A spin echo method adapted to the measurement of long nuclear relaxation times (T2) in liquids is described. The pulse sequence is identical to the one proposed by Carr and Purcell, but the rf of the successive pulses is coherent, and a phase shift of 90° is introduced in the first pulse. Very long T2 values can be measured without appreciable effect of diffusion.

The main objective of this study was to determine the important mechanisms regulating woody plant establishment in recently abandoned agricultural fields in central New York. The study focused on the early life stages, from the dispersal of seeds to the fate of seedlings at the end of the second growing season, of common old—field invaders, primarily Acer rubrum, Pinus strobus, Cornus racemosa, and Rhamnus cathartica. We investigated patterns of survivorship and causes of mortality during each of these life stages in microsites dominated by previously established herbs and in experimental openings in the herb canopy. Establishment of a tree or shrub seedling was much less likely from seeds landing among herbs than in openings. During the fall, seed predation by mice, especially under the cover of herbs (median of averages across all species = 6% in open, 85% under herbs), was an important process regulating the relative abundance and spatial distribution of seeds after dispersal. Seedling emergence rates were generally quite low, but for most species tended to be greater in the presence of perennial herbs (12%) and were lower under annuals and biennials (7%) and in bare soil (6%). For cohorts of seedlings followed for two growing seasons (Experiment 6A), some mortality occurred due to frost heaving (17% in open, 4% under herbs) and apparent heat stress or desiccation during the summer (5% in open, 0% under herbs); however, the largest source of mortality, especially under the herb canopy, was predation by rodents, especially meadow voles (6% in the open, 62% under herbs). Competition with herbs, especially aboveground competition, significantly reduced growth of woody seedlings. In spite of growth reduction in the presence of herbs, there was no significant relationship between seedling size and survivorship within any treatment, indicating that mortality could not be directly attributed to competition. For different species, and at different life stages, support was found for all three of the succession models of Connell and Slatyer (1977). The tolerance model was supported in some cases of seed predation (Experiment 2), seedling emergence (Experiments 4 and 5, in part), and seedling predation (Experiment 7). The facilitation model was supported in some cases by increased seedling emergence (Experiments 4 and 5, in part) and increased seedling survivorship in the presence of herbs during periods of environmental stress (Experiments 6A and 8). The inhibition model was also supported in some cases of decreased growth (Experiment 6B) and decreased survival of seeds and seedlings (Experiments 1, 3, 6A, and 8) in the presence of herbs. The lower survivorship under herbs was largely the result of seed and seedling predation, which were the two most important processes regulating tree and shrub colonization of old fields in this study. Reduced growth of later colonists due to competition with earlier colonists is considered direct inhibition, while increased mortality due to predation on later colonists caused by animal...

No abstract

The +4 charged dye tetramethylpyridylporphyrin, TMPyP (≡H2TMPyP4+), and the metallo ZnTMPYP are water-soluble. The cationic TMPy groups are nearly perpendicular to the plane of the porphine. When the dyes were applied to the layered inflatable clay mineral Laponite a ∼30 nm red shift in the spectrum of both TMPyP−'s adsorbed on outer surfaces and a ∼60 nm shift in the dyes trapped in interlamellar galleries were found. The negatively charged clay surface may be very acidic. In previous works, the red shifts were interpreted in terms of diprotonation of TMPyP. In this work, several possible causes of the observed red shifts were rigorously examined. These included diprotonation, redox reactions, solvent effects, π-electron interaction with the oxygen plane, and electronic changes due to the flattening of TMPyP on the clay. To facilitate in a deductive manner a possible explanation for the observed metachromasy, the charged, flat Laponite was replaced with its neutral analogue:  talc and porous silica. The electronic structure of the porphine ring is classically treated by means of the 4-level model. In charged porphyrins, such as TMPyP, the modelization is much complicated. Here, semiempirical quantum chemical PM3 modeling in a vacuum confirmed the perpendicularity of the TMPy groups. The dihedral angles were constrained toward planarity and the resulting red shifts in the spectra were calculated, then a classical (four-level) adaptation was applied. It then appears that the ∼30° twists from the vertical would cause a ∼30 nm red shift in the Soret band with a ∼3 kcal/mol rise in ground-state energy. Then ∼46° twists would cause a ∼60 nm red shift in the Soret band with a ∼11 kcal/mol rise in ground-state energy. Hence, sterically induced hindrance in adsorbed TMPyP had a profound influence on the absorption spectra, confirming our conclusions from the experimental part.

Although the structural data obtained on PF2NH2 and (CH3)2NPF2 add considerable support to a ttbonding model, it should be mentioned that a recent infrared and Raman investigation of N[P(CF3)2]3 suggests that this molecule does not have a planar NP3 skeleton.19The 16N-H coupling constant (83.2 Hz) obtained for PF215NH2 is also consistent with a planar -NH2 group. A relationship between VuN_H and the percentage 2s character in the nitrogen orbital used in the NH bond has been developed.20 This relationship predicts 30% 2s character in the nitrogen orbitals bonded to the hy-(19) P.