Electron Spin Resonance Study of PH4 Trapped in Solid Krypton

The ESR spectrum of the PH₄ radical, produced by the vacuum uv photolysis of phosphine in solid krypton, is reported for the first time. The spectrum centered at g=2.002 is consistent with four equivalent hydrogens, and shows only isotropic hyperfine splittings ($a_p\text{= 973.8 ± 2.0\hspace{0.17em}}\mathrm{G}$ and $a_H\text{= 21.0 ± 2.0\hspace{0.17em}}\mathrm{G}$ ). The observed equivalence of the hydrogens contrasts with the spectra reported previously for PF₄. A treatment of the energies of configurations for PH₄ with the CNDO molecular orbital method indicates the optimum geometry to be C_2v with HPH angles equal to 162° and 97°; additionally, the energy barrier to pseudorotation through a C_3v intermediate is estimated to be relatively low (approximately 6 kcal mole⁻¹), and this leads to the possibility that the equivalence of the hydrogens is due to internal tunneling motions.