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An Infrared Spectroscopic Study of the Acidic CH Bonds in Hydrated Clusters of Cationic Pentane Tomoya Endo, Yoshiyuki Matsuda, and Asuka Fujii J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02282 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 17, 2017

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The Journal of Physical Chemistry Letters

An Infrared Spectroscopic Study of the Acidic CH Bonds in Hydrated Clusters of Cationic Pentane. Tomoya Endo, Yoshiyuki Matsuda,* Asuka Fujii Department of Chemistry, Graduate School of Science, Tohoku University, 6-3, Aramaki AzaAoba, Aoba-ku, Sendai, 980-8578, Japan AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interests.

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ABSTRACT: Infrared spectroscopy of the hydrated clusters of cationic pentane, which are generated through the vacuum ultraviolet photoionization in the gas phase, is carried out to probe the acidic properties of their CH bonds. The mono-hydrated pentane cation forms the protonshared structure, in which the proton of CH in cationic pentane is shared between the pentyl radical and water molecule. In the di- and tri-hydrated clusters, the proton of CH is completely transferred to the water moiety, so that the clusters are composed of the pentyl radical and protonated water cluster. These results indicate two water molecules are enough to cause the proton transfer from CH of cationic pentane and thus its acidity is highly enhanced with the ionization.

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CH bonds are generally regarded as aprotic though they sometimes act as very weak proton donors in the CHLO/N interaction.1,2 On the other hand, proton transfer from positivelycharged CH bonds is frequently seen in gas phase ion-molecule reactions, radiation chemistry, and organic synthesis.3-6 However, while the acidities of cationic OH and NH have undergone a great deal of experimental and theoretical investigations7, that of cationic CH has attracted much less attentions. Recently, barrierless proton transfer from CH has been found in the ionized trimethylamine and dimethylether dimers.8,9 These prove the acidity enhancement of cationic CH and the reaction mechanism has been interpreted by delocalization of the positive charge through the hyperconjugation between the CH and nonbonding orbitals the electron of which is ejected in the ionization.10-12 Infrared (IR) spectroscopic signatures of acidity enhancement have been reported also for the cationic alkanes, which have no nonbonding orbital.13 This result implies that alkyl groups as well as alkanes can be acidic under the influence of positive charge. Because of the ubiquity of alkyl groups in chemistry, elucidation of the property of charged CH would be highly important in understandings of diverse chemical processes involving carbocation and CH activation, in which (partially) charged CH arises through charge fluctuation and environmental perturbation. To estimate the acidity of cationic CH, it will be very informative to observe intracluster proton transfer in hydrated clusters of the cation. The proton affinity of the water moiety in the cluster increases with increasing cluster size (number of the water molecules in the cluster). Therefore, the size dependence of the proton transfer in the hydrated clusters can be a measure of the acidity of the cationic CH. In this study, we carry out IR predissociation spectroscopy of the hydrated clusters of cationic pentane, which are generated through the one photon ionization by the 118 nm light.14 We investigate how many water molecules are necessary for proton transfer from CH in cationic pentane through their cluster structure analyses based on the sizeselective IR spectroscopy. Figure 1(a) shows the observed IR spectrum of [pentane-H2O]+. Figures 1(b) – 1(e) are the simulated vibrational spectra based on the energy-optimized structures of the cluster, which are schematically shown in the insets. 15, 16 All the stable isomeric structures found in our calculations are compiled in Supporting Information and they are classified to the four types.16 Each structure depicted in Figure 1 is the most stable one in each structure type. In the observed spectrum, the CH stretches (νCH) of the pentane cation moiety and the free OH stretches (ν1 and ν3) of the water moiety are seen at ~2915, 3520, and ~3640 cm-1, respectively. Both ν1 and ν3 frequencies are much lower than those of 1. (a) Observed IR spectrum of [pentane-H2O]+ and neutral water (3657 and 3756 cm-1, Figure (b)-(e) simulated vibrational spectra for its optimized structures at the ωB97X-D/6-311++G(3df,3pd) level. The relative energies are corrected with the zero point energies (ZPE). The calculated frequencies are scaled by 0.945.


