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Conformations of 1-heptene secondary ozonide as studied by low temperature FT-IR spectroscopy
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Journal of Molecular Structure 844–845 (2007) 186–192 www.elsevier.com/locate/molstruc
Conformations of 1-heptene secondary ozonide as studied by low temperature FT-IR spectroscopy R. Bariseviciute, J. Ceponkus, V. Sablinskas, L. Kimtys
*
Department of General Physics and Spectroscopy, Vilnius University, Universiteto str. 3, Vilnius 01513, Lithuania Received 27 February 2007; received in revised form 24 April 2007; accepted 28 April 2007 Available online 13 May 2007
Abstract Conformational diversity of the 1-heptene secondary ozonides (SOZ) in solid neat films as well as isolated in Ar or CO2 matrices was studied by the means of FT-IR absorption spectroscopy. The ozonization reaction was performed at 77 K in the neat films of the reactants. The spectra of the ozonide were analyzed by combining the experimental data with the results of theoretical calculations performed at B3LYP 6-311++G (3df, 3pd) level. It was found that the samples of 1-heptene secondary ozonide exist as a mixture of three dominating conformers. The most stable conformer is the one with O–O half-chair configuration of the five membered ring, the aliphatic radical attached to the ring in equatorial position and the aliphatic chain being in gauche (\OCCC 60) position. The other two stable conformers are equatorial with aliphatic chain in anti (\OCCC 180) and gauche (\OCCC 60) positions. It was found from Van’t Hoff plots that DH of the equatorial anti conformer is equal to 0.24 ± 0.03 kJ/mol. The experimental value of DH is in reasonable accordance to the calculated one – 0.5 kJ/mol. 2007 Elsevier B.V. All rights reserved. Keywords: 1-Heptene secondary ozonide; Conformations; Matrix isolation; FT-IR spectroscopy
1. Introduction An ability of ozone to break double C@C bond in olefins is known for more than five decades. Understanding of those reactions is very important in atmospheric chemistry. Alkene ozonization reaction is believed to be proceeded under so-called Criegee three-step mechanism [1]. During different steps of the reaction the primary ozonide (POZ), carbonyl oxide (COX) and the secondary ozonide (SOZ) are formed. Fate of the reaction depends on many parameters such as type of radical, conformation of alkene, temperature of the reaction and environmental effects. Despite of numerous studies of the reaction by different spectroscopic techniques the precise mechanism of the reaction is still unknown [2–5]. It is well established experimentally that POZ is highly unstable intermediate, which rapidly dissociates into a car*
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0022-2860/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2007.04.041
bonyl compound (CC) and COX often known as Criegee intermediate (CI) [5]. Further fate of the reaction depends on the conditions in which the reaction proceeds [2]. Many different reaction pathways are possible in the gas phase such as dissociation or rearrangement of the CI followed by the bimolecular reactions with CC or residual CI, which leads to the rather complicated reaction sequence. The carbonyl compound and CI readily recombine to form secondary ozonide in the condensed phase. However, some other competing processes may occur in the condensed phase too. Suenram and Lovas have reported the detection of dioxirane from ethene ozonolysis [5]. It is experimentally observed that the SOZ is more stable than POZ [6]. At room temperature it can remain from few hours up to a few days. SOZ dissociates into formic acid and aldehide. Stability of the SOZ depends on the size and configuration of the radical. Unfortunately, it is not much known about the spatial structures of the SOZs. The variety of the possible conformations of 1-alkene SOZs, are generally caused by three reasons. First, five
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membered ring of the SOZ can possess two different structures – so-called C–O half-chair conformation and O–O half-chair conformation [7]. According to Kuckowski and Bauld-Bailey [8,9] the five member ring of SOZ exists preferably in the O–O half-chair conformation. Such configuration of the ring was confirmed experimentally from the microwave spectroscopy experiments [8,10–14]. Second, an aliphatic chain of the 1-alkene ozonide can be differently attached to the ring. In the case of the 1-alkene SOZ an aliphatic chain can be attached to the ring in either so-called axial or equatorial positions. In the axial conformer the chemical bond between the ring carbon and the aliphatic carbon is perpendicular to the ring plane. This bond lies in the ring plane in the equatorial conformer. It is notable, that the C–C bond does not lie exactly parallel or perpendicular to the plane of the ozonide five member ring but these notations are still in use in the conformational analysis of SOZs [8,15]. The structure of equatorial propene SOZ conformer was determined by the microwave spectroscopy [8]. Haas [16] used ab initio calculated vibrational spectrum of the 1-hexene axial conformer for assignment of the infrared spectral bands of the 1-hexene ozonization reaction products isolated in the CO2 matrix. Samuni et al. [16] have not made any attempt to consider the possible conformational diversity of the ozonide. Third, aliphatic chain by itself can have different conformations. It is very well established that the aliphatic chain is usually planar in the most stable conformer of any saturated aliphatic compound. However, rotation around the closest to the ring C–C bond can cause formation of so-called anti and gauche conformers in the case of aliphatic chain attached to the five membered ozonide ring (COCOO). It has been experimentally and theoretically confirmed that energy difference between anti and gauche conformers are of 3.4–3.8 kJ/mol [17,18] for alkanes. This value can be different in the case of the aliphatic chain attached to the COCOO ring of the ozonide. This is due to the interactions between ring and the aliphatic radical. Our initial conformational studies of 1-alkene ozonides, using gas phase FT-IR spectroscopy, are presented in [15]. It was concluded that at least two conformers of 1-heptene were present in the gaseous samples. We were not able to make more quantitative conclusions about the number and stability of the conformers. Infrared absorption bands,
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belonging to the different conformers of the 1-heptene SOZ, in the gas phase spectra were strongly overlapped due to the broadness of the bands. The aim of this study is to define the geometrical structures and stability of the different conformers of the 1-heptene secondary ozonide by combined analysis of the low temperature experimental spectral data with the results of Density Functional Theory (DFT) calculations. 2. Experimental Oxygen (99.999%) from AGA and 1-heptene (99%) from Aldrich were used without an additional purification. Ozone was prepared from oxygen by the electric discharge. The ozonization reaction was performed in the condensed phase in the stainless steel reactor [19]. The inner part of the reactor was cooled with liquid nitrogen. 1-Heptene and ozone were consequently introduced in to the reactor in the small portions (about 15 torr · l) thus forming multi-layered film of the reactants on the walls of reactor. When the reaction was finished the reactor was slowly warmed up. Continuous pumping of the reactor during the warm up allowed us to separate the SOZ from the other reaction products. The products of the reaction started actively evaporate only at temperatures close to their melting points. For instance, formaldehyde actively evaporates starting from 140 to 160 K, hexanal – from 180 to 200 K. Only the products, which evaporate from the walls of the reactor at 275 K temperature were transferred to the vacuum system and used for preparation of amorphous film of the secondary ozonide or the matrix sample. Closed cycle Helium cryostat Leybold RW2 was used for cooling the samples. Matrix mixture (ratio of 1-heptene SOZ with Ar (or CO2) – 1:600) was deposited on CsI window of the cryostat cooled down to 20 K for Ar matrix or 65 K for CO2 matrix. The gaseous mixture was kept at room temperature during the deposition. Photolysis experiments of the matrix isolated samples were performed using conventional high pressure Hg arc lamp, providing UV radiation in the spectral range of 170–400 nm. The closed cycle helium cryostat was used for the preparation of the amorphous films of the 1-heptene SOZ. The gaseous SOZ was deposited on the optical window cooled down to 60 K. Infrared absorption spectra were recorded on BRUKER Vertex 70 FT-IR spectrometer, using 0.5 cm1 spectral resolution, glowbar source, KBr beamsplitter, and MCT detector.
