The flashcards below were created by user
on FreezingBlue Flashcards.
A proton in the NMR is subjected to both the __ and the __. If there are protons nearby, their small magnetic fields also affect the __ of the proton. The splitting of signals into multiplets, called __, results when two different types of protons are close enough that their magnetic fields influence each other. Such protons are said to be __.
- external magnetic field
- induced field of the shielding electrons
- absorption frequencies
- spin-spin splitting
- magnetically coupled
Spin-spin splitting is a __. If one proton splits another, what must happen?
In general, when aligned against the field pr in the direction of the field?..?
- reciprocal property
- the second proton must split the first
- against: shielded
- aligned with: deshielded
In general, the multiplicity (number of peaks) of an NMR signal is given by the __.
What is it?
The relative area of the __ that results are approx. given by the appropriate line of __.
If a signal is split by N neighboring equivalent protons, it will be split into N+1 peaks
- N+1 multiplet
- Pascal's triangle
Most spin-spin splitting is between __.
Protons bonded to the same carbon atom (geminal prtons) can split each other only if __. In most cases, protons on the same carbon atom are __, and __ cannot __.
- protons on adjacent carbon atoms
- they are nonequivalent
- equivalent protons
- split each other
Protons separated by more than three bonds do not __. Occassionally, such __ does occur, but these cases are unusual.
In general, a multiplet does what?
- produce observable spin-spin splitting
- long-range coupling
- leans upward toward the signal of the protons responsible for the splitting
Any two magnetically coupled protons must have __. The distance between adjacent peaks of the Hc multiplet must equal the distance between the peaks of the Hb doublet.
The distance between the peaks of a multiplet is called the __, represented by __, and the one between Ha and Hb is represented by __. In complicated spectra with many types of protons, groups of neighboring protons acn sometimes be identified by measuring their coupling constnatns. Multiplets that have the same coupling constant may arise from adjacent groups of protons that split each other.
- must have equal effects on each other
- coupling constant
The magnetic effect that one proton has on another depends on the __, but it does not depend on the __. For this reason, the __ does not vary with the field strength of the spectrometer.
- bonds connecting the protons
- strength of the external magnetic field
- coupliing constant
There are many cases of __, where signals are split by adjacent protons of more than one type, with different coupling constants.
Sometimes, a signal is split by __.
two or more different kinds of protons with similar coupling constants
Stereochemical differences often result in __for protons on th same carbon atom.
To determine whether similar-appearing protons are equivalent mentally sub another atom for each of the protons in question. If the same product is formed by imaginary replaement of either of two protons those protons are __.
If they are differnet, they are called __.
different chem shifts
__ also occurs in saturated, acyclic compounds.
In general, any type of movement or change that takes place faster than about a hundredth of a second will __. This is illustrated by the cyclohexane spectrum. In the chair comf there are two kinds of protons: the __ and __.
Becuase of __, there is an average of the axial and equatorial positiosn.
produce an averaged NMR spectrum
Like chair interconversions chemical processes often occur __.
Proton exchange occurs in most alcohols and carb acids, and in many amines and amides. If the exchange is fast, we see __. If slow, we see __. If moderately slow, we see __.
faster than the NMR can observe them
- one sharp averaged signal
- a broadened peak that is neither clealy split nor cleanly averaged
Protons on nitrogen often show __ int he NMR< bth because of moderate rates of exchange and beacuse of hte magnetic properties of the nitrogen nucleus. Depending on the rate of exchange an dother factors, NH protons may give abosrptions that are __, __, or __.
- broadened signals
- sharp and cleanly split, sharp and unsplit, or broad and spaeless
If a carbonyl group has a proton attached (aldehyde proton), the peak at __ alerts us of its presence. If the adjacent carbon has hydrogens their signals ebtween __ and __ are suggestive.
__ determines hte magnetic environoments of the carbon atoms themselves. Carbonyl carbon atoms, alkyne carbon atoms, and aromatic carbon atoms all have characteristic chem shifts in the 13C NMR spectrum.
- delta 9 or 10
- delta 2.1 and delta 2.5
- Carbon NMR
Which is weaker: carbon or proton signals. Why is carbon-13 used?
