Kenneth Hedberg

Bond lengths in free molecules of buckminsterfullerene, c60, from gas-phase electron diffraction.

Electron diffraction patterns of the fullerene C60 in the gaseous state have been obtained by volatilizing it from a newly designed oven-nozzle at 730�C. The many peaks of the experimental radial distribution curve calculated from the scattered intensity are completely consistent with icosahedral symmetry for the free molecule. On the basis of this symmetry assumption, least-squares refinement of a model incorporating all possible interatomic distances led to the values rg(C1-C2) = 1.458(6) angstroms (�) for the thermal average bond length within the five-member ring (that is, for the bond fusing five- and six-member rings) and rg(C1-C6) = 1.401(10) � for that connecting five-member rings (the bond fusing six-member rings). The weighted average of the two bond lengths and the difference between them are the values 1.439(2) � and 0.057(6) �, respectively. The diameter of the icosahedral sphere is 7.113(10) �. The uncertainties in parentheses are estimated 2σ values.

Molecular Structure of Thionyltetrafluoride, SOF4

A gaseous electron‐diffraction investigation of SOF4 has led to the discovery of four models in excellent agreement with experiment, all with molecular symmetry C2υ corresponding to replacement of an equatorial fluorine atom of a trigonal bipyramid with oxygen. The models differ largely in the relative magnitudes of the F(eq)···F(eq) and F(eq)···O distances and the F(ax)···F(eq) and F(ax)···O distances. The favored model has the following distance (ra), angle, and root‐mean‐square amplitude (la) values (parenthesized errors are 2σ): S=O=1.403 A (0.0032), S–F(eq) = 1.552 A (0.0043), S–F(ax) = 1.575 A (0.0038), F(eq)···F(eq) = 2.545 A (0.0259), F(ax)···O = 2.121 A (0.0073), F(eq)···O = 2.621 A (0.0139), F(ax)···F(eq) = 2.204 A (0.0050), F(ax)··· = 3.150 A (0.0076), ∠F(eq)SF(eq) = 110.17°(1.82), ∠F(ax)SO = 90.65°(0.42), ∠F(ax)SF(eq) = 89.63°(0.24), ∠F(eq)SO = 124.91°(0.92), lS = O = 0.0367A (0.0050), lS–F(eq) = lS–F(ax) = 0.0540A (0.0029), lF(eq)...F(eq) = 0.0468A (0.0124), lF(ax)...O = 0.0431A (0.0055), lF(...

Reinvestigation of the Molecular Structure of 1,3,5,7‐Cyclooctatetraene by Electron Diffraction

The molecular structure of 1,3,5,7‐cyclooctatetraene has been studied by a sector‐microphotometer technique using data extending to very much larger scattering angles than were obtained in earlier investigations. An application of the method of least squares to sector‐microphotometer data in electron diffraction worked out by one of us (KH) has led in three refinement stages to unusually precise values for the parameters. The following are the more interesting parameter values with standard errors. It should be noted these results do not include a possible error of up to 0.2% in the scale of the molecule because of uncertainties in the electron wavelength, nor do they include the effect of correlations among the observations on the standard errors, which we estimate might increase the standard errors by as much as the factor 2½. Molecular symmetry D2d,C=C=1.334±0.001 A,C–C=1.462±0.001 A,C–H=1.090±0.005 A,∠C=C–C=126.46∘±0.23∘,∠C=C–H=118.3∘±5.9∘,aC=C(=〈δl2〉C=C/2)=(103±7)×10−5,aC–C=(147±10)×10−5,aC1···C3=(25...

Effect of Temperature on the Structure of Gaseous Molecules. Molecular Structure of PCl3 at 300° and 505°K

The molecular structure of PCl3 has been studied by electron diffraction from the vapor at nozzle temperatures of 300° and 505°K. Both the size of the molecule and the displacement of the atoms during molecular vibration are found to be significantly greater at the higher temperature: The P–Cl and Cl···Cl distances are longer by 0.3 and 0.4%, respectively, and the rms amplitudes associated with these distances are greater by a striking 19 and 31%. Both the increased size of the molecule and increased amplitudes are in very good agreement with prediction from simple theory. The results at 300° and 505°K, respectively, are rP‐Cl=2.039±0.0014 A and 2.045±0.0016 A; rCl···Cl=3.130±0.0026 A and 3.142±0.0038 A; <Cl–P–Cl=100.27±0.09° and 100.40±0.16°; lP‐Cl=0.0501±0.0013 A and 0.0594±0.0017 A; lCl···Cl=0.0834±0.0023 A and 0.1097±0.0035 A. The interatomic distances given are ra values obtained from the third cycle of least‐squares refinement of intensity curves and differ from the equilibrium values re by small am...

Nickel tetracarbonyl, Ni(CO)4. I. Molecular structure by gaseous electron diffraction. II. Refinement of quadratic force field

The molecular structure of gaseous nickel tetracarbonyl has been investigated by electron diffraction at room temperature. The analysis, based on an assumed Td molecular symmetry with corrections for the effects of vibrational motion, led to the following bond distances (rg) and amplitudes of vibration (l) with estimated uncertainties (2σ), all in angstroms: r (C=O) =1.141(2), r (Ni–C) =1.838(2), l (C=O) =0.039(2), l (Ni–C) =0.059(3), l (Ni⋅⋅⋅O) =0.061(3), l (C⋅⋅⋅C) =0.124(17), l (C⋅⋅⋅O) =0.176(11) and l (O⋅⋅⋅O) =0.244(34). The quadratic force field was refined using our structure and vibrational wave numbers from the literature in order to permit calculation of the distance corrections arising from vibrational averaging. The force constants are generally very similar to those from a previous spectroscopic study.

