International Journal of PharmTech ResearchCODEN (USA): IJPRIF ISSN : 0974-4304

Vol.3, No.3,pp 1410-1422, July-Sept 2011

Heteronuclear Multible Bond CorrelationSpectroscopy- An Overview

Y.Vasavi1, N.Parthiban1, D.Sathis Kumar*1, David Banji1 ,N.Srisutherson1,Somsubhara Ghosh1

, M. Vinay Kumar Chakravarthy2.

1Department of Pharmaceutical Analysis,Nalanda College of Pharmacy,Nalgonda, Andhra Pradesh -508001,India.

2JNT University, Hyderabad, India.

*Corres. author: satmpdina@yahoo.co.inMobile: +919966796051

Abstract: HMBC is one of the two dimensional experiments. Two-dimensional nuclear magnetic resonance (2DNMR) provides one of the foremost contemporary tools available for the elucidation of molecular structure, function,and dynamics. HMBC is a long range correlation experiment which provides information about carbons bonded toproton which are 2-3 bonds away. HMBC has applications in every field for identifying and characterising the structureof compound. In the present review, we discussed the following, the HMBC process, its advantages over 1D NMRtechniques and comparisons with HETCOR and HMQC.Key words: Heteronuclear ,Multible Bond Correlation Spectroscopy.

Introduction:Two dimensional NMR spectroscopy can be defined asa spectral method in which the data are collected intwo different time domains: acquisition of the freeinduction decay, FID (t2), and a successivelyincremented delay (t1) to elapse before detection, thenduring this time interval (the evolution period) thenuclei can be made to interact with each other invarious ways, depending on pulse sequencesapplied.[1,2] The resulting FID is accordingly subjectedto two successive sets of Fourier transformations tofurnish a two-dimensional NMR spectrum in twofrequency axes. The major difference between one-and two- dimensional NMR methods is therefore theinsertion of an evolution time, t1. [1] For 20 years centrestage has been occupied by two-dimensional (and now

three and four-dimensional) NMR techniques. 2DNMR offers distinct advantages like relief fromovercrowding of resonance lines, as the spectralinformation is spread out in a plane or a cube ratherthan along a single frequency dimension, opportunityto correlate pairs of resonances, has features incommon with various double resonance methods andis far more efficient and versatile. [3]. A convenient wayto understand the modern 2D NMR experiment is interms of magnetisation vectors. A /2 pulse is thatwhich rotates the magnetisation vector by 900. That isthe net magnetisation vector rotating about the z-axisis now rotating in xy plane. A 900 pulse gives thestrongest signal, as the signal y-axis component is onlydetected. The figure 1 below explains this.

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Figure 1: Classical picture of a pulsed NMR experiment. Relation between précising moment (top andcentre) and the observed transverse component of the magnetization as a function of time (bottom) [ref:Affa-ur-rahman, Muhammad chaudary, Solving problems with NMR spectroscopy, Pakistan, 1991,Academic Press.]

Data are taken as a function of a single time parameter, and the relation between these data and the frequencyspectrum is Fourier transform relation.[2] The resulting time domain signal is changed to frequency domain signalby some mathematical calculations (Fourier transformations). This is represented in the following figure 2. [4]

Figure 2: The FID time signal (A) and resultant 1H frequency spectrum (B) for asingle off-resonance peak. [ref: John C.Lindon, 2000.]

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Basic principle:2D NMR essentially allows us to irradiate all of thechemical shifts in one experiment and gives us amatrix or two dimensional maps of all of the affectednuclei. There are four steps to any 2D experiment[5].Preparation, evolution, mixing and detection. [6, 7]

