Reactions of Mn with H2O and MnO with H2. Matrix-isolation FTIR and quantum chemical studies

Article abstract of DOI:10.1021/jp0043118

The reactions of Mn atoms with water and MnO with H₂ have been studied using matrix-isolation FTIR and DFT theoretical calculations. In solid argon, Mn atom reacted with H₂O molecule to form the MnOH₂ complex spontaneously

Full text of DOI:10.1021/jp0043118

J. Phys. Chem. A 2001, 105, 5801-5807
5801
Reactions of Mn with H2O and MnO with H2. Matrix-Isolation FTIR and Quantum
Chemical Studies
Mingfei Zhou,*,†,‡ Luning Zhang,†,‡ Limin Shao,†,‡ Wenning Wang,† Kangnian Fan,† and
Qizong Qin†,‡
Department of Chemistry, Fudan UniVersity, Shanghai 200433, People’s Republic of China, and Laser
Chemistry Institute, Fudan UniVersity, Shanghai 200433, People’s Republic of China
ReceiVed: NoVember 28, 2000; In Final Form: February 14, 2001
The reactions of Mn atoms with water and MnO with H2 have been studied using matrix-isolation FTIR and
DFT theoretical calculations. In solid argon, Mn atom reacted with H2O molecule to form the MnOH2 complex
spontaneously. Photolysis destroyed the MnOH2 complex and produced the inserted HMnOH molecule. In
addition, HMnOMnH, Mn(OH)2, HMnOMnOH, and posssibly HMnOMnOMnH molecules were also produced
on photolysis. Reaction of MnO with H2 gave the (H2)MnO complex without activation energy. Photolysis
of the (H2)MnO produced the inserted HMnOH molecule as well. The aforementioned species were identified
via isotopic substitutions as well as density functional theoretical frequency calculations. Potential energy
surface for the Mn + H2O S MnO + H2 reaction was also given. Our calculation results showed that all
these reaction intermediates and products are high spin molecules. The MnOH2, HMnOH and (H2)MnO
molecules have ground states with S ) 5/2, the HMnOMnH, and HMnOMnOH molecules have ground states
with S ) 10/2, and the HMnOMnOMnH molecule has ground state with S ) 15/2.
Introduction
Recently, we have performed matrix-isolation FTIR and
theoretical studies of the reactions of laser-ablated early transi-
The interaction of transition metal centers with water
molecules is of interest in a wide variety of areas, including
catalytic synthesis, surface chemistry. and biochemical systems.
Previous experimental and theoretical studies have provided a
wealth of insight concerning the reactivity of transition metal
atoms and ions with water molecules.1-14 The reaction of
manganese with water is of particular interest in biochemical
systems. It is generally thought that the reaction center of
photosystem II binds the Mn cluster that catalyzes the oxidation
of water. Photosystem II is a key enzyme located in the
thylakoid membranes in green plants, algae and cyanobacteria.15
It catalyzes the light driven electron transfer from water to the
membrane bound electron carrier plastoquinone. In the process,
water molecules are oxidized to molecular oxygen. Although
interaction of water with manganese cluster complexes has
gained intensive attention,16 the reaction of Mn atom with water
molecule has attracted much less attention. Using matrix-
isolation infrared absorption method, Kauffman et al. showed
that thermal manganese atom formed adduct with water, which
was rearranged to the inserted HMnOH molecule on photolysis.2
The H-H activation by metal monoxide provides a simple
system to study in detail, and serves as a model for other
reactions of metal oxide with organic substrates.17 Previous gas-
phase studies have demonstrated that monoxide cations of the
late row transition metals Mn, Fe, Co, and Ni are more reactive
than their bare metal cation analogues.18-24 Recent studies
showed that these MO+ ions are able to oxidize H2 and small
organic molecules at thermal energy; apparently, the oxo ligand
increased the reactivity of the bare metal cations. To our
knowledge, there is no report on the reactions of neutral
transition metal monoxides with H2 molecule.
tion metal atoms (Sc, Ti, and V group) with water molecules
in solid argon.25-27 Different reaction paths leading to the H2-
MO and MO + H2 were observed. In particular, the combination
of experimental and theoretical approaches has been extremely
beneficial in understanding and interpreting the reaction mech-
anisms and periodic trends for the early transition metal-water
reactions. Here we report a study of reactions of laser-ablated
Mn atom with water and manganese monoxide with hydrogen
in solid argon. Although this study seems parallel to that of
early transition metal atoms with water, we will show that it is
distinctly different because of the intrinsic differences between
Mn and early transition metals.
