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.