The data for KCl have been summarized in
M. Caris, F. Lewen, H. S. P. Müller, and G. Winnewisser,
Pure Rotational Spectroscopy of Potassium Chloride, KCl, up to 930 GHz
and Isotopically Invariant Analyses of KCl and NaCl,
J. Mol. Struct. 695–696, 243–251 (2004).
View abstract.
It is rather straightforward to perform an isotopically invariant fit with SPFIT/SPCAT because all the vibrational and isotopic dependences are coded explicitly. However, because this has to be done separately for each parameter, each vibrational state, and each isotopomer, the parameter file is usually rather large. In the present case, there are 360 parameters – of these are 12 independent. Therefore, it may be useful to employ a routine that generates the parameter file automatically.
The quantum numbers J and "v". The latter is the state number which is used to label both vibrational states and isotopomers. "v" = 0 – 7 designate 39K35Cl, v = 0 – 7; "v" = 8 – 15 designate 39K37Cl, v = 0 – 7; "v" = 16 – 22 designate 41K35Cl, v = 0 – 6; "v" = 23 – 28 designate 41K37Cl, v = 0 – 5; and "v" = 29 designates v = 0 of 40K35Cl.
NOTE: The parameters actually determined directly are not the Uij plus U01(BO), Δ01K, and Δ01Cl ! Instead, the Yij plus Y01(BO), Y01(K), and Y01(Cl) have been determined. However, the parameters in the publication can be derived from these parameters readily.
It may be worthwhile mentioning that the choice of parameters is a question of taste – as is often the case. In the present data set only parameters have been retained that were determined better than 5 : 1. Including several additional higher order constants in the fit, the reduced standard deviation of the fit can be lowered from 0.923 to about 0.84. Notably, the inclusion of Y41 lowers it to 0.891. However, its value of (–0.180 (43)) x 10–6 appears to be slightly too large in magnitude.