Isomeric effects on fragmentations of crotonaldehyde and methacrolein in low-energy electron–molecule collisions

Chemical Physics Letters 561–562 (2013) 24–30

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Isomeric effects on fragmentations of crotonaldehyde and methacrolein in low-energy electron–molecule collisions Arup Kumar Ghosh, Aparajeo Chattopadhyay, Anamika Mukhopadhyay, Tapas Chakraborty ⇑ Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700032, India

a r t i c l e

i n f o

Article history: Received 19 December 2012 In final form 18 January 2013 Available online 26 January 2013

a b s t r a c t Fragmentation behavior upon low energy (10–16 eV) electron molecule collision of two isomeric a,benones, crotonaldehyde and methacrolein, has been studied by quadrupole mass spectrometry. Three predominant reaction channels identified immediately above the ionization threshold are H, CO and HCO losses from the parent molecular ions, and isomeric effects are vividly manifested with respect to these channels. Signals corresponding to CH3C@O+ ion, produced due to methyl migration, are displayed only by methacrolein. The observed isomeric effects are interpreted using the energies of the transition states and thermodynamic stabilities of the products predicted by calculation at DFT/B3LYP/ 6311++G(d,p) level. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Low kinetic energy (KE) electrons can induce various chemical events when allowed to interact with molecules of different sizes and complexities [1,2]. Electrons of KE as low as 0.8 eV are known to break the DNA strands [3]. The origin of this chemistry is suggested as due to attachment of such electrons onto the p⁄ valence molecular orbitals of DNA bases. Bond cleavage and rearrangement reaction of relatively smaller molecules have also been observed with low KE electrons, e.g., the case of various aliphatic alcohols, amines, carboxylic acids, fluoroalkanes and even DNA bases [4,5]. For electron energies less than 4 eV hydrogen abstraction channel has been found to be dominant reaction channel in each case. Within 4–10 eV of electron kinetic energy (e-KE) range, other channels open up but H ion formation channel is till significant due to core excited resonances located on different sites of the molecules. For example, the H channel of methanol displays three resonances at 6.4, 7.9 and 10.2 eV, and of these the first two have been assigned to be located at the O–H site. The last one, which has been assigned to be located at the C site, is a characteristic of alkanes. For most of the carbonyl compounds the first ionization energies fall in the energy range 9.5–11 eV. Immediately above the ionization thresholds, these molecules undergo are known to important structural rearrangements such as Mclafferty rearrangement [6]. Photochemical reactions in the gas phase of volatile organic compounds (VOCs) induced by electrons and/or by solar UV radiation are very important to understand the details of their atmospheric implications [7]. In this Letter we report the ⇑ Corresponding author. Fax: +91 3324732805. E-mail address: [email protected] (T. Chakraborty). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.01.026

fragmentation behavior of two isomeric a,b-enones, crotonaldehyde (CA) and methacrolein (MA) upon electron molecule collisions of electron energy immediately above their ionization thresholds for the first time. CA is released into atmosphere as direct biological emission and partial oxidation of fuels and other VOCs [8–10], and MA is produced primarily as the oxidation product of isoprene in our atmosphere [11–13]. The ionization energy of CA has been measured to be 9.73 ± 0.01 eV using photoionization study by Watanabe et al., where H2 emission lines were used as light source for ionization [14]. The same has been reported it to be 9.86 ± 0.03 eV using ultraviolet photoelectron spectroscopy by Klessinger et al. [15]. Thus, although MA is isomeric to CA, the ionization energy of the former is 0.15 eV higher [16]. In this connection, the study of fragmentation pattern of the molecular ions generated with limited internal energy is expected to provide valuable information about the dynamics of the energized molecules that result to chemical events. Several studies were devoted in the recent past on electronic spectroscopy and UV photochemistry of these molecules in the gas phase [17–31]. However, to our knowledge, the chemical outcome following low energy electron–molecule collisions of these systems are not known. In the ground state, both the molecules can exist over two conformational forms, s-trans and s-cis. In the case of CA, the s-trans form has been found to be present predominantly (>98%) [32,33], and the calculated difference in energy between the two conformers of CA is 2.5 kcal/mol [32]. The first electronic absorption band for S1 S0 transition of the molecule appears within 250– 400 nm range in the vapor phase. The origin region of this spectrum shows several vibronic structures. The second transition (S2 S0) appears within 190–250 nm range with a maximum at about 203 nm [17–19]. Photolysis in the vapor phase results in

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70

CA

69

42

10 eV

11 eV

11 eV

41

42 41

10 eV

12 eV

70

MA

43

69

12 eV

13 eV

13 eV

14 eV

14 eV

15 eV

15 eV

16 eV

16 eV

39

39 70 eV

70 eV

Figure 1. Ionic fragments produced by electron–molecule collisions at different e-KE probed by quadrupole mass spectrometry.

