Isotope effects in liquid water by infrared spectroscopy. V. A sea of OH4 of C2v symmetry

The two water gas OH stretch vibrations that absorb in the infrared (IR) near 3700 cm−1 are redshifted to near 3300 cm−1 upon liquefaction. The bathochromic shift is due to the formation of four H-bonds: two are from the labile hydrogen atoms to neighbors and two are received from neighbors by the oxygen free electron pairs. Therefore, the water oxygen atom is surrounded by four hydrogen atoms, two of these make covalent bonds that make H-bonds and two are oxygen H-bonded. However, these permute at rate in the ps range. When the water molecules are isolated in acetonitrile (MeCN) or acetone (Me2CO), only the labile hydrogen atoms make H-bonds with the solvent. The bathochromic shift of the OH stretch bands is then almost 130 cm−1 with, however, the asymmetric (ν3) and symmetric (ν1) stretch bands maintained. When more water is added to the solutions, the oxygen lone doublets make H-bonds with the available labile hydrogen atoms from neighboring water molecules. With one bond accepted, the bathochromic shi...