NMR Spectroscopy CHEM 430 Fall 2014. FACTORS THAT INFLUENCE PROTON SHIFTS Local Fields Shielding by...

NMR Spectroscopy

CHEM 430Fall

2014

FACTORS THAT INFLUENCE PROTON SHIFTS

Local Fields• Shielding by e- that surround the resonating nuclei arise from local fields

• They are a simple function of e- density affected by induction, resonance and hybridization effects

• The magnetic field at the nucleus is altered from B0 to a quantity B0(1-s) where s is called the shielding

NMR - The Chemical Shift 3-1

CHEM 430 – NMR Spectroscopy

2


Local FieldsElectronegativity effects

• A nearby e- withdrawing atom or group with decrease e- density moving the observed resonance downfield to higher n

• Conversely, a nearby e- donating atom or group will increase e- density moving the observed resonance upfield to lower n



33

B0

B0(1 – s) sOH < sCH3

DE = hn

CH3

OH


Local Fields



44

B0

B0(1 – s) sO < sC

DE = hn



The trend follows Pauling electronegativity (EN) scale:



55

CH3F CH3O- CH3Cl CH3Br CH3I CH4 (CH3)4Si CH3Li

EN 4.0 3.5 3.1 2.8 2.5 2.1 1.8 1.0

d of H 4.26 3.40 3.05 2.68 2.16 0.23 0.0 -0.4



• The effect is cumulative:

• The effect drops sharply with distance:



66

CH3Cl CH2Cl2 CHCl3

d of H 3.05 5.30 7.27

-CH2Br -CH2CH2Br -CH2CH2CH2Br

d of H 3.30 1.69 1.25



• The effect is a useful tool in quickly deducing simple aliphatic chains:



77

O

HO


Local FieldsResonance effects

• Donation or withdrawal of e- through resonance will have shielding or deshielding effects, respectively:



88

H2C CH2reference

5.28

resonance withdrawaldeshielding effect

resonance donationshielding effect


Local FieldsHybridization effects

• Hybridization of the carbon atom influences e- density

• As proportion of s-character increases (sp3 sp2 sp) bonding electrons move toward C and away from hydrogen - deshielding

• This effect however is secondary to the influence of the p-cloud of electrons from the unhybridized p-orbitals as we will see



99


Nonlocal Fields• Purely magnetic effects from a neighboring group can influence nuclear

shielding

• As we saw earlier the combined effects of local and nonlocal fields:

• Nonlocal fields have a major influence on chemical shift only if the group has a non-spherical shape



1010


Nonlocal Fields• The electrons in a spherical substituent also precess in the applied field,

creating a non-local field

• As the molecule tumbles the lines of magnetic force will remain lined up with the applied field, but the position of the attached nuclei will change

• The effect cancels itself out leaving only the effect on the local field by the substituent



1111


Nonlocal Fields• The electrons in a non-spherical substituent also precess in the applied

field, creating a non-local field

• In benzene, the 6-p-orbitals overlap to allow full circulation of electrons; as these electrons circulate in the applied magnetic field they oppose the applied magnetic field at the center:



1212

B0


Nonlocal Fields• At the ring periphery, the effect is opposite and the protons are

deshielded :



1313


Nonlocal Fields• The benzene system is not spherical, but an oblate ellipsoid

• As the molecule tumbles in in solution there is either the effect of shielding inside the ring or deshielding outside the ring if the ring is perpendicular to the applied field

OR• No effect if the ring is parallel to the applied field



1414


Nonlocal Fields• Groups that have appreciably different currents induced by B0 resulting

from different orientations in space are said to have diamagnetic anisotropy

• Just as the local effect can result in shielding (electron donation) or deshielding (electron withdrawal) the nonlocal effect can result in either permutation

• Regions of shielding are indicated by (+) and deshielding (-)



1515


Nonlocal Fields• The effect was modeled quantitatively by McConnell

• The following equation relates shielding to the influence of a magnetic dipole on the point in space where a proton(nuclei) resides:

• At q = 54o 44’ the expression (3cos2q – 1) goes to zero

• On either side of this ‘magic angle’ sA changes sign



1616

sA – shielding for a proton at (r, q)cL and cT are the diamagnetic susceptibilities of the group longitudinal and transverse to B0


Nonlocal Fields• The protons of benzene reside on the periphery of the ring within the

deshielding cone

• Molecules have been constructed for the purpose of confirming the shielding effect predicted by the model:



1717

H2C

CH2 -1.0 d

2.0 d

HH9.3

-3.0

H H-0.5


Nonlocal Fields• 4n + 2 p-electrons (aromatic) result in the diamagnetic circulation of e-s

• Pople demonstrated that 4n p-systems have the opposite or paramagnetic circulation:



1818

4n = 16 electrons

HH5.2

10.3


Nonlocal Fields• For a prolate ellipsoid it is sometimes not as clear as to which

arrangement has the stronger induced current

• The acetylene system provides a simple example



1919


Nonlocal Fields• We say the shift for the terminal acetylene proton experiences magnetic

anisotropy. Usual shifts for this H are d 1.8-3.0. For reference sp3 ethane is at 0.86 and sp2 ethene at 5.28



2020


Nonlocal Fields• Alkanes do not possess the same degree of electron circulation as alkynes

but do exert nonlocal fields on adjacent nuclei

• The C—C s-bond shields a proton on its side more than its end



2121


Nonlocal Fields• The result of which is a deshielding of equatorial protons in

conformationally locked systems:

• Even simple alkane systems show anisotropy:



2222


Nonlocal Fields• The result of which is a deshielding of equatorial protons in

conformationally locked systems:

• Even simple alkane systems show anisotropy:



2323


Nonlocal Fields• The highly shielded position of cyclopropane resonances may be

attributed either to an aromatic-like ring current or to the anisotropy of the bond that is opposite to a group in the three- membered ring:

• The effect is much larger than the indicated 1.2 ppm , because the cyclopropane carbon orbital to hydrogen ( compared with the orbital in cyclohexane) deshields the proton.

• A cyclopropane ring also can shield more distant hydrogens:



2424

Heq 1.2 less than Hax


Nonlocal Fields• Most common single bonds (C-N, C-O) have shielding properties that

parallel those of the bond, although the geometry is more complex than that for the C-C bond.

• Lone electron pairs can have a special effect:

• In N-methylpiperidine the axial lone pair shields the vicinal Hax by an n s* interaction without any effect on Heq. As a result, Hax increases to about 1.0 ppm or more in similar systems.



2525


Nonlocal Fields• The anisotropy of double bonds is more difficult to assess, because they

have three nonequivalent axes (the McConnell equation, with only two axes, does not apply).

• Protons situated over double bonds are, in general, more shielded than those in the plane both for alkenes and for carbonyl groups

• The position of the methylene protons in norbornene may be explained in this fashion since the syn and endo protons, respectively, are shielded with respect to the anti and exo protons.



2626


Nonlocal Fields• The highly deshielded position of aldehydes ( ca. 9.8) is attributed to a

combination of a strong polar effect and the diamagnetic anisotropy of the carbonyl group.



2727


Nonlocal Fields• The nonspherical array of lone pairs of e- may exhibit diamagnetic

anisotropy, although, alternatively, the effect may be considered a perturbation of local currents.

• A proton that is H-bonded to a lone pair is invariably deshielded.

• For example: the -OH proton in ethanol • CCl4 resonates at 0.7 (dilute no H-bonding)

• CD3CD2OH resonates at 5.3 (H-bonding)

• Carboxylic protons resonate at extremely high frequency (11– 14), because every proton is H-bonded within a dimer or higher aggregate.



2828


Nonlocal Fields• Lone- pair anisotropy also has been invoked to explain trends in ethyl

groups :For XCH2CH3:

• The resonance position of –CH2- attached to X is explained by the polar effect

• The trend for the more distant –CH3 group is opposite; as X increases size, the lone pair moves closer to the –CH3 group and deshields it.



