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We measured the temperature dependence (from +32 to -50 °C) of charge-recombination rates between contact radical ion pairs in isopropyl ether. In the systems selected for this study, aromatic hydrocarbon cations are the electron acceptors and the fumaronitrile anion is the electron donor. Nearly quantitative electron transfers occur at all temperatures. The charge recombinations have excess exothermicities of -60 kcal mol-1 and exhibit a very weak temperature dependence. Our observations emphasize the absence of solvent effects and the relevance of nuclear tunneling in charge recombinations.

We calculate energy barriers of atom- and proton-transfer reactions in hydrogen-bonded complexes in the gas phase. Our calculations do not involve adjustable parameters and are based on bond-dissociation energies, ionization potentials, electron affinities, bond lengths, and vibration frequencies of the reactive bonds. The calculated barriers are in agreement with experimental data and high-level ab initio calculations. We relate the height of the barrier with the molecular properties of the reactants and complexes. The structure of complexes with strong hydrogen bonds approaches that of the transition state, and substantially reduces the barrier height. We calculate the hydrogen-abstraction rates in H-bonded systems using the transition-state theory with the semiclassical correction for tunneling, and show that they are in excellent agreement with the experimental data. H-bonding leads to an increase in tunneling corrections at room temperature.

Fluorescence from the second excited singlet state (S2) of 3-hydroxyflavone (3HF) has been observed for the first time in supercritical carbon dioxide (sc-CO2) environment. Steady-state experiments reveal that the intramolecular proton transfer is less effective from the S2 state of 3HF compared to that from the S1 state. http://www.sciencedirect.com/science/article/B6TFN-4BVP7G9-5/1/fe6fe6a663b47e1a622241a93ef55a87

Submitted by Isabel Ferreira (iferreira@ft.lisboa.ucp.pt) on 2016-04-06T15:27:25Z No. of bitstreams: 1 V0330102-637-648.pdf: 297127 bytes, checksum: 8e03777dce1b7bd030a0a3a305f90d94 (MD5) Approved for entry into archive by Isabel Nazareth (iqn@lisboa.ucp.pt) on 2016-04-06T15:47:27Z (GMT) No. of bitstreams: 1 V0330102-637-648.pdf: 297127 bytes, checksum: 8e03777dce1b7bd030a0a3a305f90d94 (MD5) Made available in DSpace on 2016-04-06T15:47:27Z (GMT). No. of bitstreams: 1 V0330102-637-648.pdf: 297127 bytes, checksum: 8e03777dce1b7bd030a0a3a305f90d94 (MD5) Previous issue date: 2003

The low-temperature matrix isolation technique in combination with FTIR spectroscopy, has been used to study the trans-cis photoisomerization of p-(dimethylamino)-‚-chlorostyrene (DMACS). The highly dominant form of the compound immediately after deposition in argon matrix has been identified, by comparison of its experimental infrared spectrum with that calculated at the DFT(B3LYP)/6-31G** level, as the trans isomer. Upon UV irradiation of the matrix, a photoreaction was observed resulting in the appearance of a new IR spectrum corresponding to the photoproduct. By comparison with the theoretically predicted spectra, the photoproduct was identified as the cis form of DMACS. Cross-sections of the potential energy surfaces (PES) along the C1-C10dC11-Cl torsional coordinate in the ground (S0) as well as in the lowest excited singlet (S1) states were generated using the semiempirical (AM1) method. Although the relative energies of the two isomers are quite close (4.6 kJ mol -1 , MP2/6-31G** calculation) in the ground state, the PES reflects that attainment of the equilibrium in the distribution of the two isomers is restricted because of a large activation barrier. The simulated PES, however, corroborates a trans f cis photoisomerization process involving the participation of the S1 state.

Máthesis, n. 8 (1999)

Submitted by Isabel Ferreira (iferreira@ft.lisboa.ucp.pt) on 2015-10-06T15:07:29Z No. of bitstreams: 1 V0290102-453-472.pdf: 443194 bytes, checksum: b60d35ba2e4a7840b8dcec73aa5aa7cc (MD5) Approved for entry into archive by Isabel Nazareth (iqn@lisboa.ucp.pt) on 2015-10-07T10:29:13Z (GMT) No. of bitstreams: 1 V0290102-453-472.pdf: 443194 bytes, checksum: b60d35ba2e4a7840b8dcec73aa5aa7cc (MD5) Made available in DSpace on 2015-10-07T10:29:13Z (GMT). No. of bitstreams: 1 V0290102-453-472.pdf: 443194 bytes, checksum: b60d35ba2e4a7840b8dcec73aa5aa7cc (MD5) Previous issue date: 1999