NMR - 4 KEY Rules

¹³C NMR

# of carbons in different environments

¹H NMR

# of hydrogens in different environments

1HNMR Short Answer Rundown

1) signals ____________ 2) splitting pattern - mention the region 3) ** if both [1] and [2] are the same mention chemical shift - which is downfield? (deshielding explained)

Ortho

1,2 position

Meta

1,3 position

Para

1,4 position

NMR 4 Rules

1. Number of Signals 2. Position of Signals 3. Intensity of Signals (integration/ratio) 4. "spin spin" Splitting Pattern

Gameplan: NMR Signals

1. Read the question! Is it asking for ¹³C NMR or ¹H NMR? 2. Locate similarities in the environment --> you could always find a plane of symmetry, consider what common I guess goal they lead to Think: Are our neighbors the same? If they are, whose next? If theres any difference in the bigger picture then theyre not the same signal, if there is no difference then they are! Also, they have to be coming from the same place, say they meet at a methyl but came from different environments, this wont make them the same signal

Gameplan: Spin Spin Splitting

1. look at the carbon right next to the carbon that the hydrogen is connected to 2. count how many hydrogens are attached to that carbon 3. add to 1 4. slay

Details of ¹³C NMR: What details give the most structural information?

1. number of signals observed 2. chemical shifts of those signals

¹³C NMR Example

1. this is asking for carbons 2. the carbons at the very end are the same based on the same logic, theyre connected to the same thing on the left and right - they have the same neighbords!

¹H NMR Example

1. this is asking for hydrogens 2. the blue CH3 are the same, notice how theyre both racing to meet the same carbon There are 6 signals

How do we use NMR to distinguish constitutional isomers from each other?

1. write out the signals and splitting patterns 2. notice differences in their HNMR splitting patterns, explain what this means ***IF splitting patterns are the same look at the chemical shift (look for most downfield signal splitting - closest to more EN atom) Template: We can distinguish between two constitutional isomers by using 1HNMR in the following ways Considering the signals of the protons remain the same for both constitutional isomers (state the region, like if its benzene its 6.5-8), we must compare the splitting patterns of each. **elaborate on differences you see and youre done**

Protons on Benzene Rings

Benzene: six equivalent deshielded protons and exhibits a single peak in the Monosubstituted Benzenes: contain five deshielded protons that are no longer equivalent apperance of these signals is highly variable, depending on the identity of Z

VERY SAD but what table do you have to commit to memory?

Characteristic Chemical Shifts - I have a separate quizlet for this!

Nonequivalent Protons

Hydrogens in the same signal

Are wedges and dashes the same signal?

Nope, they count as different signals but only consider this when theyre explicitly written out

Practice Example: (1) Number of Signals (2) Integration and (3) Spin spin Splitting

Okay so, 1. theres two signals, remember that when there is symmetry the hydrogens on opposite ends are in the same signal 2.A because we cant include the equivalent hydrogens, which share the same signal, there are six nonequivalent hydrogens on the red hydrogen = septet 2.B again, we cant include equilavent OR hydrogens that arent on the neighboring carbon, hence there are two nonequivalent hydrogens on the blue hydrogen = triplet

Big Takeaway: Disheilding vs. Shielding

Shielding is a hydrogen minding its business Deshielding is having a EN neighbor come and push you to down the ground bc he hates you

Example 1: Shielding and Chemical Shift - Comparing two different compounds Which is going to be more downfield?

The second, it has an oxygen getting in the way

Big Takeaway: How do we find different environments?

What is connected from left to right? Where are they coming from and where are they going toward?

¹H NMR - Splitting Pattern Rule [2]

a set of n nonequivalent protons splits the signal of a nearby proton into n + 1 peaks (n = hydrogens on neighboring carbon)

Shielding Effects

an electron shields the nucleus, the absorption shifts upfield the hydrogen happily has its protons

Lack of Splitting of ¹³C NMR

consequence of low natural abundance of ¹³C splitting occurs when two NMR active nuclei (protons) are close to each other the chance of two ¹³C atoms being bonded to each other is very small, appears as a singlet

Deshielding Effects

decreased electron density deshields a nucleus and the absorption shifts downfield if there is an EN atom directly neighboring the hydrogen, then itll be greedy and shove it

Magnitude of the Coupling Constant (J)

depends on the arrangement of hydrogen atoms

Use ¹H NMR Data to Determine a Structure [1]

determine the number of different kinds of protons - the number of NMR signals equals the number of different types of protons

