Volume 60, Issue 8, 15 April 1974

Rydberg states of polyatomic molecules using model potentials
View Description Hide DescriptionWe have used a simple model potential to calculate the Rydberg series in carbon dioxide, nitrogen dioxide, nitrous oxide, acetylene, propyne, formaldehyde, allene, butadiene, and butene. The model potential for a molecule is taken as a sum of atomic model potentials. The results agree well with experiment and allow us to assign and interpret almost all the Rydberg series in the molecules studied. In addition to various applications we can identify several Rydberg series which are perturbed by interloping valence transitions, e.g., in H_{2}CO and CO_{2}.

Mass spectrometry utilizing collisional ionization of cesium: Maleic anhydride and succinic anhydride
View Description Hide DescriptionThe translational energy dependence of the relative cross section for production of negative ions by collisions of fast cesium atoms with maleic anhydride (C_{4}H_{2}O_{3}) and succinic anhydride (C_{4}H_{4}O_{3}) has been studied from threshold up to approximately 20 eV (c.m.). Accurate measurements of the threshold energy for the creation of Cs^{+} and from collisions of cesium atoms with maleic anhydride yield the electron affinity of maleic anhydride to be 1.4±0.2 eV. A number of fragment negative ions are observed from both compounds and the results are compared with our recently reported electron impact studies. ions are observed to be unstable with respect to autodetachment, and the autodetachment lifetimes are measured to be 62±10 and 71±10 μsec for from maleic and succinic anhydride, respectively. In both cases the lifetime of is independent of the collision energy from threshold up to 15 eV.

Classical dynamics: The study of vibrational and rotational excitation in Li^{+}, H_{2}
View Description Hide DescriptionA classical study of translational, rotational, and vibrational energy exchange in the systen Li^{+}, H_{2} is presented. The potential surface used was the ab initio Lester surface appropriate for rotational and vibrational energy transfer studies. The effect of initial energy configuration and orientation variables on the energy transfer processes was studied. A central force calculation of the deflection function was made for comparison with that obtained from an anisotropic potential. The primary mode of energy transfer to the internal degrees of freedom is from translational energy.

Diffusion and field‐gradient effects in NMR Fourier spectroscopy
View Description Hide DescriptionIn NMRFourier transform spectroscopy, rf pulses are periodically applied to a spin system to generate a sequence of free induction decays which form the basis of spectroscopic measurements. In inhomogeneous magnetic fields, distortions of the free induction decay arise from superimposed spin‐echo effects. They are strongly dependent on molecular diffusion which is also responsible for the nonexponential envelope of the echo train observed in a stopped pulse experiment. These effects are investigated by means of two independent methods, the Fourier expansion method and the partition method. The results are discussed and used to explain measurements made on a one‐spin system. The influence of echoes on the sensitivity of Fourier transform spectroscopy is considered, and it is concluded that the achievable gain does not normally outweigh the complications arising from retention or enhancement of echoes. Experimental parameters are derived for efficient suppression of echoes by means of a periodically pulsed magnetic field gradient.

Infrared spectroscopic evidence for matrix isolated ``nitrosyl hypofluorite,'' an isomer of nitryl fluoride
View Description Hide DescriptionThe in situphotolysis of molecular fluorine and NO_{2} molecules in nitrogen matrices at 8°K yields the FN‐bonded nitryl fluoride, FNO_{2}, as the primary reaction product. However, controlled fluorine atom diffusion at 20°K, in the absence of photolytic radiation, results in the formation of the highly reactive, FO‐bonded, nitrosyl hypofluorite, ONOF. Infrared absorptions at 1716.4, 1199.9, 702.3, and 411.9 cm^{−1} were assigned to this species, which was shown from ^{15}N and ^{18}O isotopic studies to contain two nonequivalent oxygen atoms. The ONOF molecule undergoes intramolecular photorearrangement at wavelengths below 400 nm to produce the more stable FNO_{2} form.

Chemically induced dynamic electron polarization. V. A possible theory for alkyl radicals produced in radiolyses
View Description Hide DescriptionThis work describes the spontaneous emission and enhanced absorption pattern of the initial spin polarization of alkyl radicals, XH_{2} and XH_{3}, produced in radiolyses. It is assumed that the various total spin multiplets (electron and nuclear) have different probabilities of formation; this is an extension of our earlier work on the H and D atoms. The mode‐spontaneous emission or enhanced absorption‐for a given ESR line depends on two factors: first, which total spin multiplet is the more probable and second the sign of the isotropic hyperfine splitting constant.

