http://doc.rero.ch View metadata,citationandsimilarpapersatcore.ac.uk hc hudb ie orfrt thiswork. which shouldbecitedtorefer Published in"AngewandteChemieInternationalEdition45(40):6616-6619,2 T nirmtct ylbtdee·thermochemistry · cyclobutadiene · antiaromaticity Keywords: suso aromaticity/antiaromaticity to of dedicated were issues CBD, to references atfv er w nieteai issues the of thematic in entire two and years 2002, five last and year 2000 per in papers appeared of number as highest wane, to the about sign is no CBD in is interest that There complexes. metal derivatives there and on more and many span, probably time are this in of appeared year have every general in butdienes n localization/delocalization, and Bally* Thomas Paradigm? Antiaromatic The Cyclobutadiene: rtedge fataoaiiy tis It antiaromaticity. of degree the or its extent of the delocalization to relate of which mole- of most fascinating cule, this surround many myths paragraph, opening the in mented the which with fraught. is pitfalls discussion resulting the on out to reflect and point field to this occasion in developments this recent take antiaromaticity, I of and consequences thermochem- the ical on foundation discussion solid the a for provided have they raohr ihteprn compound parent C the with form another, has one or in average, as deal, which On years publications 40 16 (CBD). past the cyclobutadiene consis- over so chemists tently theoretical and tal experimen- fascinated has that molecule civdb h ru fKass. been of group has the CBD by of achieved formation en- of the thalpy of determination accurate ably debate the to CBD. central on are that topics is, * rf .Bally T. Prof. [*] 4 H eei rbbyn te single other no probably is here Homepage: [email protected] E-mail: (Switzerland) Fribourg 1700 Fribourg of University Department Chemistry hmclReviews Chemical http://www-chem.unifr.ch/tb/bally/ nsieo h ra ciiydocu- activity great the of spite In eetytefrtdrc n reason- and direct first the Recently 4 n 0wihda ihcyclo- with deal which 40 and ohwt many with both , p lcrn and/ electrons [3,4] [2] Thus that [1] ugsin yPuigadLondsdale, and Pauling Following by polyenes. suggestions from compounds aromatic distinguishes which its exaltation, and susceptibility diamagnetic anisotropy the of the magnetic particular to in relates properties, first The “beacon” features. as emerged have aromaticity for qualify to molecule distinction. a this for met be have to criteria these as- of several generally that was sumed it and smell unpleasantly), should token, compounds same the antiaromatic by thus, impor- criterion; an tant as considered been one have at must time odor worth pleasant a perhaps that itself implies “aromatic” is term the (it that recalling not or is molecule aromatic a whether judge to time over debate. this of aspects herein some how comment on will course assess I not properties. of should these or should is one exactly issue Another (anti)aromatic- ity. with associated are properties that the affects is this debate extent what continuing to the of issues the one of and compounds, two the same in the meaning “bond have of not notion does the delocalization” Thus obviously case. is the this not bonds, double sin- and gle localized strongly having structure rectangular its with CBD, in whereas try, egt othe equal to with weights reso- contribute Kekul structures be- two nance the seem, benzene might in cause it as straightforward antiaromaticity. of paradigm delo- cyclically four calized its with conclusion CBD, the to that rush to tempted is one 4 the Hckel of famous view cyclically In aromaticity. six of paradigm its with delocalized benzene, disputed) hardly that (and accepted widely w ye fcaatrsi risof traits characteristic of types Two proposed been have criteria Various oee,ti ocuini o as not is conclusion this However, p lcrn,i hrfr the therefore is electrons, p lcrn,rpeet the represents electrons, D n 6 h /4 1 qiiru geome- equilibrium n + lcrnrule, electron 2 0 yrgntossol ufrfo any the from of suffer other should hydrogenations these the from evident result or that no dienes conjugated one is why there reason con- As of choice formations). the on depends number hmclsit”(ISvalues) (NICS shifts” independent chemical “nuclear the stand debate tahdt rmtco antiaromatic or respectively, rings, aromatic protons to of attached shielding or to deshielding contribute the currents ring what these to extent and whether about years recent y2 kJmol its 22 endothermic that by fact is the in (mono)hydrogenation expresses example, for which