erich hückel, 9 august 1896 - 16 february...

1980Erich Hückel, 9 August 1896 - 16 February

H. Hartmann and H. C . Longuet-Higgins, F. R. S.

1982, 153-162, published 1 November281982 Biogr. Mems Fell. R. Soc.

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ERICH H U C K E L

9 A ugust 1896 — 16 F e b ru ary 1980

E lected F o r .M e m .R .S . 1977

By H . H a r t m a n n a n d H . C. L o n g u e t - H i g g i n s , F .R .S .

E r i c h H u c k e l , w ho was elected a F o re ign M em b er o f the Royal Society on 21 A pril 1977, d ied at the age of 84 on 16 F e b ru a ry 1980 in M a rb u rg , w here he was P ro fesso r E m eritu s o f the U n ivers ity . H e and his wife, A nne H uckel, had lived for 43 years in th e ir house in R oserstrasse .

T h is m em o ir first gives an accoun t o f H iick e l’s life, and th en discusses his special m erits and his p ioneer w ork in the theo ry of m a tte r and especially in the q u a n tu m theo ry of m olecules.

A t the age of 79 E rich H uckel w ro te an au to b io g rap h y en titled Ein Gelehrtenleben— Ernst und Satire; th is th row s in te restin g ligh t on his ch arac te r and is essential read ing for anyone w ish ing to learn abou t h im . In ad d itio n to revealing m any in te restin g op in ions, it describes his way of life, and ind icates the influence his friends exerted on h im , th u s m aking im p o rta n t c o n tr ib u tio n s to the s tudy o f the psychology of scien tists and the genesis of scientific ideas.

S ince the au to b iog raphy was w ritten and p ub lished in G erm an it m ay be useful to give here an ab ridged account of the im p o rtan t periods in H u c k e l’s life.

H is fa ther, A rm an d H uckel, was a m edical doc to r w ho had an especial in te rest in chem istry . H is fam ily cam e from Alsace and he was bo rn in S tu ttg a rt. O ne of A rm and H u ck e l’s ancestors, Josef G a rtn e r (1732— 1791), was a fam ous b o tan is t w ho becam e a Fellow of the Royal Society in 1761. A rm and H uckel at first had w anted to becom e a un iversity p ro fesso r b u t abandoned th is idea later, as his financial s itua tion allow ed him to live as a p riva te sc ien tist. T h u s he had p len ty of tim e to superv ise the education of his th ree sons, W alter H uckel (1895-1973), E rich H uckel and R udi H uckel (1899-1949). T h e youngest son, R udi, later becam e a m edical doc to r and W alter H uckel, a u th o r of the fam ous book Theoretische Grundlagen der Organischen Chemie (3. Auflage 1940), was subsequen tly p rofessor of pharm aceu tica l chem istry at T u b in g en .

A rm and H uckel exerted a very strong influence on his sons. In his

153



154 Biographical Memoirs

lifetim e great progress had been achieved in the natu ra l sciences, w hile on the o th er hand at the end of the 19th cen tu ry the influence of tran scen dental idealism on the general way of th ink ing had nearly d isappeared .

H e therefo re guided his sons’ in terests to the field of the exact sciences. T h e critical, rational and stric tly an ti-m etaphysica l k ind of th ink ing w hich was always typical of E rich H iickel apparen tly had its orig in in th is pa ternal influence.

