IR Spectroscopy

• Spectroscopy describes how electromagnetic radiation interacts with molecules.
• At room temperature molecules have energy, and some of this energy is stored in bond vibrations.

IR spectroscopy

• An infrared absorption (IR) spectrum displays bond vibration frequencies in a molecule.
• There are usually many different bond vibrations causing many different peaks in an IR spectrum.
• IR bond vibrations often happen on bonds or groups of atoms in functional groups.
• Wavenumber in cm⁻¹ units measure different bonds vibrating at different frequencies
• Stronger bonds have higher vibration frequencies.
• Bonds involving lighter atoms have higher vibration frequencies

Approximate Bond Vibration Regions in the IR Spectrum

• Bonds to H, the lightest atom are stronger bonds
• Triple Bond regions are stronger bonds
• Single bond regions (fingerprint) vibrations are "mixed"

Absorption Peak Strength

• When the bond is polar, bond vibrations have larger peaks on an IR spectrum.
• The more polar the bond, the larger the peak in an IR spectrum

Vibrations of C–H bonds around 3000 cm⁻¹

• Bonds to H atoms tend to be high frequency with large v, ca. 2700 to 3500 cm⁻¹.
• Stronger C-H bonds have higher vibration frequencies, weaker C–H bonds vibrate with lower frequencies.
• Stronger C-H bonds have higher frequency absorption in IR spectroscopy

Additional Facts regarding Bonds

• C(sp³)-H bonds are weaker than C(sp²)–H and C(sp)H bonds and vibrate at frequencies lower than 3000 cm⁻¹.
• C(sp)-H bonds vibrate with frequencies around 3300 cm⁻¹ and are sharp.
• Bonds to electronegative elements are stronger than C-H bonds and vibrate with higher frequencies.
• N-H and O-H bonds also have large dipole moments, making their IR absorption peaks strong.
• The absorption strength of one O-H bond is equivalent to many C-H bonds combined.
• O-H bond vibrations occur at ca. 3300 cm⁻¹, and the IR absorption is broad due to hydrogen bonding.
• Hydrogen bonding "pulls" electron density from the O-H bond resulting in a lower frequency vibration.
• A distribution in hydrogen bonding results in a distribution in frequencies, which results in a broad absorption band.
• N-H stretching vibrations of Amine are also broad due to hydrogen bonding, however N-H hydrogen bonding is weaker than O-H Hydrogen bonding.
• Some non-hydrogen-bonded N-H vibrations can be observed as small sharp peaks (spikes) on top of the broad absorption.
• There are usually two small (non-hydrogen bonding) peaks for a primary amine with two N-H bonds.
• There is usually one small (non-hydrogen bonding) peak for a secondary amine with one N-H bond.
• Aldehydes have two small peaks around 2730 and 2820 cm⁻¹ for the C-H bond on the C=O.
• Aldehyde vibrations range between ca. 2720 to 2740 and ca. 2810 to 2830 cm⁻¹, but the aldehyde also has the strong C=O stretching vibration at ca. 1700 cm⁻¹.
• Observation of both vibrational features (C=O and C-H) helps to identify an aldehyde.
• An aldehyde C-H stretching vibration has a lower frequency than other C-H bonds due to electron withdrawal from the C-H bond by the electronegative oxygen.
• Carboxylic acids have a broad O--H peak because of hydrogen bonding, but hydrogen bonding is stronger in carboxylic acids leading to a lower frequency O–H vibration.
• Bonds to atoms heavier than hydrogen vibrate with lower frequencies, the strongest of these are triple bonds.

Vibrations around 2500 cm⁻¹

• Only two kinds of vibrations are observed in this region: the C-N triple bond of the nitrile functional group and the C-C triple bond of the alkyne functional group.
• Carbon-carbon triple bond absorptions tend to be weaker and observed only for the asymmetrical terminal alkynes.
• The C-N triple bond absorptions are strong because the dipole moment associated with this bond is large.
• IR peaks get assigned to individual bonds first, and functional groups second
• IR peaks assigned to vibrations of a C-C triple bond and C(sp)-H indicate an alkyne functional group's presence

Vibrations around 1700 cm⁻¹

• C=O bonds are stronger than C=C bonds and vibrate at higher frequencies.
• C=O bonds have larger bond dipole moments than C=C bonds, and have strong absorptions.
• Vibrations of the C=O bond in aldehydes and ketones are strong and are close to 1700 cm⁻¹, often ca. 1710 to 1715 cm-1.
• Aldehydes can be distinguished from ketones because aldehydes also have the two peaks due to C-H vibration at ca. 2730 and 2820 cm⁻¹.
• Electronegative oxygen connected to the C=O bond in the ester makes all of the bonds stronger; thus, ester C=O vibrations occur at relatively high frequencies, around 1720 to 1730 cm-1.
• Minor resonance contributors for conjugated aldehydes and ketones give the C=O bond single bond character.
• Conjugated aldehydes and ketones thus have vibration frequencies around 1680 cm⁻¹ compared to nonconjugated aldehydes and ketones (ca. 1710 cm⁻¹).
• The electrons involved in resonance are nonbonding on nitrogen, and are thus more "available," the minor contributor in the amide is even more important.
• The C=O bonds in amides have even more single bond character leading to vibration frequencies are further decreased to ca. 1640 cm-1. Amides will have N-H vibrations in the 3300 cm⁻¹ region if they have N-H bonds.

Molecular vibrations closer to 1600 cm⁻¹

• C=C double bonds tend to have small dipole moments and are usually weak (small) absorptions; they are also weaker than C=O bonds and vibrate with lower frequencies, ca. 1620 cm⁻¹. A
• The peak is often weak because of a small dipole moment, but it is usually sharp and close to 1600 cm-1.
• There is one final vibration that is unusual in that it is not of a single bond, but is associated with a stretching motion of an entire benzene ring
• If there is a benzene ring, there should almost always also be C(sp²)-H bond vibrations, which is observed at >3000 cm-1.
• One must distinguish "real" peaks from impurity or other artifact peaks, such as contaminating water, which is often seen as a weak peak around 3300 cm⁻¹ or any “real” peaks in this region would be strong.
• The strong peak at 1710 cm⁻¹ must be an aldehyde or a ketone and is assigned to the vibration of the C=O bond first, and an aldehyde/ketone second. In this case, it must be a ketone because the two C-H aldehyde peaks at 2720 to 2830 cm⁻¹ are not be seen. The broad peak centered around 3400 cm⁻¹ can be assigned to the vibration of the O-H bond first, and an alcohol second.
• A peak in an IR spectrum is always assigned to the vibration of a specific bond first, and a functional group second.