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The relationship between the electronic-nuclear level structure, the electronic couplings, and the dynamics of hole hopping transport in DNA is explored. The exploration of the range of applicability and the breakdown of the superexchange mechanism in DNA lay the foundations for the scrutiny of the universality.

In this paper we present a theoretical study of the predissociation dynamics of the nf(N+=2) (with the principal quantum numbers n=40–95) and the np(N+=0) (n=70–125) Rydberg series of NO, which exhibit a marked lifetime dilution (lengthening) at n>=65 for the f series and at n>=116 for the p series [M.J.J. Vrakking and Y. T. Lee, J. Chem. Phys. 102, 8818 (1995)]. The multichannel effective Hamiltonian with several doorway (for excitation) and escape (for decay) states was constructed using experimental information on the quantum defects and on the decay width constants incorporating both intramolecular coupling and exterior electric field coupling between high Rydbergs. The analysis of the intramolecular Rydberg electron–core dipole long range coupling (HR-D) in conjunction with the energy gaps between proximal pairs of energy levels, which are subjected to appropriate selection rules, reveals that (i) for low l(≤3) core-penetrating Rydbergs only a small number of accidental near-resonances are exhibited, and (ii) for high l(>=3) nonpenetrating Rydbergs the electron-core dipole coupling decreases fast with increasing l, i.e., (HR-D)∝l-7. The general characteristics of the high l(>=3) manifold establish a bottleneck effect, which precludes intramolecular l mixing, implying that high Rydberg lifetime dilution effects can be induced only by exterior electric field coupling (HSTARK). Parameter-free multichannel effective Hamiltonian calculations were conducted under narrow-band excitation conditions, which interrogate the electric field induced mixing in the energetic vicinity of the doorway state. The electric field induced l mixing model accounts semiquantitatively for the electric field dependence of the energy-resolved line shapes of the nf(N+=2) series and for the n and electric field dependence of the lifetimes of the nf(N+=2) and the np(N+=0) series. Accidental near-resonant simultaneous intramolecular and electric field... [ABSTRACT FROM AUTHOR]

In this paper we present a theoretical study of the autoionization dynamics of high 2P1/2np′[3/2]1 Rydbergs (with the principal quantum numbers n=100–280) of Ar in weak homogeneous electric fields (F=0.01–1.0 V/cm), which were experimentally interrogated by time-resolved zero-electron kinetic energy (ZEKE) spectroscopy [M. Mühlpfordt and U. Even, J. Chem. Phys. 103, 4427 (1995)], and which exhibit a marked dilution (i.e., ∼2 orders of magnitude lengthening) of the lifetimes relative to those inferred on the basis of the n3 scaling law for the spectral linewidths of the np′ (n=12–24) Rydbergs. The multichannel effective Hamiltonian (Heff) with several doorway state(s) (for excitation and decay) and pure escape states (for decay) was advanced and utilized to treat the dynamics of the mixed Stark manifold of the ZEKE Rydbergs. Heff of dimension 2n-1 is then constructed for a n Rydberg manifold using independent experimental information on the (l dependent) quantum defects δ(l) and the (l, K, J dependent) decay widths, which are of the form Γ0(lKJ)/(n-δ(l))3, with Γ0(lKJ) being the decay widths constants. Here, l, K, and J are the azimuthal, the electronic and the total electronic angular momentum quantum numbers, respectively. Two coupling ranges are distinguished according to the strength of the reduced electric field F(n,p′)=(F/V cm-1)n5/ 3.4×109[δ(p′)(mod1)].Range (A); The onset of the effective coupling of the doorway and escape states, i.e., 0.7≤F(n,p′)≤2. Range (B); The strong mixing domain F(n,p′)≥3. The lifetimes in range (B) can be well represented by a nearly democratic mixing of all the doorway and escape states (lKJ), with the average value <τ(n)>[bar_over_tilde:_approx._equal_to]<τSM(n)>= 2n4h/[J(lJK)Γ0(lJK)]. In range (B) <τ(n)> increases with increasing n and is only weakly F dependent... [ABSTRACT FROM AUTHOR]

