J. Phys. Chem. 1993,97, 7465-7470
1465
LIF Excitation Spectrum of Dibromocarbene in a Supersonic Free-Jet Expansion Songlin Xu and Marlin D. Harmony' Department of Chemistry, University of Kansas, Lawrence. Kansas 66045 Received: March 16, 1993; In Final Form: May 7, 1993
CBrz has been produced by pyrolysis in a supersonic free-jet expansion and investigated by laser-induced fluorescence-excitation spectroscopy. Observations of the vibrational isotope effect for several bending progressions have led to a reassignment of the earlier gas-phase UZ' numbering and have consequently cleared up some earlier discrepancies. For the 79BrC81Brspecies, values of several vibrational frequencies (referred to the lowest vibrational level ofeach electronic state) are uy = 475 cm-l, u: = 185 cm-l, and u y = 599 cm-l; and the origin of the 'B1 X 'A1 electronic transition is placed at TOO= 15 091.5 cm-l. Partially resolved rotational sub-band structure yields so-me structural results. In particular, the BrCBr bond angle increases 21" upon excitation from the X to the A state. The electronic origins, TOO, and the argon matrix shifts of TOO, are shown to satisfy physically reasonable additivity relationships for CBrz, CC12, and CBrCl.
A
-
Introduction Simple and complex carbenes are important well-known intermediates and reactants in numerous organic thermal and photochemical The physical and chemicalproperties of these reactive species, which generally exist in either singlet or triplet states, have been extensively investigated over the years by various direct and indirect means. The prototype carbene, methylene, has been the subject of many years of both experimental4 and theoretical5-' investigation in both its singlet and triplet states. Interest in substituted carbenes has continued strong in recent years with the development of new techniques for experimental gas-phase observationsand enhanced capability for theoretical investigation. The singlet ground-state halocarbenes have been the subject of numerous studies during the past 10 years. For example, CHF8and CHC19have been studied by CW dye laser excitation spectroscopy in flowing gas systems as well as by CW laser excitation spectroscopy in a crossed beam apparatus.10 CCl2 has been investigated by both pulsed and CW laser spectroscopy in supersonic free-jet expansions from hot nozzles,l1J2 while CFBr and CBrCl have been observed in free jets from an electrical discharge ~ o u r c e . ~ ~ J ~ CBrzhas been the subject of severalspectroscopicinvestigations beginning with the matrix studies of Andrews and Carver15and Tevault and Andrews.16 Its laser excitation spectrumwas observed somewhat later in a matrix by Bondybey and English.'' Still more recently, Zhou et a1.18obtained CBrzspectra in their crossed beam apparatus, but bandwidths (fwhm) of -40 cm-' limited the spectral information and precision. We recently reported19 efficient production of several halocarbenes (including CBrz) in free jets emanating from a specially constructed pyrolysis nozzle of the type described by Chen and co-workers.20 In the work reported here, we present a detailed description and analysis of the pulsed laser excitation spectrum of the A 'BI X 'A1 electronic transition of CBrz observed in a supersonic free jet.
-
Experimental Section As reported earlier,19 CBrz was efficiently produced in a supersonic free jet by the a-elimination of HBr from bromoform (CHBr3, Aldrich Chemical Co.) in a ceramic nozzle heated to approximately 1000 "C. The precursor vapor was seeded at the 1% level into flowing argon at a total pressure of typically two atmospheres. The precursor carrier gas mixture was pulsed into the ceramic nozzle by a Lasertechnics pulsed valve operating at a repetition rate of 10 Hz and a pulse width of 1.5 ms. A modified brass quick-connector was sealed to the pulsed valve face plate 0022-365419312097-7465$04.00/0
to permit attachment of the 1 mm i.d. thin-walledceramic (AlzOs, Johnson-Matthey Co.) nozzle. The ceramic tubing, approximately 4.5 cm in length, was cemented into a brass or stainless steel fitting which mated with the brass quick-connector. A tantalum wire heating coil was wrapped around the final 1.5 cm of the ceramictube and was insulated with severallayers of ceramic paper. In early experiments a Pt/Pt-Rh thermocouple was attached to the nozzle assembly for temperature measurements. It was found to be more convenient for routine operation to simply monitor the power input to the oven and to relate this to temperature by a calibration curve. Nozzles of this type, which are based on the design of Chen et a1.,20 yield temperatures up to approximately 1300 "C with power inputs of less than 25 W. In the present experiments, a heating power of -14 W yielded an apparent temperature of 1000 OC. The very modest heat generation is easily dissipated by a water-cooled radiation shield and by water cooling of the brass quick-connector assembly, so that the pulsed-valve remains at temperatures below 