Infrared Spectra of Boron-Ammonia Reaction Products in Solid Argon

May 23, 1995 - Porter photolyzed H3BNH3 in solid argon and observed the three strongest ... absorptions along with the band profiles of isotopic mixtu...
0 downloads 0 Views 1MB Size
Download PDF
J. Phys. Chem. 1995,99, 13839-13849

13839

Infrared Spectra of Boron Atom-Ammonia Reaction Products in Solid Argon Craig A. Thompson and Lester Andrews* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901

Jan M. L. Martint?*and J. El-Yazalt Department SBG, Limburgs Universitair Centrum, Universitaire Campus, B-3590 Diepenbeek, Belgium, and Department of Chemistry and Institute for Materials Science, University of Antwerp, Universiteitsplein 1, B-2610 Wilrtjk, Belgium Received: May 23, 1995; In Final Form: July 3, 1995@

Pulsed laser ablated boron atoms were reacted with ammonia during condensation at 10 K with excess argon. Reaction products were identified from matrix infrared spectra by isotopic substitution ('OB, I 'B, " 3 3 , N D 3 ) , photolysis behavior, and comparison to ab initio calculated frequencies. The major primary reaction is insertion to form the [HBNH2]* intermediate, which either dissociates to BH and NH2, eliminates H or H2 to give products, or relaxes in the matrix. The major primary product is iminoborane, HBNH, a reactive molecule isoelectronic with acetylene, and a minor primary product is the photosensitive BNH radical. A second boron atom can react with primary products, and the major secondary reaction products are cyclic B2N radical and linear BNBH. These studies show that atomic boron reacts readily with ammonia.

Introduction Laser ablation matrix isolation spectroscopy has proven to be a valuable tool for studying boron atom reactions with small molecules such as 02, N2, and H2.1-4 In addition to producing novel molecules, this method gives valuable information for identifying mechanistic pathways. Previous boron-methane experiments showed the primary reaction of to be insertion of atomic boron into a C-H bond followed by dehydrogenation to give novel HBCH2 and HBCH products,5s6 and reactions with H20 followed a like mechanism to give the HBO and BO molecules.' Reaction of a second boron atom produced the new molecules HBCBH and BOB. The analogous boron atom reaction with ammonia was done to test this mechanism and to synthesize new reactive boron-nitrogen-hydrogen intermediate species. Since the average N-H bond energy is less than the average C-H bond energy, it is expected that ammonia will be even more reactive with boron than methane. Boron atoms react with acetylene to give both addition and insertion products so secondary reactions to give diboron species are also expected in this s y ~ t e m . ~ ? ~ The anticipated major product molecule iminoborane (HBNH) is isoelectronic with acetylene and has been characterized in two experimental and several theoretical studies.I0-l6 Lory and Porter photolyzed H3BNH3 in solid argon and observed the three strongest infrared absorptions of HBNH.'O The B-N stretching mode has been measured at 1786.2 cm-' for HBNH in a B2Hd NH3 discharge using diode laser spectroscopy and the HBNH lifetime found to be a few hundred milliseconds." The related BNH radical has been characterized by matrix ESR spectroscopy as a product of the laser ablated boron atom reaction with ammonia." Unfortunately, the reported infrared detectionlo of this molecule is not correct. The reactive HBNH molecule is a precursor to the six-membered ring compound borazine, which is isoelectronic with benzene. The reactivity of iminoboranes and their stabilization by substituents have been reviewed by Paetzold.I8 Products expected from the reaction of atomic boron

' Limburgs Universitair Centrum. * University of Antwerp.

@Abstractpublished in Advance ACS Abstracts, August 1, 1995.

0022-3654/95/2099-13839$09.00/0

with ammonia are of interest, in part, since these molecules will be isoelectronic with analogous carbon species. Furthermore, there is considerable recent interest in new forms of boron nitride grown by chemical m e a n ~ . ' ~ - ~ l Here we present a series of matrix infrared experiments with laser ablated boron and ammonia. Relatively strong product absorptions along with the band profiles of isotopic mixtures, photolysis behavior, and quantum chemical calculations of product isotopic vibrational frequencies have facilitated assignment of the remaining two modes of HBNH, as well as the infrared spectrum of BNH. The insertion reaction product HBNH2 and a new secondary reaction product molecule BNBH were observed and characterized as well. Experimental Section The apparatus for pulsed laser-ablation matrix infrared spectroscopy has been described earlier.'-2 Mixtures of k N H 3 (0.5-1%) were codeposited at 2 mmoyh for 4-8 h onto a 10 f 1 K substrate with laser ablated boron atoms using the first harmonic (1064 nm) of a Nd:YAG laser (40-80 mllpulse) focused onto a rotating boron target. Natural abundance NH3 samples (Matheson) were employed as well as isotopically enriched I 5 N H 3 and ND3 (MSD Isotopes). Natural abundance natB(Aldrich, 80.4% I'B, 19.6% log) and isotopically enriched boron samples (93.8% 'OB and 97.5% IlB, Eagle Pitcher Industries) were used to prepare targets for laser ablation. Following deposition, matrix samples were subjected to broad band mercury arc photolysis (Philips 175 W) and annealing, and more spectra were recorded at 0.5 cm-' resolution and f 0 . 2 cm-' accuracy on a Nicolet 750 Fourier transform infrared (FlTR) spectrometer using a liquid N2 cooled mercury cadmium telluride (MCT) detector. Results Infrared spectra for reaction products of different isotopic modifications of boron and ammonia will be reported. Ammonia precursor absorptions were strong in these experiments and appeared as reported previously22for concentrations in the argon/ammonia = 100/1 to 200/1 range where monomer 0 1995 American Chemical Society

13840 J. Phys. Chem., Vol. 99, No. 38, 1995

Thompson et al.

P

1.0

O.Oe4

A

h

n

I/

I

> L F

0.0 38W

37w

3750

3650

-I

I

2840

3800

I 2820

+

+

+

+

I 2760

Figure 2. Infrared spectra in the 2850-2750 cm-' upper B-H stretching region for pulsed laser ablated B atoms codeposited with argodammonia samples described in Figure 1. Lines connect 'OB-"B counterparts. D

0.051

dominated, but dimer and trimer were also observed. In addition to new product absorptions weak BO, B02, (B0)2, BOz-, and HBO bands'$7were present owing to trace amounts of boron oxide on the target surface. loB 14NH3. Figure l a shows the spectrum for the best 'OB experiment in the hW stretching region using laser power on the lower end of the range employed here. Two strong bands at 3712.6 and 3701.9 cm-' (marked A) gave similar (+20%) growth on photolysis and are matrix sites of the same product species. A weaker band at 3676.3 cm-' (marked C) was accompanied by a band centered at 3683.5 cm-' (marked B,C). Decrease on photolysis revealed this band to be the overlap of absorptions, a matrix site of the C band and a new species B. A weak band at 3743.9 cm-' was also associated with C by substantial (-90%) decrease on photolysis. Two absorptions at 3598.6, 3588.4 cm-' (marked D) are assigned to matrix sites of another new species. In addition, a weak band at 3125.8 cm-' is due to NH23,and a new weak band was observed at 3088.6 cm-I. Figure 2a shows the spectrum for the same experiment in the high BH stretching region. A broad absorption at 2826.9 cm-' is marked E, and two sharper bands at 2808.9 and 2796.9 cm-' are marked F and A. Figure 3a shows the spectrum for the same experiment in the low BH stretching region; new bands at 2438.6, 2430.6, and 2405.3 cm-' (marked D and F) were observed. The boron-nitrogen stretching region is shown in Figure 4a for the same 1°B experiment discussed above. A new band appeared at 1894.5 cm-' (marked F) and the strongest product band at 1876.4 cm-' (marked E). A weaker band at 1879.9 cm-' (marked C) gave isotopic counterparts and tracked with the C band (-90%) in the NH stretching region on photolysis. Two sharp bands at 1826.3 and 1819.9 cm-' tracked with the

I 2780

Wavenumbers (cm.')

