Infrared Spectroscopic Investigation of the Acidic CH Bonds in

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Infrared Spectroscopic Investigation of the Acidic CH Bonds in Cationic N-Alkanes# Pentane, Hexane, and Heptane Min Xie, Yoshiyuki Matsuda, and Asuka Fujii J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b05567 • Publication Date (Web): 28 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

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The Journal of Physical Chemistry

Infrared Spectroscopic Investigation of the Acidic CH Bonds in Cationic N-Alkanes：： Pentane, Hexane, and Heptane

Min Xie,† Yoshiyuki Matsuda,* Asuka Fujii*

Department of Chemistry, Graduate School of Science, Tohoku University, 6-3, Aramaki Aza-Aoba, Aoba-ku, Sendai, 980-8578, Japan

*

Corresponding authors’ E-mail: YM: [email protected]; AF: [email protected]; Fax: +81(0)22 795 6785; Tel: +81(0)22 795 6573 † Present address: Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu, Taiwan 300, R. O. C.

1 ACS Paragon Plus Environment


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Abstract Radical cations of n-alkanes (pentane, hexane, and heptane) in the gas phase are investigated by infrared predissociation spectroscopy with the argon or nitrogen tagging. All trans and gauche-involving conformers are identified for these cations by comparisons of observed infrared spectra and vibrational simulations. Intense CH stretch bands are observed in the frequency region lower than the normal alkyl CH stretch frequency. These low frequencies and high intensities of the CH stretch bands are caused by the CH bond weakening and the enhanced positive charge of the hydrogen atoms through the delocalization of the σ electron in the CH bonds. These effects of the delocalization of the σ electron result in the enhanced acidity of the CH bonds. The conformation as well as alkyl chain length dependence of the acidity of the CH bonds is demonstrated by the CH stretch frequency shift trend.


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1. Introduction Except the case of alkyne, CH bonds generally polarize very weakly and they are regarded as aprotic in neutral molecules. However, in cations, proton transfer reactions from CH bonds have been reported in the gas phase collision experiments and mass spectrometric studies.1-6 Recently, barrierless proton transfer from CH bonds has been spectroscopically demonstrated for cationic trimethylamine and dimethylether.7,8 These barrierless proton transfer reactions indicate high proton donor ability of cationic CH bonds.

Vibrational

spectral signatures of acidity enhancements of cationic CH bonds have also been observed for alcohols, ethers, benzene derivatives, and so on.9-14 In their infrared (IR) spectra, stretching vibrational bands of acidic CH bonds show remarkable frequency reduction and intensity enhancement.

The acidity enhancements as well as these IR spectroscopic

characteristics of the CH bonds have been rationalized through theoretical analyses of cationic diethylether, cyclic ethers, and alcohols.12-14 In these systems, delocalization of the σ electrons of the CH bonds is induced by hyperconjugation with the nonbonding orbital of which electron is ejected in their ionization. The acidity enhancements as well as the shifts to low frequency and intensity enhancements of the CH stretch bands are attributed to this hyperconjugation. The correlation between acidity and stretch band features of CH bonds indicates that IR spectroscopy is a useful tool to probe acidity of cationic CH bonds. Alkanes are composed only of carbon and hydrogen atoms.

Since they have no

nonbonding electron, a σ electron is ejected in their ionization to the ground cationic state. Therefore, CH bond properties of alkane radical cations would differ from those of



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alcohols, ethers, and amine, in which the ejection from the nonbonding orbital of the hetero atom preferentially occurs in ionization and hyperconjugation with the nonbonding orbital plays a key role in the properties of the acidic CH bond. Alkane radical cations are often seen as precursors and reaction intermediates in synthetic chemistry, electrochemistry, radiation chemistry, and interstellar chemistry.15-19

Therefore, microscopic level

understanding of their properties would contribute to elucidate such chemical processes involving alkane cations.

