Spectroscopy:
IR & \( ^1\text{H} \) NMR

Learn to identify functional groups with infrared spectroscopy and determine hydrogen environments using proton NMR: the two essential tools for molecular structure determination.

Organic Chemistry 1

What Is Spectroscopy?

Spectroscopy is the study of how matter interacts with electromagnetic radiation. Different types of radiation probe different molecular properties: bond vibrations, electronic transitions, or nuclear spin states.

IR

Infrared light excites bond vibrations: stretching and bending

\( ^1\text{H} \) NMR

Radio waves flip nuclear spins of hydrogen atoms in a magnetic field

How IR Spectroscopy Works

IR radiation is absorbed when its frequency matches a bond's natural vibration frequency. Each functional group absorbs at a characteristic wavenumber (\( \tilde{\nu} \), in \( \text{cm}^{-1} \)).

Key IR Absorptions to Memorize

Bond	Wavenumber (\(\text{cm}^{-1}\))	Appearance
O-H (alcohol)	3200-3600	Broad, strong
O-H (carboxylic acid)	2500-3300	Very broad, strong
N-H	3300-3500	Medium, 1-2 peaks
C-H (\(\text{sp}^3\))	2850-3000	Strong
C=O	1650-1780	Strong, sharp
C=C	1600-1680	Medium
C-O	1000-1260	Strong

How \( ^1\text{H} \) NMR Works

In a strong magnetic field, hydrogen nuclei absorb radio-frequency energy at different frequencies depending on their electronic environment. This frequency difference is called the chemical shift (\( \delta \)), measured in ppm.

Chemical Shift (\( \delta \))

Position on the x-axis: tells you the type of hydrogen environment

Integration

Area under a peak: tells you the relative number of equivalent H atoms

Splitting

Number of sub-peaks: tells you how many neighbors (\( n+1 \) rule)

Chemical Shift Ranges

The \( n+1 \) Splitting Rule

A hydrogen signal is split into \( n+1 \) peaks by \( n \) equivalent neighboring hydrogens on adjacent carbons. This is called spin-spin coupling.

Singlet (s)

\( n = 0 \) neighbors: 1 peak

Doublet (d)

\( n = 1 \) neighbor: 2 peaks

Triplet (t)

\( n = 2 \) neighbors: 3 peaks

Quartet (q)

\( n = 3 \) neighbors: 4 peaks

IR vs NMR: Key Distinction

Feature	IR Spectroscopy	\( ^1\text{H} \) NMR
Radiation type	Infrared light	Radio waves
What it probes	Bond vibrations	Nuclear spin states
X-axis unit	\( \text{cm}^{-1} \) (wavenumber)	\( \delta \) (ppm)
Primary info	Functional groups present	H environments, connectivity
Key question	"What groups are there?"	"How are the H atoms arranged?"

How to Interpret a Spectrum

1

Start with IR: check for O-H (broad, 3200-3600), C=O (sharp, ~1700 \(\text{cm}^{-1}\)), N-H (3300-3500)

2

Determine the molecular formula from mass spec or given data: calculate the degree of unsaturation

3

Count NMR signals: the number of peaks tells you how many distinct H environments exist

4

Read integration ratios: the relative areas give the ratio of H atoms per environment

5

Analyze splitting patterns: apply the \( n+1 \) rule to determine neighbor connectivity

6

Match chemical shifts to expected ranges and propose a structure

Example 1: IR of Butanone

Example 2: NMR of Diethyl Ether

Diethyl ether has the formula \( \text{CH}_3\text{CH}_2\text{-O-CH}_2\text{CH}_3 \). Let's predict its \( ^1\text{H} \) NMR.

Example 3: Identifying Acetic Acid

A compound with formula \( \text{C}_2\text{H}_4\text{O}_2 \) shows the following spectral data. Identify the structure.

IR Data

Very broad absorption 2500-3300 \(\text{cm}^{-1}\); strong sharp peak at 1710 \(\text{cm}^{-1}\)

NMR Data

Singlet at \( \delta \) 2.1 (3H); broad singlet at \( \delta \) 11.4 (1H)

Integration and Equivalent Hydrogens

Equivalent hydrogens are H atoms in the same chemical environment: they appear as a single NMR signal. Integration measures the area under each peak, giving the ratio of H atoms.

1

Symmetry check: Hydrogens related by a mirror plane or rotation axis are equivalent

2

Methyl groups: all 3 H in \( \text{CH}_3 \) are equivalent (free rotation around C-C bond)

3

Integration ratio example: if two peaks integrate as 3:2, the molecule has H atoms in a 3:2 ratio

Carbonyl Types by IR Position

The exact position of the C=O stretch tells you which carbonyl-containing functional group is present. Each type absorbs in a narrow, characteristic range.

