How to reproduce typesetting of coupling constant in chemmacros

Question

I am using the fonts libertine with newtxmath and beramono together with the chammacros package to typeset experimental data. This works very well and I get the desired results. This package manages to typeset the coupling constant in a way that its prescript is very close to the letter J. When I try to achieve the same result text, I fail, even though I am using packages like mathtools of leftidxbut they don't meet my needs it their standard version.

Here is a MWE

\documentclass[a4paper,10pt,bibliography=totoc,listof=totoc]{scrreprt}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{libertine}
\usepackage[scaled=.83]{beramono}
\usepackage[libertine]{newtxmath}

\usepackage{chemmacros}
\chemsetup[nmr]{
  delta = (ppm) ,
  pos-number = side ,
  use-equal,
  format = \bfseries,
  list=true
}
\usepackage{siunitx}
\sisetup{
  separate-uncertainty ,
  per-mode = symbol ,
  range-phrase = -- ,
  detect-mode = false ,
  detect-weight = true ,
  mode = text ,
  text-rm = \libertineLF % use libertine with lining figures
}

\ExplSyntaxOn
\cs_set_protected:Npn \__chemmacros_nmr_coupling:w (#1;#2)
  {
    \tl_set:Nn \l__chemmacros_nmr_coupling_bonds_tl { #1 \! }
    \tl_set:Nn \l__chemmacros_nmr_coupling_nuclei_tl
      {
        \c_math_subscript_token
          { \chemmacros_chemformula:n { #2 } }
      }
    \__chemmacros_nmr_coupling_aux_i:w
  }
\cs_set_protected:Npn \chemmacros_nmr_number:n #1
  {
    $ #1 $ \, % put the number in math-mode for lining figures
    \chemmacros_atom:V \g__chemmacros_nmr_element_tl
  }
\ExplSyntaxOff

\begin{document}
Good way:
\begin{experimental}
  \NMR(400)[C6D6] \val{2.01} (d, \J(1;CH)[Hz]{25.0}, \#{24}, \pos{5})
\end{experimental}
Not so good way:
$\prescript{1}{}{J}$ coupling

\end{document}

I believe chemmacros is using mathtools internally from what I remember from the documentation, but appparently in a smarter way than I do.

Answer 1

chemmacros uses normal math mode for this: $^{1}J$. You have a modified version which reduces the space a bit: $^{1\!}J$.

Below I use v4.4 of chemmacros which also uses \! and adds a possibility to decide how the different numbers are typeset (see the extended options in the example below) so the redefinitions are not needed anymore.

\documentclass[a4paper,10pt,bibliography=totoc,listof=totoc]{scrreprt}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{libertine}
\usepackage[scaled=.83]{beramono}
\usepackage[libertine]{newtxmath}

\usepackage{chemmacros}[2014/01/29] % use version 4
\chemsetup[nmr]{
  delta = (ppm) ,
  pos-number = side ,
  use-equal,
  format = \bfseries,
  list=true ,
  coupling-pos-cs = \ensuremath , % <<< added
  atom-number-cs = \ensuremath    % <<< added
}
\usepackage{siunitx}
\sisetup{
  separate-uncertainty ,
  per-mode = symbol ,
  range-phrase = -- ,
  detect-mode = false ,
  detect-weight = true ,
  mode = text ,
  text-rm = \libertineLF % use libertine with lining figures
}

\begin{document}

Good way:
\begin{experimental}
  \NMR(400)[C6D6] \val{2.01} (d, \J(1;CH)[Hz]{25.0}, \#{24}, \pos{5})
\end{experimental}
Same way:
$^{1\!}J$ coupling

\end{document}