Molecular structure and hydrogen bonding in 1,2-diaminopropane and 1,3-diaminopropane and their binary mixtures with water studied by NIR spectroscopy and DFT calculations

The molecular structure and hydrogen bonding in liquid 1,2-diaminopropane (12DAP) and 1,3-diaminopropane (13DAP) have been studied as a function of temperature and water content by using near-infrared (NIR) spectroscopy and density functional theory (DFT) calculations. Detailed analysis of the experimental data was achieved with a two-dimensional (2D) correlation approach and chemometrics methods. As is evident from present studies, in the water-poor region (XH2O<0.5)(XH2O<0.5) the molecules of diamines are in the same environment as those in the pure liquid diamines, where cyclic associates dominate. Molecules of water do not break these associates, but they act as double donors to different associates of diamines. This way, addition of water strengthens the structure of liquid diamines. Only a small amount of water is singly bonded, but the population of these species increases with the temperature rise. Most of the water molecules in the mixtures with diamines are involved in stronger hydrogen bonding than those in bulk water. The significance of water–water interactions increases at higher water content (XH2O>0.3)(XH2O>0.3). The present studies reveal that the intramolecular hydrogen bonding for both diamines does not appear or is very weak. In diluted CCl4 solution the molecules of diamines predominantly remain non-bonded, whereas at higher concentrations they prefer to create the intermolecular hydrogen bonds.