Lawrence S. Bartell

Influence of Nonbonded Interactions on Molecular Geometry and Energy: Calculations for Hydrocarbons Based on Urey—Bradley Field

A modified Urey—Bradley potential energy function comprised of quadratic terms for bond stretches, bond‐angle bends, and torsional displacements together with analytical expressions for pairwise nonbonded interactions was chosen to represent the force field for hydrocarbon molecules. Quadratic constants were taken from the spectroscopic U–B analyses of Schachtschneider and Snyder [Spectrochim. Acta 19, 117 (1963)], while the nonbonded functions adopted were those proposed by Bartell [J. Chem. Phys. 32, 827 (1960)]. Reference bond angles for the quadratic terms were taken to be 109.5° or 120° for tetrahedral or trigonal coordination, respectively. Reference single‐bond lengths and the torsional constant were adjusted to fit the experimental data for CH4 and C2H6. Double bonds and ring bonds in cyclopropyl compounds were considered to be rigid. The above selections served to establish a universal model force field for hydrocarbons with no remaining adjustable parameters. The potential energy functions for a...

Refined Procedure for Analysis of Electron Diffraction Data and Its Application to CCl4

A refined procedure for obtaining the structure of free molecules from electron diffraction data is described which compensates for the interference arising from non‐nuclear scattering. The procedure is applied to CCl4 using somewhat more extensive rotating sector data than has hitherto been published for this molecule. Estimates are made for the first time in electron diffraction results of the effect of anharmonicity of vibration on the measurement of internuclear distance and of the effect of the failure of the Born approximation on the measurement of amplitudes of vibration. A method of estimating the reliability of the results is described.

Molecular Structure of XeF6. II. Internal Motion and Mean Geometry Deduced by Electron Diffraction

The distribution of internuclear distances in gaseous XeF6 exhibits unusually diffuse XeF6 bonded and F–F geminal nonbonded peaks, the latter of which is severely skewed. The distribution proves the molecule cannot be a regular octahedron vibrating in independent normal modes. The instantaneous molecular configurations encountered by the incident electrons are predominantly in the broad vicinity of C3υ structures conveniently described as distorted octahedra in which the xenon lone pair avoids the bonding pairs. In these distorted structures the XeF bond lengths are distributed over a range of approximately 0.08 A with the longer bonds tending to be those adjacent to the avoided region of the coordination sphere. Fluorines suffer angular displacements from octahedral sites which range up to 5° or 10° in the vicinity of the avoided region.Alternative interpretations of the diffraction data are developed in detail, ranging from models of statically deformed molecules to those of dynamically inverting molecu...

Electron diffraction study of gaseous tetrahydrofuran

Ah&met-The gas phase molecular structure of tetrahydrofuran has been reinvestigated. Experimental diffraction intensities are well accounted for by a freely pseudo-rotating molecule. The mean value of the pseudo-rotational puckering coordinate was found to be qr = @38 f W2 A, a value slightly lower than the spectroscopic q0 value reported in a far IR study. A definite distinction between free pseudo-rotation and the presence of one or more static puckered conformations could not be made from the electron diffraction intensities. Mean bond lengths and amplitudes of skeletal vibrations were determined and found to be normal.

Electron‐Diffraction Structural Study of Polymeric Gaseous Hydrogen Fluoride

Electron diffraction patterns with molecular interference features extending beyond s = 35 A−1 were obtained for gaseous self‐associated HF introduced under its own vapor pressure at −19° and at +22°C through a conventional nozzle system into a 40‐kV electron beam. The diffraction patterns and their dependence on temperature are best explained with the hypothesis that the monomer and a puckered, cyclic hexamer were the only appreciable constituents of the scattering vapors. Mean FFF angles in the hexamer were found to be only about 104°, in contrast with the 120° angles reported for the infinite planar zigzag chains in crystalline HF at −125°C. This pucker may simply be a consequence of thermal bending of the extremely flexible ring. Indeed, the experimental radial distribution function is so smeared out by large ring‐bending amplitudes that the data cannot distinguish between boatlike and chair conformations. It is likely that the free (HF)6 molecules sweep randomly through both conformations in their th...

Molecular structure of n-butane: calculation of vibrational shrinkages and an electron diffraction re-investigation

Abstract A normal coordinate analysis was carried out based on the force field of Schachtschneider and Snyder in order to calculate all amplitudes of vibration and shrinkage corrections for n-butane. The results are tabulated to aid diffraction analyses of related substances. A vapor-phase electron diffraction reinvestigation of n-butane led to experimental measurements of the principal amplitudes of vibration and to the following molecular parameters (± 3σ ): rg(C-C) = 1.531(2)A, rg(C-H)= 1.117(5)A, ∠CCC (trans. gauche average) = 113.8(4)°, ∠CCH (ave) = 111.0(5)° , gauche CCCC dihedral angle 65(6)°, % trans conformer = 54 ± 9%, and ΔG° (gauche— trans) = 497 ± 220 cal mol−1.

