Hindered amine stabilizes

 By

 Awad Nasser Albalwi

(June,2010)

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CONTENTS

Abstract Introduction The project proposal Procedure Results Discussion Conclusion

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Abstract

A B3LYP, HF, AM1 and PM2 computational studies of the reaction of hindered amine (HALS) has been perfumed. Four different theories were used to calculate the bond dissociation energy (BDE). In two molecules studied the nitrogen were protonated and not protonated. BDE were calculated when aromatic rings were substituted with NO2 and OCH3. B3LYP was the best theoretical calculation level, The BDE was grater when nitrogen in HALS was protonated. There was no big significant difference in BDE when aromatic ring of hindered amine was substituted with NO2 and OCH3.

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Introduction Hindered amine light stabilizers (HALS) are among the most efficient polymer stabilizers

known. Bis (2,2,6,6-tetramethyl-4-piperidinyl) sebacate, is a typical (HALS). Since the early 1970s, HALS have become a highly important class of light stabilisers for

polymers. They stabilise wide range of commercial polymers and are particularly effective for stabilization of polyolefins when use where resistance to deterioration by light and weathering are important.

Hinder amines have also been used as stabiliser against light induced degradation of polymers such as polyolefin and polyurethane1&4.

Polypropylene is an example of a major commercial polymer which would never have achieved any practical use without the development of a good stabiliser system.

Polyolefin needs protection in all the stages of its life cycle. In order for an antioxidant to improve the long-term weathering performance of an automotive clearcoat/basecoat paint which is a polymer system, it must inhibit clearcoat photo-oxidation at the onset of exposure and sustain the inhibition for many years.

While there is sample evidence that hindered amine light stabilizer (HALS) additives can inhibit the photo-oxidation of automotive clearcoats polymer,

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Introduction Polyolefin needs protection in all the stages of its life cycle. In order for an antioxidant

to improve the long-term weathering performance of an automotive clearcoat/basecoat paint which is a polymer system, it must inhibit clearcoat photo-oxidation at the onset of exposure and sustain the inhibition for many years.

While there is ample evidence that hindered amine light stabilizer (HALS) additives can inhibit the photo-oxidation of automotive clearcoats polymer,

HALS acts as a scavenger for free radicals that would otherwise degrade or discolour HALS are efficient inhibitors of the photooxidation of polyolifins HALS act as scavengers for free radicals that would otherwise degrade or discolour the polymer coating.

Hindered amine has been employed in the automotive and wood coating sectors of the surface coatings industry for many years.

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The purpose of this paper

The purpose of this paper to examine the following hypothesis:

There is correlation / relationship between an increase the size of group (R) and an increase the Bond dissociation energy of Hindered amine (HALS).

There is no effect of substitution on the aromatic ring of HALS with OCH3&NO2 in various positions.

There are differences in BDE between none & protanated nitrogen of Hindered amine (MO1).

There is no relationship between change of such group (OCH3, NO2) on the same position on the aromatic ring (HALS) & BDE changes.

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In this project the molecular MO1 and MO2 is refer to this structure:

R R

MO1 Non-protonated

MO1(+) protonated

R R

MO2(+) Protonated

MO2 Non-protonated

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Procedure

Firstly, the calculations were performed with the GAUSSIAN09 (G09) programme in order to select the best level of theory to calculate the Bond Dissociation energy (BDE). Four level of theories (B3LYP, AM1, HF & MP2) at the basis set 3-21(G) were used to calculate the BDE of these reactions (Scheme3&4):

Scheme3: breaking reaction of the bond O-R of Hindered amine MO1 .

