J. Phys. Chem. 1982, 86, 728-730
728
identity and oscillator strength of the excitation mode accessed by the COz laser differ from those of the parent, would also be valuable, and are currently under way in our laboratory.
Acknowledgment. This research was supported, in part, by U.S. Air Force Office of Scientific Research Grant No. 78-3725. Helpful discussion with Dr. H-L. Dai, and comments by Dr. A.W. Pryor, are gratefully acknowledged.
Photodissociation of a Molecule with Two Chromophores. CH,IBr S. J. Lee and R. Bersohn' Department of Chemistry, Columbia University, New York, New York 10027 (Received: Ju& 31, 1981; In Final Form: October 9, 1981)
When CH,IBr is dissociated with light in its first absorption band, which peaks at 258 nm, it is found that 86% of the fragmentations yield I atoms and 14% Br atoms. The anisotropy parameter, (3, in the angular distribution is 1.42. This fact leads to two conclusions: (1) the dissociation process is direct, and (2) the Br atoms are formed as a result of a weaker absorption to a different upper state than that reached in the main absorption.
CH,IBr is a prototype of a molecule with two possible dissociation pathways: CH21Br CH2Br + I (1)
-
CHJBr
-
CHJ
+ Br
(2)
On thermally dissociating such a molecule, one would expect a high degree of selectivity based on the fact that the C-I bond energy is substantially less than the C-Br bond energy (47 vs. 69 kcal/mol). Unimolecular decomposition on the ground-state surface should in principle be very selective for such large differences in bond energy. What determines selectivity, i.e., branching ratio, in the upper states? Clearly the nuclear potential surface of each electronic state must extend to regions of both dissociations. However, the vertical transition of the ground-state molecule to the upper state brings it to a region in which the potential may be repulsive in a specific coordinate, e.g., C-I or C-Br. Nevertheless, crossings are possible in which a system may cross from an excited surface repulsive in one coordinate to one repulsive in another coordinate. Thus, the dissociation of even such a small molecule as CH2fBr may be surprisingly complex. Here we make a simple beginning by investigating the branching ratio and the fragment anisotropy when exciting in the first absorption band.
Experimental Section The photofragment spectrometer which is used to measure the photofragment mass spectrum and the angular distribution of photofragments is described in detail elsewhere.' A molecular beam is crossed by light from a 1-kW high-pressure Hg-Xe lamp which passes through a filter solution (a mixture 0.3 M in NiCl, and 0.1 M in CoSO,) to cut off the shorter wavelength region and the infrared. The photofragment mass spectrum is obtained by using a quadrupole mass spectrometer. The angular distribution is measured by inserting a chopper and a polarizer in the light path. The signal from the mass spectrometer is amplified by a lock-in amplifier tuned to the chopping frequency and measured as a function of the laboratory angle 0 between the direction of detection and the photon polarization direction. The Polacoat polarizer (1) M. J. Dzvonik and S. Yang, Reu. Sci. Instrum., 45, 750 (1974).
produces a degree of polarization p = 0.83 at 280 nm. The CHzIBr sample reservoir was kept at -33 "C but the molecular beam was warmed to 60 OC to produce a pressure in the reaction chamber around 5 X lo4 torr. Methylene bromoiodide2 was synthesized by refluxing an equimolar mixture of CH212and CH,Br2 under nitrogen at 140 "C (bp of CH,IBr a t 1 atm) for 1 week. The resulting mixture was separated by spinning band vacuum distillation and the desired distillate was collected at 71 "C at 128 torr. The purity of CHJBr was checked by VPC (5% SE-20 column) and NMR (6 = 4.56 ppm for CHJBr, 3.9 ppm for CH212,and 4.9 ppm for CH2Br2). The NMR spectrum was a single line a t 6 = 4.56 ppm.
Results Ultraviolet Absorption Spectrum and the Electronic Configurations of CH21Br. The spectrum of a 2 X lo4 M solution of CH21Brin hexane is shown in Figure 1. There are two bands with maxima at 268 nm (e = 0.93 X lo4 M-l) and at 213 nm (e = 2.88 X lo4 M-' cm-'1. Recalling that the first absorption maxima of CH31and CH3Br are at 258 and 202 nm, respectively, we see that the lower energy band is mainly due to the C-I chromophore and the second band mainly to the C-Br chromophore. The red shift and intensification of both bands as compared to the corresponding bands of the monohalides reminds us that there is moderate coupling between the two chromophores. The n u* nature of the first optical transitions of the hydrogen and alkyl halides, in which a nonbonding electron mostly localized on a a-type "lone pair" orbital of the halogen is promoted to an antibonding u* orbital of the C-X bond, has been extensively discussed3 and experimentally substantiated. In this model the electronic configurations of the ground and first two sets of excited states of CH21Br are
-
N: 3JQ:
U&B,U&-~T&T~ u&-B,u&-~T~,*~u&I 3 4 1 +-BP&-ITB~TI U C - B ~
(3) (4)
The Q notation, due to Mulliken,, means that each of the (2) G. S. Forbes and H. H. Anderson, J. Am. Chem. SOC.,67, 1911 (1945). (3) R. S. Mulliken, J. Chem. Phys., 8, 382 (1940).
