Rotation, partial

For 20- [11] and 40-membered [12] macrocyclic di- and tetra-Schiff bases being derivatives of (R)-BTNOL, the chemical shift differences (AS) measured for diastereotopic methylene protons of BINOL unit in CDCI3 have suggested the presence of the partial rotation around the Ar—O—CH2 bond.27 The splitting of some 1H signals in acetone-d6 was explained as a result of a dynamic interconversion process on the NMR time-scale. 

As mentioned earlier, we usually encounter two characteristic secondary relaxations in polymethacrylates and polyacrylates (below the glass transition temperature) which are assigned to side-chain motions1,12,13,15 The p relaxation due to partial rotation of... 

The partial rotation of side chains12,13) COOR sets in (Fig. 4) at about Ip = 280 K, if R = CH3 QH5 C3H7 C4H9 consequently. Tp of poly(alkyl methacrylates) is not... 

The second type of the diluent-induced alterations in the relaxation pattern is characteristic of e.g. PMAAc149 (Fig. 16), PAAm136 (Fig. 17), and PHEA127 (Fig. 18). Incorporation of water (no other diluent has been used) accounts for a continuous decrease in the temperature and increase in the area of the existing / or /T loss peak. Since no new peak is produced, it seems reasonable to infer that the mobility of diluent molecules sets in along with the partial rotation of the COOR side chains. However, it is possible to... 

The temperature position of the secondary fi relaxation (about 290 K 1 Hz), generally attributed to partial rotations of the side chains COOR, is only slightly affected by the polarity and volume of the substituent R but decreases markedly (by 120 K) on removal of the a-methyl group on the main chain. The experimental data obtained contradict the assumption that there is a certain relationship between this temperature and the glass transition temperature. Nevertheless, we can infer that the pertinent molecular mechanism in polymethacrylates differs from that in polyacrylates, probably due to the different participation of the main chains. The values of the individual contributions to the activation energy were estimated by employing a procedure similar to that used in the y relaxation process, and their sum was found to agree approximately with the experimental values. 

The y-dispersion is solely due to water and its relaxational behavior near about 20 GHz. A minor additional relaxation (6) between 3 and y-dispersion is caused in part by rotation of amino acids, partial rotation of charged side groups of proteins, and the relaxation of protein bound water which occurs somewhere between 300 and 2000 MHz. 

The successful application of either microwave or dielectric energy for drying requires that water, as it is evaporated, be removed from the paper and book. Air with low relative humidity is forced through the chamber to facilitate water vapor removal from the object. Since interaction of the water molecules at different locations in the chamber may not be uniform, each frozen flood-damaged item was inverted and partially rotated between each burst of energy. Radiation leakage from both types of equipment is possible therefore, special precautions for com-... 

The effect of organic solvents on the rate constant for amide rotation in Af,A -dimethylacetamide (DMA) has also been investigated (Drakenberg et ai, 1972). As the solvent is changed from water to acetone to cyclohexane, first-order rate constants for rotation increase from 0.025 to 0.33 to 1.5 sec . This observation that nonpolar solvents increase reaction rates indicates that the transition state for amide rotation is nonpolar relative to the reactant state and, thus, is stabilized in nonpolar solvents. This transition state is presumably characterized by partial rotation about the amide bond. In this transition state, polar resonance structures for the amide bond no longer exist and, thus, the transition state is less polar than the reactant state. The 60-fold rate acceleration that accompanies transfer of DMA from water to cyclohexane will provide an important clue in understanding enzymatic prolyl isomerization (see below). 

The available data support a mechanism involving catalysis by distortion in which the enzyme binds and stabilizes a transition state that is characterized by partial rotation about the C-N amide bond. The energy that is required to distort this bond out of planarity with the C=0 bond, thereby destroying the resonance stabilization of the amide linkage, is supplied by favorable transition state binding interactions between enzyme and substrate. As Lumry states (1986), mechanical distortion as a source of small-molecule reactivity is attractive as a basis for enzymatic catalysis. It is quite realistic to assume that a distorted substrate will have enhanced reactivity, either because its ground state or the activated complex for its chemical reaction or both are altered by strain and stress in the protein conformation. However, as mentioned previously, this distortion need not be the result of mechanical deformation but could also be the result of desolvation or electrostatic destabilization. In fact, the current data support contributions from all three mechanisms for distortion. 

Wasserman, E. Yaga-, W. A. Kuck, V. J. EPR of CH2 a substiantiaUy bent and partially rotating ground state triplet, ChertL Phys. Lett. 1970, 7,409-413. 

The nomenclature has not been consistent in attempts to describe deviations from the idealized configurations. We will generally refer to a deviation in 0 simply as partial rotation. The term twist has been used for this, but has often been used to describe rotational distortion about the C=C axis. Partial rotation about the remaining axis has been called skewing. The stereochemically nonrigid process of alkene rotation primarily involves changes in 9, but would also... 

The vibrational spectrum of the tetramethylammonium cation in the region 150 -550 cm l contains botii torsional and vibrational modes. The vg and V19 vibrational modes of E and T2 symmetry involve C-N-C bond angle bending. These modes are Raman active and have been studied for TMA+ in several zeolite environments, although little change in frequency is observed (51). The V4 and V12 torsional modes involve partial rotation about C - N bonds and form respectively a singlet (A2) and a triplet (Ti) which are both Raman inactive. These torsional modes are directly observed in the HNS spectra and prove to be sensitive to the character of the TMA+ cation (see Table 1) environment(52). 

To obtain the best possible fit, a full regression analysis will be necessary, and results of this are not yet available. The outstanding question to be answered is whether the broad absorption is to be regarded as a relaxation process, or whether it is a resonance process, a molecular libration (partial rotation within a potential well) such as is responsible for submillimetre-wavelength absorption bands in many liquids it is possible that both processes contribute. A resonance process would produce an effect in the real permittivity similar to an anomalous dispersion a relaxation process merely produces a monotonic fall in the real permittivity. 

Stereochemistry of the Epoxidation Reaction. The stereochemistry of the ethylene epoxidation reaction has been studied using cis- and trans- dideuterio-ethylene. The retention of configuration was found to be either 70% or about 55%, implying that at least partial rotation is possible at some... 

Fig. 18. (A) Top partial rotation about the CH-2-C-2-0-2-C(—O) torsion angle of 17 E0. Note the apparent TS at point 20, which occurs after passing CH-2-C-2-0-2-C(—O) — about —90°. The four structures correspond to points 1, 12, 20, and 22 respectively. Also note the apparent proton transfer to 0-4 after ring opening.

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