SCF MO calculations show that 2-phenyl-1,3-thiazole exists
in planar configuration, while protonated 2-phenyl-1,3-thiazole
tends to stabilize in a non-planar form

M. Farkas

N. Bodor

N. Trinajstić

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CR 0 AT IC A CHEMIC A ACT A 43 (1971) 107 CCA-649 539.19:547.789 Original Scientific Paper SCF MO Calculations for 1,3-Thiazole and 2-Phenyl-1,3-thiazole and Rotational Barriers in 2-Phenyl-1,3-thiazole and Its Protonated Form N. Bodora=F, M. Farkasb*, and N. Trinajsticc Chemical-Pharmaceutical Research Institute, Cluj, Romania [a], Department of Chemistry, Pharmacy, Medico-Pharmaceutical Institute, Cluj, Romania [b], and Institute »Ruder Boskovic«, Zagreb, Croatia, Yugoslavia [ c] Received December 21, 1970 SCF MO calculations show that 2-phenyl-1,3-thiazole exists in planar configuration, while protonated 2-phenyl-1,3-thiazole tends to stabilize in a non-planar form. The reactivity and aromaticity of the 1,3-thiazole, a five-membered aro-matic compound containing two different heteroatoms (S,N), is a problem of current interest1. The reactivity of 2-phenyl-1,3-thiazole was also the object .of extended studies2-4, ,particularly the transmission of the effects of the swbsti-tuents from one ring to the other, which is considered as a measure of the mutual conjugation 1between the two rings. In order to obtain further infor-mation about the n-interaction between the rings in 2...,phenyl-1,3-thiazole, we carried out molecular orbital calculations for rotational barrier in this molecule and its protonated form. THEORETICAL APPROACH Calculations were carried out using recent Dewar's variant5,6 of Pople's SCF MO method7• In Dewar's variant the essential parameter of the theory: one-electron core resonance integral (Bc. )is given as follows: lJ ~fi = kSii I .+ I. k=k.. - ' __ J IJ 2 (1) (2) where Su is the overlap integral estimated from the Slater-Zener orbitals, while kii is a parameter characteristic for a particular bond. The following bond parameters were used in the present work: kc-C = 0.6207 kc- N = C.5507 kc _ s = o.9240 (3) (4) (5) =l= Present address: Department of Chemistry, University of Texas, Austin, Texas 78712, U.S. A. * To whom correspondence in connection with this paper should be addressed. 108 N. BODOR, M. FARKAS, AND N. TRINAJSTIC Since the standard n-approximations are unable to calculate the strain and non-bonded interactions, the problem of rotational barrier cannot be studied by this kind of approach, which is, therefore, limited to planar structures. In the case of two n-electron rings linked together by one bond, the n-interaction between the rings tends to keep the molecule coplanar; the non-bonded inter-actions, on the other hand, are smallest when the terminal units are perpendi-cular to each other. It is, therefore, necessary to include both effects in order to obtain a more realistic estimate for the potential function of rotation. In the present work a procedure similar to the one used in the case of biphenyl5 was made use of, i. e. the effects of non-planarity in the n-bonding energy was estimated by assuming that the core resonance integral shows a cosine de-pendence on the angle of twist (0): A c =Ac COS 0 "e ,,ii (6) The energy minimization was carried out as a function not only of the twist angle but also of the inter-ring distance. For both molecules we carried out the minimization twice: first we started with a very long distance [1.540 A] , and then with a rather short one [1.350 A]. In both minimization procedures we obtained for 2-phe:nyl-1 ,3-thiazole the same value of the inter-ring distance of 1.466 A, while this distance for the case of the protonated 2-phenyl-1,3--thiazole was 1.426 A. Similarly, the energies converged to the 1same values of 56.62 eV for 2-phenyl-1 ,3-thiazole and to 57.75 eV for the protonated form of 2-phenyl-1,3-thiazole. This procedure was repeated throughout our calcu-lations for various twist angles. The contribution of non-bonded interactions iJn the case of the protonated 2-phenyl-1,3-thiazole was estimated using the expressions of Bartell8 for non-bonded interactions between carbon-cavbon, carbon-hydrogen, and hydro.gen-hydrogen. The geometry was first calculated from the Dewar SCF MO treatment using (6) for each value of 0 and then the non..,bonded interactions were estimated from Bartell's potential functions by using the calculated geometries; the C-H bond lengths were assumed to be 1.082 A. DISCUSSION AND RESULTS The method used5•6 here gives good estimates of heats of atomization, as well as of geometries of conjugated molecules. The caloulated heats of ato-mization (-/',.Ha) for investigated molecules together with the calculated values ·Of total energy (Et01), vertical ionization potentials [(IP)vl and aromatic stabilization (A8) are given in Table I. The calculated bond lengths and electron densities of thiophene, 1,3-thiazole, 2-phenyl-1,3-thiazole and protonated forms of 1,3-thiazole and 2-phenyl-1,3-thiazole are given in Fig. 1. Unfortunately there are as yet no experimental data available for 1,3-thiazole and 2-phenyl--1,3-thiazole to test our calculations. On the other hand, the agreement with experiment for the thiophene is very good indeed. Our calculations imply that the aromaticity of 1,3-thiazole is of the order of the one of thiophene; 1,3-thiazole having a slightly higher value of aromatic stabilization than thiophene. Aromatic stabilization is calculated in the manner d~scribed earliers,s,9• Calculated vertical ionization potentials of thiophene and thiazole are almost identical. It was pointed out earlier10 that the stabilization of isocori.jugated molecules reflects in the higher value of Compound Thiophene (1) 1,3-Thiazole (2) Protonated 1,3-Thiazole (3) 2-Pheny 1-1,3-thiazole (4) Protonated 2-phenyl-1,3-thiazole (5) SCF MO CALCULATIONS TABLE I Calculated Energy Values -/o,.H,.. at 25°C -Etot (eV) (eV)" Cale. I Obs. 40.32 40.32° 22.57 35.05 21.74 22.21 87.68 56.62 57.75 I 109 (IP)v (eV)" A (kcal) s mole Cale. I Obs. 8.93 I 9.0" I 6.5 8.94 7.2 8.54 28.8 • E,0 , = E" + E ,. ; " From Koopmans theorem ; c G. Waddington, J. W. Know 1-t on, D . W . Scott, G.D. Oliver, S.S. Todd, W. N . Hubbard, J.C. Smith, and H . M . Huffman, J . Am. Chem. Soc. 71 {1949) 797 ; S. Sunn er, Acta Chem. Scand. I (1955) 847; d H . D . Hart o ugh, in The Chemistry of Heterocyc!ic Compounds, Vol. 3, A. Weissberger, Editor, Interscience Publishers, Inc., New York 1952, p . 94. 0·8994 1.3111 ® /H ::- 1.6680 ... '::> 7• 1.1395 1.1395 0.7651 1.&2&7 1.6036 (1) (2) (3) 0.8936 1.Jeo.t. 0-9'65 1.1460 0.9736 0.&224 1.0082 0-9971 (4) ( 5) Fig. 1. M,olecular orbital diagrams (bond lengths and charge densities) of thiophene, thiazole, protonated thiazole, 2-phenylthiazole, and protonated 2-phenylthiazole. The observed bond lengths of thiophene [B. B a k, D . Christensen, J. Rast r up - And er s en, and L . Tannenbaum, J . Chem. Phys. 25 (1956) 892] are given in bracketts 110 N. BODOR, M. FARKAS, AND N _ TRINAJSTIC the first n:-ionization potential. Therefore, our calculated value of vertical ionization potential for thiazole of 8.94 eV might be quite close to the experi-mental value; the deviation from experiment may be of the order of 0.1-0.2 eV. Note that the sum (29.8 kcal/mole) of aromatic stabilization for 1,3-thiazole (7.2 kcal/mole) and benzene (22.6 kcal/mole) is nearly equal to the calculated value for 2-phenyl-1,3-thiazole (28.8 kcal/mole) and this is in agreement with the earlier prediction11 that if aromatic components A and B are linked together by an essential single bond (1.46 A), the following relationship is valid: ~~-me~W+~~ 00 The validity of th e above relation was earlier confirmed for biphenyl5, and 2,2' -bithioph ene6. Table II shows the results of calculation for the rotation of 2-phenyl-1,3--thiazole, where th e non~bonded interactions are neglected, because no H-atom is attached to the N or S atoms and there is also no potential function .presently available for the interaction of hydrogen with a lone pair. The planar configu-ration of 2-phenyl-1,3-thiazole is the most stable form of this molecule, sho-winig a difference of 9.2 kcal/mole from the perpendicular configuration (0 = 90°). Apparently, 2-phenyl-1,3-thiazole is more stable in planar confi-guration than, for example, biphenyl, because of the lack of any direct hydro-gen-hydrogen non-bonded interactions. These interactions are in the case of biphenyl over balancing conj.ugation and keeping bi1phenyl in configuration with the dihedral angle of 42° between two rings. TABLE II Rotation of Phenyl Group Around the Central Carbon-Carbon Bond in 2-Phenyl-- 1,3- thiazole e· - En (eV)" I - E tot (eV)c i /:J. E tot (eV)" oo 14.6002 56.6156 - 0.3969 10° 14.5781 56.4904 -0.3717 20° 14.5193 56.5230 -0.3043 30° 14.4421 56.4340 -0.2153 40° 14.3673 56.3475 -0.1288 jjOO 14.3103 56.2810 -0.0623 60° 14.2761 56.2411 -0.0244 70° 14.2612 56.2236 -0.0049 80° 14.2557 56.2191 - 0.0004 goo 14.2557 56.2187 0.0000 a dihedral angle of rotation; " n-bonding energy for each dihedral angle of rotation; c total energy for each dihedral angle of rotation; " stability of each conformation relative to the perpendicular configuration (0 = 90') . Similar calcu lations for the protonated 2-phenyl-1,3-thiazole are reported in Table III. The calculated angle of the twist in the protonated 2~phenyl-1,3--thiaQ:ole is about 10° and is expected to be of lower value than the angle of twist deduced for biphenyl (42°) by Bastiansen et al.12 Perhaps the experimental value is somewhat greater because the non-bonded interactions with lone pairs on the sulphur atom were neglected. Note the slow change of energy at the minimum (around 10°), implying that torsional vibrations should have large amplitudes. SCF MO CALCULATIONS 111 TABLE III Rotation of Phenyl Group Around the Cen tral Carbon-Carbon Bond i n Protonated 2-Phenyl-1,3-thiazole e· I - E tot (eV)" oo I 57.8091 -0.8961 0.4771 - 0.4179 10° 57.7571 -0.8441 0.4177 -0.4264 20° 57.6154 - 0.7024 0.2863 - 0.4161 30° 57.4219 - 0.5089 0.1574 - 0.3515 40° 57.2254 - 0.3124 0.0772 - 0.2352 50° 57.0678 -0.1548 0.0339 -0.1209 60° 56.9701 -0.0571 0.0137 -0.0434 70° 56.9262 -0.0132 0.0045 -0.0087 80° 56.9143 -0.0013 0.0009 - 0.0004 goo 56.9130 -0.0000 0.0000 0.0000 a d ihedral a n gle of rotation; b total e nergy fo r each dihedral angle of rota tion ·; c stability of each configura tion relative to the perpendicu lar configuration (8 = 90') ; d non-bonded inter -actions calcula ted using Bartell's expressions. Acknowledgement. We would like to thank Dr. L. Klasinc (Zagreb) for his interest in this work and helpful discussions. REFERENCES 1. R. Ph a n - Tan - Luu, L. Bou s ca s s e , E. J. Vincent, and J . Met z ger, Bull. Soc. Chim. France [1969] 1149; J. Metzger, Congres international du chimie heterocyclique, Monpellier, J'uly 1969. 2. A. S i 1 b erg, I. Simi ti, M. F a r k a s, S . S i 1 berg, and H. Ma n t s ch, Rev. Roumaine Chim. 7 (1962) 513. 3. I. Simi ti and M. F a r k as, Chem. Ber. 98 (1965) 3441. 4. I. Simi ti and M. Far k as, Bull. Soc. Chim. France (1968) 3862. 5. M. J. S . D ew ar and A. J . Har g e·t , Proc. Roy. Soc. (London) A 315 (1970) 443, 457. 6. M. J . S . Dewar and N. Trin a j st i c, J. Am. Chem. Soc. 92 (1970) 1453. 7. J . A. Pop 1 e , Trans. Faraday Soc. 49 (1953) 1375. 8. L. S . Barte 11, J. Chem. Phys. 32 (1960) 827. 9. M. J . S. Dewar and N. Trina j st i c, Theoret. Chim. Acta (Berlin) 17 (1970) 235. 10. M . J. S. D e war, A. J. H a r get, N. Trina j st i c, and S. D . W or 1 e y , Tetrahedron 26 (1970) 4269. 11. M. J. S. Dewar, The Molecular Orbital Theory of Organi c Chemistry , Mc · Graw-Hill, New York, 1969. 12. A . A 1 men n in gen, 0. B as ti ans en, and M. Tr a e t t b erg, Acta Chem. Scand. 12 (1958) 1221. IZVOD SCF MO racuni za 1,3-tiazol i 2-fenil-1,3-tiazol i rotacijske barijere u 2-fenil-1,3--tiazolu i protoniranom 2-fenil-1,3-tiazolu N. Bodor, M. Farkas i N. Trinajstic SCF MO racuni pokazuju da 2-fenil-1 ,3-tiazol postoji u planarnoj konfiguraciji, dok se protonirani 2-fenil-1,3-tiazol nastoji stabilizirati u neplanarnoj formi. CHEMICAL-PHARMACEUTICAL RESEARCH INSTITUTE, CLUJ, ROMANIA, I INSTITU T »RUDER BOSKOVIC« ZAGREB Primlj eno 21. prosinca 1970. 

SCF MO Calculations for 1,3-Thiazole and 2-Phenyl-1,3- thiazole and Rotational Barriers in 2-Phenyl-1,3-thiazole and Its Protonated Form

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CR 0 AT IC A CHEMIC A ACT A 43 (1971) 107 CCA-649 539.19:547.789 Original Scientific Paper SCF MO Calculations for 1,3-Thiazole and 2-Phenyl-1,3-thiazole and Rotational Barriers in 2-Phenyl-1,3-thiazole and Its Protonated Form N. Bodora=F, M. Farkasb*, and N. Trinajsticc Chemical-Pharmaceutical Research Institute, Cluj, Romania [a], Department of Chemistry, Pharmacy, Medico-Pharmaceutical Institute, Cluj, Romania [b], and Institute »Ruder Boskovic«, Zagreb, Croatia, Yugoslavia [ c] Received December 21, 1970 SCF MO calculations show that 2-phenyl-1,3-thiazole exists in planar configuration, while protonated 2-phenyl-1,3-thiazole tends to stabilize in a non-planar form. The reactivity and aromaticity of the 1,3-thiazole, a five-membered aro-matic compound containing two different heteroatoms (S,N), is a problem of current interest1. The reactivity of 2-phenyl-1,3-thiazole was also the object .of extended studies2-4, ,particularly the transmission of the effects of the swbsti-tuents from one ring to the other, which is considered as a measure of the mutual conjugation 1between the two rings. In order to obtain further infor-mation about the n-interaction between the rings in 2...,phenyl-1,3-thiazole, we carried out molecular orbital calculations for rotational barrier in this molecule and its protonated form. THEORETICAL APPROACH Calculations were carried out using recent Dewar's variant5,6 of Pople's SCF MO method7• In Dewar's variant the essential parameter of the theory: one-electron core resonance integral (Bc. )is given as follows: lJ ~fi = kSii I .