Relative energies of different intermediates for parent molecule.
Abstract
Formation of 2-(N-arylamino)benzothiazole takes place, when N,N′-diphenylthioureas are treated with polymer-supported tribromide or with iodine-alumina as catalyst under solvent free conditions. However, when N-substituted-N′-benzoylthioureas are treated with polymer-supported tribromide or with iodine-alumina as catalyst either under various conditions or under solvent free conditions, decomposition takes place to give the respective benzamides and thiobenzamides. Mechanistic study of the formation of these compounds is studied using DFT calculations. It is found that electron donating group at the para-position of the aryl group of benzoylthiourea favors the formation of benzamide whereas the presence of electron withdrawing group at para-position of the aryl group of benzoylthiourea, formation of thiobenzamide takes place. When the catalyst is changed to diacetoxyiodobenzene (DIB) under similar reaction conditions, benzoxazole amides are formed; expected benzothiazoles or the decomposition products are not obtained. Mechanistic study of the reaction using DFT calculation again shows that the reaction followed through carbodiimide intermediate undergoes the formation of C-O bond in benzoxazole moiety, instead of the expected C-S bond formation of benzothiazole moiety via a sequential acylation and deacylation process.
Keywords
- mechanism
- benzoxazoles
- benzothiazoles
- decomposition
- thioureas
- DFT calculations
1. Introduction
Heterocyclic chemistry is the most complex and intriguing branch of organic chemistry, and heterocyclic compounds constitute the largest and most unique family of organic compounds [1, 2, 3]. Nitrogen, oxygen, and sulfur are the most common heteroatoms but some other heterocyclic compounds containing selenium, tellurium, phosphorus, arsenic, silicon, boron, etc., are also widely known. Heterocyclic compounds are present in many natural and non-naturally occurring compounds. Some examples of such compounds are alkaloids, vitamins (vitamin B series and vitamin C), antibiotics, amino acids, hemoglobin, hormones, pigments, and a large number of synthetic drugs and dyes. Several natural drugs such as morphine, codeine, quinine, penicillin, papaverine, atropine, emetine, reserpine, procaine, theophylline, etc., are examples of heterocyclic compounds. Some of the synthetic drugs have shown several therapeutic uses such as antidiabetic, antitubercular, antidepressant, antitumor, anti-HIV, anthelmintic, antibacterial, antifungal, antiviral, antimalarial, antileishmanial, analgesic, anti-inflammatory, anticonvulsant, anticancer, muscle relaxants, lipid peroxidation inhibitor, herbicidal, trypanocidal, fungicidal, and insecticidal activities. Thus, heterocyclic compounds are receiving more and more significance in recent years, particularly owing to their pharmacological as well as synthetic potential.
In recent years, green chemistry has become one of the most important philosophies in chemistry, since it represents a major change in the way we think about practicing chemistry and using chemicals. The emerging area of green chemistry envisages minimum hazard as the performance criteria while designing new chemical processes. The search for new environmentally benign solvents and catalysts that operate efficiently in them and can be easily recycled is of significant academic and industrial interest. There have been several approaches to access to this problem, e.g., the developments of neat reactions that proceed under various conditions such as microwave irradiation, thermal heating, grinding, sonication, etc., or in organic or inorganic solid-media, or in ionic liquid-media under organic solvent-free reactions. Among the proposed solutions, solvent free conditions are becoming more and more popular and it is often claimed that the best solvent from an ecological point of view is, without a doubt, no solvent. The formation of various compounds from thioureas and their derivatives under different catalysts in solvent free condition is highlighted.
The density functional theory (DFT) method has become one of the most prevalent and efficient tools, as compared to the conventional
2. 2-Aminobenzothiazoles from thioureas
Benzothiazoles are an important class of heterocycles that possess a broad range of biological activities [4]. They were studied extensively for their anti-allergic, anti-inflammatory, antitumor, antimicrobial, and analgesic activities. Among those 2-substituted benzothiazole derivatives, the 2-aminobenzothiazoles are one of the most important structural motifs in pharmaceutically active compounds and natural products [5]. A large number of 2-aminobenzothiazole derivatives are also found to be anticancer active and the 2-aminobenzothiazole moieties act as a privileged pharmacophores as well as valuable reactive intermediates [6, 7, 8]. For example,
The main objectives of benzothiazoles synthesis are not only for the development of more diverse and complex bioactive compounds for biological activity and structure-activity relationship (SAR) studies but also for other applications, such as preparation of dyes. There are several methods for the synthesis of 2-aminobenzothiazoles. The most versatile and economical method involves the treatment of various substituted arylthioureas (which are synthesized
Recently, several methods have been reported which utilize bromine as catalyst. Basically, cyclization with bromine is achieved by oxidation of aniline, substituted aniline, and arylthiourea in acid or chloroform with alkali thiocyanate. Hugerschoff, in early 1900s, synthesized 2-aminobenzothiazole and found that 1, 3-diarylthiourea can be cyclized with liquid bromine in chloroform to form a 2-aminobenzothiazoles (Scheme 1) [14, 15].
