Reactions of laser-ablated Y and La atoms with H2O infrared spectra and density functional calculations of the HMO, HMOH and M(OH)2 molecules in solid argon

Article abstract of DOI:10.1016/S0301-0104(01)00450-5

Reactions of laser-ablated Y and La atoms with water molecules have been studied using matrix-isolation FTIR spectroscopy and density functional calculations. The reaction products were identified based on isotope labeled experiments and density functiona

Full text of DOI:10.1016/S0301-0104(01)00450-5

Chemical Physics 272 ꢀ2001) 27±36
www.elsevier.com/locate/chemphys
Reactions of laser-ablated Y and La atoms with H2O
Infrared spectra and density functional calculations of the
HMO, HMOH and MꢀOH)2 molecules in solid argon
Luning Zhang, Limin Shao, Mingfei Zhou *
Department of Chemistry, Laser Chemistry Institute, Fudan University, Shanghai 200433, China
Received 26 April 2001
Abstract
Reactions of laser-ablated Y and La atoms with water molecules have been studied usingmatrix-isolation FTIR
spectroscopy and density functional calculations. The reaction products were identi®ed based on isotope labeled ex-
periments and density functional calculations. For both Y and La atom reactions with H2O, the major reaction
products are the HMOH, MO and MꢀOH)2 ꢀM ˆ Y and La) molecules. In addition, HMO and HMꢀOH)2 were
produced on matrix sample annealingor UV broadband photolysis. Evidence is also presented for the formation of the
‡
HYOH species. The potential energy surfaces for the Y, La ‡ H2O reactions have been calculated to interpret the
reaction mechanisms. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction
in the gas phase have been extensively studied both
experimentally and theoretically [10±17]. It was
‡
‡
Transition metal centers in both bare metals
and metal oxides play important roles in catalytic
processes. There have been numerous studies on
the gas phase reactions between metal atoms and
metal oxides with small molecules containingC±H
[1±6], C±C [1±6], N±H [7±9], H±H and O±H bonds
[10±17]. These reactions can serve as models for
understandingthe reactivity pattern of transition
metal based catalysts in condensed phase.
found that early transition metal cations ꢀSc , Ti ,
‡
V ) are more reactive than their oxides, and the
‡
formation of the low lyingstates MO ‡ H2 is the
only exothermic process [12±14]. The reactions of
neutral transition metal atoms with H2O have re-
ceived relatively less attention. Lin and Parson
reported that atomic Sc reacted with water to give
ScO in the gas phase [18]. Matrix isolation com-
biningwith infrared spectroscopy is very powerful
for studyingthe reactions of transition metals with
H2O. Representative work by Kau€man et al.
[19,20] showed that thermally evaporated Sc, Ti
and V atoms could react with water to form the
insertion products spontaneously, while the late
transition metal atoms reacted with water to
form M±OH2 adducts. These adducts rearranged
to the insertion molecules upon photolysis. Quite
Reaction between the ®rst row transition metal
centers and H2O molecules is one of the most
‡
‡
studied systems. The M ‡ H2O ! MO ‡ H2 ꢀM
denotes transition metals) and its reverse reactions
* Correspondingauthor. Fax: +86-21-65102777.
E-mailaddress: mfzhou@fudan.edu.cn ꢀM. Zhou).
0301-0104/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S0301-0104ꢀ01)00450-5

28
L. Zhang et al. / Chemical Physics 272 *2001) 27±36
recently, we have performed a series of matrix-
isolation FTIR and theoretical studies on the re-
actions of Sc [21], Ti [22], V [23], Mn and Fe metal
atoms with H2O. Compared to ordinary thermal
atom reactions, some new reaction channels were
observed on laser ablation experiments. In particu-
lar, the combination of matrix-isolation FTIR
spectroscopy and quantum chemical calculations
is powerful in elucidatingthe reaction mechanism.
Reactions of second and third row transition
metals with water have gained much less attention.
