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

4.8.4 · D5 · Chemistry › Spectroscopy & Analysis (Intro) › ¹H NMR — chemical shift, multiplicity (n + 1 rule), integrat

Yeh ek rapid-fire self-test hai jo the ¹H NMR topic ke traps se banaya gaya hai. Har ::: line ka right side cover karo aur apne aap ko justify karne par majboor karo, sirf guess mat karo. Yahan har jawab reasoning hai, koi bare label nahi.

Symbols jo traps se pehle zaroori hain

Test se pehle, neeche har symbol plain words mein samjhaya gaya hai. Agar koi question ise use kare, toh iska matlab bilkul yahi hai.

Definition Khelne wale chaar symbols

$ν$ (nu) — radio frequency (Hz mein) us electromagnetic wave ki jise ek proton absorb karta hai jab woh flip karta hai. Ise us particular proton ka exact "radio station" samjho jis par woh tune hai.
$B_{0}$ — spectrometer ke magnet dwara provide kiya gaya bada external magnetic field (tesla mein). Yeh ek given machine ke liye fixed hota hai aur har energy gap ka scale set karta hai. "300 MHz" machine aur "600 MHz" machine bilkul apne $B_{0}$ mein alag hote hain.
$δ$ (delta), the chemical shift — ek peak ki position, jo reference frequency subtract karke aur machine ki operating frequency $ν_{0}$ se divide karke define hoti hai: $δ = \frac{ν _{sample} - ν _{ref}}{ν _{0}} \times 1 0^{6} ppm$ jahan $ν_{sample}$ proton ki absorption frequency hai, $ν_{ref}$ TMS ki hai, aur $ν_{0}$ operating (spectrometer) frequency hai. $ν_{0}$ se divide karne par har $ν$ ke andar chhupa $B_{0}$ cancel ho jaata hai, isliye $δ$ (ppm mein) kisi bhi machine par same number hota hai.
$J$ , the coupling constant — ek multiplet ki split lines ke beech spacing, Hz mein measure ki gayi. Yeh measure karta hai ki do protons ek doosre ki spin ko kitni strongly feel karte hain aur, $δ$ ke unlike, $B_{0}$ ke saath nahi badalti.

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

True ya false — justify karo

True ya false: same chemical shift wale do protons hamesha same functional group se aate hain.

False — shift ki equivalence matlab hai magnetic environment ki equivalence, jo alag-alag groups mein coincidentally match ho sakti hai; identical

δ

same group prove karne ke liye necessary hai lekin sufficient nahi.

True ya false: chemically equivalent protons ek doosre ko multiplet mein split karte hain.

False — equivalent protons ke identical energy levels hote hain, isliye unka mutual coupling koi observable line splitting produce nahi karta; ek akela isolated CH₃ ek singlet hota hai.

True ya false: zyada chemical shift (

δ

) ka matlab hamesha taller peak hota hai.

False —

δ

(position) environment report karta hai jabki peak area (dekho integration) proton count report karta hai; 9.7 ppm par ek 1H aldehyde tiny ho sakta hai.

True ya false: 300 MHz se 600 MHz spectrometer switch karne par

δ

value ppm mein badal jaati hai.

False — operating frequency

ν_{0}

se divide karne par

B_{0}

cancel ho jaata hai, isliye

δ

ppm mein field-independent hota hai (yahi ppm ka poora point hai).

True ya false: coupling constant

J

(Hz mein) higher-field magnet par zyada bada ho jaata hai.

False —

J

ek fixed through-bond interaction hai jo

B_{0}

se independent hai; sirf

δ

separation (Hz mein) field ke saath badhta hai, isliye high-field spectra zyada first-order lagte hain.

True ya false: zyada shielded proton higher ppm par resonate karta hai.

False — shielding ka matlab hai proton

B_{0}

se kam feel karta hai, isliye woh upfield par lower

δ

par resonate karta hai; deshielding use downfield pe higher

δ

par push karta hai.

True ya false:

n

neighbours wale proton ke liye Pascal-triangle intensities ka sum

2^{n}

hota hai.

