J. Phys. Chem. 1995,99, 6356-6366
6356
Reactions of Laser Ablated Titanium, Zirconium, and Hafnium Atoms with Oxygen Molecules in Condensing Argon George V. Chertihin and Lester Andrews* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 Received: November 23, 1994; In Final Form: January 31, 1995@
Reactions of laser ablated Ti, Zr, and Hf atoms with oxygen molecules in a condensing argon stream gave the MO, MOz, and M202 molecules and MOz- molecular anions, which were identified from oxygen isotopic splittings and shifts. Simple self-consistent field, ECP calculations support the assignments to MOZ and MO2- molecular species. The metal dioxide molecules have 113 f 5" valence angles, while the molecular anions have 128 f 5" valence angles. The M202 molecules are probably nonplanar in contrast to the analogous more ionic alkaline earth metal species. A strong similarity between zirconium and hafnium oxide spectra was found. The observation of HfO and HfO2 stretching modes only 1-4 cm-' lower than ZrO and ZrOz modes is due to a combination of lanthanide contraction and relativistic effects for hafnium.
Introduction Reactions of oxygen with transition metals are of chemical interest as these oxides are involved in catalysis and chemisorption processes. However, little experimental data are available for titanium, zirconium, and hafnium oxides. Previous experimental and theoretical works have been mostly devoted to metal monoxide molecules due to their astrophysical interest.'-I7 Although the earlier work employed thermally heated oxide samples, more recent studies have utilized pulsed-laser evaporation methods. The Ti0 molecule has 3Aand ZrO and HfO have IX+ ground states for which gas phase and matrix vibrational fundamentals have been determined.',3,s,'0,'2-'4Also, rotational spectra have been observed for the diatomic molecules, and electronic transition moments have been measured in optical emission spectra and calculated the~retically.'~-'~ Four spectroscopic and one theoretical works have been done for Ti02 and Zr02.25'7-20 These molecules were evaporated from the solid oxides at about 2500 K, and V I and v3 were observed in Ar and Ne matrices; it was concluded2 from Ti isotopes that Ti02 has a valence angle near 110". Early molecular beam experiments observed a dipole moment for Ti02 and therefore a bent structure,18 and recent theoretical calculations obtained a bond angle in the 114- 120" range, depending on the level of theory.*O The Hf02 molecule has not been previously characterized. In addition, two mass spectrometric studies have been devoted to titanium-oxygen The vapor over EuTiO3 and CoTiO3 contained TiO, TiOz, Ti203, and Ti204 molecules, and the concentration of the two latter molecules increased with increasing temperature; the Ti202+ and Ti20+ fragment ions were also observed. It is important to note that previous experimental work has been done by thermal evaporation of solid^.',^,'^ But it is wellknown that the method of matrix synthesis with laser ablation of metals can produce more chemistry than ordinary evaporation. Thus, it was decided to reinvestigate the M 0 2 (M = Ti, Zr, and Hf) systems because the formation of other species besides MO and MO2 was expected. Also, the new HfOz molecule should be produced because of the similarity of Zr and Hf.
+
Experimental Section The apparatus for laser ablation and matrir isolation has been described in detail p r e v i o u ~ l y The . ~ ~Ti ~ ~(Goodfellow ~ Metals, @
Abstract published in Advance ACS Abstracts, March 15, 1995.
99.6%), Zr (Johnson-Matthey), and Hf lumps (Johnson-Matthey) targets were mounted on a rotating (1 rpm) rod. The Nd:YAG laser fundamental (1064 nm, 6-Hz repetition rate, 10-ns pulse width) was focused (10-cm focal length) on the target through a hole in CsI cryogenic window. Laser power ranged from 40 to 100 &/pulse at the target. Metal atoms were codeposited with 0.5 and 1% 0 2 (and isotopic modifications) in argon at 2-4 mmolh for 1-3-h periods. Further experiments were done by codepositing ablation products from the metal targets, Ti02 (Strem Chemicals), and Zr02 (Zr-23 zirconia disk, 5% CaO, Johnson-Matthey) with argon. FTIR spectra were recorded with 0.5-cm-' resolution on Nicolet 750 and 60 SXR instruments; the frequency accuracy is &O. 1 cm-' for sharp bands. Samples were temperature cycled, and more spectra were recorded; selected samples were subjected to broad-band photolysis by a medium pressure mercury arc (Philips, 175 W) with the globe removed. Results
Infrared spectra of Ti, Zr and Hf+O2 in argon will be presented in turn. Ti 0 2 System. Spectra of the Ti 0 2 system in argon are illustrated in Figures 1 and 2 and listed in Table 1. With low laser power the major products were a triplet 989.71987.81 985.5 cm-' and sharp 946.9- and 917.1-cm-' bands (Figure la), which are due to Ti0 and Ti02, based on earlier work.2 Broad-band photolysis (Figure lb) destroyed 0 4 - 25 at 953.7 cm-I, decreased TiO, and increased Ti02 4-fold. In addition, weak 878.4- and 736.5-cm-' bands increased, and new bands appeared at 956.8, 706.5, and 683.8 cm-I. A subsequent annealing to 25 K (Figure IC) sharpened the latter bands to 704.2 and 680.1 cm-I, increased Ti02 bands, decreased Ti0 bands, and increased very weak bands at 863.2 and 753.5 cm-I. An experiment with higher laser power (Figure Id) gave a spectrum similar to that in Figure IC; annealing to 25 K decreased Ti0 and increased Ti02 and the 863.3-, 753.6-cm-' and 704.2-, 680.1-cm-I band pairs. Further annealing to 35 and 45 K decreased TiO, TiO2, and the latter pair, while the former pair increased stepwise. Comparison of the low and high laser power spectra (Figure la,d) provides evidence for the metal atom count in the product molecules as titanium atom concentration (and radiation) increases with laser power. Bands that increase proportional to Ti0 and Ti02 involve a single Ti atom, and
+
0022-365419512099-6356$09.0010 0 1995 American Chemical Society
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J. Phys. Chem., Vol. 99, No. 17,1995 6357
Ti, Zr, and Hf Atom Reactions with Oxygen Molecules
0.22
I
0.20
j
0.12
-
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TiO, I
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Wavenumbers (cm-I 1 Figure 1. Infrared spectra in the 550-1050-cm-' region for reactions of pulsed-laser ablated Ti and 0 2 (0.5% in argon) after deposition at 10 iz 1 K: (a) low power ablation for 1.5 h, (b) after 20-min broad-band photolysis, (c) after annealing to 25 K, and (d) higher power ablation for 1.5 h.
