IR Spectroscopic Investigation on Isomerization of Ionized Ethylene

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IR Spectroscopic Investigation on Isomerization of Ionized Ethylene Glycol Yoshiyuki Matsuda,* Ayumu Matsuura, Takahiro Kamiyama, and Asuka Fujii Department of Chemistry, Graduate School of Science, Tohoku University, 6-3, Aramaki-Aza-Aoba, Aoba-ku, Sendai, 980-8578 Miyagi, Japan

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S Supporting Information *

ABSTRACT: It has been known that photoionization of ethylene glycol generates protonated methanol when the ionization energy is in the vicinity of the vertical ionization energy. Although two different isomerization paths have been proposed for the protonated methanol production, the isomerization path has not yet been identified. To investigate the isomerization of ionized ethylene glycol, infrared (IR) predissociation spectroscopy based on vacuum ultraviolet photoionization is carried out for neutral and cationic ethylene glycol and partially deuterated isotopomer (HOCD2CD2OH). The IR spectroscopic results indicate that ionized ethylene glycol isomerizes to the protonated methanol−HCO complex, and the isomerization path involving the double proton transfer is identified. This isomerization path is also supported by the theoretical isomerization path search, which demonstrates that several reaction pathways are mutually intercommunicated.

1. INTRODUCTION An understanding of isomerization and dissociation following ionization of chemical compounds has been an important subject in mass spectrometry, interstellar chemistry, radiation chemistry, and so on.1−8 Various chemical processes are caused by radical cations produced in the secondary process of ionization. However, ionization-induced isomerization and dissociations tend to be highly complicated even in simple molecules, and they frequently make mass spectrometric observations puzzling. Numerous studies have therefore been carried out to elucidate ionization-induced isomerization/ dissociation mechanisms.1−8 Ethylene glycol (HOCH 2 CH2 OH) is one of the fundamental ethane derivatives and is the prototype of diols. Neutral ethylene glycol energetically favors the gauche form, in which the intramolecular hydrogen bond is formed between the two OH groups.9−13 It has been reported that the protonated methanol fragment is generated upon photoionization of ethylene glycol with the photon energy higher than 10.4 eV, which is near the vertical ionization energy.14−21 Because a double proton transfer reaction is necessary to generate protonated methanol from ionized ethylene glycol, this ionization-induced isomerization/dissociation process has attracted much interest. (Here, we should note that the reaction is, strictly speaking, proton and hydrogen transfer. It is, however, difficult to identify which process is proton transfer and which is hydrogen transfer because both of them have both the characters. Therefore, in this paper, we use the term “double proton transfer” for simplicity.) Its mechanism has therefore undergone various investigations with mass spectrometry, photo© 2019 American Chemical Society

electron spectroscopy, collision induced dissociation (CID), threshold photoelectron−photoion coincidence spectroscopy (TPEPICO), theoretical computations, and so on.14−21 The vertical ionization energy of ethylene glycol has been determined to be 10.55 eV by photoelectron spectroscopy.19−21 For the isomerization process of ionized ethylene glycol to generate protonated methanol, mechanism I, shown in Figure 1a, has been proposed in the mass spectrometric and theoretical investigation by Burgers et al.16 In this mechanism, the double proton transfer reaction from OH to the other O

Figure 1. Proposed isomerization mechanisms of ionized ethylene glycol to generate the protonated methanol fragment (refs16,18). Received: March 21, 2019 Revised: May 20, 2019 Published: May 28, 2019 5122

