Infrared spectra of water. I. Effect of temperature and of H/D isotopic dilution

Abstract

The implementation of a cell operating with the technique of Attenuated Total Reflection (ATR) enabled us to obtain good quality IR spectra of water for the whole region above 750 cm⁻¹ (which is the limit of the effect of the cutoff wave number of the crystal) and up to 5000 cm⁻¹. ATR spectra have been recorded as a function of temperature ranging from −5 °C to 80 °C and of H/D molar concentration c(0<cc and T are investigated and analyzed. It is shown that any experimental spectrum in ε‘ of a mixture of normal and heavy water is a superposition of three spectra due to pure H₂O, pure D₂O, and a spectrum that is only slightly dependent upon isotopic concentration c and is consequently mainly due to HDO. Analysis of this slight dependence upon c allows precise conclusions concerning vibrational interactions for both bending bands δ and stretching bands ν_s to be reached. Evolution of spectra with temperature unambiguously shows that all spectra at temperature T may be decomposed into a low temperature spectrum and a high temperature spectrum. The coefficient defining this decomposition [Eqs. (11) and (14)] displays a quadratic variation with T. It is the same for H₂O, D₂O, and HDO within the precision of the experiment. A low temperature spectrum corresponding to a (slightly distorted) H‐bond network with no other defects, as well as a high temperature spectrum due to ‘‘defects’’ cannot, however, be determined without any further assumption. Limits for values of the concentration of defects α(T) may nevertheless be established.