The electronic spectra and vibrational assignments of carbonyl chloride and formyl fluoride

JOI‘RNAI, OF MOLECCLAR

8, 323337

SPECTROSCOPY

(1962)

The Electronic Spectra and Vibrational Assignments Carbonyi Chloride and Formyl Fluoride”? L. E. Department

JR.

GIDDIXGS,

01 (‘hemisi,y,

SKD

I’anderbilt

Ii.

KEITH

~‘niwrsity,

of

INNES

Na&cille,

Tennessee

The absorption spert,rum of COCl, in the region 2100-3150 A was studied at high dispersion and for gas temperatures from -70” to +25O”C. Absorbing paths were 0.005 to 5 m-atmos. The two hundred strongest sharp heads below 3050 A are interpret,ed as Q-branches of d-type asymmetric top bands and are fit,ted into a new vibrat,ional scheme. Two vibrational frequencies of the escit,ed state are established: v6 = 430 cn-l and ~6 = 580 cm-i. The analysis thus indicates that the electronic transition moment8 is perpendicular to the CO bond and that, in the excited state, the molecule does not have a plane of symmetry. A striking temperature-sensitive continuum of phosgene is discussed. An absorbing path of 5 m-atmos of HCOF extended to longer wavelengths t,he spectrum recently published. The new bands make possible new assignment~s which give a value of ~6’ (570 CII~-~) that is reasonable by compari.-on with H&O and COCI,. I. INTIIOI~UCTIO~

The earliest thorough analysis of an ultrariolet spectjrum of a polyatomir molecule appears to be that of Henri and Howell for the 3000-A absorption by COCl,

(1). In that pioneering

paper 263 observed

to within t’wo or three wave number fundamental

frequencies,

wits

$22 and 254 cm-’

c’n-I for t.hr lower state. Despik now seems ripe for coneideration.

sharp band heads were fitt’ed

by an equation

containing

only four

for the upper state and .X2 and ~02

the impressiveness The reasons

of t,he equation,

are: (1) The infrared

the sp&rtmi and Raman

specka of phosgene hare been analyzed (2-/,) so that, one may hope to intrrprct vibrational differences of the electronic spect,rum in terms of changes in geometry of t’he molecule;

(2) The geometric

structure

of the gromld

state

moltcult

has

been fomld t,o be t’hat of a planar asymmetric top with inertial vonstant,s 0.2622, 0.11.59, and 0.0805 vm-’ (5). Thus, if (1) is successful, the excited-st,at,e molevttlar shape

is determined.

CHaCHO

hare

* Sonlt~ of the the Internat ional t Supported by Research Projects

been

(3)

The

discussed

analogous recently

transitions (H-8)

so that

of H&O, comparison

HCOI;,

and

of a rather

results presented here were report,ed at the August, 1961, meciing of Union of Pure and Applied Chemistry in Montreal. the Office of Ordnanre Research with funds supplied by the Atlv:~ncctl Agency. 32x

ELECTRONIC

SPECTRA

OF COCl, AND

HCOF

329

different carbonyl compound is interesting and perhaps helpful in clearing up remaining difficulties with t’he former spectra. The corresponding spectrum of each of the other molecules is dominated by progressions in Y& of about llOO1200 cm-‘. Henri and Howell did not find such a large difference for phosgene. Few transitions originating from excited vibrational levels (“hot” bands) have been found for the related molecules. Henri and Howell assigned 80% of t’heir bands as hot. Points such as these may be checked more easily in light of recent experience. (4) It might be thought that the corresponding emission of phosgene could be used to check the Henri and Howell conclusions, but the emission spectrum has not yet been obtained (9). Since most of the absorption spectrum is somewhat diffuse, the failure to emit may be due to predissociation. The new analysis found here for t,he phosgene bands suggested improvements of our recent treatment of the corresponding formyl fluoride bands. Experiments with longer absorbing paths of HCOF have supported those improvements. The new bands and revised analyses given here for the two molecules indicate that in each case the electronic transition moment is about perpendicular to the CO bond and that the excited state molecule has no axis or plane of symmetry. II. EXPERIMENTAL