This reaction worked well for symmetrical thioureas giving exclusively one product. But, when the same reaction is performed using unsymmetrical 1,3-diaryl thioureas, there is always uncertainty as to on which aryl ring the intramolecular electrophilic substitution would take place to give aminobenzothiazole. Kamel
Jordan
Liu
The palladium-catalyzed intramolecular cyclization of 2-bromophenylthioureas to synthesize 2-substituted benzothiazoles was also reported. Castillon
Recently, the transition-metal (copper or iron)-catalyzed one-pot tandem reactions of 2-halobenzenamines with isothiocyanates for the synthesis of 2-aminobenzothiazoles have received considerable attention because of their efficiency and low costs. For example, Wu
Meanwhile, the ligand-free copper-catalyzed one-pot tandem reactions of 2-halobenzenamines and isothiocyanates were also reported [24, 25]. However, it should be noted that the copper or iron catalyzed one-pot tandem reactions of 2-halobenzenamines with isothiocyanates generally involve organic solvents such as DMSO, DMF, and toluene which are environmentally unfriendly. Moreover, the reactions which are described above might proceed efficiently; they usually suffer from the use of highly toxic and corrosive reagents, high-costing metal catalysts, and specific ligands. There is also possibility to leave toxic traces of metals in the products. More recently, Jiang
Recently, Patel
Very recently, polymer-supported tribromide has been used as a new solid phase and recyclable catalyst for the one-pot synthesis of 2-(
The probable reaction mechanism for the formation of 2-(
3. Decomposition of benzoylthioureas
When the reaction of
Instead, the decomposition of benzoylthioureas to benzamides and thiobenzamides in a single route using iodine-alumina as catalyst under solvent-free condition takes place. When electron donating group, such as methyl or methoxy group, is present at the
Amides and thioamides are an important class of building blocks in modern organic synthesis, with broad applications in advanced materials, pharmaceuticals, agrochemicals, and polymers, etc. They are used for the synthesis of various natural products as well as intermediates of organic compounds. Generally, amides are prepared from their corresponding ketoximes by Beckmann rearrangement, and thioamides are prepared by thionation of the corresponding amide analogues by Lawesson’s reagent. Liana Allen
Gelens
Very recently, Rajeshwer Vanjari
Different synthetic methods have been discovered for the synthesis of thioamides. Among these strategies, thionation of amide analogues with Lawesson’s reagent is the most common, but this reaction cannot be classified as an atom economical approach because of crucial limitations: only one oxygen atom is replaced by a sulfur atom, and no other new bond was created. Thus, it is worthwhile to provide a practical and environmentally benign method to synthesize thioamides. Recently, some three component reactions have nicely exploited the use of benzylamine [33], aldehydes [34], and alkyne [35], in combination with elemental sulfur and amine for the synthesis of thioamides (Scheme 17).
More recently, Guntreddi
The Beckmann rearrangement generally requires a strong acid, high reaction temperature, harsh reaction conditions, and production of unwanted by-products. Several methodologies to check the reaction conditions, such as, in liquid phase, in vapor phase, in supercritical water, and in ionic liquids have been developed. However, the drawbacks in such methods are the use of toxic solvents, expensive reagents, long reaction times, low yields, and the production of considerable amounts of by-products. Literature survey reveals that there were many reports for the synthesis of amides and its sulfur containing analogue, thioamides; however, there is no report for the simultaneous synthesis of benzamides and thiobenzamides from benzoylthiourea [29].
