Raman spectra and structure of hydrogen‐bonded water oligomers in tetrahydrofuran–H2O binary solutions

Raman spectra of hydrogen‐bonded water oligomers have been resolved from concentration‐dependent Raman spectra of tetrahydrofuran (THF)–water binary solutions with the use of multivariate curve resolution with alternating least‐squares (MCR‐ALS) analysis, assisted by hypothetical addition multivariate analysis with numerical differentiation (HAMAND) procedure. A total of 38 Raman spectra acquired in the broad spectral range of 700–3810 cm−1 were analyzed in a multistep procedure to explain the spectral variations in terms of four components: pure THF, Spectra A, Spectra B, and Spectra C. A four‐component MCR‐ALS analysis was performed with initial guesses by HAMAND analysis to refine the spectral components and determine their concentration dependence. Of the MCR‐ALS resolved three spectra, A was identified as THF(H2O), B as THF(H2O)2 with two hydrogens attached to THF, and C as THF(H2O)n having two or more linearly hydrogen‐bonded oligomers of water (n = 2–4). Observed spectral changes of the OH stretch vibrations of water and those of the C–C/C–O stretches of THF were well reproduced by quantum chemical calculations. Formation of hydrogen‐bonded water oligomers in THF–water complexes has thus been revealed. The present study provides a new insight into the basics of hydrogen bonding formation in aqueous systems. It also provides experimental methodology for extracting concentration‐dependent minor spectral components contained in complex condensed‐phase Raman spectra. Raman spectra of hydrogen‐bonded water oligomers are resolved from concentration‐dependent Raman spectra of tetrahydrofuran(THF)–water binary solutions with the use of multivariate curve resolution with alternating least‐squares (MCR‐ALS) analysis, assisted by hypothetical addition multivariate analysis with numerical differentiation (HAMAND) procedure.