| Autor: |
Ballester M; Department of Chemistry and Physics, Nova Southeastern University, Fort Lauderdale, Florida 33314-7796, United States., Ravotto L; Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States.; Department of Chemistry, School of Arts and Sciences, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States., E Quirke JM; Department of Chemistry and Biochemistry, Florida International University, Miami, Florida 33199, United States., López de la Vega R; Department of Chemistry and Biochemistry, Florida International University, Miami, Florida 33199, United States., Shelnutt JA; Center for Integrated Nanotechnologies, Albuquerque, New Mexico 87185-1315, United States., Cheprakov AV; Department of Chemistry, Moscow State University, Moscow 117899, Russia., Vinogradov SA; Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States.; Department of Chemistry, School of Arts and Sciences, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States., Medforth CJ; Department of Chemistry, University of California, Davis, California 95616, United States. |
| Abstrakt: |
Herein, we report the first calorimetric study of the protonation of planar and nonplanar free-base porphyrins: H 2 OETPP (strongly saddled by its substituents), H 2 T(tBu)P (strongly ruffled by its substituents), and the nominally planar porphyrins (npPs) H 2 OEP, H 2 TPP, H 2 T(nPe)P, and H 2 T(iPr)P. The observed enthalpies of protonation in solution (Δ H protsoln ) for formation of the dications in 1,1,2,2-tetrachloroethane with 2% trifluoroacetic acid are -45 ± 1 kcal mol -1 for the npPs, -52.0 kcal mol -1 for H 2 T(tBu)P, and -70.9 kcal mol -1 for H 2 OETPP. The corresponding enthalpies of protonation (Δ H DFT ) obtained from DFT calculations (-27 ± 5, -42, and -63 kcal mol -1 , respectively) reproduce this trend. The much more negative enthalpy of protonation seen for H 2 OETPP is consistent with this molecule being pre-deformed into the saddle structure favored by porphyrin dications. Except for OETPP, the calculated enthalpies of the first protonations (Δ H 1 ) are significantly more positive than the enthalpies of the second protonations (Δ H 2 ). In addition, the structural strain energies for the first protonations (Δ E st (1)) are also significantly more positive than Δ E st (2). According to the calculations, the monocations thus have higher proton affinities than the corresponding free-base porphyrins due to a structural strain effect, which is consistent with the generally elusive nature of the porphyrin monocation. The recent observations of monocations for free-base porphyrins with a high degree of saddling can be rationalized in terms of Δ H 1 and Δ H 2 being similar; so, the monocation is no longer an unstable intermediate. |
