Chemical Physics Letters 598 (2014) 91–95

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Chemical Physics Letters

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On the stability of [Pb(Proline)]2+ complexes. Reconciling theorywith experiment

http://dx.doi.org/10.1016/j.cplett.2014.03.0060009-2614/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: mm.montero@uam.es (M.M. Montero-Campillo).

Fernando Aguilar-Galindo, M. Merced Montero-Campillo ⇑, Manuel Yáñez, Otilia MóDepartamento de Química, Módulo 13, Universidad Autónoma de Madrid, Campus de Excelencia (UAM-CSIC), Cantoblanco, 28049 Madrid, Spain

a r t i c l e i n f o

Article history:Received 23 December 2013In final form 4 March 2014Available online 12 March 2014

a b s t r a c t

Salt-bridge and canonical charge solvated complexes of neutral proline (Pro) and Pb(II), [Pb(Pro)]2+, arediscussed for the first time. Although thermochemically stable with respect to their corresponding depro-tonated forms [Pb(Pro-H)]+, the dicationic complexes are not observed experimentally. Indeed, for thedeprotonated complexes a disagreement between IRMPD results and theoretical calculations wasreported. We perform an exhaustive DFT assessment to correctly predict the experimental findings,and to rationalize why [Pb(Pro)]2+ complexes are not observed. The deprotonation is likely to occurthrough a highly exergonic proton transfer between [Pb(Pro)]2+ and a water molecule resulting in theobserved [Pb(Pro-H)]+ singly charged ion.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Lead is one of the chemical elements whose effects for humanhealth have been studied for a very long time [1]. Its harmful activ-ity has a direct relationship, among other factors, with the interac-tion that this metal is able to establish with amino acids, thebuilding elements of the protein architecture [2–5]. The study ofthe interaction of metal cations with aminoacids is widely repre-sented in the literature, many papers being devoted to complexesbetween proline (Pro) and mono and dications from an experimen-tal point of view [6–12]. From the whole family of natural amino-acids, Pro presents some unique characteristics, some of themrelated to its quite rigid structure and its secondary amine group[13]. Theoretical and computational chemistry has offered manyinsights about Pro-cation complexes, as is the case of alkalinemonocations (Li+, Na+, K+, Rb+, Cs+), alkaline-earth dications (Be2+,Mg2+, Ca2+), or transition metals (Cu2+, Mn2+, Zn2+, Ag+, Pb2+) [14–19]. As far as the interactions between Pro and Pb(II) are concernedthe first significant finding is that Electrospray Ionization/MassSpectrometry (ESI–MS) experiments showed that in these com-plexes Pro is deprotonated, so only [Pb(Pro-H)]+ singly chargedspecies are detected [20]. Similar deprotonation processes havebeen described also in the literature when Pb2+ interacts with otherrelevant biochemical systems, such as thiouracil derivatives [21] oruridine-50-monophosphate [22] among many others. In somecases, suitable mechanisms for these deprotonation process havebeen also suggested [21,22]. Also very recently the structure of

bare and hydrated [Pb(Pro-H)]+ monocations have been character-ized using Infrared Multiple Photon Dissociation Spectroscopy(IRMPD) [20,23]. Quite importantly, however, theoretical calcula-tions performed by the same authors seem not to be in good agree-ment with the experiments, since the theoretically predicted moststable structure has an IR spectrum which does not match theexperimental one. In this Letter we aim at providing a betterknowledge of the Pb(II)-Pro systems and a reliable theoretical pro-tocol to study them. We will focus first our attention on the struc-ture of [Pb(Pro)]2+ complexes and on their stability with respect tothe different coulomb explosions they can undergo. To the best ofour knowledge this question has never been investigated before,and it would help to explain why these doubly charged specieswere never detected in the gas phase. Secondly, we will try to solvethe apparent dichotomy between experiment and theory with re-gards to the most stable conformation of the [Pb(Pro-H)]+ system.Electronic structure calculations of heavy elements are often chal-lenging due to both relativistic effects and large electron correla-tion contributions, so that the appropriate choice not only of themethod but of the basis set may be crucial, as our group has re-cently shown [24,25]. Hence unavoidably, the first step trying tosolve the aforementioned dichotomy, requires an assessment ofthe theoretical model.