Preparation is usually just a 90o pulse which excites allof the sample nuclei simultaneously. Duringpreparation nucleus, A (when two nuclei A and B areconsidered) gets excited creating magnetisation in x-yplane. During evolution, chemical shift of the nucleusA is measured. [8] During mixing, transfer ofmagnetisation from nucleus A to the nucleus B via J orNOE. Detection is simply recording an FID andfinding the frequency of nucleus B by Fouriertransformation. All possible pairs of nuclei in thesample go through this process at the same time. Toget a second dimension, the chemical shift of nucleusA have to be measured before it passes its

magnetization to nucleus, B. This is accomplished bysimply waiting a period of time (called t1, theevolution period) and letting the nucleus, Amagnetization rotate in the x-y plane. The experimentis repeated many times over (for example, 512 times),recording the FID each time with the delay time t1incremented by a fixed amount. The time course of thenucleus, A magnetization as a function of t1 is used todefine how fast it rotates and thus its chemical shift.Mixing is a combination of RF pulses and/or delayperiods which induce the magnetization to jump fromA to B as a result of either a J coupling or an NOEinteraction. Different 2D experiments (e.g., NOESY,COSY, HETCOR, etc.) differ primarily in the mixingsequence, since in each one we are trying to define therelationship between A and B within the molecule in adifferent way.[9]

The general pulse sequence for the two dimensionalexperiment is as follows. [1]

Figure 3: 2D NMR pulse experiment. Time axis is divided into preparation andan evolution period, t1. [ref: Affa-ur-rahman, University of Karachi, Karachi, Pakistan andMuhammad chaudary, 1991.].

The actual view of 2D NMR plot can be seen below in figure 4. [3]

Figure 4: Data from 2D pulse sequence. Spectraresulting from a) Fourier transformation withrespect to t2 axis at various t1, (b) Rotation of axesin (a) to show modulation in signal amplitude as afunction of t1. (c) Signal after second Fouriertransform with respect to t1 showing a single peakat coordinates, (d) Contour plot, showing the peakfrom (c) [ref: Edwin D. Becker,2000]

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2D NMR classification:2D NMR studies can be classified into HomonuclearCorrelation studies and hetero nuclear correlationstudies. Homonuclear correlation studies are donebetween similar nuclei (like proton-proton correlation,etc). This can be further classified depending onwhether the correlation (magnetisation transfer) isthrough bond or through space into two types. 2DCOSY and 2D TOCSY are through bond and NOESYand ROESY are through space. Coming toheteronuclear correlation studies, magnetisationtransfer is between two dissimilar nuclei, may bebetween C13 and H1 or N15 and H1, etc. This is againclassified based on detection methods. Determinationis direct in HETCOR and LR-HETCOR, while thedetermination is indirect/ inverse in HMBC, HMQC,and HSQC. Other two types are J-Resolved correlationwhere correlation studies are based on couplingconstants (may be homo-J or hetero-J) and multiplequantum correlation which includes INADEQUATE,DQFCOSY.COSY (Correlation Spectroscopy) is good fordetermining basic connectivity via J-couplings(through bond). TOCSY (Total CorrelationSpectroscopy) same as COSY, but is also able togenerate cross peaks via intermediate spins. This usesa spin lock that produces RF heating of the sample andhence requires many steady state scans (ss). This isalso called as HOHAHA (HOmonuclear-HArtmann-HAhn spectroscopy). NOESY (Nuclear OverhauserEffect Spectroscopy) allows one to see through-spaceeffects, and is useful for investigating conformationand for determining proximity of adjacent spinsystems. This is not so useful for MWs in the 1 kDa(kilo Dalton) range due to problems arising from theNMR correlation time. Here longitudinal exchange ofmagnetisation between nuclei takes place. ROESY(Rotational Overhauser Effect Spectroscopy) is sameas NOESY, but works for all molecular weights. Heretransverse exchange of magnetisation between nucleitakes place[10]. HMQC (Heteronuclear MultipleQuantum Correlation) allows one to pair NH or CHresonances. It often uses X-nucleus decoupling andhence gives rise to RF heating, requires reasonablywell calibrated pulses and many steady state scans.Only protons directly bonded to C13 nuclei producecross peaks. HSQC (Heteronuclear Single QuantumCorrelation) provides the same information as HMQC,but gives narrower resonances for 1H-13C correlations.This also requires X-decoupling and hence a largenumber of steady state scans and is also more sensitiveto pulse imperfections. HMBC (HeteronuclearMultiple Bond Correlation) is a variant of the HMQCpulse sequence that allows one to correlate X-nucleus

