Transcript Introduction to Spectroscopy

Introduction to
Spectroscopy
The Light of Knowledge
Mass Spectrometry
Ultraviolet-Visible Spectroscopy
Infrared Spectroscopy
Nuclear Magnetic Resonance
Spectroscopy
Mass Spectrometry
A small sample of compound is ionized,
usually to cations by loss of an
electron. The Ion Source
The ions are sorted and separated
according to their mass and charge.
The Mass Analyzer
The separated ions are then detected and
tallied, and the results are displayed on a
chart. The Detector
4-methyl-3-pentene-2one
N,N-diethylmethylamine
Visible and Ultraviolet
Spectroscopy
Violet: 400 - 420 nm
Indigo: 420 - 440 nm
Blue: 440 - 490 nm
Green: 490 - 570 nm
Yellow: 570 - 585 nm
Orange: 585 - 620 nm
Red: 620 - 780 nm
lmax, nm
e
Chromophore
Example
Excitation
C=C
Ethene
p
__>
p*
171
15,000 hexane
C@C
1-Hexyne
p
__>
p*
180
10,000
Ethanal
n
p
__>
p*
__> p*
290
180
15
hexane
10,000 hexane
N=O
Nitromethane
n
p
__>
p*
__> p*
275
200
17
5,000
ethanol
ethanol
C-X X=Br
X=I
Methyl
bromide
Methyl Iodide
n
n
s*
__> s*
205
255
200
360
hexane
hexane
C=O
__>
Solvent
hexane
Terminology for Absorption Shifts
Nature of Shift
Descriptive
Term
To Longer Wavelength
Bathochromic
To Shorter Wavelength
Hypsochromic
To Greater Absorbance
Hyperchromic
To Lower Absorbance
Hypochromic
Empirical Rules for Absorption
Wavelengths of Conjugated Systems
Core
Chromophore
Woodward-Fieser
Rules for
Calculating the lmax
of Conjugated
Dienes and
Polyenes
Transoid Diene
215 nm
Substituent and Influence
R- (Alkyl Group) .... + 5nm
RO- (Alkoxy Group) .. +6
X- (Cl- or Br-) ......... +10
RCO2- (Acyl Group) .... 0
RS- (Sulfide Group) .. +30
R2N- (Amino Group) .. +60
Further p -ConjugationC=C (Double
Bond) ... +30
C6H5 (Phenyl Group) ... +60
Cyclohexadiene*
260 nm
(i) Each exocyclic double bond adds 5 nm. In the example on the
right, there are two exo-double bond components: one to ring A and
the other to ring B.
(ii) Solvent effects are minor.
* When a homoannular (same ring) cyclohexadiene chromophore is
present, a base value of 260 nm should be choosen. This includes
the ring substituents. Rings of other size have a lesser influence.
lmax (calculated) = Base (215 or 260) + Substituent Contributions
Infrared Spectroscopy
a wavelength range from 2,500 to 16,000 nm,
with a corresponding frequency range
from 1.9*1013 to 1.2*1014 Hz.
Typical Infrared Absorption Frequencies
Stretching Vibrations
Bending Vibrations
Range (nm)
Intensity
Assignment
Range (nm)
Intensity
Assignment
Alkanes
2850-3000
str
CH3, CH2 & CH
2 or 3 bands
1350-1470
1370-1390
720-725
med
med
wk
CH2 & CH3 deformation
CH3 deformation
CH2 rocking
Alkenes
3020-3100
1630-1680
med
var
=C-H & =CH2 (usually sharp)
C=C (symmetry reduces intensity)
880-995
780-850
675-730
str
med
med
=C-H & =CH2
(out-of-plane bending)
cis-RCH=CHR
1900-2000
str
C=C asymmetric stretch
Alkynes
3300
2100-2250
str
var
C-H (usually sharp)
C@C (symmetry reduces intensity)
600-700
str
C-H deformation
Arenes
3030
1600 & 1500
var
med-wk
C-H (may be several bands)
C=C (in ring) (2 bands)
(3 if conjugated)
690-900
str-med
C-H bending &
ring puckering
Functional
Class
Alcohols &
Phenols
Amines
3580-3650
3200-3550
970-1250
var
str
str
O-H (free), usually sharp
O-H (H-bonded), usually
broad
C-O
13301430
650-770
med
var-wk
O-H bending (in-plane)
O-H bend (out-of-plane)
3400-3500 (dil. soln.)
