When aqueous solutions containing Fe2+ ions are irradiated at <250 nm, photooxidation to Fe3+ occurs and molecular hydrogen is generated. This photoprocess has been studied extensively for over 60 years, but without agreement being reached about the nature of the primary step. Possible initial steps include metal-to-ligand charge transfer (MLCT), internal Fe2+ 3d → 4s absorption, direct electron photodetachment producing a partially solvated electron in a pre-existing solvent cavity, and polaron-type charge transfer to solvent (CTTS) absorption. We consider the energetics and solvent shift of the first three of these processes, concluding that the MLCT band is too high in energy, the 3d → 4s excitation could participate, and the direct photodetachment band is at the correct energy and intensity to account for all that is (as yet) observed of the absorption band. In general, a rather complicated picture of this process in inorganic complexes emerges. In this work, we apply a general method we have developed for estimating the effects of solvents on transitions of species that have strong specific interactions (e.g., hydrogen bonding) with the solvent molecules.

Structural and energetic features of the intermolecular interactions of a creatinine designed-receptor complex are investigated using ab initio electronic structure methods. Both the host and receptor can adopt different tautomeric forms and it is found that in the complex both molecules are considerably different from their gas-phase structures. A polar environment has an important role in determining the binding energy of the complex and may lead to proton transfer in the complex.

The effect of water on the energetics of the Claisen rearrangement of chorismate to prephenate has been investigated with both a continuum model(PCM) and explicit solvation within a Monte Carlo (MC) free energy perturbation (FEP) treatment, Both models are surprisingly similar in their quantitative predictions, Solvation is shown to increase the conformational flexibility in the reactant by reducing the energy difference between the diequatorial and diaxial structures, and to lead to a considerable reduction in the barrier to the reaction, The estimate of the activation barrier, 22 kcal mol(-1),dagger is very close to the experimental value of 24.5 kcal mol(-1). The implications of the various possible conformers on kinetic isotope effects are discussed.

The effect of water on the Claisen rearrangement of allyl vinyl ether is modelled using both ab initio continuum and Monte Carlo simulation techniques. The effect of electron correlation and electronic polarisation of the solute is included in both treatments. In the continuum calculations these effects are included directly in the quantum mechanical treatment, whilst in the classical simulations solute atomic charges that reflect both correlation and polarisation are employed. The results of the calculations are used to predict both the barrier lowering and the kinetic isotope effects in aqueous solution. The role of both electron correlation, solute polarisation and the need to include explicit solvent molecules are all clearly identified by the results presented.

The tautomeric equilibrium involving the amine and imine forms of creatinine are studied theoretically in both the gas phase, the aqueous environment, and the solid state. High level ab initio calculations predict the imine form to be preferred in the gas phase by 2.0 kcal mol−1. In the aqueous environment, both continuum and explicit solvent models predict the amine form to be the most stable, with solute polarization being an important contributing factor. An embedding procedure is used to model solid state effects when again the amine is preferred, in agreement with experimental crystallographic studies. The predicted solid state molecular geometry is compared to the experimental structure.

The barrier to rotation about the C-N(H-2) bond in creatinine in aqueous environments has been studied experimentally, by means of NMR spectroscopy, and theoretically, employing a number of models based upon explicit solvation and continuum descriptions. The measured barrier to rotation (13 kcal mol(-1)) is obtained only if an explicit solvent is included in calculations on a supermolecule model; the use of a continuum model alone predicts a barrier (9 kcal mol(-1)), considerably smaller than the experimental value.

A quantum chemical study of geometries, H-2 binding energies, and HD spin-spin coupling constants using self-consistent field theory, second-order Moller-Plesset theory (MP2), and density functional theory (DFT) techniques for a series of molecular hydrogen complexes [OS(NH3)(4)L-z(eta(2)-H-2)]((z+2)+) [L-z = (CH3)(2)CO, H2O, CH3COO-, Cl-, H-, C5H5N, CH3CN-, CN, NH2OH, and NH3] is described. Electron correlation was found to be of crucial importance in the description of the H-H potential and the equilibrium H-H distance. The MP2 and DFT predictions of the geometries and energetics are In reasonable agreement, but there is noticeable divergence in the predicted H-H distances for weakly bound complexes containing trans ligands with strong pi-acceptor properties. The calculated H-H distances range from 0.95 to 1.40 Angstrom and are consistent with stretched molecular hydrogen acting as a ligand rather than dissociating into two atoms bound as hydrides. The H-H distance predicted by both MP2 and DFT methods is in good agreement with that observed in the [Os(NH2C2H4NH2)(2)(CH3COO-)(eta(2)-H-2)](+) complex, the only one for which neutron diffraction data are available.

