Infra-Red Absorption of HDO in Water at High Pressuresand TemperaturesBY E. U. FRANCK AND K. ROTHInstitut fur Physikalische Chemie, Technische Hochschule,Karlsruhe, Englerstr. 1 l/GermanyReceived 16th Jamary, 1967By use of a specially designed cell with a sapphire window the absorption of the OD-stretchingvibration of HDO in H20 has been measured at frequencies from 2200 to 2900 cm-1 and at tempera-tures and pressures from 30 to 400°C and from 50 to 4000 bars respectively. At the supercriticaltemperature of 400°C and pressures below 200 bars (corresponding to a density of water of 0.1 g/cm3)the rotational structure of the vibration band of free water molecules is observed. At higher densityonly one intensive absorption maximum with simple shape is observed at all temperatures.This isconsidered as support for the continuum model for water in the liquid and in the dense supercriticalstate.New information on the structure of liquid water has recently been derived fromnear infrared and Raman spectra 1-12. The frequency and intensity of the absorptionof the oxygen-hydrogen vibrations indicate the existence and the extent of hydrogenbonding. The OD-vibration of dilute HDO in H2O is particularly suited for suchinvestigations because of the absence of interference from other frequencies and forother reasons.1, 2, 8 , 13 The temperature dependence of this vibration is of specialinterest. Wall and Hornig 1 and Falk and Ford 2 have obtained important resultswhich support the assumption of a continuum model for liquid water.The infra-red spectrum of liquid water in the overtone region above 5000 cm-1 atsaturation pressure up to the critical point at 374°C and 221 bars has been measuredby Luck.9 Other authors have also recently published spectra in this frequencyregion for liquid water at saturation pressure and elevated temperatures.10-12 TheOD-vibration of HDO, however, has only been determined up to 13OOC.2 No infor-mation about the pressure or density dependence of this vibration was available.Therefore, it was the purpose of this work, to investigate separately the influence oftemperature and density on the OD-stretching frequency of HDO.These investiga-tions had to be extended to supercritical temperatures in order to observe the variationof the absorption from very low to very high density of wster at constant temperature.It should be possible to determine the conditions under which OD-vibrationnot affected by hydrogen bonding is detectable.To obtain this information measure-ments were made to temperatures and pressures up to 400°C and 4000 bars respectively.EXPERIMENTALAn optical cell of the reflection type similar in some ways to that used by Welsh 19 wasdesigned. The cell has a single window, made of a colourless synthetic sapphire of 10 minthickness. Attached to the surface of the sapphite is a platinum-iridium mirror. Spacersof gold foil determine the distance between the mirror and sapphire. This space, fdled withthe fluid, provides a path length of twice the distance between the mirror and sapphire10E.U . FRANCK AND K. ROTH 109surfaces. Thus the path length is independent of the applied pressure. The aperture ofthe sapphire window is 8 mm. The body of the cell is made from a non-corrosive nickelalloy. It is covered by the coils of the heating wire and enclosed in a double-walled brasscase cooled with water. The cell body contains two thermocouple wells. A detaileddescription of the cell will be given elsewhere.The housing of the Perkin-Elmer model 521 grating spectrometer was flushed withdry air. It was furnished with a Micro Specular Reflectance Accessory which had beenslightly changed in order to mount the high-pressure cell outside the normal sample area.A 1 : 1 slit image could be produced inside the cell on the Pt-Ir mirror from which the beamwas reflected.The beam is refocused on the entrance aperture of the instrument by atoroidal mirror. A narrow stainless-steel capillary connects the cell to the pressure generatingunit. This unit consists of a hand pump, a pressure intensifier* and a separator vessel inwhich the pressure is transmitted from oil to water. The pressure is measured by calibratedBourdon gauges. The DzOt was 99-7 % pure ; the mixtures of D20 and H2O were preparedby weighing.0.HDO---------I I I I 8 1 I t2 8 0 0 2600 2400v [cm-11FIG. 1.