Estimation of Concentration and Bonding Environment of Water Dissolved in Common Solvents Using Near Infrared Absorptivity
Brian Dickens, Sabine H. Dickens
Integrated near infrared (NIR) absorbance has been used to determine theabsorptivity of the Ņ2 + Ņ3 combination band of the asymmetric stretch (Ņ2) and the bending vibration (n3) 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 least squares fit of a straight line to data from water concentrations between 0.01 mol/L and 0.06 mol/L. Absorptivity measured from the peak maximum of the Ņ2 + Ņ3 combination band of water varies with the type of hydrogen bonding of the water molecule because the shape of the NIR absorption envelope changes with the hydrogen bonding.Because the integrated NIR absorptivity of the Ņ2 + Ņ3 combination band ofwater is essentially constant across the absorption envelope, the NIR absorption envelope reflects the distribution of hydrogen bonding 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 molecules to easily polarized atoms. Water that is a donor in hydrogen bonds to atoms 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 hydrogen bond to an easily polarized atom (such as a water molecule oxygen) absorbs 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 environment.Hydrogen bonding of water to toluene, ketones and esters perturbs the O-H frequency of the water molecule only slightly. With little or no interaction with the host molecule, as for water in toluene, the width of the NIR absorption is approximately} 35 cm-1. For more interaction, the width of the NIR absorption increases to about 75 cm-1 to 100 cm-1, depending on the polarizability of the recipient of the hydrogen bond. Water in N-methyl pyrrolidinone appears to make one hydrogen bond to an easily polarized group which acts like the oxygen of water in perturbing the frequency of the hydrogen bond.
analysis, determination of water in solvents, hydrogen bonding distribution, near infrared spectroscopy
and Dickens, S.
Estimation of Concentration and Bonding Environment of Water Dissolved in Common Solvents Using Near Infrared Absorptivity, Journal of Research (NIST JRES), National Institute of Standards and Technology, Gaithersburg, MD, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=851486
(Accessed December 8, 2023)