M

IR Spectroscopy Vocabulary Review (Chapters 1–7 Notes)

IR Spectroscopy Notes – Functional Groups and Peak Interpretation

  • Purpose and scope

    • Infrared (IR) spectroscopy probes vibrational transitions in molecules by exposing them to electromagnetic radiation in the IR region.
    • Absorption occurs when the energy of the radiation matches the energy of a vibrational mode of a bond in the molecule.
    • Different bonds absorb at different energies, so the spectrum provides a fingerprint of the molecule’s bonds and functional groups.

  • How absorption arises in IR (vibrational modes)

    • A bond can undergo various motions when it absorbs energy:
    • Symmetric stretch
    • Antisymmetric stretch
    • Bending (scissoring, rocking, wagging, etc.)
    • In-plane and out-of-plane motions
    • An individual absorption peak corresponds to a specific vibrational mode of a bond or group of bonds.
    • Whether a given bond shows absorption depends on how the bond is structured (bond symmetry and vibrational mode), not on the overall symmetry of the molecule.

  • Key relationships: energy, wavelength, and wave number

    • Energy-wavelength relationship (qualitative):
    • Higher energy ↔ shorter wavelength; lower energy ↔ longer wavelength.
    • Energy-wavelength relationship (quantitative):
    • E = h\nu = \dfrac{hc}{\lambda} = h c \tilde{\nu}
      where \tilde{\nu} = \dfrac{1}{\lambda} is the wave number in cm$^{-1}$ (units: cm$^{-1}$).
    • Wave number and wavelength are inversely related:
    • \tilde{\nu} = \dfrac{1}{\lambda} \quad (\text{cm}^{-1})
    • Bond vibration frequency and wave number depend on two factors:
    • Bond strength (spring constant k): stronger bonds vibrate at higher frequencies.
    • Reduced mass (μ) of the two atoms attached to the bond: larger μ lowers the vibrational frequency.
    • From the diatomic oscillator model:
    • \nu = \dfrac{1}{2\pi}\sqrt{\dfrac{k}{\mu}}
    • Reduced mass for a diatomic bond: \mu = \dfrac{m1 m2}{m1 + m2}
    • Heavier masses (larger μ) lead to smaller ν and thus smaller (lower) wave numbers; larger bond stiffness (larger k) leads to larger ν and larger wave numbers.
    • Practical upshot: stronger bonds and lighter bonded partners push absorptions to higher wave numbers (higher energy), whereas heavier partners and weaker bonds push absorptions to lower wave numbers.

  • What is measured in the IR: peak properties

    • Wave number (position of the peak, in cm$^{-1}$): the primary data used to identify functional groups.
    • Transmission/absorbance: reflects how polarizable a bond is (intensity relates to dipole moment changes during vibration and to how many such bonds are present).
    • Shape of peaks: influenced by hydrogen bonding and environment; broad or jagged peaks can indicate hydrogen bonding; sharp peaks often indicate little or no hydrogen bonding.
    • Quantity effect: the number of identical bonds (e.g., many C–H bonds) can amplify peak intensity.

  • Regions of the IR spectrum

    • Functional group region (approx. above ~1500 cm$^{-1}$): where most diagnostic bonds appear and where you identify functional groups.
    • Fingerprint region (below ~1500 cm$^{-1}$): highly specific to a given molecule; useful for determining purity and confirming identity, less for initial functional-group assignment.
    • Practical guidance from the lecture: functional group region is around 1500 cm$^{-1}$ and above; fingerprint region below 1500 cm$^{-1}$ is unique to each compound and great for purity checks.

  • Four characteristic IR features to recognize (visual cues)

    • OH stretch (alcohols) – broad, smooth, around ~3400 cm$^{-1}$; in alcohols the peak is typically smooth and not jagged.
    • OH stretch for carboxylic acids – very broad, often with jagged, finger-like components due to extensive hydrogen bonding; appears around ~2500–3300 cm$^{-1}$ (broader than alcohol).
    • Carbonyl (C=O) stretch – very strong and typically centered around ~1700 cm$^{-1}$; not very broad unless conjugated or interacting with others.
    • C=C stretch (alkenes) – appears near ~1600 cm$^{-1}$; can be weak to medium depending on substitution and conjugation.
    • Note on additional pieces: esters often show a C–O stretch around ~1250 cm$^{-1}$ (and sometimes ~1050–1150 cm$^{-1}$); aldehydes show characteristic aldehydic C–H stretches near ~2720 cm$^{-1}$ and sometimes ~2820 cm$^{-1}$, in addition to the carbonyl peak.

