By making use of a novel diastereotopicity probe, namely C(CF3)(2)OH, it has been possible to measure by very low temperature F-19 NMR spectroscopy the elusive aryl aryl rotation barriers of biphenyls bearing an OH or F group in one rill position. The experimental values (5.4 and 4.4 kcal mol(-1), respectively) are matched by those from ab initio calculations (5.3 and 4.3 kcal mol(-1), respectively).