Far-infrared Domain Research Articles

Vibrational Modeling of Copper−Histamine Complexes: Metal−Ligand IR Modes Investigation

Recent reports on proteins and related models show that vibrational spectroscopy in the far-infrared domain is a promising technique to characterize metal sphere coordination in proteins. The low-frequency modes are however complex, and there is a need to develop the analysis of metal sites by means of quantum chemical calculations as a support for useful interpretation of the far-IR data. In this study, we determined vibrational properties for systems containing Cu(II)-N(Imidazole) interactions present in many biological complexes by means of quantum chemical calculations and confronted the normal mode description with available experimental data. Analysis of the [Cu(histamine)]2+ complex led us to conclude that the anharmonic contributions are negligible in the far-IR domain. Geometry optimizations and vibrational frequency calculations of [Cu(hm)]2+ and [Cu(hm)2(ClO4)2] complexes were compared using various hybrid DFT functionals and basis sets. These investigations stressed the need of potential energy distribution calculations (PED) to assign the vibrational modes, to obtain an overall description of the vibration modes, and to efficiently compare the methods. Comparison of calculation methods with the B3LYP/6-31+G(d,p) and B3LYP/6-311+G(2d,2p) methods and with available experimental data showed that the B3LYP/6-31G(d,p) level of theory provides accurate predictions of the normal mode frequencies and assignments. These comparisons also enlighten that theoretical investigations of 2H- and 65Cu-labeled [Cu(hm)2(ClO4)2] complexes give with a very good accuracy the band shifts of the labeled copper-histamine derivatives. The theoretical calculations combined with experimental data allowed us to predict and calculate with good accuracy the values and assignments of the low-frequency IR modes, notably those involving metal contribution.

Temperature Dependence Discontinuity of the Phonon Mode Frequencies Caused by a Zero-Gap State in HgCdTe Alloys

In the HgCdTe (MCT) alloys, a zero-gap state E(g)=Gamma(6)-Gamma(8)=0 may occur as the composition varies from HgTe to CdTe. This singular mechanism of the E(g) variation may be triggered by an external pressure or by a temperature. In this Letter, we present experimental data of the optical reflectivity in the far-infrared (FIR) domain in a wide interval of temperature (from 10 to 290 K) of the Hg1-xCdxTe (x=0.115) samples. Since the intensity of classical IR sources drops abruptly in this spectral region, a brilliant synchrotron radiation FIR source has been used. The results clearly show that frequencies of the optical phonon modes exhibit discontinuity in their temperature dependence when a zero-gap state occurs.

Gas-Phase Vibrational Spectroscopy and Ab Initio Study of Organophosphorous Compounds: Discrimination between Species and Conformers

Gas phase vibrational spectra of dimethyl methylphosphonate (DMMP), trimethyl phosphate (TMP), and triethyl phosphate (TEP) have been measured using FTIR spectroscopy. For DMMP, TMP, and TEP, most of the infrared active vibrational modes have been observed in the 50-5000 cm (-1) spectral range, allowing an unambiguous discrimination between the three molecules. The vibrational analysis of the spectra was performed by comparing with MP2 and B3LYP harmonic and anharmonic force field ab initio calculations. The extension to anharmonic calculations provides the best agreement for the mid-infrared and the near-infrared spectra, but they do not improve the harmonic frequency predictions in the far-infrared domain. This part of the vibrational spectra associated with collective and nonlocalized vibrational modes presents the largest frequency differences between the two lowest energy conformers of DMMP and TMP. These two conformers were taken into account in the vibrational assignment of the spectra. Their experimental evidence was obtained by deconvoluting vibrational bands in the mid-infrared and in the far-infrared regions, respectively. For TEP, the conformational landscape appears very complicated at ambient temperature, and a further analysis at low temperature is required to explain the vibrational features of each conformer.

Nonresonant structured material with extreme effective parameters

We demonstrate that a metamaterial formed by crossed metallic wires is characterized by an anomalously high index of refraction. Such property does not rely on a resonant response of the inclusions, but rather is a consequence of the ultrahigh density of the wires and of the strong electromagnetic interaction between crossed wires. It is shown that the bandwidth of such phenomenon can be quite large, and that it is mildly affected by metallic loss in the far-infrared domain. Prospective applications in the miniaturization of waveguides and other devices are discussed.

Dynamical charge tensors and infrared spectra of the crystalline α-quaterthiophene polymorph phases from first-principles calculations

Infrared intramolecular vibrations and lattice modes for both the crystalline α-quaterthiophene (4T) polymorph phases are investigated by using the modern theory of polarization combined with density functional theory. For the first time, Born effective charge tensors and far- and mid-infrared responses for both the polymorph phases of 4T have been calculated. We have also proposed an assignment of the main spectral infrared signatures of the 4T polymorph phases. These assignments could be of particular interest in the far-infrared domain for the understanding of the electronic transport of these materials.

Far- and Mid-Infrared of Crystalline 2,2‘-Bithiophene: Ab Initio Analysis and Comparison with Infrared Response

Infrared intramolecular vibrations and lattice modes in the crystalline phase of 2,2'-bithiophene (2T) are investigated using the direct method combined with density functional theory (DFT)-based total energy calculations. For the first time, the far- and mid-infrared responses have been calculated from the Gamma-point modes and the Born effective charge tensors of the 2T crystalline phase. The relative good agreement between the calculated and experimental infrared spectra allows us to assign the origin of the main features of the experimental spectra, which is of particular interest in the far-infrared domain. These assignments are useful for understanding all the properties of the 2T crystalline phase in which phonon-phonon and electron-phonon interactions play an important role.

Refractive index measurements in liquids within the frequency region 50 to 600 GHz; complex permittivity of C6H5Cl, CH3Cl and CH2Cl2

For a complete specification of molecular processes involved in rotational dynamics it is necessary to have both the absorption spectra and their dispersive counterparts for frequencies ranging from static to infrared. The authors describe an extension of their experimental set-up for measuring the absorption coefficient alpha over the frequency range 50-600 GHz to measure also the refractive index n. They are now thus able to bridge the gap in complex permittivity data between the microwave and the far-infrared domains. Refractive index is measured using the polarising Martin Puplett interferometer in monochromatic mode of operation, with accuracies of better than 1% over all the covered frequency range. Results of calibration measurements with liquid chlorobenzene (C6H5Cl) and sample absorption-dispersion data for two other liquids, chloromethane (CH3Cl) and dichloromethane (CH2Cl2), are also presented.