CONFORMATIONAL-ANALYSIS .14. THE DIHALOETHANES CLCH2CH2F, BRCH2CH2F, AND BRCH2CH2CL - INVESTIGATIONS OF THE MOLECULAR-STRUCTURES, ROTAMERIC COMPOSITIONS, ANTI AND GAUCHE ENERGY AND ENTROPY DIFFERENCES, AND BARRIERS HINDERING INTERNAL-ROTATION BY GAS-PHASE ELECTRON-DIFFRACTION AUGMENTED BY ROTATIONAL-CONSTANTS AND ABINITIO CALCULATION

The structures and conformational compositions of 1 -chloro-2-fluoroethane (CFE), 1 -bromo-2-fluoroethane (BFE), and 1 -bromo-2-chloroethane (BCE) have been investigated in the gas phase by electron diffraction, each at three nozzle-tip temperatures: CFE, -25, 95, 360 °C; BFE, 20, 150, 350 °C; and BCE, 17, 125, and 302 °C. The structure analyses were aided by inclusion of experimental values of rotational constants for CFE and BFE taken from the literature and by ab initio geometry optimizations at the HF/3-21G⋆ level (CFE) and HF/STO-3G⋆ level (BFE and BCE). Each molecule exists as a mixture of anti and gauche rotamers with the former the more stable; the standard internal energy difference is least for CFE and greatest for BCE. Three types of model that take account of torsional motion in different ways gave essentially the same values for bond distances and bond angles. One of these yielded refined values, with estimated 2σ uncertainties, of potential constants for the three-term function 2V = ΣiVi(1-cosi) for CFE, BFE, and BCE, respectively: V1 = 1.66 (108), 1.68 (116), and 4.00 (190) kcal/mol; V2 = -0.96 (90), -0.57 (114), and -1.85 (150) kcal/mol; V3 = 3.55 (64), 3.08 (134), and 3.56 (106) kcal/mol. Distances (rg/Å) and angles (∠α/deg) averaged over the three temperatures, with estimated 2σ uncertainties, are as follows: CFE, r(C-H) = 1.107 (7), r(C-C) = 1.504 (6), r(C-F) = 1.391 (3), r(C-Cl) = 1.784 (3), 〈∠CCH〉 = 110.6 (19), ∠CCFA = 108.5 (11), ∠CCFG = 111.1 (4), ∠CCClA = 109.3 (11), ∠CCClG = 111.5 (4), ∠FCCClG = 111.9 (16), (∠FCCC1A = 0); BFE, r(C-H) = 1.084 (8), r(C-C) = 1.509 (7), r(C-F) = 1.393 (4), r(C-Br) = 1.937 (4), ∠CCH = 110.0 (19), ∠CCFA= 107.6 (8), ∠CCFG = 110.1 (6), ∠CCBrA = 109.9 (7), ∠CCBrG = 112.0 (5), ∠FCCBrG = 112.2 (53); BCE, r(C-H) = 1.090 (8), r(C-C) = 1.515 (5), r(C-Cl) = 1.789 (5), r(C-Br) = 1.947 (6), ∠CCH = 111.2 (16),∠CCC1A = 109.4 (5), ∠CCClg = 111.9(24), ∠CCBrA= 109.9 (5), ∠CCBrG = 112.6 (24), ∠ClCCBrG = 108.6 (30). The structures and thermodynamic properties are discussed. © 1990, American Chemical Society. All rights reserved.