3500
J. Phys. Chem. 1993, 97, 3500-?:~13
Reaction of Boron Atoms with Carbon Dioxide, Matrix and ab Initio Calculated Infrared Spectra of OBCO Thomas R. Burkholder and Lester Andrews' Department of Chemistry, University of Virginia, Charlottesville. Virginia 22901 - 781 I Rodney J. Bartlett Quantum Theory Project, University of Florida, Cainesville, Florida 3261 1-2085 Received: November 2, 1992; In Final Form: January 4, I993
Pulsed laser evaporated boron atoms codeposited with Ar/C02 samples give a strong product band a t 1863.4 cm-I that shows boron, carbon, and oxygen isotopic shifts. A b initio calculations predict two strong infrared bands for the BOCO addition product and one strong band, which scales to the observed band position, for the O B C O insertion product. The calculated and observed frequencies exhibit isotopic shifts appropriate for an "antisymmetric" 0-B, C-0 stretching fundamental in a bent OBCO molecule.
introduction Recent investigations in this laboratory have focused on boron atom reactions with oxygen, water, sulfur dioxide, and methane.I4 Thesestudies have shown a propensity for boron atoms toundergo insertion reactions. This current investigation of the B + C02 reaction is part of a series of studies of boron atom reactions with small molecules. The reaction of boron atoms with C02 has been the subject of both experimental and theoretical studies. The gas-phase experimental work has determined one of the products to be BO with the other presumed to be C0.5 Although this reaction is exothermic by 67 f 5 kcal/mol,6 it proceeds about 650 times slower than the boron atom reaction with oxygen, which is exothermic by 72.4 f 0.2 kcal/mol. Theoretical work predicts that the gas-phase reaction product will go to BO CO via an intermediate 'trans" structure BOCO (Figure la), predicted to lie 25 kcal/mol below B + C02; the intermediate lies 36 kcal/ mol above BO + CO with an activation barrier to decomposition of 5 kcal/m01.~ This adduct is similar to the antisymmetric AlOCO product which Le Quere and co-workers* observed in equilibrium between 9 and 17 K with a C2L'symmetric adduct (Figure Id). Recent studies in this laboratory9 have also yielded a wealth of new boron and CO species including the stable BCO species characterized previously by ESR and theoretical studies.I0 Since matrix isolation conditions are ideal for stabilizing these types of intermediates for study by IR, a study of boron atom reactions with C02 was undertaken. The major product OBCO, is intermediate in bonding between the stable linear OBBO and unstable linear OCCO species.l.".I2
+
Experimental Section The apparatus used for matrix isolation experiments has been described previously;1,2laser vaporization requires only a few modifications to a standard matrix isolation vacuum chamber. A closed cycle helium refrigerator (CTI Model 22) in a custom stainless steel vacuum chamber cooled a copper mounting block for a Csl window to 12 f 1 K. Basically, the 1064 nm line of a Spectra Physics DCR-I 1 Nd:YAG laser operated in the Q switch mode at 5 Hz with 60-100 mJ/pulse of energy was used to vaporize B atoms from a piece of solid boron. Efficient collection of laser vaporized species required that the CsI cold window be oriented at 1 8 0 O ; the incident light passed through a hole in the window and was focused on the ablation target. The atoms were then codeposited with Ar/C02 = 400/1 to 200/1
172.20
Figure 1. Calculated structural isomers of BCO2. 0
?l
IC.
O'OBCO
Figure 2. Infrared spectrum in the 2150-1750-c1n-~ region for laser ablatedboronatomscodepositedwithAr/C02= 200/1: (a)ondeposition and (b) after annealing to 25 K. mixtures for several hours and the infrared spectra from 4000 to 400 cm-1 were recorded on a Nicolet 60-SXR instrument with either 0.5- or 0.2-cm-I resolution; final spectra are averages of 300-800scans. Boronsamples ("B, 80.4% "B, 19.6% I0B, Aldrich 99.7%) and enriched boron samples (93.8% 'OB, 6.2% IlB, and 97.49% I IB, Eagle Pitcher) were used without further purification as were C02 samples (bone dry, Matheson) and isotopically enriched CO2 samples (I3CO2, 99% I3C and C1*02,50% l*O, Cambridge Isotopes Laboratory, CI802,93.6% '*OMiles-YEDA). Results
Infrared spectra obtained from reaction of B atoms with CO2 during condensation in an argon matrix and results of a b initio calculations will be presented.
