Lab Report on the Determination of the Stability Constants of Molecular Complexes

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The lab report below was submitted as part of the coursework for CM2101 Principles of Spectroscopy. Please do not plagiarise from it as plagiarism might land you into trouble with your university. Do note that my report is well-circulated online and many of my juniors have received soft copies of it. Hence, please exercise prudence while referring to it and, if necessary, cite this webpage.



Aim
To determine the stability constants and molar absorptivities of the donor-acceptor complexes formed between iodine and butyl alcohol from UV-vis spectrophotometric data.

Results and calculation
Temperature of experiment = 27.5 ˚C

[I2] provided = 0.005 M
[I2] required = 0.0005M
Volume of iodine required = (0.0005 x 25.0 x 10-3) / 0.005 = 0.0025L = 2.5mL

For solution 1, volume of cyclohexane required = 25.0 – 2.5 = 22.5 mL
For solution 2 of t-butyl alcohol, c1V1=c2V2
Volume of alcohol required = (Concentration of the alcohol required x Total vol of solution) / (Conc of the given alcohol) = (0.2) x (25)/ (2.0) = 2.5mL
Volume of cyclohexane required = 25.0 – 2.5 -2.5 = 20.0 mL
Similar calculations are repeated for the other 5 solutions of t-butyl alcohol.
Table 1: Absorbance for t-butyl alcohol
Solution
Vol of I2 /mL
Vol of cyclohexane / mL
Vol of t-butyl alcohol /mL
Absorbance
l=440nm
l=460nm
l=520nm
1
2.5
22.5
0.0
0.0504
0.1168
0.4717
2
2.5
20.0
2.5
0.0951
0.1677
0.3976
3
2.5
17.5
5.0
0.1379
0.2120
0.3737
4
2.5
12.0
10.0
0.1943
0.2644
0.3284
5
2.5
7.5
15.0
0.2280
0.2949
0.2952
6
2.5
2.5
20.0
0.2631
0.3274
0.2797

The isosbestic point occurs at 491.8 nm with absorbance of 0.3293 A.



Since A =a/ [A]0, where a is the absorbance of the solution and [A]o is the initial  concentration of the acceptor (I2), the value of A can be calculated as follows:
(Solution 1 at 440nm) A = a/ [A]0 =  0.0504 / 0.0005 = 100.8 mol-1L
The absorbance, a, of the solution equals the sum of absorbances for the acceptor (A) and the complex(C): a = eA ([A]0 – [C]) + eC [C], where eA and eC are molar absorptivities of the acceptor and complex respectively. When no donors are added, no complexes are formed.
Thus [C] = 0 and a = eA [A]0
Molar absorptivity of acceptor I2, eA at  l = 440nm = a/ [A]0 =  0.0504 / 0.0005 = 100.8 mol-1 L cm-1

(Solution 2 at 440nm) A =   a/ [A]0 =  0.0951 / 0.0005 = 190.2 mol-1 L
(eA –A)/[D]0 = (100.8 – 190.2) / 0.2 = -447
Table 2:  A and (eA – A) / [D]values of t-butyl solutions at different wavelengths
Wavelength
440nm
460nm
520nm
Solution
A
eA
(ea – A) / [D]o
A
eA
(ea – A) / [D]o
A
eA
(ea – A) / [D]o
1
100.8
100.8
-
233.6
233.6
-
943.4
943.4
-
2
190.2
100.8
-447.0
335.4
233.6
-509.0
795.2
943.4
741.0
3
275.8
100.8
-437.5
424.0
233.6
-476.0
747.4
943.4
490.0
4
388.6
100.8
-359.8
528.8
233.6
-369.0
656.8
943.4
358.3
5
456.0
100.8
-296.0
589.8
233.6
-296.8
590.4
943.4
294.2
6
526.2
100.8
-265.9
654.8
233.6
-263.3
559.4
943.4
240.0

Based on the equation A= (eA – A) / (k [D]) + eC, the gradient of the graph of A against (eA – A) / [D] is 1/K and the y-intercept is eC.





At 440nm,
k
= 1/1.626
= 0.6150 mol-1 L
and
eC = 954.9 mol-1 L cm-1






At 460 nm,
k
= 1/1.172
= 0.8532 mol-1 L
and
 eC = 955.2 mol-1 L cm-1






At 520nm,
k
= 1/0.477
= 2.094 mol-1 L
and
eC = 467.0 mol-1 L cm-1


Average stability constant K = (0.6150 + 0.8532 + 2.094) / 3 = 1.187 mol-1 L
Average molar absorptivity of iodine-t-butyl alcohol complex,
eC = (954.9 + 955.2 + 467) / 3 = 792.4 mol-1 L cm-1
Similarly,

