N.S. Hush

Ionization potentials and donor properties of nucleic acid bases and related compounds

Abstract The first vertical ionization potentials of guanine, adenine, hypoxanthine, xanthine, cytosine, thymine, uracil and purine have been determined by HeI photoelectron spectroscopy. The potentials increase in the above order and are assigned to ionization from the highest π level. The experimental results are compared with valence shell SCF calculations, and the correlation between the association constants of these molecules with riboflavin and their donor properties is discussed. Detailed spectra will be presented and discussed in a forthcoming paper.

Distance dependence of photoinduced electron transfer through non-conjugated bridges

Abstract Picosecond time-resolved emission studies of a series of molecules containing an electron donor-acceptor pair interconnected by a series of rigid non-conjugated bridges reveal the occurrence of very fast photoinduced intramolecular electron transfer. The length of the bridge was varied to provide donor-acceptor centre-to-centre separations ranging from 8.1 to 13.3 A (edge-to-edge 5 to 10.2 A). At centre-to-centre separations up to 10.7 A the rate of photoinduced electron transfer exceeded 5×10 10 s −1 (τ 10 s −1 (τ = 68 ps).

Finite field method calculations. VI. Raman scatering activities, infrared absorption intensities and higher-order moments: SCF and CI calculations for the isotopic derivatives of H2O and SCF calculations for CH4

Abstract Finite-field perturbation methods are applied in calculating polarisabilities and polarisability gradients at the CI level for H 2 O, D 2 O, T 2 O, HDO, HTO and DTO. Dipole moments and dipole moment gradients are also calculated. The resulting Raman scattering activities and infrared absorption intensities are derived and compared with the SCF results in part V along with other theoretical and experimental values. Higher-order moments and polarisability tensor components are calculated for H 2 O at the SCF and CI levels and for CH 4 at the SCF level. The calculated values are compared with the results of other calculations as well as the currently available experimental values.

Finite-field method calculations. IV. Higher-order moments, dipole moment gradients, polarisability gradients and field-induced shifts in molecular properties: Application to N2, CO, CN−, HCN and HNC

Abstract In this paper, theoretical methods developed in III are applied in calculating polarisabilities, polarisability gradients and field-induced shifts, by the finite-field method. Values of dipole moment gradients and higher-order moments, calculated from the unperturbed wavefunctions, are also reported. Results for N 2 , CO, CN − , HCN and HNC have been obtained at the SCF level; some CI results for the N 2 polarisability components and moments and for the dipole moment gradients of HCN are also given. The calculated polarisability gradients and dipole moment gradients have been used to estimate the Raman scattering intensities and depolarisation ratios and the IR absorption intensities. Model calculations of field-induced shifts in bond length, vibrational levels, spectroscopic constants, force constants and dipole moment gradient are reported for N 2 and CO. The discrepancy between the SCF and experimental bond dipole moment gradients for HCN, previously noted in the literature, has been re-examined and resolved by our CI results.

Inequivalent XPS binding energies in symmetrical delocalized mixed-valence complexes

Abstract It has previously been assumed that an ESCA measurement on a mixed-valence-e.g., M(II)-M(III) - compound which yields two peaks for an inner-shell M ionization at energies close to those measured for separate M(II) and M(III) ions provides direct evidence that the complex has an unsymmetrical (trapped-valence) ground state rather than one in which the electrons are symmetrically delocalized. This assumption is incorrect. A complex which has a symmetrical ground state will have two accessible unsymmetrical photoionized states owing to electron relaxation in the strong field of the core hole. For a range of values of binding energy differences and electron coupling parameters, the photoionized states will be very nearly localized and the peak separation for a complex with delocalized ground state will be close to that for isolated M(II) and M(III) ions. The appearance of two M binding energies is thus not in itself evidence for electronic ground-state asymmetry in a mixed-valence compound. A model is proposed from which quantitative predictions are made.