0022-3654/93/2097-3500%04.00/0 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 14, 1993 3501
Reaction of Boron Atoms with Carbon Dioxide
TABLE I: Calculated and Observed Frequencies (cm-1) for Boron Insertion Products with CO2 scaled calcd linear"
bent"
bent'
bentd
bentd
SCF
SCF
SCF
SC F
SCF
DZP' (0.901)h
DZP (0.873)h
DZP (0.874)h
DZP (0.875)h
1863.4 1850.7 1847.8 1835.2 1820.8 10.9
1863.4 1834.4 1834.4 1844.2 1819.5 1.1
1863.4 1835.8 1832.6 1841.6 1819.6 1.1
1900.7 1883.2 1881.1 1876.3 1858.1 26.8 21.2
1878.4 1841.9 1869.5 1839.8 1834.1 2.4 1.9
1880.1 1843.5 1870.5 1841.2 1835.6 1.3 1.2
PVDZ'' (0.876)h
bentd MBPT(2) PVDZ (0.966)h
bentd PVDZ (0.958)h
1863.4 1839.3 1839.7 1835.8 1820.0 2.7
1863.4 1838.9 1835.3 1842.7 1820.0 2.4
1863.4 1862.6 1860.5 1829.1 1826.5 20.4
1863.4 1833.2 1830.2 1845.5 1819.1 2.5
1863.4 1836.2 1833.6 1843.4 1819.9
1884.3 1849.9 1872.2 1847.4 1839.9 3.9 3.5
1883.6 1848.4 1845.4 1872.3 1839.2 3 .O 2.4
1921.2 1919.5 1916.5 1888.7 1884.8 53.2 41.9
1877.0 1839.8 1868.8 1838.0 1832.5 4.0 3.4
1880.1 1844.6 1869.7 1843.6 1836.3
CCSD obsd
"B
OBCO OBITO
'hOBCIXO I KOBC IhO IXOBCIXO AKU~
'"B OBCO OBITO ' hOBC'KO I nOBC'hO 'nOBC'XO AKUS
(overall)
" HONDO 7.0." * Ratio 16-1 1-12-16 observed/calculated = scale factor for other isotopic calculations. Gaussian 90.14 ACESII.IS e Basis set used.
Infrared Spectra. Spectra in the 1750-2150-~m-~region of matrices formed by depositing isotopically enriched I0B and C02 (Figure 2a) showed several bands not observed in spectra of matrices containing only C02 argon. In addition to peaks due to C O and BCO species9marked in Figure 2, bands were observed at 1907.6, 1880.1, and 1863.4cm-I. The 1907.6-cm-I band was observed in previous 'OB 0 2 reactions assigned to IOB0.l Spectra recorded after annealing of the matrix to 18 K showed no changes; spectra taken after annealing to 25 K (Figure 2b) showed a decrease in the product band at 1880.1 cm-I, and an increase in broad bands at 2140 and 1850 cm-I. Further annealing to 35 K destroyed the product band completely. Spectra obtained upon deposition of isotopically enriched IB with C 0 2were similar to those obtained with 'OB; a single product band was observed a t 1863.4 cm-I in addition to the IlB/CO complexes and "BO at 1854.7 cm-I. In fact deposition of either I1B or 'OB with the isotopically enriched I3CO2or C1*02each produced only one new product band, which are listed in Table I. Natural isotopic boron reactions with CO2 produced a doublet at 1863.4 and 1880.1 cm-I with an intensity ratio of 4:l. Spectra of mixed carbon isotope C 0 2 ( A r / 1 3 C 0 2 / 1 2 C 0=2 1600/1/4) showed a doublet of product bands identical to the bands observed for the pure isotopes. This was also the case for samples containing CI602 and CI8O2. However samples containing 50 atom% I8O (Le., 25% CI602, 50% 1 6 0 C 1 8 0and , 25% C i 8 0 2 showed ) the