On the feasibility of reactions through the fullerene wall: a theoretical study of NHx@C60

We propose a new approach to the synthesis of AHx@fullerene structures via reactions through the fullerene wall. To investigate the feasibility of the approach, the step-by-step hydrogenation of the template endofullerene N@C60 up to NH4@C60 has been studied using DFT and MP2 calculations. Protonation of the endohedral guest through the fullerene wall is competitive with escape of the guest, whereas reaction with a hydrogen atom is less favorable. Each protonation step is highly exothermic, so that less active acids can also protonate the guest with less accumulation of energy. The final product, NH4@C60 is a novel concentric ion pair NH4 @C60 in which the charge-centers of the two ions coincide.

Most of the above examples of the endofullerenes were synthesized by constructing or reclosing the fullerene cage in the presence of the moiety to be incorporated.][10][11][12][13] Diatomics were inserted into C 60 and C 70 under high pressures and temperatures. 18We have therefore used the examples of NH 3 @C 60 and NH 4 @C 60 to conduct a purely theoretical study to investigate the possibility of synthesizing endohedral guests within fullerenes by allowing reagents (in this case protons and atomic hydrogens) to pass through the walls of the fullerene.][27] Here we investigate the possibility of synthesizing NH x @C 60 (x = 1-4) starting from N@C 60 by insertion of protons or hydrogen atoms through the fullerene wall.9][30] In 2008 ammonia was inserted into a chemically opened fullerene. 31However, the chemical properties of the host-guest complex obtained must differ greatly from the target endofullerene NH 3 @C 60 , since even at low temperatures (À10 1C) ammonia escapes slowly from this open-cage fullerene. 31It is known, however, that NH 3 @C 60 is thermodynamically stable, while nNH 3 @C 60 with n = 2-7 represent metastable structures and the cage finally breaks for n = 8. 30 Scheme 1 shows a suggested synthetic route to NH 3 @C 60 and NH 4 @C 60 via consecutive pronation and reduction steps starting from the known 2-4 N@C 60 , which has been suggested as a possible material for the development of the electron-spin quantum computers. 32,33We compare this route to the stepwise direct hydrogenation.Since the spin states of nitrogen hydrides vary with the number of hydrogen atoms, we also investigate all the intermediate NH x @C 60 compounds for x = 0-4 as they can be potentially interesting for spintronics applications.In addition, we investigate the electronic properties of NH 4 @C 60 .
Additional single-point (SP) calculations were performed at the MP2 52-57 level of theory with the same basis set on the DFT-optimized geometries (denoted in the following MP2).All B3LYP/6-31G(d)-and MP2-computed relative energies are corrected for zero-point vibrational energies (ZPEs) calculated at the DFT level.Unrestricted B3LYP calculations were performed for all open-shell systems.However, ROMP2 single points were performed for open-shell systems because of high spin contamination in the unrestricted calculations.All structures were visualised with ChemCraft 1.7. 58ll Hartree-Fock reference wavefunctions used in RMP2 calculations exhibit RHF/UHF instabilities for the closed-shell systems and the reference UHF wavefunctions have internal instabilities for the open-shell systems.Some, but not all, B3LYP wavefunctions also exhibit instabilities.Wavefunction instabilities cause the large relative energy differences between B3LYP and MP2 calculations in some cases.Thus, the orbital initial guesses for MP2 calculations of the endofullerenes were read from DFT checkpoint files, which lead to the numerically stable and consistent results.
The Gaussian 03 59 and 09 60 program packages were used for all calculations.The key reaction pathways along both directions from the transition structures were followed by the IRC method. 61atural population analysis [62][63][64][65][66][67][68] (NPA) was performed within the Gaussian 03 and 09 packages using the density matrices for the current methods. 69