2929

X d -CH2- d –CH3

F 4.36 1.24

Cl 3.47 1.33

Br 3.37 1.65

I 3.16 1.86


Summary • Functional group effects on proton chemical shifts are explained largely by

two general effects.:1. Local Effects: Electron withdrawal or donation by induction

(including hybridization) or by resonance alters the electron density and hence the local field around the resonating proton. • Higher electron density shields the proton (lower n, upfield) • Low electron density deshields the proton (higher ,n downfield)

2. Nonlocal Effects: Diamagnetic anisotropy of nonspherical substituents is largely responsible for the proton resonance positions of aromatics, acetylenes, aldehydes, cyclopropanes, cyclohexanes, alkenes, and hydrogen- bonded species.



3030

PROTON CHEMICAL SHIFTS AND STRUCTURE

3-2a Saturated AliphaticsAlkanes.



3131


3-2a Saturated AliphaticsFunctionalized Alkanes.

Oxygen:

Nitrogen:

Sulfur and the Halides:



3232

OHOH

3.38 3.56

OH3.85

O

3.24

O

O

3.67

NNH2

2.42 2.22N

3.33N

O

2.88

H3C CH3 reference, 0.86

S2.12 I Br Cl F

2.15 2.69 3.06 4.27


3-2a Saturated AliphaticsFunctionalized Alkanes.



3333


3-2a Saturated AliphaticsFunctionalized Alkanes. • After years of collective observation of 1H (and 13C) NMR it is possible to

predict chemical shift to a fair precision using Shoolery Tables

• These tables use a base value for 1H (and 13C) chemical shift to which are added adjustment increments for each group on the carbon atom

d = 0.23 + DX + DY + DZ

• The tables work well for methyl and methylene but diverge greatly with methine due to the increased interaction between effects



3434

X C Z

Y

methine

X C Y

H

methylene

X C H

H

methyl

H HH


3-2a Saturated AliphaticsFunctionalized Alkanes. Example Shoolery Table - Methylene



3535

X or Y Increment X or Y Increment

-H 0.34 -OC(=O)OR 3.01

-CH3 0.68 -OC(=O)Ph 3.27

-C—C 1.32 -C(=O)R 1.50

-CC- 1.44 -C(=O)Ph 1.90

-Ph 1.83 -C(=O)OR 1.46

-CF2- 1.12 -C(=O)NR2 or H2 1.47

-CF3 1.14 -CN 1.59

-F 3.30 -NR2 or H2 1.57

-Cl 2.53 -NHPh 2.04

-Br 2.33 -NHC(=O)R 2.27

-I 2.19 -N3 1.97

-OH 2.56 -NO2 3.36

-OR 2.36 -SR or H 1.64

-OPh 2.94 -OSO2R 3.13


3-2a Saturated AliphaticsFunctionalized Alkanes. Example Shoolery Table - Methine



3636

X ,Y or Z Increment X, Y or Z Increment

-F 1.59 -OC(=O)OR 0.47

-Cl 1.56 -C(=O)R 0.47

-Br 1.53 -C(=O)Ph 1.22

-NO2 1.84 -CN 0.66

-NR2 or H2 0.64 -C(=O)NH2 0.60

-NH3+ 1.34 -SR or H 0.61

-NHC(=O)R 1.80 -OSO2R 0.94

-OH 1.14 -CC- 0.79

-OR 1.14 -C=C 0.46

-C(=O)OR 2.07 -Ph 0.99

-OPh 1.79


3-2a Saturated AliphaticsFunctionalized Alkanes. • After years of collective observation of 1H (and 13C) NMR it is possible to

predict chemical shift to a fair precision using Shoolery Tables

• These tables use a base value for 1H (and 13C) chemical shift to which are added adjustment increments for each group on the carbon atom

d = 0.23 + DX + DY + DZ

• The tables work well for methyl and methylene but diverge greatly with methine due to the increased interaction between effects



3737

X C Z

Y

methine

X C Y

H

methylene

X C H

H

methyl

H HH


3-2b Unsaturated AliphaticsAlkynes.

