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A palladium-catalyzed Stille coupling reaction was employed as a versatile method for the synthesis of a novel terpyridine-pincer (3, TPBr) bridging ligand, 4'-{4-BrC6H2(CH2NMe2)2-3,5}-2,2':6',2' '-terpyridine. Mononuclear species [PdX(TP)] (X = Br, Cl), [Ru(TPBr)(tpy)](PF6)2, and [Ru(TPBr)2](PF6)2, synthesized by selective metalation of the NCNBr-pincer moiety or complexation of the terpyridine of the bifunctional ligand TPBr, were used as building blocks for the preparation of heterodi- and trimetallic complexes [Ru(TPPdCl)(tpy)](PF6)2 (7) and [Ru(TPPdCl)2](PF6)2 (8). The molecular structures in the solid state of [PdBr(TP)] (4a) and [Ru(TPBr)2](PF6)2 (6) have been determined by single-crystal X-ray analysis. Electrochemical behavior and photophysical properties of the mono- and heterometallic complexes are described. All the above di- and trimetallic Ru complexes exhibit absorption bands attributable to (1)MLCT (Ru --tpy) transitions. For the heteroleptic complexes, the transitions involving the unsubstituted tpy ligand are at a lower energy than the tpy moiety of the TPBr ligand. The absorption bands observed in the electronic spectra for TPBr and [PdCl(TP)] have been assigned with the aid of TD-DFT calculations. All complexes display weak emission both at room temperature and in a butyronitrile glass at 77 K. The considerable red shift of the emission maxima relative to the signal of the reference compound [Ru(tpy)2]2+ indicates stabilization of the luminescent 3MLCT state. For the mono- and heterometallic complexes, electrochemical and spectroscopic studies (electronic absorption and emission spectra and luminescence lifetimes recorded at room temperature and 77 K in nitrile solvents), together with the information gained from IR spectroelectrochemical studies of the dimetallic complex [Ru(TPPdSCN)(tpy)](PF6)2, are indicative of charge redistribution through the bridging ligand TPBr. The results are in line with a weak coupling between the {Ru(tpy)2} chromophoric unit and the (non)metalated NCN-pincer moiety.

In this thesis, ion stability, ion-molecule reactions and reaction rates are studied using mass spectrometry and molecular modelling. In Chapter 2 the effect of functional group substitution on neutral and ionised ketene are studied. Electron-donating substituents show a stabilising positive induction effect on the ketene ion, while electron acceptor substituents destabilise it by a negative induction effect. There exists also resonance stabilisation of the product ion, caused by electron donation from the substituent to the product ion. This stabilisation is in some cases so strong that the existence of a covalently bound substituted ketene ion is not possible. The stabilisation effects on proton-bound complexes are studied in Chapter 3. An existing linear correlation method to estimate the stability of the formed complexes is extended by inclusion of a dipole moment factor in the method. This improves the accuracy of the stability estimates in general for a range of complexes. Especially in many cases where there is a strong ion-dipole interaction contributing to the stability of the complex, the estimations are improved. Chapter 4 focuses on the dissociation reactions of protonated oxalic acid. It is found from computations that the lowest-energy reaction proceeds to the dissociation into protonated water, carbon monoxide, and carbon dioxide. This happens via a unique ter-body complex. This ter-body complex is also argumented to be the reason for the main peak in a metastable ion mass spectrum of protonated oxalic acid. Reaction paths of the minor dissociation products were also obtained. The possibility of pyrimidine formation in interstellar dust clouds from an acrylonitrile radical cation dimer is discussed in Chapter 5. Metastable ion mass spectrometric experiments on acrylonitrile exhibit a small amount of pyrimidine formation, which results from a covalently bound adduct cation of neutral and ionized acrylonitrile. From experimental and computational studies this is found to be only a minor product, while the major product is self-protonation via proton-bound complexes. A related reaction of acrylonitrile with hydrogen cyanide is studied in Chapter 6 using solely computations. Pyrimidine formation is deemed possible, in terms of energy, although kinetic studies show the adduct formation to be very slow in comparison to proton-transport catalysis reactions. The latter reactions are identified as the most favourable reactions. Protonated hydrogen cyanide formation is found to be a possible minor process as well. Chapter 7 is focused on qualitative studies using semi-classical trajectory calculations on the dissociation rate of an ion-dipole complex with a deep potential well preceding the dissociation. To reduce required computational time, a qualitative interpolation method is developed to predict the behaviour of trajectories in cases where many vibrational modes are excited. It is shown that, on average, only one-tenth of the channels open to dissociation lead directly to dissociation products for moderate excess energies (less than 10 kcal mol-1). The rest of the open channels lead either to a very slowly progressing dissociation, or to semi-periodic behaviour of the trajectory.

