J. Phys. Chem. 1994, 98, 132-135
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Emission Spectra and Effect of Molecular Orientation on Branching of Chemiluminescent Channels in the CF3CN Ar(3P) Reaction
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Dock-Chi1 Che, Toshio Kasai,' Hiroshi Ohoyama, and Keiji Kuwata' Department of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan Received: April 28, 1993; In Final Form: July 29, 1993'
The effect of molecular orientation on the branching of the chemiluminescent channels of the formation of the CN*(B), the CN*(A), and the CFs* radicals has been studied in the reaction of Ar* with the oriented CFsCN molecule. The branching ratio of the formation of the CFs* radical has been found to be very small compared to that of the CN*(B) and of the CN*(A) radicals. The CN-end attack has been observed to be more favorable than the CFs-end attack in the formation of CN*(B) and of CN*(A), and the steric asymmetries in the reaction are almost equal to each other. By applying a painted hard-sphere model, the reactivity for the CN-end attack was found to be 1.6 f 0.2 larger than for the CF3-end attack in the formation of CN*(B), and 1.5 f 0.2 in the formation of CN*(A). These values are close to the rather isotropic value of 1.5 for the CDsCN Ar* reaction, but smaller than 3.1 for the CHsCN + Ar* reaction. These results indicate that the formation of the vibrationally excited CN* radicals is significant in this reaction. Spectral analysis of the CN*(B) and of the CN*(A) emissions supports our hypothesis concerning the orientation dependence for this type of reaction. The total emission cross sections of both CN*(B) and CN*(A) as well as the vibrational energy disposal in the CN*(B) product were found to be larger than in the CH3CN Ar* reaction.
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Introduction Studies on the steric effects in the atom-molecule reactions have shown that chemical reaction depends on the orientation of reagent.'" The wavelength dependence of the BaO* chemiluminescence in the reaction of Ba with oriented N20 has also been studied.' When several products are formed in a reaction, orientation of the reactant molecule may be one of the dominant factors to determine the branching of reaction.*v9 The dissimilar alignment dependence between the formations of the CS+ ion and the CS*(A) radicals has been observed in the reaction of the metastable argon atom, Ar*, with the aligned CS2 molecule, where the molecular alignment has been achieved by removing the molecules of the unwanted alignment from the beam using a polarized laser light.lo The result suggested that the two competing reactions depend on the molecular alignment. Orientational dependence on the formation of CN*(B) has been observed in the reactions of Ar* with acetonitrile and with acetonitrile-d3.llJ2 The deuterated reactant, however, shows a smaller stereoanisotropytogether with an increased reactive cross section and vibrational disposal into CN*(B). An orientational dependence for the formation of the CF3* radical has also been observed in the reaction of Ar* with CFsH.13 The orientational dependence for the formations of radicals in these metastable rare-gas-atom reactions has been tentatively explained by the overlap between the molecular orbitals and the metastable atomic orbital. Formation of the CN*(B), of the CN*(A), and of the CF3*(2A"2, lE', and 2AfI)radicals, and also of the ArF* excimer, is energetically accessible for the reaction of Ar* with CF3CN. In the present experiments, a substantial yield of the CN*(B), the CN*(A), and the CF3* radicals could be directly detected by the chemiluminescencesof the excited radicals. This reaction appears to be closely related to the reactions of Ar* with acetonitrile and with CF,H and is considered to be suitable for the study of the relationship between reaction branching and orientation of the reactant molecule. The disposal of the vibrational energy in the products seems to be reflected on the reaction branching between the atomization and the formations of the excited radicals. Due to the weak Abstract published in Aduance ACS Abstracrs, December 1, 1993.
0022-365419412098-0132$04.50/0
intensity of the oriented molecular beam, the examination of the wavelength dependence of the emission using the oriented beam could not be carried out. Thus theunoriented ordinary molecular beams wereemployedto observe theviolet and red emission spectra for the CN(B2Z X*Z) and the CN(A2n X2Z) transitions, respectively.
