## 4 Analysis and Discussion  
In this experiment, the pure rotational spectra of tert-butyl chloride and its isotopologues (\({}^{13}\)C, \({}^{37}\)Cl) were studied in the range of 1-18 GHz by cp-FTMW spectrometer and quantum chemical calculation. From the calculation results, it can be found that the rotational constants \(A>B=C\), and moments of inertia \(I\). \(a<I\). \(b=I\). \(c\), which clearly confirm that these molecules belong to prolate symmetric top. As shown in Table 2, a total of 107 transition lines were observed and assigned, of which 24 for (CH\({}_{3}\))\({}_{3}\)C\({}^{35}\)Cl, 22 for (CH\({}_{3}\))\({}_{3}\)C\({}^{37}\)Cl, 30 for (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{35}\)Cl and 31 for (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{37}\)Cl. The root-mean-square errors of fitted transitions are no more than 2.5 kHz, indicating that the transition frequencies measured by the experiment are in good agreement with the predicted frequencies. As shown in Figures 2 and 3, after 20,000 times of averaging, the maximal signal-to-noise ratio of the spectrum was up to 2000, so that the low-order rotational transitions of each species were completely captured.  
In this work, we measured the rotational energy level transition of (CH\({}_{3}\))\({}_{3}\)C\({}^{35}\)Cl, (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{35}\)Cl in the low frequency band and obtained their exact rotational parameters by fitting and analyzing the rotational transitions, as shown in Table 2. Some data of (CH\({}_{3}\))\({}_{3}\)C\({}^{35,37}\)Cl have been published in previous work. Due to the accurate measurement of the low frequency part, the calculation of the quadrupole coupling constants of isotopologues is better. The measurement results of rotational constants and quadrupole coupling constants are accurate, and their measurement accuracy can reach about 0.002%. The standard deviation of a single parameter is about five times less than that in the literature [24], which is helpful to provide a more accurate parameter standard for the quantum chemical calculation of the molecular structure. This is owing to improvements in the hardware design and new signal acquisition technology applied for our spectrometer. However, the measurement accuracy of centrifugal distortion is about 0.02%, but still better than the results in the literature [24; 25], although the lack of millimeter-wave spectral data could result in inaccurate fitting. It can be found that, compared to the central carbon isotopes (\({}^{12}\)C and \({}^{13}\)C), halogen isotopes (\({}^{35}\)Cl and \({}^{37}\)Cl) can result in bigger difference for both rotational and quadrupole coupling constants. (CH\({}_{3}\))\({}_{3}\)C\({}^{35}\)Cl and (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{35}\)Cl with the same \({}^{35}\)Cl isotope have close rotational constants and almost equal quadrupole coupling constants. Similar conclusion can be made for (CH\({}_{3}\))\({}_{3}\)C\({}^{37}\)Cl and (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{37}\)Cl. When comparing (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{35}\)Cl and (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{37}\)Cl, the rotational and quadrupole coupling constants are off about 64.462866 MHz and -14.25952 MHz respectively. For the rotational constants, the halogen isotope can make a difference mainly due to its far off the molecule's center of mass. For the quadrupole constants, charge distribution in the halogen nuclei results in the very different spectroscopic hyperfine splitting between (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{35}\)Cl and (CH\({}_{3}\))\({}_{3}\)\({}^{13}\)C\({}^{37}\)Cl.