Is allylphosphine a carbon or a phosphorus base in the gas phase?

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Eur. J. Mass Spectrom. 9, 245 - 255 (2003)
DOI: 10.1255/ejms.558

Is allylphosphine a carbon or a phosphorus base in the gas phase?

Maria del Carmen Sicilia,^a Otilia Mó,^a Manuel Yáñez,^a Jean-Claude Guillemin,^b Jean-François Gal^c and Pierre-Charles Maria^c
^aDepartamento de Química, C-9. Universidad Autónoma de Madrid. Cantoblanco, 28049-Madrid, Spain
^bLaboratoire de Synthèse et Activation de Biomolécules, UMR-6052 CNRS, ENSCR, 35700 Rennes, France
^cChimie des Matériaux Organiques et Métalliques, Université de Nice-Sophia Antipolis, Parc Valrose, 06108 Nice Cedex 2, France

ABSTRACT:

The gas-phase basicity of allylphosphine (2-propenylphosphine) was measured by means of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry techniques. A complete survey of the allylphosphine-H⁺ potential energy surface, carried out through the use of high-level G2(MP2) and B3LYP/6-311+G(3df,2p) calculations, allows us to conclude that, under low-pressure, low-energy ICR conditions, the interaction between the protonated reference base, B_refH⁺, and allylphosphine leads to a complex in which B_refH⁺ attaches to the phosphorus atom of allylphosphine, where the electrostatic potential is strongly attractive. Hence, in the first step only the phosphorus protonated species should be formed. Its isomerization to yield the C_β-protonated form, which is the global minimum of the potential energy surface, implies a very high activation barrier that cannot be overtaken under normal experimental ICR conditions. Therefore, the main conclusion of our study is that allylphosphine behaves as a phosphorus base in the gas phase, even though the C_β-protonated structure is the most stable protonated species. We have also shown that both C_β- and C_γ-protonation triggers a cyclization of the system. An analysis of the bonding of the different protonated species as compared with that of the neutral system is presented.

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