Transcript Document

Chem 1140; Spectroscopy
• UV-VIS
• IR
• NMR
UV-VIS Spectroscopy
The Abso rption Laws
I1
I0
I
intensity of the
incident beam
Overall transmittance:
Detector
transmitted
light
I2
T=
Internal transmittance: T
Usually T0 = Ti
I2
I1
i=
I
I0
(w hat is actually measured)
(of interest to the spectroscopist)
Such differences as m ight exist can be m inim ized by using m atched
cells and setting T for the reference at 100%.
The quantity I /I0 is independent of th e intensity (I) of the source and
proportional to th e number of abs orbing molecules
Lambert-Beer Law:
log
I0
I
=
. l . c
l = path length of the absorbing solution in [cm]
c = concentration in moles/liter
Log I0/I = absorbance or optical density = A
 = molar extinction coefficient [1000 cm2 mol-1]
=A
 i s a characteristic of a g iven compound, or more accurately, the light
absorbing system of the compound, the so called chromophore.  i s
correlated to the size (Å) of the chromophore and of course wave-length
dependent.
 = f ()
 ° f (c)
(Approximation!
An accurate
determination of  r equires determining
A at v arious concentrations)
: 10 - 105 (scales with ext ended -systems)
(dyes)
UV-Vis spectra are usually plotted as A vs.  plots:
A
hyperchromic
2
hypsochromic
shifts
bathochromic
hypochromic
1

n
max
[nm]

Though there are discrete levels of electro nic excited s tates in a
molecule, we do not obs erve absorption lines but broad peaks; the
change in vibrational and rotational energy levels during absorption of
light leads to peaks containing vibrational and rotational fine
structur e. Due to additional interaction with s olvent molecules, this
fine structure is blurred out, an d a smooth c urve is observed. (vapo ur
phase: one can observe v ibrational fine structu re).
Selection Rules
The irradiation of organic compounds may or may not give rise to
excitation of electrons from one orbital to another orbital. There are
transitions between orbitals that are quantum mechanically forbidden.
Two selection rules:
- Spin-rule:
The total spin S may not change during transition
(S  S; T  T)
- Symmetry rule: e—transitions between orbitals of identical symmetries
are not allowed.
(for ex. Even/uneven with regard to inversion).
n
*
even
even

In reality, these quantum mechanical rules are not rigidly observed,
however, due to molecular vibrations, the intensities ( ) of "forbidden"
transitions are significantly reduced (and are usually of diagnostic
importance).
n  * band near 300 nm of ketones;  = 10-100
benzene 260 nm band with  = 100-1000.
Chromophores
Definition:
Rules:
Chromophore – light absorbing electron system of a compound
-  or n- orbitals that do not interact lead to a spectrum that is the
sum of the individual absorptions of the isolated chromophores.
- the longer the conjugated system, the longer the wavelength of
the absorption maximum and the higher its intensity.
- a bathochromic and hyperchromic effect is observed, when atoms
with n-orbitals are directly attached to a chromophore
(-OH, -OR, NH2, SH, SR, Hal…) = auxochromic groups.
 Isolated Chromophores
Of the once listed in table, only few are of practical significance (Vacuum-UV)


C C
max =~ 190nm

h



Conjugated Chromophores


C C
max =~ 190nm

h


 

