UV spectroscopy involves absorption spectroscopy from 160 nm to 780 nm to identify inorganic and organic species. It uses ultraviolet radiation that stimulates molecular vibrations and electronic transitions. When a molecule absorbs UV radiation, it causes excitation of electrons from occupied bonding molecular orbitals to unoccupied antibonding molecular orbitals. The absorption spectrum obtained will show "gaps" at discrete energies where particular electronic transitions match the energy of bands of UV light. Factors like conjugation, substituents, and solvent can affect the absorption spectrum by causing bathochromic or hypsochromic shifts.

1. UV spectroscopy

2. UV spectroscopy
• Ultraviolet–visible spectroscopy or ultraviolet-
visible spectro photometry (UV-Vis or UV/Vis)
refers to absorption spectroscopy or
reflectance spectroscopy in the ultraviolet-
visible spectral region.
• This means it uses light in the visible and
adjacent (near-UV and near-infrared [NIR])
ranges.
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3. UV-Visible Spectroscopy
• Absorption spectroscopy from 160 nm to
780 nm
• Ultraviolet radiation stimulates molecular
vibrations and electronic transitions.
• Measurement absorption or transmittance
• Identification of inorganic and organic
species
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4. UV Spectroscopy
Introduction
UV radiation and Electronic Excitations
1. The difference in energy between molecular bonding, non-bonding and
anti-bonding orbitals ranges from 125-650 kJ/mole
2. This energy corresponds to EM radiation in the ultraviolet (UV) region,
100-350 nm, and visible (VIS) regions 350-700 nm of the spectrum
3. For comparison, recall the EM spectrum:
4. For purposes of our discussion, we will refer to UV and VIS spectroscopy
as UV
UVX-rays IRg-rays RadioMicrowave
Visible
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5. Where in the spectrum are
these transitions?
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6. AISC/SBS/UV 6

7. Spectroscopy
Spectral Distribution of Radiant Energy
Wave Number (cycles/cm)
X-Ray UV Visible IR Microwave
200nm 400nm 800nm
WAVELENGTH(nm)
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8. Molecules containing π-electrons or non-bonding electrons
(n-electrons) can absorb the energy in the form of ultraviolet
or visible light to excite these electrons to higher anti-
bonding molecular orbitals.
The more easily excited the electrons (i.e. lower energy gap
between the HOMO and the LUMO), the longer the
wavelength of light it can absorb.
Where in the spectrum are
these transitions?
E
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9. UV Spectroscopy
The Spectroscopic Process
1. In UV spectroscopy, the sample is irradiated with the broad spectrum of
the UV radiation
2. If a particular electronic transition matches the energy of a certain band
of UV, it will be absorbed
3. The remaining UV light passes through the sample and is observed
4. From this residual radiation a spectrum is obtained with “gaps” at these
discrete energies – this is called an absorption spectrum




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10. AISC/SBS/UV 10

11. Electromagnetic Radiations
EMR consist of discrete packages of energy which are
called as photons.
A photon consist of an oscillating electric field (E) & an
oscillating magnetic field (M) which are perpendicular
to each other.
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12. AISC/SBS/UV 12

13. AISC/SBS/UV 13

14. Principle of UV
UV visible spectroscopy measure the response of
a sample to ultra violet and visible range of
electromagnetic radiation.
Molecule have either n, π (pi) or σ-sigma
electrons. These electrons absorb UV radiation
and undergoes transition from ground state to
excited state.
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15. 15AISC/SBS/UV

16. II. Instrumentation and Spectra
A. Instrumentation
1. The construction of a traditional UV-VIS spectrometer is very similar to an
IR, as similar functions – sample handling, irradiation, detection and
output are required
2. Here is a simple schematic that covers most modern UV spectrometers:
samplereference
detector
I0
I0 I0
I
log(I0/I) = A
200 700
l, nm
monochromator/
beam splitter optics
UV-VIS sources
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17. AISC/SBS/UV 17

