Ab Initio Comparative Study of the Structure and Properties of H<sub>2</sub>-Porphin and H<sub>2</sub>-Phthalocyanine. The Electronic Absorption Spectra

2003-10-23T00:00:00Z (GMT) by H. Cortina M. L. Senent Y. G. Smeyers
The structure and properties of free base porphin (PH<sub>2</sub>) and phthalocyanine (PcH<sub>2</sub>) are compared using B3LYP/6-31G(d,p) (5D) and UHF/6-31G(d,p) (5D) calculations. The two molecules show two isomers where the two central hydrogen atoms lie in trans and cis positions. The <i>C</i><sub>2</sub><i><sub>v</sub></i> cis-isomers lie 2840 cm<sup>-1</sup> (PH<sub>2</sub>) and 3183 cm<sup>-1</sup> (PcH<sub>2</sub>) over the most stable <i>D</i><sub>2</sub><i><sub>h</sub></i> trans-geometries. The tautomerization processes are hindered by transition states at 5269 cm<sup>-1</sup> (PH<sub>2</sub>) and 5107 cm<sup>-1</sup> (PcH<sub>2</sub>). The ionization potential and the electron affinity are calculated to be +6.40 and −1.11 eV for PH<sub>2</sub> and +6.03 and −1.91 eV for PcH<sub>2</sub>, from the determination of the cation and anion energies and structures. Harmonic frequencies and electric and nonlinear optical properties are also reported. An interpretation of the electronic absorption spectra is also given for both molecules. For PH<sub>2</sub>, CASPT2 calculations have been achieved using the 6-31G(d,p) (5D) base set and different active spaces. For PcH<sub>2</sub>, the spectrum has been interpreted at the CASPT2(4,4)/6-31 level. The calculated positions of the Q<i><sub>x</sub></i>, Q<i><sub>y</sub></i>, and B bands of phthalocyanine (1.61, 1.99, and 3.14 and 3.19 eV) are in a very good agreement with the experimental values (1.81, 1.99, and 3.65 eV).