HT mechanism

It can be concluded that direct evidence linking endogenous 5-HT to emesis is not convincing. On the other hand, selective effectiveness of 5-HT3 antagonists as antiemetic agents and presence of 5-HT3 receptors in the area postrema and vagus nerve do implicate 5-HT mechanisms. 

There are indications that other psychotomimetic agents also act through a central 5-HT mechanism. Four groups can be distinguished ... 

We have presented the first published results of a QM/MM wavepacket dynamics study of a photochemical reaction. The photoisomerization mechanism for the GFP chromophore that we observe has the signatures of HT motion, even in the complete absence of an environment. The HT mechanism is aborted in both the gas phase and solution, but the... 

The composition and sequence of the base pairs in DNA should have an important effect on the ET and HT mechanisms. With regard to HT, guanine (G) is much more easily oxidized than adenine A) or thymine (T) consequently, hole injection into the DNA helix will generate a G+ radical cation. The hole may now randomly... 

The classic cases of the HT mechanism concern coupling between two electronic states of different symmetry or between the different components of two degenerate states of the same symmetry. An important example of the first case occurs when electric dipole transitions to one of the two states are forbidden (e.g. the Laporte-forbidden d—d and f-f transitions). In this case, the forbidden transition may acquire absorption intensity by HT mixing with an allowed transition via a non-totally symmetric mode of appropriate symmetry (the irreducible representation of the active mode must be contained in the direct product of the irreducible representations for the two states coupled by the HT mechanism). 

The HT Mechanism. Figures 2, 3, and 4 resume the main characteristics of the HT conduction mechanism. The conduction can be ascribed to the mobility of the exchangeable cations. This is in agreement with conclusions by other authors (3, 6, 7, 19). Thus, in Figure 2, the conductivity at 19 kHz and 700 °K is represented as a function of the radius of the exchangeable ions for the X and Y zeolites. The specific conductivity of X is considerably higher than that of Y for samples... 

It has also been suggested that the reduction by 1,4-dihydropyri-dine derivatives proceeds by a one-step hydride transfer mechanism (HT-mechanism), which seems to be contrary to the ET-mechanism described above. This assertion is mostly based on the results from the reduction of cationic substrates such as quinolinium salts, acridinium salts, and protonated Schiff bases (Srinivasan et al. 1982). For example, Ostovic et al. (1983) reported that the value of the kinetic isotope effect does not change significantly for the reactions with a series of substrates with which equilibrium constants change widely. 

Table 3.1 Some transp

These observations are now understandable within the context of the HT mechanism.Two examples are illustrated below. 

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