We report ab initio calculations, at the Hartree-Fock lever of theory, of the binding energies and electric dipole polarizabilities of the linear and cyclic clusters Li2F2 and Na2F2, and of the linear cluster K2F2. Bond distances corresponding to the diatomic (denoted MF) bond lengths were used, and all calculations were done using large basis sets.The results demonstrate that four-body effects are non-negligible in calculations of the electric dipole polarizability for such species, with the geometries considered.

The effect of hydration upon the barrier to rotation in formamide has been predicted using both continuum models and those that consider the solvent molecules explicitly. The continuum calculations are based upon the polarizable continuum model, while the simulation calculations employ a Monte Carlo (MC) treatment. Variants of the MC method are studied employing classical and quantum mechanical (both semiempirical and ab initio) descriptions of the solute. All methods suggest a different transition state structure in water compared to the gas phase and show the importance of solute polarization. The calculated free energy barrier to rotation is in good agreement with the experimental data in all cases.

We report ab initio values of the various triplet incremental polarizabilities and polarizability anisotropies for the three species LiFLi+, NaFNa+ and KFK+. All calculations were done at the SCF level of theory, using large basis sets. A series of calculations was performed on linear and bent (90 degrees) conformers, covering a range of chemically important interatom distances. The classical superposition approximation generally underestimates the tripler values for geometrical arrangements using distances around the diatomic energy minima, although it does predict the correct sign.

Ab initio molecular orbital calculations on the structure and stability of the nitrate and sulfate and sulfate complexes of uranyl (UO22+) and plutonyl (PuO22+) using effective core potentials are reported. It is found that the binding energy of sulfate is greater than that of nitrate to both uranyl and plutonyl, with a slight preference for plutonyl. A method of decomposing the binding energy into electrostatic, Pauli repulsion, polarization and charge-transfer components is described which predicts that electrostatic forces are dominant. A simple molecular mechanics potential is developed by using this finding, which is successful in reproducing the ab initio results.

In search of materials which may function as molecular wires or switches, analytical models have suggested that the Brooker ions should be particularly interesting. We study them in detail using ab initio, semi-empirical and specially-designed empirical techniques, predicting molecular geometries, charge distributions, and conductivities. Provided molecular symmetry is maintained, odd polyenes and Brooker ions NH2-(CH)+ 2n - 1 - NH2 are shown to conduct significantly better than even polyenes, but the advantage becomes a simple multiplicative factor once solitons form (chains of length ca 20 A). Symmetry lowering is predicted to dramatically decrease the conductivity but introduces the possibility that the Brooker ions may function as molecular switches, having greatly enhanced, switchable, non-linear optical properties.

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A theoretical study of the stretched molecular hydrogen complexes [Os(NH3)(4)L(z)(eta(2)-H-2)]((z+2)+)(L(z) = (CH3)(2)-CO, H2O, CH3COO-, Cl-, H-, C5H5N, and CH3CN) is reported. Using SCF and MP2 methods in conjunction with effective core potentials and basis sets of triple-zeta quality on Os and double-zeta on the ligand atoms the geometries, HD spin-spin coupling constants, and binding energies have been calculated and compared with the available experimental data. The calculated H-H distances are remarkably uniform: all fall in the range 1.30-1.40 Angstrom and correspond to stretched eta 2-H-2 complexes, i.e. no cis-dihydrides have been found. The predicted H-H distance in the acetate complex is consistent with the observed distance of 1.34 Angstrom in the ethylenediamine derivative. A unique feature of these complexes is the crucial role that electron correlation has on the H-H bond lengths; this is a consequence of the unusual potential energy surfaces that are extraordinarily flat with respect to the H-H stretch. The calculated Os-H bond lengths and stretching frequencies are also consistent with experiment. The calculated HD spin-spin coupling constants (J(HD)) are of the same order of magnitude as those observed, yet the trend in the latter with changing trans ligand L(z) is not adequately reproduced by the calculations, although a reasonable correlation between experimental J(HD) and calculated H-H distance is demonstrated. In order to elucidate the nature of the Os-H-2 bonding, population analyses as well as an analysis of the binding energy of the Cl- complex have been carried out and these further emphasize the importance of electron correlation. At the simplest level, the effect of the trans ligand L(z) on the properties of the complexes can be related to its spectrochemical constant, which correlates with H-H distance and binding energy as well as HD spin-spin coupling constant.

