Acidity enhancement of unsaturated bases of group 15 by association with borane and beryllium dihydride. Unexpected boron and beryllium Brønsted acids.

International audienceThe intrinsic acidity of CH2[double bond, length as m-dash]CHXH2, HC[triple bond, length as m-dash]CXH2 (X = N, P, As, Sb) derivatives and of their complexes with BeH2 and BH3 has been investigated by means of high-level density functional theory and molecular orbital ab initio calculations, using as a reference the ethyl saturated analogues. The acidity of the free systems steadily increases down the group for the three series of derivatives, ethyl, vinyl and ethynyl. The association with both beryllium dihydride and borane leads to a very significant acidity enhancement, being larger for BeH2 than for BH3 complexes. This acidity enhancement, for the unsaturated compounds, is accompanied by a change in the acidity trends down the group, which do not steadily decrease but present a minimum value for both the vinyl- and the ethynyl-phosphine. When the molecule acting as the Lewis acid is beryllium dihydride, the π-type complexes in which the BeH2 molecules interact with the double or triple bond are found, in some cases, to be more stable, in terms of free energies, than the conventional complexes in which the attachment takes place at the heteroatom, X. The most important finding, however, is that P, As, and Sb ethynyl complexes with BeH2 do not behave as P, As, or Sb Brønsted acids, but unexpectedly as Be acids

A RRKM study and a DFT assessment on gas-phase fragmentation of formamide–M2+ (M = Ca, Sr)

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Post Transition State Dynamics in Gas Phase Reactivity : The Importance of Bifurcations and Rotational Activation.

International audienceBeyond the established use of thermodynamic vs kinetic control to explain chemical reaction selectivity, the concept of bifurcations on a potential energy surface (PES) is proving to be of pivotal importance with regard to selectivity. In this article, we studied by means of post-transition state (TS) direct dynamics simulations the effect that vibrational and rotational excitation at the TS may have on selectivity on a bifurcating PES. With this aim, we studied the post-TS unimolecular reactivity of the [Ca(formamide)]2+ ion, for which Coulomb explosion and neutral loss reactions compete. The PES exhibits different kinds of nonintrinsic reaction coordinate (IRC) dynamics, among them PES bifurcations, which direct the trajectories to multiple reaction paths after passing the TS. Direct dynamics simulations were used to distinguish between the bifurcation non-IRC dynamics and non-IRC dynamics arising from atomistic motions directing the trajectories away from the IRC. Overall, we corroborated the idea that kinetic selectivity often does not reduce to a simple choice between paths with different barrier heights and instead dynamical behavior after passing the TS may be crucial. Importantly, rotational excitation may play a pivotal role on the reaction selectivity favoring nonthermodynamic products

Unimolecular Fragmentation of Deprotonated Diproline [Pro₂-H]⁻ Studied by Chemical Dynamics Simulations and IRMPD Spectroscopy

International audienceDissociation chemistry of the diproline anion [Pro2-H]− is studied using chemical dynamics simulations coupled with quantum-chemical calculations and RRKM analysis. Pro2– is chosen due to its reduced size and the small number of sites where deprotonation can take place. The mechanisms leading to the two dominant collision-induced dissociation (CID) product ions are elucidated. Trajectories from a variety of isomers of [Pro2-H]− were followed in order to sample a larger range of possible reactivity. While different mechanisms yielding y1– product ions are proposed, there is only one mechanism yielding the b2– ion. This mechanism leads to formation of a b2– fragment with a diketopiperazine structure. The sole formation of a diketopiperazine b2 sequence ion is experimentally confirmed by infrared ion spectroscopy of the fragment anion. Furthermore, collisional and internal energy activation simulations are used in parallel to identify the different dynamical aspects of the observed reactivity

Unimolecular dissociation of peptides: statistical vs. non-statistical fragmentation mechanisms and time scales

International audienceIn the present work we have investigated mechanisms of gas phase unimolecular dissociation of a relatively simple dipeptide, the di-proline anion, by means of chemical dynamics simulations, using the PM3 semi-empirical Hamiltonian. In particular, we have considered two activation processes that are representative limits of what occurs in collision induced dissociation experiments: (i) thermal activation, corresponding to several low energy collisions, in which the system is prepared with a microcanonical distribution of energy; (ii) collisional activation where a single shock of hundreds of kcal mol−1 (300 kcal mol−1 in the present case) can transfer sufficient energy to allow dissociation. From these two activation processes we obtained different product abundances, and for one particular fragmentation pathway a clear mechanistic difference for the two activation processes. This mechanism corresponds to the leaving of an OH− group and subsequent formation of water by taking a proton from the remaining molecule. This last reaction is always observed in thermal activation while in collisional activation it is less favoured and the formation of OH− as a final product is observed. More importantly, we show that while in thermal activation unimolecular dissociation follows exponential decay, in collision activation the initial population decays with non-exponential behaviour. Finally, from the thermal activation simulations it was possible to obtain rate constants as a function of temperature that show Arrhenius behaviour. Thus activation energies have also been extracted from these simulations

