Initio and DFT Studies of Conformational Properties of Heteroatom Containing Ketene Analogues and Their Comparison with the Related Cyclic Analogues

Minimum-energy and transition state geometries of 3-thioxoprop-2enethial, 3-thioxoacrylaldehyde, 3-oxoprop-2-enethial, 3-selenoxoprop-2-enethial, 3-thioxoprop-2-eneselenal, 3-selenoxoprop-2-eneselenal, 3-oxoacrylaldehyde, 3-selenoxoacrylaldehyde and 3-oxoprop-2-eneselenal were calculated using HF, B3LYP and MP2 levels of theory and 6-31+G basis set by rotation around the related -C-Csingle bonds. In all of the above mentioned molecules, the s-trans conformation was obtained as the most stable conformer with the 180o dihedral angle, apart from 3-oxoprop-2-enethial and 3-thioxoprop-2-eneselenal which their s-cis conformers were appeared more stability than related to s-trans forms. Their perpendicular geometries, with torsional angles approximately 90o, were as transition state for conformational interconversion between the two global minima forms. Cyclic structures all of the above mentioned molecules were unstable than their linear forms.


Introduction
Ketenes are reactive intermediates with unusual physical properties and unique spectrum of chemical reactivity 1 .Their special structural forms have captured the minds of academic chemists and the potential multi-functional reactivity of these compounds and possible practical applications have attracted the attention of industrial chemists 1 .They can combine rapidly with nucleophiles such as water, alcohols and amines to give carboxylic acid, esters, and amids derivatives 2 .These compounds can also perform different intramolecular cycloaddition reactions 3 .Thioketenes have used to production N-sulfenylimines derivatives 4 .
The 2+2 cycloaddition of ketenes with ynamides has produced 3-aminocyclobutenone derivatives 5 .Reaction between fluoroketenes and imines has been led to production β-lactams 6 .Development of ketene analogous chemistry has resulted in the utilization of ketene-functionalized polymers for general materials applications 7 .

Calculations
The ab initio molecular orbital calculations were carried out using the GAUSSIAN 98 program 13 .All geometries in torsional analyses of 1-9 and 1′-9′ analogues were fully optimized at the HF/6-31+G * , B3LYP/6-31+G * and MP2/6-31+G * levels of theory.Geometry parameters were also reported according to the obtained results of the MP2/6-31+G * level in this paper.The optimized calculations at the MP2/6-31+G * level were used to evaluate the electron correlation effect in the energies and the order of stability of conformers.Frequency calculations were performed at the MP2/6-31+G * level for 1-9 and 1′-9′ on the related optimized geometries.To account for the difference between the harmonic and anharmonic, oscillations of the actual bonds scaled the harmonic vibrational frequencies and ZPVE data at the MP2/6-31+G * level (for 1-9 and 1′-9′) by 0.9135 14 .

Results and Discussion
Calculations of 1-9 and 1′-9′ structures were performed at the HF, B3LYP and MP2 methods with 6-31+G * basis set.The obtained results from MP2/6-31+G * calculations were reported in the discussions of conformational energies and the following used structural parameters.
Conceptually, the calculated molecules of 1-9 had the structures similar to propene that its methylene and methyl moieties were replaced with C=X and C=Y groups (X= O, S and Se; Y=O, S and Se).

C 3 H 2 S 2 isomers: (3-Thioxoprop-2-enethial (1) and 2H-Thiete-2-thione (1′))
The obtained results of the torsional analyses of ketene 1 are shown in Tables 1 and Figure 1.The different conformations of 1 were found by driving Ф 3125 (selected torsional angle) from 180 to -180º (Scheme 1).Ab initio calculations of 1 showed three energy minima and two transition state structures.The low-energy conformations were obtained at Ф 3125 = 180, 0 and -180.0º(Figure 1 and Table 1).Maxima energy conformations were seen at 91.5 and -91.5º dihedral angles (Figure 1 and Table 1).The s-trans conformers (180 and -180º), 1-trans and 1-trans', with symmetry plane were calculated to have the lowest energy (Table 2).This is due to the lowest steric interaction experienced by the -C=C=S and -C=S groups and the hydrogen atoms in 1-trans and 1-trans' forms.By Fixing the -C=C-C=S torsional angle at 91.5 and -91.5º, conformational changes were occurred (Table 1 and Figure 1).In these changes, 1-TS and 1-TS΄ conformations (C 1 ) were obtained as the transition state geometries.The relative energies of 1-TS and 1-TS΄ conformations were 48.6 kJ mol -1 higher than 1-trans conformation (Table 2).The instability of these transition state structures may be attributed to the unfavorable π-π repulsion between the electrons in the p orbitals of the -C=C=S and -C=S groups, due to the inappropriate torsional angles.Another energy minimum conformation, 1-cis was obtained by constraining the torsional angle at 0º (C S ), while the -C=C=S and -C=S groups were coplanar.Conformational energy of 1-cis was 2.7 kJ mol -1 higher than 1-trans conformation (Table 2).The 0º torsional angle in above conformers lead to a reduction involvement between π electron-π electron in the p orbitals of the -C=C=S and -C=S groups than 1-TS and 1-TS΄ forms (Figure 1).In this conformer, energy increase than 1-trans may be due to eclipsing of the -C=C=S and -C=S groups, which leads to increase of the repulsion between the electrons in the p orbitals at C β of the -C=C=S and -C=S groups and also repulsion of the positively charged C α carbonyl carbons of the same groups.

