Splatalogue displays two possible frequencies with each species: "Freq" and "Meas Freq" (see figure 1). In the cases of JPL, CDMS, SLAIM, and the LISA line lists, "Freq" is the theoretically predicted frequency of the molecule and is calculated based on the experimentally determined molecular constants. These constants are found in the literature and reference is given to the paper(s) from which the values are obtained. The "Meas Freq" values are the laboratory measurements that have been published in the literature. Again, reference is given to the paper(s) from which the values are obtained. The reference list and additional information about the molecule can be found by clicking on the chemical formula.
In some cases, such as with the JPL and CDMS catalogues, the laboratory measured values are given if available. Otherwise, the theoretical/calculated values are posted, but not both. In other cases, such as with SLAIM and the LISA lines, both values are posted. The predicted frequency is given and the measured frequency is posted where available. In cases such as this, the user will notice slight differences in the laboratory value and the theoretical/predicted value. It should be noted that for molecules with a well determined Hamilitonian the calculated frequencies are usually considered more accurate than the experimental laboratory measurements. For more information about how the theoretical/calculated frequencies were obtained, please refer back to the original referenced catalog website. Finally, the data provided by Frank De Lucia's lab at The Ohio State University, gives ONLY measured frequencies as measured by his FASST technique. Further information about the data from The Ohio State University can be found on the OSU Splatalogue information page.
The LOVAS line list only displays frequencies that have previously observed in space. These are displayed under the "Freq" column. More can be read here and here for information about the origin of the observed transitions.
You will also notice in figure 1 that often, many different line lists contain the same transition of a particular molecular species. In this case, which frequency should an observer use from the database? The NRAO has assigned a "flag" for the recommended rest frequency for all known interstellar molecules. This flag can be turned on under the "Miscellaneous" section of the left sidebar. You will see a checkbox that says, "Display NRAO Recommended Frequencies". Select that option and search. A "splat" () will appear next to the transition frequency that NRAO recommends (see figure 2). In the case where there is a measured and observed frequency for a given transition, refer to the paragraph above but in general, the calculated frequency is often the best one to use.
The decision making process on what line list and what frequency should be recommended went like the following: