Heavy water reactions with atomic transition-metal and main-group cations: Gas phase room-temperature kinetics and periodicities in reactivity

Article abstract of DOI:10.1021/jp072661p

Reactions of heavy water, D₂O, have been measured with 46 atomic metal cations at room temperature in a helium bath gas at 0.35 Torr using an inductively coupled plasma/selected ion flow tube tandem mass spectrometer. The atomic cations were produced at ca. 5500 K in an ICP source and were allowed to decay radiatively and thermalize by collisions with Ar and He atoms prior to reaction. Rate coefficients and product distributions are reported for the reactions of fourth-row atomic cations from K⁺ to Se⁺, of fifth-row atomic cations from Rb⁺ to Te⁺ (excluding Tc⁺), and of sixth-row atomic cations from Cs⁺ to Bi ⁺. Primary reaction channels were observed leading to O-atom transfer, OD transfer, and D₂O addition. O-Atom transfer occurs almost exclusively (≥90%) in the reactions with most early transition-metal cations (Sc⁺, Ti⁺, V⁺, Y⁺, Zr ⁺, Nb⁺, Mo⁺, Hf⁺, Ta⁺, and W⁺) and to a minor extent (10%) with one main-group cation (As⁺). OD transfer is observed to occur only with three cations (Sr⁺, Ba⁺, and La⁺). Other cations, including most late transition and main-group cations, were observed to react with D ₂O exclusively and slowly by D₂O addition or not at all. O-Atom transfer proceeds with rate coefficients in the range of 8.1 × 10^-13 (As⁺) to 9.5 × 10^-10 (Y ⁺) cm³ molecule^-1 s^-1 and with efficiencies below 0.1 and even below 0.01 for the fourth-row atomic cations V⁺ (0.0032) and As⁺ (0.0036). These low efficiencies can be understood in terms of the change in spin required to proceed from the reactant to the product potential energy surfaces. Higher order reactions are also measured. The primary products, NbO⁺, TaO⁺, MoO ⁺, and WO⁺, are observed to react further with D ₂O by O-atom transfer, and ZrO⁺ and HfO⁺ react further through OD group abstraction. Up to five D₂O molecules were observed to add sequentially to selected M⁺ and MO⁺ as well as MO₂⁺ cations and four to MO₂D ⁺. Equilibrium measurements for sequential D₂O addition to M⁺ are also reported. The periodic variation in the efficiency (k/k_c) of the first addition of D²O appears to be similar to the periodic variation in the standard free energy (ΔG°) of hydration.

Full text of DOI:10.1021/jp072661p

J. Phys. Chem. A 2007, 111, 8561-8573
8561
Heavy Water Reactions with Atomic Transition-Metal and Main-Group Cations: Gas
Phase Room-Temperature Kinetics and Periodicities in Reactivity
Ping Cheng, Gregory K. Koyanagi, and Diethard K. Bohme*
Department of Chemistry, Centre for Research in Mass Spectrometry and Centre for Research in Earth and
Space Science, York UniVersity, Toronto, Ontario, Canada, M3J 1P3
ReceiVed: April 4, 2007; In Final Form: June 20, 2007
Reactions of heavy water, D2O, have been measured with 46 atomic metal cations at room temperature in a
helium bath gas at 0.35 Torr using an inductively coupled plasma/selected ion flow tube tandem mass
spectrometer. The atomic cations were produced at ca. 5500 K in an ICP source and were allowed to decay
radiatively and thermalize by collisions with Ar and He atoms prior to reaction. Rate coefficients and product
distributions are reported for the reactions of fourth-row atomic cations from K+ to Se+, of fifth-row atomic
cations from Rb+ to Te+ (excluding Tc+), and of sixth-row atomic cations from Cs+ to Bi+. Primary reaction
channels were observed leading to O-atom transfer, OD transfer, and D2O addition. O-Atom transfer occurs
almost exclusively (g90%) in the reactions with most early transition-metal cations (Sc+, Ti+, V+, Y+, Zr+,
Nb+, Mo+, Hf+, Ta+, and W+) and to a minor extent (10%) with one main-group cation (As+). OD transfer
is observed to occur only with three cations (Sr+, Ba+, and La+). Other cations, including most late transition
and main-group cations, were observed to react with D2O exclusively and slowly by D2O addition or not at
all. O-Atom transfer proceeds with rate coefficients in the range of 8.1 × 10-13 (As+) to 9.5 × 10-10 (Y+)
cm3 molecule-1 s-1 and with efficiencies below 0.1 and even below 0.01 for the fourth-row atomic cations
V+ (0.0032) and As+ (0.0036). These low efficiencies can be understood in terms of the change in spin
required to proceed from the reactant to the product potential energy surfaces. Higher order reactions are also
measured. The primary products, NbO+, TaO+, MoO+, and WO+, are observed to react further with D2O by
O-atom transfer, and ZrO+ and HfO+ react further through OD group abstraction. Up to five D2O molecules
were observed to add sequentially to selected M+ and MO+ as well as MO2 cations and four to MO2D+.
+
Equilibrium measurements for sequential D2O addition to M+ are also reported. The periodic variation in the
efficiency (k/kc) of the first addition of D2O appears to be similar to the periodic variation in the standard free
energy (∆G°) of hydration.
1. Introduction
both in the absence and in the presence of benzene,2 and with
hexafluorobenzene,3 N2O,4 CO2,5 CS2,6 and CH3F.7 Here, we
There is much interest in fundamental aspects of the chemistry
of atomic cations, especially atomic metal cations. Bare and
ligated atomic cations generally play an important role in
physics, chemistry, and biology. Water, of course, is practically
omnipresent and so is an obvious molecular target for atomic
cations found in natural environments such as the earth’s
ionosphere but also in man-made devices such as mass
spectrometers designed to handle atomic cations in which water
is often present as a background impurity.
In the mass spectrometer developed in our own laboratory,
atomic cations can be generated almost universally in an
inductively coupled plasma (ICP) ion source, injected into a
flow tube flushed with helium gas at room temperature, and
then reacted with water vapor. With the addition of mass
selection of the ions entering and emerging from the flow
tube, the resulting inductively coupled selected ion flow tube
(ICP/SIFT) tandem mass spectrometer allows for surveys of
trends in the room-temperature chemical kinetics of reactions
with atomic cations across and down the periodic table. For
example, reactions of fourth-, fifth-, and sixth-row atomic
cations have recently been surveyed with molecular oxygen,1
report the results obtained for reactions of atomic cations with
D2O (D2O was chosen instead of H2O to relax the resolution
requirements in the downstream mass spectrometer and to
distinguish between added and background water).
The first gas phase reactions of atomic metal cations measured
with D2O were those of Na+ and K+ reported in 1971 by
Biondi’s group.8 The metal cations were produced with a hot
filament coated with the oxide of the alkali metal, and rate
coefficients for their reactions were measured in a drift tube
mass spectrometer over an energy range from thermal energies
to 5 eV (mean kinetic energy). This work was followed more
than 15 years later by various studies of the interactions of water
with other atomic metal cations, especially atomic transition-
metal cations.9-16
The thermal energy reaction of Ti+ with H2O has been
investigated with a drift tube and a fast-flow reactor with He
as the buffer gas in the pressure range from 0.2 to 0.7 Torr.12,16
The reactions of fourth-row atomic transition-metal cations of
heavy water were studied systematically as a function of
translational energy with a guided ion beam tandem mass
spectrometer by Armentrout et al.10 Experimental results were
obtained for reactions with Sc+ and Ti+,10 Fe+,17 V+,18 and
Co+.19 In these measurements, the atomic cations were produced
* Corresponding author. E-mail: dkbohme@yorku.ca; phone: (416) 736-
2100, ext 66188; fax: (416) 736-5936.
10.1021/jp072661p CCC: $37.00 © 2007 American Chemical Society
Published on Web 08/16/2007