50 "C. In the work described here, the CBrz excitation spectrum is obtained by probing the free-jet expansion -1 2 mm downstream of the nozzle by an unfocused pulsed dye-laser beam at 90" to the free-jet center line. Thevacuum, laser, and detection systems have been mostly described previously.ll Very briefly, a 1500 L/s unbaffled 6-in. oil-diffusion pump maintains a background pressure of less than 1-2 mTorr under typical free-jet conditions. The Quantel YAG-pumped pulsed dye laser is synchronized to the pulsed valve and a time delay circuit permits temporal optimization of the gas-beam/laser-beam interaction. Unresolved fluorescence is detected by a photomultiplier (EMI-9798B) at right angles to the laser and gas beam axes after passing through stray-light baffles, a collection lens, and an appropriate highpass filter. (Here we use red filters, Oriel Models 51320 and 51330.) Photomultiplier output is processed by a PARC Model 162/164 boxcar averager whose output is sent to a recorder and to a 386 microprocessor-basedcomputer. The computer controls the laser scans and pulsed nozzle synchronization and also performs various spectral data processing functions. In particular, the apparent laser wavelength or frequency is calibrated by 12 scans in each spectral region. The experimental 1 2 spectrum (obtained from a separate gas cell) is collected and compared on the microcomputer to a simulated 12 spectrum based on the precision measurements of Gerstenkorn and L U C .This ~ ~ provides a calibration curve which is stored and therefore available for converting apparent (i.e., instrument) wavelengthsto truevacuum wavelengths or wavenumbers. In this way, wavelength (wavenumber) accuracies approaching f0.15 A (0.4 cm-l) are achiev0 1993 American Chemical Society
1466 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993
Xu and Harmony
03
, ) 1 8 , 1 8 ( -
-
?
2
3
(
27
P 9
(79,79) ,
8
1
)
2
3
Wavelength (A)
Figure 1. Prominent progressions of
A
2 transition of CBr2.
able. The spectra presented in figures in this paper have not been corrected for variations in laser power. With some minor gaps, the CBr2 spectra reported here cover the range of approximately 5600 A (17 850 cm-l) to 6600 A (15 200 cm-l) using the Exciton dyes R590, R610, R640, and DCM. Rotational temperatures in the hot-nozzle expansions are near 20 K. Vibrational temperatures are much higher, as evidenced by the observation of vibrational hot bands, but are not precisely known at this time. Based upon separate experiments on molecular bromine, vibrational temperatures may be as high as 500-600 OC. It seems likely, moreover, that the vibrational cooling varies from mode to mode.
Wavelength (A) band under moderate resolution. 'RK sub-bands are identified for the isotopic species.
Figure 2. 1A2:
Resulb
Observed Band Systems. The excitationspectrum is dominated by two readily assigned band systems, 2: and 1:2:, as shown in Figure 1 for the 5600-5900-A region. The v2' spacing of approximately 185 cm-l is clearly evident in the twoprogressions. Earlier, we reported similar bands in the 58504050-Aregion.lg These band systems were first identified in a low-temperature matrix by Bondybey and Eng1i~h.l~ Later, Zhou et a1.18 observed the two band systems in the gas phase with rather low resolution. As described below (see Vibrational Isotope Effect), the quantum number assignments are unambiguously prescribed by the vibrational isotope effect. This shows then, that the u2' gasphase numbering suggested by Zhoul8 is high by one quantum. In addition to the clear Occurrence of 1:2:1 triplets arising from the vibrational isotope shifts of the C79Br2,'gBrCBlBr, and CB1Br2species, respectively, the spectra show under higher resolution clear T Rsub-band ~ rotational structure characteristic of Itransitions (C-type in this case) of a near-prolate asymmetric rotor. The isotope splitting and sub-band structure is shown for one of the bands in Figure 2. We demonstrated a similar spectrum in our earlier report.19 Near the red end of the spectrum it is possible to identify two hot band series, 1;2: and 1;2:, as demonstrated in Figure 3. These bands arise from the symmetrical stretching mode of the ground electronic state and have been unambiguously assigned with aid of the known ground-state stretching frequency of vl" = 595 cm-l reported for CBr2 by matrix IR studies.15 These progressions are very short and clearly do not belong to the principal progressions of Figure 1 . On the basis of strict numerology the bands might be assigned as the 2; and 2: hot bands of the ground-state bending frequency, which is reported16 as v2" = 196 cm-1, almost exactly one-third the VI" frequency. Such an assignment would be unreasonable on several grounds, in particular because of the absence of 2;, 2;, etc. hot bands. For the u1" = 1 series the vibrational isotope effect and rotational
I
I
I
I
I
6350
6400
6450
6500
I
Wavelength (A) Figure 3. Region containing prominent CBrz hot-band progressions.
