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Volume 62, Issue 7, 01 April 1975

Mean configurational relaxation rates in finite length polypeptides
View Description Hide DescriptionExpressions for the mean relaxation rate <r≳ and the time correlation function <ϑ (0) ϑ (t) ≳ of the helix fraction are derived for polypeptides in the helix–random coil transition region. The resulting equations are based upon a general description of the time rate of change of the configurational probabilities in the form of a set of coupled differential equations. Quantities are expressed in terms of the Zimm–Bragg equilibrium statistics and the neighbor dependent transition rates for an individual peptide unit. Both <r≳ and <ϑ (0) ϑ (t) ≳ exhibit strong chain length dependent behavior throughout the transition region. The quantities 1/<r≳ show maxima at some ϑ?0.5 within the range of the transition. As chain length increases, these curves sharpen and their maxima shift toward ϑ = 0.5, asymptotically. Results for <r≳ are identical to those calculated by an alternative method which follows from a theory developed by Schwarz. Effects of finite chain length can persist to greater than 1000 peptide units.

Electric field deflection of molecules with large amplitude motions
View Description Hide DescriptionThe Stark effect is derived for several plausible models for molecules such as XeF_{6} and IF_{7} which are believed to show pseudorotation or other large‐amplitude internal motion. These results are related to the behavior of these molecules in electric‐field‐deflection molecular beam experiments. The absence of focusing of XeF_{6} is compatible with an effective dipole moment of distorted XeF_{6} of up to 1.2 D for free pseudorotation but only 0.2 D if pseudorotation is restricted by a potential barrier with C _{4v } orientation 770 cm^{−1} above C _{3v }. Additional examples are treated for XeF_{6} as well as models appropriate to the MX_{7} molecules.

Liquid–vapor asymmetries in pure fluids
View Description Hide DescriptionThe coexistence curve and the compressibility‐like quantity (∂ρ/∂μ)_{ T } at the coexistence curve are calculated for Mermin’s decorated lattice gas. We tabulate the Padé approximants to the Ising model properties which were used in these calculations. It is shown that for the proper choice of the adjustable parameter in the model the asymmetries in the coexistence curve and in (∂ρ/∂μ)_{ T } over a large temperature range, T _{ c }/2 < T < T _{ c }, are qualitatively similar to those in real fluids. Also, similar correlations between the overall coexistence curve asymmetry and the amplitudes of divergences for the critical region occur in the model and in real fluids. Based on our model calculations, several predictions for fluid behavior near the critical point are listed: (1) The a s y m p t o t i c asymmetry in the coexistence curve and in (∂ρ/∂μ)_{ T } at the coexistence curve should be most observable for fluids with large overall asymmetry such as SF_{6}, NH_{3}, and H_{2}O; (2) the range of the asymptotic asymmetry for (∂ρ/∂μ)_{ T } is expected to extend somewhat beyond t = 1 − (T/T _{ c }) = 0.1 for such fluids; (3) in order to obtain an estimate for the compressibility exponent γ′ that is within a few percent of the true asymptotic value, experiments will have to be confined to the range t < 10^{−2}. Recent experimental data for (∂ρ/∂μ)_{ T } at the coexistence curve are found to be consistent with the model for t ≳ 10^{−2}, but are inconclusive closer to the critical point.

Radiative lifetimes in molecules studied by means of fast, molecular ion beams
View Description Hide DescriptionRadiative lifetimes of excited levels in molecules have been studied by a spectroscopic investigation of the collision processes occurring between fast, molecular ion beams and helium atoms. This technique has been used to measure the lifetimes of the A ^{2}Δ state in CH, the B ^{2}Σ^{+} state in CO^{+}, and the B ^{2}Σ^{+} states in CO_{2} ^{+} with an accuracy of 10%–20%. A comparison with previously published results and a discussion on the perspectives of the method have been carried out.

