A. Diefenbach et Fm. Bickelhaupt, Oxidative addition of Pd to C-H, C-C and C-Cl bonds: Importance of relativistic effects in DFT calculations, J CHEM PHYS, 115(9), 2001, pp. 4030-4040
To assess the importance of relativistic effects for the quantum chemical d
escription of oxidative addition reactions of palladium to C-H, C-C and C-C
I bonds., we have carried out a systematic study of the corresponding react
ions of CH4, C2H6 and CH3Cl with Pd-d(10) using nonrelativistic (NR), quasi
relativistic (QR), and zeroth-order regularly approximated (ZORA) relativis
tic density functional theory (DFT) at the BP86/TZ(2)P level. Relativistic
effects are important according to both QR and ZORA, the former yielding si
milar but somewhat more pronounced effects than the latter, more reliable m
ethod: activation barriers are reduced by 6-14 kcal/mol and reaction enthal
pies become 15-20 kcal/mol more exothermic if one goes from NR to ZORA. Thi
s yields, for example, 298 K activation enthalpies DeltaH(298)(not equal) o
f -5.0 (C-H), 9.6 (C-C) and -6.0 kcal/mol (C-Cl) relative to the separate r
eactants at ZORA-BP86/TZ(2)P. In accordance with gas-phase experiments on r
eactions of Pd with alkanes, we find reaction profiles with pronounced pote
ntial wells for reactant complexes (collisionally stabilized and observed i
n experiments for alkanes larger than CH4) at -11.4 (CH4), -11.6 (C2H6) and
-15.6 kcal/mol (CH3Cl) relative to separated reactants [ZORA-BP86/TZ(2)P].
Furthermore, we analyze the height of and the relativistic effects on the
activation energies DeltaE(not equal) in terms of the activation strain Del
taE(strain)(not equal) in of and the transition-state interaction DeltaE(in
t)(not equal) between the reactants in the activated complex, with DeltaE(n
ot equal) = DeltaE(strain)(not equal) + DeltaE(int)(not equal). (C) 2001 Am
erican Institute of Physics.