Matrix infrared spectra and density functional calculations of TiO 3 and TiO5 in solid argon

Article abstract of DOI:10.1021/jp805495d

The reaction of titanium monoxide molecules and O₂ was studied by using matrix isolation infrared spectroscopy as well as theoretical calculations. The titanium monoxide molecule reacts with O₂ to form TiO₃ spontaneously on annealing. The TiO₃ molecule is characterized to be a side-on bonded peroxo titanium monoxide complex, (η²-O₂)TiO, which has a nonplanar Cs symmetry with a ¹A′ ground state. The (η²-O₂)TiO complex can further coordinate another dioxygen to give TiO₅, a disuperoxo titanium monoxide complex, (η²-O₂) ₂TiO, which possesses a ³A″ ground state and a nonplanar C_s geometry.

Full text of DOI:10.1021/jp805495d

9758
J. Phys. Chem. A 2008, 112, 9758–9762
Matrix Infrared Spectra and Density Functional Calculations of TiO3 and TiO5 in Solid
Argon
Yu Gong and Mingfei Zhou*
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and InnoVatiVe Materials,
AdVanced Materials Laboratory, Fudan UniVersity, Shanghai 200433, P. R. China
ReceiVed: June 22, 2008
The reaction of titanium monoxide molecules and O2 was studied by using matrix isolation infrared spectroscopy
as well as theoretical calculations. The titanium monoxide molecule reacts with O2 to form TiO3 spontaneously
on annealing. The TiO3 molecule is characterized to be a side-on bonded peroxo titanium monoxide complex,
(η2-O2)TiO, which has a nonplanar Cs symmetry with a 1A′ ground state. The (η2-O2)TiO complex can further
coordinate another dioxygen to give TiO5, a disuperoxo titanium monoxide complex, (η2-O2)2TiO, which
3
possesses a A′′ ground state and a nonplanar Cs geometry.
Introduction
mental of a Nd:YAG laser (Continuum, Minilite II, 10 Hz
repetition rate and 6 ns pulse width) was focused onto a rotating
titanium dioxide or metallic titanium target through a hole in a
CsI window cooled normally to 6 K by means of a closed-
cycle helium refrigerator (ARS, 202N). The laser-evaporated
species were codeposited with O2/Ar mixtures onto the CsI
window. In general, matrix samples were deposited for 1 h at
a rate of approximately 4 mmol/h. The O2/Ar mixtures were
prepared in a stainless steel vacuum line by using standard
manometric technique. Isotopic 18O2 (ISOTEC, 99%) was used
without further purification. The infrared absorption spectra of
the resulting samples were recorded on a Bruker IFS 66V
spectrometer at 0.5 cm-1 resolution between 4000 and 450 cm-1
by using a liquid nitrogen cooled HgCdTe (MCT) detector.
Samples were annealed to different temperatures and cooled
back to 6 K for spectral acquisition.
Quantum chemical calculations were performed by using the
Gaussian 03 program.31 The three-parameter hybrid functional
according to Becke with additional correlation corrections due
to Lee, Yang, and Parr (B3LYP) was utilized.32 The 6-311+G(d)
basis set was used for the O atom, and the all electron basis set
of Wachters-Hay as modified by Gaussian was used for the
Ti atom.33 The geometries were fully optimized; the harmonic
vibrational frequencies were calculated, and zero-point vibra-
tional energies were derived.