product band split intoa quartet with intensity ratios of 25/ 18/20/26 for the I1Bproduct bands (Figure 3b) and 29/23/18/30 for the 'OB product bands (Figure 3a). Calculations. Ab initio calculations were run using three different program packages, HONDO 7.0,13Gaussian 90,14and ACES II.I5 The first two programs were only used for S C F calculations, while ACES I1 permits correlated results to be obtained at the coupled-clusterI6 and MBPTI7 levels. The first set of calculations were performed with the HONDO 7.0 program at the S C F level using a double {+ polarization basis set (DZP). Several geometries were optimized including linear and bent insertion products. The bent insertion product was found to be stable by about 57.9 kcal/mol with respect to BO and CO; the linear insertion product was found to be the next most stable, by about 50.5 kcal/mol. Both structures were found to be more stable than the 'cis" and "trans" addition products calculated in ref 7. DZP/SCF and the 6-31G'SCF geometries, the latter obtained by Gaussian 90, are shown in Table 11. Calculated S C F a b initio isotopic frequencies were obtained for the linear and bent OBCO products and are compared in Table I with the observed frequencies; the calculated isotopic frequencies were
OllBCO
m
+
z
- I
- h - .
0
b
. I
2150
2100
2650
2000
I 9 f l 9 3 3 3 a o 7 5 0
nfiVENUMOFR
Figure 3. Infrared spectrum in the 2150-1750-cm I region for laser ablated isotopic boron atoms codeposited with scrambled isotopic oxygen COz (25% ClbO2,50% C'bOinO,25% C'nO~):(a) deposition with 'OB, and (b) deposition with "B.
TABLE 11: Calculated Geometries for Bent OBCO. Hondo Gaussian Aces I1 Aces 11 7.0 90 (SCF) W F ) DZPh 6-31G* DZP PVDZ
RBO 1.188 RBC 1.567 Rco 1.159 CYOBC 172.2 CYCBO 133.5 (I
1.157 1.570 1.185 171.2 133.6
1.188 1.570 1.155 171.3 134.2
1.183 1.572 1.153 171.2 133.6
Aces I1 (CCSD) PVDZ 1.225 1.554 1.186 169.7 135.6
1.180 1.566 1.210 168.8 135.6
Distances in angstroms, angles in degree. Basis set used.
scaled using the observed 16011B12C160 frequency 1863.4 cm-1 for normalization. Geometries and frequencies for the bent OBCO molecule were also obtained at the MBPT(2) and CCSDI8correlated level using the PVDZ basisI9and the analytical CC/MBPTgradient methods in the ACES I1 program system. These calculations employed the recently developed open-shell C C analytical gradient capability in ACES I1 and arecompared with theresultsof SCFcalculations in Table III.2O Force constants were obtained from finite differences of the analytically computed gradients employing the automatic, minimum symmetry distinct displacements generated by ACES 11. The second-order perturbation theory MBPT(2) frequencies are fairly consistent with those from CCSD, except for the mode near 2000cm-I, where MBFT(2) offersonly a slight improvement over S C F and differs from the CCSD value by almost 200 cm-I.