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.

New chiral NCN-pincer palladium complexes containing bulky diphenylhydroxymethyl pyrrolidinyl moieties as chiral auxiliaries have been synthesized. Oxidative palladium addition to ligand 2 (2 = 2,6-bis{[(S)-2-(diphenylhydroxymethyl)-1-pyrrolidinyl]methyl}-1-bromobenzene) initially yielded neutral arylpalladium bromide complex 3 in a moderate yield as a consequence of the bulky pyrrolidinyl functional groups. Performing the palladation reaction under microwave irradiation for only 5 min gave 3 in 79% yield. Abstraction of the bromide ion from 3 subsequently yielded the cationic complexes [4]PF6 and [4]BF4. Palladium complexes 3, [4]PF6, and [4]BF4 are all formed as single diastereoisomers with an RNRNSCSC configuration. X-ray crystal structure determinations of [4]PF6 and [4]BF4 revealed an unusual κ5-N,C,N,O,O coordination around palladium, in which the normal meridional κ3-coordination mode of the pincer framework is complemented by two rather long Pd−O interactions (2.622(2)–2.649(2) Å). Theoretical calculations (Mulliken population analysis and “atoms in molecules” analysis) confirmed that there is a coordinative interaction between the palladium and the oxygen atoms of both hydroxyl groups despite the long Pd−O distance. Complexes [4]PF6 and [4]BF4 catalyze the aldol condensation between α-methyl isocyanoacetate and various aromatic aldehydes with enhanced regio- (up to 70%) and stereoselectivity (up to 42% ee) for the cis-oxazolines. These observations are in contrast to prior literature results on other pincer systems, where the trans-oxazolines were the main product, and point to a deeper chiral cavity pointing away from palladium toward the coordinated isocyanoacetate substrate.

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.

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.

Large energy barriers prohibit the rearrangement of solitary acrylonitrile ions, CH2CHCN+, into their more stable hydrogen-shift isomers CH2CCNH+ or CHCH–CNH+. This prompted us to examine if these isomerizations occur by self-catalysis in acrylonitrile dimer ions. Such ions, generated by chemical ionization experiments of acrylonitrile with an excess of carbon dioxide, undergo five dissociations in the μs time frame, as witnessed by peaks at m/z 53, 54, 79, 80 and 105 in their metastable ion mass spectrum. Collision experiments on these product ions, deuterium labeling, and a detailed computational analysis using the CBS-QB3 model chemistry lead to the following conclusions: (i) the m/z 54 ions are ions CH2CHCNH+ generated by self-protonation in ion–dipole stabilized hydrogen-bridged dimer ions [CH2CHCNH–C(CN)CH2]+ and [CH2CHCNH–C(H)C(H)CN]+; the proton shifts in these ions are associated with a small reverse barrier; (ii) dissociation of the H-bridged ions into CH2CCNH+ or CHCH–CNH+ by self-catalysis is energetically feasible but kinetically improbable: experiment shows that the m/z 53 ions are CH2CHCN+ ions, generated by back dissociation; (iii) the peaks at m/z 79, 80 and 105 correspond with the losses of HCN, C2H2 and H, respectively. The calculations indicate that these ions are generated from dimer ions that have adopted the (much more stable) covalently bound “head-to-tail” structure [CH2CHCN–C(H2)C(H)CN]+; experiments indicate that the m/z 79 (C5H5N) and m/z 105 (C6H6N2) ions have linear structures but the m/z 80 (C4H4N2) ions consist of ionized pyrimidine in admixture with its stable pyrimidine-2-ylidene isomer. Acrylonitrile is a confirmed species in interstellar space and our study provides experimental and computational evidence that its dimer radical cation yields the ionized prebiotic pyrimidine molecule.

