Microhydration of verbenone: how the chain of water molecules adapts its structure to the host molecule

The microsolvation of verbenone (C10H14O)·(H2O)n (n = 1, 2, 3) was experimentally investigated in a supersonic expansion using a cavity-based Fourier transform microwave spectrometer, in the 2.8-14 GHz frequency range. Thanks to computationally optimized structures at the B3LYP-D3BJ/def2-TZVP and MP2/6-311++G(d,p) levels using the Gaussian 16 software, the spectra of two mono- and two dihydrates, and that of the lowest energy conformer among the four expected trihydrates, could be assigned. A similar study replacing normal water with 18O labeled water allowed the identification of the spectra of all possible isotopomers, leading to the calculation of the substitution coordinates of water oxygen atoms, and of the effective structure of the water molecule arrangements around verbenone. The computed rotational constants and structural parameters were found to be quite close to the experimental ones both at the DFT and ab initio levels. A comparison between the structures of the hydrates of camphor previously studied by Pérez et al. [J. Phys. Chem. Lett., 2016, 7, 154-160] and of those of verbenone shows that the chain of water molecules adapt their structure according to the geometry of the host molecule. The general trend is that bond angles in the water chain are much wider in verbenone than in camphor.