Hyperfine structure, rotational spectra

Figure Bl.4.9. Top rotation-tunnelling hyperfine structure in one of the flipping inodes of (020)3 near 3 THz. The small splittings seen in the Q-branch transitions are induced by the bound-free hydrogen atom tiiimelling by the water monomers. Bottom the low-frequency torsional mode structure of the water duner spectrum, includmg a detailed comparison of theoretical calculations of the dynamics with those observed experimentally [ ]. The symbols next to the arrows depict the parallel (A k= 0) versus perpendicular (A = 1) nature of the selection rules in the pseudorotation manifold.

The longest wavelengths of the dectromagnetic spectrum are sensitive probes of molecular rotation and hyperfine structure. An important application is radio astronomy (23—26), which uses both radio and microwaves for chemical analysis on galactic and extragalactic scales. Herein the terrestrial uses of microwave spectroscopy are emphasized (27—29). 

The microwave spectra of 1//-benzotriazole and its N-D isotopomer have been studied in a heated cell. The molecule is planar. Due to the quadrupole coupling effects of the N nuclei, no hyperfine structures are observed. The dipole moment of benzotriazole obtained by microwave is 4.3 D, which is in agreement with the value determined in solution. The rotational spectrum is also assigned <93JSP(161)136>. 

In order to assign the Zeeman patterns for the three lowest rotational levels quantitatively, one must determine the spacings between the rotational levels, and the values of g/and gr-In the simplest model which neglects centrifugal distortion, the rotation spacings are simply B0. /(./ + 1) this approximation was used by Brown and Uehara [10], who used the rotational constant B0 = 21295 MHz obtained by Saito [12] from pure microwave rotational spectroscopy (see later in the next chapter). The values of the g-factors were found to be g L = 0.999 82, gr = —(1.35) x 10-4. Note that because of the off-diagonal matrix elements (9.6), the Zeeman matrices (one for each value of Mj) are actually infinite in size and must be truncated at some point to achieve the desired level of accuracy. In subsequent work Miller [14] observed the spectrum of A33 SO in natural abundance 33 S has a nuclear spin of 3/2 and from the hyperfine structure Miller was able to determine the magnetic hyperfine constant a (see below for the definition of this constant). 

This expression tells us that for a rotational transition ATV = 1, the selection rule for J is A J = 0, 1, with the A J = ATV components being the strongest. The zero-field swept-frequency rotational transitions are shown in figure 9.24 if there is no hyperfine structure, the spectrum will consist of a main doublet, with a weaker A J = 0 component. 

Figure 9.37. Section of the FIR laser magnetic resonance spectrum of CoFI (34) arising from the J = 6 - 5 rotation transition, at low resolution, showing the 59Co hyperfine structure [74].

The data show that the quadmpole hyperfine patterns of the rotational transitions are different between the two states, due to changes of the relative positions of some of the hyperfine components within the multiplet. The rotational spectrum of a pyrrole dimer is consistent with essentially a T-shaped structure, in which the planes of the two pyrrole monomers form an angle of 55.4(4)° and the nitrogen side of one ring is directed to the 7t-electron system of the other ring establishing a weak H bond <1997JCP504>. 

Figure 2 Spectrum of the J = 8-7 rotational transition of I32xe65cu35ci xiig complicated hyperfine structure arises from nuclear quadrupole interactions of Cu (7cu = 3/2) and Cl (Id = 3/2). All transitions are split into Doppler doublets as a result of the molecular expansion traveling parallel to the microwave cavity axis. For clarity of the picture, the quantum number assignments of only a few hyperfine components are given as F -F". The...

Figure 2 Spectrum of the 7 = 8-7 rotational transition of Xe Cu Cl. The compUcated hyperfine structure arises from nuclear quadrupole interactions of Cu (7cu = 3/2) and C1 (7qi = 3/2). All transitions are spUt into Doppler doublets as a result of the molecular expansion traveling parallel to the microwave cavity axis. For clarity of the picture, the quantum number assignments of only a few hyperfine components are given as Fj -F/, F -F". The angular momentum coupling scheme Fi = Icu + J F = Fi + Iq was used. The compound was produced using laser ablation of a copper rod in a molecular expansion of a mixture of 0.1% CI2, 15% Xe, and 85% Ar. The particular isotopomer was measured in its natural abundance of 6.3%. This spectmm was recorded using 15 000 averaging cycles with a total accumulation time of about 3.5 h...

A number of halogenomethanes have been subjected to other forms of molecular spectroscopy. High-resolution Stark spectra of several transitions of the V3 band of CH3F have been studied by means of a CO2 laser measurements of the hyperfine structure on certain rotational transitions in CH2F2 have been made using a molecular beam maser spectrometer the millimetre-wave spectrum of ground-state CDCla and the microwave spectrum of CD3I in excited vibrational states have also been observed. 

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