3502 The Journal of Physical Chemistry, Vol. 97, No. 14, I993
TABLE III: Calculated Frequencies and Intensities for Bent OIIBCO (SCF, MBPT(2), and CCSD Levels Using PVDZ Basis Sets) SCF M BPT( 2) CCSD (cm I)
int (km/mol)
219.4 346.7 57 1.6 715.0 2126.5 2220.0
29.7 69.8 34.2 4.8 426.2 9.7
freq
freq
(cm
I)
184.5 294.6 525.6 680.6 1928.8 2208.8
int (km/mol) 10.2 37.8 16.9 0.3 98.7 225.8
freq
(cm
I)
198.1 301.9 527.8 673.9 1944.2 2043.3
int (km/mol) 14.4 40.2 17.5 0.8 202.4 6.2
Unlike S C F or CCSD, MBPT(2) places most of the intensity into this band. Apparently, because of the two close vibrations, the interaction causes the MBPT(2) isotope shifts to be poor. The infinite order CCSD results show much improvement, redistributing the intensity into the 1944.2 mode and showing a reasonable isotope shift. The CCSD frequency is 8 1 cm-1 above the observed values, which is about what is expected.I6 Once CCSD is augmented by triple excitations, for example, the 1855 cm-I stretch in C2 is predicted to lie 32 cm-I lower at the CCSDT level compared to CCSD,21and the 1 135-cm-l symmetric stretch mode of O3is reduced by 1 15 cm-' to 1 14 1 cm-1 from CCSD to CCSDT.22
Discussion The boron-carbon dioxide reaction product will be identified and isotopic frequencies will be compared with frequencies from several quantum chemical calculations, and reaction mechanisms will be discussed. Product Identification. The product observed showed only one characteristic vibrational mode, making a definitive structure difficult to obtain from the spectrum alone. The results with natural isotopic abundance boron indicate that only one boron is involved in the vibration. From the scrambled oxygen (50 atom % I8O)COzdata, thequartet observed leadstotheconclusion that there are two inequivalent oxygen atoms involved in the normal mode. The mixed carbon isotopic experiments indicates that there is only one C02 molecule involved in the reaction product. The boron shift observed, R( 10/ 1 1) = 1.008 96, is much smaller than expected for a diatomic B-0 vibration, R( 10/ 11) = 1.0290, in the harmonic oscillator approximation; however, the lXOshift R(16/18) = 1.0238, and 13C shift, R(12/13) = 1.0192, for the 'OB product band are very close to the shifts calculated for a diatomic C O vibration, R( 16/18) = 1.0248 and R( 12/ 13) = 1.0228. The product is thus identified with the BC02 stoichiometry, and the normal mode involves B-0 and C-0 stretching motions. Another interesting observation is the coupling of the IIB isotopes relative to the coupling of the 'OB isotopes. If thecoupling were perfect, Le., the boron and carbon masses identical, the result with scrambled oxygen would be a triplet or an unresolvable quartet. Since I1B is closer in mass to I2C than 'OB, the inequivalence (splitting of the central band pair) is less for 1611-12-16 than 16-10-12-16. Ab Initio Calculations. Several models exist from theoretical work that are consistent with the observation of a CO oscillator coupled to either the boron or oxygen of a BO subgroup; candidates are the 'cis" or 'trans" BOCO, Figure la,b, respectively, and the linear and bent insertion products OBCO, Figure lc,e. The box isomer, Figure Id, which is analogous to the most stable AI-CO2 structurexis ruled out since it has two equivalent oxygens and the strongest bands should be observed in the 1200-cm-1 region.' Furthermore, ab initiocalc~lations~ predict a strong C=O stretch around 1900 cm-' and a stronger B-0 stretch around 1200 cm-1 for the cis- and trans-BOCO molecules; both should be observed in the infrared spectrum if BOCO were formed. No such peak was observed near 1200 cm-1.
