Enhanced Basicity of Push-Pull Nitrogen Bases in the Gas Phase

Nitrogen bases containing one or more pushing amino-group(s) directly linked to a pulling cyano, imino, or phosphoimino group, as well as those in which the pushing and pulling moieties are separated by a conjugated spacer (C═X)_n, where X is CH or N, display an exceptionally strong basicity. The n-π conjugation between the pushing and pulling groups in such systems lowers the basicity of the pushing amino-group(s) and increases the basicity of the pulling cyano, imino, or phosphoimino group. In the gas phase, most of the so-called push-pull nitrogen bases exhibit a very high basicity. This paper presents an analysis of the exceptional gas-phase basicity, mostly in terms of experimental data, in relation with structure and conjugation of various subfamilies of push-pull nitrogen bases: nitriles, azoles, azines, amidines, guanidines, vinamidines, biguanides, and phosphazenes. The strong basicity of biomolecules containing a push-pull nitrogen substructure, such as bioamines, amino acids, and peptides containing push-pull side chains, nucleobases, and their nucleosides and nucleotides, is also analyzed. Progress and perspectives of experimental determinations of GBs and PAs of highly basic compounds, termed as "superbases", are presented and benchmarked on the basis of theoretical calculations on existing or hypothetical molecules.

The vibrational spectra of the boron halides and their molecular complexes Part 10. The complexes of boron trifluoride with ammonia and its methyl derivatives. An ab initio study

Ab initio calculations, at the level of second order Møller-Plesset perturbation theory, and using a triple-zeta Gaussian basis set with polarization and diffuse functions on all atoms, have been carried out on the donor-acceptor complexes of boron trifluoride with ammonia and its mono-, di- and trimethyl derivatives. The structures, interaction energies and vibrational spectra of the complexes have been determined. An eclipsed and a staggered conformer have been examined for each complex, and the preferred conformer was found to be the staggered species in each case. The computed data have been compared with those for some similar complexes containing boron trifluoride and a series of oxygen and sulphur electron donors (water, hydrogen sulphide, methanol, methanethiol, dimethyl ether and dimethyl sulphide) and the effect of successive methyl substitution in all three series has been investigated.

Thermochemical aspects of proton transfer in the gas phase

The beginning of the twentieth century saw the development of new theories of acidity and basicity, which are currently well accepted. The thermochemistry of proton transfer in the absence of solvent attracted much interest during this period, because of the fundamental importance of the process. Nevertheless, before the 1950s, few data were available, either from lattice energy evaluations or from calculations using the emerging molecular orbital theory. Advances in mass spectrometry during the last 40 years allowed studies of numerous systems with better accuracy. Thousands of accurate gas-phase acidities or basicities are now available, for simple atomic and molecular systems and for large biomolecules. The intrinsic effect of structure on the Brønsted basic or acidic properties of molecules and the influence of solvents have been unravelled. In this tutorial, the basics of the thermodynamic principles involved are given, and the mass spectrometric techniques are briefly reviewed. Advances in the design and measurements of gas-phase superacids and superbases are described. Recent studies concerning biomolecules are also evoked.