W orld W ar I broke out shortly after H iickel had becom e a s tu d en t of physics in G o ttingen . D u rin g the w ar he w orked u n d e r the fam ous P rand tl in an aeronau tics labora to ry and a sim ilar in stitu tio n in W arnem iinde. W hen the w ar ended he again becam e a s tu d en t of physics and m athem atics. H is thesis was the resu lt of cooperation w ith P rofessor D ebye. It was an experim ental w ork abou t the scattering of R on tgen-rays or X -rays, and H uckel was p rom o ted D r phil. in 1921 in G o ttingen . H e later nam es the G o ttin g en professors, the chem ist W allach, the physicist P eter D ebye and the co llo id -chem ist R. Z sigm endy (who later becam e his fa ther-in -law ) as those teachers w ho had been m ost im p o rtan t for h im . All th ree G o ttin g en professors w ere or later becam e N obel prize laureates. A t his p rom o tion H uckel was exam ined in m athem atics by D avid H ilbert. H ilb ert d u rin g th a t tim e in G o ttin g en had two places for assistants. O ne of those norm ally was occupied by a m athem atic ian , the o th er norm ally by a theoretical physicist. A t the tim e of H iickel’s p rom o tion H ilb e r t’s physics assistan t was K ra tze r w ho had been a pup il of Som m erfeld . W hen K ra tzer was appo in ted to a C hair in M u n ste r, H ilb ert offered the position to E rich H uckel. W hen one of us (H . H .) visited H uckel som e years ago at his hom e in M arb u rg , he spoke about the tim e in H ilb e r t’s in stitu te . H e po in ted out th a t the co-opera tion w ith H ilb ert and later w ith M ax Born had been very happy b u t an ‘undeserved lucky chance’ for h im . D u rin g tha t tim e H uckel had also been ch ief redactory ed ito r of Physikalische Zeitschrift u n d e r the ed ito rs D ebye and M ax Born. A t the tim e w hen H uckel was p rom oted in G o ttin g en his teacher D ebye had gone to the E igenossische T echn ische H ochschu le in Z urich and no longer lived in G o ttingen . B ut only in 1922 could H uckel follow D ebye as his assistan t to Z urich . H uckel held th is new position un til 1927 as a co-w orker of P e ter D ebye.

In 1922 H uckel m ade the acquain tance of his fu tu re wife A nne Z sigm ondy, daugh te r of Professor R ichard Z sigm ondy. Z sigm ondy had m ade his consent to the m arriage conditional upon H tickel’s ob tain ing a lec tu resh ip , w hich he did at Z urich in 1925 and the m arriage took place there in the sam e year. T h e re were th ree sons and one daugh te r of the m arriage. T h e years in Z urich m arked the first peak in H iickel’s scientific activity. It was there tha t the D ebye-H uckel theo ry of strong electro lytes, of w hich m ore will be said later, was developed. M otivated by his special in terest in research in the field of strong electro lytes L ars O nsager came to the Z urich In stitu te and the two becam e close friends d u ring tha t period .



Erich Hiickel 155

In the fo llow ing tw o years, 1928 and 1929, H iickel was aw arded a scho la rsh ip by the R ockefeller F o u n d a tio n and w ent first to w ork w ith P ro fesso r D o n n an in L o n d o n and th en w ith N iels B ohr in C openhagen . O rig ina lly he had p lan n ed to w ork on the th eo ry of ad so rp tio n p h e n o m ena, b u t he aban d o n ed th is idea and w hile in L o n d o n began to s tudy the new d eve lopm en ts in the field of q u a n tu m physics. H is stay in C o p e n hagen had been very s tim u la tin g because he m et m any physic ists w orking in the field of q u a n tu m th eo ry there . T o the end of his life H uckel rem em b ered a rem ark m ade by B ohr as decisive for his ow n scientific research . B ohr had to ld h im th a t a sho rt tim e before, H e itle r and L o ndon had successfully trea ted the covalen t chem ical b ond w ith in the fram e of q u a n tu m m echan ics and th is led h im to th in k th a t a fter th is success one cou ld also s tudy the d o u b le -b o n d betw een n e ig h b o u rin g carbon atom s.

P e te r D ebye and , in p a rticu la r, M ax von L aue, had the idea th a t so m eth in g shou ld be done for H uckel and the consequence was th a t H uckel was aw arded a scho larsh ip by the D eu tsch e N o tgem einschaft w ith w hich he w ent to L eipzig in 1929 to w ork in the g roup of H e isenberg and H u n d . H e in ten d ed indeed to w ork on the d o u b le -b o n d , and the fam ous fundam en ta l p ap e r was fin ished in Jan u ary 1930 and pub lish ed in the sam e year in the Zeitschrift fu r Physik. the nex t step was the d eve lopm en t of the theo ry of benzene. H uckel says in his au tob iog raphy th a t H e isenberg and H u n d helped h im considerab ly w ith th e ir know ledge of the q u a n tu m theory . H u c k e l’s m ost im p o rta n t scientific achievem en t, the theo ry of u n sa tu ra te d and arom atic com pounds, will be d iscussed later.

