Electron Spin Resonance of Trapped PF2 and PF4 Radicals

Density functional theory predictions of isotropic hyperfine coupling constants

The reliability of density functional theory (DFT) in the determination of the isotropic hyperfine coupling constants (hfccs) of the ground electronic states of organic and inorganic radicals is examined. Predictions using several DFT methods and 6-31G, TZVP, EPR-III and cc-pVQZ basis sets are made and compared to experimental values. The set of 75 radicals here studied was selected using a wide range of criteria. The systems studied are neutral, cationic, anionic; doublet, triplet, quartet; localized, and conjugated radicals, containing 1H, 9Be, 11B, 13C, 14N, 17O, 19F, 23Na, 25Mg, 27Al, 29Si, 31P, 33S, and 35Cl nuclei. The considered radicals provide 241 theoretical hfcc values, which are compared with 174 available experimental ones. The geometries of the studied systems are obtained by theoretical optimization using the same functional and basis set with which the hfccs were calculated. Regression analysis is used as a basic and appropriate methodology for this kind of comparative study. From this analysis, we conclude that DFT predictions of the hfccs are reliable for B3LYP/TZVP and B3LYP/EPR-III combinations. Both functional/basis set scheme are the more useful theoretical tools for predicting hfccs if compared to other much more expensive methods.

Theoretical isotropic hyperfine coupling constants of third-row nuclei (29Si, 31P, and 33S)

In a previous paper (Hermosilla, L.; Calle, P.; Garcia de la Vega, J. M.; Sieiro, C. J. Phys. Chem. A 2005, 109, 1114), an adequate computational protocol for the calculation of isotropic hyperfine coupling constants (hfcc's) was proposed. The main conclusion concerns the reliability of the scheme B3LYP/TZVP//B3LYP/6-31G* in the predictions of hfcc's with low computational cost. In the present study, we gain insight into the behavior of the above functional/basis set scheme on nuclei of the third row, for which few systematic studies have been carried out up to the present date. The systems studied are neutral, cationic, anionic, localized, and conjugated radicals, containing (29)Si, (31)P, and (33)S nuclei. After carrying out a regression analysis, we conclude that density functional theory (DFT) predictions on the hfcc's of the third-row nuclei are reliable for B3LYP/TZVP by using an optimized geometry with B3LYP/6-31G* combination. By comparison with other much more computationally demanding schemes, namely, B3LYP/cc-pVTZ and B3LYP/cc-pVQZ, we conclude that the B3LYP functional in conjunction with the TZVP basis set is the most useful computational protocol for the assignment of experimental hfcc's, not only for nuclei of first and second rows, but also for those of the third row.

An Experimental and Theoretical Evaluation of the Reactions of Silver Hyponitrite with Phosphorus Halides. In Search of the Elusive Phosphorus-Containing Hyponitrites

In the reaction of F2PBr, F2P(O)Br, (C6F5)2PBr, (CH3)2P(S)Br, and (CH3)2P(O)Cl with silver hyponitrite (AgON=NOAg), nitrous oxide (N2O) and mu-oxo phosphorus species were obtained in all cases rather than the plausible hyponitrite alternative. Theoretical calculations of the geometries and expected decomposition pathways of the phosphorus-containing hypothetical hyponitrites were carried out at the B3LYP/6-311+G(2df)//B3LYP/6-31+G(d) level. The cis-hyponitrite, XON=NOX (X=PF2, OPF2), is predicted to concertedly decompose to N2 plus phosphorus-containing radicals (OPF2, O2PF2) or to N2O plus the mu-oxo phosphorus species, X-O-X, (X=PF2, OPF2) with the former pathway having a smaller activation barrier (4.6 kcal/mol, X=PF2; 10.5 kcal/mol, X=OPF2). On the other hand, trans-hyponitrite can only decompose to N2 plus the phosphorus-containing radicals, because there is a very high barrier for rearrangement to cis-hyponitrite. These results are in disagreement with experiment, because only the mu-oxo phosphorus species are observed. Reconciliation between experiment and theory is made for X=OPF2 when a silver cation is included in the calculations. In THF (as a model for neat F2P(O)Br), the silver cation is predicted to reverse the order of the two transition states through stronger interactions with the oxygen atoms in the transition state of the N2O-producing pathway. Thus, Ag(I) is predicted to be not only catalytic for X=OPF2 but also product-specific toward the mu-oxo products.

Electron Paramagnetic Evidence for the ·pf3 Radical Anion

Although ·PF2 radicals have been extensively studied there does not seem to be any spectroscopic evidence for ·PF3-radical anions. Here it is proposed that an EPR spectrum previously assigned to ·PF2 radicals is almost certainly due to ·PF3-centres. The 31P and 19F hyperfine coupling constants for these radicals are expected to be very similar. However, the ·PF3-radicals should show an extra coupling to the third 19F nucleus. This is clearly present.