The activation of the O±H bond in water by
second row transition metal atoms has only been
theoretically studied [24] besides our recent report
on Zr, Hf [22], Nb, and Ta [23]. In the present
paper, we investigate the reactions of Yttrium and
Lanthanum atoms with water molecules, which
will be compared to the analogous Sc reactions
and also complete our series studies on the reac-
tions between early transition metals and H2O.
with additional correlation corrections due to
Lee, Yangand Parr were utilized ꢀB3LYP) [26,27].
The 6-311++Gꢀd,p) basis sets were used for H
and O atoms, and the Los Alamos ECP plus DZ
ꢀLANL2DZ) were used on Y and La atoms [28,
29]. These e€ective core potentials ꢀECPs) have
incorporated Darwin relativistic e€ects into the
potentials. Recent studies have shown that density
functional theory usingthis hybrid functional
and ECPs can provide quite reliable predictions
of the energies, structures and vibrational frequen-
cies of compounds containingsecond and third
row transition metal [30±32]. Reactants, transition
states, intermediates and products were optimized.
The vibrational frequencies were calculated with
analytic second derivatives, and zero point vibra-
tional energies ꢀZPVE) were derived. Transition
state optimizations were done with the synchro-
nous transit-guided quasi-Newton ꢀSTQN) method
[33], followed by the vibrational frequency calcu-
lations showingthat the obtained structures to be
true saddle points. All energies presented include
ZPVE corrections.
2. Experimental
The experimental setup for pulsed laser ablation
and matrix infrared spectroscopic investigation
has been described previously [21±23]. Brie¯y, the
1064 nm Nd:YAG laser fundamental ꢀSpectra
Physics, DCR 150, 20 Hz repetition rate and 8 ns
pulse width) was focused onto the rotatingYttrium
or Lanthanum metal target, and the ablated spe-
cies were co-deposited with reagent gas 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
one hour at a rate of around 5 mmol/h. Typi-
cally, 5±20 mJ/pulse laser power was used. H2O,
H2 18Oꢀ96% 18O) and D2O samples were subjected
to several freeze-pump-thaw cycles before use. In-
frared spectra were recorded on a Bruker IFS113v
spectrometer at 0.5 cmÀ1 resolution usinga DTGS
detector. Matrix samples were annealed at di€er-
ent temperatures, and selected samples were sub-
jected to broadband photolysis usinga hihg
pressure mercury lamp.
3. Results and discussion
In this section we will ®rst present the infrared
absorption spectra of the Y ‡ H2O and La ‡ H2O
reactions. Assignments of the reaction products
are based on isotopic labeled experiments and
density functional calculations. Our discussion will
then focus on the reaction mechanisms and the
potential energy surfaces of the corresponding
M ‡ H2O systems ꢀM denotes Y or La).
3.1. IR spectra and calculation results
Representative infrared spectra for the reaction
of laser-ablated Y with H2O in excess argon are
shown in Fig. 1 along with the corresponding
absorptions summarized in Table 1. The results
of isotope enriched experiments usingD 2O and
H2 18O are given in Figs. 2 and 3, with the band
positions also listed in Table 1.
Quantum chemical calculations were performed
usingthe GAUSSIAN98 program [25]. The three
parameter hybrid functional accordingto Becke
Figs. 4±6 show the IR spectra from co-deposi-
tion of laser-ablated La with H2O, D2O, and

L. Zhang et al. / Chemical Physics 272 *2001) 27±36
29
Fig. 1. Infrared spectra of laser-ablated Y co-deposited with
H2O ꢀ0.1%) in Ar matrix. ꢀa) After deposition, ꢀb) after 25 K
annealing, ꢀc) after 20 min broadband Hg arc photolysis, and
ꢀd) after 35 K annealing.
Fig. 2. Infrared spectra of laser-ablated Y co-deposited with
0.5% D2O ꢀwith about 30% HDO contamination) in Ar matrix.
ꢀa) After deposition, ꢀb) after 30 K annealing, ꢀc) after 20 min
broadband Hgarc photolysis, and ꢀd) after 32 K annealing.
