An electron diffraction study of the molecular structure of gaseous cyclobutylgermane

An electron diffraction study of the molecular structure of gaseous cyclobutylgermane

Jo,,rna~ o f _bfolecular Str~Jcture, 130 (19~5) 2 8 9 - - 3 0 0 I~Isevier S c a e n c e PublLshers D V , _~msterd~.-n -- Printed in T h e ]Netherlands...

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Jo,,rna~ o f _bfolecular Str~Jcture, 130 (19~5) 2 8 9 - - 3 0 0 I~Isevier S c a e n c e PublLshers D V , _~msterd~.-n -- Printed in T h e ]Netherlands

A~N E L E C T R O N D I F F R A C T I O N STUDY OF THE MOLECULAR STRUCTURE OF GASEOUS CYCLOBUTYLGEItMANE

MARWAN D _ ~ K O URI A b t e r l ~ n g f u r P h y s i k a b ~ c h e Chern~e, Unwers~tat Ulm, 7 9 0 0 [~rn (West .~errnany)

(Received 1~ March 1985)

ABSTRACT

The molecular structure and c o n f o r m a t i o n o f cyclobutylgermmne ha~ e been de~erm.ined by ga~-phase electron diffractmn. Like ~.ts c o u n t e r p a r t cyclohutylstlane (CBS) it p o s s ~ quasx-equatorial a n d quasi-axml c o n f o r m e r s

The

most

interesting ~spect of the structure

o f CBG is the influence of the g e r ~ y l group on the ratio of equatorial to axml co-nforme-~s. The predomlnance of the quasl-equato_~al corLforrner ( ~ G = 3.1(1) kJ mol-~), the near equality o f the skeleton C--C b o n d len=~ths (C--C = 1.55"~(3) A) (ra value) and the v ~ u e s o f the puckering angles for the equatom-,A form and the axmi one o f 25 3(3 1) ~ and --20 4(3 6) ° respectively, all s u p p o r t the predmtlons made by Jonvlk and Boggs concernmg the correlation between e l e c ~ o n e g a t ~ ! t y and s~uctur, Ll parameters m four-membered rings. F r o m a conslderatmn o f these predmtmns, a c o m p a r i s o n o f the racst promme'.,t structural factors in CBG and cyclobutylsflane indmates ~ at the ge_mamum atom is more electronegattve than silicon Th~s result could be conmdered as the first s~ructur~] evidence for the previously postulated ,nversmn of the eleet~onegat, v~ty orde- w~thm group IV INTRODUCTION D u r i n g t h e l a s t f e w y e a r s s e v e r a l w e i l - l ~ o ~ m g r o u p s in t h e t w o m o s t prominent fields o f s~mactural mvestaga~ion, s p e c t r o s c o p y and electron chff_ractlon, h a v e p a i d g r e a t a t v e n t l o n t o t h e stl-'dy o f d y n a m i c p r o c e s s e s , conformations a n d m t r a - r i n g i n t e r a c t i o n s w l t h u n tb.ree-, f o u r - a n d f i v e membered rings [i--11]. Systema~c studies, inc]udLng electron dfftraction, spectroscopm met,hods and ab inlhO calculations, have oeen perfo_rmed on a w i d e v a r i e t y o f s u b s ~ , t u t e d f o u r - m e m b e r e d rungs m o r d e r t o e s t a b h s h t r e n d s i n t h e C - - C b o n d l e n g t h s , t h e r i n g - p u c k e r i n g a n g l e , d i s t o r t i o n s o f t h e CH2 g r o u p s , a n d t h e angles r e v o l v i n g t h e s u b s t i t u e n t . T h e accurac¢¢, t h e r e h a b i l i t y , a n d t h e r e p e a t e d s u c c e s s o f t h e a b m i t l o c ~ c u l a t m n s h a v e p~- o b a b l y contributed to this development [12--16]. M , ~ r e o v e r , i n t e r e s t in t h e s e molecules has focussed on the determ,nation of the potenti~ function governing the out-of-plane Fag puckering vibratton [17, 18] and the ringinversion barrier height [ 19]. Ln o r d e r vo assess m m o r e de~ail rkle poss~_ble i n t e r a c t ! o n b e t w e e n s m a l l rungs a n d --XY_~ g r o u p s ( X = S i , O e a n d Y = H . h a ] c g e n ) a n d t o c o l l e c t f u r t h e r emp~cal d a t a f o r t h e p u r p o s e o f p e r f o r m u - g ~-ystematm c o n f o r m a t i o n ~ 0022-2860/85]803.30