2. Computational details

Electronic structure calculations were carried out with theGAUSSIAN 09 code [26]. All stationary points of the potential energysurface were fully optimized and have been proved to be minimaor first-order saddle points through the corresponding analytical

92 F. Aguilar-Galindo et al. / Chemical Physics Letters 598 (2014) 91–95

second-derivatives calculation. A series of different density func-tionals, namely M06, M06-L, B3LYP, BLYP, LC-BLYP, B2PLYP,X3LYP, O3LYP, B97-D, PBE0, MPW1PW91, B3PW91 and BP86 wereused, plus some additional MP2 calculations [27–41]. This set con-tains several types of DFT functionals (pure, hybrid, double-hybrid,long-range corrected, and dispersion corrected). Lead active elec-trons have been represented by different valence basis sets plussmall-core effective relativistic potentials as defined in the GAUSSIAN

09 code and the EMSL basis set library (LANL2DZ, SDD, cc-PVTZ-PP,aug-cc-PVTZ-PP, aug-cc-PVQZ-PP DEF2-TZVPPD) [42,43]. Hydro-gen, carbon, nitrogen and oxygen atoms were defined by6-31G(d,p) and 6-311g(d,p) double and triple-zeta Pople’s basisset, respectively when LANL2DZ and SDD were used for lead. Inall other cases the same type of basis set was used for all the atoms.The nature of bonding on a given molecule can be studied by look-ing at the topology of the electron density, by locating its criticalpoints, among which the BCP values (Bond Critical Points) are ofparticular relevance. The density at BCPs provides informationabout the strength of the bond and it allows to identify and quan-tify weak interactions that are not evident by just looking at thegeometry. Also, the so-called natural atomic charges coming fromthe Natural Bond Analysis (NBO) approach were used [44]. NBOoffers a Lewis-picture of a given molecule by describing bondingin the system in terms of lone pairs and localized hybrids.

3. Results and discussion

3.1. On the structure of the [Pb(Pro-H)]+ and [Pb(Pro)]2+ complexes

Pro aminoacid has two predominant conformations, ‘‘pucker-up’’ (Cc-exo) and ‘‘pucker-down’’ (Cc-endo) [45]. These two struc-tures are separated by 2 kJ/mol at B3LYP/6-311+G(d,p) level [17].Only the ‘‘pucker-up’’ complexes will be presented here for sim-plicity. On complexing with Pb2+ cation, the most stable conform-ers are those shown in Figure 1, which contains both [Pb(Pro)]2+

and [Pb(Pro-H)]+ complexes. The first row on Figure 1 correspondsto complexes in which Pro remains intact. These complexes can beclassified as charge-solvated and salt-bridge depending on Probeing in its canonical or in its zwitterionic structure in which a pro-ton transfer from the acidic function to the amino group took place.The deprotonated complexes [Pb(Pro-H)]+, shown in the secondrow of Figure 1, have been labeled as pCS(X,Y) or pSB(X,Y) whereX, Y denote nitrogen or oxygen atoms depending on the coordina-tion of the cation, and pCS and pSB stand for pseudocharge-solvatedand pseudosalt-bridge. We decided to use the prefix pseudo, as they

Figure 1. Most stable conformers of non-deprotonated [Pb(Pro)]2+ and deprotonated [Pb

are not real charge-solvated or salt-bridge structures, but thisnomenclature is useful to establish an analogy with the corre-sponding neutral forms. It should be noted that deprotonated com-plexes were already studied by Burt and collaborators, whereasneutral complexes are reported here for the first time.

