Volume 63, Issue 11, 01 December 1975
Index of content:

The ultraviolet vapor absorption spectrum of 2‐furaldehyde: Electronic assignments and vibrational analysis
View Description Hide DescriptionThe vapor absorptionspectrum of 2‐furaldehyde has been photographed in the 20 000–50 000 cm^{−1} region. The spectrum is unusual in that it displays two separate electronic transitions with sharp structure, the origin band in both cases having a 5 cm^{−1} bandwidth. The 26 000–34 000 cm^{−1}absorption region is assigned as an Ã ^{1} A ^{ ″ }(nπ*) ←X̃ ^{1} A′ transition and the 36 000–41 000 cm^{−1} region as a B̃ ^{1} A′ (ππ*) ←X̃ ^{1}A′ transition with its transition moment direction tentatively thought to be nearly parallel to the ring–CHO bond. The assignments are in accord with a CNDO configurational interaction description by Del Bene and Jaffe [J. Chem. Phys. 48, 4050 (1956)]. A detailed vibrational analysis of the Ã–X̃ system based upon identification of the polarization of the bands by their rotational contours and a recent assignment of the fundamental vibrations in the ground state leads to the following conclusions. The intensity in the electronically allowed part of the absorptionspectrum is dominant. There is considerable vibronic intensification in the Ã←X̃ transition of 2‐furaldehyde with the activity of the out‐of‐plane ring modes being comparable to the out‐of‐plane aldehyde modes. The Herzberg–Teller activity of the ring modes indicates that the n‐electron density is delocalized into the ring. A comparison of the rotational contours of the vibronic bands in the Ã–X̃ system with those of the B̃–X̃ system suggests that the B̃–X̃ system is not the source of intensity for the Ã–X̃ system. Nearly all of the absorption bands in the B̃–X̃ system have rotational contours similar to the origin, indicating that intensity borrowing is not important. Most of the spectrum can be analyzed in terms of the combinations and progressions formed by two excited state modes: ν_{19}(180 cm^{−1}), the in‐plane wagging motion of the aldehyde against the furan ring, and ν_{9}(1271 cm^{−1}), the ring stretching mode.

High resolution proton NMR study of gypsum, CaSO_{4}⋅2H_{2}O
View Description Hide DescriptionMultiple pulse nuclear magnetic resonance techniques have been used to measure the protonchemical shifttensor in gypsum, CaSO_{4}⋅2H_{2}O, and to characterize the proton exchange occurring in this material. The orientation of the room‐temperature averaged protonchemical shifttensor is as would be expected from the H_{2}O molecular symmetry alone, and the low temperature results are consistent with a tensor of near axial symmetry along the O–H bond direction (σ_{∥}=+2.1 ppm, σ_{⊥} (in H_{2}O plane) =−18.4 ppm, and σ_{⊥} (perpendicular to H_{2}O plane) =−17.6 ppm±1 ppm). In addition, a value of 1.54±0.01 Å was determined for the interproton distance within the H_{2}O molecule in gypsum.

Excitation functions for the reactions of Ar^{+} with CH_{4}, CD_{4}, and CH_{2}D_{2}
View Description Hide DescriptionIntegral reaction cross sections as a function of initial translational energy (0.4–30 eV c.m.) are reported for isotopic variants of the exoergic ion‐molecule reaction Ar^{+}+CH_{4} → ArH^{+}+CH_{3}. The excitation functions, which maximize at about 5 eV and decrease at lower collision energies, appear to possess translational energy thresholds at about 0.1 eV. At the higher energies there is a large isotope effect favoring abstraction of H over D. The observed threshold behavior, unusual for exoergic reactions of positive ions, is discussed in terms of the formation of an ArCH_{4} ^{+} intermediate complex at low collision energies.

Macromolecular dimensions obtained by an efficient Monte Carlo method without sample attrition
View Description Hide DescriptionThe statistical dimensions of macromolecular chains of fixed contour length can be rapidly calculated by Monte Carlo methods applied to a model consisting of dynamic self‐avoiding random chains on a lattice. This ’’slithering snake’’ model involves moving the head of a chain one space in a lattice with all other elements of the chain moving forward along the old contour. Possible moves of the head are selected at random, but if such a move is precluded by double occupancy, the old configuration is retained, with head and tail interchanged, and then counted as if a move were made. This technique gives unbiased statistical results except for the effect of double cul‐de‐sacs. The method can also be applied to interacting chains, either free or confined to a box. Calculations have been made for 10‐link chains on a square planar lattice for two different concentrations in infinite space and for two concentrations in a small box.

