Photodissociative charge transfer in Ag+-pyridine complex

Article abstract of DOI:10.1016/S0009-2614(99)00378-4

Ag⁺-pyridine complex has been generated in the gas phase. Dissociative metal-to-ligand charge transfer was observed through the photodissociation technique. Two continuum absorption/dissociation bands were recorded by monitoring the pyridine cation fragments as a function of the laser wavelengths and the possible dissociative mechanisms are discussed. The binding energy of the Ag⁺-pyridine was determined to be 45 kcal/mol. Ag⁺ bonding to nitrogen atom in C₅H₅N with ion-dipole interactions is suggested.

Full text of DOI:10.1016/S0009-2614(99)00378-4

28 May 1999
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Chemical Physics Letters 305 1999 395–400
Photodissociative charge transfer in Agq–pyridine complex
)
Yuh-Sheng Yang, Chen-Sheng Yeh
Department of Chemistry, National Cheng Kung UniÕersity, Tainan 701, Taiwan
Received 11 December 1998; in final form 16 February 1999
Abstract
Agq–pyridine complex has been generated in the gas phase. Dissociative metal-to-ligand charge transfer was observed
through the photodissociation technique. Two continuum absorptionrdissociation bands were recorded by monitoring the
pyridine cation fragments as a function of the laser wavelengths and the possible dissociative mechanisms are discussed. The
binding energy of the Agq–pyridine was determined to be 45 kcalrmol. Agq bonding to nitrogen atom in C5H5N with
ion–dipole interactions is suggested. q 1999 Elsevier Science B.V. All rights reserved.
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1. Introduction
lations 2–10 . Electrostatics are found to contribute
predominantly to the bonding.
Studying metal ion–molecule complexes in the
gas phase permits us to elucidate bonding and ge-
ometries of metals to ligands and microscopic behav-
ior of chemical and physical processes, which pro-
vide relevant information in understanding catalysis,
oxidation and electron transfer involved in biological
processes, and the organometallic chemistry, without
interference from the solvent. Metal cation–aromatic
systems are particularly attractive because of their
bonding scheme involving interactions between metal
ions and the delocalized p electrons of the aromatic
structures, which are important in determining the
In addition to theoretical predictions, several tech-
niques, such as photodissociation, and collision-in-
duced dissociation, have been employed to acquire
binding energetic information. Among these experi-
ments, a photo-process known as ‘dissociative intra-
molecular charge transfer’ in metal ion–benzene
complexes was reported by Duncan and co-workers
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3–5 . By monitoring the intensity of the fragment,
C6 Hq6 , as a function of laser wavelength, the Agq–
C6 H6 complex exhibited a continuous absorptionr
dissociation spectrum. This absorption is expected to
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.
result from a charge-transfer CT electronic transi-
tion, i.e. excitation to the energy above the dissocia-
tion limit of a CT band. By measuring the threshold
of a structureless appearance combined with the IP
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structure of the biological macromolecules 1 . The
binding energies of the metal cations, e.g. alkali and
transition metals, with the simplest aromatic com-
pound, benzene, have been investigated extensively
by experimental measurements and theoretical calcu-
difference between Ag and C6 H6, the dissociation
q
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.
energy of the ground state Ag –C6 H6 representing
an upper limit can be obtained.
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.
In this Letter, pyridine C5H5N , an azabenzene,
formed a complex with Agq in the gas phase. The
metal-to-ligand CT transition associated with disso-
)
Corresponding author. E-mail: csyeh@mail.ncku.edu.tw;
fax: q886 6 274 0552
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
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PII: S0009-2614 99 00378-4

