Low-Frequency Vibrational Spectra of 4-Fluorophenol Studied by THz Spectroscopy and Solid-State Density Functional Theory.

Hydrogen bonds serve as important intermolecular interactions organizing the spatial arrangement of molecular crystals. Determining the hydrogen-bond orientations remains a challenging task. In the previous XRD study, the authors assumed a single crystal structure of 4-fluorophenol in which molecules form hexamer-ring clusters via anticlockwise hydrogen bonding. However, the existence of the other structure, which adopts clockwise hydrogen bonding, remains uncertain and warrants further exploration. We unveil distinctive fingerprint information associated with both structures by using high-resolution terahertz spectroscopy. The distinct structures result in different intramolecular geometries of 4-fluorophenol regarding the O-H bond configurations, which are manifested by a noticeable peak splitting in the 20-200 cm-1 frequency range. This result illustrates the sensitivity of THz spectroscopy to hydrogen-bond conformational polymorphism. Our findings represent a significant advancement in utilizing high-resolution THz spectroscopy to resolve hydrogen-bond orientations in molecular crystals with possible broad applicability across diverse hydrogen-bonded organic and inorganic framework materials.

Where do the Fluorine Atoms Go in Inorganic-Oxide Fluorinations? A Fluorooxoborate Illustration under Terahertz Light.

The substitution of fluorine atoms for oxygen atoms/hydroxyl groups has emerged as a promising strategy to enhance the physical and chemical properties of oxides/hydroxides in fluorine chemistry. However, distinguishing fluorine from oxygen/hydroxyl in the reaction products poses a significant challenge in existing characterization methods. In this study, we illustrate that terahertz (THz) spectroscopy provides a powerful tool for addressing this challenge. To this end, we investigated two fluorination reactions of boric acid, utilizing MHF2 (M=Na, C(NH2)3) as fluorine reagents. Through an interplay between THz spectroscopy and solid-state density functional theory, we have conclusively demonstrated that fluorine atoms exclusively bind with the sp3-boron but not with the sp2-boron in the reaction products of Na[B(OH)3][B3O3F2(OH)2] (NaBOFH) and [C(NH2)3]2B3O3F4OH (GBF2). Based on this evidence, we have proposed a reaction pathway for the fluorinations under investigation, a process previously hindered due to structural ambiguity. This work represents a step forward in gaining a deeper understanding of the precise structures and reaction mechanisms involved in the fluorination of oxides/hydroxides, illuminated by the insights provided by THz spectroscopy.

Terahertz Fingerprints of Short-Range Correlations of Disordered Atoms in Diflunisal.

This work proposes a terahertz (THz) spectroscopy approach to the investigation of one of the outstanding problems in crystallography-the structure analysis of a crystal with disorder. Form I of diflunisal, in which the two ortho sites on one phenyl ring of diflunisal show occupational disorder, was used for an illustration. THz radiation interacts with the collective vibrations of correlated disorder, thus providing a promising tool to examine the symmetry of short-range correlations of disordered atoms. Through a thorough examination of the selection rule of THz vibrations in which the disordered atoms are involved to different extents, we deduced that only four short-range correlation possibilities of disorder exist and all of them display unambiguous fingerprint peaks in the 50-170 cm-1 frequency region. We finally proposed an alternating packing model in which the correlation lengths of disorder are on the nanometer scale.

Application of THz Vibrational Spectroscopy to Molecular Characterization and the Theoretical Fundamentals: An Illustration Using Saccharide Molecules.

This work illustrates several theoretical fundamentals for the application of THz vibrational spectroscopy to molecular characterization in the solid state using two different types of saccharide systems as examples. Four subjects have been specifically addressed: (1) the qualitative differences in the molecular vibrational signatures monitored by THz and mid-IR vibrational spectroscopy; (2) the selection rules for THz vibrational spectroscopy as applied to crystalline and amorphous systems; (3) a normal mode simulation, using α-l-xylose as an example; and (4) a rigorous mode analysis to quantify the percentage contributions of the intermolecular and intramolecular vibrations to the normal mode of interest.

Elucidation of Chiral Symmetry Breaking in a Racemic Polymer System with Terahertz Vibrational Spectroscopy and Crystal Orbital Density Functional Theory.

The conservation of chiral symmetry has been used as a fundamental rule to determine polymer packing conformations in racemic systems. We have illustrated, through the interplay of polarization terahertz (THz) spectroscopy and solid-state density functional theory, that the chiral symmetry is not conserved in a poly(lactic acid) stereocomplex (scPLA) system. Poly(l-lactic acid) (PLLA) displays a weaker violation of the 31 screw symmetry than poly(d-lactic acid) (PDLA) and possesses a stronger intramolecular vibrational energy, on average, in the low-frequency gamma-point phonon modes than does PDLA. Polarization THz spectroscopy adds a new dimension to polymer crystallography through which new phenomena are expected to be revealed.

Characteristics of Low-Frequency Molecular Phonon Modes Studied by THz Spectroscopy and Solid-State ab Initio Theory: Polymorphs I and III of Diflunisal.

THz absorption spectra of two polymorphs of diflunisal, form I and form III, exhibit distinct features due to the influence of packing conformations on the frequency distributions and IR activities of gamma point phonon modes within the 100 cm(-1) region. In order to understand the origins of these THz modes, we perform a detailed mode analysis. The result shows that although the spectral features are different, these low-frequency phonon modes of the two molecular polymorphs have similar vibrational characteristics in terms of harmonic couplings of intermolecular and intramolecular vibrations.

Intramolecular vibrations in low-frequency normal modes of amino acids: L-alanine in the neat solid state.

