Pulsed laser evaporated boron atom reactions with methane. 2

Pulsed laser evaporated boron atom reactions with methane. 2. Infrared spectra of H2CBH2, H2CBH, HCBH, and HBCBH in solid argon. Parviz Hassanzadeh ...
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J. Phys. Chem. 1993,97, 6418-6424

Pulsed Laser Evaporated Boron Atom Reactions with Methane. 2. Infrared Spectra of H2CBH2, HzCBH, HCBH, and HBCBH in Solid Argon Parviz HPrrcranzadeh,+ Yacine Hamachi,* and Lester Andrews' Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 Received: December 29, 1992; In Final Form March 19, 1993

Codeposition of pulsed laser evaporated boron atoms with a mixture of methane/argon onto a 12 K cold window produced several new organoborane species. Substitution of the isotopic reagents I1B, l0B, CH4, 13CH4, CD4, and CH2D2 has characterized H2CBH2, H2CBH, HCBH, and HBCBH as the major reaction products. These identifications are supported by ab initio quantum chemical frequency calculations. The anticipated primary insertion product CHsBH radical was tentatively identified by its strongest calculated infrared band, a B-H stretching mode at 2561 cm-I. The rearrangement product H2CBH2 was characterized by four infrared bands. The H elimination product HzCBH was identified from a B-H stretching mode a t 2724.6 cm-I, a C - B stretching mode a t 1469.7 cm-l, and BH and CH2 deformation modes at 902.6, 705.7, and 611.8 cm-I, and the H2 elimination product HCBH radical exhibited diagnostic fundamentals at 3248.8,2743.4, and 1475.3 cm-I. The secondary reaction product HB==C=BH, with two equivalent boron atoms and carbon-boron double bonds, was identified from a natural boron isotopic triplet at 1895.2, 1883.9, and 1872.0 cm-l. Atomic boron is extremely reactive with hydrocarbons.

Pulsed laser evaporated metal atom reactions with small molecules produce novel moleculeip and new transient species which can be trapped in an argon matrix and characterized by FTIR ~pectrcwcopy.~-~ Of particular interest, the insertion of boron into a C-H bond in CH4 to produce CH3BH from the reaction of concentrated methane samples with pulsed laser evaporated boron atoms has been attempted.6 The laser evaporated boron atom-methane reaction has been reinvestigated with more dilute mixtures of methane and argon, complete isotopic substitution, higher resolution measurements, and quantum chemical calculations. Several new species have been identified: the novel linear HBCBH molecule with two carbon-boron double bonds has been characterized in a preliminary report.' In addition, the simple organoborane molecules H&BH and HCBH also exhibit carbonboron doublebondsand structural similarities to the fundamental hydrocarbons ethylene and acetylene. Here follows a description of matrix infrared spectra and supporting ab initio quantum chemical calculations of several new organoboranespecies in the boron-methane system.

W)photolysis and annealing on a Nicolet 60SXR instrument at 0.5-cm-I resolution. RWdb

Pulsed laser evaporation produced enough boron atoms for product detectionwith minimal methane dissociation by the laser plume. Nevertheless, CH3 radical* was observed as a medium intensity band at 612-619 cm-I in these experiments, and annealing produced C2H6 absorptionsg at 1466 and 822 cm-Lat the expense of methyl radical. At lower precursor concentrations, the sharp species three product band dominates the CH3 radical absorption. Typically a 21100 mixture of methane/argon and laser power of 40 mJ/pulse at the target were employed although 1/400 mixtures gave the same but weaker product absorptions. The isotopic counterpart for each band was identified based on the spectral band shape and matrix site splitting, and changes on photolysis and annealing. In addition weak (A < 0.005) absorptions' due to the boron oxides BO*, BOz-, BO, and HBO were observed in each experiment. Isotopichbstitption. The infrared spectra for "B C&, loB CH4, "B I3CH4, and "B CD4 are contrasted in Figures 1 and2 for the 1900-500and 3300-1900~m-~ regions,respectively. The sets of bands at 972.4, 1239.8,1414.7,2522.8 cm-* (species 2), 611.8, 705.7, 706.5, 902.6, 1469.7, 2724.6 cm-l (species 3), 1475.3, 2725.8, 3248.8 cm-l (species 4), 894.3, 1137.3, 2609.7 cm-I (species 5 ) , and 1872.0, 1883.9,1895.2 cm-l (species 6) for natural isotopes showed constant integrated intensity ratios in all experiments as well as on photolyses and annealings; several remaining bands could not be correlated. The new boron-11 product absorptions from the CH4 reaction with natural boron and with enriched boron-10 are collected in Table I. The isotopic data including I3CH4, CD4, and CH2D2 for the species 2, 3, 4, and 6 bands are given in Tables 11-VI, respectively. In general, CD4 gave the same organoborane species and a lower yield of CD3 radicals at 454 cm-l, but radiation from the laser plume allowed for the formation of Ar,D+ at 644 cm-l.lo In the CHzD2 experiments, both CHzD and CHD2 radicals* were observed at 558 and 507 cm-l independent of boron isotope as noted in Figure 3 along with mixed HID organoborane products. Photolysis and AMepung. The spectra from the reaction of natural boron with CH4 before and after photolysis and annealing

