oubled output of a Nd: YAG-pumped dye laser following a 300 ms collection and cooling period, before being ejected for mass analysis and signal averaging. This gave a total duty cycle of 1.0 s for each period of ion injection

Figure 2. Mass spectrum of [Pb(Benzene)2]2+ at oven temperature of 890˚C.

and excitation. Photon absorption led to fragmentation, and the intensities of the precursor and all fragment ions were monitored as a function of photon energy. The cycle of trapping and laser excitation was repeated 200 times to yield a photofragment mass.

Figure 3 denotes a typical photofragment mass spectrum of [Pb(Benzene)2]2+. Three parent ion peaks were identified [Pb(Benzene)2]2+ at 181.5 amu the parent ions picking one and two molecule(s) of water from the background gas as it circulates within the trap forming [Pb(Benzene)2(H2O)]2+ at 190.5 amu and [Pb(Benzene)2(H2O)2]2+ at 199.5 amu respectively. In addition, three photofragment peaks relating to the [Pb(Benzene)]+ at 285 amu, Pb+ at 207 amu and Benzene+ ions were also observed. Compared to the previous work at the same average photon energy of 45,355 cm−1 two additional distinct weak peaks of [Pb(Benzene)]+ and Pb+ were observed in this current work [18] .

Figure 4 presents the mass spectrum of methane activation at oven temperature of 890˚C. Four new distinct ion peaks were observed namely, [Pb(Benzene)2(H2O)(Me)2]2+ = 206.5 amu, [Pb(Benzene)2(H2O)(Me)]2+ = 198.5 amu, [Pb(Benzene)(Me)3]2+ = 166.5 amu and [Pb(Benzene)(Me)]2+ = 150.5 amu. The important point about the fragmentation mass spectrum is that even without inputting energy via the laser, the complex ions are also able to fragment via collision induced dissociation (CID).

A typical photofragmentation mass spectrum of the methane activation with [Pb(Benzene)2]2+ under laser irradiation at wavelength of 42,283 cm−1 is shown in Figure 5. The spectra presented the parent fragment [Pb(Benzene)2]2+ at 181.5 amu; a characteristic photofragmentation route of losing a molecule of benzene+ resulting in the appearance of the lead monocation complexes [Pb(Benzene)]+ at 285 amu. The ability of the parent ion to pick a molecule of water to form [Pb(Benzene)2H2O]2+ was observed at 190.5 amu. In addition various metal benzene hydrated dication complex with methane molecule such as [Pb(Benzene)2H2OCH4]2+

Figure 3. Photofragmentation Mass spectrum of [Pb(benzene)2]2+ by ion trap mass spectrometer recorded under laser irradiation at an average photon energy of 45,355 cm−1.

Figure 4. Collision induced dissociation mass spectrum of [Pb(C6H6)2CH4]2+.

Figure 5. Photofragmentation Mass spectra [Pb(Benzene)2CH4]2+ (Photon energy = 42,283 cm−1).

at 198.5 amu, [Pb(Benzene)2(H2O)2(CH4)]2+ at 207.5 amu, [Pb(Benzene)2(H2O)2(CH4)2]2+ at 215.5 amu and [Pb(Benzene)2(H2O)3(CH4)]2+ at 216.5 amu were identified. Comparing the nature of laser induced fragmentation (LIF) to CID (Figure 4 and Figure 5), it was observed that the peak intensities of the fragments in CID depreciated enabling the appearance of the daughter photofragments benzene+ and [PbBenzene]+ with appreciably ion peak intensities in LIF.

Photofragmentation of [Pb(BENZENE)2]2+

Photofragmentation of [Pb(Benzene)2]2+ in the ion trap can occur in three possible reaction routes as illustrated below:

[ Pb ( Benzene ) 2 ] 2 + Pb 2 + + 2Benzene (4)

[ Pb ( Benzene ) 2 ] 2 + [ Pb ( Benzene ) ] + + Benzene + (5)

[ Pb ( Benzene ) 2 ] 2 + [ Pb ( Benzene ) ] 2 + + Benzene (6)

[ Pb ( Benzene ) 2 CH 4 ] 2 + [ Pb ( Benzene ) 2 ] 2 + + CH 4 (7)

Pb ( Benzene ) 2 ( CH 4 ) 2 ] 2 + [ Pb ( Benzene ) 2 CH 4 ] 2 + + CH 4 (8)