D u rin g the follow ing eigh t years the H uckel fam ily had to live on the incom e from a so-called ‘L e h ra u f tra g ’. T h is was no t a regu lar position at the T ech n ica l H igh School of S tu ttg a rt. T h ese years becam e in fact w ith the S tu ttg a rt ‘H a b ilita tio n ’ w ith the paper on benzene as H iick e l’s ‘H a b ilita tio n ssc h rif t’. All the im p o rtan t papers on m olecules w ith n- elec tron system s, includ ing the im p o rtan t sum m ary pub lished in the ZeitschriftfiirElektrochemie in 1937, w ere w ritten d u rin g th is period w hen

H uckel was in g rea t financial difficulties and was finding political cond itions m ore and m ore unbearab le . In 1937 H uckel was appo in ted E x trao rd in a ry P rofessor of T heo re tica l Physics at the U n iversity of M a rb u rg . In 1961 th is position was changed to an o rd inary p ro fesso rsh ip and H uckel finally left the un iversity in 1962. H is vital energy after he had fin ished his g rea t p roductive period was no t solely influenced by the general cond itions of life. In 1930 on the occasion of a lecture on his d o u b le -b o n d w ork given at the m eeting of the G erm an Physical Society in K on isberg he had been d isappo in ted . T h e re he had to realize tha t physicists in th a t co u n try in any case w ere no t yet ready to accept investiga tions abou t m ore com plicated chem ical bond phenom ena as a typical co n trib u tio n of a physicist.

Still m ore difficult was his re la tionsh ip to the chem ists. Before W orld




W ar II , especially in the A nglo-Saxon coun tries , chem ical physics and w ith in th a t field q u a n tu m chem istry also was accepted by bo th physicists and chem ists as an in te resting new field of science. C hem ists in G erm any , on the o th er hand m ain ta ined th a t chem istry is w hat chem ists do. T h ey did not do q u an tu m chem istry and therefo re th is k ind of science did no t belong to chem istry . W hen the first au th o r in 1951 gave a lecture abou t H iicke l’s electron theory d u rin g a session of the C hem ical Society in M a rb u rg and po in ted especially to new applications of H u ck e l’s ideas in organic chem istry , E rich H iickel was overjoyed. A few physical chem ists w ere in terested b u t from the m ajority of chem ists one could feel a definite rejection. In 1957 the first au tho r, who had to organize the annual m eeting of the B unsen-G esellschaft in K iel, succeeded in g ratify ing P rofessor Hiickel once m ore in a general lecture. A t th a t tim e, how ever, th ro u g h w ar, disease and d isap p o in tm en t he was really exhausted . Im p o rta n t acknow ledgem ents cam e ra th e r late. In 1965 the O tto -H ah n M edal and the O tto -H ah n Prize w ere given to h im . In 1966 the T echn ica l U n iversity of S tu ttg a rt conferred upon him an honorary doctorate . In the sam e year he was elected a m em ber of the L eopold ina A cadem y. H e received an honorary docto ra te in 1973 from the U n iversity of U ppsala. In the sam e year he was elected a m em ber of the A cadem ie des Sciences Q uan tiques M oleculaires. F inally in 1977 he was elected a Fore ign M em b er of the Royal Society.

S t r o n g e l e c t r o l y t e s

T h e first im p o rtan t period of scientific activ ity in H iickel’s life was the tim e w hich he spen t as an assistan t to P eter D ebye, who at th a t tim e was con stru c tin g a novel theory of strong electro lytes. O ne can see th is period as a m ethodological p repara tion for H u ck e l’s later scientific m asterp iece, his theo ry of 7i-electron system s.

T h e D ebye-H uckel theory , as it cam e to be called, m ade its first appearance in two papers in the Physikalische Zeitschrift in 1923. T h e first paper dealt w ith the colligative p roperties of s trong electro lytes, and the second w ith th e ir tran sp o rt p roperties . T h e central idea of the theory , tha t of the ‘ionic a tm o sp h ere ’ was p robab ly P eter D eb y e ’s, b u t in such a co llaboration it is risky to a ttem p t to assign cred it in detail. Broadly speaking, the activ ity coefficient of an ion decreases w ith increasing concen tra tion , and in the D ebye-H uckel theory th is effect is a ttr ib u ted to a low ering of the free energy by the neighbou ring ions of opposite charge. A sim ilar hypothesis accounts for the reduced m obility of the ions w hen the concen tra tion is raised: in th is case the electric field pulls a given ion in one d irection b u t im pels the ne ighbouring coun ter-ions in the opposite d irection , thus im peding its progress.

