n-π∗ transitions in carbonyl group

IOCRNAL

OF MOLECULAR

SPECTROSCOPY

19,

-&(?2

n--7~* Transitions

(l%%)

in Carbonyl

Group

Electronic Spectrum of Carbonyl

Cyanide

*J. PHOCHOROW AND A. TRAMEK Instit,ufe of Physics,

Department

of Inorganic

Polish Academy

Chemistry,

of Sciences,

Warsaw

WTarsaw, Poland

I.niclersity,

b’arsaw,

Poland

The absorption and fluorescertre spectrum of the B* + n(‘.‘lz + tdl) trattsitiott itt rarbonyl cyanide CO(CN), was investigated at the high and itttermediary dispersion. The oscillator strength estimated from integrated itttensity measurements is f = 0.85~10-4. The (0, 0) band is missing attd all progressions originate at vibrottic levels involving nontotally symmetric vibrations. The molecule is supposed to preserve it.s plattar configuration in the excited stat.e. Intensity distribution suggests a pronounced lengthening of the C-O bond and a change in valence angles at’ t.he central carbott atom in the .I, state. Broadening of higher vibrat~iottal levels is due to predissociatiott, the predissociation limit depending on t.he number of quattt,a of the bending vibrat.iott. w,,,, = 23 550 cm-‘, the prottouttced red shift of carbotryl absorptiott being

caused

by the conjugation

and

inductive

efiect)s.

The &c%ronic sl)e&urn of carbonyl compounds and esl)ecially that of formaldehyde has been discussed by a number of aut,hors ( l--12). Walsh ( 3) and Brand (4) proposed an interpretation of the absorption and fluorescence spectrum of C’H& on the basis of the assumption of a nonplanar excited stat,e and this assumpt,ion was confirmed in furt.her works ( 7, 9). The authors suggest.ed that this devi:kion from planarity is responsible for the breaking-off of the A, - AI selection rule and for the relatively high intensity of the a* + II transition. On the other hand, Pople and Sidman (IO) gave a theory of the intensity-borrowing mechanism in t,he case of formaldehyde, involving nontot.ally symmetric vibr:lt ions and the cboupling of elect,ronic and rotakional motion. The study of other carbonyl cornl)ounds seems, c~onsequently, a subjwl of high interest. Recent studies of unsaturated aldehydes (6, 11, 12) have show1 t l’r‘esettl ettces

address: Ittstitute WarsarcPoland.

of Biophysics

and

Biochemistry,

Polish

.4cademy

of Sc.i

46

PROCHOROW,

TRAMER,

AND

WIERZCHOWSKl

that these molecules probably conserve their planar configuration in the excited state. Carbonyl cyanide CO(CN)2 is an interesting example of a symmetrically conjugated carbonyl compound. It belongs to the same symmetry group (C,,) as formaldehyde, and the a* +- n transition is rigorously excluded by electronic selection rules. Its electronic spectrum shows a well-resolved vibrational structure (1~) and its vibration spectrum in the ground state has been the subject of our previous study (14). EXPERIMENTAL

Samples of carbonyl cyanide’ were purified by vacuum distillation into a receiver cooled wit#h liquid nitrogen. The absorption spectrum in the gas phase was investigated in a l-m silica cell, the vapor pressure varying between 3 and 120 mm Hg. The spectrum was photographed with a Hilger E 492 spectrograph or registered by means of Zeiss SPM-1 monochromator equipped with an EM1 6256B photomult,iplier. Extinction coefficients were determined from photoelectric measurement’s under the assumption that the density of vapors is that of perfect gas in the same conditions. The temperature dependence of the spectrum was studied photoelectrically with the same cell equipped pith an electrically heated jacket. Very weak absorption bands in 44004000 A region were measured with vapor density of 5.83 g/l at 90°C. This region was reinvestigated in the presence of oxygen (p = 140 atm). The apparatus used was that described by Grabowska (15). The long-wave part of the absorption spectrum in the 4100-3200 8 region was d,hotographed in the first order of 6-m grating. The linear dispersion was 1.2 A/mm, the effective resolution and accuracy in frequencies of t.he order of 1-2 cm-‘. Absorption spectra of hexane solutions were investigated by means of a SP500 Unicam spectrophotometer. In ethyl ether, benzene, and other donor solvents the weak CO( CN)2 absorpt’ion is completely covered by much stronger chargetransfer bands (16). The fluorescence spectrum of vapors was excited by 3650-w Hg lines from a high-pressure mercury arc and photographed with a Huet BII spectrograph. Because of the weakness of the emission spectrum it was necessary to take 40-hour exposures with a slit of 0.1 mm. The accuracy of frequency determination was of the order of Ti cnl-I. The strong fluorescence of frozen n-hexane solutions was measured by both photographical and photoelectric methods. Optical orientation experiments were performed by means of a Bauer-Rozwadowski polarimeter (17) ; the rigid solution used was that of methyl-cyclohexaneiso-pentane glass. 1Samples of carbonyl cyanide were obtained from the Laboratory of Warsaw University owing to the kindness of Prof. 0. Achmatowicz.

of Organic Chemistry

Species

’ Coordinate YC’N

“(‘6n Y(‘c:

~ RI,,,

I&:,.

