874
0.HERSTAD, G. A. PRESSLEY, Jr. AND F.E, STAFFORD
resulting intrusion of large volumes of products at this location might be expected to cause considerable disruption of the film compared with growth from the film-gas interface which would occur if a Wagner-
type mechanism prevailed. Further work which should establish whether in the case of halogen molecules, the growth occurs a t the halogen-silver halide or the silver halide-silver interface, is planned.
Mass Spectrometric Investigation of the Fragmentation Pattern and the Pyrolysis of Borane Carbonyl’
by 0. Herstad,2G. A. Pressley, Jr., and F. E. Stafford Department of Chemistry and the Materials Research Center, Northwestern University, Evanston, Illinois (Received J u l y 14, 1969)
60801
Borane carbonyl, BH3C0,and its pyrolysis have been investigated using a mass spectrometer as the detector of a molecular beam issuing from a flow reactor. The high resolution of the instrument and a movable beam defining slit made it possible to distinguish between residual background, background caused by the effluent from the reactor, and the molecular beam molecules (Le., pyrolysis effects in the ion source were eliminated) and also to discern reactive intermediates. The first mass spectrum of BHsCO free of pyrolysis products is reported as well as a clear-cut identification of borane, BHI, formed in the pyrolysis. Pressures of BH3 about six times greater than that of the other product, B~HB, were obtained. This confirms the identification and the mass saectrum of BH9formed from B2Hs. The ion BHO+ was observed at high temperature and may be due to BH~OH.
Introduction Borane, BHa, has long been postulated as a reactive intermediate in formation and dissociation processes for polyboranes and other borane compounds. 3~ However, the only direct identifications of BH, have been by mass s p e c t r ~ m e t r p . ~ - ~Of these, only one paper reports a mass spectrum of BH3, in that case produced from B2H8.9 To our knowledge, at least three laboratories have attempted to use various optical spectrometric techniques including matrix isolation to identify BHs produced by pyrolysis or flash techniques from a variety of compounds; none has yet been successful enough to publish. In addition, mass spectrometry involves ionization, fragmentation, and possibly excitation. In particular, the process B2Hs*(excited) + BHs+ could account for the results of Baylis, et aLg Therefore, means were sought to produce BH3 in high yield and to replicate the mass spectrum of BH3 produced from a source other than B2Ha. Borane carbonyl has been extensively investigatedst10-14and seemed to be a propitious starting material because of its reported pyrolysis rnechani~m’~-’~
BH3C0 K’,BH3 The Journal o j Physical Chemistry
+ CO
--f
(1)
and because of a previous mass spectrometric studys of its pyrolysis from which was reported formation of
BH3. Neither a mass spectrum of BH3CO free of pyrolysis (1) Supported by the United States Atomic Energy Commission, Document No. COO-1147-31. The acquisition and maintenance of the mass spectrometer facility was made possible by grants from the Materials Research Center, the AEC, and the University. Presented a t 3rd Great Lakes Regional Meeting of the American Chemical Society, Northern Illinois University, DeKalb, Ill., June 5, 1969. (2) Institute of Inorganic Chemistry, Technical University of Norway, Trondheim, Norway. (3) R. M. Adams, “Boron, Metallo-Boron Compounds and Boranes,” Interscience Publishers, New York, N. Y., 1964. (4) R, T. Holzmann, “Production of the Boranes and Related Research,” Academic Press, New York, N. Y . ,1967. (5) W. N. Lipscomb, “Boron Hydrides,” W. A. Benjamin, Ino., New York, N. Y., 1963. (6) A. E. Stock, “Hydrides of Boron and Silicon,” Cornel1 University Press, Ithaca, N. Y., 1933. (7) T. P. Fehlner and W. S. Koski, J . Amer. Chem. Soc., 86, 2733 (1964). (8) T. P. Fehlner and W. S. Koski, ibid., 87,409 (1965). (9) A. B. Baylis, G. A. Pressley, Jr., and F. E. Stafford, ibid., 88, 2428 (1966). (lo) A. B. Burg, ibid., 74,3482 (1952). (11) S. H. Bauer, (bid.,78,5775 (1956). (12) Y . C. Fu and G. R. Hill, ibid., 84,353 (1962). (13) M. E. Garabedian and 8. W. Benson, ibid., 86, 176 (1964). (14) G . W. Bethke and 1LI. K. Wilson, J . Chem. Phys., 26, 1118 (1957).