8562 J. Phys. Chem. A, Vol. 111, No. 35, 2007
Cheng et al.
with a variety of ion sources including surface ionization,
electron impact, and flow tube ion sources, in part to try to
discern the influence of electronic excitation of the atomic
cations on reactivity. The ground-state reactions of Sc+, Ti+,
and V+ were shown to be exothermic at thermal energy, whereas
the reactions with Fe+ and Co+ exhibited endothermic behavior.
These authors also investigated the solvation of metal cations
by water and reported sequential bond energies for M+(D2O)n
with n ) 1-4 and M ) Na+, Mg+, or Al+20 and transition-
metal cations from Ti+ to Cu+.21 Measurements of the energetics
of hydration of alkali ions also were reported by Kebarle et al.,
who used high-pressure ion source (HPIS) mass spectrometry.22-24
Systematic theoretical investigations using ab initio calculations
designed to understand the reactivity of transition-metal cations
with water have been reported by Ugalde et al.25 These authors
have addressed quantum mechanical constraints introduced by
spin-forbidden crossings of the potential energy surfaces that
are involved in the formation of oxide cations in the reactions
of water with Sc+ and V+,26 Cr+,27 Mn+,27 Fe+,27,28 Co+, Ni+,
and Cu+.25 Similar surface crossings have been predicted very
recently for the reaction of W+ with water.29 As far as we are
aware, with the exception of the reaction with Hf+,30 none of
the reactions of water with the atomic metal cations from the
fifth and sixth row of the periodic table appears to have been
studied previously.
In the measurements reported here, fourth-period atomic
cations from K+ to Se+, fifth-period atomic cations from Rb+
to Te+ (excluding Tc+), and sixth-period atomic cations from
Cs+ to Bi+ were generated in one source, an ICP, which has
been found to be suitable for virtually any element on the
periodic table (with the exception of ions with a mass less than
Ar).31
Higher-order chemistry initiated by the primary reactions of
the atomic metal cations also can be monitored with the
ICP/SIFT technique. In the experiments reported here with D2O,
the primary MO+ product ions reacted further with D2O to form
MO2+ or MO2D+ or simply hydrate with D2O. These observa-
tions of higher-order chemistry also will be reported.
Mo+, Ru+, Hf+, Ta+, W+, and Pt+ having intermediate
distributions at 5500 K, with Os+ not known.
The ions emerging from ICP are injected through a differ-
entially pumped sampling interface into a quadrupole mass
spectrometer and, after mass analysis, introduced through an
aspirator-like interface into flowing helium carrier gas at 0.35
( 0.01 Torr and 295 ( 2 K. The ions are allowed to react with
added D2O a few milliseconds downstream of the flow tube.
After extraction from the ICP, the plasma ions may experience
both radiative electronic-state relaxation and collisional electronic-
state relaxation. The latter may occur already with argon as the
extracted plasma cools upon sampling and then by collisions
with He atoms in the flow tube (ca. 4 × 105 collisions with
He) prior to the reaction region, but the actual extent of
electronic relaxation (either radiative or collisional) is not known
and is difficult to assess. Almost all of the electronic states of
the transition-metal ions have positive parity; electric dipole
transitions between states of the same parity are forbidden
(Laporte rule).37 This means that radiative transitions between
different states in metal cations can be achieved only by either
magnetic dipole or electric quadrupole radiation. The prob-
abilities for these transitions are very low, and the resulting
radiative lifetimes are of the order of seconds or larger. The
time interval in the ICP-SIFT experiments between the exit of
the ICP source and the entrance in the reaction region is ca. 20
ms, and therefore, no major modification of state distributions
can occur in this time interval by forbidden radiative decay.
That having been said, there were no indications of excited-
state effects in our previous measurements of reactions of N2O
with atomic cations derived from the same ICP source operated
in the same manner, except with Pt+.4 The many collisions
experienced by the atomic cations with the quite polarizable
Ar atoms as they emerge from the ICP and the ca. 4 × 105
collisions with He atoms in the flow tube (the helium buffer
gas pressure was 0.35 ( 0.01 Torr) appear to be sufficient to
provide for the thermalization of the excited states and ensure
that the atomic ions reach a translational temperature equal to
the tube temperature of 295 ( 2 K prior to entering the reaction
region. However, the exact extent of electronic relaxation is
uncertain. Clues to the presence of excited electronic states of
the atomic ions in the reaction region can be found in the product
ions observed and the shape of the semilogarithmic decay of
the reacting atomic ion upon the addition of a neutral reactant.
Curvature will appear in the measured atomic ion decay when
the ground state and excited state react at different rates even
when they give the same product ions. An excited-state effect
cannot be seen when the products and reaction rates are the
same for both the ground and the excited states, but in this case,
the measured atomic ion decay defines the ground-state kinetics.
Our growing experience has shown that excited states can reveal
themselves when the ground state of the atomic ion reacts only
slowly by termolecular addition and when excited states react
rapidly in a bimolecular fashion. Our previous studies indicate
that that the collisions with Ar and He ensure that most atomic
ions reach a translational and internal temperature equal to the
tube temperature of 295 ( 2 K prior to entering the reaction
region.1-6,38,39
2. Experimental Procedures
The experimental results reported here were obtained with
the SIFT tandem mass spectrometer described in detail else-
where.32,33 This instrument was modified to accept ions gener-
ated in an ICP torch (ELAN series, PerkinElmer SCIEX)
through an atmosphere-vacuum interface. The ICP ion source
and interface have also been described previously,31,34 as has
the preparation of the solutions containing the atomic salt of
interest that were peristaltically pumped via a nebulizer into
the plasma.5
Atomic ions emerge from the ICP at a nominal ion temper-
ature of 5500 K with corresponding Boltzmann state distribu-
tions. These distributions have been derived from available
optical spectra35,36 and reported by us previously for the two
electronic spin states with the highest population at 5500 K.4
Energy levels as high as 3.7 eV (3 × 104 cm-1) were included
in the calculations. The calculations show that excited states of
the main-group elemental cations except Ba+ are high in energy
and contribute little (never more than 10%) to the total ion
2
population at 5500 K. The ground S state of Ba+ contributes
Reactant and product ions were sampled still further down-
stream with a second quadrupole mass spectrometer and were
measured as a function of added reactant. The resulting profiles
provide information about product ion distributions, reaction
rate coefficients, and reaction molecularity. Rate coefficients
for the primary bimolecular ion-molecule reactions are deter-
mined from the rate of decay of the reactant ion intensity with
44% and the excited 2D state 55% at 5500 K. The state
distributions are more variable for the transition-metal cations.
The excited states at 5500 K contribute 20% or less toward the
populations of Cr+, Mn+, Ni+, Cu+, Zn+, Rh+, Pd+, Ag+, Cd+,
Re+, Au+, and Hg+ and 50% or more toward the populations
of Ti+, Y+, Zr+, Nb+, La+, and Ir+ with Sc+, V+, Fe+, Co+,

