Hydrogen Trapping Ability of the Pyridine–Lithium+ (1:1) Complex

Article pubs.acs.org/JPCA

Hydrogen Trapping Ability of the Pyridine−Lithium+ (1:1) Complex Saparya Chattaraj,† K. Srinivasu,† Sukanta Mondal,‡ and Swapan K. Ghosh*,† †

Theoretical Chemistry Section, Bhabha Atomic Research Centre, Mumbai 400085, India Department of Chemistry and Center for Theoretical Studies, Indian Institute of Technology, Kharagpur 721302, India

‡

ABSTRACT: Theoretical studies have been carried out at different levels of theory to verify the hydrogen adsorption characteristics of pyridine−lithium ion (1:1) complexes. The nature of interactions associated with the bonding between pyridine and lithium as well as that between lithium and adsorbed molecular hydrogen is studied through the calculation of electron density and electron-density-based reactivity descriptors. The pyridine−lithium ion complex has been hydrogenated systematically around the lithium site, and each lithium site is found to adsorb a maximum of four hydrogen molecules with an interaction energy of ∼−4.0 kcal/mol per molecule of H2. The fate of the hydrogen adsorbed in a pyridine−lithium ion complex (corresponding to the maximum adsorption) is studied in the course of a 2 ps time evolution through ab initio molecular dynamics simulation at different temperatures. The results reveal that the complex can hold a maximum of four hydrogen molecules at a temperature of 77 K, whereas it can hold only two molecules of hydrogen at 298 K.

1. INTRODUCTION Among the different alternate energy systems proposed, molecular hydrogen is considered to be one of the most important candidates due to the fact that when it burns, it produces energy with water as the only byproduct, in comparison to other fuels.1 However, at present, the energy expense to produce and store hydrogen is higher than that of the retrieved energy.2 Experimental and theoretical works are going on worldwide on the development of more reliable hydrogen production methods as well as on better hydrogen storage technologies. Designing an effective hydrogen storage material that can store hydrogen at ambient conditions with proper adsorption/desorption properties is challenging. Though a large number of materials have been investigated, none of the materials could achieve the targets set by the Department of Energy (DOE, U.S.A.).3 Lithium-decorated lightweight materials are established as a promising hydrogen storage material as it is observed that lithium can adsorb molecular hydrogen with a reasonable adsorption enthalpy. It is reported that lithium-decorated materials like carbon nanotubes, fullerenes, carboranes, graphene, graphyne and graphdiyne, boranes, and metal clusters are good hydrogen storage materials.4−22 Chandrakumar and Ghosh theoretically predicted that the Li ion can adsorb molecular hydrogen with high interaction energy (−30 to −13.5 kcal/mol).4 Sun et al. showed that Li atoms can be adsorbed strongly by the surface of C60, and Li12C60 is a promising system to store hydrogen in molecular form with high gravimetric and volumetric densities.5 Srinivasu et al. carried out a study on organo−alkali metal complexes in the search for hydrogen storage material and reported that simple organic molecular systems (CnHn, n = 4, 5, 6, 8) (aromatic or nonaromatic) form complexes with a Li atom, which can adsorb molecular hydrogen with high interaction energy.6 Blomqvist et al. showed increased hydrogen adsorption in the Li-decorated © 2015 American Chemical Society

metal−organic framework (MOF-5) in comparison to the undoped one.10 In a couple of papers, Srinivasu et al. studied Li-decorated (alkali metal decoration) graphyne, graphdiyne, and borane systems and discussed their stability as well as applicability as hydrogen trapping materials.11−13 Chattaraj and co-workers studied the hydrogen storage ability of lithiumdecorated boron hydrides and also star-like as well as other molecular motifs.14,17−19,22 From these studies, it is clear that Li can adsorb molecular hydrogen more efficiently when it is adsorbed or complexed to some chemical systems. With the same will, here in this article, we are presenting a theoretical study on the stability of the pyridine−lithium ion (Py−Li+) (1:1) complex20,21 and its hydrogen trapping potential. The character of the pyridine−lithium bond as well as the interaction between lithium and adsorbed hydrogen molecules is studied through the assessment of electron density and electron-density-based reactivity descriptors. The stability of the possible Py−Li+ (1:1) complex and their hydrogentrapped analogues at different temperatures is investigated in the course of time using ab initio molecular dynamics23 simulations through an atom-centered density matrix propagation (ADMP)24−26 technique.

