The B3LYP density functional method was used to determine the optimized geometries, harmonic vibrational frequencies, and anharmonic vibrational frequencies of HPO2 and the trans- and cis-isomers of HOPO. Additionally, the isotropic hyperfine splitting (hfs) constants and the vibrationally averaged hfs were determined for the radical anions of each species. cis-HOPO is predicted to be the lowest energy isomer of the neutral systems, with trans-HOPO lying 3.0 kcal mol(-1) higher and HPO2 lying 11.9 kcal mol(-1) higher than that. The pyramidal HPO2-radical is the lowest energy anion, and the cis- and trans-isomers lie similar to5 and similar to8 kcal mol(-1), respectively, above HPO2-. Higher lying sigma radicals of HOPO- resemble PO2- and free H. The B3LYP/6-311+G(2d,2p) values of A(iso)(H-1) = 1211.98 MHz (trans) and 1199.90 MHz (cis) confirm that the (2)A' HOPO anion dissociates to PO2-+ H. H2PO was also investigated because of its similarity to HPO2-. The computed hfs constant for H-1 (109.64 MHz) is fortuitously in excellent agreement with the experimental value, while that for P-31 (950.29 MHz) differs significantly from experiment, and this deviation is attributed to the lack of a basis set for P that provides a good description of the P 1s core. Vibrational averaging affects the computed hfs in H2PO by 5-6 MHz for A(iso)(P-31) but only by similar to2 MHz in HPO2-. The effect of vibrational averaging is small (1-2 MHz) for A(iso)(H-1) but more significant (8-20 MHz) in HPO2-. A(iso).(O-17) is relatively unaffected by vibrational averaging in both H2PO and HPO2- Solvent effects were evaluated using the conductor-like polarizable continuum model (CPCM), and these hfs values for HPO2- [A(iso)(P-31) = 1123.42 MHz, A(iso)(H-1) = 243.51 MHz] and H2PO [A(iso)(P-31) = 1003.54 MHz, A(iso)(H-1) = 109.27 MHz] are in closer agreement with experiment. Convergence of the Fermi contact terms for P-31 computed using B3LYP with the aug-cc-pCVXZ and aug-cc-pV(X+d)Z families of basis sets is erratic and slow. On the other hand, with the same basis sets, CCSD(T) produces hfs constants for the P atom that are in close agreement with experiment.