Chapter 6 — Seeing Molecules: Introduction to Spectroscopy (IR, MS, and the Logic of Structure Determination)

"The spectrum is the molecule's voice. We just have to learn to listen." — Apocryphal, attributed to several chemists over the decades

Every chapter so far has assumed you know the structure of the molecule you are working with. Real chemistry rarely has that luxury. A chemist running a reaction gets a bottle of product back from the lab and has to figure out what it is. A forensic chemist has a sample of unknown powder and has to identify it. An environmental chemist takes a water sample and has to determine what contaminants are present. A graduate student running their first successful reaction has to prove the expected product really did form.

All four scenarios are solved the same way — by spectroscopy. Spectroscopic instruments shine light (or another energetic probe) at the molecule, record how the molecule responds, and output a spectrum — a plot of response vs. wavelength or mass. From the spectrum, a trained chemist can infer the structure.

This chapter introduces the first two spectroscopic tools a chemist reaches for:

• Infrared (IR) spectroscopy — identifies functional groups by their characteristic vibrations.
• Mass spectrometry (MS) — determines molecular weight and gives clues about fragmentation.

Chapter 9 will introduce the third major tool: nuclear magnetic resonance (NMR) spectroscopy. After Chapter 9 you will have the three most powerful structural-determination techniques in the organic chemist's arsenal.

6.1 The electromagnetic spectrum

Spectroscopy uses electromagnetic radiation (light, broadly construed). The key relationships:

• Wavelength $\lambda$ and frequency $\nu$ are related by $c = \lambda \nu$, where $c$ is the speed of light.
• Energy per photon is $E = h \nu = hc/\lambda$, where $h$ is Planck's constant.
• In IR spectroscopy, we use wavenumber $\tilde{\nu} = 1/\lambda$, measured in cm⁻¹. Higher wavenumber = higher frequency = higher energy.

A small number of regions matter in organic chemistry:

Organic spectroscopy occupies several of these regions:

• IR — when molecules absorb infrared photons, specific bonds vibrate more energetically. Different bonds absorb at different wavenumbers. Reading an IR spectrum is reading which bonds are present.
• NMR (Chapter 9) — radio-frequency photons flip nuclear spins in a magnetic field. Different nuclei in different environments flip at different frequencies.
• UV-Vis (introduced briefly here, returned to in Ch 19): visible/UV photons excite π electrons. Diagnostic for conjugated systems and aromatic compounds.
• Mass spectrometry is not a light-based technique but a mass-measuring one; included here because it is used together with IR routinely.

This chapter handles IR and MS in detail. NMR is deferred to Chapter 9, where it gets a dedicated treatment.

6.2 Infrared spectroscopy

What the experiment looks like

An IR spectrometer passes infrared light through a sample and records how much gets through at each wavelength. Bonds in the sample absorb photons of the right energy to excite them into higher vibrational states. The output is a plot of transmittance (0-100%) vs. wavenumber (with wavenumber usually shown from ~4000 on the left to ~400 on the right — higher-energy on the left).

Figure 6.1 — A schematic IR spectrum. Wavenumber on the x-axis (right-to-left, descending); transmittance on the y-axis (100% at top). Each absorption is a "dip" where bonds in the sample have absorbed IR photons to vibrate more energetically. Broad, intense dips indicate strong absorbers (usually polar bonds like $O-H$, $N-H$, $C=O$). Narrow, sharp dips are often less intense bonds or specific C-H stretches. The range from 4000 to 1500 cm⁻¹ is the diagnostic region where characteristic functional-group bonds appear. The range from 1500 to 400 cm⁻¹ is the fingerprint region — crowded with bond-bending modes that uniquely identify a molecule but are hard to assign individually.

Modern IR instrumentation: FTIR

Most modern IR spectrometers use Fourier transform infrared (FTIR) technology. Instead of scanning one wavelength at a time, FTIR uses an interferometer to collect all wavelengths simultaneously, then computes the spectrum by Fourier transform. Advantages:

• Faster: an FTIR scan takes seconds; older grating-based scans took minutes.
• More sensitive: better signal-to-noise ratio.
• Higher resolution: can resolve close peaks better.

