J. Phys. Chem. 1993, 97, 5839-5847
5839
Pulsed Laser Evaporated Boron Atom Reactions with Acetylene. Infrared Spectra and Quantum Chemical Structure and Frequency Calculations for Several Novel BCzH2 and HBCz Molecules Lester Andrews' and Parviz Hassanzadeht Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901
Jan M. L. Martin* and Peter R. Taylor San Diego Supercomputer Center, P.O.Box 85608, San Diego, California 92186-9784 Received: December 26, 1992
Pulsed laser evaporated boron atoms react with C2H2 to form new organoborane products during condensation with excess argon. Isotopic frequencies have been obtained for the IlB, I0B, C2H2, 13C2H2,and C2D2 reactions. Several structures of the HBC2 and BCzH2 molecules have been studied a b initio using large basis sets and augmented coupled cluster methods. Computed isotopic shifts are in excellent agreement with experiment for two BC2H2 species (bent HBCCH insertion and ring BCzHz addition products) and three HBC2 species (linear HBCC, linear HCCB, and C2" HBC2). The BC2H2 borirene radical is photosensitive, forms spontaneously on annealing above 18 K, and exhibits calculated bond lengths appropriate for delocalized 7r bonding in the BC2 ring system. The linear HBCC and HCCB species are formed by the reaction of hyperthermal B atoms and C2H2 during matrix deposition. Thermochemistry has been addressed by a b initio calculations.
The evaporated boron atoms were codeposited with a mixture of argon/acetylene in the 200/1 to 800/1 range. Natural ("B) Pulsed laser evaporated boron atom reactions with small (Aldrich) and enriched boron- 10 (log)(Eagle-Pitcher Industries) molecules have produced new reactive boron species, which can and carbon- 13and deuterium enriched acetylene (MSD Isotopes) be trapped in solid argon and characterized by FTIR specwere used. Infrared spectra were collected before and after tros~opy.l-~ The insertion of atomic boron into a C-H bond in medium pressure mercury arc (Philips H39KB, 175W) photolysis methane studied by two groups is relevant to the present s t ~ d y . ~ - ~ and annealing on a Nicolet 60SXR instrument at 0.5-cm-1 Boron atoms react with methane to give several new products resolution. including H2CBH2, HzCBH, HCBH, and HBCBH, which have been identified from matrix infrared spectra by isotopic substiComputational Methods tution and quantum chemical calculations of isotopicvibrational All calculations were done either with the GAUSSIAN 9219 spectra.' or the ACES I12C-22 program systems, running on the CRAY The boron-acetylene reaction is of particular interest owing Y-MP 8/864 at the San Diego Supercomputer Center. to competition between insertion into a C-H bond and addition First, some preliminary geometry optimizations and harmonic to the M C bond. Metal atom reactions with C2H2 have formed frequency calculations were carried out at the Hartree-Fock level u or 7~ complexes (Al,8,9Cu, Ag, Au,IoJ1and Li12) for the most using either the standard Pople-type 6-31G** 23-2s basis set or part, but Fe appears to insert into a C-H bond.13 The BC2H2 the Huzinaga-Dunning DZP (double-t plus polarization) basis system has been studied theoreticallyby two groups; one addressed set.26927 The relative stabilities of the structures obtained were the acetylene vinylidene rearrangement in the presence of then assessed using single-point calculations at the CCSD(T) boron,I4and the other reported a study of the BC2H2 system and leve128-30 with the above basis set. (This is a coupled cluster the interaction of boron with the C=C and C-H bonds of method with all single and double excitations (CCSD)31-33and acetylene.Is Furthermore, the B C2H2 reaction is an important a quasiperturbative