Oxidative addition of Pd to C-H, C-C and C-Cl bonds: Importance of relativistic effects in DFT calculations

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.