Heavy H2O Reactions with Cations
J. Phys. Chem. A, Vol. 111, No. 35, 2007 8563
TABLE 1: O-Atom Affinities (D0(M+-O) (kcal mol-1)) and Ionization Energies (IE(M) (eV))40 for Fourth-, Fifth-, and
Sixth-Row Atomic Cations
M+
OA(M+)
IE(M)
M+
OA(M+)
IE(M)
M+
OA(M+)
IE(M)
K+
3a
4.34
6.11
6.56
6.83
6.75
6.77
7.43
7.90
7.88
7.64
7.72
9.39
6.00
7.90
9.81
9.75
Rb+
Sr+
7a
4.18
5.70
6.22
6.63
6.76
7.09
Cs+
Ba+
La+
Hf+
Ta+
W+
Re+
Os+
Ir+
14a
3.89
5.21
5.58
6.83
7.55
7.86
7.83
8.44
8.97
8.96
9.23
10.44
6.11
7.42
7.29
Ca+
Sc+
Ti+
77.2a
71.4a
92.8a
164.6 ( 1.4b
158.6 ( 1.6b
134.9 ( 3.5b
85.8 ( 2.8b
68.0 ( 3.0b
80.0 ( 1.4b
74.9 ( 1.2b
63.2 ( 1.2b
37.4 ( 3.5b
38.5 ( 1.2b
5.6a
Y+
167.0 ( 4.2c
178.9 ( 2.5c
164.4 ( 2.5c
116.7 ( 0.5c
206 ( 4d
173 ( 5a
188 ( 15a
166 ( 10f
115 ( 15g
100 ( 12h
59a
Zr+
Nb+
Mo+
Tc+
Ru+
Rh+
Pd+
Ag+
Cd+
In+
V+
Cr+
Mn+
Fe+
Co+
Ni+
Cu+
Zn+
Ga+
Ge+
As+
Se+
87.9 ( 1.2e
69.6 ( 1.4e
33.7 ( 2.5e
28.4 ( 1.2e
7.36
7.46
8.34
7.58
8.99
5.79
7.34
8.64
9.01
Pt+
77,i 75 ( 2j
Au+
Hg+
Tl+
Pb+
Bi+
Po+
81.8a
Sn+
Sb+
Te+
75.1a
96.6a
53.2a
41.6a
147 ( 2k
92k
a Reference 51. b Reference 52. c Reference 53. d Reference 55. e Reference 54. f Reference 59. g Reference 56. h Reference 57. i Reference 58.
j Reference 60. k Reference 4.
TABLE 2: OH Affinities of Bare Metal Cations
TABLE 4: Sequential Binding Energies for M+(H2O)n (n e
4) (kcal mol-1 a
(D0(M+-OH) (kcal mol-1))a
)
M+ D0(M+-OH) M+ D0(M+-OH)
M+
D0(M+-OH )
M+
1
2
3
4
Ca+
106
Sc+
Ti+
V+
87.8
113
107
74.3
Mn+
Fe+
Co+
Ni+
82
K+
17.9b
28c
16.1b
24c
13.2b
21.5c
11.8b
18.7c
Sr+
105 ( 5b
127 ( 7b
30.4
85.3
72.2
42.2
Ca+
Sc+
Ti+
Ba+
Zn+
34.5d
31.4d
Cr+
37.7 ( 1.4
35.8 ( 1.2
31.7 ( 2.1
29.1 ( 1.4
31.5 ( 1.2
39.3 ( 1.4
43.9 ( 0.8
38.4 ( 1.8
32.5d
32.6 ( 1.2
36.0 ( 2.3
34.0 ( 1.8
21.5 ( 1.2
39.3 ( 1.0
38.8 ( 1.6
40.2 ( 1.8
40.7 ( 1.6
22.2d
16.6 ( 1.6
16.2 ( 1.1
12.1 ( 1.2
25.9 ( 1.4
18.3 ( 0.9
15.5 ( 1.1
16.2 ( 1.5
13.7 ( 1.8
19.9 ( 1.8
16.0 ( 1.8
12.1 ( 1.4
11.8 ( 1.2
11.8 ( 1.6
13.7 ( 1.4
12.3 ( 1.5
12.8 ( 1.0
V+
a With a few exceptions, the values for D0(M+-OH) were taken
from ref 9. b Determined form ∆Hf° found in ref 51.
Cr+
Mn+
Fe+
Co+
Ni+
Cu+
Zn+
Rb+
Sr+
TABLE 3: H-Atom Affinities (HA(M+)) of Bare Metal
Cations (HA(M+) ) D0(M+-H) (kcal mol-1))a
M+
HA(M+)
M+
HA(M+)
50.9b
61.2 ( 1.9
52.1 ( 1.9
M+
HA(M+)
15.2e
13.5e
12.8e
12.8e
Ba+
Y+
34.5f
30.5f
25.7f
22.3f
Sc+
Ti+
56.2 ( 2.2
53.3 ( 2.6
La+
Hf+
57.1 ( 2.2
54.9b
Cs+
Ag+
Au+
Pb+
13.8e
11.8e
12.0e
14.7e
Zr+
32.0 ( 1.9g
40.1 ( 2.3h
22.4f
30.4 ( 1.9g
45.0 ( 2.5h
16.9f
15.1 ( 2.2g
23.1 ( 4.6h
12.2f
12.1 ( 2.9g
20.8 ( 4.6h
10.8f
45.4 ( 2.5c
52.3 ( 1.2
55.0 ( 1.4
44.5b
V+
47.3 ( 1.4
31.5 ( 2.2
47.6 ( 3.4
Nb+
Mo+
Tc+
52.6 ( 1.7
39.7 ( 1.4
50.7d
Ta+
W+
Re+
Cr+
Mn+
a Unless indicated otherwise, the values for the binding energies of
M+(H2O)n (n e 4) are taken from ref 21. b Reference 22. c Reference
68. d Reference 69. e Reference 70. f Reference 71. g Reference 72.
h Reference 73.
52.8 ( 1.6e
56.2b
Fe+
48.8 ( 1.4
45.7 ( 1.4
Ru+
Rh+
37.3 ( 1.2
38.5 ( 1.0
Os+
Ir+
Co+
65.8b
71.9 ( 1.4f
64.8 ( 1.2
33.4b
Ni+
38.7 ( 1.9
21.0 ( 3.1
Pd+
47.8 ( 1.0
9.6 ( 1.4
Pt+
Cu+
Ag+
Au+
ratio of the concentration of the adduct ion over that of the bare
ion against the flow of the added reactant. This ratio will curve
upward as equilibrium is approached and increase linearly with
flow when equilibrium is achieved. Linearity in the ratio plot
provides a measure of the equilibrium constant, K, and so the
standard free energy change, ∆G°T, for the addition reaction
since ∆G°T ) -RT ln K. D2O was introduced into the reaction
region of the SIFT as a dilute mixture in helium (∼2.5%) and
was obtained commercially with high purity (Aldrich, isotopic
purity >99.75%).
50.0 ( 1.8g
48.6b
Zn+
54.5 ( 3.1
Cd+
42b
Hg+
a With few exceptions, the values for HA(M+) were taken from the
review of ref 61. b Reference 63. c Reference 64. d Reference 62.
e Reference 65. f Reference 66. g Reference 67.
an uncertainty estimated to be less than (30%.32,33 The
uncertainty may be somewhat larger for slow reactions for which
the measured decay is significantly less than an order of
magnitude. Often, slow adduct formation is observed exclu-
sively, and this can introduce curvature into the primary ion
decay due to the occurrence of the reverse reaction (the extent
of curvature will depend on the strength of the adduct bond
being formed). Weak adduct bonding can lead to early curvature
in the ion decay and so prevent the proper definition of the
forward rate coefficient; only the determination of a lower limit
to the rate coefficient is possible under such conditions. The
approach to equilibrium can be monitored with a plot of the
3. Results and Discussion
The reactions of D2O were measured with 46 atomic metal
cations, including 29 transition-metal and 17 main-group cations,
at a helium buffer gas pressure of 0.35 ( 0.01 Torr and a
temperature of 295 ( 2 K. Both the primary and the higher-
order chemistry was monitored. The primary reactions exhibited
a wide range in reactivity with measured rate coefficients from
<5 × 10-13 (for K+) to 9.5 × 10-10 cm3 molecule-1 s-1 (for