True — har neighbour ek independent spin-up/spin-down choice contribute karta hai, jo

2^{n}

total spin combinations deta hai, jo row sum

\sum_{k} (k n)

hai (

k

ko 0 se

n

tak sum karte hue).

True ya false: OH aur NH protons

n + 1

rule ko C–H protons jitni reliably follow karte hain.

False — OH/NH typically fast proton exchange undergo karte hain, broad singlet ke roop mein variable

δ

par appear hote hain aur often na split hote hain na split kiye jaate hain.

True ya false: Tetramethylsilane (TMS) ko

δ = 0

set kiya gaya hai kyunki iske koi protons nahi hain.

False — TMS mein 12 equivalent protons hain jo ek sharp reference peak dete hain; woh 0 par isliye baithta hai kyunki Si electropositive hai, un protons ko unusually shielded banata hai.

True ya false: aromatic protons mainly carbon atoms ki electronegativity ki wajah se deshielded hote hain.

False — dominant effect ring current hai, ek circulating

π

-electron field jo ring ke edge par

B_{0}

ko reinforce karta hai, un H's ko ~7 ppm tak deshield karta hai.

Error dhundho

Error: "Ethanol mein CH₃ ek quartet hai kyunki iske 3 protons hain."

Multiplicity neighbours se aati hai, proton ke apne count se nahi; CH₃ ke 2 vicinal CH₂ neighbours hain, isliye

n = 2

→ triplet.

Error: "

n + 1

ke liye, main molecule mein har hydrogen count karta hoon."

Sirf vicinal (adjacent-carbon) protons count karo jo observed proton se inequivalent hain; door ke aur equivalent H's use split nahi karte.

Error: "

δ = \frac{ν _{sample} - ν _{ref}}{ν _{sample}} \times 1 0^{6}

Denominator operating frequency

ν_{0}

honi chahiye, sample frequency nahi; sirf woh choice har

ν

ke andar chhupe

B_{0}

ko cleanly cancel karti hai.

Error: "Ek quartet ka intensity pattern 1:2:2:1 hota hai."

Ek quartet (

n = 3

) Pascal row 1:3:3:1 follow karta hai; teen equivalent neighbours binomial coefficients

(k 3)

dete hain

k = 0..3

ke liye.

Error: "

C H C l_{2} - C H_{2} C l

mein CH₂ ek triplet hai kyunki iske 2 protons hain."

Iska multiplicity single CHCl₂ neighbour (

n = 1

) par depend karta hai → doublet; iske apne 2 protons integration determine karte hain, splitting nahi.

Error: "Deshielded protons spectrum ke right (upfield) side par appear hote hain."

Deshielded protons downfield, left (high

δ

) par appear hote hain; upfield/right shielded, low-

δ

region hai.

Error: "Kyunki carboxylic acid H exchange karta hai, yeh kabhi 11 ppm par nahi dekha ja sakta."

Exchange broadens karta hai aur use variable banata hai, lekin yeh routinely far downfield (10–12 ppm) par observe hota hai strong deshielding aur H-bonding ki wajah se; ise remove karne ke liye D₂O shake chahiye.

Error: "Do peaks jinki

δ

separation unke

J

ke comparable hai woh phir bhi clean 1:3:3:1 quartets dete hain."

Jab

δ

gap (Hz mein)

J

se bahut zyada bada nahi hota, system second-order (strongly coupled) hota hai aur intensities distort ho jaati hain (leaning "roofing"), tidy Pascal pattern tod ke.

Why questions

Hum TMS ke against reference kyun karte hain sirf raw absorption frequency

ν

report karne ki jagah?

Raw

ν

B_{0}

par depend karta hai aur machines ke beech drift karta hai; ek fixed peak par reference karna aur operating frequency

ν_{0}

se divide karna ek portable, field-independent

δ

yield karta hai.

Ek neighbouring proton do lines kyun produce karta hai na ki ek broadened single line?

Woh neighbour ek magnet hai jo

B_{0}

ke saath ya ulta point kar raha hai, local field ko do discrete values mein split karta hai, isliye observed proton do distinct frequencies par absorb karta hai (ek doublet)

J

se separated.

Ethanol ka CH₂ apne CH₃ se zyada downfield kyun hai?