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respectively) and are rather close to the OH stretch frequencies (3490 and 3550 cm-1) of the hydronium ion, H3O+.17, 18 This suggests that the water moiety in [pentane-H2O]+ has the hydronium ion character due to the incomplete proton transfer. In Structures 1 – 3, the proton of one of CHs in the pentane moiety is shared with the water moiety, and the remarkable elongation of the proton-donating CH bond occurs. The difference among Structures 1 – 3 is difference of the proton-donating site. On the other hand, in Structure 4, the protons contacting with the water moiety are clearly localized to the pentane moiety. The simulated ν1 and ν 3 frequencies of the water moiety in Structures 1 – 3 agree well with the observed ones, while those of Structure 4 do not match with the observed ones. Furthermore, the intense band at 2800 cm-1 (stretch of CH interacting with the water moiety) simulated for Structure 4 is not observed. Therefore, we conclude that [pentane-H2O]+ forms a proton-shared type structure such as Structures 1 – 3. Because of their larger stabilities, Structures 1 and 2 would be more probable than Structure 3. The frequency of the shared proton vibration in Structures 1 – 3 is calculated at ~1300 cm-1 and this is out of the observed frequency range. Figure 2(a) shows the observed IR spectrum of [pentane-(H2O)2]+. Figures 2(b) – 2(e) are simulated spectra for the most stable structures in the four structure types (See Supporting Information for the other isomers). In the observed spectrum, there appears a broad feature in the frequency region lower than 2800 cm-1, in addition to free OH bands and νCH. Structures 1– 3 are the proton-transferred type structures, in which a proton of the pentane moiety is completely transferred to the water moiety. They are much more stable than the non-protontransferred type, Structure 4, which is excluded from the spectral carrier because of the lack of its simulated intense hydrogen(H)-bonded νOH at 3470 cm-1 in the observed spectrum. For Structures 1 – 3, two H-bonded νOH bands of the protonated water moiety are predicted in the 2100 – 2500 cm-1 region. It has been wellknown that H-bonded νOH bands of a protonated water site are extremely broadened and their center frequencies are much lower

Figure 2. (a) Observed IR spectrum of [pentane-(H2O)2]+ and Figure 3. (a) Observed IR spectrum of [pentane-(H2O)3]+ and (b)-(e) simulated vibrational spectra of its optimized (b)-(f) simulated vibrational spectra of optimized structures structures at the ωB97X-D/6-311++G(3df,3pd) level. The at the ωB97X-D/6-311++G(3df,3pd) level. The relative relative energies are corrected with ZPE. simulated are corrected with ZPE. The calculated frequencies ACSThe Paragon Plusenergies Environment frequencies are scaled by 0.945. are scaled by 0.945.

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than the harmonic frequencies because of their large anharmonicity.7,8,14 The broad feature in the low frequency region of the observed spectrum is assigned to the H-bonded νOH bands in Structures 1 – 3. Therefore, proton-transferred structures are concluded for [pentane-(H2O)2]+. This means that two water molecules are enough to induce the proton transfer from CH of cationic pentane. Figure 3(a) shows the observed IR spectrum of the [pentane-(H2O)3]+. Figures 3(b) – 3(f) show the simulated vibrational spectra for Structures 1 – 5, respectively. The observed spectral features are similar to those of [pentane(H2O)2]+, but the broad absorption in the low frequency region becomes more remarkable. This broad band is assigned to the overlap of three modes of H-bonded νOH of the hydronium ion in Structures 1 – 4. The simulated spectrum of the non-protontransferred form, Structure 5, does not agree with the observed one, because of the missing of the simulated intense H-bonded νOH at The spectral features ~3500 cm-1. demonstrate that [pentane-(H2O)3]+ also forms proton-transferred structures, such as Structures 1 – 4. Figures 4(a) – 4(c) show the potential energy curves along the proton transfer coordinate from the C1-H2 bond of the pentane moiety to the O3 atom of the water moiety in [pentane-(H2O)1-3]+, respectively. These curves are calculated by fixing the C1-H2 bond length to each value and optimizing all the other structural parameters. The energy diagrams of their isomerization pathways from the verticallyionized structures are shown in the Supporting Information. The potential minima in the figures are their most stable structures, which correspond to Structures 1 in Figures 1 – 3, respectively. In the minimum structure of [pentane-H2O]+, the O3-H2 distance (1.16 Å) is smaller than the Figure 4. Potential energy curves along the C1-H2 bond C1-H2 distance (1.48 Å). The O3-H2 distance of cationic pentane hydrated with (a) H2O, (b) distance is, however, still too long for the (H2O)2, and (c) (H2O)3. All potential energies are calculated at the ωB97X-D/6-311++G(3df, 3pd) level by fixing the C1complete O3-H2 covalent bond formation. H2 distance at each value and optimizing all the other With increasing the number of water structural parameters. The most stable structure in each molecule, the O3-H2 bond at their stable cluster size is employed in the calculation. The shortest bond length is set to 1.09 Å in all cases, which is the simulated C1structures shortens, while the C1-H2 bond H2 bond length of neutral pentane. elongates. This is explained well by the increase of the proton affinity of the water


