Estimation of Electron Spectra Transitions of Free-Based Porphin and Mg-Porphin Using Various Quantum Chemical Approaches

International Journal of Molecular Sciences

Josef �²  Jaroslav V. Burda¹  Veronika Brázdová² 

[1]Department of Chemical Physics and Optics, Faculty of Mathematics and Physics,Charles University, Ke Karlovu 3, 121 16 Prague 2, Czech RepublicE-mail: Jaroslav Burda &l

关键词: porphin;    magnesium-porphin;    excitation  spectra;    CIS;    RPA;    CASSCF;    TDDFT;    ZINDO;   

DOI  :  10.3390/i5040196

来源: mdpi

【 摘 要 】

For optimized molecules of free-base porphin and magnesium-porphin (at Hartree- Fock level and 6-31G* basis set) excitation spectra were determined using several ab initio methods:   CIS,   RPA,   CASSCF,   and   TDDFT.   Obtained   values   were   compared   with semiempirical ZINDO method, other calculations found recently in literature and experimental data. It was demonstrated that for qualitatively correct spectra description the AO basis must include both the polarization and diffuse functions. The later play an important role in formation of Rydberg MOs. Estimated energies of the spectra transitions using the CIS method remain relatively far from the measured values. RPA method can be already considered as a quantitatively accurate method when sufficiently large basis set is used. For CASSCF approach, it was shown that even the lowest energy transitions are insufficiently described in CAS formalism and much larger active space or inclusion of more inactive orbitals in correlation treatment would be necessary for obtaining sufficient accuracy. It can be stated that without sufficiently large correlation contributions, the determined spectra are not able to reach quantitative agreement with experimental data. From the methods treated in this study, only TDDFT can be considered as a useful tool for spectra prediction, at least for calculations of lower excited states. It is relatively fast and feasible for calculation of middle-size molecules. ZINDO  approximation  is  also  relatively  successful  for  such  large  systems.  Acceptable predictions of experimentally observed energy transitions in the range of Q and B bands were obtained. Until higher (UV) part of spectra is examined where the excitations to Rydberg orbital will happen, it can be considered as a good candidate for electron spectra calculations.

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