Water as a special case

The importance of H2O to cloud chemistry is seen from the reaction sequence  

What this means for the structures of clouds is not presently clear. It is possible that the clouds are fluffy— that UV radiation penetrates deeper into the clouds than previously thought. There is some indication that the ionization fractions for the clouds are higher than we would have expected. 

It is possible, however, to determine the D/H ratio through a different avenue, the isotopic shift caused by the mass ratio of D/H in the rotational and vibrational spectrum of the H2 molecule compared with HD. This method is not free from difficulties, though, because of the many possible routes available for depletion of HD through molecular formation. The reaction energies for the isotopic species are slightly different, by several tens of degrees (hundredths of an electron volt), which at interstellar temperatures produces substantial differences in reaction rates. A detailed calculation of the isotope s mobility and reaction among the different species is required. 

Both H2 and HD are formed on grain surfaces. The subsequent reactions that involve these molecules can enhance the D abundance in the more complex species which are formed, and also open up new channels. For example. 

One well observed species, HCO, is observed to yield a higher than expected D/H ratio. This appears to be due to the protonation reaction chain 

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An illustration of Lowry s concept of acid catalysis is given in Figure 7, where S is the molecule to be converted (rearranged) to T. The proton donators cited as examples are the oxonium ion an undissociated acid molecule HA, and water as a special case of HA, while the proton acceptors cited as examples are water (transformed to H30 ), and the acetate ion (transformed to acetic acid). Contrary to the concept of Arrhenius, Lowry s concept can explain why water as a proton donator and acetate ions as proton acceptors represent a power-... 

Water as a Special Case Let us now consider a special case of the acid-base pairs where the solvent water is the reaction partner and where the pairs limit the range of potentials of the weakly and moderately acidic and alkaline pairs from that of the... 

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