Issue 13, 2007

Reliable structures and energetics for two new delocalized π⋯π prototypes: cyanogen dimer and diacetylene dimer

Abstract

Two new prototype delocalized π⋯π complexes are introduced: the dimers of cyanogen, (N[triple bond, length as m-dash]C–C[triple bond, length as m-dash]N)2, and diacetylene, (HC[triple bond, length as m-dash]C–C[triple bond, length as m-dash]CH)2. These dimers have properties similar to larger delocalized π⋯π systems such as benzene dimer but are small enough that they can be probed in far greater detail with high accuracy electronic structure methods. Parallel-slipped and T-shaped structures of both cyanogen dimer and diacetylene dimer have been optimized with 15 different procedures. The effects of basis set size, theoretical method, counterpoise correction, and the rigid monomer approximation on the structure and energetics of each dimer have been examined. MP2 and CCSD(T) optimized geometries for all four dimer structures are reported, as well as estimates of the CCSD(T) complete basis set (CBS) interaction energy for every optimized geometry. The data reported here suggest that future optimizations of delocalized π⋯π clusters should be carried out with basis sets of triple-ζ quality. Larger basis sets and the expensive counterpoise correction to the molecular geometry are not necessary. The rigid monomer approximation has very little effect on structure and energetics of these dimers and may be used without consequence. Due to a consistent cancellation of errors, optimization with the MP2 method leads to CCSD(T)/CBS interaction energies that are within 0.2 kcal mol−1 of those for structures optimized with the CCSD(T) method. Future studies that aim to resolve structures separated by a few tenths of a kcal mol−1 should consider the effects of optimization with the CCSD(T) method.

Graphical abstract: Reliable structures and energetics for two new delocalized π⋯π prototypes: cyanogen dimer and diacetylene dimer

Article information

Article type
Paper
Submitted
22 Nov 2006
Accepted
03 Jan 2007
First published
07 Feb 2007

Phys. Chem. Chem. Phys., 2007,9, 1550-1558

Reliable structures and energetics for two new delocalized π⋯π prototypes: cyanogen dimer and diacetylene dimer

B. W. Hopkins, A. M. ElSohly and G. S. Tschumper, Phys. Chem. Chem. Phys., 2007, 9, 1550 DOI: 10.1039/B616878G

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