Wiseman Alexander, Sims Lacey A, Snead Russell, Gronert Scott, Maclagan Robert G A R, Meot-Ner Mautner Michael
Department of Chemistry, Virginia Commonwealth University , Richmond, Virginia 23284-2006, United States.
J Phys Chem A. 2015 Jan 8;119(1):118-26. doi: 10.1021/jp506913r. Epub 2014 Dec 26.
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.
多环氮杂环化合物（PANHs）在质谱分析的气相中、酸性和生物环境的溶液中以及如果存在的话在星际云中都可以被质子化。通过其气相质子化热化学可以观察到对PANH碱度的内在分子效应。我们通过动力学括值法，利用质子转移反应到参考碱，测定了尺寸和极化率不断增加的原型单氮、3至5环PANH化合物的气相碱度/质子亲和能（GBs/PAs）。实验质子亲和能从1环（吡啶，222.2）增加到2环（喹啉，227.8），再到3至5环化合物，为227 - 234 kcal mol⁻¹。我们还在M06 - 2X/6 - 311 + G**//B3LYP/6 - 31g水平上计算了GB/PA值。计算得到的PAs在实验不确定度范围内与以NIST GB/PA数据库上限为基准的实验值一致。具体而言，对于非芳香族氮参考碱和1至5环PANHs，计算得到的PAs比实验值小1.4 ± 0.9 kcal/mol，与环数、芳香性和分子大小无关。因此，一种计算PANH化合物质子亲和能的有用方法可以是使用M06 - 2X/6 - 311 + G//B3LYP/6 - 31g计算得到的PAs + 1.4 ± 0.9 kcal mol⁻¹。与实验的一致性在该精度范围内支持了NIST数据库，上限可达235 kcal mol⁻¹，尽管在该范围内没有直接的绝对实验基准点。对于天体化学应用，测量得到的PAs允许计算（PANH）⁺• + H₂ → （PANH）H⁺ + H•反应的能量，该反应可能将自由基离子转化为反应性较低的11电子离子。对于3至5环离子，这些反应是吸热的或接近热中性的，并且在低温下会非常缓慢，使得反应性的（PANH）⁺•自由基离子能够在星际云中持续存在。
