π-Hydrogen Bonding in Liquid Water

Abstract

Experimental MethodsRaman Spectral Measurements: Raman spectra were obtained using a home-built, micro-Raman system similar to that used in previous studies (1). The system used in the present studies includes an Ar-Ion laser source (514.5nm, ~ 50 mW power at the sample) and a thermoelectrically cooled CCD detector (Princeton Instruments Inc., Pixis 400, 1340x400 pixel) mounted to a 300 mm focal length imaging spectrograph (SpectraPro300i, Acton Research Inc.), with a 300 g/mm grating, such that the dispersion is approximately 5 cm -1 per CCD pixel. Liquid samples were analyzed in spectroscopic 1 cm glass cuvettes contained within a thermoelectric, temperature-controlled, translating cell holder (Quantum Northwest). The solution temperature was controlled to within ± 0.01 °C over a 0°C to 60°C temperature range. Such temperature control was required in order to avoid spurious spectral features in the solute-correlated (SC) spectra resulting from small differences in temperature between the solution and pure water samples. A helium lamp was placed behind the sample so that two He lines at 587.562 nm and 667.815 nm were visible in each Raman spectrum (on either side of the OH stretch band). The He lines were used to correct for small wavelength drifts (resulting from small changes in ambient temperature and pressure) which, if uncorrected, would also produce spurious features in the SC spectra within the OH stretch band. The frequency shifts were corrected by introducing a sub-pixel shift the wavelength axis of the solution spectra so as to precisely match the He peak positions in the solution and pure water spectra. The latter shifts were performed using IgorPro (Wavemetrics Inc.) which facilitates duplication of waves with sub-pixel shifts, using a command such as wave1=wave0(p+d), where p is the pixel number (variable) d is a real constant whose magnitude is typically less than 0.1, and wave1 and wave0 are the shifted and un-shifted solution spectra, respectively.Benzene: Spectrophotometry grade benzene (99.93 % assay, EMD Chemicals Inc., Germany) was used without further purification. Water was ultra-purified (Milli-Q UF plus, Millipore Inc.) to an electrical resistance of 18.2 MΩ •cm. Saturated benzene in water was prepared by thoroughly stirring a sample consisting of water with a small excess benzene for 5 min. The solution was then allowed to equilibrate at the desired temperature for at least one day (in equilibrium with the excess benzene phase). Raman spectra were collected from the aqueous phase of the resulting solution (with an excess benzene layer at the top) as well as from a pure water sample (at the same temperature). All samples were equilibrated for at least 5 min in the temperature controlled sample cell holder, and measurements were made in the same cell position. Total (un-polarized) Raman scattering spectra of each solution of benzene in water

Molecular Shape and the Hydrophobic Effect

“…(69) In combination with computational studies, their results revealed the OH–π bond is ∼20% weaker and more flexible than bulk HBs. Furthermore, their results suggest that dangling HBs are more entropically favored but less enthalpically favored than water-water HBs.…”

Solubilidade das substâncias orgânicas

“…15 A conversão de uma ligação de hidrogênio água-água (Figura 5a) em uma ligação de hidrogênio água-benzeno (nuvem π) (Figura 5b) é favorecida pela entropia (∆S > 0), mas desfavorecida pela entalpia (DH > 0). Apesar de incomum, este tipo de ligação pode ter relevância em processos biológicos.…”

“…Apesar de incomum, este tipo de ligação pode ter relevância em processos biológicos. 15 É importante ressaltar que a interação eletrons π -hidrogênio não afeta a solubilidade do benzeno em água (0,07 g/100 g de H 2 O a 23 o C), 4 portanto, o seu comportamento é o apresentado, em geral, pelos hidrocarbonetos: pouco solúveis em água e solúveis em solventes apolares.…”

IR spectroscopy of pyridine–water structures in helium nanodroplets

“…This conformer was found for the benzene-water heterodimer in previous experiments in the gas phase 33 and in the liquid phase. 34 Using DFT calculations, we found a structure in which one hydrogen is H bonded to the p electron network and the second hydrogen atom of the water molecule is bonded to the nitrogen. However, the expected field dependency for the D-O stretch for the band at 2776.2 cm À1 predicted for this structure does not match the experimental results (VTMA 46.11).…”