Proton NMR and Infrared (IR) Spectroscopy Reference Values

Comprehensive vocabulary flashcards covering characteristic proton NMR chemical shifts and infrared (IR) spectroscopy diagnostic regions and functional group absorption ranges.

Methyl Proton ($\text{R}-\text{CH}_3$)

A primary aliphatic proton that typically displays a $\text{ }^1\text{H}\text{ NMR}$ chemical shift of approximately $\delta \sim 0.9$.

Methylene Proton ($-\text{CH}_2-$)

A secondary aliphatic proton that typically displays a $\text{ }^1\text{H}\text{ NMR}$ chemical shift of approximately $\delta \sim 1.2$.

Methine Proton ($-\text{CH}-$)

A tertiary aliphatic proton that typically displays a $\text{ }^1\text{H}\text{ NMR}$ chemical shift of approximately $\delta \sim 1.7$.

Allylic Proton

A proton attached to a carbon directly adjacent to a carbon-carbon double bond ($\text{C}=\text{C}-\text{C}\mathbf{H}$), displaying a chemical shift of approximately $\delta \sim 2$.

Alkynyl Proton ($\text{R}-\text{C}\equiv\text{C}-\text{H}$)

A terminal alkyne proton that appears in the $\text{ }^1\text{H}\text{ NMR}$ spectrum at a chemical shift of approximately $\delta \sim 2.5$.

Benzylic Methyl Proton ($\text{Ar}-\text{CH}_3$)

A methyl proton on a carbon directly attached to an aromatic ring, appearing at a chemical shift of approximately $\delta \sim 2.3$.

Alkyl Halide Proton ($\text{H}-\text{C}-\text{X}$)

A proton bonded to a carbon bearing an electronegative halogen atom, yielding a chemical shift in the range of $\delta = 2\text{--}4$.

Alcohol Proton ($\text{R}-\text{O}-\text{H}$)

A hydroxyl proton of an alcohol, which characteristically resonates in the $\text{ }^1\text{H}\text{ NMR}$ chemical shift range of $\delta = 2\text{--}5$.

Vinylic Proton ($\text{C}=\text{C}-\text{H}$)

An alkene proton attached directly to an $sp^2$-hybridized double-bonded carbon, appearing at a chemical shift of $\delta = 4.5\text{--}6.5$.

Aryl Proton ($\text{Ar}-\text{H}$)

An aromatic ring proton that appears deshielded in the chemical shift range of $\delta = 6.5\text{--}8$.

Aldehyde Proton ($\text{R}-\text{CHO}$)

A proton attached directly to a carbonyl carbon, producing a characteristic downfield $\text{ }^1\text{H}\text{ NMR}$ signal at approximately $\delta \sim 10$.

Carboxylic Acid Proton ($\text{R}-\text{COOH}$)

A highly deshielded acidic proton bonded to a carboxyl group, typically appearing at a chemical shift of approximately $\delta \sim 12$.

Bonds-to-Hydrogen IR Region

The region of the infrared spectrum from $4000\text{--}2700\,\text{cm}^{-1}$ that contains stretching frequencies for bonds such as $\text{O}-\text{H}$, $\text{N}-\text{H}$, and $\text{C}-\text{H}$.

Triple-Bond IR Region

The infrared spectral region spanning $2300\text{--}2100\,\text{cm}^{-1}$, where stretching absorptions for $\text{C}\equiv\text{C}$ and $\text{C}\equiv\text{N}$ bonds occur.

Double-Bond IR Region

The infrared spectral region spanning $1850\text{--}1600\,\text{cm}^{-1}$, containing stretching absorptions for $\text{C}=\text{O}$, $\text{C}=\text{N}$, and $\text{C}=\text{C}$ bonds.

Single-Bond IR Region

The infrared region below $1600\,\text{cm}^{-1}$ (spanning $1600\text{--}400\,\text{cm}^{-1}$), also referred to as the fingerprint region, where single-bond vibrations ($\text{C}-\text{C}$, $\text{C}-\text{O}$, $\text{C}-\text{N}$) occur.

