10088
J. Phys. Chem. 1996, 100, 10088-10099
Infrared Spectra and Quantum Chemical Calculations of Group 2 MO2, O2MO2, and Related Molecules Lester Andrews,* George V. Chertihin, Craig A. Thompson, Janet Dillon, and Susan Byrne Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22901
Charles W. Bauschlicher, Jr.* STC-230-3, NASA Ames Research Center, Moffett Field, California 94035 ReceiVed: December 19, 1995; In Final Form: April 12, 1996X
Laser-ablated group 2 metal atoms have been reacted with O2 in condensing N2 to complement earlier Ar studies owing to different relaxation dynamics of N2 and Ar with respect to excited metal atoms and ionic product molecules. In the case of Ca + O2, the reaction in condensing Ar gives primarily the 3B2 open bent OCaO dioxide product, but the reaction in condensing N2 favors the 1A1 cyclic CaO2 peroxide species. Three fundamentals are observed with 18O and 44Ca substitution for CaO2, and isotopic frequencies are in excellent agreement with the predictions of quantum chemical calculations. Although DFT/B3LYP frequencies are slightly higher than MP2 and CASSCF values, a similar pattern is calculated. Ionic molecules interact more strongly with a nitrogen matrix than with argon, and calculations of N2MOx and ArMOx molecules can be used to explain matrix shifts. Several N2MO2 species are formed spontaneously from MO2 molecules in solid nitrogen, and a match is found for observed matrix and DFT calculated isotopic frequencies. By comparison, the new molecule ArBeO2 is identified in earlier argon matrix experiments. The new metal disuperoxide molecules, O2MO2, are also identified here. Calcium disuperoxide, O2CaO2, is characterized as a D2d species with +1.12 charge on Ca and -0.28 on each O, in contrast to calcium peroxide, CaO2, a C2V molecule with +1.05 charge on Ca and -0.525 on each oxygen atom.
Introduction Alkaline-earth metal atoms can react with molecular oxygen depending on the initial reagent kinetic energy to form three different MO2 products qualitatively described as dioxide 2+ 1 2+ 2(3Σg , O -M -O ), peroxide ( A1, M O2 ) and super3 + 1,2 Theoretical studies on these species oxide ( A2, M O2 ). reveal different relative stabilities for each electronic structure as a function of metal atom.1 It is of interest to determine what parameters govern the product selectivity so that these different structures can be formed. The use of a N2 carrier gas increases the rate of the thermal Ca + O2 reaction in the gas phase,3 and CaO2 product species can be formed in condensing nitrogen but not in condensing argon.4-6 Recent infrared studies of pulsed laser evaporated alkaline-earth metal atoms and oxygen in argon have shown that the dioxide is the major product.2 However, in preliminary nitrogen studies, calcium peroxide appears to be the major product formed, and a complete study of group 2 atom reactions with O2 in nitrogen has been done in order to more fully characterize these peroxide molecular species. Complimentary quantum chemical calculations have been done to further characterize these novel product species. Experimental Section Pulsed laser evaporated alkaline-earth metal atoms were reacted with O2 in nitrogen gas during condensation at 10 ( 1 K using the same apparatus and methods described for previous work.2,7,8 Infrared spectra were recorded on a Nicolet 750 FTIR instrument at 0.5 cm-1 resolution. In addition complementary thermal experiments were done using Knudsen effusion4-6 of Ca at 500 °C into condensing gas on a 12 ( 1 K substrate. X
Abstract published in AdVance ACS Abstracts, May 15, 1996.
S0022-3654(95)03763-4 CCC: $12.00
Infrared spectra were recorded on a Perkin-Elmer 983 grating instrument at 0.5 cm-1 resolution. Results Reactions of laser ablated group 2 metal atoms with O2 in condensing nitrogen will be presented. Be. Figure 1 shows the spectra for Be atoms co-deposited with 1% 16O2, 18O2, and 16,18O2 in excess nitrogen, and Table 1 lists the observed bands. Most of these bands can be identified by comparison with the argon matrix study.8 Two exceptions are the sharp weak 1428.9, 1422.1 cm-1 bands, the 1214.7, 1212.1 cm-1 doublet, and a 928.9 cm-1 band. The former showed no intermediate component with 16,18O2, but the latter two exhibited 1/2/1 triplets. An experiment with a 15N2 matrix displaced the latter doublet to 1213.8, 1211.2 cm-1, the 928.9 cm-1 band to 928.7 cm-1, and altered no other Table 1 bands. Absorptions due to Be/N2 species shifted with nitrogen isotopic substitution.9 The new triplets sharpened on annealing but did not grow, whereas (BeO)2 increased 50% on annealing. In contrast, photolysis increased the new triplet bands by 40 and 100%, respectively, and also increased (BeO)2 by 100%. An experiment with 16O2/18O2 gave only the product absorptions for the 16O2 and 18O2 reagents. Mg. Experiments with Mg gave bands at 946, 924, 844.2, and 829.8 cm-1 that grew on annealing and were observed previously,7,10 and new weak bands at 723.6 and 628.5 cm-1 that decreased on annealing. Substitution with 18O2 gave bands at 799 and 784 cm-1 and weak absorptions at 706.4 and 623.4 cm-1. Generally product yields were low. Ca. Co-deposition of laser evaporated Ca atoms with 1% O2 in nitrogen gave a strong group of bands at 742.1, 555.7, and 500.5 cm-1 (labeled 6) in Figure 2 to conform to the nomenclature employed in ref 2. Weaker bands were observed © 1996 American Chemical Society
Group 2 MO2, O2MO2, and Related Molecules
J. Phys. Chem., Vol. 100, No. 24, 1996 10089
Figure 1. Infrared spectra in the 1580-860 cm-1 region for laserablated Be atoms co-deposited with 1% oxygen in nitrogen: (a) 16O2, (b) 16,18O2, and (c) 18O2.
TABLE 1: Absorptions (cm-1) Observed for Be + O2 Reaction Products in Condensing Nitrogen 16
O2
1552 br 1428.9 1422.1 1408.3 1214.7 1212.1b 1118.0 970.8 928.9b 892.4 792.4 539.1
18O 2
16,18O 2
1526 br 1401.3 1394.8 1367.4 1192.8 1203.9 1189.6 1201.4 1097.8 1106.0 958.8 965.1 905.9 917.1 877.1 885.2 748.2 sextet 529.8 534.4
R(16/18) anna 1.0170 1.0197 1.0196 1.0299 1.0184 1.0189 1.0184 1.0125 1.0254 1.0174 1.0591 1.0176
+ 0 0 + 0 + +
assign
argon 16O2
BeOBeO (N2)xBeO (N2)xBeO BeOBe NNBeO2 (N2)BeO2 (BeO)2 (OBeO)+c NNBeO2 (BeO)2 O3(BeO)2
(1573, 1549)d (1526.1)e 1412.3 (1288.9)f (1264.1)f 1131 988.6 866 803 522
a Annealing behavior. b Shifts 14N to 15N matrix: 1212.1 f 1211.2 2 2 cm-1; 928.9 f 928.7 cm-1. c Tentatively assigned to anion in ref 8; the present DFT calculations suggest cation is better assignment. d Reassigned here to BeOBeO based on DFT calculations. e Assigned to Ar-BeO in ref 8. f Reassigned here to Ar-BeO2 based on DFT calculations.
