Acetonitrile is an extremely important solvent and cosolvent. Despite this, we have no general picture of the nature of mixed liquids containing acetonitrile applicable across-solvent families. We consider the properties of acetonitrile dissolved in 33 solvents, focusing on interpretation of the environment-sensitive solvent shift, Δν, of its CN stretch frequency, ν2. The two major models (dispersive and specific solvation) which have been proposed to interpret Δν are based on diverse experiments with incompatible conclusions. We ascertain the robust features of these models and combine them into a new one in which solvent−solvent and solvent−solute forces compete to determine the structure of the solution and hence Δν. First, Δν is analyzed in terms of solvent repulsive and dielectric effects combined with specific solvation effects. To interpret this specific solvation, 95 MP2 or B3LYP calculations are performed to evaluate structures and CN frequency shifts for CH3CN complexed with one molecule of either water, methanol, ethanol, 2-propanol, tert-butyl alcohol, phenol, benzyl alcohol, acetic acid, trifluoroacetic acid, 2,2,2-trifluoroethanol, 1,1,1,3,3,3-hexafluoro-2-propanol, acetonitrile, chloroform, carbon tetrachloride, tetrahydrofuran, formamide, pyridine, or Cl-, as well as 45 parallel calculations for the solvent monomers or dimers. The results are then convolved using known structural properties of the various solutions and/or related neat liquids, leading to an interpretation of the observed solvent shifts. Also, we measure Δν for acetonitrile in aqueous solution using Fourier transform Raman spectroscopy and show that the results are consistent with, but require modification of, microheterogeneity theories for the structure of acetonitrile−water solutions. Although such theories are still in their infancy, we suggest that microheterogeneity could also account for most known properties of acetonitrile−alcohol solutions and, in fact, be a quite general phenomenon.

Various Green’s-function-based formalisms which express the current I as a function of applied voltage V for an electrode–molecule–electrode assembly are compared and contrasted. The analytical solution for conduction through a Hückel (tight binding) chain molecule is examined and only one of these formalisms is shown to predict the known conductivity of a one-dimensional metallic wire. Also, from this solution we extract the counter-intuitive result that the imaginary component of the self-energy produces a shift in the voltage at which molecular resonances occur, and complete analytical descriptions are provided of the conductivity through one-atom and two-atom bridges. A method is presented by which a priori calculations could be performed, and this is examined using extended-Hückel calculations for two gold electrodes spanned by the dithioquinone dianion. A key feature of this is the use of known bulk-electrode properties to model the electrode surface rather than the variety of more approximate schemes which are in current use. These other schemes are shown to be qualitatively realistic but not sufficiently reliable for use in quantitative calculations. We show that in such calculations it is very important to obtain accurate estimates of both the molecule–electrode coupling strength and the location of the electrode’s Fermi energies with respect to the molecular state energies.

Over 100 oligoporphyrin (porphyrin molecules fused to each other through rigid acene-type bridges) molecules have now been synthesized, their long rigid π-bonded structures making them very suitable as molecular wires while their synthetic flexibility offers the possibility of tailoring their structural and electronic properties to match specific needs. To examine their basic operational principles and to explore synthetic possibilities, we optimize the geometry of 85 oligoporphyrin and related molecules including porphyrin dimers and trimers using the accurate B3LYP density-functional technique. Also, a scheme is developed by which accurate geometries of oligoporphyrins of arbitrary size can be estimated, and this is applied to determine the geometries of a further 13 porphyrin trimers and tetramers. At these geometries we analyze SCF orbital properties in order to determine the superexchange electronic couplings within the oligoporphyrins. Couplings are monitored for bridge-length dependence and interpreted in terms of a detailed description involving bridge-porphyrin orbital resonances, as well as in terms of a simpler picture in which π-electron delocalization is seen as a prerequisite for strong intramolecular coupling. Variations of the coupling with the nature of the bridge (e.g., naphthalene, anthracene, free-base or protonated 1,4,5,8-tetraazaanthracene, tetracene, pyrene, coronene, biphenylene, dicyclobuta [a,d]benzene, dicyclobuta[b,g]naphthalene, dicyclobuta[b,h]biphenylene, and bridges additionally fused to porphyrin meso positions) and porphyrin (e.g., porphyrin or bacteriochlorin, β-substituents such as methoxy and cyano, Mg, Zn, Ru(CO) 2 , and free-base porphyrins) units are considered, and the physical origin of quinonoid switching is determined. Terminal "alligator clips" such as fused phenanthroline, here complexed with Cu I Cl 2 , are also considered.

The design rationale is described as are some experimental and theoretical developments concerning rigidly fused porphyrin molecular wires. These materials consist of porphyrin units fused to acene‐type bridges and have been synthesized in a range of topologies including linear porphyrin octamers of length ca. 120 Å. Next we demonstrate, for some linear oligoporphyrins, how the electronic coupling between the end porphyrin units can be modulated by simple (possibly in situ) chemical modulation of the bridging units. Specifically, the chemical systems considered involve either pH‐controlled protonation of bridge azines or conversion of bridge quinone or quinone dioxime rings to or from benzenoid or hydroquinone rings. In the most general terms, the electronic coupling through oligoporphyrin molecular wires is discussed in terms of a simple model in which complete end‐to‐end π electronic delocalization is required in order to provide strong long‐range interactions. Computationally, we monitor interorbital coupling using an appropriate mixture of density functional (B3LYP) and ab initio SCF computational schemes. Finally, we examine bridge modulation of the intermetallic coupling in three homovalent bis‐metallic oligoporphyrin systems, two in which Ru(CO)2 is complexed within terminal porphyrin rings, the other in which [Cu(I)Cl2]− is tethered using phenanthroline end groups. Results are obtained both using an effective two‐level model, appropriate for spectroscopic properties, and using a more general scheme, appropriate for molecular conduction.

No abstract