DOI: 10.1021/acs.jpca.9b02591 J. Phys. Chem. A 2019, 123, 5122−5128

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neutral, the IR light was irradiated ∼20 ns prior to VUV photoionization. On the other hand, the injection of the IR light was delayed by ∼20 ns from VUV photoionization for IR spectroscopy of the cation. The VUV light at 118 nm was generated by tripling the third harmonic output (355 nm) of the Nd/YAG laser through the Xe/Ar = 1:10 gaseous mixture. The IR light (2600−3800 cm−1) was generated by the difference frequency generation (DFG) of the second harmonic output (532 nm) of another Nd/YAG laser and the dye laser output with the DCM/methanol solution. Neutral structures were initially searched by the GRRM method mounted on the Gaussian 09 and 16 programs at the PBE1PBE/6-31+G* level.22−24,27,28 The obtained stable structures are re-optimized at the ωB97X-D/6-311++G(3df, 3pd) level. In the reaction path search of ionized ethylene glycol, the intrinsic reaction coordinate (IRC) calculation was applied to the vertically ionized structure of ethylene glycol at the PBE1PBE/6-31+G* level. The IRC result was used as an initial structure for the GRRM reaction path search calculation at the PBE1PBE/6-31+G* level. In the GRRM search, the large anharmonic downward distortion (ADD) finding approach, which limits the exploration area to low-energy parts of the potential energy surface, was applied under the condition in which the 30 largest ADDs (isomerization paths) were explored for 30 minima. The GRRM search calculations with the same condition were also started from other 6 minima. Additionally, the full ADD search around 9 minima was also carried out. All the obtained structures of the cation were re-optimized at the ωB97X-D/6-311++G(3df, 3pd) level. The harmonic vibrational calculations were also carried out at the ωB97X-D/6-311++G(3df, 3pd) level, and the simulated frequencies were scaled by 0.93.

atom and from CH to the other C atom occurs between the two CH2OH moieties. As a result, the CH3OH2+−OCH complex is formed. On the other hand, in the CID study performed by Audier et al. for partially deuterated ethylene glycol-d2, DOCH2CH2OD, it has been observed that ionized DOCH2CH2OD dissociates to CH2DOHD+.17 This result indicates another isomerization path, mechanism II, shown in Figure 1b, exists. In this mechanism, the protons of CH and OH (OD) in the donor CH2OH (CH2OD) moiety are transferred to the O and C atoms in the acceptor moiety, respectively. Li and Baer have performed TPEPICO and observed the breakdown diagrams of ethylene glycol and its fully deuterated isotopomer, C2D6O2, as a function of the VUV photon energy.18 The Rice−Ramsperger−Kassel−Marcus (RRKM) analysis involving the tunneling corrections for the breakdown diagram implies that proton tunneling would be responsible for the slow dissociation rate of ionized ethylene glycol to protonated methanol. The role of the tunneling has also been supported by the difference of appearance energies of CH3OH2+ and CD3OD2+ in the photoionization of ethylene glycol and its deuterated isotopomer, respectively.18 However, in the theoretical calculation at the B3LYP/6-311+G(d,p)// B3LYP/6-31G(d) level, all the simulated energy barriers in mechanism II have been estimated to be lower than the vertical ionization energy of ethylene glycol.18 This means that the tunneling cannot be the main factor of the observed slow reaction rate when mechanism II is supposed. Therefore, the proposal of the RRKM analysis conflicts with mechanism II. Hence, the isomerization mechanism of ionized ethylene glycol to generate protonated methanol has not yet been identified. In the present study, to investigate the isomerization path of ionized ethylene glycol, we carry out infrared (IR) predissociation spectroscopy of neutral and cationic of ethylene glycol and partially deuterated ethylene glycol-d 4 (HOCD2CD2OH) through the photoionization detection at 118 nm. These spectroscopic results indicate that the ionized ethylene glycol isomerizes to the CH3OH2+−OCH complex through mechanism II. The extensive isomerization path search calculations with the global reaction route mapping (GRRM) method22−24 also support mechanism II for the ionized ethylene glycol at 118 nm.

3. RESULTS AND DISCUSSION Figure 2 shows (a) the observed IR spectrum of neutral ethylene glycol and (b−d) the simulated vibrational spectra of

2. EXPERIMENTS AND CALCULATIONS Ethylene glycol was purchased from TCI Chemicals and deuterated ethylene glycol-d4 (HOCD2CD2OH) purchased from Aldrich was used without further purification. The sample was heated at ∼363 K in a sample holder attached to a supersonic jet valve and was supersonically expanded into a vacuum chamber with the helium carrier gas. IR spectra of the neutral and cation of ethylene glycol were observed by IR predissociation spectroscopy based on the VUV photoionization detection at 118 nm.25,26 The sample concentration in the jet expansion was controlled to totally suppress cluster ion signals of ethylene glycol. The mass spectra of ethylene glycol and ethylene glycol-d4 are shown in the Supporting Information. The protonated methanol fragment (m/z = 33) was detected for IR predissociation spectroscopy of both the neutral and cation. IR spectra of the deuterated sample were observed also detected by the protonated methanol fragment, which appeared at m/z = 36. For both ethylene glycol and ethylene glycol-d4, only the protonated methanol signal was enhanced through IR predissociation of the parent ions. In IR spectroscopy of the

