Index of content:
Volume 68, Issue 9, 01 May 1978

Laser magnetic resonance spectroscopy of the ν_{3} fundamental band of HO_{2} at 9.1 μm
View Description Hide DescriptionThe ν_{3} fundamental band (0–0 stretch; ν_{0}=1097.626 cm^{−1}) of the hydroperoxyl radical HO_{2} has been studied using the technique of laser magnetic resonance, in which molecular transitions are tuned into resonance with fixed laser lines by means of the Zeeman effect. The HO_{2} was produced by reacting O atoms with methyl or allyl alcohol in a flow system incorporating an absorption cell inside the CO_{2}laser cavity. Over 200 resonances involving different M _{ J } components of a‐type HO_{2} transitions with 1⩽N⩽7 and 0⩽K _{ a }⩽4 were assigned, and from an analysis of the spectra the band origin and excited state rotational, centrifugal distortion, and spin–rotation interaction parameters were determined. The possible application of these results to the remote spectroscopic detection of HO_{2} is discussed.

Effects of H and D order on the thermal conductivity of ice phases
View Description Hide DescriptionThermal conductivities (λ) of nine ice phases were determined by the transient hot wire method in a temperature range of 100–300 K and under pressures up to 2.2 GPa. Pure D_{2}O as well as D_{2}O–H_{2}O mixtures were found to have values of λ close to those of pure H_{2}O, except in phase VIII. For this phase λ was found to be about 9% lower in the case of pure D_{2}O as well as in 1:1 and 1:3 mixtures of D_{2}O:H_{2}O. The hydrogen ordered phases systematically exhibit higher λ than the disordered ones, and in particular the presence of ice VI′ could be detected by a change of slope at low T in the curve of ln λ versus ln T for phase VI. If the results are fitted to the function λ=C T ^{−n }, the values of n fall into two groups. One group with n<1 contains only paraelectric phases, whereas the other group, having higher values of n, contains antiferroelectric phases and phase Ih. The latter phase is exceptional in that λ has a negative pressure coefficient. The results are interpreted in terms of dynamic decoupling of the hydrogen atoms from the lattice and in terms of induced oxygen disorder.

A model calculation of vibration–translation energy transfer
View Description Hide DescriptionBy employing a simple step potential as the interaction potential in a collision between an atom and a diatomic molecule, we develop a full quantum mechanical treatment of vibrational energy transfer in a collinear collision for which the resulting coupled differential equations can be solved exactly. We find that for all but the weakest coupling strengths, the trend of the solutions to their converged value is not smooth and monotonic but rather shows large oscillations. We also locate a region in the parameter space in which the solutions are a very sensitive function of the coupling strength showing something analogous to ’’resonant’’ behavior.

Vaporization, thermodynamics, and dissociation energy of gadolinium monosulfide: Systematics of vaporization of the rare earth monosulfides
View Description Hide DescriptionMass spectrometric studies of the congruent, near‐stoichiometric sublimation of gadolinium monosuflide and of the isomolecular gaseous reaction GdS+Y=Gd+YS have been carried out in the range 2005 to 2290 K. Sublimation occurs simultaneously to GdS molecules and to atoms Gd+S. Partial pressures are given by log P _{Gd}(atm) =−(29873±182)/T+(7.254±0.084) and log P _{GdS}(atm) =−(30343±221)/T+(7.281±0.102). The indicated uncertainties are standard deviations; the absolute pressures may be low by as much as a factor 2. Enthalpies derived from combined second and third law considerations, with estimated maximum uncertainties due to cross section ratios, ground state degeneracies, and the use of the YS/GdS equilibrium for pressurecalibration are (all in kcal/mole): GdS(s) =GdS(g), ΔH°_{2150}=140±3, ΔH°_{0}=147±4; GdS(s) =Gd(g)+S(g), ΔH°_{2150}=270±4, ΔH°_{0}=272±4; GdS(g) =Gd(g)+s(g), ΔH°_{2150}=129.8±2.5, D°_{0}(GdS) =125.0±2.5; Gd(s)+S(rh) =GdS(s), ΔH°_{298}=−111±4. Data on the vaporization behavior and thermodynamics of the rare‐earth monosulfides from LaS to GdS are assembled and/or estimated and discussed. Heats of atomization of MS(s) and of dissociation of MS(g) follow closely the trend of heats of sublimation of the metals. Heats of formation and sublimation of MS(s) are essentially equal for all the elements.

