12214
J. Phys. Chem. 1996, 100, 12214-12221
Reactions of Laser Ablated Be Atoms with H2O: Infrared Spectra and Density Functional Calculations of HOBeOH, HBeOH, and HBeOBeH Craig A. Thompson and Lester Andrews* Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22901 ReceiVed: March 6, 1996; In Final Form: April 14, 1996X
Reactions of laser-ablated Be atoms and H2O molecules during condensation with excess argon at 10 K gave three new molecules. The direct insertion product HBeOH was characterized by matrix infrared observation of isotopic frequencies and density functional theory calculations. The major product, HOBeOH, was formed by reaction of water with BeO, made from HBeOH decomposition. Another product, HBeOBeH, was formed by the reaction of a second Be atom with HBeOH.
Introduction Recently laser ablation has proven to be an effective means for producing novel beryllium-containing molecules including OBeO, ArBeO, NNBeNN, and HBeH.1-4 Molecules formed by reactions of beryllium with water have been examined by calculations;5,6 however, beryllium atom reactions with H2O have not been studied experimentally, presumably due to the high temperatures required to evaporate beryllium, the relative unreactivity of the metal, and the toxicity of its oxide. In contrast, photochemical reactions of thermally evaporated Mg, Ca, Sr, and Ba atoms with H2O have all produced new molecules for study with matrix isolation spectroscopy,7 as did reactions of laser-ablated B atoms with H2O.8 Here, spectra for reactions of laser-ablated beryllium atoms with water are presented along with density functional theory (DFT) calculations of several novel molecules in order to help with product identification. These products are of interest, in part, because relatively few small beryllium-containing molecules have been identified. Experimental Section The apparatus for pulsed-laser-ablation matrix isolation spectroscopy has been described earlier.9,10 Ar/H2O mixtures were deposited at 2 mmol/h onto a 10 ( 1 K substrate with metal atoms ablated using a Nd:YAG laser (1064 nm) focused (10 cm focal length) on a rotating 1 cm by 1 cm square beryllium metal target (Johnson-Matthey). Laser energies ranged from 40 to 80 mJ/pulse at the target, which gave dim blue and bright white plumes, respectively; spectra reported here were recorded in experiments using 70 mJ/pulse. Samples were co-deposited for 1-2 h periods and subjected to mercury arc photolysis (Philips 175 W) and annealing cycles, and more spectra were recorded at 0.5 cm-1 resolution and (0.1 cm-1 accuracy on a Nicolet 550 FTIR using a liquid N2 cooled MCT detector. Water vapor was added to the matrix gas (argon stream) through a needle valve, and deposition rates were monitored primarily by yields of isolated water monomer, dimer, and trimer bands.11 Distilled H2O and isotopic H218O (99% 18O, Cambridge Isotope Laboratories) and D2O (Aldrich) samples were outgassed by pumping on frozen samples prior to thawing for use. For mixed H216O/H218O experiments, separate water samples were evaporated into the argon stream. X
Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00692-2 CCC: $12.00
Figure 1. Infrared spectra from 3900 to 3650 cm-1 for laser-ablated Be + H2O reaction products in solid argon at 10 ( 1 K: (a) H2O in argon, (b) Be + H2O in argon, (c) H218O in argon, and (d) Be + H218O in argon.
Results Reactions were carried out for laser-ablated beryllium atoms and combinations of H216O, H218O, and D216O in a series of matrix isolation experiments. In order to simplify nomenclature, product bands in Figures 1-6 are labeled with their molecular assignments. Be + H216O in Argon. The H2O stretching region shown in Figure 1 includes the product spectrum for a 2 h co-deposition of Be with H216O in argon (Figure 1b) and a 0.5 h co-deposition of H2O in argon (Figure 1a). The Q branch of nonrotating water11 is marked W for comparison with product molecules, which do not rotate in the matrix environment. In addition to several known water bands (marked w), two new absorptions were observed. A band at 3841.8 cm-1 (marked HBeOH) was © 1996 American Chemical Society
Reactions of Laser Ablated Be Atoms with H2O
J. Phys. Chem., Vol. 100, No. 30, 1996 12215
Figure 4. Infrared spectra from 600 to 440 cm-1 for Be + H2O reaction products in solid argon at 10 ( 1 K: (a) Be + H2O in argon and (b) Be + H218O in argon. Figure 2. Infrared spectra from 2150 to 2060 cm-1 for Be + H2O reaction products in solid argon at 10 ( 1 K: (a) Be + H2O in argon and (b) Be + H218O in argon.
Figure 5. Infrared spectra from 2850 to 2675 cm-1 for Be + D2O reaction products in solid argon at 10 ( 1 K: (a) D2O in argon and (b) Be + D2O in argon. Figure 3. Infrared spectra from 1510 to 1350 cm-1 for Be + H2O reaction products in solid argon at 10 ( 1 K: (a) Be + H2O in argon and (b) Be + H218O in argon.
the highest frequency new absorption, and a sharp band at 3805.9 cm-1 (marked HOBeOH) adjacent to water absorptions was the strongest product band observed in this region. Other weak, sharp OH species were observed at 3547.9 and 3451.9
cm-1,12 and weak beryllium nitrogen product bands were found in agreement with earlier work.3 In addition, BeH2 was observed at 2159.4 and 696.8 cm-1; however, the BeH absorption at 1970.0 cm-1 was not detected.4 New product bands were observed in the BeH stretching region. Figure 2a shows a band at 2117.7 cm-1, marked HBeOH, and a strong absorption at 2092.4 cm-1, marked HBeOBeH.
12216 J. Phys. Chem., Vol. 100, No. 30, 1996
Thompson and Andrews TABLE 1: Isotopic Absorptions (cm-1) and Frequency Ratios for Be + H2O Reaction Products in Solid Argon
cm-1
Figure 6. Infrared spectra from 1590 to 1450 for laser-ablated Be + water reaction products in solid argon at 10 ( 1 K: (a) Be + H2O in argon, (b) Be + D2O in argon, and (c) Be + H2O-HOD-D2O in argon.
