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International audience; We present a combined experimental and theoretical study of the fragmentation of singly and doubly N-methylated glycine (sarcosine and N,N-dimethyl glycine, respectively) induced by low-energy (keV) O6+ ions. Multicoincidence mass spectrometry techniques and quantum chemistry simulations (ab initio molecular dynamics and density functional theory) allow us to characterise different fragmentation pathways as well as the associated mechanisms. We focus on the fragmentation of doubly ionised species, for which coincidence measurements provide unambiguous information on the origin of the various charged fragments. We have found that single N-methylation leads to a larger variety of fragmentation channels than in no methylation of glycine, while double N-methylation effectively closes many of these fragmentation channels, including some of those appearing in pristine glycine. Importantly, the closure of fragmentation channels in the latter case does not imply a protective effect by the methyl group.

The scientific community and regulatory bodies worldwide, currently promote the development of non-experimental tests that produce reliable data for pesticide risk assessment. The use of standard quantum chemistry methods could allow the development of tools to perform a first screening of compounds to be considered for the experimental studies, improving the risk assessment. This fact results in a better distribution of resources and in better planning, allowing a more exhaustive study of the pesticides and their metabolic products. The current paper explores the potential of quantum chemistry in modelling toxicity and environmental behaviour of pesticides and their by-products by using electronic descriptors obtained computationally. Quantum chemistry has potential to estimate the physico-chemical properties of pesticides, including certain chemical reaction mechanisms and their degradation pathways, allowing modelling of the environmental behaviour of both pesticides and their by-products. In this sense, theoretical methods can contribute to performing a more focused risk assessment of pesticides used in the market, and may lead to higher quality and safer agricultural products. © 2017 Society of Chemical Industry.

We present an extension of the structural motif model [Cioslowski et al., J. Am. Chem. Soc., 2000, 122, 8265], originally proposed to predict the relative stability of medium-sized neutral fullerenes (C

We present a combined experimental and theoretical study of the fragmentation of multiply-charged γ-aminobutyric acid molecules (GABA

The scientific community and regulatory bodies worldwide, currently promote the development of non-experimental tests that produce reliable data for pesticide risk assessment. The use of standard quantum chemistry methods could allow the development of tools to perform a first screening of compounds to be considered for the experimental studies, improving the risk assessment. This fact results in a better distribution of resources and in better planning, allowing a more exhaustive study of the pesticides and their metabolic products. The current paper explores the potential of quantum chemistry in modelling toxicity and environmental behaviour of pesticides and their by-products by using electronic descriptors obtained computationally. Quantum chemistry has potential to estimate the physico-chemical properties of pesticides, including certain chemical reaction mechanisms and their degradation pathways, allowing modelling of the environmental behaviour of both pesticides and their by-products. In this sense, theoretical methods can contribute to performing a more focused risk assessment of pesticides used in the market, and may lead to higher quality and safer agricultural products. © 2017 Society of Chemical Industry.

Predicting and understanding the relative stability of exohedral fullerenes is an important aspect of fullerene chemistry, since the experimentally formed structures do not generally follow the rules that govern addition reactions or the making of pristine fullerenes. First-principles theoretical calculations are of limited applicability due to the large number of possible isomeric forms, for example, more than 50 billion for C60X8. Here we propose a simple model, exclusively based on topological arguments, that allows one to predict the relative stability of exohedral fullerenes without the need for electronic structure calculations or geometry optimizations. The model incorporates the effects of π delocalization, cage strain, and steric hindrance. We show that the subtle interplay between these three factors is responsible for (i) the formation of non-IPR (isolated pentagon rule) exohedral fullerenes in contrast with their pristine fullerene counterparts, (ii) the appearance of more pentagon–pentagon ad...

We report a DFT study on the self-assembly of the fullerene derivative PCBM on the Au(111) surface. Recent STM experiments (Angew. Chem. 2007, 119, 8020-8023 [1] ) show a coverage-dependent transition of the adsorption and self-assembly of PCBM on this surfaces. To understand the origin of this observation, we compute the geometries and relative energies of ten PCBM dimers and four tetramers. The calculations show that the self-assembly of PCBM at high coverage is mainly controlled by hydrogen bonding between the PCBM tails. Due to the large size of the fullerene cage, the hydrogen bonds are formed far away from the surface; hence they are very similar to those found in the gas phase. This picture successfully explains the observed site-insensitive adsorption at high coverage and the 2D arrangement of PCBM on the surface.

Unlike traditionally used acyclic 1,2-disubstituted alkenes, the reaction of alpha,beta-unsaturated phenyl sulfones with aryl iodides under Heck reaction conditions takes place mainly by means of a four-component domino process, involving one unit of the alkene and three units of the aryl iodide, affording substituted 9-phenylsulfonyl-9,10-dihydrophenanthrenes. We report here the results of a computational study on the mechanism of this domino arylation reaction. Based on these results we can explain why vinyl sulfones, unlike other electron-deficient alkenes such as enones, preferentially follow this domino pathway instead of the usual Heck pathway. The key step is a C-H activation process in which a five-membered palladacycle is formed. The greater ability of vinyl sulfones, relative to enones, to reach the transition state that leads to the formation of the initial palladacycle makes the difference.

