Quantitative Temperature Dependence of the Microscopic Hydration

May 22, 2017 - To discuss the temperature effect on microscopic hydration structures in clusters, relative populations of the isomers having different...
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Quantitative Temperature Dependence of the Microscopic Hydration Structures Investigated by Ultraviolet Photodissociation Spectroscopy of Hydrated Phenol Cations Haruki Ishikawa,* Itaru Kurusu, Reona Yagi, Ryota Kato, and Yasutoshi Kasahara Department of Chemistry, School of Science, Kitasato University, Minami-ku, Sagamihara 252-0373, Japan S Supporting Information *

ABSTRACT: To discuss the temperature effect on microscopic hydration structures in clusters, relative populations of the isomers having different hydration structures at welldefined temperatures are quite important. In the present study, we measured ultraviolet photodissociation spectra of the temperature-controlled hydrated phenol cation [PhOH(H2O)5]+ trapped in the 22-pole ion trap. Two isomers having a distinct hydration motif with each other are identified in the spectra, and a clear change in the relative populations is observed in the temperature range from 30 to 150 K. This behavior is quantitatively interpreted by statistical mechanical estimation based on density functional theory calculations. A ring with tail-type hydration motif is dominant in cold conditions, whereas a chain-like motif is dominant in hot conditions. The present study provides very quantitative information about the temperature effect on the microscopic hydration structures.

H

be investigated at well-defined temperature conditions. Theoretical studies on the temperature dependence of the hydration structure in molecular clusters have also been carried out in recent years.40−45 Thus, quantitative information about such a problem is crucial for direct comparison between experimental and theoretical results. To obtain the quantitative temperature dependence of the hydration structure of clusters, we carried out ultraviolet photodissociation (UVPD) spectroscopy of a hydrated phenol cation, [PhOH(H2O)5]+, trapped in a temperature-controlled 22-pole ion trap. Although detailed hydration structures are not assigned directly from UV spectra, much plausible information about relative populations is expected to be obtained. We have succeeded in observing a clear temperature dependence of relative populations between isomers of [PhOH(H2O)5]+ having distinct hydrogen-bond structures. The possible hydrogen-bond structures were obtained by density functional theory (DFT) calculations. In the present Letter, we discuss the temperature effect on the hydration structures based on a quantitative comparison between the experimental and theoretical results. In the present study, we carried out UVPD spectroscopy using our temperature-variable ion trap apparatus.10,11,13 It consists of tandem-type quadrupole mass filters and a temperature-variable 22-pole ion trap filled with a temperature-controlled He buffer gas in between them. Details of the experimental conditions are described in the Supporting Information. In the present experiment, we monitored yields

ydrogen bonds play important roles in various processes involving chemical reactions, biological processes, and so on. To understand microscopic behaviors of hydrogen-bond networks, a large number of spectroscopic studies on gas-phase hydrogen-bonded molecular clusters have been carried out.1−5 One of characteristic features of hydrogen-bond networks is their flexible structures. Because structural flexibility is related to entropic effects, temperature effects on the hydration structures are very important to understand the nature of hydrogen-bond networks. One of the temperature effects on hydration structures in molecular clusters is a change in relative populations among isomers. In recent years, mainly two cooling techniques are utilized for molecular cluster ions; one is a buffer gas cooling method using a cryogenically cooled ion trap,6−24 and the other is a so-called “tagging” method,25−39 in which the internal energy of the cluster is restricted. However, the number of the studies aiming to observe a temperature dependence of the hydration or hydrogen-bond structures is not so large.7,19,22,34,38,39 Recently, a clear change in relative populations between ring- and chain-type isomers of protonated methanol clusters was reported.39 In this study, internal energies of the tagged clusters are controlled by proper selection of the tag species. However, quantitative evaluation of the relative populations among the isomers was not carried out. To our knowledge, only one paper reported a clear temperature dependence of the hydration structures of I−(H2O)2.19 The structural change observed was simple dissociation of a hydrogen bond between two water moieties. For a further understanding of the microscopic nature of the hydrogen-bond network, the temperature dependence of a more complicated system where structural changes of isomers involve not only dissociation but also rearrangements of hydrogen bonds should © 2017 American Chemical Society