-One set of original curves of the % transmittance as a function of frequency v in cm-1 at400OC and 3900 bars (0.9 g/cm3) ; path length, 35 p. 1, background absorption of H20 andsapphire; 2, absorption of a 8.5 mole % solution of HDO in H2O; 3, zero line, representinglight reflected by the water-sapphire surface and light emitted by the hot cell body ; --- region of C02absorption with reduced reliability.The accuracy of the temperature measurements, at the highest temperatures is f5"C.The pressure is correct within f 5 bars below 250 bars and within 340 bars beyond loo0bars.The water density has been calculated using the VDI-steam-tables 21 up to 1000 barsand the experimental PVT-data of Maier and Franck 14 at higher pressures. The spectro-meter was calibrated using the water vapour bands, the C02 doublet and polystyrene foilwith the micro-reflectance unit and the empty cell in position. The frequencies are correctto within f2 cm-1.The path length was determined using the interference fringes of theempty cell. For every transmittance curve of HDO in HzO the spectrum of pure H20 atequal temperatures and pressures had to be determined. Fig. 1 gives an example for 400°Cand 3900 bars. Curve 1 is the transmittance of pure H20 and sapphire ; curve 2 representsthe solution ofHDO in H20 with a path length as before. Curve 3 was determined withoutthe Pt-Ir mirror; it represents the black-body radiation of the heated cell filled with the*Hamood EngineeringTFarbwerke Hoechs110 INFRA-RED ABSORPTION OF HDOHDO+H20 mixture together with the light reflected by the inner surface of the sapphire.From such curves the extinction coefficient IC of HDO and the integrated band intensity Bhave been evaluated using the relationsv2 .=I Icdv.Vi -M is the average molecular weight of the mixtures, X ~ O the mole fraction of HDO usingan equilibrium constant of 3~96~3~ 1 5 ~ ~ 2 0 denotes the density of water in g/cm3 at temperatureT and pressure P. d is the path length in cm and JO and J in % are the transmittance valuesaccording to fig.1.RESULTSThe HDO absorption was determined in the range 2200-2900cm-1. At 30, 100,200 and 300"C, different pressures selected to produce water densities of 1.1, 1.0 and0.9 g/cm3 were applied. At 400°C nine different pressures between 48 and 3900 barswere used. Table 1 gives a compilation of experimental conditions together withthe main properties of the spectra observed.TABLE ABSORPTION OF THE OD-VIBRATION OF HDO (8.5 MOLE % IN H20). EXPERI-MENTAL CONDITIONS, BAND MAXIMA, HALF-BANDWIDTHS, MOLAR EXTINCTION COEFFICIENTSAND INTEGRATED INTENSITIESTOC4004004004004004004004004003002003002001003010030Pbar489614419324128010652055390021 8060050002800100010044003080.Pg/cm30-0 1 650.0360.060.0950.150.3 10.70.80.90.90.91.01.01.01.01.11.1dcm0.10-10.10.10.13.5 x 10-33.5 x 10-33.5 x 10-33.5 x 10-33.5 x 10-33.5 x 10-33.5 x 10-33.5 x 10-33.5 x 10-33-5 x 10-33-5 x 10-33.5 x 10-3v maxcm-12719271 526602655264226372619261 3260525952578258725682540250725352505A v tcm-1I-157148152150157167-153195195168-K X 10-3cm*/mole-4.5610152122232835293342554556BX 10-5cm/mole78.510121828384145-58708599Fig. 2 demonstrates the variation of the OD-absorption with temperature atconstant density.The decrease of frequency and maximum extinction was alreadyexpected from the isobaric infra-red 2, 5 and Raman 1, 6 observations. At densitiesof 0.9 g/cm3 and higher, no trace of a second