The BeO stretching region included both new and familiar bands. Two sharp new absorptions at 1559.2 and 1541.3 cm-1 will be illustrated with D2O experiments. A strong new band at 1493.7 cm-1 shown in Figure 3a is marked HOBeOH along with a sharp band marked BeOBe at 1412.4 cm-1 from previous Be + O2 experiments.1 The characteristic HO2 feature was observed at 1388.4 cm-1.13 In addition, a sharp new absorption was observed at 1403.1 cm-1, marked HBeOBeH. Bands for the rhombic (BeO)2 dimer were observed at 1131.2, 866.2, and 522.4 cm-1.1 Weaker, sharp bands at 1245.5 and 1206.9 cm-1 (not shown) gave oxygen and deuterium isotopic shifts as listed in Table 1. Bands in the region below 600 cm-1 are expected to belong to bending modes of X-BeOH and X-BeH species. The spectrum for a Be + H2O sample in Figure 4a reveals three major product bands. The sharp band at 578.7 cm-1, marked HBeOBeH, has a side band at 581.0 cm-1. Similarly an intense band at 559.0 cm-1 with a side band at 552.0 cm-1 is marked HBeOH. Finally, a weaker band at 456 cm-1 is labeled HOBeOH. Under the conditions described here, photolysis and annealing produced similar results: growth of weak Be + N2 aggregate bands observed earlier and broadening of sharp new product bands.3 No new bands were observed on photolysis or annealing. Be + H218O in Argon. Isotopic substitution with H218O was done to assist with band assignments. The spectrum for H218O in argon is given in Figure 1c along with Be + H218O co-deposition with argon in Figure 1d. Isotopic counterparts were observed at 3829.2 cm-1, marked HBe18OH, above the sharp band at 3793.7 cm-1, marked H18OBe18OH. Water (H218O) bands are labeled w with the nonrotating water band at 3715.7 cm-1 labeled W. Two new bands in the BeH stretching region showed small oxygen-18 isotopic shifts. Figure 2b shows a band at 2117.0 cm-1 marked HBe18OH, which shifted only 0.7 cm-1, as well as a band at 2091.7 cm-1 marked HBe18OBeH, which also shifted 0.7 cm-1.
assgn
H216O
H218O
D2O
16O/18O
HBeOH HOBeOH (aggregate) H 2O ? very weak ? very weak HBeOH HBeOBeH (H2O)-BeO (H2O)x-BeO Ar-BeO HOBeOH BeOBe HBeOBeH (BeOH) HBeOH (BeO)2 (BeO)2 HBeOBeH shoulder (H2O)(HBeOH) HBeOH (BeO)2 HOBeOH
3841.8 3805.9 3793.4 3730.1a 2707.1 2638.4 2117.7 2092.4 1559.3b 1541.3c 1526.1d 1493.7e 1412.4 1403.1f 1245.5 1206.9 1131.2 866.2 578.7 565 559.0 552.0 522.4 456
3829.2 3793.7 3780.0 3715.7 2692.8 2623.5 2117.0 2091.7 1536.3 1515.8 1497.4 1478.1 1371.3 1361.9 1223.4 1188.6 1109.0 849.0 575.1 562 557.8 550.0 512.3 453
2834.2 2805.4 2796.9 2769.7
1.003 29 1.003 22 1.003 54 1.003 88 1.005 31 1.005 68 1.000 33 1.005 14 1.014 97 1.016 82 1.019 17 1.010 55 1.029 97 1.030 25 1.018 06 1.015 40 1.020 02 1.020 26 1.006 26 1.005 3 1.002 15 1.003 64 1.019 72 1.006 6
2027.8 1663.6 1642.7 1554.1 1533.5 1477.3 1279 1098.3 1131.2 849.0 481 467 449 522.4
a Corresponds to ν3 of nonrotating water marked W in Figures 1 and 4.11 b Additional absorption at 1557.0 cm-1 in samples containing high concentrations of HOD; no new bands in mixed H216O-H218O samples. c Additional absorption at 1536.5 cm-1 in samples containing high concentrations of HOD; no new bands were seen in mixed H216OH218O samples. d Ar-BeO observed in extremely dilute water experiments, which generally resembled Be + O2 experiments due to oxygen contamination.2 e Additional absorptions at 1485.1 cm-1 in samples containing high concentrations of HOD and 1486.1 cm-1 in samples containing high concentrations of both H216O and H218O. f Additional absorption at 1348 cm-1 in samples containing high concentrations of HOD.