bands that increase substantially more involve two or more Ti atoms. Table 1 also lists the Ti count for product molecules. Further studies with higher laser power led to the spectra shown in Figure 2. The two Ti02 bands remained the most intense in the spectra, and other bands appeared in the 980950-cm-' region and below 900 cm-', but 0 4 - was barely detected at 953.7 cm-' and 03- was observed at 804.2 cm-'.25326 These samples were subjected to broad-band medium pressure mercury arc photolysis for 20-60-min periods. The only change was a 25% reduction in the weak 1014.3-, 1012.9-cm-' band and a 5% growth in TiO2. Annealing to 25-30 K decreased intensities of the 989.7/987.8/985.5- and 736-cm-' bands. Other bands increased for 25 < T < 30 K and decreased for T > 30 K, including those for TiO2. The 999.91996.9-cm-' band appeared in the spectra only after annealing above 20 K; annealing behavior is also summarized in Table 1. Oxygen isotopic substitution was employed for the identification of bands (Figure 2). The band 946.9, 917.1, 878.4, 863.2, 704.2, and 680.1 cm-I produced triplets in the experiments with scrambled 1 6 , 1 8 0 2 oxygen. Other bands produced doublets. In experiments the 16061802,the bands at 946.9 and 917.1 cm-' also gave triplets with weak middle components of isotopic structure. The band 704.2 and 680.1 cm-I again produced triplets with strong middle components, almost as strong as in 1 6 , 1 8 0 2 experiments. Notice also that weak bands at 987.8, 946.9, 917.1, and 704.2 cm-' were observed even in the experiments with l e 0 2 . All isotopic spectra were enriched by
these I6O product bands. The Ti0 and Ti02 bands were also observed in blank experiments where metal atoms were codeposited with argon. Ablation is always accompanied by the production of Ti0 and Ti02 from metal surface contamination or reaction with trace 0 2 impurity, as has been noted in reports of Ti reactions with N2, H2, and CO from this l a b ~ r a t o r y . ~ ' - ~ ~ Separate laser ablation studies were done with Ti02 pellets and argon. The spectra were similar to those in Figures l a and 2a, including the observation of minor bands as shown in Figure 3a. The relative yield of Ti0 and Ti02 depended upon laser power; higher power favored Ti02 presumably from decomposition of the substrate to elements with subsequent reaction as in Figures la and 2a. Laser ablation studies were also done with the Ti target and argon. After deposition, only Ti0 and Ti02 bands were observed in the spectra. The relative yield of Ti0 was always smaller than that for TiO2. When the target was kept under vacuum, the yield of oxides decreased from one experiment to the next due to cleaning of the surface by the ablation process. Experiments were done with increased 0 2 concentration. With 10% 02, ozone bands were increased, the 999-cm-' band replaced TiO, sharp 972.1-, 946.9-, 917.1-, and 878.4-cm-I bands were observed, as in Figure 2a, and a new broad band was observed at 801 cm-I. Annealing produced Ti0 and strong new 813- and 829-cm-' bands and destroyed the 946.9-, 917.1-, and 878.4-cm-' bands. In pure 0 2 , the major products were 0 3 and new bands at 999 and 805 cm-I; annealing produced broad 980- and 812-cm-I bands.
Chertihin and Andrews
6358 J. Phys. Chem., Vol. 99, No. 17, 1995
I
0.8 -
TiO,
1
0.3
!
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Ti0
I
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W
L L I
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Wavenumbers (cm-1) Figure 2. Infrared spcztra in the 550-1050-~m-~region for Ti
+
+
+
0 2
(1%) in argon at 10 ir 1 K: (a) I6O2, (b) 16.1802, (c)
Zr 02 System. Product absorptions from the Zr 0 2 reaction are listed in Table 2. As in the Ti system, deposition with low laser power gave only three bands, namely, 958.6, 884.3, and 818.0 cm-' in the spectrum. Deposition with higher laser power gave the spectra shown in Figure 4. Annealing revealed three new absorptions at 915.4,808.7/805.3, and 796.3/ 790.6 cm-', and other bands decreased. The bands at 761.4 and 703.8 cm-' decreased, while the bands at 652.4/644.8 and 593.1 cm-I grew on annealing to 25 K and then decreased at higher temperature. Isotopic substitution with scrambled oxygen gave triplets for the 884.3-, 818.0-, 761.4-, 652.4-, and 593.1-cm-' bands. Bands near 800 cm-' gave rather complicated isotopic structures, which were overlapping with the intense 818.0-, 793.2-, and 780.5cm-' bands. Reaction of ' 6 0 2 / ' 8 0 2 mixtures produced a different picture: the middle components of isotopic structures from the 884.3-, 818.0-, and 761.4-cm-I absorptions were weak, while middle components for the 652.4- and 593.1-cm-I bands were strong. Also, like the previous system, anomalous distribution of isotopic intensities was observed due to the presence of ZrI602 from the zirconium metal surface. This was again confirmed by metalhlank argon experiments, which contained ozone, 0 4 - , and 958.6-, 884.3-, and 818.0-cm-l bands. Pulsed-laser ablation experiments were done with a zirconia ceramic target and argon, and the spectrum is shown in Figure 3b. As with titanium, the spectrum was similar to that for the Zr i0 2 reaction, which can be seen by comparing Figures 3b and 4a. Note the increased yield of 958.6 cm-' relative to 884.3-, 818.0-cm-' absorption and weak bands at 747, 581, and
1602
-t I8O2, and (d)
"02.
5 16 cm-' due to CaO and (CaO)z, respe~tively,~~ from the CaO present as a stabilizer in the ceramic sample. Higher laser power favored the 884.3-, 8 18.0-cm-' absorptions. Hf 0 2 System. Spectra for the Hf 0 2 system, shown in Figure 5 and listed in Table 3, are very similar to those obtained for the Zr 0 2 system. The most intense new bands after deposition with medium and high laser power were 958.3, 883.4, and 814.0 cm-'. Note that two doublets (one strong and one weak) were observed in each region: 886.0/883.4 and 879.3/ 876.6 cm-' and 816.0/814.0 and 807.6B05.3 cm-I. With low laser power the 958.3-cm-' band was the strongest band in the spectrum. Annealing to 20 K increased bands in the 1050-750-cm-' region, particularly 958.3, 883.4, and 814.0 cm-I. On further annealing the 893.6-, 801.1-, and 792.3-cm-' bands increased in intensity; these bands appeared in the spectra only after annealing to 25 K. Below 750 cm-I spectra were much more complicated. The most intense bands in this region were 747.9 and 684.4 cm-I. The first band grew on annealing to 20 K and slowly decreased with further increasing temperature, and the second band decreased progressively with increasing temperature. The results of oxygen isotopic substitution are also presented in Table 3. The doublets near 886-876 and 815-805 cm-' produced triplet isotopic bands with scrambled 1 6 , 1 8 0 2 (Figure 5c). Due to overlapping it was difficult to determine middle components of isotopic structure for the 801.1-/798.9-cm-' bands. The 958.3-cm-' band gave a single oxygen-18 counterpart at 908.3 cm-I. The 684.4- and 649.6-cm-' bands
+
+
+
Ti, Zr,and Hf Atom Reactions with Oxygen Molecules TABLE 1: Infrared Absorptions (cm-') Observed for Ti 1039.6 1033.1 1014.3 1012.9 999.9 996.9 989.7 987.8 985.5 972.1 958.9 956.8 953.7 95 1.O 948.9 946.9 944.9 943.1 925.9 923.2 920.1 917.1 914.1 911.3 909.5 895.0 892.9 885.0 881.6 878.4 875.4 872.4 866 sh 863.2 842.4 813 804.2 790.2 753.5 736.5 704.2 695.9 680.1 625.0 611.1 596.5 561.6
982.3 976.2 971.3 969.9 955.6 952.6 948.0 946.0 943.9 932.0 918 sh 916.9 901.6 904.5 902.3 900.4 890.2 888.1 884.8 881.7 878.6 875.7 874.9 860.7 858.7 853.1 899.7 846.3 843.0 840.0 827 819.8 808.0 769 759.0 746.2 719.5 704.8 673.7 666.1 652.7 594.7 586.0 574.5 535.2
J. Phys. Chem., Vol. 99, NO. 17, 1995 6359
+
0 2
in Solid Argon
1039.4, 1032.8, 1025.4 1018.1, 1016.5, 1010.2, 1005.9,999.5, 991.4, 985.3, 982.2,976.2
989.7 987.7, 946.0 985.5 972.1, 969, 935, 932.0 958.9,918 956.8,916.9
1.0583 1.0583 1.0443 1.0443 1.0464 1.0465 1.0440 1.0442 1.0441 1.0430 1.044 1.0435
953.4
1.0578
1.@69 1.0472 1.0474 1.0401 1.0395 1.0399 1.0401 1.0404 1.0497 1.0395 1.040 1 1.0398 1.0374 1.0375 1.0379 1.0284 1.0386 1.0472 1.0529 1.0425 1.0572 1.0596 1.0590 1.0473 1.0450 1.0453 1.0447 1.0420 1.0511 1.0428 1.03829 1.0493
946.9, 935.9, 904.5 944.9, 902.3 943.1, 900.4 925.9, 898.4, 890.2 923.2, 895.3, 888.1 920.1, 892.4, 884.8 917.1, 8892, 881.7 914.1, 885.6, 878.6 91 1.3, -, 875.7 909.5, -, 874.9 868.6 866.5 -, 856.0, 846.4
863.2 819.8 842,833,821,808
753.5, 719.5 736.5, 704.8 704.2, 689.4, 673.7 680.1, 652.7 ? ? 1
a Behavior on annealing to 20-25 K: power experiments (Figure 1).