Species

C’oordinate

~ 2248 s 2242 s 1709 vs 1714 s ) 710 m 1 712 m

5ti5 w 300

m

~475 VW 255 II,

:’ CMcul:ttrd from overtorle frequellcies.

We have rc-investigated the infrared spectrum of CO( CN)? vnl~ors in fhc -lOO%$500 cm-’ region by means of an IJR 10 Zeiss spectrometer. The :lhsorl)tiorr I)ath was 10 cm, t,he vapor pressure 10 - 120 mm Hg. \.IBBATION

SPECT11IM

IN THE

(:ROIJNI)

STATE

The set of fundnmental frequencies given in Table I is t~akcn from our previous work (14 ) with some refinements introduced on the basis of our new me:tsur(‘ments of t)he infrared spectrum. The symmetry notation is that commonly usccl for c’r, molecules, i.e., Ri is antjisymmc~tric: with respect t)o the molecular I&LIIP. The envelopes of bands corresponding to A, vibrations [R-type for :~symmet ric. (Atylw) : we ; :ud to R, vihrz t,op molecules (,18)]: wt , 2we ; to B, vibrations tions (C-type) : wll have distinct’ly tliffcrent forms. From the frequencies of w2 = 1714 :md 2~~ = :ilOS cnC’ the :tllh:unloflic,i1!, con&nt is cvaluat’ed as xz2 = 10 cm ~‘.

The absorption spcckum at a low tlispersioll :LSwell as a schematic diagram of the fluorescence spectrum of the val)or is given in E’ig. 1 and 2. (‘orrcsl)onding d:lta for n-hexane liquid (absorption ) and solid (emission 1 solutions :Ire rq)r(‘scnted in P’ig. 2 and 3. The nbsorl)t,ion spect,rum consists of two distinct electronic transitions. The stSrong quasi-continuous absoq)tion beginning at -I3 000 NC’ I)rob:rbl\ :Zt its corresponds t)o t,he g* + ?r ( lL4, - l-4,) or Cry + ni ‘I?, + ‘dI) transilioli. long-wave side several diffuse bands with a spncing of -MI0 cn-’ m:ly be (lis(*errled. The weak absorption in the -&100-‘600 Ai region [oscillator strength est~imatcd to he j = i0.M f 0.15) .10-4] corresl,onds to the forhiddetl r* - ?I l.42 t- ‘_4] t rnnsition. The well-developed bands form the main ( A- ) progression involving 1wo frequencies: -12.50 cm-’ (by analogy to ot,her curbonyl com~~ounds it may

PROCHOROW, TRAMER, AND WIERZCHOWSKI

48

I,O-. 25

25

24

32

3i

2’6

33

27

34

2&

3’s

&7

39

I 31

3’6

b FIG.

1. Absorption spectrum of CO(CN)2 vapors.

be assigned to the C=O stretching frequency in the excited state) and 125 cm-l. The intensity distribution suggests that the transition is accompanied by a pronounced change in the C-O distance. At a high dispersion (see Fig. 4) there appear a number of weaker bands. Most of these also form 11~.1250 + n. 125 progressions, but with different active origins (B, C, D, and E progressions). Several weak bands camlot be assigned. The rotational constants of CO( CN) 2 can be estimated from the approximate values of interatomic distances and valence angles in the ground state. We obtain A = 0.245, B = 0.089, and C = 0.065 cm-‘. The rotational structure of bands cannot be resolved, but some bands show a fine struct,ure due to the maxima of &-branches. The absorption spectrum of the n-hexane solution is only slightly different from the gas-phase one. The w2progressions are distinct, but the bands are broadened

SPECTRUM

FIG. 2. Absorption

OF CARBONYL

spectrum

of CO(CN),

CYANIDE

in hexane solution.

FIG. 3. Fluorescence spectrum of CO(CN)? in frozen hexane solution schematic diagram of the fluorescence spectrum of vapors.

(T = 77”Bj

and

and their structure, due to ws vibrations, is diffused. The spectrum shows :L very slight red shift: AV = -300 cnl’. The intensities are much more sensitive t,o environment effects. Extinction coefficients are 4-5 times higher in the liquid; this difference is probably dur to donor-acceptor interactions with solvent molecules.

50

PROCHOROW,

FIG. 4. Microphotometer

tracing

TRAMER,

AND

WIERZCHOWSHI

of the vapor absorption

spectrum

at the high dispersion.

