Vibrational Spectrum of 1-Octyne

Display 1-octyne as a ball-and-spoke model. Examine the individual vibrations ( 1400 cm only), and classify the bands in the experimental infrared spectrum as due to specific motions, e.g., CH, CC and C=C stretching, and HCH and CCC bending motions. 

Each atom may vibrate along tliree different coordinates (giving rise to 3N vibrations). However, the origin of the coordinate system and the orientation of the molecule in the coordinate system are arbitrary. This eliminates six vibrations, leaving 3N-6. Linear molecules require only two variables to specify their orientation. Here, only five vibrations are eliminated, leaving 3N-.S. 

Calculated vibrational frequencies will not exactly match measured frequencies (they are typically too large by about 12%). For example, a measured frequency of 1700 cm would correspond to a calculated frequency of around 1900 cm-.  

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Since these molecules are both simple hydrocarbons with mostly the same bonds, the two spectra are very similar. There are various C-H stretching vibrations just below 3000 cm and C-H bending vibrations between 1300 and 1500 cm". We should note, however, that the fingerprint regions of the spectra, though similar, are not identical. Next, consider Figures 5.52 and 5.53 with the spectra of 1-octene and 1-octyne. In Figure 5.52, notice the band for an sp C-H stretch above 3000 cm" and the C=C stretch at 1640 cm". In the spectrum of 1-octyne, we note a very prominent and sharp sp C-H stretch at 3315 cm, the C C stretch... 

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