Marcela Hurtado

The ever-surprising chemistry of boron: enhanced acidity of phosphine.boranes.

The acidity-enhancing effect of BH(3) in gas-phase phosphineboranes compared to the corresponding free phosphines is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. Thus, the enhancement of the acidity of protic acids by Lewis acids usually observed in solution is also observed in the gas phase. For example, the gas-phase acidities (GA) of MePH(2) and MePH(2)BH(3) differ by about 118 kJ mol(-1) (see picture).The gas-phase acidity of a series of phosphines and their corresponding phosphineborane derivatives was measured by FT-ICR techniques. BH(3) attachment leads to a substantial increase of the intrinsic acidity of the system (from 80 to 110 kJ mol(-1)). This acidity-enhancing effect of BH(3) is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. This indicates that the enhancement of the acidity of protic acids by Lewis acids usually observed in solution also occurs in the gas phase. High-level DFT calculations reveal that this acidity enhancement is essentially due to stronger stabilization of the anion with respect to the neutral species on BH(3) association, due to a stronger electron donor ability of P in the anion and better dispersion of the negative charge in the system when the BH(3) group is present. Our study also shows that deprotonation of ClCH(2)PH(2) and ClCH(2)PH(2)BH(3) is followed by chloride departure. For the latter compound deprotonation at the BH(3) group is found to be more favorable than PH(2) deprotonation, and the subsequent loss of Cl(-) is kinetically favored with respect to loss of Cl(-) in a typical S(N)2 process. Hence, ClCH(2)PH(2)BH(3) is the only phosphineborane adduct included in this study which behaves as a boron acid rather than as a phosphorus acid.

Gas‐Phase Interactions between Lead(II) Ions and Cytosine: Tandem Mass Spectrometry and Infrared Multiple‐Photon Dissociation Spectroscopy Study

Gas-phase interactions between Pb(2+) ions and cytosine (C) were studied by combining tandem mass spectrometry, infrared multiple photon dissociation spectroscopy, and density functional theory (DFT) calculations. Both singly and doubly charged complexes were generated by electrospray. The [Pb(C)-H](+) complex was extensively studied, and this study shows that two structures, involving the interaction of the metal with the deprotonated canonical keto-amino tautomer of cytosine, are generated in the gas phase; the prominent structure is the bidentate form involving both the N1 and O2 electronegative centers. The DFT study also points out a significant charge transfer from the nucleobase to the low-lying p orbitals of the metal and a strong polarization of the base upon complexation. The various potential energy surfaces explored to account for the fragmentation observed are consistent with the high abundance of the [PbNH2](+) fragment ion.

On the origin of the enhanced acidity of chalcocyclopentadienes (cyclopentadiene chalcogenols) in the gas phase.

The intrinsic acidity of chalcocyclopentadienes (CpXH; X=O, S, Se, Te) is investigated by high-level G3B3 and G2 ab initio as well as B3LYP DFT calculations, which show that, independent of the nature of the heteroatom, all chalcocyclopentadienes are stronger acids in the gas phase than cyclopentadiene. However the acidity does not increase regularly down the group, and the acidity enhancement for Te derivatives is five times larger than for O derivatives, but only twice that of S-containing compounds. The most favorable deprotonation process corresponds to loss of the proton attached to the heteroatom, with the sole exception of the 5-substituted 1,3-cyclopentadienes, for which the O and S derivatives are predicted to behave as carbon acids. No matter the nature of the heteroatom, the 1-substituted 1,3-cyclopentadienes are the strongest acids. The intrinsic acidity of all isomers, namely, 1-substituted, 2-substituted, and 5-substituted 1,3-cyclopentadienes, increases with increasing aromaticity of the anion formed on deprotonation, and therefore the Te compound is the strongest acid for the three series. However, the intrinsic acidity of chalcocyclopentadienes is not dictated by aromaticity, so that, in general, the most stable deprotonated species do not coincide with the most aromatic ones.

