Estimation of Microwave and Infrared Spectral Parameters of Some Molecular Ions from Ab-Initio Electronic Structure Calculations
The observation of the microwave spectra of new molecular ions has been a principal objective of the author’s research program for a long time. Successful detection of the spectrum of a new ionic species may lead to a precise determination of its molecular structure, allow radioastronomical observation of it in the interstellar medium, and/or provide useful spectroscopic probes of the dynamics of plasmas in which it may be located. To search over a sufficiently wide spectral range at high enough sensitivity to detect a new ion, however, is a tedious task, which may require weeks of effort. To minimize this experimental burden accurate advance predictions of molecular geometric structures and thus rotational constants (reciprocals of moments of inertia) are highly desirable. With the computer facilities and public programming packages that are currently available we have found that ab initio calculations of sufficient quality to be very useful in this regard are quite practical. In this paper we will briefly describe results that we have obtained for several molecular cations and anions of interest using the Gaussian 82 programs of Pople and coworkers1 with rather large basis sets and extensive treatment of electron correlation. The emphasis has been (1) to employ calculations that were of the highest quality that seemed practical, (2) to calculate all the spectroscopic constants, e.g., vibration-rotation interaction and anharmonicity, from the molecular potential function, and (3) to use identical calculations on known systems (typically neutrals) for calibration purposes to the maximum extent possible.
KeywordsRotational Constant Spectroscopic Constant Quadrupole Coupling Constant Microwave Spectrum Bend Force Constant
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