Electron transfer and energy transfer through bridged systems. I. Formalism

Abstract A time-dependent formalism is developed for reactions in which energy (vibrational or electronic excitation, electron or hole transfer, etc.) is transferred coherently between centres through a bridge. This approach is inspired by the Robinson and Frosch model of energy transfer within two-level systems. This formalism yields a completely general algorithm which, in particular limits, reduces to a generalised form of both Fermis golden rule and Rabis rate equation, and, in so doing, unifies many existing theories. It is shown that, only in the limit of the bridge states being non-resonant with the initial and final states, can the full problem be represented by an effective two-level model. Existing methods based upon Lowdin diagonalization are shown to be appropriate only when this limit applies, and ambiguities which arise from the ad hoc nature of these methods are resolved. Also, it is typically only in this limit that the transfer of energy proceeds exponentially in time and can be described by a simple single-parameter rate constant. Only problems which can be modelled using a single set of quantum numbers are treated in this paper. Applications and more general problems are treated in subsequent papers.

The coupled-pair approximation in a basis of independent-pair natural orbitals

Abstract A computational method based on a rapidly convergent form of the unlinked cluster expansion is presented. Ciźeks coupled-pair approximation (CPA) is derived in a basis of partially non-orthogonal orbitals which transform each pair function to diagonal form; this produces a simple (non-variational) set of equations from which may be extracted the energy and coefficients of a wavefunction constructed from the Hartree-Fock function, all double excitations and all unlinked clusters of these. The relationship of the CPA to simpler treatments is developed using the results of Hurley, and numerical results of a simple illustrative study of the BH 3 molecule are given.

Unlinked cluster effects in molecular electronic structure. I. The HCN and HNC molecules

Extensive calculations on the molecules HCN and HNC have been performed using our recently proposed ’’coupled‐pair approximation’’ (CPA) in a basis of nonorthogonal independent‐pair natural orbitals. A number of linear geometries are used for both systems, allowing prediction of equilibrium geometry, rotational constants and force constants for the stretching vibrational modes. The CPA values are in substantially better agreement with experimental results (where available) than those obtained from variational CI calculations including all double excitations, and can be generated with little extra computational effort. In addition, several approximate coupled‐pair techniques, which require no more effort than a CI calculation, are investigated in order to estimate their accuracy relative to the full coupled‐pair method. Using the bond‐stretching potentials, we have calculated vibrational energy levels and transition energies. Again, the values obtained by the CPA method are in better agreement with experim...

Finite field method calculations. V. Raman scattering activities and infrared absorption intensities for H2O, D2O, CH4 and CD4

Abstract Finite-field perturbation methods are applied in calculating polarisabilities, polarisability gradients, dipole moments and dipole moment gradients at the SCF level for H 2 O, D 2 O, CH 4 and CD 4 . The corresponding Raman scattering activities and infrared absorption intensities are calculated and compared with experimental and other theoretical determinations.

The effects of couplings to symmetric and antisymmetric modes and minor asymmetry on the spectral properties of mixed-valence and related charge-transfer systems

Abstract The most common methods used to describe the energy levels of charge-transfer systems (including mixed-valence systems) are the linear response approach of Rice and co-workers and the essentially equivalent PKS model described initially by Piepho, Krausz, and Schatz. While these methods were quite successful, in their original form they omitted the effects of overall symmetric vibrations. As a consequence, in particular they were not capable of adequately describing the electronic band width in the strong-coupling limit: Hush and later Ondrechen et al. demonstrated that symmetric modes are essential in this case, and modern versions of these models now include them. Here, we explore the relationship between symmetric and antisymmetric modes, concentrating on how this is modified by the presence of weak (e.g., environmentally or substitutionally induced) asymmetry. For the symmetric case, we show that when the electronic Hamiltonian operators are transformed from their usual localized diabatic representation into a delocalized diabatic representation, the effects of the symmetric and antisymmetric modes are interchanged. The primary effect of weak asymmetry is to mix the properties of the various modes, and possible consequences of this for the spectroscopy of bacterial photosynthetic reaction centre and substituted Creutz—Taube cations are discussed. We also consider the problem from an adiabatic Bom—Oppenheimer perspective and examine the regions in which this approach is appropriate.