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

Abstract

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 Å (0.0106), r(B–B) = 1.702 Å (0.0692), r(B2···Cl2) = 3.000 Å (0.0494), r(Cl1···Cl2) = 3.011 Å (0.0081), r(Cl2···Cl4) = 4.087 Å (0.0400), ∠ Cl1BCl2 = 118.65° (0.66), l(B–Cl) = 0.0562 Å (0.0081), l(B–B) = 0.0500 Å (assumed), l(B2···Cl2) = 0.1223 Å (0.1022), l(Cl1···Cl2) = 0.0616 Å (0.0080);SiCl4:r(Si–Cl) = 2.019 Å (0.0088), r(Cl···Cl) = 3.299 Å (0.0241), l(Si–Cl) = 0.0539 Å (0.0122), l(Cl···Cl) = 0.0732 Å (0.0185. The ra and la are, respectively, inter-atomic distances and root-mean-square amplitudes of vibration; they differ slightly from the equilibrium values re and le. The form of the potential hindering internal rotation in B2Cl4 was investigated by comparing the experimental distribution of rotation-sensitive distances at each of the five temperatures with theoretical distributions calculated from the functions V = 12V0(1 − cos2θ) and V = V0θ2. In each case the cosine function gave better agreement. The barrier (V0) averaged for the five temperatures has the value 1.85 kcal/mole for the cosine function.