Experimental vibrational spectra of small molecules adsorbed on single crystal metal surfaces are explored using DFT calculations. By combining high sensitive and high resolution (@>=@ 0.5 cm@super -1@) surface infrared spectroscopy and density functional calculations details in the spectra can be detected and analyzed. Strategies for analysis at both the harmonic and anharmonic levels are discussed. Overtone and combination modes in the absence and presence of Fermi resonances are also explored. Both cluster and extended structure calculations are utilized for investigation of the coordination site of the adsorbate on the surface. Methoxy and ethoxy adsorbed on Cu(100) and W(110) surfaces are used to show the interplay between experiments and calculations.