A theoretical study of Li_{2}H. I. Basis set and computational survey of excited states and possible reaction paths
View Description Hide DescriptionSCF potential curves for C_{2v } (Li_{2}+H) and C_{∞v } (LiH+Li) symmetries have been calculated for the 26 lowest lying states of Li_{2}H. The curves are estimated to be ≲0.1 eV above the Hartree–Fock–Roothaan limit. Correlated MCSCF/CI curves were computed for the lowest states of ^{2} A _{1}, ^{2} B _{2}, and ^{2}Σ symmetries. All low‐lying states of the complex Li_{2}H involve substantial charge transfer. The most stable of these, ^{2} A _{1}, lies ∼20 kcal/mole below the ground state LiH+Li asymptote. The first excited state of the complex, ^{2} B _{2}, is but weakly bound relative to the same asymptote. Electron‐jump geometries and vertical spectra were computed for all of these low‐lying states. Possible low‐energy paths for the reaction Li_{2}+H→LiH+Li were determined, and all involve charge‐transfer intermediates.

Density and temperature effects on motional dynamics of SF_{6} in the supercritical dense fluid region
View Description Hide DescriptionThe NMR^{19}F spin–lattice relaxation times and self‐diffusion constants have been measured in the dense fluid of SF_{6} as a function of pressure and temperature over the density range 0.4⩽ ρ/ρ_{ c }⩽2.6 and the temperature range 0.8⩽T/T _{ c }⩽1.5. Densities have also been determined under the same experimental conditions. The relaxation mechanism for the fluorine nuclei is dominated by the spin–rotation interactions and therefore the analysis of the T _{1} data yields the angular momentumcorrelation time τ_{ J }. On the other hand, the self‐diffusion data provide information about the behavior of the linear momentum correlation time τ_{ P }, in SF_{6}. Interpretation of the experimental data based on the hard sphere model of liquids allows us to draw several conclusions of general character about the molecular dynamics of the spherical molecule of SF_{6} in the dense fluid region: (a) The angular momentum relaxation can be approximated as occurring through a sequence of uncorrelated binary collisions; (b) the estimated hard sphere diameter is temperature dependent and in reasonable agreement with the theoretical prediction; (c) for fluid densities lower than twice the critical density one finds considerable deviation between experiment and theory; (d) due to many body correlations the linear momentum behavior differs from the Enskog theory predictions in agreement with the results of the molecular dynamics calculations.

Heat capacity, entropy, enthalpy, and Gibbs energy of ^{242}PuF_{4} from 10 to 350 °K
View Description Hide DescriptionHeat capacity measurements have been made on a 20.9 g sample of ^{242}PuF_{4} from 10 to 350 °K. The measurements from 20 to 350 °K were made by intermittent electrical heating with corrections based on the observed temperature drifts. Heat capacities determined in this way were compared with temperature drifts without electrical heating in the range 20 to 56 °K to evaluate the radioactive self‐heating rate as (1.214±0.004) ×10^{−4} W/g Pu. From 10 to 20 °K heat capacities were obtained from this value of the radioactive self‐heating rate and the temperature drifts without electrical heating. The heat capacity curve had the normal sigmoid shape. At 298.15 °K the heat capacityC°_{ p }, entropyS °, enthalpyH °−H °_{0}, and tempered Gibbs energy (G °−H °_{0})/T are, respectively (116.19±0.29) J °K^{−1}⋅ mole^{−1}, (147.25±0.37) J °K^{−1}⋅ mole^{−1}, (22 385±56) J mole^{−1}, and −(72.17±0.18) J °K^{−1}⋅ mole^{−1}. The changes in various thermodynamic functions for the formation of PuF_{6}(g) from PuF_{4}(c) and F_{2}(g) are evaluated at 550 and 298.15 °K.

