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A series of η6,η1-heterobimetallic complexes have been prepared in which a [Ru(η6-arene)(C5R5)]+ fragment (R = H or Me) and an η1-NCN-pincer platinum fragment are combined within the same molecule. In complexes [2]+ and [3]+, the ruthenium and platinum centers are η6 and η1 coordinated, respectively, to the same arene ring, whereas in [4A]+ and [5A]+ they are coordinated to two different arene rings that are linked with a covalent bond ([4A]+) or an ethyl bridge ([5A]+). Upon changing the organic manifold between both metal centers, very strong ([2]+) to very weak ([5A]+) ruthenium- to-platinum interactions are obtained. Experimentally, X-ray crystal structures show an increaing steric hindrance when the Ru–Pt distance diminishes, and electrochemical and 195Pt NMR spectroscopic studies show a decreasing electron density on platinum from [5A]+ to [2]+. Theoretical DFT calculations were undertaken, which show an increasing charge on platinum from [5A]+ to [2]+. Our theoretical analysis shows that the particularly strong ruthenium-to-platinum electronic interactions in [2]+ and [3]+ do not come from binding of ruthenium to platinum, but from the pincer Cipso sharing its electron density between both metal centers, which decreases the σ donation to platinum, and from increased backdonation of the platinum d electrons into the π system of the arene ring.

In this work, the complexation of the bapbpy ligand to zinc dichloride is described (bapbpy = 6,6′-bis(2-aminopyridyl)-2,2′-bipyridine). The water-soluble, colorless complex [Zn(bapbpy)Cl]Cl·2H2O (compound 2·H2O) was synthesized; its X-ray crystal structure shows a mononuclear, pentacoordinated geometry with one chloride ligand in apical position. Upon excitation of its lowest-energy absorption band (375 nm) compound 2 shows intense emission (Φ = 0.50) at 418 nm in aqueous solution, and an excited state lifetime of 5 ns at room temperature. Photophysical measurements, DFT, and TD-DFT calculations prove that emission arises from vibronically coupled Ligand-to-Ligand Charge Transfer singlet excited states, characterized by electron density flowing from the lone pairs of the non-coordinated NH bridges to the π* orbitals of the pyridine rings. Monofunctionalization of the ligand with one long alkyl chain was realized to afford ligand 3, which can be inserted into dimyristoylphosphatidylglycerol (DMPG) or dimyristoylphosphatidylcholine (DMPC) unilamellar vesicles. For negatively charged DMPG membranes the addition of a zinc salt to the vesicles leads to an enhancement of the fluorescence due to zinc coordination to the membrane-embedded tetrapyridyl ligand. No changes were observed for the zwitterionic DMPC lipids, where binding of the Zn ions does not take place. A modest binding constant was found (5 × 10(6) M(−1)) for the coordination of zinc cations to bapbpy-functionalized DMPG membranes, which allows for the detection of micromolar zinc concentrations in aqueous solution. The influence of chloride concentration and other transition metal ions on the zinc binding was evaluated, and the potential of liposome-supported metal chelators such as ligand 3 for zinc detection in biological media is discussed.

The SO2-binding properties of a series of h6,h1-NCN-pincer ruthenium platinum complexes (NCN = 2,6-bis[(dimethylamino)methyl]phenyl anion) have been studied by both UV-visible spectroscopy and theoretical calculations. When an electron-withdrawing [Ru(C5R5)]+ fragment (R = H or Me) is h6-coordinated to the phenyl ring of the NCN-pincer platinum fragment (cf . [2]+ and [3]+, see Scheme 1), the characteristic orange coloration (pointing to h1- SO2 binding to Pt) of a solution of the parent NCN-pincer platinum complex 1 in dichloromethane upon SO2-bubbling is not observed. However, when the ruthenium center is h6-coordinated to a phenyl substituent linked in para-position to the carbon-to-platinum bond, i.e. complex [4]+, the SO2-binding property of the NCN-platinum center seems to be retained, as bubbling SO2 into a solution of the latter complex produces the characteristic orange color. We performed theoretical calculations at the MP2 level of approximation and TD-DFT studies, which enabled us to interpret the absence of color change in the case of [2]+ as an absence of coordination of SO2 to platinum. We analyze this absence or weaker SO2-coordination in dichloromethane to be a consequence of the relative electron-poorness of the platinum center in the respective h6-ruthenium coordinated NCN-pincer platinum complexes, that leads to a lower binding energy and an elongated calculated Pt–S bond distance. We also discuss the effects of electrostatic interactions in these cationic systems, which also seems to play a destabilizing role for complex [2(SO2)]+.

