engleski

An AM1 Semiempirical Study of Host-Guest Complexation in Hemicarcerand Complexes

Semiempirical AM1 calculations have been carried out on host/guest complexes of model hemicarcerands with methyl feet. The justification for the choice of the AM1 Hamiltonian was based on a comparison between reported X-ray data for the smaller tetrabromocavitand and computational results obtained using several different Hamiltonians. The complexation behaviour of hemicarcerands have been compared with experimental results reported by Cram et al. on hemicarcerands with phenethyl feet. Based on this comparison, a criterion for predicting guest encapsulation was developed, Ecomplexation, which relies on the calculation of AM1 heats of formation for host, guest and hemicarceplex. If Ecomplexation is lower than 10 kcal/mol, then a guest will be encapsulated, while if it is greater than 20 kcal/mol, a guest will not be encapsulated. Structural comparison was also made for the pyrazine carceplex where the AM1 host geometry was found to be in excellent agreement with reported X-ray data. For larger hosts (natoms > 200) with long, flexible bridges, as is the case for hemicarcerand, Ecomplexation becomes of little use as a predictor of host-guest complexation. The use of constrained-path AM1 optimizations to determine the energy barriers to guest entry and exit from the host was found to be a far more discriminating tool for predicting suitable host/guest combinations especially when the Ecomplexation criteria does not hold. We have computed the barriers to exit of N, N-dimethylformamide (dmf) and furan from the hemicarcerand, the former has been compared with experiment and shows excellent agreement Based on the success of the above methods in predicting which hosts-guest combinations will form stable hemicarceplexes we have synthesized a new target hemicarceplex with furan.

semiempirical calculations ; host-guest chemistry ; modelling ; complexes ; supramolecular

nije evidentirano