Critical evaluation of molecular mechanics

Engler, E. M.; Andose, Joseph D.; Schleyer, Paul von Ragué

doi:10.1021/ja00805a012

Cited by 622 publications

(223 citation statements)

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“…This is very similar to the 3.51 5 0.25 kcal/mol difference in AH,(g), and 3.4 kcal/mol difference in AHH,(&) between 2-methyl-2-butene and 3-methyl-1-butene, again suggesting that the cyclic compounds differ in double bond substitution rather than conformational stability. The 5.60 + 0.34 kcal/mol more exothermic AH,(g) of vinylcyclohexane relative to allylcyclopentane is essentially identical to the ethylcyclohexaiie-propylcyclopentane difference ( 9 , and is mainly a reflection of the 6.1 kcal/mol greater strain in the cyclopentane ring (12).…”

Section: Resultsmentioning

confidence: 84%

Heats of vaporization and gaseous heats of formation of some five- and six-membered ring alkenes

Fuchs¹,

Peacock²

1979

Can. J. Chem.

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Dc,pcrriritetti c!f Clr~ttti.si~:\*, Utri~r~'siiy oJ'Hor,.siott. Horr.siori, TX 7700-1, U . S . A Received January 16, 1979 RICHARD FUCHS and L. ALAN PEACOCK. Can. J. Chem. 57.2302Chem. 57. ( 1979. The heats of vaporization of I-methylcyclopentene, 3-methylcyclopentene, ethylidenecyclopentane, 1-ethylcyclopentene, methylenecyclohexane, allylcyclopentane, vinylcyclohexane, ethylidenecyclohexane, allylcyclohexane, 3,3-diethylpentane, 2,2,4,4-tetramethylpentane, and trnns-2,2,5,5-tetramethyl-3-hexene have been measured by the gas chromatography -calorimetry method. These values have been combined with previously reported liquid heats of formation to give gaseous values of AH,. The results indicate that the internal double bond is favored by about 0.5 kcal over the exo in both 5-and 6-membered rings, but the endo-exo differences are much smaller than previously believed. Several of the liquid heat capacities that were measured were not well predicted by group additivity schemes.RICHARD FUCHS et L. ALAN PEACOCK. Can. J. Chem. 57.2302Chem. 57. (1979. Utilisant une niethode combinant la chromatographie en phase gazeuse et la calorimetric, on a mesure les chaleurs de vaporisation des hydrocarbures suivants: niCthyl-1 cyclopentene, methyl-3 cyclopentene, ethylidenecyclopentane, ethyl-1 cyclopentene, mCthyltnecyclohexane, allylcyclopentane, vinylcyclohexane, tthylidenecyclohexane, allylcyclohexane, diethyl-3,3 pentane, tetramethyl-2,2,4,4 pentane et tttramtthyl-2,2,5,5 hexene-3 trnns. On a combine ces valeurs avec les chaleurs deformation de ces liquides qui avaient Ctt rapporttes anterieurement afin d'en tirer les valeurs de AH, en phase gazeuse. Les resultats indiquent que les doubles liaisons internes sont favorisees par environ 0.5 kcal/mol par rapport aux doubles liaisons exo dans les cycles a 5 et a 6 chainons; toutefois la difference entre ettdo et exo est beaucoup plus faible que ce qu'on croyait anterieurement. En se basant sur les schkmas d'additivite des groupes, il aurait ete impossible de predire adtquatement plusieurs des capacites calorifiques mesurkes.[Traduit par le journal]A much discussed conceDt more than two decades ago was the idea that the internal (endocyclic) double bond is thermodynamically favored over the exocyclic double bond in six-membered ring systems, whereas the reverse was believed to be true in fivemembered rings (1). However, Turner and Garner (2) measured the heat of hydrogenation (acetic acid solution, 25°C) of 1-methylcyclohexene, methylenecyclohexane, 1-methylcyclopentene, and methylenecyclopentane, and found that hydrogenation of the exo double bond is more exothermic in both the five-membered ring (by 3.7 kcal/mol) and sixmembered ring (2

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Section: Resultsmentioning

confidence: 84%

Heats of vaporization and gaseous heats of formation of some five- and six-membered ring alkenes

Fuchs¹,

Peacock²

1979

Can. J. Chem.

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“…Like the cyclic alkanes, these are molecules with rigid conformations which can be predicted very successfully by standard methods of molecular mechanics (cf. Engler, Andose &von Schleyer, 1973;Jeffrey & Park, 1979;Ceccarelli, Ruble & Jeffrey, 1980). The molecules have a higher ratio of ether oxygens to hydroxyls than the pyranoses and pyranosides, which influences the intermolecular hydrogen bonding as illustrated by the crystal structure of 1,6-anhydro-fl-D-galactopyranose (Ceccarelli et al, 1980).…”

Section: The Anhydro Sugarsmentioning

confidence: 99%

Crystallographic studies of carbohydrates

Jeffrey¹

1990

Acta Crystallogr B Struct Sci

151

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The monosaccharides which constitute the monomer units of many important industrial and biological macromolecules are well represented among the 2000 crystal structures of the carbohydrate class 45 of the Cambridge Structural Database. There are few examples of crystal structure analyses of the corresponding acids, but many of their calcium salts and calcium salt complexes. With the exception of the disaccharides and cyclodextrins, the oligosaccharides are not well represented, with less than ten trisaccharides, one tetrasaccharide and one hexasaccharide-iodide complex. Two important conformational factors are the Hassel-Ottar effect and the anomeric effect, both of which have been studied using crystallographic data. Hydrogen bonding is ubiquitous in carbohydrate crystals and generally involves all the hydroxyls as both donors and acceptors, and some of the ring and glycosidic oxygens as acceptors. These hydrogen bonds tend to form finite or infinite chains. In hydrates, these chains are linked through the water molecules to form networks. Cyclic hydrogen-bond systems are observed in the cyclodextrins. Long-chain alkylated carbohydrates provide a large class of thermotropic and lyotropic liquid crystals, and some non-ionic surfactants which have been shown to be useful for membrane-protein solubilization and crystallization.

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“…[1][2][3] Due to their inherent ring strains, [4][5][6][7][8][9][10][11] interesting preparative aspects specific to these ring compounds have been developed. 12) With regard to the preparative perspective, the synthesis of optically active cyclobutane ring systems is particularly important since many compounds with such ring systems not only occur in nature 13,14) but are also key intermediates [15][16][17][18] in the synthesis of naturally occurring or biologically important target molecules.…”

Section: Introductionmentioning

confidence: 99%