The a* +- n absorption in the hexane solution is followed by and partially superimposed on the strong structureless absorption beginrung at -3000 .k This band may correspond either to the r* + ?r (or (T* + n) transition showing an extremely strong red shift or, what seems much more probable, t’o the chargetransfer absorption of CO( CN)z-hexane contact charge-transfer complexes. The vibrational strucbure of the emission spectrum in the 4300-7000 8 region is quite analogous to that observed in absorption: the main progression (F-) and the supplementary one (G-) are formed by the frequencies corresponding Both frequencies were found to the same vibrational modes: 1714 and 126 cm. in the Raman spectrum of t’he compound and assigned (14) as w~(A~)v~~ = 1714 and ws(A1)Gs_c_c-c_N = 125 cm? (142 cm-’ in the liquid). The latter is a characteristic frequency of geminal nitriles (19). The emission spectrum in frozen hexane solutions, in MP glass and that, of crystalline (freshly distilled) compound at liquid nitrogen temperature has the same general form: w2 progression is preserved, the G-bands appear as shoulders at the long-wavelength side of F-bands. The red shift is of -2.50 cm’. Liquid solutions are not fluorescent. Vibrational analysis of the absorption and emission specka of vapors shows that the (0, 0) band is missing and that t’he absorption and emission progressions have no common origins. The very weak absorption bands (H-bands) corresponding to the first group in emission do not fit into any absorpt’ion progression. Two alternative interpretations can be proposed : (1) Absorption and emission correspond to the same ‘AZ ~ ‘AI system. The (0, 0) transition is forbidden by electronic selection rules; the active origins of progressions are X0” = woo+ w=’ (in absorption) or Xoo = woo- w: (in emission), where woois the frequency of the purely electronic transition and wZare frequencies of nontotally symmetric (a2 , bl , 62 in CZ,, symmetry group) vibrations. (2) The upper level in the emission spectrum is different than that involved in absorption. Since ‘AZ is the lowest singlet, state of carbonyl compounds, the emission would be considered as the 3A, +- ‘A,( T, +- So) phosphorescence. The lattrr alternat’ive does not seem, however, very probable:

SPECTRUM

OF CARBONYL

CYANIJ)E

.i I

( 1) The “12 t ’ ill t,ransition was observed in formaldehyde (4, 8), hut’ only it) t Iw :thsorl)tion sl)ectrum and never itr emission. The internal conversion of all exeit~ctl molcc~lrs to the t)riplet state in the gas is highly improbable and so is the st rang 1)hosI)horescence of a pure solid. OII the contrary, if the emission is considered as fluorescence, its low yieltl in the gas and in liquid solut,ions can be easily cxl)lained. The transition being strongly forbidden (the lifetime corresponding to the observed value of oscillator strength is 2. 10m7 SW), the cluemhing l)roc+esses in fluid media may be very efficient. (2) The absorpt)ion bands coinciding with the first group of bands in emission would be assigned as “hot bands” in t)he first case, and as a l’ - AS’absorption it1 the sec+ond. Their int,ensity ~~0.001 is t,oo high for an intersystem transitions and they seem to be temperature del)mdent. Moreover, the spect,rum in hit region remains l)ractically unchanged at a high c*oncentration of oxygen ( I’,,? = 110 atm ), i.e., in conditions where the 7’ - 5’ absorption is usually SttY)Jl&’

~~tlh:LIlWd

( 20).

I,unlineseence excited in the scc~ontl (25 400 mC’ ) and third (26 600 (~1 ’ ) absorl)tion band with 1)olnrized light, shows :t strong, posit’ivc polaris:~tion; the degree of l)olarization, ext,rapolated to infinite dilution, is p = 0.18 f 0.02. Taking int,o acclount t’he existence of some depolarizing fact,ors this value (~1 be considered as the lower limit of the true value. Transition moments of strongest bands in absorption ant1 emission are, then, parallel, while t’he T--S bands in formaldehyde spectrum are 1ongit)idunal (8 ) and the h-8 ones transvcrsal ( 1 ) . 0~1 this basis, t,hc emission sI)ectrum (in be assigned to the fluorescent (A’ + S ) tratisition. ( 13)

17BRATIONAL

ANALYSIS

OF THE ELECTRONIC

SPECTRUM

Since the a* G- ‘n transition in carbony c~on~l~ounds is forbidden by ele,rtronic selec%ion rules, transitions from t’he vibrat’ionless level of the ground state are allowed orlly when t,he over-all symmetry of the excited vibronic level is A, , R, , or H, . It tleclessitates the excitat,ion of an odd number of quanta of a2 , b2 , or bl vibrations, respec+ively. The same is true for the fluoresc*ence spect~rum where terminal levels are vibrational levels of the ground state involving an odd number of quanta of unsymmetric vibrations (lo). hs 110 frequency int’ervals corresponding t,o two quanta of unsymmetric: vibrxtions were found in absorpdion or emission, WC an assume t’hat, any progression is allowed by a single quantum of an unspmmetric~ vibration in the ground or excited electronic stak. The general formula representing frequencies of vihronic transibions takes in this (‘:ise the following form:

