Transition state characteristics of planar singlet 2,4,6,8-cyclononatetraenylidenes and its halo derivatives, via ab initio M.Z. Kassaee * , M. Koohi, S. Arshadi Department of Chemistry, Tarbiat Modarres University, P.O. Box 14155-4838, Tehran, Iran Received 30 October 2004; revised 31 January 2005; accepted 31 January 2005 Available online 20 April 2005 Abstract Calculations are carried out at HF/STO-3G, HF/6-31G*, MP2/6-31G*, B3LYP/6-31G* and B3LYP/6-311CG* levels of theory, for conversion of three series of cyclic conjugated non-planar allenes: 4-, 5-, and 6-X-1,2,4,6,8-cyclononapentaenes (1 X , 2 X , and 3 X ) to their mirror images (1 0 X , 2 0 X , and 3 0 X ) through the corresponding singlet planar conjugated carbenic transition states: 3-, 4-, and 5-X-2,4,6,8- cyclononatetraenylidenes (1* X , 2* X , and 3* X ), where XZH, F, Cl and Br. The employed halogens (F, Cl and Br) are chosen not to be directly attached to the allenic centers. For Series 1, the relative ease of racemization of 1 X to 1 0 X through 1* X is: 1* H O1* Br O1* F R1* Cl . For Series 2, the relative ease of racemization of 2 X to 2 0 X through 2* X is: 2* H O2* F R2* Cl R2* Br . Finally, for Series 3, the relative ease of racemization of 3 X to 3 0 X through 3* X is: 3* H O3* F R3* Cl R3* Br . Hence, the lowest racemization energy barrier encountered (26.8 kcal/mol) is for XZH, where 1 H (Z2 H Z3 H ) is converted to its mirror image 1 0 H (Z2 0 H Z3 0 H ) through 1* H (Z2* H Z3* H ) as a transition state. q 2005 Elsevier B.V. All rights reserved. Keywords: Carbene; Allene; Racemization; Enantiomerization; C 9 H 7 Br; C 9 H 7 Cl; C 9 H 7 F; 4-, 5-, or 6-Halo-1,2,4,6,8-cyclononapentaene; 3-, 4-, or 5-Halo- 2,4,6,8-cyclononatetraenylidene; Transition state; Ab initio 1. Introduction The theoretical and experimental reports on the possible racemization of cyclic conjugated non-planar allenes to their potential valance isomeric, planar, carbenes have made the chemistry of cumulated alkadienes of great interest [1–3]. It is now established that cyclic conjugated planar singlet carbene C 7 H 6 (middle structure in Scheme 1) is not an energy minimum. It has a negative force constant. So, it is rather a transition state for the inter-conversion of its corresponding cyclic conjugated non-planar, chiral allene (left structure in Scheme 1) to its mirror image (right structure in Scheme 1) [4–31]. Many interesting reports have appeared on different aspects of cyclic conjugated C 7 H 6 species, with allenic, and/or carbenic (singlet or triplet) forms. In contrast, very few reports have surfaced on its most interesting unsaturated, fully conjugated C 9 H 8 allenic analogue: 1,2,4,6,8-cyclononapentaene, 1 H [32–35] (Fig. 1). Previously, we have reported that a cyclic conjugated planar, C 2v constrained version of singlet 1* H (a p 2 carbene) is much less stable than its corresponding cyclic conjugated non-planar allene 1 H (or 1 0 H ) [32]. The vibrational force constant calculation for C 2v singlet 1* H shows one negative force constant. Hence, it is not an intermediate. It transforms to lower energy C 2 structure allene, 1 H (or 1 0 H ), with no activation energy (Fig. 1). In this paper, characteristics of planar singlet 3-, 4-, or 5-X-2,4,6,8- cyclononatetraenylidenes (XZH, F, Cl and Br), as a transition state, in mirror image conversions of their corresponding non-planar allene analogues are reported using ab initio calculations (Fig. 1). 2. Computational methods Ab initio calculations are carried out, at HF/STO-3G, HF/6-31G*, MP2/6-31G*, B3LYP/6-31G* and B3LYP/ 6-311CG* levels of theory, for mirror image conversions of three series of non-planar allenic C 9 H 7 X (where XZH, F, Cl and Br) through their corresponding singlet planar conjugated carbenic C 9 H 7 X transition states (Fig. 1). Series 1 reveals racemizations of Journal of Molecular Structure: THEOCHEM 724 (2005) 61–71 www.elsevier.com/locate/theochem 0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2005.02.025 * Corresponding author. Tel.: C98 912 100 0392; fax: C98 21 800 6544. E-mail address: kassaeem@modares.ac.ir (M.Z. Kassaee).