But it is one th ing to propose qualita tive explanations of physical phenom ena, and qu ite ano ther th ing to m ake these explanations



Erich Hiickel 157

q u a n tita tiv e . T h e tour de force by w hich D ebye and H uckel tu rn e d th e ir th eo ry in to a q u a n tita tiv e one was th e ir fam ous ‘P o isso n -B o ltzm an n e q u a tio n ’. T h e d is tr ib u tio n of co u n te r-io n s ro u n d a cen tral ion was assum ed to be d e te rm in ed by the B o ltzm ann d is tr ib u tio n in the field of the cen tra l ion (ap p ro p ria te ly m odified by the d ielectric constan t); the e lectrical p o ten tia l was assum ed to satisfy P o isso n ’s equa tion in w hich the charge d ensity was regarded as a co n tin u o u s fu n c tio n of position in the n e ig h b o u rh o o d of the cen tra l ion. In the resu ltin g th eo ry a cen tral role is p layed by the ‘ionic s tre n g th ’ —a w eigh ted sum of the con cen tra tio n s of the ind iv idual ions, w eigh ted by the squares o f th e ir charges— and th is is th e q u a n tity upon w hich the activ ity coefficient o f each ion and its m o b ility o u g h t to dep en d . T h e ag reem en t be tw een th is p red ic tio n and the available m easu rem en ts was a clear tr iu m p h for the theo ry .

T h e D ebye-H iicke l th eo ry of s tro n g e lec tro ly tes is a ‘m odel th e o ry ’ in th e classical tra d itio n — a theo ry based on a bo ld sim plify ing co n cep t— in th is case th a t of the ionic a tm osphere . T h e sim plification w hich m akes the th eo ry trac tab le lies in the assu m p tio n th a t the ionic a tm o sp h ere is a static en tity , in w hich density fluc tuations can be ignored , and it was no t long before F ow ler was able, in 1925, to ju stify th is a ssum p tion for very d ilu te so lu tions on the basis of statistical m echanical reasoning . B ut the second p a rt o f the theo ry , hav ing to do w ith the tra n sp o rt p ro p ertie s of s trong e lec tro ly tes, stood up less well to su b seq u en t sc ru tiny . In 1928, five years a fter it was p u b lish ed , O nsager d rew a tten tio n to a lack o f sym m etry in D ebye and H iick e l’s assu m p tio n s abou t the m otion of an ion and the m o tions of its ne ighbou rs; h is su b seq u en t m odification o f th e ir co n d uc tance theo ry , w hich cam e to be know n as the D eby e-H iick e l-O n sag er theo ry , has rem ained v irtua lly unchallenged ever since.

T t-E L E C T R O N SYSTEM S

V aluable as H u ck e l’s co n tr ib u tio n to the theo ry of e lectro ly tes u n d o u b ted ly was, he w ould p robab ly have w ished to be rem em bered p rinc ipally for his en tire ly original and p ro found ly im p o rtan t co n trib u tio n s to theo re tical o rganic chem istry . It was he w ho first show ed, in a series of papers da ting from 1930, how the pecu liar s tab ility o f the benzene ring and o th er arom atic system s could be in te rp re ted by an elegant app lication of the in d ep en d en t-e lec tro n m odel orig inally p ro posed by F. Bloch for the descrip tion of crystals. W ith H u n d and M ulliken , H uckel is now accepted as one o f the founders of the m olecu lar o rb ita l theo ry , w hich is now universally em ployed by chem ists for in te rp re tin g the physical and chem ical p ro p ertie s of com plex m olecules.

