Matrix-isolation fourier transform infrared and theoretical studies of laser-ablated Sc atom reactions with water molecules

Article abstract of DOI:10.1021/jp001836r

Laser-ablated Se atoms have been reacted with water molecules during condensation with argon at 11 K. In agreement with previous thermal atom reactions, absorptions at 1482.6 and 713.0 cm^-1 are assigned to the HScOH molecule formed via an inser

Full text of DOI:10.1021/jp001836r

8882
J. Phys. Chem. A 2000, 104, 8882-8886
Matrix-Isolation Fourier Transform Infrared and Theoretical Studies of Laser-Ablated Sc
Atom Reactions with Water Molecules
Luning Zhang, Jian Dong, and Mingfei Zhou*
Laser Chemistry Institute, Fudan UniVersity, Shanghai 200433, P.R.China
ReceiVed: May 17, 2000; In Final Form: July 17, 2000
Laser-ablated Sc atoms have been reacted with water molecules during condensation with argon at 11 K. In
agreement with previous thermal atom reactions, absorptions at 1482.6 and 713.0 cm-1 are assigned to the
HScOH molecule formed via an insertion reaction. Photolysis of the HScOH produces the metal monoxide
ScO. In addition, new absorption at 765.6 cm-1 is also observed and is assigned to the ScOH molecule. The
ScOH molecule undergoes photoinduced rearrangement to the HScO molecule, which is characterized by
Sc-H and Sc-O stretching vibrations at 1391.1 and 922.3 cm-1. Density functional theoretical calculations
have been performed for the aforementioned species, and important transition states on the reaction paths
have been obtained.
Introduction
a hole in a CsI window, and the ablated metal atoms were
codeposited with H2O in excess argon onto a 11 K CsI window,
which was mounted on a cold tip of a closed-cycle helium
refrigerator (Air Products, model CSW202) for 1 h at a rate of
4-5 mmol/h. Typically, a 5-10 mJ/pulse laser power was used
and focused into a spot of about 0.25 mm2 on the target. At
this controlled laser fluence, we managed to generate a plume
containing enough hot metal atoms and electrons for reaction
and also prevent excessive high-energy UV photons to deplete
the products. H2O, H218O(96%18O), and D2O were subjected
to several freeze-pump-thaw cycles before use. Infrared
spectra were recorded on a Bruker IFS113v spectrometer at 0.5
cm-1 resolution using a DTGS detector. Matrix samples were
annealed at different temperatures, and selected samples were
subjected to broadband photolysis using a high-pressure mercury
lamp.
Quantum chemical calculations were performed using the
Gaussian 98 program.20 The three-parameter hybrid functional
according to Becke with additional correlation corrections due
to Lee, Yang, and Parr were utilized (B3LYP).21,22 Recent
calculations have shown that this hybrid functional can provide
accurate results for the geometries and vibrational frequencies
for transition metal containing compounds.12-14,23,24 The
6-311++G(d,p) basis sets were used for H and O atoms, and
the all electron basis sets of Wachters-Hay as modified by
Gaussian were used for the Sc atom.25,26 Reactants, various
possible transition states, intermediates, and products were
optimized. The vibrational frequencies were calculated with
analytic second derivatives, and zero point vibrational energies
(ZPVE) were derived.
The interaction of metal atoms and cations with water
molecules has attracted considerable experimental1-9 and theo-
retical attention.10-14 The reactions of transition metal cations
and water have been studied in the gas phase and the formation
of MO+ + H2 was found to be exothermic for early transition
metal cations, while for middle transition metal cations, no
exothermic products were observed.1-3 Several theoretical works
have also been carried out concerning the reactivity of transition
metal cations with water molecules, which provided new insights
into likely reaction intermediates, various possible transition
states, and products.10-14
Lin and Parson reported that atomic Sc reacted with water
to give ScO in the gas phase.5 Matrix isolation infrared
absorption studies showed that thermal Sc atoms reacted with
water to form the insertion products spontaneously, while the
metal monoxides were formed on photolysis.7 To our knowl-
edge, there is no theoretical study on Sc atom reactions with
water, and the reaction pathway concerning metal atom reactions
with water are not clear.
It is well-known that laser ablation of the metal target can
produce ground-state metal atoms as well as excited-state atoms
with high kinetic energy. Recent investigations of laser-ablated
transition metal atom reactions with small molecules have shown
very rich chemistry due to higher reactivity of the ablated metal
species, and more products were observed as compared with
thermal atom reactions.15-17 In this paper, we present a
combined matrix-isolation FTIR and theoretical investigation
on the reactions of laser-ablated Sc atoms with water molecules.