A series of η6,η1-heterobimetallic complexes have been prepared in which a [Ru(η6-arene)(C5R5)]+ fragment (R = H or Me) and an η1-NCN-pincer platinum fragment are combined within the same molecule. In complexes [2]+ and [3]+, the ruthenium and platinum centers are η6 and η1 coordinated, respectively, to the same arene ring, whereas in [4A]+ and [5A]+ they are coordinated to two different arene rings that are linked with a covalent bond ([4A]+) or an ethyl bridge ([5A]+). Upon changing the organic manifold between both metal centers, very strong ([2]+) to very weak ([5A]+) ruthenium- to-platinum interactions are obtained. Experimentally, X-ray crystal structures show an increaing steric hindrance when the Ru–Pt distance diminishes, and electrochemical and 195Pt NMR spectroscopic studies show a decreasing electron density on platinum from [5A]+ to [2]+. Theoretical DFT calculations were undertaken, which show an increasing charge on platinum from [5A]+ to [2]+. Our theoretical analysis shows that the particularly strong ruthenium-to-platinum electronic interactions in [2]+ and [3]+ do not come from binding of ruthenium to platinum, but from the pincer Cipso sharing its electron density between both metal centers, which decreases the σ donation to platinum, and from increased backdonation of the platinum d electrons into the π system of the arene ring.

We discuss Ab Initio approaches to calculate the energy lowering (stabilisation) due to aromaticity. We compare the valence bond method and the block-localised wave function approaches to calculate the resonance energy. We conclude that the valence bond approach employs a Pauling–Wheland resonance energy and that the block-localised approach employs a delocalisation criterion. The latter is shown to be more basis set dependent in a series of illustrative calculations.

The molecular geometry and the normal modes properties of coronene are investigated by means of DFT B3LYP and restricted/Hartree–Fock calculations utilizing basis sets of triple zeta +polarization quality. The interpretation of the infrared and Raman spectra of coronene, especially in solid state, is critically revised. The phantom bands in the solid state, previously not understood, are readily assigned after considering a minute out-of-plane molecular distortion from D6h to C2h.

A new scheme, called “list of nonredundant bonds”, is presented to record the number of bonds and their positions for the atoms involved in Kekulé valence structures of (poly)cyclic conjugated systems. Based on this scheme, a recursive algorithm for generating Kekulé valence structures has been developed and implemented. The method is general and applicable for all kinds of (poly)cyclic conjugated systems including fullerenes. The application of the algorithm in generating Valence Bond (VB) wave functions, in terms of Kekulé valence structures, is discussed and illustrated in actual VB calculations. Two types of VBSCF calculations, one involving Kekulé valence structures only and the second one involving all covalent VB structures, were performed for benzene, pentalene, benzocyclobutadiene, and naphthalene. Both strictly local and delocalised p-orbitals were used in these calculations. Our results show that, when the orbitals are restricted to their own atoms, other VB structures (Dewar structures) also have a significant contribution in the VB wave function. When removing this restriction, the other VB structures (Dewar and also the ionic structures) are accommodated in the Kekulé valence structures, automatically. Therefore, at VBSCF delocal level, the ground states of these systems can be described almost quantitatively by considering Kekulé valence structures only at a considerable saving of time.

The Atoms in Valence Bond (AiVB) method, a new approach in the Valence Bond, is introduced. This approach combines the ideas behind the early Atoms in Molecules (AIM) developments, e.g. by Moffit [21], to understand a molecular wave function in terms of proper atomic wave functions, with the available framework of the VBSCF [17,18] as implemented in TURTLE [19]. The fundamental theoretical tools, to explain the AiVB concept, are shown and the initial test results are presented.

We consider delocalization in small molecules. The valence bond technology allows an arbitrary division of the 1-electron space in strictly separate, but nonorthogonal, spaces. Allowing or obstructing orbital mixing between subspaces during the SCF procedure is associated with incorporating or eliminating the hyperconjugation effect. We show an example for tert-butyl and trimethylsilyl cations and radicals. For H2, for which really extensive basis sets are feasible, we extend the basis so far that the delocalization or hyperconjugation effect has nearly disappeared

