EXPERIMENTAL AND THEORETICAL STUDY OF WEAK INTERACTIONS IN SIMPLE MOLECULAR SOLIDS
Giancarlo Cicero, Bartolomeo Civalleri, Roberto Orlando, Piero Ugliengo and Davide Viterbo1 and Riccardo Bianchi2
1Dip. di Chimica IFM, University, Via P.
Giuria 7, I-10125 Torino; (viterbo@ch.unito.it),
2CSRSRC, CNR, Via C. Golgi 19, I-20133
Milano, Italy
Keywords: hydrogen bond, weak interactions, charge density, ab-initio calculations.
The aim of this work is to gain some understanding on the nature of hydrogen bonds and other weak intermolecular interactions in some simple molecular crystals, by comparing the experimental results obtained by diffraction methods with those derived from ab-initio theoretical calculations performed using the program CRYSTAL [1], which is designed to treat infinite periodic systems.
Three molecular systems were chosen: i) formamide, a good model for the hydrogen bond structure in DNA, in which each molecule is linked to three other molecules by two types of hydrogen bonds; ii) boric acid, an inorganic solid in which each molecule forms six hydrogen bonds with three neighbouring molecules; iii) NH3-BF3 Lewis adduct, with the dative NB bond of great interest within acid-base theories. The charge density study by diffraction methods of the first two compounds has already been carried out [2,3], while for the third we have collected data at 120K up to sinq/l = 1.18Å-1 and the charge density analysis is under completion.
On the first two systems, we have carried out CRYSTAL calculations with different Pople standard basis sets (6-21G, 6-31G, 6-311G, also with polarisation functions). The basis set 6-31G(d) turned out to represent the best compromise between accuracy and computing time. The main physical observables derived were: the electron density, the electrostatic potential, the structure factors and the binding energies. The prevailing electrostatic nature of hydrogen bonds is confirmed. In formamide we have analysed the two different types of interactions: the ones leading to the formation of linear chains and the ones connecting the chains. In boric acid the three non-equivalent hydrogen bonds around each molecule are almost identical, with formation of layers held together by weak non specific interactions. The comparison of the Bader critical point analysis of the experimental and theoretical electron density maps will be presented, together with the most recent results obtained by performing CRYSTAL calculations with optimisation of some geometrical parameters.
The theoretical analysis of the Lewis adduct is still at a
preliminary stage.