Leavitt Christopher M, Oomens Jos, Dain Ryan P, Steill Jeffrey, Groenewold Gary S, Van Stipdonk Michael J
Department of Chemistry, Wichita State University, Wichita, Kansas 67260-0051, USA.
J Am Soc Mass Spectrom. 2009 May;20(5):772-82. doi: 10.1016/j.jasms.2008.12.023. Epub 2009 Jan 9.
Anionic group II metal nitrate clusters of the formula M(2)(NO(3))(5), where M(2) = Mg(2), MgCa, Ca(2), and Sr(2), are investigated by infrared multiple photon dissociation (IRMPD) spectroscopy to obtain vibrational spectra in the mid-IR region. The IR spectra are dominated by the symmetric and the antisymmetric nitrate stretches, with the latter split into high and low-frequency components due to the distortion of nitrate anion symmetry by interactions with the cation. Density functional theory (DFT) is used to predict geometries and vibrational spectra for comparison to the experimental spectra. Calculations yield two stable isomers: the first one contains two terminal nitrate anions on each cation and a single bridging nitrate ("mono-bridging"), while the second structure features a single terminal nitrate on each cation with three bridging nitrate ligands ("tri-bridging"). The tri-bridging isomer is calculated to be lower in energy than the mono-bridging one for all species. Theoretical spectra of the tri-bridging structure provide a better qualitative match to the experimental infrared spectra of Mg(2)(NO(3))(5) and MgCa(NO(3))(5). However, the profile of the low-frequency nu(3) band for the Mg(2) complex suggests a third possible isomer not predicted by theory. The IRMPD spectra of the Ca(2) and Sr(2) complexes are better reconciled by a weighted summation of the spectra of both isomers suggesting that a mixture of structures is present.
研究了化学式为[M₂(NO₃)₅]⁻的阴离子型II族金属硝酸盐簇，其中M₂ = Mg₂、MgCa、Ca₂和Sr₂，通过红外多光子解离（IRMPD）光谱获得中红外区域的振动光谱。红外光谱以对称和反对称硝酸根伸缩振动为主，由于硝酸根阴离子对称性因与阳离子相互作用而扭曲，后者分裂为高频和低频成分。采用密度泛函理论（DFT）预测几何结构和振动光谱，以便与实验光谱进行比较。计算得出两种稳定的异构体：第一种在每个阳离子上包含两个末端硝酸根阴离子和一个单桥连硝酸根（“单桥连”），而第二种结构在每个阳离子上具有一个末端硝酸根和三个桥连硝酸根配体（“三桥连”）。计算得出，对于所有物种，三桥连异构体的能量低于单桥连异构体。三桥连结构的理论光谱与[Mg₂(NO₃)₅]⁻和[MgCa(NO₃)₅]⁻的实验红外光谱提供了更好的定性匹配。然而，Mg₂配合物低频ν₃带的轮廓表明存在理论未预测到的第三种可能异构体。Ca₂和Sr₂配合物的IRMPD光谱通过两种异构体光谱的加权求和能更好地拟合，这表明存在结构混合物。
