Chemical Applications of Density-Functional Theory - American

Chapter 25

Copper Corrosion Mechanisms of Organopolysulfides Anne M . Chaka , John Harris , and Xiao-Ping Li 1

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Lubrizol Corporation, 29400 Lakeland Boulevard, Wickliffe, OH 44092-2298 Biosym/Molecular Simulation, 9685 Scranton Road, San Diego, CA 92121-3752

Downloaded by UNIV ILLINOIS URBANA on March 17, 2013 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0629.ch025

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Organopolysulfide lubricant additives are effective antiwear agents which protect ferrous metal components, but also cause corrosion of copper-based alloys such as bronze and brass in the same mechanical systems. In commercial organopolysulfides of the type R - ( S ) - R , the corrosive behavior of polysulfides dramatically increases when n ≥4. Three possible reasons for this behavior are examined using local and nonlocal density functional theory as well as post-Hartree-Fock theory at the M P 2 level. In addition we present some of the first results using a new density functional program, Fast_Structure, based on the Harris functional. n

Organopolysulfide lubricant additives which effectively passivate and protect ferrous metals often corrode copper-containing metal alloys such as bronze and brass. This, is a serious limitation as ferrous and non-ferrous metals are commonly used to fashion different parts of the same mechanical system. The ultimate goal is to minimize the corrosive behavior of the additives while preserving their effective wear protection performance. In commercial organopolysulfides of the type R - S - R , where « = 2 - 6 , the longer sulfide chains are known empirically to be much more corrosive with respect to copper than the shorter chains where η < 3. Currently little is known about the corrosion mechanisms involved which can explain this difference in reactivity. W e propose three hypotheses i n an effort to determine why the corrosive behavior of polysulfides dramatically increases when η > 4. The first hypothesis proposes that the S-S bonds are weaker and hence more reactive in the longer polysulfides. The second hypothesis suggests that the longer polysulfides are more corrosive because they are capable o f removing copper atoms from the surface via a chelation mechanism. The third hypothesis considered is that the steric bulk of the hydrocarbon side chains can inhibit corrosion by limiting the contact of the sulfur with the surface i n the shorter polysulfide chains, but not for the inner sulfur atoms in the longer chains. In this study we present some of the first results utilizing a new density functional program Fast_Structure (FS) (7), based on the Harris functional with trial densities constructed from spherically symmetric site-densities. These results are compared with Kohn-Sham (2) density functional theory with and without nonlocal gradient corrections, as well as post-Hartree-Fock results at the M P 2 level (3). n

0097-6156/96/0629-0368$15.00/0 © 1996 American Chemical Society In Chemical Applications of Density-Functional Theory; Laird, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

25.

CHAKA E T AL.

Copper Corrosion Mechanisms

ofOrganopolysulfides

369


Methodology Polysulfides are linear molecules with dihedral angles of approximately 90°. Disulfides can exist in either d or / conformations, and polysulfides can form either right or left-handed helices. Chains with at least five atoms can be described as either cis or trans depending on whether the two terminal atoms are on the same or opposite side of the plane formed by the three central atoms. Fibrous sulfur S*, exists in the a l l trans conformation, forming a helical structure (4). In this study we use polysulfides in the all-irans conformation, analogous to S«,, and with twofold symmetry. Starting geometries for all neutral species in this study were obtained using Fast_Structure. Additional refinement was performed using Hartree-Fock self consistent field ( S C F ) theory and M P 2 methodology as implemented in H O N D O 94.8,(5) and K o h n - S h a m D F T i n D M o l 2.3.5 ( R S - C u O H + R S »

In Chemical Applications of Density-Functional Theory; Laird, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

25.

CHAKA E T AL.

Table X I I . Bond Dissociation Energies for C H S C H 3

SCF

W

3

DMol (VWN) 89.7

DMol (BLYP) 60.4

(kcal/mol)

CH3S-SCH3

29.4

MP2/ DZP 59.6

CH3S-SSCH3

21.6 22.5 22.8 22.4

48.0 50.3 50.6 50.5

68.9 71.1 64.8 69.1

49.7 49.2 47.4 45.3

46

11.8 13.5

34.8 37.4

46.1 48.4

37.8 37.2

36.6

CH3SS-SSSCH3 CH3SSS-SSSCH3

14.4

39.7

50.7

36.7

CH3S-SSSCH3 CH3S-SSSSCH3 CH3S-SSSSSCH3 CH3SS-SSCH3


377

Copper Corrosion Mechanisms of Organopolysulfides

Expt. 67-73.2

a

b

e

e x p e r i m e n t a l S-S bond dissociation energies for C H 3 S - S C H 3 in kcal/mol: 67 (29), 69 (24), 70.7 (21) 73.2 (29). R e f . 30. R e f . 31.

