Nature and Strength of the π‐Hole Chalcogen Bonded Complexes between Substituted Pyridines and SO₃ Molecule

Abstract

A theoretical study has been carried out on the π‐hole based chalcogen bonding interaction between SO3 and various pyridines (XC5H4N), at the MP2=full/aug‐cc‐pVTZ level, to obtain a better insight into the nature and strength of N…S bond. Our calculations predict two different types of chalcogen bonded complexes: (1) Type‐A complex is formed via lone pair‐ π‐hole (N…S) interaction, and (2) Type‐B complex is stabilized through π‐hole‐π‐electron interaction. The binding energy of the Type‐A complexes varies between −109.78 and ‐81.56 kJ/mol whereas it ranges between −28.93 and −17.82 kJ/mol for Type‐B complexes. The nature of the N…S bond and the changes in its properties on the influence of electron donating and electron withdrawing substituents have been discussed with the help of various computational tools like AIM, NCI, SAPT and NBO analysis. The binding energy of the Type‐A complexes are found to correlate with the basicity parameters such as PA and Vs,min of the substituted pyridines. The binding energies of the pyridine complexes with SF2, SO2, CS2 and OCS are also computed for finding the effect of variation of Vs,max value of the S‐atom on the binding energy. The 2nd order hyperconjugation interaction energy between LP(N) and σ*(S−O)/LP*(S) is well correlated to the binding energy as: −ΔE=0.11E(2) −20.08. The variation in the bond distances and their corresponding vibrational frequencies of both the interacting partners (XC5H4N and SO3) are discussed in detail.

Strongly Bound π‐Hole Tetrel Bonded Complexes between H2SiO and Substituted Pyridines. Influence of Substituents

“…The stronger complexes are characterized with higher geometrical distortion (DFE) and vice versa. A similar type of variation in DFE ranging between 47.11 and 36.02 kJ/mol was predicted for the complexes between substituted pyridines and SO 3 [40] . Scheiner et al [26] .…”

“…Therefore, the relative effect of substitution at Lewis base on binding energy diminishes with the increase of π‐hole strength. As observed for many other systems, [40–42,49] the binding energies of Y‐Pyr.H 2 SiO complexes are found to be well correlated with the PA values of substituted pyridines. …”

“…The large difference in electronegativity between the Si and O atoms would polarize the Si−O bond, with accumulation of electron density towards the O‐atom. A detailed discussion on various properties such as PA, IP and V s,min of substituted pyridines has already been provided in our previous work, and the same values are used for our present discussion [40] . As expected, the substitution of EDS enhances the PA value (and basicity) of pyridine, whereas EWS reduces the PA value.…”

“…The MEP diagram indicates (shown in Fig.1) the presence of a minimum (V s,min ) on the nitrogen atom along the C2 axis of pyridines. This V s,min value varies from −179.8 kJ/mol (NH 2 ‐Pyr) to −109.7 kJ/mol (NO 2 ‐Pyr) [40] …”

“…Therefore, the influence of substituent in pyridine is not large, as a variation of only 27 kJ/mol is obtained for a change of 102 kJ/mol in PA value (from NH 2 ‐Pyr to NO 2 ‐Pyr) and 70 kJ/mol in V s,min value. A similar substitution effect varying within 28.22 kJ/mol was predicted for Y‐Pyr.SO 3 complexes as well [40] . However, the relative effect of substituent at Lewis base (pyridine) on the binding strength of π‐hole based chalcogen bond and π‐hole based TB complexes diminishes with the increase of π‐hole strength.…”