Predicted bond energies in peroxides and disulfides by density functional methods

We have performed self-consistent linear combination of Gaussian-type orbitals-density functional calculations for the molecules YY, RY, RYY, RYR', RYYR, and RYYR' with Y=O,S and R,R'=H,CH3. The structures were optimized within the local spin density approximation while the Y--Y, Y--C, and Y--H bond dissociation energies (BDE) were calculated with both a local exchange-correlation energy functional and a gradient corrected functional. Comparison of results obtained with the local and gradient corrected exchange-correlation functionals provides more experience on the successes and failures of gradient corrections. Trends in BDEs and the nature of bonding in oxygen and sulfur containing analog molecules are analyzed on the basis of two observations: (1) the O atom is more electronegative than S, C, and H atoms; (2) a S atom can have a valency larger than two and has a greater ability for multiple bonding than oxygen. Finally, comparison with a number of experimental results suggest that the C--S BDE in CH3S, the S--S BDE in CH3SSCH3, and the enthalpy of formation of CH3S should be reexamined