Showing papers in "Journal of Physical Chemistry A in 1998"

Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model

Abstract: A new implementation of the conductor-like screening solvation model (COSMO) in the GAUSSIAN94 package is presented. It allows Hartree−Fock (HF), density functional (DF) and post-HF energy, and HF and DF gradient calculations: the cavities are modeled on the molecular shape, using recently optimized parameters, and both electrostatic and nonelectrostatic contributions to energies and gradients are considered. The calculated solvation energies for 19 neutral molecules in water are found in very good agreement with experimental data; the solvent-induced geometry relaxation is studied for some closed and open shell molecules, at HF and DF levels. The computational times are very satisfying: the self-consistent energy evaluation needs a time 15−30% longer than the corresponding procedure in vacuo, whereas the calculation of energy gradients is only 25% longer than in vacuo for medium size molecules.

Refinement and Parametrization of COSMO-RS

Abstract: The continuum solvation model COSMO and its extension beyond the dielectric approximation (COSMO-RS) have been carefully parametrized in order to optimally reproduce 642 data points for a variety of properties, i.e., ΔG of hydration, vapor pressure, and the partition coefficients for octanol/water, benzene/water, hexane/water, and diethyl ether/water. Two hundred seventeen small to medium sized neutral molecules, covering most of the chemical functionality of the elements H, C, N, O, and Cl, have been considered. An overall accuracy of 0.4 (rms) kcal/mol for chemical potential differences, corresponding to a factor of 2 in the equilibrium constants under consideration, has been achieved. This was using only a single radius and one dispersion constant per element and a total number of eight COSMO-RS inherent parameters. Most of these parameters were close to their theoretical estimate. The optimized cavity radii agreed well with the widely accepted rule of 120% of van der Waals radii. The whole parametriza...

A Bond Path: A Universal Indicator of Bonded Interactions

Abstract: The quantum mechanics of proper open systems yields the physics that governs the local behavior of the electron density, ρ(r). The Ehrenfest force F(r) acting on an element of ρ(r) and the virial field ν(r) that determine its potential energy are obtained from equations of motion for the electronic momentum and virial operators, respectively. Each is represented by a “dressed” density, a distribution in real space that results from replacing the property in question for a single electron with a corresponding density that describes its average interaction with all of the remaining particles in the system. All bond paths, lines of maximum density linking neighboring nuclei in a system in stable electrostatic equilibrium, have a common physical origin in terms of F(r) and ν(r), regardless of the nature of the interaction. Each is homeomorphically mirrored by a virial path, a line of maximally negative potential energy density linking the same nuclei. The presence of a bond path and its associated virial path...

The Proton's Absolute Aqueous Enthalpy and Gibbs Free Energy of Solvation from Cluster-Ion Solvation Data

Abstract: A method is presented to determine the absolute hydration enthalpy of the proton, ΔHaq°[H+], from a set of cluster-ion solvation data without the use of extra thermodynamic assumptions. The absolute proton hydration enthalpy has been found to be ∼50 kJ/mol different than traditional values and has been more precisely determined (by about an order of magnitude). Conventional ion solvation properties, based on the standard heat of formation of H+(aq) set to zero, have been devised that may be confusing to the uninitiated but are useful in thermochemical evaluations because they avoid the unnecessary introduction of the larger uncertainties in our knowledge of absolute values. In a similar strategy, we have motivated the need for a reassessment of ΔHaq°[H+] by the trends with increased clustering in conventional cluster-ion solvation enthalpy differences for pairs of oppositely charged cluster ions. The consequences of particular preferred values for ΔHaq°[H+] may be evaluated with regard to cluster-ion prop...

Characterization of a Dihydrogen Bond on the Basis of the Electron Density

Abstract: A new type of hydrogen bond, called a dihydrogen bond, has recently been introduced. In this bond a hydrogen is donated to another (hydridic) hydrogen. We apply a set of criteria developed in the context of the theory of “atoms in molecules” that were previously successfully used to study conventional hydrogen bonds. This method enables one to characterize the dihydrogen bond on the basis of the electron density only. We investigated a dimer structure of BH3NH3 at the ab initio level which contains two dihydrogen bonds that differ in strength. The combination of a theoretical density with our hydrogen-bonding criteria turns out to be a valuable new and independent source of information complementary to techniques such as NMR, IR, and structural crystallography.

