Meng-Sheng Liao

Electronic structure and bonding in metal phthalocyanines, Metal=Fe, Co, Ni, Cu, Zn, Mg

Electronic structure and bonding in metal phthalocyanines (Metal=Fe, Co, Ni, Cu, Zn, Mg) is investigated in detail using a density functional method. The metal atoms are strongly bound to the phthalocyanine ring in each case, by as much as 10 eV. The calculated orbital energy levels and relative total energies of these D4h structures indicate that Fe and Co phthalocyanines have 3A2g and 2Eg ground states, respectively, but that these states are changed upon interaction with strong-field axial ligands. The valence electronic structures of Fe and Co phthalocyanines differ significantly from those of the others. The HOMOs in Fe, Co, and Cu phthalocyanine are metal 3d-like, whereas in Ni and Zn phthalocyanines, the HOMO is localized on the phthalocyanine ring. The first ionization removes an electron from the phthalocyanine a1u orbital in all cases, with very little sensitivity of the ionization energy to the identity of the metal. Whereas the first reduction in Fe and Co phthalocyanine occurs at the metal, i...

Electronic structure and bonding in metal porphyrins, metal=Fe, Co, Ni, Cu, Zn

A systematic theoretical study of the electronic structure and bonding in metal meso-tetraphenyl porphines MTPP, M=Fe, Co, Ni, Cu, Zn has been carried out using a density functional theory method. The calculations provide a clear elucidation of the ground states for the MTPPs and for a series of [MTPP]x ions (x=2+, 1+, 1−, 2−, 3−, 4−), which aids in understanding a number of observed electronic properties. The calculation supports the experimental assignment of unligated FeTPP as 3A2g, which arises from the configuration (dxy)2(dz2)2(dxz)1(dyz)1. The calculated M–TPP binding energies, ionization potentials, and electron affinities are in good agreement with available experimental data. The influence of axial ligands and peripheral substitution by fluorine are in accord with the experimental observation that not only half-wave potentials (E1/2) of electrode reactions, but also the site of oxidation/reduction, may be dependent on the porphyrin basicity and the type of axial ligand coordination.

Comparative study of metal‐porphyrins, ‐porphyrazines, and ‐phthalocyanines

A theoretical comparative study of complexes of porphyrin (P), porphyrazine (Pz), and phthalocyanine (Pc) with metal (M) = Fe, Co, Ni, Cu, and Zn has been carried out using a DFT method. The calculations provide a clear elucidation of the ground states for the MP/Pz/Pc molecules and for a series of [MP/Pz/Pc]x− and [MP/Pz/Pc]y+ ions (x = 1, 2, 3, 4; y = 1, 2). There are significant differences among MP, MPz, and MPc in the electronic structure and other calculated properties. For FeP/Pz and CoP/Pz, the first oxidation occurs at the central metal, while it is the macroring of FePc and CoPc that is the site of oxidation. The smaller coordination cavity results in a stronger ligand field in Pz than in P. However, the benzo annulation produces a surprisingly strong destabilizing effect on the metal‐macrocycle bonding. The effects of Cl axial bonding upon the electronic structures of the iron(III) complexes of P, Pz, and Pc were examined, as was the bonding of pyridine (py) to NiP, NiPz, and NiPc. The porphinato core size plays a crucial role in controlling the spin state of FeIII in these complexes. FePc(Cl) is predicted to be a pure intermediate‐spin system, whereas NiPz(py)2 and NiPc(py)2 are metastable in high‐spin (S = 1) states. The NiPz/Pc(py)2 binding energy curve has only a shallow well that facilitates decomposition of the complex. The NiP(py)2 bond energy is small, but the relatively deep well in the binding energy curve ought to make this system stable to decomposition.

Performance Assessment of Density-Functional Methods for Study of Charge-Transfer Complexes

Various density functionals are applied to a number of weakly bound intermolecular π–π charge‐transfer (CT) complexes. Most functionals, including the recently developed mPWPW91 and mPW1PW91, grossly underestimate experimental excitation energies; good agreement is obtained only with the half‐and‐half hybrid BH&HLYP functional. PW91PW91 provides the best agreement with intermolecular distances measured in crystal, while the BH&HLYP values are about 0.1 Å too long. Various hybrid functionals with nonlocal exchange correction provide binding energies that compare favorably with the experimental heats of formation measured in solution.

Assessment of the performance of density-functional methods for calculations on iron porphyrins and related compounds.

The behaviors of a large number of GGA, meta‐GGA, and hybrid‐GGA density functionals in describing the spin‐state energetics of iron porphyrins and related compounds have been investigated. There is a large variation in performance between the various functionals for the calculations of the high‐spin state relative energies. Most GGA and meta‐GGA functionals are biased toward lower‐spin states and so fail to give the correct ground state for the high‐spin systems, for which the meta‐GGA functionals show more or less improvement over the GGA ones. The GGA functionals that use the OPTX correction for exchange show remarkably high performance for calculating the high‐spin state energetics, but their results for the intermediate‐spin states are somewhat questionable. A heavily parameterized GGA functional, HCTH/407, provides results which are in qualitative agreement with the experimental findings for the iron porphyrins [FeP, FeP(Cl), FeP(THF)2], but its relative energies for the high‐spin states are probably somewhat too low. The high‐spin state relative energies are then even more underestimated by the corresponding meta‐GGA functional τ‐HCTH. For the hybrid‐GGA functionals, the Hartree‐Fock (HF)‐type (or exact) exchange contribution strongly stabilizes the high‐spin states, and so the performance of such functionals is largely dependent upon the amount of the HF exchange admixture in them. The B3LYP, B97, B97‐1, and τ‐HCTH‐hyb functionals are able to provide a satisfactory description of the energetics of all the systems considered.

