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Halogens are among the most electronegative elements, and the variations in size and polarizability of halogens require different descriptions of the intermolecular bonds they form. Here we use the inelastic tunneling probe (itProbe) to acquire real-space imaging of intermolecular-bonding structures in the two-dimensional self-assembly of halogenbenzene molecules on a metal surface. Direct visualization is obtained for the intermolecular attraction and the "windmill" pattern of bonding among the fully halogenated molecules. Our results provide a hitherto missing understanding of the nature of the halogen bond.

SummaryA reinvestigation of 2-methylacetophenone (1) by ns flash photolysis has provided detailed evidence for the reaction sequence of photoenolization. The triplet reaction proceeds adiabatically from the lowest excited triplet state of the ketone, 3K (l), to the enol excited triplet state, 3E (l), which decays both to enol and ketone ground state. The Z-and E-isomers of the photoenol, Z-E (1) and E-E (1) are formed in about equal yield by the triplet pathway, while direct enolization from the lowest excited singlet state of 1 yields (predominantly) the 2-isomer. Intramolecular reketonization from 2-E (1) to 1 proceeds at a rate of ca. loss-' in cyclohexane, but can be retarded to ca. 104s-' in hydrogen-bond-acceptor solvents. The proposed mechanism is summarized in Scheme 1 and rationalized on the basis of a state correlation diagram, Scheme 2. 3,3,6,8-Tetramethyl-l-tetralone (2) was used as a reference compound with fixed conformational position of the carbonyl group, and some results from a brief investigation of 2,4-dimethylbenzophenone (3) are also reported.We have recently found [ 11 that the photoenolization of 5-methyl-l,6naphtho-quinone takes place with high efficiency and that the thermal reketonization to starting material is very fast at room temperature. The great retardation of the back reaction from a rate of ca. 105s-' in cyclohexane to ca. lo's-' in hexamethyl phosphoric acid triamide (HMPA) was striking but not surprising: The solvation of the enolic proton in hydrogen-bond-acceptor (HBA) solvents obviously provides an I)

Magnetic single atoms and molecules are receiving intensifying research focus because of their potential as the smallest possible memory, spintronic, and qubit elements. Scanning probe microscopes used to study these systems have benefited greatly from new techniques that use molecule-functionalized tips to enhance spatial and spectroscopic resolutions and enable new sensing capabilities. We demonstrate a microscopy technique that uses a magnetic molecule, Ni(cyclopentadienyl)2, adsorbed at the apex of a scanning probe tip, to sense exchange interactions with another molecule adsorbed on a Ag(110) surface in a continuously tunable fashion in all three spatial directions. We further used the probe to image contours of exchange interaction strength, revealing angstrom-scale regions where the quantum states of two magnetic molecules strongly mix. Our results pave the way for new nanoscale imaging capabilities based on magnetic single-molecule sensors.

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