2. THEORY AND COMPUTATION To study the stability and reactivity of the chemical systems, various electronic structure principles are found to be useful.27−38 For an N-electron system, the conceptual density functional theory (CDFT)-based reactivity descriptor, hardness37−39 (η), can be defined as the second derivative of the energy E as Received: December 26, 2014 Revised: February 23, 2015 Published: February 23, 2015 3056

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The Journal of Physical Chemistry A ⎛ ∂ 2E ⎞ η = ⎜ 2⎟ ⎝ ∂N ⎠ν( r ⃗)

(1)

where ν(r)⃗ is the external potential. A finite difference approximation to eq 1 can be expressed as

η=I−A

(2)

where I is the ionization potential and A is the electron affinity of the chemical system and have been calculated here by using Koopmans’s theorem,40 thus making η the same as the HOMO−LUMO gap. The validity of Koopmans’s theorem is within the Hartree−Fock theory, but one can use the same approach with the help of Janak’s theorem41 in Kohn−Sham density functional theory as well. All of the geometry optimizations and energy calculations have been performed using the ab initio electronic structure theory based program GAMESS.42 We have employed density functional theory (DFT) with different functionals, namely, B3LYP, PBE, M06, and ωB97X-D, as well as the second-order Møller−Plesset (MP2) perturbation method for all of the systems considered.43−50 We have used the extensive splitvalence basis set along with the polarization functions, 6-311+ +G(d,p). All of the initial guess structures are generated, and the results obtained are analyzed by using the graphical software GABEDIT51 and GaussView 5.52 To check the nature of bonding between pyridine and Li+ in the complexes I and II, energy decomposition analysis (EDA)53−56 is carried out at the BLYP-D3/TZ2P level by taking the optimized geometries obtained at the MP2/6-311++G(d,p) level. The binding energy/H2 (Eb) is calculated for all of the Py− Li+−nH2 complexes using the following formula (eq 3) Eb =

⎛1⎞ ⎜ ⎟[E + − (E Py−Li+ + nE H2)] ⎝ n ⎠ Py−Li −nH2

Figure 1. Minimum-energy structures of Py−Li+ complexes optimized at the MP2/6-311++G(d,p) level of theory.

complex I is mostly ionic in nature because the ΔEelstat term (63.95%) is the dominating term toward the total attraction. In complex II, ΔEorb contributes 89.06% toward the total attraction, implying the bonding between lithium and pyridine ring as mostly of covalent type. Next, we studied the hydrogen adsorption ability of Py−Li+ complexes by systematically hydrogenating the complexes, as shown in Figure 2. Optimizations of hydrogen-adsorbed Py− Li+ complexes I (Py−Li+−(H2)n) give stationary points, and successive calculation of vibrational frequencies shows them to be minima on the potential energy surface. It has been observed that the Py−Li+ complex can adsorb a maximum of four hydrogen molecules around the Li ion center, as shown in Figure 2. Binding energies are calculated using different density functionals, hybrid and meta-GGA exchange−correlation functionals, and the second-order Møller−Plesset (MP2) perturbation method and are given in Table 2. There is a little variation in binding energy with the change in the method of calculation. The binding energies obtained by using PBE and ωB97X-D functionals are slightly higher than that obtained from the MP2 method, whereas the corresponding results from B3LYP and M06 functionals are found to be lower as compared to the MP2 results. The calculated binding energy of the first hydrogen molecule using the MP2 method is found to be −4.64 kcal/mol, and the Li−H distance is found to be 2.073 Å. The calculated MP2 binding energies for the adsorption of the second, third, and fourth hydrogen are found to be −4.36, −4.13, and −3.61 kcal/mol, respectively. As the number of adsorbed hydrogen molecules around the Li site increases, the binding energy per molecular hydrogen is found to decrease. The Mulliken charge (Table 2) on the Li center decreases with the adsorption of hydrogen molecules; thereby, the lithium (partially charged) induced dipolar charge on the hydrogen molecule decreases. Thus, the attractive interaction between the Li site and molecular hydrogen decreases, which results in a decrease in binding energy. As the number of hydrogen molecules adsorbed around the Li site increases, the Li−N distance is also found to increase (from 1.934 Å in the Py−Li+ case to 1.958 Å in the Py−Li+−4H2 case), which can be attributed to the decrease in the positive charge on the Li site upon hydrogenation. Calculated HOMO−LUMO energy gaps (in eV) of hydrogen-adsorbed systems are also given in Table 2. The HOMO−LUMO energy gap for all of the hydrogenated systems is calculated to be around 6.0 eV, and the gap is found to increase slightly with the number of hydrogenation molecules adsorbed, indicating the increased stability of the hydrogenated systems. Hydrogen adsorption properties of the π-complex (Py−Li+II) have also been investigated, and the optimized geometries of