Electron diffraction studies of supersonic Jets. IV. Conformational cooling of n-butane

Expansions through small tapered nozzles (∼10−2 cm inlet diameter) have produced conformational cooling of gas phase n‐butane to estimated conformational temperatures as low as 180 K. Relaxation into the lower energy trans form was seen with neat butane and with addition of up to ∼30% helium or neon. Thin plate nozzles of comparable diameter do not seem to produce the same effects, presumably because the more rapid cooling they bring about is accompanied by many fewer collisions. Conformational analyses carefully checked for and took into account butane cluster scattering, which if present and ignored, artificially increases the apparent trans mole fraction. At higher concentrations of monatomic carrier gas the cluster scattering becomes strong enough to interfere seriously with the determination of conformational composition. Analysis of the present data and a reanalysis of earlier, conventional, gas electron diffraction data both gave the room temperature trans mole fraction as 64% (3σ=9%) in agreement ...

Electron‐Diffraction Study of Ammonia and Deuteroammonia

The gas‐phase structures of NH3 and ND3 molecules were determined by the sector‐microphotometer method of electron diffraction. The following internuclear distances rg and mean amplitudes le with estimated standard errors were obtained: For NH3, rg(N − H) = 1.0302 ± 0.002 A, rg(H − H) = 1.662 ± 0.010 A, le(N − H) = 0.0731 ± 0.002 A, le(H − H) = 0.125 ± 0.006 A, and for ND3, rg(N − D) = 1.0266 ± 0.003 A, rg(D − D) = 1.654 ± 0.008 A, le(N − D) = 0.0611 ± 0.002 A, le(D − D) = 0.101 ± 0.006 A, with the parameter κ representing bond‐stretching anharmonicity fixed at 1.0 × 10−5 and 0.5 × 10−5 A3 for N–H and N–D, respectively. Effects of anharmonicity and isotope differences in the structural parameters analogous to those in CH4 and CD4 were observed. The rα0 and re bond distances calculated from the above rg distances are found to be consistent with the corresponding rz and re distances derived from the spectroscopic rotational constants of Benedict and Plyler. The isotope effects reported by Bell and by Halevi...

Structures of the strained molecules hexamethylethane and 1,1,2,2-tetramethylethane by gas-phase electron diffraction

Abstract Hexamethylethane has bond lengths of r g (C-C central) = 1.582 ± 0.01 A, r g (C-C terminal) = 1.542 ± 0.002 A, r g (C-H) = 1.113 ± 0.004 A, and bond angles of ∠C c C c C t = 111.0 ± 0.3° and ∠CCH = 111.5 ± 1.4° (uncertainties 2σ). It suffers a mean twist from D sd symmetry of 5 ± 4°. Tetramethylethane is approximately 60 % gauche , 40 % trans , in composition in the gas-phase with bond lengths r g (C-C central, trans ) = 1.544 ± 0.006 A, r g (C-C central, gauche )- r g (C-C central, trans ) = 0.002 A (assumed), r g (C-C terminal) = 1.539 ± 0.002 A, r g (C-H ave) = 1.115 ± 0.004 A, and angles are distributed around the average angle of 111.3 5 ± 0.4° in accord with a picture of steric interactions. The gauche conformer is twisted 65 ± 5° from the eclipsed configuration. Amplitudes of vibration were determined for both molecules. The structure of (CH 3 ) 3 BN( CH 3 ) 3 is considered in the light of (CH 3 ) 3 CC( CH 3 ) 3 results and it is concluded that the B-N length is intermediate between the values proposed by Lide and by Geller.

Electron Diffraction Study of Monomethyl‐ and Dimethylphosphine

The structural parameters of gaseous monomethyl‐ and dimethylphosphine were determined by the sector‐microphotometer method of electron diffraction. Center of gravity bond distances and standard errors for the two molecules were, respectively: rCP=1.858±0.003 A and 1.853±0.003 A; rCH=1.094±0.008 A and 1.097±0.007 A; rPH=1.423±0.007 A and 1.445±0.02 A. The angles P–C–H were 109.6±1° and 109.8±0.7°. In dimethylphosphine the angle C–P–C was 99.2±0.6°. The methyl groups were found to be in staggered conformations. The distances and root‐mean‐square amplitudes of vibration agreed well with the values determined in recent studies of phosphine and trimethylphosphine.