In this paper we explore the level structure, the optical excitation modes and the dynamics of a mixed Stark manifold of very high Rydberg states (with principal quantum numbers n=80–250) of large molecules, e.g., 1,4 diaza bicyclo [2,2,2] octane (DABCO) and bis (benzene) chromium (BBC) [U. Even, R. D. Levine, and R. Bersohn, J. Phys. Chem. 98, 3472 (1994)] and of autoionizing Rydbergs of atoms [F. Merkt, J. Chem. Phys. 100, 2623 (1994)], interrogated by time-resolved zero-electron kinetic energy (ZEKE) spectroscopy. We pursue the formal analogy between the level structure, accessibility and decay of very high Rydbergs in an external weak (F[bar_over_tilde:_approx._equal_to]0.1–1 V cm-1) electric field and intramolecular (interstate and intrastate) relaxation in a bound molecular level structure. The onset n=nM of the strong mixing (in an external field F and in the field exerted by static ions) of a doorway state, which is characterized by a low azimuthal quantum number l, a finite quantum defect δ, and a total nonradiative width Γs[bar_over_tilde:_approx._equal_to]Γ0/n3, with the inactive high l manifold is specified by nM[bar_over_tilde:_approx._equal_to]80.6δ1/5(F/V cm-1)-1/5. At n≥nM the level structure and dynamics are characterized by the product γρ, where ρ is the density of states and γ=ΓsD(n) is the average decay width of the eigenstates, with the dilution factor D(n)≊n-2 for (lml) mixing and D(n)[bar_over_tilde:_approx._equal_to]n-1 for (l) mixing, whereupon γρ=(Γ0/4δR)(nM/n)5, being independent of D(n).The sparse level structure is realized for γρ<1, while the dense level structure prevails for γρ>=1, resulting in two limiting situations; (a) a dense limit for n≥nM and a sparse limit for n>nM, and (b) a sparse limit for all n≥nM. The experimental... [ABSTRACT FROM AUTHOR]

In this paper we explore the interrelationship between the energetics and the thermodynamic properties of molecular clusters. We advance simple models for the energy spectrum, which are used to derive analytical results for the thermodynamic properties of these clusters. The energy spectrum is characterized by the distribution of the energies of the local minima of the nuclear potential energy hypersurface, i.e., the inherent structures. On each of these energy levels the vibrational density of states of the particular inherent structure is superimposed. The energy spectra were specified in terms of the energy gap, Δ, between the (single) ground state and the excited-state inherent structures, the number, R, of the inherent structures and their energetic spread W. Four classes of energy spectra were considered. (1) A large energy gap with nearly degenerate excited-state manifold, i.e., Δ>W. (2) A large energy gap with a considerable spread of the excited-state manifold, i.e., Δ≪W and W/R≪Δ. (3) A gapless spectrum with W/R>=Δ. (4) A multiple bunched spectrum with several energy gaps. Explicit analytical relations for the temperature dependence of the internal energy were derived for energy spectra of types (1), (2), and (3) both for the canonical and for the microcanonical ensembles. For energy spectra of types (1) and (2) the caloric curve exhibits a single inflection, which marks the ‘‘transition’’. A unified description of multistate isomerization with large R, which corresponds to rare-gas clusters, and of molecular isomerization with R[bar_over_tilde:_approx._equal_to]1, which prevails for alkali-halide clusters, was provided. For energy spectra of type (3) the transition disappears, while for energy spectra of type (4) hierarchical isomerization is exhibited. Our analytical models have established the ensemble dependence of the transition for types (1) and (2), which is manifested by... [ABSTRACT FROM AUTHOR]

Explores the energetic control of sequential and superexchange electron transfer (ET) mechanisms. Quantum-mechanical simulations and calculations for long-range ET in DBA systems; Separation of the donor and the acceptor by a bridge; ET dynamics in a Franck-Condon (FC) system; Multi-dimensional displaced harmonic potential surfaces; Superposition of superexchange and sequential mechanisms.

Explores the temporal vibrational coherence effects in nonadiabatic radiationless transitions between two electronic states in a large molecule or in the condensed phase. Role of the vibrational quasicontinuum of the final states; Treatment of the final evolution of the wave packet of states.