The electronic and geometric structures of simple titanium
oxide molecules and small clusters have been the subject of
numerous experimental and theoretical studies.1-25 Among the
TiOx series, the titanium monoxide and dioxide molecules are
the most widely investigated candidates both in the gas phase
and in solid matrixes,1-12 and a number of theoretical calcula-
tions were performed on the structure and bonding of these
simple diatomic and triatomic systems and their clusters.13-25
Compared with the rich investigations on titanium monoxide
and dioxide, few studies have been carried out on the TiOx
species with x higher than two. As for the TiO3 species,
Chertihin and Andrews assigned a 972.1 cm-1 absorption to
the end-on bonded OTiOO complex in the reaction of laser-
ablated titanium atoms with dioxygen in solid argon.9 Wu and
Wang performed an anion photoelectron spectroscopic study
on TiOx with x ) 1-3. The neutral TiO3 molecule was proposed
to contain a peroxide unit.10 Theoretical calculations indicate
that the ground state of TiO3 possesses a nonplanar Cs symmetry
with the peroxo O2 unit bound to TiO in a side-on fashion.18,19,21a
Studies on the next member in the TiOx series, TiO4, are mainly
limited to the corresponding tetrahedral TiO4 anion.26-28
Recently, two interconvertible oxo-superoxo-titanium-ozonide
complexes with TiO6 stoichiometry in solid argon were prepared
and characterized in this laboratory.29
4-
In this paper, we provide a joint matrix isolation infrared
spectroscopic and theoretical investigation on the formation and
characterization of the TiO3 and TiO5 molecules. These titanium
oxide species with odd number of oxygen atoms were formed
via the reactions of laser-evaporated titanium monoxide and O2
in solid argon. The TiO3 molecule was characterized to be a
side-on bonded peroxo titanium monoxide complex, whereas
TiO5 was characterized to be a side-on bonded disuperoxotita-
nium monoxide complex.
Results and Discussions
Infrared Spectra. A series of experiments were performed
by using a bulk titanium dioxide target with different O2
concentrations. Laser ablation of the TiO2 target under controlled
laser energy (approximately 3 mJ/pulse) followed by co-
condensation with pure argon formed TiO and TiO2 as the major
products.9 The Ti2O4 cluster absorptions were produced on
sample annealing at the expense of TiO2.34 Distinct new product
absorptions were observed when O2/Ar was used as reagent gas.
The infrared spectra in selected regions from codeposition of
laser evaporated titanium oxide species with 0.5% O2 in argon
are shown in Figure 1, and the product absorptions are listed in
Table 1. Besides the TiO, TiO2, and Ti2O4 absorptions, two
groups of new absorptions were evolved upon sample annealing
as shown in Figure 1, traces b and c. The first group includes
Experimental and Computational Methods
The experimental setup for pulsed laser-evaporation and
matrix isolation infrared spectroscopic investigation has been
described in detail previously.30 Briefly, the 1064 nm funda-
* E-mail: mfzhou@fudan.edu.cn.
four absorptions centered at 971.9, 863.3, 624.8, and 611.1 cm-1
,
10.1021/jp805495d CCC: $40.75
2008 American Chemical Society
Published on Web 09/11/2008

TiO3 and TiO5 in Solid Argon
J. Phys. Chem. A, Vol. 112, No. 40, 2008 9759
monoxide complex. The band position is quite close to those
of previously characterized TiO containing species.29,34,36,37 The
natural titanium isotopic splittings on this mode can clearly be
resolved, which proves that only one titanium atom is present
in this species. The spectra from the experiments using 16O2 +
18O2 and 16O2 + 16O18O + 18O2 mixtures with bulk titanium
dioxide target revealed that only one TiO fragment is involved
in this mode (Figure 2, traces c and d). The next absorption
lies at 863.3 cm-1 with the pure 18O counterpart at 819.9 cm-1
.
The spectral feature in the experiments with 16O2 + 16O18O +
18O2 mixture implies that two equivalent oxygen atoms are
involved in this mode. The 16O/18O isotopic ratio for this
vibration (1.0529) suggests that this is not a pure normal O-O
stretching mode; it may be coupled with the TidO stretch
vibration. The 624.8 and 611.1 cm-1 absorptions are attributed
to the Ti-O2 vibrations.
Both the (η2-18O2)Ti16O and (η2-16O18O)Ti18O absorptions
were observed in the spectrum with 18O2/Ar and normal titanium
dioxide target (Figure 2, trace b). This indicates that the oxygen
atoms of the peroxo ligand can be interconverted with the
oxygen atom of the monoxide subunit during the association
reaction process.
Figure 1. Infrared spectra in the 1150-850 and 650-570 cm-1 regions
from codeposition of laser-evaporated titanium oxides with 0.5% O2
in argon. (a) 1 h of sample deposition at 6 K, (b) after 25 K annealing,
and (c) after 35 K annealing. The asterisk denotes the absorption of
TiO6.