A
A


n

H
H
S
S=Auxochrome
S
 Benzene and aromatic compounds
The UV spectra of benzenes are characterized by three major bands which have
been given a variety of names.
Only  is allowed. B band is forbidden (loss of symmetry due to molecular vibrations;
shows vibrational fine structure).
MO Diagram of Ferrocene
Fe
FeCp2
2 Cp-
e1u
a1g
4p
a2u, e1u
e2g
a2u
e2u
e2g, e2u
4s
a1g
UV Spectrum of Ferrocene
300
250
4p
a1g, e1g, e2g
extinction [cm-1/M]
200
150
e1u
a1g
e2g
100
e1g, e1u
50
e1u
0
e1g
-50
200
300
400
500
600
700
800
a2u
lambda [nm]
Ferrocene has a molar extinction coefficient of 96 M-1cm-1 at 442 nm
a1g
a1g, a2u
Nomenclature of Electronic Transitions; Symbols of Symmetry Classes
Symbols of symmetry classes:
A: sym. (according to a Cn operation)
B: antisym. (according to a Cn operation)
E: 2-fold degenerate state
T: 3-fold degnerate state
Examples:
1A
1B
1B
1E
2
1u
2u
1u
1A
1
1A
1g
1A
1g
1A
1g
Indices:
g: sym. (according to an inversion operation)
u: antisym. (according to an inversion operation)
1: sym. (according to a C2 axis that is orthogonal to a Cn axis)
2: antisym. (according to a C2 axis that is orthogonal to a Cn axis)
‘: sym. (according to a plane of symmetry sn that is orthogonal to a Cn axis)
‘: antisym. (according to a plane of symmetry sn that is orthogonal to a Cn axis)
Calculation of spectra:
◦ Bonus Problem: Calculate UV and IR
Spectra of Ferrocene and Acetylferrocene, and
Compare to Experimental Data; can you
design a Ferrocene derivative that is greencolored?
Infrared Spectroscopy
After considering ultraviolet and visible radiation (200-800 nm)
which is energetic enough to affect the electronic levels in a
molecule, we shall now consider radiation which has a longer
wavelength: infrared radiation which extends beyond the visible
into the microwave region and is capable of affecting both the
vibrational and the rotational energy levels in molecules.
Range of commercial instruments:
2500 nm to 16’000 nm
physical chemists:
25000 – 160’000 Å
analytical chemists:
2.5 – 16 microns ( )
organic chemists:
4000 – 625 cm-1
wavenumber
 = 1/ = /c
"normalized frequency"
Use: simple, ra pid, r eliable means for functional group identification.
Vi brational modes
For a molecule comprised of N atoms, there are 3N-6 normal
modes of vibration (3N-5 for linear molecules). To a good approximation,
however, some of these molecular vibrations are associated with the
vibrations of individual bonds or functional groups (localized vibrations)
while others must be considered as vibrations of the whole molecule.
Localized v ibrations are:
Stretching modes:
simple
coupled:
H
C
H
H
C
symmetric
H
H
C
asymmetric
Bending modes:
H
H
H
C
C
scissoring (sym)
H
H
H
C
w agging (sym)
in plane deformations
rocking (asym)
H
H
C
tw isting (asym)
out of plane def ormations
Transmission
O H
C C
C C
N H
C N
C O
X Y Z
stretching
C H
stretching
other stretching
C N
N O
stretching
N H
bending
Absorbance
2500
4000
3000
bending and
combination
bands
FINGERPRINT
REGION
1500
2000
1000
cm-1
wave numbers
Selection rules
1. In order to observe an absorption, the dipole moment of the
excited vibrational state must differ from that of the ground state.
Reason: oscillating dipole interacts with oscillating electric vector of h
O
H C C H
O
H C
C H
observed
not observed
Analysis of IR - Spectra of Unknown Compounds
1. frequency, shape, intensity of an absorption band have all to be considered
in the interpretation
2. all characteristic absorption frequencies of a functional group
have to be considered (band can be missing or is caused by another function)
3. first the obvious absorptions should be identified:
X-H, C=O, C=C, out of plane C-H
4. subsequently the strong absorptions in the fingerprint region should be
analyzed
5. possible structures can now be proposed and have to be checked with
reference spectra (or data from other spectroscopic techniques)
An Introduction to NMR Spectroscopy
1H
NMR
13C
NMR
The types of information accessible via high resolution NMR include:
1. Functional group analysis (chemical shifts)
2. Bonding connectivity and orientation (J coupling),
3. Through space connectivity (Overhauser effect)
4. Molecular Conformations, DNA, peptide and enzyme sequence and structure.
5. Chemical dynamics (lineshapes, relaxation phenomena).
http://www.chem.ucla.edu/%7Ewebspectra/
Nuclear Spin: The proton is a spinning charged particle and has also
a magnetic moment.
1
H:
- nuclear spin quantum number
m = 1/2
B
- such a nucleus is is described as
having a nuclear spin I of 1/2
m = +1/2
lower
energy
m = -1/2
higher
energy
Because nuclear charge is the opposite of electron charge, a nucleus
whose magnetic moment is parallel to the magnetic field has the lower
energy.
The difference in energy is given by:
 = h B0/2
 = magnetogyric ratio ( a constant, typical for a nucleus, which essentially reflects
the strength of the nuclear magnet)
Bo = strength of the applied magnetic field
h = Planck’s constant (3.99 x 10-13 kJ s mol-1)
Note that as the field strength increases, the difference in energy between
any two spin states increases proportionally.

nuclear spin quantum #
m =-1/2
Ei = -mhBo/2

= -mNBo
O
degenerate
m = +1/2
O
Bo
[values of = 1/ were picked for m, so that the difference in energy between two
neighboring states will always be an integer multiple of Bo (h/2)].
The number of nuclei in the low energy state (N) a nd the number in
the high energy state (N) will differ by an amount determined by the
Boltzmann distribution:
N/N = e(-/kT)
k = 1.381 x 10-23JK-1
When a radio frequency (RF) signal is applied, this distribution is changed if
the radio frequency matc hes .
 = h = h B0/2
 = resonance fr equency =  B0/2
 i s therefore dependent upon both the applied field strength and the
nature of th e nucleus.
1
H: in a 2.35 T field (earth magnetic field = 0.00006 T) = 0.999984
~ 1 in 106
( = 100 MHz)
- The difference in population of t he two states i s exceed ingly small, in
the order of few parts per million. (even smaller in 13C, because  is
smaller).
- Relatively low sensitivity o f NMR compared to IR or UV
- Large Bo needed to increase the population differ ence (usually given in
MHz of 1H resonance fr equency).
SUMMARY
- Nuclear spin is a property characteristic of each isotope and is a
function of Z and N.
- Each isotope with I ° 0 has a characteristic magnetogyric ratio ( ) that
determines the frequency of its precession in a magnetic field of strength
B0