52

PROCHOROW,

TRAMER,

AND

TABLE

WIERZCHOWSKI

II

FLUORESCENCE SPECTRUM OF CO(CN)% Notation FO,-2 FO4 F 0.0 F 0.1 F 032 Fll.3 F 014 F0.5 G“,2 G0,s G084 G0, I, FL-2 FL_1 Fl.0 PI,1 F1.2 Ft.3 F1.4 FL5 FI 16 G 181 G1.2 G1.3 G1,4 GI.5 FM n 1ntensit.y

Frequencvi Calc. 23 23 22 22 22 22 22 22 22 21 21 21 21

219 093 967 841 715 589 463 337 010 884 758 632 511

21 21 21 21 20 20 20 20 20 20 20 20

385 259 133 OOG 881 755 629 503 428 302 176 050

19 924 19 825

estimation

where X00 are “active vibrations :

-- Intens.”

Ohs. 23 23 22 22 22 22 22 22 22 21 21 21 21 21 21 21

225 095 967 839 716 596 463 332 008 869 751 635 507 395 261 136

21 20 20 20 20 20 20 20 20 19 19

007 882 756 627 502 434 304 173 043 908 820

within

VAPORS

Frequency

Notation

Calc. 19 19 19 19 19 19 18 18 18 18 18 18 17 17 17 17 17 16 1B 16 16 16 16 16 16 16

F2.d FZ.lJ F 2,L

1 2 5 10 8 6

Fz,z F?,z F2.4

F 2.5

4 2 0 1 0 0 2

FZ.0 G 2.1 Gz.z G 2,s

F3.-1 F 3.0 F a,1

4 7 10

Ft,2

F,,3

10 7 5

F3,7 c‘3,2 G 3,P

2 1 0

G,,4 G3.s

F4._-I

1 1

F4.0

F 4.1

0 0

F4.2 F4.3

Ohs.

702 576 450 324 198 072 946 820 745 619 493 045 919 793 667 541 019 958 832 706 580 532 406 280 154 028

Intens.

19 19 19 19 19 19 18

709 575 448 324 196 072 945

3 6 9 10 7 4 2

18 18 18 18 18 17 17 17 17 17 16 16 16

819 748 620 493 051 923 798 673 548 027 964 831 708

1 1 1 1

2 6 10 10 6 1 2 1 1

16 552 16 16 10 16

405 280 155 034

1

every group of hands.

origins”

corresponding

to different

unsymmetric

normal

(2) wx are frequencies of unsymmetric symmetric vibrations, and (&Ix are vibrational intervals for different progressions. progressions. Vibrational

analysis

vibrations,

subscripts

i, j number

the totally

= woi + Gx

(3)

in the X-progression which may be slightly different, Anharmonicity constant,s .rij must be identical in all

of the fluorescemae

spectrum

(see

Table

JI

j is faeilitaM

by

SPECTRUM

OF Cz4RBONYL

:hc knowledge of vibrational frequencies in the ground state. hands of t hc more intense (F-j progression may be represented F,.,~~,,,,~ = Foe those of the weaker

170SU,N -

3

CYBNIDE

Frequcnc~ies of by thcl formul:r

126r,N + l!k13’ + U&l5 ,

14,

ones by

170s is the frcquenry 6~~” of the C=O stretching vihratiou; :I con~l)arison of W2” frequenc*ies deduced from the infrared and from the fluorescence slwc*trunl shows that the first) group of fluorescent hands (2X000c&J undoubtedly NW Th(l difference brtwwl t h CO?” responds to the (P?’ = 0 + 1’2 ” = 0) transition. frcqucncies determined from the infrared and the fluorewellw >lwt run1 : 1714 - 170s = 6 cm-’ gives t,he value of the int’er:ic+tion caonstant .r:.,- whew _Y tlenot,cs the still unidentified unsymnetric vibration. The anhnlnlollicity (YH__ is, within the error limit’s, t,he same as in the infrared slwtrunl. stant SC The difference between active origins C:,,, - F,),, = 705 mm1 is obviously tlw frequcncay of the totally symmetric 6~:~~vibration. Roth 1)rogressious ~:IT’(’ :I common ac%ive origin, i.e., arc allowed hy t,he same unsyrnnletri~ vibration. The det~orminntion of act’ive origins is, however, mnpli~atcd by the fact that the w5frequency has almost the same value in the ground and in the cwitc(l stat (1. The 1)opulation of excited 7~~5 vibrat,ional levels in hoth states may bc quit(> important~ and the hot’ fluorescence bands must aplwar in the s~wctrunl with the S;,?IIWit~tervals, as in normal progression. The :wcw:wy of frequency tlet wnlin:tt ion is noi suflicieutj to dist)inguish between w5’ and w5”, or to detcrnlinc~ tlw :~l~harmonic,ity of c+,” vibrations and to find F,, ill this way. The intensity relations in the first, (33000cw-' ) groul) of emission h:u~tls :III~ iti the c*orrrsl)onding “hot” absorption H-bands may ht, of sonic aid IIPW. If t Iw Illirlo~-synlrlletry rule holds in this mw, the :wtive origin of hoth lwogrc’ssions MII hc wt:~blished as

