J. Phys. Chem. 1996, 100, 8131-8138
8131
Characterization of the Hydrogen-Bonded Cluster Ions [Phenol-(H2O)n]+ (n ) 1-4), (Phenol)2+, and (Phenol-Methanol)+ As Studied by Trapped Ion Infrared Multiphoton Dissociation Spectroscopy of Their OH Stretching Vibrations Takahiro Sawamura, Asuka Fujii,* Shin Sato, Takayuki Ebata, and Naohiko Mikami* Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai 980-77, Japan ReceiVed: September 7, 1995; In Final Form: February 21, 1996X
OH stretching vibrations of hydrogen-bonded cluster ions of phenol (PhOH), [PhOH-(H2O)n]+ (n ) 1-4), (PhOH)2+, and (PhOH-methanol)+ have been observed with infrared photodissociation spectroscopy in combination with an ion-trapping technique. Cluster ions were efficiently generated by ionization of phenol followed by a jet expansion and were mass-selectively stored by the radio frequency ion trap method, which allows us to observe infrared multiphoton dissociation yield spectra of size-selected cluster ions. For [PhOH(H2O)n]+, the OH stretching vibrations of the water moieties strongly suggested that the n g 3 cluster ions exhibit the proton-transferred form, [PhO-H3O+(H2O)n-1], while the n ) 1 and 2 ions are of the nontransferred form, [PhOH+-(H2O)n]. As for (PhOH)2+, the infrared spectra indicate that the dimer ion is characterized as the open form, in which the phenol ion acts as a proton donor and the neutral phenol as an acceptor through their single hydrogen bond. The similar open form is also found for (PhOH-methanol)+, in which the phenol ion acts as a proton donor.
I. Introduction Proton transfer is one of the most fundamental chemical processes, and it plays an essential role in acid-base reactions occurring in solution. Hydrogen-bonded clusters produced in supersonic jet expansions provide us with detailed microscopic information concerning the processes in solution.1 In this respect, clusters of hydroxy aromatic molecules, such as phenol and various base molecules, have extensively been investigated in many studies as typical examples of their solution with hydrophilic solvents. In these studies, the dependence of proton transfer on cluster size is of particular interest. The cluster size dependence of proton transfer was first demonstrated by Cheshnovsky and Leutwyler for neutral naphthol-ammonia clusters.2 They showed that the stepwise clusterization of ammonia induces proton transfer from the naphthol to the ammonia site at a particular cluster size. This is because the basicity of the ammonia cluster increases with the cluster size. Such a change of the cluster structure is considered to be an essential step in solvation of an isolated molecule. Solvation of a molecular cation can also be investigated with spectroscopy of cluster cations produced in supersonic jet expansions. Because of strong charge-multipole interactions, cluster cations generally have structures and dynamics that are different from those of corresponding neutral clusters. Compared the case for with neutral clusters, however, less definitive information on proton transfer was obtained. This is because photofragment ion mass spectroscopy was the only method for cluster ion studies, while in neutral clusters spectroscopic investigations using multiphoton ionization, fluorescence excitation, and dispersed fluorescence techniques gave unambiguous evidence for proton transfer.2-7 Since a complicated potential surface for the dissociation pathway of cluster ions often generates unexpected fragments and may lead to ambiguity of their structure analysis, such a restriction in the investigation method makes the analysis difficult for intracluster proton transfer in cluster ions. X
Abstract published in AdVance ACS Abstracts, April 15, 1996.