As it could perhaps be easily anticipated, the global minima ofthe [Pb(Pro)]2+ PES corresponds to structure SB(O,O) in whichthe doubly-charged metal ion bridges between the two oxygenatoms of the carboxylate group of the zwiterionic form of Pro,whereas the different CS structures lie significantly higher in en-ergy (see Table 1).

It is interesting to note that Pb2+ is not symmetrically bound toboth oxygen atoms in SB(O,O), because as it is shown by the corre-sponding molecular graph (see Figure 2) the oxygen atom closer tothe amino group acts as a hydrogen bond acceptor and conse-quently is a weaker electron donor with respect to Pb2+. This pic-ture is also consistent with the second order NBO interactionenergies which show that the charge donation form this oxygenatom towards Pb2+ is lower than from the other oxygen atom(see Table S1 of the Supporting information).

Also interestingly, the attachment of Pb2+ to the two oxygens ofthe carboxylate group does not lead to the most stable structure forthe [Pb(Pro-H)]+ complexes, the global minimum being conformerpCS(NH,O), in which the metal interacts simultaneously with theamino group and one of the O group of the carboxylate group. Notethat a similar structure is not possible for [Pb(Pro)]2+ because theamino nitrogen is necessarily protonated. Rather close in energythere is a second pCS structure, namely pCS(N,O). The large stabil-ity of this complex can be understood because, although the C@Ogroup of the carboxylic acid is a poorer electron donor than thatof the carboxylate group, the Lewis basicity of the deprotonatedamino N is significantly enhanced with respect to that of the NHgroup. These differences are clearly seen in the electron densitiesat the corresponding BCPs and in values of the NBO second orderinteraction energies, between the lone pairs of the NH basic siteand the empty p orbital of Pb2+ (see Table S1 of the Supportinginformation).

3.2. Thermochemical stability

Our group has studied through ab initio and DFT calculations inthe recent years several biomolecule-cation charged systems incomparison with experimental findings. Their stability is stronglycorrelated to the nature of the interacting cation and the possibilityof proton transfer reactions in the media. With ESI–MS techniquesas those used for proline–Pb(II) systems, it was found for example

(Pro-H)]+ complexes of proline with Pb2+ cation at the B3LYP/6-31+G(d,p)/SDD level.

Table 1Relative Gibbs energy of deprotonated and neutral Pro complexes with Pb(II) at theB3LYP/6-31+G(d,p)/SDD level of theory in kJ/mol.

[Pb(Pro-H)]+ DGa

(kJ/mol)[Pb(Pro)]2+ DG

(kJ/mol)

pCS(N,O) 1.4 SB(O,O) 0.0pCS(NH,O) 0.0 CS1(N,O) 57.4pSB(O,O) 33.7 CS2(N,O) 139.9pCS(N,OH) 53.1 CS(O,O) 189.1

a These values agree with those reported in Ref. [20].Scheme 1. Generic potential energy surface of a AB2+ system.

Table 2Calculated Gibbs energies of different reactions involving Pb(II)-Pro complexes.SB(O,O) is the reference for [Pb(Pro)]2+ and pCS(N,O) is the reference for [Pb(Pro-H)]+

at B3LYP/6-31+G(d,p)/SDD. The "*" denotes Proline radical.

Reaction DG(kJ/mol) – B3LYPa

[1] [Pb(Pro)]2+ ? Pro + Pb2+ +560.1[2] [Pb(Pro)]2+ ? [Pb(Pro-H)]+ + H+ +586.6[3] [Pb(Pro)]2+ ? Pb+ + Pro+⁄ +14.8[4] [Pb(Pro)]2+ + Pro ? [Pb(Pro-H)]+ + ProH+ �335.4[5] [Pb(Pro)]2+ + H2O ? [Pb(Pro-H)]+ + H3O+ �69.3

a M06 with same basis set results for reactions [2–4] with the same basis set are+583.9, +14.3 and �330.3 kJ/mol, respectively.