Experimental and Theoretical Methods. The experimental
setup for pulsed laser ablation and matrix infrared spectroscopic
investigation has been described previously.25-28 The 1064 nm
Nd:YAG laser fundamental (Spectra Physics, DCR 150, 20 Hz
repetition rate, and 8 ns pulse width) was focused onto the
rotating manganese metal or dioxide targets through a hole in
a CsI window, and the ablated species were co-deposited with
reagent gas in excess argon onto a 11K 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 2-4 mmol/
h. Typically, 5-20 mJ/pulse laser power was used. 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.29 The three parameter hybrid functional
according to Becke with additional correlation corrections due
to Lee, Yang, and Parr were utilized (B3LYP).30,31 Recent
† Department of Chemistry.
‡ Laser Chemistry Institute.
10.1021/jp0043118 CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/30/2001

5802 J. Phys. Chem. A, Vol. 105, No. 24, 2001
Zhou et al.
TABLE 1: Infrared Absorptions (cm-1) from Co-Deposition of Laser-Ablated Mn Atoms with H2O Molecules in Excess Argon
H2O
D2O
H218O
H2O + HDO + D2O
1663.4,1197.8
1644.4,1182.4
H216O + H218O
assignment (mode)a
1663.4
1644.4
1638.5
1197.8
1182.4
1175.3
1163.4
1644.6
1638.6
HMnOH (Mn-H)
HMnOMnOH (Mn-H)
HMnOMnH
1641.8,1638.5, 1179.5, 1175.3
(asy-MnH)
(HMnOMnOMnH)
(asy-MnH)
1627.7
1627.8
1576.6
884.8
1166.0
884.6
1570.0
841.9
1576.6, 1388.5, 1166.0
1576.6,1570.0
884.8,841.9
MnOH2 (H2O bending)
HMnOMnOH
(asy-MnOMn)
(HMnOMnOMnH)
(asy-OMnO)
875.1
870.5
875.2
870.3
835.0
827.6
870.5,827.6
HMnOMnH
(asy-MnOMn)
MnO (Mn-O)
Mn(OH)2
833.3
709.0
833.3
693.6
796.7
687.3
833.3,796.7
709.0,699.3, 687.3
709.0,701.9, 693.6
(asy-Mn-OH)
HMnOH (Mn-OH)
648.6
628.7
624.3
648.6,624.3
a sym ) symmetric; asy ) asymmetric.
Figure 2. Infrared spectra in the 1670-1560 and 895-635 cm-1
regions from co-deposition of laser-ablated Mn with 0.4% H2O in argon.
(a) 1 h sample deposition at 11 K, (b) after 25 K annealing, (c) after
20 min photolysis, and (d) after 30 K annealing.
Figure 1. Infrared spectra in the 1670-1560 and 895-635 cm-1
regions from co-deposition of laser-ablated Mn with 0.2% H2O in argon.
(a) 1 h sample deposition at 11 K, (b) after 25 K annealing, (c) after
20 min photolysis, and (d) after 30 K annealing.
1663.4, 1576.6, and 648.6 cm-1. Annealing to 25 K increased
the 1576.6 cm-1 absorption with no obvious change on 1663.4
and 648.6 cm-1 absorptions. Photolysis destroyed the 1576.6
cm-1 absorption but greatly enhanced the 1663.4 and 648.6
cm-1 absorptions. In addition, new absorptions at 1644.4,
1638.5, 884.8, 870.5, and 709.0 cm-1 were also produced on
photolysis. Figure 2 shows the spectra with 0.4% H2O in argon
using slightly higher laser power. The new product absorptions
in Figure 1 were also observed here but with higher intensities.
In contrast to Figure 1, weak new absorptions at 1627.7 and
875.1 cm-1 were produced on photolysis.
Isotopic substitution was employed for band identification,
and the results are also listed in Table 1. Figures 3-5 show the
mixed H2O + HDO+D2O and H216O + H218O spectra in
selected regions, and the isotopic splittings will be discussed
below.
calculations have shown that this hybrid functional can provide
very reliable predictions of the state energies, structures and
vibrational frequencies of transition metal containing com-
pounds.32-36 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 Mn atom.37,38 The
geometries were fully optimized, and harmonic vibrational
frequencies calculated with analytic second derivatives, and zero
point vibrational energies (ZPVE) were derived. Transition state
optimizations were done with the synchronous transit-guided
quasi-Newton(STQN) method at the B3LYP/6-311++G(d,p)
level,39 followed by the vibrational frequency calculations
showing that the obtained structures to be true saddle points.
In addition, the IRC calculations were performed to verify the
transition states connecting the reactant and product on the
potential energy surface.
