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enough that overtones with only four and five stretching atures up to the critical point. As the temperature is in-quanta can wntribute to absorptiou intensity in the visi- creased, hydrogeu bonding decreases in importance to theble. DzO is colorless, a t least for path lengths on the order point that a t 375OC the liquid peaks are only an average ofof meters, because a minimum of six stretching quanta 80 cm-' below the gas phase transitions of the table. Ourwould be required for any transition in the visible region. assignment in the table of the higher energy liquid phasetransitions from 600-800 nm is specnlative and should notGas Phases versus Liquid PhasesI t is interesting to compare the gas phase spectrum ofwater, for which all overtone and combination bands areassigned, with the liquid phase spectrum that appears tobe less completely described with respect to the i~r igm f

infrared and vislble vibrational bands. This is important inthat i t clarifies the role of hydrogen bonding in ih e liquidphase spectrum, a matter of some confusion in the severalsources that address the question of why water i s blue.Note in the table th at the liquid phase OH-stretchingband centered a t about 3400m-'(3) is red shifted from theeas ohase values of v, and VP bv several hundred.wavenumbem. This shi fti s the result of hydro-gen bonding in the liquid. The shift to lower energy in-duced by hydrogen bonding is seen perhaps most clearly ina comoarison of the stretchine frequencies observed for-monomeric and dimeric water in a solid nitrogen matrix.Tursi and Nixon (4)onfirmed earlier matrix isolationwork by Van Thiel et al. (5) o establish that, while themonomer OH-stretching bands vl and v3 were slightly redshifted (- 25 em-') from gas phase energies by the matrix,the frequency of the dimer OH stretch involved in a hydro-gen bond was red shifted to 3547 cm-'. That is 105 and 209em-' below the gas phase values of vl and v3, respectivelyHydrogen bonding shifts the overtone and combinationband transitions to lower energy a s well (see the table). I nfad, each of the near-IR bands of liquid water shifts tohigher energy as the temperature is raised (6). This hasbeen attributed by several workers (6, 7) o the decreasingimportance of hydrogen bonding with increasing tempera-ture. As expected from such arguments the near IR absorp-tion bands of ice are the most red shifted of all (6).The assignment in th e table of the four liquid-phasebands from 5000 to 11000 cm-' seems almost certain andaerees with that of Nassau (8)and of Luck (6). We ar e con-A c e d of the assignments by the workof Luck (6) n which

the near-IR absorptions were observed in water a t temper-Vibrational Tansitions of Gaseou s and L iquid(20 "C)WaterGasa Assignmenta Liquid Shift

16.899is Ref. 1:

5.15'6.90'8.40'

10.3'13.1 6(760 nm)13.51 (740 nm)'15.1 5(660 nm)15.1 5(660 nm)16.53(605 nm)

iq Ref 7.. ..'is Ref. 2: s=shouider; Ail liquid phase assignm ents of gas phase transitionswhich lie above 10,61 3 cm-' are based on the figure and arespeculative.Transition energies and shin are in units of l ~ ~ c m - ' .ransitions that inw iveexcitations of the bending mode vn areomined except when stretch plus bendcombination bands ap pear in the liquid spectrum. Aisoomined are several otherweak qas Dhase transitions that do not avwar to give rise to structure in the

be taken too seriouslv. The mai n- ~o in ts to show tha t thered absorption whicg gives rise & the blue color of liquidwater can be ascribed dausib lv to hieh e ue rw vibrationalovertone and comb i~at ion ands Ghich, f i e the otherbands of the table, have been shifted to lower energy byhydrogen bonding. We also have measured HzO spedralike those of the figure a t 1T nd at 51T. s he temper-ature is lowered, the band near 760 nm shifts to lower en-ergy but also broadens enough to increase the intensitynear 700 nm.However, the changes are small enough tha tthe color of water should not vary significantly with tem-perature between 0 and 50 'C.

