.
In general,
polyatomic radical molecules such as
H3.,
H2F.,
H3O.,
NH4.,
CH5. have been
called Rydberg molecules by G.Herzberg
[Herzberg 87] .
These molecules are viewed by Herzberg to
possess, in their ground state,
a lone electron which orbits around a positive core, on a low-lying
yet diffuse orbital.
The (H3O.)
radical has a long and tumultuous career.
It has been found, lost, and found again quite a number of times.
This species has been first hypothetized to exist
in 1907
[Goldsmith 1907] .
In several experiments in the thirties,
the H3O. was
claimed to have been found.
In 1964, Magee
[Magee 64]
suggested that
the H3O.
radical should be an important transient species
in radiation chemistry, formed by the recapture process of a
quasifree electron by H3O+ .
Sworki
[Sworski 64] ,
from kinetics data, suggested that, in the
Gamma-radiolysis of water, H3O. should be the main precursor of H2.
After an 85 eV or 100 eV electron radiolysis, in gaz-phase,
Melton and coworkers
[Melton 66] ,
[Melton 67] ,
claimed to find
evidence of H3O..
In their experiments, it was
H3O+ which was
detected by mass spectrometry, after ionization
of the neutral or negatively charged parent species.
It was
supposed that the main source of H3O+
is H3O. ionization.
H3O+ might be also a
direct product of radiolysis.
The measured IP is 10.9 eV. Melton and coworkers hinted that
this high value
could correspond to the IP of a hydronium radical in an excited state.
Nevertheless, Melton and coworkers
were able to detect D3O-
directly [Melton 67] .
Kongshaun and coworkers
[Kongshaun 71]
irriadiated acidic solutions
with X-rays,Gamma-Rays, H. and Ar atoms generated by an
electric discharge.
They concluded that H3O.
must have been formed.
Martin and Swift
[Martin 71]
claimed to have isolated the H3O.
and D3O. radicals
in UV-irradiated aqueous
matrices, and presented an ESR quartet spectrum.
However their experiment was not reproducible
[Wargon 72] ,
and was latter
explained in terms of
CH3.
impurities
[Noda 73] .
At last, in 1984, using a neutralized ion beam spectroscopy technique,
Gellene and Porter
confirmed a preliminary study
[Williams 80] ,
and brought solid experimental evidence
[Gellene 84]
of
the metastability of D3O..
They found also
D3O(D2O).
and H3O(H2O).,
but found no trace of H3O..
They reported an ionization potential (IP) of 4.3 eV for D3O. ,
and suggested a 3.4 eV IP lower limit for the hydronium hydrate
radical.
For comparison, the IP for a sodium atom is 5.1 eV.
Latter, Raksit and Porter observed an intriguing oxygen atom isotope
effect.
[Raksit 87].
With its most naturally abundant isotope
16O,
D3O. is metastable.
When substituted with the heavier
18O,
D3O.
is no longer detectable while
D518O
2.
can still be detected.
These surprising isotop effects were
recently confirmed
in further experiments
[Hudgins 94] .
In 1987,
Griffith, Harris and Beynon
[Griffith 87] ,
using a neutralization-reionization spectroscopy technique,
claimed that the metastability of
H3.,
NH4.,
H3O. was established.
In particular, the lifetime of
H3O. was claimed
to extend up to 1 microsecond.
However, after reinterpretation of their own data,
these authors soon retracted their claims.
Picosecond laser spectroscopy experiments
were conducted in mixed water/alcohol
solvents,
with the help of water-specific fluorescent probe molecules,
in Robinson's laboratory
[Lee 85]
[Moore 85] .
It was found that a (H2O)4+/-1 cluster
was the lowest size water acceptor structure,
required both for proton hydration and "hydrated electron" formation.
Latter on, Robinson disregarding the traditional
hydrated electron cavity model, proposed instead
a semi-ionic pair model
( H3O. -- e-)aq
of the "hydrated electron",
where the hydronium radical plays
a major role
[Robinson 87] ,
[Hameka 87] .