Difficulties in removing the last traces of SO2 from the commercial product persuaded us to prepare phosgene by the direct reaction of carbon monoxide and chlorine (10). Formyl fluoride was prepared as described previously by Giddings et al. (7). Several absorption cells were used, varying from a IO-cm all quartz cell to a 500-cm monel tube with quartz windows waxed on. Provision was made for cooling the cells with solid COz as well as for heating with split-tube furnaces and heating tape, to 300°C. The continuous background for the absorption photographs was the Hanovia 150~watt high-pressure xenon lamp. Eastman Kodak SA-1 photographic plates were used. A model 14 Cary spectrophotometer gave low dispersion recordings of t#he spectra. All high dispersion data were obtained with a 3.4-meter plane grating spectrograph, using second and third orders, respectively, for phosgene and formyl fluoride. The dispersion was l.-3 A to 0.7 A/mm and the band heads were measured against iron lines of second order. No obvious fine st,ructure was resolved but the heads are about 0.1-0.2 A wide for phosgene; the strong bands certainly can be measured to better than 1 cm-l for phosgene and 5 or 10 cm-l for formyl fluoride. The lower dispersion measurements for phosgene by Henri and Howell (1) agree with the present ones to about f 3 cm-l. We list in Tables I and II only those bands ?~otobserved in the earlier work. In addition we note t,hat phosgene bands listed by Henri and Howell at 33,097, 33,329, 33,531, are probably SOS bands. At frequencies higher than 34,225 cm-l a number of phosgene bands were observed by Henri but, not by us. Most of these are quite weak. Since they are in a region where some bands have begun to appear diffuse it is

TABLE

I

PHOSGENE BAND-HEADS NOT OBSERVED BY HENRI AND HOWELL (Estimated relatk’i%k%)ies 32,327(l)* 32,434(O)* 32,439(O)* 32,X8 (00) * 32 )522 (00) * 32,542(O)* 32,564(00)* 32,568(00)* 32,G04(00)* 32,625 (00) * 32,714(1vh)* 32,730(1vb)* 32,747(1vb)* 32,764(1vb)* 32,873(2)* 32,878(3)* 32,905(3b)+ 32,977(2)* 32,98313j* 33,010(4j* 33,019(2)* 33,044(2)* 33,049(l)* 33,119(3)* X3,123(2)+ 33,128(4)* 33,1-Q(1)* 33,147(l)* 33,157(2)* X3,164(2)* 33,185(2)* 33,189(2)* 33,205(2j* 33,207(3)* 33,211(2)* * Bands measured room temperature.

33,219(l)* 33,225(O)* 33,230(O)* 33,235(O)* 33,240(O)* 33,246(l)* 33,260(2)* 33,263(2)* 33,267(2)* 33,277(2)* 33,284(l)* 33,292(3)* 33,298(3)* 33,309(3)* 33,31614)* 33,342(4b)* 33,350(2)* 33 355(0?)* 33:375(l)* 33,385(l)’ 33,406(3)* 33,413(3)* 33,421(3)* 33,442(3b)* 33,453(3)* 33,45815)* 33,472 12vb)* 33,503(o)* 33,511 (oj* 33,556(3)* :33,600(2)* 33,609(1b)* 33,624(2)*

for gas temperature

in parentheses) 33,631(3) 33,639 (1) 33,643 (2) 33,665(1?) 33,699 (3 ) 33,704(4) 33) 708 (2) 33,710(2) 33,739(3) 33,745(3) 33,766(1?) 33,768(1?) 33,789(3) 33,800(2) 33,806(l) 33,811(3) 33,817(1?) 33,827(2) 33,830(1?) 33,8+2(2) 33,847(3) 33,852(11) 33,858(l) 33,867(l) 33,874(l) 33,879(l) 33,889 (2) 33,931!1b) 33,940(0?) 33,948ili) 33,955(11) 33,994 (4) 34.001(1 j 34,050(2b)

of 185” C. Other bands

34,120(l) 34,125(l) 34,13010) 34,154(2) 34,ll(iO(2) 34, l(ili(3) 34,214(2) 34,220t3) 34,23512) 34,246(2) 34,274(l) 34,280~:~) 34,283(1’) 34,286(Z) 34,289(3) 31,356(1 1 34,3(il i I 1 34,3lii11) 34,X113) 34.380(2) 34,385(l) 34,388(l) 34.398(13 34,401(l) 34,403(l) 34,40X(2) 34,411(l) 34,413(l) 34,416(2) 34,422(2) 34,430(2) 34,441(l) 34,4,52(l) 34,4lil(l 1

obtained

frolu g:ts at,

assumed that they have been missed because of t’he higher dispersion of t,hc present work. III.