Table 3: Absorbance for n-butyl alcohol
Solution
Vol of I2 /mL
Vol of cyclohexane / mL
Vol of n-butyl alcohol /mL
Absorbance
l=440nm
l=460nm
l=520nm
1
2.5
22.5
0.0
0.0104
0.0996
0.4579
2
2.5
20.0
2.5
0.0532
0.1707
0.4114
3
2.5
17.5
5.0
0.0837
0.2142
0.3660
4
2.5
12.0
10.0
0.2140
0.2786
0.3204
5
2.5
7.5
15.0
0.2562
0.3129
0.2771
6
2.5
2.5
20.0
0.2855
0.3374
0.2518

The isosbestic point occurs at 490.8 nm with absorbance of 0.3146 A.
Table 4:  A and (eA – A) / [D]values of n-butyl solutions at different wavelengths
Wavelength
440nm
460nm
520nm
Solution
A
eA
(ea – A) / [D]o
A
eA
(ea – A) / [D]o
A
eA
(ea – A) / [D]o
1
100.8
100.8
-
199.2
199.2
-
915.8
915.8
-
2
190.2
100.8
-447.0
341.4
199.2
-711.0
822.8
915.8
465
3
275.8
100.8
-437.5
428.4
199.2
-573.0
732.0
915.8
459.5
4
388.6
100.8
-359.8
557.2
199.2
-447.5
640.8
915.8
343.8
5
456.0
100.8
-296.0
625.8
199.2
-355.5
554.2
915.8
301.3
6
526.2
100.8
-265.9
674.8
199.2
-297.3
503.6
915.8
257.6





At 440 nm,
k
= 1/1.626
= 0.6150 mol-1 L
and
eC = 954.9 mol-1 L cm-1





At 460nm,
k
= 1/0.825
= 1.212 mol-1 L
and
eC = 919.0 mol-1 L cm-1



At 520nm,
k
= 1/1.345
= 0.7435 mol-1 L
and
eC = 159.0 mol-1 L cm-1


Average stability constant K = (0.615 + 1.212 + 0.7435) / 3 = 0.8568 mol-1 L
Average molar absorptivity of iodine-n-butyl alcohol complex,
eC = (954.9 + 919 + 159) / 3 = 677.6 mol-1 L cm-1

Discussion

Ultraviolet-visible spectroscopy

The absorption of Ultraviolet-visible (UV-vis) radiation may cause electrons to transit to higher energy states. Through analysis of the resultant spectra, the identities and concentrations of compounds may be determined.

The possible electronic transitions that light may cause are shown on the left diagram.

http://www.chemguide.co.uk/analysis/uvvisible/jumps2.gif
In this experiment, when the alcohol reacts with iodine, alcohol act as a donor and iodine act as acceptor to produce the alcohol-iodine (donor-acceptor) complex. When a photon is absorbed, an electron in the highest occupied molecular orbital (HOMO) of alcohol gains enough energy to jump to the lowest occupied molecular orbital (LUMO), anti-bonding orbital (s*) of iodine. The energy of the photon corresponds to the energy gap between the HOMO and LUMO.

Theory behind experiment

Iodine shows a single maximum absorption at 520nm, in inert solvent cyclohexane. This wavelength of absorbed light corresponds to the energy require to promote electrons from ground state to excited state. When iodine forms a complex with a butyl alcohol donor, the maximum absorption occurs at a shorter wavelength. More energy is required to transfer an electron from the donor to an orbital associated with the acceptor. Molar absorptivities from charge-transfer absorption are large, proven from the results of more than 600 mol-1 L cm-1.

Spectra

From the spectroscopic data of solution 1, it can be observed that iodine absorbs at a higher wavelength. However, as butyl alcohol concentration increases, the concentration of the resultant iodine-alcohol complex increases too. This causes absorption peaks to occur at lower wavelengths and overall absorbances to decrease. The slope of the spectrum becomes increasingly gentle too.

Precautions

Water was not allowed to be present in the volumetric flasks. This is because water is a potential ligand and may form complex with iodine and hence affecting the stability constant. Thus, all apparatus used in the experiment were dry. Inert solvent, cyclohexane, was used to ensure no side interactions with the reactants.
25.0 mL solution was prepared first by adding iodine, followed by cyclohexane then butyl alcohol so as to prevent iodine from reacting rapidly with the alcohol. This decreases the enthalpy of formation and bonding forces, leading to increased accuracy o absorption spectrum. In addition, the solutions were left to stand for about an hour for all the reactions to reach equilibrium in order to achieve better absorption spectrum. Thus, the concentration of complexes formed will be higher and as absorption is dependent on concentration according to Beer-Lambert Law, the resultant spectra will be more accurate too.
Care was taken to ensure that the cuvettes inserted into the spectrometer were free of fingerprints on the transparent sides before being inserted into the spectrophotometer as fingerprints may scatter the radiation passing through, resulting to inaccuracies. The readings were taken starting from the least concentrated solution to the higher concentrations. This was done to prevent significant changes in the concentrations of solutions in the cuvette which may lead to inaccuracies. In addition, the cuvettes were rinsed three times with the solution that is to be measured before placement into the spectrophotometer.

