Twisting modes

Molecular Origins of the Resonance Raman Twisting Modes of... 

MOLECULAR ORIGINS OF THE RESONANCE RAMAN TWISTING MODES OF ANTENNA XANTHOPHYLLS... 

In SmA and lamellar phases, all twist modes are forbidden due to the relatively large values of twist elastic constant (K22), resulting in a different frequency dependence for 2-dimensional DF. Thus, the spectral density J u>) due to 2-dimensional DF (or layer undulations) is given ( T33 [Pg.102]

K.ey I = relative intensity p = depolarization ratio vs = symmetrical stretching-mode vas = asymmetrical stretching-mode 8 = bending mode and r = twisting mode. b Taken as reference. 

The current method of determining the energy properties of polyurethane is the Dynamic Thermal Mechanical Analyzer (DTMA). This instrument applies a cyclic stress/strain to a sample of polyurethane in a tension, compression, or twisting mode. The frequency of application can be adjusted. The sample is maintained in a temperature-controlled environment. The temperature is ramped up over the desired temperature range. The storage modulus of the polyurethane can be determined over the whole range of temperatures. Another important property closely related to the resilience, namely tan delta (8), can also be obtained. Tan (8) is defined in the simplest terms as the viscous modulus divided by the elastic modulus. 

Independently, Nakamoto, Margoshes, and Rundle (1488) also concluded that each type of H bond has a distinct Ai -R relationship. Their figure, which is reproduced in Fig. 3-9, includes data for a wide variety of bond types. These authors reach the important conclusion that m/ramolecular H bonds do not have the same Aj/j-R dependence as do intermolecular bonds. Magat also compiles Ai/.-R data (1316). More novel is Pirenne s attempt to investigate the dependence of the bending and twisting modes on R (1641). (See also 282.)... 

Fig. 4.12. Far infrared spectrum of 2,5-dihydrofuran. P = 60 torr pathlength = 30 cm. The satellite series, originating from the first excited state of the ring twisting mode, is clearly visible shifted to higher frequency.

Separation of the observed modes to Twist, Wag and Rock, however, are not easy since in a crystal these modes would not be independent. Our assignments are based on the following assumptions. (1) Referring the MD calculation for ice Ih, the modes in the middle part of spectra in Fig. 5 are the twist modes. (2) Twist motions are rather independent, where as Wag and Rock are coupled each other. It is because, as shown in Fig.6 (b), twist of B molecule would not significantly affect the motion of A and C. 

The downward shift of the Mo=0 vibration, observed in the Raman and infrared spectra of the catalyst, was concomitant with the development of the water twisting mode observed in the INS spectrum. The alternative possibility, that adsorption of water on calcined molybdate/ alumina was dissociative, was not consistent with the spectra. Dissociative chemisorption implies the formation of a Mo-OH group. Such groups have been identified by INS of hydrogen molybdenum... 

Table 6 Assignment of the absorption bands of the spectrum in Fig. 16. The symbol r refers to the rocking mode and the symbol t refers to the twisting mode...

To evaluate the increment to the isotope effect caused by each vibrational motion in the S + C2H4 reaction, the magnitude of the isotope effect associated with each normal mode was calculated separately. The results obtained for the asymmetrical model of the C2D4/C2H4 reactant pair are collected in Table II. The most important contribution to the isotope effect comes not from the out-of-plane CH bendings but from a single vibration—namely the asymmetric twist mode of the thiirane molecule which is absent in the ethylene. 

To estimate the wave-number of the twisting mode of ethylene from measured values of the heat capacity and the known wave-numbers of the remaining eleven modes. 

The normal vibrational modes of ethylene, numbered according to the convention of Herzberg ( Infra-red and Raman spectra , van Nostrand, 1945) are shown in table 1. Since the molecule has a centre of symmetry no mode symmetrical with respect to this centre can be active in the infra-red and no antisymmetrical mode can be active in the Raman spectrum. The twisting mode number 4 is inactive in both. The most recent assignment (Crawford, Lancaster, and Inskeq>, J. Chem. Phys. 1953, 21, 678) of wave-numbers for the remaining eleven modes are given. 

We shall tentatively use the alternative values 0 = 1400 deg and 04 = 1500 deg for the twisting mode. With these values we calculate the contribution of this mode to the heat capacity and add this to the contribution of the other 11 modes given in table 1. 

T C/R exp 11 known modes twisting mode CjR calc twisting mode CjR calc... 

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