Wavenumbers (cm.')

Figure 1. Infrared spectra in the 3800-3550 cm-l N-H stretching region for pulsed laser ablated B atoms codeposited with argon/ ammonia samples. (a) 1°B I4NH3, (b) 'OB ''NH#5NHs = 5/4, (c) ""B I4NH3, and (d) natB I4NHdl5NH3= 2/3. Lines connect I4NI5N counterparts.

I

2800

+

A b S 0

r b

a n C

e

0.v

1 LA

7 ll/l

F

0.0 I

2440

I 2420

2400

2380

Wavenumbers (cm.')

Figure 3. Infrared spectra in the 2450-2370 cm-I lower B-H stretching region for samples described in Figure 1. Lines connect 'OB "B counterparts.

A bands in the upper regions (+20%) on photolysis. The radical BNB was also observed at 1769.1 cm-I. New absorptions were also observed in the 1700-800 cm-I region and are listed in Table 1. A band at 1556.9cm-I tracked

Boron-Ammonia Reaction Products in Solid AI

J. Phys. Chem., Vol. 99, No. 38, 1995 13841

+

TABLE 1: Absorptions (cm-l) for "B JNH3 (x = 10,ll; y = 14,15) Reaction Products in Solid Argon assignment 10/14/1 10/15/1 11/14/1 11/15/1 pa

E

0.7

I

E 0.6

-

BNB

-1

d

IE

0.5

F

A b S

0.4

A

1I

, I

?

NH2 X-NH2 NH X-NH BNBH (E) HiBNBH (F) HBNH (A) H2BNBH (F) HBNH2 (D) HiBNBH (F) (H2)BH BH

r b a I

0.3

E

E

e 0.2

HBNH (A) HBNH (A) HNBBNH (B) BNH (C) BNH (C) BNH (C) HBNH2 (D) HBNH2 (D)

F

0.1

3712.6 3701.9 3683.0 3743.9 3683.5 3676.3 3598.6 3588.4 3544 3216.5 3183.6 3125.8 3088.6 2826.9 2808.9 2796.9 2438.6 2430.6 2405.3 2306.9 2268.3

3690.3 3673.3 3705.1 3669.2 3662.4 3589.4 3579.6 3532 3212.1 3178.8 31 19.2 3082.3 2825.9 2807.8 2796.2 2438.1 2429.9 2404.9 2306.9 2268.3

? a 0.0 I

1

1850

3

I

I

1800

1750

Wavenumbers (cm.')

Figure 4. Infrared spectra in the 1900-1720 cm-I B-N stretching region for samples described in Figure 1. Lines connect boron isotopic multiplets. with D, a weak band at 1538.8 cm-' with G, and a weak band at 1136.4 cm-' with E on photolysis. Absorptions were o b ~ e r v e dfor ~ ~NH;? . ~ ~at 1494.9 cm-' and for HNNH26at 1282.5 cm-I. Figure 5a shows the lowest region for the 'OB experiment discussed above. A weak absorption at 754.5 cm-' is marked E. Bands at 683.1 and 678.1 cm-' (marked A) and at 608.7 cm-' (marked D) are associated with the same species observed in the upper regions. A weak photosensitive 596.5 cm-' band is marked G. An intense band at 464.8 cm-' with a site splitting at 462.4 cm-I (marked A) and weaker bands at 433.1 cm-' (marked C) and 413.6 cm-I (marked B) tracked with absorptions of the upper region. Photolysis results were helpful in grouping absorptions that belonged to the same species. The result of broad-band photolysis for each band is given in Table 1. Note that, in general, all of the bands A and B increased -20% on photolysis while bands C were -90% destroyed. This different behavior was helpful in the separation of the B and C bands in the 36753745 cm-' region of Figure la. In contrast, D products increased 20%, E products increased 5%, and F bands increased 20%. In addition to B NH3 product bands, reaction products identified in B N2 and B f HZ experiments were also observed. Two bands at 2306.9 and 2268.3 cm-' have been identified as (H2)BH and BH, respectively, and BH3 was observed at 2601.5 cm-' ( A = 0.002) and at 1140.5 cm-I. Strong bands at 2068.5 and 901.5 were also observed for B2N radical. Isotopic counterparts observed for the N2 and Hz reaction products are listed in Table 1 in agreement with previous observation^.^,^ 'OB 14NHf15NH3. Exchange of adsorbed I4NH3in sample containers resulted in isotopic ammonia samples that ranged from approximately 1:2 to 2:l (I4NH3:l5NH3). The signature 14:15 ratios (easily obtained from ammonia bands) assisted in

+

+

+

B2N ?

HzBNBH (F) BNH (C) HNBBNH (B) BNBH (E) HBNH (A) HBNH (A) BNB HBNH2 (D) BNH2 (GI X-NH2 NHz ?

HNNH ?

BNBH (E) B2N BNBH(E) HBNH (A) HBNH (A) HBNH2 (D) BNH2 (GI ?

HBNH (A) HBNH (A) BNH (C) HNBBNH (B) a

2068.5 1936.0 1894.5 1879.9 1877.7 1876.4 1826.3 1819.9 1769.1 1556.9 1538.8 1513.3 1494.9 1425.1 1282.5 1221.4 1136.4 901.5 754.5 683.1 678.1 608.7 596.5 540.6 464.8 462.4 433.1 413.7

2048.0 1904.5 1861.6 1858.3 1855.2 1843.3 1804.6 1798.3 1735.1 1551.7 1527.1 1509.3 1490.9 1421.5 1279.6 1216.9 1134.2 881.8 753.5 681.5 676.4 604.6 592.5 539.0 462.2 431.1 41 1.7

37 10.5 3700.5 3682.8 3693.7 3686.3 3598.4 3588.4 3544 3216.5 3183.6 3125.8 3088.4 2805.6 2788.3 2775.3 2426.5 2418.2 2393.2 2298.3 2259.4 2034 1998.0 1894.5 1864.2 1829.6 1827.4 1846.3 1788.7 1782.8 1736.4 1556.4 1533.2 1513.3 1494.8 1388.6 1282.5 1221.4 1089.3 882.1 746.2 676.9 671.6 608.5 596.0 538.0 463.3 461.0 432.5 413.3

3689.6 3673.3 3681.3 3589.2 3579.6 3532 3212.1 3178.8 3119.2 3082.2 2804.6 2787.0 2774.4 2426.0 2417.4 2392.8 2298.3 2259.4 2009 1977.0 1859.5 1830.4 1806.7 1804.9 1812.2 1766.2 1760.3 1705.1 1551.3 1521.9 1509.3 1490.8 1884.7 1279.6 1216.9 1087.2 862.1 745.2 675.2 669.9 604.4 592.0 536.4 460.8 430.4 411.1

+20

+20 +5 -90 -90 -90 1-20 +20 +5 -20 0 -10 +5

+5 +20 +20 $20 $20 +20 +20 +20 0 +10 +100 +20 -90 +5 +5 f20 130 -100 +20 -90 0 -20 -0 +30 0 +5 +10 f50 f20 +20 +20 -90 +20

$20 +20 -90

+5

Percent change on broad band mercury arc photolysis.

the recognition of product bands. Figure lb shows the spectrum for 1°B codeposited with a 5:4 enriched l4NH3:I5NH3sample in argon using laser power on the higher end of the range. Careful inspection reveals the most intense A band isotopic counterpart at 3690.3 cm-' (marked A) as well as the two bands observed earlier. Note that the higher frequency nitrogen-15 site band is coincident with the lower frequency nitrogen-14 site band slightly broadening the band profile. Site bands of B and C are more resolved in the I5N counterparts. Two sites of the C band are present again at 3683.5 and 3676.3 cm-I. In addition, two new C bands are observed at 3669.2 and 3662.4 cm-I for I5N substitution. A weak isotopic C analog was present at 3705.1 cm-I. The B band is again observed at 3682.9 cm-' for I4N (based on photolysis behavior) and at 3673.3 cm-' for 15N. In the lower region new counterpart bands at 3589.4 and 3579.6 cm-I were observed. Figure 2b shows the high BH stretching for the same experiment. The product band E in this region has shifted

13842 J. Phys. Chem., Vol. 99,No. 38, 1995

0.8

; I s

Thompson et al.

