Chapter 12: Infrared Spectroscopy

Vocabulary practice flashcards covering fundamental concepts, diagnostic stretching frequencies, and functional group identifiers in Infrared (IR) Spectroscopy.

Spectroscopy

A technique used to determine the structure of a compound.

Infrared (IR) Spectroscopy

An analytical technique that measures bond vibration frequencies in a molecule and is used to determine functional groups.

Wavenumbers ($cm^{-1}$)

Reciprocal centimeters, which serve as the common unit of measurement for frequency in infrared spectroscopy.

Enantiomers (IR Spectrum Exception)

The only exception to the rule that no two distinct molecules will give exactly the same IR spectrum.

Fingerprint Region

The spectral region between $600$ and $1400\,cm^{-1}$ containing the most complex vibrational patterns.

Functional Group Region ($1600\text{--}3500\,cm^{-1}$)

The region between $1600$ and $3500\,cm^{-1}$ that exhibits the most common bond vibrations and provides information about specific functional groups.

$3000\,cm^{-1}$ Diagnostic Boundary

A reference threshold in IR spectra where unsaturated $C=C-H$ ($sp^2$) bonds absorb to the left (above $3000\,cm^{-1}$) and saturated $C-C-H$ ($sp^3$) bonds absorb to the right (below $3000\,cm^{-1}$).

Alkane $C-H$ Bend

A medium-intensity absorption band occurring in the range of $1450\text{--}1470\,cm^{-1}$.

Alkene $C=C$ Stretch

A characteristic stretching absorption appearing around $1640\text{--}1660\,cm^{-1}$ (such as $1642\,cm^{-1}$ in 1-hexene), which decreases in frequency upon conjugation.

Alkene Unsaturated $=C-H$ Stretch

A stretching band located just above $3000\,cm^{-1}$ (such as $3080\,cm^{-1}$ in 1-hexene), appearing to the left of $3000\,cm^{-1}$.

Terminal Alkyne (IR Characteristics)

An alkyne possessing a sharp, typically strong $\equiv C-H$ stretch around $3300\text{--}3313\,cm^{-1}$ and a $C\equiv C$ stretch just below $2200\,cm^{-1}$ (such as $2119\,cm^{-1}$ in oct-1-yne).

Internal Alkyne (IR Inactivity)

A symmetrically or near-symmetrically substituted triple bond (such as in oct-4-yne) that lacks a terminal $\equiv C-H$ stretch and shows no visible $C\equiv C$ stretch due to the lack of dipole moment change.

Alcohol $O-H$ Stretch

A broad, intense absorption band centered around $3300\,cm^{-1}$ characterized by a rounded tip.

Primary Amine ($RNH_2$) Stretch

A broad absorption centered around $3300\,cm^{-1}$ that features two sharp spikes.

Secondary Amine ($R_2NH$) Stretch

A broad absorption centered around $3300\,cm^{-1}$ that features a single sharp spike (as seen in dipropylamine).

Tertiary Amine ($R_3N$) in IR

An amine type that produces no $N-H$ stretching absorption around $3300\,cm^{-1}$ because it lacks any hydrogen atoms bonded to the nitrogen.

Ketone Carbonyl ($C=O$) Stretch

A strong, sharp absorption appearing at approximately $1710\text{--}1718\,cm^{-1}$ (such as $1718\,cm^{-1}$ in heptan-2-one).

Aldehyde Diagnostic Absorptions

A characteristic profile consisting of a $C=O$ stretch around $1710\text{--}1725\,cm^{-1}$ along with two distinct $C-H$ stretch bands appearing at $2720\,cm^{-1}$ and $2820\,cm^{-1}$.

Carboxylic Acid Diagnostic Absorptions

A unique profile requiring both a very broad $O-H$ absorption between $2500$ and $3500\,cm^{-1}$ caused by strong hydrogen bonding, and a $C=O$ carbonyl stretch around $1710\,cm^{-1}$.

Amide Diagnostic Absorptions

An absorption profile featuring a strong $C=O$ stretch shifted to a lower frequency of $1640\text{--}1680\,cm^{-1}$ (around $1650\,cm^{-1}$) and $N-H$ stretches around $3300\,cm^{-1}$ if hydrogens are present on the nitrogen.

Nitrile ($C\equiv N$) Stretch

An intense, sharp absorption centered around $2200\text{ to }2300\,cm^{-1}$ (just above $2200\,cm^{-1}$, such as $2249\,cm^{-1}$ in butyronitrile).

Nitrile vs. Alkyne Triple Bond Polarity

The reason nitriles produce stronger IR absorptions than alkynes around $2200\,cm^{-1}$, as carbon-nitrogen triple bonds are more polar than carbon-carbon triple bonds.

Ester Carbonyl ($C=O$) Stretch

A carbonyl absorption occurring at a higher frequency than ketones and aldehydes, at approximately $1735\,cm^{-1}$.

$C-O$ Stretch Region

An absorption band located between $1000$ and $1200\,cm^{-1}$, commonly observed in ethers, esters, and alcohols.

Conjugation Effect on IR Frequencies

The structural factor that lowers the stretching absorption frequencies of both carbonyl ($C=O$) and alkene ($C=C$) bonds.