+, increase; 0, no change; -, decrease.
produced triplets with 16,1802. The 583.3-cm-' band was weak, and only its '*O counterpart was measured. A metal blank experiment was done with the Hf target and condensing argon. Owing to the ablation of surface oxides, the spectrum shown in Figure 3c is similar to that in Figure 5a except for reduced product yield, in particular the 747.9-cm-I band. Subsequent annealing had less effect in general and gave much smaller growth of satellite bands likely due to 0 2 complexes. Calculations. Simple all-electron self-consistent field (SCF) calculations were done for the Ti02 and TiOz- molecules using the ACES II program system.31a The Dunning DZP basis set (in the ACES I1 program) was used for oxygen and Wachters' uncontracted GTO 14s9p5d basis set3Ib for Ti. The Ti02 molecule was found to be a bent singlet state, in agreement with SCF and CI calculations of Ramana and Philips.20 The ROHF procedure was employed for the doublet state molecular anion. The anion is 5 kcdmol more stable than the molecule based on total all-electron energies. In addition, simple SCF calculations were done with electron core potentials and the
+ +-
0 3
(TiO+)
+ +-
1
-
1
0
1
+
2
+ + + + ++ + + + +++ ++ + + + + + +
-
0
0 -
2 2 1
-
1 2 2 2
++-
+ + + -
TiO(Oz), 1
1
1
site Ti0 site OTiOO site Ti203 04-
&Ti02 47Ti02 48Ti02(vI) 49~i0~ 5qio2 TiO2, site 46Ti02 47~i0~ 48Ti02(v3) 49x0~
1 1
5qio2 TiOz, site TiOz(x) TiO2(x) 46~i0~47~i0~4 8 ~ i 0 ~(Y3) 45x0~-
5qio2Ti,O, Ti,O, 0 3 - site 03-
2
Ti03 Ti203 Ti20 Ti202 Ti,O, Ti202 Ti@,
+ +
Relative number of Ti atoms in species based on lowhigh laser
GAMESS program32afor the same Ti and Zr species. The StevenslBaschKrauss (SBK) effective core potentials32bemployed the 12 outer electrons for each metal. The Stevens/ BaschKrauss split valence basis set for each metal and the McLedChandler triple split (12s9p)/[6s5p] basis set with additional diffuse p function (ap= 0.059) and polarization d function (Q = 0.8) for oxygen in the GAMESS program were used. It is interesting to note that the anions are more stable than the neutral molecules by 39 and 37 kcdmol in the case of Ti and Zr, respectively, based on total pseudopotential energies. The above calculations with Hf failed to localize minima on the potential surface, but calculations with a xenonlike effective core pseudopotential without spin-orbit operators and the original basis set valence split to (3~3p4d)/[2~2pld]~*~ gave results that are compared in Table 4 with those for the Ti and Zr species. The calculations for Zr and Hf are not comparable, but metal dioxide and molecular anion parameters are comparable for each metal. For additional comparison SBK ECP calculations for the 'Z ground state of ZrO gave a 1.701-A bond length and 1063-
Chertihin and Andrews
6360 J. Phys. Chem., Vol. 99, No. 17, 1995 0.40
0.35
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Wavenumbers (cm-1)
Figure 3. Infrared spectra in the 550-1050-cm-' region for samples laser ablated into condensing argon at 10 and (c) Hf with oxide coating.
+
TABLE 2: Infrared Absorptions (cm-') Observed for Zr 0 2 in Solid Argon I6O2 I8O2 '60z/160180/~80z R ( ' 6 0 / 1 8 0 ) A" assgnmt 958.6 941.1 915.4 884.3 870.9 818.0 814.6 808.7 805.3 796.3 790.6 761.4 703.8 652.4 644.8 615 593.1
912.6 895.8 871.3 840.4 828.2 780.5 777.1 764.1 761.1 751.5 745.7 727.6 671.9 621.5 613.5 565.6
0 -
808.7, ?, 763.9 805.3, ?, 760.8 796.3, ?, 751.5 790.6, ?, 745.7 761.4, 738.9, 727.7 687.5 653.0.637.9, 621.8 644.6,628.8, 613.5
1.0504 1.0506 1.0506 1.0524 1.0516 1.0480 1.0482 1.0584 1.0581 1.0596 1.0602 1.0465 1.0475 1.0502 1.0506
594.6, 5743,565.6
1.0513
+
958.6,912.6 941.1, 895.8 915.4,871.3 884.3, 868.5, 840.4 870.9, ?, 828.2 813.0, 793.2, 780.4
++ + + + ++ ++ ++ ++ -
-t
-
ZrO OZrOO ZrO(O2), ZrOz ( V I ) ZrOz(O2) ZrOz ( ~ 3 ) site ZrO3 ZrO2Zr204 Zr202 site Zr,O, Zr202
Annealing behavior at 20 K.