FRAGMENTATION PATTERN AND THE PYROLYSIB OF BORANE CARBONYL or other impurities (except for a preliminary result from this laboratory15) nor that of BH3 produced from it has been reported. Subsequent to the start of our work Fehlner and M a p p e P reported a reinvestigation of the BHBCO pyrolysis a t higher pressures and longer contact times than those to be reported on below. Diborane was observed in large quantities, but BHa was barely discernible. In this paper is reported a reinvestigation of the pyrolysis of BHaCO in a flow reactor. The dissociation of BHaCO and the reaction products, including intermediates, have been studied as a function of temperature by means of molecular beam mass spectrometry. In add.ition to the study of the dissociation process, the molecular beam fragmentation pattern (Le., that free of effects of pyrolysis) of BHaCO has been determined and is reported for the first time.
Experimental Section
875
All ion current readings at “centered” and “displaced” shutter settings were time averaged. These averages and the instrument settings were read directly into a DEC (Digital Equipment Corporation, Maynard, Mass.) PDP-S/S digital computer which was programmed to calculate total ion current (amperes), net molecular beam ion current, shutter percentage,21 “grid” currents, and gain. These data were immediately printed out on a Teletype, and were also stored in core or on a disk memory so they could later be put out on tape for further calculations in the University’s central computer facility. Data and Results Fragmentation Pattern of BHDCO. The fragmentation pattern of ‘OBHaCO was determined from the mass spectra obtained with the reactor at temperatures of 290-450°K, over which range it remained nearly constant and where no thermal decomposition could be observed . Shutterable mass peaks were observed in three mass regions: (1) m/e = 42, 41, 40, 39, and 38; (2) m/e = 24, 23, and 22; (3) m/e = 13, 12, 11, and 10. That part of the mass spectrum assigned to BHsCO, and in some cases stripped of contributions from trace amounts of B2H6,is given in Table I. The entire mass range m/e = 43 to 75 was searched. No peaks were observed; our detectability limit was 0.05% of the reference peak. Region (1) can then be assigned to the ions BHpCO+, BH:CO+, BHCO+, and BCO+. The major molecular beam mass peak in this group is a t m/e = 39 (mainly ‘OBHCO+); m/e = 42 is due to the ion species llBHal2C0+ and 10BH313CO+.The measured ratio I(42 +)/ 1(41+) was 0.047, compared to 0.049 calculated from the isotopic contributions of l1BZ3and (4% ‘lB and 1.1%lac).
