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For the first time, a theoretical study has been performed on the prototypical decathio[10]circulene (C(20)S(10)) species, which is an analogue of the novel octathio[8]circulene "Sulflower" molecule (C(16)S(8)). Examinations of the singlet and triplet states of C(20)S(10) were made at the B3LYP/6-311G(d) level. Local minima of C(2) and C(s) symmetry were found for the lowest singlet and triplet states, respectively. The stability of C(20)S(10) was assessed by calculating the ΔH°(f) of C(16)S(8) and C(20)S(10) and the ΔH(o) for their decomposition into C(2)S units. Frontier molecular orbital plots show that structural adjacent steric factors along with the twist and strain orientations of C(20)S(10) do not disturb the aromatic π-delocalizing effects. In fact, C(20)S(10) maintains the same p(z) HOMO character as C(16)S(8). These similarities are further verified by density-of-states characterization. Calculated infrared spectra of C(16)S(8) and C(20)S(10) show broad similarities. Molecular electrostatic potential results reveal that eight of the peripheral sulfur atoms are the most electronegative atoms in the molecule, while the interior ten-membered ring exhibits virtually no electronegativity.

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.

We investigated the relaxation dynamics of bis(2-phenylpyridinato-)(2,2'-bipyridine)iridium(III), [Ir(ppy)(2)bpy](+) using the technique of time-resolved spectroscopy. In the visible emission spectra this molecule exhibits triple phosphorescence: displaying blue, green, and orange bands. From the dependence of spectral shifts with polarity of solvent, decay lifetimes, and the results of calculations using time-dependent density functional theory, we assigned these three emitting states to be triplet interligand charge-transfer ((3)LLCT), metal-to-ligand ppy charge transfer ((3)MLCT(ppy)), and metal-to-ligand bpy charge transfer ((3)MLCT(bpy)) states. The blue states were formed promptly after excitation at wavelength 355 nm; the one lying at higher energy decaying with a time coefficient 0.79-2.56 ns is assigned to be a triplet MLCT, and the other at lower energy decaying in 1.5-2.8 μs is assigned to (3)LLCT(A), A symmetry. This decay time coefficient of (3)LLCT(A) decreases with increasing dielectric constant of the solvent indicating this state mixing of some MLCT character. The green state (3)MLCT(ppy) decays in 0.13-4.8 ns to a nearby intermediate state either (3)MLCT(ppy) or (3)MLCT(bpy). The orange state (3)MLCT(bpy) is coupled to the intermediate state to have a rise time about 0.36-0.84 ns and decays in 425-617 ns. Although many triplet states exist in a small energy range, they couple weakly to display triple emission. All (3)LLCT and (3)MCLT states are coupled to the singlet (1)LLCT manifold directly and/or indirectly and contribute to the emission in the visible range.

Coupled-cluster calculations with extended basis sets that include noniterative connected triple excitations (CCSD(T)) have been used to study the FOOOF isomer of F(2)O(3). Second-order Moller-Plessett perturbation theory (MP2) and density-functional theory (B3LYP functional) calculations have also been performed for comparison. Two local minima of similar energy, namely, conformers of C(2) and C(s) symmetry have been located. Structures, harmonic vibrational frequencies, and standard enthalpies and free energies of formation have been calculated. The calculated bond lengths of F(2)O(3) are more characteristic of those in F(2)O and a "normal" peroxide than the unusual bond lengths in F(2)O(2). Both conformers have equal F-O and O-O bond lengths, contrary to a recent suggestion of an unsymmetrical structure. The harmonic vibrational frequencies can aid possible identification of gaseous F(2)O(3). The calculated Δ(f)H° and Δ(f)G° are 110 and 173 kJ mol(-1), respectively. These values are based on extrapolation of CCSD(T) results with augmented triple- and quadruple-ζ basis sets and are expected to be within chemical accuracy (i.e., 1 kcal mol(-1) or 4 kJ mol(-1)). F(2)O(3) is calculated to be stable to decomposition to either FO + FOO or F(2) + O(3), but unstable to decomposition to its elements, to F(2)O(2) + (1)/(2)O(2), and to F(2)O + O(2).