8564 J. Phys. Chem. A, Vol. 111, No. 35, 2007
Cheng et al.
TABLE 5: Rate Coefficients (cm3 molecule-1 s-1), Reaction Efficiencies (k/kc), Primary Product Ions and Their Distributions,
and Higher-Order Product Ions Measured for Reactions of Atomic Ions M+ with D2O in Helium at 0.35 ( 0.01 Torr and 295 (
2 K Ordered According to Row in the Periodic Table
primary
products
higher-order
product ions
M+
ka
kcb
k/kc
PD (%)c
K+
<5.0 × 10-13
2.7 × 10-12
1.6 × 10-10
4.1 × 10-11
1.1 × 10-11
2.0 × 10-12
<5.0 × 10-13
6.0 × 10-12
1.7 × 10-12
1.7 × 10-12
9.0 × 10-12
7.0 × 10-12
<5.0 × 10-13
<5.0 × 10-13
8.1 × 10-12
2.42 × 10-9
2.42 × 10-9
2.37 × 10-9
2.34 × 10-9
2.32 × 10-9
2.32 × 10-9
2.30 × 10-9
2.29 × 10-9
2.28 × 10-9
2.28 × 10-9
2.26 × 10-9
2.25 × 10-9
2.23 × 10-9
2.22 × 10-9
2.22 × 10-9
<2.1 × 10-4
1.1 × 10-3
6.8 × 10-2
1.8 × 10-2
4.7 × 10-3
8.6 × 10-4
<2.2 × 10-4
2.6 × 10-3
7.5 × 10-4
7.5 × 10-4
4.0 × 10-3
3.1 × 10-3
<2.2 × 10-4
<2.2 × 10-4
3.6 × 10-3
Ca+
Sc+
Ti+
V+
Ca+(D2O)
ScO+
100
100
100
70
Ca+(D2O)2
ScO+(D2O)1-5
TiO+(D2O)1-5
VO+(D2O)1-3
V+(D2O)2,3
TiO+
VO+
V+(D2O)
Cr+(D2O)
Mn+(D2O)
Fe+(D2O)
Co+(D2O)
Ni+(D2O)
Cu+(D2O)
Zn+(D2O)
30
Cr+
Mn+
Fe+
Co+
Ni+
Cu+
Zn+
Ga+
Ge+
As+
100
100
100
100
100
100
100
Fe+(D2O)2
Co+(D2O)2-4
Ni+(D2O)2,3
Cu+(D2O)2
AsO+
10
90
AsO+(D2O)
As+(D2O)2-3
As+(D2O)
Se+(D2O)
Rb+(D2O)
SrOD+
Se+
Rb+
Sr+
<5.0 × 10-13
4.6 × 10-12
3.0 × 10-12
2.20 × 10-9
2.19 × 10-9
2.18 × 10-9
<2.2 × 10-4
2.1 × 10-3
1.4 × 10-3
100
50
50
100
100
Sr+(D2O)
YO+
Y+
9.5 × 10-10
2.18 × 10-9
0.44
0.34
YO+(D2O)1-5
ZrO2D+(D2O)0-3
ZrO+(D2O)1-4
NbO2+(D2O)0-4
NbO+(D2O)1-3
Nb+(D2O)2-4
MoO+(D2O)
Zr+
7.4 × 10-10
2.17 × 10-9
ZrO+
Nb+
3.0 × 10-11
2.2 × 10-11
2.17 × 10-9
2.16 × 10-9
1.4 × 10-2
NbO+
95
Nb+(D2O)
MoO+
5
90
10
100
100
100
100
Mo+
1.0 × 10-2
Mo+(D2O)
Ru+(D2O)
Rh+(D2O)
Pd+(D2O)
Ag+(D2O)
MoO2+Mo+(D2O)2
Ru+(D2O)2
Ru+
Rh+
Pd+
Ag+
Cd+
In+
1.0 × 10-12
3.3 × 10-12
7.2 × 10-13
7.0 × 10-13
<5.0 × 10-13
<5.0 × 10-13
<5.0 × 10-13
<5.0 × 10-13
<5.0 × 10-13
5.0 × 10-13
3.0 × 10-12
3.0 × 10-10
2.16 × 10-9
2.15 × 10-9
2.15 × 10-9
2.14 × 10-9
2.14 × 10-9
2.13 × 10-9
2.13 × 10-9
2.12 × 10-9
2.12 × 10-9
2.11 × 10-9
2.11 × 10-9
2.11 × 10-9
4.6 × 10-4
1.5 × 10-3
3.3 × 10-4
3.3 × 10-4
<2.3 × 10-4
<2.3 × 10-4
<2.3 × 10-4
<2.3 × 10-4
<2.3 × 10-4
2.3 × 10-4
1.4 × 10-3
0.14
Rh+(D2O)2-5
Pd+(D2O)2
Sn+
Sb+
Te+
Cs+
Ba+
La+
Cs+(D2O)
BaOD+
LaO+
100
100
80
20
100
BaOD+(D2O)
LaO+(D2O)1-5
LaOD+(D2O)
HfO+(D2O)
LaOD+
HfO+
Hf+
7.5 × 10-10
2.08 × 10-9
0.36
HfO2D+ ((D2O)0-4
TaO2+(D2O)0-4
WO2+(D2O)0-5
Ta+
W+
Re+
Os+
Ir+
2.6 × 10-10
2.0 × 10-11
<5.0 × 10-13
<5.0 × 10-13
1.4 × 10-11
4.7 × 10-12
5.8 × 10-12
8.1 × 10-12
<5.0 × 10-13
<5.0 × 10-13
1.3 × 10-12
2.07 × 10-9
2.07 × 10-9
2.07 × 10-9
2.07 × 10-9
2.07 × 10-9
2.07 × 10-9
2.07 × 10-9
2.06 × 10-9
2.06 × 10-9
2.06 × 10-9
2.06 × 10-9
0.13
TaO+
100
100
100
9.7 × 10-3
<2.4 × 10-4
<2.4 × 10-4
6.8 × 10-3
2.3 × 10-3
2.8 × 10-3
3.9 × 10-3
<2.3 × 10-4
<2.3 × 10-4
6.3 × 10-4
WO+
Re+(D2O)
Ir+(D2O)
Pt+(D2O)
Au+(D2O)
Hg+(D2O)
100
100
100
100
Ir+(D2O)2-5
Pt+(D2O)2
Au+(D2O)2
Hg+(D2O)2
Pt+
Au+
Hg+
Tl+
Pb+
Bi+
Bi+(D2O)
100
Bi+(D2O)2
a Measured reaction rate coefficient with an estimated accuracy of (30%. Rate coefficients with values of 5 × 10-12 cm3 molecule-1 s-1 or less
may have a higher uncertainty. b Calculated capture rate coefficient (see text). c PD ) product distribution expressed as a percentage, with an
estimated accuracy of (10%.
Y+). Three reaction channels were observed, and these are
indicated in reaction 1.
of D2 to form heavy water, OA(D2), is 117.5 kcal mol-1 40
the DO-D bond energy of D2O is 121.3 kcal mol-1 40
Available
, while
.
O-atom affinities (Table 1) indicate that only 11 O-atom transfer
reactions are exothermic out of the 46 that are possible with
the atomic cations investigated. Available OD affinities are listed
in Table 2, and only Ba+ has an affinity for OD, 127.4 kcal
mol-1, that is higher than the DO-D bond energy. H-atom
affinities are available for most metal cations and are listed in
Table 3. All these values are much lower than the DO-D bond
M+ + D2O f MO+ + D2
(1a)
f MOD+ + D
f M+(D2O)
(1b)
(1c)
The bimolecular channels correspond to O-atom transfer
(channel 1a) and OD transfer (channel 1b). The O-atom affinity
energy of D2O ) 121.3 kcal mol-1 40
, which means that D-atom

Heavy H2O Reactions with Cations
J. Phys. Chem. A, Vol. 111, No. 35, 2007 8565
Figure 1. Periodic variations observed in the reaction efficiencies, k/kc (solid circles), for reactions of atomic cations with D2O. Measured reaction
rate coefficient is represented by k, and kc is the calculated collision rate coefficient (see text). Also indicated are the observed reaction channels
and electronic configurations of atomic cations. Numbers in parentheses indicate the number of sequential events observed for the process indicated.
The bar graphs indicate the reaction branching ratios and are scaled to 1.0 full scale.
transfer is endothermic with all of these atomic metal cations.
Electron transfer was not observed to any of the 46 atomic
cations since their recombination energies are much smaller than
IE(D2O) ) 12.6395 ( 0.0003 eV.40 Channel 1c is assumed to
occur in a termolecular reaction with helium acting as the third
body that carries away the surplus energy in the reaction. Known
sequential binding energies of M+(H2O)n (n e 4) are listed in
Table 4.
the experimentally measured rate coefficient and kc is the capture
or collision rate coefficient computed using the algorithm of
the modified variation transition-state classical trajectory theory
developed by Su and Chesnavich41 with R(D2O) ) 1.26 × 10-
-24
cm3,42 and µd(D2O) ) 1.8545 D.43 Figure 1 displays the
data in Table 5 and shows the overall trends in reaction
efficiencies and primary product distributions on a periodic
table.
Among the 46 atomic cations investigated, seven (Sc+, Ti+,
Y+, Zr+, Hf+, Ta+, and W+) exhibit the bimolecular channel
1a exclusively with measured rate coefficients in the range from
2.0 × 10-11 (for W+) to 9.5 × 10-10 cm3 molecule-1 s-1 (for
Y+), one cation (Ba+) exhibits channel 1b exclusively with a
rate coefficient of 3.0 × 10--12 cm3 molecule-1 s-1, and 21
cations exhibit only channel 1c with measured rate coefficients
in the range from <5 × 10-13 (for K+) to 1.4 × 10-11 cm3
molecule-1 s-1 (for Ir+). Eleven cations showed no products at
all in the flow region investigated. Among the remaining six
metal cations, four (V+, As+, Nb+, and Mo+) were observed to
proceed by both channel 1a and channel 1c, one (La+) by
channels 1a and 1b, and one (Sr+) by channels 1b and 1c. No
attempt was made to measure the pressure dependence of
channel 1c since a large range in pressure was not experimen-
tally accessible. No obvious excited-state effects seemed present
in the data. The semilogarithmic decays of the atomic ions Ti+,
Y+, Zr+, Nb+, and Ir+, all of which were calculated to have
excited-state populations in the Ar plasma of g50%, did not
exhibit obvious curvature (see Figures 2-4). The La+ reaction
did produce a minor (20%) channel that could not be unambigu-
ously assigned to the La+ (3F, 5d2) ground state as is discussed
next (see section 3.5.).
3.1. Fourth-Row Atomic Ions. With the exception of the
nonreaction of K+, Ga+, and Ge+, only O-atom transfer and
D2O addition were observed as primary reaction channels for
the fourth-row atomic metal cations. O-Atom transfer occurs
with the very early transition-metal cations Sc+ and Ti+. D2O
addition was observed to compete with O-atom transfer to V+
and the main-group cation As+. The other fourth-row metal
cations were observed to proceed only by D2O addition.
Secondary and higher-order D2O addition was observed with
both MO+ and M+(D2O) ions as seen in Figure 2, which
presents the data for Sc+, Ti+, Fe+, and As+. Higher-order D2O
addition according to reaction 2 was observed for the fourth
row cations M+ ) Ca+ (n ) 1), V+ (n ) 1), Fe+ (n ) 1), Co+
(n ) 1-3), Ni+ (n ) 1, 2), Cu+ (n ) 1), and As+ (n ) 1-2).
The Cr+, Mn+, Zn+, and Se+ cations only added one D2O in
the range of D2O flow that was investigated. Reaction 3 was
observed for MO+ ) ScO+ (n ) 0-4), TiO+ (n ) 0-4), VO+
(n ) 0), and AsO+ (n ) 0) in the range of D2O flow that was
investigated.
M+(D2O)n + D2O f M+(D2O)n+1
(2)
(3)
MO+(D2O)n + D2O f MO+(D2O)n+1
Table 5 summarizes the measured rate coefficients, reaction
efficiencies, observed primary products, and product distribu-
tions, as well as the observed higher-order products. The reaction
efficiency is taken to be equal to the ratio of k/kc, where k is
A rate coefficient for the reaction of Sc+ with D2O has been
reported by Armentrout et al.10 (k ) 1.3 ( 0.4 × 10-11 cm3
molecule-1 s-1), and the reactions of Ti+ with H2O and D2O