Borane carbonyl was synthesized using the method of Burg and Schle~inger,’~ as modified by J. C. Carter,’* by treating B2H6 with CO at 20 atm. Diborane (96% ‘OB and 40/,”B) was prepared by the LiAIH4 reduction of loBF3.Et20. Borane carbonyl thus produced was purified by trap-to-trap distillation in an all-glass apparatus and stored at -196’ (liquid N2). During the experiments, the sample was kept in a Pyrex container at - 160.5’ (isopentane slush), at which temperature the pressure of BHaCO is 4 X T0rr.l’ At the entrance of the inlet system the pressure was reduced by a metering valve to 7 X loq4Torr, as determined from the ratio of ion intensities at the opening used and at the full opening of the valve. The rest of the inlet system, the furnace, the movable beam defining slit (called the “shutter”), and the ion source region were the same as described earlierv9 The only (15) Private communication. Earlier measurements on BHaCO done change was that the reactor in this case was made of at this laboratory by 8. M. Schildcrout. molybdenum (instead of stainless steel) and had a lid (16) Private communication. Preliminary report on the thermal with a knife-edged slit of 0.2 cm2, giving a c a l ~ u l a t e d ~ ~decomposition of borane carbonyl, BHxCO, by T. P. Fehlner and G. W. Mappes, Aug 1968; J . Phys. Chem., 73,873 (1969). contact time of 1.8msec. (17) A. B. Burg and H. I. Schlesinger, J . Amer. Chem. SOC.,59, 780 The molecular beam effusing from the reactor usually (1937). was ionized by 70-eV electrons except when appearance (18) J. C. Carter, Ph.D. Thesis, University of Michigan, Ann Arbor, Michigan, 1961. potential measurements were made. The ions formed (19) S. Dushman, “Scientific Foundations of Vacuum Technique,” were accelerated through 4000 V, mass analyzed, ac2nd ed, revised by J. M. Lafferty, John Wiley & Sons, Inc., New York, celerated through an additional 3200 V, and detected N. Y.,1962. (20) R. J. Loyd and F. E. Stafford in “Mass Spectrometry in Inorby a 20-stage Cu-Be secondary electron multiplier, ganic Chemistry,” Advances in Chemistry Series, No. 72, American Appearance potentials (AP) were measured by the Chemical Society, Washington, D. C., 1968,pp 127-136, vanishing current method, using ionization efficiency (21) The movable beam defining slit (“shutter”), located between the curves made automatically with an X-Y recorder.20 crucible orifice and the ion source, permitted differentiation of species originating from the crucible, the crucible lip, the radiation shields, Argon, leaked directly into the mass spectrometer, was and the residual background gases. The ratio of molecular beam to total ion intensity a t a given mass peak is called the “percentage shut,used as the calibrant. ter effect.” Highly reactive species, which are expected to be deIdentifications of ions and their neutral progenitors stroyed by every collision with the vacuum chamber walls, have high shutter effects (-loo%), while stable species have small ones (e.g., were accomplished using mass number, mass defect, 5-10% for Ar at these temperatures). isotopic ratios, shutter effect,21*22 and intensity variation (22) F. E. Stafford, G. A. Pressley, Jr., and A. B. Baylis, ref 20, as a function of temperature. PP 137-152. Volume 74, Number .G February 19,1970
0 , HERSTAD, G. A. PRESSLEY, Jr., AND F. E. STAFFORD
876
Table I : Observed and Stripped Molecular Beam Mass Spectra of BH&O w e
42 41 40 39 38 24 23 22 13 12 11 10
96% log, re1 intensity
Ion
2.6 62 . O 83.0 100.0 0.86 0.15 1.2 0.6 1.1 28.1 14.5 6.4
10BHaCO+ BHiCO + BHCO+ BCO+ BHaC BHC + BC + BHI + BHa + BII+ B+
Re1 inta
+
59.
79
I
100.
0.86 0.10 1.2 0.6 -Ob
28. 14.3 6.4
a Calculated from column 2 assuming 47, llB. See ref 23. 'See text.