S0022-3654(95)02622-0 CCC: $12.00
Recently, we developed trapped ion photodissociation (TIP) spectroscopy to investigate stable structures of cluster ions.8-12 In this method, a radio frequency (RF) ion trap cell was used to hold mass-selected cluster ions, and their electronic transitions were measured by observing the photodissociation yield spectra. By using TIP spectroscopy, we observed electronic spectra of phenol-water cluster cations, [PhOH-(H2O)n]+ (n ) 1-4).12 The electronic spectra of the n ) 3 and n g 4 cluster ions were very similar to that of the phenoxy radical (C6H5O), indicating that these cluster ions have the proton-transferred form which is composed of the phenoxy radical and hydrated hydronium ions;
C6H5O-H3O+(H2O)n-1 (n g 3) On the other hand, the n ) 1 and n ) 2 cluster ions showed broad and structureless spectra which are due to the phenol ion chromophore. This means that the proton transfer does not occur in these cluster ions, and they are composed of the phenol ion and water molecules:
C6H5OH+-(H2O)n (n e 2) Though we inferred the size dependence of the proton transfer in the cluster ions from their electronic spectra, the vibrational spectra of OH stretching vibrations are expected to provide more direct evidence for the proton transfer. This is because the hydrogen bond formation directly affects these vibrations. Schwarz performed infrared spectroscopy of hydrogen-bonded cluster ions for the first time, though the size selection and spectral resolution were limited.13 Later, difference frequency mixing and/or optical parametric oscillation techniques for the intense and high-resolution infrared light allowed more precise application of infrared spectroscopy. In infrared spectroscopy of clusters, we should overcome two difficulties: the first is the size selection of clusters intermingled in jet expansions, and the second is the detection method for the weak infrared absorption. Conquering the difficulties, Lee and co-workers © 1996 American Chemical Society
8132 J. Phys. Chem., Vol. 100, No. 20, 1996 established infrared spectroscopy of cluster ions.14-17 They used a sector magnet for size selection of cluster ions. Size-selected ions were held in an octapole ion trap cell, and infrared multiphoton dissociation spectroscopy was utilized to observe weak infrared absorption. They measured vibrational spectra of hydrated hydronium ions [H3O+(H2O)n]14,15,17 and ammoniated ammonium ions [NH4+(NH3)n].16,17 By using similar infrared spectroscopic techniques, the structure of solvated metal ions was investigated by Lisy et al.18 and intracluster reactions in clusters of NO+ and of NO2+ with water molecules were investigated by Okumura et al.19,20 In the present work, we observed infrared spectra of the sizeselected [PhOH-(H2O)n]+ cluster ions (n ) 1-4) to investigate the size dependence of the intracluster proton transfer. A cylindrical RF ion trap cell was used for the size selection as well as for storage of the size-selected ions. Infrared spectra of the cluster ions were measured by detecting fragment ions produced by infrared multiphoton dissociation. Observed infrared spectra of the n g 3 clusters showed the OH stretching vibration bands which are characteristic in hydrated hydronium ions [H3O+(H2O)n], strongly suggesting that these clusters have the protontransferred form, [PhO-H3O+(H2O)n-1]+. This result was consistent with that inferred from the electronic spectra. Infrared spectra of the phenol dimer ion, (PhOH)2+, and phenol-methanol cluster ion, (PhOH-metanol)+, were also measured. These spectra not only were helpful for the assignment of vibrations of the [PhOH-(H2O)n]+ ions but also were useful for the structure analysis of these two dimer ions. II. Experimental Section The experimental apparatus of the trapped ion photodissociation spectroscopy was described in previous papers.8-12 The only difference in the present work is that infrared light was used for photodissociation spectroscopy. The hydrogen-bonded cluster ions were prepared by using a channel nozzle system, which was also described in a previous paper.21 A gaseous mixture of helium with vapors of phenol and solvent molecules (water or methanol) was expanded through a pulsed nozzle with a channel orifice 1 mm in radius and 15 mm in length. The stagnation pressure of helium gas was typically 3 atm. The channel orifice had a side hole of 2 mm diameter, which crossed the channel at right angles. An ultraviolet laser beam was focused into the side hole, so that phenol was photoionized at the crossing point. The laser wavelength was fixed at the 0-0 band of the S1-S0 transition of phenol. The cluster ions were formed by collision among phenol ions, solvent molecules, and helium atoms in the collisional region in the channel. The cluster ions were cooled down through the expansion into vacuum. A typical background pressure in the experimental region was 1 × 10-5 Torr. The stagnation pressure of helium gas was typically 3 atm, and the expansion duration was about 800 ms. The partial pressures of phenol and solvent (water or methanol) were about 10 and 20 Torr, respectively. This method for the production of cluster ions is much more efficient than the direct photoionization of neutral clusters in jet expansions. Cluster ions cooled by the expansion from the nozzle were introduced to a cylindrical ion trap cell, whose detail was described in previous papers.8-12 The ion trap cell was used for the size selection as well as for the ion storage. The cell was composed of a cylindrical cage electrode, and two end caps were used as the other electrode. A radio frequency (RF) electric potential was applied on the cylindrical cage electrode with respect to the end cap electrode which was grounded. The mass selection of cluster ions was performed by selecting
