On the molecular structures of the prevailing anti conformers of gaseous cis-1 and trans-1-chloro-1,3-butadiene

Abstract

The molecular structures of the title compounds have been investigated by ab initio MO calculations and by analyses of room-temperature electron diffraction data alone, as well as by conjoint use of these data and literature ground-state rotational constants of eleven isotopic species. Models for the structures were specified in terms of geometrically consistent r α-parameters under the assumption of C s molecular symmetry. Literature molecular force fields were used to calculate vibrational amplitude parameter quantities needed for vibrational corrections and for making the three methods compatible. Results of the computed structures were incorporated into the structure analyses so as to reduce the geometry-defining variables for the hydrogen positions from ten to four, and ultimately to constrain the difference parameter Δ a(CC) = r a(C 1C 2) - r a (C 3C 4), to 0.37 and 0.40 pm respectively for the cis and trans isomer. The final structures obtained by conjoint consideration of computational, electron diffraction and spectroscopic evidence then have the following principal distances ( r a in pm) and angle (∠C α in degrees) with estimated standard deviations in parentheses: the cis isomer, r(C 1C 2) = 134.2(2), r(C 3C 4) = 134.5(2), r(C 2C 3) = 146.6(3), r(C-Cl) = 173.0(2), ∠C 1C 2C 3 = 125.5(2), ∠C 4C 3C 2 = 122.3(4), ∠C 2C 1Cl = 123.9(2); the trans isomer, r(C 1C 2) = 134.0(2), r(C 3C 4) = 134.4(2), r(C 2C 3) = 146.1(3), r(CCl) = 172.8(2), ∠C 1C 2C 3 = 122.5(4), ∠C 4C 3C 2 = 123.3(3), ∠C 2C 1Cl = 122.5(2). It is shown that the computational results and the GED structures are not in conflict with the rotational constants used to obtain the literature r s-structures which are however not in good agreement with the present results.