+ I. k=k.. - ' __ J IJ 2 (1) (2) where Su is the overlap integral estimated from the Slater-Zener orbitals, while kii is a parameter characteristic for a particular bond. The following bond parameters were used in the present work: kc-C = 0.6207 kc- N = C.5507 kc _ s = o.9240 (3) (4) (5) =l= Present address: Department of Chemistry, University of Texas, Austin, Texas 78712, U.S. A. * To whom correspondence in connection with this paper should be addressed. 108 N. BODOR, M. FARKAS, AND N. TRINAJSTIC Since the standard n-approximations are unable to calculate the strain and non-bonded interactions, the problem of rotational barrier cannot be studied by this kind of approach, which is, therefore, limited to planar structures. In the case of two n-electron rings linked together by one bond, the n-interaction between the rings tends to keep the molecule coplanar; the non-bonded inter-actions, on the other hand, are smallest when the terminal units are perpendi-cular to each other. It is, therefore, necessary to include both effects in order to obtain a more realistic estimate for the potential function of rotation. In the present work a procedure similar to the one used in the case of biphenyl5 was made use of, i. e. the effects of non-planarity in the n-bonding energy was estimated by assuming that the core resonance integral shows a cosine de-pendence on the angle of twist (0): A c =Ac COS 0 "e ,,ii (6) The energy minimization was carried out as a function not only of the twist angle but also of the inter-ring distance. For both molecules we carried out the minimization twice: first we started with a very long distance [1.540 A] , and then with a rather short one [1.350 A]. In both minimization procedures we obtained for 2-phe:nyl-1 ,3-thiazole the same value of the inter-ring distance of 1.466 A, while this distance for the case of the protonated 2-phenyl-1,3--thiazole was 1.426 A. Similarly, the energies converged to the 1same values of 56.62 eV for 2-phenyl-1 ,3-thiazole and to 57.75 eV for the protonated form of 2-phenyl-1,3-thiazole. This procedure was repeated throughout our calcu-lations for various twist angles. The contribution of non-bonded interactions iJn the case of the protonated 2-phenyl-1,3-thiazole was estimated using the expressions of Bartell8 for non-bonded interactions between carbon-cavbon, carbon-hydrogen, and hydro.gen-hydrogen. The geometry was first calculated from the Dewar SCF MO treatment using (6) for each value of 0 and then the non..,bonded interactions were estimated from Bartell's potential functions by using the calculated geometries; the C-H bond lengths were assumed to be 1.082 A. DISCUSSION AND RESULTS The method used5•6 here gives good estimates of heats of atomization, as well as of geometries of conjugated molecules. The caloulated heats of ato-mization (-/',.Ha) for investigated molecules together with the calculated values ·Of total energy (Et01), vertical ionization potentials [(IP)vl and aromatic stabilization (A8) are given in Table I. The calculated bond lengths and electron densities of thiophene, 1,3-thiazole, 2-phenyl-1,3-thiazole and protonated forms of 1,3-thiazole and 2-phenyl-1,3-thiazole are given in Fig. 1. Unfortunately there are as yet no experimental data available for 1,3-thiazole and 2-phenyl--1,3-thiazole to test our calculations. On the other hand, the agreement with experiment for the thiophene is very good indeed. Our calculations imply that the aromaticity of 1,3-thiazole is of the order of the one of thiophene; 1,3-thiazole having a slightly higher value of aromatic