Structural and vibrational characterisation of 3-amino-1-propanol a concerted SCF-MO ab initio, Raman and infrared (matrix isolation and liquid phase) spectroscopy study

Abstract

Results obtained for the isolated and liquid 3-amino-1-propanol by a concerted molecular orbital and vibrational spectroscopic approach are reported. The relative energies and both structural and vibrational data of the different conformers of the studied compound were calculated using the extended 6-31G* basis set both at the HF-SCF and MP2 ab initio levels of theory and the theoretical results used to interpret Raman and infrared experimental data. In the gaseous phase and for the molecule isolated in an Argon matrix, monomeric 3-amino-1-propanol exists as a mixture of conformers, the first and second lowest energy forms corresponding to conformers which exhibit an intramolecular OH-N hydrogen bond (forms I and II). On the other hand, in the pure liquid, where intermolecular H-bonding occurs, the monomeric unit within the aggregates assumes a conformation similar to that of the third most stable form found for the isolated molecule situation (form III), which is characterised by having a weak intramolecular NH-O bond. The experimental data obtained for the pure liquid also reveals the presence of monomeric form I in this phase, a result that is in consonance with the strongly stabilizing OH-N intramolecular hydrogen bond that is present in this conformer.

A combined matrix-isolation infrared spectroscopy and MO study of 1-amino-2-propanol

“…In agreement with the strongest intramolecular OH Á Á Á N hydrogen bond present in the conformational ground state of 3AP, which reduces its tendency to aggregate, the spectroscopic data obtained for this molecule clearly show that monomers assuming this conformation are also present in detectable amounts in the pure liquid [4], while for 2AE and 2AP, where the intramolecular OH Á Á Á N hydrogen bonding is considerably weaker than in 3AP, no experimental evidence could be found for the presence of monomeric species in the pure liquids [3,5].…”

“…As for 2AE and 2AP [3,5], and contrary to what was found for 3AP [4], temperature variation studies carried out on the pure liquid show that there are no detectable amounts of free monomeric molecules in the pure liquid 1AP. The different behavior of 3AP, when compared with the remaining aminoalcohols, has been correlated with the larger conformational exibility in 3AP, which leads to a more favorable OH Á Á Á N intramolecular hydrogen bonding interaction geometry [4,5,27].…”

“…A plot of nOH versus the OH Á Á Á N distance in the previously studied molecules was found to obey to a straight line correlation and demonstrated a progressive strengthening of the intramolecular OH Á Á Á N hydrogen bond in the most stable conformer of each molecule in the order 2AE < 2AP < 3AP [5]. The better ability of 3AP to take part in hydrogen bonding could be ascribed to the greater conformational¯ex-ibility of this molecule which leads to a more favorable interaction geometry [4,5]. On the other hand, the presence of the extra methyl group attached to the carbon atom bearing the amino group in 2AP was pointed out as the main cause justifying the stronger OH Á Á Á N intramolecular hydrogen bond in this compound when compared with 2AE, since it leads to an increased basicity of the amino group [5].…”

“…The different behavior of 3AP, when compared with the remaining aminoalcohols, has been correlated with the larger conformational exibility in 3AP, which leads to a more favorable OH Á Á Á N intramolecular hydrogen bonding interaction geometry [4,5,27]. Indeed, in general the most fundamental factors leading to energetically more favorable intermolecular hydrogen bonding relatively to intramolecular hydrogen bonding are geometrical requirements, which can be easier attained in the case of the ®rst type of interaction.…”

“…The molecules already studied include 2-aminoethanol (2AE) [3], 3-amino-1-propanol (3AP) [4] and 2-amino-1-propanol (2AP) [5], for which we were able to identify the most stable conformers and characterize the most important intramolecular interactions that are responsible for their relative stability. It could be well established that, for the isolated molecules, OH Á Á Á N and NH Á Á Á O intramolecular hydrogen bonding may occur, the former being considerably stronger than the latter and being present in the most stable conformers of all molecules studied [3±5].…”