Estimation of concentration and bonding environment of water dissolved in common solvents using near infrared absorptivity

Integrated near infrared (NIR) absorbance has been used to determine the ab sorptivity of the upsilon(2) + upsilon(3) combination band of the asymmetri c stretch (upsilon(2)) and the bending vibration (upsilon(3)) for water in several organic solvents. Absorptivity measured in this way is essentially constant across the absorption envelope and is found to be 336 L mol(-1) cm (-1) with a standard deviation of 4 L mol(-1) cm(-1) as estimated from a le ast squares fit of a straight line to data from water concentrations betwee n 0.01 mol/L and 0.06 mol/L. Absorptivity measured from the peak maximum of the upsilon(2) + upsilon(3) combination band of water varies with the type of hydrogen bonding of the water molecule because the shape of the NIR abs orption envelope changes with the hydrogen bonding. Because the integrated NIR absorptivity of the upsilon(2) + upsilon(3) comb ination band of water is essentially constant across the absorption envelop e, the NIR absorption envelope reflects the distribution of hydrogen bondin g of the water. The shape and location of the absorption envelope appear to be governed mostly by the number of hydrogen bonds from the water molecule s to easily polarized atoms. Water that is a donor in hydrogen bonds to ato ms which are not easily polarized (such as the oxygen of a typical carbonyl group) absorbs near 5240 cm(-1) to 5260 cm(-1). Water that donates one hyd rogen bond to an easily polarized atom (such as a water molecule oxygen) ab sorbs near 5130 cm(-1) to 5175 cm(-1), and water that donates two hydrogen bonds to easily polarized atoms is estimated to absorb near 5000 cm(-1) to 5020 cm(-1). Water donating two hydrogen bonds to other water molecules may be said to be in a water-like environment. In no case does a small amount of water absorbed in a host material appear to have a water-like environmen t.