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ExperimenW Section The closed-cycle helium refrigerator (CTI Cryogenics Model 22), vacuum chamber, and pulsed YAG laser evaporation apparatus have been described previ~usly.~.~ A piece of boron was epoxy glued to the end of a glass rad and rotated at 1 rpm and the fundamental of the YAG laser beam was focused by a IO-cm focal length quartz lens onto the target; laser powers of 1 0 4 5 mJ/pulse at the target were employed. The evaporated boron atoms were deposited with methane/argon mixtures ranging from 1:400 to 5: 100at a total rate of 2 mmol/h. Natural ("B) and enriched boron-10 ('OB) (Eagle-Pitcher Industries) and carbon-13and deuterium-enriched methane (Cambridge Isotope Laboratories) were used. Infrared spectra were collected before and after medium-pressure mercury arc (Philips H39KB, 175 Present address: Department of Chemistry, College of Sciences, Shiraz University, Shiraz, Iran. Present address: Laboratoire de Photophysique and Photochimie Moleculaire, Universite de Bordeaux I, 351, Cours de la liberation, F-33405 Talence Cedex (France); on leave from Laboratoire de Spcctrochimie Moleculaire (CNRSURA 508) U.P.M.C., Paris (France).

0022-3654/93/2097-6418S04.00/0

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0 1993 American Chemical Society

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The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 6419

Boron Atom Reactions with Methane

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Figure 1. Infrared spectra in the 1900-500-~m-~ region for laser ablated boron atoms deposited with Ar/methane = 100/2 samples at 12 K. (a) natural B (80% IlB, 20% ]OB) + CH4, (b) ‘OB (94%) + CH4, (c) natural B I3CH4,and (d) natural B + CD4. The CD4 sample contained C02 impurity.

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1

1

cu4

-

0 3 h

3eoo

sic0

3boo

2630

zboo

zioo

zboc

WRVENLIYSER

z~oc r~oc

2300

zzoc

zloc

2600

1900

Figure 2. Infrared spectra in the 330&1900-~m-~region for laser ablated boron atoms codeposited with &/methane = 100/2 samples at 12 K. (a) natural B (80% llB, 20% log) + CH4, (b) loB (94%) + CHI, (c) natural B + ”CH4, and (d) natural B + CD4.

at 20,30, and 40 K are shown in Figure 4 for the 50&1900-~m-~ region. Species 2 and 3 bands increased by 10% on 254-nm photolysis and decreased slightly on 20 K annealing, but species 1 bandsdecreased by only 10%on 30 K annealing whereas species 3 decreased by 40% and about 25% of species 1 remained on 30

K annealing which destroyed species 3. Species 4 increased 20% on photolysis and decreased 30% on annealingto 30 K. Annealing also destroyed CH3 radical and produced C2Ha as noted in Figure 3e. Species 5 increased by 50% on photolysis and decreased on annealing to 30 and 40 K,and species 6 increased by 20% on

6420 The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 TABLE I: Boron Isotopic Absorptio~(cm-I) Observed for Pulsed h r Evaporated Boron Atom Reactions with Muted in Argon

a

a

lOB

LIB

identification'

617.7 706.5 792.7 902.1 913.4 98 1.8 983.9 987.3 1040.0 1159.6 1160.5 1251.6 1255.2 1264.4 1413.5 1416.3 1487.9 1489.2 1497.4 1895.2 2452.1 2466.4 2480.7 2526.2 2540.0 2554.8 2575.3 2624.7 2626.7 2742.7 2760.2 2770.7 2979.5 3245.6 3249.9 3266.0

611.8 705.7 788.3 894.3 902.6 970.3 972.4 976.3 1027.5 1137.3 1138.2 1235.6 1239.8 1260.3 1412.0 1414.7 1466.4 1469.7 1475.3 1872.0 2434.5 2447.4 2460.0 2506.4 2522.8 2540.2 2560.6 2609.7 2612.0 2724.6 2743.4 2751.0 2979.2 3244.7 3248.8 3265.1

species 3, HzCBH species 3, HzCBH U' species 5, (HBCHBH) species 3, H2CBH species 2, site species 2, H2CBH2 species 2, site

U species 5, (HBCHBH) species 5, site species 2, site species 2, H2CBH2 species ? species 2, site species 2, H2CBH2 species 3, site species 3, H2CBH species 4, HCBH species 6, HBCBH U U U U species 2, site species 2, HzCBHz species 1, CH3BH species 5, (HBCHBH) species 5, site species 3, H2CBH species 4, HCBH species 4, site species 1, CHJBH species 4, site species 4, HCBH species 4, site

Unidentified bands are noted by U.

