Cl⁽⁻⁾ Exchange S_N2 Reaction inside Carbon Nanotubes: C–H···π and Cl···π Interactions Govern the Course of the Reaction

Abstract

The carbon nanotube (CNT)-confined chloride exchange S_N2 reaction for methyl chloride has been examined using either a full quantum mechanical (QM) DFT approach based on the M06-2X functional or a hybrid approach where a (6,6) CNT is satisfactorily described by the molecular mechanics (MM) UFF force field and the substrate by the M06-2X functional (M06-2X/UFF approach). We found that inside the CNT the reaction is disfavored with respect to the gas phase, the intrinsic reaction barrier <i>E</i>_a (difference between the preliminary complex <b>I</b> and transition state <b>TS</b>) being 17.9 kcal mol^–1 (13.2 kcal mol^–1 in the gas phase). The augmented barrier, with respect to the gas phase, can be ascribed to a complex interplay between Cl···π and C–H···π interactions (i.e., interactions of the two Cl atoms and the C–H bonds of the substrate with the carbon electron cloud of the tube wall). While the Cl···π interactions behave like a molecular glue which sticks the two Cl atoms to the tube wall and remain approximately constant in <b>I</b> and <b>TS</b>, the importance of the stabilizing C–H···π interactions is significantly lower in <b>TS</b> with a consequent increase of the barrier. The barrier increases with the increase of the tube length to reach the asymptotic value of 19.9 kcal mol^–1 for tube length larger than 24.4 Å. This value is the minimum length of a (6,6) CNT model system that can emulate the CNT-confined S_N2 reaction and provides useful suggestions to build reliable model systems for other S_N2 reactions and, in general, different chemical processes. Furthermore, the activation barrier <i>E</i>_a is strongly affected by the tube radius. Because of the reduced volume inside the tube causing a strong structural distortion in <b>TS</b>, <i>E</i>_a is very large for small tube radii (34.4 kcal mol^–1 in the (4,4) case). When the volume increases enough (tube (5,5)) to avoid the distortion, the barrier suddenly decreases and remains approximately constant (about 20 kcal mol^–1) for tubes in the range (5,5) to (8,8). The activation barrier grows for a (9.9) tube, and the value again remains approximately constant (about 22 kcal mol^–1) for larger tubes