absorption is detectable. At lowertemperatures the shape of the bands is almost Gaussian, a fact which has been empha-sized by Falk and Ford.2 Above 200°C, however, the shape becomes increasinglyasymmetric. The wings of the bands seem to disappear always in the same frequencyregion ; only the maxima are shifted to higher frequency with increasing temperatureE .U. FRANCK AND K . ROTH 111The isothermal behaviour of the OD-absorption at one supercritical temperatureis shown by fig. 3. At the lowest density, which is 28 times the density of water--50 X 10’I . I * l . l . l . , 2200 2800 2600 2400r. _ _ _ I iv25001 1 , , 1 , , , , , 1 1 ,2aoo 2 6 0 0 2400 2 200v [cm-11FIG. 2.-Molar extinction coefficient K in crnz/mole of the OD-stretching vibration as a function offrequency v in cm-1. Absorption curves at constant densities of water of 0.9, 1.0 and 1-1 g/crn3and at temperatures of 30, 100,200,300 and 400°C.2e00 2 6 0 0 2 4 0 0K2 8 0 0 2600 2400v [cm-11FIG. 3.-Molar extinction coefficient K in cm2/mole of the OD-stretching vibration as a function offrequency v in cm-1 at a constant temperature of 400°C and at different densities of water (glcm3).Note the different base lines of the individual curves.vapour at normal boiling conditions, the R-, Q- and P- branches of the OD-vibration band are clearly observable.The peak of the Q-branch is at 2719 cm-1.Benedict, Gailar and Plyler give 2720 & 5 cm-1 for dilute HDO gas at room tempera-ture. This Q-branch was no longer observable in the present work at densities highe112 INFRA-RED ABSORPTION OF HDOthan 0-1 g/cm3, e.g., at pressures beyond 200 bars. The structures in the P- andR-branches at 400°C and 0.0165 g/cm3 (48 bars) are in accord with the contours ofthe vibration-rotation band of the free HDO molecule observed in the gas at atmos-pheric pressure.16. 17DISCUSSIONThe temperature dependence of the frequency Vmax, of maximum absorption andof the integrated intensity B is shown in fig.4. The results for Vmax at a density of1.0 g/cm3 corroborate the frequency increase found by Falk and Ford 2 at isobaricconditions. The decrease of B with temperatures was expected from earlier observa-tions. Fig. 5 presents the density dependence of Vmax and of B at 400°C. The curvesare almost linear between 0.2 and 0-9 g/cm3. No effect of the critical density (0.32g/cm3) is observable.0 I 2 0 0 400 IvrE:XQI IT K IFIG. 4.-Frequency vmax (in cm-1) of the absorption maximum and integrated intensity B (in cm/mole) as a function of temperature ; --- results of Falk and Ford 2 at atmospheric pressure. Thecurves are for constant density of water of 0.9, 1.0 and 1-1 g/cm3 respectively.The curves in fig.4 and fig. 5 suggest that a simple relation between 7max andBmay exist. This assumption is verified by the plot of fig. 6. All the experimentalpoints, although extending over nearly 400°C and over a wide region of densities,are lying within the range of experimental uncertainty on one curve, which is onlyslightly curved. The extrapolated value of V m a = 2670 cm-1 for B = 0 is lowerthan the frequency of maximum absorbance of the observed Q-branch of free watermolecules at the lowest density investigated (see fig. 3). One might presume that theextrapolated value of Vmax reflects the combined influence of the surrounding polarmolecules on the OD-groups without hydrogen bonding.Other authors haveobtained curves of the type of fig. 6 by compiling frequency shifts and intensitychanges for several different hydrogen-bonded compounds in one diagrain.2E. U. FRANCK AND K. ROTH2 7 0 0 -.1 -113** - 0 -branch 4 0 0°C-2700u €2 ’ 2 6 0 0 -1 1 1FIG. 5.-Frequency Vmm (in cm-1) of the absorption maximum and integrated intensity B (in cmlmole) as a function of density of water (in glcm3) at 400°C. In the upper left corner the maxima ofthe Q-branch at densities of 0.0165 and 0-036 g/cm3 are indicated (see fig. 3).