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COVER STORY

CHEMICAL BONDINGis the heart ofchemistry, according to Alexander I. Bold-yrev, a chemistry professor at Utah StateUniversity who spends his time thinkingabout how molecules are put together.But it is still not well-defined. We need

to develop the breadth of the concept andmake it as rigorous as we can, yet as simpleas we can, so it is more powerfuland more useful.

Boldyrev put thatchallenge to a group ofsome 50 of the worldsleading experts onchemical bonding.The occasion was theopening session of theInternational Conferenceon Chemical Bonding, whichwas held on the Hawaiian is-

land of Kauai during thefinal week of July. Its ameeting Boldyrev helpedstart last year to bringtheoreticians, spec-troscopists, and experi-mentalists together toforget business as usualand think about branchingfurther out into underexploredfields in bonding such as biochemis-try and materials science.

Chemists have developed manybonding models over the years in

trying to comprehend the tusslebetween attractive and repulsive forc-es that hold atoms together in molecules.These models help researchers visualizechanges in atomic and molecular orbitalsthat give rise to the many types of bonds.

Armed with knowledge of bond energiesand other parameters, chemists can betterorchestrate chemical reactions, determinehow biomolecules respond to myriadchemical signals in the body, and monitormaterials to see how they answer to exter-

nal forces such as light or an applied cur-rent or magnetic field.

The end goal, Boldyrev emphasized, isthe ability to better understand chemi-cal behavior and advance the design and

synthesis of chemicals and materials.Wasting no time, Frank A. Weinhold of

the University of Wis-consin, Madison, led

off the conferenceby asking an often-repeated question:What is a hydrogen

bond? Its a ques-tion fraught with

controversy that Wein-hold nevertheless feelscompelled to pursue.

H-bonds give waterits room-temperatureliquid properties,lower or raise energy

barriers and thusspeed or slow re-actions, and hold

together the three-dimensional shapes of

DNA, proteins, and syn-thetic materials such as some typesof polymers. Better understanding of

these weak interactions could enablechemistsin particular those in

pharmaceutical researchto useH-bonding as a catalyst design tool

or to design drugs for better inter-actions with their targets.

But current chemical bonding descrip-tions for H-bonds leave a lot to be desired,Weinhold believes. The dipole-dipoleconcepts that fill current textbooks and areused for molecular dynamics simulations

PUSHING THE LIMITS OF

CHEMICAL BONDINGConference highlights how computational and experimental chemistsare applying BONDING MODELSto more complex systems

STEPHEN K. RITTER, C&EN WASHINGTON

MORE ONLINEHow would you explain a bond? Check out bonding

specialists descriptions at http://cenm.ag/explain.

NANOWHEEL Chemists usedthe AdNDP theoretical model tocalculate the bonding orbitals forTaB10

(left), an observed gas-phaseplanar cluster.

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of H-bonding are inadequate, he said. Still,just mentioning H-bonding raises the dan-der of many bonding specialists who cantagree on whether it is a true bonding phe-

nomenon or not. Weinhold would like toresolve the issue once and for all.

An H-bond is typically describedas a weak electrostatic attractionbetween an electronegative

atom or a polar molecule anda hydrogen atom that is cova-lently bonded to an electronega-tive atom, typically oxygen or

nitrogen. The interaction is notstrong enough to be considered

a conventional covalent bond, butsome scientists contend that bonding

models as well as spectroscopic evi-dence indicate that H-bonds must containsome covalent character. Others think it is

purely an electrostatic interactionandnot a bond at all.

To address this disparity, in 2011 a com-mittee of the International Union of Pure& Applied Chemistry proposed a broaderdefinition of H-bonding along with a setof criteria that can be used as evidencefor H-bonds. And last year, researchersused atomic force microscopy to capturestunning images of H-bonding for the firsttime. The wispy lines of electron de nsitybetween 8-hydroxyquinoline molecules ona copper surface lend credence to a higherlevel of bonding.

For his part, Weinhold created a modelfor a surprising class of H-bonded complex-es between ions of like charge, somethingso unthinkable that people havent lookedfor them before. These anion-anion andcation-cation interactions exhibit the char-acteristics of H-bonding despite the power-ful opposing electrostatic forces, Weinholdsaid. If H-bonds still occur in these com-plexes, he reasoned, there must be sometype of covalent interaction involved.

Weinhold used the Natural Bond Or-bital analysis method he developed toanalyze potential energy curves for the

ion-ion interactions. Natural bond orbit-

als are designed orbitals with propertiesintermediate to the atomic orbital proper-ties of atoms and the combined molecularorbitals of molecules. They are used to cal-culate the distribution of electron densityand spot bonds in molecules.