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Y.-S. Yang, C.-S. YehrChemical Physics Letters 305 1999 395–400
ciation occurred and two absorption bands were ob-
served with this system. The photochemical pro-
cesses are more complicated than the Agq–benzene
mentioned above. To our knowledge, there is no
report on Agq–Py complex in the solution or in the
gas phases. Different from benzene, pyridine can be
treated as a p-type complex or as an onium-type
complex due to the lone-pair electron on N atom as
metal plasma region, where the complexation and
condensation took place to form Agq–Py, by a
Jordan valve with 0.75 mm diameter nozzle. The
metal ion complexes were then expanded and
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.
skimmed 1 mm diameter into a reflectron time-of-
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.
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.
flight TOF mass spectrometer R.M. Jordan, USA .
The positive ions were extracted by applying the
pulsed voltages, i.e. 1460 V of repeller plate and
1350 V of extraction plate. The parallel plates that
served as the mass gate are 46 cm above the extrac-
tion place. The mass gate performed the mass-
selected function. When the desired ion packet
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discussed by Mulliken 11 , although the latter is
favored to form complex. Therefore, it will be inter-
esting to inspect the structure and the bond energy of
the metal ion–pyridine species.
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.
reaches the mass gate, the voltage q300 V is
pulsed to ground. The selected ions traveled in the
field-free region and reached a reflector at the end of
the flight tube. In photodissociation experiments, the
dissociation laser crossed the ion beam at the turning
point in the reflector. The fragment and parent ions
were detected by a dual microchannel plate detector.
The experimental setup and performance is shown
schematically in Fig. 1.
2. Experimental
Silver ion–pyridine complexes were generated by
laser vaporization method combined with supersonic
molecular beam. A rotating and translating silver rod
Y
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99.9%, 1r4 diameter was irradiated using 532 nm
wavelength of the second harmonic output of a
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Nd:YAG laser Quantel Brilliant operated at 10 Hz
Photodissociation action was accomplished by
doubling the frequency of the tunable dye laser
Ž
with 2 mJ per pulse. The pyridine vapor 20.7 Torr
. w
x
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.
at 258C 12 was mixed with helium carrier gas 6
Continuum ND6000 pumped by the second or third
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harmonic of a Nd:YAG pulsed laser Continuum
.
atm backing pressure , and was injected into the
Fig. 1. Schematic diagram of the experimental performance using the reflectron TOF mass spectrometer.

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Y.-S. Yang, C.-S. YehrChemical Physics Letters 305 1999 395–400
397
.
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Surelite . Wavelength-dependent spectrum was ob-
which was developed by Duncan 13 , has been
utilized to produce ion clusters. In this configuration,
minimum collisions between Ag ions and pyridine
molecules are achieved to diminish the clustering. If
the nozzle source has a channel after the vaporiza-
tion point for the cluster growth rather than for
immediate gas expansion, the number of the pyridine
molecules up to 6 coupled with silver ions has been
synthesized. Fig. 2b displays photodissociative mass
spectrum from mass-selected Agq–pyridine at 286.5
nm. The depletion signal represents the parent ions
tained by monitoring the fragment ion intensities as a
function of the dissociation laser wavelengths. The
laser fluence was kept at 2 mJrcm2 throughout the
experiments. Fragment power dependence studies re-
vealed that the signals were attributed to a one-pho-
ton absorption.
3. Results and discussion
q
Ž
.
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.
Ag – pyridine complexes are shown in Fig. 2a.
with two silver isotopes 107, 109 and is accumu-
lated based on a data acquisition cycle; photodissoci-
ation laser on subtracted from laser off. One daugh-
ter ion was observed and assigned as pyridine frag-
ment. Silver has lower ionization potential than that
x
It is apparent that Agq–pyridine has dominant inten-
sity in the mass spectrum. To acquire this feature, a
rod holder so called ‘cutaway’ source designed,
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x
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of pyridine. IP values are 7.57 14 and 9.26 eV 12
for Ag and pyridine, respectively. Hence, the posi-
tive charge is expected to reside on Ag, once the
complex is formed in the source. Many beam experi-
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x
ments 3–5,8,9,13 , pertaining to the formation of
the metal ion–organic molecule or –rare gas clus-
ters, have provided this evidence. Under similar
experimental conditions as made for Agq–pyridine,
we have produced Agq–benzene and acquired ben-
zene CT channel by photodissociation. Moreover, it
can be seen that one single fragment rather than two
isotopes’ peaks resulting from Ag indicates pyridine
appearing in Fig. 2b. Our other experiments have
shown that the fragment isotopes can be resolved
Ž
clearly after photodissociation, for example Cu 63,
.
65 . Therefore, it is concluded that intramolecular
metal-to-ligand charge transfer has occurred in
Agq–pyridine complex.
Photoinitiated CT process was observed from 341
to 237 nm, as shown in Fig. 3. During search signal,
the parent ions needed to be kept as constant as
possible. From the Fig. 3, it shows that the experi-
ments were all operated using the UV light; the
frequency doubled output from the red dyes pumped
by 532 nm to the blue dyes pumped by 355 nm.
Detailed continuously scan was not performed for
whole spectrum, whereas the spectral ranges from
343 to 340 nm and between 300 and 295 nm have
been scanned by tuning dye laser. The spectrum has
been checked twice and more wavelengths than the
data points of the figure were inspected for the
intensity of the pyridine ion. A continuous signal and
TOF mass spectrum of the Agq – pyridine com-
Ž .
Ž .
b
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.
Fig. 2.
plexes.
a
x
Dissociation mass spectrum of the mass-selected
Agq –pyridine at 286.5 nm wavelength with 2 mJrcm2.

