Probing structure and phase-transitions in molecular crystals by terahertz time-domain spectroscopy

The spectrum of Ih ice using terahertz time-domain spectroscopy

Here, we report the frequency-dependent spectrum of ice Ih in the range of 0.2-2 THz. We confirm the presence of a feature that blue-shifts from around 1.55-1.65 THz with a decreasing temperature from 260 to 160 K. There is also a change in the trend of the refractive index of ice corresponding to a dispersion, which is also around 1.6 THz. The features are reproduced in data acquired with three commercial terahertz time-domain spectrometers. Computer-simulated spectra assign the feature to lattice translations perpendicular to the 110 and 1̄10 planes of the ice Ih crystal. The feature's existence should be recognized in the terahertz measurements of frozen aqueous solution samples to avoid false interpretations.

THz spectroscopy on the amino acids L-serine and L-cysteine

We present a detailed study on the temperature-dependent THz spectra of the polycrystalline amino acids, L-serine and L-cysteine, for wavenumbers from 20 to 120 cm-1 and temperatures from 4 to 300 K. Even though the structure of these two amino acids is very similar, with a sulfur atom in the side chain of cysteine instead of an oxygen atom in serine, the excitation spectra are drastically different. Obviously, the vibrational dynamics strongly depend on the ability of cysteine to form sulfur-hydrogen bonds. In addition, cysteine undergoes an order-disorder type phase transition close to 80 K, documented by additional specific heat experiments, with accompanying anomalies in the THz results. On increasing temperatures, well-defined vibrational excitations exhibit significant shifts in the eigenfrequencies with concomitant line-broadening yielding partly overlapping modes. Interestingly, several modes completely lose all their dipolar strength and are unobservable under ambient conditions. Comparing the recent results to the published work utilizing THz, Raman, and neutron-scattering techniques, as well as with ab initio simulations, we aim at a consistent analysis of the results ascribing certain eigenfrequencies to distinct collective lattice modes. We document that THz spectra can be used to fine-tune the parameters of model calculations and as fingerprint properties of certain amino acids. In addition, we analyzed the low-temperature heat capacity of both the compounds and detected strong excess contributions compared to the canonical Debye behavior of crystalline solids, indicating soft excitations and a strongly enhanced phonon-density of states at low frequencies.

Wide temperature range studies of the low frequency THz spectrum of benzoic acid using FTIR spectroscopy

Fourier Transform Infrared Spectroscopy was employed to measure the low frequency terahertz transmission spectrum of benzoic acid over a wide temperature range. One broad absorption peak was observed at room temperature but more peaks appeared as the sample was cooled. All the peaks shifted to higher frequencies with the decrease in temperature and they displayed a rate of change in the range of (2–5.6) x 10⁻²cm⁻¹K⁻¹. An anomalous behavior was observed in the peak width and the peak intensity for some absorption peaks and this was attributed to the peaks being composite. There is a good agreement between the model of benzoic acid crystal and the experiment. The QUANTUM ESPRESSO code utilizing the PBEsol functional proved to be slightly better than the GAUSSIAN 03 method which employed NC-PBE functional in predicting the absorption peaks. A comparison was made of these results and the previous studies done using the THz-TDS. These results proved the ability of the FTIR technique in performing low frequency THz measurements.

Insights into the water status in hydrous minerals using terahertz time-domain spectroscopy

The determinations of water status incorporated in hydrous minerals are of considerable significances in geoscience fields. Coincidentally, the aqueous sensitivity of terahertz radiation has motivated numerous explorations in several cross-domain applications. Terahertz time-domain spectroscopy is employed as a major probing technique coupling of traditional detecting methods to uncover the mask of water status in copper sulfate pentahydrate as well as mineral quartz in this article. Based on the quantitative identification of water status in copper sulfate pentahydrate, the water incorporated in mineral quartz is verified qualitatively. Notable differences of optical constants originating from the water content are obtained for copper sulfate pentahydrate and mineral quartz. These present works indicate that terahertz technology can be considered as a promising method to satisfy the ever-increasing requirements in hydrous mineral analyses.

A comparative evaluation of the activities of thiol group and hydroxyl group in low-frequency vibrations using terahertz spectroscopy and DFT calculations

Low-frequency vibrations of biomolecules govern many biological processes like allostery of proteins, binding interaction and solvation. Revealing how involved are different groups of molecules in low-frequency vibrations is of significance to understand the behaviors of biomolecules. To compare the activities of hydroxyl group and thiol group in low-frequency vibrations, we conducted THz measurements and DFT calculations of l‑serine and l‑cysteine. We performed quantitative analyses on their low-frequency vibrations to study their difference. Statistical analyses on the vibrational modes in our studied range suggest the thiol group shows a higher level of activity in low-frequency vibrations than hydroxyl group.

H-bond network in amino acid cocrystals with H2O or H2O2. The DFT study of serine-H2O and serine-H2O2

The structure, IR spectrum, and H-bond network in the serine-H(2)O and serine-H(2)O(2) crystals were studied using DFT computations with periodic boundary conditions. Two different basis sets were used: the all-electron Gaussian-type orbital basis set and the plane wave basis set. Computed frequencies of the IR-active vibrations of the titled crystals are quite different in the range of 10-100 cm(-1). Harmonic approximation fails to reproduce IR active bands in the 2500-2800 frequency region of serine-H(2)O and serine-H(2)O(2). The bands around 2500 and 2700 cm(-1) do exist in the anharmonic IR spectra and are caused by the first overtone of the OH bending vibrations of H(2)O and a combination vibration of the symmetric and asymmetric bendings of H(2)O(2). The quantum-topological analysis of the crystalline electron density enables us to describe quantitatively the H-bond network. It is much more complex in the title crystals than in a serine crystal. Appearance of water leads to an increase of the energy of the amino acid-amino acid interactions, up to ~50 kJ/mol. The energy of the amino acid-water H-bonds is ~30 kJ/mol. The H(2)O/H(2)O(2) substitution does not change the H-bond network; however, the energy of the amino acid-H(2)O(2) contacts increases up to 60 kJ/mol. This is caused by the fact that H(2)O(2) is a much better proton donor than H(2)O in the title crystals.