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Single molecular devices have attracted strong interest as a new element of electronic devices. One of the serious problems in the realization of feasible single molecular devices is how to control their current-voltage (I-V) characteristics. This problem is related with poor reproducibility of measured I-V data (junction-to-junction fluctuation), which probably originates from the fluctuation of the adsorption structure of a molecule on surface. Thus, unambiguous determination of adsorption sites and molecular shape between electrodes is urgently required. Although the inelastic electron tunneling spectroscopy (IETS) has been employed successfully to determine the species of molecules attached to electrodes, vibration-mode dependence of its intensity and poor performance for the determination of adsorption structure limit the application range of IETS. Therefore we propose a novel nuclear magnetic resonance (NMR) measurement using the scanning tunneling microscope (STM), STM-NMR, for the determination of adsorption structures in single molecular devices, and evaluate its effectiveness using ab initio Green's function method incorporated into the GAUSSIAN03 package. The calculation for 1H-NMR chemical shifts of Al(electrode)-H2-Al(electrode) and Au(electrode)-H2-Au(electrode) junctions are performed using the gauge invariant atomic orbital (GIAO) method and the local density approximation (LDA) of the density functional theory (DFT). Significant dependence of the STM-NMR on the adsorption angle of the hydrogen molecule is found in both cases. The maximum difference of 1H-NMR chemical shift in Al(electrode)-H2-Al(electrode) is calculated to be 2 ppm between the adsorption configurations parallel and perpendicular to the one-dimensional electrodes. We also calculate the IETS intensity on the basis of the Green's function derivative approach with the LDA-DFT. In contrast to the STM-NMR case, the adsorption angle dependence is not found in the IETS simulations, which clearly shows the effectiveness of the STM-NMR measurement. |