A Valence Bond Study of the Low‐Lying States of the NF Molecule

The electronic structures of the three lowest‐lying states of NF are investigated by means of modern valence bond (VB) methods such as the VB self‐consistent field (VBSCF), breathing orbital VB (BOVB), and VB configuration interaction (VBCI) methods. The wave functions for the three states are expressed in terms of 9–12 VB structures, which can be further condensed into three or four classical Lewis structures, whose weights are quantitatively estimated. Despite the compactness of the wave functions, the BOVB and VBCI methods reproduce the spectroscopic properties and dipole moments of the three states well, in good agreement with previous computational studies and experimental values. By analogy to the isoelectronic O₂ molecule, the ground state ³Σ^? possesses both a σ bond and 3‐electron π bonds. However, here the polar σ bond contributes the most to the overall bonding. It is augmented by a fractional (19 %) contribution of three‐electron π bonding that arises from π charge transfer from fluorine to nitrogen. In the singlet ¹Δ and ¹Σ⁺ excited states the π‐bonding component is classically covalent, and it contributes 28 % and 37 % to the overall bonding picture for the two states, respectively. The resonance energies are calculated and reveal that π bonding contributes at least 24, 35 and 42 kcal mol^?1 to the total bonding energies of the ³Σ^?, ¹Δ and ¹Σ⁺ states, respectively. Some unusual properties of the NF molecule, like the equilibrium distance shortening and bonding energy increasing upon excitation, the counterintuitive values of the dipole moments and the reversal of the dipole moments as the bond is stretched, are interpreted in the light of the simple valence bond picture. The overall polarity of the molecule is very small in the ground state, and is opposite to the relative electronegativity of N vs F in the singlet excited states. The values of the dipole moments in the three states are quantitatively accounted for by the calculated weights of the VB structures.     

Oriented Electric Fields Accelerate Diels–Alder Reactions and Control the endo/exo Selectivity

Herein we demonstrate that an external electric field (EEF) acts as an accessory catalyst/inhibitor for Diels–Alder (DA) reactions. When the EEF is oriented along the “reaction axis” (the coordinate of approach of the reactants in the reaction path), the barrier of the DA reactions is lowered by a significant amount, equivalent to rate enhancements by 4–6 orders of magnitude. Simply flipping the EEF direction has the opposite effect, and the EEF acts as an inhibitor. Additionally, an EEF oriented perpendicular to the “reaction axis” in the direction of the individual molecule dipoles can change the endo/exo selectivity, favouring one or the other depending on the positive/negative directions of the EEF vis‐à‐vis the individual molecular dipole. At some critical value of the EEF along the “reaction axis”, there is a crossover to a stepwise mechanism that involves a zwitterionic intermediate. The valence bond diagram model is used to comprehend these trends and to derive a selection rule for EEF effects on chemical reactions: an EEF aligned in the direction of the electron flow between the reactants will lower the reaction barrier. It is shown that the exo/endo control by the EEF is not associated with changes in secondary orbital interactions.     

A combined kinetic-quantum mechanical model for assessment of catalytic cycles: application to cross-coupling and Heck reactions

The efficiency of catalytic cycles is measured by their turnover frequency (TOF). The degree of TOF control determines which states contribute most to the rate of the cycle, and thus indicates the steps that have the highest impact on the cycle. A kinetic model developed by Christiansen (Christiansen, J. A. Adv. Catal. 1953, 5, 311) for catalytic cycles is implemented here in a form that utilizes state energies. This enables one to assess the efficiency of quantum mechanically computed catalytic cycles like the palladium-catalyzed cross-coupling and Heck reactions, to test alternative hypotheses, and to make some predictions. This implementation can also account for effects such as Sabatier's volcano curve for heterogeneous catalysis. The model leads to a dependence of the TOF for any cycle on the "corrected" energy span quantity, deltaE, whose precise expression depends on the location of the summit and trough of the cycle in the step sequence of the cycle. Thus, knowing the highest energy transition state, the most abundant reaction intermediate, and the reaction energy enables one to make quick predictions about relative efficiency of cycles. At the same time, the degree of TOF control determines which states contribute most to the rate of reaction, and thus indicates the values to be included in the calculation of the energetic span and the steps that may be tinkered with to improve the cycle.     