Figure 2. (a) Observed IR spectrum and (b−d) simulated vibrational spectra of neutral ethylene glycol. The simulated spectra are calculated for the optimized structures inserted in the figure at the ωB97X-D/6-311++G(3df, 3pd) level. The simulated frequencies are scaled by 0.93.

its optimized structures at the ωB97X-D/6-311++G(3df, 3pd) level. The optimized structures are inserted in the figure. In the observed spectrum (a), in addition to the CH stretching bands at ∼2900 cm−1, the weakly hydrogen (H)-bonded OH and free OH stretch bands are seen at 3635 and 3690 cm−1, respectively. We observed the IR spectrum in the region from 3800 to 2600 cm−1. We could not cover the lower frequency region because of the restriction of the output power 5123

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Figure 3. Simulated isomerization paths of ionized ethylene glycol at the ωB97X-D/6-311++G(3df, 3pd) level. The details for isomerization path search calculations are described in the main text. The red line shows the path of mechanism II, and the blue lines show those of mechanism I. The black and red lines are located below the 10.5 eV energy from structure A of neutral ethylene glycol. The energies of the path of the blue lines are higher than 10.5 eV from structure A. The relative energies of the minima and TSs are listed in Tables 1 and 2, respectively.

of DFG. The IR spectrum of the ethylene glycol vapor at 318 K has been previously reported by Buckley and Giguère.29 They have also assigned the two bands at 3644 and 3677 cm−1 to the stretching vibrations of free and weakly H-bonded OH bonds, respectively. Two main CH stretching bands at 2883 and 2933 cm−1 in the present spectrum have been observed at 2878 and 2941 cm−1 in the spectrum of the warm vapor.29 The small differences of the band frequencies would come from the contribution of hot bands and temperature dependences of the rotational contours. The observed bands in the spectrum at 318 K are much more broadened than the present jet-cooled spectrum.29 In the two most stable structures (A and B) shown in Figure 2, the intramolecular H-bond is formed between the two OH groups. The simulated spectra of these two structures (Figure 2b,c) reproduce well the observed spectrum. The main structural difference between them is the internal rotational angles of the OH groups. Structure C, shown in Figure 2, has no intramolecular H-bond, and its energy is higher by ∼2.5 kcal/mol than structures A and B. In addition, its simulated spectral features (Figure 2d) do not match with the observed spectrum. Therefore, structure 3 is excluded from the spectral carrier of the observed spectrum. We, therefore, conclude that neutral ethylene glycol forms an H-bonded structure such as structures A or B. Figure 3 shows the simulated isomerization paths of ionized ethylene glycol. The same figure of the expanded size is also shown in the Supporting Information. Both the IRC calculations started from the vertically ionized structures of neutral structures A and B at the PBE1PBE/6-31+G* level and lead to cationic structure 1-1 in Figure 3. The GRRM reaction path search calculations were therefore performed from structure 1-1 at the PBE1PBE/6-31+G* level. The searched minima (stable structures) and transition state (TS) structures are re-optimized at the ωB97-XD/6-311++G(3df, 3pd) level, and their re-calculated energies are listed in Tables 1 and 2, respectively. In Figure 3, the red line indicates the isomerization path in mechanism II, while the blue lines show the isomerization paths in mechanism I. In each case, the double

Table 1. Simulated Relative Energies (eV) of the Stable Structures of the Ethylene Glycol Cationa structure numberb structure structure structure structure structure structure structure structure structure structure structure structure structure structure structure structure structure structure structure structure structure

1-1 1-2 1-3 1-4 1-5 1-6 2-1 2-2 2-3 2-4 2-5 2-6 2-7 3-1 3-2 3-3 4 5 6 7-1 7-2

relative energy (eV) from structure A of neutral 9.46 9.49 9.49 9.48 9.46 9.46 10.17 10.17 10.21 10.21 10.18 10.20 10.15 10.05 10.04 10.05 10.15 9.87 9.69 10.08 10.41

a

All the energies are relative to structure A of neutral ethylene glycol. The calculations were performed at the ωB97X-D/6-311++G(3df, 3pd) level. bSee Figure 3 or the Supporting Information for the numbering of the stable structures.