F. Aguilar-Galindo et al. / Chemical Physics Letters 598 (2014) 91–95 93

that uracil-Ca2+ complexes are detected, but only deprotonated[(uracil-H)(uracil)-Cu]+ together with UracilH+ have been observedwhen the interaction involves Cu2+ rather than Ca2+ [46–49]. Thefact that uracil-Cu2+ is not observed depends on its relative stabil-ity with respect to coulomb explosions and on the barriers that thesystems should overcome to reach the products (see Scheme 1).

Considering [Pb(Pro)]2+ system, our calculations at B3LYP/6-31+G(d,p)/SDD level of theory (see Table 2) show that the free en-ergy of dissociation of the most stable SB(O,O) complex intoPro+Pb2+ is highly positive (reaction [1]) and that the expectedcoulomb explosions (reactions [2] and [3]) are clearly endergonicand therefore should not be observed in the gas phase. This meansthat [Pb(Pro)]2+ is thermodynamically stable, and not onlykinetically stable, with respect to all these fragmentation reactions.Note that in particular the deprotonation process yielding themost stable [Pb(Pro-H)]+ complex through the reactionSB(O,O) ? pSB(O,O) + H+ is the most endergonic at 298 K. Then,it is reasonable to ask why only the singly-charged deprotonatedspecies are observed but not the doubly-charged complexes.

It is obvious that the strong charge transfer from Pro towardsPb2+ implies a drastic reorganization of the electron density ofPro and a significant increase of its intrinsic Bronsted acidity. Asa consequence, a proton transfer from [Pb(Pro)]2+ towards eithera second Pro molecule or to a solvent water molecule in thedeclustering process occurring during the electrospray ionizationare both highly favored (See Table 1), yielding the [Pb(Pro-H)]+

singly charged complex. Both processes are compatible with usualobservation of the protonated base in the source [21,22]. It must benoted however, that in the electrospray experiments the residualpressure is very small and, in principle, the probability of observingreaction [4] is rather small, so under the experimental conditionsin which the experiments of Ref. [20] have been carried out,

Figure 2. Molecular graphs of the [Pb(Pro)]2+ and [Pb

reaction [5] is the most likely to occur. The high exergonicity ofreaction [4] indicates however that should the [Pb(Pro)]2+ be pro-duced in the gas-phase through different techniques, the probabil-ity of observing these doubly charged species should be stillnegligible.

3.3. Experiment-theory dichotomy

Since only [Pb(Pro-H)]+ can be found in the gas phase as dis-cussed in the previous section, this one is devoted to try to solvethe apparent disagreement between theory and experiment re-ported in Ref. [20] which led to the authors to conclude that theorywas not reliable for this particular problem: ‘The fact that simplychanging a basis set produces different minimum energy structuresdemonstrates the importance that experiments have in revealingstructural information. One cannot solely rely on calculations’.According to the calculated energy gap between pCS(NH,O) andpCS(N,O) reported in that reference (See Table 1) both species

(Pro-H)]+ complexes. Electron densities are in a.u.

Table 3Free energy difference between pCS(NH,O) and pCS(N,O) complexes in kJ/molcalculated with B3LYP and different basis sets.a The total number of primitive GAUSSIAN

basis functions is also provided.

Basis set Number of basis functions DG(kJ/mol)

LANL2DZ (Pb), 6-31+G(d,p) 192 �5.5SDD (Pb), 6-31+G(d,p) 197 �1.4SDD (Pb), 6-311+G(d,p) 237 �1.3DEF2-TVZPPD (Pb), 6-311+G(d,p) 280 0.2cc-pVTZ-pp (all atoms) 391 3.8aug-cc-pVTZ-pp (all atoms) 607 3.1

a See also ‘‘comment on Table 3’’ (Supp. info).

Table 5Free energy gaps (kJ/mol) between pCS(NH,O) and pCS(N,O) complexes calculatedwith M06 and different basis sets. The total number of primitive GAUSSIAN basisfunctions is also provided.