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Y.-S. Yang, C.-S. YehrChemical Physics Letters 305 1999 395–400
3
q
˚
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x
10.3 A 12 of C6 H6. If Ag –C5H5N has same
electrostatic interactions, i.e. charge-induced dipole,
both complexes should have close binding energies.
Using the DY0 ground state of the Ag –C6 H6 or
values less than this combined with the DIP between
the silver atom and pyridine, this will place the
photoexcitation energies measured above the dissoci-
ation limit of the lowest CT electronic state. Another
possibility is due to the absorption of the molecular
q
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.
chromophore. It is known that the allowed electronic
1
Ž1
.
n–pU transition B1 § A1 starts at 287 nm for
q
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pyridine molecule 15 . Formation of C5H5N can
be carried out through the excitation of the Agq–
Fig. 3. Photodissociation spectrum for the Agq –pyridine. The
arrows indicate the thresholds of two continuous absorption bands.
qŽ1
.
C5H5N to the state corresponding to the Ag S q
C5H5N B1 surface, followed by the dissociation
Ž1
.
Ž2
.
.
via electronic curve crossing to the Ag S q
qŽ1
C5H5Nq asymptote. In this case, the Ag S q
a structureless dissociation spectrum were detected.
Different from the observations of the Agq–benzene,
two absorption bands were observed and the appear-
Ž1
.
C5H5N B1 potential surface will be shifted to the
ance of the CT process covers with narrower energy
y1
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.
range 237–341 nm; 12868 cm
than the benzene
y1
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.
complex does 260–418 nm; 14538 cm . The
pyridine cations begin to emerge near 340 nm and
reveal typical absorptionrdissociation behavior
which grows gradually, then falls off in the higher
energy region. The threshold of the spectrum is
determined at 341 nm. Nevertheless, the signals
increase again around 260 nm and display second
continuum band with smaller intensity.
Without the detailed information of this complex,
such as ground state and excited states, it will be
difficult to interpret the exact photochemical pro-
cesses yielding both absorptions. We have attempted
to give the possible mechanisms by referring to Fig.
4. The lowest excited state is the CT band correlating
Ž2
.
q
to the Ag S qC5H5N states. These atomic states
are 1.69 eV, calculated from the IP difference of the
Ag and C5H5N, above the Ag S qC5H5N A1
qŽ1
.
Ž1
.
constructing the complex ground state. From elec-
tronic excitation energy, it is reasonable to suggest
that the lower energy band is due to the CT elec-
tronic transition. The complex absorbing photon en-
ergies exceeds the dissociation limit of the CT band
to cause the formation of the pyridine fragment.
Considering the bonding, the charge-induced dipole
attraction is predominant in the Agq–benzene with
Fig. 4. Energy level diagram showing the asymptote states corre-
sponding to the silver atom and pyridine molecule. The ground-
state potential depth of the Agq –pyridine is 1.95 eV, which is
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1.58 eV binding energy 10 . The polarizability of
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.
determined from the relation between the threshold 341 nm
3
˚
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C5H5N is 9.5 A 12 , which is slightly smaller than
measured and IP difference of the Ag and C5H5N.