Mass peak 23 was the most intense peak in mass region (2). The intensity of mass peak 25 was not measurable except during the first experiment after changing to a new sample of IOBHaCO. Since mass peak 25 is the major peak in the molecular beam mass spectrum of 10BzHe,gthis behavior indicates that a small amount of diborane was present initially as an impurity. Consequently mass peaks 24, 23, and 22 were assigned to the ionic species l0BHzC+ (isoelectronic with CN+ and C2), IoBHC+, and loBC+, respectively, These assignments agree with those of Fehlner and Koski,a who were unable to report relative intensities. In mass region (3), the ratios 1(10+)/1(39+), 1(11+)/1(39+)) 1(12+)/1(39+), and 1(13+)/1(39+) were nearly constant below 500"K, indicating that these mass peaks are due t o the fragments B+, BH+, and BM2+ from BHaCO. The BH3+ion seems not to be formed by fragmentation of BH&O since the measured ratio 1(13+)/1(12+) = 0.037 is smaller than that calculated from the nominal isotopic contribution of 4% llB, specified by the sup~ l i e r . The ~ ~ monoisotopic molecular beam mass spectrum calculated, assuming 4.0% llB, of 'OBHaCO is given in Table I, column 4. Thermal Dissociation of BH3C0. The thermal dissociation of BH3C0 started a t about 500"K, as seen from the drop in the intensity of m/e = 39 ('"BHCO+) in Figure 1. Above this temperature the intensities of the mass peaks 38,39,40,41, and 42 all dropped steadily with increasing temperature. However, a slight change (Figure 2) in the ratio 1(41+)/1(39+) was observed just below the temperature where the pyrolysis started, dropping from 0.62 (the value obtained in the low temperature range) to about 0.55 in the pyrolysis range. Similar changes were not observed for the other intensity ratios as seen from Figure 2. Above 500"K, where the intensities of the fragments from BH&O were decreasing, the intensities of the mass peaks 22, 23, 24, 25, and 26 increased, reached a maxiThe JOUTTW~ of Physical Chemistry
T
O K
Figure 1. Variation in ion current X d T / T o with reactor temperature. T is the variable temperature and To is a reference temperature (404'K). The factor is introduced to correct for changing molecular beam velocity with temperature.
Z/G
mum at about 700°K, and fell off rapidly at higher temperatures as shown in Figure 3. The measured ratios 1(24+)/1(25+) = 0.48 and 1(22+)/1(25+) = 0.56 are in fair agreement with the respective values 0.44 and 0.52 obtained for diborane at this l a b o r a t ~ r y . ~ The observed ratio 1(26+)/1(25+) S 0.2 over the range 500-740°K. This is about three times that reported for d i b ~ r a n e indicating ,~ possibly that a small amount of another neutral species contributed to mass peak 26. However, the low absolute intensity of mass peak 26 makes measurement of its relative intensity uncertain. Thus, the molecular beam ion intensities at mass peaks 22, 23, 24, 25, and probably 26 are assigned to ion fragments from diborane in the pyrolysis region above 500'K. Mass peak 27 also was shutterable above 500°K, and increased rapidly with temperature without reaching a maximum a t 700°K as shown in Figure 3 (see also the figure caption). The ratio 1(27+)/1(26+) increased from about 2 at 500°K to 23 at 739°K a t which temperature it has a 30% shutter effect. This shows that the neutral species giving rise to the ion current at m/e (23) Analysis given by the supplier, United States Atomic Energy Commission, Oak Ridge, Tenn., claims a minimum of 96% ]OB, Therefore, the concentration of *lB may be less than 4% in some samples. (24) "Chart of the Nuclides," Knolls Atomic Power Laboratory, Naval Reactors, U.S. Atomic Energy Commission 9th ed, Revised to July 1966.
FRAGMENTATION PATTERN AND THE PYROLYSIS OF BORANE CARBONYL
877
r
1
90
+
I++
-
+ ++ + +&-y+ ++ +-
+-+-+
80
I, I- 27+ I
f
#
-1
+
I
I I I
70!-
I
I
+++
‘tt I
R
I (38+$ (39+
+
+ ++++#++++
400
f
600 T
O K
Figure 2. Relative ion intensities a t m/e 38, 40, and 41 as percentages of mle 39 vs. temperature.
