Infrared Spectroscopy An Instrumental Method for Detecting Functional Groups

Transcription

1 Infrared Spectroscopy An Instrumental Method for Detecting Functional Groups 1 The Electromagnetic Spectrum Infrared Spectroscopy I. Physics Review Frequency, υ (nu), is the number of wave cycles that pass through a point in one second. It is measured in Hz, where 1 Hz = 1 cycle/sec. Wavelength, λ (lambda), is the length of one complete wave cycle. It is often measured in cm (centimeters). Wavelength and frequency are inversely related: Energy is related to wavelength and frequency by the following formulas: Wavenumbers and energy: υ = c λ E = hυ = hc λ

2 II. IR Spectroscopy A. Bond Vibrations There are two types of molecular vibrations, stretching and bending. 2 You may have come to think of a molecule as having rigid bond lengths and bond angles, as when you work with your molecular model sets. This is not the actual case, since bond lengths and angles represent the average positions about which atoms vibrate. One way of visualizing bond vibrations is to treat the bond as two balls (atoms) connected by a spring. The stretching frequency of a bond can be approximated by Hooke s Law. In this approximation, two atoms and the connecting bond are treated as a simple harmonic oscillator composed of 2 masses (atoms) joined by a spring:

3 According to Hooke s law, the frequency of the vibration of the spring is related to the mass and the force constant of the spring, k, by the following formula for the case of a diatomic molecule: 3 _ o υ is the wavenumber (cm -1 ) o k is the force constant o μ is the reduced mass (proportional to the masses of the atoms) Important Relationships in Hooke s Law 1) Masses of Atoms: as the mass of the atoms increases, the frequency of vibration decreases. C-H C-C C-O C-Cl C-Br C-I 3000 cm cm cm cm cm cm -1 2) Force Constant: as the strength of the spring increases, the frequency of the vibration increases. Type of Bond -C-H (sp 3-1s) =C-H (sp 2-1s) C-H (sp - 1s) Approximate Wavenumber Force Constant 5 x 10 5 dyne/cm 2900 cm x 10 5 dyne/cm 3100 cm x 10 5 dyne/cm 3300 cm -1 B. Other types of Bond Vibrations Vibrational modes are often given descriptive names, such as:

4 4 C. The Absorption Process The absorption of energy is a quantized process. A molecule absorbs only selected energies (frequencies) of IR radiation. In the absorption process, those frequencies of IR radiation which match the natural vibrational frequencies of the molecule are absorbed in a process called resonance. o The energy absorbed increases the amplitude of the vibrational motions (modes) of the bonds in motion. However, not all bonds in a molecule are capable of absorbing IR radiation, even if the frequency of the IR radiation matches the bond vibrations. Only bonds that possess a dipole moment are capable f absorbing IR radiation. When a polar bond vibrates there is a slight change in the dipole moment of the bond. The changing electrical dipole of the bond can then couple with the incoming electromagnetic field of the IR radiation. D. The IR Spectra The absorption frequencies are specified as wavenumbers in units of reciprocal centimeters (cm-1) The spectrum is a plot of wavenumber [(wavelength) -1 ] on the horizontal axis versus strength of absorption on the vertical axis. The IR is a fingerprint (~500 to 1500 cm -1 ) of the molecule because of the unique and large number of peaks seen for a particular molecule.

5 5 E. Interpreting IR Spectra 1) Compare your IR to a reference IR if possible. 2) Note the presence and the absence of peaks. 3) Identify Key Peaks Characteristics: The wavenumber where the absorption occurs The peak intensity relative to others in the spectra (stong, medium, weak) The peak shape (spike, broad, very broad) Acetone Ethanol Acetic Acid 4) Consult correlation tables and know the peaks listed below in number 5 5) Know the following four key IR-absorption bands and band properties. a) sp 3 -C-H stretch slightly less than 3000 cm -1 sp 2 =C-H stretch is slightly greater than 3000cm -1 ( k = wave #) sp C-H stretch is much greater than 3000cm -1 ( k = wave #) b) C=C stretch is around cm -1. Peaks are sharp and of medium intensity c) C=O stretch in the vicinity of 1720 cm -1. Present in aldehydes, ketones, carboxylic acids, esters, amides. Conjugation moves the peak to a lower wavenumber. If present, it is usually the most intensive peak. d) O-H stretch is around 3000 cm -1. Intermolecular H-bonding makes this a very broad peak when the sample is more concentrated In carboxylic acids, this peak is very broad.

6 6 Bond Type Type of Vibration Frequency (cm 1 ) Intensity C H Alkanes (stretch) s CH 3 (bend) 1450 and 1375 m CH 2 (bend) 1465 m Alkenes (stretch) m (out-of-plane bend) s Aromatics (stretch) s (out-of-plane bend) s Alkyne (stretch) 3300 s Aldehyde 2850 and 2750 w C C Alkane Not useful (too many) C=C Alkene m-w Aromatic 1600 and 1475 m-w CC Alkyne m-w C=O Aldehyde s Ketone s Carboxylic acid s Ester s Amide s Anhydride 1810 and 1760 s Acid chloride 1800 s Carbon dioxide 2400 w C O Alcohols, ethers, esters, carb acids s O H Alcohols, phenols Free m H-bonded (broad) m Carboxylic acid (broad) m N H Primary and secondary amines, amides (stretch) m (bend) m-s C N Amines m-s C=N Imines and oximes w-s CN Nitriles m N=O Nitro 1550 and 1350 s S H Thiols 2550 w C X Fluoride s Chloride s Bromide, iodide <667 s