Class Notes (809,509)
Canada (493,754)
Chemistry (602)
CHEM 205 (62)
All (12)

Spectroscopy Lecture 21

8 Pages
Unlock Document

University of British Columbia
CHEM 205

CHEM 205: more on NMR: J coupling and Splitting of signals • Why does 1 signal split? o Described by N+1 rule where N is the number of equivalent H bonded to an adjacent carbon o Due to magnetic nuclei nearby that exert internal B field • For lecture, will be focusing on vicinal coupling of freely rotating structures (sp3 hybridized) • Simplest example: AX spin system • o Notice for this system:  Each signal is split into 2, since the possibility of the neighboring H is only spin up or down. The two spins are equal and opposite, thus producing 2 split peaks of equal intensity. Hx is to the left due to the more electronegative chlorine which causes increased shift.  Also notice that the frequency distance in Hz between the peaks in each doublet is called the spin-spin coupling constant J. In H NMR, J is 8Hz in alkyl groups. • J-coupling values are constants (since magnetic fields within the molecule are constant) depending ONLY on the molecule and independent of B unlike larmor frequencies o However when converted to chemical shift, it will depend. o AX2 spin system  Eg: • Splitting is not seen when proton is rapidly exchanged (eg. H in –OH group while in solution) • Back to the N+1 rule and finding the relative intensities of split lines. Can use pascal’s triangle. Note, max N=9 for CH(CH3)3 • Other notes: o Protons don’t split if they are separated by more than 3 sigma bonds o Labile Hs can exchange with solvent which cause line broadening and no splitting • Take a look at notes on notability for actual examples of identifying NMR (Lec 21 slides) • The scenario gets trickier when involving fixed, planar functional groups such as aromatic rings o Eg. For toluene, the H bound to the aromatic ring have high chem shifts (as discussed last lecture) and strong J coupling between those hydrogens. But splitting disappears and concentrated into single tight multiplet since their chem shifts are quite close to each other. • When sketching NMR of a given molecule o Determine the number of signals (how many non-equivalent H present?) o Determine chem shift values of these Hs from the table o De
More Less

Related notes for CHEM 205

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.