CHEM 2211 1st Edition Lecture 12 Outline of Last Lecture I II III Stereoisomers of cyclic compounds Meso Compounds are optically inactive How to Name Isomers with more than one Asymmetric Center Outline of Current Lecture I II III IV Brief intro to HNMR Shielding effects of NMR Number of Signals in HNMR Chemical Shif Current Lecture I II III A brief Intro to H NMR A H NMR stands for nuclear magnetic resonance and is used to identify the carbonhydrogen bonding of an organic molecule B All nuclei have a magnetic spin that respond differently to an applied magnetic field i Nuclei that align to the field are lower in energy and called spin state ii Those that align against the field and have higher energy are called spin state Shielding effects on the NMR A Shielding refers to the electron density around a hydrogen i A high electron density partly shields the nuclei from the applied magnetic field ii The amount of magnetic field the nucleus actually receives is called the effective magnetic field iii Nuclei in a dense electron environment experience diamagnetic shielding and also require a lower frequency to go into resonance iv 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 downfield Number of signals in HNMR A Chemically equivalent protons have the same environment B Each chemically equivalent set of protons produces separate signals in the HNMR spectrum 2 signals 1 signal 3 signals C In rigid molecules like pi bonded carbons or cyclic molecules two protons on the same molecule might not be equivalent Bromoethene chlorocyclobutane IV Chemical shif A 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 scale ii Chemical shif is calculated using the following formula distance downfield chem ical shift ppm TMS Hz operating frequency of spectrometer MHz B The scale is the scale most used for HNMR because it measures chemical shif independent of the operating frequency of the HNMR C Remember for chemical shif Protons in electron poor environments Deshielded protons Downfield High frequency Large values Type of proton Protons in electron dense environments Shielded protons Upfield Low frequency Small values ppm frequency Chemical Shif for HNMR Approximate Type of proton chemical shif ppm Approximate chemical shif ppm 0 85 2 5 4 1 20 2 5 4 1 55 3 4 1 7 4 4 5 2 1 1 5 4 2 3 2 5 2 4 4 7 3 3 6 5 8 4 7 9 0 10 5 3 10 12 5 8
View Full Document
Unlocking...