The assignment of the (η2-O2)TiO complex is strongly
supported by density functional calculations. The DFT/B3LYP
calculations revealed that the most stable structure of TiO3 is
the side-on bonded (η2-O2)TiO complex. The end-on bonded
OTi-OO isomer was predicted to be less stable than the side-
on bonded structure by about 44.8 kcal/mol. The (η2-O2)TiO
complex possesses a singlet ground state with a nonplanar Cs
symmetry, in which the terminal oxygen atom bends out of the
Ti(O2) plane (Figure 4), in agreement with previous theoretical
predictions.18,19,21a The four experimentally observed modes
were computed at 1038.4, 909.5, 636.8, and 636.6 cm-1 with
309/124/35/36 km/mol IR intensities (Table 2). As listed in
Table 3, the calculated isotopic frequency ratios are also in very
good agreement with the experimental values.
The O-O bond length of (η2-O2)TiO was calculated to
be 1.471 Å. Although the O-O and TidO vibrations are
coupled, the calculated O-O bond length implies that (η2-
O2)TiO is a peroxo complex of titanium monoxide.38 It can
be regarded as a titanium monoxide dication coordinated by
an O22- dianion, [(O2)2-(TiO)2+]. The titanium center is in its
formal +4 oxidation state. Similar transition metal peroxide
complexes have been prepared and characterized in this labora-
tory.39
which were produced upon 25 K annealing (Figure 1, trace b)
at the expense of the TiO absorptions. The second group
involves four absorptions with band positions at 1124.1, 1116.6,
996.8, and 596.4 cm-1. This group of absorptions also appeared
on 25 K annealing and doubled upon 35 K annealing during
which the first group of absorptions decreased slightly. In
addition, the absorptions due to TiO6 and O3 were also produced
on sample annealing.29,35
To help the product assignments, experiments were repeated
with the isotopic labeled 18O2 sample and the 16O2 + 18O2 and
16O2 + 16O18O + 18O2 mixtures. The spectra in selected regions
from codeposition of laser-evaporated titanium oxides with
different isotopic samples taken after 1 h of sample deposition
followed by 35 K annealing are shown in Figures 2 and 3,
respectively. The new product absorptions are shifted and
splitted as listed in Table 1.
Complementary experiment on the reaction of laser-evapo-
rated titanium atoms and 0.5% dioxygen in argon was also
performed with a titanium metal target. The TiO2, Ti2O4, and
TiO6 absorptions dominate the spectrum, but the TiO absorption
is weak. The new 971.9, 863.3, 624.8, 611.1, 1124.1, 1116.6,
996.8, and 596.4 cm-1 absorptions were also produced on
sample annealing, but their IR intensities are much weaker than
those in the experiment with bulk TiO2 target. Similar experi-
ment with the Ti target was done by using the 18O2/Ar sample
to get the pure O-18 counterparts of the newly observed
absorptions (Table 1).
Recent experimental and theoretical investigations suggest
that the titanium peroxide species is an important intermediate
in the process of O2 adsorption and photoreduction on TiO2
surface.40,41 In the experiments using multiple internal reflection
infrared spectroscopic (MIRIR) technique, Nakamura et al.
assigned an absorption at 943 cm-1 to the surface adsorbed
peroxide species, Ti(O2), which can further be transferred to
the TiOOH hydroperoxo species via capture of H+ cations in
aqueous solution.40 The adsorbed peroxo dianion was rational-
ized to be located on a 5-fold coordinated surface Ti4+ cation.41
The O-O stretch frequency of the isolated (η2-O2)TiO complex
is about 80 cm-1 lower than that of the surface adsorbed peroxo
species, whereas the predicted O-O bond length for the surface
adsorbed O22- dianion of 1.46 Å,41 is only slightly shorter than
that of (η2-O2)TiO (1.471 Å). The electron transfer from TiO
to O2 in forming the (η2-O2)TiO complex is in accord with the
mechanism of electron capture by adsorbed dioxygen molecule
on metal oxide surface during photocatalytic process.