MnO2+H2/Ar. First, an experiment was performed with a
MnO2 target and pure argon gas, and the spectrum after
deposition is shown in Figure 6a. The major product absorptions
are 833.3, 947.8, and 816.3 cm-1 due to MnO and MnO2; weak
Results and Discussion
Experimental Results. Mn + H2O/Ar. Experiments were
performed with different laser power and water concentrations;
the observed product absorptions and assignments are listed in
Table 1. Representative spectra in the Mn-H stretching and
Mn-O stretching vibrational frequency regions using lower laser
power and 0.2% H2O in argon are shown in Figure 1. The major
new product absorptions observed after sample deposition were
absorptions of MnO2- (858.3 cm-1) and MnOMn (808.3 cm-1
)
were also observed.40 MnO + H2 reactions were performed with
laser ablation of MnO2 in 1.0% H2 in argon, the product
absorptions are listed in Table 2, and infrared spectra in the
Mn-O stretching vibrational frequency region are shown in
Figure 6b-d. The MnO, MnO2, MnO2-, and MnOMn absorp-

Reactions of Mn with H2O and MnO with H2
J. Phys. Chem. A, Vol. 105, No. 24, 2001 5803
Figure 6. Infrared spectra in the 900-620 cm-1 regions from co-
deposition of laser-ablated MnO2 with (a) pure argon, 1 h sample
deposition at 11 K, (b) 1.0% H2 in argon, 1 h sample deposition at 11
K, (c) 25 K annealing of sample (b), and (d) after 20 min photolysis of
sample (c).
Figure 3. Infrared spectra in the 1670-1560 and 1210-1160 cm-1
regions from co-deposition of laser-ablated Mn with 0.6%(H2O + HDO
+ D2O) in argon. (a) 1 h sample deposition at 11 K, (b) after 20 min
photolysis, (c) after 28 K annealing, and (d) after 32 K annealing.
TABLE 2: Infrared Absorptions (cm-1) from Co-Deposition
of Laser-Ablated MnO2 with H2 Molecules in Excess Argon
assignment (mode)a
1663.4
947.8
858.3
849.7
833.3
816.3
808.3
648.6
HMnOH (Mn-H)
MnO2 (asy-MnO2)
MnO2- (asy-MnO2)
(H2)MnO (Mn-O)
MnO (Mn-O)
MnO2 (sym-MnO2)
MnOMn (asy-MnOMn)
HMnOH (Mn-OH)
a sym ) symmetric; asy ) asymmetric.
Figure 4. Infrared spectra in the 900-610 cm-1 region from
co-deposition of laser-ablated Mn with 0.6%(H2O + HDO + D2O) in
argon. (a) 1 h sample deposition at 11 K, (b) after 20 min photolysis,
(c) after 28 K annealing, and (d) after 32 K annealing.
Figure 7. B3LYP/6-311++G(d,p) calculated geometric parameters
(bond length in angstroms, bond angle in degrees) and relative stability
(in kcal/mol) of the MnH2O isomers and the transition states in the
Mn + H2O reaction path.
TABLE 3: Calculated Vibrational Frequencies (cm-1) and
Intensities (km/mol) for the MnOH2, HMnOH, (H2)MnO,
and the Transition States (TS1, TS2) on the Mn + H2O
Reaction Path
Figure 5. Infrared spectra in the 1670-1560 and 900-610 cm-1
regions from co-deposition of laser-ablated Mn with 0.6%(H216O +
H218O) in argon. (a) 1 h sample deposition at 11 K, (b) after 25K
annealing, (c) after 20 min photolysis, and (d) after 28 K annealing.
MnOH2
HMnOH
(H2)MnO
TS1
TS2
3824(58, a′′) 4000(142, σ) 4089(9, a1)
3803(36)
1773(206)
1721(2)
619(83)
261(130)
245(125)
3705(99, a′) 1729(204, σ) 905(209, a1) 1379(13)
1597(83, a′) 691(143, σ) 847(29, b2) 673(104)
478(125)
213(0.2, b1) 336(152)
198(0, b2)
tions were observed as in Figure 6a, and new absorption at 849.7
cm-1 was observed after sample deposition, which slightly
increased on 20K annealing. Photolysis destroyed the 849.7
cm-1 absorption and produced two new absorptions at 1663.4
317(0.4, a′′) 290(286, π) 462(1, a1)
181(182, a′) 44(318, π)
141(1, a′)
1142i(961) 316i(11)
and 648.6 cm-1
.
and intensities are listed in Table 3. At the B3LYP/6-311++G-
Calculation Results. DFT calculations were performed on the
potential products observed in present experiments. Calculations
were performed on three isomers of MnH2O, namely, the
MnOH2 complex, the inserted HMnOH molecule, and the (H2)-
MnO complex. The optimized geometric parameters and relative
stability are shown in Figure 7, and the vibrational frequencies
(d,p) level of theory, all three MnH2O isomers were calculated
6
to have sextet ground states. The Σ+ HMnOH is the global
minimum, the 6A′ MnOH2 is 27.2 kcal/mol higher and the 6A1
(H2)MnO is 48.3 kcal/mol higher than the global minimum.