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Procedure

Scheme4: breaking reaction of the bond O-R of Hindered amine MO1(+)

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Procedure In order to calculate the BDE, the reactants and products structures were built by drawing

each of them on the built molecule page in the job manager (GAUSSIAN09 (G09)). After constructing the molecule, comprehensive cleanup using idealized Geometry &

Mechanics was used to get the best molecular structures. . In addition the theory level was basis set, optimize + Vibe freq calculation, charge and

multiplicity were selected from Configure Gaussian Job Options page. After the calculation was done successfully, the electronic energy for every molecule was

determined from the final block of output of (G09). The BDE was calculated by using the formula:  

BDE = ∑ reactants energy -∑ products energy Comparison between the 4 levels of theories was done. Comparison of the four level of theories depends on how long every theory takes and how

accurate they are.by using results from research papers and experimental data After selecting the suitable theory, the comparison between different basis sets of the

selected theory, in calculation time and BDE results were done.

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Procedure By using the B3LYP 3-21G data sets (Optimize + Vib Freq - Gaussian )., BDE

of breaking reaction of the bond O-R, when the substituting the aromatic ring with different groups such as OCH3 & NO2 in various position (meta, Ortho & Para) were calculated. The calculation was applied when the nitrogen is protonated & non protonated (scheme 5&6).

Scheme 5: Substituting the aromatic ring with group NO2 in various position (meta, Ortho & Para).

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Procedure

Scheme 6: Substituting the aromatic ring with group OCH3 in various position (meta, Ortho

& Para).

The results of this project were compared with experimental data and different level theories from other research papers.

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Results: Table.1 Comparison between experimental and calculated BDE (O_R) for HALS Molecular 1 (MO1)

[kJmol_1] BDE 3LYP BDE AM1 BDE HF BDE MP2 BDE exp

(from research

BDEPM3

paper)

BDEDFT

M1-OH= M1-O.+H. 271.65 162.34 199.10 185.15 291 296 279

M1-OCH3= M1-O. + CH3 140.46 120.76 111.62 96.04 197 178 185

MO1-OC(CH3)3 = MO1-O. +.C(CH3)3

186.53 172.98 100.71 74.72 n/a 94 n/a

Graph.1.: Comparison between experimental and calculated BDE (O-R) for HALS Moleculer1.

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Results

Graph.2: Comparison of BDE when an increase of R from H to CH3 between experimental, PM3, DFT from research paper & calculated with HF, B3LYP, MP2 and AM1.14

Results

Graph.3: The comparison of calculation time of energy with different levels of theories15

Results

Graph. 5: Comparison of BDE changes with an increase the basis sets of B3LYP for HALS Molecular No.1 with different R group ( R=H, CH3 & C(CH3)3). 16

Results

Graph.6: Comparison of BDE between none & protanated nitrogen of Hindered amine (MO1) with different group of R ( H, CH3 & C(CH3)3. 17

Results

Graph.7: Comparison between BDE change with HALS molecular No1 with different group of R ( H, CH3, CH2CH3, CH(CH3)2 , C(CH3)3 ).

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Results

KJ/mol

Graph.8: comparison between change the group (OCH3) on the aromatic ring of HALS molecular No2 (MO2) – none protonated Nitrogen -and BDE changes at B3LYP/3-21(G). 19

Results

Graph.9: comparison between change the group (OCH3) on the aromatic ring of HALS molecular No.2 (MO2(+)) - protonated Nitrogen -and BDE changes at B3LYP/3-21(G).20

Results

Graph.10: comparison between change the group (NO2) on the aromatic ring of HALS molecular No2 (MO2) – none protonated Nitrogen -and BDE changes at B3LYP/3-21(G).

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Results

Graph.11: comparison between change the group (NO2) on the aromatic ring of HALS molecular No2 (MO2) – proton ted Nitrogen -and BDE changes at B3LYP/3-21(G).22

Results

Graph.12: comparison of BDE between different groups (NO2 & OCH3) on the aromatic ring of HALS molecular No2 (MO2) – none- protonated Nitrogen -at B3LYP/3-21(G). 23

Discussion It is observed that very few experimental BDEs of HALS have been reported in literature. BDE of O-R in HALS compound have been investigated for various group ( R= H, CH3 & C

(CH3)3 using different level of theories and different data sets, Calculated BDE from research papers were compared with calculated BDE in this paper (Table .1 and figure. 1) .

