peak. Both the splitting constants and spectral shapes show that PFPE is present as a n organized assembly. In contrast, the motion of the probe in water-saturated C 0 2 (without surfactant) is isotropic.A t COZ densities >13 M , the apparent A N in the surfactant solution drops to near that in neat C02. Thus, the observed AN

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The results of the first crystal structure determination of [W(CH 3 ) 6 ] are confirmed: its structure is strongly distorted towards a trigonal-prismatic C 3v symmetry, while [Re(CH 3 ) 6 ] has a regular trigonal-prismatic symmetry, and [Ta(CH 3 ) 6 ] À and [Nb(CH 3 ) 6 ] À show a slight distortion from ideal trigonal prismatic. This distortion is similar but much weaker than that in [W(CH 3 ) 6 ]; the structure is additionally influenced by one strong CH 3´´´L i contact.

COMMUNICATIONSTable 3. 13C-i3C coupling constants 'Jc.c [Hz] of incarcerated [13C6]l at -98'C. c1-Cl' c 1 -C2/Cl'-C2' C2-C3/C2-C3' C3-C3' 'Jc,c [a1 177.9 (k0.7) 75.7 (k0.9) 50.9 (t0.8) 71.0 (i0.8) ~~ [a] The 95%-confidence limits are given in parentheses.interference effect between chemical shift anisotropy and dipole-dipole relaxation."The coupling constant Jc,,cl, is in the frequency range expected for C-C triple bonds: for example, Jcl,c2 =I85 Hz for diphenyl acetylene 13, and Jcl,cz = JCl,,,--, for 1 is smaller than the sp-C-spz-C coupling in 13 (Jcl,c3 = 91 Hz).''*l We attribute the difference to the ring strain, ~-c ' ' c z~ which increases the p character of the hybridization of C1 and C1' in 1 relative to 13. However, 1Jc3,c3, is of the order of an olefinic bond (for ex-

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