Burkholder et al. Other possibilities are the insertion products, linear and bent OBCO; the bent structure is predicted to be more stable by 7 kcal/mol than the linear product. These species are calculated to have a very intense 'CO stretch" near 1900 cm-1with the next most intense infrared absorption a factor of 20 weaker for the linear product and a factor of 15 weaker for the bent product. The motion is predicted to be an 'antisymmetric stretch", an out-of-phase coupling of the BO and C O bond stretches which is strongest for linear molecules and varies as the cosine of the angle between the bonds. When scaled by a factor of approximately 0.9, the SCF frequencies obtained for linear OBCO are in excellent agreement with the observed isotopic frequencies for the I1Bproduct bands; even the scrambled oxygen splittings are in reasonably close agreement. The agreement is not nearly as good for the 'OB isotopic bands; the frequencies are too high, as are the frequencies for the 13Cisotope, indicating that thecoupling through the B-C bond was not correctly determined. Furthermore, the lack of an observable B-0stretching fundamental is in agreement with the theoretical model for OBCO. The scaled a b initio frequencies for the geometry optimized bent OBCO structures are in much better agreement than with the linear OBCO model. The root-mean-square (RMS) deviation for all 9 isotopic combinations (excluding OilBCO) is 21.2 cm-1 for the calculated linear frequencies and less than 3.5 cm-1 for the S C F frequencies for bent OBCO. The CCSD frequencies were almost as good when scaled but the unscaled frequencies were within 5% of the observed frequencies. The I3C shift and the I8O shifts are predicted within a few wavenumbers of the observed shifts. This can be seen by the ARMSvalues in Table I. These frequency calculations for the bent OBCO showed that the coupling of the BO and CO oscillators through the B-C bond is determined correctly by all the methods used. The observed motion is the C A I vibration coupled strongly to the B-Ovibration. Reaction Mechsnisms. TheOBCO product is made by insertion of a B atom into CO2. When mixed Ci602/C1802samples were used, no bands were observed due to scrambled '60C180. The product molecule may decompose a t 25 K to give BO and CO, which are the gas phase reaction p r o d ~ c t s .This ~ is difficult to prove since both C O and BO diffuse through the matrix at this temperature and can complex or react with other species in the matrix. The molecule might also form larger clusters which have broad weak absorptions undetectable in the concentrations employed. The insertion reaction probably occurs in the gas phase or on the surface of the matrix and the product is quenched by the matrix. Evidence for this includes the fact that BO and CO were produced by reagent deposition (some C O could be produced by laser plume photolysis), and the observation that diffusion of B atoms at 18 K produces no further reaction and no increase in the product band intensity. Activation energy for the insertion reaction is provided by hyperthermal boron atoms produced in the laser vaporization process. This insertion behavior has also recently been observed for SO2reactions with boron..' The B SO2 reaction proceeds on annealing to 25 K, which is accord with the gas-phase observation that SO2 is more reactive than C 0 2 with boron.5 Aluminum and boron atoms form similar complexes with CO, but aluminum atoms do not insert into CO?under the conditions of the thermal evaporation experiments.8 A1 forms a AI-OCO complex with a C2, structure similar to transition metalcarboxylate anion complexes. This symmetric form is interconvertible by photolysis with a nonsymmetric form; Ga forms similar complexes with C02.23 Boron stands in contrast to both A1 and Ga; there is no experimental evidence for either of these latter addition complexes with boron. Furthermore, theoretical studies predict that the bent boron insertion product is more stable. Why does boron but not A1 or Ga insert into CO?? The simple answer may be that the 2p orbital on B has better overlap with one of
+
Reaction of Boron Atoms with Carbon Dioxide the T * antibonding orbitals on COZthan the larger 3p orbitals on A1 and 4p orbitals on Ga. The other explanation is that the insertion mechanism may require a three membered ring transition state with the incoming boron as the apex of the triangle. Aluminum and gallium are too large to fit without ring strain making the transition state inaccessible. And finally, the insertion process may require more activation energy than present in thermally evaporated aluminum and gallium atoms.