C4H5O+

+

H•

+

CO

+

HCO •

m/z 69

C4H6O•+

C3H6•+

m/z 70

m/z 42

C3H5+ m/z 41

Figure 2. Fragmentation channels corresponding to formation of m/z 69, 42 and 41 ions arising from CA and MA.

CO and propylene as the major products along with small amounts of C2H4, H2, CH4, allene, propyne and cyclopropane [30,31]. The molecule also undergoes photoisomerisation upon excitation within 245–400 nm range to produce 3-buten-1-al, ethylketene (EK) and the enol form of CA [31]. In inert argon matrix, photo-isomerization resulting in formation of the isomeric s-cis conformer is the major outcome of UV excitation [34]. It has been suggested that small amounts of CO and EK are also formed under such conditions. Such conformational isomerisation in the cold matrix has also been observed for MA [34]. Shu et al. have used photofrag-

ment translation spectroscopy (PTS) to investigate the 193 nm (ArF excimer laser) photodissociation of CA and identified three dissociation channels corresponding to loss of H atom, methyl group and CO [35]. It has been suggested that H loss occurs in the ground state potential energy surface, while the CO loss takes place following isomerisation of CA into EK. The occurrence of a fourth reaction channel corresponding to HCO and/or C2H5 loss is also suggested. In contrast, vapor phase photolysis or photochemistry of MA is less known. From the viewpoint of atmospheric importance, only the reaction of this molecule with OH radical has been studied, experimentally as well as theoretically [36,37]. In the following sections, the observed fragmentation behaviors of CA and MA upon interaction with low energy electrons are presented, and these are analyzed with the aid of the prediction of electronic structure calculation. 2. Experiment The experimental set up has been described elsewhere [38,39]. Briefly, the sample vapor was introduced in the ionization region via an effusive nozzle of diameter 100 lm and a working pressure

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pole mass spectrometer (Extrel, Model MAX500) which is operated at a radiofrequency of 1.2 MHz and the rod diameter of the quadrupole is 19 mm. The detector assembly consists of a conversion dynode and a Channeltron electron multiplier, both negatively biased at 5 and 2 kV, respectively. For data acquisition and processing, software package Merlin Automation provided by Extrel was used. CA (99%) and MA (95%) were purchased from LobaChemie Pvt. Ltd. and Sigma–Aldrich, respectively. These were purified by vacuum distillation for further experiments.

0.6 0.5 0.4 0.3

70CA 70MA

0.2

3. Theoretical calculations 0.1

The structures of the molecular ion, fragments and the transition states were optimized using density functional theory (DFT) method with B3LYP functional and 6-311++G(d,p) basis set. All structures of closed shell electronic configurations were optimized using restricted B3LYP (RB3LYP) while those with open shell configurations were optimized using unrestricted B3LYP (UB3LYP) option. The transition state structures confirmed by harmonic frequency calculations (one imaginary frequency) are shown to connect the reactant and product via intrinsic reaction co-ordinate (IRC) calculations. All the calculations were performed using GAUSSIAN G09 program suite [40].

0.0 0.16

0.12

0.08

69CA 69MA

Intensity (arb. units)

0.04

4. Results and discussions 4.1. Isomeric effect on fragmentation channels

0.00 0.40

0.32

0.24

0.16

42CA 42MA

0.08

0.00

0.48 0.40 0.32 0.24 0.16

41CA 41MA

0.08 0.00 10

20

30

40 e-KE (eV)

50

60

70

Figure 3. Relative ion yields of different fragment ions of CA (black) and MA (red), at different e-KE. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of 3.0  106 mbar was maintained in the mass filter region. The KE of the impinging electrons was varied in the range 10–16 eV. The generated fragment ions were mass analyzed using a quadru-