photolysis and decreased on annealing to 20 and 30 K, and almost vanished at 40 K. Species 5 also increased 10% on photolysis, decreased by 30% on 30 K annealing, but species 4 increased 20% on photolysis and decreased by 40% on 30 K annealing. All of the IlB and loB bands listed in Table I except the 1895.2-cm-l band (species 6) showed the 4: 1 intensity ratio for natural isotopic abundance boron and only the higher frequency loB bands were observed with the enriched boron-10 sample. The 1895.2-cm-l band showed a triplet isotopic pattem at 1872.0, 1883.0, and 1895.2cm-l with 16:8:1 intensityratiosfor thereactionofnatural isotopicboron with natural methane (Figure la); the 94%enriched boron-10 showed only the 1883.9 and 1895.2 cm-l bands with a 1:8 intensity ratio (Figure 1b). These bands showed 20% growth on photolysis and decreased on annealing (Figure 3). The v3 fundamental and combination modes of methane covered most of the B-H stretching region. The E D stretching region in the reaction of deuterated methane with natural isotopic boron and enriched boron-10 revealed new bands at 2190.0,2213.1,2230.7 cm-1 with a 16:8:1 intensity ratio and 2213.1, 2230.7 cm-l with a 1:8 intensityratio,res@vely(Figure Id). Thesebandstracked with the 1727.4, 1729.8, and 1732.2 cm-I absorptions and grew 30% on full arc photolysis and decreased on annealing above 30 K. All product bands revealed doublet isotopic patterns with a 1:l mixture of 12CH4and 13CH4in argon as well as a 1:l mixture of CH4 and CD4 in argon. No new intermediate components were observed for the incorporation of two methane molecules into a single product species. Natural boron as well as enriched boron-10 were also reacted with the CH2D2/Ar sample. The 972.4 c m - I band of the species 1 also showed a triplet structure with 1:4:1 intensity ratio at 960.2,887.7, and 845.3 cm-l (Figure 3); the product absorptions from the reaction of natural boron and enriched boron-10 are

Hassanzadeh et al. collected in Table 11. The 706.5- and 902.6-cm-l bands of the species 2 were not observed; intead, two sets of bands at 679.7, 611.9, 587.9 and 877.0, 871.2, 749.9 cm-'appeared (Figure 3). The product absorptions from the reaction of natural boron and enriched boron-10 are collected in Table 111. The upper bands of species 6 (1872.0, 1883.9, and 1895.2 cm-l) showed multiplet pattemsforbothboron-11 andboron-10whicharelistedinTable V. A new band at 2152.4 cm-1 which was shifted to 2182.2 cm-l with enriched boron-10 tracked with the upper species 6 bands (Table VI). Ab Initio Calculations. Quantum chemical calculations were performed in order to facilitate the identification of new products of the boron atom reaction with methane using the ACES I1 program." The calculations have been carried out at both self consistent field (SCF) and second order multibody perturbation theory (MBPT(2)) levels of theory using the Dunning-Huzinaga doubletplus polarization (DZP) basis set constructed as (9s5pld/ 4~2pld)for B and c with ad(C) 0.75, (Td(B) 0.70 and as (4slp/2slp) for H with a,(H) = 1.O. Geometrieswere optimized by means of the gradient technique in the ACES I1 program. Vibrationalfrequencies and IR intensitieswere calculated at both levels within the double harmonic approximation by finite differences of analytical gradients. Spin-restricted theory was used for closed shell and spin-unrestricted theory was used for open-shell species. The complete results of these calculations will be published separately;12however, the calculated isotopic frequencies for the major product absorptions observed here are included in the tables for comparison and to aid in making vibrational assignments.

Mscmrsion The new species produced from the reaction of methane with the laser evaporated boron atoms will be identified from isotopic shifts, isotopic intensity distribution patterns, and ab initio calculations. Unfortunately, the previous experiments of JBK6 were complicated by boron oxides' from the reaction of B and 0 2 impurity and the lack of sharp product bands owing to poor isolation conditions. The broad 2500-cm-l feature observed by JBK is resolved here into 6 strong bands and 4 weaker site splittings. The 971- and 894-cm-l bands assigned by JBK to CH3BH were observed here and are assigned to different species with the aid of quantum chemical calculations. In addition, the 822-cm-l band assigned by JBK to CH3BH is in fact due to C2H6.9 Finally, the 2004-, 2012-cm-l bands assigned by JBK to bridged B-H species are probably due to BCO species.13 The reaction of B CH4 is expected to produce CH3BH as a primary reaction product. This reaction is exothermic by approximately the energy of a C-B single bond since B-H and C-H bonds have about the same energy. Quantum chemical calculations at the CCSD/DZP level place CHoBH below B CH4 by about 57 kcal/mol.12 The question, then, is if (CH3BH)* can be relaxed by the condensing matrix before rearrangement or hydrogen elimination to give other products as illustrated in Scheme I. The rearrangement product radical CH2BH2 is 7.7 kcal/mol lower in energy than the CHpBH radical.12 Typically B-H stretching frequencies based on diborane as a model fall in the 2500-cm-I region.14 The large number of product bands observed between 2434 and 2579 cm-l show that at least twoorganoborane products are formed. These bands show smaller (less than 1 cm-I) carbon- 13shifts and large (10-20 cm-l) boron10 shifts and very large deuterium (600 cm-l) shifts and are clearly due to B-H stretching funadamentals. However, these bands cannot be assigned to a specific organoborane product without the aid of quantum chemical frequency calculations. Species1: CHJBH. The CH3BH molecule is calculated (SCF level/DZP basis set) to have a very strong B-H Stretching fundamental at 2725 cm-l which is 4 times stronger than any other fundamental;12thiscalculated valueisexpected to b e d l o % too high.15 Thus, it will be difficult to identify CH3BH from the

