mained the same as before heating. When deuterium is added to adsorbed isobutew, d_'H ban& become rapidly weak with the appc'ararrce oi" dJl?i nnd 011 bands. E. ~ ~ - ~ e t h ~ ~ ~ 'I'he u ~ espectrra n e - ~ obtnmcd , from the adsorption of ~ ~ ~ ~ ~ t h yaw ~ ahovc b u IJ t i ~r li'iganre ~n ~ - ~ 3b. Considerable spectral chsnges en evacuation are again attributa bPe to the removal ol physiealiy ndslnrbd olefin. Addition of hydrogen led to t h isopentanc in the gas phase; on cvaeii~.I. gena bicm si nri llar nn tensity c b argm other bul,enes. KOspecific forand for tdw arisorhrd r3rpec:im b~cxizrc tained were rather weak, and t b hilities for an adsorbed hgdrocarb owever, S ~ M Pmethane, in a d wa3 observed in Ibc gas phase a f gexiatcd specie; tu 150". Acilnoi/ibedgl.nen2.
One 06
11s
(A. K
B.P.(I%t*riiicds) L1 d,, for a main
is grai,ei'ul i o ec grant duv ing
the course of the work. (11) A . Rrnit, Doctoral Thesis, Vniver&ty of Ct'treoht, 1972
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Solvent ~ t ~Spectroscopy ~ v ~
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ox and E. Hayon" Pionrering Research Laboratory, U.S. Army Natick Laboratories, Naticlc, Massachusetts 01760 (Rem ived April 19, 1972) PubEzmtion costs assisted by the U.S. A r m y Natick Laboratories
A systematic study of
the far-ultraviolet spectroscopy of some polar liquid solvents has been carried out with the object of extending the range a t which the spectroscopy of certain solutes can be determined. The solvents examined include water, D20, methyl alcohol, CHzOD, CD,OD, methyl cyanide, ethyl cyanide, n-propyl cyenide, trifluoroethyl alcohol, hexafluoro-&propyl alcohol, dimethoxyethane, and acetone. The effect of temperature on the absorption spectra of these solvents was examined. A considerable blue shift was found for most solvents with decrease in temperature. The most significant temperature effects were found for hydroxylic solvents. Absorption spectra a t different temperatures are given, and the transmission limits of the above solvents down to 60,500 cm-1 ( ~ 1 6 nm) 5 have been tabulated.
One of the principal limitations of far-ultraviolet solution spectra is the absorption of the solvent itself, Thus aqueous sohition spectra are limited by water absorption, for which1 k -- 1.46 em-' a t 54,090 cm-I (185 rim) and 298"K, These limitations are also overe m p ~ a s ~ in z esome ~ e w e s on inspection of "uv eut-off'' charts for spectroscopic solvents. Such charts usually refer io ambient temperatures and 1-lQ-mm sample
thickness. As part of a program to study the electronic absorption spectra of inorganic and organic molecules into the far-uv r e g i ~ n ,we ~ , ~report the spectra of a number of solvents determined over a wide temperature range and at small sample thickness (optical path). (1) J. Barrett and A. L. Mansell, Nature (London), 187, 138 (1960).
(2) M. F. Fox and E. Hayon, Chem. Phys. Lett., 14, 442 (1972). (3) M. F. Fox and E. Rayon, t o be submitted for publication.
The Journal of Physical Chemistry, Vol. 76, N o . 19, 1972
$1. F.Fox AND E. HAYON
2704
Experimental Section Materials. Solvontr were purified following niethods describcd in the l i t e r a t u r ~ . ~Once the specified physical propertics of the solvents u w e attained, the criterion of purity was t akrn as “best” whcn no further drcreasc. in thr ultraviolet absorption waa obtained on further treatment . Water wits distilled three timw, the second and third di>tillation,3 from permanganate and dichromate, respc.ctively. This \+atcr was then irradiated uith y rays and substquently photolyzed with a 2537-A mercury lamp. LIZ@ Mas distilled from allralinc permanganate undthr rdtrogcn. 1Iethyl aicotiol (Eastman Kodak Spectroscopic grade) \+as alloacd to stand in contact with calcium hydridc for 24 hr atid mas then distilled undcr nitrogvn. Isdbutyl alcohol, 2,2,2-trifluoroethyl alcohol (both Eastman Kodab), arid 1,1,1,3,:~,:?-hexafluoro-~-propyl alcohol (PCR, fnc , Gainesvillc, 1~1:~) w r c similarly t rtia ted . Spectroscopic grad(. mcthyl cyanidr arid ethyl arid n-propyl cyanidrr (isonitrilr-frec, I3astman 1Codak) w t w distillrd from Drieritc undcr nitrogcn aftw allo\+irig t o starid for 24 hr. Spectroscopic grade acctoric and dimethos1 &hanth (ICastman and Matheson Colt.man and Ucll) ivrrc similarly trcatcd. Spectrol?hIJ10711etr.y. A Cary 15 low-uv recording spcctrophotomctclr libtmlly flushed with very dry gasc’ous nitrogen (using Lindr-Division, Union Carbidc liyuid nitrogrn) \vas used to obtain thc spectra of the absorption cdl, arid then of the cell with the solvc~ntin thc sample bcain. Solvent spectra N crr obtained by subtraction 111thr region of interest, ccll absorption varicd onlj .rlightly with wave numbcr. For rach detrrmination t h r refcrmce beam containcd a similar, blit empty, ccil to balanccl light scattered at the airsilica Intcrfacc.. Cylindrical cells of 1.0- and 0.1-mm path lcngths (hminco) a i d 25- or 7 X 10-3-mni path length deniountable crlls (L‘V-01 Bcckman ccll) werr used. The lattrr n’m’ uscd as flon-through cells, a low flow rate of solvmt boing maintained using a syringe backed bjr a pump. No difficulty was experienced in maintaining, ovw a wid(. tcmperature range, a solvent film in the TTV-01 cc4s using the flow-through technique In the rangc 273-32XoIi, the temperature was controlled by aquwus g!ycol circulated from a thermostatcd (and rvfrigwated) bath, through a Cary ccll holder. Tliermal contact between cell and holder was maintained using copper plates and 1% edges. Below 273°K a VI,T-2 variable temperature dcwar unit with TE3I-ICI coiitroller (Bcckman) was used with solvent samplcs hcltl in an FH-01 cell (Bwkman). The VLT-2 unit \+as ponitroned in the sample compartment of t h t Cary 15 nith th(b outcr \~indowsremoved. The samplr compartmcvit was kept at atmospheric pressurc as thtb
solvent film rapidly disappeared hen vacuum was applied. The molar absorption coefficients given here have been corrected for thermal drnsity variations, using litcraturc values where possible and otherwisc by extraor interpolation. All the solvent spectra were detcrmined in the absence of air by flushing prepurificd nitrogen (AIatheson) through thc~soiwnts. Mlliporc filters (hlillipore Corp ) wrre usrd to rcmove all particlrs from thc nitrogrxn uscd to flush t h e Cary 15 spectrophotometer Con4dcrable care was taken to keep continuously all thc optical paths u ithin thr spectrophotomcter clean and in an inert at moqphcrr.
