Concomitant polymorphism and the martensitic-like transformation of an organic crystal.

Crystalline polymorphism is a phenomenon that occurs in many molecular solids, resulting in a diverse range of possible bulk structures. Temperature and pressure can often be used to thermodynamically control which crystal form is preferred, and the associated transitions between polymorphic phases are often discontinuous and complete. N-Methyl-4-carboxypyridinium chloride is a solid that undergoes an apparent continuous temperature-dependent phase transition from an orthorhombic to a monoclinic polymorph. However, a hybrid characterization approach using single-crystal X-ray diffraction, terahertz time-domain spectroscopy, and solid-state density functional theory reveals the transformation to be actually a slowly changing ratio of the two discrete polymorphic forms. The potential energy surface of this process can be directly accessed using terahertz radiation, and the data show that a very low barrier (43.3 J mol-1) exists along the polymorph transformation coordinate

Quantification of cation-anion interactions in crystalline monopotassium and monosodium glutamate salts.

Crystalline salt compounds composed of metal cations and organic anions are becoming increasingly popular in a number of fields, including the pharmaceutical and food industries, where such formulations can lead to increased product solubility. The origins of these effects are often in the interactions between the individual components in the crystals, and understanding these forces is paramount for the design and utilisation of such materials. Monosodium glutamate monohydrate and monopotassium glutamate monohydrate are two solids that form significantly different structures with correspondingly dissimilar dynamics, while their chemistry only differs in cation identity. Crystals of each were characterised experimentally with single-crystal X-ray diffraction and terahertz time-domain spectroscopy and theoretically using solid-state density functional theory simulations, in order to explain the observed differences in their bulk properties. Specifically, crystal orbital overlap and Hamiltonian population analyses were performed to examine the role that the individual interactions between the cation and anion played in the solid-state structures and the overall energetic profiles of these materials

The significance of the amorphous potential energy landscape for dictating glassy dynamics and driving solid-state crystallisation.

The fundamental origins surrounding the dynamics of disordered solids near their characteristic glass transitions continue to be fiercely debated, even though a vast number of materials can form amorphous solids, including small-molecule organic, inorganic, covalent, metallic, and even large biological systems. The glass-transition temperature, Tg, can be readily detected by a diverse set of techniques, but given that these measurement modalities probe vastly different processes, there has been significant debate regarding the question of why Tg can be detected across all of them. Here we show clear experimental and computational evidence in support of a theory that proposes that the shape and structure of the potential-energy surface (PES) is the fundamental factor underlying the glass-transition processes, regardless of the frequency that experimental methods probe. Whilst this has been proposed previously, we demonstrate, using ab initio molecular-dynamics (AIMD) simulations, that it is of critical importance to carefully consider the complete PES - both the intra-molecular and inter-molecular features - in order to fully understand the entire range of atomic-dynamical processes in disordered solids. Finally, we show that it is possible to utilise this dependence to directly manipulate and harness amorphous dynamics in order to control the behaviour of such solids by using high-powered terahertz pulses to induce crystallisation and preferential crystal-polymorph growth in glasses. Combined, these findings provide compelling evidence that the PES landscape, and the corresponding energy barriers, are the ultimate controlling feature behind the atomic and molecular dynamics of disordered solids, regardless of the frequency at which they occur

The 2017 Terahertz Science and Technology Roadmap

Science and technologies based on terahertz frequency electromagnetic radiation (100GHz-30THz) have developed rapidly over the last 30 years. For most of the 20th century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to “real world” applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2016, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 17 sections that cover most of the key areas of THz Science and Technology. We hope that The 2016 Roadmap on THz Science and Technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies

ENVIRONMENTALLY INDUCED CHANGES IN THE BARRIERS TO INTERNAL MOTIONaMOTION^{a}

Author Institution: Department of Chemistry, University of PittsburghThe rotationally resolved $S_{1} \leftarrow S_{0}$ fluorescence excitation spectra of N-methyl-, 3-methyl-, and 5-methylindole and their Ar van der Waals complexes have been obtained. Analyses of these spectra show that the torsional barrier for methyl group rotation is significantly influenced by the attachment of a single Ar atom to the isolated molecule. For example, N-methylindole has $V_{3}(S_{0}) = 244 cm^{-1}$ and $V_{3}(S_{1}) = 115 cm^{-1}$; upon complexation, these values changes to $V_{3}(S_{O}) = 303 cm^{-1}$ and $V_{3}(S_{1}) = 126 cm^{-1}$. Possible reasons for this behavior will be discussed; including (a) simple steric effects, (b) Ar atom induced changes in the electronic distribution of the parent molecule, and/or (c) changes in the nature of the motion itself.$^{a}$ Work supported by NSF

DETERMINATION OF EXCITED STATE DIPOLE MOMENTS OF GAS PHASE MOLECULES. PART IIaII^{a}

$^{a}$ Work supported by NSF.Author Institution: Department of Chemistry, University of PittsburghThe rotationally resolved $S_{1} \leftarrow S_{0}$ fluorescence excitation spectra of aniline, phenol, styrene, and indole have been obtained in the presence of an external electric field. The Stark effect on the rotational energy level structure allows us to accurately determine the values for the different components of the dipole moment vector in the $S_{1}$ electronic state. Electronic excitation can have a considerable effect upon the dipole moment magnitude. An example is aniline, where in $S_{o}$, $\mu_{a} = 1.129 D$ and in $S_{1}$, $\mu_{a} = 2.796 D$. The large increase in the dipole moment is a result of changes in the electron distribution near the nitrogen upon absorption of an ultraviolet photon. Time permitting, preliminary results on molecular clusters will also be presented