C 3 H 2 OS isomers: 3-Thioxoacrylaldehyde (2), 2H-oxete-2-thione (2′), 3-oxoprop-2enethial (3) and 2H-thiet-2-one (3′)
In structure 2, the most stable conformations corresponded to s-trans forms (2-trans and 2-trans') with C s point group; and Φ 3124 = 180 and -18oº (Tables 1 and 2).Stability in these conformers was clearly due to the absence of steric effects.Upon rotation of the dihedral angle to 0º, 2-cis conformation with C s symmetry was obtained which was 2.6 kJ mol -1 higher in energy than 2-trans conformer (Table 2).This difference in energy could be the result of eclipsing between the two -C=C=S and -C=O groups and possible increasing repulsion between the electron-deficient C α carbons in 2-cis form.In 3 molecule, the most stable conformations corresponded to s-cis form (3-cis) with C s point group and Φ 5213 = 0º (Tables 1 and 2).Energy of s-trans conformations in this molecule by Φ 5213 = 180 and -18oº (3-trans and 3-trans') were very close to 3-cis form.Energy surface of 3-trans and 3-trans' conformations was 0.3 kJ mol -1 higher than 3-cis form (Table 2).The highest energy conformations in 2 and 3 corresponded to their perpendicular conformers (in 2 molecule Φ 3124 = 90 and -9oº (2-TS and 2-TS'), and in 3 molecule Φ 5213 = 92.1 and -92.1º (3-TS and 3-TS'), respectively (Tables 1 and 2).The relative energies of transition states in 2 molecule, 2-TS and 2-TS΄ (C 1 ), were found 42.3 kJ mol -1 more than 2-trans conformation (Table 2).Energy surface of 3-TS and 3-TS' conformers were obtained 52.8 kJ mol -1 more than 3-cis form (Table 2 and Figure 1).Its highly probable that the energy increase in the transition states is due to the unfavorable π electron-π electron repulsion between the p orbitals at the two -C=C=X and -C=Y groups (X= O or S and Y= O or S), as a steric hindrance and electronic effects.

C 3 H 2 SSe Isomers: 3-Selenoxoprop-2-enethial (4), 2H-thiete-2-selenone (4′), 3-thioxoprop -2-eneselenal (5) and 2H-selenete-2-thione (5′)
The calculated results of torsional analyses of 4 and 5 analogues are given in Tables 1 and 2. The obtained energy profiles of 3 and 4 showed three energy minima and two transition state structures.The s-trans conformations (4-trans and 4-trans'; 5-trans and 5-trans') with the torsional angle 180 and -180º were calculated to be of the lowest energy (Tables 1 and 2), as global minimum conformations.By constraining the dihedral angles to about Ф 3125 = 91.2 and -91.2º for 4 and at Ф 3125 = 91.8 and -91.8º for 5 were obtained transition state structures (4-TS and 4-TS'; 5-TS and 5-TS'), respectively, which had C 1 symmetry.Transition state structures in 4 and 5 were appeared 45.8 and 66.3 kJmol -1 energy higher than corresponded to s-trans conformations (Tables 1 and 2), respectively.Energy increase in the 4-TS and 5-TS conformers were reflected to the presence of the unfavorable torsional angle and unfit interactions between π electron-π electron of the -C=C=X and -C=Y (X and Y= S and Se) groups (Figure 1).Upon further driving of the same torsional angles in 4 and 5, the second energy minimum conformations were obtained as the s-cis conformation (4-cis and 5-cis) with the 0º torsional angle and C S symmetry (4-cis and 5-cis were a local energy minimum).The energy value of 4-cis and 5-cis were 4.0 and 8.2 kJ mol -1 more than related to their strans forms, respectively.This may be due to two reasons (a and b): a) Eclipsing of hydrogen atoms and the -C=C=X and -C=Y (X and Y= S and Se) groups as steric effect.b) Increase of the repulsion between the electrons in the p orbitals at C β of the -C=C=X and -C=Y (X and Y= S and Se) groups and also repulsion of the positively charged C α carbonyl carbons of the same groups.Linear isomers in both of them had more stability than their own cyclic forms.For example in 4 molecule, 4-trans was appeared 24.6 kJ mol -1 more stability than related to cyclic form (Table 2).5-trans was also shown 17.0 kJ mol -1 more stability than corresponded cyclic form (Table 2).Because the cyclic forms were four-membered ring containing heteroatom and double bond with more angle strain than linear conformers.
C 3 H 2 Se 2 Isomers: 3-Selenoxoprop-2-eneselenal ( 6) and 2H-selenete-2-selenone (6′) In order to determine the conformational potential energy surface of 6, calculations were performed by rotation around the C-C single bond (Figure 1).The most stable conformations corresponded to the s-trans conformations (6-trans and 6-trans'), which had plane symmetric with Φ 1234 = 180 and -180º to avoid eclipsing (Table 1 and Figure 1).Upon constraining the torsional angle to about 91.7 and -91.7º, the transition state structures with 6-TS and 6-TS' conformations (C 1 symmetry) appeared.The energy of 6-TS and 6-TS' conformers were obtained 62.8 kJ mol -1 more than 6-trans form (Table 2 and Figure 1).This is due to the unfavorable torsional angles and increase π electron-π electron interactions of the -C=C=Se and -C=Se groups.Upon rotation of the same torsional angle to 0º, the s-cis conformation (6-cis) was obtained as the other energy minimum conformations with C 1 symmetry.Energy surface in the 6-cis conformation was about 8.6 kJ mol -1 higher relative to 6-trans form (Table 2).It seems that interactions between π electron-π electron in the -C=C=Se and -C=Se groups in this conformer were not very important.But, eclipsing the C=C=Se and -C=Se groups was led to increase of the repulsion between the electrons in the p orbitals as a electronic effect.

