NMR Spectoscopy Flashcards by Sarah King

determines molecular weight and molecular formula

Mass spectrometry (ms)

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identifies functional groups and bond type

Infrared spectroscopy (IR)

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Identifies carbon–hydrogen framework

Nuclear magnetic resonance spectroscopy (NMR)

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Determines presence of conjugated pi electrons

Ultraviolet and visible spectroscopy (UV-Vis)

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NMR technique ____________ MS and IR by providing a “map” of the C-H (and C-C) framework.

Complements

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What are the two most common types of NMR?

1^H NMR (aka proton NMR),

13^C NMR (aka carbon NMR)

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H NMR allows us to determine the….

Number of types of hydrogens in a substance

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^13C NMR Is used to….

Get the number and kinds of carbons that are in an organic compound

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Basic principle behind NMR

Nuclear spins
Behavior of nuclear spins in the presence of an external magnetic field
Radio frequency, spin flip, and resonance
The applied field and RF
Magnetic - non-magnetic nuclei

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Nuclear spins stem from the spinning of _________ nuclei in Atoms. The spinning nuclei act as __________. as a result, they can interact with an external magnetic field, Bo

Positive,

Tiny bar magnets

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When there is no external field acting, nuclear spins orient themselves _________.

Randomly, no Bo

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In the presence of a strong external magnetic field, Bo, a spinning H-1 (or C-13) nucleus aligns itself……

Either with the field or against it

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The parallel orientation in a magnetic field is called the ______ state. This represents a _________ Energy state that is more favorable (more stable).

Alpha, lower

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When nuclear spins orient themselves with the applied feild they are said to have adopted a __________.

Parallel orientation

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Anti-parallel orientation in the external magnetic field is known as the ________ state. This is a _________ energy state that is less favorable (less stable).

Beta, higher

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When nuclear spins align themselves against the applied field there said to have an __________ orientation.

Anti-parallel

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This phenomenon is when the nuclear spins in the alpha state go from this ground state to the higher energy level, the beta state.

“ spin flip”

At this point the nuclei are said to be in resonance with the applied field

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By using a technique called ____________, The energy of relaxation of nuclei (returning from Beta to Alpha State) is measured.

Fourrier-Transform

(this is what MNR is all about)

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In MNR, as energy increases the energy gap gets…….

Wider. This is what allows you to do NMR

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In the absence of a magnetic field, nuclear spin states have ______ energies.

In the presence of a magnetic field, spin states have _______ energies.

equal

Unequal

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Technique in which spinning nuclei in a strong magnetic field absorb radio frequency to go from a lower energy level to a higher energy level (resonance)

NMR

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The RF (entertainment of light) used in MNR depends on the strength of the applied field in the identity of the _______. Stronger magnetic fields require higher frequencies and vice versa.

Nuclei

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The applied field, Bo, is directly proportional to the ________ of the RF required to bring the nuclei into resonance.

Frequency

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What nuclei exhibit magnetic properties?

All nuclei with an odd number of protons and all nuclei with an odd number of neutrons.

(1^H, 19^F, 31, 31^P)

(2^H, 13^C, 14^N)

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What are the six parts of the NMR spectrometer (or spectrophotometer)?

1. RF source 2. Prism or grating 3. Sample cell wrapped in a magnetic field 4. Amplifier 5. Detector 6. Recorder

The sample for NMR that is to be analyzed is dissolved first in……

Deuterated chloroform or CDCL3

the _________ magnetic field is the actual field (or portion of the applied field) Felt by a spinning nucleus

Effective (B-eff)

And Adams was spinning nuclei, they exist also spinning __________ moving around the nuclei. They act as tiny bar magnets, as a result they create their own local magnetic fields. These fields act in opposition to the applied field (Bo)

Electrons

B-effective =

B-applied - B-local [Bo - B-local]

Nuclei that feel a high B-eff require ______ frequencies to bring about residence. On the other hand, nuclei that feel a low B-eff require ______ frequencies.

High, low

Local fields created by the spinning electrons “shield” or protect spinning nuclei from the full effect of the applied field nuclei are said to be….

- “Shielded” (next O, N, halogen), | Or “desheilded” when there are no electrons

Deshielded nuclei have a ____ Beff and ______ frequency

High, high

Nuclei that have the same electronic environment, same electron density around them are said to be….

Chemically equivalent, this means that these nuclei are shielded to the same extent. As a result they appear as a single absorption sharp peak on the NMR spectrum.

What does it mean if there is a single absorption peak on the NMR spectrum from two nuclei?

They are chemically equivalent ( shielded the same)

Protons that are not shielded are located where? | Highly shielded is located where?

Downfield (left side), larger effective magnetic fields, higher frequency Upfield (right side), smaller effective magnetic fields

Nuclei that are deshielded overall feel a _________ effective magnetic field (Beff)

Larger | Small B-local

The positions of the peaks on the NMR spectrum or measured relatively to the position of a reference ____________ or TMS, for both 13^C and 1^H NMR

Tetramethylsilane (CH3)4Si

The ___________ shift of a nucleus is the position of its absorption peak from TMS

Chemical

The chemical shaft of TMS is arbitrarily set to….