3300-3400 (dil. soln.)
1000-1250
wk
wk
me
d
N-H (1°-amines), 2 bands
N-H (2°-amines)
C-N
15501650
660-900
medstr
var
NH2 scissoring (1°-amines)
NH2 & N-H wagging
(shifts on H-bonding
Aldehydes &
Ketones
Carboxylic Acids
& Derivatives
Nitriles
Isocyanates,Isoth
iocyanates,
Diimides, Azides
& Ketenes
2690-2840(2 bands)
1720-1740
1710-1720
med
str
str
C-H (aldehyde C-H)
C=O (saturated
aldehyde)
C=O (saturated
ketone)
1690
1675
1745
1780
str
str
str
str
2500-3300 (acids)
overlap C-H
1705-1720 (acids)
1210-1320 (acids)
str
str
medstr
O-H (very broad)
C=O (H-bonded)
O-C (sometimes 2peaks)
1785-1815 ( acyl halides)
1750 & 1820 (anhydrides)
1040-1100
1735-1750 (esters)
1000-1300
1630-1695(amides)
str
str
str
str
str
str
C=O
C=O (2-bands)
O-C
C=O
O-C (2-bands)
C=O (amide I band)
2240-2260
med
C@N (sharp)
2100-2270
med
-N=C=O, -N=C=S
-N=C=N-, -N3, C=C=O
13501360
14001450
1100
str
str
me
d
a-CH3 bending
a-CH2 bending
C-C-C bending
13951440
me
d
C-O-H bending
15901650
15001560
me
d
me
d
aryl ketone
a,b-unsaturation
cyclopentanone
cyclobutanone
N-H (1¡-amide) II
band
N-H (2¡-amide) II
band
Nuclear Magnetic
Resonance Spectroscop
1. A spinning charge generates a magnetic field, as
shown by the animation on the right.
The resulting spin-magnet has a magnetic moment (m)
proportional to the spin.
2. In the presence of an external magnetic field (B0),
two spin states exist, +1/2 and -1/2.
The magnetic moment of the lower energy +1/2 state is
alligned with the external field, but that of the higher
energy -1/2 spin state is opposed to the external field.
Note that the arrow representing the external field points
North.
3. The difference in energy between the two spin states is dependent on the
external magnetic field strength, and is always very small. The following
diagram illustrates that the two spin states have the same energy when the
external field is zero, but diverge as the field increases. At a field equal to Bx
a formula for the energy difference is given (remember I = 1/2 and m is the
magnetic moment of the nucleus in the field).
A Model for NMR
Spectroscopy
magnetic
moment
m
A Spinning
Gyroscope
in a Gravity Field
A Spinning Charge
in a Magnetic Field
Chemical Shift
Proton Chemical Shift Ranges
Region
Low
Field
Region
High
Field
Region
* For samples in CDCl3 solution. The d scale is relative to TMS at d = 0.
p-Electron Functions
Anisotropy effect
Spin-Spin Interactions
1,2-dichloroethane
1,1-dichloroethane
Magnitude of Some Typical Coupling
Constants
Carbon NMR Spectroscopy
Many obstacles needed to be overcome before carbon nmr
emerged as a routine tool :
i) As noted, the abundance of 13C in a sample is very low
(1.1%), so higher sample concentrations are needed.
ii) The 13C nucleus is over fifty times less sensitive than a
proton in the nmr experiment, adding to the previous
difficulty.
iii) Hydrogen atoms bonded to a 13C atom split its nmr
signal by 130 to 270 Hz, further complicating the nmr
spectrum.
13C
Chemical Shift Ranges*
Low Field
Region
*
For samples in CDCl3 solution. The d scale is relative to TMS at d=0.
High
Field
Region