Infrared Spectroscopy of Warm and Neutral PhenolâWater Clusters

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Infrared Spectroscopy of Warm and Neutral Phenol – Water Clusters Takuto Shimamori, and Asuka Fujii J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp512495v • Publication Date (Web): 02 Feb 2015 Downloaded from http://pubs.acs.org on February 6, 2015

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The Journal of Physical Chemistry

Infrared Spectroscopy of Warm and Neutral Phenol – Water Clusters Takuto Shimamori and Asuka Fujii* Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan

*) email

[email protected]

Abstract Although many studies have been reported on structures of neutral water clusters, most of experimental information has been restricted to their most stable structures. With elevation of temperature, however, transient structures as well as higher energy stable structures can be formed, as recently demonstrated in small-sized neat water clusters (Zischang and Suhm, J. Chem. Phys. 2014, 140, 064312). In the present study, we performed infrared spectroscopy of warm phenol-(H2O)2, which is an analogue of (H2O)3 concerning the hydrogen bond structure, in order to overcome the size uncertainty in the neat water cluster study. The strict size selection was achieved by infrared-ultraviolet double resonance spectroscopy combined with mass spectrometry. The temperature control of the cluster was accomplished by the reduction of the

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stagnation pressure of the jet expansion and the internal energy selective detection of the cluster. A remarkable shift to higher frequency of the phenolic OH stretch band was observed with elevation of temperature, suggesting deformation of the cluster from the most stable cyclic structure to the transient chain-type structure.

I.

INTRODUCTION

Recent studies on hydrogen-bonded clusters have greatly developed by means of various advanced spectroscopic techniques and high precision quantum chemical calculations.1 In most of experimental studies on neutral clusters, clusters are generated by using a supersonic jet expansion. As a result, vibrational temperature of the produced cluster is typically cooled down below 100  150 K (though the cluster in the jet is not strictly in equilibrium).2,3 Thus, the cluster tends to be exclusively distributed at the global minimum in the potential landscape and experimentally available information is practically restricted to that on the most stable isomer. However, a variety of stable isomer structures, which are different from the most stable one, can be formed.

With

elevation of temperature, the cluster is distributed to higher energy stable structures and it also temporarily stays in transient structures. 4-7 Such population switching among structural isomers is driven by the entropy factor, and it can be regarded as a


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microscopic phase change.4 Experimental observations of structural switching in gas phase clusters have been reported for some charged clusters by using a temperature variable ion trap or inert gas tagging for controlling internal energy of the clusters.8-21 However, such observations are still rare because of some restrictions in this type of experiments; the former approach requests a highly complicated apparatus. In the latter approach, tuning range of internal energy is restricted and irregular preference of higher energy isomers sometimes occurs.15-17 Moreover, both the techniques for internal energy control essentially cannot be applied to neutral clusters.

Therefore,

experimental observations of temperature dependence of neutral cluster structures have been very scarce. Here we should note that influence of excess charge (e.g., an excess proton) to cluster structures is significant, especially in the small cluster size region.22 Therefore, observations in neutral clusters are highly requested even if temperature dependence of similar charged clusters has been well studied. In the present study, we focus on structural changes of small water hydrogen bond (H-bond) networks with elevation of temperature (internal energy). Structures of small water clusters have been widely investigated to understand H-bond networks of water.23-43 Their most stable structures have been established both by spectroscopic studies and high level quantum chemical calculations.


However, chemically or


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biologically important roles of water are practically carried by water at near room temperature. Therefore, following the establishment of the water cluster structures at 0 K, it is strongly requested to explore higher energy structures in warm clusters and their structural switching with elevation of temperature. Some theoretical studies have been performed for this purpose.44,45 For example, the potential hypersurface search of (H2O)8 by Maeda and Ohno has predicted structural switching from the well-known cubic structures to bicyclic structures at around 300 K.45

As described above, however,

there have been few experimental reports on temperature control for neutral clusters because of technical difficulties. Recently, Zischang and Suhm have performed a pioneering work on warm water clusters.46