B etw een 1923, w hen the D ebye-H iickel theo ry was first pub lished , and 1930, w hen H uckel pub lished his first paper on benzene, physics had undergone a m ajor revo lu tion . T h e new q u an tu m theory had been




p ro p o u n d ed and b rillian tly confirm ed, and in 1927 the existence of the chem ical bond had been accounted for by the w ork of H eitle r and L ondon on the hydrogen m olecule. T h e H e itle r-L o n d o n theory was ano ther ‘m odel th eo ry ’, ow ing its success to the sim plic ity of its approx im ations. H uckel m ust have realised th a t the m olecules of u n sa tu rated and arom atic hydrocarbons are such com plex system s tha t they w ould be unlikely to yield to any q u an tum -m echan ica l theo ry w hich did not rely upon bold initial assum ptions. H uckel therefo re d iv ided the electrons of such m olecules into tw o sub-system s, w hich he called the electrons of ‘the first k in d ’ and those of ‘the second k in d ’. E lectrons of the first k ind are tigh tly bound , b u t those of the second k ind— now u n iv ersally know n as Tt-electrons— are d is tr ib u ted over one-e lec tron states— now know n as m olecular o rb ita ls— w hich are m ade up from atom ic p orb ita ls w ith axes perp en d icu la r to the p lane of the m olecule.

It w ould be natu ra l to suppose th a t E rich H iickel’s ideas abou t the s tru c tu re of benzene and sim ilar m olecules were influenced by his b ro th e r W alter, b u t in later years E rich could not rem em ber such an influence having been exerted by his b ro th e r. H e th o u g h t th a t d u rin g th a t tim e in Leipzig it was p robab ly H u n d w ho had had m ost influence on him ; H u n d was at the tim e w orking on d iatom ic m olecules and had in troduced the sym bols a and n for classifying th e ir m olecular orb itals.

In his trea tm en t of p lanar organic m olecules such as benzene and ethy lene H uckel used two d ifferen t q u an tu m m echanical approx im ation m ethods. H aving assigned the electrons of ‘the first k in d ’ to a states w hich w ere sym m etric abou t the m olecular plane, he considered two kinds of wave function for the 7t-electrons. T h e first type of wave function was analogous to the H eitle r—L ondon wave function for the hydrogen m olecule, and its application to organic m olecules is now of m erely h istorical in terest. T h e second type of wave function was one in w hich each pair of 7t-electrons was assigned to a separate m olecular o rb ita l, and each m olecular orb ital was rep resen ted as a linear com bination of carbon 2 p o rb ita ls. T h u s in e thylene, apart from the core electrons of the two carbon atom s, there w ould be 10 electrons of the first k ind and ju s t two of the second kind; these two electrons could e ither be assigned a H e itle r—L ondon wave function or allocated to a m olecular o rb ita l w hich was an tisym m etric about the m olecular plane b u t sym m etric abou t the o ther two planes of sym m etry of the m olecule. In the case of benzene there are six e lectrons of the second kind, and the m olecular orbital approach assigns them in pairs to th ree orb ita ls construc ted from the six atom ic p o rb ita ls w hich are an tisym m etric w ith respect to the p lane of the m olecule.

H iickel’s theory of the p lanar u nsa tu ra ted hydrocarbons was, in fact, an application of the m ethod of m olecular one-e lec tron states invented in 1928 and 1929 by H und , H erzberg and M ulliken . T h e basic assum ption of H u ck e l’s theory can be sta ted extrem ely sim ply. If a m olecular orbital



Erich Hiickel 159

is rep re sen ted in the fo rm

= C l 4 > i + C 2 4 > 2 + • • • + c n il/ r » ( 1 )

th en its o n e-e lec tro n energy E and the values o f the coefficients cx, . .. ,c„ are jo in tly given by the e igenvalue eq u a tio n

n

Z Hij Cj = Ect= 1 , . . . , n), (2)j = i

w here the e lem en t H tj is assigned the value a if if i and j refer toad jacen t a tom s, o r 0 o therw ise . T h u s for benzene equa tion (2) becom es

' ol - E f t f t ' ■*r 'O'f t ol - E f t 0

f t o l - E f t _f t o f t

f t ol - E f tf t f t ol - E T 6 . 0

in w hich a m ay be th o u g h t o f as the energy o f an e lec tron in an isolated ca rbon p o rb ita l, and OL + ft is the energy o f one of the 7r-electrons of e thy lene. F o r equa tion (3) to hold E m u st satisfy the s ix th -o rd e r equa tion o b ta ined by se tting the d e te rm in a n t o f the 6 x 6 m atrix equal to zero; its roo ts are

E = tx + 2P, OL + ft (tw ice), a — (tw ice), a —2ft (4)

Since ft is a negative q u a n tity the th ree low est energy m olecu lar o rb ita ls have energ ies equal to a + 2/?, OL + ft and A ssigning tw o 7r-electrons to each of these o rb ita ls we ob tain a total energy equal to 6a + 8p, w hich differs by an am o u n t 2ft from the energy of the 7t-electrons in th ree e thy lene m olecules. T h e “ delocalization e n e rg y ” 2ft is negative and accounts for the relative stab ility of the benzene m olecule in com parison w ith th ree separate e thy len ic system s.