Similar calculations were done for Mn(OH)2, HMnOMnH,
HMnOMnOH and HMnOMnOMnH molecules. The optimized

5804 J. Phys. Chem. A, Vol. 105, No. 24, 2001
Zhou et al.
TABLE 4: Calculated Vibrational Frequencies (cm-1) and Intensities (km/mol) for the Structures Described in Figure 8
molecule
frequency(intensity, mode)
Mn(OH)2
3945.4(13, a), 3944.9(183, b), 734(237, b), 608(6, a), 411(82, a),
381(242, a), 133(14, b), 127(2, b), 110(111,a)
(6A)
HMnOMnH
1709(0, σg), 1700(575, σµ), 850(794, σµ), 332(0, σg), 265(0, πg),
(11Σµ
)
261(652, πµ), 91(2, πµ)
+
HMnOMnOH
3947(103,a′), 1701(275,a′), 865(837,a′), 673(83,a′), 400(171,a′),
311(5, a′), 260(178, a”), 253(171, a′), 144(2, a”), 138(15, a′),
68(21, a′′), 59(2, a′)
(11A′)
HMnOMnOMnH
1698(0,σg), 1696(590, σµ), 869(0, σg), 849(1910, σµ), 405(7,σµ),
260(670, πµ), 259(0, πg), 229(0, σg), 142(40, πµ), 85(0, πg),29(2, πµ)
(16Σµ
)
+
of MnO2 in pure argon, suggesting a reaction product of laser-
ablated species with H2. Although it is hard to make a definitive
assignment without isotopic substitution, in light of the above
behavior and the only 16.4 cm-1 blue shift from the MnO, we
suggest assignment of this band to the (H2)MnO complex by
comparison with the (O2)MnO in the Mn + O2 system.40 The
assignment gained further support from DFT calculations. The
6
(H2)MnO molecule was predicted to have a A1 ground state
with C2V symmetry that correlated to the ground state 6Σ+
6
MnO.41 Although the A1 state (H2)MnO is higher in energy
than the MnOH2 and HMnOH isomers, it is a true minimum
on the sextet potential energy surface. The Mn-O and H-H
stretching vibrations were predicted at 905 and 4089 cm-1 with
209:9 relative intensities. The H-H stretching vibration is too
weak to be observed in present experiments.
Figure 8. B3LYP/6-311++G(d,p) calculated geometric parameters
(bond length in angstroms, bond angle in degrees) of the HMnOMnH,
Mn(OH)2, HMnOMnOH, and HMnOMnOMnH molecules.
HMnOH. In both the Mn + H2O and MnO2 + H2 experi-
ments, sharp bands at 1663.4 and 648.6 cm-1 greatly enhanced
in concert on photolysis, suggesting different vibrational modes
of the same molecule. The increase upon photodestruction of
MnOH2 (Mn + H2O experiments) or (H2)MnO (MnO2 + H2
experiments) implies that this molecule is generated from
MnOH2 or (H2)MnO The 1663.4 cm-1 band showed no oxygen-
18 shift, but shifted to 1197.8 cm-1 with D2O sample. The
isotopic H/D ratio of 1.3887 indicates that this band is due to
a Mn-H stretching vibration. The 648.6 cm-1 band shifted to
628.7 cm-1 with D2O and to 624.3 cm-1 with H218O, which
define a H/D ratio of 1.0317 and a 16O/18O ratio of 1.0389.
Isotopic substitution showed that this band is Mn-OH in
vibrational character. The mixed H2O + HDO+D2O and H216O
+ H218O isotopic spectra revealed that only one H atom is
involved in the upper mode and one OH is involved in the low
mode. These two absorptions are assigned to the Mn-H and
Mn-OH stretching vibrations of the HMnOH molecule. The
assignment is in good agreement with previous thermal atom
experiments.2 The HMnOH was predicted to have a 6Σ+ ground
state, with configuration of (core)σ2π4δ2σ2π2σ1, where the five
unpaired electrons are mainly of Mn 3d-based character. The
geometry of HMnOH ground state is predicted to be linear with
H-Mn, Mn-O and O-H bond lengths of 1.660, 1.814 and
0.953 Å, respectively. The vibrational frequencies of Mn-H
and Mn-OH stretching modes were calculated to be 1729 and
691 cm-1, which require scaling factors of 0.962 and 0.939 to
fit the experimental values.
