Hydrocarbon Spectral Database, Introduction

Introduction

These tables represent the third part of a series of critical reviews on molecular rotational spectra in the microwave frequency region. The present review on hydrocarbon species is a partial revision of the previous tabulation on polyatomic species [1]. 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 hydrocarbon species observed in the centimeter and millimeter wavelength region of the spectrum. There are two objectives that this review hopes to achieve; first, to provide an up-to-date and complete tabulation of the microwave spectra for hydrocarbon 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 All unpublished data communicated privately have been included, and the open literature has been searched through September 1987.

1. General Description of the Tables

Two types of tables are presented for each molecular species. The first tables contain the derived molecular constants for each isotopic form which has been studied, and the table following these constant tables contains the observed microwave spectral transition frequencies for all isotopic forms and vibrational states for which data are available. The ordering of the tables follows an alphabetic sequence in empirical molecular formulas. The sequence of the tables is indicated in the List of Tables shown in the Contents.

The molecular spectral data tables of Sec. 2 are followed in Sec. 3 with the references to these data. 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 earlier parts of this series [2, 3] these reference numbers follow the sequence established in the earlier work.

1.a. Molecular Constant Tables

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 Sec. 2. In order to reduce the occurrence of misprints, these tables were photographically reproduced from the Original final typewritten copy.

1.b. Spectral Line Tables

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 observed transition frequency and its estimated uncertainty. 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. The reference to the original source of the data is given in the last column. Since the maximum number of significant figures beyond the decimal point was fixed at three for the transition frequency and 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 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. A hash mark (#) located in front of the uncertainty denotes that the frequency is the hyperfine free center of the frequency. An effort was made to locate all of the essential references through mid-1987.