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The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 6421

Boron Atom Reactions with Methane

TABLE II: observed and ab Initio Calculated Vibrational Frequencies (cm-') for Species 2, the HzCBHt Molecule in C., Symmetry' CH4 v(BHz)oWW v(BH2)cal v(BHz)obs(ad v(BHz)cal v(CHz)obs(al) v(CHz)d v(B-H2)0bs(b1) v(B-H&d a

"CH4

O 'B

IIB

lOB

IlB

983.9 1087.2 1255.2 1344.3 1416.3 1582.9 2575.3 2773.4

972.4 1073.3 1239.8 1328.6 1414.7 1582.9 2560.6 2757.3

981.3 1085.1 1254.3 1343.5 1406.5 1574.9 2575.1 2773.4

969.7 1071.2 1239.2 1328.0 1405.4 1574.0 2560.2 2757.3

808.9 888.0

794.8 871.2

1071.9

1056.8

1270.2

1258.3

2088.3

2065.4

855.0,899.9,971.8

841.2,887.7,960.2

916.1,993.0, 1081.9

902.5,977.8, 1068.0

Calculated SCF frequencies using DZP basis set.

TABLE IIk Observed and ab Initio Calculated vibrational Frequencies (cm-') for Species 3, the HzCBH Molecule in CZ, Symmetry* ~~~~

CH4 v(BH)obs(bz) v(BH)cal v(CHz)~bs(bz) v(CHz)&l v(BH)obs(bl) v(BH)cal v(B=C)obs(al) V(B=C)Cal v(B-H)obs(al) v(B-H)cal (I

"CH4

log

IlB

617.7 639.9 706.5 743.8 913.4 963.4 1489.2 1533.6 2742.7 2942.1

611.8 634.8 705.7 741.4 902.6 951.2 1469.7 1513.4 2724.6 2924.9

lOB 614.0 634.5 704.2 742.6 910.3 959.5 1463.8 1506.6 2738.4 2941.9

CD4

IlB 608.1 629.5 703.2 740.0 899.5 947.2 1443.3 1586.7 2721.9 2924.9

CHzDz IlB

log

lOB

532.4 537.1 557.3 752.4 798.8

526.8 537.1 553.5 739.1 794.0

1403.7 2093.8 2224.4

1387.0 2070.2 2199.6

IlB

533.9, 591.5,611.9,679.7 645.0,694.5,700.1,714.0 763.0, 853.9,882.1,887.8 809.0,845.1,930.8,935.3

532.5, 587.9,611.9,679.7 634.0,694.5,693.8,709.2 749.9,845.3, 871.1,877.0 795.0, 831.4,918.2,923.0

1467.1, 1482.1, 1518.3, 1520.5

1434.4, 1451.5, 1483.8, 1489.1

Frequencies calculated at MBPT(2) level with DZP basis set.

TABLE Iv: Observed and ab Initio Calculated Vibrational Frequencies (cm-I) for Species 4, the Linear HCBH Molecule4 CH4 lOB 1497.4 1652.8 2760.2 2995.0 3249.9 3555.0

v(BSC)obs v(B=C)cal v(B-H)obs v(B-H)cal v(C-H)ObS v(C-H)=l

CD4

13cH4

IlB 1475.3 1618.9 2743.4 2976.5 3248.8 3554.4

lOB 1471.7 1627.1 2759.8 2994.7 3237.1 3541.0

IlB 1449.3 1592.4 2743.0 2976.2 3236.2 3540.5

CHzD2

lOB

IlB

lOB

IlB

lOB

IlB

1500.0 2125.7 2285.1 2469.4 2674.5

1480.5 2097.8 2254.3 2464.3 2669.4

1595.5 2448.7 2652.4 2738.6 3002.6

1562.1 2448.1 2651.9 2722.3 2982.9

1546.8 2149.0 2311.1 3247.7 3554.4

1528.3 2116.2 2274.9 3247.7 3554.0

Calculated SCF frequencies using DZP basis set.

TABLE V

Observed and ab Initio Calculated Vibrational Frequencies (cm-') for Species 6, the Linear HBCBH Molecule4 CH4

v(a BCB)obs v(a BCB)cal ratio v(a BH)obs v(a BH)cal 0

lOB

10,llB

IlB

1895.2 2034.2 0.9317 b 3006.3

1883.9 2022.2 0.9316 b 3001.3

1872.0 2009.6 0.9315 b 2985.9

"CH4 10,llB 1837.9 1971.5 0.9322 b 2999.9

LOB 1849.7 1984.0 0.9323 b 3004.4

CD4 IlB 1825.4 1958.2 0.9322 b 2984.3

lOB

10.llB

IlB

1732.2 1855.4 0.9336 2230.7 2399.4

1729.8 1853.4 0.9333 2213.1 2380.8

1727.4 1851.2 0.9331 2 190.9 2356.1

Frequencies calculated at MBPT(2) level with DZP basis set. Covered by methane.

