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Model compound vulcanization studied by XANESTo cite this article W Taweepreda et al 2009 J Phys Conf Ser 190 012150

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Model Compound Vulcanization Studied by XANES

W Taweepreda12

R Nu-Mard1 W Pattanasiriwisawa

3 and P

Songsiriritthigul34

1Membrane Science and Technology Research Center Polymer Science Program

Faculty of Science Prince of Songkla University Hat-Yai Songkhla 90112 Thailand

2NANOTEC Center of Excellence at Prince of Songkla University Hat-Yai Songkhla

90112 Thailand

3Synchrotron Light Research Institute 111 University Avenue Muang Nakhon

Ratchasima 30000 Thailand

4School of Physics Suranaree University of Technology Muang Nakhon Ratchasima

30000 Thailand

E-mail wirachtpsuacth

Abstract Squalene has been used as a model compound for the investigation of sulphur

crosslink in the vulcanization process The effects of the accelerator on the crosslink were

deduced from the sulfur K-edge absorption spectra The majority of the crosslinks for the

squalene vulcanized with ZDEC or TMTD is likely disulfidic while that vulcanized with CBS

or MBTS is monosulfidic

1 Introduction Natural rubber consisting of cis-14-polyisoprene exhibits outstanding properties such as high

elasticity high toughness and high abrasion resistance The properties of natural rubber can further be

improved by introducing sulfur crosslink to natural rubber in a vulcanization process By choosing

proper vulcanization conditions required properties of natural rubber suitable for specific applications

may be obtained Although different kinds of natural rubber products are widely and commercially

available some products may still need further improvement Thus better understanding of the

vulcanization in a molecular scale is necessary To explore the complicated sulfur crosslink in

vulcanized natural rubber solid-state NMR [1-2] and X-ray absorption near-edge spectroscopy (XANES) [3-7] have been employed as characterization techniques Because of the difficulties to

work with the complex polymer network in the vulcanized natural rubber simpler model compounds

with molecular structure similar to natural rubber have been used to try to understand the

vulcanization process For example squalene (C30H50) has been used in the model compound

vulcanization (MCV) for the investigations of the by-products [8] adhesion [9] reversion [10-11] and

vulcanization mechanism dissertation [12] To our knowledge there was no report on XANES study

of vulcanized squalene Thus it is interesting to carry out XANES investigations of sulfur crosslink in

vulcanization system with squalene as a model compound In this preliminary MCV study squalene

was vulcanized with only four different sulfur-containing accelerators Sulfur (S8) was not added in the

14th International Conference on X-Ray Absorption Fine Structure (XAFS14) IOP PublishingJournal of Physics Conference Series 190 (2009) 012150 doi1010881742-65961901012150

ccopy 2009 IOP Publishing Ltd 1

mixture for the vulcanization Sulfur K-edge absorption spectra of the pure accelerators and of the

vulcanized squalene were measured and studied

2 Experiment

Squalene with purity of 999 as per GC was purchased from Merck Co Ltd Germany Four

vulcanization systems were prepared with the squalene-accelerator-ZnO phr ratio of 10011 The

accelerators were N-cyclohexylbenzothiazole sulfenamide (CBS) 22 dibenzothiazyl disulfide

(MBTS) tetramethylthiuram disulfide (TMTD) and zinc diethyldithiocarbamate (ZDEC) The

vulcanization reaction was carried out at 140 degC for 24 hours in closed vials to avoid oxidation of the

double bonds of squalene A continuous stirring was required to assure the homogeneity of the

reaction mixture To stop the reaction the vials were taken out from the heating bath and left to cool

down to room temperature

XANES measurements were carried out at the beamline 8 of the Synchrotron Light Research

Institute (SLRI) in Thailand [13] The energy scan was carried out using the Si(111) double crystal

monochromator to cover the K-edge of sulfur The white line of zinc sulfate (ZnSO4) powder was used

as the reference energy of 24814plusmn01 eV [15] The reproducibility of this energy in our measurements

is better than 007eV The step width of the energy scan used for in this work is 01 eV For the

vulcanized squalene samples the measurements were performed in a fluorescence mode using a 5-grid

ionization Lytle fluorescence detector The current of the incident beam I0 was measured by using an

ionization chamber located in front of the sample The spectra were performed in the liquid phase by

dropping the vulcanized squalene on a piece of sulfur-free paper For the pure accelerators the

