5798
J. Phys. Chem. 1995, 99, 5798-5801
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Orientation Dependence of the CF3* Formation in Collisional Energy Transfer Reactions: CF3Cl Ar(3P) CF3* C1 Ar and CF3Br Kr(3P) CF3* Br Kr
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H. Ohoyama, H. Makita, T. Kasai," and K. Kuwata Department of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan Received: August 17, 1993; In Final Form: March 28, 1994@
The orientation dependence of the CF3* formation was investigated for two collisional energy transfer reactions, CF3C1 AI-(~P)and CF3Br Kr(3P>,and the results were compared with that of a related CF3H Ar(3P) reaction which was previously studied. For the three systems studied, the reactivity was found to be the greatest for collisions at the CF3 end of the molecules. The CF3C1 Ar(3P) system exhibited a steric opacity Kr(3P) system showed a broad steric opacity function as function with two reactive sites. The CF3Br compared with the other two systems. This broadness of the steric opacity function is discussed in terms of the effect of the center-of-mass shift in the molecular frame and/or of impact parameter broadness.
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1. Introduction
2. Experimental Section
In the recent decades, direct experimental studies on the steric effect in gas-phase reactions have been performed using the electric hexapole fields.' Most of these studies are concerned with reactants in the ground states. Reactants in the excited states are also expected to exhibit stereoselectivity in reaction. Since collisional energy transfer by the metastable atoms is considered to be a fundamental process in molecular excitation, its stereoselectivity has been investigated in several methods. For instance, the alignment dependence of the reactivity for two competing channels of CS2+ and CS(A) formations has been studied in the Ar* CS2 reaction using the aligned molecular beam produced by a linearly polarized laser.2 Using an electric hexapole state selection, the orientation dependences of the radical formations in the dissociative energy transfer reactions have been observed in our lab~ratory,~ in which it was commonly recognized that the spatial distribution of the reactant molecular orbitals can qualitatively explain the experimentally observed stereoanisotropies. However, one-to-one comparison of the steroselectivity with a related molecular orbital of different reaction systems is difficult, because more than one molecular orbital is involved in the reaction in many cases. In order to clarify the relationship between the spatial distribution of the molecular orbital and the stereoselectivity in the collisional energy transfer, it is desirable to compare steric opacity functions of analogous reaction systems which consist of only one type of relevant molecular orbital. A series of experimental and theoretical studies for the orientation dependence of CF3* formation in the CF3H Ar(3P) system4-' have indicated that the CF3* formation via the electron exchange interaction between the 6a1 molecular orbital of CF3H and the 3p empty core orbital of Ar(3P) may account for the observed steric opacity function with two reactive sites located at the CF3 end and the H end. In the present study, we investigate the orientation dependence of CF3* formation in two analogous reactions to CF3H Axf3P): CF3Cl Ar(3P) and CF3Br Kr(3P).4,5 Since the collisional excitation is expected to involve only one HOMO molecular orbital, which has a1 symmetry, and it is localized on the C-X bond of CF3X (X = H, C1, Br),* the relationship between the steric opacity function and the spatial distribution of the molecular orbital could be significantly examined.
