The Role of Electron Transfer in the Fragmentation of Phenyl and Cyclohexyl Boronic Acids.

Ana Lozano,Paulo Limão-Vieira,Filipe Ferreira Da Silva,Mónica Mendes,Gustavo García,Beatriz Pamplona,Tymon Kilich,Marta Łabuda,Pedro Gois,João Pereira-Da-Silva

doi:10.3390/ijms20225578

Abstract

In this study, novel measurements of negative ion formation in neutral potassium-neutral boronic acid collisions are reported in electron transfer experiments. The fragmentation pattern of phenylboronic acid is comprehensively investigated for a wide range of collision energies, i.e., from 10 to 1000 eV in the laboratory frame, allowing some of the most relevant dissociation channels to be probed. These studies were performed in a crossed molecular beam set up using a potassium atom as an electron donor. The negative ions formed in the collision region were mass analysed with a reflectron time-of-flight mass spectrometer. In the unimolecular decomposition of the temporary negative ion, the two most relevant yields were assigned to BO− and BO2−. Moreover, the collision-induced reaction was shown to be selective, i.e., at energies below 100 eV, it mostly formed BO−, while at energies above 100 eV, it mostly formed BO2−. In order to further our knowledge on the complex internal reaction mechanisms underlying the influence of the hybridization state of the boron atom, cyclohexylboronic acid was also investigated in the same collision energy range, where the main dissociation channel yielded BO2−. The experimental results for phenyl boronic acid are supported by ab initio theoretical calculations of the lowest unoccupied molecular orbitals (LUMOs) accessed in the collision process.

Highlights

During the last decades, the international scientific community has been paying considerable attention to understanding the role of different molecules used as drug-precursors within physiological environments
We focused on two different boronic acid-containing compounds: the phenyl boronic acid C6H5B(OH)2 (PBA) and the cyclohexyl boronic acid C6H11B(OH)2 (CHBA)
In order to confirm this rationale, we have investigated electron transfer in a cyclohexylboronic acid (CHBA) molecule, since it has no π electrons (Figure 1), i.e., the carbon atoms are sp3-hybridized

Summary

Introduction

The international scientific community has been paying considerable attention to understanding the role of different molecules used as drug-precursors within physiological environments. The negative ions BO− and BO2− are the most intense fragments observed in the TOF mass spectra of PBA at all collision energies and account for 40% of the total anion yield This can be expected considering the high electron affinities of their neutrals (2.51 and 4.46 eV) [15]. Given that EA(C6H5) = (1.0960 ± 0.0060) eV and the electron affinity of B(OH) is unknown but certainly lower than EA(BO2) = (4.460 ± 0.030) eV (see Table 2), in the low-collision energy regime (typically < 100 eV) with longer collision times, initial electron capture to the phenyl ring may be transferred to the boron site as long as the nuclear wavepacket survives long enough for efficient diabatic crossing of the π* and σ* states This is an assertion of the efficient intramolecular charge transfer since the collision complex formed by the potassium cation and the TNI may enhance such a process, preferentially yielding BO− and BO2−. FFiinnaallllyy,, frfroommthtehecoclolilsliiosnio-nin-dinudcuecdeddisdsoiscsioactiioantioonf CoHf BCAH,BsAix,teseinxtedeifnferdeinffteraenniot nasnhioanvse bheaevne abseseignneadssiignnTedablien 1Tawbilteh 1sowmiethofsothmeemonfotthbeemingnodtetbeecitnegd idnetPeBctAed’s iTnOPFBmAa’sssTsOcaFnsm, ainssclusdcainngs, Cin3cHlu4−d/CinBgOCH3−H(4m−//zC4B0O),HC−5H(m10/BzO40−/)C, C4H5H6B1O0B2−O(m−//Cz49H76),BaOn2d−C(m6H/z119O7−),(man/dz 9C96)H. 11O− (m/z 99)

Experimental Methods

Findings

Theoretical Method