Applications of NMR Spectroscopy in Inorganic Chemistry

7/27/2019 Applications of NMR Spectroscopy in Inorganic Chemistry

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Short Notes on Application of NMR Techniques to Inorganic Molecules

Dr. D. Ilangeswaran, M.Sc., M.Phil., Ph.D., 1

The Chemical Shift

An isolated nucleus such as hydrogen nucleus has a precessional frequency of 100

MHz at field strength of 2.3487 T. But all nuclei are associated with electrons, which revolve

around them. As a charged particle in circular motion, the electrons also produce a magnetic

moment, and this is called as secondary or induced magnetic field.When placed in an applied magnetic field, the induced magnetic field of the electrons

may oppose the net magnetic field experienced by the nucleus i.e., the electrons may circulate

in such a way its magnetic field is opposing the applied field. This circulation is known as

diamagnetic circulation. In diamagnetic circulation the electrons and the nucleus are spinning

in opposite direction to each other. On the other hand if both are spinning in the same

direction the circulation may be paramagnetic one.

Thus the nucleus can be shielded from the applied field by diamagnetic circulation of

electrons. The extent of shielding will be constant for a given atom in an isolated condition,

but will vary with the electron density about an atom in a molecule. The above equation maybe generalized as

Bi = B0 (1- i) where Biis the net magnetic field felt by a particular nucleus i its shielding

constant is i For example we know that oxygen atom is more electro negative than carbon

and hence the electron density about the H atom in C-H bonds should be considerably higher

than that in O-H bonds.

We can expect CH> OH and hence

BCH = B0 (1- CH) < BOH = B0 (1- OH)

i.e., the C-H protons are highly shielded and felt less applied field. But, the O-H protons are

less shielded and felt more applied field. Due to this reason the C-H proton precess with a

smaller Larmor frequency than that of O-H proton.

Thus in order to come

into resonance with a radiation

of particular frequency

(~100MHz at a field strength

of 2.3487T), a C-H proton

requires a greater applied field

than the O-H proton. For

example when we record the1H N.M.R. spectrum for

methanol we can see that the

O-H proton will give the signal

first i.e., at comparatively

lower field than the C-H

proton. The C-H proton signal is obtained at higher field to that of O-H proton.


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In N.M.R. spectrum thus the identical nuclei can give rise to different absorption

positions when present in different chemical surroundings as in the case of methyl and

alcohol protons in methanol. The separation between absorption peaks is usually referred to

as chemical shift.

Internal StandardFor the measurement of chemical shift we have to take tetramethylsilane, Si(CH3)4

(TMS) as an internal standard. With reference to the signal of TMS we have to find out the

shift in the position of other signals.

Since TMS is immiscible with water for aqueous samples we may use

(CH3)3SiCH2CH2CH2SO3Na as an internal standard. For13C spectrum also these two

substances may be used as a reference. The advantages of TMS are

1. Its resonance is sharp and intense since all 12 H nuclei are equivalent and hence absorb at

same position.

2. The resonance position of TMS is too high field of almost all other H absorptions in

organic molecules and hence can be easily recognized.

3. It is a low boiling liquid (b.P.27oC) and so can be readily removed from the most samples

after use.

Conventionally N.M.R. spectra are displayed with the field increasing from the left,

which places TMS signal to the extreme right. Two measurement scales are used, scale and

scale. In scale, the TMS signal is marked as 10p.p.m. and the chemical shift value is

decreasing towards left. In scale the TMS signal is marked as 0p.p.m. and the chemical shift

value is increasing towards left. The relationship of these two scale is =10 - .

The Coupling Constant

Generally the peaks of proton N.M.R. spectrum are splitting into two or more & thedistance between the adjacent peaks in a multiplet is known as coupling constant. The reason

for this is the spin interaction between the two protons when they are present very close

proximity to each other. There are several kinds of spin interactions. When compared to

gaseous molecules the spin-spin interaction will be more in solid substances and takes place

to a smaller extent in liquid molecules.