¹³C NMR Spectrum Splitting

easier to analyze than ¹H because signals arent split, each carbon appears as a single peak

¹H NMR - Splitting Pattern Rule [1]

equivalent protons do not split each others signals (hydrogens of the same signal dont count)

(3) "spin spin" Splitting Pattern

further information of the neighboring environment for various hydrogens in a molecule

Number of Signals of ¹³C NMR

gives the number of different types of carbon atoms in a molecule since the signals arent split, the number of signals is equal to the number of lines in the spectrum

Big Takeaway: Counting

how many hydrogens are on each?? youre just counting broski

Proximity and Splitting

i its not in the carbon right next to it, dont involve it (more than 3 bonds = no)

Aromatic Deshielding

in a magnetic field, six pi electrons in benzene circulate around the ring, create a ring current magnetic field induced by these moving electrons reinforces the applied magnetic field in the vicinity of the protons

(2) Position of Signals

indicates what types of hydrogen the molecule contains

(3) Intensity of Signals - Integration

information about the relative number of protons based on area under the peak usually corresponds to Hydrogens of the same type; a ratio!!! Disclaimer: Doesnt have to equal # of hydrogens

Alkene Chemical Shifts

loosely held pi electrons of the double bond create a magnetic field that reinforces the applied field in the vicinity of the protons protons feel a stronger magnetic field, requires a higher frequency for resonance (protons are deshielded and absorb downfield)

Summary of π Electron and Chemical Shift

meow look at the table and listen to whatever aguilar has to say about it

Example 3: Shielding and Chemical Shift - What if theres TWO EN atoms on a compound?

more downfield with fluorine because its more electronegative the more electronegative atom pushes the hydrogen more downfield

¹H NMR - Spin Spin Splitting

occurs only between hydrogens that arent in the same signal and are on the neighboring carbon

Alkyne Chemical Shifts

pi electrons of carbon-carbon triple bonds are induced to circulate, BUT the induced magnetic field opposes the applied magnetic field (B0) proton feels a weaker magnetic field, lower frequency needed for resonance the nucleus is shielded and the absorption is upfield

Diamagnetic aniosotropy

protons feel a stronger magnetic field and a higher frequency is needed for resonance, meaning they are deshielded and absorb downfield

Coupling Constant for Alkenes

protons on C=C bonds give characteristic splitting patterns a disubstituted double bond can have: 1. two geminal protons 2. two cis protons 3. two trans protons when these protons are different, each proton splits the NMR signal, each proton appears as a doublet

Chemical Shifts in ¹³C NMR

small range of chemical shifts in ¹H NMR, BUTTTTT ¹³C NMR occur over a much broader range chemical shifts depend on the same effects as the chemical shifts in protons in the ¹H NMR

¹H NMR - Splitting Pattern Rule [3]

splitting is observed for nonequivalent protons on the same carbon or adjacent carbons

Example 2: Shielding and Chemical Shift - Comparing different signals on the same compound Which signal going to be more downfield?

the second has an oxygen as a direct neighbor I know the first also has an oxygen close by but dont let that fool you, the second has an oxygen as a closer neighbor and to push you down the neighbor needs to be close right?

But what about alcohol???? ¹H NMR - OH Protons

under usual conditions, an OH and NH hydrogen does not split the NMR signal of neighboring hydrogens itll be a singlet

Use ¹H NMR Data to Determine a Structure [4]

use chemical shift data to complete the structure as in, put the structure together in a manner that preserves the splitting data and is consistent with the reported chemical shifts

Use ¹H NMR Data to Determine a Structure [3]

use individual splitting patterns to determine what carbon atoms are bonded to each other

Use ¹H NMR Data to Determine a Structure [2]

use the integration data to determine the number of H atoms giving rise to each signal 1. total number of integration units (add given numbers on chart) 2. total number of protons (add singlet, triplet and quartet) 3. divide (1) by (2), these are the units per proton 4. divide each integration value by the units per proton **three equivalent H = CH3 **two equivalent H = CH2

Diastereotopic Protons

when substitution of two H atoms by Z forms diastereomers the two H atoms are not equivalent and give two NMR signals

Enantiotopic Protons

when substitution of two H atoms by Z forms enantiomers these two H atoms are equivalent and give a single NMR signal, these are enantiotopic protons

(1) Number of Signals

¹H NMR or ¹³C NMR - indicates the number hydrogen or carbon in a molecule in different environments