Received: May 10, 2017 Accepted: May 22, 2017 Published: May 22, 2017 2541

DOI: 10.1021/acs.jpclett.7b01165 J. Phys. Chem. Lett. 2017, 8, 2541−2546

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The Journal of Physical Chemistry Letters of the [PhOH(H2O)]+ in PD spectroscopy of the [PhOH(H2O)5]+ to avoid signals originating from the [PhOH(H2O)4]+, which was generated by collision-induced dissociation in the trap. It was confirmed that the [PhOH(H2O)4]+ dissociates exclusively into PhOH+ after UV excitation. Local minimum structures of the clusters were obtained by DFT calculations at the ωB97X-D/6-311++G(3df, 3pd) level using the Gaussian 09 program package.46 In addition, vibrational wavenumbers obtained were used for Gibbs energy calculations. Gibbs energies for each isomer at different temperatures were estimated statistical mechanically using a script written by Irikura.47 In this calculation, a harmonic oscillator approximation was used. Vibrational wavenumbers were scaled by 0.9315 so that the OH stretching wavenumber of the phenol monomer cation calculated coincided with the experimental value of 3565 cm−1.48 The structural optimization and the vibrational analysis for the excited state were obtained by time-dependent DFT (TD-DFT) calculations at the ωB97X-D/6-311++G(d,p) level. Before reporting results of the PD spectroscopy of the [PhOH(H2O)5]+ cluster, vibrational temperatures of the cluster ions, Tvib, in the ion trap were evaluated based on the relative intensity of hot bands in spectra of the phenol-trimethylamine cation, [PhOH-TMA]+. Details are described in the Supporting Information. As a result, Tvib of the ions is almost the same as the temperature of the trap, TTrap, when TTrap ≥ 30 K. The lowest value of Tvib achieved is 30 K, even when TTrap is set below 30 K. Such behavior can be interpreted by heating by the RF electric field. Hereafter, the vibrational temperatures of the ions, Tvib, are simply denoted as T. Figure 1a shows a typical PD spectrum of [PhOH(H2O)5]+ at T = 30 K in the present study. UVPD spectroscopy of

in the case of [PhOH-TMA]+, the band pattern of this spectrum basically represents that of the phenoxy radical. As will be discussed later, positions of the vibronic bands are affected by the hydration environment at the oxygen atom of the phenoxy radical in the cluster. If there were several isomers having distinct hydration structures, the number of bands in the spectrum of [PhOH(H2O)5]+ would be larger compared with that for [PhOH-TMA]+. The fact that the similarity between the spectra in panels (a) and (b) indicates that only one isomer of [PhOH(H2O)5]+ exists at T = 30 K. Hereafter, the isomer appearing in Figure 1a is referred to as isomer A. Then we recorded PD spectra of [PhOH(H2O)5]+ at different temperatures. The spectra are shown in Figure 2, where all of the spectra are normalized by the height of the 000 band of isomer A. It was found that the spectra measured at T = 30 and 50 K are almost the same as each other. When the temperature becomes 80 K, the band pattern slightly changes. A small band marked as B in the figure appears at 25416 cm−1, about 67 cm−1 to the higher wavenumber side of band A. At 100 K, band B is now clearly recognized. As the temperature becomes higher, the newly appearing band B gains intensity. The spectral carrier of band B is referred to as isomer B. At 150 K, band B is now stronger than band A. An inset in Figure 2 shows the same spectra displayed at a different normalization condition. The 000 bands of isomers A and B are fitted with Lorentz profiles, and the spectra are normalized by the sum of the integrated intensities. Assuming that the absorption coefficients and dissociation yields of these isomers are the same as each other, relative integrated intensities correspond to relative populations of the isomers of the [PhOH(H2O)5]+ at each temperature. Table 1 lists the relative populations of isomers A and B at each temperature. This result provides a quantitative temperature dependence in the relative population among the isomers that is quite important information and is directly comparable to theoretical estimations. As the intensity of band B increases, vibronic bands of isomer B also appear at the higher wavenumber side of the vibronic bands of isomer A. The arrows in Figure 2 indicate the shift of each band of isomer B from that of isomer A. The amounts of shift are 65−70 cm−1, except for that of the intermolecular stretch band, σ01. The shift of σ01 is 150 cm−1, which is much larger than those for the other bands. The vibrational wavenumber of σ in the excited state is 160 and 250 cm−1 for isomers A and B, respectively. This fact will be discussed in assignment of the hydration structure. To discuss the possible hydration structures of the [PhOH(H2O)5]+, we carried out DFT calculations on this system. We obtained about 45 isomers in the present calculation. To describe detailed structural features of all of the isomers is verbose. Thus, representative hydration structures or hydration motifs are selected in Figure 3 and discussed here. The potential energies and the zero-point-level energies for these isomers are listed in Table 2. Detailed results obtained in the DFT calculations are summarized in Table S1 in the Supporting Information. In the majority of the isomers, the proton of the phenol moiety is transferred to the water cluster side and the phenol moiety exists as a phenoxy radical. Hydration structures obtained are grouped into several motifs such as ring (R), ring with tail (Rt), bicyclic (Bc), and chain (C) motifs. Similar groupings are reported in previous studies on hydrogen-bonded clusters.34,38,39,41 In addition to this grouping, another classification is introduced here. In the present study, we observed the electronic transition of the [PhOH-

