Conformational preferences of the axial ligands in some metalloporphyrins. A theoretical study

The conformational preferences of the axial ligands have been determined for several metalloporphyrins MPL and MPLL′ (M = Mo, Fe; P = porphine dianion; L and L′ being the axial ligands). For MoP(C₂H₂) a qualitative analysis indicates that the conformation with the acetylenic bond eclipsing two Mo-N bonds will be favored. Ab initio SCF calculations indicate that:

1. iron porphyrins with an axial imidazole ligand show a flat potential energy curve for the rotation of the imidazole ligand;
2. iron porphyrins with a dioxygen ligand prefer the staggered conformation with the O-O bond projecting along the bisectors of the Fe-N bonds;
3. in the cis-dinitrosyl molybdenum porphyrin, the nitrosyl ligands should be eclipsed with respect to the Mo-N_pyr bonds.

These theoretical predictions are compared with the experimental structures from the literature.     

Bonding analysis and electronic structure of transition metal-benzoquinoline complexes: A theoretical study

The geometric and electronic structures of a series of hypothetical compounds of the types CpM(C₁₃H₉N) and (CO)₃M(C₁₃H₉N) (M = first row transition metal and C₁₃H₉N = 7,8-benzoquinoline) have been investigated by means of density functional theory (DFT). The benzoquinoline ligand can bind to the metal through η¹-η⁶ coordination modes, adopting structures of types a, b and c, in agreement with the electron count and the nature of the metal. In the investigated species, the most favored closed-shell count is 18-MVE, except for the Ti and V models which prefer the open-shell 16-MVE configuration. This study has shown the difference in the coordination ability of this heteropolycyclic ligand and coordination of the inner C₆ ring is less favored than the outer C₆ and C₅N rings, in agreement with the π-electron density localization.     

A theoretical and structural investigation of thiocarbon anions

Density functional theory energies, geometries, and population analyses as well as nucleus-independent chemical shifts (NICS) have been used to investigate the structural and magnetic evidence for cyclic CnSn(2-) and CnSn (n = 3-6) electron delocalization. Localized molecular orbital contributions to NICS, computed by the individual gauge for localized orbitals method, dissect pi effects from the sigma single bonds and lone pair influences. CnSn(2-) (n = 3-5) structures in Dnh symmetry are minima. Their aromaticity decreases with increasing ring size. C3S3(2-) is both sigma and pi aromatic, while C4S4(2-) and C5S5(2-) are much less aromatic. NICS(0)pi, the C-C(pi) contribution to NICS(0) (i.e., at the ring center), decreases gradually with ring size. In contrast, cyclic C6S6(2-) prefers D2h symmetry due to the balance between aromaticity, strain energy, and the S-S bond energies and is as aromatic as benzene. The theoretical prediction that C6S6(6-) has D6h minima was confirmed by X-ray structure analysis. Comparisons between thiocarbons and oxocarbons based on dissected NICS analysis show that CnSn(2-) (n = 3-5) and C6S6(6-) are less aromatic in Dnh symmetry than their oxocarbon analogues.     

A structural and spectroscopic study on para-aminohippuric acid with experimental and theoretical approaches

In this work, the molecular conformation, vibrational and electronic analysis of para-aminohippuric acid (pAHA, C(9)H(10)N(2)O(3)) were presented for the ground state using experimental techniques (FT-IR, FT-Raman and UV) and density functional theory (DFT) employing B3LYP exchange correlation with the 6-311++G(d,p) basis set. FT-IR and FT-Raman spectra were recorded in the regions of 400-4000cm(-1) and 50-4000cm(-1), respectively. The UV absorption spectra of the compound that dissolved in ethanol and water solution were recorded in the range of 190-400nm. Potential energy curve was computed by means of scanning NCCO torsion angle. The geometry optimization and the energies associated possible four conformers (C1-C4) were computed. The computational results diagnose the most stable conformer of pAHA as the C1 form. Optimized structure of compound was interpreted and compared with the earlier reported experimental values. The complete assignments of fundamental vibrations were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. A study on the electronic properties, such as frontier molecular energies, absorption wavelengths and oscillator strengths, were predicted by time-dependent DFT (TD-DFT) approach, while taking solvent effects into account. To investigate non-linear optical properties: polarizability, anisotropy of polarizability and molecular first hyperpolarizability of molecule were computed. Thermodynamic properties (heat capacity, entropy and enthalpy) of the title compound at different temperatures were calculated.     