TABLE VI. Observed and ab Initio Calculated Vibrational Frequencies (cm-I), and the Statistical Intensity Distribution for Linear HBCBH Molecule Formed from the Reaction of Natural Isotopic Boron with CH&* stat wt*

-1 HBCBH u(aBCB)obs u(aBCBka1 -, u(sBH)obs u(sBH)cal

.--

1-10-12-10-1 1895.2 2034.2 c 3092.7

8

16

1-11-12-10-1 1883.9 2022.2

1-11-12-11-1 1872.0 2009.6

C

C

2978.6

2974.8

2

1-10-12-10-2 1805.9 1936.8 2182.2 2349.4

8

8

2-11-12-10-1 1805.1 1936.3

1-11-12-10-2 1793.4 1923.6

2311.6

2348.4

-1

32 1-11-12-11-2 1792.6 1922.8 2152.4 2310.5

8

16

2-10-12-10-2 1932.2 1855.4

2-11-12-10-2 1729.8 1853.4

2-11-12-11-2 1727.4 1851.2

C

C

C

2277.8

2258.9

2246.0

0 Frequencies calculated at MBPT(2) level with DZP basis set. Statistical weights for isotopic HBCBH moleculea formed from IlBlOB = 4 1 and H:D = 1:1, the underlined isotopic mixtures were observed only with the enriched boron-10experiment whereas all thers were observed with the natural isotopic boron with the expected intensity distributions. E Inactive or too weak to bc observed.

infrared spectrum. The 2561-cm-' band is tentatively assigned to CHsBH based on isotopic shifts. The boron 10/11 isotopic ratio 2575.3/2560.6 = 1.0057 is sfightly larger than the harmonic BH diatomic ratio (1.0041). The very small (0.2-0.4 cm-') carbon-13 shift showsminimalmixingwithcarbonand is in accord with the calculated shift (0.0 cm-l). The H/D ratio 2561/1935

= 1.324 is appropriate for a B-H stretching mode. The weak, sharp 2979.2-cm-l band which showed a boron-10 shift to 2979.5 cm-1 and a 7-cm-' carbon- 13 is probably due to a C-H stretching mode in species 1. On the other hand the sharp pairs of bands at 2522.8, 2506.4 cm-l and 2447.4, 2434.5 cm-I exhibit slightly larger boron 10/11 ratios (1.0068 to 1.0084) and slightly larger

6422 The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 N

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Figure 3. Infrared spectra in the 100&SOO-cm-l region for laser ablated boron atoms codeposited with Ar/CH2D2 = 100/2 samples at 12 K. (a) natural B (80% IlB, 20% log) + CHzD2, (b) loB (94%) + CHzDz.

carbon-13 shifts (0.7-1.3 cm-I). The larger boron 10/11 ratios identify antisymmetric -BH2 stretching modes and point to product molecules containing the -BHz functional group. Species 2 H2C-BH2. The 972.4, 1239.8, 1414.7, and 2522.8 cm-l bands belong to the major species produced from the laser evaporated boron atom reaction with methane. The 972.4- and 1239.8-cm-' bands show 11.5- and 15.4-cm-I boron isotopicshifts and only 2.6- and 0.6-cm-l carbon isotopic shifts, respectively. Both bands show large hydrogen/deuterium isotopic shifts of 175and 63 cm-l, respectively. T h e isotopic shifts characterize the motion of a BHz subgroup. The 1414.7-cm-l band, on the other hand, shows a 1.4-cm-' boron-10 shift but a 9.8-cm-I carbon13 shift; hence, a CH2 bending motion is identified. The 2522.8-cm-' band shows only a small 0.3-cm-I carbon isotopic shift but a larger 17.2-cm-I boron isotopic shift and decreases to 1923cm-1 on deuteration, which characterizes a B-H2 stretching mode. These observations suggest the rearrangement product HzCBH2, which is predicted to be more stable than CHsBH by 7.7 kcal/mol. Quantum chemical calculations at the SCF/DZP level reveal a planar Cb structure for HzCBH2. The most intense infrared absorptions involve the BH2 substituent. Ab initio frequency calculationsfor H2C-BH2 radical predict BH2 motions at 1073.3, 1328.6, and 2757.3 cm-I, and CH2 bending at 1582.9 cm-' as intense bands. The observed bands are converted to the experimental values of 972.4, 1239.8, 1414.7, and 2522.8 cm-I by scaling factors of 0.91,0.93,0.89, and 0.91, respectively,which are appropriate for SCF calculation^.^^ Furthermore, the ab initio predicted isotopic frequency values for each mode convert to the experimental data with the same scalingfactors. Finally, as shown in Figure 3, the 972.4-cm-l band shows a triplet mixed H/D isotopic pattern at 841.2, 887.7, 960.2 cm-1 for boron-11 (each withboron-locounterpartsat4:l relativeintensities)andat 855.0, 899.9, 971.8 cm-' for boron-10 with 1:4:1 intensity ratio when C H ~ DisZreacted with boron (Table 11). These isotopic patterns are in goad agreement with the calculated values for H~C-BDZ, HDC-BHD and D2C-BH2. Free rotation along the C-B single bond removes the stereo isomers for HDC-BHD and allows a statistical weight of 1:4:1 for the three mixed H2/D2 product species. In support of this observation, frequency calculations predict less than 2-cm-' separation between the strongest bands for HDCBHD and HDCBDH in this region. Species 3: HzC=BH. The 611.8-, 705.7-, 902.6-, 1469.7-, and 2724.6-cm-l bands also belong to a major product species. The 1469.7-cm-l band shows 19.5- and 26.4-cm-1 boron and carbon isotopic shifts, respectively. The frequency and isotopic shifts are appropriate for a C-B stretching vibration. The 902.6-cm-' band shows 10.8-cm-' boron isotopic shift, 3. I-cm-I