Table 1
Observed infrared absorptions ꢀcmÀ1) for co-condensation of
laser-ablated Y atoms with H2O in excess argon at 11 K
H2O
D2O
H2 18O
Assignment
‡
1534.2
1399.1
1368.6
1316.2
872.1
843.1
809.6
695.1
662.2
641.5
616.9
1099.6
1002.9
981.6
945.8
872.1
843.1
807.9
681.6
651.2
623.1
593.4
HYOH ꢀm Y±H)
1399.1
1368.6
1316.2
HYOH ꢀm Y±H)
HYꢀOH)2 ꢀm Y±H)
HYO ꢀm Y±H)
‡
YO
YO
802.6
771.0
HYO ꢀm Y±O)
‡
HYOH ꢀm Y±OH)
YOH
HYOH ꢀm Y±OH)
HYꢀOH)2 ꢀm asym Y±
OH)
597.9
578.0
YꢀOH)2 ꢀm asym Y±OH)
H2 18O in excess argon, respectively, and the pro-
duct absorptions are summarized in Table 2.
DFT calculations were performed for the po-
tential products expected here in order to provide a
guide for vibrational band assignments. The opti-
mized structures and geometric parameters are
shown in Figs. 7 and 8, and the calculated vibra-
tional frequencies are summarized in Tables 3 and
4.
Fig. 3. Infrared spectra of laser-ablated Y co-deposited with
H2 16O/H2 18O ꢀ0.5%) in Ar matrix. ꢀa) After deposition, ꢀb)
after 25 K annealing, ꢀc) after 20 min broadband Hg arc pho-
tolysis, and ꢀd) after 27 K annealing.
LaO molecules based on previous matrix infrared
spectroscopic studies [34±36]. These absorptions
were observed after sample deposition and in-
creased on annealing. The 872.1 and 838.2 cmÀ1
bands produced on UV broadband photolysis
3.2. Product assignment
‡
‡
are assigned to the YO and LaO cations. The
band positions of these oxide cations are in good
agreement with a recent report on the reaction of
Y and La with O2 [36].
‡
3.2.1. MO and MO
Absorptions at 843.1 cmÀ1 in Y ‡ H2O and
796.7 cmÀ1 in La ‡ H2O are due to the YO and

30
L. Zhang et al. / Chemical Physics 272 *2001) 27±36
Fig. 4. Infrared spectra of laser-ablated La co-deposited with H2O ꢀ0.1%) in Ar matrix. ꢀa) After deposition, ꢀb) after 28 K annealing,
ꢀc) after 20 min broadband Hg arc photolysis, ꢀd) after 31 K annealing, and ꢀe) after 35 K annealing.
Fig. 5. Infrared spectra of laser-ablated La co-deposited with 0.5% D2O ꢀwith about 30% HDO contamination) in Ar matrix. ꢀa) After
deposition, ꢀb) after 26 K annealing, ꢀc) after 30 K annealing, and ꢀd) after 35 K annealing.

L. Zhang et al. / Chemical Physics 272 *2001) 27±36
31
Fig. 7. Calculated geometries of various reaction products and
transition states for the Y ‡ H2O reactions.
Fig. 6. Infrared spectra of laser-ablated La co-deposited with
H2 18O ꢀ0.65%) in Ar matrix. ꢀa) After deposition, ꢀb) after 25 K
annealing, and ꢀc) after 28 K annealing.
Table 2
Observed infrared absorptions ꢀcmÀ1) for co-condensation of
laser-ablated La atoms with H2O in excess argon at 11 K
H2O
D2O
H2 18O
Assignment
1263.7
1229.0
1180.2
838.2
796.7
758.6
576.4
554.3
905.8
880.3
753.1
HLaOH ꢀm La±H)
HLaꢀOH)2 ꢀm La±H)
HLaO ꢀm La±H)
LaO
LaO
‡
756.8
720.1
HLaO ꢀm La±O)
HLaOH ꢀm La±OH)
HLaꢀOH)2 ꢀm asym La±
OH)
559.7
534.5
519.3
491.2
517.3
488.2
LaꢀOH)2 ꢀm sym La±OH)
LaꢀOH)2 ꢀm asym La±
OH)
470.6
Fig. 8. Calculated geometries of various reaction products and
transition states in the La ‡ H2O reactions.