Sawamura et al. appropriate voltage and frequency of the RF potential. The cluster ions of interest were trapped in the cell by a suitable condition of the RF potential, and the other cluster ions were removed from the cell during the storage. After about 1 ms storage, a tunable infrared light beam, of which wavelength was scanned, was introduced into the trap cell. Because hydrogen bond energies of the cluster ions are larger than the infrared photon energy (3000-4000 cm-1), onephoton absorption of infrared light is not good enough to induce the photodissociation of these cluster ions. For high-frequency vibrations such as OH and CH stretching modes, however, infrared multiphoton absorption is expected to be efficient because of the high density of states. Dissociation following multiphoton absorption is also enhanced when the infrared light is resonant with the vibrational transitions of the cluster ions. Thus, the infrared absorption spectra of the cluster ions can be observed by monitoring the yield of the dissociation fragments. The photodissociation fragments were spontaneously ejected from the trap cell. Then, they were mass selected by a quadrupole mass filter and detected by a channel multiplier. The ion current was amplified and recorded by a boxcar integrator and computer. The ultraviolet laser beam for the ionization of phenol was a second harmonic of an output of a dye laser (Lambda Physik Scanmate 2E) pumped by a XeCl excimer laser (Lambda Physik Lextra 50). The typical power of the ultraviolet laser was about 1 mJ, and the laser beam was focused by a lens of f ) 500 mm. The infrared light for photodissociation was generated with a LiNbO3 crystal by a differential frequency mixing between a second harmonic of a Nd:YAG laser (Spectra Physics GCR230) and an output of a Nd:YAG laser pumped dye laser (Spectra Physics PDL3).22 The power of the infrared light was typically 0.3 mJ, and the spectral resolution was about 1 cm-1. Infrared spectra presented in the following are not power normalized, because the infrared light power fluctuation makes artificial features at the flat base line. However, the relative peak intensities are hardly perturbed by the laser power because of the almost constant laser power in the observed range. The infrared beam was focused by a lens of f ) 250 mm. III. Results and Discussion A. Size Dependence of Proton Transfer in [PhOH(H2O)n]+. 1. Cluster Size Separation of the Infrared Spectra. Figure 1 shows the infrared spectra of the [PhOH-(H2O)n]+ cluster ions in the region of 2900-3800 cm-1. In the Figure , a-d represent the spectra of the n ) 1-4 ions, respectively, each of which was obtained by monitoring the intensity of the [PhOH-(H2O)n-1]+ fragment ion as a function of the wavelength. All the spectra show relatively sharp bands in the 36003800 cm-1 region and an extremely broad one below 3500 cm-1. The sharp bands are easily found to be associated with OH stretches of the cluster ions. The assignments of the broad bands below 3500 cm-1 will be discussed later. The 3600-3800 cm-1 regions of the spectra are reproduced in an expanded scale in Figure 2, and the corresponding mass spectra of the trapped cluster ions in the cell are shown in Figure 3. Although the proper performance of the mass separation of the ion trap cell is good enough to select the difference of single H2O, the number of ions trapped at high mass resolution was too small for infrared multiphoton dissociation spectroscopy. In this experiment, thus, we were forced to reduce the mass selectivity of the cell for the sensitivity. As seen in the mass spectra of the trapped ions in Figure 3, a mixture of differently sized cluster ions was trapped in the cell.23 Since the spectra appear to be quite different from each other, however, infrared
Characterization of Hydrogen-Bonded Cluster Ions
Figure 1. Infrared spectra of [PhOH-(H2O)n]+: (a) n ) 1, (b) n ) 2, (c) n ) 3, (d) n g 4. Each spectrum was obtained by monitoring the intensity of the [PhOH-(H2O)n-1]+ fragment. For discussion of the broad bands labeled by A and B, see the text.
J. Phys. Chem., Vol. 100, No. 20, 1996 8133
Figure 3. Mass spectra of the cluster ions trapped under the different conditions of the ion storage. Each mass spectrum a-d corresponds to the infrared spectrum a-d in Figures 1 and 2.
A
B
Figure 2. Expanded infrared spectra in the region of 3600-3800 cm-1 of [PhOH-(H2O)n]+: (a) n ) 1, (b) n ) 2, (c) n ) 3, and (d) n g 4. The sharp dips in spectra a and c and the band splitting in spectra b and d are due to strong absorption by water vapor along the optical path of the infrared light.
spectra shown in Figures 1 and 2 are considered to be due to the dominant size cluster ions. This difference also arises from the following reason. Since the [PhOH-(H2O)n]+ spectra were obtained by detecting the fragment ions [PhOH-(H2O)n-1]+, the spectra are due to infrared absorption of the single size cluster ions, as far as the ejection of a single water molecule, i.e.
[PhOH-(H2O)n]+ f [PhOH-(H2O)n-1]+ + H2O is the major dissociation process. We confirmed this assumption by another experiment, where we trapped the single species of [PhOH-H2O]+ generated by resonance-enhanced multiphoton ionization via the S1 state of
Figure 4. (A) Mass spectra of the trapped ions corresponding to the infrared spectra a and b in B. (B) Comparison between the infrared spectra of [PhOH-H2O]+ obtained under two different ion trap conditions. (a) The single species of [PhOH-H2O]+ was produced in a free jet expansion and was held in the cell. (b) The mixture of the n ) 1-3 cluster ions was produced by the channel nozzle, and they were held in the cell. Spectrum b is the same as given in Figure 2a. Both the spectra were obtained by detecting phenol fragment ions as functions of the infrared light wavelength.