Some interesting features of non-covalent interactions

Interactions between closed-shell systems exhibit some common features, four of which are particularly strong for beryllium bonds: geometrical distortion, cooperativity, changes in intrinsic reactivity and changes in the magnetic properties of the interacting subunits, which reflect the perturbations of their electron densities through polarization effects. Structural changes lead to interaction energies that can only be adequately accounted for when the effects of the distortion on the intrinsic reactivity of the system, and not only its deformation energy, are taken into consideration. Self-assembling of ditopic systems may lead to n-mers stabilized by strong cooperative effects. Chemical shifts and coupling constants also reflect the perturbations of the electron density and accordingly cooperative effects. These four features are common to any interaction involving two closed-shell systems, one acting as Lewis acid and the other as Lewis base, and the only difference between the nature of the interactions is quantitative.Peer Reviewe

Chemical dynamics simulations of CID of peptide ions: comparisons between TIK(H⁺)₂ and TLK(H⁺)₂ fragmentation dynamics, and with thermal simulations

International audienceGas phase unimolecular fragmentation of the two model doubly protonated tripeptides threonine–isoleucine–lysine (TIK) and threonine–leucine–lysine (TLK) is studied using chemical dynamics simulations. Attention is focused on different aspects of collision induced dissociation (CID): fragmentation pathways, energy transfer, theoretical mass spectra, fragmentation mechanisms, and the possibility of distinguishing isoleucine (I) and leucine (L). Furthermore, discussion is given regarding the differences between single collision CID activation, which results from a localized impact between the ions and a colliding molecule N2, and previous thermal activation simulation results; Z. Homayoon, S. Pratihar, E. Dratz, R. Snider, R. Spezia, G. L. Barnes, V. Macaluso, A. Martin-Somer and W. L. Hase, J. Phys. Chem. A, 2016, 120, 8211–8227. Upon thermal activation unimolecular fragmentation is statistical and in accord with RRKM unimolecular rate theory. Simulations show that in collisional activation some non-statistical fragmentation occurs, including shattering, which is not present when the ions dissociate statistically. Products formed by non-statistical shattering mechanisms may be related to characteristic mass spectrometry peaks which distinguish the two isomers I and L

Can an amine be a stronger acid than a carboxylic acid? The surprisingly high acidity of amine-borane complexes.

International audienceThe gas-phase acidity of a series of amine-borane complexes has been investigated through the use of electrospray mass spectrometry (ESI-MS), with the application of the extended Cooks kinetic method, and high-level G4 ab initio calculations. The most significant finding is that typical nitrogen bases, such as aniline, react with BH(3) to give amine-borane complexes, which, in the gas phase, have acidities as high as those of either phosphoric, oxalic, or salicylic acid; their acidity is higher than many carboxylic acids, such as formic, acetic, and propanoic acid. Indeed the complexation of different amines with BH(3) leads to a substantial increase (from 167 to 195 kJ mol(-1)) in the intrinsic acidity of the system; in terms of ionization constants, this increase implies an increase as large as fifteen orders of magnitude. Interestingly, this increase in acidity is almost twice as large as that observed for the corresponding phosphine-borane analogues. The agreement between the experimental and the G4-based calculated values is excellent. The analysis of the electron-density rearrangements of the amine and the borane moieties indicates that the dative bond is significantly stronger in the N-deprotonated anion than in the corresponding neutral amine-borane complex, because the deprotonated amine is a much better electron donor than the neutral amine. On the top of that, the newly created lone pair on the nitrogen atom in the deprotonated species, conjugates with the BN bonding pair. The dispersion of the extra electron density into the BH(3) group also contributes to the increased stability of the deprotonated species

BODIPY as electron withdrawing group for the activation of double bonds in asymmetric cycloaddition reactions

In this work we have found that a BODIPY can be used as an electron withdrawing group for the activation of double bonds in asymmetric catalysis. The synthesis of cyclohexyl derivatives containing a BODIPY unit can easily be achieved via trienamine catalysis. This allows a new different asymmetric synthesis of BODIPY derivatives and opens the door to future transformation of this useful fluorophore. In addition, the Quantum Chemistry calculations and mechanistic studies provide insights into the role of BODIPY as an EWGSpanish Government (CTQ2015-64561-R, CTQ2016-76061-P), CONACYT (project supported by the Fondo Sectorial de Investigación para la Educación) and PRODEP (Mexico) are acknowledged. We acknowledge allocation of computing time at the CCC-UAM. A. G. C. thanks MINECO (FPI) and T. J. P. CONACYT for PhD fellowships, respectively. A. M. S. thanks CAM for a postdoctoral contract (2016-T2/IND-1660). The authors wish to thank ''Comunidad de Madrid'' for its support to the FotoArt-CM Project (S2018/NMT-4367) through the Program of R&D activities between research groups in Technologies 2013, co-financed by European Structural Fund