•Anisotropy of C≡C results in a low frequency for terminal-H (1.8 – 3.0)

Alkenes. •Anisotropy of C=C results in a higher frequency for alkenylic (vinyl)-H

•Range is very large (4.5 – 7.7) and highly subject to other groups on C=C

•Reference value used for ethene is 5.28



3838


3-2b Unsaturated AliphaticsAlkenes. • Conjugation usually increases the observed frequency

• Angle strain increases s-character and therefore moves the resonance to higher frequency

• The presence of a C=O group w/d electrons by both resonance and induction:



3939


3-2b Unsaturated AliphaticsAlkenes.



4040


3-2b Unsaturated AliphaticsAlkenes. • These effects were quantified by Tobey and of Pascual, Meier, and Simon,

• Using an empirical approach they tabulated a series of geminal, cis and trans substituents relative to the proton being observed:

d = 5.28 + Zgem + Zcis + Ztrans

• Although the parameters incorporate inductive and resonance effects, steric effects can cause deviations from observed positions.



4141


3-2b Unsaturated AliphaticsAlkenes.



4242


3-2c Aromatics•Diamagnetic anisotropy from aromatic ring current shifts the protons to high frequency – benzene is used as a reference (7.27)

•Rings with only aliphatic substituents tend to bunch the resonances about this frequency (toluene, ~ 7.2)

•Conjugating substituents tend to spread the aromatic resonances based on contributing resonance structures and inductive effects:



4343


3-2c Aromatics• The a-protons in aromatic heterocycles are shifted due to the polar effect

of the heteroatom:

• As with the alkane and alkene systems a systematic observation has been made of aromatic H-resonances. They can be predicted from the following:

d = 7.27 + SSi



4444


3-2c Aromatics



4545


3-2c AromaticsAromatics:



4646


3-2c AromaticsHeteroaromatics:



4747


3-2d Protons on Oxygen and Nitrogen•Chemical shifts of protons attached to highly electronegative atoms such as O or N are influenced strongly by acidity, basicity, and hydrogen-bonding

Protons on Oxygen• For –OH, minute amounts of acidic or basic impurities can bring about

rapid exchange

• They are averaged with other exchangeable protons, either in the same molecule or in other molecules, including the solvent.

• Only a single resonance is observed for all the exchangeable pro-tons at a weighted-average position and no coupling is observed.

• The resonance varies from sharp to slightly broadened, depending on the exchange rate.



4848


3-2d Protons on Oxygen and Nitrogen•-OH may be determined by exchange experiments described earlier (shaking the NMR sample with D2O)

• At infinite dilution in (no H-bonding), the OH resonance of alcohols may be found at about 0.5.

•Under more normal conditions of 5% to 20% solutions, hydrogen bonding results in resonances in the 2– 4 range.

•More acidic phenols (ArOH) have resonances downfield at 4– 8. Interaction with an ortho group shifts this to 10 or higher.



4949


3-2d Protons on Oxygen and Nitrogen•Most carboxylic acids exist as H-bonded dimers or oligomers, even in dilute solution.

•Hydrogen bonding coupled with the strong deshielding of the carboxlate group places the acid protons resonate far downfield (11– 14)

•Likewise, other highly H-bonded acidic protons also may be found in this range, such as sulfonic, phosphonic and enolic protons



5050


3-2d Protons on Oxygen and NitrogenProtons on nitrogen •Similar properties to -OH

•Lower electronegativity of N results upfield shifts than those OH protons: • 0.5– 3.5 for aliphatic amines, - NH2

• 3– 5 for aromatic amines ( anilines), Ar-NH2

• 4– 8 for amides, pyrroles, and indoles• 6– 8.5 for ammonium salts, -NH3

+ (amino acid protons can exchange rapidly with solvent or other exchangeable protons to achieve an averaged position)

•The most common nuclide 14N is quadrupolar and possesses unity spin – (could split the resonance of attached protons into a 1: 1: 1 triplet) - rapid relaxation of quadrupolar 14N averages the spin states usually appearing as a broadened resonance

•Broadening may render NH resonances almost invisible.



5151


3-2d Protons on Oxygen and Nitrogen



5252


3-4 Factors That Influence Carbon Shifts• Carbon is the defining element in organic compounds, but its major

nuclide 12C has a spin of zero.