Hassanzadeh et al. carbon isotopic shift, and 163.5-~m-~ deuterium shift. These isotopic values suggest a B-H in-plane deformation motion. The 705.7-cm-1 band reveals only 0.8-cm-I boron isotopic shift, 2.5-cm-1 carbon isotopic shift and 168.6-cm-' deuterium shift and is appropriate for CH2 out-of-plane deformation. These functional group vibrations define a H2C=BH molecule. Quantum chemical calculations predict a planar C b structure for HzCBH. The HzCBH molecule has nine vibrational modes, and the five strongest modes are observed here. These include out-of-plane B-H deformation, out-of-plane CH2 deformation, in-plane B-H deformation, C-B stretching, and B-H stretching modes. Ab initio frequency calculations for H2C-BH at the MBPT(2) level with the DZP basis set predict the strongest bands at634.8,741.4,951.2,1513.4,and2924.9cm-I (TableIIQThese calculated fundamentals are converted to the observed 61 1.8-, 705.7-, 902.6-, 1469.7-, 2724.6-cm-' bands by scaling factors of 0.963,0.952,0.949,0.971, and0.937, respectively. The predicted isotopic frequencies are also converted to the observed values using the same scale factors for each normal mode, respectively. Finally, the 705.7- and 902.6-cm-' bands were split into 532.5-, 587.6-,611.9-,679.7-~m-~and749.9-, 845.3-, 871.2-, 877.0-~m-~ quartets, respectively, for boron-1 1 when the CHzD2 sample was used (Figure 3). Similar isotopic patterns were observed for the boron-locounterpart (TableIII). ThesefourmixedH/Disotopic H2QBH products follow from the three mixed H2/Dz isotopic HzC-BHz precursors in the CHzDz reaction. species 4: HO=BH, The 1475.3/2743.4/3248.8 cm-' absorptions increased 20% on photolysis and decreased 30% on annealing to 30 K and are assigned to the same product species. These bands appear in the diagnostic B=C, B-H, and C-H stretchingregions, respectively, and identify the H-BH radical. The 3248.8-cm-' band is in the region of v3, the antisymmetric C-H stretching fundamental, of H m H at 3289 cm-I in solid argon.I6 The 12.6-cm-l carbon-13 shift is appropriate for a C-H stretching fundamental, but the 1.1-cm-' boron- 10shift indicates a smaN amount of coupling to boron. The large deuterium shift to 2461.5 cm-l (3248.8/2464.3 = 1.318) is appropriate for a C-H stretching mode. The 2743.4-cm-l band, on the other hand, shows a 0.4-cm-' carbon- 13shift, a 16.8-cm-I boron- 10shift, and a deuterium shift to 2097.8 cm-l(2743.4/2097.8 = 1.308), which are indicativeof a B-H stretchingfundamentalwith somecoupling rocarbon. The 1475.3-m-l bandshowslarger (25.7cm-')carbon13and boron-10 (22.1 cm-l) shifts and is appropriate for a B=C stretching mode. The CHzD2 experiments provide additional support for the HCBH identification (Table IV). In addition to DCBD at 2463.3 and 2097.8 cm-I, two new sets of sharp bands were observed, at 3247.7 and 2116.2 cm-1 for HCBD and at 2722.3 and 2448.1 cm-l for DCBH. The lack of HCBH in the CH2D2 experiments is probably due to a kinetic isotope effect favoring H elimination from the initial excited CBH2D2 species. Finally, ab initio calculations(SCF)predict the HCBH radical tobelinear and toexhibit strong fundamentalsat 1618.9,2976.5, and 3554.4 c m - I (Table IV). It is important to note that scale factors of 0.92 f 0.01 relate the calculated and observed isotopic frequencies. This agreement between scaled calculated and observed isotopicfrequenciesc o n f i i thevibrational assignments and identification of the HCBH radical. The HC==BH radical spectrum exhibits matrix site splittings similar to acetylene.16 It is possible that a (HC=BH) (H2) complex from dissociation of energized species 1 gives rise to the extra HC=BH site absorptions. A comparison between SCF calculated frequencies for HO-BH radical and H-BH molecular anion" is of interest. The radical frequencies are higher in all cases except for the B-C stretch, which is of course B=C for the anion and B-C for the radical. The most noteworthy differenceis for the B-H stretching fundamental, which is about 200 em-' lower for the anion than for the radical. Species5 (HBCHBH). The894.3-, 1137.3-,and2609.7-cm-l bands belong to a minor product species that is formed at higher

The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 6423

Boron Atom Reactions with Methane

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F i e 4. Infrared spectra in the 190Q-S00-cm-1 region for laser ablated boron atoms codeposited with Ar/CH4 = 1OO/S sample at 12 K. (a) after 6-h sample codeposition, (b) after photolysis with the 254-nm mercury arc radiation for 30 min, and (c, d, e) after annealing at 20, 30, and 40 K, rcsptctively

.