In contrast to bands in the BeH stretching region, new absorptions in the BeO stretching region showed large oxygen18 shifts. Bands at 1536.3 and 1515.8 cm-1 are oxygen-18 counterparts of bands at 1559.3 and 1541.3 cm-1. Absorptions in the 1350-1510 cm-1 region are shown in Figure 3. A strong sharp band observed at 1478.1 cm-1 marked H18OBe18OH was accompanied by an isotopic intermediate band at 1486.1 cm-1. A sharp band at 1371.3 cm-1, identified earlier1 as Be18OBe, is accompanied by a new sharp band at 1361.9 cm-1, marked HBe18OBeH. The 1493.7, 1486.1, 1478.1 cm-1 triplet was observed in several experiments, with varying relative intensities depending on the relative amounts of H216O and H218O metered into the argon carrier gas. Bands in the region below 600 cm-1 exhibited small shifts with 18O, as shown in Figure 4b. The sharp band at 575.1 cm-1 marked HBe18OBeH shifted 3.6 cm-1 from its 16O counterpart in Figure 4a. Similarly the band at 577 cm-1 marked HBe18OH shifted 2 cm-1, while H18OBe18OH at 453 cm-1 shifted 3 cm-1. Be + D2O in Argon. A series of experiments was done with increasing deuterium enrichment, and mixed H2O/D2O experiments were carried out to enhance intermediate mixed H/D band intensity. Deuterium isotopic bands exhibited large shifts but were weaker due to isotopic intensity effects. Figure 4 shows D2O in argon along with Be + D2O in argon, which reveals analogous products to those in the 3600-3900 cm-1 region of Figure 1. A broad band at 2834.2 cm-1 (marked DBeOD) is above the sharp band at 2805.4 cm-1 (marked DOBeOH). D2O bands are again marked w with nonrotating D2O at 2769.7 cm-1 marked W. Isotopic intermediates were not observed for product bands in the water stretching region. New bands were
Reactions of Laser Ablated Be Atoms with H2O
J. Phys. Chem., Vol. 100, No. 30, 1996 12217
TABLE 2: Comparison of Hartree-Fock, Configuration Interaction, and DFT/B3LYP Calculated Frequencies (cm-1) for Beryllium Oxides and Hydroxide BeO (1Σ)
σg
RHF CISD B3LYP
TABLE 3: Energies (au) for Be + H2O Reaction Products Calculated with Becke’s Hybrid DFT Method Using the Lee, Yang, and Parr Functional (B3LYP) and 6-311G* Basis Sets atom/molecule
1728.8 1575.8 1546.8
OBeO(3Σg)
σu
σg
πu
πu
ROHF CISD B3LYP
2610.0 1499.0 1382.2
820.3 777.1 734.1
93.3 222.6 241.9
91.1 222.0 241.9
BeOBe (3Σ+ u)
σu
σg
πu
πu
ROHF CISD B3LYP
1479.5 1429.1 1425.6
1076.0 1044.4 1033.0
160.3 85.2 147.3
160.3 85.2 147.3
BeOH (2A′)
a′
a′
a′
ROHF CISD B3LYP
4339.7 4117.8 4015.6
1318.6 1304.8 1293.9
360.6 317.8 307.2
also observed in the BeD stretching region (not shown): a sharp band at 1642.7 cm-1 corresponds to DBeOBeD, and a similar band a 1663.6 cm-1 correlates with DBeOD. Product spectra in the BeO stretching region in Figure 6 show the effect of deuterium isotopic substitution in the region expected for BeO stretching modes. A spectrum for Be + H2O is given in Figure 6a; the sharp band at 1493.7 cm-1 is the same band shown in Figure 3. In addition, the broad 1570 cm-1 absorption has been recently identified as BeOBeO based on DFT/B3LYP calculations.14 The Ar-BeO band observed earlier at 1526.4 cm-1 in Be + O2 experiments1 is absent owing to competition for complex formation with H2O. A distinct band at 1559.3 cm-1, marked H2O-BeO, is accompanied by a similar band and 1541.3 cm-1, marked (H2O)x-BeO. Figure 6b shows a 2 h co-deposition spectrum for Be + D2O. A sharp band at 1477.3 cm-1 marked DOBeOD is accompanied by a weak band at 1485.3 cm-1. Deuterium counterparts at 1554.1 and 1533.5 cm-1 are marked D2O-BeO and (D2O)x-BeO, respectively. A mixed H2O-HOD-D2O experiment adjusted to optimize HOD is shown in Figure 6c. Enhanced yield of the 1485.3 cm-1 HOBeOD band is evident as are intermediate bands at 1557.0 and 1536.5 cm-1. Weaker deuterium counterpart bands for the 1403.1 cm-1 HBeOBeH band were observed at 1348 and 1279 cm-1, corresponding to DBeOBeH and DBeOBeD, respectively, and a weak band at 1098.3 cm-1 is associated with DBeOD. Bands in the bending mode region were complicated by large isotopic shifts and band overlap. A broad band centered at 481 cm-1, observed in Be + D2 experiments,4 corresponds to the deuterium isotopic counterpart DBeOBeD. A broad band at 449 cm-1 had the same profile as the 559 cm-1 band in H2O experiments. The deuterium isotopic counterpart for the 456 cm-1 band was not observed as this band shifted beneath the 400 cm-1 lower limit of the instrument. Quantum Chemical Calculations. Calculations for a number of Be + H2O product molecules were performed using Gaussian 94,15 running on a Cray C90 supercomputer at the San Diego Supercomputer Center. For molecules with more than three atoms, geometry optimizations and frequency calculations were carried out using density functional theory (DFT). The DFT calculations employed Becke’s three parameter hybrid method with the Lee, Yang, and Parr correlation functional (collectively abbreviated B3LYP),16,17 and the 6-311G* basis sets for all atoms. Hartree-Fock calculations were restricted;
symmetry, state
energy (au) -0.502 156 -1.176 632 -14.671 184 -14.580 757 -15.264 396 -75.085 386 -76.433 933 -89.929 767 -90.597 764 -90.508 150 -91.251 242 -104.077 057 -105.423 413 -106.769 226 -150.364 786 -165.163 234 -166.453 907 -166.567 259 -527.553 19 -617.500 93
2
S D∞h, 1Σg+ 1 S 3P C∞V, 2Σ 3 P C2V, 1A1 C∞V, 1Σ Cs, 2A′ C∞V, 2Σ Cs, 1Σ D∞h, 3Σu+ C∞V, 2Σ D∞h, 1Σg+ D∞h, 3ΣgD∞h, 3ΣgCs, 1A′ C2, 1A 1 S C∞V, 1Σ
H H2 Be Be BeH O H 2O BeO BeOH HBeO HBeOH BeOBe HBeOBe HBeOBeH O2 OBeO H2O-BeO HOBeOH Ar Ar-BeO
TABLE 4: Frequencies (cm-1), Intensities (km/mol), Bond Lengths (Å), and Bond Angles (deg) of H2O, BeOH, and HBeO Calculated Using B3LYP and 6-311G* Basis Sets ω HOH DOD DOH
