Reaction of halogens with laser-ablated boron: infrared spectra of BXn

F2, Cl2, Br2, and 12, trapped in solid argon at 12 f 1 K, and characterized by infrared absorption spectroscopy. The relative band intensities observe...
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J. Phys. Chem. 1993, 97, 4910-4915

4910

Reaction of Halogens with Laser-Ablated Boron. Infrared Spectra of BX, (X = F, C1, Br, I; n = 1, 2, 3) in Solid Argon Parviz Hassanzadeb*J and Lester Andrews' Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 Received: November 2, 1992; In Final Form: February 4, 1993 BX, (X = F, C1, Br, I; n = 1,2, 3) species were produced by the reaction of laser-ablated atomic boron with F2, Cl2, Br2, and 12, trapped in solid argon at 12 f 1 K, and characterized by infrared absorption spectroscopy. The relative band intensities observed for BX, BX2, and BX3 species depended on the method of production. Enclosing the boron target inside a tube and depositing boron atoms with halogen molecules enhanced the primary reaction products BX and BX2, whereas passing the halogen through the tube and coaxially mixing with atomic boron promoted the final reaction product BX3. Very low laser power favored the BX and BX2 radicals over the BX3 molecules. These studies provide further chemical evidence for the intermediate BX2 radicals.

Introduction Infrared absorption spectra of the stable boron trihalides are well-known.'" However, spectroscopic information on the BXZ free radical species is limited. The dihalide BF2 produced from radiolysis of BF3 has been detected by ESR in solid xenon,' and BCll and BBrzproduced from radiolysis and vacuum ultraviolet photolysis of BC13and BBr3,respectively, have been observed by matrix infrared spectroscopy.8 The simple diatomic BX species have been studied extensively by electronic absorption/emission spectroscopy? In the present work boron halide species are produced directly from the reaction of laser-ablated elemental boron with halogen molecules, trapped in solid argon, and characterized by infrared absorptionspectroscopy. The following study was conducted to explore the boron/halogen reaction under several mixing conditions and to prepare boron dihalide radicals by a different synthetic method.

Experimental Section The vacuum system and chamber for matrix-isolation studies have been described previously.lO A closed-cycle refrigerator (CTI Cryogenics, Model 22) and an indicator/controller were used to cool and monitor the temperature of the CsI window. FTIR spectra were recorded on a Nicolet 5DXB instrument at 2-cm-I resolution with an accuracy of 0.5 cm-I and a Nicolet 60SXR instrument at 0.25-cm-I resolution with an accuracy of 0.1 cm-1. FTIR spectra were recorded before and after annealing and photolysiswith a 175-W (Philips) medium-pressure mercury arc lamp. The laser ablation arrangement is similar to those described previ~usly.l~-~~ The first harmonic (1064 nm) of a Q-switched Nd:YAG (Quanta-Ray DCR-11) laser was focused on the boron target to generate atomic boron. The boron target was epoxy glued to a 6-mm-0.d. glass rod and rotated at 1 rpm. Natural boron with IIB (80.2%) and 1°B(19.8%)isotopes (Aldrich Chem. Co.) and enriched boron-10 material with 93.8% IOB (Eagle-Pitcher Inc.) were used. Typically a laser power of about 40 mJ/pulse at the sample with pulse duration of about 5-10 ns gave sufficient boron atoms for observation of reaction products. Fluorine (Matheson) and chlorine (Matheson) were handled in a passivated stainless steel manifold,14 bromine (Mallinckrodt) was purified by vacuum distillation, and crystallineiodine (Fisher) was placed in a 3-L Pyrex bulb. Laser ablation has proven to be a powerful means of producing a large amount of atomic vapor for reaction with gaseous Present address: Department of Chemistry, College of Sciences, Shiraz University, Shiraz, Iran. +

0022-3654/93/2091-4910~04.00/0

molecules. However, due to the high energy of the laser-ablated atoms, the reaction can be extensive, and the laser-produced plasma radiation can photolyze the precursor and the reaction products. In order to overcome such problems and differentiate between the primary and final reaction product bands, as well as the bands due to the reaction of dissociated species with laserablated atoms, the technique has been modified to limit the reaction to the matrix surface or to promote complete reaction to give the more stable final products. As shown in Figure 1 with configurationA (conventional two source deposition) the reagent and the laser effluent react above the surface before being frozen in solid argon. In configuration B (blind deposition or surface reaction) the outer tube has been moved in close to the cold window thereby the reagent contacts laser-evaporated atoms only on the matrix surface. With this configuration the reaction products are limited mainly to primary unstable species with minimum contribution from the secondary dissociation-recombination or photodissociationspecies. In configuration C (concentricreactive evaporation)the reagent passes around thelaser plume, the reagent mixes more efficiently with the laser-evaporated atoms, and the reaction is promoted toward stable final products.