TABLE 5: The Scaling Factors and the Observed and
Calculated Isotopic Vibrational Frequency Ratios of the
Major Reaction Products
H/D
obsd calcd
16O/18O
scaling
factor
molecule
MnOH2
modea
obsd calcd
OH2 bend
Mn-H
Mn-OH
asy-Mn-H
0.988 1.3521 1.3635 1.0042 1.0044
0.962 1.3887 1.4140 1.0000 1.0000
0.939 1.0317 1.0258 1.0389 1.0430
0.964 1.3947 1.4023 1.0000 1.0000
HMnOH
HMnOMnH
asy-MnOMn 1.025 1.0002 1.0002 1.0518 1.0525
Mn(OH)2
HMnOMnOH Mn-H
asy-Mn-OH 0.966 1.0222 1.0214 1.0316 1.0324
0.967 1.3907 1.4016 1.0000 1.0000
asy-MnOMn 1.023 1.0002 1.0002 1.0510 1.0514
a sym ) symmetric; asy ) asymmetric, bend ) bending.
geometries are shown in Figure 8, and the vibrational frequen-
cies and intensities are summarized in Table 4.
Detailed structural parameters, absolute energies, vibrational
frequencies for different spin state molecules and atomic spin
density on Mn centers for the ground-state molecules are given
as Supporting Information.
Product Identification. MnOH2. The 1576.6 cm-1 band in
the Mn + H2O/Ar experiments is assigned to the MnOH2
complex, in agreement with previous assignment.2 This band
produced a triplet with H2O + HDO+D2O sample and a doublet
with H216O + H218O sample as required for a product with one
H2O unit. The isotopic H/D ratio of 1.3521 and 16O/18O ratio
of 1.0042 indicate a H2O bending vibration. DFT calculations
on MnOH2 predicted that the molecule have a sextet ground
HMnOMnH. The 870.5 cm-1 band in the Mn + H2O
experiments was produced only on photolysis. This band
exhibited H218O counterpart at 827.6 cm-1. In the mixed H216O
+ H218O experiments, only pure isotopic counterparts were
observed, indicating that only one O atom is involved in this
mode. The 16O/18O ratio of 1.0518 is quite higher than the
diatomic MnO ratio of 1.0460, suggesting an antisymmetric
MnOMn stretching mode. This band showed 0.2 cm-1 deute-
rium isotopic shift, which indicates that this mode is slightly
coupled by H atoms. The 870.5 cm-1 band is associated with a
state (6A′ under Cs symmetry) that correlated to the Mn S
6
ground state. The calculated H2O bending frequency, 1597 cm-1
,
is in quite good agreement with the experimental value. As
shown in Table 5, the calculated isotopic frequency ratios agree
well with the experimental values.
(H2)MnO. The 849.7 cm-1 band in the laser ablation of MnO2
and H2 experiments increased on annealing, but was destroyed
on photolysis when the HMnOH absorptions were produced.
This band was not observed in the experiment of laser ablation

Reactions of Mn with H2O and MnO with H2
J. Phys. Chem. A, Vol. 105, No. 24, 2001 5805
1638.5 cm-1 band. This band underwent a large deuterium shift,
but no apparent oxygen shift. In the mixed H2O + HDO+D2O
experiment, two intermediates at 1641.8 and 1179.5 cm-1 were
produced, the mixed isotopic spectra are typical for linear
H-M-H type molecules. Accordingly, the 870.5 and 1638.5
cm-1 bands are assigned to the antisymmetric MnOMn and
H-MnOMn-H stretching vibrations of the HMnOMnH mol-
ecule. The 1641.8 and 1179.5 cm-1 bands in the mixed
experiment are due to H-Mn and D-Mn stretching vibrations
of the HMnOMnD molecule.
