Fortes A Dominic, Parker Stewart F
ISIS Neutron and Muon Facility, STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Chilton, Oxfordshire OX11 0QX, U.K.
J Am Chem Soc. 2022 Sep 28;144(38):17376-17386. doi: 10.1021/jacs.2c01469. Epub 2022 Sep 16.
We have re-investigated the structure and vibrational spectroscopy of the iconic molecule iron pentacarbonyl, Fe(CO), in the solid state by neutron scattering methods. In addition to the known 2/ structure, we find that Fe(CO) undergoes a displacive ferroelastic phase transition at 105 K to a 1̅ structure. We propose that this is a result of certain intermolecular contacts becoming shorter than the sum of the van der Waals radii, resulting in an increased contribution of electrostatic repulsion to these interactions; this is manifested as a strain that breaks the symmetry of the crystal. Evaluation of the strain in a triclinic crystal required a description of the spontaneous strain in terms of a second-rank tensor, something that is feasible with high-precision powder diffraction data but practically very difficult using strain gauges on a single crystal of such low symmetry. The use of neutron vibrational spectroscopy (which is not subject to selection rules) has allowed the observation of all the fundamentals below 700 cm for the first time. This has resulted in the re-assignment of several of the modes. Surprisingly, density functional theory calculations that were carried out to support the spectral assignments provided a poor description of the spectra.
我们通过中子散射方法对标志性分子五羰基铁（Fe(CO)₅）在固态下的结构和振动光谱进行了重新研究。除了已知的2/m结构外，我们发现Fe(CO)₅在105 K时经历了一个位移型铁弹相变，转变为1̅结构。我们认为这是由于某些分子间接触距离变得短于范德华半径之和，导致静电排斥对这些相互作用的贡献增加；这表现为一种打破晶体对称性的应变。对三斜晶体中应变的评估需要用二阶张量来描述自发应变，这对于高精度粉末衍射数据是可行的，但对于如此低对称性的单晶使用应变仪实际上非常困难。中子振动光谱（不受选择定则限制）的使用首次使得能够观测到低于700 cm⁻¹的所有基频。这导致了对几个振动模式的重新归属。令人惊讶的是，为支持光谱归属而进行的密度泛函理论计算对光谱的描述很差。
