Protonation Sites, Tandem Mass Spectrometry and Computational Calculations of o-Carbonyl Carbazolequinone Derivatives.

Maximiliano Martínez-Cifuentes,Pamela Zuñiga-Hormazabal,Braulio Aranda,Boris Weiss-López,Graciela Clavijo-Allancan,Andrés Barriga,Ramiro Araya-Maturana

doi:10.3390/ijms17071071

Abstract

A series of a new type of tetracyclic carbazolequinones incorporating a carbonyl group at the ortho position relative to the quinone moiety was synthesized and analyzed by tandem electrospray ionization mass spectrometry (ESI/MS-MS), using Collision-Induced Dissociation (CID) to dissociate the protonated species. Theoretical parameters such as molecular electrostatic potential (MEP), local Fukui functions and local Parr function for electrophilic attack as well as proton affinity (PA) and gas phase basicity (GB), were used to explain the preferred protonation sites. Transition states of some main fragmentation routes were obtained and the energies calculated at density functional theory (DFT) B3LYP level were compared with the obtained by ab initio quadratic configuration interaction with single and double excitation (QCISD). The results are in accordance with the observed distribution of ions. The nature of the substituents in the aromatic ring has a notable impact on the fragmentation routes of the molecules.

Highlights

Quinones are a class of compounds with high structural diversity and widely present in nature [1,2,3]
Protonation sites were obtained on the basis of molecular electrostatic potential (MEP) [65], local Fukui indexes [66], Parr functions [58], proton affinity (PA) and gas-phase basicities (GB) among several protonated forms
A series of four new o-carbonyl carbazolquinones were synthetized from anilinquinones previously prepared

Summary

Introduction

Quinones are a class of compounds with high structural diversity and widely present in nature [1,2,3]. We observed that CQ1, unsubstituted in the aromatic ring, is obtained in 52% yield for the cyclization reaction. This yield increases to 77% with the 2-methyl substituted compound (CQ2) and decreWaseedobtsoerv2e3d% thaantdC3Q91%, ufnosrubcostmitpuoteudndins tCheQa3roamndatiCc Qri4ng(,4i–sBorbatanidned4–iCnO5O2%Et ysiuelbdstfioturttehde, rtcdsheuyesicgcsplrgikeezecaianststitediviodneongltfyortc)eh2,ya3escc%ultiiigmzoagannpte.idosoTtrn3ithn9arig%nsecatyfehcoiteeroilodcfimnoti.mhnpecoprrsoetuuaabsnnesdctsesittCuoofeQ7nt73ht%eainnswdutihbtChesQtitath4rueo(e4m2n–-taBmtirniecattnhrhdiynel4gas–urfCoobOmrstOtaihttEuiecttersreduinabccgsottimfivtouiprttyoethudienn, rdretehs(apCicseQtckivt2iiniv)tdyealnyiond)f, 2c.y2c.lDizeatteiromnirneaaticotnioonf.Most Favorable Protonation Site. Wwee ccaallccuulated thermodynamic parameters such as proton affifinity (PAA)) aanndd ggaass--pphhaasse basicity (GB) [60,61,62], ffoorr tthhee pprroottoonnaattiioonn ooff aallll ooxxyyggeenn aanndd nniittrrooggeenn aattoommss iinntthheemmooleleccuulleess. Comparison of PA from these CQs with PA from 1,4-Bq shows an increase of around 50 kcalmol for CQs, indicating that the effect of the fused carbazole moiety on BQ favors the protonation. Given that the above calculations were carried out without considering the activation of the ion, it is possible that additional proton migration, induced by collisions, can occur following the initial protonation step [63]

Fragmentation Pathways of Carbazolquinone Derivatives

Mass Spectrometry

Computational Methodology

Findings

Conclusions