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Y.-S. Yang, C.-S. YehrChemical Physics Letters 305 1999 395–400
399
red at least 5500 cmy1 from the n–pU transition.
The resulting energy shifted is acceptable for the
species with mainly electrostatic bonding, as ex-
pected in this system. For the second maximum band
located from 260 to 230 nm, the onset is placed at
265 nm. The CT mechanism can be originating from
tion energy of the ground state can be deduced from
the following procedure:
hn0DY0 qDIP .
Ž
.
With the threshold 341 nm determined and the IP
difference of the Ag and C5H5N, the binding energy
Ž
.
was derived to be 45 kcalrmol 1.95 eV on the
the higher energy states, such as the p–pU transition
1
Agq–pyridine complex. For Agq–benzene with pri-
Ž1
.
A1 § A1 of the solvated pyridine, accompanied
mary ion-induced dipole interactions, the DY0 bind-
ing energy values were investigated 35 " 5
Ž
with dissociation through a repulsive surface to the
.
Ž2
.
q
Ag S qC5H5N . However, this process will re-
quire a solvating energy of 1.5 eV shifted to this
region. On the other hand, instead of considering the
kcalrmol by experimental and theoretical studies
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x
3–5,7,10 . Apparently, the pyridine complex is more
strongly bound than the benzene complex. Because
qŽ1
.
higher states, the state correlating to the Ag S q
Ž1
.
of a lone-pair electron on nitrogen atom, pyridine
C5H5N B1 asymptote is involved, followed by
electronic predissociation. In such way, the binding
in the excited state is less than it is in the ground
Ž .
molecule owns a 2.19 debye D of the dipole mo-
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ment 12 . Therefore, bonding to the nitrogen atom
and attractions predominantly resulting from
charge–dipole are considered in Agq–pyridine sys-
tem. Our preliminary calculations have shown that
the most stable structure is Agq toward nitrogen
atom with a C2v symmetry. However, it is worth to
mention that pyridine molecules adsorbed on Ag
surface via p-bonding scheme at low coverage con-
ditions, but it transformed into nitrogen-bonding by
the ‘squeezing’ effect, once the coverage increased
state, then the electronic transition will be blue shifted
y1
Ž
.
Ž
2900 cm
.
relative to the pyridine absorption n–
pU . If this dissociative route gives rise to it, the
stronger absorption peak in the lower energy region
is expected to be attributable to the direct CT transi-
tion. Since the photodissociation spectrum reveals
different intensities of both bands, we have made
cross-section measurements. The cross-sections are
estimated based on the relation
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in surface experiments 16 . The nitrogen-bonding to
the Ag surface was estimated to be less bound to the
p-bonding.
IrI0 seysf
,
where I and I0 represent the parent ion intensities
for dissociation laser on and off, respectively, s is
the cross-section, and f is the photon flux. The
cross-sections are 3.1=10y17 cm2 at 286.5 nm and
1.3=10y17 cm2 at 253.5 nm. The Franck–Condon
factor and the electronic state density of the excita-
tion energy region are important in determining the
dissociative efficiency. Unfortunately, without the
information of all the electronic states in the Agq–
pyridine complex, it is difficult to make detailed
analysis. However, both values are slightly larger
4. Conclusions
Pyridine, an aromatic species other than benzene,
combined with Agq displayed dissociative CT be-
havior. As a result of the binding energy, charge–di-
pole bonding between Agq and C5H5N is suggested
to be dominant. Compared with Agq–C6 H6, pyri-
dine complex has approximate 10 kcalrmol more in
dissociation energy. Theoretical calculations in
q
Ž
.
M –pyridine MsCu, Ag and experimental mea-
surements in complexes containing Cu are in
progress.
than the Agq–benzene cross-section, 10y18 cm2 4 .
w x
It should be noted that the exact overlap between the
laser beam and the complex ions was unable to be
determined in our experimental configuration. The
laser power and spot diameter were measured out-
side of the beam machine.
Acknowledgements
Either a CT electronic transition or an absorption
of the pyridine chromophore for the lower energy
structureless band, the upper bound of the dissocia-
We gratefully acknowledge National Science
Council of the Republic of China to support this
work under Grant No. NSC88-2113-M-006-008.

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Y.-S. Yang, C.-S. YehrChemical Physics Letters 305 1999 395–400
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P. Weis, P.R. Kemper, M.T. Bowers, J. Phys. Chem. A 101
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