© 1985 Elsevmr Scmnce Publr~_ers B V

290 stuches on ~hese monosubsti~uted s m a l l r~_ng m o l e c u l e s , w e h a v e s y n t h e s i z e d additlon'al representatives of the series (C,I-L,_ ,)XY3 (n = 3, 4, 5) [20, 21]_ T h e molecn]sr structures a n d conformations of five of these molecules have be_en ,determined. D a k k o u r i a n d O b e r h s m m e r [22] reported o n the mo]ecu]lar strac'~ure of cycloburylsL1ane a n d discn~sed the influence of the silyl group on the n n g g e o m e t r y a n d conformation. T h e present study of cyclobutylge,rnr.ne ( C B G ) w a s carned out in order to obtain addltSonal, m o r e pertinent results w h i c h m a y permit a m o r e systematic c o m p a r i s o n of structural effects of substituents. E X P E RIDI ~_J~"T_A_L C B G w a s s y n t h e s m e d in t h e a u t h o r ' s l a b o r a t o r y . I t w a s p r e p a r e d b y t r e a t ment of cyclobutyl-Gngnard w i t h G e C I ~ a n d f o l l o w i n g r e a c t i o n w i t h T,i AAH4. The sample was purified using a 30 cm spmnLng-band fractionation column. T h e b o i l i n g p o i n t wa~, 7 3 - - 7 4 ° C a t 7 1 5 T o r t . T h e p u r i t y - w a s c h e c k e d b y I ~ snd mass spectroscopy. Details of prepazation and purificaraon axe given elsewhere [21]. The electron diffraction scattering Lntensltms were recorded on Kodak e l e c t r o n i m a g e p l a t e s , usLng t h e B a l z e r s G a s D i f f r a c r ~ _ p h KD-G2 at the University of Tubmgen [ 2 8 ] . D a t a w e r e o b t a i n e d a t t w o c a m e r a dis%ances, 50 and 25 cm (nominal). Expenment~ c o n d i t i o n s a r e s u m m a r i z e d in. T a b l e 1. The electron waveleng~ was dete~nSned by analysis of the dJf£ractmn patterns of ZnO. The tranS.=hi,ion values were measured with a modified Elsean--9500 znlerodensi~-orneter [24]. These values were zeduc~-] to ~ne m o d ~ f m d m o l e c u l a r i n t e n s ! t i e s 8 * M ( s ) in intervals o f A s = 0 . 2 A - ' b~ r o u t i n e methods [25] for each photographm plate separately The averaged molecular i n t e n s i t i e s a r e p r e s e n t e d in F i g . i , t o g e t h e r w i t h t h e i n t e n s i t y / u n c t ! o n calculated for the final model. STRUOTURE

ANALYS!9

Based on a prevlous struc~ural study of cyclobutylsflane [22] I started the structure calculations vnth a mixture of quas]-equator~l s_n_d q u a s i - a x i a l TABLE I List of experm~emtal condltlon_~ Camera distance (ram) 500 250

Temperature (°C) Sample --17 --z4

Nozzle 10 20

aAccelerating voltage ~ 60 k V

Camera pressure (Tort) 5 x 1 0 -'s

1.5 x I 0 -s

Exposure tlme (s) 5---10

20--55

WaveieDg*.h (x)

s-Scale (A-*)

0.04872~(16)

1 4--17

0 048767(24)

6.6--35 0

8

291

CYCLOBUTYLGKRt4ANE ~IXTURE ~=m A X = 7 7

k_/

10

A

29

v

~

,

Fig 1 Experimental (~AA) and c_,~ lated (

v

23

A

v

3,0

SEA] 4

) molecular i~ten_~itJes a_nd dlfferences (x 5 )

conformers. In order to reduce the large n u m b e r of Lndependent parameters required to describe the two models for both conformez~ prope_rly (see Fig 2 for atomlc numbering used in defining structural parameters) the following assumptions were made: (1) T h e geometric pa_,~r~eters for both conformers are equal, except for the m o s ~ promanenz parameter, ~-le dhhedra] angle 8, the angle between the C~e---CI b o n d and the C2CiC4-plane and the L C 1 - G e H an gj e. ( 2) B y a n a l o g y ~ d t h hhe s i t u a t i o n in c y c l o b u t y l s i l a n e t h e d i f f e r e n c e b e t w e e n t h e O - - C z,_ng dis ~tances, A = C,C=--C2Cs, is c o n m d e r e d t o b e an i n d e p e n d e n t par-c~meter. ( 3) _All C - - H b o n d l e n g t h s a n d H C H angl es a r e e q u a l . ( 4 ) T h e tfi~, t w i s t a n d w a g angl es f o r t h e CH~ g r o u p s a r e z e r o . (5) S e v e r a l v i b r a u o n a l a m p h t u d e s w e r e f i x e d co r e a s o n a b l e v a l u e s . T h e s e w e r e m a i n l y t h o s e f o r n o n - b o n d e d i n v e r a t o m l c d i s t a n c e s w h i c h are a f f e c t e d b y s h r i n k a g e e f f e c t s a n d _~ng p u c k e r i n g ( T a b l e 2). The structural refinements were carried out based on molecular intensitms. T h e p r o c e d u r e d e s c r i b e d in [ 2 6 ] w as f o l l o w e c ' D m g o n a l w e i g h t s w e r e e m p l o y e d in t h e l e a s t s q u a r e s p r o c e d u r e as d e s c n b e d m r e f . 2 6 [Fne scavtering a m p l i t u d e s and phases o f Haase [27] were used t h r o u g h o u t . M o s t p r o b a b l y as a r e s u l t o f t h e i n t r o d u c t i o n o f t3ae r e s t r i c ~ o n s m e n ~ a o n e d above no considerable dffflcultms were met during the refinement The mare difficulty during the further !east-squares an~ysis, however, arose with the f i ~ i n g o f t h e H - - G e - - H a n g l e a n d t h e H C H angl e. I n i ~ a l l y t h e H C H a n g l e t o o k u p more_ o r less u n r e a s o n a b l y s m a l l v a l u e s b e h w e e n .°,5° a n d I 0 0 ° . w i t h high u n c e r t a i n t y . H o w e v e r , b y k e e p i n g s o m e of the pazan~eters c o n s t a n t and v a z T i n g o t h e r s , a n d g o i n g s y s t e m a d c a l ! y t h r o u g h all c o m b i n a t m n s w h i c h s e e m e d n e c e s s a r y , this a n g l e u n p r o v e d t o t h e s a t i s f a c t o l ~ ] v a l u e g i v e n in T a b l e 2. B e c a u s e o f h i g h c o n ' e l a t i o n s , t h e C - - G e - - H a n ~ e f o r t h e a x i a l c o n f o r m e r was fixed m t h e final stage o f r e f i n e m e n t to 1 1 0 ° . T h e final s t r u c t u r a l

