How can f-block mono-cations behave as Mono-Cations of d-block transition metals ?

International audienceThe electronic structures of LnNH + is studied by DFT (B3LYP) quantum calculation for the Ln = La, Eu and Gd 4f-block elements (lanthanides). Ln ≡N triple bonds of essentially d-character are formed for La and Gd explaining why La + and Gd + behave like d-block elements as experimentally evidenced by mass spectrometry, and why the Ln + reactivity is correlated with its electron-promotion energy: the present theoretical study is a support to such correlation and qualitative knowledge. The Ln + + NH 3 → LnNH 3 + → transition state → HLn=NH 2 + → transition state → Ln≡NH + + H 2 reaction pathway is calculated. The formation of HLn=NH 2 + corresponds to the formation of new covalent bonds associated with more electron pairing, and corresponding lowering of the spin multiplicity-spin crossing reaction. It is in this step that low electron-promotion energy is required to promote an Ln 4f electron onto an Ln 5d orbital as typically for La + and Gd +. Similar geometry, bonding and electronic cofiguration are calculated for NpNH +-an actinide complex observed by mass spectrometry-with higher participation of 5f-valence orbitals (20% and 25% for the σ and π bonds) as compared to the 4f-valence orbitals (3% and 8%) of GdNH + : Gd + and Np + are the only lanthanide and actinide cations with two non-f-valence electrons-one s and one din their ground states. ____________ [a

Experimental and computational study of the energetics of hydantoin and 2-thiohydantoin

This work reports an experimental and a theoretical study of two imidazolidine derivatives, hydantoin (CAS No. 461-72-3) and 2-thiohydantoin (CAS No. 503-87-7). The standard (p degrees = 0.1 MPa) molar energies of combustion of hydantoin and 2-thiohydantoin were measured by static and rotating bomb combustion calorimetry, respectively. The standard molar enthalpies of sublimation, at T = 298.15 K, were derived from the temperature dependence of the vapour pressures of these compounds, measured by the Knudsen-effusion technique, and from high temperature Calvet microcalorimetry. The conjugation of these experimental results enables the calculation of the standard molar enthalpies of formation in the gaseous state, at T = 298.15 K, which are discussed in terms of structural contributions. We have also estimated the gas-phase enthalpy of formation from high-level ab initio molecular orbital calculations at the G3MP2B3 level of theory, being the computed values in good agreement with the experimental ones. Furthermore, this composite approach was also used to obtain information about the gas-phase basicities, proton and electron affinities and adiabatic ionization enthalpies

Reactivity of Lanthanoid Mono-Cations with Ammonia : a Combined Inductively Coupled Plasma Mass Spectrometry and Computational Investigation

International audienceThe behavior of La+, Sm+, Eu+ and Gd+ with NH3(g) and ND3(g) was studied to understand gas phase chemical reactions used for separations in the reaction cell of a quadrupole inductively coupled plasma-mass spectrometer (ICP-MS). For Ln+ = La+ and Gd+, the primary reaction channel is the formation of the LnNH+ protonated nitride leading to H2 elimination. The LnNH(NH3)1-5+ ammonia complexes of the Ln protonated nitride are further generated. Sm+ and Eu+ are less reactive: the protonated nitride is not detected, and only small amounts of Ln(NH3)0-6+ are observed. Quantum chemical calculations at the DFT, MP2, CCSD(T) and CASPT2 levels of theory were employed to explore the potential energy surfaces. For the La+ and Gd+ ions of f-block elements, the reaction pathways are composed of three steps: first the formation of LnNH3+, then the isomerization to HLnNH2+, and finally the loss of H2 associated with the formation of an Lnsingle bondN triple bond in the final product LnNH+. On the other hand, the isomerization leading to triple bond formation with H2 loss did not proceed for Sm+ and Eu+ ions

Molecular Dynamics and Room Temperature Vibrational Properties of Deprotonated Phosphorylated Serine

International audienceThe local structure of phosphorylated residues in peptides and proteins may have a decisive role on their functional properties. Recent IRMPD experiments have started to provide spectroscopic signatures of such structural details; however, a proper modeling of these signatures beyond the harmonic approximation, taking into account temperature and entropic effects, is still lacking. In order to bridge this gap, DFT-based Car-Parrinello molecular dynamics simulations have been carried out for the first time on a phosphorylated amino acid, gaseous deprotonated phosphoserine. It is found that all vibrational signatures are successfully reproduced, and new deconvolution techniques enable the assignment of the vibrational spectrum directly from the dynamics results and the comparison of vibrational modes at several temperatures. The lowest energy structure is found to involve a strong hydrogen bond between the deprotonated phosphate and the acid with relatively small free energy barriers to proton transfer; however, we find that proton shuttling between the two sites does not occur frequently. Anharmonicities turn out to be important to reproduce the frequencies and shapes of several experimental bands. Comparison of room temperature and 13 K, effectively harmonic dynamics, allows insight to be obtained into vibrational anharmonicities. In particular, a significant blue-shift and broadening of the C=O stretching frequency from 13 to 300 K can be ascribed to intrinsic anharmonicity rather than to anharmonic coupling to other modes. On the other hand, significant couplings are found for the stretching motions of the hydrogen bonded P-O bond and of the free P-OH bond, mainly with modes within the phosphate group

Computational study of peptide bond formation in the gas phase through ion–molecule reactions

International audienceA computational study of peptide bond formation from gas-phase ion-molecule reactions has been carried out. We have considered the reaction between protonated glycine and neutral glycine, as well as the reaction between two neutral glycine molecules for comparison purposes. Two different mechanisms, concerted and stepwise, were studied. Both mechanisms show significant energy barriers for the neutral reaction. The energy requirements for peptide bond formation are considerably reduced upon protonation of one of the glycine molecules. For the reaction between neutral glycine and N-protonated glycine the lowest energy barrier is observed for the concerted mechanism. For the reaction between neutral glycine and protonated glycine at carbonyl oxygen, the preferred mechanism is the stepwise one, with a relatively small energy barrier (23 kJ mol -1 at 0 K) and leading to the lowest-lying protonated glycylglycine isomer. In the case that the reaction could be initiated by protonated glycine at hydroxyl oxygen the process would be barrier-free and clearly exothermic. In that case peptide bond formation could take place even under interstellar conditions if glycine is present in space