Be16OH Be18OH Be16OD Be18OD
I
ω
I
ω
I
H2O (C2V, 1A1), RBeH ) 0.9625, ∠HOH ) 106.0 3879.7 13 3766.2 0 1707.4 2844.4 11 2711.5 0 1251.2 3825.1 8 2776.0 3 1497.3
88 47 77
BeOH (Cs, 2A′), RBeO ) 1.392, ROH ) 0.9511, ∠BeOH ) 146.3, 〈S〉2 ) 0.7509 4015.9 118 1293.9 97 307.2 4001.5 114 1274.0 91 304.7 2931.4 80 1275.8 104 236.1 2911.1 76 1255.2 100 233.1
214 212 113 111
HBeO (C∞V, 2Σ), RBeH ) 1.325 63, RBeO ) 1.4136, 〈S〉2 ) 0.7535 HBeO 2144.2 8 1137.3 120 552.3 2 × 162 DBeO 1654.8 36 1062.4 92 449.5 2 × 108 HBe18O 2143.8 8 1112.8 115 551.1 2 × 162 DBe18O 1652.1 33 1039.7 89 448.0 2 × 107
however, at higher levels of theory unrestricted calculations were done. Very little artificial spin-orbit contamination was observed in the calculations presented here, and 〈S〉2 values are listed for multiplet state DFT calculations in Tables 3-8. Table 2 lists frequency calculations for four molecules comparing Hartree-Fock, configuration interaction (CISD), and density functional theory (B3LYP). The BeO fundamental is 1463.7 cm-1 in the gas phase;18 however, in argon matrix experiments the formation of an Ar-BeO complex blue shifted the BeO stretching vibration to 1526.4 cm-1.1 In the case of BeO, all three levels of theory presented in Table 2 overestimate the stretching frequency with DFT, giving the lowest calculated value. The antisymmetric stretch of linear OBeO was observed in earlier Be+ O2 studies at 1413.2 cm-1, and the assignment, supported by MBPT(2) calculation of this frequency at 1422.5 cm-1.2 The beryllium dioxide molecule is predicted to have the 3Σg- ground state from SA-CASSCF calculations of Bauschlicher et al.19 Comparison of the three levels of theory again indicates that DFT calculations give very good agreement. A recent study of alkaline earth metal dioxide molecules shows that B3LYP produces excellent results; a 1372.1 cm-1 frequency was obtained for OBeO using slightly different basis sets.14 The antisymmetric stretch of BeOBe was observed at 1412.4 cm-1 in an argon matrix, and the assignment, supported by MBPT-
12218 J. Phys. Chem., Vol. 100, No. 30, 1996
Thompson and Andrews
TABLE 5: Frequencies (cm-1), Intensities (km/mol), Bond Lengths (Å), and Bond Angles (deg) of HBeOH Calculated Using B3LYP and 6-311G* Basis Sets ω
ω
I
ω
I
HBeOH (Cs, A′), RBeH ) 1.327, ∠BeOH ) 146.3, RBeO ) 1.392, ROH ) 0.951, ∠HBeO ) 179.1 HBe16OH 4031.3 137 2189.4 172 1244.5 64 595.1 193 592.9 237 321.0 186 DBe16OD 2942.6 97 1710.9 170 1129.4 31 486.8 162 483.1 134 238.2 92 HBe16OD 2942.7 98 2118.5 169 1226.3 70 594.9 198 592.1 224 244.0 98 DBe16OH 4031.3 137 1714.8 171 1144.8 25 489.4 188 483.3 129 314.0 166 HBe18OH 4017.3 133 2188.8 170 1221.2 61 593.9 190 591.3 233 318.9 185 DBe18OD 2922.3 92 2188.1 168 1205.3 68 593.5 196 590.6 222 241.4 97 HBe18OD 4017.3 133 1710.9 166 1124.3 25 487.1 184 481.9 126 312.5 167 DBe18OH 2922.2 91 1707.7 166 1110.6 30 484.7 159 481.7 132 236.0 92 BeH BeD
BeH (Cs, 2045.6 1518.0
RBeH ) 1.3461, 136 75
〈S〉∞
) 0.7512
TABLE 6: Frequencies Intensities (km/mol), Bond Lengths (Å), and Bond Angles (deg) of HOBeOH and OBeO Calculated Using B3LYP and 6-311G* Basis sets I
ω
I
ω
I
HOBeOH (C2, 1A), RBeO ) 1.424, ROH ) 0.954, ∠BeOH ) 130.3, ∠OBeO ) 179.0, H-OBeO-H (dihedral) ) 86.0 H16OBe16OH 3960.0 (21) 3958.3 (130) 1550.4 (349) 752.5 (4) 552.4 (117) 538.5 (374) 337.9 (30) 331.1 (106) 237.1 (136) D16OBe16OD 2886.1 (14) 2885.6 (100) 1536.3 (381) 730.5 (0) 473.4 (123) 461.6 (205) 291.0 (10) 285.7 (102) 174.2 (73) H18OBe18OH 3946.7 (20) 3945.0 (125) 1534.2 (336) 714.8 (7) 545.0 (111) 533.1 (369) 335.4 (28) 328.7 (103) 236.1 (136) H16OBe18OH 3959.2 (69) 3945.8 (79) 1542.5 (343) 733.6 (6) 549.4 (125) 535.2 (361) 336.7 (30) 329.8 (104) 236.6 (136) H16OBe16OD 3959.2 (76) 2885.9 (57) 1543.4 (365) 741.4 (5) 467.0 (173) 546.1 (232) 334.7 (63) 288.1 (63) 208.0 (116) CISD, HOBeOH, RBeO ) 1.422, ROH ) 0.945, ∠BeOH ) 129.3, ∠OBeO ) 178.7, H-OBeO-H (dihedral) ) 90.2 4116.4 (25) 4112.7 (145) 1585.6 (377) 767.3 (4) 586.3 (128) 573.6 (406) 348.2 (49) 335.0 (122) 237.7 (153) 16OBe16O 18
OBe18O 18 OBe16O
OBeO (D∞h, 3Σg-), RBeO ) 1.434, 〈S〉2 ) 2.053 1382.2 (322) 734.1 (0) 241.9 1365.2 (314) 692.0 (0) 238.9 1373.9 (318) 713.0 (0) 240.4
(2 × 114) (2 × 112) (2 × 113)
(2) calculation of this mode at 1418.2 cm-1.2 The B3LYP value, 1425.6 cm-1, is in very good agreement. Recent higher level calculations20 predict that the 1Σg+ state may be slightly lower in energy than the 3Σu+ state; however, the calculated ν3 frequencies are nearly the same, and therefore, the experimental observation of BeOBe does not allow for selection of the ground state. Comparisons of three levels of theory are also given for the simplest beryllium-oxygen-hydrogen species, BeOH. The bending modes of BeOH and MgOH are of interest because of quartic character in the bending potential surfaces and covalent character in the bonding.21,22 Several reaction product molecules were investigated with calculations. The energies for structures optimized at the DFT/ B3LYP level are given in Table 3. Since product molecules exhibited similar O-H stretching modes as water, the results
ω
I
ω
I
HBeOBeH (D∞h, Σg), RBeH ) 1.325, RBeO ) 1.409 HBe16OBeH 2199.4 0 2192.4 496 1399.1 979.3 0 622.2 2 × 406 578.4 123.5 0 18 HBe OBeH 2199.4 0 2191.2 484 1355.8 979.3 0 619.1 2 × 403 578.4 120.2 1 DBe16OBeD 1732.4 648 1695.0 0 1287.0 898.9 0 518.8 2 × 272 466.0 107.7 0 DBe16OBeH 2195.9 247 1715.7 356 1345.3 934.8 0 605.2 2 × 268 487.6 115.5 1
I
1
Be16OBe Be18OBe
BeOBe (D∞h, 3Σu), RBeO ) 1.4087, 〈S〉2 ) 2.001 1425.6 553 1033.0 0 147.3 1382.9 521 1033.0 0 142.9
385 0 366 0 139 0 231 71
2×2 2×2
