The double donor/acceptor role of the NH3 group: microwave spectroscopy of the aminoethanol-ammonia molecular complex

The most stable conformation of the 1:1 adduct of aminoethanol and ammonia has been characterized by free jet absorption microwave spectroscopy and quantum chemical calculations at the MP2/6-311++G** level of theory. The observed rotational spectrum corresponds to an arrangement where ammonia is bridged across the aminoethanol molecule and acts as a proton acceptor from the O-H group of aminoethanol and a proton donor to its amino group. The observed configuration of the aminoethanol moiety in the adduct is distorted with respect to the most stable gauche conformation of the isolated molecule. As a consequence of the weak nature of the interactions, we observe evidence of internal rotation of the ammonia moiety, even in the presence of multiple bonds. Analysis of the motion gives clear indications concerning the potential energy surface related to the non-bonding interactions.