11524
J. Phys. Chem. A 2001, 105, 11524-11530
A Computational Study of the Structures of the p-Methoxyphenethylamine(H2O)2-4 Complexes In˜ igo Unamuno, Jose A. Ferna´ ndez, Asier Longarte, and Fernando Castan˜ o* Departamento de Quı´mica-Fı´sica, Facultad de Ciencias, UniVersidad del Paı´s Vasco. Apart. 644, 48080 Bilbao, Spain ReceiVed: August 7, 2001; In Final Form: October 11, 2001
A computational investigation of the hydrated complexes of p-methoxyphenethylamine(H2O)2-4 (hereafter referred to as either MPEA(H2O)2-4 or simply 1:2-4) has been conducted at the B3LYP/6-31+G* level in order to find out the stable structures and the isomer binding energies (BEs). As expected, the calculations supply a large number of stable geometries, built up from one of the seven MPEA conformers and a chain of solvent molecules attached to a number of sites of the chromophore molecule. Within each complex stoichiometry, some isomers have very close binding energies, in fact, smaller than that of the basis set superposition error (BSSE). Despite the seven MPEA conformer precursors, the number of complex isomers recognized in two-color mass-resolved time-of-flight (TOF) and “hole burning” spectra are scarce: one for both 1:2 and 1:3 complexes, and two for the 1:4 complex. Notwithstanding the difficulty to identify the participant conformer within a specific complex, the computation shed light on a number of significant features of their structures, for example, that the first water molecule is always bound to the nitrogen atom of the NH2 group; in the 1:2 complex, the last arrived solvent attaches to the water already bound and to the hydrogen atom of the NH2 pointing away from the aromatic ring; the addition of the third solvent results in the formation of a water daisy chain around the NH2 moiety, and the addition of the fourth water gives rise to two structures of close stability. These results are interpreted as the systematic presence of two isomers for all 1:1-4 complexes but that their spectra overlap for 1:2 and 1:3 at the resolution of the experiments. The calculated energies for the addition of one water molecule at a time to the 1:n complex match fairly well the experimental measurements (within ∼200-300 cm-1,