• Advent of pulsed FT-methods allowed practical examination of the low- abundance nuclide 13C ( 1.11%)

• The low probability of having two adjacent nuclei (0.01%) in a single molecule removes complications from carbon–carbon couplings.

• When C-H couplings are removed by decoupling techniques, the spectrum is essentially free of all spin–spin coupling, and one singlet arises for each distinct type of carbon.

• Integration is rarely done as 13C has a much larger range of relaxation times than 1H and because the decoupling field perturbs intensities



5353


3-4 Factors That Influence Carbon Shifts• Analysis therefore is simpler for 13C than for 1H spectra – but…

• Diamagnetic shielding (sd) which is responsible for 1H chemical shifts, is caused by circulation of the electron cloud about the nucleus – 1Hs are surrounded solely by s electrons ( lacking angular momentum) and consequently exhibit only diamagnetic shielding

• In carbon 2p electrons have angular momentum that can hinder free circulation - hindrance to free electron circulation creates an additional mechanism called paramagnetic shielding (sp)

• Because sp serves to reduce sd the two mechanisms have opposite signs

• 13C nuclei (and almost all other nuclides as well) additionally are surrounded by p electrons and exhibit both forms of shielding.



5454


3-4 Factors That Influence Carbon Shifts•The paramagnetic component can be quite large!

•Chemical-shift range for 1H is only a few ppm, while paramagnetic shifts for other nuclei can extend over a range of hundreds or even thousands of ppm.

•Qualitatively, angular momentum can arise from excited electronic states and from bonding. The effects are larger when electron density about the nucleus increases.

•These three considerations were gathered by Ramsay, Karplus, and Pople into the simple empirical relationship…



5555


3-4 Factors That Influence Carbon Shifts

• DE = average energy of excitation of certain electronic transitions, such as the n p* transition for many 13C and 15N nuclei.

• The radial term includes the average distance r from the nucleus of the 2p electrons - this term serves as a measure of electron density.

• SQij represents p-bonding to carbon. The negative sign in the equation indicates that paramagnetic shielding is in the opposite direction from sd.

• Structural changes can affect all three components of the equation.



5656



• The quantity DE represents the weighted- average energy difference between the ground and certain excited states.

• Because of symmetry considerations, the p p* transition is often excluded

• Low-lying excited states (with small DE) make the largest contribution, since DE appears in the denominator.



5757



•Saturated molecules, such as alkanes, typically have no low-lying excited states ( and hence possess a large DE), so that sp is small and alkane carbon resonances are found upfield

*Note that paramagnetic shielding causes shifts to high frequency, whereas diamagnetic shielding causes shifts to low frequency

•On the other hand carbonyl carbons have a low-lying excited state involving the movement of electrons from the oxygen lone pair to the antibonding p orbital that generates a paramagnetic current.

•This n p* transition causes the large shift to high frequency that characterizes carbonyl groups— up to 220 ppm



5858



• The radial term is responsible for effects related to electron density that parallel polar effects on proton chemical shifts.

• sP is larger when the p-electrons are closer to the nucleus. Thus, substituents that donate or withdraw electrons influence the paramagnetic shift. • e- donation increases repulsion between electrons, which can be

relieved by an increase in r, sP decreases, causing an upfield shift• e-withdrawal permits electrons to move closer to the nucleus,

increasing sP ,causing an upfield shift



5959



• SQij is related to charge densities and bond orders and can be considered a measure of multiple bonding. The greater the degree of multiple bond-ing, the greater the downfield shift

• This term provides a rationale for the series:CH3CH3 ( 6) < H2C=CH2 (123) < H2C=C=CH2 ( 214)

• Arene shifts are similar to those of alkenes (benzene, 129) - The effects of diamagnetic anisotropy on carbon chemical shifts are similar in magnitude to the effects on protons, but are small in relation to the range of carbon shifts.



6060



•Alkynes do not follow this pattern, but are at an intermediate position (72 for acetylene), because their linear structure has zero angular momentum about the axis.

