Electron Transfer in Pnicogen Bonds

Abstract

As a new type of noncovalent interactions, pnicogen bond between a VA group element (N, P, and As) and an electron donor (Lewis base) has grabbed attention in recent several years. Here we employ the block-localized wave function (BLW) based energy decomposition scheme to probe the bonding nature in a series of substituted phosphines X(n)PH(3-n) complexed with ammonia. As the BLW method can derive the optimal monomer orbitals in a complex with the electron transfer among monomers quenched, we can effectively examine the HOMO-LUMO interaction in these pnicogen bonding systems. Among various energy components, electron transfer energy together with the polarization energy dominates the pnicogen bonding energy. Although usually it is assumed that the electron transfer from ammonia to substituted phosphines occurs in the form of n → σ*(XP) hyperconjugative interaction, we identify a kind of new pathway when X = NO2 and CN, i.e., n → dπ*, which results from the interaction between the π orbital of cyano or nitro substituent and d orbitals on P. But still this picture of electron transfer using a single pair of orbitals is greatly simplified, as the electron density difference (EDD) maps corresponding to the overall electron transfer processes show the accumulation of electron density on the P side opposite to the X-P bond, with insignificant or even negligible gain of electron density on the substituent group side. Thus, the EDD maps tend to support the concept of σ-hole in pnicogen bonds.

The origins of the directionality of noncovalent intermolecular interactions#

“…Though the nature of hydrogen bonds is still somewhat controversial, the current consensus is that hydrogen bonds are predominantly electrostatic with minor covalent characters, and it is the latter that is responsible for the bond directionality. The covalency is conferred by the charge transfer (CT) (or hyperconjugation) effect, which results from the overlap between an occupied orbital on one end and a virtual orbital on another end, and is directional …”

“…Apart from hydrogen bonds, other types of weak bonds have been identified and explored, including interactions of Lewis bases with molecules containing elements from group VIIA (hence, halogen bonds), between Lewis bases and VIA group elements (chalcogen bond) and between Lewis bases and group VA elements (pnicogen bonds). Different from hydrogen bonds where the Lewis base approaches the positively charged hydrogen atom, a halogen bond is formed between a Lewis base and a bonded halogen atom, which carries a partial negative charge.…”

“…From a completely different direction, some of us (LG and YM) used the block‐localized wavefunction (BLW) method, which is a variant of the ab initio valence bond theory and is able to differentiate the electrostatic, polarization, and electron transfer (hyperconjugation) interactions, to probe the bonding nature in a series of substituted phosphines X n PH 3−n complexed with ammonia . Our BLW energy decomposition analyses of the pnicogen bonds in these complexes showed that the orbital interaction, which involves both the polarization and CT, dominates the binding, and the strong CT is in accord with the high degree of anisotropy (directionality) of the pnicogen bond.…”

Description of pnicogen bonding with the help of vibrational spectroscopy—The missing link between theory and experiment

“…has been intensively investigated where most of the work has been based on quantum chemical calculations [1][2][3][4][5][6][7][8][9][10][11]. Only scattered experimental results based on X-ray diffraction [12] and NMR measurements [13] could be reported providing more or less indirect or incomplete information on pnicogen interactions of the type E· · ·E .…”

Tin···Oxygen Tetrel Bonding: A Combined Structural, Spectroscopic, and Computational Study

“…9,10,11,12,13,14 Over the past few years other non-conventional noncovalent interactions have gained attention, such as tetrel bonding, 15,16,17,18,19 pnictogen bonding, 20,21,22,23,24,25,26,27,28,29,30,31 chalcogenide bonding, 32,33,34,35,36,37,38,39,40,41,42,43,44 and even aerogen bonding. 45 All follow the same bonding scheme, namely a -hole, 46 which can be viewed comparably to hydrogen bonding: X-D … A, where X is any atom, D is the donor atom and A is the acceptor atom.…”