shifts that are typically 2-4 bonds away from aproton.[9,11] This is Popular among the NMRspectroscopists in organic labs/ Parma industries. Forsmall molecules generally homonuclear 2D techniquesare sufficient for structure elucidation. However, whenthere is extensive overlap even in the 2D spectrum ithelps to do heteronuclear correlation experiments. It isa technique which helps to determine which 1H of amolecule is bonded to which X nucleus in themolecule. Transfer of magnetization takes placebetween nuclei of different types. The two axes showthe chemical shift of the respective type of nucleus. Ifa transfer has taken place (due to coupling), a signalappears at the intersection of the two frequencies. Thefirst step in processing a 2D dataset is to Fouriertransform each of the FIDs in the array. The resultingspectra are loaded into a matrix with the rowsrepresenting individual spectra. The horizontal axis islabelled F2 and the vertical axis is labelled F1

[7]. Table1 expressed different types of correlation spectroscopyand its differentiation.

Table1: Types of correlation spectroscopyName F1

nucleusMixing F2

nucleusCOSY 1H1 J 1HTOCSY 1H J,J,J.. 1HNOESY 1H NOE 1HHETCOR 1H 1JCH 13CHMQC 13C 1JCH 1HHMBC 13C 2,3JCH 1H

HMBC:HMBC (Heteronuclear Multiple Bond Correlation)spectroscopy is a modified version of HMQC suitablefor determining long-range 1H-13C connectivities.Since it is a long-range chemical shift correlationexperiment, HMBC provides the information about thechemical shift of carbon atoms that are about 2-3bonds away from the proton to which they correlate.Hence also quaternary carbon atoms will be detected.[8] 1D protein spectra are far too complex forinterpretation as most of the signals overlap heavily.By the introduction of additional spectral dimensionsthese spectra are simplified and some extrainformation is obtained. The invention ofmultidimensional spectra was the major leap in NMRspectroscopy apart from the introduction of FT-NMR.Consequently, both techniques were acknowledged bya Nobel Prize.

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Figure 5: The pulse sequence for the HMBC experiment. [ref: Affa-ur-rahman,Muhammad chaudary, 1991].

Process and pulse sequence:If the delays Δ1, Δ2 are adjusted so they correspond toJCH of 130-150 Hz, then only the direct one bondcouplings between sp2 and sp3 hybridised C13-H1 bondswill be seen. If the value of delay is adjusted tocorrespond to JCH of 5-10 Hz, long range couplings canbe seen. These cross peaks will then correspond tocoupling interactions between H1 and C13 nucleiseparated by 2, 3 even 4 bonds. In such a long rangeheteroscopy experiment, the one bond magnetisationtransfer is suppressed and long range heteronuclearcouplings observed. [1] The figure 5 explains the pulsesequence for HMBC experiment for both proton andcarbon irradiation. First 90 pulse of C13 nucleusoccurs at 1/(2 1JXH) after the first 1H 90° pulse, servesas a low-pass J-filter to suppress one-bond correlationsin the 2D spectrum. It does this by creatingheteronuclear multiple quantum coherence for 1H’sdirectly coupled to a 13C nucleus, and this unwantedcoherence is removed from the 2D spectrum by phasecycling the first 13C 90° pulse with respect to thereceiver.[12] Hence cross peaks due to directconnectivities do not appear, allowing long range C13-H1 connectivities to be recorded. Second 90 Ψ C13

pulse creates the desired zero and double quantumcoherences for 1H’s J-coupled to a 13C nucleus 2 or 3bonds away. This is followed by evolution time. Dueto the 180 x H1 pulse, placed halfway through t1, theabove created zero and double quantum coherences areinterchanged (removes the effect of 1H chemical shiftfrom the t1 modulation frequency). The final 13C 90°pulse occurs directly after the evolution period, and isfollowed immediately by the detection period t2. Afterthe last 90 C13 pulse, the H1 signals resulting fromC13-H1 multiple-quantum coherence are modulated byC13 chemical shifts and homonuclear protoncouplings.[1,13] Because of phase modulation, the finalspectrum has peaks which are a combination of