CH₂ electronegative oxygen se bonded hai (dekho Electronegativity), jo electron density ko pull karta hai, use ~3.7 ppm ki taraf deshield karta hai jabki CH₃ ~1.2 ppm par hota hai.

Splitting intensity pattern Pascal's triangle kyun follow karta hai na ki equal weights?

Har line ek net neighbour-spin arrangement correspond karta hai, aur

k

-th arrangement tak pahunchne ke tareekon ki sankhya binomial coefficient

(k n)

hai — zyada tareekon ka matlab taller line.

Integration directly absolute proton counts kyun nahi deta?

Peak areas sirf proton number ke proportional hain; internal standard ke bina tum reliable ratios padhte ho, phir smallest whole-number set tak scale karte ho.

Ek aldehyde –CHO proton sirf ek H hone ke bawajood ~9–10 ppm ke paas kyun baithta hai?

Yeh doubly deshielded hota hai — adjacent electron-withdrawing C=O se aur sp² carbon se — isliye iska environment, iska abundance nahi, position set karta hai.

¹³C NMR koi attached hydrogen na hone wale carbon ka signal kyun dikh sakta hai, jabki ¹H NMR nahi?

¹H NMR sirf protons detect karta hai, isliye ek quaternary carbon invisible hota hai; ¹³C NMR carbon nucleus ko itself detect karta hai chahe attached H's hon ya na hon.

Edge cases

Edge case: zero neighbours wala proton kaunsi multiplicity dikhata hai?

Ek singlet —

n + 1 = 0 + 1 = 1

line; koi coupling partners nahi hone par resonance undivided hoti hai.

Edge case: ek proton jo do inequivalent sets of neighbours se coupled hai (maan lo 2 type A ke aur 1 type B ke, alag

J

) — kya simple

n + 1

rule apply hota hai?

Nahi; simple rule ek equivalent set assume karta hai, isliye tum ek multiplet-of-multiplets paate ho (jaise ek doublet of triplets), alag J values se governed.

Edge case: same CH₂ par do diastereotopic protons (ek stereocentre ke next carbon) — kya yeh ek doosre ko couple karte hain?

Haan — inequivalent hone ki wajah se, unke alag

δ

hote hain aur woh ek doosre ki real geminal (

^{2} J

) splitting dikhate hain, equivalent CH₂ protons ke unlike jo nahi karte.

Edge case: same carbon par do enantiotopic/equivalent protons — kya yeh ek doosre ko split karte hain?

Nahi — magnetically equivalent hone ki wajah se, unka mutual coupling koi observable splitting cause nahi karta chahe woh chemically real protons hon.

Edge case: kya protons chaar bonds door (jaise allylic ya aromatic meta pairs) kabhi couple kar sakte hain?

Haan — long-range

^{4} J

coupling exist karta hai (chhota, ~0–3 Hz)

π

systems ya W-shaped saturated paths se hoke, naive vicinal-only rule se chhuutti fine splitting add karta hai.

Edge case: jab do coupled protons ka

δ

bahut similar ho toh spectrum ka kya hota hai?

Yeh strongly coupled / second-order ban jaata hai: line spacings aur intensities skew ho jaate hain (peaks ek doosre ki taraf "lean" karte hain) aur clean

n + 1

Pascal picture ab valid nahi rehti.

Edge case: D₂O ke saath sample shake karne par OH signal ka kya hota hai?

Exchangeable OH proton deuterium se swap ho jaata hai aur iska peak disappear ho jaata hai, ek exchangeable OH/NH group confirm karta hai — ek standard diagnostic.

Edge case: agar ek molecule fully symmetric hai aur saare protons equivalent hain (jaise benzene, TMS), toh kitne ¹H signals appear hote hain?

Bilkul ek, kyunki har proton same environment share karta hai; ek single peak jiska area saare equivalent H's account karta hai.

Edge case: ek proton ke liye tum kya observe karoge jiska sirf neighbour ek rapidly exchanging OH hai?

Often ek simple singlet, kyunki fast exchange coupling ko average out kar deta hai aur OH ek fixed spin partner maintain nahi kar sakta use split karne ke liye.