To test his idea, Weinhold calculatedthe interactions between fluoride (F) andbicarbonate (HOCO2

). It seems improb-able, he said, but short-range resonance-type covalent charge-transfer interactionsbetween the anions overcome the long-range electrostatic opposition expectedbetween ions of like charge. The small netbond energy observed signifies that thereis a bonding interaction holding the atomstogether. And the complex formed satis-fies all the accepted criteria for authenticH-bonding, Weinhold said. Cation-cationH-bonded complexes, such as hydronium

ion with protonated aminomethanol, arealso feasible.

THESE ANTIELECTROSTATICH-bondcomplexes may finally put to rest thesuperficial quasiclassical conceptionsof H-bonding and other resonance-typephenomena that have too long held swayin the molecular and supramolecular sci-ences, Weinhold concluded.

Weinhold identifies, likely correctly,that H-bonds can exist between anions,noted Martin P. Head-Gordon of the Uni-versity of California, Berkeley, who also

studies H-bonding. The H-bonded dianioncomplexes are not the overall (global)energy minimum, which would representinfinitely separated anions, Head-Gordonnoted. But they do represent a local energyminimum that is trapped from reachingcomplete disso-ciation by a largeenough barrier sothat the H-bondedcomplexes couldpossibly be observedexperimentally.

But Head-Gor-

don took friendly

BONDING NUGGETS

These digests highlight a selection of the

research presented during the Interna-

tional Conference on Chemical Bonding.

C2 Stays SingleThom H. Dunning Jr.

Pacific Northwest National Laboratory

Dunning related how generalized valence

bond theory, which draws on properties

of both atomic orbitals and molecular

orbitals, provides a simple yet richer

description of bonding than molecular or-

bital theory alone for diatomic molecules

such as C2, N2, and P2 and hypervalent

molecules such as PF5 and SF6. In C2, for

example, Dunning explained that bond-

ing might best be viewed as only a single

bond, rather than involving multiple

bonding as previous models have sug-

gested. The remaining six valence elec-

trons reside in the leftover carbon orbitals

in a high-spin coupled state.

B40 FullereneLai-Sheng Wang

Brown University

Wang reviewed the

progress his group has

made using computa-

tional studies supported

by photoelectron spec-

troscopy to analyze gas-

phase all-boron clusters.Molecules with up to 24 boron atoms form

planar species, after which larger mol-

ecules start forming 3-D structures. For

example, Wang and his colleagues recently

discovered B40, an unprecedented all-

boron fullerene-like cage molecule, which

is a potential semiconducting material.

Two-Prong AnalysisHelmut Schwarz

Technical University of Berlin

By combining computational chemistry

and mass spectrometry, Schwarz has

been breaking new ground in understand-

ing the activation sites of heterogeneouscatalysts in gas-phase reactions. Mass

spectrometry on isolated species pro-

vides a means of probing the energetics

and kinetics of reactions in an unper-

turbed environment, which is typically

not possible in solution-phase reactions.

The approach allowed Schwarz to study

the role of electron density in intermedi-

ates of metal-mediated CC bond forma-

NEGATIVE H-BOND

These calculated2-D contour and 3-Doverlap diagramsdepict unexpectedFHOCO2

antielectrostatic

hydrogen bonding.

FRANKWEINHOLD

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12CEN.ACS.ORG SEPTEMBER 22, 2014

exception to Weinholds argu-ment that the possible exis-tence of these ionic H-bondsshows the essential irrel-evance of electrostatic con-tributions to H-bonding.He also disagreed that Wein-holds results should lead toskeptical reassessmentof energy decompositionanalysis of H-bonding, amethod Head-Gordonuses that offers partialsupport to the electro-static view.

The fact that likecharges repel explainsthe overall minimumwhere there is no H-bond,Head-Gordon added. But in my view

it does not necessarily mean electrostat-ics are inessential. In fact, good energydecomposition analysis methods, whichsplit complex bonding interactions upinto permanent and induced electro-statics, dispersion, and charge-transfercontributions, can provide support foror contradict Weinholds assertion aboutelectrostatics, Head-Gordon told C&EN.

Weinholds view that electrostatics arealso not relevant for types of H-bondingbeyond those found in ion-ion interac-tions is still open for debate, Head-Gor-don suggested. On the basis of his own

research, Head-Gordon thinks H-bondingtypically has contributions from bothcharge transfer and electrostatics, withthe relative importance of each varying,depending on details of the system. Itwill be fascinating to see where this goesover the next few years, he said.

As debate about H-bonding continuedoff-line, other chemists took the floor todiscuss advancing the synergy betweenelectronic structure calculations andstudying the properties of catalytic sur-faces and solid-state materials.

Chemical bonding theory tradition-

ally has been in the realm of molecularchemistry, especially organic chemistry,conference cofounder Anastassia N. Al-exandrova of UCLA told C&EN. But that

topic is perhaps considered ar-chaic and long since done.

