excess of unity and thus the formation of very loose transition-state complexes. This appears to be typical of iodine-atom or I2-molecule reactions and thus far quite unique.