Characterizing Complexes with F-Li+-F Lithium Bonds: Structures, Binding Energies, and Spin-Spin Coupling Constants




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Characterizing Complexes with F-Li+-F Lithium Bonds: Structures, Binding Energies, and Spin-Spin Coupling Constants
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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.

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