Discrimination between Protonation Isomers of Quinazoline by Ion Mobility and UV-Photodissociation Action Spectroscopy

Abstract

The influence of oriented electric fields on chemical reactivity and photochemistry is an area of increasing interest. Within a molecule, different protonation sites offer the opportunity to control the location of charge and thus orientation of electric fields. New techniques are thus needed to discriminate between protonation isomers in order to understand this effect. This investigation reports the UV-photodissociation action spectroscopy of two protonation isomers (protomers) of 1,3-diazanaphthalene (quinazoline) arising from protonation of a nitrogen at either the 1- or 3-position. It is shown that these protomers are separable by field-asymmetric ion mobility spectrometry (FAIMS) with confirmation provided by UV-photodissociation (PD) action spectroscopy. Vibronic features in the UVPD action spectra and computational input allow assignment of the origin transitions to the S1 and S5 states of both protomers. These experiments also provide vital benchmarks for protomer-specific calculations and examination of isomer-resolved reaction kinetics and thermodynamics.

Laser Photodissociation Action Spectroscopy for the Wavelength-Dependent Evaluation of Photoligation Reactions

“…Herein, UV photodissociation action spectroscopy (PDAS) is employed on an ion-trap mass spectrometer coupled to a tunable laser system to evaluate the wavelength dependence of nitrile imine formation from substituted tetrazoles. Combining the analytical benefits of spectroscopy and mass spectrometry, the resulting action spectra reveal absorption events that produce a specified molecular mass change, − which can guide the design of new substrates with desirable photodissociation profiles . We show that the efficiency of nitrile imine formation as a function of irradiation wavelength can be readily surveyed in the gas phase, independent of dipolarophile cycloaddition.…”

Measurement of the Population of Electrosprayed Deprotomers of Coumaric Acids Using UV–Vis Laser Photodissociation Spectroscopy

“…The influence of electrospray ionization (ESI) conditions on the location of protonation and deprotonation sites of electrosprayed ions is a topic of keen debate. − Acid–base reactions are of key importance throughout chemistry and biology, so correctly identifying the structures of protomers and deprotomers can be crucial to understanding reactive processes. While ESI has been successfully employed across analytical chemistry for many years, it is increasingly being used to probe solution-phase reactions and reactive intermediates for both chemical and biochemical systems. − The role of the electrospray process in determining the location of protonation and deprotonation sites is therefore of key chemical interest.…”

Effect of protonation on the photophysical properties of 4-substituted and 4,7-disubstituted quinazoline push-pull chromophores

“…These findings are consistent with a very recent report that the stabilities of two experimentally observed protonation isomers is indistinguishable based both on similar DFT and wave-function calculations. [48] After protonation the change in the frontier molecular orbitals is insignificant and they present a similar topology (see Figure 1 for 1a and 2a), although both the HOMO and LUMO levels are stabilized. This stabilization is more pronounced for the LUMO orbital located on the quinazoline moiety on which the protonation takes place, thus resulting in a slight decrease of density on position 4 for H + (1) and on positions 2 and 4 for H + (3).…”