TABLE 8: Frequencies (cm-1), Intensities (km/mol), Bond Lengths (Å), and Bond Angles (deg) of H2O-BeO, Ar-BeO, and BeO Calculated Using B3LYP and 6-311G* Basis Sets ω
(cm-1),
ω
ω
I
1
2Σ),
TABLE 7: Frequencies (cm-1), Intensities (km/mol), Bond Lengths (Å), and Bond Angles (deg) of HBeOBeH and BeOBe Calculated Using B3LYP and 6-311G* Basis Sets
I
ω
I
ω
I
RBeO ) 1.332, RH2O-BeO ) 1.616, RHO ) 0.963, H2O-BeO (Cs, ∠OBeO ) 178.2, ∠HOBe ) 122.0, ∠HOH ) 111.0, HOBeO (dihedral) ) 76.4 H216OBe16O 3883.0 252 3798.2 97 1726.2 171 1615.9a 63 765.7 44 556.9 1 358.1 214 221.7 6 171.4 169 D216OBe16O 2851.7 128 2734.9 59 1620.7 11 1266.6 58 616.1 25 538.2 0 344.6 120 201.4 8 134.8 91 HD16OBe16O 3842.5 182 2791.4 87 1623.7 139 1518.7 67 675.2 32 544.9 1 350.9 162 211.8 10 153.4 130 H218OBe18O 3866.1 251 3790.8 95 1717.3 161 1590.1 67 760.0 45 529.8 2 352.7 208 219.7 5 170.5 173 H216OBe18 3883.0 252 3798.2 97 1725.7 164 1590.4 1 765.5 43 545.9 66 356.5 211 219.9 5 170.8 171 1A′),
Ar-Be16O Ar-Be18O Be16O Be18O
Ar-BeO (C∞V, 1Σ), RBeO ) 1.321, RArBe ) 2.086 1601.3 108 264.0 3 187.6 1571.0 106 257.5 3 185.8
2 × 59 2 × 55
BeO (C∞V, 1Σ), RBeO ) 1.321 1546.8 11 1515.5 7
a Adjacent H O bending and Be-O stretching modes showed 2 coupling; the mode corresponding predominantly to the Be-O stretch is underlined.
of B3LYP calculations for water including isotopic frequencies are given in Table 4 along with the possible HBeO and BeOH products. Many calculations on BeOH have been published, including SCF, configuration interaction (CISD), and MollerPlesset (MP2, MP3, and MP4) studies.20-22 Although BeOH has been detected from the reaction of thermally generated Be atoms with HOOH using ESR,23 vibrational spectra for this molecule are not known. The beryllium reacting in these systems is probably excited 3P atoms,3,14 which can form addition and insertion products 1 with water. In the case of Be and O2, direct insertion gave the 3 Σg molecule OBeO.2 In similar reactions of boron and ammonia, the primary insertion product HBNH2 was energized and subject to further decomposition including hydrogen atom elimination to form HBNH.24 Accordingly the insertion reaction of beryllium with water to give HBeOH is likely, and the HBeO and BeOH decomposition products must be considered. Molecules observed previously, namely, BeH, OBeO, BeO, and
Reactions of Laser Ablated Be Atoms with H2O ArBeO, were also investigated for comparison of similar modes in the anaticipated product molecules. The geometries and frequencies calculated here for HBeOH, BeOH, and HBeO are compatable with those calculated at higher MP2 and QCISD levels of theory.20 For the identification of a molecule like HBeOH, isotopic frequencies are required, and these calculations are presented in Table 5. The molecule HOBeOH was considered because the major reaction product contained two equivalent oxygen atoms. A structure with a dihedral angle resembling HOOH was found, and the strongest infrared band was blue shifted from OBeO (Table 6); a CISD calculation is also included for comparison with DFT/B3LYP. New bands adjacent to the BeOBe molecule indicated a product with two beryllium atoms. Calculations on HBeOBeH reveal a linear structure and are given in Table 7 along with B3LYP calculations for BeOBe. Previous SCF calculations predicted the same symmetry and similar geometric parameters as B3LYP calculations, but frequencies were not obtained in the earlier work.6 Calculations were also done for the H2O-BeO complex with several structures, and the Cs structure given in Table 8 was the most stable. Again isotopic frequencies were determined for comparison with experimental results. Discussion The new beryllium-oxygen product molecules will be identified, and reaction mechanisms will be discussed. HOBeOH. The signature of HOBeOH in these experiments is the strong 1493.7 cm-1 band in the OBeO stretching region. This absorption above the ν3 mode of OBeO at 1413.2 cm-1 gives an oxygen isotopic triplet (Figure 3), confirming that it also is primarily an OBeO antisymmetric stretch involving two equivalent oxygen atoms. In addition slight participation of two equivalent hydrogen atoms is demonstrated by the deuterium shift and isotopic pattern (Figure 6). The simplest molecule having two equivalent oxygen atoms and two equivalent hydrogen atoms bonded in such a way as to have an OBeO stretching mode is HOBeOH. Calculations in Table 6 show that HOBeOH is expected to have an antisymmetric OBeO stretch above OBeO, in agreement with the band observed here. It is clear from comparison of calculations that DFT/B3LYP underestimates the antisymmetric stretching frequency of OBeO and overestimates this mode for HOBeOH. Furthermore, the 1493.7 cm-1 band shows a small deuterium shift, as predicted by the calculations. There is good agreement between the calculated and observed ratios for 16O/18O (1.0056 calculated, 1.01055 observed) and H/D (1.00918 calculated, 1.01110 observed) and the position of mixed 16O, 18O, and H,D isotopic absorptions. The molecule HOBeOH is also expected to have a mode in the O-H stretching region, and B3LYP calculations predict a stretching mode about 79 cm-1 blue shifted from H2O. Using the same theoretical/experimental scale factor derived from H2O (1.0203) on the calculated OH stretching vibration for HOBeOH predicts an absorption at 3805.9 cm-1 precisely the band observed in these experiments. The DOBeOD band is likewise observed at 2805.4 cm-1. Locating the correct bending mode for this molecule in the region below 600 cm-1 is not straightforward. Three considerations are important: (1) the order of calculated frequencies for the major products, (2) the oxygen isotopic shifts observed and calculated for the major product bands, and (3) comparison of major product band intensities over a wide range of reagent concentrations, including trace water impurity in oxygen and hydrogen experiments.2,4 All of the above correlates the 456