When magnetic nuclei are placed in a uniform magnetic field and irradiated with a pulse of radio frequency close to their resonant frequency, what happens? This precession of many nuclei at slightly different frequences does what?
carbon NMR signals
because it has an odd number ofnuetrons
the nuclei absorb some of the energy and preess like little tops at their resonant frequencies.
produces a complex signal that decays as the nuclei lose the energy they gained from the pulse
This signal is called a __ and it contains all the info needed to calculate a spectrum.
As in proton NMR, many 13C signals are __. Carbon chem shifts are usually about 15-20 times larger than comparable proton chem shifts, which makes sense because the carbon atom is one atom closer to a __ than its attached hydrogen.
free induction decay (or transient)
- deshielded by electron-withdrawing substituents
- shielding or deshielding group
Explain the relation between NMR and Carbon 13 NMR.
__ is a common solvent for 13-C NMR because the spectrometer can __.
Because chem shift effects are larger in C-13 NMR, an __ has a substantial effect on the chem shift of a cabron atom beta to the group.
if HNMR is at delta 9.5, the C is fifteen to twenty times that
- lock onto the signal from deuterium at a different frequency from carbon
Difference of C-13 from proton
1) Operating frequency: the gyromagnetic ratio for 13C is about __ of the proton, so the resonance is also about __.
2) Peak areas: areas in C-13 aren't really proportional to the number of carbons giving rise to the peaks. Carbon atoms with two or three protons usually give __, and carbons with no protons tend to give __.
- strongest absorptions
- weak absorptions
Only __ of the carbon atoms in C NMR are __, so there is a small probability than an observed 13C nucleus is adjacent to another. So, __ can be ignored. __ is common as most carbon atoms are bonded directly to __ or are colse to __ for __ to be observed. Extensive __ produces splitting patterns that are complicated and difficult to observe.
- one percent
- carbon carbon splitting
- carbon hydrogen coupling
- carbon hydrogen spin-spin coupling
- C-H coupling
To simplify 13C NMR, they are commonly recorded using __,, where the protons are continuously irradiated with a broadband proton transmitter. As a result, all the protons are continuously __, and they rapidly _. The carbon nuclei see an -_ of the possible combos of proton spin states. Each carbon signal appears as a single, unsplit peak becuase any C-H splitting has been __.
- proton spin decoupling
- in resonance
- flip their spins
__ produces spectra that are very simple, but some valuable info is lost in the process. __ simplifies the spectrum but allows some of the splitting info to be retained. With __, the 13C nuclei are split only by the __. The __ applies, so a carbon with one proton appears as a __, etc. __ are easily recognized by the appearance __ as a quartet at O ppm, split by the tree protons of each methyl group.
- proton spin decooupling
- off-resonance decoupling x2
- protons directly bonded to them
- off-resonance-decoupled spectra
- of TMS
__ is more recent and provides the same info as __. It givs better __ and avoids overlapping multiplets because all the peaks remain __.
Each C-13 nucleus is magnetically coupled to the protons bonded to it. Under the right circumstances, this magnetic coupling allows __. The number of protons bonded to he C-13 nucleus determines how this polarization transfer occurs.
- off res decoupling
- decoupled singlets
- the transfer of polarization from the protons to the carbon nucleus
A Dept experiment usually includes three spectral scans.
the normal decoupled scan: each type of 13C nucleus appears as a signlet
the DEPT-90 scan: only the CH carbons bonded to exactly one proton appear
the DEPT-135 scan: the CH3 and CH groups appear normally, and CH2 give negative peaks
This info allows us to distinguish among carbons bonded to none, one, two, or three H atoms.
C with 0 appear only in normal spectrum, but not in either DEPT
Methine carbons give normal positive peaks in all three spectra
Methylene carbons give normal peaks in teh normal spectrum, no peaks in the DEPT-90 spectrum, and negative peaks int eh DEPT-135 spectrum
Methyl carbons give normal peaks int he normal spectrum, no peaks in the DEPT-90 spectrum, and normal peaks in the DEPT-135 spectrum
what does the 13C NMR spectrum tell us?
the number of different signals implies how many different types of carbons are present
the chemical shifts of those signals suggest what types of functional groups contain those carbon atoms
the splitting of signals in the off-resonance-decoupled spectrum or the DEPT 90 and DEPT 135 spectra indicate how many protons are bonded to each atom.