Nonbonded potentials for azahydrocarbons: the importance of the Coulombic interaction

IntroductionMolecular-packing analyses of nine molecular complexes between hexafluorobenzene or fluoranil and methylated benzenes or aromatic amines have been performed using lattice-energy calculations in the atom-to-atom approach. Energy minimization with molecular orientations and positions and cell parameters as variables gave, for most of the complexes, structures somewhat different from the experimental structures. Minimization of energy was also performed with only the crystallographic axes as variables. For the experimental structures and the structures obtained after the energy minimizations the contributions to the lattice energy from dispersion, repulsion and Coulombic interactions within one single stack and between different stacks were calculated. The results indicate that for some complexes there are stacking interactions in addition to those predicted by the analytical potentials, which have a substantial effect on the molecular packing. For other complexes, where the interactions in the stacks are weak, the molecular packing seems to be influenced by additional interactions between different stacks, probably directional F---H interactions.0108-7681/90/020283-06503.00 Lattice-energy calculations based on analytical atomto-atom potentials show that for many compounds the molecular packing giving the lowest calculated energy agrees fairly well with that of the experimental structure (Williams & Cox, 1984). In molecular complexes, however, there are usually special kinds of interactions, not included in the analytical potentials, in addition to the traditional van der Waals interactions. These additional interactions may cause differences between the experimental and lowest-energy structures. The purpose of the present work was to analyse such differences in complexes containing hexafluorobenzene (HFB) or tetrafluorop-benzoquinone (fluoranil) and from the observed trends try to draw conclusions concerning special kinds of interactions in these complexes. The nature of the molecular complexes containing HFB has been a matter of dispute (Swinton, 1974). In the crystalline state the partner molecules are stacked alternately in infinite columns as in ordinary charge-transfer complexes, but the importance of charge-transfer interactions is doubtful because of the low electron affinity of HFB (Wentworth,

supporting

confidence: 61%

Molecular-packing analysis of molecular complexes containing hexafluorobenzene or fluoranil

Dahl¹

1990

“…In our opinion, this high contribution is a qualitatively significant feature, in spite of the approach and the potential parameters used and the method of estimating effective atomic charges. Generally, it was tacitly assumed that the Coulombic contribution to the lattice energy is negligible but some previous results indicate that this assumption is not valid in all hydrocarbon crystals, reaching a value of 29% of the total lattice energy of benzene and up to 59% in other cases (Williams & Cox, 1984).…”

Section: Resultsmentioning

confidence: 99%

“…In this model each atom-pair interaction is written as a sum of three terms: a short-range strong repulsive energy due to overlapping electron clouds of filled shells; a weak longer-range attractive dispersion energy; and a very long-range Coulombic energywhich can be either repulsive or attractive -between site electrical charges. For hydrocarbons it is generally assumed that the Coulombic-energy term is negligible, but there are organic molecules for which this energy term can be quite important (Williams & Cox, 1984). However, the force effect of the electrostatic term should be small relative to the energy effect because its functional form causes the energy to vary less with distance than the other energy terms.…”

Section: Introductionmentioning

confidence: 99%

Molecular-packing analysis of some glucofuranoimidazolidine crystals

Estrada¹,

Conde²,

Márquez³

1987

Lattice-energy minimization for seven glucofuranoimidazolidine crystals was carried out in the atom-atom approach using an (exp-6-1) form for atom-pair interactions. Molecular rotation and translation and also internal rotations were optimized along with cell parameters starting from experimental structures. The fit between optimized and observed structures can be considered satisfactory in spite of the existence of hydrogen-bond interactions in the crystals. The contribution of the Coulombic term to the estimated lattice energy was found to be very important but its effect on structural parameters was small.

“…The parameters in the above equation were obtained by Williams et al (Williams & Starr, 1977;Cox, Hsu & Williams, 1981;Williams & Cox, 1984) from a least-squares fitting to known selected structures and energies. The parameters are therefore optimized to compensate within the limits of the chosen functional form for the inadequacies of the representation.…”

Section: Methodsmentioning

confidence: 99%

Investigation of the intermolecular potential hypersurface of N,N-dimethyl-p-nitroaniline

Higgins¹

1989

Therefore, P00 = 5, P20 = (4/27) 1/2, and the other Ptm's are zero.For O atoms, the bonding axis is taken to be X(Pl = I), and the population of the lone-pair orbitals is assumed to be two (P2 = P3 = 2) and zc-bonding-orbital population Pz is one, then P00 = 6, ell+ --__(~)1/2, P20 = --(16/243) 1/2, P22 + = -2/37r, P11-= P22---0.References COLAPIETRO, M. & DOMENICANO, A. (1982). Acta Cryst. B38, 1953-1957 AbstractAtom-atom potentials are used to calculate the lattice energy of N,N-dimethyl-p-nitroaniline for a selection of space-group symmetries. The aim is to demonstrate the viability of computer models to select crystal forms of interest in nonlinear optical applications.