有人能帮我一下吗？告诉我为什么table:S1和table:S3（参见下图）不在页面中央，就像table:S2？我在这里画了个空白。这是
以下是我用来制作此文档的 MWE：
\documentclass{book}
% *****************************************************************************
% Preamble
% *****************************************************************************
% Additional Packages
\usepackage[T1]{fontenc} % to deal with those pesky ASCII symbols!
\usepackage{siunitx} % use this package module for SI units
\usepackage{booktabs}
\usepackage{enumitem}
\usepackage{threeparttable}
\usepackage{lipsum}
\usepackage[version=4]{mhchem}
% *****************************************************************************
% *****************************************************************************
% End of Preamble
% *****************************************************************************
% *****************************************************************************
\begin{document} % Fun time!
% --> Custom commands <--
\newcommand{\tn}[1]{\textsuperscript{#1}}
\newcommand{\TN}[1]{\makebox[0pt][l]{\tn{#1}}}
% -- End of custom commands --
% ==> Case in point <==
\section{My first fully compiled MWE in TeX!}
\lipsum[1] % Generating dummy text!
\begin{threeparttable}[!htbp]
\caption{Experimentally determined spectroscopic parameters of the detected rotamer of PG}
\label{table:S1} % should go after \caption
\centering
\sisetup{table-align-text-post=false}
\begin{tabular}{@{\extracolsep{\fill}} l S[table-format=4.1(2)] S[table-format=3.4(2)] S[table-format=3.4(2)] }
\toprule
& A \tnote{a} & B & C \\
\midrule
\ce{^{13}C} (1)& 3075.3 (31)\tnote{b}& 733.2825 (30)& 697.0386 (27) \\
\ce{^{13}C} (2)& 3086.3 (15) & 736.5933 (22)& 699.4114 (17) \\
\ce{^{13}C} (4)& 3055.3 (22) & 736.9399 (26)& 698.2864 (24) \\
\ce{^{13}C} (5)& 3064.7 (23) & 731.8158 (29)& 693.8030 (27) \\
\ce{^{13}C} (6)& 3085.0 (27) & 728.5381 (35)& 692.0961 (30) \\
\ce{^{13}C} (7)& 3052.6 (32) & 732.9437 (36)& 694.8316 (36) \\
\ce{^{13}C} (8)& 3056.3 (35) & 737.5525 (40)& 699.6777 (51) \\
\ce{^{15}N} & 3042.1 (12) & 733.0221 (15)& 697.1036 (21) \\
\bottomrule
\end{tabular}
\begin{tablenotes}[flushleft]\footnotesize
\item[a] $A$, $B$, and $C$ are the rotational constants (in \si{MHz});
$\chi_{aa}$, $\chi_{bb}$, and $\chi_{cc}$ are elements of the $^{14}$N nuclear quadrupole coupling tensor (in \si{MHz}).
\item[b] Standard errors indicated in parentheses in units of the last digit.
\end{tablenotes}
\end{threeparttable}
\begin{table}[!htbp]
\centering
\caption{Observed frequencies and residuals (in \si{MHz}) for the observed transitions of \ce{^{13}C} (\ce{C_{10}}) isotopic species of phenylglycinol.}
\label{table:S2}
\sisetup{table-align-text-post=false}
\begin{tabular}{@{\hspace{\tabcolsep}\extracolsep{\fill}} cccccc S[table-format=5.4] S[table-format=1.4] }
\toprule
{J\textasciiacute} &
{K\textasciiacute$_{-1}$} &
{K\textasciiacute$_{+1}$} &
{J\textacutedbl} &
{K\textacutedbl$_{-1}$} &
{K\textacutedbl$_{+1} $} &
{$ \nu_{obs} $} &
{$ \nu_{obs} - \nu_{cal} $} \\
\midrule
5 & 1 & 5 & 4 & 1 & 4 & 7084.9419 & 0.0170 \\
5 & 0 & 5 & 4 & 0 & 4 & 7173.7289 & 0.0526 \\
5 & 2 & 4 & 4 & 2 & 3 & 7182.1213 & 0.0253 \\
5 & 1 & 4 & 4 & 1 & 3 & 7277.2026 & -0.0193 \\
6 & 0 & 6 & 5 & 0 & 5 & 8603.3344 & 0.0928 \\
6 & 2 & 5 & 5 & 2 & 4 & 8617.6872 & 0.0424 \\
6 & 1 & 5 & 5 & 1 & 4 & 8731.3175 & 0.0139 \\
7 & 0 & 7 & 6 & 0 & 6 & 10030.0312 & -0.0240 \\
7 & 2 & 6 & 6 & 2 & 5 & 10052.6810 & -0.0383 \\
