J. Phys. Chem. 1981, 85,3817-3820
resonances for the D + FD mass combination. The minimum-energy-pathvibrationally adiabatic model provides a reasonable explanation of all six resonances. The lowest-energy resonance for each case is particularly interesting because it occurs far below the nonresonant threshold. As a consequence it strongly dominates the very low-temperature rate constants. Microcanonical variational theory with the minimumenergy-path vibrationally adiabatic ground-state model for including quantal effects on reaction-coordinate motion reproduces the accurate quantal rates for H + FH within 7% over the 200-2400 K temperature range. The unified statistical model, with the same transmission coefficient, gives the same 7% level of accuracy up to the highest
3817
temperature studied, 7000 K. Canonical variational theory gives similar results to microcanonical variational theory for this case, but conventional transition-state theory with unit transmission coefficient has a 132% error a t 200 K and a 36% error at 7000 K. For both the resonance energy and the transmission coefficient, the quantum-mechanical treatment of reaction-coordinate motion is significantly more accurate than the semiclassical treatment. Acknowledgment. This work was supported in part by the U S . Department of Energy through contract no. DEAC02-79ER10425 to the University of Minnesota and through the Los Alamos Scientific Laboratory.
Infrared Multiphoton Dissociation of the BC13*CH3SHComplex Yo-ichi Ishikawa,
Osamu Kurlhara, Shigeyoshi Arai, and Ryohel Nakane
The Institute of Physical and Chemical Research, Hirosawa, Wako-shl, Saitama 35 1, Japan (Received: March 20, 198 1;
In Final Form: August 17, 198 1)
A mixture of BC13 and CH3SH in the gas phase was found to have an infrared absorption with three peaks at around 1000 cm-l. The absorption was tentatively assigned to the BC13CH3SH complex. The infrared multiphoton dissociation of the complex has been examined by using a COz TEA laser. The threshold laser fluence required for the dissociation was about 0.5 J cm-2,being smaller than that for BCl, itself by an order of 2. The dissociation rate of %Cl3.CH3SH was 10 times larger than that of 11BC13CH3SHfor the laser irradiation at 944 cm-’. The dependence of the specific dissociation rate on the absorbed energy suggests that two reaction paths with different activation energies exist; the paths are the C1 atomic and HC1 molecular eliminationfrom vibrationally excited complex molecules.
Introduction There have been fairly many studies on the boron isotope separation by means of infrared multiphoton dissociation of boron trichloride BC13. The v3 vibration modes of 1°BC13and llBC1,(B-C1 antisymmetric stretching) have intense absorption bands at 995 and 956 cm-l, respectively, showing an isotope shift as large as 39 cm-l. Since both bands overlap the tunable region of COz laser radiation, BCl, has been used first in infrared laser isotope separati0n.l Although the irradiation of pure BCl, with C02 laser pulses yields the visible fluorescence due to excited fragment radicals,2 the final concentration does not change from the initial value. When so-called radical scavengers are added to BCl,, however, an appreciable decrease in concentration can be observed after irradiation. Therefore, the reproduction of the parent molecule should take place through the recombination of fragments in pure BC13 The effects of scavengers on threshold laser fluence, specific dissociation rate, and selectivity have been examined for BCl, containing HBr, 02,C2D4, NO, H2, and H2S.,14 In the case of the dissociation of BC13requires a laser fluence of about 100 J cm-2, which is comparable to the 02,516
(1) R. V. Ambartzumian, V. S. Letokhov, E. A. Ryabov, and N. V. Chekalin, JETP Lett. (Engl. Transl.), 20, 273 (1974). (2) R. V. Ambartzumian, N. V. Chekalin, V. S. Letokhov, and E. A. Ryabov, Chem. Phys. Lett., 36, 301 (1975). (3) R. V. Ambartzumian, Yu. A. Gorokhov, V. S. Letokhov, G. N. Makarov, E. A. Ryabov, and N. V. Chekalin, Sou. J. Quantum Electron. (Engl. Transl.), 5, 1196 (1976). (4) K. Takeuchi, 0. Kurihara, and R. Nakane, Chem. Phys., 54, 383 (1981).