Basis set Basis functions DG(kJ/mol)

SDD (Pb), 6-31+G(d,p) 197 4.2SDD (Pb), 6-311+G(d,p) 237 4.7DEF2-TVZPPD (Pb), 6-311+G(d,p) 280 4.8cc-pVTZ-pp (all atoms) 391 7.1aug-cc-pVTZ-pp (all atoms) 607 7.1cc-pVQZ-pp (all atoms) 1178 7.6

Table 6

94 F. Aguilar-Galindo et al. / Chemical Physics Letters 598 (2014) 91–95

should be observed on the experimental IR spectrum, the latterbeing the dominant one (64%). However, the experimental spec-trum does not show the characteristic N–H stretching mode, whichshould be located on the 3410–3440 cm�1 range, indicating thatthe pCS(N,O) is not present in the sample, and that therefore thetheoretically predicted relative stabilities should not be correct.Taking into account that similar theoretical models normallyreproduce quite accurately the main features of IRMPD spectra ofsimilar molecular ions, [22] it seems necessary to analyze in fur-ther detail the possible origin of the malfunction of the theoreticalmodel.

Besides the fact that the energy gap between both structures islikely small, two possible sources of error to get the right stabilityorder can be envisaged, the basis set and the density functionalused. In order to have a first estimate of the possible effect of thebasis set we have recalculated this energy gap using increasinglylarge basis sets with the same functional. For this survey theB3LYP functional was chosen and the results obtained (see Table 3)indicate that at least a triple-zeta basis set for small atoms plus abetter representation of the lead atom is needed in the optimiza-tion process, to predict pCS(N,O) to be more stable than pCS(NH,O).This seems to point out that one of the possible deficiencies of themodel was the use of a too small SDD/6-31+G(d,p) basis set.

In order to analyze the possible effect of the functional, in a sec-ond test we decided to explore the performance of the rather com-plete series of functionals mentioned in the Computational Detailssection, maintaining the relatively small SDD/6-31+G(d,p) basis setused in Ref. [20]. Table 4 shows that indeed the energy gap changeswithin a rather small range (10.8 kJ/mol).

The use of the small base leads to the wrong stability orderwhen some functionals, such as B97-D, LC-BLYP. . . are used,whereas some others as O3LYP, B3PW91. . . correctly reproduce iteven with this small basis set. Taking into account that the sign

Table 4Relative Gibbs energy gap (kJ/mol) between pCS(N,O) andpCS(NH,O) complexes using 6-31+G(d,p)/SDD basis setwith different DFT functionals.

Functional DG(kJ/mol)

B97-D �6.2C-BLYP �5.6BLYP �2.5B2PLYP �2.3B3LYP �1.4X3LYP �0.4BP86 1.3O3LYP 1.9M06-L 2.6B3PW91 2.8mPW1PW91 2.8PBE0 2.9M06 4.2

of the gap changes with the size of the basis set when B3LYP func-tional is used, we decided to study what would be the predictionsof these latter functionals if the basis set used was more flexible.For this last test we have chosen the M06 functional because it isthe one that predicts a larger positive gap when the small basisset is used. The results obtained in this scrutiny have been summa-rized in Table 5.

These values clearly indicate that the energy gap seems to con-verge to a value around 7 kJ/mol, since negligible variations are ob-served at the cc-pVTZ level when diffuse functions are included inthe basis set, and the increase of the gap is only 0.5 kJ/mol on goingfrom the aug-cc-pVTZ-pp to the cc-pVQZ-pp basis set. A free en-ergy gap of 7.6 kJ/mol would be consistent with the dominanceof the pCS(N,O), since, assuming a Boltzman type distribution at298.2 K a mixture of 98% of pCS(N,O) and only a 2% of pCS(NH,O)in the gas phase should be expected, in nice agreement with theexperimental evidence.