Table 1 DFT calculated (B3LYP\6-311++G(3df, 3pd)) relative potential energies DH (calculated taking in to account ZPE) of the four most stable conformers of 1-heptene SOZ Conformer I II III IV
DH, kJ/mol (sOCCC, sCCCC) O–O O–O O–O O–O
half-chair half-chair half-chair half-chair
equatorial gauche equatorial anti axial anti equatorial gauche
0 (sOCCC = 66.4, sCCCC = 179.1) 0.5 (sOCCC = 178.5, sCCCC = 177.8) 2.4 (sOCCC = 174.8, sCCCC = 180) 2.5 (sOCCC = 57.0, sCCCC = 178.2)
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Table 2 DFT calculated (B3LYP\6-311++G(3df, 3pd)) normal vibrations of four most stable 1-heptene SOZ conformers in the spectral region of the ring vibrations Equatorial anti
mcalc*(cm1)
I (a.u.)
mcalc*
6.7
824 –– 878 –– –– 889 903 –– –– 946 962 –– –– 1004 1014 –– 1036 –– 1054 –– 1065 1105 1129 1140 1167
820 –– 874 885 896 –– –– 931 –– –– 986 990 –– 1033 1045 –– 1056 –– 1062 –– 1105 1130 1139 1160 *
6.8 8 13.5
16.6
14.5 10 6.2 14.5 76.9 22.1 131.8 12.2 9 2.7
The calculated wavenumbers are scaled by 0.99.
1
(cm )
Axial anti I (a.u.)
mcalc*
0.4
830 –– 860 881
4.9
3.9 4.4
12.4 47.6
12.9 11.3 2.3 11.6 98.2 117.2 12.9 8.5 6.8
892 –– –– –– 949 958 –– –– 994 1014 –– 1035 –– 1054 –– 1061 1103 1121 1142 1155
1
(cm )
Equatorial gauche (sOCCC = 57.0) 1
Assignment
I (a.u.)
mcalc*
1.3
818 863 878
4.4 9 5.6
CH2 twist O(1)–C(2)–C(3) def O–O stretch CH2 twist
895 –– 908 –– –– 957 –– –– 997 1013 –– 1037 –– 1054 –– 1063 1121 1125 1137 1156
6.5
CH3 rock CH2 twist, CH3 rock
11.1 1.5 5.2
10.6 57.2
11.2 2 0.8 10 37.2 216.3 12.8 9 10.2
(cm )
I (a.u.)
9.7
33.2
29.5 1.5 9.6 8.7 78.3 87.7 15.8 4.5 16.2
O–C–O asym. stretch, C(2)–O(3) stretch O(1)–C(2), C(1)–O(2) stretch O–C–O asym. stretch CH2 twist, CH3 rock C–C–C in plain def CH2 twist, CH3 rock C–C–C in plain def C–C–C in plain def C–C stretch O–C–O sym. stretch C–C–C in plain def ring vibr O–C–O sym. stretch O–C–O sym. stretch C–C–C in plain def CH2 rock C–C–C out of plain def
R. Bariseviciute et al. / Journal of Molecular Structure 844–845 (2007) 186–192
Equatorial gauche (sOCCC = 66.4)
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3. Results and discussion 3.1. Calculations Theoretical calculations of the vibrational spectra and the structure of the staggered conformers of the SOZ were performed with Gaussian 03 program package, using the DFT method B3LYP with 6-311++G (3df, 3pd) basis set. The input parameters for the program were set in such a way to look on the potential surface of the SOZ only for equatorial and axial staggered conformers formed only due to the rotation of the aliphatic radical around the C–C bond nearest to the five membered ring. The conformers formed due to the rotation around the other @C–C-bonds are much less stable and therefore could be excluded from the studies. Results of the calculations for the title ozonide are presented in the Tables 1 and 2. The structures of the four most stable 1-heptene SOZ conformers are presented in Fig. 1.
3.2. 1-Heptene SOZ in Ar and CO2 matrixes Spectral region of the ozonide ring vibrations at 800–1200 cm1 was chosen for the conformational analysis taking in to account the results of the theoretical calculations (Table 2). According to the calculations the spectral bands belonging to the different conformers are separated
189
from each other by 5–20 cm1 in this spectral region, while they are completely or partly overlapped in the other regions. Infrared absorption spectrum of the 1-heptene SOZ isolated in Ar matrix is presented in Fig. 2a. The spectral bands of the matrix isolated 1-heptene SOZ are surprisingly broad and not suitable for the conformational analysis. Broadness of the spectral bands of 1-heptene SOZ isolated in Ar matrix can be explained by large number of the 0trapping sites. The lattice constant of the Ar crystal [10] and the length of 1-heptene SOZ molecule – ca. is 5.3 A 0 . The SOZ molecule does not fit in to the cells of 10 A the Ar crystal, and only various multi-substitutional trapping sites are possible. Overlapping of the spectral bands of the SOZs surrounded by slightly different environment results their broadening. The crystalline cage of the CO2 matrix is slightly larger than that of Ar matrix. Even more, CO2 matrices obtained by deposition of CO2 gas at temperatures as low as 20 K are more amorphous than crystalline [16]. This makes the environment more uniform and it results the narrower spectral bands. Therefore the experiments with the SOZ isolated in the CO2 matrix were carried out in this work. FT-IR absorption spectrum of 1-heptene SOZ isolated in CO2 matrix is presented Fig. 2b. As seen in the figure the spectral bands of 1-heptene SOZ isolated in CO2 matrix is narrower comparing to those of the ozonide isolated in the Ar matrix. The theoretically calculated spectra of four conformers of 1-heptene SOZ together with the
Fig. 1. Theoretically (B3LYP 6-311++G (3df, 3pd)) calculated structures of four the most stable 1-heptene SOZ conformers: (I) equatorial gauche (\OCCC = 66.4); (II) equatorial (\OCCC = 178.5); (III) axial anti (\OCCC = 174.8); (IV) equatorial gauche (\OCCC = 57).
