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111.
It is proved that a natural generalization of chess to an n × n board is complete in exponential time. This implies that there exist chess positions on an n × n chessboard for which the problem of determing who can win from that position requires an amount of time which is at least exponential in . 相似文献
112.
113.
DielsAlder Cyclization of a Dihydropyridine: NMR Spectroscopy,X‐Ray Crystallography,and DFT Computations. Bent Aromatic Dimeric Clusters in the Solid Phase 下载免费PDF全文
Gideon Fraenkel Jinhua Song Albert Chow Judith C. Gallucci 《Helvetica chimica acta》2014,97(10):1365-1382
Structural studies of the N‐(2,4‐dinitrophenyl) derivative of a Diels? Alder‐cyclized 1,2‐dihydropyridine both unequivocally established the polycyclic framework and revealed interesting distortions of aromatic structure and unique dimeric clustering of the aromatic entities in the solid state. 相似文献
114.
Rieth S Yan Z Xia S Gardlik M Chow A Fraenkel G Hadad CM Badjić JD 《The Journal of organic chemistry》2008,73(13):5100-5109
A molecular basket, composed of a semirigid C3v symmetric tris-norbornadiene framework and three pyridine flaps at the rim, has been shown to coordinate to a Cu(I) cation and thereby fold in a multivalent fashion. The assembly was effective (Ka = 1.73 +/- 0.08 x 10(5) M(-1)) and driven by enthalpy (DeltaH(o) = -7.2 +/- 0.1 kcal/mol, DeltaS(o) = -0.25 eu). Variable temperature (1)H NMR studies, assisted with 2D COSY and ROESY investigations, revealed the existence of Cu(I)-folded basket 10b with a molecule of acetonitrile occupying its interior and coordinated to the metal. Interestingly, 10b is in equilibrium with Cu(I)-folded 10a , whose inner space is solvated by acetone or chloroform. The incorporation of a molecule of acetonitrile inside 10a was found to be driven by enthalpy (DeltaH(o) = -3.3 +/- 0.1 kcal/mol), with an apparent loss in entropy (DeltaS(o) = -9.4 +/- 0.4 eu); this is congruent with a complete immobilization of acetonitrile and release of a "loosely" encapsulated solvent molecule during 10a/b interconversion. From an Eyring plot, the activation enthalpy for incorporating acetonitrile into 10a was found to be positive (DeltaH(double dagger) = 6.5 +/- 0.5 kcal/mol), while the activation entropy was negative (DeltaS(double dagger) = -20 +/- 2 eu). The results are in agreement with an exchange mechanism whereby acetonitrile "slips" into an "empty" basket through its side aperture. In fact, DFT (BP86) calculations are in favor of such a mechanistic scenario; the calculations suggest that opening of the basket's rim to exchange guests is energetically demanding and therefore less feasible. 相似文献