Rotational spectra molecular structure definition

The principal application of the Kraitchman equations [Eq. (9)1 is for the determination of the atomic coordinates, at, bSi and cs. From a study of the rotational spectrum of the parent and of a species with single isotopic substitution the coordinates of the substituted atom may be determined. These coordinates are referred to as substitution coordinates or rs coordinates. Each new species yields new coordinates, and since all of the coordinates are in the same coordinate system, the calculation of substitution or rs bond distances and bond angles is a simple process. Costain,s demonstrated that there are definite advantages to the use of the Kraitchman equations to obtain molecular parameters. These advantages are sufficient to make the use of Kraitchman s equations the preferred method of structure determination from ground-state rotational constants. 

The simulation of the EPR-spectrum of N Cgi(COOEt)2 (Fig. 39) taking into account a fine structure interaction is in nice accordance with the experimental spectrum. The simulation was carried out with the hyperfine interaction and g-factor of unmodified N Cgo and the fine structure interaction 0 2=2 G and E = 0.13 G (non-axial term). The shape of the extra lines definitely requires the inclusion of a non-axial term E, indicating that the molecular structure of the adduct induces some non-axiality which is not averaged out by fast axial rotation. The non-axial term was also observed at a measurement at 100°C showing that axial motional averaging does not take place even at this temperature. These results show that like He Cgo the new endohedral compound N Cgo can be used as a probe to monitor exohedral chemical addition reactions. Due to higher sensitivity, EPR requires less material than NMR.3He Cgo and N Cgo are complementary probes since different interactions are measured. 

The choice of the effective Hamiltonian is often far from straightforward indeed we have devoted a whole chapter to this subject (chapter 7). In this section we give a gentle introduction to the problems involved, and show that the definition of a particular molecular parameter is not always simple. The problem we face is not difficult to understand. We are usually concerned with the sub-structure of one or two rotational levels at most, and we aim to determine the values of the important parameters relating to those levels. However, these parameters may involve the participation of other vibrational and electronic states. We do not want an effective Hamiltonian which refers to other electronic states explicitly, because it would be very large, cumbersome and essentially unusable. We want to analyse our spectrum with an effective Hamiltonian involving only the quantum numbers that arise directly in the spectrum. The effects of all other states, and their quantum numbers, are to be absorbed into the definition and values of the molecular parameters . The way in which we do this is outlined briefly here, and thoroughly in chapter 7. 

After hydrolysis, separation of LiBr, and distilling off the solvent and volatile by-products, compound 65 remains in the residue. In the nmr spectrum of 65 two clearly separated peaks are found for the me2Si groups. These are due to hindrance of free rotation around the molecular axis caused by the phenyl groups. A complete correlation of all peaks in the nmr spectrum is very difficult. Derivatives were found useful in the determination of the structures, changing the substituents produces a definite chemical shift in the nmr spectra. 

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