inclusion of connected triple excitationsnZ8 source of borirenes-novel27r electron aromatic The Extensive ~omparisons3~~~s have shown that the method yields present paper reports matrix infrared spectra and ab initio correlation energies very close to the n-particle space limit even calculations of structure and vibrational spectra for six B + C2H2 for problematic molecules.) These latter calculations,in the small reaction products, including the borirene radical species. basis sets considered, took only a few minutes each on the CRAY yet enabled elimination of a number of high-energy structures. Experimental Methods For the remaining structures, geometries and harmonic frequencies were refined at the MP2 level with the DZP basis set, The closed-cycle helium refrigerator (CTI Cryogenic Model using analytical second derivatives.3G38From the force constant 22), vacuum chamber, and pulsed YAG laser evaporation matrices thus obtained, isotopic shifts were calculated for all apparatus have been described previously.'J A piece of boron combinationsof naturally occurringisotopes. Previousexperience was epoxy glued to the end of a glass rod and rotated at 1 rpm at this or lower levels of theory for carbon clusters39and boronand the fundamental of the YAG laser beam was focused by a nitrogen clusters4 indicates that very good to excellent agreement 10.0-cm focal length quartz lens onto the target. With approxwith experiment is obtained unless there is an intrinsic problem imately 40 mJ/pulse of laser power at the boron target, the boron with the wave function: i.e., it is not well described by a single atom concentration in the argon matrix was on the order of 0.1%. reference determinant. (Under these circumstances, MP2 may grossly exaggerate correlation effects.) In these instances, the Present address: Department of Chemistry, Collge of Sciences, Shiraz frequencies were recalculated, after reoptimizing the geometry, University, Shiraz, Iran. t On leave from: Department SBG, Limburgs Universitair Centrum, by finite differences of QCISD (quadratic configuration interUniversitaire Campus, 8-3590 Diepenbeek, Belgium, and Department of action41 with all singles and doubles) gradient^.^^^^^ (The latter Chemistry, Institute for Materials Science, University of Antwerp (UIA), method is a slightly simplified v e r s i ~ n of ~ ~CCSD.) .~~ Universiteitsplein 1, B-2610 Wilrijk, Belgium.
Introduction
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0022-36S4/93/2097-5839$04.00/0
0 1993 American Chemical Society
5840 The Journal of Physical Chemistry, Vol. 97, No. 22, 19'93
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E m
Figure 1. Infrared spectra in the 2700-2600-, 2100-1900-, and 1210-810-cm-' regions for the "Batom reaction with Ar/C2H2 = 400/1 sample: (a) with 6-h codeposition; (b) after annealing to 26 k 1 K, (c) after annealing to 32 k 1 K.
In order to resolve the assignment of the closely spaced frequencies for the various structures of HBC2, more accurate calculations were necessary. Because these are closed-shell systems, and because of the lower numbers of basis functionsand degrees of freedom, this was technically feasible. Frequencies were calculated here by using finite differences of CCSD(T) using the larger TZ2P (triplerplus two polarization functions48)Dunning-Huzinaga basis set. To obtain a definitive assessment of the relative energetics, the equilibrium geometry and correspondingenergy for all three structures was additionally obtained with the even larger cc-pVTZ "correlation consistent" basis set,49which is a [4s3p2dlfl contraction of a (10s5p2dlf) primitive set. Frequency calculationswere not carried out at this level due to cost considerations.