Electronic structure of CaO. I
View Description Hide DescriptionElectronic wavefunctions for 15 states of CaO arising (nominally) from six electron occupancies are calculated at the single configuration (sc) SCF level and analyzed qualitatively in terms of Mulliken populations and an approximate overlap criterion. The accuracy of the sc approximation is assessed and shown to be highly inadequate in several cases. Approximate T _{ e }’s are extracted from these results when warranted by the quality of the sc wavefunctions. An improved two configuration wavefunction for the lowest ^{1}Σ^{+} state is also determined and discussed in detail.

Molecular symmetry. II. Gradient of electronic energy with respect to nuclear coordinates
View Description Hide DescriptionSymmetry methods employed in the HONDO a b i n i t i o polyatomic SCF program are extended to the analytic computation of energy gradients. Validity of the Hellmann–Feynman theorem is not assumed, i.e., all two‐electron contributions to the gradient are included explicitly. The method is geared to the efficient computation of entire blocks of two‐electron integrals. Just one of a set of symmetrically related blocks must be computed. The gradient contribution from each unique block is multiplied by q _{4}, the number of equivalent blocks, and added into a ’’skeleton gradient vector,’’ all other blocks are simply omitted. After processing molecular integrals, the true gradient vector is generated by projecting the symmetric component out of the skeleton vector. The analysis is based on Eqs. (26) and (33) which are valid for many variational wavefunctions including restricted closed shell and unrestricted open shell self‐consistent field functions. We also extend the use of translational symmetry proposed previously by Morokuma e t a l. To illustrate the method, the gradient of the restricted SCF energy is computed for eclipsed ethane using a Pople‐type 63lG** basis and D _{3h } symmetry. The same calculation is repeated using various subgroups of D _{3h }. Computation times for SCF and for the gradient are each roughly inversely proportional to the order of the group, and for a given symmetry, the gradient computation takes about two and a half times as long as SCF.

Effective core potentials for the cadmium and mercury atoms
View Description Hide DescriptionA b i n i t i o effective core potentials have been obtained for the cadmium and mercury atoms by the methods of Kahn e t a l., [J. Chem. Phys. 65, 3826 (1976)]. Both two and twelve valence electron representations of Cd and Hg were tested for various atom state‐configurations by comparison with all‐electron calculations. The generated potentials were used to obtain the equilibrium bond distances and molecular binding energies for the dichloride and dimethyl compounds of both atoms from single and optimum‐double configuration self‐consistent field calculations.

A search for the HF_{2} and HClF neutral free radicals isolated in argon matrices
View Description Hide DescriptionA systematic search has been carried out for the neutral species FHF and FHCl isolated in argon matrices at 14 K. The synthesis of these species was attempted by the photolysis of F_{2} in the presence of either HF or HCl, either concurrent with, or after deposition into the low temperature matrix. Infrared spectra of these samples gave no evidence for the formation of a species of the form FHF, while evidence was demonstrated for a weakly bound complex between HF and Cl, but with a very different spectrum and structure than the corresponding FHCl^{−} anion. These results are in general agreement with the detailed theoretical calculations that have been performed on these systems.

Generalized effective Hamiltonians: Time scale separation within a semiclassical formalism
View Description Hide DescriptionA theory for collision‐induced transitions in the subset of quantum states ‖a _{ e }〉 is developed, where the full molecular basis is ‖a _{ e } a _{ u }〉. The only assumption is that changes in a _{ u } occur much more quickly than those in a _{ e }. The choice of a _{ e } and a _{ u } is in general arbitrary, and it is shown how to make this separation based on the type of experimental measurement under consideration. For example, an evaluation of an atom–vibrotor problem (i.e., a _{ e }=n, a _{ u }=j m _{ j }) is considered. In conjuction with this illustration an analysis of the breathing sphere (BS) approximation is carried out. It is concluded that a modified BS method (i.e., use of the diagonal rotational potential) should be valid when the random phase approximation holds for the rotational scattering amplitude.