proton transfer mechanism was confirmed by the TS structure in the path. The red and black paths are located below 10.5 eV (118 nm), which was used as the ionization light in this study, from the energy of neutral structure A in Figure 2. All the TS energies on the blue paths are higher than 10.5 eV. The stable structures in the isomerization paths are categorized into the 7 types. The representative structure in each type is shown in Figure 4 with its IR spectral simulation. In structure 1 type [six 5124

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The Journal of Physical Chemistry A Table 2. Simulated Relative Energies (eV) of the TSs of Ionized Ethylene Glycola TS numberb TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

relative energy (eV) from the neutral structure A 9.57 9.55 9.57 10.03 9.54 10.03 9.96 10.47 9.95 10.26 10.07 10.05 10.06 10.25 10.24 10.21 10.21 10.20 10.18 10.48 10.25 10.36 10.37 10.40 9.98 10.15 10.41 11.04 11.45 11.01 11.08

Figure 4. (a) Observed IR and (b−h) simulated vibrational spectra of ionized ethylene glycol. Spectra (b−h) are simulated for the optimized structures at the ωB97X-D/6-311++G(3df, 3pd) level. The optimized structures and their relative energies (in kcal/mol) are inserted in the figure. The simulated frequencies are scaled by 0.93.

peak.18 The slow component has been attributed to the ions having the low internal energy. With the analysis of the breakdown diagram based on the RRKM theory and the difference of the appearance energies between ethylene glycol and deuterated ethylene glycol-d6, they have interpreted that this slow component originates from the proton tunneling in the isomerization. They have therefore concluded that the generation mechanism of protonated methanol from the low energy ion is not mechanism II but other mechanism involving proton tunneling.18 This conclusion is, however, inconsistent with the CID results supporting mechanism II.17,18 In the RRKM analysis by Li and Baer, they have employed the onedimensional double minimum potential curve. For mechanism II, the minima correspond to structures 1-1 and 6, and the TS between them is TS 8 or 9. In the RRKM analysis for mechanism I, the minima correspond to structures 1-1 and 5, and the TS is TS 28. Structure 1-1 is more stable than structure 6 by 5.5 kcal/mol at the ωB97X-D/6-311++G(3df, 3pd) level. On the other hand, structure 6 is more stable than structure 1 type in Li and Baer’s calculation at the B3LYP/631G(d)//B3LYP/6-311+G(d,p) level.18 Because structure 6 is a precursor of the generation of protonated methanol, the relative energies of structures 1-1 and 6 are the key factors in the RRKM analysis and also for the dissociation rate to protonated methanol and HCO. Table 3 shows the relative energies calculated at the MP2/6-311++G(3df, 3pd), ωB97XD/6-311++G(3df, 3pd), and PBE1PBE/6-311++G(3df, 3pd) levels for structures 1-1 and 6. The calculated relative energies depend heavily on the level of theory. This result suggests that the reliability of the previous RRKM analysis is equivocal. Furthermore, the isomerization paths simulated in this study are highly complicated. On the isomerization potential energy surface, the isomers mutually intercommunicate even below

a

All the energies are from the energy of structure A of neutral ethylene glycol. The calculations were performed at the ωB97X-D/6311++G(3df, 3pd) level. bSee Figure 3 or the Supporting Information for the numbering of the TSs.

similar structures, structures 1−X (X = 1−6)], the two CH2OH moieties interact with each other between the C atoms. In structures 2−X (X = 1−7), the CH2OH moieties interact with each other between the C and O atoms. In structure 3−X (X = 1−3), the proton is shared between the O atoms, while the proton is shared between the O and C atoms in structure 4. Structures 5 and 6 have the H-bonded structures of the CH3OH2+−OCH complex. The proton is shared between the O and C atoms in structure 5, and the proton is shared between the O atoms in structure 6. Structure 7−X (X = 1 and 2) is the intramolecular proton-transferred structure, in which the proton of OH is transferred to the other O atom with the CC bond being maintained. In the B3LYP/631G(d)//B3LYP/6-311+G(d,p) level calculations reported by Li and Baer, mechanism I has the energy barrier higher than 10.5 eV, and the barriers in mechanism II are lower than 10.5 eV.18 Our result at the ωB97X-D/6-311++G(3df, 3pd) level is consistent with their result, although we found the four paths with the similar energy barriers in mechanism I. Hence, these simulations support mechanism II for the generation of protonated methanol with the photoionization at 10.5 eV. Around this photon energy, Li and Baer have observed the slow and fast components in the protonated methanol mass 5125