at 706.9 cm-1 (labeled 3), 559.5 and 497.0 cm-1 (labeled 4), 484.7 cm-1 (labeled 1), and 449.5 cm-1 (labeled 7). Most of these bands were observed in earlier thermal Ca reactions.4 Photolysis with full mercury arc radiation destroyed the group 6 bands, decreased groups 3 and 4, and markedly increased the 497.0 and 484.7 cm-1 group 1 bands. Annealing to 22 K slightly decreased the group 1 bands, slightly increased group 7, and markedly increased the group 2 bands. Another experiment produced the same initial spectrum, and photolysis (λ > 380 nm) reduced the group 6 bands by 20% and increased the 4 and 1 bands by a similar amount. Annealing to 20 K produced new 806.1 and 801.3 cm-1 bands, doubled the group 6 bands, and increased the weak 449.5 cm-1 band. A similar Ca reaction with 18O2 gave the counterparts listed in Table 2. The deposited sample is dominated by group 6 bands at 702.5, 536.8, and 475.6 cm-1, although some of the 536.8 cm-1 band is due to species 4 also absorbing at 477.5 cm-1. Photolysis (λ > 320 nm) virtually destroyed species 6, increased species 4, markedly increased species 1 bands at 481.2
and 469.7 cm-1, and slightly increased species 7 at 440.4 cm-1. The Ca atom reaction with 16,18O2 gave some band overlap; however, changes on photolysis allowed identification of group 6 triplet bands at 742.1, 722.9, 702.5 cm-1, 555.7, 546.6, 536.8 cm-1, and 500.5, 486 sh, 475.6 cm-1 (Figure 3a) and group 1 and 4 triplet bands as listed in Table 2. Note the resolved quintet at 449.5, 447.6, 445.3, 443.0, 440.4 cm-1 for group 7. Photolysis (λ > 470 nm) had little effect; however, photolysis (λ > 380 nm, Figure 3b) decreased group 6 bands and slightly increased group 4, 6, and 7 bands, whereas further photolysis (λ > 290 nm, Figure 3c) had little effect. Finally, annealing to 22 K slightly decreased group 1 and markedly increased group 2 bands. Complementary thermal experiments were done with Ca using lower Knudsen cell temperature (500 °C) and a colder window (12 K) than earlier studies.4,6 The spectrum Figure 4a is similar to that reported by Ault and Andrews, but much less Ca was co-deposited in the present effusion and ablation experiments. Note the weak species 1 band at 484.7 cm-1 in Figure 4a. Full arc photolysis decreased group 7 and virtually destroyed group 4 and 6 bands (Figure 4b) but increased the species 1 band at 484.7 cm-1 and its counterpart at 497.0 cm-1 along with other broader bands. Annealing (Figure 4c) decreased species 1 and increased species 2 bands as before, left species 7 unchanged, and increased an aggregate band at 520 cm-1. Experiments with 10% O2 in N2 gave no species 4, weak species 1 and 6, and strong species 2 bands, and the species 7 band was the strongest absorption in the spectrum. Sr. Several experiments were done with strontium, and the product absorptions are listed in Table 3. New bands were observed at 729.8, 618.6, 502.7, 496.1, and 473.1 cm-1 (Figure 5a); these bands were observed, along with others, in the earlier thermal work.4 As with calcium, photolysis λ > 380 nm slightly reduced group 6 and increased group 1 bands (Figure 5b), and full arc photolysis continued that trend (Figure 5c). Annealing to 20 K further increased the 496.1 cm-1 band and produced group 2 bands (Figure 5d). Final annealing to 30 K markedly increased group 2 bands, and decreased the group 1 band. A similar experiment with 18O2 gave analogous results. Another experiment with 16,18O2 gave 1/2/1 triplet group 6 bands and photolysis produced a sharp 1/2/1 triplet group 1 band as listed in Table 3. Ba. One experiment was done with barium to compare with earlier thermal reactions, and the major product bands were observed in Figure 2b of ref 4 and here at 792.6 (group 2), 751.2(6), 613.0(3), 565.4(1), 539.9(1), 485.0(4), and 446 cm-1 (6). Photolysis λ > 290 nm slightly decreased group 6 bands, slightly increased group 4, but doubled group 1 bands, and full arc photolysis continued this trend. Annealing to 20 K slightly increased group 6 bands but increased the major group 1 band at 539.9 cm-1 by 50%, and final annealing to 30 K reduced the group 1 absorptions and increased the group 2 band. Theoretical Calculations Geometries are optimized and harmonic frequencies computed for Group 2 oxide species using density functional theory (DFT) in conjunction with the hybrid B3LYP functional.11 Calibration calculations are performed for CaO2, SrO2, and BeO2 using the complete-active-space self-consistent-field (CASSCF) approach, second-order Moller-Plesset (MP2) perturbation theory,12 and/ or the coupled-cluster singles and doubles method,13 including a perturbational estimate of the triple excitations14 [denoted CCSD(T)]. For the open-shell systems, the B3LYP, MP2, and CCSD(T) approaches are based on a spin-unrestricted reference. The B3LYP, MP2, and CCSD(T) calculations are performed
10090 J. Phys. Chem., Vol. 100, No. 24, 1996
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Figure 2. Infrared spectra in the 850-420 cm-1 region for laser-ablated Ca atoms codeposited with 1% 16O2 in nitrogen. (a) sample deposited for 2 h, (b) after full mercury arc photolysis, and (c) after annealing to 22 K.
TABLE 2: Absorptions (cm-1) Observed for Ca + O2 Reaction Products in Condensing Nitrogen 16
O2
806.6 801.3 742.1 706.8 633.9 559.5 555.7 500.5 497.0 497.0 484.7 449.5
18O
16,18O
2
861.8 756.7 702.5 678.3 599.7 536.6 536.8 475.6 481.2 477.4 469.7 440.4
2
sextet 722.9 doublet multiplet 548.6 546.6 486 sh 489.4 484.6 477.8 447.6, 445.3, 443.0
R(16/18)
assign
1.0588 1.0589 1.0564 1.0420 1.0570 1.0427 1.0352 1.0524 1.0328 1.0409 1.0319 1.0207
CaO3 (2) CaO3 (2) CaO2 (6) CaO (3) CaO3 (2) (CaO)2 (4) CaO2 (6) CaO2 (6) OCaO (1) (CaO)2 (4) OCaO (1) O2CaO2 (7)
using Gaussian 92/DFT,15 while the CASSCF calculations use SIRIUS/ABACUS.16 We use the 6-31+G** basis sets of Pople and co-workers17 with the following exceptions: (1) The Ar basis is the 6-311+G** set.15 (2) For Sr we use the relativistic effective core potential (RECP) of Hay and Wadt18 in conjunction with a (5s 6p 5d)/[4a 3p 3d] valence basis. (3) The Ca (12s 8p 5d)/[8s 6p 3d] basis set19,20 is used. (4) A (10s 7p 1d)/[5s 4p 1d] set is used for oxygen in the CaOxNy and SrO2 calculations. This oxygen set along with the Ca and Sr sets are described.21 These basis sets are used in all of the production calculations and should yield reasonable results for the strongly bound systems, especially for the B3LYP method, as DFT tends to have smaller one-particle basis set requirements than more traditional methods. To accurately describe very weakly bound
systems, much larger basis sets are required. Therefore while these basis sets can describe systems such as ArBeO, which has an Ar-BeO binding energy of 7 kcal/mol,22 they are too small to accurately treat the more weakly bound ArMgO. The CaO2 production results are calibrated using a much larger basis set; the oxygen set is the augmented correlationconsistent polarized-valence triple-ζ set,23 with the diffuse f function deleted. The (20s 15p 5d 3f)/[8s 8p 4d 3f] Ca set is derived from a primitive set of Partridge.24 The Sr RECP is tested by using an (26s 19p 10d)/[13s 12p 6d] all-electron basis set that is derived from primitive set of Partridge and Faegri.25 More details of the Ca and Sr sets can be obtained from one of the authors (C.W.B.). The metal oxides are poorly described by the SCF treatment, in large part because it underestimates electron affinities of O and O2. Neglect of electron correlation can lead to symmetry breaking. It is therefore important to use methods that include electron correlation. The CASSCF approach can account for near-degeneracy effects, which lead to symmetry breaking, but might not contain sufficient dynamical correlation to yield an accurate description of these systems. In addition, the CASSCF approach can lead to prohibitively large calculations for large active spaces. For example, while full valence CASSCF calculations are possible for CaO2, they would not be practical for N2CaO2. Thus using the CASSCF approach, it will be difficult to find an accurate and equivalent description for all systems of interest. Single-reference-based methods for including electron correlation, such as MP2 and B3LYP, can be used to treat all systems of interest equivalently. However, it is not
Group 2 MO2, O2MO2, and Related Molecules
J. Phys. Chem., Vol. 100, No. 24, 1996 10091
Figure 3. Infrared spectra in the 850-400 cm-1 region for laser-ablated Ca atoms codeposited with 1% 16,18O2 nitrogen. (a) sample deposited for 2 h, (b) after photolysis λ > 380 nm, (c) after photolysis λ > 290 nm for 30 min, and (d) after annealing to 22 K.
a priori obvious if they can describe these systems accurately. The CCSD(T) method, the highest quality single-reference-based approach, is expected to describe these systems accurately; however, it is too computationally intensive to study all the systems of interest. Our first series of calculations study three representative systems, CaO2, SrO2, and BeO2 and compare the calculated frequencies at various levels of theory with recent and present vibrational assignments. In Table 4 we summarize the CaO2 calibration calculations. The CASSCF calculations include the Ca 4s and 4p and oxygen 2p orbitals in the active space. The MP2 calculations that only correlate the valence electrons are denoted “val”, while those that include the Ca 3s and 3p electrons are denoted “core”. The CCSD(T) calculations also correlate the Ca 3s and 3p electrons as well as the valence electrons. For the 1A1 state, all methods yield similar values for the frequencies and intensities. The valence MP2 results are in the best agreement with the experimental fundamentals. The CASSCF results are in slightly better agreement with experiment than the B3LYP values. The Ca-O2 dissociation energy is reported at the bottom of Table 4. The CCSD(T) approach using the big basis set is our highest level of theory. From the difference in the CCSD(T) results using the small and large basis sets and from the results of previous calculations,1 even the CCSD(T) De value from the large basis set is expected to be too small by 5, and perhaps as much as 10 kcal/mol. The B3LYP value is therefore clearly too high, but should be in reasonable agreement with the true De. The MP2 values are too small.
