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

Couplings between molecular and crystal vibrational modes in imidazole have been investigated and the influence of the hydrogen bond on these modes has been studied. In normal‐mode calculations a program has been used which allows all possible couplings between vibrations of the molecules and those of the crystal lattice. In this way molecular modes coupled to lattice vibrations and lattice modes describing predominantly hydrogen‐bond vibrations have been isolated.

Calculation of normal vibrations and intra‐ and intermolecular force constants in crystalline imidazole

The influence of hydrogen bonding on the vibrational spectra of 1,2,4‐triazole has been studied using infrared and Raman spectroscopy and normal coordinate calculations. The infrared vapor phase fundamentals have been assigned. The polarized infrared and Raman spectra of triazole single crystals (Pbca,Z=8) have been investigated in the 4000 to 10 cm−1 region. An assignment of the 18 intramolecular fundamentals is given with help of −d1 and −d3 isotopic derivatives and normal coordinate calculations. Correlation field splitting is discussed. Thirty‐two of the 45 lattice modes have been assigned to their symmetry species and in terms of rotational, translational, and hydrogen bond vibrations. They are compared to those calculated on the ground of the rigid body and flexible molecule models assuming appropriate intermolecular atom–atom potentials. The variation of the NH stretching and bending force constants upon hydrogen bond formation is correlated with the hydrogen bond force constant (0.35 mdyn/A), N–N ...

1,2,4‐Triazole: Vibrational spectra, normal coordinate calculations, and hydrogen bonding

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.

Raman and infrared spectroscopic studies of benzoic acid dimers and crystals have been made. Molecular and crystal normal modes have been calculated. An assignment of the observed spectra has been proposed and some specific modes discussed.

Vibrational spectra and normal mode calculations of benzoic acid single crystals

Die Frequenzen und Bewegungsformen der in den Infrarot- und Raman-Spektren des kristallinen Thioharnstoffs auftretenden Schwingungen der Elementarzellen werden unter Berucksichtigung der Schwingungszustande der Molekule und des Gitters berechnet. Die Potentialfunktionen enthalten fur die inneren Schwingungen der Molekule Kraftkonstanten eines allgemeinen Valenzkraftfeldes, fur die auseren Molekulschwingungen (Gitterschwingungen) werden lediglich Valenzkraftkonstanten in Richtung aller zwischenmolekularen Atomabstande mit Werten kleiner als 3,4 A zugrundegelegt. Die nach unseren Berechnungen ermittelten Kraftkonstanten fur die H···H-Abstande stimmen mit denjenigen uberein, die von Shimanouchi theoretisch begrundet und angewandt wurden [1, 3]. Die Schwingungsfrequenzen des Thioharnstoffs im Bereich von 700 bis 1370 cm−1 konnen keinen bestimmten inneren Koordinaten einzeln zugeordnet werden. Sie verteilen sich uber die C – N- und C – S-Valenzschwingungen sowie Deformationsschwingungen des Gerustes und der NH2-Gruppen. Auch bei den Schwingungen mit niedrigen Frequenzen (<550 cm−1) kann man klar zwischen den inneren Schwingungen und denjenigen, die zum uberwiegenden Teil von zwischenmolekularen Kraften abhangen, unterscheiden, obwohl die Lagen einiger innerer Schwingungen auch von zwischenmolekularen Kraften abhangig sind und die Frequenzen der Gitterschwingungen von inneren Kraftkonstanten der Molekule mitbestimmt werden. Die beobachteten Temperaturabhangigkeiten sowie die Faktorgruppen-Aufspaltungen werden von der Theorie qualitativ gut wiedergegeben.



The infrared and Raman active vibrations of the thiourea unit cells are calculated considering the inner vibrations and the lattice forces. The potential functions for the inner vibrations contain force constants of the general valence force field; for the lattice modes, valence force constants for all intermolecular atom-atom distances less than 3,4 A are used. The calculated H···H force constants are equal to those explained theoretically and applied by Shimanouchi [1,3]. Some of the S···H (– N) force constants confirm the existence of hydrogen bonds in crystalline thiourea. The vibrations in the region 700-1370 cm−1 cannot be assigned to specific inner coordinates. They are distributed over the CS and CN stretching and bending coordinates of the skeleton and the NH2 groups. For the low frequency vibrations, inner and lattice vibrations can be distinguished clearly. Nevertheless the frequencies of some inner vibrations are dependent on intermolecular force constants. On the other hand the frequencies of lattice vibrations are determined in part by molecular force constants. The observed temperature dependence and the factor group splittings are qualitatively well reflected by the calculations.

Molekül‐ und Gitterschwingungen des orthorhombischen und ferroelektrischen Thioharnstoffs: II. Berechnung der innermolekularen und zwischenmolekularen Kraftkonstanten

B. Schrader

Papers

Molekül‐ und Gitterschwingungen des orthorhombischen und ferroelektrischen Thioharnstoffs: II. Berechnung der innermolekularen und zwischenmolekularen Kraftkonstanten