02 Spectroscopic Structure Elucidation Free Online FlashCards

What does each major spectroscopy method contribute to structure elucidation?

MS constrains molecular mass and formula; IR identifies bond types and functional groups; NMR reveals atom environments and how atoms are grouped.

How is degree of unsaturation calculated from a molecular formula?

Use $\mathrm{DBE}=\frac{2C+2+N-H-X}{2}$, where $X$ counts halogens. Oxygen and sulfur are omitted.

How do rings, double bonds, and triple bonds contribute to DBE?

Each ring or double bond contributes one DBE; each triple bond contributes two. A benzene ring contributes four.

What IR feature commonly indicates an alcohol or phenol $\mathrm{O-H}$ bond?

An alcohol or phenol $\mathrm{O-H}$ stretch is typically broad and usually strong, appearing around $3200\text{–}3600\ \mathrm{cm^{-1}}$.

How does a carboxylic-acid $\mathrm{O-H}$ stretch typically appear in IR?

A carboxylic-acid $\mathrm{O-H}$ absorption is very broad and typically spans about $2500\text{–}3300\ \mathrm{cm^{-1}}$, often overlapping $\mathrm{C-H}$ stretches.

What IR absorption can indicate a nitrile?

A nitrile $\mathrm{C\equiv N}$ often gives a distinct, medium-to-strong IR absorption around $2210\text{–}2260\ \mathrm{cm^{-1}}$.

What does a strong IR carbonyl band establish—and what does it not establish?

A strong band around $1650\text{–}1800\ \mathrm{cm^{-1}}$ supports a carbonyl. Its position alone may not identify the type; conjugation often lowers its frequency.

What is the IR fingerprint region useful for?

The fingerprint region is approximately $1500\text{–}500\ \mathrm{cm^{-1}}$. Its many overlapping bands are especially useful for comparing a sample with a reference spectrum.

What generally shifts an NMR signal downfield?

Electronegative atoms and nearby $\pi$ systems generally move NMR signals downfield, to higher ppm.

What is the simple proton NMR splitting rule?

In simple cases, coupling to $n$ neighboring nonequivalent hydrogens splits a signal into $n+1$ lines.

What proton NMR pattern commonly identifies an ethyl group?

An ethyl group commonly gives a $\mathrm{CH_3}$ triplet and a $\mathrm{CH_2}$ quartet, with integrations in a $3:2$ ratio.

What does the number of signals in broadband-decoupled $^{13}\mathrm{C}$ NMR represent?

A broadband-decoupled $^{13}\mathrm{C}$ NMR spectrum usually shows one singlet per distinct carbon environment. Molecular symmetry can reduce the number of signals.