The SAC(symmetry adapted cluster)/SAC-CI method is applied to the calculations of the ground, excited, and ionized states of the free base porphin. The electronic spectrum of porphin is well reproduced and new assignments for the B (Soret), N, L, and M bands are proposed. The present result shows that the four-orbital model is strongly perturbed for the B and N bands by the excitations from the lower 4b1u MO and that the σ electron correlations are important for the description of the excited states. The absorption peaks in the ionization spectrum are assigned and the reorganization effect is found to be large especially for the n and σ electron ionizations.

Excited states of free base chlorin (FBC), free base Bacteriochlorin (FBBC), pheophytin a (Pheo a), and chlorophyll a (Chlo a), which are derivatives of free base porphine (FBP), were calculated by the SAC (symmetry adapted cluster)/SAC-CI (configuration interaction) method. The results reproduced well the experimentally determined excitation energies. The reduction of the outer double bonds in the porphine ring in the order of FBP, FBC, and FBBC causes a breakdown of the symmetry and a narrowing of the HOMO-LUMO gap, which result in a red shift of the Q x band and an increase of its intensity. In the change from Pheo a to Chlo a, the Mg coordination reduces the quasidegeneracy in the Q x state and then increases the spectral intensity. The disappearance of the Q y humps from the absorption spectrum of Pheo a, compared with that of Chlo a, is due to the red shift of the Q y state.

No abstract

It is postulated that the copper(I) nitrite complex is a key reaction intermediate of copper containing nitrite reductases (Cu-NiRs), which catalyze the reduction of nitrite to nitric oxide (NO) gas in bacterial denitrification. To investigate the structure-function relationship of Cu-NiR, we prepared five new copper(I) nitrite complexes with sterically hindered tris(4-imidazolyl)carbinols [Et-TIC = tris(1-methyl-2-ethyl-4-imidazolyl)carbinol and iPr-TIC = tris(1-methyl-2-isopropyl-4-imidazolyl)carbinol] or tris(1-pyrazolyl)methanes [Me-TPM = tris(3,5-dimethyl-1-pyrazolyl)methane; Et-TPM = tris(3,5-diethyl-1-pyrazolyl)methane; and iPr-TPM = tris(3,5-diisopropyl-1-pyrazolyl)methane]. The X-ray crystal structures of all of these copper(I) nitrite complexes were mononuclear eta(1)-N-bound nitrite complexes with a distorted tetrahedral geometry. The electronic structures of the complexes were investigated by absorption, magnetic circular dichroism (MCD), NMR, and vibrational spectroscopy. All of these complexes are good functional models of Cu-NiR that form NO and copper(II) acetate complexes well from reactions with acetic acid under anaerobic conditions. A comparison of the reactivity of these complexes, including previously reported (iPr-TACN)Cu(NO2) [iPr-TACN = 1,4,7-triisopropyl-1,4,7-triazacyclononane], clearly shows the drastic effects of the tridentate ligand on Cu-NiR activity. The copper(I) nitrite complex with the Et-TIC ligand, which is similar to the highly conserved three-histidine ((His)3) ligand environment in the catalytic site of Cu-NiR, had the highest Cu-NiR activity. This result suggests that the (His)3 ligand environment is essential for acceleration of the Cu-NiR reaction. The highest Cu-NiR activity for the Et-TIC complex can be explained by the structural and spectroscopic characterizations and the molecular orbital calculations presented in this paper. Based on these results, the functional role of the (His)3 ligand environment in Cu-NiR is discussed.

The proximal heme axial ligand plays an important role in tuning the reactivity of oxoiron(IV) porphyrin π-cation radical species (compound I) in enzymatic and catalytic oxygenation reactions. To reveal the essence of the axial ligand effect on the reactivity, we investigated it from a thermodynamic viewpoint. Compound I model complexes, (TMP(+•))Fe(IV)O(L) (where TMP is 5,10,15,20-tetramesitylporphyrin and TMP(+•) is its π-cation radical), can be provided with altered reactivity by changing the identity of the axial ligand, but the reactivity is not correlated with spectroscopic data (ν(Fe═O), redox potential, and so on) of (TMP(+•))Fe(IV)O(L). Surprisingly, a clear correlation was found between the reactivity of (TMP(+•))Fe(IV)O(L) and the Fe(II)/Fe(III) redox potential of (TMP)Fe(III)L, the final reaction product. This suggests that the thermodynamic stability of (TMP)Fe(III)L is involved in the mechanism of the axial ligand effect. Axial ligand-exchange experiments and theoretical calculations demonstrate a linear free-energy relationship, in which the axial ligand modulates the reaction free energy by changing the thermodynamic stability of (TMP)Fe(III)(L) to a greater extent than (TMP(+•))Fe(IV)O(L). The linear free energy relationship could be found for a wide range of anionic axial ligands and for various types of reactions, such as epoxidation, demethylation, and hydrogen abstraction reactions. The essence of the axial ligand effect is neither the electron donor ability of the axial ligand nor the electron affinity of compound I, but the binding ability of the axial ligand (the stabilization by the axial ligand). An axial ligand that binds more strongly makes (TMP)Fe(III)(L) more stable and (TMP(+•))Fe(IV)O(L) more reactive. All results indicate that the axial ligand controls the reactivity of compound I (the stability of the transition state) by the stability of the ground state of the final reaction product and not by compound I itself.