A systematic study of the structural and electronic properties of the 17 Si38 fullerene cage isomers, which are constructed by making all possible permutations among their pentagons and hexagons, is presented. For the full optimization of these structures, a tight-binding molecular-dynamics method is firstly applied, and the resulting structures were further optimized with two more accurate but more time-consuming methods, namely, the generalized tight-binding molecular dynamics and a density-functional theory calculation at the B3LYP level. In addition, the Si20 fullerene cage, optimized with the same methods, is also presented for comparison. The results of all these methods are in good agreement with each other, and they all predict the same isomer as the energetically most stable structure among these 17 isomers. In all the fullerene isomers, half of the atoms are nearly coplanar with their three neighbors, and half lie on sharp corners of the polyhedral cage, coinciding with a distinction between half of the atoms adopting an sp2-like hybridization and the other half using essentially pure p orbitals in their bonding to nearest neighbors.

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.

2,6-Bis(oxazolinyl)phenylnickel bromide complexes [NiBr(R,R'-Phebox)] (2) were synthesized via two synthetic routes (A and B). In route A, selective bis-ortho lithiation of [R,R'-PheboxBr], followed by a transmetalation reaction with [NiBr2(PEt3)2], yielded not only complexes 2 with an 3-mer-N,C,N'-bonded Phebox ligand but also [NiBr(R,R'-Phebox)(PEt3)2], 7, where the nickel center is 1-C bonded to the intra-annular Cipso of the Phebox ligand. Coordination of two PEt3 ligands completes the square-planar coordination sphere of the Ni center in 7. When R = t-Bu, R' = H, only complex 7c was formed. Alternatively, when route B (oxidative addition with [Ni(cod)2], cod = cyclooctadiene) was followed, selective formation of complexes 2 was observed. X-ray crystal structures were obtained for [NiBr-(i-Pr,H-Phebox)] (2b) and [NiBr(bis(ketimine)phenyl)] (3). The Ni centers have square-planar geometries with a planar, 3-mer-N,C,N' coordination of the terdentate ligand systems. Complexes 2 were found to be inactive as catalysts in the atom-transfer radical polymerization (ATRP) reaction of methyl methacrylate (MMA) and in the atom-transfer radical addition (ATRA, Kharasch addition) reaction of CCl4 to MMA. This is ascribed to the relatively high oxidation potential of NiII-Phebox complexes, which excludes the (reversible) formation of a d7-NiIII-Phebox complex, a crucial condition for subsequent reactions. Cyclovoltammetry (CV) experiments ((n-Bu)4NBr as supporting electrolyte) showed no electrochemical waves between -1.00 and +1.50 V (Ag/AgCl reference electrode, (n-Bu)4NBr as supporting electrode). Theoretical calculations showed that the energy (Eox) needed for the oxidation reaction occurring during the CV experiments is considerably higher for [NiBr(Me,Me-Phebox)] (1.87 eV) and [NiBr(bis(ketimine)phenyl)] (1.90 eV) than for [NiBr(NCN)] (1) (1.45 eV).

