<< Chapter < Page Chapter >> Page >

We can then use Δ size 12{Δ} {} E to give us some information about the nature of the atoms to which a particular hydrogen atom is attached.

The position of a signal in the NMR spectrum is known as its chemical shift. It has become conventional to reference the NMR signals for hydrogen atoms to the signal found for TMS (tetramethylsilane, Si ( CH 3 ) 4 size 12{ ital "Si" \( ital "CH" rSub { size 8{3} } \) rSub { size 8{4} } } {} ), which has been assigned a value of 0. This scale gives the chemical shifts for other atoms in terms of δ size 12{δ} {} which has frequency units of parts per million (ppm). These values are constant regardless of the field strength of the magnet being used. Most hydrogen atoms attached to organic molecules will resonate in the region δ size 12{δ} {} = 0 to 10 ppm. Often, the more acidic the proton is, the more positive this value becomes (acid protons in carboxylic acids will often be seen at>+10 ppm). This means that the proton is experiencing less electron density around it, a phenomenon known as deshielding. The less electron density there is about a given hydrogen atom, the more deshielded it is. On the opposite extreme, the more electron density a hydrogen atom experiences, the more shielded it is. While most hydrogen atoms are found in the NMR spectra at positive δ size 12{δ} {} values, some may be found at negative δ size 12{δ} {} values. This often occurs when protons are attached to metals (metal hydrides). In these cases, it is typical to find hydrogen signals with δ size 12{δ} {} = 0 to -30 ppm. Chart 1 gives a general guideline for where certain types of protons are found in NMR spectra. 

Chart 1. NMR Chemical Shift Assignments 

Integration

One of the most useful aspects of NMR spectroscopy is that the signal intensities are directly proportional to the amount of a particular type of H atom present. This can be used in two ways.

First, it can be tell us the number of hydrogen atoms of a given type in relation to other hydrogen atoms present in the molecule. In order to make use of this information, we need to realize that symmetrically equivalent H atoms will have identical chemical shifts.

Thus, if we have an ethyl group, there are three protons from the methyl ( CH 3 ) size 12{ \( ital "CH" rSub { size 8{3} } \) } {} group and two protons from the methylene ( CH 2 ) size 12{ \( ital "CH" rSub { size 8{2} } \) } {} group. We expect to see two signals - one for the methyl and one for the methylene - with relative peak areas of 3:2. The process of determining the peak areas is known as integration and the resultant peak areas are known as the integral of the spectrum.

In order to use this information, you need to determine what types of atoms are the same and what types are different within a molecule. Atoms may appear the same for two reasons.

  • One is that they are related by symmetry. That means that the molecule has some symmetry element such as a mirror plane, rotation axis, inversion center, etc. that will make the atoms equivalent.
  • The other possibility is that there is some molecular motion which causes the atoms to adopt an averaged environment. For example, in the ethyl group above, we can draw a static structure which makes the H atoms on the CH 3 size 12{ ital "CH" rSub { size 8{3} } } {} group unequal because their proximity to the X group would be different. However, there is free rotation about the C-C bond, so all of the H atoms are whizzing around at very fast speeds, resulting in an averaged environment. There is only one signal observed for this methyl group.

Questions & Answers

what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
How can I make nanorobot?
Lily
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
how can I make nanorobot?
Lily
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Good
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!





Source:  OpenStax, Chem 215 spring08. OpenStax CNX. Mar 21, 2008 Download for free at http://cnx.org/content/col10496/1.8
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Chem 215 spring08' conversation and receive update notifications?

Ask