Mechanism of proton penetration and nitrogen escape
Our calculations start from the appropriate exo-protonated NH x-1 @C 60 endofullerenes and proceed according to Scheme 1.Any study of these systems is complicated by their many possible spin states.Thus, the first reaction step (step 1 in Scheme 1) begins from N@C 60 1, which can exist in high-(spin 3/2) and low-spin (spin 1/2) states.It has been shown in previous experimental 2, [70][71][72] and theoretical 4,73,74 studies that the ground state of 1 is high spin.Our current study supports this conclusion, since 4 1 is more stable than 2 1 (see Scheme 2) by 26.0 kcal mol À1 Scheme 1 Proposed approach for step-by-step synthesis of NH 4 + @C 60 À (13).The C 60 cage is represented as circles for clarity.Different pathways considered are designated with lower case characters a-i (see Results and discussion).and 79.2 kcal mol À1 at the B3LYP and MP2 levels, respectively.Moreover, although the formation of 4 1 from a free nitrogen atom and C 60 is found to be slightly endothermic (by 1.3 kcal mol À1 ) at the B3LYP level, earlier UB3LYP/D95*//PM3 calculations, 4 found it to be exothermic by 0.9 kcal mol À1 and our MP2 calculations predict the formation of 4 N@C 60 to be favorable by À6.8 kcal mol À1 .Thus, our further discussion of step 1 (Scheme 1) will be concerned with the quartet potential-energy surface (PES).
Starting from 2a + , the proton can reach the nitrogen atom by breaking either a [5,6]-or a [6,6]-bond of C 60 (TS1a + and TS1b + , respectively, Fig. 2).The more favorable of these two transition states is 4 TS1a + for migration by breaking a [5,6]-bond, with calculated barriers of 90.0 and 90.1 kcal mol À1 relative to 4 2a + at the B3LYP and MP2 levels, respectively.No pathways that involve direct passage of the proton through the hexagonal or pentagonal rings were found.An attempted transition-state optimization for the first case without symmetry constraints leads to complex 2a + , and in the second case to TS1a + .
In addition, a previous DFT study of proton migration on the C 60 surface, 75 which should behave very similarly to that on the surface of NH x @C 60 H + , showed that transition states in which the proton lies above the centers of five-or six-membered rings are those for proton migration over the C 60 surface.Nevertheless, transition states for these two processes were computed using symmetry constraints and found to be highly unfavorable relative to proton migration above [5,6]-and [6,6]-bonds. 75 mechanism analogous to He-insertion into C 60 , which occurs through a ''window'' made by opening two C-C bonds, 27 was also considered.However, the transition state for this process, 4 TS1c + lies much higher in energy than 4 TS1a,b + (Fig. 2).Another study 25 suggested that the most favorable pathway of He-insertion should be to open a window by breaking three-bonds.However, we found that the transition state for this process, 4 TS1d + is the least favorable of those studied here.
In addition to the pathways discussed above (Fig. 2), we have also considered possible lower-lying ones that occur via the formation of endo-NH x @C 60 H + intermediates at [5,6]-and [6,6]aza bridges.Protonating the C 60 cage causes a drastic increase in the number of possible isomeric endofullerenes with aza-bridges.However, due to the stabilizing interaction between the nitrogen lone pair and the positively charged carbon atoms adjacent to the C-H moiety, the three endo-N@C 60 H + isomers 2b-d + shown in Fig. 2 are expected to be the most favorable.This was confirmed partially by calculating two other endo-N@C 60 H + isomers in which the nitrogen atom is farthest from the C-H moiety.2b + is the most stable endo-N@C 60 H + isomer, but the nitrogen atom does not form an aza-bridge and is rather covalently bound to one carbon atom (denoted ''endohedrally bound'' below) with a C-N bond length of 1.53 Å.The nitrogen atom has a negative charge of À0.136 e according to an NPA analysis.2b + can be formed with a relatively low barrier (TS1g + , 19.4 and 30.1 kcal mol À1 , at the B3LYP and MP2 levels, respectively, Fig. 2) from 2a + .This barrier is much lower than that found for N@C 60 4 because of the interaction of the nitrogen lone pair with the protonated C 60 cage.Analogously to TS1a + and TS1b + , we found TS1e + and TS1f + , which correspond to the transition states for the reaction paths starting from 2b + , in which the proton is inserted through the [5,6]-and [6,6]-bonds, respectively.However, they lie too high in energy to play a role in the reaction (Fig. 2).In contrast, N-escape becomes possible from the 2b + intermediate through both the [5,6]-and [6,6]-bonds (TS1h + and TS1i + , respectively).The latter is more favorable, as also found for N@C 60 . 4TS1i + Scheme 2 Schematic energy profile for N insertion into C 60 , relative energies in kcal mol À1 at the B3LYP (black) and MP2 (red) levels.lies 81.8 kcal mol À1 higher in energy than 2a + on the PES at the B3LYP level and thus lower than TS1a + (90.0 kcal mol À1 ).However, at the MP2 level, this ordering is reversed: TS1i + lies slightly higher in energy than TS1a + (90.6 vs. 90.1 kcal mol À1 ).Thus, nitrogen escape and nitrogen protonation can be competitive processes.
We only considered insertion pathways through the [5,6]-and [6,6]-bonds via transition states of the types TS1a + and TS1b + , respectively, for the subsequent steps 2-4 (Scheme 1).These pathways are the most favorable for step 1 and the remaining steps appear to be very similar in geometries and barriers heights (see below).The designations a and b used for transition states TS2 + -TS4 + have the same meaning as for the transition states, TS1 + , for the first step.No stable minima were found for endo-NH@C 60 in which NH forms aza-bridges to a nearby C-H moiety.
All such starting geometries optimized to NH@C 60 H + with NH at the center of the C 60 cage.We therefore did not investigate pathways for further protonation of the nitrogen-containing moiety via endo-NH x @C 60 H + intermediates for steps 2-4.

