Microwave Spectra and Molecular Structures of Organic Molecules and Hydrogen Bonded Dimers

Abstract

The microwave spectra were measured in the 4-15 GHz regime for cyclopropanecarboxylic acid, 1,2-cyclohexanedione, maleimide, phthalimide, and 4a,8a-azaboranaphthalene. Doubly hydrogen bonded dimers formed with formic acid were also measured with the molecules cyclopropanecarboxylic acid, 1,2-cyclohexanedione, maleimide, and tropolone. Measurements were made using a pulsed beam Fourier transform microwave spectrometer. Rotational and centrifugal distortion constants were determined from the microwave spectra. In the case of the systems that exhibit electric quadrupole coupling interactions, the electric quadrupole coupling strengths were also determined from the analysis of the hyperfine structure in the spectra, yielding additional electronic structure information for the molecules studied. The spectra were also measured for a number of unique, singly substituted isotopologues under natural abundance concentrations. This isotopologue data is crucial in order to obtain key gas phase molecular structure parameters of the molecules and complexes studied. Many theoretical computations, using ab initio and DFT methods, were also performed to obtain optimized electronic structures of the systems studied. These computations aid in the search and assignments of the rotational transitions measured. Comparisons between the theory and the experimental results are described in greater detail in the respective chapters for those systems. The experimental results for the organic systems studied agreed well (within a few percent) with the gas phase optimization computations performed.