•There are exceptions, the most prominent being the effect of heavy atoms. The series ( CH3Br (10), CH2Br2 (22), CHBr3 (12), and CBr4 (-29) defies any explanation based on electronegativity, unlike the analogous series given before for chlorine.

•This so- called heavy- atom effect has been attributed to a new source of angular momentum from spin–orbit coupling.



6161


3-5 Carbon Chemical Shifts and Structure• You should know the most basic of shift tables:



6262


3-5 Carbon Chemical Shifts and Structure



6363


3-5 Carbon Chemical Shifts and Structure



6464


3-5a Saturated Aliphatics• Replacement of H on carbon (a) or an adjoining carbon (b) will shift the

resonance by 9 ppm

• The replacement at the g-position however shifts -2.5 ppm



6565


3-5a Saturated Aliphatics• For this reason 13C shifts lend themselves readily to empirical analysis:

• Each Ai or substituent parameter is added to -2.5 ppm (shift for CH4) for a maximum set of five carbons:



6666


3-5a Saturated Aliphatics• In general –CH3 resonances appear at 5-20, -CH2- at 15-35 and –CH- at 25-

45



6767


3-5a Saturated Aliphatics• Some complications: Corrections must be applied if branching is present

• The 13C methyl is corrected for the presence of an adjacent 3o carbon by adding -1.1 and for an adjacent 4o carbon by adding -3.4.

• The 13C methylene has corrections of and -2.5 and -7.2, respectively, for adjacent 3o and 4o carbons.

• The 13C methine has respective corrections of -3.7, -9.5 and -1.5 for adjacent 2o, 3o, and 4o carbons.

• 4o carbons have corrections of -1.5 and -8.4 for adjacent 1o and 2o carbons. Corrections for adjacent tertiary and quaternary carbons undoubtedly are significant, but are not known accurately.



6868


3-5a Cyclic Alkanes. • For small cycloalkanes: -3.8 , cyclopropane has the most upfield resonance

of hydrocarbons. Cyclobutane and cyclopentane are higher.

• Larger cycloalkanes generally resonate within 2 ppm of cyclohexane, at 27.7.

• The fixed stereochemistry of cyclohexane requires a set of empirical parameters that depend on the axial or equatorial nature of the substituents, as well as on the distance from the resonating carbon.



6969


3-5a Cyclic Alkanes. • The following table lists parameters for methyl substitution are added to the

value for cyclohexane ( 27.7).

• The substituent parameter for g -axial methyl is large and negative , reflecting the pure gauche stereochemistry between the perturbing and resonating carbons.

• A g - equatorial group represents g - anti effect and has little perturbation



7070


3-5a Cyclic Alkanes. • Corrections again are needed for branching.

• Geminal methyls: a = -3.4, b = -1.2

• Example: the calculated resonance for C2 of 1,1,3- trimethylcyclohexane :

27.7 + 5.2 + (8.9 * 2) - 1.2 = 49.5 ( observed 49.9)

• Other examples:



7171


3-5a Functionalized Alkanes. • The replacement of a hydrogen atom on carbon with a heteroatom or an

unsaturated group usually results in downfield shifts due to polar effects on the radial term.

• The effect parallels the same structural change on 1H chemical shifts but arises from a different mechanism.



7272


3-5a Functionalized Alkanes. Alkyl Halides. •Strongly EWGs have large positive effects. In the halogen series:

CH3F 75.4 CH3Cl 25.1 CH3Br 10.2 CH3I -20.6

•Multiple substitution results in larger effects— 77.7 for CHCl3

•Recall that the effect of heavy atoms such as iodine or bromine is influenced by a spin– orbit mechanism and hence may not follow the simple order of electronegativity.

•The general range for the halogen effect in hydrocarbons extends from the values given above for the simple systems to about a 25- ppm downfield shift for CH2X and CHX systems, since the a and b effects of the unspecified hydrocarbon pieces contribute to the downfield shift



7373


3-5a Functionalized Alkanes. Alcohols and Ethers: •The range for HO- substituted carbons is 49– 75. (MeOH at 49.2)

•The range for RO-substituted carbons is 59– 80. (CH3OCH3 at 59.5)

•The ether range is translated a few ppm downfield from alcohols, because each ether must have one additional b-effect with respect to the analogous alcohol.