cm-' harmonic frequency. The bond length is close to the 1.712-A gas phase value6 and 1.723-A length calculated at a higher lever of theory;I6 the calculated frequency must be scaled by 0.90 to match the observed matrix frequency. Similar scale factors, 0.88 and 0.89, are required to fit the calculated and observed V I and v3 values of ZrO2. The xenon-like ECP gave a somewhat shorter bond length for HfO than the observed value, as listed in Table 5 , but the frequency is reasonable. These
* 1 K for 2 h: (a) TiOz, (b) ZrO:,
simple calculations do not work as well for Ti0(3A) because the level of theory is not adequate for triplet molecules. Discussion Infrared spectra of the Ti, Zr, Hf, and 0 2 systems have a lot in common. More chemistry was found in the case of Ti, which is the most active metal in group 4. Product bands will be identified, and the chemistry of the group 4 oxides will be compared. MO Diatomic Molecules. TiO. The triplet 989.7/987.8/ 985.5 cm-' is assigned to the Ti0 molecule in different trapping sites in solid argon. This band was observed after deposition even with low laser power and decreased upon annealing, which indicates a first product of reaction. It produced only one counterpart in all experiments with isotopic oxygen and the 16/ 18 ratio 1.0442 is very close to the diatomic ratio 1.0446. This assignment is in a good agreement with previous work, including 1005- and 1000.O-cm-' neon matrix and gas phase fundamentals, 1,Z.I3,33a A weak band at 1014.3 cm-' decreased on photolysis and on annealing. The band exhibited an oxygen-18 counterpart at 971.3 cm-' and no intermediate component. The 16/18 ratio 1,0443 is that expected for a T i 0 diatomic oscillator. An 0 2 complex with Ti0 at 999.9 cm-' increased on annealing. We note that similar recent experiments33bwith Zr and Hf gave electron spin resonance (ESR) observable quantities of ZrOf and HfO' and suggest consideration of the weak 1014.3-cm-' band for TiO+. Although the present observations are consistent
Ti, Zr,and Hf Atom Reactions with Oxygen Molecules
J. Phys. Chem., Vol. 99, No. 17,1995 6361
0.24 0.22 0.20 0.18
A b S
0.16 0.14
0
r b a n
0.12 0.10
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e
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0.04 0.02 0.00 I
I
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800 Wavenumbers (cm-I)
900
'
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700
600
+
Figure 4. Infrared spectra in the 550-1050-~m-~region for the Zr 0 2 system in argon with sample deposition at 10 & 1 K for 2 h: (a) ( 0 2 ) = 1%; (b) after annealing to 25 K; (c) (1602) = 0.25%, ('60'80) = 0.5%, and ( 1 8 0 2 ) = 0.25%; and (d) = 1%.
with the TiO+ possibility, the infrared observation is not a definitive identification of TiO'. ZrO. The band at 958.6 cm-' was observed in the Zr 0 2 spectra even with deposition at low laser power and low oxygen concentration and in Z r O 2 and metal blank experiments. It decreased after annealing to 25 K and produced only one counterpart in isotopic experiments with the 16/18 ratio 1.0504, which is very close to the diatomic ratio 1.0507. The 958.6cm-I band is assigned to the ZrO molecule in accord with previous work.'s2 It is in good agreement with the neon matrix value 975 cm-' and the 969-cm-' gas phase value.1,2,'0Note that LinevskyI7 and further Weltner and McLeod2 assigned this molecule at 958 and 965 cm-l, respectively, in solid argon; however, the present experiments gave no bands in the 965cm-' region. HfO. The 958.3-cm-' band was observed in the spectra immediately after deposition, increased on annealing to 20 K and decreased at higher annealing temperatures. It produced only one counterpart in isotopic experiments at 908.3 cm-' with the 16/18 ratio 1.0550; the corresponding harmonic diatomic ratio is 1.0554. Previously Linevsky assigned a band at 960 cm-I in solid argon to this molecule, which is in good agreement with the present results; later Weltner and McLeod measured this band in solid neon at 974 cm-I. It is interesting to compare gas phase, neon, and the present argon matrix fundamentals for TiO, ZrO, and HfO, which are summarized in Table 6. Note that the neon values are higher by 5-8 cm-' and the argon values are lower by 9.4-12.2 cm-I
+
than the gas phase values. The small red shifts in solid argon (1.2- 1.O%) are in accord with molecules having small dipole moments. In contrast the CaO, SrO,and BaO molecules with larger dipole moments have larger matrix shifts ranging from f 2 4 to -32 c ~ - ' . ~ O The observed oxygen isotopic 16/18 ratios are slightly lower than the calculated harmonic values as expected for a small cubic contribution to anharmonicity in the observed frequencies. The MO molecules are made by reactions 1, which are exothermic for these stable molecules.6,21*22 In several dilute
+ 0, -T i 0 + 0 Zr + 0, - ZrO + 0 Ti
Hf+O,-HfO+O
AE = -42 k c d m o l
(la)
AE = -63
(lb)
kcal/mol
AE=-71kcal/mol
(IC)
Ti/O;! experiments, annealing slightly increased the yield of TiO; it is possible for reaction la to proceed without activation energy, but the union of Ti and 0 atoms on diffusion can also explain the growth of T i 0 on annealing. OMOO. The band at 972.1 cm-I appeared in the spectra only when the T i 0 band was strong, and it appeared to be first order in Ti concentration. An increasing of the oxygen concentration led to an increase of the 972.1-cm-' band intensity. This band grew after annealing at the expense of Ti0 and produced a partially resolved quartet in isotopic experiments with the 16/18 ratio 1.0430. The 972.1-cm-' band is assigned
Chertihin and Andrews
6362 J. Phys. Chem., Vol. 99, No. 17,I995 0.26 1 0.24 ;
7
-
0.22 -
Hfo2
,
Hfo
0.20 1 0.18 3
A b
u
0.16 1
S
0.14 1
0
r 0.12
a n c e
; 0,
0.10 y
0.06
0.02 0.00
, 1000
900
Figure 5. Infrared spectra in the 550-1050-~m-~region for the Hf I =
0.5%; (b) after annealing to 25 K; (c)
(I6O2) = 0.125%,
+
+
945.8 893.6 886.0 883.4 879.3 876.6 866.3 816.0 814.0 807.6 805.3 801.1 798.8 792.3 788.6 747.9 684.4 651.3 649.6 641.5 627.5 583.3
958.3, 908.3 953.7,940.3, 928.2, 924.8, 914.1, 901.6
1.0550 1.0578
+/-1-
HfO
+/847.3 838.9 836.5 832.7 830.1
893.6, 847.3 886.0, 869.7, 838.9 883.4, 866.4, 836.5 879.3, ?, 832.7 876.6, 859.5, 830.1
1.0546 ' 1.0561 1.0561 1.0560 1.0560
o/+
OHfOO HfO(On)x
774.6 772.7 766.7 764.5 760.3 758.2 751.1 748.8 710.7 649.1 618.2 616.5 608.9 594.6 553.3
816.0,789.0, 774.6 814.0, 786.8, 772.7 807.6,781.0, 766.7 805.3, 778.5, 764.5 801.1, ?, 760.3 798.9, ?, 758.2 791.3,766.5,751.1
1.0534 1.0534 1.0533 1.0534 1.0537 1.0535 1.0535 1.0532 1.0523 1.0538 1.0535 1.0540 1.0536 1.0553 1.0542
747.9,725.6, 710.7 684.4, 667.8, 649.1 651.3, 636.2, 618.2 649.6, 634.3,616.3 641.5, 626.7,608.9 627.5, 616.0,594.6
04-
-I+/+ HfOz ( V I ) +/+/+ +/- Hf,O, -1+/+ HfO2 ( ~ 3 ) +/+/+ +/+ +/+ +/+ +/+ +/-/-1+I+I+ -/+/-
600
0 2 system in argon with sample deposition at 10 f 1 K for 2 h: (a) (I6O2) (1601BO) = 0.25%, and ( 1 8 0 2 ) = 0.125%; and (d) (1802) = 0.5%.