A semiclassical, nonperturbative approach to collision‐induced transitions between rotational levels for the N_{2}–Ar system
View Description Hide DescriptionA semiclassical and nonperturbative approach in which coupling between translational and rotational degrees of freedom is also taken into account is developed to compute rotationally inelastic transition probabilities of linear molecule–atom collisions. This approach is applied to N_{2}–Ar collisions. The transfer of rotational, translational, and interaction energies is also computed during collisions. The results show that if the coupling between rotational and translational degrees of freedom is neglected, the computed transition probabilities change significantly for strong collisions, although there are no significant changes in the corresponding trajectories.

The A+B_{ x } condensation reaction: Crossed nozzle beams of Br_{2} and (Cl_{2})_{ x } or (NH_{3})_{ x } clusters
View Description Hide DescriptionNozzle beams of Br_{2} and Cl_{2} or NH_{3} have been crossed in a molecular beamscattering apparatus; the Cl_{2} or NH_{3}beam contained (Cl_{2})_{ x } or (NH_{3})_{ x } clusters distributed such that the intensity of a given cluster, F_{ x }, decreased with increasing x for 1⩽x⩽∼50. Mass, angular, and time‐of‐flight spectra of the scattered neutral species all establish that the A+B_{ x }→AB_{ x }* bimolecular condensationreaction is being observed. However, working from the data, it is not possible to distinguish between detection of a long‐lived AB_{ x }* metastable complex or of a decomposition product formed with low recoil velocity. Product angular distributions are confined to a small region of laboratory scattering angle Θ and peak at small but positive Θ (Θ=0° and 90° defined by cluster and Br_{2}beam directions, respectively). It is pointed out that this sharp peaking at small Θ is due to a number of experimental factors, including a Jacobian factor varying as sin^{−2}Θ, and should be a universal characteristic of such condensationreactions in crossed beams. In the data it is indicated that there will be a high probability of fragmentation into small daughter ions upon electron bombardment (EB) ionization of an AB_{ x } or B_{ x } cluster for the range in x most sensitive to the measurements (∼10⩽x⩽∼50). This in turn implies that the concentration of neutral clusters in the beam can be seriously underestimated if the cluster ion mass spectra produced by EB ionization of the nozzle beam are assigned assuming that fragmentation is inconsequential.

The generalized valence bond description of O_{2}
View Description Hide DescriptionA b i n i t i o calculations using the generalized valence bond (GVB) method have been carried out for the lowest triplet and singlet states of O_{2} at internuclear distances (R) between 2a _{0} and 6a _{0}. In contrast to other orbital descriptions, GVB leads correctly to ground state oxygen atoms as the bond length is increased to infinity. This proper behavior requires optimization of the spatial orbitals themselves and of the permutational coupling between them as well. Analysis of the results as a function of R is straightforward. Constructing a simple configuration interaction (CI) wavefunction using the GVB orbitals leads to excellent potential curves, accounting for 94% of the bonddissociation energy. The calculated adiabatic separation of the singlet and triplet states is 1.09 eV, which is 0.11 eV above the experimental T _{ e }.

Emission spectra of XeBr, XeCl, XeF, and KrF
View Description Hide DescriptionThe emission spectra of XeBr, XeCl, XeF, and KrF at high pressure are reported and discussed. The spectra were obtained by observing spontaneous emission from electron beam excited mixtures of argon containing lesser amounts of xenon with the halogens or krypton with fluorine. The emitting state in these species is best described as an ionic species Xe^{+}X^{−} or Kr^{+}F^{−}. The wavelengths of these emission bands are in good agreement with a theoretical model in which the ionic binding energy of the noble gas halide ion pair is roughly equal to that of the nearest alkali halide. Our high pressurespectra imply that the lowest potential energy curve for XeF is bound.

Rotationally inelastic diffraction of molecular beams: H_{2}, D_{2}, HD from (001) MgO
View Description Hide DescriptionIn the coherent scattering of molecules from a crystal surface, energy of molecular rotation can be transferred to translational motion. This interchange has been studied for thermal beams of H_{2}, D_{2}, and HD diffracted from (001) surfaces of MgO prepared by cleavage in an ultrahigh vacuum. Strong new diffraction peaks are identified for in‐plane scattering, beyond those predicted for elastic interactions. The location of these peaks is in agreement with calculations based on the assumption that phonons are not excited during the collision with the lattice. In the experiments, only those rotational transitions are detected for which there are extrema in the variation of the scattering angle with the wavenumber of the incident molecules. Scattering at these critical angles is analyzed and it is shown that for beams formed by effusion, these extrema lead to sharply peaked scattering maxima, which facilitate detection of rotational–translational interchanges. The efficiency of rotational transitions is found to be high, comparable to elasticscattering, and to depend strongly upon the angle between the incident beam and the scattering surface.

