Effects of Heterogeneity in Small π-Type Dimers: Homogeneous and Mixed Dimers of Diacetylene and Cyanogen

Abstract

The homo- and heterogeneous dimers of diacetylene (H-C≡C-C≡C-H) and cyanogen (N≡C-C≡N) were studied using ab initio electronic structure computations to probe the effects of heterogeneity on noncovalent interactions between systems with delocalized π electron networks. Full geometry optimizations and harmonic vibrational frequencies were performed using the robust coupled-cluster with single and double and perturbative triple excitations (CCSD(T)) method with the triple-ζ plus 2 sets of polarization functions TZ2P(f,d)++ basis set. Seven basic configurations were examined for each dimer (cross, stacked, parallel-slipped, parallel-tipped, linear, T-shaped and Y-shaped), but only four stationary points were identified on the potential energy surfaces (PESs) of the homogeneous cyanogen dimer and the mixed diacetylene/cyanogen dimer. Six previously characterized stationary points on the diacetylene dimer PES were re-examined with the CCSD(T) method and the TZ2P(f,d)++ basis set for consistency. Second-order Møller-Plesset perturbation theory (MP2) and CCSD(T) complete basis set (CBS) limit interaction energies were estimated using the explicitly correlated MP2-F12 and CCSD(T)-F12 methods in conjunction with the VQZ-F12 basis set. On the cyanogen dimer PES, the C2v T-shaped structure is the only minimum, with an average electronic interaction energy, Eint, of -1.96 kcal mol(-1) at the CCSD(T) CBS limit. The Cs Y-shaped structure (in agreement with previous results) is the global minimum on the diacetylene dimer PES, having a mean CCSD(T) CBS limit Eint of -1.75 kcal mol(-1). Three low-lying minima have been identified on the diacetylene/cyanogen dimer PES, a C∞v linear, a C2v cross, and a Cs parallel-slipped structure with average CCSD(T) CBS limit interaction energies of -2.00, -2.16, and -2.45 kcal mol(-1), respectively.

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.…”

Characterization of NTCDI supra-molecular networks on Au(111); combining STM, IR and DFT calculations

“…The organic p-type systems are interesting candidates to achieve a well-ordered organization onto surface since they are capable of forming molecular clusters 13,14 and supramolecular assemblies [15][16][17] in which the non-covalent interactions like hydrogen bonding interactions 18,19 and electronic dispersion interactions play an important role in their stabilization.…”

Big Changes for Small Noncovalent Dimers: Revisiting the Potential Energy Surfaces of (P2)2 and (PCCP)2 with CCSD(T) Optimizations and Vibrational Frequencies

“…In (PCCP) 2 , the individual fragments tend to be slightly bowed (nonlinear) in most of the MP2- and CCSD­(T)-optimized structures. These slight distortions are also observed in the dimers of cyanogen and diacetylene. , …”

“…These differences are far more pronounced than those for other small dispersion bound dimers such as the N 2 dimer and the acetylene dimer . For noncovalent complexes containing first-row atoms, the deviation between MP2 and CCSD­(T) interaction/binding energies increases when both fragments contain delocalized π-electron clouds, starting off near 1.0 kcal mol –1 in the diacetylene dimer − and the cyanogen dimer and growing as large as 2.0 kcal mol –1 in face-to-face configurations of the benzene dimer . It is worth noting that MP2 was shown to overbind the P 2 dimer by as much as 1.5 kcal mol –1 at the CBS limit relative to that with CCSD­(T) even though there is no delocalized π-electron system …”