They have measured the direct infrared

(IR) absorption of neat water clusters by using Fourier transform infrared (FTIR) spectroscopy. With the reduction of the stagnation pressure of the jet expansion, they have observed IR spectra of warm water trimer and tetramer and have concluded the structural change from the most stable cyclic type to the (transient) open chain type. To our best knowledge, this study has been the unique experimental observation of the temperature change of small neutral water clusters so far. In this study, however, the free OH region, which is very informative to determine water cluster structures, was severely contaminated by intense monomer bands, and the size selection of the warm


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clusters was not necessarily free from ambiguity because of the intrinsic restriction of direct absorption spectroscopy. In the present study, we are inspired by the pioneering study by the Suhm’s group, and we explore structural changes in warm water H-bond networks by using size-selective IR spectroscopy. To perform strict size selection, we introduce an ultraviolet (UV) chromophore to water clusters and employ the IR-UV double resonance scheme.47 Phenol-water mixed cluster (PhOH-(H2O)n) are suitable for this strategy since the H-bond network structures of neutral PhOH-(H2O)n clusters have been shown to be essentially equivalent to those of neat (H2O)n+1 clusters.48-56 Here, temperature (or internal energy) of clusters is controlled by two methods. One is reduction of cooling efficiency of clusters by reduction of the stagnation gas pressure of the supersonic jet expansion. The other is, as mentioned in detail later, selective detection of warm components of the cluster by the choice of the UV probe laser frequency. In this paper, we report IR spectroscopy of the warm and neutral PhOH-(H2O)2 cluster. This cluster (an analogue to neat (H2O)3) is at the minimum size to form a cyclic (triangle) structure, which is composed by three OH…O H-bonds. Though this cyclic structure is the minimum energy structure, its H-bonds are highly distorted. Therefore, it



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is expected that the cyclic structure is relatively easily distorted to a chain-like structure by thermal excitation.

II.

EXPERIMENTAL

PhOH-(H2O)n clusters were generated in a supersonic jet of a gaseous mixture of phenol, water, and helium, which is expanded into a vacuum chamber through a pulsed nozzle (General Valve Series 9). In order to control the stagnation pressure of the jet, we introduced a reservoir tank to store the He gas. By controlling the He gas pressure at the reservoir tank, the collision efficiency between the samples and He carrier gas in the jet expansion was tuned, and the reduction of cooling efficiency of clusters was achieved. In the present study, we tuned the stagnation pressure of the He gas in the range of 1  4 atm. The jet expansion was skimmed to form a molecular beam and the molecular beam was introduced into the interaction region. For measurements of IR spectra of the PhOH-(H2O)2 cluster in the electronic ground state, we adopted the IR-UV double resonance technique. The ground state population of the cluster was probed by monitoring the [PhOH-(H2O)2]+ ion intensity produced by two-color resonant two–photon ionization (R2PI) of the cluster. Produced ions were mass-selected and detected by a Wiley-McLaren type time-of-flight


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mass spectrometer. An IR pulse was introduced about 50 ns prior to the UV pulse and its wavelength was scanned. When an IR transition occurs, the population of the cluster is reduced, resulting in depletion of the ion intensity. The first UV light was the second harmonic of a dye laser output (Laser Analytical Systems (LAS), Coumarin 575 dye) pumped by the second harmonic of a Nd:YAG laser (Surelite III, Continuum). Also the second UV light was prepared by the second harmonic generation of another dye laser system (LAS) with the DCM dye.

The first and second UV pulses were spatially

overlapped without delay time. An IR light was generated by difference frequency mixing between the second harmonic of a Nd:YAG laser (PowerLite8000, Continuum) and the fundamental output of a dye laser (ND6000, Continuum, DCM dye). Density functional theory (DFT) calculations for structural optimization and vibrational frequencies were also performed at the B3LYP/6-31+G(d) level, which has been proved to provide highly reliable estimations of structures and IR spectra of H-bonded systems.57

All the calculations were performed by the Gaussian 09 program

suite.58

III.

