Spectroscopic Signatures of Resonance Inhibition Reveal Differences in Donor-Bridge and Bridge-Acceptor Couplings.

The torsional dependence of the ground state magnetic exchange coupling (J) and the corresponding electronic coupling matrix element (HDA) for eight transition metal complexes possessing donor-acceptor (D-A) biradical ligands is presented. These biradical ligands are composed of an S = 1/2 metal semiquinone (SQ) donor and an S = 1/2 nitronylnitroxide (NN) acceptor, which are coupled to each other via para-phenylene, methyl-substituted para-phenylenes, or a bicyclo[2.2.2]octane ring. The observed trends in electronic absorption and resonance Raman spectral features are in accord with a reduction in electronic and magnetic coupling between D and A units within the framework of our valence bond configuration interaction model. Moreover, our spectroscopic results highlight different orbital mechanisms that modulate coupling in these complexes, which is not manifest in the ferromagnetic JSQ-B-NN values. The work provides new detailed insight into the effects of torsional rotations which contribute to inhomogeneities in experimentally determined exchange couplings, electron transfer rates, and electron transport conductance measurements.

Transferrable property relationships between magnetic exchange coupling and molecular conductance.

Calculated conductance through Au n -S-Bridge-S-Au n (Bridge = organic σ/π-system) constructs are compared to experimentally-determined magnetic exchange coupling parameters in a series of TpCum,MeZnSQ-Bridge-NN complexes, where TpCum,Me = hydro-tris(3-cumenyl-1-methylpyrazolyl)borate ancillary ligand, Zn = diamagnetic zinc(ii), SQ = semiquinone (S = 1/2), and NN = nitronylnitroxide radical (S = 1/2). We find that there is a nonlinear functional relationship between the biradical magnetic exchange coupling, J D→A, and the computed conductance, g mb. Although different bridge types (monomer vs. dimer) do not lie on the same J D→A vs. g mb, curve, there is a scale invariance between the monomeric and dimeric bridges which shows that the two data sets are related by a proportionate scaling of J D→A. For exchange and conductance mediated by a given bridge fragment, we find that the ratio of distance dependent decay constants for conductance (ß g) and magnetic exchange coupling (ß J) does not equal unity, indicating that inherent differences in the tunneling energy gaps, Δε, and the bridge-bridge electronic coupling, H BB, are not directly transferrable properties as they relate to exchange and conductance. The results of these observations are described in valence bond terms, with resonance structure contributions to the ground state bridge wavefunction being different for SQ-Bridge-NN and Au n -S-Bridge-S-Au n systems.

Magnetic circular dichroism and electronic structure of [Re2X4(PMe3)4]+ (X = Cl, Br).

Magnetic circular dichroism (MCD) and electronic absorption spectroscopies have been used to probe the electronic structure of the classical paramagnetic metal-metal-bonded complexes [Re2X4(PMe3)4](+) (X = Cl, Br). A violation of the MCD sum rule is observed that indicates the presence of ground-state contributions to the MCD intensity. The z-polarized δ → δ* band in the near-IR is formally forbidden in MCD but gains intensity through a combination of ground- and excited-state mechanisms to yield a positive C term.

(13)C and (63,65)Cu ENDOR studies of CO dehydrogenase from Oligotropha carboxidovorans. Experimental evidence in support of a copper-carbonyl intermediate.

We report here an ENDOR study of an S = 1/2 intermediate state trapped during reduction of the binuclear Mo/Cu enzyme CO dehydrogenase by CO. ENDOR spectra of this state confirm that the (63,65)Cu nuclei exhibits strong and almost entirely isotropic coupling to the unpaired electron, show that this coupling atypically has a positive sign, aiso = +148 MHz, and indicate an apparently undetectably small quadrupolar coupling. When the intermediate is generated using (13)CO, coupling to the (13)C is observed, with aiso = +17.3 MHz. A comparison with the couplings seen in related, structurally assigned Mo(V) species from xanthine oxidase, in conjunction with complementary computational studies, leads us to conclude that the intermediate contains a partially reduced Mo(V)/Cu(I) center with CO bound at the copper. Our results provide strong experimental support for a reaction mechanism that proceeds from a comparable complex of CO with fully oxidized Mo(VI)/Cu(I) enzyme.

Electronic and exchange coupling in a cross-conjugated D-B-A biradical: mechanistic implications for quantum interference effects.