The synthesis, structure, and catalytic activity of a series of bimetallic η6,η1-NCN-pincer ruthenium palladium complexes, [3]+−[5]+, have been studied, in which η6-coordination of [Ru(C5R5)]+ (R = H or Me) is realized either directly to the central arene ring of the NCN-pincer palladium complex ([3]+ and [4]+) or to the para-phenyl substituent ([5]+). The X-ray structures of [4]+ and [5]+, as well as the regioselectivity observed in the synthesis of [5]+, provide clear evidence of intramolecular steric interactions in [4]+ between the amine arms of the pincer moiety and the methyl groups of the cyclopentadienyl ligand. Cyclovoltammetry data point to strong electronic interactions between the two metal fragments in [3]+ and [4]+ and only limited interactions in [5]+. Catalytic studies in the cross-coupling reaction between trans-phenylvinylboronic acid and vinyl epoxide show that the catalytic activity of the palladium center is enhanced by η6-coordination of the ruthenium fragment. These modifications of the catalytic activity of palladium are not correlated to the decreased electron density on palladium, as confirmed by DFT calculations. We hence propose a mechanism in which transmetalation is the rate-determining step

The B3LYP/DZP++ adiabatic electron affinity (AEA) of nine (non)-alternant polycyclic aromatic hydrocarbons are reported and discussed. Calculations became feasible for molecules this size by projecting out the near-linearly dependent part of the one-electron basis. Non-alternant PAH consisting of an alternant PAH core modified by peri-annulation with unsaturated five-membered rings (CP-PAHs) possess markedly enhanced AEAs.

The resonating block localize wave function (RBLW) method is introduced, a resonating modification of the block localized wave functions introduced by Mo et al. [Mo, Y.; Peyerimhoff, S. D. J. Chem. Phys. 1998, 109, 1687].This approach allows the evaluation of resonance energies following Pauling’s recipe. The method is tested on two model molecules, hexagonal H6 and benzene. Calculations have been done with (local) and without local restrictions (delocal). Resonance energies for both molecules have been obtained for each type of calculation, in agreement with Pauling’s concept. From a comparison of the resonance energies obtained from RBLW and standard valence bond calculations, the resonating block localize wave functions prove to yield resonance energies close to standard valence bond delocal calculations.

Infrared and Raman spectra have been calculated for several molecular cluster models for the silica-supported monomeric vanadium oxide catalysts that are proposed in the literature: the pyramid model, the umbrella model, and a model containing two bonds to the support, a V=O group and an OH group. A related model with one bond to the support, a V=O and two OH groups, will also be discussed. From the comparison with literature, it is concluded that two models, the umbrella model and the model with two bonds to the support, are realistic descriptions of actual systems. The presence of a particular compound depends on the method of preparation. The internal V-O distances by themselves are not enough to distinguish between the presented models.

The structure of the cross-conjugated compound 2,3-diphenylbutadiene was investigated by single-crystal X-ray diffraction and computational methods. In the crystal structure the central butadiene fragment adopts an s-gauche geometry [-55.6(2)° torsion angle φ around the essential single bond], whereas the styrene moieties are close to planarity. MP2/6-311G* calculations show that the s-gauche conformation represents the global minimum along the φ coordinate, but also revealed the existence of an s-trans local minimum. While the crystal structure seems to reflect dominance of styrene- like conjugation, the MP2/6-311G* calculations indicate that conjugation in both the styrene and butadiene π- systems is important. An NBO orbital deletion study shows that the structure is primarily determined by (hyper)conjugation and that steric effects play a minor role.

We present a new method for the generation of potential grids for protein-ligand docking. The potential of the docking target structure is obtained directly from the electron density derived through an ab initio computation. A large subregion was selected to allow the full ab initio treatment of a the Isocitrate Lyase enzyme. The electrostatic potential is tested by docking a small charged molecule (succinate) into the binding site. The ab initio grid yields a superior result by producing the best binding orientation and position, and by recognizing it as the best. In contrast the same docking procedure, but using a classical point-charge based potential, produces a number of additional incorrect binding poses, and does not recognize the correct pose as the best solution.

