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

Abstract

Two new prototype delocalized pi[dot dot dot]pi complexes are introduced: the dimers of cyanogen, (N[triple bond]C-C[triple bond]N)(2), and diacetylene, (HC[triple bond]C-C[triple bond]CH)(2). These dimers have properties similar to larger delocalized pi...pi 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 pi[dot dot dot]pi clusters should be carried out with basis sets of triple-zeta 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.

Characterization of the potential energy surfaces of two small but challenging noncovalent dimers: (P2)2 and (PCCP)2

“…The configurations are shown in Figure and include the PS structure with C 2 h symmetry, the T‐shaped structure (T) with C 2 v symmetry, the perpendicular X‐shaped structure ( X) with D 2 d symmetry, the rectangle structure (Rec) with D 2 h symmetry, the linear structure (Lin) with D ∞h symmetry, and the X‐shaped structure (X) with D 2 symmetry. In most configurations, the monomers are not linear and bow slightly, a characteristic shared with the cyanogen and diacetylene dimers . The X configuration has not been previously identified on the (PCCP) 2 PES.…”

“…In comparison, MP2 overestimates the interaction energy in face‐to‐face configurations of the benzene dimer by about 2 kcal mol −1 at the CBS limit relative to the corresponding CCSD(T) value, whereas MP2 and CCSD(T) tend to provide similar energetics for smaller systems like the N 2 dimer or the acetylene dimer . As with acetylene, diacetylene, and cyanogen, the relatively diminutive size of P 2 and PCCP facilitates the thorough characterization of stationary points on the corresponding dimer potential energy surfaces (PESs) with rigorous ab initio electronic structure methods. It is also feasible to examine electron correlation effects beyond the CCSD(T) level.…”

Reliable Electronic Structure Computations for Weak Noncovalent Interactions in Clusters

“…For systems with even smaller cluster-induced geometrical perturbations, such as p-type van der Waals interactions, the RMA has almost no discernable effect on the interaction energies. 93 Note that the RMA must, by definition, decrease the magnitude of the interaction energy (for variational electronic structure methods). This result is readily illustrated with Figure 3.…”

Probing the effects of heterogeneity on delocalized π⋯π interaction energies

“…The geometries of the Di-Di and Cy-Cy dimers as well as the Di and Cy monomers were obtained from the literature. 32 Bz-Bz interaction energies were taken from ref. 24 and 36. All computations were carried out with the Gaussian 03, 48 PSI3, 49 MPQC 2.3.1 [50][51][52][53][54] and SAPT2006 55,56 quantum-chemistry software packages.…”

“…[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] It was recently noted that the delocalized pÁ Á Áp dimer consisting of two diacetylene molecules, (H-CRC-CRC-H) 2 or Di-Di, behaves very much like the benzene dimer and can also serve as a useful prototype for p-type interactions. 32 While Bz-Bz and Di-Di are extremely useful models, p-type interactions in real systems such as biomolecules and nanomaterials tend to be more intricate due to substituent effects (e.g. aromatic amino acids) and the presence of heteroatoms (e.g.…”