Electronic matrix elements for hole transfer between adjacent nucleobases in DNA are calculated. The calculated intrastrand hole transfer matrix elements between adjacent thymines, v+(T, T)=0.16 eV, is seen to be in line with the results of the experiments.

We address a central question in the realm of the dynamics of high-n ( = 40- 250) Rydberg states of diatomics and large molecules. What is the coupling responsible for the 'global' l mixing, which results in the breakdown of the n8 scaling law for the non-radiative lifetimes and for the lifetime lengthening (by two to four orders of magnitude) of these states? To explore the implications of intramolecular interactions on l mixing and on electronic-rotational energy exchange we analysed the intramolecular couplings of the ion core dipole, quadrupole and (anisotropic) polarizability with a non-penetrating (l 3) Rydberg electron, in conjunction with the energy gaps between proximal pairs of energy levels. Calculations of the energy gaps and the couplings were performed for the high-n non-penetrating Rydberg states of NO and for model 'light' (B = 19 cm-1) and 'heavy' (B = 0.05 cm-1) polar molecules. All the intramolecular interactions are of the form of a power law proportional to l-eta, with eta being determined by the nature of the long-range coupling, by the l dependence of the quantum defects for multipole couplings and by the l dependence of angular integrals. We established a bottleneck effect for the intramolecular couplings between non-penetrating (l 3) states. For n- and N+-changing dipole, quadrupole and polarizability interactions the energetics ofthe proximalpairs of levels,inconjunctionwiththe bottleneck effect,prohibit the n, l, N+, N> - n', l', N+', N>, l ( 3) couplings (with n n) and the electronic-rotational energy exchange. For n- and N+-conserving quadrupole and polarizability interactions, the l ( 3) mixing (which prevails only for l N++ N) is also prohibited by the bottleneck effect. 'Global' intramolecular l mixing (with both n n' and n = n') in diatomics and in large molecules is precluded, implying that the dramatic lengthening of the non-radiative lifetimes of high-n Rydberg states can be induced only by exterior electric field coupling. [ABSTRACT FROM AUTHOR]

Demonstrates that the solvent dependence of the radiative recombination rates in donor-bridge-acceptor molecules can be quantitatively accounted for in terms of the intensity borrowing from locally excited states. Energy gap dependence of the radiative lifetimes; Oscillator strength for charge-transfer excitation; Electronic coupling between local excitation and the charge transfer state.