A theoretical interpretation of free volume at glass transition

We device a relaxed lattice model (RLM) to study the mechanism of glass transition,which unifies the cageeffects from particle-particle interaction and entropy.By analyzing entropy in RLM with considering the influence of interactions on equilibrium,we demonstrate that glass transition is a second-order phase transition.For a perfect onedimensional linked particle system like linear polymer under normal pressure,the free volume at glass transition is rigorously deduced out to be 2.6％,which provides a theoretical basis for the iso-free volume of 2.5％ given by Willian,Landel and Ferry (WLF) equation.Extending to system with dead particles linked with higher dimensions like branched or cross-linked chains under positive or negative pressure,free volume at glass transition is varied,based on which we construct a phase diagram of glass transition in the space of free volume-dead particle-pressure.This demonstrates that free volume is not the single parameter determining glass transition,while either dead particles like cross-linked points or external force fields like pressure can vary free volume at the glass transition.     

A theoretical study of spectroscopic properties and transition moments of HBr

Relativistic configuration interaction calculations are performed for twelve electronic states of the HBr molecule. Ground-state spectroscopic properties and electronic dipole moment function are calculated and compared with theoretical and experimental data. Electric dipole moments for eleven excited states are presented and discussed. Electronic transition moments between the ground state and seven excited states are presented in the intermediate coupling scheme.     

General theoretical analysis of three-connected polyhedral molecules and their capped derivatives

The synthesis of the half-sandwich compound Na[(C₅H₅)Ni{P(S)(CH₃)₂}₂] is described. The anions [(C₅H₅)Ni{P(S)R₂}₂]^?, 1a (R = OCH₃) and 1b (R = CH₃) react as bidentate sulfur ligands with [Ni₂(C₅H₅)₃]⁺, giving nickelocene and weakly paramagnetic dinuclear complexes of the type [(C₅H₅)Ni{P(S)R₂}₂Ni(C₅H₅)] (2a,b). In these compounds, the P(S)R₂ units form NiPSNi bridges in such a fashion as to generate a (C₅H₅)NiP₂ and a (C₅H₅)NiS₂ unit. A temperature-dependent singlettriplet spin equilibrium is observed, which is essentially localized on the (C₅H₅)NiS₂ side. Accordingly, the position of the cyclopentadienyl peak of the (C₅H₅)Ni unit bound to the two sulfur donor centers displays a very large temperature dependence in the ¹H NMR spectra. MO model calculations (EHT) for P(S)H₂^?, [(C₅H₅)Ni{P(S)H₂}₂]^? (1c), [(C₅H₅)Ni{P(S)H₂}₂Ni(C₅H₅)] (2c) and its isomer 3c allow the observed spin crossover to be explained as a consequence of the pronounced π-donor properties of the sulfur centers and allow predictions for related complexes.The green complexes 2a,b isomerize completely and irreversibly in a first-order reaction to yield the diamagnetic red compounds [{(C₅H₅)NiP(S)R₂}₂] (3a,b), in which each (C₅H₅)Ni unit is coordinated to one P and one S donor atom. The rate constant of isomerization of 2a, k (7.6 ± 0.3) × 10^?4s^?1 at 306 K, and the energy of activation, E_a 76 kJ mol^?1, have been determined. The rate of isomerization is independent of the solvent, and crossover experiments verify that the isomerization is an intramolecular process without involvement of the monomeric units [(C₅H₅)NiP(S)R₂].     