Sample preparation is also typically simpler now thanks to attenuated total reflection (ATR) sampling — the sample is pressed against a crystal (diamond, ZnSe, or Ge), and IR light reflected within the crystal interacts with the sample at the surface. ATR-FTIR requires no preparation: just place a drop or solid sample on the crystal, press, and acquire. This has revolutionized day-to-day IR work.

Why different bonds absorb at different wavenumbers

A vibrating bond behaves like a tiny harmonic oscillator. The vibrational frequency depends on two things:

1. The spring constant $k$ — the stiffness of the bond. Stiffer bonds (double and triple bonds, bonds to hydrogen) vibrate faster.
2. The reduced mass $\mu$ — the effective mass of the two atoms. Lighter atoms vibrate faster.

Hooke's law gives:

$$\tilde{\nu} = \frac{1}{2\pi c}\sqrt{\frac{k}{\mu}}$$

Larger $k$ (stiffer bond) → higher $\tilde{\nu}$. Smaller $\mu$ (lighter atoms) → higher $\tilde{\nu}$. This explains most of what you see in an IR spectrum:

• C-H stretches (around 2900-3100 cm⁻¹): high because $\mu$ is small (hydrogen is light).
• O-H stretches (3200-3600 cm⁻¹): also involve a hydrogen; slightly higher than C-H because $k_{O-H} > k_{C-H}$.
• N-H stretches (3300-3500 cm⁻¹): similar.
• C=O stretches (1680-1780 cm⁻¹): much lower than C-H (heavier atoms), but higher than C-C (stiffer bond).
• C=C stretches (1620-1680 cm⁻¹): similar region to C=O but usually weaker (less polar).
• C-C stretches (below 1200 cm⁻¹, often in fingerprint region): heavier atoms, single bond — low frequency.

Stretching vs bending modes

A bond can vibrate in multiple ways: - Stretching: atoms move along the bond axis (longer-shorter); two atoms have one stretching mode. - Bending: atoms move perpendicular to the bond axis (changing bond angle). Three atoms (e.g., a CH₂ group) have multiple bending modes (scissoring, rocking, wagging, twisting).

Stretching modes are usually stronger and at higher wavenumber than bending modes. The diagnostic region (4000-1500 cm⁻¹) is dominated by stretches; the fingerprint region (1500-400 cm⁻¹) contains many bending modes.

For a non-linear molecule with N atoms, the total number of vibrational modes is 3N − 6 (for linear molecules, 3N − 5). This is why molecules with many atoms have crowded fingerprint regions.

Selection rules: when does a bond absorb?

Not every bond vibration shows up in the IR spectrum. The selection rule:

A vibration is IR-active if it changes the molecule's dipole moment.

• Polar bonds (C=O, O-H, C-O, C-X) have large dipole changes during vibration → strong IR signals.
• Nonpolar bonds (C-C in symmetric molecules, sometimes C=C) have small dipole changes → weak IR signals.
• Symmetric vibrations in symmetric molecules (e.g., symmetric C≡C in 2-butyne) can give zero dipole change → IR-inactive (no peak).

A complementary technique, Raman spectroscopy, uses a different selection rule (change in polarizability, not dipole). Raman is the inverse: nonpolar bonds (C=C, C-C) are often Raman-active even when IR-inactive. Both techniques together give complementary information.

Characteristic IR frequencies — the table to memorize

The diagnostic region (4000-1500 cm⁻¹) is where you identify functional groups. Memorize at least these:

This list is worth committing to memory. You will use it in every problem for the rest of the book.