Foe= 22 967cm-‘, although I;,,, = 23 093 cannot8 be excluded with full caertainty. The vibrat,ional analysis of the absorption slwct,rum (see Table III ) is lwrformed in it,s long-wavelength part, the one which is less affected hy ~mclissoc*iat ion cffecats, The accuracy of vibrational parameter determination i.Glimited by the unresolved complex structures of hands or by their diffuse char:lcater; thcb fornlulas are therefore established not for zero frequcrwies, but, for band heads or average frequencies of &-branches. The error is of the order of 1 MY’, but in sonw (-.,ses deviat,ions of calculated and observed frequencies amount to 10 m-‘. .I11 hut :L few absorption bands may he rel)rrsentc~d as rnemhers of five wddcvr~lopc~d lwogressions: il, R, C, D, and E. Very weak hands iu 24 000 ~111 ’ ( ohstwd only at high vapor pressure at high t,enlperature) form WI E-l-lw)gw axon ~ww+~~~.~ding probably to the transitions from excited vibrational

54

PROCHOROW,

TRAMER,

AND WIERZCHOWSKI Tables

Absorption Spectw Notation

FlXguenog CPlC. Obs.

of CO/CN/2 vapouro

a/, Intera.

227%

0

HO,-1

22840

0

HO,0

22970

1

HO,1

23099

1

Ho,2

23221

0

HO,-2

EO,-1

23971

23976

0

DO,0

24014

24015

1

Eo.0

24098

24101

1

DO,1

24140

Do,2 ?

DO,3 ?

Do,4 ?

2 3

24224

24226

5

24260

24270

5

2431j

6

24329

7

Intcns.

do.-2

25158,5

25159,2

0

El,-1

25198,O

25200.8

0

BO,-1

25245,0

25243,0

0

*o,-1

25285.5

25285,O 287~7

1 1

El.0

25525.0

25322.1 332,4

1 1 1

BO,O

25372,O

25x9,7

0

%,l

25S9,O

?5387,5

1 1

390,O 7

2i402,l

0

*o,o

25412.5

25411,T 414,6

4 2

417,9

1

El.1

25451,0

25449,8 456.4 45912

1 0 1

24352

24350

24398

24393

7

2444c

8

2%71,7

0

24462

9

25496,5

25495,b

1

24402

24481

10

25517,O

25515,6

245%

24521

9

51697

1 2

24574

8

25527,6

2

2kOl

7

24614

6

H1,2 Eo,3

24140 24203

Hl,l EO,B

!

Frequency CdC. Obs.

33692

Al,0 EO,l

Notation

%3

9

25538,5

255%,7

2 4 8

*0,-s

24778

24778

V.l.

*0,-4

24905

24909

V.".

53617 537.5 5%1 541,5

24991

24997

V.W.

25551,9

1

*0,-3

25032

25033

V.W.

25580.1 586,o

1 0

y-1

25071

25071

V.W.

1 0 0 1 0

Eo,4

Eo,-2

B0,2

D1,3

24614

25622,5

25519.6 622,3 625.6

1 2 1

25647,0

25645.7 647.5 650.9

1 2 1

25659r7

2

2%65,1 667.4

3 6

? %2

25665.5

25579,0

10 2

Al*-1

26513~5

590,o 594,0 26513,l 517.7 52Q,3

*2,0

26547~0

26543,')

1

y-1

2659695

2659&j 602,6

1 1

D2.1

26618.0

26614,7 618,7 62395

1 1 2

SPECTRUM

OF CARBONYL

CYANIDE I’

N.tatioa

Bo,3

'0.3

Bo.4

bo,4

FrequenccJ CdC. Ohs.

b750.5

25796,5

25880.5

?5928,5

Intens.

668.2

10

670,4 671,9 674,5

4 4 1

25749.5 752.3 756,3 758.9

1 1 1 1

25794.9 798,5

2 3

802.5 805.2 w3,9

3 2 1

25879,l

0

883.1 as,3 891,O

1 1 0

25924.3 928,s 93311

26640.5

26636.9 639.7

2

2

D2,2

26746.0

26742.8

3

Co,2

x755,5

X756.9

8

Al.1

X766.5

4 4 4

1 1 1

X763,5 765.7 769.8 779,2

E2.2

26801,O

26796.9 799.9

1 4

El,1

26849,5

26&+5,4

1 1 2

2

850,8 85597

1 0

D2,j

26876,O

26877.0

5

26883.5

26885,4

I.0

26894,5

26892,7

6

2b5%,5

26387,0

0

76505,5

26503,4

L

t1,2

2b931,O

1

2

:0,3

26914,6

3 2 2

642,0 644.7 647,7

26731,5

ho,5

-. 2,;

5

Al.0

26719.1

1

E

266SO.3

=723,5

1

Al,-2

2=29,5

B1.0 7

26055.9

Co.0 7

Co,1

3 1

26019,7

26375.0

Iltere.