Prediction of Hydrophobic (Lipophilic) Properties of Small Organic Molecules Using Fragmental Methods: An Analysis of ALOGP and CLOGP Methods

Abstract: Molecular hydrophobicity (lipophilicity), usually quantified as log P (the logarithm of 1-octanol/water partition coefficient), is an important molecular characteristic in drug discovery. ALOGP and CLOGP are two of the most widely used methods for the estimation of log P. This work describes an extensive reparametrization of the atomic log P values and a detailed comparison of the performance of ALOGP and CLOGP methods on the Pomona Medchem database. Only the “star list” compounds having precisely measured log P values were used in this analysis. While the overall results with both methods are similar, analysis shows that the CLOGP method is better for very small molecules in the range of 1−20 atoms. The two methods are almost comparable in the range of 21−45 atoms, while the ALOGP method has better accuracy for molecules with more than 45 atoms. Although the rms deviation and the correlation coefficient for CLOGP predictions were marginally better than those for corresponding ALOGP predictions, the latte...

Thermodynamic Model of the System H+−NH4+−SO42-−NO3-−H2O at Tropospheric Temperatures

Abstract: A multicomponent mole-fraction-based thermodynamic model is used to represent aqueous phase activities, equilibrium partial pressures (of H2O, HNO3, and NH3), and saturation with respect to solid phases (H2SO4 and HNO3 hydrates, (NH4)2SO4(cr), (NH4)3H(SO4)2(cr), NH4HSO4(cr), (NH4)2SO4·2NH4NO3(cr), (NH4)2SO4·3NH4NO3(cr), and NH4HSO4·NH4NO3(cr)) in the system H+−NH4+−SO42-−NO3-−H2O. The model is valid from 328 to <200 K, dependent upon liquid-phase composition. Parameters for H2SO4−H2O, HNO3−H2O, and (NH4)2SO4−H2O interactions were adopted from previous studies, and values for NH4NO3−H2O obtained from vapor pressures (including data for supersaturated solutions), enthalpies, and heat capacities. Parameters for ternary interactions were determined from extensive literature data for salt solubilities, electromotive forces (emfs), and vapor pressures with an emphasis upon measurements of supersaturated H2SO4−(NH4)2SO4−H2O solutions. Comparisons suggest that the model satisfactorily represents partial pressures...

New Evidence against Hydroxyl Radicals as Reactive Intermediates in the Thermal and Photochemically Enhanced Fenton Reactions

Abstract: During the oxidative degradation of 2,4-dimethylaniline (2,4-xylidine) by means of the H2O2/UV method, a series of hydroxylated aromatic amines are formed, this result confirming the role of the hydroxyl radical as an initiator of the oxidative chain reaction. Thermal or photochemically enhanced Fenton reactions in the presence of 2,4-dimethylaniline (2,4-xylidine) yield primarily 2,4-dimethylphenol as an intermediate product, the genesis of which may only be explained by an electron transfer mechanism. Experimental evidence for such a mechanism is presented, and values for the quantum yields of the photochemically enhanced reduction of iron(III) to iron(II) in aqueous solutions of 2,4-xylidine are given.

Hydration of Beryllium, Magnesium, Calcium, and Zinc Ions Using Density Functional Theory

Abstract: In order to study the coordinative behavior of doubly charged metal ions in water, a few representative metals have been chosen for theoretical studies. These are the group 2 metal ions beryllium, magnesium, and calcium and the group 12 zinc ion. The density functional method B3LYP has been used with very large basis sets. It is found that the water dipole moment and polarizabilities, which are critical for the accuracy of the binding energies, are very well reproduced provided that the basis set on the metal is included in the calculations. One of the main points of the present investigation has been to study the boundary between the first and second hydration shells. Trends of binding energies and structures are also discussed.

Direct Scaling of Primitive Valence Force Constants: An Alternative Approach to Scaled Quantum Mechanical Force Fields

Abstract: We present an alternative approach to the derivation of scaled quantum mechanical (SQM) force fields involving the direct scaling of individual primitive valence force constants from a full set of redundant valence coordinates. Our approach is completely general and more flexible than previous SQM schemes. Optimal scaling factors for various primitive stretching, bending, and torsional force constants are derived from a training set of 30 molecules containing C, O, N, H, and Cl and used to scale force constants for a further 30 molecules. Calculated vibrational frequencies are compared with experimental values for over 1500 fundamentals. Using the hybrid three-parameter B3-LYP density functional with the split-valence 6-31G* basis set, our scaling procedure gives an average error of less than 8.5 cm-1 in the scaled frequencies. The average percentage error is under 1%.