1

0.7

a

0.2

+

-"I-,

1

1

1

0.1

0.0 800

700

600

500

Wavenumbers (cm")

Figure 5. Infrared spectra in the 800-400 cm-] bending region for samples described in Figure 1.

slightly to 2825.9 cm-I. Similarly, the F and A bands shifted to 2807.8 and 2796.2 cm-I. In the low BH stretching regions nitrogen isotopes also shifted product absorptions only slightly as given in Table 1. In the BN stretching region isotopic counterparts were observed for five band systems. Figure 4b shows a sharp band at 1843.3 cm-' marked E. Note the 5:4 I4NH3:I5NH3ratio is shown by the I4E:l5Eband intensities. An isotopic counterpart of the F band was also observed at 1861.6 cm-I. Isotopic counterparts for the A and C bands of Figure 4b were also observed at 1804.6 and 1798.3 cm-' (HloBISNH)and at 1858.3 cm-I. The BI5NB isotopic counterpart is also shown in Figure 4b. In the low region shown in Figure 5c I5N isotopic counterparts were observed for the E species at 753.5 cm-' as well as the A species at 681.5 and 676.4 cm-I. The most intense band of the lower region belonging to the A band system is partially resolved at 462.2 cm-I. Nitrogen-15 counterparts for C and B are also observed at 431.1 and 411.7 cm-'. Photolysis of the samples again assisted in the grouping of band systems. l*B NH3 and 14NHf15NH3. Experiments with "B provided isotopic counterparts for the above bands, as well as an interesting comparison to natBexperiments for species that contain more than one boron atom. Isotopic counterparts observed for product bands in each region are listed in Table 1; however, the spectra for pure IlB experiments are not shown. The most notable difference from the natural abundance spectra shown was the absence of 1°B and mixed isotopic boron multiplets. natB NH3. Natural boron experiments are shown in Figures lc-5c using laser power on the lower end of the range. As expected, the products in the NH stretching region showed little shift with natB. Figure I C shows product bands at 3710.5 and 3700.5 cm-I; note the band contour revealing unresolved 'OB bands. Photolysis behavior allowed identification of the species B counterpart band at 3682.8 cm-I. In contrast Fermi resonance

+

+

coupling (to be discussed later) gave two blue-shifted boron11 band counterparts for C at higher frequencies than the boron10 absorptions. Figure 2c shows the same experiment in the high BH stretching region. Note that unlike the NH stretching region of Figure IC absorptions shifted significantly and that 1:4 boron isotopic doublets can be identified. New bands were observed at 2805.6,2788.3, and 2775.3 cm-I. Similarly, relatively large shifts were observed for the bands of the low BH stretching region. Figure 3c shows new isotopic bands at 2426.5, 2418.2, and 2393.2 cm-I. Isotopic bands for IlB were observed in the BN stretching and are provided in Table 1. Notice, however, for the natB N H 3 experiment shown in Figure 4c two boron isotopic quartets were observed. Bands at 1876.4, 1871.8, 1851.5, and 1846.3 cm-' (collectively marked E) and at 1894.5, 1891.0, 1868.2, and 1864.2 cm-' (collectively marked F) revealed boron isotopic quartets belonging to two different species. Bands at 1788.7 and 1782.8 cm-' (H"BI4NH) were observed along with Hl0Bl4NH bands from Figure la present here in natural abundance. Isotopic counterparts were also observed for bands in the lower region. Figure 4c shows a weak band at 746.2 cm-' and two weak bands at 676.9 and 671.6 cm-I. A small shift in the lowest A bands was observed at 463.3 and 461.0 cm-I. Similar shifts were observed for B and C bands at 432.5 and 413.3 cm-I. "'B 14NHf15NH3. Nitrogen-15 counterparts for the boron11 product absorptions are illustrated for 2:3 I4NH3/l5NH3 sample in Figures 1-4d using laser power on the higher end of the range. The important information with the natural boron sample is observation of boron isotopic multiplets. Note in Figure 4d boron isotopic doublets for the A and C bands for both I5N and I4N isotopic ammonia reagents. Also note the 1:4:4:16 boron isotopic quartets for I5N and I4N counterparts of E and F. B ND3. Deuterated ammonia was handled in a special manifold used exclusively for ND3 to minimize D/H exchange contamination; even so the major H product bands were detected. Deuterium isotope bands typically show large shifts and weaker isotopic intensities. Counterpart bands were identified by their profiles, photolytic behavior and isotopic shifts. To identify deuterium isotopic product bands, reactions with boron isotopes were carried out. Figure 6a shows the region expected for ND stretches for 1°B reactions with ND3. Notice the similar band pattern for the two bands at 2802.0 and 2794.6 cm-I, which are isotopic analogs of the A bands in Figure la. Two weak bands at 2774.8 and 2780.6 cm-' were destroyed on photolysis and are isotopic counterparts of the C bands in Figure la. Weak bands at 2691 and 2682.1 cm-' were observed to be isotopic analogs of the D bands in Figure la. Deuterium counterparts for the E and A bands were identified at 2263.2 and 2231.3 cm-' (not shown) and are listed in Table 2. In the BN stretching region shown in Figure 7a, a weak band at 1825.1 cm-' is the isotopic analog of band C in Figure 4a. Similarly, a strong band at 1729.0 cm-' is the deuterium isotopic counterpart of the band E in Figure 4. Diatomic BD was observed at 1695.2 cm-I. Weak isotopic counterparts of the two A bands were observed at 1616.2 and 1610 cm-I. The ND:! radical24 was observed at 1106.8 cm-I. The hydrogen product bands in the 413-464 cm-' region were deuterium shifted below the 400 cm-I limit of the instrument. Figure 6b shows the spectrum for IIB ND3; boron isotopic counterparts of the bands seen in the 1°B ND3 work were observed for all species discussed above. Two bands at 2797.0 and 2789.8 cm-' are the boron-11 deuterium counterparts of

+

+

+

+

Boron-Ammonia Reaction Products in Solid Ar

J. Phys. Chem., Vol. 99, No. 38, 1995 13843 E

m

0.25

0.20

A b s

uvvvd

0.15

c n

o.lol, 1

--

b a n C

e

e

I

-

I

0.10

0.05

A

C

0.0

I 2800

I 2750

I 2700

11

I

Wavenumbers (cm.')

region for pulsed laser ablated B atoms codeposited with argon/ deuteriated ammonia samples. (a) loB ND3, (b) IlB ND3, and (c) natB ND3.

+

+

+

TABLE 2: Frequencies (cm-l) for XB NzH3 (x = 10,ll; z = 1,2) Reaction Products in Solid Argon assignment

10/14/1

11/14/1

10/14/2

11/14/2

HBNH (A) HBNH (A) BNH ( C ) BNH ( C ) HBNH? (D) HBNH2 (D) NH BNBH (E) HBNH (A) HBNH (A) BH BNH (C) BNBH (E) HBNH (A) HBNH (A) NHz BNBH (E)

3712.6 3701.9 3683.5 3676.3 3598.6 3588.4 3125.8 2826.9 2808.9 2796.9 2268.3 1879.9 1876.4 1826.3 1819.9 1494.9 1136.4

3710.5 3700.5 3693.7 3686.3 3598.4 3588.4 3125.8 2805.6 2788.3 2775.3 2259.4 1829.6 1846.3 1788.7 1782.8 1494.9 1089.3

2802.0 2794.6 2780.6 2774.8 269 1 2682.1 2313.0 2263.2

2797.0 2789.8 2777.4 2772.1 2685.9 2678.7 2313.0 2218.7 2224.8 2189.3 1782.8 1776.1 1717.6 1600.6 1595 1106.8

2231.3 1695.2 1825.1 1729.0 1616.2 1610 1106.8 1062.4

the A bands. Similarly, a band at 2772.1 cm-' is marked C. Two isotopic bands at 2685.9 and 2678.7 cm-I are marked D. In the BN stretching region the boron-11 deuterium counterpart of band C is observed at 1776.1 cm-I again based on photolysis behavior. Similarly, the E band was observed at 1717.6 cm-'. Experiments with natB ND3 confirmed isotopic counterpart assignments for bands comprised of boron isotopic multiplets as shown in Figure 6c. Notice that unlike the NH stretching region shown in Figure la, the boron isotopic shift is resolved even thought the mode is predominantly an ND stretch. Only the pure isotopic bands for C and D are observed; however, Figure 7c shows a quartet at 1729.0, 1723.8-1723.2 (unresolved), 1717.6 cm-' for the E band. In addition, a 1:4 doublet was observed at 2231.3, 2189.3 cm-l, and three new bands at 2262.3, 2219.7, and 2218.6 cm-l show 4:4:16 relative intensi-

+

I

I

1750

1700

Wavenumbers (cm.')