All‐electron nonempirical study of the ground state of MgF_{2}
View Description Hide DescriptionLCAO–MO–SCF calculations on the ground state of MgF_{2} are reported for a number of geometries (both linear and bent). The results indicate a linear molecule with an equilibrium bond length (Mg–F) of 1.77 Å. All three force constants for the molecule are predicted. Least squares fitting of the data to second order potential functions gives force constants f _{ r }=3.21, f _{ r r }=−0.029, and f _{ϑ}/r ^{2} _{0}=0.188 mdyne/Å. Dipole moment calculations for a number of geometries, and the diamagnetic susceptibility tensor,quadrupoletensor, and other molecular properties for the lowest energy conformation are reported.

Microwave spectrum, dipole moment, and the conformation of 7‐oxabicyclo[4.1.0]hept‐3‐ene
View Description Hide DescriptionThe microwave spectrum of 7‐oxabicyclo[4.1.0]hept‐3‐ene has been studied and the rotational constants for the ground state are (in MHz) A=4073.15±0.01, B=3394.54±0.01, C=2174.07±0.01. Stark effect measurements have allowed determination of the dipole moment components μ_{ a }=0.775±0.002 D and μ_{ c }=1.770±0.005 D for a total dipole moment μ=1.932±0.005 D. Analysis of these data indicates the molecule has a boatconformation.

Evidence against a strong critical anomaly in the depolarized Rayleigh line of nitrobenzene:n‐hexane
View Description Hide DescriptionThe critical behavior of the depolarized Rayleigh linewidth Γ of the liquid system nitrobenzene:n‐hexane is in dispute. Atakhodzhaev e t a l. [Zh. Eksp. Teor. Fiz. Pis. Red. 17, 95 (1973)] reported a 20‐fold change in Γ on approaching T _{ c }, while Beysens e t a l. [Opt. Commun. 15, 436 (1975)] found only a weak anomaly. We report a further study of Γ for 20?T−T _{ c } ?−17 K. We find at most a weak anomaly in Γ, in accord with the results of Beysens e t a l. Multiple scattering apparently is not responsible for the discrepancy.

Dynamic effects of pair correlation functions on spin relaxation by translational diffusion in liquids. II. Finite jumps and independent T _{1} processes
View Description Hide DescriptionHwang and Freed have previously given solutions for the relative diffusion of molecules that include the proper boundary condition (i.e., an excluded volume due to a distance of minimum approach) which has usually been neglected in spin relaxation theories. In this work their results are extended to include effects of (1) one type of spin that is rapidly relaxing, (2) diffusion by jumps of finite size, and (3) frequency‐dependent diffusion coefficients in the theory of spin relaxation by intermolecular dipolar interactions. These results are mathematically simpler and sounder than those commonly employed. In particular, it is shown that for case (2) measurements of J (O), the zero‐frequency spectral density cannot solely be used to determine the jump size, in constrast to the Torrey theory, which did not consider the boundary‐value problem.

Intrachain dynamics and forward depolarized scattering
View Description Hide DescriptionThe quasielastically scattered spectrum from a dilute polymer solution in the forward depolarized configuration is shown to be generally a superposition of uniformly weighted multiple Lorentzians. This is true irrespective of the dynamic models provided that one samples only the long wavelength modes of the intrachain dynamics, there exists a normal coordinate system for the chain configuration, and the polymer chain is long enough that its mean square end‐to‐end distance is proportional to the number of chain elements.

Vibrational excitation of N_{2} during optical pumping of Hg by 253.7 mμ resonance radiation
View Description Hide DescriptionThe observed decay time and the population density of the metastable 6 ^{3} P _{0} state of mercury in a N_{2}:Hg gas mixture are markedly reduced with increased duration of an intense 253.7 mμ resonance radiation pulse. We show that this phenomenon is due to a vibrational excitation of the nitrogen in the spin–orbit relaxation decay of the 6 ^{3} P _{1} to the ^{3} P _{0} state of Hg which is followed by the quenching of the ^{3} P _{0} state. Such quenching takes place by diffusion and deactivation of ^{3} P _{0} at the walls or by a three‐body excimer formation in the gas. Observations are confirmed by a numerical solution of the rate equations for the different excited states in the system. The impact of these results on the performance of the optically pumped Hg laser is discussed.

