[texhax] Chemarrow problems

[John Simmie]

Chemarrow.sty usefully defines a no. of arrows/harpoons for writing 
chemical reactions
but the horizontal spacing between items is too short, cf. 1st two lines

$$ \mathrm{CH_3CH_3+O_2 \chemarrow CH_3\dot{C}H_2 + H\dot{O}_2} $$
$$ \mathrm{CH_3CH_3+O_2 \rightarrow   CH_3\dot{C}H_2 + H\dot{O}_2} $$

Also the text above or below the extensible arrow is too large and needs to be
2 sizes down (?)

$$ \mathrm{A+B \autorightarrow{$k_2$}{{\scriptsize 1,000 K}} C+D} $$

is there a more elegant way to redefine \autorightarrow etc to incorporate 
these
changes?

                Dr. John M. Simmie
Chemistry Department & Environmental Change Institute
    National University of Ireland, Galway::Ireland
      Phone: +353-91-750388::Fax: +353-91-525700