26671,9 679,6

Z6062.5

x378,5

Frequency C4lC. Obo.

26673,O

26012,5

Co,-1

b 1 . 3 /continued a/

Notation

E2.1

%5

062.8

P

55

1

E3,1

?7891,0

899,b *7ti90,1 501,4

2

B2,o

27737.0

27929,l

1 2

8 4

X928,6

2

2696&O

1

26977.5

26983,9

2

27013,5

27008,8

2

'j *2

27955.0

nsi3,ti

2

Co.4

27017.2 030.2 034,3

4 5 2

Cl,2

27968,O

27969,l

6

*1,3

2'iO24,5

*2,1

27981,O

1

27974.2 97991 983,9 992.0

5 I.0 6 2

E-. 7,2

28319.0

28017,2

3

E2.1

2806>,0

=049,9 066, j

1 1

cl,7

28096,O

28096,8

4

82,2

2alo9,o

28108,l 112,2 121,2 156,;

7 lo 2 2

B2,2

28191,O

28194,4

1 _

7 B1.2

=2,4

27X3,0

27072,O

Al,4

27156,5

27150 /a/

?

.41,5

27290,5

27288 /a/

1

B2,-4

27435.0

27421,5 4Z.O,b

b2 .-3

27560,O

2755194 55997

93a,s

1 1 1 1

Cl,-1

27591,o

27589,7

1

*; t-2 EL c,-2

27601,o

27601.4

1

27685.0

27677,l 685,3

1 2

105.3

3

PROCHOROW,

56

TRAMER,

AND

WIERZCHOWSKI

T a b 1 e 3 /continued b/ Notation

Cl,0

Frequency Ohs. C&Z. 27718,O

Inten&

an1e.a

Notation

FlYJ$LenCy

Calc. 2

Cl,4 *

Inteno.

Ohs.

28266,O

28219 /a/

r

28239,O

2d241 /d/

5

258 /a/ 271 /a/

2 1 3

27722,9 728.0 733.5

2 3 2

A2,3

277b4,6

1

*2,4

2dj71,O

28373 /a/

28505.0

2W5/d/

2

813.7

1 2

*2,5 *2,6

28641,O

28645/d/

i

27827,O

27826.8

3

c2,-1

28787,O

28781 /a/

0

Cl,1

77842,O

27842,8

>

*?I-2

28302,o

28798 /4/

0

d2,o

27855,O

?7852,7

7

A.3,-l

28929,O

28924 /a/

1

855.2 858,l

5 5

E4.0

28977,O

2a977 /a/

0

865.1

1

c2,1

29041,O

29027 /d/

3

AZ,-1

E3,0 B2,-1

D>,l

li3,0

E;ii,l c1.2 *3,1

27728,O

27765,O 27812.0

27804.4

29056.0

Zr;lOj,O

1

29218 /dJ

1

29230/d/

2 7

078 /al

2

E4,2

2q231,O

29102 /d/

1

c2,3

29295,o

29298 /a/

29310,o

29506 /a/ 10 317 /a/ 10

29440,O

2%53/d/

.29167,0

24171 /af

6

*3,2

29182.0

29178 /df 6 182 /d./ 6

AS,3

189 /a/

6

6

levels in the ground state. (The H-progression has a common origin with the F-progression in fluorescence.) The determination of the active origins of other progressions is more difficult’. The spectjrum in the 24 000-cm? region consists uniquely of D and E bands. The strongest, A-progression starts in -25 400-cm-’ region. Strong C-bands appear only in the third (-26 600-cm-‘) group of bands. The weak absorption at the shoulders of A bands in the second (25 400~cm-‘) group might be due to C-bands, but such an assignment would necessitate a drastic change in the I,/I, intensity ratio between two neighbor regions which seems highly improbable. Nothing can be said about the origin of the weakest B-progression. Moreover, the assignment of bands corresponding to AUS = 0 transitions is uncertain because of the equality ~5~ = ~5’ and of the strong population of excited ~1~”> 0 levels. The Av5 sequences are superimposed and even at high dispersion we cannot, differentiate the maxima corresponding to different Qbranches and those of superimposed “hot” bands. At a low dispersion every observed band of the main (A-) progression must be considered as a cluc+er of all the Us” = n --_)vg’ = 12 + k (k = const) transitions. If the Franck-Condon factors are assumed to be identical for )L + 71 + k

