Gas phase dissociation energies of saturated AHn*+ radical cations and AHn neutrals (A = Li-F,Na-Cl): dehydrogenation,deprotonation, and formation of AHn-2*+ - H2 complexes

The dissociation energies corresponding to the two possible A-H cleavages of A (A = Li-F and Na-Cl) radical cations (loss of a H(+) and loss of a H(.)) have been computed at the CCSD(T)/ 6-311++G(3df,2pd) level of theory and compared to those of their neutral precursors. Removing an electron from AH(n)() decreases dramatically its deprotonation energy, especially for the A molecules (C and ), which become one of the most acidic species of the row, their acid character being only exceeded by FH(.+) and ClH(.+), respectively. However, dehydrogenation energies only decrease for the systems on the left side of the row (up to C and SiH(4)(.+)) for which the electron is removed from a A-H bonding orbital. Nevertheless, the loss of hydrogen is the more favorable cleavage in all cases except FH(.+). Ionization of SiH(4) leads to a Jahn-Teller distorted structure that corresponds to a Si - H(2) complex. Other - eta(2)H(2) complexes in the doublet spin state have also been found to be stable for A = Be, Mg, Al, and P, the hydrogen molecule complexes being more stable than their corresponding radical cations, for Be, Mg, and Al.     

Adsorption of NH(3) and H(2)O in acidic chabazite. Comparison of ONIOM approach with periodic calculations

The adsorption of NH(3) and H(2)O in acidic chabazite has been studied with the B3LYP method within the cluster approach (5T, 48T clusters) and the periodic approach adopting a Si/Al = 11/1 chabazite and a basis set of polarized double-zeta quality. The 5T cluster has been treated fully ab initio at the B3LYP level whereas the 48T cluster has been treated with the ONIOM2 scheme using B3LYP as the high level of theory and the MNDO, AM1, and HF/3-21G methods as low levels of theory. Periodic calculations show that the adsorption of NH(3) in acidic chabazite takes place through an ion pair (NH(4)(+)-CHA(-)) structure, the computed adsorption energy being -32 kcal/mol. The adsorption of H(2)O leads to a hydrogen bonded (H(2)O-HCHA) complex with the computed adsorption energy of -20 kcal/mol. All ONIOM combinations provide similar structures to those obtained with periodic calculations. Adsorption energies, however, are sensitive to the low level used, especially for NH(3). The ONIOM B3LYP:HF/3-21G method is the one that provides more satisfactory results. Present results show that, for larger zeolites, the ONIOM scheme can be successfully applied while drastically reducing the cost of a fully ab initio treatment.     

Gas-phase proton-transport self-catalysed isomerisation of glutamine radical cation: The important role of the side-chain

The gas-phase isomerisation reaction of glutamine radical cation from [NH₂CH (CH₂CH₂CONH₂) COOH ]^+• to [ NH₂C (CH₂CH₂CONH₂) C (OH)₂]^+• has been studied theoretically using the MPWB1K functional approach. The [ NH₂ C (CH₂CH₂CONH₂) C (OH)₂]^+• diol species has been found to be the most stable isomer for glutamine radical cation. Moreover, it has been observed that glutamine has a long enough side-chain with basic groups that acts as a solvent molecule favouring the proton-transfer from C _α to COOH group. This fact reduces dramatically the isomerisation energy barriers compared to the same process for glycine radical cation in gas phase. Thus, this reaction can be considered as an example of gas-phase proton-transport catalysed reaction in which the proton-transport is carried out by the reactant molecule itself instead of any solvent. Contribution to the Serafin Fraga Memorial Issue.     

Aluminosilicate surfaces as promoters for peptide bond formation: an assessment of Bernal's hypothesis by ab initio methods

The role in prebiotic chemistry that Br?nsted and Lewis sites, both present at the surface of common aluminosilicates, may have played in favoring the peptide bond formation has been addressed by ab initio methods within a cluster approach. B3LYP/6-31+G(d,p) free energy potential energy surfaces have been fully characterized for the model reaction glycine + NH3 --> 2-NH2 acetamide (mimicking the true 2 Gly --> GlyGly one) occurring on (i) a Lewis site, (ii) a Br?nsted site, and (iii) a combined action of Lewis/Br?nsted sites. Compared to the gas-phase (gp) activation free energy of 50 kcal/mol, the Lewis site alone reduces the gp barrier to 41 kcal/mol, whereas the activation by the Br?nsted site dramatically reduces the barrier to about 18 kcal/mol. Nevertheless, formation of the prereactant complex in this latter case will rarely occur, since water will easily displace the glycine molecule interacting with the Br?nsted site. However, if a realistic feldspar surface with neighboring Br?nsted and Lewis sites is considered, the proper prereactant complex is highly stabilized by a simultaneous interaction with the Lewis and the Br?nsted sites, in such a way that the Lewis site strongly attaches the glycine molecule to the surface whereas the Br?nsted site efficiently catalyzes the condensation reaction, showing that the interplay between Lewis/Br?nsted sites is an important issue. The free energy barrier computed for the realistic feldspar surface model is 26 kcal/mol. The role of dispersive interactions on the free energy barrier and the stabilization of the final product, not accounted for by the B3LYP functional, have been estimated and shown to be substantial. Speculations about further elongation of the formed dipeptide have been put forward on the basis of the relatively strong interaction energy of the formed GlyGly dipeptide with the aluminosilicate surface.