Thermodynamic Model of the System H+-NH4+-Na+-SO42--NO3--Cl--H2O at 298.15 K

Abstract: A multicomponent mole-fraction-based thermodynamic model of the H+−NH4+−Na+−SO42-−NO3-−Cl-−H2O system is used to represent aqueous-phase activities, equilibrium partial pressures (of H2O, HNO3, HCl, and NH3), and saturation with respect to 19 solid phases ((NH4)2SO4(cr), (NH4)3H(SO4)2(cr), NH4HSO4(cr), NH4NO3(cr), NH4Cl(cr), Na2SO4·10H2O(cr), Na2SO4(cr), Na3H(SO4)2(cr), NaHSO4·H2O(cr), NaHSO4(cr), NaH3(SO4)2·H2O(cr), NaNO3(cr), NaCl(cr), NH4HSO4·NH4NO3(cr), (NH4)2SO4·2NH4NO3(cr), (NH4)2SO4·3NH4NO3(cr), (NH4)2SO4·Na2SO4·4H2O(cr), Na2SO4·NaNO3·H2O(cr), 2NaNO3·NH4NO3(cr)). The model is valid for concentrations from infinite dilution to saturation (with respect to the solid phases) and to about 40 mol kg-1 for acid sulfate systems which can remain liquid to concentrations approaching the pure acid. Parameters for H2SO4−H2O interactions were adopted from a previous study, and values for other binary (water−electrolyte) and ternary (water and three ions) interactions were determined from extensive literature da...

Molecular and Atomic Polarizabilities: Thole's Model Revisited

Abstract: Thole's modified dipole interaction model for constructing molecular polarizabilities from effective, isotropic atomic polarizabilities is reviewed and extended. We report effective atomic polarizabilities for H, C, N, O, S, and the halogen atoms, independent of their chemical environment. They are obtained by fitting the model both to experimental and calculated molecular polarizabilities, the latter to enable one to model ab initio polarizabilities for various basis sets.

A generalized hybrid orbital (GHO) method for the treatment of boundary atoms in combined QM/MM calculations

Abstract: A generalized hybrid orbital (GHO) method has been developed at the semiempirical level in combined quantum mechanical and molecular mechanical (QM/MM) calculations. In this method, a set of hybrid orbitals is placed on the boundary atom between the QM and MM fragments, and one of the hybrid orbitals participates in the SCF calculation for the atoms in the QM region. The GHO method provides a well-defined potential energy surface for a hybrid QM/MM system and is a significant improvement over the “link-atom” approach by saturating the QM valencies with hydrogen atoms. The method has been tested on small molecules and yields reasonable structural, energetic, and electronic results in comparison with the results of the corresponding QM and MM approximations. The GHO method will greatly increase the applicability of combined QM/MM methods to systems comprising large molecules, such as proteins.

Spectroscopic and Theoretical Study of (CF3SO2)2N- (TFSI-) and (CF3SO2)2NH (HTFSI)

Abstract: The vibrational properties of the TFSI- anion solvated in a polymer or in water have been studied by comparing its IR and Raman spectra with those of the HTFSI molecule. Ab initio self-consistent field Hartree−Fock calculations have also been performed on the free ion or molecule to investigate their Mulliken charges, equilibrium geometry, and internal force constants. Both experimental and theoretical approaches confirm a pronounced delocalization of the negative charge on the nitrogen and oxygen atoms and a marked double-bond character of the SNS moiety for the anion. This double-bond character is decreased for the HTFSI molecule, leading to rather distinct frequencies for some specific vibrations such as the stretching motions of the SO2 and SNS groups. The agreement between experimental and calculated spectra is much better for HTFSI than for TFSI-. Tentative explanations are proposed for this difference.

Quantum-Mechanical Study of Thermodynamic and Bonding Properties of MgF2

Abstract: The structural and thermodynamic properties of MgF2 have been investigated in a wide range of pressures (0−80 GPa) and temperatures (0−850 K) by coupling quantum-mechanical ab initio perturbed ion ...