Subsequently, Muguet proposed an itinerant radical model
[Muguet 92] ,
[Muguet 94] for the
"hydrated electron".
The 1.7 eV absorbing species, traditionally known as the
"hydrated electron", is no longer viewed as an electron
solvated in liquid water cavities, but as a hydrated
hydronium radical.
The hydronium radical is
considered as an itinerant species, constituted
with ever-changing oxygen and hydrogen atoms. It is a topological
locus, moving across the H-bond network.
H3O. is neutral, but because of the high
polarizablity of the unpaired electron cloud, it moves
like a negatively charged species, in presence of an
electric field.
Within femtosecond laser spectroscopy experiments performed in Gauduel's
research group
[Gauduel 93] ,
[Gauduel 94a] ,
[Gauduel 94b] ,
( see also our
WATOC96 WWW poster )
after a two-photon
photoexcitation (2 x 4 eV = 8 eV)
of pure liquid water,
three spectroscopic species have been discriminated :
A short-lived infrared band
(" pre-hydrated electron") species.
A long-lived 1.7 eV band " hydrated electron" (e-)aq
species.
A 1.45 eV centered band species.
with a large kinetic isotopic H/D substitution effect:
In pure light water,
the 1.45 eV band begins to appear within the pump pulse, and reaches its maximum 500 fs after
and has disappeared in 2 ps.
According to pseudo-first order kinetics
after pump/probe signal deconvolution,
the appearance time (T=1/k ) is 130 fs and the
mean lifetime is 340 fs, with an error bar of +/- 20 fs.
In pure deuterated water,
the 1.45eV band begins also to appear within the pump pulse, and but
reaches
its maximum 750 fs after and has disappeared in 4 ps.
The appearance time (T=1/k) is 320 fs while the mean
lifetime is 750 fs
In concentrated acidic solutions
[Gauduel 90] ,
a near IR band in the 1.45 eV region has also been
observed, which has been written conveniently as
an
( H3O+ -- e-)hyd
encounter pair while no hypothesis concerning this latter
structure were proposed.
The hydronium radical has also been
the object of numerous theoretical investigations.
Conway,Bockris and Linton
[Conway 55]
theorized that the radical
H3O.
as a transient species must exist in order to explain proton
conductance.
Bernstein
[Bernstein 63]
tried to show that HA radicals might be
thermodynamically stable, where A is a saturated proton acceptor.
He suggested reactions with H. atoms, in order to form such
radicals.
With a one-center STO basis set, Bishop
[Bishop 66]
computed H3O.
to be thermodynamically
unstable.
Melton and Joy
[Melton 67] ,
also with a one-center STO expansion, found
an equilateral triangle geometry for
H3O.
which was unstable
against dissociation.
At the ROHF level,
Gangi and Bader
[Gangi 71]
found a pyramidal C3v geometry with
a small inversion barrier of 2 kcal/mole, and an IP of 4.6eV.
Their basis set was complemented by a single diffuse s-type gaussian
centered on the oxygen atom.
They predict that the
H3O.
radical might be marginally metastable
at low temperature with a dissociation barrier of only
6.6 kcal/Mole.
At the INDO level
[Efskind 72] ,
a planar geometry was obtained, and
the IP was found to be 7.45 eV. Furthermore
the H3O. radical was
predicted to be
stable against dissociation ( -40 kcal/mole).
ESR parameters were
computed at the INDO and CNDO levels
[Chuvylkin 72] .
At the UHF level,
M.C.R. Symons and coworkers
[Claxton 73]
found a pyramidal geometry
and also found large positive hyperfine coupling constants on the protons.
Magee
[Magee 64]
proposed
a pathway for the formation of the hydrated electron involving the
formation of H3O..
The hydronium radical is first formed
through the capture of a quasifree electron by
H3O+.
