## 3 Results  
The pure rotational spectra of tert-butyl chloride and its isotopologues in natural abundance were measured by cp-FTMW spectrometer in the frequency range of 1-18 GHz. The quanta \(J\) = 1\(\leftarrow\)0 to \(J\) = 3\(\leftarrow\)-2 rotational energy level transitions of four substances ((CH\({}_{3}\))\({}_{3}\)C\({}^{35}\)Cl, (CH\({}_{3}\))\({}_{3}\)C\({}^{37}\)Cl, (CH\({}_{3}\))\({}_{3}\)\({}_{3}\)\({}^{13}\)C\({}^{35}\)Cl, (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{37}\)Cl have been observed, as shown in Figure 3. Figure 4 displays the hyperfine splitting spectra of \(J\) = 1\(\leftarrow\)0 transitions arising from Cl nuclei of each species. By using [PERSON]'s SPCAT/SPFIT (spectral fitting analysis software) [29], the observed rotational spectral lines were assigned and high precision spectral parameters (including rotational constant \(A\), \(B\), \(C\), centrifugal distortion constant \(D_{J}\), \(D_{JK}\) and quadrupole coupling constant \(eQq\)) of each species were well determined, as shown in Table 2.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{5}{c}{**Tert-Butyl Chloride and Its Isotopologues**} \\ \cline{2-5}
**Spectral Parameter** & **(CH\({}_{3}\))\({}_{3}\)C\({}^{35}\)Cl ** & **(CH\({}_{3}\))\({}_{3}\)C\({}^{37}\)Cl ** & **(CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{35}\)Cl ** & **(CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{37}\)Cl ** \\ \hline \(A\)(MHz) ** & 4557.36740 & 4557.70485 & 4571.70485 \\ \(B\)(MHz)  & 3035.35346 & 2970.78357 & 3025.45983 & 2960.51347 \\ \(C\)(MHz)  & 3035.35068 & 2970.78091 & 3025.45734 & 2960.51109 \\ \(I.\)\(a\)(amu-Å\({}^{2}\))  & 110.892750 & 110.892750 & 110.544977 & 110.544977 \\ \(I.\)\(b\)(amu-Å\({}^{2}\))  & 166.497580 & 170.116399 & 167.042048 & 170.706538 \\ \(I.\)\(c\)(amu-Å\({}^{2}\))  & 166.497732 & 170.116552 & 167.042185 & 170.706575 \\ \(P.\)\(a\)(amu-Å\({}^{2}\)/s)  & 111.051277 & 114.670097 & 111.769630 & 115.434120 \\ \(P.\)\(b\)(amu-Å\({}^{2}\)/s)  & 55.446453 & 55.446453 & 55.272556 & 55.272556 \\ \(P.\)\(c\)(amu-Å\({}^{2}\)/s)  & 55.44645301 & 55.446301 & 55.5272419 & 55.272419 \\ \hline \end{tabular}
\end{table}
Table 1: The rotational constant, moment of inertia and rotational angular momentum of (CH\({}_{3}\))\({}_{3}\)C\({}^{35}\)Cl, (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{35}\)Cl, (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{37}\)Cl predicted by Gaussian03 [27] and PMIFST [28].  
Figure 2: Three-dimensional structure diagram (upper left) and plane projection (A-B, A-C, B-C) of tert-butyl chloride calculated by Gaussian03 software at the MP2/6-311++G (D, P) theory level.  
Figure 4: Diagram of measured hyperfine splitting of ground state (\(J\) = 1\(\leftarrow\)0) energy level transitions (\(J^{\prime}\)\({}_{K^{\prime}F}\)\(\leftarrow\)\(J^{\prime}\)\({}_{K^{\prime}F}\)) of (CH\({}_{3}\))\({}_{3}\)C\({}^{35}\)Cl, (CH\({}_{3}\))\({}_{3}\)C\({}^{37}\)Cl, (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{35}\)Cl, (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{37}\)Cl.  
Figure 3: Diagram of the \(J\) = 1\(\leftarrow\)0 to \(J\) = 3\(\leftarrow\)2 rotational transitions of (CH\({}_{3}\))\({}_{3}\)C\({}^{35}\)Cl, (CH\({}_{3}\))\({}_{3}\)C\({}^{37}\)Cl, (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{35}\)Cl, (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{37}\)Cl detected in the 1–18 GHz frequency region.