TABLE 3: Infrared Absorptions (cm-') Observed for Hf 0 2 in Solid Argon I6O2 I 8 0 2 1602/160180/1802 R(160/180) A" assgnmt 958.3 908.3 953.7 901.6
700
800 Wavenumberr (cm-1)
Hf02(02)x HfOzHf204 Hf202 site Hf202 Hf,O, Hf,O, Hf202
Annealing behavior at 20 K/30 K.
to the OTiOO molecule, where slight coupling to a second inequivalent oxygen atom gives rise to the quartet.
TABLE 4: SCF Calculated Geometries, Frequencies ( V I , vz, and v3, cm-l), and Infrared Intensities (0for M02 and M01- Species molecule R ( A ) a(deg) V I (Z) v2(Z) v2 (Z) calc Ti02 TiO2Ti02 TiO2ZrOz ZrO2HfO2 HfO2-
1.597 1.636 1.619 1.656 1.762 1.802 1.652 1.693
115 118 119 120 112 114 120 121
1193 (125) 1077(265) 1127(2) 1041 (6) 1001 (3) 929 (6) 977(1) 915(3)
393 (50) 1143 (895) 337 (18) 1073 (860) 322(1) 1051 (21) 320(0.2) 964(15) 296(0.7) 909(16) 289 (0.4) 836 (11) 271 (1) 826(14) 293(1) 778(12)
a a b b b b
c c
a All-electron; infrared intensities (kdmol). SBK ECP; infrared intensities (D2/(amu A2)). Xenon-like ECP; infrared intensities D2/ (amu A2)).
TABLE 5: SCF Calculated Bond Lengths and Harmonic Frequencies for MO Diatomic Molecules R (A)
w (cm-I)
molecule
calc
obs"
calc
obs"
calc
3A T i 0
1.671 1.701 1.569
1.620 1.712 1.723
987 1063 1043
1009 970 974
b b
'XZro 'I:HfO a
c
Reference 6. SBK ECP method. Xenon-like ECP.
The 941.5-cm-I band was observed in the spectra together with the band of ZrO. When the concentration of oxygen was 1% or less, the 941.5-cm-' band was weaker than the ZrO band, but increasing oxygen concentration up to 2% increased intensity. After isotopic substitution the 941.5-cm-' band gave
Ti, Zr, and Hf Atom Reactions with Oxygen Molecules TABLE 6: Vibrational Fundamentals (em-') for TiO, ZrO, and HfO .gasa Neb Ar(16) Ar(18) obs 16/18 harm 16/18 Ti0 1OOO.O 1005 987.8 946.0 1.0442 1.0446 ZrO 969 975 958.6 912.6 1.0504 1.0507 HfO
967.7
974
958.3
908.3
1.0550
1.0554
J. Phys. Chem., Vol. 99, No. 17, 1995 6363 TABLE 7: Vibrational Fundamentals and Bond Angles for the Metal Dioxides, Metal Dioxide Anions, and Metal Oxide Dimers of Ti, Zr, and Hf molecule v (cm-I) v (cm-I) anglea (den) 946.9 917.1 111 zk 3-115 f 4 878.4 124 i 4-131 zk 4 884.3
References 6, 10, 13, and 14. Reference 1.
883.4
a doublet with a 16/18 ratio (1.051) close to the diatomic value, which is appropriate for perturbed ZrO and assignment to the OZrOO molecule. An analogous 945.8-cm-' band is tentatively assigned to the OHfOO molecule following the two previous systems. Unfortunately it was always weak and the isotopic shift could not be measured. M0(02),x > I . The 996.9-cm-I band was observed with high 0 2 concentration and after annealing in the titanium system. The 16/18 ratio was 1.0432, and no middle component was found in mixed isotopic spectra. This band is clearly due to perturbed TiO, and 0 2 is the likely source of this perturbation. A broad band at 915.4 cm-' appeared in the Zr spectra only after annealing, and its intensity increased with increasing oxygen concentration. The band gave a doublet upon mixed isotopic substitution with the ratio 1.0506, which is appropriate for a Zr0(02), species. In the Hf 0 2 system, the 893.6cm-I band behaved analogously, and it can be associated with HfO(O2), species. The weakness of such bands in metal blank experiments attests to their identification as 0 2 complexes. M02 Dioxide Molecules. Ti02. Bands at 946.9 and 917.1 cm-' are assigned to the V I and v3 vibrations of the Ti02 molecule based on the results of oxygen and natural titanium isotopic substitution and previous matrix isolation work.2 These bands grew together upon broad-band photolysis and annealing to 25 K (Figure 1). The lower frequency bending mode is calculated to be below the spectrometer limit (Table 4). The present assignments are substantiated by oxygen- 18 substitution. The 16/18 ratios for the 946.9- and 917.1-cm-l bands, 1.0469 and 1.0401, respectively, are above and below the diatomic ratio and are appropriate for the v1 and v3 modes of the C2" Ti02 molecule. In addition, the triplet pattems observed in the scrambled isotopic experiment verify two equivalent oxygen atoms for this molecule. Furthermore, the 160Ti1s0stretching modes are displaced exactly 10.2 cm-I above and below the medians for the TiI602 and TiI802 values of each mode owing to interaction between the stretching modes for the lower symmetry 160Ti180 isotope. The resolved titanium isotopic splittings on the v3 fundamental are of appropriate relative intensity for the five naturally abundant Ti isotopes34 in a vibration involving a single Ti atom as reported earlier.2 Note that the Ti isotopic shifts are smaller (1.8-2.1 cm-') for V I , which has more oxygen motion, than for v3 (2.8-3.1 cm-I), which has less oxygen character in the vibrational motion. Previous workers determined a 110 f 15" lower limit to the valence angle from titanium isotopic v3 fundamentak2 In the present work the v3 bands have 0.6-0.8-cm-' full widths at half-maximum and can be measured to fO.1 cm-'. Lower limits calculated for 10 titanium isotopic pairs with TiI602 and 10 pairs with a TiI8O2 range from 109" to 114" and average 111" for both TiI602 and TiI802. Upper limits to the valence angle were found to range from 113" to 119" for 20 oxygen isotopic ratios with different titanium isotope pairs, giving a 115" average, and from 114.1 to 114.6' for 5 oxygen isotopic ratios with the same titanium isotope. Earlier valence angle calculations of this type have shown that the lower limit-upper limit average provides a good measure of the valence angle for heavy atoms with small differences in isotopic anharmonici-
+
704.2 652.4 65 1.3
818.0 761.4 814.0 747.9 680.1 593.1 583.3
-113&5 -128 i 4 -115 If 5 -132 zk 4
a Lower limit-upper limit range calculated from metal and oxygen isotopic v3 frequencies, respectively.
