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The strength of the "second bond" in H3P=CH2 has been obtained in different ways at the MP4/6-3 1 1G**/ /MP2/6-31 lG**+ZPE level of theory. The calculated bond strengths of 33.7 and 38.4 k c d m o l are comparable to those obtained for the A3-P=C bond (38-49 kcaymol). The similarity in bond strengths could explain the similar bond lengths of the two systems. The rotational barrier about the P-C bond in H3P=CH2 obtained at the QCISD/6-31 I+G**+ZPE level of theory is 0.9 kcaymol. The low barrier is, however, not necessarily an indication of a weak double bond but is probably the consequence of the similar bond strength of the "second bond" for both rotational conformers. The bonding MO-model evaluated here for the description of the less stable conformer can be derived from the n-MO of the A3-P=C bond; the resulting electronic structure is similar to an allylic n-system. Topological electron density analysis has indeed shown that the electron density and ellipticity at the bond critical point of the somewhat less stable conformer can be related to those of HP=CH;, and similarly to those of allyl ion and ethane.

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