J. Phys. Chem., Vol. 100, No. 30, 1996 12219 cm-1 band with the other HOBeOH absorptions. Calculations at the B3LYP level predict a strong Be-O-H bending mode at 538 cm-1 in the harmonic approximation. However, the BeO-H bending mode has strong quartic character,21,22 and the neglect of anharmonicity in the present calculations will no doubt affect agreement with experiment. On the basis of correlation of band intensities the 456 cm-1 band is grouped with the 3805.9 and 1493.7 cm-1 bands, which are assigned to HOBeOH. The HOBeOH molecule is calculated to have a C2 structure with bond and dihedral angles near those in hydrogen peroxide. Although HOBaOH, HOCaOH, and HOMgOH have been characterized in earlier work,7,25 HOBeOH is a new molecule with an interesting structure. HBeOH. Bands at 3841.8, 2117.7, and 1206.9 cm-1 are grouped together on the basis of common behavior from concentrated to dilute water experiments. HBeOH is a likely reaction product in these experiments because it arises from the simple insertion of a beryllium atom into water and is energetically favored with respect to the Be-H2O complex or smaller molecules formed by the ejection of hydrogen atoms. The HBeOH molecule (Table 5) is calculated to have an OH stretching mode at 4031 cm-1, blue-shifted ∼150 cm-1 from H2O. The band at 3841.8 cm-1 is a likely candidate since it is significantly blue-shifted from H2O (3730.1 cm-1) and the calculated 16O/18O isotopic ratio (1.00348) compares favorably to the observed value (1.00322). The band at 1206.9 cm-1 has an isotopic ratio indicating primarily a BeO stretching mode; however, the band is significantly red-shifted (258 cm-1) from the gas-phase fundamental of BeO at 1463.7 cm-1.18 Analogous behavior is found for the HBeOH band calculated at 1244.5 cm-1 below the BeO fundamental calculated at 1547 cm-1 using the same method. A third band in the BeH stretching region also tracked with the two above absorptions. The band at 2117.7 cm-1 is 148 cm-1 above matrix-isolated BeH (1970 cm-1).4 This is in agreement with the BeH stretch of HBeOH calculated at 2189.4 cm-1, which is 143 cm-1 above the BeH fundamental calculated at 2045.6 cm-1 using the same method. Assignment of the 2117.7 cm-1 Be-H stretching absorption shows that this molecule is not BeOH. The calculated frequencies (Table 5) are complicated by slight overestimation of mode mixing in the vibrational frequencies of HBeOH, DBeOD, and HBe18OH; however, trends in the calculated frequencies are in agreement with the observed spectra. Accordingly the above bands are assigned to HBeOH based on the observation of blue-shifted OH and BeH stretching modes along with a red-shifted BeO stretching mode in agreement with frequency calculations. Calculations predict HBeO and out-of-plane HBeOH bending modes just above bending modes for HOBeOH and below the bending mode for HBeOBeH. The broad 559 cm-1 band most likely contains more than one absorption, and the sharp 552.0 cm-1 shoulder is tentatively assigned to HBeOH. Hydrogen bonding is expected to blue shift this bending motion, and the broad 559 cm-1 band is probably due to the (H2O)(HBeOH) complex. Such a dimer can be formed by the reaction of Be with (H2O)2. On the other hand, red-shifted O-H stretching modes in the complex are probably masked by water absorption. Finally, the Be-O-H bending mode calculated by theory increases with level of theory from 321 cm-1 (DFT) to 413 cm-1 (QCISD).20 Following this mode for BeOH, some quartic character is expected and harmonic calculations may be in error. Although HMgOH has been observed in thermal experiments,7 HBeOH is a new molecule, which has been investigated by theoretical calculations. The B3LYP calculation in Table 5 predicts the molecule to be bent (Cs) in agreement with earlier
12220 J. Phys. Chem., Vol. 100, No. 30, 1996 reports.5,6,20 SCF frequencies provided by Sakai and Jordan were considerably higher than B3LYP calculations for all modes.6 In contrast, MP2 and QCISD calculations by Boldyrev and Simons20 predict a similar structure but calculate the OH stretching mode higher by ∼30 cm-1, the BeO stretching mode lower by ∼70 cm-1, and BeH stretching and bending modes comparable to the B3LYP calculations presented here. BeOH. The weak band at 1245.5 cm-1, which shifts to 1223.4 cm-1 with 18O, is tentatively assigned to BeOH on the basis of B3LYP prediction of this mode about 50 cm-1 above the HBeOH value. As BeOH is potentially a minor reaction product, no other bands were observed. HBeOBeH. New bands at 2092.4, 1403.1, and 578.7 cm-1 are grouped on the basis of relative intensities in experiments with varying concentrations of H2O. The presence of a BeOBe subunit in the molecule is indicated by the strong band at 1403.1 cm-1 with a similar large 16O/18O isotopic ratio and red shift from BeOBe by 10 cm-1 (Figure 3). The 1403.1 cm-1 band exhibits HD and D2 isotopic counterparts, which verify the involvement of two equivalent H atoms and a mixed oxygen isotopic doublet for the vibration of a single oxygen atom. HBeOBeH is a likely candidate since it is a stable molecule with a similar BeOBe subunit structure, and the HMgOMgH molecule has been previously observed.7 Calculations in Table 7 show that HBeOBeH is expected to have a strong absorption red-shifted from BeOBe in agreement with the 1403.1 cm-1 band observed here. The strong band at 2092.4 cm-1 (Figure 2) agrees with the calculation of a strong band in the BeH stretching