IRMPD Spectra of Protonated Hydroxybenzaldehydes: Evidence of Torsional Barriers in Carboxonium Ions.,ChemPhysChem

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IRMPD Spectra of Protonated Hydroxybenzaldehydes: Evidence of Torsional Barriers in Carboxonium Ions.
ChemPhysChem ( IF 2.3 ) Pub Date : 2020-03-11 , DOI: 10.1002/cphc.202000041
Barbara Chiavarino ₁ , Otto Dopfer ₂ , Maria Elisa Crestoni ₁ , Davide Corinti ₁ , Philippe Maître ₃ , Simonetta Fornarini ₁

Affiliation

Protonation at the formyl oxygen atom of benzaldehydes leading to the formation of carboxonium ions yields two distinct isomers, depending on the relative orientation of the proton either cis or trans with respect to the hydrogen atom on the adjacent carbon. In this context, the IR multiple photon dissociation (IRMPD) spectra of protonated ortho, meta, and para‐hydroxybenzaldehydes (OH−BZH⁺), delivered into the gas phase by electrospray ionization of hydro‐alcoholic solutions, are reported in the 3200–3700 cm⁻¹ spectral range. This range is characteristic of O−H stretching modes and thus able to differentiate cis and trans carboxonium isomers. Comparison between IRMPD spectra and DFT calculations at the B3LYP/6‐311++G(2df2p) level suggests that for both p‐OH−BZH⁺ and m‐OH−BZH⁺ only cis conformers are present in the ion population analyzed. For o‐OH−BZH⁺, IRMPD spectroscopy points to a mixture comprising one trans and more than one cis conformers. The energy barrier for cis–trans isomerization calculated for each OH−BZH⁺ isomer is a measure of the degree of π‐electron delocalization. Furthermore, IRMPD spectra of p‐OH−BZH⁺, m‐OH−BZH⁺ and protonated phenol (this last used as reference) were recorded also in the fingerprint range. Both the observed C−O and O−H stretching vibrations appear to be a measure of π‐electron delocalization in the ions.

中文翻译：

质子化的羟基苯甲醛的IRMPD光谱：碳离子中扭转壁垒的证据。

苯甲醛在甲酰氧原子上的质子化导致形成碳鎓离子，产生两种不同的异构体，这取决于质子相对于相邻碳原子上氢原子的顺式或反式的相对取向。在这种情况下，质子化的IR多个光子解离（IRMPD）光谱邻，间，和对位-hydroxybenzaldehydes（OH-BZH ⁺），由水-醇溶液的电喷雾离子化输送到气相中，在3200-报告3700 cm ^-1光谱范围。这个范围是特性-O-H的伸缩振动，从而能够区分顺式和tran的碳鎓异构体。在B3LYP / 6-311 ++ G（2df2p）水平上对IRMPD谱图和DFT计算进行比较表明，对于p- OH-BZH ⁺和m- OH-BZH ^+而言，所分析的离子群中仅存在顺式构象异构体。对于o -OH-BZH ⁺，IRMPD光谱指向包含一个tran和一个以上顺式构象异构体的混合物。为每个OH-BZH ⁺异构体计算的顺式-反式异构化的能垒是π电子离域度的度量。此外，p-的IRMPD光谱OH-BZH ⁺，m- OH-BZH ⁺和质子化苯酚（最后用作参考）也记录在指纹范围内。观察到的C-O和O-H拉伸振动似乎都是对离子中π电子离域的一种度量。

更新日期：2020-03-11

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