Experimental and Theoretical Methods
Results and Discussion
The experimental setup for pulsed laser ablation and matrix
infrared spectroscopic investigation has been described previ-
ously18 and is very similar to the technique employed earlier
by the Andrews group.19 The 1064 nm Nd:YAG laser funda-
mental (Spectra Physics, DCR 150, 20 Hz repetition rate and 8
ns pulse width) was focused onto the rotating Sc target through
Infrared Spectra. Infrared spectra for the reaction of laser-
ablated Sc atoms with water molecules in excess argon in the
1520-1380, 980-900, and 780-680 cm-1 regions are il-
lustrated in Figure 1, and the absorptions are listed in Table 1.
The stepwise annealing and photolysis behavior of these product
absorptions are also shown and will be discussed below. D2O
and H218O samples were employed for product identification
through isotopic shift and splitting, the isotopic counterparts
* Corresponding author. E-mail: mfzhou@srcap.stc.sh.cn.
10.1021/jp001836r CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/08/2000

Sc Atom Reactions with Water Molecules
J. Phys. Chem. A, Vol. 104, No. 39, 2000 8883
Figure 1. Infrared spectra in the 1520-1380, 980-900, and 780-680 cm-1 regions from codeposition of laser-ablated Sc atoms with 0.5% H2O
in argon: (a) 1 h sample deposition at 11 K; (b) after 25 K annealing; (c) after 20 min λ > 400 nm photolysis; (d) after 20 min λ > 250 nm
photolysis; (e) after 30 K annealing.
TABLE 1: Infrared Absorptions (cm-1) from Codeposition
of Laser-Ablated Sc Atoms and Water Molecules in Excess
Argon at 11 K
H2O
H218O
D2O
assignment
1485.3
1482.6
1418.3
1391.1
954.7
922.3
765.6
729.6
715.7
713.0
1485.2
1482.6
HScOH site
1068.5
HScOH (Sc-H str)
HSc(OH)2 (Sc-H str)
HScO (Sc-H str)
ScO
HScO (Sc-O str)
ScOH (Sc-OH str)
HSc(OH)2 (Sc-(OH)2 asy str)
HScOH site
1390.8
915.0
884.3
736.9
709.5
692.1
689.1
1004.2
954.7
922.3
750.5
713.3
696.9
693.6
HScOH (HSc-OH str)
are also listed in Table 1. The spectra with mixed H2O + D2O
in the 1100-980 and 780-680 cm-1 regions are shown in
Figure 2. Figure 3 shows the spectra in the 1000-850 cm-1
region using a H2O + H218O sample.
Calculation Results. B3LYP calculations were first done on
scandium monoxide and hydride, and the results are listed in
2
Table 2. The ScO was predicted to have a Σ+ ground state
Figure 2. Infrared spectra in the 1100-980 and 780-680 cm-1 regions
from codeposition of laser-ablated Sc atoms with 0.1% H2O + 0.2%
HDO + 0.1%D2O in argon: (a) 1 h sample deposition at 11 K; (b)
after 25 K annealing; (c) after 20 min λ > 250 nm photolysis; (d) after
30 K annealing.
with a bond length of 1.659 Å. Two states of the ScH molecule
were calculated to be very close in energy, a 3∆ triplet state is
about 1.7 kcal/mol lower in energy than a Σ+ state. Previous
1
experiments and calculations reported the 1Σ+ to be the ground
state,27,28 our B3LYP/6-311++G(d,p) calculations predicted the
relative stability of these two states in error.
of theory, the 1A′ HScO is about 6.8 kcal/mol more stable than
1
the Σ+ ScOH molecule.
Calculations were done on two [Sc,O,H] isomers, namely,
HScO and ScOH. The calculated geometric parameters are
shown in Figure 4, and the vibrational frequencies and intensities
Calculations were also done for two isomers of ScH2O on
the doublet potential energy surface, namely, the inserted
HScOH molecule with Cs symmetry and the ScOH2 complex
with C2V symmetry. For HScOH and ScOH2, the values of S2
are 0.752 and 0.778, respectively, suggesting negligible mild
spin contamination. The geometric parameters of HScOH and
ScOH2 are shown in Figure 4, and the vibrational frequencies
with IR intensities are listed in Table 4. The inserted HScOH
1
are listed in Table 3. The HScO has a A′ ground state with
bent geometry ( HScO ) 119.2°), with Sc-H and Sc-O bond
lengths of 1.879 and 1.671 Å. The ScOH molecule was predicted
to have a 1Σ+ ground state with a 3∆ state lying only about 0.7
kcal/mol higher in energy. At the B3LYP/6-311++G(d,p) level

8884 J. Phys. Chem. A, Vol. 104, No. 39, 2000
Zhang et al.