In silico design of a mutant of cytochrome P450 containing selenocysteine

A mutant of P450(cam), in which the cysteine ligand was replaced by selenocysteine, was designed theoretically using hybrid QM/MM (quantum mechanical/molecular mechanical) calculations. The calculations of the active species, Se-CpdI (selenocysteine-Compound I), of the mutant enzyme indicate that Se-Cpd I will be formed faster than the wild-type species and be consumed more slowly in C-H hydroxylation. As such, our calculations suggest that Se-Cpd I can be observed unlike the elusive species of the wild-type enzyme (Denisov, I. G.; Makris, T. M.; Sligar, S. G.; Schlichting, I. Chem. Rev. 2005, 105, 2253-2277). Spectral features of Se-Cpd I were calculated and may assist such eventual characterization. The observation of Se-Cpd I will resolve the major puzzle in the catalytic cycle of a key enzyme in nature.     

Two-state reactivity, electromerism, tautomerism, and "surprise" isomers in the formation of compound II of the enzyme horseradish peroxidase from the principal species, compound I

QM and QM/MM calculations on Compound II, the enigmatic species in the catalytic cycle of the horseradish peroxidase enzyme, reveal six low-lying isomers. The principal isomer is the triplet oxo-ferryl form (PorFe(IV)=O) that yields the hydroxo-ferryl isomer (PorFe(IV)-OH+). These are the only forms observed in experimental studies. Theory shows, however, that these are the least stable isomers of Compound II. The two most stable forms are the singlet and triplet states of the Por+*Fe(III)-OH electromer. In addition, theory reveals species never considered in heme enzymes: the singlet and triplet states of the Por+*Fe(III)-OH2 electromer. The computational results reproduce the experimental features of the known isomers and enable us to draw relationships and make predictions regarding the missing ones. For example, while the "surprise" species, singlet and triplet Por+*Fe(III)-OH2, have never been considered in heme chemistry, the calculated Fe-O bond lengths indicate that these isomers may have, in fact, been observed in one of the two opposing EXAFS studies reported previously. Furthermore, these ferric-aqua complexes could be responsible for the reported 18O exchange with bulk water. It is clear, therefore, that the role of Compound II in the HRP cycle is considerably more multi-faceted than has been revealed so far. Our suggested multi-state reactivity scheme provides a paradigm for Compound II species. The calculated M?ssbauer parameters may be helpful toward eventual characterization of these missing isomers of Compound II.     

The ground and excited states of polyenyl radicals C2n-1H2n + 1 (n = 2-13): a valence bond study.

The semiempirical valence bond (VB) method, VBDFT(s), is applied to the ground states and the covalent excited states of polyenyl radicals C2n - 1H2n + 1 (n = 2-13). The method uses a single scalable parameter with a value that carries over from the study of the covalent excited states of polyenes (W. Wu, D. Danovich, A. Shurki, S. Shaik, J. Phys. Chem. A, 2000, 104, 8744). Whenever comparison is possible, the VB excitation energies are found to be in good accord with sophisticated molecular orbital (MO)-based methods like CASPT2. The symmetry-adapted Rumer structures are used to discuss the state-symmetry and VB constitution of the ground and excited states, and the expansion to VB determinants is used to gain insight on spin density patterns. The theory helps to understand in a coherent and lucid manner the properties of polyenyl radicals, such as the makeup of the various states, their geometries and energies, and the distribution of the unpaired electrons (the neutral solitons).     

