Microsolvation of myrtenal studied by microwave spectroscopy highlights the role of quasi-hydrogen bonds in the stabilization of its hydrates.

Hydrates of myrtenal (C10H14O) · (H2O)n (n = 1, 2, 3) were experimentally investigated in a molecular jet using a cavity-based Fourier transform microwave spectrometer in the 2.6 GHz-15 GHz frequency range. The assignment of the spectra was made possible, 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 those of the lowest energy conformer among the two expected trihydrates could be assigned. A similar study replacing normal water by 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 that of the effective structure of the water molecule arrangements around myrtenal, except for the trihydrate. The structure of the latter species was nevertheless confirmed by the analysis of the spectrum of the isotopomer with three H2 18O molecules. The computational rotational constants and structural parameters were found quite close to the experimental ones at the density functional theory B3LYP-GD3BJ/def2-TZVP and ab initio MP2/6-311++G(d,p) levels. Symmetry adapted perturbation theory calculations reveal that the aldehyde hydrogen atom strongly interacts with water oxygen atoms in the case of di- and trihydrates.