indicating that only one H atom is involved in this vibrational
mode. The 884.8 cm-1 band underwent a very small deuterium
shift (0.2 cm-1), but shifted to 841.9 cm-1 with H218O, and
gave an isotopic 16O/18O ratio of 1.0510. This ratio suggests an
antisymmetric MnOMn stretching mode analogous to the 870.5
cm-1 band. In light of the above, assignment to HMnOMnOH
is suggested. The bonding of HMnOMnOH molecule is quite
similar to the HMnOMnH molecule. DFT calculations predicted
that the linear HMnOMnOH molecule has a 11Σ+ ground state,
but has an imaginary frequency. Therefore, we search for
minimum energy structure in bent geometry, as shown in Figure
8, the bent structure with an MnOH angle of 137.8° is about
0.3 kcal/mol lower in energy than the linear structure and has
no imaginary frequency. As the potential surface for bending
of the MnOH unit is very flat, it is difficult to determine whether
this molecule is linear or not. The Mn-H and antisymmetric
MnOMn stretching vibrations for the 11A′ HMnOMnOH
molecule were predicted at 1701 and 865 cm-1. The MnOH
stretching vibration was calculated at 673 cm-1 with one-tenth
intensity of the antisymmetric MnOMn stretching mode, and is
under noise level. The calculated isotopic shifts are also in very
good agreement with experimental values (see Table 5).
(HMnOMnOMnH). The 1627.7 and 875.1 cm-1 bands were
presented only at higher laser power and medium water
concentration experiments. These two bands appeared together
on photolysis. The mixed isotopic structures could not be
resolved due to isotopic dilution. These two bands are tentatively
assignedtotheantisymmetricH-MnOMnOMn-HandO-Mn-O
stretching vibrations of the HMnOMnOMnH molecule. Our
As ground-state Mn has 3d54s2 valence electron configuration,
one can draw a convenient Lewis structure that satisfies the
valence of H and O:
H-.M. . n. .-O-.M. . n. .-H
In this structure, the 4s electrons of Mn are used to form covalent
bonds with H and O atoms, and the Mn 3d electrons remain
unpaired. This suggests that HMnOMnH should have a ground
state with S ) 10/2. Consistent with this simple formulation,
our DFT calculations predicted that the molecule has a
+
11Σµ ground state with linear geometry, and the 10 unpaired
metal-based electrons occupying the nonbonding or antibonding
dπ, dδ, and dσ orbitals. The antisymmetric MnOMn and
H-MnOMn-H stretching vibrations of the ground-state mol-
ecule were calculated at 850 and 1700 cm-1, with 794:575
relative intensities. As listed in Table 5, the calculated isotopic
frequency ratios are in excellent agreement with the observed
values.
DFT calculations predicted the HMnOMnOMnH to have a 16Σµ
+
Mn(OH)2. The 709.0 cm-1 band appeared on photolysis, and
is favored by slightly higher water concentrations. It shifted to
693.6 cm-1 with D2O and to 687.3 cm-1 with H218O, giving a
H/D ratio of 1.0222 and a 16O/18O ratio of 1.0316. The mixed
H2O + HDO + D2O and H216O + H218O isotopic spectra
revealed triplet isotopic structure and clearly indicate that two
equivalent O and H atoms are involved in this mode. The
isotopic shifts and mixed isotopic splittings suggest an assign-
ment to the Mn(OH)2 species. The above assignment is
confirmed by DFT calculations. As shown in Figure 8, the Mn-
ground state with linear geometry. The bonding of this molecule
is quite similar to the HMnOMnH and HMnOMnOH molecules.
The 4s electrons of Mn atoms are used to form covalent bonds
to satisfy the valence of H and O atoms, and the unpaired
electrons are located on the three Mn centers. The two
vibrational modes were calculated at 1695 and 849 cm-1
.
Reaction Mechanism. Laser-ablated Mn atoms co-deposited
with water to form the MnOH2 complex at low temperature,
and the MnOH2 absorption increased on annealing, suggesting
that reaction 1 requires no activation energy. The MnOH2
absorption was destroyed on photolysis. The increase of
HMnOH absorptions upon photodestruction of the MnOH2
implies that the HMnOH molecule is generated from MnOH2
via reaction 2, which was predicted to be exothermic by about
27.2 kcal/mol.
6
(OH)2 is predicted to have a A ground state, with the five
unpaired electrons mainly located on the Mn center which are
mainly Mn 3d in character. The ground state is predicted to be
nonlinear ( OMnO ) 173.6° and MnOH ) 137.8°); the two
H atoms are slightly out of OMnO plane. The linear geometry
is predicted to be slightly higher in energy and has one
imaginary frequency. The antisymmetric and symmetric Mn-
OH stretching vibrations in the ground-state molecule were
calculated at 734 and 608 cm-1 with 237:6 relative intensities.
The symmetric stretching vibrational frequency is lower than
that of the antisymmetric vibration and is not observed in our
experiments due to weakness. The calculated isotopic frequency
ratios of the observed antisymmetric stretching mode (H/D:
1.0214, 16O/18O:1.0324) are in excellent agreement with the
experimental values.