The fragmentation spectra of the two molecules, CA and MA, upon collisions with electrons of KE immediately above the ionization threshold (10–16 eV) are shown in Figure 1. The lowermost panel depicts the spectra obtained for use of 70 eV electrons, typically used in conventional mass spectrometry for analytical purposes. The displayed traces show clearly that KE variation has a significant effect on fragmentation pattern of the two isomeric molecules only at low values of electron kinetic energy (e-KE) and the differences are lost at 70 eV. Thus the two mass spectra in the lowermost panel look identical, where the intensity of molecular ion peaks (m/z 70) is much smaller compared to the two fragment ions of m/z 41 and 39. In the top panel (at 10 eV), on the other hand, the parent molecular ion peaks are most intense but the relative intensities of the fragment ions for the two molecules are distinctly different. With increase in e-KE, the parent ion intensity of molecular ion peaks decreases, and the fragment ion m/z 41 becomes the dominant feature in the spectra of both molecules, and the fragment peak at m/z 42 eventually disappears. MA+ displays a higher tendency for fragmentation and the parent molecular ion remains as base peak for e-KE of only up to 13 eV, whereas, the same for CA+ is 15 eV. The dissociation channels corresponding to 69, 42 and 41 fragment ions (m/z) are shown in Figure 2. We suggest these as the primary dissociation channels, and variation of the relative ion yields of the three primary fragments as a function of e-KE are shown in Figure 3. It is notable that a fragment of m/z 43 appears only in the spectra of MA but absent in the spectra of CA which can be corresponded only to CH3CO+. Below we argue that for generation of CH3CO+, MA+ must undergo structural reorganization to produce CH3CO+ (cation of methyl vinyl ketone). However, such rearrangement is not favored in case of CA. The other distinct isomeric effect is manifested in the relative intensities of m/z 69 peak between the spectra of the two molecules for e-KE in the range of 12–16 eV. In the case of CA, the intensity of this ion is nearly twice as large compared to that of MA. We have rationalized these differences in terms of structures, energies of the transition states and thermodynamic stabilities of different reactions.

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Ha C3

C1 C2

Ha

C2

1.504

C1

C3 C1

C3 C4

C4

1..537

C2

MA + (9.68)

C4

MVK+ (9.43)

CA + (9.49) 1.131 C1

1.372

1.483 C3

C3 C4

C1 C2

EK + (8.92)

1.169

DMK

Ionization

C4 + (8.63)

Ionization

Relative Energy (eV)

C2

1.166 C1

1.310

C3

1.518

C2

C3

C1 C4

C2 C4

EK (0.17)

Ha

DMK (0.15)

C3

C1 Ha

C2

C3

1.501

C1 C4

C4

C2

MA (0.03) CA (0.0) Figure 4. Geometries of CA, EK, DMK, MA, their respective ions and MVK cation optimized at DFT/B3LYP-6311++G(d,p) level. The C–C and C–O bond lengths are mentioned in Å units.

4.2. Structure and energetics of several ionic species 4.2.1. Molecular ion (C4H6O+ m/z 70) for z = 1 The preferred conformations in neutral and ionic forms of CA and MA in the respective ground electronic states are shown in Figure 4. Out of several possible conformations, which could arise due to internal rotations about two C–C single bonds, the s-trans is of the lowest energy in both neutral and cationic forms. According to our calculation at DFT/B3LYP/6311++G(d,p) level, the energy of the s-cis conformation of the neutral CA and MA form are 2.03 and 3.22 kcal/mol higher, respectively. The same theory also predicts that in the neutral ground state, both the molecules in s-trans conformation are nearly isoenergetic. Calculation predicts that the ionization energy of MA is 0.22 eV larger than CA, and is consistent with the experimentally measured ionization energies of the two molecules, which for CA and MA are 9.49 and 9.68 eV, respectively [14–16]. Some geometric parameters of neutral and cation of the two molecules are shown in Figure 4. Natural Bond Orbital (NBO) analysis predicts that the ionic charge is distributed over the entire

molecular ion. The natural negative charge on the oxygen atom of CA is reduced by 0.507, and that for MA is reduced by 0.496. These isomeric ions could in principle be produced if the original molecules are formed with excess internal energies via structural rearrangements before fragmentation. The optimized structures and relative energies of several possible isomers of the parent ions of both the molecules are also shown here. Thus, the isomeric ion EK+ could be produced from CA+ via H migration from C1 to C3 position and in doing so gains further stability by 0.57 eV. Similar rearrangement of MA+ could produce DMK+ gaining a stability of 1.05 eV. The experimental evidence for formation of these ions is that these structures are suitable to release CO. The spectra presented in Figure 1 indicate that CO loss is the most preferred channel at low values of e-KE of both molecules. Likewise, methyl vinyl ketone cation (MVK+) can be formed from MA+ via methyl migration from C2 to C1 along with simultaneous exchange of the aldehydic H atom. The spectra presented in Figure 1 indicates occurrence of such rearrangement. The generated MVK+ can undergo a-cleavage to produce CH3CO+ ion at m/z 43. Such methyl