ReSults Dilute mixtures of Ar/C2H2 were codeposited with laser evaporated boron atoms. Product band absorbances increased with reagent concentration and were characterized by isotopic substitution, annealing, photolysis behavior, and quantum chemical calculations. Isotopic Substitution. Matrix infrared experimentswere done by reacting "B and 1°B with C2H2 to search for products with boron isotopic shifts. Figure l a is an example of an Ar/C2H2 = 400/ 1 experiment with "B; new product absorptions are given in Table I for IIB (from nB) and IOB isotopic reactions. Clearly a number of new organoborane species were produced. The 2700-2600-cm-I region shows new bands at 2665 and 265 1cm-1; annealingdecreasesa 2665.2-cm-1 satellite and leaves a sharp 1:4 doublet at 2663.6 and 2651.4 cm-I. The 12001000-cm-1 region reveals two new E bands at 1175.3, 1170.6 cm-I, a sharp F band at 1122.7 cm-I, and a G absorption at 1007.8 cm-I. The strongest product bands labeled A and B from the 2100-1900-~m-~ region areillustratedin Figure 2, wheredifferent reagent isotopes are compared. ThenB reaction product spectrum inFigure2bshowsstrong IlB bandsat 1995.2,2039.3,and2080.4 cm-I (labeled A, B, and C, respectively) with loBbands at 2002.4, 2041.7, and 2084.1 cm-I having 4 to 1 relative intensities appropriatefor natural boron isotopes. The latter bands are strong in the ]OB spectrum of Figure 2a, where the IlB bands are weak due to 6% IlB present in the 94% IOB sample. The bands show large carbon-13shifts with the I3C2H2reagent in Figure 2c. Weak
Andrews et al. intermediatebands are noted for reaction with HI2Cl3CHpresent in the carbon-13 enriched sample. Deuterium shifts were observed for all product absorptions; a representative spectrum is shown in Figure 3a for the C2D2 reaction. The strong D band shifted from 2651.4 cm-I (Figure la) to 2003.7 cm-I (Figure 3a), and a sharp new A doublet was observed at 2173.0 and 2205.2 cm-1 which tracked with the A doublet at 1907.7 and 1910.7 cm-I. The observed isotopic frequencies are given in Table I. Boron isotopic reactions were done for a mixed C2H2/C2HD/ C2Dzsample; all product bands were in agreement with the above C2H2and C2D2reactions. Likewise an experiment with 12C2H2/ '3C2H2 mixturesgave only the bands listed in Table I. In addition very weak bands (A C 0.005) were observed for the boron oxide species,l%2and a very weak C2H4 band was detected at 947.4 cm-I, which indicates very little sample photolysis by the laser plume. Annealing. Warming the matrix allows trapped atoms and small molecules to diffuse and react. For example the !]BO2 band at 1274.6 cm-l (not shown) increased from A = 0.002 to A = 0.004 on annealing to 26 K in the 400/ 1 experiment illustrated in Figure 1 while (C2H2),, clusters near 1970 cm-I increased at the expense of C2H2 monomer. The organoborane products are characterized by annealing behavior in Figure 1. The sharp A and B bands decreased by 20% on annealing to 26 and 32 K, while the sharp C band increased by 25% on annealing to 26 and 32 K, whereas the D bands increased by 15%on each annealing. The E bands at 1170.6 and 896.7 cm-I more than doubled on the first annealing, while the F and G bands at 1122.7 and 1007.8 cm-1 were unchanged, but the D satellite at 1119.7 cm-I increased. The first annealing to 30 K in the C2D2experiment (Figure 3c) showed analogous changes, Le. small decreases in A and B and a large increase in E. A final annealing to 40 K (Figure 3d) showed marked growth of species C' and (C2D2),,clusters with the survival of some species D and traces of species E and G. Photolysis. Photolysis dissociates molecules or stimulates the reaction of precursor molecules and atoms trapped in adjacent matrix sites, and as might be expected, photolysis behavior depended somewhat on reagent concentration. In the Ar/C2H2 = 800/1 experiment, the A band increased lo%, the B band was unchanged, C decreased by 75%, E and F decreased by 60%, and G was increased by 25% on A > 254-nm photolysis. Subsequent annealing to 18 and 25 K had the same effect on these bands, as shown in Figure 1; the E band growth was noteworthy. In another 400/ 1 experiment with C2H2 and "B, the A and B bands increased