Molecular orbital theory of the hydrogen bond. XIV. Disubstituted carbonyl compounds as proton acceptors
View Description Hide DescriptionA b i n i t i o SCF calculations with a minimal STO‐3G basis set have been performed to determine the ground state equilibrium structures and energies of dimers in the series HOH...OCR_{2}, where R is CH_{3}, NH_{2}, OH, or F. The equilibrium structures of the water–acetone, water–urea, and water–carbonyl fluoride dimers are similar to those of the water–acetaldehyde, water–formamide, and water–formyl fluoride dimers, respectively, in which the substituents are ’’c i s’’ to water with respect to the carbonyl CO bond. No equilibrium water–formic acid dimer of this type was found, and no equilibrium water–carbonic acid dimer exists in which water is the proton donor molecule. There is a greater differentiation of dimer stabilities in the series having the disubstituted carbonyl compounds as proton acceptors, which may be related to enhanced pi‐donating and sigma‐withdrawing effects of the substituents. Thus, the order of increasing dimer stability in the combined series HOH...OCRR′ is F_{2}<H,F<H_{2}<H,CH_{3}< (CH_{3})_{2}<H,NH_{2}< (NH_{2}) _{2}. CI calculations show that the dimer singlet n→π* transition energies are well approximated as the sum of the n→π* transition energy in the monomer R_{2}CO and the hydrogen bondenergy in the corresponding dimer HOH...OCR_{2}, suggesting that the hydrogen bond is broken in the dimer n→π* state.

A configuration interaction study of the ground state molecular properties of NO_{2}
View Description Hide DescriptionAn a b i n i t i o SCF–CI wavefunction for the ^{2} A _{1}ground state of NO_{2} has been calculated using a ’’double zeta plus polarization’’ quality basis set. The CI energy, −204.374346 a.u., is the lowest which has been reported for this molecule. The CI wavefunction is discussed and Mulliken population analyses are presented. Several one‐electron properties, including the magnetic hyperfine splitting parameters, are reported for both the SCF and CI wavefunction.

Gaussian basis sets suitable for accurate valence‐shell calculations using the model potential method
View Description Hide DescriptionPople’s extended Gaussian‐type basis, usually termed 4‐31G, was adapted for use with the model potential method. Molecular calculations were performed successfully for N_{2}, H_{2}O, CH_{4}, NH_{3}, HCN, PH_{3}, H_{2}S, and ClF. Substantial savings were achieved in computing cost.

PNO–CI (pair‐natural‐orbital configuration interaction) and CEPA–PNO (coupled electron pair approximation with pair natural orbitals) calculations of molecular systems. IV. The molecules N_{2}, F_{2}, C_{2}H_{2}, C_{2}H_{4}, and C_{2}H_{6}
View Description Hide DescriptionSCF, IEPA–PNO, CEPA–PNO, and PNO–CI calculations have been performed for the molecules N_{2} and F_{2} at their experimental equilibrium distances with two basis sets, a ’’small’’ basis that contains one d set per atom and a ’’standard’’ basis with two d sets and one f set. Potential curves of these molecules are calculated with the small basis sets. The molecules C_{2}H_{2} and C_{2}H_{4} are calculated with the small basis (which contains additionally one p set on H) and with a ’’hydrocarbon’’ basis that is smaller in the s,p part, but includes the same polarization functions. For C_{2}H_{6} in both its staggered and eclipsed forms only the hydrocarbon basis is used. The computed correlation energies are analyzed in terms of quantities defined in Part I, in particular, in terms of the IEPA pair correlation energies ε_{μ} ^{IEPA} and the error ΔE _{IEPA} of the IEPA energy. A comparison is made between the results in the canonical and the localized representations and a partially localized description in which the σ–π separation is preserved. The IEPA error is rather large for all of these molecules, especially for N_{2}. The IEPA error in the localized representation changes smoothly in the series C_{2}H_{2} to C_{2}H_{6}; whereas in the canonical representation it varies by almost two orders of magnitude. A new geometry optimization for staggered and eclipsed ethane is carried out, since previous optimizations turned out to be unsatisfactory. Correlation, however, does not affect the geometries significantly; its effect on the rotational barrier is very small. For N_{2} and F_{2} in the neighborhood of the equilibrium distances the CEPA potential curves turn out to be very close to the exact ones (and yield good equilibrium distances and force constants); whereas the PNO–CI curves (and, of course, more so the SCF and IEPA curves) are unacceptable. At large internuclear distances, where the weight of the leading determinant in the full wavefunction becomes smaller than ∼0.8 the CEPA curve is unsatisfactory.