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moiety. This O3-H2 bond shortening is reflected by the blue-shifts of the modes involving the H-bonded O3-H2 stretch. In all the potential energy Table 1. The dissociation energies (kcal/mol) of [pentane-(H2O)n]+ curves, no energy are simulated for Structures 1, which are shown in Figures 1-3 in the barrier exists along the main text. proton transfer (C1Dissociation channels Dissociation energy H2) coordinate from the neutral C1-H2 pentane+ → C5H11 + H+ 180.21 bond length at 1.09 Å + → C5H12+ + H2O 14.10 to their stable [pentane-H2O] structures. The [pentane-H2O]+ → C5H11 + H3O+ 29.63 position of the + + 32.24 transferred proton pentane-(H2O)2] → C5H12 + (H2O)2 reflects the balance [pentane-(H2O)2]+ → C5H11 + H+(H2O)2 16.36 between the proton + + 45.31 affinities of the pentyl [pentane-(H2O)3] → C5H12 + (H2O)3 radical and the water [pentane-(H2O)3]+ → C5H11 + H+(H2O)3 10.98 moiety. These energies correspond to the gas phase acidities. Table 1 lists the *The H+ and H+(H2O)n dissociate through the bond cleavages of the dissociation energies C1 and H2 atoms. The C1 and H2 atoms are numbered in Figure 4 of the cations of bare in the main text. and hydrated pentane. The dissociation energy of the proton from the C1-H2 bonds drastically decreases with increasing the number of the water molecule. The dissociation of H2O is energetically more favorable than that of H3O+ in the [pentane-H2O]+ cluster cation, although the O3-H2 bond is shorter than the C1-H2 bond in the cluster cation as seen in Figure 4. Therefore, we regard it as the proton-shared structure, in which the proton is not completely transferred. In [pentane(H2O)n]+, n=2 and 3, the O3-H2 bond distance is less than 1.05 Å. Moreover, the dissociation of H3O+(H2O)n-1 is more favorable than that of H2O in [pentane-(H2O)n]+, n=2, 3. Thus, we have concluded that their structures are the proton-transferred structures. Both the observed IR spectral features and DFT calculations indicate that two water molecules are enough to induce the proton transfer from cationic pentane. The proton affinity of the water dimer has been reported to be 808 kJ/mol-1. 19 Cationic phenol, which is known as a strong acid, needs more than two water molecules to transfer its hydroxyl proton.20 Therefore, the acidity of cationic pentane could be higher than cationic phenol, and cationic pentane can be regarded as strong acid. As seen in Figure 4, the proton is transferred from CH without the energy barrier. Therefore, we have concluded that the pentane cation has the acidic CH bonds. The acidity enhancements of CH in cationic pentane, hexane, and heptane have been suggested by the CH stretch frequency shifts of their monomer cations.13 The present results support that cationic alkanes as well as cationic pentane are highly acidic. In this study, the high acidity of CH in cationic pentane has been elucidated by the IR spectroscopy and theoretical calculations for [pentane-(H2O)n]+ (n = 1-3). The acidity of CH in the pentane cation is remarkably enhanced and its magnitude would be higher than that of OH in cationic phenol. Although neutral alkanes (alkyl groups) are normally aprotic, the acidic character of positively-charged CH, demonstrated in this study would emerge through charge fluctuation, delocalization, and separation even in neutral systems. Moreover, positively-charged


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alkanes often appear as reactants in ion-molecule reactions, radiation chemistry, and so on. The acidic character of positively-charged CH would contribute to diverse chemical processes.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Methods (Experimental and theoretical), Optimized geometries of [pentane-(H2O)n]+, n=1-3, Isomerization pathways of vertically-ionized pentane-(H2O)n, n=1-3, complete author list of reference (15) (PDF) AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We thank Prof. T. Maeyama and Dr. Min Xie for their helpful discussions. T. E. appreciates DIARE in Tohoku University for the fellowship. Y. M. acknowledges supports from the Grantin-Aid for Scientific Research on Innovative Area [2507] (Project No. 16H00930) from MEXT Japan and the Grand-in-Aid for Scientific Research (Project No. 16K05640) from JSPS.