DISCUWION L I Lb

Henri and Howell (I) observed that the absorpt’ion by phosgene is weak at its origin war 33,500 cm-’ and t#hat it grows steadily stronger at, short,er wave-

ELECTRONIC

SPECTRA

OF COClz AND

TABLE HCOF

BANDS

OBSERVED

ONLY

WITH

II ABSORBING

5 m-atmos

kc

(cm-*)

38,190 37,850 37, 760b 37 )405b 37,050(?) 36, 940b 36,835h 36,490

331

HCOF

PATHS

GREATER

THAN

.B Prohahle Assignment 38,877-687 39,696-1846 38,877-1117 38,500-1095 38,877-1827 38,036-1094 37,938-l 103 37,852-1362

a The faint band at 36,650 cm-l, list,ed previously, was not observed in the present experiment. b Bands which extend to lower energies the Y:, progressions of Ref. 7. The other bands listed are assumed to originate in the excited vibrational levels noted.

lengths, where it also grows diffuse. We find this to be correct, except that the absorption reaches a maximum at about 43,100 cm-l. From the displacement of the maximum from the origin we infer that a large change of geometry occurs which is intermediate in magnitude between those for the corresponding transitions of H&O and HCOF (6, 7). It is interesting to note that the origin is also intermediate in position bet’ween those of H&O and HCOF. Little help in determining the nature of the change of geometry of phosgene can be expected from rotational structure. The molecule is a strongly asymmet’ric top, and resulting complex band structures should be further complicated by overlapping lines of corresponding COC~Z~~and COC135C137bands (of relative intensity 3 and 2, respectively). This expectation is verified by the absence of rotational fine structure even when resolving powers of 150,000 to 200,000 have been employed. We therefore turn t’o the vibrational analysis and the FranckCondon Principle as guides to the change of geometry. The key to the vibrational assignment,s is t’he observation of the effect of temperature on relative band intensit.ies. The best dat’a obtained for phosgene were at -80°C and +9O”C bet’ween 34,900 and 35,600 cm-’ and at 25°C and 170°C between 33,400 and 34,000 cm-l (See Fig. 1). Only four or five bands of several dozen studied definit,ely originated in excited vibrational levels. Three of these are evident in the figure, near 33,600 cm-l, while fewer are found at higher energies. The approximate Boltzmann distributions for lower state levels of phosgene at t,hese two pairs of temperatures are given in Table III. The intensity of any hot band relative to a neighboring ‘Lcold” band should have been changed by more than a factor of two by each temperature change. It seems likely that most, hot bands would have been noted, and certainly those hot by more than 600 cm-1 would have been recognized. Spect,rophotometer studies of t,he wider