R. Bariseviciute et al. / Journal of Molecular Structure 844–845 (2007) 186–192
Absorbance →
190
a
♦
b 800
* 850
900
* * 950
1000
1050
1100
1150
1200
Wavenumber, cm-1
Fig. 2. IR absorption spectra of 1-heptene SOZ isolated in: (a) argon matrix; (b) CO2 matrix. *, 1-heptene spectral bands, , formic acid spectral band.
experimental spectrum in 930–975 cm1 spectral regions are presented in Fig. 3. The band at 936 cm1 can be assigned to the equatorial gauche ( 60) conformer (I), the bands at 947 and 965 cm1 – to equatorial anti conformer (II) and the band at 956 cm1 – to equatorial gauche ( 60) conformer (Table 2). Separation of the spectral bands of different conformers of 1-heptene SOZ isolated in CO2 matrix is sufficient for the qualitative conformational analysis, but it is not enough for the quantitative analysis – many spectral bands belonging to different conformers are still overlapped. It is widely accepted in the conformational analysis, that temperature evolution of the vibrational spectral bands of fully amorphous solid can be also used for the quantitative estimations of stability of the conformers. Therefore additional experiments with amorphous solid film of neat 1-heptene SOZ were carried out. 3.3. 1-Heptene SOZ in amorphous film
Absorbance →
The amorphous film of the 1-heptene SOZe was formed by deposition of the gaseous 1-heptene SOZ on the optical
I II III IV 930
935
940
945
950
955
960
965
970
975
Wavenumber, cm-1
Fig. 3. IR absorption spectrum of 1-heptene SOZ isolated in CO2 matrix (top). Theoretically calculated spectra of: (I) – equatorial gauche (s = 66.4); (II) – equatorial anti; (III) – axial anti; (IV) – equatorial gauche (s = 57) conformers of 1-heptene SOZ.
window cooled to 90 K and following annealing at 160 K. The annealing of the film at temperature higher than 160 K leads to the crystallization of the film. The spectra of the film annealed for 10 min. at 160 K and cooled down to 150, 120, and 70 K are presented in Fig. 4. The spectral bands of amorphous 1-heptene SOZ are rather narrow, what allows their assignment to the different conformers. We were able to explain the main spectral features of amorphous 1-heptene SOZ taking in to account the results of theoretical calculations (Table 2) and assuming that amorphous film of 1-heptene SOZ contains three conformers – I, II, and IV. Surprisingly, the spectral bands of conformer III which stability is expected to be nearly the same as conformer IV were not found in the spectra. In order to use amorphous film for determination of the stability of different conformers one has to be sure that the film is fully amorphous and crystallization was not yet started. Much longer time up to 10 h was used for the annealing of the amorphous film at 160 K, but the spectra look the same as in the case of the annealing for 10 min. This experimental fact indicates that the 1-heptene SOZ film annealed at temperature not exceeding 160 K remains fully amorphous and the conformers in the film are in thermodynamical equilibrium defined only by the temperature of the film. This is not surprising, taking in to account the fact that the melting point of the 1-heptene SOZ is at 203 K, e.g. – much higher that the annealing temperature. Final prove for the correct assignment of vibrational spectral bands to different conformers in conformational analysis is crystallization of amorphous solid [20]. Usually, only one type conformers persist in the crystal cell after crystallization of amorphous solid, containing different conformers. Conformers of two types can persist in the crystal cell when two different conformers due to some sterical reasons can be closely packed. The last is rare case and also can be used as an argument for the correct assignment of vibrational spectral bands to different conformers. The conformational ratio is usually 1:1 in such crystals. In order to crystallize the amorphous film temperature of the film should be set slightly below the melting point. The crystallization process depends on many experimental parameters such as film deposition rate, annealing temperature and annealing time, therefore relative intensity of the bands belonging to different conformers in different experiments were found to be different. The spectra of the film annealed at 190 K for 20 and 120 min. together with the spectrum of fully amorphous film at 160 K are presented in Fig. 5 The film after annealing at 190 K for short time becomes partly crystalline, containing mainly conformer I. The spectral bands of conformer II completely disappear from the spectrum. Prolonging the annealing time up to 120 min. leads to complete crystallization of the film. The spectral bands assigned as a mixture of conformer I and conformer IV, decrease dramatically during the prolonged annealing of the film at 190 K. It is notable that the spectral bands of conformer IV in the amorphous film nearly completely overlap either with those of conformer I or
R. Bariseviciute et al. / Journal of Molecular Structure 844–845 (2007) 186–192
191
I+II
0.24±0.03 kJ/mol
0.6 II+IV
I+IV
I+IV
I+II+IV
0.4 I+II+IV
I+IV II
I
I
I+II+IV
-ln(Astable /Aless stable)
Absorbance →
II
II
II
I+IV
?
a
b
850
0.0
0.24±0.02 kJ/mol
-0.2
c 800
0.2
900
950 1000 1050 Wavenumber, cm-1
1100
1150
1200
-0.4
Fig. 4. IR absorption spectra of 1-heptene SOZ amorphous film at (a) 70 K; (b) 120 K; (c) 150 K temperature. Notation of the bands belonging to the different conformers is the same as in the Table 1.
0.0008
0.0010
0.0012
0.0014
0.0016
0.0018
1/T×R, J×mol-1×K-1 Fig. 6. Van’t Hoff plots for the 1-heptene SOZ conformers in amorphous film. Band pairs: d 923 (I) and 1016 cm1 (II); j 923 (I) and 944 cm1 (II).
I+II II+IV
I+IV
I+IV
I+II+IV II
I+II+IV
I+IV II
Absorbance
↑
I
I
II
I+II+IV II I+IV
a
b
c
d e 800
850
900
950
1000 1050 Wavenumber, cm-1
1100
1150
1200
Fig. 5. IR absorption spectrum of 1-heptene SOZ: (a) amorphous at 90 K; partly crystalline after annealing of the amorphous film at 190 K: (b) for 20 min., (c) for 120 min; (d) and (e) theoretically calculated spectra of the conformer I and conformer IV.