Electronic structure of Li–H_{2}O and related neutral molecular complexes, including Al–H_{2}O
View Description Hide DescriptionA b i n i t i o molecular electronic structure theory has been applied to a novel class of molecular complexes, of which the neutral Li–H_{2}O may be considered the prototype. Near the Hartree–Fock limit Li–H_{2}O is bound by 10.4 kcal/mole relative to separated Li+H_{2}O. Electron correlation was accounted for by large configuration interaction, and this increases the predicted dissociation energyD _{ e }(Li–H_{2}O) to 11.7 kcal. A number of related molecular systems have been investigated including Al–H_{2}O, predicted to be bound by 4.4 kcal.

Quantum mechanical reactive scattering via exchange kernels: Application to the collinear H+H_{2} reaction
View Description Hide DescriptionA formulation of quantum mechanical reactive scattering given by Miller is applied to the collinear H+H_{2}reaction. The approach is the direct analog to the Hartree–Fock method of electronic structuretheory, and it obviates the need for specialized (e.g., ’’natural’’ collision) coordinates. The rearrangement process takes place via an explicit exchange interaction (cf. electron exchange in Hartree–Fock theory), and closed channels are incorporated via a square‐integrable set of correlation functions. Agreement with results obtained by others using other methods is excellent, showing this approach to quantum mechanical reactive scattering to be a viable one.

Vibrational energy relaxation of diatomic molecules isolated in rare‐gas matrices: Role of orientational motions
View Description Hide DescriptionA quantum‐mechanical model which accounts for the dynamical role of orientational motions in vibrational energy relaxation (VER) of diatomic impurities isolated in rare‐gas solids is developed. A simple closed expression for the VER rate constant is derived upon the following principal assumptions: (i) the lattice is planar, the diatomic a plane vibrotor; (ii) dynamical participation of local translational and bulk lattice modes is neglected. Results for the systems NH and ND in Ar, OH and OD in Ne, HCl and DCl in Ar and CO in Ar are in reasonable agreement with experiment. In particular, observed isotopic effects seem to be well described by the model. However, there are a couple of discrepancies between calculated and observed ratios of VER rate constants for the systems NH, ND/Kr and HCl, DCl/Ar. It is surmised that these are due to features not included in the model, e.g., lattice imperfections, van der Waals molecule formation and/or rotational–translational coupling.

The Jahn–Teller effect in the lowest charge transfer state of UF_{6}
View Description Hide DescriptionThe neat and mixed crystal emission spectra of UF_{6} are analyzed. It is found that long uniquantal progressions appear in the ν_{5} (t _{2g }) vibration built on the origin, ν_{6} (t _{2u }), and n _{1}ν_{1} (a _{1g }). The maximum intensity in all these series falls around n _{5}=2. These data are interpreted as being indicative of a substantial molecular Jahn–Teller interaction in the excited charge transfer state. Using a modified Franck–Condon analysis for a three‐dimensional vibrational space, it is found that D _{5} ∼2.02 and that the excited state is distorted along each of three ν_{5} coordinates by roughly 0.09 Å.

Microwave optical double resonance spectrum of NH_{2}. IV. Observation of an unusual rotational transition in ? ^{2} B _{1}
View Description Hide DescriptionTwo rotational transitions in the ground ^{2} B _{1} state of NH_{2}—3_{31}, J=7/2←4_{13}, J=7/2 and 3_{31}, J =7/2←4_{13}, J=9/2—have been observed by microwave optical double resonance. The transitions are magnetic dipole allowed as a result of the spin–rotation interaction mixing of 4_{13}, J=7/2 with 3_{31}, J=7/2. An analysis of the mixing when combined with the previously determined spin–rotation centrifugal distortion parameters allows the determination of five of the six linearly independent spin–rotation centrifugal distortion constants. The resulting constants (in MHz) are η_{ a a a a }=30.7(6), η_{ b b b b }=0.624(14), η_{ a a b b }=−2.7(14), η_{ b b a a }=−13.9(14), and (η_{ a b a b }+η_{ a b b a })/2=3.4(2), where the quoted error in parentheses is three standard deviations.