Study of the theoretical dipole moment function and infrared transition matrix for the X ^{1}Σ^{+} state of the HF molecule
View Description Hide DescriptionThe dipole moments as a function of the internuclear distance are obtained from the Hartree‐Fock‐with‐proper‐dissociation (H–F PD) wavefunctions for the ground state of the HF molecule from R = 1.25 to 10.0 bohr. The computed vibrationally averaged dipole moment for the v = 0 level is 1.828 D (H^{+}F^{−}), to be compared with the experimental value of 1.827 D. The first and second derivatives of the dipole momentfunction are computed to be 0.692 D/bohr and −0.680 D/bohr^{2}, respectively, comparing with the experimentally deduced values of 0.805 D/bohr and −0.076 D/bohr^{2}. The infrared transition matrix is reported and compared with the available experimental data. The line strengths for the P and R branches are given for the 0–1, 0–2, 0–3, and 1–2 infrared bands.

Temperature dependent ^{13}C NMR and ir studies in Fe(CO)_{5−x }(PF_{3})_{ x }, X = 0, 1, 2, 3, and 4
View Description Hide DescriptionThe ^{13}C NMRspectra for members of the series Fe(CO)_{5−x }(PF_{3})_{ x }, where x = 0, 1, 2, 3, and 4 were studied for temperatures between + 25° and − 100 °C and carbon‐phosphorus and carbon‐fluorine spin coupling constants evaluated. The infrared spectra were also studied within the same temperature limits in the CO‐stretching region and mole fractions for the different isomers within each composition x were evaluated at each temperature. The magnitude of the observed J _{CP} spin‐coupling constants can be accounted for by using the experimentally determined mole fractions and the trans, cis, and vicinal coupling constant increments. The magnitude of these three coupling constant increments were determined by using a set of observed coupling constants and the mole fractions. By using the mole fractions together with the time‐averaged ^{13}C chemical shifts, the chemical shift difference δσ^{0} between the axial and equatorial sites is found to be 17.7±1.5 ppm. Using this value and relaxation rates determined in earlier work, the frequency of exchange between axial and equatorial sites is calculated to be 1.1×10^{10} sec^{−1} in the five‐coordinate nonrigid Fe(CO)_{5} molecule. This is the first time such a surprisingly fast flip frequency has been measured for such a large molecule.

Study of the electronic structure of molecules XXII. Additional ab initio computations for the barrier to internal rotation in polynucleotide chains
View Description Hide DescriptionIn a previous paper of this series (paper XIX) we reported an ab initio computation for the barrier to the internal rotation in the sugar‐phosphate‐sugar complex, C_{10}H_{19}O_{8}P; the barrier corresponds to the configuration with ω″ = 45°, ω′ = Ψ′ = Ψ″ = φ′ = 0°, and φ″ varies from 0° to 360°. Here we repeat the same study with the C_{10}H_{18}O_{8}P^{−} complex (where one proton of the phosphate group has been removed). The new potential curve shows essentially a parallel displacement in total energy relative to the previously computed one. In addition we report two new computations of the barriers obtained by varying φ′ from 0° to 360°, while keeping ω″ = 45° and ω′ = Ψ′ = Ψ″ = φ″ = 0° (using Olson and Flory's notation). Again the Hartree‐Fock model predicts similar features for the rotational barriers of the two complexes C_{10}H_{19}O_{8}P and C_{10}H_{18}O_{8}P^{−}. The calculated barrier heights are 4.0, 3.2, and 7.0 kcal/mole for the φ″ variation in the C_{10}H_{19}O_{8}P complex, and 5.7, 3.2, and 8.2 kcal/mole in the C_{10}H_{18}O_{8}P^{−} complex. The barrier heights for the φ′ variation are 18.8 and 30.3 kcal/ mole for C_{10}H_{19}O_{8}P, and 34.8 and 35.2 kcal/mole for C_{10}H_{18}O_{8}P^{−}. Finally, the total energy for the internal rotation of φ″ is decomposed into a sum of contributions, representing either the electronic rearrangement following the rotation for a number of fragments composing the C_{10}H_{18}O_{8}P^{−} complex, or the pairwise interaction of such fragments during the rotation. The location of the potential minima and maxima is predicted by the pairwise interaction of the fragments. The energy for the rearrangement within each fragment is large.

Preparation and thermal, structural, and photoelectric properties of glassy and crystalline Te_{2.0}Br_{0.75}I_{0.25}
View Description Hide DescriptionIt has been found that mixed crystals of the general composition Te_{2.0}Br_{1−x }I_{ x }, where x ≤0.75 form glasses when cooled from the melt. We present the results of the thermal, structural, and photoemission experiments on Te_{2.0}Br_{0.75}I_{0.25}, which is representative of the materials of the general composition above. The radial distribution function of the glassy form determined from x‐ray diffraction indicates that the short range order is similar to that of the crystalline phase. Differences in the density of valence states of elemental Te and Te_{2.0}Br_{0.75}I_{0.25} as determined with uv and x‐ray photoemission show that the electronic configuration characteristic of twofold coordinated Te is absent in the mixed crystals. The sharp core level spectra observed in the mixed crystal demonstrates that each type of element is in a similar Coulombic environment.

