Ionic liquids: not only structurally but also dynamically heterogeneous.

In recent years, the complex and heterogeneous structure of ionic liquids has been demonstrated; however, the consequences on the dynamics have remained elusive. Here, we use femtosecond IR spectroscopy to elucidate the local structural dynamics in protic alkylammonium-based ionic liquids. The structural relaxation after an ultrafast temperature increase, following vibrational excitation and subsequent relaxation of the N-D (or N-H) stretching vibration, is found to vary substantially between the ionic and hydrophobic subdomains. The dynamics in the ionic domains are virtually unaffected by the alkyl chain length and is, therefore, decoupled from viscosity. Equilibration within the hydrophobic subdomains, as evident from the dynamics of the C-H stretching vibration, is faster than that in the ionic domains and shows a remarkably low thermal activation.

[Study on THz spectra and vibrational modes of benzoic acid and sodium Benzoate].

Terahertz time-domain spectroscopy was employed to measure the terahertz absorption spectra of benzoic acid and sodium benzoate at room temperature. The origins of the measured features of benzoic acid were summarized based on previous study. Density functional theory was used to compute and analyze the molecular structure and vibrational modes of sodium benzoate in monomer. Based on the obtained results, the authors found that the THz spectral features can be used to distinguish benzoic acid and sodium benzoate totally; the essential reason for the THz spectral difference between benzoic acid and sodium benzoate is that the electrovalent bond of sodium benzoate affects the values of covalent bond lengths and bond angles, as well as the molecular interactions and arrangement in unit cell; the measured features of benzoic acid and sodium benzoate come from the collective vibrations except the peaks located at 107 cm-1 of benzoic acid and 54 cm-1 of sodium benzoate.

[Terahertz spectroscopy of DNA nucleobases: cytosine and thymine].

The absorption features of DNA nucleobases cytosine and thymine were measured by terahertz time-domain spectroscopy (THz-TDS) from 0.1 to 3.5 THz. Our experimental results clearly show that these important biomolecules exhibit distinctive absorption features in THz region. To the best of our knowledge, the subtle absorption peak of cytosine at 2.53 THz is reported for the first time. Moreover, geometry optimizations and lattice dynamic calculations on cytosine crystal were also performed with the pseudo-potential plane wave method of density functional theory by taking periodic boundary conditions into account. All measured terahertz absorption features of cytosine were assigned successfully and its absorption spectrum was reproduced according to our calculations. Furthermore, our results show that absorption features of cytosine below 3.5 THz arise from external modes in translation and rotation motions, which are dominated by the intermolecular hydrogen bonds.

First principles investigation of L-alanine in terahertz region.

Terahertz absorption spectrum (0.5-4.0 THz) of L-alanine in the solid phase was measured by terahertz time-domain spectroscopy at room temperature. Simulations utilizing gaseous-state and solid-state theory were performed to determine the origins of the observed vibrational features. Our calculations showed that the measured features in solid-state materials could be well understood by considering the crystal packing interactions in a solid-state density functional theory calculation. Furthermore, intermolecular vibrations of L-alanine are found to be the dominating contributions to these measured spectral features in the range of 0.5-4.0 THz, except that located at 3.11 THz.