RESULTS AND DISCUSSION

Figure 1 provides an overview of the potential energy curves and excitation schemes of



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PhOH-(H2O)2.

48-56

Ebata et al. have performed size-selective IR spectroscopy of

PhOH-(H2O)2 and they have established that the most stable structure of this cluster is cyclic in the electronic ground (S0) state.48

On the other hand, their dispersed

fluorescence study of the cluster has indicated that remarkable structural changes occur in the first electronic excited (S1) state, and chain-type structure formation has been suggested.59 This structural change upon the electronic excitation is rationalized by the following reason; the cyclic structure in the S0 state is due to the balance between the maximum number of H-bonds (advantage) and much distortion in each H-bond (disadvantage). Upon the electronic excitation, the acidity of the phenolic OH is largely enhanced while its proton affinity is weakened. Therefore, in the S1 state, the optimization of the hydrogen bond from the highly acidic phenolic OH group to the acceptor water is more advantageous in the total energy even though the H-bond from the water donor to the phenol acceptor cleaves. In the S1-S0 electronic transitions of PhOH-(H2O)n clusters, the strong and broadened band (~36230 cm-1) near the origin band of the phenol monomer (36348 cm-1) is the lowest frequency band of the cold PhOH-(H2O)2 cluster. However, this band is assigned to vibronic bands of intermolecular vibrations in the Franck-Condon region rather than the origin band (see Fig. 1(a)).59 The origin band from the S0 cyclic structure to the chain-like structure in S1


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is expected to be in the much lower frequency region but is practically forbidden because of the poor Franck-Condon factor between the largely different structures. If intermolecular vibrations are thermally excited in S0 by elevation of cluster temperature, it will loosen the cyclic structure and finally open the access to the chain-type structure in the S1 state because of the better Franck-Condon overlap (see Fig. 1(b)).

Such an

electronic transition is expected to appear in the frequency region lower than that of the Franck-Condon transitions of the cold cluster. Thus, we would be able to selectively measure IR spectra of warmer clusters by tuning the UV probe frequency to the lower frequency region. Two-color R2PI spectra of the PhOH-(H2O)2 cluster measured by monitoring the [PhOH-(H2O)2]+ mass channel under the different stagnation pressure conditions are shown in Fig. 2.

In these measurements, the second UV laser frequency for the

ionization was fixed at 31254 cm-1 to suppress the dissociation after the ionization. Spectrum (A) in Fig. 2 was observed at the higher stagnation gas pressure (4 atm), and the spectrum mainly reflects the most stable structure of the cold cluster. The somewhat broadened band appearing at ~36230 cm-1 is assigned to the lowest Franck-Condon allowed vibronic band of the most stable cyclic structure of PhOH-(H2O)2.

The

UV-UV hole-burning study has demonstrated that this broadened band is attributed to



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intermolecular vibrations of a single isomer.60

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A sharp peak (asterisked band) at the

higher frequency side is assigned to the origin band of PhOH-(H2O)3, which appears at the [PhOH-(H2O)2]+ channel because of the dissociation after the ionization.48,61 Its much narrower band width indicates that large rearrangement of the H-bond structure such as that in PhOH-(H2O)2 does not occurs upon the electronic excitation of PhOH-(H2O)3 since the larger H-bond ring size in PhOH-(H2O)3 largely reduces distortion in each H-bond. With the reduction of the stagnation pressure to 1 atm, the major band of PhOH-(H2O)2 becomes broader and its tail to the lower frequency is enhanced (spectrum (B) in Fig. 2). These spectral changes suggest that the reduction of the stagnation pressure reduces the collisional cooling efficiency of the cluster and the warmer cluster has a better Franck-Condon overlap to the chain-type structure in S1. We chose five frequency points ((b) to (f) in Fig. 2(B)) as the UV probing wavelengths in the measurement of IR spectra of the warm cluster.

A mass spectrum obtained by the

excitation at point (b) is shown in Fig. S1 (in the Supporting Information).

