Structure and UV-induced photochemistry of 2-furaldehyde dimethylhydrazone isolated in rare gas matrices

In this work, a combined matrix isolation FTIR and theoretical DFT(B3LYP)/6-311++G(d,p) study of 2-furaldehyde dimethylhydrazone (2FDH) was performed. According to calculations, two E and two Z conformers exist, the E forms having considerably lower energy than the Z forms. The absence of relevant sterical hindrance between the two substituents around the C=N bond (dimethylamino and 2-furyl) in the E structures and an extended pi-p electron delocalization in the hydrazone moiety determines the higher stability of these species relatively to the Z structures. In the lowest energy form (E-AG) the O-C-C=N and C=N-N-Lp (Lp = lone electron pair of amine nitrogen atom) dihedral angles are predicted by the calculations to be -177.2 degrees and 93.7 degrees, respectively. The weak (N=C)-H center dot center dot center dot O hydrogen bond type interaction (H center dot center dot center dot O distance: 252.2 pm) in form E-AG, together with the absence in this form of the destabilizing interaction between the lone electron pairs of the oxygen and nitrogen atoms existing in E-SG, explains its lower energy in comparison with this latter form. Both E-AG and E-SG conformers could be trapped from room temperature gas phase in low temperature argon and xenon matrices. The high E-SG -> E-AG energy barrier (>25 kJ mol(-1)) explains that, upon increasing the temperature of the matrices no conformational isomerization could be observed. After irradiation of 2FDH with UV-light at lambda > 328 and lambda > 234 nm, two different photochemistries were observed. Irradiation at lower energy (lambda > 328 nm) induced the E-AG -> E-SG isomerization. Further irradiation at higher energy (lambda > 234 nm) led to a quick consumption of 2FDH and production of furan and dimethylisocyanide.