Abstract
The CH3Cl molecule has been used in several studies as an example purportedly to demonstrate that while Cl is weakly negative, a positive potential can be induced on its axial surface by the electric field of a reasonably strong Lewis base (such as O=CH2). The induced positive potential then has the ability to attract the negative site of the Lewis base, thus explaining the importance of polarization leading to the formation of the H3C–Cl···O=CH2 complex. By examining the nature of the chlorine’s surface in CH3Cl using the molecular electrostatic surface potential (MESP) approach, with MP2/aug-cc-pVTZ, we show that this view is not correct. The results of our calculations demonstrate that the local potential associated with the axial surface of the Cl atom is inherently positive. Therefore, it should be able to inherently act as a halogen bond donor. This is shown to be the case by examining several halogen-bonded complexes of CH3Cl with a series of negative sites. In addition, it is also shown that the lateral portions of Cl in CH3Cl features a belt of negative electrostatic potential that can participate in forming halogen-, chalcogen-, and hydrogen-bonded interactions. The results of the theoretical models used, viz. the quantum theory of atoms in molecules; the reduced density gradient noncovalent index; the natural bond orbital analysis; and the symmetry adapted perturbation theory show that Cl-centered intermolecular bonding interactions revealed in a series of 18 binary complexes do not involve a polarization-induced potential on the Cl atom.
Highlights
Murray, Politzer, and their colleagues have analyzed the surface reactivity of several molecular systems using the molecular electrostatic surface potential (MESP) model [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]. They utilized density functional theory (DFT) with a variety of functionals (B3LYP, B3PW91, M06-2X) and a standard double/triple-ζ quality Gaussian basis set to compute the electrostatic potential [1,2,3,4,5,6,7,8,9,10]. They concluded that DFT, together with an 0.001 a.u. isodensity envelope on which to compute the potential, is adequate to reveal the nature of the electrostatic potential on the surface of any atom in a molecule [7,8]
Such a provocative view led to the suggestion that the MESP model is superior to other computational methods such as the second-order natural bonding orbital analysis (NBO) [30], the quantum theory of atoms in molecules in molecules (QTAIM) [31,32,33], and the density functional theory symmetry adapted perturbation theory energy decomposition analysis (DFT-SAPT-EDA) [34,35]
We argue that combining an inappropriate theoretical method with an arbitrarily chosen isodensity envelope can be misleading insofar as the sign of the potential on the axial portion of the Cl atom is concerned, and when such a result is used for the interpretation of the origin of an intermolecular interaction, misleading conclusions can be reached
Summary
Murray, Politzer, and their colleagues have analyzed the surface reactivity of several molecular systems using the molecular electrostatic surface potential (MESP) model [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]. To explain what causes the formation of a H3 C–Cl···O=CH2 complex, it was argued that despite the potential on the outer axial surface of the Cl atom in H3 C–Cl being weakly negative in the isolated molecule, this can be transformed and become positive through the electrostatic polarizing effect of the negative site interacting with it [5,7,10,28,29] Such a provocative view led to the suggestion that the MESP model is superior to other computational methods such as the second-order natural bonding orbital analysis (NBO) [30], the quantum theory of atoms in molecules in molecules (QTAIM) [31,32,33], and the density functional theory symmetry adapted perturbation theory energy decomposition analysis (DFT-SAPT-EDA) [34,35]. We argue that combining an inappropriate theoretical method with an arbitrarily chosen isodensity envelope can be misleading insofar as the sign of the potential on the axial portion of the Cl atom is concerned, and when such a result is used for the interpretation of the origin of an intermolecular interaction, misleading conclusions can be reached
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