GIAO nuclear magnetic shielding tensors in free base porphyrin and in magnesium and zinc metalloporphyrins

The gauge including atomic orbital (GIAO) method at the self-consistent fie ld (SCF) level of theory was used to calculate magnetic shielding tensors i n free base porphyrin (H2P) and in magnesium (MgP) and zinc (ZnP) metallopo rphyrins, as well as in the MgP(H2O) and MgP(H2O)(2) complexes. All princip al components of the calculated shielding tensors are reported. We have rec orded new solution NMR spectra for all three molecules under identical cond itions, to ensure the reliability of small differences in their NMR shifts. The calculated isotropic values are compared with these values and with lo w-temperature solid-state shifts, which resolve the tautomerism in H2P. Agr eement with experiment is good. It is encouraging that the calculations cor rectly reproduce most of the the small differences in the chemical shifts. A comparison of the observed and calculated NMR shifts of MgP leads to the conclusion that the species observed in tetrahydrofuran solution is either MgP(H2O)(2) or MgP(THF)(2). We have calculated the effect of the nut-of-pla ne displacement of the metal ion in metalloporphyrins, with a view to using NMR as a probe of nonplanarity. Only the N-15 shifts show any sensitivity.