Pb ( Benzene ) 2 ( CH 4 ) 2 ] 2 + [ Pb ( Benzene ) 2 ] 2 + + 2CH 4 (9)

[ Pb ( Benzene ) 2 CH 4 H 2 O ] 2 + [ Pb ( Benzene ) 2 H 2 O ] 2 + + CH 4 (10)

[ Pb ( Benzene ) 2 CH 4 ( H 2 O ) 2 ] 2 + [ Pb ( Benzene ) 2 ( H 2 O ) 2 ] 2 + + CH 4 (11)

[ Pb ( Benzene ) 2 ( CH 4 ) 3 ( H 2 O ) 2 ] 2 + [ Pb ( Benzene ) 2 ( H 2 O ) 2 ] 2 + + 3CH 4 (12)

[ Pb ( Benzene ) 2 ( CH 4 ) 3 ( H 2 O ) 3 ] 2 + [ Pb ( Benzene ) 2 ( CH 4 ) 3 H 2 O ] 2 + + 3CH 4 (13)

The total binding energy is given by Equation (4) while Equation (5) defines the binding energy relative to charge transfer and Equation (6) can best be described as the incremental binding energy reaction. However, Equations (7)-(13) represented electrostatic interaction of lead benzene dication complex ion with methane molecules at various stages of the activation process as identify by the ion peaks. From the thermodynamic point of view the photofragmentation mechanisms of [Pb(Benzene)2]2+ leading to the formation of the products are highly feasible and does not necessarily depend on electronic excited state to occur. This is due to the fact that the first ionization energy of lead is lower than that of Benzene [IE (Pb) = 7.42 eV and IE (Bz) = 9.24 eV] a charge transfer reaction could be spontaneous. Puskar et al. in the application of the pickup technique in association with high-energy electron impact ionization to form complexes in the gas phase between Pb2+ with a wide range of ligands; observed that Pb dissimilar itself from many other metal dication complexes. The subsequent of collisional activation is that very slight chemical reactivity is demonstrated [17] . Hence such reactions are initiated using the energy difference between M2+ + e → M+ and L → L+ + e, which normally is ~5 eV and are mostly promoted via charge transfer. The resultant effect of this energy difference is the appearance of L+ and the loss of a substantial fraction of the residual ligands as neutral species in the Pb2+ complexes. In most occasions Pb+ appears as a charge-transfer product [17] .

3. Theory

The density functional theory as implemented in Gaussian 09 [21] were used to calculate structures and binding energies of [Pb(benzene)2]2+ and lead dication complex ions with methane. The local density approximation (LDA) [22] together with the gradient-corrected exchange of Becke [23] and the correlation correction of Perdew [24] (BVP86) were applied on geometry optimization and frequency analysis. Structural minima were verified by the absence of imaginary vibrational modes. These calculations were compared with results calculated using the metahybrid functional of Tao, Perdew, Staroverov, and Scuseria (TPSSh) [25] . A 6-311++G(d,p) basis set was used for all atoms except Pb2+, for which the standard SDD relativistic pseudopotential (ECP78MWB) was used [26] . All energies presented are zero point energy corrected. The two optimised geometries observed on [Pb(Benzene)2]2+ were the C2V eclipse (Figure 6) and C2 staggered (Figure 7). The resultant effect was that the rings of the sandwich complexes were not parallel with angle of 180˚, rather bent hemi-directed geometries with centroid-Pb-centroid angle of ~167˚ however, this angle was slightly larger than the 1620 obtained previously [18] . The Pb-Benzene bonds were typically found to be focused all the way through only part of the coordination sphere for lead dication complexes, suggesting possibility of the calculated bent structures being expression of this effect.

From Table 1 it is clear that the total binding energy defined as [Pb(Benzene)2]2+ → Pb2+ + 2Benzene has the highest calculated binding energy follow by the incremental binding energy, defined as [Pb(Benzene)2]2+ → [Pb(Benzene)]2+ + Benzene, with the calculated binding energy relative to charge transfer defined as [Pb(Benzene)2]2+ → [Pb(Benzene)]+ + Benzene+ recording the lowest. The accessibility of the staggered and eclipsed conformers of [Pb(benzene)2]2+ in this experiment was demonstrated by the approximately the same calculated binding energy values observed with BVP86 and TPSSH.