XANES spectra were measured in the powder form in the standard transmission mode

3 Results and Discussion

Figure 1 shows sulfur K-edge XANES spectra of vulcanized squalene with four different accelerators

ie CBS MBTS ZDEC and TMTD and the unreacted accelerators It should be noted that the

absorption spectrum of pure squalene shows no absorption K-edge of sulfur confirming that there is

no sulfur in squalene molecule (the spectrum is not shown) This is one of the reasons for choosing

squalene instead of natural rubber for this study While natural rubber contains sulfur in L-metionine

(CH3-S-CH2-CHNH+3-CO-

2) which is one of important amino acids in natural rubber latex [16] It is

obvious that the spectra of the vulcanized samples are different from those of the unreacted

accelerators used in the vulcanization process indicating that the surroundings of the sulfur atoms in

all accelerators have been changed during the vulcanization process though sulfur (S8) was not used It

is interesting that sulfate by-product was not observed in the spectra of the vulcanized samples

Fig 2 shows the zoom-up XANES spectra of the vulcanized squalene with the four accelerators

Vulcanized squalene with ZDEC or TMTD accelerator show absorption peak lower than that with

CBS or MBTS accelerator The position of the peak in the absorption spectra may be used as the

indicator for the type of the σ resonance The assignment of the σ resonance peaks referred in this

work are based on the reports of Hitchcock [17] Sze [18] and George [19] There has also been the

reports that the absorption peak shifts towards higher photon energy when the number of sulfur in C-

Sx-C (x=1234) chains decreases [46] The absorption peak of the vulcanized squalene with CBS and

MBTS are observed at 24731 and 24730 eV respectively These peaks correspond to the resonance

of σ (S-C) The XANES spectra of the vulcanized squalene with ZDEC and TMTD accelerators shift

towards lower photon energy with the peak position at 24726 eV and 24723 eV respectively These

peaks may be assigned to the resonance of σ (S-S) This indicates that ZDEC and TMTD accelerators

yield disulfidic crosslink in vulcanized squalene The higher photon energy of the peak in the XANES

spectra of squalene vulcanized with CBS or MBTS accelerator indicates that the majority of the

crosslink may be monosulfidic It is interesting to point out from the measured XANES spectra that

14th International Conference on X-Ray Absorption Fine Structure (XAFS14) IOP PublishingJournal of Physics Conference Series 190 (2009) 012150 doi1010881742-65961901012150

2

the effects of CBS and TMTD accelerators on the vulcanized squalene are similar to that of the

vulcanized natural rubber previously reported [7]

Fig 1 Sulfur K-edge of XANES spectra of vulcanized squalene with (a)

CBS (b) MBTS (c) ZDEC and (d) TMTD accelerators

Fig 2 The zoom up XANES spectra in figure 1 illustrating the position of

the sulphur absorption peak

14th International Conference on X-Ray Absorption Fine Structure (XAFS14) IOP PublishingJournal of Physics Conference Series 190 (2009) 012150 doi1010881742-65961901012150

3

4 Conclusion We have used squalene as a model compound for the study of the vulcanization The effects of CBS

and TMTD accelerators on the crosslink in the vulcanization process of squalene were found to be

similar to that of natural rubber It was found that vulcanization of squalene with ZDEC and TMTD

accelerators yield disulfidic crosslink while with CBS and MBTS accelerator result in monosulfidic

crosslink It is interesting to point out that sulfate by-product was not observed after the vulcanization

process

5 Acknowledgement The authors would like to acknowledge Prof Josef Hormes for useful discussion One of the authors

(RN) would like to thanks SLRI for the scholarship (contract number 255003-GS-50-M03) This

work is supported by the National Science and Technology Development Agency of Thailand

(contract number F-31-102-18-01)

References [1] Boochathum P and Chiewnawin S 2001 Eur Ploym J 37 429-434

[2] Boochathum P and Prajudtake W 2001 Eur Ploym J 37 417-427

[3] Chauvistreacute R Hormes J Bruumlck D Sommer K and Engels H W 1992 Kautsch Gummi Kunstst

45 808-813

[4] Chauvistreacute R Hormes J Hartmann E Etzenbach N Hosch R and Hahn J 1997 Chem Phys

