CHEM 2211 1st Edition Lecture 12 Outline of Last LectureI. Stereoisomers of cyclic compoundsII. Meso Compounds are optically inactiveIII. How to Name Isomers with more than one Asymmetric CenterOutline of Current LectureI. Brief intro to HNMRII. Shielding effects of NMRIII. Number of Signals in HNMRIV. Chemical ShifCurrent LectureI. A brief Intro to H NMRA. H NMR stands for nuclear magnetic resonance and is used to identify the carbon-hydrogen bonding of an organic moleculeB. All nuclei have a magnetic spin that respond differently to an applied magnetic fieldi. Nuclei that align to the field are lower in energy and called α-spin stateii. Those that align against the field and have higher energy are called β-spinstateII. Shielding effects on the NMRA. Shielding refers to the electron density around a hydrogeni. A high electron density partly shields the nuclei from the applied magnetic fieldii. The amount of magnetic field the nucleus actually receives is called the effective magnetic fieldiii. Nuclei in a dense electron environment experience diamagnetic shieldingand also require a lower frequency to go into resonanceiv. On the HNMR, the more shielded nuclei appear on the right hand side with the lower frequencies known as upfield and the least shielded appear on the lef hand side with the higher frequencies known as downfieldIII. Number of signals in HNMRA. Chemically equivalent protons have the same environmentB. Each chemically equivalent set of protons produces separate signals in the HNMRspectrum2 signals1 signal3 signalsC. In rigid molecules, like pi bonded carbons or cyclic molecules, two protons on thesame molecule might not be equivalentBromoethenechlorocyclobutaneIV. Chemical shifA. The chemical shif measures how far the signal is from the reference compound (generally tetramethylsilane (TMS))i. The reference compound is determined to be zero on the HNMR δ-scaleii. Chemical shif is calculated using the following formuladistance downfieldδ=chem ical shift(ppm)=¿ TMS(Hz)¿operating frequency of spectrometer (MHz)B. The δ-scale is the scale most used for HNMR because it measures chemical shif independent of the operating frequency of the HNMRC. Remember for chemical shif Protons in electron poor environmentsDeshielded protonsDownfieldHigh frequencyLarge δ valuesProtons in electron-dense environmentsShielded protonsUpfieldLow frequencySmall δ values δ ppm frequencyChemical Shif for HNMRType of protonApproximatechemical shif (ppm)Type of protonApproximatechemical shif (ppm)0.85 2.5-41.20 2.5-41.55 3-41.7 4-4.52.1 1.5-42.3 2-52.4 4-73.3 6.5-84.7 9.0-105.3
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