How to increase the length of the arrows/increase space in schemes in chemfig?

Question

I have a reaction scheme as shown in figure below:

MWE

\documentclass{article}
\usepackage{chemfig}

\begin{document}

\schemestart
A \arrow(a--b){->[*{0}$\mathrm{-HO}$][*{0}$\mathrm{+O_{2}}$]}[-90] B
\arrow(@b--c){->[][]} C
% \subscheme{\arrow{->}\lewis{1.,R}}%
\arrow(@b--d){<=>[][*{0}$\mathrm{+O_{2}}$]}[-90] D
\arrow(@d--f){} F
\arrow(@d--f){->[$\mathrm{-\lewis{1.,HO}}$][]}
\arrow(@b--@f){->[$\mathrm{+O_{2}}$][$\mathrm{-\lewis{1.,HO}}$]}
\arrow(@d--e){<=>[][]}[-90] E
\arrow(@e--g){->[$\mathrm{-\lewis{3.,OH}}$][]} G
\arrow(@e--@f){->[$\mathrm{-\lewis{1.,HO}}$][]}
\schemestop

\end{document}

However the area around product 'F' appears to be cramped. To fix this I add some space as shown in figure:

MWE after adding space

\\documentclass{article}
\usepackage{chemfig}

\begin{document}

\schemestart
A \arrow(a--b){->[*{0}$\mathrm{-HO}$][*{0}$\mathrm{+O_{2}}$]}[-90] B
\arrow(@b--c){->[][]} C
% \subscheme{\arrow{->}\lewis{1.,R}}%
\arrow(@b--d){<=>[][*{0}$\mathrm{+O_{2}}$]}[-90] D
\arrow(@d--f){} \hspace{3em} F
\arrow(@d--f){->[$\mathrm{-\lewis{1.,HO}}$][]}
\arrow(@b--@f){->[$\mathrm{+O_{2}}$][$\mathrm{-\lewis{1.,HO}}$]} \hspace{7em}
\arrow(@d--e){<=>[][]}[-90] E
\arrow(@e--g){->[$\mathrm{-\lewis{3.,OH}}$][]} G
\arrow(@e--@f){->[$\mathrm{-\lewis{1.,HO}}$][]}
\schemestop

\end{document}

Now, two problems exists:

1. The length of the arrow from 'D' to 'F' does not increase.
2. The arrows are not exactly aligned with 'F'

How to fix the above two issues with schemes in chemfig?

Answer 1

The length of an arrow can be set by adding a multiplier after the angle:

\arrow(@d--f){}[0, 2] F

where the 2 indicates that the arrow has twice the standard length. Note also that you should replace f by @f in the next line.

\documentclass{article}
\usepackage{chemfig}

\begin{document}

    \schemestart
    A \arrow(a--b){->[*{0}$\mathrm{-HO}$][*{0}$\mathrm{+O_{2}}$]}[-90] B
    \arrow(@b--c){->[][]} C
    % \subscheme{\arrow{->}\lewis{1.,R}}%
    \arrow(@b--d){<=>[][*{0}$\mathrm{+O_{2}}$]}[-90] D
    \arrow(@d--f){}[0, 2] F
    \arrow(@d--@f){->[$\mathrm{-\lewis{1.,HO}}$][]}
    \arrow(@b--@f){->[$\mathrm{+O_{2}}$][$\mathrm{-\lewis{1.,HO}}$]}
    \arrow(@d--e){<=>[][]}[-90] E
    \arrow(@e--g){->[$\mathrm{-\lewis{3.,OH}}$][]} G
    \arrow(@e--@f){->[$\mathrm{-\lewis{1.,HO}}$][]}
    \schemestop

\end{document}