1 (426)

Prévia do material em texto

418 12MAGNETIC RESONANCE
coupling between H4 and H5 is the same as that between H6 and H5, there-
fore the test for magnetic equivalence is satis�ed: H4 and H6 are magnetically
equivalent.
Cl
H5
Cl
H4
Cl
H6
E12B.13(b) �e molecule SF−5 is square pyramidal.�e four equatorial �uorine atoms, Fe,
are related by a 90○ rotation about the Fa–S bond: these four atoms are therefore
chemically equivalent.�e axial �uorine atom, Fa, is unique.
Each Fe has the same coupling to Fa, thus satisfying the test formagnetic equiv-
alence: the four equatorial �uorine atoms are magnetically equivalent.
S
Fa
Fe
FeFe
Fe
-
E12B.14(b) Coalescence of two NMR lines due to rapid nuclei exchange occurs when the
condition given in [12B.16–505], τ = 21/2/πδν, is satis�ed; in this expression τ
is the lifetime of an environment, and δν is the di�erence between the Larmor
frequencies of the two environments. Using [12B.5–494],
δν = ν2 − ν1 = [ν○ + (
νspec
106
) δ2] − [ν○ + (
νspec
106
) δ1] = (
νspec
106
) (δ2 − δ1)
For a proton jumping between two sites with �rst-order rate constant k, k = 1/τ
δν = (
νspec
106
) (δ2 − δ1) = (350 × 10
6
106
)(5.5 − 4.2) = 455 Hz
k = 1
τ
= πδν
21/2
= π × (455 Hz)
21/2
= 1.0 × 103 s−1
Solutions to problems
P12B.2 �e 19FNMR spectrumhas twomultiplets: a doublet and a quintet.�e former
indicates coupling to one spin- 12 nucleus, the latter indicates coupling to four
equivalent spin- 12 nuclei.