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

Abstract

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.

Dependence of Intramolecular Hydrogen Bond on Conformational Flexibility in Linear Aminoalcohols

“…In addition, the red shift of 249 cm –1 in n = 4 at 65 °C is much larger than those of 116 and 191 cm –1 in n = 2 and n = 3, respectively. Therefore, the intramolecular H-bond in n = 4 is much stronger than that in n = 2 and n = 3, in good agreement with AIM and NCI analyses and microwave investigation under supersonic jet cooling conditions that the strongest intramolecular H-bond exists in n = 4 . The largest red shift means the strongest H-bond, but it does not mean the most favorable formation probability in n = 4 since the latter is dependent on the temperature at which the molecules are.…”

“…Therefore, the intramolecular H-bond in n = 4 is much stronger than that in n = 2 and n = 3, in good agreement with AIM and NCI analyses and microwave investigation under supersonic jet cooling conditions that the strongest intramolecular H-bond exists in n = 4. 26 The largest red shift means the strongest H-bond, but it does not mean the most favorable formation probability in n = 4 since the latter is dependent on the temperature at which the molecules are. As seen from the OH bonded intensity of n = 2−4 in Figure 2(a), it can be seen that the intramolecular H-bond probability for n = 2 and n = 3 is much larger than that for n = 4, whereas from a view of intramolecular H-bond strength, n = 4 and n = 3 are much stronger than n = 2.…”

“…It occurs in n = 4 under low temperature T < 200 K, whereas it occurs in n = 3 at 200 K ≤ T < 400 K, and occurs in n = 2 at T ≥ 400 K. For this reason, n = 4 was observed as a single conformer with the strongest intramolecular H-bond under supersonic jet cooling conditions. 26 Since our experimental temperature ranges from room temperature 298 to 338 K, the Gibbs free energy optimum occurs in n = 3 according to the calculations in Table 2.…”

“…Intramolecular H-bonds in aminoalcohols NH 2 (CH 2 ) n OH have been the subject of many experimental and theoretical studies. The levels of theory range from Hartree–Fock, MP2, B3LYP and CCSD­(T), − and spectral techniques include microwave spectroscopy, low-temperature matrix infrared spectroscopy, and gas-phase infrared spectroscopy. − It was shown that only the conformer with the O–H···N intramolecular hydrogen bond was identified in the gas phase although theoretical calculation predicted that another conformer with the N–H···O intramolecular H-bond should exist. In addition, compared to n = 2, the O–H···N intramolecular H-bond is much stronger in n = 3, indicating that the strength of intramolecular H-bond increases as the backbone chain gets longer.…”

“…A more flexible chain will be more favorable for orientating two functional groups to maximize the intramolecular H-bond strength. Recently, using microwave spectroscopy under supersonic jet cooling condition of 2 K rotational temperature, Lavrich et al conducted a series of investigations on intramolecular H-bonded conformations of linear aminoalcohols n = 2–5, showing that n = 4 yields the strongest intramolecular H-bond. ,,, In addition, using high-level G4 ab initio calculations, Lamsabhi et al investigated the characteristics of intramolecular H-bonds in aminoalcohols of n = 2–7 and suggested that the strength of intramolecular H-bonds goes through a maximum at n = 4 with a seven-membered ring . It is known that conformational distributions under supersonic jet cooling conditions are different from room temperature, and accurate description of the characteristics of weak noncovalent interaction is still a challenge for quantum chemistry calculation.…”