Potential Curves and Large Amplitude Motions

Potential Curves and Large Amplitude Motions. The study of non-rigid molecules is closely related to the study of conformational equilibria. The subject is discussed in recent review papers, but the topic seems so important that a short discussion is given here cf. also Chapter 4). The idea of using electron diffraction to study internal motion is far from new some of the earlier studies are given in ref. 81. 

F re 11 The proposed potential function for the CCC bending in C Oj (2a = 180° — / CCQ. The uncertainty is too large to exclude completely a potential function with no barrier at a. = 0°. The electron-effraction ta seem, however, to rule out a nearly harmonic potential as obtained by molecular orbital calculations (see J. R. Sabin and H. Kim, J. Chem. Phys., 1972, 56, 2195) 

Several investigations have shown that the electron-diffraction results may usually be interpreted by assuming a classical density distribution for the co-ordinate, q, describing the large amplitude motion, 

The potential curve may be determined by assuming an analytical form for V(q) and refining parameters in the expression by the least-squares method, or just by comparing experimental and theoretical RD curves for various potentials. It is a considerable advantage if u %q) can be calculated from spectroscopic data by conventional methods. In some cases constants in an expression for u (q) are also refined. If possible, structural and potential parameters should be refined simultaneously. 

For simple molecules it is not possible to determine rotational barriers with an accuracy which can compete with that of spectroscopic methods. However, it is possible to get qualitative and semi-quantitative information about potential curves in suitable compounds, especially if exp[—V q)/RT] is not too small (say not less than 0.05) for maxima in V(q). 

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