9-SUBSTITUTED TRIPTYCENE AS A PROBE FOR THE STUDY OF INTERNAL-ROTATION AROUND A C-PH BOND IN THE SOLID-STATE - A SINGLE-CRYSTAL EPR STUDY AT VARIABLE-TEMPERATURE

To measure the rotation barrier around an R(3)C-(PH)-P-. bond in the s olid state; 9-phosphinotriptycene(50) 2 has been synthesized and its c rystal structure has been determined. It is shown, by EPR, that the ra diogenic radical 3, which results from a homolytic scission of a P-H b ond, can indeed be trapped in the crystal matrix. Its g-tensor togethe r with its P-31 and H-1 hyperfine coupling have been measured at 300 a nd 77 K. These tensors show that free rotation around the C-P bond occ urs at room temperature but is blocked at liquid nitrogen temperature. The temperature dependence of the EPR spectra has been analyzed using the density matrix formalism and has led to a rotation barrier of abo ut 2.5 kcal mol(-1). This result and the various hyperfine couplings h ave been compared with the values predicted by ab initio methods for t wo isolated model radicals: the tert butylphosphinyl radical 4 and the barrelenophosphinyl radical 5.