Article pubs.acs.org/JPCA
Ground State Proton Transfer in Phenol−(NH3)n (n ≤ 11) Clusters Studied by Mid-IR Spectroscopy in 3−10 μm Range Mitsuhiko Miyazaki,† Ayako Kawanishi,† Iben Nielsen,‡ Ivan Alata,‡ Shun-ichi Ishiuchi,† Claude Dedonder,‡,§ Christophe Jouvet,‡,§ and Masaaki Fujii*,† †
Chemical Spectroscopy Division, Chemical Resources Laboratory, Tokyo Institute of Technology, 4259-R1-15, Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan ‡ Institut des Sciences Moléculaires d’Orsay and Centre Laser de l’Université Paris Sud, Université Paris-Sud 11, 91405 Orsay Cedex, France § PIIM−UMR CNRS 7345, Aix Marseille Université, Avenue Escadrille Normandie-Niémen, 13397 Marseille Cedex 20, France ABSTRACT: The infrared (IR) spectra of size-selected phenol− ammonia clusters, PhOH−(NH3)n (n ≤ 11) in the 3−10 μm wavelength region were measured to investigate the critical number of solvent molecules necessary to promote the ground state proton transfer (GSPT) reaction. While the N−H stretching vibrations did not provide clear information, characteristic changes that are assigned to the GSPT reaction were observed in the skeletal vibrational region. The production of phenolate anion (PhO−), which is a product of the GSPT reaction, was established from the appearance of characteristic bands assignable to C−C stretching and C−H bending vibrations of PhO− and from the corresponding disappearance of C−O−H bending vibration of PhOH at n = 9. The mid-IR spectroscopy directly proves the structural change induced by the deprotonation from the O−H bond and thus establishes the GSPT reaction as complete at n = 9. No such absorptions were observed for n ≤ 5 in line with a previous report. For n = 6−8, both the proton transferred and the nontransferred signatures were observed in the spectra, showing coexistence of both species for the first time.
1. INTRODUCTION Understanding bulk phase chemical reactions from the microscopic molecular point of view has been one of the central topics of cluster science.1−6 Investigation of cluster size dependence on the reaction mechanism is a notable advantage of utilization of gas phase clusters. Proton transfer (PT) and hydrogen transfer (HT) reactions from a solute molecule to solvents have occupied the central topic because of their fundamental importance in a variety of chemistry. Particularly, the minimum number of solvents that can induce ionic dissociation of the proton from an acidic functional group of the solute moiety has attracted much attention. To this end, many cluster systems have already been studied. Water clusters are directly linked to the bulk phase acid−base reactions. PT reaction has been studied so far mainly by hydrated radical cations like phenol+ (PhOH+),7,8 benzene+,9 and aniline+10 and protonated species such as H+(HNO3),11 H+(CH3)2O,12 and H+(CH3OH).13,14 In these systems, size dependence of PT reactions was discussed based on a competition of proton affinities (PA) between the solute and solvent cluster moieties as a first approximation. For neutral systems, recently, the deprotonation from hydrogen chloride was reported for the HCl−(H2O)n (n ≥ 4) clusters isolated in a He nanodroplet by infrared (IR) spectroscopy.15,16 However, such a size selected PT study in neutral hydrated clusters has limitations because of the relatively low proton affinity of water clusters. Ammonia © 2013 American Chemical Society
(NH3) solvent, instead, has been frequently employed for this purpose based on its larger PA than that of H2O.17 Especially, complexes combined with so-called photoacid molecules such as aromatic alcohols have been extensively investigated, e.g. phenol,18−39 naphthol,40−52 7-hydroxyquinoline,5,53−56 and 7azaindole.57,58 One of the typical examples of such studies on PT reactions has been carried out on phenol−ammonia clusters, PhOH− (NH3)n. Since the pKa value of PhOH was estimated to be largely reduced by photoexcitation,59 an excited state proton transfer (ESPT) reaction was expected to occur in the clusters. This ESPT reaction has attracted great attention over the 20 past years as a model for an acid−base reaction from a microscopic point of view.18−39,60−62 While many researchers had tried to explain their experimental results with the ESPT model, many inconsistencies remained. Finally, contrary to the initial expectation, the controversy has been settled by the introduction of a new concept in the cluster reactivity: an excited state hydrogen transfer (ESHT) reaction takes place for n ≤ 5, and the ESPT reaction is not observed.30,32−39 The ESHT reaction can be observed only if PhOH is solvated with Received: December 7, 2012 Revised: January 25, 2013 Published: January 25, 2013 1522
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signature of the GSPT reaction investigating both the X−H stretching to the skeletal vibrational regions using a newly developed table top mid-IR system.