PROCHOROW, TRAMER,

58 26 600-cm-’

region corresponding to A, There are then two two alternative sets

AND WIERZCHOWSKI

(A- and C-bands). This result shows that all transitions C, and F bands must belong to the same symmetry species. possible tiPN frequencies: U: (bl) or w: (61), and we obtain of frequencies:

(A) wFN = us’ = 567 cm

-1

,

WA’ = we’ = 1875 cm-‘,

woo

=

23 534 cm-‘,

W’ = we’ + wl’ = 1880 + 1090

and (B) OF” = agV = 306 cm-‘, W*l = Q’ = 2139 cm-‘,

woo = 23 273 cm-‘, wc’ = we’ + wl’ = 2140 + 1090,

where the frequencies of the symmetric and antisymmetric CkK stretching are assumed as almost equal also in the excited state. If the first set of frequencies is chosen, reasonable values for wD’ and wE’ are obtained: WD’ = 451 cm-l,

oE’ = 564 cm-‘,

very close to ugN and $1 . No other set of frequencies can be applied, and the excited-state fundamentals as compared to the ground-state change in the same way as in the case of propynal (11). This result is, however, surprising in two points : (1) No strong absorption band was found at the presumed frequency of woo+ us’; this frequency would be active only with one quantum excitation of ml’. (2) All intense bands correspond to transitions to az.bl = Bz vibronic levels, i.e., their intensity is borrowed from a transition to the high-lying Bz (T, c*) electronic state while the intensity-borrowing mechanism involving the mixing of the Az(n, T*) state with the A,(s, T*) and BI(n, u*) stat’es by means of a2 and b2 vibrations seems much more effective (10). PREDISSOCIATION

Carbonyl cyanide, as shown by one of us (21) is photodecomposed by ultraviolet radiation, with CO and (CN), as the final products of react,ion, .COCN and .CN radicals probably being formed in the first stage. The absorption spectrum of the compound shows predissociation effects very clearly, the predissociation limit depending strongly on the quantum number of bending frequency ~5’. All A-bands are diffuse for ~2’ = 4 (w g 30 000 cm-‘), but higher members of every 02’ = const group have predissociative character. So, for v2’ = 1, the predissociation limit corresponds to vg’ = 3 (27 030 cm-l) ; for ~2’ = 2, to vg’ = 2 (28 110 cm-‘); and for v2’ = 3, to ~6’ = 1 (29 050 cm-‘). This dependence of the predissoriation limit on t’he quantum number of the bending vibration is obvious if the equilibrium values of the valence angles on

SPECTRUM

OF CARBONYL

CYANIDE

r FIG. 5. Schematic ative state.

representation

of potent.ial

curves

1’ = F(8) for the i12 and dissoci-

the central wrbon are different in tlw A, excited state and in state. In I’ig. 5 :I schematic diagram of the I)otcntial energy as a SC_,._, angle is given for the dissociative state and for different A, state. _k can be seen, intersection l)ointa dcpcwl both on quantum numbers.

the dissoGtivc1 furlction of the ~1~’levels of t.lw the ~2’ and Pi’

Carbony cyanide shows a very strong red shift of t)he slwtrum whw (*o111-. pared with formaldehyde (woo = 28 2.52 m? j :md even with unsaturafe(1 aldehydes. [w,,”is 26 162 cm-’ in propynal (II ), 25 MO cn-] in propenal ( I), (S).] This shift’ can be only partially CCand 26 327 cmml in crotonaldehyde plained by conjugation effects, so the inductive influence of C-Y groups IWISI also be taken into account. This influence increases the cffwtive positiw charge on t’he wntral carbon atom and lowers t#he energy of the a and cspwially of t 11~ T* orbital, while the energy of the oxygen n-electrons is practically not, affecatcd. The quite specific chemical reactivity of carbonyl cyanide (.%9), its strong clt~tron-acceptor properties (16), and the red shift of ?r* - n absorption are ohviously due to this reason. The rot:Itional and vibrational analysis of the absorl)tion spectra proves that

60

PROCHOROW,

TRAMER,

AND

WIERZCHOWSKI

formaldehyde has a pyramidal struct’ure in the first excited state (4, ‘7, 9), while the excited-state configuration of unsaturated aldehydes is planar. In the case of CO(Ch’), the rotational structure is not resolved, but the vibrational analysis points &rongly to a planar configuration of the excited molecule: (1) The (r0 transition is missing; (2) no inversion-doubling indices are found; and (3) no intervals in the absorption and fluorescence progressions correspond to the single or double quanta of the out-of-plane (b,) vibrations. The planar configuration of t’he excited molecule is obviously due to the conjugation effect. The striking feature of the CO( CN)z spectrum is the extreme weakness of the 'A,-'A, transition. (The oscillator st’rength is only one-third of that of formaldehyde.) This fact may be connected with the relatively low intensity of A" + A' transitions in propenal and propynal (11,lZ) (about twice that of formaldehyde) which are not’ strictly forbidden. Two alternative reasons for such behavior can be suggested: (1) the enhancement’ of formaldehyde absorption due to the distorted configuration of the excited molecule, as primarily prol)osed by Brand (4), and (2) the smaller overlap in the case of conjugated molerules between the no and t’he highly delocalized T* orbitals. Some geometrical factors of the excited molecule can be determined from the Franck-Condon analysis of its spectrum. We used for this purpose the fluorescence spectrum, where little doubt persists concerning t’he assignment of vibronic levels and the complete set of ground-level fundamentals is known. The strongest band corresponds to the v2’ = 0 + v: = 2 transition. From the relation 5fiWzC=