absorption and dispersion line shapes. It is not possibleto phase correct the spectrum so that the peaks arepurely absorptive, and so the spectrum must bepresented in magnitude mode. If more than one long-range 1H-13C connectivity is detected for one particularproton, the relative intensities of the correspondingresonances are directly related to the magnitude of thecoupling constant[12]. The difference between the twohetero nuclear correlation experiments, HETCOR andHMBC is that in HETCOR F1 is proton domain and F2is C13 domain. This can be explained as follows. It iseasier to obtain a higher digital resolution in 2Dexperiments in the F2 domain than in the F1 domain,since doubling the acquisition time t2 results in littleoverall increase in the experiment time. This is sobecause the increase can be compensated bydecreasing the relaxation delay between successiveexperiments. However, if the maximum time reachedduring t1, is doubled, the number of t1 experimentsshould be doubled, there by not only increasing thetotal time taken for the experiment but also suffering aloss of signal due to relaxation during t1. This problemis avoided in inverse experiment, because C-Hcorrelation is detected via the proton spectrum and thecrowded proton region now lies in the F2 dimension,which has a high digital resolution, where as the betterdispersed C13 spectrum, lies in F1 dimension, whichrequires a lower digital resolution. [13]

The HMBC experiment, like the 2D COLOC(correlation spectroscopy for long range coupling)experiment, is particularly useful for locating thequaternary carbons by identifying various protonsinteracting with them through two-bond, three-bondoccasionally four-bond coupling interactions. [1]

However some common pitfalls and artefacts that maylead to unsatisfactory results. For 2D experiments,spinning should always be turned off, as the spinningmay introduce artefacts leading to criss-cross noise in

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the spectra. Note the adjusting of the z and eventuallyalso the z2 shim, if sample rotation is stopped.Therefore it is best to run already the preparatory 1Dexperiment without spinning. If the acquisition time isnot long enough or the number of increments is toosmall, the echo signal will be clipped. This may lead tounreliable peaks in the spectrum, while the real longrange peaks may be very weak or missing. The numberof scans and dummy scans needs to be a multiple ofthe minimum given in the pulse program. If that is notthe case, the phase cycle is not completed which leadto auxiliary diagonals may parallel to the real diagonal.[8]

Acquisition and processing: [11]

The preliminary steps include the following. Insert thesample in the magnet. Lock the spectrometer. Re-adjust the Z and Z2 shims until the lock level isoptimized. Tune and match the probe head for 1Hobservation 13C decoupling. Reference spectrum forboth H1 and C13 should be run separately[14].

H1 reference spectrum: Since HMBC is a 1H-observeexperiment, the first step is to obtain a reference 1H

spectrum of the sample. This reference spectrum willbe used to determine the correct parameters for 1H andfor the F2 dimension, and can also be used as the F2projection of the HMBC spectrum. [11]

13C reference spectrumIt can be assumed that the sample used for an inverseexperiment such as HMBC has too small a 13C signalto make it practical to obtain a 13C reference spectrum.Thus, the user will need to make an educated guess asto the appropriate values of parameters for the F1dimension. Note that, because HMBC is a multiplebond correlation experiment, one can expect to detectsignals from 1H’s coupled to quaternary 13C’s, inaddition to primary, secondary and tertiary 13C’s. Thus,the 13C spectral width should be larger than that usedfor HMQC. Then a series of steps should be followedlike create a new file directory for the 2D data set,change to 2D parameter mode, Set up the acquisitionparameters, Acquire the 2D data set, Set up theprocessing parameters, Process the 2D data set, Adjustthe contour levels, Phase correct the spectrum, Plot thespectrum, and finally interpretation.[11]

Interpretation of HMBC spectra: Let us take a simple spectrum and understand its interpretation,

HMQC and HMBC of

1. Ipsenol interpretation [15]

Figure6: structure of ipsenol. (Ref: Robert M. Silverstein, Francis X. Webster, David J.Kiemle, 2005).