0022-3654/81/2085-3817$01.25/0
threshold of the visible fluorescence in pure BC13 Hence, oxygen probably reacts with fragments after the dissociation of vibrationally excited BC13 On the other hand, the dissociation occurs at smaller fluences in the case of H2S. Furthermore, the relation between the specific dissociation rate and the absorbed energy gives an apparent activation energy of 28 f 10 kcal mol-l,4 which is considerably lower than the B-Cl bond energy of 106.1 kcal mol-’.’ Hydrogen sulfide may react with BC13 molecules at relatively low vibration levels. This paper describes infrared spectra, the threshold laser fluence, the relation between specific dissociation rate and absorbed energy, and the isotope effect in the infrared multiphoton dissociation of ‘OBCl,, l’BCl,, and natural BCl,, when methylmercaptan (CH,SH) is used as a scavenger. Being different from the other scavengers studied previously, this compound produces a complex in the presence of BCl,, which dissociates efficiently in the C02 laser field. Experimental Section The infrared light source was a Lumonics Model 103-2 C02TEA laser, which generated pulses with 100-ns fwhm and 1.0 J cm-2 fluence at 944 cm-’ when mixed gases of ( 6 ) V. N. Bourimov, V. S. Letokhov, and E. A. Ryabov, J. Photochem., 5, 49 (1976). (6) Y. Ishikawa, 0. Kurihara, R. Nakane, and S. Arai, Chem. Phys., 52, 143 (1980). (7) D. R. Stull and H. Prophet, “JANAF Thermochemical Tables”, 2nd ed., Natl. Stand. R e f . Data Ser. ( U S . Natl. Bur. Stand.), No. 37 (1971).
@ 198 1 American Chemical Society
3818
The Journal of Physical Chemistry, Vol. 85, No. 25. 7987
vlcrn-'
Flgure 1. Infrared absorption spectra of 'OBCI, (a: 1oBC13,1.00 torr) and 'QBC13.CH3SH(b: 'OBCI3, 1.00 torr; CH,SH, 10.28 torr; mixture) at room temperature.
1200
1000
Y /ern-'
Flgure 2. Infrared absorption spectra of "BCI, (a: "BCl,, 1.03 torr) and "BCI,CH,SH (b: "BCI3, 1.03 torr; CH,SH, 10.18 tori.; mixture) at room temperature.
He and C 0 2were used as lasing medium. The pulse profile did not show any appreciable tail as nitrogen was not added to the medium. The laser was tuned to the P(20) line of the COz 10.6-pm band at 944 cm-l, unless mentioned otherwise, and operated at a repetition rate of 0.7 Hz. The beam was focused mildly by a BaFzlens with a focal length of 1280 mm after passing a Teflon iris (4 = 11mm). When we wished to reduce a fluence, we used either metal meshes or KBr windows. The laser energy was measured with a Scientech 364 disk calorimeter. The concentratious of the BCl,CH,SH complex before and after irradiation were determined from infrared absorption intensities by using a Jasco Model A-102 infrared spectrometer. The reaction cell was a Pyrex cylindrical tube with two KBr windows in the middle for infrared optical measurement and two KBr windows at both ends for laser irradiation. The IR monitoring was done transversely to the COz laser irradiation axis. Its total volume was about 60 cm3. The optical path lengths were 5 cm in the direction of infrared measurement and 10 cm in the direction of laser irradiation. Natural BC13 (299.9%) purchased from Matheson Co. and CH3SH (-98.5%) from Nakarai Chemical Co. were distilled several times in vacuo before use. 1°BC13(298%) and 11BC13(297%) were produced from 1°BF3and 11BF3 through contact with aluminum chloride. The 1°BF3was kindly supplied from Mitsubishi Kinzoku Co. and the 11BF3was obtained from Union Carbide Co.
Results Infrared Spectra of BCZ3.CH3SH. The addition of CH,SH to 1°BC13or 11BC13gives rise to a new infrared absorption with three peaks at around 1000 cm-l. Figures 1and 2 present the spectra obtained by adding 10 torr of CH3SH to 1torr of I0BCl3and 1 torr of 11BC13,together with those for 1°BC13 and 11BC13. Further addition of CH3SH caused no essential change in spectral shape within the region of 650-4000 cm-l. The spectrum of 40 torr of CH,SH measured by using the same cell showed weak absorption bands in the 900-1 150- and 1250-1600-~m-~ regions and a relatively sharp band at 2600 cm-l, being
Ishikawa et al.
