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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 Å (0.0083), rNO = 1.190 Å (0.0018), ∠ ONO = 135.4o (0.58),  lNN = 0.0816  Å  (0.0178), lNO = 0.0381 Å (0.0019), lo1o2 = 0.0493 Å (0.0040), lN···O = 0.0729 Å (0.0061), lo1o1′ = 0.0970 Å (0.0167), and lo1o2′ = 0.0730 Å (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.

The structure of iron pentacarbonyl, Fe(CO)(5), was reinvestigated by gas-phase electron diffraction using an experimental rotational constant available from the literature as a constraint on the structural parameters. The study utilized a B3LYP/6-311+G(d) ab initio quadratic force field, scaled to fit observed infrared wavenumbers, from which were calculated corrections for the effects of vibrational averaging on distances and certain other quantities useful for the structural analysis. The results confirm that the equatorial Fe-C bonds are longer than the axial ones, an important difference with the structure in the crystal where the equatorial Fe-C bonds are the shorter. Some distance (r(g)/A) and vibrational amplitude (l(alpha)/A) parameter values with estimated 2sigma uncertainties based on assumption of D(3h) symmetry are [r(Fe-C)] = 1.829(2), r(Fe-C)(eq) - r(Fe-C)(ax) = 0.032(20), [r(C=O)] = 1.146(2), r(C=O)(eq) - r(C=O)(ax) = 0.006(27), r(Fe-C)(ax) = 1.810(16), r(Fe-C)(eq) = 1.842(11), r(C=O)(ax) = 1.142(23), r(C=O)(eq) = 1.149(16), l(Fe-C)(ax) = l(Fe-C)(eq) = 0.047(5), and l(C=O)(ax) = l(C=O)(eq) = 0.036(3).

Abstract. C15HzxA106; orthorhombic, Pna21, a = 15.699 (3), b=32.546 (7), c= 13.369 (2) A; Z=16; Din= 1"25 (1) (by flotation), Dx= 1.261 g cm -a. Pale yellow transparent eight-sided prisms were obtained from commercially available material (Aldrich Chemical Co. Inc.). The c~ and ~, forms differ primarily in the relative orientations of the molecules. The average molecular structure is not significantly different from that observed in the ~ form.

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