Gas-phase structure of borneol and isoborneol monomers and their microsolvated complexes.
The Borneo camphor molecule (C10H18O) is a naturally occurring monoterpene present in many essential oils, with applications in medicine, food or perfume industry. Its structure is similar to that of camphor with a hydroxyl group instead of a ketone, and the different disposition of the OH group gives rise to two isomers, borneol and isoborneol. The microwave spectra of borneol, isoborneol and their microsolvated complexes with two of the most common solvents, water and DMSO, were generated in a supersonic expansion and characterized using chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy in the 2-8 GHz frequency range. The search for experimental species was guided by quantum-chemical calculations, which also helped with the elucidation of the structures and the interaction points between the molecules. For both borneol and isoborneol monomers, three low energy forms are predicted, from which only one was experimentally identified in the gas phase. This could be due to relaxation processes coming from collisions with the carrier gas.
In the microwave spectra of both monomers the lowest energy forms for the 1:1 borneol-water and 1:1 isoborneol-water complexes were also found due to the ubiquity of water. In both cases, the complexes are stabilized by a hydrogen bond in which the water molecule acts as a hydrogen bond donor and the OH group acts as a hydrogen bond acceptor. In the experiments with DMSO, the lowest energy conformations for the 1:1 borneol-DMSO and 1:1 isoborneol-DMSO complexes were detected. Those are also predicted to be stabilized by hydrogen bonds, however, in this case the OH group of borneol or isoborneol acts as a hydrogen bond donor, while the DMSO carbonyl group acts as a hydrogen bond acceptor.