chemfig double bond gap at joint

Question

I'm using chemfig to draw chemical figures. However, when drawing double bonds as side group onto a carbon chain, I get a little gap. This behaviour is different e.g. in chemdraw, where it looks better in my opinion.

This is my mwe:

\documentclass[border=2pt]{standalone}
\usepackage{chemfig}

\usepackage{mathpazo}
\renewcommand*{\familydefault}{\sfdefault}
\renewcommand*{\printatom}[1]{\ensuremath{\mathsf{#1}}}

\setdoublesep{0.35700 em}  % 'Bond Spacing'
\setatomsep{1.478500 em}    % 'Fixed Length'
\setbondoffset{0.18265 em} % 'Margin Width'
\setbondstyle{line width = 0.06642 em}
\setbondstyle{thick,cap=round}

\begin{document}
    \chemfig{
        -[:30](=[2]O)-[:-30]
    }
\end{document}

How can I remove this gap and "attach" the boule-bond onto the backbone? Drawing Carbon-Atoms is not the desired solution in this case.

Answer 1

With some help I found a solution:

 -[:30](=[2,,,,shorten <=-1pt]O)-[:-30]

So what is going on here? The fifth argument is tikz-code. shortenis a tikzcommand for e.g. lines in a \draw commands and will shorten one end of line by the specified amount. As we specify the amount to be negative, it will make the line(s) longer. With the <= part we are able to select one of the two lines end, which should be shortened (or in this case prolonged).

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or another code, a little shorter:

-[:30](=#(-1pt)[2]O)-[:-30]

This one is explained in the chemfig-manual. Bond-spacing can be altered directly with the #()-command (see manual section 2, Different types of bonds). Keep in mind: if used within a macro, #can make problems.

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A little more manual-digging could have helped me :-)