Theoretical Survey of the Potential Energy Surface of Ti⁺ + Methanol Reaction

Abstract

The gas-phase reaction of Ti(+) ((4)F and (2)F) with methanol is investigated using density functional theory. Geometries and energies of the reactants, intermediates, and products involved are calculated. The approach of Ti(+) toward methanol could form either a "classical" O- or a "nonclassical" eta(3)-methyl H-attached complex. The reaction products observed in the experiment (Guo, Kerns, Castleman J. Phys. Chem. 1992, 96, 4879) are produced via the classical association rather than the nonclassical complex. All possible pathways starting with C-O, C-H, and O-H activation are searched. Methane and methyl loss products (TiO(+) and TiOH(+)) are produced via the C-O activation; the O-H activation accounts for the H(2) and H elimination (producing TiOCH(2)(+) and TiOCH(3)(+)); and the C-H activation is unlikely to be important. Through the bond insertion (H shift) reductive elimination mechanism, the products of a closed-shell molecule (H(2) or methane) elimination could take place on both the quartet and doublet PESs owing to a spin inversion occurring in the course of initial bond insertion, whereas only the quartet products are produced adiabatically via the simple bond insertion-reductive elimination mechanism for the loss of a radical-type species (H or CH(3)). The computational results are in concert with the available experimental information and add new insight into the details of the individual elementary steps.

Discrepancy Between Experimental and Theoretical Predictions of the Adiabaticity of Ti++CH3OH

“…[11] This result is in agreementw ithc alculationso fd ecreasingS OC alongt he reactionp ath, [12] and was recently well-reproduced by trajectory calculations [13] employing am odel for surface hopping dependentu pon SOC. [14] Here we present evidence, based on experimental resultsa nd kinetic modeling, of adiabatic behavior in at ransition metal-activated reactioni ns tark contrastt op redictions based on calculations [15] of the Massey parameter.…”

“…The present work considers the reaction of Ti + with methanol (reaction 1, see Figure ), which has been shown to proceed at 300 K by some combination of reactions 1a–d, with energetics calculated by Zhang et al . using the B3LYP functional. …”

“… Calculated reaction coordinates relevant at thermal energies (see text) for C−O (orange) and O−H (blue) activation; adapted from ref. . Black dots indicate minimum energy crossing points between the quartet (solid lines) and doublet (dashed lines) surfaces.…”

“…showed their calculations to reproduce several experimental thermochemical values, including the doublet to quartet excitation energy. Zhang et al . also calculated the relevant reaction coordinates (Figures and Figure ), finding both major channels (1a and 1b) take place by C−O activation.…”

“…The preference is due to entropic effects: 4 TS CO 1 contains more lower energy vibrational modes than 2 TS OH1 primarily because the former is bent along the Ti‐ O ‐C backbone, resulting in modes with more “bending” and less “stretching” character than in the doublet, which is linear. A crossing point (CP CO ) was calculated along the reaction coordinate slightly past 4 TS CO 1 . TiOH + (1b) can be formed from direct dissociation of either 2 I CO 1 or 4 I CO 1 via adiabatic and diabatic pathways, respectively.…”