Abstract
In the present work, a number of R–X⋯NH3 (X = Cl, Br, and I) halogen bonded systems were theoretical studied by means of DFT calculations performed at the ωB97XD/6-31+G(d,p) level of theory in order to get insights on the effect of the electron-donating or electron-withdrawing character of the different R substituent groups (R = halogen, methyl, partially fluorinated methyl, perfluoro-methyl, ethyl, vinyl, and acetyl) on the stability of the halogen bond. The results indicate that the relative stability of the halogen bond follows the Cl < Br < I trend considering the same R substituent whereas the more electron-withdrawing character of the R substituent the more stable the halogen bond. Refinement of the latter results, performed at the MP2/6-31+G(d,p) level showed that the DFT and the MP2 binding energies correlate remarkably well, suggesting that the Grimme’s type dispersion-corrected functional produces reasonable structural and energetic features of halogen bond systems. DFT results were also observed to agree with more refined calculations performed at the CCSD(T) level. In a further stage, a more thorough analysis of the R–Br⋯NH3 complexes was performed by means of a novel electron localization/delocalization tool, defined in terms of an Information Theory, IT, based quantity obtained from the conditional pair density. For the latter, our in-house developed C++/CUDA program, called KLD (acronym of Kullback–Leibler divergence), was employed. KLD results mapped onto the one-electron density plotted at a 0.04 a.u. isovalue, showed that (i) as expected, the localized electron depletion of the Br sigma-hole is largely affected by the electron-withdrawing character of the R substituent group and (ii) the R–X bond is significantly polarized due to the presence of the NH3 molecule in the complexes. The afore-mentioned constitutes a clear indication of the dominant character of electrostatics on the stabilization of halogen bonds in agreement with a number of studies reported in the main literature. Finally, the cooperative effects on the [Br—CN]n system (n = 1–8) was evaluated at the MP2/6-31+G(d,p) level, where it was observed that an increase of about ~14.2% on the complex stability is obtained when going from n = 2 to n = 8. The latter results were corroborated by the analysis of the changes on the Fermi-hole localization pattern on the halogen bond zones, which suggests an also important contribution of the electron correlation in the stabilization of these systems.
Highlights
Halogen bond refers to the interaction between a nucleophile, Nu, and the heavy halogen atom belonging to a R–X bond (X = Cl, Br or I, and R = an organic moiety) [1,2,3,4,5,6,7]
basis set superposition (BSSE)-corrected binding energies computed at the DFT are reported in Table 1 where it is observed that all the complexes under investigation are stable with BEDFT values ranging from 1.0 to 16.4 kcal/mol, being the CH3 Cl· · · NH3 system the only exception with a modest negative value of −0.1 kcal/mol
Two clear tendencies are observed in Table 1: (i) for a given R substituent group, the relative stability of the complexes follows the Cl < Br < I trend indicating that in agreement with previous reports [1], the less electronegative the halogen bond-donor, the larger the stability of the halogen bond complex; and (ii) for any halide atom acting as halogen bond donor, the relative stability with respect to the R substituent groups follows the –X > –CF3 > –CHF2 > –CH2 F > –CH3 trend, suggesting that, apart from the –X substituent, the higher the electron-withdrawing character of R the larger the stability of the complex
Summary
Halogen bond refers to the interaction between a nucleophile, Nu, and the heavy halogen atom belonging to a R–X bond (X = Cl, Br or I, and R = an organic moiety) [1,2,3,4,5,6,7]. It has been experimentally determined that halogen bond exhibit redMoreover, it has been experimentally determined that halogen bond exhibit red-shifts in the R–X shifts in the R–X stretching frequency upon complexation [13,14,15]; blue-shifts have stretching frequency upon complexation [13,14,15]; blue-shifts have been observed in some been observed in some cases The latter is of significant interest because it indicates that this cases. It has to be considered that the IR detection of considered that the IR detection of halogen bonds is by no means a trivial task due to the strong halogen bonds is by no means a trivial task due to the strong coupling of the R–X stretching vibration coupling of the R–X stretching vibration with other normal modes; in most of the cases, no with other normal modes; in most of the cases, no “pure” R–X vibrational modes exist, a fact “pure” R–X vibrational modes exist, a fact that could affect the experimental measurements
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