Resuits Boron atom reactions (nB and 'OB) will be described in turn for the four molecular halogen precursors with three different reaction geometries. Fluorine. Mixtures of F2/Ar (1:400) deposited with laserablated boron by using configuration B gave strong new infrared bands with 4/1 intensity ratios at 1441.0/1492.1 cm-I, 1389.9/ 1438.1 cm-I, and 1373.7/ 1415.5 cm-I (Figure 2a). Weaker bands with 4/1 intensity ratios were observed in the lower region at 1147.2/ 1177.1 cm-I, 921.1 /938.0 cm-I, 675.9/703.2cm-I, 522.4/ 528.0 cm-I, and 476.9/478.7 cm-I. Boron oxides, water, and the impurities in commercial fluorine CF,, OCFz, and SiF4 gave very weakabsorptions.12J4 In another experimentusing configuration A, the intensity distribution of the bands was changed; the 1441.0-cm-1 band, which was the strongest band with configuration B, decreased and became comparable with the 1389.9-cm-l band. Irradiation with the full output of a medium-pressure mercury arc lamp (A > 254 nm) did not affect the spectrum. A similar experiment with configuration C slightly favored the 1389.9-cm-1 absorption over the 1441.0-cm-' band. In all experiments the 1373.7/1415.5-cm-l bands were observed with half the intensity of the 1389.9/1438.1-cm-I bands. A similar experiment was carried out with configuration B and enriched boron- 10 to confirm the bands due to the boron- 10 isotope (Figure 2b); the weak 1376.2-cm-l band is probably due Q 1993 American Chemical Society

Reaction of Halogens with Laser-Ablated Boron

The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 4911

laser beam

laser beam

laser beam

Th Figure 1. Schematic diagram of different arrangements for controlled reaction of laser-ablated boron atoms with halogen molecules in an argon stream. Angular codeposition (configuration A), blind codeposition for surface reaction (configuration B), and coaxial codeposition to promote the reaction (configuration C).

TABLE I: Infrared Absorption Bands of the Reaction Products of Laser-Ablated Boron Atoms with Fluorine Molecules in h i i d Argon

/Bil

"B

'OB

1447.2 1444.7 1443.0 1441.0" 1440.4 1420.3 1389.9" 1387.7 1373.7" 1370.6 1151.2 1147.2" 1146.0 921.1 677.5 675.9" 524.7 522.4" 521.8 476.90 477.4

1498.2 1495.9 1494.1 1492.1 1491.6 1449.2 1438.1 1435.9 1415.5 1412.4 1181.2 1177.1 1176.0 938.0 704.9 703.2 530.5 528.0 527.3 478.7 478.2

assignment

The strongest absorbing site.

a

50

i500

;+SO

:$00

:350

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Fipre2. Infrared spectra at 0.25-cm-1 resolution in the 1350-1 550-cm-1 region after codeposition of F2/Ar (1:400) with laser-ablated boron. Spectrum of the reaction product bands using (a) configuration B with natural isotopic boron and (b) configuration B with enriched B-lo sample.

to HF2- isolated in solid argon.Is The boron fluoride.product bands are collected in Table I. The 1441.0- and 1492.1-cm-I bands with a relative intensity of 4/1 increased on consecutive matrix annealings at 20,25, and 30 K, decreased on further annealing at 30 and 35 K, and finally disappeared at 40 K. The 1389.9-cm-1 band increased slightly at 20 K, decreased upon further annealing at 25,30, and 35 K, and finally disappeared at 40 K. The 1373.7-cm-I band showed a similar annealing behavior to the 1389.9-cm-l band. The 675.9/703.2- and 476.9/478.7-cm-I bands followed the 1441.O/ 1492.1-cm-I bands on annealing.

