2528
J. Phys. Chem. 1980, 84, 2528-2531
Effects of Thermal Reaction on the Boron Isotope Enrichment in a BC1,/02 Mixture by TEA C02 Laser Hiroyuki Kojima, * Toshio Fukumi, Kiyoshi Fukui, and Kazuo Naito Government Industrial Research Institute, Osaka Mldorlgaoka 1, Ikeda, Osaka 563,Japan (Recelved: March 10, 1980)
The isotope selectivity of boron through a multiphoton dissociation process in a BC13/02mixture was studied by means of multiphoton absorption spectra, nonresonant dissociation, and effects of a buffer gas. The measurement of the multiphoton absorption spectra showed resonant characteristics in the excitation process. Studies of nonresonant dissociation suggested that the Bz03solid which deposited on the cell windows reacted with BC13 on irradiation. The dependence of the selectivity on the pressure of a buffer gas showed that the selectivity first increased and then decreased with an increase in pressure. It was explained in that either intermediates which were selectively generated by photodissociation or plasma which was nonselectivelygenerated from a trace impurity played the role of a heat source to cause the thermal reaction. Introduction Since much interest has developed in the multiphoton dissociation by a TEA COPpulsed laser, considerable effort has been directed toward boron isotope enrichment in the system of a BC13/02 m i ~ t u r e . l - ~Although BC13 has a strong u3 vibration absorption with a large isotope shift (39 cm-l) in the tuning range of the C02laser, both dissociation efficiency and isotope selectivity have been rather poor. The poor dissociation efficiency may be understood in terms of strong B-C1 bonds and the occurrence of backreactions of the bond dissociation. The bond dissociation energy is equivalent to 40 photons at 10.6 ym wavelength, which is considerably large compared with that of, for example, SF6(29 photons).6 The effect of the back-reaction is diminished by the addition of the scavenger 02. Instead, the latter dissipates the excitation energy via collisional processes. On the other hand, the reason for poor selectivity remains ambiguous. Enrichment factors so far reported have been as low as around 8, even at relatively low pressures of BC13.2 The reaction in the BC13/02mixture shows a rather complicated mechanism, giving B203as the final product. Although the mechanism of multiphoton dissociation is not fully understood, a general scheme admitted so far is the following: a molecule is excited first through several transitions to a vibrational quasicontinuum state, where the absorbed energy is rapidly dumped into all other modes via intramolecular V-V transfer. Thus it becomes possible to pump energy into the molecule so effectively as to lead to selective dissociation before the energy is lost through collisional proce~ses.~ This scheme, however, is for a unimolecular reaction. A reaction which can occur only by the aid of a scavenger should involve additional factors. Other various interactions of excited states with a strong laser field may cause a transition to a less stable state leading to dissociation of the molecule at a frequency where no apparent absorption occurs.2ss Recently, Lau presented a theoretical model for nonresonant dissociation via a predissociation mechani~m.~ In fact, Isenor et al. observed the effective dissociation of SiF4which has very little absorption at the radiation frequency.1° In principle, BCl, can be dissociated by radiative excitation of the v3 vibration on account of vibrational predissociation via the vz ~ibration.~ Lyman and Rockwood pointed out a possible overlapping of the combination band v1 + vz of 1°BC13with the v3 vibration of 11BC13.6 We present here the multiphoton absorption spectra used to examine the resonant characteristics of the 0022-3654/80/2084-2528$0 1.OO/O
BC13/02system. We discuss the reason for the poor selectivity in terms of nonresonant dissociation and the effects of a buffer gas to study the contribution from the thermal reaction after the photodissociation. Experimental Section The experiments were done with a Lumonics Type 103 TEA COz pulsed laser with a typical 70-11s pulse width and 50-MW peak power at 10.6 ym. Infrared wavelengths were measured with an Optical Engineering Model 16A C02 laser spectrum analyzer. The radiation energies which ranged from 1 to 5 J/pulse were determined with a Lumonics Model 50 D pyroelectric detector. The laser beam was reduced to a circular area of 2-cm diameter by means of a slate mask in order to eliminate outer regions of low energy density. A fairly good homogeneity was confirmed by the burn patterns produced on thermal recorder papers. Two types of cells were used. One was for the enrichment reaction and had a 10-cm length and 4-cm diameter as shown in Figure 1. The laser beam was focused at the center of