A lthough its theo re tical po ten tia l took m any years m ore to be app rec ia ted , H iickel’s theo ry of the stab ility of benzene led im m ediately to an exp lanation of a p h enom enon of long fam iliarity to organic chem ists, nam ely the stab ility of the ‘arom atic se x te t’. If one com pares the th ree neu tra l ring m olecules C 5H 5, C 6H 6 and C 7H 7 then benzene is the m ost stable of the th ree . B ut the m ost stable of the positive ions derived from these m olecules is C 7H 7 , and of th e ir negative ions C 5H J has a special stability ; C5H5 and C 7H f are unknow n. I t appears, in short, th a t a cyclic m olecule is p articu la rly stable if it has exactly six Tt-electrons, and th is resu lt em erges d irectly from H u ck e l’s theory : the low est m olecular o rb ita l is invariab ly non -degenera te , w hereas the h igher o rb ita ls occur in degenera te pairs w ith equal energies. If n such pairs of o rb ita ls are filled (as well as the low est o rb ita l), we ob tain a closed shell of exactly 4« + 2 Tt-electrons, and for progressively h ig h er values of n th is form ula gives



160

closed shells w ith 2, 6, 10, . . . , Tt-electrons. It is a well know n fact that cyclic m olecules w ith these nu m b ers of Tt-electrons exhib it the peculiar stab ility often described as ‘arom atic ch a ra c te r’; the cyclic hydrocarbon ^ 1 8 ^ 1 8 is one of the m ore recently d iscovered m olecules of th is type.

It is no t w ithou t in te rest th a t H u ck e l’s m olecular o rb ita l theory p red ic ts th a t C 4H 4 should have a zero delocalization energy, b u t his ‘first p ro ce d u re ’ -analogous to the H e itle r-L o n d o n m ethod and p u rsu ed for a tim e by S later and Pau ling— a ttrib u tes a finite ‘resonance energ y ’ to th is m olecule, w hich for m any years afterw ards defied all a ttem p ts to p repare it. In the years follow ing his fundam enta l work on the double bond H iickel always p referred the m olecular o rb ita l theo ry , and one m ay regard th is as a sign of his sound chem ical instincts. T h e re is a sense in w hich the theory allows one to regard p lanar conjugated system s as a na tu ra l class of substances, each m em ber of the class being characterized by a s tru c tu ra l descrip tion of elegance and sim plic ity— the H uckel m atrix . T h e p roperties of these m atrices and the eigenstates to w hich they can rise have been the subject of a very great deal of theoretical work in the last 50 years; it is aston ish ing how m any of the signposts to such research areas w ere set up by H uckel him self. H e show ed, for exam ple, tha t his wave functions for benzene could be used as the basis for a p e rtu rb a tio n a l calculation of the effects of su b stitu en ts on the d is tr ib u tion of e lectrons over the atom s of the ring, thereby supp ly ing a partial in te rp re ta tio n of the chem ical reactiv ity of su b s titu ted benzenes. In later papers he calculated the m olecular orb ita ls of larger arom atic h y d ro carbons such as naph tha lene and an th racene , and show ed how their delocalisation energies could be calculated as num erical m ultip les of the s tandard resonance in tegral /?. T h e concept of delocalisation energy enabled him to in te rp re t the low heat of dissociation of hexaphenyle thane in to a pair of tripheny lm ethy l radicals, and he show ed that the lowest unoccupied m olecular o rb ita ls of arom atic system s could in p rincip le accept additional electons, thereby accounting for the existence of the negative ions w hich are p roduced by reaction betw een arom atic h y d ro carbons and alkali m etals.

