Theoretical Studies On σ-hole Interaction In Ylides

Noncovalent interactions play a critical role in chemistry, biology and physics, especiallyin supramolecular chemistry, as molecular assemblies are usually held together through weakinteractions. Interest in new types of intermolecular interactions, such as σ-hole and π-holeinteractions has recently grown. Recently, we noticed a region of positive local maximumelectrostatic potential (σ-hole) in the phosphorus side when we studied P-ylide. As a kind ofimportant organic reactive intermediates, ylides play a great role in promoting the progress oforganic chemistry. The existence of σ-hole makes ylides and nucleophiles can be formedσ-hole interactions. These σ-hole interactions are directional noncovalent interactions, it canbe highly significant in organic synthesis.1The positive electrostatic potentials (σ-hole) have been found in ylides CH2XH3(X=P,As, Sb) and CH2YH2(Y=S, Se, Te), on the outer surfaces of Group VA and VIA atoms,approximately along the extensions of the C–X and C–Y bonds, respectively. Theseelectrostatic potentials suggest that the above ylides can interact with nucleophiles to formweak, directional noncovalent interactions similar to halogen bonding interactions.MP2/aug-cc-pVDZ calculations have confirmed the formation of CH2XH3···HM complexes(X=P, As, Sb; M=BeH, ZnH, MgH, Li, Na). The interaction energies, interaction distances,topological properties (electron density and its Laplacian), and energy properties (kineticelectron energy density and potential electron energy density) at the X(1)···H(10) bond criticalpoints are all correlated with the most negative electrostatic potential value of HM, indicatingthat electrostatic interactions play an important role in these weak X···H interactions. Similarto the halogen bonding interactions, weak interactions involving ylides may be significant inseveral areas such as organic synthesis, crystal engineering, and design of new materials.2Cooperativity between H···C or Cl···C bonds of XH···CH2PH3and XCl···CH2PH3andthe P···N interaction of CH2PH3···NH3in XH···CH2PH3···NH3and XCl···CH2PH3···NH3(X=F,N3, CN, CCCN, CCF) were investigated by perturbation theory and quantum theorycalculations. Formation of the hydrogen/haologen bonds greatly enhances the size and themost positive electrostatic potential of the σ hole outside the P atom. This increases P···Ninteraction energies, electron density at P···N bond critical points, electrostatic character ofP···N interactions, and charge transfer from NH3to CH2PH3and decreases P···N interactiondistances. Formation of the hydrogen/halogen bonds greatly enhances the P···N interactionand vice versa. Enhancement of the P···N interaction by the hydrogen or halogen bonds is much greater than that of hydrogen or halogen bonds by the P···N interaction. In thetermolecular complexes, CH2PH3acts as electron acceptor in the P···N interaction andelectron donor in the hydrogen/haologen bonds, resulting in positive cooperativity of P···Ninteractions and the hydrogen/halogen bonds.3The substituent YH2and ZH effects on the σ-hole out of X atom in molecules ofXH3YH2(X=C, Si, Y=N, P, As, Sb) and XH3ZH(X=C, Si, Z=O, S Se, Te) have beeninvestigated by means of second-order M ller Plesset perturbation theory (MP2) calculations,Natural bond orbital (NBO) analyses and “quantum theory of atoms in molecules”(QTAIM)in the part three. By analyzing of ESP, we can find the σ-hole out of X atom. The value ofσ-hole becoming weaker as the electronegativity of the substituent YH2and ZH increases.According to NBO analyses, with the increasing of atomic number of substituent YH2and ZH:i) NBO charge on X atom will be more and more little, the X atom be more negative, thevalues of σ-hole be more small. ii) The shifting of the bonding electrons from X toward Y/Zscaled-up. The X atom more negative, the values of σ-hole be more small. The Analysis resultof QTAIM indicates, in going from the lighter to the heavier atoms in Y/Z Group of theperiodic table, electron density at σXY/Zbond critical points become smaller and smaller. Thepolarizability of the Y/Z atom decreases as σXY/Zbond becoming weaker. Charge transferfrom X to Y/Z decreases, resulting in the decrease values of the σ-hole.