| Popis: |
A study of NMR coupling constants, 1J(13C13C), of compounds containing a CF3 group has revealed that the values for the coupling of the CF3 carbon with the neighbouring carbon are exceptionally large [1]. In a series of compounds in which the CF3 group is bonded to a spa3, sp2, and sp-carbon the 1J(CC) values show a very good linear correlation with the product of the s-characters of the two orbitals forming the CC bond. However, this correlation differs considerably from the analogous FREI-BERNSTEIN relationship [2] for hydrocarbons: F3CC cpds: 1J(CC) = 874 × S1(CF3) × S2(C2) + 8.8 [1] H3CC cpds: 1J(CC) = 575 × S1 (Cl) × S2 (C2) - 3.4 [2] The 1J(CC) values of compounds of type RR' do not correlate with (a) the product of the s-characters of the two carbon orbitals forming the CC bond, (b) the ionicity of the CC bond, and (c) the CC bond length, however, a fairly good correlation with the sum of the group electronegativities of the two electronegative groups is found. It can be concluded that the inductive effect of the substituents has a major influence on the 1J(CC) coupling and that a small but significant non-contact contribution to the 1J(13C13C) coupling exists. The results suggest a contribution to the CC coupling from a relatively large variation of the valence-shell s-orbital density at the carbon nucleus, Sc2 (0), due to the contraction of the s orbitals. Hitherto, this term was usually assumed to be constant. Similarly, coupling constants 1J(13C77Se), 1J(13C125Te) and 1J(13C113Cd) of CF3 substituted compounds show much larger values than their CH3 analogues [3, 4]. |