the n ) 1 neutral cluster in a free jet expansion, and compared the spectra with Figure 2(a). Figure 4 shows a comparison of two spectra obtained for two different preparation conditions; spectrum a is obtained for the n ) 1 cluster ion generated by direct ionization of PhOH-H2O, and spectrum b which is reproduced from Figure 2a represents the n ) 1 cluster ion contaminated by the n ) 2 and 3 ions. In both cases the spectra were obtained by monitoring phenol ion as the photofragment
8134 J. Phys. Chem., Vol. 100, No. 20, 1996 species. As seen in Figure 4, two spectra are essentially identical, indicating that the contamination of a small amount of larger size clusters is not a serious problem for the spectrum of the major species. The features of spectra b in Figures 1 and 2 are completely different from those of spectra a and c. This proves that spectra b are due to the n ) 2 cluster ions. In the same way, spectra c are attributed to the n ) 3 cluster ions. As for spectra d in Figures 1 and 2, we cannot define the cluster size because spectra of n g 5 cluster ions are not known and the possibility of sequential dissociation cannot be excluded. Therefore, we regard this spectrum as due to the n g 4 cluster ions. 2. Assignments of the 3600-3800 cm-1 bands. In a previous paper,24 we reported that the OH stretching vibration of bare phenol ion occurs at 3535 cm-1. It is well-known that OH bands of proton-donating moieties exhibit substantial red-shifts from the hydrogen bond-free OH bands. In [PhOH-(H2O)n]+, since it is evident that the phenol moiety acts as a proton donor, the band due to the phenolic OH stretch should appear on the lowfrequency side of 3535 cm-1, which is out of the 3600-3800 cm-1 region. Thus, all the bands shown in Figure 2 are due to OH stretches of the water moiety of the cluster ions, but the phenolic OH stretch is not involved in this region. Therefore, the assignments of the IR spectra of [PhOH-(H2O)n]+ are presented on the basis of the above results. 3. [PhOH-H2O]+. The infrared spectrum of the water moiety of the n ) 1 cluster ion, [PhOH-H2O]+, is shown in Figure 2a. Two intense bands are seen at 3625 and 3710 cm-1. These frequencies are very close to those of bare water, in which two vibrations at 3657 and 3756 cm-1 are due to the symmetric and antisymmetric OH stretches, respectively. On the other hand, the H3O+ ion has no fundamental vibrations in this region. The asymmetric OH stretches of H3O+ appear at 3519 and 3536 cm-1, and the symmetric OH stretch frequency, which has not yet been observed, is estimated to be around 3400 cm-1.25,26 It is unlikely that the frequencies of the hydronium ion show large blue-shifts by the cluster formation with the phenoxy radical. Therefore, the spectrum indicates that the structure of the water moiety is very similar to that of bare water. Consequently, the n ) 1 cluster ion is considered to have a non-proton-transferred form, [C6H5OH+-H2O]. The non-proton-transferred form is consistent with the result of the electronic spectrum measurement of the n ) 1 cluster ion by the trapped ion photodissociation method.12 The observed electronic spectrum was broad and was similar to the structureless spectrum of the phenol ion. Hobza et al.27 performed an ab initio calculation of [PhOH-H2O]+ to interpret the zero kinetic energy photoelectron spectra by Dopfer et al.28 This calculation also predicted a non-proton-transferred structure, though only low-frequency vibrations (below 1200 cm-1) were evaluated. In spite of strong interaction expected in the hydrogen bond formation with the phenol ion, the frequency shifts of the water moiety from a bare molecule are very small. Recent vibrational spectroscopic studies of various hydrogen-bonded clusters demonstrated that the same phenomena are commonly observed in proton-accepting sites.22,29-31 On the other hand, it is noticed that there is a significant difference in intensities. The intensity of the OH symmetric stretch is very weak in bare water, while it is remarkably enhanced in the water moiety of the cluster. Although the accuracy in the band intensity measurement in the present work is limited because of a saturation effect and low signal/noise ratio, the observed spectra show qualitatively their intensity. Similar intensity enhancement of the symmetric OH stretches of water moieties was observed in the [NO+(H2O)n] cluster ions by Choi et al., who also utilized infrared
Sawamura et al. multiphoton dissociation spectroscopy.19 They found that the symmetric OH stretch is dominant over the antisymmetric stretch in the water moiety of [NO+(H2O)]. On the basis of ab initio calculations of this cluster ion, they showed that the symmetric and antisymmetric OH stretches have almost the same intensities and both are considerably enhanced compared to those of a bare water molecule, though their frequency shifts are small. Actually, only the symmetric OH stretch was seen in their observed spectrum of [NO+(H2O)]. They concluded that the intensity anomaly is due to the difference in dissociation efficiency of the vibrational levels. The dissociation efficiency depends on the absorption cross sections of the second IR photon and intramolecular vibrational redistribution (IVR) rates. The anomalous intensity of the symmetric OH stretch band of [PhOH- H2O]+ can also be caused by the similar reasons encountered in the case of [NO+(H2O)]. 