The MP2 results for the 3A2 state are dramatically different from the CASSCF or B3LYP results, which are quite similar to each other. The b2 mode, for example, has the lowest frequency at the B3LYP and CASSCF levels, but has the highest frequency at the MP2 level. When the Ca core is correlated, the MP2 results are even worse. The extremely large intensity is typical of a mode that is not well described and suggests that there is a symmetry-broken solution nearby. The MP2 approach is inappropriate for this state and therefore of questionable value for these systems, despite yielding the best results for the 1A1 state. While the CASSCF and B3LYP approaches yield similar frequencies and intensities, the CASSCF method incorrectly places the 3A2 state below the 1A1 state. Thus while the CASSCF can describe the near-degeneracy effects, such that the b2 mode is correctly described, it does not account for sufficient dynamical correlation to correctly position the 3A2 state, which has Ca+O2- character, with 1A1 state, which has Ca2+O22- character. The B3LYP 3A2-1A1 separation is larger than that from the CCSD(T) treatment and suggests that the B3LYP separation is too large. We should note that the problem of correctly positioning states with different ionic contributions to the bonding is not unique to CaO2; the B3LYP approach predicts the 3Π state of MgO to be 4.2 kcal/mol below the 1Σ+ state, while from experiment27 it is known to be 7.5 kcal/mol higher. The 3B2 state appears to be easier to describe than the 3A2 state, and there is reasonable agreement among the methods. The CASSCF approach is in the best agreement with the experimental fundamental. Even the 3B2-1A1 separation is
10092 J. Phys. Chem., Vol. 100, No. 24, 1996
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Figure 4. Infrared spectra in the 850-700 and 600-440 cm-1 regions for thermal Ca atoms at 500 °C co-deposited with 2% O2 in N2 at 12 ( 1K. (a) Sample deposited for 6 h, (b) after full arc photolysis, and (c) after annealing to 25 K.
TABLE 3: Absorptions (cm-1) Observed for Sr + O2 Reaction Products in Condensing Nitrogen 16
O2
806.2 729.8 618.6 617.3 502.7 496.1 473.1 (423)
18O 2
16,18O 2
R(16/18)
assign
761.4 689.4
sextet 710.0
1.0588 1.0586
SrO3 (2) SrO2 (6) SrO (3) SrO3 (2) (SrO)2 (4) OSrO (1) SrO2 (6) (SrO)2 (4)
582.8 478.3 475.5 452.6 (403)
486.8 467
1.0592 1.0510 1.0433 1.0453 1.0496
relatively insensitive to level of theory, because both the 3B2 and 1A1 states have significant Ca2+ character. We report the SrO2 calibration calculations in Table 5. The results are similar to those for CaO2. Namely the methods are in reasonable agreement except for the b2 mode of the 3A2 state, which is poorly described at the MP2 level. The all-electron and RECP results are in good agreement. One disappointing feature of these results is the b2 mode of the 3B2 state, where the computed harmonic frequency is 128 cm-1 smaller than experiment. This is in contrast to CaO2, where the B3LYP b2 mode of the 3B2 state was 35 cm-1 larger than experiment. Thus while the B3LYP result is much superior to the MP2, the B3LYP approach is giving only qualitative results to aid in the interpretation of the experiment. The final calibration is for the symmetric linear OBeO neutral, cation and anion systems. The active space includes the Be 2s and 2p orbitals and the oxygen 2p orbitals, which leads to two σg, two σu, two πu, and one πg orbitals. A second πg orbital is added to the active space to correlate the oxygen πg lone pair electrons. For the neutral molecule the CASSCF and B3LYP methods yield relatively similar results. For the σu mode, the CASSCF is in the best agreement with experiment. For this mode, the MP2 is significantly too large. We note that previous MP2 calculations8 based on a spin-restricted approach yielded a harmonic frequency in much better agreement with experi-
ment. The results for the ions show an even larger difference between the methods. The MP2 shows symmetry breaking for the negative ion and the CASSCF method shows symmetry breaking for the positive ion for all of the active spaces tested. On the basis of these calibration calculations, we can rule out the MP2 approach. Excluding the ions, the performance of the B3LYP and CASSCF approaches seem to be similar. In light of the much smaller computational requirements for the B3LYP, this is the approach that will be used in further work. Accordingly, Tables 7-9 collect B3LYP results on BeO2, MgO2, CaO2, and related new species considered in these experiments. Discussion The reaction products of group 2 metal atoms and O2 molecules in condensing nitrogen will be characterized from isotopic shifts and comparison with quantum chemical calculations. The realization that the dominant Ca reaction product in condensing argon was 3B2 open bent OCaO and in condensing nitrogen was 1A1 cyclic CaO2 suggested that 3P excited atoms dominate reactions in condensing argon while 1S ground-state atoms are the major reagent in condensing nitrogen. Be. Two products are identified in the Be + O2 system in solid nitrogen from isotopic shifts and proximity to these product absorptions in solid argon,8 namely, BeOBe and (BeO)2. In the former case the Ar-N2 matrix shift is small (4 cm-1) and the 16/18 ratios are 1.0300 ( 0.0001, which is appropriate for the linear BeOBe molecule. A very recent theoretical study28 confirms the earlier calculated and observed8 ν3 mode for linear BeOBe near 1400 cm-1. The three observed fundamentals of (BeO)2 are red- and blue-shifted 13-26 cm-1 between matrixes. Is any BeO trapped in the N2 matrix analogous to the ArBeO species observed in solid argon?8 Calculations using DFT/ B3LYP predict the Be-O fundamental in NN-BeO to be very weak and blue shifted the same as in Ar-BeO, but such a
Group 2 MO2, O2MO2, and Related Molecules
J. Phys. Chem., Vol. 100, No. 24, 1996 10093
Figure 5. Infrared spectra in the 850-400 cm-1 region for laser-ablated Sr atoms codeposited with 1% 16O2 in N2. (a) Sample deposited for 2 h, (b) after photolysis λ > 380 nm, (c) after full arc photolysis, and (d) after annealing to 20 K.
TABLE 5: SrO2 Calibration Calculationsa
TABLE 4: CaO2 Calibration Calculations MP2
MP2
val ωe
core I
ωe
67 122 78
526 615 763
B3LYP I
ωe
I
500 620 824
45 91 64
CASSCF ωe
I
b
ωe
expt ν
1A
b2 486 a1 541 a1 765
68 99 69
538 91 571 74 679 110
505.5(N2) 555.7(N2) 742.1(N2), 739.2(Ar)
3A 2
b2 960 2083 1982 40799 369 a1 349 12 422 81 422 a1 820 90 1066 4 1192 3
B2 a1 65 a1 436 b2 546
106 21 368
83 445 522
MP2
96 34 243
0.1 368 68 419 9 1101
125 78 492 50 550 149
0.5 94 1
80 92 444 39 515 229
515.7(Ar), 497.0(N2)
core
small
big
B3LYP
1
41.50
43.01
37.25
47.80
59.34
3A
21.4 12.0
4.1 19.1
-0.2 7.7
3.8 10.3
10.4 13.3
CASSCF
De Te 2 3B 2
ωe
I
ωe
56 119 50
471 528 806
33 93 57
446 33 514 89 801 61
1726 24668 358 63 1074 5
339 360 1182
0 54 10
342 0.1 361 51 1184 9
118 413 429
55 99 55
128 59 404 93 424 54
exptb ν
I
1
473.1(N2) 729.8(N2)
3
A2 b2 a1 a1 3B 2 a1 b2 a1
80 428 407
69 199 61
532.4(Ar), 496.0(N2)
MP2
B3LYP(RECP)
B3LYP(AE)
1
67.6
53.8
59.8
2 3B 2
6.4 24.0
8.1 14.1
9.6 13.9
De 1A
Te
CCSD(T)
val A1
I
b2 483 a1 526 a1 759
1A 1
B3LYP(RECP) B3LYP(AE)
3A
a The frequencies are in cm-1, the intensities are in km/mol, and the De and the Te values are in kcal/mol. b This work and ref 2.