  • How to interpret peak intensities and shapes in practice

    • Intensity relates to bond polarizability and to how many such bonds exist (e.g., many C–H bonds give strong peaks for CH stretches).
    • A peak can be weak if the bond is less polarizable or if the bond motions don’t lead to a large dipole change.
    • A peak can be broad if hydrogen bonding is strong (e.g., carboxylic acid OH broadens due to extensive H-bonding).
    • The presence or absence of specific peaks helps distinguish similar functional groups (e.g., alcohol vs carboxylic acid via the OH region; aldehyde vs ketone via the aldehyde C–H band at ~2720 cm$^{-1}$).

  • How to identify common functional groups from IR (typical cues)

    • Alcohol (R–OH, non-carboxylic):
    • OH stretch: around ~3400 cm$^{-1}$, smooth and relatively sharp for simple alcohols; for primary/secondary alcohols the band is broad but not jagged.
    • Carboxylic acid (R–COOH):
    • Broad, strong OH stretch around ~2500–3300 cm$^{-1}$ (very broad and irregular);
    • A very strong C=O stretch around ~1700 cm$^{-1}$ (often the most intense peak).
    • The combination of a broad OH and a strong, centered carbonyl confirms carboxylic acid.
    • Carbonyl-containing groups (R–C(=O)–R’ in general):
    • Very strong C=O stretch near ~1700 cm$^{-1}$; the exact position shifts slightly with conjugation and substitution (e.g., aldehydes often at ~1720–1740 cm$^{-1}$, ketones around ~1705–1725 cm$^{-1}$, esters around ~1735–1750 cm$^{-1}$).
    • Aldehydes vs Ketones (carbonyl-containing, but aldehyde has a distinct C–H band):
    • Aldehyde C–H stretches near ~2720 cm$^{-1}$ and ~2820 cm$^{-1}$ are diagnostic.
    • If you see a carbonyl near 1720–1740 cm$^{-1}$ plus aldehydic C–H bands near 2700–2900, you can assign an aldehyde.
    • Ketones lack the aldehyde C–H bands; they show the carbonyl peak but no aldehyde C–H bands.
    • Esters (R–C(=O)–OR’):
    • Carbonyl peak near ~1735–1750 cm$^{-1}$ (often slightly higher than ketones).
    • A characteristic C–O stretch in the ~1250 cm$^{-1}$ region (sometimes also observed ~1050–1150 cm$^{-1}$). The presence of a strong peak around ~1250 cm$^{-1}$ with a carbonyl peak supports an ester.
    • Alkenes (C=C):
    • The C=C stretch around ~1600 cm$^{-1}$; usually moderate intensity.
    • Amines (R–NH2, R2NH, R3N):
    • N–H stretches around ~3300–3500 cm$^{-1}$.
    • Primary amines show two N–H stretches (two peaks) due to two N–H bonds;
    • Secondary amines show one N–H stretch; tertiary amines show little to no N–H stretch in this region.

  • Practical examples (how these cues come together)

    • Example A: Carboxylic acid
    • OH stretch: broad, around 2500–3300 cm$^{-1}$ (jagged).
    • C=O stretch: strong around ~1700 cm$^{-1}$.
    • Conclusion: Carboxylic acid present (OH + C=O).
    • Example B: Alcohol (not carboxylic)
    • OH stretch: around ~3400 cm$^{-1}$, broad but smoother than carboxylic acid.
    • No C=O at ~1700 cm$^{-1}$ (or absent).
    • Conclusion: Alcohol present.
    • Example C: Ester
    • C=O stretch near ~1735–1750 cm$^{-1}$ (strong).
    • A C–O stretch around ~1250 cm$^{-1}$ (prominent).
    • No broad OH around 2500–3300 cm$^{-1}$ (unless a free OH is present elsewhere in the molecule).
    • Conclusion: Ester present.
    • Example D: Aldehyde vs Ketone (carbonyl-containing, no OH)
    • Carbonyl around ~1720–1740 cm$^{-1}$ (aldehydes can be ~1720–1740 depending on substitution).
    • Aldehyde CH stretches at ~2720 cm$^{-1}$ (and sometimes ~2820 cm$^{-1}$) present.
    • If aldehyde CH bands are observed, assign aldehyde; otherwise, ketone.
    • Example E: Alkene-containing molecule
    • C=C stretch around ~1600 cm$^{-1}$; use this with other peaks to classify the molecule as containing an alkene.
    • Example F: Amine
    • N–H stretch(s) around ~3300–3500 cm$^{-1}$ with the number of peaks indicating primary (two peaks), secondary (one peak), or tertiary (no N–H peak).