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The Journal of Physical Chemistry A Table 3. Simulated Relative Energies (kcal/mol) between Structures 1-1 and 6 at the Different Levels of Theory structure 1-1 structure 6

PBE1PBE/6-311++G(3df, 3pd)

ωB97X-D/6-311++G(3df, 3pd)

MP2/6-311++G(3df, 3pd)

0 7.1

0 5.5

1.7 0

the 118 nm ionization energy. In the complicated isomerization process, ionized ethylene glycol would roam among structures 1, 2, 3, and 4 types before it reaches to structures 5 and 6. The slow component observed in the protonated methanol mass channel can be explained by a roaming time in the isomerization paths prior to the formation of structures 5 and 6. To experimentally confirm the isomerization path to structures 5 and 6, we have carried out IR predissociation spectroscopy of ionized ethylene glycol. Figure 4a shows the IR spectrum of ionized ethylene glycol, which is generated by the VUV photoionization at 118 nm (10.5 eV). Figure 4b−h represents the simulated vibrational spectra of the 7 representative isomers of ionized ethylene glycol at the ωB97X-D/6-311++G(3df, 3pd) level. The simulated spectra of all the stable isomers found in the GRRM search are shown in the Supporting Information. In the observed spectrum, a free OH stretching band is seen at 3538 cm−1 and several CH stretching bands appear near 2950 cm−1. In addition, a heavily broadened band is observed from ∼3200 cm−1 to the lower frequency region. This broad band is attributed to a strongly H-bonded OH bond. This is because H-bonded OH-stretching bands in cations generally have wide bandwidths and appear in such a low-frequency region.25,26 Structures 3, 4, 5, 6, and 7-1 have H-bonded OH or intermolecularly shared proton. The band of the shared proton vibration in structures 3 and 4 is predicted to be in the 1400−1700 cm−1 region, whereas that in structure 5 is predicted to be at 2354 cm−1. In structure 6, the in-phase and out-of-phase modes of H-bonded OH and CH of the HCO moiety are predicted to be at 2711 and 2661 cm−1, respectively. The out-of-phase mode of the H-bonded OH stretch and the CH stretches in structure 7-1 are predicted to be at 2732 cm−1. The frequencies of the shared proton vibration in structures 3 and 4 are too low in comparison with the observed broad band position, although frequencies of stretching vibrations of such strongly H-bonded OHs might not be well reproduced by harmonic vibrational calculations. Structures 5 and 6 are more stable than structures 3, 4, and 7. Therefore, the broad component in the observed spectrum is attributed to the vibration of the shared proton of structures 5 and 6. This assignment is supported also by the production of protonated methanol in the ionization of ethylene glycol with the ionization energy higher than ∼10.4 eV.18 Moreover, the present IR spectrum was observed by detecting the fragmentation to protonated methanol (m/z = 33). At the ωB97X-D/6-311++G (3df, 3pd) level, structure 1-1 is simulated as the most stable structure and thus is more stable than structures 5 and 6. However, as shown in Table 3, the energy order of structures 1-1 and 6 is reversed at the MP2 level. It is therefore difficult to definitely determine the energetic superiority among the isomers. Partial contributions of structures 1 and 2 type isomers to the observed free OHstretching band cannot be excluded, although the observed broad component is uniquely attributed to structures 5 and 6. Further information is provided by IR spectroscopy of the partially deuterated isotopomer of ethylene glycol.