Chemical accelerator studies of reaction dynamics: Ar^{+} + CH_{4} → ArH^{+} + CH_{3}
View Description Hide DescriptionChemical accelerator studies on isotopic variants of the reaction Ar^{+} + CH_{4} → ArH^{+} + CH_{3} are reported. Velocity and angular distributions of the ionic product as a function of initial translational energy have been measured over the energy range 0.39–25 eV centerofmass (c.m.). The asymmetry of the product distribution with respect to the center of mass indicates that the reaction is predominantly direct over the energy range studied. The dynamics of the reaction are approximated by the spectator stripping model: The reaction exothermicity appears as product internal energy and product excitation increases with collision energy at the rate predicted by this model. The internal degrees of freedom of the neutral product have little effect on reactiondynamics, and product excitation appears to reside principally in the ionic product. Deviations from the spectator stripping model suggest the existence of a basin in the potential energy hypersurface for this reaction; the ArCH_{4} ^{+} complex which may be formed at low collision energies, however, preferentially decomposes via reaction channels other than that resulting in ArH^{+} formation.

Chemiluminescence and photoluminescence of diatomic iron oxide
View Description Hide DescriptionBand emission from the Infrared, Orange, and Blue systems of FeO is observed in chemiluminescent flames resulting from the reaction of Fe with O_{3}, N_{2}O, NO_{2}, and discharged O_{2}. Flames are produced at a relatively low temperature (700 K) in flowing inert gas at pressures of 10^{−3} atm. Bands not previously reported are observed and ascribed to FeO. Reaction of Fe with O_{2} produces continuumlike emission of unidentified origin; however, laser photoluminescence studies confirm ground state FeO is present. Photon yields for the Orange systems of FeO formed in reactions with O_{3} and N_{2}O are found to be on the order of 2% and 0.2%, respectively. Photoluminescence of FeO is excited with a krypton ion laser and a cw, tunable dye laser.Ground state vibrational constants of ω^{″} _{ e } = 875.8 cm^{−1} and ω^{″} _{ e } x ^{″} _{ e } = 4.6 cm^{−1} are determined from the laser excited photoluminescence spectra. Using the tunable dye laser, a rotational constant for the upper state of the B 0,0 band (λ = 579 nm) is found to be B′_{0} = 0.497 cm^{−1}. Radiative lifetimes of the Orange system upper electronic levels are measured using a pulsed, tunable dye laser excitation source. Lifetimes are found to be on the order of 450±100 nsec.

A study of the magnetic state of O_{2} ^{+} in fluoride salts
View Description Hide DescriptionStatic magnetic susceptibilitymeasurements for 4°K < T < 300°K are reported for the following dioxygenyl salts: O_{2}AsF_{6}, O_{2}SbF_{6}, O_{2}Sb_{2}F_{11}, and O_{2}PtF_{6}. In contrast to previous studies, we find b o t h spin–orbit coupling and crystal field effects to be important. While the As and Sb compounds are paramagnetic at 4.2°K and have small O_{2} ^{+}–O_{2} ^{+} exchange, O_{2}PtF_{6} is ordered at 4.2°K, probably due to O_{2} ^{+}–Pt(V) exchange. ESR studies confirm the susceptibility results, except in O_{2}PtF_{6} where a resonance was not detected, and show that the O_{2} ^{+} magnetic state is described by an axial spin Hamiltonian with g _{⊥} ∼ 1.98 and g _{∥} ∼ 1.74. The ESRlinewidth of O_{2}AsF_{6} below 70°K is ∼ 80 G. Above 100°K the linewidth rapidly increases so that above 180°K the line is unobservable. This increase may be due to O_{2} ^{+} motion. NMR shows that the AsF_{6} groups in O_{2}AsF_{6} are also rotating above 200°K, probably in some hindered form.

^{19}F relaxation in paramagnetic O_{2}AsF_{6}
View Description Hide DescriptionThe properties of the fluorine spins of O_{2}AsF_{6} are dominated by the coupling with dioxygenyl unpaired electrons. Study of the longitudinal relaxation shows that a phase transition takes place at 255±3°K. Motions of the dioxygenyl cations are shown to take place and are discussed.

Direct measurement of the radiative lifetimes of the A ^{2}Σ^{+} (V′ = 0) states of OH and OD
View Description Hide DescriptionThe fluorescent decay of the A ^{2}Σ^{+} (V′ = 0) state of OH and OD has been measured for N′ ? 8 for OH and N′ ? 9 for OD using pulsed dye laser excitation and at total gas pressures ? 1 mtorr. For the rotationless molecule, a radiative lifetime of 0.688 ± 0.021 μsec for OH and 0.692 ±0.021 μsec for OD was obtained, where the error bars are three standard deviations. In addition, OH exhibited a 8 ± 2% lengthening of the radiative lifetime between N′ = 1 and N′ = 8.

