Rotational spectra nuclear quadrupole coupling

Legon, A.C., Soper, P.D., and Flygare, W.H. (1981) The rotational spectrum, H-19F nuclear spin-nuclear spin coupling, D nuclear quadrupole coupling, and molecular geometry of a weakly bound dimer of carbon monoxide and hydrogen fluoride. J. Chem. Phys., 74, 4944-4950. 

The resulting spectrum represents a weighted average over the rotational states and a careful analysis of it yields the nuclear quadrupole coupling constant. Molecular beam electric resonance is complementary to pure rotational spectroscopy since transitions between the Stark levels of the rotational states (AJ = 0) are observed (sometimes AJ= 1 transitions are also studied). Specially constructed maser spectrometers55 that can detect transitions of rotationally selected molecules have been used to determine very small coupling constants, such as those for deuterium compounds. Again, molecular beam resonance is currently limited to the study of small molecules. 

A. C. Legon, P. D. Aldrich, and W. H. Flygare,/. Am. Chem. Soc., 104, 1486 (1982). The Rotational Spectrum, Chlorine Nuclear Quadrupole Coupling Constants, and Molecular Geometry of a Hydrogen-Bonded Dimer of Cyclopropane and Hydrogen Chloride. 

While the magnitudes of NQR frequencies are for the most part a consequence of the nature and the electronic structure of the molecules that are under study, they are modified by the fact that these molecules are present in the solid state. The nuclear quadrupole coupling tensor for an isolated molecule is accessible from the study of its pure rotational spectrum but when the molecule is incorporated in a crystalline solid its observed NQR frequencies are slightly different from those that would have been expected from its quadrupole coupling tensor, even if the forces that retain the molecule in the solid are weak van der Waals forces or even the physical entrapment that characterizes many inclusion complexes. Furthermore the resonance frequencies are temperature-dependent. As will be shown below, it is in these solid state effects and in the temperature-dependence of the resonance frequencies that resides the utility of the NQR technique for the study of inclusion complexes. 

As we shall see, each of these two terms, one for each nucleus, describes a second-rank scalar interaction between the electric field gradient at each nucleus and the nuclear quadrupole moment. De Santis, Lurio, Miller and Freund [44] included two other terms which involve the nuclear spins. One is the direct dipolar coupling of the 14N nuclear magnetic moments, an interaction which we discussed earlier in connection with the magnetic resonance spectrum of D2 its matrix elements were given in equation (8.33). The other is the nuclear spin-rotation interaction, also discussed in connection with H2 and its deuterium isotopes. It is represented by the term... 

Legon, A. C. and Suckley, A. R, Br nuclear quadrupole and H,Br nuclear-spin-nuclear-spin coupling in the rotational spectrum of H20 HBr, Chem. Phys. Lett. 150, 153-158 (1988). 

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...

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