3.4. Infrared spectra

Since the experimental characterization of the [Pb(Pro-H)]+

was based on the characteristics of their IRMPD spectra, and in par-ticular on the position of the OH stretching mode, we have inves-tigated in this section which of the functionals included in ourassessment better reproduce the experimental value without aposteriori corrections.

Not surprisingly, the values summarized in Table 6 reveal thatpure functionals such as BLYP and BP86 give excellent results onpredicting the position of the main peak with almost negligible er-rors (see also Figure S1). Interestingly, BP86 functional was able topredict Hg-H stretching modes with similar small errors [24]. To

Raw frequency (m, cm�1) values of the O–H stretching mode peak in pCS(N,O)complex with SDD/6-31+G(d,p) basis set and different DFT functionals, along withtheir relative error with respect to the experimental value (3570.0 cm�1). Also, scaledvalues are provided.

Functional m Erel

(%)m scaleda Erel

(%)

B97-D 3637.6 1.9 – –LC-BLYP 3785.4 6.0 – –BLYP 3565.8 0.1 3557.7 0.3B2PLYP 3735.2 4.6 – –B3LYP 3715.9 4.1 3585.1 0.4X3LYP 3723.3 4.3 – –BP86 3589.8 0.6 3567.9 0.103LYP 3738.9 4.7 3607.3 1.0M06-L 3748.6 5.0 – –B3PW91 3747.3 5.0 3598.2 0.8mPW1PW91 3779.0 5.9 3602.1 0.9PBE0 3773.7 5.7 3602.8 0.9M06 3769.9 5.6 – –

a Values corrected according to scale factors given in Ref. [50] for 6-31+G(d,p)basis set for the corresponding functionals.

Table 7Wavenumbers (m, cm�1) of the N–H and O–H stretching mode peaks on SB(O,O) andCS1(N,O) complexes with SDD/6-31+G(d,p) basis set and BLYP functional. B3LYPcorrected values (scale factor 0.9648) are given in parentheses for comparison.

m SB(O,O) CS1(N,O)

N–H stretch 3300.0 (3282.2) 3367.3 (3354.1)3379.4 (3359.1)

O–H stretch – 3513.9 (3529.4)

F. Aguilar-Galindo et al. / Chemical Physics Letters 598 (2014) 91–95 95

obtain a similar result with B3LYP, an a posteriori scale factor of0.955 is used by Burt et al. Table 6 also includes scaled frequencyvalues, which is a standard approach for harmonic calculations.Although Pb atom is represented by a pseudopotential, we chosescale factors obtained for 6-31+G(d,p) basis set, used in our calcu-lations for C, N, O and H atoms [50]. As long as scale factors forBLYP and B386 are very near to 1, corrected values are quite similarand still those with the smallest errors. However, corrections arecrucial for the rest of the functionals, and particularly for B3LYP.

For the sake of completeness we present in Table 7 the mainpeaks of the IR spectra of the two more stable [Pb(Pro)]2+ com-plexes obtained by means of the BLYP functional which was theone exhibiting a better performance as far as Pb(Pro-H)]+ com-plexes were concerned. B3LYP corrected values are also providedfor comparison.

4. Conclusions

Salt-bridge and canonical charge solvated complexes of neutralPro and Pb2+ are discussed for the first time. The relative stabilityof the deprotonated Pb(Pro-H)]+ complexes is correctly repro-duced by several functionals, provided that a flexible enough basisset is used for both the base and the metal dication. It is importantto emphasize that, although is not surprising to find variations inthe stability order of systems with similar stability, this does notinvalidate the theoretical approach and only indicates that themodel should be appropriately assessed. Of course the availabilityof experimental information can be very helpful indeed in thisassessment, but not strictly necessary if it is proved that the theo-retical predictions converged, in the sense that substantialimprovements in the basis set have a negligible effect on the calcu-lated values. We have shown that this is the case in particularwhen the M06 functional was used to describe Pb(Pro-H)]+ com-plexes. We have also found that pure functionals such as BLYPand BP86 gave infrared data for this system with very small errorswithout needing an a posteriori scaling correction. B3LYP gives avery similar result using the corresponding scale factor. Theoreticalcalculations allowed to explain the absence of the most stable[Pb(Pro)]2+ species in the experiments, which is likely due to a pro-ton transfer between the complex and a water molecule resultingin the observed deprotonated complex [Pb(Pro-H)]+. The highexergonicity of the proton transfer between [Pb(Pro)]2+ and Proindicates that even assuming that the doubly charged speciescould be generated in the gas phase by different experimentaltechniques, the probability of observing it should be negligible.