B0
2
It is this frequency that must be matched by the incident electromagnetic
radiation for absorption to occur.
- When a collection of nuclei with I ° 0 is immersed in a strong magnetic
field, the nuclei distribute themselves among 2I + 1 spin states, the relative
population of which is determined by the Boltzmann distribution, usually
being near unity
N
N
=e
(-E/kT)
- If two (or more) spin state populations become equal, the system is said to be
saturated.
Obtaining an NMR Spectrum
Magnet
Source of RF radiation
Detector + amplifier
Plotter, sample
The magnet:
permanent
cheap, stable,
fixed field
1.4T
electromagnet
more expensive,
stronger, variable
field
superconducting
expensive,
stronger, variable
field
18T (24T)
Strength of magnetic field shifts: lock necessary
(= substance with strong, defined NMR
signal) Older: reference
internal, external
CDCl3
TMS: 0.0 ppm singlett
Once a stable field is established, t he question remains as to whether
that field is completely ho mogeneous throughout th e region between
the pole faces of the magnet.
N
S
lines of magnetic flux
not uniform
Sample
sample has to be placed
near the center of the pole
gap
For 2.35 T, to achieve a precision of +/- 1 Hz (10 ppb at 100 MHz) the field must be
homogeneous to the extent of +/- 2.35 x 10-8 T! Such a phenomenal uniformity,
even at the center of the field , can be achieved only by means of two additional
techniques:
Spinning of the sample ("averages" out small inhomogeneities)
Variation of the contour of the field by passing extremely small currents through shim
coils wound around the magnet itself: Shimming (manually, automatically)
Paradox: Large sample in order to have as many nuclei as possible, small sample to
increase uniformity of the field.
narrow bore tubes
The Pulsed Fourier Transform Technique
Further advances in S/N ratio improvement had to await the development of faster
computer microprocessors: ~1970’s.
- RF radiation is supplied by a brief but powerful pulse of RF current through the
transmitter coil. The spectral width of the pulse is chosen to cover absorption of all
nuclei of interest.
The duration of the pulse (tp) determines
the frequency range covered (Heisenberg's
uncertainty principle: t >_ h)
SW ~ tp-1 ; tp
2SW
_
>
RF
intensity
(4SW)-1
Frequency
o
Optimum tp are obtained by trial and error and are usually in the order of 10 s
for  = 90° for best S/N ratio.
The next step in the PFT process is to monitor th e induced AC
receiver s ignal. Digital data collection gives us the modulated free
induction decay (function of M xy ). FID because the c urrent intensity
decreases with time. T his decay is the result of T2 (spin-spin)
relaxation.
M
the microprocessor samples the
voltage in the receiver coil at a regular
interval, called dwell time, td.
Voltage
td > (2SW)-1
td
t0
t0
Time
In a set of nuclei with different  precession and T1/T2, the digital
FID curve becomes very complex:
CH3
At this point it becomes necessary for the computer to
recognize the patterns mathematically and extract the
signal frequencies and relative intensities for each set of
nuclei. This analysis is performed by a Fourier transformation of the FID date.

f( x)  ao   an cos2nso x  bn sin 2nso x 
n 1
where:
a0 = constant;
an = amplitude;
x = period;
so = fundamental frequency;
xo = 1/so; and
n = order of harmonic
The parent function is constructed
by summing together a series of
sine waves.
line width: uncertainty principle:
t > 1
1/2 >
1
T2*
Nuclei that are slow to relax give sharp signals,
nuclei that relax rapidly give broad signals (solids).
1
2
3
paramagnetic residues
0
frequency
line broadening
- Wink, D. J. J. Chem. Ed., 1989, 66, 810. Spin-Lattice Relaxation Times in 1H NMR Spectroscopy.
- Glasser, L. J. Chem. Ed., 1987, 64, A228. Fourier Transforms f or Chemists.
- King, R. W.; Williams, K. R. J. Chem. Ed., 1989, 66, A213, A243. The Fourier Transform in
Chemistry.
Summary:
A typ ical example of t he generation of a PFT spectrum:
t p:
t acq:
pulse time ( sec)
the length of the time a given FID signal is actually monitored
(resolution, the ability to distinguish two nearby signals, is inversely
prop. to t
acq.
R = (t acq )-1
3 sec  0.3 Hz
tw: delay time, to allow for equilibrium distribution
tw =
3T1 - tacq.
+ dead time (
adding up, FT, spectrum
(for 1H no waiting time)
phasing necessary) phase correction
Taking an NMR – Practical Consideration
- Use 5 mm tube filled with ~ 0.5 mL of solution containing 1-5 mg of sample (1H
NMR).
- common deuterated solvents:
CCl4, CDCl3, C6D6, DMSO-d6, D2O, CD3CN, CD2Cl2, d6-acetone, CD3OD
(because of HCl formation, do not leave sample in CDCl3!)
- peak listings in ppm and/or Hz.
- paramagnetic metal ion  broad peaks
Chemical shift
H depends on Bo  therefore relative frequencies are reported:
 
