Experimental and theoretical study of the structures and binding energies of eugenol $(H2O)n,$ $n=0–2$

Eugenol (4-Allyl-2-methoxyphenol), a phenol-derivative with an intramolecular –$OH⋯OCH3$ hydrogen bond (H bond), has been studied in a supersonic expansion using a number of complementary laser spectroscopic techniques. The mass-resolved excitation spectrum of eugenol and its water complexes are reported for the first time. The most intense set of bands on the resonantly enhanced multiphoton ionization (REMPI) spectrum of eugenol originate in a conformer whose $S1←S0$ transition is at 35 202 cm⁻¹ and the ionization threshold at $(I0←S0)$ 62 544±150 cm⁻¹ (7.755±0.019 eV). In addition, two low intensity features redshifted with respect to the $000$ transition have been identified as due to a second, less stable conformer. Ab initio calculations show that the potential energy landscape depicts at least three minima associated with one folded and two extended conformers, one of which is the most stable. Clusters of eugenol/water were prepared in a supersonic expansion by seeding eugenol and water in noble gas He and examined by two-color REMPI (R2PI) and IR-UV double resonance spectroscopies. Only one single isomer was observed for both 1:1 and 1:2 complexes, in contrast with the several stable conformers provided by the computations. The dissociation energies of the 1:1 and 1:2 complexes have been determined by the fragmentation threshold method and the results compared with those from ab initio calculations conducted at the B3LYP and MP2 levels with a variety of basis sets.