Coupled proton vibrations between two weak acids: the hinge complex between formic acid and trifluoroethanol

Journal article

Raman and FTIR spectra of an acid–alcohol complex show complementary signatures from acidic and alcoholic OH stretching, proving its existence.When formic acid and 2,2,2-trifluoroethanol are co-expanded through a slit nozzle into vacuum, a single dominant, hinge-like 1 : 1 complex is formed in significant amounts and its two OH stretching fundamentals separated by 100 cm −1 can be unambiguously assigned by a combination of infrared absorption and Raman scattering. Quantum chemical calculations at different levels reproduce this finding in a satisfactory way and suggest that in-phase (Raman-sensitive and lower wavenumber) OH stretch excitation more or less along the concerted degenerate proton transfer coordinate in the hydrogen-bonded ring stays below the barrier for this concerted exchange. Anharmonic calculations indicate only weak intensity sharing with dark states coming into reach due to the hydrogen bond downshift of the OH stretching vibration. This well-behaved system sets the stage for acid combinations with more basic alcohols, where the in-phase OH stretching vibration is more difficult to detect, possibly due to fast intra-complex vibrational dynamics. It thus provides a benchmark point from which one can explore the evolution of vibrational resonances when the acidic proton meets a more electron-rich alcoholic oxygen.Deutsche Forschungsgemeinschaft https://doi.org/10.13039/50110000165

Authors: Schweer, SM; Nejad, A; Suhm, MA

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Royal Society of Chemistry
• Journal: Physical Chemistry Chemical Physics
• Volume: 24
• Issue: 43
• Pages: 26449-26457
• Publication date: 2022-11-09
• DOI: 10.1039/d2cp04176f
• EISSN: 1463-9084
• ISSN: 1463-9076
• Language: English
• Keywords: Formic acid; Chemistry; Organic chemistry; Raman spectroscopy; Proton; Optics; Physics; Alcohol; Fourier transform infrared spectroscopy; Hinge