Alcohol $\text{O}-\text{H}$ Stretch

A characteristic broad infrared absorption band appearing between $3200\text{--}3600\,\text{cm}^{-1}$.

Carboxylic Acid $\text{O}-\text{H}$ Stretch

An extremely broad infrared absorption band appearing across $2200\text{--}3600\,\text{cm}^{-1}$ that characteristically overlaps the aliphatic $\text{C}-\text{H}$ stretching region.

Amine/Amide $\text{N}-\text{H}$ Stretch

An infrared absorption band typically found in the range of $3350\text{--}3500\,\text{cm}^{-1}$.

$sp\,\text{C}-\text{H}$ Stretch

The stretching absorption of a terminal alkyne $\text{C}-\text{H}$ bond ($\equiv\text{C}-\text{H}$), appearing as a sharp peak at approximately $3300\,\text{cm}^{-1}$.

$sp^2\,\text{C}-\text{H}$ Stretch

The stretching absorption of an alkene or aromatic $\text{C}-\text{H}$ bond ($=\text{C}-\text{H}$), appearing just above $3000\,\text{cm}^{-1}$ in the range of $3000\text{--}3100\,\text{cm}^{-1}$.

$sp^3\,\text{C}-\text{H}$ Stretch

The stretching absorption of an alkane $\text{C}-\text{H}$ bond ($-\text{C}-\text{H}$), appearing just below $3000\,\text{cm}^{-1}$ in the range of $2850\text{--}3000\,\text{cm}^{-1}$.

Aldehyde $\text{C}-\text{H}$ Stretch

A distinct pair of infrared absorption peaks (two signals) appearing at approximately $2750\,\text{cm}^{-1}$ and $2850\,\text{cm}^{-1}$.

Alkyne $\text{C}\equiv\text{C}$ Stretch

An infrared stretch occurring in the triple-bond region between $2100\text{--}2200\,\text{cm}^{-1}$.

Nitrile $\text{C}\equiv\text{N}$ Stretch

An infrared stretch appearing in the triple-bond region between $2200\text{--}2300\,\text{cm}^{-1}$.

$\text{C}=\text{O}$ vs. $\text{C}=\text{C}$ IR Comparison

The $\text{C}=\text{O}$ bond is stronger and absorbs at a higher wavenumber ($1650\text{--}1850\,\text{cm}^{-1}$) than $\text{C}=\text{C}$ ($1600\text{--}1700\,\text{cm}^{-1}$) due to the greater electronegativity of oxygen and the resulting dipole strength.

Aromatic Ring IR Absorptions

Infrared signals produced by aromatic rings, consisting of $\text{C}=\text{C}$ ring stretching bands in the $1450\text{--}1600\,\text{cm}^{-1}$ region and overtone combinations in the $1650\text{--}2000\,\text{cm}^{-1}$ region.

IR Absorption Intensity

The strength of an infrared peak, which depends directly on the magnitude of the dipole moment change during molecular vibration; greater dipole changes produce stronger signals, while vibrations with little or no dipole change produce weak or inactive signals.

Effect of Bond Strength on IR Frequency

A higher force constant ($k$) causes molecular vibration and absorption to occur at a higher wavenumber ($\text{cm}^{-1}$), whereas a lower force constant results in absorption at a lower wavenumber.

Homotopic Protons

Protons related by a proper axis of rotational symmetry ($C_n$) that share identical chemical environments and resonance frequencies under all conditions; replacing each individually with another atom yields identical structures.

Enantiotopic Protons

Protons related by an alternating axis of symmetry or reflection plane ($\sigma$) that are chemically equivalent in achiral media but give non-superimposable mirror-image stereoisomers upon separate substitution.

Diastereotopic Protons

Non-equivalent protons lacking symmetry relationships (commonly adjacent to a stereocenter) that display distinct chemical shifts and mutual spin-spin splitting; separate substitution yields non-mirror-image stereoisomers.

Heterotopic Protons

Protons occupying constitutionally distinct positions with different atomic connectivity, resulting in different chemical shifts across an NMR spectrum.