We tuned

the jet expansion condition so that ion signals of clusters higher than [PhOH-(H2O)2]+ are enough suppressed to avoid the cluster size contamination of IR spectra. First, we measured an IR spectrum of the cold and cyclic PhOH-(H2O)2 cluster.


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The spectrum is shown in Fig. 3(a). The UV probe wavelength was fixed at the center of the vibronic band of the cold cluster (position (a) in Fig. 2(A)) and the IR wavelength was scanned in the OH stretching vibrational region. In this spectrum, the dotted line is the baseline and the upward signal represents depletion (IR absorption). This spectrum agrees very well with the previously reported spectrum of the cluster.48 The most stable structure of PhOH-(H2O)2 is cyclic and its schematic representation is shown in Fig.4. Four remarkable bands labeled by P (3388 cm-1), W1 (3505 cm-1), W2 (3553 cm-1), and F (3725 cm-1) are observed in the spectrum. These bands are mainly attributed to H-bonded phenolic OH stretching vibration, H-bonded OH stretching vibration of water moiety (I), H-bonded OH stretching vibration of water moiety (II), and free (dangling) OH stretching vibrations of the water moieties, respectively.

These assignments are

based on the IR simulation of the energy-optimized cyclic cluster at the B3LYP/6-31+G(d) level (see Fig.4) and the normal mode analyses shown in the Supporting Information. Here we should note that each normal mode of the H-bonded OH stretch bands in PhOH-(H2O)2 is rather localized in one of the OH bonds, reflecting different strength of the three H-bonds because of the presence of phenolic OH. This situation is different from that in neat (H2O)3, in which three H-bonds are basically equivalent.

Water moiety (I) is the water molecule accepting the H from phenol and



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donating its H to the other water. Water moiety (II) is that accepting the H of water (I) and donating its H to the phenol moiety (see Fig. 4).

Because phenol is the strongest

proton donor but the weakest proton acceptor, band P is at the lowest frequency and band W2 is at the highest frequency among the H-bonded OH stretch bands. Figures 3(b) to 3(f) show the IR spectra of warm PhOH-(H2O)2 clusters obtained by probing at points (b) to (f) shown in Fig. 2(B), respectively. Also in these spectra, the free, H-bonded, and phenolic OH stretching vibrational bands appear. A small enhancement signal (downward signal below the baseline of the spectra) is also observed in the low frequency region and this is attributed to absorption of larger-sized clusters, PhOH-(H2O)n>2, which dissociate into the [PhOH-(H2O)2]+ channel. Band P (phenolic OH stretching band) is largely shifted to higher frequency with probing at the lower frequency electronic transition. Significant band broadening is also seen with lowering the UV probing frequency. On the other hand, the shift trend of the H-bonded OH stretch bands of the water moieties (W1 and W2) is rather unclear. However, judging from the extension of the higher frequency tail and suppression of the relative peak intensity of band W2 with lowering the UV probing frequency, it seems that shift to higher frequency and significant broadening simultaneously also occurs at least in band W2. Moreover, clear disappearance of the bands does not occur even with the


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probing the lowest UV frequency (spectrum (f)), though the band broadening is most significant. Structural switching of the cluster from the cyclic-type to the chain-type is expected with elevation of temperature (this would be detected by the change of the probing UV frequency, i.e., range of the magnitude of the intermolecular vibrational excitation). This structural switching should be associated with destruction of one H-bond (the weakest one which binds water (II) and the phenol moieties is most probable) and result in disappearance of one H-bonded OH band. In addition, appearance of new free OH bands (1 and 3) of the terminal water moiety in the H-bond chain is also expected in the lower (1) and higher (3) frequency sides of the dangling OH stretch band (band F).

