| Popis: |
At its simplest, chemical bonding involves a combination of two dominant contributions: direct electrostatics (ionic) and electron sharing (covalent). The relative importance of these contributors has been the subject of signif- icant study in primary (intramolecular) chemical interactions. For example, the relevance and importance of covalent contributions has been a primary focus of transition metal chemistry for decades. For weaker secondary chem- ical interactions such as hydrogen bonding (HB) and halogen bonding (XB), the prevailing view in the literature is that electrostatic interactions are so dominant that covalent contributions are negligible. A notable exception is that of so-called symmetric hydrogen bonds, which exhibit large covalent contributions. With X-ray Absorption Spectroscopy (XAS), we have provided the first direct experimental evidence of covalency in XB. From such studies, we ob- serve that XB exhibit a significantly higher degree of covalency compared with HB counterparts of similar bond strength. Notably, the degree of co- valency in certain XBs is equivalent to that observed in transition metal halides. Our studies provide information of the electronic changes that oc- cur in both the charge donor and charge acceptor in model systems, affording us a unique experimental view of these weak interactions. We also demon- strate the importance of covalent contributions in XBs by showing the effect of covalency in the electron transfer properties in XB-modified dye sensi- tised solar cells. These results lead us to conclude that XBs should more generally be classified as coordinate bonds (and thus identified using an ar- row) to distinguish them from significantly less covalent HBs and other weak interactions. |