0022-3654/82/2086-0728$0 1.2510 0 1982 American Chemical Society
Photodissociation of CHJBr
The Journal of Physical Chemistty, Vol. 86, No.
5, 1982 729
Scheme I
fragmentation and the ion is then detected. Pi,j will be the probability that fragment i is dissociatively ionized to ion j and subsequently detected. A series of equations can be written down for the observables of the problem. If 4Br and q51 are the quantum yields for dissociation into Br and I atoms, then A [nmi
4
~ + ~2I ~ ' ~c H ~~B ~ B ~ + = 0.14 41p1+ ~BJ'CH~I-I+
(CH2Br+)
~IPCH~B~ = 0.40 @PI + ~BQCH~I-I+
(Br?
Figure 1. Absorption spectrum and excited population density of CH21Br. A fitter solution is used to excite selectively with wavelengths in the region of the first absorption band.
a=--
I
P=
(I+)
-
-
(I+)
cH,ial
y=
(CH,I+) =--~BJ'CH~I - 0.41 (CH2Br+) ~ I P C H ~ B ~
a = PCH21-I+/PCH21
(5) (6)
(7) (8)
= 0.20
b = P c H ~ B ~ B ~ += /0.07 P c H ~ B ~ (9)
c = P B r / P I = 0.69
50
150
100 M O S S number
200
(m/ei
Figure 2. Photofragment mass spectra of CHJBr. When the light is off, all peaks disappear.
excited configurations gives rise to a mixture of four states, 3&1,3&1,3&0+ and 'Q1.Magnetic circular dichroic spectra5 have shown that the major transaction in the first band of CH31is N q0+ which is polarized parallel to the C-I bond.6 Consistent with the smaller spin-orbit coupling of the Br atoms, the first bandof CH3Br is mainly the N 'Q1transition which is in principle polarized perpendicular to the C-Br bond. Photofragment Mass Spectrum of CH21Br. The distribution of photon energies used in the photodissociation of CH21Bris shown in Figure 1 together with its absorption spectrum. The relative excited population density is determined by folding the lamp spectral irradiance into the absorption spectrum of the molecule. It can be seen that the filter solution confines the excitation to the first absorption band. The photofragment mass spectrum Figure 2 contains a large I+ peak, a small Br+ peak, but no peak at masses corresponding to IBr+. Hence, in this photon energy region the energetically possible dissociation to CH, and IBr is negligible. The ratio of the area under the Br+ peak to that under the I+ peak in 0.14. However, this is not the ratio of bromine to iodine atoms produced. Two effects must be considered. First of all, the ionization cross sections for the two atoms are not the same. When IBr was photolyzed with the same apparatus, the ratio of the Br+ to the I+ peak was 0.69, which is the ratio of the probabilities of detecting a bromine and an iodine atom, Le., P B r / P I . There is an additional complication shown in Scheme I. The electron ionizer of the mass spectrometer can cause dissociative ionization of the radical fragments. Let Pi be the probability that a fragment i is ionized without further
-
-
(4) R.S. Mulliken, J. Chem. Phys., 3, 513 (1935). (5) A. Gedanken and M. D. Rowe, Chem. Phys. Lett., 34, 39 (1975). ( 6 ) M. Dzvonik, S. Yang, and R. Bersohn, J. Chem. Phys., 61,4403 (1974).
(10) The numerical values of the ratios a, P, and y were obtained by weighing areas under the different masses of Figure 2 and have a 10% probable error. The values of a and b were obtained by assuming that the cracking pattern of CH,I(Br) is similar to that of CH,I(Br);' i.e., we assume (X+)/(CH2X+)from an experiment on a beam of CH2X fragments is about the same as (X+)/(CH,X+) obtained from a beam of CH3X molecules. Fortunately, the probability of dissociative ionization is much less than that of nondissociative ionization particularly for the more abundant radical CH,Br. Solving eq 5-10, one finds
4Br _ 41
a-bb
4 1 - aPr) = 0.168
(11)
4 ~ , 0.14 and 41 = 0.86. Angular Distribution of the Iodine Atoms. Direct photodissociation by polarized light produces an anisotropic flux of fragments. This anisotropy is experimentally fitted to an angular distribution of the form f(e) = (1/4r)u + PP,(~- e,)) (12) wher 0 is the angle between the velocity u' of the fragment in the lab system and the electric vector of the light wave. The phase shift 8, is the sum of a phase shift sin-l (lq/lu'l), where c' is the velocity of the parent molecule in the molecular beam and a phase shift produced by the lock-in amplifier. The uncorrected experimental anisotropy parameter Po = 1.04 f 0.09 is corrected for the finite time constant of the lock-in amplifier and the finite speed of rotation of the polarizer to give PI = 1.12 f 0.11. The latter value is corrected for the incomplete polarization (0.83) of the incident light to give the final value, p2 = 1.42 0.14. The angular distribution is shown in Figure 3.
