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NMR Dr Harisadhan Ghosh Dept of Chemistry, Surendranath College, Kol-9 ______Introduction; nuclear ; NMR active molecules; basic principles of Magnetic Resonance; choice of solvent and internal standard; equivalent and non-equivalent ; and factors influencing it; ring current effect; significance of the terms: up-/downfield, shielded and deshielded protons; spin coupling and coupling constant (1st order spectra); relative intensities of first-order multiplets: Pascal’s triangle; chemical and magnetic equivalence in NMR ; anisotropic effects in alkene, alkyne, aldehydes and aromatics; NMR peak area, integration; relative peak positions with coupling patterns of common organic compounds (both aliphatic and benzenoid-aromatic); rapid proton exchange; interpretation of NMR spectra of simple compounds. Introduction Nuclear magnetic resonance (NMR) is a spectroscopic technique that detects the energy absorbed by changes in the nuclear spin state.

NMR spectroscopy is a form of absorption spectrometry.

Most absorption techniques (e.g.: Ultraviolet-Visible and Infrared) involve the , in the case of NMR, it is the nucleus of the which determines the response.

An applied (magnetic) field is necessary for the absorption to occur. THEORY v The theory behind NMR comes from the spin of a nucleus and it generates a . v Without an external applied magnetic field, the nuclear spin are random in directions. But when an an external magnetic field (B ), is v 0 present the nuclei align themselves either with or against the field of the external magnet. v When a compound have a proton, proton spin around it’s own axis & act as a magnet. v If kept it an external magnetic field it align according to external magnetic field. Nuclear spin (I) • Subatomic particles (electrons, protons and ) can be imagined as spinning on their axes. • (such as 12C) has no overall spin. • atoms (such as 1H and 13C) has an overall spin.

Each spin-active nucleus has a number of spins defined by its spin , I. NUCLEAR SPIN The nuclei of some atoms have a property called “SPIN”.

These nuclei behave as if they were spinning.

….. we don’t know if they actually do spin!

This is like the spin property of an , The spinning charged nucleus generateswhich can havea magnetic two spins: +1/2 and field -1/2 .

Each spin-active nucleus has a number of spins defined by its spin quantum number, I.

SIRaJ/MSc/NMR 8 Is all nuclei are NMR active? The angular of spinning nucleus is described in terms of spin quantum number I The spin quantum number I is a characteristic constant of a nucleus, and is dependent on the number of protons and neutrons.

In general three rules apply to the nuclear spins. Nuclei with odd mass number and odd or even no. of protons have half – integral spin such as 1/2, 3/2, 5/2 etc. Nuclei with even mass number and odd no. of protons have integral spin such as 1, 2, 3 Nuclei with even mass number and even no. of protons always have zero spin (Due to Pairing of oppositely directed spins in the nucleus) Theory of NMR

Spin quantum number (I) is related to the atomic and mass number of the nucleus I Z A Eg;

Half integer Odd Odd 1H (1/2)

Half integer Odd Even 13C (1/2)

Integer Even Odd 2H(1)

Zero Even Even 12C (0)

Elements with either odd mass or odd atomic number have the property of nuclear “spin”

3 MagnMeatgince tPic rPoropperrtiteise osf Noufc leNi uclei

Magnetic Properties of Nuclei l Magnetic Properties of Nuclei l l l

Nucleii SpinningSpinning charged charged NucleiiNucleii Spinningparticle charged is a magnet spin + charge particleparticle is a magnet is a magnet spinspin ++chargecharge The spinning of positively charged particle produces: The spinning of positively charged particle produces: The spinning (1of) Spin positively charged or particleSpin quantum produces: numberSpinning (I) charged Nucleii (1) Spin angular( 2momentum) or Spin (m quantum) along the numberaxis of spin (I) (1) Spin angular momentum or Spin quantum number (I) (2) Magnetic moment(3) Electric (m) alongquadrupole the axis moment of spin (Q) particle is a magnet spin + charge (as a result of non-spherical distribution of nuclear charge) ((32) )Electric Magnetic quadrupole moment moment (m) along (Q) the axis of spin SIRaJ/MSc/NMR 9 ( 3 )(as Electric a result quadrupole of non-spherical moment distribution (Q) of nuclear charge)

SIRaJ/MSc/NMR 9 (as a result of non-spherical distribution of nuclear charge)

The spinning of positively chargedSIRaJ/MSc/NMR particle produces: 9 (1) Spin angular momentum or Spin quantum number (I) (2) Magnetic moment (m) along the axis of spin (3) Electric quadrupole moment (Q) (as a result of non-spherical distribution of nuclear charge)

SIRaJ/MSc/NMR 9 The spin state of a nucleus is affected by an applied magnetic field The spin state of a nucleus is affected by an applied magnetic field If an External magnetic Field is Applied-

