The molecular structure and conformational behaviour of 2-chloro-1,3-butadiene (chloroprene) studied by gas-phase electron diffraction and ab initio MO calculations

• Published in: Journal of molecular structure 1991-03, Vol.243 (3), p.385-409
• Main Authors: Gundersen, Grete, Thomassen, Hanne G., Boggs, James E., Peng, Chunyang
• Format: Article
• Language: English
• Subjects: Atomic and molecular physics; Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics); Exact sciences and technology; Molecular properties and interactions with photons; Physics; Properties of molecules and molecular ions
• ISSN: 0022-2860; 1872-8014
• DOI: 10.1016/0022-2860(91)87053-K

MO calculations establish a global minimum for chloroprene at the planar anti conformation (θ 1 = 180°). A subsidiary minimum appears at θ 2 = 40°, 8.3 kJ mol −1 above anti and 4.2 kJ mol −1 below planar syn with an rotational barrier of 20 kJ mol −1 at about 105°. Analyses of GED data for chloroprene at 655,565, and 298 K gave, in addition to a planar anti conformer (θ 1 = 180°), 25 (6), 18 (5), and 7(4)%, respectively, of a co-existing second form with θ 2=27(11)°. These quantities correspond to Δ E= E 2- E 1=6.4(12) kJ mol −1 and Δ S c = S 2– S 1 = − 6(3) J K −1 mol −1. The value of θ 2 was determined from the 655 -K data using a flexible backbone structure upon rotation as extracted from the MO results. Rigid rotation gave θ 2=41 (8)°, Δ E= 6.8(13) kJ mol −1, and Δ S c= -5(3) J K −1 mol −1. Some MO constraints were used in defining the positions of hydrogen atoms and for a difference r(C1C2) - r (C3C4) parameter. Important geometrical parameters ( r a,∠ α) of anti chloroprene as obtained in such refinements based on 298-K GED data and literature rotational constants are: r(C1C2)=134.4(2), r(C3C4)=134.4(2), r(CC)=146.9(3) and r(CCl)=174.2(2) pm; ∠CC2C = 123.5(1), ∠CC3C = 125.6 (2) and ∠CCCl = 117.2(1)°. Refinements based on the GED data alone gave the following changed parameters: r(CC)=146.0(3) pm; ∠CC2C = 123.9 (2), ∠CC3C = 126.2 (2)°.