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Localization and delocalization of electrons is a key concept in chemistry, and is one of the important factors determining the efficiency of electron transport through organic conjugated molecules, which have potential to act as "molecular wires". This, in turn, substantially influences the efficiencies of organic solar cells and other molecular electronic devices. It is also necessary to understand the electronic energy landscape and the dynamics that govern electron transport capabilities in one-dimensional conjugated chains so that we can better define the design principles for conjugated molecules for their applications. We show that nitrile ν(C≡N) vibrations respond to the degree of electron localization in nitrile-substituted organic anions by utilizing time-resolved infrared detection combined with pulse radiolysis. Measurements of a series of aryl nitrile anions allow us to construct a semiempirical calibration curve between the changes in the ν(C≡N) infrared (IR) shifts and the changes in the electronic charges from the neutral to the anion states in the nitriles; more electron localization in the nitrile anion results in larger IR shifts. Furthermore, the IR line width in anions can report a structural change accompanying changes in the electronic density distribution. Probing the shift of the nitrile ν(C≡N) IR vibrational bands enables us to determine how the electron is localized in anions of nitrile-functionalized oligofluorenes, considered as organic mixed-valence compounds. We estimate the diabatic electron transfer distance, electronic coupling strengths, and energy barriers in these mixed-valence compounds. The analysis reveals a dynamic picture, showing that the electron is moving back and forth within the oligomers with a small activation energy of ≤kBT, likely controlled by the movement of dihedral angles between monomer units. Implications for the electron transport capability in molecular wires are discussed.

The paramagnetic [1:1] encounter complex ( TCNE ) 2 - is established as the important precursor in the kinetics and mechanism of electron-transfer for the self-exchange between tetracyanoethylene acceptor (TCNE) and its radical-anion as the donor. Spectroscopic observation of the dimeric complex ( TCNE ) 2 - by its intervalence absorption band at the solvent-dependent wavelength of λIV ∼ 1500 nm facilitates the application of Mulliken–Hush theory which reveals the significant electronic interaction extant between the pair of cofacial TCNE moieties with the sizable coupling of HDA = 1000 cm−1. The transient existence of such an encounter complex provides the critical link in the electron-transfer kinetics by lowering the classical Marcus reorganization barrier by the amount of HDA in this strongly adiabatic system. Ab initio quantum-mechanical methods as applied to independent theoretical computations of both the reorganization energy (λ) and the electronic coupling element (HDA) confirm the essential correctness of the Mulliken–Hush formalism for fast electron transfer via strongly coupled donor/acceptor encounter complexes.

The standard heterogeneous electron-transfer rate constants (k(n)0) between substrate gold electrodes and the ferrocene redox couple attached to the electrode surface by variable lengths of substituted or unsubstituted oligophenyleneethynylene (OPE) bridges as constituents of mixed self-assembled monolayers were measured as a function of temperature. The distance dependences of the unsubstituted OPE standard rate constants and of the preexponential factors (An) obtained from an Arrhenius analysis of the unsubstituted OPE k(n)0 versus temperature data are not monotonic. This surprising result, together with the distance dependence of the substituted OPE preexponential factors, may be assessed in terms of the likely conformational variability of the OPE bridges (as a result of the low intrinsic barrier to rotation of the phenylene rings in these bridges) and the associated sensitivity of the rate of electron transfer (and, hence, the single-molecule conductance which may be estimated using An) through these bridges to the conformation of the bridge. Additionally, the measured standard rate constants were independent of the identity of the diluent component of the mixed monolayer, and using an unsaturated OPE diluent has no effect on the rate of electron transfer through a long-chain alkanethiol bridge. These observations indicate that the diluent does not participate in the electron-transfer event.

Coupling between donor and acceptor orbitals for optically-induced intervalence electron transfer processes has been considered for a series of rigid mixed-valent dinuclear tris(2,2‘-bipyridine)iron complexes. Each of the four complexes considered contains three saturated bridges which link the two tris(2,2‘-bipyridine)iron moieties. The bridging linkages are −CH2CH2−, −CH2CH2CH2−, −CH2OCH2−, and −CH2SCH2−. Despite differences in the composition of the bridges X-ray diffraction and/or molecular dynamics calculations show that the metal−metal separation and relative bipyridine orientations among all four complexes are nearly identical. Consequently, the only factor which differs significantly among these complexes and which might affect the donor−acceptor coupling in the mixed-valent forms is their connectivity. These complexes thus provide a unique opportunity to focus on potential superexchange coupling in the absence of ambiguities introduced by other structural and energetic considerations. Theories de...