Figure 6. Infrared spectra in the 2830-2650 cm-I B-D stretching

+

I

18W

Figure 7. Infrared spectra in the 1850-1670 cm-' B-N stretching region for argodND3 samples described in Figure 6. Lines connect boron isotopic multiplets. ties. The 1:8:16 triplet for B2N at 901.5, 891.1, 882.2 cm-I (A = 0.7) was very strong in this experiment. Calculations Ab initio calculations were completed for a variety of expected product molecules. Self-consistent-field (SCF) calculations were done using the 6-31 lG* basis set with the ACES 11 package.27 All SCF calculations were restricted; however, for singlet states frequency calculations were solved analytically from the second derivatives of the energy, whereas for multiplet states numeric differentiation of an analytic gradient was utilized. For smaller molecules complete active space self-consistentfield energy calculations (CASSCF) were done using the SIRIUS/ABACUS program system.28 When possible, full valence active spaces were used: where this was no longer technically feasible, the nitrogen 2s orbitals were constrained to be doubly occupied. Dunning's consistent polarized valence double-zeta (cc-pVTZ) basis set vas used.29 It has been shown that this is perhaps the smallest acceptable basis set for correlated frequency calculations and that the basis set incompleteness error with this basis set is about 26 cm-' on average, Le., smaller than the error caused by neglect of extemal correlation in CASSCF.30 Energy calculations for several product molecules are listed in Table 3. The remaining correlated calculations were carried out at the CCSD(T) level3' using the ACES 11program system.27 This method is known to yield energies32 (and harmonic frequen~ies)~~ close to an exact solution within the given finite basis set, as long as no pathological nondynamical correlation effects are present. Ab initio calculations have assisted in the assignment of bands that might otherwise be indistinguishable due to their close proximity or similar behaviors with photolysis and annealing; furthermore, the identification of probable reaction pathways from previous has significantly reduced the number of

13844 J. Phys. Chem., Vol. 99, No. 38, 1995

Thompson et al.

TABLE 3: Energy and Geometry Calculations for B N H Species Full Valence CASSCF Level with cc-pVDZ Basis Set species

state

energy (au)

NBH BNH BNH2 HBNH HBNH2

2l-I 2Z+ ‘AI 2A’

-79.748 -79.784 -80.380 -80.445 -80.984

BNBHb

‘Z+

‘Z+

geometry (A, deg) ~ N B =

1.325, QH = 1.174 1.253, = 1.007 @N=1.401, mH = 1.024, & N H = 115 rHB= 1.193, r B N = 1.252, mH= 1.005 & I B = 1.223, rBN = 1.398, INH = 1.024, mH = 1.020“ OHBN = 123.5, & N H = 123.2, &NH = 113.7 rBN= 1.410, rNB= 1.276, rBH= 1.170

68 08 26 16 37

~ B N=

- 104.525 06

RHF and ROHF Levels with 6-31 1G* Basis Set species

state

energy (au)

BNH BBNH B2NH BNBH‘ HBNBH

2Zf

2A’

-79.662 -104.285 -104.301 -104.381 -104.974

H2BNBH

]AI

-105.610 82

HNBBNH

’Z+

- 159.484 22

lZ+ ‘AI

lZ+

14 57 30 53 34

geometry (A, deg) 1.219, “H= 0.980 Q B = 1.759, ~ B N = 1.223, “ ~ = 0 . 9 8 1 TBB = 2.046, rBN= 1.405, “H= 1.009,@BNH=114.9 Q N = 1.369, “B= 1.248, T B H = 1.166 rHB= 1.193, rBN= 1.385, mB = 1.240, Q H = 1.167, & B N = @BNs = 176.2, 6 B N H = 179.8 rHB = 1.196, TBN= 1.406, rNB= 1.237, rBH= 1.168 &BH= 119.8 THN = 0.980, GVB = 1.220, ~ B = B 1.650 ~BN=

117.1,

a H atom cis to lone (J electron. BNBH was partial valence calculation excluding the lowest valence orbital and its antibonding counterpart = 1.386, m~ = 1.270, and from complete active space leaving a “10 in 11” calculation. MBPT(2)/6-311G* calulation gave -104.739 18 au, aN rBH = 1.172.

molecules for which ab initio calculations are required. The primary reaction pathway for boron-methane reactions was found to be insertion of boron into a C-H bond followed by the ejection of hydrogen atoms or H2 molecule^.^ Accordingly, the products of reactions of boron with ammonia were expected to be similar insertion reactions to produce HBNH2 followed by decomposition to give HBNH, BNH2, and BNH. Calculated and observed vibrational frequencies for specific product molecules are compared in Tables 4-9.

TABLE 4: Observed and CASSCF Calculated Frequencies (cm-I) for Linear HBNHa

calculated observed ratio I(observed)b I(km/mol)’

3796.7 3700.5 (0.9747) [0.30] [I961

Discussion

calculated observed ratio

3785.4 3689.6 (0.9747)

calculated observed ratio

3797.3 3701.9 (0.9749)

calculated observed ratio

3785.9 3690.3 (0.9747)

calculated observed ratio

2836.7 2789.8 (0.9835)

calculated observed ratio

2841.9 2794.6 (0.9834)

Product absorptions will be identified from isotopic spectra and ab initio calculations, and the reaction mechanism for these product molecules will be presented. Species A: “€I.Three intense absorptions were observed for species A. The 3701.9 cm-’ band exhibits a 1 cm-’ boron isotopic shift, an 11 cm-’ nitrogen isotopic shift and a 907 cm-’ deuterium shift, which is indicative of an N-H stretching mode slightly coupled to boron. The 1788.7 cm-’ band exhibits boron and nitrogen isotopic shifts expected for a B-N stretching motion. The strongest A band in the lower region at 463.3 cm-’ has small nitrogen and boron isotopic shifts, identifying a H-B -N-H bending mode. Where components are resolvable, isotopic doublets in mixed isotopic experiments show that species A contains single B and N atoms. The A bands are in excellent agreement with two previous experimental and ab initio frequency calculation^'^ for iminoborane, HBNH. Careful inspection of regions expected for the two other modes of HBNH revealed weaker bands that can be assigned to the B-H stretching and bending modes based on isotopic shifts. Comparisons between CASSCF calculated and observed frequencies are given in Table 4. Note that the ratio of observed to calculated harmonic frequencies is 0.97-0.99 for NH, BH, and BN stretching modes; however, the bending modes are not as well described and have 0.93-0.94 ratios. The ratios are slightly higher for DBND, owing to less anharmonicity in the observed frequencies. Comparison of the ratios gives a quantitative measure of the agreement between calculated and observed frequencies. For the four HBNH isotopes, the average deviation in the v(N-H) ratios is f0.0001; this represents f 0 . 4 cm-’ agreement between calculated and observed isotopic

v(N-H)

v(B-H)

v(B-N)

