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Atoms found to interact unexpectedly

July 2, 2008
World Science staff

A new­found type of in­ter­ac­tion be­tween at­oms may change our un­der­stand­ing of cer­tain chem­i­cal re­ac­tions in the at­mos­phere and in our bod­ies, sci­en­tists say.

Im­ag­ine a sim­ple mol­e­cule con­sist­ing of two at­oms as be­ing like two balls linked by a spring. If an at­om strikes one side of the mol­e­cule, the spring com­presses and you’d ex­pect the mol­e­cule to jump back­wards.

A still from an anima­tion of atom­ic scat­ter­ing. in the new study (Cour­tesy Eck­art Wrede, Uni­ver­s­ity of Dur­ham)  
How­ev­er, the new re­search sug­gests that sur­pris­ing­ly, in cer­tain con­di­tions the mol­e­cule jumps for­wards, not back­wards. 

The re­ac­tion amounts to a new form of ener­gy-trans­fer, ac­cord­ing to the in­ves­ti­ga­tors, who re­ported the find­ings in the July 3 is­sue of the re­search jour­nal Na­ture.

They stud­ied fast hy­dro­gen at­oms col­lid­ing with cooled mol­e­cules built of deu­ter­i­um, a var­i­ant of hy­dro­gen. When the col­li­sion does not re­sult in a chem­i­cal re­ac­tion, the hy­dro­gen at­oms scat­ter. 

In these so-called in­e­las­tic pro­cesses, the hy­dro­gen at­om nor­mally scat­ters back­wards. But in this case, the team found that the pro­cess un­ex­pectedly led mainly to for­ward scat­tering.

“The re­ac­tion un­der study is the sim­plest chem­i­cal re­ac­tion pos­si­ble and yet it still con­tin­ues to sur­prise us, even af­ter 80 years” of anal­y­sis, said chem­ist Stu­art Greaves at the Uni­ver­s­ity of Bris­tol, U.K., a co-author of the re­port. “Our work pro­vides an­oth­er vi­tal piece of the jig­saw for un­der­stand­ing the me­chan­ics of chem­i­cal re­ac­tions, such as those go­ing on in the at­mos­phere.”

The find­ings have “changed a very sim­ple idea that we cher­ished”—that to make a mol­e­cule vi­brate strongly, “you bas­ic­ally had to crush it, squeeze it, hit it over the head. Com­press some bond and the mol­e­cule would snap back,” said Rich­ard Zare, a chem­ist at Stan­ford Uni­ver­s­ity in Cal­i­for­nia who led the re­search. “We found quite the op­posite.”

The ex­plana­t­ion of the events is that even if the hy­dro­gen at­om flies past the mol­e­cule—which con­sists of two deu­ter­i­um at­oms—in a “graz­ing col­li­sion,” this can tug on the deu­ter­i­um at­om near­est to it, Greaves said. This stretches the bond link­ing the two deu­ter­i­um at­oms, caus­ing the mol­e­cule to move for­wards.

The au­thors sug­gest that this “tug-of-war” be­hav­iour may come in­to play when­ev­er a strong at­trac­tion de­vel­ops be­tween the col­lid­ing part­ners, just as Moon’s gravita­t­ion “pulls” at wa­ter on Earth.

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A newfound type of interaction between atoms may change our understanding of certain chemical reactions in the atmosphere and in our bodies, scientists say. Imagine a simple molecule consisting of two atoms as being like two balls linked by a spring. If an atom strikes one side of the molecule, the spring compresses and you’d expect the molecule to jump backwards. However, the new research suggests that surprisingly, in certain conditions the molecule jumps forwards, not backwards. The reaction amounts to a new form of energy-transfer, according to the investigators, who reported the findings in the July 3 issue of the research journal Nature. They studied fast hydrogen atoms colliding with cooled molecules built of deuterium, a variant of hydrogen. When the collision does not result in a chemical reaction, the hydrogen atoms scatter. In these so-called inelastic processes, the hydrogen atom normally scatters backwards. But in this case, the team found that the process unexpectedly led mainly to forward scattering. “The reaction under study is the simplest chemical reaction possible and yet it still continues to surprise us, even after 80 years” of analysis, said chemist Stuart Greaves at the University of Bristol, U.K., a co-author of the report. “Our work provides another vital piece of the jigsaw for understanding the mechanics of chemical reactions, such as those going on in the atmosphere.” The findings have “changed a very simple idea that we cherished”—that to make a molecule vibrate strongly, “you basically had to crush it, squeeze it, hit it over the head. Compress some bond and the molecule would snap back,” said Richard Zare, a chemist at Stanford University in California who led the research. “We found quite the opposite.” The explanation of the events is that even if the hydrogen atom flies past the molecule—which consists of two deuterium atoms—in a “grazing collision,” this can tug on the deuterium atom nearest to it, Greaves said. This stretches the bond linking the two deuterium atoms, causing the molecule to move forwards. The authors suggest that this “tug-of-war” behaviour may come into play whenever a strong attraction develops between the colliding partners, just as Moon’s gravitation “pulls” at the water on Earth.