排序方式: 共有3条查询结果,搜索用时 0 毫秒
1
1.
David A. Hrovat Richard Vaughan Williams Alan C. Goren Weston Thatcher Borden 《Journal of computational chemistry》2001,22(13):1565-1573
B3LYP/6‐31G* calculations on the degenerate rearrangements of substituted semibullvalenes spuriously predict the relative enthalpies of the bishomoaromatic TSs to be lower than the experimental values. However, the calculations do make the useful and experimentally testable prediction that the two cyano and two phenyl substituents in 2,6‐dicyano‐4,8‐diphenylsemibullvalene ( 9d ) are more likely than four cyano substituents in 2,4,6,8‐tetracyanosemibullvalene ( 9f ) or the four phenyl substituents in 2,4,6,8‐tetraphenylsemibullvalene ( 9g ) to produce a semibullvalene that has a bishomoaromatic equilibrium geometry in the gas phase. The major reason for the surprising finding that 9d is more likely to be bishomoaromatic than 9g is shown to be steric interactions between the phenyl groups at C‐2 and C‐8 and at C‐4 and C‐6 in bishomoaromatic structure 10g . These interactions inhibit the conjugative stabilization of 10g ; but they are absent in bishomoaromatic structure 10d , where cyano groups replace the phenyl groups at C‐2 and C‐6 in 10g . © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1565–1573, 2001 相似文献
2.
Helmut Quast Markus Heubes Anita Dunger Hans-Heinrich Limbach 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,134(2):236-244
The first carbon-13 shift thermometer for the temperature range of 100–300 K is based on the very rapid equilibration of a pair of semibullvalene valence tautomers. The temperature dependence of the equilibrium constant is reflected in strongly temperature-dependent shift differences Δδ between averaged signals, e.g.,d(Δδ)/dT= 0.051 ppm K−1at 300, 0.087 ppm K−1at 200, and 0.175 ppm K−1at 110 K for the quaternary carbon atoms C2 and C6. At 37 temperaturesT, which were measured with calibrated platinum resistance thermometers, shift differences Δδ were taken from nondecoupled carbon-13 spectra recorded from solutions of 1 in mixtures of chlorodifluoromethane and deuterated dimethyl ether without spinning. The least-squares fit of these Δδ vsTdata to a polynomial equation of the fourth degree (Eq. [5],r2= 0.9999) allows the calculation of temperatures from measured shift differences with a standard deviation of 0.46 K and an estimated error of about 1 K. The heating effects of WALTZ-16 decoupling and the influence of solvents on Δδ are investigated. A comparison with existing NMR thermometers demonstrates the superior performance of the new carbon-13 shift thermometer with respect to precision and the accessible temperature range. 相似文献
3.
Griffiths PR Pivonka DE Williams RV 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(33):9193-9199
Infrared spectra recorded for 1,5-dimethyl-2,8:4,6-semibullvalenetetracarboxylic acid dianhydride (12) in the condensed and vapor phase clearly prove that in the vapor phase the dianhydride 12 is a homoaromatic ground state semibullvalene. 相似文献
1