Figure 3. Infrared absorption spectra of the white, powdery compound obtained by mixing natural BC13and CH,SH at 77 K. Measurement was done with KBr method at room temperature.
consistent with the previous measurement by Pierson et a1.* For the mixtures we observed only a trace of absorption due to CH3SH in the region other than that given in Figure 1or 2. Three peaks are located within an error limit of f 2 cm-l at 957,989, and 1010 cm-l for a mixture of 1°BC13and CH3SH and at 923,954, and 988 cm-l for a mixture of 11BC13 and CH3SH in the highly resolved spectra recorded on a Perkin-Elmer Model 180 spectrometer. Gouben and Wittmeier have reported that the white, powdery compound assigned to BC12SCH3was produced by mixing BCl, and CH,SH in the liquid phase? We were also able to obtain a similar white, powdery compound upon warming gently BC13 and CH3SH condensed at 77 K. The spectrum of Figure 3 observed for the compound is, however, quite different from those of Figures 1 and 2. (The powdery mixture of the compound and KBr was transformed into a relatively transparent plate by compression. The infrared optical measurement was carried out with the plate.) Martin has found the BC13-H2Scomplex with a freezing point of -35.3 f 0.4 OC in a cryogenetic study of BC13 and H2S mixtures.1° Thereafter, Britton and Martin obtained the white solid assignable to BC13. CH3SH in the reaction of BCl, with CH3SH at 0 and 28.6 OC.ll However, its unstability at room temperature prevented definite identification and further elucidation of physical and chemical characteristics. Judging from absorption frequencies and the boron isotope shift, we attribute the observed absorption with the three peaks to the weakly binding complex produced from BC13 and CHBSH. The equilibrium constant of the reaction, BCl, + CH3SH F! BC13CH3SH,was estimated from quantitative changes in the IR absorption spectrum when CH3SH was added to 3 torr of BCl,. The constant was about 1at room temperature. The absorption due to loBCl3or 11BC13decreased rapidly by the addition of CH3SH. The peaks at 957 and 1010 cm-' for loBCl3.CH3SHor the peak at 923 cm-' for 11BC13CH3SHgrew as CH3SH was increased. The peak at 954 cm-l for l1BCl3.CH3SHwas also found to grow with increasing CH,SH, if the overlapping absorption due to 11BC13was taken into consideration. On the other hand, the peak at 989 cm-' for l0BCl3.CH3SHor the peak at 988 cm-l for 11BC13CH3SHwas always very small, although the absorption due to 1°BC13overlapped the peak in the case of l0BC1,.CH3SH complex at low pressures of CH3SH. These weak absorptions could be still observed as a shoulder, when 120 torr of CH3SH was added to 3 torr of 1°BC13or 11BC13. Similar infrared spectral changes, holding the large (8) R. H. Pierson, A. N. Fletcher, and St. C. Gantz, Anal. Chem., 28, 1218 (1956). (9) V. J. Goubeau and H. W. Wittmeier, Z . Anorg. Allg. Chern., 270, 16 (1952). (IO) D.R. Martin, J. Am. Chem. SOC.,67, 1088 (1945). (11) G. A. Olah, "Friedel-Crafts and Related Reactions", Interscience, New York, 1963, p 490.
The Journal of Physical Chemistty, Vol. 85, No. 25, 1981 3819
Infrared Multiphoton Dissociation of BC13.CH,SH
TABLE I: Infrared Absorption Peak Frequencies for 'BC1, Thiols and t Thioethers
+
peak frequency/cm-' 'BCl, 'OBC1, (1)' "BClj (1) "BC1, (10) "BC1, (10) "BC1, (10) llBcl; ( i o j "BC1, (10) "BC1, (10) "BCI; ( l o ) "BCl, (10) "BC1, (10) "BCI, (10)
sulfur compd
,,I
CH,SH
1010 954 1002 971
C,H,SH ( 1 0 5 ) (CH,I,CHSH (68) . _ I _
,
I
CH,CH,CH,SH ( 6 7 ) (CH,),S ( 1 0 5 ) (C,H,),S (18)
a
1002 967 995 956 780 748 790 748
,,I!
b
989 988 990 985 990
1000 1000 995 1010 1010 d
,,I!!
a
957 923 963 926 960 923 950 915 676 657 653 626
0'
(12)E. Gore and S. S. Danyluk, J . Phys. Chem., 69, 89 (1965). (13) K. J. Wynne and J. W. George, J. Am. Chem. SOC.,87, 4750 (1965).