Asterisk identifies most prominent unassigned band.
sub-band structure are clearly discernible. It is significant to note that the 1;2;f progression and to a lesser extent the 1$2;5 progression were also prominent in the CCl2 pyrolysis-nozzle spectrum reported by Clouthier et a1.I2 Thus, there appears to be no substantial uncertainty in these assignments. Perhaps the most surprising feature of the spectrum is that no bending-state hot bands, 2; or 2;, have been detected. These bands were observed with low intensities for CC12 for relatively small n values.12 On the basis of the matrix fluorescence spectrumI6a ground-state CBr2 bending frequency near 196 cm-1 seems well established. This value, along with the excited-state bending frequency of approximately 185 cm-l, requires that the 2y1and 2;+2 series appear red-shifted from the corresponding 2,"progressions by approximately 1 1 and 22 cm-l, respectively. Very careful study of the spectra (especially for small n values, for which the bands should be most prominent) fails to yield any conclusiveevidenceofthesebands. On theother hand,it is possible that the bands appear weakly and are obscured by the more intense band structure of the 2: bands of higher n. It is certainly true that there are unassigned weak features throughout the spectrum which have not been identified. The most intense unassigned features are two bands near the red end of the spectrum, one appearing =26 cm-l to the blue of the 2; and one =32 cm-1 to the blue of the 2; bands. (One of these unassigned bands is indicated in Figure 3.)
LIF Excitation Spectrum of Dibromocarbene
The Journal of Physical Chemistry, Vol. 97, No.29, 1993 1461
TABLE I: Rotational Sub-band Measurements for Several Members of the 25 Progression for 'nBrC?lBr (cm-l)' 2:
'& 'RI 'Rz 'R3 'R4 '&
'R1 'Rz 'R3 '&
*16017.8 (-O.l)b 16025.1 (0.0) 16035.4 (0.1) 16048.6 (0.0)
2; 16753.6 (-0.3) 16761.7 (0.2) 16772.5 (0.1) 16786.8 (0.1) 16804.1 (-0.2)
2: ,16201.4 (-0.2) 16209.2 (0.2) 16219.6 (0.1) 16232.9 (-0.1) 2;o 16937.7 (-0.1) 16945.5 (0.1) 16956.6 (0.0) 16971.0 (0.0)
TABLE III. Band Origins of the 1:2: Progressions (cm-1)'
2: *16569.2 (-0.1) 16576.6 (-0.2) 16588.1 (0.3) 16602.1 (0.1) 16619.4 (-0.1)
22 17121.4 (0.0) 17129.3 (0.0) 17140.4 (-0.1) 17155.3 (0.0)
@; 1i2; 12: li2; 1i2: 1i2: 1i2i0 l;gl 12;'
*15935.1 (0.4)b *16118.5 (-0.1) *16302.3 (-0.1) *16485.9 (-0.2) 16669.8 (0.2) 16853.5 (0.4) 17036.0 (-0.4) 17219.1 ( - 0 . 5 ) 17402.1 (-0.6) 17585.3 (-0.4) 17769.7 (1.1)
16663.0 (-0.3) 16847.0 (1.1) 17028.0 (-0.3) 17210.3 (-0.4) 17392.5 (-0.5) 17574.9 (-0.3) 17758.1 (0.7)
16675.4 (0.0) 16860.2 (0.4) 17044.3 (0.2) 17227.6 (-0.7) 17412.0 (-0.4) 17596.0 (-0.5) 17781.3 (0.9)
"Values preceded by an asterisk identify subbands which are substantially overlappedby other bands. Parentheticalvalues represent (observed - calculated) frequencies from the least-squares fits.
Values preceded by an asterisk were not obtained by rotational subband analysis. See text. Parenthetical values represent (observed calculated) using the parameters of Table V.
