Theoretical and experimental study of resonance-assisted hydrogen bond in o-hydroxyl diaryl schiff bases

Abstract

In this paper, the intramolecular hydrogen bond (IMHB) energies (EHB) in fifty 2-OH-XArCH=NArY (2-OH-XBAY) compounds containing resonance-assisted hydrogen bond (RAHB) system and fourteen XArCH=NAr-2′-OH (XBA-2′-OH) compounds without RAHB system were estimated by molecular tailoring approach (MTA) and cis-trans method (CTM) at the theoretical level of DFT B3LYP/6–311++G(d, p). The above compounds were synthesized and their 1H NMR and 13C NMR spectra were determined, then the chemical shifts of H δH on OH and chemical shifts of C δC on CH=N were assigned. The relationship between the δH of the bridged hydrogen atom on OH, the δC of the carbon atom on the CH=N bond in the RAHB ring and the estimated EHB was investigated. The results show that: (1) There is a good linear relationship between the IMHB energy calculated by CTM and that calculated by MTA. (2) There is a good linear relationship between the calculated O-H bond length dO-H and the N···H distance dN···H. (3) For 2-OH-XBAY compounds with fixed X and changing Y, the chemical shift of the bridge hydrogen atom δH(OH) has a good linear relationship with the IMHB energy ECTM, but when both X and Y change, the linear relationship is not good. (4) There is no good linear relationship between the chemical shift of C on CH=N δC(CH=N) and the IMHB energy ECTM. (5) The IMHB energy ECTM of 2-OH-XBAY cannot be expressed by only employing δH(OH), and it is necessary to employ both δH(OH) and δC(CH=N) for expressing their ECTM. (6) As a comparison, there is no RAHB in XBA-2′-OH, and in such case, it is unsuitable to use δH(OH) to express the EHB.