Oriented (Local) Electric Fields Drive the Millionfold Enhancement of the H‐Abstraction Catalysis Observed for Synthetic Metalloenzyme Analogues

This contribution follows the recent remarkable catalysis observed by Groves et al. in hydrogen‐abstraction reactions by a) an oxoferryl porphyrin radical‐cation complex [Por^?+Fe^IV(O)L_ax] and b) a hydroxoiron porphyrazine ferric complex [PyPzFe^III(OH)L_ax], both of which involve positively charged substituents on the outer circumference of the respective macrocyclic ligands. These charge‐coronated complexes are analogues of the biologically important Compound I (Cpd I) and synthetic hydroxoferric species, respectively. We demonstrate that the observed enhancement of the H‐abstraction catalysis for these systems is a purely electrostatic effect, elicited by the local charges embedded on the peripheries of the respective macrocyclic ligands. Our findings provide new insights into how electrostatics can be employed to tune the catalytic activity of metalloenzymes and can thus contribute to the future design of new and highly efficient hydrogen‐abstraction catalysts.     

Topology of Electron Charge Density for Chemical Bonds from Valence Bond Theory: A Probe of Bonding Types

Covalent, ionic, or something new? A new interpretation of the topology of the electron density at the bond critical point is proposed to characterize covalent, ionic, and charge‐shift bonding from the density point of view (see figure). The topological properties of the density representation confirm the reality of charge‐shift bonds, in which the covalent contribution is weak or repulsive, and most of the bonding is due to the covalent–ionic resonance energy.

     

Relaxation dynamics in glycerol-water mixtures. 2. Mesoscopic feature in water rich mixtures

The relaxation dynamics of water-rich glycerol-water mixtures is studied by broadband dielectric spectroscopy (BDS) at 173-323 K and differential scanning calorimetry (DSC) at 138-313 K. These data indicate the existence of the critical concentration of 40 mol % glycerol. In the studied temperature range for water-rich glycerol mixtures, the two states of water (ice and interfacial water) are observed in addition to water in the mesoscopic 40 mol % glycerol-water domains. The possible kinetics of water exchange between different water states is discussed in order to explain the mechanism of the broad melting behavior observed by DSC.     

Weak polyelectrolytes tethered to surfaces: Effect of geometry,acid–base equilibrium and electrical permittivity

The structural and thermodynamical properties of weak polyelectrolytes end-tethered to surfaces of arbitrary geometry are studied using a molecular theory. The theory is based on writing down the free energy functional of the system including all the basic interactions and the explicit acid–base equilibrium for the chargeable groups of the polymer. The theory explicitly includes the size, shape, conformations, and charge distribution of all the molecular species. The electrostatic interactions include a density-dependent dielectric function, modeled with the Maxwell–Garnett mixing formula, to account for the composition-dependent permittivity. The minimization of the free energy leads to the distribution of all molecular species and their dependence on bulk pH and salt concentration. We apply the theory to polymer chains end-tethered to planar, cylindrical, and spherical surfaces. The radius of the curved surfaces is small to enhance the curvature effect. We find that when the grafting surfaces are uncharged, the approximation of a constant dielectric function works very well for both structural and thermodynamic properties. The structure of weak polyelectrolytes tethered on cylindrical and spherical surfaces is different from that of polymers tethered on planar surfaces due to the available volume as a function of the distance from the surface. Specifically, the degree of dissociation increases with increasing curvature of the surface. This is a manifestation of the coupling between the local density of protons, counterions, and polymer segments. The results can be interpreted in terms of the local Le Chatelier principle for the acid–base equilibrium, with proper account of the three local contributions: counterions, protons, and chargeable groups. We find that one can achieve local changes of pH between one to two units within 1–2 nm. The thickness of the tethered layers as a function of bulk pH shows a large increase when the pH is equal to the bulk pK. However, the variation with salt concentration is different for the different geometries. The largest swelling is found for cylindrical surfaces. The predictions from scaling theories of a maximum in the thickness of the film as a function of salt concentration is found for planar films, but not for curved surfaces. Finally, the interactions between cylinders with tethered polyelectrolytes is very different from the equivalent planar surfaces. These results are important for the interpretation of force measurements with nanoscale AFM tips. The implications of the results for the rational design of responsive tethered polymer layers is discussed together with the limitations of the theoretical approach. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2638–2662, 2006