3.2.2. HMOH
HYOH molecule. Our DFT calculations predicted
2
In the reaction of Y with H2O, absorptions at
1399.1 and 597.9 cmÀ1 tracked through all the
experiments. As shown in Fig. 1, this set of bands
increased upon matrix annealingand decreased
on broadband photolysis. The 1399.1 cmÀ1 band
shifted to 1002.9 cmÀ1 upon deuterium ꢀD2O)
substitution with a H/D ratio of 1.3951. This isÀa1
typical Y±H stretchingmode. The 597.9 cm
band shifted to 576.4 cmÀ1 with D2O and to 575.0
cmÀ1 with H2 18O, and is due to a Y±OH stretching
vibration. We assign these two absorptions to the
Y±H and Y±OH stretchingvibrations of the
the HYOH molecule to have a A0 ground state
with a bent structure as shown in Fig. 7. The Y±H
and Y±OH vibrations were calculated at 1453.9
and 642.5 cmÀ1 ꢀTable 3 gives other calculated
vibrational modes of HYOH).
In the La ‡ H2O reaction, similar bands at
1263.7 and 576.4 cmÀ1 were assigned to the La±H
and La±OH stretchingvibrations of the HLaOH
molecule. The correspondingIR spectra are shown
in Fig. 4. DFT calculations predicted this molecule
to have a 2A0 ground state with La±H and La±OH
À1
stretchingvibrations at 1347.6 and 578.0 cm
,

32
L. Zhang et al. / Chemical Physics 272 *2001) 27±36
Table 3
Calculated vibrational frequencies ꢀcmÀ1) and IR intensities
ꢀkm/mol) of the structures described in Fig. 7
3.2.3. HMO
In the reactions of Y ‡ H2O, two bands at
1316.2 and 809.6 cmÀ1 are weak after sample de-
position. These bands increased together on broad-
band photolysis; and were sharpened on 30 K
annealing. The 1316.2 cmÀ1 band shifted to 945.8
cmÀ1 with D2O, and gave a H/D ratio of 1.3916,
suggesting a Y±H stretching vibration. The 809.6
cmÀ1 band showed a very small deuterium isotopic
shift ꢀ1.7 cmÀ1), and a large oxygen-18 shift ꢀ38.6
cmÀ1) as can be seen in Fig. 3. The 16O/18O ratio
of 1.0501 is very close to the diatomic YO ratio
ꢀ1.0505) observed in Ar matrix [36], indicatingthat
this band is mainly due to a terminal Y±O vibra-
tion. Accordingly, the 1316.2 and 809.6 cmÀ1
bands are assigned to the Y±H and Y±O stretching
vibrations of HYO. Our B3LYP calculations
predicted 1389.8 and 839.2 cmÀ1 Y±H and Y±O
stretchingfundamentals for HYO with 1A0 ground
state.
Molecule
Frequency ꢀintensity)
YO
Y±OH2
858.8ꢀ177)
3719.5ꢀ5), 3603.2ꢀ611), 1569.1ꢀ179),
330.5ꢀ9), 280.4ꢀ14), 108.8ꢀ59)
HYOH
3969.0ꢀ214), 1453.9ꢀ392), 642.5ꢀ136),
375.2ꢀ124), 338.4ꢀ126), 321.9ꢀ100)
3882.2ꢀ431), 1631.2ꢀ199), 740.7ꢀ158),
517.4ꢀ270), 463.9ꢀ113), 405.0ꢀ184)
1389.8ꢀ347), 839.2ꢀ235), 411.4ꢀ135)
3967.8ꢀ273), 684.1ꢀ73), 356.4ꢀ98 Â 2)
3971.3ꢀ125), 3970.4ꢀ205), 622.0ꢀ61),
619.3ꢀ175), 425.3ꢀ136), 419.3ꢀ0),
410.3ꢀ214), 375.0ꢀ136), 100.0ꢀ1)
3974.0ꢀ84), 3973.1ꢀ215), 1424.8ꢀ476),
635.7ꢀ368), 612.2ꢀ89), 466.1ꢀ424),
457.9ꢀ25), 439.6ꢀ42), 376.7ꢀ275),
320.0ꢀ110), 148.3ꢀ99), 88.6ꢀ171)
3855.9ꢀ81), 1352.7ꢀ97), 677.9ꢀ198),
469.5ꢀ42), 136.7ꢀ106), 1396.5iꢀ1922)
1795.0ꢀ59), 1369.3ꢀ238), 1208.0ꢀ23),
789.1ꢀ168), 744.9ꢀ98), 1321.8iꢀ803)
‡
HYOH
HYO
YOH
YꢀOH)2
HYꢀOH)2
TS1
TS2
Similarly, bands at 1180.2 and 758.7 cmÀ1 in
the La ‡ H2O reactions are assigned to HLaO. In
likely fashion, as shown in Fig. 5, the 1180.2 cmÀ1
band showed large deuterium isotopic shift ꢀ427.1
cmÀ1) and is due to a La±H stretchingmode.
