Homolysis diagram in LaTeX

Question

I want to write the following equations; especially the two red arrow in the left part.

Is it possible with chemfig ?
Thanks for your help :)

Roland · Accepted Answer

Yes it can be done.
documentclass{article}
usepackage{chemfig}

begin{document}
    
    schemestart
    setchemfig{bond offset=2pt}
    chemfig{@{a}A-[,0.5]@{b}-[,0.2]@{b2}-[,0.5]@{c}B}
    arrow
    chemfig{charge{55=.}{A}+charge{55=.}{B}}
    schemestop
    chemmove{draw[-{Latex[length=2mm, width=1mm]},red,shorten >=2pt](b)..  controls +(85:0.5cm) and +(85:0.5cm)..  (a);}
    chemmove{draw[-{Latex[length=2mm, width=1mm]},red,shorten >=2pt](b2)..  controls +(95:0.5cm) and +(95:0.5cm)..  (c);}
    
end{document}

Or more like your picture:
documentclass{article}
usepackage{chemfig}

begin{document}

schemestart
    setchemfig{bond offset=2pt}
    chemfig{@{a} A-[,0.3]@{b}-[,0.1]@{b2}-[,0.3]B @{c}}
    arrow
    chemfig{charge{55=.}{A}+charge{55=.}{B}}
    schemestop
    chemmove{draw[-{Stealth[right][length=2mm, width=1mm]},red,shorten >=3pt](b)..  controls +(90:0.4cm) and +(80:0.4cm)..  (a);}
    chemmove{draw[-{Stealth[left][length=2mm, width=1mm]},red,shorten >=3pt](b2)..  controls +(90:0.4cm) and +(100:0.4cm)..  (c);}
    
end{document}

Edson Marcon · Answer

documentclass[margin={2mm 4mm}, width=25cm]{standalone}
usepackage[utf8x]{inputenc}
usepackage{stix}
usepackage{chemfig}
usepackage[version=3]{mhchem}
usepackage{amssymb}
setatomsep{1.8em}
usetikzlibrary{calc,arrows.meta}
tikzset{myedge/.style={->, -{Latex[#1]}}}

begin{document}


schemestart
%
chemfig{@aA-[@b]-[@c]@dB}
 arrow{->}
  %
  chemfig{lewis{0.,A}}
  %
  +
   %
   chemfig{lewis{4.,B}}
%
chemmove{
    draw[myedge={right},red,shorten <=1pt,shorten >=1pt](b).. controls +(90:6mm) and +(60:6mm)..(a);
    draw[myedge={left},red,shorten <=1pt,shorten >=1pt](c).. controls +(90:6mm) and +(120:6mm)..(d);
}
schemestop


end{document}

Answered by Edson Marcon on March 5, 2021

polyn · Answer

documentclass{article} usepackage{chemfig} begin{document} schemestart chemfig{@{a}A(-[@{bd2,0.6},,,,draw=none]phantom{B})-[@{bd1,0.4}]@{b}B} arrow{->} chemfig{charge{0=.}{A} + charge{180=.}{B}} chemmove{ draw[red,-{Stealth[right,width=4pt]},shorten <=1pt,shorten >=1pt] (bd1) .. controls +(90:4ex) and +(85:3ex) .. (a); draw[red,-{Stealth[left,width=4pt]},shorten <=1pt,shorten >=1pt] (bd2) .. controls +(90:4ex) and +(95:3ex) .. (b); } schemestop end{document}

Leeser · Answer

I know I'm a bit late to the party but another way of doing it is:

documentclass[border=1cm]{standalone}
usepackage{tikz}
usetikzlibrary{arrows}
usepackage{chemfig}
begin{document}
schemestart
chemfig{@{a}A(-[@{bd2,0.6},,,,draw=none]phantom{B})-[@{bd1,0.4}]@{b}B}
arrow
chemfig{charge{0=.}{A}  + charge{180=.}{B}}
schemestop
chemmove{
draw[red,thick,-right to,shorten <=3pt,shorten >=3pt]
(bd1.north) ..controls +(90:5mm) and +(90:5mm) .. (a.north) ;
draw[red,thick,-left to,shorten <=3pt,shorten >=3pt]
(bd2.north) ..controls +(90:5mm) and +(90:5mm) .. (b.north) ;
}
end{document}

The -right to and -left to harpoons are provided by the arrows library in usepackage{tikz} usetikzlibrary{arrows} commands.

Changing the anchor positions can give a better look, try replacing ..(a.north) with ..(a.west) and ..(b.north) with ..(b.east).

Curved harpoons or curved half-head arrows are the proper way of depicting the movement of one electron in chemistry. A full headed arrow depicts the movement of two electrons.

I'm sure you are aware of this but just informing texperts who may not have a chemistry background and may not have seen the subtle difference in the arrow heads.

Homolysis diagram in LaTeX

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