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

The dinuclear complex [(tpy)RuII(PCP?PCP)RuII(tpy)]Cl2 (bridging PCP-PCP ) 3,3',5,5'-tetrakis(diphenylphosphinomethyl) biphenyl, [C6H2(CH2PPh2)2-3,5]2 2-) was prepared via a transcyclometalation reaction of the bis-pincer ligand [PC(H)P-PC(H)P] and the Ru(II) precursor [Ru(NCN)(tpy)]Cl (NCN ) [C6H3(CH2NMe2)2-2,6]-) followed by a reaction with 2,2':6',2''-terpyridine (tpy). Electrochemical and spectroscopic properties of [(tpy)RuII(PCP?PCP)RuII- (tpy)]Cl2 are compared with those of the closely related [(tpy)RuII(NCN-NCN)RuII(tpy)](PF6)2 (NCN?NCN ) [C6H2(CH2- NMe2)2-3,5]2 2-) obtained by two-electron reduction of [(tpy)RuIII(NCN?NCN)RuIII(tpy)](PF6)4. The molecular structure of the latter complex has been determined by single-crystal X-ray structure determination. One-electron reduction of [(tpy)RuIII(NCN?NCN)RuIII(tpy)](PF6)4 and one-electron oxidation of [(tpy)RuII(PCP?PCP)RuII(tpy)]Cl2 yielded the mixed-valence species [(tpy)RuIII(NCN?NCN)RuII(tpy)]3??and [(tpy)RuIII(PCP?PCP)RuII(tpy)]3+, respectively. The comproportionation equilibrium constants Kc (900 and 748 for [(tpy)RuIII(NCN?NCN)RuIII(tpy)]4+ and [(tpy)RuII(PCP??PCP)RuII(tpy)]2+, respectively) determined from cyclic voltammetric data reveal comparable stability of the [RuIII-RuII] state of both complexes. Spectroelectrochemical measurements and near-infrared (NIR) spectroscopy were employed to further characterize the different redox states with special focus on the mixed-valence species and their NIR bands. Analysis of these bands in the framework of Hush theory indicates that the mixed-valence complexes [(tpy)RuIII(PCP-PCP)RuII(tpy)]3+ and [(tpy)RuIII(NCN-NCN)RuII(tpy)]3+ belong to strongly coupled borderline Class II/Class III and intrinsically coupled Class III systems, respectively. Preliminary DFT calculations suggest that extensive delocalization of the spin density over the metal centers and the bridging ligand exists. TD-DFT calculations then suggested a substantial MLCT character of the NIR electronic transitions. The results obtained in this study point to a decreased metal?metal electronic interaction accommodated by the double-cyclometalated bis-pincer bridge when strong s-donor NMe2 groups are replaced by weak s-donor, p-acceptor PPh2 groups.

The 12π cation (3) and 14π anion (4) derived from the phenalenyl radical (2) support diatropic ("aromatic") perimeter ring currents, but isoelectronic replacement of the central atom by either boron (5) or nitrogen (6) leads to paratropic ("antiaromatic") current; the ipsocentric approach to molecular magnetic response accounts for all four patterns in terms of competition between translationally and rotationally allowed virtual π-π* excitations.

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.

Apparent contradictions between ipsocentric, Nucleus-Independent Chemical Shift (NICS) and Gauge-Including Magnetically Induces Current (GIMIC) evaluations of ring current aromaticity in Al4 2- are resolved in favour of the descripion of this all-metal system as sigma aromatic on the magnetic criterion.

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.

To establish the link between the aromaticity descriptors based on the Pauling resonance energy and the molecular properties, the electric (polarizability) and magnetic (magnetizability) field response properties haven been calculated using the valence bond approach for various molecules and their individual Kekule resonance structures. The results show that there is no direct relationship between the Pauling resonance energy and the properties; the response properties are weighted averages of the properties of the individual structures. According to the aromaticity criteria based on molecular properties, one-structure benzene would be aromatic; thus, concerning molecular properties, spin-coupled bonds do not behave like localized bonds in Lewis structures, with which they are usually associated.

Complete ¹H and ¹³HC NMR assignments of the (di-)cyclopenta-fused pyrene congeners, cyclopenta[cd]- (2), dicyclopenta[cd,fg]- (3), dicyclopenta[cd,jk]- (4) and dicyclopenta[cd,mn]pyrene (5), respectively, are archieved using two-dimensional (2D) NMR spectroscopy. The experimental ¹³C chemical shift assignments are compared with computed ab initio CTOCD-PZ2/6-31G** ¹³C chemical shifts; a satisfactory agreement is found. Substituent-induced chemical shifts in the pyrene core induced by annelation of cyclopenta moieties are discussed. Effects of dicyclopenta topology on electric structure are illustrated for 3-5.

Previous studies have shown that the solitary ketene-water ion CH2=C(=O)OH2+ (1) does not isomerize into CH2=C(OH)2+ (2), its more stable hydrogen shift isomer. Tandem mass spectrometry based collision experiments reveal that this isomerization does take place in the CH2=O loss from low-energy 1,3-dihydroxyacetone ions (HOCH2)2C=O+. A mechanistic analysis using the CBS-QB3 model chemistry shows that such molecular ions rearrange into hydrogen-bridged radical cations [CH2C(=O)O(H)-H...OCH2]+ in which the CH2O molecule catalyzes the transformation 1->2 prior to dissociation. The barrier for the unassisted reaction, 29 kcal mol-1, is reduced to mere 0.6 kcal mol-1 for the catalysed transformation. Formaldehyde is an efficient catalyst because its proton affinity meets the criterion for facile proton-transport catalysis.