The experimental apparatus and the procedure have been described in detail e l ~ e w h e r e . ~Briefly, .~ a 3-ms pulsed CF3X (X = Br, C1) beam was produced by a supersonic expansion from a pulsed valve at the stagnation pressure of 450 Torr. The CF3X beam was rotationally state-selected by a 60-cm electric hexapole field and then oriented at the beam intersection in an orientating field (100 V cm-'). A 1.5-ms pulsed Rg(3P) (Rg = Kr, Ar) beam was produced by an electric glow discharge. The velocity distributions of CF3X and Rg(3P) were determined by a conventional time of flight (TOF) method, and the TOF profiles were simulated by a standard two-parameter expression for the shifted Maxwellian distribution.'O The velocity parameters for the stream velocity and the most probable velocity after the nozzle expansion were determined to be vs = 420 ms-', a, = 80 ms-' for CF3C1 and v, = 370 ms-', a, = 70 ms-' for CF3Br. The CF3* emission from the beam crossing zone in the visible wavelength region was collected by a concave mirror through a long-pass filter (420-nm cut wavelength) and focused on a photomultiplier. The signal was fed into a gated photoncounting system. The background counts were subtracted from the crossed beam signal at every pair of consecutive beam pulses. To obtain an acceptable signal-to-noise ratio, the difference counts were accumulated over 40 000 x 2 pulses. In order to change the molecular orientation, the direction of the orienting field was switched cyclically to positive, negative, and zero for every 300 beam pulses. Figure 1 shows the dependences of the CF3Cl and of the CF3Br beam intensities on the hexapole rod voltage (viz, the focusing curves). The beam intensity was measured by a quadrupole mass spectrometer tuned to the CFf fragment peak (mle = 31). The experimental error was within the open circles. The solid lines were obtained by Monte Carlo computer simulations, from which the orientational distributions of the reactant supersonic beams were calculated by assuming a thermal distribution for the rotation." The rotational temperatures thus determined were 45 f 5 K for both CF3Cl and CF3Br reactant beams. Figure 2 shows calculated orientational distributions, W(rE), of the state-selected molecular beam of CF3Br at several hexapole voltages Vo, where the vector r in the parentheses indicates the direction of the molecular axis and E is the direction of the orienting electric field. A good degree of orientation was achieved even at the highest applied voltage of
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Abstract published in Advance ACS Abstracts, April 1, 1995.
0022-365419.512099-5798$09.00/0
0 1995 American Chemical Society
CF3* Formation in Collisional Energy Transfers
J. Phys. Chem., Vol. 99, No. 16, 1995 5799
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CF+r I
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I
d I
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observed
observed
5 10 Voltage / kV
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5 10 Voltage / kV
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Figure 1. Dependences of the CF3Cl (left panel) and of the CF3Br (right panel) beam intensity on the hexapole voltage. The experimental error is within the open circles. The solid lines were obtained by the Monte Carlo computer simulations. TABLE 2: Dependence of the CF3* Emission Intensity upon the Molecular Orientations emission intens (lo-* count pulse-') reac syst voltage (kV) X end random CF3 end CF3Cl-l- AI(~P) 14 3.1 2.2 3.6" CF~BC K I ~ ~ P ) 9 6.5 7.7 8.7" 11.5 15.5 17.2 19.1b 13 23.7 25.5 27Sb
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Fl
The standard deviation is estimated to be within 9%. The standard deviation is estimated to be within 5%. parameters such as the geometrical parameters of the hexapole field and the velocity distribution of the incident beam.
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r.E Figure 2. Calculated quantal orientational distributions, W(rE), of the CF3Brbeam after the hexapole state selection. The W(rE) of the CFK1 beam at 14 kV was similar to that for CF3Br at 11.5 kV. TABLE 1: Legendre Moments for the Orientational Distributions of the CFjCl and CFiBr Beams hexapole voltage, V, (kV) CF3Cl CSBr Legendre moment 14 9 11.5 13 0.682 0.653 0.631 (Pl) 0.655 0.298 0.250 0.215 0%) 0.250 0.071 0.035 0.012 (P3) 0.034 0.005 -0.009 -0.015 (P4) -0.010 0.001 -0.001 0.OOO (P5) -0.001 0.001 0.002 0.m (p6) 0.002 14 kV due to the small dipole moments of the molecules (i.e., 0.50 D for CF3C1; 0.639 D for CF3Br). The calculated Legendre moments of the orientational distributions were summarized in Table 1. Since the Legendre expansion of W(rE) was found to converge rapidly, the expansion up to n = 6 turned out to be adequate for obtaining such proper orientational distributions. The typical undertainty of the calculated Legendre moments was less than 10%; this was estimated by changing the fitting