HI ’BI4NH 2777.6 1799.2 2775.3 1782.8 (0.9992) (0.9909) [0.01] [0.06] 1201 ~ 3 1 HI IB I5NH 2776.7 1776.0 2774.4 1760.3 (0.9992) (0.9911) H’OB14NH 2800.9 1835.8 2796.9 1819.9 (0.9986) (0.9913) H10B15NH 2799.8 1813.6 2796.2 1798.3 (.9987) (0.9916) D”BI4ND 2186.2 1586.9 2189.3 1595 (1.0014) (1.0051) D’OBI4ND 2231.1 1599.4 2231.3 1610 (0.9999) (1.0066)

6(B-H)

6(N-H)

719.0 671.6 (0.9341) [0.01] [2 x 21

492.9 461.0 (0.9353) [0.25] [2 x 1031

718.9 489.7 669.9 460.8 (0.9318) (0.9410) 725.6 494.7 678.1 462.4 (0.9345) (0.9347) 725.4 491.6 676.4 462.2 (0.9325) (0.9402) 558.0

387.5

567.4

388.4

a cc-pVDZ basis set. Absorbances for major site band. Calculated infrared intensities.

frequencies at the CASSCF level of theory. Note also good agreement between calculated and observed relative band intensities. Iminoborane is a reactive linear moleculeI0-l6 that is isoelectronic with acetylene. The molecule is a precursor for borazine, H3N3B3H3, which is isoelectronic with benzene; however, b ~ r a z i n ewas ~ ~ not detected in these experiments as HBNH was trapped and isolated in solid argon. Species C: BNH. The decrease of C bands upon photolysis (nearly 90%) was the most notable characteristic and helped group the bands in different regions, especially with isotopic

Boron-Ammonia Reaction Products in Solid Ar

J. Phys. Chem., Vol. 99, No. 38, 1995 13845

TABLE 5: Observed and CASSCF Calculated Frequencies (cm-9 for BNHaa v(N-H) v(B-N) 6(N-H)

ratio

"B14NH 3783.1 1862.3 3693.7 1829.6 (0.9764) (0.9824) [201 [I851 "B15NH 3771.7 1839.0 3681.3 1806.7 (0.9760) (0.9824) 'OBI4NH 3783.6 1914.8 3683.5 1879.9 (0.9735) (0.9818) 10B'SNH 3772.2 1892.2 3669.2 1858.3 (0.9727) (0.9821) I 'B I4ND 2821.5 1800.1 2777.4 1776.1 (0.9844) (0.9867)

calculated observed ratio

2824.9 2780.6 (0.9843)

calculated observed ratio I (Wmol)'

calculated observed ratio calculated observed ratio calculated observed ratio calculated observed

482.3 432.5 (0.8967) [2 x 931 479.4 430.4 (0.8978 483.4 433.1 (0.8959) 480.5 431.1 (0.8972) 377.8

loBI4ND 1850.3 1825.1 (0.9864)

379.2

cc-pVDZ basis set. A similar calculation for BN gave a 1509.4 cm-' harmonic fundamental, 0.46 kdmol intensity, and 1.342 8, bond length. See also: Martin, J. M. L.; Lee, T. J.; Scusena, G. E.; Taylor, P. R. J. Chem. Phys. 1992, 97, 6549. Calculated infrared intensities. TABLE 6: Ab Initio Frequency Calculations (cm-I) for Linear HNBBNH CCSD(TY SCFb 1- 14-11- 1 1- 14-1 1- 14-11-11-14-1 1- 14-10-10-14-1 1- 15-1 1- 11-15-1

3860.5 [O] 4108.5 [O] 3857.1 [483] 4104.2 [533] 2025.0 [O] 2215.1 [O] 1850.1 [85] 2032.7 [299] 632.1 [O] 643.2 [O] 563.2 [2 x 3811 489.4 [2 x 01 473.8 [2 x 2071 555.9 [2 x 01 410.2 [2 x 01 503.7 [2 x 01 179.4 [2 x 0.31 210.7 [2 x 0.11

4109.1 4104.6 2290.3 2090.0 652.2 564.4 558.2 523.6 216.3

4096.4 4092.3 2195.4 2007.1 628.9 560.0 552.5 503.0 208.7

Using the cc-pVDZ basis set, all elkctrons correlated and Cartesian d functions used for technical reasons; infrared intensities [kdmol]. Using the 6-311G* basis set; a like calculation for BNH (11-14-1) gave 4108.8, 2045.8, and 707.1 cm-'. samples. As with HBNH, three absorptions observed for C correspond to N-H stretching, B-N stretching, and BNH bending motions based on isotopic shifts; however, no B-H stretching mode was observed for the C molecule. Frequencies calculated at the CASSCF level for BNH are in excellent agreement with the C isotopic bands as listed in Table 5. For the B-N stretching mode the average deviation in the observed calculated ratios is f0.0003, which corresponds to f 0 . 6 cm-l in frequency fit. For the BHN bending mode the average deviation in ratios is f0.0001 or 0.5 cm-I. The ratios for BND are slightly higher owing to lower anharmonicity in the deuterated species. Accordingly, the C bands are assigned to BNH. Small discrepancies in the observed and calculated harmonic 14-15 shifts for v(N-H) and the coincidence of 2 x v(B-N) required consideration of Fermi resonance interaction in BNH. In the case of l0Bl4NH, interaction between 2v(B-N) at 3743.9

cm-' and v(N-H) at 3683.5 cm-' causes a lower observed frequency for the latter fundamental. Likewise, in the case of l'%15NH,stronger interaction between 2v(B-N) at 3705.1 cm-I and v(N-H) at 3669.2 cm-I forces the fundamental even lower. This gives rise to a 14.3 cm-' observed 14-15 shift, which exceeds the scaled calculated harmonic 11.1 cm-' shift. For the "BNH isotopes, the overtone is lower than the fundamental, and a weaker interaction gives a small blue displacement in the fundamental. In this case, the 12.4 cm-' observed 14-15 shift is just above the calculated harmonic 11.1 cm-I shift, and the weaker interaction in "BNH does not give 2v(B-N) observable intensity. Accordingly, the Fermi resonance interaction in BNH causes v(N-H) to be 10 cm-' higher for the IlB than for the 1°B isotopic molecule. Previous photolysis experiments with H3BNH3 produced vibrational spectra for HBNH and a minor product, which was assigned to BNH radical.I0 The 3675 cm-' band first assigned to v(N-H) for 'OBNH and "BNH is in the correct regjon, but does not exhibit the above Fermi resonance. Although good agreement was found between experiment and ab initio calculations for the B-N stretch of HBNH, there is a major discrepancy between the broad 2035 cm-I band assignedIOto BNH and the earlieri4 and present calculated values; however, the present 1879.9 cm-I assignment is in excellent agreement with calculations for six isotopic molecules. Furthermore, the strong BNH bending mode was not observed in the earlier product spectrum. All of the above lead to the conclusion that Lory and Porterlo did not trap BNH, and suggest that their 3675 cm-' band was likely due to perturbed HBNH. The BNH radical is an extremely reactive species, and like HBNH, the nitrogen lone pair is involved in bonding to boron giving a formal triple bond. The photolysis of BNH most likely involves photodissociation of H, which diffuses through the matrix cage. The BN product is unfortunately not detected owing to very low infrared intensity. At the CASSCF level the infrared intensity of BN is 1/40 of that for v(B-N) of BNH; hence BN produced from BNH is not detectable in these experiment^.^^ Frequencies were also calculated for the ground *Ilstate of NBH at the CASSCF level: 2951.8 cm-' (7 kndmol), 1563.5 cm-I (9 kndmol), and 716.3 cm-' (58 ludmol). We found no evidence for a NBH bending mode, other than those assigned to BNBH and HBNH, in the region below 800 cm-I. Species B: HNBBNH. Species B is similar to BNH in that it also has three observed modes (strong v(N-H) and d(N-H) and weak v(B-N)) in close proximity to the modes for BNH; however, unlike BNH radical, species B is not destroyed on photolysis. Previous experiments have shown that dimer formation is a common reaction pathway for radicals before trapping in the matrix. The linear dimer HNBBNH is suggested; this molecule is more stable than two BNH radicals by 126 =k 2 kcal/mol at the CCSD(T) level and is isoelectronic to diacetylene. Frequency calculations at the SCF level predict a strong antisymmetric N-H stretching mode for HNBBNH about 13 cm-' below HBNH and 4 cm-I below BNH, which supports assignment of the B band to the former molecule. A strong deformation mode is also predicted with isotopic shifts in agreement with the 413.3 cm-l band. Here the SCF calculation is high for this anharmonic bending mode, but the CCSD calculation predicts a lower value more in accord with the observed value. A weaker B-N stretching mode is identified in Table 1. Further comparisons between the HCCH, HCCCCH, HBNH and HNBBNH molecules support this identification. The antisymmetric C-H stretching modes are 3289 and 3327 cm-I, respectively, for the a c e t y l e n e ~ and ~ ~ . the ~ ~ N-H stretching

Thompson et al.