the cell by a concave mirror. The inward flanges of the glass cylinder on both ends serve to prevent the light from scattering to the packing rubbers. The other was for measurement of the multiphoton absorption spectra and had a 40-cm length and 4-cm diameter as shown in Figure 2. A GaAs lens of 25.4-cm focal length was set in front of the cell so that the beam of 3.14-cm2cross section could be focussed at the center of the cell. The energy absorptions (J/pulse) were measured by the pyroelectric detector with reference to the empty cell in both focussed and unfocussed conditions a t each frequency of the C02 laser. Both cells were baked out in vacuo at over 140 "C for 1 h to exclude water which was adsorbed on the cell walls. Gas mixtures at desired ratios were prepared by using,a MKS Type 220 Baratron pressure gauge. Boron trichloride was purified by pumping HC1 out at the CS2 slush temperature. The changes of the concentration of BCl, were measured by IR spectroscopy, using the v3 vibration at 994 cm-' for 1°BC13and at 956 cm-l for 11BC13. Results and Discussion Multiphoton Absorption Spectra. Multiphoton absorption spectra in both unfocussed and focussed conditions were studied to examine the selectivity in the multiphoton excitation process of BCl,. Figure 3 gives the spectra of 2 torr of BCl, in unfocussed (curve a) and focussed (curve b) conditions. Curve c shows an ordinary IR spectrum. By limiting the tuning range of the COz laser, the spectra were restricted to the region of 925-975 0 1980 American Chemical Society
Thermal Reaction Effects on Boron Isotope Enrichment
The Journal of Physical Chemistry, Vol. 84, No. 20, 1980 2529
quasicontinuum ~ t a t e . ~ Bachmann !~J~ et al. reported the dependence of the product yield on the radiation wavelength of the C02 laser for the laser-induced ligand exchange reaction16 2BC13 + B(CH3)3 laser beam
Flgure 1. Reaction cell: (a)glass cylinder, (b) metal frame, (c) concave mirror, (d) NaCl windows, and (e) rubber packings.
p-,
Pyroelectric Energy Detector
\Nindow
Irraco,I+ Laser
[lo
640 crri
Flgure 2. Block diagram for multiphoton absorption measurements.
hu (C02 laser)
under irradiation of a COzlaser. The maximum yield was obtained at 925-940 cm-l which coincides with the absorption maxima in Figure 3. In comparison between curves a and b, there is a tendency that a higher fluence gives a broader absorption contour and a larger shift. Such a tendency was also obtained in SF6." On the other hand, the absorption at -970 cm-l has no direct correspondence in the linear absorption spectrum. It is not due to the shift of the v3 vibration of 1°BC13because the shift (25 cm-l) is too large compared with that of 'lBC13. There are some possibilities to be considered in the multiphoton absorption process. The vibrational anharmonicity might cause a multiply degenerate level to split into many levels in an upper excited state.31 The rotational compensation of the anharmonicity might lead to an absorption split because of the various choices of the rotational level^.^ However, the observed shift in Figure 3 is too large when one considers that the P-Q separation is 3.5-3.8 cm-l.17 A possible assignment in this case may be the third overtone of the v4 vibration of which the fundamental wavenumber is 255 cm-l. Its anharmonicity is so large that the second overtone 3v4 could clearly be observed at 732 cm-l for lo BC13 and at 729 cm-l for 11BC13 with medium intensity in an ordinary IR spectrum.l7J8 Therefore it is possible that the third overtone 4v4 may interact with the v3 vibration of 11BC13(956 cm-l) by Fermi resonance, resulting in an enhancement of the intensity.lg It was reported that ethylene gave new absorptions under irradiation by a C021aser.ll Optoacoustic studies revealed that Fermi resonance was the origin of these new absorptions.21722On the other hand, the possibility of the combination band v1 + v 2 of 1°BC13seems negative because radiation at this wavenumber did not result in the selective dissociation of 1°BC13 The yield spectrum by Bachmann et al. never gave a new peak at 940 cm-l.16 Presumably, the contribution to the reaction from the absorption of the third overtone may be very small. Regardless of the definite assignment of the new absorption Figure 3 may suggest that the multiphoton absorption process has resonant characteristics in BC13 systems. Nonresonant Dissociation. The changes of the partial pressure of each isotope molecule under irradiation were measured in terms of In n(O)/nvs. t where n ( 0 ) and n are the concentrations of residual isotope molecules before and after irradiation by t shots of laser pulses. Figure 4 shows a typical result in nonresonant condition using the P(24) line of 9.5-pm band which is far enough to get off the resonant absorption of either 11BC13or 10BC13 It was found