27 makes little or no contribution to the current at mass peak 26. I n experiments in which ‘OBH3CO and “B2He where admitted simultaneously (see below), this peak was observed to grow in just down of mass llBzH~+. Accordingly, this peak a t m/e 27 formed from a pyrolysis product of 10BHSCOis assigned to the ion ‘OBHOf. The excess intensity observed at m/e = 26 (relative to 1(25+)) may be due to logo+, but this must be formed independently of l0BHO+. The molecular beam intensities at m/e 10,11,12, and 13 increased both absolutely and relatively. The ratio 1(13+)/1(12+) increased from 0.037 to 0.36. Since the only known borane that gives any appreciable BHa+ on fragmentation is BHs, this is strong evidence for the formation of monoborane. I n addition to the increasing intensities of the mass peaks in the monoborane region, their shutter percentagesZ1increased strongly with temperature as shown in Figure 4. This also points clearly to BHa neutral. The shutter percentage for mass peak 13 is expected to be 100% if no other ion species contribute to that peak. I n this case, however, 11BH2+ from BHsCO was not resplved from the l0BH~+. By using the known mass spectra of BzH8and BH&O to subtract out the contributions due to fragmentation of BHaCO and BzH6 we deduce the mass spectrum of BHa. The monoisotopic mass spectrum of ‘OBHZ is obtained by further subtracting out a 4% isotopic23 contribution from llB. These residual intensities of ‘OBH+, loBHz+, and loBH3+are shown in Figure 5 as a function of temperature. The average mass spectrum of BHa, obtained for the temperature range 640-740°K,
T
OK
Figure 3. Variation in the ion currents a t m/e 22, 24, 25, 26, and 27 as a function of temperature. Not shown on the graph is the ion current for 27+ equal to 4.5 X 10-11 at 739°K; % shutter effect = 30%, while for m/e 22-26 it is 6-775 a t this temperature.
500
T
700 O K
Figure 4. Shutter percentages for m/e 11, 12, and 13 8,s 8 function of temperature.
is given in Table 11. As shown in the table, this mass spectrum of I0BH3is in good agreement with the only other reported maBs spectrum. Volume 74,Number
4 February 18,1870
0.HERSTAD, G. A. PRESSLEY, Jr., AND I?. E. STAFFORD
878
f
L
cn
a
5 =2 X
I-
5
$2 3 0
4 I
500
T
700 OK
Figure 5. Ion currents of loBH+, l0BH2+,and l0BH3+ as a function of temperature. The contributions due to fragmentation of BH$O and BzHe at these masses have been subtracted out. Note that these ion currents are substantially higher than those in the diborane region as shown in Figure 3.
Table I1 : Comparison of Monoisotopic Molecular Beam Mass Spect.rurn of 1°BH3Formed from BH3C0 and from B2H6 Starting material
Ion
BHaCO
BzHs"
37 100 16 9
31 100 16 8
From the mass spectra of "B2H6 (96% l"B) and it can be calculated that at equal ratios of 10BH3/ 'OB2Hs and "BHa/"B2H6 for the respective diboranes, the intensity ratio 1(14+)/1(27+) (for "B2H6) should be 1.6 times that of 1(13+)/1(25+) (for loB2€I6, 96% lag). Mass peak 27 from "B2H6 was observed a t the high mass side of 'OBHO+ from the pyrolysis of loBH,CO, and the two ion peaks were completely resolved. "BzHa was admitted in such an amount that its shutter,. able intensity at m/e 27 was about the same as that for "B2H6 a t m/e 25. The measured ratio 1(14+)/1(27+) = 4.05 X for the pyrolysis of " B & 3 is to be compared with I('%&+)/1(25+) = 2.1, the latter obtained for the pyrolysis of 1°BH3C0just before "B2H6 was introduced. The small value for I(14+)/I(27+) shows that very little BH3 is being formed from B ~ H under B the experimental conditions (reactor, material, pressures, contact time) used. Therefore, the BH3 observed is formed directly from the pyrolysis of BH3CO. The agreement between the mass spectra for BH3 obtained in this work and that reportedg earlier confirms that the latter also was due to BH3 neutral. The appearance potential for BH3 + was measured using higher resolution than usual. With the exit slit 12 X in., the ions loBH3+and 11BH2+were resolved. The electron energy scale was calibrated by setting the measured AP(Ar+) = IP(Ar) = 15.75 eV.25 The corrected value for AP(BH3+)was found to be 14.0 2 eV. The large uncertainty is due to the low molecular beam intensity of BH3+. The obtained AP(BH3+) is 2-2.5 eV higher than earlier reported values, 11.4 0.2' and 12.32 0.1,26 and probably is due to the low beam intensity. Yield of BH3 and B2H6.The ratio between mono- and diborane is nearly constant in the pyrolysis region, as
*
*
*
13
shown in Figure 6 (top) where the ratio b = 26
I(i+)is plotted
us. temperature.