13846 J. Phys. Chem., Vol. 99, No. 38, 1995 modes are 3702 and 3683 cm-', respectively, for the iminoboranes, both in close agreement. The strong deformation mode of diacetylene is 15% lower than for acetylene, and this mode is 11% lower for HNBBNH than for HBNH. A very strong bending mode observed for diacetylene at 226 cm-I is just above the calculated values for HNBBNH. Species D: HBNHz. It is clear from isotopic shifts that the D bands in Figure la-d are N-H stretching vibrations; however, the D bands are weaker than the HBNH bands. The bands marked D in Figure 1 are in the region expected for antisymmetric NH2 stretching modes. Relative yields and behavior on photolysis were sufficient to group bands in the BH stretching region, the BN stretching region and the NH bending region with the D band system. Unscaled CASSCF calculations predict the strongest N-H2 mode to be 3611.3 cm-I compared to 3588.4 cm-I observed (0.993 observedcalculated ratio). The unscaled CASSCF calculations also predict a BH stretching mode at 2525.2 cm-' compared to 2430.6 cm-' (0.963 observedcalculated) and a BN stretching mode at 1631.6 cm-I compared to 1556.9 cm-l observed (0.955). In the lower region the strongest calculated band at 565.8 cm-' compared to 608.7 cm-I observed (1.08). The seemingly large scale factor for the lower mode is attributed to the anharmonic nature of this out-of-plane bending motion. The H3N donor-B acceptor interaction gives a weak complex at the CASSCF level with a calculated N-B distance of 1.914 A. A small (14 cm-') blue shift is calculated for the v2 mode of ammonia in the complex, which would be difficult to observe in the presence of (NH3)2 in the NH3 spectrum.22 Hence, we have no evidence for the H3N---B complex. Species E: BNBH. The characteristic feature of species E was an intense 1:4:4:16 isotopic quartet in the B-N stretching region at 1876.4, 1871.8, 1851.5, and 1846.3 cm-' (A = 0.16). This natural boron isotopic quartet in Figure 4c shows that species E contains two inequivalent boron atoms. The molecule produces a nitrogen isotopic doublet indicating the involvement of only one nitrogen atom. Absorptions in the region are adjacent to linear BNB, a radical observed in earlier boronnitrogen reactions; however, ND3 experiments with natural isotopic boron gave a strong quartet at 1729.0, 1723.8, 1723.2, and 1717.6 cm-I (Figure 7c, A = 0.14), clearly showing the involvement of hydrogen. Increased vibrational interaction with the B-D stretching mode also revealed a weaker 1:4:4:16 quartet at 2263.2, 2262.3, 2219.7, and 2218.7 cm-I (A = 0.05) for this molecule. The counterpart bands in the B-H stretching region gave a weaker 1:4 doublet at 2826.9, 2805.6 cm-I (A = 0.004). The strongest E band was detected (A = 0.002) in B/N2 experiments where BNB radical was also a p r ~ d u c t . ~ Ab initio calculations showed BNBH to be a stable linear molecule with calculated frequencies in excellent agreement with E bands as shown in Table 7. Note that calculations predict intensification of the B-D stretching fundamental and two weaker modes B-N-B stretching and B-H bending that are observed in the spectrum. Again, observedcalculated frequency ratios show little variation among isotopic molecules. This variation in eight isotopic ratios for the antisymmetric B-N-B stretching mode is f0.0003, which corresponds to f 0 . 6 cm-'. For the three stretching modes these ratios are 0.93 f 0.01, but for the one observed bending mode the ratio drops to 0.88 as the harmonic model is a poorer approximation for the linear bending mode. Accordingly species E is identified as BNBH. Finally, the spectrum of the molecule CCBH made by insertion of B atoms into acetylene followed by H elimination is of interest as CCBH and BNBH are isoelectronic molecule^.^^^ The strongest mode of CCBH at 1995 cm-I shifts to 1907 cm-I

TABLE 7: Observed and Calculated Frequencies (em-') for Linear BNBH CASSCFkc-pVDZ (Partial Valence) 2979.4 [62] 2910.2 [351 2981.4 2805.6 0.9410 [94] [0.004] 2981.4 2805.6 3004.9 2826.9 3004.9 2826.9 11141 2980.3 2805.1 [88] 2980.3 2805.1 3003.5 2826.2 3003.5 2826.2 2368.8 2218.7 [447] [0.053] 2369.3 2219.7 2418.6 2262.3 2419.1 2263.2

1866.0 1045.1 [696] [163] MBPT(2)/6-311G* 1876.2 1076.7 [6511 11351 RHF/6-3 11G* 1988.9 1159.3 1846.3 1089.3 0.9283 0.9396 [947] [118] [0.160] [0.014] 1993.9 1196.1 1851.5 2016.0 1173.7 1871.4 2020.5 1210.9 1876.4 1136.4 [957] [133] 1951.1 1157.5 1812.3 1087.2 [903] [121] 1956.0 1194.7 1817.2 1979.5 1171.4 1838.8 1983.9 1209.0 1843.3 1134.2 1833.3 1134.9 1717.6 1062.4 [615] [97] [0.140] [0.015] 1839.7 1169.9 1723.8 1837.5 1147.4 1723.2 1074.0 1843.6 1182.8 1729.0

787.6 [2 x 131

133.8 [2 x 0.0011

751.1 [2 x 111

93.6 [2 x 0.11

847.0 746.2 0.8810 [2 x 241 [0.009] 847.0 746.2 857.0 754.5 857.0 754.5 [2 x 261 846.0 745.2 [2 x 241 846.0

158.2

855.9

155.9

855.9 753.5 677.4

157.5

[2 x 251 [0.017] 677.4

[2 x 01

689.9

150.9

689.9

152.6

[2 x 0.31 159.9 158.9 160.6 [2 x 0.33 155.1 [2 x 0.31 156.8

150.6

152.4

in CCBD upon interaction with the B-D stretching mode at 2173 cm-', which is analogous to the behavior of the strong 1846 cm-I band of BNBH. Species F: H9NBH. Band F is characterized by its position relative to BNBH and comparable isotopic ratios. Isotopic experiments reveal a boron isotopic quartet similar to BNBH. The proximity to BNBH and similarity of isotopic band splitting implies a common subunit with inequivalent boron atoms. Species F is further blue-shifted than BNBH relative to BNB, implying a species with more hydrogen atoms. The HBNBH radical was considered, but calculations predict a strong BNB mode 6eZow'BNBHand a stronger B-H stretching mode, which are not in agreement with the observed bands. A possible product is singlet HzBNBH. Calculations on H2BNBH are in good agreement with observed bands, (Table 8), including terminal B-H and B-H2 stretching modes, all of which support the identification of HzBNBH. Species G: BNH2. Two weak bands identified as G near 1530 and 596 cm-I are almost completely destroyed by broadband photolysis. These bands are in the regions of NH2 bending and deformation modes and the nitrogen isotope shifts (Table 1) support this characterization; but small boron isotopic displacements show that boron is also involved in these vibrational modes. CASSCF calculations show that BNH2 is less stable than "Hby 41 kcal/mol, but nevertheless, a minor amount of BNH2 could be formed by H atom elimination from the initial insertion product. Vibrational frequency calculations at the CASSCF level are compared with the observed frequencies in Table 9. The strongest al mode calculated at 1579.1 cm-l exhibits isotopic frequencies in excellent agreement with

