Conformational flexibility and hydrogen bonding in 5-aminopentanol

Abstract

Rotational spectra of two conformers of 5-aminopentanol have been recorded using a Fourier-transform microwave spectrometer. For conformer C-1, eighty-two hyperfine components from the twenty-four a-, b-, and c-type transitions measured were fit to the quadrupole coupling constants, χaa = −2.954(2) MHz, χbb = 2.386(3) MHz. For conformer C-2, the fit of the seventy-six hyperfine components from the twenty-four a- and b-type transitions measured yielded χaa = −3.636(1) MHz and χbb = 2.087(2) MHz. Rotational and centrifugal distortion constants determined from fits of the resulting unsplit line centers to the Watson A-reduction Hamiltonian are A = 3322.169(1) MHz, B = 1958.7382(9) MHz, C = 1402.5957(8) MHz, ΔJ = 0.60(2) kHz, ΔJK = −0.21(8) kHz, ΔK = 0.99(3) kHz, δJ = 0.203(7) kHz, and δK = 1.1(1) kHz for conformer C-1 and A = 3249.2215(6) MHz, B = 2027.9327(3) MHz, C = 1432.5846(3) MHz, ΔJ = 0.545(6) kHz, ΔJK = 0.25(2) kHz, ΔK = 0.39(5) kHz, δJ = 0.18(4) kHz, and δK = 0.96(3) kHz for conformer C-2. The two experimental conformations are consistent with the two lowest energy ab initio MP2/6-311++G(d,p) structures. Both conformations of 5-aminopentanol are stabilized by an intramolecular hydrogen bond from the alcohol proton to amino nitrogen. The flexibility introduced by the five carbons in the alkyl group separating the amino and alcohol functional groups resulted in the first appearance of multiple low energy conformers being detected relative to the four, three, and two carbons in 4-aminobutanol, 3-aminopropanol, and 2-aminoethanol respectively in which only one experimental conformation was observed.