7474


3-5a Functionalized Alkanes. Amines:• The lower EN of nitrogen moves the amine range somewhat upfield

(CH3NH2 at 28.3), the range for amines extending some 30 ppm higher

• The amine range is larger than the alcohol range because nitrogen can carry up to three substituents, with more a and b effects possible

Other EN groups:• CH3SCH3 at 19.5, CH3CN at 1.8, and CH3NO2 at 62.6, with the respective

ranges for thioalkoxy, cyano, and nitro substitution extending some 25 ppm downfield

• The anomalous low- frequency position for -CN substitution is related to the cylindrical shape of the group and its reduced angular momentum.



7575


3-5a Functionalized Alkanes. Attached C=C or C=O•An attached double bond has only a small effect on a methyl group.

•For example, the position for the methyls of trans-2-butene is 17.3, and that for the methyl of toluene is 21.3.

•The range for carbons on double bonds is about 15– 40.

•Methyls on carbonyl groups are at a slightly downfield: 30.8 for acetone and 31.2 for acetaldehyde, with a range of about 30– 45.



7676


3-5a Functionalized Alkanes. • Introducing heteroatoms or unsaturation into alkane chains requires

completely new sets of empirical parameters that depend on the substituent, on its distance from the resonating carbon ( , , a b g), and on whether the substituent is terminal or internal:

• These parameters on the next slide represent the effect on a resonating carbon of replacing a hydrogen atom with X:



7777


3-5a Functionalized Alkanes.



7878


3-5a Functionalized Alkanes. • With the exception of cyano-, acetyleno-, and the heavy atom iodine, the

a-effects are determined largely by the ENof the substituent.

• The b-effects are all positive and generally of similar magnitude ( 6 to 11 ppm) and that the g-effects are all negative and generally of similar magnitude (-2 to -5 ppm).

• Although the details are not entirely understood, it is clear that simple polar considerations do not dominate the b- and g-effects.



7979


3-5a Functionalized Alkanes. Examples:

• To use the substituent parameters given in the table, one adds the appropriate values to the chemical shift of the 13C in the unsubstituted hydrocarbon analogue (already having the C-effects):

• In the first example 16 is the base value for the 1-carbon in propane and 27 is the base value for the carbons in cyclopentane



8080


3-5b Unsaturated Compounds. • The effects of diamagnetic anisotropy on a carbon and a proton have

similar magnitudes, but the much larger paramagnetic shielding renders the phenomenon relatively unimportant for carbon.

• Thus, benzene (128.4) and the alkenic carbon of cyclohexene (127.3) have almost identical carbon resonance positions, in contrast to the situation with their protons.

• The full range of alkene and aromatic resonances is about 100– 170.



8181


3-5b Unsaturated Compounds. Alkenes•Alkenic carbons that bear no substituents resonate at 104– 115 for hydrocarbons (isobutylene [ (CH3)2C=CH2] at 107.7)

•Alkenic carbons that have one substituent resonate in the range 120– 140 (trans-2-butene at 123.3)

•Finally, disubstituted alkenic carbons resonate the most downfield at 140– 165 (isobutylene [ (CH3)2C=CH2] at 146.4)

•Polar substituents on double bonds, especially those in conjugation, can alter the resonance position appreciably. Unsaturated ketones, such as 3- 33 and 3- 34, have lower- frequency a resonances and higher- frequency b resonances. The effect is d (“ CRR ¿ ) d d d (“ CHR) ( CH3) 2C“ CH2 d d (“ CH2).



8282


3-5b Unsaturated Compounds. Alkenes• Polar substituents on double bonds, especially those in conjugation, can

alter the resonance position appreciably; e- donation or w/d alters the radial term through resonance.

• Unsaturated ketones have more upfield a resonances and more downfield b resonances.

• The effect is reduced in acyclic molecules

• Electron donation in enol ethers reverses the trend:CH2=CHOCH3 (a = 153.2, b = 84.2)



8383


3-5b Unsaturated Compounds. Alkenes• Alkene chemical shifts may be estimated from substituent parameters

added to the shift for ethene ( 123.3).

