Analytical Tools: UV-Vis, IR (fingerprint region) & NMR principles; applications in identifying organic functional groups

UV-Vis, IR & NMR — Principles and Functional-Group Identification

1. UV-Vis spectroscopy (electronic transitions)

Measures absorbance vs wavelength (200–800 nm). Transitions in order of energy: $\sigma\to\sigma^\*$ (far UV) > $n\to\sigma^\*$ > $\pi\to\pi^\*$ > $n\to\pi^\*$ (near UV/visible, lowest).

• Chromophore: group absorbing UV-Vis (C=C, C=O, NO₂, aromatic ring). More conjugation → longer $\lambda_{max}$ (bathochromic/red shift): ethylene 165 nm, butadiene 217 nm, β-carotene ~450 nm (visible colour).
• Auxochrome: group that shifts/intensifies absorption (–OH, –NH₂, –OCH₃) without absorbing itself.
• Uses: concentration by Beer–Lambert, conjugation check, reaction kinetics, purity of drugs.

2. IR spectroscopy (vibrational transitions)

Range 4000–400 cm⁻¹ (wavenumber $\bar{
u}$). Each functional group has a characteristic stretching frequency from Hooke's law — light atoms + strong bonds absorb high.

Group	Absorption (cm⁻¹)	Notes
O–H (free) / N–H	~3600 / ~3300 broad	H-bonded O–H broad 3200–3600
C–H (alkane/alkene/alkyne)	2850–2960 / ~3100 / ~3300	C≡C–H sharp at 3300
C≡N / C≡C	~2250 / ~2150	sharp, medium
C=O (ketone/ester/acid)	~1715 / ~1735 / ~1710	strongest peak in spectrum
C=C	~1650	medium
C–O	1000–1300	strong

• Fingerprint region (1500–400 cm⁻¹): bending vibrations, unique pattern for every molecule — identity proof by matching with standard spectrum.
• Functional-group region (4000–1500) tells what groups; fingerprint tells which exact molecule.

3. NMR principle (¹H NMR essentials)

Nuclei with spin ($^1H$, $^{13}C$) act as tiny magnets. In external field $B_0$ they absorb radio-frequency energy at resonance:

where $\sigma$ = shielding by surrounding electrons.

• Chemical shift $\delta$ (ppm) $= \frac{
  u_{sample} -
  u_{TMS}}{
  u_{spectrometer}} \times 10^6$, TMS = 0 ppm reference.
• Deshielded protons (near O, aryl, aldehyde) appear at high $\delta$ (downfield, left).
• n+1 rule: peak splits into $n+1$ lines by $n$ equivalent neighbours (CH₃–CH₂: triplet + quartet); area ratio = proton count.

Proton type	$\delta$ (ppm) approx
Alkane CH₃/CH₂	0.9–1.5
CH next to O/N/X	3–4.5
Alkene / aromatic	5–6 / 7–8
Aldehyde H	9–10
Acid O–H	11–12

4. Combined identification strategy

1. IR: C=O present? O–H present? triple bond?
2. UV: conjugated/aromatic?
3. NMR: how many proton environments, splitting, total H count.

Example: IR 1715 cm⁻¹ + no broad O–H + NMR single peak ~2.1 ppm (6H singlet) → acetone (CH₃COCH₃).

Exam Points

• Characteristic IR frequencies (C=O, O–H, C≡N, C–H types) + fingerprint definition (3 marks, very frequent).
• UV shifts with conjugation, chromophore vs auxochrome (2–3 marks).
• NMR: define chemical shift, n+1 rule, assign 2–3 δ values (5 marks favourite); one combined-structure problem is almost guaranteed.