M. Decouzon et al., STERIC EFFECTS IN CROWDED MOLECULES IN THE GAS-PHASE - POLYMETHYL-SUBSTITUTED BENZOIC-ACIDS, New journal of chemistry, 21(5), 1997, pp. 561-573
The gas-phase acidities of seven polymethyl benzoic acids have been me
asured by Fourier transform ion cyclotron resonance IFT-ICR) spectrome
try, and estimated from semiempirical AMI and PM3 calculations. Togeth
er with the derivatives investigated previously, complete data are now
available for all methyl-substituted benzoic acids. AMI calculations
reproduce reasonably well the relative enthalpies of formation provide
d ortho derivatives are considered separately, and can be used to dete
rmine conformations and approximate conformational equilibria. However
, only the trends in the gas-phase acidities are reproduced by the AMI
calculations. Empirical correlations have been carried out using two
approaches. The additivity scheme approach distributes the errors equa
lly among all derivatives but requires special correction terms that a
ccount for the steric crowding. The substituent effect approach starts
from monomethyl derivatives and attempts to separate polar and steric
effects. In a further step, the buttressing effect (BE) is defined as
the excess energy in dimethyl and polymethyl derivatives. Both the st
eric effect and BE have been obtained independently, either from entha
lpies of formation or from gas-phase acidities, with concordant result
s. The final conclusion is that the global steric effect of two substi
tuents is always larger than the sum calculated from effects in mono d
erivatives. Nevertheless, in more substituted compounds, this excess o
f steric energy tends to approach a limit.