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1.121 1.103 1.305

TS1 1.98 eV

1.964 1.359

1.366

TS2 1.06 eV 0.94 eV

1.412

1.365

0.13 eV CA ion

TS3 1.47 eV

1.13 eV 1.02 eV

TS4

1.217 1.113 1.461

-0.1 eV

MA ion

Figure 5. Energy profile diagram showing optimized geometries of the molecular ions, transition states and products resulting from CA (upper trace) and MA (lower trace).

Table 1 Calculated (DFT/B3LYP/6-311++G(d,p)) thermodynamic parameters (DH and DG) for the three fragmentation channels of CA+ and MA+. Fragment ion

CH3CHCHCO+(69) CH3 CHCHþ 2 (42) CH3CHCH+ (41)

Neutral

Crotonaldehyde

Methacrolein

DH° (298 K)kcal/mol (eV)

DG° (298 K)kcal/mol (eV)

DH° (298 K)kcal/mol (eV)

DG° (298 K)kcal/mol (eV)

H (1) CO (28)

21.7 (0.94) 5.1 (0.22)

14.3 (0.62) 7.1 (0.31)

23.5 (1.02) 0.7 (0.03)

15.22 (0.66) 13.1 (0.57)

HCO (29)

45.7 (1.98)

42.4 (1.84)

38.3 (1.66)

24.9 (1.08)

A.K. Ghosh et al. / Chemical Physics Letters 561–562 (2013) 24–30

migration is less favorable for CA+ and consequently no peak is observed at the mass channel m/z 43 for this molecule. 4.2.2. C4H5O+ (m/z 69) for z = 1 This fragment ion is produced due to loss of an H atom from the parent molecular ion. It is likely that the aldehydic hydrogen is lost more easily than from other positions. In an earlier theoretical work, Fang has suggested that in the excited electronic state of the analogous molecule acrolein, loss of the aldehydic H atom is a barrierless process in spite of the process being endothermic [41]. The optimized structures of the ion (C4H5O+) formed both from CA and MA cations are shown in Figure 4. Figure 5 depicts the energetic parameters associated with H loss process from the molecular ions of these two molecules. Similar to the case of electronic excited acrolein, H atom loss in the ground state of the two molecular ions is endothermic by 1 eV. Secondly, the barriers of both the molecules are nearly comparable with endothermicity of the reaction. It has been pointed out before that in the e-KE range of 12– 16 eV, CA+ displays a higher tendency to react via this channel compared to MA+. In consequence, the relative intensity of m/z 69 peak in the spectra of CA is nearly twice larger compared to that of MA. Such higher probability of this ion formation for CA implies that H atom loss could occur from other sites as well, most likely this occurs from C2 position. However, the loss of C2 hydrogen must occur with relatively larger energy barrier compared to the aldehydic H atom, and the process becomes facile only at a relatively higher value of e-KE. The ion yield variation data with eKE shown in Figure 3 indeed shows a maximum for this ion yield around 13 eV. As C2 hydrogen is absent in MA, e-KE variation does not have effect on ion yield and the overall ion yield is also half of the former. The yield of this ion monotonically decreases with increase in e-KE because of opening up of other favorable channels. þ 4.2.3. C3 Hþ 6 (m/z 42) and C3 H5 (m/z 41) for z = 1 These two fragment ions are produced due to CO and HCO losses from the molecular ions. Theoretically calculated enthalpy and free energy values of the corresponding reactions are presented in Table 1. From energetic viewpoint CO loss is the most favored reaction channel of both molecules. This prediction is consistent with the fragmentation spectra shown in Figure 1 which shows that CO loss is most facile at lowest value of e-KE used