Electronic excitation spectra of furan and pyrrole are reinvestigated by the symmetry-adapted cluster configuration-interaction method. The 47 and 46 lowest singlet and triplet electronic states are computed for furan and pyrrole, respectively. Two series ͑1a 2 and 2b 1 ͒ of low-lying Rydberg states and the valence-* excited states strongly influence each other in both furan and pyrrole. The present calculations give detailed and satisfactory theoretical assignments of the vacuum ultraviolet spectra and the electron energy-loss spectra of the two molecules. The similarities and differences in the electronic excitations between furan and pyrrole are discussed in detail. The accuracy and assignments of recent theoretical studies, i.e., complete active space second-order perturbation, multireference Møller-Plesset perturbation, second-order algebraic-diagrammatic construction, multireference double configuration interaction, and CC3, are compared.

An accurate determination of the effective electric field (E eff ) in YbF is important, as it can be combined with the results of future experiments to give an improved new limit for the electric dipole moment of the electron. We report a relativistic coupled-cluster calculation of this quantity in which all the core electrons were excited. It surpasses the approximations made in the previous reported calculations. We obtain a value of 23.1 GV/cm for E eff in YbF with an estimated error of less than 10%. The crucial roles of the basis sets and the core excitations in our work are discussed.The electric dipole moment (EDM) of a nondegenerate system arises from violations of both the parity (P) and the time-reversal (T) symmetries [1]. T violation implies charge parity (CP) violation via CPT theorem [2]. In general, CP violation is a necessary condition for the existence of the EDMs of physical systems, and, in particular, atoms and molecules. Paramagnetic atoms and molecules are sensitive to the EDM of the electron (eEDM) [3], which is an important probe of the physics beyond the standard model [4]. The eEDM arising from CP violation could also be related to the matter-antimatter asymmetry in the universe [5]. A number of studies using atoms have been performed during the past few decades to extract an upper limit for the eEDM [6]. In general, for heavy polar molecules, the effective electric field experienced by an electron (E eff ) obtained from relativistic molecular calculations can be several orders of magnitude larger than that in atoms [7]. Therefore, the experimental observable (i.e., the shift in energy because of the interaction of the electric field with the eEDM) is also several orders of magnitude larger. Owing to the high sensitivity of the eEDM in molecules, there has been a considerable increase in interest in this field during the past decade The aim of the present work is to calculate E eff in YbF using a rigorous relativistic many-body method, which is more accurate than the methods used in the previous calculations. The method we have chosen is the four-component relativistic coupled-cluster (RCC) method, which is arguably the current gold standard for calculating the electronic structure of heavy atoms and diatomic molecules [18].The electron EDM interaction Hamiltonian in a molecule can be written as [19] Here, d e is the eEDM of an electron,  is one of the Dirac matrices, and  are the Pauli spin matrices. i is the index of summation labelling for electrons and N e is the total number of electrons. E int is the electric field acting on an electron in a molecule. The quantity that is of experimental interest in the search for the eEDM is an energy shift (E) of a particular state owing to the interaction Hamiltonian given in Eq.(1). This can be expressed as

Singlet excited states and ionized states of aniline are studied by the symmetry adapted cluster/ configuration interaction method. Absorption bands of states that have mainly -* nature are assigned as 1 AЉ (ϳ 1 B 2 ), 1 AЈ (ϳ 1 A 1 ), 1 AЉ (ϳ 1 B 2 ), 1 AЈ (ϳ 1 A 1 ), 1 AЉ (ϳ 1 B 2 ) in increasing-energy order. An s-Rydberg state is predicted to lie between the first and second valence states, in agreement with recent experimental results. The lowest band has a charge-resonance character with a slight charge-transfer ͑CT͒ character ͑CT is defined as NH 2 →C 6 H 5 ͒; third and fifth valence bands have back-CT ͑BCT͒ nature, and second and fourth are local excitations within the benzene ring. The extent of CT of excited states depends on amino group conformation. In the planar form, CT characters of several states were altered; however, spectral shapes are very similar to that of the equilibrium form. On the other hand, amino group twisting altered both the spectrum and nature of excited states. Third and fourth lowest valence states exhibited strong CT character, while fifth to eighth states are of the strong BCT type, implying that the CT nature of excited states of aniline can be changed by amino group twisting. For ionized states, the lowest three states are assigned to 2 AЈ (ϳ 2 B 1 ), 2 AЉ (ϳ 2 A 2 ), 2 AЈ (ϳ 2 B 1 ) in increasing-energy order, all being -ionizations. The sixth one is also due to -ionization (ϳ 2 B 1 ) and the others are -ionizations.Ordering was the same as Koopmans' case.