Thomas F. Magnera

Two-dimensional supramolecular chemistry with molecular Tinkertoys

For some time now (1–5), we have worked on the development of a “molecular construction kit,” analogous to the childrens Tinkertoy† construction set, which permits the assembly of complicated objects from a limited set of rods, connectors, and other simple building elements. The idea is to do civil engineering with individual molecular components ranging in size from a few to several dozen A. A somewhat related concept of “molecular Lego,” based on a different set of structural elements, was proposed (7) and developed (8) independently by Stoddard. Both belong to the wider category of “modular chemistry,” in which a small number of mid-sized rigid molecular structural elements are combined into complex structures (9). At the start of our project, a very limited selection of types and lengths of straight terminally functionalized molecular rods and connectors was available. We synthesized additional ones based on the oligomers of [1.1.1]propellane ([ n ]staffanes) (10) and on oligomeric 10-vertex and 12-vertex p -carboranes (11). Other laboratories produced polycubyls (12), oligomers of [2.2.2]propellane (13), additional oligomers of 12-vector carboranes (14), and many rods constructed from the more common p -phenylene and acetylene subunits. A combination of several types of such structural elements permits one to achieve an accurate match to a desired rod length. A recent comprehensive review (15) makes it abundantly clear that after a decade of effort, considerable flexibility exists in the choice of molecular rod lengths and properties. After some initial experiments with point (4) connectors, which function by forming bonds from a central atom to the termini of several rods, we turned our attention to star (4) connectors, which function by forming a bond between each of several star arm termini to the terminus of a rod or of another star arm. Various suitable star connectors such …

The first twenty-eight gas-phase proton hydration energies☆

Abstract The successive binding energies Δ E n −1, n ( n =2−28) of protonated water clusters H + (H 2 O) n , produced by fast-atom bombardment of ice, have been determined by collision-induced dissociation in a triple-quadrupole mass spectrometer. They decrease dramatically initially, and after n =9 increase slowly up to n =28. However, the twenty-first water molecule stands out as bound about 2 kcal/mol more strongly than expected from a simple interpolation, accounting for the well-known “magic” number, n =21. The trend in Δ E n −1, n shows that hydrogen bonding quickly becomes a dominant force after the first nine water molecules surround a proton. The Δ E n −1, n values were used in thermodynamic cycles to show that the “magic” cluster, n =21, already contains 78% of the total aqueous proton hydration energy.

Cluster ions from keV-energy ion and atom bombardment of frozen gases

Abstract A brief survey is given of the mass spectra obtained from frozen gases by bombardment with keV-energy ions and atoms. The internal chemical constitution of the observed secondary cluster ions, which bears no simple relation to the molecular structure of the solid, has been established by observations of collision-induced dissociation, laser-induced dissociation and metastable decay. It has been correlated with the chemical composition of the residual bombarded solid, deduced from spectroscopic observations. These results, as well as preliminary results on sputtering yields for impact of 1–4 keV rare gas ions on solid argon, are compatible with the previously proposed mechanistic model for the formation of the cluster ions based on the flow of supercritical gas from the elastic collision spike region.

“Magic” losses of one to five N2 molecules from metastable N+2n clusters

Abstract SIMS and FAB of solid N 2 generated large N 2n + clusters (N 4 + -N + 54 ) which showed three types of metastable decay: (i) loss of a single N 2 molecule, assigned to ordinary (“thermal”) metastability, (ii) nearly total disintegration, assigned to radiationless decay of an electronically excited state, and (iii) “magic” losses of very specific small numbers of N 2 molecules, assigned to N 2 ( v = 1) vibrational relaxation by use of isotope effects. Average N 2 binding energies are estimated.

The dielectric response of chloromethylsilyl and dichloromethylsilyl dipolar rotors on fused silica surfaces

We have measured the dielectric response of monolayer films of surface mounted chloromethyl- and dichloromethylsilyl dipolar rotors on fused silica at frequencies in the 1 kHz range and temperatures from 4 to 300 K. The torsional potentials, calculated from molecular mechanics, show an asymmetrical three-fold barrier to rotation with a barrier height sufficient to hinder motion of the rotor at experimental temperatures. A broad distribution of barrier heights is observed experimentally, consistent with calculated results showing that the intrinsic barrier of the rotor is modified by interactions with the underlying substrate. For a series of samples with differing concentrations of the rotor, the observed signal strength varies in proportion to the rotor coverage measured by Auger spectroscopy; however, the absolute strength of the signal is about three times larger than expected.