I(i+)/
i=10
If the secondary
i=22
'Reference 9. See this paper for 5 comparison with BXs (X = halogen) and other M"1X8 mass spectra.
It now must be shown that the BH3 spectrum reported above does indeed come from BH3 neutral produced from BH3C0, and not from the B2H6 produced as a side product in the reaction. I n order t o resolve this important problem, "&He, containing the normal isotopic composition of boron24 (20% loB and 80% llB), was leaked into the mass spectrometer together with 1OBH3CO (96% log)a t 769°K (the highest temperature used in the earlier experiments). Since the major peak in the mass spectrum of "B2H60is at m/e = 27 and "BH3 from it is observed at m/e = 14, it was possible to study the pyrolysis of "&He in presence of the pyrolysis products from l0BH3CO. The Journal of PhysicaZ Chemistry
electron multiplier gains are equal and the cross section27 for the ionization of diborane is about twice that of monoborane, the ratio p(BHs)/p(BzHe) is about 5.6. If anything, the secondary electron multiplier gain for B2H,+ ions is greater than that for BH,+ ions, changing this ratio in favor of BH3. The formation of BH3 and B2H6 relative to the loss of BH3C0 is shown in Figure 6 (bottom), where (25) C. E. Moore, "Atomic EnergV Levels," Vol. 1 and 2, National Bureau of Standards, Circular 467, U. S.Government Printing Office, Washington, D. C., 1949, 1952. (26) J. H. Wilson and H. A. McGee, Jr., J. Chem. Phys., 46, 1444 (1967). (27) S.-S. Lin and F. E. Stafford, ibid., 47, 4664 (1968); the arguments of this paper show that the arguments given by F. E. Stafford (ref 20, p 153) for the validity, and for the small deviations from the "additivity rule" for cross sections are roughly valid also for the Z(ri*) method of calculating cross sections.
FRAGMENTATION PATTERN AND THE PYROLYSIS OF BORANE CARBONYL
879
[ 5 I(i+>+ c I ( i + ) ] d E 26
6-22
a=
i- 10
I(39’)To
-
-
T
1(39+)45
is plotted us. temperature. T is the variable temperature, and TO = 404°K is a reference temperature. As seen from Figure 6, the loss of BHICO to products other than mono- and diborane rapidly increases up to about 600”K, and then becomes constant up to 740°K. By assuming 100% yield at 535”K, we find that a t the maximum intensity of BH3 and B z H ~at , about 7OO0K, the yield is only 20% or less. The maximum yield of boranes from the total amount of BHaCO introduced is obtained at about 740”K,also being 20% or less. This low yield shows that other products must be formed. One possibility was indicated in an earlier result from our laboratory,’5 when some evidence for the formation of higher boranes was observed using higher inlet pressure and contact time for the reactor, which in that case was made of stainless steel. I n the present work, attempts were made to replicate the earlier result using the same experimental conditions, except that the reactor was made of molybdenum. No or very little evidence for higher boranes was observed, even when a mixture of BzHG and BHaCO was introduced and reacted a t temperatures between 450 and 600°K. The earlier result15 may be due to a catalytic effect from stainless steel (
