Laser-induced fluorescence excitation spectrum of ... - ACS Publications


Jul 8, 1991 - Instituí für Physikalische und Theoretische Chemie der TV München, D-8046 ... cm"1 for the C-Cl and C-Br ground-state stretching freq...
0 downloads 0 Views 364KB Size


7132

J. Phys. Chem. 1991, 95, 7132-7134

Laser- Induced Fluorescence Excitation Spectrum of Supersonically Cooled Bromochlorocarbene In the Gas Phase R. Schlachta, G. Lask, A. Stangassinger, Institut fur Physikalische und Theoretische Chemie der TU Miinchen, 0-8046 Garching, Lichtenbergstrasse 4, Germany

and V. E. Bondybey* Institut fiir Physikalische und Theoretische Chemie der TU Miinchen, 0-8046 Garching, Lichtenbergstrasse 4, Germany, and Department of Chemistry, Ohio State University, Columbus, Ohio 43210 (Received: July 8, 1991)

A visible laser excitation spectrum of gas-phase CBrCl in a supersonic free jet has been obtained by using a novel pulsed discharge technique recently developed in our laboratory. The excited state was found to have an origin of To = 16 191 cm-' and vibrational frequencies q' = 246 cm-' and wj) = 532 cm-I, in good agreement with the matrix isolation data. The spectrum of CBrCl exhibits in an Ar matrix a red shift of 145 cm-I.

Introduction Carbenes, and in particular the halogen-substituted species, are important chemical intermediates."' Their spectroscopy and photochemistry have been the subject of considerable interest4q5 in view of the role of halogens in the stratospheric chemistry and the destruction of ozone. Carbenes are also challenging subjects from the theoretical point of view!.' In spite of their importance for both synthetic and stratospheric chemistry, the available spectroscopic information about halogenated carbenes is still rather fragmentary! So far, the CBrCl carbene has been studied only in rare-gas matrices. In the first report, Andrews and Carver produced it by reaction of alkali-metal atoms with tetrahalomethanes in a solid argon matrix? Using IR-absorption spectroscopy, they obtained values of 739 and 612 cm-l for the C-Cl and C-Br ground-state stretching frequencies. Maltsev and Nefedov'O produced CBrCl by pyrolysis of C6H5HgCBrCI2 and trapped the fragments in solid argon. They observed strong IR absorptions in agreement with the results of Andrews. Tevault and Andrews studied CBrCl in solid argon by electronic absorption and emission spectroscopy and obtained a value of 257 cm-' for the ground-state bending mode." In a later work, bromochlorocarbene was produced by in situ photolysis of bromochloromethane in solid argon and studied b using the time-resolved, laser-induced fluorescence technique. IY The laser excitation spectra established the excited-state fundamental frequencies q' = 246 cm-I, wj' = 526 cm-',and somewhat tentatively wl' = 669 cm-l. In a recent publication, we have described a simple dc discharge technique, suitable for the production of supersonically cooled ions, radicals, and other reactive intermediates." In the present (1) Schoeller, W. W. Carbene. In Merhoden der organfschen Chemic; Houben-Weyl, Regitz, M., Ed.;Georg Thieme Verlag: Stuttgart, 1990, Bd E 19b, and references therein. (2)Wentrup, C. Reakrive Zwischenstufen; Gcorg Thieme Verlag: Stuttgart, 1979 Vol. 1. (3)Moss, R. A.; Jones, M. Carbenes; Wiley-Interscience: New York, 1975. (4)Stolarski, R. S.Sci. Am. 1988, 258, 20. (5) Anderson, J. G. Annu. Rev. Phys. Chem. 1987,38, 489. (6)Liebman, J. F. In Molecular Srrucrure and Energerics; Liebman, J. F., Greenberg, A., Eds.; VCH Publishers: New York, 1986. (7) Carter, E. A.; Goddard 111, W. A. J . Chem. Phys. 1988. 98, 1752. (8)Jacox, M.E. J . Phys. Chem. Re/. Dara 1990, 19, 1387. (9)Andrews, L.;Carver, T. G. J . Chem. Phys. 1968,19, 896. (10)Maltsev, A. K.; Nefedov, 0. M. J . Phys. Chem. 1971, 75. 3984. ( I 1) Tevault, D.E.; Andrews, L. J . Am. Chem. Soc. 1975, 97, 1707. (12) Bondybey, V. E.; English, J. H. J . Mol. Specrrosc. 1980,79, 416.