However, such features are not seen in the

observed spectra. Moreover, the evolving spectra very much look like warmer spectra of the same cluster structure. Therefore, the observed spectra suggest that the cyclic structure might be loosened by the thermal excitation but most of them are not transformed into the chain-type structure and they still keep the cyclic H-bond framework. In the following, we consider implications of the OH frequency shift trends upon the temperature elevation. The loosening of the cyclic structure to the chain-type



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structure may cause opposite effects on the frequency shifts of the H-bonded OH stretching vibrations. The elongation of the H-bond lengths by thermal excitation weakens the hydrogen bond strength. This would induce shifts to higher frequency. The weakest H-bond in the cyclic structure is the one from water moiety (II) to the phenol moiety. Then if this H-bond bond is most loosened, the cooperative effect in the phenolic OH is largely reduced. This may cause further enhancement of the shift to higher frequency of the phenolic OH. On the other hand, once the cyclic structure is loosened to the chain-type structure, it reduces the distortion in the two H-bonds which survive in the chain-type structure (their H-bond angles can be more optimized in comparison with the cyclic structure). This factor may cause shifts to lower frequency of the bands. Directions of the actual band shifts are determined by the balance among these effects. In the observed IR spectra, a remarkable shift to higher frequency is seen for band P. Thus, the effect of the weakening of the hydrogen bond by thermal excitation and reduction of the cooperative effect is superior in the phenolic OH.

In

the two H-bonded OH bands of the water moieties, the intensity of band W2 (that is the more weakly H-bonded OH) relative to W1 becomes weaker and the band width is more significantly broadened to the higher frequency side with probing lower UV frequency. On the other hand, the peak position of band W1 does not show a clear shift. This


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different band behaviors of the water moieties suggest that the weaker H-bond is more loosened and its H-bonded OH band (W2) shifts to higher frequency resulting in the much weaker peak intensity and long tail to the higher frequency, while the stronger H-bond (middle in the chain) of water (I) moiety is not largely affected since its cooperative enhancement of the H-bond (due to the H-bond from the phenolic OH) is still kept even in the chain structure.

The shift to higher frequency of the H-bonded

OH bands with elevation of temperature has also been observed for neat (H2O)3 and (H2O)4 by Zischang and Suhm, and this supports the generality of the shift direction in the smallest cyclic H-bonded water networks.46

In the present observation, the

size-selection of the cluster is more rigorous than the direct absorption study.

It should

be, however, also noted that the presence of the non-equivalent H-bond (phenolic OH) makes the observation of the shift easier. For further support of the above discussion, we performed DFT calculations of geometrical optimizations and frequency calculations for the cyclic and chain-type structures

of

PhOH-(H2O)2.

All

calculations

were

performed

with

the

B3LYP/6-31+G(d) level and vibrational frequencies were scaled by the factor 0.9749 to fit to the free OH stretching vibration band of cold PhOH-(H2O)2. Although a cyclic structure well converges in the geometry optimization, any chain-type structures are not



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stable and their initial structures converge into the cyclic structure or run out in the optimization. This calculation result is supported by the fact that a clear origin band (corresponding to the transition between the chain-type structures) has never been found for the S1 - S0 transition of PhOH-(H2O)2.59 Missing of stable chain-type structures has also been reported for the analogous system, (H2O)3, by the Suhm’s group.46

Then, as

a simulation of the chain-type structure, we carried out a partial geometry optimization of PhOH-(H2O)2, in which we added the terminal water molecule (water (II)) to the most stable structure of PhOH-(H2O)1 and optimized this chain-type PhOH-(H2O)2 cluster under the restriction that all the oxygen atoms are in a plane. This is essentially the same method as the Suhm’s group employed in their analyses of the warm water clusters.46 Figure 4 indicates the results of the structure optimizations and vibrational simulations. Two spectra in the upper box are the observed spectra of cold (blue trace) and warm (red trace) clusters (reproduction of Figs. 3 (a) and (f)). The calculated stick spectra are shown in the lower box. The observed shift trend of the phenolic OH (band P) and the H-bonded OH band of the water (I) moiety (band W1) is well reproduced by the calculated spectra. The band shift of the phenolic OH is more remarkable, and this demonstrates that the reduction of the cooperative effect between the water (II) and phenol moieties is very effective to the shift of the phenolic OH band (band P). As


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mentioned above, however, there still remains the H-bonded OH stretching vibrational band of water (II) moiety (band W2) in the observed spectrum of the warm cluster, and the 3 band of the terminal water molecule in the chain-type structure is not seen in the observed spectrum. Comparison with the simulated IR spectra also supports that the observed warm cluster dominantly keeps the third H-bond and hardly stay in the transient chain-type structure. To confirm the above interpretation of the spectra of warm PhOH-(H2O)2, IR spectra for PhOH-(H2O)1 were also measured under the same conditions as those for PhOH-(H2O)2.