Using B3LYP density functional theory, first and second ionization potentials as well as dissociation energies for neutral, singly and doubly charged fullerenes with sizes between 20 and 70 atoms have been evaluated. Comparison with available experimental data is good except for the doubly charged species. The results show that neutral fullerenes with a magic number of atoms, namely C32, C50, C60 and C70, have the largest stability against ionization and C2 evaporation. A similar large stability is observed for the corresponding singly and doubly charged magic fullerenes, except for C32+ and C322+. Neutral and positively charged C62 is found to be rather unstable. Also, C2+ emission is shown to become competitive with C2 emission for sufficiently small doubly charged fullerenes. The origin of these and other properties is discussed in detail.

Computational studies on the cyclization reactions of some polycyclic aromatic hydrocarbons (PAHs) were performed at the DFT level. Compounds C26H14 and C24H14, which show the connectivity of C60 fullerene fragments, were chosen as suitable models to study the formation of curved derivatives by six- or five-membered ring formation, upon oxidation to their radical cations. Four possible pathways for the cyclization process were considered: a) initial C-C bond formation to afford a curved derivative, followed by dehydrogenation; b) homolytic C-H cleavage prior to cyclization; c) initial concerted H2 elimination and subsequent cyclization; and d) deprotonation of the radical cations prior to cyclization. Computed reaction and activation energies for these reactions show that direct cyclization from radical cations (pathway a) is the lowest-energy mechanism. The formation of five-membered rings is somewhat more favourable than benzannulation. After new cycle formation, homolytic C-H dissociation to afford the corresponding cations is the most favourable process. These cations react with H* without barrier to give H2* Intermediate deprotonations are strongly disfavoured. The relatively low activation energies compared with carbon cage rearrangements suggest that ionization of PAHs can be used for the tailored preparation of nonplanar derivatives from suitable precursors.

The structure, vibrational spectra and electronic properties of the neutral, singly and doubly charged C52 fullerenes were studied by means of the Hartree-Fock method and density functional theory. Different isomers were considered, in particular those with the lowest possible number (five or six) of adjacent pentagons, and an isomer with a four-atom ring. For neutral and singly charged species, the most stable isomer is that with the lowest number of adjacent pentagons, namely five. However, for C(52)2+, the most stable structure has six adjacent pentagons. This finding, which contradicts the pentagon adjacency penalty rule, is a consequence of complete filling of the HOMO pi shell and the near-perfect sphericity of the most stable isomer. The simulated vibrational spectra show important differences in the positions and intensities of the vibrations for the different isomers.

The structures and relative stabilities of the complexes between Cu2+ and uracil, 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil were investigated by B3LYP/6-311+G(2df,2p)//B3LYP/6-31G* DFT calculations. In those systems in which both types of basic centers, that is, a carbonyl and a thiocarbonyl group, are present, association of Cu2+ with the oxygen atom is systematically favored, in contrast to what was found for the corresponding Cu+ complexes. This can be understood by considering that association of Cu2+ is immediately followed by oxidation of the base, which accumulates the negative charge at the oxygen atoms. Similarly, for 2,4-dithiouracil the most basic site for Cu+ attachment is the sulfur atom at the 4-position, while for association of Cu2+ it is sulfur at the 2-position. In contrast, differences between uracil-Cu+ and uracil-Cu2+ complexes are very small, and in both cases the oxygen atom at the 4-position is the most basic. Cu2+ binding energies are about 4 and 1.2 times larger than Cu+ binding energies and proton affinities, respectively. Uracil- and thiouracil-Cu2+ complexes are thermodynamically unstable but kinetically stable with respect to their dissociation into uracil*+ + Cu+ or thiouracil*+ + Cu+. The Cu2+ binding energies vary with the difference between the second ionization potential of the metal and the first ionization potential of the base. regardless of the reference acid (H+, Cu+, Cu2+) the basicity trend is 2,4-dithiouracil > 4-thiouracil > 2-thiouracil > uracil.

Density functional theory (DFT) calculations have been carried out to explore the potential energy surface (PES) associated with the gas-phase reactions between ethylenediamine (En) and Cu + . The structures and bonding characteristics of the different stationary points of this PES have been investigated at the B3LYP/ 6-31 lG(d,p) level. Final energies were obtained by means of B3LYP/6-311+G(2df,2p) single point calculations. En strongly binds Cu + by forming a chelated structure in which the metal cation binds to both amino groups. Different mechanisms leading to the loss of H 2 , NH 3 , and CuH are analyzed in terms of the topology of the PES. The most favorable mechanism corresponds to the loss of H 2 , through a process in which the transition metal cation acts as a carrier, connecting a hydrogen atom from a methylene group with a hydrogen atom of one of the amino groups. The product ion is a five-membered ring in which Cu + bridges between N atoms of the H 2 N-CH 2 -CH-NH moiety. The loss of ammonia is less favorable, because all mechanisms involve higher activation barriers. The most favorable of these mechanisms implies hydrogen shift between the two methylene groups that triggers a spontaneous hydrogen shift between the two amino groups, favored by the existence of a strong intramolecular hydrogen bond. These mechanisms explain the experimental results involving fully C-deuterated species, where only a loss of HD and NH 3 are observed. The loss of HCu is also discussed.

