When the pyridine molecule approaches the Au clusters, the IR peaks of three vibrational modes (ring breathing and two C-H wags) within 1000∼1300 cm -1 become increasingly more intense as the distance reduces from 5 to 3 Å. The second C-H wag at 1238 cm -1 is predicted by full QM calculations to have the largest enhancement of 24 times (from 2.7 to 48.7 km/mol) for the S configuration, 33 times for V configuration, and 24 times for Py-Au 32 , as shown in the panels (j), (k), and (l) of Fig. , respectively. In contrast, some peaks are significantly weakened after the binding to Au clusters. This is especially obvious with the two out-of-plane C-H bends at 744 and 780 cm -1 , which have comparable intensities in the gas phase. While only the IR peak at 744 cm -1 is reduced in Py-Au 18 -S (3Å) and Py-Au 18 -V (3Å) complexes, both peaks (744 cm -1 and 780 cm -1 ) lose some strength in the Py-Au 32 (3Å) structure. No.