The protonation and hydration of carbon suboxide (O=C=C=C=O) were studied b
y ab initio molecular orbital methods. While the geometries of the stationa
ry points were optimized using MP2/6-31G(d,p) calculations, relative energi
es were estimated using QCISD(T)/6-31G(d,p) and 6-311++G(d,p)+ZPE. The beha
viour of carbon suboxide was compared with that of carbon dioxide and keten
e. The protonation at the beta-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(-1) (experimental: 817 +/- 3 kJ mo
l(-1)). The PAs at oxygen amount to 654, 641 and 542 kJ mol(-1) (experiment
al: 548 kJ mol(-1)) for C3O2, H2CCO and CO2, respectively. Using the approa
ch of one and two water molecules to model the hydration reaction, the calc
ulated 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 i
s about 31 kJ mol(-1) smaller than that of C=O addition. The C=C addition i
n C3O2 is inherently favoured owing to a smaller energetic cost for the mol
ecular distortion at the transition state, and a higher thermodynamic stabi
lity of the acid product. Molecular deformation of carbon suboxide is in fa
ct a fairly facile process. A similar trend was observed for the addition o
f H-2, 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 rea
ction mechanisms could be rationalized in terms of Fukui functions for both
nucleophilic and electrophilic attacks, Copyright (C) 2000 John Wiley & So
ns, Ltd.