-15.0 9.9
The frequencies are in cm-1, the intensities are in km/mol, and the De and Te values are in kcal/mol. b This work and ref 2. a
species is not observed in solid nitrogen. However, the weak 1428.9, 1408.3 cm-1 bands show the appropriate diatomic BeO 16/18 ratio (1.0197) and these bands are tentatively assigned
to the more highly coordinated species (N2)xBeO in solid nitrogen. A major reaction product in solid argon,8 the linear 3Σg OBeO molecule absorbing at 1412.4 cm-1, is clearly not observed in solid nitrogen. The new 1212.1 cm-1 band in solid nitrogen exhibits a 1/2/1 mixed isotopic triplet for two equivalent oxygen atoms and a 16/18 ratio appropriate for the ν2 (sym
10094 J. Phys. Chem., Vol. 100, No. 24, 1996
Andrews et al.
TABLE 6: BeO2 Calibration Calculationsa CASSCF MP2 ωe OBeO πu σg σu OBeOπu σg σu OBeO+ πu σg σu
294 767 1599 517.5 789 c
B3LYP
small
exptb
big
I
ωe
I
ωe
I
335 0 8
227 722 1372
268 0 311
323 680 1428
361 0 231
1956 0
395 739 1219
213 0 826
434 691 1550
235 0 3759
196 602 996
305 0 676
185 602 10006i
324 0.0
ωe
ν
I
1413.2(Ar)
205 586 1868i
198 0
a The frequencies are in cm-1, and the intensities are in km/mol. b Reference 8. c The imaginary frequency is so large in magnitude that is not printed by the program.
TABLE 7: Calculated Frequencies (cm-1) for BeO, BeO2, BeO3, and BeO4 Molecules and Ar and N2 Complexes Using DFT/B3LYPa BeO, 1Σ BeO, 3Π ArBeO, 1Σ NNBeO, 1Σ OBeO 3 Σg (E ) 0)c 1.441 Å BeO2 1A (E ) +11)c 1.668, 1.425 Åd 1 BeO2 3A (E ) +29)c 1.362, 1.586 Åd 2 (Ar)OBeO(Ar)e cross ArBe16O2 singlet C2V ArBe16O18O ArBe18O2 NNBeO2, singletf end-NN, side O2 (E ) 0) N2BeO2, triplet end-NN, side-O2 (E ) 24) N2BeO2, triplet side-N2, side-O2 (E ) 25) OBeO2, triplet side O2, C2V (E ) 0)c Be16O3 singlet cyclic C2V (E ) +29) Be16O3 triplet cyclic C2V (E ) +72) Be18O3 16O Be16O , triplet 2 2 side, side D2d 1.365, 1.585 Åd 18 18 O2Be O2 (BeO)2 singlet D2h (E ) 0.0) Be16OBe16O linear, triplet (E ) +44) Be18OBe18O (BeO)2 triplet C2V ring (E ) +65) 16 OBe16OBe16O linear, triplet 1.365, 1.585Å 18 OBe18OBe18O
566.1 a1 (4) 275.7 (66) 319.4 b1 (.1) 305.7 460.0 a1 (0) 433.6 530.2 b3u (192) 170.2 (5) 165.8 379.6 a1 (23)
1529.4 (20)b 1105.5 (93) 1583.3 (161) 1583.1 (1) 226.7 π (134) 559.7 a1 (33) 538.2 b2 (.5) 273.4 b1u (256) 606.2 a1 (44) 590.9 575.2 634.4 a1 (2) 642.5 b2 (17) 607.2 b2 (12) 597.5 b2 (17) 497.5 (51) 396.5 b (27) 385.8 585.0 e (2 × 17) 567.5 813.3 ag (0) 357.8 (116) 351.1 522.7 a1 (.2) 477.7 π (400) 465.3 σ (0)
a
f
267.8 (3) 477.9 (21) 722.0 σg (0) 1060.3 b2 (67) 908.6 a1 (116) 359.7 b3u (113) 1026.2 b2 (53) 1012.9 1000.4 985.6 b2 (57) 1111.0 a1 (53) 1096.2 a1 (123) 1109.2 a1 (1) 644.8 (15) 689.0 a1 (67) 624.6 1089.6 b2 (103) 1039.8 858.1 b3g (0) 361.7 (103) 355.4 611.1 b2 (884) 490.2 σ (0) 466.8 π (196)
1372.1 σu (311) 1347.2 a1 (113) 1177.8 a1 (20) 681.3 ag (0) 1336.4 a1 (263) 1324.9 1312.6 1334.6 a1 (37) 1295.2 a1 (334) 1236.9 a1 (266) 1348.4 a1 (141) 869.4 (1) 795.7 a1 (45) 757.1 1150.7 a1 (0) 1084.8 900.5 b2u (356) 630.5 (2) 602.9 799.3 a2 (0) 855.7 σ (107) 811.0 σ (90)
1398.3 b2u (251) (804)
1085.6 (109) 898.0 b2 (132) 850.6 1286.4 b2 (329) 1261.9 1163.1 b1u (512) 1231.2 (28) 1213.3 832.7 b1 (.2) 1385.7 σ (0) 1376.1 σ (0)
1217.9 (86) 1051.0 a1 (9) 993.7
1186.2 ag (0) 1548.4 (738) 1517.6 1025.7 a1 (14) 1468.0 σ (521) 1437.8 σ (519)
Highest frequencies listed. b Intensities in km/mol. c Relative energies of isomers in kcal/mol. d R(O-O) and R(Be-O), respectively. Side-N2, side-O2 singlet collapses to end-NN.
Be-O2) stretching mode of a cyclic BeO2 species, but calculated frequencies for cyclic 1A1 and 3A2 BeO2 are not in this region (Table 4). Likewise the 928.9 cm-1 band forms a triplet for two equivalent O atoms but exhibits a 16/18 ratio appropriate for the ν3 (antisym Be-O2) mode of a cyclic BeO2 species, again not in this region.
e
Triplet.
Four N2BeO2 species were calculated: singlet and triplet states with end and side-on N2 bonding to cyclic BeO2. The side, singlet collapsed to end, singlet, which was the lowest energy species. However, the strongest infrared band was calculated at 985.6 cm-1 (b2, 57 km/mol), which is compatible with the 928.9 cm-1 band. The calculated 18O2 shift for the
Group 2 MO2, O2MO2, and Related Molecules
J. Phys. Chem., Vol. 100, No. 24, 1996 10095
TABLE 8: Calculated Frequencies (cm-1) for MgO, MgO2, MgO3, and MgO4 Molecules and Ar and N2 Complexes Using DFT/B3LYPa MgO, 1Σ (E ) +4) MgO, 3Π (E ) 0) ArMgO, 1Σ NNMgO, 1Σ OMgO 3Σ-g (E ) 0)c 1.870 Å MgO2 1A1 (E ) 0)c 1.643, 1.827 Åd MgO2, 3A2 (E ) +25)c 1.360, 2.011 Åd NNMgO2 end-tee triplet (E ) 0)c N2MgO2 side-side (E ) +17) OMgO2 triplet side-O2 C2V, (E ) 0)c MgO3 (E ) +10) C2V, triplet ring 16 O224Mg16O2, triplet, D2d side-side, 1.366 Åd, triplet, 1.968 Åd 16 O226Mg16O2
786.2 (10)b 606.5 (46) 790.9 (18) 823.1 (9) 534.5 σg (0) 654.1 b2 (24) 383.9 b2 (3) 518.6 (7)
23.3 (2) 120.0 (1) 91.1 π (118 × 2) 580.7 a1 (41) 520.1 a1 (84) 197.8 (95)
734.1 σu (85) 820.8 a1 (13) 1150.3 a1 (.3) 767.7 (11)
437.4 (2) 431.8 b2 (2) 302.6 b2 (69) 382.0 a1 (0) 382.0
456.4 a1 (1) 398.2 a1 (69) 435.1 e (2 × 3) 432.8
674.7 (132) 773.2 a1 (187) 701.0 a1 (20) 706.5 b2 (152) 686.6
18O 26Mg18O 2 2
360.1
411.2
675.2
(MgO)2 singlet D2h (E ) 0) MgOMgO linear, triplet (E ) +39)
273.9 b3u (145)
436.8 ag (0) 51.4 (0.1 × 2)
554.9 ag (0) 172.6 (103 × 2)
a
1138.6 (.2) 1135.0 a1 (0) 898.4 b2 (160) 1137.1 a1 (0) 1137.1 a1 (0) 1071.9 a1 (0) 584.5 b1u (179) 392.4 (14)
1077.6 a1 (1) 1140.2 b2 (1) 1139.9 1075.0 646.0 b2u (173) 684.7 (21)
657.6 ag (0) 942.7 (337)
Highest frequencies listed. b Intensities in km/mol. c Relative energies of isomers in kcal/mol. d R(O-O) and R(Mg-O), respectively.