(10 eV). The isomeric effect is most vividly manifested with respect to this channel under this condition, and the peak intensity in the spectra of MA+ is visibly larger compared to that from CA+. In addition to thermodynamic parameters, other factors like energy barrier and stability of isomeric intermediates, EK+ and DMK+, contribute to the reaction. Figure 5 depicts the transition state configuration for CO loss channels of the two molecular ions. Along with C1–C2 bond breaking it involves C1–Ha bond breaking and formation of the new C2–H bond. According to Figure 5, the calculated energy barrier for the process is 2 eV for CA (TS1) and 1.5 eV for MA (TS3). On the other hand, as C3 Hþ 5 formation requires direct cleavage of C1–C2 bond of the parent molecular ion and there is no obvious intermediate that can facilitate HCO loss, overall energy barrier for breaking of this bond determines the relative tendency of the process. Figure 6 depicts the calculated barrier to break C1–C2 bond of CA+ and MA+. It is seen that CA+ requires nearly 1 eV more energy to break the bond. Thus, the prediction is consistent with the experimental observation at 10 eV of e-KE when m/z 41 peak of CA is barely visible, the intensity of this peak in the mass spectrum of MA is significantly large. 5. Summary and conclusion In this Letter we report for the first time the fragmentation behavior upon low-energy electron–molecule collisions of two isomeric molecules of an a,b-enone, CA and MA, which differ with respect to the position of a methyl group. In the neutral ground electronic state, the two isomeric molecules are nearly isoenergetic. However, the yield of various ionic fragments for e-KE a few eVs above the ionization threshold of the two molecules are markedly different. This difference is lost for e-KE of 70 eV. The observations have been interpreted with the aid of the theoretically predicted (DFT/B3LYP/6-311++G(d,p)) kinetic and thermodynamic parameters of different fragmentation channels of the two molecular ions. The calculated data indicate that the energy required to split MA molecular ion into HCO and C3H5+ fragments is 1 eV larger compared to that for CA+, and this difference is vividly manifested in the fragmentation spectra of the two molecules. Likewise, the energy barrier for CO loss from CA+ is predicted to be 0.5 eV larger compared to that of MA+. CO loss has been found to be the most favored dissociation channel of both molecular ions and this points to isomerizations to other isomers which favor CO loss. Acknowledgements

2.5

The authors sincerely thank the Atomic Energy Commission, Government of India for the financial support to T.C. A.K.G. thank CSIR, Government of India for Senior Research Fellowship and A.C. for Junior Research Fellowship to carry out the research presented here.

2.0

Relative Energy (eV)

29

1.5

References 1.0 CA+ C3 C1

0.5

C3

C1

MA+

C4

C2

C2 C4

0.0 1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

C1-C2 Bond Length (Å) Figure 6. Energy required to break the C1–C2 bond of CA+ and MA+, calculated at (DFT/B3LYP/6-311++G(d,p)) level of theory.

[1] V.S. Prabhudesai, A.H. Kelkar, D. Nandi, E. Krishnakumar, Phys. Rev. Lett. 95 (2005) 143202. [2] S. Ptasinska, S. Denif, V. Grill, T.D. Mark, E. Illenberger, Phys. Rev. Lett. 95 (2005) 093201. [3] F. Martin, P.D. Burrow, Z. Cai, P. Cloutier, D. Hunting, L. Sanche, Phys. Rev. Lett. 93 (2004) 068101. [4] V.S. Prabhudesai, D. Nandi, A.H. Kelkar, E. Krishnakumar, J. Chem. Phys. 128 (2008) 154309. [5] H.A. Carime, S. Gohlke, E. Illenberger, Phys. Rev. Lett. 92 (2004) 168103. [6] A.N.H. Yeo, R.G. Cooks, D.H. Williams, Chem. Commun. (1968) 1269. [7] A. Lee, A.H. Goldstein, J.H. Kroll, N.L. Ng, V. Varutbangkul, R.C. Flagan, J.H. Seinfeld, J. Geophys. Res. 111 (2006) D17305. [8] T.E. Graedal, D.T. Hawkins, L.D. Claxton, Atmospheric Chemical Componds: Sources, Occurrence and Bioassay, Academic Press, Orlando, 1986.