itself, ther- stability up). special modynamic a mixed enjoys being benzene often Clearly, two (the kinetic ones and/or (anti)aromaticity, thermochemical namely of that characterictic properties are “beacon” of class other only have current, that trons. ring systems the in of the even size of indication and good direction the a above provides ) ring 1 (usually distance at some tensor and NICS) (or sign magnetic-shielding the the of that magnitude seems it However, nue ntecceof cycle currents the of in measure valid induced a not does provide rings (anti)aromatic of center i yaot14kJmol 114 about exother- by is which mic hexatriene, of that to 1 (1928–2003) Masamune Satoru of memory In eoecerta h rgnlpropos- original has al the the It that in molecule. clear become point a any surrounding at space computed be can h ytmo ylclydelocalized cyclically of p in- system in are field magnetic that the external currents the by ring duced of terms effects in these for accounts that model appeal- ing very a 1937 in proposed London M pcr.A h ou fthis of focus the At spectra. NMR H [9] electrons. hr a enavgru eaein debate vigorous a been has There oee,Iwl ou eeno the on herein focus will I However, ocluaeNC ausa the at values NICS calculate to [10] [5] 1 Shm ) ncontrast in 1), (Scheme [6–8] p opnn fthe of component smnfse in manifested as provided byRERODOCDigitalLibrary 1 p teexact (the brought toyouby electrons. [9] s elec- that CORE http://doc.rero.ch hc a engtv rpstv,is ener- gy. positive, resonance Dewar or second, termed negative usually The be may antiaromatic. which or nonaro- aromatic, matic, is un- consideration species der the whether of lowered), irrespective is energy the is, (that negative htetape faoiaino for- or atomization of enthalpies that usin ieauewihhsbeen has controversial which elsewhere. reviewed very literature this question, with literature of dealing body enormous construct. a an is theoretical There as a real a or take either molecule be should could of which one question reference, exactly the upon what hits one precisely, invariably to more energy” wants (de)sta- bilization “(anti)aromatic one the if quantitate However, no problem. is there level, semiquantitative a such for similar butadiene, a example). to (relative of this compound indication of destabilization an thermochemical high is of the hydrogenation CBD token, the same of exothermicity the By hexa- to triene. compared benzene special of some stability of result a kJmol must as 136 difference the regarded strain-free, be because essentially may and compounds both destabilization, special kJmol in compounds 1. Scheme h rgnlPuigWeadreso- energy, Pauling–Wheland nance original the resonance. cyclic for but not conjugation, energy” for accounts “conjugation energies it because termed two be these cyclic may of being difference conjugation The the of virtue and lost) (or conjugation gained energy cyclic) the bonds or double (linear putting energy into by gained total is the that between distinguish carefully should one that is point portant [15,16] sln steagmn ean on remains argument the as long As h oa nrygi orsod to corresponds gain energy total The tdrvsfo h observation the from derives It et fhdoeaino various of hydrogenation of Heats [14] n ti invariably is it and 1 . [13] [11] oee,a im- an However, xlsvl by exclusively [12] 1 . ru nrmns(1 kJmol (113 increments group mut o8 kJmol 87 to amounts nryo 316 destabilization whopping of a energy with left is rmeprmnal ae bond composed based be experimentally can from polyenes of mation eemndha ffraino CBD of recently (429 formation the of compares heat determined one If CBD. in of sum radii). the Waals der than van larger their are atoms gen hydro- nonbonding between separations nirmtcdsaiiaineeg of energy destabilization antiaromatic above the dissect number to way unambiguous that did) co-workers and Kass results which destabilization. and antiaromatic from from strain, results of energy form this some out of figure to part how which of problem the faces (all strain 120 sp of of for value ideal free the have be angles bond to be compounds assumed to such are needs because that only considered, the effect is this that nonadditive fa- fact the greatly by is the cilitated energy of resonance assessment Dewar the hydrocarbons) comparable standards. theoretical) or (experimental to procedures—relative complicated quan- more much same by accord tity—usually the good of in determinations indeed with are results hydrocarbons. resonance The benzoid aromatic in the transparent stabilization assess most to and