292

H t

°°W~

~F'z3

JV" --N= hb~ " ] ~

H 12

F~g 2 Molecular models and atom numbenng for quasi-equatorial (left) end quam-axla] (right) conformer of CBG r e s u l t s o b t a i n e d f r o m t h e le;,st-squares r e f i n e m e n t are s u m m m n z e d in T a b l e 2. E r r o r l i m i t s a~e b a s e d o n 3 a v a l u c s a n d systemat~_e e r r o r s d u e t o c o n s t r a i n t s ar e i n c l u d e d . As m e n t i o n e d b e f o r e , w e i n t r o d u c e d t h e d i f f e r e n c e b e t w e e n t.he s k e l e t o n c a r b o n - - c a r b o n d i s t a n c e s , A , as a p a r m m e t e r . R e g a r d l e s s o f t h e s t a r t i n g v a l u e thin p a r a m e t e r a l w a y s r e d u c e d t o v e r y s m a l l val ues, v e r y c l o s e t o z e r o . DISCUSSION C B G p o s s e ~ e s , l i ke its c o u n t e r p a r t c y c l o b u t y l s d m ~ e , p s e u d o - e q u a t o r i a l a n d p s e u d o - a x i a l conforme~-s. T h e m o s t i n t e r e s t i n g a s p e c t o f t h e s t r u c t u r e o f C B G ~s t h e i n f l u e n c e o f t h e g e r m y l g r o u p s o n t h e r a t i o o f e q u a t o r i a l ~o a x i a l c o n f o r m e r s . T h i s r a t i o o f 7 7 : 2 3 is c o n s i d e r a b l y h l g h e r t h a u t h a t o f 5 9 : 4 1 in c y c l o b u t y l s i l a n e . T h e dete_n~una~ion o f t h i s r a t i o was m - ~ n l y e n a b l e d b y t h e relatave ar ea s o f t h e " p e a k s " a t 3 . 9 a n d ~.5 A in Lhe r a d i a l d m t n b u t i o n f u n c t i o n (Fig. 3). T h e n o n - b o n d e d CI . . . . C3 = 2 . 1 6 a n d C2 . . . . C4 ~ 2 . 2 A distances for the equatorial c o n f o r m e r indlcate that the ring m less disvorted t h a n in cyclobutylsilane (C I.... Cs = 2,14 a n d C~ .... C 4 = 2.20 A ) T h e s e distances for ~he ax,_~ c o n f o r m e r in b o t h C B G a n d cyclobut-ylsilane are nearly

t h e s a m e : Cl . . . . C3 = 2 1 8 a n d C2 . . . . C4 = 2 . 2 A. F u r t h e r , t h e e q u a t o r i a l c o n f o r m e r in C B G is less p u c k e r e d t h a n c y c l o b u t a n e ( 2 7 7 ( 1 . 3 ) = [ 8 ] ) , a n d ~he s a m e c o n f o r m e r in C B S ( 3 i . 8 ( 3 0) ° ) is m o r e p u c k e r e d t h a n Lhe m o t h e r m o l e c u l e . M o r e o v e r , t h e C - - C b o n d l e n g t h s m C B G d o n o t d i f f e r g r e a t l y f r o m t h e s a m e d i s t a n c e ( 1 . 5 5 1 ( 0 . 0 0 1 ) A ) ,_n cyclobutane [8]. B o t h t h e pueken_ng a n g l e 8 a n d t h e e q : a x r a t i o F-roved t o b e a s e n s l h v e i n d m a t o r f o r t h e e x p e c t e d m t e r a c t a o n b e t ~ v e e n s u b s t i t u e n ~ s ar.d r i n g s k e l e ~on in m o n ~ s u b s t i t u t e d f o u r - m e m b e r e d r i n g - s y s t e m s . Tbds has b e e n shova-. i m p r e s s i v e l y b y ~_xtended a b m i t i o c a l c u l a t i o n s c a r r i e d o u t b y J o n v i k a n d B o g g s [ 1 4 , 1 5 ] . 1~ t/les.: ~b i n i D o s t u d i e s i t was p r e d i c t e d t h a t Lhe l a r g e r t h e e l e c t x o n e g a t a v i t y o f fine s u b s t i ~ u e n t : (1) t h e m o r e Lhe !engLh o f C1--Cz in c o m p a r m o n ~ u t h Ce--C3 deerem~es, ( 2 ) t h e l a r g e r *,he p u c k e r i n g a n g l e s o f t h e

293 TABLE 2 F i . . l s t r u c t u r a l r e s u l t s f o r c y c l o b u t y l g e r m a n e (CBG)-mixl-.tre ( D i s t a n c e s (ra) a n d alnplit u d e s o f v i b r a t i o n (l) in A , angles m d e g r e e s . A t o m numbeH_ng is given in F i g 2 F o r e r r o r h m i t s see t e x t . ) Parameter

Equatorial conformer I

r~

(a) I n d e p e n d e n t p a r a m e t e r s C,---C -. 1 557 (3) C=--C= 1 558 c Ge--C 1 9 4 8 (4) C--H : O85 (4) Ge--H 1 513 (S) /_C=C,C~ 89 S (0 7) LC,GeH 1 0 3 . 7 (2 9) LHCH 105 9 (3 2) L~._C,C,, 131.2 (I I) Oa