Meanwhile, materials scien-tists have typically used eithermacroscopic qualitative descrip-tors or nondescriptive electronicband structure concepts and notbenefited from the rationale and

design principles that arisefrom traditional bonding the-

ory, Alexandrova explained.

Applying bonding theory tosolid-state materials has been underex-plored. But even as capable models have be-come available, they are not garnering muchattention. Breaking this stagnation is oneof the conferences goals, she said.

To set an example, Alexandrovas groupis collaborating with Mark Saeys at GhentUniversity, in Belgium, to study the chem-istry of unexpected square-planar carbonspecies that form on cobalt catalyst sur-faces treated with synthesis gas. The tet-racoordinated carbon takes on the square-planar geometry as opposed to the typical

tetrahedral structure, and the aromaticityof Co4C building blocks drives formationof nanocluster islands on the catalyst sur-face that facilitate the reaction.

In industrial reactions, catalyst surfacesoften undergo massive reconstructionsas the catalytic sites form and re-form viaself-organization. Saeys and Alexandrova,in partnership with Shell, are now able tounderstand the unusual bonding leading tosurface reconstruction of commercial co-balt catalysts. This bonding insight couldlead to improved catalyst stability and se-lectivity in the Fischer-Tropsch synthesis

of fuels and chemicals.In another example of analyzing struc-

ture-property relationships in materials,Mark E. Eberhart of Colorado School ofMines described the versatility of charge-density analysis based on an extendedform of the Bader Atoms in Moleculestheory of molecular structure.

Through calculations, Eberhart is ableto glue together the electronic parametersof sets of atoms into an extended frame-work and create charge-density maps. The

COVER STORY

tion and oxygen-centered radicals in CH

activation.

Phase-Switch BondingMatthias Wuttig

RWTH Aachen University, Germany

Phase-change materials such as

Ge2Sb2Te5 rapidly switch between amor-

phous and crystalline states, changing

their optical and electrical properties

and making them useful for optical data

storage. Wuttig showed that the crystal-

line state is characterized by resonant,

directional covalent bonding rather than

delocalized bonding as in the amorphous

state. This information can be used to

create phase diagram treasure maps

for selecting alloy compositions in the de-

sign of new phase-change materials.

Sturdy MicroalloysWilliam A. Goddard

California Institute of Technology

By studying the multicentered bonding

in borons many polymorphs, Goddard

has been able to design boron alloys with

carbon, oxygen, and other elements. For

example, B4C is a known material that is

very hard. But its also very brittle, which

prevents its use in engineering applica-

tions. Goddard described how his team

has used computational methods to ex-

plore the mechanism of the structural de-formation that leads to B4Cs brittleness.

The researchers are designing microalloys

with improved properties by further add-

ing hydrogen, silicon, or nitrogen atoms,

with silicon looking the most promising.

Whats the Highest Oxidation State?Jun Li

Tsinghua University, China

Oxidation state is a central concept in

chemistry, and the question of highest

possible oxidation state has long been

a controversial topic. For elements in

neutral compounds, the highest state is

thought to be 8+, which occurs in sometransition-metal compounds, such as

RuO4. Li showed through

new calculations that, de-

spite their f electron shell,

such high oxidation states

are not stable in actinides.

Because of mass-driven

relativistic effects leading

to contracted 5f electron

BOND BUNDLES

This charge-densitymap depicts across section of the

interface betweeniron (red atoms)

and a metal-carbide

additive (green andblack) in steel. The redand yellow regions inthe middle are the most

important in controllinginterfacial strength.

Alloying elements thatdecrease the number

of electrons in the redregion improve the steels

fracture resistance.

INORG.

CHEM.

BONDING NUGGETS CONTINUED

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topology of these maps provides che micalinformation, with peaks (atoms), valleys(nonbonding areas), and ridges (bondingareas) illustrating where bonds exist andhow reactive these bonds might be.

To demonstrate how the process works,Eberhart described an analysis of high-strength steel alloyed with dispersions oftitanium carbide to determine the pro-pensity of metal-carbide interfaces in thematerial to fracture and fail. The topolo-gies enable Eberharts team to determineoptimum positions in the materials latticewhere inserting other metals such as nickelwould alter the number of electrons inspecific regions and thereby allow themto improve the adhesive properties of thealloys. This capability will be particularlybeneficial in solids, Eberhart said, wherethe nature of charge-density evolution as-

sociated with even simple processes, suchas fracturing, is understudied.

YET ANOTHERbonding model discussedat the conference is Adaptive NaturalDensity Partitioning (AdNDP), which wasdeveloped by Utah States Boldyrev andhis colleagues. An extension of WeinholdsNatural Bond Orbital approach, AdNDPis geared toward preserving the classicalLewis two-center, two-electron bondingpicture as much as possible. But it relies ex-tensively on multicentered, two-electrondelocalized bonding.