Both metal-to-ligand charge transfer (MLCT) and oxidation (ionization) processes have been investigated for (NH3)5RuL2+ complexes with L = pyridine, pyrazine, and protonated pyrazine. Calculations ...

The recently developed generalized Mulliken-Hush approach for the calculation of the electronic coupling matrix element for electron-transfer processes is applied to two rigidly linked donor-bridge-acceptor systems having dimethoxyanthracene as the donor and a dicarbomethoxycyclobutene unit as the acceptor. The dependence of the electronic coupling matrix element as a function of bridge type is examined with and without solvent molecules present. For clamp-shaped bridge structures solvent can have a dramatic effect on the electronic coupling matrix element. The behavior with variation of solvent is in good agreement with that observed experimentally for these systems. 23 refs., 2 tabs.

The kinetics of electron transfer between a substrate gold electrode and a self-assembled monolayer formed from CH{sub 3}(CH{sub 2}){sub n-1}SH and ({eta}{sup 5} C{sub 5}H{sub 5})Fe ({eta}{sup 5}-C{sub 5}H{sub 4})CO{sub 2}(CH{sub 2}){sub n}SH were studied as a function of n, the number of methylenes in the alkyl chain tethering the ferrocene moiety to the electrode, using the indirect laser-induced temperature jump method (ILIT). For 5 {

Using a corresponding orbital approach implemented computational with the INDO method of Zerner et al, the authors have evaluated and analyzed the electron-transfer matrix elements (H{sub if}) for the ferrocene/ferrocenium and cobaltocene/cobaltocenium (Cp{sub 2}M/(Cp{sub 2}M){sup +}) electron exchange processes. Taking due account of the relative energies of different orientations of the reactants, they obtain good agreement with estimates of H{sub if} inferred from experimental kinetic data, as well as a detailed interpretation of the coupling based on a fully specified effective one-electron model. This model reveals distinct electronic mechanisms for the M = Fe and M = Co cases, corresponding, respectively, to superexchange of the 'electron' and 'hole' type. Variationally defined effective donor and acceptor orbitals are shown to be essentially equivalent to appropriately selected canonical molecular orbitals of the Cp{sub 2}M/(Cp{sub 2}M){sup +} transition-state complex. Departures from a simple one-electron model affect the calculated H{sub if} values by less than 10% (i.e., the many-electron core overlap integral is {approx gt} 0.9) even though the wave functions properly reflect the appreciable degree of electronic relaxation which is known to accompany the oxidation of the Cp{sub 2}M molecules. {sigma} as well as {pi}-type orbitals of the Cp rings are shown tomore » play a significant role in facilitating the electronic coupling between reactants.« less

This work provides a detailed account of the application of our multichromophoric Förster resonance energy transfer (MC-FRET) theory (Phys. Rev. Lett. 2004, 92, 218301) for the calculation of the energy transfer rate from the B800 unit to the B850 unit in the light harvesting complex 2 (LH2) of purple bacteria. The model Hamiltonian consists of the B800 unit represented by a single bacteriochlorophyll (BChl), the B850 unit represented by its entire set of BChls, the electronic coupling between the two units, and the bath terms representing all environmental degrees of freedom. The model parameters are determined, independent of the rate calculation, from the literature data and by a fitting to an ensemble line shape. Comparing our theoretical rate and a low-temperature experimental rate, we estimate the magnitude of the BChl-Qy transition dipole to be in the range of 6.5-7.5 D, assuming that the optical dielectric constant of the medium is in the range of 1.5-2. We examine how the bias of the average excitation energy of the B800-BChl relative to that of the B850-BChl affects the energy transfer time by calculating the transfer rates based on both our MC-FRET theory and the original FRET theory, varying the value of the bias. Within our model, we find that the value of bias 260 cm-1, which we determine from the fitting to an ensemble line shape, is very close to the value at which the ratio between MC-FRET and FRET rates is a maximum. This provides evidence that the bacterial system utilizes the quantum mechanical coherence among the multiple chromophores within the B850 in a constructive way so as to achieve efficient energy transfer from B800 to B850.

