Experimental and Theoretical Study on Dissociative Photoionization of Cyclopentanone

Zhao-hui LiYe-peng YuXuan LinJun ChenHang ZhangYan-bo LiHuan-huan WangQing-hui MengRui-rui SunXiao-bin ShanFu-yi LiuLiu-si Sheng

National Synchrotron Radiation Laboratory,University of Science and Technology of China,Hefei 230029,China

Key words:Cyclopentanone,Synchrotron radiation,Photoionization and dissociation,ab initio calculations

I.INTRODUCTION

The production of excellent alternative clean fuels from raw biomass,which is generally known as lignocelluosic bio-re finery,has attracted more and more attention in view of the urgent demand of sustainable and clean fuels[1?3].Cyclopentanone,a lignocellulosic platform compound,is one of the selective hydrogenation products of furfural and is regaining attention as a building block for the synthesis of high-density renewable fuels[4?6].Hence,a better understanding of the energetics for cyclopentanone is clearly desirable.In this work,we report a quantitative study on the photoionization and dissociative photoionization of cyclopentanone.

Wanget al.[7]studied the dissociation dynamics of cyclopentanone in intense 788 nm,90 fs pulses of radiation using a time-of- flight(TOF)ion mass spectrometer.The ionization rate constants and branching ratios were investigated according to quantum chemical computations.Wuet al.[8]performed a combined experimental and theoretical study on the photoionization/dissociation of cyclopentanone,and proposed several possible reaction channels. Price and co-workers[9]measured the absolute photoionization cross sections of cyclopentanone via a multiplexed photoionization mass spectrometer(PIMS)equipped with synchrotron radiation source in the energy range of 8?11 eV.More recently,Pastoorset al.[10]theoretically and experimentally investigated the photoionization and the thermal decomposition mechanisms of cyclopentanone using an imaging photoelectron photoion coincidence spectroscopy(iPEPICO)apparatus with VUV synchrotron radiation source which was tuned from 8.0 eV to 11.7 eV.For the dissociative photoionization,the fragmentation of the molecule ions is dominated by loss of CO,C2H4,and C2H5to form C4H8+,C3H4O+,and C3H3O+,respectively.They calculated possible structures of three main dissociative fragments along with their respective appearance energies and a model for the possible fragmentation mechanism was constructed.

As mentioned above,despite that considerable experimental and theoretical work was performed on neutral and cationic cyclopentanone,the detailed mechanisms for the formation of fragment ions are still not well understood. In the present study,we utilized tunable VUV photoionization TOF mass spectrometry combined withab initiomolecular orbital calculations to investigate the possible reaction mechanism in the dissociative photoionization of cyclopentanone. The ionization energy(IE)of cyclopentanone and the appearance energies(AEs)for major fragment ions were obtained by measuring their photoionization efficiency curves.Additionally,the possible mechanisms of the dissociation pathways are discussed with the aid ofab initiocalculations.

II.EXPERIMENTS AND CALCULATED METHODS

Experiments were performed using the Atomic and Molecular Physics Beamline(U14A)of National Synchrotron Radiation Laboratory in Hefei,China.Photoionization mass spectra and PIE curves of cyclopentanone for fragment ions were obtained in the energy range of 9.0?15.5 eV.Only a brief summary of the experimental apparatus is given here,the more details of this apparatus have been described elsewhere[11?13].Synchrotron radiation generated by an undulator at U14 beamline from 800 MeV electron storage ring at NSRL,and a high-resolution spherical-grating monochromator were employed to select the VUV light.The grating was installed in the chamber,covering the energy ranges from 7.5 eV to 22.5 eV with the energies resolving power(E/?E)about 1000.A Si photodiode was used for measuring the photo flux of synchrotron VUV.The average photo flux was measured to be 5×1013photons per second at the ionization region.Argon(IE=15.759 eV)as the filter gas was utilized for eliminating the higher harmonic produced by the undulator.

Cyclopentanone sample was purchased from Alfa Aesar(≥99%purity),and used directly without further treatment.Cyclopentanone was contained in a stainless evaporator,which is connected to the molecule expansion chamber by a 6 mm diameter stainless steel pipeline.We chose He(purity 99.99%)as the carrier gas and the stagnation pressure was about 0.15 MPa.After the sample was introduced into the beam source chamber,the gaseous cyclopentanone molecules were introduced into the ionization chamber by supersonic expansion through a 70μm diameter nozzle and one skimmer with diameter of 1.5 mm.Then the cold skimmed molecular beam was injected into the ionized region to perpendicularly intersect the monochromatic VUV radiation.Subsequently,the produced ions were massanalyzed using a homemade RTOF-MS.

