An attempt has been made to refine electron diffraction data collected at 130°for cyclodecane in terms of a simplified boat-chair-boat (BCB) model of C2h symmetry. The resulting structural parameters were: rg(C-C)£V = 1.545 ± 0.003 A, rg(C-H)av = 1.115 ± 0.003Á, ZCCH(av) = 108.7 ± 1.0°, /CAA = 114.6 ± 1.2°, and ZCiC2C3 = ZC2C3C4 = 116.9 ± 0.9°. The resolution factors for the short and long camera distances were 0.99 and 0.97, respectively. It is readily conceivable that the excellence of the fit of this model is fortuitous and misleading, however, since electron diffraction analyses rarely yield a unique structure for a molecule of this size and flexibility. An alternative analytical approach has been adopted in which detailed structures and selected thermodynamic properties have been calculated for 12 stable conformations of cyclodecane. Theoretical radial distribution curves have been generated for the four conformations of lowest energy. Least-squares analysis of the experimental radial distribution curve utilizing the four theoretical curves has indicated the following composition: BCB 49 ± 3%, twist-boat-chair (TBC) 35 ± 3%, TBCC 8 ± 4%, and BCC 8 ± 4%. The two methods of analysis have been critically discussed, particularly from the point of view of the analysis of multiconformational molecules of cyclodecane's complexity.(1) (a) North Dakota State University; (b) Indiana University.(2) See, for example, E.

No abstract

The structure of t-butyl chloride has been determined by electron diffraction. The results obtained are in good agreement with the previous structure determination by Momany, Bonham, and Druelinger. This structure is shown to be consistent with the microwave rotational constants determined by Lide and Jen in spite of the fact that the rg distance for the C–Cl bond (1.828±0.005 Å) is quite different from the rs microwave distance (1.803±0.002 Å). The remaining structural parameters were determined to be C–C (1.528 Å), C–H (1.102 Å), ∠ ClCC (107.3o), ∠ CCH(110.8o), ∠CCC (111.6o), and ∠ HCH (108.1o). A simultaneous investigation of the structure of the deuterated species suggests slight secondary isotope effects on the C–C (ΔC–C=0.0010±0.0007-Å) and C–Cl (ΔC–Cl=0.0010±0.0015-Å) distances. Observed mean amplitudes of vibration are found to be consistent with values calculated from the harmonic force field of Hüttner and Zeil.

No abstract

4514(151 For comparison. Debar et a, ' state tnat the optimirea C, symmetrical trans;tion state (w tn 2 X 2 CI) ties 40 kcal/mol aDove the unsymmetrical (161 Mclver has suggesteo tnat transition states of cycioaaoitions (and many other react'ons. as well) may genera ly oe nonsymmetrc the symmetric (C,, for example, for the Diels-Alder reaction) trans;tion state representing a stattonary po.nt wtth fwo negative force constants (J W. Mclver. Jr., J.Am Chem. SOC.. 94, 4782 (1972); Acc. Chem. Res.. 7, 73 (1974)). While this has oeen proven to De the case in MIND012 and MlhD013 ca.culations on the Diels-Alder transition state, the results reported here and those of Salem and coworkers suggest that the force constant for ant symmetric distortion (one new bond lengthen'ng wh'le the other shortens) 's slightly positive. in contraoostinction to the MIND0 results where tnis force constant is nega! ve. (171 R. S Mu liken. Because of the nLmDer of parameters involve0 in tnese techniques it IS d.ff'cuit to prove bnicn interactions are treated unrealistically. M. V. Bas-ilevsKy. Adv Chem Phys. 33, 345 (1975). nas commented on possible ,nderest mation of repJlslve forces at transit on-state distances by MlhDO 2 On tne otner hand, Goroon and borr(ers found that MIND0/3 grassy .ngerestimates ong-range Interactions of tne type wh ch resLlt n me pnenomenon of spiroconjJgation : M D Gordon, T. FLkJnaga. and H. E. Simmons. J. Am. Chem. Soc.. 98,8401 (1976). ThLs. one short-range interaction such as that present in the asynchronous transition states may well oe overestimate0 relative to tne two ,ong-range interactions present on the synchronoLs transdion states. We thank Dr. Gordon for a preprint of rhese resdts (211 Ep,ot s and Yates nave shoan that ' ar0mat.c ' molecLles have less c osezi-sne repulslon tnan 'nonaromatic ' (ana oy extension. antiaromatic molecules will have the most1 hc D. Epiotis and R L. Yates. J. Am. Chem. Soc, 98, 461 (19761. Our treatment of transition states IS stm.lararomalic ~s)nchrono,s a lowed) transition states hav ng less closed-shell repJ ston than ' nonaromat c' (nonsynchronous) (221 MlhDO, 3 gives a symmetrca transition state for the etnylene-singlet oxygen reactoon: M J S. Debar and W Thie , J Am. Chem Soc., 97,3978 (19751. 0-1 th s transition state lacks signilicant closed-shell repulsions. For OrDital-symmetry-forDidoen reactions sLch as [,2, + .2,] reaction, th s type of reason ng leaos to the expectation tnat MINDO. or other overlap-neglected. calculat ons wi I tena to give more symmetrical transition states !han over,ap-.nc lroeo calculations A R. Gregory ana M. N Paddon-Row have recently shown that CNDOlP and INDD calculations predict staoil ty for mo ecJies wnch we feel snould experience serious closed-shell repulsions: J. Am Chem. Soc.. 98, 7521 (1976) (231 We thank Professor Lionel Salem for a nelpful 0 scbssion of tnis and related points (24) Alter smmiss on of th s work Basi