7 & 1 & 6 & 6 & 1 & 5 & 10184.6218 & 0.0138 \\
8 & 0 & 7 & 7 & 0 & 7 & 11453.7239 & -0.0258 \\
8 & 2 & 7 & 7 & 2 & 6 & 11487.1502 & -0.0909 \\
\bottomrule
\end{tabular}
\end{table}
\begin{table}[!htbp]
\centering
\caption{Experimentally determined spectroscopic parameters of the detected rotamer of PG}
\label{table:S3} % should go after \caption
\sisetup{table-align-text-post=false}
\begin{tabular}{@{\hspace{\tabcolsep}\extracolsep{\fill}} c S[table-format=4.1(2)] S[table-format=3.4(2)] S[table-format=3.4(2)] }
\toprule
& A \tn{a} & B & C \\
\midrule
\ce{^{13}C} (1)& 3075.3 (31)\tn{b}& 733.2825 (30)& 697.0386 (27) \\
\ce{^{13}C} (2)& 3086.3 (15) & 736.5933 (22)& 699.4114 (17) \\
\ce{^{13}C} (4)& 3055.3 (22) & 736.9399 (26)& 698.2864 (24) \\
\ce{^{13}C} (5)& 3064.7 (23) & 731.8158 (29)& 693.8030 (27) \\
\ce{^{13}C} (6)& 3085.0 (27) & 728.5381 (35)& 692.0961 (30) \\
\ce{^{13}C} (7)& 3052.6 (32) & 732.9437 (36)& 694.8316 (36) \\
\ce{^{13}C} (8)& 3056.3 (35) & 737.5525 (40)& 699.6777 (51) \\
\ce{^{15}N} & 3042.1 (12) & 733.0221 (15)& 697.1036 (21) \\
\bottomrule
\end{tabular}
\raggedright\footnotesize
\begin{enumerate}[label=\tn{\alph*},leftmargin=*]
\item $A$, $B$, and $C$ are the rotational constants (in \si{MHz});
$\chi_{aa}$, $\chi_{bb}$, and $\chi_{cc}$ are elements of the $^{14}$N nuclear quadrupole coupling tensor (in \si{MHz}).
\item Standard errors indicated in parentheses in units of the last digit.
\end{enumerate}
\end{table}
\end{document}
多谢！
更新- 毕竟，这个问题还是有道理的！正确的 LaTeX 代码格式也是必要的！显然，一个空行可以带来很大的不同！到目前为止，这是我的幼稚结论。
我自己就经历过很多失败，并且我了解到如果你没有经历很多失败，那么你可能就没有发挥出你应该有的创造力 - 你没有发挥你的想象力。 - J. Backus
答案1
第一个表未居中（水平），因为您没有将
threeparttable环境嵌入环境内table。然后，移动\centering指令外部环境threeparttable。要理解为什么第三个表没有居中，有必要了解
\centering和\raggedright（以及\raggedleft）的工作原理。本质上，它们可以完成它们的工作在段落末尾\end{tabular}。在您的代码中，和之间没有空行，也没有其他类型的段落分隔符\raggedright。您可以（应该能够？）猜测\raggedright，而不是才是\centering（逻辑）段落末尾的有效内容。
这三个表格位于同一页上，均水平居中。
\documentclass{book}
\usepackage[margin=1in,letterpaper]{geometry}
\usepackage{array} % for \newcolumntype macro
\newcolumntype{C}{>{$}c<{$}}
\usepackage{siunitx}
\usepackage{booktabs}
\usepackage{enumitem}
\usepackage{threeparttable}
\usepackage[version=4]{mhchem}
\usepackage[skip=0.333\baselineskip]{caption}
\begin{document}
\newcommand{\tn}[1]{\textsuperscript{#1}}
\newcommand{\TN}[1]{\makebox[0pt][l]{\tn{#1}}}
\begin{table}[h!]
\centering % Now placed *before* \begin{threeparttable}
\begin{threeparttable}[!htbp]
\caption{Experimentally determined spectroscopic parameters of the detected rotamer of PG}
\label{table:S1}
\sisetup{table-align-text-post=false}
\begin{tabular}{@{} l S[table-format=4.1(2)]
S[table-format=3.4(2)]
S[table-format=3.4(2)] @{}}
\toprule
& {$A$\,\tnote{a}} & {$B$} & {$C$} \\
\midrule
\ce{^{13}C} (1)& 3075.3 (31)\tnote{b}& 733.2825 (30)& 697.0386 (27) \\
\ce{^{13}C} (2)& 3086.3 (15) & 736.5933 (22)& 699.4114 (17) \\
\ce{^{13}C} (4)& 3055.3 (22) & 736.9399 (26)& 698.2864 (24) \\
\ce{^{13}C} (5)& 3064.7 (23) & 731.8158 (29)& 693.8030 (27) \\
\ce{^{13}C} (6)& 3085.0 (27) & 728.5381 (35)& 692.0961 (30) \\
\ce{^{13}C} (7)& 3052.6 (32) & 732.9437 (36)& 694.8316 (36) \\
\ce{^{13}C} (8)& 3056.3 (35) & 737.5525 (40)& 699.6777 (51) \\
\ce{^{15}N} & 3042.1 (12) & 733.0221 (15)& 697.1036 (21) \\
\bottomrule
\end{tabular}
\begin{tablenotes}[flushleft]
\footnotesize
\item[a] $A$, $B$, and $C$ are the rotational constants (in \si{\mega\hertz}).