Stability constant

Iodine-butyl alcohol complexes differ in their stabilities which may be measured from their stability constants, K which is directly proportional to complex concentration but inversely proportional to donor and acceptor concentrations. A large K value means a larger concentration of complex in equilibrium compared to the product of the concentration of reactants, thus, the complex is more stable and less likely to dissociate into iodine and alcohol.


Theoretically, the stability constant of iodine-t-butyl alcohol complex should be larger than iodine-n-butyl alcohol complex. A tertiary alcohol, with 3 electron-donating methyl groups which can donate electron density inductively to the oxygen atom, forms a more stable complex than n-butyl alcohol which has only 1 electron-donating propyl group. The lone pair in t-butyl alcohol has a higher electron density and is more available for donation to iodine than that in n-butyl alcohol, thereby forming a more stable complex. The experimental K values – higher for iodine-t-butyl alcohol complex than iodine-n-butyl alcohol complex – reaffirm this.

Conclusion
The average stability constant of iodine-t-butyl alcohol complex is 1.187 mol-1 L while that of iodine-n-butyl alcohol complex is 0.8568 mol-1 L. The average molar absorptivity of iodine-t-butyl alcohol complex is 792.4 mol-1 L cm-1 while that of iodine-n-butyl alcohol complex is 677.6 mol-1 L cm-1. Isosbestic point of spectroscopic data with t-butyl alcohol donor occurs at 491.8 nm with absorbance of 0.3293 A while that with n-butyl alcohol donor occurs at 490.8 nm with absorbance of 0.3146 A.

The results indicate that t-butyl alcohol forms a more stable complex with iodine than n-butyl alcohol.

Bibliography

1. Pavia et al.. Introduction to Spectroscopy. Brooks/ Cole. 2001.
2. Blinder, S. M. Introduction to quantum mechanics: in Chemistry, materials science, and biology. Elsevier Academic Press. 2004. p. 177
3. “Understanding Chemistry: Complex Ions ” www.chemguide.co.uk/inorganic/complexmenu.html
4. Inorganic Chemistry 4th Edition, Shriver & Atkins, Oxford University Press
5.  Banwell and McCash, Fundamentals of Molecular Spectroscopy, 4th edition
6. G. D. Christian, J. E. O’Reilly. Instrumental Analysis. Allyn and Bacon. 1986.




Answers to exercises

1.       t-butyl alcohol forms a more stable complex with iodine. A tertiary alcohol, with 3 electron-donating methyl groups which can donate electron density inductively to the oxygen atom, should form a more stable complex than n-butyl which has only 1 electron-donating propyl group. The lone pair in t-butyl alcohol has a higher electron density and is more available for donation to iodine than that in n-butyl alcohol.

On the other hand, t-butyl alcohol being branched has higher steric hindrance than the more linear n-butyl alcohol. Oxygen atom in t-butyl alcohol may be hindered from interacting with iodine.  

The higher K values for the t-butyl alcohol-iodine complex indicate that t-butyl alcohol forms a more stable complex with iodine than n-butyl alcohol. It suggests that the significance of electronic effects outweigh that of the steric interference for these molecular complexes.

              

2.       The isosbestic point is defined as the wavelength at which two species have the same molar absorptivity during a reaction.

From Beer-Lambert Law, A= εcl (where ε is molar absorptivity, c is the concentration of sample and l is the path length), the total absorbance of a system, AT is given to be AT= ε1c1l + ε2c2l. It is assumed that alcohol does not absorb in the wavelengths (400 nm – 580 nm) in this experiment.  At isosbestic point, ε1= ε2, Thus it is extremely unlikely that a third species will have the same molar absorptivity. The equation can be rewritten as AT= εl (c1+c2) where ε=ε1 + ε2.

Hence, the isosbestic point indicates that the total absorbance of a reaction solution depends on the total concentration of the solution, but not on the concentrations of the individual species in the solution. The presence of isosbestic point implies that only 2 dominant species, reactant and product, are in the solution.
3.       The absorption maximum occurs at the wavelength at which the species absorbs the most radiation and shifts upon a change in the relative concentrations of donor, acceptor used and complex produced. Absorbance reflected in the spectra is the sum of the absorbance of the individual species.

The complex absorbs more at lower wavelengths while iodine absorbs at higher wavelengths. As the concentration of donor increases, more iodine-alcohol complex forms and thus the wavelength of maximum absorption will decrease.

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