bluuebunny/crossref_metadata_2025_split huggingface dataset:
Analysis of Carr—Purcell Spin-Echo NMR Experiments on Multiple-Spin Systems. II. The Effect of Chemical Exchange
Authors: Allerhand Adam , Thiele Everett 
Published: January 2005

A general matrix formulation for the treatment of Carr—Purcell spin-echo (CPSE) nuclear magnetic resonance experiments on systems undergoing chemical exchange between r sites is presented. Indirect spin coupling is ignored. A simple similarity transformation is given, which makes the matrix L (a basic quantity in the theory) Hermitian, and clearly exposes its positive definite character. As a consequence, it is shown that the general solution for the complex transverse magnetization in the rotating frame is, at the nth echo, a sum of r terms of the form [A+(−1)nB]exp(−ntCPR), where tCP is the interval between 180° pulses, and R is a real positive function of tCP. A computer program calculates all these quantities. In many cases of interest, such as fast exchange, one exponential term dominates in this sum.
For fast exchange, a second-order perturbation treatment can be used to derive an explicit equation for the dominant decay time constant R1. This equation reduces to the Luz—Meiboom equation in the special case considered by those authors. The second-order treatment also gives an expression for the steady-state high-resolution linewidth. This equation reduces to the Piette—Anderson equation under the same special conditions for which the Luz—Meiboom equation holds. Detailed calculations which illustrate the use of the second-order equations are exhibited for the cases of the hydride shift in the 2-norbornyl cation and the degenerate Cope rearrangement in bullvalene.
The second-order equation takes a particularly simple form when applied to the study of a nucleus A coupled to a quadrupolar nucleus X. The result shows that one can extract both the magnitude of the coupling constant JAX and the relaxation time TX from the CPSE experiment on A, even when the high-resolution spectrum shows a single line.
Calculations on πunp Rydberg Terms of the Nitrogen Molecule
Authors: Duncan A. B. F. , Damiani Antonio 
Published: January 2005

A more thorough calculation is made of the lowest terms in the Worley—Jenkins Rydberg series in molecular nitrogen. A complete variation of the energy is made with respect to the screening parameters in the Rydberg functions for n=3, 4 and limited variation with parameters for functions with n=5, 6, 7. In every case investigated, a stable minimum in the term energy was found. Terms with n=5, 6, 7 agree with experimental terms to about 10%, and differences appear to decrease slightly with increasing n.
Some variation was made in parameters of the core function and the effect of this limited variation on the Rydberg terms is discussed. A brief discussion is made of results on computed Rydberg defects.
Effect of Excluded Volume on Phase Transitions in Biopolymers
Authors: Fisher Michael E. 
Published: January 2005

The effects of excluded volume on the existence and nature of a phase transition in an infinitely long double-stranded helical polymer are discussed in relation to the recent treatment of Poland and Scheraga, which showed that, if the entropy of a closed polymer loop of 2l links (corresponding to a diassociated length of l polymer units) varies as S2l/kB=al−clnl+b  (l→∞),then a genuine phase transition should occur if c&gt;1 but that θ, the fraction of ordered units, would be continuous at the transition unless c&gt;2. The standard Markoffian models of a polymer chain neglect the excluded volume and yield c=½d for a general d-dimensional system. Numerical data on self-avoiding lattice chains and rings are reviewed and it is concluded that c≃1.46 and 1.75 are better estimates for d=2 and 3, respectively.
The transition should thus remain continuous but it becomes sharper since it is shown that θ varies as (Tt—T)(2−c)/(c−1) when the transition temperature Tt is approached from below (and 1&lt;c&lt;2).
Viscoelasticity of Networks Consisting of Crosslinked or Entangled Macromolecules. I. Normal Modes and Mechanical Spectra
Authors: Chompff A. J. , Duiser J. A. 
Published: January 2005

