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 wellunderstood, much remains to be lea ed 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, CH3F, 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 complexes 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 Ahlrichs38 qzp basis set was used on 13C and 19F, the qz2p basis on 35Cl, and the hybrid basis set on 7Li.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 1J(F-Li), 1liJ(Li-F) and 2liJ(F-F), consistent with the designations 1J(X-H), 1hJ(H-Y), and 2hJ(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 program41 on the Itanium cluster at the Ohio Supercomputer Center. It should be noted that a single-reference treatment of the F2 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 F2 except for the lithiated homodimer F2 · · · Li+ · · · F2, where it drops to less than 0.10. The large amplitudes are associated with the description of the F2 molecule itself and do not appear to give rise to any anomalies in the properties of complexes with F2 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 and the H+ binding energy is defined similarly. Experimental gas-phase proton affinities are from the NIST Web site.42 From Table 1 it can be seen that both the lithium ion and proton binding energies of these fluorine bases vary dramatically and decrease in the order LiF > CH3F > HF > ClF > F2. The electronic lithium ion and proton binding energies of these fluorine bases are linearly related as Table 2 summarizes the F-Li, Li · · · F, and F-F distances across the F-Li+ · · · F bonds, the Li-Fd-Fa angle which measures the nonlinearity of the lithium bond, and the binding energies of these complexes. The lithium ion complexes are listed in Table 2 according to the lithium ion affinity of the base. Thus, the first set of complexes has the strongest base (LiF) lithiated (LiFLi+) and acting as the acid; the acceptor bases are listed in order of decreasing base strength. The first complex, LiF · · · Li+ · · · FLi, has D∞h symmetry with a symmetric F · · · Li+ · · · F lithium bond and the highest binding energy of 51.4 kcal/mol. The LiF · · · Li+ · · · FLi complex has been detected experimentally,43,44 and theoretical studies of its conformational space have been carried out.43-45 These studies concluded that the linear D∞h conformation is the global minimum on the potential surface. Reference 43 reported a computed binding energy and 0 K enthalpy of 56.9 and 55.7 kcal/mol, respectively, for this complex, using a different method and basis set than used in this work. Our values of 51.4 and 50.3 kcal/mol, respectively, are consistent with but lower than those of ref 43. To the best of our knowledge, no other complexes with F · · · Li+ · · · F bonds have been detected experimentally, although several reports have shown the possibility of obtaining metallic salts containing HF as a ligand.46-48 From Table 2 it can be seen that as the difference between the Li+ affinities of the two bases increases, the binding energy decreases, as observed previously for hydrogen-bonded complexes formed from second-period bases.49 The F-Li, Li · · · F, and F-F distances also vary systematically. It is apparent that complex formation increases the F-Li distance from 1.689 Å in the isolated ion Li-F-Li+, to 1.694 Å in the complex with the weakest base F2, and to 1.739 Å in the complex with the strongest base (LiF) and the symmetric lithium bond. The Li · · ·F and F-F distances change in the reverse order, with the shortest distances found in the complex that is most strongly bound and the longest distances in the most weakly bound complex. All of the lithium bonds in this series are essentially linear, except for the complex with F2. In this complex, the F-Li+ bond of the acid points to the midpoint of the F-F bond. This is a consequence of the absence of a dipole moment for F2 and its poor electron-donating ability through a lone pair of electrons. For all complexes, there is only a single minimum across the Li+ transfer coordinate. The variations in the F-Li+ and F-F distances observed in the series of complexes with Li-F-Li+ as the acid are also characteristically seen in related series of hydrogen-bonded complexes.50 In the hydrogen-bonded complexes, if a symmetric hydrogen bond is found in a protonated homodimer, it is referred to as a proton-shared symmetric hydrogen bond. As the difference between the proton affinities of the hydrogen-bonded bases increases, the proton-shared character of this bond decreases, until a traditional (normal) hydrogen bond is formed. By analogy, we might call the lithium bond in a lithiated homodimer a symmetric “lithium-shared bond”. And similarly, the lithium-shared character of this bond decreases as the difference between the lithium ion affinities of the two bases increases. This concept will be useful when describing variations in spin-spin coupling constants in these complexes. The second group of complexes listed in Table 2 has CH3FLi+ as the Li+ donor. The lithiated homodimer (CH3-F · · · Li+ · · · F-CH3) has a symmetric F · · · Li+ · · · F lithium bond and a binding energy of 24.5 kcal/mol. Once again, as the strength of the acceptor base decreases, the binding energy also decreases and is only 5.9 kcal/mol for CH3-F-Li+ · · · F2. The structure of this complex is similar to that of the Li-F-Li+ · · ·F2 complex and is shown in Figure 1. The patte of energy and distance changes apparent in complexes with LiFLi+ as the donor is also seen in complexes which have CH3F-Li+ and H-F-Li+ as donors. In all lithiated homodimers, the F · · · Li+ · · · F bond is symmetric. For complexes with LiF, CH3F, and HF as the base, the angle Li+-F-X, where X is the atom covalently bonded to the acceptor F, tends toward 180°, as illustrated for the CH3- F-Li+ · · ·FH complex in Figure 2. This is a consequence of the large electrostatic component of the binding energy and the preference for a head-to-tail arrangement of the F-Li+ bond dipole moment with the dipole moment vector of the base. However, the structures of complexes with ClF and F2 as acceptor bases are different. As seen in Table 1, the dipole moment vector of ClF is relatively small at 0.92 D, and the angle Li+-F-Cl in complexes with ClF as the base is approximately 130°, as illustrated in Figure 3 for Cl-F · · · Li+ · · · F-Cl. This structure reflects the greater involvement of the lone pair of electrons on F in the formation of the F-Li+-F bond. The extreme case is F2, which has no dipole moment and is a poor electron donor. The complexes of LiFLi+, CH3FLi+, and HFLi+ with F2 have the F-Li+ bond pointing to the midpoint of the F-F bond, as illustrated in Figure 1. The complex Cl-F-Li+ · · · F2, illustrated in Figure 4, does not have this orientation of F2. Rather, F2 is displaced from its perpendicular position to give a trans arrangement of Cl and F with respect to the linear F-Li+ · · · F bond, thereby reducing the longrange repulsion between these two atoms. The final complex is F2 · · · Li+ · · · F2, which has D2h symmetry and is shown in Figure 5. This complex has a dramatically different structure and is the most strongly bound complex with F2 as the acceptor base. This again illustrates the rule that binding energies increase as the difference between the lithium ion affinities of the two bases decreases.
