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.

Automated summary

Raman spectra of hydrogen-bonded water oligomers have been successfully resolved from the Raman spectra of tetrahydrofuran (THF)-water binary solutions using multivariate curve resolution with alternating least-squares (MCR-ALS) analysis, assisted by hypothetical addition multivariate analysis with numerical differentiation (HAMAND) procedure, providing new insights into hydrogen bonding formation in aqueous systems.