A molecular theory is developed to describe quantitatively the mechanical behavior of entanglement networks of linear, randomly coiling molecules. The theory is based on the model of Rouse for a single molecule and is a generalization of the theory of Duiser and Staverman for chemically crosslinked networks.
A new model for an ``entanglement point'' is suggested, which accounts for the frictional force due to the velocity difference between two entangled molecules at that point. This leads to a modification of the diffusion equation in the Rouse theory.
It turns out that the relaxation spectrum of a network corresponds to that of an assembly of ``decoupled'' equivalent molecules in which the ends of some submolecules have a mobility which is smaller by a slip factor δ. This parameter may have values from zero (crosslinked molecules) to unity (free molecules in dilute solution).
The relaxation spectra of the decoupled equivalent molecules can be obtained by applying a property of the Sturm sequence in a computer program, without actually solving for the eigenvalues of the modified matrix in the Rouse diffusion equation.
Application of the theory to experimental data yields the number of elastically effective network chains of an entanglement network by integration of the area under the curve in the long-time region of the relaxation spectrum.
Pseudonatural Orbitals as a Basis for the Superposition of Configurations. I. He2+
Authors: Edmiston C. , Krauss M. 
Published: January 2005

The use of pseudonatural orbitals (PNO) is proposed to improve the rate of convergence in the superposition of configurations (SOC). Natural orbitals are determined for selected electron pairs in the Hartree—Fock field of the n−2 electron core and are then used as the basis for the total SOC calculation. Since these natural orbitals are not natural for the n-electron system they are considered false or pseudonatural orbitals when used in the n-electron problem.
The PNO basis has been applied to He2+ and H3 to test the convergence. Complete results are reported here only for He2+. The PNO's are quite successful in speeding up the convergence of the SOC and rendering the calculation of correlation energy quite practical in general. Gaussian-type orbitals (GTO) are used throughout and were not a serious impediment to obtaining quantitative accuracy. In fact the large number of unoccupied Hartree—Fock orbitals consequent upon the use of a GTO basis permit a straightforward determination of the PNO orbitals.
Relationship Between Pair Potentials and Distribution Functions
Authors: Strong S. L. , Kaplow Roy 
Published: January 2005

It is demonstrated that the relationship between pair potentials and distribution functions is experimentally ambiguous for dense fluids. Calculations in the Percus—Yevic approximation show that qualitatively different potentials can be chosen which predict pair distributions which are virtually indistinguishable, and vice versa.
In addition, if the nearest-neighbor interaction is predominant, it appears that the assumption of symmetric interactions may not be valid in liquids such as mercury and tin.
Spectroscopic Properties of the Lower-Lying Excited States of Fluorenone
Authors: Yoshihara Keitaro , Kearns David R. 
Published: January 2005

The spectroscopic properties of the lower-lying electronically excited states of fluorenone have been investigated. A test for n—π* transitions, based upon the effect of oxime formation on the absorption spectra, is discussed. Application of the oxime test to fluorenone, in conjunction with an examination of the emission properties, the solvent effects on the absorption spectrum, the vibrational structure of the fluorescence spectrum, the absorption spectra of related molecules, and the results of MO calculations clearly establish that the lowest excited singlet state of fluorenone is a (π, π*) state, rather than an (n, π*) state, as previously assigned.
The principal emission observed from fluorenone in solution at room temperature and at 77°K, and from crystalline fluorenone, is demonstrated to be fluorescence. Both the fluorescence spectrum and the quantum yield for fluorescence were found to be solvent and temperature dependent. At room temperature in solution, the fluorescence quantum yield is about 1% and increases to about 12% upon cooling to 77°K. The fluorescence quantum yield from fluorenone crystals is nearly temperature independent, but decreases by a factor of 14 when the crystal is melted. The relation between the quenching of the fluorescence in fluid solutions and the concommitant shift of the emission spectra is discussed in terms of the interaction between excited state fluorenone molecules and polar solvent molecules.
Very weak delayed fluorescence is observed from concentrated rigid-glass solutions and zone-refined crystals of fluorenone. In fluorenone crystals doped with 3.7×10−2 mole/mole pyrene, the delayed fluorescence is considerably stronger than in the ``pure'' crystals, and strong pyrene phosphorescence is observed.
Normal phosphorescence from fluorenone could not be observed from thoroughly purified material, but estimates of the singlet—triplet splitting, combined with the pyrene-doping experiments and solution absorption spectra obtained using heavy-atom solvents, indicate that the lowest triplet state of fluorenone is located near 560 to 570 mμ. Anomalous phosphorescence (τ=3.6 sec, λ0–0=435 mμ, Φ≃0.01) has been observed, but only upon excitation of the fluorenone to a high-lying excited singlet state (&lt;330 mμ). It is proposed that this anomalous phosphorescence originates from some new chemical species which is produced photochemically in its excited triplet state from fluorenone.
Energy Storage and Luminescence in KI:Tl at Low Temperatures
Authors: Hadley Wayne B. , Polick Stanley , Kaufman R. Gilbert , Hersh Herbert N. 
Published: January 2005