TABLE 11: Band Origins of the 2: Progression (cm-1)'
TABLE Iv: Band Origins of the Observed Hot-Band Progressions (cm-1)'
79BrC81Br
2; 2; 2; 2; 2; 2: 2: 2: 2; 2A0 2:' 2i2 2i3 2f
'15275.5 (-0.8)b *15461.3 (0.3) *15646.5 (0.9) * 15829.7 (-0.4) 16015.0 (0.5) 16198.8 (0.0) ,16382.7 (-0.2) 16566.3 (-0.7) 16750.9 (0.0) 16934.8 (0.1) 17118.4 (0.0) 17302.0 (0.1) 17485.4 (0.0) 17669.0 (0.3)
CS'Br2
C79Br2
79BrC*1Br
16009.9 (-0.1) 16192.7 (-0.5)
16019.3 (0.1) 16204.2 (-0.1)
16559.3 (-0.1) 16743.0 (0.6) 16925.7 (0.4) 17108.3 (0.1) 17291.0 (0.1) 17473.3 (-0.3) 17656.0 (-0.2)
16574.0 (-0.2) 16759.5 (0.5) 16944.0 (0.2) 17128.3 (-0.2) 17313.1 (0.0) 17497.5 (-0.1) 17681.7 (-0.3)
a Values preceded by an asterisk were not obtained by rotational subband analysis. See text. Parenthetical values represent (observed calculated) using the parameters of Table V.
Band Origin Determination. Preliminary simulations of the expected rotational sub-band structure were performed in order to assess the most reliable and accurate means of determining the vibrational band origins. These simulations suggested for the C-type rotational selection rule! of th_e near-prolate rotor ( K -0.998 and K -0.999 for the X and A states, respectively) that the band origins should be accurately obtained by fitting the sharp bandheads of the IRK sub-bands to the near-prolate N
N
symmetric rotor expression.
= yo + (K + l)'(A'-B') - K'(A''-B) (1) For the majority of the stronger bands the K = 0 to K = 3 subY
bands were observable for all three isotopic species. In a few cases the K = 4 sub-bands were observed, while for the doubly excited hot bands only the K = 0, 1, and 2 heads of the 79BrC81Br species could be reliably measured. As evident from Figure 2, some overlapping of sub-bands of different isotopomers exists, but the band origins are nevertheless quite well determined. Table I presents for illustration purposes some typical sub-band data for several members of the 2," series for the 79BrC81Br species. Tables I1 and I11 summarize the YO values obtained from leastsquares fits of the subband frequencies to eq 1 for the two principal progressions, and Table IV presents analogous results for the hot-band progressions. Some details of the fitting procedure will be described in a later section. Also included in the tables are some less accurate band origins for several weaker transitions whose sub-bands could not be clearly observed. These have been obtained by measuring simply the maximum of the strong complex sub-band consisting of P- and Q-branch transitions clustered about
1y2: 1y2; 1y2: 172; 172: 1y2:
1:2: 1!2? 1!2i1
*15231.8 (-0.8)b 15417.7 (0.7) 15601.6 (0.4) 15785.5 (0.1) 15969.2 (-0.2) 16153.2 (-0.1) 15190.6 (-0.5) 15374.6 (0.5) 15559.9 (0.8) 15743.8 (0.9) *15925.7 (-0.8) '16109.2 (-0.9)
CSLBrz 15414.1 (0.7) 15596.7 (0.1) 15779.7 (-0.1) 15962.7 (-0.1) 16145.3 (-0.5) ,15185.6 (-0.6) *15368.5 (-0.7) *15552.9 (0.7) *15735.8 (0.6)
C79Brz 15421.0 (0.1) 15606.2 (0.2) 15791.0 (0.0) 15976.2 (0.3) 16160.0 (-0.7) *15195.5 (-0.9) *15380.5 (-0.9) *15567.2 (0.9) *15751.9 (0.9)
a Values preceded by an asterisk were not obtained by rotational sub band analysis. See text. Parenthetical values represent (observed calculated) using the parameters of Table V.