conformer II (see Fig. 5a and b), what does not allow to affirm for sure about presence of the conformer IV in the sample. Nevertheless of this overlapping behavior of the intensities of the overlapping spectral bands during the annealing enables us to make conclusion about existence of conformer IV in the film. Despite of much higher stability of conformer II than conformer IV in amorphous film, the conformer II is getting less abundant faster than conformer IV during the crystallization. Perhaps, due to the similarity of structure of the conformer I and the conformer IV, the last one appears more stabilized during the crystallization. Our experimental finding about presence of three conformers in the amorphous film of 1-heptene SOZ is not in full accordance with results of the
theoretical calculations, which predict existence of four conformers. Nevertheless, this prediction we failed to observe experimental evidences about existence of conformer III (O–O half-chair equatorial gauche) in the low temperature solid samples of the ozonide. It is notable that the three conformers experimentally found in the amorphous film are equatorial, while this additional one, predicted by the theoretical calculations, is axial. The theoretical calculations of the stability of a molecule containing strained not planar five membered COCOO ring is a challenging task and the results of the calculations has to be treated with care. It might be, that the calculations underestimate strong interaction between the ring and the closest to the ring CC bond in the axial conformer, what finally gives DH = 2.4 kJ/mol. Perhaps, in reality this value is much higher, what makes this axial conformer not observable in the1-heptene SOZ amorphous film. The temperature evolution of the spectral bands belonging to the different conformers of 1-heptene SOZ fully amorphous film can be used for the estimation of the stability of the conformers. The Van’t Hoff plots for the two pairs (923/944 and 923/1016) of the spectral band belonging to conformer I and conformer II are presented in Fig. 6. The DH values calculated from the both plots coincide. The experimental value of DH equals to 0.24 ± 0.03 kJ/mol is in reasonable accordance to the calculated one – 0.5 kJ/mol. Unfortunately, the spectral bands attributed to the conformer IV cannot be used to the delta H estimation, due to the overlapping with the spectral bands of other conformers. 4. Conclusions The FT-IR spectra of 1-heptene SOZ isolated in Ar and CO2 matrixes and its neat amorphous film were recorded. Spectra were analyzed combining experimental results with
192
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the theoretical calculations performed at B3LYP 6311++G (3df, 3pd) level. It was found that conventional matrix media – Ar and CO2 in case of 1-heptene SOZ is lacking the main matrix advantage – narrowing of the vibrational spectral bands. This is due to the size of the SOZ molecules, which excides the size of the matrix cage. It was experimentally proved that the neat film of 1-heptene SOZ at 90–160 K temperature range remains completely amorphous with the conformational mixture defined by the temperature of the film. It was found that the samples of amorphous 1-heptene secondary ozonide exist as a mixture of three dominating conformers. The most stable conformer is the one with O–O half-chair configuration of the five membered ring, the aliphatic radical attached to the ring in equatorial position and the aliphatic chain being in gauche (\OCCC 66.4) position. The other two stable conformers are equatorial with aliphatic chain in anti (\OCCC 180) and gauche (\OCCC 60) positions. The conformer III predicted by the theoretical calculations was not observed in the amorphous film of 1-heptene SOZ. Perhaps, the calculations underestimate interaction between the ring and the closest to the ring CC bond in the axial conformer. This interaction is smaller in the equatorial conformers due to orientation of the closed to the ring CC bond in respect to the ring what makes three staggered equatorial conformers (I, II, and IV) more stable and abundant in the amorphous samples of 1-heptene SOZ. The ordered crystal of 1-heptene SOZ was obtained for the first time. The crystal cage contains only one type of the SOZ – conformer I. A closer look to the crystallization process revealed that the conformer IV (DHcalc = 2.5 kJ/mol) persists in the partly crystalline sample until very end of completing of the crystallization while much more stable conformer II (DHcalc = 0.5 kJ/mol) vanishes from the sample at the very beginning of the crystallization. This experimental fact might be because of the similarity of the structures of conformer I and conformer IV – both are O–O half-chair equatorial gauche with only difference – the torsional angle \OCCC being equal to 66.4 and +57, respectively. Relative stability of the conformer II was estimated from the Van’t Hoff plots for the two pairs (923/944 and 923/1016) of the spectral band belonging to conformer I and conformer II at temperature range 90–160 K. The
obtained experimental value of DH equal to 0.24 ± 0.03 kJ/mol is found to be in reasonable accordance to the calculated one – 0.5 kJ/mol. Unfortunately, the spectral bands attributed to the conformer IV cannot be used for the DH estimation, due to their overlapping with the spectral bands of the other conformers. In order to overcome this difficulty a proper matrix material has to be found. The matrix cage should be big enough to fit large 1-heptene SOZ molecule in it. The narrowing effect of the SOZ spectral bands can be expected in such a matrix. Searching for a matrix material better suited for the conformational studies of 1-heptene SOZ than Ar and CO2 is underway. References [1] R. Criegee, Rec. Chem. Prog. 18 (1957) 110. [2] R. Fajgar, J. Vitek, Y. Haas, J. Pola, J. Chem. Soc. Perkin Trans. 2 (1999) 239. [3] R. Atkinson, E.C. Tuazon, S.M. Aschmann, Environ. Sci. Technol. 29 (1995) 1860. [4] P. Neeb, O. Horie, G.K. Moortgat, Tetrahedron Lett. 37 (1996) 297. [5] R.D. Suenram, F.J. Lovas, J. Am. Chem. Soc. 16 (1978) 5117. [6] J.Z. Gillies, C.W. Gillies, R.D. Suenram, F.J. Lovas, J. Am. Chem. Soc. 110 (1988) 7991. [7] N.L. Bauld, J.A. Thompson, C.E. Hudson, P.S. Bailey, J. Amer. Chem. Soc. 90 (1968) 1822. [8] R.P. Lattimer, R.L. Kuczkowski, C.W. Gillies, J. Am. Chem. Soc. 96 (1974) 348. [9] P.S. Bailey, T.M. Ferrell, J. Am. Chem. Soc. 100 (1978) 899. [10] C.W. Gillies, R.L. Kuczkowski, J. Am. Chem. Soc. 94 (1972) 6337. [11] R.P. Lattimer, C.W. Gillies, R.L. Kuczkowski, J. Am. Chem. Soc. 95 (1973) 1348. [12] M.S. LaBarge, H. Keul, R.L. Kuczkowski, M. Wallasch, D. Cremer, J. Am. Chem. Soc. 110 (1988) 2081. [13] K.W. Hillig, R.L. Kuczkowski, J. Phys. Chem. 86 (1982) 1415. [14] C.W. Gillies, S.P. Sponseller, R.L. Kuczkowski, J. Phys. Chem. 83 (1979) 1545. [15] R. Bariseviciute, J. Ceponkus, A. Gruodis, V. Sˇablinskas, Cent. Eur. J. Chem. 4 (2006) 1. [16] U. Samuni, Y. Haas, R. Fajgar, J. Pola, J. Mol. Struct. 449 (1998) 177. [17] M.G. Loudon, in: Organic Chemistry, fourth ed., Oxford University Press, Oxford, 2002. [18] J. Dale, in: Stereochemistry and Conformational Analysis, Universitetsforlaget, Oslo, 1978. ˇ eponkus, V. Sˇablinskas, Cent. Eur. J. Chem. 5 [19] R. Barisevicˇi ute_, J. C (2007) 1. [20] P. Klaeboe, Vibrational Spectroscopy 9 (1995) 3.