223 293-302

[5] Modrow H Zimmer R Visel F and Homes J 2000 Kautsch Gummi Kunstst 53 328-337

[6] Hormes J and Modrow M 2001 Nucl Instum Meth A 467-468 1179-1191

[7] Pattanasiriwisawa W Siritapetawee J Patarapaiboolchai O and Klysubun W 2008 J

Synchrotron Rad 15 510ndash513

[8] Vidal-Escales E and Borros S 2004 Talanta 62 539ndash547

[9] Y M Tsai F J Boerio W J van Ooij D K Kim and K Dong 1997 J Adhesion 62 127ndash150

[10] R N Datta A H M Schotma A J M Weber F G H Van Wijk P J C Van Haeren J W Hofstraat A G Talma and A G V D Bovenkamp-Bouwman 1997 Rubber Chem Technol 70 129-145

[11] R N Datta 1995 Rubber World 212 24

[12] S Rodracuteıguez C Masalles N Agulloacute S Borroacutes L Comellas F Broto 1999 Kautch Gummi

Kunst 52 438ndash444

[13] Klysubun W Sombunchoo P Wongprachanukul N Tarawarakarn P Klinkhico S Chaiprapa J

and Songsiriritthigul P 2007 NuclInstrum Methods Phys Res 582 87ndash89

[14] Prange A Chauvistreacute R Modrow H Hormes J Truumlper H G and Dahl C 2002 Microbiology

148 267ndash276

[15] Ravel B and Newville M 2005 J Synchrotron Rad 12 537ndash541

[16] Blackley D C 1996 Polymer Latice Science and Technology V2 type of latice 2nd ed

Chapman and Hall London

[17] Hitchcock A P Bodeur S and Tronc M 1987 Chem Phys 115 93-101

[18] Sze K H Brion C E Tronc M Bodeur S and Hitchcock A P 1988 Chem Phys 121 279-297

[19] George G N and Gorbaty M L 1989 J Am Chem Soc 111 3182-3186

14th International Conference on X-Ray Absorption Fine Structure (XAFS14) IOP PublishingJournal of Physics Conference Series 190 (2009) 012150 doi1010881742-65961901012150

4

mixture for the vulcanization Sulfur K-edge absorption spectra of the pure accelerators and of the

vulcanized squalene were measured and studied

2 Experiment

Squalene with purity of 999 as per GC was purchased from Merck Co Ltd Germany Four

vulcanization systems were prepared with the squalene-accelerator-ZnO phr ratio of 10011 The

accelerators were N-cyclohexylbenzothiazole sulfenamide (CBS) 22 dibenzothiazyl disulfide

(MBTS) tetramethylthiuram disulfide (TMTD) and zinc diethyldithiocarbamate (ZDEC) The

vulcanization reaction was carried out at 140 degC for 24 hours in closed vials to avoid oxidation of the

double bonds of squalene A continuous stirring was required to assure the homogeneity of the

reaction mixture To stop the reaction the vials were taken out from the heating bath and left to cool

down to room temperature

XANES measurements were carried out at the beamline 8 of the Synchrotron Light Research

Institute (SLRI) in Thailand [13] The energy scan was carried out using the Si(111) double crystal

monochromator to cover the K-edge of sulfur The white line of zinc sulfate (ZnSO4) powder was used

as the reference energy of 24814plusmn01 eV [15] The reproducibility of this energy in our measurements

is better than 007eV The step width of the energy scan used for in this work is 01 eV For the

vulcanized squalene samples the measurements were performed in a fluorescence mode using a 5-grid

ionization Lytle fluorescence detector The current of the incident beam I0 was measured by using an

ionization chamber located in front of the sample The spectra were performed in the liquid phase by

dropping the vulcanized squalene on a piece of sulfur-free paper For the pure accelerators the

XANES spectra were measured in the powder form in the standard transmission mode

3 Results and Discussion

Figure 1 shows sulfur K-edge XANES spectra of vulcanized squalene with four different accelerators

ie CBS MBTS ZDEC and TMTD and the unreacted accelerators It should be noted that the

absorption spectrum of pure squalene shows no absorption K-edge of sulfur confirming that there is

no sulfur in squalene molecule (the spectrum is not shown) This is one of the reasons for choosing

squalene instead of natural rubber for this study While natural rubber contains sulfur in L-metionine