Photodissociation of Arn−mN+2m clusters (n=2 or 3, m=0−n)☆

Abstract Laser-induced photodissociation cross sections of Ar 2 + , ArN 2 + , N 4 + , Ar 3 + , Ar 2 N 2 + , ArN 4 + , obtained by sputtering of solid argon—nitrogen mixtures, have been measured in the 270–650 nm range. All the spectra exhibit a band in the UV (305–335 nm) and those of the trimeric ions, except for N 6 + , also have a band in the visible (465–525 nm). The photodissociation products depend on the choice of wavelength in a readily rationalized fashion.

Sputtering yields of condensed rare gases

Abstract Improved experimental techniques were employed to measure the initial sputtering yields of condensed rare gas targets under 0.75–5 keV ion bombardment. These sputtering yields show a strong nonlinear dependence on the total stopping power. The experimental results for condensed Ar, Kr, and Xe targets are consistent with a systematic change from a linear collision cascade to evaporation and to a gas-flow mechanism as the deposited energy density increases. For condensed Ne sputtered by heavy projectiles. the yields are larger than can be explained by any existing quantitative model. The sputtering yields for all targets are found to approach a plateau values as sputtering progresses. This is attributed to surface roughening and the initial slope is used to determine the size of the impact damage area.

The organometallic ‘molecular tinkertoy’ approach to planar grid polymers

A general approach is outlined for the synthesis of thin-layer materials of arbitrarily configured structure (‘designer solids’), based on confinement to two dimensions. The synthesis of a building module, the tetra-n-butylammonium salt of lanthanum(III)bis[5,10,15,20-tetrakis(4-pyridyl)porphyrinate] (1), is described. It is shown by grazing incidence IR spectroscopy that this cross-shaped molecule adheres firmly to a mercury surface, with the porphyrin rings parallel to the surface and the pyridine rings carried by the upper porphyrin ring ready for linear coupling. The proposed surface-confined synthesis of a square grid, which would define a first layer of a designer solid, can now be tested.

Kev-energy ion and atom bombardment of low-temperature solids: Radiation damage, sputtering, and mechanism

Abstract Experimental evidence available for the phenomena induced by the low energy bombardment of frozen gases with keV energy ions and atoms is summarized. The results can be understood qualitatively in terms of a model incorporating (i) sputtering and primary damage cente. formation in the collision cascade regime, (ii) conversion of primary to secondary damage centers in the elastic collision spike which develops subsequently, accompanied by flow of a portion of the hot gas from the spike region into vacuum and the freezing of the remainder on the crater walls, and (iii) selective evaporation of the least firmly held components of the solvation shells surrounding those ejected secondary damage centers which are charged. New results are reported for keV-energy sputtering yields of the solid rare gases and for spectroscopically observed radiation damage in several other frozen gases. They are compatible with the model.

Self-assembly of a metal-ion-bound monolayer of trigonal connectors on mercury: an electrochemical Langmuir trough.

The adsorption of the trigonal connector, 1,3,5-tris[10-(3-ethylthiopropyl)dimethylsilyl-1,10-dicarba-closo-decaboran-1-yl]benzene (1), from acetonitrile/0.1 M LiClO4 on the surface of mercury at potentials ranging from +0.3 to −1.4 V (vs. aqueous Ag|AgCl|1 M LiCl) was examined by voltammetry, Langmuir isotherms at controlled potentials, and impedance measurements. No adsorption is observed at potentials more negative than ∼ −0.85 V. Physisorption is seen between ∼ −0.85 and 0 V. At positive potentials, adsorbate-assisted anodic dissolution of mercury occurs and an organized surface layer is formed. Although the mercury cations are reduced at −0.10 V, the surface layer remains metastable to potentials as negative as −0.85 V. Its surface areas per molecule and per redox center are compatible with a regular structure with the connectors 1 woven into a hexagonal network by RR′S→Hg\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{2}^{2+}}}\end{equation*}\end{document}←SRR′ or RR′S→Hg2+←SRR′ bridges. The structure is simulated closely by geometry optimization in the semiempirical AM1 approximation.