4. Hydrogen Bond Structures and IR Spectra. Before we examine the IR spectra of the n g 2 cluster ions, we will discuss the characteristic spectral features of both the proton-transferred and the non-proton-transferred forms. The difference is quite essential for the characterization of the hydrogen bonds in the cluster ions. Recent ab initio calculations of hydrogen-bonded clusters of PhOH-(H2O)n in the neutral state have showed that the cyclic structure is the most stable form at least for the n ) 2-4 clusters,32-37 as the same is true for the pure water clusters, (H2O)n+1.38,39 Infrared spectroscopy of the neutral PhOH(H2O)n clusters for n ) 1-4 has confirmed that their stable forms for n ) 2-4 are the cyclic structure and no other isomers such as noncyclic or chain forms are identified for the jet-cooled clusters.22,40 The results indicate that the cyclic forms are considered to be the most stable structure in the neutral clusters. When [PhOH-(H2O)n]+ has a non-proton-transferred form, the similar cyclic structure is also expected for the stable form of the cluster ion, as illustrated in Figure 5a. This is because of following reasons: the positive charge is inferred to be localized in the phenol moiety in the non-proton-transferred cluster ion, as was confirmed for [PhOH-H2O]+ by an ab initio calculation.27 The charge localization in the phenol moiety indicates that the nature of neutral water is still held in the cluster ion and the basic structure of the water moiety is not severely perturbed by ionization, though, of course, interaction between the phenol and water moieties becomes stronger. In such a cyclic form involving water molecules, one of the OH bonds of each of the water moieties is in the ring of hydrogen bonds and the other is protruding from the ring. The IR bands due to stretching vibrations of the OH oscillators protruding from the ring are known to occur at around 3720 cm-1, which are very close to the bands due to antisymmetric OH stretches of bare water. The IR bands due to the OH oscillators in the ring are also known to exhibit large red-shifts and to appear below 3600 cm-1. As a result, in IR spectra of the cyclic forms there is a characteristic “window” region of 3600-3700 cm-1, in which no band is allowed.22,40,41 A schematic view of the window region is illustrated in Figure 5a. On the other hand, when the [PhOH-(H2O)n]+ cluster ion has a proton-transferred form, it is regarded as a cluster composed of a phenoxy radical and hydrated hydronium ion, [PhO-H3O+(H2O)n-1]. It has been well-known that small size hydrated hydronium ions (n e 4) have radial structures in which H3O+ is centered and water molecules are bound at each site of H3O+.42 The structure of the water moiety of the protontransferred cluster ions is also expected to be radial, as schematically shown in Figure 5b. The structural difference among hydrated hydronium ions and neutral water clusters is
Characterization of Hydrogen-Bonded Cluster Ions
Figure 5. Schematic representations of expected structures and corresponding infrared spectra of [PhOH-(H2O)n]+ (n g 2): (a) Nonproton-transferred form, and (b) proton-transferred form. Note that these structures, vibrational modes, and spectra are not exact but are schematic.
caused by the fact that a hydronium ion can act only as a proton donor in the cluster but neutral water can play both roles of proton donor and acceptor. In infrared spectra of hydrated hydronium ions, no window region is seen but characteristic bands appear around 3600 cm-1.14,15,17 These bands originate from the terminal water molecules in the cluster and are attributed to symmetric OH stretches. The difference between the expected structures of the protontransferred and non-proton-transferred clusters gives us a remarkable characteristic for the identification of the intracluster proton transfer in [PhOH-(H2O)n]+ (n g 2). When a window region is observed in the 3600-3700 cm-1 region in the infrared spectrum, it indicates a cyclic structure of the cluster ion. This strongly suggests that the proton transfer does not occur. On the other hand, when OH stretch bands are observed around 3600 cm-1, a radial structure is evident, indicating that proton transfer takes place in the cluster ion. It should be noted that this criterion for the intracluster proton transfer not only is based on the simple similarity among the spectra of the phenol-water cluster ion and other clusters but also arises from an essential difference between the nature of a neutral water molecule a and hydrated hydronium ion. As for [PhOH-H2O]+, the criterion is not applied because this cluster ion cannot be a cyclic form. 5. [PhOH-(H2O)n]+ (n g 2). On the basis of the criterion described above, we examine the size dependence of the proton transfer in [PhOH-(H2O)n]+ for n g 2. The spectra of the n ) 3 cluster ion reproduced in Figure 2c show two intense bands at 3637 and 3730 cm-1, which are evidently characteristic of the radial structure of the water cluster moiety. The similar two bands of the n g 4 spectrum shown in Figure 2d are also characteristic of the radial structure, though the relative intensity of the lower frequency band is weaker than the corresponding one in the n ) 3 ion. The results indicate that the protontransferred forms are realized in those cluster ions. Very recently, the proton-transferred structure of these cluster ions has been confirmed by observing electronic spectra of trapped ions.12 The electronic spectra of the n ) 3 and n g 4 cluster ions are very similar to the spectrum of phenoxy radical, PhO, indicating that the radical is the chromophore of the electronic transition in the cluster ions and that these cluster ions consists of the radical and protonated water site. Thus, both the vibrational and the electronic spectroscopies confirmed that the proton-transferred form is evident for the n g 3 cluster ions. The infrared spectrum of the n ) 2 cluster ion, shown in Figure 2b, shows a distinct band with its peak at 3685 cm-1. A
