Etalon-Assisted Study of the Strong CO Ligand Vibrations of the fac-[Re(CO)3(bpy)(CH3CN)]+ Octahedral Complex

The strong CO ligand vibrations of an octahedral complex, fac-[Re (CO)3(bpy)­(CH3CN)]+, in acetonitrile are observed at 2040 and 1932 cm–1. Facial rhenium tricarbonyl systems offer very strong and isolated CO vibrations with the potential for interactions between these vibrations. This work first identifies the dominant ion-pair species using attenuated total reflection infrared (ATR-IR) absorption spectra on a dilution series and then determines the strength of these CO ligand vibrations (as isolated vibrations) with a combination of ATR-IR and etalon-based measurements that determine the absolute complex index of refraction of the solution. Finally, the etalon experiments are modeled to study the interaction between vibrations, which is a property not embedded in the solution’s complex index of refraction. The ATR-IR spectra are accomplished on a dilution series as well as a larger set of spectra as these solutions evaporated. The A′(1) CO ligand band at 2040 cm–1 is fit with a sum of three Lorentzian functions characterizing the distribution of free, solvent-separated, and contact ion pairs of this octahedral complex vs concentration. The other CO ligand band at 1932 cm–1 is broader and complicated by the dynamics of vibrational interactions, the unresolved splitting of the A′(2) and A″ CO vibrations, and ion-pair speciation. The etalon transmission measurements vs angle were on a 0.029 M solution, and Rabi splittings of 19 and 38 cm–1 were observed for the A′(1) CO vibration and the unresolved A′(2) + A″ CO vibrations, respectively. The great strength of the CO ligand vibrations is evident despite the use of a dilute solution. Integrated band intensities are reported in comparison to hybrid density functional calculations for isolated vibrations. Then, the observed Rabi splittings are modeled to obtain the coupling strength of the CO ligand vibration with etalon cavity modes and with each other. In summary, this work develops a method to determine the concentration of these solutions from the ATR-IR spectrum, characterizes the ion-pairing, shows that the index of refraction is not constant in the IR spectral region of interest, and develops an interaction Hamiltonian that characterizes cavity–vibration and vibration–vibration coupling.在乙腈溶剂中，八面体配合物面式-[铼(CO)₃(联吡啶(bpy))(乙腈)]⁺的强CO配体振动峰位于2040 cm⁻¹和1932 cm⁻¹处。面式三羰基铼体系具有极强且孤立的CO振动模式，且这些振动之间存在相互作用的可能性。本研究首先通过稀释系列的衰减全反射红外（attenuated total reflection infrared, ATR-IR）吸收光谱，确定了主导性的离子对物种；随后结合衰减全反射红外与基于标准具（etalon）的测量手段，测定了这些CO配体振动（作为孤立振动）的强度，该测量可获得溶液的绝对复折射率（complex index of refraction）。最后，通过对标准具实验进行建模，研究了振动之间的相互作用——这一性质无法通过溶液的复折射率体现。本研究的衰减全反射红外光谱数据采集既包含稀释系列样本，也包含随溶剂蒸发过程采集的多组光谱。位于2040 cm⁻¹处的A′(1)型CO配体吸收带，通过三个洛伦兹函数（Lorentzian function）的叠加进行拟合，以表征该八面体配合物的自由离子对、溶剂分隔离子对与接触离子对的浓度分布。位于1932 cm⁻¹处的另一CO配体吸收带则更宽，且受到振动相互作用动力学、A′(2)与A″型CO振动的未分辨劈裂以及离子对物种分布的共同影响而变得复杂。针对0.029 M的溶液开展了标准具透射光谱随角度变化的测量，分别在A′(1)型CO振动以及未分辨的A′(2)+A″型CO振动处观测到19 cm⁻¹与38 cm⁻¹的拉比劈裂（Rabi splitting）。即便使用稀溶液，仍可观测到CO配体振动的极强信号。本研究报道了积分带强，并将其与孤立振动的杂化密度泛函（hybrid density functional）计算结果进行了对比。随后，通过对观测到的拉比劈裂进行建模，得到了CO配体振动与标准具腔模之间，以及各CO振动之间的耦合强度。综上，本研究建立了一种通过衰减全反射红外光谱测定该溶液浓度的方法，表征了体系的离子对行为，证实了目标红外光谱区域内折射率并非恒定值，并构建了用于描述腔-振动耦合与振动-振动耦合的相互作用哈密顿量（Hamiltonian）。