H uckel did not h im self study the p rob lem of light abso rp tion by u nsa tu ra ted and arom atic hydrocarbons, though he m ust have appreciated the analogy betw een optical transitions in atom s and those taking place in m olecules. C uriously enough the first calculation of the near u ltrav io let abso rp tion frequency of benzene was perfo rm ed by F o rste r (1937) on the basis of H iickel’s ‘first p ro ced u re ’. P erhaps H uckel was too well aware of the unreliab ility of excitation energy calculations to a ttem p t to calculate them by an in dependen t-e lec tron m odel. It was not until the 1940s and 1950s th a t theo rists acqu ired sufficient experience and confidence in dealing w ith e lectron repu lsion , to be able to in te rp re t near u ltrav io let spectra w ith any reliability .

P articu larly since the advent of the e lectronic com puter, H iickel’s

Biographical Memoirs



Erich Hiickel 161

orig inal sim ple th eo ry has spaw ned a considerab le n u m b er of varian ts, d is tin g u ish e d by equally m any acronym s. T h e relative m erits o f these m eth o d s have been w idely d iscussed , b u t possib ly the m ost in fluential, at least am ong organ ic chem ists , was the so-called ‘ex tended H iickel m e th o d ’ w hich L o n g u e t-H ig g in s and R oberts first in tro d u ced (though not by th a t nam e) to accoun t for the e lec tron ic s tru c tu re s of the B6 o c tah ed ro n and the B 12 icosahedron . W oodw ard and H offm ann; w ho co ined the nam e, used the ideas of the theo ry w ith g rea t elegance and effectiveness for in te rp re tin g the stereospecificities of a n u m b er of o rgan ic rea rran g em en ts , in te rm s of little m ore th an the sym m etries of the m o lecu lar o rb ita ls involved . It could be well c la im ed th a t no o th er sc ien tis t of P ro fesso r H iick e l’s genera tion has had a g rea te r influence u p o n the su b seq u en t d eve lopm en t of theo re tical o rganic chem istry .

T h e p h o to g ra p h rep ro d u ced was taken by T ita Binz, M an n h eim , in1965.

B i b l i o g r a p h y

Strong electrolytes1923 (With P. D ebye) Z ur Theorie der Elektrolyte. I. Gefrierpunktsern iedr igung und verwandte

Erscheinungen. Phys. Z . 24, 185; II. Das Grenzgesetz fur die elektrische Leitfahigkeit. Phys. Z . 24, 305.

1924 (W ith P. D ebye) Bemerkungen zu einem Satze uber die kataphoretische W anderungsgesch- windigkeit suspendierter Teilchen. Phys. Z . 25, 49.Die Kataphorese der Kugel. Phys. Z . 25, 204.Z ur T heorie der Elektrolyte. Ergebn. exakt. N aturw . 3, 200.

1925 Z ur T heorie konzentrierter wasseriger Losungen starker Elektrolyte, Phys. Z . 26, 93.Z ur T heorie der Membrangleichgewichte . Zsigmondy-Fest-schrif t , E rganzungsband zu K olloidzeitschrift 36, 204.Berichtigung hierzu. Kolloidzeitschrift 36, 319.

1955 (W ith G. Krafft) U ntersuchungen uber den Einfluss der endlichen Ionengrossen auf das therm odynam isehe Verhalten von Elektrolvtlosungen. Z . phys. Chem. (Neue Folge) 3, 135. Notiz hierzu. Z . phys. Chem. 5, 305.

1959 (W ith H. Schaaf) U ntersuchung uber die Konzentra tions- und Tem pera turabhangigkeit der Zahigkeit wasseriger Losungen starker Elektrolyte. IV. Mitte ilung. Z. phys. Chem. (Neue Folge) 21, 326.

n-Electron systems1930 Z ur Quantentheorie der D oppelbindung. Z. Phys. 60, 423.

Z ur Quantentheorie der D oppe lb indung und ihres stereochemischen Verhaltens. Z. Elektro- chem. angew. phys. Chem. 36, 641.

1931 Quantentheoretische Beitrage zum Benzolproblem.I. Die Elektronenkonfiguration des Benzols und verwandter Verbindungen. Z. Phys. 70, 204.II. Quantentheorie der induzierten Polaritaten. Z. Phys. 72, 310.

1932 III . Quantentheoretische Beitrage zum Problem der aromatischen und ungesatt igten Verbindungen. Z. Phys. 76, 628.

1933 IV. Die freien Radikale der organischen Chemie. Z. Phys. 83, 632.1936 U b e r die C -C -B indung in Hexaphenvlathan. Kritische Bemerkungen zu Arbeiten von H. E.