A combination of variable-temperature EPR spectroscopy, electronic absorption spectroscopy, and magnetic susceptibility measurements have been performed on Tp(Cum,Me)Zn(SQ-m-Ph-NN) (1-meta) a donor-bridge-acceptor (D-B-A) biradical that possesses a cross-conjugated meta-phenylene (m-Ph) bridge and a spin singlet ground state. The experimental results have been interpreted in the context of detailed bonding and excited-state computations in order to understand the excited-state electronic structure of 1-meta. The results reveal important excited-state contributions to the ground-state singlet-triplet splitting in this cross-conjugated D-B-A biradical that contribute to our understanding of electronic coupling in cross-conjugated molecules and specifically to quantum interference effects. In contrast to the conjugated isomer, which is a D-B-A biradical possessing a para-phenylene bridge, admixture of a single low-lying singly excited D → A type configuration into the cross-conjugated D-B-A biradical ground state makes a negligible contribution to the ground-state magnetic exchange interaction. Instead, an excited state formed by a Ph-NN (HOMO) → Ph-NN (LUMO) one-electron promotion configurationally mixes into the ground state of the m-Ph bridged D-A biradical. This results in a double (dynamic) spin polarization mechanism as the dominant contributor to ground-state antiferromagnetic exchange coupling between the SQ and NN spins. Thus, the dominant exchange mechanism is one that activates the bridge moiety via the spin polarization of a doubly occupied orbital with phenylene bridge character. This mechanism is important, as it enhances the electronic and magnetic communication in cross-conjugated D-B-A molecules where, in the case of 1-meta, the magnetic exchange in the active electron approximation is expected to be J ~ 0 cm(-1). We hypothesize that similar superexchange mechanisms are common to all cross-conjugated D-B-A triads. Our results are compared to quantum interference effects on electron transfer/transport when cross-conjugated molecules are employed as the bridge or molecular wire component and suggest a mechanism by which electronic coupling (and therefore electron transfer/transport) can be modulated.

Spectroscopic studies of bridge contributions to electronic coupling in a donor-bridge-acceptor biradical system.

Variable-temperature electronic absorption and resonance Raman spectroscopies are used to probe the excited state electronic structure of Tp(Cum,Me)Zn(SQ-Ph-NN) (1), a donor-bridge-acceptor (D-B-A) biradical complex and a ground state analogue of the charge-separated excited state formed in photoinduced electron transfer reactions. Strong electronic coupling mediated by the p-phenylene bridge stabilizes the triplet ground state of this molecule. Detailed spectroscopic and bonding calculations elucidate key bridge distortions that are involved in the SQ(π)(SOMO) → NN-Ph (π*)(LUMO) D → A charge transfer (CT) transition. We show that the primary excited state distortion that accompanies this CT is along a vibrational coordinate best described as a symmetric Ph(8a) + SQ(in-plane) linear combination and underscores the dominant role of the phenylene bridge fragment acting as an electron acceptor in the D-B-A charge transfer state. Our results show the importance of the phenylene bridge in promoting (1) electron transfer in D-Ph-A systems and (2) electron transport in biased electrode devices that employ a 1,4-phenylene linkage. We have also developed a relationship between the spin density on the acceptor, as measured via the isotropic NN nitrogen hyperfine interaction, and the strength of the D → A interaction given by the magnitude of the electronic coupling matrix element, H(ab).

Hyperfine interaction, spin polarization, and spin delocalization as probes of donor-bridge-acceptor interactions in exchange-coupled biradicals.

Computations and EPR spectroscopy are used to probe the spin distribution of donor-bridge-acceptor (D-B-A) biradical complexes: Tp(Cum,Me)Zn(SQ-NN) (1), Tp(Cum,Me)Zn(SQ-1,4-Ph-NN) (2), Tp(Cum,Me)Zn(SQ-2,5-TP-NN) (3), and Tp(Cum,Me)Zn(SQ-2,5-Xyl-NN) (4) (SQ = orthosemiquinone and NN = nitronylnitroxide). These complexes are ground-state analogs of the charge-separated excited states formed in photoinduced electron transfer reactions. The intraligand magnetic exchange interaction (J) in these complexes is mediated by the bridges and has been found to stabilize the triplet ground states of 1 and 2. Detailed spectroscopic and bonding calculations have been used to elucidate the role of the bridge fragment (B) and its conformation relative to donor (SQ) and acceptor (NN) on spin density distributions. The computed results correlate well with experimental nitrogen hyperfine coupling constants.