J. Phys. Chem., Vol. 100, No. 20, 1996 8135 weak band at 3640 cm-1 and a shoulder around 3730 cm-1 are probably due to a slight contamination of the n ) 3 species. The peak position of the 3685 cm-1 band is almost in the middle of the expected symmetric and antisymmetric OH stretching vibrational frequencies. Thus, it was difficult to judge whether there is a window region or not. Although the result of the vibrational spectroscopy is ambiguous, our recent electronic spectra measurement has supported the non-proton-transferred form for the n ) 2 cluster ion.12 The electronic spectra of the n ) 2 cluster ion showed a broad structure similar to that of the phenol ion, which represents the key chromophore of the cluster ion. We, therefore, inferred that the n ) 2 cluster ion has the non-proton-transferred form. To summarize the size dependence of the intracluster proton transfer in [PhOH-(H2O)n]+, we concluded that the n ) 1 and 2 cluster ions have the non-proton-transferred forms and the n g 3 cluster ions have the proton-transferred forms. The same size dependence was derived from both of the vibrational and electronic spectroscopic measurements, except for the n ) 2 cluster ion whose infrared spectrum was ambiguous. The transformation from the non-proton-transferred form to the transferred form with increase of the cluster size is quite reasonable because the proton affinities of the water clusters increase with their size.43 B. Infrared Spectra of [PhOH-(H2O)n]+ below 3600 cm-1. 1. Assignments of the Broad Bands. As seen in Figure 1, a characteristic feature in the 2900-3400 cm-1 region is the presence of two extremely broad bands. The peak position of one of the broad bands, denoted by A, depends on the cluster size. In the n g 4 spectrum, another broad band denoted by B is seen. The following three candidates can be possible for the assignment of band A: (a) CH stretches of the phenol moiety of the cluster ions; (b) hydrogen-bonded OH stretches of the phenol and water moieties; (c) two-photon electronic transitions of the cluster ions. CH stretches of the phenyl ring are usually observed in the 3000-3100 cm-1 region, though their infrared absorption intensities are much weaker than those of OH stretches.22 It is clear from the present spectra that the integrated absorption intensity of the broad band A is much stronger than those of the OH stretches of the water moieties. Therefore, it is hard to regard the broad band A as associated being with the CH stretches. To confirm this, we measured the infrared spectrum of the deutrated phenol-water cluster ion, [C6D5OH-H2O]+, because the CD stretches of the phenyl ring are found in the 2200-2300 cm-1 region. Figure 6 shows the observed infrared spectrum with the mass spectrum of the trapped cluster ions. The infrared spectrum is almost the same as that of [C6H5OHH2O]+ shown in Figure 1a. The spectrum of [C6D5OH-H2O]+ also has a broad band at around 3000 cm-1, indicating that the band cannot be attributed to the CH stretches. Infrared spectroscopic studies of neutral clusters established that hydrogen-bonded OH stretches are also expected to appear in the 2900-3600 cm-1 region and that hydrogen bond formation largely enhances infrared absorption intensities of OH stretches of hydrogen-donating sites.22,29-31,40 Also in the ionic clusters involving water, the strong hydrogen-bonded OH bands were observed in this region.17,19,20 Therefore, it seems to be quite reasonable for band A to be assigned as hydrogen-bonded OH band(s). However, there are some difficulties for the assignment of an exclusive contribution of the OH bands. This is because of the following reasons. One significant reason is the bandwidth, which is about 200 cm-1 and is too large to be attributed to one or two OH bands in the case of the n ) 1 and
8136 J. Phys. Chem., Vol. 100, No. 20, 1996
Figure 6. (a) Infrared spectrum of [C6D5OH-H2O]+, obtained by detecting the C6D5OH+ fragment. (b) Mass spectrum of the trapped cluster ions for the infrared spectrum a.
Figure 7. (a) Infrared spectrum of (PhOH)2+, obtained by detecting the PhOH+ fragment. (b) Mass spectrum of the trapped cluster ions for the infrared spectrum a.
n ) 2 cluster ions. This difficulty becomes much clearer in the infrared spectra of the phenol dimer ion and the phenolmethanol cluster ion, shown in Figures 7 and 8, respectively. Though the details will be given later, both spectra have the similar broad bands at around 3000 cm-1, and their widths are too wide to be attributed to one hydrogen-bonded OH stretch of the donor site. Another reason is the peak position of band A; it has been observed for various neutral solvated phenol clusters that red-shifts of hydrogen-bonded OH bands are closely correlated with proton affinities (PAs) of acceptor molecules.29-31 The peak positions of band A of the 1-1 cluster ions with water and methanol and of the phenol dimer ion show no apparent correlation with these acceptors, though there are substantial difference among PAs of water (PA ) 165 kcal/mol), methanol (PA ) 182 kcal/mol), and phenol (PA ) 196 kcal/mol).44 This fact also throws doubt on the exclusive contribution of the hydrogen-bonded OH vibrations to the band. Now, let us examine the last candidate, two-photon electronic transition. In monosubstituted benzene of C2V symmetry, the highest occupied π orbital (e1g) of benzene splits into two orbitals of b1 and a2. The ground state phenol cation has the
Sawamura et al.
