Chapter 14 NMR

NMR purpose

structure determination 

Nuclear spin

• odd number of protons or neutrons

• spin = rotating sphere of charge that generates a magnetic moment

alpha spin state

protons aligned with magnetic field

• lower energy

beta spin state

proton against magnetic field

• higher energy

nucleus in resonance

• Absorption of electromagnetic radiation causes nucleus to flip to beta spin state (higher in energy)

do all nuclei absorb same frequency of rf

no bc nucleus surrounded by electrons

diamagnetism

• In presence of an external magnetic field, e^- density circulates, which produces a local (induced) magnetic field that opposes the external magnetic field

• all materials have this property

deshielded electrons

• electron deficient areas

• higher in energy

• downfield

• experience a net magnetic field strength greater than external magnetic field 

shielded electrons

• surrounded by electrons

• lower in energy

• upfield

• experiences a net magnetic field strength smaller than external magnetic field 

magnitude of energy gap between spin states depends on…

strength of external magnetic field

energy gap increases with …

increases magnetic field strength

strength of magnetic field (in tesla) determines …

range of frequencies (in MHz) that must be used  

Solvents Used in NMR

• can’t have protons

• use deuterated substances

three protons of a CH₃ group are ….

always chemically equivalent

two protons of a CH₂ group are ….

generally equivalent if compound has no chiral centers

chiral center

bonded to four different atoms

Chemical Shift (δ)

• location of signal

• indicates electronic environment of protons giving rise to signal

• δ = observed shift from TMS in hertz / operating frequency of the instrument in hertz 

• constant regardless operating frequenct

• in ppm

• left is downfield = deshielded

• right is upfield = shielded

inductive effects

• Electronegative atoms like halogens can withdraw electron density from neighboring atoms making protons deshielded and pushed downfield 

• additive effect

• more electronegative atom = stronger effect

where are inductive effects most significant?

• position closest to electronegative atom

  • alpha positions 

• positions 2 away from electronegative atom are beta positions and are only slightly affected

  • feel 1/5 of effect alpha protons feel

Anisotropic effects

• protons outside of ring (aromatic positions) experience a deshielding effect and protons shifted downfield

• protons inside of ring are shielded

• protons vinylic to pi bond (attached to pi system) are more deshielded

ortho positions

next to each other

para positions 

across 

meta

separated by one group 

integration

• area under each signal

• number of protons giving rise to signal

• gives relative number or ratio of protons

how to calculate integration value

• divide by all integration values by smallest number

• once ratio of who numbers obtained, check molecular formula to determine actual amounts

multiplicity

• number of peaks in signal

• shape

• indicates number of neighboring protons

• if two H atoms are not equivalent and within 3 sigma bonds apart, will split one another = spin-spin coupling

• can be singlet, doublet, triplet, etc

spin-spin coupling or coupling (multiplicity)

• if two H atoms are not equivalent and within 3 sigma bonds apart, will split one another

• n+1 gives multiplicity

  • n = number of neighboring protons

  • neighboring protons must be chemically equivalent

do equivalent protons split each other

no

coupling constant

• J value

• distance between individual peaks

• neighboring protons always split each other with equivalent J values

• value is independent of operating frequency

Complex splitting

• proton split by two different neighbors

• shape will depend on J values

• whichever has greater J value will give primary multiplicity 

  • ex: If J_ab is greater than J_bc and H_a is a doublet and H_c is a triplet, signal will appear as a doublet of triplets 

no complex splitting

• if structure lacks rigid double bonds, J values will be generall similar 

¹³C NMR spectrum

• only chemical shift is reported

• usually no splitting between neighboring C bc low abundancde of ¹³C

• ¹³C-¹H splitting occurs causing an unreadable spectra

  • splitting suppressed through broadband decoupling