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Re: Hill notation

Paula Fitzgerald (paula_fitzgerald@Merck.Com)
Wed, 1 Nov 95 15:00:26 EST 

Frances Bernstein writes:

>     The mmCIF dictionary says, under _chem_comp.formula:
> 
>                4. The order of elements with the formula should be: C, then H,
>                   then the other elements in alphabetical order of their
> symbol.
>                   This is the 'Hill' system used by Chemical Abstracts.
> 
> The PDB uses C, H, N, O, then others alphabetically.  Clearly it is simple
> to convert one to the other but I got curious and looked at Chem Abstracts
> for a description of the Hill system.  Apparently the ordering is as presented
> in the mmCIF dictionary if carbon is present.  If there is no carbon, then
> the ordering is strictly alphabetic; i.e., hydrogen is not given first.
> Perhaps the description given in the dictionary should be revised to cover
> the case where no carbon is present.

The definition has been enhanced, to wit:

               4. The order of elements depends on whether or not carbon is
                  present.  If carbon is present, the order should be: C, then
                  H, then the other elements in alphabetical order of their
                  symbol.  If carbon is not present, the elements are listed
                  purely in alphabetic order of their symbol.  This is the 
                  'Hill' system used by Chemical Abstracts.

Paula

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