Ab Initio Study Of Hydrogen-Bonded Complexes Of Small Organic Molecules With Water

Abstract: Hydrogen bonding between water and a series of small organic molecules was examined via electronic structure calculations. Several computational methods were examined, including both a hybrid density functional procedure (Becke3LYP) and second-order Moller−Plesset theory (MP2) coupled with a double-ζ basis set augmented by diffuse polarization functions on heteroatoms. The agreement between Becke3LYP and MP2 energies was generally good, as was the agreement with energies obtained using more sophisticated and costly methods. The energies and structures of 53 hydrogen-bonded complexes of water with various small organic molecules, including alcohols, thiols, ethers, thioethers, carboxylic acids, esters, amines, amides, nitriles, and nitro compounds, were then examined systematically using the Becke3LYP and MP2 procedures. The hydrogen bond geometries were generally linear, and acceptor sites corresponded closely to the positions of lone pairs as predicted by simple hybridization arguments. Structures with s...

Hydrolysis of ferric ion in water and conformational equilibrium

Abstract: Reported here are results of theoretical calculations on Fe(H2O)63+, Fe(H2O)5(OH)2+, three isomers of Fe(H2O)4(OH)2+, and Fe(H2O)3(OH)2+, which investigate the molecular mechanisms of hydrolysis of ferric ion in water. The combination of density functional electronic structure techniques and a dielectric continuum model for electrostatic solvation applied to the Fe(H2O)63+ complex yields an estimate of −1020 kcal/mol (experimental values −1037 to −1019 kcal/mol) for the absolute free energy of the aqueous ferric ion. The predicted free energy change for the first hydrolysis reaction is surprisingly close to the experimental value (2 kcal/mol predicted compared to 3 kcal/mol experimental). For the second hydrolysis reaction, we found an unexpected low-energy isomer of Fe(H2O)4(OH)2+ with five ligands in the inner sphere and one water outside. The hexacoordinate cis and trans isomers are, respectively, slightly lower and higher in energy. Calculations on the pentacoordinate species Fe(H2O)3(OH)2+ suggest th...

Electron Attachment Energies of the DNA Bases

Abstract: Injection of electrons into the empty π* molecular orbitals of uracil and the DNA bases creates short-lived anion states whose energies have been determined by electron scattering. A common range of attachment energies into the lowest orbitals is observed in all the bases. Evidence for nuclear motion during the lifetimes of the anions is found in all the compounds except adenine. These properties of the bases as bridge sites along the π-stack of DNA, namely, the effective degeneracy of the anion energies and the strong excitation of vibration, are key parameters for theories of electron-transfer rate, some of which lead to inverse rather than exponentially decreasing bridge-length dependences.

Ultrafast Charge Transfer in Amino-Substituted Boron Dipyrromethene Dyes and Its Inhibition by Cation Complexation: A New Design Concept for Highly Sensitive Fluorescent Probes

Abstract: The photophysical behavior of a newly synthesized aza crown-substituted boron−dipyrromethene (BDP) dye and its dimethylamino analogue were investigated with steady-state and time-resolved fluorometry and compared to a reference compound. In solvents more polar than hexane, excitation of the dyes leads to a fast charge transfer from the locally excited (LE) state to a weakly emissive charge-transfer (CT) state. The donor-substituted compounds show dual emission from the LE and CT state, both fluorescence quantum yields being low. The rate constant of excited-state charge separation, calculated from the global analysis of time-resolved emission data, was determined to 1.6 × 1011 s-1 in 1,4-dioxane. The crowned compound forms 1:1-complexes with various alkali and alkaline-earth metal ions, which exist as two conformers in solution. In these complexes, coordination of the cation to the nitrogen donor atom of the crown inhibits the charge-transfer process, leading to a cation-dependent enhancement of the LE em...

Anti-Hydrogen Bond in the Benzene Dimer and Other Carbon Proton Donor Complexes

Abstract: A new type of bonding, termed anti-hydrogen bond, is identified in the benzene dimer and other carbon proton donor complexes from correlated ab initio computations. Gradient optimization of the benzene dimer at the MP2/6-31G* and MP2/6-31G** levels shows a shortening of the C−H bond of the proton donor and a blue-shift of the corresponding C−H stretching frequency. The harmonic C−H stretching vibrational frequency shift agrees well with that evaluated for various anharmonic approaches. The blue-shift of the C−H stretching frequency was also found in the case of benzene complexes with other carbon proton donors, CH4 and CHCl3. The anti-H-bonds are expected to be very significant for the structure and dynamics of biomolecules.

