CONFORMATIONAL STUDIES OF PHENYL-TRIARYLMETHYLCARBENIUM AND (1-PYRENYL)TRIARYLMETHYLCARBENIUM IONS - SEMIEMPIRICAL CALCULATIONS AND NMR INVESTIGATIONS UNDER STABLE ION CONDITIONS

Highly stable crowded carbenium ions such as (1-pyrenyl)diphenylmethyl carbenium ion (2) and 1,6-(3) and 1,8-bis(diphenylmethylenium)pyrene [ dication] (4) and their dibrominated analogues (3Br and 4Br) have been studied at low and at ambient temperatures. 2 shows a conventional tw o-ring flip (ph,ph), whereas the disubstituted pyrene derivatives show one-ring flips (py) and two-ring flips (ph,ph) with a higher rotation al barrier in agreement with AM1 calculations. A series of calculation s show that the AM1 method gives the transition-state energies in best agreement with experiment. The propeller-shaped geometry of these mol ecules is reflected in characteristic low-frequency resonances of the phenyl rings. At low temperature, 3 and 4 exist as rotational isomers with C-2 and C-s (or C-i) symmetry. In 4, steric interaction makes the two rotamers slightly different in energy (1 kcal). For 4 or 4Br, the C-13 chemical shift differences between the C-2 and the C-s species o f 4 or 4Br correlate roughly with the calculated charge differences be tween the C-2 and the C-s species. The charge at the C+ carbon is most extensively delocalized in 2, whereas in 3 and 4 with two C+ groups t he pyrene moieties are less effective in charge delocalization.