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In a recent paper [/. Phys. Chem. 1995, 99, 5325] we presented a model characterizing the activation hardness in terms of the activation energy of rotational isomerization reactions. We present in this paper an extension of that model for obtaining a direct relation between hardness and potential energy profiles. The hardness is related to the potential energy through a two-term equation in which the leading term is a linear dependence on the potential energy, and the second term can be seen as a correction occurring in cases where the critical points for hardness and energy profiles do not match. It is shown that the hardness and energy profiles behave in opposite ways that can be rationalized in terms of two parameters. A qualitative proof of the maximum hardness principle (PMH), within the frame of the model potential employed, is given, and its consistency with the Hammond postulate is discussed in the light of results for different molecules presenting rotational isomerization.

A set of substituted (sulfonate, amino) nickel porphyrin derivatives such as phthalocyanine and phenylporphyrin was studied by spectroscopic (UV-vis, FTIR, XPS) and quantum-chemical methods. The Q and Soret bands were identified in the UV-vis spectra of aquo solutions of the tetrasulfo-substituted complexes and in DMF and ACN solutions of the amino-substituted phenylporphyrin and phthalocyanine Ni(II) complexes, respectively. In all the complexes the frontier molecular orbitals predict that the oxidation and reduction sites are localized on the ligand rather than in the metal atom. A natural bonding orbital (NBO) analysis of all the complexes showed that a two-center bond NBO between the pyrrolic nitrogens (Npyrr) and the nickel atom does not exist, the Npyrr...Ni interaction occurring instead by a delocalization from one lone pair of each Npyrr toward one lone pair of the nickel atom, as estimated by second-order perturbation theory. The calculated values of electronic transitions between the frontier molecular orbitals are in good agreeement with the UV-vis data. At the theoretical level, we found that while the ligand effect is more important in the Q-band (approximately 16 kcal/mol), the substituent effect is more significant in the Soret band (approximately 9 kcal/mol). A good agreement was also found between the experimental and calculated infrared spectra, which allowed the assignment of many experimental bands. The XPS results indicate that the Ni(II) present in the phenylporphyrin structure is not affected by a change of the substituent (sulfonate or amino).