Local Softness and Hardness Based Reactivity Descriptors for Predicting Intra- and Intermolecular Reactivity Sequences: Carbonyl Compounds

Abstract: The DFT-based reactivity descriptors “local softness” and “local hardness” are used as reactivity indices to predict the reactivity sequences (both intramolecular and intermolecular) of carbonyl compounds toward nucleophilic attack on them. The finite difference approximation is used to calculate local softness, whereas local hardness is approximated by −Vel/2N, where Vel is the electronic part of the molecular electrostatic potential. Both aldehydes and ketones, aliphatic and aromatic, have been selected as systems. Critical cases, e.g., C6H5CHCHCHO, CH3CHCHCHO, and CH2CHCHO, where a CC double bond is in conjugation with the CO group, are also considered. Two new reactivity descriptors are proposed, “relative electrophilicity” (sk+/sk-) and “relative nucleophilicity” (sk-/sk+), which will help to locate the preferable reactive sites. Our results show that local hardness can be used as a guiding parameter when constructing intermolecular reactivity sequences.

Matrix-Isolation Infrared and Theoretical Studies of the Glycine Conformers

Abstract: The IR spectra of nonionized glycine and its deuterated derivatives isolated in the low-temperature argon matrices have been measured, and for the first time the infrared spectral characteristics of the three most stable conformers have been determined and assigned. Correlated level ab initio and density functional theory (DFT) calculations of IR frequencies and intensities with extended basis sets were performed and their results were employed to separate the bands of the glycine conformers in the experimental spectra and to assist the assignment. The intramolecular interconversion, conformer III ⇒ conformer I, which is observed in the matrices at temperatures higher than 13 K, was found to cause a significant decrease of the band intensities of conformer III in the spectra. This phenomenon was used to distinguish the vibrational bands of this conformer from the bands of the other conformers. The reliability of the Moller−Plesset second-order perturbation theory (MP2) method and the DFT method with the t...

Classifying the Photophysical Dynamics of Single- and Multiple-Chromophoric Molecules by Single Molecule Spectroscopy

Abstract: The single molecule fluorescence spectroscopy of various isolated single-chromophoric dye molecules and multiple-chromophoric conjugated polymer molecules has been investigated. For each system the transient fluorescence “intensity”, ICW(t) (i.e., detected photons/dwell time), has been recorded with continuous wave (CW) irradiation. ICW(t) has been analyzed to yield an occurrence histogram for the different “intensities”, H(I), and an intensity time-autocorrelation function CI(t). The histograms H(I) for the various examples show highly diverse behavior with one, two, or even three peaks as well as “flat regions”. The different features in the histograms are shown to arise from distinct photophysical processes. From the study of model systems, characteristic features in the intensity histograms and autocorrelation functions are shown to result from photon shot noise, “blinking” due to triplet bottlenecks, spectral diffusion due to environmental fluctuations, and interchromophoric energy transfer. Classifi...

New Class IV Charge Model for Extracting Accurate Partial Charges from Wave Functions

Abstract: We propose a new formalism, Charge Model 2 (CM2), to obtain accurate partial atomic charges from a population analysis of wave functions by a parametrized mapping procedure, so that the resulting charges reproduce highly accurate charge-dependent observables. The new method, which produces class IV charges, is illustrated by developing CM2 mappings of Lowdin charges obtained from semiempirical and ab initio Hartree−Fock theory and density functional theory, in particular AM1, PM3, HF/MIDI!, HF/6-31G*, HF/6-31+G*, BPW91/MIDI!, BPW91/6-31G*, B3LYP/MIDI!, and BPW91/DZVP calculations. The CM2 partial charges reproduce experimental dipole moments with root-mean-square errors that are typically a factor of 7 better than dipole moments computed from Mulliken population analysis, a factor of 3 better than dipole moments computed by Lowdin analysis, and even a factor of 2 better than dipole moments computed from the continuous electron denisty. At the HF/6-31G* and B3LYP/MIDI! levels, the new charge model yields r...