0022-3654/91/2095-7132$02.50/0

TABLE I: Deslrndrea T8ble of the Excitation Bad M8wlrm of CBrCl ( c d

vi 1

20'

2 3 4

16682 16928 17 172 17417 17662 17906 18 150 18 395 18 640 18878

5 6 7 8 9 10 11 12 13 14 15 16

19 362 19608 19 847 20087

20'30~ 16967 17 210 17450 17 693 17936 18 184 18 427 18671 18 91 1 19 155

v302 17968 18212 18 701 18 934

19637 19882

publication we apply this technique to fragment bromochloromethane and report the first gas-phase spectroscopic observation of CBrCI.

Experimental Section The laser systems, detection electronics, and pulsed electrical discharge source have been described in detail elsewhere.') Briefly, CH,BrCl (Aldrich) in 3-4 bar of argon expands through a pulsed electromagneticvalve (Bosch) and flows through a narrow channel in a Teflon block. In the presence of the gas pulse an electrical discharge takes place between two electrodes in the flow channel, resulting in ionization and fragmentation of the parent compound. The carrier gas with the discharge products expands further on through a small orifice into an evacuated chamber. The molecules cooled by the adiabatic, supersonic expansion are then excited by the pulses of an excimer (EMG500, Lambda Physik) pumped dye laser (1.3 mJ/pulse, 14-11s pulse duration, resolution 1 cm-I, FL 2000,Lambda Physik). Dyes rhodamine 6G and coumarin 153 and 307 (Radiant Dyes) were used to cover the 608-479-nm spectral range. Laser excitation spectra are recorded by monitoring the undispersed fluorescence intensity as a function of laser wavelength. The laser-induced fluorescence signal is detected by a photomultiplier tube (R 928,Hamamatsu) and digitized in a waveform recorder (2262,LeCroy). The signal is averaged and processed (13)Schlachta, R.; Lask, G.; Bondybey, V. E. Chem. Phys.. in press. Q 1991 American Chemical Society

The Journal oj‘Physica1 Chemistry, Vol. 95, No. 19, 1991 7133

Letters

TABLE 11: Molecular Constants of CBrClO

To

17850

18050

18250

18650

18450

w AVENUMBER [ c m-‘] Figure 1. Part of the excitation spectrum of the CBrCl free radical produced by dc discharge of approximately 4%CH2BrC1/3 bar of argon in a supersonic free jet apparatus. The dye laser bandwidth was about 1 cm-I, and the spectrum was not corrected for laser-power variation. The band marked with a triangle is partially overlapped by CC1, carbene. The circles indicate the respective bending mode progression for CBr3’CI. 0

>E

‘”I

-20

LL

wjl

w,”

Andrews and Carver9 (Ar) Maltsevb and Nefedovlo (Ar) TevaultCand Andrews” (Ar) Bondybeydand 16046 669 245 528 Englishi2 (Ar) 16 190 246 532 this work (gy

739

wI’

W;

W;

UP 612

743.9 61 1.4 739.5 744 257 612 744

260 612

= 1:0.34; AW,”(’~B~OAll values in cm-I. I,,3,9(3sCI):1739,s(37CI) *OBr) = 0.7 cm-I. ‘Using wl”and 0211 from ref I O the anharmonic Y23” = -2.5 cm-l, Xi? = -5 cm-l, and X33)1= -1 terms x2[ = 0 cm-’,, cm-’ were obtained. dLifetime T = 5.6 ps; refit: X22/ = 0.01 cm-I, X23) = -1.0 cm-’, and XI;= 2.3 cm-’. = -0.2 cm-l, X2,’ = -1.0 cm-I, and X3,’ = -4.2 cm-I. TABLE 111: Comparison of the Band Origins of Some HalocrrbenesO band origin Tn species (ref) gas phase Ar matrix shift CF, (17: lac) 37 226 36 878 348 25 287 24985 302 CFCl (19: 209 CFBr (14: 21‘) 20906 23 300 -2394? CC12 (15: 16‘) 17256 17 092 164 16 191 16045 145 CBrCl (& 12‘) CBr2 (22: 12‘) 14885 14962 -77? aAll values in cm-’. bGas phase. ‘matrix isolation. 4This work.

ii

-50 -60

1

0

,

1

2

3

4

I

1

5

6

I

7 8 9/10 QUANTA OF 0 2

I

I

11

12

T

13

14

i 15

Figure 2. Plot of the vibronic band maxima isotope shifts (CBr3’CICBr3Cl) versus the number of quanta of d2for the ZOn progression in the spectrum of CBrC1.

in a Hewlett-Packard QS20 computer controlling the experiment.