We are not the first to call attention to the vibrationalorigin of water's blue wlor. However, Nassau, in his gener-ally excellent book (81, The Physics and Chemistry of Color,credits hydrogen bonding in water with strengthening thebonding and thus raising the frequency of high overtoneand combination bands. Such frequency increases wouldshift Hz0 monomer (gas phase) transit ions from the nearIR into the visible thus increasing the visible absorption ofwater. However. as we see from the table. hvdroeen bond-ing causes the stretching frequencies of GO& shift tolower, not hicher frequencies. Atkins, too, invokes hydro-gen bonding& emcia to the visible color of water and ice(9). nstead, it appears to us that the hypothetical liquidwithout any hydrogeu bonds still would be colored, per-haps even more intensely than i s actual water. Dera, too,invokes vibrational overtones as the oriein of the red ab-sorption by water. His work is notable for& thorough com-nilation of visible soectra of water (10). Haooilv. the ab-. ..:option coefficient: that he tabulates for water sampledfrom around the elnbe show that the ahsorotion seen in thefigure is cbarac tks tic of most oceans. ~b llut iou as notaltered the color of the earth 's great seas (10).

Still one may well ask, "Why when one looks a t a body ofwater, not through it. does one often see a blue color?"~ o h r e i11)has answered tha t question in his highly rec-ommended book, Clouds in a Glass of Beer. He makes itclear tha t any simple answer is bound to mislead. It turnsout that contributions to the observed color are made bothby reflected skylight and by the intrinsic absorption ofwater described above (11). Light scattering by suspendedmatter i s required in order that the blue light produced bywater's absorptiou can return to the surface and be ob-served. Such scattering also can shift the spectrum of theemerging photons toward the green, a color often seenwhen water laden with sus ~e nd ed articles is observed.Furthermore, as Bohren shdws (111,the relative contribu-tion of reflected skvlieht and the light scattered back fromthe depths is ~tron "~1i de~e nden tn observation angle.

The absomtion s~ ec tr umf ice (6. 12) is similar to tha tofthe liquid;xceptihat hydrogen bondingcauscs all peaksto shift to lower enerm.An elegant description of thecolorstransmitted by snowand ice also has been given by Bohren(13).His paper should be consulted for the fascinatingstory of how multiple scattering combined with red lightabsorption in snow gives rise to a "vivid blueness" beneathsnow's surface th at exceeds in ouritv "that of the bluestsky". Bohren also discusses (13)how ;he larger grain sizesof bubblv ice allow deeo oenetration of incident lieht and areflectei hue that c a i ;aw from blue-green to-blue de-Dendine on the color of the surface t ha t underlies the ice.

Students usually are fascinated by the topic of color. Be-cause the mechanism is unique, the story of the vibrational.liquid

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origin of the color of water and ice should attract their cu - Literature Citedrious minds. I. ~ ~ ~ ~ b ~ ~ ,.~ ~ f ~ ~ d ~ ~ d ~ ~ ~ ~ ~ ~ ~ ~9 4 5 : ~81.2. Curcio, J . A.; Petty,C . C .J. h m Phys. 1951,41,302.3. Marecha1.Y. Hydmgen-Bandad Ligulda: Dore J. C. : Teixeira, J . Eds.; NATO AS1Series, 1989.329, p 237.Acknowledgment 4. n r a i , A. J.; ~ i ~ ,. R. . them. P ~ Y * . 970.52,1521.5. Van'hisl , M.; B~ke r ,E . .;Pimentel,G .C . J. Ch em. Ph yg 1957,27,486.6. L uck , W A P B e r B u n s e w s P h ys ik . Ch am . 1965.69.626.Grant DE-5G02-86ER13592 the Office Of Basic En- 7. Thomas, M. R.; ehersga H.A.; Sehrier ,E. . : J P h ys . C k m . I885,69,3'l22; Buijs,ergy Sciences, United States Department of Energy pm- K; happin,G.R. J. c h . hys . 1 8 6 s , 6 ~ , 2 0 3 ~ .vided partial support of this work. comments from 8. N ~ S ~ U . ph ys i c s ch mis t r y or color ; N ~ W orh

1983:oo 71-74,many of our colleagues a re gratefully acknowledged. 9. ~t& w M O ~ C U ~ ~ ~ :dentific~ m d - ~ i b r ~ :~ Work, 987,P 24.10. Ders, J.MminaPhys&;Elsevjer: Amsterdam. 1992. p 155ffCraig F. Bobe n of The Pennsylvania State University pro- IL ~ ~ h ~ ~ ~ ,r a ig F C I O ~ An c las s o r~ee r ;~ I W : ~ W Y O ~ ~ ,=?: pp15~170.vided a particularly helpful &tique of an earliermanu- 12. Grenfel l ,T C .; Perovich, D. K J CIophys. Res. 1981.86.7441;Warren.S G . A p p l .Optics 1984. 1206.Script. 13. Bohren,C. F.J.Opt S o c h . 1983,73,1646.

614 Journal of Chemical Education