4. Benzoxazole amides from benzoylthioureas
When
Benzoxazoles are a class of heterocyclic compounds exhibiting therapeutical activities (Figure 2), such as, antifungal agents [38, 39, 40], cytotoxic compounds [41], as anti-inflammatory agents [42], as HIV-1 protease inhibitor [43], as an antibiotic [44], as
Various methods have been reported in the literature for the synthesis of benzoxazoles starting from 2-aminophenol precursors with carboxylic acid derivatives, such as carboxylic acids, acid chlorides, acid anhydrides, and amides (Scheme 20), or by reacting 2-aminophenols with aldehydes followed by oxidation (Scheme 21) [48, 49, 50, 51].
In most cases, 2-aminophenols are used as the starting materials for the preparation of 2-arylbenzoxazoles. However, the synthesis of
5. DFT calculations
With the help of density functional theory (DFT), the electronic structure of organic compounds could be expressed by electron density functional. DFT calculation is recently applied to the study of various reaction mechanisms,
5.1. DFT calculations for the formation of benzamides and thiobenzamides
The mechanism for the decomposition of benzoylthioureas to benzamides and thiobenzamides in a single route using iodine-alumina as catalyst under solvent-free condition was studied with DFT calculations; all the structures were optimized by hybrid density functional B3LYP [62, 63] using the segmented all-electron relativistically contracted Def2-TZVP(−df) basis set with the help of ORCA [64]. The DFT calculation shows that the formations of both benzamides and thiobenzamides with by-products,
The plausible mechanism for the formation of benzamides and thiobenzamides is shown in Scheme 22. To understand the mechanistic pathway, three most probable iodide intermediates
The results show that the intermediate (
To study the possibility of breaking the molecular backbone, the strength of different bonds were considered based on the Mayer bond order [65], which indicates a number of electron pairs that constitute a bond. When considering the backbone structure, C1-N2 has the least Mayer bond order in intermediate
Intermediates | Mayer bond order | ||||
---|---|---|---|---|---|
C1-N2 | N2-C3 | C3-N4 | N4-C5 | C5-C6 | |
0.9092 | 1.1663 | 1.5143 | 1.3996 | 0.9238 | |
1.1027 | 1.8061 | 0.9953 | 1.1750 | 0.9085 | |
1.0134 | 1.1414 | 1.0373 | 1.9322 | 0.9493 |
In case of the
For other substituted molecules also, C5-C6 has the least bond order in the intermediate
Intermediates | Mayer bond order | ||||
---|---|---|---|---|---|
C1-N2 | N2-C3 | C3-N4 | N4-C5 | C5-C6 | |
0.9138 | 1.1550 | 1.5238 | 1.3918 | 0.9254 | |
1.1197 | 1.7945 | 1.0017 | 1.1714 | 0.9103 | |
1.0194 | 1.1321 | 1.0436 | 1.9265 | 0.9495 | |
0.9047 | 1.1728 | 1.5131 | 1.4038 | 0.9261 | |
1.1200 1.0081 | 1.8032 1.1453 | 0.9944 1.0359 | 1.1719 1.9327 | 0.9076 0.9498 | |
0.9585 | 1.1536 | 1.5098 | 1.4015 | 0.9236 | |
1.1379 | 1.7861 | 0.9979 | 1.1722 | 0.9090 | |
0.9087 | 1.1397 | 1.0361 | 1.9337 | 0.9413 | |
1.0052 | 1.1535 | 1.5311 | 1.3810 | 0.9264 | |
1.0588 1.0440 | 1.8130 1.1511 | 1.0024 1.0501 | 1.1710 1.9346 | 0.9111 0.9517 |
Thus, the DFT studies showed that the formation of benzamide was due to the migration of the aryl group (in intermediate
5.2. DFT calculations for the formation of benzoxazoles
Density functional calculations were performed at a B3LYP/Def2-TZVP(−df) level of theory using ORCA to study the reaction mechanism for the formation of benzoxazole amides from benzoylthioureas in presence of DIB as catalyst and the role of substitution with electron withdrawing/donating group at different positions of the phenyl ring on the reaction. The plausible mechanism for the formation of
