1 Vilnius University Faculty of Physics, Dept. of General Physics and Spectroscopy CONFORMATIONAL ANALYSIS OF 1-ALKENE SECONDARY OZONIDES BY MEANS OF MATRIX ISOLATION FTIR SPECTROSCOPY S. STRAZDAITE, J. CEPONKUS and V. SABLINSKAS

2 Why should we care about the ozonides? Ozonides are considered to participate in forming of aerosols and smog through chemical reactions. Despite of numerous studies of the reaction by different spectroscopic techniques the precise mechanism of the reaction is still unknown. Final fate of the ozonization reaction depends on many parameters such as type and conformation of the radical. Unfortunately it’s not much known about spatial structures of ozonides.

3 The aim of this study To define the geometrical structures and stability of the different conformers of the 1-butene and 1-heptene secondary ozonides by combined analysis of the matrix isolation FTIR spectral data with the results of Density Functional Theory (DFT) calculations

4 Is amount of unsaturated hydrocarbons in troposphere significant? Alkenes are reactive hydrocarbons emitted from plants and during combustion Annual global emissions by plants 1250  10 6 t/year Guenther et al. (2006) Similar figures are for hydrocarbon release due to combustion…

5 Criegee mechanism breaking double bond Unstable product Primary Ozonide More stable product Secondary Ozonide R. Criegee The course of ozonization of unsaturated compounds (1957) Y.Haas et al. Formation of secondary ozonides in the gas phase low-temperature ozonation of primary and secondary alkenes (1999)

6 Conformations of secondary ozonides 1.Ring conformations- twist type C-O half chair and O-O half chair C.W. Gillies and R.L. Kuczkowski (1974) O-O half chair C-O half chair

7 1-butene SOZ O-O half chair axial Conformations of secondary ozonides 2. Conformations due to position of aliphatic radical: (a) equatorial and axial (b) rotation about single CC bond 1-butene SOZ O-O half chair equatorial

8 What is done? 1-butene SOZ1-heptene SOZ Ab-initio calculations Matrix isolation low temperature experimental data in various matrixes Photolysis experiment Crystallization of sample at higher temperatures Temperature evolution of the bands intensities Determination of conformations

9 Ab-initio calculations GAUSSIAN B3LYP G (3df, 3pd) Conformer  H, kJ/mol (  OCCC ) Rel. conc., % T=300 K Rel. conc., % T=40 K I O-O half chair equatorial 0 (-66,1 o )3171 II O-O half chair equatorial 0,3(-177,5 o )28 III O-O half chair equatorial 1,6 (56,2 o )160,7 IV O-O half chair axial 2,1 (175,5 o )130,3 V O-O half chair axial 3,2 (62,6 o )80 VI O-O half chair axial 6,2 (-60,6 o )40 1-butene SOZ

10 Ab-initio calculations Conformer 1-butene SOZ  H, kJ/mol (  OCCC ) 1-heptene SOZ  H, kJ/mol (  OCCC ) I O-O half chair equatorial 0 (-66.1 o )0 (-66.3 o ) II O-O half chair equatorial 0.3( o )0.3 ( o ) III O-O half chair equatorial 1.6 (56.2 o )2.3 (57.3 o ) IV O-O half chair axial 2.1 (175.5 o )2.5 (174.6 o ) V O-O half chair axial 3.2 (62.6 o )3.6 (62.9 o ) VI O-O half chair axial 6.2 (-60.6 o )6.9 (-60.4 o )

11 Ab-initio calculations GAUSSIAN B3LYP G (3df, 3pd) 1-butene SOZ ~10-15 cm -1 Absorbance 

12 What is done? 1-butene SOZ1-heptene SOZ Ab-initio calculations Matrix isolation low temperature experimental data in various matrices Photolysis experiment Crystallization of sample at higher temperatures Temperature evolution of the bands intensities Determination of conformations

13 How is the matrix formed? Matrix gas 1-alkene REACTOR VACUUM SYSTEM (1-alkene SOZ/matrix gas CsI O3O3 KBr

14 FTIR absorption spectrum of 1-butene SOZ isolated in various matrices 20 K 60 K Most efficient matrix isolation for both secondary ozonides Absorbance 

15 Photolysis experiments

K Wavenumber, cm K Neat film I Absorbance  Crystallization of amorphous neat film of 1-heptene

17 Temperature evolution of 1-butene SOZ spectral bands Wavenumber, cm K 60 K Neat Film Absorbance 

18 What is done? 1-butene SOZ1-heptene SOZ Ab-initio calculations Matrix isolation low temperature experimental data in various matrixes Photolysis experiment Crystallization of sample at higher temperatures Temperature evolution of the bands intensities Determination of conformations

19 Determination of 1-butene SOZ conformations Absorbance 

20 Determination of 1-heptene SOZ conformations 150 K Wavenumber, cm K Neat film I I I+II+III I II+III II Absorbance 

21 Conclusions N 2 matrix is the most efficient isolation media for the title ozonides. Potential barrier for rotation of alkyl radical around C radical -C ring bond in the title ozonides exceeds 10 kJ/mol. Gaseous 1-butene SOZ at 300 K exists as mixture of four conformers while 1-heptene SOZ – as mixture of three conformers.

22 Vilnius University Faculty of Physics, Dept. of General Physics and Spectroscopy CONFORMATIONAL ANALYSIS OF 1-ALKENE SECONDARY OZONIDES BY MEANS OF MATRIX ISOLATION FTIR SPECTROSCOPY S. STRAZDAITE, J. CEPONKUS and V. SABLINSKAS