Weak hydrogen bonds (π⋯HOH, CH⋯OH2) in aqueous medium and their dependence on solute’s charge polarity

Weak hydrogen bond interactions (π⋯HOH and CH⋯OH₂) of benzene (Bz) and π-electron containing molecular ions tetraphenylborate (TPB⁻) and tetraphenylphosphonium (TPP⁺) are investigated in aqueous medium using Raman difference spectroscopy with simultaneous curve fitting analysis (Raman-DS-SCF), molecular dynamic simulation and density functional theory (DFT) calculation. The OH stretch spectra of the hydration water, extracted from their experimental Raman spectra of Bz, TPB⁻, and TPP⁺ (20 – 40 mM), show distinct π-hydrogen bond (π⋯HOH) with the phenyl ring, which is assignable to the water that approaches the phenyl ring axially (C₆ axis). The strength of the π⋯HOH bond varies as TPP⁺ < Bz < TPB⁻. The interaction of water with the phenyl-CH (equatorial hydration along C₂ axes) is reflected in the blue-shift of the C–H stretch, called blue-shifted hydrogen bond (CH⋯OH₂). Strength of the (phenyl)CH⋯OH₂ interaction follows an opposite order to that of the π⋯HOH (TPP⁺ > Bz > TPB⁻). These hydration shell interactions are in agreement with the MD simulation and DFT results. The 3200 cm⁻¹ region of the OH stretch band is enhanced in hydration shell of TPP⁺ but diminished in Bz and TPB⁻, revealing reinforced collective vibration of water in the vicinity of TPP⁺ and loss of the same in the vicinity of Bz and TPB⁻.