Figure 5a shows the observed IR spectrum of neutral deuterated ethylene glycol-d4 (HOCD2CD2OH). In the

Figure 5. (a) Observed IR spectrum of neutral deuterated ethylene glycol-d4 (HOCD2CD2OH) and (b) observed and simulated IR spectra of its cation at the ωB97X-D/6-311++G(3df, 3pd) level. The simulated frequencies are scaled by 0.93. Ethylene glycol-d4 is ionized by 118 nm light. The middle spectrum of (b) is the simulated one for the CD3OH2−OCD complex generated through mechanism I upon the ionization of HOCD2CD2OH. The bottom spectrum of (b) is the simulated one for the CD2HOHD−OCD complex generated through mechanism II.

spectrum, only the OH-stretching band involving the Hbonded OH and free OH-stretching modes is observed. Although we measured the spectrum with the reduced IR power to avoid the saturation broadening, the H-bonded and free OH bands were not well separated. With deuteration, the CH-stretching bands shift to low frequency beyond the observed region, and they disappear in the spectrum. Figure 5b presents the observed IR spectrum (top) of the ionized ethylene glycol-d4 with the 118 nm light (10.5 eV), and the simulated vibrational spectra of structure 6 formed through mechanism I (middle) and mechanism II (bottom) from ethylene glycol-d4. In the observed spectrum of the cation, only the free OH-stretching band is seen in the measured region; and the broad band as well as CH-stretching bands is not observed. Disappearance of the broad band indicates that the deuteron is shared between the protonated methanol and formyl radical moieties. In the observed spectrum, CHstretching bands are not also observed, although the CD2HOHD−OCD composition has one CH bond. Because the simulated CH stretching band intensity is very low as seen in Figure 5b, this band cannot be seen within the signal to noise ratio of the observed spectrum. On the other hand, several CH bands are observed on the broad absorption of the shared proton vibration in the IR spectrum of the ionized 5126

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ethylene glycol in Figure 4a, though the simulated CHstretching band intensities are as low as that of the partially deuterated isotopomer. The observation of CH stretches with the high intensities in the spectrum of Figure 4a could be explained by the intensity borrowing from the intense broad band of the shared proton vibration through multiple anharmonic couplings. Upon the ionization of ethylene glycol-d4, the deuteron of CD and the proton of OH are transferred to the CD2 group and the OH group between the two CD2OH moieties in mechanism I, respectively. In mechanism II, the deuteron of CD of the donor moiety is transferred to OH of the acceptor moiety, and the proton of OH is transferred to CD2. Hence, the two different isomerization paths of the ionized ethylene glycol-d4 generate the different isotopomers. As shown in the insets in Figure 5b, mechanism I generates structure 6 of CD3OH2−OCD, while CD2HOHD−OCD is generated through mechanism II. The same isotopomer pattern is also supposed for structure 5. The shared proton in structures 3, 4, and 7-1 is the H atom. This is because the D atom is shared in structures 5 and 6 only after the deuterium atom transfer from CD to OH in mechanism II. Therefore, the carrier of the observed IR spectrum is unequivocally assigned to the CD2HOHD−OCD cation formed through mechanism II. We therefore conclude that ethylene glycol ionized at 118 nm isomerizes to the CH3OH2−OCH complex through mechanism II.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yoshiyuki Matsuda: 0000-0001-7757-8626 Asuka Fujii: 0000-0002-6854-9636 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. T. Maeyama for his helpful discussions. Y.M. acknowledges the Grant-in-Aid for Scientific Research (Project No. 26108504 on Innovative Area [2507]) from MEXT Japan. A.F. acknowledges the Grant-in-Aid for Scientific Research (project no. 18H01931) from JSPS.

