The enthalpies of combustion, heat capacities, enthalpies of sublimation and enthalpies of formation of 2-methylbenzimidazole (2MeBIM) and 2-ethylbenzimidazole (2EtBIM) are reported and the results compared with those of benzimidazole itself (BIM). Theoretical estimates of the enthalpies of formation were obtained through the use of atom equivalent schemes. The necessary energies were obtained in single-point calculations at the B3LYP/6-311+G(d,p) on B3LYP/6-31G* optimized geometries. The comparison of experimental and calculated values of benzenes, imidazoles and benzimidazoles bearing H (unsubstituted), methyl and ethyl groups shows remarkable homogeneity. The energetic group contribution transferability is not followed, but either using it or adding an empirical interaction term, it is possible to generate an enormous collection of reasonably accurate data for different substituted heterocycles (pyrazole-derivatives, pyridine-derivatives, etc.) from the large amount of values available for substituted benzenes and those of the parent (pyrazole, pyridine) heterocycles.

We acknowledge the financial support of the DGI/MCyT (project nos. BQU-2003-00976, 01251 and 05827). This work has been partially supported by the DGI project no. BQU-2003-00894. A generous allocation of computational time at the CCC of the Universidad Auto´noma de Madrid is also gratefully acknowledged. Thanks are also due to Instituto de Cooperac¸a˜o Cientı´fica e Tecnolo´gica Internacional (ICCTI), Lisbon, Portugal, and Consejo Superior de Investigaciones Cientı´ficas (CSIC), Madrid, Spain, for a joint research project ICCTI/CSIC. MLPFA thanks Fundac¸a˜o para a Cieˆncia e Tecnologia (FCT), Lisbon, Portugal, for the award of a postdoctoral fellowship (PRAXIS XXI/BPD/16319/98) and MT thanks MECD/SEEU (AP 2002-0603), Spain, for financial support.