1430 J. Phys. Chem. A, Vol. 102, No. 8, 1998
Taha et al.
their trifluoroacetamide counterparts. The activation parameters,
in kcal mol-1, for N,N-dimethylformamide, N,N-diethylforma-
mide, and N,N-diisopropylformamide are 19.4,2419.6,12 and
19.0,12 respectively. We observe no difference in the activation
barriers between MPF and N,N-dimethylformamide and a minor
Although the stability and reactivity of liquid amides have
been extensively investigated,11,27,29 the conformational dynam-
ics of gaseous asymmetric systems have not. Solvents clearly
influence conformer stability and free energy differences. The
differences between gas- and liquid-phase ∆G°’s are significant.
The rotational barriers also change with phase, though these
effects have been well documented.12-14,24 The gas-phase
rotational barriers are consistently lower than in the liquid phase.
difference with MBTFA. ∆Gq for MIF is lower than the
298
activation barrier for N,N-diethylformamide and higher than that
for N,N-diisopropylformamide by 0.1 kcal mol-1, demonstrating
the weaker influence of substituent bulk on the barrier compared
to the trifluoroacetamide systems.
In conclusion, the conformational equilibria and the internal
rotation barriers of the asymmetric amides studied are primarily
influenced by the nature of the substituent on the carbonyl
carbon and by the medium. The conformational equilibria are
relatively insensitive to the nature of the alkyl substituent. The
rotational barriers, however, are influenced by the alkyl sub-
The effect of substituent electronegativity on amide internal
rotation barriers has been investigated in both the gas16 and
liquid phases.15,27-31 The transition state is more affected by
carbonyl substituent electronegativity effects because the car-
bonyl moiety in the transition state is more electrophilic than
in the ground state. In trifluoroacetamides, the repulsive
interaction between the positively charged trifluoro carbon and
the positively charged carbonyl carbon results in a destabilization
of the transition state, which should raise the rotational barrier.
Our results show the formamide internal rotation barriers are
substantially higher (≈2.5 kcal mol-1) than the corresponding
trifluoroacetamide barriers, indicating a much higher degree of
ground-state destabilization due to steric interactions in the
trifluoroacetamides. Close inspection reveals that the most
bulky substituent (the isopropyl group) serves to lower the liquid
internal rotational barriers to an even greater extent.
stituent, yielding smaller ∆Gq values with increased sub-
298
stituent bulk.
Acknowledgment. We are pleased to acknowledge the
National Science Foundation (CHE 93-21079) for support of
this research. We would also like to thank Robert K. Bohn for
the gift of the N-methyl-N-propylformamide.
References and Notes
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Table 2 shows that in the gas phase the population of the
syn conformer is ca. 55% for the formamides studied and ca.
30% for the trifluoroacetamides studied. The percentage is not
very sensitive to the length of the alkyl chain or whether the
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the alkyl substituent. The temperature-dependent population
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carbonyl group and the relatively nonpolar alkyl group to
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less available to solvent molecules. This effect is eliminated
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