Conformational equilibria, non-covalent interactions and large amplitude motions in molecules and small molecular clusters revealed by rotational spectroscopy

In this dissertation, the introduction of the basic knowledge of microwave spectroscopy including the fundamental theory of the rotational spectroscopy and the experimental techniques was reviewed firstly. Then the work that I have performed during my PhD career is discussed. Pulsed-jet Fourier transform microwave (PJ-FTMW) spectroscopy has been used to study the monomers, and the bimolecular clusters formed with hydrogen bond or weakly non-covalent bonds. Difluoromethane and water oligomers have been studied by Chirped-Pulse Fourier transform microwave (CP-FTMW) spectroscopy. The spectroscopic work in this dissertation includes three parts: 1) Isolated molecules with large amplitude motions. Three subjects are involved in this part, which are the rotational studies of indan, 1,2-dimethoxylethane, and 1-methylcyclohexanol. The transitions were split due to ring puckering motion, the internal rotations of methyl groups, and the rotation of the hydroxyl group, respectively. 2) Clusters formed by the cooperation of different classes of HBs. Four subjects are involved in this chapter. Firstly, formic acid clustering with dimethyl ether and cyclobutanone, respectively, indicate that both complexes are formed with one typical O-H···O HB and two C-H···O weak hydrogen bonds (WHBs). Secondly, the investigation of the complex of indan with trifluoromethane showed a cage structure based on the cooperative effects of C-H···π and C-H···F WHB interactions. Lastly, The study of oligomers of (CH2F2)m-Wn, (m,n = 1,2) has broaden our knowledge of the cooperation of different HBs in a cluster. 3) Another interesting result is the observation of the competition between reactivity and the pre-reactivity in the mixtures of carboxylic acids and alcohols, which is a basic phenomenon in organic synthesis. The experimental results on the primary and secondary alcohols mixed with carboxylic acids have shown that the esterification reaction occurs, whereas only the spectra of the adducts could be identified by mixing the tertiary alcohols and formic acid.