Fine structure of carbonyl region

The carbonyl region (1680-1780 cm⁻¹) contains the strongest IR absorption in many molecules. Within this region, you can often distinguish carbonyl types by exact wavenumber:

• Conjugated C=O (e.g., α,β-unsaturated ketone): shifts to lower wavenumber (~1675 cm⁻¹) because conjugation weakens the C=O bond (less double-bond character).
• Strained C=O (e.g., cyclopentanone, cyclobutanone): shifts to higher wavenumber. Cyclobutanone is at ~1780; cyclopentanone at 1745; cyclohexanone at 1715.
• Hydrogen-bonded C=O (e.g., carboxylic acid dimer): shifts to lower wavenumber.

These shifts are small but diagnostic. With practice, the carbonyl wavenumber can identify which type of carbonyl is present.

Reading an IR spectrum: the procedure

When you look at an unknown's IR spectrum:

1. Scan the O-H / N-H region (3200-3600 cm⁻¹). If you see a broad peak here, you likely have an alcohol, carboxylic acid, or amine/amide. If not, you probably don't.
2. Scan the C-H region (2850-3100 cm⁻¹). Almost every molecule has $sp^3$ C-H stretches. Look for sp² (3030-3100) indicating alkene or aromatic.
3. Scan the triple-bond region (2100-2260 cm⁻¹). A peak here signals a nitrile (2210-2260) or terminal alkyne (2100-2260, often weak).
4. Scan the C=O region (1680-1780 cm⁻¹). A strong sharp peak here identifies a carbonyl. The exact wavenumber refines the carbonyl type: - 1720-1740: aldehyde - 1705-1725: ketone - 1735-1750: ester - 1630-1690: amide - 1700-1725: carboxylic acid - 1780-1815: acid chloride
5. Scan the C=C region (1450-1680 cm⁻¹). Aromatic signatures come in pairs around 1500 and 1600; alkenes are single peaks around 1650.
6. Scan the fingerprint region (400-1500 cm⁻¹). This is crowded and hard to assign; use it mostly to confirm or rule out specific C-X stretches (C-O around 1100; C-N around 1000-1200).

With a little practice, you can identify the major functional groups of a molecule from its IR in about 30 seconds.

Worked Problem 6.1 — Identify the functional group

An unknown compound has an IR spectrum showing: - Broad peak centered at 3350 cm⁻¹ - Sharp peaks 2900-2960 cm⁻¹ - No peak around 1700 cm⁻¹ - C-O stretch around 1050 cm⁻¹

The broad peak at 3350 is O-H (H-bonded). The 2900-2960 are $sp^3$ C-H. No C=O rules out aldehyde, ketone, ester, carboxylic acid, amide. The 1050 cm⁻¹ C-O is consistent with an alcohol.

Conclusion: the compound is an alcohol. Maybe an ether, but the broad O-H at 3350 points to alcohol rather than ether (which has no O-H).

Worked Problem 6.2 — Distinguish carbonyl types

Three unknowns each show a strong C=O peak. Their carbonyl peaks are at: (A) 1740 cm⁻¹ (B) 1715 cm⁻¹ (C) 1670 cm⁻¹

A is most likely an ester (1735-1750). B is a ketone (1705-1725). C is most likely an amide (1630-1690), with the relatively high value perhaps reflecting an N-substituted amide.

6.3 Mass spectrometry

Mass spectrometry determines the mass of molecules (and fragments). The output is a plot of ion abundance vs. mass-to-charge ratio ($m/z$).

How a mass spectrometer works

Three essential steps:

1. Ionization. A molecule is ionized, typically by one of several methods: - Electron ionization (EI): high-energy electrons (70 eV) knock an electron off the molecule, producing a radical cation (M⁺•). Often fragments extensively. Common in older instruments and forensics. - Chemical ionization (CI): a protonated reagent transfers a proton to the molecule, giving (M+H)⁺. Gentler than EI; less fragmentation; useful for confirming molecular ion. - Electrospray ionization (ESI): a common modern technique, particularly for biomolecules. Produces charged ions that typically resemble (M+H)⁺ or (M+Na)⁺. Nondestructive; works for small molecules and proteins. - MALDI (matrix-assisted laser desorption/ionization): laser desorbs sample from a matrix; produces (M+H)⁺ for proteins and peptides; very gentle. - APCI (atmospheric pressure chemical ionization): similar to CI but at atmospheric pressure; common for LC-MS of small drugs.

