Infrared spectroscopy skeletal vibrations

Infrared (IR) spectroscopy is perhaps the most convenient complementary technique for use with NMR. For example, we show in Fig. 3(a) (61) an IR spectrum of a soluble PHEMA. The polymer contains hydroxyls (3400 cm-1), saturated hydrocarbon functionality (circa 3900 cm-1 and 1500 1300 cm-1), and ester functionality at 1725 cm-1. Deuterium exchange brought about by exposure to d4 methanol vapor may be used to show that the in chain C-C skeletal vibration of PMMA at 1070 cm-1 which has been associated with atactic polymer, (79) has an analogue in PHEMA at 1080 cm-1 (Fig. 3b). Spectral subtraction after deuteration reveals also the primary alcohol C-O stretch of PHEMA at 1025 cm-1. 

The application of infrared spectroscopy to the solution of such solvation problems is hampered to a certain extent by the fact that complex formation leads to the appearance of new, skeletal vibrations, the coupling of which with the vibrations of the original molecules makes the vibrational spectrum more complicated. The situation may similarly be complicated by other effects, such as changes in the orbital hybridization, back-coordination, etc. In the coordination of the solvent acetonitrile to metal ions, for example, if only the effect of coordination causing a decrease in the electron density on the nitrogen is taken into consideration, the frequency of the C—vibration would be expected to decrease. In fact, in the course of this process the coordination increases the a character of the C—bond, which is accompanied by an increase in the frequency of the C—vibration [Be 61]. In many cases, it is difficult to assign the infrared bands to the corresponding vibrations the conclusions drawn from the spectral data may therefore be uncertain. 

After the crystallinity dependence has been established, the heat capacity of the solid at constant pressure, Cp, must be changed to the heat capacity at constant volume C, as described in Fig. 2.31. It helps in the analysis of the crystalline state that the vibration spectrum of crystalline polyethylene is known in detail fromnonnal mode calculations using force constants derived from infrared and Raman spectroscopy. Such a spectrum is shown in Fig. 2.47 [22]. Using an Einstein function for each vibration as described in Fig. 2.35, one can compute the heat capacity by adding the contributions of all the various frequencies. The heat capacity of the crystalline polyethylene shown in Fig. 2.46 can be reproduced above 50 K by these data within experimental error. Below 50 K, the experimental data show increasing deviations, an indication that the computation of the low-frequency, skeletal vibrations cannot be carried out correctly using such an analysis. 

---