Electron diffraction patterns of the fullerene C(60) in the gaseous state have been obtained by volatilizing it from a newly designed oven-nozzle at 730 degrees 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 r(g)(C(1)-C(2)) = 1.458(6) angstroms (A) for the thermal average bond length within the five-member ring (that is, for the bond fusing five- and six-member rings) and r(g)(C(1)-C(6)) = 1.401(10) A 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) A and 0.057(6) A, respectively. The diameter of the icosahedral sphere is 7.113(10) A. The uncertainties in parentheses are estimated 2sigma values.

No abstract

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 Å (0.0032), S–F(eq) = 1.552 Å (0.0043), S–F(ax) = 1.575 Å (0.0038), F(eq)···F(eq) = 2.545 Å (0.0259), F(ax)···O = 2.121 Å (0.0073), F(eq)···O = 2.621 Å (0.0139), F(ax)···F(eq) = 2.204 Å (0.0050), F(ax)··· = 3.150 Å (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 = 0.0367Å (0.0050), lS–F(eq) = lS–F(ax) = 0.0540Å (0.0029), lF(eq)...F(eq) = 0.0468Å (0.0124), lF(ax)...O = 0.0431Å (0.0055), lF(eq)...O = 0.0758Å (0.0192), lF(ax)...F(eq) = 0.0962Å (0.0074), and lF(ax)...F(ax) = 0.0611Å (0.0122).

Electron-diffraction patterns of the fullerene C 70 in the gaseous state at 810-835 °C have been recorded in experiments similar to those recently described for C 60 . The radial distribution curve calculated from the scattered intensity is entirely consistent with a molecule of D 5h symmetry. With assumption of this symmetry, 12 parameters are required to specify the structure. Reliable values are thus much more difficult to obtain for these parameters than for C 60 whose structure is completely defined by only two parameters. Six different models were found that give excellent fits to the diffraction data. The models may be divided into two types characterized either by a shorter (1.4+ Å) or a longer (1.5+ Å) equatorial bond. Despite this difference, however, the aVerage length of the eight bonds is similar in all models (1.434 Å; average deviation 0.006 Å). Since no model could be favored on the basis of the electron-diffraction data alone, a best model was selected from considerations of theoretical energies (BP86/TZP level of density functional theory) and by comparison of computed 13 C NMR chemical shifts (gaugeincluding atomic orbitals, GIAO-SCF/TZP) with those from experiment. This model is in good agreement with structures determined in the crystal by neutron and X-ray diffraction, and with ab initio calculated structures (BP86/ TZP), with one important difference: the equatorial bond is some 0.06 Å longer. Based on assumed D 5h symmetry, and designating the five circles of atoms from the top (capping) pentagon to the equator as a, b, c, d, and e, the bond lengths (r a /Å) are as follows: r(a-a) ) 1.461(8), r(a-b) ) 1.388(16), r(b-c) ) 1.453(11), r(c-c) ) 1.386(25), r(c-d) ) 1.468(11), r(d-d) ) 1.425(14), r(d-e) ) 1.405(13), r(e-e) ) 1.538(19). The equatorial diameter of the ellipsoid is 7.178(50) Å, and the distance between the polar pentagons is 7.906(64) Å; quantities in parentheses are 2 esd.

The structure and conformational properties of oxalyl chloride, which experiences internal rotation about the C-C bond, have been reinvestigated by electron diffraction from the gas at 0,80, and 190 "C and by extensive ab initio calculations. Complete structure optimizations at a very high level (MP2/TZ2P, 166 basis functions) revealed, in addition to the anti form at LClCCCl = 180", a second stable form (gauche) with LClCCCl = 89.8" characterized by a very shallow minimum in the energy; earlier theoretical results for oxalyl chloride had been inconsistent with the existence of a second form known from experiment to be present. The electron diffraction analysis was based on dynamic models that comprised a set of pseudoconformers spaced at regular intervals around the torsional coordinate @ = LClCCCl and Boltzmann weighted according to a three-term torsional potential V(@) = I/2ClV1[ 1 -cos i ( 180 -@)I. For the more elaborate model results from the ab initio calculations were incorporated in the form of distance and angle differences among the pseudoconformers; in a second, simpler model these differences were omitted so that the structures of the pseudoconformers differed only in their torsion angles. A theoretical force field for the anti form was also evaluated ab initio, scaled to fit the observed wavenumbers, and used in each model to calculate the usual corrections for vibrational averaging. The results for the analysis of the 0 "C data for the more elaborate (preferred) model are as follows (rg/& Lddeg with 2 0 uncertainty estimates): r(C=O) = 1.184(2), r(C-C) = 1.548(8), r(C-C) = 1.749(3), LCCO = 123.8(4), LCCCl = 111.8(3), and LClCCCl,,,t,, = 76(18) where 0" corresponds to cis; results at the other temperatures are similar. Values of the potential constants, which should be temperature independent, are found in the ranges (kcaymol) 1.45 I Vi I 1.99, -0.40 d V2 d 0.03, and 0.43 I V3 I 1.05; the average values are V I = 1.59(83), V2 = -0.1 1(38), and V, = 0.74(39). The estimated mole fractions of the anti form at 0, 80, and 190 "C are 0.67, 0.62, and 0.43, from which the internal energy difference AUo = is calculated to be 0.75(50) kcal mol-' and the entropy difference ASo = $ + R In 2 --to be 1.31(148) cal mol-' K-I. The simpler model gives similar results.