A theoretical comparative study of a series of five- and six-coordinate iron porphyrins, FeP(L) and FeP(L)(O(2)), has been carried out using DFT methods, where P = porphine and L = imidazole (Im), 1-methylimidazole (1-MeIm), 2-methylimidazole (2-MeIm), 1,2-dimethylimidazole (1,2-Me(2)Im), 4-ethylimidazole (4-EtIm), or histidine (His). Two ligated "picket-fence" iron porphyrins, FeTpivPP(2-MeIm) and FeTpivPP(2-MeIm)(O(2)), were also included in the study for comparison. A number of density functionals were employed in the computations to obtain reliable results. The performance of functionals and basis set effects were investigated in detail on FeP, FeP(Im), and FeP(Im)(O(2)), for which certain experimental information is available and there are some previous calculations in the literature for comparison. Many subtle distinctions in the effects of the different imidazole ligands on the structures and energetics of the deoxy- and oxy iron porphyrins are revealed. While FeP(2-MeIm) is identified to be high spin (S = 2), the ground state of FeP(1-MeIm) may be an admixture of a high-spin (S = 2) and an intermediate-spin (S = 1) state. The ground state of FeP(L)(O(2)) may be different with different L. A weaker Fe-L bond more likely leads to an open-shell singlet ground state for the oxy complex. The 2-methyl group in 2-MeIm, which increases steric contact between the ligand and the porphyrinato skeleton, weakens the Fe-O(2) bond, and thus iron porphyrins with 2-MeIm mimic T-state (low affinity) hemoglobin. The calculated FeP(2-MeIm)-O(2) bonding energy is comparable to the FeTpivPP(2-MeIm)-O(2) one, in agreement with the fact that the picket-fence iron porphyrin binds O(2) with affinity similar to that of myoglobin but different from the result obtained by the CPMD scheme. Im and 4-EtIm closely resemble His, the biologically axial base, and so future computations on hemoprotein models can be simplified safely by using Im.

Ab initio theoretical calculations have been used to study the influence of phosphorus substituents, Y, on the tautomerism between the vinylphosphine XP(H)C(CH(3))=CH(2) and the phosphaalkene XP=C(CH(3))(2) (X = H, F, Cl, Br, OH, and Ar(F); Ar(F) = 2,6-(CF(3))(2)C(6)H(3)) and on the acidity of the aforementioned vinylphosphine. The stabilization of the phosphaalkene and the increased acidity of the vinylphosphine by Ar(F) are possible factors in the successful synthesis of certain isolable phosphaalkenes. In this work, the properties of Ar(F) are assessed theoretically. Density functional theory using the B3LYP functional has been used for all substituents. In addition, coupled-cluster singles and doubles with noniterative triples (CCSD(T)) has been used for X = H, F, Cl, Br, and OH. The phosphaalkene is favored over the vinylphosphine for all substituents, with F having the strongest stabilizing effect. Cl, Br, and OH have stronger stabilizing effects than Ar(F). In contrast, the most acidic vinylphosphine is that with Ar(F). To aid in the interpretation and analysis of future experimental work, CCSD(T) calculations were used to provide structures and vibrational frequencies for the series XP=C(CH(3))(2) (X = H, F, Cl, Br). The influence of the substituent on geometries and C=P and X-P stretching frequencies was examined, and comparisons were made with the CH(2)=PX series.

A theoretical study has been made on six isomers of H2SO2 using coupled-cluster singles and doubles with noniterative triple excitations (CCSD(T)). The isomers studied are sulfoxylic acid (S(OH)2; C2 and Cs conformers), sulfinic acid (HS(=O)OH; 2 C1 conformers), dihydrogen sulfone (H2SO2; C2v), sulfhydryl hydroperoxide (HSOOH; C1), thiadioxirane (Cs), and dihydrogen persulfoxide (H2SOO; Cs). Molecular geometries, harmonic vibrational frequencies, and infrared intensities of all species were obtained using the CCSD(T) method and the 6-311++G(2d,2p) basis set. All aforementioned species were found to be local minima, with the exception of thiadioxirane, which has one imaginary frequency. A prior possible infrared observation of sulfinic acid was reassessed on the basis of the present data. In agreement with previous MP2 results, the present CCSD(T) data provide support for at most 4 of the 8 observed frequencies. The CCSD(T) frequencies and intensities should be of assistance in future identification of H2SO2 isomers by vibrational spectroscopy. Relative energies were calculated using the CCSD(T) method and several larger basis sets. As found previously, the lowest energy species is C2 S(OH)2, followed by Cs S(OH)2, HS(=O)OH, H2SO2, HSOOH, thiadioxirane, and H2SOO. Expanding the basis set significantly reduces the relative energies of HS(=O)OH and H2SO2. The CCSD(T) method was used with extended basis sets (up to aug-cc-pV(Q+d)Z) and basis set extrapolation in two reaction schemes to calculate the DeltaH degrees t (25 degrees C) of C2 S(OH)2. The two reaction schemes gave -285.8 and -282.7 kJ mol-1, which are quite close to a prior theoretical estimate (-290 kJ mol-1).