A Systematic Failing of Current Density Functionals: Overestimation of Two-Center Three-Electron Bonding Energies

Abstract: Several density functional theories, chosen among the most popular, are shown to systematically overestimate three-electron bond dissociation energies in a series of model systems covering the full range of homonuclear three-electron X∴X bonds (X ) He, N, O, F, Ne, P, S, Cl, Ar) that can be found in organic cation radicals. The errors range from 15 to 54 kcal/mol with the SVWN local spin density method, from 6 to 45 kcal/mol with the gradient-corrected BLYP, BP86, and BPW91 functionals, and from 3 to 31 kcal/mol with the B3LYP, B3P86, and B3PW91 three-parameter hybrid functionals. The errors follow some regular tendencies according to the place of the X atom in the periodic table. The geometries and frequencies are also in error, the bond lengths being systematically too long and the frequencies too low. The errors are tentatively interpreted as consequences of electron self-interaction, leading to overstabilization of the Coulombic terms relative to the exchange-correlation terms in this type of bonds. At variance with these systematic errors, the BH&HLYP functional displays overall better results but still severely fails in some particular cases. Some lines of thought for devising modified DFT functionals are proposed.

Born-oppenheimer ab initio qm/mm dynamics simulations of na+ and k+ in water : from structure making to structure breaking effects

Abstract: The description of nonadditive contributions in the first hydration shell of Na+ and K+ has been improved by performing molecular dynamics simulations based on combined ab initio quantum mechanical...

Local Treatment of Electron Correlation in Molecular Clusters: Structures and Stabilities of (H2O)n, n = 2−4

Abstract: The structures and stabilities of small water clusters are studied by local electron correlation methods. It is demonstrated that the local treatment eliminates basis set superposition errors (BSSEs) to a large extent and thus allows BSSE-free geometry optimizations. Results for various basis sets are presented which show that the interaction energies and structural parameters obtained by local second-order Moller−Plesset perturbation theory (LMP2) without counterpoise correction are in close agreement with counterpoise-corrected conventional MP2 results. Furthermore, a partitioning of the LMP2 energies of (H2O)n, n = 2−4, into different excitation classes is reported, which underlines the importance of ionic contributions as well as intramolecular correlation for hydrogen-bonded clusters. The results of this analysis are compared with previous data obtained by symmetry-adapted perturbation theory (SAPT).

Bifurcated Hydrogen Bonds: Three-Centered Interactions

Abstract: The nature of bifurcated or three-centered hydrogen bonds (HB) has been investigated. Different families of compounds were chosen: monomers with intramolecular three-centered HB, dimers with a HB donor (HBD) and a molecule with two HB acceptor (HBA) groups, and trimers with one HBD and two HBAs. All the systems were optimized at the B3LYP/6-31G* level, and, in the case of the complexes, the interaction energies were evaluated and corrected with the basis set superposition error (BSSE). The electronic nature of these three-centered HBs was analyzed by means of the atoms in molecules (AIM) approach. The present study indicates the existence of bifurcated bond paths in the AIM analysis with electron densities that can be classified as follows: (i) compounds with symmetric three-centered HBs presenting two symmetric bond critical points with equal values of electron density; (ii) compounds with asymmetric three-centered HBs presenting two bond critical points with different values of electron density; (iii)...

Hydration energies and entropies for mg2+, ca2+, sr2+, and ba2+ from gas-phase ion-water molecule equilibria determinations

Abstract: The sequential enthalpies , free energies , and entropies for the hydration reaction M(H2O)n-12+ + H2O = M(H2O)n2+ were determined in the gas phase for M = Mg, Ca, Sr, Ba. The gas-phase ion hydrate...

Cobalt Porphyrin Catalyzed Reduction of CO2. Radiation Chemical, Photochemical, and Electrochemical Studies

Abstract: Several cobalt porphyrins (CoP) have been reduced by radiation chemical, photochemical, and electrochemical methods, in aqueous and organic solvents. In aqueous solutions, the CoIP state is stable at high pH but is shorter lived in neutral and acidic solutions. Stable CoIP is also observed in organic solvents and is unreactive toward CO2. One-electron reduction of CoIP leads to formation of a species that is observed as a transient intermediate by pulse radiolysis in aqueous solutions and as a stable product following reduction by Na in tetrahydrofuran solutions. The spectrum of this species is not the characteristic spectrum of a metalloporphyrin π-radical anion and is ascribed to Co0P. This species binds and reduces CO2. Catalytic formation of CO and HCO2- is confirmed by photochemical experiments in acetonitrile solutions containing triethylamine as a reductive quencher. Catalytic reduction of CO2 is also confirmed by cyclic voltammetry in acetonitrile and butyronitrile solutions and is shown to occur ...