(CH3-S-CH2-CHNH+3-CO-

2) which is one of important amino acids in natural rubber latex [16] It is

obvious that the spectra of the vulcanized samples are different from those of the unreacted

accelerators used in the vulcanization process indicating that the surroundings of the sulfur atoms in

all accelerators have been changed during the vulcanization process though sulfur (S8) was not used It

is interesting that sulfate by-product was not observed in the spectra of the vulcanized samples

Fig 2 shows the zoom-up XANES spectra of the vulcanized squalene with the four accelerators

Vulcanized squalene with ZDEC or TMTD accelerator show absorption peak lower than that with

CBS or MBTS accelerator The position of the peak in the absorption spectra may be used as the

indicator for the type of the σ resonance The assignment of the σ resonance peaks referred in this

work are based on the reports of Hitchcock [17] Sze [18] and George [19] There has also been the

reports that the absorption peak shifts towards higher photon energy when the number of sulfur in C-

Sx-C (x=1234) chains decreases [46] The absorption peak of the vulcanized squalene with CBS and

MBTS are observed at 24731 and 24730 eV respectively These peaks correspond to the resonance

of σ (S-C) The XANES spectra of the vulcanized squalene with ZDEC and TMTD accelerators shift

towards lower photon energy with the peak position at 24726 eV and 24723 eV respectively These

peaks may be assigned to the resonance of σ (S-S) This indicates that ZDEC and TMTD accelerators

yield disulfidic crosslink in vulcanized squalene The higher photon energy of the peak in the XANES

spectra of squalene vulcanized with CBS or MBTS accelerator indicates that the majority of the

crosslink may be monosulfidic It is interesting to point out from the measured XANES spectra that

14th International Conference on X-Ray Absorption Fine Structure (XAFS14) IOP PublishingJournal of Physics Conference Series 190 (2009) 012150 doi1010881742-65961901012150

2

the effects of CBS and TMTD accelerators on the vulcanized squalene are similar to that of the

vulcanized natural rubber previously reported [7]

Fig 1 Sulfur K-edge of XANES spectra of vulcanized squalene with (a)

CBS (b) MBTS (c) ZDEC and (d) TMTD accelerators

Fig 2 The zoom up XANES spectra in figure 1 illustrating the position of

the sulphur absorption peak

14th International Conference on X-Ray Absorption Fine Structure (XAFS14) IOP PublishingJournal of Physics Conference Series 190 (2009) 012150 doi1010881742-65961901012150

3

4 Conclusion We have used squalene as a model compound for the study of the vulcanization The effects of CBS

and TMTD accelerators on the crosslink in the vulcanization process of squalene were found to be

similar to that of natural rubber It was found that vulcanization of squalene with ZDEC and TMTD

accelerators yield disulfidic crosslink while with CBS and MBTS accelerator result in monosulfidic

crosslink It is interesting to point out that sulfate by-product was not observed after the vulcanization

process

5 Acknowledgement The authors would like to acknowledge Prof Josef Hormes for useful discussion One of the authors

(RN) would like to thanks SLRI for the scholarship (contract number 255003-GS-50-M03) This

work is supported by the National Science and Technology Development Agency of Thailand

(contract number F-31-102-18-01)

References [1] Boochathum P and Chiewnawin S 2001 Eur Ploym J 37 429-434

[2] Boochathum P and Prajudtake W 2001 Eur Ploym J 37 417-427

[3] Chauvistreacute R Hormes J Bruumlck D Sommer K and Engels H W 1992 Kautsch Gummi Kunstst

45 808-813

[4] Chauvistreacute R Hormes J Hartmann E Etzenbach N Hosch R and Hahn J 1997 Chem Phys

223 293-302

[5] Modrow H Zimmer R Visel F and Homes J 2000 Kautsch Gummi Kunstst 53 328-337

[6] Hormes J and Modrow M 2001 Nucl Instum Meth A 467-468 1179-1191

[7] Pattanasiriwisawa W Siritapetawee J Patarapaiboolchai O and Klysubun W 2008 J

Synchrotron Rad 15 510ndash513

[8] Vidal-Escales E and Borros S 2004 Talanta 62 539ndash547

[9] Y M Tsai F J Boerio W J van Ooij D K Kim and K Dong 1997 J Adhesion 62 127ndash150

[10] R N Datta A H M Schotma A J M Weber F G H Van Wijk P J C Van Haeren J W Hofstraat A G Talma and A G V D Bovenkamp-Bouwman 1997 Rubber Chem Technol 70 129-145

