Rotational spectrum and bending potential of LiOH: A semirigid bender analysis

Some 29 microwave transitions in the millimeter region have been measured in the high-temperature vapor of LiOH, ranging up to J=5, including lines from the isotopomers⁷Li¹⁶OH,⁷Li¹⁶OD in both ground and excited bending states, and⁷Li¹⁸OH in the vibrational ground state only. An analysis of these transitions, together with nine previously known radio-frequency transitions, based on the semirigid bender model of vibration-rotation interaction has been used to investigate the form of the bending potential and to measure the semirigidity parameters that show the variation in the bond lengths that accompany the large-amplitude bending motion. The molecule was found to be linear in its equilibrium configuration, with an essentially harmonic bending potential. The experimentally derived quadratic semirigidity parameter for the Li-O bond was found to agree to within less than 5% with that from an ab initio calculation at the MP3/6-311G^** level.