Experiments were done in nitrogen matrices with 1% and 2% Fz to check for HBFz,which has not been observed in solid argon.I6 Three major boron fluoride species with relative intensity of 4/1 were observed at 1439.4 (with a matrix site at 1443.4)/1490.4 (site at 1494.8)-cm-I, 1384.8/1433.0 cm-I, and 1371.0/1412.9 cm-I. Weak HBF2 bands were detected at 1156.8, 1148.6 tuz), and 918.0 tug) cm-I; the u4 bands expected at 1385.1 and 1433.7 cm-l for boron-11 and boron-10, respectively, were covered by the strong new bands at 1384.8 and 1433.0 cm-I. The 1371.0/1412.9-~m-~ bands decreased on annealing to 25 K and 30 K and vanished above 35 K. The 1433.0/1384.8-cm-l bands grew slightly at 25 K in the 1% F2 experiment, decreased at 30 K, andvanishedabove 35 K. The 1439.4 (siteat 1443.4)/1490.4 (site at 1494.8) cm-l bands grew on annealing to 25 K and 35 K and decreased thereafter. The HBFz bands at 1156.8 and 918.0 cm-I decreased 60% on annealing to 25 K and vanished at 30 K. In addition, products of the reaction of B and N2 were also observed.13 Chlorine. Argon-diluted chlorine (Clz/Ar 1:400) was codeposited with the laser-ablated boron by using configuration A.

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Hassaneadeh and Andrews

The Journal of Physical Chemistry, Vol. 97, No. 19, 1993

TABLE II: Infrared Absorption Bands of the Reaction Products of Laser-Ablated Boron Atoms with Chlorine Molecules in Solid Argon "B 1580.4"~~ 1579.0 1420.4 1417.9 1415.3 1082.7 968.2d 965.6 963.4 961.3 945.5" 945.0 942.7 941.6 912.6 815.1 809.8 725% 724.7 723.2 721.7 690.8 687.8 684.7 68 1.7

'OB

assignment

1621.0 1619.7 1479.3 1476.9 1474.4 1098.6 1006.9 1004.2 1002.1 1000.1 984.0 983.5 98 1.4 980.3 844.9 839.8 750.0 749.1 747.7 746.4 713.0 710.0 707.0 704.0

The strongest absorption. Mixed I1/l0Babsorptions at 1599.8 and 1598.4cm-I. U isunidentified. Matrixsite. MixedlI/lOBabsorptions are 737.1, 736.0, 734.6, and 733.2 em-'.

The spectrum in the B-Cl stretching region showed new bands, which are collected in Table 11. The 815.1/844.9-~m-~, 965.6 (with a satelliteat 963.4)/ 1004.2(with a satelliteat 1OO2.4)-cm-l, and 945.5(with a shoulder at 942.7)/984.0 (with a shoulder at 981.4)-cm-l bands showed 411 relative intensities. The 815.1and 844.9-cm-I bands showed resolved splittingswith 3/1 intensity ratios at 809.8 and 839.8 cm-I, respectively. Weak HBC12 bands8-I7at 886.3/884.1/881.9 cm-I with 91611 intensity ratio for boron-11 and a trace of HC12- at 696 cm-' were also observed.8J8In another experiment the C12/Arsamplewas passed through the tube (configurationC). The945.51984.0-cm-l band pairs increased by a factor of 6 compared to the 965.6/1004.2-~m-~ bands. Reaction with configuration B decreased the 945.51 984.0-cm-I band intensities compared to the 965.611004.2-cm-I bands. A similar experiment carried out with enriched boron-10 exhibited bands at 1004.2 (with a shoulder at 1002.1), 983.5 (with a shoulder at 981.4), 844.9, and 839.8 cm-I; the 844.91839.8-cm-I bands showed a clear 31 1 intensity ratio. The 945.51984.0-cm-I bands grew on annealing to 20,25,30, 35, and 40 K and decreased on further annealing to 45 and 50 K. The 965.6/1004.2-cm-l bands grew at 20 K and decreased on further annealing to 45 K and disappeared at 50 K. The 815.1 1844.9-cm-I bands also grew slightly on annealing to 20 K, decreased on further annealing to 35 K, and disappeared at 40 K.

A 1/400mixtureof Clz/Ar wascodeposited with laser-ablated natural isotopic boron and spectra were taken at 0.25-cm-I resolution to observe the chlorine isotopic patterns. The spectra before and after annealing at 30 and 40 K for natural isotopic boron are shown in Figure 3, and the data are also collected in Table 11. The bands due to the matrix sites disappeared upon annealing the matrix to 30 K. The 965.6/963.4/961.3- and 1004.2/1002.1/1000.1-cm-~bands and weaker bands at 687.8/ 684.7/681.7 and 710.0/707.0/704.0 cm-1 were resolved into 91611 triplet patterns; the 815.1/809.8- and 844.9/839.8-cm-l bandsshowed 3: 1 intensityratios. A relatively broad band peaked at 912.6 cm-I and three sets of structured bands between 720 and 750 cm-l appeared on annealing to 30 K and grew at 40 K.

