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 shieldings. The shielding does not vary uniformly with ring size, but rather the smallest ring (n = 2) has the highest shielding while the next smallest (n = 3) has the lowest shielding. Hartree–Fock calculations in the gauge-including atomic orbital (GIAO) approach on the related hydrogen derivatives have reproduced this trend in shielding and allow a qualitative understanding of the experimental observations. With respect to the relatively unstrained n = 4,5 ring systems, the unusual behavior of the n = 2 and 3 molecules can be understood in terms of the differences in the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) gaps and the p-character of the phosphorus lone pair. The HOMO/LUMO gap is largest for phosphirane (n = 2) but smallest in phosphetane (n = 3). The hybrid character of the lone pair in phosphirane (n = 2) is almost sp while that for phosphetane (n = 3) is essentially sp2.