Journal of Molecular Structure 844–845 (2007) 186–192 www.elsevier.com/locate/molstruc
Conformations of 1-heptene secondary ozonide as studied by low temperature FT-IR spectroscopy R. Bariseviciute, J. Ceponkus, V. Sablinskas, L. Kimtys
*
Department of General Physics and Spectroscopy, Vilnius University, Universiteto str. 3, Vilnius 01513, Lithuania Received 27 February 2007; received in revised form 24 April 2007; accepted 28 April 2007 Available online 13 May 2007
Abstract Conformational diversity of the 1-heptene secondary ozonides (SOZ) in solid neat films as well as isolated in Ar or CO2 matrices was studied by the means of FT-IR absorption spectroscopy. The ozonization reaction was performed at 77 K in the neat films of the reactants. The spectra of the ozonide were analyzed by combining the experimental data with the results of theoretical calculations performed at B3LYP 6-311++G (3df, 3pd) level. It was found that the samples of 1-heptene secondary ozonide exist as a mixture of three dominating conformers. The most stable conformer is the one with O–O half-chair configuration of the five membered ring, the aliphatic radical attached to the ring in equatorial position and the aliphatic chain being in gauche (\OCCC 60) position. The other two stable conformers are equatorial with aliphatic chain in anti (\OCCC 180) and gauche (\OCCC 60) positions. It was found from Van’t Hoff plots that DH of the equatorial anti conformer is equal to 0.24 ± 0.03 kJ/mol. The experimental value of DH is in reasonable accordance to the calculated one – 0.5 kJ/mol. 2007 Elsevier B.V. All rights reserved. Keywords: 1-Heptene secondary ozonide; Conformations; Matrix isolation; FT-IR spectroscopy
1. Introduction An ability of ozone to break double C@C bond in olefins is known for more than five decades. Understanding of those reactions is very important in atmospheric chemistry. Alkene ozonization reaction is believed to be proceeded under so-called Criegee three-step mechanism [1]. During different steps of the reaction the primary ozonide (POZ), carbonyl oxide (COX) and the secondary ozonide (SOZ) are formed. Fate of the reaction depends on many parameters such as type of radical, conformation of alkene, temperature of the reaction and environmental effects. Despite of numerous studies of the reaction by different spectroscopic techniques the precise mechanism of the reaction is still unknown [2–5]. It is well established experimentally that POZ is highly unstable intermediate, which rapidly dissociates into a car*
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0022-2860/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2007.04.041
bonyl compound (CC) and COX often known as Criegee intermediate (CI) [5]. Further fate of the reaction depends on the conditions in which the reaction proceeds [2]. Many different reaction pathways are possible in the gas phase such as dissociation or rearrangement of the CI followed by the bimolecular reactions with CC or residual CI, which leads to the rather complicated reaction sequence. The carbonyl compound and CI readily recombine to form secondary ozonide in the condensed phase. However, some other competing processes may occur in the condensed phase too. Suenram and Lovas have reported the detection of dioxirane from ethene ozonolysis [5]. It is experimentally observed that the SOZ is more stable than POZ [6]. At room temperature it can remain from few hours up to a few days. SOZ dissociates into formic acid and aldehide. Stability of the SOZ depends on the size and configuration of the radical. Unfortunately, it is not much known about the spatial structures of the SOZs. The variety of the possible conformations of 1-alkene SOZs, are generally caused by three reasons. First, five
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membered ring of the SOZ can possess two different structures – so-called C–O half-chair conformation and O–O half-chair conformation [7]. According to Kuckowski and Bauld-Bailey [8,9] the five member ring of SOZ exists preferably in the O–O half-chair conformation. Such configuration of the ring was confirmed experimentally from the microwave spectroscopy experiments [8,10–14]. Second, an aliphatic chain of the 1-alkene ozonide can be differently attached to the ring. In the case of the 1-alkene SOZ an aliphatic chain can be attached to the ring in either so-called axial or equatorial positions. In the axial conformer the chemical bond between the ring carbon and the aliphatic carbon is perpendicular to the ring plane. This bond lies in the ring plane in the equatorial conformer. It is notable, that the C–C bond does not lie exactly parallel or perpendicular to the plane of the ozonide five member ring but these notations are still in use in the conformational analysis of SOZs [8,15]. The structure of equatorial propene SOZ conformer was determined by the microwave spectroscopy [8]. Haas [16] used ab initio calculated vibrational spectrum of the 1-hexene axial conformer for assignment of the infrared spectral bands of the 1-hexene ozonization reaction products isolated in the CO2 matrix. Samuni et al. [16] have not made any attempt to consider the possible conformational diversity of the ozonide. Third, aliphatic chain by itself can have different conformations. It is very well established that the aliphatic chain is usually planar in the most stable conformer of any saturated aliphatic compound. However, rotation around the closest to the ring C–C bond can cause formation of so-called anti and gauche conformers in the case of aliphatic chain attached to the five membered ozonide ring (COCOO). It has been experimentally and theoretically confirmed that energy difference between anti and gauche conformers are of 3.4–3.8 kJ/mol [17,18] for alkanes. This value can be different in the case of the aliphatic chain attached to the COCOO ring of the ozonide. This is due to the interactions between ring and the aliphatic radical. Our initial conformational studies of 1-alkene ozonides, using gas phase FT-IR spectroscopy, are presented in [15]. It was concluded that at least two conformers of 1-heptene were present in the gaseous samples. We were not able to make more quantitative conclusions about the number and stability of the conformers. Infrared absorption bands,
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belonging to the different conformers of the 1-heptene SOZ, in the gas phase spectra were strongly overlapped due to the broadness of the bands. The aim of this study is to define the geometrical structures and stability of the different conformers of the 1-heptene secondary ozonide by combined analysis of the low temperature experimental spectral data with the results of Density Functional Theory (DFT) calculations. 2. Experimental Oxygen (99.999%) from AGA and 1-heptene (99%) from Aldrich were used without an additional purification. Ozone was prepared from oxygen by the electric discharge. The ozonization reaction was performed in the condensed phase in the stainless steel reactor [19]. The inner part of the reactor was cooled with liquid nitrogen. 1-Heptene and ozone were consequently introduced in to the reactor in the small portions (about 15 torr · l) thus forming multi-layered film of the reactants on the walls of reactor. When the reaction was finished the reactor was slowly warmed up. Continuous pumping of the reactor during the warm up allowed us to separate the SOZ from the other reaction products. The products of the reaction started actively evaporate only at temperatures close to their melting points. For instance, formaldehyde actively evaporates starting from 140 to 160 K, hexanal – from 180 to 200 K. Only the products, which evaporate from the walls of the reactor at 275 K temperature were transferred to the vacuum system and used for preparation of amorphous film of the secondary ozonide or the matrix sample. Closed cycle Helium cryostat Leybold RW2 was used for cooling the samples. Matrix mixture (ratio of 1-heptene SOZ with Ar (or CO2) – 1:600) was deposited on CsI window of the cryostat cooled down to 20 K for Ar matrix or 65 K for CO2 matrix. The gaseous mixture was kept at room temperature during the deposition. Photolysis experiments of the matrix isolated samples were performed using conventional high pressure Hg arc lamp, providing UV radiation in the spectral range of 170–400 nm. The closed cycle helium cryostat was used for the preparation of the amorphous films of the 1-heptene SOZ. The gaseous SOZ was deposited on the optical window cooled down to 60 K. Infrared absorption spectra were recorded on BRUKER Vertex 70 FT-IR spectrometer, using 0.5 cm1 spectral resolution, glowbar source, KBr beamsplitter, and MCT detector.