by 35% on 254-nm photolysis for 30 min and another 30% on photolysis for 45 min more, while the other bands behaved as described above; the E bands increased markedly on annealing at 27 K. In a third 400/ 1 experiment with C2H2andnB,photolysis with A > 380-, 320-, and 290-nm filters was performed. The first A > 380-nm photolysis produced a 5% decrease in E and F bands and a 10%increase in A. The A > 320-nm irradiation decreased E and F bands by 20%, decreased C by lo%, and increased the G band by 30%. Further irradiation at A > 290 nm decreased E and F bands by 40% and decreased C by 20%. The final A > 254-nm photolysis almost destroyed E and F bands, slightly increased D, and increased A by 35%and B by 10%. A subsequent annealing to 18 K increased the E band, and annealing to 28 K restored the 1170.6-cm-1band. In a 400/ 1experiment with C2H2 and log, photolysis after annealing to 30 K had the same effect described above for the "B experiment. The C' band, which appeared on annealing, decreased on photolysis with the C band. The E band grew 4-fold on annealing to 30 K, was almost destroyed by photolysis, and was restored to an intermediate intensity on subsequent 30 K annealing. The F band was not changed on annealing but decreased on photolysis. Photolysis of the Ar/C2D2 = 200/1 sample with "B in Figure 3 had a similar effect as that describedabove for "Band Ar/C2H2
Pulsed Laser Evaporated Boron Atom Reactions
The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 5841
TABLE I: Product Absorptions (cm-I) in the Reaction of Boron Atoms with Acetylene in a Condensing Argon Stream "B/C2H2
'OB/C2H2
I 'B/l3C2H2
IoB/"C2H2
265 1.4 2080.4 2065.8 2045.3 2039.3 2006.4 1995.2 1175.3 1 170.6 1188 1122.7 1119.7 1007.8 896.7 835.1
2663.6 2084.1 2069.7 2047.9 2041.7 2013.3 2002.4 1202.4 1197.0
2650.8 2010.4 1994 1973.9 1968.1"
2663.2 2013.9 1998 1978.2 1971.0"
1926.9b 1152.5 1147.7( 1176 1106.2 1102 994.2 889.3
1935.1b 1177.6 1172.2 1134.9 1130 1020.6 900.1 -
1151.4 1146.5 1033.6 907.0 870.5
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'OB/"C2D2 2173.3 2004.0 1940.3 1940 br 1925.6 1918.8 1910.7 1907.7 1173.6 1169.4 1113 1075.7
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885.2 77 1.4 -
'OB/C*D2 2205.2 2026.1 1945.3 1945 br 1923.0 1909.2 1199.4 1196.0 1150 1092.7 913.7 783.7 -
ident A D C C' B site B A site A E site E aggregate F D G E ?
"Additional weak bands were observed with I3C sample at 1994.5 and 2016.7 cm-I for 'B and at 1997.2 cm-I for 'OB. Additional weak bands were observed with I3C sample at 1945.8 and 1977.1 cm-I for "B and 1953.4 cm-1 for 'OB. Additional weak band observed at 1161.9 for the I2Ci3C species.
"A 10
"B
1:
I
0
1:
"m 0
ll
? U W
6" a?
"m 0
Q
N 0
B
A
Figure 3. Infrared spectra in the 2250-1750- and 1250-750-~m-~ regions for the nB atom reaction with Ar/C2D2 = 200/1 sample: (a) with 4-h codeposition; (b) after photolysis h < 254 mm for 45 min; (c) after annealing to 30 1 K; (d) after annealing to 40 1 K.
*
10
9 0
Figure 2. Expanded scale infrared spectra in the 2100-1900-~m-~ region comparing isotopic reagents: (a) I0B (94% 'OB + 6% "B) + C2H2; (b) nB (20% 'OB 80%"B); (c) nB '3C2H2(92.5%carbon-13,i.e. 85.5% "CZH2, 13.9% "CI3CH2, 0.6% I2C2H2).
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= 400/1 reagents; the A and B bands increased by lo%, the D and G bands increased by 20%, and the E and F bands decreased by 80%. BCzH2Calculations. For the B CzHzadditionstoichiometry, only the three most stable structures found by Flores and Largo's were considered (Figure 4): a BC2 ring with hydrogens on both carbons (CZu symmetry), a BC2ring with hydrogens on boron and one of the carbons (C, symmetry), and an HBCCH chain which is approximatelylinear except for the HBC angle (C, symmetry). As witnessed by Table 11, structures 1 and 2 lie fairly closely together in energy (largely due to the comparable CH and BH bond strengths). Structure 3, which is highly spin-contaminated, lies somewhat higher up but might still be detectable. Because of this high spin contamination,the MP2 geometry and harmonic frequencies are unrealistic. To remedy this problem, we have recomputed the harmonic frequencies at the QCISD/DZP level.