X irradiation of KI:Tl at 78°K results in storage of energy which can later be released as light by thermal or by optical means. Observation of stimulation spectra, absorption spectra, and emission spectra together with light sum and bleaching experiments indicate that the storage centers are I2− and Tl+ + trapped holes and Tl0 trapped electrons. The first thermal glow results from the mobility of I2− holes, while the second thermal glow and stimulated emission result from the release of electrons from Tl0 centers.
Three distinct emission modes correspond to the three recombination processes. Stimulated electrons returning to I2− centers produce emission bands at 302 and 370 mμ. Electrons recombine with Tl+ + holes to give a 425-mμ emission identical with that from uv excited fluorescence. I2− holes migrating to Tl0 centers produce a broader emission at 425 mμ.
This material is compared to other alkali halide phosphors and also to phosphors in general.
Photochemistry of Benzene Vapor at 1849 Å
Authors: Shindo Kotaro , Lipsky Sanford 
Published: January 2005

The benzene-disappearance quantum yield at 1849 Å and 1 torr has been determined to be 0.25±0.02 and independent of the number of absorbed quanta to ∼15% conversion. Nitrogen pressures from 0.1 to 50 atm reduce the disappearance yield. At 1 atm N2, the yield is reduced approximately fourfold. Addition of 200 torr propane reduces the yield more than one order of magnitude. At 2 and 3 torr benzene, the quantum yield is reduced to 0.13 and 0.10, respectively, and extrapolates at zero pressure to ∼1.0. Only two major products are found. One is volatile with structured absorption in the region 2600 to 2100 Å. Seventeen peaks are resolved in the spectrum with constant separation of ∼500 cm−1. Assuming that the initial yield of the product is equal to the disappearance yield of benzene, an oscillator strength of 0.2 is calculated for the band with maximum absorptivity of 8300 liters/mole·cm at 2350 Å. The compound is tentatively identified as an isomer of benzene. The second product is of high molecular weight and arises by polymerization of the volatile compound.
It is suggested that the mechanism for benzene disappearance involves internal conversion to the ground electronic state with subsequent rearrangement to produce the benzene isomer.
Electron Spin Resonance of Met-Myoglobin: Field Dependence of g⊥eff
Authors: Eisenberger P. , Pershan P. S. 
Published: January 2005

The spin Hamiltonian for high-spin Fe3+ in single crystals of met-myoglobin has the form Hs=D(Sz2−25/4)+g| |βHzSz+g⊥β(HxSx+HySy). In view of the large value of D the only ESR signal that can be observed is the transition from the ms=—½ to ms=+½ states. The resonance condition for the ground doublet is described by geffβH=hν. When the magnetic field is applied along the z axis g∥eff=g∥ and when the magnetic field is applied perpendicular to the z axis g⊥eff=3g⊥[1–2(g⊥βH)2(2D)−2]. Through measurement of g⊥eff and g∥eff at 13 and 35 Gc/sec the spin-Hamiltonian parameters were found to be g∥=2.002±0.001, g⊥=1.985±0.002, and 2D=8.76±1.2, cm−1.
These results are interpreted by replacing the pure 3d wavefunctions from which the (3d)5 electronic configuration of Fe3+ is usually formed by covalent molecular orbitals which are only part 3d. The values for g∥, g⊥, and D can be explained in terms of a single low-lying pure antibonding state of 4A2 symmetry 2000 cm−1 above the 6A1 ground state and coupled to it by the spin—orbit interaction. Numerical agreement between theory and experiment requires molecular orbits which have an average 3d character of only 68%. These highly covalent orbitals are in very good agreement with the theoretical results of Zerner and Gouterman.
Luminescence of Pyrimidines, Purines, Nucleosides, and Nucleotides at 77°K. The Effect of Ionization and Tautomerization
Authors: Longworth J. W. , Rahn R. O. , Shulman R. G. 
Published: January 2005