Figure 8. (a) Infrared spectrum of (PhOH-methanol)+, obtained by detecting the PhOH+ fragment. (b) Mass spectrum of the trapped cluster ions for the infrared spectrum a. 2B symmetry with a hole in the b orbital. The excitation of 1 1 an a2 electron to the b1 orbital creates the first electronic excited state 2A2. Though this π r π transition has not yet been directly observed, its transition energy is estimated to be about 0.69 eV from photoelectron studies of phenol.45 The electronic structure of the ground state of the phenoxy radical is similar to that of the phenol cation which has a hole in the b1 orbital. The hole transfer from the π orbital to the nonbonding orbital (b1) localized in the oxygen atom results in the first excited state 2B (refs 46-48). Recent photoelectron spectroscopy of phe1 noxide anion (C6H5O-) showed that the first exited state 2B1 of phenoxy radical lies about 0.8 eV above the ground state.49 Both phenol cation and phenoxy radical, thus, are known to exhibit the lowest electronic excited state in the 6000-6500 cm-1 region. If we assume two-photon processes for the spectra, band A is very close in energy to the transition to their lowest electronic state. Difficulties in the attribution to the two-photon electronic transitions are the following two points. One is that much weaker transition intensities than the OH stretch vibrations are expected for the two-photon electronic transitions. The other difficulty is the reason why the hydrogen-bonded OH bands are so weak to be observed. Both of the difficulties might be explained by the fact that the observed spectra were not direct absorption spectra but dissociation yield spectra. The intensity in the spectra are strongly affected by the dissociation yield of the pumped level. Much higher dissociation yield can be expected for the electronic excitation than for the OH fundamental excitation. Moreover, since the anharmonicity of the hydrogen-bonded OH oscillator is much larger than that of the free OH oscillator, the fast IVR might take place preferentially in the hydrogen-bonded OH oscillators, leading to the reduction of the dissociation yield of the intermolecular hydrogen bonds. In addition, the anharmonicity reduces the resonance effect for the second infrared photon absorption. At the present stage, however, we cannot determine which candidate, namely, the hydrogen-bonded OH stretch or two-photon electronic transition, is responsible for the broad band A. Of course, the overlap of the both transitions is also possible. Further studies are needed to clarify this problem. 2. Structure of [PhO-H3O+(H2O)n-1]. In the spectrum of [PhOH-(H2O)n]+ (n g 4) shown in Figure 1d, another broad and intense band with a peak at 3370 cm-1 is seen, which is
Characterization of Hydrogen-Bonded Cluster Ions denoted by B. This appears only in the spectrum of n g 4 but not in that of n ) 3. Such a size dependence cannot be explained by a similar assignment to an electronic transition in the phenoxy radical site. Since the ground electronic state of the phenoxy radical is of π radical type, as described above, the PA of the radical is not very different from that of H2O. Actually, PAs of phenoxy radical and H2O in the gas phase have been reported to be 204.4 and 165 kcal/mol, respectively.43,44,50 In this respect, the phenoxy radical in the cluster ions can be replaced by H2O without significant change in structure. In other words, the proton-transferred form can be represented by [PhO-H3O+(H2O)n-1], as it is expressed by [H3O+(H2O)n]. As was described in section III.A.4., the latter is known to have the structure such that three H2O molecules are bound at each of three OH oscillators of H3O+. The first solvation shell is closed with three solvent molecules, and the remaining H2O molecule(s) are bound at the H2O sites of the first shell, leading to the second solvation shell. The OH stretches of the first shell H2O without the second shell are known to appear only at 36003750 cm-1. This spectral feature is in accord with the observed spectrum of the n ) 3 cluster ion (Figure 1c), in which the protonated acceptor site is considered to be a radial structure similar to [H3O+(H2O)3]. The second shell formation leads to a new band due to the OH oscillator of the hydrated first shell. The new bands appear in 3250-3400 cm-1 with an extremely broad band width, as reported by Crofton et al.17 The spectral feature is quite similar to that of [PhO-H3O+(H2O)3] as shown in Figure 1d. Therefore, we conclude that the broad and intense band B observed in [PhOH-(H2O)n]+ (n g 4) is attributed to the hydrated OH oscillator of the first solvation shell of the H3O+ moiety. C. Structures of (PhOH)2+ and (PhOH-Methanol)+. The structure of the neutral phenol dimer, (PhOH)2, has been examined for many years.29,31,51,52 Though the cyclic form was first suggested, the trans-planar structure has been proposed among various open structures. Recently, its rotational constants were determined with rotational coherence spectroscopy by Connell et al., and the nonplanar