Predicted geometries of porphyrin excited states and radical cations and anions
- Princeton Univ., NJ (United States)
Semiempirical calculations have been carried out on zinc porphine (ZnP), which has an a[sub 1u] HOMO and on zinc mesotetrafluoroporphine (ZnTFP), which has an a[sub 2u] HOMO, using the MNDO/3 Hamiltonian supported in the AM1 computational package. Geometry optimization proceeded to convergence with no symmetry instability (previously encountered in calculations on free-base porphine), whether the symmetry was constrained to D[sub 4h] or not. The optimized bond distances differed very little between ZnP and ZnTFP but were slightly longer than the crystallographically determined bond lengths of zinc meso-tetraphenylporphine (ZnTPP). Extraction of an electron from ZnTFP, giving an a[sub 2u] radical cation, increased the C[sub [beta]]C[sub [beta]] and C[sub [alpha]]C[sub m] lengths and decreased the C[sub [alpha]]C[sub [beta]] lengths, leading to a slight core expansion, while extraction of an electron from ZnP, giving an a[sub 1u] radical, decreased the C[sub [beta]]C[sub [beta]] lengths and increased the C[sub [alpha]]C[sub [beta]] lengths (all changes ca. 0.01 [angstrom]). These changes are from the orbital pattern of the HOMO's, which have previously been used to explain qualitatively the observed up and down shifts of the [nu][sub 2] C[sub [beta]]C[sub [beta]] stretching modes in resonance Raman (RR) spectra upon formation of a[sub 1u] and a[sub 2u] radicals, respectively. These up and down shifts were modeled satisfactorily by applying bond distance/force constant scaling to the recently developed empirical force fields for OEP (octaethylporphyrin) and TPP. Interestingly, the MNDO/3 calculation failed to converge for ZnP[sup +] but not for ZnTFP[sup +], when the symmetry was restricted to D[sub 4h]. Calculation of the ZnP[sup [minus]] radical anion produced a B[sub 1g] distortion, consonant with EPR results. Modeling of the frequency shifts observed in ZnTPP[sup [minus]] RR spectra, using bond distance/force constant scaling, was only partially successful.
- DOE Contract Number:
- AC02-81ER10861
- OSTI ID:
- 6923816
- Journal Information:
- Journal of Physical Chemistry; (United States), Journal Name: Journal of Physical Chemistry; (United States) Vol. 95:24; ISSN JPCHAX; ISSN 0022-3654
- Country of Publication:
- United States
- Language:
- English
Similar Records
Influence of porphyrin radical type on V double bond O bond strength in vanadyl porphyrin cation radicals: Implications for heme protein intermediates
Resonance Raman spectra of metallooctaethylporphyrin cation radicals with a/sub 1u/ and a/sub 2u/ orbital character
Related Subjects
140505 -- Solar Energy Conversion-- Photochemical
Photobiological
& Thermochemical Conversion-- (1980-)
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
ANIONS
CARBOXYLIC ACIDS
CATIONS
CHARGED PARTICLES
COMPARATIVE EVALUATIONS
COMPLEXES
DATA
ELECTRON SPIN RESONANCE
ENERGY LEVELS
EVALUATION
EXCITED STATES
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
INFORMATION
IONS
JAHN-TELLER EFFECT
MAGNETIC RESONANCE
MATHEMATICAL MODELS
MOLECULAR STRUCTURE
NUMERICAL DATA
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PORPHYRINS
RADICALS
RESONANCE
THEORETICAL DATA
VIBRATIONAL STATES
ZINC COMPLEXES