Acknowledgements

This work has been partially supported by the Ministerio deEconomía y Competitividad (Project No. CTQ2012-35513-C02-01), by the CMST COST Action CM1204, by the Project MADRISO-LAR2, Ref.: S2009PPQ/1533 of the Comunidad Autónoma deMadrid, and by Consolider on Molecular Nanoscience CSC2007-00010. F.A.-G. thanks Ministerio de Educación, Cultura y Deportefor his undergraduate Collaboration Research Grant. M.M.M.-C.

acknowledges financial support from the MADRIDSOLAR2 Project.Computing time at Centro de Computación Científica of the Uni-versidad Autónoma de Madrid is also acknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cplett.2014.03.006.

References

[1] G. Flora, D. Gupta, A. Tiwari, Interdiscip. Toxicol. 5 (2) (2012) 47.[2] S. Magyar, T.C. Weng, C.M. Stern, D.F. Dye, B.W. Rous, J.C. Payne, J. Am. Chem.

Soc. 127 (2005) 9495.[3] N. Burford, M.D. Eelman, W.G. LeBlanc, Can. J. Chem. (2004) 1254.[4] C.D.L. Saunders, L.E. Longobardi, N. Burford, M.D. Lumsden, U. Werner-

Zwanziger, B. Chen, R. McDonalds, Inorg. Chem. 50 (2011) 2799.[5] J.C. Payne, M.A. ter Horst, H.A. Godwin, J. Am. Chem. Soc. 121 (1999) 6850.[6] R.C. Dunbar, N.C. Polfer, J. Oomens, J. Am. Chem. Soc. 129 (2007) 14562.[7] R.M. Moision, P.B. Armentrout, J. Phys. Chem. A 110 (2006) 3933.[8] A.S. Lemoff, M.F. Bush, E.R. Williams, J. Phys. Chem. A 109 (9) (2005) 1903.[9] C. Kapota, J. Lemaire, P. Maitre, G. Ohanessian, J. Am. Chem. Soc. 126 (2004)

1836.[10] M.K. Drayss, D. Blunk, J. Oomens, M. Schäfer, J. Phys. Chem. A 112 (2008)

11972.[11] P.B. Armentrout, Y. Chen, M.T. Rodgers, J. Phys. Chem. A 116 (2012) 3989.[12] T. Shoeib, A.C. Hopkinson, K.W.M. Siu, J. Phys. Chem. B 105 (2001) 12399.[13] A. Lesarri, S. Mata, E.J. Cocinero, S. Blanco, J.C. López, J.L. Alonso, Angew. Chem.

Int. Ed. 41 (2002) 4673.[14] M.H. Khodabandeh, H. Reisi, M.D. Davari, K. Zare, M. Zahedi, G. Ohanessian,

ChemPhysChem 14 (2013) 1733–1745.[15] M.K. Drayss, P.B. Armentrout, J. Oomens, M. Schäfer, Int. J. Mass Spectrom. 297

(2010) 18–27.[16] G.J. Fleming, P.R. McGill, H. Idriss, J. Phys. Org. Chem. 20 (2007) 1032.[17] G.-Y. Lee, J. Korean Chem. Soc. 53 (3) (2009) 257–265.[18] A. Gholami, T.D. Fridgen, J. Phys. Chem. B 117 (2013) 8447.[19] M.B. Burt, T.D. Fridgen, J. Phys. Chem. A 117 (2013) 1283.[20] M.B. Burt, S.G.A. Decker, C.G. Atkins, M. Rowsell, A. Peremans, T.D. Fridgen, J.

