Giammanco Chiara H, Kramer Patrick L, Fayer Michael D
Department of Chemistry, Stanford University , Stanford, California 94305, United States.
J Phys Chem B. 2015 Feb 26;119(8):3546-59. doi: 10.1021/jp512426y. Epub 2015 Feb 16.
Dihydrogen bonding occurs between protonic and hydridic hydrogens which are bound to the corresponding electron withdrawing or donating groups. This type of interaction can lead to novel reactivity and dynamic behavior. This paper examines the dynamics experienced by both borohydride and its dihydrogen-bound water solvent using 2D-IR vibrational echo and IR pump-probe spectroscopies, as well as FT-IR linear absorption experiments. Experiments are conducted on the triply degenerate B-H stretching mode and the O-D stretch of dilute HOD in the water solvent. While the B-H stretch absorption is well separated from the broad absorption band of the OD of HOD in the bulk of the water solution, the absorption of the ODs hydrogen bonded to BHs overlaps substantially with the absorption of ODs in the bulk H₂O solution. A subtraction technique is used to separate out the anion-associated OD dynamics from that of the bulk solution. It is found that both the water and borohydride undergo similar spectral diffusion dynamics, and these are very similar to those of HOD in bulk water. Because the B-H stretch is triply degenerate, the IR pump-probe anisotropy decays very rapidly, but the decay is not caused by the physical reorientation of the BH₄⁻ anions. Spectral diffusion occurs on a time scale longer than the anisotropy decay, demonstrating that spectral diffusion is not yet complete even when the transition dipole has completely randomized. To prevent chemical decomposition of the BH₄⁻, 1 M NaOH was added to stabilize the system. 2D-IR experiments on the OD stretch of HOD in the NaOH/water liquid (no borohydride) show that the NaOH has a negligible effect on the bulk water dynamics.
质子氢和氢负离子氢之间会形成双氢键，它们分别与相应的吸电子基团或供电子基团相连。这种相互作用类型可导致新的反应活性和动力学行为。本文使用二维红外振动回波和红外泵浦 - 探测光谱学以及傅里叶变换红外线性吸收实验，研究了硼氢化物及其与双氢键合的水溶剂所经历的动力学。实验针对水溶剂中三重简并的B - H伸缩振动模式以及稀HOD的O - D伸缩进行。虽然B - H伸缩吸收与大量水溶液中HOD的OD宽吸收带很好地分开，但与BHs氢键合的ODs的吸收与大量H₂O溶液中ODs的吸收有很大重叠。采用一种减法技术将与阴离子相关的OD动力学与大量溶液的动力学区分开来。研究发现，水和硼氢化物都经历类似的光谱扩散动力学，并且这些动力学与大量水中HOD的动力学非常相似。由于B - H伸缩是三重简并的，红外泵浦 - 探测各向异性衰减非常迅速，但这种衰减不是由BH₄⁻阴离子的物理重新取向引起的。光谱扩散发生的时间尺度比各向异性衰减长，这表明即使跃迁偶极子已经完全随机化，光谱扩散仍未完成。为防止BH₄⁻发生化学分解，添加了1 M的NaOH以稳定系统。在NaOH/水液体（无硼氢化物）中对HOD的OD伸缩进行的二维红外实验表明，NaOH对大量水的动力学影响可忽略不计。