18. UV Spectroscopy Instrumentation and Spectra
Source:- Two sources are required to scan the entire UV-VIS band:
• Deuterium lamp – covers the UV – 200-330
• Tungsten lamp – covers 330-700
Sample container:- quartz or fused silica cells are used
As with the dispersive IR, the lamps illuminate the entire band of UV or visible
light; the monochromator (grating or prism) gradually changes the small
bands of radiation sent to the beam splitter
The beam splitter sends a separate band to a cell containing the sample
solution and a reference solution
The detector measures the difference between the transmitted light through
the sample (I) vs. the incident light (I0) and sends this information to
the recorder
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19. Decreasing wavelength in nm
Increasingabsorbance*
Beer Lambert Law
A = .c.l
cuvette
source
slit
detector
19AISC/SBS/UV

20. Beer-Lambart’s Law
• Beer-Lambart’s Law stated that absorbance of a sample directly
proportional to its concentration (c) and the path length of sample
container i.e. cuvette
A α c x l
A = .c.l
Whrere A = absorbance; c = concentration in moles l-1;
l = path length in cm ;
 = molar absorptivity (also known as molar extinction coefficient)
which has units of moles-1 L cm -1.
Glass cell filled with
concentration of solution (C)
II
Light
0
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21. Absorbance (A)
Absorbance or optical density is defined as the log of the ratio of
intensity of incidence light and transmitted light.
A = log (Io /I )
From the spectrometers point of view, absorbance is the inverse of
transmittance:
Glass cell filled with
concentration of solution (C)
II
Light
0
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22. Vacuum ultraviolet spectroscopy
• UV techniques cannot be used below 200nm,becouse the radiation
are absorbed by Air due to presence of moisture and also due to
electronic transition in atmospheric Oxygen, Nitrogen & carbon
dioxide.
• In order to study transition below 200nm one must use vacuum
apparatus.
• So UV region below 200nm is also known as vacuum region
spectroscopy.
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23. Electronic Excitation
Absorption of radiation of wavelength 200-750nm
in visible and UV region.
These cause excitation of electrons from occupied
bonding molecular orbital (lower energy) to
unoccupied antibonding molecular orbital (higher
energy) This excitation is called electronic
excitation.




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24. Here is a graphical representation of energy
level diagram for MO.
Energy
s

s

n
Atomic orbitalAtomic orbital
Molecular orbitals
Occupied levels
Unoccupied levels
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25. Observed electronic transitions
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 compds.
O, N, S, halogens
carbonyls
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26. 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 √
The absorption of electromagnetic radiation by organic molecules in UV and
visible region involves promotion of electron from ground state to higher
energy level.
Some transitions and their wavelength are given below.
s

s

n
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27. Types of electronic transitions
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28. Absorption: Physical Basis
Absorption occurs when the energy contained in a photon is
absorbed by an electron resulting in a transition to an
excited state
Since photon and electron energy levels are quantized, we
can only get specific allowed transitions
~ 115 nm
~ 200 – 400 nm
~ 150-250 nm
~ 400 - 700 nm
E=h (h = 6.626*10-34 Js)
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29. AISC/SBS/UV 29

30. AISC/SBS/UV 30

31. AISC/SBS/UV 31

32. AISC/SBS/UV 32

33. AISC/SBS/UV 33

34. AISC/SBS/UV 34

35. AISC/SBS/UV 35

36. Plot-
Absorption Vs Wavelength
• We know that UV radiation are more energetic.
• Absorption of these radiations by Organic compound bring electronic
transitions.
• The process of electronic transition is accompanied by large number
of vibration and still large number of rotational change.
• It is hardly possible to resolve all these spectral bands which is
mixed together. So bands in UV spectrum are relatively broad.
AISC/SBS/UV 36

37. Absorption: Lineshape

h
*
So, our absorption spectrum
should probably look like this:
But they don’t…
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38. Absorption: Lineshape
This is because molecules are always rotating and vibrating.
Each rotational or vibrational state slightly changes the
energy of the transition.
Distrubtion of these states is… a random walk.
So the lineshape of our
absorption spectra is…
normally distributed
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39. E1
V0
V1
V2
V3
R0
R1
R2
R3
E2
E
R0
R1
R2
R3
e
-