8566 J. Phys. Chem. A, Vol. 111, No. 35, 2007
Cheng et al.
Figure 2. Composite of ICP/SIFT results for the reactions of the fourth-row metal cations Sc+, Ti+, Fe+, and As+ with D2O in helium buffer gas
at 0.35 ( 0.01 Torr and 295 ( 2 K.
have been measured by Castleman et al.12,16 and Armentrout et
al.10 The Ti+ results were somewhat controversial with sug-
gestions that excited states might play a role. Our result for
Ti+ (k ) 4.1 × 10-11 cm3 molecule-1 s-1) is in quite good
agreement with Castleman’s most recent flow reactor result
obtained with H2O in He buffer between 0.2 and 0.6 Torr (k )
6.1 × 10-11 cm3 molecule-1 s-1), but our results for Sc+ and
Ti+ (1.8 × 10-10 and 4.1 × 10-11 cm3 molecule-1 s-1) are an
order of magnitude larger than Armentrout’s results (k ) 1.3
The cross sections measured under single collision conditions
apply to a reacting ion population that is not truly thermalized
and does not have a Maxwell-Boltzmann population at a well-
defined temperature; as such, the rate coefficients derived from
these cross-sections are at best phenomenological.
The reaction of V+ with D2O has been measured previously
by our group using ICP-SIFT (k ) 1.0 ( 3.0 × 10-11 cm3
molecule-1 s-1 14
) and by Schwarz et al. using FTICR with H2O
(k ) 8.3 ( 2.5 × 10-12 cm3 molecule-1 s-1).14 These results
both are in line with our observation of slow VO+ formation in
the reaction with D2O (k ) 1.1 ( 3.0 × 10-11 cm3 molecule-1
s-1). However, experiments by Armentrout’s group with a flow
tube ion source did not show a measurable formation of VO+
from the reaction of ground-state V+(5D) with D2O, although
the experimental results with a surface ionization source appear
not to be as definitive about this nonreactivity.18 Here, too,
pressure effects may contribute to the apparent discrepancy.
We also have reported previously a nonreaction of Fe+ with
H2O (k < 1 × 10-13 cm3 molecule-1 s-1), in which Fe+ was
produced by electronic impact on ferrocene and for which the
addition of H2O could not be demonstrated.15 The previous rate
coefficient of <1 × 10-13 cm3 molecule-1 s-1 is much lower
( 0.4 × 10-11 and 3 ( 1 × 10--12 cm3 molecule-1 s-1
)
obtained with D2O at much lower pressures. There is no clear
indication of a role for excited Sc+ and Ti+ states in our data
shown in Figure 2, and the presence of excited states appears
to have been ruled out for the low-pressure data. One reviewer
suggests that the discrepancies in reaction rate coefficients may
be a consequence of the difference in pressure (single vs multiple
collisions), which may lead to a difference in the lifetime of
the reaction intermediate against back dissociation due to the
collisional quenching of the intermediate (with less back
dissociation at higher pressures). However, this proposal is not
supported by the pressure independence of the rate coefficient
for the reaction of Ti+ with H2O in He buffer between 0.2 and
0.6 Torr observed by Castleman et al.12 Also, the validity of
extracting room-temperature rate coefficients from cross-
sectional measurements at low energies under single-collision
conditions as described by Chen et al.10 may be questioned.
than that reported here, k ) 6 × 10-12 cm3 molecule-1 s-1
,
although this value may well be an upper limit (see Figure 2).
Also, the formation of some Fe+(D2O) was observed in the
experiments reported here (see Figure 2). The two results can

Heavy H2O Reactions with Cations
J. Phys. Chem. A, Vol. 111, No. 35, 2007 8567
Figure 3. Composite of ICP/SIFT results for the reactions of the fifth-row metal cations Y+, Zr+, Nb+, and Mo+ with D2O in helium buffer gas
at 0.35 ( 0.01 Torr and 295 ( 2 K.
be brought in line if the water adduct in the earlier experiments
was collisionally dissociated in the sampling process; less adduct
formation would be observed, and the decay of the Fe+ signal
would be reduced with the resulting regeneration of Fe+. The
difference probably is too large to be attributed to a kinetic
isotope effect.
with M+ ) Nb+ (n ) 1-3), Mo+ (n ) 1), Ru+ (n ) 1), Rh+
(n ) 1-4), and Pd+ (n ) 1). Type 3 reactions were observed
with MO+ ) YO+ (n ) 0-4), NbO+ (n ) 0-2), and MoO+
+
(n ) 0). Reactions 6 were observed with NbO2 (n ) 0-3).
Reactions 7 were observed with ZrO2D+ (n ) 0-2) and can
be seen in Figure 3.
3.2. Fifth-Row Atomic Ions. With the exception of the
nonreactions with Cd+, In+, Sn+, Sb+, and Te+ and the OD
group transfer with Sr+, O-atom transfer and D2O addition also
predominate in the reactions of the fifth-row metal cations with
D2O. O-Atom transfer was observed exclusively with Y+ and
Zr+ and D2O addition with Rb+, Ru+, Rh+, Pd+, and Ag+. The
D2O addition was observed to compete with O-atom transfer
to Nb+ and Mo+ and to compete with OD group transfer with
Sr+.
Figure 3 displays the primary and higher-order chemistry
observed with Y+, Zr+, Nb+, and Mo+. The secondary O-atom
transfer according to reaction 4 was observed for the metal
cations M+ ) Nb+ and Mo+, and OD group transfer to metal
oxides according to reaction 5 was observed for Zr+.
+
+
MO2 (D2O)n + D2O f MO2 (D2O)n+1
MO2D+(D2O)n + D2O f MO2D+(D2O)n+1
(6)
(7)
3.3. Sixth-Row Atomic Ions. With the exception of the
nonreactions with Os+, Tl+, and Pb+, and the OD group transfers
with Ba+ and La+, O-atom transfer and D2O addition are still
the dominant reactions with the sixth-row metal cations. O-Atom
transfer was observed exclusively with Hf+, Ta+, and W+ and
D2O addition with Cs+, Re+, Ir+, Pt+, Au+, Hg+, and Bi+. The
O-atom transfer was observed to compete with the OD group
transfer with La+.
Figure 4 displays the primary and higher-order chemistry with
Hf+, Ta+, W+, and Ir+. Secondary O-atom transfer according
to reaction 4 was observed with M+ ) Ta+ and W+ to form
TaO2+ and WO2+, respectively, and OD group transfer to metal
oxide according to reaction 5 was observed with Hf+ to form
HfO2D+. Sequential D2O addition was seen with all three of
these secondary ions with the addition of up to four D2O
MO+ + D2O f MO2+ + D2
MO+ + D2O f MO2D+ + D
(4)
(5)
Secondary and higher-order D2O addition was observed with
M+, MO+, MO2+, and MO2D+. Type 2 reactions were seen