It was found that the BDEs of O-R (HALS) were decreasing from H> CH3> C (CH3)3 using HF & MP2 theories. However , the BDE was random from H> C(CH3)3 > CH3 using B3LYP & AM.

It was found that in most cases B3LYP/3-21(G) calculations were slightly closer to BDE experimental value when R= H& CH3 .It is also observed that the results coming from HF & MP2 were more reasonable, thus the stability of these groups were increasing from C (CH3)3

> CH3>H. The stability of those group lead to decrease the BDE of O-R in HALS.

It is interesting to note that the calculated BDE using B3LYP /3-21(G) of this paper was in agreement with the experimental values . Graph.2 has shown that The B3LYP /3-21(G) was closer R2=0.86 to the experimental value & DFT (R2 = 0.99) level theory from journal article than other theoretical calculation (HF, AM1 & MP2) 6. Thus , It has chosen the B3lYP/3-21(G) to calculate the BDE for various structures in this project . In addition, The B3LYP/3-21(G) takes short calculation time Graph 3&4.

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Discussion Figures 8 and 9 show the effect of substituting the aromatic ring with OCH3 in Meta, .or tho

and Para positions, there was no significant change in BDE when OCH3 was substituted on all three positions of the aromatic ring.

Figures 8 and 9 have shown that there was difference in BDE when the nitrogen is proton ted and not protonted.

In protonated Nitrogen the BDE is greater than that of non protonated by about 7% .

figures 10and 11 show the substitution of NO2 on the aromatic ring, In figure .10 there was no change in the BDE when NO2 was substituted in ortho and para positions. However BDE decreased significantly in meta position and this is not normal compare to other positions.

Figure 12 shows comparison between 2 different substitutions ie NO2 and OCH3.on aromatic rings. There were no different in BDE in ortho and para positions when NO2 and OCH3 were substituted on the aromatic rings.

However the BDE of OCH3 was three times greater than that of NO2 in meta position figure.12.

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Discussion Figure. 6 has indicated that protonated Nitrogen of HALS gives an increase in BDE than

non protonated . Thus the HALS (MO1) with protonated Nitrogen might be more stable than non protonated Nitrogen

In addition , protonated Nitrogen of HALS might lead to increase the lifetime of the paint that contains the HALS Molecule.

From the result , it can be said , the increase the size back rings of HALS is not significant in an increase the stability of HALS in comparison between non protonated nitrogen of MO1 & MO2.

How ever, in protonated Nitrogen of MO1(+) &MO2(+) cases , the MO1(+) was greater in BDE than MO2(+) (graph 6,9&11). Thus , the MO1(+) is more stable than MO2(+).

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Conclusion

Computational analysis now show that there is a relationship between the size of R and BDE of HALS. cause When R increases, BDE decreases. It is also observed that there was no significant change in BDE when OCH3 was substituted on all the three positions of the aromatic rings of HALS. Computational calculations also show that there was difference in BDE between protonated and non protonated nitrogen of HALS.

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References:

1-Possi, Aventurini and A Zedda J. AM Chem .SCI (1999)121,,7914-7917 2-F,.Gugumus Polymer Degradation and Stability (1995) 50, 101-116 3- P.P. Klemchuk , M.E Gande Polymer Degradation and Stability (1988),22,241-274 4- T.A. Lowe, M.R.L Paine,D.L.Marshall.L.A.Hick,J.A.Boge,P.J.Barker , S.J.Blanksby

J Mass Spec (2010) 45(5) 486-496   5- G.Geuskens ,M.N.Kanda Polymer Degradation and Stability (1996),51, 227-232. 6- A Gaudel,S., D. Siri, P.Tordo ,ChemPhysChem,(2006),7,430-438

******************************** Paine, M. R. L., Barker, P. J. and Blanksby, S. J. "Desorption Electrospray Ionisation

Mass Spectrometry Reveals In Situ Modification of a Hindered Amine Light Stabiliser Resulting From Direct N-OR bond cleavage" Analyst 2011, 136 (5), 904-912. [Cover Article]

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