D.Sathis Kumar et al /Int.J. PharmTech Res.2011,3(3) 1416

In HMBC spectrum of ipsenol, the ellipse and diamond shaped area represents absence of direct bond coupling of3rd and 5th carbon with corresponding protons.

Figure 7: DQFCOSY spectrum of ipsenol. (Ref: Robert M. Silverstein, Francis X. Webster,David J.Kiemle, 2005).

DQF COSY HETCOR HMQC HMBCA homo nuclearcorelationspectroscopy whereplot is between twoprotons moleculeswhich are attacheddirectly or a littleaway.

A heteronuclearcorelation spectroscopywhere plot is betweeentwo dis similar nucleiwhich are directlyattahced to each other.

A highly resolvedinverse detected heteronuclear correlationspectroscopy whereplot is between twodissimilar nucleiattached to each other(single bond coupling).

A highly resolved inverse detectedheteronuclear correlationspectrosocpy where plot is betweentwo dissimilar nuclei attached atlonger distance(2 or 3 bondscouplings).

Proton of 3rd carbonshows 3 cross peaks,two for the protonsattached attached toprotons of 3rd carbonand one for protonattached to 5th carbon.

3rd carbon shows twocross peaks whichrepresent protons attachedto 3rd carbon directly.

3rd carbon shows twocross peaks whichrepresent protonsdirectly coupled to 3rd

carbon through singlebond.

3rd carbon shows five cross peakswhich represent 2 or 3 bondcouplings of 3rd carbon with variousprotons.2JCH – cross peak for coupling of 3rd

carbon with proton of 2nd and protonof 4th carbon.3JCH – cross peak for coupling of 3rd

carbon with proton of 1st and protonof 4th carbon.

Proton of 4th carbonshows five crosspeaks, for the protonsattached to 3rd and 5th

carbons (doublets), 4th

carbon (singlet).

4th carbon shows a singlecross peak whichrepresents correlationbetween 4th carbon andits protons.

4th carbon shows asingle peak which aredirectly bonded to it.

4th carbon shows five cross peaks,2JCH- cross peaks for coupling withproton of 3rd and 5th carbon.3JCH – cross peak for coupling withproton of2nd carbon.

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Figure 8: HETCOR spectrum of ipsenol. (Ref: Robert M. Silverstein, Francis X. Webster,David J.Kiemle, 2005).

Figure 9: HMQC spectrum of Ipsenol (Ref: Robert M. Silverstein,Francis X. Webster, David J.Kiemle, 2005).

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Figure 10: HMBC spectrum of Ipsenol (Ref: Robert M. Silverstein,Francis X. Webster, David J.Kiemle, 2005).

Thus the spectral differences for a compound are thatthere cannot be direct coupling peaks in HMBCspectrum which can only be seen in HMQC spectrum.And also for a quaternary carbon there occurs no crosspeaks in HMQC spectrum while through long range

coupling cross peaks can be seen in HMBC spectrum.This is shown in spectrum of Ipsenol by differentmarkings.

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2. Thymol interpretation: [15]

Figure 11: structure of Thymol (Ref: Robert M. Silverstein, Francis X. Webster, David J.Kiemle, 2005).

Figure 12: DQF COSY spectrum of Thymol compound. (Ref: Robert M. Silverstein,Francis X. Webster, David J.Kiemle, 2005).

D.Sathis Kumar et al /Int.J. PharmTech Res.2011,3(3) 1420

Figure 13: HMQC spectrum of Thymol. (Ref: Robert M. Silverstein,Francis X. Webster, David J.Kiemle, 2005).

As described in the previous example, lines weredrawn parallel to proton axis and interpreted. But nowunlike previous example, spectrum is studied withreference to proton spectrum here. The proton attachedto seventh carbon shows a single cross peak in HMQC

spectrum, as that is the only possibility. Similarlysingle cross peaks were observed for 3, 4, 6 and 10carbons. The following figure 11 shows HMBCspectrum of the same compound.

Figure 14: HMBC spectrum of Thymol. (Ref: Robert M. Silverstein, Francis X. Webster,David J.Kiemle, 2005).