40. AISC/SBS/UV 40

41. AISC/SBS/UV 41

42. Absorption: Lineshape
Absorption is an additive property, that is, the spectrum is
the sum of the Gaussians associated with each transition
normal distribution.mw
This can make looking at complex solutions quite hard,
especially if we are trying to say something about intensity
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43. AISC/SBS/UV 43

44. Terms used in UV spectroscopy
• Chromophore:- (chroma- colour phoros- bearing)
According to Witt’s theory some groups are responsible for absorption
of radiation So chromophore is defined as an isolated functional
group capable of absorbing UV radiation
Some important chromophore are:
-NO2,-N=O, -N=N-, -C=O,-C=N , C=C, -C=S etc.
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45. List of simple chromophores
• chromophores
– only molecular moieties likely to absorb light in the 200 to 800 nm region
– pi-electron functions
– hetero atoms having non-bonding valence-shell electron pairs.
– The oxygen non-bonding electrons in alcohols and ethers do not give rise to absorption above
160 nm. Consequently, pure alcohol and ether solvents may be used for spectroscopic studies.
• The presence of chromophores in a molecule is best documented by UV-Visible
spectroscopy
• but the failure of most instruments to provide absorption data for wavelengths below 200
nm makes the detection of isolated chromophores problematic.
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46. Auxochrome:-
• It is a functional group which itself does not absorb radiation in near
UV region but when attached to chomophore, shift the absorption to
longer wavelength i.e. increase the colour intensity
• Substituent's that increase the intensity and often wavelength of an
absorption are called auxochromes
• Auxochrome is a Greek word arising from two word roots; ‘auxo’
meaning “to increase” and ‘chrome’ meaning “color”
• An auxochrome is a functional group of atoms with one or more lone
pairs of electrons which, when attached to a chromophore, alters
both the wavelength and intensity of absorption.
Some important auxochrome:-
-OH, -OCH3, -NH2,-NHR, -SH etc.
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47. OH NH2
Benzene Phenol Aniline
255 nm 270 nm 280 nmλmax
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48. Conjugated dienes
As conjugation is increased in a molecule,
more delocalization (stability) of the 
electrons results. The effect of this
delocalization is to decrease the energy level
gap of molecular orbital.
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49. AISC/SBS/UV 49

50. UV Spectroscopy
III. Chromophores
C. Substituent Effects
2. Conjugation – Alkenes
When we consider butadiene, we are now mixing 4 p orbitals giving 4
MOs of an energetically symmetrical distribution compared to ethylene
Y2*
pY1
Y1
Y2
Y3
*
Y4
*
DE for the HOMO LUMO transition is reduced
ethylene
butadiene
octatetraene
50

51. UV Spectroscopy
Chromophores
C. Substituent Effects
2. Conjugation – Alkenes
Extending this effect out to longer conjugated systems the energy gap
becomes progressively smaller:
Energy
ethylene
butadiene
hexatriene
octatetraene
Lower energy =
Longer wavelenghts
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52. Effect of conjugation on position of UV bands
O
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53. UV Spectroscopy
III. Chromophores
C. Substituent Effects
2. Conjugation – Alkenes
Similarly, the lone pairs of electrons on N, O, S, X can extend conjugated
systems – auxochromes
Here we create 3 MOs – this interaction is not as strong as that of a
conjugated p-system
Y2
p
Y1
A
p*
nA
Y3
*
Energy
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54. Conjugations / resonance
increase the bathochromic
shift
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55. Effect of Conjugation and
Aromaticity of Chromophores
As conjugation is increased in a molecule,
more delocalization (stability) of the p
electrons results. The effect of this
delocalization is to decrease the p*
molecular orbital. The result is a decrease
in transition energy from p-p* and thus a
red or bathochromic shift. The molar
absorptivity will increase in this case and
better quantitative analysis will be
achieved.
AISC/SBS/UV 55

