Abstract
Calculations are carried out on the à state of HO~2~, CH~3~O~2~, and CH~3~CH~2~O~2~ and 10 isomers and conformers of the isoprene‐OH‐O~2~ peroxy radicals derived from OH addition to isoprene (2‐methyl‐1,3‐butadiene). In addition to calculating vertical and adiabatic excitation energies, we consider the effect of excitation on molecular structure, and examine the OO stretching frequencies, which are known to be major features in the absorption spectra of the à states of the smaller radicals. The two methods used are the configuration interaction with single excitations (CIS) method and time‐dependent density functional theory (TD‐DFT), both with a range of basis sets up to 6‐311++G(2df,2pd). TD‐DFT overestimates excitation energies considerably, while CIS tends to underestimate them slightly. TD‐DFT does seem to capture the trend in excitation energy vs. size for the smaller peroxy radicals. Conformation and configuration strongly affect the excitation energies of the peroxy radicals from isoprene. CIS calculations indicate that the intramolecular OHO hydrogen bonds, present in the ground state of some peroxy radicals from isoprene, are weakened or broken in the excited state, while TD‐DFT calculations suggest they are retained. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 836–845, 2005