The standard heterogeneous electron-transfer rate constants between substrate gold electrodes and either ferrocene or pentaaminepyridine ruthenium redox couples attached to the electrode surface by various lengths of an alkanethiol bridge as a constituent of a mixed self-assembled monolayer were measured as a function of temperature. The ferrocene was either directly attached to the alkanethiol bridge or attached through an ester (CO(2)) linkage. For long bridge lengths (containing more than 11 methylene groups) the rate constants were measured using either chronoamperometry or cyclic voltammetry; for the shorter bridges, the indirect laser induced temperature jump technique was employed to measure the rate constants. Analysis of the distance (bridge length) dependence of the preexponential factors obtained from an Arrhenius analysis of the rate constant versus temperature data demonstrates a clear limiting behavior at a surprisingly small value of this preexponential factor (much lower than would be expected on the basis of aqueous solvent dynamics). This limit is independent of both the identity of the redox couple and the nature of the linkage of the couple to the bridge, and it is definitely different (smaller) from the limit derived from an equivalent analysis of the rate constant (versus temperature) data for the interfacial electron-transfer reaction through oligophenylenevinylene bridges between gold electrodes and ferrocene. There are a number of possible explanations for this behavior including, for example, the possible effects of bridge conformational flexibility upon the electron-transfer kinetics. Nevertheless, conventional ideas regarding electronic coupling through alkane bridges and solvent dynamics are insufficient to explain the results reported here.

The recent investigation of the apparently anomalous attenuation factor (beta1.5 A(-1)) for photoinduced hole injection into DNA duplexes modified by protonated 9-amino-6-chloro-2-methoxyacridine (X+) led to the conclusion that in addition to the electronic couplings, the activation energy must also be distance-dependent. In this communication we report the verification of this postulate by direct measurements of the activation energies for a series of (X+)-modified DNA duplexes which sample an appreciable range of donor-acceptor distances (approximately 4-11 A). The resulting changes in thermal activation energy can be explained within the framework of a distance-dependent reorganization energy.

Equilibrium O…O separations ( r e ) and bond energies ( D e ) are calculated for the water dimer, with electron correlation included at the Moller—Plesset (MP2 or MP3) level. Inclusion of corrections for basis set superposition error allows the experimental data for r e and D e to be bracketed. The r e for the dimer is significantly larger than the mean O…O distance in liquid water.

Calculations using effective potentials demonstrate sensitivity of the SiOSi bond angle to basis set. The important polarization functions for obtaining reasonable bond angles are oxygen d functions have greater population. No significant angular variation of the pπ-dπ bonding is obtained; rather, coulombic and steric repulsions open the SiOSi angle.

The relatively large differences in D e and R e obtained at the STO-3G level for hydrogen bonded dimers of water, methanol and dimethyl ether, and which correlate monotonically with atomic charges, are strongly attenuated with extended basis sets, and relationships among D e , R e , and electron populations are reversed. Ratios of D e R e products for pairs of proton donors (acceptors) with a common acceptor (donor), however, are constant for a given basis set.

Single crystal neutron diffraction studies are reported for cubic dinitrogen tetroxide (space group Im3; Z = 6). The crystals were grown from NO2 vapor in situ on the diffractometer by precise cryostatic control. The lattice parameter, measured at seven temperatures, increases from 7.6937(6) A at 20 K to 7.7925(6) A at 140 K. The nuclear positional and thermal parameters were refined using diffraction data (sin ϑ/λ⩽0.79 A−1) measured at 20, 60, and 100 K. Final fit indices R(F2) are 0.028, 0.034, 0.037, respectively. The observed N–N bond length and O–N–O angle are invariant between 20 and 100 K at values 1.7562(±4) A and 134.46(±6)° with individual e.s.d.’s of 0.001 A and 0.1°. The observed N–O bond length increases linearly from 1.1855(9) A at 100 K to 1.1893(5) A at 20 K; the extrapolated zero‐point value is 1.191 A. Ab initio self‐consistent field calculations using a two‐configuration wave function which allows partial occupation of the σ* MO (antibonding with respect to the two nitrogens) yields a r...

