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 À1 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 fourcomponent 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(H 2 O), B as THF(H 2 O) 2 with two hydrogens attached to THF, and C as THF(H 2 O) 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.
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