New semiclassical treatments of rotational and vibrational transitions in heavy‐particle collisions. I. H–H_{2} and He–H_{2} collisions
View Description Hide DescriptionTwo new semiclassical methods—the multistate orbital treatment and the multichannel eikonal treatment—are proposed for the description of rotational and vibrational excitation in heavy‐particle collisions. The first method includes appropriate trajectories determined from a certain optical potential designed to couple the response of the internal structure, which is described by a quantal multistate expansion, to the orbit for the relative motion and vice versa. While this approach is, in general, valid when the quantal imprecision in the classical trajectories is small (as for heavy particles) the second method based on the use of a straight‐line eikonal for the relative motion, of different local momenta in the various channels and of a multistate expansion for the internal motions, is valid for scattering mainly about the forward direction. These procedures are applied to representative rotational transitions in H–H_{2} and He–H_{2} collisions at 0.25–1.5 eV and yield angular distributions and integral cross sections in very close accord with corresponding quantal results. The methods are particularly valuable at higher impact energies when the inclusion of the resulting many rotational and vibrational channels by full quantal treatments is prohibitively difficult. Various approximate schemes—the perturbed‐rotating‐atoms approximation and the effective potential method—are also investigated.

Cluster‐type calculations of electronic structures of crystals by the method of linear combinations of atomic orbitals
View Description Hide DescriptionFirst‐principle cluster‐type calculations of the electronic energy structures for the silicon and lithium fluoride crystals by the method of linear combinations of atomic orbitals (LCAO) are presented. The Hamiltonian is that of the infinite crystal (with a Slater‐type approximation for electron exchange) and the basis functions are linear combinations of localized functions centered at the atomic sites within a cluster. By means of the Gaussian technique all the multicenter integrals associated with the Hamiltonian and overlap matrix elements are readily evaluated. For the localized functions in the basis set, we use (i) atomiclike orbitals, expanded in terms of the Gaussian‐type orbitals, corresponding to the core and valence states of the free atoms and (ii) single Gaussians with various exponents. With exclusively atomiclike basis orbitals we show that one can reproduce the valence bands of the infinite crystals reasonably well using a cluster of eight or ten shells. However, to obtain representative conduction bands, particurly energy band gaps, we find it necessary to supplement the atomiclike orbitals by a few single Gaussians. In these cluster calculations, no energy levels are found in the crystal band gap region because the Hamiltonian is that of an infinite crystal with no physical surfaces present.

Hyperfine and superhyperfine EPR spectra of Tc(IV) and Re(IV) in tin dioxide single crystals
View Description Hide DescriptionThe electron paramagnetic resonance spectra of Tc(IV) and Re(IV) in tin dioxide have been observed at X‐ and Q‐band frequencies at liquid helium temperature. Both ions with a spin ground state of S=3/2 occupy lattice sites of D _{2h } symmetry. The hyperfine structures show large anisotropies, especially in the case of Re(IV), which can be explained by the large crystal field parameters. The spectra fit with a conventional spin Hamiltonian for orthorhombic symmetry correspond to the electron transition 1/2↔−1/2. The obtained parameters for the c‐axis and the magnetic z‐axis coincident are Besides the hyperfine lines, both systems exhibit a partly resolved superhyperfine structure due to an interaction of the unpaired electrons with adjacent tin nuclei.

Molecular polarizabilities and susceptibilities from Frost‐model wavefunctions
View Description Hide DescriptionAverage polarizabilities and susceptibilities of a number of molecules are computed from Frost‐model wavefunctions using a form of symmetry‐adapted double perturbation theory. The anisotropy of α and χ is found for a few molecules using the elliptical Gaussian form of the Frost model. The results obtained are in reasonable agreement with experiment and other calculated values.