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.

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.

Surprisingly, the anti-aromatic paratropic ring current of the planar D4h transition state of 1,3,5,7-cyclooctatetraene (COT) survives almost all the way to the equilibrium tub-shaped D2d COT structure (plane-to-plane distance, d = 0.76 A); the current vanishes at ca. 80% of the geometric change and ca. 90% of the energy change from planar to tub geometry.

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.

The differences between the molecular struturs of the PCP-pincer complex [RuCl{C6H3(CH2P(C6H5)2)2- 2,6}(PPh3)] ([RuCl(PCP H)(PPh3), 1) and its tetrakis-pentafluorophenyl substituted analogue [RuCl{C6H3(CH2P(C6F5)2)2- 2,6}(PPh3)] (RuCl(PCP F20)(PPh3)], 2) have been rationalised by performing calculations on the cations [Ru(PCP H)(PPh3)]+ (1cat) and [Ru(PCP F20)(PPh3)]+ (2cat). The molucular interactions between the chloride ligand and the axial rings, as found in 1 and 2, respectively, have been studied computationally in the model system [(C6X5PH2)2CL-](X = H, F). The calculations on 2cat show that in 2 it is most likely the attractive electrostatic interaction between the chloride ligand and the fluorinated phenyl rings that forces the C ipso atom to occupy an axial postion rather than an equatorial one in the observed (X-ray of 2) square pyramidal arrangement. In 1, however, repulsive steric hinderance forces the PPh3 ligand to take the apical postion. The applicability of the TD-DFT method for the calculation of the electronic spectra of the PCP-pincer compounds 1 and 2 has been tested. The results indicate that the excitation energies calculated for both complexes are in a reasonable agreement with the experimental absorption maxima. However, for 1, all the calculated transition energies are underestimated.

The predominant dissociation process observed for metastable protonated oxalic acid ions HOOC-C(OH)2+ (generated by self-protonation) leads to H3O++CO+CO2. We have traced the mechanism of this intriguing reaction using the CBS-QB3 model chemistry. Our calculations show that a unique ter-body complex, O=C=O ...H3O+...CO, plays a key role in the rearrangement process. This complex can also dissociate to the proton bound dimers [H2O...H...O=C=O]+ and [H2O...H...CO]+ which are minor processes observed in the metastable ion mass spectrum. A further minor process leads to the proton bound dimer O=C=O...H+...CO which is formed by water extrusion from the ter-body complex. Argments are provided that the ter-body complex is also generated in the ion source by the collision encounter between neutral and ionized oxalic acid.

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.

In this Letter, the formulation and implementation of a parallel response property code for non-orthogonal, valence bond wave-functions are described. Test calculations on benzene and cyclobutadiene show that the polarisability and magnetisability tensors obtained using valence bond theory are comparable to those obtained at the full p-CASSCF level of theory. The properties are a weighted average of those of its individual resonance structures. Resonance has no relevance for the polarisability and magnetisability.

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.

Putative s-aromaticity of the clusters H3+ and Li3+ is investigated by computation of ring-current maps in the ab initio ipsocentric approach. Although H3+ shows a marked diatropic ring-current and can be considered s aromatic on the magnetic criterion, Li3+ shows no global current and is non-aromatic on this criterion, in spite of its electron count and negative NICS value. The difference in magnetic response is interpreted in terms of orbital energies using the ipsocentric model: in Li3+ the diatropic HOMO-LUMO excitation is opposed by paratropic excitations to higher unoccupied orbitals; in H3+ this cancellation does not occur.

We present the results of a study on the structural and electronic properties of the Si38 fullerene isomers, which are constructed by making all possible permutations among their pentagons and hexagons. These structures were firstly fully optimized with a tight binding molecular dynamics method and the resulting structures were further optimized with two more accurate, but more time consuming, methods, namely the generalized tight binding molecular dynamics and a DFT calculation at the B3LYP level. For comparison, the optimum structure of the Si20 fullerene cage is also presented, optimized with the same methods.