SCHEME1 H

I

H-B,

trearrange

I

H- B

H-B=C=B-H 6

B/CH4 ratios. The 894.3-cm-1 band reveals 7.8-cm-' boron, 3.1-cm-l carbon, and 184.9-cm-l deuterium shifts which define a vibration containing a BH subgroup. The 1 137.3-cm-1 band shows 22.3-cm-1 boron and 17.9-cm-1 carbon isotopic shifts and defmeacarbon-boronvibration. The 1137.3-cm-1 bandis halfway between single and double carbon-boron bond values, based on an average B-C stretching mode for B(CH3)3.18 The 2609.7-cm-l band shifts of 2624.7 cm-l with boron-10 and to 2608.9 cm-l with carbon-13 which are indicative of a B-H stretching mode. The 2624.712609.7 = 1.005 74 ratio is appropriate for a single B-H motion. Thus, species5 contains a C-B-H subgroup. A possible molecule consistent with the above observation is the secondary reaction product 5 in Scheme I, which contains the C-B-H

\ C s B. - H

H/

subgroup. The present observations, however, do not provide a definitive identification of species 5. Species 6: HB=C=BH. The 1872.0-, 1883.9-, and 1895.2-cm-l bands with 16:8:1 relativeintensity for naturalboron andonly 1883.9- and 1895.2-cm-l bandswith 1:8 relativeintensity for enriched boron-10 are indicative of the participation of two equivalent boron atoms in the vibrational motion. Similar intensitydistributionpatterns at lower frequenciesfor the reactions of '3CH4 and CD4 with natural isotopicboron and enriched boron10suggest that this speciescontains boron, carbon, and hydrogen. Spectra from the reaction of mixtures of C& and 13CH4with boron are an exact superimposition of the spectra from reaction of pure CH4 with boron and pure 13Ch with boron which indicates

Hassanzadeh et al.

6424 The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 a species with only one carbon atom. The isotopic distribution pattern observed from the reaction of CHzD2 with natural isotopic boron (Table VI) shows that two equivalent hydrogen atoms remain in the product species. This follows from the observation of the pure H and pure D product species and one new mixed H/D species with double the intensity of the pure H and pure D species. The participation of onecarbon, two equivalent boron, and two equivalent hydrogen atoms in the vibrational mode identifies the product HBCBH.’ The ab initio calculationspredict a linear structure for HBCBH with vibrational frequencies listed in Tables V and VI. The calculated antisymmetric BCB stretching v(aBCB) and BH stretching v(aBH) frequencies for all isotopes are converted to theobserved frequency values by constant scaling factors of 0.932 and 0.930, respectively. An average 0.9319 scale factor (ratio) for the six hydrogen isotopic values of v(aBCB) predicts frequencies within f0.6 cm-l of the observed values. The three deuterium isotopic values require a slightly higher scale factor owing to slight differences in anharmonicity for the H and D isotopic species. Parent molecule absorptions interfere with the antisymmetric BH stretching (v(aBH)) mode in the lZCH4and 13CH4 experiments and these bands were observed only in the CD4experiments. The excellent agreement between the predicted and observed frequencies for isotopic molecules (Tables V and VI), as demonstrated by the constant scale factor required to relate the calculated and observed frequencies, confirms the identification of H-B=C=B-H. Isotopic substitutions change the relative intensities of the antisymmetric B-C-B stretching, the symmetricB-H stretching, and the antisymmetric B-H stretching modes. For HBCBH, the antisymmetric B-C-B stretching is the strongest absorption with an intensity of at least four times more than the antisymmetric B-H mode and the symmetric B-H mode is inactive. For DBCBD, the antisymmetric E D stretching is stronger than the antisymmetric B-C-B mode and the symmetric E D stretching remains inactive. For DBCBH, the symmetric B-H(D) stretching mode is as intense as the antisymmetric B-C-B stretching mode, and the antisymmetric B-H stretching mode is weaker by a factor of 3-4. These calculationspredict a quartet pattern for symmetric B-H(D) stretching (v(sBH)) absorptions for natural boron and mixed hydrogen/deuterium isotopes with the mixed isotopicboron molecules being resolved from the pure boron- 11 and pure boron10 isotopic molecules by less than a wavenumber. The statistical intensity distribution pattern with natural isotopic boron is 32: 8:8:2, and the absorptions observed at 2152.4 and 2182.2 cm-l in the experiments with natural boron and enriched boron-10 isotope, respectively, are in good agreement with the calculated values and the intensity distributions. Other Possible Products. The major unidentified bands near 2450 cm-l are clearly due to an antisymmetric -BH2 stretching vibration. Although the bands near 2450 cm-l behave like the HzC-BHz bands near 2520 cm-l, the former are due to a different -BH2 containingspecies. Calculations predict the strongest mode for HzC-BH~anion at 2452 cm-l, some 300 cm-l lower than predicted for HzC-BHz radical so the anion possibility is ruled out. Another possibility is the rearranged secondary reaction product HzBCHBH. Reaction Mechanism. Insertion of a boron atom into a C-H bond in methane is believed to proceed without activation energylg to give the CH3BH radical with at least 50 kcal/mol of excess internal energy.12 Quenching of this intemal energy by the matrix before rearrangement or decomposition is necessary for trapping the CH3BH radical, species 1, in Scheme I. However, the major reaction products characterized here are HzC-BHz, a 7.7 kcal/ mol more stable rearrangement product of CHSBH, and H2C=BH and HC=BH, which are produced by hydrogen elimination from CH3BH. Another hydrogen atom elimination product CH3B is calculated to be less stable than HzCBH by 29 kcal/mol. Although CH3B was not observed here, it wouid be difficult to detect without the BH subgroupand with CH4 masking >

