or may simply begin to participate in the cis-trans equilibration which dominates the cmr and pmr spectra at temperatures above -25°. The large 1450-Hz spectral range over which the resonances are spread causes no signals to be seen at -12°.We find no other cmr signals which might correspond to the intermediate 5. A recent cmr study15 of over 20 (/z6-C5H5)Fe(CO)2X derivatives, where X represents a wide variety of substituents, showed that all CO resonances lie within 7.2 ppm of Fe(CO)5, thus assuring that the signal at -50.2 ppm is not from 5.Studies of Fe3(C0)i2 by cmr have been limited by solubility to a -10 to 50°t emperature range. Only one sharp resonance is measured at -20.1 ppm vs.CS2, whereas the crystal structure16 has both bridging and terminal carbonyls, indicating that this molecule is either a triangle of iron atoms with each iron being bonded to four carbonyls or a fluxional molecule undergoing some sort of bridging-terminal CO interconversion of the type discussed above. Our research group has also obtained spectra at -110°for Fe(CO)3, [(C6H5)sP]Fe(CO)4, and (norbornadiene)Fe(CO)3. No structural interconversion was observed.We are presently undertaking solvent dependent cmr studies of 1 as well as preparing the 1SC enriched dimer and Fe3(C0)12.
Absorption and excitation spectra for Os4+:K2SnCl6, K20sCl6, and K20sBr6 are discussed. At low temperatures crystals of these systems undergo phase transitions to lower symmetry structures. Our data indicate that the effects of lowered symmetry are in general small and that the spectra can be interpreted in a manner paralleling our earlier studies of Os4+ in cubic environments. In pure K20sC16, new transitions were located which cannot be assigned to intraconfigurational transitions of Os4+. These may be due to either impurities or interconfigurational effects.
Site-specific isotopic labeling of molecules is a widely used approach in IR spectroscopy to resolve local contributions to vibrationalm odes.T he inducedf requency shift of the corresponding IR band depends on the substituted masses, asw ell as on hydrogen bondinga nd vibrational coupling. The impact of these differentf actors was analyzed with ad esigned three-stranded b-sheet peptidea nd by use of selected 13 Ci sotope substitutions at multiple positionsi n the peptide backbone. Single-strand labels give rise to isotopically shiftedb ands at different frequencies,d epending on the specific sites;t his demonstrates sensitivity to the local environment. Cross-strand double-and triple-labeled peptides exhibited two resolved bands that could be uniquely assigned to specific residues, the equilibrium IR spectrao fw hich indicatedo nly weak local-mode coupling. Te mperature-jump IR laser spectroscopy was appliedt o monitors tructural dynamics and revealed an impressive enhancement of the isotope sensitivity to both local positions and coupling between them, relative to that of equilibrium FTIR spectroscopy.S ite-specific relaxation rates were altered upon the introductiono fa dditional cross-strand isotopes. Likewise, the rates for the global b-sheet dynamics were affected in am anner dependent on the distinct relaxation behavior of the labeled oscillator.T his study reveals that isotope labels providen ot only local structural probes, but rather sense the dynamic complexityo ft he molecular environment.[a] Dr.[c] Prof. Dr.H.C hi Jiangsu Food and PharmaceuticalS cience College Huai'an (P.R.China)Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.1002/chem.201904497.I tc ontains NMR structure solution data,computational details,DFT simulations of afully minimized threestranded structure, acomparison of turn variation and distance between the strands, complete IR absorption and thermal transition data,s imulated spectra for one representative solution structure, an example for transient relaxation,a nd ac ompleteoverview for T-jump relaxation times.
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