Figure 3. Infrared spectra in the 1000-850 cm-1 region from
codeposition of laser-ablated Sc atoms with 0.1% H2O + 0.4% H218O
in argon: (a) 1 h sample deposition at 11 K; (b) after 25 K annealing;
(c) after 20 min λ > 250 nm photolysis; (d) after 30 K annealing.
TABLE 2: Calculated Bond Length and Vibrational
Frequencies of ScH and ScO
bond length (Å)
frequency (intensity)
ScH(3∆)
1.849
1.751
1.659
0.744
0.962
1483.8 (295)
1626.3 (275)
999.5 (211)
4423.3 (0)
ScH(1Σ+)
ScO(2Σ+)
+
H2(1Σg
)
H2O(1A1)a
3922.2 (57), 3817.0 (9), 1602.5 (67)
a Bond angle: 105.1°.
Figure 4. Calculated geometric parameters for the Sc + H2O reaction
products and transition states (bond length in Å, bond angle in degree,
relative energies for different isomers are given in parentheses in kcal/
mol).
2
molecule was predicted to have a A′ ground state with cis
configuration. The ScOH2 complex was predicted to have a 2A1
ground state, which is about 48.0 kcal/mol higher in energy
than the 2A′ HScOH molecule. As can be seen in Table 4, there
is one imaginary frequency for the 2A1 ScOH2 molecule,
suggesting that it is a first-order saddle point. Geometry
optimization on H2ScO converged to ScO + H2.
TABLE 3: Calculated Vibrational Frequencies (cm-1) and
Intensities (km/mol) for ScOH, HScO, and the Transition
State (TS2) on the ScOH f HScO Reaction Path
molecule
frequency (intensity)
ScOH (1Σ+)
ScOD
3958.8 (349), 778.8 (120), 322.5 (204)
2886.7 (233), 761.6 (112), 244.3 (114)
3945.4 (337), 748.5 (111), 320.0 (202)
3976.3 (178), 691.3 (100), 303.9 (212)
1459.2 (317), 977.4 (302), 391.0 (203)
1046.1 (121), 975.7 (335), 294.0 (103)
1459.0 (320), 936.8 (281), 389.3 (202)
1536.7 (86), 932.9 (182), 1342.0i (1910)
Finally, similar calculation was also done on the HSc(OH)2
molecule, as shown in Figure 4, this molecule was predicted to
Sc18OH
ScOH(3 ∆)
HScO(1A′)
DScO
1
have a A1 ground state with planar geometry.
Assignment. ScO. The 954.7 cm-1 band is assigned to the
ScO molecule based on previous work. Present DFT calculations
predicted a 2Σ+ ground state with the Sc-O stretching vibration
at 999.5 cm-1, which is in good agreement with previous
work.7,29
HSc18O
TS2
TABLE 4: Calculated Vibrational Frequencies (cm-1) and
IR Intensities (km/mol) for the HScOH, ScOH2, and the
Transition State (TS1) on the HScOH f H2 + ScO Reaction
Path
HScOH. The 1482.6 cm-1 absorption increased on annealing
and was totally eliminated on full arc mercury lamp photolysis.