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Experimental Section The apparatus for the measurement of orientation dependence of radical formation with the oriented molecular beam and for the spectroscopic measurement with the unoriented molecular beam are the same as described b e f ~ r e . ' ~ In J ~both types of experiment, the pulsed beam of pure CF3CN (PCR. Inc., purity 97%) was produced at the stagnation pressure of 60 Torr, and the beam of the metastable argon atom, Ar*, was produced by electron impact. The spectroscopicmeasurements of theviolet and red emissions from the excited CN radicals with the ordinary pulsed beam of CF3CNwere carried out by use of a 30-cm monochromator with the F number of 3. The emissions were accumulated up to 1000 times at each wavelength and swept with the 0.25-nm interval in the wavelength region from 375 to 395 nm for the violet emission and 800 times in the wavelength region from 580 to 620 nm with 0.5-nminterval for the red/CF3* emission. Theobservedemission spectra were simulated using parameters of the pupulations of the vibrational states in the usual manner.I6J7 For the rotational population, a Boltzmann distribution was assumed. The measurements of the orientational dependenceof the total emissions from the excited radicals of CN*(B), CN*(A), and CF3* were carried out by use of the oriented molecular beam. The oriented CF3CN molecules were prepared by a 60-cm electric hexapole field, a guiding field, and a uniform electric field of 150 V cm-1. The molecules with unwanted orientations and clusters in the molecular beam were completely blocked out by a beamstop set in front of the hexapole field. The emissions from the beam intersection were isolated from others by a band-pass filter (389 nm, X l / 2 = 20 nm) for the violet emission and a low-pass = 450 nm) for the red emission. The filter (Toshiba UV-45, but band-pass filter only transmits the CN(B X) chemiluminescence, while the low-pass filter transmits the CN(A X) and
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0 1994 American Chemical Society
Branching of Chemiluminescent Channels
The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 133
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385 395 WAVELENGTHhm Figure 1. CN(B X) emission spectrum of the violet band (Au = 0) with a spectral resolution of 0.9 nm. The shaded area is the observed spectrum. The solid line is the simulated spectrum. The rotational temperature is estimated to be 4200 K.
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the CF3* chemiluminescences. The measurements of the orientation dependenceof the emissions were carried out with a rod voltage of the hexapole of 10.1 kV. The direction of the uniform field was switched periodically to alter the collisional geometry from the CN-end, the CF3-end, and the random orientations. The random orientation is prepared with the uniform field off. The signals for the CN-end, the CF3-end, and the random orientationswere accumulated by a microcomputer. The emissions were accumulated up to 5700 pulses for the violet emission and 20 700 pulses for the red emission in order to get the standard deviations within 5%.
Results and Discussion Emission Spectra of the CN* Radicals from the Unoriented CFJCN. The emission spectrum of the violet bands of CN*(B) formed in the crossed-beam reaction of Ar* resulting from an unoriented CF3CN is shown by the shaded area in Figure 1, and the result of simulation is displayed with the solid line. The intensity of the total emission of the CN(B X) emission in this reaction was 4 times larger than that in the reaction with CH3CN. The intensity of the total emission of CN*(B) increased by the heavy-atom substitution in the methyl group. This tendency of the emission intensity to increase can be understood as follows. The mechanism of the formation of CN*(B) from CF3CN may be divided into two steps.12 The electronically excited CF3CN*, where CF3CN* would be a Rydberg state, is formed at first by the energy transfer from Ar*. In the second step, the CN*(B) radical is produced via the dissociation of CF3CN*. In the first step, the probability of the formation of CF3CN* in the reaction of CF3CN with Ar* is expected to be bigger than that in the reaction of CH3CN with Ar*, because of the larger polarizability of the CF3CN molecule.I* In the second step, the rate of the fluorine atom formation is smaller than that of the hydrogen atom formation (atomization) becauseofthelowerfrequencyoftheC-X (X= For H)stretching vibration. Thesubstitutionby the heavy atom may likely influence the vibrational distribution of CN*(B), as in the case of the deuterium substitution of CH3CN.12 As a result, the branching to the channel of the CN*(B) formation, which competes with the atomization channel, increases. The vibrational distribution of the CN*(B) radicals obtained by the spectral simulation of Figure 1 is shown in Figure 2. The vibrational distributionspreviously obtained for the reactions of Ar* with CH3CN and with CD3CN are also shown. Figure 2 shows that all those distributions exhibit a Boltzmann character. There is a tendency for the energy disposal in the vibration of the CN*(B) radical to increase as the weight of the methyl group increases. A similar tendency has also been observed in the photoexcitation by the vacuum-ultraviolet light.19 The emission spectrum in the wavelength region from 580 to 620nm is shown in Figure 3. The CN(A X) and the CF3( 1E’ and/or 2Aff2 lA’1) emissions may appear in this wavelength
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Figure 2. Vibrational distribution of the CN*(B) radical in the reaction of Ar* with CF3CN (circles), CD3CN (squares), and CH3CN (triangles). The vibrational distribution resulted in a Boltzmann character in all cases (solid line and dashed lines). The vibrational temperatures are determined to be 10 000 K for CF3CN, 7000 K for CD3CN, and 6000 K for CH3CN. A
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Figure 3. CN(A X) emission spectrum of the red band with a spectral resolution of 1.1 nm. The shaded area is the observed spectrum. The solid line is the simulated spectrum. The base line of the emission is set at zero on the ordinate axis. The rotational temperature is estimated to be 4000 K.
