Identification of halogen atoms in scanning tunneling microscopy images ofsubstituted phenyl octadecyl ethers

A homologous series of para-substituted phenyl octadecyl ethers (X-POEs, wh ere X = H, Cl, Br, I) was prepared using the Williamson ether synthesis and characterized by H-1 NMR and GC/MS. Scanning tunneling microscopy images a cquired from these ethers revealed a bias-dependent contrast corresponding to electron density contours of various X-POE molecular orbitals. Images re flecting the electron density contour of the highest occupied molecular orb ital exhibited four bright spotsone for the halogen atom, two representing the pair of lobes of the phenyl ring, and one for the oxygen/(alpha -carbon atoms. For each X-POE, the intensities (Z(max)) of the spots for the halog en atom and the lobe of the phenyl ring closest to the halogen were measure d and their ratio was calculated (Cl-POE 0.49 +/- 0.06; Br-POE 0.59 +/- 0.0 6; I-POE 0.75 +/- 0.07). Analysis of variance at the 95% confidence level r evealed that the intensity ratios were consistent from molecule to molecule , image to image, and day to day. According to the Student t-test, the aver age Z(max) ratios for Cl-POE, Br-POE, and I-POE are different at the 95% co nfidence level. In addition, they follow a trend that corresponds favorably with that of the atomic radii of the halogens. The probability of classify ing a single X-POE molecule as Cl-POE, Br-POE, or I-POE is variable and dep ends on the magnitude of the Z(max) ratio.