Nitrosamines: A challenge for theory and experiment

The electronic structure of 1-nitrosopyrrolidine has been investigated by HeI and HeII UV photoelectron spectroscopy and DFT/CCSD/OVGF calculations. Even high-levels of theory and large basis sets failed to reproduce the measured molecular structure or vertical ionization energies in a consistent manner. The unexpectedly large intensity enhancement of the band corresponding to HOMO ionization was observed in the HeII photoelectron spectrum of 1-nitrosopyrrolidine. Our results indicate that the HOMO and SHOMO ionizations in N-nitrosamines have σ and π characters, respectively rather than the reverse as was previously thought. We relate the electronic structure to the genotoxic, carcinogenic properties of N-nitroso compounds.