p-Difluorobenzene-Argon Ground State Intermolecular Potential Energy Surface † Jose ´ Luis Cagide Fajı ´n and Berta Ferna ´ ndez* Department of Physical Chemistry, Faculty of Chemistry, UniVersity of Santiago de Compostela, E-15782 Santiago de Compostela, Spain Peter M. Felker Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, California 90095-1569 ReceiVed: July 15, 2005; In Final Form: October 4, 2005 The ground state intermolecular potential energy surface for the p-difluorobenzene-Ar van der Waals complex is evaluated using the coupled cluster singles and doubles including connected triple excitations [CCSD(T)] model and the augmented correlation consistent polarized valence double- basis set extended with a set of 3s3p2d1f1g midbond functions. The surface minima are characterized by the Ar atom located above and below the difluorobenzene center of mass at a distance of 3.5290 Å. The corresponding binding energy is -398.856 cm -1 . The surface is used in the evaluation of the intermolecular level structure of the complex. The results clearly improve previously available data and show the importance of using a good correlation method and basis set when dealing with van der Waals complexes. I. Introduction van der Waals complexes are well-known for playing a key role as models in the study of processes so crucial as the solvation or adsorption of molecules. 1,2 Series of systems constituted by an aromatic molecule to which a successive number of rare-gas atoms are added were studied as models for specific solvation steps. 2 van der Waals complexes consti- tuted by aromatic molecules and rare-gas atoms have been studied intensely in the past. 3 In previous work we have studied the benzene-argon ground, 4,5 excited S 1 , 4,6 and excited T 1 7 states, evaluating highly accurate intermolecular potential energy surfaces (IPESs) using the coupled cluster singles and doubles (CCSD) model including connected triple corrections [CCSD- (T)] and the aug-cc-pVDZ basis set extended with a set of 3s3p2d1f1g midbond functions (denoted 33211). 4 For the excited states 4,6,7 we used the CCSD method. In the benzene-argon ground state equilibrium configuration the Ar atom is located at (3.5547 Å above and below the benzene plane on the benzene C 6 axis and has a dissociation energy of 386.97 cm -1 . Recently, we extended this work by considering the effect of a slight modification of the benzene ring, studying complexes such as the chlorobenzene- 8 and the fluorobenzene-argon. 9 In the case of the latter, we obtained a binding energy of 391.1 cm -1 and an equilibrium geometry with the argon atom located at a distance of 3.562 Å from the fluorobenzene center of mass, with an angle of 6.33° with respect to the axis that passes through the fluorobenzene center of mass and is perpendicular to the fluorobenzene plane. For all the studied complexes the vibrational levels obtained from the ground state IPESs agreed very well with the experimental data available and in several cases were able to correct some of the assignments. For the two excited states considered 4,6,7 the results were also satisfac- tory. In the present study we are going to continue this work, and apply the same method and basis set to study the p-difluoroben- zene-Ar complex. This complex was selected because of three main reasons: first, to provide an accurate ground state IPES, as a first step in the evaluation of the S 1 excited state IPESs, work that has been requested by several authors to be able to interpret their results; 12 second, to check the accuracy of the MP2/aug-cc-pVDZ surface of ref 13; and third, to be able to assess the effect on the IPES of a slight modification of the benzene ring by the introduction of a second fluorine atom in the para position. The p-difluorobenzene-Ar has been studied from the theo- retical point of view using the second-order Møller-Plesset (MP2) method. In the first of these studies, Hobza et al. 10 used the 6-31+G* basis set to describe the p-difluorobenzene and a [7s4p2d] basis set for the argon atom. They carried out calculations for three structures given by intermolecular dis- tances of 3.5 Å (333 cm -1 ), 3.6 Å (342 cm -1 ), and 3.7 Å (336 cm -1 ). Tarakeshwar et al. 11 using the MP2 method and a [7s4p2d1f/4s3p1d/3s1p] basis set obtained a binding energy of 408 cm -1 (D 0 ) 364 cm -1 ) and a position for the argon atom on the C 2 axis perpendicular to the ring at a distance of 3.578 Å from the p-difluorobenzene plane. Recently, Moulds et al. 12 have studied this complex using the MP2 method and the aug-cc-pVDZ basis set. Unconstrained geometry optimization was carried out for all the stationary points. They obtained a binding energy of 377 cm -1 and an internuclear equilibrium distance of 3.366 Å. The MP2 method is found to overestimate the energy barriers, and this error is corrected by comparison with previous CCSD(T) results for the benzene-argon complex (refs 3-5). The energy barrier for the movement of the argon atom around the ring is estimated as e204 cm -1 in the S 0 state and as e225 cm -1 in the S 1 state. But anomalous fluorescence is observed from the 240 cm -1 level, and the authors concluded that the evaluation of a coupled cluster S 1 IPES is necessary to be able to interpret the results. The most recent theoretical study on the complex has been carried out also using the MP2 method and the aug-cc-pVDZ and aug-cc-pVDZ(-d,-2p) bases. 13 The latter basis set was † Part of the special issue “Jack Simons Festschrift”. 11602 J. Phys. Chem. A 2005, 109, 11602-11608 10.1021/jp0538969 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/01/2005