Greet Raspoet

Contrasting mechanism of the hydration of carbon suboxide and ketene. A theoretical study

The protonation and hydration of carbon suboxide (O=C=C=C=O) were studied by ab initio molecular orbital methods. While the geometries of the stationary points were optimized using MP2/6-31G(d,p) calculations, relative energies were estimated using QCISD(T)/6-31G(d,p) and 6-311aaG(d,p)a ZPE. The behaviour of carbon suboxide was compared with that of carbon dioxide and ketene. The protonation at the b-carbon is consistently favoured over that at the oxygen; the proton affinities (PA) are estimated to be PA(C3O2) = 775 15 and PA(H2CCO) = 820 10 kJ mol ˇ1 (experimental: 817 3k J mol ˇ 1 ). The PAs at oxygen amount to 654, 641 and 542 kJ mol ˇ1 (experimental: 548 kJ mol ˇ1 ) for C3O2 ,H 2CCO and CO2, respectively. Using the approach of one and two water molecules to model the hydration reaction, the calculated results consistently show that the addition of water across the C=O bond of ketene, giving a 1,1-ethenediol intermediate, is favoured over the C=C addition giving directly a carboxylic acid. A reverse situation occurs in carbon suboxide. In the latter, the energy barrier of the C=C addition is about 31 kJ mol ˇ1 smaller than that of C=O addition. The C=C addition in C3O2 is inherently favoured owing to a smaller energetic cost for the molecular distortion at the transition state, and a higher thermodynamic stability of the acid product. Molecular deformation of carbon suboxide is in fact a fairly facile process. A similar trend was observed for the addition of H2, HF and HCl on C3O2. In all three cases, the C=C addition is favoured, HCl having the lowest energy barrier amongst them. These preferential reaction mechanisms could be rationalized in terms of Fukui functions for both nucleophilic and electrophilic attacks. Copyright

Mechanism of the Beckmann rearrangement in sulfuric acid solution

Ab initio calculations have been performed to probe the mechanism of the Beckmann rearrangement of formaldehyde oxime in concentrated sulfuric acid or in oleum solution (H2SO4 + S2O7 ). In the gas phase, the most favoured reaction path is: protonation of oxime → N-protonated oxime → O-protonated oxime → fragmentation products, in which the 1,2-H-shift connecting both protonated forms constitutes the rate-determining step. Reaction field calculations using two different models [Onsager self-consistent reaction field (SCRF) and polarizable continuum model (PCM)] indicate that the non-specific interaction of the solvent exerts only a small effect on both the energetic and geometrical parameters of the considered reaction path. Formation of the sulfate ester, H2CN–O–SO3H, also appears to play a negligible role in marginally affecting the 1,2-H-shift. In contrast, a specific interaction between solvent molecules and substrates seems to be the dominant factor in reducing substantially the energy barrier to 1,2-H-shift. Using a neutral H2SO4 molecule as a simple model for solvent molecules, MP2/6-311G(d,p) energy calculations based on HF/6-31G(d)-geometries of the supermolecule reveal that the barrier to 1,2-H-shift is decreased by 115 kJ mol-1 with respect to the gas phase value, when a H2SO4 molecule interacts specifically with the protonated oxime and thereby assists the hydrogen migration. The calculated results thus suggest a strong case of active solvent participation in which the solvent molecules exert a catalytic effect.

IMPORTANT ROLE OF THE BECKMANN REARRANGEMENT IN THE GAS-PHASE CHEMISTRY OF PROTONATED FORMALDEHYDE OXIMES AND THEIR [CH4NO](+) ISOMERS

The [CH4NO]+ potential energy surface has been (re)examined in detail using ab initio molecular orbital calculations. Geometries of the stationary points were optimized at the MP2/6-31 G(d,p) level. On the basis of MP4SDTQ/6-311 ++ G(2d,2p) electronic energies with zero-point corrections, the following heats of formation at 0 K could be proposed: ΔfHo0 in kJ mol–1: H2CNOH: 31 ± 12; H3C–NOH+: 903 ± 12; H2CNH–OH+: 759 ± 12; H2CN–OH2+: 838 ± 12; H3C–NHO+: 840 ± 12 and cyclic H2[graphic omitted]H+: 918 ± 12. The proton affinities could also be evaluated: EPA(H2CNOH)= 799 ± 12 kJ mol–1 and EPA(CH3–NO)= 763 ± 12 kJ mol–1. Energies of the transition structures of several unimolecular rearrangements and fragmentations obtained using MP4SDTQ/6-311 ++ G(d,p)+ ZPE calculations suggest that the following sequence of transformations is the most energetically favoured route: protonation of formaldehyde oxime →N-protonated oxime →O-protonated oxime → fragmentation products (HCN + H3O+). The 1,2-H-shift connecting both protonated forms constitutes the rate-controlling step. The classical Beckmann rearrangement of the O-protonated formaldehyde oxime is the most facile reaction of all the paths considered and should thus play an important role in the gas phase unimolecular chemistry of the [CH4NO]+ ion isomers. The CH4+ NO+ reaction has also been examined as a simple model for the electrophilic substitution of aliphatic hydrocarbons. While the insertion of NO+ into a C–H bond can be established, the evidence recently reported for preferential attack of NO+ on the carbon atom could not be confirmed.