Phase separation in monolayers of adsorbed ions
View Description Hide DescriptionNumerical calculations are performed on a dilute electrolytesolution in equilibrium with a charged surface, using the theory developed by Buff and Stillinger. The concentrations of adsorbed cations in the double layer (in the case of a negatively charged surface) are computed as a function of the electrostatic potential in the adsorption zone. It is found that a critical value of the potential exists, such that, for values of the potential more negative than the critical value, a phase transition in the double layer occurs to yield an almost close‐packed layer of counterions. The results are discussed in the light of similar effects found in polyelectrolyte theory as well as of experimental results from nuclear relaxation studies. It is demonstrated that in the case of a mixed electrolytesolution (two monovalent cations and a common monovalent anion) the adsorbing surface will selectively adsorb the smallest cation below the threshold potential, even though the larger ion may predominate in the bulk phase. This effect is due largely to the short range (hard sphere) interactions.

Proton magnetic resonance absorption and relaxation of dimethylamine in the condensed and clathrate deuterate phases
View Description Hide DescriptionProtonmagnetic resonance second moment and spin‐lattice relaxation data are reported for polycrystalline solid dimethylamine and dimethylamine clathrate deuterate from 77 K up to the melting points of the respective compounds. The second moment results for solid dimethylamine indicate a hindered rotation of the methyl groups around their threefold axes, while the protonT _{1} data show a broad distribution of reorientational correlation times among the methyl groups. A quantitative interpretation of this distribution based on the Davidson‐Cole distribution function is given, and a reorientational energy barrier of 2.70±0.20 kcal mole^{−1} is obtained from the relaxation data using a Davidson‐Cole distribution parameter δ=0.8. In the dimethylamine clathrate deuterate, a pseudothreefold axis (c‐axis) reorientation of dimethylamine is observed at higher temperatures. Proton spin‐lattice relaxation in this case is nonexponential over most of the temperature range studied, and an analysis of the ``null'' time (t _{0}) data in terms of the Hilt‐Hubbard theory leads to an activation energy barrier of 2.26 ± 0.02 kcal mole^{−1} for reorientation around the c axis. Measurements of the rotating frame spin‐lattice relaxation (T _{1ρ}) in the temperature region above the t _{0} minimum for the clathrate disclose the onset of slow tumbling and lattice diffusion processes, governed by an activation energy of 14.0 ±0.6 kcal mole^{−1}.

Semiclassical treatment of bound state systems. II. Trajectory calculations using the exact quantum momentum
View Description Hide DescriptionEquations of motion based upon the exact solution of the Milne equation are used to perform trajectory calculations. Using the trajectory information we calculate charge densities for N fermions in a harmonic oscillator potential in a semiclassical approximation. The charge densities so obtained are in almost exact agreement with exact calculation within the outermost density maxima. Outside this region the differences between the approximate and exact densities are essentially independent of the number of fermions in the system.

Model of pre‐existing traps for electrons in polar glasses
View Description Hide DescriptionA model of pre‐existing traps for electrons in polar glasses is presented. It is assumed that the condensed medium consists of a large number of cells and that each cell is composed of a certain number of molecular dipoles. Two specific cases are investigated. In one case each cell consists of four tetrahedrally located dipoles. In the other it consists of six hexahedrally located dipoles. However, it is assumed that the orientation of the dipoles is totally random. In both cases it is found that the electron is bound inside the cell if the sum of the radial components of the dipole moments in the cell is greater than 3.78 D. The energy of the bound state is calculated as a function of the sum of the radial components of the dipole moments. The relative number of cells with the sum of the radial components of the dipole moments lying between q and q + d q is calculated as a function of q. From these two quantities the relative number of pre‐existing traps with the binding energy of the electron lying between E and E + d E is calculated as a function of E.

Collisions of F(^{2} P _{1/2}) with H_{2}
View Description Hide DescriptionThe role of spin‐orbit and nonadiabaticeffects in the reaction of F atoms with H_{2} is discussed, Ground and excited FH_{2}potential energy surfaces and the required interactions between them are computed by the diatomics‐in‐molecules method. Using an approximate semiclassical procedure, thermal rate constants for reaction and quenching of F(^{2} P _{1/2}) are estimated to be 1.2 and 8.4×10^{−12} cm^{3} mol^{−1} · sec^{−1}, respectively. Since the former is about an order of magnitude smaller than previously reported calculations of the rate constant for reaction of F(^{2} P _{3/2}) with H_{2}, these results predict a substantial difference in reactivity between the ^{2} P _{3/2} and ^{2} P _{1/2} fine structure states of atomic fluorine.

