Effect of solvent on molecular conformation: Microwave spectra and structures of 2-aminoethanol van der Waals complexes

Abstract

Rotational spectra of the C13 isotopomers of the 2-aminoethanol monomer have been recorded in natural abundance using a Fourier-transform microwave spectrometer. The two sets of C13 isotopomer rotational constants were used to complete the 2-aminoethanol substitution structure. Rotational spectra of the van der Waals complexes 2-aminoethanol–water and 2-aminoethanol–argon were also recorded. Sixteen a-, b-, and c-type transitions were fit to the Watson A-reduction Hamiltonian for 2-aminoethanol–argon yielding A=4986.762(2) MHz, B=1330.693(3) MHz, and C=1143.933(3) MHz. Fifteen a- and b-type transitions for 2-aminoethanol–water were fit to A=4886.451(5) MHz, B=3356.038(2) MHz, and C=2311.715(2) MHz. The spectra are assigned to the lowest-energy ab initio [MP2/6-311++G(d,p)] structures of the two complexes. The conformation of 2-aminoethanol is unchanged in the argon complex, and the argon is 3.775 Å from the monomer center of mass. A network of intermolecular hydrogen bonds in the 2-aminoethanol–water complex replaces the intramolecular hydrogen bond of the monomer, and formation of the network requires the OCCN torsional angle to increase from 57° to 75° and the O–N distance to increase from 2.796 to 3.100 Å.