Boron-Ammonia Reaction Products in Solid Ar

J. Phys. Chem., Vol. 99, No. 38, 1995 13847

TABLE 8: Observed and Calculated Frequencies (cm-l) for HaNBH at the RHF/6-311G* Level Hz"B14N'1BH

I (kdmol) observed H2'0B14N11BH observed H2"B14N10BH observed H2'0B'4N'oBH observed H2"B"N"BH observed H2"B"N"BH observed H2"B"N"BH observed H2'0B'5N'oBH observed

2969.5 [891 2788.3 2969.5 2788.3 2994.2 2808.9 2994.2 2808.9 2968.1 2787.0 2968.1 2787.0 2992.4 2807.8 2992.4 2807.8

2696.1 [2341 2426.5 2712.1 2438.6 2696.1 2426.5 2712.1 2438.6 2696.1 2426.0 2712.1 2426.5 2696.1 2426.0 2712.1 2426.5

2634.4 ~291 2393.2 2640.3 2405.3 2634.4 2393.2 2640.3 2405.3 2634.4 2392.8 2640.3 2404.9 2634.4 2392.8 2640.3 2404.9

2023.3" [5641 1864.2 2026.4 1868.2 2052.4 1891.0 2055.2 1894.5 1985.8 1830.4 1988.9 1834.2 2016.4 1858sh 2019.1 1861.6

a Additional calculated frequencies and intensities are 1361.1 [145], 1108.7 [102], 1055.6 [31], 1034.0 [51], 850.3 [25], 805.8 [7],249.5 [14], and 208.7 cm-' [14 kdmol].

TABLE 9: CASSCF (Full Valence)/cc-pVDZ Frequency Calculations (cm-') for Planar BNH2 11-14-1-1 calc" I [kdmol] observed ratio 11-15-1-1 obs ratio 10- 14-1-1 observed ratio 10- 15-1-1 observed ratio

a(N-Hd

&N-Hz)

1579.1 [I381 1533.2 (0.9709) 1567.8 1521.9 (0.9707) 1584.2 1538.8 (0.9713) 1572.3 1527.1 (0.9713)

404.4 [I841 596.0 (1.4738) 401.5 592.0 (1.4745) 404.8 596.5 ( 1.4736) 401.9 592.5 (1.4742)

a Weaker a1 modes: 3496.2 [59], 1232.9 [56]. bl modes: 3619.9 [73], 642.8 [8].

the observed 1533.2 cm-' absorption; the average deviation in the ratios (observedkalculated) is 60.0003, which represent f0.5cm-'. The strong out-of-plane b2 mode calculated at 404.4 cm-I exhibits an even better fit to the isotopic frequency pattern; the average deviation in the ratios f0.0005represents a frequency fit of f0.3 cm-'. Note, however, that the harmonic outof-plane bending frequency is predicted at 0.68 of its observed anharmonic value, which is attributed to inadequacy in modeling such as anharmonic out-of-plane motion. On the basis of excellent isotopic frequency fit for the two strongest R absorptions predicted by CASSCF calculations, species G is identified as BNH2, a less stable isomer of the primary HBNH product.37 Other Absorptions. Other weak absorptions with reagent isotopic shifts are observed and listed in Table 1. Although functional groups are implicated by the band positions, these new species cannot be identified without more information. Weak BO2 as well as B02- bands are observed in the spectrum. This indicates that electrons are produced in the ablation process. No molecular anions were identified in the NH, system; in particular, no evidence was found for NH2-.25*39 The absorption for Ar,D+ was observed at 643.4 cm-I in ND3 experiments, again noting that some hard radiation is produced by the ablation process.40 Only two absorptions in these experiments show evidence of two nitrogen atoms. The first is diimide, HNNH, at 1282.5 cm-1.26 The second absorbs at 1425.1 cm-I with log, which shifts to 1388.6 cm-' with IlB and gives a 1:4 doublet in natB

studies for a single B atom. These bands exhibit triplets with mixed I4NH3/I5NH3at 1425.1, 1423.4, and 1421.5 cm-' and at 1388.6, 1386.6, and 1384.7 cm-I, which is appropriate for two equivalent N atoms. A species of general formula H2NBNH2 is suggested to account for this mixed NH2 bending, N-B-N stretching mode. Finally, several known molecules that might be expected in the B/NH3 reaction system include hydrazine (the dimer of NH2),33 aminoborane BHzNH2,4' and borazine (the trimer of HBNH);42however, these molecules were not detected here. The most surprising absence is diatomic BN; this molecule has been observed by electronic absorption in solid neon following photolysis of H3BNH3.43 The failure to detect BN here is probably due to high reactivity and low infrared intensity. Reaction Mechanisms. The reactions that occur in the argon carrier gas during condensation on the cold window are summarized in Scheme 1. Total atomization energies for the molecules involved were obtained at the CCSD(T) level using Dunning's correlation consistent polarized valence triple-zeta (cc-pVTZ) basis set29corrected for residual basis set incomp l e t e n e ~ s .These ~ ~ energies, which will be reported elsewhere,44 allow changes accurate to f 2 kcdmol to be determined for the reactions considered here. The primary insertion reaction 1 of B with N H 3 is presumed to require activation energy as B*

+ NH, - [HBNH,]* *HBNH,

(1)

product bands decrease on warming the sample 10-20-3040 K were cold boron atoms diffuse and have the opportunity to react. Evidence for hyperthermal laser-ablated atoms has been presented in previous work with aluminum.45 Reaction 1 is calculated to be exothermic by 89 f 2 kcdmol (not including activation energy). This excess energy can be quenched by the matrix and the insertion product H B N H 2 trapped. However, before relaxation in the condensing matrix, [HBNH2]* can decompose to give several different products, which include HBNH, BNH, BNH2, BH, and N H 2 . Based on band intensities, the major primary reaction product is the reactive iminoborane molecule, HBNH. The formation of HBNH from HBNH2, reaction 2, is endothermic by 35 f 2 kcdmol, but this energy HBNH,

-

HBNH

+H

(2)

is readily available from the insertion reaction 1. The minor product BNH requires 43 f 2 kcdmol and H2 elimination, reaction 3, which is a slower reaction. Finally, the addition reaction to give H3N---B results in a weakly bound complex (12 f 1 kcdmol), which could not be detected here. HBNH,

-

BNH

+ H,

(3)

A number of secondary reactions involving a second boron atom occur as two new products exhibit two inequivalent B atoms and two previously identified3 products, B2N and BNB, contain two equivalent B atoms. Following the example of acetylene reactions with B atom^,^^^ most of the secondary B atom reactions occur with HBNH. The insertion reaction leads to BNBH and BNB, following H elimination, and the addition reaction forms the cyclic B2N radical, following H2 elimination. The major secondary reaction 4 is exothermic by 85 f 2 kcaV mol; however, decomposition of HBNBH radical to HBNB requires 66 f 2 kcdmol, which is provided by reaction 4. The net exchange reaction (5) is exothermic by 19 f 2 kcal/mol.

+ HBNH- HBNBH B + HBNH-HBNB + H B

(4) (5)

Thompson et al.

13848 J. Phys. Chem., Vol. 99,No. 38, 1995 SCHEME 1: Reaction Mechanism for Laser Ablated Boron Atom H B ‘N -

BH + NH,

+ Ammonia Reaction

/H

H‘

H N‘

.\,\\\*I H

r

l*

-H

-B =N=B

1

/H

H‘

-I

+B

H-B=N=B

I-

---I

L

H

H-BIN-H

H

-e-/ H‘

BNH

B=N=B

HNBBNH

Note the increased yield of BNBH relative to HBNH with increased laser power in Figures 1-5b,d relative to 1-5a,c. The very high yield of B2N formed here from the secondary B HBNH reaction adds strong support for its identification as cyclic B2N from mixed isotopic ~ p e c t r a . ~The HBNBH2 molecule requires further B atom reaction with H B N H 2 probably during the relaxation process. Finally, the observation of substantial remaining ammonia precursor absorption in the presence of products with two boron atoms suggests that secondary B reactions with HBNH can proceed with less energetic B atoms after energy quenching in the condensing matrix whereas the ammonia reaction requires more energetic B atoms directly from the ablation process.