This step would be followed by an autoionization of
H3O. which would
leads to the solvation of the ionized electron into a
liquid water cavity.
Webster
[Webster 75]
investigated the ability of
the H3O+ cation
to capture an extra electron. The
resulting radical was computed to be slightly nonplanar.
Magee's suggestion
[Magee 64]
also stimulated an ab initio
study by Maurice Schwartz
[Schwartz 76]
who found that the transition to the lowest excited state
corresponded to an energy of 1.87 eV. The IP is 4.75 eV.
His basis set is supplemented by two s and two p
diffuse gaussians
on the oxygen atom, and one extra diffuse s-type and p-type
gaussians on each hydrogen atom.
Strangely enough, this interesting paper is never quoted.
Niblaeus ,Roos and Siegbahn
[Niblaeus 77]
performed several computations
both at the SCF/UHF
and CI levels.
Their largest basis set comprises a valence gaussian set with
polarization functions,
supplemented by only two s diffuse gaussians
centered on the oxygen atom.
H3O.
was determined to be thermodynamically
unstable, the dissociation
products being H2O and H..
The H3O.
radical corresponds to a local minimum,
one pathway to the formation of H
3O ,
being electron attachment to
H3O+.
It is interesting to mention absolute energies since it gives
an idea of the quality of the basis set and the method.
Their best energies
for H3O. were,
at the UHF level -76.499,800 Hartrees and with a direct CI (10,074 CSFs)
method
-76.715,324 Hartrees.
These authors computed
a dissociation barrier of
4.6 kcal/Mole at the UHF level and a dissociation barrier of only
3.4 kcal/Mole at the CI level.
They concluded that the existence of a metastable state with a
measurable lifetime was unlikeky.
Raynor and Herschbach
[Raynor 82]
computed the transition energies
as well as the transition dipole moments of electronic transitions
between the ground state and excited states as well as between
excited states for various radicals. Each electronic state
was calculated with respect to a frozen cationic core.
Their cation basis set is supplemented by five s, five
p,five d
diffuse STOs
centered on the oxygen atom.
For H3O.,
they predicted two transitions at 550 nm (2.25 eV)
and 661 nm (1.8 eV)
with large Einstein emission coefficients
6.48 107 s-1
and 5.64 107 s-1
respectively, to the ground state.
Talbi and Saxon
[Talbi 89]
performed a thorough study of the PES of
the ground state and the first four doublet excited states.
Their basis set is supplemented by two s and two p
diffuse gaussians
centered only on the oxygen atom.
In all their MCSCF and CI computations, the reference configuration
carries the most important weight (0.94-0.95).
Their ground state geometry is pyramidal, with an energy of -76.556,186
Hartrees
at the MCSCF level,
comprising up to 17000 configurations state functions (CSFs),
and of -76.727,647 Hartrees at the MRSDCI level (up to 118,000 CSFs).
An Einstein coefficient of 5.46 107 s-1
was calculated for the 3s --> 3p
transition. The radiative lifetime of the 3p excited state
was estimated around 18 ns.
The ionization potential (IP) of the 3s ground state is 5.27 eV,
while the IP of the 3p excited state is 2.92 eV.
The dissociation barrier for
H3O.
---> H2O + H. was found to be
3.58 kcal/mole.
Taking into account the zero-point
vibrational energies at the local minimum and
at the transition point, the barrier is reduced to 0.40 kcal/mole
( D3O 1.32 kcal/mole).
Talbi and Saxon could not explain
why D3O. might be metastable,
contrary to H3O..
Another question which was left unanswered is why
D318O.
is not detected in experiments, although it includes
a heavier oxygen isotope
18O nucleus
which implies a slightly lower zero-point vibrational
energy.
After establishing a correlation diagram,
Talbi and Saxon concluded
that the dissociation barrier does not arise from configuration
mixing or avoided crossing, but originates from a de-Rydbergization
process
[Mulliken 77] .
