Photoisomerization of Coordinated Diazene

We demonstrated how the photoisomerization hypothesis can be supported by accurate quantum chemical calculations (103). The experimental infrared and resonance Raman study of complex 5 led to the first determination of normal modes and force constants of diazene coordinated to a metal fragment. Isotope substitution yielding 15N- and 2H-isotopomers permitted the assignment of diazene normal modes in the experimental spectrum. Moreover, the spectra of these three isotopomers indicated that a laser-induced photoisomerization occurred in the Raman sample. However, a detailed assignment of the split bands was not possible in the experiment. 

In order to investigate the photoisomerization hypothesis we started with the two possible candidates, whose Lewis structures are given in Fig. 10. Structure optimization yielded the structures depicted in Fig. 11. 

The accuracy of the harmonic BP86/RI/TZVP force field (104) permitted that all bands, which split upon photoisomerization, could be assigned to the two possible isomers 5(A) and 5(B). The accuracy of the harmonic wavenumbers is, in general, better than 6 cm-1 for these bands (with the exception of 12 cm-1 for the mode at 1270 cm-1). 

The calculated modes at 1282, 1296, and 1306 cm-1 represent motions of the diazene moiety. The assignment of these vibrations in (101) as overtones of the modes at about 650 cm-1, which was confirmed by the experimental spectra of the isotopomers, was well reproduced by the calculations (103), in which we used the harmonic oscillator model as a zeroth-order approximation for the estimation of the overtone wavenumbers. 

The peaks at 663 cm-1 and 1455 cm-1 belong to isomer 5(A), while the peaks at 637 cm-1 and 1509 cm-1 belong to isomer 5(B). The N-N stretch vibration splits only by a very small amount so that two peaks cannot be resolved by the experimental spectrum and only a single band appears at 1365 cm-1. 

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