Observation of conformational changes in 1-propanol-water complexes by FTIR spectroscopy

Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) measurements were carried out on the 1-propanol-water (abbreviated as 1PA-W) mixtures over the entire 1-propanol molar fraction range at 298 K. The two bands at ∼1053 and ∼1068 cm^-1, assigned to the vibrational modes of the gauche (v_C-C-C-O-G) and the trans (v_C-C-C-O-T) conformational isomers, respectively, which both include C-O and C-C stretching motions, were used to monitor the structural changes of the mixtures. When the water to 1-propanol molar ratio (WPR) is smaller than 0.2, the absorbance ratio of the two bands (A_vC-C-C-O-G/A_vC-C-C-O-T) remains constant at 1.42, characteristic of the existence of the 1-propanol aggregate chains, hydrogen-bonded by the O-H groups of 1-propanol in gauche conformations. When increasing the WPR from 0.2 to 20, there is an abrupt decrease in the absorbance ratio (A_vC-C-C-O-G/A_vC-C-C-O-T) from 1.42 to 1.01, corresponding to penetration of water molecules into the gauche-aggregate chains. The penetrated water molecules disrupt the 1PA chains and transform these gauche-aggregate 1PA chains to trans-aggregate chains, which are 1PA dimers of trans-conformation. The structural change induces complicated spectroscopic changes, including the red shifts of the series of bands 1016, 1053, and 1098 cm^-1 and blue shifts of the bands 2877, 2937, and 2961 cm^-1. With further increase of WPR up to 100, the absorbance ratio of A_C-C-C-O-G/A_vC-C-C-O-T increases from 0.98 to 1.07, indicating a transformation of partial 1PA dimers to single molecules with gauche-conformation in the water hydrogen-bonding network. Together with results from quantum calculations at the B3L YP/6-31G (d, p) level, and two-dimensional infrared correlation and excess spectroscopy analysis, the structural evolution of water and 1PA molecules in 1PA-W mixtures has been inferred.