Table 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,
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 example 1Jc3,c4 = 70.0 Hz for cis-hexa-I ,3-diene) ,[191 and 1Jc2.c3 and 1JC2,,c3, are comparable with the coupling constants between two sp2-hybridized carbon atoms connected through a single bond as in buta-1,3-diene (J,,,,, = 53.7 Hz).[~*] These comparisons and the fact that 6(C2,C3) = 170.3 for butatriene is only slightly smaller than the measured S(CljC1') for 1 can be interpreted as follows: 1 has cumulenic character and is dominated by the mesomeric structure 13 Such a hypothesis requires significant bond length alternation, 18 Ib which conflicts with recent ab initio calculations even at the highest level of
The problem is the lack of suitable model compounds for comparing their NMR spectra with those of 1 and the large uncertainty of structural predictions based on empirical correlations. The exact influence of the main factors that determine the value of the coupling constants is not fully However, the experimental NMR spectra of 1 reported here, in connection with ab initio methods that simulate the observed NMR spectrum with high precision, are valuable for making reliable predictions of the geometry and the electronic properties of this interesting molecule.[22g1
We have demonstrated that 1 can be stabilized in solution by incarceration in the unique protective inner cavity of a molecular container and characterized by NMR spectroscopy in solution. Additional compounds with highly strained multiple bonds and other reactive intermediates should be stabilized, and their structural and electronical properties examined.