This document provides an overview of UV-Visible spectroscopy. It begins with an introduction that describes UV radiation and electronic excitations. It explains how UV spectroscopy works and the types of electronic transitions that are observed. It discusses chromophores, substituent effects, and selection rules. The document then covers using UV spectroscopy for structure determination, including discussing dienes and applying the Woodward-Fieser rules to determine structures of cyclic and acyclic dienes. In the final section, it discusses common errors in applying the Woodward-Fieser rules.

1. Dr. L. Sakthikumar M.Sc., M.Phil., Ph.D
Assistant Professor in Chemistry
Saiva Bhanu Kshatriya College
Aruppukottai – 626101
Tamilnadu , India
ORGNAIC SPECTROSCOPY

2. UV-VISIBLE
INFRARED
NMR (1H &13C)

3. 3
UV Spectroscopy
I. Introduction
UV radiation and Electronic Excitations
 The difference in energy between molecular bonding, non-bonding and
anti-bonding orbitals ranges from 125-650 kJ/mole
 This energy corresponds to EM radiation in the ultraviolet (UV) region,
100-350 nm, and visible (VIS) regions 350-700 nm of the spectrum
 For comparison, recall the EM spectrum:
 Using IR we observed vibrational transitions with energies of 8-40 kJ/mol
at wavelengths of 2500-15,000 nm
 For purposes of our discussion, we will refer to UV and VIS spectroscopy
as UV
UVX-rays IRg-rays RadioMicrowave
Visible

4. 4
UV Spectroscopy
I. Introduction
The Spectroscopic Process
 In UV spectroscopy, the sample is irradiated with the broad spectrum of
the UV radiation
 If a particular electronic transition matches the energy of a certain band
of UV, it will be absorbed
 The remaining UV light passes through the sample and is observed
 From this residual radiation a spectrum is obtained with “gaps” at these
discrete energies – this is called an absorption spectrum





5. 5
UV Spectroscopy
I. Introduction
Observed electronic transitions
 The lowest energy transition (and most often obs. by UV) is typically that
of an electron in the Highest Occupied Molecular Orbital (HOMO) to the
Lowest Unoccupied Molecular Orbital (LUMO)
 For any bond (pair of electrons) in a molecule, the molecular orbitals are
a mixture of the two contributing atomic orbitals; for every bonding
orbital “created” from this mixing (s, ), there is a corresponding anti-
bonding orbital of symmetrically higher energy (s*, *)
 The lowest energy occupied orbitals are typically the s; likewise, the
corresponding anti-bonding s orbital is of the highest energy
 -orbitals are of somewhat higher energy, and their complementary anti-
bonding orbital somewhat lower in energy than s*.
 Unshared pairs lie at the energy of the original atomic orbital, most often
this energy is higher than  or s (since no bond is formed, there is no
benefit in energy)

6. 6
UV Spectroscopy
I. Introduction
C. Observed electronic transitions
 Here is a graphical representation
Energy
s

s

n
Atomic orbitalAtomic orbital
Molecular orbitals
Occupied levels
Unoccupied levels

7. 7
UV Spectroscopy
I. Introduction
C. Observed electronic transitions
7. From the molecular orbital diagram, there are several possible electronic
transitions that can occur, each of a different relative energy:
Energy
s

s

n
s
s

n
n
s


s

alkanes
carbonyls
unsaturated cmpds.
O, N, S, halogens
carbonyls

8. 8
UV Spectroscopy
I. Introduction
C. Observed electronic transitions
 Although the UV spectrum extends below 100 nm (high energy), oxygen
in the atmosphere is not transparent below 200 nm
 Special equipment to study vacuum or far UV is required
 Routine organic UV spectra are typically collected from 200-700 nm
 This limits the transitions that can be observed:
s
s

n
n
s


s

alkanes
carbonyls
unsaturated cmpds.
O, N, S, halogens
carbonyls
150 nm
170 nm
180 nm √ - if conjugated!
190 nm
300 nm √

9. 9
UV Spectroscopy
I. Introduction
D. Selection Rules
 For an electron to transition, certain quantum mechanical constraints
apply – these are called “selection rules”
 For example, an electron cannot change its spin quantum number during
a transition – these are “forbidden”
 Other examples include:
 the number of electrons that can be excited at one time
 symmetry properties of the molecule
 symmetry of the electronic states
 To further complicate matters, “forbidden” transitions are sometimes
observed (albeit at low intensity) due to other factors

10. 10
UV Spectroscopy
I. Introduction
E. Band Structure
 It would seem that since the electronic energy levels of a pure sample of
molecules would be quantized, fine, discrete bands would be observed for
atomic spectra, this is the case
 In molecules, when a bulk sample of molecules is observed, not all bonds
(read – pairs of electrons) are in the same vibrational or rotational energy
states
 This effect will impact the wavelength at which a transition is observed –
very similar to the effect of H-bonding on the O-H vibrational energy
levels in neat samples