3. Results
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The emission cross section for the CF3Br Kr(3P) reaction was found to be 5 times greater than that for the CF3C1 Ar(3P) reaction (Le., the reaction cross sections were estimated to be 1 .& for CF3C1 and 5 A* for CF3Br); therefore, we carried out the orientation-dependence measurement of the CF3* emission at three V, for the former reaction and only one V, for the latter reaction. The dependences of the CF3* emission intensity for both reaction systems on the molecular orientation are summarized in Table 2. The reactivity at the CF3 end was found to be the greatest, similar to that in the CF3H reaction. 3.1. CF3Cl Ar(3P) System. A remarkable feature of the orientation dependence for the CF3C1 Ar(3P) system is that the CF3* emission in random orientation gave the weakest emission intensity, which has not been observed in the sequel of reactions we have studied. The present result explicitly suggests a very small reactivity for the sideways attack. In order to estimate this contribution from the sideways attack, the orientation dependence of the CF3* emission was analyzed by means of the step-function model in which the CF3C1 molecule was divided into three reactive zones by 60" of the angle attack y and a zero-impact-parameter approximation was assumed since both systems have small reaction cross sections.'*
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5800 J. Phys. Chem., Vol. 99, No. 16, 1995
Ohoyama et al. means that the sideways attack has a nonzero reactivity this time, or two reactive sites at both ends of the molecule merge into the sideways area. Therefore, it may be difficult to recognize two separated reactive sites for the CF3Br system within the present the experimental accuracy.
180°
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CF3 end
Figure 3. Schematic illustration of the step-function model employed for three reactive sites, in which the CF3X molecule is regard as a hard
sphere with three reactive zones. The reaction probabilities are designated by a h , a,, and at, for the CF3 end, the sideways region, and the X end, respectively.
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4. Discussions
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4.1. CF3CI AI-(~P)System. The analogy to the CF3H Ar(3P) system suggests that the dissociative energy transfer in the present two reaction systems could be determined by the electron exchange mechanism. In this mechanism, an electron (designated by 1) of a molecule is donated to the vacant p orbital of Rg(3P) and a back-donation of the s electron (designated by 2 ) of Rg(3P) to a Rydberg state of the molecule. In this case, the probability Px is expressed by the following exchange interaction.
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Figure 4. Steric opacity function deconvoluted using the step-function model. The steric opacity function for CF3Cl+ Ar(3P) (the solid line) which has not reactivity for the sideways attack, and that for CF3Br Kr(3P) (the dashed line).
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Figure 3 schematically illustrates the employed step-function model. The reaction probabilities in three reactive zones are designated by a h , us,and ut, for the CF3 end, the sideways region, and the C1 end, respectively. In this model, the observed emission intensity, I , is represented by the total intensity of three reactive zones, as the following equation shows.
j-:o:otW(cos
y ) d cos y
where W(cos y ) is the orientational distribution of the CF3C1 molecular beam after the state-selection (the same as W(rE) in the previous section). In random orientation, the emission intensity should be equal to (Oh 2u, 4- ut)/4,and it is set as unity. Then these Oh, us, and at can be determined by reproducing the normalized emission intensities, which are defined as the emission intensities for each orientation divided by that for the random orientation. The obtained reaction probabilities are summarized below, and they are also represented by the solid histograms in Figure 4.
