Table 1 provides the identity of molecular species detected in astronomical sources in the radio, microwave, millimeter and submillimeter region. For a number of the species one or more isotopically substituted forms have been observed and these are listed as well in Table 1. In Table 1 and Table 2 the species are listed in alphabetic sequence according to empirical formula (Hill system) in the first column along with the common names of the molecule in the second column, and molecular formula in the third column. The reference(s) given in the last column of Tables 1 and 2 are for the original detection in the astronomical source. Table 3 provides the code and identification of the telescope used in the astronomical reference for each transition in Table 4.
The major emphasis of the present work is to provide the most accurate transition frequencies available for all of the astronomically observed spectral lines which are listed in Table 4. In Table 4 the recommended frequency is listed in column (1), followed by an asterisk in the case of calculated values, and its expanded uncertainty (k = 2 or 2σ) is shown in units of the least significant digit(s). Uncertainties  for calculated frequencies are Type A with coverage factor k=2 (two standard deviations). For measured frequencies, the uncertainties are Type B and taken directly from the reference cited. The chemical formula for each molecular species is given in column (2), the chemical name in column (3), and the quantum number labels are shown in column (4). Columns (5), (6), and 7) present astronomical information: antenna temperature
(Tr* or Ta*) or integrated intensity (full line width at half intensity times peak intensity), molecular cloud for the observation and abbreviation for the telescope employed (see Table 2 for a list of telescopes referenced), respectively. Most often the molecular cloud listed is Orion A (OriMC-1), Sagittarius B2 (SgrB2), Taurus Molecular Cloud 1 (TMC-1), or the circumstellar envelope of the infrared star IRC+10216, since these are the richest molecular sources and often provide the most intense emission lines. In column (8) the reference abbreviation for the astronomical observation is given and column (9) shows the reference to measured (or calculated) frequencies when taken from the literature. The reference code is based on the first three letters of the lead author's last name, plus the last two digits of the year of publication. If no laboratory reference appears, the frequencies presented are calculated in the present work. The reference list for Tables 1, 2, and 4 then follows Table 4.