YO. We estimate that the latter values have uncertainties of =f 1.0 cm-l, while the results obtained from eq 1 yield uncertainties of eh0.5 cm-I. Vibrational Isotope Effect. The band origin shifts originating from the vibrational isotope effect permit unambiguous assignment of the u2' quantum numbers for the observed band systems. Neglecting higher order terms, the isotopic band-origin shift for the 2: progressions is given by
+
Avo = ATm (AO:)U; (2) Graphs of eq 2 for two sets of CBrz isotope data are presented in Figure 4. The intercept AT, is expected to be very small since it represents essentially the isotope shift of the difference i n zero-point vibrational energies for the upper (A)and lower (X) states. Least-squares fits for lines a and b in Figure 4 lead to AT, = 0.12 f 0.26 cm-', Aw? = -0.94 f 0.03 cm-I and AT, = 0.36 f 0.36 cm-1, AUT = 0.89 f 0.04 cm-1, respectively. The slopes, A@, have values entirely in agreement with expectations based on the mass changes, and the intercepts have values near zero. If the vibrational quantum number assignmentwere shifted up by one quantum as proposed by Zhou et a1.,18 the intercepts of the two lines a and b would shift to values of 1.06 and -0.53 cm-l, respectively. These latter values are much larger than For example, Clouthier and reasonably expected for AT,. KarolczakI2 report a value of AT, = -0.22 cm-' in their very accurate study of the two most abundant CClz isotopomers. Actually, a somewhat smaller value is expected for CBrz due to the lower frequencies and smaller isotope shifts. Recognizing the uncertainties in the ATmvalues for our preferred assignment, it is clear nevertheless that the results arein substantial agreement with expectations.
7468 The Journal of Physical Chemistry, Vol. 97, No. 29, 19193 I
’
”
’
”
”
f
.
,
’
’
’
l
Xu and Harmony
TABLE V: Molecular Parameters of CBrz (cm-1) 79BrC81Br CE’Br2 C79Brz Too
@
&’ &’ x:,” a
5
0
”
15
10 2’
Figure 4. Isotope shifts for 2: progressions. The lower curve (a) represents uo(81-81) - uo(79-81) and the upper curve (b) represents ~o(79-79) - ~o(79-81).
As discussed later, the assignment deduced here also clears up an apparent discrepancy in the reported matrix shifts for the hal~carbenes.~~J* Rotational Band Structure Analysis. The band-origin determination by eq 1 invclves the combination rotational constant (A-B) for the X and A states. The experimental ‘RKbandhead data were found to be very insensitive to the ground-state value (A”-B”), and consequently this value was constrained in the least-squares fitting procedure. The chosen value of this parameter, (A”-B”) = 1.30 cm-l, was selected to be in substantial agreement with the ab initio structure reported by Bauschlicher.22 (A’-B’) and YO were then allowed to vary freely in the leastsquares fit of each band, providing in this way the YO values of Tables 11-IV and (A’-B’) values for each excited state. The YO (band-origin) values obtained in this fashion are very immune to the assumed value of A”-B”. Consequently, any expected error in this value will have a minor effect upon the tabulated YO values. For this same reason we use the iden-tical (A”-B’) value not only for those transitions arising from X(OO0) but $so for the hotband transitions arising from the X(100) and X(200) states. In addition to providing the band origins, the least-squares fits providevaluesof (A’-B’) for each of the excited states, A(ul’u2)O). Although the values are not highly precise, they are nevertheless sufficiently accurate to nicely characterize the excited states. In particular, the values for each progression increase with u2) in complete accord with expectations based upon increased vibrational amplitude and corresponding increase in the vibrationally averaged bond angle (02) 1/2. For example, when the results for the 2: series are fit to the empirical expression
the best values (A’-B)o = 2.71 f 0.03 cm-l and a = 0.033 f 0.007 cm-1 result. These results will be shown later to be in excellent accord with-expect_ations based upon the structural changes between the X and A states. Vibrational Constants. The band-origin data of Tables 11,111, and IV have been used to determine vibrational frequencies (w,), anharmonic constants (x,,), and the origin of the electronic transiti_on (Too = u ~ )all, referred to the lowest vibrational levels of the X 1Al and A 1B1 states. Explicitly, the band origins, YO, are fit to the equation uo = T,
+ @ul’ + wTu; + x;u;2
+ xY;ul‘u; @ul”
- XY1”U1”Z (4)
where the single primes reJer to the upper (A) state and the double primes to the lower (X) state. Table V presents the results
15091.5 (0.5)“ 475.0 (0.5) 184.9 (0.1) -0.058 (0.009) -0,706 (0.067) 599.2 (0.6) -1.62 (0.29)
15092.7 (1.6) 473.8 (1 -2) 183.6 (0.4) -0.038 (0.020) -0.615 (0.125) 598.0 (0.7) -1.47 (0.34)
15092.5 (1.7) 474.8 (1.2) 185.5 (0.4) 4 , 0 4 0 (0.021) -0,622 (0.130) 600.2 (0.7) -1.91 (0.35)
Parenthetical values represent 1 standard deviation.