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oh
= 2.2 f 0.4,
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This result reveals two isolated reactive sites: one at the CF3 end and the other at the C1 end. An alternative statement is that there remains a nonreactive zone for the sideways attack as the raw data explicitly showed. 3.2. CF3Br KI-(~P)System. The same step-function model was applied for the CF3Br system. The reaction probabilities in three reactive zones were determined as follows:
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o h = 1.15f0.1, a,= l.OfO.1, u t = 0 . 8 5 f 0 . 1 In contrast to the CF3C1 system, the steric opacity function in Figure 4 gives only a small heads-to-tails stereoanisotropy. This
where ~3,tid and qfinalare the initial and final wave functions, and 112 is the distance of electrons 1 and 2. Because of the radial decay of the orbital function, the transition matrix K increases exponentially as 112 becomes small. Thus, the transition probability could be approximated as the value at the tuming point of the reactant approach. Because the Rydberg orbital of CF3X overlaps efficiently with the s orbital of Rg(3P), this probability is further approximated by the electron density at the tuming point. Here, the electron near the tuming point is called the exterior e1ectr0n.l~ A series of experimental and theoretical studies on the CF3H Ar(3P) system have suggested that the CF3* formation via the exchange interaction between the 6al molecular orbital of CF3H and the 3p orbital of Ar(3P) gives two reactive sites at CF3 end and the H end and that the main feature of the exterior electron certainly reflected on the electron exchange probability, P,. These results indicate that the spatial distribution of the exterior electron is a good measure for approximating the orientation dependence of the electron exchange probability.' For molecules of CF3X (X = H, C1, Br), studies have commonly attributed the formation of CF3* to the excitation of the HOMO orbital electron to the Rydberg states, which has a1 symmetry and is localized on the C-X bond.* Since the shape of the molecular orbital localized at the C-X bond would be similar to each other in analogy to the 6a1 orbital for C S H , this moleuclar orbital would give two reactive sites at both ends of the molecule. Thus, the two reactive sites in the CF3Cl reaction system would not contradict the assertion that a steric opacity function in many cases reflects the spatial distribution of the exterior electron of the HOMO molecular orbital. 4.2. CF3Br Kr(3P) System. Because of the present experimental inaccuracy, it was difficult to confirm the existence of two reactive sites, as shown in Figure 4. However, it still remains a possibility that the CF3Br system could have two reactive sites, but they might be coagulated in the middle reactive zone in the step-function model. An explanation for the nonresolved two reactive sites for CF3Br may be a simple effect of the center-of-mass shift upon the steric opacity function, as is shown in Appendix. Another possibility for the nonresolved stereoanisotropy in the CF3Br system may be an effect of the impact parameter broadness. Although the zero-impactparameter approximation was appropriate for the CF3H and CF3C1 systems which have the small reaction cross sections of less than 1 A2, this approximation would not be appropriate for the CF3Br system with a larger cross section of 5 A2, indicating a possible contribution from collisions with nonzero-impact
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J. Phys. Chem., Vol. 99, No. 16, 1995 5801
CF3* Formation in Collisional Energy Transfers
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in the molecular frame, receives an influence from the shift of the center-of-mass location. In order to compare opacity functions of different reactant systems, it is necessary to take into account this discrepancy due to the different definitions. For CF3X molecules, the shifts of the center of mass are schematically shown in Figure 5. It was found that the center of mass shifts toward the X atom side as X becomes heavier. As a result, the nominal position of the reactive site located at the X end in the molecular frame moves to the sideways region in the steric opacity function. This effect may explain the broadness for the CF3Br system due to the shift of the Br end site toward the sideways region.
Figure 5. Shift of the center of mass for CF3X (X = H,Br, C1) molecules. The filled circles indicate the center of mass for CSX. The center of mass shifts toward the X atom as the molecule becomes heavier. The shaded region schematically indicates the exterior electron for the C-X located molecular orbital fixed in the molecular frame, which is assumed to be equal to that for CF3H. The nominal position of the reactive site at the X end moves toward the sideways region as the X atom becomes heavier. The direction of a reactive site in the steric opacity function is illustrated by the arrow.
parameters. Nonetheless, it may be said that the present two reactions can be explainable by the same electron exchange mechanism, but at the same time further experimental and theoretical studies may be necessary to clarify the validity of the relationship between stereoselectivity and the spatial distribution of molecular orbitals.
5. Conclusions (1) The reactivity was found to be the greatest for collision at the CF3 end of the molecule for the CF3Cl -t Atf3P) and CF3Br -t KI-(~P)systems. (2) The steric opacity function for CF3* formation in the CF3Cl system revealed two reactive sites, similar to those of the CF3H AI-(~P)system. (3) Two reactive sites in the CF3Br Kr(3P) system did not appear to be clear at this stage within the limit of the present experimental accuracy, but this broadness may be due to the effects of the center-of-mass shift onto the steric opacity function and/or of the impact parameter broadness.
