Abstract
The π–π interactions between benzene and the aromatic nitrogen heterocycles pyridine, pyrimidine, 1,3,5-triazine, 1,2,3-triazine, 1,2,4,5-tetrazine, and 1,2,3,4,5-pentazine are systematically investigated. The T-shaped structures of all complexes studied exhibit a contraction of the C—H bond accompanied by a rather large blue shift (40–52 cm−1) of its stretching frequency, and they are almost isoenergetic with the corresponding displaced-parallel structures at reliable levels of theory. With increasing number of nitrogen atoms in the heterocycle, the geometries, frequencies, energies, percentage of s character at C, and the electron density in the C—H σ antibonding orbital of the complexes all increase or decrease systematically. Decomposition analysis of the total binding energy showed that for all the complexes, the dispersion energy is the dominant attractive contribution, and a rather large attraction originating from electrostatic contribution is compensated by its exchange counterpart.
Highlights
Interactions between p systems have been the focus of attention for a long time because they play key roles in many fields such as supramolecular chemistry, drug design, biochemistry, crystal engineering, and many other new cross-disciplines associated with molecular science.[1,2,3,4] As a prototype of p–p interactions, the benzene dimer has been studied both theoretically and experimentally,[5,6,7,8] and these studies have greatly improved our understanding of the fundamental physics of p–p interactions
Vibrational frequencies, natural bond orbitals, and binding energies, we conclude with the following remarks: 1) The CÀH bond length in the T-shaped dimer is contracted on complex formation
This contraction is accompanied by a large blue shift of the CÀH stretching frequency
Summary
Interactions between p systems have been the focus of attention for a long time because they play key roles in many fields such as supramolecular chemistry, drug design, biochemistry, crystal engineering, and many other new cross-disciplines associated with molecular science.[1,2,3,4] As a prototype of p–p interactions, the benzene dimer has been studied both theoretically and experimentally,[5,6,7,8] and these studies have greatly improved our understanding of the fundamental physics of p–p interactions. The very electronegative heteroatom nitrogen withdraws electron density from the aromatic heterocycle. With increasing number of nitrogen atoms in the ring, the p electron density in the ring decreases and the molecular electrostatic potential (MEP) becomes more positive. The MEP value is negative above the center of the benzene ring but positive in the aromatic nitrogen heterocycles pyridine, pyrimidine, 1,3,5-triazine, 1,2,3-triazine, 1,2,4,5-tetrazine, and 1,2,3,4,5-pentazine. The negative MEP of benzene is responsible for the existence of the T-shaped benzene dimer. The absence of the negative MEP above the center in the other compounds suggests that their dimers might not be T-shaped like the benzene dimer. The interactions between the benzene molecule and pyridine, pyrimidine, 1,3,5-triazine, 1,2,3-triazine, 1,2,4,5-tetrazine, and 1,2,3,4,5-pentazine should be stronger than that in the benzene dimer.
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