less than 6 ammonia molecules, suggesting the absence of proton/hydrogen transfer in S0 for these sizes. Though the ESHT reaction has been established for clusters with n ≤ 5, no clear spectroscopic features in the electronic transition have been obtained for much larger sized clusters, and the reaction products of the ESHT reaction also suddenly disappear at n = 6.32 A ground state proton transfer (GSPT) reaction was proposed as a possible scheme to explain this observation.29 This was supported by a large low-energy shift of the ionization potential (IP) between n = 5 and 6 measured by vacuum ultraviolet (UV) single photon ionization efficiency curves. Ionization of the system results in the deprotonation from the O−H group and in the production of phenoxy radical (PhO•), [PhOH−(NH3)n]+ → PhO•−H+(NH3)n. The low energy shift of IP was explained by better Franck−Condon geometry overlap between the cationic state and the ion paired neutral ground state produced by the GSPT reaction, PhOH− (NH3)n → PhO−−H+(NH3)n. The size dependence of the reaction enthalpy in the GSPT reactions was also estimated based on PA values of phenolate anion (PhO−) and (NH3)n, and it was concluded that the enthalpy becomes negative, i.e., exothermic reaction, for n ≥ 6. Recently, ab initio geometry optimizations up to n = 10 was reported,35 and the proton transferred ion-paired structures are more stable than the nontransferred structures for n ≥ 6 in agreement with former studies.60,62 This conclusion, however, was based on rather indirect information on the electronic states, i.e., change in IP between n = 5 and 6. Therefore, structural information for the PhOH− (NH3)n (n ≥ 6) clusters is desired. IR spectroscopy is a powerful tool to establish structural changes because it can directly monitor the nature of chemical bonds.63−66 The structures of hydrogen bonded networks formed by the NH3 molecules around the phenolic OH group are probed by the O−H and N−H stretching vibrations in the 3 μm region. These vibrations, in fact, have successfully been interrogated in nonreactive hydrated systems like PhOH−(H2O)n to reveal the growth of the hydrogen bonded network structure on the basis of characteristic band shifts of the O−H stretching vibrations.67−69 The O−H stretching vibration, however, shows extensive broadening in the case of strong hydrogen bonding as can be found in reactive systems, which results in a loss of structural information.8,70,71 In addition, the N−H stretching vibrations are not as convenient as O−H stretches because of the overlap due to the number of the N−H bonds, the small shifts, weak band intensities, and anharmonic splitting caused by the bending mode ν4. Thus, solvation structures of NH3 are not determined easily if only the N−H stretching vibrations are analyzed. However, the IR spectroscopy in the skeletal vibrations is widely used to investigate structures and reactions because spectral features in this region remain sharp and can provide a lot of structural information even in the bulk. This advantage should also be true for gas phase clusters in which a reaction is expected. In the present case of the GSPT reaction, the O−H bond dissociation is expected to have large effects on the IR spectra in several vibrational bands. For example, the C−O−H bending mode should disappear, the C− O stretching mode should change to reflect larger double bond character, the C−C ring modes are expected to reflect the change in the aromaticity of the ring, and so on. In this study, we applied infrared (IR) spectroscopy for size selected PhOH− (NH3)n (n = 0−11) clusters to systematically investigate the transition of the solvation structures and detect the spectral
2. EXPERIMENTAL METHODS The IR spectra were recorded by using IR-UV depletion spectroscopy. The experimental scheme was different depending on the cluster size: size selective IR spectroscopy was implemented for small clusters with n ≤ 5 that show ESHT reaction in the S1 states, while for n ≥ 6, there is incomplete size selection. The excitation schemes for each case are shown in Figure 1. For n ≤ 5, the ESHT reaction products, H·(NH3)n
Figure 1. Excitation schemes of the IR-UV depletion spectroscopy employed for (a) 2 ≤ n ≤ 5 and (b) n ≥ 6 clusters. νexc was set to resonance of each cluster for panel a and 285 nm for panel b. The wavelength and delay of νion were set to 355 nm and 200 ns from νexc for panel a and 470 nm and 50 ns from νexc for panel b, respectively. νIR was introduced 50 ns before νexc for both the cases. ESHT means an excited state hydrogen transfer reaction from the S1 state of PhOH−(NH3)n clusters.