The 758.7 cmÀ1 band exhibited an 16O/18O isotopic
ratio of 1.0535 and is due to a La±O stretching
mode ꢀrefer to Fig. 6). The HLaO molecule was
also predicted to have 1A0 ground state. The La±H
and La±O stretchingvibrations were predicted at
1301.3 and 755.5 cmÀ1. Structure of HLaO is
shown in Fig. 8.
Table 4
Calculated vibrational frequencies ꢀcmÀ1) and IR intensities
ꢀkm/mol) of the reaction products in the reactions of La with
H2Oa
Molecule
Frequency ꢀintensity)
LaO
La±OH2
775.0ꢀ187)
3713.2ꢀ9), 3600.6ꢀ798), 1565.3ꢀ209),
325.1ꢀ15), 2682ꢀ20), 149.4ꢀ44)
HLaOH
3852.4ꢀ177), 1347.6ꢀ586), 578.0ꢀ163),
320.0ꢀ91), 283.6ꢀ133), 264.8ꢀ151)
1301.3ꢀ607), 755.5ꢀ225), 402.1ꢀ129)
3943.7ꢀ268), 616.7ꢀ97), 383.6ꢀ94 Â 2)
3948.3ꢀ103), 3947.7ꢀ152), 1336.9ꢀ664),
560.6ꢀ154), 550.1ꢀ315), 437.7ꢀ292),
414.9ꢀ84), 406.2ꢀ66), 364.6ꢀ258),
317.7ꢀ88), 236.2ꢀ199), 103.7ꢀ20)
3946.8ꢀ163), 3946.1ꢀ158), 570.9ꢀ121),
547.0ꢀ155), 421.9ꢀ103), 402.4ꢀ197),
400.6ꢀ0), 352.9ꢀ163), 114.2ꢀ2)
HLaO
LaOH
HLaꢀOH)2
3.2.4. M*OH)2
In the reactions of Y ‡ H2O, sharp band ap-
peared at 597.9 cmÀ1 on sample annealing. As
shown in Fig. 1, this band dominates the spectrum
on high temperature annealing ꢀ35 K). It shifted
to 578.0 cmÀ1 with D2O sample, the band position
and isotopic ratio suggest a Y±OH stretching vi-
bration. In mixture H2O/HDO/D2O spectra, a
quartet at 597.9, 596.2, 578.0 and 576.4 cmÀ1 was
produced as illustrated in Fig. 2. The 597.9 cmÀ1
band is assigned to the antisymmetric Y±OH
LaꢀOH)2
TS1
TS2
3874.9ꢀ153), 1298.3ꢀ306), 656.6ꢀ135),
436.4ꢀ62), 167.4ꢀ83), 1464.9iꢀ2854)
1731.3ꢀ47), 1572.5ꢀ377), 1174.0ꢀ19),
732.8ꢀ128), 682.1ꢀ196), 1394.4iꢀ807)
a Refer to Fig. 8 for geometries.
stretchingvibration of the YꢀOH) molecule. The
2
respectively, which agree well with the experi-
mental values.
