Two-dimensional 31P Exchange NMR Study of Adsorbed Phosphine Layers

Adsorption of organic phosphines on acidic surfaces produces phosphonium ions, which can be detected by 31 P NMR, since their chemical shift is different from that of the free phosphine. In the presence of excess adsorbed phosphine, lines from both phosphine and phosphonium are observed. Two-dimensional exchange NMR is applied to trimethyl- and triphenylphosphines adsorbed on SiO 2 −Al 2 O 3 and demonstrates that free phosphine and phosphonium ions are undergoing chemical exchange at ambient temperatures. There are two populations of PMe 3 H + which exchange with different rate constants, 7 × 10 4 and 800 s - 1, respectively. PPh 3 H + exchanges much more slowly with PPh 3, having a rate constant of 14 s - 1 .