Mt. Nguyen et al., Necessity to consider a three-water chain in modelling the hydration of ketene imines and carbodiimides, J CHEM S P2, (4), 1999, pp. 813-820
Citations number
24
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2
A theoretical study of the hydration of a model ketene imine (R2C--C=NH) an
d carbodiimide (RN=C=NR) has been undertaken. The detailed hydration mechan
ism of the simplest cumulenes by water and water dusters (HX=C=NH + n H2O--
>H2XCONH2 + (n - 1) H2O, n = 1, 2, 3 and X = CH, N) was modelled using high
-level ab initio MO methods. Geometric and energetic parameters were determ
ined for two possible reaction channels involving water attack across both
C=C and C=N bonds of ketene imine. Using one and two actively participating
water molecules to model the hydration, calculated results consistently sh
ow that the C=N addition, giving first an amide enol, is favoured over the
C-C yielding immediately the amide product. A reverse situation occurs when
a chain of three water molecules is used. Since attack in two different pl
anes is possible in the latter case, reducing the unfavourable distortion o
f the methylene group, the C-C addition becomes easier to perform than the
C=N, with an energy barrier of 48 kJ mol(-1) found at the CCSD(T)/6-31G(d,p
) level, the lowest barrier of all the calculated water chain models. These
findings are consistent with experimental evidence for direct formation,of
C=C products in non-hindered ketene imines. Thus, water oligomers higher t
han the dimer seem to make a primordial contribution to the rate of the hyd
ration and are really needed to perform a concerted reaction. These gas-pha
se results are confirmed when the effect of the solvent bulk is:taken into
account in PCM calculations. Hydration pf the analogous carbodiimide, in wh
ich addition can only occur across the C=N:bond, was also studied. The C=N
addition with the aid of a three-water cluster is rate-determining followed
by a tautomerization of the primary adduct leading to urea. Carbodiimide h
ydration turns out to be easier to achieve than ketenimine hydration.