Figure3. Infrared spectra at 0.25-em-' resolution in the 700-1 lOO-cm-' region (a) after a 4-h codeposition of C12/Ar (1:4OO) with natural boron laser ablated at 40 mJ/pulse, (b) after annealing at 30 f 2 K, and (c) after annealing at 40 i 2 K.

TABLE IIk Infrared Absorption Bands of the Reaction Products of Laser-Ablated Boron Atoms with Bromine Molecules in Solid Argon "B

10B

833.4" 83 1.7" 813.0 666.5 665.7

870.1 868.3 849.5 694.6 693.7

assignment BBr2 BBr2 BBr3 B79Br B8IBr

Matrix sites at 0.25-em-' resolution.

Bromine. Similar experiments were conducted with mixtures of bromine and argon, and several new bands were observed in the B-Br stretching region (Table 111). With natural isotopic boron and configuration A, new bands at 6661694 cm-I, 832/ 869 cm-I, and 8 131849cm-I showed 4:l intensity ratios. A weak HBr2- absorption8J9was also observed at 728 cm-I (Figure 4a). With configuration B the 8 131849-cm-' band intensitiesdecreased a factor of 4 compared to the 8321869-cm-I band intensities (Figure 4b). For configuration C, the 813/849-cm-I bands were a factor of 2 stronger than the 832/869-cm-I bands (Figure 4c). With enriched boron-10 only the 694,849, and 869-cm-l bands were observed (Figure 4d). The 8 131849-cm-l bands grew on consecutive annealing to 45 K and decreased at 50 K. The 832/869-cm-I band intensities increased slightly on annealing to 20 K and decreased on further annealing to 45 K and disappeared at 50 K. The 6661694-cm-1 bands also increased slightly at 20 K but decreased on further annealing to 45 K and vanished at 50 K. The 6661694-cm-1 bands decreased faster than the 832/869-cm-I bands in these annealing processes. In another series of experiments using configuration C, the Brl/Ar concentration was varied from 1:200 to 1:2O00. At high concentration of Brz/Ar (1:200), the 813/849-cm-l bands dominated the 832/869- and 666/694-cm-l bands by a factor of 5. At intermediate Brz/Ar concentrations of 1/600, the 8 13/849-cm-l bands were stronger than the 832/869-cm-I bands by a factor of 2. At lower Br2/Ar concentration (1:lOOO) the relative intensityfor the 8 131849to 8321869-cm-1bands remained at 2:l; however, the 666/694-cm-I bands dominated the other two sets of bands by a factor of 2. At the extreme dilution of Brz/Ar (1 :2000), the 6661694-cm-1band intensitieswere further enhanced by a factor of 4 over the 8321869-and the 8 131849-cm-1 bands. At low concentration of Br2/Ar (1/ lOOO), low laser power (10 mJ/pulse at the target), and instrumental resolution of 0.25

Reaction of Halogens with Laser-Ablated Boron

The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 4913

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WAVENUMBER

Figure 4. Infrared spectra at 2-cm-’ resolution in the 600-1000-cm-’ region after d e p o s i t i o n of Brz/Ar (1:400) with laser-ablated boron. Spectrum of the reaction product bands with (a) configuration A, (b) configuration B, (c) configuration C, (d) configuration C with enriched boron- 10 isotope.

TABLE I V Infrared Absorption Bands of the Reaction Products of Loser-Ablated Boron Atoms with Iodine Molecules in Solid Argon “B

’OB

745.5 739.5 736.0 705.2 701.1 689.8 563.9

779.2 773.3 769.4 740.4 730.7 720.7 588.7

assignment

850 750 Si0 WAVENUMBER

550

Figure 5. Infrared spectra at 2-cm-! resolution in the 550-950-cm-’ region after d e p o s i t i o n of I2/Ar (1:400) wiht laser-ablated boron. Spectrum of the reaction product bands with (a) configuration A, (b) configuration C, (c) configuration A with high I2/Ar flow rate, and (d) configuration A with enriched boron-10 isotope.

isotopic boron- 10 the upper bands of boron isotopic pairs dominated the spectrum (Figure 5d). Annealing behavior of the product bands provides further information. The 563.9- and 588.7-cm-I bands decreased markedly at 40 K and vanished at 45 K. The 745.5,739.5,736.0 and 779.2,773.3,769.4 cm-l bands first increased then decreased on annealing to 40 K and vanished thereafter. On the other hand, the 705.2-, 701.1-, and 689.8-cm-I bands increased at the expense of other absorptions.