30

A.K. Ghosh et al. / Chemical Physics Letters 561–562 (2013) 24–30

[9] J.J. Orlando, G.S. Tyndall, J.B. Burkholder, S.B. Bertman, W. Chen, Abstr. Pap. Am. Chem. Soc. 223 (096-ENVR PART 1) (2002). [10] E. Zervas, X. Montagne, J. Lahaye, Environ. Sci. Technol. 36 (2002) 2414. [11] E.C. Tuazon, R. Atkinson, Int. J. Chem. Kinet. 22 (1990) 1221. [12] J.D. Crounse, F. Paulot, H.G. Kjaergaard, P.O. Wennberg, Phys. Chem. Chem. Phys. 13 (2011) 13607. [13] A. Guenther, T. Karl, P. Harley, C. Wiedinmyer, P.I. Palmer, C. Geron, Atmos. Chem. Phys. 6 (2006) 3181. [14] K. Watanabe, J. Chem. Phys. 26 (1957) 542. [15] M. Klessinger, E. Gunkel, Tetrahedron 34 (1978) 3591. [16] P. Masclet, G. Mouvier, J. Electron Spectrosc. Relat. Phenom. 14 (1978) 77. [17] F.E. Blacet, W.G. Young, J.G. Roof, J. Am. Chem. Soc. 59 (1937) 1937. [18] A.D. Walsh, Trans. Faraday Soc. 41 (1945) 498. [19] I. Magneron, R. Thévenet, A. Mellouki, G. Le Bras, G.K. Moortgat, K. Wirtz, J. Phys. Chem. A 106 (2002) 2526. [20] F.E. Blacet, J.G. Roof, J. Am. Chem. Soc. 58 (1936) 73. [21] D.H. Volman, P.A. Leighton, F.E. Blacet, R.K. Brinton, J. Chem. Phys. 18 (1950) 203. [22] A.G. Harison, F.P. Lossing, Can. J. Chem. 37 (1959) 1696. [23] C.A. McDowell, S. Sifniades, J. Am. Chem. Soc. 84 (1962) 4606. [24] E. Grosjean, D. Grosjean, Int. J. Chem. Kinet. 30 (1998) 21. [25] E. Grosjean, D. Grosjean, J. Atmos. Chem. 27 (1997) 271. [26] M. Ullerstam, E. Ljungström, S. Langer, Phys. Chem. Chem. Phys. 3 (2001) 986.

[27] R. Thévenet, A. Melllouki, G.L. Bras, Int. J. Chem. Kinet. 32 (2000) 676. [28] J. Albaladejo, B. Ballesteros, E. Jiménez, P. Martín, E. Martínez, Atmos. Environ. 36 (2000) 3231. [29] B. Cabañas, S. Salgado, P. Martín, M.T. Baeza, E. Martínez, J. Phys. Chem. A 105 (2001) 4440. [30] E.R. Allen, J.N. Pitts Jr., J. Am. Chem. Soc. 91 (1969) 3135. [31] J.W. Coomber, J.N. Pitts Jr., J. Am. Chem. Soc. 91 (1969) 4955. [32] H. Mackle, L.E. Sutton, Trans. Faraday Soc. 47 (1951) 691. [33] W.G. Fateley, R.K. Harris, F.A. Miller, R.E. Wiltkowski, Spectrochim. Acta 21 (1965) 231. [34] D.E. Johnstone, J.R. Sodeau, J. Chem. Soc. Faraday Trans. 88 (1992) 409. [35] J. Shu, D.S. Peterka, S.R. Lenone, M. Ahmed, J. Phys. Chem. A 108 (2004) 7895. [36] J.D. Crounse, H.C. Knap, K.B. Ørnsø, S. Jørgensen, F. Paulot, H.G. Kjaergaard, P.O. Wennberg, J. Phys. Chem. A 116 (2012) 5756. [37] H.G. Kjaergaard, H.C. Knap, K.B. Ørnsø, S. Jørgensen, J.D. Crounse, F. Paulot, P.O. Wennberg, J. Phys. Chem. A 116 (2012) 5763. [38] A. Mukhopadhyay, M. Mukherjee, A.K. Ghosh, T. Chakraborty, J. Phys. Chem. A 115 (2011) 7494. [39] A. Mukhopadhyay, A.K. Ghosh, M. Mukherjee, T. Chakraborty, Int. J. Mass Spectrom. 309 (2012) 192. [40] M.J. Frisch et al., GAUSSIAN 09, Revision C.01, Gaussian, Inc., Wallingford, CT, 2010. [41] W. Fang, J. Am. Chem. Soc. 121 (1999) 8376.