25 3 (3 1)

% Rd

77 (3 ! ) 0 04 (v)

Axizl conformer re

I

o o 5 9 (2)

0 059 (2 ~ 0 070 (3) 0 0~s (4)

0 073 (9) 110 b 1 2 6 . 4 (1.9) --20.4 (3.6)

(b) I m p o r u a n t n o n b o n d e d d t s t a n c e s a n d vi ~ r a t i o n a l a z n p h ~ d e s C,--C~ 2 158 0 0 6 2 (5) 2 176 C~--C 4 2.195 0 060 b 2 195

C;--C.-Te

0 0 6 2 (5) 0 060 b

C.,--Ge

3 009 3 904

0.107 (9) 0 135 (32)

2 962 3.503

0 . 0 5 9 (16) 0 . 1 2 0 (50)

Ge--HL~ Ge--Hto C 3- -H~ C:--H 7

3 3 3 3

0 128 (29) 0 170 ¢ 0.163 b 0 124 ( 1 4 )

2.971 3.846 4.5!1 3.510

0 128 (29) 0 170 c 0 130 b 0.125 (15)

071 650 695 419

a p u c k e m n g angle, b N o t r e f - m e d ment factor

C C o n s t r a i n e d in t h e final l e a s t - s q u a r e s r e f i n e m e n t , d A g r e e -

equatorial forms, (3) the smaller the puckering anises of the axial forms, and (4) the greater the stabihty of equatorial conformers B e f o r e d , s c u s s i n g t h e s t r u c t u r a l r e s u l t s o f CBC4 i n t h e h g h t o f t h e s e r e l a tionships I would hke to make some remarks on the concept of electronegatwity There are numerous scales for electronegatixdty, and many methods o f p r o d u c L n g t h e s e s c a l e s [ 2 8 ] . T h i s r e s u ,Vs i n t h e c o n t = m u o u s g e n e r ation of new definitions for this notion. Controversies have aIisen between a number of authors about the interpretation of the concept of electronegativity [29--3!]. All of them claim to have evaluated the mos~ precise d e f h ~ . i t i o n f o r i t T h i s m a y be_ t h e m a i n r e a s o n f o r t h e w h o l e c o n c e p t being r e g a r d e d w i t h s o m e s u s p m , _ o n . I b e l i e v e t h a t i t i s d i f f i c u l t t o d e t - m e t~hls c o n cept precisely. However, +.he m o s t s a t i s f a c t o r y definition seems to be that %he e l e c t r o n e g a t i v l t y i s t-he p o t e n ~ a l w i t h w h i c h ~m e l e c t r o n is a t e x a c z e d i n t o an atonnc orbital of the bonded atom. This Dotentaa/ res~flts not only from

294

I

CvCLOBUTYLGERMANE b!,XTORE EQ A X = 7 7

FI

23

F I

I

t II

[ t-

I,

/

i

F i g 3 E x p e r m a e n t a l (A•) a n d c~dculated (

) r a d m l d~s~ribution f d n c t i o n .

t h e a t t r a c t i o n o f t h e a t o m u n d e r c o n s i d e r a t a o n b u t also f r o m t h e i n ~ e r a c t z o n s o f a~l t h e o t h e r e l e c t r o n s a n d a t o m s o f t h e m o l e c u l e [ 3 2 , 3 3 ] . A c c o r d i n g t o this defLnition I suggest using the g r o u p e l e c t r o n e g a t i v i t y , if the substStuent is a g r o u p , a n d n o t o n l y t h e e l e c ~ o n e g a t i v i t y o f t h e a t o m u n d e r c o n s i d e r a t i o n m th is g r o u p . T.hAs s h o u l d b e d o n e b e c a u s e , as is a p p a r e n t from_ T a b l e 3, a t o m a n d g r o u p electro~_egativities d i f f e r s i g n i f i c a n t l y Ln m o s t cases a n d o n l y Jr. a f e w cas e s is t h e d , f ? e r e n c e s mal l . T a b l e 3 s u m m a r i z e s s o m e s e l e c t e d e x a m p l e s o f t h e d i s c r e p a n c y betwee_n a t o m e l e c t r o n e g a t i v z t y a n d t h e el ectronegatavity of groups including that atom. As an i l l u s t r a t a o n o f t h e l a t t e r p o i n t t h e u s a g e o f g r o u p e l e c t r o n e g a t i v i t , _ e s in r e l e v a n t c a s es w o u l d l e a d t o a s h g h t l y d i f f e r e n t r e s u l t f o r a p l o t o f E ~ x E ~ a g a i n s t elec~or_.egatiw~y t h a n t h a t p r e s e n t e d b y J o n v i k a n d B o g g s [ 1 5 ] . T h i s p r e s e n t w o r k ,~as~,cally c o n s i d e r s t h e s~rne d a t a as J o n v l k a n d Boggs p l u s t h o s e o f C B G h u t in o r d e r t o r e m o v e all ambiguit3" d a t a f o r m o l e c u l e s ~ t h m o r e t h a n t w o c o n f o r m e r s h a s b e e n o,-~Litted. T h e vm-ious possibilitSes f o r t h e r e p r e ~ n ~ a t i o n o f t h e c a l c u l a t e d a n d expezamenf4al]y d e t e r m i n e d d a t a f o r t h e - - G e H 3 , --CH3 a n d - - S i l l 3 s u b s t a t u t e d c y c l o b u t a r _ e s are ~A~own in Fig. 4. T h e a p p a r e n t i n v e r s i o n o f t h e - - O H 3 a n d --SiI-I 3 p o s i t i o n s m t h i s ~_~zaph zs d u e t o exp~.~i~--~enta! unce_-ff~q~ties I n g e n e r a l i t c a n b e s e e n t h a t t h e use o f g r o u p elecvronegativit~¢ p r o d u c e s , m m o s t cases, b e t t e r f~e t o t h e p~;dzcted knea~ rela~onshxp b e t w e e n ~ E and electronegat2x~tT. C o n s i d e r agai n t h e reiat2on~'alps d e v e l o p e d b y J o n v i k a n d B o g g s a n d ~o o u r r e s u l t s . A s zs a p p a r e n t f r o m T a b l e 4, C B G s e e m s ~o # e p r e s e n ~ a v a l i d i v y test for those relatzonships. Tne value for the difference of the skeletal C - - C d~stances Ln C B G £i~s v e ~ - w e l l w i t h i n t h i s list o f t h e s - y s t e m a t i c decrease_ m t h e C~--C2 b o n d d i s t a n c e -with z e s p e c t t o t h e C2--03 b o n d len~oT,h in s o m e