Molecular Rydberg transitions. I. Low‐energy Rydberg transitions in methyl halides
View Description Hide DescriptionThe energies of the rare gas transitions p ^{6}; ^{1} S → (p ^{5}; ^{2} P _{ J }) n l; Γ, where p is the highest occupied p AO of the ground configuration and n l is the nth virtual l AO, exhibit a significant linear correlation, for fixed n and l, with the ionization energies p ^{6}; ^{1} S → p ^{5}; ^{2} P _{ J }. Similar correlations exist for Rydberg transitions in CH_{3}X molecules, X = Cl, Br, I. The assignment capability of the combined interatomic and intermolecular correlations is equivalent, at least in the case of the methyl halides, to that of the Rydberg equation for n ? 5.

Equilibrium constants of hydrogen–deuterium–tritium self‐exchange reactions in water vapor as studied with a pulsed molecular‐beam quadrupole mass filter
View Description Hide DescriptionWe measured the equilibrium constantsK _{HDO} = P ^{2} _{HDO}/P _{H} _{2} _{O}⋅P _{D} _{2} _{O}, K _{HTO} = P ^{2} _{HTO}/P _{H} _{2} _{O}⋅P _{T} _{2} _{O}, and K _{DTO} = P ^{2} _{DTO}/P _{D} _{2} _{O}⋅P _{T} _{2} _{O} at 295.6°K (22.4°C), assuming the mass spectrometric sensitivities of the three species cancel in each expression of the equilibrium constant. The results obtained were K _{HDO} = 3.82±0.05, K _{HTO} = 3.66±0.06, and K _{DTO} = 3.99±0.07. These values agreed with the current theoretical values of K _{HDO} = 3.84, K _{HTO} = 3.69, and K _{DTO} = 3.97 at the same temperature. We discuss the discrepancy of the present value of K _{HDO} as compared with previous experimental determinations, and we conclude that a direct measurement of the mass spectrometric sensitivity ratio for K _{HDO} is still necessary to resolve the disagreement. We prepared the water mixtures that contained tritium by reacting the appropriate hydrogenic gases with oxygen over a platinum sponge catalyst.

Electronic spectroscopy of propadiene (allene) by electron impact
View Description Hide DescriptionThe electron impact excitation of propadiene (allene) has been studied experimentally at impact energies of 20, 40, and 60 eV and scattering angles from 6° to 80°. Two transitions with maxima at 4.28 and 4.89 eV are identified as singlet → triplet excitations. The magnitude of the splitting between these transitions is a measure of the interaction between the two perpendicular π molecular orbitals. The significance of these triplet excited states in the interpretation of previous electronic energy transfer experiments and sensitized photochemical studies is briefly discussed. Two very weak transitions are observed between 5.0 and 6.5 eV. The angular dependence of the corresponding cross sections indicates that these are spin‐allowed but symmetry‐forbidden transitions. A weak singlet → singlet transition is seen with a maximum at 6.74 eV. This feature has also been optically observed and attributed to an electric dipole‐allowed ? ^{1} A _{1}→1 ^{1} E transition. This assignment is discussed in the light of the present results. The first strong transition, the ? ^{1} A _{1}→1 ^{1} B _{2} π→π* transition, appears with a maximum at 7.24 eV. Higher energy‐loss features between 7.95 and 10 eV probably involve excitations to Rydberg states. A previously unreported transition to a superexcited state is observed with a maximum at 11.25 eV.

Molecular complexes of I_{2}: A Mössbauer effect study
View Description Hide DescriptionMolecular complexes of I_{2} with different organic solvents, in frozen solution, were studied using Mössbauer effect in ^{129}I. The results obtained for the hyperfine constants indicate that the complexes may be divided into three main groups with increasing bond strength, according to the organic solvent used: (a) π solvents, (b) alcohols and ether, and (c) amines. The same classification was also used in the analyasis of various properties of the I_{2} complexes. Some general conclusions on the electronic nature of the I_{2}‐complexant bond are derived and discussed.