HMnOMnOH. The 884.8 and 1644.4 cm-1 bands were
produced upon photolysis, these two bands showed exactly the
same annealing behavior and maintained the same relative
intensities in different experiments, suggesting different vibra-
tional modes of the same molecule. These bands were favored
with slightly higher water concentrations and higher laser power.
The 1644.4 cm-1 band shifted to 1182.4 cm-1 with D2O, but
no apparent shift with H218O, and is assigned to a Mn-H
stretching mode. In the mixed H2O + HDO+D2O isotopic
spectra, no obvious intermediate absorption was observed,
Mn(6S) + H2O(1A1) f MnOH2(6A′)
∆E ) -34.6 kcal/mol (1)
MnOH2(6A′) f HMnOH(6Σ+)
∆E ) -27.2 kcal/mol
(2)
The reactions of early transition metal atoms with water
molecules have been recently studied in this laboratory.25-27
For Sc, Ti, and V, the inserted HMOH molecules were produced
spontaneously, and broadband photolysis produced the co-
valently bonded H2MO molecules as well as the MO monoxides.
In present Mn + H2O reaction, no H2MnO molecule was
produced. Although the MnO molecule absorption was observed,
the MnO absorption did not enhance on photolysis. In the H218O
experiments, no Mn18O absorption was observed, indicating that
the observed MnO molecule was not the product of water
reaction.
Although dehydrogenation process is not observed for Mn
+ H2O reaction, co-deposition of laser-ablated MnO with H2
formed the (H2)MnO spontaneously. On photolysis, the HMnOH

5806 J. Phys. Chem. A, Vol. 105, No. 24, 2001
Zhou et al.
distances were predicted to be quite long (2.050 Å). This
minimum has no Mn-H covalent bonding character, and should
be considered as a complex. The Mn-O and H-H bond lengths
in this complex are about 0.003 and 0.019 Å longer than that
of diatomic MnO and H2.
The transition state 2 lies 26.1 kcal/mol higher in energy than
the separated Mn + H2O reactants. Reaction from HMnOH to
(H2)MnO is predicted to be endothermic by about 48.3 kcal/
mol; there is about 87.9 kcal/mol energy barrier. Apparently,
this reaction is hardly to be observed in present experimental
conditions. In contrast, the reverse reaction: (H2)MnO f
HMnOH is exothermic, and the energy barrier is only about
39.6 kcal/mol. This reaction is observed upon photolysis in the
MnO + H2 reaction system.
In the Mn + H2O reaction system, the HMnOMnH absorp-
tions appeared on photolysis, suggesting that Mn atoms can
further react with HMnOH via reaction 5. This reaction was
predicted to be exothermic by about 65.3 kcal/mol.
Figure 9. Potential energy surface following the Mn + H2O f MnO
+ H2 reaction path. Energies given are in kcal/mol and are relative to
the separated ground-state reactants: Mn(6S) + H2O(1A1).
absorptions were produced at the expense of the (H2)MnO.
Certainly the growth of the (H2)MnO molecule on annealing
suggests a spontaneous reaction 3, although this reaction was
predicted to be exothermic by only about 1.3 kcal/mol. The
isomerization reaction 4 was predicted to be exothermic by about
48.3 kcal/mol, this reaction requires activation energy.
HMnOH(6Σ+) + Mn(6S) f HMnOMnH(11Σµ
)
+
∆E ) -65.3 kcal/mol (5)
The Mn(OH)2 absorption was also produced on photolysis, this
molecule is most probably formed by reaction of Mn atom and
water dimer via reaction 6. This reaction was calculated to be
exothermic by about 82 kcal/mol.
MnO(6Σ+) + H2(1Σg+) f (H2)MnO(6A1)
∆E ) -1.3 kcal/mol (3)
(H2)MnO(6A1) f HMnOH(6Σ+)
∆E ) -48.3 kcal/mol (4)
Mn(6S) + (H2O)2 f Mn(OH)2(6A) + H2(1Σg )
+
∆E ) -82.0 kcal/mol (6)
Figure 9 shows the potential energy surface starting from the
separated Mn + H2O and leading to the MnO + H2 for the
sextet state at the B3LYP/6-311++G(d,p) level of theory. The
first step of Mn and water interaction is the formation of the
MnOH2 complex. The MnOH2 complex has a 6A′ ground state
with nonplanar geometry. Becausethe separation between the
3d54s2 ground state and the 3d64s1 excited state of Mn atom is
very large (49 kcal/mol),42 the 4s-3dσ hybridization to reduce
the σ repulsion is not possible. The only mechanism for Mn to
reduce the repulsion is by 4sp polarization away from water.
