Department of Chemistry and Biochemistry, University of California, San Diego, California 92093, United States.
Institute of Chemistry and Fritz Haber Research Center, Hebrew University of Jerusalem, Jerusalem 91904, Israel.
J Phys Chem A. 2020 Jun 25;124(25):5071-5080. doi: 10.1021/acs.jpca.0c01016. Epub 2020 Jun 16.
Recent studies have shown that pyruvic acid can produce higher-molecular-weight compounds upon irradiation in the aqueous phase. These compounds can contribute to the formation of secondary organic aerosols. There have been several previous studies on the effect of ionic strength on the photochemistry of pyruvic acid; however, few of them investigated the effects of marine relevant salts such as NaCl and CaCl. In this study, we examine the effect of NaCl and CaCl, namely, containing the coordinating cations Na and Ca, on the speciation, absorption properties, and photoreactivity of pyruvic acid in aqueous solutions of varying pH. NMR shows that both Ca and Na further deprotonate pyruvic acid and decrease the diol to ketone ratio of pyruvic acid than in pure water at the same pH, especially at more acidic pH (pH less than 4). The absorption spectrum shows a strong red shift in the peak maxima for the n → π* transition of pyruvic acid in NaCl/CaCl solutions. This dependence is much more pronounced for divalent cations (Ca) compared to monovalent cations (Na). Vertical excitation energy calculations of the anionic ketone form of pyruvic acid confirm the same red shift on the n → π* transition peak in the presence of Ca. In addition, the presence of NaCl/CaCl suppresses the photolysis rate of pyruvic acid, which could be due to the deprotonation of pyruvic acid by the cations and the lower photochemical reactivity for pyruvate, the deprotonated form.
最近的研究表明，丙酮酸在水相辐照下会产生高分子量化合物。这些化合物可能有助于二次有机气溶胶的形成。已有一些关于离子强度对丙酮酸光化学反应影响的先前研究；然而，其中很少有研究海洋相关盐类（如 NaCl 和 CaCl）的影响。在这项研究中，我们研究了 NaCl 和 CaCl（即含有配位阳离子 Na 和 Ca）对不同 pH 值水溶液中丙酮酸的形态、吸收特性和光反应性的影响。NMR 表明，与在纯水中相同 pH 值下相比，Ca 和 Na 进一步使丙酮酸脱质子化，降低了丙酮酸的二醇到酮的比例，特别是在更酸性的 pH（pH 小于 4）下。吸收光谱显示，在 NaCl/CaCl 溶液中，丙酮酸的 n→π跃迁的峰最大值发生了强烈的红移。与单价阳离子（Na）相比，这种依赖性在二价阳离子（Ca）中更为明显。丙酮酸阴离子酮形式的垂直激发能量计算证实了在 Ca 存在下 n→π跃迁峰的相同红移。此外，NaCl/CaCl 的存在抑制了丙酮酸的光解速率，这可能是由于阳离子对丙酮酸的去质子化以及去质子化形式丙酮酸的较低光化学活性所致。
