Department of Chemistry, Memorial University, St. John's, Newfoundland and Labrador, A1B 3X7, Canada.
J Phys Chem A. 2010 Mar 18;114(10):3449-56. doi: 10.1021/jp9098683.
Complexes between adenine and the alkali metal ions Li(+), Na(+), K(+), and Cs(+) have been investigated by infrared multiple photon dissociation (IRMPD) spectroscopy between 2800 and 3900 cm(-1), as have some singly hydrated complexes. The IRMPD spectra clearly show the N-H stretching and the NH(2) symmetric and asymmetric stretching vibrations of adenine; and for the solvated ions, the O-H stretching vibrations are observed. These experimental spectra were compared with those for a variety of possible structures, including both A9 (A9 refers to the tautomer where hydrogen is on the nitrogen in position 9 of adenine, see Scheme 1) and A7 adenine tautomers, computed using B3-LYP/6-31+G(d,p). By comparing the experimental and the simulated spectra it is possible to rule out various structures and to further assign structures to the species probed in these experiments. Single-point calculations on the B3-LYP/6-31+G(d,p) geometries have been performed at MP2/6-311++G(2d, p) to obtain good estimates of the relative thermochemistries for the different structures. In all cases the computed IR spectrum for the lowest energy structure is consistent with the experimental IRMPD spectrum, but in some cases structural assignment cannot be confirmed based solely upon comparison with the experimental spectra so computed thermochemistries can be used to rule out high-energy structures. On the basis of the IRMPD spectra and the energy calculations, all adenine-M(+) and adenine-M(+)-H(2)O are concluded to be composed of the A7 tautomer of adenine, which is bound to the cations in a bidentate fashion through N3 and N9 (see Scheme 1 for numbering convention). For the hydrated ions water binds directly to the metal ion through oxygen, as would be expected since the metal contains most positive charge density. For the hydrated lithium cation-bound adenine dimer, the water molecule is concluded to be hydrogen bonded to a free basic site of one of the adenine monomers, which is also bound to the lithium cation. Experimental and theoretical results on adenine-Li(+)-H(2)O suggest that the electrosprayed adenine-Li(+) resembles the lowest-energy solution phase ion rather than the lowest-energy gas-phase ion, which is the imine form.
已通过红外多光子解离 (IRMPD) 光谱在 2800 至 3900cm(-1) 范围内研究了腺嘌呤与碱金属离子 Li(+)、Na(+)、K(+) 和 Cs(+) 的复合物，以及一些单水合复合物。IRMPD 光谱清楚地显示了腺嘌呤的 N-H 伸缩和 NH(2)对称和不对称伸缩振动；对于溶剂化离子，观察到 O-H 伸缩振动。这些实验光谱与各种可能的结构进行了比较，包括 A9（A9 是指腺嘌呤中位置 9 的氮上的氢的互变异构体，参见方案 1）和 A7 腺嘌呤互变异构体，使用 B3-LYP/6-31+G(d,p) 进行了计算。通过比较实验和模拟光谱，可以排除各种结构，并进一步将结构分配给这些实验中探测到的物种。在 MP2/6-311++G(2d, p) 上对 B3-LYP/6-31+G(d,p) 几何结构进行了单点计算，以获得不同结构的相对热化学的良好估计值。在所有情况下，计算出的最低能量结构的 IR 光谱与实验的 IRMPD 光谱一致，但在某些情况下，仅基于与实验光谱的比较无法确认结构分配，因此可以使用计算出的热化学来排除高能结构。基于 IRMPD 光谱和能量计算，所有腺嘌呤-M(+)和腺嘌呤-M(+)-H(2)O 均被认为由腺嘌呤的 A7 互变异构体组成，该互变异构体通过 N3 和 N9 以双齿方式与阳离子结合（有关编号约定，请参见方案 1）。对于水合离子，水通过氧直接与金属离子结合，这是意料之中的，因为金属包含最高的正电荷密度。对于水合锂离子结合的腺嘌呤二聚体，得出结论认为水分子通过氢键与一个腺嘌呤单体的游离碱性位点结合，该单体也与锂离子结合。腺嘌呤-Li(+)-H(2)O 的实验和理论结果表明，电喷雾的腺嘌呤-Li(+)类似于最低能量的溶液相离子，而不是最低能量的气相离子，即亚胺形式。