8568 J. Phys. Chem. A, Vol. 111, No. 35, 2007
Cheng et al.
Figure 4. Composite of ICP/SIFT results for the reactions of the sixth-row metal cations Hf+, Ta+, W+, and Ir+ with D2O in helium buffer gas
at 0.35 ( 0.01 Torr and 295 ( 2 K.
molecules to form HfO2D+(D2O)4, TaO2+(D2O)4, and WO2
-
molecule-1 s-1. If the OH transfer channel is due to the reaction
of the low-lying 4F excited-state of Hf+, then the channel
discrepancy may be due to a higher proportion of this state
produced in the laser ablation of the Hf foil in the FTICR
experiments. There is better agreement on the higher-order
chemistry as both experiments show further reaction of HfO+
to form HfO2H+(HfO2D+) and then its hydrate, but our rate
coefficient of 6.4 × 10-10 cm3 molecule-1 s-1 for the reaction
of HfO+ is again larger than that of 1.6 × 10-10 cm3 molecule-1
s-1 reported by Beauchamp and Irikura.30
+
(D2O)4. Ir+ is seen to add up to five D2O molecules to form
Ir+(D2O)5. The faster second addition of D2O to Ir+(D2O) is
clearly rate limited by its slower production from the addition
of D2O to Ir+.
Secondary and higher-order D2O addition is generally ob-
served with the sixth-row metal species M+, MO+, MOD+,
MO2+, and MO2D+. Type 2 reactions were seen with M+
)
Ir+ (n ) 1-4), Pt+ (n ) 1), Au+ (n ) 1), Hg+ (n ) 1), and
Bi+ (n ) 1). Type 3 reactions were seen with MO+ ) LaO+ (n
) 0-4) and HfO+ (n ) 0). Type 6 reactions were seen with
3.4. O-Atom Transfer (Dehydrogenation). 3.4.1. Thermo-
dynamics and Efficiency. According to the O-atom affinities
listed in Table 1, only metal cations in groups 3B to 6B have
higher O-atom affinities than OA(D2) ) 117.8 ( 0.1 kcal
+
+
+
MO2 ) TaO2 (n ) 0-3) and WO2 (n ) 0-4). Type 7
reactions were seen with MO2D+ ) HfO2D+ (n ) 0-3). Type
8 reactions were seen with MOD+ ) BaOD+ (n ) 0) and
LaOD+ (n ) 0).
mol-1 40
and this means that only these cations are thermody-
,
namically allowed to abstract an O-atom from D2O, viz. to cause
dehydrogenation and release D2. Indeed, only these metal cations
were observed to dehydrogenate D2O. Dehydrogenation was
observed with the group 3B (Sc+, Y+, and La+), group 4B (Ti+,
Zr+, and Hf+), group 5B (V+, Nb+, and Ta+), and group 6B
(Mo+ and W+) transition-metal cations and the main-group
cation As+. Rate coefficients for O-atom transfer are in the range
MOD+(D2O)n + D2O f MOD+(D2O)n+1
(8)
Beauchamp and Irikura30 previously investigated the reaction
of Hf+ with H2O using FTICR with the ion produced by laser
ablation of a metal foil, but their results are quite different from
ours in that they show the two reaction channels of O-atom
transfer and OH group transfer with a product distribution
estimated as I[HfO+]/I[HfOD+] ) 1.6:1 and a measured rate
coefficient of 3.8 × 10-10 cm3 molecule-1 s-1. In comparison,
our experiments indicate an exclusive O-atom transfer reaction
channel with a larger rate coefficient of 7.5 × 10-10 cm3
of 8.1 × 10-13 (As+) to 9.5 × 10-10 (Y+) cm3 molecule-1 s-1
.
OD transfer was observed to compete with O-atom transfer in
the reactions of La+ (20%), and D2O addition was observed to
compete with O-atom transfer with V+ (33%), As+ (90%), Nb+
(5%), and Mo+ (10%).

Heavy H2O Reactions with Cations
J. Phys. Chem. A, Vol. 111, No. 35, 2007 8569
Figure 5 displays the dependence of the efficiency of O-atom
transfer on the known O-atom affinity of the metal cation. It is
interesting to note in Figure 5 that O-atom transfer from D2O
to Mo+ is slightly endothermic and almost thermoneutral, ∆H298
) 1.1 ( 0.6 kcal mol-1. Also, given that O-atom transfer
generally predominates when exothermic or nearly thermoneu-
tral, we can assign an upper limit to OA(Re+), OA(Re+) < 117.8
kcal mol-1, since only D2O addition was observed with Re+.
This limit improves the value of OA(Re+) ) 115 ( 15 kcal
mol-1 in Table 1 to 109 ( 9 kcal mol-1
.
3.4.2. Spin Control and Reaction Mechanism. Figure 5 shows
that the reactions with D2O often proceed with efficiencies
below 0.1 when O-atom transfer is exothermic, even below 0.01
for the fourth-row atomic cations V+ (0.0032) and As+
(0.00036). Low efficiencies for exothermic O-atom transfer are
interesting and, given our previous experience with O-atom
transfer from N2O to atomic cations,4 are likely to speak to the
role of electronic spin conservation in the formation of the
monoxide cation. O-Atom transfer from D2O is not overly
exothermic, by less than 88 kcal mol-1, as compared to O-atom
transfer from N2O that can be exothermic by as much as 168
Figure 5. Dependence of the reaction efficiency, k/kc, on the O-atom
affinity, OA, of the atomic cation M+. Measured reaction rate coefficient
is represented by k, and kc is the calculated collision rate coefficient.
Reactions on the right of the dashed line are exothermic for O-atom
transfer, while those on the left are endothermic. Circles refer to fourth-
row cations, squares to fifth-row cations, and triangles to sixth-row
cations. Open symbols designate the observation of D2O addition, while
solid symbols designate the observation of O-atom transfer.
kcal mol-1
.
We note, first of all, that the exothermic O-atom transfer
reactions were observed to be inefficient (<0.1) in our experi-
ments with D2O, viz. the reactions of Sc+ (0.068), Ti+ (0.018),
V+ (0.0032), and As+ (0.00036) in the fourth row, Nb+ (0.013)
and Mo+ (0.0090) in the fifth row, and W+ (0.0097) in the sixth
row, where efficiencies for the O-atom transfer channel are given
in parentheses. The one or two electronic states of these atomic
cations with the highest population within the ICP are all high-
spin states: Sc+(3D, 3F), Ti+(4F)), V(5D, 5F), Nb+(5D, 5F), Mo+-
O-atom transfer reaction 15 is exothermic, spin-forbidden and
inefficient.
As+(3P) + D2O(1A1) f
AsO+(1Σ) + D2(1Σg )
0.0036
-29 (15)
+
6
(6S, D), and W+(6D).4 Their reactions with D2O by O-atom
The appearance of small amounts of AsO+ at low flows of D2O
recorded in Figure 2 may well be the result of the spin-allowed
reaction of excited As+(1D), which has been estimated to be
formed in the Ar plasma at levels of ca. 6% of the total As+
ion population.4
transfer to produce metal-oxide cations in their ground states
(ScO+(1Σ), TiO+(2∆), VO+(3Σ), NbO+(3Σ), MoO+(4Σ), and
WO+(4Σ))4 are all spin-forbidden (see reactions 9-14 for which
the measured reaction efficiencies and reaction enthalpies in
kcalories per mol are also indicated).
Other reactions that are spin-forbidden for the formation of
ground-state products appear to be sufficiently exothermic to
achieve a high reaction efficiency with the formation of spin-
allowed excited products. These include the reactions with the
Sc+(3D, 3F) + D2O(1A1) f
ScO+(1Σ) + D2(1Σg )
0.068
0.018
-47 (9)
-41 (10)
-17 (11)
-47 (12)
+1 (13)
+
3
high-spin states of Y+(3D), Zr+(4F), La+(3F, D), Hf+(2D,4F),
Ti+(4F) + D2O(1A1) f
3
and Ta+(5F, F), for which the monoxide cations have ground
TiO+(2∆) + D2(1Σg )
+
states of YO+(1Σ), ZrO+(2Σ), LaO+(1Σ), HfO+(2Σ), and TaO+-
(3∆).4 However, collisional de-excitation to the ground state
(e.g., Y+(1S)) or a low-lying state (e.g., La+(1D)) can promote
spin-allowed O-atom transfer and so cannot be ruled out. In
V+(5D, 5F) + D2O(1A1) f
+
VO+(3Σ) + D2(1Σg )
0.0032
0.013
2
the case of Hf+, the ground D state is already sufficiently
Nb+(5D, 5F) + D2O(1A1) f
NbO+(3Σ) + D2(1Σg )
populated (48%)11 within the ICP to possibly account for the
observed O-atom transfer efficiency (0.36).
+
The potential-energy surface for the exothermic but spin-
forbidden dehydrogenation of D2O with ground-state V+(5D)
has been described previously by Armentrout et al.18 and Ugalde
et al.26 The reaction path involves the initial formation of the
ion dipole complex V+‚OD2, which then isomerizes by insertion
to form D-V+-OD, evolves through a four-centered transition
state into VO+‚D2, and then separates into the products
VO+(3Σ) + D2. The overall transformation involves a crossing
to a low-spin triplet state located between the intermediate
D-V+-OD and the V+‚OD2 complex and lies above the initial
energy of the reactants.
Mo+(6S, 6D) + D2O(1A1) f
+
MoO+(4Σ) + D2(1Σg )
0.0097
0.0097
W+(6D) + D2O(1A1) f
WO+(4Σ) + D2(1Σg )
-48 (14)
+
Apparently, the rates of these reactions are constrained by the
change in spin required to proceed from the reactant to the
product potential energy surfaces, as has been proposed previ-
ously by Armentrout et al.10,18 for the reactions of Sc+, Ti+,
and V+.
Armentrout et al.18 and Ugalde et al.26 also have considered
the mechanism for metal-oxide production in the reaction of
D2O with the ground states Sc+(3D) and Ti+(4F). The mecha-
nisms are qualitatively similar to that proposed for the analogous
The reaction of D2O with As+ may be a special case
among the main-group atomic cations in that the ground-state