11. 11
UV Spectroscopy
III. Chromophores
C. Substituent Effects
General – Substituents may have any of four effects on a chromophore
i. Bathochromic shift (red shift) – a shift to longer l; lower energy
ii. Hypsochromic shift (blue shift) – shift to shorter l; higher energy
iii. Hyperchromic effect – an increase in intensity
iv. Hypochromic effect – a decrease in intensity
200 nm 700 nm
e
Hypochromic
Hypsochromic
Hyperchromic
Bathochromic

12. 12
UV Spectroscopy
IV. Structure Determination
A. Dienes
For acyclic butadiene, two conformers are possible – s-cis and s-trans
The s-cis conformer is at an overall higher potential energy than the s-
trans; therefore the HOMO electrons of the conjugated system have less
of a jump to the LUMO – lower energy, longer wavelength
s-trans s-cis

13. 13
UV Spectroscopy
IV. Structure Determination
A. Dienes
1. General Features
The Y2  Y3
* transition is observed as an intense absorption (e =
20,000+) based at 217 nm within the observed region of the UV
While this band is insensitive to solvent (as would be expected) it is
subject to the bathochromic and hyperchromic effects of alkyl
substituents as well as further conjugation
Consider:
lmax = 217 253 220 227 227 256 263 nm

14. 14
UV Spectroscopy
IV. Structure Determination
A. Dienes
2. Woodward-Fieser Rules
Woodward and the Fiesers performed extensive studies of terpene and
steroidal alkenes and noted similar substituents and structural features
would predictably lead to an empirical prediction of the wavelength for
the lowest energy   * electronic transition
This work was distilled by Scott in 1964 into an extensive treatise on the
Woodward-Fieser rules in combination with comprehensive tables and
examples – (A.I. Scott, Interpretation of the Ultraviolet Spectra of Natural
Products, Pergamon, NY, 1964)
A more modern interpretation was compiled by Rao in 1975 – (C.N.R.
Rao, Ultraviolet and Visible Spectroscopy, 3rd Ed., Butterworths, London,
1975)

15. 15
UV Spectroscopy
IV. Structure Determination
A. Dienes
2. Woodward-Fieser Rules - Dienes
The rules begin with a base value for lmax of the chromophore being
observed:
acyclic butadiene = 217 nm
The incremental contribution of substituents is added to this base value
from the group tables:
Group Increment
Extended conjugation +30
Each exo-cyclic C=C +5
Alkyl +5
-OCOCH3 +0
-OR +6
-SR +30
-Cl, -Br +5
-NR2 +60

16. 16
UV Spectroscopy
IV. Structure Determination
A. Dienes
2. Woodward-Fieser Rules - Dienes
For example:
Isoprene - acyclic butadiene = 217 nm
one alkyl subs. + 5 nm
222 nm
Experimental value 220 nm
Allylidenecyclohexane
- acyclic butadiene = 217 nm
one exocyclic C=C + 5 nm
2 alkyl subs. +10 nm
232 nm
Experimental value 237 nm

17. 17
UV Spectroscopy
IV. Structure Determination
A. Dienes
3. Woodward-Fieser Rules – Cyclic Dienes
There are two major types of cyclic dienes, with two different base values
Heteroannular (transoid): Homoannular (cisoid):
e = 5,000 – 15,000 e = 12,000-28,000
base lmax = 214 base lmax = 253
The increment table is the same as for acyclic butadienes with a couple
additions:
Group Increment
Additional homoannular +39
Where both types of diene
are present, the one with
the longer l becomes the
base

18. 18
UV Spectroscopy
IV. Structure Determination
A. Dienes
3. Woodward-Fieser Rules – Cyclic Dienes
In the pre-NMR era of organic spectral determination, the power of the
method for discerning isomers is readily apparent
Consider abietic vs. levopimaric acid:
C
O
OHC
O
OH
levopimaric acidabietic acid

19. 19
UV Spectroscopy
IV. Structure Determination
A. Dienes
3. Woodward-Fieser Rules – Cyclic Dienes
For example:
1,2,3,7,8,8a-hexahydro-8a-methylnaphthalene
heteroannular diene = 214 nm
3 alkyl subs. (3 x 5) +15 nm
1 exo C=C + 5 nm
234 nm
Experimental value 235 nm

20. 20
UV Spectroscopy
IV. Structure Determination
A. Dienes
3. Woodward-Fieser Rules – Cyclic Dienes
C
O
OH
heteroannular diene = 214 nm
4 alkyl subs. (4 x 5) +20 nm
1 exo C=C + 5 nm
239 nm
homoannular diene = 253 nm
4 alkyl subs. (4 x 5) +20 nm
1 exo C=C + 5 nm
278 nm
C
O
OH