(3)

where n is the number of adsorbed H2 molecules. The Mulliken charge on the Li center and the HOMO−LUMO gap are calculated at the same level of theory. Minimum-energy structures obtained at the ωB97X-D/6311++G(d,p) level (obtained using the electronic structure theory based program G0957) are taken as the initial structures for ADMP simulation. The simulations are done at the same level of theory and at four different temperatures to see the dynamical behavior of the 4H2@Py−Li+ complex in the gaseous state. Initial nuclear kinetic energies of the systems are generated by using a Boltzmann distribution. The temperature throughout the simulation is maintained by using a velocity scaling thermostat. A default random number generator seed is used as implemented in G0957 to initiate the initial mass-weighted velocities. The simulation is done by keeping the fictitious electronic mass as 0.1 amu.

3. RESULTS AND DISCUSSION Two different isomers of Py−Li+ (1:1) complexes as shown in Figure 1 have been considered, and their structures are optimized at the MP2/6-311++G(d,p) level of theory. The optimized energy values of the Py−Li+ (1:1) complexes reveal that complex I with the Li ion bonding to the nitrogen atom is energetically more stable as compared to complex II where the Li ion is situated above the six-membered ring by ∼15.9 kcal/ mol. EDA splits the total interaction energy (ΔEint) into Pauli repulsion (ΔEPauli), electrostatic (ΔEelstat), orbital (ΔEorb), and dispersion energy (ΔEdisp) terms (Table 1). The N−Li bond in 3057

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The Journal of Physical Chemistry A Table 1. EDA Results of Complex I and Complex II Studied at the MP2/6-311++G(d,p)//BLYP-D3/TZ2P Levela systems

ΔEint

ΔEPauli

ΔEelstat

ΔEorb

ΔEdisp

Py−Li+ complex I Py−Li+ complex II

−48.03 −27.97

18.01 8.15

−42.23(63.95%) −3.67(10.16%)

−23.02(34.86%) −32.17(89.06%)

−0.79(1.20%) −0.28(0.78%)

The values are in kcal/mol. The values given in parentheses are the percentage contributions toward the total attractive interaction ΔEelstat + ΔEorb + ΔEdisp. a

Figure 2. Minimum-energy structures of Py−Li+−(H2)n (n = 1−4) complexes at the MP2/6-311++G(d,p) level of theory.

Table 2. Binding Energy (kcal/mol/H2), Mulliken Charge on the Li Center, and HOMO−LUMO Energy Gap (in eV) of Hydrogen-Adsorbed Py−Li Complexes at Different Levels of Theory (with the 6-311++G(d,p) Basis Set) Binding Energy (Eb) (in kcal/mol/H2)

Charge on Li Center (Mulliken Calculations) methods

systems

B3LYP

PBE

M06

MP2

ωB97X-D

B3LYP

PBE

M06

MP2

ωB97X-D

HOMO−LUMO gap (in eV) (B3LYP)

Py−Li+−H2 Py−Li+−2H2 Py−Li+−3H2 Py−Li+−4H2

−4.57 −4.17 −3.65 −2.95

−4.78 −4.58 −4.22 −3.45

−3.96 −3.85 −3.93 −3.69

−4.64 −4.36 −4.13 −3.61

−4.94 −4.67 −4.58 −4.39

0.403 0.236 0.204 0.194

0.402 0.226 0.193 0.164

0.516 0.306 0.175 0.070

0.538 0.394 0.332 0.236

0.543 0.422 0.316 −0.045

6.06 6.09 6.10 6.11

Figure 3. Minimum-energy structures of Py−Li+-II−(H2)n (n = 1−3) complexes at the MP2/6-311++G(d,p) level of theory.