Figure 1. Photodissociation spectra of (a) [PhOH(H2O)5]+ and (b) [PhOH-TMA]+ measured at T = 30 K.

hydrogen-bonded clusters of the phenol cation was extensively carried out by Mikami and co-workers.49−51 Spectral features of the spectrum are essentially the same as that reported in the previous study. The strongest band at 25349 cm−1 is assigned as an origin band of the [PhOH(H2O)5]+. In this cluster, the proton of PhOH is transferred to the water moiety. Therefore, the phenoxy radical becomes a chromophore of this cluster. The transition observed corresponds to the D3−D0 transition of the phenoxy radical.52 For comparison, the PD spectrum of [PhOH-TMA]+ measured at the same conditions is also shown in Figure 1b. It is clearly seen that band patterns in both spectra are quite similar to each other. Because there is only one isomer 2542

DOI: 10.1021/acs.jpclett.7b01165 J. Phys. Chem. Lett. 2017, 8, 2541−2546

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Figure 2. Photodissociation spectra of [PhOH(H2O)5]+. The arrows indicate the shift of each band of isomer B from that of isomer A. An inset displays the spectra normalized based on the summed integrated intensities. To clearly exhibit the differences, the inset spectra are smoothed.

Table 1. Temperature Dependence in the Relative Populations of Isomers A and B of [PhOH(H2O)5]+ Obtained by PD Spectroscopya T/K

isomer A

isomer B

30 50 80 100 120 150

1.0 1.0 0.96 0.74 0.56 0.42

0.0 0.0 0.04 0.26 0.44 0.58

Table 2. Differences in the Potential Energy, Zero-PointLevel Energy, and Gibbs Energy at 50, 100, and 150 K Measured from the Most Stable Isomers at Each Condition for the Isomers of [PhOH(H2O)5]+ Shown in Figure 3 Obtained by ωB97X-D/6-311++G(3df,3pd) Calculation along with Wavenumbers of each 000 Band, ν (000), Estimated by TD-DFT Calculation at the ωB97X-D/6-311+ +G(d,p) Levela ΔG

The uncertainty of the temperature is ±10 K, whereas that of the relative populations is estimated to be ±0.01.

a

a

isomer

ΔE

ΔE0

50 K

100 K

150 K

ν (000)

C-I-a C-I-b C-I-c Bc-III-a Rt-II-a Rt-II-b Rt-II-c Rt-II-d R-II-a

1333 1252 1306 0.0 375 425 651 313 1487

463 465 418 99 0 113 136 46 1010

335 399 318 182 0 110 157 74 1062

136 279 140 352 0 124 179 129 1166

0 230 21 664 99 245 292 299 2684

27228 27255 27246 26917 27129 27122 27142 27116 27151

All values are in units of cm−1.

atom of the phenoxy radical is hydrated by two neutral water molecules, whereas it is hydrated by one neutral H2O and one H3O+ in scheme III. In scheme IV, the phenoxy radical is hydrated by one neutral H2O as a proton acceptor. By combining the classification based on the hydration structural motifs and the hydration schemes at the O atom of the phenyl radical, the hydration structures of the [PhOH(H2O)5]+ are classified into nine groups: R-II, Rt-I, Rt-II, Rt-III, Rt-IV, Bc-II, Bc-III, C-I, and C-IV. In the present study, we discuss the temperature effect on the hydration structures. Thus, the Gibbs energies of the isomers at various temperatures are evaluated statistical mechanically. At every 10 K from 10 to 300 K, relative populations are evaluated based on the Boltzmann distribution using the Gibbs energies. The Gibbs energies of the isomers shown in Figure 3 at 50, 100, and 150 K are also listed in Table 2. Among all of the isomers, at most seven isomers belonging to the C-I and Rt-II motifs shown in Figure 3 are found to exhibit appreciable contributions to the relative populations. As mentioned above, we observed the electronic transitions of the phenoxy moiety in the clusters. The vibronic bands of the isomers having the same hydration scheme around the phenoxy radical should be overlapped in our observation. Actually, the wavenumbers of the 000 bands of the isomers belonging to

Figure 3. Typical hydration structures of [PhOH(H2O)5]+. The eight isomers belonging to the C-I and Rt-II motifs exhibit major contributions to the relative populations.