that the amount of nonresonant dissociation varied, depending upon the condition of the cell windows. Lines a and b are examples when the windows were carefully cleaned with a polishing plate and were baked enough just before the experiment. Curve c shows an example in nonresonant condition after the several experimental runs in on-resonant condition. In this case the amount of nonresonant dissociation is very large. After irradiation the intensity of the absorption at 1280 cm-l due to Bz0322 was greatly increased. The result may be explained in that the Bz03solid which deposited on the cell windows reacts with BC1,. The broad absorption characteristic of the B203 solid makes it possible to be heated by the irradiation,
950 " ' 975i ' " 1000 ' i ' ' ' ' J 09 Wave Number (Cm") Figure 3. Multiphoton and linear absorption spectra of BCi3. Curves a and b are the multiphoton absorption spectra in unfocussed and focussed conditions, respectively. The vertlcal axis ( n ) denotes the average number of photons absorbed per molecule. The incident light energy is 1.3 J/pulse and the pressure of BCI, is 2 torr. Curve c is a linear absorption spectrum with an arbitrary scale for the vertical axis.
cm-l where the v3 viblration of 11BC13is mainly located. The incident energy at each line was adjusted to 1.3 J/pulse. The average number of photons absorbed per molecule was calculated by using eq 1, where Eabis the absorbed energy
( n ) = Eab,/(Nhv) (1) and N is the number of potential absorbers in the beam. N was simply calculated from the volume of the optical path by estimating the beam geometry. Since the absorption in the focussed condition significantly involves the contribution from the off-focal region, ( n )of curve b may be just for qualitative comparisons. The vertical axis for curve c is arbitrarily scaled. Clearly there are two absorptions at -940 and -970 cm-I in curves a and b. The former is due to the v3 vibration of 11BC13although it shifts toward a low wavenumber by 10-15 cm-' compared with that for linear absorption. The shift of the absorption maximum toward the red side is a common characteristics in multiphoton absorption spectra, being observed in many O S O ~CF31,12113 , ~ ~ and other systems which include CH3NHz.14 It is explained in terms of rotational compensation of vibrational anharmonicity in the successive transitions for the several first vibrational levels to a
3B(CH3)C12
2530
The Journal of Physical Chemistry, Vol. 84, No. 20, 1980
' I
I
&/ I
0
100 200 Number of pulses
I
Flgure 4. Nonresonant dissociation of BCI, with irradiation by the P(24) line of the 9.6-pm band. The incident energy is 4.0 J/pulse and the partial pressure of BCIBis 0.4 torr. Lines a and b are with windows which are freshly cleaned. The initial compositions are BCI3/O2= 114 (U)., and BCI$O2/N = 1/4/10 (A& where the white and dark marks denote 'lBCI, and "BCI3, respectively. Curve c is after several runs of resonant dissociatlon with BCI,/02 = 114 (09). 4. 1
:[
I
3.
Kojima et al.
induced by the increase in pressure under a very intense radiation field by the mechanism of cascade ionization which begins through the ionization of a trace impurity.24 If the pressure is low, the cascade ceases to some extent, while if it is high enough, the cascade develops into an explosion. Since the plasma which is generated by the optical breakdown can become a heat source for the thermal reaction, the selectivity should be decreased depending upon the extent of optical breakdown. On the other hand, the increase in CY at lower pressures in Figure 5 has to be attributed to an effect of the buffer gas totally different from the optical breakdown. Considering that either photoexcited BC13molecules or active intermediates in the selective photochemical process may also play the role of a heat source, the thermal reaction should occur in a secondary process. Then, the addition of the buffer gas serves to quench the thermal reaction through collisional processes. In laser isotope enrichment in the BC13/Ozmixture, Ambartzumian et al. reported a delayed luminesceme relative to the laser pulse.4 It was also observable in the weak field off the focal region. They found BC1 and BO radicals as the intermediates. The emission from the latter in the excited state especially had a long duration of up to 1.2 ys. These facts suggest that the thermal reaction really occurs to reduce the selectivity after the photodissociation. Geiss and Froschle measured the rate of the reaction 5 at -1000 K by the diffusion tube method. The rate equation may be expressed by
Pressure of N2 ( t o r r )
-d[BClJ/dt = k[BC13][02]2
(6)
Figure 5. Dependence of the selectivity factor CY on the pressure of added N, gas. The incident energy is 4.0 J/pulse and the partial pressures of BC13 and O2 are 0.4 and 1.6 torr, respectively.