[11] R N Datta 1995 Rubber World 212 24

[12] S Rodracuteıguez C Masalles N Agulloacute S Borroacutes L Comellas F Broto 1999 Kautch Gummi

Kunst 52 438ndash444

[13] Klysubun W Sombunchoo P Wongprachanukul N Tarawarakarn P Klinkhico S Chaiprapa J

and Songsiriritthigul P 2007 NuclInstrum Methods Phys Res 582 87ndash89

[14] Prange A Chauvistreacute R Modrow H Hormes J Truumlper H G and Dahl C 2002 Microbiology

148 267ndash276

[15] Ravel B and Newville M 2005 J Synchrotron Rad 12 537ndash541

[16] Blackley D C 1996 Polymer Latice Science and Technology V2 type of latice 2nd ed

Chapman and Hall London

[17] Hitchcock A P Bodeur S and Tronc M 1987 Chem Phys 115 93-101

[18] Sze K H Brion C E Tronc M Bodeur S and Hitchcock A P 1988 Chem Phys 121 279-297

[19] George G N and Gorbaty M L 1989 J Am Chem Soc 111 3182-3186

14th International Conference on X-Ray Absorption Fine Structure (XAFS14) IOP PublishingJournal of Physics Conference Series 190 (2009) 012150 doi1010881742-65961901012150

4

the effects of CBS and TMTD accelerators on the vulcanized squalene are similar to that of the

vulcanized natural rubber previously reported [7]

Fig 1 Sulfur K-edge of XANES spectra of vulcanized squalene with (a)

CBS (b) MBTS (c) ZDEC and (d) TMTD accelerators

Fig 2 The zoom up XANES spectra in figure 1 illustrating the position of

the sulphur absorption peak

14th International Conference on X-Ray Absorption Fine Structure (XAFS14) IOP PublishingJournal of Physics Conference Series 190 (2009) 012150 doi1010881742-65961901012150

3

4 Conclusion We have used squalene as a model compound for the study of the vulcanization The effects of CBS

and TMTD accelerators on the crosslink in the vulcanization process of squalene were found to be

similar to that of natural rubber It was found that vulcanization of squalene with ZDEC and TMTD

accelerators yield disulfidic crosslink while with CBS and MBTS accelerator result in monosulfidic

crosslink It is interesting to point out that sulfate by-product was not observed after the vulcanization

process

5 Acknowledgement The authors would like to acknowledge Prof Josef Hormes for useful discussion One of the authors

(RN) would like to thanks SLRI for the scholarship (contract number 255003-GS-50-M03) This

work is supported by the National Science and Technology Development Agency of Thailand

(contract number F-31-102-18-01)

References [1] Boochathum P and Chiewnawin S 2001 Eur Ploym J 37 429-434

[2] Boochathum P and Prajudtake W 2001 Eur Ploym J 37 417-427

[3] Chauvistreacute R Hormes J Bruumlck D Sommer K and Engels H W 1992 Kautsch Gummi Kunstst

45 808-813

[4] Chauvistreacute R Hormes J Hartmann E Etzenbach N Hosch R and Hahn J 1997 Chem Phys

223 293-302

[5] Modrow H Zimmer R Visel F and Homes J 2000 Kautsch Gummi Kunstst 53 328-337

[6] Hormes J and Modrow M 2001 Nucl Instum Meth A 467-468 1179-1191

[7] Pattanasiriwisawa W Siritapetawee J Patarapaiboolchai O and Klysubun W 2008 J

Synchrotron Rad 15 510ndash513

[8] Vidal-Escales E and Borros S 2004 Talanta 62 539ndash547

[9] Y M Tsai F J Boerio W J van Ooij D K Kim and K Dong 1997 J Adhesion 62 127ndash150

[10] R N Datta A H M Schotma A J M Weber F G H Van Wijk P J C Van Haeren J W Hofstraat A G Talma and A G V D Bovenkamp-Bouwman 1997 Rubber Chem Technol 70 129-145

[11] R N Datta 1995 Rubber World 212 24

[12] S Rodracuteıguez C Masalles N Agulloacute S Borroacutes L Comellas F Broto 1999 Kautch Gummi

