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1.
E. M. S. Castanheira J. M. G. Martinho 《Journal of photochemistry and photobiology. A, Chemistry》1994,80(1-3):151-156
Solvatochromic shifts of pyrene (Py) and naphthalene (Np) excimers were obtained in polar and non-polar solvents. The observed shifts for both excimers are explained by changes in the polarizability between the excimer and the dissociative ground state. The magnitudes of the shifts in the pyrene excimer are larger, indicating that the pyrene excimer is more polarizable than the naphthalene excimer. 相似文献
2.
J. P. S. Farinha J. M. G. Martinho H. Xu M. A. Winnik R. P. Quirk 《Journal of Polymer Science.Polymer Physics》1994,32(9):1635-1642
Cyclization of a polystyrene chain (Mn = 10,600; Mw/Mn = 1.09) both ends labeled with 4-(1-pyrenyl)butanoamide groups was studied in cyclohexane between 25 and 95°C. The amide groups (peptide bonds) at both ends can form an intrachain hydrogen bond between the amide hydrogen at one chain end and the carbonyl oxygen at the other. The presence of two sets of conformers, random coils, and chains cyclized through hydrogen bonding, complicates the data analysis. The pyrene excimer kinetics of this polymer is well described by a model composed of two monomers (hydrogen bonded and nonbonded chains) and one excimer, in equilibrium. The cyclization rate constant for hydrogen-bonded chains is larger than the one for nonhydrogen-bonded chains. The pyrene excimer binding energy (ca. 1.6 kcal/mol) is lower than the published value for nonhydrogen-bonded chains (~ 9 kcal/mol), suggesting that intrachain hydrogen bonding hinders the stabilization of the excimer. © 1994 John Wiley & Sons, Inc. 相似文献
3.
J. Salgado E. Martinho I. F. Gonçalves 《Journal of Radioanalytical and Nuclear Chemistry》2004,260(2):317-320
The resonance neutron self-shielding factor, G
res, is required in neutron metrology and activation data analysis. In a previous paper, the authors have shown that a dimensionless
variable can be introduced which converts the dependence of G
res on the physical and nuclear properties of the material samples into an universal curve, valid for the isolated resonances
of any nuclide. This work presents a methodology based on the universal curve, which enables to calculate G
res for a group of isolated resonances by weighting its individual contributions. A good agreement was reached with results calculated
by the MCNP code and with experimental values for Mo foils and wires.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
Martinho M Banse F Bartoli JF Mattioli TA Battioni P Horner O Bourcier S Girerd JJ 《Inorganic chemistry》2005,44(25):9592-9596
The green complex S=1 [(TPEN)FeO]2+ [TPEN=N,N,N',N'-tetrakis(2-pyridylmethyl)ethane-1,2-diamine] has been obtained by treating the [(TPEN)Fe]2+ precursor with meta-chloroperoxybenzoic acid (m-CPBA). This high-valent complex belongs to the emerging family of synthetic models of Fe(IV)=O intermediates invoked during the catalytic cycle of biological systems. This complex exhibits spectroscopic characteristics that are similar to those of other models reported recently with a similar amine/pyridine environment. Thanks to its relative stability, vibrational data in solution have been obtained by Fourier transform infrared. A comparison of the Fe=O and Fe=(18)O wavenumbers reveals that the Fe-oxo vibration is not a pure one. The ability of the green complex to oxidize small organic molecules has been studied. Mixtures of oxygenated products derived from two- or four-electron oxidations are obtained. The reactivity of this [FeO]2+ complex is then not straightforward, and different mechanisms may be involved. 相似文献
5.
P. Cabral do Couto R. C. Guedes B. J. Costa Cabral J. A. Martinho Simes 《International journal of quantum chemistry》2002,86(3):297-304
We are reporting ab initio and density functional theory (DFT) calculations for the phenol O–H bond dissociation energy in the gas phase and in phenol–water clusters. We have tested a series of recently proposed functionals and verified that DFT systematically underestimates the O–H bond dissociation energy of phenol. However, O–H bond dissociation energies in water clusters are in reasonable agreement with experimental data for phenol in solution. We have evaluated electronic difference densities in phenol–water, phenoxy–water, and water, and we are suggesting that the representation of this quantity gives an interesting picture of the electronic density rearrangement induced by hydrogen bond interactions in phenol–water clusters. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 相似文献
6.
L.M. Ilharco A.M. Santos M.J. Silva J.M.G. Martinho 《Journal of Sol-Gel Science and Technology》1997,8(1-3):877-882
The influence of the acid catalyst concentration on the structural evolution of a sol-gel system was studied by doping TEOS
based starting solutions with two fluorescent probes: a polystyrene chain (M
n
=1700) and a much shorter alkane chain (M=172), both of them labeled at both ends with 1-pyrenyl. For this purpose, each probe was incorporated in two TEOS∶H2O∶C2H5OH mixtures (molar ratios 1∶4∶1), one at pH 1.2 and the other at pH 2.5 (respectively below and above the isoelectric point
of silica). Very low concentrations of the probes were used (≤10−6
M), so the pyrene dimmers and excimers were formed only intramolecularly. The ratios of excimer to monomer fluorescence intensities
at excitation wavelength of 360 nm (where mainly the ground state pyrene dimmers are excited) were studied as a function of
time. Different evolutions of these ratios were observed, which allowed us to predict that the silica structure developsvia the formation of primary particles, even at pH values below the isoelectric point of silica, where it is not possible to
directly detect their formation. 相似文献
7.
8.