TiO3. The 971.9, 863.3, 624.8, and 611.1 cm-1 absorptions
are due to different vibrational modes of the same species based
on their behavior upon annealing at different experiments. These
absorptions depend strongly on the TiO absorption, which led
us to propose that these absorptions are due to a reaction product
of TiO. The 971.9 cm-1 absorption was observed in the previous
experiment on the reaction of laser-ablated titanium atom with
dioxygen and was assigned to the TidO stretch vibration of an
end-on bonded OTi-OO complex.9 The observation of three
additional absorptions helps us to reassign these absorptions to
a side-on bonded titanium-monoxide-dioxygen complex. The
971.9 cm-1 absorption is about 20 cm-1 red-shifted from that
of diatomic TiO. It shifted to 931.7 cm-1 with Ti + 18O2. The
band position and O-18 isotopic shift indicates that this
absorption is due to a terminal TidO stretch mode of a titanium

9760 J. Phys. Chem. A, Vol. 112, No. 40, 2008
Gong and Zhou
assignment
TABLE 1: Infrared Absorptions (cm-1) from Co-Deposition of Laser-Evaporated TiO with O2 in Solid Argon
16O2
18O2
16O2 + 18O2
16O2 +16O18O+ 18O2
18O2
a
977.5
974.6
971.9 971.7, 931.8
969.3
(η2-O2)46TiO
(η2-O2)47TiO
971.9, 931.8
971.9, 931.9
931.7 (η2-O2)48TiO TidO str
(η2-O2)49TiO
966.9
(η2-O2)50TiO
863.3 842.0, 820.4
624.8 611.5, 594.5
611.1 595.9, 586.7
863.3, 842.0, 820.4
624.8, ...,b 594.5
611.1, ...,b 586.7
863.3, 842.8, 820.4
624.8, ...,b 594.5
611.1,b 586.7
819.9 (η2-O2)TiO O-O str
594.4 (η2-O2)TiO asym Ti-O2 str
586.1 (η2-O2)TiO sym Ti-O2 str
1061.0 (η2-O2)2TiO sym O-O str
1124.1 1089.6, 1060.9 1124.1, 1120.2, 1116.6
1124.1, 1120.2, 1116.6
1116.6 1058.0, 1054.4 1089.6, 1060.9, 1058.3, 1054.4 1093.2, 1089.6, 1086.1, 1060.9, 1058.3, 1054.4 1054.4 (η2-O2)2TiO asym O-O str
996.8 996.8, 955.5
596.4 584.0, 581.2
996.8, 955.5
596.2, 589.5, 584.1, 581.3
996.8, 955.5
594.3, 591.8, 589.6, 584.4, 581.4
955.5 (η2-O2)2TiO TidO str
580.8 (η2-O2)2TiO asym Ti-(O2)2 str
a Absorptions observed in the reaction of Ti and 18O2. b Absorptions unresolved because of band overlap.
Figure 4. Optimized structures (bond lengths in angstrom and bond
angles in degree) of the observed products.
revealed that these absorptions are favored with higher O2
content relative to the TiO3 absorptions, which suggests that
the absorber should involve more oxygen than that in TiO3.
The 996.8 cm-1 absorption is the strongest one in this group,
which shifted to 955.5 cm-1 with 18O2. This absorption is
attributed to a terminal TidO stretch mode based upon its band
position and oxygen isotopic shift following the example of
(η2-O2)TiO. The 1124.1 and 1116.6 cm-1 absorptions exhibit
similar isotopic ratios which are characteristic of O-O stretch
vibrations. In addition to the absorptions observed in the experi-
ments with 16O2 and 18O2 separately, two intermediate absorptions
at 1120.2 and 1058.3 cm-1 were observed in the experiment
using a mixed 16O2 + 18O2 sample (Figure 3, trace c). This
experimental observation demonstrates that the 1124.1 and
1116.6 cm-1 absorptions are due to the symmetric and anti-
symmetric O-O stretch vibrations originating from a species
involving two equivalent O2 subunits. On the basis of the
experimental observations, we assign the 1124.1, 1116.6, 996.8,
and 596.4 cm-1 absorptions to the (η2-O2)2TiO complex. The
596.4 cm-1 absorption is due to a Ti-(O2)2 stretch vibration.
To further support the assignment, theoretical calculations
were carried out on this TiO5 species. As shown in Figure 4,
the most stable configuration for the (η2-O2)2TiO complex was
found to have a nonplanar Cs geometry with a 3A′′ ground state.