295

3

TABLE

Comparison groups

between

Atom

atom-

and

group

electronegativn~ie_¢, o f s o m e

E_N

Group

(i) C S: Ge N P

selected

atoms

and

EN

(II)

(IH)

2 3 2 2

2.27 2 21 2.32 2 61

0

2 5 1.74 2 02 3.07 2.06 3.5

---CH~ --SiH~ ---~eH~ --NH= --PH: ---OH

3 35 2_13 3 7

S

2 44

--SH

2 8

2 32

F

4.1

--CF,

3 35

3 46

CI

2 83

-<~Cl 3 ~ N

3.0

2 84

Set (I) fromref

3 51

3.3

3 4 , S e t ( I I ) e m p i r i c a l v a i u e s f r o l n rex 3 5 , S e t ( H I ) f r o m r e f . 3 8 ( c a l c u l a t e d

v~ues).

/

8

4

--

r ...... /CI G e H 3,:_= . ~ _ .

,~- ~--: ,,.. /,~o_o__j C H 3

M,I

<3

_ __ o_j

0

-4 0

....

3

J

I

i

I

I

2

3

4

ELECTRONEGATZVITY

(_V)

F i g 4 E a x - - E e q = ~ E (-'n k J t o o l -~) 7 s e l e c ~ o n e g a ~ i v l t y o f tJ:e s u b s t i t u e n t (~) C alculased A E vB XaLom, (X) e x p e r - ~ r n e n~nl~. , _ . ~ d e t e r m i n e d A E -vs Xa*.~m; ( 0 ) c a l c u l a t e d _ ~ _ E ' s Xgroup, (a) experk-nentalIy de~rmlned ~E vs X~oup

296 TABLE 4 C - - C B o n d " - * h difference a n d p u c k e n n g cyc]t but~--nes

angles 0 (~n degrees} in s o m e

e.CC a

C,H~ F C4H , CI C,H_ Br C 4 H , C-ell3 C,H~ C H ~

C,H~ SlH~

o eq

--0.025

--13 4 --

--~).016

--0 008 ---0 0 0 i i ~0.0 0 002 0 016 0.0!6

C , H ~ L* a.~cc gRe[

8 ax

0 032

= C.C..---C~C 3 in A 15 nRef 22

b(

monosuh~£ltUf.ed

Method

26 9

ab initm c

--20.3

20 (1) b 25 1

},~-1-9(r0) d a b mitlo c

---20.4 (3 6) b --19 7 --23 3 (4.1)b --20 3

-25 3 (3 I)b 24.1 31 ~ (3 0) b 21 8

IR ~

--26

23 7

ab mitlo ¢

)uncertainties

5 CRef

14

dRef

36

eRef.

ED f a b m,t~og ED h ab init~oh

37

fThis

~ork.

monosubstitut~d cyclobutanes. Furthermore, Table 4 shows that if we accept t h e a r r a n g e m e n t given m t hi s t a b l e , t h e r e is a d e f i n i t e t e n d e n c y f o r t h e p u c k e r i n g an gl e 8 zo d e c r e a s e in t h e a ~ l f o r m a n d f o r t h e s~rpe angl e m t h e e q u a t o r i a l f o r m t o i n c r e a s e Th~s agrees v n t h rahe s e c o n d a n d th,_rd r u l e f r o m the relationships derived by J o n v i k and Boggs C o n s i d e r a t i o n of t h e same c o m p o u n d s h s t e d kn T a b l e 4, again a c c e p t i n g th~s o r d e r , w i t h r e s p e c t t o t h e p e r c e n t a g e o f t h e e q u a t o r i a l c o n f o r m e r ( T a b l e 5) i n d i c a t e s u n a m b i g u e u s l y t_he v a l i d i t y o f t h e f o u r t h r u l e in t h o s e r e l a t i o n s h i p s . T a b l e 6 c o m b i n e s t h e r e s u l t s o f T a b l e s 4 rand 5 a n d c o m p a r e s t h e m w i t h t h e a p p r o p n a t e e l e c t r o n e g a ~ i v i t y v a l u e s ( m t hi s T a b l e t h e ~ r o u p e l e c t r o negat~_vlty is u s e d i f t h e s u b s t l t u e n t is a ~ o u p ) a n d l e ~ d s t o t h e f o l l o w i n g c o n c l u s i o n : T h e s y s t e m a t i c d e c r e a s e in e l e c t r o n e g a t i ~ d t y f r o m +.he g e r m y l g r o u p t o m e t h y l - a n d t o sflyl g r o l i p s c a l c u l a t e d b y H u h e e y [ 3 8 ] undoubtedly correct Furthermore, ~cause of the substand~ difference b e t w e e n t h e p e r c e n t a g e s a n d A E o f th~ e q u a t o m a l f o r m s f o r c y c l o b u t y l s f l a n e a n d c y c l o b u t y l g e r m a n e , it can be p o s t u l a t e d t h a t t h e g e r m a n i u m a t o m is m o r e e l e c t r o n e g a t i v e t h a n t h e s i l i c o n a t o m . T h i s in t u r n c l e a r l y s u p p o r t s t h e n e w e l e c t r o n e g a t a v l t y o r d e r f o r t h e g r o u p I V e l e m e n t s e v a / u a t e d b y Allr~ -~_ and Rochow [29] and others [39]. They presented m seve~/~apers extensive chem_ic~ a n d N M R , N Q R s p e c t r o s c o p m e v i d e n c e f o r t h e v a h d i t y o f this n e w o r d e r " X= > X c ~ > X ~ ~ XsnLet