'3
1.0
1.5
'
laser fluencelJ cm-*
0
Flgure 4. Relations between average number of absorbed photons per molecule in the irradiation zone (n) and specific decomposition rate of "BC13.CH3SH dagainst laser fluence Cp .
v--?
d a An estimated error is i 10 cm". The values of V" are not determined clearly because of the weak intensity. Values in parentheses are pressures (torr) of compounds. Not determinable because of t h e intense overlapping absorption of (C,H,),S around 9 9 0 cm-'.
boron isotope shifts, have been observed for the mixtures of "C13 and several thiols (C2H5SH, (CH3)2SH,and C3H7SH)or thioethers (CH3SCH3and C2H5SC2H6), where i denotes either 10 or 11. The frequencies corresponding to the three peaks of the iBC13-CH3SHcomplex are summarized in Table I. Intensities of the bands classified as v' and Y"' in the table increased with increasing pressures of sulfur compounds, while intensities of v" bands were considerably weaker than those of 'v and '"v bands even at high pressures. The 'v and v"' bands of 'BC13.thioether complexes showed large shifts from antisymmetrical iB-C1 stretching modes of iBC1,. This fact suggests the strong interaction of thioethers with "C13 in the complexes. On the other hand, small differences in frequency between 'BC13,and iBC13.thiolssuggest the weak interaction of thiols with "BCl,. The complex formation has been reported for BC13.0(CH3)2,12BC1343e(CH3)2,13 BC13-S(CH3)2,11 and BC1,aother thioether.ll Multiphoton Dissociation of BCl3*CH3SHComplex. The specific dissociation rate d of infrared multiphoton dissociation corresponds to -In ([XI,/ [ X ] , ) / nprovided the conversion is kept less than about 30%, where [ X I , and [XI, are the concentrations of X before and after irradiation of n laser pulses. The error limit of the specific decomposition rate depends on the number of laser pulses and the concentration of a reactant. It is estimated to be f0.4 X pulse-' in the typical experiment where the number of pulses is 1000 and the pressures of 'BC13 and CH3SH are 2 and 10 torr, respectively. Table I1 tabulates d for 'OBC13-CH3SHand 11BC13.CH3SH,both complexes being produced by mixing 10 torr of CH3SH and 2 torr of 1°BC13and "BC13. The frequencies of the laser lines used here were as follows: P(36), 929 cm-'; P(20), 944 cm-l; and R(12), 970 cm-'. The power ratio among these three lines was P(36):P(20):R(12)= 85:100:85. Because of a relatively short cell length, the laser beam may be regarded to be parallel inside the cell. The fluence of the P(20) line was, in fact, approximately 2.5 J cm-2 in the center of the cell and 2.4 J cm-2 a t the entrance or exit window. The neat SWD of the table is the ratio between d for l0BCl3-CH3SH and d for 'lBC1&H3SH a t each laser line. The value corresponds to the selectivity, when the multiphoton dissociation of 10BC13CH3SH proceeds independently of that of "BC13.CH3SH in their mixture. The maximum
L',35
\
F
i 0
U
L
-
04 E&/mol
.02
L
-
.06 08 kcal-'
Figure 5. Relation between specific decomposition rate d and absorbed ener y ,Eabs obtained in 10BC13(2 torr)-CH,SH (10 torr) system (0)and in BCI3 (2 torr)-H,S (10 torr) system (A)by COP laser lrradiation at 944 cm-'.
