## 1 Introduction  
In the field of molecular rotational spectroscopy, high-resolution laboratory experiments are mostly combined with quantum chemical calculations to facilitate spectroscopic assignments for researchers engaged in chemical detection [1; 2; 3; 4; 5]. The majority of molecules have a unique set of rotational spectra in the microwave to terahertz band, i.e., the so-called molecular fingerprints. On the basis of quantum chemical calculation, molecular rotational spectroscopy can fit extremely accurate three-dimensional structures of free molecules and describe the local electric field gradient distribution caused by electron arrangement [6; 7]. Therefore, it is quite important to capture pure rotational high-precision energy level transitions of molecules. Generally, the Fourier transform detection technology is used to quickly capture the hyperfine rotational spectra of gaseous substances. At present, two types of Fourier transform spectrometers are mainly used in the microwave band, one is the narrowband microwave spectrometer based on the Fabry-Perot cavity, while the other is the broadband microwave spectrometer based on chirped-pulse linear frequency modulation. The narrowband microwave spectrometer was designed and built by Professor [PERSON] of the University of Illinois in the early 1980s [8], with high sensitivity and high resolution. The broadband microwave spectrometer was successfully developed by the [PERSON]'s team at the University of Virginia in 2008 [9]. Compared with the narrow-band microwave spectrometer, the single scanning bandwidth of the broadband type can be improved by several orders of magnitude, but with lower resolution and sensitivity. Nowadays, many laboratories use both broadband and narrowband microwave spectrometers to improve molecular detection efficiency. This combination means that firstly samples are quickly scanned by a broadband microwave spectrometer, and then are rescanned by a narrowband microwave spectrometer in specific frequency regions with high resolution. In recent years, with the continuous updating of microwave electronic devices [10; 11; 12], Fourier transform microwave spectroscopy detection technology has developed rapidly. Researchers not only improved the scanning bandwidth, but also took the high sensitivity and high resolution into account at the same time, and provided a series of auxiliary sample preparation technologies for spectrometers, such as laser photolysis [13] and high voltage discharge [14]. In addition to dealing with problems such as molecular structure and intermolecular interactions [15; 16], FTMW spectroscopy is also applied to study the dynamic process of complex chemical reaction systems [17], and sensitively detect chiral compounds [18]. Therefore, it plays a significant role in the fields of chemical analysis, pharmaceutical detection and radio astronomy [19; 20]. Tert-butyl chloride is a symmetric top molecule with a high degree of symmetry (C\({}_{3\text{v}}\) symmetry). The existence of a large number of similar trimethyl compounds has made relevant studies attractive [21]. Due to the nuclear coupling of halogen and the torsional oscillation or internal rotation of methyl groups, their microwave spectra become complex. In 1950, [PERSON] and [PERSON] first studied the millimeter wave rotational spectra of tert-butyl bromide, tert-butyl chloride and tert-butyl iodine, and obtained their rotational constants, moments of inertia and carbon-halide bond lengths [22]. Subsequently, [PERSON] and [PERSON] successively analyzed the hyperfine structure of nuclear quadrupole coupling of (CH\({}_{3}\))\({}_{3}\)C [37; 35]Cl, and gave the quadrupole coupling constants of chlorine [23; 24]. What is more, [PERSON] and her coworkers also analyzed the rotational spectra of tert-butyl chloride, tert-butyl bromide and their isotopologues (\({}^{13}\)C, D\({}_{3}\) full deuterium) and redetermined the bond length of C-X (x = Cl, Br) [25].