596.2 and 576.4 cmÀ1 bands in mixed spectra are
due to the antisymmetric and symmetric Y±OH

L. Zhang et al. / Chemical Physics 272 *2001) 27±36
33
stretchingvibrations of the YꢀOH)ꢀOD) molecule.
Although the symmetric Y±OH stretching mode
was not observed for YꢀOH)2, interaction between
the stretchingmodes within the lower symmetry
YꢀOH)ꢀOD) molecule activates the symmetric
stretchingmode. DFT calculations support our
assignment. Our B3LYP calculations predicted the
examples of HScꢀOH)2 and HYꢀOH)2. Both
HYꢀOH)2 and HLaꢀOH)2 were predicted to have
1A0 ground state with non-planar Cs symmetry as
illustrated in Figs. 7 and 8.
‡
3.2.6. HYOH
2
In the reactions of Y ‡ H2O, new IR absorp-
tions at 1534.2 and 695.1 cmÀ1 appeared on broad-
band photolysis. These bands shifted to 1099.6 and
681.6 cmÀ1 with D2O, and gave isotopic H/D ra-
tios of 1.3952 and 1.0198, respectively. The isoto-
pic H/D ratios suggest that the upper mode is an
Y±H stretchingvibration and the lower mode is an
Y±OH stretchingvibration. These two bands are
135.1 and 53.6 cmÀ1 higher than the Y±H and Y±
OH stretchingvibrations of neutral HYOH, so
cation species should be considered. Previous stud-
ies have shown that Sc group atom containing
species could be easily photoionized upon UV
photolysis to produce cationic species in solid
argon matrix [36,37]. The 1534.2 and 695.1 cmÀ1
ground state of YꢀOH)2 molecule as A1 ꢀFig. 7)
with a strongantisymmetric vibration at 619.3
cmÀ1 and a weaker symmetric vibration at 622.0
cmÀ1 ꢀ175:61 km/mol, see Table 3). For compari-
son, the relative intensities of antisymmetric and
symmetric vibrations of YꢀOH)ꢀOD) were pre-
dicted to be 119:113 km/mol.
Similar absorptions at 519.3 and 491.2 cmÀ1
were produced on annealingin La ‡ H2O experi-
ments, and are assigned to the symmetric and
antisymmetric HO±La±OH stretchingmodes of
2
the LaꢀOH)2 molecule. These two modes for A1
ground state LaꢀOH)2 were calculated at 570.9 and
547.0 cmÀ1 with 121:155 km/mol relative intensi-
ties.
‡
bands are accordingly assigned to the HYOH
cation.
DFT calculations lend further support to the
‡
3.2.5. HM*OH)2
cation assignment. The HYOH molecule has a
1A0 electronic ground state ꢀsee Fig. 7 for struc-
ture). The Y±H and Y±OH stretchingvibrations
In the Y ‡ H2O experiments, absorptions at
1368.6 and 616.9 cmÀ1 increased together on an-
nealingand decreased upon broadband photolysis.
The upper mode shifted to 981.6 cmÀ1 with D2O
and gave a H/D ratio of 1.3943. The H/D ratio
suggests a Y±H stretching vibration. The 616.9
cmÀ1 band falls into the Y±OH stretchingfre-
quency region and gave triplet feature at 613.6,
603.2, 593.4 cmÀ1 with H2O/DHO/D2O sample,
indicatingthat two OH units are most probably
involved in this vibration. Analogous to HScꢀOH)2
reported previously [21], we assign the above two
absorptions to the Y±H and antisymmetric Y±OH
were predicted to be at 1631.2 and 740.7 cmÀ1
,
which are in good agreement with the experimen-
tal values.
3.2.7. Other absorptions
Weak absorption at 662.2 cmÀ1 appears in the
Y ‡ H2O experiments. It shifted to 651.2 cmÀ1 with
D2O. This band is tentatively assigned to YOH.
Our DFT calculations found a linear structure in
‡
stretchingvibrations of HYꢀOH) .