This paper presents a theoretical analysis of the low-frequency phonons of L-alanine by using the solid-state density functional theory at the Γ point. We are particularly interested in the intramolecular vibrations accessing low-frequency phonons via harmonic coupling with intermolecular vibrations. A new mode-analysis method is introduced to quantify the vibrational characteristics of such intramolecular vibrations. We find that the torsional motions of COO(-) are involved in low-frequency phonons, although COO(-) is conventionally assumed to undergo localized torsion. We also find the broad distributions of intramolecular vibrations relevant to important functional groups of amino acids, e.g., the COO(-) and NH3(+) torsions, in the low-frequency phonons. The latter finding is illustrated by the concept of frequency distribution of vibrations. These findings may lead to immediate implications in other amino acid systems.

Terahertz spectroscopy and solid-state density functional theory calculation of anthracene: effect of dispersion force on the vibrational modes.

The phonon modes of molecular crystals in the terahertz frequency region often feature delicately coupled inter- and intra-molecular vibrations. Recent advances in density functional theory such as DFT-D(*) have enabled accurate frequency calculation. However, the nature of normal modes has not been quantitatively discussed against experimental criteria such as isotope shift (IS) and correlation field splitting (CFS). Here, we report an analytical mode-decoupling method that allows for the decomposition of a normal mode of interest into intermolecular translation, libration, and intramolecular vibrational motions. We show an application of this method using the crystalline anthracene system as an example. The relationship between the experimentally obtained IS and the IS obtained by PBE-D(*) simulation indicates that two distinctive regions exist. Region I is associated with a pure intermolecular translation, whereas region II features coupled intramolecular vibrations that are further coupled by a weak intermolecular translation. We find that the PBE-D(*) data show excellent agreement with the experimental data in terms of IS and CFS in region II; however, PBE-D(*) produces significant deviations in IS in region I where strong coupling between inter- and intra-molecular vibrations contributes to normal modes. The result of this analysis is expected to facilitate future improvement of DFT-D(*).

Atomistic nucleation sites of Pt nanoparticles on N-doped carbon nanotubes.

The atomistic nucleation sites of Pt nanoparticles (Pt NPs) on N-doped carbon nanotubes (N-CNTs) were investigated using C and N K-edge and Pt L3-edge X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure (EXAFS) spectroscopy. Transmission electron microscopy and XANES/EXAFS results revealed that the self-organized Pt NPs on N-CNTs are uniformly distributed because of the relatively high binding energies of the adsorbed Pt atoms at the imperfect sites. During the atomistic nucleation process of Pt NPs on N-CNTs, stable Pt-C and Pt-N bonds are presumably formed, and charge transfer occurs at the surface/interface of the N-CNTs. The findings in this study were consistent with density functional theory calculations performed using cluster models for the undoped, substitutional-N-doped and pyridine-like-N-doped CNTs.

Localized Gaussian type orbital-periodic boundary condition-density functional theory study of infinite-length single-walled carbon nanotubes with various tubular diameters.

The detailed geometrical structures of zigzag and armchair type single-walled carbon nanotubes (SWCNTs) with infinite tubular length were investigated using localized Gaussian type orbital-periodic boundary condition-density functional theory (LGTO-PBC-DFT) method. The structures of (n, 0) zigzag SWCNTs were optimized for n = 5-21, (n, n) armchair SWCNTs for n = 3-12. For comparison, the optimized geometry of a two-dimensional graphite sheet was also calculated. It was found that the optimized structures of the SWCNTs showed two C-C bond lengths that decrease with an increase in the tubular diameter. More specifically, the two bond lengths converged with those found in the two-dimensional graphite sheet. We also found a degeneracy in the highest occupied crystal orbitals if identical bond lengths were employed for the zigzag SWCNTs and the two-dimensional graphite sheet. This implies that the two different bond lengths found in the zigzag SWCNTs and the two-dimensional graphite sheet are probably due to the Jahn-Teller effect. The armchair SWCNTs show two slightly different bond lengths if the diameter is less than 12 A; otherwise they are almost identical, approaching the longer bond length of the two-dimensional graphite sheet. This can be due to the fact that the armchair SWCNTs do not have degeneracy in occupied crystal orbitals for identical C-C bond lengths. The crossing point of the conducting and valence bands of each armchair SWCNT were also calculated and show a diameter dependence in which the deviation from 2pi/3a decreases as diameter increases.

Excited-state dynamics of trans,trans-distyrylbenzene: transient anisotropy and excitation energy dependence.

We report a study of excited-state dynamics of trans,trans-distyrylbenzene in hexane solution with femtosecond two-color transient absorption spectroscopy. A consistent model of two distinct excited states, S1 and X, connected by the 10 ps dynamics is proposed with the support from the analysis of excited-state anisotropy. An investigation on the 10 ps dynamics with varying excitation energy has been also conducted. In the assumption of fast intramolecular vibrational redistribution, a dependence of this nonradiative 10 ps process on intramolecular temperature in the S1 state has been analyzed. We have found that an effective mode of approximately 1270 cm-1 is responsible for the 10 ps dynamics. The analysis of both anisotropy and pump-dependent results further implies that a distribution of rotamers may exist in the X state. The result indicates a drastic different excited-state relaxation pathway than that of trans-stilbene.

Atomic-scale deformation in N-doped carbon nanotubes.

We present the N-doping induced atomic-scale structural deformation in N-doped carbon nanotubes by using density functional theory calculations. For substitutional N-doped nanotube clusters, the N dopant with an excess electron lone pair exhibits the high negative charge, and the homogeneously distributed dopants enlarge the tube diameter in both zigzag and armchair cases. On the other hand, in pyridine-like N-doped ones, the concentrated N atoms result in a positively curved graphene layer and, thus, can be responsible for tube wall roughness and the formation of interlinked structures.