the systematically hydrogenated complex using the MP2/6311++G(d,p) level of theory are reported in Figure 3. From the optimized structures, it is found that the hydrogenated Li site is moving toward the N site and finally converted to complex I, indicating the stability of complex I over complex II. We have also calculated the free energies at different temperatures using the MP2 method, and the results are reported in Table 3. From the free energies at different temperatures, it is evident that the free energies are negative for all four hydrogen molecules at 77 K, whereas at 100 K, the free energy of adsorption for the fourth hydrogen molecule is

Table 3. Gibbs Free-Energy Change for Hydrogen Adsorption (kcal/mol/H2) in Py−Li+−(H2)n Complexes at Different Temperatures Calculated at the MP2/6-311+ +G(d,p) Level of Theory T (K)

3058

n

77

100

150

200

298

1 2 3 4

−2.00 −1.24 −0.97 −0.47

−1.61 −0.70 −0.45 0.07

−0.75 0.50 0.71 1.28

0.11 1.73 1.88 2.51

1.79 4.08 4.17 4.89

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The Journal of Physical Chemistry A positive. At 150 K, only one H2 is found to have negative free energy, and at 200 K and above, all four hydrogen molecules have positive adsorption free energy, indicating that no hydrogen can be in the bound form. 3.1. Nature of Bonding. Adsorption of the hydrogen molecule on the Py−Li+ complex occurs due to the interaction between the Li ion and hydrogen molecules. To get more insight into the nature of bonding, we have generated plots of electron density, the Laplacian of electron density, and the ELF by taking Py−Li+−(H2) as a case study, with the aid of the Multiwfn package.58 The surface map of the electron density and contour plots of the Laplacian of the electron density are shown in Figures 4 and 5, respectively.

Figure 5. Contour plot of the Laplacian of the electron density of the Py−Li+−(H2) complex generated at the MP2/6-311++G(d,p) level of theory. The area of charge depletion (∇2ρ(r) > 0) is shown by green solid lines, and blue dotted lines show areas of the charge concentration (∇2ρ(r) < 0). The bond paths are shown by the brown solid lines connecting the atomic nuclei, and the solid bold blue lines separating the atomic nuclei indicate the zero-flux surfaces in the molecular plane. Where the bond paths and zero-flux surfaces cross each other are the bond critical points (BCPs), which are shown as light blue spheres.

Figure 4. Shaded surface map with projection of the electron density of the Py−Li+−(H2) complex generated at the MP2/6-311++G(d,p) level of theory.

The upper section of the electron density plot (Figure 4) having different heights and colors of the shaded surface corresponds to different values of the electron density, whereas the lower section shows the colored filled map for the same. Figure 4 shows that the electron density is accumulated on the highly electronegative N atom with almost no electron density in the N−Li bond and in between Li and the H2 unit. An analysis of the Laplacian of electron density, ∇2ρ(r), provides additional insights into the bonding situation (Figure 5). The blue dotted lines represent areas of charge concentration (∇2ρ(r) < 0), while the green solid lines show areas of charge depletion (∇2ρ(r) > 0).59 Further, the area of charge concentration between two atoms implies the presence of a covalent bond. Figure 5 shows the presence of a noncovalent interaction in the N−Li bond and in between Li and the H2 unit. The interaction between nitrogen and the Li atom is of ionic type, having hardly any covalent contribution, and the same is true for the Li···H2 interaction. The electron localization function (ELF) provides an idea about the localized electrons. Thus, from the ELF plot (Figure 6), it is clear that there is hardly any localized electron in the N−Li bond and in between Li and the H2 unit. To get a detailed picture of bonding in the N−Li bond and in between Li and the H2 unit, the electron density descriptors (the local kinetic energy density (G(rc)), local potential energy density (V(rc)), and local electron energy density (H(rc)) are

Figure 6. Plot of the ELF of the Py−Li+−(H2) complex generated at the MP2/6-311++G(d,p) level of theory.

calculated (provided in Table 4) at the bond critical points (BCPs). Both the Py−Li+ and Py−Li+···H2 interactions are noncovalent in nature according to Cremer et al.52 as the values of ∇2ρ(rc) as well as H(rc) are positive. The negative ratio of G(rc) and V(rc) indicates the existence of covalency in bonding.60,61 As the value of −G(rc)/V(rc) is greater than 1 Table 4. Electron Density Descriptors (au) at the BCPs of Hydrogen and the Py−Li+ Complex Obtained from the Wave Functions Generated at the MP2/6-311++G(d,p) Level of Theory

