NoteNew measurements and global analysis of rotational spectra of Cl-, Br-, and I-benzene: Spectroscopic constants and electric dipole moments
Introduction
The first studies of the fundamental single halogen benzene derivatives by rotational spectroscopy were carried out over 40 years ago, Refs. [1], [2], [3], [4] for F-, Cl-, Br-, and I-benzene, respectively. Many further investigations have been made since then including room-temperature cm-wave studies of hyperfine resolved transitions for the three heaviest molecules [5], [6]. Nevertheless, it is only for fluorobenzene [7] that the high-resolution results from contemporary supersonic expansion Fourier transform microwave spectroscopy (FTMW) have been combined with high-J measurements in the mm-wave (MMW) rotational spectrum. For chlorobenzene some MMW [8], but not FTMW results are available, whereas for bromobenzene [9], [10] and iodobenzene [10] the FTMW and not the MMW spectrum has been studied. We presently systematically extend the experimental information to allow determination of spectroscopic constants for all monohalogenobenzenes at the level of precision allowed by contemporary rotational spectroscopy.
Section snippets
Experimental details
All measurements were carried out on the two rotational spectrometers in Warsaw, the supersonic expansion FTMW spectrometer [11] and the broadband MMW spectrometer [12]. Measurements for chlorobenzene were made up to 312 GHz, while for bromo- and iodobenzene up to 220 GHz, corresponding to maximum values of J″ of 125, 129, and 164, respectively, for the three molecules. Stark effect measurements were carried out using the electrode arrangement designed for obtaining a uniform electric field under
Rotational spectrum
The high-J rotational spectra of the studied monohalogenobenzene molecules show features typical of planar molecules, as illustrated for iodobenzene in Fig. 1. The properties of the type-II+ bands visible in this spectrum are well known [16], [17] and allow ready assignment of the mm-wave spectrum, as has previously been used to advantage for chlorobenzene [8] and fluorobenzene [7]. The measurements of the mm-wave rotational spectrum for chlorobenzene are presently extended up to 314 GHz, while
Excited vibrational states
All of the studied molecules have several low-frequency vibrational modes, so that in the mm-wave spectrum the prominent high-J band for the ground state is accompanied by several vibrational satellite bands, as visible for iodobenzene in Fig. 1. The lowest frequency normal modes are due to the motion of the halogen relative to the phenyl ring and are: the out-of-plane bend of the halogen in relation to the ring, ν30(B2), the corresponding in-plane bend, ν24 (B1), and the CX stretch, ν11(A1).
Electric dipole moments
Electric dipole moment for each molecule was determined by performing Stark shift measurements on many different Stark lobes belonging to several hyperfine components of rotational transitions with low values of Ka. The results are summarised in Table 3 and the primary data files are in Table S9. The exact agreement between the dipole moment values derived for the two isotopologues of bromobenzene confirms self-consistency of the experimental method. The current results are compared in Fig. 2
Conclusion
The present work reports the hitherto most precise rotational, centrifugal distortion, and nuclear quadrupole splitting constants for the ground states of chlorobenzene, bromobenzene and iodobenzene. The new values have been derived by combining new measurements of rotational transitions with the use of global fits to all data available from rotational spectroscopy. The ground state estimates of the ratios of nuclear quadrupole moments for the isotopic nuclei given by χaa(35Cl)/χaa(37Cl) =
Acknowledgment
Financial support from the Institute of Physics of the Polish Academy of Sciences is gratefully acknowledged.
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