the strongest calculated band at 1433 cm-l. The reaction of a second B atom with the same CH4 precursor gives H-B=C=B-H with two equivalent boron atoms. Additional minor reactions occur but other organoborane products cannot be identified with confidence. It is clear that atomic boron is extremely reactive with methane.

CoocIusions Pulsed laser evaporated boron atoms react readily with methane to produce a variety of new organoborane products, which can be trapped in solid argon for FTIR analysis. The substitution of log, 13C, and D and isotopic mixtures enables the observed vibrational motions to be characterized. Ab initio quantum chemical calculations predict structures and isotopic frequencies for the major product molecules. With appropriate scale factors the calculated isotopic frequencies match the observed values. Thus, matrix infrared spectroscopy and quantum chemistry cooperate to identify the new organoborane species HzC-BHz, HzC=BH, HC-BH, and H-B=C=B-H. This work characterizes three new species with C=B double bonds, which are small, simplemolecules compared to the carbonboron double bond species with large substituents described in the Typical C - B double bond lengths fall in the 1.39-1.44-A range from experimentzO-z2and theoretical calcul a t i o n ~ ? ~The . ~ present SCF/DZPcalculated C=B bond lengths for HBCBH (1.356 A), HCBH (1.350 A), and HzCBH (1.384 A) are in this range and slightly longer than the calculated triple bond value (1.32-1.34 A).17 These simple organoboranes have structural similarities with the simple hydrocarbons allene, ethylene, and acetylene. Acknowledgment. We gratefully acknowledge the financial support from N.S.F. Grant CHE 91-22556 and C.N.R.S./N.S.F. Grant 91N92/0072. References aad Notes (1) Burkholder, T. R.; Andrews, L. J. Chem. Phys. 1991, 95, 8697. (2) Andrews, L.; Burkholder, T. R. J. Phys. Chem. 1991,95, 8554. (3) Hassanzadeh, P.; Andrews, L. J. Phys. Chem. 1992,96, 9177. (4) Andrews, L.; Burkholder, T. R.; Yustein, J. T. J . Phys. Chem. 1992, 96, 10182. (5) Burkholder, T. R.; Yustein, J. T.; Andrews, L. J. Phys. Chem. 1992, 96, 10189. (6) Jeong, G. H.; Boucher, R.; Klabunde, K. J. J . Am. Chem. Soc. 1990, I 12,3332. (7) Hassanzadeh,P.; Andrews, L. J . Am. Chem. Soc. 1992,114,9239. (8) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1967, 47, 5146. (9) (a) Hansen, G. E.; Dennison, D. M. J. Chem. Phys. 1952,20,313. (b) Davis, S. R.; Andrews, L. J . Am. Chem. Soc. 1987,109,4769. (10) Milligan, D. E.; Jacox, M.E. J. Mol. Spectrosc. 1973, 46, 460. Andrews, L.; Auk, B. A,; Grzybowski, J. M.; Allen, R. 0. J. Chem. Phys. 1975, 62, 2461. (1 1) Stanton, J. F.; Gauss, J.; Watts, J. D.; Lauderdale, W. J.; Bartlett, R. J. ACES 11, Quantum Theory Project, University of Florida, Gainsville, FL,1992. (12) Hannachi, Y.; Hassanzadeh, P.; Andrews, L., to be publiihcd. (13) Burkholder, T. R.; Andrews, L. J. Phys. Chem. 1992, 96, 10195. (14) Smith, W. L.; Mills, I. M. J . Chem. Phys. 1961,41, 1479. (15) Hehre, W. J.; Radom, L.; Schleyer, P. von R.; Pople, J. A. Ab Initfo Molecular Orbital Theory; Wiley: New York, 1986. (16) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J. Phys. Chem. 1982,86, 3374. (17) Alberta, I. L.;Schaefer 111, H. F. Chem. Phys. Letts. 1990,165,250. (18) Bccher, H. J.; Bramsiepc, F. Spectroehim. Acta 1979, 3 5 4 53. (19) Lebrilla, C. B.; Maier, W. F. Chem. Phys. Lett. 1984, Z05, 183. (20) Klusic, H.; Bemdt, A. Angew. Chem., Int. Ed. Engl. 1983,22,877. (21) Glascr,E.;Hanecher,E.;Noth,H.;Wager,E. Chem.Ber. 1987,120, 659. (22) Boese, R.; Paetzold, P.; Tapper, A. Chem. Ber. 1987, 120, 1069. (23) Cook,C. M.; Allen, L. C. Organometallics 1982, I , 246. (24) Frenking, G.; Schaefer 111, H. F. Chem. Phys. Lett. 1984,109,521.