ties.35,36In addition, v3 changes of +0.1 cm-I in lighter and -0.1 cm-I in heavier isotopes lead to an increase of only 5" in the calculated angle. Thus, we conclude that the Ti02 valence angle is 113 f 5". ZrOz. Bands at 884.3 and 818.0 cm-I are assigned to the V I and v3 vibrations of the ZrO2 molecule in agreement with the 884- and 819-cm-I assignments of Linevsky.I7 The 16/18 ratios 1.0524 and 1.0480 are in accord with values for the v1 and v3 modes. Furthermore, the 1/2/1 triplets observed with 1631e02 verify that two equivalent oxygen atoms are involved in the vibrations. Again coupling between the two stretching modes for 160Zr180 separates these frequencies more than the medians of the pure oxygen isotopic values for V I and for v3 of ZrO2. The valence angle upper limit was estimated from oxygen-18 substitution; the corresponding value, 113 f 5', is within experimental error of the Ti02 value. Hf02. Four doublets 886.0/883.4 and 879.3/876.6 cm-I and 816.0/814.0 and 807.6B05.3 cm-I are assigned to the V I and v3 modes of the hafnium dioxide molecule in two different matrix sites. The 16/18 ratios 1.0561 and 1.0534 bracket the diatomic value, as expected for V I and v3 of the HfO2 molecule. Furthermore, the triplet absorptions with verify two equivalent oxygen atoms for this molecule. Note the 6.5 f 0.1-cm-I separation between stretching modes for 160Hf'80 from the median of the HfI602 and HfI802 values, which arises from interaction between modes for the lower symmetry mixed isotopic molecule. The 0-Hf-0 valence angle upper limit calculated from the 1.0534 ratio is 115 f 5', within experimental error of the values for ZrO2 and TiO2. Frequencies and angles for the metal dioxide molecules are summarized in Table 7. The MO2 molecules are made here by the insertion reactions 2, which are highly exothermic.2'x22The absorptions for each 1631802
Ti
+ 0, -Ti02 AE = -183 kcaUmol Zr + 0, -.Zr02 Hf + 0, - Hf02
(2a)
(2c)
MO2 molecule increased on annealing, which suggests that reactions 2 proceed without activation energy. A small increase in Ti02 absorptions on broad-band W-vis photolysis could also be due to faster reaction of nearby photoexcited Ti atoms37 and 0 2 in the matrix cage. Finally, in the case of HfO2, photolysis increased the 886.0- and 8 16.0-cm-I absorptions due to one matrix environment, whereas annealing decreased these absorptions in favor of the 883.4- and 814.0-cm-I bands due to a slightly different argon matrix environment or "malrix site". M02 Dioxide Complexes. Bands at 895.0 cm-I, just below v3 of TiO2, and at 801.1 and 791.3 cm-I, just below v3 of HfOz, must be considered for 0 2 complexes with the metal dioxide.
6364 J. Phys. Chem., Vol. 99, No. 17, 1995 First, the shifts are relatively small, 16-26 cm-I, and the 16/ 18 ratios are the same as the metal dioxide. The bands in the analogous Z r O 2 region (790-808 cm-I), however, exhibit larger (1.058-1.060) 16/18 ratios and are due to ZrO3 species. MO2 Dioxide Anions. The sharp band at 878.4 cm-' in Ti/ 0 2 experiments behaves much like the sharp 917.1-cm-I band. The 16/18 ratio, 1.0379, is smaller, and the Ti isotopic splittings are correspondingly larger (3.0-3.4 cm-I). The intermediate component at 856.0 cm-I denotes the vibration of two equivalent oxygen atoms, and the five Ti isotopic bands again denote a single metal atom. With higher power ablation (Figure 2) broadband photolysis slightly increased Ti02 and had no effect on the 878.4-cm-' band, but with lower power ablation (Figure 1) broad-band photolysis almost destroyed a substantial 0 4 - band, decreased 03-, doubled the 878.4-cm-' band, halved TiO, and increased Ti02 4-fold. Similar bands were observed at 761.4 cm-' in Zr and at 747.9 cm-I in Hf experiments. These bands gave mixed isotopic triplets and exhibited 16/18 ratios slightly smaller than the metal dioxides (Tables 2 and 3). In the case of Hf, photolysis diminished 0 4 - with little effect on the 747.9-cm-I band. The above bands are appropriate for assignment to the metal dioxide molecular anions. These absorptions are 39, 57, and 66 cm-' below v3 of TiO2, Z r o z , and HfOz, respectively, which is too far for complexes. For CzVspecies the 16/18 ratios give valence angle upper limits of 131", 128" and 132", respectively, for TiOz-, ZrOz-, and HfOz-. For TiI602-, 10 Ti isotope pairs give a 124 f 4" lower limit to the valence angle. Taken together, the metal dioxide anions have valence angles of 128 f 5", just 15" larger than the neutral molecules. The observation of molecular anions is not without precedent in pulsed laser ablation experiments, particulary with 0 2 in the sample to facilitate the formation of Of course lower laser power minimizes the photodissociation of 04- once formed. Isolated 0 4 - was observed in alkaline earth metal atom experiments Mg-Ba and was especially strong in Li reactions with 02.30,38,39 We note here that the Ti, Zr, and Hf ionization energies are intermediate between Mg and Ca ionization energies, and it is reasonable to expect electrons to be ablated along with atoms. The electrons may be captured by molecules in the sample or neutralize cations. Molecular ions are present in these samples, but charged species play a minor role in the overall chemistry as the TiOz- v3 fundamental is 3-10% of the Ti02 v3 fundamental intensity. In the case of BO2 and BOz-, the latter gave a v3 fundamental absorption 10% as strong as the former.40 What is the approximate electron affinity (EA) of Ti02? The TiO2- absorption at 878.4 cm-I is stable to broad-band UVvis photolysis that destroys 04-,and in one experiment TiO2increased slightly at the expense of isolated 03- at 804.2 cm-I. This suggests EA(Ti02) 1 EA(03) and the latter has been measured as 2.1 eV.41 Although the electron affinity of titanium is small (0.08 eV), oxygen is larger (1.46 eV), and, in the case of Cu and CuO, the monoxide electron affinity (1.77 eV) is higher than EA for the metal atom by about 0.6 eV.42,43The ECP calculations suggest an EA(Ti02) near 1.6 eV. It is thus reasonable for EA(Ti02) to be in the 2-eV range. The SCF calculations reported in Table 4 support the present assignments to Ti02 and TiOz-, although much higher levels of theory will be required for better agreement with experiment. Scale factors for the all-electron SCF calculations for the V I and v3 modes of Ti02 are 0.793 and 0.802, respectively. The calculated angle, 115", is in agreement with the 113 f 5" experimental value. The strongest molecular anion fundamental is predicted, on this basis in the 860-cm-I region, and the 878.404-.30538339
Chertihin and Andrews cm-I band (scale factor 0.818) is in excellent agreement. The 118" angle calculated for TiO2- is lower than the 128 f 5" experimental value. The ECP calculations give the same general trends, and calculated frequencies are closer to the observed values. Note that the calculated stretching fundamentals overestimate the difference between the observed Ti02 and ZrO2 values by 40-60 cm-'. M202 Molecules. Cyclic Ti202. Two bands at 704.2 and 680.1 cm-' are observed after deposition with high laser power. These bands are weak in the metal blank and low power experiments, but they grow together on annealing. These bands gave triplets with 1602/1802and 16,1802 isotopic substitution, which denotes two equivalent oxygen atoms. The isotopic ratios were 1.0453 and 1.0420, respectively. The first ratio is just above the diatomic ratio, but the second is lower. No higher frequency absorptions correlated with this pair. The 700-cm-I region is appropriate for a bridged Ti-0-Ti vibration, and these absorptions are assigned to the cyclic Ti202 molecule. The 16/ 18 ratios differ from the "diatomic" f $ ~ ) PO) value required for the planar rhombus structure, which suggests that this cyclic molecule is nonplanar. It was mentioned above that in the experiments with I6O2/ I8O2 the