  • Quick decision flow for IR-based functional-group identification (practical steps)
    1) Scan for a carbonyl peak around ~1700 cm$^{-1}$.
    2) If a carbonyl is present, check the OH region:

    • OH broad and jagged around ~2500–3300 cm$^{-1}$ with a strong C=O suggests carboxylic acid.
    • If no OH broad band in this region, but carbonyl is present, consider ketone, aldehyde, or ester depending on other peaks.
      3) If an OH band around ~3400 cm$^{-1}$ is present without a carbonyl, assign an alcohol.
      4) If a band near ~1250 cm$^{-1}$ plus a carbonyl is seen, consider an ester.
      5) Look for aldehyde-specific C–H bands near ~2720 cm$^{-1}$ to distinguish aldehydes from ketones.
      6) Check the N–H region (~3300–3500 cm$^{-1}$) for amines and note the number of peaks to distinguish primary/secondary/tertiary amines.
      7) Look at the ~1600 cm$^{-1}$ region for C=C to identify alkenes and their influence on peak positions.

  • Connections to broader chemistry concepts

    • The IR approach aligns with Gross molecular properties:
    • Bond strength and atomic masses determine vibrational energies, linking spectroscopy to molecular structure.
    • Hydrogen bonding alters peak shapes and intensities, connecting spectroscopy to intermolecular interactions and solvent effects.
    • The fingerprint region is highly molecule-specific, making IR useful for identity verification and purity checks when combined with other data.
    • The relationships between energy, wavelength, and wave number reflect fundamental physical chemistry concepts (quantum vibrational energy, dipole transitions, and mass–spring models).

  • Quick reference values (typical ranges, approximate)

    • OH stretch – Alcohol: ~3400 cm$^{-1}$ (broad, smooth).
    • OH stretch – Carboxylic acid: ~2500–3300 cm$^{-1}$ (very broad, jagged).
    • C=O stretch – Carbonyls (ketones, aldehydes, esters, acids, amides): ~1700 cm$^{-1}$ (intense; exact position shifts with context).
    • C=C stretch – Alkenes: ~1600 cm$^{-1}$ (medium).
    • Ester C–O stretch: ~1250 cm$^{-1}$ (and other C–O bands ~1050–1150 cm$^{-1}$).
    • Aldehyde C–H stretches: ~2720 cm$^{-1}$ (and ~2820 cm$^{-1}$).
    • N–H stretches – Primary amines: two peaks around ~3300–3500 cm$^{-1}$; Secondary amines: one peak; Tertiary amines: none.

  • Summary takeaways

    • IR spectra provide diagnostic peaks that map to specific bonds and functional groups.
    • The position of a peak (wave number) reflects bond strength and reduced mass; intensity reflects bond polarizability and the number of such bonds.
    • Hydrogen bonding dramatically affects peak shape, especially for OH-containing groups.
    • Practical identification relies on combining signals: OH and C=O together point to carboxylic acids; a smooth OH with a single C=O suggests alcohol and ketone; an ester shows a carbonyl plus a distinctive C–O band; aldehydes show carbonyl plus aldehydic C–H bands; amines show N–H bands with counts indicating substitution level.

  • Appendix: sample deduction exercise (illustrative)

    • Spectrum with a very broad OH around 2500–3300 cm$^{-1}$ and a very strong peak near 1700 cm$^{-1}$ → carboxylic acid.
    • Spectrum with a strong peak around 1700 cm$^{-1}$ and no broad OH in 2500–3300 → likely a ketone or aldehyde; check aldehyde C–H bands near ~2720 cm$^{-1}$ to decide aldehyde vs ketone.
    • Spectrum with a sharp, smooth OH around ~3400 cm$^{-1}$ and no carbonyl peak → alcohol.
    • Spectrum with a carbonyl near ~1735–1750 cm$^{-1}$ and a strong peak near ~1250 cm$^{-1}$ → ester.