¹H NMR — chemical shift, multiplicity (n + 1 rule), integration; common ranges

4.8.4 · Chemistry › Spectroscopy & Analysis (Intro)

Teen pieces of information ek ¹H spectrum se nikalti hain, aur tum teeno padhte ho:

Chemical shift ( $δ$ ) — peak kahaan hai → kaunsa environment hai (electron-rich vs electron-poor).
Integration — peak ke neeche ka area → us type ke kitne H hain.
Multiplicity (splitting) — peak mein lines ki sankhya → uske kitne neighbours hain.

1. Chemical Shift ( $δ$ )

Intuition WHY divide karo aur ×10⁶? (Scratch se derivation)

Absorption frequency $ν$ , nucleus jo field feel karta hai uske proportional hoti hai: $ν = \frac{γ}{2 π} B_{local}, B_{local} = B_{0} (1 - σ)$ jahaan $γ$ gyromagnetic ratio hai aur $σ$ shielding constant hai (electrons nucleus ko kitna screen karte hain). Problem: $ν$ $B_{0}$ par depend karta hai, isliye 300 MHz aur 600 MHz ki machine ek hi proton ke liye alag-alag frequencies deti hain. Labs ke beech compare karne ke liye yeh bekar hai. Fix: reference se difference lo aur spectrometer frequency se divide karo. Numerator aur denominator mein $B_{0}$ cancel ho jaata hai: $δ = \frac{( γ /2 π ) B _{0} ( σ _{ref} - σ _{sample} )}{( γ /2 π ) B _{0}} \times 1 0^{6} = (σ_{ref} - σ_{sample}) \times 1 0^{6}$ To $δ$ field-independent hai — yeh sirf shielding report karta hai. $\times 1 0^{6}$ sirf chote number ( $\sim 1 0^{- 6}$ ) ko convenient "ppm" units mein scale karta hai.

Figure — ¹H NMR — chemical shift, multiplicity (n + 1 rule), integration; common ranges

Formula Common chemical-shift ranges (woh 20% jo 80% kaam deta hai)

Proton type	$δ$ (ppm)	Kyun
TMS reference	0	Si electropositive hai → bahut zyada shielded
Alkane C–H (R–CH₃, R–CH₂)	0.5–1.5	sirf C, H neighbours
Allylic / C=O ke paas	2.0–2.5	halka deshielding
C–H jo O ya halogen ke paas ho	3.0–4.5	electronegative atom e⁻ kheenchta hai
Vinyl =CH	4.5–6.5	$π$ deshielding
Aromatic Ar–H	6.5–8.0	ring current strongly deshield karta hai
Aldehyde –CHO	9–10	C=O aur O-pulling carbon se attached
Carboxylic acid –COOH	10–12	extreme deshielding + H-bonding

2. Integration — protons ginana

3. Multiplicity — $n + 1$ rule

$n$	naam	pattern (Pascal)
0	singlet	1
1	doublet	1 1
2	triplet	1 2 1
3	quartet	1 3 3 1
4	quintet	1 4 6 4 1

Common Mistakes (Steel-man + fix)

Active Recall

Recall Quick self-test (answers dhako)

Peak ka area kya batata hai? → protons ki sankhya (ratio).
3 equivalent neighbours wala proton → kitni lines? → 4 (quartet), pattern 1:3:3:1.
Aromatic H ~7 ppm par kyun hota hai? → ring current usse deshield karta hai.
$δ$ paane ke liye spectrometer frequency se kyun divide karte hain? → $B_{0}$ cancel ho jaata hai, field-independent (ppm) ban jaata hai.
Kya equivalent protons ek doosre ko split karte hain? → Nahi.

Recall Feynman: ek 12-saal ke bachche ko samjhao

Socho ki har hydrogen atom ek bachcha hai jo ek chota sa compass pakde hua hai. Hum unhe sab ko ek bade magnet mein rakhte hain aur ek radio bajate hain. Har bachcha radio ko thoda alag station par "jawab" deta hai is baat par depend karte hue ki woh kahaan khada hai — ek bachcha jo bahut saare electrons ke peeche chupa hai woh dial ke ek end ke paas jawab deta hai, ek bachcha jo khule mein hai woh doosre end par. Isliye dial par kahaan buzz sunai deta hai yeh batata hai ki bachcha kis kamre mein hai (chemical shift). Buzz kitna tez hai yeh batata hai ki kitne bachche wahan saath khade hain (integration). Aur har bachcha ka compass uske bilkul paas khade bachchon ke compasses se nudge hota hai: 1 neighbour uski buzz ko 2 choti buzzes mein split kar deta hai, 2 neighbours se 3, aur aise aage — yahi $n + 1$ rule hai. Teeno clues padho aur tumhe molecule ka poora layout pata chal jaata hai.

¹H NMR mein δ = 0 kaun sa reference compound define karta hai?

Tetramethylsilane (TMS)

Chemical shift Hz ki jagah ppm mein kyun report ki jaati hai?

Spectrometer frequency se divide karne par

B_{0}

cancel ho jaata hai, jo

δ

ko magnet strength se independent banata hai.

Deshielded protons spectrum par kahaan dikhte hain?

Downfield (left), high

δ

par — yeh zyada

B_{0}

feel karte hain kyunki electron density kam hoti hai.

n + 1

rule batao.

n

equivalent neighbouring (vicinal) protons wala proton

n + 1

lines mein split hota hai.

Peak integration kya measure karta hai?

Us signal dene wale protons ki relative sankhya (area ∝ number of H).

Splitting pattern intensities kis cheez ko follow karti hain?

Pascal's triangle / binomial coefficients C(n,k); row sum =

2^{n}

Equivalent protons ek doosre ko kyun split nahi karte?

Unke energy levels identically shift hote hain, isliye koi observable splitting nahi hoti.

Ethanol ka ¹H pattern predict karo.

CH₃ triplet (3H, ~1.2), CH₂ quartet (2H, ~3.7), OH broad singlet (1H, variable).

Aromatic C–H ka typical

δ

6.5–8.0 ppm (ring-current deshielding).

Aldehyde –CHO ka typical

δ

9–10 ppm.

OH/NH proton ko experimentally kaise identify karte hain?

Broad, variable singlet; D₂O exchange par gayab ho jaata hai.

Quartet mein split hone wale signal ka matlab kitne neighbours hain?

3 equivalent neighbouring protons.

Connections

Tetramethylsilane (TMS) — yeh ideal reference kyun hai (shielded, inert, single peak)
Spin-spin coupling (J) — coupling constants aur second-order spectra
¹³C NMR — sister technique, kam splitting issues
Electronegativity — shielding/deshielding drive karta hai
Aromaticity & ring current — 7 ppm aromatic protons explain karta hai
Pascal's triangle — multiplet intensities ki origin
Integration in IR/MS — complementary structure-determination tools

1. Chemical Shift (δ)

2. Integration — protons ginana

3. Multiplicity — n+1 rule

Common Mistakes (Steel-man + fix)

Active Recall

Connections

Concept Map

1. Chemical Shift ( $δ$ )

3. Multiplicity — $n + 1$ rule