Electronic matrix elements for hole transfer between Watson-Crick pairs in desoxyribonucleic acid (DNA) of regular structure, calculated at the Hartree-Fock level, are compared with the corresponding intrastrand and interstrand matrix elements estimated for models comprised of just two nucleobases. The hole transfer matrix element of the GAG trimer duplex is calculated to be larger than that of the GTG duplex. "Through-space" interaction between two guanines in the trimer duplexes is comparable with the coupling through an intervening Watson-Crick pair. The gross features of bridge specificity and directional asymmetry of the electronic matrix elements for hole transfer between purine nucleobases in superstructures of dimer and trimer duplexes have been discussed on the basis of the quantum chemical calculations. These results have also been analyzed with a semiempirical superexchange model for the electronic coupling in DNA duplexes of donor (nuclobases)-acceptor, which incorporates adjacent base-base electronic couplings and empirical energy gaps corrected for solvation effects; this perturbation-theory-based model interpretation allows a theoretical evaluation of experimental observables, i.e., the absolute values of donor-acceptor electronic couplings, their distance dependence, and the reduction factors for the intrastrand hole hopping or trapping rates upon increasing the size of the nucleobases bridge. The quantum chemical results point towards some limitations of the perturbation-theory-based modeling. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this contribution we advance and explore the thermally induced hopping (TIH) mechanism for long-range charge transport (CT) in DNA and in large-scale chemical systems. TIH occurs in donor-bridge-acceptor systems, which are characterized by off-resonance donor-bridge interactions (energy gap DeltaE > 0), involving thermally activated donor-bridge charge injection followed by intrabridge charge hopping. We observe a "transition" from superexchange to TIH with increasing the bridge length (i.e., the number N of the bridge constituents), which is manifested by crossing from the exponential N-dependent donor-acceptor CT rate at low N (< N(X)) to a weakly (algebraic) N-dependent CT rate at high N (>N(X)). The "critical" bridge size N(X) is determined by the energy gap, the nearest-neighbor electronic couplings, and the temperature. Experimental evidence for the TIH mechanism was inferred from our analysis of the chemical yields for the distal/proximal guanine (G) triplets in the (GGG)(+)TTXTT(GGG) duplex (X = G, azadine (zA), and adenine (A)) studied by Nakatani, Dohno and Saito [J. Am. Chem. Soc. 2000, 122, 5893]. The TIH sequential model, which involves hole hopping between (GGG) and X, is analyzed in terms of a sequential process in conjunction with parallel reactions of (GGG)(+) with water, and provides a scale of (free) energy gaps (relative to (GGG)(+)) of Delta = 0.21-0.24 eV for X = A, Delta = 0.10-0.14 eV for X = zA, and Delta = 0.05-0.10 eV for X = G. We further investigated the chemical yields for long-range TIH in (G)l(+)Xn(G)l (l = 1-3) duplexes, establishing the energetic constraints (i.e., the donor - bridge base (X) energy gap Delta), the bridge structural constraints (i.e., the intrabridge X-X hopping rates k(m)), and the kinetic constraints (i.e., the rate k(d) for the reaction of with water). Effective TIH is expected to prevail for Delta less than or approximately equal to 0.20 eV with a "fast" water reaction (k(d)/k(m) approximately 10(-3)) and for Delta < 0.30 eV with a "slow" water reaction (k(d)/k(m) approximately 10(-5)). We conclude that (T)n bridges (for which Delta approximately equals 0.6 eV) cannot act in TIH of holes. From an analysis based on the energetics of the electronic coupling matrix elements in G(+)(T-A)n(GGG) duplexes we conclude that the superexchange mechanism is expected to dominate for n = 1-4. For long (A)n bridges (n > or approximately equal to 4) the TIH prevails, provided that the water side reaction is slow, raising the issue of chemical control of TIH through long (A)n bridges in DNA attained by changing the solution composition.

We explore charge migration in DNA, advancing two distinct mechanisms of charge separation in a donor (d)-bridge ([Bj])-acceptor (a) system, where [Bj] = B1,B2, . , BN are the N-specific adjacent bases of B-DNA: (i) two-center unistep superexchange induced charge transfer, d*[Bj]a --> d[Bj]a+/-, and (ii) multistep charge transport involves charge injection from d* (or d+) to [Bj], charge hopping within [Bj], and charge trapping by a. For off-resonance coupling, mechanism i prevails with the charge separation rate and yield exhibiting an exponential dependence approximately exp(-betaR) on the d-a distance (R). Resonance coupling results in mechanism ii with the charge separation lifetime tau approximately Neta and yield Y approximately (1 + Neta)-1 exhibiting a weak (algebraic) N and distance dependence. The power parameter eta is determined by charge hopping random walk. Energetic control of the charge migration mechanism is exerted by the energetics of the ion pair state dB1+/-B2 . BNa relative to the electronically excited donor doorway state d*B1B2 . BNa. The realization of charge separation via superexchange or hopping is determined by the base sequence within the bridge. Our energetic-dynamic relations, in conjunction with the energetic data for d*/d- and for B/B+, determine the realization of the two distinct mechanisms in different hole donor systems, establishing the conditions for "chemistry at a distance" after charge transport in DNA. The energetic control of the charge migration mechanisms attained by the sequence specificity of the bridge is universal for large molecular-scale systems, for proteins, and for DNA.

In this paper we propose that the reduction of the bacteriochlorophyl dimer cation (P(+)) by cytochrome c in the photosynthetic bacteria Rps. viridis and Chromatium vinosum proceeds via two parallel electron transfer (ET) processes from two distinct cytochrome c molecules. The dominating ET process at high temperatures involves the activated oxidation of the high-potential cytochrome c at closest proximity to P, while the dominating low-temperature process involves activationless ET from a low-potential cytochrome c, which is further away from P. The available data for the effects of blocking the low-potential cytochrome c on ET dynamics are consistent with this model, which results in reasonable nuclear reorganization and electronic coupling parameters for the parallel cytochrome oxidation processes. The lack of universality in the cytochrome oxidation in reaction centres of various bacteria is emphasized.