It shifted to 1068.5 cm-1 with D2O and gave an isotopic H/D
ratio of 1.3876. A band at 713.0 cm-1 exhibited the same
annealing and photolysis behavior with the 1482.6 cm-1 band,
it shifted to 689.1 cm-1 with D2O and to 693.6 cm-1 with H218O
reagent. These two bands are assigned to the Sc-H and Sc-
OH stretching vibrations of the HScOH molecule, in agreement
with the Hauge group.7 Present DFT calculations predicted that
HScOH(2A′)
DScOD
HSc18OH
ScOH2(2A1)
TS1
3956.1 (230) 2883.3 (167) 3942.8 (219) 3710.8 (4)
1885.3 (99)
1529.5 (516) 1093.8 (269) 1529.4 (516) 3602.0 (533) 1670.8 (194)
717.1 (212) 700.0 (189) 690.0 (201) 555.0 (189) 1224.2 (21)
368.6 (162) 279.0 (95)
347.1 (160) 265.3 (89)
365.8 (160) 322.3 (17)
344.7 (156) 322.0 (10)
911.6 (205)
892.3 (132)
325.3 (82)
236.1 (45)
324.1 (83)
150.1i (59) 1347.3i (743)
2
the HScOH molecule has a A′ ground state with the Sc-H
and Sc-OH stretching vibrations at 1529.5 and 717.1 cm-1
,
deposition and increased on broadband photolysis. The 1391.1
cm-1 band exhibited a very small shift when the isotopic H218O
sample was used but was shifted to 1004.2 cm-1 with D2O, the
isotopic H/D ratio 1.3853 indicating that it is due to a Sc-H
stretching vibration. In concert, the 922.3 cm-1 band showed
no isotopic shift with the D2O sample but was shifted to 884.3
when the H218O sample was used. The isotopic 16/18 ratio
just 46.9 and 4.1 cm-1 higher than the observed values, which
provides excellent support for the assignment.
HScO. The 1391.1 and 922.3 cm-1 bands were not reported
on thermal Sc atom reactions.7 These two bands tracked through
all the experiments, suggesting different vibrational modes of
the same molecule. These two bands were observed after sample

Sc Atom Reactions with Water Molecules
J. Phys. Chem. A, Vol. 104, No. 39, 2000 8885
1.0430 is very close to the diatomic ScO ratio (1.0434) observed
in solid argon, suggesting that this band is due to a terminal
Sc-O stretching vibration. Accordingly, these two bands are
assigned to the Sc-H and Sc-O stretching vibrations of the
HScO molecule. The assignment is further supported by DFT
calculations. Present calculations predicted that the HScO
molecule has a 1A′ ground state with a HScO angle of 119.2°.
The Sc-H and Sc-O stretching vibrations were predicted at
1459.2 and 977.4 cm-1, which must be scaled by 0.953 and
0.977 to match the experimental values. The higher energy mode
was calculated to shift by 413.1 and 0.2 cm-1 with D2O and
H218O, respectively, while the lower mode was calculated to
shift by 1.7 and 40.6 cm-1 with D2O and H218O, which are in
good agreement with the observed argon matrix values.
ScOH. A weak band at 765.6 cm-1 was produced on sample
deposition, it decreased on annealing and was destroyed on
broadband photolysis. This band was not observed in previous
thermal Sc atom experiments; it underwent oxygen-18 and
deuterium shifts of 28.7 and 15.1 cm-1, respectively. Mixed
isotopic sample experiments indicted that only one H and one
O atom are involved in this vibrational mode. This band is
assigned to the Sc-OH stretching vibration of the ScOH
molecule. DFT calculation predicted that the ScOH molecule
Figure 5. Potential energy surface for the Sc + H2O reaction at the
B3LYP/6-311++G(d,p) level. Energies given are in kcal/mol and are
relative to the separated ground-state reactants: Sc(2D) + H2O(1A1).
starting from Sc + H2O were calculated at the B3LYP/6-
311++G(d,p) level, as shown in Figure 5. Recent studies on
Sc+ + H2O reactions have shown that the first step of this
reaction is the formation of a Sc+OH2 ion-molecule complex;
then through a transition state, one hydrogen atom is passed
from oxygen to the metal, leading to the insertion HScOH+
molecule.13 For Sc atom reactions, our DFT calculations
predicted that the ScOH2 complex is a first-order saddle point,
and no evidence was presented for the ScOH2 complex in our
experiments. The ground-state Sc atom reactions with H2O
molecules to form the inserted HScOH molecules proceed
spontaneously without energy barrier. The HScOH molecule
undergoes H2 elimination through a transition state (TS1). This
transition state has a four-centered structure; it is about 28.8kcal/
mol higher in energy than the HScOH molecule. As can be seen
in Figure 4, the H-H distance is 1.007 Å; the two Sc-H
distances 1.975 and 1.865 Å and the Sc-O distance 1.752 Å
are significantly longer than that in the HScOH molecule. The
one imaginary frequency corresponds to the H-H bond forma-
tion and the O-H bond breaking.
1
has a Σ+ ground state with a Sc-OH stretching vibration at
778.8 cm-1. The calculated isotopic frequency ratios (16O/18O:
1.0405, H/D:1.0226) are in good agreement with observed
values (16O/18O:1.0389, H/D:1.0201). Previous thermal Sc +
D2O study assigned a 699.2 cm-1 band to the ScOD molecule,7
we believe that this identification is in error.