TABLE 9: Calculated Frequencies (cm-1) for CaO2, CaO3, and CaO4 Molecules and N2CaO2 Species Using DFT/B3LYP a1 16O
Ca 1A 1
2
NNCaO2 end, side OCaO 3 B2 CaO2 3A 2 OCaO2 triplet side O2, C2V (E ) 0) CaO3 singlet non-planar (E ) +12) CaO3 triplet C2V (E )+39) O2CaO2 triplet D2d a
45.1° 1.518 Åb 1.978 Å
135.7° 2.095 Å NNCaO2 end-tee 35.6° 1.340 Åb 2.191 Å 91.6 b2 (75) 422.2 (24) 219.0 (2) 208.5 D2d 1.344 Å 2.226 Å
a1
b2
isotope
824.0 802.3 779.6 822.1 777.3 825.8 (151) 492.1 (50) 479.1 (67) 1192.1 (9)
620.4 (91) 610.1 598.7 609.2 587.4 595.7 (23) 125.3 (78) 164.4 (54) 443.4 (68)
499.7 (45) 486.2 474.3 497.3 471.7 496.4 (42) 549.5 (149) 536.6 143 369.0 (0)
40-16-16 40-16-18 40-18-18 44-16-16 44-18-18 NN 40-16-16 16-40-16 NN (16-40-16)
379.3 a1 (27) 472.2 (18) 308.1 (50) 300.1 339.8 a (0) 320.3 339.8
397.9 b2 (13) 606.2 (66) 642.7 (22) 606.2 402.6 e (2 × 7) 381.7 401.0
546.9 a1 (181) 701.2 (106) 866.3 (139) 816.7 498.4 b2 (231) 487.6 484.3
(64)a
40-16-16 1183.0 a1 (29) 862.2 (23) 1067.2 (1) 1006.1 1177.6 b2 (21) 1110.1 1177.6
162-40-16 40-163 40-183 162-40-162 182-40-182 162-44-162
Intensity in km/mol. b R(O-O) and R(Ca-O), respectively. c Relative energies of isomers in kcal/mol.
latter band (23.3 cm-1) is in excellent agreement with the observed (23.0 cm-1) value; likewise the calculated 15N2 shift (0.2 cm-1) is in agreement with the observed (0.2 cm-1) value. The end, triplet was higher energy (+24 kcal/mol), but the strongest infrared band calculated at 1295.2 cm-1 (a1, 334 km/ mol) had too much 15N2 shift (3.0 cm-1) and not enough 18O2 shift (14.9 cm-1). Although the side-on N2 triplet was higher (+25 kcal/mol), the most intense infrared band for the triplet state of the side-on N2, side-on O2 species is calculated at 1236.9 cm-1; the calculated 18O2 shift is 23.3 cm-1 (observed 22.5 cm-1); the calculated 15N2 shift is 1.2 cm-1 (observed 0.9 cm-1). This excellent agreement between observed and calculated band
position and isotopic shift supports the identification of two N2BeO2 species. The photosensitive 970.8 cm-1 band is more intense in solid N2 than its 988.6 cm-1 solid argon counterpart. Both bands exhibit 1/2/1 isotopic triplets and the 16/18 ratios for a linear OBeO species. The destruction on photolysis suggested a charged species, and since the anion is expected to be more stable than the cation, a tentative anion assignment was offered.8 The present DFT calculations predict strong ν3 fundamentals for the linear anion at 1219 cm-1 and cation at 996 cm-1. The latter is clearly in better agreement with the observed band and suggests assignment of the 988.6 and 970.8 cm-1 bands to the
10096 J. Phys. Chem., Vol. 100, No. 24, 1996 cation (OBeO)+. The electron-deficient cation has longer bonds (1.535 Å) than the anion (1.438 Å) and neutral (1.441 Å) based on DFT/B3LYP calculations. The presence of a cation requires an anion and O3- is also observed at 792.4 cm-1 in the solid nitrogen matrix and O4- was observed in the argon matrix studies.2 The present DFT calculations (Table 7) provide more theoretical support for identification of several bands observed in the earlier Be + O2 reactions in condensing argon.8 Two bands labeled K at 1288.9 and 1264.1 cm-1 increased on photolysis, revealed 1/2/1 mixed isotopic triplets for two equivalent O atoms, and exhibited 16/18 ratios (1.0168, 1.0175) appropriate for a symmetric Be-O2 stretching mode.29 The obvious molecule, cyclic 1A1 BeO2, was considered, but a calculated spectrum for the high-energy BeBeO2 species fit the observed bands better. However, the present DFT calculations predict the strongest band for 1A1 BeO2 at 1347.2 cm-1 with a 16/18 ratio of 1.0189. The ArBeO2 species gives the strongest band at 1336.4 cm-1, a 16/18 ratio of 1.0181, and an argon binding energy of 8 kcal/mol. Accordingly, the K bands are reassigned to singlet ArBeO2. Calculated charge distributions (Be0.42+O2-0.42 and Ar0.21+Be0.47+O2-0.68) show that cyclic 1A1 BeO2 is stabilized by the argon interaction, and like ArBeO, ArBeO2 is a neutral argon-containing molecule owing to the positive charge on beryllium. The growth on photolysis may be due to direct reaction with 1P1 Be reached by photoexcitation at 236 nm:30 236 nm
Ar + Be + O2 98 ArBeO2 The Ar2BeO2 (rhombus, cross) was investigated, and the strongest mode was calculated at 1398.3 cm-1, blue-shifted 26.2 cm-1 from triplet linear OBeO, the charge distribution O0.30-Be0.60+O0.30+ was found, and two argon atoms were bound with a total of 7 kcal/mol. The OBeO molecule has been identified at 1412.4 cm-1 in solid argon, and the
Ar (OBeO) Ar model may more correctly represent the observed species. Calculations show that the most stable BeO3 isomer is triplet OBeO2 < singlet BeO3 < triplet BeO3 with all C2V structures. Although the analogous OMgO2 species is observed in thermal Mg + O3 and laser-ablated Mg + O2 experiments,7,10 the OBeO2 molecule cannot be identified in the 1300 cm-1 region in the Be + O2 work. The species J band at 871.8 cm-1 increased on annealing, exhibited a mixed isotopic multiplet and decreased on photolysis as expected for an ozonide. Although calculations for a singlet BeO3 species8 did not fit the observed spectrum, the higher energy triplet Be+O3- species calculated here (Table 7) does predict the strong ν3 ozonide mode in the correct region. The strongest band calculated here for the triplet Be+O3 species at 898.0 cm-1 exhibits a 1.0557 ratio for 16/18 in good agreement with the observed 1.0565 value. Apparently O2 can add only to the oxygen side of ArBeO during annealing. The present calculations have identified BeO4 in the previous argon matrix experiments.8 A sharp weak band appeared at 1307.4 cm-1 on annealing (Figure 5c, ref 8); a similar 1282.9 cm-1 band appeared on annealing with Be and 18O2. In 16,18O2 experiments, a pentet at 1307.4, 1301.6, 1295.3, 1289.2, 1282.9 cm-1 with 1/4/6/4/1 relative intensities appeared on annealing. The strongest band calculated for the triplet D2d O2BeO2 species at 1286.4 cm-1 with 18O2Be18O2 counterpart at 1261.9 cm-1 (16/18 ratio 1.0194) is in excellent agreement with the observed
Andrews et al. 16/18 ratio (1.0191) and band position. This excellent agreement and the observed pentet mixed isotopic structure confirm the identification of the new O2BeO2 species. The O2BeO2 molecule is more stable than OBeO + O2 by 52 kcal/mol, based on DFT calculations, and its formation from further reaction with O2 and OBeO on annealing is expected. Analogous O2MgO2 and O2CaO2 molecules will be identified below.
