Protonation Energies of 1–5-Ring Polycyclic Aromatic Nitrogen Heterocyclics: Comparing Experiment and Theory

Abstract

Polycyclic nitrogen heterocyclic compounds (PANHs) can be protonated in the gas phase in mass spectrometry, in solution in acidic and biological environments, and if present, in interstellar clouds. Intrinsic molecular effects on PANH basicities can be observed by their gas phase protonation thermochemistry. We determined the gas phase basicities/proton affinities (GBs/PAs) of prototype one-nitrogen, 3-5-ring PANH compounds of increasing sizes and polarizabilities by kinetic bracketing, using proton transfer reactions to reference bases. The experimental proton affinities increase from 1-ring (pyridine, 222.2); to 2-ring (quinoline, 227.8); to 3-5-ring compounds, 227-234 kcal mol(-1). We also calculated the GB/PA values at the M06-2X/6-311+G**//B3LYP/6-31g* level. The computed PAs agree, within the experimental uncertainty, with the experimental values anchored to the upper range of the NIST GB/PA database. Specifically, the computed PAs are smaller than the experimental values by 1.4 ± 0.9 kcal/mol for nonaromatic nitrogen reference bases and for 1-5-ring PANHs, independently of the number of rings, aromaticity, and molecular size. Therefore, a useful method to calculate proton affinities of PANH compounds can use M06-2X/6-311+G**//B3LYP/6-31g* computational PAs + 1.4 ± 0.9 kcal mol(-1). The agreement with experiment supports the NIST database within this accuracy, in the upper range up to 235 kcal mol(-1), even though there are no direct absolute experimental anchor points in this range. For astrochemical applications, the measured PAs allow calculating the energies of the (PANH)(+•) + H2 → (PANH)H(+) + H(•) reactions that may convert the radical ions to less reactive 11-electron ions. The reactions are endothermic or nearly thermoneutral for the 3-5-ring ions and would be very slow at low temperatures, allowing reactive (PANH)(+•) radical ions to persist in interstellar clouds.

Coumarin Antifungal Lead Compounds from Millettia thonningii and Their Predicted Mechanism of Action

“…In order to shed light on this phenomenon, thermochemical density functional theory (DFT) calculations were performed to determine bond dissociation energies (BDE) for the bonds X and Y, and proton affinities (PA) for derivative 4 . It has been shown that BDEs and PAs can be reliably calculated using the DFT method [35,36,37,38]. PAs of the carbonyl and methoxy oxygen atoms were calculated to determine the preferred protonation site due to the acidic nature of the reaction medium.…”

2,4-Bis(arylethynyl)-9-chloro-5,6,7,8-tetrahydroacridines: synthesis and photophysical properties

“…Acridine derivatives have exhibited a range of biological activities [15][16][17][18][19][20] and have been particularly explored in chemotherapeutic protocols against several types of tumors [21][22][23][24][25][26][27][28][29][30][31] . In recent years, much attention has been devoted to acridines in materials science due to their attractive photophysical and electrochemical properties [32][33][34][35]. They have been investigated in organic electronic devices [36][37][38][39] and were reported to be promising candidates for potential use as organic light emitting diodes [40].…”

Synthesis and investigation on optical and electrochemical properties of 2,4-diaryl-9-chloro-5,6,7,8-tetrahydroacridines

“…Within this background, acridines represent an interesting class of heteroaromatic π-conjugated compounds [11][12][13][14] which show great perspectives in medicinal chemistry [15][16][17][18][19][20][21][22][23], dye industries [24][25][26], and metal chemo sensing [27][28][29]. More recently, acridines have received growing attention in organic electronics [30][31][32], due to their strong electron-donating ability [33,34] and remarkable optoelectronic properties [32,[35][36][37]. Acridines, as aza-analogues of anthracene have obtained much attention in the field of organic light emitting diodes [38][39][40][41].…”