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The reactions of ¹S and ³D Cu⁺ ions with N2O are studied by means of ab initio methods. The structure of all stationary points along the reaction coordinate is optimized with density functional theory (DFT), using the B3LYP functional. The reaction and binding energies and the activation barriers are evaluated with DFT-B3LYP and also with the coupled cluster method CCSD(T). Three reaction paths were found, depending on the orientation of the approaching N2O molecule. If N2O approaches the Cu⁺ ions via its O-side, this leads to formation of CuO⁺ and N2. On the other hand, if N2O attacks via its N-end, the reaction products are CuN2⁺ and an O atom. Finally, it was found that the ³D Cu⁺ ions can also react with the central N of N2O, with formation of the insertion product [OCuN2]⁺. The most exothermic path is the insertion reaction of Cu⁺ into the N2−O bond. The calculations indicate that bare ¹S Cu⁺ ions are relatively inert with respect to N2O:  high energy barriers are calculated. The ³D Cu⁺ ions, on the other hand, are strong activators of the N2−O bond, and the reactions proceed without energy barrier, in agreement with experiment.

The reactions of 3d transition metal atoms with N2O, producing the metal oxide and N2, have been studied by means of density functional theory and the coupled cluster method CCSD(T). The importance of charge transfer in the reaction mechanism has been investigated. For Sc, Ti, and V, the transition state is very reagent-like, and almost no charge transfer occurs. On the other hand, charge transfer from the metal 4s orbital into the N2O LUMO becomes more important when moving to the right in the 3d series. The reactions with Sc, Ti, and V proceed almost without energy barriers, whereas for Mn, Fe barriers around 9 kcal/mol are calculated. For transition metal atoms with a 3dⁿ4s² ground-state configuration, a correlation is found between the activation barriers and the binding energy of the formed metal oxide. The 3dⁿ⁺¹4s¹ configuration gives rise to a higher reactivity than the 3dⁿ4s² configuration.

The optical spectra of the cyclometalated complexes Pd,Pt(thpy)2, with thpy^-, the deprotonated form of 2-(2-thienyl)pyridine, have been calculated using a multiconfigurational second-order perturbation treatment based on a complete active space reference wave function, i.e., the CASSCF/CASPT2 method. The calculations were performed on optimized structures within C2v symmetry, obtained using density functional theory (DFT). The structure and lowest excited states of free protonated thpyH were also calculated. On the basis of the calculated results for excitation energies and oscillator strengths, the most important bands in the experimental absorption spectra of the complexes were assigned as excitations to states of symmetry ¹A1 or ¹B2, all of mixed charge-transfer/ligand-centered character. The lowest triplet excited state was calculated to be the a ³A1 state, closely followed by the ³B2 state. Both excitations were found to be almost purely ligand-centered, with a very limited charge-transfer contribution in Pd(thpy)2 and an only slightly larger contribution in Pt(thpy)2. The differences between the position and character of the singlet excited states in the absorption spectra of both molecules are also discussed. The results are found to be consistent with the trends obtained from previous experimental measurements.

Density functional and complete active space (CAS) SCF and CASPT2 calculations have been performed to investigate structural and electronic features of p-phenylbisphosphinidene and the analogous p-phenylbisnitrene, p-phenylbismethylene, p-phenylbis(fluoromethylene), and p-phenylbis(chloromethylene) molecules. Similar to the other species investigated, with the exception of p-phenylbis(fluoromethylene), p-phenylbisphosphinidene exhibits an open-shell singlet diradical ground state with a quinone-like structure. The singlet−triplet energy gap, computed with CASPT2 on CASSCF(10,10)/6-31+G* optimized structures, amounts to 1.78 kcal/mol. At the DFT (B3LYP/6-31+G*) level a value of 5.14 kcal/mol is obtained. Due to the highly multiconfigurational character of the p-phenylbisphosphinidene singlet and triplet wave functions, the DFT treatment becomes less accurate. Going to p-phenylbisnitrene, p-phenylbis(chloromethylene), p-phenylbis(fluoromethylene), and p-phenylbismethylene energies are better described by DFT calculations as the triplet wave functions have only one and the singlet wave functions exactly two leading configurations. The calculated singlet−triplet energy gaps are found to lie in a range of 1.3−2.2 kcal/mol at both DFT and CASPT2 levels of theory. p-Phenylbis(fluoromethylene) is a special case in the series as it has a closed-shell singlet ground state. The multiconfigurational treatment shows, however, that the singlet diradical state is very close in energy to the closed-shell singlet, even though the stability of the diradical is somewhat overemphasized by this method. The relative energies and the respective IR spectra are also discussed.