Alkanes have undergone photoelectron spectroscopy and

theoretical investigations.20-23 Low energy features in their photoelectron spectra have been interpreted as ejection of an electron from delocalized orbitals through pseudo π orbitals (πtype orbitals) of CH bonds and σ orbitals of C-C bonds. These orbital profiles indicate that the positive charge in an alkane cation delocalizes through the singly occupied molecular orbital (SOMO) which is composed by CH pseudo π orbitals and C-C σ bonds.

Such a

character of SOMO would influence properties of alkane cations. In the present study, to investigate properties of alkane cations, we carried out IR spectroscopy of cationic n-pentane, n-hexane, and n-heptane in the CH stretch region, and the alkyl chain length dependence of the cationic CH bond properties was examined. Because IR dissociation spectroscopy combined with mass spectrometry is hardly applicable to these bare cations due to low fragmentation yield with IR excitation, we employed argon (Ar) atom or nitrogen molecule (N2) tagging to the cations. The CH bond properties of these alkane cations were analyzed with IR spectroscopic results and theoretical calculations.


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2. Experiments and Calculations IR spectra of the alkane cations were measured by the IR predissociation spectroscopy of vacuum ultraviolet (VUV)-pumped ion (IRPDS-VUV-PI) by use of the tandem type quadrupole mass spectrometer. Details of the experimental setup and the spectroscopic scheme have been described elsewhere.24, 25

Ar tagging was used for cationic pentane,

while Ar and N2 tagging was used for cationic hexane and heptane. The VUV light at 118 nm used for the ionization was introduced into the collisional region of the supersonic jet expansion. Therefore, generated ions undergo the collisional cooling and the tagging. IR predissociation spectroscopy was carried out for mass-selected cations after passing through the first quadrupole mass filter. IR spectra were recorded by monitoring fragment cations (loss of the tag) through the second quadrupole mass filter. The 118 nm light was generated by tripling the third harmonics of a Nd:YAG laser (Continuum, Surelite-III). The output of the IR-optical parametric oscillator (LaserVision) pumped by the fundamental of a Nd:YAG laser (Continuum, Powerlite-8000) was used for the IR spectroscopy. Optimized geometries, harmonic vibrational frequencies, spin densities, and atomic charges were calculated with the Gaussian 09 program package involving the natural bond orbital (NBO) ver. 3.1.26, 27 Calculation results were visualized by the GaussView 5.0.28 Geometry and frequency calculations were performed at the ωB97X-D/6-311++G(3df, 3pd), MP2/6-311++G(3df, 3pd), and B3LYP/6-311++G(3df, 3pd) levels. The calculated frequencies at ωB97X-D/6-311++G(3df, 3pd) were scaled by 0.945, and were mainly



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employed for comparison with observed spectra.

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Spin density and atomic charge

calculations were done only at the ωB97X-D/6-311++G(3df, 3pd) level. Dissociation energy of the proton from the most acidic CH bond (gas phase acidity) was also calculated.

3. Results and Discussion 3.1 IR spectrum of cationic pentane Figure 1(a) shows the observed IR spectrum of the Ar-tagged pentane cations in the CH stretch region. Four main features of CH stretching vibrations are seen at 2605, 2796, 2816, and ~2990 cm-1.

Figures 1(b)-(e) display the harmonic vibrational simulations of

four stable conformers of the trans-trans (tt), trans-gauche (tg), and two gauche-gauche (gg1 and gg2) structures at the ωB97X-D/6-311++G(3df, 3pd) level. Optimized structures are also depicted in the figures.

