Interactions involving aromatic rings are important in molecular/biomolecular assembly and engineering. As a consequence, there have been a number of investigations on dimers involving benzene or other substituted pi systems. In this Feature Article, we examine the relevance of the magnitudes of their attractive and repulsive interaction energy components in governing the geometries of several pi-pi systems. The geometries and the associated binding energies were evaluated at the complete basis set (CBS) limit of coupled cluster theory with singles, doubles, and perturbative triples excitations [CCSD(T)] using a least biased scheme for the given data set. The results for the benzene dimer indicate that the floppy T-shaped structure (center-to-center distance: 4.96 A, with an axial benzene off-centered above the facial benzene) is isoenergetic in zero-point-energy (ZPE) corrected binding energy (D0) to the displaced-stacked structure (vertical interplanar distance: 3.54 A). However, the T-shaped structure is likely to be slightly more stable (D0 approximately equal to 2.4-2.5 kcal/mol) if quadruple excitations are included in the coupled cluster calculations. The presence of substituents on the aromatic ring, irrespective of their electron withdrawing or donating nature, leads to an increase in the binding energy, and the displaced-stacked conformations are more stabilized than the T-shaped conformers. This explains the wide prevalence of displaced stacked structures in organic crystals. Despite that the dispersion energy is dominating, the substituent as well as the conformational effects are correlated to the electrostatic interaction. This electrostatic origin implies that the substituent effect would be reduced in polar solution, but important in apolar media, in particular, for assembling processes.

The edge-to-face interactions for either axially or facially substituted benzenes are investigated by using ab initio calculations. The predicted maximum energy difference between substituted and unsubstituted systems is approximately 0.7 kcal/mol (approximately 1.2 kcal/mol if substituents are on both axially and facially substituted positions). In the case of axially substituted aromatic systems, the electron density at the para position is an important stabilizing factor, and thus the stabilization/destabilization by substitution is highly correlated to the electrostatic energy. This results in its subsequent correlation with the polarization and charge transfer. Thus, the stabilization/destabilization by substitution is represented by the sum of electrostatic energy and induction energy. On the other hand, the facially substituted aromatic system depends on not only the electron-donating ability responsible for the electrostatic energy but also the dispersion interaction and exchange repulsion. Although the dispersion energy is the most dominating interaction in both axial and facial substitutions, it is almost canceled by the exchange repulsion in the axial substitution, whereas in the facial substitution, together with the exchange repulsion it augments the electrostatic energy. The systems with electron-accepting substituents (NO2, CN, Br, Cl, F) favor the axial substituent conformation, while those with electron-donating substituents (NH2, CH3, OH) favor the facial substituent conformation. The interactions for the T-shape complex systems of an aromatic ring with other counterpart such as H2, H2O, HCl, and HF are also studied.

Cation-pi and the corresponding anion-pi interactions have in general been investigated as binary complexes despite their association with counterions. However, a recent study of the ammonia channel highlights the important but overlooked role of anions in cation-pi interactions. In an effort to examine the structural and energetic consequences of the presence of counterions, we have carried out detailed ab initio calculations on some model cation-pi-anion ternary complexes and evaluated the nonpair potential terms, three-body contributions, and attractive and repulsive energy components of the interaction energy. The presence of the anion in the vicinity of the pi system leads to a large redistribution of electron density and hence leads to an inductive stabilization. The resulting electronic and geometrical changes have important consequences in both chemical and biological systems. Compared to cation-pi-anion ternary complexes, the magnitude of the cation-pi interaction in pi-cation-anion ternary complexes is markedly lower because of charge transfer from the anion to the cation.

