In a feat of modern-day alchemy, atom tinkerers have fooled hydrogen atoms into accepting a helium atom as one of their own. The camouflaged atom behaves chemically like hydrogen, but has four times the mass of normal hydrogen, allowing predictions for how atomic mass affects reaction rates to be put to the test.
A helium atom consists of a nucleus containing two positively charged protons and two neutrons, encircled by two orbiting electrons which carry a negative charge. A hydrogen atom has just one proton and one electron. Donald Fleming of the University of British Columbia in Vancouver, Canada, and colleagues managed to disguise a helium atom as a hydrogen atom by replacing one of its orbiting electrons with a muon, which is far heavier than an electron.
Because it is so heavy, the muon sits 200 times closer to the helium nucleus than the electron it replaces and cancels out one of the nucleus's positive charges. The remaining electron then behaves as if it were orbiting a nucleus with just one positive charge, just like the electron in a hydrogen atom. The difference is that the nucleus is 4.1 times heavier than normal.
Fleming and his colleagues used this "super-heavy hydrogen", to test the effects of mass on chemical reaction rates. A lone hydrogen atom can form a new hydrogen molecule by stealing one of the two atoms from a pre-existing hydrogen molecule – but for that to happen there has to be enough energy available to break the bond holding the existing molecule together.
According to quantum mechanics, it is not always necessary to climb over this energy barrier: instead, particles can "tunnel" through it. But the heavier the particles are, the harder it is to tunnel, so the less frequently the partner-swapping reaction takes place.
Two hydrogen isotopes, containing one or two neutrons and with two or three times the mass of normal hydrogen, can be used to test this. An even heavier isotope, with three neutrons, exists but decays too quickly. Muonic helium, which has the same mass as this isotope, lasts long enough to react with a hydrogen molecule.
Fleming's team shot muons produced at the TRIUMF accelerator in Vancouver into a cloud of helium, molecular hydrogen and ammonia. The helium atoms captured the muons, then pulled hydrogen atoms away from the molecular hydrogen and bonded with them.
The team compared how long this took with the rate of the same reaction using normal hydrogen, and with a reaction rate recorded in 1987 when a type of ultra-light hydrogen, called muonium, was used. Chemists formed this by replacing the proton in a hydrogen atom with an antimuon, the muon's positively charged antimatter partner.
As expected, the reaction with the disguised helium was the slowest, followed by normal hydrogen, then the light hydrogen. The rates perfectly matched predictions from quantum mechanical calculations led by Fleming's teammate Donald Truhlar of the University of Minnesota in Minneapolis.
The way any physical system changes with time can, in theory, be predicted from the quantum states of its particles. Most reactions involve far too many particles for this to be practical, but Truhlar says the hydrogen reaction was just simple enough.
Millard Alexander of the University of Maryland in College Park calls it "very sexy nuclear chemistry".
Journal reference: Science, DOI: 10.1126/science.1199421
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