"Long before it's in the papers"
January 27, 2015


“Quantum chemistry” a new window into lives of molecules

Feb. 12, 2010
Courtesy NIST
and World Science staff

Phys­i­cists have de­tected mol­e­cules in­ter­act­ing with each oth­er near the cold­est pos­si­ble tem­per­a­ture, where by the “clas­si­cal” laws of phys­ics they should be mo­tion­less, ac­cord­ing to a re­port.

The chem­i­cal re­ac­tions are ex­plained by the bi­zarre rules of quan­tum me­chan­ics, which gov­ern mat­ter at the sub-microscopic lev­el. These rules al­low par­t­i­cles to go in­to places where they don’t have enough en­er­gy to go, by very briefly “bor­row­ing” en­er­gy seem­ingly from no­where. Quan­tum me­chan­ics al­so re­quires par­t­i­cles to be some­times con­sid­ered as waves rath­er than as sol­id grains. 

A map of the density of a mo­lec­u­lar gas in which each mol­e­cule is in its low­est pos­si­ble en­er­gy state. The gas has just been re­leased from a trap cre­at­ed by la­sers. The mol­e­cules are near ab­so­lute ze­ro, a tem­per­a­ture at which quan­tum prop­er­ties reign. The im­age -- made by de­tect­ing the ab­sorp­tion of la­ser light by the mol­e­cules -- re­veals their spa­tial dis­tri­bu­tion, with den­si­ty in­di­cat­ed by peak height and false col­or. (Cred­it: D. Wang/JI­LA).

Phys­i­cists don’t know what it is about at­oms that makes them man­i­fest them­selves in such bi­zarre ways, but none­the­less, based on many dec­ades of ex­pe­ri­ments, they do. The more sen­si­ble “clas­si­cal” laws of phys­ics, which reigned in sci­ence un­til the 20th cen­tu­ry, apply only to more ordinary-sized ob­jects. 

The new find­ings, de­scribed in the Feb. 12 is­sue of the re­search jour­nal Sci­ence, will help sci­en­tists un­der­stand pre­vi­ously un­known as­pects of how mol­e­cules in­ter­act, a key to ad­vanc­ing bi­ol­o­gy, cre­at­ing new ma­te­ri­als, pro­duc­ing en­er­gy and oth­er re­search ar­eas, phys­i­cists said.

The work has prac­ti­cal ap­plica­t­ions for chem­ists in that it shows chem­i­cal re­ac­tion rates can be con­trolled us­ing quan­tum me­chan­ics, added the re­search­ers, from JILA, a joint in­sti­tute of the U.S. Na­tional In­sti­tute of Stan­dards and Tech­nol­o­gy and the Uni­vers­ity of Col­o­rad­o at Boul­der.

It’s “rea­son­a­ble to ex­pect that when you go to the ul­tra­cold re­gime there would be no chem­is­try to speak of,” be­cause at­oms should be mo­tion­less, said phys­i­cist Deb­o­rah Jin, lead­er of one JILA group in­volved in the ex­pe­ri­ments. But “this pa­per says no, there’s a lot of chem­is­try go­ing on.”

The ex­pe­ri­ments ex­am­ined mol­e­cules near ab­so­lute ze­ro, the the­o­ret­ic­ally low­est pos­si­ble tem­per­a­ture, in which mol­e­cules would have no en­er­gy to move. Ab­so­lute-ze­ro con­di­tions can’t be cre­at­ed in prac­tice, but phys­i­cists can get there to with­in a ti­ny frac­tion of a de­gree. Ab­so­lute ze­ro lies at mi­nus 273 de­grees Cel­si­us, mi­nus 460 de­grees Fahr­en­heit, or ze­ro on the so-called Kel­vin scale.

Sci­en­tists have long known how to con­trol mol­e­cules’ in­ter­nal states, such as the en­er­gies that de­ter­mine their rota­t­ion and vibra­t­ion. In ad­di­tion, the field of quan­tum chem­is­try has ex­isted for dec­ades to study the ef­fects of the quan­tum be­hav­ior of sub­a­tom­ic par­t­i­cles, com­po­nents of mol­e­cules.

But un­til now sci­en­tists have been un­able to ob­serve di­rect con­se­quenc­es of quan­tum me­chan­i­cal mo­tions of whole mol­e­cules on the chem­i­cal re­ac­tion pro­cess, ac­cord­ing to the sci­en­tists. Cre­at­ing sim­ple mol­e­cules and chilling them al­most to a stand­still makes this pos­si­ble by pre­sent­ing a sim­pler and more plac­id en­vi­ron­ment that can re­veal sub­tle, pre­vi­ously un­ob­served chem­i­cal events, ac­cord­ing to Jin and col­leagues.