3059

BCP

ρ(rc)

∇2ρ(rc)

G(rc)

V(rc)

H(rc)

−G(rc)/ V(rc)

Py−•− Li+(H2) Py−Li+−•− H2

0.038

0.245

0.054

−0.047

0.007

1.149

0.011

0.062

0.013

−0.010

0.003

1.300

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Figure 7. Variation in the total energy and HOMO−LUMO gap with time for the Py−Li+−(H2)n complex during the simulation obtained at the ωB97X-D/6-311++G(d,p) level of theory.

for both Py···Li+ (H2) and Py−Li+···H2, the interactions are purely noncovalent in nature. 3.2. Ab Initio Simulation. An ab initio simulation is done at four different temperatures for the Py−Li+−(H2)4 complex to check the kinetic stability. In this section, we discuss the change in energy and the HOMO−LUMO gap with structural changes of the complex up to 1.75 ps. Here, we report the simulation up to 1.75 ps as after that, no structural change is observed up to 2 ps. Figure 7 depicts the energy and HOMO− LUMO gap profiles of the Py−Li+−(H2)4 complex obtained at four different temperatures. 3.2.1. Simulation at 298 K. As the simulation starts from the minimum-energy structure, all of the hydrogen molecules start moving near the Li atom and the N−Li bond, which increases the energy of the system, and the HOMO−LUMO gap decreases (Figure 7). At 108 fs, the distant hydrogen molecule

becomes ∼6.472 Å away from the Li center, and the remaining three hydrogen molecules stay at 2.475, 2.100, and 2.651 Å away from the Li atom (Figure 8). At this point, the energy of the system becomes maximum, and the HOMO−LUMO gap decreases to minimum due to an unstable orientation of the three hydrogen molecules around Li and the N−Li bond and also a possible increase in the translational energy of the H2 molecules. After that, the distant hydrogen molecule becomes more than ∼8 Å away (144 fs). The distant hydrogen molecule leaves the electrostatic attraction field of the Py−Li+−(H2)3 system, and as time proceeds, another hydrogen molecule (among the remaining three) starts moving away from the proximity of the Li site, and at 1076 fs, we observe another maximum in the energy profile. The distant hydrogen molecule becomes ∼10 Å away from the Li atom. After that, a minimum and maximum are observed, respectively, in the energy profile, 3060

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Figure 8. Simulated structures of the Py−Li+−(H2)n complex at 298 K at the mentioned times obtained at the ωB97X-D/6-311++G(d,p) level of theory.

Figure 9. Simulated structures of the Py−Li+−(H2)n complex at 200 K at the mentioned times obtained at the ωB97X-D/6-311++G(d,p) level of theory.

and the reverse is observed in the profile of the HOMO− LUMO gap due to the movement of the hydrogen molecule around the Li atom and pyridine complex. However, the energy profile gradually proceeds to a minimum due to the plodding movement of the distant hydrogen molecule from the electrostatic attraction field of the Py−Li+−(H2)2 complex, and at 1177 fs, the Py−Li+ complex contains only two hydrogen molecules. After that, a small trough and crest are observed in the energy profile due to the change in orientation of the adsorbed H2 molecule. At 1750 fs, we observe two hydrogen molecules with the Py−Li+ complex at a distance of 2.006 and 2.107 Å from the Li center. 3.2.2. Simulation at 200 K. Although the basic features remain more or less similar, the increment in energy in the case of the 200 K simulation is smaller than that obtained in the case of the 298 K simulation due to the decrease in nuclear kinetic energy. From the beginning of the simulation, one of the H2 molecules starts to move away from the adsorbent (lithium center). There is a maximum in the total energy plot (Figure 7) at 185 fs, which accounts for the fact that one of the adsorbed hydrogen molecules becomes ∼8 Å away from the Li atom (whereas other three remain in the proximity). With the progress of time, the distant H2 molecule becomes more than 10 Å away from the Li atom and leaves the electrostatic attraction field of the Py−Li+−(H2)3 complex, which explains the minimum observed at 296 fs. The remaining three hydrogen molecules remain adsorbed up to 1750 fs. The trough and crest in the energy profile are observed due to the movement of the H2 molecules around the N−Li bond and Li center. All of the snapshots taken during the 200 K simulation are given in Figure 9. 3.2.3. Simulation at 150 and 77 K. In the case of the 150 K simulation, one of the hydrogen molecules starts moving away from the adsorbent (Li center), and the total energy of the system increases further, after 78 fs. At 181 fs, the distant hydrogen molecule becomes 6.382 Å (Figure 10) away, and at

