A molecular orbital multicenter bond index I_ABC…K has proven to be useful in discriminating between strong and usual hydrogen bonds [1]. It has been the subject of more general treatments through the formalism of General Population Analysis [2] and applied successfully to electron delocalization and aromaticity [3]. It is here used, for the first time, in the description of dihydrogen bonds.

where P is the charge-bond order matrix, S the overlap matrix, η a normalization constant and the permutation operator Γ_i interchanges basis functions labels.

The indices are calculated for: i) dimers of FH and XH_n (X = Li, Na, Be, Mg); ii) the complexes appearing in Ref. [5]; iii) the BH₃NH₃ dimer [6]. While they correlate satisfactorily with the binding energy, their relation to the blue-shift is not equally clear.

[1] M. Giambiagi, M. S. de Giambiagi, K. C. Mundim, Struct. Chem. 1990, 1, 423.

[2] R. Ponec, F. Uhlik, Croat. Chem. Acta 1996, 69, 941.

[3] P. Bultinck, R. Ponec, S. Van Damme, J. Phys. Org. Chem. 2005, 18, 706.

[4] S. J. Grabowski, J. Phys. Chem. A 2000, 104, 5551.

[5] I. Alkorta, J. Elguero, O. Mó, M. Yánez, J. E. Del Bene, J. Phys. Chem. A 2002, 106 1325.

[6] Y. Meng, Z. Zhou, C. Duan, B. Huang, Q. Zhong, J. Mol. Struct. (Theochem) 2005, 713, 135.

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1. Measuring Electron Delocalization in Aromatic Molecules