Use of # in align block

I have a chemfig command which involves the use of #. It compiles properly on itself.

\chemfig{**[135,405,dash pattern=on 2pt off 2pt]6(----#(,0pt)\chembelow[2.5em]{}{+}(-#(0pt)[3]H)(-#(0pt)[1]NO_3)-#(0pt)-)}


But when I put it into the align block, pdfLaTeX gives error.

\begin{align*}
\chemfig{**[135,405,dash pattern=on 2pt off 2pt]6(----#(,0pt)\chembelow[2.5em]{}{+}(-#(0pt)[3]H)(-#(0pt)[1]NO_3)-#(0pt)-)}
\end{align*}


The error message is here: http://pastebin.com/3FLzX4RP

MWE:

\documentclass[10pt]{article}
\usepackage[usenames]{color} %used for font color
\usepackage{amssymb} %maths
\usepackage{amsmath} %maths
\usepackage[utf8]{inputenc} %useful to type directly diacritic characters
\usepackage{chemfig}

\pagestyle{empty}
\begin{document}

\begin{align*}
\chemfig{**[135,405,dash pattern=on 2pt off 2pt]6(----#(,0pt)\chembelow[2.5em]{}{+}(-#(0pt)[3]H)(-#(0pt)[1]NO_3)-#(0pt)-)}
\end{align*}

\end{document}


In chemfig, # is used to adjust the bond shortening. The following example is from the chemfig documentation, pg. 6

I've tried using \#(0pt), but instead of adjusting the bond, it outputs the string directly.

• Welcome to TeX.SX! Please help us to help you and add a minimal working example (MWE) that illustrates your problem. It will be much easier for us to reproduce your situation and find out what the issue is when we see compilable code, starting with \documentclass{...} and ending with \end{document}.What does # do in chemfig? Perhaps it's required literally, i.e. \# within align? – user31729 Feb 22 '15 at 11:28
• @ChristianHupfer , thank you for the reminder. I have updated the the question. – Eana Hufwe Feb 22 '15 at 11:54
• Related, for the plus sign. By the way, aren't the angles 150,390? – Manuel Feb 22 '15 at 12:04

The \chemfig command relies on category code changes, in particular it changes # to an “other character” (category code 12). Such commands cannot appear in the argument to another command.

While equation and equation* are safe on this respect, align* isn't: the multiline alignment environments of amsmath absorb their content as the argument to a command. You'd have the same problem trying \fbox{\chemfig{...}}.

Using align* as a substitute for equation* is not recommended, so in the example case equation* would do. But if you really need a multiline environment, here's a trick:

\documentclass{article}
\usepackage{amsmath,chemfig}

\makeatletter
\newcommand{\chemfigon}{\begingroup\@makeother\#}
\let\chemfigoff\endgroup
\makeatother

\begin{document}

\chemfig{**[135,405,dash pattern=on 2pt off 2pt]6(----#(,0pt)\chembelow[2.5em]{}{+}(-#(0pt)[3]H)(-#(0pt)[1]NO_3)-#(0pt)-)}

\chemfigon
\begin{align*}
\chemfig{**[135,405,dash pattern=on 2pt off 2pt]6(----#(,0pt)\chembelow[2.5em]{}{+}(-#(0pt)[3]H)(-#(0pt)[1]NO_3)-#(0pt)-)}
\end{align*}
\chemfigoff

\end{document}


This makes # an “other character” before the input is grabbed as an argument.

• \chemfig{**[150,390,dash pattern=on 2pt off 2pt,late options={name=arccenter,label=center:+}]6(----#(,0pt)(-#(0pt)[3]H)(-#(0pt)[1]NO_3)-#(0pt)-)} this is more elegant in my opinion. – Manuel Feb 22 '15 at 12:20
• And this one, without #, should be prefered: \chemfig{**[135,405,dash pattern=on 2pt off 2pt]6(----(-[3]H)(-[1]NO_3)(-[6,,,,draw=none]+)--)} – unbonpetit Feb 22 '15 at 15:34

In theory, \chemfig{<code>} can be written in the argument of a macro since \CF@chemfig@iv does a \scantokens of the <code>. Unfortunately, there is a bug because via \scantokens, # becomes ## in the argument of a macro and this behaviour is not taken into account by \chemfig.

For example, if you write \fbox{\chemfig{A-#(0pt)B}}, the code of the molecule becomes A-##(0pt)B, where # is catcode 12. The bug is in \CF@grab@bondoffset: this macro expects a ( juste after the first #. And this is not the case when \chemfig is in the argument of a macro.

Here is a fix:

\documentclass{article}
\usepackage{chemfig}
\makeatletter
\def\CF@grab@bondoffset#1(#2)#3\@nil{%
\ifx\@empty#2\@empty
\else
\CF@if@instr{#2},%
{\CF@analyse@bondoffset#2\@nil}%
{\def\CF@start@offset{#2}}%
\fi
\def\CF@remain@afterbond{#3}%
}
\makeatother
\begin{document}
\fbox{\chemfig{A-#(0pt)B}}

\chemfig{A-#(0pt)B}
\end{document}