region, which shifts to 2091.7 cm-1 with 18O and to 1642.7 cm-1 with D2O in accord with calculated isotopic frequencies. Calculations also predict a very strong bending mode at 622.2 cm-1 with a 16/18 ratio of 1.005 00 and a H/D ratio of 1.199; the 578.7 cm-1 band exhibits a 16/18 ratio of 1.006 26 and a H/D ratio of 1.203 in excellent agreement. Finally the 2092.4, 1403.1, and 578.7 cm-1 band absorbances are 0.055, 0.032, and 0.048, respectively, in the cleanest experiment; although the calculated intensities favor the bending mode slightly, the relative intensity agreement is very good. The observation of BeOBe stretching and BeH stretching and bending modes in excellent agreement with B3LYP calculated frequencies and isotopic shifts is sufficient to assign these bands to HBeOBeH with confidence. H2O-BeO and (H2O)x-BeO. In addition to the bands previously discussed, two new complexes of BeO were observed. Bands at 1559.3 and 1541.3 cm-1 both produced oxygen isotopic doublets and isotope ratios consistent with BeO stretching modes. The bands are higher in frequency than the Ar-BeO complex observed in earlier Be + O2 experiments.1,2 Furthermore, the complexes gave small isotopic shifts in experiments with D2O, but shifts were not seen in earlier Be + H2 work.4 Note the isotopic triplet for the 1559.3 cm-1 band with mixed H2O-HOD-D2O (Figure 6c). Calculated frequencies for BeO, Ar-BeO, and H2O-BeO predict the complex H2O-BeO to blue shift from Ar-BeO by 15 cm-1. The calculation of a blue-shifted BeO stretching frequency relative to Ar-BeO and the coupling of H2O to form hydrogen-deuterium isotopic multiplets are sufficient to assign these bands to the water complexes: H2O-BeO and (H2O)xBeO. The observation of a sharp intermediate isotopic band for the 1559 cm-1 absorption is the basis of assignment to H2OBeO. Similarly, the 1541.3 cm-1 band produced a broad intermediate band that is likely the result of overlapping intermediates and is accordingly assigned to (H2O)x-BeO. Note that, in the isotopic calculations for H2O-BeO, coupling of hydrogen to the BeO stretch accounts for the H-D isotopic
Thompson and Andrews triplet in Figure 6c; however, coupling of the BeO stretching mode to the water bending mode is overestimated, resulting in the frequency being highest for HOD-BeO in contrast to the present observation. Water bending and stretching modes were not observed for these complexes, but B3LYP calculations show that these modes are expected to be very close to the water absorptions and likely coincident. Isotopic data are also consistent with an H2 complex; however, failure to observe the bands in Be + H2 work,4 where Ar-BeO absorptions were present, are consistent with H2O-BeO and not H2-BeO complex. Reaction Mechanisms. Mechanisms of formation can be deduced from products observed in this study and in previous experiments with beryllium and oxygen.1,2 Earlier studies with laser-ablated beryllium suggest participation of metastable Be (3P1) in these reactions.3,14 The excited state is 61 kcal/mol higher in energy than ground state Be and likely provides activation energy for the reactions observed here; however, energy changes for reactions are calculated using the ground state Be atom energy. HOBeOH is expected from OBeO or BeO reactions in these experiments; however, the high yield of H18OBe18OH and Be18O complexes and the absence of 18OBe18O in the spectra indicate that the major reaction likely involves BeO, formed by the reaction of atomic beryllium with water. The initial insertion of Be into water forms an energy-rich species:
Be + HOH f [HBeOH]* f HBeOH ∆E ) -92 kcal/mol (1) Some of the insertion product is relaxed by the matrix and is trapped; however, a fraction dissociates to form BeO:
Be + HOH f HBeOH* f BeO + 2H ∆E ) +107 kcal/mol (2a) Be + HOH f HBeOH* f BeO + H2 ∆E ) -1 kcal/mol (2b) Be(3P1) + HOH f HBeOH* f BeO + H2 ∆E ) -62 kcal/mol (2c) Be + HOH f HBeOH* f BeOH + H ∆E ) +3 kcal/mol (2d) The energy requirement for reaction 2a is high even when considering Be(3P1) and implies a rearrangement such as (2b). For the Be(3P1) reagent, reaction 2c is exothermic by 62 kcal/ mol, and the large yield of BeO-containing products observed here attests to the role of Be(3P1) in these reactions. Although reaction 2d is slightly endothermic, metastable Be(3P1) can easily provide this energy requirement, and BeOH was possibly detected in these experiments. The higher energy HBeO isomer (by 56 kcal/mol) was not observed. Once formed, BeO can insert into water in a highly exothermic reaction:
BeO + HOH f HOBeOH
∆E ) -128 kcal/mol (3a)
H2O + BeO f H2O- -BeO
∆E ) -57 kcal/mol (3b)
Ar + BeO f Ar-BeO
∆E ) -11 kcal/mol
(3c)
Considering the exothermicity of reaction 2c, the BeO reagent participating in reaction 3a probably contains considerable internal energy, which serves as activation energy. Reaction 3a accounts for the large yields of DOBeOD and H18OBe18OH in isotopic experiments. If, on the other hand, BeO is relaxed,
Reactions of Laser Ablated Be Atoms with H2O
J. Phys. Chem., Vol. 100, No. 30, 1996 12221
it may form a complex with H2O, reaction 3b, which is more stable than the Ar-BeO complex, reaction 3c, in agreement with the Frenking et. al calculation (∆E ) -7 kcal/mol).27 In earlier Be + H2 experiments, OBeO formed from reaction with trace O2 impurity probably inserted into hydrogen; again the OBeO so formed contains considerable excess internal energy at birth. The analogous magnesium species HOMgOH was identified in laser-ablated Mg atom studies.25,26 The absence of 18OBe18O and high yield of H18OBe18OH indicates that reaction 4 is not the major reaction pathway in the water experiments.