By pre­cisely con­trolling the mol­e­cules’ in­ter­nal states while al­so con­trolling the mo­lec­u­lar mo­tions at the quan­tum lev­el, sci­en­tists al­so say they can study how the mol­e­cules scat­ter or in­ter­act with each oth­er quan­tum me­chan­ic­ally. 

The phys­i­cists could ob­serve, they ex­plained, how the quan­tum ef­fects of the mol­e­cule as a whole dic­tate re­ac­ti­vity. This win­dow in­to mo­lec­u­lar be­hav­ior has al­lowed the ob­serva­t­ion of long-range in­ter­actions in which quan­tum me­chan­ics de­ter­mines wheth­er two mol­e­cules should come to­geth­er to re­act or stay apart. Thus the work ex­pands the stand­ard con­cep­tion of chem­is­try, ac­cord­ing to the group.

The ex­pe­ri­ments are done with a gas con­tain­ing up to a tril­lion mol­e­cules with­in a space the size of a small die. Each mol­e­cule con­sists of one po­tas­si­um at­om and one ru­bid­i­um at­om. The mol­e­cules have a neg­a­tive elec­tric charge on the po­tas­si­um side and a pos­i­tive charge on the ru­bid­i­um side, so they can be con­trolled with elec­tric fields. 

By meas­ur­ing how many mol­e­cules are lost over time from a gas con­fined by lasers with­in a trap, at dif­fer­ent tem­per­a­tures and un­der var­i­ous oth­er con­di­tions, the team found ev­i­dence of heat-pro­duc­ing chem­i­cal re­ac­tions in which the mol­e­cules must have swapped at­oms, bro­ken chem­i­cal bonds, and forged new bonds. The­o­ret­i­cal cal­cula­t­ions of long-range quan­tum ef­fects agree with the ob­serva­t­ions, they not­ed.

In con­ven­tion­al chem­is­try at room tem­per­a­ture, mol­e­cules can col­lide and re­act to form dif­fer­ent com­pounds, re­leas­ing heat. 

In the ul­tra­cold ex­pe­ri­ments, quan­tum me­chan­ics reigns and the mol­e­cules spread out as ethe­real rip­pling waves in­stead of act­ing like sol­id grains. They don’t col­lide in the con­ven­tion­al sense. Rath­er, as their wave as­pects overlap, the mol­e­cules “sense” each oth­er from as much as 100 times far­ther apart than would be ex­pected normal­ly. At this dis­tance the mol­e­cules ei­ther scat­ter from one anoth­er or, if con­di­tions are right, swap at­oms. Sci­en­tists ex­pect to be able to con­trol long-range in­ter­actions by cre­at­ing mol­e­cules with spe­cif­ic in­ter­nal states and “tun­ing” their re­ac­tion en­er­gies with elec­tric and mag­net­ic fields.

The JILA team pro­duced a dense mo­lec­u­lar gas and found that, al­though mol­e­cules move slowly in the cold, re­ac­tions can oc­cur quick­ly. But they can be sup­pressed us­ing quan­tum me­chan­ics. For in­stance, a cloud of mol­e­cules in the low­est-en­er­gy elec­tron­ic, vibra­t­ional and rota­t­ional states re­acts dif­fer­ently if the nu­clear “spins” of some mol­e­cules are op­po­site. If the mol­e­cules are di­vid­ed 50/50 in­to two dif­fer­ent nu­clear spin states, re­ac­tions pro­ceed up to 100 times faster than if all mol­e­cules have the same spin. Thus, by pre­par­ing all mol­e­cules in the same spin state, sci­en­tists can de­lib­er­ately sup­press re­ac­tions.

The ex­pe­ri­men­tal team at­tributes these re­sults to the fact the mol­e­cules are fermions, one of two types of quan­tum par­t­i­cles found in na­ture. (Bosons are the sec­ond type.) Two iden­ti­cal fermions can’t be in the same place at the same time. This quan­tum be­hav­ior of fermions man­i­fests itself as a sup­pression of the chem­i­cal re­ac­tion rate in the ul­tralow tem­per­a­ture gas. That is, mol­e­cules with iden­ti­cal nu­clear spins are less likely to ap­proach each oth­er and re­act than are par­t­i­cles with op­po­site spins.

“We are ob­serving a new fun­da­men­tal as­pect of chem­is­try—it gives us a new ‘knob’ to un­der­stand and con­trol re­ac­tions,” said in­sti­tute phys­i­cist Jun Ye, lead­er of a sec­ond group in­volved in the re­search.