The molecular structures of WF6, ReF6, OsF6, IrF6, and PtF6 have been measured by electron diffraction from the gases, the last from both PtF6 itself and from a vapor assumed to consist of a mixture of O2 and PtF6 obtained by heating the salt O2PtF6. For models of Oh symmetry the bond lengths in the first three members of the series are essentially identical, but the Ir-F and Pt-F bonds are respectively about 0.01 and 0.02 A longer. Models of D4h symmetry were also tested for ReF6, OsF6, and IrF6 in which operation of the Jahn-Teller effect is thought possible. For these models the same trend was seen in the average bond-length values. The effect of three-atom multiple scattering was also investigated, and experimental estimates of the effects of vibrational averaging ("shrinkage") on the distances were obtained. Normal-coordinate analyses based on the observed wavenumbers yielded stretching force constants consistent with the usual inverse bond-length/force-constant relationship. Ab initio molecular orbital optimizations of the molecules constrained to Oh symmetry were carried out at several levels of theory and basis-set size. Less extensive optimizations of ReF6, OsF6, and IrF6 with D4h symmetry were also carried out. The best overall agreement with both the experimental values and the distance trend for Oh symmetry was obtained with the Hay-Wadt (n+1)VDZ basis on the metals and the aug-cc-pVTZ on the fluorines at the MP2 level, but these bases with B3P86 and B3PW91 density functional theory were nearly as good and with B3LYP only slightly worse. The D4h structures for ReF6, OsF6, and IrF6 with the cited bases at the B3P86 level were slightly more stable (respectively 0.8, 2.6, and 1.4 kcal/mol) with the axial bonds shorter by about 0.04 A in ReF6 and 0.07 A in OsF6, but about 0.05 A longer in IrF6. The significance of these values is uncertain. The experimental bond lengths (rg/A) with estimated 2sigma uncertainties for the models of Oh symmetry are W-F = 1.829(2), Re-F = 1.829(2), Os-F = 1.828(2), Ir-F = 1.839(2), and Pt-F = 1.852(2); the Pt-F value from the O2PtF6 sample was 1.851(2) A. Although the experimental data neither confirm nor refute the existence of the Jahn-Teller effect in ReF6, OsF6, and IrF6, they ensure that if present the distortion from Oh symmetry must be small.

Gas-phase electron-diffraction (GED) data together with results from ab initio molecular orbital and normal coordinate calculations have been used to determine the structures of the aluminum trihalides AlX3 (X = Cl, Br, I) and the chloride and bromide dimers Al2Cl6 and Al2Br6. No monomeric species were detected in the vapors of Al2Cl6 at the experimental temperature of 150 °C, nor in Al2Br6 at167 °C, but the vapors of AlCl3 at 400 °C and AlBr3 at 330 °C contained respectively 29 (3)% and 7 (4)% dimer and the AlI3 at 300 °C about 8% I2. The known equilibrium symmetry of the dimers is D 2 h , but the molecules have a very low-frequency, large-amplitude, ring-puckering mode that lowers the thermal average symmetry to C 2 v . The effect of this large-amplitude mode on the interatomic distances was handled by dynamic models of the structures which consisted of a set of pseudoconformers spaced at even intervals along the ring-puckering angle 2Φ. The ring-puckering potential was assumed to be V(Φ) = V 4 0Φ4 + V 2 0Φ2, and the individual pseudoconformers were given Boltzmann weights. The structures were defined in terms of the geometrically consistent r α space constraining the differences between corresponding bond distances and bond angles in the different pseudoconformers to values obtained from ab initio calculations at the HF/6-311G(d) level. Results for the principal distances (r g/Å), angles (∠α,θ/deg), and potential constants (V i 0/kcal mol deg-1) from the combined GED/ab initio study for Al2Cl6/Al2Br6 with estimated 2σ uncertainties are Al−Xb = 2.250(3)/2.433(7), Al−Xt = 2.061(2)/2.234(4), XbAlXb = 90.0(8)/91.6(6), XtAlXt = 122.1(31)/122.1(31), 〈θ〉 = 180 − 2Φ = 165.5(59)/158.2(91), V 4 0 = 0.0/75.0 (assumed), V 2 0 = 25.0/0.0 (assumed). The potential constants could not be refined; although the single-term values listed provide good fits, in each case combinations of quadratic and quartic terms also worked well. For the monomers AlCl3, AlBr3, and AlI3 (D 3 h symmetry assumed in r α space) the distances (r g/Å) with estimated 2σ uncertainties are Al−Cl = 2.062(3), Al−Br = 2.221(3), and Al−I = 2.459(5) Å. Vibrational force fields were evaluated for all molecules. The experimental, theoretical, and vibrational results are discussed.

Schmidt Structure und Bonding in Transition‐Metal Carbonyls and Nitrosyls.