D.Sathis Kumar et al /Int.J. PharmTech Res.2011,3(3) 1421

Considering the same seventh proton, in HMBCspectrum, this shows four cross peaks of which none isshowing directly coupled carbon. This proton has threecarbons in alpha position and two carbons in betaposition. As 8, 9 carbons are equivalent they showsingle cross peak in the spectrum, thus this proton hastwo non equivalent carbons in alpha position (8, 9, and2) and two in beta position (1, 3). HMBC spectrum hasno cross peak in seventh position. Similar studiesshould be done for every proton to understand itsenvironment and its correlation with remaining carbonatoms. Consider the quaternary carbons 1, 2, 5; theyhave no cross peaks in HMQC while many cross peakscan be seen in HMBC spectrum. By comparing thesetwo spectrum, one can conclude that 1, 2, 5 carbonshave no attached protons (or quaternary carbons). Forelucidating the structure of a compound completely allthe spectrum needed is 1D proton spectrum, 1D C13

spectrum, COSY, DQF-COSY, HETCOR, HMQC,and HMBC.

Applications:1. Microstructure determination of poly

(acrylonitrile-co-methyl methacrylate-co-methylacrylate) terpolymers by 2D HMBC:[16]

2. Recent progress in heteronuclear long-rangeNMR of complex carbohydrates: 3D H2BC andclean HMBC:[17]

3. Use of heteronuclear multiple bond coherenceNMR spectroscopy to monitor nitrogenmetabolism in a transformed root cultureof Datura stramonium:[18]

4. Valproic acid intoxication is identified by H1 –C13 correlated NMR spectroscopy from urinesamples.[19]

5. Using HMBC studies on cholesteryl acetate,cyclosporine and isopropylidene glycerol,structures have been elucidated.[20]

6. 1H–15N HMBC as a valuable tool for theidentification and characterization of nitrones. [21]

7. Use of 15N-HMBC NMR techniques to determinethe orientation of the steroidal units in ritterazineA1[22]

8. Determination of a symmetrical dimer structure inbenzo[c]phenanthridine alkaloids by pulsed-field-gradient HMBC[23]

9. Differentiation of HMBC two- and three-bondcorrelations: A method to simplify the structuredetermination of natural products [24]

10. Variable temperature gradient 1H, 1H–13C GE-HSQC and GE-HMBC NMR studies of flavonolsand flavones in organic and aqueous mixtures[25]

11. Structural investigation of poly (methyl acrylate)by 2D HMBC NMR.[26]

Conclusion:For the structural elucidation of a compound, a simple1D NMR (H1 and C13) is not sufficient. Advancedtechniques like correlation spectroscopic methods,when understood properly give the perfect way todetermine the structure for a given formula. Thecombination of all the spectra DQFCOSY, DEPT,HETCOR, HMQC, HMBC are all very useful inidentification and characterization of the structure, andorientation of bonds in a molecule. By using HMBCstudies, large molecules can also be elucidated easily.

References:1. Available at http://ebookee.org/Solving-

Problems-with-NMR-Spectroscopy-Repost-_691136.html Retrieved on 06-03-2011.

2. Bernard C.Gerstein, Encyclopedia of physicalScience and Technology, Analytical chemistry,Iowa State University, 701-720.

3. Edwin D. Becker, High resolution NMR - Theoryand Chemical Applications, Academic Press,Maryland, 2000, 251-277.

4. John C.Lindon, George E. Trater, John L.Holmes, Encyclopedia of spectroscopy andspectrometry, Academic Press, 2000, 1545-1567.

5. Available athttp://chemistry.uah.edu/Faculty/vogler/CH421/ch421-nmr-part3.pdf on retrieved on 28-11-2010.

6. Lucio Frydman, Adonis Lupulescu and TaliScherf, Principles and Features of Single-ScanTwo-Dimensional NMR Spectroscopy, Journal ofAmerican chemical society, 2003, 10, 109-116.

