Energy Decomposition Analysis of the P…N interaction energy between different halo-dienophiles and trivalent phosphorus-based catalyst at the transition state of a cycloaddition reaction

The energy decomposition analysis [1] of the noncovalent intermolecular P…N interaction energy (Eint) between different the dienophiles of X2C=NNH2, where X = F, Cl, and Br and trivalent phosphorus-based catalyst (PHCl2) was performed using with the Amsterdam Density Functional program package, ADF2009.01 [2] at the M06-2X/TZ2P-(ZORA)//M06-2X /def2-TZVPP level of theory. The size effect of the X substitutions of dienophile molecule on the interaction energy (Eint) and its decomposed components [electrostatic (Eele), Pauli (EPauli), and interactions between the occupied andunoccupied molecular orbitals energy (Eorb)] were studied. The aim was to investigate the relation and nature of the P...N bonding at the transition state (TS) of aza-diels-alder [4π+2π] cycloaddition reaction [3] with the size effect of the X substitutions of dienophile molecule at the TS. The calculations showed that the Eint between the dienophile molecules and PHCl2 catalyst shows considerable changes with the size of the X substitutions of dienophile molecule. Also, it was found that the value of the Eelec is nearly insensitive to the size of the X substitutions of dienophile molecules, especially for the F2C=NNH2 and Cl2C=NNH2 molecules, while for the Br2C=NNH2 observed a small dependency. Similarly, the EPauli showed a nearly size independent value for the F2C=NNH2 and Cl2C=NNH2 molecules while it was some size-dependent for Br2C=NNH2 molecules.