Table 1 DFT calculated (B3LYP\6-311++G(3df, 3pd)) relative potential energies DH (calculated taking in to account ZPE) of the four most stable conformers of 1-heptene SOZ Conformer I II III IV
DH, kJ/mol (sOCCC, sCCCC) O–O O–O O–O O–O
half-chair half-chair half-chair half-chair
equatorial gauche equatorial anti axial anti equatorial gauche
0 (sOCCC = 66.4, sCCCC = 179.1) 0.5 (sOCCC = 178.5, sCCCC = 177.8) 2.4 (sOCCC = 174.8, sCCCC = 180) 2.5 (sOCCC = 57.0, sCCCC = 178.2)
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Table 2 DFT calculated (B3LYP\6-311++G(3df, 3pd)) normal vibrations of four most stable 1-heptene SOZ conformers in the spectral region of the ring vibrations Equatorial anti
mcalc*(cm1)
I (a.u.)
mcalc*
6.7
824 –– 878 –– –– 889 903 –– –– 946 962 –– –– 1004 1014 –– 1036 –– 1054 –– 1065 1105 1129 1140 1167
820 –– 874 885 896 –– –– 931 –– –– 986 990 –– 1033 1045 –– 1056 –– 1062 –– 1105 1130 1139 1160 *
6.8 8 13.5
16.6
14.5 10 6.2 14.5 76.9 22.1 131.8 12.2 9 2.7
The calculated wavenumbers are scaled by 0.99.
1
(cm )
Axial anti I (a.u.)
mcalc*
0.4
830 –– 860 881
4.9
3.9 4.4
12.4 47.6
12.9 11.3 2.3 11.6 98.2 117.2 12.9 8.5 6.8
892 –– –– –– 949 958 –– –– 994 1014 –– 1035 –– 1054 –– 1061 1103 1121 1142 1155
1
(cm )
Equatorial gauche (sOCCC = 57.0) 1
Assignment
I (a.u.)
mcalc*
1.3
818 863 878
4.4 9 5.6
CH2 twist O(1)–C(2)–C(3) def O–O stretch CH2 twist
895 –– 908 –– –– 957 –– –– 997 1013 –– 1037 –– 1054 –– 1063 1121 1125 1137 1156
6.5
CH3 rock CH2 twist, CH3 rock
11.1 1.5 5.2
10.6 57.2
11.2 2 0.8 10 37.2 216.3 12.8 9 10.2
(cm )
I (a.u.)
9.7
33.2
29.5 1.5 9.6 8.7 78.3 87.7 15.8 4.5 16.2
O–C–O asym. stretch, C(2)–O(3) stretch O(1)–C(2), C(1)–O(2) stretch O–C–O asym. stretch CH2 twist, CH3 rock C–C–C in plain def CH2 twist, CH3 rock C–C–C in plain def C–C–C in plain def C–C stretch O–C–O sym. stretch C–C–C in plain def ring vibr O–C–O sym. stretch O–C–O sym. stretch C–C–C in plain def CH2 rock C–C–C out of plain def
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Equatorial gauche (sOCCC = 66.4)
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3. Results and discussion 3.1. Calculations Theoretical calculations of the vibrational spectra and the structure of the staggered conformers of the SOZ were performed with Gaussian 03 program package, using the DFT method B3LYP with 6-311++G (3df, 3pd) basis set. The input parameters for the program were set in such a way to look on the potential surface of the SOZ only for equatorial and axial staggered conformers formed only due to the rotation of the aliphatic radical around the C–C bond nearest to the five membered ring. The conformers formed due to the rotation around the other @C–C-bonds are much less stable and therefore could be excluded from the studies. Results of the calculations for the title ozonide are presented in the Tables 1 and 2. The structures of the four most stable 1-heptene SOZ conformers are presented in Fig. 1.
3.2. 1-Heptene SOZ in Ar and CO2 matrixes Spectral region of the ozonide ring vibrations at 800–1200 cm1 was chosen for the conformational analysis taking in to account the results of the theoretical calculations (Table 2). According to the calculations the spectral bands belonging to the different conformers are separated
189
from each other by 5–20 cm1 in this spectral region, while they are completely or partly overlapped in the other regions. Infrared absorption spectrum of the 1-heptene SOZ isolated in Ar matrix is presented in Fig. 2a. The spectral bands of the matrix isolated 1-heptene SOZ are surprisingly broad and not suitable for the conformational analysis. Broadness of the spectral bands of 1-heptene SOZ isolated in Ar matrix can be explained by large number of the 0trapping sites. The lattice constant of the Ar crystal [10] and the length of 1-heptene SOZ molecule – ca. is 5.3 A 0 . The SOZ molecule does not fit in to the cells of 10 A the Ar crystal, and only various multi-substitutional trapping sites are possible. Overlapping of the spectral bands of the SOZs surrounded by slightly different environment results their broadening. The crystalline cage of the CO2 matrix is slightly larger than that of Ar matrix. Even more, CO2 matrices obtained by deposition of CO2 gas at temperatures as low as 20 K are more amorphous than crystalline [16]. This makes the environment more uniform and it results the narrower spectral bands. Therefore the experiments with the SOZ isolated in the CO2 matrix were carried out in this work. FT-IR absorption spectrum of 1-heptene SOZ isolated in CO2 matrix is presented Fig. 2b. As seen in the figure the spectral bands of 1-heptene SOZ isolated in CO2 matrix is narrower comparing to those of the ozonide isolated in the Ar matrix. The theoretically calculated spectra of four conformers of 1-heptene SOZ together with the
Fig. 1. Theoretically (B3LYP 6-311++G (3df, 3pd)) calculated structures of four the most stable 1-heptene SOZ conformers: (I) equatorial gauche (\OCCC = 66.4); (II) equatorial (\OCCC = 178.5); (III) axial anti (\OCCC = 174.8); (IV) equatorial gauche (\OCCC = 57).