2. Mass separation. Ions are accelerated through electric/magnetic fields that deflect them according to their mass-to-charge ratio. Different $m/z$ values arrive at the detector at different times or positions. - Quadrupole mass analyzer: variable electric field selects one $m/z$ at a time. - Time-of-flight (TOF): ions are accelerated and timed; lighter ions arrive first. - Orbitrap, FT-ICR: ions trapped in electric/magnetic field; their oscillation frequency is measured by Fourier transform; ultra-high resolution.

3. Detection. A detector counts the number of ions at each $m/z$. The output: a mass spectrum.

Figure 6.2 — Schematic mass spectrum. The x-axis is mass-to-charge ratio ($m/z$); the y-axis is abundance (relative intensity), typically normalized to the most intense peak (the base peak) at 100%. The molecular ion ($M^+$) at the highest significant $m/z$ reveals the molecular weight. Lower-$m/z$ peaks are fragments produced by bond cleavages within the cation.

The molecular ion

The peak at highest $m/z$ (where some ions survive unfragmented) is the molecular ion peak, $M^+$. Its mass equals the molecular weight of the compound (in EI mode, where $M$ just loses an electron).

The molecular weight alone is often enough to narrow possibilities to a handful of compounds. Combined with the IR data, it often uniquely identifies the structure.

Remember: for most organic molecules with only C, H, N, O, halogens — all with even atomic weights except hydrogen (1) and nitrogen (14) — the molecular weight of the neutral molecule is even if there is no nitrogen or an even number of nitrogens, and odd if there is an odd number of nitrogens. This is the nitrogen rule:

If $M^+$ is odd, the molecule contains an odd number of nitrogens. If $M^+$ is even, the molecule has 0, 2, 4, ... nitrogens.

Isotope patterns

Many elements have isotopes — atoms with the same number of protons but different numbers of neutrons. Most organic-chemistry isotopes have very low natural abundance (12C vs. 13C: 99% vs. 1%; 1H vs. 2H: 99.99% vs. 0.01%). The isotope pattern near the $M^+$ is often subtle.

But two elements produce very diagnostic isotope patterns:

• Chlorine (35Cl:37Cl in ratio 3:1). A molecule with one chlorine shows an $M^+$:$(M+2)^+$ ratio of 3:1. With two chlorines, the pattern is 9:6:1 at $M$, $M+2$, $M+4$.
• Bromine (79Br:81Br in ratio 1:1). A molecule with one bromine shows a 1:1 doublet at $M$ and $M+2$.

If you see a doublet at the molecular ion with roughly 1:1 intensity separated by 2 mass units, there is a bromine in the molecule. If 3:1 in that same pattern, there is a chlorine. These are pieces of evidence you will use.

Counting carbons from M+1

The natural abundance of ¹³C is about 1.1%. Therefore the (M+1) peak, due to molecules with one ¹³C, has intensity ~1.1% × n_C of the M peak (where n_C is the number of carbons). Counting carbons:

$$n_C \approx \frac{[\text{M+1}]/[\text{M}]}{0.011}$$

For example, if the M+1 is 9% of M, then n_C ≈ 9/1.1 ≈ 8 carbons. Combined with molecular formula, this can give the carbon count directly.

Common fragmentations

EI mass spectra often show fragments arising from preferred bond cleavages. Some diagnostic patterns:

• Loss of 15 mass units (a methyl, $CH_3$): common for compounds with methyl groups.
• Loss of 18 (water, $H_2O$): common for alcohols.
• Loss of 28 (CO or N₂ or CH₂CH₂): common for carbonyls (CO loss) or aromatic NHN compounds (N₂ loss).
• Loss of 29 (either $CHO$ or $C_2H_5$): aldehydes often lose 29 (formyl, $HCO$); ethyl esters/ethers often lose 29 ($C_2H_5$).
• Loss of 31 (methoxy, $OCH_3$): methyl esters.
• Loss of 43 (either $C_3H_7$ or $CH_3CO$): isopropyl, propyl, or acetyl.
• Base peak 43 at $m/z$ 43: often the acylium cation $CH_3CO^+$ from methyl ketones.
• Base peak 91 at $m/z$ 91: the tropylium cation ($C_7H_7^+$), an aromatic 7-membered ring cation; diagnostic for benzyl groups (Ph-CH₂-X).

