The present tabulation of recommended transition frequencies for interstellar molecular species is the fourth version and is based on previously published tables [1-3]. Since the last revision in 2004, 19 new molecular species (CF+, HCP, SiNC, CNCHO, H2CCNH, CCP, c-C3H2O, CH2CHCH3, CH3CH2CHO, C3N-, C4H-, HC4N, CH2CCHCN, C6H-, CH3C4CN, CH3C6H, and C8H-, PO, O2) and approximately 2000 new transitions have been added to the recommended rest frequency table, which now lists more than 12400 entries. This report updates the previous summaries, provides a current source of radio-astronomical molecular line observations, and improves the accuracy for many previously tabulated transition frequencies, important for determining physical properties of the molecular clouds investigated. The literature has been surveyed through the Spring of 2008.
The present tabulation covers the astrophysical literature through June 2007. The 134 molecular species listed in Table 1 have now been identified in interstellar and circumstellar astronomical sources by means of their microwave spectra. The 17 additional interstellar species in Table 2, identified by their infrared or ultraviolet spectra provide a total of 151 interstellar species, and 8 additional molecular species found only in comets are listed in Table 2. Since no microwave transitions of these species have been reported yet, there will be no entries in Table 4 for these species.
The sources of the transition frequencies selected are as follows: laboratory measurements and predictions from the literature data, previously published tabulations of spectral frequencies [4-30], spectral predictions of transition frequencies from reanalysis of the literature data carried out in the present work, and web-based catalogs [31,32]. The primary criterion for selection of the transition frequencies is the magnitude quoted for the estimated uncertainty in the measured frequency or the standard deviation of calculated frequencies. For well-behaved species, i.e. those whose
spectra are well fit by established Hamiltonians, the calculated frequencies are often more accurate than individual measurements, thus many of the entries in Table 4 are calculated values and are identified with an asterisk (*) following
the frequency entry. In entries where it was determined that the measured value had the lowest uncertainty value, a reference to the literature value is given. For many of the interstellar species the previously published tabulations of critically evaluated laboratory data [4-30] were the source of both measured and calculated frequencies cited here. In many cases for species treated in the publications indicated above, new spectral data have been reported and have been combined with the earlier data sets and reanalyzed to provide predicted frequencies employed here. Some of the earlier reviews also have a frequency limit of 200 GHz or 300 GHz, while new interstellar observations range up to 900 GHz and even higher for small molecules. Thus, for most of the smaller species (diatomic, triatomic, etc.) calculations were extended to higher frequencies with new laboratory data included where available. The earlier reviews on CH3OH6,
CH3CHO12, and HCOOCH319 are out-dated and newer reviews were used [28,27,31]. In a number of cases, e.g., for radical species such as C2H, C3N, C4H, etc., the on-line catalogs developed at the Jet Propulsion Laboratory (JPL)  and at the University of Köln  were used as the source of calculated frequencies.