1 SUPPLEMENTARY DATA 1.3 Small organic molecules as corrosion inhibitors

Molecular Ionization and Deprotonation Energies as Indicators ofFunctional Coating Performance

Michael Breedon,∗ Manolo C. Per, Ivan S. Cole, and Amanda S. Barnard

1 Supplementary Data

1.1 Experimental determination of corrosioninhibition efficacy

For comparison with experimentally determined corrosion in-hibition efficacy, the results from Harvey et al. are presentedalongside the calculated values. Within the Harvey data setcorrosion inhibition efficacy is referenced against the industrialchromate standard, with the inhibitor efficiencies calculated us-ing the weight loss of the respective alloys in 0.1 M NaCl as abaseline as defined in reference1:

I% =w−w′

w×100 (1)

where, I% is the inhibitor efficiency (%); w is weight loss ofpanels after 4 weeks immersion in 0.1 M NaCl solution alone;and w′ is the weight loss of panels after 4 weeks immersionin 0.1 M NaCl + 1 mM inhibitor. Thus a compound with 0%inhibitor efficiency would show the same degree of corrosionas 0.1 M NaCl; and the inhibitor would be considered to be100% efficient if the mass loss was equivalent to the chromatestandard. Conversely, compounds with negative experimentalinhibition efficacy act as a corrosion accelerator rather than asan inhibitor.

1.2 Basis set dependenceBefore analysing the entire set, we first examine the uncer-tainties associated with the computational method using threemolecules with experimentally determined IP values (Thiophe-nol, Pyridine, 2-mercaptobenzothiazole). It is clear from Table1, that the calculated molecular IP values of thiophenol, pyri-dine and 2-mercaptobenzothiazole are basis set choice depen-dant. Minimal basis sets such as STO-3G seriously underes-timate molecular IP, and also, to a lesser extent, the SIESTADZP basis set; which lacks diffuse functions. It can be summa-rized from these results that the choice of basis set does affectmolecular IP, and that introducing polarization and/or diffusefunctions, can improve the numerical prediction of molecularIP. While the 6-311++G basis set does provide closely match-ing molecular IP energies for thiophenol and pyridine, it fails toaccurately predict the molecular IP of 2-mercaptobenzothiazole(2-MBT). Considering that the 6-311++G** basis set gives re-sults within a few percent of the experimental values for the

basis set ThiophenolIP (eV)

PyridineIP (eV)

2-MBT†

IP (eV)NIST reference 8.302 9.262 7.992

DZP ′ 7.42 8.67 7.40STO-3G 5.99 7.80 6.023-21G 8.10 9.10 8.246-21G 8.06 9.08 8.216-31G 8.03 9.09 8.196-311G 8.18 9.24 8.336-311+G 8.20 9.31 8.356-311++G 8.20 9.31 8.356-31G** 7.90 9.15 7.866-31++G** 8.06 9.34 8.026-311++G** 8.09 9.38 8.05cc-pVDZ 7.97 9.21 7.91aug-cc-pVDZ 8.07 9.33 8.02cc-pVTZ 8.05 9.30 7.99aug-cc-pVTZ 8.08 9.35 8.03

Table 1 NIST IP reference and vertical IP dependance on basis setchoice. ′ denotes a SIESTA calculation. † 2-mercaptobenzothiazole(MBT)

sub-set of molecules examined, it was selected for all futurecalculations.

Thiophenol and pyridine molecular IPs as calculated bySIESTA were within 11% of the NIST (eval) reference en-ergy, Gaussian (6-311++G**) results were significantly moreaccurate, <3% of the reference energy. It can be general-ized from the data, that with the exception of molecule 4-mercaptobenzoate, that SIESTA consistently underestimatesmolecular IP, as expected of a minimal basis set. This under-estimation may also be related to the use of pseudopotentialsin SIESTA calculations, c.f. Gaussian09 (all electron) calcula-tions.

1.3 Small organic molecules as corrosion in-hibitors

The contextual use of five selected corrosion inhibitor candi-date molecules, along with their presented optimised geome-tries are presented below.