*
Discussion The electronic spectrum of CH21Br exhibits a red shift and a considerable intensification of the first two spectral bands as compared to CHJ and CH3Br. Both observations (7) E.Stenhope, S. Abrahamson, and F. W. McLafferty, Eds., "Atlas of Mass Spectral Data", Interscience, New York, 1969.
730
The Journal of Physical Chemistry, Vol. 86, No.
5, 1982
Lee and Bersohn
. , . -
I
0.5-
I
... 0. 45
35
33
53
?*
i b :-qe
Figure 3. Angular distribution of the iodine atom fragment in the photodissociationof CH,IBr.
may be due to the fact that, in the dihalomethane, the central carbon atom has more positive charge than in the monohalomethanes. Thus, the transitions which promote an electron largely on the halogen to an antibonding orbital have a larger charge-transfer character. This transfer reduces the positive charge associated with the upper state, lowering its energy and therefore producing a red shift. The experimental anisotropy may be compared to that of CH31in the same wavelength region, 1.81 f 0.33.6 The value for CH21Br is sufficiently large to prove that the molecule dissociates in a time short compared to a rotation period, 7. The latter may be adequately estimated as follows. Let us neglect the mass of the CH, group and consider CH,IBr to be a pseudodiatomic with moment of inertia I perpendicular to the IBr axis. Using the equi/ 2kT, we find 7 = 6.3 ps. The partition theorem I ( 2 ~ / 7 ) ~ = time for dissociation may well be 1 order of magnitude shorter than this upper limit, suggesting that the dissociation process is a relatively uncomplicated descent on a monotonically repulsive surface. By analogy with CH318 the large value of P also argues that the iodine atoms are produced mainly in the upper 2P1/2state and that the first excited state has A' symmetry. (The point group of CHJBr is C8.) A quantitative interpretation of the anisotropy parameter which is only barely justified by its accuracy might proceed as follows. The measured value of 1.42 f 0.14 was obtained for a warm gas of rotating molecules. What we are really interested in is the anisotropy in the molecular axis system. Assuming again that the molecule is diatomic, the 0 , of a nonrotating molecule can be expressed in terms of PrOt= 1.42 for the rotating molecule:9
( E T ) ,the average translational energy of the fragments was not measured in our experiments but is probably not far from the value of 7.5 kcal/mol found a t 266 nm for CH212.10 Substituting in eq 13, we find P, = 1.75 f 0.17. A classical calculation shows that P,, = 2P2(x)where x is the angle between the transition dipole direction and the direction of dissociation. According to this relation x is 17 f 5". The molecule CHJBr has a plane of symmetry, i.e., the ICBr plane, and thus all electric dipole transitions will be polarized perpendicular or parallel to that plane. Let us assume that the transition in the C-I chromophone is largely N 3Q0and polarized parallel to the C-I axis and that the transition in the C-Br chromophore is largely N 'Q1and therefore polarized perpendicular to the C-Br bond. There are two perpendicular directions, one in and the other normal to the plane of symmetry. It will turn out that only the latter choice is quantitatively reasonable. Let us take the ICBr bond angle to be 114O, a little larger than the value observed for CH2Br2. The angle between the C-I bond and the direction perpendicular to the C-Br bond in the reflection plane is then 24'. A resultant angle of 17 f 5O is possible only is the C-I and C-Br bonds made comparable contributions to the resultant transition dipole moment. If this were true, we would expect comparable yields of Br and I atoms, contrary to observation. If we assume that the transition dipole of the C-Br bond is normal to the ICBr plane, the absorption band would be a mixture of an A' A' transition localized on the C-I A" transition localized on the C-Br bond and an A' bond. We then find pnr/2 = ( P ~ ( ~ = )(1) - t ) ~ 2 e)( +~ E ~ ~~ 2 (T/2)1 [ ~ = ~ ~ (1.75 f 0.17)/2
-
-
--
and thus t = 0.083 f 0.057. The reduction of the asymmetry parameter from its maximum value of 2 is here seen to be due to a small admixture of a transition of different A' first absorption symmetry to the predominant A' band. A level crossing does not have to be invoked to explain the small yield of bromine atoms. Levy and Simons" have reported an interesting example of selectivity in the photodissociation of CH,I. Irradiation in the 258-nm band produces only iodine atoms, yet irradiation in the range 140-170 nm leads to hydrogen atoms and CHJ. In each range of photon energies, a separate region of the small molecule is being excited and different bonds are broken in each case.
-
Acknowledgment. This work was supported by the National Science Foundation, (9) C. Jonah, J. Chem. Phys., 55, 1915 (1971). (10) P. M. Kroger, P. C. Demou, and S. J. Riley, J. Chem. Phys., 65, 1823 (1976).
~
~
~~
(8) S. L. Boughcum and S. R. Leone, J . Chem. Phys., 72,6531 (1980).
(11) M. R. Levy and J. P. Simons, J. Chem. SOC.,Faraday Trans 2, 71,561 (1975).