us

assume

*.ahat n o

drastlc

compesmg

electronlc

change--, o n

%he

ger-

manlum or silicon a t o m s c a u s e d b y effects other t h a n electT_onegativ,ty difference are posqlble. W e c a n c o n c l u d e thaE o u r results for C B G a n d cyclobuty]2flane, es[ _~cis!ly the p e r c e n t a g e of the equatorial c o n f o r m e r , s u p p o r e. the o r d e r of electronegatlvlties s u g g e s t e d b y Allred a n d R o c h o w . To our knowledge +~rlis is ~he flrst structural e v l d e n c e for the inverted o r d e r in e!ec~onegativlr~y values v.nt_hin the g r o u p I V e l e m e n t s . A n o t . h e r possible

297 5

TABLE

Percentage of equatorial forms and energy difference f o r m e r s in s o m e m o n o ~ u b s t ~ t u ~ e d c_vclobutanes

bet~ een

equatorie/

and

~,w~al c o n -

Equatorial conformer (~)

z,E(aG) (K~ moP' )

Method

97 98

~ 6 C.l

C . H ~ C'-

S5

C+H~ Br

84

4.3 4 9.

FIR a ab ,mtio b a b ~nit~ob IRe

C~H~ F

C,H~

GeH~

7 7 (7)

C.H~ CH,

S~

C,H~

SiI-I,

5 9 (5)

C~H.

L~

54 32

~, 0 (I) ! q 0 8 (0 4)

ED d

a b Ynh;io e ED f

0 4 --] .8

a b in,t,o* a b in,t o b

TABL]~I 6 Cornpar,son between e]ecbronegah~,ty m o n o s u b s t . t u t e d cb c l o ~ u t a m e s

and

some

~mportant

1£1~cza'on e F ~ v i t y o f su]ostJtuent

Eq-coz~f or-J~er (%)

C~H~ F C . H - C1 C.M- B_"

4 I0 a 2_8fla 2 74 a

98 85 84

4-3

--~ 0 1 6

4 2

---0 0 0 8

C,H. C ~

2.27 b

66

1 7

59 (5)

0 8 (0 ~-)

C.H~ I2.

0.97 a

32

A~"('kG) (KJ mo/-~ ,

9 1

--i 8

-~'cc

structu-,-al

clara '-n s o m e

8ax

9eq

Method

--!3 4 --ZO 3 --

25 9 ~-5 ! --

ab m _ ~ o c ab LnaZaoc

0 002

--19 7

~

al~ mxt~o f

0 016 0 032

--23 3 "4 I)

3!.8 (3 0)

--2S 5

23 7

(~.) --

1

IR d

EDg ab L-_tuo c

i n t e r p r e E a t i o n f o r t h e r e s u l t s d i s c u ~ e d h e r e c o u l d be r/hat t h e g e r m a n i u m a t o m is m o r e ~-eleclxon acceptor and less G-donoz than sillcon a t o m Bearing in m i n d f/aat electronega£ivil;y is the potential wi~h which an electron i'~ e l e c t ~ n ~ e x c h a n g e ~ , in, rrry o p i n i o n , n ~ h m g m o r e ~han t h e ~ u ~ of elec~on~g~+a-~, dgfexen~e. Hargi~tai and Bliefert have recently reported ~nth s o m e reservatic_ £.at the values obtained from the estimation of elecbconegativlties of gro ~p IV e]~rn~ ~m~ ~=C C~.~ ~C ~ m ~i~t~ ~m~K~ ~ j ~ < K ~- E~.~ , G ~ indic~ ~ u p p o ~ 2or b~e g a ~ b ~ a t ~ h~'er~on o~ ~he e~_ec~'oneg'ade~-X- order v ~ k h m group IV [40]. A modified Schomaker--Stevenson equatien suggested by Rob~ette [41] was used for thelr estimation of the eiectronegativity values.

