1. Quantum Mechanical
Modeling of Organic-Oxide
Surface Complexation
Reactions
UNDERGRADUATE SENIOR THESIS
DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING
UNIVERSITY OF CONNECTICUT
BRIANNA DATTI

2. Organic pollutants in the environment are a
growing concern.
Agriculture
Industry
Pharmaceutical

3. We can expand on current models to increase the
understanding of adsorption of contaminants in
the environment.
(Kung & McBride, 1989)

4. The model utilizes quantum mechanics to predict
binding energies of adsorption.
∆𝐺 = 𝑓(Ebinding, Enonbinding)
Schrödinger equation: 𝐸𝛹 = Ĥ𝛹
∆ 𝑟 𝐺°
(298𝐾) = (𝜀0+𝐺𝑐𝑜𝑟𝑟) 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠 − (𝜀0+𝐺𝑐𝑜𝑟𝑟) 𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠
∆𝐺° = −𝑅𝑇𝑙𝑛(𝐾)

5. The adsorption of organic acids to iron oxide was
investigated.
Figure 1. Adsorption of organic acids to iron oxide. The following are the organic acids: (A) meta-hydroxybenzoic acid; (B)
ortho-hydroxybenzoic acid; (C) Carboxybenzoic acid; (D) Methylbenzoic acid; (E) Methoxybenzoic acid; (F) Malonic acid;
(G) Lactic acid; (H) Phthalic acid; (I) Aminobenzoic acid; (J) Nitrobenzoic acid; (K) Bisulfide benzoic acid.

6. Results: Thermodynamic favorability of adsorption
Compound ΔrG°(298K) (KJ/mol) ΔrH°(298K) (KJ/mol)
Para-hydroxybenzoic acid -40.7038 -40.0609
Carboxybenzoic acid -47.5986 -47.5227
Methylbenzoic acid -42.3473 -41.9869
Meta- hydroxybenzoic acid -41.6365 -41.391
Ortho- hydroxybenzoic acid -40.9961 -40.7976
Nitrobenzoic acid -53.2728 -53.3361
Aminobenzoic acid -39.8923 -39.3759
Methoxybenzoic acid -40.281 -39.9421
Bisulfide-benzoic acid -45.8215 -45.6282
Malonic acid -413.5957 -413.2736
Lactic acid -42.0809 -42.2158
Phthalic acid -57.5008 -57.1641
Para-hydroxybenzoic acid with
bidentate binuclear iron oxide
-48.0664 -45.0541
Para-hydroxybenzoic acid with
bidentate mononuclear iron oxide
125.2468 127.0777

7. The existence of para-hydroxybenzoic acid
adsorbed to bidentate mononuclear iron oxide has
been debated.
Bidentate mononuclear
VS
Bidentate binuclear

8. Adsorption strength is correlated to Hammet
constants, but has little correlation to pKa values.
R² = 0.853
0
10
20
30
40
50
60
-1 -0.5 0 0.5 1
|ΔG|
Hammet Constant
R² = 0.1251
0
10
20
30
40
50
60
70
0 5 10 15
|ΔG|
pKa

9. 05001000150020002500300035004000
Intensity
Wavenumber (cm-1)
Meta-hydroxybenzoic acid
Ortho-hydroxybenzoic acid
Para-hydroxybenzoic acid
Theoretical spectra indicate bonding geometries
and allow comparisons to experimental data.
05001000150020002500300035004000
Intensity
Wavenumber (cm-1)
Bisulfide-benzoic acid
Nitrobenzoic acid
Aminobenzoic acid
Methylbenzoic
acid
Methoxybenzoic
acid
Carboxybenzoic
acid

10. 05001000150020002500300035004000
Intensity
Wavenumber (cm-1)
Bidentate mononuclear
iron oxide Bidentate binuclear iron oxide
Theoretical spectra indicate bonding geometries
and allow comparisons to experimental data.
05001000150020002500300035004000
Intensity
Wavenumber (cm-1)
Phthalic acid
Lactic acid
Malonic acid

11. Shift in spectra peaks from aqueous to adsorbed
structures represent inner-sphere complexes.
05001000150020002500300035004000
Intensity
Wavenumber (cm-1)
Ortho-hydroxybenzoic acid
Meta-hydroxybenzoic acid
Para-hydroxybenzoic acid
Aqueous Structures
05001000150020002500300035004000
Intensity
Wavenumber (cm-1)
Meta-hydroxybenzoic acid
Ortho-hydroxybenzoic acid
Para-hydroxybenzoic acid
Adsorbed Structures

12. Combining quantum mechanical modeling, as
presented, with molecular dynamics simulations will
provide a greater scope of knowledge concerning
contaminant fate.
Molecular Dynamics Simulation:
para-hydroxybenzoic acid
Inner-Sphere complexation:
∆𝐺𝑠𝑜𝑟𝑏 ≅ 6.5
𝑘𝑐𝑎𝑙
𝑚𝑜𝑙
= 27.2
𝐾𝐽
𝑚𝑜𝑙

13. Conclusions
• Thermodynamic favorability of the investigated organic acids sorption to iron oxides
o Except para-hydroxybenzoic acid adsorbed to bidentate mononuclear iron oxide
• Adsorption increases with increasing Hammet constants; electron withdrawing group
substituents having the greatest sorption
• Quantum mechanical modeling results validated by comparison of theoretical spectra
to experimental IR spectra
o Theoretical spectra indicate presence of inner-sphere and outer-sphere complexes,
with inner-sphere complexes being dominant for para substituted benzoic acids
• Combining quantum mechanical modeling and molecular dynamics simulations can
expand the study of adsorption to a whole new class of chemicals

14. Acknowledgments
Major Advisor: Dr. Chad Johnston
Fellow Thesis Students:
• Grant Bedard
• Luke McNaboe
• Stefanie Shea

15. Questions?

16. References
Kung, K. H., McBride, M. B. (1989). Adsorption of Para-substituted Benzoates on Iron Oxides. Soil Science Society of
America Journal, (53), 1673-1678.
Ochterski, J. W. (2000). Thermochemistry in gaussian. Gaussian Inc, Pittsburgh, PA, 1-17.
Gaussian 09, Frisch M. J., G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V.
Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino,
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Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J.
Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L.
Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö.
Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT, 2009.
Chad Johnston, personal communication, April 29, 2015