Range, Strength and Anisotropy of Intermolecular Forces in Atom-Molecule Systems: an Atom-Bond Pairwise Additivity Approach

F. Pirani, D. Cappelletti and G. Liuti

INFM and Dipartimento di Chimica and Dipartimento di Ingegneria Civile ed Ambientale

Universita’ di Perugia, 060123-Perugia, Italy

A new method is proposed for calculations of intermolecular interactions in van der Waals complexes which is of general applicability and gives accurate results with a negligible effort in terms of computing time. The method is based on correlation formulas between the polarizability of the interacting partners and the main interaction parameters. Our attention was originally addressed to spherical atom--closed shell atom systems [1,2,3] bound by typical van der Waals forces, where the attractive and repulsive component of the interaction were modeled in terms only of the polarizabilities of the interacting partners. This represented the foundation for a proper description of ion--atom [4] ion--ion [5], and open shell atom--closed shell particle

systems [6,7], where in addition to van der Waals forces, additional interaction components operate.

The present work [8] is concerned with the extension of the method to systems involving diatoms or polyatomic molecules interacting with a closed shell atom. The basic idea is to consider the molecular size as determined by the bonds nearest to the approaching atom and to delocalize the dispersion centers on the molecule by using additively bond polarizabilities. The method, tested mainly on homonuclear diatomics-rare gas and simple hydrocarbon-rare gas complexes for which experimental and theoretical information is available, is used to make predictions for systems of higher complexity up to a graphite plane. Tables 1-4 below illustrate the quality of the agreement and some example of the applications. This effort is worthwhile for an advanced and systematic study of phenomena like collisional alignment, broadening and shift of spectral lines of molecules in very different environments, steric effects in elementary processes, chemistry and physics of aggregates, transport properties in gas phase and physical absorption on surfaces. Moreover, as for atom--atom cases, the results of this work could be crucial for a proper rapresentation of systems with increasing complexity where in addition to typical van der Waals forces, induction, electrostatic and chemical contributions can operate.

Table 1 – Interaction features for some homonuclear diatom-Ar systems in the two limiting parallel and perpendicular configurations.

	parallel		perpendicular
	Rm(Å)	e (meV)	Rm(Å)	e (meV)
H2-Ar	3.32	3.32	3.35	3.24	present
	3.35	3.08	3.28	2.78	Exp. (LeRoy)
O2-Ar	4.11	9.07	3.61	13.6
	4.00	9.50	3.56	14.0	Exp. (Pirani)
N2-Ar	4.07	9.27	3.68	13.0
	4.28	7.6	3.68	13.2	Exp. (Casavecchia)

Table 2 – Interaction features for benzene-rare gas systems in the perpendicular configurations.

	Rm(Å)	e (meV)
C6H6-He	3.24	10.3	present
	3.17		Exp. (Smalley)
	3.3	8.4	Calc. (Hobza)
C6H6-Ne	3.28	21.0
	3.30		Exp. (Bauder)
	3.32	19.8	Calc. (Bauder)
C6H6-Ar	3.55	46.0
		<46.4	Exp. (Meijer)
	3.50		Exp. (Bauder)
	3.55	47.9	Calc. (Makarewitz)
C6H6-Kr	3.69	55.9
	3.68	<57	Exp. (Neusser)
	3.66		Exp. (Gutowski)
	3.7	60	Calc. (Hobza)
C6H6-Xe	3.89	63.3
	3.77		Exp. (Bauder)
	3.9	75	Calc. (Hobza)

Table 3 – Interaction features for coronene-rare gas, N₂ and O₂ systems.

	Rm(Å)	e (meV)
C23H12-He	3.43	13
C23H12-Ne	3.43	29
C23H12-Ar	3.61	75
C23H12-Kr	3.73	96
C23H12-Xe	3.90	116
C23H12-N2	3.63	75
C23H12-O2	3.60	76

Table 4 – Interaction energy (in meV) for rare gas atoms interacting with a single layer of graphite (G).

	present	experiment
G-He	15	17
G-Ne	31	33
G-Ar	74	96
G-Kr	93	125
G-Xe	112	162

References

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[2] V.Aquilanti, G.Liuti, F.Pirani and F.Vecchiocattivi, J.Chem.Soc.Faraday Trans., 2 85 (1989) 955.

[3] R.Cambi, D.Cappelletti, G.Liuti, and F.Pirani, J. Chem. Phys., 95, 1852 (1991).

[4] D.Cappelletti, G.Liuti and F.Pirani, Chem. Phys. Lett., 183 (1991) 297.

[5] V. Aquilanti, D. Cappelletti, F.Pirani, Chem. Phys, 209, 299 (1996).

[6] V. Aquilanti, D. Cappelletti, F.Pirani, Chem. Phys. Lett., 271, 216 (1997).

[7] F.Pirani, A. Giulivi, D. Cappelletti, and V. Aquilanti, Mol. Phys., 98, 1749 (2000).

[8] F. Pirani, D.Cappelletti, G. Liuti, submitted for publication (2001).