Results The present absorption spectroscopy work for water and DzO bridges the gap between t h e lo\r-cnc~gyspectra’,‘ and that obtained from thin film studim using a vacuum spectrograph.G Excellent agrcrnicnt dolt n to c55,000cm-’ is found bcttvccn thiq work ( w c I’igurc> 1) and thc temperature studies of flalmann, et a/.,5 and Barrett, et al.,’ for both water and D20. Dtwcmc in temperaturc and/or substitution of D2O for H20 shifts th(h absorption edge to higher energies (1;igurc. 1). T h r grltphical data of Verrall and Scnior6 have brcn photograplid and enlargcd to obtain comparison with our data for HzO and LhO in the 53.000-61,000-crn-’ region. Good agreement w a y obtairird as far as th(. limits of mea~urcmcntfor this work One parameter for the percentage of hydrogen bonds “broken” in watcr has been taltcn7 as thr ratio bet\{wn the. molar absorption intcnfiiticbs of liquid water and rvatcr vapor at the same n a w nuinber. Such a comparison is incorrect sinccl the. rcspectiw absorption maxima are separated by 10,000 cm-lG and perhaps accounts for thr rcsult of O.ln/;, of hydrogen bonds broken at 274°K. When integrated inteiisities arc’ compared, the opposite result is obtained, thr ratio indicating bctwwn 50 and 100% hydrogcn bonds ar(-’ broken. Tht. qam(’ shift of the absorption cdgr to highrr cncrgies on deuteration has been found in the spectra of methyl alcohol, mcthyl alcohol-dl, and methyl-& alcohol-& (Figure 2 ) . Ilecreasc in tcmpcraturc. again shifts the absorption edge of methyl alcoho! to higher encrgicu. The large liquid rang’ of organic solvents, particularly a t iow temperaturrs, 13 infrcquently exploited : cy., at IY3”K methyl alcohol is essentially tramparent doun to .%,OOO crn-’ arid has the‘ same limit as mt%hyl-da alcohol-dl at 273°K. The molar (4) See, e.g., “Techniques of Organic Chemistry,” L’ol. V I I , A . Weissberger, Ed., Interscience, New York, N. Y., 1955; J. A . ltiddick and W. €3. Bunger, “Techniqu?s of Chemistry,” Vol. 11, Interscience, New York, N . Y . , 1970. (5) hI. IIalrnariri arid I. l’liitzner, J . I’hua. Chem., 70, 580 (1966). (6) It. E. L‘errdl and W.8 . Senior, .I. Chem.f’hYS., 50, 2746 (1969). (7) D. 1’. Stevenson, J . I’hys. Chcm., 69, 2145 (1965).
Figure 3. Molar absorption coefficients of trifluoroethyl alcohol (TFE) and hexafluoro-2-propy~alcohol (HFP) both a t 298 and at 274°K ~
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Figure 1. Molar absorption coefficients of liquid DzO (curves and c a t 278, 298, and 32S°K, respectively) and HzO (curves d, e, arid E at 276, 298, and 328'K, respectively). a, b,
Vvwmmber, IO-' cm-'
Figure 2. Molar absorption coefficients of CHaOH a t 328, 298, 273, and 198°K.. For comparison, the spectra for CH30D and C:D&JDboth a t 2'13°K are shown.
cient of methyl alcohol a t 54,090 cm-l (185 nm) and 298'K obtained in this work, 8.8 M-I cm-l, is close t o Ihe value of Weeks, et a1.,88.4 M - 1 6m-l. There is a possibility of a low-intensity absorption band under the low-temperature (198°K) absorption edge (see Figure 2 ) .
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Attention has been drawngto the enhanced transmittance of fluoroalkanes relative to alkanes. A similar situation exists for the fluorinated 8116ohok4, 2,%,2-triAuoroethyl alcohol, and 1 , 1 , 1 , 3 , 3 , 3 - h e x a A ~ o r o - ~ - ~ ~ o ~ y ~ alcohol (compare Figures 2 and 3). The spectra are temperature sensitive and the limit for ~ , ~ ~ ~ - t ~ ~ f l ~ o ethyl alcohol i n a O.l-rnm thiclc cell at 274°K .is 60, cm-l (Figure 3) ; the limit for h e x a ~ ~ o r ~ - 2ai-~ro~~~ coho1 lies beyond 61,000 cm-1 (
