IR

An IR spectrum usually contains more peaks than we can assign, or even need to assign. It is important to learn to focus in on key regions of an IR spectrum. To get a big picture of where these key regions are located, use Figure 14.33 to list the major types of structural units and their associated range of wavenumbers.

3600-2850 cm-1: Single-bond stretching O-H, N-H, C-H 2300-2100 cm-1 triple-bond stretching CC and CN 1850-1620 cm-1 Double-bond stretching C=C, C=N, C=O 1500-500 cm-1 Single-bond stretching and bending C-C, C-N, C-O, C-H bending

The x-axis on IR spectra goes from high-energy to low-energy. What is the corresponding range of wavenumbers shown on IR spectra?

4000 cm-1 is the high-energy end of the scale and 625 cm-1 is the low-energy end. [The x-axis of an IR spectrum goes from 4000-500 cm-1.]

What type of vibrational modes are shown for a CH2 group? List the type of vibration and the mode of vibration.

CH2 groups can stretch symmetrically and antisymmetrically. CH2 groups can also bend by modes of scissoring, rocking, wagging, and twisting.

Why is IR spectroscopy an important tool?

It's useful for identifying functional groups within a molecule.

Our lab spectrometers use ATR coupled with FT data analysis to take spectra with little to no sample preparation. Briefly describe other methods used for taking IR spectra of solids and liquid

Sodium chloride ["salt"] plates - A drop of liquid sample can be placed between two sodium chloride plates and then placed in the IR for determination. Solids can be dissolved in a solvent and also placed between the NaCl plates for determination. Potassium bromide - Another method to determine IR of solids is to mix the solid with potassium bromide and press it into a wafer [or "pellet"] which is then placed in the IR for determination.

What is the basis of IR spectroscopy?

Structural units within a molecule vibrate characteristically and the sensitivity to these group vibrations is the basis of IR spectroscopy.

What is the fingerprint region and where is it found on an IR spectrum?

The fingerprint region is the range of wavenumbers from 1500-500 cm-1 where the pattern of peaks varies most from compound to compound. [Peaks from 4000-1600 cm-1 are most useful for our purposes in identifying functional groups.] Note: The fingerprint region is the "fingerprint" of a molecule in that if two spectra are superimposable including their fingerprint regions, they are identical compounds.

The region of infrared radiation (see Figure 14.1) falls between which two types of electromagnetic radiation?

The range of wavelengths for IR falls between those of UV-vis and microwaves.

Is the IR spectrum of a compound unique to that compound? What is the evidence?

Two IR spectra of the same compound are superimposable [while any two different compounds are NOT superimposable]. This is evidence that the IR spectrum of a compound is unique.

What are wavenumbers and how are they expressed (i.e. what units are associated with the wavenumber)?

Wavenumbers are inverse wavelengths and the units are expressed as cm-1. [Wavenumber is the x-axis of an IR spectrum and are most useful for functional gruop determination.]

Table 14.3 (pg. 560) is called a correlation table; it lists the characteristic absorption frequencies for a variety of organic functional groups. Another correlation can be found on the fold-over back cover of your lab notebook; use this correlation table to describe the OH - NH region of an IR spectrum.

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