α-cleavage of carbonyl compounds

Carbonyl compounds preferentially cleave bonds α to the $C=O$, producing acylium-like fragments:

$$\text{R-C(=O)-R'} \to \text{R}^+ + \text{R'-C}\equiv\text{O} \quad \text{or} \quad \text{R-C}\equiv\text{O}^+ + \text{R'}$$

The acylium ion is stabilized by the C=O+ resonance (R-C≡O⁺ ↔ R-C⁺=O). The smaller fragment is usually the cation; the larger is lost as neutral.

For acetone (CH₃COCH₃), m/z 58: α-cleavage gives loss of CH₃ → m/z 43 (CH₃CO⁺ = acetylium). The acetylium at 43 is the base peak.

McLafferty rearrangement

For carbonyl compounds with a γ-hydrogen, the McLafferty rearrangement is diagnostic:

$$\text{Aldehyde or ketone with } \gamma\text{-H} \to \text{enol radical cation} + \text{neutral alkene}$$

The γ-H migrates to the carbonyl O via a 6-membered cyclic transition state; a β-bond cleaves; the result is an even-electron enol cation (m/z = ?) and a neutral alkene. The McLafferty fragment has m/z corresponding to the smaller piece.

For 2-pentanone (CH₃COCH₂CH₂CH₃, MW 86), the McLafferty rearrangement gives: - Neutral alkene CH₂=CH₂ (28) leaves. - m/z 58 fragment is the enol cation (CH₃C(OH)=CH₂⁺• → m/z 58).

McLafferty is diagnostic of carbonyls with γ-H — the peak at M-28 (loss of ethylene) or M-42 (loss of propene) etc. is a flag.

Degrees of unsaturation

Given a molecular formula, the degrees of unsaturation (DoU), also called the index of hydrogen deficiency (IHD), tells you the total number of rings plus multiple bonds:

$$\text{DoU} = \frac{2C + 2 + N - H - X}{2}$$

where $C$ = number of carbons, $H$ = hydrogens, $N$ = nitrogens, $X$ = halogens. Oxygens are ignored in this formula.

• Each ring contributes 1 DoU.
• Each double bond contributes 1 DoU.
• Each triple bond contributes 2 DoU (two $\pi$ bonds).
• An aromatic ring (benzene) contributes 4 DoU (3 double bonds + 1 ring).

Worked Problem 6.3 — Putting it all together

A compound has molecular formula $C_8H_{10}O$ (MW = 122). Its IR shows: - Strong, broad peak at 3380 cm⁻¹ (O-H) - Peaks 3000-3100 cm⁻¹ ($sp^2$ C-H) - Peaks 2880-2960 cm⁻¹ ($sp^3$ C-H) - Peaks 1490 and 1600 cm⁻¹ (aromatic C=C) - Peak 1055 cm⁻¹ (C-O stretch)

Its MS shows $M^+ = 122$ and a base peak at $m/z = 107$ (loss of 15 = $CH_3$).

DoU for $C_8H_{10}O$: $(2 \cdot 8 + 2 - 10)/2 = 4$. Consistent with an aromatic ring (4 DoU).

The IR tells us: aromatic ring, alcohol, $sp^3$ CH somewhere. No carbonyl, no alkene, no nitrile.

The MS fragmentation loss of 15 ($CH_3$) suggests a methyl group, probably on the ring.

Possible structures consistent with $C_8H_{10}O$ with aromatic + alcohol + methyl + $sp^3$ CH: methylbenzyl alcohol (CH₃-C₆H₄-CH₂OH): has aromatic ring, a methyl on ring, and a CH₂-OH group. MW = 122 ✓. Loss of 15 (methyl) ✓.