Rotational excitation in the small‐angle scattering of protons from diatomic molecules
View Description Hide DescriptionData for the small‐angle scattering of protons from several polar and nonpolar molecules in the energy range 10 to 30 eV indicate that the presence of a dipole moment in the neutral molecule greatly enhances the rotational excitation process in ion‐molecule collisions. Within our experimental energy resolution, no rotational excitation was observable for proton collisions with H_{2}, HD, D_{2}, or N_{2}. There is, however, some evidence for a small amount of rotational excitation of the dipolar molecule CO, and for HCl and HF large amounts of rotational excitation were observed. The inelasticity was found to be greater for HF than for HCl, consistent with the relative dipole moments and moments of inertia of the two molecules. The average excitation energies for HF measured at fixed scattering angle are qualitatively in agreement with a classical perturbation calculation for pure rotational excitation. For HCl, the classical perturbation calculation gives a rotational excitation energy much lower than the average observed translational energy loss, indicating that concurrent vibrational excitation, though not resolved in the presence of rotational excitation, may be relatively more important for HCl than for HF.

Electron energy dependence of the energy and angular distributions of O^{−} from dissociative ion pair formation in O_{2}
View Description Hide DescriptionThe energy and angular distributions of O^{−} produced from O_{2} by dissociative ion pair formation have been measured at selected incident electron energies from threshold to 100 eV. The kinetic energy distributions show well‐defined maxima near 2.0 and 3.3 eV with appearance potentials, respectively, at 20.0 and 23.0 eV. The angular distributions are peaked in the forward and backward directions relative to electron beam and exhibit nondipolar structure. The shape of the distributions depends on electron energy, becoming more isotropic with increasing energy. An attempt has been made to explain the behavior of the angular distributions in terms of a superposition of final states using a multipole expansion of the differential cross section which includes the effects of higher‐order partial waves.

Breakdown of the dipole‐Born approximation for predicting angular distributions of dissociation fragments
View Description Hide DescriptionA general expression is given for the form of the angular distribution of molecular fragments produced by electron‐induced dissociation of diatomic molecules. A model for parametrizing angular distributions is introduced which allows investigation of higher‐order multipole corrections to the simple dipole‐Born approximation. It is found that higher‐order terms are significant and even dominate the form of the angular distribution at electron energies near thresholds involving large momentum transfer. The model is useful in explaining previously observed deviations from the dipolar form and is applied to recent measurements of the angular distribution of O^{−} produced from O_{2} by dissociative ion pair formation. When the effect of molecular rotation is included, the degree of anisotropy is found to diminish with increasing lifetime of the dissociating state, although general features of the distribution are preserved.

Modification of the exact finite method for boundary tension calculations on a hard sphere lattice gas
View Description Hide DescriptionThe exact finite method, developed largely by Runnels, is modified so that calculations can be made of the properties of a single phase in equilibrium with other phases. The modification also allows the calculation of some boundary properties. Application of the method is made to the hard sphere lattice gas on a triangular lattice with first and second nearest neighbor exclusions. The results in this paper are in general agreement with earlier work on the same model: ρ/ρ_{0}=0.78±0.01 and 0.69±0.01; lnz_{ t } = 1.715±0.02; P_{t}/kT =0.475±0.004. Two boundaries are investigated, the surface between two crystalline phases of different order, and a free surface. Boundary tensions and excess surface densities indicate this model has more complicated surface structures than those found earlier in another hard sphere lattice gas (square lattice‐nearest neighbor exclusions).

Angular correlations in linear copolymers from the compositional dependence of their dipole moments. I. Random copolymerization
View Description Hide DescriptionA theory is developed, without recourse to a detailed model, for evaluating certain linear combinations of the average cosines of the angles between dipole moments of pairs of structural units that are closely spaced along the chain contour. Measurements of the compositional dependence of the effective dipole moment per structural unit together with a knowledge of the compositional dependence of the probabilities for various types of sequences of structural units provide the required information. The theory offers a possibility for testing the validity of certain restrictive assumptions that are implicit in earlier treatments of the problem. Additionally, the theory provides a basis for determining unequivocally from experimental measurements whether angular correlations exist among the structural units that contribute to the orientational part of the polarization and whether the angular correlation parameters, if they exist, depend on the types of structural units involved.