Our method (parts I-III1-3) for estimating solvent shifts of species which have strong specific interactions (e.g., hydrogen bonding) with the solvent is applied to inorganic charge transfer spectra; As the simulation of the structure of ions in solution is not completely straightforward, a number of aspects of the simulation procedure are investigated, concentrating on solvent shift sensitivity. Specifically, we investigate the ultraviolet absorption spectrum of aqueous Fe2+(H2O)(6); for centrosymmetric systems such as this, it is found to be most important to correctly represent the structure of the second coordination shell. Assumptions such as that of rigidity of the inner shell and the particular choice of boundary conditions are of minor consequence. In a process studied extensively over several decades, ultraviolet light absorption results in electron ejection, leading to the photochemical decomposition of water. Several mechanisms for the primary process have been suggested in the past, without consensus being achieved. These include initial metal to ligand charge transfer (MLCT), metallic 3d --> 4s absorption, direct electron photodetachment producing a partially solvated electron in a preexisting solvent cavity, and charge transfer to solvent absorption (CTTS). We consider the energetics and solvent shift of the first three of these processes, concluding that the MLCT band is too high in energy, the d --> s band could participate, and the photodetachment band is at the correct energy and intensity to account for all that is (as yet) observed of the absorption band. In general, a rather complicated picture of this process in inorganic complexes emerges.

The results of an ab initio quantum chemical study, carried out largely at the MP2 level of theory, are reported for the trimethylamine—hydrogen chloride complex. The resulting geometry, harmonic frequencies and properties such as dipole moment and electric field gradients at the Cl and N nuclei are compared with those of the parent monomers as well as those of the trimethyl-ammonium ion. The computed N…Cl distance, intermolecular stretching frequency and field gradients agree with experiment. The asymmetric NHCl stretch is found to be quite anharmonic and consequently vibrational averaging with respect to that mode has a significant effect on the dipole moment and electric field gradients. The calculated structure and frequencies point to a large degree of proton transfer and hence ionic character. This is confirmed by an analysis of the SCF binding energy by the constrained spatial orbital variation method as well as the results of population analyses that also suggest significant covalency.

Both the anti hydrogen bonded and hydrogen bonded dimers of C1F, HF are studied at the MP2/6-311G∗∗ level. The minima for both dimers was found, the zero-point dissociation energies being 13.0 and 11.0 kJ mol−1 respectively. Apparent disagreement with the experimentally observed frequencies can be resolved if it is assumed that both dimers were present in the experimental study.

Ab initio molecular orbital theory has been used to study the binary complex formed between the benzene and ClF molecules. The 4-31G and 6-31G** basis sets were employed and also electron correlation effects were assessed by second order Moller-Plesset perturbation theory with the 6-31G** basis set in single-point calculations at the SCF optimized geometry. Basis set superposition errors have been evaluated by the counterpoise correction method. It was found that the Cl-F subunit interacts with the pi electrons of the benzene ring through the Cl end, and also that there is a significant deviation from the C6v symmetry expected.

A brief outline is given of a new method for the calculation of electronic band-centre shifts of molecules in condensed phases. It is easy to implement using existing technologies, and is readily applicable to transitions involving from quite small to large degrees of charge transfer. A two-step process is involved: first, simulations are performed in order to determine the equilibrium structure of the medium around the chromophore. These configurations are then sampled and the frequency shift deduced using perturbation expansions of the quantum-chemical interactions. The method is applied here to the 1(n, π*) spectra of dilute pyridine in water. Observed thermodynamic and spectroscopic properties are reproduced.

The equilibrium geometry and infrared spectra for the five isotopomers of the linear weakly bound complex between the C2H2 and CO monomeric units have been calculated at the MP2 (Møller—Plesset second-order perturbation theory) level employing the [5s4p1d/3s1p] basis set. The energy barrier for interconversion between the DCCH…CO and HCCD…CO configurations is evaluated with the HCCD…CO isotopomer predicted to be the more stable form. The dipole moment and nuclear—quadrupole coupling constant are also calculated. There is good agreement with a recent experimental study.