The two O2 groups roughly form a plane, which is perpendicular
to the TiO fragment. Compared with the (η2-O2)TiO complex,
the terminal TiO bond length of the (η2-O2)2TiO complex is
shortened by 0.019 Å upon coordination of a second O2
molecule. The two O-O bonds exhibit the same bond length
which was calculated to be 1.320 Å, much shorter than that of
the (η2-O2)TiO complex. The O-O bond length of (η2-O2)2TiO
is very close to those of the recently characterized transition
metal superoxide complexes.39,42 Therefore, the (η2-O2)2TiO
complex can be described as a disuperoxo titanium monoxide
complex, [(O2-)2(TiO)2+], which also stratifies the +4 oxidation
state of Ti. Consistent with this notion, the (η2-O2)2TiO complex
has a triplet ground state with the two unpaired electrons located
on the two O2 fragments (1.06 e each). On the basis of the
calculation results, the four experimentally observed vibrational
modes for the (η2-O2)2TiO complex were predicted at 1198.4,
Figure 2. Infrared spectra in the 1020-816 cm-1 region from
codeposition of laser-evaporated titanium oxides with isotopic-labeled
dioxygen in excess argon. Spectra were taken after 1 h of sample
deposition followed by 35 K annealing. (a) 0.5% 16O2, (b) 0.5% 18O2,
(c) 0.2% 16O2 + 0.2% 18O2, and (d) 0.15% 16O2 + 0.3% 16O18O +
0.15% 18O2.
Figure 3. Infrared spectra in the 1140-1050 and 635-570 cm-1
regions from codeposition of laser-evaporated titanium oxides with
isotopic-labeled dioxygen in excess argon. Spectra were taken after
1 h of sample deposition followed by 35 K annealing. (a) 0.5% 16O2,
(b) 0.5% 18O2, (c) 0.2% 16O2 + 0.2% 18O2, and (d) 0.15% 16O2 + 0.3%
16O18O + 0.15% 18O2.
TiO5. The 1124.1, 1116.6, 996.8, and 596.4 cm-1 absorptions
increased together upon sample annealing after the (η2-O2)TiO
absorptions. Experiments with different O2 concentrations

TiO3 and TiO5 in Solid Argon
J. Phys. Chem. A, Vol. 112, No. 40, 2008 9761
TABLE 2: DFT/B3LYP Calculated Total Energiesa, Frequencies (cm-1), and Intensities (km/mol) of the Products
molecule
energy
frequency (intensity)
(η2-O2)TiO (1A′, Cs)
(η2-O2)2TiO (3A′′, Cs)
-1075.188968
-1225.620590
1038.4 (309) 909.5 (124) 636.8 (35) 636.6 (36) 234.3 (36) 205.6 (21)
1198.4 (16) 1184.8 (78) 1068.8 (244) 587.1 (112) 499.0 (10)
453.4 (2) 453.3 (20) 222.2 (19) 219.8 (3) 159.0 (0) 137.7 (4) 96.7(30)
a In Hartree, after zero point energy corrections.
TABLE 3: Comparison Between the Observed and
Calculated Vibrational Frequencies (cm-1) and Isotopic
Frequency Ratios of the Product Molecules
molecule can be regarded as only one-fold coordinated;
therefore, it can either coordinate one O2 molecule in forming
the peroxo complex or coordinate two O2 molecules to give
the disuperoxo complex to satisfy the +4 oxidation state of Ti.
freq
calcd
(η2-O2)TiO (1A′, Cs)
16O/18Oa
molecule
mode
obsd
calcd obsd
With the two complexes characterized here, a series of
complexes between titanium monoxide and small molecule
ligands have been reported.37,43,44 The (CH4)TiO complex was
found to be very weakly bound with a binding energy of only
about 1.1 kcal/mol.37 Different from the spherical methane
molecule, the water molecule can serve as a σ donor owing to
the existence of two pairs of lone-pair electrons. Hence, the
TiO molecule was able to coordinate with water molecule to
form the (H2O)TiO complex with a predicted binding energy
of 16.1 kcal/mol, much higher than that of (CH4)TiO.43 The
(C2H2)TiO and (OC)TiO complexes were predicted to have large
binding energies of 38.344a and 23 kcal/mol (DFT/BP86
method).45 This may be attributed to the existing of both σ
donation and π back-donation interactions between TiO and CO
or C2H2. The above-characterized (η2-O2)TiO and (η2-O2)2TiO
charge transfer complexes have the largest binding energies in
this set (80.7 kcal/mol for the former and 60.7 kcal/mol per O2
for the latter).
