The Changing Landscape of Physical Chemistryat the Beginning of the 21st Century

G. A. Somorjai*,† and R. D. Levine‡

Department of Chemistry and Lawrence Berkeley NationalLaboratory, UniVersity of California, Berkeley,California 94720-1460, and Department of Chemistry andBiochemistry, UniVersity of California,Los Angeles, California 90095.

ReceiVed: December 10, 2004

There are major changes occurring in the way research isperformed in physical chemistry. This is in part due to oursuccess in providing an ever-increasing science component toexisting and emerging technologies that accelerates their needfor even more. Our ability to study the science of chemical com-plexity permitted us to target major scientific and societal prob-lems that require an interdisciplinary approach. These includeenvironmental chemistry, problems of size reduction in micro-electronics that led to the rise of nanoscience and nanotech-nologies, and the design of drugs and implant devices that extendhuman life span and sustain the health of the human body.

How did this come about? Our success in devising instru-ments for molecular-level studies of systems in the gas phase,in condensed phases, and at surfaces produced new science. Italso permitted studies of systems in ever-widening temperatureand pressure ranges, with constantly improving time resolution,spatial resolution, and energy resolution. With the aim ofunderstanding simple model systems on the molecular scale atfirst, the structure and dynamics of small molecules, monatomicand diatomic solids and their surfaces, were studied. Theseinvestigations then opened the door for the exploration of morecomplex systems such as proteins, molecular machines, andcomplex catalytic reactions. The remarkable progress in experi-ment has stimulated and has benefited from the coming-of-ageof applied theoretical chemistry. It is nowadays possible forquantum chemistry to provide quantitative predictions ofstructure and energetics for systems of realistic chemical interest,and dynamicists and statistical mechanicians are able to use thisas input for the quantitative understanding of reactivity and theformation and temporal response of molecular assemblies.

Biological or environmental systems are complex. Mosttechnologies that produce polymers, drugs, catalysts, micro-electronic circuits, or magnetic storage disks are complex. Theirfabrication and development require an integrated quantitativeapproach involving many fields of chemistry: synthesis,characterization, and function, or physical property studies, alongwith applications of solid-state physics, electronics, and biology.The pursuit of nanosciences, one of the frontier fields of physicalchemistry, definitely requires and benefits from this approach.

(a) The increasing science component in most technologiesthat drives them to become high technology, (b) our ability totackle chemical complexity, and (c) the interdisciplinary ap-proach necessary for success in producing new science in manyfronts of physical chemistry are all causing us to rethink theway research is performed in the field. The use of singleinstruments to perform measurements and the compartmental-

ization of synthesis, characterization, and studies of propertiesno longer fit the purpose which is to solve major science ortechnology problems. The division of physical chemists togroups that investigate molecular behavior in the gas phase orin the condensed phase is no longer possible, as most problemsthat involve physical chemistry have both gas-phase andcondensed-phase components. Interfaces are where much of thecurrent interest is, whether the coupling of a molecule to anelectrode or the biocompatibility of implant devices andmolecular recognition in general. The increasing human powercost of research, which rises more rapidly than the costs ofinstruments, requires that each investigator be knowledgeablein the use of several instruments. The same researcher shouldbe able to carry out characterization and property studiesrequiring a broader and more interdisciplinary education. Whenshort pulse lasers can be used at the diffraction limit, we wouldneed all the integration that is humanly possible.

Physical chemistry has gone through one period of integrationfollowed by a period of decentralization before. In the beginningof the 20th century, the rise of quantitative analytical techniques,thermodynamics, and macroscopic kinetics spawned chemicaltechnologies ranging from ammonia and methanol synthesis tothe internal combustion engine. The rise of quantum mechanicsopened the door for studies of molecular structure and bondingand led to the development of many instruments for spectro-scopic studies of gas-phase molecules and diffraction studiesin the condensed phase. Decentralization and specialization haveserved us well to reach a molecular understanding of manysimple chemical systems. By the 1950s, this knowledge permit-ted the studies of complex molecules. Pauling published thestructure of topaz determined by X-ray diffraction in 1928. In1951, his eight publications in a single issue of PNASestablished the helical structure of proteins, a great increase inthe complexity of chemistry that could be tackled on themolecular level. The increasing complexity of systems that canbe explored is also evident from NMR studies, as shown byrecent Nobel Prizes. The continuous push for improved time,spatial, and energy resolutions opened up new domains ofphysical chemistry research. From the 1950s, the discovery ofthe transistor provided the thrust for molecular surface chem-istry, because this and other technologies developed in subse-quent years improved by size reductionsthe smaller they arethe more efficient they become. The techniques of modernsurface chemistry permit the atomic level studies of 1013-1015

atoms per cm2 at the surface in the background of 1022 per cm3

in the condensed phase. The simple, single-component modelsurface systems that were studied at first are being substitutedby more complex surfaces, polymers, biopolymers, electrodes,and multicomponent catalysts as our molecular-level under-standing improves.

Now enter the nanosciences, which again are also driven bythe needs of technologies, which provide challenges to learnthe manipulation of matter on the nanoscale: connectingmolecules and studying their self-assembly, optical, chemical,electronic, magnetic, and mechanical properties. The centralizingthemes of physical chemistry again become dominant at thestart of the 21st century, just as they were dominant in the earlydecades of the 20th century.

Up to the later 20th century, our thinking in physicalchemistry was conditioned by the concept of a fairly rigidmolecular structure. When we allowed displacements from

* To whom correspondence should be addressed. Phone: (510) 642-4053/Fax: (510) 643-9668. E-mail: somorjai@socrates.berkeley.edu.

† University of California, Berkeley.‡ University of California, Los Angeles.

9853J. Phys. Chem. B2005,109,9853-9854

10.1021/jp040754q CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 04/27/2005

equilibrium, they were taken to be small enough that there wasa restoring force bringing the system back. The barriers toisomerization or to reaction were taken to be larger thankT.When reaction did take place, the potential-energy landscapewas assumed to be simple enough that there was a clear reactionpath leading to the products. Unusual cases such as the van derWaals molecules or clusters were exceptions to the norm. Butthe complex systems of current concern force us to recognizefluxionality as the norm. There are numerous energetically low-lying isomers so that weak interactions, many of which can bethought of in mechanical terms, are essential for the action ofthe system.

There are more technology challenges to come! Energyconversion and storage require learning how to producehydrogen, the development of fuel cells, and learning how tosequester CO2 and mine methane hydrates in the continentalshelf and convert them to liquid fuels. The development ofpolymers as structural materials and of biopolymers that are

biocompatible with the human body is within our reach. Theencapsulation of nuclear waste in a way that lasts for centuriesusing glasses, ceramics, and composites is a major challenge.The development of defense science that uses sensors andremote sensors to improve personal safety and the explorationof outer space all require new science and an interdisciplinaryapproach to tackle the chemical complexity that is involved.Catalysts must have 100% selectivity for desired reactions toeliminate byproducts (waste). Green chemistry requires anunderstanding of the molecular ingredients that control selectiv-ity. Enzymes do exhibit this kind of selectivity, and syntheticcatalysts must be able to attain their level of selectivity.

Physical chemistry is a vibrant ever-changing central fieldof science. The push of high technology, nanosciences, increas-ing labor costs, and the need to understand biological complexityand many other complex systems on the molecular level drivethe integration of disciplines at presentsand we suspect formany years to come.

9854 J. Phys. Chem. B, Vol. 109, No. 19, 2005 Comments