Bent and M itarbeitern. Z. phys. Chem. Abt. B, 34, 335.Z ur Theorie des Magnetism us sogenannter Biradikale. Z. phys. Chem. Abt. B, 34, 339.

1937 Kritische Betrachtungen zur Theorie der Substitutionsreaktionen an substi tuierten Ben- zolen. Z. phys. Chem. 35, 163.

1934 T heory of free radicals in organic chemistry. Tram . Faraday Soc. 30, 40.




1934 Aromatic and unsaturated molecules. Contributions to the problems of their constitution and properties, ln t. Conf. o f Physics, London vol. II, 9.

1935 Composes aromatiques et non satures. Conferences devant la Societe de Physique, p. 347.1936 Die Bedeutung der neuen Quantentheorie fur die Chemie. Z. Elektrochem. angew. phys. Chem.

42, 657.1937 Grundziige der Theorie ungesattiger und aromatischer Verbindungen. Z. Elektrochem. angew

phys. Chem. 43, 752.1951 (With B. Krause) Uber die relative Stabilitat von Methylacenen und M ethylendihydro-

acenen. Z. N aturf. 6a, 112.Nachtrag hierzu. Z. N aturf. 6a, 514.

1957 Zur modernen Theorie ungesatt igter und aromatischer Verbindungen. Z. Elektrochem. angew. phys. Chem. 61, 866.

Other fields1917 (With M. M i nk) Systematische Untersuchungen an Flugelprofilen. Ber. Flugzeug-

meisterei Bd. I, Nr. 5, 148. 1, no. 5, 148.1918 Weitere Gottinger Profiluntersuchungen, ibid. 2, no. 3, 407.1918 Der Profilwiderstand von Tragfliigeln, ibid. 2, no. 3, 451.1921 Zerstreuung von Rontgenstrahlen, Dissertation Gottingen.1923 (With M. Born) Zur Quantentheorie mehratomiger Molekule. Phys. Z . 24, 1.1925 (With R. Zsigmondy) U ber Reduktionsgeschwindigkeit und das W achstum kleiner Goldteil-

chen bei der Herstellung kolloidaler Goldlosungen. Z. phys. Chem. 116, 291.1928 Theorie der Beweglichkeit des Wasserstoff- und Hydroxylions in wasseriger Losung.

Z. Elektrochem. angew. phys. Chem. 34, 546.1932 Theory of heat evolved in capillary condensation. Trans. Faraday Soc. 28, 382.1933 Bemerkungen zur Bedeutung der sogenannten spezifischen Exaltation der Molrefraktion und

Moldispersion. Z. phys. Chem. A, 163, 67.1936 Bemerkung zur Thomsonschen Theorie der Kondensation an Ionen. Phys. Z . 37, 137.1944 Zur Theorie der natiirlichen optischen Aktivitat gasfbrmiger und flussiger isotroper Stoffe.

Z. Elektrochem. angew. phys. Chem. 50, 13.1948 Zur Theorie der S treuung langsamer N eutronen an freien Protonen. Z. N aturf. 3a, 134.1948 Berichtigung hierzu. Z. N aturf. 3a, 134.1951 (With K. F. Born) Die nicht-relativistische zweite Naherung der Bornschen Stosstheorie fur

das Yukawa-Potential . Naturwissenschaften 38, 302.1951 (With W. Bingel) Ein quantenmechanisches eindimensionales Vlodell fur spezielle lineare

Vlolekulketten (als denkbares Modell fur Kraftwirkungen zwischen Genmolekulen im Protoplasma). A nn In. Phys. 6. Folge, 8, 391.

1957 (With E. Ganssauge) Uber die Druck- und Temperaturabhangigkeit der Dielektrizitat- skonstanten von Fliissigkeiten. Z. phys. Chem. 12, 110.

1956 Die Entwicklung der Denkmodelle in der Chemie. N aturw . Rdsch. 3, 92.1970 Chemiker im Gesprach (Interview). Chem. Unserer Zeit. 4, 180.1972 Erinnerungen an Peter Debye und meine Lehrjahre. Phys. Bl. 28, 53.1928 Adsorption und Kapillarkondensation. Akademische Verlagsgesellschaft, Leipzig 1928.



erich hückel, 9 august 1896 - 16 february...

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