At the ωB97X-D/6-311++G(3df, 3pd) level, the tg

conformer is more stable by 0.21 kcal/mol than the tt conformer. However, at the MP2/6311++G(3df, 3pd) and B3LYP/6-311++G(3df, 3pd) levels, the tt conformer is most stable among these conformers. Because the energy difference between the tt and tg conformers at all the calculation levels is quite small, both the conformers would be present in the conformer population and contribute significantly to the observed spectrum. Through the comparison of the observed and simulated spectra, the most intense band at 2816 cm-1 is assigned to the antisymmetric stretch vibration of the in-plane (the plane is defined by the five carbon atoms) CH bonds of the tt conformer. The tg conformer would


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primarily contribute to the bands at 2605 and 2796 cm-1. These two bands would undergo minor contribution from the higher energy gg1 and gg2 conformers. The band at 2605 cm-1 is assigned to the stretch vibration of the in-plane CH bond next to the out-of-plane methyl group (the plane is composed by the other four carbon atoms) of the tg conformer. The band at 2796 cm-1 is also attributed to the stretch band of the terminal in-plane CH bond of the tg conformer. The band at 2990 cm-1 is the stretch vibrations of the out-of-plane CH bonds of the tt and tg conformers, and minor contribution of the other conformers would coexist. The frequencies of the bands at 2605 and 2796 cm-1 are lower than the normal CH stretch frequency (2850~3050 cm-1) of neutral alkyl CH bonds. In addition, the intensities of the bands at 2605 and 2796 cm-1 are largely enhanced in comparison with the out-ofplane CH bands at ~2990 cm-1. Similar features of the CH stretch band have been seen in the radical cations of ethers, alcohols, and so on.9-14 In these systems, the characteristics of such a CH stretch band have been rationalized by delocalization of σ electrons of the CH bond through hyperconjugation with SOMO at the hetero atom,12-14 and the acidity enhancement of the CH bond has also been demonstrated. This is because CH bond weakening and enlargement of the positive charge of the H atom through the hyperconjugation necessarily induce the acidity enhancement of the CH bond. Thus, the CH stretch frequency and band intensity correlate with the acidity of the cationic CH bond. The observed low frequency CH bands at 2605, 2796, and 2816 cm-1 in cationic pentane also imply the enhancement of the acidity of the CH bonds, although pentane includes no hetero atom (nonbonding orbital). These CH frequencies would reflect the



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acidity of the corresponding CH bonds. The calculated gas-phase acidity of the terminal inplane CH bonds in the tg and tt conformers are 187.5 and 190.8 kcal/mol, respectively. They well correlate with the observed stretch band frequencies. Very recently, in the diand tri-hydrated pentane cations, we have found that proton transfer preferentially occurs from the CH bond of the pentane cation to the water moiety.29 This indicates the high proton donor ability of the CH bond in the pentane cation. To investigate the mechanism of the CH acidity enhancement in the pentane cation, the spin densities of the tt and tg conformers were simulated, and are depicted in Figure 2(a). These can be regarded as the distribution of the positive charge as well as the profile of SOMO.

In the tt conformer, the spin density distributes to the two terminal in-plane CH

bonds and the four CC bonds, as assigned to the delocalized orbital composed by the pseudo π orbitals of the CH bonds and the σ orbitals of CC bonds in the previous photoelectron study.20 The delocalization of the spin density to the two terminal CH bonds indicates the delocalization of the positive hole (charge) to the CH bonds and, in other words, the delocalization of the bonding σ electron of the CH bonds. The delocalization of the σ electrons of the CH bonds contributes to weakening of the bond strengths and increase of the positive charge of the hydrogen atoms, and then the acidity enhancement of the CH bonds. Figure 3(a) shows the atomic charges of the tt conformer simulated by the NBO analysis at the ωB97X-D/6-311++G(3df, 3pd) level. The positive charges (0.28) of the hydrogen atoms in the in-plane CH bonds are higher than those (0.23) of the other hydrogen atoms. In the tg conformer, the positive charge is distributed to the fewer number of orbitals