radicals, are exclusively produced from PhOH−(NH3)n clusters without NH3 evaporation and can be probed size specifically as H+(NH3)n by soft photoionization. Since the S1−S0 absorption spectra obtained by monitoring the H+(NH3)n products show well-separated, sharp transitions,26,36,37 one can achieve cluster size and isomer selective IR spectroscopy of PhOH−(NH3)n (n ≤ 5) clusters by probing the product ion intensities.36,37 For n ≥ 6, no sharp electronic absorption has been observed. Photoionization via such a broad UV absorption is still applicable as a probe of IR spectroscopy of solvated clusters. In fact, Fujii and co-workers have used a nonsize specific broad UV absorption for the IR spectroscopy of large sized hydrated PhOH−(H2 O) n clusters and succeeded to extract the information on size-dependent growth of the hydration shell.67,68 The key point in this case is to estimate the number of solvent molecules that evaporate during the probe process. The UV excitation and ionization via such broad absorptions induce transitions to a high energy portion of the potential surface. The large excess energy implemented by the excitation processes causes evaporation and results in the loss of the size selectivity. To suppress the excess energy, we adopted twocolor two-photon ionization whose total energy was set as close as possible to the reported ionization potential of the clusters. This procedure should enhance the size selectivity of the measurement. A detailed description of the apparatus has already been given elsewhere, and we only briefly go over the main points.72 PhOH vapor was diluted in a gas mixture of Ne gas that contains a small amount of NH3, and the mixture was expanded into a vacuum chamber through the orifice of a pulsed nozzle to 1523
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make a supersonic jet. The stagnation pressure was about 3 bar, and two concentrations of NH3, 0.5% and 5%, were used to control the cluster size distribution in the jet. The jet expansion was skimmed and introduced into the acceleration region of a time-of-flight mass spectrometer, where the PhOH−(NH3)n clusters were excited and ionized by laser pulses, and the ionic species monitored as a probe of the ground state population. The probed species were the ESHT reaction products H+(NH3)n for n ≤ 5 and the parent clusters [PhOH− (NH3)n]+ for n ≥ 6, respectively. The PhOH−(NH3)n (2 ≤ n ≤ 5) clusters were excited to the S1 states via their respective resonant transitions, and the resulting ESHT products were ionized by another UV pulse, 200 ns after the excitation.36,37 The PhOH−(NH3)n (n ≥ 6) clusters were electronically excited via their broad UV absorption around 285 nm and then ionized by a visible light (470 nm), 50 ns after the excitation (Figure 1b). The PhOH monomer and the n = 1 cluster were ionized by usual 1 + 1 two-photon ionization via their respective resonant transitions.25 Prior to the UV excitation pulse, an IR laser pulse was introduced in a coaxial and counter propagating manner to the UV light with a given delay time. When the IR wavelength is resonant on a vibrational transition of the target clusters, the ground state population is depleted, which results in a reduction of the signal on the probed ionic species. Thus, by scanning the IR wavelength while monitoring the ion signal, an IR spectrum of the electronic ground states of the clusters is obtained as an ion depletion spectrum. The UV excitation pulse was generated by the second harmonic generation (SHG) of an output of a Nd3+:YAG laser pumped dye laser. The fundamental or SHG output of a dye laser pumped by another Nd3+:YAG laser was used as ionization pulse. Tunable IR light in the 3 μm region was generated by difference frequency generation (DFG) in a LiNbO3 crystal between the second harmonic of a Nd3+:YAG laser and the output of a dye laser pumped by a part of the second harmonic. Tunable mid-IR light was generated by DFG in ZnGeP2 crystals between a 2 μm light produced by a degenerate optical parametric process of the fundamental of a Nd3+:YAG laser and the tunable 3 μm light described above.73,74 The spectral resolution of the mid-IR laser has been improved to be 10: Structural Strains in Hydrogen Bond Networks of Neutral Water Clusters. J. Phys. Chem. A 2009, 113, 12134−12141. (69) Watanabe, T.; Ebata, T.; Tanabe, S.; Mikami, N. Size-Selected Vibrational Spectra of Phenol−(H2O)n (n = 1−4) Clusters Observed by IR-UV Double Resonance and Stimulated Raman-UV Double Resonance Spectroscopies. J. Chem. Phys. 1996, 105, 408−419. (70) El-Nasr, E. a. E.-H. A.; Fujii, A.; Yahagi, T.; Ebata, T.; Mikami, N. Laser Spectroscopic Investigation of Salicylic Acids Hydrogen Bonded with Water in Supersonic Jets: Microsolvation Effects for Excited Sate Proton Dislocation. J. Phys. Chem. A 2005, 109, 2498− 2504. (71) Kleinermanns, K.; Janzen, C.; Spangenberg, D.; Gerhards, M. Infrared Spectroscopy of Resonantly Ionized (Phenol)(H2O)n. J. Phys. Chem. A 1999, 103, 5232−5239. (72) Saeki, M.; Ishiuchi, S.; Sakai, M.; Fujii, M. Structure of the JetCooled 1-Naphthol Dimer Studied by IR Dip Spectroscopy: 1529
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