V.B.4.c Frequencies
The power of the QM analyses (and also the reason for their necessity) is that they allow the calculation of many spectral lines based on an accurate measurement of a smaller subset. It has long been know that the accuracy of these calculations is much higher for lines that represent an interpolation of quantum numbers than for those based on an extrapolation. Indeed, one of the primary issues for laboratory spectroscopist when simulating spectra for astronomical use is how far to take these extrapolations. In excited vibrational states that have local perturbations this problem is exasperated even for states well within the bounds of the analyses. Figure V.B.4.c -1 shows such an example in an excited vibrational state of vinyl that has been partially analyzed. For molecules such as methanol, even in regimes for which there is a nearly complete set of experimental measurements for a given vibrational state, the difficulty of obtaining a model that will fit the experimentally observe lines results in small off sets. Examples of this can be seen in Fig. V.B.4.b-4.
Figure V.B.4.c-1