of the least-squaresfits, which have been performed independently for each isotopomer. The quality of the fits is quite good overall. For the ground-state progressions of Tables 11 and 111, the “observedminus calculated”values (in parentheses) are generally in the f0.5-cm-l range, while for the hot-band series (Table IV) and a few of the Table I1 and 111 entries the deviations are as large as f l cm-1. The excited-state bending frequency w2) has been especially well determined from the extensive progressions and shows the expected isotopeeffect, which is incidentally in accord with results of Figure 4. The symmetric stretching frequency q ’ i s somewhat less well determined, and it is noted that the mixed isotopicspecies value does not lie intermediate to the values for the heavy and light isotopomers. Of course, because our data involved only the u1’ = 1 state, noanharmonic terms (e.g., xll’) could be determined. Consequently, the w1’ values are of intrinsically poorer quality than the wz’ values since they are contaminated with uncorrected anharmonic contributions. On the other hand, the ground-state symmetrical stretching frequency wl” has been rather accurately determined (along with the first anharmonic correction) from the hot-band data. Theisotopicvariationis strikinglysymmetrical as expected, perhaps fortuitously so considering the standard deviations of 0.7 cm-I. Finally, we note that the electronic origin Too is especially well determined for the most abundant isotopic species. The isotope shifts in TOO for the light and heavy isotopomers are inconsistent and larger than those deduced from the Figure 4 data. However, the statistical uncertaintiesof u = 1.6 cm-1 negates the significance oftheapparentshifts. ItseemslikelythatthevalueToo= 15 091.5 cm-l with a 2u uncertainty of f l .Ocm-1 will encompass the true values for all three isotopomers. Molecular Structure. The available data for the rotational constant (A’-B’) does not permit a precise specification of the molecular geometry in the excited state, but it does permit a 8ood estimation of the most probable geometry changes for the X A transition. As mentioned earlier, (A”-B”) was fixed at avalue of 1.30cm-l on the basis of the theoretical study of Bauschlicher.22 This value is consistent with a CBr bond distance of 1.865 A and a bond angle of 0 = 110.7O. Bauschlicher actually computed C-Br = 1.875 A (and 0 = llO.lo), but the bond length value is expected to be somewhat too long due to minor basis set deficiencies. If the C-Br distance in the excited (A) state decreases by the same fractional amount as observed for the C-Cl distance in CClz (from 1.716 to 1.652 A, or 3.7%), we would expect the A state bond length to be 1.796 A. The (A’-B’) value of 2.71 cm-l leads then to a bond angle of 0 = 131.3O for the A state. We note that the large increase in angle of 20.6O is entirely in accord with the corresponding increases for CF2 (1 7.4O)23 and CC12(22.20).12 The computed bond angle increase depends only slightly upon the assumed C-Br distance in the excited state. Thus, if no decrease in the C-Br distance occurred, the excitedstate value of 0 would be 133.2O, an increase of 22.5O from the ground state. There can be no substantial doubt, therefore, that the CBrz bond angle 0 increases =21° upon excitation from the ‘A1 to A lB1 state.
-
The Journal of Physical Chemistry, Vol. 97,No. 29, I993 1469
LIF Excitation Spectrum of Dibromocarbene
TABLE VI: Vibrational Parameters of Bromo and Chloro Carbeaes (cm-1)’
A
2 03
01
02
wj
TOO
335
745*
634
303
-
17256
774*
260*
612.
669,
246
532
599
196*
641’
475
185
-
w1
w2
CC12
730
CBrCl CBr2
ref
gasb matrixc 16191 gasd matrixef 15092 gasd matrixh
I, Matrix values are indicated with an asterisk. Gas-phase values are given when available. Tabulated frequencies are matrix IR fundamentals in a few cases. In general, data refer to the most abundant isotopomer. Reference 12 in text. c Bondybey, V. E. J . Mol. Spectrosc. 1977, 64, 180. d Reference 14 in text. Reference 17 in text. f Maltsev, A. K.; Nefedov, 0. M.; Hauge, R. H.;Margrave, J. L.; Seyferth, D. J . Phys. Chem. 1971, 75, 3984. *Present work. See Table V, 79BrC81Br. *References 15, 16 and 17 in text.