region, although some band peaks in the spectrum could be assigned to the progressionsof the CN(A- X) transition. Under the present spectral resolution, the unresolved broad background in the spectrum has tentatively been assigned to both the CN(A X) and the CF3 emissions.20 In order to estimate the contributionof theCF3 emission, a spectral simulationwascamed out. The result of the simulation is shown by the solid line in the figure. The contribution of the CF3 emission has been estimated to be less than 10% of the total emission. This result indicates the presence of a small branching ratio for the formation of CF3* in the reaction of CF3CN with Ar*. The intensity of the total emission for theCN(A- X) transition is also larger in thereaction of CF3CN with Ar* than that in the reaction of CH3CN. Orientational Dependenceof the Formationof the CN*(B) and of the CN*(A) Radicals. The orientational dependences of the totalemission for theCN(B-X) and theCN(A-,X) transitions have been investigated using an oriented CF3CN beam. For a quantitative determination of the orientational dependence of the formation of the CN* radicals, the orientational distribution of the CF3CN molecules in the reactant beam should be known. It is desirable to determine directly the orientational distribution of oriented molecules, but it is difficult.21*22Thus it is usually done by computer simulations of focusing curves. Figure 4 shows the focusing curve of the CF3CN beam. The beam intensity was measured using a quadrupole mass spectrometer tuned to the (CF2CN)+ fragment peak of m / e = 76, which is known to be the strongest fragment peak of CF3CN by
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134 The Journal of Physical Chemistry, Vol. 98, No.
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Che et al. cross sections for the CN-end attack and for the CF3-end attack in the formation of CN*(A) was found to be 1.5 f 0.2. This valueisessentially thesameasthat in the formation oftheCN*(B) radical. Mechanism for the Formationof the CN*(B)and of the CN*(A) Radicals. The reaction of the metastable argon atom with the molecule is likely to be initiated by the step of an electron exchange in the energy-transfer process.29 In the electron exchange, an electron of a molecule is transferred to the vacant 3p orbital of the metastable argon atom, and simultaneously,an electron from the 4s orbital of the argon atom is transferred to some vacant orbitals of the molecule. In the reaction of CH&N with Ar*, the CN-end attack has been shown to be more efficient in the formation of CN*(B) than the methyl-end attack by a factor of 3, and the 7a1 nonbonding orbital localized at the CN-end has been suggested to make the CN-end attack favorable in the formation of CN*(B).ll Similar stereoanisotropywas seen in the CF3CN + Ar* reaction; however, the stereoanisotropy of the formations of CN*(B) and CN*(A) in this reaction became smaller than that in the CH3CN Ar* reaction. In the reaction of the CD3CN with Ar*, the dissimilar orientation dependence of the formation of CN*(B) has been explained by the contribution of the isotropic 2e molecular orbital to the formation of CN*(B).12 In addition, the 2e orbital has been postulated to produce more vibrationally excited CN*(B) radicals than the 7a1 orbital. The 7al and 2e orbitals of the CF3CN molecule have been reported to have almost the same energy from the study of a photoabsorption experiment.19 Thus the decrease of the stereoanisotropy in the formations of CN*(B) and CN*(A) may be due to the additional contribution of the 2e orbital. This is consistent with the result of the analysis of the CN* emission spectra, which showed the increase of the vibrational energy disposal in CN*(B) and the increase of the reaction cross sections of the formations of CN*(B) and CN*(A) as compared to that in the reaction of CH3CN. The electronically excited CF3CN is formed by the electron exchange between the atomic orbital of Ar* and the molecular orbital of the molecule (excitation process). And the second step is the dissociation of the excited CF3CN molecule to form CN*(B) or CN*(A) (branching process). It has been known that the Rydberg transitions arising from the 7al and 2e orbitals of CF3CN are almost similar.19 Although the electronically excited state of the CF3CN molecule could not be assigned in our experimental investigation,the singlet and the triplet excited states of the molecule should be formed by the electron exchange with CFSCN, which also follows for Ar*.*9 This characteristic differs from that of the photoexcitati0n.1~ In the photoexcitation experiment, it has been shown that CN*(B) was formed through a long-lived excited m01ecule.l~If the lifetime of the excited CF3CN molecule is long enough to redistributethe internal energy in the molecule, the iritemal energy distribution of the excited product does not depend on the collisional geometry and leads to an absence of orientational dependence in the branching process of the excited CF3CN molecule. Thus, the orientational dependence of the radical formation should originate in the excitation process. The similar orientation dependences of the formations of CN*(B) and of CN*(A) indicate that both radicals originate from the same electronically excited state of CF3CN. The triplet excited state of CF3CN is mainly formed in the reaction of Ar* with CFpCN, because the excitation by the metastable rare-gas atom favors the triplet electronic state.)O A small difference between the stereoanisotropies in the formations of CN*(B) and CN*(A) might be due to the minor contribution of the CF3* emission to the dominant CN*(A) emission. The relationship between the molecular orientation and the branching of reaction provide new insights into the control of the reaction.
