3 Electromagnetic Spectrum : Frequency (v): the number of complete wave cycles in a second.Given in hertz(Hz)Wavelength(λ): distance between any two peaks or troughs of the wave.C: speed of light (3x10 m/sec)Energy of the waves: It is proportional to its frequency and inversely proportional to its wavelength.8E = ____hcλ
5 Infrared Region (IR)IR photons do not have enough energy to cause electronic transition, but they can cause groups of atoms to vibrate with respect to the bonds that connect them. Each trough is caused because energy is being absorbed from that particular frequency of infra-red radiation to excite bonds in the molecule to a higher state of vibration.
6 Infrared Region (IR) Molecular vibration: the covalent bond between two atoms acts like a spring.1-StretchingThe bond will stretched or compressed.Symmetrical stretchingAsymmetrical stretching
7 Infrared Region (IR) 2-Bending: In- plane bending (scissoring and rocking )Out of plane bending (wagging and twisting)scissoringrockingwaggingtwisting
8 Infrared Spectra: 4000-2500 cm-1 single bonds N-H, C-H, O-H N-H broad with spikesO-H broadC-Hcm-1 triple bonds C≡C ,C ≡ NC≡C moderate or weakcm-1 double bond C=C ,C=OC=O ≈1750C=CBelow 1500 cm-1 “fingerprint” region
9 Infrared Spectra:Fingerprint region: (from cm¯¹) Contains very complicated series of absorptions, mainly due to all manner of bending vibrations.
10 Infrared Spectra: C-H stretching (2800 – 3300) cm -1 Alkenes : C=C (1640 – 1680) cm -1Alkynes : C≡C (2100 – 2200) cm -1