The carbodiimide thus formed is acylated at carbodiimide carbon which after rearrangement gives the acylated intermediate (
To determine which route the reaction follows, one needs theoretical consideration of each step of deacylation then cyclization (Route 1) or cyclization then deacylation (Route 2). One way to determine which route the reaction follows is to compare the energetics of each step undergoing in either route (Table 6). Comparison of energies of reactions for the first step through Route 1 (deacylation) and Route 2 (cyclization by oxidation of DIB) shows that the initial deacylation (Route 1) is exothermic and thermodynamically favored over oxazole cyclization through Route 2 which is endothermic. The Route 1 is favored more on substitution with both electron withdrawing as well as donating groups at all positions, that is,
Entry number | Energy of reaction (kcal/mole) | |||||
---|---|---|---|---|---|---|
Route 1 | Route 2 | |||||
C to D | D to D′ | D′ to 2 | C to E | E to E′ | E′ to 2 | |
1 | −22.81 | 52.02 | −79.23 | 52.23 | −84.65 | −17.60 |
2 | −24.66 | 57.52 | −81.42 | 55.53 | −86.40 | −17.70 |
3 | −24.55 | 53.96 | −81.50 | 52.12 | −86.46 | −17.75 |
4 | −24.61 | 56.30 | −79.75 | 54.34 | −84.67 | −17.72 |
5 | −24.57 | 56.41 | −80.15 | 53.78 | −84.54 | −17.56 |
6 | −24.88 | 57.55 | −79.68 | 55.48 | −85.48 | −17.02 |
7 | −24.91 | 57.46 | −80.33 | 55.33 | −85.27 | −17.84 |
8 | −28.23 | 57.48 | −81.85 | 52.68 | −86.76 | −18.52 |
9 | −25.05 | 57.63 | −80.15 | 55.72 | −85.48 | −17.81 |
The deacylation of
Entry number | Mulliken charge of hydrogen atom bonded to | |
---|---|---|
N2 | N4 | |
1 | 0.2982 | 0.2679 |
2 | 0.3002 | 0.2657 |
3 | 0.2984 | 0.2686 |
4 | 0.301 | 0.2654 |
5 | 0.3011 | 0.2653 |
6 | 0.3016 | 0.2675 |
7 | 0.3018 | 0.2671 |
8 | 0.3061 | 0.2658 |
9 | 0.3073 | 0.2671 |
Upon cyclization to form oxazole ring of
The aromaticity is regained upon subsequent deprotonation of
There is further interesting result from the reaction when there is substitution at the
When
Thus, the density functional calculations showed the reaction followed through carbodiimide intermediate formed by the oxidation of
References
- 1.
Katritzky AR, Boulton JA. Advances in Heterocyclic Chemistry. Vol. 1-27. New York: Academic Press; 1963-1980 - 2.
Weissberger A. The Chemistry of Heterocyclic Compounds. Vol. 1-29. New York: Wiley Interscience; 1950-1975 - 3.
Elderfield RC. Heterocyclic Chemistry. Vol. 1-9. New York: Wiley; 1950-1967 - 4.
Serdons K, Terwinghe C, Vermaelen P, Van LK, Kung H, Mortelmans L, Bormans G, Verbruggen A. Journal of Medicinal Chemistry 2009; 52 :1428-1437 - 5.
Bondock S, Fadaly W, Metwally MA. European Journal of Medicinal Chemistry. 2010; 45 :3692-3701 - 6.
Yoshida M, Hayakawa I, Hayashi N, Agatsuma T, Oda Y, Tanzawa F, Iwasaki S, Koyama K, Furukawa H, Kurakata S. Bioorganic & Medicinal Chemistry Letters. 2005; 15 :3328-3332 - 7.
Ma D, Xie S, Xue P, Zhang X, Dong J, Jiang Y. Angewandte Chemie, International Edition. 2009; 48 :4222-4225 - 8.
Ding Q, He X, Wu J. Journal of Combinatorial Chemistry. 2009; 11 :587-591 - 9.
Van Heusden J, Van Ginckel R, Bruwiere H, Moelans P, Janssen B, Floren W, van der Leede BJ, van Dun J, Sanz G, Venet M, Dillen L, Van Hove C, Willemsens G, Janicot M, Wouters W. British Journal of Cancer. 2002; 86 :605-611 - 10.
Catalano A, Carocci A, Defrenza I, Muraglia M, Carrieri A, Bambeke FV, Rosato A, Corbo F, Franchini C. European Journal of Medicinal Chemistry. 2013; 64 :357-364 - 11.
