02 Spectroscopic Structure Elucidation

How the techniques work together

Identifying an unknown organic compound requires a structure that accounts for the full set of evidence. Mass spectrometry helps constrain molecular mass and formula, infrared spectroscopy identifies likely bond types and functional groups, and NMR reveals the number and environments of hydrogen and carbon atoms. A proposed structure is strongest when all major observations agree.

Start with the formula and unsaturation

Begin by establishing a molecular formula, then calculate the degree of unsaturation (DBE), also called the index of hydrogen deficiency:

Here, $X$ represents the number of halogen atoms; oxygen and sulfur are omitted. Each ring or double bond accounts for one DBE, while each triple bond accounts for two. A benzene ring accounts for four DBE: three double bonds and one ring.

Next, check the IR spectrum for diagnostic functional groups, interpret NMR signals to build fragments, and combine those fragments so that their atoms, formula, and DBE agree. Use mass-spectrometric fragments and isotope patterns as supporting evidence.

Infrared spectroscopy: identify bond types

Infrared spectroscopy detects absorptions when molecular vibrations change the molecule’s dipole moment. Frequencies are commonly reported in $\mathrm{cm}^{-1}$. Reference ranges are approximate and can shift with conjugation, hydrogen bonding, and measurement conditions.

Useful diagnostic regions include:

• Alcohol or phenol O–H: $3200\text{–}3600\ \mathrm{cm}^{-1}$, typically broad and strong.

• Carboxylic-acid O–H: about $2500\text{–}3300\ \mathrm{cm}^{-1}$, very broad and often overlapping C–H stretches.

• Amine or amide N–H: about $3300\text{–}3500\ \mathrm{cm}^{-1}$. Primary amines often show two bands, secondary amines one, and tertiary amines none.

• Alkane C–H: $2850\text{–}2960\ \mathrm{cm}^{-1}$; alkene or aromatic C–H: about $3000\text{–}3100\ \mathrm{cm}^{-1}$.

• Nitrile C≡N: about $2210\text{–}2260\ \mathrm{cm}^{-1}$, often distinct. Alkyne C≡C: about $2100\text{–}2260\ \mathrm{cm}^{-1}$, sometimes weak, especially in symmetrical alkynes.

• Carbonyl C=O: about $1650\text{–}1800\ \mathrm{cm}^{-1}$, usually strong. Esters commonly absorb at higher frequency than amides, while conjugation often lowers the carbonyl frequency.

• Alcohol, ether, or ester C–O: about $1000\text{–}1300\ \mathrm{cm}^{-1}$, useful as supporting evidence.

The region from about $1500$ to $500\ \mathrm{cm}^{-1}$ is often called the fingerprint region. Its many overlapping bands can help compare a sample with a reference spectrum. A strong carbonyl band suggests a C=O group, but its position alone may not uniquely identify the carbonyl type. Aldehydes may also show weak C–H bands near $2700\text{–}2850\ \mathrm{cm}^{-1}$.

Takeaway: Use IR to identify likely groups, then confirm their presence and context with the formula and NMR evidence.

NMR spectroscopy: map atom environments

NMR chemical shifts are reported in parts per million (ppm) relative to a reference, commonly tetramethylsilane. Electronegative atoms and nearby π systems generally move signals downfield, to higher ppm. Equivalent nuclei produce the same signal, so molecular symmetry can make the number of observed signals smaller than the number of atoms.

Proton NMR

Proton NMR provides three complementary clues:

• Chemical shift suggests the proton’s surroundings.

• Integration estimates the relative number of hydrogens represented by a signal.

• Splitting often indicates neighboring nonequivalent hydrogens. In simple cases, a signal coupled to $n$ neighboring hydrogens is split into $n+1$ lines. This is a guide rather than a universal rule; overlapping signals, exchangeable O–H or N–H protons, and more complex coupling can produce exceptions.

Approximate proton-shift regions include alkyl C–H at $0.8\text{–}2.0\ \mathrm{ppm}$; C–H near a carbonyl, aromatic ring, or other π system at $2.0\text{–}3.0\ \mathrm{ppm}$; C–H attached to or near O, N, or a halogen at $3.0\text{–}4.5\ \mathrm{ppm}$; alkene C–H at $4.5\text{–}6.5\ \mathrm{ppm}$; aromatic C–H at $6.5\text{–}8.5\ \mathrm{ppm}$; aldehyde C–H at $9\text{–}10\ \mathrm{ppm}$; and carboxylic-acid O–H at $10\text{–}12\ \mathrm{ppm}$. O–H and N–H shifts can vary substantially with solvent, concentration, and temperature.