OBeO + O2 f O2BeO2
∆E ) -52 kcal/mol
Additional calculations on BeOBeO and OBeOBeO predict the highest Be-O stretching mode at 1548.4 cm-1 for the former and at 1468.0 cm-1 for the latter molecule. Calculated 16/18 ratios for these modes, 1.0203 and 1.0210, are indicative of Be-O bond vibrations. Calculations for the mixed isotopic species reveal bands at 1548.4, 1544.9, 1521.7, 1517.6 cm-1 for the former and 1468.0, 1466.7, 1465.2, 1441.1, 1439.6, 1437.8 cm-1 for the latter molecule and predict doublet experimental product spectra with bandwidths of about 3 cm-1. The bands observed at 1572 and 1465 cm-1 grow on annealing and are appropriate for aggregate species. These bands are reassigned here, respectively, to the linear BeOBeO and OBeOBeO species. Although singlet (BeO)2 ring is more stable than triplet BeOBeO by 44 kcal/mol, both are produced on Be + O2 codeposition, but only the latter is formed on annealing the cold solid. Both are sufficiently stable for BeO to displace argon from ArBeO. However, the most likely diffusing species on
ArBeO + BeO f Ar + (BeO)2 f Ar + BeOBeO Be + OBeO f BeOBeO
∆E ) -150 kcal/mol ∆E ) -106 kcal/mol ∆E ) -78 kcal/mol
annealing is ground-state (1S) Be atoms, and only the Be reaction with triplet OBeO and not with singlet ArBeO2 gives product on annealing in solid argon. Mg. Two new absorptions were observed in these experiments. The sharp, weak 723.6 cm-1 band has the appropriate 16/18 ratio (1.0244) for the 3Σg linear insertion product OMgO observed at 767.7 cm-1 in solid argon (16/18 ratio 1.0243).7 Earlier CASSCF calculations1 predicted the ν3 fundamental for OMgO at 796 cm-1 and the present DFT calculations predict 734 cm-1. The NN-perturbed OMgO tee is predicted at 767.7 cm-1. The weak 628.5 cm-1 band shows only a 5.1 cm-1 18O2 shift which is small for a MgO2 species. However, the side-N2, side-O2 triplet state with perpendicular N2Mg, MgO2 planes has by far the strongest calculated band at 674.7 cm-1 with a predicted 18O2 shift of 6.8 cm-1, which agrees well with the 628.5 cm-1 band. The NN-(OMgO) tee and N2MgO2 side-side complexes are more stable than linear, triplet OMgO + N2 by 32 and 15 kcal/mol, respectively, so these species will form spontaneously from OMgO in solid N2. Calculations performed here for OMgO2 confirm the earlier assignment of a 731.3 cm-1 band in Mg + O3 reactions to this species based on the doublet of triplets with 16,18O3 reagent.10 The strongest band calculated at 773.2 cm-1 (Table 8) reveals 16/18 vs 26 ratio 1.0244 and 24/26 vs 16 ratio 1.0237 in excellent agreement with the observed ratios 1.0252 and 1.0225 for this O2-perturbed Mg-O stretching mode. The bands earlier assigned to Mg+O3- at 848 and 823 cm-1 based on large 16/ 18 ratios (1.058) for the predominately O-O motions in the perturbed ozonide anion are substantiated by the strong calculated 898.4 cm-1 b2 mode for the C2V triplet Mg+O3- ring species.
Group 2 MO2, O2MO2, and Related Molecules DFT calculations also confirm the previous identification7 of linear MgOMgO, a higher energy isomer of (MgO)2, which is formed here by addition of Mg to linear OMgO. Additional calculations done here (Table 5) for the new D2d O2MgO2 species, which is stable to dissociation to OMgO and O2 by 78 kcal/mol, match isotopic shifts and structure for a 669.9 cm-1 band previously identified as MgxOy.7,10 The 2616 isotopic band7 at 652.3 cm-1 enables the 26-18 isotopic band from earlier experiments10 to be calibrated as 640.6 cm-1. The experimental problem with this species was isotopic CO2 masking the 24-18 isotopic product. However, in the 26Mg + 16,18O experiment (Figure 4c, ref 10) three sharp bands were 3 resolved at 650.0, 647.0, 644.0 cm-1, which are the strongest bands of a mixed isotopic pentet. The calculated isotopic ratios 669.9/652.3 ) 1.0267 and 652.3/640.6 ) 1.0183 for 24/26 with 16 and 16/18 with 26 are in very good agreement with calculated 1.0290 and 1.0169 ratios, respectively, for this antisymmetric O2-Mg-O2 stretching mode. Furthermore, the calculated spectra for 16O226Mg18O2 and 16O18O26Mg16O18O give the same 681.1 cm-1 band, with 16O18O26Mg16O2 at 683.9 and 18O 26Mg16O18O at 678.2 cm-1, which justifies the pentet 2 structure with the strongest central peak. This calculated and observed pentet structure for the scrambled isotopic species verifies the observation of the D2d disuperoxide O2MgO2. Ca. The major difference between N2 and Ar experiments with Ca and O2 is marked enhancement of group 6 bands at the expense of group 1 in N2. The group 6 bands photolyze together with radiation between 380 and 470 nm, which increases the group 1 bands at 497.0 and 484.7 cm-1 (Figure 3b). The case was made for the sharp group 1 band at 515.7 cm-1 in solid argon to be due to a bent OCaO dioxide molecule with valence angle upper limit of 150°.2 That assignment is reaffirmed by the quantum chemical calculations summarized in Table 4, which predict a strong ν3 fundamental in the 549514 cm-1 range and valence angle in the 135-145° range for 3B OCaO. Furthermore, the 16/18 ratio predicted from 2 CASSCF harmonic frequencies is 1.0344 as compared to the observed 1.0335 ratio. The nitrogen matrix environment red shifts OCaO to 497.0, 484.7 cm-1 and slightly decreases the 16/18 ratio (1.0328, 1.0319) which may imply a more nearly linear OCaO molecule in solid nitrogen. Triplet OMgO, is linear, but OCaO is more stable in the bent (∼140°) 3B2 structure, which requires the participation of d orbitals on Ca that are not available for Mg. A weak group 6 band at 736.2 cm-1 with shoulder at 739.2 cm-1 in solid argon was assigned to cyclic CaO2 in the 1A1 peroxide state based on agreement with CASSCF calculations;1,2 unfortunately the Ca-O2 stretching mode was masked by stronger 592 cm-1 absorptions due to a water reaction product.31 The larger group 6 band yield in solid nitrogen provides all three fundamentals at 742.1, 555.7, 500.5 cm-1 in excellent agreement with predicted harmonic values (Table 6). Furthermore, the 12/25/9 relative intensities are in good agreement with theoretical predictions. The present thermal Ca experiments observed the same group 6 bands. Earlier thermal experiments also measured the strongest band at 556.0 cm-1 and the 44Ca counterpart at 545.5 cm-1. In these experiments the upper band was measured at 742.5 and 740.7 cm-1 for 40Ca and 44Ca, respectively, giving a 1.8 cm-1 shift. Likewise, the upper band was measured at 703.0 and 701.0 cm-1 for 40Ca and 44Ca with 18O2 giving a 2.0 cm-1 shift indicating a small amount of O-O and Ca-O2 stretching mode mixing. Agreement between calculated and observed isotopic ratios confirms the identification of 1A1 cyclic CaO2. For the ν1 (a1)
