Eisuke ITOa, Hiroshi OJIa, Tohru ARAKIb, Kazuyoshi OICHIa, Hisao ISHII, Yukio OUCHIa Toshiaki OHTAc, Nobuhiro KOSUGI, Yusei MARUYAMAd, Toshio NAITOe, Tamotsu INABEe and Kazuhiko SEKIa (aNagoya Univ.y, bKEK, cUniv. Tokyo, d Hosei Univ. and eHokkaido Univ.)
[J. Am. Chem. Soc. 119, 6336 (1997)]
The tautomerism in solid phase was studied by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the N K- and O K-edges and X-ray photoelectron spectroscopy (XPS) in the N1s and O1s regions for five N-salicylideneaniline derivatives: N,N'-disalicylidene-1,6-pyrenediamine (DSPY), N,N'-disalicylidene-p-phenylenediamine (BSP), N-(2-hydroxy-1-naphthylidene)-1-pyrenylamine (NPY), N,N'-di(2-hydroxy-1-naphthylidene)-p-phenylenediamine (DNP), and the complex between DNP and 7,7,8,8-tetracyanoquinodimethane (DNP-TCNQ). The NEXAFS spectral features were assigned by (1) the comparison with the core excitation spectra of reference compounds and (2) ab initio MO calculations of the core-excited states by the improved virtual orbital method using the relaxed Hartree-Fock orbitals. The tautomeric structures were deduced by the intensity of peaks characteristic of the tautomers in OH- and NH-forms. The XPS spectra gave structures consistent with those by NEXAFS, and also allowed quantitative estimation of the relative tautomer populations. The results range from the dominance of the OH-form in DSPY and BSP to comparable mixture of the OH- and NH-forms in DNP and DNP-TCNQ. These structures are consistent with the previous estimation by X-ray diffraction (XD), except for DNP-TCNQ where XD suggested the NH-form dominance. The good correlation of the population by XPS with the C-O bond length by XD supports the present conclusion for DNP-TCNQ. These results demonstrate the usefulness of NEXAFS as well as XPS for studying the tautomerism in hydrogen bonded systems.