IR Spectroscopy Vocabulary Review (Chapters 1–7 Notes)

Vocabulary flashcards covering key IR spectroscopy concepts from the notes, including peak positions, regions, functional groups, and vibrational modes.

Wave number

The position of an IR absorption peak, measured in cm^-1; inversely related to wavelength and related to bond strength and vibrational energy.

Functional group region

IR region around 1500 cm^-1 and higher used to identify functional groups.

Fingerprint region

Below ~1500 cm^-1; region used for impurity checks and is unique to each compound.

C=O carbonyl stretch

A strong IR absorption around ~1700 cm^-1 due to C=O bonds in carbonyl-containing groups (e.g., aldehydes, ketones, esters, carboxylic acids, amides).

Alcohol OH stretch

OH stretch around ~3400 cm^-1; broad and smooth, indicating an alcohol.

Carboxylic acid OH stretch

OH stretch for carboxylic acids around ~2500-3300 cm^-1; very broad and jagged due to hydrogen bonding; accompanies a carbonyl peak.

C=C stretch

Alkene C=C stretching vibration around ~1600 cm^-1; typically weak to moderate.

Aldehyde C-H stretch

Aldehyde C–H stretch around ~2700-2750 cm^-1; helps identify aldehydes.

Esters: carbonyl and C–O stretches

Esters show C=O stretch near ~1735-1750 cm^-1 and a C–O stretch near ~1250 cm^-1 (R–O).

Primary amine

Two N–H stretches around 3300-3500 cm^-1 indicating two N–H bonds.

Secondary amine

One N–H stretch around 3300-3500 cm^-1.

Tertiary amine

No N–H stretch observed in IR for tertiary amines.

Hydrogen bonding effect on OH peak

Hydrogen bonding broadens or distorts OH peaks; COOH OH is jagged, alcohol OH tends to be smooth.

Symmetric stretch

Vibrational mode where bonds stretch in phase; produces IR peak when there is a change in dipole moment.

Antisymmetric stretch

Vibrational mode where bonds stretch out of phase; produces IR peak depending on dipole change.

In-plane bending

Bending motion within the molecular plane (e.g., scissoring).

Out-of-plane bending

Bending motion out of the molecular plane (e.g., wagging, twisting).

Wigging and wagging

Torsion-like motions of substituents; types of bending vibrations seen in IR.

Reduced mass

μ = (m1*m2)/(m1+m2); vibrational frequency increases as the reduced mass decreases; heavier atoms lower frequency.

Bond strength

Energy required to vibrate a bond; stronger bonds vibrate at higher frequencies (higher wave numbers).