Infrared spectroscopic investigation International Symposium on Molecular Spectroscopy 71st Meeting - June 20-24, 2016 - Champaign-Urbana, Illinois Infrared spectroscopic investigation on CH bond acidity in cationic alkanes Yoshiyuki Matsuda, Min Xie, and Asuka Fujii Department of Chemistry, Graduate School of Science, Tohoku University, Japan Acknowledgements: The Grant-in-Aid for Scientific Research from JSPS The Grant-in-Aid for Scientific Research on Innovative Area from MEXT 1

Introduction In radical cations In neutral state, CH bonds are normally regarded as aprotic, because of their low acidity. In radical cations Acidity enhancement of CH bonds (barrierless proton transfer from CH bonds) have been found in cluster cation of amine, ether, and so on. (ex. PCCP 16, 9619 (2014)) Potential energy curve of ionized diethylether dimer in IRC In the dimer of diethylether, proton-transfer from CH occurs without the energy barrier from the vertically-ionized structure Vertically ionized Proton transferred CH of cationic diethylether is highly acidic UPBE1PBE/6-31+G(d) 2 2

Introduction: IR spectrum of diethylether cation proton donor site JPCA, 119, 4885 (2015). typical alkyl CH stretch range a A very intense CaH stretch band is seen at 2700 cm-1, which is out of the typical alkyl CH stretch frequency range. nCH of CaH CaH is proton donor site in ionized dimer. ωB97X-D/6-311++G(3df,3pd) level 2400 　2800 　 3000　　 3400 Wavenumber / cm-1 scaled by 0.94 e- Hyperconjugation between the σ orbital of CaH and SOMO of the O atom σ orbital of CaH Delocalization of σ electrons Weakening CH bond strength Increase of positive charge of the H atoms The hyperconjugation enhances the acidities of the CH bonds. SOMO (nonbonding)

Introduction: IR spectrum of diethylether cation proton donor site JPCA, 119, 4885 (2015). typical alkyl CH stretch range a A very intense CaH stretch band is seen at 2700 cm-1, which is out of the typical alkyl CH stretch frequency range. nCH of CaH CaH is proton donor site in ionized dimer. ωB97X-D/6-311++G(3df,3pd) level IR spectroscopy is a good tool to examine an acidity of CH bond. 2400 　2800 　 3000　　 3400 Wavenumber / cm-1 scaled by 0.94 e- Hyperconjugation between the σ orbital of CaH and SOMO of the O atom σ orbital of CaH Delocalization of σ electrons Weakening CH bond strength Increase of positive charge of the H atoms The hyperconjugation enhances the acidities of the CH bonds. SOMO (nonbonding)

Aims This study pentane hexane heptane Alkanes are composed by only C and H atoms. (No nonbonding orbital) hexane Aims We want to explore: The acidity of the CH bonds in cationic alkanes . The correlation of the acidic CH and the length of the alkyl chain. heptane IR spectroscopy of n-alkane (pentane, hexane, and heptane) cations with different alkyl chain lengths

Experiments & calculations Argon tagged IR predissociation spectroscopy of VUV-pumped ions (IRPDS-VUV-PI) Tandem Q-mass spectrometer V’ dissociation ion signal Dt IR VUV S0 IE experimental setups PCCP 11, 1279 (2009) Calculations All conformers were obtained from the GRRM* (Global Reaction Route Mapping) programs at the PBE1PBE/6-31+G(d) level. They are recalculated at the ωB97X-D/6-311++G(3df,3pd) level with Gaussian09. Atomic spin density was analyzed by Multiwfn 3.3.8†. *Ohno and Maeda, CPL 384, 277 (2004). †Lu and Chen, J. Comp. Chem. 33, 580 (2012).