First, stagnation pressure dependence of the S1 - S0 electronic spectrum

is shown in Fig. 5.

Since PhOH-(H2O)1 is uniquely in the chain-type structure both in

S1 and S0, no extremely broad component is seen in the electronic spectra even under the warm condition (stagnation pressure 1 atm).

Two UV frequencies, indicated by

arrows (a) and (b) in the electronic spectrum, were selected to measure IR spectra of cold and warm PhOH-(H2O)1 clusters, respectively. The observed IR spectra are shown in Fig. 6. In both the IR spectra, two bands are observed, namely free OH stretch (3) of the water moiety and H-bonded phenolic OH stretch at 3750 cm-1 and 3524 cm-1 (under the cold condition), respectively.48 The phenolic OH shows a small shift to higher frequency and line broadening in the spectrum of the warm cluster. But their magnitude



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is much less in comparison with PhOH-(H2O)2, though they were measured under the same jet expansion condition.

This implies that in PhOH-(H2O)2, the H-bond structure

deformation to the chain-type structure (H-bond ring opening) and resulting suppression of the cooperative effect is more crucial for the frequency shift of the phenolic OH band than the simple elongation of the H-bonds by the thermal excitation. Finally, we briefly discuss on the temperature estimation of the cluster. We simply estimate temperature in the jet (molecular beam) at the interaction region based on the relative intensity of the hot band in the R2PI spectrum of bare phenol. Figure 7 shows the R2PI spectra under the two different stagnation pressures, which were fixed at the same conditions as those for the observations of the PhOH-(H2O)2 clusters. A weak band is seen at the 67 cm-1 lower frequency of the 0-0 transition. This band is the 16b11 hot band.62 With the reduction of the stagnation pressure, the intensity of the hot band is largely enhanced. We roughly estimate temperature of bare phenol by the Maxwell-Boltzmann distribution, exp where

and

mean the populations of molecule in the vibrational excited and

ground levels, respectively, (503 cm-1), and

,

and

is the vibrational energy of the mode 16b in the S0 state are the Boltzmann constant and temperature, respectively.


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We supposed that the Franck-Condon factor of the 16b11 hot band is the same value as the 0-0 band.

Then, this estimation yields the temperature of 140 K and 175 K for the

stagnation gas pressures 4 and 1 atm, respectively, and the elevation of temperature with the reduction of the stagnation gas pressure is confirmed. Here we should note the following points; (1) this measurement is performed for the pulsed supersonic jet expansion, and thermal equilibrium is not necessarily achieved in the jet. Therefore, temperature of PhOH-(H2O)2 is also not necessarily same as that of bare phenol. (2) The condition is optimized for the production of PhOH-(H2O)2 and the temperature of bare phenol at the stagnation pressure 4 atm seems rather higher than that expected for a typical jet expansion. This might be because the partial thermalization of the jet due to the reflection from the skimmer. (3) In the present observation of the IR spectra, as described above, the magnitude of the internal energy of the cluster was selected by the choice of the UV laser frequency. Therefore, effective temperature of the observed cluster in the spectra cannot be determined.

In other words, the internal energy

selective detection by the double resonance scheme is more essential to observe the “warm” cluster in the present study.

IV.

SUMMARY



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In this study, we measured IR spectra of the warm and neutral PhOH-(H2O)2 cluster by controlling the stagnation gas pressure for the jet expansion and the internal energy-selective detection scheme.