The structure of the local Co(II) six-ring oxygen environment in zeolite A and the corresponding ligand field spectrum have been studied using large cluster models, including all six surrounding Si or Al tetrahedra terminated by either H or OH groups. Structures were optimized by means of density functional theory (DFT), using a nonlocal (BP86) approach and keeping the orientation of all dangling bonds frozen at the X-ray diffraction (XRD) positions. Electronic spectra were calculated using multiconfigurational perturbation theory based on a CASSCF wave function (CASPT2). It is shown that, in all cases, the presence of the Co(II) ion induces a local distortion of the zeolite surface, resulting in an oxygen coordination number of 3, 4, or 5, depending on the Si/Al ratio. This distortion is reflected in the calculated electronic spectra, showing an increased splitting of the Co²⁺ free-ion ⁴F and ⁴P states as compared to the (average) XRD structures. A new general assignment of the spectrum is proposed, different from earlier assignments based on ligand field theory. The calculated excitation energies of the optimized structures are in excellent agreement with the experimental band positions, thus proving the strength of the present combined DFT−CASPT2 approach. Our results further suggest that the experimentally observed splitting of the main band in the spectrum is due to the presence of asymmetric coordination sites, rather than to Jahn−Teller effects or spin−orbit coupling. The latter may, however, at least partly be responsible for the splitting of the weak feature at 25 000 cm^-1.

The molecular and electronic structures of Co(II)-substituted azurin have been investigated using several realistic models of the metal coordination sphere. The geometry of the models was optimized using the hybrid density functional B3LYP method and compared to the structures obtained for similar Cu(II) models. It is found that Co(II) prefers a distorted tetrahedral structure with four strong bonds to two histidine nitrogens, the cysteine sulphur, and the backbone carbonyl group. This is in contrast to Cu(II), where two weak axial bonds to methionine and the backbone oxygen are found, combined with three strong bonds to the histidines and cysteine in the equatorial plane of a trigonal bipyramidal structure. The optimal structure of the models conforms with experimental crystal data, indicating that the active-site structure in these proteins is determined by the preferences of the metal ion and its ligand and not by protein strain. The electronic structure and spectrum of the Co(imidazole)2(SH)(SH)2(HCONH2)⁺ model have been investigated in detail using multiconfigurational second-order perturbation theory based on a complete active-space wavefunction (CASPT2). Nine ligand-field transitions and six Scys → Co charge-transfer transitions have been calculated, and all experimentally observed absorption bands in the absorption spectrum of Co(II) azurin have been assigned. It is shown that the Co−Scys bond is more ionic than the Cu−Scys bond and that this causes the blue shift and weakening of the charge-transfer states in the spectrum of Co(II)-substituted azurin compared to native copper protein.

The structures and interconversion pathway between the [Cu2(μ-η²:η²-O2)]²⁺ and [Cu2(μ-O)2]²⁺ isomers of model systems with three ammonia ligands per copper center are investigated using both density functional theory with a B3LYP functional (B3LYP-DFT) and multiconfigurational perturbation theory (CASSCF/CASPT2). Both methods lead to thoroughly different results for the relative energy of both isomers. The CASPT2 results reveal an intrinsic stabilization of the [Cu2(μ-O)2]²⁺ isomer, thus indicating that the presence of a [Cu2(μ-η²:η²-O2)]²⁺ core in the respiratory proteins must be brought back to the presence of bulky capping ligands and/or to external effects caused by solvents and counterions. Both isomers are found to be diamagnetic. For the [Cu2(μ-η²:η²-O2)]²⁺ isomer an antiferromagnetic coupling constant, −2J, of 4209 cm^-1 is calculated at the CASPT2 level. On the other hand, in the [Cu2(μ-O)2]²⁺ isomer, the lowest ³Bu state is calculated at 9316 cm^-1, but is found to correspond instead to an oxygen π* → σ* excitation.