8570 J. Phys. Chem. A, Vol. 111, No. 35, 2007
Cheng et al.
(1D, 5d6s), which contributes 11%, and La+(3D), which
contributes 23% in the ICP plasma at 5500 K.11 These two states
both have s1 orbitals available for bonding as is the case of Sr+
and Ba+.4
TABLE 6: Measured Equilibrium Constants (K) and
Derived Standard Free Energy Changes (∆G° (kcal mol-1))
for Sequential D2O Addition to Metal Cations at 0.35 ( 0.01
Torr and 295 ( 2 K
K (∆G°)
The OD transfer channel was easily observed by Armentrout
et al.10,17,18,44 in reactions of the fourth-row transition-metal
cations with D2O at the higher energies available in a guided
ion beam instrument. All the OD transfer reactions were found
to be endothermic at room temperature and so are expected not
to be observed in our ICP/SIFT instrument. Studies of the
reactions of Ba+, Sr+, and La+ with D2O were not reported.
3.6. D2O Addition (Hydration). Addition of D2O was
observed exclusively in the reactions of the fourth-row cations
Ca+, Mn+, Fe+, Co+, Ni+, Cu+, Zn+, and Se+, the fifth-row
cations Rb+, Ru+, Rh+, Pd+, and Ag+, and the sixth-row cations
Cs+, Re+, Ir+, Pt+, Au+, Hg+, and Bi+. Beside these, D2O
addition was observed to compete with O-atom transfer in the
ractions with V+, As+, Nb+, and Mo+ and to compete with OD
group transfer in the reactions with Sr+. All these addition
reactions are presumed to occur by the collisional stabilization
reaction 16 at 0.35 Torr of He (however, pressure-dependent
studies were not performed).
M+
M+(D2O)
M+(D2O)2
M+(D2O)3
Ca+
V+
g8.2 × 104 (e-6.7)
g6.7 × 104 (e-6.6)
g3.1 × 104 (e-6.1)
g1.7 × 104 (e-5.8)
2.1 × 106 (-8.6)
Cr+
Mn+
Fe+
Co+
Ni+
Cu+
Zn+
As+
Sr+
g2.2 × 105 (e-7.3) g2.4 × 107 (e-10.1)
g1.0 × 105 (e-6.8) g9.6 × 106 (e-9.5) 6.9 × 105 (-8.0)
g4.1 × 105 (e-7.6) g2.3 × 107 (e-10.0) 1.4 × 107 (-9.8)
g4.1 × 105 (e-7.6)
g1.3 × 105 (e-7.0)
g1.2 × 106 (e-8.3)
g2.0 × 104 (e-5.9)
Mo+
Ru+
Rh+
Pd+
Ag+
Cs+
Ir+
g7.3 × 104 (e-6.6)
g2.5 × 105 (e-7.4)
1.3 × 105 (-7.0)
4.5 × 106 (-9.1)
2.2 × 106 (-8.6)
1.4 × 107 (-8.6)
g3.7 × 104 (e-6.2) g1.1 × 106 (e-8.2)
g1.0 × 105 (e-6.8)
3.3 × 104 (-6.2)
g-7.3 (e2.3 × 105)
g3.9 × 104 (e-6.3)
g1.8 × 105 (e-7.2)
g7.9 × 104 (e-6.7)
g2.1 × 104 (e-5.9)
2.3 × 107 (-10.2)
Pt+
Au+
Hg+
Bi+
M+ + D2O + He f M+ D2O + He
(16)
D2O addition reactions were observed to proceed with
relatively small effective bimolecular rate coefficients, k e 1.4
× 10-11 cm3 molecule-1 s-1, presumably due the short lifetime
of the intermediate complex that is expected from the weak
bonding of the atomic ion with water and the small degrees of
freedom available for energy partitioning. No products at all
were observed in the reactions of K+, Ga+, Ge+, Cd+, In+, Sn+,
Sb+, Te+, Os+, Tl+, and Pb+ with D2O.
Two D2O molecules were observed to add sequentially to
the first-row cations Ca+, Fe+, and Cu+, the second-row cations
Mo+, Ru+, and Pd+, and the third-row cations Pt+, Au+, Hg+,
and Bi+. V+, Ni+, and As+ were observed to add three D2O
molecules. Co+ and Nb+ were observed to add four D2O
molecules, and only Ir+ and Rh+ were observed to add five
D2O molecules in the D2O flow range that was investigated.
In general, the higher-order rate coefficients for addition are
higher than the primary ones, and this can be attributed to the
increased number of degrees of freedom in the higher-order
reaction intermediates, which leads to longer lifetimes and so
higher rates of collisional stabilization. Furthermore, the second
water molecule often binds as strongly as, or more strongly
than,9,45,46 the first (see Table 4), and this is also kinetically
favorable for water addition since the lifetime of the intermediate
also increases with increasing binding energy.
Equilibrium analyses were performed on the kinetic results
by plotting product/reactant ion signal ratios as a function of
reactant flow. Equilibrium is achieved when such a plot achieves
linearity, viz. when [M+(D2O)n]/[M+(D2O)n-1] ) Keq [D2O].
The equilibrium analyses show that equilibrium was not
achieved with most cations, and so most of the reported values
for Keq are lower limits. Calculated equilibrium constants, Keq,
and the corresponding changes in standard free energy, ∆G°,
are presented in Table 6. The apparent bimolecular efficiencies
for hydration are presented in Table 7. According to the formula
∆G° ) ∆H° - T∆S° and that ∆S° for all the metal cations
clustering with D2O is nominally 20 eu,22,47 there is a constant
difference of about 6 kcal mol-1 between ∆G° and ∆H° at 298
K. For certain cations, we can make sure that equilibrium has
been achieved, and the values for ∆G° in Table 6 can agree
with the values of ∆H° in Table 4. For example, for Cs+, the
reaction with V+(5D) and also involve crossings of different
spin states. The exothermicities for oxide formation are in the
order Sc+ > Ti+ > V+ as are the efficiencies observed by us.
All three reactions are spin-forbidden overall and relatively
inefficient, 0.068 > 0.018 > 0.0031, for MO+ formation. VO+
formation is by far the least efficient of the three (6× less
efficient than TiO+ formation), is the only one of the three that
competes with D2O addition, and also is the only one for which
calculations predict that the spin crossing lies above the energy
of the high-spin reagents. The potential energy surface computed
for exothermic WO+ formation exhibits a low-lying sextet/
quartet crossing and subsequent barrier on the quartet surface
of only 5.1 kcal mol-1, and this is consistent with the observed
efficiency of 0.0097.
3.5. OD Transfer. Primary OD transfer was observed
exclusively (100%) only in the reaction of Ba+ with D2O and
competed with O transfer in the reaction with La+ (20%) and
with D2O addition in the reaction with Sr+ (50%).
The DO-D bond energy of D2O is quite high, 121.3 kcal
mol-1 40
Available OD affinities for atomic metal cations are
.
listed in Table 2; they are known for only 12 of the 46 atomic
cations that were investigated. Of these, only Ba+ appears
to have an affinity, D0(Ba+-OD) > D(DO-D), with
D0(Ba+-OD) ) 127 ( 7 kcal mol-1. We did observe OD
transfer in the reaction of Ba+ with D2O, but with a low
efficiency of about 1.4 × 10-3, perhaps due to a very slight
endothermicity. The heavy hydroxide cation also was observed
as a product in the reaction with Sr+ but in this case in
competition with adduct formation. In this case, however, we
must attribute the heavy hydroxide formation to reaction with
excited state Sr+(2D, 4d) since D0(Sr+-OD) ) 105 ( 5 kcal
mol-1 so that OD transfer is endothermic by at least 10 kcal
mol-1 for the reaction with ground-state Sr+. The maximum
excited-state population of Sr+ should be about 10%,11 and this
could easily account for the observed 50% occurrence of OD
transfer. The OD affinity of La+ appears not to be available
but should be higher than D(DO-D) for OD transfer to be
exothermic with ground-state La+(3F). In this case also, we
cannot exclude the possible role of excited states such as La+-

