Updated 28.10.2010
University of Iceland
Science faculty ,
EFN512M, Litrófsgreiningar sameinda og hvarfgangur efnahvarfa
EFN010F, Litrófsgreiningar sameinda og hvarfgangur efnahvarfa /
Molecular spectroscopy and reaction dynamics;
FT-NMR;
Framkvæmd (sjá nánar neðar):
-fer fram í VRI-121 (1) sýnagerð)og húsnæði Raunvísindastofnunar Háskólans (2)
sýnikennsla og mæling undir handleiðslu Sigríðar Jónsdóttur), Dunhaga 3 .
Tilraunin felst í ítarlegri NMR greiningu (1H-NMR, C-NMR, 2D-NMR, FT …mælingum, gagnaúrvinnslu og hermun) á völdu efni (sjá neðar).
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English:
Procedure (see more detailed below):
The demonstration involves 1) sample preparation (VRI-121) and 2) measurements
(Science Institute, Dunhaga 3, basement; Sigríður Jónsdóttir): An organic sample (see
below) will be thoroughly analysed (1H-NMR, 13C-NMR, 2D-NMR, FT …measurements, analysis and spectra simulations).
INTRODUCTION
A. When a sample is in a magnetic field, atom nuclei will spin around an axis parallel to
the magnetic field (See Fig. 1; The direction of the magnetic field is up). Nuclei with spin
quantum number, either spin up ( in Fig. 1; projection quantum number, mI =
+1/2) or down (in Fig. 1; projection quantum number, mI = -1/2). The energies differ
due to different interactions of the magnetic fields of the spinning nuclei with the
surrounding magnetic field. nuclei are more energetic by factor   or gI  B
(See Fig. 2) where  is the "magnetogyric ratio and gI is the g-factor, which depends on
the atom nuclei.  (= h/(2)) and (nuclear magneton) are constants.
In an NMR equipment electromagnetic waves in the radio frequency region are absorbed
causing transitions of  to  in case when the photon energy () equals the energy
difference between  and    , i.e.
h =   B
(1)
The effective magnetic field, close to a nucleus, depends on its surrounding
(environment). According to Eq. (1) nuclei with different environment (different B)
therefore absorb electromagnetic waves of different frequencies. Therefore absorption
peaks for analogous nuclei but different surrounding appear in different positions on the
NMR spectral scale.
B. When hydrogen bonds are formed or broken the surrounding of hydrogen nuclei,
involved, will change with respect to electron density. Therefore, corresponding
absorption peak will shift.
C. A nucleus which is either situated in a reagent or in a product molecule for a reaction
in an equilibrium can show two different absorption peaks depending on the difference in
it surrounding in the two molecules involved.
D. A nucleus (X in Fig. 3) van interact with a neighbour electron in a chemical bond
(Fermi interaction) affecting its spin direction. Second electron, in the chemical bond,
will spin in the opposite direction (Pauli principle). That electron will affect the magnetic
field in the neighbourhood of a second nuclei (Y in Fig. 3). Therefore, the magnetic field
close to Y effectively depends on the spin direction of nucleus X. In other words,
“information” concerning the spin direction of X are handed over to Y by the electrons
in the chemical bond between X and Y. The effective magnetic field close to Y will be
different for different spin directions of X. Therefore Y will show two absorption peaks.
The difference between the peaks shows the interaction strength and determines the
coupling constant (J).
Mynd 1. Segulvægisvektorar atómkjarna velta / spinna umhverfis ás í stefnu álagðs
segulsviðs. Veltingurinn / spuninn er ýmist í stefnu með(a) eða á móti (b) álagða
segulsviðinu. / Fig. 1 The nuclei spin (magnetic field) vectors rotate around an axis
parallel to the surrounding magnetic field. The average spin direction is either parallel
() or antiparallel() to the surrounding magnetic field.
Mynd 2. Orka atómkjarna með mismunandi spunastefnur verður ólík við álagningu
segulsviðs./ Fig. 2, The energies of  and  nuclei differ.
Mynd 3. Kúplun milli kjarna X og Y gerist með tilstuðlan víxlverkana kjarna við
rafeindir í efnatengjum. / Fig. 3, Coupling between nuclei occurs via interactions with
electrons in bonds between the nuclei.
INNGANGUR
A. Þegar sýni er sett í segulsvið taka atómkjarnar að spinna umhverfis ása samsíða álagða
segulsviðinu (Mynd 1; stefna segulsviðsins vísar upp á myndinni). Atómkjarnar með
spinnskammtatöluna I = 1/2 spinna ýmist í stefnu með ( á mynd 1;
ofanvarpsskammtatala, mI = +1/2) eða á móti ( á mynd 1; ofanvarpsskammtatala, mI = 1/2) viðkomandi segulsviði. Orka þessara kjarna er mismunandi vegna mismunandi
víxlverkunar segulvægja kjarnanna við álagða segulsviðið sem er háð innbyrðis afstöðu /
stefnu viðkomandi vektora í rúminu.  kjarnar eru orkuhærri sem nemur   B (eða gI
B) (sjá Mynd 2) þar sem  er "magnetogyric ratio" og gI er "g-stuðull" sem eru háðir
atómkjörnum.  (= h/(2)) og ("nuclear magneton") eru fastar. B er segulsviðið í
nánd við kjarnann.
Í NMR tæki á sér stað gleypni rafsegulbylgna á útvarpsbylgjusviðinu samfara tilfærslum
á kjörnum úr  í  þegar orka rafsegulbylgjunnar (E = h) er jöfn orkumuninum milli
skammtaþrepanna (E =   B) , þ.e.
h =   B
(1)
Segulsviðið sem rýkir í nánd við viðkomandi kjarna er háð nánasta umhverfi hans.
