Perrin Charles L, Dong Yanmei
Department of Chemistry 0358, University of California-San Diego, La Jolla, California 92093-0358, USA.
J Am Chem Soc. 2007 Apr 11;129(14):4490-7. doi: 10.1021/ja069103t. Epub 2007 Mar 15.
Secondary deuterium isotope effects (IEs) on acidities have been accurately measured by an NMR titration method applicable to a mixture of isotopologues. Deuteration definitely decreases the acidity of carboxylic acids and phenols, by up to 0.031 in the DeltapK per D. For aliphatic acids, the IEs decrease as the site of deuteration becomes more distant from the OH, as expected, but a surprising result is that IEs in both phenol and benzoic acid do not decrease as the site of deuteration moves from ortho to meta to para. The experimental data are supported by ab initio computations, which, however, substantially overestimate the IEs. The discrepancy does not seem to be due to solvation. The IEs originate in isotope-sensitive vibrations whose frequencies and zero-point energies are lowered upon deprotonation. In the simplest case, formate, the key vibration can be recognized as the C-H stretch, which is weakened by delocalization of the oxygen lone pairs. For the aromatic acids, delocalization cannot account for the near constancy of IEs from ortho, meta, and para deuteriums, but the observed IEs are consistent with calculated vibrational frequencies and electron densities. Moreover, the ability of the frequency analysis to account for the IEs is evidence against an inductive origin.
通过适用于同位素异构体混合物的核磁共振滴定法，已精确测量了酸度的二级氘同位素效应（IEs）。氘化确实会降低羧酸和酚类的酸度，每一个氘的ΔpK值降低可达0.031。对于脂肪族酸，正如预期的那样，随着氘化位点离OH越来越远，同位素效应会降低，但一个令人惊讶的结果是，当氘化位点从邻位移动到间位再到对位时，苯酚和苯甲酸中的同位素效应并未降低。实验数据得到了从头算计算的支持，然而，从头算计算大大高估了同位素效应。这种差异似乎不是由于溶剂化造成的。同位素效应源于对同位素敏感的振动，其频率和零点能在去质子化时会降低。在最简单的甲酸根的情况下，关键振动可被识别为C-H伸缩振动，它因氧孤对电子的离域而减弱。对于芳香族酸，离域不能解释来自邻位、间位和对位氘的同位素效应的近乎恒定，但观察到的同位素效应与计算出的振动频率和电子密度一致。此外，频率分析解释同位素效应的能力证明了其并非源于诱导作用。
