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A: Spectroscopy, Molecular Structure, and Quantum Chemistry
Infrared Spectroscopy of Protonated Phenol-Water Clusters Marusu Katada, and Asuka Fujii J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b04446 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 23, 2018
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The Journal of Physical Chemistry
Infrared Spectroscopy of Protonated PhenolPhenol-Water Clusters Marusu Katada and Asuka Fujii* Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan *Corresponding author E-mail:
[email protected] ORCID
Asuka Fujii: 0000-0002-6854-9636
Abstract To explore the microhydration structures of protonated phenol, size-selective infrared spectroscopy of protonated phenol-(water)n clusters (n = 1 – 5) was performed. Protonation of phenol can occur either at the phenyl ring or the hydroxy group. The coexistence of the two isomer types separated by the high isomerization barrier was reconfirmed for bare protonated phenol. Preferential hydration of the hydroxy group initially occurs in both the two isomer types of protonated phenol. Development of the water hydrogen-bond network is localized around the hydroxy group up to n = 2. Intracluster proton transfer from the phenol moiety to the water moiety was observed in
n ≥ 3~4. The water moiety with the H3O+ ion core resides on the phenyl ring, and the water moiety is bound to the phenyl ring with a π-hydrogen bond. Such a structure is 1
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in striking contrast to those of phenol+-(water)n radical cation clusters, in which the water moiety is located away from the phenyl ring even when intracluster proton transfer occurs.
1. Introduction Protonated aromatic molecules have attracted much interest in the diverse fields of chemistry and biochemistry.
1-7
Protonated aromatics are well-known crucial
intermediates in electrophilic aromatic substitution reactions.
1
The presence of
protonated aromatics in interstellar space has been suggested. 2,3 Moreover, protonated aromatics are expected to play important roles in biochemical processes since the proton is one of the major carriers of positive charge in such systems.
4
Therefore,
characterization of protonated aromatic molecules and their microsolvated clusters in the gas phase has been extensively performed by spectroscopic, mass spectrometric, and theoretical approaches.
5-45
Not only the structure determination, e.g., identification of
the protonated site and their microsolvation structures, but also (excited state) proton transfer mechanisms have been studied. The simplest prototype of protonated aromatics is protonated benzene.
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Protonated benzene has the arenium (benzenium) ion structure, in which the
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The Journal of Physical Chemistry
protonated carbon atom has the sp3 hybridization and the resonance among the π orbitals is partly broken. This type of protonated species is also called σ-complex or Wheland intermediate. Though the proton affinity (PA) of benzene (750 kJ/mol) is larger than water (PA = 691 kJ/mol), 46 the hydration of protonated benzene with only a single water molecule induces the proton transfer reaction to form protonated water (hydronium ion, H3O+).
13
This is due to the large solvation energy of the hydronium
ion by benzene with the strong charge-induced dipole interaction. On the other hand, in substituted benzene molecules,
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the protonation of the
substituent competes with the protonation of the phenyl ring, and the former process can be exclusive when the “local” proton affinity of the substituent is much larger than that of the phenyl ring moiety. Such a case has been found in benzaldehyde.
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In
this context, protonated phenol, H+PhOH, is of great interest because competition between the protonation of the substituent (hydroxy (OH) group) and the phenyl ring has been observed.
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spectroscopy to H+PhOH.
Solcà and Dopfer have applied infrared (IR) dissociation 19-21
They found that at least two isomers of H+PhOH coexist
under the jet-cooled condition. In the most stable structure, the excess proton is located at the para (p-) position of the phenyl ring, resulting in an arenium ion. In the second stable structure, the excess proton is located at the ortho (o-) position of the phenyl ring.
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Protonation of the meta (m-) and ipso positions is much less stable. Since the arenium ions produced by the protonation at the p- and o-positions are difficult to distinguish by spectroscopy, hereafter we call them both Ph-type. Solcà and Dopfer have reported that the excess proton can also be located at the lone pair electrons of the OH group and this forms an oxonium ion.
Hereafter, this isomer is called O-type.
Though the
relative energy of the O- type is over 70 kJ/mol higher than that of the most stable
Ph-type, Solcà and Dopfer have observed the competitive production of the O-type isomer under the supersonic jet expansion cooling condition, and they attributed it to the high potential energy barrier (~160 kJ/mol) in the isomerization from the O-type to the Ph-type. The microsolvation of H+PhOH is also of great interest. Solcà and Dopfer performed IR spectroscopic studies on the microsolvation by inert gas species (Ne, Ar, and N2).
