Nature of the water/aromatic parallel alignment interactions.

The water/aromatic parallel alignment interactions are interactions where the water molecule or one of its O-H bonds is parallel to the aromatic ring plane. The calculated energies of the interactions are significant, up to ΔE(CCSD)(T)(limit) = -2.45 kcal mol(-1) at large horizontal displacement, out of benzene ring and CH bond region. These interactions are stronger than CH···O water/benzene interactions, but weaker than OH···π interactions. To investigate the nature of water/aromatic parallel alignment interactions, energy decomposition methods, symmetry-adapted perturbation theory, and extended transition state-natural orbitals for chemical valence (NOCV), were used. The calculations have shown that, for the complexes at large horizontal displacements, major contribution to interaction energy comes from electrostatic interactions between monomers, and for the complexes at small horizontal displacements, dispersion interactions are dominant binding force. The NOCV-based analysis has shown that in structures with strong interaction energies charge transfer of the type π → σ*(O-H) between the monomers also exists.

Parallel water/aromatic interactions of non-coordinated and coordinated water.

The parallel interactions of non-coordinated and coordinated water molecules with an aromatic ring were studied by analyzing data in the Cambridge structural database (CSD) and by using quantum chemical calculations. The CSD data show that water/aromatic contacts prefer parallel to OH/π interactions, which indicates the importance of parallel interactions. The results reveal the influence of water coordination to a metal ion; the interactions of aqua complexes are stronger. Coordinated water molecules prefer a parallel-down orientation in which one OH bond is parallel to the aromatic ring, whereas the other OH bond points to the plane of the ring. The interactions of aqua complexes with parallel-down water/benzene orientation are as strong as the much better known OH/π orientations. The strongest calculated interaction energy is -14.89 kcal mol(-1) . The large number of parallel contacts in crystal structures and the quite strong interactions indicate the importance of parallel orientation in water/benzene interactions.

Investigation of interactions in Lewis pairs between phosphines and boranes by analyzing crystal structures from the Cambridge Structural Database.

The interactions between phosphines and boranes in crystal structures have been investigated by analyzing data from the Cambridge Structural Database (CSD). The interactions between phosphines and boranes were classified into three types; two types depend on groups on the boron atom, whereas the third one involves frustrated Lewis pairs (FLPs). The data enabled geometric parameters in structures to be compared with phosphine-borane FLPs with classical Lewis pairs. Most of the crystal structures (78.1%) contain BH3 as the borane group. In these systems, the boron-phosphorus distance is shorter than systems where the boron atom is surrounded by groups other than hydrogen atoms. The analysis of the CSD data has shown that FLPs have a tendency for the longest boron-phosphorus distance among all phosphine-borane pairs, as well as different other geometrical parameters. The results show that most of the frustrated phosphine-borane pairs found in crystal structures are bridged ones. The minority of non-bridged FLP structures contain, beside phosphorus and boron atoms, other heteroatoms (O, N, S for instance).

Orientations of axially coordinated imidazoles and pyridines in crystal structures of model systems of cytochromes.

Many properties of cytochromes and model systems depend on orientations of axial ligands. In this work, we elucidated the role of porphyrin substituents on orientation of axial ligands in model systems of cytochromes. The orientations of axially coordinated imidazoles and pyridines in crystal structures of model systems of cytochromes were analyzed and data were compared with previous quantum-chemical calculations. The results show that eight ethyl groups on porphyrin ring strongly favor parallel orientation, hence, in all these complexes axial ligands, pyridines or imidazoles, are mutually parallel. Four phenyl or mesityl groups at meso-carbons also favor parallel orientation but less strongly. Hence, in most of the bis-imidazole complexes the orientation is parallel, while in bis-pyridine complexes the orientation of pyridines depends on oxidation state of Fe. In bis-pyridine Fe(II) complexes orientation is parallel, in Fe(III) it is orthogonal. This analysis is in agreement with previous quantum-chemical calculations.

C-H. . .pi interactions in the metal-porphyrin complexes with chelate ring as the H acceptor.

Specific C-H. . .pi interactions with the pi-system of porphyrinato chelate ring were found in crystal structures of transition metal complexes from the Cambridge Structural Database and statistical analysis of geometrical parameters for intramolecular and intermolecular interactions was done. By density functional theory calculations on a model system it was evaluated that an interaction energy is above 1.5 kcal/mol and that the strongest interaction occurs when the distance between hydrogen atom and the center of the chelate ring is 2.6 A. This prediction is in good agreement with the distances for intermolecular interactions found in the crystal structures. In many cases the intramolecular interaction distances are much shorter than 2.6 A, and these short distances are caused by geometrical constrains. The C-H. . .pi interactions with chelate ring of porphyrinato ligand can influence the structure, contribute to its stability, and play some role in the function of biomolecules with metalo porphyrins.

CH/π interactions in metal-porphyrin complexes with pyrrole and chelate rings as hydrogen acceptors.

CH/π interactions in metal porphyrinato complexes were studied by analyzing data in crystal structures from the Cambridge Structural Database (CSD) and by quantum chemical calculations. The analysis of the data in the CSD shows that both five-membered pyrrole and six-membered chelate rings form CH/π interactions. The interactions occur more frequently with five-membered rings. The analysis of distances in crystal structures and calculated energies show stronger interactions with six-membered chelate rings, indicating that a larger number of interactions with five-membered rings are not the consequence of stronger interactions, but better accessibility of five-membered pyrrole rings. The calculated energies of the interactions with positions in six-membered rings are -2.09 to -2.83 kcal/mol, while the energies with five-membered rings are -2.05 to -2.26 kcal/mol. The results reveal that stronger interactions of six-membered rings are the consequence of stronger electrostatic interactions. Substituents on the porphyrin ring significantly strengthen the interactions. Substituents on the six-membered ring strengthen the interaction energy by about 20%. The results show that CH/π interactions play an important role in molecular recognition of metalloporphyrins. The significant influence of the substituents on interaction energies can be very important for the design of model systems in bioinorganic chemistry.

XH/pi interactions with the pi system of porphyrin ring in porphyrin-containing proteins.

Searching structures of porphyrin-containing proteins from the Protein Data Bank revealed that the pi system of every porphyrin ring is involved in XH/pi interactions, with most of the porphyrins having several interactions. Both five-membered pyrrole rings and six-membered chelate rings are involved in XH/pi interactions; the number of interactions with five-membered rings is larger than the number of interactions with six-membered rings. We found interactions with C-H and N-H groups as hydrogen-atom donors; however, the number of CH/pi interactions is much larger than the number of NH/pi interactions. The amino acids involved in the interactions show a high conservation score. Our results that every porphyrin is involved in XH/pi interactions and that amino acids involved in these interactions are highly conserved demonstrate that XH/pi interactions play an important role in porphyrin-protein stability.

Electron delocalization mediates the metal-dependent capacity for CH/pi interactions of acetylacetonato chelates.

CH/pi interactions between the coordinated acetylacetonato ligand and phenyl rings were analyzed in the crystal structures from the Cambridge Structural Database and by quantum chemical calculations. The acetylacetonato ligand may engage in two types of interactions: it can be hydrogen atom donor or acceptor. The analysis of crystal structures and calculations show that interactions with the acetylacetonato ligand acting as hydrogen atom donor depend on the metal in an acetylacetonato chelate ring; the chelate rings with soft metals make stronger interactions. The same trend was not observed in the interactions where the acetylacetonato chelate ring acts as the hydrogen atom acceptor.