GIDDINGS

0 B $

AXD

INNES

ELECTRONIC

SPECTRA

OF COC12 AND

TABLE

HCOF

333

III

APPROXIMATE BOLTZMANN DISTRIBUTIONS FOR LOWER STATE LEVELS OF PHOSGENE

0 285 440 580 850 1827

2:l l:&! 1:2 I:4 1:10 1:200

2:l.O 1:0.8 1:l.O 1:1.3 1:2.0 1:8.0

region 2740-2900 A, at phosgene temperat’ures of 25” and 2OO”C, confirm the paucity of hot bands. Since Henri and Howell interpreted most of the bands of these regions as hot by more than 600 cm-l, a need for a new analysis is indicated. Most of the sharp heads appear in pairs, a strong component, and a weaker component 6 to 10 cm-’ to the red (see Fig. 1). Henri and Howell (1) interpreted these doublets as corresponding bands of COC1235and COC135C137, respectively. This has been supported by selective photodecomposition of COC1,35 using the 2816.2-A line of the aluminum arc (11). Further verification is fomld in the fact that the doublet components respond equally to temperature changes. However, Henri and Howell ignored their general conclusion in making detailed assignments. For example, the isotopic pair at 35,483 and 35,492 cm-l were assigned as “hot” by 604 cm-l and “cold”, respect#ively. The need for reassignment is clear. Since most of the phosgene transitions seem to originate in the ground vibrational state, most observed band intervals should represent vibrational differences of the excited electronic state. Two such differences, 580 cm-’ and 430 cm-l, dominat’e the strongest bands. There is some evidence of a difference of about 2 X 580 cm-’ which is the expected CO stretching frequency of the excited state, but the 580-cm-1 interval and the onset of diffuseness at about 36,200 cm-l make it difficult to confirm the 1160~cm-’ frequency. The fundamental frequencies found from the COC1,35 band heads are Y: = 430, Y; = 581, Y! = 438, and vi = 587 cm-l, and the system origin is at 32730 cm-l. The resulting assignments include most strong bands (see Fig. 1) and are shown in Table IV. Band-heads of COC135C137 are each, within experimental error, 7 cm-l to the low energy side of one of the heads shown in Table IV. A very slow trend to larger separations can be seen toward higher energies, but the isotope shifts of the active vibrations are clearly very small. Addit#ional broad features occurring with many of the isotopic doublets are pointed out in Fig. 1 and discussed below. Assignments could be made easily for the many remaining weak bands in the spectrum. Such assignments would be very tentative and do not seem worthwhile at this stage.

The absolute numbering for Table IV was foulld from the results of the high temperature experiments and, as with diatomic molecules, by noting point’s at, which rather abrupt changes of t’he intervals and the character of t’hc table occur. For example, the most active difference of Table IV decreases from the top to thr bottom of the table from 4-20 cm-’ to about 400 cnl-I. It has IKYII assumed t#hat differences -MO and XX cm-’ are lowrr state differences bec*an~~ they both correspond to t#heRaman line at 439 cm-l wit,hin experimcnt,al t’rror. The next difference, 130 cm-l, seems so low t’hat it’ must be associated with the excited stat’e. The temperat#ure dat’a of Fig. I are in agreement’ with this rouelusion. According to current interpretations of the infrared and Raman spclctra of phosgcne (2-b) neit#her 582 nor 439 cm-’ corresponds to a tot,ally symmetric mot,ion. The former is assigned to the out-of-plane motion and the lattrr t20 an unsymmetrical in-plane bending motion (4). These would not be expect,ed to give isot’ope shifts as large as those of CC1 stretching motions. However, long progressions in the corresponding upper state frequencies could occllr only if the molecule loses both of its planes of symmetry by the transition. That ih, t.hc phosgene molecule is evidently strongly nonplanar and exhibit#s &ikingly different, OCCl angles in its electronically excit,ed &ate. These conclusions abotlt t,he active frequencies and the changes of geometry arc quite analogous to thosct drawn earlier for HCOF. In the HCOF paper (Giddings et al. (7)) it was emphasizrd that, t,hr>barrirr to invrrsion between the potential minima is high compared to t,hat#of FT,( X ) (6). A high inversion barrier is more easily underst’ood in thr case of phosgcnc. For phosgene, two further potent,ial minima arise from thr nonrcllliv:ll(,n(, ehlorinr atoms. The long regular progression in t*he frequency that givrs rise>to thr nonrcluivalmce indicates a second very high barrier. With somewhat less cert#ainty we suggest that the CXZZbond is grc>atly lengthened by the transition. For HCOF, & was a vihrat,ion prominent as a11 interval iu the spectrum at high frequencies. As explained above the analogous bands of COCl~ are not’ easy to locat’e. However, support, for this position is folmd ill thr striking temperature-sen&ve continuum first observed (tho\lgh not interpreted) by Xlmasy and Wagner-Juaregg (12). Our expcrimen& have amply vrrifird thrir results. For example, a sample appropriate for phot,ographing sharp bands in the region 2800-2870 X if the gas is at 90°C absorbs t,he sam( amount of light totally when the gas is at 160°C. This dots not arise from enhancrment8 of int)ensity of obsrrved bands. Our interpretat’ion is that, a continuum arises from transit#ions which originate in levels of 1827 cm+ (&) or greater and terminat’r in the predissociatrd statrs at 36,200 cm-’ and grrat,cr. Thcsc hot, hands involving ~6, presumably art relatively st,rong because r&, is grcatrr than r&. The Roltzmann distributions already noted are consistrnt wit,h this view. This coincidence of factors makes it, easy to understand why such a st,rong t$fec~t of temperature is very rare or unique.