way best the the and rep- resent energy procedure resonance Dewar this resulting that believe h aetvrino esntp group increments, Benson-type of uses version one latest If the the reference. of polyene hypo- its thetical and formation scrutiny under or compound atomization of enthalpy differ- experimental the the between with ence resonance equated Dewar then is the energy) is, (that way this benzene. of case reso- the in equivalent structures for nance allows that double of bonds arrangements cyclic the from energy but conjugation the all a from benefits of that energy polyene cyclic the hypothetical) (purely calculate to one allow increments. group yadn orplei C polyenic four adding by oee,ti scerynttecase the not clearly is this However, ti eyipratt eonz (as recognize to important very is It benzenoid (or benzene of case the In h xr nrygie rls in lost or gained energy extra The not 2 hbiie abnaos n the and atoms, carbon -hybridized 6kJmol 16 h pca otiuinresulting contribution special the foewnst uniaethe quantitate to wants one If . [18] hseeg difference energy this 6kJmol 16 1 ) [17] [3] 2 ihta obtained that with uhincrements Such 1 nbnee I benzene. in 1 n then One . hr sno is there d 1 -(H)(C ), [18] [15] one or d ) 8 nC in 4 of delocalization h taneeg nrae yamere a by kJmol increases 4 energy strain the that fact The consideration. some merits u rgnlyb oizre al., et Politzer by originally out a at arrive two the to in compounds. “strain” invoked of picture be complete to explanations need above the cyclobutane. of both Perhaps and be- energies cyclopropane surprisingly strain tween the in difference for small account pro- been to have posed be they cannot assessed, effect directly this of consequences nices ntesrnt fteC the of strength the by in balanced increase largely an the the is that between compounds strain two argues angular in first increase dis- this The for crepancy. account to advanced have been explanations different Two property this alone. for account (Baeyer cannot deformation strain) angular of cost ic energet- the that indicates cyclopropane nlyavne yDewar, by advanced orig- inally proposal, the to refers explanation oyn.Tefc htteC the other that fact any The in polyene. than position proximate two CBD in p that observation the cerns h nhlycag o h isodesmic the (1). for Reaction change enthalpy by given the antiaro- is energy the destabilization case increases matic this energy In cyclobutane/cyclo- butene/CBD. strain series the in the linearly that rea- to be seem most would assumption The such energy. sonable strain its assumption about some make must one CBD, ISvalues). NICS by ring destabi- lized is the cyclobutane above whereas plane), values in NICS example, negative for itself, manifests (which so-called this by stabilized tblzto rdsaiiain respec- tively. destabilization, or stabilization uaei ahrnormal. rather therefore is cyclo- butane of is that whereas low, cyclopropane strain anomalously the of that and energy atoms, the carbon of ring rehybridization to owing bonds, od r ocdit uhmore much a into forced are bonds oee,teaoeassumption above the However, nadtoa opiain pointed complication, additional An od a lola oaromatic to lead also may bonds C [10] 1 codnl,ccorpn is cyclopropane Accordingly, ngigfo ylbtn to cyclobutane from going on s [3] nirmtct (positive antiaromaticity [10] lhuhteenergetic the Although n + 2or4 [19] s [20] n h second The aromaticity s htcyclic that electrons bond C [21] con- H http://doc.rero.ch lcrncsaeadtefrtexcited first the state. singlet and ground state the electronic strong between to coupling attributed vibronic be in also length may bond CBD pro- in the alternation and nounced repulsion, this quantitate two the p between a of repulsion expression substantial an be may compounds, the whereas butadiene, C in than longer B?Teinzto nryo CBD in of eV energy (8.16 antiaromaticity ionization The CBD? with associated share. to view tend be I a that molecule”, unique should the a as it as regarded CBD paradigm, of antiaromatic interpretation tional 4kJmol 44 clyrcnldsi i above-cited his fact, paper in In concludes Schleyer properties. its on thermochemical based as as nonaromatic, considered essentially enormous be its must of destabilization, spite in CBD, that tion to attributed is p number lower this even er wheth- to unclear is It cyclobutane CBD. to from cyclobutene going property not on this changes how therefore and why is clear priori it a b) be repulsion; and/or cannot steric deformation it angular under- to and reduced property, to this chemists