SIRaJ/MSc/NMR 10 SIRaJ/MSc/NMR 10 Behavior of Magnetic Nuclei BehaviorBehavior of of Magnetic Magnetic Nuclei Nuclei For nuclei with spin I = ½ BehaviorBehaviorTwo ForForof possible nucleiof Magneticnuclei Magnetic with orientations with spin Nucleispin I =Nuclei ½ I = ½ asTwo per possibleequation orientations 2I + 1 TwoFor nuclei possible with orientationsspin I = ½ BehaviorFor of as nuclei Magneticper equation with spin 2Nuclei II =+ ½1 Two asTwo possible per possible equationE lorientationsect rorientationsomag 2neI t+ ic R1a diation in R F range with energy E = Ep Randomly oriented nuclear For as nuclei per as perequation with equation Espinle 2cIt rI+o =2 m1 I½ a+g 1n eticRadiation in spRinasn odfo emqluy aol reiennetregdy ninu cthleea r Two possible orientationsR FE rlaencgtero wm itha genertgicyR Ea =di aEtpion in absence of any magnetic field E_ le1ctromagneticRadiation in Randospminlys oofr eieqnutael den neurgcyl eina rthe as per equationElec 2trRIo + mF 1a r gannegticeR waditihat ieonne inrg y E _= 1 Ep R 2F_ r1ange with energy E = Ep RaanbRdsaoenmndlcoye mo orlyfie aonnrtyiee dmn tnaeugdcn nleutaicrcl efiaerld R F range with energy E = Ep 2_ 1 spins of equal energy in the 2 spinssp oinfs e oqfu eaql eunael regnye ring yth ine the ElectromagneticRadiation in absence of any magnetic field Ho EP _ 1_ 1 2 abseanbcse nocfe a onfy amnayg mnaegtinc eftieicld field R F r_an1ge with energy E = Ep _ 1 _ 1 Randomly oriented nuclear E 2 2 _ 1 Ho P 2 2 2 spins of equal energy in the 1 2 _ +1 1 absence of any magnetic field Ho EP _ 1 HoHo EPEP 2 1 + 2 + 2 1 2 2+ Precisely oriented nuclear 1spins 2 Ho EP 1 + 1 in Pthre cpisreeslye nocriee notfe Md+a ngunceltei2acr f 1isepldins 1 + 2 + + 2 1 in the presence of Magneti2c field 2 + 1 2 PrecPirseeclyis oerlyie onrteiedn nt+eudc lneuacr lseparin sspins 1 2 + in Pthirnee pcthries sepelryne csoeer noicef enM toaefg dMn eantguicnc efliteeicald rfi eslpdins 2 Precinis tehlye o prireensteedn ncuec olefa Mr sapginnsetic field in the presence of Magnetic field In NMR, we are measuring the energy required for the flipping of the nucleus In NMR, we are measuring the energySIRaJ/MSc/NMR required for the flipping of the nucleus 11 In NMR, we are measuring the energySIRaJ/MSc/NMR required for the flipping of the nucleus 11 In NMR, we are measuring the energy required for the flipping of the nucleus SIRaJ/MSc/NMR 11 SIRaJ/MSc/NMR 11 InIn NMR, NMR, wewe are measuring measuring the the energy energy required required for the for flipping the flipping of the nucleus of the nucleus SIRaJ/MSc/NMRSIRaJ/MSc/NMR 11 11 Nuclear NuclearMagnetic Magnetic Resonance Resonance Spectroscopy Spectroscopy Nuclear spins are oriented randomly in the absence (a) of an external Nuclear magneticspinsNuclear are orientedfield spins but randomlyare have oriented ain specific therandomly absence orientation in (a)the of absence an externalin the(a) ofpresence magnetic an external (b) magnetic of an field but havefield a specific but have orientation a specific in orientation the presence in the(b) ofpresence an external (b) of field, an external B field, B external field, 0 0 • Some nuclear• Some spins nuclear areB 0aligned spins parallel are aligned to the parallel external to fieldthe external field – Lower energy– orientationLower energy orientation – More• Some likely nuclear – More likelyspins are aligned parallel to the external field • Some– nuclearLower• Some spins energy nuclear are orientation–aligned spins antiparallel are aligned More to antiparallellikely the external to fieldthe external field – – Higher energy orientation Higher• Some energy nuclear orientation spins are aligned antiparallel to the external field – Less likely – Less likely – Higher energy orientation– Less likely

In the presence of a strong magnetic field, the tiny magnetic field due to spinning charged particles aligns to be either with or In the presence of a strong magnetic field, the tiny magnetic field againstdue to spinning the charged magnetic particles field aligns. to be either with or against the magnetic field.

SIRaJ/MSc/NMR 13 • More will be in the lower energy state aligned with the magnetic field. • More• Anucleons will canbe in absorb the lower a energy aligned of energy with the in magneticthe field. radio frequency range and align against • A nucleon can absorb a quantum of energy in the radio frequency range andthe align magnetic against the field. magnetic field. • It • emitsIt emits a radio a frequencyradio frequency when it drops when back it todrops its original back position. to its original position.

SIRaJ/MSc/NMR 14 NUCLEAR SPIN STATES - NUCLEUS

NUCLEAR SPIN STATES - HYDROGEN NUCLEUS

The spin of the positively m charged nucleus generates a magnetic moment vector, m.

+ +

The two states m are equivalent in energy in the + 1/2 - 1/2 absence of a magnetic or an TWO SPIN STATES electric field. SIRaJ/MSc/NMR 15

The axis of the nuclear magnet is oriented exactly parallel or anti parallel with applied magnetic field, there will be a certain force by the external field to so oriented it. But because the nucleus is spinning the effect is that its rotation Precessional motionaxis draws out a circle perpendicular to the applied field. This motion is called precession (example is the gyroscopic motion of The axis of the nuclear magnet isthe oriented spinning top) exactly parallel or anti parallel withThe applied precessional frequency of the nucleus depends upon the magnetic field, there will be a certain force by the external field to so oriented it. strength of the applied magnetic field and the nature of the But because the nucleus is spinningnucleus. the effect is that its rotation axis draws out a circle perpendicular to the applied field. This motion is called precession (example is the gyroscopic motion of the spinning top) The precessional frequency of the nucleus depends upon the strength of the applied magnetic field and the nature of the nucleus. spinning top Precession is a change in direction of the axis, but without a change in tilt. SIRaJ/MSc/NMR 24 The behavior of a nuclear magnet in a magnetic field

HO

w o m

Nuclear magnet Precessional orbit H

SIRaJ/MSc/NMR 25

To be Continued