The problematic C2H4+F2 reaction barrier

Abstract

The C(2)H(4)+F(2) reaction is investigated through the most rigorous electronic structure methods currently feasible, using a focal point approach to converge toward the ab initio limit. Explicit computations were executed with basis sets as large as aug-cc-pV5Z and correlation treatments as extensive as coupled cluster through full triples with a perturbative inclusion of quadruple excitations [CCSDT(Q)]. Auxiliary core correlation, diagonal Born-Oppenheimer, and first-order relativistic corrections were included. All optimized geometries and vibrational frequencies were determined completely at the CCSD(T)/aug-cc-pVQZ level. The final C(2)H(4)+F(2) reaction barrier from theory (8.0 kcal mol(-1)) is significantly higher than the recently reported experimental barrier (5.5+/-0.5 kcal mol(-1)). Our computations also yield a new enthalpy of formation of the fluoroethyl radical, Delta(f)H(298) degrees(C(2)H(4)F)=-13.2+/-0.2 kcal mol(-1), whose uncertainty is an order of magnitude less than previous experimental values.

What are the most efficient basis set strategies for correlated wave function calculations of reaction energies and barrier heights?

“…The Gaussian-n approach by Pople, Curtiss, Raghavachari, and co-workers [32][33][34][35] was the first popular example of such a scheme. Hobza and coworkers used a similar strategy, but without following the precise scheme of G-n methods 28,36 Focal point analysis of Allen and co-workers 31,37 and multicoefficient correlation methods (MCCMs) (Refs. [39][40][41][42] by our group are based on a similar idea and may also be considered to be elaborations of the older SEC (Ref.…”

Characterizing the BP Stretching Vibration in Phosphorus‐Substituted Phosphine Boranes

“…Note that the TED components can be negative within the decomposition scheme employed in INTDER, owing to coupling among coordinates 46. 47 As such, it is not impossible for a component to exceed 100 %, because their fractional contributions must sum to 1. Optimized Cartesian coordinates, 3 N −6 simple internal coordinates, and TEDs at the M06‐2X/6‐311G(2df, 2pd) level of theory are provided in the Supporting Information.…”

Elemental F2 with Transannular Dienes: Regioselectivities and Mechanisms

“…The more realistic C s symmetric (close to C 2 v ) transition structure ( TS1 , Figure 1) for the F 2 attack on the CC bond was located at many levels. However, all DFT and high‐level ab initio methods are not able to reproduce the experimental barrier (from +1.8 kcal mol −1 at B3LYP to +6.3 kcal mol −1 at BHH with aug‐cc‐pVTZ basis set),12 and overestimate it by 1.6 and 2.5 kcal mol −1 at CASPT29 and CCSD(T),13 respectively. The problem may arise from the fact that due to low dissociation energy of fluorine (37 kcal mol −1 ) the F 2 +H 2 CCH 2 reaction proceeds through the molecule‐induced homolysis, that is, with the formation14 of two radical species (F .…”

“…This mechanism, which was confirmed9 for the gas phase by crossed beam experiments, is difficult to validate computationally. Although, it was demonstrated that single‐reference computational approaches such as CCSD, CCSD(T), and MP2 are trustworthy,9, 13 the open‐shell states may develop on route to the F 2 attack and cause discrepancies between the experimental and theoretical data. Whereas the attack of F 2 on H 2 CCH 2 occurs on the singlet surface cleanly (our TS1 reoptimized15 as the open‐shell singlet is identical geometrically and energetically to the closed‐shell singlet structure), the situation with other olefins is clearly different.…”