Stability trends and tautomerization of chalcocyclopentadienes. The role of aromaticity

The stability trends and tautomerization processes within the family of chalcocyclopentadienes (CpXH, X = O, S, Se, Te) have been investigated through the use of high-level G3B3 and G2 ab initio, as well as B3LYP density functional theory calculations. All methods predict the 5-substituted derivative to be the least stable tautomer, with the only exception of the Te derivative. For X = O, Se the dominant forms are the 2- and the 1-substituted derivatives, respectively, whereas for S, an equal distribution of the two forms is predicted. The enhanced stability of the 5-substituted compound on going from O to Te is due to a parallel increase of the aromaticity of the system, through a charge donation from the C–X bond toward the five membered ring. Independently of the nature of the chalcogen heteroatom, the global minimum of the PES is the asymmetric keto structure 2-cyclopenten-1-one. Its enhanced stability with respect to the symmetric keto form 3-cyclopenten-1-one is due to a more favorable conjugation between the CX and the CC π-systems. This interaction steadily increases on going from O to Te. The barriers connecting the keto and the enolic forms are rather high, as those connecting the enolic forms among each other. Hence, an important conclusion is that, in the gas-phase, if the 5-substituted derivative is formed, it should be observable no matter the nature of the chalcogen heteroatom.

The role of hyperconjugative π-aromaticity in the enhanced acidity of methyl-, silyl and germylcyclopentadienes

The relative stability of the different isomers of cyclopentadienyl derivatives CpXH3 (X = C, Si, Ge) and their intrinsic acidities have been investigated by means of B3LYP/6-311+G(3df,2p)//CCSD/6-311+G(d,p) density functional theory calculations. Whereas for the methylcyclopentadiene the 1- and 2- substituted isomers are almost equally stable and much more stable than the 5-substituted isomer, for the germyl derivatives the 5-substituted compound is the global minimum, due to the stabilization of the system through a hyperconjugative π-aromaticity effect, which is the larger the more electropositive the XH3 substituent is. As a consequence CpXH3 (X = Si, Ge) are more aromatic than cyclopentadiene. The silyl and germyl derivatives are more fluxional than the methyl derivative, the 1,2-XH3 shift activation barriers being around 60 kJ mol−1. For all the isomers, the most favourable deprotonation process corresponds to the loss of the proton attached to the sp3 carbon atom of the five membered ring. For Si and Ge containing compounds this behaviour differs from that observed for saturated and α,β-unsaturated compounds, which behave as Si or Ge acids in the gas phase. CpXH3 (X = C, Si, Ge) compounds are predicted to be stronger acids in the gas phase than the unsubstituted parent compound, due to a significant anionic hyperconjugation effect which reinforces the C–X bond upon deprotonation and favours the conjugation of the C–X π-bond with the π-system associated to the five membered ring.

Conformational analysis, NMR properties and nitrogen inversion of N-substituted 1,3-oxazines

The conformational preference of a set of selected N-substituted oxazines has been investigated at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. From the ΔG0 calculated values, it can be concluded that when the substituent is methyl, ethyl and propyl the axial conformation is preferred in the gas phase. When the substituent is isopropyl or tert-butyl the equatorial conformer is the most abundant in the gas phase. This situation does not change in solution provided the solvent has very low polarity as Cl4C, in good agreement with the experimental findings. However, when the polarity of the solvent increases, the stabilization of the equatorial conformer is quite significant, due to its large dipole moment, and it becomes the dominant form in CHCl3 and CH2Cl2solvents. Quite importantly, the percentage of axial conformer obtained from the values of the 1H chemical shift are in good agreement with the percentage obtained from the calculated free energy. The nitrogen inversion barrier of N-substituted 1,3-oxazines decreases with the size of the substituent. The largest substituent stabilizes the nitrogen lone-pair to the greatest extent in the transition state and is the one that least interacts with the ring when it becomes planar.