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REFERENCES

(1) Hoffmann, E.; Stroobant, V. Mass Spectrometry: Principles and Applications, 3rd ed.; John Wiley & Sons, 2007. (2) Watson, J.; Sparkman, O. D. Introduction to Mass Spectromety: Instrumentation, Applications, and Strategies for Data Interpretation; John Wiley & Sons, 2013. (3) Ferguson, E. E. Ion-Molecule Reactions. Annu. Rev. Phys. Chem. 1975, 26, 17−38. (4) Franklin, J. Ion-Molecule Reactions; Springer Science & Business Media, 2012; Vol. 1. (5) Carrascosa, E.; Meyer, J.; Wester, R. Imaging the Dynamics of Ion-Molecule Reactions. Chem. Soc. Rev. 2017, 46, 7498−7516. (6) 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. (7) Wishart, F. J.; Rao, B. S. M. Recent Trends in Radiation Chemistry; World Scientific, 2010. (8) Streitwieser, A.; Taft, R. W. Progress in Physical Organic Chemistry; John Wiley & Sons, 2009. (9) Kristiansen, P.-E.; Marstokk, K.-M.; Møllendal, H.; Parker, L. Microwave Spectrum of HOCH2CD2OH and the Assignment of a Second Hydrogen-Bonded Conformation of Ethylene Glycol. Acta Chem. Scand. 1987, 41a, 403−414. (10) Caminati, W.; Corbelli, G. Conformation of ethylene glycol from the rotational spectra of the nontunneling O-monodeuterated species. J. Mol. Spectrosc. 1981, 90, 572−578. (11) Walder, E.; Bauder, A.; Günthard, H. H. Microwave Spectrum and Internal Rotations of Ethylene Glycol. I. Glycol-O-d2. Chem. Phys. 1980, 51, 223−239. (12) Frei, H.; Ha, T.-K.; Meyer, R.; Gu̅nthard, H. H. Ethylene Glycol: Infrared Spectra, Ab Initio Calculations, Vibrational Analysis and Conformations of 5 Matrix Isolated Isotopic Modifications. Chem. Phys. 1977, 25, 271−298. (13) Buckley, P.; Giguère, P. A. Infrared Studies on Rotational Isomerism. I. Ethylene Glycol. Can. J. Chem. 1967, 45, 397−407. (14) Holmes, J. L.; Lossing, F. P. Heats of Formation of Organic Radicals from Appearance Energies. Int. J. Mass Spectrom. Ion Processes 1984, 58, 113−120. (15) Biermann, H. W.; Morton, T. H. Reversible tautomerization of radical cations. Photoionization of 2-methoxyethanol and 3-methoxy1-propanol. J. Am. Chem. Soc. 1983, 105, 5025. (16) Burgers, P. C.; Holmes, J. L.; Hop, C. E. C. A.; Postma, R.; Ruttink, P. J. A.; Terlouw, J. K. The isomeric [C2H6O2]·+ hydrogenbridged radical cations [CH2−O(H)···H···O=CH2]·+, [CH3−O···H··· O=CH2]·+, and [CH3−O(H)···H···O=CH]·+: Theory and Experiment. J. Am. Chem. Soc. 1987, 109, 7315−7321. (17) Audier, H. E.; Milliet, A.; Leblanc, D.; Morton, T. H. Unimolecular decompositions of the radical cations of ethylene glycol and its monomethyl ether in the gas phase. Distonic ions versus ionneutral complexes. J. Am. Chem. Soc. 1992, 114, 2020−2027.

4. CONCLUSIONS In this study, IR spectroscopy of the neutrals and cations of ethylene glycol and ethylene glycol-d4 and the theoretical reaction path search were carried out to investigate the isomerization path of the ionized ethylene glycol. The simulated isomerization paths to the CH3OH2−OCH complex are complicated and support mechanism II, in which the protons of CH and OH in the donor CH2OH moiety are transferred to the O and C atoms in the acceptor CH2OH moiety. The IR spectroscopic results of ionized ethylene glycol and its deuterated isotopomer experimentally supported the formation of the CH3OH2−OCH complex through mechanism II in the ionization of ethylene glycol at 118 nm. The spontaneous dissociation to protonated methanol upon VUV photoionization is not experimentally examined in this study. Possibilities of contributions of other mechanisms to the spontaneous dissociation upon the VUV photoionization cannot be excluded. An experiment with tunable VUV would be more informative for this problem and is a future subject.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.9b02591. Mass spectra of ethylene glycol and ethylene glycol-d4, expanded figure of isomerization paths, simulated vibrational spectra of all isomers of the ethylene glycol cation, optimized structural coordinates and energies of stable and TSs of the ethylene glycol cation, vibrational calculational results (frequencies and intensities), and complete author list of ref 27 (PDF) 5127

DOI: 10.1021/acs.jpca.9b02591 J. Phys. Chem. A 2019, 123, 5122−5128

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DOI: 10.1021/acs.jpca.9b02591 J. Phys. Chem. A 2019, 123, 5122−5128