Discussion cm-I, partiallyresolveddoubletswereobservedat 666.5 and665.7 cm-I for boran-1 1 and 694.6 and 693.7 cm-I for boron-10. Each of the 832- and 869-cm-I bands at 2-cm-1resolution were resolved into two matrix sites at 833.4, 831.7 and 870.1, 868.3 cm-I at 0.25-cm-’ resolution. Iodine. A constant pressure of 100 Torr of argon gas was maintained over crystalline iodine in a 3-L Pyrex bulb to prepare a mixture of iodine vapor/argon (approximately 1:400). This mixture was codeposited with laser-ablated boron by using configuration A. The spectrum in the B-I stretching region showed a strong band at 563.91588.7 cm-l with a 4:l intensity ratio, HI2- absorption20 at 682 cm-I, weak bands at 705.2,701.1, 689.81740.4, 730.7, 724.7, 720.7 cm-I, strong bands at 745.5, 739.5,736.0/779.2, 773.3, 769.4 cm-I with a 4:l intensity ratio (Table IV), and a broad band at 815.2 cm-l (Figure 5a). A similar intensity distribution was obtained with configuration B. When I2/Ar was passed around the laser-evaporation boron plasma in configuration C, all except the 705.2-, 701.1-, and 689.8-cm-I bands were reproduced (Figure 5b). In another experiment with configurationA, the I2/Ar flow rate was increased by a factor of 3, and the 705.2-, 701.1-, and 689.8-cm-1 band intensities were markedly enhanced (Figure 5c). With enriched

The new absorptionsproduced by boron atom-halogen molecule reactions will be identified for each halogen in turn. All the species showed matrix sites and only the strongest absorbing site will be used for discussion. The boron halide BX, species were formed by the B XZreaction with higher n species favored by reaction configurationsA and C and on matrix annealing to allow diffusion and reaction of trapped atoms. All product bands showed 114 doublets with natural isotopic boron and only the upper band was observed with boron- 10,which identifiesproducts containing a single boron atom. Fluorine. BF3. The 1441.O/ 1492.1-cm-l bands were produced with the same integrated intensity as the 1389.9/1438.1-cm-I bands when the reaction was confined to occur on the matrix surface by using configuration B; however, the 1441.0-, 1492.1-cm-I band intensities increased over the others when further reaction was promoted by using configuration A. The 1441.O/ 1492.1-cm-’bands increased on annealing to 30 K, while the other bands decreased,suggestingthat the former bands belong to the stable reaction product BF3. These bands are slightly red shifted from the 144511496-cm-I vnvalues obtained for BF3 earlier in solid argon*and the 145411505-cm-I gas-phase values.’ The band pairs at 675.91703.2 and 476.91478.7 cm-1, which follow