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Vo I k V Figure 4. Dependence of the CF3CN beam intensity on the hexapole rod voltage. The experimental error is smaller than the size of each filled circle. The solid line shows the best fit for the Monte Carlo trajectory simulation with rotational temperature of 61 K. The calculated quantal orientational distribution of CF3CN at the beam intersection is shown in the inset. The hexapole rod voltage is 10.1 kV. The W(cos 0 ) distribution was simulated with 2 X los trajectories.
TABLE 1: Orientation Dependences of tbe Violet and Red Emissions of the CN* Radicals' total emission total emission intensity of CN*(A) intensity of CN*(B) orientation per count pulse-1 per count pulse-' 0.21 0.12 CN-end 0.15 0.09 CF3-end 0.18 0.10 random Thegatewidthofthephotoncountwas 3 ms. Theestimatedstandard deviations were -5%. electron impact. The result of the best fit of the Monte Carlo trajectory calculations to the observed points is shown by the smooth solid line in the figure, and the rotational temperature of the CFpCN beam was determined to be 61 f 10 K.23 The inset of Figure 4 shows thecalculated quantal orientational distribution of the CF3CN beam at the beam intersection after the hexapole state-selection with a rod voltage of 10.1 kV.24.2sThe ensemble average of cos 6 of the CF3CN beam was 0.55, which is better than the value of 0.23 of the CHpCN beam with the same rod voltage. The determination of the orientational dependences of the violet and the red emissions were all carried out with 10.1 kV. Both orientational dependences of the emission intensities for the violet and the red bands of CN* are listed in Table 1. As shown in Table 1, the CN-end attack was more favorable than the CF3-end attack in both cases. The tendency is the same as that in the case of the formation of the CN*(B) radical in the CH3CN + Ar* and the CD3CN + Ar* reactions.I2 The ratio of the reaction cross sections for the CN-end attack to that for theCF3-endattackwasdeterminedtobe 1.6f0.2in theformation of CN*(B) on the basis of the analysis using a painted hardsphere model which has been used in the reaction of Ar* with acetonitrile so far.11JGZ8 This value is almost equal to that for the CD3CN + Ar* reaction but smaller than the value of 3.1 obtained for the CH3CN Ar* reaction.12 Similarly, the orientation dependence of the formation of the CN*(A) radical was measured, and the ratio of the reaction
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Branching of Chemiluminescent Channels
Acknowledgment. Theauthors wish to thankMr. Y.Fukunishi of Kyoto University for the calculation of the dipole moment of CF3CN. They acknowledge the Ministry of Education, Science, and Cultureof Japan, through a Grant-in-AidScientificResearch 63606003,for financial support. References and Notes (1) Brooks, P. R.; Jones, E. M. J . Chem. Phys. 1966,45, 3449. (2) Beuhlcr, R. J., Jr.; Bernstein, R. B.; Kramer, K. H. J . Am. Chem. Soc. 1966,88, 5331. (3) Broob, P. R. Science 1976, 193, 11. (4) Stolte, S.Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 413. (5) Bernstein, R. B.; Herschbach, D. R.; Levine, R. D. J. Phys. Chem. 1987, 91, 5365. (6) Parker, D. H.;Bemstein, R. B. Annu. Reu. Phys. Chem. 1989,40,
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(7) Jalink, H.; Stolte, S.;Parker, D. H. Chem. Phys. Lcrt. 1987, 140, 215. (8) Rettner, C. T.; &re, Z . N. J . Chem. Phys. 1982, 77, 2416. (9) deVries, M. S.;Srdanov, V. I.; Hanrahan, C. P.; Martin, R. M. J . Chem. Phvs. 1982. 77. 2688. (IO) d h k , M.S.;Tyndall, G. W.; Cobb, C. L.; Martin, R. M. J . Chem. Phys. 1987.86, 2653. (11) Kasai, T.; Che, D.-C.; Ohashi, K.; Kuwata, K. Chem. Phys. Lett. 1989, 163, 246. (12) Che,D.-C.; Kasai,T.;Ohoyama, H.;Ohashi, K.;Fukawa,T.;Kuwata, K. J. Phys. Chem. 1991,95, 8159.
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