- 2 6 0 00 so I0033 x 1 O-5[cm/mole]FIG. 6.-Frequency Vma (in cm-1) of the absorption maximum plotted against the integrated intensityB (in cmlmole).I, points at 400°C and different densities of water : *, points at different tempera-tures and density 1*0g/cm3; +, maxima of the Q-branch at densities of 0.0165 and 0.036 g/cm3(see fig. 3) ; ---, extrapolation114 INFRA-RED ABSORPTION OF HDOThe frequency increase of the maximum absorption of hydrogen bonded OH-and OD-vibrations has been related to an increase of the 0-0 distance of thehydrogen bond. If such a relation does apply here, the curves of fig. 2 and fig. 5imply, that the 0-0 distance increases with increasing temperature although thedensity of the water remains constant.At all densities of water higher than 0.1 g/cm3 and in the whole region of tempera-tures up to 400"C, there is only one absorption maximum for the OD-vibration ofHDO in H20 having a simple shape and no shoulder.Its frequency graduallyincreases from 2505 cm-1 at 30°C and 1.1 g/cm3 to 2655 at 400°C and 0.095 g/cm3.If one considers this absorption as being characteristic for hydrogen-bonded OD-groups and assumes that free OD-groups should be indicated by a separate absorptionaround 2700cm-1, then almost all of the oxygen-hydrogen groups in this rangeshould be to some extent hydrogen bonded. Only at densities below 0.1 g/cm3 at400°C is the occurrence of free water molecules clearly demonstrated by the rotationalstructure of the water spectrum. It is reasonable to assume a wide distribution ofhydrogen bonds with different energies and 0-0 distances corresponding to thebroadness of the absorption.One might presume that the character of the spectrumat higher density is entirely a consequence of the increased dipole interaction becauseof closer intermolecular approach. This, however, would not account for theincrease of the integrated intensity of the band by a factor of 14 when proceedingfrom 0.0165 to 0.9 g/cm3 at 400°C. For HCl the density increase caused only atwofold or threefold rise of intensity.18Thus, the infra-red spectrum of the OD-vibration of HDO in H20 gives noindication of non-hydrogen-bonded OD groups or of defined small clusters of watermolecules at a density higher than 0.1 g/cm3. In accordance with the conclusion ofWall and Hornig 1 and of Falk and Ford 2 these spectra are considered as supportfor the continuum model of liquid and of dense supercritical water.1 T.T. Wall and D. F. Hornig. J . Chem. Physics, 1965, 43, 2079.2 M. Falk and T. A. Ford, Can. J. Chem., 1966,44, 1699.3 C. A. Swenson, Spectrochim. Acta, 1965, 21, 987.4 J. G. Bayly, V. B. Kartha and W. H. Stevens, Infra-red Physics, 1963, 3, 221.5 K. A. Hartmann, J. Physic. Chem. 1966,70,270.6 G. E. Walrafen, J. Chem. Physics, 1966,44, 1546; 1964,40, 3249.7D. P. Stevenson, J. Physic. Chem., 1965, 69, 2145.8 R. E. Weston, Spectrochim. Acta 1962, 18, 1257.9 W. A. P. Luck, Ber. Bunsenges. Physik. Chem., 1965,69,626.10 W. C. Waggener, A. J. Weinberger and R. W. Stoughton, 149th Nat. Meeting A.C.S., 1965.11 R. Goldstein and S. S. Penner, J. Quant. Spectr. Rad. Transfer, 1964, 4, 359,441.12 M. R. Thomas, H. A. Scheraga and E. E. Schrier, J. Physic. Chem., 1965,69,3722.13 R. D. Waldron, J. Chem. Physics, 1957, 26, SO9.14 S. Maier and E. U. Franck, Ber. Bunsenges. Physik. Chem., 1966,70,639.15 R. E. Weston, J. Chem. Physics, 1965, 42,2635.16 W. S. Benedict, N. Gailar and E. K. Plyler, J. Chem. Physics, 1956, 24, 1139.l7 E. F. Barker and W. W. Sleator, J. Chem. Physics, 1935,3, 660.18 W. West, J. Chem. Physics, 1939,7, 795.19 W. F. J. Hare, and H. L. Welsh, Can. J. Physics, 1958, 36, 88.20 G. C. Pimentel and A. L. McClellan, The Hydrogen Bond, (W. H. Freeman and Comp. San21 VDI-Steam Tables, Springer-Verlag and Verlag R. Oldenbourg, Berlin-Gott ingen-Heidelberg-Francisco and London, 1960), p. 96.Miinchen 6th ed., 1963