57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

In the past year, we focused on understanding the nature of the cation-p interaction between benzene (C₆H₆) molecule and the calcite (CaCO₃) surface. With the help of a collaborator, quantum chemical calculations were performed for the benzene-calcite interaction at the density functional theory (DFT) level. Using the program package Gaussian, a benzene molecule was placed on top of the  cleavage surface of a calcite slab consisting of 18 CaCO₃ units. The position and structure of the benzene molecule were optimized with the M06 functional and a 6-31G* basis set. During optimization, the calcite surface was structurally constrained. The result of the optimization was the structure of the calcite-BTE complex associated with the global minimal total energy. As shown in Fig. 1, after optimization the benzene molecule was arranged by the cation-p interaction with its aromatic ring facing the exposed surface calcium ions. Each surface calcium ions were connected with 5 oxygen atoms from 4 surface carbonate ions and 1 carbonate ion underneath; therefore, each of them had an empty orbital from the 4s³d⁵-hybridization with a nominal charge of 1/3 of an elementary charge. The aromatic ring of the benzene molecule was oriented most perpendicular to the surface so that the p electrons can enter the empty orbital of the surface calcium atom. The aromatic ring was not centered over the calcium atoms but rather shifted toward the recessed surface oxygen atom bonded to the calcium atom.

The energy of the cation-p interaction was estimated based on the formation energy of calcite-benzene complex calculated from quantum chemical calculations, which was E = 58.4 kJ mol^-1. We further attempted to separate the interaction energy into two contributions with one from the charge-quadrupole electrostatic force and the other from the hydrogen bonding between the aryl hydrogen atoms and the calcite oxygen atoms. The contribution from the cation-π interaction is estimated using the classic quadruple-charge interaction. As shown in Fig. 2, the potential field of the quadrupole is:

where the quadruple moment Q = -2ds², ε₀ = 8.85x10^-12 C² J^-1 m^-1, d is the distance from the testing point P to the center of the aromatic ring, and q is angle formed by the line connecting P and the ring center and the z axis. This gives the energy of the cation-p interaction to be:

where q is the charge of the surface calcium cation. Because each calcium ion has six bonds in calcite, the nominal charge of surface cations is:

where e = 1.6x10^-19 C. Combining the above equations gives

.

The values of Q for BTE molecules can be obtained from the literature.

Recognizing d is the distance from the center of the benzene ring to the calcium cation underneath, we estimate the contribution from the charge-quadrupole electrostatic force to be E₁ = 18.2 kJ mol^-1. The contribution from hydrogen bonding was then computed as E₂ = E - E₁ = 40.2 kJ mol^-1. The calculation above indicates that energetically, the cation-p interaction between benzene and calcite is only 31% electrostatic but mainly (i.e., 69%) due to hydrogen bonding. This insight regarding noncovalent cation-p interactions has not been reported previously in the literature. We are currently verifying this finding by using quantum mechanical methods to directly separate the contributions from the charge-quadrupole electrostatics and the hydrogen bonding without the use of classical approximation.

This grant has made an important and positive impact on my career. It has provided me an opportunity to pursue an idea that is deemed too risky for a junior faculty and would otherwise have been shelved.