I need some quick help figuring out how to center text in a `longtable` where I use `bigdelim/multirow`.

An MWE:

```
\documentclass[11pt,letterpaper,twoside]{book}
\usepackage[utf8]{inputenc}

%---------- Page Style ----------
\usepackage[top=2.5cm,bottom=2.5cm,left=2.5cm,right=2.5cm]{geometry}
%------------ Tables ------------
\usepackage{dcolumn}
\usepackage{longtable}
\usepackage{multirow,bigdelim}                                          % for curly braces in tables
\usepackage{booktabs}                                                   % redefines thickness of lines for table frames
%-------- Chemistry -------------
\usepackage[version=4]{mhchem}

%--------------- END OF PREAMBLE ---------------------%

\begin{document}

\begin{longtable}{@{} l c c c c @{}}
\caption[]{Constants and parameters (modern-day standards).} \\

\hline \addlinespace
Parameter & Definition & Value & Units & Ref \\ \midrule
$R$ & ideal gas law constant & 8.314 & $m^3$Pa/Kmol & \\
$\text{Pa}_\text{atm}$ & pascals per atmosphere & $1.023\times10^5$& Pa & \\
$\kappa_\text{0,O2}$ & \rdelim\}{5}{250pt}[ Henry's Law constants at $T_0$ ]  \ldelim\{{5}{35pt}[] & $1.2\times10^{-5}$ & mol/m$^{3}$Pa & S15\\ 
$\kappa_\text{0,CO2}$ &    & $3.3\times10^{-4}$ & mol/m$^{3}$Pa & S15\\ 
$\kappa_\text{0,CH4}$ &    & $1.4\times10^{-5}$ & mol/m$^{3}$Pa & S15\\ 
$\kappa_\text{0,N2}$  &    & $6.4\times10^{-6}$ & mol/m$^{3}$Pa & S15\\ 
$\kappa_\text{0,NH3}$ &    & $5.9\times10^{-1}$ & mol/m$^{3}$Pa & S15\\  
$D_\text{O2}$ & \rdelim\}{5}{250pt}[ Diffusion constants ]  \ldelim\{{5}{22pt}[]& $1.78\times10^{-5}$ &  cm/s & CRC\\ 
$D_\text{CO2}$ &    & $1.45\times10^{-5}$ & cm/s & CRC\\ 
$D_\text{CH4}$ &    & $1.55\times10^{-5}$ & cm/s & CRC\\ 
$D_\text{N2}$ &     & $2\times10^{-5}$ & cm/s & CRC\\ 
$D_\text{NH3}$ &    & $1.5\times10^{-5}$ & cm/s & CRC\\
$K_\text{O,rm}$ & half-saturation uptake values for \ce{O2} in remineralization & $12\times10^{-6}$ & mol/L & L17\\
\bottomrule
\end{longtable}

```

For instance, I want the text "Henry's Law constants" to be centered between the two curly braces.