Factors Influencing Coupling Constant, J Values

1. Geminal Coupling

The protons connected to the same atom are separated only by 2 chemical bonds are

known as geminal protons. The geminal coupling constant values varies from +5Hz to -30Hzdepending on the bond angles between these 2 protons.

2. Vicinal Coupling

The protons present on the adjacent atoms are separated by 3 chemical bonds are

known as vicinal protons. Usually the vicinal coupling constant values vary from 0 to 9 Hz

depends on the dihedral angle. When the dihedral angle, increases from 0 to 90 the J value

decreases from 9 to 0 and again it increases if the value rises from 90 to 180. Karplus

equation given below helps to calculate the vicinal coupling constant, if value is provided.

J = 8.5 cos2 - 0.28 (if = 090o)

J = 9.5 cos

2

- 0.28 (if = 90180

o

)


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3. Long Range Coupling

If protons are separated by more than 3 chemical bonds the J value is very small.

13C NMR Spectra

For a

13

C nucleus, the value of I = and has a precessional frequency of 20 MHz inan external magnetic field strength of 1.9 tesla. But at the same magnetic field strength the

precessional frequency of proton is 80 MHz. Hence the magnetic moment of 13C nucleus is

1/4th the magnetic moment of H nucleus. At the same time the natural abundance of 13C

nucleus is 1.1% only. So, the NMR signal of13C nucleus is very weak.

The problem of weak signal in simple molecules can be overcome by synthesizing 13C

enriched samples. But this is very difficult in complex molecules. In practice by using pulsed

FT method 13C NMR spectra are recorded with natural abundance of13C, with the sensitivity

enhanced by summation of several spectra.

Chemical shift values vary from 0 to 200 ppm. As both 13C and H nucleus have I = ,

we can expect 13C - 13C and 13C 1H couplings. The probability of 2 13C atoms being

together in the same molecule is very low and so 13C - 13C coupling is not usually observed.

Due to 13C 1H coupling, the 13C spectrum will be more complex. This can be

avoided by proton decoupling. The 1H nucleus can be decoupled by double irradiation at its

resonance frequency (80 MHz at 1.9 T). This is an example of hetero nuclear decoupling. The

following table gives the chemical shift values of some carbon atoms.

Type of C atoms Chemical Shift () ppmsp carbon 080

sp carbon 80150

sp carbon 7090sp (aromatic) 110140

CX 1080

Alcoholic, ether, CO 4080

CN 2080

C = O 180 - 200

Here also internal standard is tetra methyl silane (TMS)

19F and

31P NMR

The abundance of

19

F is 100 % and its value of I = . Its precessional frequency is56.46 MHz at 1.4 tesla. Internal standard is CFCl3 or CF3COOH and value varies from 0 to

240 ppm. Geminal FF coupling ranges from 43370 Hz and vicinal coupling varies from

0 to 39 Hz. Coupling between 1H19F also strong with geminal coupling value between 42

80 Hz and vicinal coupling varies between 1.229 Hz.31

Pnucleus also has I = with = 24.3 MHz at 1.4 T. For example, (CH3)2PCF2CH3

has 84 lines. First the signal of P split into 3 by 2 F atoms, then each line into 4 by single CH3

protons. Finally each of 12 lines into 7 by 2 CH3 protons.


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Problems

1. HPF2: The31Pnmr spectrum shows a sextet (three doublets). The two F atoms first split the

phosphorous signal into a triplet and then the H atom split each peak into a doublet. Two

triplets may result if it were JP.H > JP.F (When there are two interacting magnetic nuclei

nearby a sample nucleus the nucleus with larger coupling constant value will first cause thesplitting of the signal of the sample nucleus followed by the other nucleus).

WhenJP-F> JP-H three doublets

WhenJP-H > JP-FTwo triplets

Spli tting due to one H atom spli tting due to two F atoms

2. Protons attached to metal ions are very highly shielded, the peaks often occurring 5 to 1 5

ppm on the high field side of TMS. It is due to non-polar nature of M-H bond. The H nmr of

HRh(CN)53- is a doublet (IRh = 1/2) which occurs at 10.6 ppm on the high field side of TMS.

