Conformational Studies of Phenyl- and (1-Pyrenyl)triarylmethylcarbenium Ions: Semiempirical Calculations and NMR Investigations under Stable Ion Conditions

Abstract

Highly stable crowded carbenium ions such as (1-pyrenyl)diphenylmethylcarbenium 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 two-ring flip (ph,ph), whereas the disubstituted pyrene derivatives show one-ring flips (py) and two-ring flips (ph,ph) with a higher rotational barrier in agreement with AM1 calculations. A series of calculations show that the AM1 method gives the transition-state energies in best agreement with experiment. The propeller-shaped geometry of these molecules is reflected in characteristic low-frequency resonances of the phenyl rings. At low temperature, 3 and 4 exist as rotational isomers with C2 and Cs (or Ci) symmetry. In 4, steric interaction makes the two rotamers slightly different in energy (1 kcal). For 4 or 4Br, the 13C chemical shift differences between the C2 and the Cs species of 4 or 4Br correlate roughly with the calculated charge differences between the C2 and the Cs species. The charge at the C+ carbon is most extensively delocalized in 2, whereas in 3 and 4 with two C+ groups the pyrene moieties are less effective in charge delocalization.