middle component of isotopic structure always was strong. This isotopic distribution requires dimerization of TiO, 2Ti0 Ti202, rather than reaction of Ti02 with a second Ti atom. Furthermore, this observation suggests that the Ti0 concentration may exceed the Ti02 concentration in these samples in spite of band intensities. Cyclic Zi-202. Two bands at 652.4 and 593.1 cm-I are assigned to the cyclic Zr202 molecule on the basis of the results of isotopic substitution, concentration and annealing temperature dependencies and on comparison with the analogous bands in the Ti 0 2 system. Both bands gave the isotopic ratios 1.0502 and 1.0514, which are very close to the diatomic ratio. This cyclic molecule may accordingly be nearly planar. The middle components of isotopic structures were strong in the experiments with 1602/1802,as in the case with Ti202, which again indicates that dimerization of ZrO is the means of formation of dizirconium dioxide. Cyclic HfiOz. Two bands at 649.6 and 583.3 cm-I were observed in the Hf 0 2 spectra only after deposition with high laser power. Both increased on annealing to 20 K and then decreased together on further annealing. The first band produced a triplet with mixed isotopic substitution, and the second was too weak to observe. The ratios for both bands (1.0540 and 1.0542, respectively) are very close to the diatomic ratio, which indicates that the molecule may be nearly planar. It is interesting to compare the cyclic Ti202, zI202, and Hf202 dimers with their CazO2, Sr202, and Ba202 analogues. The later metal oxide monomers are highly ionic,& and the dimers are rhombic rings (with 16/18 ratios just below the diatomic value as expected).30 Dipole moments for TiO, ZrO, and HfO are much smaller,I2 and these molecules exhibit less than half of the ionic character of CaO, SrO, and BaO. Accordingly, the cyclic Ti202,Zr202, and Hf202 molecules are expected to exhibit substantial covalent character. With two extra d electrons on the transition metal, the cyclic dimetal dioxides are nonplanar, although the Zr and Hf compounds are probably closer to planar than the Ti compound. This is in contrast to GezOz, with an extra s2 electron pair on germanium, which, on the basis of isotopic frequency ratios, is a rhombic ring45 presumably of covalent character. An analogous structural effect is found for Ti in the OCaO, OTiO, and OGeO series, where O c a 0 is almost linear, OTiO is clearly bent, and OGeO is linear.30%46
+
I
-
+
+
J. Phys. Chem., Vol. 99, No. 17,1995 6365
Ti, Zr, and Hf Atom Reactions with Oxygen Molecules
Higher Oxides. The sharp band at 956.8 cm-' was observed in the spectra after Ti deposition with high laser power and when the Ti0 band was strong. The band increased on annealing to 25 K. The 16/18 ratio of 1.0436 denotes a terminal Ti0 vibration, but the 955.6-cm-' splitting with 16,1802 indicates coupling to a second inequivalent oxygen atom. The 753.5cm-' band behaves in like manner with increased laser power and on annealing. The latter band gives a broad doublet with 1 6 , 1 8 0 2 and a 16/18 ratio of 1.0473, which denotes a Ti-0-Ti vibration. Although the 956.8- and 753.5-cm-' bands could be due to TiOTiO, two points favor OTiOTiO-a presumably zig-zag Ti203 molecule. First, the more stable Ti202 structure is surely cyclic, and second, the lack of evidence for the Ti02 Ti reaction and the abundance of Ti0 suggest the T i 0 Ti02 reaction product. The Zr and Hf spectra have one band between the MO2 and M202 absorptions that remains to be identified. The bands at 703.8 and 684.8 cm-I, respectively, give triplet mixed oxygen isotopic absorptions and 16/18 ratios near the MO2 v3 modes, and decrease slightly on annealing. These bands are appropriate for assignment to M204 molecules in a (M02)2 structure which perturbs but retains the character of the v3 M02 mode. Such a configuration might be 02M-02M where one metal has a coordination number of 4. In contrast. the Ti spectra show no analogous band. Absorptions at 866, 863.2, and 842.4 cm-' exhibit different characteristics: the 863.2-cm-' band involves a single 0 atom with a large 16/18 ratio, the 842.4-cm-' band involves at least two inequivalent 0 atoms and a 16/18 ratio appropriate for a terminal Ti-0 vibration, and the broad 866-cm-' band grows on annealing. The best candidate for Ti204 is the 842.4-cm-' band, and if this identification is correct, the structure is clearly different from the above described Zr204and Hf204 species. A cyclic Ti202 ring with terminal 0 atoms added to Ti is a possible Ti204 product species. Other Absorptions. Several other weaker bands in the tables cannot be identified and are labeled M,O,. In the case of Zr, the bands 808.7/805.3and 796.3/790.6 cm-' became strong after annealing. Their isotopic structures were not resolved due to overlapping and broadening but isotopic ratios obtained in the 1 8 0 2 experiments indicate predominately 0-0 vibrations. Since this is the ozonide region, these bands are probably due to Zr03 molecules in different matrix environments. A final sharp absorption at 736.5 cm-' is due to a species unique to Ti. (This band is not due to C2H2, as other C2H2 bands were not detected and the band increased on photolysis in the low power experiment, which is not appropriate for C2H2.) The 736.5-cm-' band is strong when Ti0 is strong, but it does not grow on annealing. The 16/18 ratio (1.0450) is slightly higher than the diatomic value. The observation of a mixed isotopic doublet identifies a single 0 atom species, and the preference with higher laser power suggests more than one Ti atom. The 736.4-cm-' band is assigned to the obtuse bent molecule TiOTi. The 16/18 ratio for the v3 assignment predicts a 92" lower limit to the valence angle. The failure to observe the analogous Zr and Hf species may be due to lower atom concentrations in the matrix from these less volatile metals.
+
+
Conclusions Pulsed laser evaporated Ti, Zr, and Hf atoms react with 0 2 molecules to produce the known MO and MO2 molecules, the MO2- molecular anions, the new M202 molecules, and evidence for M204 species. The yield of the latter two species depends strongly on deposition conditions in all systems. Valence angles calculated from isotopic v3 fundamentals increased from 113 f 5" for the MO2 molecules to 128 f 5" for the MO2-
molecular anions. The latter anions are stable under the conditions of these experiments, which suggests electron affinities in the 2-eV range for the M02 molecules. SCF ECP calculations support the present assignments to M02 and MOzspecies. It is interesting to compare the cyclic Ti202,Zr202, and Hf202 and the cyclic CazOz, SrzO2, and Ba.202 dimer molecules. The latter monomers are more ionic, and their dimers are rhombic rings based on oxygen isotopic shifts, whereas the former monomers are much less ionic and give more covalent rings with nonplanar structures owing to d2 electron configurations although the heavier species may be close to planar. The most extensive chemistry was observed in the Ti 0 2 system, where evidence was found for the Ti203 and Ti20 molecules. Thermal evaporation of solids produces mainly the MO and MO2 molecule^.'^^-'^ In matrix synthesis experiments the MO and MOz yield is sufficient to allow the formation of Mz02 and M2O4 molecules. In the experiments with ablation of metal targets or Ti02 or ZrOz pellets, the yield is also large due to laser dissociation of surface oxides during ablation. In these experiments reactions between metal and oxygen atoms on the matrix surface lead to high MO and MO2 yield and subsequent M202 and M204 formation. Finally, the strong similarity between zirconium and hafnium oxide spectra is due to a combination of lanthanide contraction and relativistic effects for hafnium.47
+
Acknowledgment. We are grateful for NSF support from Grant CHE 91-22556 and helpful correspondence with P.
