Deciphering the non-covalent interactions in the furan⋯hexane complex using rotational spectroscopy and theoretical analyses.

The rotational spectrum of a binary complex formed between furan and n-hexane was investigated using a chirped pulse Fourier transform microwave spectrometer in the range of 2-6 GHz. While furan has only one conformer, n-hexane exists in multiple conformations. The conformational landscape of the binary complex was systematically explored by using a semiempirical conformational search tool, namely CREST. The CREST conformational candidates were subjected to further geometry optimization and harmonic frequency calculations at the B3LYP-D3BJ/def2-TZVP level of theory, resulting in 34 minima within an energy window of 5 kJ mol-1. The three most stable furan⋯hexane minima all contain the most stable n-hexane conformer subunit and are separated by relatively low conformational conversion barriers. Additional calculations were carried out to support the conclusive identification of the global minimum structure responsible for the set of assigned rotational transitions. These include calculations at the B3LYP-D3BJ level with the aug-cc-pVTZ and 6-311++G(d,p) basis sets and the MP2/def2-TZVP level, as well as the single point energy calculations at the CCSD(T)-F12/cc-pVDZ level. Further non-covalent interaction and principal interacting orbital analyses show that the synergy of the πfuran → σ*hexane and σhexane → π*furan interactions plays an important role in stabilizing the observed furan-hexane conformer.