Theoretical Study of the Oxidation Catalyst N-Hydroxyphthalimide (NHPI): Thermochemical Properties, Internal Rotor Potential, and Gas- and Liquid-Phase Bond Dissociation Energies

Abstract

N-Hydroxyphthalimide (NHPI) has received significant attention in recent years as a catalyst of oxidation synthesis reactions. In this study, we use quantum chemical methods to calculate the molecular geometries, vibrational frequencies, and thermochemical properties of NHPI and the species corresponding to OH hydrogen loss in NHPI, the phthalimide N-oxyl radical (PINO). We also study an analogue of NHPI, N-hydroxymaleimide (NHMI), and we demonstrate that NHPI and NHMI have similar thermochemical properties. G3B3 and CBS-APNO calculations with bond-isodesmic work reactions provide O−H BDEs of 83.3 and 83.5 kcal mol-1 (348.5 and 349.4 kJ mol-1) for NHPI and NHMI, respectively, in the gas phase. Liquid-phase bond energy calculations are performed using the solvents benzene, carbon tetrachloride, acetone, ethanol, DMSO, and acetonitrile with the PCM solvent model. From these liquid-phase G3B3 calculations, we calculate a NHPI O−H BDE of between 83.3 and 83.7 kcal mol-1 (348.5 and 350.2 kJ mol-1) in a variety of polar and nonpolar solvents. We propose that the favorable catalytic properties of NHPI are, in part, due to its O−H bond being close in energy to typical ROO−H bonds and resonantly stabilized C−H bonds.