b

c

For this and subsequent reactions energies in this work involving metal containing clusters, we report nonlocal DFT-based results due to greater computational efficiency, although both M P 2 and B L Y P have exhibited trends consistent with each other and with the majority of experimental data. A s can be seen from the results in Table ΧΠΙ, considerable energy from the bond dissociation is recovered from the interaction of the radical fragments with C u O H . For D M o l V W N , this amount ranges between 55 and 77 kcal/mol depending upon which fragment interacts with the surface, making the reaction very exothermic even i f only one of the two fragments is stabilized by the surface. For both fragments, the amount of energy recovered would be twice that amount. For D M o l B L Y P , the amount of energy recovered ranges between 37 and 52 kcal/mol. If the uncaptured fragment is the methylsulfide radical C H 3 S the bond dissociation reaction is still endothermic by 5.4 to 10.4 kcal/mol, but becomes exothermic when the uncaptured fragment is the disulfide radical C H 3 S S * . If both fragments are 'recovered' by the 'surface', then the bond dissociation becomes spontaneous i n all cases i n D F T . T o design a finite cluster to include more than one copper atom, several considerations are involved. Ideally one would like a cluster which models the reactivity, electron density, and the symmetry of the periodic surface, yet is sufficiently small to be computationally feasible. In addition, one would like to eliminate dangling bonds, except at the surface, yet maintain electronic neutrality, as electrostatic forces are very long range and can easily distort the charge density of the cluster. In the crystal structure of C u 0 shown in Figure 2, one can see that each oxygen atom is tetrahedrally coordinated in a diamond-like structure, with linearly coordinated copper atoms serving as spacers between each pair of oxygen atoms. Hence, a logical cluster would be one shown in Figure 3a, with hydrogen atoms being used to replace the terminal shell of copper atoms. Assuming each copper atom forms one covalent and one Lewis acid bond, and each oxygen forms two covalent and two L e w i s base bonds, the cluster w i l l have a net +5 charge. A neutral cluster can be constructed by using only four terminating hydrogen atoms instead of nine as shown in Figure 3b. The positions of the terminating hydrogens are optimized, as are the e

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378

CHEMICAL APPLICATIONS O F DENSITY-FUNCTIONAL

THEORY

Figure 2. Crystal structure of C u 0 showing tetrahedrally coordinated oxygen atoms and linear coordinated copper. 2

Figure 3. Copper oxide clusters: (a). [ C u 0 4 H ] 4

9

+ 5

(b). C u 0 H 4

4


4

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CHAKA ET AL.

Copper Corrosion Mechanisms

379

of Organopolysulfides

Table X I I I . Reaction Energy for RS-SR* + C u O H -> R S - C u O H + R S * (kcal/mol)

3

DMol (VWN) 12.3 8.8 4.6 10.1 2.6

DMol (BLYP) 7.8 5.4 6.8 10.4 7.0

S* CuOH + *SSCH SS* CuOH + *SSCH SSS* CuOH + *SSCH SSSS* CuOH + *SSCH

3

-8.6 -13.9 -18.1 -12.7

-2.8 -6.5 -5.2 -1.5

CH SSS* CuOH + *SSSCH

3

-15.8

-5.8

Reaction Product


CH CH CH CH CH CH CH CH CH

3

3

3

3

3

3

3

3 3

S* CuOH + *SCH SS* CuOH + *SCH SSS* CuOH + *SCH SSSS* CuOH + *SCH SSSSS* CuOH + *SCH 3

3

3

3

3

3

3

3

central copper and its adjacent oxygen atom, while the remainder of the atoms in the cluster are fixed. M u l l i k e n population analyses performed with D S o l i d (34) on C u 0 with threedimensional periodic boundary conditions shows a charge of +0.41 on copper and -0.81 on oxygen i n the crystal. This charge on copper is reproduced almost exactiy in C u O H (0.41) and very closely in the neutral C u 0 H cluster (0.38). The effect of the +5 charge on the cluster in Figure 3a is apparent when compared to the electronically neutral C u O H and C u 0 H clusters when each forms a complex with •SH. F r o m the geometries in Table X I V and the Mulliken populations i n Table X V , it can be clearly seen that the +5 charge results in a slight lengthening of the C u - S bond (0.05Â), a shortening of the C u - 0 bond (0.05Â), and considerable shifting of electron density away from the reactive site. In the +5 charged cluster, the sulfur atom has a charge of +0.31, whereas in the neutral clusters C u O H and C u 0 H , it has charges of -0.33 and -0.44, respectively, indicating an additional transfer of approximately 2/3 of an electron from the sulfur atom to the copper oxide cluster. The additional charge resides almost exclusively on the terminal hydrogen and peripheral oxygen atoms. The charge on the central copper atom in all three S H complexes is relatively unaffected by the total charge, with populations ranging from 0.26 to 0.31. For the neutral clusters, a smaU amount of charge is actually transferred from the cluster to the sulfur atom, 0.08e for the C u O H cluster and O.lSe for the C u 0 H cluster. There is some reference in the literature (35) to reductive cleavage of polysulfides by metal surfaces i n which an electron is transferred from the metal to the sulfur to produce the sulfide anion: 2

4

4

4

4

4

4

4

4

4

4

4

4

R - S - S - R ' -> R - S * + R ' S : metal

For a passivated metal surface such as copper oxide where the copper is i n a formal +1 oxidation state, such reductive cleavage is clearly not the case. Examination of the electronic structure shows that the very stable d electronic configuration of the copper atom remains intact. Surprisingly, whether or not the cluster is neutral has a negligible effect on the binding energy despite the rather considerable effect on the charge distribution. The binding energy of H S * to the charged cluster is -83.7 kcal/mol, only 0.5 kcal/mol lower than the-83.2 kcal/mol obtained for the neutral C u 0 H U cluster. The-108.9 1 0

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CHEMICAL APPLICATIONS OF DENSITY-FUNCTIONAL THEORY

380

Table X I V . Geometry of HSCuOX complexes HS-CuOH HS*Cu 0 H [HS*Cu 04H ]


4

r(HS)A r(SCu) Â r(CuO)À

1.371 2.072 1.753

1.384 2.161 1.930

Chemical Applications of Density-Functional Theory - American

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