Longtable with specific columnwidth

Question

I have been trying to resolve this for days and now I surrender. I need a table that can span multiple pages, so I started using longtable instead of tabularx. I tried to do a fixed width column (the first one) but I still get it too wide. when I try to shrink it to 3 or 4cm, it doesn't work.

\section{Main structural statistical parameters for the conformers calculated}
\begin{landscape}
\setlength\LTleft{-2cm}
\setlength\LTright{-4cm}
     \begin{longtable}{r{4cm} |c|c|c|c|c|}
\caption[Chemical shift list of PG-T4 in \ce{H2O}, pH 3.5] {Main structural statistical parameters for the ensemble of the 20 lowest target function conformers calculated for PG-T4, PG-C6C15, PG-C8C13 in \ce{H2O} and 30 mM DPC, pH 3.5}\\
\toprule
 &
\multicolumn{3}{c}{\textbf{\ce{H2O}}}   &
\multicolumn{3}{c}{\textbf{30 mM DPC}}   \\
\cmidrule(lr){2-4}
\cmidrule(lr){5-7}
          & 
        \textbf{PG-T4} & 
        \textbf{PG-C6C15} & 
        \textbf{PG-C8C13} & 
        \textbf{PG-T4} & 
        \textbf{PG-C6C15} & 
        \textbf{PG-C8C13} 
\\

\endfirsthead
\toprule
         & &
        \textbf{PG-T4}  & 
        \textbf{PG-C6C15}  & 
        \textbf{PG-C8C13}& 
        \textbf{PG-T4}  & 
        \textbf{PG-C6C15}  & 
        \textbf{PG-C8C13}
        \\
\midrule
\endhead
\endhead
\bottomrule
\multicolumn{7}{r}{\footnotesize\itshape continue on the next page} % <-- added
\endfoot
\endlastfoot

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Upper Limit distance restraints (from NOEs)             &       &       &       &       &       &       \\  
        \tab Total  &   numbers &   numbers &   numbers &   numbers &   numbers &   numbers \\  
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    \bottomrule
\end{longtable}
\end{landscape}
\clearpage

If you can help me to find what's happening here, I would be very thankful.

I tried your solutions, @koleygr and this is the result:

Answer 1

with ltablex, which is combination of longtable and tabularx, you can write long table in tabularx format.

an example (based on mwe in koleygr answer) anticipated that "numbers" will ones be real numbers, for which i suggest to use S column type from siunitx package, is:

\documentclass{article}
\usepackage{geometry}
\usepackage{pdflscape}
\usepackage{booktabs, makecell, ltablex, tabularx}
\renewcommand\theadfont{\small\bfseries}
\renewcommand\theadgape{}
\keepXColumns
\usepackage{mhchem}
\usepackage{siunitx}

\begin{document}
    \begin{landscape}
    \setlength\LTcapwidth{\linewidth}
    \begin{tabularx}{\linewidth}{>{\raggedright\arraybackslash}X *{6}{S[table-format=2.3]}}
\caption[Chemical shift list of PG-T4 in \ce{H2O}, pH 3.5]
        {Main structural statistical parameters for the ensemble of the 20 lowest target function conformers calculated for PG-T4, PG-C6C15, PG-C8C13 in \ce{H2O} and 30 mM DPC, pH 3.5}
\label{tab:long-table}  \\
    \toprule
    &
\multicolumn{3}{c}{\thead{\ce{H2O}}}    &   \multicolumn{3}{c}{\thead{30 mM DPC}}   \\
    \cmidrule(lr){2-4}  \cmidrule(lr){5-7}
        & {\thead{PG-T4}}   & {\thead{PG-C6C15}}    & {\thead{PG-C8C13}}
        & {\thead{PG-T4}}   & {\thead{PG-C6C15}}    & {\thead{PG-C8C13}}            \\
    \midrule
\endfirsthead
\caption{Main structural statistical parameters for the ensemble of the 20 lowest target function conformers calculated for PG-T4, PG-C6C15, PG-C8C13 in \ce{H2O} and 30 mM DPC, pH 3.5 (cont.)}\\
    \toprule
    &
\multicolumn{3}{c}{\thead{\ce{H2O}}}    &   \multicolumn{3}{c}{\thead{30 mM DPC}}   \\
    \cmidrule(lr){2-4}  \cmidrule(lr){5-7}
        & {\thead{PG-T4}}   & {\thead{PG-C6C15}}    & {\thead{PG-C8C13}}
        & {\thead{PG-T4}}   & {\thead{PG-C6C15}}    & {\thead{PG-C8C13}}            \\
    \midrule
\endhead
    \midrule[\heavyrulewidth]
\multicolumn{7}{r}{\footnotesize\itshape continue on the next page}                 \\
\endfoot
    \bottomrule
\endlastfoot
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Upper Limit distance restraints (from NOEs)
        &           &           &           &           &           &               \\
Total   & 12.345    & 22.345    & 32.345    & 42.345    & 52.345    & 62.345        \\
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    \end{tabularx}
\end{landscape}

\end{document}

after three times compilation you will obtain:

Answer 2

1) There was 2 problems in the parameters of columntypes that will be used

First: less columns (6 instead of 7)

Second: r column doesn't take argument of length (except if you redefined it first) I used p columntype that takes argument and breaks the lines automatically (You may try m instead or >{\arraybackslash\raggedleft}p{4cm} if you really need it aligned right)

2) There was an extra separator in last line

\documentclass{article}
\usepackage{lscape}
\usepackage{longtable}
\usepackage{array}
\usepackage{mhchem}
\usepackage{booktabs}
\def\tab{}
\begin{document}
\section{Main structural statistical parameters for the conformers calculated}
\begin{landscape}
\setlength\LTleft{-2cm}
\setlength\LTright{-4cm}
     \begin{longtable}{p{4cm} cccccc}
\caption[Chemical shift list of PG-T4 in \ce{H2O}, pH 3.5] {Main structural statistical parameters for the ensemble of the 20 lowest target function conformers calculated for PG-T4, PG-C6C15, PG-C8C13 in \ce{H2O} and 30 mM DPC, pH 3.5}\\
\toprule
 &
\multicolumn{3}{c}{\textbf{\ce{H2O}}}   &
\multicolumn{3}{c}{\textbf{30 mM DPC}}   \\
\cmidrule(lr){2-4}
\cmidrule(lr){5-7}
          & 
        \textbf{PG-T4} & 
        \textbf{PG-C6C15} & 
        \textbf{PG-C8C13} & 
        \textbf{PG-T4} & 
        \textbf{PG-C6C15} & 
        \textbf{PG-C8C13} 
\\

\endfirsthead
\toprule
         & 
        \textbf{PG-T4}  & 
        \textbf{PG-C6C15}  & 
        \textbf{PG-C8C13}& 
        \textbf{PG-T4}  & 
        \textbf{PG-C6C15}  & 
        \textbf{PG-C8C13}\\
\midrule
\endhead
\endhead
\bottomrule
\multicolumn{7}{r}{\footnotesize\itshape continue on the next page} % <-- added
\endfoot
\endlastfoot
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Upper Limit distance restraints (from NOEs)             &       &       &       &       &       &       \\  
        \tab Total  &   numbers &   numbers &   numbers &   numbers &   numbers &   numbers \\  
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    \bottomrule
\end{longtable}
\end{landscape}
\clearpage
\end{document}