Ferromagnetic nanoscale electron correlation promoted by organic spin-dependent delocalization.

We describe the electronic structure and the origin of ferromagnetic exchange coupling in two new metal complexes, NN-SQ-Co(III)(py)(2)Cat-NN (1) and NN-Ph-SQ-Co(III)(py)(2)Cat-Ph-NN (2) (NN = nitronylnitroxide radical, Ph = 1,4-phenylene, SQ = S = (1)/(2) semiquinone radical, Cat = S = 0 catecholate, and py = pyridine). Near-IR electronic absorption spectroscopy for 1 and 2 reveals a low-energy optical band that has been assigned as a Psi(u) --> Psi(g) transition involving bonding and antibonding linear combinations of delocalized dioxolene (SQ/Cat) valence frontier molecular orbitals. The ferromagnetic exchange interaction in 1 is so strong that only the high-spin quartet state (S(T) = (3)/(2)) is thermally populated at temperatures up to 300 K. The temperature-dependent magnetic susceptibility data for 2 reveals that an excited state spin doublet (S(T) = (1)/(2)) is populated at higher temperatures, indicating that the phenylene spacer modulates the magnitude of the magnetic exchange. The valence delocalization within the dioxolene dyad of 2 results in ferromagnetic alignment of two localized NN radicals separated by over 22 A. The ferromagnetic exchange in 1 and 2 results from a spin-dependent delocalization (double exchange type) process and the origin of this strong electron correlation has been understood in terms of a valence bond configuration interaction (VBCI) model. We show that ferromagnetic coupling promoted by organic mixed-valency provides keen insight into the ability of single molecules to communicate spin information over nanoscale distances. Furthermore, the strong interaction between the itinerant dioxolene electron and localized NN electron spins impacts our ability to understand the exchange interaction between delocalized electrons and pinned magnetic impurities in technologically important dilute magnetic semiconductor materials. The long correlation length (22 A) of the itinerant electron that mediates this coupling indicates that high-spin pi-delocalized organic molecules could find applications as nanoscale spin-polarized electron injectors and molecular wires.

Synthesis, characterization, and spectroscopy of model molybdopterin complexes.

The preparation and characterization of new model complexes for the molybdenum cofactor are reported. The new models are distinctive for the inclusion of pterin-substituted dithiolene chelates and have the formulation Tp(*)MoX(pterin-R-dithiolene) (Tp(*)=tris(3,5,-dimethylpyrazolyl)borate), X=O, S, R=aryl. Syntheses of Mo(4+) and (5+) complexes of two pterin-dithiolene derivatives as both oxo and sulfido compounds, and improved syntheses for pterinyl alkynes and [Et(4)N][Tp(*)Mo(IV)(S)S(4)] reagents are described. Characterization methods include electrospray ionization mass spectrometry, electrochemistry, infrared spectroscopy, electron paramagnetic resonance and magnetic circular dichroism. Cyclic voltammetry reveals that the Mo(5+/4+) reduction potential is intermediate between that for dithiolenes with electron-withdrawing substituents and simple dithiolates chelates. Electron paramagnetic resonance and magnetic circular dichroism of Mo(5+) complexes where X=O, R=aryl indicates that the molybdenum environment in the new models is electronically similar to that in Tp(*)MoO(benzenedithiolate).

Closing of the fingers domain generates motor forces in the HIV reverse transcriptase.

Using the force sensor of an atomic force microscope, motor forces of the human immunodeficiency virus-1 reverse transcriptase were measured during active replication of a short DNA transcript. At low load forces the polymerase is mechanically slowed, whereas at high force (approximately 15 piconewton) it stalls. From recordings of estimated polymerase turnover velocity versus load force, an approximate force-velocity curve has been constructed. The shape of the curve suggests that load force strongly inhibits the rate-limiting step of the polymerase turnover cycle and that the combined effect of load on all steps involves an effective motion of about 1.6 nm. Earlier results from pre-steady-state kinetics experiments have identified the rate-limiting step as the closing of the fingers domain to form a tight catalytic complex. Together these findings indicate that the closing of the fingers domain is a major force-generating step for human immunodeficiency virus reverse transcriptase and, by extension, for all DNA polymerase machines.