羰桥双共轭体系结构效应的研究 Ⅰ.羰桥两边支链的共轭竞争

A Study of Structural Effect in Carbonyl Bridged Di-conjugative Systems Ⅰ. Conjugative Competition for Carbonyl Bridge

为了定量地研究羰桥双共轭体系中的结构效应,制备T_mB_n、B_mB_n和T_mT_n系列共32个羰桥化合物,测定了电子吸收光谱。发现:电子光谱频率与由总双键数所构成的同系因子不成直线关系,而是不规则的散布点,表明分子中没有贯通的共轭基干,羰基对整个分子的共轭极化有隔断作用。羰桥两边两个分支对羰桥的共轭作用是互相竞争的,当共轭竞争的优势从一分支转移到另一分支时,同系直线就分为斜率不同的两段,其间出现转折点。随着两个共轭分支链长的改变,拐点出现有规则的移动。在羰桥化合物的同分异构体中,两个分支的共轭竞争也很明显,其电子吸收峰与一定的m+n值表现勺V形曲线。

In order to study the structural effect in carbonyl bridge, 32 compounds of three series (T_mB_n, B_mB_n, T_mT_n) have been prepared. The electronic absorption spectra in alcohol and cyclohexane were determined. It has been found that: instead of a homologous line a plot of scattered points was obtained in each case, indicating that there is no single conjugated system. The conjugative palarization on the molecular chain is blocked by the carbenyl bridge, as shown by the turning point on the homologous line. The two branches on both sides of the carbonyl bridge are in conjugative competition for the carbonyl group. The position of the turning point for different series shifts regularly with increase of length of one branch. When the wave length of absorption peaks for isomeric compounds are ploted against their chain number (m+n), a 'V' shaped curve with lowest point at the middle was found. These give further support to the conclusion on conjugative competition of the two branches on the carbonyl bridge.