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 CN bond (dimethylamino and 2-furyl) in the E structures and an extended π-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-CN and CN-N-Lp (Lp=lone electron pair of amine nitrogen atom) dihedral angles are predicted by the calculations to be -177.2° and 93.7°, respectively. The weak (NC)-H⋯O hydrogen bond type interaction (H⋯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 λ>328 and λ>234 nm, two different photochemistries were observed. Irradiation at lower energy (λ>328nm) induced the E-AG→E-SG isomerization. Further irradiation at higher energy (λ>234 nm) led to a quick consumption of 2FDH and production of furan and dimethylisocyanide.

NMR nuclear magnetic shielding anisotropy of linear molecules within the linear response within the elimination of the small component approach.

The influence of the spin-Zeeman (SZ) operator in the evaluation of the spin-orbit effect on the nuclear magnetic shielding tensor in the context of the linear response within the elimination of the small component approach is critically discussed. It is shown that such term yields no contribution to the isotropic nuclear magnetic shielding constant, but it may be of great importance in the determination of individual tensor components, and particularly of the tensor anisotropy. In particular, an interesting relation between the SZ and orbital Zeeman contributions to the spin-orbit effect for the case of linear molecules is shown to hold. Numerical examples for the BrH, IH, and XeF(2) molecules are presented which show that, provided the SZ term is taken into account, results of the individual shielding tensor components and the tensor anisotropy are in good agreement with those obtained by other theoretical methods, and particularly by the Dirac-Hartree-Fock approach.