stand by invented com- were that effects is nonadditive several of a which posed is property “strain” multifaceted a) considerations: above 3 kJmol 230 four its of conjugation p cyclic the of tue vir- by ther- solely vary is destabilized CBD mochemically opinions much how on recent enormously that surprising to strain. towards or as towards antiaromaticity counted be arises should this question whether CBD, of tantalizing destabilization the the to icantly If ring. the of s size the and on depends rings it how four-membered CBD) in (as planar and cyclobutane) in (as puckered hto ezn 92 V shge than higher whereas is eV) eV), (9.25 (9.08 benzene of butadiene that of that eghi B c.15 codn to according calculations 1.57 high-level (ca. CBD in length = nirmtct oscnrbt signif- contribute does antiaromaticity od.Hwvr ti ifcl to difficult is it However, bonds. euso,oei etwt h predic- the with left is one repulsion, lcrn;etmtosrnefrom range estimations electrons; odlnt ssmlri h two the in similar is length bond C htohrpoete ih be might properties other What w oolre olwfo the from follow corollaries Two nve falteaoei snot is it above the all of view In s nirmtct stesm for same the is antiaromaticity [13] [25] ht“nta fteconven- the of “instead that 1 snal Vlwrthan lower eV 1 nearly is ) 1 [23] M n Schleyer and (Mo Dnze al. et (Deniz [22] snal . 0.1 nearly is ) [24] [13] )to .If ). loe rniin(1 transition allowed adosre t30nm 300 at observed absorption unambiguously band An been experiment. by not assessed has CBD of spectrum absorption the that is problem t30ad25n.Acrigt our to calculations, According CASPT2 nm. 285 own and 360 (vertically) occur at transitions two these htcrepn oteectto fone of (1 excitation the to correspond that near- two transitions has electric-dipole-forbidden UV CBD rectangular tions, acltos hc r surprisingly are scarce. which calculations, observed. too be albeit to band region, a for this weakly in absorb must 2 h aea hto w tyeemole- ethylene (4 total two of cules the that as to same the over carries p trend conjugation, course Of this benzene. cyclic in lowered by is it while raised the is of energy CBD the that is indicates comparison similar this a What by amount. eV) (8.29 hexatriene of that ieacetylene to nm give 350–420 at irradiation of upon cleavage CBD but such, as confirmed been fLvhnoadKrylov and Levchenko of calculations coupled-cluster recent The bv V( eV 7 above UOexcitation. HOMO– LUMO simple by benzene described of be the cannot state the singlet to MOs, excited owing frontier lowest that, the of fact ham- degeneracies the is by Un- comparison pered compounds. this two fortunately, the the in of very be different also energies should states the singlet lowest reason, that For potential). ionization the HOMO is, the (that energy with correlate HOMO–LUMO should gaps their hydrocarbons, (anti)aro- of maticity. expression an is potential about by hexatriene 1 of that than 2 lower by molecules ethylene three of total p the whereas 0.5 butadiene, of nearly that than and terms) orbital ina rud15e oe nenergy in lower benzene. eV in 1.5 than around at tion excita- HOMO–LUMO must involve CBD indeed of photocleavage near-UV observed the Thus, region. vacuum-UV 1 b A nry hc ntecs fCDis CBD of case the in which energy, nryo ezn slwrta that than lower is benzene of energy 1 .I hsprpcieteionization the perspective this In ). A hs ems eyo excited-state on rely must we Thus, sCDadbneeaealternant are benzene and CBD As g rmteHM oteLUMO. the to HOMO the from ) g ! [29] 1 a 1 B codn oteecalcula- these to According + 1g 4 rtoeetos(1 electrons two or ) < b [28] 8 m,ta s nthe in is, that nm), 180 nHce molecular Hckel in [26] niae htCBD that indicates 3 1 A oee,tereal the However, g ! [22] [27] 1 p [30] 1 rdc that predict B OOof HOMO a never has 3u b h first the occurs ) b higher 1 A (and g ! rcs ttmeaue eo 0 below temperatures automerization at process the would dominate mechanism actually tunnelling the that i experiments, his from deduced had he which entropy activation negative of the for account exactly to done that fact in had In Carpenter tunnelling. 