+

4914 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993

the 1441.0/1492.1-cm-I bands, are slightly red shifted from the gas-phase values for v2 and v4 of BF3.132 BF. The 1373.7/1415.5-cm-l band pair decreased more on annealing than the other band pairs, which suggests that these bands belong to a primary unstable intermediate species. These bands show a boron isotopic ratio Rlolll = 1415.5/1373.7 = 1.0304, which is in good agreement with the R l ~ / l=l 1.0310 value calculated for the B-F harmonicoscillator, and are assigned to diatomic BF. The present 1373.7-cm-I band is slightly red shifted from the gas-phase value of 1378.4 cm-1 deduced from the emission s p e ~ t r u m . ~ , ~ ~ BFz. The 1389.9/1438.1-cm-I band pairs decreased on annealing above 20 K as BF3 bands increased, suggesting that these bands belong to unstable intermediate reaction products. The possible contribution of HBF2 to the 1389.9/1438.1-cm-' bands is difficult to determine. Observation of weak HIlBF2 bands at 1147.2, 921.1, and 522.4 cm-I argues for a HIlBF2 contribution in the 1385-1440-cm-I region, and the 1391.7-cm-I shoulder may be due to HBF2. In the N2 matrix experiment, weak HBF2 bands were observed,16 but the 1384.8- and 1371.0-~m-~ product bands are appropriate for BF2 and BF in solid N2. We conclude that the major absorption at 1389.9/ 1438.1cm-I in solid argon is due to the BF2 radical. A 1390-cm-1 band observed in the matrix radiolysis study8 and assigned to HBF2 could be due to either species. The major bands showed a boron isotopic ratio of Rlop I = 1438.1/1389.9 = 1.0347,which is expected for the antisymmetricstretching vibration of the F-B-F radical. The boron isotopic ratio for v 3 can also be used to predict a lower limit for the apex angle;Z2 the BF2 data give cos a = -0.412 (a = 114O). This angle is in agreement with the 112O value deduced from the ESR study in solid xenon.7 In view of the BC12apex angle calculations to follow, the apex angle of BF2 is probably 120 f 5O. Finally, no evidence was found for BzF4 in these experiment^,^,^ which is consistent with the low concentration of boron employed here. Chlorine. Bas. The 945.5/984.0-cm-I bands were enhanced over the other bands when the reaction was carried out with configuration C and diminished when the reaction was carried out on the matrix surface by using configuration B. The fact that these bands grew on annealing to 45 K suggests assignment to the stable molecule BCl3. The 945.5/984.0-cm-I bands are in agreement with the BC13bands previously observed in solid argon at 946.0 and 984.0 cm-I and are red shifted from the gas-phase values of 956 and 995 cm-1.193.8 BCl. The 815.1/844.9-and809.8/839.8-~m-~ bandsappeared with a 4:l intensity ratio in all experiments with natural boron as expected for a single boron containing species, and the 8 15.1/ 809.8- and 844.9/839.8-cm-I bands also showed a 3:l intensity ratio as expected for a single chlorine containing species with natural chlorineisotopes. These bands were enhanced in reaction limited to the matrix surface by using configuration B and were decreased on annealing, suggesting a primary reaction product. The boron isotopic ratios Rl&35/ll-35 = 844.9/815.1 = 1.0366 and R10-37/~~-37 = 839.8/809.8 = 1.0371 are in reasonable agreementwiththeRI~35/11-35= 1.0372andR1&37/11-37= 1.0376 ratios calculated for the B-Cl harmonic oscillator. The chlorine isotopic ratios are Rll-35/11-37 = 815.1/809.8 = 1.0065 and Rl(r3s/la.37 = 844.9/839.8 = 1.0061, which are also in good agreementwiththeRII-35/11-37 = 1.0065andR10-35/1~37 = 1.0061 values calculated for the B-Cl harmonic oscillator. The boron11 fundamental of 8 15.1 cm-' is red shifted from the gas-phase value of 828.9 cm-l deduced from the emission study.9 BCl2. The965.6,963.4,961.3cm-~and1004.2,1002.1,1001.1 cm-I triplets appeared with 4:l relative intensities in all experiments involving natural boron. Enriched boron- 10 isotope produced only the 1004.2, 1002.1, and 1000.1 cm-1 bands, indicatingthatthe965.6,963.4,961.3and 1004.2,1002.1,1000.1 cm-I tripletsbelongtoboron-1 1and boron-lOcontainingchlorides,