Collisions between systems of Coulomb particles. I. Small‐angle scattering for time‐dependent fields
View Description Hide DescriptionCollisions between stationary systems of charged particles, i.e., systems in a dynamic equilibrium, are investigated in a so‐called small‐angle approximation. The interaction of unperturbed systems, described in terms of time‐varying electric multipoles (with the dependance on time given by Fourier analysis) is discussed in detail. It is found that the exponential cutoff for the interaction is a characteristic feature of systems with fields periodically varying in time. Relations important for development of the low energy atomic collisiontheory are derived.

Classical theory of atomic collisions. II. Low energy scattering
View Description Hide DescriptionIt is shown that large number of low energy scattering phenomena can be consistently explained if time‐varying electric multipoles are ascribed to the atomic shell. Scattering experiments allow for estimation of the atomic multipoles and their absolute values and frequencies. It is discovered that atoms of the same chemical properties have multipoles of the same order and only their absolute values and their frequencies are different. From the whole analysis the conclusion is drawn that unperturbed atomic field must be

Low energy scattering and the ’’free‐fall’’ atomic model
View Description Hide DescriptionThe free‐fall atomic model has been applied for quantitative analysis of atomic and molecular collision processes. The model yields results which are in agreement with the measurements. It describes properly the low energy scattering of electrons (with respect to the energy dependence, as well as to the absolute cross section values) and van der Waals interaction and gives some insight into the nature of chemical bonds. The dynamic dipole was found to be a characteristic feature of π bonds.

Multicentric expansions for isosceles H^{++} _{3} configurations
View Description Hide DescriptionThree, four, and ten center expansions, using Gaussian orbitals have been used to study the isosceles energy maps of the H^{++} _{3} ion. Results confirm the inexistence of a bound state, and are compared with those of other authors.

ESR studies of [RhCl_{6}]^{4−} in various matrices
View Description Hide DescriptionAn ESR study of the [RhCl_{6}]^{4−} center in single crystals of AgCl has been made. The g values and chlorine hyperfineinteractions observed for the axial chlorines are consistent with a d ^{7} configuration and localization of the odd electron largely in the 4d _{ z } _{ 2 } orbital. The same species has been studied in γ‐irradiated single crystals of NaCl doped with [RhCl_{6}]^{3−} and the ESR parameters found were quite similar. PolycrystallineNa_{3}RhCl_{6}⋅12H_{2}O, after γ irradiation at room temperature, appears to give the same species, although only g _{⊥} can be obtained from the spectrum. Molecular orbital theory has been used to interpret the chlorine hyperfineinteractions in terms of bonding parameters for the complex. The delocalization found is smaller than that for [RhBr_{6}]^{4−}.

Effects of collisions on the magnetic quenching of positronium
View Description Hide DescriptionA theory describing the behavior of positronium (Ps) in a magnetic field is presented including the effects of Ps collisions. Such effects are especially important if the Ps is formed in a medium containing unpaired electrons since Ps collisions with these electrons can provide an effective spin exchange mechanism. By integrating the equation of motion for the density matrix, it is shown that the dependence of any annihilation yield N on the static magnetic fieldx is of the form N (x) = (g + h x + k x ^{2}) / (l + m x ^{2}) for a wide class of quenching mechanisms with time independent reaction rates. The coefficients g, h, k, l, and m are evaluated explicitly for the 2γ and 3γ yields from Ps formed from polarized positrons assuming that the quenching mechanisms can be characterized by an effective spin exchange rate and an effective pickoff rate. The yields are also evaluated assuming zero positron polarization in formulas which include the interesting Ps chemical reaction rates directly. The formulas derived can be used to obtain information about such chemical reaction rates from magnetic quenching data. Diagrams which permit the quick estimation of magnetic quenching effects as a function of the quenching rates are presented. The formalism has also been extended to include an rf magnetic field and is used to compute the shift of the measured Ps hyperfine structure interval as a function of quenching and microwave power. The shifts are found to be extremely small as expected.

Thermomagnetic force at low pressures
View Description Hide DescriptionThe longitudinal component of the thermomagnetic force on a disk with surface perpendicular to the heat flux between parallel plates has been measured in N_{2} and HD at pressures near the observed maximum in the thermal and thermomagnetic forces. A comprehensive study of the thermomagnetic force in N_{2} was conducted as a function of pressure, magnetic field strength, disk size, disk thermal conductivity, and applied thermal gradient. The data on N_{2} and HD are in agreement with a recent theory derived for the Waldmann limit. In addition, a preliminary measurement of the thermomagnetic force on a sphere is reported.