cis Versus trans -Azobenzene: Precise Determination of NMR Parameters and Analysis of Long-Lived States of 15 N Spin Pairs.

We provide a detailed evaluation of nuclear magnetic resonance (NMR) parameters of the cis - and trans -isomers of azobenzene (AB). For determining the NMR parameters, such as proton-proton and proton-nitrogen J -couplings and chemical shifts, we compared NMR spectra of three different isotopomers of AB: the doubly 15 N labeled azobenzene, 15 N,15 N'-AB, and two partially deuterated AB isotopomers with a single 15 N atom. For the total lineshape analysis of NMR spectra, we used the recently developed ANATOLIA software package. The determined NMR parameters allowed us to optimize experiments for investigating singlet long-lived spin states (LLSs) of 15 N spin pairs and to measure LLS lifetimes in cis -AB and trans -AB. Magnetization-to-singlet-to-magnetization conversion has been performed using the SLIC and APSOC techniques, providing a degree of conversion up to 17 and 24% of the initial magnetization, respectively. Our approach is useful for optimizing the performance of experiments with singlet LLSs; such LLSs can be exploited for preserving spin hyperpolarization, for probing slow molecular dynamics, slow chemical processes and also slow transport processes.