 act.   TMS 83.4Hz
6


1.39
x10
1.39 ppm
6
o
60 x10
1.39 ppm =  = downfield from TMS.
2
ppm 
1
0
downfield
upfield
deshielded
shielded
Integration
Area under absorption peak ~ # of nuclei resonating at that 
But: nuclei must relax to equilibrium between pulses, not generally true of 13C
NMR!
First-Order Spin-Spin-Coupling
1
H - 1H
From previous discussions one could have g otten the impression th at
a ty pical 1H NMR spectru m exhibits just one signal for each set of
equivalent 1H-nuclei and that the s ame thing is true for 13C spectra,
as well as for spectra of any ot her isotope. However, the re are many
more lines in a s pectrum, and while these extra lines do make a
spectru m more complex, th ey a lso offer valuable structural
information th at com plements the chemical shift d ata.
CH3CH2OH: The spin states of two hydrogens (methylene group):
B0
M=-1
0
0
1
Total Magnetization
Three spin states with population ratio of 1:2:1
For methyl hydrogens the net experienced field will depend on the
magnetization of the neighboring methylene group!
The methyl signal will be split into three lines with intensity ratio 1 : 2 : 1 (=
spin-spin-coupling,homonuclear coupling because the coupling is between nuclei
of the same isotope). Triplet.
Accordingly, fo r the methylene signal, th e possible spin states of the
methyl group determine its multiplic ity (number of lines in th e
signal).
The spin states of th ree hydrogens:
M=-3/2
M=3/2
4 spin states with population
ratio of 1 : 3 : 3 : 1
M=1/2
M=-1/2
Quartet
Accordingly, a doublet is observed for hydr ogens that are coupled to
a methine (CH) proton.
The multiplicity of a given resonance = n+1 (n=# of neighboring
equivalent nuclei). The relative intensities of the multiplet f ollow
Pascal’s triangle.
J
J
J = spacing between lines.
The slight difference in energy b etween the resonances is the
coupling constant J [Hz]. J’s are independent of instrumental
parameters!
Consider a 3-spin system with/without equivalent n uclei:
Ha Hb Hc
C C C
Ha will be a doublet with 3Jab
Hb will be a “doublet of do ublets” with 3Jab and 3Jbc
Hc will be a doublet with 3Jbc
Jbc
Jab
Jab
Ha
Hb
Jbc
Hc
13
C NMR
• 12C (98%) has I = 0  no NMR
• 13C (1.1.%) has I = 1/2  NMR
• ( = h = hBo/2  gyromagnetic ratio such that obs  1/4 that of 1H
(300 MHz  75 MHz)
• Observe ty pically 0 –
230 ppm (rel. to TMS)
Typically deco uple the protons by sat urating them with a s econd
broadband RF puls (double resonance technique, second
transmitter co il; “white n oise”  if the irradiating field is strong
enough, not only will the 1H nuclei approach saturation, but v irtually
all the 1H magnetization will be tipped into t he x1y p lane. Since the 1H
nuclei are no longer aligned with ( or against) the applied field (which
is along th e z ax is) they can no longer augment or diminish the
magnetic field exp erienced by the carbons. As a result, th e coupling
interaction disappears, and each 13C multiplet co llapses to a singlet!
(D coupling not e ffecte d!)
 causes all 13C resonances to be singlets
 affords Nuclear Overhauser Effect
 makes integration of 13C spectra unreliable
Aromatics: not eff ected by r ing current
128.5 ppm
substitute d C’s are typically of lower intensity
--
++
176.8
176.8
209.0
102.1
85.3
Carbonyls:
O
O
205.1
H
199.6
O
O
OH
177.3
OEt
169.5
O
RO
OR
Cl
168.6
O
~ 100 - 110
O
CH3
N
H
174.9
I
158.9
Heavy atom effect.