Phys. Chem. B 115 (2011) 11506.[21] J.-Y. Salpin, S. Guillaumont, D. Ortiz, J. Tortajada, A.M. Lamsabhi, J. Am. Soc.

Mass Spectrom. 20 (2009) 359.[22] J.-Y. Salpin, S. Guillaumont, D. Ortiz, J. Tortajada, P. Maitre, Inorg. Chem. 50

(2011) 7769.[23] L. Banu, V. Blagojevic, D.K. Bohme, Int. J. Mass Spectrom. 316–318 (2012) 23.[24] M.M. Montero-Campillo, A.M. Lamsabhi, O. Mó, M. Yáñez, Theor. Chem. Acc.

132 (2013) 1328.[25] J.-Y. Salpin, J. Tortajada, M. Alcamí, O. Mó, M. Yáñez, Chem. Phys. Lett. 383

(2004) 561.[26] M.J. Frisch et al., GAUSSIAN 09, Revision C01, Gaussian Inc, Wallingford CT, 2009.[27] Y. Zhao, D.G. Truhlar, Theor. Chem. Acc. 120 (2008) 215.[28] Y. Zhao, D.G. Truhlar, J. Chem. Phys. 125 (2006) 194101.[29] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.[30] C. Lee, W. Yang, R. Parr, Phys. Rev. B 37 (1988) 785.[31] A.D. Becke, Phys. Rev. A 38 (1988) 3098.[32] I. Iikura, T. Tsuneda, T. Yanai, K. Hirao, J. Chem. Phys. 115 (2001) 3540.[33] S. Grimme, J. Comput. Chem. 27 (2006) 1787.[34] X. Xu, W.A. Goddard III, Proc. Natl. Acad. Sci. USA 101 (2004) 2673.[35] A.J. Cohen, N.C. Handy, Mol. Phys. 99 (2001) 607.[36] C. Adamo, V. Barone, J. Chem. Phys. 110 (1999) 6158.[37] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865.[38] C. Adamo, V. Barone, J. Chem. Phys. 108 (1998) 664.[39] J.P. Perdew, in: P. Ziesche, H. Eschrig (Eds.), Electronic structure of solids,

Akademie Verlag, Berlin, 1991.[40] K. Burke, J.P. Perdew, W. Yang, in: J.F. Dobson, G. Vignale, M.P. Das (Eds.),

Electronic density functional theory: recent progress and new directions,Springer, Heidelberg, 1998.

[41] J.P. Perdew, Phys. Rev. B 33 (1986) 8822.[42] D. Feller, J. Comput. Chem. 17 (13) (1996) 1571.[43] K.L. Schuchardt, B.T. Didier, T. Elsethagen, L. Sun, V. Gurumoorthi, J. Chase, J. Li,

T.L. Windus, J. Chem. Inf. Model. 47 (3) (2007) 1045.[44] A.E. Reed, L.A. Curtiss, F. Weinhold, Chem. Rev. (1988) 899.[45] J. Behre, R. Voigt, I. Althöfer, S. Schuster, Naturwissenschaften 99 (2012) 789.[46] O. Brea, M. Yáñez, O. Mó, A. Lamsabhi, Org. Biomol. Chem. 11 (2013) 3862.[47] C. Trujillo, A. Lamsabhi, O. Mó, M. Yáñez, J.-Y. Salpin, Int. J. Mass Spectrom. 306

(2011) 27.[48] A. Lamsabhi, M. Alcamí, O. Mó, M. Yáñez, J. Tortajada, J.-Y. Salpin,

ChemPhysChem 8 (2007) 181.[49] O.Y. Ali, T.D. Fridgen, ChemPhysChem 13 (2012) 588.[50] J.P. Merrick, D. Moran, L. Radom, J. Phys. Chem. A 111 (2007) 11683.