Calculations on members of the oligo(cyclohexylidene) series [1(n), n = 1-5)] and related tetrahydro-4H-thiopyran end-capped analogues [2(n), n = 1-4)] show a strong through-bond coupling between their π bonds and sulfur lone pairs (LpπS). This coupling is mediated by an interaction between the H(ax)-C-C-H(ax) structural sub-units and the π bonds connecting the cyclohexyl moieties. A comparison of the length dependency of the through-bond coupling via an oligo(cyclohexylidene) and an alkane bridge [divinyl alkanes 3(n)] shows that oligo(cyclohexylidenes) are more efficient in mediating through-bond couplings over large distances. Oligo(cyclohexylidene) bridges exhibit molecular wire characteristics

We consider the role that large-scale electronic structure computations can now play in the modelling of the condensed phase. To structure our analysis, we consider four distict ways in which today's scientific targets can be re-scoped to take advantage of advances in computing resources: 1. time to solution-performing the same calculation, with delivery of the simulation in shorter elapsed time; 2. Size-applying today's methods to a more extensive problem; 3. Accuracy-replacing current physical models with more accurate ones; 4. Sampling-simultaneously studying more chemical or conformational states. Each of these offer some scientific rewards, but all present thenological challenges and it is likely that a mixture of approaches will be needed to make the best use of capability computing. We discuss some aspects of our work in each of these areas, including replicated and distributed data parallel implementations of GAMESS-UK, and approaches incorporating multi-level parallelism. Examples of the latter include pathway optimization using replica methods and task farming approaches to global optimization problems. We consider a typical application of GAMESS-UK to heterogeneous catalysis, and the role that large-scale DFT vibrational frequency calculations have played in the study of partial oxidation catalysts incorporating supported VOx species.

An earlier-proposed model for the molecular structure of VO₄ species on alumina was tested and compared with the pyramid model on a silica support. The model can be described as a chemisorbed Osupport-V=O(O₂) species with a similar geometry as on alumina. Results of DFT calculations on this model are consistent with the experimental Raman and EXAFS data collected on low-loaded silica-supported vanadium oxide catalysts. The band observed at 915 cm ¯¹ is assigned to an O-O stretch vibration. The thermal motion of the bound O₂ molecule can explain the broadness of this band. These findings, in combination with our previous work, demonstrate that the umbrella model is a viable, internally consistent model for supported vanadium oxide catalysts at least at low loadings.

Ab initio valence bond (VB) calculations give direct evidence for the stabilistaion and closed-shell character formation of formal diradicals by neighbouring group interactions. By allowing the singly occupied p-orbitals to delocalise during the orbital optimisation, neighbouring group effects are implicitly included in the wavefunction. By constraining the orbitals in the wavefunction to remain strictly atomic, through-bond effects are excluded from the wavefunction. These effects can then be included in the wavefunction by expanding the wavefunction using more valence bond structures. The choice of these structures used to built the wavefunction determines which interactions will be accounted for. For the compounds considered here, 2,4-dioxo-1,3-cyclobutanediyl (1), 2,4-bis(methylene)-1,3-cyclobutanediyl (2), 1,3-cyclobutanediyl (3), and 1,1,3,3-tetrahydro-1,3,2,4-diphosphadiboretane-1,3-diyl (4), the calculations show that the most important VB structures for mediating interactions between the opposing radical centres are those that withdraw electrons from the ring, thereby creating an aromatic, 2pi-electron four-membered ring. For the 1,3-cyclobutanediyl analogues (3 and 4), the PH2 group functions similar to the C=O group, and is a better electron-accepting substituent than the CH2 group. The energy differences between the strictly atomic and delocal orbital optimisation models show that the molecules with less diradical character are stabilised more by through-bond orbital interactions.

In Memory of Jaap G. Snijders (1951-2003)

We describe the procedure to start an SCF calculation of the general type from a sum of atomic electron densities, as implemented in GAMESS-UK. Although the procedure is well-known for closed-shell calculations and was already suggested when the Direct SCF procedure was proposed, the general procedure is less obvious. For instance, there is no need to converge the corresponding closed-shell Hartree-Fock calculation when dealing with an open-shell species. We describe the various choices and illustrate them with test calculations, showing that the procedure is easier, and on average better, than starting from a converged minimal basis calculation and much better than using a bare nucleus Hamiltonian.

The previously proposed atomic zeroth-order regular approximation (ZORA) approch, which was shown to eliminate the gauge dependent effect on gradients and to be remarkably accurate for geometry optimization, is tested for the calculation of analytical second derivatives. It is shown that the resulting analytic second derivatives are indeed exact within this approximation. The method proves to yield frequencies that are remarkably close to the experimental frequency for uranium hexafluoride but less satisfactory for the gold dimer.

An extension of the CEPA1 method to the multi-reference case is presented, the MR-CEPA1 approach. The method takes the variationally included terms into account as in MRDCEPA and corrects for the exclusion principle violating terms as in closed shell CEPA1. It is shown that this method yields potential energy curves that are close to those of the multi-reference coupled cluster method and parallel the full CI results quite well. The size consistency of the method is as good as the MRDCEPA method and much beter than approaches that ignore the VI terms like MR-ACPF and MR-AQCC. A simpler method, where the EPV terms are not corrected on an individual basis for the doubly occupied orbitals, dubbed the multi-reference averages CEPA1 method (MR-ACEPA), which is akin to methods previously suggested by Szalay et al., is comparable in performance.