3. The 31Pnmr ofPhosphorous acid, HPO(OH)2 is a doublet and that ofHypophosphorous

acid, H2PO(OH) is a triplet. Therefore, the structures are as given below. There is no

coupling due to -OH proton as they are far removed.

P

OH

OH

O

HP

OH

O

H

H

4. The 31Pnmr of P4S3 shows two peaks with 3:1 ratio. The more intense peak is a doublet and

the less intense peak is a quartet. S does not couple as IS = 0. This spectrum indicates that

there are three equivalent and one unique P atoms.

S

P

P

S

P

S

P


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5. Solutions of 1:1 TiF62- and TiF4 in ethanol give

19Fnmr consisting of two peaks with

intensity ratio 4:1 (ITi48 = 0). The low intensity peak is a pentet and the high intensity peak is

a doublet. Therefore the structure is [TiF5(OHC2H5)].

6. Ammonia dissolved in water undergoes very fast proton exchange with water. The NMR

spectrum consists of only one peak, which is the average resonance frequency of all theprotons on nitrogen and oxygen.

7. The l9Fnmr spectrum of TiF4 in donor solvents at30oC consists of two triplets of equal

intensity. This corresponds to the cis-structure. However, the spectrum at 0 oC shows a single

fluorine peak. This may correspond to the trans structure. But actually the change in the

spectrum at higher temperature is not due to cis - trans conversion but it is due to the

dissociation reaction of type: TiF4.2B TiF4.B + B

At high temperature, this dissociation is fast enough as to make all the four fluorine atoms

equivalent.

8. Nuclear quadrupole relaxation changes the splitting pattern. When the relaxation is rapid

the spin state of the nucleus is rapidly changed and as a result splittings do not occur. Slow

relaxation rates cause normal splittings. Intermediate exchange rates often result in a

broadening of the peaks. Relaxation effects are often encountered for nuclei, which have

quadrupole moment because these nuclei are very efficiently relaxed by the fluctuating

electric field gradients, which arise from the thermal motion of the polar solvent and solute

molecules. For N14H3 (I=1), three broad HNMR signals are obtained and forN15 H3 (I=1/2; no

quadrupole relaxation), a sharp doublet is obtained.

9. Spin-spin coupling depends also on the number of intervening bonds. In saturated

molecules of light elements JH_H falls off rapidly as the number of bonds between the two

interacting nuclei increases and usually is negligible for coupling of nuclei separated by morethan three bonds.

10. Long-range couplings are observed in unsaturated compounds. Here the spin-spin

coupling is transmitted through -bonds, which are delocalized over the entire molecule.

11. Long-range coupling occurs through space instead of or bonds in cases where

coupling involves an atom other than hydrogen. For the compound SF5.CF2CF3, JF-F (of the

CF2 group and trans- F) is -5 cps while JF_F (of CF2 group and the cis-F) is -16 cps. In the

latter case there is through-space coupling.


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12. F19 nmr of CH2=CF2 shows four lines of equal intensity. The two F and two H atoms are

non - equivalent. The two fluorines have the same chemical shift. This peak is split by the

TWO HATOMS one by one into four lines of equal intensity (it is not 1:2:1 as If the 2F or 2H

were equivalent).

C C

Ha

Hb

Fa

Fb

cis H-F coupling

trans H-F coupling

13. C1F3: Two fluorines are of one type and one F is of other type. The F

19nmr shows a (1F,

triplet) and a (2F, doublet).

Cl

F

F F(14) P4O13

6-: Two peaks viz., (2P, triplet) and (2P, triplet).

(15) P4S3: Two peaks viz., (3P, doublet) and (1P, quartet)

16. If the molecules being studied are undergoing very rapid exchange reactions, the nmr

spectrum drastically affected. A mixture of CH3COOH and H2O does not show two separateO-H resonances from water and acid but instead shows only one. The -OH protons are

exchanging positions very rapidly between the acid and the water.

17. In N.N-dimethylacetamide, the methyl groups a,b,c are non-equivalent at room

temperature as the double bond character of C-N bond restricts the rotation about C-N axis.

As a result, the methyl groups b and a are cis and trans to O-atom and give rise to two peaks.