The luminescence of purines and their corresponding nucleosides and nucleotides has been studied at 77°K for different states of ionization and for some alterations of tautomeric forms. To a first approximation, the excitation spectra always coincided with the absorption spectra. Furthermore the emission profiles were independent of the exciting wavelength and, except for the protonated forms of guanine and its derivatives, phosphorescence always consisted of a single exponential decay. The naturally occurring pyrimidines, uracil, thymine, and cytosine, fluoresced over a wide pH range but only phosphoresced at high pH where they have lost a proton. From a study of methylated pyrimidines it was shown that the lactim forms fluoresced at 300°K and phosphoresced at neutral pH at 77°K. In these two respects they differed from the natural pyrimidines and support the commonly accepted lactam structures for the natural pyrimidines.
The purines, adenine and guanine, and their derivatives fluoresced and phosphoresced in neutral and basic solutions at 77°K. Upon protonation at pH∼4 the emission of AMP was completely quenched, adenosine had its phosphorescence considerably reduced, while the luminescence of adenine was essentially unchanged. The quantum yields, luminescence spectra, and phosphorescence decay times were measured for all the compounds.
Spectroscopy of Carbon Vapor Condensed in Rare-Gas Matrices at 4°K. III
Authors: Weltner William , McLeod Donald 
Published: January 2005

For C3, controlled warming of neon matrices containing carbon vapor caused the multiple sites for C3 molecules to disappear. The assignments of vibronic transitions in the 1IIu←X1Σg+ system could then be made more definite using the parameters of Gausset, Herzberg, Lagerqvist, and Rosen. Two unaccounted-for bands were attributed to transitions involving v3′≅840 cm−1. This low value was rationalized by comparison with other similar molecules. The 1Σu+←X1Σg+ transition of C3 in a neon matrix was not observed in the ultraviolet out to 1950 Å.
For C2, the spectrum of carbon—neon matrices, before and after warming to 12°K, indicated that the absorption bands in the region of 4000–6000 Å are probably not the C2 Swan system but may be due to a larger carbon molecule. A band at 2320 Å in neon was attributed to the (0, 0) band of the Mulliken system (1Σu+←x1Σg+ of C2. These and other observations indicate that the trapping of C2 in the excited 3IIu state in matrices is doubtful.
CNN is formed in neon matrices containing nitrogen by the reaction of C atoms in carbon vapor with N2. The proportion of C atoms in the vapor and the formation of CNN can be enhanced by vaporization of carbon from thin tungsten or tantalum cells because of the formation and vaporization of the solid carbide.
For Cn, carefully controlled diffusion of carbon molecules relatively concentrated in a neon matrix led to the appearance of many new vibrational bands. These were analyzed in terms of two extreme linear chain models. It is concluded, for example, that C4 is probably best written as ·C≡C–C≡C·, in agreement with theory.
Chemical-Shift Data for Water and Aqueous Solutions
Authors: Rüterjans Heinrich H. , Scheraga Harold A. 
Published: January 2005

Since the chemical shift δ of the proton magnetic resonance signal of water can be assumed to be approximately proportional to the degree of hydrogen bonding, xHB, the temperature dependence of the chemical shift is calculated using theoretical values of xHB vs t from various theories of water structure. These theoretical curves of δ are compared to the experimental temperature dependence of the chemical shift.
In addition, the chemical shift of water in some alcohol and carboxylic acid solutions is measured and discussed.
Influence of the Nature of Ionizing Particles on the Specific Luminescence of Organic Scintillators
Authors: Voltz R. , da Silva J. Lopes , Laustriat G. , Coche A. 
Published: January 2005

The relationship between the specific luminescence dS/dx of organic scintillators and characteristic quantities of the ionizing particle is investigated. Considering the existence of the two scintillation components and the effect of δ rays, it is shown that dS/dx depends on the specific energy loss dE/dx and on the charge number z of the incident particle: For a given differential energy loss, the ion having the greatest charge number gives rise to the highest light output; for a given particle with increasing dE/dx, the specific luminescence goes through a maximum.
The theoretical conclusions are in agreement with experimental results obtained using α particles of 1 to 8 MeV, 3He ions in the energy range 1–3 MeV and protons from 0.3 to 3 MeV. An experimental device is described, which allows a direct measurement of the energy loss ΔE of particles passing through a thin organic scintillator, and of a quantity proportional to the number ΔS of emitted photons.
Liquid Isochores and Derived Functions of n-C7F16, c-C6F11CF3, c-C4Cl2F6, n-2,2,3C4Cl3F7, CCl2F·CCl2F, and CCl4
Authors: Smith E. Brian , Hildebrand Joel H. 
Published: January 2005