stabilization than thiophene. Aromatic stabilization is calculated in the manner d~scribed earliers,s,9• Calculated vertical ionization potentials of thiophene and thiazole are almost identical. It was pointed out earlier10 that the stabilization of isocori.jugated molecules reflects in the higher value of Compound Thiophene (1) 1,3-Thiazole (2) Protonated 1,3-Thiazole (3) 2-Pheny 1-1,3-thiazole (4) Protonated 2-phenyl-1,3-thiazole (5) SCF MO CALCULATIONS TABLE I Calculated Energy Values -/o,.H,.. at 25°C -Etot (eV) (eV)" Cale. I Obs. 40.32 40.32° 22.57 35.05 21.74 22.21 87.68 56.62 57.75 I 109 (IP)v (eV)" A (kcal) s mole Cale. I Obs. 8.93 I 9.0" I 6.5 8.94 7.2 8.54 28.8 • E,0 , = E" + E ,. ; " From Koopmans theorem ; c G. Waddington, J. W. Know 1-t on, D . W . Scott, G.D. Oliver, S.S. Todd, W. N . Hubbard, J.C. Smith, and H . M . Huffman, J . Am. Chem. Soc. 71 {1949) 797 ; S. Sunn er, Acta Chem. Scand. I (1955) 847; d H . D . Hart o ugh, in The Chemistry of Heterocyc!ic Compounds, Vol. 3, A. Weissberger, Editor, Interscience Publishers, Inc., New York 1952, p . 94. 0 ·8994 1.3111 ® /H ::- 1.6680 ... '::> 7• 1.1395 1.1395 0.7651 1.&2&7 1.6036 (1) (2) (3) 0.8936 1.Jeo.t. 0-9'65 1.1460 0.9736 0.&224 1.0082 0-9971 (4) ( 5) Fig. 1. M,olecular orbital diagrams (bond lengths and charge densities) of thiophene, thiazole, protonated thiazole, 2-phenylthiazole, and protonated 2-phenylthiazole. The observed bond lengths of thiophene [B. B a k, D . Christensen, J. Rast r up - And er s en, and L . Tannenbaum, J . Chem. Phys. 25 (1956) 892] are given in bracketts 110 N. BODOR, M. FARKAS, AND N _ TRINAJSTIC the first n:-ionization potential. Therefore, our calculated value of vertical ionization potential for thiazole of 8.94 eV might be quite close to the experi-mental value; the deviation from experiment may be of the order of 0.1-0.2 eV. Note that the sum (29.8 kcal/mole) of aromatic stabilization for 1,3-thiazole (7.2 kcal/mole) and benzene (22.6 kcal/mole) is nearly equal to the calculated value for 2-phenyl-1,3-thiazole (28.8 kcal/mole) and this is in agreement with the earlier prediction11 that if aromatic components A and B are linked together by an essential single bond (1.46 A), the following relationship is valid: ~~-me~W+~~ 00 The validity of th e above relation was earlier confirmed for biphenyl5, and 2,2' -bithioph ene6. Table II shows the results of calculation for the rotation of 2-phenyl-1,3--thiazole, where th e non~bonded interactions are neglected, because no H-atom is attached to the N or S atoms and there is also no potential function .presently available for the interaction of hydrogen with a lone pair. The planar configu-ration of 2-phenyl-1,3-thiazole is the most stable form of this molecule, sho-winig a difference of 9.2 kcal/mole from the perpendicular configuration (0 = 90°). Apparently, 2-phenyl-1,3-thiazole is more stable in planar confi-guration than, for example, biphenyl, because of the lack of any direct hydro-gen-hydrogen non-bonded interactions. These interactions are in the case of biphenyl over balancing conj.ugation and keeping bi1phenyl in configuration with the dihedral angle of 42° between two rings. TABLE II Rotation of Phenyl Group Around the Central Carbon-Carbon Bond in 2-Phenyl-- 1,3- thiazole e· - En (eV)" I - E tot (eV)c i /:J. E tot (eV)" oo 14.6002 56.6156 - 0.3969 10° 14.5781 56.4904 -0.3717 20° 14.5193 56.5230 -0.3043 30° 14.4421 56.4340 -0.2153 40° 14.3673 56.3475 -0.1288 jjOO 14.3103 56.2810 -0.0623 60° 14.2761 56.2411 -0.0244 70° 