No abstract

Figure 1. Part of the NH-CJH region of the 300-MHz NOESY spec-trum (2K X 2K data matrix) of the decapeptide LHRH. The two framed regions 1 and 2 showing severe overlaps are also plotted on expanded scales. The corresponding regions from the NOESY TR spectrum (700 f, samples zero filled to 2K X 2K) exhibiting homonuclear decoupling in the uq dimension are shown in the frames V and 2'. No undesired cross peaks (see text) appeared in the NH-CJH region so suppression10 was unnecessary. N = 4 (see text) was sufficient in this experiment since the Ca protons can have at maximum three coupling partners, leading to the seven-step cycle ¡0°;0.

The laser-induced fragmentation behavior of positive carbon cluster ions has been investigated by tandem time-of-flight techniques for the jet-cooled clusters up to 80 atoms in size. Two distinct photophysical regimes were found. The first applies to clusters with 34 atoms or more, all of which dissociate to produce even numbered fragments. Large even clusters fragment by the loss of the high energy species C2, odd ones lose a C atom. The second regime applies to clusters composed of 31 or less atoms, all of which fragment by the loss of C3. These two regimes are sharply separated by C+32 which fragments to produce small cluster ions in the 10–19 atom size range. Fragmentation of the large clusters occurs on a microsecond or faster time scale only at very high levels of excitation (>12.8 eV). These photophysical results are interpreted as consequences of the large even clusters having edgeless, spheroidal cage structures while the small ones have linear chain or ring structures.

Reactions on the surface of a variety of transition metal clusters have been studied in the gas phase at near room temperature using a newly developed fast-flow reaction device. Initial examples of the use of this device are provided by survey studies of the reactivity of iron, cobalt, nickel, copper, and niobium clusters in contact with low concentrations of D2, N2 and CO. Dissociative chemisorption of D2 is found to occur with dramatic sensitivity to cluster size in the cases of iron, cobalt, and niobium clusters, the detailed pattern of reactivity differing markedly for each metal. The corresponding reaction is also observed with nickel clusters, but here the reactivity shows only a slow, steady increase with cluster size. Copper clusters are found to be completely unreactive to H2 chemisorption under these conditions. Molecular nitrogen is found to chemisorb readily to clusters of cobalt and niobium, with a reactivity pattern very similar to that observed with D2. Iron clusters are found to show slight reactivity with N2; only a small amount of chemisorption is observed on the most reactive clusters at high N2 concentration, but the pattern of this reactivity with cluster size is consistent with that observed in D2 chemisorption. In contrast to these highly structured reactivity patterns of D2 and N2, carbon monoxide is found to show only a slow, monotonic increase in reactivity with cluster size. It is suggested that these dramatic reactivity patterns for chemisorption on metal clusters provide stringent tests for future theories as to the nature of chemisorption on metal surfaces at a detailed, molecular level.

No abstract

No abstract

No abstract

No abstract