and 2H-oxet-2-one (7′)
Analysis conformational curve of 7 was obtained by "driving" Φ 4213 , from 180 to -180º (Figure 1).Global minima for 7 were appeared at the torsional angle 0º (Figure 1), with plane symmetry.The dihedral angle 180º energy in 7, the s-trans conformer (7-trans) was obtained 1.7 kJ mol -1 higher than its s-cis conformation (Table 2).By constraining the dihedral angles to Ф 4213 = 89.8 and -89.8º were appeared transition state structures (7-TS and 7-TS΄), which had C 1 symmetry.Transition state structures energy in 7 were obtained 49.4 kJ mol -1 higher than their s-cis conformation (Table 2).This may due to the unfavorable torsion angles and significant involvement between p orbitals of -C=C=O and -C=O groups.

C 3 H 2 OSe Isomers: 3-Selenoxoacrylaldehyde (8), 2H-oxete-2-selenone (8′), 3-oxoprop-2-eneselenal (9) and 2H-selenet-2-one (9′)
Conformational analyses for interconversion of the global energy minimum conformations of 8 and 9 are investigated, and the results are shown in Figure 1 and Tables 1 and 2. The energy profile of 3-selenoxoacrylaldehyde (8) and 3-oxoprop-2-eneselenal (9) proved three energy minima at 180, 0.0 and -180º of the related torsional angle with s-trans (8-trans and 9-trans, 180º and 8-trans' and 9-trans', -180º), s-cis (8-cis and 9-cis, 0º) forms.The 8 analog had two conformations corresponding to potential energy maxima at Φ 3125 = 90.0 and -90.0º (Figure 1 and Table 1).The 9 analog had also two conformations related to potential energy maxima at Φ 3125 = 92.4 and -92.4º (Figure 1 and Table 1).The s-trans conformations 8 (8-trans and 8-trans') with C S symmetry had 4.6 kJ mol -1 higher stability than s-cis conformer (8-cis, C S ) (Table 2).The s-trans conformations 9 (9-trans and 9-trans') with C S symmetry had also 5.0 kJ mol -1 more stability than related to s-cis conformer (9-cis, C S ) (Table 2).The most stable of the s-trans forms 8 and 9 were reflected on minimized steric effect.In s-cis conformers, eclipsing between the -C=C=X and -C=Y (X and Y= O and Se) groups and also repulsion of the positively charged C α carbonyl carbons of the same groups are very significant and these cause are reason instability of the s-cis forms than the s-trans conformations in 8 and 9 molecules.With rotation around the C-C axe to about 90.0 and -90.0º in the 8 analog, transition state forms (8-TS and 8-TS΄) appeared with C 1 symmetry and about 41.0 kJ mol -1 more energy than 8-trans conformer (Figure 1 and Table 2).Similarly in the 9 analog, with rotation around the C-C axe to about 92.4 and -92.4º, transition state forms (9-TS and 9-TS΄) obtained with C 1 symmetry and about 67.0 kJ mol -1 more energy than 9-trans conformer (Figure 1 and Table 2).It seem that π electron-π electron repulsion between in the p orbitals of the -C=C=X and -C=Y (X and Y= O and Se) groups is important.

Conclusion
In conclusion, ab initio calculations provided a picture of the conformations of 1-9 from both structural and energetic points of view.The s-trans conformation was the most stable form for 1-2, 4-6 and 8-9, but in 3 and 7 the s-cis conformer was shown higher stability than other conformers.The s-cis form of 1-2, 4-6 and 8-9 was 2.6-8.6 kJ mol -1 less stable than s-trans.In 3 and 7, the s-trans form was 0.3 and 1.7 kJ mol -1 less stable than s-cis conformers, respectively.Interconversion of these conformations can take place via a energy barrier of 42.3-67.0kJ mol -1 .The cyclic forms were four-membered rings containing heteroatom and double bond with more energy and angle strain than corresponded linear conformers.