For example, an ethyl group commonly gives a triplet for its $\mathrm{CH_3}$ group because it has two neighboring $\mathrm{CH_2}$ hydrogens, and a quartet for its $\mathrm{CH_2}$ group because it has three neighboring $\mathrm{CH_3}$ hydrogens. Their integrations are usually in a $3:2$ ratio.

Carbon-13 NMR

A broadband-decoupled Carbon-13 NMR spectrum usually shows one singlet for each distinct carbon environment. Approximate regions are $0\text{–}50\ \mathrm{ppm}$ for many saturated alkyl carbons, $40\text{–}90\ \mathrm{ppm}$ for many carbons bonded to O or N, $100\text{–}160\ \mathrm{ppm}$ for alkene and aromatic carbons, and $160\text{–}220\ \mathrm{ppm}$ for carbonyl carbons. Carbonyl carbons are typically farthest downfield. The wider carbon shift range can help distinguish environments that overlap in proton spectra.

Takeaway: Use proton shifts, integrations, and splitting to build fragments; use carbon signal count and shifts to check how many distinct carbon environments the structure requires.

Mass spectrometry: constrain mass and formula

A mass spectrum plots ion abundance against mass-to-charge ratio, written $m/z$. In electron-ionization spectra, the molecular ion $\mathrm{M^{+\bullet}}$, if visible, corresponds to the molecule after loss of an electron. It may be weak or absent because the molecule fragments. The base peak is simply the most abundant ion; it is not necessarily the molecular ion. Softer ionization methods may instead produce ions such as $[\mathrm{M+H}]^+$, so identify the ion type before inferring the neutral molecule’s mass.

High-resolution mass spectrometry can help distinguish formulas with similar nominal masses. Isotope patterns also provide clues: chlorine often gives an $M$ and $M+2$ pair in an approximate $3:1$ ratio, while bromine often gives an approximately $1:1$ pair. A small $M+1$ peak commonly arises mainly from naturally occurring carbon-13, and its relative size tends to increase with the number of carbon atoms.

Takeaway: Establish which ion is being observed before using its mass, and treat fragments and isotope patterns as checks on the proposed formula.

Worked example: identify ethyl acetate

Consider an unknown with formula $\mathrm{C_4H_8O_2}$, an electron-ionization molecular-ion peak at $m/z=88$, and the following observations:

• Unsaturation: $\mathrm{DBE}=(2\times4+2-8)/2=1$, consistent with one double bond or one ring.

• IR: A strong carbonyl absorption and strong C–O-region bands support an ester. The absence of broad alcohol or carboxylic-acid O–H absorption is consistent with this interpretation.

• Proton NMR: A quartet integrating to $2\mathrm{H}$ near $4.1\ \mathrm{ppm}$, a singlet integrating to $3\mathrm{H}$ near $2.0\ \mathrm{ppm}$, and a triplet integrating to $3\mathrm{H}$ near $1.3\ \mathrm{ppm}$ indicate an $\mathrm{O-CH_2-CH_3}$ group and a carbonyl-adjacent methyl group.

• Carbon-13 NMR: Four distinct environments are expected: an ester carbonyl carbon, an O-bound $\mathrm{CH_2}$, and two different $\mathrm{CH_3}$ carbons.

• Mass spectrometry: The molecular-ion mass agrees with the formula’s nominal mass of $88$.

Together, these observations support ethyl acetate, $\mathrm{CH_3COOCH_2CH_3}$. No single observation proves the structure; the formula, carbonyl, oxygenated methylene, ethyl splitting pattern, and carbon-signal count reinforce one another.

Synthesize the evidence

A reliable structure should account for the molecular mass and formula, the degree of unsaturation, the diagnostic IR absorptions, and the NMR signal patterns and counts. Treat reference ranges as approximate, and use each technique to test the conclusions drawn from the others. Accept a proposed structure only when the full set of evidence agrees.