Hassanzadeh and Andrews respectively. Furthermore, each set of bands was resolved into the 9/6/1 intensity triplet expected for two equivalent chlorine atoms. These bands decreased on annealing, which further supports their assignment to a reactive intermediate species, particularly in view of the growth of B2C14. The relative intensity of these bands to the BC13 bands increased when reaction was limited to the surface in configuration B. The weak 731-cm-I band in the radiolysis experiments associated with the 965-cm-I multiplet8was observed at 73 1.7 cm-l in the present studies and is probably due to v1 of HBC12.I7 Thestrong 965.6-cm-1multiplet is assigned to BCl2 in agreement with the radiolysis experiments.* The boron isotopic ratios of Rlo/ll = 1004.2/965.6 = 1.0400 and Rlo/ll = 1000.1/961.3 = 1.0404 are expected for the antisymmetric stretching vibration of the Cl-B-Cl radical. The chlorine isotopic ratios of R35/37 = 965.6/961.3 = 1.0045 for boron-1 1 and R35/37 = 1004.2/1000.1 = 1.0040 for boron-10 are appropriate for the BCl2 radical. The boron and chlorine isotopic ratios predict lower and upper limit bond angles22of 115' (cos a = -0.429) and 127' (cos a = -0.605), respectively, in agreement with the 115 f 5 O lower limit value predicted previously.8 The mean value, 121O, which averagesanharmoniceffects and provides a reliable bond angle determination, is in excellent agreement with the 122O angle calculation from the ESR spectrum.23 B2Q. The broad band at 912.6 and three structured bands at (725.8, 724.7, 723.2, 721.7)/(737.1, 736.0, 734.6, 733.2)/ (750.0, 749.1, 747.7, 746.4) cm-l, which have an integrated intensity ratio of 10/5/1 and grow together on annealing, are in agreement with the v7 and v5 modes of B2C14.4,6The growth of B2Cl4 on annealing provides further support for the presence of the BCl2 radical in the matrix. Bromine. BBr3. The 813.0/849.5-cm-I bands were more intense with configuration C where the reaction was promoted to final products and were in much lower yield with configuration B where the reaction occurred mainly on the matrix surface. These bands were also enhanced over the other bands as the Brz/Arconcentration increased from 1:20oOto 1:200onannealing to 45 K. The 813.0/849.5-cm-I bands are assigned to BBr3, in agreement with previously reported bands in solid argon at 812.7/ 849.2 cm-1,8which are red shifted from the gas-phase values of 819.6/856.1 cm-'.' BBr. The (666.5,665.7)/(694.6,693.7)-cm-Ibands decreased on annealing and dominated the others as Br2/Ar concentration decreased from 1:200 to 1:2000, suggesting a primary reaction product. These partially resolved 1:1doublets gave boron isotopic ratios R I O ~=I I694.6/666.5 = 1.0422 for bromine-79 and Rlo/ll = 693.7/665.7 = 1.0421 for bromine-81, which are inagreement with isotopic ratios of R1~79/11-79 = 1.0428 and R1&81/11-81 = 1.0429, respectively,calculated for the B-Br harmonic oscillator. The 665.7-cm-1 value is red shifted from the gas-phase fundamental of 677.3 cm-l deduced from the BBr emission spectrum.9 BBrz. The remaining 832/869-cm-I product bands were favored by configuration B and decreased on annealing, which suggests an intermediate species. The present bands are in agreement with 833/87O-cm-l bands produced by proton radiolysis and argon resonance photolysis of BBr3 and assigned to the antisymmetric stretching vibration of the BBr2 radical$ however, the 597/619-cm-I bands produced from radiolysis of BBr3 and assigned to the V I of BBr2 were not observed here. The latter bands have been reassigned to H B B c ~ .Valence ~~ angle calculations for the antisymmetricstretchingvibration for BBr2predict a lower limit of 112O. On the basis of calculations for BC12, the valence angle of BBr2 is close to 120O. Finally, recent matrix photochemical studies of H2/BBr3 mixtures have also produced the bands assigned here to the BBr2 radical.24 Iodine. BIJ. The 705.2-, 689.8/740.4-, and 720.7-cm-1 bands increased at higher flow rate of 12/Ar and on &maling to 45 K, which is indicativeof a stable final product. The 705.2/740.4-cm-I bands are in agreement with the previously reported gas-phase

Reaction of Halogens with Laser-Ablated Boron values of 704/737 cm-1 for BI3 and assignment to the antisymmetric stretching vibration of BI3 follows.1 The 689.8- and 705.2-cm-I absorptions are probably the same molecule trapped in different matrix sites or perturbed by nearby species. BI. The 563.9/588.7-cm-I bands decreased on annealing, suggesting an unstable species. The boron isotopic ratio of RlollI = 588.7/563.9 = 1.0440 is in agreement with the calculated value of Rl&127/11-127 = 1.0448 for the B-I harmonic oscillator. The 563.9-cm-I matrix value is red shifted from the 569.9-cm-1 vibrational mode of ground-state diatomic BI deduced from the emission spe~trum.~ BIZ. The (745.5, 739.5, 736.0)/(779.2, 773.3, 769.4)-cm-I bands also showed 4:l relative intensityratios and only the 779.2-, 773.3-, and 769.4-cm-l bands wereobserved withenriched boron10 isotope. These bands decreased on annealing, indicating involvement of an unstable species. The boron isotopic ratios of Rlojll = 779.2/745.5 = 1.0452, Rlo/ll = 773.3/739.5 = 1.0457, and RlollI = 769.4/736.0 = 1.0454 are appropriate for the antisymmetric stretching vibration for the I-B-I radical. The boron isotopic data predict lower limit angle values of 105 f 6O for BIZradicals trapped in three different matrix sites.

Conclusion Laser-evaporatedboron atom reactions with molecular halogens diluted in argon produced BX, BX2, and BX3 products. The BX and BX2 spedies were favored by carrying out the reaction on the matrix surface, which facilitates the reaction pathway B plus X2 giving BX X and BX2. The trihalide BX3 dominated when the reaction was promoted in the gas phase as well as on annealing to allow diffusion and secondary reaction of BX2 and X atoms to give BX3. The identification of BX, species was confirmed from boron and chlorine isotopicsplittingsin B Clz experiments. Formation of BX and BX2 from the elemental componentsinstead of dissociation of BX3 eliminatedspectral interferencesfrom strong BX3 bands and allowed confirmation of previous BCl2 and BBr2 assignments. Lower limit valence angle predictions for BX2 radicals from boron isotopic data ranged between 105 and 115O, which suggests that the BX2 radicalvalence angles are near 120°. The best data obtained here for BC12 from boron and chlorine isotopic measurements predict a 121O angle for the BC12 radical.

