Aligning xilene isomeri with chemfig

Question

Ho can vertical align this scheme to the red line?

\documentclass{article}
\usepackage[margin=1.5cm,top=1cm,headheight=16pt,headsep=0.1in,heightrounded]{geometry}
\usepackage{chemfig}
\setchemfig{%
%       %scheme debug=true,
        arrow offset=9pt,
        arrow coeff=0.7,
%       compound sep=5em,
        + sep left=0.6em,
        + sep right=0.6em,
        atom sep=1.25em,
        fixed length=true
}
\begin{document}
\begin{tikzpicture}
\draw[red](0,-.59) --++ (12,0);
\schemestart
    \rotatebox{-90}{\chemfig{**6(---(-)-(-)--)}}
    \arrow{<=>}
    \rotatebox{-90}{\chemfig{**6(--(-)--(-)--)}}
    \arrow{<=>}
    \rotatebox{-90}{\chemfig{**6(-(-)---(-)--)}}
    \schemestop
\end{tikzpicture}
\end{document}

Answer 1

You can use chemfig only. The nvisible bonds adjust the molecule position.

\documentclass{article}
\usepackage[margin=1.5cm,top=1cm,headheight=16pt,headsep=0.1in,heightrounded]{geometry}
\usepackage{chemfig}
\setchemfig{%
    %       %scheme debug=true,
    arrow offset=9pt,
    arrow coeff=0.7,
    %       compound sep=5em,
    + sep left=0.6em,
    + sep right=0.6em,
    atom sep=1.25em,
    fixed length=true
}

\begin{document}
\schemestart 
\chemfig{[:-30]**6(--(-)-(-)--(-[,,,,,draw=none])-)}
%
\arrow{<=>}
%
\chemfig{[:-30]**6(-(-)--(-)--(-[,,,,,draw=none])-)}
%
\arrow{<=>}
%
\chemfig{[:-30]**6((-)---(-)---)}
\schemestop
\end{document}