Guest Partitioning in Carbon Monoxide Hydrate by Raman Spectroscopy

Gas hydrates are inclusion compounds composed of a H-bonded water network forming cages, inside of which gaseous (guest) molecules are encapsulated. Depending on the nature and partitioning of the guest molecules, various types of clathrate structures may be formed. In this work we have elucidated the guest partitioning of the CO hydrate, using high-resolution Raman microspectroscopy, and investigated the impact of pressure–temperature (<i>P</i>–<i>T</i>) conditions on this partitioning. For the first time, vibrational signatures of CO molecules encapsulated in a large cage and small cage are identified. It is also shown that the large cages of the CO hydrate have the ability to easily catch or release CO guest molecules, while the small cages remain singly occupied. Moreover, the study of the <i>P</i>–<i>T</i> dependence of the Raman signature demonstrates not only the CO stretching frequency dependence with the cage filling but also the tuning effect of the cage filling by the <i>P</i>–<i>T</i> conditions of treatment

Magnetism and Molecular Nonlinear Optical Second-Order Response Meet in a Spin Crossover Complex

The quadratic hyperpolarizability of two inorganic Schiff base metal complexes which differ from each other by the nature of the central metal ion (Fe^II or Zn^II) is estimated using hyper-Rayleigh light-scattering (HRS) measurements. The investigated Fe^II microcrystals exhibit a thermal spin-crossover (SCO) from a diamagnetic to a paramagnetic state centered at <i>T</i>_1/2 = 233 K that can be reproduced by the HRS signal whose modest intensity is mainly due to their centrosymmetric packing structure. Diamagnetic Zn^II microcrystals even lead to much weaker (∼400 times) HRS intensities which are in addition temperature-independent. These observations allow us to ascribe the change in HRS of the Fe^II complex to two contributions, namely, the molecular SCO phenomenon and the crystal orientation with respect to the light polarization. A connection between the SCO and a nonlinear optical property has thus been demonstrated for the first time, with potential future applications in photonics

Raman Spectroscopic Investigation of Individual Single-Walled Carbon Nanotubes Helically Wrapped by Ionic, Semiconducting Polymers

Raman-active vibrational modes of (6,5) chirality-enriched single-walled carbon nanotubes (SWNTs), helically wrapped by semiconducting poly­[2,6-{1,5-bis­(3-propoxysulfonic acid sodium salt)}­naphthylene]­ethynylene (PNES), are described in great detail. At an irradiation wavelength of 568.2 nm, the extent to which the environment impacts the nanotube vibrational signature can be probed; in particular, the absence of a G band shift for PNES–[(6,5) SWNT] samples relative to benchmark surfactant-coated nanotubes indicates the lack of any significant charge transfer between the PNES strand and the SWNT skeleton, but electronic spectra provide compelling evidence for polymer-to-SWNT energy transfer. At an irradiation wavelength of 457.9 nm, vibrational modes associated with PNES chains that wrap (6,5) SWNTs are conspicuously enhanced. Under 514.5 nm irradiation, PNES–[(6,5) SWNTs] are not excited in resonance but G and G′ bands associated with these nanohybrids are strongly enhanced, reflecting the excitation of a multiphonon-mediated vibronic transition of the (6,5) SWNT backbone. At a 488.0 nm irradiation wavelength, Raman spectral signatures of both the PNES polymer and the vibronically excited (6,5) SWNT skeleton through one-phonon-assisted processes are pronounced, demonstrating that a specific SWNT chirality and the corresponding semiconducting polymer helically wrapped about its surface can be probed using an excitation wavelength that does not resonantly excite the SWNT structure