4006
C O M M U N I C A T I O N S TO THE E D I T O R
The Near-Infrared Spectra of Water and Heavy Water a t Temperatures between 25 and 390”
Sir: We have recently examined the near-infrared region of the HzO and DzO spectra at temperatures between 25 and 390”. We have okserved two new absorption bands at 6000 and 6600 A for HzO and three new bands a t 7200, 8000, and 8500 for DzO having molar absorptivities of The transition energies and the intensities of the five water bands between 5900 and 10,500 increase as the temperature of the liquid water increases, but these parameters cease to change for water above the critical point where the single-phase sample is restricted by the constant cell volume; i.e., the sample is a t constant molar concentration. The same changes were observed for the vapor phase above the liquid phase, and again the changes ceased when only one phase was attained. We found the changes in the transition energies and intensities of the water absorption bands to be linear functions of the molar concentration of water over the range of 55 to 4 M whether the state of the water is liquid or vapor. (Lower concentrations were not investigated because the observed band intensities would have been too low for accurate determination.) This indicates that the association of H20 molecules is continuously dependent upon the molar concentration regardless of the physical state, and essentially independent of temperature. The extrapolation of the band parameters to zero molar concentration gives values for the absorption band parameters of completely dissociated water molecules (Table I). These results disagree with some conclusions based upon spectral changes of liquid water through the 25 to 375” range that water molecules are dissociated near the critical point,l where the molar concentration is 17.5.
Table I : The Parameters of Some ru’ear-Ir Bands of Water at 55 and 0 M Concn, M
55
0
Transition energy, om-1
16540 15060 13520 11940 16900 15380 13800 12170
At 4 M and 330” the near ir bands of HzO vapor separate into fine structure, and this resolution of fine structure further indicates significant association of the water molecules at the critical point and a t even lower concentrations. The changes in the near ir bands of DzO over the 55 to 4 M range are very similar to those changes for the water spectra with comparable changes in the transition energies, the band intensities, and the resolution of fine band structure at the low concentrations. (1) W. A. P. Luck, Ber. Runsenges. Phys. Chern., 69, 626 (1965).
* T o whom correspondence should be addressed. DEVELOPMENT SECTION A CHEMICAL TECHNOLOGY DIVISION OAKRIDGENATIONAL LABORATORY OAKRIDGE,TENNESSEE 37830
J. T. BELL*
CHEMIC.4L
N.A. KROHN
R E C E I V ~DECEMBER D 8, 1969
Comments on “Near-Infrared Spectra of Water and Heavy Water,” by Bell and Iirohn
Sir: The observations of Bell and Krohn that the spectra of water above the critical temperature T , are density dependent are in accord with earlier ~ o r k . l - ~ I n addition, the heat content of water vapor shows that the interaction energy between the water molecules under saturation conditions at T , is 3.6 kcal/moL6 This induces a large disturbance of the rotation structure of the ir bands. I think we have to differ between the H-bond interaction (-8 kcallmol) and the normal intermolecular interaction (-3.6 kcallmol) .6 Both disturb the ir band-the H bonds strongly, the normal intermolecular forces less.7 An exception is the disappearance of the rotation structure of the vapor spectra owing to dispersion forces, but a quantitative analysis of this disappearance of the rotation structure is complicated,8 although in the case of H F we could show that H bonds
Molar absorptivity ( l o b ) , 1. mol-’ cm-1
1
0.5 20 30 7 10 100 135
The Journal of Physical Chemistry, Vol. 7 4 , N o . $8, 1070
E.U.Frank and K. Roth, Discuss.Faraday SOC.,43, 108 (1967). (2) W. A . P. Luck, ibid., 43, 135, 141 (1967). (3) W. A. P. Luck in “Physico-Chemical Processes in Mixed Aqueous Solvents,” Heinemanns, London 1967, p 28 and Figure 2.28. (4) W. A . P. Luck and W. Ditter, Rer. Bunsenges., 72, 371 (1968). (5) W. A. P. Luck and W. Ditter, 2. Naturforsch., 24b, 487 (1969). (6) W. A. P. Luck, Discuss.Faraday Soc., 43, 115 (1967). (7) W. A. P. Luck, J. Phys. Chem.,in press. ( 8 ) W. A . P. Luck, Z. ?!raturforsch., 6a, 191 (1951). (1)