In this study,the high-accuracyab initiomethod was utilized for obtaining the most stable con figuration of the cyclopentanone and its fragments. Previous studies have indicated that theωB97X-D[14]functional can significantly reduce self-interaction errors and has been widely used to provide reliable results.The coupled cluster theory with single and double excitations and perturbative estimate of triple excitations CCSD(T)[15]can obtain more accurate singlepoint energy.Therefore,geometry optimizations of the cyclopentanone,parent cations,transition states(TS),intermediate(INT)ions and fragments ions,were carried out with theωB97X-D theoretical functional using the 6-31G(d,p)[16]basis set and the reliable singpoint energies were obtained at the CCSD(T)level using the cc-pVTZ basis set.The unscaledωB97X-D zero-point vibrational energy(ZPVE)was used to correct all CCSD(T)energies.In order to further validate the transition states connecting the desired reactants and products,internal reaction coordinate(IRC)[17]calculations were carried out at the same level with geometry optimization.All these computational calculations were performed using the Gaussian 09 suite of programs on the Supercomputing Center of University of Science and Technology of China[18].The adiabatic ionization energy(AIE)of cyclopentanone is defined as,whererefers the total electronic energy of theis the total electronic energy of the

FIG.1 Photoionization mass spectra of cyclopentanone at the photon energy of 9.5,13.0,15.5 eV.

III.RESULTS AND DISCUSSION

A.VUV photoionization mass spectra

In this work,the photoionization mass spectra were collected continuously by changing the photon energy between 9.0 and 15.5 eV at 298 K.FIG.1 depicts the typical photoionization mass spectra of cyclopentanone at 15.5,13.0,and 9.5 eV,respectively.At the low photon energy of 9.5 eV,only the molecular ion(m/z=86)is observed.With photon energy increasing to 13.0 eV,two strong fragments,namely(m/z=56)by CO-loss or(m/z=56)byloss,and(m/z=55)byloss,were yielded.In addition,more fragment ions atm/z28,40,41,42,43,44,69,83 are detected.At the photon energy of 15.5 eV,ionic fragments atm/z28,41,42,55,56 have become stronger.Meanwhile,two weak fragments,namely(m/z=29)and(m/z=33)are also observed.All observed fragments are considered to be originated from dissociation of parent ion since no signal at mass greater than that of(m/z=84)is detected.The ion peak ofm/z=55 is the strongest one in the dissociative photoionization of cyclopentanone,which indicates thation is the dominated channel of cyclopentanone ion.

FIG.2 The PIE curves of parent ion(a)and the main fragments(e),and

TABLE I Experimental and calculated ionization energies(IEs)of cyclopentanone and appearance energies(AEs)of the major fragments.

The photoionization efficiency curves of cyclopentanone cation and its fragment ionswere obtained by integrating the area of each mass spectral peak at each photon energy. FIG.2 is the PIE curves of cyclopentanone and its main fragment ions.The appearance energies of all ions were determined from the PIE curves,detailed methods on analyzing the PIE curve have been reported elsewhere previously[19,20].Table I presents the AEs of all ions and summarizes the calculated energies of related species,as well as possible formation pathways for the dissociation processes. For the parent ion,the measured IE value is(9.23±0.03)eV,which is in good agreement with previously reported data of(9.28±0.03)eV[8],(9.30±0.05)eV[9],(9.28±0.01)eV[21]and(9.25±0.02)eV[22].Cyclopentanone has a nonplanar five membered ring conformation with C2symmetry.FIG.3 shows the calculation structures for the ground state neutral and ionized molecules,and it is found that the C1?O distance is shortened from 1.210? to 1.189? after photoionization.Meanwhile,both the lengths of theα-C?C bonds(C1?C2 and C1?C5)connected to the carbonyl group change from 1.524? to 1.552?.This suggests that the initial ionization result is an electron removing from theσnetwork of the five membered ring.The results are consistent with a previous study by Priceet al.[9]who proposed that the initial ionization is caused by removal of an electron from a bonding orbital localized on theαandα′carbons’sigma bonds and antibonding C?Oπorbital.

FIG.3 The optimized ground state structures of(a)neutral and(b)ionic cyclopentanone at the ωB97X-D/6-31G(d,p)level.

B.Dissociation mechanisms

With the increasing of photon energy,the parent ion will undergo a series of dissociative reactions to generate fragments.Detailed dissociation pathways of the cyclopentanone cation are established with the aid of calculations at theωB97X-D/6-31+G(d,p)level.The fragmentation pathways,the relative energies and structures of each species are shown in FIGs.4?7.

The proposed pathway for this reaction is described in FIG.4.Firstly,parent ion undergoes a ring-open process by C?C bond cleavage via TS1 to form INT1.Afterward,a H atom migration step from C5 to C2 occurs to produce INT2 via TS2 with the energy barrier of 0.44 eV.Then,INT2 undergoes a H migration to produce INT3 via transition state TS3.Subsequently,C5H7O+is produced via a H atom elimination in C2 atom.The calculated energy barrier for P1,10.87 eV,matches perfectly with experimental value of(10.88±0.05)eV.The most possible con figuration ofis CH3CH2CHCHOH+.