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than that of the tt conformer (see Figures. 2 and 3). The charge distribution is limited to the one CC bond, the terminal in-plane CH bond, and the in-plane CH bond next to the out-ofplane methyl group at the other end. Therefore, the spin density of in-plane atoms in the tg conformer tends to be higher than that of the tt conformer. (See Figure S1 in Supplementary Information.) This means that the influence of the delocalization of the σ electrons of the in-plane CH bonds, that is, partial loss of the σ electrons from the bonds, is stronger in the tg conformer than in the tt conformer. These spin densities correlate with the magnitude of the reduction of the CH stretch frequencies and the simulated atomic charge shown in Figure 3(b). The atomic charges (0.32 and 0.29) of the hydrogen atoms of the two in-plane CH bonds are higher than those of the other hydrogen atoms in the tg conformer and all of those in the tt conformer. These results indicate that the acidity of the in-plane CH bonds of the tg conformer is higher than that of the tt conformer. The acidity of CH bonds of cationic pentane correlates with its conformation because the profile of SOMO depends on the conformation.

3.2 IR spectrum of cationic hexane Figures 4(a) and (b) show the observed IR spectra of Ar-tagged and N2-tagged hexane cations, respectively, and (c)-(i) are the simulated spectra of the optimized structures depicted in the figures. The calculations were performed at the ωB97X-D/6-311++G(3df, 3pd) level. The difference of the qualities of spectra (a) and (b) comes from the difference of the generation efficiencies of these tagged cations. The N2-tagged cation was employed



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in this measurement to clearly observe the minor bands while the observed cation would be warmer than the Ar-tagged cation. The most intense band at 2862 cm-1 is assigned to the anti-symmetric stretch vibration of the in-plane terminal CH bonds of the most stable ttt conformer. The band at 2757 cm-1 is attributed to the ttg conformer. The minor feature at 2577 cm-1 observed in spectrum (b) would be attributed to several higher energy conformers such as gtg1, gtg2, ggg, and so on. The features around 2900 cm-1 would be due to the ttg conformer. The band at 2976 cm-1 are assigned to stretches of out of plane CH bonds of ttt, ttg, and other conformers. The intensity profiles of the observed bands indicate that the ttt conformer is dominantly formed and the ttg conformer is secondary dominant. This agrees with the calculated relative energies of the conformers. The calculated gas-phase acidity of the acidic CH bonds of the ttt and ttg conformers is 195.2 and 190.9 kcal/mol, respectively. This order correlates with that of the frequencies of their in-plane CH stretches at 2862 and 2757 cm-1. This correlation is explained by the spin density, which is interrelated with the delocalization of the positive charge, depicted in Figure 2(b).

Similar to the pentane cation, the spin density in all trans conformer of ttt

delocalizes more widely, compared with the ttg conformer. The positive charge as well as the spin density in the in-plane H atoms is, therefore, higher in the ttg conformer than that of the ttt conformer. This causes the difference of the acidity and stretch frequency of their in-plane CH bonds between the ttt and ttg conformers. Thus, their acidity depends on the conformation. The low frequency CH stretches are simulated at around 2700 m-1 for the gtg, ggg, and tgg type conformers. This implies that their acidity would be higher than that of the ttt and ttg conformers.

The gas phase acidity of gtg1 conformer is calculated to be


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190.5 kcal/mol, and this indicates its higher acidity than the ttt and ttg conformers.

3.3 IR spectrum of cationic heptane Figure 5 shows the observed IR spectra of (a) Ar- and (b) N2-tagged heptane cations, and (c)-(i) simulated spectra for the optimized structures, of which relative energies are less than 4 kcal/mol from the most stable tttt conformer, at the ωB97X-D/6-311++G(3df,3pd) level. The totally eleven conformers involving much higher energy conformers and their simulated spectra are shown in Supplementary Information.

The intense modes are

simulated in the frequency region lower than 2850 cm-1 for all the conformers except the tttt conformer. The relative intensities of the bands at 2907, 2961, and 2982 cm-1 are much higher than the bands observed in the frequency region lower than 2850 cm-1. The spectral carrier of these intense bands is mainly attributed to the most stable tttt conformer. The tttt conformer would also dominantly contribute to the bands at ~3010 cm-1. The bands at 2961, 2982, and ~3010 cm-1 are mainly attributed to the stretch bands of the out-of-plane CH bonds (the plane is defined by the carbon atoms.).