56. 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 shift
iv. Hypochromic shift
200 nm 700 nm
e
Hypochromic
Hypsochromic
Hyperchromic
Bathochromic
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57. Bathochromic shift (Red shift)
• An absorption to a longer wavelength is known as bathochromic
shift or red shift
It is produced by
1.Change in medium i.e. solvent,solvent polarity.
2.When auxochrome is attached to chromophore system
e Bathochromic
Examples:
phenol to its phenoxide ion results in
bathochromic shift
And
P-nitrophenol shows red shift in alkaline
medium
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58. Conversion of phenol to its phenoxide ion
results in bathochromic shift
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59. P-nitrophenol shows red shift in alkaline medium
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60. Hypsochromic shift (Blue shift)
• Hypsochromic shift or Blue shift is a shift of absorption toward a
shorter wavelrngth.
• The increase in solvent polarity causes blue shift
• Loss of non-bonding pair of electron also lower the wavelength
absorption
• Examples:
1) Aniline shows hypsochromic shift in acidic medium
e
Hypsochromic
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61. Aniline shows blue shift in acidic medium
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62. Hyperchromic & Hypochromic shift
• Hyperchromic shift :
when intensity of absorption increase i.e. e value increase, it is known
as hyperchromic shift.
• Hypochromic shift:
when intensity of absorption decrease i.e. e value decrease, it is
known as hyperchromic shift.
e
Hypochromic
Hyperchromic
62

63. Care must be taken when choosing a solvent, because many
solvents absorb in the ultraviolet region.
63

64. Optically clear solvent concept
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65. Polar solvent Vs Non-polar Solvent
The excited states of most π→π* transitions are more
polar than their ground states because a greater charge
separation is observed in the excited state.
If a polar solvent is used the dipole–dipole interaction
reduces the energy of the excited state more than the
ground state,
hence the absorption in a polar solvent such as ethanol
will be at a longer wavelength (lower energy, hence lower
frequency) than in a non-polar solvent such as hexane
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66. The reverse is also observed if the excited state reduces the
degree of hydrogen bonding.
Here the transitions are n→π* and the shift of wavelength is
due to the lesser extent that the solvent can hydrogen bond
to the excited state.
Carbonyl groups in particular hydrogen bond to their solvent.
For example changing from hexane to water as the solvent
for propanone, the absorption maximum moves from 280 to
257nm
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67. AISC/SBS/UV 67

68. A net increase in energy required for a n-*
transition is thus observed in protic
solvents; like water or alcohols. Therefore,
an increase in energy will reflect a decrease
in transition wavelength, or what is called
hypsochromic shift or blue shift.
On the other hand, a -* transition is affected
in an opposite manner with solvent polarity.
In presence of a polar solvent, the more polar
p* orbital will be more stabilized than the 
orbital leading to a net decrease in the
transition energy. This results in an increase
in transition wavelength or what is called a
bathochromic shift (red shift).
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69. Calculation of ƛmax by Woodward and Fieser Rule.
• Extensive correlation betweem UV spectra and
the organic compounds lead to simple
empirical rules for calculation of wavelength
maxima. these rules are known as
“Woodward-Fieser Rule”
• This quantification is referred to as the
Woodward-Fieser Rules which we will apply to
three specific chromophores:
» Conjugated dienes
» Conjugated dienones
» Aromatic systems
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70. Terms used in Woodward-Fieser Rule:
1.A linear conjugation or acyclic conjugated diene.
2.Transoid double bond:
if acyclic diene exist in trans geometrical isomerism, it is termed as
transoid.
3.Exocyclic double bonds:
if any doule bond of diene is out side to ring is called exocyclic
double bond.
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71. Hetroannular & Homoannular diene
Hetroannular diene:
in hetroannular diene, two double bonds are confined to two
different rings. However, these double bond exocyclic to ring
• Homoannular diene:
In homoannular diene, two double bonds are present in same ring..
?
AISC/SBS/UV 71