structure was deduced, in which two phenyl rings make large interactions.52 In addition, vibrational spectroscopy of the OH stretching region provides an unambiguous result indicating that the neutral phenol dimer has the open form, in which one of the phenol molecules acts as a proton donor and the other as an acceptor.29,31 As for the (PhOH)2+ ion, zero kinetic energy photoelectron spectroscopy (ZEKE-PES) was performed by Dopfer et al.53 Their ZEKEPES study was concentrated to the low-frequency vibrations including the intermolecular vibrations in the cluster ion and indicated a large change in the geometry on ionization. They discussed the structure of (PhOH)2+ on the assumption of the structure of the neutral dimer deduced by rotational coherence spectroscopy. The infrared spectra of (PhOH)2+ and (PhOH-methanol)+ shown in Figures 7 and 8 give us information on the structure of these cluster ions. Both the cluster ions have two OH oscillators, and two types of structure are possible. One type is the cyclic structure, in which two OH oscillators form hydrogen bonds with each other. The other is an open structure, in which one of the OH oscillators is free from hydrogen bond formation. The spectra of both (PhOH)2+ and (PhOHmethanol)+ show a single sharp band at 3620 and 3660 cm-1, respectively. The frequencies of these bands are very close to those of the OH stretches of bare phenol and of bare methanol, which are known to be 3657 and 3680 cm-1, respectively. It is well-known that the frequency shift induced by hydrogen bond
J. Phys. Chem., Vol. 100, No. 20, 1996 8137 formation is usually small for the OH oscillator acting as proton acceptor, while the frequency of the proton donating OH oscillator is largely reduced. In this respect, the spectra shown in Figures 7 and 8 indicate that both the bands are due to the OH oscillators acting as proton acceptor. The results, therefore, mean that the neutral phenol site of (PhOH)2+ behaves as a proton acceptor and that the methanol site of (PhOHmethanol)+ acts as a proton acceptor. The infrared spectrum measured in the present work is the first direct evidence of the open form of the (PhOH)2+ ion, that is
Of course, information deduced from the present infrared spectra is still insufficient to infer detailed structure of the cluster ion. Further experiments and ab initio calculations are needed. The structure of the neutral (PhOH-methanol) cluster is less certain than that of (PhOH)2. Very recently, our group confirmed the open form of (PhOH-methanol) by infrared spectroscopy of the OH stretches.54 Ab initio calculations indicated that the p-cresol-methanol cluster has a trans-linear form, and the similar structure was assumed to the neutral (PhOH-methanol) cluster.55,56 Though the trans-linear (planar) structure was denied for (PhOH)2, this structure may be still a good approximation for (PhOH-methanol) because of the small interaction between the phenyl and methyl groups. As for (PhOH-methanol)+ Wright et al. measured ZEKE photoelectron spectra.56 The measured spectra showed long progressions of intermolecular vibrations similar to those of (PhOH)2+ and suggested a large structural change upon ionization, although no direct evidence for the geometry of the cluster ion was obtained. The present infrared spectrum is the first evidence for the open form of (PhOH-methanol)+. IV. Concluding Remarks In this work, we developed trapped ion infrared multiphoton dissociation spectroscopy for study of hydrogen-bonded cluster ions of phenol. The size dependence of proton transfer in the [PhOH-(H2O)n]+ cluster ions (n ) 1-4) was investigated by infrared spectroscopy of OH stretches of the water moieties. The observed infrared spectra indicated that proton transfer occurs in the cluster ions of n g 3. This size dependence was consistent with the result of our study by using electronic spectra of the cluster ions. The infrared spectra of (PhOH)2+ and (PhOH-methanol)+ were also measured, and direct evidence of their open form was obtained. An important problem in this study is the assignment of the broad bands below 3600 cm-1 discussed in section III.B.1. Dissociation spectroscopy of overtones of the OH stretch in the cluster ions and experiments using D2O are expected to give useful information on this problem. Though the infrared spectra gave us the qualitative information to characterize structures of the cluster ions, our knowledge is still limited. As vibrational analysis of molecular clusters in their neutral ground states has progressed extremely well with theoretical investigations based on SCF calculations, ab initio calculations should be the most powerful tools to clarify the detailed structure of the cluster ions. Reliable theoretical calculations of the cluster ions are strongly desired for analysis of their intermolecular structure. Acknowledgment. This work is partly supported by Grantsin-Aids for Scientific Research (No. 06554024 and 06640642) from the Ministry of Education, Science, and Culture.
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