• For , a b and g carbons on the same end of the double bond as the resonating carbon, respective increments of 10.6, 7.2, and -1.5 are added.

• For , a b and g carbons on the opposite end of the double bond from the resonating carbon, respec-tive increments of -7.9, -1.8 and -1.5 are added.

• An increment of -1.1 is added if any two substituents are cis to each other.



8484


3-5b Unsaturated Compounds. Alkenes



8585


3-5b Unsaturated Compounds. Alkynes and Nitriles•Terminal alkyne carbon generally resonates in the narrow range 67– 70.

•An alkyne carbon that carries a carbon substituent resonates at a slightly higher frequency ( 74– 85), because of a and b effects from the R group.

•Effects of conjugating, polar substituents expand the range to 20– 90.

•Nitriles resonate in the range 117– 130 (CH3C≡N at 116.9). The n p * transition pushes the range downfield.



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3-5b Unsaturated Compounds. Aromatics•Alkyl substitution has its major effect on the ipso carbon.

•Because this carbon has no attached proton, its relaxation time is much longer than those of the other carbons, and its intensity is usually lower.

•Conjugating substituents like –NO2 have strong perturbations on the aromatic resonance positions, as the result of a combination of traditional , a b and g effects and changes in e- density through delocalization



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3-5b Unsaturated Compounds. Aromatics• Heteroaromatics display a similar interplay of effects:

• Aromatic resonances may be calculated empirically by adding increments to the benzene chemical shift (128.4) for each substituent that is ipso, ortho, meta, or para to the resonating carbon as seen on the following table:



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3-5b Unsaturated Compounds. Aromatics



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9090





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3-5c Carbonyl Groups General•Carbonyl groups have no direct representation in 1H NMR spectra, so 13C NMR provides unique information for their analysis.

•The entire carbonyl chemical shift range, 160– 220, is well removed to high frequency from that of almost all other functional groups, on account of the effect of the n p* transition on the magnitude of the paramagnetic shift.

•Like aromatic ipso carbons and nitriles, carbonyl carbons other than those in aldehydes carry no attached protons and hence relax more slowly and tend to have low intensities.



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3-5c Carbonyl Groups Aldehydes and Ketones•Aldehydes resonate toward the middle of the carbonyl range, at about 190– 205 (acetaldehyde at 199.6)

•Unsaturated aldehydes, in which the carbonyl group is conjugated with a double bond or phenyl ring, are shifted upfield (benzaldehyde at 192.4)

•The a, b and g effects of substituents on ketones add to the carbonyl chem-ical shift and hence are found on the downfield end of the carbonyl range from195– 220 (acetone at 205.1 and cyclohexanone at 208.8)

•Again, adjacent unsaturation shifts the resonances to lower frequency.



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3-5c Carbonyl Groups Carboxylic Acid Derivatives•Carboxylic derivatives fall into the range 155– 185.

•The resonances for the series carboxylate , carboxyl , and ester often are well defined:

NaOAc (181.5), HOAc(177.3), and CH3OAc(170.7)

•The range for esters is about 165– 175, and that for acids is d 170– 185.

•Acid chlorides are at slightly upfield at 160– 170, (168.6 for AcCl) .

•Anhydrides have a similar range: 165– 175 (167.7 for Ac2O).



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3-5c Carbonyl Groups Carboxylic Acid Derivatives•Lactones overlap the ester range, with the six- membered lactone at 176.5.

•Amides have a similar range: 160– 175 (172.7 for AcNH2) .

•Oximes have a larger range, from 145– 165.



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3-5c Carbonyl Groups



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3-5c Carbonyl Groups



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NMR Spectroscopy CHEM 430 Fall 2014. FACTORS THAT INFLUENCE PROTON SHIFTS Local Fields Shielding by...

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Transcript of NMR Spectroscopy CHEM 430 Fall 2014. FACTORS THAT INFLUENCE PROTON SHIFTS Local Fields Shielding by...