Microwave spectroscopy and quantum chemical investigation of nine low energy conformers of the 15-crown-5 ether.

Crown ether macrocyles constitute a central class of selective substrates in Supramolecular Chemistry. The backbone of crown ethers is particularly flexible which makes the spectroscopic and computational investigations challenging. In this work, we present a systematic investigation of the low energy conformational landscape of the benchmark macrocycle 15-crown-5 ether (15c5) under isolated conditions. Molecular beam Fourier transform microwave spectroscopy is employed to measure a broad ensemble of rotational transitions within the range of 4-11 GHz. The recorded spectra allow us to determine the rotational constants of eight individual 15c5 rotamers which are identified among the nine lowest energy conformers predicted by ab initio MP2 computations. The study illustrates well the variety of conformers of prolate and oblate character that the 15c5 molecule displays within a narrow range of energies. Implications for the supramolecular behavior of 15c5 are discussed.