OBeO + H2 f HOBeOH
∆E ) -143 kcal/mol (4)
The formation of BeOBe is in agreement with earlier Be + O2 experiments,2 where Be adds to BeO in a secondary reaction.
BeO + Be f BeOBe
∆E ) -106 kcal/mol
(5)
The great stability of BeOBe has been discussed, and ∆E for reaction 5 at the QCISD(T) level of theory (-117 kcal/mol) is in excellent agreement.20 The stable linear molecule HBeOBeH is formed in these experiments. There are two apparent routes for the formation of HBeOBeH, and the straightforward reaction 6a is more likely as the yield of HBeOBeH was low in Be+H2 experiments.
Be + HBeOH f HBeOBeH
∆E ) -97 kcal/mol (6a)
BeOBe + H2 f HBeOBeH
∆E ) -82 kcal/mol (6b)
Conclusions The novel products identified here provide interesting comparisons to analogous species with Mg, Ca, and Sr and further characterize the extensive chemistry of BeO and beryllium. Laser-ablated Be atoms react with water to form an insertion product [HBeOH]* that may either relax to form HBeOH, a new molecule, or dissociate to form BeO, whose chemistry is of interest owing to its ability to form strong complexes such as Ar-BeO.1,2 Once formed, BeO further reacts with water to produce HOBeOH, a new molecule whose BeOH bond and dihedral angles resemble those of H2O2. Complexes of H2OBeO and (H2O)x-BeO are also identified by a blue-shifted Be-O stretching vibration, which showed H2, HD, and D2 isotopic structure and accounted for decreased yields of ArBeO, owing to competititon for BeO complex formation. BeO also reacts with another Be atom to form the stable BeOBe molecule observed in previous Be + O2 experiments.2 Some of the HBeOH also undergoes a secondary reaction with Be to form the stable linear HBeOBeH molecule. The new molecules reported here, HBeOH, HBeOBeH, and HOBeOH, are identified
through a comparison of observed and calculated isotopic frequencies using DFT with the B3LYP functional. Acknowledgment. We gratefully acknowledge financial support from NSF Grant CHE 91-22556, computer time at the San Diego Supercomputer Center and helpful discussions with G. P. Kushto. References and Notes (1) Thompson, C. A.; Andrews, L. J. Am. Chem. Soc. 1994, 116, 423. (2) Thompson, C. A.; Andrews, L. J. Chem. Phys. 1994, 100, 8689. (3) Thompson, C. A.; Andrews, L.; Davy, R. D. J. Phys. Chem. 1995, 99, 7913. (4) Tague, T. J., Jr.; Andrews, L. J. Am. Chem. Soc. 1993, 115, 12111. The product band at 1493.4 cm-1 must be reassigned to HOBeOH. (5) Curtiss, L. A.; Frurip, D. J. Chem. Phys. Lett. 1980, 75, 69. (6) Sakai, S.; Jordan, K. D. Chem. Phys. Lett. 1986, 130, 103. (7) Kaufman, J. W.; Hauge, R. H.; Margrave, J. L. High Temp. Sci. 1984, 18, 97. (8) Andrews, L.; Burkholder, T. R. J. Phys. Chem. 1991, 95, 8554. (9) Burkholder, T. R.; Andrews, L. J. Chem. Phys. 1991, 95, 8697. (10) Hassanzadeh, P.; Andrews, L. J. Phys. Chem. 1992, 96, 9177. (11) Reddington, R. L.; Milligan, D. E. J. Chem. Phys. 1962, 32, 2162. Ayers, G. P.; Pullin, A. D. E. Spectrochim. Acta 1976, 32A, 1629. (12) Suzer, S.; Andrews, L. J. Chem. Phys. 1988, 88, 916. Cheng, B.M.; Lee, Y.-P.; Ogilvie, J. F. Chem. Phys. Lett. 1988, 151, 109. (13) Jacox, M. E.; Milligan, D. E. J. Mol. Spectrosc. 1972, 42, 495. Smith, D. W.; Andrews, L. J. Chem. Phys. 1974, 60, 81. (14) Andrews, L.; Chertihin, G. V.; Thompson, C. A.; Dillon, J.; Byrne, S.; Baushlicher, C. W., Jr. J. Phys. Chem. 1996, 100, 10088. (15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; HeadGordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 92 and Gaussian 94, Revision B.1; Gaussian, Inc.: Pittsburgh, PA, 1995. (16) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (17) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (18) Huber, K. P.; Herzberg, G. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979. (19) Bauschlicher, C. W., Jr.; Partridge, H.; Sodupe, M.; Langhoff, S. R. J. Phys. Chem. 1992, 96, 9259. (20) Boldyrev, A. I.; Simons, J. J. Phys. Chem. 1995, 99, 15041. (21) Palke, W. E.; Kirtman, B. Chem. Phys. Lett. 1985, 117, 424. (22) Bauschlicher, C. W., Jr.; Langhoff, S. R.; Partridge, H. J. Chem. Phys. 1986, 84, 901. (23) Brom, J. M.; Weltner, W., Jr. J. Chem. Phys. 1976, 64, 3894. (24) Thompson, C. A.; Andrews, L.; Martin, J. M. L.; El-Yazal, J. J. Phys. Chem. 1995, 99, 13839. (25) Tague, T. J., Jr.; Andrews, L. J. Phys. Chem. 1994, 98, 8611. (26) Andrews, L.; Yustein, J. T. J. Phys. Chem. 1993, 97, 12700. (27) Frenking, G.; Koch, W.; Gauss, J.; Cremer, D. J. Am. Chem. Soc. 1988, 110, 8007.
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