J. Phys. Chem., Vol. 100, No. 24, 1996 10097 mode the observed and calculated 16/18 ratios are 1.0564 and 1.0570, and the 40/44 ratios are 1.00243 and 1.00231, respectively. For the ν2(a1) mode the observed and calculated 16/18 ratios are 1.0352 and 1.0362, and the 40/44 ratios are 1.0192 and 1.0184. For the ν3(b2) mode the observed and calculated 16/18 ratios are 1.0536 and 1.0524. Finally, note that the ν1 and ν2 (a1) mode mixing in cyclic CaO2 reflected in the oxygen and calcium isotopic shifts is reproduced by the DFT isotopic frequency calculations. Calculations were done for N2CaO2 species to rationalize argon-to-nitrogen matrix shifts. In both 1A1 and 3B2 cases, the side-on N2 species was unstable relative to the end-on NN complexes. The singlet NNCaO2 molecule (end NN, side O2) is calculated to be stable by 2.1 kcal/mol relative to N2 and 1A1 CaO2, and the ν1 mode of CaO2 is blue shifted 1.8 cm-1 with the ν2 mode red shifted by 25 cm-1 in NNCaO2. Hence, the relative position of 1A1 CaO2 modes in solid argon and nitrogen is rationalized. The thermal experiments (Figure 4) gave a higher yield of group 4 (CaO)2 bands at 559.5, 497.0 cm-1 relative to group 6. The (CaO)2 molecule is obviously formed by the addition of a second Ca atom to CaO2. This shows that thermal evaporation generated a higher Ca atom concentration than pulsed laser ablation in these matrix experiments. The 449.5 cm-1 band was observed previously (at 451 cm-1) and the large 44Ca shift noted.6 Owing to a lack of resolution in 16,18O2 experiments, the band was assigned to CaOx. This species was observed as a sharp, weak 484.4 cm-1 band in solid argon, but a broad 485-475 cm-1 band was observed with 16,18O .2 However, the present experiments reveal a partially 2 resolved 1/4/6/4/1 pentet with 16,18O2, which suggests two equivalent O2 molecules. The D2d structure should exhibit a 1/4/4/2/4/1 sextet; here the two central components form a single band in agreement with calculations for O2MgO2. Furthermore, the marked enhancement of the 449.5 cm-1 band relative to the 556.0 cm-1 CaO2 band with increasing O2 concentration denotes a higher dioxygen species. DFT calculations predict a stable D2d triplet O2CaO2 species bound by 54 kcal/mol with respect to O2 and CaO2 (1A1), which will form readily from CaO2 and O2. This molecule is predicted to have one very strong fundamental at 498.4 cm-1 for normal isotopes, at 484.3 cm-1 for 44Ca and 16O, and at 487.7 cm-1 for 40Ca and 18O. The observed and calculated 16/18 ratios are 1.0207 and 1.0221, and the observed and calculated 40/44 ratios are 1.0297 and 1.0291, respectively. These ratios are in excellent agreement and confirm the identification of the new calcium disuperoxide molecule O2CaO2 in these samples. It is interesting to compare calculated bond lengths for 1A1 and 3A2 Ca(O2) with triplet O2CaO2. The 1A1 peroxide species has Ca-O ) 1.978 Å, O-O ) 1.518 Å, and Ca1.05+(O2)1.05-; the 3A2 superoxide has Ca-O ) 2.191 Å, O-O ) 1.340 Å and Ca0.54+(O2)0.54-; the triplet disuperoxide has Ca-O ) 2.226 Å, O-O ) 1.344 Å and (O2)0.56-Ca1.12+(O2)0.56-. The O2CaO2 species is a true disuperoxide based on calculated atomic charges and O-O bond lengths. In contrast, the potassium and cesium species are due to a different M+O4- structure with intermolecular (O2-O2)- bonding.32-35 The analogous O2SrO2 and O2BaO2 species are expected to be formed, but assignments cannot be made from the observed spectra. The group 2 bands are due to Ca+O3- calcium ozonide as attested by the large 16/18 ratios for pure oxygen motions and the increased relative yield at higher O2 concentration. The 806.6 cm-1 band is due to ν3 and the 633.9 cm-1 band to ν2 of the ozonide anion. Analogous bands have been observed for Na+O3- and Cs+O3-.36 Although calculations predict triplet
10098 J. Phys. Chem., Vol. 100, No. 24, 1996 OCaO2 and singlet CaO3 at lower energy than triplet CaO3, the latter is clearly observed here.4-6 Note that Ca+O3- is a true ozonide based on infrared, optical, and resonance Raman spectra.4-6,37 Sr. Strontium follows calcium in the major difference between N2 and Ar experiments with the same preference for group 6 bands in N2 at the expense of group 1 bands. Again near ultraviolet photolysis converted the cyclic SrO2 species to the open (probably linear) OSrO molecule, and the ozonide SrO3 increased on annealing. The assignments to cyclic SrO2 are in excellent agreement with calculations. As comparisons of Tables 3 and 5 show, MP2 calculations predict the ν1(O-O) stretching and ν2(sym Sr-O2) stretching modes 29 and 15 cm-1 higher than the observed values in solid argon. The calculated 16/18 ratios, 1.0599 and 1.0442, are also in excellent agreement with the observed ratios, 1.0589 and 1.0434. In solid nitrogen the ν2 mode is red-shifted 36 cm-1 by interaction with nitrogen, but the ν1 mode is not shifted. DFT calculations give almost the same ν1 frequency but slightly lower ν2 and ν3 values. The assignments to open OSrO are, however, not as well predicted by theory. The ν3 fundamental of OSrO in argon, 532.4 cm-1, is actually higher than the OCaO value, 515.7 cm-1, but theory predicts a value 95 cm-1 lower. Ba. Barium also follows strontium and calcium with the preference for group 6 BaO2 bands over group 1 OBaO bands in solid nitrogen in contrast to argon. Again near ultraviolet photolysis converted cyclic BaO2 to open OBaO. Reaction Mechanisms. The very high yield of triplet-state open dioxide product molecules with laser-ablated Be, Mg, and Ca atoms in condensing argon invites consideration of a direct insertion reaction involving metastable 3P metal atoms produced by laser ablation. It is known from earlier thermal Mg and Ca studies with O2 in condensing argon that no reaction occurs with ground-state Mg and Ca atoms;4,5,10 although reactions have not been studied with thermal Be atoms, the smallest group 2 metal atom is expected to be even less reactive. The radiative lifetime of 3P1 Ca is 331 ( 8 µs, 3P1 Mg is 2.3 ( 0.2 ms38 and the 3P1 Be lifetime is expected to be longer still. At 500 °C thermal effusion described here, the velocity (rms) of Ca atoms is 6.9 × 104 cm/s. Even at this very low temperature, only 29 µs is required for Ca to travel 2 cm, and with an order of magnitude longer 3P lifetime, most 3P metastable atoms will survive the flight. Of course, laser ablated Ca is much hotter and faster and the majority of 3P Ca atoms produced by laser ablation can reach the condensing matrix. The same can be said for laser-ablated Mg and Be atoms with longer triplet lifetimes. Although the radiative lifetime of 3P1 Sr is shorter (19 ( 1 µs),38 laser ablation will produce 3P Sr capable of surviving the 2 cm flight. The open OSrO triplet insertion product dominates the laser ablated Sr + O2 reaction in condensing argon although more 1A1 cyclic SrO2 product is observed than for Ca in argon.2 However, in condensing N2, cyclic 1A1 SrO2 is favored. The lifetime of 3P1 Ba (1 µs)38 is probably too short for laser-ablated metastable Ba atoms to contribute to the product yield; however, ground-state Ba is sufficiently reactive to undergo addition or insertion reactions with O2 as thermal Ba matrix experiments have made both products.2,4 Furthermore, N2 is more effective for collisonal quenching of 3P Ca and Sr atoms than krypton (and argon).39 Hence the dominance of 1A1 cyclic CaO2 in laser-ablation nitrogen matrix experiments is due to reaction of ground-state 1S atoms, the same as obserVed with thermal Ca atoms by Knudsen effusion.
