Isocyanates: group frequencies

To study the influence of the laser repetition rate on the decomposition of Kapton, a SiC disk with abraded Kapton was irradiated with 80 mj cm"2 at a repetition rate of 0.086 Hz. This is well below the frequency where accumulative heating from the laser is important [296]. The spectra match very well the spectra from the experiments using 10 Hz. The only difference is an enhanced absorption of the band at 2270 cm"1, suggesting that in the experiments with higher repetition rates additional thermal decomposition of the isocyanate group takes place. 

These spectra were plotted from runs on a Jarrell-Ash 25-300 Raman spectrophotometer with a 4880 A argon ion laser. In some spectra the region from 4000 to 2000 cm" has been plotted so that the intensity is 0.5 times its true value compared to the rest of the spectrum. These are marked xO.5. Like the infrared spectra, these Raman spectra illustrate a group frequencies which are labeled directly on the spectra. Groups illustrated include alkanes in spectra 1-6, cyclohexanes 7-8, aromatics 9-12,15,17,18,20,21,25, 32-34, double bonds 13,14,24, isocyanate 15, triple bond 16, nitrile 17,18, carbonyls 19-26, alcohols 27-29, ether 30, amines 31, 32, nitro 33, C—Cl 34, C Br 35, and mercaptan 36. A molecular formula index of the Raman spectra follows. 

The first of these bands has a frequency very similar to that of a diazo group and of the C=N group in isocyanates and other compounds. 

Cyanates and Related Species. Theoretical calculations of the electronic spectrum of a number of isocyanates HNCO, FNCO, CINCO, and LiNCO have been performed by means of serai-empirical MO methods. The NCO group is found to be non-linear, in agreement with recent experimental results. The results of these calculations have also been used to assign the u.v. photoelectron spectra of isocyanic acid (HNCO). A theoretical analysis of the vibrations of co-ordinated isocyanate (NCO ) and fulminate (CNO ) groups has been carried out the changes in the stretching vibrational frequency of these groups in metal co-ordination compounds have been explained. 

Polyurethanes are a broad class of polymers produced by the polyaddition reaction of a diisocyanate or a polymeric isocyanate with a polyol, in the presence of suitable catalysts and additives. Under the name of polyurethanes, a practically unlimited number of structures can be involved. The only necessary condition is in general reduced to the presence of the urethane group, (-NHC0-0-) on the macromolec-ular chain with a more or less frequency. The urethane group is usually formed by reaction between isocyanate and hydroxyl groups, although alternative routes such as from bischloroformates and amines are used in special cases. 

Trimeric isocyanates also contain the CO—N—CO system. AUcyl derivatives show their main band around 1700—1680 cm" with a weaker shoulder at 1715—1710 cm". Aromatics have a higher frequency with the main band near 1715 cm" and the weaker shoulder near 1780 cm" This is parallel to the raised carbonyl frequencies of anilides as compared with alkyl amides. Isocyanate dimers are of some theoretical interest in that the carbonyl groups are in direct opposition so the symmetric mode is forbidden in the infra-red. Only a single carbonyl band is therefore seen, and this is near 1780 cm"... 

Aliphatic isocyanurates (isocyanate trimers) have a strong C=0 band at 1700-1680 cm with a weak shoulder near 1755 cm" Aromatic isocya-nurates have a higher C—0 frequency at 1715-1710 cm with a weak shoulder near 1780 cm" because of the electron withdrawing group on the nitrogen. 

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