* * *

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Physicists detected molecules interacting with each other near the coldest possible temperature, where by the “classical” laws of physics they should be motionless, according to a report. The chemical reactions are explained by the bizarre rules of quantum mechanics, which govern matter at the sub-microscopic level. These rules allow particles to go into places where they don’t have enough energy to go, by very briefly “borrowing” energy seemingly from nowhere. Quantum mechanics also requires particles to be sometimes considered as waves rather than as solid grains. Physicists don’t know what it is about atoms that makes them manifest themselves in such bizarre ways, but nonetheless, based on many decades of experiments, they do. The more sensible “classical” laws of physics, which reigned in science until the 20th century, apply only to more ordinary-sized objects. The new findings, described in the Feb. 12 issue of the research journal Science, will help scientists understand previously unknown aspects of how molecules interact, a key to advancing biology, creating new materials, producing energy and other research areas, physicists said. The work has practical applications for chemists in that it shows chemical reaction rates can be controlled using quantum mechanics, said the researchers, from JILA, a joint institute of the National Institute of Standards and Technology and the University of Colorado at Boulder. “It’s perfectly reasonable to expect that when you go to the ultracold regime there would be no chemistry to speak of,” because atoms should be motionless, said institute physicist Deborah Jin, leader of one JILA group involved in the experiments. But “this paper said no, there’s a lot of chemistry going on.” The experiments examined molecules near a temperature called absolute zero, the theoretically lowest possible temperature, in which molecules would have no energy to move. Absolute-zero conditions can’t be created in practice, but physicists can get there to within a tiny fraction of a degree. Absolute zero lies at minus 273 degrees Celsius, minus 460 degrees Fahrenheit, or zero on the so-called Kelvin scale. Scientists have long known how to control molecules’ internal states, such as the energies that determine their rotation and vibration. In addition, the field of quantum chemistry has existed for decades to study the effects of the quantum behavior of subatomic particles, components of molecules. But until now scientists have been unable to observe direct consequences of quantum mechanical motions of whole molecules on the chemical reaction process, according to the scientists. Creating simple molecules and chilling them almost to a standstill makes this possible by presenting a simpler and more placid environment that can reveal subtle, previously unobserved chemical events, according to Jin and colleagues. By precisely controlling the molecules’ internal states while also controlling the molecular motions at the quantum level, scientists also say they can study how the molecules scatter or interact with each other quantum mechanically. The physicists could observe, they explained, how the quantum effects of the molecule as a whole dictate reactivity. This window into molecular behavior has allowed the observation of long-range interactions in which quantum mechanics determines whether two molecules should come together to react or stay apart. Thus the work expands the standard conception of chemistry, according to the group. The quantum chemistry experiments are performed with a gas containing up to a trillion molecules within a space the size of a small die. Each molecule consists of one potassium atom and one rubidium atom. The molecules have a negative electric charge on the potassium side and a positive charge on the rubidium side, so they can be controlled with electric fields. By measuring how many molecules are lost over time from a gas confined within a trap using lasers, at different temperatures and under various other conditions, the team found evidence of heat-producing chemical reactions in which the molecules must have swapped atoms, broken chemical bonds, and forged new bonds. Theoretical calculations of long-range quantum effects agree with the observations, they noted. In conventional chemistry at room temperature, molecules can collide and react to form different compounds, releasing heat. In the ultracold experiments, quantum mechanics reigns and the molecules spread out as ethereal rippling waves instead of acting like solid grains. They don’t collide in the conventional sense. Rather, as their wave aspects overlap, the molecules “sense” each other from as much as 100 times farther apart than would be expected normally. At this distance the molecules either scatter from one another or, if conditions are right, swap atoms. Scientists expect to be able to control long-range interactions by creating molecules with specific internal states and “tuning” their reaction energies with electric and magnetic fields. The JILA team produced a dense molecular gas and found that, although molecules move slowly in the cold, reactions can occur quickly. But they can be suppressed using quantum mechanics. For instance, a cloud of molecules in the lowest-energy electronic, vibrational and rotational states reacts differently if the nuclear “spins” of some molecules are opposite. If the molecules are divided 50/50 into two different nuclear spin states, reactions proceed up to 100 times faster than if all molecules have the same spin. Thus, by preparing all molecules in the same spin state, scientists can deliberately suppress reactions. The experimental team attributes these results to the fact the molecules are fermions, one of two types of quantum particles found in nature. (Bosons are the second type.) Two identical fermions can’t be in the same place at the same time. This quantum behavior of fermions manifests as a suppression of the chemical reaction rate in the ultralow temperature gas. That is, molecules with identical nuclear spins are less likely to approach each other and react than are particles with opposite spins. “We are observing a new fundamental aspect of chemistry—it gives us a new ‘knob’ to understand and control reactions,” said institute physicist Jun Ye, leader of a second group involved in the research.