Using basis-set extrapolation schemes for a given data set, we evaluated the binding energies and geometries at the complete basis set (CBS) limit at the levels of the second order Møller-Plesset perturbation theory (MP2) and the coupled cluster theory with singles, doubles, and perturbative triples excitations [CCSD(T)]. The systems include the hydrogen bonding (water dimer), aromatic interaction (benzene dimer), pi-H interaction (benzene-water), cation-water, anion-water, pi-cation interaction (cation-benzene), and pi-anion interaction (anion-triazine). One extrapolation method is to exploit both BSSE-corrected and BSSE-uncorrected binding energies for the aug-cc-pVNZ (N = 2, 3, 4, ...) basis set in consideration that both binding energies give the same CBS limit (CBS(B)). Another CBS limit (CBS(C)) is to use the commonly known extrapolation approach to exploit that the electron correlation energy is proportional to N(-3). Since both methods are complementary, they are useful for estimating the errors and trend of the asymptotic values. There is no significant difference between both methods. Overall, the values of CBS(C) are found to be robust because of their consistency. However, for small N (in particular, for N = 2, 3), CBS(N)(B) is found to be slightly better for water-water interactions and cation-water and cation-benzene interactions, whereas CBS(N)(C) is found to be more reliable for bezene-water and anion-water interactions. We also note that the MP2 CBS limit value based on N = 2 and 3 combined with the difference between CCSD(T) and MP2 at N = 2 would be exploited to obtain a CCSD(T)/CBS value for aromatic-aromatic interactions and anion-pi interactions, but not for cationic complexes.

Owing to the utility of redox phenomena of silver in many chemical systems, it is important to understand the coordination chemistry of Ag+ ion and hence the hydration structure. The lowest-energy conformations of Ag+(H2O)1–6 are sensitive to the calculation method employed. The coordination number (Nc) of Ag+(H2O)n is predicted to be 2 for n=2–6 at the density functional theory level, while the Nc for n=3–5 is 3, and that for n=6 is 4 at the second-order Møller–Plesset perturbation level. Further accurate analysis based on coupled-cluster singles and doubles theory with perturbative corrections for triple excitations agrees with the MP2 results except that Nc of 4 is also as competitive as Nc of 3 for n=5. To identify the correct Nc, it would be useful to facilitate the IR experimental characterization. We thus provide the OH spectra for various possible structures. It is interesting to note that the hydration chemistry of Ag+ ion is somewhat different from that of alkali metal ions.

Using the computer-aided molecular design approach, we recently reported the synthesis of calix[4]hydroquinone (CHQ) nanotube arrays self-assembled with infinitely long one-dimensional (1-D) short hydrogen bonds (H-bonds) and aromatic-aromatic interactions. Here, we assess various calculation methods employed for both the design of the CHQ nanotubes and the study of their assembly process. Our calculations include ab initio and density functional theories and first principles calculations using ultrasoft pseudopotential plane wave methods. The assembly phenomena predicted prior to the synthesis of the nanotubes and details of the refined structure and electronic properties obtained after the experimental characterization of the nanotube crystal are reported. For better characterization of intriguing 1-D short H-bonds and exemplary displaced pi-pi stacks, the X-ray structures have been further refined with samples grown in different solvent conditions. Since X-ray structures do not contain the positions of H atoms, it is necessary to analyze the system using quantum theoretical calculations. The competition between H-bonding and displaced pi-pi stacking in the assembling process has been clarified. The IR spectroscopic features and NMR chemical shifts of 1-D short H-bonds have been investigated both experimentally and theoretically. The dissection of the two most important interaction components leading to self-assembly processes would help design new functional materials and nanomaterials.

The intermolecular interaction driven structural change is vital to molecular architecturing. In the Cambridge Structural Database (CSD), we find that the preference for geometrical conformations of electron-deficient π systems is different from those of electron-rich π systems. Indeed, ab initio calculations find that electron-deficient π ring systems should involve different structures and energetics, consistent with the CSD search, due to the electric multipole moments and the decrease in the spatial extent of π-electron density.

Hydrogen-bond assisted enormous broadening of infrared spectra of phenol-water cationic cluster: An ab initio mixed quantum-classical study J. Chem. Phys. 126, 074304 (2007) To understand the hydration phenomena of noble transition metals, we investigated the structures, hydration energies, electronic properties, and spectra of the Cu + ͑H 3 O͒ 1-6 and Au + ͑H 2 O͒ 1-6 clusters using ab initio calculations. The coordination numbers of these clusters are found to be only two, which is highly contrasted to those of Ag + ͑H 2 O͒ n ͑which have the coordination numbers of 3-4͒ as well as the hydrated alkali metal ions ͑which have the coordination numbers of ϳ6͒. For the possible identification of their interesting hydration structures, we predict their IR spectra for the OH stretch modes.