A structural and theoretical study of intermolecular interactions in nicotinohydrazide dihydrochloride

In the title compound [systematic name: 3‐(azaniumylcarbamoyl)pyridinium dichloride], C₆H₉N₃O²⁺·2Cl⁻, the ions are connected by N—H...Cl hydrogen bonds to form layers and C—H...Cl interactions expand the layers into a three‐dimensional net. The energies of the N—H...Cl interactions range from typical for very weak interactions (0.17 kcal mol⁻¹) to those observed for relatively strong interactions (29.1 kcal mol⁻¹). C—H...Cl interactions can be classified as weak and mildly strong (energies ranging from 2.2 to 8.2 kcal mol⁻¹). Despite the short contacts existing between the parallel aromatic rings of the cations, π–π interactions do not occur.     

Theoretical analysis of core size effect in metalloporphyrins

Density functional theory has been applied to a series of unsubstituted planar metalloporphyrins (MPs) to elucidate how geometry and frequencies correlate with the metal-nitrogen distance, referred to as the core size. Different transition metals can invoke expansion or contraction of the porphyrin core due to electronic effects resulting from the amount of d-electron pairing as well as occupancy of the d(x(2)(-y(2))) orbital. A full vibrational analysis consisting of all in-plane and out-of-plane frequencies was carried out, and the resulting modes were plotted against core size for a linear analysis and grouped within symmetry blocks. The modes were separated according to planarity, and all modes with a large slope and best fit greater than 0.8 were considered sensitive to metal-nitrogen distances. All planar skeletal modes above 1450 cm(-1), including the pyrolle ring deformations, are found to be core-size sensitive. The most significant out-of-plane modes sensitive to core size are gamma(8) and gamma(9), which are infrared active and grouped within the A(2u) symmetry block. The present work also opens possible quantitative applications for the correlation of spectroscopic properties of MPs and heme proteins with actual structural parameters.     

Photophysics and spectroscopy of the higher electronic states of zinc metalloporphyrins: a theoretical and experimental study

The photophysics of the S2 and S1 excited states of zinc porphyrin (ZnP) and five of its derivatives (ZnOEP, ZnTBP, ZnTPP, ZnTFPP, ZnTCl8PP) have been investigated by measuring their steady-state absorption and fluorescence spectra, quantum yields and excited state lifetimes at room temperature in several solvents. The radiative and radiationless decay constants of the fluorescent excited states accessible in the visible and near UV regions of the spectrum have been obtained. Despite the similarities in the Soret spectra of these compounds, their S2 excited state radiationless decay rates differ markedly. Although the S2-S1 electronic energies of a given zinc porphyrin vary linearly with the Lippert (refractive index) function of the solvent, the S2 radiationless decay rates of the set of compounds do not follow the energy gap law of radiationless transition theory. Calculations, using time-dependent density functional theory (TDDFT), of the energies and symmetries of the complete set of excited states accessible by 1- or 2-photon absorption in the near UV-visible have also been carried out. Substitution on the porphyrin macrocycle framework affects the ground state geometry and alters the electron density distributions, the orbital energies and the relative order of the excited electronic states accessible in the near UV-blue regions of the spectrum. The results are used to help interpret both the nature of the electronic transitions in the Soret region, and the relative magnitudes of the radiationless transition rates of the excited states involved.     

A theoretical conformational analysis of morpholine

The CNDO and INDO methods were used for a theoretical conformational analysis of morpholine. The agreement between the methods is very satisfactory. Some results are also compared with the available experimental data.     