At high temperatures, rotation becomes free and as a result there is no distinction between cis

and trans methyl groups. Hence, band c give single peak.


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18. NMR spectrum of methylammonium chloride in water at pH = I shows

(i) a quartet methyl peak (split by three NH3 protons); (ii) a sharp water peak; (iii)

three broad peaks from NH3 protons (IN=1). Broadening is due to quadrupole nature of

Nitrogen. No fine structure due to CH3 protons is observed because of quadrupole

broadening by nitrogen.

When pH is increased, all bands begin to broaden. At pH = 5, the spectrum shows:(i) one sharp peak due toCH3; and (ii) a broad peak due to all other protons.

19. B3H8- : B11nmr spectrum is a nonet which results from a splitting of three equivalent

borons by eight equivalent protons. Because of the fast intramolecular hydrogen exchange all

B and all H become equivalent.

20. P3N3Cl4F2: The19Fnmr spectrum shows it is a AB2X2 system. The two F atoms are X2,

the P atom to which the fluorines are attached is A and the other two P atoms are B 2; 1.1-

difluoride-3.3.5,5-tetrachloride.

P

N

N

P

P

N

Cl

Cl

Cl

Cl

F F 21. P4N4Cl6(NHC6H5)2: 31Pnmr shows two triplets of equal intensity. The structure is given

below.

P

N

N

P N

P

P

N

Cl

Cl Cl

Cl

Cl

Cl

NHC6H5

H5C6HN


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22. NF3: The Fl9nmr shows a sharp singlet at -205 oC. As the temperature is raised the peak

broadens and at 20 C, a sharp triplet (IN14 = 1) is obtained. This change is opposite to that

normally obtained for exchange processes. At lower temperatures, the slow molecular

motions are most effective for quadrupole relaxation of N14. At higher temperature, the

relaxation is not as effective and the life-time of a given state for N

14

nucleus is sufficient tocause spin-spin splitting.

23. The N14nmr of azoxybenzene exhibits only a singlet. The field gradient at the N-O

nitrogen is so large as to make this resonance unobservable. Only the other nitrogen shows

the signal.

N+N

O-

24. 31Pnmr spectrum of PF2H(I5NH2)2 shows 90 lines as explained below.

P

F

F

H

NH2

NH2

One P31signal gets split into a doublet by one HEach line in the doublet is split into a triple by two F's (2 X 3 = 6 lines)

Each of these six lines is split into triplet by two 15Ns (IN15 = ) (6 X 3 = 18 lines)

Each of these 18 lines is split into a quintet by the four NH2protons (18 X 5 = 90 lines)

25. The proton noise decoupled P31 spectrum of a mixture of trans - [PtCl4(PEt3) and trans -

[PtBr4(PEt3) shows six lines (each withl95Pt satellites), showing the existence of six

complexes viz, [PtBr4Cl(PEt)3], [PtBr3Cl(PEt)3], [PtBr2Cl2(PEt)3] two isomers,

[PtBrCl3(PEt)3] and [PtCl4(PEt)3].

Effect of Quadrupole Moments in NMRNuclei with quadrupole moments undergo spin-lattice relaxation rapidly. Therefore

the signal of a nucleus, which is attached to a quadrupole nucleus, is extensively broadened.

The nmr signals of a quadrupole nucleus are broadened so extensively that no spectrum is

obtained.

The quadrupole relaxation process is due to the interaction of electric field gradient

with the quadrupole moment. The field gradient is caused by the asymmetry of the electronic

cloud.

(1) In halide ion, the charge distribution is spherical and gives rise to only small field gradient

at the nucleus (longer).Hence, halide ions give sharp peaks.


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(2) Solutions of I- (I127= 5/2) show a NMR signal. When iodine is added the triiodide, I3-, is

formed destroying cubic symmetry of the iodide ion so that quadrupole broadening becomes

effective and the signal disappears. Broadening depends on the amount of iodine added and

hence the rate constant for the reaction I- + I2 I3-can be calculated.