'9
value of d for 'OBC13.CH3SHwas obtained with the P(20) line, which is located at the red side of the absorption due to the complex. This fact is consistent with the tendency that the multiphoton absorption spectrum is more or less red shifted from the one photon spectrum. Taking the absorption into consideration, one expects the dissociation of 11BC13.CH3SHto occur more efficiently a t frequencies lower than 923 cm-l. At the same frequency and fluence (944 cm-l and 2.5 J cm-2)as Table 11, the dissociation rates were also measured for 2 torr of "C13 10 torr of O2 and 2 torr of "BCl, + 10 torr of H2Smixtures. The P(20) line (944 cm-l) is resonant with the v3 mode of l'BC13. In contrast to the large rates obtained with iBC13 CH3SH mixtures, these mixtures showed no appreciable dissociation under the irradiation conditions. The dissociation rate of 1.5 X pulse-' for the l0BCl3-CH3SHcomplex was about 80-100 times larger than that for the I1BCl3 H2S or 11BC13+ O2mixtures. Parallel to the results, the absorbed energy in the iBC13 + H2S or "'C13 + O2mixture was found to be at least less than 20 kcal mol-' in comparison with the energy of 65 kcal mol-' in the 1°BC13CH3SHcomplex. Infrared absorption spectra of irradiated samples showed the formation of hydrogen chloride as a product. However, other products could not be observed in the gas phase. Although the deposition of colorless powder was seen on the glass wall of the reaction cell after a number of irradiation pulses, we were unable to identify the compound positively owing to its small amount. Relation between Specific Dissociation Rate and Absorbed Energy. The dependences of absorbed energy E,,, and specific dissociation rate d on laser fluence 4 were examined for the mixture of 2 torr of 1°BC13 10 torr of CH3SH. The laser line used was the P(20) line of the C02 10.6-pm band at 944 cm-l. Then, the average number of absorbed photons per molecule in the irradiation zone ( n ) was determined from Eah Figure 4 shows the plots of ( n ) and d against 4. The absorbed photon increases gradually with Cp, while d increases sharply above 0.6 J cm-l. The pseudo-Arrhenius plot of d against Eab;l is shown
+
+
+
+
3820
The Journal of Physical Chemistry, Vol. 85, No. 25, 198 1
Ishikawa et al.
TABLE 11: Specific Dissociation Ratesa for 'OBCl,.CH,SH and "BCl,.CH,SH by CO, Laser Irradiation a t R(12), P(20), and P(36) of 10.6-pm Band and SMPD CO, laser line
R ( 1 2 ) ( 9 7 0 cm-')
P(20) ( 9 4 4 cm-')
1.8x 10-4 0.5 X 3.6
d for 'uBC1,CH,SH d for "BCl,.CH,SH SM PD
P(36) (929 cm-') 1.7 x 0.4 X 4.6
15.4 X 1.4 X 11.2
a Unit: pulse-'.
TABLE 111: Isotope Selectivity in 'OBC1,*CH,SII.and ''BCl,.CH,SH Mixtures run
"BCl,/ torr
"BCl,/ torr
1 2 3 4' 5d
2.00 2.13 3.21 3.10 3.00
0.99 1.01 1.01 1.01 1.00
CH,SH/ torr
10.0 15.6 20.6 20.5 20.0
103dAa 103dRb
2.02 1.48 1.07 5.96
1.55 1.12 0.89 5.75
s 1.31 1.32 1.20 1.04 0.92
a d for 'oBCI,CH,SH in pulse-'. d for "BCl,.CH,SH in pulse-'. Breakdown. Thermal reaction.
in Figure 5. The plot is clearly biphasic, and the slope values are approximately 30 and 260 kcal mol-l, respectively. The solid line of Figure 5 can be expressed by the following equation: d = 0.1 eXp(-260/Eab,) + 2.5 X low4 exp(-30/Eab,). The previous results reported for BC13 H2S are also shown in the same f i g ~ r e .The ~ data fit the line with the gradual slope. Isotope Effects. The dissociation rates were measured for iBC13+ CH3SH mixtures containing both loB and I1B. The results are summarized in Table 111. The concentrations of the 'OBC13CH3SHand l1BCl3.CH3SHcomplexes were determined from infrared peak intensities at 1010 and 923 cm-l, respectively. The selectivity s is defined as the ratio between the dissociation rates of the two complexes. The table includes the selectivities obtained with laserinduced breakdown and thermal dissociation, where both processes should be nonselective. These results indicate that there is a meaningful difference between the dissociation rates of 10BC13CH3SHand l1BCl3-CH3SHcomplexes.
