The molecular structure and conformation of trichloronitromethane as determined by gas-phase electron diffraction and theoretical calculations Q. Shen a, * , J.W. Brown a , A.D. Richardson b , K. Hagen c a Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, NY 13346, USA b Department of Chemistry, Oregon State University, Corvallis, OR 97331, USA c Department of Chemistry, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway Received 1 May 2006; received in revised form 3 August 2006; accepted 3 August 2006 Available online 18 September 2006 Abstract The molecular structure of trichloronitromethane has been studied in the gas phase using electron diffraction data. The molecules are found to undergo low barrier rotation about the CAN bond with a planar CANO 2 moiety in agreement with HF/MP2/B3LYP/6- 311G(d,p) calculations. The experimental data are consistent with a dynamic model using a potential function for the torsion of V =(V 6 /2)(1  cos 6s). The major geometrical parameters (r g and \ a ) for the eclipsed form, obtained from least squares analysis of the data are as follows: r(NAO 3 )= r(NAO 4 ) = 1.213(2) A ˚ , r(CAN) = 1.592(6) A ˚ , r(CACl) av = 1.749(1) A ˚ , \Cl 5 CN/\Cl 6 CN = 109. 6°/106.3°(2), \O 3 NC/\O 4 NC = 117. 6°/114.1°(4), sCl 5 C 1 N 2 O 3 = 0.0°, and V 6 = 0.20(25) kcal/mol. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Conformational analysis; Theoretical calculations; Torsional potential 1. Introduction The molecular structure of trichloronitromethane has been previously studied by electron diffraction [1] and the CANO 2 moiety was found to be non-planar by 15°. The threefold rotation barrier (V 3 ) about the CAN bond was estimated to be 3 kcal/mol. These results, however, are inconsistent with results obtained for nitromethane and trifluoronitromethane, where sixfold torsional potential functions with low barriers have been observed, V ¼ðV 6 =2Þð1  cos 6sÞ ð1Þ The V 6 values for nitromethane [2] and trifluoronitrome- thane [3] are 6.03(3) · 10 3 and 74.4(5) · 10 3 kcal/mol, respectively. An IR spectroscopic study of trihalonitrome- thanes [4], and microwave investigations of trifluoroni- tromethane [5] and trichloronitromethane [6] have all been reported. The trichloronitromethane was reported to have a low sixfold rotational barrier (V 6 = 73.24(17) · 10 3 kcal/mol). Stephenson and Macdonald [5] used vibra- tional coupling between the NO 2 wagging and the low fre- quency torsion modes to reconcile the discrepancies between the microwave and electron diffraction results for trifluoronitromethane. The apparent non-planar CANO 2 moiety observed in the earlier electron diffraction investigation [1] could be the result of ignoring a possible low frequency tor- sional motion about the CAN bond in data analysis. In this article, we are reporting our gas-phase electron diffraction reinvestigation and ab initio molecular orbi- tal and DFT-calculated results for trichloronitrome- thane, with particular attention paid to the planarity of the CANO 2 moiety and the barrier of the CAN torsion. 0022-2860/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.08.004 * Corresponding author. Tel.: +1 315 2287243; fax: +1 315 2287935. E-mail address: mshen@mail.colgate.edu (Q. Shen). www.elsevier.com/locate/molstruc Journal of Molecular Structure 830 (2007) 204–207