E

_

which

whioh

‘I Bands

C Bands

do not increase

when

higher

in intensity

in intensity

(1)

(4)

when

gaseous

419

(3) 579

(3) 584

(4) 586

(4) 583

phosgene

HEADS

for those

higher

(-1

(-)

(-1 582

(-) 681

(4) 578

(-) 590

34,500 CIII-1.

36,833 427 37,260 405 37,665 415 38,080 S.96 38,476 410 38,886

36.3Yl (-) 44f

than

(-) 575

(-) 569

(-1 591

(1) 575

(6) 581

(-) 587

38,730 (-)

419

36,216 435 36,679 413 37,092 9.97 37,480 418 37,907 404 38,311

35.378 (1) 4% 35,814 (1) 577 4SB

frequencies

35.654 (2) 592 428 36,082 (5) NY 416 36,498 (5) 594 416 36,914 (1) 575 411 37,325 (-) 581 426 37,751 (-1 560 434 38,185 (-1 545 $95 38,580 (-) 414 38,994 (-)

35,220 (1) 594 4%

are listed

35,058 (5 )a96 434 35,492 (5) 590 42s 35,915 (4) 5&Y 419 36,334 (5) 580 419 36,753 (5) 571” 4la 37,165 (-) 586 414 37,579 (-) 606 408 37,987 (-) 59J S96 38,383 (-) 611 996 38,779 (-)

34,182 (3) 445 34,627 (4) 593 431

BAND

IV

. cm-‘)*

mensurements

is heated.

ia heated.

Hem-i’s

37.005 (4) 574 408 37,413 (2) 574 997 37,810 (-1 575

36,000 CIC’,

(5) 591

(2) 585

(4) 596

(2) 589

(5) 591

(2) 586

(5) 580

phosgene

about

gaseous

than

(-1 576

(3) 577

(1) 577

(4)VSS

(4) 581

(3) 580

(3)“5@

(5J ‘5%

(5) 678

34,480 487 34,907 485 35,332 416 35,748 421 36,169 417 36,586

(3) 58f

33,8!?8 439 34,327 419 31,746 411 35,157 483 35,580 410 35,990 4to 36,410 ‘$12 36,822

(-1) 58P

33.316 429 33.745 421 34,166 410 34,576 481 34,997 417 35,413 4to 35.833 413 36,246 597 36,643 499 37,046

at frequencies

(I)

(I) 588

(-1 579

(5) $85

(2) 574

(2) 580

(2) 581

(4) 568

(IJ’J78

(2, 581

(1) 586

increltse

are diffuse

a Bends

35,071 (5jbsss 40s 35,474 (3) 590

3i2A6 (2) 587

119

33,010 (4) 578 40s 33,413 (3) 581 414 33,827 (2) 589

32,130 434 33.164 4% 33,588 406 33,994 419 33,416 417 34.833 486 35,259 402 35,661 403 36,064 S.96 36,460 397 36,857

33,600 (4)%88 437 34,037 (5) 590 441

33.019 (3) 581 4% 33,158 (U”579 440

bv,,

COCl:”

TABLE STRONG

32.439 (0) 580 439 32.8;s (3) 580 49s

THE

37,423 415 37,838 408 38,246 416 38,662

(-)

(-)

(-)

(-) 58f

38,004 (-)