(H2O)5]+ clusters, whose chromophore is the phenoxy radical. The hydrogen-bonding scheme around the oxygen atom in the phenoxy moiety should be reflected in the electronic transition. Thus, the hydrogen-bonding scheme of the phenoxy moiety in the clusters is examined. The O atom of the phenoxy radical can form up to two hydrogen bonds as a proton acceptor. Thus, four hydrogen-bonding schemes exist. In scheme I, the phenoxy radical acts as a single-proton acceptor. In scheme II, the O 2543

DOI: 10.1021/acs.jpclett.7b01165 J. Phys. Chem. Lett. 2017, 8, 2541−2546

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The Journal of Physical Chemistry Letters the same motif are expected to be within 30 cm−1, expected by TD-DFT calculations. The 000 band wavenumbers are also listed in Table 2. In addition, as the temperature elevates, isomerizations among the isomers can occur. Especially, the isomers in the C-I group should easily isomerize with each other. Thus, the relative populations of the isomers belonging to the same motif are summed up and examined here. Figure 4

respectively. The other isomers belonging to the C-I motif also have similar wavenumbers of mode σ. Thus, the results of the TD-DFT calculation strongly support the assignment that isomer B belongs to the C-I motif. At 150 K, the difference between the experimental and the theoretical relative populations is larger than that at other temperatures. The possible reason is limitation of the harmonic approximation used in our calculation. The hydration structures belonging to the C motif are very flexible. Thus, effects of the anharmonicity and the isomerization among the isomer should be included at a higher temperature than 150 K. In summary, we recorded UV photodissociation spectra of a temperature-controlled [PhOH(H2O)5]+ cluster in the trap. A clear temperature dependence of the spectra indicated that the relative populations between the two isomers strongly depend on temperature. The observed and theoretically estimated changes in the relative population between the hydration motifs having distinct structures exhibit very good agreement with each other. Our result is the first quantitative information about the temperature dependence of the hydration structures of clusters, which should be a benchmark sophisticated theoretical study. We are planning to carry out IR spectroscopy to make a definitive determination of the hydration structures of the isomers. In addition, isomerization paths among the isomers have to be investigated in the future.

Figure 4. Temperature dependence of the relative population of the isomers of [PhOH(H2O)5]+. The solid line indicates the estimation based on the DFT calculation at the ωB97X-D/6-311++G(3df,3pd) level. Open squares and circles indicate the observed relative populations of isomers A and B, respectively.

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exhibits the temperature dependence of the summed relative population of the isomers in the C-I, Rt-II, and Bc-III. The temperature dependence is easily interpreted by considering the flexibility of the hydration structures. The chain-type hydration structure is much more flexible than the ring-type one. Thus, the C motif is the most flexible and expected to exhibit a large entropic effect, whereas the Bc motif is the most rigid and exhibits less entropic effect. The Rt motif is more flexible than the Bc because of the addition of the flexible chaintype tail to the ring structure. In cold conditions, a contribution of entropy is less important. On the other hand, the entropy term greatly contributes to the Gibbs energy in hot conditions. Thus, in general, the Bc motif is expected to be dominant in cold conditions, whereas the C motif gains its population in hot conditions. The Rt motif should exhibit a large contribution in a middle temperature range. In the present case, although the BcIII-a isomer corresponds to the global minimum on the potential surface, the Rt-II-a isomer becomes the most stable at the zero-point level due to a difference in the zero-point energy. Because the entropy of the Rt motif is larger than that of the Bc motif, the Bc motif cannot get an appreciable population in all of the temperature range. Similar temperature dependence of the hydrogen-bond structure is reported for protonated methanol clusters.34,38,39 Finally, the experimental result of the temperature dependence of the relative populations is discussed in comparison with the theoretical calculations. In our experiment, we observed two isomers or motifs in the temperature range from 30 to 150 K. The relative populations obtained based on the integrated intensities are also displayed in Figure 4. The theoretical results agree very well with our experimental observation. Thus, isomer A is assigned as ring with tail-type structure, while isomer B is the chain-type one. As pointed out before, the vibrational wavenumbers of mode σ of isomer A in the excited state is 160 cm−1, while that of isomer B is 250 cm−1. We consider this fact to be support for our assignment. Thus, we carried out time-dependent DFT calculations of some isomers. The TD-DFT calculations showed that the wavenumbers of mode σ for Rt-II-a and C-I-a are 199 and 257 cm−1,

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01165. Experimental details, characterization of the temperature of the ions, complete author list of ref 46, and hydration structures of [PhOH(H2O) 5]+ obtained by DFT calculation at the ωB97XD/6-311++G(3df,3dp) level (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Haruki Ishikawa: 0000-0003-1499-9436 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI Grant Number JP25104008. REFERENCES

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DOI: 10.1021/acs.jpclett.7b01165 J. Phys. Chem. Lett. 2017, 8, 2541−2546

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DOI: 10.1021/acs.jpclett.7b01165 J. Phys. Chem. Lett. 2017, 8, 2541−2546