1. For comprehensive reviews see G.A. Olah and P.v.R. Schleyer (Eds.), Carbonium Ions, Wiley Interscience, New York, London, Vol. I, 1968; Vol. II, 1970; Vol. III, 1972; Vol. IV, 1973; Vol. V, 1976.
2. G.A. Olah, G.K.S. Prakash and J. Sommer, Superacids, Chapter III, Wiley Interscience, New York, 1985, Ch. 3.
3. P. Vogel, Carbocation Chemistry, Elsevier Science Publishers, Amsterdam, New York, 1985.
4. J.B. Stothers, Carbon-13 NMR Spectroscopy, Academic Press Inc., New York, 1972, pp. 217.
5. P. Ahlberg, G. Jonsall and C. Engdahl, Adv. Phys. Org. Chem., 19(1983)223.