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Appendix In a steric opacity function, the angle of attack y is defined as the collisional angle toward the center of mass of the target molecule, while the molecular orbital is defined in the molecular frame. Therefore, a nominal position of a reactive site (i.e., the exterior electron position), which is localized at the X end
References and Notes (1) Brooks, P. R. Science 1976, 193, 11. Stolte, S. Dynamical StereochemistryIssue. J. Phys. Chem. 1987, 91. Stolte, S. Ber. BunsenGes. Phys. Chem. 1982, 86, 413. Parker, D. H.; Bernstein, R. B. Annu. Rev. Phys. Chem. 1989,40, 561. Bernstein Memorial Issue on Molecular Dynamics. J. Phys. Chem. 1991, 95. (2) deVries, M. S.; Tyndall, G. W.; Cobb, C. L.; Martin, R. M. J. Chem. Phys. 1987, 86, 2653. (3) Che, D.-C.; Kasai, T.; Ohoyama, H.; Ohashi, K.; Fukawa, T.; Kuwata, K. J. Phys. Chem. 1991,95,8159. Kasai, T.; Che, D.-C.; Ohashi, K.; Kuwata, K. Chem. Phys. Lett. 1989,163,246. Che, D.-C.; Kasai, T.; Ohoyama, H.; Kuwata, K. Bull. Chem. SOC. Jpn. 1944, 67, 1265. Che, D.-C.; Kasai, T.; Ohoyama, H.; Kuwata, K. J. Phys. Chem. 1994,98, 135. Fukunishi, Y.; Kasai, T.; Kuwata, K. Chem. Phys. 1993, 177, 85. Kasai, T.; Mastunami, T.; Fukawa, T.; Ohoyama, H.; Kuwata, K. Phys. Rev. Lett. 1993, 70, 3864. (4) Ohoyama, H.; Kasai, T.; Ohashi, K.; Kuwata, K. Chem. Phys. 1992, 165, 155. (5) Ohoyama, H.; Iguro, T.; Kasai, T.; Kuwata, K. Chem. Phys. Lett. 1993,209, 361. (6) Takahashi, H.; Ohoyama, H.; Kasai, T.; Kuwata, K.; Nakano, M.; Yamaguchi, K. Chem. Phys. Lett. 1994,224,445. (7) Takahashi, H.; Ohoyama, H.; Kasai, T.; Kuwata, K.; Nakano, M.; Yamaguchi, K. Chem. Lett. 1994, I , 1985. (8) Suot, M.; Lee, L. C. J. Chem. Phys. 1983, 79, 1127. Lee, L. C.; Han, J. C.; Ye, C.; Suto, M. J. Chem. Phys. 1990, 92, 133. Doucet, J.; Sauvageau, P.; Sandorfy, C. J. Chem. Phys. 1973, 58, 3708. Cvitas, T.; Gusten, H.; Klasinc, L. J. Chem. Phys. 1977,67,2687. Doucet, J.; Gilbert, R.; Sauvageau, P.; Sandorfy, C. J. Chem. Phys. 1975, 62, 366. (9) Ohoyama, H.; Kasai, T.; Ohashi, K.; Kuwata, K. Chem. Phys. Lett. 1987, 136, 236. (10) Hagena, 0. F.; Varma, A. K. Rev. Sci. Znstrum. 1968, 39, 47. Anderson, J. B.; Fenn, J. B. Fluids 1965,8,780. Anderson, J. B.; Andres, R. P.; Fenn, J. €3. Advan. Chem. Phys. 1966, 10, 275. (1 1) Kasai, T.; Ohashi, K.; Ohoyama, H.; Kuwata, K. Chem. Phys. Lett. 1986, 127,581. (12) Stolte, S.; Chakravorty, K. K.; Bemstein, R. B.; Parker, D. H. Chem. Phys. 1982, 71, 353. (13) Ohno, H.; Mutoh, H.; Harada, Y. J. Am. Chem. SOC. 1983, 105, 4555. Hotop, H.; Niehaus, A. 2. Phys. 1969, 228, 68. Miller, W. H.; Morgner, H. J. Chem. Phys. 1977, 67,4923.
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