Quantifying hexafluoroisopropanol's hydrogen bond donor ability: infrared photodissociation spectroscopy of halide anion HFIP complexes

Abstract

We report on the gas phase vibrational spectroscopy (3500–950 cm⁻¹) of halide anion complexes with 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) and its partially deuterated analogue (HFIP-d₁). Infrared photodissociation spectra of messenger-tagged X⁻(HFIP/HFIP-d₁), with X⁻ = Cl⁻, Br⁻, and I⁻, together with electronic structure calculations reveal O–H(D) stretching fundamentals that are red-shifted twice as much as those for the corresponding complexes with isopropanol and water, directly reflecting HFIP's enhanced hydrogen-bond donor ability. The harmonic analysis of the bands in the fingerprint region reveals that HFIP assumes a synperiplanar conformation in the complexes. The consideration of anharmonic effects is necessary to recover the efficient coupling between stretching and bending modes in the OH stretching region. An energy decomposition analysis shows that the roughly twice as large binding energy in the HFIP complexes vs. i-PrOH and water is determined mainly by differences in the electrostatic attraction. The observed red-shifts, which reflect the extent of charge transfer along the coordinate of the proton transfer reaction X⁻ + HM → XH + M⁻, correlate qualitatively with the difference in the proton affinities ΔPA = PA(X⁻) − PA(M⁻). A more quantitative agreement requires also considering differences in the hydrogen bond angle.