where
According to the synthesis by Blaner and Farber,23B,03 reacts with BC13 a t 500-800 K by the reaction
It should be noted that the activation energy is considerably lower than the bond dissociation energies, since the individual bond dissociation energies BCl241,BC1-C1, and B-C1 are reported to be 110.2,78.2, and 127 + 1kcal/mol, respectively.26 The exothermicity of the reaction 5 (AH" = -109.7 kcal/m01)~~ facilitates further thermal reaction. From the viewpoint of a model given for a unimolecular reaction, the key for the selective multiphoton dissociation is the collisionless process in which molecules can be heated selectively. On the other hand, in a reaction which can occur only in the presence of a scavenger, collisions with scavenger molecules are absolutely necessary, that is, the activation energy is partially derived from the collisional process. Thus the molecule does not need all the energy for the dissociation threshold from the photons absorbed, and, instead, it becomes possible that molecules which are unexcited can be dissociated by the collisional process. Several reagents other than O2 have so far been proposed as scavengers, including H2,6 H2S,28CH4,29and HBr.l Relatively high values for the enrichment factors (-20) were obtained with HBr by Ambartzumian et al.' A problem in this system was the presence of the backreaction in the ligand replacement reaction
B203(1) + BC13(g) -+ (BOC1)kd
(3)
The product trichloroboroxine is unstable a t room temperature and decomposes to B203in the presences of OPz2 One can easily expect a similar reaction under infrared laser irradiation. The result in Figure 4 suggests that the effect of the deposition on the cell windows on the selectivity is very important, although the involvement of intrinsic nonresonant dissociation which can be induced by a strong laser field8 may not fully be ruled out. Resonant Dissociation. Selective dissociation of "BC13 was carried out with the P(20) line of the 10.6-ym band which is overlapped with the v3 absorption of 11BC13. The isotope selectivity was evaluated in terms of the selectivity factor a by the equation (4) In [nl/nl(0)l = CY In [nzlnz(O)l where the subscript 1denotes the isotope molecule which is selectively excited and the subscript 2 denotes the other isotope molecule.6 Figure 5 shows the dependence of CY on the pressure of the buffer gas Nz in the 0.4-torr BC131.6-torr O2 system. Values of a first increase from 1.4 to 2.4 with an increase of the Nzpressure. The further increase of the N2 pressure results in a decrease in CY and eventually optical breakdown at Nz pressures over 12 torr. The result in Figure 5 suggests that the thermal reaction 2BC13 + (3/2)O2 Bz03 + 3c12 (5) -+
contributes significantly to the dissociation under irradiation. The decrease in the selectivity at higher pressures in Figure 5 may be attributed to partial or entire optical breakdown. The optical breakdown is well-known to be
k(atm-2 s-') = 4.5
BC13
X
+ HBr
10l6 exp[-89,9(kcal)/RT]
BC1,Br
+ HC1
(7)
(8)
Considering the analogy of hydrogen pseudo-halogenides to HBr, we expected that HCN might undergo a similar reaction, being accompanied with little b a c k - r e a ~ t i o n . ~ ~ The result, however, turned out to be unsuccessful,because BC13 catalyzed the polymerization of CN- at the moment of mixing. Preferable systems for the enrichment should be those in which the photochemical processes are entirely completed in the intramolecular phase without the presence of a scavenger.