298 I t ~Auou]d b e n o t e d t h a t t h e d e c r e a s e in t h e dih.edral ~ngle f o r t h e ~ i a l c o n f o r m e r f r o m O= = 2 3 3 ° in c y c l o b u t y l s i l a n e t o 0 = = 2 0 4 ° m C B G ¢~hows a n e x c e l l e n t a g r e e m e n t w i t h t:ne t h i r d z"~de o f t h e relaifonsb_lps p o s t u l a t e d b y J o n v i k a n d B oggs . H o w e v e r , t h e b e h a v i o u r o f t hi s a n g l e f o r t h e e q u a t o r i a l c o n f o r m e r s b o w s a c o n s i d e r a b l e d i s a g r e e m e n t ,~4th t h e s e c o n d r u l e o f t h e s e r e l a t i o n s h i p s . I t d r o p s fl ) m 0 ~ = 3 1 . 8 ° f o r c y c l o b u t y l s i l a n e t o 0 ~ = 2 5 . 3 ° f o r C B G . I h a v e n o e x p l ~ , , a t i o n f o r t hi s b e h a v i o u r . T h e a u t h o r s w o r k i n g g r o u p has c a r r i e d o u t a n u m b e r o f s v r u c t u r a l s t u d i e s o n c y c h c m o l e c u l e s o v e r t h e p a s t f e w y e a r s a n d it h a s f r e q u e n t l y b e e n o b s e r v e d t h a t r e l a t i v e l y s h g h t va_riaticns in e x o c y c l i c H C H angles e n g e n d e r i n m o s t cases c o n s i d e r a b l e n o n - s y s t e m a t i c d ~ s t o r t l o n o f i m p o _ r t a n t n n g par~--~eters. ~'or e x a m p h . , t h e r a h a b _ o n o f t h e H C H angl e in C B G be~-ween 1 0 3 ° a n d 1 1 2 ° c a u s e s t h e p u c k e r i n g a n g l e f o r t_he a x m l c o n f o r m e r t o r e d u c e f r o m 29 ° ~o %9° a n d f o r t h e e q u a ~ o r m l c o n f o r m e r f r o m 2 6 ° t o 2 2 °. C o r r e s p o n d i n g l y , t h e ' C G e H ) ~ a n g l e c h a n g e s f r o m 1 0 8 ° t o 1 1 7 °. T h i s e f f e c t is in i t s e l f n o t p a r t i c •-flarly s u r p r i s i n g b e c a u s e i t r e s u l t s m g e n e r a l f r o m r i n g s t r a t a ( B a e y e r strm_n) a n d a t t r a c t r v e [ 4 2 ] mud r e p u l s i v e n o n - b o n d e d i n t e r a c t i o n s w i t h i n t h e w h o l e m o l e c u l e T h e s t r u c t u r a l a n d energet,_c c o n s e q u e n c e o f thL~ a n g u l a r d e f o r m a t i o n has b e e n e x t e n s i v e l y m v e s t u g a t e d t h e o r e t i c a l l y [ 4 3 , 4 4 ] . O u r d a m p r o v i d e n o s u p p o ~ f o r t h e p o s t u l a t e d c o r r e s p c n d e n c e b e ~ w e e n an i n c r e a s e m t h e C - - H b o n d l e n g t h s w i t h d e c r e a s e in t h e e x o c y c l i c H C H angl e [ 4 3 ] . T h e r e is still a n o t h e r c o o p e r a t i v e e f f e c t b e t w e e n m ug p u c k e r i n g a n d t h e c l o s e r a p p r o a c h o f C, a n d C3- T h i s is t h e " D u n i t z - - S c h o m a k e r s t r a t a " [ 4 5 , t 6 ] c a u s e d b y 1,3 c a r b o n / c a r b o n a n d 1,3 c a r b o n / h y d r o g e n r e p u l s i o n s T h e structur~ results of CBG do not yield clear evidence for a posslble contribut i o n o f thin st_~am t o t h e n e t s t r a i n o f f~he c y c l o b u t y l rung. O n l y t h e s m a l l d e v i a t i o n o f t h e p u c k e r i n g angl e a n d t h e s h g h t l e n g t h e n i n g o f t h e C--C b o n d d m t a n c e s in c o m p a m s o n w i t h r_he s a m e p a r a m e t e r s m t_he m o t h e r m o l e c u l e (0 = 2 7 7 ° a n d C - - C = I 5 5 1 [ 8 ] ) m a y r e f l e c t t hi s s t r a t a . Wit.b_out o v e r e s t u n a t m g t h e " D u n i t z - - S c h o m a k e r s t r a t a " I b e l i e v e t h a t t i ns c o m p o nent con~butes to the net strata m CBG but the interaction of the electron e g a t i v e g e r m y ! g r o u p ~ i t h t h e c y c l o b u t a n e rung m a y b e r e s p o n s i b l e f o r a partual c o m p e n s a t ! o n o f s u c h c r o s s - r i n g r e p u l s i o n . CONCLUSION F r o m t h e f o r e g o i n g i t c a n be d e d u c e d t h a t m C B G as in c y c l o b u t y l s f l a n e t h e p u c k e r i n g a n ~ e , t h e C--C b o n d l e n g t h d i f f e r e n c e a n d t h e e q : a x r a t i o are consaderably affected by the substatuent group. Therefore, it seems clear that t h e s e p a r ~ r o e t e r s a r e suiLable ~o s e r ve as a " d e t e c v o r ' " f o r a n y g e o m e t r i c a l o r e l e c t r o m c va~_at!ons r e s u l t i n g f r o m s u b s t i t u e n t ~ r - u g i n t e r a c v i o n w i t h i n m o n o s u b s t l t u t e d f o u r - m e m b e r e d rings T h e s,~ructural r e s u l t s o f C B G a n d c y c l o b u t y l s f l a n e e v u d e n t l y s u p p o r t Gae r e l a t i o n s h i p s d e r i v e d b y J o n v u k a n d B o g g s c o n c e r n i n g Ene correla*~lon b o t w e e n e i e c t ~ o n e g a t i v i t y o f s u b s t i ~ e n t s a n d r i n g p a r a m e t e r s . T h e c o r r e l a t i o n is g e n e r a l l y i m p r o v e d i f g r o u p