Figure 6. Optimized C2v using BVP86, 6-311++G(d,p) as basic sets.

Figure 7. Optimized C2 using BVP86, 6-311++G(d,p) as basic sets.

Table 1. Binding Energies for the Pb2+ Complexes with respect to various Products formed and Calculated using both BVP86/6311++G(d,p) and TPSSh/6311++G(d,p).

At the initial geometry of [Pb(Benzene)2(CH4)]2+ with methane the benzene-lead-benzene bond was 180˚; all the hydrogen-carbon-hydrogen bonds in methane were observed to be 109.50˚ and the two benzene-lead-methane angles were observed to be 90˚ and 89.79˚ for the top and down benzene molecules respectively. However, at the optimised geometry of [Pb(Benzene)2(CH4)]2+ (Figure 8) all the initial angles was observed to be distorted; for instance benzene-lead-benzene bond was distorted from 180˚ to 166.01˚, the top benzene-Pb-methane bond is distorted to 101.10˚ while the down benzene-Pb-methane bond was distorted to 107.49˚. The angles of 109.5˚ in methane were distorted to 107.11˚, 108.94˚, 113.01˚, 107.10˚ with the remaining two angles distorted to equal values of 110.25˚.

In order to represent the observed charge separation reaction qualitatively a one-dimensional potential energy curve model was plotted (Figure 9). From the curves, it can be seen that the photo induced charge transfer to give [Pb((Benzene)2]+ and CH 4 + of [Pb(Benzene)2(CH4)]2+ was also not observed because this reaction was endothermic as evidenced by observing that the repulsive energy curve (blue) lies above the attractive curve (red).

At the optimised geometry of [Pb(Benzene)2(CH4)]2+ the lead-methane distance (Pb-C) (Figure 9) was 2.24 Ǻ corresponded to 0.114 eV (11.00 kJ/mol in photon energy) on the PEC. Comparing with the calculated DFT of 8.20 kJ/mol it was clear that the calculated PEC value was higher by 2.80 kJ/mol than calculated DFT value. This difference can be accounted for by the charge differences on the lead metal centre, while calculated charge on the Pb in the optimised geometry was 1.68 the charge of Pb = 2 was considered in the PEC calculation.

Figure 8. The Optimized geometry of [Pb(Benzene)2(CH4)]2+ at BSVP86/6311++G(d,p) with C1 Symmetry.

Figure 9. Potential energy curve model showing attractive and repulsive curves of [Pb(Benzene)2CH4]2+.

4. Conclusions

The UV spectra of [Pb(Benzene)2]2+ complexes have been recorded in the gas phase from ions that have been held and cooled in an ion trap which was then activated successfully with methane molecules at a selected photon energy. Photofragmentation of [Pb(Benzene)2]2+ complexes yielded Pb+ and benzene+ contrary to the previous work of Ma et al. [18] .

The calculated binding energies on the optimised geometry of [Pb(Benzene)2]2+ revealed that the values obtained were slightly lower than the previous values obtained at same levels of theory of zero point energy probably due to a slightly larger distortion angle observed in the optimised geometry in this current work. The PEC calculated binding energy of methane to lead benzene dication complex ion was approximately 25.45% higher than the value recorded on DFT; this difference can be accounted for by the charge differences on the lead metal centre. The actual calculated charge on the Pb in the optimised geometry was 1.68 while the charge of Pb = 2 was considered in the PEC calculation. The incremental addition of methane to the metal dication complex revealed approximately 39.15% difference in binding energy between [Pb(Benzene)2(CH4)]2+ and [Pb(Benzene)2(CH4)2]2+ for the calculated DFT values.

Acknowledgements

The author would like to thank Prof. Anthony Stace of University of Nottingham School of Chemistry, UK for all the sacrifice and support to see this research through, Dr Lifu Ma of University of Nottingham School of Chemistry for his contributions Prof Hazel Cox of the University of Sussex UK, Computational chemistry Department for the Software and computer time.

Funding

The author is indebted to British government for granting him “International Excellence Research Award” which made the financial assistance for this research in the Nottingham University UK possible.

Cite this paper
Koka, J. (2019) Gas Phase Activation of Methane Molecule with Lead Benzene Dication Complex Ion, [Pb(Benzene)2]2+. Materials Sciences and Applications, 10, 105-117. doi: 10.4236/msa.2019.102009.
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