IR spectrum of cationic pentane typical CH stretch range tt Observed IR spectral carriers are assigned to the two conformers, tt and tg. tg Low frequencies and high intensities of the bands indicate the high acidity of the CH bonds. tt Two in-plane CH stretch frequencies of tg conformer are lower than those of the acidic CH bonds of the tt conformer. tg 2400 2600 2800 3000 3200 Wavenumber / cm-1 scaled by 0.945 ωB97X-D/6-311+G(3df,3pd) The tg conformer is more acidic. Acidities of CH bonds correlate with the conformation. tt (0.21 kcal/mol) tg (0.0 kcal/mol)

Mechanism of acidiy enhancement of CH typical alkyl CH stretch range tt Delocalization of spin density tg Delocalization of positive charge Delocalization of s electron of in-plane CH bonds tt Acidity enhancement tg In the tt conformer, the spin density (positive charge) delocalizes more widely. 2400 2600 2800 3000 3200 Wavenumber / cm-1 scaled by 0.945 ωB97X-D/6-311+G(3df,3pd) Magnitude of delocalization of s electron is small. 0.056 0.056 0.058 0.08 The CH bond in the tg conformer is more acidic. tt (0.21 kcal/mol) tg (0.0 kcal/mol) Spin density (isoval = 0.007) of cationic pentane conformers

IR spectrum of cationic hexane typical alkyl CH stretch range ttt Similar to pentane, two conformers coexist. The low frequency CH bands are assigned to the in-plane acidic CH bonds. ttg ttt ttg 2400 2600 2800 3000 3200 scaled by 0.945 Wavenumber / cm-1 ωB97X-D/6-311+G(3df,3pd) The ttg conformer is more acidic than the ttt conformer. ttt (0.0 kcal/mol) ttg (0.80 kcal/mol)

IR spectrum of cationic heptane tttt Two conformers coexist. (similar to pentane and hexane) tttg (a) obs. IR The low frequency CH bands are assigned to the in-plane acidic CH bonds. (b) calc. tttt The tttg conformer is more acidic than the tttt conformer. (c) calc. tttg 2400 2600 2800 3000 3200 Wavenumber / cm-1 scaled by 0.945 ωB97X-D/6-311+G(3df,3pd) level The same trends for all the alkane cations. tttt (0.0 kcal/mol) tttg (0.72 kcal/mol)

Blue shift of CH stretch tt Pentane cation Acidic CH bands in both the types of isomers show blue shifts with increasing alkyl chain length. tg ttt Hexane cation ttg tttt The acidity of the CH bonds decreases with increasing the alkyl chain length. Heptane cation tttg 2400 2600 2800 3000 3200 Wavenumber / cm-1

Mechanism of blue shift tt 0.056 The positive charge is mainly shared by in-plane orbitals. The positive charge is shared more widely in the longer alkyl chains. 0.056 tg tt ttt 0.041 ttg 0.041 ttt tttt 0.03 0.03 Stretch bands of the acidic CH bonds shift to higher frequency with increase of the alkyl chain length. tttg tttt 2400 2600 2800 3000 3200 Spin density (isoval = 0.007) of cationic alkanes Wavenumber / cm-1

Summary IR spectroscopic investigations of the cationic n-alkanes. IR spectra indicate enhancement of CH acidities in cationic 　 　　 alkanes. 2) Internal rotation changes the acidity of CH in the alkyl cations. CH3CH2 rotation acidity increase Pentane cation, tt Pentane cation, tg 3) The acidity of CH tends to decrease with increase of the alkyl chain length. longer alkyl chain acidity decrease Pentane cation Hexane cation

TI01 : Pentane+-(H2O)n TI01 : INFRARED SPECTROSCOPIC STUDY FOR THE HYDRATED CLUSTERS OF PENTANE CATION TOMOYA ENDO, YOSHIYUKI MATSUDA, ASUKA FUJII Pentane+-(H2O)n, n=2,3 form the proton-transferred structures. Pentane+-(H2O)2 Proton-trasnferred These structures are formed though the barrierless proton transfer. Pentane+-(H2O)3 Proton-trasnferred CH of pentane cation is also highly acidic, although it has no nonbonding orbital. 14 14