19-21 They
showed that the solvation begins with hydrogen (H-) bond formation
to the acidic OH group(s) and then proceeds to π-bond formation, while intermolecular binding to the aliphatic CH2 group in the protonated phenyl ring is not preferred. On the other hand, studies on the microsolvation of H+PhOH by water have been very scarce in spite of its special importance in relation to the protonation of tyrosine in biological environments. H-bonded structures of H+PhOH-(water)n (H+PhOH-Wn)
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The Journal of Physical Chemistry
clusters and the size dependence of their intracluster proton transfer have been studied by quantum chemical computations and energy-resolved collision-induced dissociation (CID) measurements.
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However, the computations were performed for only a small
number of hydrated structures based on the Ph-type isomer.
24
It is also difficult to
definitively infer H-bonded structures of clusters from CID experiments because rearrangements of clusters can occur prior to dissociation. On the other hand, neutral phenol-water clusters (PhOH-Wn) and phenol-water radical cation clusters (PhOH+-Wn) have been extensively studied in the gas phase. 47-66 Neutral PhOH-Wn has been investigated by size-selective IR spectroscopy as a model of water H-bond networks.
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It has been demonstrated that the H-bond network of
PhOH-Wn develops from ring structures to cage structures as the cluster size increases. 47-57
Further processes to form bulk-like structures including fully solvated water
molecules have been discussed.
58,59
The H-bonded structures of PhOH+-Wn have also
been investigated by IR and electronic spectroscopies, and the occurrence of intracluster proton transfer from the phenol cation to the water moiety has been confirmed in n ≥ 3 ~ 4.
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We should note that both the H-bonded networks of neutral and cationic
hydrated phenols develop around the phenolic OH group and a direct interaction between the water network and the phenyl ring has not yet been observed in these
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systems. As mentioned above, the two isomer types of H+PhOH coexist under the jet-cooled condition.
19-21
The differences in the microhydration structures between these two
types and interconversion between the isomers with the development of hydration are of great interest.
In the present study, we apply IR dissociation spectroscopy to the
H+PhOH-Wn (n = 1- 5) clusters to explore the development of the H-bond network structures of these clusters with the help of quantum chemical calculations and by comparison with related systems. The IR spectrum of the H+PhOH monomer was measured by the Ar-tagging method to determine the isomer ratio under the experimental conditions as well as to confirm the presence or absence of radical cation species. 67-69
2. Experimental Experimental and Computational Methods Size-selective IR spectra of H+PhOH-Ar and H+PhOH-W1-5 in the OH and CH stretching vibrational region (2800 - 3900 cm-1) were measured by IR dissociation spectroscopy. The mass selection of the clusters was performed by a tandem-type quadrupole mass spectrometer.
68, 69
Protonated clusters were generated by electron ionization with an
electron gun (Omegatron Co.) followed by supersonic jet expansion cooling. The sample
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The Journal of Physical Chemistry
of phenol (Kanto Chemical Co.) was used without further purification. Phenol was heated to ~280 K in the sample container of a high pressure supersonic jet valve (Even−Lavie valve).
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The phenol vapor was seeded in the carrier gas of He/Ar/H2
(80:10:10) or He/H2 (95:5) containing trace water vapor. The total stagnation pressure of the jet expansion was 60-80 atm. The electron beam accelerated at a voltage of 200 V was crossed with the jet expansion in the collisional region. The protonated cluster of interest was selected by the first quadrupole mass filter and the cluster was introduced into the octopole ion guide. The size-selected cluster was irradiated by an IR laser in the ion guide. A fragment ion is generated via predissociation when the IR frequency is resonant on a vibrational transition of the cluster. The second quadrupole mass filter was set to pass only the fragment ion and the IR spectrum of the cluster was measured by monitoring the intensity of the fragment ion while scanning the IR frequency. The IR light was generated by an OPO/OPA system (LaserVision), and the typical IR power was ∼2 mJ/pulse. The IR spectrum of the H+PhOH monomer was measured by using the He/Ar/H2 carrier gas and by the Ar-tagging method. On the other hand, the IR spectra of the H+PhOH-Wn clusters were measured by using the He /H2 carrier gas and by monitoring the water-loss channel. The dissociation of PhOH was not detected in the present
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measurements. In the present experiment, the protonated cluster ion intensity was not high enough to achieve a mass resolution of ∆m/z