Energetics of the step-by-step formation of NH 4
+ @C 60

À
The energetics of all four steps shown in Scheme 1 are given in Table 1 and in Scheme 3, where energies relative to 4 2a + and relative energies within a step are shown.All reactions are exothermic, by 7-56 kcal mol À1 at B3LYP and by 18-109 kcal mol À1 at MP2.The barriers for each type of pathway hardly vary for the different steps and multiplicities.Thus, for step 1 the doublet PES lies almost parallel to the quartet one.Since doublet 2a + lies higher in energy than quartet 2a + , and 1 exists in the Fig. 2 Structures and energies relative to 2a + in kcal mol À1 at the B3LYP (black) and MP2 (red) levels for proton migration from 2a-d + to 3 + via the alternative quartet transition states TS1a-f + , and for the N-escape from 2b-d + via alternative quartet transition states TS1h + by breaking a [5,6]-bond and TS1i + by breaking a [6,6]-bond.TS1a,e + corresponds to proton migration by breaking a [5,6]-bond; TS1b,f + -by breaking a [6,6]-bond; TS1c + -by breaking two bonds and TS1d + by breaking three bonds.TS1g + corresponds to the formation of 2b + from 2a + .quartet state (see above) the entire reaction most likely proceeds on the quartet PES.Similarly, the second step should proceed on the triplet, rather than on the singlet or quintet PES, because 4 + is by far most stable in the triplet state (Table 1).
The endofullerenes NH x + @C 60 all have high electron affinities (from 111 to 164 kcal mol À1 (4.83-7.11eV) at B3LYP and from 97 to 211 kcal mol À1 (4.23-9.16eV) at MP2, Table 2) and thus they can be readily reduced to the neutral endofullerenes NH x @C 60 , e.g. using gas-phase neutralization as has been demonstrated for other endofullerenes. 11,12he total energy gain of all transformations starting from 1 and ending with 13 according to eqn (1) is 1555.0kcal mol À1 at B3LYP and 1530.6 kcal mol À1 at MP2.
Although the barriers for protonating endohedral nitrogen hydrides through the fullerene cage are too high to be observable in solution, the entire process involves a continuous decrease in energy, so that each step is possible in the gas phase.The calculated proton affinities of NH x @C 60 in the gas phase (Table 3) are very similar to that of C 60 itself (211 and 196 kcal mol À1 at the B3LYP and MP2 levels of theory, respectively, compared with the experimental range 76 of 204 to 207 kcal mol À1 and a further calculated value 75 of 202 kcal mol À1 ).The calculated proton affinities for the endohedral nitrogen-containing species lie in the range between 207 and 213 kcal mol À1 with B3LYP and between 194 and 198 kcal mol À1 with MP2.Thus, the protonated species NH x @C 60 H + possess adequate energy immediately after their formation to cross the calculated barriers for protonation through the C 60 cage.Therefore, a protonation-rearrangement cascade from NH xÀ1 @C 60 to NH x + @C 60 is possible.However, as the rearrangements to NH x + @C 60 are mildly exothermic, the product is even hotter than the protonated fullerene precursor, so that thermal energy would have to be dissipated at the product stage.Using less energy-rich acids such as H 3 + and CH 5 + , which are common protonating agents in ion cyclotron resonance spectrometry, [77][78][79] would render the initial proton transfer to NH x @C 60 less exothermic.The relevant heats of reaction are shown in Table 3.Generally, the energy gained from protonation by CH 5 + is slightly less than the barriers for transferring the proton through the cage to nitrogen.On the other hand, proton transfer from H 3 + releases slightly more energy than is necessary to overcome the barrier.Thus, H 3 + is a promising candidate for the individual through-cage protonation steps.