299

e l e c t r o n e g a t i v i t a e s are u s e d i n s t e a d o f t h e e l e c t r o n e g a t i ~ 4 t y o f t h e c e n t r a l atom within the group. The geometric~ parameters obtained m botl monosubsti~uted cyclob u t a n e s w h i c h w e h a v e s*.uched p r o v i d e v a l u a b l e m f o r m a ~ o n f o r b e t t e r u n d e r s t a n d m g o f d y n ~ m , c p r o c e s s e s m f o u r - m e m b e r e d r i n g s y s t e m s a n d o f elect r o n i c m t e r a c t a o n b e t w e e n s u b s t i t u e n t s a n d rzr,g a t o m s . H o w e v e r , m o r e expeLl_mental d a t a f r o m r o t a ~ o n a l a n d v l b r a t m n a ] s p e c t r o s c o p y axe r e q u i r e d to confirm the correlations discussed above. Analysis of these spectroscopic d a t a l e a d s t o b a r n e r s t o i n t e r n a l r o t a t i o n o f the, - - X H 3 g r o u p a n d t o t h e barrier between equatorial--axml conformers. These barriers were very r e c e n t l y d e t e r m m e d f o r c y c l o b u t y l s i l a n e t o be ~ 5 5 c m -' a n d 7 6 3 cm-'respecuvely [47]. An analogous study on CBG has just been started Addit i o n a l l y , w e a r e n o w m t h e p r o c e s s o f p r e p a x m g ,hid s t u d y i n g t h e s t r u c t u r e o f c y c l o b u t y l s t a n n a n e . T h i s is o f m t e r e s t t o e n a b l e bet+mr d~scussion o f t h e concept of group electronegat~xSty and to provide add:tional lmpo~t data to rest the va/ichty o f the suggested inverted 3rder o f e l e c t r o n e g a t k u t y wlthm the group IV elements. REFERENCES 1 C o m p r e h e n m v e rev,ews of far-mfrared and Ram.an studies of small ring molecules can he fCund !,~-refs 2 and 3 respect,% ely. 2 J R D u n g ( E d ) Vibratlona/Spectra and Structure, V o s I--IZ, Elsevier, 1972--1983 3 R J. H. C|azk and R E Eester (Eds }, Advances in Infrared and R a m a n Spectroscopy~ Vols. I--I0, Heyden, 1975--1983 4 For example. (a) J Laane and R C Lord, J Mol. Spectrosc., 39 (1971) 3 4 0 (b) J. Laane, Q. R e v C h e m . Soe , 25 (1971) 533 and r~J'erencesthereto (c) J R Durig, T S. Little ~_nd Y S- L~, J C h e m . Phys., 76 t!982) 384_9. 5 M . D H a r m o n y , Tenth Austin S y m p o s l u m o n ~olecular Structure, Austin, Texas, paper WIVIB, 1 9 8 4 6 (a) J R Durig, C. A C~rreire and %¥ J Lafferty, J. i%,~IolS~ruc~ , 46 (1978) 187 (b) M Y. F o n g and i%~ D H a r m o n y , J. C h e m Phys , 58 (1973) 4 2 S 0 7 0 Bastia-nsen, K Kveseth and H ?4~ilendR] In Topics m Current C h e m ~ t r y , Vol 81, Springer-Verlag, N e w York, 1979, C h a p 2 and references therein 8"['. Egawa~ S Yarnamoto, F Takabayashi, K. Karnbara, ~'. F u k u y a m m , T U e d a a n d K. Kuehltsn, m press W e express our tha_nks to Proi. H. Kuchltsu fo~ m a k m g the results a-~ai]able prior £o p u t 3 1 1 c a t l o n 9 L . H e d b e r g , K . H e d b e r g a n d J E B o g g s , J. C h e m . P h y s , 7 7 ( 1 9 8 2 ) 2 9 9 6 1 0 L S B a r t e l / a n d B. A n d e r s e n , J. C n e m S o e C n e m C o m m u n . , ( 1 9 7 3 ) 7 8 6 1 1 R L H i l d e r h r a n d r , H . L e a v i t t a n d Q. S h e n , J M o l S ~ r u c t . , 1 1 6 ( 1 9 8 4 ) 2 9 . 1 2 P P u l a y , G- F o r g a r a s i , F . P a n g a n d J E B o g g s , J A m . C h e m S o c . , 1 0 1 ( 1 9 7 9 ) 2 5 5 0 13 H Oberharnmer and J E Boggs, J. Mol. Su-uct, 57 (1979) 175 1 4 T J o n v i k a n d J. E B o g g s , J . _Mol S t r u c t . . 8 5 ( 1 9 8 1 ) 2 9 3 . 1 5 T J o n v i k a n d J_ E B o g g s , J _MoL S m a c t , 1 0 5 ( 1 9 8 3 ) 2 0 1 1 6 D C r e m e r , J. Axn C h e m S o c , 9 9 ( 1 9 7 7 ) 1 3 0 7 . 17 J. R. Durig and A_ C Morrmsey, J. C h e m . Phys , 46 (1967) 4 8 5 4 I S J R. D u n g , J N. %¥zl/isand W H ~reen, J C h e m . Phys , 54 (1971) 1 5 4 7 19 For example (a) J Laane and R C Lord, J Cqnem Phys, 48 ( 1 9 6 8 ) 1 5 0 8 (b) V F K~asinaky, G A. Guirgis a~nd J R Durig, J. M o l Skruct , 39 (1977) 51

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