Molecule 1H, Thiophenol also known as benzenethiol

1

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2014

REFERENCES REFERENCES

(C6H6S), is a small organic molecule which has been reportedto protect iron surfaces in an acidic medium3. On AA-2024-T3it has been experimentally determined to exhibit similar corro-sion inhibition to chromate1 and is an effective AA-2024-T3corrosion inhibitor, featuring only a thiol functional group.

Molecule 1I, Pyridine (C5H5N), is commonly used as anorganic solvent and is an excellent example of a terrible AA-2024-T3 corrosion inhibitor, however, as an ubiquitous organicsolvent a wealth of experimental data is available for compari-son, hence its inclusion. This is an excellent example of a lowmolecular weight molecule which is also a heterocyclic com-pound, and has been reported to actually accelerate corrosionfor some alloy formulations1.

Molecule 2B, Benzotriazole (C6H5N3), has been reported asa very effective corrosion inhibitor on a variety of materials in-cluding AA-2024-T31, AA-7075-T61, and Cu4. It is a classicexample of an N containing heterocyclic compound. It alsofinds utility as an additive in aircraft deicing and for protect-ing silverware during dishwashing, however it and some of itsderivatives have been identified as being toxic to aquatic life5.

Molecule 1C, Diethyl(dithiocarbamate) (C5H10NS2), iscommonly used as a copper chelating agent when prepared as asodium salt6 and has been reported to exhibit almost equivalentcorrosion inhibition on both AA-2024-T3 and AA-7075-T6 al-loys1. It has established inhibition properties on cold rolledsteel7, copper8, and brass9. This molecule is an example of acorrosion inhibitor which is a well established copper chelatingagent, and incidently occupies a relatively large volume givenits molecular weight.

Molecule 1W, Mercaptopropionate (C3H5O2S), was recentlyidentified as an equivalent corrosion inhibitor to the chromatestandard when assessed via mass loss measurement on AA-2024-T31. Within the calculated data set, this molecule isunusual as it features both a carboxylate and thiol functionalgroups in a linear conformation.

For these five inhibitor molecules their respective geometry,calculated bond angles and lengths are presented in Figure 1.Here, experimental bond lengths and angles were directly com-pared wherever possible, however in some instances where un-available. Such values were obtained from analogue molecules,and are denoted by an asterisk eg. thiophenol ΘCCC was takenfrom the corresponding angle in nitrobenzene. As can be seenfrom Figure 1, the bond lengths and angles of molecules 1H,1I, 1C, 1W and 2B are in good agreement with experimentalvalues10.

References[1] T. G. Harvey, S. G. Hardin, A. E. Hughes, T. H. Muster, P. A. White, T. A.

Markley, P. A. Corrigan, J. Mardel, S. J. Garcia, J. M. C. Mol and A. M.Glenn, Corrosion Science, 2011, 53, 2184–2190.

[2] S. Lias, Ionization Energy Evaluation, NIST Chemistry WebBook, NIST

Standard Reference Database Number 69, http://webbook.nist.gov, (re-trieved August 15, 2013).

[3] M. Bouayed, R. H., A. Srhiri, J.-Y. Saillard, A. Ben Cachir andA. Le Beuze, Corrosion Science, 1999, 41, 501–517.

[4] M. Finsgar and I. Milosev, Materials and Corrosion, 2011, 62, 956–966.

[5] A. Seeland, M. Oetken, A. Kiss, E. Fries and J. Oehlmann, EnvironmentalScience Pollution Research, 2012, 19, 1781–1790.

[6] C. J. Williams, G. Andrew and N. M. H., Electrochimica Acta, 2010, 55,5947–5958.

[7] L. Li, Q. Qu, W. Bai, F. Yang, Y. Chen, S. Zhang and Z. Ding.

[8] Q. Liao, Z. Yue, D. Yang, Z. Wang, Z. Li, H. Ge and Y. Li, CorrosionScience, 2011, 53, 1999–2005.

[9] Q. Liao, Z. Yue, Y. Li and H. Ge, Corrosion Science, 2010, 66, 125002–125006.

[10] NIST, http://cccbdb.nist.gov (accessed Aug, 2013).