Ab initio SCF and CI calculations are reported for the sulfur nitride, SN, and disulfur dinitride, S2N2, molecules. Calculations on SN, varying the internuclear distance, are performed using a variety of basis sets to determine a suitable basis for S2N2. The total energy, internuclear distance, and force constant of SN are reported for several choices of basis. Extended basis computations on S2N2 are performed, and midbond polarization functions are found to contribute significantly to a description of the ground state, affecting the higher MO’s and the charge density. A prominent feature of the virtual spectrum is the existence of a low energy π* orbital which could play a role in low lying excited states, the polymerization process, and polymer conductivity. CI studies of the ground and excited states of S2N2 show several low lying singlet and triplet states of the type π→σ*, π→π*, and σ→π*, and possible identifications with experiment are discussed. A possibility of relating the observed paramagnetism ...

The MNDO approximation is employed to compute normal modes of vibration for the proposed C/sub 60/ isomer known as buckminsterullerene. Group theoretical invariance theorems are derived to aid in the interpretation of the normal modes. One particularly interesting mode (the sole A/sub u/ vibration) consists of a rotary oscillation of the pentagonal rings of C/sub 60/, with all rings rotating in the same direction.

Ab initio configuration interaction calculations have been carried out for important regions on the potential energy surfaces of the C + H2 system. The implications of the results for the reactivity of both thermal and energetic 3P, 1D, and 1S carbon atoms are discussed. Preliminary study does not reveal any low-energy pathways for the C(3P) + H2 reaction.

Ab initio molecular‐orbital wavefunctions for ethylene, butadiene, and benzene have been analyzed in terms of the localized orbitals (LMO's) defined by minimizing molecular exchange energy. The effect of conjugation on the LMO's is studied in terms of the delocalization of LMO's and the curvature of the energy surface which determines the LMO's. The degree of preference for bent double‐bond LMO's over σ–π double‐bond LMO's is also examined. In all cases, the former LMO's represent the correct solutions, while the latter LMO's, obtained by imposing the constraint of σ–π separation, correspond to saddle‐point solutions. The double‐bond LMO's for ethylene and butadiene are quite similar, whereas in the case of benzene, appreciably greater delocalization is found, and the preference for bent bonds over σ–π bonds is greatly reduced.

Molecular SCF orbitals of B2H6 have been computed from optimized minimum basis sets which employ isotropic or anisotropic atomic 2p orbitals. These SCF wavefunctions have been transformed to localized MO's which maximize the self‐energy, D = Σi(φiφi | φiφi). This objective procedure strongly supports the three‐center bond for each BHB bridge. The resulting hybrids are sp2.5, with ∠Ht–B–Ht=125° and ∠Hb–B–Hb=93° for terminal and bridge H's, respectively.

Ab initio calculations at the SCF and Cl level have been carried out for the singlet ground state of ortho-benzyne (1,2,-dehydrobenzene) at a variety of C 2v molecular geometries. The principal features of the equilibrium geometry are: (1) an “acetylenic” C 1 C 2 bond (1.22 A): (2) a C 4 C 5 bond slightly elongated (1.42 A) with respect to benzene; (3) no elongation of the C 2 C 3 and C 1 C 6 bonds, due to the high s-character and angular deviation of the hybrid orbitals. Extended basis SCF calculations lead to an estimate of ≈ 120 kcal/mole for the Δ H 298° f of o -benzyne.

Under appropriate collisional conditions the mass spectrum of carbon fragments produced by laser vaporization of graphite is dominated by C/sub 60/ and (to a lesser extent) C/sub 70/ clusters. The discoverers of this phenomenon have noted that the carbon valence requirements can be satisfied in closed, hollow structures. For C/sub 60/ they suggest an icosahedral soccer ball network, which they call buckminsterfullerene and we abbreviate as BF. Experimental support has come from studies with lanthanum-impregnated graphite. The resulting mass spectra show intense C/sub 60/La peaks, but no C/sub n/La/sub 2/ or C/sub n/La/sub 3/ peaks. Subsequent experiments have demonstrated the inertness of C/sub 60/ and, indeed, other large C/sub 2n/ clusters under NO attack. We report here the results of quantum calculations which were prompted by the experiments cited above and other earlier work. Our purpose has been to test the intrinsic stability of BF and related polyhedral species and to compare their stability with that of planar graphite fragments. The latter have the advantage of being strain free, but suffer from dangling valences on their perimeters. We also make comparisons with linear carbon chains.