R. Bariseviciute et al. / Journal of Molecular Structure 844–845 (2007) 186–192
Absorbance →
190
a
♦
b 800
* 850
900
* * 950
1000
1050
1100
1150
1200
Wavenumber, cm-1
Fig. 2. IR absorption spectra of 1-heptene SOZ isolated in: (a) argon matrix; (b) CO2 matrix. *, 1-heptene spectral bands, , formic acid spectral band.
experimental spectrum in 930–975 cm1 spectral regions are presented in Fig. 3. The band at 936 cm1 can be assigned to the equatorial gauche ( 60) conformer (I), the bands at 947 and 965 cm1 – to equatorial anti conformer (II) and the band at 956 cm1 – to equatorial gauche ( 60) conformer (Table 2). Separation of the spectral bands of different conformers of 1-heptene SOZ isolated in CO2 matrix is sufficient for the qualitative conformational analysis, but it is not enough for the quantitative analysis – many spectral bands belonging to different conformers are still overlapped. It is widely accepted in the conformational analysis, that temperature evolution of the vibrational spectral bands of fully amorphous solid can be also used for the quantitative estimations of stability of the conformers. Therefore additional experiments with amorphous solid film of neat 1-heptene SOZ were carried out. 3.3. 1-Heptene SOZ in amorphous film
Absorbance →
The amorphous film of the 1-heptene SOZe was formed by deposition of the gaseous 1-heptene SOZ on the optical
I II III IV 930
935
940
945
950
955
960
965
970
975
Wavenumber, cm-1
Fig. 3. IR absorption spectrum of 1-heptene SOZ isolated in CO2 matrix (top). Theoretically calculated spectra of: (I) – equatorial gauche (s = 66.4); (II) – equatorial anti; (III) – axial anti; (IV) – equatorial gauche (s = 57) conformers of 1-heptene SOZ.
window cooled to 90 K and following annealing at 160 K. The annealing of the film at temperature higher than 160 K leads to the crystallization of the film. The spectra of the film annealed for 10 min. at 160 K and cooled down to 150, 120, and 70 K are presented in Fig. 4. The spectral bands of amorphous 1-heptene SOZ are rather narrow, what allows their assignment to the different conformers. We were able to explain the main spectral features of amorphous 1-heptene SOZ taking in to account the results of theoretical calculations (Table 2) and assuming that amorphous film of 1-heptene SOZ contains three conformers – I, II, and IV. Surprisingly, the spectral bands of conformer III which stability is expected to be nearly the same as conformer IV were not found in the spectra. In order to use amorphous film for determination of the stability of different conformers one has to be sure that the film is fully amorphous and crystallization was not yet started. Much longer time up to 10 h was used for the annealing of the amorphous film at 160 K, but the spectra look the same as in the case of the annealing for 10 min. This experimental fact indicates that the 1-heptene SOZ film annealed at temperature not exceeding 160 K remains fully amorphous and the conformers in the film are in thermodynamical equilibrium defined only by the temperature of the film. This is not surprising, taking in to account the fact that the melting point of the 1-heptene SOZ is at 203 K, e.g. – much higher that the annealing temperature. Final prove for the correct assignment of vibrational spectral bands to different conformers in conformational analysis is crystallization of amorphous solid [20]. Usually, only one type conformers persist in the crystal cell after crystallization of amorphous solid, containing different conformers. Conformers of two types can persist in the crystal cell when two different conformers due to some sterical reasons can be closely packed. The last is rare case and also can be used as an argument for the correct assignment of vibrational spectral bands to different conformers. The conformational ratio is usually 1:1 in such crystals. In order to crystallize the amorphous film temperature of the film should be set slightly below the melting point. The crystallization process depends on many experimental parameters such as film deposition rate, annealing temperature and annealing time, therefore relative intensity of the bands belonging to different conformers in different experiments were found to be different. The spectra of the film annealed at 190 K for 20 and 120 min. together with the spectrum of fully amorphous film at 160 K are presented in Fig. 5 The film after annealing at 190 K for short time becomes partly crystalline, containing mainly conformer I. The spectral bands of conformer II completely disappear from the spectrum. Prolonging the annealing time up to 120 min. leads to complete crystallization of the film. The spectral bands assigned as a mixture of conformer I and conformer IV, decrease dramatically during the prolonged annealing of the film at 190 K. It is notable that the spectral bands of conformer IV in the amorphous film nearly completely overlap either with those of conformer I or
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191
I+II
0.24±0.03 kJ/mol
0.6 II+IV
I+IV
I+IV
I+II+IV
0.4 I+II+IV
I+IV II
I
I
I+II+IV
-ln(Astable /Aless stable)
Absorbance →
II
II
II
I+IV
?
a
b
850
0.0
0.24±0.02 kJ/mol
-0.2
c 800
0.2
900
950 1000 1050 Wavenumber, cm-1
1100
1150
1200
-0.4
Fig. 4. IR absorption spectra of 1-heptene SOZ amorphous film at (a) 70 K; (b) 120 K; (c) 150 K temperature. Notation of the bands belonging to the different conformers is the same as in the Table 1.
0.0008
0.0010
0.0012
0.0014
0.0016
0.0018
1/T×R, J×mol-1×K-1 Fig. 6. Van’t Hoff plots for the 1-heptene SOZ conformers in amorphous film. Band pairs: d 923 (I) and 1016 cm1 (II); j 923 (I) and 944 cm1 (II).
I+II II+IV
I+IV
I+IV
I+II+IV II
I+II+IV
I+IV II
Absorbance
↑
I
I
II
I+II+IV II I+IV
a
b
c
d e 800
850
900
950
1000 1050 Wavenumber, cm-1
1100
1150
1200
Fig. 5. IR absorption spectrum of 1-heptene SOZ: (a) amorphous at 90 K; partly crystalline after annealing of the amorphous film at 190 K: (b) for 20 min., (c) for 120 min; (d) and (e) theoretically calculated spectra of the conformer I and conformer IV.