Kjarnar með mismunandi umhverfi (mismunandi B) m.t.t. afstöðu til annarra atóma í
sameind eða kjarnar sem verða fyrir mismunandi áhrifum segulsviðs frá
umhverfinu gleypa því rafsegulbylgjur af mismunandi tíðni () skv. jöfnu (1). Því koma
gleypitoppar eins kjarna með mismunandi umhverfi / mismunandi B, fram á mismunandi
stöðum á litrófsskala NMR rófs.
B. Þegar vetnistengi rofna eða myndast breytist nánasta umhverfi viðkomandi
vetniskjarna m.t.t. rafeindaþéttleika í nánd við kjarnann og NMR gleypitoppur hliðrast.
C. Sami atómkjarni sem ýmist kemur fyrir í sameindum hvarfefna eða myndefna í
jafnvægisefnahvörfum getur sýnt tvo mismunandi gleypitoppa vegna mismunandi
umhverfis í hvarfefni og myndefni.
D. Nærliggjandi kjarni (X á Mynd 3) getur víxlverkað við næstu rafeind í efnatengi
(Fermi víxlverkun) þannig að spunastefna hennar ræðst af spunastefnu kjarnans.
Önnur rafeind í sama sameindasvigrúmi efnatengisins hefur andstæða spunastefnu við
fyrrgreindu rafeindina (skv. Pauli víxlverkun). Sú rafeind getur orsakað segulsvið í nánd
við annan kjarna (Y á Mynd 3). Það segulsvið ræðst því í reynd af spunastefnu
nágrannakjarnans (X) og segja má að "boð um spunastefnu kjarna X berist til kjarna Y" í
gegnum rafeindir efnatengisins. Segulsviðið í nánd við Y er því mismunandi, háð
spunastefnu X. Y sýnir því tvo gleypitoppa. Umrædd hrif svara til kúplunar milli
kjarnanna X og Y. Bil milli viðkomandi toppa (í Hz) er mælikvarði á styrkleika
kúplunarinnar (kúplingsfasti, J).
PROCEDURE:
Detailed FT-NMR analysis (1H-NMR, 13C-NMR, 2D-NMR, FT …- measurements,
analysis and spectra simulations (gNMR)) will be performed for one of the following
compounds:
Three or two spin systems: /
/ Explain (assign) spectra. Determine couplings constants and chemical shift values from
the 1H-NMR spectra (i.e. 1st order approximation). Use your 1st order approximation
values to calculate 1H-NMR spectra by use of g-NMR and compare calculated and
experimental spectra. Vary chemical shift and/or couplings constant parameters in your
calculation procedure until you obtain satisfactory spectra fits.
See detaild instructions relevant to
a) – calculations of NMR spectra:
http://www.hi.is/~agust/NMR/gNMR/instr.ppt
b) – transfer of calculated spectra by gNMR) to IGOR (to compare with
experimental spectra):
http://www3.hi.is/~agust/kennsla/ee09/vee09/VEE-NMR-09/calc-to-IGOR.ppt
NB: You might (?) be able to transfer your experimental spectra to gNMR, analogous
to the procedure for transferring 60MHZ spectra from the teaching lab (VRI)
described in http://www3.hi.is/~agust/kennsla/ee09/vee09/VEE-NMR-09/gNMR_leidbeiningar-09ksk.doc
Shorter instructions for (a)-(b) (above):
a)
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b)
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Use of the program gNMR to calculate NMR spectrum. Example: etanol: CH3CH2-OH; see: http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi
NB: Demo version of gNMR can be downloded from the web:
http://www.adeptscience.co.uk/products/lab/gnmr/. Alternatively an old PCversion can be copied via ÁK. gNMR is to be found on PC computers in VRI-121
Go to folder (gNMR) which includes gNMR program
Double click gNMR
Press “Run”
Press ”Start with an empty molecule”:
Type in appropriate values for ethanol: For example as:
No
protone n
1
CH3
3
2
OH
1
3
CH2
2
Assign. Shift(ppm)
A 3.687 B 2.61 C 1.226
Press “Spectrum”:
For example: Expand CH2 spectrum: move curse on left side of CH2 spectrum.
Click and drag to the right side of the CH2 spectrum. Release and click
Right click
For example: Integrate
Instructions about how to transfer a calculated NMR spectrum (calculated with
gNMR) to IGOR: for example for comparison with experimental spectra or for
general manipulation of the data.
Inside gNMR: Settings -> File..
Make sure the spectrometer Frequency is correct, e.g. 60 MHz (see File settings)
File-> Export -> As ASCII..
File name, for example: “ediksyra.txt” -> Save (in apropriate place)
See file: double click on “ediksyra.txt”
Insert file into IGOR
Open IGOR
Inside IGOR: Data -> Load Waves -> Load General Text...
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Open “ediksyra.txt”
Load
Display wave1 (or name of array)
Now x-axis needs to be corrected (see information in txt file above), i.e.:
here Total sweep width is 221.925 Hz (according to information in data file)
According to display of wave1 in table:Here total number of points are 2000.
Make new wave for x-axis
Table -> Append Columns to Table...
Select wave2
Do it
wave2 = (-221.925/2000)*x+221.925 , i.e.:
display wave1 vs wave2
Double click on x-axis scale
Add Min Value = 221.925; Max Value: 0
Do it
Scale is now / Hz scale