J. fhys. Chem. 1980, 84, 2531-2535
Acknowledgment. The authors are indebted to Mr. R. Makabe for his expert aid in the laser experiments. References and Notes
(13) (14)
(1) R. V. Ambartzumian, N. V. Chekalin, Yu. A. Gorokhov, V. S.Letokhov, G. N. Makarov, and E. A. Ryabov, "Laser Spectroscopy", S.Haroche, J. C. Pebay-Peyrouila, T. W. Hansch, and S. E. Harris, Ed., Springer-Verlag, New York, 1975, p 120. (2) V. S. Letokhov, "Friontiers in Laser Spectroscopy", Vol. 2, R. Baiian, S. Haroche, and E;. Liberman, Ed., North-Holland Publishing Co., Amsterdam, 1977, p 771. (3) R. V. Ambartzumian, N. V. Chekalin, V. S.Doljikov, V. S.Letokhov, and E. A. Ryabov, Chem. Pbys. Lett., 25, 515 (1974). (4) R. V. Ambartzumian, V. S,Doizhikov, V. S. Letokhov, E. A. Ryabov, and N. V. Chekalin, Sov. Pbys. JETP, 42, 36 (1976). (5) S. D. Rockwwd andi S.W. Rabdeau, Los Ahmos Scientific Laboratory Report LA-5761-SR, Nov 1974. (6) J. L. Lyman and S. D. Rockwood, J. Appl. Phys., 47, 594 (1976). (7) See, for example, N. Bloembergen and E. Yablonovitch, "LaserInduced Processes in Molecules", K. L. Kompa and S. D. Smith, Ed., Springer-Veriag, New York, 1979, p 117. (8) See, for examples, E. 0. Degenkolb, J. I., Steinfield, E. Wasserman, and W. Kiemperer, J. Cbem. Phys., 51, 615 (1961); J. D. Campbell, G. Hancock, J. 8. Halpern, and K. H. Welge, Ctmm. fhys. Lett., 44, 404 (1976). (9) A. M. F. Lau, fhys. Rev. A , 18, 172 (1978). (10) N. R. Isenor, V. Merchant, R. S. Hallswarth, and M. C. Richardson, Can. J. Pbys., 51, 1281 (1973). (11) V. N. Bagratashvili,I.N. Knyazev, V. S.Letokhov, and V. V. Lobko, Opt. Commun., 18, 525 (1976); S. S.Alimpiev, N. V. Karlov, S. M. Nikiforov, A. M. Prokhorov, B. G. Sartakov, E. M. Khokhlov, and A. L. Shtarkov, Opt. Isommun., 31, 309 (1979). (12) R. V. Ambartzumian, N. P.Furzikov, Yu. A. Gorokhov, V. S. Letokhov, G. N. Makarov, anci A. A. Puretzky, Opt. Lett., 1, 22 (1977); R. V.
(15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) ,
(30)
(31)
2531
Ambartzumian, V. S.Letokhov, G.N. Makarov, and A. A. Puretzky, Opt. Commun., 25, 69 (1978). I. N. Knyazev, Yu. A. Kudryavtsev, N. P. Kuz'mina, and V. S.Letokhov, Sov. Phys. JETf, 49, 650 (1979). G. Hancock, R. J. Hennessy, and T. Vlllis, "Laser-InducedProcesses in Molecules", K. L. Kompa and S.D. Smith, Ed., Springer-Verlag, New York, 1979, p 190. V. S.Letokhov, "Multiphoton Processes", J. H. Eberly and P. Lambropoulos, Ed., Wiley, New York, 1978, p 331. F. Bachmann, H. Noth, R. Rinck, W. Fuss, and K. L. Kompa, Ber. Bunsenges. Pbys. Chem., 81, 313 (1977). I. W. Levin and S. Abramowitz, J . Cbem. Pbys., 43, 4213 (1965). R. E. Scruby, J. R. Lacher, and J. D. Park, J. Chem. fhys., 19, 386 (195 1). S. Mizushima and T. Shimanouchi, "Sekigaisen Kyushu to Raman Koka", Kyoritu Shuppan, Tokyo, 1955, p 78. T. Fukumi, Opt. Commun., 30, 351 (1979). D. S. Frankel, Jr., and J. T. Manuccia, Chem. fhys. Lett., 54, 451 (1978). D.J. Knowles and A. S. Buchanan, Inorg. Cbem., 4, 1799 (1965). J. Blaner and M. Farber, J. Chem. Pbys., 39, 158 (1963). J. F. Ready, "Effects of HigkPower Laser Radiation", Academic Press, New York, 1971, p 213. V. Gsiss and E. Froschle, J. Electrocbem. Soc., 123, 133 (1976). W. H. Johnson, R. G. Miller, and E. J. Prosen, J. Res. Natl. Bur. Stand., 62, 213 (1959). V. H. Dibeler and J. A. Walker, Inorg. Chem., 8, 50 (1969). S. M. Freund and J. J. Ritter, Chem. Pbys. Lett., 32, 255 (1975). T. J. Manuccla, M. D. Clark, and E. R. Lory, J. Cbem. Phys., 68, 2271 (1978). K. Kawai and I.Kanesaka, Spectrocbim. Acta, Part A , 25, 1265 (1969). C. D. Cantrell, H. W. Galbraith, and J. Ackerhalt, "Multiphoton Processes", J. H. Eberly and P. Lambropoulos, Ed., Wiley, New York, 1978, p 307.