The former two bands, however,

involves the in-plane CH stretch components so that these bands undergo the intensity enhancements.

The most intense band at 2907 cm-1 is mainly assigned to the anti-

symmetric stretch band of the terminal in-plane CH bonds of the tttt conformer. The bands at 2787 and 2889 cm-1 would be attributed to several conformers such as tttg conformer. The tttg conformer would mainly contribute to these bands. This is because this conformer is most stable among those having bands in this region. Here, we should note that the band



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intensities of the tttt conformer (at 2907, 2961, and 2982 cm-1) are much larger than that of the 2787 cm-1. This means that the small energy difference at 0.72 kcal/mol between the tttt and tttg conformers effectively affects their population. Therefore, contributions of other conformers to the observed spectra would be much smaller because of their higher relative energies. The main spectral carrier of the band at 2787 cm-1 is then assigned to the in-plane CH stretch in the tttg conformer, although other isomers such as gttg1 and gttg2 may contribute to the band. The spectral carrier of the very weak features at ~2600 cm-1 would be the gggg conformer. Similar to the cases of the pentane and hexane cations, the frequency difference of stretch bands of the tttt and tttg conformers implies the difference in their acidity. This is also explained by the difference of the magnitude of the σ electron delocalization, which is indicated by the spin density depicted in Figure 2(c) (see also Figure S1 in Supplementary Information).

3.4 Influence of alkyl chain lengths on acidities of CH bonds Figure 6 compares the observed IR spectra of the Ar-tagged pentane, hexane, and heptane cations.

They are same as those in Figures 1, 4, and 5.

The antisymmetric stretch

frequencies of the terminal in-plane CH bonds of the all trans conformers (tt, ttt, and tttt) of the pentane, hexane, and heptane cations are 2816, 2862, and 2889 cm-1, respectively. The stretch frequencies of the in-plane CH bond next to the out-of-plane methyl group of the other stable conformers (tg, ttg, and tttg), which have one gauche bond, are 2605, 2757,


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and 2787 cm-1, respectively. Hence, these CH stretches gradually shift to higher frequency as the alkyl chain extends. These frequency shifts indicate the decrease of their acidity with increase of the alkyl chain length. This change of the acidity as well as that of the stretch frequency is explained by the spin density that represents the magnitude of the delocalization of the positive charge.

As seen in Figure 2, the spatial distribution of the

spin density becomes wider with increasing alkyl chain length in the both the series of conformers.

Therefore, the spin density tends to be lower with elongation of the alkyl

chain. (see Figure S1 in Supplementary Information.) This trend means that the positive charge in the in-plane CH bond is reduced with elongation of the alkyl chain. Thus, the acidity of alkane cations decreases with the elongation of the alkyl chains.

4 Conclusions In this study, we investigated cationic pentane, hexane, and heptane with the IR spectroscopy. were identified.

The generations of the all trans and single-gauche involving conformers The stretch bands of their in-plane CH bonds appear in the frequency

region lower than the normal alkyl CH stretch frequencies. This observation of the low frequency CH stretch bands indicates the remarkable enhancement of the CH acidity in the alkane cations. The dependence of the CH acidity on the conformation and the alkyl chain length was also demonstrated. The acidity of alkane cations varies with internal rotation about the C-C bonds, and it decreases with extension of the alkyl chain length.



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5 Acknowledgements We thank Dr. T. Maeyama for his helpful discussions. MX appreciates the Guangzhou Elite Project for the Ph.D. scholarship. YM acknowledges support from the Grant-in-Aid for Scientific Research on Innovative Area [2507] (Project No. 26108504 and 16H00930) from MEXT Japan and the Grant-in-Aid for Scientific Research (Project No. 16K05640) from JSPS. AF acknowledges the Grant-in-Aid for Scientific Research (Project No. 26288002) from JSPS.

