Frequency of rotational transitions

Fig. 4.5 Frequencies of rotational transitions for molecules in the o "0 vibrational level of a diatomic molecule.

The frequencies of rotational transitions are much smaller than vibrational frequencies, which means that the rotational motion is slower than the vibrational one. For a free molecule, the period of rotational motion is within 10 12-10 9 s. In condensed media the rotational motion is even slower, its period being respectively greater. At this stage it is more correct to speak of the relaxation time of the molecules. The latter essentially depends on the phase state of the medium. For example, in liquid water the relaxation time of molecular dipoles in an external electric field is about 10 11 s, whereas in ice (at 0°C) it is — 1 () 5 s. 

The frequencies of rotational transitions can be determined from the microwave spectrum with great precision and reproducibility. This is illustrated in Table 1 by some frequencies of rotational transitions of dimethyl sulphone as obtained in two independent studies. At the same time, it is also true that microwave spectroscopy is capable to deteimiine only relatively simple structures. Concerning the sulphone molecules, the isotopic species occuring in their natural abundances do not provide sufficient data for a complete structural determination even of the simplest molecules. For improving this situation one possibility is to synthesize and investigate further isotopic... 

Frequencies of rotational transitions of dimethyl sulphone from two independent microwave spectroscopic studies... 

Table 5.1 Frequencies and wavenumbers of rotational transitions of CO observed in the millimetre wave region...

Figure 8.15. Correlation diagram between levels of a rigid rotor K = 0 (water dimer with Cs symmetry in the nontunneling limit), a rotor with internal rotation of the acceptor molecule around the C2 axis (permutation-inversion group G ), and group G16. The arrangement of levels is given in accordance with the hypothesis by Coudert et al. [1987], The arrows show the allowed dipole transitions observed in the (H20)2 spectrum. The pure rotational transitions E + (7 = 0) - E (J = 1) and E (7 = 1) <- E + (/ = 2) have frequencies 12 321 and 24 641 MHz, respectively. The frequencies of rotationtunneling transitions in the lower triplets AI (7 = 1) <- A,+ (7 = 2) and A," (7 = 3) <- A,+ (7 = 4) are equal to 4863 and 29 416 MHz. The transitions B2(7 = 0)<- B2(7 = l) and BJ(7 = 2) <- B2 (7 = 3) with frequencies 7355 and 17123 MHz occur in the higher multiplets.

The microwave magnetic resonance spectrum of SO was recorded at a resonance frequency of 8762 MHz and involved Zeeman components of rotational transitions between the N = 1 and 2 rotational levels. We first describe the effective Hamiltonian and its matrix elements, and then the observed spectrum and its analysis. 

Linear hydrocarbon radicals have been the subject of intensive laboratory spectroscopic and radio-astronomical research since the early 1980s. In recent years, a considerable number of rotational spectroscopic studies of medium to longer hydrocarbon chains such as C5H, CeH, CgH, and ChH have been carried out using a pulsed molecular beam FTMW spectrometer. The high resolution offered by such a spectrometer allowed the detection of the hyperfine sphtting of rotational transitions. These measurements improved fine and hyperfine coupling constants and provided rest frequencies with accuracies better than 0.30 km s in equivalent radial velocity up to 50 GHz. Indeed, some of the small C H radicals with n < 9 have subsequently been detected in space, in molecular cloud cores, and in certain circumstellar shells. These hydrocarbon chains are among the most abundant reactive space molecules known. 

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