21. 21
UV Spectroscopy
IV. Structure Determination
A. Dienes
3. Woodward-Fieser Rules – Cyclic Dienes
Be careful with your assignments – three common errors:
R
This compound has three exocyclic
double bonds; the indicated bond is
exocyclic to two rings
This is not a heteroannular diene; you would
use the base value for an acyclic diene
Likewise, this is not a homooannular diene;
you would use the base value for an acyclic
diene

22. IR – SPECTROSCOPY- REF. ANIMATION
 Range 4000 – 400 Cm-1
 Vibrational change makes the bond become
dipolar in nature.
 More useful in the determination of functional
groups of an organic compound
 To confirm the presence of inter and intra
molecular H-Bonding
 To confirm the identity of two similar
molecules using finger print region

23. TABLE OF CHARACTERISTIC IR ABSORPTIONS
frequency, cm–1
bond functional group
3640–3610 (s, sh) O–H stretch, free hydroxyl alcohols, phenols
3500–3200 (s,b) O–H stretch, H–bonded alcohols, phenols
3400–3250 (m) N–H stretch 1˚, 2˚ amines, amides
3300–2500 (m) O–H stretch carboxylic acids
3330–3270 (n, s) –C≡C–H: C–H stretch alkynes (terminal)
3100–3000 (s) C–H stretch aromatics
3100–3000 (m) =C–H stretch alkenes
3000–2850 (m) C–H stretch alkanes
2830–2695 (m) H–C=O: C–H stretch aldehydes
2260–2210 (v) C≡N stretch nitriles
2260–2100 (w) –C≡C– stretch alkynes
1760–1665 (s) C=O stretch carbonyls (general)
1760–1690 (s) C=O stretch carboxylic acids
1750–1735 (s) C=O stretch esters, saturated aliphatic
1740–1720 (s) C=O stretch aldehydes, saturated aliphatic
1730–1715 (s) C=O stretch α, β–unsaturated esters
1715 (s) C=O stretch ketones, saturated aliphatic
1710–1665 (s) C=O stretch α, β–unsaturated aldehydes, ketones

24. 1680–1640 (m) –C=C– stretch alkenes
1650–1580 (m) N–H bend 1˚ amines
1600–1585 (m) C–C stretch (in–ring) aromatics
1550–1475 (s) N–O asymmetric stretch nitro compounds
1500–1400 (m) C–C stretch (in–ring) aromatics
1470–1450 (m) C–H bend alkanes
1370–1350 (m) C–H rock alkanes
1360–1290 (m) N–O symmetric stretch nitro compounds
1335–1250 (s) C–N stretch aromatic amines
1320–1000 (s) C–O stretch alcohols, carboxylic acids, esters, ethers
1300–1150 (m) C–H wag (–CH2X) alkyl halides
1250–1020 (m) C–N stretch aliphatic amines
1000–650 (s) =C–H bend alkenes
950–910 (m) O–H bend carboxylic acids
910–665 (s, b) N–H wag 1˚, 2˚ amines
900–675 (s) C–H “oop” aromatics
850–550 (m) C–Cl stretch alkyl halides
725–720 (m) C–H rock alkanes
700–610 (b, s) –C≡C–H: C–H bend alkynes
690–515 (m) C–Br stretch alkyl halides
m=medium, w=weak, s=strong, n=narrow, b=broad, sh=sharp

25. 1H & 13CNMR SPECTRA
 Used to study the protonic environment,
carbon skeleton and its spin – spin
interaction in an organic molecule when
introduced in a strong magnetic field
 Useful in the determination of cis-trans
isomerism
 Useful in the determination of presence of –
OH group

26. 1HNMRChemical Shift Data
Shown below is a chart of where some common kinds of protons appear in the d
scale. Note that most protons appear between 0 and 10 ppm. The reference,
tetramethylsilane (TMS) appears at 0 ppm, and aldehydes appear near 10 ppm.
Note that these are typical values and that there are lots of exceptions!
d ppm
TMS
CH3CH3
RONR2
CH3OCH3
RO
HR
R R
HH
RO
Ph CH3
HR
Cl
CH3
Ph
OH
OH
R
NH
R
Upfieldregion
of the spectrum
Downfieldregion
of the spectrum
TMS = Me Si
Me
Me
Me
012345678910
CH3HO
(R)

28. 13CNMR – CHEMICAL SHIFT VALUES

29. 29
1H NMR—Spin-Spin Splitting
1H NMR spectra for the alkenyl protons of
(E)- and (Z)-3-chloropropenoic acid

30. 30
The MRI image of the lower back

31. THANK YOU