To probe the effect of external cyclopenta-fusion on a naphthalene core, ab initio valence bond (VB) calculations have been performed, using strictly atomic benzene p-orbitals and p-orbitals that are allowed to delocalize, on naphthalene (1), acenaphthylene (2), pyracylene (3), cyclopenta[b,c]-acenaphthylene (4), fluoranthene (5), and cyclopenta[c,d]fluoranthene (6). For the related compounds 1-4 and 5,6 the total resonance energies (according to Pauling's definition) are similar. Partitioning of the total resonance energy in contributions from the possible 4n+2 and 4n pi-electron conjugated circuits shows that only the 6 pi-electron conjugated circuits (benzene-like) contribute to the resonance energy. The results show that cyclopenta-fusion does not extend the pi system in the ground state; the five-membered rings act as peri-substituents. As a consequence, the differences in (total) resonance energy do not coincide with the differences in thermodynamic stability. Notwihstanding, the relative energies of the Kekule structures can be estimated using Randic's conjugated circuits model.

Magnesium hydride is cheap and contains 7.7 wt % hydrogen, making it one of the most attractive hydrogen storage materials. However, thermodynamics dictate that hydrogen desorption from bulk magnesium hydride only takes place at or above 300 degrees C, which is a major impediment for practical application. A few results in the literature, related to disordered materials and very thin layers, indicate that lower desorption temperatures are possible. We systematically investigated the effect of crystal grain size on the thermodynamic stability of magnesium and magnesium hydride, using ab initio Hartree-Fock and density functional theory calculations. Also, the stepwise desorption of hydrogen was followed in detail. As expected, both magnesium and magnesium hydride become less stable with decreasing cluster size, notably for clusters smaller than 20 magnesium atoms. However, magnesium hydride destabilizes more strongly than magnesium. As a result, the hydrogen desorption energy decreases significantly when the crystal grain size becomes smaller than ~ 1.3 nm. For instance, an MgH2 crystallite size of 0.9 nm corresponds to a desorption temperature of only 200 degrees C. This predicted decrease of the hydrgen desorption temperature is an important step toward the application of Mg as a hydrogen storage material.

Valence bond (VB) theory and ring-current maps have been used to study the electronic structure of inorganic benzene analogues X6H6 (X = C (1), Si (2)), X6 (X = N (3), P (4)), X3Y3H6 (X,Y = B,N (5), B,P (6), Al,N (7), Al,P (8)), and B3Y3H3 (Y = O (9), S (10)). It is shown that the homonuclear compounds possess benzene-like character, with resonance between two Kekule-like structures and induced diatropic ring currents. Heteronuclear compounds typically show localization of the lone pairs on the electronegative atoms; Kekule-like structures do not contribute. Of the heteronuclear compounds, only B3P3H6 (6) has some benzene-like features with a significant contribution of two Kekule-like structures to its VB wave function, an appreciable resonance energy, and a discernible diatropic ring current in planar geometry. However, relaxation of 6 to the optimal nonplanar chair conformation is accompanied by the onset of localization of the ring current.

In this paper we present the first application of the ZORA (Zeroth Order Regular Approximation of the Dirac Fock equation) formalism in Ab Initio electronic structure calculations. The ZORA method, which has been tested previously in the context of Density Functional Theory, has been implemented in the GAMESS-UK package. As was shown earlier we can split off a scalar part from the two component ZORA Hamiltonian. In the present work only the one component part is considered. We introduce a separate internal basis to represent the extra matrix elements, needed for the ZORA corrections. This leads to different options for the computation of the Coulomb matrix in this internal basis. The performance of this Hamiltonian and the effect of the different Coulomb matrix alternatives is tested in calculations on the radon en xenon atoms and the AuH molecule. In the atomic cases we compare with numerical Dirac Fock and numerical ZORA methods and with non relativistic and full Dirac basis set calculations. It is shown that ZORA recovers the bulk of the relativistic effect and that ZORA and Dirac Fock perform equally well in medium size basis set calculations. For AuH we have calculated the equilibrium bond length with the non relativistic Hartree Fock and ZORA methods and compare with the Dirac Fock result and the experimental value. Again the ZORA and Dirac Fock errors are of the same order of magnitude.