14.2612 56.2236 -0.0049 80° 14.2557 56.2191 - 0.0004 goo 14.2557 56.2187 0.0000 a dihedral angle of rotation; " n-bonding energy for each dihedral angle of rotation; c total energy for each dihedral angle of rotation; " stability of each conformation relative to the perpendicular configuration (0 = 90') . Similar calcu lations for the protonated 2-phenyl-1,3-thiazole are reported in Table III. The calculated angle of the twist in the protonated 2~phenyl-1,3--thiaQ:ole is about 10° and is expected to be of lower value than the angle of twist deduced for biphenyl (42°) by Bastiansen et al.12 Perhaps the experimental value is somewhat greater because the non-bonded interactions with lone pairs on the sulphur atom were neglected. Note the slow change of energy at the minimum (around 10°), implying that torsional vibrations should have large amplitudes. SCF MO CALCULATIONS 111 TABLE III Rotation of Phenyl Group Around the Cen tral Carbon-Carbon Bond i n Protonated 2-Phenyl-1,3-thiazole e· I - E tot (eV)" oo I 57.8091 -0.8961 0.4771 - 0.4179 10° 57.7571 -0.8441 0.4177 -0.4264 20° 57.6154 - 0.7024 0.2863 - 0.4161 30° 57.4219 - 0.5089 0.1574 - 0.3515 40° 57.2254 - 0.3124 0.0772 - 0.2352 50° 57.0678 -0.1548 0.0339 -0.1209 60° 56.9701 -0.0571 0.0137 -0.0434 70° 56.9262 -0.0132 0.0045 -0.0087 80° 56.9143 -0.0013 0.0009 - 0.0004 goo 56.9130 -0.0000 0.0000 0.0000 a d ihedral a n gle of rotation; b total e nergy fo r each dihedral angle of rota tion ·; c stability of each configura tion relative to the perpendicu lar configuration (8 = 90') ; d non-bonded inter -actions calcula ted using Bartell's expressions. Acknowledgement. We would like to thank Dr. L. Klasinc (Zagreb) for his interest in this work and helpful discussions. REFERENCES 1. R. Ph a n - Tan - Luu, L. Bou s ca s s e , E. J. Vincent, and J . Met z ger, Bull. Soc. Chim. France [1969] 1149; J. Metzger, Congres international du chimie heterocyclique, Monpellier, J'uly 1969. 2. A. S i 1 b erg, I. Simi ti, M. F a r k a s, S . S i 1 berg, and H. Ma n t s ch, Rev. Roumaine Chim. 7 (1962) 513. 3. I. Simi ti and M. F a r k as, Chem. Ber. 98 (1965) 3441. 4. I. Simi ti and M. Far k as, Bull. Soc. Chim. France (1968) 3862. 5. M. J. S . D ew ar and A. J . Har g e·t , Proc. Roy. Soc. (London) A 315 (1970) 443, 457. 6. M. J . S . Dewar and N. Trin a j st i c, J. Am. Chem. Soc. 92 (1970) 1453. 7. J . A. Pop 1 e , Trans. Faraday Soc. 49 (1953) 1375. 8. L. S . Barte 11, J. Chem. Phys. 32 (1960) 827. 9. M. J . S. Dewar and N. Trina j st i c, Theoret. Chim. Acta (Berlin) 17 (1970) 235. 10. M . J. S. D e war, A. J. H a r get, N. Trina j st i c, and S. D . W or 1 e y , Tetrahedron 26 (1970) 4269. 11. M. J. S. Dewar, The Molecular Orbital Theory of Organi c Chemistry , Mc · Graw-Hill, New York, 1969. 12. A . A 1 men n in gen, 0. B as ti ans en, and M. Tr a e t t b erg, Acta Chem. Scand. 12 (1958) 1221. IZVOD SCF MO racuni za 1,3-tiazol i 2-fenil-1,3-tiazol i rotacijske barijere u 2-fenil-1,3--tiazolu i protoniranom 2-fenil-1,3-tiazolu N. Bodor, M. Farkas i N. Trinajstic SCF MO racuni pokazuju da 2-fenil-1 ,3-tiazol postoji u planarnoj konfiguraciji, dok se protonirani 2-fenil-1,3-tiazol nastoji stabilizirati u neplanarnoj formi. CHEMICAL-PHARMACEUTICAL RESEARCH INSTITUTE, CLUJ, ROMANIA, I INSTITU T »RUDER BOSKOVIC« ZAGREB Primlj eno 21. prosinca 1970. 

SCF MO Calculations for 1,3-Thiazole and 2-Phenyl-1,3- thiazole and Rotational Barriers in 2-Phenyl-1,3-thiazole and Its Protonated Form

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