Electronic structure and bonding in unligated and ligated FeII porphyrins

The electronic structure and bonding in a series of unligated and ligated FeII porphyrins (FeP) are investigated by density functional theory (DFT). All the unligated four-coordinate iron porphyrins have a 3A2g ground state that arises from the (dxy)2(dz2)2(dπ)2 configuration. The calculations confirm experimental results on Fe tetraphenylporphine but do not support the resonance Raman assignment of Fe octaethylporphine as 3Eg, nor the early assignment of Fe octamethyltetrabenzporphine as 5B2g. For the six-coordinate Fe–P(L)2 (L=HCN, pyridine, CO), the strong-field axial ligands raise the energy of the Fe dz2 orbital, thereby making the iron porphyrin diamagnetic. The calculated redox properties of Fe–P(L)2 are in agreement with experiment. As models for deoxyheme, the energetics of all possible low-lying states of FeP(pyridine) and FeP(2-methylimidazole) have been studied in detail. The groundstate configuration of FeP(2-methylimidazole) was confirmed to be high-spin (dxy)2(dz2)1(dπ)2(dx2−y2)1; FeP (pyri...

Effects of Peripheral Substituents on the Electronic Structure and Properties of Unligated and Ligated Metal Phthalocyanines, Metal = Fe, Co, Zn

The effects of peripheral, multiple -F as well as -C2F5 substituents, on the electronic structure and properties of unligated and ligated metal phthalocyanines, PcM, PcM(acetone)2 (M = Fe, Co, Zn), PcZn(Cl), and PcZn(Cl(-)), have been investigated using a DFT method. The calculations provide a clear explanation for the changes in the ground state, molecular orbital (MO) energy levels, ionization potentials (IP), electron affinities (EA), charge distribution on the metal (QM), axial binding energies, and in electronic spectra. While the strongly electron-withdrawing -C2F5 groups on the Pc ring change the ground state of PcFe, they do not influence the ground state of PcCo. The IP is increased by ∼1.3 eV from H16PcM to F16PcM and by another ∼1.1 eV from F16PcM to F48PcM. A similar increase in the EA is also found on going from H16PcM to F48PcM. Substitution by the -C2F5 groups also considerably increases the binding strength between PcM and the electron-donating axial ligand(s). Numerous changes in chemical and physical properties observed for the F64PcM compounds can be accounted for by the calculated results.

Structure, bonding, and linear optical properties of a series of silver and gold nanorod clusters: DFT/TDDFT studies.

DFT/TDDFT calculations have been carried out for a series of silver and gold nanorod clusters (Ag(n), Au(n), n = 12-120) whose structures are of cigar-type. Pentagonal Ag(n) clusters with n = 49-121 and hexagonal Au(n) clusters with n = 14-74 were also calculated for comparison. Metal-metal distances, binding energies per atom, ionization potentials, and electron affinities were determined, and their trends with cluster size were examined. The TDDFT calculated excitation energies and oscillator strengths were fit by a Lorentz line shape modification, which gives rise to the simulated absorption spectra. The significant features of the experimental spectra for actual silver and gold nanorod particles are well reproduced by the calculations on the clusters. The calculated spectral patterns are also in agreement with previous theoretical results on different-type Ag(n) clusters. Many differences in the calculated properties are found between the Ag(n) and Au(n) clusters, which can be explained by relativistic effects.

Relativistic effects in iron-, ruthenium-, and osmium porphyrins

Abstract Nonrelativistic and relativistic DFT calculations are performed on four-coordinate metal porphyrins MP and their six-coordinate adducts MP(py)2 and MP(py)(CO) (py=pyridine) with M=Fe, Ru, and Os. The electronic structures of the MPs are investigated by considering all possible low-lying states with different configurations of nd-electrons. FeP and OsP have a 3 A 2 g ground state, while this state is nearly degenerate with 3 E g for RuP. Without relativistic corrections, the ground states of both RuP and OsP would be 3 E g . For the six-coordinate adducts with py and CO, the strong-field axial ligands raise the energy of the M dz2-orbital, thereby making the MII ion diamagnetic. The calculated redox properties of MP(py)2 and MP(py)(CO) are in agreement with experiment. The difference between RuP(py)(CO) and OsP(py)(CO), in terms of site of oxidation, is due to relativistic effects.

Assessment of dispersion corrections in DFT calculations on large biological systems

The performance of empirical dispersion corrections in DFT calculations has been assessed for several large, genuine biological systems that include MbAB, H64L(AB), and V68N(AB) (AB = CO, O2), where Mb stands for a wild-type myoglobin, H64L is the (histidine64 → leucine) mutated myoglobin, and V68N is the (valine68 → asparagine) mutated myoglobin. The effects of the local protein environment are accounted for by including the five nearest surrounding residues in the calculated systems and they are examined by comparing the binding energies of AB to the myoglobin and to the porphyrin (Por) without residues. Three versions of Grimmes dispersion correction methods, labeled as DFT-D1, DFT-D2, and DFT-D3, were all tested. In the first version (-D1), the dispersion correction (Edisp) is calculated only for noncovalent interactions between molecular fragments and Edisp within a covalent molecule is not calculated. For the DFT functionals, for which the calculated Por–AB binding energies are already too large, only further overestimation occurs when a dispersion correction is made. The geometry optimizations show that the DFT-D2 and DFT-D3 approaches give too short distances between the residues and the heme moiety in the myoglobins and their calculated relative binding energies ΔEbind(myoglobin-AB/Por–AB) are in poor agreement with experiment in most cases. DFT-D1 performs very well, ensuring structural and energetic features in close agreement with experiment.