2

REFERENCES REFERENCES

Figure 1 Comparison of bond lengths and angles of selected organic corrosion inhibitor molecules; yellow=sulphur, red=oxygen,blue=nitrogen, grey=carbon, white=hydrogen.

3

1.4 Adiabatic SIESTA results REFERENCES

1.4 Adiabatic SIESTA results

4

REFERENCES 1.4 Adiabatic SIESTA results

IDM

olec

ule

IP (eV

)E

A(e

V)

FG (eV

)A

v.D

E(e

V)

DE

Ran

ge(e

V)

Exp

t.(%

)

1A4,

5-D

iam

ino-

2,6-

dim

erca

ptop

yrim

idin

e6.

22-1

.32

7.54

16.4

215

.65

-16.

9087±

01B

4,5-

Dia

min

opyr

imid

ine

6.62

-1.7

48.

3617

.47

16.4

0-1

9.08

47±

31C

diet

hyl(

dith

ioca

rbam

ate)

6.49

0.92

5.57

16.9

916

.87

-17.

0997±

11D

2-M

erca

ptop

yrim

idin

e7.

84-1

.24

9.09

17.8

316

.14

-18.

4989±

41E

Pyri

mid

ine

8.04

-1.4

79.

5118

.69

18.3

9-1

9.00

-153±

11G

Ben

zoat

e8.

921.

857.

0716

.71

16.5

7-1

6.83

-80±

141H

Thi

ophe

nol

7.33

-1.5

88.

9118

.19

16.0

6-1

8.80

93±

51I

Pyri

dine

8.21

-1.8

110

.03

18.8

218

.59

-19.

06-1

39±

181J

4-ph

enyl

benz

oate

8.14

1.95

6.19

16.6

016

.50

-16.

73-7

2±

61K

4-hy

drox

yben

zoat

e8.

111.

746.

3816

.48

15.2

3-1

6.93

-34±

51L

4-m

erca

ptob

enzo

ate

7.76

1.88

5.87

16.2

714

.76

-16.

7997±

21M

6-m

erca

pton

icot

inat

e8.

082.

036.

0616

.04

14.7

8-1

6.54

94±

01N

Nic

otin

ate

8.39

2.05

6.34

16.4

016

.23

-16.

55-1

07±

111O

Ison

icot

inat

e8.

552.

136.

4216

.32

16.1

1-1

6.53

-12±

11P

Pico

linat

e8.

521.

726.

8016

.63

16.4

6-1

6.80

58±

01Q

3-m

erca

ptob

enzo

ate

7.70

1.95

5.75

16.6

016

.44

-16.

7616±

81R

Salic

ylat

e8.

131.

586.

5516

.54

15.1

0-1

7.05

-175±

321S

2-m

erca

ptob

enzo

ate

7.84

2.12

5.72

16.0

814

.55

-16.

6188±

01T

2-m

erca

pton

icot

inat

e8.

032.

235.

8015

.83

14.5

6-1

6.44

83±

21U

2,3-

mer

capt

osuc

cina

te6.

932.

734.

2014

.30

13.6

2-1

4.81

82±

11V

Mer

capt

oace

tate

7.42

2.07

5.35

14.9

714

.76

-15.

3896±

11W

Mer

capt

opro

pion

ate

8.27

1.77

6.50

15.8

515

.11

-16.

0810

0±

01X

Ace

tate

10.1

91.

218.

9816

.77

16.7

7-1

6.77

-12±

82A

2,5-

dim

erca

pto-

1,3,

4-th

iadi

azol

e7.

17-0

.94

8.11

15.4

215

.40

-15.

4426±

52B

Ben

zotr

iazo

le8.

24-1

.13

9.37

18.0

316

.20

-18.

6898±

02C

2-m

erca

ptob

enzi

mid

azol

e6.

90-1

.12

8.02

17.7

715

.48

-18.

9590±

02D

6-am

ino-

2-m

erca

ptob

enzo

thia

zole

6.10

-1.2

67.

3517

.39

15.5

4-1

8.48

89±

42E

2-m

erca

ptob

enzo

thia

zole

7.13

-0.8

07.

