Alignment of text in siunitx package

Question

I made a table using siunitx package. In one of the column there is text depicting chemical formulae. Is it possible to align only this column to the left.

\documentclass{article}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{booktabs}
\usepackage{siunitx}
\begin{document}
\begin{table}
\caption{ Comparison of new data to previously reported \label{Transition_delG_HD} }
\begin{tabular}{SSSSSSSS}
    \toprule

    &   \multicolumn{3}{c}{${\alpha}$} & & \multicolumn{3}{c}{$\beta$}   \\ \cline{2-4} \cline{6-8} 
    &   \multicolumn{1}{c}{$Our$} & \multicolumn{1}{c}{$Ref. $} &  {$\Delta$} & & \multicolumn{1}{c}{$Our$} & \multicolumn{1}{c}{$Ref. $} & {$\Delta$}\\  \cline{2-4} \cline{6-8}

         {C\textsubscript{2}H\textsubscript{5}OH}     & 25    & 25.5  & -0.5  &       & 25    & 25.5  & -0.5 \\
            {CH\textsubscript{3}OH}       & 26    & 26.12 & -0.12 &       & 26    & 26.12 & -0.12 \\
          {H\textsubscript{2}O}        & 24    & 24.23 & -0.23 &       & 24    & 24.23 & -0.23 \\
            {HF}        & 24    & 24.23 & -0.23 &       & 24    & 24.23 & -0.23 \\


    \bottomrule
    \multicolumn{6}{l}{$\Delta^{a}$ : From Ref. 5} \\

\end{tabular}
\end{table} 
\end{document}

Following image shows the issue.

The left-most column with the chemical formulae, I was hoping it can be left aligned.

Answer 1

Here are some improvements to your table: you don't need an empty column to have distinct \clines if you replace them with \cmidrules and crop them on both sides. Also, as daleif pointed, the first column can to be left-aligned rather than S type. You have three different number formats for your S columns, and it's better to differentiate them. Last, don't end a tabular by a \\: it adds an unwanted vertical spacing.

I loaded the cellspace package to give some vertical padding to cells in the first column; the column specifier has to be prefixed with the letter S in general, and Cif siunitx, as here, is loaded. I aalso loade the chemformula to simplify typing of chemical compounds.

\documentclass{article}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{array, booktabs, cellspace}
\setlength\cellspacetoplimit{3pt}
\setlength\cellspacebottomlimit{3pt}
\usepackage{siunitx}
\usepackage{chemformula}

\begin{document}

\begin{table}
\centering
\caption{ Comparison of new data to previously reported \label{Transition_delG_HD} }
\begin{tabular}{Cl*{2}{S[table-format=2.0]S[table-format=2.2]S[table-format=-1.2]}}
    \toprule
    & \multicolumn{3}{c}{${\alpha}$} & \multicolumn{3}{c}{$\beta$} \\
 \cmidrule(lr){2-4} \cmidrule(lr){5-7}
    & {Our} & {Ref. } & {$\Delta$} & {Our} & {Ref.} & {$\Delta$}\\
 \cmidrule(lr){2-4} \cmidrule(lr){5-7}
         \ch{C2H5OH} & 25 & 25.5 & -0.5 & 25 & 25.5 & -0.5 \\
            \ch{CH3OH} & 26 & 26.12 & -0.12 & 26 & 26.12 & -0.12 \\
          \ch{H2O} & 24 & 24.23 & -0.23 & 24 & 24.23 & -0.23 \\
            HF & 24 & 24.23 & -0.23 & 24 & 24.23 & -0.23 \\
    \bottomrule
    \addlinespace
    \multicolumn{6}{l}{\footnotesize$\Delta^{a}$ : From Ref. 5}
    \end{tabular}
\end{table}

\end{document}