Reinvestigation of the Structure of Dinitrogen Tetroxide, N2O4, by Gaseous Electron Diffraction

A reinvestigation of the structure of N2O4 in the gas phase at −21oC has given results in good agreement with an earlier study so far as the molecular shape is concerned, but the size of the molecule appears to be about 0.9% larger than originally thought. The results for the coplanar (D2h symmetry) model are: rNN = 1.782 A (0.0083), rNO = 1.190 A (0.0018), ∠ ONO = 135.4o (0.58),  lNN = 0.0816  A  (0.0178), lNO = 0.0381 A (0.0019), lo1o2 = 0.0493 A (0.0040), lN···O = 0.0729 A (0.0061), lo1o1′ = 0.0970 A (0.0167), and lo1o2′ = 0.0730 A (0.0114). These distances and root‐mean‐square amplitudes are ra and la values; the parenthesized values are 2σ and include estimates of systematic error. The −NO2 groups have a structure very similar to that of NO2 itself. The very long N–N bond and its large amplitude of vibration imply a weak link and are in accord with the low dissociation energy.

Reinvestigation of the molecular structure of gaseous p‐benzoquinone by electron diffraction

The molecular structure of gaseous p‐benzoquinone has been studied by electron diffraction at a nozzle‐tip temperature of 110–125°C. The molecule has D2h symmetry to within experimental error. The results for the more important distance (ra), bond angle, and rms amplitude (l) parameters are rC–H=1.089 A (0.011), rC=O=1.225 A (0.002), rC=C=1.344 A (0.003), rC–C=1.481 A (0.002), ∠C2C1C6=118.1° (0.3), ∠C3C2H=121.4° (assumed), lC–H=0.077 A (assumed), lC=O=0.0424 A (0.0020), lC=C=0.0446 A (0.0024), and lC–C=0.0546 A (0.0024). These values are in good agreement with those found in an early electron‐diffraction study by Swingle, and except for the length of the carbon‐carbon double bond, in excellent agreement with those found in the crystal by Trotter.

Molecular Structure of Dicyclopentadienylnickel (C5H5)2Ni

The structure of gaseous nickelocene has been determined at 110°C by electron diffraction using two independent sets of data. The molecule has the familiar ferrocenelike sandwich shape, but it differs from ferrocene in having weaker metal–carbon bonds and greater amplitudes of vibration for all distances affected by motion of the cyclopentadienyl rings as units. The simplest interpretation of the large inter‐ring amplitudes leads to the conclusion that the rings are rotating much more freely than in ferrocene. There is some slight evidence that the Ni–C bonds may not all be equivalent, corresponding to a structure that is not strictly symmetric (D2h or D2d), or alternatively that the molecular motions affecting the Ni–C bonds are unusual in a way which does not give rise to the expected essentially harmonic Ni–C amplitudes, either of which suggests the faint possibility of a Jahn–Teller effect. Based upon a symmetric sandwich conformation with freely rotating rings the principal distance (ra) and mean amp...

Effect of Temperature on the Structure of Gaseous Molecules. II. An Electron‐Diffraction Investigation of the Molecular Structures of B2Cl4 and SiCl4. The Potential Function for Internal Rotation in B2Cl4

Electron‐diffraction data for B2Cl4 with SiCl4 present as an impurity have been gathered at nozzle temperatures of 251°, 295°, 331°, 376°, and 423°K. Complete structural analyses of both molecules were carried out at the lowest and the highest temperatures by least squares based upon intensity curves, but converged results were obtained only at the lowest. Based upon molecular symmetry D2d for B2Cl4, the results (parenthesized values are 2σ) at 251°K are as follows: B2Cl4:r(B–Cl) = 1.750 A (0.0106), r(B–B) = 1.702 A (0.0692), r(B2···Cl2) = 3.000 A (0.0494), r(Cl1···Cl2) = 3.011 A (0.0081), r(Cl2···Cl4) = 4.087 A (0.0400), ∠ Cl1BCl2 = 118.65° (0.66), l(B–Cl) = 0.0562 A (0.0081), l(B–B) = 0.0500 A (assumed), l(B2···Cl2) = 0.1223 A (0.1022), l(Cl1···Cl2) = 0.0616 A (0.0080);SiCl4:r(Si–Cl) = 2.019 A (0.0088), r(Cl···Cl) = 3.299 A (0.0241), l(Si–Cl) = 0.0539 A (0.0122), l(Cl···Cl) = 0.0732 A (0.0185. The ra and la are, respectively, inter‐atomic distances and root‐mean‐square amplitudes of vibration; they diff...

Centrifugal Distortion of Bond Distances and Bond Angles

A simple method has been developed to estimate the changes of bond distances and bond angles produced by the centrifugal force arising from rotational motions of molecules. The method has been applied to the PCl3 molecule and the predicted effect found to be small but significant in comparison to the effect of vibrational motion. The theoretical results for PCl3 at 300° and 505°K are in good agreement with experimental results from gaseous electron diffraction.