72. • Extended conjugation or extra conjugation:
• If there is extra double bond in cojugation with parent diene, it is
called extented conjugation/s
Cross conjugation system:
in such system, more reliable prediction comes from chromophore that is
highly substituted or have extended conjugation.
AISC/SBS/UV 72

73. Woodward-Fieser Rules for Dienes
• The rules begin with a base value for max of the
chromophore being observed:
acyclic butadiene = 214 nm
• The incremental contribution of substituent's is
added to this base value from the group tables:
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74. Table of values
AISC/SBS/UV 74

75. UV Spectroscopy
A. Dienes
1. General Features
Two possible p p* transitions can occur for butadiene Y2 Y3
* and Y2
Y4
*
The Y2 Y4
* transition is not typically observed:
• The energy of this transition places it outside the region
typically observed – 175 nm
• For the more favorable s-trans conformation, this transition is
forbidden
The Y2 Y3
* transition is observed as an intense absorption
s-trans s-cis
175 nm –forb.
214 nm 253 nm
175 nm
Y4
*
Y2
Y1
Y3
*
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76. Polyenes, and Unsaturated Carbonyl
groups;an Empirical triumph
Homoannular, base 253
nm
heteroannular, base 214 nm
Acyclic, base 217 nm
Attached group increment, nm
Parent base value 214 nm
Homoannular Ring +39
Extend conjugation +30
exocyclic bonds Addn +5
Alkyl +5
O-Acyl 0
O-R +6
Cl, Br +5
S-alkyl +30
NR2 +60
Total = ______ nm
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77. 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
Examples of dienes
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78. Woodward-Fieser Rules for enone system
(α-β unsaturated carbonyl compound)
O
O
O
O
O
O
H
O
O
AISC/SBS/UV 78

79. Enones
O
x

bb
X=H 207
X=R 215
X=OH 193
X=OR 193
215 202 227
Base Values, add these increments…
Extnd C=C +30
Add exocyclic C=C +5
Homoannular diene +39
alkyl +10 +12 +18 +18
OH +35 +30 +50
OAcyl +6 +6 +6 +6
O-alkyl +35 +30 +17 +31
NR2
S-alkyl
Cl/Br +15/+25 +12/+30
 b g d,+
O
O
O
79

80. AISC/SBS/UV 80

81. Some examples – keep in mind these are more complex than dienes
cyclic enone = 215 nm
2 x b- alkyl subs.(2 x 12) +24 nm
239 nm
Experimental value 238 nm
cyclic enone = 215 nm
extended conj. +30 nm
b-ring residue +12 nm
d-ring residue +18 nm
exocyclic double bond + 5 nm
280 nm
Experimental 280 nm
O
R
O
AISC/SBS/UV 81

82. Some Worked Examples
O
Base value 217
2 x alkyl subst. 10
exo DB 5
total 232
Obs. 237
Base value 214
3 x alkyl subst. 30
exo DB 5
total 234
Obs. 235
Base value 215
2 ß alkyl subst. 24
total 239
Obs. 237AISC/SBS/UV 82

83. Aromatic compounds values
R
O
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84. AISC/SBS/UV 84

85. AISC/SBS/UV 85

86. Application of UV spectroscopy
• UV spectroscopy has many appication in the field of organic
chemistry
1.Determination of structure
2.Study of geometric isomerism
3.Study of chemical reaction
4.Determination of stereochemistry
5.Determination of strength of Hydrogen bonding
6.Study of charge transfer spectra
7.In Quantitative and Qualitative analysis
AISC/SBS/UV 86

87. • So UV and visible spectroscopy provide us information about
structure of the molecule that contain double bond or triple
bond or conjugated bonds.
• This spectroscopy can distinguish
•
1)between conjugated and isolated double bonds.
2) between diene and trienes
3) between carbonyl compound and a-b unsaturated
double bond
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88. • Examples to Solve
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89. AISC/SBS/UV 89