Center align italic number in siunitx S-column

Question

Here is a table with the siunitx S column type. I would like the italic value (the command with swapshift) to be still centered on the decimal point. The superscript can be variable, but will almost always be just "a" or "b", maybe "c", never more than 1 letter in length.

\documentclass{memoir}
\usepackage{siunitx}
\usepackage{multirow}
\sisetup{inter-unit-product={}\cdot{},sticky-per=true,multi-part-units=single,separate-uncertainty=true,list-units=single,range-units=single,detect-all,detect-weight=true,detect-inline-weight=math}
\usepackage{etoolbox}
\newcommand{\thirteenc}{\textsuperscript{13}C}
\newcommand{\oneh}{\textsuperscript{1}H}
\robustify\bfseries
\usepackage{bpchem}
\usepackage[tracking=bpchem]{chemstyle}
\setcounter{BPCno}{200}

\newcommand{\mcol}[3]{\multicolumn{#1}{#2}{#3}}
\newcommand{\mrow}[3]{\multirow{#1}{#2}{#3}}
\newcommand{\onemultcol}[2]{\mcol{1}{#1}{#2}}
\newcolumntype{g}{>{\rule{1.2em}{0em}}r@{ }l}
\newcommand{\greek}[1]{$#1$}
\newcommand{\range}[1]{\mcol{1}{c}{\bfseries #1}}
\newcommand{\swapshift}[2]{\mcol{1}{c}{\textit{#1}\textsuperscript{#2}}}
\newenvironment{onehTable}[2]{%
\begin{tabular}{gS[table-format=1.2]S[table-format=1.2]S[table-format=1.2]S[table-format=1.2]} \toprule%
\mcol{2}{c}{\mrow{2}{*}{Position}} & \mcol{2}{c}{Computed} & \mcol{2}{c}{Experimental} \\%
& & \mcol{1}{c}{\textbf{#1}} & \mcol{1}{c}{\textbf{#2}} & \mcol{1}{c}{\textbf{#1}} & \mcol{1}{c}{\textbf{#2}} \\%
\cmidrule[\lightrulewidth](r){1-2} \cmidrule[\lightrulewidth](lr){3-4} \cmidrule[\lightrulewidth](l){5-6}}%
{\end{tabular}}

\begin{document}
\CNlabelsubnoref{carvomenthone}{cis}
\CNlabelsubnoref{carvomenthone}{trans}
\begin{table}
    \centering
    \caption{Calculated and Experimental \oneh{} Shifts (in ppm) for carvomenthones \CNref{carvomenthone}}\label{tab:SI:carvomenthone1H}
    \begin{onehTable}{\CNrefsub{carvomenthone}{cis}}{\CNrefsub{carvomenthone}{trans}}
    2 & & 2.47 & 2.48 & 2.45 & 2.33 \\
    3 & \greek{\alpha} & 1.87 & 1.29 & \swapshift{1.87}{a} & 1.30 \\
    3 & \greek{\beta} & 1.70 & 2.12 & \range{1.60--1.75} & 2.10 \\
    10 & Me & 1.06 & 0.92 & 1.10 & \swapshift{1.01}{b} \\ \bottomrule
    \end{onehTable}
\end{table}
\end{document}

Answer 1

Simplifying your example a bit, I think you want something like

\documentclass{article}
\usepackage{booktabs,etoolbox,multirow,siunitx}
\robustify\itshape
\begin{document}
\begin{table}
  \sisetup{table-format = 1.2, detect-all}
  \begin{tabular}
    {
      >{\rule{1.2em}{0em}}
      r@{ }
      l
      S
      S
      S[table-space-text-post = \textsuperscript{a}]
      S[table-space-text-post = \textsuperscript{b}]
    }
    \toprule
      \multicolumn{2}{c}{\multirow{2}{*}{Position}} &
        \multicolumn{2}{c}{Computed} &
        \multicolumn{2}{c}{Experimental} \\
       & &
         {\textbf{201a}} & {\textbf{201b}} & {\textbf{201a}} & {\textbf{201b}} \\
       \cmidrule[\lightrulewidth](r){1-2}
         \cmidrule[\lightrulewidth](lr){3-4}
         \cmidrule[\lightrulewidth](l){5-6}
       2 &          & 2.47 & 2.48 &         2.45                       & 2.33 \\
       3 & $\alpha$ & 1.87 & 1.29 & \itshape 1.87{\textsuperscript{a}} & 1.30 \\
       3 & $\beta$  & 1.70 & 2.12 & {\bfseries 1.60--1.75}             & 2.10 \\
      10 & Me       & 1.06 & 0.92 &         1.10                      &
        \itshape 1.01{\textsuperscript{b}} \\
    \bottomrule
  \end{tabular}
\end{table}
\end{document}

The key here is to use \itshape to alter the formatting while retaining siunitx S column alignment, plus setting up so space for the superscript using table-space-text-post.

I've also cut down on the use of \multicolumn, as {...} will escape thing from siunitx more clearly. siunitx treats anything inside a wrapping set of braces as 'text', so for example

& {\bfseries 1.60--1.75} &

typesets the material without parsing the number: the alignment of 'text' is governed by the table-text-alignment key. (You could alternatively use \multicolumn and \tablenum, but that seems overly complex here.)

The table-space-text-post setting reserves 'some space' for additional material after the number. It needs to be wide enough when typeset to hold the largest item you will have after the text in a column. Usually, this is simply a case of picking something representative, although if the letter width varies a lot in your font then that may be an issue. I've used superscript a and b as they are the only things in those columns: I guess if you have an m or w to fit in you'll need to use them as they are wider!