Andrews et al. A similar argument can be made for 1A1 cyclic SrO2 in laser ablation nitrogen matrix experiments. The mechanism of 380-470 nm photolysis by which 1A1 CaO2 is converted to 3B2 OCaO must be considered. It appears likely that dissociation of 1A1 CaO2 to 1S Ca and O2 occurs (De ) 59 kcal/mol, B3LYP or 48 kcal/mol, CCSD(T)). The strong 1P r 1S resonance transition is excited in solid nitrogen at 405, 415 nm40 from which the 3P state can be populated for insertion to form 3B2 OCaO. In the case of Be and Mg the 3P-1S energy gap is larger, the ablated atoms are faster, and quenching by N2 does not relax all of the ablated 3P Be and Mg atoms to the unreactive 1S ground states, but the overall product yield is much lower in condensing N2 than in argon. The stable triplet linear OBeO molecule is not observed in solid N2. Triplet linear and cyclic BeNN and BeN2 species are stable with respect to 3P Be atoms and N2 by 43 and 41 kcal/mol, respectively (DFT calculations) but unstable by 14 and 16 kcal/mol relative to 1S Be and N2. These triplet molecules can react readily with O2 to form singlet NNBeO2 and triplet N2BeO2 products, which are stable even to 1S Be, O2 and N2 by 89 and 64 kcal/mol, respectively. If formed in condensing N2, linear OBeO collapses to NNBeO2 (calc ∆E ) -11 kcal/mol) but not to N2BeO2 (calc ∆E ) +14 kcal/mol). In argon matrix reactions with laser ablated Be, the 3P Be reaction is spontaneous to give triplet linear OBeO some of which decomposes to BeO. The BeO diatomic is trapped as Ar-BeO, and the Ar-BeO binding energy is calculated by DFT to be 11 kcal/mol. Linear OBeO is also solvated by argon, but the perturbation on the OBeO molecule is small. However some OBeO is relaxed as singlet Ar-BeO2; this molecule is also produced by photoexcitation of Be near O2 in the argon matrix.8 In addition, DFT calculations identify the stable O2BeO2 disuperoxide molecule and characterize the exothermic formation reaction from OBeO + O2 on annealing. Conclusions Pulsed laser-ablated group 2 metal atoms react with O2 in condensing N2 to give a different product distribution than in condensing Ar. This is attributed to faster relaxation of ablated metastable 3P metal atoms by N2. The major product for Ca in argon is the 3B2 open bent OCaO dioxide but in nitrogen the major product is the 1A1 cyclic CaO2 peroxide. Three fundamentals for the latter with 44Ca and 18O substitution are in excellent agreement with quantum chemical isotopic frequency calculations, which confirm this identification of CaO2. Several N2MO2 molecules have been characterized by matrix infrared and DFT calculated isotopic spectra, which account for nitrogen matrix shifts. The singlet ArBeO2 molecule is identified from previous argon matrix experiments with the help of DFT isotopic frequency calculations; DFT calculations predict Ar-BeO2 binding energy of 8 kcal/mol just less than the ArBeO binding energy of 11 kcal/mol. Finally, the triplet disuperoxide molecules O2BeO2, O2MgO2, and O2CaO2 have been identified from matrix infrared and DFT calculated isotopic spectra; these molecules are true disuperoxides, based on calculated charge distributions, in contrast to the K+O4species.32-35 The successful collaboration of quantum chemistry and matrix infrared spectroscopy for the identification of new molecular species is clearly demonstrated. Acknowledgment. We gratefully acknowledge support by N.S.F. Grant CHE 91-22566 and preliminary N2BeO2 calculations performed by R. D. Davy.
Group 2 MO2, O2MO2, and Related Molecules References and Notes (1) Bauschlicher, C. W., Jr.; Partridge, H.; Sodupe, M.; Langhoff, S. R. J. Phys. Chem. 1992, 96, 9259. (2) Andrews, L.; Yustein, J. T.; Thompson, C. A.; Hunt, R. D. J. Phys. Chem. 1994, 98, 6514. (3) Nein, C.-F.; Rajasckhar, B.; Plane, J. M. C. J. Phys. Chem. 1993, 97, 6449. (4) Ault, B. S.; Andrews, L. J. Chem. Phys. 1975, 62, 2312, 2320. (5) Thomas, D. M.; Andrews, L. J. Mol. Spectrosc. 1974, 50, 220. (6) Andrews, L.; Ault, B. S. J. Mol. Spectrosc. 1977, 68, 114. (7) Andrews, L.; Yustein, J. T. J. Phys. Chem. 1993, 97, 12700. (8) Thompson, C. A.; Andrews, L. J. Am. Chem. Soc. 1994, 116, 423; J. Chem. Phys. 1994, 100, 8689. (9) Thompson, C. A.; Andrews, L.; Davy, R. D. J. Phys. Chem. 1995, 99, 7913. (10) Andrews, L., Prochaska, E. S.; Ault, B. S. J. Chem. Phys. 1978, 69, 556. (11) Stevens, P. J.; Devlin, F. J.; Chablowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623 and references therein. (12) Pople, J. A.; Binkley, J. S.; Seeger, R. Int. J. Quantum Chem. Symp. 1976, 10, 1. (13) Bartlett, R. J. Annu. ReV. Phys. Chem. 1981, 32, 359. (14) Raghavachari, K.; Trucks, G. W.; Pople, J. A.; Head-Gordon, M. Chem. Phys. Lett. 1989, 157, 479. (15) Gaussian 92/DFT, Revision G.2, M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. W. Wong, J. B. Foresman, M. A. Robb, M. Head-Gordon, E. S. Replogle, R. Gomperts, J. L. Andres, K. Raghavachari, J. S. Binkley, C. Gonzalez, R. L. Martin, D. J. Fox, D. J. Defrees, J. Baker, J. J. P. Stewart, and J. A. Pople; Gaussian, Inc.: Pittsburgh PA, 1993. (16) SIRIUS is an MCSCF program written by H. J. Jensen, H. Ågren, and J. Olsen; ABACUS is an MCSCF energy derivatives program written by T. Helgaker, H. J. Jensen, P. Jorgensen, J. Olsen, and P. R. Taylor. (17) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265 and references therein. (18) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
J. Phys. Chem., Vol. 100, No. 24, 1996 10099 (19) Pettersson, L. G. M.; Siegbahn, P. E. M.; Ismail, S. Chem. Phys. 1983, 82, 355. (20) Roos, B.; Veillard, A.; Vinot, G. Theor. Chim. Acta 1971, 20, 1. (21) Bauschlicher, C. W., Jr.; Sodupe, M.; Partridge, H. J. Chem. Phys. 1992, 96, 4453. (22) Frenking, G.; Koch, W.; Gauss, J.; Cremer, D. J. Am. Chem. Soc. 1988, 110, 8007. (23) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007. Kendall, R. A.; Dunning, T. H.; Harrison, R. J. J. Chem. Phys. 1992, 96, 6796. (24) Partridge, H. J. Chem. Phys. 1989, 90, 1043. (25) Partridge, H.; Faegri, K. Theor. Chim. Acta 1992, 82, 207. (26) Bauschlicher, C. W.; Langhoff, S. R.; Partridge, H. J. Chem. Phys. 1994, 101, 2644. (27) Ikeda, T.; Wong, N. B.; Harris, D. O.; Field, R. W. J. Mol. Spectrosc. 1977, 68, 452. (28) Boldyrev, A. I.; Simons, J. J. Phys. Chem. 1995, 99, 15041. (29) The 18O2 counterpart of 1264.1 should be 1242.4 cm-1. (30) Brom, J. M., Jr.; Hewitt, W. D., Jr.; Weltner, W., Jr. J. Chem. Phys. 1975, 62, 2133. (31) Kauffman, J. W.; Hauge, R. H.; Margrave, J. L. High Temp. Sci. 1984, 18, 97. (32) Andrews, L. J. Chem. Phys. 1971, 54, 4935. (33) Jacox, M. E.; Milligan, D. E. Chem. Phys. Lett. 1972, 14, 518. (34) Andrews, L.; Hwang, J.-T.; Trindle, C. J. Phys. Chem. 1973, 77, 1065. (35) Thompson, W. E.; Jacox, M. E. J. Chem. Phys. 1989, 91, 3826. (36) Spiker, R. C., Jr.; Andrews, L. J. Chem. Phys. 1973, 59, 1851, 1863. (37) Ault, B. S.; Andrews, L. J. Mol. Spectrosc. 1972, 65, 437. (38) Husain, D. J. Chem. Soc., Faraday Trans. 2 1989, 85, 85. (39) Husain, D.; Roberts, G. J. Chem. Soc., Faraday Trans. 2 1985, 81, 101. (40) Andrews, L.; Duley, W. W.; Brewer, L. J. Mol. Spectrosc. 1978, 70, 41.
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