2D NMR Spectroscopy

  • View
    217

  • Download
    0

Embed Size (px)

Text of 2D NMR Spectroscopy

  • 8/22/2019 2D NMR Spectroscopy

    1/21

    11/30/20

    Two Dimensional (2D) NMRS ectrosco

    Correlation NMR

    The two important parameters obtained from NMR spectraare;

    a. Chemical shiftb. Spin-spin coupling constant

    Large molecules with numerous atoms nuclear magnetic

    fundamental parameters easily.

    Some 1D spectra are far too complex for interpretationbecause signals overlap heavily e.g. cholesterols, proteinspectra

    ntensity

    Chemical shift, ppm

    I

    1D spectrum of a protein

  • 8/22/2019 2D NMR Spectroscopy

    2/21

    11/30/20

    Nonequivalent proton groups can have nearly the samechemical shift and/or complex splitting patterns making1H NMR spectra complicated even for relatively simplemolecules.

    The introduction of additional spectral dimensions simplifies thespectra and provides more information.

    Two-dimensional (2D) NMR techniques can be used to solvesuch sophisticated structural problems.

    2-D spectra simplify the complexity arising from overlappingof peaks.

    David E. Alonso* and Steven E. Warren, NMR Analysis of Unknowns: An Introduction to 2D NMR

    Spectroscopy, J ournal of Chemical E ducation 82,1385 (2005)

    Simplification of NMR spectra makes their interpretationeasier and sometimes the only way possible.

    The interaction of nuclear spins (1H with 1H, 1H with 13C, etc.)are plotted in two dimensions

    Examples:

    COSY information concerning coupled (homonuclear) systems.

    HETCOR, HMBC connectivity between protons and carbons.

    NOSEY and ROSEY configuration of a molecule.

    INADEQUATE constitution of a molecule without 1H-NMR.

    Common Pulse sequences 1-D:

    1D-HNMR

    2 2t /T0 2M M sin(2 t )e

    1D-13C-NMRDecoupled

    1-D spectra are plots of intensity vs a frequency(chemical shift). In 2-D spectra the intensity is plotted asa function of two frequencies, usually represented as F1and F2.

    There are two ways to present 2D spectra; stack plotsand contour plots.

    2-D spectra are presented usually as a contour plot,where, the intensity of the peaks are represented bycontour lines (recall topographical maps).

  • 8/22/2019 2D NMR Spectroscopy

    3/21

    11/30/20

    F1

    F2

    General Presentation of Correlation Spectra

    Two frequency axes. F1 and F2 are Fourier transformedfrequency axis from a time domain signal.

  • 8/22/2019 2D NMR Spectroscopy

    4/21

    11/30/20

    H-H Correlation Spectroscopy (COSY)

    In a COSY experiment, the chemical shift range of the protonspectrum is plotted on both axes.

    The Diagonal of a H-H COSY is its 1-D H-NMR spectrum !

    Each peak is specified by the two frequency co-ordinates(F1, F2). 2-D NMR spectra are always arranged so that theF2 co-ordinates of the peaks correspond to those found inthe normal 1-D spectrum. (Often 1-D spectrum is presentedon the horizontal F2 axis).

    F1 co-ordinates of the peaks also correspond to those of thenormal 1-D spectrum (1-D spectrum plotted on the F1 axis)in H-H COSY.

    HX

    HX

    H

    H

    Cl

    H

    X

    COSY Spectrum

    X

    F1X

    COSY spectrum of a molecule containing just one type

    of protons HX.

    F2

    HAHX

    HA

    Cl

    R2

    H

    R1

    R

    H

    R3

    A X

    COSY Spectrum

    X AA

    F1 HX

    X

    COSY spectrum of a hypothetical molecule containing just

    two protons, HA and HX, which are not coupled, is shown.

    F2

  • 8/22/2019 2D NMR Spectroscopy

    5/21

    11/30/20

    HAHX

    HA

    R3

    H

    R2

    H

    R1

    R

    A X

    COSY SpectrumJ AX

    X AA

    F1 HX

    X

    COSY spectrum of a hypothetical molecule containing justtwo types of protons, HA and HX, which are coupled is shown.

    COSY spectrum has some symmetry about the diagonal, F1=F2,which shown above.

    F2

    In 2-D spectra the idea of a multiplet consists of an array ofindividual peaks often forming of a square or rectangular outline.Multiplets form a square or rectangle with two vertices on thediagonal.

    HA

    X AA

    HX

    F1

    F2

    Diagonal multipletscentered around sameF1 and F2.

    Cross-peak multiplets

    centers around differentF1 and F2 co-ordinates.

    X

    In a homonuclear COSY spectrum, the presence of a cross-peakmultiplet F1 =A, F2 =X indicates that the two protons A and Xat chemical shifts A and X are scalar coupled.

    If there had been no coupling, their magnetizations would nothave given rise to off-diagonal peaks.

    COSY s ectrum shows which ai rs in a molecule are

    coupled (thro bond coupling, hence connectivity).