McDonnell ME, Vera MD, Blass BE, Pelletier JC, King RC, Fernandez-Metzler C, Smith GR, Wrobel J, Chen S, Wall BA, Reitz AB. Bioorganic & Medicinal Chemistry. 2012; 20 :5642-5648 - 12.
Cheah BC, Vucic S, Krishnan AV, Kiernan MC. Current Medicinal Chemistry. 2010; 17 :1942-1999 - 13.
Song EY, Kaur N, Park MY, Jin Y, Lee K, Kim G, Lee KY, Yang JS, Shin JH, Nam KY, No KT, Han G. European Journal of Medicinal Chemistry. 2008; 43 :1519-1524 - 14.
Patel NB, Khan IH, Pannecouque C, De Clercq E. Medicinal Chemistry Research. 2013; 22 :1320-1329 - 15.
Patel NB, Khan IH, Rajani SD. European Journal of Medicinal Chemistry. 2010; 45 :4293-4299 - 16.
Sprague JM, Land AH. In: Elderfield RC, editor. Heterocyclic Compounds. Vol. 5, Chapter 8. New York: Wiley; 1957. pp. 484-721 - 17.
Jordan AD, Luo C, Reitz AB. The Journal of Organic Chemistry. 2003; 68 :8693-8696 - 18.
Feng E, Huang H, Zhou Y, Ye D, Jiang H, Liu H. Journal of Combinatorial Chemistry. 2010; 12 :422-429 - 19.
Benedi C, Bravo F, Uriz P, Fernandez E, Claver C, Castillon S. Tetrahedron Letters. 2003; 44 :6073-6077 - 20.
Inamoto K, Hasegawa C, Hiroya K, Doi T. Organic Letters. 2008; 10 :5147-5150 - 21.
Ding Q, He X, Wu J. Journal of Combinatorial Chemistry. 2009; 11 :587-591 - 22.
Qiu JW, Zhang XG, Tang RY, Zhong P, Li JH. Advanced Synthesis and Catalysis. 2009; 351 :2319-2323 - 23.
Ding Q, Cao B, Liu X, Zong Z, Peng YY. Green Chemistry. 2010; 12 :1607-1610 - 24.
Shen G, Lv X, Bao W. European Journal of Organic Chemistry. 2009:5897-5901 - 25.
Guo YJ, Tang RY, Zhong P, Li JH. Tetrahedron Letters. 2010; 51 :649-652 - 26.
Zhao J, Huang H, Wu W, Chen H, Jiang H. Organic Letters. 2013; 15 :2604-2607 - 27.
Yella R, Patel BK. Journal of Combinatorial Chemistry. 2010; 12 :754-763 - 28.
Lokendrajit N, Brajakishor CS, Warjeet LS. Journal of Heterocyclic Chemistry. 2015; 52 :267-272 - 29.
Lokendrajit N, Chipem FAS, Brajakishor CS, Warjeet LS. New Journal of Chemistry. 2015; 39 :2240-2247 - 30.
Allen CL, Chhatwal RA, Williams JM. Journal of the Chemical Society, Chemical Communications. 2012; 48 :666-668 - 31.
Gelens E, Smeets L, Sliedregt LAJM, van Steen BJ, Kruse CG, Leurs, R, Orrua RVA. Tetrahedron Letters 2005; 46 :3751-3754 - 32.
Vanjari R, Guntreddi T, Singh KN. Organic Letters. 2013; 15 :4908-4911 - 33.
Nguyen TB, Ermolenko L, Al-Mourabit A. Organic Letters. 2012; 14 :4274-4277 - 34.
Xu H, Deng H, Li Z, Xiang H, Zhou X. European Journal of Organic Chemistry. 2013; 31 :7054-7057 - 35.
Nguyen TB, Tran MQ, Ermolenko L, Al-Mourabit A. Organic Letters. 2014; 16 :310-313 - 36.
Guntreddi T, Vanjari R, Singh KN. Organic Letters. 2014; 16 :3624-3625 - 37.
Lokendrajit N, Chipem FAS, Brajakishor CS, Warjeet LS. Organic & Biomolecular Chemistry. 2016; 14 :7735-7745 - 38.
Ertan T, Yildiz I, Tekiner-Gulbas B, Bolelli K, Temiz-Arpaci O, Ozkan S, Kaynak F, Yalcin I, Aki E. European Journal of Medicinal Chemistry. 2009; 44 :501 - 39.