conformer II (see Fig. 5a and b), what does not allow to affirm for sure about presence of the conformer IV in the sample. Nevertheless of this overlapping behavior of the intensities of the overlapping spectral bands during the annealing enables us to make conclusion about existence of conformer IV in the film. Despite of much higher stability of conformer II than conformer IV in amorphous film, the conformer II is getting less abundant faster than conformer IV during the crystallization. Perhaps, due to the similarity of structure of the conformer I and the conformer IV, the last one appears more stabilized during the crystallization. Our experimental finding about presence of three conformers in the amorphous film of 1-heptene SOZ is not in full accordance with results of the
theoretical calculations, which predict existence of four conformers. Nevertheless, this prediction we failed to observe experimental evidences about existence of conformer III (O–O half-chair equatorial gauche) in the low temperature solid samples of the ozonide. It is notable that the three conformers experimentally found in the amorphous film are equatorial, while this additional one, predicted by the theoretical calculations, is axial. The theoretical calculations of the stability of a molecule containing strained not planar five membered COCOO ring is a challenging task and the results of the calculations has to be treated with care. It might be, that the calculations underestimate strong interaction between the ring and the closest to the ring CC bond in the axial conformer, what finally gives DH = 2.4 kJ/mol. Perhaps, in reality this value is much higher, what makes this axial conformer not observable in the1-heptene SOZ amorphous film. The temperature evolution of the spectral bands belonging to the different conformers of 1-heptene SOZ fully amorphous film can be used for the estimation of the stability of the conformers. The Van’t Hoff plots for the two pairs (923/944 and 923/1016) of the spectral band belonging to conformer I and conformer II are presented in Fig. 6. The DH values calculated from the both plots coincide. The experimental value of DH equals to 0.24 ± 0.03 kJ/mol is in reasonable accordance to the calculated one – 0.5 kJ/mol. Unfortunately, the spectral bands attributed to the conformer IV cannot be used to the delta H estimation, due to the overlapping with the spectral bands of other conformers. 4. Conclusions The FT-IR spectra of 1-heptene SOZ isolated in Ar and CO2 matrixes and its neat amorphous film were recorded. Spectra were analyzed combining experimental results with
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the theoretical calculations performed at B3LYP 6311++G (3df, 3pd) level. It was found that conventional matrix media – Ar and CO2 in case of 1-heptene SOZ is lacking the main matrix advantage – narrowing of the vibrational spectral bands. This is due to the size of the SOZ molecules, which excides the size of the matrix cage. It was experimentally proved that the neat film of 1-heptene SOZ at 90–160 K temperature range remains completely amorphous with the conformational mixture defined by the temperature of the film. It was found that the samples of amorphous 1-heptene secondary ozonide exist as a mixture of three dominating conformers. The most stable conformer is the one with O–O half-chair configuration of the five membered ring, the aliphatic radical attached to the ring in equatorial position and the aliphatic chain being in gauche (\OCCC 66.4) position. The other two stable conformers are equatorial with aliphatic chain in anti (\OCCC 180) and gauche (\OCCC 60) positions. The conformer III predicted by the theoretical calculations was not observed in the amorphous film of 1-heptene SOZ. Perhaps, the calculations underestimate interaction between the ring and the closest to the ring CC bond in the axial conformer. This interaction is smaller in the equatorial conformers due to orientation of the closed to the ring CC bond in respect to the ring what makes three staggered equatorial conformers (I, II, and IV) more stable and abundant in the amorphous samples of 1-heptene SOZ. The ordered crystal of 1-heptene SOZ was obtained for the first time. The crystal cage contains only one type of the SOZ – conformer I. A closer look to the crystallization process revealed that the conformer IV (DHcalc = 2.5 kJ/mol) persists in the partly crystalline sample until very end of completing of the crystallization while much more stable conformer II (DHcalc = 0.5 kJ/mol) vanishes from the sample at the very beginning of the crystallization. This experimental fact might be because of the similarity of the structures of conformer I and conformer IV – both are O–O half-chair equatorial gauche with only difference – the torsional angle \OCCC being equal to 66.4 and +57, respectively. Relative stability of the conformer II was estimated from the Van’t Hoff plots for the two pairs (923/944 and 923/1016) of the spectral band belonging to conformer I and conformer II at temperature range 90–160 K. The
obtained experimental value of DH equal to 0.24 ± 0.03 kJ/mol is found to be in reasonable accordance to the calculated one – 0.5 kJ/mol. Unfortunately, the spectral bands attributed to the conformer IV cannot be used for the DH estimation, due to their overlapping with the spectral bands of the other conformers. In order to overcome this difficulty a proper matrix material has to be found. The matrix cage should be big enough to fit large 1-heptene SOZ molecule in it. The narrowing effect of the SOZ spectral bands can be expected in such a matrix. Searching for a matrix material better suited for the conformational studies of 1-heptene SOZ than Ar and CO2 is underway. References [1] R. Criegee, Rec. Chem. Prog. 18 (1957) 110. [2] R. Fajgar, J. Vitek, Y. Haas, J. Pola, J. Chem. Soc. Perkin Trans. 2 (1999) 239. [3] R. Atkinson, E.C. Tuazon, S.M. Aschmann, Environ. Sci. Technol. 29 (1995) 1860. [4] P. Neeb, O. Horie, G.K. Moortgat, Tetrahedron Lett. 37 (1996) 297. [5] R.D. Suenram, F.J. Lovas, J. Am. Chem. Soc. 16 (1978) 5117. [6] J.Z. Gillies, C.W. Gillies, R.D. Suenram, F.J. Lovas, J. Am. Chem. Soc. 110 (1988) 7991. [7] N.L. Bauld, J.A. Thompson, C.E. Hudson, P.S. Bailey, J. Amer. Chem. Soc. 90 (1968) 1822. [8] R.P. Lattimer, R.L. Kuczkowski, C.W. Gillies, J. Am. Chem. Soc. 96 (1974) 348. [9] P.S. Bailey, T.M. Ferrell, J. Am. Chem. Soc. 100 (1978) 899. [10] C.W. Gillies, R.L. Kuczkowski, J. Am. Chem. Soc. 94 (1972) 6337. [11] R.P. Lattimer, C.W. Gillies, R.L. Kuczkowski, J. Am. Chem. Soc. 95 (1973) 1348. [12] M.S. LaBarge, H. Keul, R.L. Kuczkowski, M. Wallasch, D. Cremer, J. Am. Chem. Soc. 110 (1988) 2081. [13] K.W. Hillig, R.L. Kuczkowski, J. Phys. Chem. 86 (1982) 1415. [14] C.W. Gillies, S.P. Sponseller, R.L. Kuczkowski, J. Phys. Chem. 83 (1979) 1545. [15] R. Bariseviciute, J. Ceponkus, A. Gruodis, V. Sˇablinskas, Cent. Eur. J. Chem. 4 (2006) 1. [16] U. Samuni, Y. Haas, R. Fajgar, J. Pola, J. Mol. Struct. 449 (1998) 177. [17] M.G. Loudon, in: Organic Chemistry, fourth ed., Oxford University Press, Oxford, 2002. [18] J. Dale, in: Stereochemistry and Conformational Analysis, Universitetsforlaget, Oslo, 1978. ˇ eponkus, V. Sˇablinskas, Cent. Eur. J. Chem. 5 [19] R. Barisevicˇi ute_, J. C (2007) 1. [20] P. Klaeboe, Vibrational Spectroscopy 9 (1995) 3.