Thermodynamics and photochemical properties of alpha, beta, and gamma-hexabromocyclododecanes: a theoretical study.

The thermodynamics and photochemical properties of -alpha, +beta, and -gamma-hexabromocyclododecanes (HBCDs) have been investigated by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. The optimized geometries of HBCDs are in agreement with recently published X-ray crystallographic data. The thermodynamic properties of three HBCDs indicate that alpha-HBCD is the most stable one. The TD-DFT calculations obtain the UV absorption spectra of the three HBCDs and provide a detailed assignment of the UV spectral features. Inspections of the frontier molecular orbitals reveal the n-->sigma* nature of the lowest-lying transition and predict the photodegradation and photostereoisomerization trends of HBCDs under the UV illumination with wavelengths shorter than 240 nm.