Temiz-Arpaci O, Yıldız Y, Ozkan S, Kaynak F, Akı-Sxener E, Yalcin I. European Journal of Medicinal Chemistry. 2008; 43 :1423 - 40.
Jauhari PK, Bhavani A, Varalwar S, Singhal K, Raj P. Medicinal Chemistry Research. 2008; 17 :412 - 41.
Ueki M, Ueno K, Miyadoh S, Abe K, Shibata K, Taniguchi M, Oi SJ. Antibiot. 1993; 46 :1089 - 42.
Seth K, Garg SK, Kumar R, Purohit P, Meena VS, Goyal R, Banerjee UC, Chakraborti AK. ACS Medicinal Chemistry Letters. 2014; 5 :512-516 - 43.
Hohlfeld K, Tomassi C, Wegner JK, Kesteleyn B, Linclau B. ACS Medicinal Chemistry Letters. 2011; 2 :461-465 - 44.
Martin DD, Kotecha NR, Ley SV, Mantegani S, Menéndez JC, Organ HM, White AD, Banks BJ. Tetrahedron. 1992; 48 :7899 - 45.
Gorla SK, Kavitha M, Zhang M, Chin JEW, Liu X, Striepen B, Makowska-Grzyska M, Kim Y, Joachimiak A, Hedstrom L, Cuny GD. Journal of Medicinal Chemistry. 2013; 56 :4028-4043 - 46.
Boyer J, Arnoult E, Médebielle M, Guillemont J Unge J, Jochmans D. Journal of Medicinal Chemistry. 2011; 54 :7974-7985 - 47.
Aiello S, Wells G, Stone EL, Kadri H, Bazzi R, Bell DR, Stevens MFG, Matthews CS, Bradshaw TD, Westwell AD. Journal of Medicinal Chemistry. 2008; 51 :5135-5139 - 48.
Wang H, Wang L, Shang J, Li X, Wang H, Gui J, Lei AW. Chemical Communications. 2012:76 - 49.
Shelkar R, Sarode S, Nagarkar J. Tetrahedron Letters. 2013; 54 :6986 - 50.
Bala M, Verma PK, Sharma U, Kumar N, Singh B. Green Chemistry. 2013; 15 :1687 - 51.
Mayo MS, Yu X, Zhou X, Feng X, Yamamoto Y, Bao M. The Journal of Organic Chemistry. 2014; 79 :6310 - 52.
Kim BJ, Kim J, Kim YK, Choi SY, Choo HYP. Bulletin of the Korean Chemical Society. 2010; 31 :1270-1274 - 53.
Hana L, Org TL. Biomolecular Chemistry. 2017; 15 :5055-5061 - 54.
Han L, Zhang X, Wang X, Zhao F, Liu S, Liu T. Organic & Biomolecular Chemistry. 2017; 15 :3938-3946 - 55.
José Aurell M, Domingo LR, Arnó M, Zaragozá RJ. Organic & Biomolecular Chemistry. 2016; 14 :8338-8345 - 56.
Liu L, Wu Y, Wang T, Gao X, Zhu J, Zhao Y. The Journal of Organic Chemistry. 2014; 79 :5074-5081 - 57.
Bagno A, Kantlehner W, Kress R, Saielli G, Stoyanov E. The Journal of Organic Chemistry. 2006; 71 :9331-9340 - 58.
Šakić D, Hanževački M, Smith DM, Vrček V. Organic & Biomolecular Chemistry. 2015; 13 :11740-11752 - 59.
Zheng L, Tang M, Wang Y, Guo X, Wei D, Qiao Y. Organic & Biomolecular Chemistry. 2016; 14 :3130-3141 - 60.
Zhang W, Yang W, Wei D, Tang M, Zhu X. Organic & Biomolecular Chemistry. 2016; 14 :6577-6590 - 61.
Ray JK, Paul S, Ray P, Singha R, Rao DY, Nandi S, Anoop A. New Journal of Chemistry. 2017; 41 :278-284 - 62.
Becke ADJ. Chemical Physics. 1993; 98 :5648-5652 - 63.
Lee C, Yang W, Parr RG. Physical Review B. 1988; 37 :785-789 - 64.
Neese F. WIREs Computational Molecular Science. 2012; 2 :73-78 - 65.
Mayer I. Chemical Physics Letters. 1983; 97 :270-274