Comparison of Molecular Structures Determined by Electron Diffraction and Spectroscopy. Ethane and Diborane

TLDR

The study used literature electron‑diffraction and spectroscopic data, converting them into average structures, and enhanced diborane diffraction resolution by incorporating calculated B–H mean amplitudes. Gas‑phase average structures of ethane and diborane were determined, showing that electron‑diffraction yields more precise internuclear distances than rotational‑constant‑derived structures, while spectroscopy provides more accurate moments of inertia, and the complementary use of both methods clarifies the relative merits of rg and rz representations.

Abstract

Gas-phase average structures for the ground-vibrational state (rz) for ethane and diborane have been determined by a critical comparison of the experimental results obtained from electron diffraction (average internuclear distances rg) and those obtained from high-resolution infrared and Raman spectroscopy (rotational constants Bz(α)). Experimental values have been taken from the recent literature and converted into the average structure (rzor rα0). The rg and rα0 distances determined from electron diffraction carry uncertainties less than those in the rz distances determined from rotational constants, because the latter structures are very sensitive to assumptions about the unknown isotope differences in the structures. On the other hand, the average moments of inertia from spectroscopy are much more precise than those calculated from diffraction internuclear distances. Examinations of the data have led to the following rz structures with standard errors: For C2H6, rz(C–H) = 1.0957 ± 0.002 Å, rz(C–C) = 1.5319 ± 0.002 Å, and ∠C–C–H = 111.5° ± 0.3°; for C2D6, rz(C–D) = 1.0941 ± 0.002 Å, rz(C–C) = 1.5300 ± 0.002 Å, and ∠C–C–D = 111.4° ± 0.3°; and for B2H6, rz(B–Ht) = 1.192 ± 0.01 Å, rz(B–Hb) = 1.329 ± 0.005 Å, rz(B–B) = 1.770 ± 0.005 Å, ∠Ht–B–Ht = 121.8° ± 3°, and ∠Hb–B–Hb = 96.5° ± 0.5°. It was possible to increase the resolving power of the diffraction analysis of diborane by inclusion of calculated B–H mean amplitudes. The effective complementary use of electron-diffraction and spectroscopic data for determining reliable gas-phase structures and the relative merits of the two alternative representations of the average structure (rgand rz) have been discussed.