When analyzing a molecule, the re-searchers first localize electron pairs intwo-center bonds or as lone pairs on singleatoms. They then place the remaining elec-tron pairs in multicentered bonds localizedon the fewest atoms possible, which couldbe as many as all the atoms in the system.In doing so, the method invokes both and aromatic and antiaromatic bonding.

For me, a chemical bond is no longer anelectron pair sitting between two atoms,Boldyrev said. It is more simply a two-electron blob that can connect two, three,five, or more atoms. We allow the pairs to

run over whatever part of the moleculethey want to occupy.

Beyond common molecules, this ap-proach is proving useful for determiningthe bonding in metal clusters and mo-lecular systems that are too complex fortraditional bonding models. For example,Boldyrevs team has used AdNDP to obtainbonding patterns in all-boron naphtha-lene (B16

2), inorganic complexes such asTc3Cl9

2, and gold nanoclusters. Boldyrevsgroup has also developed an AdNDP ver-

sion for extended molecular structuresin bulk materials and used it to analyzethe bonding in graphene, all-boron planarsheets, MgB2with alternating magnesiumand boron layers, and intercalated inor-ganic materials such as Na8BaSn6.

I believe this is a method that eventuallywill enable us to draw structures of solids byhand based on their formula, as we do nowfor organic molecules, UCLAs Alexandro-va speculated. From there, we might even-tually arrive to the intuitiveinterpretation of chemicalreactivity and other proper-ties of solid materials.

That is exactly whereBoldyrev hopes the methodwill lead. Like physicistswho want a unified theoryto explain all matter, we

chemists would like a uni-fied bonding theory that works seamlesslyfor solids, gases, and solutions and helpsexplain the behavior of organic and in-organic compounds, biomolecules, andnanomaterials, Boldyrev said.

Chemists currently must solve struc-tural problems for different types of com-pounds by sifting through and choosingamong different models, he explained.The Lewis electron-pair model is simple,easy to understand, and teach, Boldyrevsaid. However, it quickly runs into prob-lems if we go beyond simple molecules.

What is the Lewis structure for the tetra-hedral Au20 cluster, the molecular TaB10

nanowheel, or the FeSMo cluster thatserves as the cofactor of nitrogenase?

Molecular orbitals can be obtainedfor any chemical compound, Boldyrevcontinued. But they are delocalized overthe whole molecule, which is not alwaysdescriptive of the actual bonding, and getvery complicated as the number of elec-trons in the system increases.

Weinholds Natural Bond Orbital meth-od, Baders Atoms in Molecules approach,and the AdNDP model are all headed to-

ward a unified model, Boldyrev thinks. Forexample, Boldyrev has successfully usedAdNDP to decode the bonding in Au20 andTaB10

, though his group hasnt yet resolvedthe bonding in the nitrogenase cofactor.But it should only be a matter of time be-fore bonding models evolve to tackle thatproblem as well as more complex ones.

That makes Boldyrev optimistic aboutthe future of his field. I dont think chem-ists are abandoning the concept of chemi-cal bonding anytime soon.

shells, actinides such as plutonium tend to

form superoxo (O2) rather than oxo (O2)

complexes and max out at an oxidation

state of 5+, such as in PuO4.

Enzyme Metal MysteriesCrystal E. Valdez

University of California, Los Angeles

When it comes to designing catalysts,

chemists often look to

enzymes as models.

But enzymes dont al-

ways work efficiently

outside their cellular

environment, so

chemists often tweak

enzymes to make

artificial versions withthe appropriate metal charge and size,

coordination geometry, and redox poten-

tials needed for chemical reactions. Valdez

described using a new approach, quantum

mechanics/discrete molecular dynamics

(QM/DMD) simulations, to explore what

makes a difference when switching one

metal for another in metalloproteins. In

two related natural amide hydrolases (one

shown) different metalszinc versus nick-

elare used to catalyze similar reactions

proceeding through identical mechanisms.

Predicting Crystal Structures

Artem R. OganovStony Brook University, SUNY

Researchers have long wished for the

ability to predict crystal structures of ma-

terials before they are synthesized, which

could significantly accelerate chemical

discoveries. Oganov described a computa-

tional tool he developed to fulfill that wish.

Called USPEX, the method includes an

evolutionary algorithm to search through

multidimensional energy landscapes at

different temperatures and pressures to

find stable combinations of selected ele-

ments. Oganovs group has used USPEX

to predict a new phase of boron (-B)and an insulating and

optically transpar-

ent form of sodium,

among other materials.

The researchers have

prepared some of the

materials, including

salt with unusual ratios

of sodium and chloride,

such as NaCl3 shown.

J.

PHYS.

CHEM.

B

BONDING NUGGETS CONTINUED