Binding Energy, Specificity, and Enzymic Catalysis: The Circe Effect

This paper reviews the experimental studies of displacive phase transitions in solids. Primary emphasis is upon inelastic light scattering and neutron scattering; related infrared reflectivity measurements, as well as x-ray and EPR analyses are also summarized. Several prototype structures are considered in detail: (1) the rocksalt IV-VI semiconductors PbTe, SnTe, and GeTe; (2) the ferroelectric perovskites exemplified by PbTi${\mathrm{O}}_{3}$ and BaTi${\mathrm{O}}_{3}$; (3) perovskites which exhibit cell-doubling transitions, such as LaAl${\mathrm{O}}_{3}$, SrTi${\mathrm{O}}_{3}$, and KMn${\mathrm{F}}_{3}$; (4) crystals having the $\ensuremath{\alpha}$-quartz structure, including Ge${\mathrm{O}}_{2}$, Si${\mathrm{O}}_{2}$, and AlP${\mathrm{O}}_{4}$; (5) the "improper ferroelectrics" ${\mathrm{Gd}}_{2}$${(\mathrm{M}\mathrm{o}{\mathrm{O}}_{4})}_{3}$ and ${\mathrm{Tb}}_{2}$${(\mathrm{M}\mathrm{o}{\mathrm{O}}_{4})}_{3}$; (6) the V-VI-VII semiconductors typified by SbSI; (7) the hydrogen-bonded ferroelectrics of the K${\mathrm{H}}_{2}$P${\mathrm{O}}_{4}$ family; (8) Jahn-Teller systems such as DyV${\mathrm{O}}_{4}$ and RbCo${\mathrm{F}}_{3}$, in which structural distortions occur as secondary effects; (9) order-disorder systems such as NaN${\mathrm{O}}_{2}$ and the ammonium halides (N${\mathrm{H}}_{4}$Cl, N${\mathrm{H}}_{4}$Br), in which no "soft mode" occurs in the spectral region ($\ensuremath{\omega}g{10}^{11}$ Hz) probed by ir, Raman, and neutron spectroscopy; (10) $\ensuremath{\beta}$-tungsten ($A\ensuremath{-}15$) structures such as ${\mathrm{V}}_{3}$Si and ${\mathrm{Nb}}_{3}$Sn, which exhibit high-temperature superconductivity. These crystal categories are used to illustrate several phenomena of current physical interest: Specifically, we discuss harmonic and anharmonic mode coupling; "critical exponents" $\ensuremath{\beta}$ differing from one-half in the temperature dependences of the order parameter $\ensuremath{\phi}(T)={\ensuremath{\phi}}_{0}{(T\ensuremath{-}{T}_{0})}^{\ensuremath{\beta}}$ and of the soft-mode frequency $\ensuremath{\omega}(T)={\ensuremath{\omega}}_{0}{(T\ensuremath{-}{T}_{0})}^{\ensuremath{\beta}}$; and the recently discovered "central" modes centered at zero frequency, which grow in intensity as the transition temperature ${T}_{0}$ is approached from above or below. The review covers the period 1940-1972. A few 1973 works are mentioned for which the author had preprints in 1972 or very early 1973. This review is in no sense a comprehensive survey of ferroelectricity. Readers are referred to the following earlier reviews on that subject: Silverman (1966, 1969), Cochran and Cowley (1967), Blinc (1968), Murzin et al. (1968), Nettleton (1970), and Blinc and Zaks (1972).

Soft-mode spectroscopy: Experimental studies of structural phase transitions

Vibrational spectra of zinc tris(thiourea) sulphate (ZTS), a new semiorganic non-linear optical crystal, were record- ed to determine the symmetries of molecular vibrations. The observed Raman and infrared bands are assigned to molecular vibrations of thiourea, sulphate groups and $zn(thiourea)_3SO_4$ tetrabedra. The Raman spectra are inter- preted in the light of crystal structure data. Modifications in the IR spectra of thiourea due to metal complexations are discussed.