0 ppm

The delta scale is an arbitrary scale defined as…..

1 delta = 1ppm of the Spectrometer operating frequency

What is directly proportional to the applied frequency?

1 ppm of a 200-MHz is 200 Hz; therefore, for this spectrometer, 1 delta = _____ Hz.

200 *If the applied frequency is 500 MHz, 1 delta = 500 Hz.

observed chemical shift in hertz divided by spectrometer frequency in megahertz

= delta

The chemical shift in Delta for a given nucleus is the same for all NMR spectrometers, regardless of……

The applied frequency. * analogy: The distance between two cities is always the same regardless of the speed of travel. One driver can drive slower but that does not change the distance between the cities (in NMR the first city is TMS)

The general chemical shift range for 1^H NMR is ______.

0-10 delta

The general chemical chef range for 13^C NMR is ______.

0-220 delta

Delta of TMS is ….

A set of equivalent hydrogens gives a ______ sharp peak

Single

What are the four different types of hydrogens or protons?

- unrelated hydrogens or protons - Homotopic hydrogens or protons - enantiotopic hydrogens or protons - diastereotopic hydrogens or protons

Unrelated protons have different ____________ and therefore give different NMR signals.

Chemical environments

Homotropic protons have the same _____________. As a result they have the same 1^H NMR signal.

Electronic environment

Enantiotropic protons are ___________ but have the _____ electronic environment. Therefore, they will have the same NMR signal.

Different, same * The replacement of one of the protons creates a new stereogenic center (replacing one proton will give you S enantiomer and replacing the other proton will give you R enantiomer)

Diastereotopic protons are _____ chemically or electronically equivalent.

Not * they show different NMR signals * the replacement of one of the protons creates a second stereogenic center

The more alkyl groups, the _________ the chemical shift.

Higher *note: Substituted SP3 carbon protons have higher chemical shift values than methane because carbon is more electronegative than H

I’m SP2 protons, The loosely held pi electrons induce a local magnetic field that ______ to the external applied magnetic field, Bo. What is the equation?

Adds Beffective = Bo + Blocal

Are SP2 protons de-shielded or shielded?

De-shielded

SP2 protons feel a larger __________ magnetic field.

Effective (requires a higher frequency)

In the presence of an external magnetic field, the 6 delocalize pi electrons in benzene circulate around the ring. This creates a *ring current* that induces a _______ magnetic field that ______ to the external magnetic field.

Local, adds (Beffective = Bo + Blocal) * benzene feels a higher effective magnetic field as a result

With SP protons, they also create a ring current in the presence of an external magnetic field. However, the induced local magnetic field that results does _______ to Bo.

Not add, (it acts in opposition to the applied field) Beffective = Bo - Blocal

SP protons are de-shielded or shielded?

Shielded (absorb upfield)

The height of a peak in H NMR is proportional to the area ________ which is itself directly proportional to the number of hydrogens that give rise to the peak.

Under a peak * this allows you to determine the number of hydrogens responsible for the peaks. This is called integration of the H NMR spectrum

Total number of units =

(Summation symbol) integration units

Number of units per proton = Number of protons per signal =

Total number of units divided by /number of protons Integration value divided by / number of units per proton

The absorption signal of a proton or equivalent protons is sometimes split into two or more peaks. This phenomenon is called….

Spin-spin splitting or spin coupling * it occurs with non-equivalent protons on the same carbon or adjacent carbons

What causes spin-spin splitting?

Local magnetic fields of neighboring protons

_____ is the distance between peaks in a H NMR signal

J- The coupling constant * it is expressed in the unit of frequency, hertz.

J is the ______ for coupled protons.

Same

Do you chemically equivalent protons split each other?

No *splitting rule

What is the n + 1 rule?

If a proton has n equivalent neighboring protons, it’s NMR signal is split into n+1 peaks*

If there are more than seven peaks in a signal, we have a __________.

Multiplet

What is the n and m rule?

The signal of a proton or equivalent protons “sandwiched” between n and m non-equivalent protons is split into (n+1) (m+1) peaks. Example on pg.144

How does the coupling constant, J, differ for geminal (on same carbon), Cis and trans?

J-geminal < J-cis < J-trans

OH and NH protons are not split by neighboring protons. They appear as…..

Single peaks or singlets

How do the signals appear for protons in cyclohexane conformers?

It cannot distinguish between axial and equatorial hydrogens so the signal is a singlet because NMR sees an average environment

How does the signal appear for protons in benzene?

All six protons appear as singlet since they are equivalent but deshielded because of the ring current

How can proton NMR be used?

- to identify the structure of an unknown or the products of a chemical reaction - to support a reaction mechanism