The likely structure: a methylbenzyl alcohol (e.g., 4-methylbenzyl alcohol). Additional data (NMR, Chapter 9) would distinguish between the ortho, meta, and para isomers.

High-resolution mass spectrometry (HRMS)

HRMS measures m/z to 4-5 decimal places (compared with low-resolution's 1-2 decimal places). Why does this matter?

Different molecular formulas can have the same nominal mass. For example: - C₂H₄O = 44.026 - CH₄N₂ = 44.037 - CO₂ = 43.990 - C₃H₈ = 44.063

Low-resolution MS just sees "44." HRMS distinguishes them. HRMS at 5 decimal places can determine the exact molecular formula, often unambiguously. This is now standard for confirming new compound structures in research papers.

The exact mass of an atom is its monoisotopic mass (the mass of the most abundant isotope). For organic molecules, you compute the molecular formula's exact mass: - C = 12.0000 (by definition) - H = 1.0078 - N = 14.0031 - O = 15.9949 - Cl-35 = 34.9689 - Br-79 = 78.9183

HRMS measurements are usually reported as [M+H]⁺ exact mass; the difference from theory (in millimass units, mDa or ppm) is the error.

6.4 UV-Visible spectroscopy (preview)

While IR probes vibrations and MS measures mass, UV-Vis spectroscopy probes electronic transitions. When a molecule absorbs UV or visible light, an electron is excited from a bonding/non-bonding to an antibonding orbital (typically π → π or n → π).

Key features: - Range: 200-800 nm (200-400 = UV; 400-800 = visible). - Wavelength of maximum absorption (λ_max) depends on the conjugated π system. - Beer's law: $A = \epsilon c l$, where ε is the molar absorptivity, c is concentration, l is path length.

Diagnostic for: - Conjugated dienes/polyenes (Ch 19): λ_max increases with extent of conjugation. - Aromatic compounds (Ch 20): characteristic π → π transitions. - Carbonyls: weak n → π near 280 nm. - Dyes (Ch 22): visible absorption gives color.

UV-Vis is especially useful for: - Concentration measurement (Beer's law for known compounds). - Following reactions (changes in λ_max as conjugation changes). - Bioanalysis (DNA at 260 nm; proteins at 280 nm).

We'll return to UV-Vis in Chapter 19 (conjugated systems) and Chapter 22 (dyes).

6.5 Why spectroscopy is a "storyline"

The introduction to this book promised that spectroscopy would be a continuous storyline — used in every chapter, not crammed into two chapters near the end. This is the chapter where the storyline starts.

From here on, every chapter that introduces a new functional group or a new reaction will include a Spectroscopy Clue callout showing you how to recognize that functional group or reaction in an IR, MS, or NMR spectrum. By Chapter 20, you will have accumulated a practical toolkit — not just knowing individual functional-group absorbances but being able to piece together unknowns from the combined data.

The progression:

• This chapter (6): IR and MS for simple molecules.
• Chapter 9: NMR spectroscopy — the most powerful of the three, and the basis for almost all modern structure elucidation.
• Chapter 15+: using all three to identify alkenes, alcohols, carbonyls, and other products in context.
• Chapter 31: using spectroscopy to verify synthesis products and propose structures of complex compounds.

If you finish this chapter with a habit of looking at an IR or MS and asking "what can I learn about this structure?" — rather than just reading the peaks — you are exactly where you need to be.

Computational Exercise 6.1

Download the IR spectrum of a compound you have at home (ethanol, acetone, vinegar — all have spectra published on the Spectral Database for Organic Compounds, SDBS, at sdbs.db.aist.go.jp). Print it. Using the table in this chapter, label every significant peak. Hand it in (or save it). This is the kind of exercise a student can do in 20 minutes that teaches more about IR than an hour of reading.