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The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 4915

Ackwwledgmeat. We gratefully acknowledge fmancialsupport from N.S.F.Grants CHE 88-22556 and CHE 91-22556. References and Notes (1) (a) Nakamoto, K.Infrared Spectra of Inorganic and coordination Compounds,4thed.; John Wiley & Sons, Inc.: New York, 1986. (b) Wentink, T., Jr.; Tiensuu, V. H. J. Chem. Phys. 1958,28,826. (2) (a) Bassler, M.; Timms, P. G.; Margrave, J. L. J . Chem. Phys. 1966, 45,2704. (b) Levin, I. W.; Abramowitz, S.Chem. Phys. Lett. 1971,9,247. (3) (a) Jones, L. H.; Swanson, B. I.; Ekberg, S. E. Chem. Phys. Lett. 1979,68,499.(b) Holland, R.; Maier, W. B., 11; Freund, S. M.; Beattie, W. H. J . Phys. Chem. 1983, 78,6405. (4) Nimon, L. A.; Seshadri, K. S.; Taylor, R. C.; White, D. J . Chem. Phys. 1970,53,2416. (5) Finch, A.; Hyams, I.; Steele, D. Spectrochim. Acto 1965,21,1423. (6) (a) Linevsky, M. J.; Shull, E. R.; Mann, D. E.; Wartik, T. J. Am. Chem. Soc. 1953,75,3287. (b) Mann, D. E.; Fano, L. J. Chem. Phys. 1957, 26, 1665. (7) Nelson, W.; Gordy, W. J . Chem. Phys. 1969,51, 4710. (8) Miller, J. H.; Andrews, L. J . Am. Chem. SOC.1980,102,4900. (9) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure, IV. Constants of Diatomic Molecules; Van Nostrand Reinhold Company: New York, 1979. For BI, see: Coxon, J. A.; Naxakis, S. J. Mol. Spectrosc. 1987, 121,453. (10) (a) Brabson, G. D.; Mielke, Z.; Andrews, L. J. Phys. Chem. 1991, 95,79. (b) Mielke, Z.; Brabson, G. D.; Andrews, L. J . Phys. Chem. 1991, 95,7 5 . (1 1) Jeong, G. H.; Boucher, R.; Klabunde, K.J. J . Am. Chem. Soc. 1990, 112, 3332. (12) (a) Andrews, L.; Burkholder. T. R. J . Phys. Chem. 1991,95,8554. (b) Burkholder, T. R.; Andrews, L. J. Chem. Phys. 1991, 95,8697. (13) Hassanzadeh, P.;Andrews, L. J . Phys. Chem. 1992,96,9177. (14) Andrews, L.; Lascola, R. J . Am. Chem. SOC.1987, 109,6243. (15) Hunt, R. D.;Andrews, L. J . Chem. Phys. 1981,87,6819. (16) (a) Porter, R. F.;Porter, S.K . J . Phys. Chem. 1965,69,2208.(b) Shirk, A. E.; Shirk, J. S. Inorg. Chem. 1983,22, 72. (17) Bass,C. D.; Lynds, L.; Wolfram,T.; DeWames, R. E. J. Chem. Phys. 1964,40, 361 1. (18) Milligan, D. E.;Jacox, M. E. J . Chem. Phys. 1970,53,2034. (19) Millinan, D. E.;Jacox, M. E. J. Chem. Phvs. 1971. 55. 2550. (20) (a) Noble, P. N. J . Chem. Phys. 1972,56,2b88.(b) Ellison, C. M.; Auk, B. S. J . Phys. Chem. 1979,83,832. (21) (a) Lovas, F. J.; Johnson, D. R. J . Chem. Phys. 1971,55,41.(b) Onaka, R. J . Chem. Phys. 1957,27, 374. (22) Allavena, M.; Rysnik, R.; White, D.; Calder, V.; Mann, D.E. J . Chem. Phys. 1969,50,3399. (23) Franzi, R.; Geoffroy, M.; Lucken, A. C. J . Chem. Phys. 1983,78, 708. ..

(24) Moroz, A.; Sweany, R. L. Inorg. Chem. 1992,31, 5236.