Occurrence of hydrogen

The occurrence of hydrogen exchange and double-bond migration in heterogeneous catalytic hydrogenation means that the hydrogenation does not necessarily... 

Repeat the last problem using hydrogen gas instead of helium. Explain why the occurrence of hydrogen gas in diatomic molecules is so important. 

The occurrence of hydrogen sulfide in the sewer atmosphere is an important example for illustrating odor problems and other negative effects associated with sulfide that will be further dealt with in Section 6.2.6. According to Table 4.1, HB2S = 563 atm (mole fraction)-1, and H2S, therefore, observes the... 

In addition to odor, several human health-related problems are potentially associated with the occurrence of hydrogen sulfide at concentration levels relevant for sewer networks. In this respect, it is interesting to compare values from Table 4.5 with Figure 4.2. The levels indicated in Table 4.5 depend on human sensitivity and time of exposure. 

Problems related to the occurrence of hydrogen sulfide have been intensively reported in the literature (cf. this chapter and Section 6.2.6). Although a great number of factors affect the relation between the occurrence and the problems identified, Table 4.6 can be considered a relevant approach. The table gives a simplified estimate, and indication of medium problems must not be considered equivalent to no need for control. ... 

Indisputable evidence for the occurrence of hydrogen spill-over exists although the mechanism whereby it occurs is still not clear. Benson et al. 

Figure 1. Carbon abundance as a function of mass for both components of a close binary system at the onset of mass transfer. The region from Mx=0 to Mr=Mgi=8.1 Mo corresponds to the originally less massive component (gainer), whereas the carbon distribution of the loser is plotted from 8.1 Mo (surface) to 17.1 M (center). The first occurrence of hydrogen depleted layers (Xat<0.7) and the end of the Roche Lobe Overflow are indicated.

At this stage of our knowledge, the occurrence of hydrogen bonding between hydroxyl groups and anionic oxygen in carbohydrate alcoholates has not been shown conclusively. That it probably does occur is strongly... 

Nangia, A., Desiraju, G. R., Pseudopolymorphism occurrences of hydrogen bonding organic solvents in molecular crystals. Chem. Commun. 1999, 605-606. 

For most carbonyl compounds, we expect to have two near-lying excited states in the triplet manifold, which are either n-n or tc-tc in character. The n-7T states frequently show radical-like behavior. Benzophenone is an example of such a molecule which has an n-7t triplet state in which we see occurrences of hydrogen abstraction and very efficient intersystem crossing. When the lowest state is the 71-71 state, largely centered on the aromatic part of the molecule, as in the case of p-methoxyacetophenone, the reactivity decreases significantly (8,9). With the nature of lignin and the nature of the model we have chosen, we are mostly interested in molecules which have this type of behavior. 

Equations (1) to (3) explain the occurrence of hydrogen evolution over the substrate. The presence of phosphorus is a result of the reaction between hypophosphorous acid with a hydrogen radical, according to... 

An NMR study of enamines 32 also showed the occurrence of hydrogen bonds62 in the major form 33 when R1 = R2, R3 = H, R = Ph. 

R3 = H R4 = Bu, t-Bu) failed to conclusively demonstrate the occurrence of hydrogen bonds except, perhaps, in CDC13, where the chemical shift of the NH resonance is low (ca 4-5 ppm)63a. On the other hand, recently reported 170-NMR data seem to point to the presence of hydrogen bonds. However, the energy of these bonds must be much lower than for those of other enamines mentioned above64. 

Some authors70 have developed a rather different approach for investigating the occurrence of hydrogen bonds and for determining their strength using isotopic effects on nuclear shielding. The model equation is equation 29,... 

The occurrence of hydrogen bonds cannot be detected in 6-methylcarbamoyl-tetrahydro-4/f-pyrido[l,2-[Pg.721]

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