Competing Intramolecular Hydrogen Bond Strengths and Intermolecular Interactions in the 4-Aminobutanol-Water Complex

We seek to determine the effect of competing intermolecular hydrogen bonds from water on the preferred conformation of 4-aminobutanol (4AB) monomers stabilized by intramolecular hydrogen bonds. Toward this end, the rotational spectrum of the 4-aminobutanol-H₂O complex was recorded using Fourier transform microwave spectroscopy and fit to the rotational, quadrupole coupling, and centrifugal distortion constants of the Watson S-reduction Hamiltonian. The experimental results are consistent with a 4AB-water complex that preserves the intramolecular hydrogen bond within the 4AB monomer and forms a single intermolecular bond with water acting as a donor. The experimental monomer structure agrees well with the lowest energy conformation calculated at the MP2/6-311++G(d,p) level of theory. Upon complex formation and the introduction of competing intermolecular bonds from water, only small changes in the OH···N intramolecular hydrogen bond and backbone torsional angles of the 4-aminobutanol monomer are observed. Similar small changes were observed for the shorter chain 3-aminopropanol amino alcohol monomer when complexed with water, in contrast to the 2-aminoethanol-H₂O complex. In the latter, a large change in the backbone torsional angle and a breaking of the intramolecular hydrogen bond were observed. Thus, extending the methylene chain results in an increase in the strength of the intramolecular hydrogen bond in unbranched amino alcohols.