TidO str. (a′)
O-O str. (a′)
1038.4 971.9 1.0438 1.0431
909.5 863.3 1.0540 1.0529
636.8 624.8 1.0517 1.0511
636.6 611.1 1.0433 1.0427
asym Ti-O2 str. (a”)
sym Ti-O2 str. (a′)
(η2-O2)2TiO (3A′′, Cs)
sym O-O str. (a′)
asym O-O str. (a”)
TidO str. (a′)
1198.4 1124.1 1.0607 1.0595
1184.8 1116.6 1.0606 1.0590
1068.8 996.8 1.0439 1.0432
asym Ti-(O2)2 str. (a”) 587.1 596.4 1.0257 1.0269
a Oxygen-18 refers to Ti18O3 and Ti18O5.
1184.8, 1068.8, and 587.1 cm-1 with the calculated isotopic
16O/18O ratios in good agreement with the experimental values
(Table 3). These four modes were predicted to have the largest
IR intensities; all the other modes are either out of the spectral
range or have very low IR intensities (Table 2).
Reaction Mechanism. Laser evaporation of bulk titanium
dioxide produces substantial amount of titanium monoxide
molecules, which are trapped and isolated in solid argon. When
the as-deposited sample containing TiO and O2 was annealed,
the reactants can diffuse and react to form the (η2-O2)TiO and
(η2-O2)2TiO complexes. According to the DFT/B3LYP calcula-
tions, the formation of the (η2-O2)TiO complex from the ground
state of TiO and O2 is exothermic by 80.7 kcal/mol (reaction
1). The formation of the (η2-O2)TiO complex upon annealing
indicates that reaction 1 requires negligible activation energy.
Conclusions
The reaction of titanium monoxide molecules and O2 was
investigated by using matrix isolation infrared spectroscopy and
theoretical calculations. Laser evaporation of bulk titanium
dioxide produces substantial amount of titanium monoxide
molecules, which are trapped and isolated in solid argon. When
the as-deposited sample containing TiO and O2 was annealed,
the reactants can diffuse and react to form the (η2-O2)TiO
complex, which is characterized to be a peroxo titanium
monoxide complex, (O2)2-(TiO)2+, with a nonplanar Cs sym-
3
(
-
TiO(3∆) + O
f (η2-O )TiO(1A′)
∑
)
2
2
1
metry and a A′ ground state. The (η2-O2)TiO complex is able
g
to further react with another molecular oxygen to give the
(η2-O2)2TiO complex, which can be described as a disuperoxo
titanium monoxide complex, (O2-)2(TiO)2+. The (η2-O2)2TiO
∆E ) -80.7 kcal ⁄ mol (1)
The (η2-O2)2TiO complex absorptions increased after the
(η2-O2)TiO absorptions. This suggests that the (η2-O2)TiO
complex can further react with another molecular oxygen to
give the (η2-O2)2TiO complex, reaction 2. The spontaneous
formation of the (η2-O2)2TiO complex upon sample annealing
implies that reaction 2 is also a barrierless reaction, which was
predicted to be exothermic by 40.7 kcal/mol.
3
complex was predicted to possess a A′′ ground state and a
nonplanar Cs geometry. These simple superoxo and peroxo
titanium monoxide complexes may serve as simple model in
understanding the adsorbed species on titanium oxide surface.
Acknowledgment. This work is supported by National Basic
Research Program of China (2007CB815203) and National
Natural Science Foundation of China (20433080).
3
(
-
(η2-O )TiO(1A′) + O
f (η2-O ) TiO(3A″)
∑
)
2
2
2 2
g
∆E ) -40.7 kcal ⁄ mol (2)
References and Notes
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in understanding the adsorbed species on titanium oxide surface.
However, the Ti site on the titanium oxide surface is 5-fold
coordinated; it can only adsorb one O2 molecule in forming
the charge transferred superoxo and peroxo species. The TiO
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