Halogen bridging in the 2-chloroethyl radical

Volume 18, number 1

CHEMICAL

INDO calcuktions predict an unsymmetrically Amost perpendicular to the radical plane.

PHYSICS LETTERS

bridged structure

The question of bridging in &halogen substituted a&y1 radicals is of coriti~uing interest [ 11. Conclusive evidence for symmetrically bridged radicals is lacking but recent ESR studies have indicated that there is often a strong conformations preference for an unsymmetrically bridged species. The ESR spectrum of the Zchloroethyl radical E2] is of particular relevance in this respect; the magnitude and tenl~~rature dependence of the hyperfine coupling constants indicate that the chlorine atom prefers to occupy ii position eclipsing the odd electron orbital and we have suggested that there is some distortion away from a regular geometry with the chlorine moving towards a bridging position and the P-protons moving towards the molecular plane, We have now estimated the equilibrium geometry of this important radical using an INDO program described elsewhere [3]. The minimum energy conformation was determined by systematically varying all the atomic co-ordinates apart from the C-H bond lengths which were taken as 1.03 a. The calculations support our previocs conclusions about cH,CHZCl. As shown in fig. 1, the geometry around C, is essentially normal sp’ but there are substantial distortions around C, with the Cz-Cl bond aimost perpendicular to the pkne of the radical. The C2-Cl bond is slightly Ionger than normal for a C-Cl sin&e bond; the C-Cl distance is only 2.34 K. The angle C, C2H3 has opened up to 120” as the Fhydrogens approach the nodal plane of the odd electron orbital on Cl. We have also estimated the barrier to intern& rotation in CHzCH2C1’by calculating the energy as a func-

1 hww

for the 2-chtoroethyl

1973

radkxl with the C-Cl bond

tion of tkaangle of twist about the C, -C, bond. \Ve did not attempt to fully optimise the geometry for each angie of twist but the energy was minimised with respect to LC, C,CI in each conformation. The angle LC, C,C1 was found to increase with the angle of rotation and approached the normal tetrahedral acg!e of 109.5” when the chlorine atom was in the radicaf plane. This behaviour is consistent with decreased hyperconjugation [4] as overlap between the odd electron orbita and the C-Cl CJbond decreases and ~1s~ decreased p-p homo~oIljt;g3tion [5]. The estimated barrier to internal rotation of 16 kc21 mole-l is quite sufficient H,-ii

I ISO

H,

C&Cl

92’

C&-H,

I x>-!”

I-L-<-H,

IlO”

PLANAR

abaut

c,

Pig. 1.

45

:-

Voiume 18, number, 1

CHEMZCAL

PHYSICS

io expla+ the tfans effect found in the addition of halogens to double bonds and other chemical evidence. ,:. We shall discuss ekewhere the relationship of these 1 resuults tb teniperature dependent ESR data on 1 CH~CiiI,C1 and several other @-chloroa!kyl radicak

References [I]

p.S. Skell and K.J. Shea,

Israel I. Chem. IO (19723 493.

LETl-ERS

[Z] A.3. Eowles, A. Hudson and R.A. Jackson,

1 January 1973

Chem. Phys Letters 5 (1970) 552. 131H.G. Benson and A. Hudson, Theoret. Chim. Acta 23 (1970) 259; 1. Biddies and A. Hudson, ,Mol. Phys., submitted for pubiicatio il. [41 A.R. Lyons and h!.C.R. Symons, J. Am. Chem. Sot. 93 (19713 7330. T. Kawrnura, D.J. Edge and J.K. Kochi, J. Am. Chem. sot. 94 (1972) 1752.