Abstract
Heterodimers are constructed containing imidazolium and its halogen-substituted derivatives as Lewis acid. N in its sp3, sp2 and sp hybridizations is taken as the electron-donating base. The halogen bond is strengthened in the Cl < Br < I order, with the H-bond generally similar in magnitude to the Br-bond. Methyl substitution on the N electron donor enhances the binding energy. Very little perturbation arises if the imidazolium is attached to a phenyl ring. The energetics are not sensitive to the hybridization of the N atom. More regular patterns appear in the individual phenomena. Charge transfer diminishes uniformly on going from amine to imine to nitrile, a pattern that is echoed by the elongation of the C-Z (Z=H, Cl, Br, I) bond in the Lewis acid. These trends are also evident in the Atoms in Molecules topography of the electron density. Molecular electrostatic potentials are not entirely consistent with energetics. Although I of the Lewis acid engages in a stronger bond than does H, it is the potential of the latter which is much more positive. The minimum on the potential of the base is most negative for the nitrile even though acetonitrile does not form the strongest bonds. Placing the systems in dichloromethane solvent reduces the binding energies but leaves intact most of the trends observed in vacuo; the same can be said of ∆G in solution.
Highlights
The hydrogen bond (H-bond) is the noncovalent force that has arguably received the greatest attention over the years [1,2,3,4,5,6,7], the halogen bond (XB) is not far behind and its study continues to grow apace [8,9,10,11,12,13,14,15,16]
Of the positively charged imidazolium is replaced by a halogen atom? Does the pattern of enhanced binding noted for neutral Lewis acids remain in force for these ionic species? Again in the context of ionic Lewis acids, how do the various halogen bonds compare with the H-bond of the unsubstituted imidazolium Can any of these interactions be enhanced by extra conjugation if the imidazolium species is connected with a phenyl ring? In connection with the Lewis base, how does the hybridization of the N atom on the electron donor affect the halogen bond? What is the nature of any alkylation effect associated with substitution of N? Given the competition that may be present between H-bonds and
H-bonds illustrated in Figure 1a,b deviate by some 14◦ from full linearity
Summary
The hydrogen bond (H-bond) is the noncovalent force that has arguably received the greatest attention over the years [1,2,3,4,5,6,7], the halogen bond (XB) is not far behind and its study continues to grow apace [8,9,10,11,12,13,14,15,16] These interactions have been studied in numerous situations [17], varying from gas phase [18,19,20], to solution and solid state [21,22,23,24,25,26,27], superfluid He droplets [28], self-assembled nanostructures [29,30,31], clathrate cages [32], and on a solid/liquid interface [33]. In the context of ionic Lewis acids, how do the various halogen bonds compare with the H-bond of the unsubstituted imidazolium Can any of these interactions be enhanced by extra conjugation if the imidazolium species is connected with a phenyl ring? How strong a halogen bond can arise when the H of the positively charged imidazolium is replaced by a halogen atom? Does the pattern of enhanced binding noted for neutral Lewis acids remain in force for these ionic species? Again in the context of ionic Lewis acids, how do the various halogen bonds compare with the H-bond of the unsubstituted imidazolium Can any of these interactions be enhanced by extra conjugation if the imidazolium species is connected with a phenyl ring? In connection with the Lewis base, how does the hybridization of the N atom on the electron donor affect the halogen bond? What is the nature of any alkylation effect associated with substitution of N? Given the competition that may be present between H-bonds and XBs [67,68,69,70,71,72], and the sensitivity of this competition to the nature of the solvent [73], it is judicious to examine these issues in a fundamental way in the gas phase, and within solvent
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