The latter utilizes the large structural difference

between the electronic ground and excited states of the cluster. It was demonstrated that with elevation of temperature (increase of the internal energy) of the cluster, the H-bonded structure of the cluster tends to be deformed from the stable cyclic-type to the transient chain-type. However, evidence of full breaking of an H-bond was not found in the present observation. In addition, it was revealed that in the cyclic-chain conversion of the H-bond structure, loss of the cooperative effect in the cyclic form is superior to the H-bond angle optimization in the chain structure, and this results in the remarkable weakening of the H-bond of the phenolic OH.

Acknowledgements

This study was supported by the Grant-in-Aid for Scientific Research (Project No. 26288002 from JSPS).

The authors thank Dr. Toshihiko Maeyama and Dr. Yoshiyuki

Matsuda for their helpful discussion.

Supporting Information Available. 20 ACS Paragon Plus Environment

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Mass spectrum of PhOH-(H2O)n clusters under the measurement condition of the IR spectra. Normal modes of the OH stretching vibrations of PhOH-(H2O)2. Complete authors list of Reference 58. This material is available free of charge via the Internet at http://pubs.acs.org.

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of Ground and First Excited State Vibrations in Phenol. Chem. Phys. 2000, 252, 247-256.



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Figure 1.

Schematic potential and excitation diagrams of (a) cold and (b) warm

PhOH-(H2O)2 clusters. (a) The cold cluster is localized at the global minimum corresponding to the cyclic structure. In its electronic transition to the S1 state of the chain-type structure, only vibronic bands appear because of the poor Franck-Condon factor for the origin band region. (b) In the warm cluster, thermal excitation of intermolecular vibrations would open the access to the chain-like structure in the S1 state because of the better Franck-Condon overlap.

It should be noted that the

transition to the S1 chain-type structure is largely red-shifted from the vertical transition (vibronic band) of the cold cluster.


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Figure 2.

Two-color R2PI spectra of the neutral PhOH-(H2O)2 cluster produced

under the different stagnation pressure conditions of the jet expansion. The spectra were measured by monitoring the [PhOH-(H2O)2]+ ion intensity. frequency for the ionization was fixed at 31254 cm-1.

The second UV laser

Arrows in the figures indicate

the probe frequencies of the UV laser for the measurement of IR spectra ((a) 36232, (b) 36232, (c) 36211, (d) 36146, (e) 35915, and (f) 35834 cm-1). The asterisked band is the origin band of PhOH-(H2O)3 (see text).



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Figure 3.

IR spectra of neutral PhOH-(H2O)2 cluster observed at the different

UV probe frequencies. The UV frequencies to measure spectra (a) – (f) were indicated by arrows (a) – (f) in Fig. 2, respectively.

Labels for the band assignments are

phenolic OH (P), two H-bonded OHs (W1 and W2) of the water moieties, and free OH (F) stretching vibrational bands. Horizontal dotted lines indicate the baseline of the spectra.

The spectra are plotted as an upward signal represents depletion. 34 ACS Paragon Plus Environment

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Figure 4.

Comparison between the experimental and calculated spectra. In the

upper box, the observed IR spectra of cold (blue trace; reproduction of Fig.3(a)) and warm (red trace; reproduction of Fig.3(f)) PhOH-(H2O)2 are displayed. In the lower boxes, the calculated stick spectra of the cyclic and chain-type structure are shown. The chain-type structure is transient in S0 (see text).



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Figure 5.

Two-color R2PI spectra of the neutral PhOH-(H2O)1 cluster produced

under the different stagnation pressure conditions of the jet expansion. The second UV laser frequency for the ionization was fixed at 31254 cm-1. Arrows in the figure indicate UV probe laser frequencies for measurements of the IR spectra ((a) 35995 and (b) 35979 cm-1).


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Figure 6. frequencies.

IR spectra of PhOH-(H2O)1 observed at the different UV probe Spectra (a) and (b) were measured by probing the UV frequencies

indicated by arrows (a) and (b) in the R2PI spectrum (Fig. 5(B)), respectively.



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Figure 7.

Two-color R2PI spectra of bare phenol measured under the different

stagnation pressure of the jet expansion.


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TOC Figure


Infrared Spectroscopy of Warm and Neutral PhenolâWater Clusters

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