Alternative approach using hydrogenation by hydrogen atoms
In addition, we considered the corresponding hydrogenation of nitrogen inside C 60 1 through the buckminsterfullerene wall by atomic H to compare barriers with those described above for protonation by the bare proton H + (Scheme 1).Three possible spin states (quintet, triplet and singlet) were taken into account.
The energetics of the computed pathway are summarized in Table 4. Notations of species are the same as above with the difference that all further discussion will refer to neutral species rather than positively charged ones.Unlike 2a + with nitrogen located at the center of the protonated C 60 cage (Fig. 2), neutral N@C 60 H 2a is not the most stable isomer.The most favorable one is singlet 2e (Table 4 and Fig. 3).In 2e nitrogen forms covalent bonds with three neighboring carbons of a hexagon and the fourth carbon is saturated with hydrogen atom.Such a structure is so strongly preferred for the singlet state that no 2b can be located: any attempts to find 2b end in 2e.
Moreover, 1 2e is closely followed in energy by the most stable triplet isomer of 2 (2b) and by quintet 2a (Fig. 3), which are less favorable by 0.1 and 2.2 kcal mol À1 at DFT and by 8.1 and 5.9 kcal mol À1 at MP2, respectively.Thus, the higher spin state, the lower ability of nitrogen to form covalent bonds with the inner surface of C 60 cage.This can be seen clearly from Step 2 (triplet PES) 3 5 + 0.0 À393.9 0.0 À376.9 À201.9 À995.9 À212.9 À980.8 Step 4 (singlet PES) the geometries of 5 2a, 1 2e and 3 2b (Fig. 3): nitrogen is located at the center of the C 60 cage for the quintet 2a, it is covalently bound with only one carbon atom in triplet 2b and with three carbon atoms in singlet 2e.
In contrast to the protonation, nitrogen escape appears to be more favorable than hydrogen insertion through the C 60 cage for all spin states (Table 4 and Fig. 3).The most favorable transition state is singlet TS1i, i.e. nitrogen escape via breaking the [6,6]-bond (Fig. 3).The barrier to this escape is 69.4 and 80.5 kcal mol À1 at DFT and MP2, respectively.N-escape through a [5,6]-bond breaking via 1 TS1h is less than 2 kcal mol À1 higher in energy.Nitrogen escape for the triplet and quintet PESs proceeds via the corresponding TS1i with barriers of 76.9 an 95.9 kcal mol À1 at DFT and of 81.0 and 98.8 kcal mol À1 at MP2, respectively.They are followed up by the TS1m, in which nitrogen displaces the carbon atom (Fig. 3).
Hydrogen penetration through the cage on the singlet PES is highly unfavorable.Moreover, as in the case of minimum 1 2e, nitrogen covalent bonding to carbons is so strong that no 1 TS1a,b were found. 1 TS1j and 1 TS1k (Fig. 3) were located instead and rather than 1 TS1e,f.The TSs for hydrogenation of nitrogen through the fullerene cage for triplet and quintet PESs are similar to those for protonation, i.e.TS1a,b,e,f were found.However, hydrogenation of the N-atom is less favorable than N-escape for the triplet PES by 25.7 and 54.2 kcal mol À1 at DFT and MP2, respectively.Nevertheless, barriers of hydrogenation and N-escape are much closer in energy for the quintet PES: hydrogenation is less favorable by 5.0 and 2.0 kcal mol À1 at DFT and MP2, respectively.
The reaction 1 2e -1 4 is endothermic by 16.4 and 25.5 kcal mol À1 , while 3 2b -   39.5 kcal mol À1 and 5 2a -5 4 is also exothermic by 0.1 and 16.3 kcal mol À1 at DFT and MP2 (Table 4), respectively.However, hydrogenation of 4 1 to 1 2e, 3 2b and 5 2a is exothermic by only 44.0, 43.5 and 41.8 kcal mol À1 at DFT and 30.8, 22.7 and 24.9 kcal mol À1 at MP2, respectively.This energy gain is ca.30-50 kcal mol À1 less than is necessary to overcome the barrier of nitrogen escape through the cage of C 60 (for the singlet PES).This is in contrast to the case of protonation through the cage, when initial protonation of NH x @C 60 leads to an energy release larger than that required to overcome the barrier to proton insertion through the C 60 cage.Thus, hydrogenation by protonation is expected to be the only way for the synthesis of nitrogen hydrides inside C 60 .