Quadratic nonlinearity of one- and two-electron oxidized metalloporphyrins and their switching in solution

We report the quadratic nonlinearity of one- and two-electron oxidation products of the first series of transition metal complexes of meso-tetraphenylporphyrin (TPP). Among many MTPP complexes, only CuTPP and ZnTPP show reversible oxidation/reduction cycles as seen from cyclic voltammetry experiments. While centrosymmetric neutral metalloporphyrins have zero first hyperpolarizability, beta, as expected, the cation radicals and dications of CuTPP and ZnTPP have very high beta values. The one- and two-electron oxidation of the MTPPs leads to symmetry-breaking of the metal-porphyrin core, resulting in a large beta value that is perhaps aided in part by contributions from the two-photon resonance enhancement. The calculated static first hyperpolarizabilities, beta0, which are evaluated in the framework of density functional theory by a coupled perturbed Hartree-Fock method, support the experimental trend. The switching of optical nonlinearity has been achieved between the neutral and the one-electron oxidation products but not between the one- and the two-electron oxidation products since dications that are electrochemically reversible are unstable due to the formation of stable isoporphyrins in the presence of nucleophiles such as halides.     

Absorption spectra of first-row transition metal complexes of bacteriochlorins: a theoretical analysis

A theoretical study on a family of divalent transition metal bacteriochlorin complexes (M-BC, where M = Mn, Fe, Co, Ni Cu, and Zn) has been carried out to elucidate their potentialities as active molecules in photodynamic therapy (PDT). To draw a complete picture of their electronic properties, both for the ground and excited states, these complexes have been studied by the means of density functional theory (DFT). The time-dependent DFT (TDDFT) approach was used to interpret the electronic spectra, while solvent effects were taken into account by explicitly considering both two water molecules coordinated to the central metal atom and the contribution from the solvent bulk. Particular attention has been devoted to the analysis of the so-called Q bands, since these can be particularly important for medical applications. Metal substitution and environment (solvent) effects have been analyzed, and good agreement is found between computed and available UV-vis spectra. These theoretical data, especially those relative to the metallobacteriochlorins not yet completely characterized at the experimental level, could give some hints for future medical applications.     

Magnetic field induced Freedericksz transition of a planar aligned liquid crystal doped with metalloporphyrins FeTPPCl,MnTPPCl and ZnTPP

We observed that the planar aligned nematic liquid crystal (5CB) doped with a volume fraction of 1% of FeTPPCl [5,10,15,20-tetraphenylporphyriniron(III)chloride] or MnTPPCl [5,10,15,20-tetraphenylporphyrinmanganese(III)chloride] dramatically decreased the critical magnetic field for the magnetic field induced Freedericksz transition, while 5CB doped with ZnTPP [5,10,15,20-tetraphenylporphyrinzinc(II)] revealed no such effect, when compared with pure 5CB. In the guest-host (5CB) system, FeTPPCl and MnTPPCl as guests are both strong paramagnetic materials with an interaction through coordination of the -CN group in 5CB onto the metal ion of the porphyrin. As a result, the 5CB molecules are dragged to reorientate under a static magnetic field, while ZnTPP is a diamagnetic material without such a property. This phenomenon concerning magneto-optical components could be useful in liquid crystal displays.     

Magnetic field induced Freedericksz transition of a planar aligned liquid crystal doped with metalloporphyrins FeTPPCl, MnTPPCl and ZnTPP

We observed that the planar aligned nematic liquid crystal (5CB) doped with a volume fraction of 1% of FeTPPCl [5,10,15,20-tetraphenylporphyriniron(III)chloride] or MnTPPCl [5,10,15,20-tetraphenylporphyrinmanganese(III)chloride] dramatically decreased the critical magnetic field for the magnetic field induced Freedericksz transition, while 5CB doped with ZnTPP [5,10,15,20-tetraphenylporphyrinzinc(II)] revealed no such effect, when compared with pure 5CB. In the guest-host (5CB) system, FeTPPCl and MnTPPCl as guests are both strong paramagnetic materials with an interaction through coordination of the -CN group in 5CB onto the metal ion of the porphyrin. As a result, the 5CB molecules are dragged to reorientate under a static magnetic field, while ZnTPP is a diamagnetic material without such a property. This phenomenon concerning magneto-optical components could be useful in liquid crystal displays.