Efficient Prediction of Isotropic Hyperfine Coupling in Radicals Containing Phosphorus. A Systematic Comparison of UHF, PUHF, and UMP2 Spin Densities

Cramer, C. J.; Lim, M. H.

*J. Phys. Chem.* **1994**, *98*,
5024.

Isotropic hyperfine couplings have been calculated and compared to
experiment for 25 radicals containing phosphorus using the 6-311G**
basis set. At HF/6-31G** optimized geometries, Fermi contact integrals
derived from UMP2 spin-density matrices give the best correlation with
experiment (20 data points for ^{31}P, 8 data points for
^{19}F, 7 data points for ^{35}Cl, and 5 data points
for ^{1}H hyperfine couplings, rms error of 22.9 G with
the data spanning a range of about 1650 G); PUHF spin-density matrices
are somewhat less useful (rms error of 33.6 G), and calculation
at the UHF level gives poor results (rms error of
45.6 G). Hyperfine couplings calculated at the UMP2 level should
be scaled by 102% to bring them into still closer agreement with
experiment. Even though geometric changes on going to the MP2 level
may be sizable, better correlation with experiment is obtained when
HF/6-31G** optimized geometries are used instead of MP2/6-31G**
optimized geometries (rms error of 36.1 G). Including diffuse
functions in the calculation of the hyperfine couplings for anionic
radicals has in general only marginal effect. Statistical trends
within specific subsets of the data are also discussed.

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