Values of the function, (∂P/∂T)v have been determined for six liquids through ranges of 1–60 atmos and initial temperature ranges as high as 25°. The values at 1 atmos and 25°C in atmos deg—1 and the compressibilities×104 calculated from them are, respectively, C7F16 7.10, 2.34; c—C6F11CF3 7.53, 2.18; c—C4Cl2F6 8.61, 1.76; n—C4Cl3F7 8.97, 1.41; CCl2F·CClF2 8.67, 1.50; CCl4 11.22, 1.23.
Since (∂2P/∂T2)v = 0 in the range of the observations, (∂E/∂V)T is a pure volume function, but v2(∂E/∂V), (v is molal volume) is not strictly constant, as it would be for a van der Waals liquid, but the drift is substantially reduced, and the relation between ∂E/∂V and ΔEvap, the energy of vaporization per mole, are both consistent with the relations: vn+1(∂E/∂V)=nvnΔEvap=const with values of n having the following values, in the order of liquids given above: 1.49, 1.44, 1.35, 1.36, 1.19, 1.09.
These relations are explained qualitatively in terms of the expression derived by Hildebrand and Wood for the potential energy of a liquid in terms of its distribution function and intermolecular pair potentials.
Proton Magnetic Resonance of the CH3 Group. V. Temperature Dependence of T1 in Several Molecular Crystals
Authors: Stejskal E. O. , Woessner D. E. , Farrar T. C. , Gutowsky H. S. 
Published: January 2005

The proton T1 has been observed by rf pulse techniques in acetonitrile, neopentane, t-butyl chloride, 2,2-dichloropropane, and methyl chloroform from their melting points to temperatures about —150°C. Activation energies of 5 to 6 kcal/mole were obtained for self diffusion in the high-temperature solid phase of neopentane, t-butyl chloride, and 2,2-dichloropropane. Activation energies ranging from 1 to 4.5 kcal/mole were obtained for molecular tumbling and from 3 to 4 kcal/mole for CH3 group reorientation in the series of tetrasubstituted methanes, neopentane to methyl chloroform. The effects of intermolecular forces on CH3 group reorientation are shown by the activation energy of 2.1 kcal/mole found for acetonitrile, which has no internal barrier. The various results confirm that a tunneling mechanism is plausible for the CH3 group reorientations.
Other results include: the Bloch decay after a 90° pulse in solid methyl chloroform has a ``slow beat'' which is the Fourier transform of the triplet line shape found in steady-state experiments. The dependence of T1 upon resonance frequency in the low-temperature phase of methyl chloroform exhibits systematic effects attributable to proton-chlorine spin exchange induced by reorientations of the CCl3 group. The T1 is constant in the super-cooled high-temperature solid phase of methyl chloroform. There is an anomalous increase in the T1 at the low-temperature transition in t-butyl chloride, indicating a higher molecular mobility during the phase transition than in either stable solid phase. A 2:1 mixture of CH2DCCl3 and CH3CCl3 gave proton T1 values only 1.3 to 1.6 times as long as those in normal CH3CCl3, whereas the tunneling mechanism suggests a 20-fold increase. Spin exchange between protons in neighboring CH2DCCl3 and CH3CCl3 molecules accounts for the results and it is proposed that spin exchange may be investigated directly by observing T1 in appropriate isotopic mixtures of known composition.
Microwave Spectrum and Structure of Propynal (H–C≡C–CHO)
Authors: Costain C. C. , Morton J. R. 
Published: January 2005

The moments of inertia IB0 and IC0 of fifteen isotopic species of propynal have been determined from the frequencies of certain lines in the type a spectra. With the aid of type b transitions, IA0 and the inertial defects Δ have been obtained for some of the isotopic species. Using equations relating the atomic coordinates to the change in the moments of inertia on isotopic substitution, three independent sets of molecular parameters have been obtained (two for C=O). The agreement between the three sets of data is within the probable experimental error. The mean values of the molecular parameters are as follows: rs(C–H) (acet.) = 1.0553A, rs(C–H) (ald.) = 1.1064A, rs(C≡C) = 1.2089A, rs(C–C) = 1.4446A, rs(C=O) = 1.2150A, ≰CCO = 123°47′, ≰CCC = 178°24′, ≰CCH (ald.) = 113°54′, ≰CCH (acet.) = 180°0′.
The error in the bond lengths is estimated to be ±0.001 A, and in the angles ±10′. A slight deviation from linearity (1°36′±10′) was apparent in the carbon chain.