37,565 (-) 439

:3:x

GIDDINGS

AND

Ih’NES

It seems worthwhile to comment on the phosgene band types. There is litAt> doubt t,hat the sharp features of phosgene are collected Q-branches of A-type bands of an asymmetric top; it. is not easy to conceive of such sharp features in R- or (‘- type bands (13). About. 30 cm-l to t’hc blue of each of the longer wavelength characteristic pairs of Q-branches is a broad feature (IPig. 1) whicah WV int,erpret as a coincidence of R-heads. These features show roughly the same temperature dependence as their respective associated doublets. The axis of smallest inert#ia, a, is perpendicular to the CO bond in thr ground state COCl, molecule. It, is t,hereforc assunled that, the electronic transitjiou moment is approximately perpendicular to the CO bond, as in the cas;cs of the corresponding t’ransitions of H&O and HCOE’. The results for phosgene indicate possible improvements of our earlier vihrational assignmrnt,s for HCOE’ and DCOF (7). By empirical arguments t& progressions were found at int#ervals of -MO and 190 cm-l (7). The corresponding were assigned to number for DCOF were UO and 120 cm-‘. These frequencies tht> ecluivalent of excit’ed-stat.r FCO-bending and out-of-plane mot#ions, rcspc~ tircly. The smaller frequencies seemed low and the isot’ope shift, somewhat large. SOW that’ it has been found t,hat# not only HL’O (6) hut COCl, (ahovc~) shows Y; = 330 VIX+, it seems wort’hwhile t’o try an analysis for H(‘OP’ using a similal freyuc,ncy. It will be noted t’hat the interval obt’ained by taking every t,hird of the 190 cm-’ spacings for IICOV gives a frequency (570 cm-‘) close t.o thoscx of H&Y1 and COCl,. Similarly, every fourt’h of t#he 120-cm-’ spacings for l)(K)I~ gives the frequency (-180 cnl-I) which seems a more normal isotope shift for th(x o&of-plane motion (6). It’ turns out’ t’o be quit,e simple to rtarrange t,hc H(3 )I< and DCOF bands taking account of these changes of vi but leaving t#hf>UO- alrd I 1OO-cm-L frequcnc~ies unchanged. For HCOTi only a change of relativ(l nt~mlIering of the five strongest v& progressions is involved. The nrw system origin is t’hen at, least MO cm-l to t,he red of t,he old (see Giddings rt al. (7)). ‘I‘h(>VO~Isistency of the new assignments is checked by t#hc bands of Table Ii wht~ it is s~tn that three z&, progressions &end to energies lower t,han that of t hc old O-O band (37,310 cm+). Since the absolutcx numbering is st,ill doubtful and tbta O-O band may lie at still lower energies, we do not’ include further dct,ail.q of IIW as;signmrnt s here. Comments analogous t’o those of the precaeding paragraphs apply also to our recent discussion of acetaldehyde (8). ACKNOWLEI)GMENTS We are indebted t)o Mr. 1’. M. Griffin who made availablr to w a Illicrol)hotolrlt,frr. Professor M. Lindauer, a iS\‘ntional Science Foundation Summer Research Participnnt at Vanderbilt in 1960, made our first, photographs and measurements of the phosgene spcct runs. The automatic reductions of data and cnomputations of Boltzmann factors were sul)port cd by thP Sational Science Foundation under Grant ?;o. NSF-(~1008.

R WKTVEI) : September

2.5, 1’361

ELECTRONIC

SPECTRA

OF COCl, AND

HCOF

337

REFERENCES 1. V. HENRI ANU 0. R. HOWELL, Proc. Roy. Sot. Al28, 178 (1930). 2. E. CATAI,ANO ANI) K. S. PITZER, J. Bm. Chem. Sot. 80, 1054 (1958); B. SCHNEIDER AND J. STOKR, Coil. Czech. Chem. Comm. 27, 1221 (1961). S. J. OVERENI) AND J. C. EVANS, Trans. Faraday Sot. 66, 1817 (1959). 4. J. OVIRENI) ANI) J. R. SCHERER, J. Chem. Phys. 32, 1390 (1960). 5.
8. 9. 10. 11. 1%. 1s.

NUOVOcimenlo [IO], 16, 861 (1960); G. GIACOMETTI, G. RIGATTI, AND G. SEMERANO, Nuovo cimento [lo], 16, 939 (1960). K. K. INNES AND L. E. GIDDINGS, JR., J. Mol. Spectroscopy 7,435 (1961). P. J. DYNE AND D. W. G. STYLE, J. Chem. Sot. p. 2122 (1952). R. E. KIRK ANU D. F. OTHMER, “Encyclopedia of Chemical Technology”, Vol. 10. Interscience, New York, 1953. W. KCHN ANU H. MARTIN, 2. phys. Chem. B21,93 (1933). F. AI,MASY AND T. WAGNER-JUAREGG, 2. phys. Own. B19.405 (1932). H. C. AUEN, JR., Phil. Trans. Roy. Sot. A263, 335 (1961).