Title
Substituent and ring effects on enthalpies of formation: 2-methyl- and 2-ethylbenzimidazoles versus benzene-and imidazole-derivatives
Date Issued
10 April 2004
Access level
metadata only access
Resource Type
journal article
Author(s)
Jiménez P.
Roux M.V.
Temprado M.
Da Silva M.A.V.R.
Da Silva M.D.D.M.C.R.
Amaral L.M.P.F.
Cabildo P.
Claramunt R.M.
Mó O.
Yáñez M.
Elguero J.
Consejo Superior de Investigaciones Científicas
Abstract
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 ΔfHmo(g) values available for substituted benzenes and those of the parent (pyrazole, pyridine) heterocycles.
Start page
711
End page
721
Volume
102
Issue
7
Language
English
OCDE Knowledge area
Química
Scopus EID
2-s2.0-3042819563
Source
Molecular Physics
ISSN of the container
00268976
Sponsor(s)
The main conclusions of the present study are the following ones: 2EtBIM (4c) shows a phase transition at 406.4 K and melts at 448.8 K (figure 1). As far as the enthalpies of formation are concerned, methyl-and ethyl-derivatives are more alike than unsubstituted ones. Although from H to CH3 and from CH3 to C2H5, there is the addition of one CH2 group, ·fHmoðgÞ decreases by −52:2 kJ mol−1 for the first step and −15:5 kJ mol−1 for the second one. It must be emphasized that, although these effects are similar to those found for the corresponding benzene and imidazole derivatives, ring and substituent effects are not independent and the energetic group contribution transferability (pure additivity) is not exactly followed. Most importantly, it could be possible to generate an enormous collection of reasonably accurate data for different substituted heterocycles (pyrazole derivatives, pyridine derivatives, etc.) from the large amount of ·fHmoðgÞ values available for substituted benzenes and those of the parent heterocycles (pyrazole, pyridine, etc.) by means of multivariate correlations, such as the ones proposed in this paper. However, further studies are needed before such a network of thermodynamic properties would be built up, because, for instance, substituent effects also depend strongly on the nature of the substituted atom, as is evident by comparing the enthalpies of formation of 2-substituted and N-substituted imidazoles (see table 8 and [4].) 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 Autónoma de Madrid is also gratefully acknowledged. Thanks are also due to Instituto de Cooperac¸ ão Científica e Tecnológica Internacional (ICCTI), Lisbon, Portugal, and Consejo Superior de Investigaciones Científicas (CSIC), Madrid, Spain, for a joint research project ICCTI/CSIC. MLPFA thanks Fundac¸ ão para a Ciência 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.
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