9317

.82

15.3

3-1

8.66

95±

5

Tabl

e2

Adi

abat

icm

olec

ular

para

met

ers

calc

ulat

edw

ithSI

EST

A(D

ZP

basi

sse

t):I

P,E

A,F

G,D

E,D

ER

ange

;exp

erim

enta

lcor

rosi

onin

hibi

tion

onA

A-2

024-

T3

take

nfr

omre

fere

nce1 .A

posi

tive

sign

conv

entio

nha

sbe

enad

opte

dfo

rall

DE

and

DE

rang

eva

lues

5

1.5 Structural representations, bond lengths and angles of all calculated molecules REFERENCES

1.5 Structural representations, bond lengthsand angles of all calculated molecules

6

REFERENCES 1.5 Structural representations, bond lengths and angles of all calculated molecules

Figure 2 Structural representations of the Harvey data set, indicating bond lengths and angles of corrosion inhibitor molecules;yellow=sulphur, red=oxygen, blue=nitrogen, grey=carbon, white=hydrogen.

Figure 3 Structural representations of the Harvey data set, indicating bond lengths and angles of corrosion inhibitor molecules;yellow=sulphur, red=oxygen, blue=nitrogen, grey=carbon, white=hydrogen.

7

1.5 Structural representations, bond lengths and angles of all calculated molecules REFERENCES

ID1A

1B1C

1D1E

1G1H

1I1J

1K1L

1M1N

1O1P

1Qd

CH

(A)

-1.10

1.101.10

1.101.10

1.101.10

1.101.10

1.101.10

1.101.10

1.101.10

dC

C(A

)1.42

1.421.52

1.401.40

1.401.40

1.401.40

1.401.40

1.401.40

1.401.40

1.40d

CO

(A)

--

--

-1.27

--

1.271.28

1.271.27

1.271.27

1.261.27

dC

N(A

)1.34

1.341.36

1.341.34

--

1.34-

--

1.341.34

1.341.34

-d

CS

(A)

1.77-

1.731.77

--

1.77-

--

1.771.77

--

-1.77

dN

C(A

)1.34

1.341.46

1.351.34

--

1.34-

--

1.331.34

1.341.34

-d

NH

(A)

1.011.01

--

--

--

--

--

--

-d

OH

(A)

--

--

--

--

0.97-

--

--

-d

SH(A

)1.38

--

1.38-

-1.37

--

1.371.37

--

-1.37

ΘC

CC◦

115115

-116

116120

120119

119121

120119

118119

118120

ΘN

CN◦

129127

-128

128-

--

--

--

--

-Θ

CN

C◦

--

117-

--

117-

--

117117

117117

-Θ

OC

O◦

--

--

-126

-126

125126

126126

126126

126Θ

SCS ◦

--

125-

--

--

--

--

--

-

Table3

Bond

lengthsand

anglesas

calculatedby

SIESTA

/DZ

P.

ID1R

1S1T

1U1V

1W1X

2A2B

2C2D

2Ed

CH

(A)

1.101.10

1.101.10

1.101.10

1.10-

1.101.10

1.101.10

dC

C(A

)1.41

1.391.41

1.531.52

1.531.52

-1.40

1.401.41

1.41d

CO

(A)

1.271.26

1.281.27

1.271.27

1.27-

--

--

dC

N(A

)-

-1.33

--

--

1.311.38

1.391.38

1.38d

CS

(A)

-1.79

1.761.82

1.821.82

-1.76

-1.76

1.761.76

dN

C(A

)-

-1.34

--

--

1.31-

1.321.30

1.30d

NN

(A)

--

--

--

-1.36

1.30-

--

dN

H(A

)-

--

--

--

-1.01

1.011.01

-d

OH

(A)

0.97-

-0.98

--

--

--

--

dSH

(A)

-1.38

1.381.38

1.371.37

-1.37

-1.37

1.371.37

ΘC

CC◦

120120

120-

--

--

116117

119118

ΘN

CN◦

--

--

--

--

-114

--

ΘN

NN◦

--

--

--

--

109-

--

ΘC

NC◦

--

118-

--

--

--

110110

ΘO

CO◦

125126

125127

126126

126-

--

--

Table4

Bond

lengthsand

anglesas

calculatedby

SIESTA

/DZ

P.

8