These tables represent the second part of a series of critical reviews on molecular rotational spectra in the microwave frequency region. The present review on triatomic molecules is a partial revision of the previous tabulation on polyatomic species, NBS Monograph 70, Vol. IV (1968) . The primary aim of the present critical review is directed at detecting errors, misprints and incomplete analyses in the literature. The coverage includes microwave rotational spectra and molecular properties derived therefrom for all the triatomic species observed in the centimeter and millimeter wavelength region of the spectrum. There are two objectives which this review hopes to achieve: first, to provide an up-to-date and complete tabulation of the microwave spectra for triatomic molecules, and second, to provide the best set of molecular properties which can be derived from the observed spectra. Although the spectral line frequencies are limited to the radio and microwave frequency region, derived molecular constants are included from a variety of sources, e.g., molecular beam electric resonance, electron paramagnetic resonance and infrared spectroscopy, in order to provide the most complete set of properties presently available. All unpublished data communicated privately have been included, and the open literature has been searched through June 1976.
Two types of tables are presented for each molecular species: 1) the derived molecular constants for each isotopic form which has been studied, and 2) the observed microwave spectral transition frequencies for all isotopic forms and vibrational states for which data are available. The appendix contains molecular structure information taken directly from the literature, and a discussion of the analysis of the F2O spectrum. Literature references are labeled with 5-digit numerals. This system has been formulated such that the first two digits refer to the year of publication of the work while the remaining three digits correspond to a chronological ordering within the year as required for referencing purposes here. Since this system was introduced in the first part of this series , these reference numbers follow the sequence established in the earlier work.
Since a uniform format could not be readily constructed for these tables, they were composed in variable format depending on the type and amount of information available. In general, the rotational constants are listed first, followed by the centrifugal distortion parameters, hyperfine structure data, electric dipole moments and rotational g-factors. In cases where extensive data are available, several individual tables are used to present the molecular constants. A more detailed description of the molecular constants and their symbols is given in section 2.
The spectral tables contain all of the data intrinsic to an assigned molecular transition. The first column of these tables contains the isotopic molecular species to which the data pertain. The next columns contain the assigned quantum numbers for the transition in the sequence: rotational and the hyperfine for the upper state. rotational and hyperfine for the lower state, and vibrational state designation. Since the vibrational state remains unchanged for nearly all transitions, a single set of quantum numbers, (v1,v2ℓ,v3), is sufficient. The next columns contain the observed transition frequency, its estimated uncertainty, and finally the reference to the original source of the data. Since the maximum number of significant figures beyond the decimal point was fixed at three for the transition frequency and two for uncertainties, in a few cases it was necessary to round off the measured data. This situation occurs primarily in the reproduction of molecular beam measurements. When the uncertainty in the measurement is less than 0.01 MHz, the exponential notation is employed. Thus, an uncertainty of 0.005 MHz appears as 5E-3, which represents 5 x 10-3 MHz. When uncertainties were not given in the original source, an uncertainty was assigned on the basis of the internal consistency of all the data available for the molecule in question. Predicted transition frequencies have been given for several isotopic forms of HCN, H2O, H2S, and OCS. In order to distinguish these from measured transitions, an asterisk (*) was placed in front of the estimated uncertainty, which is twice the standard deviation from the fit.
An effort was made to locate all of the essential references through mid-1976. In situations where multiple measurements have been reported, the sources not directly cited here are included as additional references at the end of the molecular constant tables. These additional references also include publications which are indirectly related to the frequency measurements and which contain intensity measurements.