2
1R ground state with Y±OH stretching vibration
In the reactions between La and H2O, distinct
bands at 1229.0 and 554.3 cmÀ1 appeared after
matrix deposition and increased upon annealing
and broadband photolysis. The band at 1229.0
cmÀ1 shifted to 880.3 cmÀ1 with D2O reagent. The
554.3 cmÀ1 band went to 534.5 cmÀ1 with D2O and
split into a triplet at 554.3, 545.2, and 534.5 cmÀ1
with H2O ‡ HDO ‡ D2O. These two IR absorp-
tions are assigned to HLaꢀOH)2 followingthe
at 684.1 cmÀ1. Recent gas phase electronic spectra
‡
of YOH shows that the 1R state YOH should have
ꢀ
ꢀ
1.948 A ꢀY±O) and 0.920 A ꢀO±H) bond lengths
[38]. Its m3 mode was measured at 673.83 cmÀ1
,
which is close to our IR spectrum. In La ‡ H2O
reaction, no evidence has been found for linear
‡
LaOH ꢀ1R ground state). The La±O vibration
mode is predicted to be at 616.7 cmÀ1 with DFT
calculations.

34
L. Zhang et al. / Chemical Physics 272 *2001) 27±36
3.3. Reaction mechanism
The potential energy surfaces ꢀPES) for the re-
actions between Y or La and H2O are given in
Figs. 9 and 10. Y and La atoms reacted with water
molecules to form the monoxide MO and the in-
serted HMOH molecules. All these product ab-
sorptions increased on annealing, suggesting that
reactions ꢀ1)±ꢀ4) require negligible activation
energy.
0
2
1
2
Yꢀ D† ‡ H2Oꢀ A1† ! HYOHꢀ A †
ꢀ1†
ꢀ2†
ꢀ3†
ꢀ4†
2
1
2
‡
Yꢀ D† ‡ H2Oꢀ A1† ! YOꢀ R † ‡ H2
Fig. 10. Calculated potential energy surface for the La ‡ H2O
0
2
1
2
reaction.
Laꢀ D† ‡ H2Oꢀ A1† ! HLaOHꢀ A †
2
1
2
‡
Laꢀ D† ‡ H2Oꢀ A1† ! LaOꢀ R † ‡ H2
predicted to have a 2A00 ground state with non-
planar geometry. It is 14.5 kcal/mol lower in
energy than separated Yꢀ2D) and H2O. Previous
calculations on the Y±OH2 adduct [24] predicted a
stabilization energy of 15.1 kcal/mol, which agrees
well with our calculations. This complex was not
observed in our experiments. As can be seen in
Fig. 9, Y±OH2 could rearrange to the inserted
HYOH molecule via a transition state ꢀTS1). This
transition state lies below the energy of free
The ®rst step of Y ‡ H2O interaction is the
formation of a Y±OH2 complex. This complex was
2
Yꢀ D† ‡ H2O reactants and the energy barrier was
calculated to be only 7.3 kcal/mol. This explains
why Y±OH2 was not observed. The total exo-
thermicity for Y insertion into H2O ꢀreaction ꢀ1))
is 66.4 kcal/mol, which is also in agreement with
previous report by Siegbahn et al. [24].
The HYOH molecule could further undergo
H2 elimination process to form YO ‡ H2. This
step involves a transition state ꢀTS2), which lies
34.1 kcal/mol above HYOH. Although this barrier
is signi®cant, TS2 still lies energetically below the
Yꢀ2D) and H2O reactants by 32.3 kcal/mol. Due to
the strongexothermic property of overall reaction
ꢀ2) ꢀDE ˆ À45:5 kcal/mol), reaction ꢀ2) is ther-
modynamically favored although the H2 elimina-
tion process from HYOH is endothermic by 20.9
kcal/mol. In present experiments, both HYOH and
YO absorptions increased on annealing, indicating
that part of ``hot'' HYOH formed from reaction
ꢀ1) can eciently release the excess energy and be
Fig. 9. Calculated potential energy surfaces for the M ‡ H2O
reactions. ꢀa) Y ‡ H2O, ꢀb) Sc ‡ H2O.