Electronic properties of NH 4 @C 60
The formation of NH 4 @C 60 according to is calculated to be highly exothermic (À83.9 kcal mol À1 and À156.5 kcal mol À1 at the B3LYP and MP2 levels, respectively).
We performed an NPA analysis of the target species NH 4 @C 60 13 at B3LYP both with and without an implicit representation of the solvent (benzene) to study its nature.We used a polarized continuum model (PCM) [80][81][82][83][84][85][86] to consider solvent effects.Both calculations confirmed that the NH 4 moiety carries almost a unit positive charge (+0.97 e with and without PCM corrections), while the C 60 moiety is correspondingly negatively charged (13).
The sum of Coulson charges at the AM1 level 87 leads to a similar charge of +0.96 e.The total charge of 13 is naturally zero, and the whole species 13 is a radical.Thus, NH 4 @C 60 is indeed a ''concentric ion pair'' more properly described as NH 4 + @C 60 À , in agreement with previous theoretical studies for this and related MH 4 AE @C 60 À species. 88 has a peculiar electronic structure as its metal-free cation is confined inside the C 60 anion and cannot escape from the fullerene cage, although metal containing Ca 2+ @C 60 2À has been observed experimentally 89 and M 3 N@C x concentric ion pairs are known for larger fullerenes. 90,9113 is not a classical salt with two counterions held together by electrostatic forces and is also not a zwitterion, because the oppositely charged moieties are not covalently bound.Moreover, charge centers for both the positively charged ammonium ion and the fullerene C 60 and (NH 3 + H ), [92][93][94][95][96][97][98][99][100] which is why we have explored whether these decomposition products are more or less energetically preferable inside C 60 than ion pair NH 4 + @C 60 À 13. (NH 2 + H 2 )@C 60 13a is rather unstable in comparison to 13, since its formation from 13 is highly endothermic (by far more than 50 kcal mol À1 ) and thus thermodynamically unfavorable (Fig. 4).
In addition, optimization of (NH 3 + H )@C 60 in conformation 13b at the B3LYP level, even starting from the structure with a shortened C-H bond length (1.08 Å) terminated with the structure of NH 4 + @C 60 À 13. (NH 3 + H )@C 60 (or NH 3 @C 60 H as hydrogen is covalently bound to the inner surface of fullerene) in   conformation 13c is also highly endothermic and thus very unlikely to exist.Moreover, since ammonia is known to invert readily with a barrier of 5.8 kcal mol À1 , 101 we have calculated that the barrier to ammonia inversion, which corresponds essentially to the barrier of rearrangement of 13c to 13, is À0.1 and 0.7 kcal mol À1 at the B3LYP and MP2 levels, respectively.Thus, NH 3 @C 60 H 13c obviously transforms directly into NH 4 + @C 60 À 13.The electrostatic potential created by the ammonium cation makes the fullerene a much stronger electron acceptor than parent C 60 .The vertical electron affinity (EA V ) of pure C 60 calculated at the B3LYP/ 6-311+G(d,p) [43][44][45][46][47][48][49][50][51][102][103][104] level on the B3LYP/6-31G(d) geometries is 2.59 eV (close to the experimental value of 2.68 AE 0.02 eV), 105,106 but becomes 3.12 eV larger when NH 4 + is placed inside the C 60 (Table 5). Moreove, even the second vertical electron affinity of NH 4 + @C 60 (2.71 eV) is higher than the first EA v of neutral C 60 , similarly to experimental observations for Ca 2+ @C 60 2À .89 Although all further electron affinities are negative for both compounds (Table 5), no electron is transferred to NH 4 + from the fullerene.Note that the EAs of NH 4 + @C 60 nÀ plotted vs. those of C 60 nÀ lie on a straight line (R 2 = 0.9997) with a slope of 1.0 that intersects the axis at 3.1 eV (Fig. 5).These findings are in agreement with the previous theoretical observation for MH 4 + @C 60 species that their EAs can be described by a simple charged sphere model and particular differences in structures of the endohedral guests has only relatively small effect of 0.1-0.6 eV.88 All these observations are supported by analysis of the local electron affinity (EA L , RHF-EA L 107,108 for closed-shell and Fig. 4 Relative energies at the B3LYP (first entry) and MP2 levels (second entry) in kcal mol À1 for NH 4 + @C 60 À (13), (NH 2 + H 2 )@C 60 (13a) and two conformers of NH 3 @C 60 H (13b and 13c).