McLoughin and Gellene
[McLoughin 92]
took into account vibrational energy and,
rather interestingly,
tried to explain
hydronium radical metastability with the help of the concept of
"adiabatic molecules" as set forward by Pollak
[Pollak 83] .
A dynamical barrier contribution might appear, if
the vibrational energy increases
along the dissociation pathway.
Towards this goal,
these authors
computed the energy at more than
300 different
geometries at the MP3 level, in order to establish a pointwise harmonic
fit of the
potential energy
surface (PES) in the vicinity of the dissociation pathway.
Their valence
basis set is supplemented by three s and two p
diffuse gaussians
on the oxygen atom, and three s-type and one p-type diffuse
gaussians on each hydrogen atom.
In the vibrational ground state,
the estimated tunneling lifetimes in the order
of 10-13,10-12 s are inconsistent
with experimental lifetime.
A dynamical barrier,
high enough to permit a microsecond lifetime,
is predicted to appear only when
the asymmetric strech normal mode
( 1444 cm-1 ) reaches its
fourth excited state.
In this unlikely high overtone,
H3O.
is predicted to feature a lifetime at least four times
larger than
D3O., in
contradiction with experimental data.
In regards to oxygen isotop effects, attention was brought
on the sensitivity of one cubic and one quartic potential
constants to oxygen isotopic substitution.
It is quite important to mention
that MRCI calculations
predict that the hydronium radical should be formed after
excitation of a linear water dimer.
The interplay between the
various excited states appears rather complicated
[Sosa 93] .
Recently, at a fixed geometry,
within a dielectric cavity, we have performed a series of
computations of
MCSCF-RF (Reaction Field) ground state
H3O.
wavefunctions.
Then, starting from these wavefunctions, using an electron
propagator method ( RPA approximation ), we have been able
to compute the excitation spectrum of
(H3O.)hyd.
[Muguet 96a] ,
[Muguet 96b] ,
( see also our
WATOC96 WWW poster ).
We predicted that the hydrated
(H3O.)hyd
radical should feature an absorption band,
which is, in regard to both shape and
energy range, quite similar to the experimentally observed
"hydrated electron"
absorption band. This computational result brings
support to Muguet's itinerant radical model.
Our results suggest also that the excited state radical
(H3O.)hyd*
could be assigned to the 1.45 eV absorbing species.
Very recently, at a fixed geometry,
within a dielectric cavity, we have performed a series of
computations of
MCSCF-RF (Reaction Field) ground state
H3O.
wavefunctions.
Then, starting from these wavefunctions, using an electron
propagator method ( RPA approximation ), we have been able
to compute the excitation spectrum of
(H3O.)hyd.
[Muguet 96a] ,
[Muguet 96b] ,
( see also our
WATOC96 WWW poster ).
We found that the predicted hydrated
(H3O.)hyd
radical and experimental
(e-)aq
absorption bands feature similar shapes, within the same spectral
1.7 eV (720nm) range.
Our results suggest also that the excited state radical
(H3O.)hyd*
could be assigned to the 1.45 eV absorbing species.
Therefore, it appears that, as
Talbi and Saxon concluded,
there are still "substantial gaps in our
understanding of the
H3O. system".
both in the gas phase and in the liquid phase.
We feel that
the Rydberg radical denomination brings in itself the seed of
possible misconceptions.
1/
Rydberg excited states are hydrogen-like states
[Robin 75]
which feature an atomic-like sharp spectra.
2/Highly excited Rydberg states have a quasi-classical character, and
the extra electron may be conceived as circling around the
positive core like in a planetary orbit.
Low-lying
Rydberg ground states are quite different from
highly excited Rydberg states.
In this paper, several questions are of a more specific concern :
What is the exact nature of the so called low lying Rydberg orbital ?.
Can we intuitively understand and
visualize the deRydbergisation process involved in the
dissociation process ?.
Are the basis sets employed so far, large enough ?.
