CHEM 231 : IR Spectroscopy Ranges and Types

Descriptions of visualizing the graph, the ranges, and peaks to observe!

Where is the “Fingerprint Region” located on the IR graph?

Range of 1500 cm^-1 and below

Where are the single bonds located on the IR graph? (ex. N-H, C-H, O-H)

Range is from 4000-2500 cm^-1

Where are the triple bonds located on the IR graph? (ex. C ≡ N or C ≡ C)

Range is from 2500-2000 cm^-1

Where are the double bonds located on the IR graph? (ex. C = O, C = C, C = N)

Range is from 2000-1500 cm^-1

Hybridized Region: C-H (sp)

@ 3250 cm^-1

Hybridized Region: C-H (sp2)

@ 3050 cm^-1

Hybridized Region: C-H (sp3)

@ 2950 cm^-1

C-H (for an Aldehyde)*

@ 2750 cm^-1

What is the relationship between atomic mass and the wavenumber?

As the atomic mass increases, the wavenumber decreases because the heavier atoms move slower. They vibrate at a lower frequency.

What is the relationship between bond strength and wavenumber?

As the bond strength (from single to double to triple) increases, the wavenumber increases because more energy is needed to stretch a stronger bond.

Amine (Primary) Location

In the single bond region (4000-2500 cm^-1), two bumps at 3300-3500 cm^-1

Amine (secondary) Location

In the single bond region (4000-2500 cm^-1), one bump at 3300-3500 cm^-1

Alcohol O-H Location

In the single bond region (4000-2500 cm^-1), large U-shape at 3200-3600 cm^-1

Carboxylic O-H Location

In the single bond region (4000-2500 cm^-1), large U-shape at 2500-3300 cm^-1

C-H (sp3) Location

In the single bond region (4000-2500 cm^-1), multiple peaks at 2800-3000 cm^-1 (around 2950 like mentioned before)*

C-H (sp2) Location

In the single bond region (4000-2500 cm^-1), you have the sp3 peak and then nearby would be a peak @ 3050 cm^-1. The peak would be to the LEFT of the sp3 peak.

C-H (sp) Location

In the single bond region (4000-2500 cm^-1), you have the sp3 peak and then a little further off would be a peak @ 3250 cm^-1. The peak would be to the LEFT of the sp3 peak.

C-H Aldehyde Location

In the single bond region (4000-2500 cm^-1), you have the sp3 peak and then a further off would be a small peak @ 2750 cm^-1. The small peak would be to the RIGHT of the sp3 peak.

C ≡ C (alkyne) Location (& how can you tell the difference between this and nitrile?)

In the triple bond region (2500-2000 cm^-1), strong and sharp peak at around 2300 cm^-1 (if you see an sp C-H at 3250 with this, then it is a terminal alkyne)*

C ≡ N (nitrile) Location

In the triple bond region (2500-2000 cm^-1), strong and sharp peak around 2300 cm^-1

C = O Location

In the double bond region (2000-1500 cm^-1), a peak around 1700 cm^-1

C = C Alkene Location

In the double bond region (2000-1500 cm^-1), a sharp and small peak around 1650 cm^-1