PYYW. References and Notes (1) Weltner, W., Jr.; McLeod, D., Jr. J . Phys. Chem. 1965, 69, 3488. (2) McIntyre, N. S.; Thompson, K. R.; Weltner, W., Jr. J. Phys. Chem. 1971, 75, 3243. (3) Balfour, W. J.; Tatum, J. B. J . Mol. Spectrosc. 1973, 48, 313. (4) Lauchlan, L. J.; Brom, J. M., Jr.; Broida, H. P. J . Chem. Phys. 1976, 65, 2672. ( 5 ) Hocking, W.; Gerry, M. C. L.; Merer, A. J. Can. J . Phys. 1979, 57, 54. (6) Huber, K. P.; Herzberg, G. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979. (7) Gallaher, T. N.; DeVore, T. C. High Temp. Sci. 1979, 11, 123. (8) Powell, D.; Brittain, R.; Vala, M. Chem. Phys. 1981, 58, 355. (9) Dyke, J. M.; Gravenor, B. W.; Josland, G.D.; Lewis, R. A,; Moms, A. Mol. Phys. 1984, 53, 465. (10) Simard, B.; Mitchell, S . A,; Humphries, M. R.; Hackett, P. A. J . Mol. Spectrosc. 1988, 129, 186. (11) Steimle, T. C.; Shirley, J. E. J . Chem. Phys. 1989, 91, 8000. Steimle, T. C.; Shirley, J. E.; Jung, K. Y.; Russon, L. R.; Scurlock, C. T. J . Mol. Spectrosc. 1990, 144, 27. (12) Suenram, R. D.; Lovas, F. J.; Fraser, G. T.; Matsumura, K. J . Chem. Phys. 1990, 92, 4724. (13) Gustavson, T.; Amiot, C.; Verges, J. J . Mol. Spectrosc. 1991, 145, 56. (14) Edvinsson, G.; Nylen, Ch. Phys. Scr. 1971, 3, 261. (15) Bauschlicher, C. W.; Bagus, P. S.; Nelin, C. J. Chem. Phys. Left. 1983, 101, 229. (16) Langhoff, S. R.; Bauschlicher, C. W., Jr. J . Chem. Phys. 1988,89, 2160; Astrophys. J . 1990, 349, 369. (17) Linevsky, M. J. Proceedings of the First Meeting of the Interagency Chemical Rocket Propulsion Group on Thermochemistry; New York, 1963; Vol. 1; Chemical Propulsion Information Agency: Silver Spring, MD, 1964. (18) Kaufman, M.; Muenter, J.; Klemperer, W. J . Chem. Phys. 1967, 47, 3365. (19) Gallaher, T. N.; DeVore, T. C. High Temp. Sei. 1983, 16, 269. (20) Ramana, M. V.; Philips, D. H. J . Chem. Phys. 1988, 88, 2637. (21) Balducci, G.; Gigli, G.;Guido, M. J . Chem. Phys. 1985, 83, 1909. (22) Balducci, G . ;Gigli, G.;Guido, M. J . Chem. Phys. 1985, 83, 1913. (23) Andrews, L.; Burkholder, T. R.; Yustein, J. T. J . Phys. Chem. 1992, 96, 10182. (24) Chertihin, G. V.; Andrews, L. J . Phys. Chem. 1993, 97, 10295.
6366 J. Phys. Chem., Vol. 99, No. 17, 1995 (25) Andrews, L. J. Chem. Phys. 1971, 54, 4935. Thompson, W. E.; Jacox, M. E. J . Chem. Phys. 1989, 91, 3826. Hacaloglu, J.; Andrews, L. To be published. (26) Spiker, R. C., Jr.; Andrews, L. J . Chem. Phys. 1973, 59, 1851. Andrews, L.; Auk, B. S.; Grzybowaki, J. M.; Allen, R. 0. J . Chem. Phys. 1975, 62, 2461. Andrews, L. J . Chem. Phys. 1971, 54, 4935. (27) Chertihin, G. V.; Andrews, L. J. Phys. Chem. 1994, 98, 5891. (28) Chertihin, G. V.; Andrews, L. J. Am. Chem. SOC. 1994, 116, 8322. (29) Chertihin, G. V.; Andrews, L. J . Am. Chem. SOC.1995,117, 1595. (30) Andrews, L.; Yustein, J. T.; Thompson, C. A,; Hunt, R. D. J. Phys. Chem. 1994, 98, 6514. (31) (a) Stanton, J. F.; Gauss, J.; Watts, J. D.; Lauderdale, W. J.; Bernhold, D. E.; Barlen, R. J. ACES II, Quantum Theory Project; University of Florida: Gainesville, FL, 1992. (b) Wachters, A. J. H. J . Chem. Phys. 1970, 52, 1033. (32) (a) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A,; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A,, Jr. J. Comput. Chem. 1993, 14, 1347. (b) Stevens, W. J.; Basch, H.; Krauss, M.; Jasien, P. Can. J . Chem. 1992, 70, 612. (c) Ross, R. B.; Powers, J. M.; Atashroo, T.; Ermler, W. C.; LaJohn, L. A.; Christiansen, P. A. J. Chem. Phys. 1990,93, 6654. (33) (a) The present Ti0 assignment is in disagreement with the Ti/ H20 reaction product at 1010.5 cm-I (Kauffman, J. W.; Hauge, R. H.; Margrave, J. L. J. Phys. Chem. 1985, 89, 3547) assigned to TiO. We note that the 16/18 ratio for the 1010.5 cm-I band, 1.0424, is too low for the
Chertihin and Andrews Ti0 assignment. (b) Van Zee, R. J.; Li, S.; Weltner, W., Jr. Chem. Phys. Lett. 1994, 217, 381. (34) Handbook of Chemistry and Physics; Chemical Rubber Co.: Cleveland, OH, 1966. (35) Allavena, M.; Rysnik, R.; White, D.; Calder, V.; Mann, D. E. J . Chem. Phys. 1969, 50, 3399. (36) Brabson, G. D.; Mielke, A.; Andrews, L. J. Phys. Chem. 1990, 95, 79. (37) Gruen, D. M.; Carstens, D. H. W. J. Chem. Phys. 1971,54, 5206. (38) Andrews, L.; Yustein, J. T. J. Phys. Chem. 1993, 97, 12700. (39) Andrews, L.; Saffell, W.; Yustein, J. T. Chem. Phys. 1994, 189, 343. (40) Burkholder, T. R.; Andrews, L. J . Chem. Phys. 1991, 95, 8679. (41) Novik, S. E.; Engelking, P. C.; Jones, P. L.; Futrell, J. H.; Lineberger, W. C. J. Chem. Phys. 1979, 70, 2652. (42) Hotop, H.; Lineberger, W. C. J . Phys. Chem. Re$ Data 1985, 14, 731. (43) Polak, M. L.; Gilles, M. K.; Ho, J.; Lineberger, W. C. J. Phys. Chem. 1991, 95, 3460. (44) Kaufman, J.; Wharton, L.; Klemperer, W. J . Chem. Phys. 1962, 37, 621; 1965, 43, 943. (45) Ogden, J. S.; Ricks, M. J. J . Chem. Phys. 1970, 52, 352. (46) Hassanzadeh, P.; Andrews, L. J. Phys. Chem. 1992, 96, 6181. (47) Pyykko, P. Chem. Rev. 1988, 88, 563.
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