The electronic structure of some substituted, four-coordinate iron(II) porphyrins has been investigated with DFT methods. These systems include iron tetraphenylporphine (FeTPP), iron octamethyltetrabenzporphine (FeOTBP), iron tetra(alpha,alpha,alpha,alpha-orthopivalamide)phenylporphine (FeTpivPP, also called "picket fence" porphyrin), halogenated iron porphyrins (FeTPPXn, X=F, Cl; n=20, 28), and iron octaethylporphine (FeOEP). A number of density functionals were used in the calculations. Different from the popular, intermediate-spin FeTPP, the ground states of FeOTBP, FeTPPCl28, and FeTPPF20betaCl8 are predicted to be high spin. The calculated result for FeOTBP is in agreement with the early experimental measurement, thereby changing the previous conclusion drawn from the calculations with only the BP functional (J. Chem. Phys. 2002, 116, 3635). But FeTpivPP might have an intermediate-spin ground state, a conclusion that is different from the "experimental" one. With a notably expanded Fe-N bond length, FeOEP might exist as an admixed-spin (S=1, 2) state. We also calculated the electron affinities (EAs) for the various iron porphyrins and compared them to experiment. On the basis of the calculated trends in the EAs and in the orbital energies, the experimental EAs for FeTpivPP, FeTPPF20, and FeTPPCl28 may be too small by 0.4-0.5 eV.

The electronic structure, molecular structure, and electronic spectra of lanthanide(III) mono- and bisporphyrin complexes are investigated using a DFT/TDDFT method. These complexes include YbP(acac), YbP(2), [YbP(2)](+), YbHP(2), and [YbP(2)](-) (where P = porphine and acac = acetylacetonate). To shed some light on the origin of the out-of-plane displacement of Yb in YbP(acac), unligated model systems, namely, planar D(4h) and distorted C(4nu) YbP, were calculated. For comparison, the calculations were also extended to include the C and [Ce(IV)P(2) ](+) systems. Even without an axial ligand, the lanthanide atom lies considerably above the porphyrin plane; the distortion of the YbP molecular structure from a planar D(4h) to the nonplanar C(4nu) symmetry leads to a considerable energy lowering. The axial ligand makes the metal out-of-plane displacement even larger, and it also changes the redox properties of the lanthanide monoporphyrin. The ground-state configurations of YbP(2) and YbHP(2) were determined by considering several possible low-lying states. YbP(2) is confirmed to be a single-hole radical. The special redox properties of the bisporphyrin complexes can well be accounted for by the calculated ionization potentials and electron affinities. The TDDFT results provide a clear description of the UV-vis and near-IR absorption spectra of the various lanthanide porphyrins.

The effects of peripheral substituents and axial ligands (L) on the electronic structure and properties of cobalt tetraphenylporphyrin (CoTPP) have been studied using DFT methods. Various density functionals were tested, and the ground state of each system was determined by considering several possible low-lying states. The ground states of the fully fluorinated CoTPPF28(L)2 complexes with L = THF, Py, and Im were identified to be high-spin (4E(g)) by the meta-GGA functional tau-HCTH, which contains the kinetic energy density tau, in agreement with experimental measurements. All the pure GGA functionals, including the recently developed mPBE, OPBE, and HCTH/407, show more or less overestimation of the relative energies of the high-spin states. The energy gap between the 2A(1g) and 4E(g) states is insignificant (approximately 0.1 eV) and varies in the order L = Py < L = THF < L = Im. The results and their trend are consistent with 19F NMR studies which show partial population of the 4E(g) state in CoTPPF28(THF)2 and CoTPPF28(Py)2 and a complete conversion to the high-spin state in CoTPPF28(1-MeIm)2. Upon coordination by two very strong field axial CO ligands, CoTPPF28(CO)2 becomes low-spin, as in unligated CoTPPF(x). The influence of the peripheral substituents and axial ligands on the ionization potentials, electron affinities, and CoTPPF(x)-(L)2 binding strength was also investigated in detail.

A theoretical comparative study of complexes of porphyrin (P), porphyrazine (Pz), phthalocyanine (Pc), porphycene (Pn), dibenzoporphycene (DBPn), and hemiporphyrazine (HPz) with iron (Fe) has been carried out using a density functional theory (DFT) method. The difference in the core size and shape of the macrocycle has a substantial effect on the electronic structure and properties of the overall system. The ground states of FeP and FePc were identified to be the 3A2g [(d(xy))2(d(z)2)2(d(pi))2] state, followed by 3E(g) [(d(xy))2(d(z)2)1(d(pi))3]. For FePz, however, the 3E(g)-3A2g energy gap of 0.02 eV may be too small to distinguish between the ground and excited states. When the symmetry of the macrocycle is reduced from D4h to D2h, the degeneracy of the d(pi) (d(xz), d(yz)) orbitals is removed, and the ground state becomes 3B2g [(d(xy))2(d(z)2)1(d(yz))2(d(xz))1] or 3B3g [...(d(yz))1(d(xz))2] for FePn, FeDBPn, and FeHPz. The calculations also show how the change of the macrocycle can influence the axial ligand coordination of pyridine (Py) and CO to the Fe(II) complexes. Finally, the electronic structures of the mono- and dipositive and -negative ions for all the unligated and ligated iron macrocycles were elucidated, which is important for understanding the redox properties of these compounds. The differences in the observed electrochemical (oxidation and reduction) properties between metal porphycenes (MPn) and metal porphyrins (MP) can be accounted for by the calculated results (orbital energy level diagrams, ionization potentials, and electron affinities).