Spectroscopy Clue 6.1 — The 5-second IR check

When you see an IR spectrum, do these checks in order (each takes 1 second): 1. Look at 3200-3600: is there a broad O-H? → alcohol or carboxylic acid. 2. Look at 1700-1750: is there a strong C=O? → carbonyl (and which kind from exact wavenumber). 3. Look at 2200-2300: is there a sharp C≡N or C≡C? → nitrile or alkyne. 4. Look at 1500-1600: aromatic ring? 5. Anything weird? (e.g., 1500-1600 at strong intensity = nitro group)

Common Mistake 6.1

Students sometimes confuse the broad O-H (3200-3600) with the broad N-H (3300-3500). The two regions overlap. To distinguish: - O-H is broader and typically more intense. - N-H is sharper and typically less intense. - Primary amines (R-NH₂) show two N-H bands (symmetric and asymmetric stretches); secondary amines (R₂NH) show one. This is a great diagnostic.

Common Mistake 6.2

Students sometimes interpret a small peak at the M+1 position as a separate compound. It's almost always the ¹³C isotope contribution (~1% per carbon). Ignore unless it's >5% of M.

6.6 Summary

1. Spectroscopy is how chemists determine structure. Different techniques probe different features of molecules: IR probes bond vibrations; MS measures molecular mass; UV-Vis probes electronic transitions; NMR (Chapter 9) probes nuclear environments.

2. IR spectroscopy identifies functional groups by their characteristic vibration frequencies. The diagnostic region (1500-4000 cm⁻¹) is where you identify functional groups; the fingerprint region (<1500 cm⁻¹) is for distinguishing closely-related molecules. Memorize the key wavenumbers: O-H (3200-3600), C-H (2850-3100), C≡N (2210-2260), C=O (1680-1780), C=C aromatic (1500-1600).

3. Hooke's law explains why bonds vibrate at different frequencies: stiffer bond + lighter atoms = higher wavenumber.

4. Selection rules: an IR-active vibration changes the dipole moment. Polar bonds (C=O, O-H) give strong signals; nonpolar bonds (symmetric C=C) may be IR-silent.

5. Mass spectrometry tells you molecular mass and fragmentation. The molecular ion ($M^+$) reveals MW. Isotope patterns (Cl: 3:1 $M$:$M+2$; Br: 1:1 $M$:$M+2$) reveal halogens. Common fragmentation losses (15 = methyl; 29 = CHO or $C_2H_5$; 43 = $CH_3CO$ or $C_3H_7$) reveal structural features.

6. Ionization methods: EI (hard, lots of fragmentation); CI (soft, gives M+H); ESI (soft, common for biomolecules); MALDI (proteins); APCI (small drugs in LC-MS).

7. Mass analyzers: quadrupole (variable field, low-resolution); TOF (time-of-flight, fast); orbitrap/FT-ICR (high-resolution).

8. Nitrogen rule: if $M^+$ is odd, the molecule contains an odd number of nitrogens.

9. Degrees of unsaturation: count rings plus multiple bonds from molecular formula.

10. α-cleavage and McLafferty rearrangement are characteristic carbonyl fragmentations.

11. HRMS (high-resolution MS) measures m/z to 4-5 decimal places; can determine exact molecular formula. Now standard for confirming new compounds.

12. Combined IR + MS often uniquely identifies simple organic compounds without needing NMR.

13. UV-Vis probes electronic transitions; useful for conjugated systems and concentration measurements.

Chapter 9 (in Part II) will introduce NMR — the most powerful individual tool for structure determination. Combined with Chapter 6 techniques, NMR gives you essentially the full structural-determination toolkit.

Part I ends here. You have: the structural vocabulary (Chapter 2), the acid-base framework (Chapter 3), the functional-group vocabulary (Chapter 4), the conformational and energetic framework (Chapter 5), and the diagnostic spectroscopy tools (Chapter 6).

Part II — Stereochemistry — is next. The first chapter of Part II (Chapter 7) is about chirality and the 3D arrangement of atoms. Then NMR (Chapter 9). Then the first mechanisms (Part III).

The mechanism chapters are where the book picks up speed. Make sure Part I is solid before you proceed.