Heavy H2O Reactions with Cations
J. Phys. Chem. A, Vol. 111, No. 35, 2007 8571
TABLE 7: Measured Reaction Rate Coefficients (ka (cm3 molecule-1 s-1)) and Reaction Efficiencies (k/kc) for Sequential D2O
Addition to M+ in Helium at 0.35 ( 0.01 Torr and 295 ( 2 K
M+(D2O)
M+(D2O)2
M+(D2O)3
M+
k
k/kc
k
k/kc
k
k/kc
Ca+
V+
2.7 × 10-12
3.3 × 10-12
2.0 × 10-12
<5.0 × 10-13
6.0 × 10-12
1.7 × 10-12
1.7 × 10-12
9.0 × 10-12
7.0 × 10-12
7.3 × 10-12
1.6 × 10-12
2.2 × 10-12
1.0 × 10-12
3.3 × 10-12
7.2 × 10-13
7.0 × 10-13
5.0 × 10-13
1.4 × 10-11
4.7 × 10-12
5.8 × 10-12
8.0 × 10-12
1.3 × 10-12
1.1 × 10-3
1.4 × 10-3
8.6 × 10-4
<2.2 × 10-4
2.6 × 10-3
7.5 × 10-4
7.5 × 10-4
4.0 × 10-3
3.1 × 10-3
3.2 × 10-3
7.0 × 10-4
1.0 × 10-3
4.6 × 10-4
1.5 × 10-3
3.3 × 10-4
3.3 × 10-4
2.4 × 10-4
6.8 × 10-3
2.3 × 10-3
2.8 × 10-3
3.9 × 10-3
6.2 × 10-4
1.3 × 10-10
5.7 × 10-2
Cr+
Mn+
Fe+
Co+
Ni+
Cu+
Zn+
As+
Sr+
9.0 × 10-11
2.6 × 10-10
2.8 × 10-10
3.2 × 10-11
4.1 × 10-2
0.12
2.7 × 10-10
0.13
0.13
1.5 × 10-2
2.6 × 10-11
1.2 × 10-2
Mo+
Ru+
Rh+
Pd+
Ag+
Cs+
Ir+
5.8 × 10-11
5.6 × 10-11
2.7 × 10-2
2.6 × 10-2
3.0 × 10-10
0.15
Pt+
Au+
Hg+
Bi+
2.1 × 10-10
0.10
4.8 × 10-11
2.3 × 10-2
a Effective bimolecular rate coefficient with an estimated accuracy of (50%.
D2O addition was also observed with many of the MO+,
MOD+, MO2+, and MO2D+ cations that were formed in the
reactions of the early transition-metal ions. Up to five D2O
+
molecules were observed to add sequentially to MO+ and MO2
four to MO2D+, and one with MOD+ (see Table 5).
,
+
3.7. MO2 and MO2D+ Formation. A second O-atom
transfer, reaction 4, was observed with the early transition-metal
oxides NbO+, MoO+, WO+, and TaO+, and OD group transfer,
reaction 5, was observed with ZrO+ and HfO+. The rate
coefficients measured for these reactions are summarized in
Table 8 along with calculated reaction efficiencies. O-Atom
transfer to TaO+ is the most efficient secondary O-atom transfer,
k/kc ) 0.14, and that to NbO+ is the least, k/kc ) 0.020. The
OD group transfer reactions with HfO+ and ZrO+ are both quite
efficient with k/kc equal to 0.31 and 0.12, respectively.
Available O-atom affinities for atomic metal oxide cations
are listed in Table 9. Of these, five (those for the early transition-
metal oxide cations NbO+, MoO+, TaO+, and WO+ and the
late transition--metal oxide cation IrO+) have OA(MO+) >
OA(D2). We observed O-atom transfer with all four of the early
transition-metal oxide cations but could not investigate this
reaction with IrO+. This latter ion was not generated in the
reaction of Ir+ with D2O since only in this interesting case was
OA(M+) < OA(D2), while OA(MO+) > OA(D2).
+
The structure of the MO2 ions formed in reaction 4 is
uncertain. Our multi-collision induced dissociation technique,48
which involves simply raising the potential of the sampling nose
cone, did not yield bond cleavage for MO2 and so did not
provide bond connectivity information. But, based on the
elemental composition, three different bonding isomers are
possible: the end-on metal dioxygen complex I, MOO+, the
side-on complex II, c-MO2+, and the inserted metal dioxide
complex III, OMO+. Several of these isomers may even coexist
for some of these metals. For example, structures I and III were
Figure 6. Variations in the reaction efficiency, k/kc (open circles), and
standard free energy change, ∆G° (solid circles), for single D2O addition
to bare first-, second-, and third-row atomic metal cations at room
temperature.
+
value of ∆H° in our experiment is about 12.1 kcal mol-1 based
on ∆S° ) 19.4 eu,22 which is in good agreement with the value
12.9 kcal mol-1 in Table 4. Figure 6 explores the periodic
variation in the values of the measured efficiencies and the
standard binding free energies observed from our equilibrium
measurements (see Table 6) for the addition of D2O to M+ at
295 ( 2 K.
computed to coexist as well-separated minima on different spin
+ 49
surfaces of CrO2
.
A general guide has been provided by
Schwarz et al.,50 who propose that structure III is energetically

8572 J. Phys. Chem. A, Vol. 111, No. 35, 2007
Cheng et al.
TABLE 8: Rate Coefficients (cm3 molecule-1 s-1) Measured
for O-Atom Transfer and OD Transfer Reactions of Metal
Oxide Cations with D2O in Helium at 0.35 ( 0.01 Torr and
295 ( 2 K
efficiencies, and these can be understood in terms of barriers
introduced by the change in spin required to proceed from the
reactant to the product potential energy surfaces.
Secondary and higher-order reactions lead to the further
oxidation of Nb+, Ta+, Mo+, and W+ to form MO2+ and further
OD group transfer with Zr+ and Hf+ to form MO2D+. Further
D2O addition occurs with M+(D2O), MO+, MOD+, MO2+, and
MO2D+ to form M+(D2O)n (n e 5), MO+(D2O)n (n e 5),
MOD+(D2O), MO2+(D2O)n (n e 5), and MO2D+(D2O)n (n e
4). Up to five D2O molecules add sequentially to selected atomic
M+ and MO+ as well as MO2+ and four to MO2D+. Equilibrium
measurements for sequential D2O addition to M+ indicate that
the periodic variation in reaction efficiency (k/kc) parallels the
periodic variation in the standard free binding energy (∆G°).
reaction
ka
kc
k/kc
NbO+ + D2O f NbO2+ + D2
4.3 × 10-11 2.17 × 10-9 0.020
3.0 × 10-10 2.09 × 10-9 0.14
6.0 × 10-11 2.09 × 10-9 0.029
MoO+ + D2O f MoO2+ + D2 2.2 × 10-10 2.17 × 10-9 0.10
TaO+ + D2O f TaO2+ + D2
WO+ + D2O f WO2+ + D2
HfO+ + D2O f HfO2D+ + D 6.4 × 10-10 2.10 × 10-9 0.31
ZrO+ + D2O f ZrO2D+ + D
2.6 × 10-10 2.17 × 10-9 0.12
a Reaction rate coefficient with an estimated accuracy of (30%.
TABLE 9: O-Atom Affinities of Atomic Metal Oxide
Cations (D0(MO+-O) (kcal mol-1))a
MO+
OA(MO+)
MO+
OA(MO+)
MO+
LaO+
OA(MO+)
23
Acknowledgment. Continued financial support from the
Natural Sciences and Engineering Research Council of Canada
is greatly appreciated. Also, we acknowledge support from the
National Research Council, the Natural Science and Engineering
Research Council, and MDS SCIEX in the form of a Research
Partnership grant. As holder of a Canada Research Chair in
Physical Chemistry, D.K.B. thanks the contributions of the
Canada Research Chair Program to this research.
ScO+
TiO+
VO+
40
81
90
66
66
99
YO+
41
89
132
128
79
ZrO+
NbO+
MoO+
RuO+
RhO+
TaO+
WO+
ReO+
OsO+
IrO+
140
132
65
105
125
75
CrO+
FeO+
CuO+
78
PrO+
a Reference 50.
References and Notes
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group 3 cations only have two electrons to bind the first oxygen
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