Bending frequency

Molecules like those presented in figure B 1.4.4 fonn another interesting suite of targets from a TFIz perspective. Such chains can be treated as rigid rods, and as they get longer their lowest bending frequencies move rapidly into the FIR. For example, the lowest frequencies of a variety of chains are as follows ... 

Figure 5-10 Partial MM3 Output as Related to the Vibrational Spectrum of H2O. The experimental values of the two sti etching and one bending frequencies of water are 3756, 3657, and 1595 cm. The IR intensities are all very strong (vs).

Finally, the S(CH) bending frequencies are practically independant of the physical state of the sample as are the nuclear vibration modes (Table 1-27). 

In order to test the sensitivity of the model, calculations for adsorbed proplyene were repeated with the assumed value of vf 517 cm-1. This increase of 24% in the value of vf caused vf to increase to 1220 cm-1, a change of 1% in frequency. By way of contrast, if the angle was changed by 4%, the change in the calculated value of vf was 50 cm-1 (4%). Thus, the calculations depend little on the assumed bending frequency but provide a sensitive criteria for the angle. The value of 120°, the ideal value for a ir-allyl, provides the best fit to the data. The value of the stretching... 

Infrared Spectrum. The infrared spectrum of gaseous SiF 2 has been recorded from 1050 to 400 cm"1 63 Two absorption bands, centered at 855 and 872 cm 1, were assigned to the symmetric (v j) and antisymmetric (V3) stretching modes, respectively. The assignment was rendered difficult because of the considerable overlap of the two bands. The fundamental bending frequency occurs below the instrumental range of the study, but a value of 345 cm 1 can be determined from the ultraviolet study. The vibrational frequencies were combined with data from a refined microwave study 641 and utilized to calculate force constants and revised thermodynamic functions. 

Figure 4 illustrates the infrared spectrum for a sample of PPE. The absorptions of the peaks at 3.4, 6.9 and 7.3 pm were assigned to C-H stretch and C-H bending frequencies in CH2 and CH3 (33). These absorptions are proportional to the surface density of deposited ethane (16). However, the absorptions at photons near 10 pm are attributable to OH deformations and CO stretchings of alcoholic groups and vibrations of alkyl ketones (22). They also indicate the existence of branches in unsaturated chain (33). 

Conventional infrared spectroscopy yields information about the functional features of various petroleum constituents. For example, infrared spectroscopy will aid in the identification of N-H and O-H functions, the nature of polymethylene chains, the C-H out-of-place bending frequencies, and the nature of any polynuclear aromatic systems (Speight, 1999, and references cited therein). 

Fig. 7. Shrinkage (s) versus bending frequency V2) curves calculated for a series of temperatures )...

These estimations engage the assumption that any additional effects due to bending vibrations will tend to contribute relatively weakly to the isotope effect. In fact, however, the above estimates are probably only ball park estimates, in that coupling of molecular vibrations of multiatomic molecules is ignored Another key point is that a covalent bond need not be completely broken in the transition state, and two isotopic isomers may behave slightly differently in the transition state. Jencks discusses this matter as well as the problem of nonlinear transition states, a condition that takes into account the fact that bending frequencies often lessen the developed magnitude of isotope effects. 

---