+

Conclusions Pulsed laser ablated B atoms react with ammonia to produce a large number of reactive species, which are trapped in solid argon and identified from isotopic shifts, photolysis behavior, and comparison with ab initio calculated isotopic frequencies. The primary insertion reaction is highly exothermic, and most of this initial product decomposes before matrix trapping. The major product is iminoborane, HBNH, a reactive molecule, which under normal conditions trimerizes to give H3B3N3H3. A minor product is the BNH radical. The BH and N H 2 fragment species are also observed. The reaction of a second boron leads to BNBH, BNB, and B2N secondary products. Ab initio isotopic frequency calculations are extremely helpful in making vibrational assignments. For heavy (non-hydrogen) atoms, isotopic shifts agree within &OS cm-I between calculated (scaled) and observed shifts. In order to identify new product molecules with common functional groups, it is necessary to match isotopic frequencies as a measure of the normal mode molecular fingerprint.8 Finally, these matrix studies show that atomic boron reacts readily with ammonia. Acknowledgment. The experimental work was supported by the Air Force Office of Scientific Research, and calculations were done at the San Diego Supercomputer Center. J.M. is a Postdoctoral Fellow at the National Science Foundation of Belgium (NFWO/FNRS). J.E.Y. acknowledges a graduate

fellowship from the LUC (Stimuleringsfonds). This research was partially supported by the Prime Minister’s Office for Science Policy Programming. References and Notes (1) Burkholder, T. R.; Andrews, L. J. Chem. Phys. 1991, 95, 8697. (2) Hassanzadeh, P.; Andrews, L. J. Phys. Chem. 1992, 96, 9177. (3) Andrews, L.; Hassanzadeh, P.; Burkholder, T. R.; Martin, J. M. L. J. Chem. Phys. 1993, 98, 922. (4) Tague, T. J., Jr.; Andrews, L. J. Am. Chem. SOC.1994, 116,4970. ( 5 ) Hassanzadeh, P.; Hannachi, Y.; Andrews, L. J. Phys. Chem. 1993, 97, 6418; 1994, 98, 6950. (6) Hassanzadeh, P.; Andrews, L. J. Am. Chem. SOC.1992,114,9239. (7) Andrews, L.; Burkholder, T. R. J. Phys. Chem. 1991, 95, 8554. (8) Martin, J. M. L.; Taylor, P. R.; Hassanzadeh, P.; Andrews, L. J. Am. Chem. SOC.1993, 115, 2510. (9) Andrews, L.; Hassanzadeh, P.; Martin, J. M. L.; Taylor, P. R. J. Phys. Chem. 1993, 97, 5839. (10) Lory, E.; Porter, R. J. Am. Chem. SOC.1973, 95, 1767. (1 1) Kawashima Y.; Kawaguchi, K.; Hirota, E. J. Chem. Phys. 1987, 87, 633 1. (12) Dill, J. D.; Schleyer, P. v. R; Pople, J. A. J. Am. Chem. SOC. 1975, 97, 3402. (13) Summers, N.; Tyrell, J. J. Am. Chem. SOC.1977, 99, 3960. (14) Botschwina, P. Chem. Phys. 1978, 28, 231. (15) Hout, R. F., Jr.; Levi, B. A.; Hehre, W. J. J. Comput. Chem. 1982, 3, 234. (16) DeFrees, D. J.; Binkley, J. S.; McLean, A. D. J. Chem. Phys. 1984, 80, 3720. (17) Knight, L. B., Jr.; Herlong, J. 0.;Kirk, T. J.; Amngton, C. A. J. Chem. Phys. 1992, 96, 5604. (18) Paetzold, P. Pure Appl. Chem. 1991, 63, 345; Adv. Inorg. Chem. 1987, 31, 123. (19) Silaghi-Dumitrescu, I.; Haiduc, I.; Sowerby, D. B. Inorg. Chem. 1993, 32, 3755. (20) Zhu, H.-Y.; Klein, D. J.; Seitz, W. A,; March, N. H. Inorg. Chem. 1995, 34, 1377. (21) Hamilton, E. J. M.; Dolan, S. E.; Mann, C. M.; Calijn, H. 0.; Shore, S. G. Chem. Muter. 1995, 7, 111. (22) Suzer, S.; Andrews, L. 1. Chem. Phys. 1987, 87, 5131. (23) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1964, 41, 2838. (24) Milligan, D. E.; Jacox, M.E. J. Chem. Phys. 1965, 43, 4487. (25) Suzer, S.; Andrews, L. J. Chem. Phys. 1988, 89, 5347. (26) Minkwitz, R. 2. Anorg. Allg. Chem. 1974, I , 411. Rosengren, K.; Pimentel, G. C. J. Chem. Phys. 1965, 43, 507. (27) Stanton, J. F.; Gauss, J.; Watts, J. D.; Lauderdale, W. J.; Bartlett, R. J. ACES 11, an Ab Initio System. Quantum 7’heory Project; University of Florida: Gainesville, FL, 1994. This package includes MOLECULE by J. Almlof and P. R. Taylor and ABACUS by T. Helgaker, P. Jorgensen, H. J. Aa. Jensen, P. R. Taylor.

Boron-Ammonia Reaction Products in Solid Ar (28) SIRIUS by H. J. Aa.Jensen, H. Agren, J. Alsen, and P. R. Taylor. ABACUS by T. Helgaker, P. Jorgensen, H. J. Aa. Jensen, and P. R. Taylor. (29) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007. (30) Martin, J. M. L. J. Chem. Phys. 1994, 100, 8186. (31) Raghavachari, K.; Trucks, G. W., Pople, J. A.; Head-Gordon, M. Chem Phys. Lett. 1989, 157,479. Scuseria, G. E. Chem. Phys. Lett. 1991, 176, 27. Watts, J. D.; Gauss, J.; Bartlett, R. J. J. Chem. Phys. 1993, 98, 8718. (32) Scusena, G. E.; Lee, T. J. J. Chem. Phys. 1990, 93, 5851. Lee, T. J.; Rendell, A. P.; Taylor, P. R. J. Phys. Chem. 1990, 94, 5463. (33) Kaldor, A,; Porter, R. F. Inorg. Chem. 1971, 10, 775. (34) Part of the region where BN might absorb is masked by the 1494.9 cm-' NH2 band. (35) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J. Phys. Chem. 1982, 86. 3374.

J. Phys. Chem., Vol. 99, No. 38, 1995 13849 (36) Patten, K. O., Jr.; Andrews, L. J. Phys. Chem. 1986, 90, 3910. (37) The isomer NBHz is calculated to be a transition state31with higher energy than BNHz by 43 kcdmol. No evidence was found for NBHz. (38) Nguyen, M. T. J. Chem. SOC., Chem. Commun. 1987, 342. (39) Tack, L. M.; Rosenbaum, N. H.; Owrutsky, J. C.; Saykally, R. J. J. Chem. Phys. 1986, 84, 7056; 1986, 85, 4222. (40) Andrews, L. Annu. Rev, Phys. Chem. 1979, 30,79. (41) Lascola, R.; Withnall, R.;Andrews, L. Inorg. Chem. 1988,27,642. (42) Carpenter, J. D.; Ault, B. S. J. Phys. Chem. 1991, 95, 3502. (43) Mosher, 0. A,; Frosch, R. P. J. Chem. Phys. 1970, 52, 5781. (44) Martin, J. M. L.; El-Yazal, J.; Francois, J. P. To be published. (45) Andrews, L.; Burkholder, T. R.; Yustein, J. T. J. Chem. Phys. 1992, 96, 10182.

JP95 14266