FIG.4 The formation pathways for(P2)and (P5).

FIG.5 The formation pathways for C4H8+(P3),C3H4O+(P4),C2H4O+(P7)and(P8).

The formation pathway for C4H5O+is assumed to remove a methyl directly as reaction(2). The calculated appearance energy for direct dissociation is 10.87 eV,which is lower than the experimental value of(11.03±0.06)eV.Then,we scanned the C4?C5 bond length from 1.35? to 3.5? to search the transition state,and found the TS4 for which energy barrier is 10.97 eV.Finally,the C4H5O+is generated by breaking the C4?C5 bond in INT3,with a concomitant loss of a methyl radical.In the whole reaction pathway,the highest energy step is TS4(10.97 eV),which is consis-tent with experimental value.

FIG.6 The formation pathway for(P12).

FIG.7 The formation pathways for (P10)and(P11).

There are two probable candidate structures,(P3),and C3H4O+(P4),that can correspond to the peak atm/z=56 according to calculation. For formation of(P3), firstly,rotation around the C1?C2 bond can transform INT1 into isomer INT4.Secondly,theis formed by a CO elimination step from INT4. The calculated AE ofis 10.86 eV,which is consistent with the experimental value of(10.64±0.03)eV and the other experimental data(10.44 eV[8]and 10.76 eV[10])as well.However,Priceet al.[9]gave the experimental AE value of(9.75±0.05)eV for,the reason for this deviations is not known.

The ion C3H4O+is formed by lossing C2H4from parent ion(reaction(4)).In this pathway, firstly,INT1 proceeds to undergo bond cleavage of C3?C4 via transition state TS6 and the barrier is located to be 10.90 eV above neutral cyclopentantone.Then C3H4O+(P4)and C2H4are produced by the bond fission of C3?C4.The calculated overall energy barrier is 11.01 eV,which is in good agreement with the observed threshold value(11.25±0.08 eV).This suggests that C4H8+is formed at low energy while the isomer C3H4O+may be generated at high energy threshold.

4.Formation pathway of C3H3O+(m/z=55)

As the dominant dissociation product from C5H8O+,the fragment ion C3H3O+(P5)is considered to be produced by the direct bond fission of C3?C4 in INT3(FIG.4).The corresponding AE is calculated to be 10.89 eV,which is close to the experimental value of(11.18±0.04)eV and the previous value(11.14 eV)reported by Pastoorset al.[10]as well.

It should be noted that,initially we thought that the species atm/z=55 was C4H7+.However,the reaction barrier for this pathway is computed to be at least 11.65 eV,which is higher than the experimental AE of 11.18 eV.

5.Formation pathway of C4H6+(m/z=54)

C3is produced by C2H2O elimination from INT1.As shown in FIG.5,the reaction(8)is derived from the C?C bond cleavage in INT1.The C2?C3 bond length has been scanned from 1.45? to 3.4? and no transition state is found.The total barrier of this process is calculated to be 11.73 eV,which is in excellent agreement with the experimental value of(11.78±0.03)eV.

The detailed formation pathway ofis also shown in FIG.7.First,2-propenyl cationundergoes a 1,2-hydrogen shift to generate the INT9 by overcoming a barrier of 1.22 eV.Then,H migration toward the terminal carbon atom in 2-propenyl cation leads to the formation of intermediate INT10 via transition state TS14 with an energy barrier of 1.47 eV.Finally,two H atoms of methyl group in INT10 get close to each other by passing through a transition state(TS15)located at 14.37 eV,leading to the formation of C3H3+and hydrogen molecule(reaction(9)).This pathway is consistent with the literature results[23,24].8.Formation pathways of C(m/z=28)

IV.CONCLUSION

The photoionization and dissociation of cyclopentanone have been investigated experimentally using reflection time-of- flight mass spectrometer with the tunable vacuum ultraviolet synchrotron radiation as the ionization source.The ionization energy and appearance energies for cyclopentanone and 12 fragment ions are obtained from their PIE curves.The IE and AEs for cyclopentanone and fragments,are determined to be 10.88,11.03,10.64/11.25,11.18,12.05,12.32,11.78,12.32,14.33,12.98,and 12.44 eV,respectively.The dissociative photoionization mechanisms of C5H8O are proposed with the help of theab initiocalculations at theωB97X-D/6-31+G(d,p)level.Ring opening and hydrogen migrations are the predominant processes in the fragmentation pathways of cyclopentanone.

V.ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China(No.91544105,No.U1532137,No.U1232209,and No.11575178). The authors are grateful to the Supercomputing Center of University of Science and Technology of China for generous allocation of computing resources.

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