Ab Initio and density functional theory study of the interaction in formamide and thioformamide dimers and trimers

Cabaleiro-Lago, Enrique M.; Otero, Jesu´s Rodrı´guez
July 2002
Journal of Chemical Physics;7/22/2002, Vol. 117 Issue 4, p1621
Academic Journal
In this work, clusters consisting of two and three formamide or thioformamide molecules were subjected to ab initio and density functional theory calculations using the aug-cc-pvdz/cc-pvdz basis set. Formamide and thioformamide dimers were both found to exhibit five different minima on their potential surfaces involving hydrogen bonds of the N-H···X=C or C-H···X=C (X=O, S) type. The most stable structure in both cases is a cyclic configuration of Csub 2h] symmetry involving two identical N-H···X=C bonds. The interaction energy for such a structure is -60 and -48 kJ/mol for formamide and thioformamide, respectively. Based on the calculations, each N-H···X=C bond contributes - 30 kJ/mol to it in formamide and - 24 kJ/mol in thioformamide. On the other hand, each N-H···X=C bond contributes -9.7kJ/mol in formamide and -ll.7kJ/mol in thioformamide. The interaction causes appreciable distortion in the molecules, particularly in the N-H groups involved in a hydrogen bond, which are lengthened by up to 0.019 and 0.013 Å in formamide and thioformamide, respectively. The trimer structures identified on the potential surfaces of formamide and thioformamide are cyclic configurations capable of establishing 3 or 4 hydrogen bonds. While formamide tends to adopt planar configurations (the most stable of which possesses an interaction energy of -105 kJ/mol), thioformamide forms preferentially nonplanar structures (the most stable being a nonplanar cyclic configuration with an interaction energy of - 88 kJ/mol). The contribution of nonadditive pairwise terms is not particularly significant in either compound, which suggests the absence of substantial cooperative phenomena in the trimers. However, this contribution is crucial with a view to determining the stability sequence for the trimers, where the most stable structures result from the contribution of nonadditive...


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