Bond Properties and Absorption Trends

Let us now consider how bond strength and the masses of the bonded atoms affect the infrared absorption frequency. For the sake of simplicity, we will restrict the discussion to a simple hetero-nuclear diatomic molecule (two different atoms) and its stretching vibration. 

A diatomic molecule can be considered as two vibrating masses connected by a spring. The bond distance continually changes, but an equilibrium or average bond distance can be definedi When-ever the spring is stretched or compressed beyond this equilibrium distance, the potential energy of the system increases. 

As for any harmonic oscillator, when a bond vibrates, its energy of vibration is continually and periodically changing from kinetic to potential energy and back again. The total amount of energy is proportional to the frequency of the vibration, 

Z is a constant that varies from one bond to another. As a first approximation, the force constants for triple bonds are three times those of single bonds, whereas the force constants for double bonds are twice those of single bonds. 

Two things should be noticeable immediately. One is that stronger bonds have a larger force constant K and vibrate at higher frequencies than weaker bonds. The second is that bonds between atoms of higher masses (larger reduced mass, jd) vibrate at lower frequencies than bonds between lighter atoms. 

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The Modes of Stretching and Bending 18 Bond Properties and Absorption Trends 20 The Infrared Spectrometer 23... 

In solid state physics, the sensitivity of the EELS spectrum to the density of unoccupied states, reflected in the near-edge fine structure, makes it possible to study bonding, local coordination and local electronic properties of materials. One recent trend in ATEM is to compare ELNES data quantitatively with the results of band structure calculations. Furthermore, the ELNES data can directly be compared to X-ray absorption near edge structures (XANES) or to data obtained with other spectroscopic techniques. However, TEM offers by far the highest spatial resolution in the study of the densities of states (DOS). 

Hydrophylicity of amide linkage leads to moisture absorption by all aramids. The skin-core structure of the p-aramid fibers plays an important role in the moisture absorption, which is critical for many structural applications of the FRP on their basis. Thus, Fhkuda and Kawai found that the ultrahigh modulus Kevlar 149 has a moisture uptake of 1% (20°C, 55% relative humidity), while in the regular brand Kevlar 29 it is -7% under the same conditions [34], Apparently, above a certain concentration, the water molecules could upset the intermolecular hydrogen bond formation (Figure 8.2) and affect the mechanical properties, as seen by the comparison of the mechanical properties of Kevlar 29 and Kevlar 149. The same authors found that the diffusion coefficient through the skincore structure of the Kevlar fiber is also of importance in the skin the trend is Kevlar 149 > Kevlar 29 > Kevlar 49 and in the core it is reversed Kevlar 29 > Kevlar 49 > Kevlar 149. 

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