Supporting Information Available:

Spin densities of the pentane, hexane, and heptane cations. Optimized structures of the heptane cation. Simulated vibrational spectra of the heptane cation. These materials are available free of charge via the Internet at http://pubs.acs.org.


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(20) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Ionization energies, Ab initio assignments, and valence electronic structure for 200 molecules in Handbook of HeI Photoelectron Spectra of Fundamental Organic Compounds, Japan Scientific Soc. Press, Tokyo, 1981. (21) Liu, Y. –J.; Huang, M. –B. The asymmetric structure of the n-pentane radical cation: a theoretical study” by Y.-J Liu, M.-B Huang Chem. Phys. Lett. 2000, 321, 89–94. (22) Bouma, W. J.; Poppinger, D.; Radom, L. The ionization of alkanes, Israel J. Chem. 1983, 23, 21-36. (23) Zuilhof, H.; Dinnocenzo, J. P.; Reddy, A. C.; Shaik, S. Comparative study of ethane and propane cation radicals by B3LYP density functional and high-level ab initio methods, J. Phys. Chem. 1996, 100, 15774-15784. (24) Matsuda, Y.; Mori, M.; Hachiya, M.; Fujii, A.; Mikami, N. Infrared spectroscopy of size-selected neutral clusters combined with vacuum-ultraviolet-photoionization mass spectrometry, Chem. Phys. Lett. 2006, 422, 378-381. (25) 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. (26) Gaussian 09, Revision C01, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian, Inc., Wallingford CT, 2009. (27) NBO Version 3.1, E. D. Glendening, A. E. Reed, J. E. Carpenter, and F. Weinhold. (28) GaussView, Version 5, Dennington, R.; Keith, T.; Millam, J. Semichem Inc., Shawnee Mission, KS, 2009. (29) Tomoya, Endo.; Matsuda, Y.; Fujii,A. Manuscript in preparation. 17 ACS Paragon Plus Environment


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Figure 1 (a) Observed IR spectrum of the Ar-tagged pentane cation and (b)-(e) simulated vibrational spectra of the pentane cation. The optimized structures corresponding to the simulations are depicted in the figure. The calculations were performed at the ωB97XD/6-311++G(3df, 3pd) level. Frequencies of the simulated spectra are scaled by 0.945. The numbers in the parentheses are relative energy in kcal/mol.


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Figure 2 Spin density (isoval = 0.007) of (a) tt and tg conformers of the pentane cation, (b) ttt and ttg conformers of the hexane cations, and (c) tttt and tttg conformers of the heptane cations.



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Figure 3 Atomic charges (atomic units) of (a) tt and (b) tg conformers of the pentane cations. The atomic charges were evaluated by the NBO analysis at the ωB97X-D/6-311++G(3df, 3pd) level.


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Figure 4 Observed IR spectra of (a) Ar-tagged and (b) N2-tagged hexane cations, and (b)-(i) simulated vibrational spectra of the hexane cation at the ωB97X-D/6-311++G(3df, 3pd) level. The optimized structures corresponding to the simulations are depicted in the figure. Frequencies of the simulated spectra are scaled by 0.945. The numbers in the parentheses are relative energy in kcal/mol.



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Figure 5 Observed IR spectra of (a) Ar-tagged and (b) N2-tagged heptane cations, and (c)-(i) simulated vibrational spectra of the heptane cation at the ωB97X-D/6-311++G(3df, 3pd) level. The optimized structures corresponding to the simulations are depicted in the figure. Frequencies of the simulated spectra are scaled by 0.945. The numbers in the parentheses are relative energy in kcal/mol.


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Figure 6 Comparison of observed IR spectra of Ar-tagged (a) pentane, (b) hexane, and (c) heptane cations.



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TOC/Abstract Graphics


Infrared Spectroscopic Investigation of the Acidic CH Bonds in

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