HSc(OH)2. The 729.6 cm-1 band was also observed in
previous thermal atom experiments.7 This band appeared on
annealing. Isotopic substitution experiments indicated that this
is a Sc-OH stretching vibration and two OH units are involved.
This band was tentatively assigned to the HSc(OH)2 molecule
previously. A weak band at 1418.3 cm-1 tracked with the 729.6
cm-1 band and is due to a Sc-H stretching vibration. Our DFT
1
calculations on HSc(OH)2 find a A1 ground state with planar
geometry (see Figure 4). The antisymmetric HO-Sc-OH,
symmetric HO-Sc-OH, and Sc-H stretching vibrations were
calculated at 756.7, 683.6, and 1546.4 cm-1 with 431:96:468
relative intensities. The symmetric HO-Sc-OH stretching
vibration was not observed due to weakness.
There is no evidence for scandium hydride molecules.
Scandium monohydride has been the subject of several gas-
phase spectroscopic and theoretical studies.27,28 The ground state
The HScO absorptions were markedly enhanced on photoly-
sis; there are two major possible reactions to form this molecule,
1
of ScH in the gas phase has been determined to be Σ+ with a
HScOH(2A′) f HScO(1A′) + H
ScOH(1Σ+) f HScO(1A′)
(2)
(3)
3∆ state lying slightly higher in energy. Our DFT calculations
predicted a vibrational fundamental of 1483.8 cm-1 for the 3∆
1
state and 1626.3 cm-1 for the Σ+ state.
Reaction Mechanism. Co-condensation of laser-ablated Sc
atoms and water molecules in excess argon resulted in the
insertion reaction (1) to form HScOH as the major product. Our
Reaction 2 is predicted to be endothermic, while reaction 3 is
slightly exothermic. If reaction 2 takes place in the matrix, the
hydrogen atom will back-react to form the HScOH molecule
due to the matrix cage effect. As can be seen in Figure 1, visible
range photolysis eliminated the ScOH absorption with no
obvious effect on the HScOH absorptions, while the HScO
absorptions greatly increased. In previous thermal Sc atom
experiments,7 the HScO molecule was not formed on photolysis,
as no ScOH was present. All this evidence suggests that reaction
3 is the dominant process for the formation of HScO under
photolysis. Figure 6 shows the doublet potential energy surface
of the ScOH f HScO reaction. The transition state (TS2) lies
above the ScOH molecule by about 28.9 kcal/mol at the B3LYP/
6-311++G(d,p) level of theory. This transition state has Sc-
H, Sc-O, and O-H bond lengths of 1.832, 1.717, and 1.473
Sc(2D) + H2O(1A1) f HScOH(2A′)
(1)
DFT calculation showed that this reaction is exothermic by about
61.5 kcal/mol. Weak ScO, ScOH, and HScO molecules were
also observed on sample deposition, indicating that some
HScOH molecules were decomposed in the laser ablated plume
before deposition at the low temperature.
The HScOH absorptions increased on matrix sample anneal-
ing, suggesting that the insertion reaction require no activation
energy. On broadband photolysis, the HScOH absorptions were
destroyed, while the ScO absorption greatly increased. To have
a better understanding of this reaction, potential energy surfaces

8886 J. Phys. Chem. A, Vol. 104, No. 39, 2000
Zhang et al.
References and Notes
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Conclusions
The reactions of Sc atoms with water molecules have been
reinvestigated using the laser-ablation technique and density
functional theoretical calculations. In agreement with previous
thermal atom work, the HScOH molecules are formed by direct
insertion reaction that requires no activation energy. New
absorption at 765.6 cm-1 is also observed and is assigned to
the ScOH molecule. The ScOH molecule undergoes photoin-
duced rearrangement to the HScO molecule, which is character-
ized by Sc-H and Sc-O stretching vibrations at 1391.1 and
922.3 cm-1. UV-visible photolysis of the HScOH causes H2
elimination from a four-centered transition state and produces
the metal monoxide ScO. The aforementioned species have been
identified by isotopic substitution experiments as well as density
functional theoretical calculations. Potential energy surfaces for
the above-mentioned reactions have also been calculated, and
important transition states on the reaction paths have been
obtained.
Acknowledgment. We acknowledge contributions from
Prof. Q. Z. Qin. This work is supported by the Chinese
NKBRSF.