Functional Group	C=O Range (\(\text{cm}^{-1}\))	Other IR Clues
Ester	1735-1750	Two C-O stretches (1000-1300)
Aldehyde	1720-1740	Two C-H stretches near 2720, 2820
Ketone	1705-1725	No O-H, no aldehyde C-H
Carboxylic Acid	1700-1725	Very broad O-H (2500-3300)
Amide	1630-1690	N-H stretch (3300-3500)

Common Mistakes to Avoid

✕ Confusing O-H Shapes

Alcohol O-H is broad (3200-3600). Carboxylic acid O-H is very broad (2500-3300) and overlaps C-H. Don't mix them up.

✕ Ignoring Symmetry

Forgetting that molecular symmetry reduces the number of NMR signals. Always check for equivalent H groups before counting peaks.

✕ Miscounting Neighbors

The \( n+1 \) rule counts H on adjacent carbons only, not on the same carbon. Also, O-H and N-H often do not show splitting.

Summary: IR and NMR at a Glance

Concept	IR	\( ^1\text{H} \) NMR
What you measure	Bond vibration frequencies	H nuclear spin environments
X-axis	Wavenumber (\( \text{cm}^{-1} \))	Chemical shift (\( \delta \), ppm)
Y-axis	% Transmittance	Signal intensity
Key features	Peak position and shape	Shift, integration, splitting
Tells you	Which functional groups	How H atoms are connected
O-H detection	Broad dip (3200-3600)	Broad singlet (variable \( \delta \))
C=O detection	Sharp dip (~1700)	Adjacent H shifts downfield

Question 1 of 5

A strong, sharp IR absorption near 1715 \(\text{cm}^{-1}\) most likely indicates which functional group?

The C=O stretch absorbs as a strong, sharp peak in the 1650-1780 \(\text{cm}^{-1}\) region. O-H is broad (3200-3600), C=C is medium near 1600-1680, and N-H appears at 3300-3500.

Question 2 of 5

In the \( ^1\text{H} \) NMR of \( \text{CH}_3\text{CH}_2\text{Br} \), the \( \text{CH}_2 \) group appears as a:

The \( \text{CH}_2 \) has 3 neighboring H atoms on the adjacent \( \text{CH}_3 \). By the \( n+1 \) rule: \( 3 + 1 = 4 \) peaks, which is a quartet.

Question 3 of 5

A very broad IR absorption from 2500-3300 \(\text{cm}^{-1}\) combined with a strong peak at 1710 \(\text{cm}^{-1}\) suggests:

The very broad O-H from 2500-3300 \(\text{cm}^{-1}\) (overlapping C-H) combined with a C=O at ~1710 is the hallmark of a carboxylic acid. Alcohols have O-H at 3200-3600 (no C=O), and ketones have C=O but no broad O-H.

Question 4 of 5

A \( ^1\text{H} \) NMR signal at \( \delta \) 7.3 ppm most likely corresponds to:

Aromatic (Ar-H) protons resonate at \( \delta \) 6.5-8.5 ppm due to the ring current deshielding effect. Alkyl C-H appears at 0.5-2.0, aldehyde C-H at 9.5-10.0, and ether O-CH at 3.3-4.0.

Question 5 of 5

How many distinct \( ^1\text{H} \) NMR signals would you expect for \(\text{(CH}_3\text{)}_3\text{CH}\) (2-methylpropane)?

2-Methylpropane (isobutane) has two types of H: the 9 equivalent \( \text{CH}_3 \) hydrogens (all related by the molecule's three-fold symmetry) and the 1 \( \text{CH} \) hydrogen. The \( \text{CH}_3 \) signal appears as a doublet (split by 1 CH neighbor, \( n+1 = 2 \)), and the \( \text{CH} \) signal appears as a multiplet (split by 9 equivalent H neighbors). The answer is 2 distinct environments.

Spectroscopy:IR & \( ^1\text{H} \) NMR

What Is Spectroscopy?

How IR Spectroscopy Works

Key IR Absorptions to Memorize

How \( ^1\text{H} \) NMR Works

Chemical Shift Ranges

The \( n+1 \) Splitting Rule

IR vs NMR: Key Distinction

How to Interpret a Spectrum

Example 1: IR of Butanone

Example 2: NMR of Diethyl Ether

Example 3: Identifying Acetic Acid

Integration and Equivalent Hydrogens

Carbonyl Types by IR Position

Common Mistakes to Avoid

Summary: IR and NMR at a Glance

Question 1 of 5

Question 2 of 5

Question 3 of 5

Question 4 of 5

Question 5 of 5

Spectroscopy:
IR & \( ^1\text{H} \) NMR