$\chi_{aa}$, $\chi_{bb}$, and $\chi_{cc}$ are elements of the $^{14}$N nuclear
quadrupole coupling tensor (in \si{\mega\hertz}).
\item[b] Standard errors indicated in parentheses in units of the last digit.
\end{tablenotes}
\end{threeparttable}
\end{table}
\begin{table}[!h]
\centering
\caption{Observed frequencies and residuals (in \si{MHz}) for the observed transitions of \ce{^{13}C} (\ce{C_{10}}) isotopic species of phenylglycinol.}
\label{table:S2}
\sisetup{table-align-text-post=false}
\begin{tabular}{@{} CCCCCC S[table-format= 5.4]
S[table-format=-1.4] @{}}
\toprule
J' & K'_{-1} & K'_{+1} & J'' & K''_{-1} & K''_{+1}
& {$\nu_{\mathrm{obs}}$}
& {$\nu_{\mathrm{obs}} - \nu_{\mathrm{cal}}$} \\
\midrule
5 & 1 & 5 & 4 & 1 & 4 & 7084.9419 & 0.0170 \\
5 & 0 & 5 & 4 & 0 & 4 & 7173.7289 & 0.0526 \\
5 & 2 & 4 & 4 & 2 & 3 & 7182.1213 & 0.0253 \\
5 & 1 & 4 & 4 & 1 & 3 & 7277.2026 & -0.0193 \\
6 & 0 & 6 & 5 & 0 & 5 & 8603.3344 & 0.0928 \\
6 & 2 & 5 & 5 & 2 & 4 & 8617.6872 & 0.0424 \\
6 & 1 & 5 & 5 & 1 & 4 & 8731.3175 & 0.0139 \\
7 & 0 & 7 & 6 & 0 & 6 & 10030.0312 & -0.0240 \\
7 & 2 & 6 & 6 & 2 & 5 & 10052.6810 & -0.0383 \\
7 & 1 & 6 & 6 & 1 & 5 & 10184.6218 & 0.0138 \\
8 & 0 & 7 & 7 & 0 & 7 & 11453.7239 & -0.0258 \\
8 & 2 & 7 & 7 & 2 & 6 & 11487.1502 & -0.0909 \\
\bottomrule
\end{tabular}
\end{table}
\begin{table}[!htbp]
\centering
\caption{Experimentally determined spectroscopic parameters of the detected rotamer of PG}
\label{table:S3}
\sisetup{table-align-text-post=false}
\begin{tabular}{@{} l S[table-format=4.1(2)]
S[table-format=3.4(2)]
S[table-format=3.4(2)] @{}}
\toprule
& {$A$\,\tn{a}} & {$B$} & {$C$} \\
\midrule
\ce{^{13}C} (1)& 3075.3 (31)\tn{b}& 733.2825 (30)& 697.0386 (27) \\
\ce{^{13}C} (2)& 3086.3 (15) & 736.5933 (22)& 699.4114 (17) \\
\ce{^{13}C} (4)& 3055.3 (22) & 736.9399 (26)& 698.2864 (24) \\
\ce{^{13}C} (5)& 3064.7 (23) & 731.8158 (29)& 693.8030 (27) \\
\ce{^{13}C} (6)& 3085.0 (27) & 728.5381 (35)& 692.0961 (30) \\
\ce{^{13}C} (7)& 3052.6 (32) & 732.9437 (36)& 694.8316 (36) \\
\ce{^{13}C} (8)& 3056.3 (35) & 737.5525 (40)& 699.6777 (51) \\
\ce{^{15}N} & 3042.1 (12) & 733.0221 (15)& 697.1036 (21) \\
\bottomrule
\end{tabular}
\raggedright % leave the preceding line blank!
\footnotesize
\begin{enumerate}[label=\tn{\alph*},leftmargin=*]
\item $A$, $B$, and $C$ are the rotational constants (in \si{MHz}).
$\chi_{aa}$, $\chi_{bb}$, and $\chi_{cc}$ are elements of the $^{14}$N nuclear
quadrupole coupling tensor (in \si{\mega\hertz}).
\item Standard errors indicated in parentheses in units of the last digit.
\end{enumerate}
\end{table}
\end{document}