Figure 10. Simulated structures of the Py−Li+−(H2)n complex at the mentioned temperatures and times obtained at the ωB97X-D/6-311+ +G(d,p) level of theory.

that instant, we noticed a maximum in the total energy profile and a minimum in the HOMO−LUMO gap profile (Figure 7). A minimum is observed at 262 fs due to further movement of the distant hydrogen molecule (it becomes ∼8 Å away) as well as to the change in orientation of the remaining three hydrogen 3061

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The Journal of Physical Chemistry A molecules (Figure 7). At 406 fs, a maximum is observed in the total energy profile because two adsorbed hydrogen molecules become distant from the Li center. After that, up to 1750 fs, further liberation of the hydrogen molecule is not observed. Shallow maxima and minima in the total energy profile as well as in the HOMO−LUMO gap profile are observed thereafter due to the change in orientation of the adsorbed hydrogen molecules. It is noted that at 1750 fs, the adsorbed hydrogen molecules are at 2.146, 2.178, and 2.335 Å away from the Li atom, which reveals that at 150 K, the Py−Li+ complex binds three hydrogen molecules strongly. The total energy profile of the 77 K simulation differs from other higher-temperature simulations due to a decrease in the nuclear kinetic energy. Here, we have observed that all of the hydrogen molecules remain adsorbed up to 1750 fs. A maximum at 500 fs is observed due to the distant position of one of the adsorbed H2 molecules at 3.845 Å, which is supported by the corresponding HOMO−LUMO gap profile (Figure 7). At 1750 fs, it is observed that the adsorbed hydrogen molecules are at 2.112, 2.181, 2.316, and 2.440 Å away from the Li atom (Figure 10). 3.2.4. Simulation at 500 K. We found out the temperature and the time at which complete desorption of the hydrogen molecule (removal of all hydrogen molecules) occurs from the Py−Li+ ionic complex. It is observed that at 500 K and at 1750 fs, all of the adsorbed H2 molecules leave the electrostatic attraction field of the Py−Li+ ionic complex.

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ACKNOWLEDGMENTS

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REFERENCES

One of us (S.C.) would like to thank the Indian Science Academies for a Summer Research Fellowship to work at BARC. The work of S.K.G. is supported by J.C. Bose Fellowship of DST, India and also RRF, DAE, India.

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4. CONCLUSIONS The complex Py−Li+ formed by interaction of pyridine with Li+ ion is shown to be a candidate for adsorption of hydrogen the molecular form. Among the two Py−Li+ (1:1) complexes considered, in complex I, with Li bonded to N, the interaction between Li+ and N is mostly ionic in nature, whereas in complex II, where Li is above the pyridine ring, the interaction between Li+ and the pyridine ring is mostly of covalent type. Complex I is found to be energetically more stable as compared to complex II. The single-hydrogen-adsorbed complex (Py− Li+−H2) is more stable in comparison to other multiplehydrogen-adsorbed complexes {Py−Li+−(H2)n} toward the elimination of the H2 molecule. From thermodynamic (freeenergy) considerations, it is clear that the Py−Li+ complex can hold four hydrogen molecules at 77 K, whereas at 100 K, it can adsorb three H2. At 150 K, it can hold only one H2, whereas at 200 K and above, the formation of {Py−Li+−(H2)n} complexes is thermodynamically not favorable. Ab initio molecular dynamics study up to 1.75 ps reveals the kinetic stability of {Py−Li+−(H2)4} complexes. The Py−Li+ complex can adsorb two hydrogen molecules at room temperature, and as the temperature decreases, the hydrogen trapping ability (kinetic stability of {Py−Li+−(H2)4}) of the Py−Li+ complex increases. Thus, it can hold a maximum of four hydrogen molecules at 77 K and three hydrogen molecules at 150 K as well as at 200 K, as revealed by simulation up to 1.75 ps. No hydrogen molecule remains in the electrostatic attraction field of the Py−Li+ complex after 1.75 ps at 500 K.

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