Contact Shifts (NMR of Paramagnetic Species)

The unpaired electron present in the paramagnetic compounds broadens the spectrum

by both dipolar and electron spin-nuclear spin coupling mechanisms. The magnetic moment

of an electron is ~10000 times larger than the nuclear magnetic moment. Therefore,

paramagnetic ions produce large magnetic fields which make dipolar-spin lattice relaxation

very rapid.

If the electron relaxation is very slow, then splittings due to s = 1/2 will be observed.

If the relaxation is fast, the magnetic nucleus senses only the time-averaged magnetic field of

the two spin orientations and a single peak is observed. Intermediate relaxation rates will

broaden the spectrum. The presence of unpaired electrons makes the electron relaxation very

rapid; this causes a very large chemical shift (~3000-5000 cps) of the resonance in the NMR

spectrum. This is referred to as contact nmr shi ft.

The relationship between contact shift and magnetic field for proton magnetic

resonance is given as:

Where, e gyromagnetic ratio for the electron, N is that for the magnetic nucleus, S is the

electron spin multiplicity, H is (Hcomplex - Hligand) and v = (vcomplex - vligand), is Bohr

megneton and g is the ratio of magnetic moment to the total angular momentum of the

electron.

The condition requisite to observing a contact shift in nmr is

e is electronic relaxation time, s is chemical exchange time and AN is the contact interaction

constant (electron spin - nuclear spin coupling constant).

Example: In nickel(ll)aminotroponeiminate, one electron is transferred to Ni(II) from the

ligand. As a result, the unpaired electrons present at and positions are aligned with

magnetic field while that at is anti-aligned with the field. and carbons have positive

spin density and carbon negative spin density, i.e., and protons are shielded and

proton is deshielded.


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Fluxional Behaviour

These molecules possess more than a single conformation representing an energyminimum. Several such minima may be present and accessible with ordinary thermal

energies. NMR techniques are very useful in studying fluxional molecules.

The fluxional behaviour is studied by taking NMR spectrum at lower and higher

temperatures. At lower temperature, the rate of fluxional behaviour will be slow and we can

obtain individual lines for each site. At higher temperature, due to interconversion, a time

averaged spectrum will result. For example:

1)A complex formed between tetramethylallene and iron carbonyl

Below -60 C the H nmr shows three peaks in the ratio 1:1:2 representing the three cis

proton, three trans-protons and six protons in a plane perpendicular to CFe bond. As thetemperature is raised, the spectrum collapses to a single resonance for the average

environment of the 12 protons as the iron migrates around the allene -system.

C

CH3CH3

CH3 CH3

Fe(CO) 4

C

CH3CH3

CH3 CH3

Fe(CO) 4

2) 3 -allyl complex

The HNMR spectrum at low temperature shows two doublets, representing the cis-

and trans- protons and one multiplet, corresponding to the non-terminal hydrogen. Upon

warming, the spectrum changes with collapse of the two doublets into one, establishing that

there are four terminal hydrogen atoms and one non-terminal hydrogen atom. This change is

due to the rapid inter-conversion, which makes Hs and Ha indistinguishable.

CC C

Ha

Hb

Ha

H

b

H

M

C

C CHa

Hb

Ha

H

b

HM

CC C

Ha

Hb

Ha

Hb

H

M


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3)Mercury cydopentadienyl compound, cp2Hg

This complex shows two conformers (a) and (b). In (a) the divalent Hg is bonded to

two 3-C5H8 rings. In (b) it is an ionic compound, a

5-complex. Structure (a) shows 3

protons resonances with intensity ratio 1:2:2, whereas all the protons are equivalent in (b).

The Hnmr spectrum at -70

o

C shows a single peak.

Hg

H

H Hg2+

(a)(b)

4)Ferrocenophane

A sharp singlet for the methylenic protons and two narrow multiplets for the and protons.

5) P31nmr spectrum of PF4NMe2 shows triplets of triples (two equatorial and two axial F's

interact separately with P31 signal) at low temperature and on warming, it gives a regular

quintet (all the four F's become equivalent).

P

F

F

N

F

F

CH3

CH3

Applications of NMR Spectroscopy in Inorganic Chemistry

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