Electronic Effects of Polar Substituents at the Acyl Carbon. The Pyrolysis Kinetics of Several Isopropyl Esterst Mai,a A. Garcia de Sarmiento, Rosa M. Dominguez, and Gabriel Chuchani" Centro de Q h i c a , Instituto Venezolano de Investlgaciones Cienfificas, Caracas, Venezuela (Received: February 19, 1980)
The gas-phase pyrolysis of several isopropyl esters has been determined in the temperature range of 279.8-351.8 " C and the pressure range of 26.5-250 torr. The reactions in a static system, seasoned with allyl bromide, and in thie presence of an inhibitor are homogeneous and unimolecular and follow a first-order law. The rate coefficients are given by the Arrhenius equations: for isopropyl trichloroacetate, log k(s-') = (13.55 f 0.16) - (178.3 f 1.6) kJ mol-' (2.303R7'-l; for isopropyl dichloroacetate, log k(s-') = (12.78 f 0.45) - (176.2 f 5.0) = (12.57 f 0.21) - (180.8 f 2.3) kJ mol-' kJ miol-l (2.303RT)-'; and for isopropyl P-chloropropionate, log W') (2.303RT)-'. The present data together with those reported in the literature give a good correlation of log krel vs. u*(z) values with p* = 0.464, r = 0.963, and intercept = 0.044 at 330 "C. This linear relationship implies that electronic factors of polar substituents affect the rate of elimination of these type of compounds. Because of these results,the pyrolysis kinetics of isopropyl a-haloacetateshad to be redetermined. The Arrhenius equations are the following: for isopropyl fluoroacetate, log k(s-') = (12.73 f 0.56) - (180.6 f 6.1) k J mol-' (2.303RT)-'; for iriopropyl chloroacetate, log h(s-') = (13.04 f 0.58) - (183.6 f 6.2) kJ mol-' (2.303RT)-l;for isopropyl bromoacetate, log k(s-') = (12.87 f 0.31) - (181.2 f 3.5) kJ mol-l (2.303RT)-'; and for isopropyl iodoacetate, log h(s-') = (13.84 f 0.41) - (193.5 f 4.6) kJ mol-' (2.303R7'-'. Yet, with regard to the alkyl substituents at the acyl carbon, steric rather than electronic factors appear to be determinant.
Introduction The several works describing the effect of substituents at the acyl carbon in the gas-phase pyrolysis of esters have presented different arguments with regard to the factors affecting the rate of elimination of this type of organic compound. Bailey and Hewitt' studied the effect of 'Taken from the theisis of Maria A. Garcia de Sarmiento, submitted on the Faculty of IVIC, June 1979, in partial fulfillment of the requirements for the Degree of Magister Scientiarum. 0022-3654/80/2084-2531$01 .OO/O
changes in the acid portion of twelve esters of 4-methyl2-pentanol. They found that the extent of yield percent in the pyrolysis was directly proportional to the pK, values of the corresponding barboxylic acid. Smith and Wetze12 demonstrated qualitatively a direct relationship between the pyrolyses of m- and p-substituted cyclohexyl benzoates with the strength of the corresponding acid. A copyrolysis technique has suggested that substituents which decrease the electron density a t the acyl carbon of the ester will increase the rate of elimination and in acid dis~ociation.~ 0 1980 American Chemical Society