Vibrational spectroscopic study of zinc tris(thiourea) sulphate, a new organometallic non-linear optical crystal

Growth and characterization of nonlinear optical crystal zinc tris (thiourea) sulphate in presence of l-arginine

Potential energy surfaces (PES) for rotation about the N−C(sp3) or N−C(aryl) bond and energies of stationary points on PES for rotation about the C(sp2)−N bond are reported for methylthiourea, ethylthiourea, isopropylthiourea, tert-butylthiourea, and phenylurea, using the MP2/aug-cc-pVDZ method. Analysis of alkylthioureas shows that conformations, with alkyl groups cis to the sulfur atom, are more stable (by 0.4−1.5 kcal/mol) than the trans forms. All minima adopt anti configurations with respect to nitrogen pyramidalization, whereas syn configurations are not stationary points on the MP2 potential surface. In contrast, analysis of phenylthiourea reveals that a trans isomer in a syn geometry is the global minimum, whereas a cis isomer in an anti geometry is a local minimum with a relative energy of 2.7 kcal/mol. Rotation about the C(sp2)−N bond in alkyl and phenyl thioureas is slightly more hindered (9.1−10.2 kcal/mol) than the analogous motion in the unsubstituted molecule (8.6 kcal/mol). The maximum bar...

Conformational preferences and internal rotation in alkyl- and phenyl-substituted thiourea derivatives.

Die Infrarot- und Ramanspektren von Thioharnstoff-Einkristallen bei 300 und 120°K werden mit polarisierter Strahlung aufgenommen. Zusatzlich werden die Spektren der deuterierten Verbindung an polykristallinen Proben und die Fern-Infrarot-Spektren an pulverformigen Proben gewonnen. Damit ist eine vollstandige Zuordnung der inneren und der Gitterschwingungen moglich. Durch Kopplung uber zwischenmolekulare Kraftkonstanten und durch Dipol-Wechselwirkung spalten die inneren Schwingungen um ca. 10 — 40 cm−1 auf. Die Umwandlung in die ferroelektrische Phase ist an Aufspaltungen, Intensitatsveranderungen und Frequenzsprungen sichtbar. Aus den beobachteten Daten wird fur den Bereich 25 bis 350 °K die Entropie berechnet, sie ist in guter Ubereinstimmung mit experimentellen Werten.



Infrared and Raman spectra of single crystals of thiourea at 300 and 120°K are recorded using polarized radiation. Furthermore the spectra of the deuterated polycrystalline thiourea are obtained. A complete assignment of all optically active molecular and lattice vibrations is given. Vibrational coupling by intermolecular atom-atom force constants and by dipole interaction lead to a factor group splitting to the order of 10–40 cm−1. The phase transition into the ferroelectric modification is seen from band splittings, variations of intensity and frequency jumps. From the observed data the entropy is calculated for the region from 25 to 350°K. Results are in good agreement with observed values.

Molekül- und Gitterschwingungen des orthorhombischen und ferroelektrischen Thioharnstoffs I. Temperaturabhängigkeit der Infrarot- und Ramanspektren und der berechneten Entropie

Molekuel- und gitterschwingungen des orthorhombischen und ferroelektrischen thioharnstoffs 1. mitt. temperaturabhaengigkeit der infrarot- und ramanspektren und der berechneten entropie 2. mitt. berechnung der innermolekularen und zwis

W. Meier

Papers

Molekül- und Gitterschwingungen des orthorhombischen und ferroelektrischen Thioharnstoffs I. Temperaturabhängigkeit der Infrarot- und Ramanspektren und der berechneten Entropie

Molekuel- und gitterschwingungen des orthorhombischen und ferroelektrischen thioharnstoffs 1. mitt. temperaturabhaengigkeit der infrarot- und ramanspektren und der berechneten entropie 2. mitt. berechnung der innermolekularen und zwis