Results and Discussion Vibrational Analysis. Figure 1 shows a part of the CBrCl excitation spectrum. The vibrational structure of the spectrum is similar to that observed in solid argon matrix and analogous to the spectra of other halocarbenes.Icl6 It exhibits several progressions of bands with an approximate spacing of 244 cm-’. With the help of the solid argon matrix data these progressions can be assigned to the 2d, 20“-2301,and 20‘T‘302transition series. The relatively complex appearance of the spectrum and the formation of “clusters* of bands is caused by the near degeneracy of the symmetric stretch (0;= 532 cm-’) and an overtone of the bending frequency (2*0; = 492 cm-I). An additional complication in interpreting the bands resulted from a partial overlapping of the bands with the dichlorocarbene. CC12 is probably a product of a secondary reaction, or it results from a CH2C12impurity in the CH2BrCl sample. By modification of the discharge conditions, its production could be suppressed but not completely eliminated. A careful comparison with the spectra of CC12 in this spectral range, obtained under similar conditions from CC14, allowed the overlapped bands to be clearly identified. In Figure 1 triangles mark areas of overlapping CCI2 bands. A Deslandres table summarizing the observed transitions (band maxima) and their assignments is presented as Table I. A weaker satellite band to lower frequency can be observed for each of the strong CBrCl excitation maxima. These are readily identified as the isotopic bands due to 37Cl,present in the natural (14) Schlachta. R.; Lask, G.; Bondybey, V.

E. Chem. f h y s . Lett. 1991,

180, 215.

(15) Clouthier, D. J.; Karolczak, J. J . Chem. fhys. 1991, 94, 1. ( I 6 ) Bondybey, V. E. J . Mol. Specrrosc. 1977.64, 180.

abundance of about 25%. A plot of this red shift as a function of the quantum number u ’ ~shown in Figure 2 gives a linear relationship, with an increasing red shift of approximately -3.4 cm-’/quantum, in fair agreement with what could be expected based on the reduced ma- for the bending vibration of CK-Br. The observation of the isotopic bands permits identification of the value of the vibrational quantum number and absolute assignments of the observed progressions. The first observed band of the 20”progression is assigned as 202. An extrapolation back to u’= 0 yields the unobserved, Franck-Condon forbidden electronic origin band near 16 190 cm-l and results in a 37Cl isotopic shift of -0.08 f 0.5 cm-I. Since the vibrational constants decrease somewhat in the excited electronic state, a small blue shut would actually be predicted for the 0-0 band of the heavier isotope. The observed value is also consistent with the isotope effect of +1.4 cm-’ observed for the 0-0 band of the CC12 carbene in the interesting investigation of C10uthier.I~ Each of the carbehe bands is further doubled, this doubling being caused by the bromine isotopes 79Brand *lBr. The bromine splitting is naturally smaller and therefore not clearly resolved in our spectrum. It is partially obscured by the rotational structure to be described below. Qualitatively the splitting again increases linearly with the vibrational quantum number approximately -0,8 cm-l/quantum. The observed vibronic bands were least-squares fitted to the standard vibrational H a m i l t ~ n i a n .The ~ ~ results of the fit and the molecular constants are summarized and compared with the matrix data in Table 11. The standard deviation of the least squares fit is about 2 cm-I. A comparison of our present data with the matrix isolation results indicates a moderate To red shift of 145 f 2 cm-l in solid argon. This is in good agreement with most of the data available for the other halocarbenes, as shown in Table 111. The To of the excited electronic state of CF, shifts in solid Ar by 348 cm-’ to lower energies, and the shifts decrease slowly as one goes to the heavier, more polarizable halogens, C1 and Br. The data for CBrF for which a blue shift of 2394 cm-’ has been reported stands out clearly from this smooth trend. This is probably due to an incorrect vibrational assignment. A reinvestigation of the matrix isolated CFBr would be desirable. To some extent CBr2 does not fit into the general trend either. In this case, the matrix vibrational numbering is well established, but it is possible that the vibrational assignme& of the gas phase

7134

J. Phys. Chem. 1991, 95,7 134-7 136 CBrCl

SIMULATION

17140

17180

17220

WAVENUMBER

17260

[cm-'l

Mgure 3. Comparison of the observed spectrum (lower trace) and the simulated contours of the 20' band at 17 172 cm-' of CBrCI. The following structural parameters were assumed: r'$-Br = 1.875 A, r'hxl = 1.716,O" = lI0.9O, rh+ = 1.85 A, rhKl = 1.746 A, W = 134.1O. The rotational temperatures was assumed to be 15 K. The bands marked with circles are for CBr3'CI. CBrz is in error by one quantum. Rotational A ~ l y s i s .A higher resolution (