+
Discussion The multiphoton dissociation of the BC13CH3SHcomplex occurs much more efficiently as compared with the dissociation of the BC13 + O2 and BC13 H2S mixtures. The absorbed energy in the 1°BC13.CH3SHcomplex is 3 times larger than that in the BC13 + O2 or BC13 + H2S mixture. Large polyatomic molecules have high energy level densities due to increasing vibrational modes and their mutual combinations. Such molecules find easily allowed transitions which are in resonance with the laser radiation in the multiphoton absorption process. The energy absorption process is reflected on the high dissociation rate obtained with low threshold fluences.14J5 An extremely efficient COz laser-induced reaction has been reported for a uranyl complex which consists of 44 atoms. Black et a1.16 and Grunwald et al.17have proposed that the relation between specific dissociation rate d and ab-
+
~~
~
(14) D. M. Coxand and E. T. Mass, Jr., Chem. Phys. Lett., 71, 330 (1980). (16)A. Kaldor, R. B. Hall, D. M. Cox, J. A. Horseley, P. Rabinowitz, and G. M. Kramer, J. Am. Chem. SOC.,101, 4465 (1979). (16) J. G. Black, E. Yablonovitch, N. Bloembergen, and S. Mukamel, Phys. Rev. Lett., 38, 1131 (1977). (17) E. Grunwald, D. F. Dever, and P. M. Keehn, "Megawatt Infrared Laser Chemistry", Wiley, New York, 1978, p 54.
sorbed energy Eebscan be expressed as follows: = A exp[-sEact/Eabsl (1) where A is a constant and EeCtis an apparent activation energy. Although s means the degree of freedom of molecular vibration, it is generally considered to be an arbitrary parameter depending on a relevant molecule. The relation obtained with the I0BCl3-CH3SHcomplex seems to fit well eq I, if two different values are assumed as sE,& Since the BC13CH3SHcomplex decomposes easily at fluences as low as 0.2 J cmw2,we were able to examine the decomposition rate by using almost parallel laser beam. The dependence of the rate on Eabs in Eab < 40 kcal mol-I was largely different from that in ,Tabs> 40 kcal mol-l, suggesting the occurrence of different reactions in the regions. This result may be interpreted in terms of two paths; one has a lower activation energy and the other a higher one. We propose the following mechanism: BCl,
+ CH3SH
(BCl3-CH3SH)+ nhv
+
(BC13.CH3SH) mhv i BC I3.C H 3SH )* ( B C I3.CH 3 S H
i""
F
--
(BC13CH3SH)
( 1)
(BC13.CH3SH)*
(2)
(BCl3.CH3SH)**
(3)
BCl3
+
+
HCI
(BCIz.CH3SH)
+
BCIzSCH3
(4)
CH3SH
(5)
(6)
CI
Where m > n. Reaction 5 can occur through lower vibrational energy states of the BC13.CH3SH complex but reaction 6 through only higher ones. The decomposition of the complex in reaction 4 cannot be observed apparently because of the reproduction of the complex in reaction 1. The above mechanism can explain HCl production, higher threshold energy for the C1 atomic elimination, and the relation shown in Figure 5. The relation between d and Eabs for the BC13 H2S mixtures in the previous stud? coincides with the similar relation for 'OBC13CH3SH. This fact suggests that the vibrationally excited BC13 results in the formation of the transient complex BCl3-H2Sfollowed by the decomposition similar to reactions 4-6. Although the formation of the l0BCl3.CH3SHcomplex reduces markedly the threshold energy of decomposition and the molecular HC1 elimination is expected to take place in its decomposition at low Eat,,, the high isotope selectivity could not be obtained in the mixture of 10BC13CH3SHand "BC13CH3SH. The selectivity was comparable to those for other additives such as O2 and H2S. The reason cannot be obviously understood at this moment. The laser-heated reaction may contribute to the nonselective decomposition of these complexes due to the low activation energy of decomposition.
+
Acknowledgment. We thank Dr. H. Honma for his measurement of the infrared absorption spectra on a Perkin-Elmer spectrometer.