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Virtually all the spin contamination comes from a single contaminant, which means it should be handled quite well by methods like CCSD and QCISD, which are essentially unaffected by a single spin contami~~ant.~~*s] (Both structures 1 and 2 are nearly pure doublet states.) All three structures have a number of fingerprint bands in the IR: isotopic shifts are given in Tables
111-v. From the computed structures, we can see that the lengths for CH, BH, and BC bonds are close to what is expected for the corresponding single bonds: the CC bond is close to an ethylenic length in the ring structures 1 and 2, but closer to an acetylenic one in structure 3. HBC2. The B + C2Hz addition reaction may give a hydrogen atom and an HBC2 species with sufficient incident B kinetic energy. Four structures immediately come to mind linear HBCC, linear HCCB, and a BC2ring with a hydrogen attached either to the boron atom or one of the carbon atoms. The latter arrangement, which has only C, symmetry, rearranges to the HCCB structure upon optimization. On the singlet surface, three structures thus remain (Figure 5 ) . As seen in Table VI, these are quite close together in energy. Both linear structures 4 and 5 are minima on the potential surface: the Cz,structure 6 is a transition state, with the reaction coordinate leading to HBCC in both directions. This hence corresponds to a migration of the HB group from one carbon
Andrews et al.
5842 The Journal of Physical Chemistry, Vol. 97, No. 22, 19'93
.
1
MP2fDZP
MP2/DZP
1.1719
1.4978
H
H
',
MPZDZP
7
127.84''
179.50"
1.4883
1.2376
1.0697
H, l14,440%,..,,
1.1748
/ ( y 8 7
/c1.372413850\
H
MPYDZP
180.010
H
1 1.0898
Mp'Dz~c\S210
/C-1 9 1 5 4 + . 0 7 8 7
1.0832
H
H
149.91
H
Figure 4. Structures calculated for BC2H2, BC2H3, and C3H4.
TABLE Ik Overview of Stationary Points for H2BC2.
Ground-State Harmonic Frequencies for C2H.4, HJBC2, and
CJH4Given for Comparison
MP2/DZP frequencies, CCSD(T)/DZP//MPZ/DZP energies 1 ( ' A l ) -101.81094 734 (bl, 50), 911 (a], 16), 926 (bz, 31), 1010 (82, O), 1201 (bz, 3), 1215 (a], 65), 1506 (a!, 2), 3289 (b2, 2), 3313 (al, 4 ) 2 (IA') -101.815 163 733 (a", 6), 754 (af, 18), 849 (a', 21), 874 (a", 4 9 , 1014 (a', 61), 1180 (a', 75), 1610 (a', 4), 2873 (a', 58), 3313 (af, 1) 3 (IA') -101.803 208 304 (a", 3), 351 (af, 6), 818 (a', 89), 853 (a', lo), 904 (a', 44), 944 (a', 27), 2760 (a', 90), 2954 (af, 104), 3685 (a', 24) 542 (rg, O), 729 (ru,9 9 , 1 9 5 0 (ug,0), C2H4 3464 (uu,91), 3548 (ug,0) 808 (b2g, 01,829 (b2.u. 11,957 (b3ur 105), 1063 (au, 0), 1259 (b3g,0), 1390 (ag, 0), 1503 (biu,8), 1696 (ag, 0), 3221 (blu,1 l), 3244 (ag, 0), 3329 (b3g,0), 3357 (23, bzu) 670 (bi, 26), 832 (b2,6), 871 (bl, 48), H3BC2 919 (al, 9), 932 (b2,31), 1009 (az, O), 1215 (b2,8), 1216 (a,, 42), 1516 ( a l , 2), 2846 (al, 76), 3284 (b2,4), 3310 (al, e l ) 563 (bl, 95), 804 (bz, 17), 808 (a2, O), C3H4 944 (ai, 7), 1027 (a*, 0). 1047 (bt, 26), 1090 (bz, 31), 1132 (b,, l), 1183 (a!, < l ) , 1556 (a], l), 1684 (al, 17), 3158 (al, 52), 3250 (bl, 39), 3327 (b2, l ) , 3374 (al,