*
Discussion The principal results of the present study are the molecular vibrational parameters of Table V. The ground-state (X ‘AI) symmetrical stretching frequency, wl” = 599 cm-’, has been determined in the gas phase for the first time. Previously, the only available value was the infrared fundamental value of 595 cm-1 observed by Andrews and Carver for argon matrix isolated CBr2. Using our anharmonic constant xll’, we can predict the fundamental frequency to be 597 cm-> in close agreement with the infrared value. The excited-state (A lB1)frequencies wl’ and w{ have been reported recently by Zhou et a1.18 to have values of 460 cm-l and 189 cm-l, respectively, from low-resolution gasphase studies. Zhou’s work failed to resolve the various isotopic molecules and exhibited bands with widths of =40 cm-l. In addition, the present work has shown the vibrational numbering of Zhou et al. to be in error by one quantum. Thus, the present determination represents a substantial improvement, particularly in the value of w1’. We should note that the w1’ and w{ values had been determined also by Bondybey and English some years ago in an argon matrix, where the frequencies were 468 and 186 cm-1, respectively.17 In Table VI we summarize comparative data for the related halocarbenes CC12, CBrCl, and CBr2. Entries which represent argon matrix values are indicated with an asterisk and in the case of infrared studies we do not distinguish possible differences between w values and fundamental frequencies Y. It is seen that the gas-phase data are most complete for CC12, for which only two frequencies remain unobserved. For both CCl2 and CBr2 the excited-stateantisymmetricalstretching mod, remains undetected in either gas or matrix. Overall, the X and A state data for CC12 and CBr2 are very consistent as expected.-We note in particular that the changes in w1 and w2 between the X and A states are very similar for the two molecules; namely, w1 decreases substantially (96 and 124cm-l, respectively) whileo2 decreases a much smaller amount (32 and 11 cm-l, respectively). Moreover, the unsymmetrical mixed carbene CBrCl follows similar trends for w1 and w2, namely, decreases of 75 and 14 cm-I, respectively, in going from the X to the A state. From these comparisons it appears that the electronic structural changes (or chemical bonding changes) ?f the three molecules-are substantially similar in going from the X ‘A1 (or A’) to the A 1Bl (or A”) states. Finally, we note that wl” (the C-Cl stretching frequency) and 0 3 ” (the C-Br stretching frequency) of CBrCl are very similar to wl” for CCl2 and w1” for CBr2, respectively. A significant feature of the present work is that the band assignments, which have been shifted one quantum in vi from the assignments of Zhou et al.,l* now eliminate some apparent discrepancies in the electronic band origin data for CC12, CBr2, and CBrCl. First, the gas-phase values of TOOin Table VI are seen to follow closely a reasonable additivity-typerelation, namely
+
Tm(CBrC1) = 1/2[ Tm(CCl2) Tm(CBr2)l. Such behavior would be expected in the first approximation if the CBr bond energies and the CClbond _energieswere transferable between molecules in both the X and A states. Stated differently, the result implies essentiallythat AHf”(CBrCl) = 1/2[AHfo(CC12)+ AHfo(CBr2)] in both the 2 and A states. The value of Too reported by Zhou et a1.I8 for CBr2, viz., 14 885 cm-’, is in much poorer agreement with the additivity relationship. In addition to bringing order to the gas-phase TOO values, the band reassignment also brings about a pleasing order to the argon matrix shifts of the TOO values. The reported argon-matrix TOO values for CC12, CBrCl, and CBr2 are 17 092,16 045, and 14 962 cm-’, respecti~ely?~.~~ Combining thesevalueswith those of Table VI for the gas-phase, we obtain matrix shifts [ Tm(matrix) - Too (gas)] of-164,-146, and-130cm-I, respectivelyfor CCL, CBrCl, and CBr2. The excellent additivity agreement here [1/2( ‘64 130) = 1471 suggests that the matrix perturbations of the X and A states are themselves additive properties for these structurally and electronicallyanalogous halocarbenes. We note that if Zhou’s value’*of Tm is used for gas-phase CBr2 the matrix shift becomes +77 cm-’, in serious disagreement with physically reasonable expectations. Finally, it should be emphasized t t a t precise structural results for the CBr2 molecule in the X and A states will require studies at higher resolution. As described in the earlier section, however, the data obtained here show clearly that the molecule undergoes a sukstantial increase in bond angle (-21 ”) upon excitation from the X A state. This structural feature has been discussed by Clouthier and Karolczak12for CCl2 and the results are expected to be comparable for CBr2. The only previously reported experimental structural results for CBr2 are those of Ivey, et al.,25who proposed structures for both singlet and triplet CBr2 from electron diffraction studies of the 1200 OC pyrolysis products of CBr4. In this work, the triplet and singlet carbenes were assigned with abundances of 18 and 996, respectively, along with substantial amounts of other species such as CBr3and C2Br6. The quoted structures of the two carbenes (singlet and triplet) showed C-Br distances near 1.74 A for both species, and bond angles of -1 14O and 150” or greater for the singlet and triplet, respectively. While these structural parameters are not completely unreasonable, they do not agree well with theoretical expectations22or with our experimental results (Le., A-B values). Moreover, it is not clear that formation of the triplet and singlet in a 2: 1 ratio is kinetically or thermodynamically sensible,since the triplet is =8 kcal/mol less stable.22 Considering the difficulties in deconvoluting the diffraction patterns when a mixture of severalcarbon-bromine species is present, the electron diffraction results must still be considered inconclusive.
