Rotational effects in complex-forming bimolecular substitution reactions: A quantum-mechanical approach

Nucleophilic Substitution (SN 2): Dependence on Nucleophile, Leaving Group, Central Atom, Substituents, and Solvent

The reaction potential energy surface (PES), and thus the mechanism of bimolecular nucleophilic substitution (SN 2), depends profoundly on the nature of the nucleophile and leaving group, but also on the central, electrophilic atom, its substituents, as well as on the medium in which the reaction takes place. Here, we provide an overview of recent studies and demonstrate how changes in any one of the aforementioned factors affect the SN 2 mechanism. One of the most striking effects is the transition from a double-well to a single-well PES when the central atom is changed from a second-period (e. g. carbon) to a higher-period element (e.g, silicon, germanium). Variations in nucleophilicity, leaving group ability, and bulky substituents around a second-row element central atom can then be exploited to change the single-well PES back into a double-well. Reversely, these variations can also be used to produce a single-well PES for second-period elements, for example, a stable pentavalent carbon species.

Mode-Specific SN2 Reaction Dynamics

Despite its importance in chemistry, the microscopic dynamics of bimolecular nucleophilic substitution (SN2) reactions is still not completely elucidated. In this publication, the dynamics of a prototypical SN2 reaction (F(-) + CH3Cl → CH3F + Cl(-)) is investigated using a high-dimensional quantum mechanical model on an accurate potential energy surface (PES) and further analyzed by quasi-classical trajectories on the same PES. While the indirect mechanism dominates at low collision energies, the direct mechanism makes a significant contribution. The reactivity is found to depend on the specific reactant vibrational mode excitation. The mode specificity, which is more prevalent in the direct reaction, is rationalized by a transition-state-based model.

Reaction cross sections and thermal rate constant for Cl(-) + CH3Br → ClCH3 + Br(-) from J-dependent quantum scattering calculations

Employing dimensionality-reduced time-independent quantum scattering theory and summation over all possible total angular momentum states, initial-state selected reaction cross sections for the exothermic gas-phase bimolecular nucleophilic substitution (SN2) reaction Cl(-) + CH3Br → ClCH3 + Br(-) have been calculated. The carbon-halogen bonds and the rotation of the methyl halides are taken into account. In agreement with previous calculations for J = 0, initial rotational motion of CH3Br decreases the reaction probability and consequently the cross sections. The experimentally obtained thermal rate constant for 300 K is reproduced within the experimental error. For lower temperatures, it is calculated to be below the experimental values but shows the same strong increase for T → 0.

Mechanisms of SN2 reactions: insights from a nearside/farside analysis

A nearside/farside analysis, performed for the first time for a complex-forming polyatomic reaction, reveals details of the reaction mechanism.

Chemical dynamics simulations of X- + CH3Y → XCH3 + Y- gas-phase S(N)2 nucleophilic substitution reactions. Nonstatistical dynamics and nontraditional reaction mechanisms

Extensive classical chemical dynamics simulations of gas-phase X(-) + CH(3)Y → XCH(3) + Y(-) S(N)2 nucleophilic substitution reactions are reviewed and discussed and compared with experimental measurements and predictions of theoretical models. The primary emphasis is on reactions for which X and Y are halogen atoms. Both reactions with the traditional potential energy surface (PES), which include pre- and postreaction potential energy minima and a central barrier, and reactions with nontraditional PESs are considered. These S(N)2 reactions exhibit important nonstatistical atomic-level dynamics. The X(-) + CH(3)Y → X(-)---CH(3)Y association rate constant is less than the capture model as a result of inefficient energy transfer from X(-)+ CH(3)Y relative translation to CH(3)Y rotation and vibration. There is weak coupling between the low-frequency intermolecular modes of the X(-)---CH(3)Y complex and higher frequency CH(3)Y intramolecular modes, resulting in non-RRKM kinetics for X(-)---CH(3)Y unimolecular decomposition. Recrossings of the [X--CH(3)--Y](-) central barrier is important. As a result of the above dynamics, the relative translational energy and temperature dependencies of the S(N)2 rate constants are not accurately given by statistical theory. The nonstatistical dynamics results in nonstatistical partitioning of the available energy to XCH(3) +Y(-) reaction products. Besides the indirect, complex forming atomic-level mechanism for the S(N)2 reaction, direct mechanisms promoted by X(-) + CH(3)Y relative translational or CH(3)Y vibrational excitation are possible, e.g., the roundabout mechanism.

Electronic structure theory study of the F(-) + CH(3)I → FCH(3) + I(-) potential energy surface

MP2 and DFT electronic structure theories, with the functionals OPBE, OLYP, HCTH407, BhandH, and B97-1 for the latter, were used to investigate stationary point properties on the F(-) + CH(3)I → FCH(3) + I(-) potential energy surface (PES). The aug-cc-pVDZ and aug-cc-pVTZ basis sets for C, H, and F, with Wadt and Hay's 3s3p valence basis and an effective core potential (ECP) for iodine, were employed for both MP2 and DFT. Single-point CCSD(T) calculations were also performed to obtain the complete basis set (CBS) limit for the stationary point energies. The CCSD(T)/CBS reaction exothermicity is only 5.0 kJ/mol different than the experimental value. MP2 and DFT do not predict the same stationary points on the PES. MP2 predicts the C(3v) F(-)-CH(3)I and FCH(3)-I(-) ion-dipole complexes and traditional [F-CH(3)-I](-) central barrier as stationary points, as well as a C(s) hydrogen-bonded F(-)-HCH(2)I complex and a [F-HCH(2)-I](-) transition state connecting this latter complex to the F(-)-CH(3)I complex. A CCSD(T)/CBS relaxed potential energy curve, calculated for the MP2 structures, shows that going from the F(-)-CH(3)I complex to the [F-CH(3)-I](-) TS is a barrierless process, indicating these two structures are not stationary points. This is also suggested by the DFT calculations. The structures and frequencies for CH(3)I and CH(3)Cl given by MP2 and DFT are in overall good agreement with experiment. The calculations reported here indicate that the DFT/B97-1 functional gives the overall best agreement with the CCSD(T) energies, with a largest difference of only 7.5 kJ/mol for the FCH(3)-I(-) complex.