mixture was 4.3% by 13C NMR. The carbon spectrum showed two peaks in the carbonyl carbon region of nearly equal intensities at −151.6 °C, with E-2 (48%) absorbing downfield of the major Z-2 (52%). The large population of E-2 confirms that electron-withdrawing groups R‘ in RCO2R‘ enhance the populations of the E-isomers. The free-energy barriers for 2 of 6.24 (E-to-Z) and 6.26 kcal/mol (Z-to-E) were determined
甲酸(1)的低温1 H和13 C NMR光谱显示在含有氢键受体二
甲醚的溶剂中E和Z构象的单独信号。在相同溶剂中,E - 1的种群(在3:1:1 CHClF 2 / CHCl 2 F /(CH 3)2 O中为6.2%)大于在13 C标记的
甲酸甲酯中的E - 1种群(0.2%),指出相对种群不是由空间效应决定的。E和Z构象之间的自由能差为1为0.9kcal / mol。在1:3 CD 2 Cl 2 /(CH 3)2 O溶剂混合物中,
甲酸和
六氟丙酮与HCO 2 C的加合物在低温下通过1 H和13 C NMR鉴定出E和Z构型的峰。(CF 3)2 OH(2)。的人口ë - 1在该溶剂混合物为4.3%的13 C NMR。碳谱在-151.6°C处的羰基碳区域中显示两个强度几乎相等的峰,其中E - 2(48%)吸收了主要Z - 2(52%)的低场。E - 2的大量存在证实了RCO 2 R'中的吸电子基团R'增强了E-异构体的数量。由在-143