Hyttinen Noora, Otkjær Rasmus V, Iyer Siddharth, Kjaergaard Henrik G, Rissanen Matti P, Wennberg Paul O, Kurtén Theo
Department of Chemistry, University of Helsinki , P.O. Box 55, FI-00014 Helsinki, Finland.
Department of Chemistry, DK-2100, Copenhagen Ø, University of Copenhagen , Copenhagen, Denmark.
J Phys Chem A. 2018 Jan 11;122(1):269-279. doi: 10.1021/acs.jpca.7b10015. Epub 2017 Dec 22.
High pressure anion chemical ionization is commonly used for the detection of neutral molecules in the gas phase. The detection efficiency in these measurements depends on how strongly the reagent ion binds to the neutral target molecule. We have calculated the binding strength of nitrate (NO), acetate (CHC(O)O), lactate (CHCH(OH)C(O)O), trifluoroacetate (CFC(O)O), trifluoromethanolate (CFO), bromide (Br), and iodide (I) reagent ions to ten different products derived from the OH radical-initiated oxidation of butadiene. We found that the binding of these oxidation products to the reagent ions depends almost linearly on the number of oxygen atoms in the target molecule, with the precise chemical identity of the compound (e.g., the number and relative position of hydroxyl or hydroperoxy groups) playing a more minor role. For acetate, the formation free energy decreases on average by around 4 kcal/mol when the number of oxygen atoms in the sample molecule increases by one. For the other reagent ions the corresponding decrease is around 3 kcal/mol. For all of the molecules studied, acetate forms the most stable clusters and I the least stable. We also investigated the effect of humidity on the chemical ionization by calculating how strongly water molecules bind to both the reagent ions and the ion-molecule clusters. Water binds much more strongly to the reagent ion monomers compared to the reagent ion "dimers" (defined here as a cluster of the reagent anion with the corresponding neutral conjugate acid, e.g., HNO(NO)) or the ion-molecule clusters. This likely leads to a stronger humidity dependence when using reagent ions that are not able to form reagent ion dimers (such as CFC(O)O, CFO, Br, and I).
高压负离子化学电离常用于气相中中性分子的检测。这些测量中的检测效率取决于试剂离子与中性目标分子结合的强度。我们已经计算了硝酸根（NO）、乙酸根（CHC(O)O）、乳酸根（CHCH(OH)C(O)O）、三氟乙酸根（CFC(O)O）、三氟甲醇根（CFO）、溴离子（Br）和碘离子（I）试剂离子与丁二烯羟基自由基引发氧化产生的十种不同产物的结合强度。我们发现，这些氧化产物与试剂离子的结合几乎线性地取决于目标分子中的氧原子数量，而化合物的确切化学特性（例如羟基或氢过氧基的数量和相对位置）起的作用较小。对于乙酸根，当样品分子中的氧原子数量增加一个时，生成自由能平均降低约4千卡/摩尔。对于其他试剂离子，相应的降低约为3千卡/摩尔。对于所有研究的分子，乙酸根形成最稳定的簇，而碘离子形成最不稳定的簇。我们还通过计算水分子与试剂离子和离子 - 分子簇的结合强度来研究湿度对化学电离的影响。与试剂离子“二聚体”（这里定义为试剂阴离子与相应中性共轭酸的簇，例如HNO(NO)）或离子 - 分子簇相比，水与试剂离子单体的结合要强得多。当使用不能形成试剂离子二聚体的试剂离子（如CFC(O)O、CFO、Br和I）时，这可能导致更强的湿度依赖性。