    Recognition of the preceding fact is the essence for theanalysis COSY spectra.

    From a single COSY spectrum it is possible to trace out thewhole coupling network in the molecule.

    H-H COSY

  • 8/22/2019 2D NMR Spectroscopy

    6/21

    11/30/20

    Preparation Evolution, t1 Detection, t2

    (/2)x (/2)x

    Mixing time

    Prototype pulse sequence 2D NMR Prototype pulse sequence 2D NMR

    1 2t /TM M sin(2 t e

    1 2 22t /T0

    t /T21M sin(2[M sin(2 t )e t e] )

    Data acquired at theend of the an acquisition(after acquisition pulse)is labeled with the timevariable labeled t .

    The generation of a 2D experiment:

    In addition to preparation and detection (done in the 1Dexperiment) the 2D experiment has an indirect evolutionand a mixing sequence, time t1.

    a. Do something with the nuclei (preparation)b. let them precess freely (evolution) t1. 1

    e. and detect the result (detection, of course).

    After preparation the spins can precess freely for a given time t1.During this time the magnetization is labeled with the chemical

    shift of the first nucleus.

    The basic 2D spectrum would involve repeating a multiplepulse 1D sequence with a systematic variation of the evolutionand mixing times, t1, and then plotting Fourier transformed FID.

    This generates two time domains, one of which, t2, is theac uisition time thata ears durin the ac uisition as usualand the other time domain originates from the variable delaypart, t1.

  • 8/22/2019 2D NMR Spectroscopy

    7/21

    11/30/20

    t1=t

    t1=2t

    O

    frequency data (FID) in one axis (f2, fromt2),

    t1=0t2

    t1

    A(t1)

    Time domain data in t1. It isperiodic; a pseudo FID created foreach of the frequencies in f2.

    t1

    t1

    f2 (t2)

    Decay not shown.

    Note all tops form one FID, ..

    t1

    O

    t1

    stacked plot

    Appearance:

    On the 2D-NMR spectra an additional chemical shift(homonuclear or heteronuclear) is recorded on the third axis.

    http://www-keeler.ch.cam.ac.uk/lectures/understanding/chapter_7.pdf

  • 8/22/2019 2D NMR Spectroscopy

    8/21

    11/30/20

    f2

    1

    Two dimensional FT yields the 2D spectrum with two

    In a real molecule where J coupling exist, during the mixingmagnetization can be transferred from one nucleus to asecond one. Mixing sequences utilize two mechanisms formagnetization transfer, namely scalar coupling or dipolarinteraction (NOE).

    requency axes. e spec rum s omonuc ear s gnas othe same isotope - say 1H, are detected) the spectrumwould have a characteristic symmetric topology.

    COSY:

    1D- double resonance experiment that is often used to findrelationships between protons, the protons are irradiated oneby one.

    COSY generates all information from a series of doubleresonance experiments in one output (2D spectrum).

    The pulse sequence for a COSY experiment contains avariable delay time as well as an acquisition time. Theexperiment is repeated with different and incremented delaytimes, and the data collected during the acquisition are storedin the computer. The value of the delay time is increased by

    regular, small intervals for each experiment, so that the datathat collected consist of a series of FIDs collected during theacquisition, each with a different value of delay time.

  • 8/22/2019 2D NMR Spectroscopy

    9/21

    11/30/20

    In the COSY experiment, the magnetization is transferred byscalar coupling.

    In the COSY spectrum of a molecule where all possibleoff-diagonal peaks are generated; the result is a completedescription of the coupling partners in a molecule.

    ome mes e coup ng e ween pro ons a are more anThree chemical bonds apart can be seen.

    Signals on the diagonal divides the spectrum in two equalhalves. Signals symmetrical to the diagonal called crosssignals (peaks).

    The diagonal results from contributions of the magnetizationthat has not been changed by the mixing sequence.

    The cross signals originate from nuclei that exchangedmagnetization during the mixing time. They indicate aninteraction of these two nuclei. The cross signals containthe information of 2D NMR spectra.

    time - time

    time - frequency

    0 200 400 600 800 1000t 2 pt s

    t2

    t1

    Pulegone

    frequency - frequency400 500

    f 2 pt s

    500 600 700 800 900f 2 pt s

    f2

    f2

    1

    f1

    http://tonga.usp.edu/gmoyna/NMR_E N/NMR_ lectures.html

    Contour plot

    f2

    f1

    http://tonga.usp.edu/gmoyna/NMR_EN/NMR_ lectures.html

  • 8/22/2019 2D NMR Spectroscopy

    10/21

    11/30/20

    1 2 3 4

    d s d-d d

    Stick diagram;

    1-H NMRSpectrum

    4 types of H

    1 2 3 4

    3

    4

    axis

    Each circle represents thecenter of the multiplet.

    1

    2

    1

    2 axis

    H4 H3H1

    H2 no coupling

    C3H8O; U =0

    CH3 CH2 CH2 OH

    peak label

    4 3 2 1

    2H 1H 2H 3H

    Pick multi