Photochemistry of BrNCO and INCO

Abstract

The absorption spectra of BrNCO and INCO in the region 160–400 nm are continuous in nature, and photolysis of these molecules with pulsed excimer lasers leads to dissociation. Photolysis of BrNCO and INCO at 308 and 351 nm, respectively, produces vibrationally cold ground state NCO radicals. Weak NCO A 2Σ+→X 2Π emission results from two-photon absorption processes at these wavelengths. Photolysis of BrNCO and INCO at 193 and 222 nm, respectively, produces intense NCO A 2Σ+→X 2Π emission. The NCO(A) is a direct one-photon photofragment in each case, and has considerable vibrational excitation. The yield of NCO(A) from BrNCO photolysis at 193 nm is approximately 16%; the yield of NCO(A) from INCO photolysis at 222 nm is near 13%. From the production of NCO(A) at these wavelengths and the short wavelength limit of the NCO A 2Σ+→X 2Π emission, limits on the heats of formation of these molecules are found to be ΔHf(BrNCO)≥−1.7 kcal mol−1 and ΔHf(INCO)≥10.9 kcal mol−1. These results demonstrate the preference of the halogen isocyanates for dissociation to doublet fragments by scission of the nitrogen–halogen bond, in contrast to the analogous halogen azides which photodissociate to singlet or triplet fragments.