Roderick M. Macrae

Paramagnetic muonium states in elemental sulfur

Zero-point vibrational corrections are calculated by abinitio molecular orbital methods for the energy and the reduced hyperfine tensor of the muonated sulfanyl radical SMu 2Π. Only a small α-isotope effect is found for the reduced isotropic muon–electron hyperfine coupling constant of SMu relative to SH. Interstitial muonium in S8 is also investigated. The results are discussed in the light of recent observations on muonium in elemental sulfur.

Isotopes and analogs of hydrogen--from fundamental investigations to practical applications.

Hydrogen has a central role in the story of the universe itself, and also in the story of our efforts to understand it. This paper retells the story of the part played by hydrogen and its stable isotope deuterium in the primordial synthesis of the elements, then goes on to describe how the spectrum of atomic hydrogen led to insights into the laws governing matter at the most fundamental level, from the quantum mechanics of Schrödinger and Heisenberg, through quantum electrodynamics, to the most recent work investigating the underlying structure of the proton itself. Atomic hydrogen is unique among the elements in that the concept of isotopy – atoms having the same nuclear charge but different masses – is stretched to its limit in the isotopes of hydrogen, ranging from the well-known isotopes deuterium and tritium to exotic species such as muonium, muonic helium, and positronium. These atoms, or atom-like objects, have much to tell us about fundamental aspects of the universe. In recent years the idea of utilizing hydrogen either as an energy source (through nuclear fusion) or as an energy storage medium (bound in hydrides or other materials) has attracted much attention as a possible avenue to a post-oil energy future. Some of the more interesting recent developments are described here.

Competitive muonium addition to 3-methyl-2-butenal

We report the observation of muonium-substituted radicals formed in 3-methyl-2-butenal. The reduced hyperfine coupling constants (A′µ) at 273 K are 59.96 MHz for addition to the C—C bond and 2.89 MHz for muonium attachment to CO.

Vibrational Bonding: A New Type of Chemical Bond Is Discovered

A long-sought but elusive new type of chemical bond, occurring on a minimum-free, purely repulsive potential energy surface, has recently been convincingly shown to be possible on the basis of high-level quantum-chemical calculations. This type of bond, termed a vibrational bond, forms because the total energy, including the dynamical energy of the nuclei, is lower than the total energy of the dissociated products, including their vibrational zero-point energy. For this to be the case, the ZPE of the product molecule must be very high, which is ensured by replacing a conventional hydrogen atom with its light isotope muonium (Mu, mass = 1/9 u) in the system Br – H – Br, a natural transition state in the reaction between Br and HBr. A paramagnetic species observed in the reaction Mu + Br2 has been proposed as a first experimental sighting of this species, but definitive identification remains challenging.

Puzzles in Bonding and Spectroscopy: The Case of Dicarbon

The unstable molecule C2 has been of interest since its identification as the source of the “Swan band” features observable in the spectra of flames, carbon arcs, white dwarf stars, and comets, and it continues to serve as a focal point for experimental and theoretical discovery. Recent spectroscopic work has identified a quintet state of the molecule for the first time, while new insights into the bond order of C2 in its ground state have been provided by sophisticated computational methods based on valence bond theory. This article gives a review of spectroscopic and computational work on C2 including both historical background and the most recent discoveries.

Order–disorder transition in anthracene/tetracyanobenzene probed by muonium-substituted radicals

Abstract ALC- μSR measurements on polycrystalline anthracene / tetracyanobenzene (A / TCNB) from 25 K to above room temperature reveal Δ M=1 transitions corresponding to several adducts of anthracene; the strongest of these show temperature-dependent behaviour related to orientational order and split at the phase transition temperature. The complexity of the lineshapes and the existence of the splitting indicate that intermolecular effects are important in determining the isotropic and anisotropic components of A μ in solid state systems of this kind. Parallel orientation-dependent TFμSR investigations on a single-crystal sample confirm that the main ALC signals observed correspond to separate radicals and reveal a room-temperature hyperfine tensor in which the molecular-frame anisotropy is partially averaged out. Ab initio calculations using Hartree–Fock and density functional methods are used to assign the radical signals.

The first observation of a muonium–carbonyl adduct with a negative muon coupling constant

Muonium atom addition to cyclopent-4-ene-1,3-dione I leads to the adduct radical II; in contrast with all other muonium adducts of carbonyl (CO) compounds so far studied, in which the isotropic muon coupling is of positive sign, the temperature dependence of the muon coupling in II reveals that its sign is negative; this may be explained as a consequence of the electronic ‘push–pull’ interaction within the MuO–C–CC–CO system, which reduces the out-of-plane vibrational amplitude of the muon by increasing the MuO–C/MuOC partial π-bond character.