THE CHARACTERIZATION OF NMR SHIELDING IN MONOCYCLIC PHOSPHINES

The phenyl-substituted saturated monocyclic phosphines, PhP(CH2)(n), n = 2-5, show an interesting variation in their phosphorus NMR shieldin gs. The shielding does not vary uniformly with ring size, but rather t he smallest ring (n = 2) has the highest shielding while the next smal lest (n = 3) has the lowest shielding. Hartree-Fock calculations in th e gauge-including atomic orbital (GIAO) approach on the related hydrog en derivatives have reproduced this trend in shielding and allow a qua litative understanding of the experimental observations. With respect to the relatively unstrained n = 4,5 ring systems, the unusual behavio r of the n = 2 and 3 molecules can be understood in terms of the diffe rences in the highest occupied molecular orbital/lowest unoccupied mol ecular orbital (HOMO/LUMO) gaps and the p-character of the phosphorus Zone pair: The HOMO/LUMO gap is largest for phosphirane (n = 2) but sm allest in phosphetane (n = 3). The hybrid character of the lone pair i n phosphirane (n = 2) is almost sp while that for phosphetane (n = 3) is essentially sp(2). (C) 1997 John Wiley & Sons, Inc.