Apparently, this is not very efficient, thus that water tilts to
further reduce the Mn-water lone pair repulsion.
From MnOH2 complex, one hydrogen atom is passed from
oxygen to the Mn, leading to the HMnOH intermediate through
TS1. This process involves the breaking of one H-O bond,
and the formation of a Mn-H bond and a Mn-O bond by
overcoming an energy barrier of 24.5 kcal/mol. The TS1
transition state lies 10.1 kcal/mol below the energy of the
ground-state reactants. Geometric parameters of this transition
state can be found in Figure 7. This transition state has nonplanar
geometry, and the one imaginary frequency reflects the H-O
bond breaking and the Mn-H bond formation. The IRC
calculation also indicates that TS1 connects the MnOH2 complex
and the HMnOH molecule on the potential energy surface.
From linear HMnOH, the second hydrogen transfers from
oxygen to form the (H2)MnO through transition state 2. This
process involves the breaking of the Mn-H and O-H bonds,
and the formation of a H-H bond and a Mn-O π bond. The
geometric parameters of transition state 2 are also given in
Figure 7. This transition state shows quite long Mn-O bond
distance (1.885 Å), suggesting a Mn-O single bond. The two
Mn-H bond distances 1.581 and 1.632 Å indicate that both
Mn-H are mainly covalent bonds. From the (H2)MnO, the loss
of H2 proceeds without transition state to the MnO + H2. This
(H2)MnO intermediate has planar C2V symmetry; the Mn-H
In higher laser power experiments, photolysis also produced
the HMnOMnOH and possibly the HMnOMnOMnH absorp-
tions, suggesting that Mn atoms can further react with Mn(OH)2
molecules via reactions 7 and 8, which were predicted to be
quite exothermic:
Mn(OH)2(6A) + Mn(6S) f HMnOMnOH(11A′)
∆E ) -66.0 kcal/mol (7)
HMnOMnOH(11A′) + Mn(6S) f HMnOMnOMnH(16Σµ
)
+
∆E ) -65.8 kcal/mol (8)
In previous thermal Mn atom and water reaction studies, only
the HMnOH and HMnOMnH molecules were produced on
photolysis with 300-340 nm using a 100W medium-pressure
mercury lamp.2 To have a better understanding of the photolysis
behavior, experiments were performed with different wavelength
range high-pressure mercury lamp photolysis. The HMnOH
molecules were produced after 20 min photolysis with a 400
nm long-wavelength pass filter. The Mn(OH)2, HMnOMnH and
HMnOMnOH absorptions were produced on photolysis with λ
> 290 nm photolysis, and the HMnOMnOMnH absorptions
were mainly produced on full arc photolysis.
The results on Mn + H2O system are different from the early
transition metal Sc, Ti and V +H2O systems.25-27 As Sc has
only three valence electrons, the only exothermic reaction path
observed is the formation of ScO + H2. For Ti and V systems,
two exothermic reaction paths have been observed: the forma-
tion of H2MO and MO + H2. (M ) Ti, V). Although the Mn
+ H2O f MnO + H2 reaction was predicted to be slightly
exothermic (-12.2 kcal/mol), the MnO was not formed, as proof
of the existence of the high energy barrier for the transformation
of HMnOH to (H2)MnO. The H2MnO was not formed either.
Our calculations showed that the ground state of H2MnO is a
quartet state, and is higher in energy than the sextet (H2)MnO.

Reactions of Mn with H2O and MnO with H2
J. Phys. Chem. A, Vol. 105, No. 24, 2001 5807
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Conclusions
The reactions of Mn atoms with water and MnO with H2
have been studied using matrix-isolation FTIR and DFT
theoretical calculations. The MnOH2, HMnOH and (H2)MnO
intermediates in the Mn + H2O S MnO + H2 reaction path
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aforementioned species were identified via isotopic substitutions
as well as density functional theoretical frequency calculations.
Scheme 1 summarizes the observed chemistry:
The spectroscopic studies of these molecules in conjunction
with DFT calculations underscore the palpable differences
between the Mn chemistry and the analogous chemistry with
early transition metals. This work also provides a good example
in demonstrating the significance of the combination of experi-
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Acknowledgment. We acknowledge support for this re-
search from NSFC (Grant 20003003) and the Chinese NKBRSF,
and Mr. J. Dong and H.Lu for assistance with experiments.
Supporting Information Available: Detailed structural
parameters, absolute energies, vibrational frequencies for dif-
ferent spin state molecules, and atomic spin density on Mn
centers for the ground-state molecules. This material is available
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