Fluorescence of the single tryptophan of cutinase: temperature and pH effect on protein conformation and dynamics 总被引:1,自引:0,他引:1
Martinho JM Santos AM Fedorov A Baptista RP Taipa MA Cabral JM 《Photochemistry and photobiology》2003,78(1):15-22
The cutinase from Fusarium solani pisi is an enzyme with a single L-tryptophan (Trp) involved in a hydrogen bond with an alanine (Ala) residue and located close to a cystine formed by a disulfide bridge between two cysteine (Cys) residues. The Cys strongly quenches the fluorescence of Trp by both static and dynamic quenching mechanisms. The Trp fluorescence intensity increases by about fourfold on protein melting because of the disruption of the Ala-Trp hydrogen bond that releases the Trp from the vicinity of the cystine residue. The Trp forms charge-transfer complexes with the disulfide bridge, which is disrupted by UV light irradiation of the protein. This results in a 10-fold increase of the Trp fluorescence quantum yield because of the suppression of the static quenching by the cystine residue. The Trp fluorescence anisotropy decays are similar to those in other proteins and were interpreted in terms of the wobbling-in-cone model. The long relaxation time is attributed to the Brownian rotational correlation time of the protein as a whole below the protein-melting temperature and to protein-backbone dynamics above it. The short relaxation time is related to the local motion of the Trp, whose mobility increases on protein denaturation. 相似文献
9.
Pinto SS Diogo HP Guedes RC Costa Cabral BJ Minas da Piedade ME Martinho Simões JA 《The journal of physical chemistry. A》2005,109(42):9700-9708
The energetics of the phenolic O-H bond in the three hydroxybenzoic acid isomers and of the intramolecular hydrogen O-H- - -O-C bond in 2-hydroxybenzoic acid, 2-OHBA, were investigated by using a combination of experimental and theoretical methods. The standard molar enthalpies of formation of monoclinic 3- and 4-hydroxybenzoic acids, at 298.15 K, were determined as Delta(f)(3-OHBA, cr) = -593.9 +/- 2.0 kJ x mol(-1) and Delta(f)(4-OHBA, cr) = -597.2 +/- 1.4 kJ x mol(-1), by combustion calorimetry. Calvet drop-sublimation calorimetric measurements on monoclinic samples of 2-, 3-, and 4-OHBA, led to the following enthalpy of sublimation values at 298.15 K: Delta(sub)(2-OHBA) = 94.4 +/- 0.4 kJ x mol(-1), Delta(sub)(3-OHBA) = 118.3 +/- 1.1 kJ x mol(-1), and Delta(sub)(4-OHBA) = 117.0 +/- 0.5 kJ x mol(-1). From the obtained Delta(f)(cr) and Delta(sub) values and the previously reported enthalpy of formation of monoclinic 2-OHBA (-591.7 +/- 1.3 kJ x mol(-1)), it was possible to derive Delta(f)(2-OHBA, g) = -497.3 +/- 1.4 kJ x mol(-1), Delta(f)(3-OHBA, g) = -475.6 +/- 2.3 kJ x mol(-1), and Delta(f)(4-OHBA, cr) = -480.2 +/- 1.5 kJ x mol(-1). These values, together with the enthalpies of isodesmic and isogyric gas-phase reactions predicted by density functional theory (B3PW91/aug-cc-pVDZ, MPW1PW91/aug-cc-pVDZ, and MPW1PW91/aug-cc-pVTZ) and the CBS-QMPW1 methods, were used to derive the enthalpies of formation of the gaseous 2-, 3-, and 4-carboxyphenoxyl radicals as (2-HOOCC(6)H(4)O(*), g) = -322.5 +/- 3.0 kJ.mol(-1) Delta(f)(3-HOOCC(6)H(4)O(*), g) = -310.0 +/- 3.0 kJ x mol(-1), and Delta(f)(4-HOOCC(6)H(4)O(*), g) = -318.2 +/- 3.0 kJ x mol(-1). The O-H bond dissociation enthalpies in 2-OHBA, 3-OHBA, and 4-OHBA were 392.8 +/- 3.3, 383.6 +/- 3.8, and 380.0 +/- 3.4 kJ x mol(-1), respectively. Finally, by using the ortho-para method, it was found that the H- - -O intramolecular hydrogen bond in the 2-carboxyphenoxyl radical is 25.7 kJ x mol(-1), which is ca. 6-9 kJ x mol(-1) above the one estimated in its parent (2-OHBA), viz. 20.2 kJ x mol(-1) (theoretical) or 17.1 +/- 2.1 kJ x mol(-1) (experimental). 相似文献
10.
Dr. Sara Realista Janaína C. Almeida Sofia A. Milheiro Dr. Nuno A. G. Bandeira Dr. Luis G. Alves Dr. Filipe Madeira Prof. Dr. Maria José Calhorda Dr. Paulo N. Martinho 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(50):11670-11679
Three CoII octaazacryptates, with different substituents on the aromatic rings (Br, NO2, CCH), were synthesised and characterised. These and the already published non-substituted cryptate catalysed CO2 photoreduction to CO and CH4 under blue visible light at room temperature. Although CO was observed after short irradiation times and a large range of catalyst concentrations, CH4 was only observed after longer irradiation periods, such as 30 h, but with a small catalyst concentration (25 nm ). Experiments with 13C labelled CO2 showed that CO is formed and reacts further when the reaction time is long. The CCH catalyst is deactivated faster than the others and the more efficient catalyst for CH4 production is the one with Br. This reactivity trend was explained by an energy decomposition analysis based on DFT calculations. 相似文献