REFERENCES (1) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond; OXFORD UNIVERSITY PRESS: Oxford, U.K.; 2001. (2) Wahl,M. C.; Sundaralingam, M. C-H⋯O Hydrogen Bonding in Biology. Trends Biochem. Sci. 1997, 22, 97-102. (3) Matsuoka, S.; Ikezoe, Y. Ion-Molecule Reactions and Thermal Decomposition of Ions in Nitrogen-Oxygen Alkane (C2-C8) Mixtures Studied by Time-Resolved Atmospheric Pressure Ionization Mass Spectrometry. J. Phys. Chem. 1988, 92, 1126-1133. (4) de Hoffmannnn, E.; Stroobant, V. Mass Spectrometry : Principles and Applications. 3rd ed.; John Wiley & Sons: New York; 2007. (5) Tagawa, S.; Hayashi, N.; Yoshida, Y.; Washio, M.; Tabata, Y. Pulse Radiolysis Studies on Liquid Alkanes and Related Polymers. Radiat. Phys. Chem. 1989, 34, 503-511. (6) Singh, M. S. Reactive Intermediates in Organic Chemistry: Structure, Mechanism, and Reactions; John Wiley & Sons: New York; 2014.


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(7) Lisy, J. M. Infrared Studies of Ionic Clusters: The Influence of Yuan T. Lee. J. Chem. Phys. 2006, 125, 132302-1-19. (8) Matsuda, Y.; Nakayama, Y.; Mikami, N.; Fujii, A. Isomer-Selective Infrared Spectroscopy of the Cationic Trimethylamine Dimer to Reveal Its Charge Sharing and Enhanced Acidity of the Methyl Groups. Phys. Chem. Chem. Phys. 2014, 16, 9619-9624. (9) Yoder, B. L.; Bravaya, K. B.; Bodi, A.; West, A. H. C.; Sztaray, B.; Signorell, R. Barrierless Proton Transfer Across Weak CH⋯O Hydrogen Bonds in Dimethyl Ether Dimer. J. Chem. Phys. 2015, 142, 114303-1-9. (10) Matsuda, Y.; Endo, T.; Mikami, N.; Fujii, A.; Morita, M.; Takahashi, K. The Large Variation in Acidity of Diethyl Ether Cation Induced by Internal Rotation about a Single Covalent Bond. J. Phys. Chem. A 2015, 119, 4885-4890. (11) Mosley, J. D.; Young, J. W.; Huang, M.; McCoy, A. B.; Duncan, M. A. Infrared Spectroscopy of the Methanol Cation and Its Methylene-Oxonium Isomer. J. Chem. Phys. 2015, 142, 114301-1-9. (12) Xie, M.; Matsuda, Y.; Fujii, A. Infrared Spectroscopic Investigation of PhotoionizationInduced Acidic C-H Bonds in Cyclic Ethers. J. Phys. Chem. A 2015, 119, 5668-5675. (13) Xie, M.; Matsuda, Y.; Fujii, A. Infrared Spectroscopic Investigation of the Acidic CH Bonds in Cationic n-Alkanes: Pentane, Hexane, and Heptane. J. Phys. Chem. A 2016, 120, 63516356. (14) Matsuda, Y.; Mikami, N.; Fujii, A. Vibrational Spectroscopy of Size-Selected Neutral and Cationic Clusters Combined with Vacuum-Ultraviolet One-Photon Ionization Detection. Phys. Chem. Chem. Phys. 2009, 11, 1279-1290. see Supporting Information for the details of experimental and theoretical methods. (15) Frisch, M. J. et al. Gaussian 09, Revision C.01, see the Supporting Information. (16) Varetto ,U. Molekel 5.4.0.8; Swiss National Supercomputing Centre: Manno, Switzerland, 2009. (17) Fraley, P. E.; Narahari Rao, K. High Resolution Infrared Spectra of Water Vapor: ν1 and ν3 Band of H216O. J. Mol. Spectrosc. 1969, 29, 348-364. (18) Okumura, M.; Yeh, L. I.; Myers, J. D.; Lee, Y. T. Infrared Spectra of the Solvated Hydronium Ion: Vibrational Predissociation Spectroscopy of Mass-Selected H3O+ ⋅ (H2O)n ⋅ (H2)m. J. Phys. Chem. 1990, 94, 3416-3427. (19) Goebbert, D. J.; Wenthold, P. G. Water Dimer Proton Affinity from the Kinetic Method: Dissociation Energy of the Water Dimer. Eur. J. Mass Spectrom. 2004, 10, 837-845. (20) Sato, S.; Mikami, N. Size Dependence of Intracluster Proton Transfer of Phenol-(H2O)n (n = 1−4) Cations. J. Phys. Chem. 1996, 100, 4765-4769.


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