Kunst 52 438ndash444

[13] Klysubun W Sombunchoo P Wongprachanukul N Tarawarakarn P Klinkhico S Chaiprapa J

and Songsiriritthigul P 2007 NuclInstrum Methods Phys Res 582 87ndash89

[14] Prange A Chauvistreacute R Modrow H Hormes J Truumlper H G and Dahl C 2002 Microbiology

148 267ndash276

[15] Ravel B and Newville M 2005 J Synchrotron Rad 12 537ndash541

[16] Blackley D C 1996 Polymer Latice Science and Technology V2 type of latice 2nd ed

Chapman and Hall London

[17] Hitchcock A P Bodeur S and Tronc M 1987 Chem Phys 115 93-101

[18] Sze K H Brion C E Tronc M Bodeur S and Hitchcock A P 1988 Chem Phys 121 279-297

[19] George G N and Gorbaty M L 1989 J Am Chem Soc 111 3182-3186

14th International Conference on X-Ray Absorption Fine Structure (XAFS14) IOP PublishingJournal of Physics Conference Series 190 (2009) 012150 doi1010881742-65961901012150

4

4 Conclusion We have used squalene as a model compound for the study of the vulcanization The effects of CBS

and TMTD accelerators on the crosslink in the vulcanization process of squalene were found to be

similar to that of natural rubber It was found that vulcanization of squalene with ZDEC and TMTD

accelerators yield disulfidic crosslink while with CBS and MBTS accelerator result in monosulfidic

crosslink It is interesting to point out that sulfate by-product was not observed after the vulcanization

process

5 Acknowledgement The authors would like to acknowledge Prof Josef Hormes for useful discussion One of the authors

(RN) would like to thanks SLRI for the scholarship (contract number 255003-GS-50-M03) This

work is supported by the National Science and Technology Development Agency of Thailand

(contract number F-31-102-18-01)

References [1] Boochathum P and Chiewnawin S 2001 Eur Ploym J 37 429-434

[2] Boochathum P and Prajudtake W 2001 Eur Ploym J 37 417-427

[3] Chauvistreacute R Hormes J Bruumlck D Sommer K and Engels H W 1992 Kautsch Gummi Kunstst

45 808-813

[4] Chauvistreacute R Hormes J Hartmann E Etzenbach N Hosch R and Hahn J 1997 Chem Phys

223 293-302

[5] Modrow H Zimmer R Visel F and Homes J 2000 Kautsch Gummi Kunstst 53 328-337

[6] Hormes J and Modrow M 2001 Nucl Instum Meth A 467-468 1179-1191

[7] Pattanasiriwisawa W Siritapetawee J Patarapaiboolchai O and Klysubun W 2008 J

Synchrotron Rad 15 510ndash513

[8] Vidal-Escales E and Borros S 2004 Talanta 62 539ndash547

[9] Y M Tsai F J Boerio W J van Ooij D K Kim and K Dong 1997 J Adhesion 62 127ndash150

[10] R N Datta A H M Schotma A J M Weber F G H Van Wijk P J C Van Haeren J W Hofstraat A G Talma and A G V D Bovenkamp-Bouwman 1997 Rubber Chem Technol 70 129-145

[11] R N Datta 1995 Rubber World 212 24

[12] S Rodracuteıguez C Masalles N Agulloacute S Borroacutes L Comellas F Broto 1999 Kautch Gummi

Kunst 52 438ndash444

[13] Klysubun W Sombunchoo P Wongprachanukul N Tarawarakarn P Klinkhico S Chaiprapa J

and Songsiriritthigul P 2007 NuclInstrum Methods Phys Res 582 87ndash89

[14] Prange A Chauvistreacute R Modrow H Hormes J Truumlper H G and Dahl C 2002 Microbiology

148 267ndash276

[15] Ravel B and Newville M 2005 J Synchrotron Rad 12 537ndash541

[16] Blackley D C 1996 Polymer Latice Science and Technology V2 type of latice 2nd ed

Chapman and Hall London

[17] Hitchcock A P Bodeur S and Tronc M 1987 Chem Phys 115 93-101

[18] Sze K H Brion C E Tronc M Bodeur S and Hitchcock A P 1988 Chem Phys 121 279-297

[19] George G N and Gorbaty M L 1989 J Am Chem Soc 111 3182-3186

14th International Conference on X-Ray Absorption Fine Structure (XAFS14) IOP PublishingJournal of Physics Conference Series 190 (2009) 012150 doi1010881742-65961901012150

4