Conclusions
We have demonstrated the possibility in principle of a new approach to the synthesis of endofullerenes via molecular ''assembly'' from ''template'' endofullerenes rather than insertion of the whole molecule into the fullerene cage or one-pot formation.N@C 60 1 was chosen as the ''template'' for the present study, which was hydrogenated step-by-step up to ammonia inside C 60 10 and the ''concentric ion pair'' NH 4 + @C 60 À 13 according to Scheme 1.Note that such an approach would allow us to obtain NH@C 60 and NH 2 @C 60 , which are open-shell systems and thus potentially interesting for spintronics.NH 4 + @C 60 À is an end product with electron affinity similar to that of C 60 .
The rate-determining steps of the approach are proton penetrations through the C 60 cage.The most favorable pathways are proton-insertion via [5,6]-bond breaking with barriers about 90 kcal mol À1 .The competitive pathway for the first step N@C 60 H + -NH + @C 60 is nitrogen escape, the barriers of which are very close in energy.Meanwhile, energy gains during proton transfer to NH x @C 60 from H 3 + as proton carrier are about 30 kcal mol À1 larger than the subsequent barriers.Hydrogenation rather than protonation of nitrogen through the C 60 wall leads to nitrogen escape from the fullerene cage, rather than to the formation of nitrogen hydrides at C 60 .
Of course, the proposed approach cannot only be used for the case of N@C 60 studied here, but for other endofullerenes too.Interestingly enough, if we start from CO@C 60 we can end up with methanol inside buckminsterfullerene CH 3 OH@C 60 and CH 3 OH 2 + @C 60

À
. We note at this point that we use theory to investigate a fascinating possibility for experiments and that we make no attempt at experimental validation, which would be outside our expertise.The levels of theory are adequate that we can be confident of the general features of the calculated energy landscape and can draw conclusions about the feasibility of the approach that we suggest.We can only speculate as to possible experimental realization of the reaction sequence described here.Protonation of the intermediate endohedral species and penetration of the fullerene wall by protons should be achievable under conditions that are well established [77][78][79]111 for ion-molecule reactions. The ubsequent reduction step can be performed either by established gas-phase neutralization techniques 11,12 or after isolating cation intermediates, possibly in a reducing matrix, before proceeding to the next step.
Additional experimental studies are necessary for further investigation of this interesting approach.

Fig. 1
Fig. 1 Structures and relative energies in kcal mol À1 at the B3LYP (black) and MP2 (red) levels for the quartet minima 2a-d + .
Scheme 3 Energetics of the four-step synthesis of NH 4 + @C 60 À 13 via the most favorable transition states and spin states.Energies in kcal mol À1 within a step vs. (/) relative to 4 2a + at the B3LYP (black) and at the MP2 (red).

À 4 + @C 60 À
radical anion coincide with the geometrical and mass centers of the C 60 cage.The ammonium ion is thus forced to reside at the center of the C 60 , since otherwise the centers of positive and negative charges would be displaced, and the resulting electrostatic attraction returns NH 4 + to the C 60 À origin.Indeed, the dipole moment of NH is essentially zero at the B3LYP level of theory.It results in an absence of charge separation and the additional stabilization of the system.On the other hand, it is known that the naked Rydberg radical [(NH 4 + )(e À ) Rydberg ] readily decomposes into (NH 2 + H 2 )

Table 1
Energetics of the four-step synthesis of NH 4

Table 3
Energetics of protonation of the species NH x @C 60 , x = 0-3 (1, 4, 7 and 10, respectively) and of the proton transfer to them from the proton

Table 4
Energetics of the formation of NH@C 60 4

Table 5
EAs of NH 4