Conclusions Boron atoms react with CO2 in the gas phase or the fluid phase on the matrix surface to make an intermediate reaction product, which is then trapped in the matrix. Isotopic substitution shows one boron, one carbon, and two inequivalent oxygen atoms in the product. Quantum chemical calculations predict spectra for the BOCO addition product and the OBCO insertion product; the observed data match the calculated insertion product spectrum. The insertion reaction did not proceed on annealing the matrix, which suggests that activation energy from hyperthermal boron atoms was required. The new OBCO molecule is bent, which makes it different from either OBBO,' which is linear, or OCCO, which is predicted to be linear but weakly bonded.12 Several quantum chemical calculations predicted isotopic frequencies, which scaled to the observed values and confirmed identification of the bent OBCO insertion product.
Acknowledgment. We are grateful to J. D. Goddard for performing Gaussian 90 calculations and P. Szalay for helping get ACES I1 on line at Virginia. This work was supported by N.S.F. Grants C H E 88-20764 and 91-22556 and US.Air Force Contract AFOSR-F49620-92-5-0141. References and Notes ( I ) Burkholder, T. R.; Andrews, L. J . Chem. Phys. 1991, 95, 8697. (2) Andrews, L.; Burkholder, T. R. J . Phys. Chem. 1991, 95, 8554.
The Journal of Physical Chemistry, Vol. 97, No. 14, 1993 3503 Burkholder, T. R.; Andrews, L. Chem. Phys. Lefts. 1992,199,455. Hassanzadeh, P.; Andrews, L. J. A m . Chem. Soc. 1992, 96, 9177. DiGiuseppe, T. G.; Davidovits, P. J. Chem. Phys. 1981, 74, 3287. Chase, M. W., Jr.; Davies. C. A.; Downey. J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. J. Phys. Chem. Re/. Data 1985, Suppl No. I. (7) Marshall, P.;O'Connor, P. B.; Chan, W.-T.; Kristof, P. V.; Goddard, J. D.In Gas-Phose Mefal Reactions; Fontyn, A,, Ed.; Elsevier: Amsterdam, (3) (4) (5) (6)
1992. (8) LeQuere, A. M.; Xu, C.; Manceron, L. J . Phys. Chem. 1991, 95, 3031. (9) Burkholder, T. R.; Andrews, L. J . Phys. Chem. 1992, 96, 10195.
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( I I ) Sommer, A.; White, D.; Linevsky, M. J.; Mann, D. E. J . Chem. Phys. 1963, 38, 87. (12) Raine, G.P.; Schaefer, H. F., I l l ; Hadden, R. C. J. A m . Chem. Soc. 1983, 105, 194. (13) Dupuis, M.; Watts, J. D.; Villar, H. 0.;Hurst, G.J . B. HONDO 7.0; IBM Corp.: Kingston, NY, 1987. (14) Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Forseman, J. B.;
Schlegel, H. B.; Raghavachari, K.; Robb, M.A.; Binkley, J. S.;Gonzalez, C.; Defrees, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J . J. P.; Topiol, S.;Pople. J. A. GAUSSIAN 90; Gaussian, Inc.: Pittsburgh, PA, 1990. (15) Stanton, J. F.; Gauss, J.; Watts, J. D.; Lauderdale, W. J.; Bartlett, R. J . ACES II, an Ab Initio System for Coupled Cluster and Many-Body Perturbation Theory Methods. Quantum Theory Project; University of Florida: Gainesville, FL, 1992. (16) Bartlett, R. J. J. Phys. Chem. 1989, 93, 1697. (17) Bartlett. R. J. Annu. Reus. Phys. Chem. 1981, 32. 359. (18) Purvis, 111, G . D.; Bartlett, R. J. J . Chem. Phys. 1982, 76, 1910. (19) Dunning, T. H. J. Chem. Phys. 1989, 90, 1009. (20) Gauss, J.; Stanton, J. F.; Bartlett, R. J. J. Chem. Phys. 1991, 95, 2623. (21) Watts, J. D.; Bartlett, R. J. J . Chem. Phys. 1992, 96, 6073. (22) Watts, J. D.; Bartlett, R. J. Chem. Phys. Letr. 1992, 190. 19. (23) LeQuere, A. M.; Xu,C.; Manceron, L.; Burkholder, T. R.; Andrews,
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