We report here the results of a computational study on the the mechanism of the Oppolzer cyclization. These results lead us to conclude that the insertion of olefins in Pd-allyl complexes probably takes place directly from the eta(3)-allyl species. The presence of a phosphane ligand in the reagents plays the role of enhancing the electron density on the Pd atom; this makes the alkene moiety more reactive towards insertion by back-donation from the metal. The results also indicate that the configuration of the new stereogenic centers is fixed in the insertion of the alkene into the (eta(3)-allyl)palladium species.

We illustrate the potential of ab initio and density functional theory (DFT) methods to model and gain some understanding of the intrinsic reactivity of molecules. We show how the use of high-level ab initio or DFT calculations permit to rationalize (a) basicity and acidity trends along a series of homologous compounds, namely thiocarbonyl and selenocarbonyl derivatives, α,β-unsaturated amines, phosphines and arsines, and α,β-unsaturated alkanes, silanes, germanes and stannanes, (b) the role of intramolecular hydrogen bonds in intrinsic acidities and basicities, (c) the incidence of induced proton transfer process effects on the intrinsic basicity of compounds stabilized by intramolecular hydrogen bonds, (d) hybridization effects on intrinsic basicities, (e) the role of non-classical structures in intrinsic basicities and (f) the observation of bond cleavage associated with gas-phase protonation and the influence of these dissociative proton attachment mechanisms on the intrinsic basicity of the system. Copyright © 2002 John Wiley & Sons, Ltd.

In this review, we present a brief summary of the theoretical methods most frequently used in gas-phase ion chemistry. In subsequent sections, the performance of these methods is analyzed, paying attention to the reliability of geometries, vibrational frequencies, energies, and entropies. The possible pathologies of the different methods, in the form of instabilities of the wave function or spin contamination problems, are discussed. Several examples are presented to illustrate the usefulness of ab initio or density functional theory (DFT) methods to predict the existence of elusive molecules and/or to characterize non-conventional structures, and to rationalize the charge redistributions normally associated with ion-molecule interactions and which result in bond-weakening or bond-reinforcement effects. Finally, the role of non-classical structures in ion-molecule interactions is also illustrated with different examples.

Ab initio molecular orbital calculations have been used to study the most important features of the potential energy surfaces corresponding to Li+ association to C2v and D3h ozone. For this purpose highly correlated techniques [CASSCF, QCISD, and QCISD(T)] have been used. Our results confirm that these highly correlated techniques are unavoidable in so far as a correct description of ozone–Li+ complexes is needed. Good agreement between CASSCF and QCI results is attained for C2v ozone when the UHF function is taken as the reference function and triple excitations are considered in the QCI treatment. Results for D3h ozone are in agreement only when a proper description of the π correlation is included in the CASSCF treatment. Interactions with Li+ are stronger for open chain ozone than for the cyclic isomer. Thus on complex formation the energy gap between C2v and D3h ozone increases. There exist three structures which are predicted to lie very close in energy. The global energy minimum corresponds to an i...

Hartree—Fock calculations were performed to investigate the structure and relative stabilities of complexes between halogen cations and first- and second-row bases. It is shown by means of both a qualitative perturbation molecular orbital treatment and a topological analysis of the electronic charge density that second-row bases present enhanced halogen cation basicities compared with first-row bases. In this respect the results predict that although the fluorine cation basicity of water is smaller than its proton affinity, the fluorine cation basicities of SH2, thioether, phosphine and trimethylphosphine are considerably higher than their proton basicities. Similarly, phosphine and trimethylphosphine should have chlorine cation basicities greater than their proton affinities, in contrast with ammonia and trimethylamine. The results also show that fluorine and chlorine cation basicities are more sensitive to methyl substitution than proton affinities.

We have theoretically evaluated the fragmentation of excited and ionized polycyclic aromatic hydrocarbons (PAH) and clusters of PAHs. In particular the stability of neutral and positively charged coronene has been studied as a prototype of PAH. We present in this communication vertical and adiabatic ionization potentials with charge up to 6 and dissociation energies and fragmentation paths of several channels: H/H, H2, H+/H and H+/H+ loss. We present as well results corresponding to the fragmentation dynamics of singly-ionized clusters of pyrene.

We study dissociative ionization of water clusters by impact of fast Ni ions. Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) is used to obtain information about stability, energetics and charge mobility of the ionized clusters. An unusual stability of the (H2O)4 H + ion is observed, which could be the signature of the so called Eigen structure in gas phase water clusters. High charge mobility, responsible for the formation of protonated water clusters that dominate the mass spectrum, is evidenced. These results are supported by CPMD and TDDFT simulations, which also reveal the mechanisms of such mobility.