Ab initio multiconfiguration reference perturbation theory calculations on the energetics of low-energy spin states of iron(III) porphyrins

Abstract

Although a major goal of inorganic spectroscopy is to determine the energetics of the low-lying spin states of transition metal complexes, surprisingly little has been accomplished in this respect by means of accurate ab initio calculations. Against this context, we present ab initio multiconfiguration reference perturbation theory (CASPT2) calculations with large basis sets on the low-lying spin states of Fe(III)(P)Cl and [Fe(P)Cl](+) (P(2-)=porphinato). The CASPT2 results on the energetics of various low-lying spin states studied differ significantly, sometimes even dramatically, from those obtained from density functional theory calculations.

Spectroscopic identification of reactive porphyrin motions

“…65 The ground state is experimentally established as a sextet, [60][61][62][63][64] but the lowest lying sextet and quartet states are predicted to lie close in energy. [65][66][67] Even after correcting for the larger zeropoint energy of the quartet state, our calculations predict comparable energies for the quartet and sextet states in all ferric porphyrin chlorides that we have considered (see Table I). …”

“…Although experimental measurements indicate a sextet ground state for the halide complexes, [60][61][62][63][64] predicted quartet state energies lie close to or slightly below the sextet energies, as found in previous calculations. [65][66][67] The energies listed in Table I include the zero-point energies α¯ω α /2 calculated from the predicted normal mode frequencies ω α , which favor the sextet state. The small resulting sextet/quartet energy splittings are on the order of energy differences resulting from minor variations in convergence criteria, and thus cannot be considered significant.…”

Towards quantifying the role of exact exchange in predictions of transition metal complex properties

“…For a balanced treatment of both static correlation and dynamic correlation also in the absence of self-interaction error, multireference wavefunction techniques have been used [43][44][45][46][47][48][49][50] to study spin crossover complexes up to around 45 atoms in size. 48 The predominant wavefunction method employed in the study of spin-crossover complexes is CASPT2, and, while it scales more expensively than density functional theory approaches, recent improvements in scaling 44 have made larger systems tractable.…”

The performance of nonhybrid density functionals for calculating the structures and spin states of Fe(II) and Fe(III) complexes

“…Matters are improved by the inclusion of exact HF exchange in hybrid functionals like B3LYP, but in its original formulation, B3LYP still tends to favor high-spin configurations. 22,36,38 This has prompted a reparameterization of the B3LYP hybrid functional to modify the proportion of exact exchange and thus improve agreement with experimental data for spin-state energy differences in a variety of Fe(II) complexes. 10,36,38 The important result of these studies is that the difference between high and low spin states is a linear function of the amount of exact exchange, and that the difference between the predictions of different functionals is approximately constant.…”

“…[22][23][24]37 In general, local and gradient corrected density functionals favor low spin configurations, while the HF theory favors high spin. Matters are improved by the inclusion of exact HF exchange in hybrid functionals like B3LYP, but in its original formulation, B3LYP still tends to favor high-spin configurations.…”