+
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Acknowledgment. We are grateful to the Research Corp. for their support of this research. References and Notes Moss,R. A.; Jones, M. Carbenes; John Wiley: New York, 1973. (2) Scaiano, J. C. Handbook of Organic Photochemistry; CRC Press: Boca Raton, FL, 1989; Volume 2, Chapter 9. (3) Carey, F. A,; Sundberg,R. J. In Advanced Organic Chemistry, Parr E, 3rd ed.; Plenum Press: New York, 1990. (4) For numerous references to earlier work, see: McKellar, A. R. W.; Bunker, P.R.; Sean,T. J. J. Am. Chem. Soc. 1983, 79, 5251. (5) Goddard, 111, W. A. Science 1985, 227, 917. (6) Schaefer 111, H. F. Science 1986, 231, 1100. (7) Carter, E. A.; Goddard 111, W. A. J . Chem. Phys. 1988,88, 1752. ( 8 ) Kakimoto,M.;Saito,S.; Hirota, E. J. Mol. Spectrosc. 1981,88,310. (9) Kakimoto,M.;Saito,S.; Hirota, E. J . Mol. Spectrosc. 1983,97,194. (10) Qin, Y.;Zhou, S.; Shi, J. Chem. Phys. Lett. 1987, 136, 93. (1 1) Choe, J.-I.; Tanner, S.R.; Harmony, M. D. J . Mol. Spectrosc. 1989, (1)
138, 319. (12) Clouthier, D. J.; Karolczak, J. J . Chem. Phys. 1991, 94, 1. (13) Schlachta, R.;Lask, G.; Bondybey, V. E. Chem., Phys. Lett. 1991, 180, 275.
1470 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 (14) Schlachta, R.; Lask, G.; Stangassinger,A.; Bondybey, V. E. J. Phys. Chem. 1991,95, 7132. (15) Andrews, L.; Carver, T. G. J. Chem. Phys. 1968,19, 896. (16) Tevault, D. E.; Andrews, L. J. Am. Chem. Soc. 1975, 97, 1707. (17) Bondybey, V. E.; English, J. H. J. Mol. Spcrrosc. 1980, 79,416. (18) Zhou, S. K.;Zhan, M. S.;Shi, J. L.; Wang, C. X. Chem. Phys. Lerr. 1990,166, 547. (19) Xu, S.;Harmony, M. D. Chem. Phys. Lerr. 1993,205, 502.
Xu and Harmony (20) Minsek, D. W.; Chen, P. J. Phys. Chem. 1990,94,8399. Clauberg, H.; Minsek, D. W.; Chen, P. J. Am. Chem. Soc. 1991,114,99. (21) Gemtenkom, S.; Luc, P. J. Phys. (Paris) 1985,16, 867. (22) Bauschlicher, Jr., D. W. J. Am. Chem. Soc. 1980, 102, 5492. (23) Matthcws, C. W. Can. J. Phys. 1%7,45,2355. (24) Bondybey, V. E. J. Mol. Specrrosc. 1977, 64, 180. (25) hey, R. C.; Schulze, P. D.;Leggett, T. L.; Kohl, D. A. J. Chem. Phys. 1974,60, 3174.
