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NMR spectrum provides info about what?
__ and __ are most useful because hydrogen and carbon are moajor components of organic compounds.
structure of the compound
proton (1H) and carbon-13 (13C
A nucleus with an __ or an __ has a __ that can be observed by the NMR spec. A __ is the simplest nucleus, and its odd atomic number of 1 implies it has a spin.
- odd atomic number
- odd mass number
- nuclear spin
When a small bar magnet is placed in the field of a larger magnet, it __. The same occurs with a proton.
Quantum mechanics requires the proton's magnetic moment to what?
twists to align itself with the field of the larger magnet--a lower energy arrangement than an orientation against the field
be aligned with either the external field or against the field
The lower-energy state with the proton aligned with the field is called the __. The higher energy state with the proton aligned against the external magnetic field is called the __.
- alpha-spin state
- beta-spin state
In the absence of an external magnetif field, the proton magnetic moments have random orientations. When an external magnetic field is applied, each proton in a sample assumes the __ or the __. Because the __ is lower in energy, there are more __ than __
- alpha state
- Beta state
- alpha-spin state
- alpha spins
- beta spins
In a strong magnetic field, the energy difference between the two spin states is __. In fact, the energy difference is proportional to the __, as expressed in the equation __.
- larger than it is in a weaker field
- strength of the magnetic field
- dE= energy difference between alpha and ebta states
- h= Planck's constant
- B0= strength of external magnetic field
- y= gyromatgnetic ratio
What is the gyromagnetic ratio?
Magnetic fields are measured in __.
a constant that depends on the magnetic moment of the nucleus under study.
The energy difference between a proton's two spin states is __. For a strong external magnetic field of 25,000 gauss, it is only about 10^-5 kcal/mol. Even this small energy difference can be __. When a proton interacts with a photon with just the right amount of electromagnetic energy, what can happen? A nucleus aligned with the field can do what?
- detected by NMR
- the proton's spin can flip from allpha to beta or vise versa
can absorb the energy needed to flip and become aligned against the field
When a nucleus is subjected to the right combo of magnetic field and Electromagnetic radiation to __, it is said to be __, and its absorption of energy is detected by the __. This is the origin of the term __.
- flip its spin
- in resonnce
- NMR spectrometer
- nuclear magnetic resonance
For the fields of currently available magnets, proton resonance frequencies occur in the __ of the spectrum. __ are usually designed for hte most powerful magnet that is practical for the price range of the spectrometer, and the radio frequency needed for __is calculated based on the field.
- radio-frequency region
- NMR spectrometers
A more powerful magnet makes dE __, and it __ between signals, giving spectra that are more clearly resolved and easier to interpret.
- larger and more easily detected
- increases the frequency difference
In reality, protons are not __, but instead are __, which generate a small __ that opposes the externally applied field.
In a molecule the electron clound around each nucleus acts like a loop of wire, rotating in response to teh external field. This induced rotation is a __ whose magnetic field opposes the __. The result is that the magnetic field at the nucleus is weaker than the external field., and we say that the nucleus is __.
The effective magnetic field __ is always weaker than the external field, so the applied field must be increased for resonance to occur at a given frequency.
- surrounded by electrons that partially shield them from the external magnetic field
- induced magnetic field
- circular current
- external field
- at the shielded proton
Equation for B
True or False:
All proteins are shielded in the same amount.
If a proton is not shielded as much by other protons, therefore absorbing at a lower field than the methyl protons, what is this considered?
False: they are shileded by different amounts
A careful measurement of the field strengths required for resonance of the various protons in a molecule provides us with two important types of info:
- the number of different absorptions: implies how many different types of protons are present
- the amount of shielding shown by these absorptions: implies the electronic structure of the molecule close to each type of proton
Two other aspects of the NMR spectrum we will consider are the _ and the __.
- the intensities of the signals: imply how many protons of each type are present
- the splitting of the signals gives info about other nearby protons
Summarize what happens in an NMR spectrometer.
- protons are placed in a magnetic field where they aligh either with or against a field
- While still in the magnetic field, the protons are subjected to radiation of a frequency they can absorb by changing the orientation of their magnetic moment relative to the field. If protons were isolated, they would all absorb at the same frequency, proportional to the magnetic field
Summarize what happens in an NMR spectrometer cont.
but protons in a moelcule are partially shielded, whcih dependson the environmnet, which causes different absorption of radiation
What lies on the X axis of the graph?
Explain where shielded and deshielded lie?
applied magnetic field (higher are upfield toward the right/ lower are downfield toward the left)
shielded protons appear upfield toward the right; deshielded appear downfield