冬月 世馬

We report measurements of the ultraviolet absorption cross sections of (OCS)-S-32, (OCS)-S-33, (OCS)-S-34 and (OCS)-C-13 from 195 to 260 nm. The OCS isotopologues were synthesized from isotopically-enriched elemental sulfur by reaction with carbon monoxide. The measured cross section of (OCS)-S-32 is consistent with literature spectra recorded using natural abundance samples. Relative to the spectrum of the most abundant isotopologue, substitution of heavier rare isotopes has two effects. First, as predicted by the reflection principle, the Gaussian-based absorption envelope becomes slightly narrower and blue-shifted. Second, as predicted by Franck-Condon considerations, the weak vibrational structure is red-shifted. Sulfur isotopic fractionation constants ((33)epsilon, (34)epsilon) as a function of wavelength are not highly structured, and tend to be close to zero on average on the high energy side and negative on the low energy side. The integrated photolysis rate of each isotopologue at 20 km, the approximate altitude at which most OCS photolysis occurs, was calculated. Sulfur isotopic fractionation constants at 20 km altitude are (-3.7 +/- 4.5)parts per thousand and (1.1 +/- 4.2)parts per thousand for (33)epsilon and (34)epsilon, respectively, which is inconsistent with the previously estimated large fractionation of over 73 parts per thousand in (34)epsilon. This demonstrates that OCS photolysis does not produce sulfur isotopic fractionation of more than ca. 5 parts per thousand, suggesting OCS may indeed be a significant source of background stratospheric sulfate aerosols. Finally, the predicted isotopic fractionation constant for S-33 excess (E-33) in OCS photolysis is (-4.2 +/- 6.6)parts per thousand, and thus photolysis of OCS is not expected to be the source of the non-mass-dependent signature observed in modern and Archaean samples.

The fractionation of sulfur isotopes in the gas-phase reaction of OCS with OH was calculated by ab initio methods. The first reaction step occurs through a carbon bonded OH-OCS transition state. The activation barrier is 18.4 kJ mol(-1) (CCSD(T)/aug-cc-pVDZ level plus zero-point energy) relative to the reactants. Fractionation constants are -1.4, -2.6 and -6.3 parts per thousand for S-33, S-34 and S-36 respectively. These results bring new insight regarding the origin of stratospheric sulfate aerosols, including the identification of characteristic isotopic fractionations in the lower and middle stratosphere. (C) 2007 Elsevier B.V. All rights reserved.

¹³C NMR chemical shielding and XPS of cellulose and chitosan were analyzed by deMon DFT calculations using the model dimers. The calculated ¹³C chemical shifts of (α-<small>D</small>-glucose, β-<small>D</small>-glucose, and β-<small>D</small>-glucosamine) and cellobiose with DZVP basis are in considerably good accordance with the experimental values in the average absolute deviations (AAD) of ±3.1 and 2.0 ppm, respectively. The calculated shifts of the dimer models for cellulose and chitosan also correspond well to the experimental ones of both solid biopolymers in the AAD of ±3.1 ppm. In order to simulate the valence XPS and to calculate core-electron binding energies (CEBE)s of cellulose and chitosan, we used the restricted diffuse ionization (rDI) and generalized transition-state (GTS) methods, respectively, due to Slater's transition-state (TS) concept. The simulated valence spectra of the dimer models showed good agreement with the experimental ones of cellulose and chitosan. We also estimated as 5.9 and 5.7 eV for WD (work function and the other energies) values of cellulose and chitosan, respectively from the differences between calculated CEBE values for the model molecules and experimental ones on the solid polymers.