7. Available athttp://www.chem.arizona.edu/facilities/nmr/manuals/gen2d.pdf Retrieved on 19-02-2011.

8. Sandra Loss, Till Kühn, Bruker Biospin AG,Fällanden, Introduction to 1- and 2- dimensionalNMR, 2005, 1.

9. Availableathttp://web.mit.edu/speclab/www/Facility/TIPS/2D%20NMR%20Quick%20Overview.pdfRetrieved on 27-11-2010.

10. Available at http://nmr.chem.umn.edu/2Dnew050211.pdf retrieved on 28-11-2010.

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11. Available athttp://www.uwyo.edu/wheelernmr/NMR/avanceuser/hmbc.pdf retrieved on 27-11-2010.

12. Willliam D. Wheeler, Two DimensionalHomonuclear Corelation Spectroscopy,University of Wyoming, 1999.

13. Ad bax, Kathleen A. Farley, and Gregory S.Walker, Increased HMBC Sensitivity forCorrelating Poorly Resolved Proton Multiplets toCarbon-13 Using Selective or Semi-selectivePulses, Journal of magnetic resonance,1996, 119,134–138.

14. 2D NMR Spectroscopy Quick Overview,available athttp://web.mit.edu/speclab/www/Facility/TIPS/2D%20NMR%20Quick%20Overview.pdfretrieved on 27-11-2010.

15. Robert M. Silverstein, Francis X. Webster, DavidJ. Kiemle, Spectrometric identification of organiccompounds, John Wiley and Sons , 2005, 258 –372.

16. Sunita Hooda, Ashok Kumar Goyal and A.S.Brar, Journal of Molecular Structure, 2009, 920,2274-227.

17. Sebastian Meier, Bent O. Petersen, Jens O.Duus and Ole W. Sorensen, CarbohydrateResearch, 2009, 16, 2274-227.

18. Ophelie Fliniaux , Francois Mesnard, SophieRaynaud, Sylvie Baltora, Richard J. Robins andMarc-Andre Fliniaux,, Comptes Rendus del’Academie des Sciences Chemistry, 2001, 10,775-778.

19. Azarowal. N, Imbenotle M, Cartigny. B,Leclerc. F, Vallee. L, Lhermitte. M, Veneersch.G, Magnetic Resonance Materials in physics,biology, medicine,2000, 10, 177-182.

20. Furrer. J, Chem Commun (Camb) - A robust,sensitive, and versatile HMBC experiment forrapid structure elucidation by NMR: IMPACT-HMBC, 2010, 19, 3396-8.

21. Ignacio Delso and Tomas Tejero, 1H–15N HMBC as a valuable tool for theidentification and characterization of nitrones,Tetrahedron Letters, 2007, 23, 4101-410.

22. Seketsu Fukuzawa, Shigeki Matsunaga andNobuhiro Fusetani, Use of 15N-HMBC NMRtechniques to determine the orientation of thesteroidal units in ritterazine A1, TetrahedronLetters, 1996, 9, 1447-1448.

23. Radek Marek, Vladimir Sklenar Ji, Dostal i andJi, Slavik i, Determination of a symmetrical dimerstructure in benzo[c]phenanthridine alkaloids bypulsed-field-gradient HMBC, TetrahedronLetters, 1996, 10, 1665-1658.

24. Matthias Kock, Bernd Reif, William Fenical andChristian Griesinger, Differentiationof HMBC two- and three-bond correlations: Amethod to simplify the structure determination ofnatural products, Tetrahedron Letters,1996, 37,363-366.

25. Vassiliki Exarchoua, Anastasios Troganisb,Ioannis P. Gerothanassis, Maria Tsimidouc andDimitrios Boskouc, Do strong intramolecularhydrogen bonds persist in aqueous solution?Variable temperature gradient 1H, 1H–13C GE-HSQC and GE-HMBC NMR studies of flavonolsand flavones in organic and aqueous mixtures,Tetrahedron letters, 2002, 37, 7423-7429.

26. Brar A. S., Ashok Kumar Goyal and SunitaHooda, Structural investigation of poly (methylacrylate) by 2D HMBC NMR, Journal ofMolecular Structure, 2008,885, 1-3.

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