Characterizing Complexes with F-Li + -F Lithium Bonds: Structures, Binding Energies, and Spin-Spin Coupling Constants Janet E. Del Bene,* ,† Ibon Alkorta, ‡ and Jose ´ Elguero ‡ Department of Chemistry, Youngstown State UniVersity, Youngstown, Ohio 44555, and Instituto de Quı ´mica Me ´dica, CSIC, Juan de la CierVa, 3, E-28006 Madrid, Spain ReceiVed: March 8, 2009; ReVised Manuscript ReceiVed: May 26, 2009 Ab initio MP2/aug-cc-pVTZ calculations have been performed to determine the structures and binding energies of complexes with F-Li + -F bonds formed from the fluorine bases LiF, CH 3 F, HF, ClF, and FF. There is only a single minimum across the Li + transfer coordinate, and in each series, the lithiated homodimer is stabilized by a symmetric F ··· Li + ··· F bond. Complexes having LiF, CH 3 F, and HF as the base have similar structures, with linear F-Li + -F bonds and a head-to-tail alignment of the F-Li + bond dipole with the dipole moment vector of the base. In each series with a given acid, the binding energy decreases as the difference between the lithium ion affinities increases. EOM-CCSD coupling constants 1 J(F-Li), 1li J(Li-F), and 2li J(F-F) have also been evaluated. In complexes with essentially linear bonds, 2li J(F-F) values are small and positive and increase quadratically as the F-F distance decreases. 1li J(Li-F) and 1 J(F-Li) also vary systematically with distance. Comparisons are made between structural, energetic, and coupling constant properties of these complexes and corresponding complexes stabilized by F-H + -F hydrogen bonds. Introduction If one were to ask a group of chemists what is the most important tenet of their discipline, their answers would vary, but surely a common theme would be “the chemical bond”. 1 Despite its long history, the concept of the chemical bond is continuously changing and expanding, making it a subject which is forever young. Although covalent bonds present in organic molecules and ionic bonds formed in solids are generally well- understood, much remains to be learned about organometallic bonds, bonds involving atoms found in the lower portion of the periodic table, and intermolecular bonds. Our own interests are in intermolecular bonds, as evidenced by our studies of neutral and cationic hydrogen bonds, 2-15 dihydrogen bonds, 16-19 and more recently, halogen bonds. 20,21 We have characterized the complexes stabilized by intermolecular bonds in terms of their structures, binding energies, and spin-spin coupling constants. In this paper we extend our investigations to complexes stabilized by cationic lithium bonds. Some studies of such bonds have been published previously. 22-28 For our study we have selected five neutral fluorine bases: LiF, CH 3 F, HF, ClF, and FF; the corresponding lithiated ions; and the 15 complexes arising from the formation of F-Li + -F intermolecular lithium bonds. Of particular interest are the structures, binding energies, and spin-spin coupling constants across these lithium bonds and the similarities and differences between these com- plexes and complexes stabilized by hydrogen bonds. Methods The fluorine bases, their lithiated ions, and the complexes formed from these bases and ions have been optimized at second-order Møller-Plesset perturbation theory (MP2) 29-32 with the Dunning aug-cc-pVTZ basis set. 33,34 Vibrational frequencies were computed to ensure that each structure is an equilibrium structure on its potential surface. These calculations were carried out with the Gaussian-03 suite of programs. 35 Spin-spin coupling constants were computed using the equation-of-motion coupled cluster singles and doubles method in the CI (configuration interaction)-like approximation with all electrons correlated. 36,37 The Ahlrichs 38 qzp basis set was used on 13 C and 19 F, the qz2p basis on 35 Cl, and the hybrid basis set on 7 Li. 39 The Dunning cc-pVDZ basis set was placed on all H atoms. 33,34 Coupling constants across the F-Li + ··· F lithium bonds are designated 1 J(F-Li), 1li J(Li-F) and 2li J(F-F), consistent with the designations 1 J(X-H), 1h J(H-Y), and 2h J(X-Y) for coupling across X-H ··· Y hydrogen bonds. In the Ramsey approximation, 40 the total coupling constant (J) is a sum of four terms: the paramagnetic spin-orbit (PSO), diamagnetic spin-orbit (DSO), Fermi-contact (FC), and spin-dipole (SD). All terms have been evaluated for all monomers and complexes. Coupling constants were computed using the ACES II program 41 on the Itanium cluster at the Ohio Supercomputer Center. It should be noted that a single-reference treatment of the F 2 molecule produces a large CCSD t2 amplitude of 0.16, indicative of the multireference character of this molecule. This amplitude remains high, although it is slightly reduced to 0.14 in all of the complexes with F 2 except for the lithiated homodimer F 2 ··· Li + ··· F 2 , where it drops to less than 0.10. The large amplitudes are associated with the description of the F 2 molecule itself and do not appear to give rise to any anomalies in the properties of complexes with F 2 as the base. Results and Discussion Structures and Binding Energies. Table 1 presents the dipole moments, electronic Li + binding energies, and the electronic H + binding energies of the fluorine bases. The Li + binding energy is the negative electronic energy for the reaction * Corresponding author. E-mail: jedelbene@ysu.edu. † Youngstown State University. ‡ Instituto de Quı ´mica Me ´dica. J. Phys. Chem. A XXXX, xxx, 000 A 10.1021/jp9020917 CCC: $40.75  XXXX American Chemical Society Downloaded by AUSTRIA CONSORTIA on July 6, 2009 Published on July 1, 2009 on http://pubs.acs.org | doi: 10.1021/jp9020917