1983 heavy-atom invokes if reconciled one be only can facts two These 10 than higher be inter- must of constant conversion rate cryogenic the IR that in matrices from measurements deduced NMR al. and et Orendt while ftemanuscript. reading the of critical for of Boulder) (University Colorado, Michl Josef of and University Cardiff) now University, Carpenter Barry (Cornell Profs. to indebted am I ground. at solid rests on destabilization least its to vari- effects of ous contributions the spec- purported about ulation any the antiaromaticity, CBD, of paradigm of formation of heat well- the for a available is that number founded Now molecules. anti- or aromatic aromatic ambiva- of sometimes manifestations the to lent of with sense up come make chemists that theoretical models and different benchmark for ground a testing as certainly serve will to it continue and chemists fascinate date. to not has dispelled it been scepticism, of met deal great originally a with proposal this Although nryo ciaino esthan less of activation kJmol 4 of energy olebten2 n 2kJmol 32 and 27 between lie barrier to the predict calculations initio ab 5K, 25 i ewe . n a4 kJmol 40 ca and 6.7 must between barrier lie the Car- that and trapping concluded Whitman penter From solution, in limits. studies and rough estimates provided only far so Ex- has CBD. periment of the structures of rectangular barrier two automerization and rate 4 h nhlyo omto fcyclobuta- of formation of enthalpy The [4] Kass, R. S. Tian, Z. Liz, L. Fattahi, A. [3] [2] [1] hs ylbtdeecniusto continues cyclobutadiene Thus, ial hr steqeto fthe of question the is there Finally in a enpeiul eie from derived previously been had diene Ed. Int. Chem. Chem. Angew. hm Rev. Chem. hm Rev. Chem. [30] hc rnltsit free a into translates which 1 oee,tebs high-level best the However, . 2005 2001 [29] 2006 2006 , , 105(10) 101(5) n econcluded he and , , 45 118 4984. , 1115. , 5106; , 3433. , Angew. 1 8 . C. [31,32] 1 3 , [33] [29] at http://doc.rero.ch 1]D oa,M aoaa,T Heine, T. Manoharan, M. Moran, D. [10] 1]Bsdo nhliso omto from: formation of enthalpies on Based [11] 1]Ntihtnigrpae n continu- and repeated Notwithstanding [12] 9 .vnR clyr .Mekr A. Maerker, C. Schleyer, R. von P. [9] Stanger, A. [8] 6 .S anr,P o .Schleyer, R. von P. Wannere, S. C. [6] 7 .Vgin,R aai .Lazzeretti, P. Zanasi, R. Viglione, R. [7] 5 .C London, C. F. [5] .vnR Schleyer, R. von P. 6317. uain rmtercn oko Kass co-workers. of and work recent cyclo- the of from formation butadiene of enthalpy the and and MD,(http://webbook.nist.gov/chemistry), Standards Gaithersburg, of Technology, Mallard), Institute G. W. 69 National Lonstrom, No J. P. Database (Eds.: Reference Standard Stein E. S. Liebman, in F. J. Afeefy, Y. H. rnfl,H io .J .vnEikema van R. Hommes, J. N. Jiao, H. Dransfeld, 837. r.Lett. Org. Lett. n list h otay seilyby especially contrary, the to claims ing hi eutn siaeof estimate resulting reaction, their this of products pre- side and the this cursor to of regard with properties made thermochemical be to had that assumptions various to [24]). reference owing However, (see photo- precursor cyclobu- tadiene polycyclic the a during of decomposition liberated heat the iiatytohigh. too nificantly 477 ITCeityWbok NIST WebBook, Chemistry NIST 2003 6kJmol 46 , .A.Ce.Soc. Chem. Am. J. 2004 5 .Og Chem. Org. J. 605. , , .Ce.Phys. Chem. J. [3] 6 1 2265. , undott esig- be to out turned r.Lett. Org. D 2005 f H 2003 8 1996 (CBD) , 1937 70 , , 883. , 5 2005 Org. 118 23. , , = 5 , , 1]E .Dmlk,E .Hearing, S. E. Domalski, S. E. [18] 1]S .Benson, W. S. [17] 1]R .Bc,O Dmitrenko, O. Bach, D. R. [19] 1]L .Sha,B .Hess, A. B. Schaad, J. Llano, L. de [16] C. Dewar, S. J. M. [15] 1]L .Puig .W Wheland, W. G. Pauling, C. L. [14] Schleyer, R. von P. Mo, Y. [13] 2]M .S Dewar, S. J. M. [20] y nrae yaot3 kJmol 35 about by increases enthal- py) dissociation their is, (that bonds acltos h tegho h C the of strength the calculations, ie,NwYork, New Wiley, Soc. hm e.Data Ref. Chem. 2001 2006 Phys. therein. ref- standard. or nonaromatic erence a to is, recourse without that “absolutely”, (anti)aromaticity measure to impossible is it theoreticians, aet eaot7 kJmol 70 about be to have 106 matter. non- delicate a dissecting is effects that additive more once This shows cyclobutane! in than cyclopropane aet eoe 0 kJmol 200 over would be strain to angular have the is, that group, tano arwn h -- bond C-C-C 90 the from angles angular narrowing in on increase the strain are, sim- they as as compounds ilar two the strain of overall cyclopro- the energies keep to to order In cyclobutane pane. from going hm Soc. Chem. 669. , , , 101 2004 1933 12 09adrfrne cited references and 2009 , 1465. , , , 1 1969 126 362. , hrohmclKinetics Thermochemical 8 .A.Ce.Soc. Chem. Am. J. 44 codn to According 4444. , , o60 to 4 1993 91 1976 789. , , 8 . 22 ol therefore would 1963. , hm u.J. Eur. Chem. .A.Chem. Am. J. hm Rev. Chem. 1 1 ihrin higher .Chem. J. e CH per .Phys. J. .Am. 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