lJfd&z = j-a&a” + .fbbqb’+

fabqaqb

,

(8)

+ qccz] and (from the first-order calculations qa = qco, qb = 1/d2iqcc, taking into account only these two coordinates) fbb = 5.54, faa = 11.14, fab = 0.09 (in lo5 dyn.cm-‘), and qb = -0.281 q. , the amplitude of the C-O vibrat.ion in the v2” = 2 stat’e is obtained as where

qoco

=

0.135 8.

If we assume the C=O distance in the ground state to be equal to ~a, = 1.20 8, its equilibrium value in the A, state would be v&, = 1.33.5 A. Nearly the same value may be deduced from the Morse equation for the free C=O oscillator: &, In the vz progression the of and us” = 2 final levels are only the interaction of 0,_,_, fe+ = 0.09 (in lo-” dynecm)

= ?$o(w”/o’)1’3

= 1.33.

(9)

fluorescence spectrum, transitions to the ugN = 1 the strongest. First-order calculat’ions involving and &C--C--Nangles give fee = 0.50, f++ = 0.99, and 0 = 1.254. Hence, A0 = 3.S”.

SPECTRUM

OF

CAHBONYL

CIYANIl)E

1.1,

‘:Iz (m,%,= 23 275)

22-12 1711 1% 22P2 1115 567

1880 12.50 1X 1880 I O!)O 565 or HJ 1 .:1:<5 .1 llfi or 1%’

The authors would like to express their gratitude to Prof. W. Kemttla, who brclttght I IIC to Prof. 0. Achmatowirz fur making samples of CO(CS ).’ presetlt subject to our at tention, available 1I) us, attd to Prof. Borowski for his permission to use in 1his work the UI: 10 spectrometer in his laboratory. Thanks are also due I jr. A. Grabowskn and 1)r. Il. Bauc~~ for their suggestions and aid in high-pressure and ~mluriz:ttion measurements and 111 ottc of the referees for his critical retnarks.

RE~IEIVED

:

Icchruary

10, 1965 REFERENCES

1. 2. s. 4, 6. 6’. 7.

8. Y. IO.

I’. J. I )I-SE, J. Chew&. Phys. 20, 811 (1954). A. 1). %LSH, J. Chem. Sot. p. 2318 (1953). A. I). WALSH, J. Chem. Sot. p. 2306 (1!)53). J. C. I). BKLND, .J. Chem. Sot. p. 858 (1956). W. A. NOYES, (4. B. PORTER, AND J. E. JOLLY, C’hcm. /
31.

PROCHOROW,

62

TRAMER,

AND

WIERZCHOWSKI

11. J. C. D. BRAND,

J. H. CALLOMON, AND J. K. G. WATSON, Disc. Faraday Sot. 36, 176 (1963). 12. J. C. D. BRAND AND D. G. WILLIAMSON, Disc. Faraday Sot. 36, 184 (1963). 13. W. KEMULA AND K. L. WIERZCHOWSKI, Roczniki Chem. 27, 522 (1963). 14. A. TRAMER AND K. L. WIERZCHO~SKI, Bull. Acad. Polon. Sci., Cl. III, 6,411,417 (1957).

15. 16. f7. 18.

A. A. R. G.

GRABOWSKA, Trans. Faraday

Sot. (in press). Sci., Ser. Sci., Mat., A&on. BAUER _~ND M. ROZ~ADOWSKI, Optik 18, 37 (1961).

TRAMER, Bull. Acad.

HERZBERG, “Infrared

19. T. TAKENAK~

Polon.

and Raman

Spectra.”

van Nostrand

AND 8. HAYASHI, Bull. Chem. Sot. Japan

Phys. 12, 669 (1964). Co., London,

1949.

37, 1216 (1964).

20. D. F. EVANS, J. Chem. Sot. pp. 1351 and 3885 (1957). .2f. W. KEMULS AND K. L. WIERZCHOWSKI, Roczniki Chem. 27, 524 (1953). .f%. 0. ACHMATO~ICZ, 0. AC~MATOWICZ, JR., K. BELNIAK, AND J. WR~BEL, Roczniki 36, 783 (1961).

Chem.