L. Zhang et al. / Chemical Physics 272 *2001) 27±36
35
0
‡
1
1
stabilized in argon matrix, part of them ended up
in YO ‡ H2 fragments. The detailed PES shown
for Sc ‡ H2O system also completes our previous
study on the reaction of laser-ablated Sc with
water molecules [21]. In a word, the reaction PES
of Y ‡ H2O resembles that of Sc ‡ H2O as illus-
trated in Fig. 9.
Y ‡ OH ! YOHꢀ R † ! HYOꢀ A †
ꢀ9†
0
‡
1
1
La ‡ OH ! LaOHꢀ R † ! HLaOꢀ A †
ꢀ10†
‡
The HYOH cations are most likely formed by
photoionization of the HYOH molecules via re-
action ꢀ11), which was predicted to be endothermic
by about 141.1 kcal/mol. However, no similar
As shown in Fig. 10, the potential energy sur-
‡
2
2
‡
face for Laꢀ D† ‡ H2O ! LaOꢀ R † ‡ H2 is very
HLaOH cations were observed in the La ‡ H2O
‡
2
2
similar to that of Yꢀ D† ‡ H2O ! YOꢀ R † ‡ H2
reaction. The major di€erence is that reaction ꢀ3) is
calculated to be exothermic by 41.6 kcal/mol,
about 20 kcal/mol lower than the values for the
Y ꢀDE ˆ À66:4 kcal/mol) and Sc ꢀDE ˆ À61:5
kcal/mol) reactions.
reactions, probably due to low yield of the
HLaOH molecule in La experiments.
0
‡
HYOHꢀ A † ! HYOH ‡ eÀ
ꢀ11†
2
The YꢀOH)2 and LaꢀOH)2 molecules are prob-
ably formed by reactions of metal atoms with
water dimers via reactions ꢀ5) and ꢀ6), which were
predicted to be exothermic by about 108.9 and
100.1 kcal/mol, respectively. Both the YꢀOH)2 and
LaꢀOH)2 absorptions markedly increased on an-
nealing, suggesting that reactions ꢀ5) and ꢀ6) re-
quire negligible activation energy.
4. Conclusions
Reactions between laser-ablated Y and La
atoms and H2O molecules have been investigated
by matrix-isolation FTIR spectroscopy and den-
sity functional calculations. Y and La atoms
reacted with H2O spontaneously to form the in-
serted HMOH molecules, MO monoxides and the
MꢀOH)2 ꢀM ˆ Y and La) molecules. Similar with
the Sc ‡ H2O reaction, the HMO and HMꢀOH)2
molecules were also produced on matrix sample
annealingor on broadband photolysis. In addi-
tion, evidence is also presented for the formation
2
1
2
Yꢀ D† ‡ 2H2Oꢀ A1† ! YꢀOH†2ꢀ A1† ‡ H2
ꢀ5†
ꢀ6†
2
1
2
Laꢀ D† ‡ 2H2Oꢀ A1† ! LaꢀOH†2ꢀ A† ‡ H
Upon broadband photolysis from a high pres-
sure Hglamp, the HYOH and HLaOH absorp-
tions greatly decreased with co-currently growth of
‡
of the HYOH species. The potential energy sur-
faces for the M ‡ H2O ! MO ‡ H2 reactions have
been calculated and the reaction mechanisms dis-
cussed.
‡
‡
‡
HYOH , YO , HYO, LaO , and HLaO. There
are mainly two possible reactions to form the HYO
and HLaO molecules: ꢀa) photodissociation from
HYOH and HLaOH via reactions ꢀ7) and ꢀ8); ꢀb)
metal atom reactions with OH radicals via YOH
and LaOH intermediates, as shown in reactions ꢀ9)
and ꢀ10). The OH radicals were formed during
deposition process. Reactions ꢀ7) and ꢀ8) were
predicted to be endothermic, while reactions ꢀ9)
and ꢀ10) were calculated to be exothermic. There-
fore, from the thermodynamics point of view, re-
actions ꢀ9) and ꢀ10) are probably the dominant
process for the formation of HYO and HLaO.
Acknowledgements
We gratefully acknowledge support from NSFC
ꢀGrant no. 20003003) and the Chinese NKBRSF.
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