a
IC
0.015
Rb CS
0I025
a o and ~
0 048 I
0.27 0.26 0.29
X arc in units of atomic gas-phase hyperfine coupling
constants.
preciable deviations from linearity may be expected for D < 3. Returning to the ion pairs, we now estimate the amount of electron transfer by substituting D = 3 instead of D = 1. This leads to values of the order of 1% Tor the total amount of eIectron transfer, a much
Nitrogen and
more reasonable value than the 30Oj, previously estimated. Values QE a"Na and X are listed j~ Table 1 for some systems which adhere to eq 6. Unfortunately, insufficient data are available a t present to pick out trends in these quantities which may correlate with solvent structure, hydrocarbon structure, or metaI. However, the efficacy of the analpis presented here may be tested by studying variations in a~ over wider ranges of D by varying both Lemperuture arid solvent. Obtaining resuhs for a homologous series of EOYV~KYLS may be particularly useful in this respect because may be changed a great deal \n;Ethou~:it the ssnit, time changing specific effects which relate to the molecular structure o€ the solvent.
A ~ ~ ~ n o w l This ~ ~ ~research ~ e n ~ as . supported in part through a grant from the National Science Foundation (CP15517).
oron Spin-Lattice Relaxation in Borazol el
by George M. Whitesides,* Steven L. Regen,2 John B. Lisle, and Robert Mays Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02159 (Rmeived January 7 , 1972) Publication costs assisted by the National Institutes of Health
The observed line shapes of the IH nuclear magnetic resonance spectra of neat borazole and of borazole in toluene-dg, dimethyl-& sulfoxide, chloroform-d, and cyclohexane-dlz solutions are dominated by broadening due to the electric quadrupole-dependent nuclear spin-lattice relaxation of nitrogen and boron. Comparisons of observed spectra for borazole with those calculated for varying values of the nitrogen and boron spinlattice relaxation rates establish that a parameter f = [.(ey)z]~/[~(eq)2]2r'characteriaingthe relative response of the boron and nitrogen nuclei to rotational diffusion (eq 3) is indistinguishable from unity (j" = 1.0 jr 0.1) over a range of experimental conditions in these media. These observations are rationalized satisfactorily by eq 1 and provide empirical support for the usefulness of this equation in describing the influence of molecular motion on the rates of spin-lattice relaxation of quadrupolar nuclei. Spin-lattice relaxation of nitrogen is more rapid than that of boron in solutions containing the stable free radical di-tert-butylnitroxyl, presumably due to weak specific interactions between the borazole molecule and the nitroxyl radical. Combination of spin- lattice relaxation times inferred from the 1H spectra of borazole with an estimate of its mean rotational correlation time in solution leads to approximate values for the quadrupole coupling constants : (eSyQ/h) 7.6 i 2.9 MHz (LOB); 3.6 i 1.3 MHz (IIB); 1.4 =t0.5 MHz (I*N).
In~roductio~i The determination of spin-lattice relaxation times for yuadrupolar nuclei is a technique of increasing importance in the study of molecular motions in solution 3--5 Interpretation of these relaxation time measurements in terms of molecular motions rests on the assumption that relaxation is dominated by quadrupolar interactions and that contributions from dipoledipole, anisotropic shielding, and spin-rotation interactions are unimportant. With this assumption,
equat'ions having the form of eq 1 are used to relate re-
--(">( 1 I
Tlx
40
21
+3
)
P ( 2 1 - 1)
laxation times to molecular motion. Here TI, is the (1) Acknowledgment is made to the donors of she Petroleum Research Fund, administered by the American Chemical Society (Grant No. 4032), and t o the National Institutes of Health (Grant No. GM16020) for support of this research. ( 2 ) A. D. Little Fellow, 1969-1970.
The Journal of Phgsical Che'hemistr.y, Vol. 76,~1'0. 20, 1972
(3b) tical to make a reliable a priori estimate of the ratio (eqB/egN)2; however, it is possible to eatimabe the ratio TB/TN, The nitrogen and bvroia ncaclei occupy symmetry-equivalent positions in the borazole skeleton. (3) Review: W. T. Huntress, Jr.$ Advan. IWagn. Resonance, 4, 2 (1970); J . Chem. Phys., 48, 3524 (1968); R. A. Dw(ek and R. E. Richards, Discuss. Faraday Soc., 43, 196 (1967); J. A. Pople, ibid., 192 (1967). (4) T. T.Bopp, J . Chem. Phys., 47, 3621, (1967); T.D. Alger and H. S. Gutowsky, ibid., 48, 4625 (1968); R. K. Harris and Pa. C. Pyper, Mol. Phys., 20, 467 (1971); and references in each. (5) R. P. Hangland, L. Stryer, T‘. R. Ston,gle, and J. D. Baldeschwieler, Biochemistrv. 6, 498 (1967): R. 6 . Brvant, J~ Amer. Chem. Sac., 89, 2496 (1967) : Ch. ’Brevard and J. M.‘Lehn, ibid., 92, 4987 (1970). (6) A. Abragam, T h e Principles of Nuclear. LMagnetism,” Oxford University Press, London, 1961, Chapters VI1 and YIII; C. P. Slichter, “Principles of Magnetic Resonance,” Harper and Row, New Yorlr, N. Y . , 1963, Chapten3 5 and 6. (7) H. S. Gutowsky, R. L. Vold, and E. d. Well.s, d . @hem. Phys., 43, 4107 (1965), and references thetein. (8) E. W. Randall arid D. Shaw, Spectrochim. Acta, Part A, 23, 1235 (1966); D. W. Aksnes, S. M. Hutchinson, and IC. 5. Packer, Mot. Phys., 14, 301 (1968); M. Arnold and K. J. Packer, ibid., 18, 141 (1966); C. Deverell, I). J. Frost, and R. E. Richitrds. dbid., 9, 564 (1965). (9) W. A. Stoele, J . Chem. Phys., 38, 2404 (1963); P. S. Nubbard, ibid., 53, 985 (1970); 5. G. Brush, Chem. Rev., 62, 513 (1962); A. Rondi, J . Amer. Chem,. Soc., 88, 2131 (1966). (10) D. E. Woessner, J. Chem. Phyci., 40, 29-41. (1964); K. T. Gillen and J. H. Noggle, ibid., 52, 4905 (1870); W. B. Moniz and XI. S Gutowsky, ibid., 38, 1155 (1963); E. Bhi.mixur, ihid., 40, 756 (1964)‘. (11) For a review of boraeoles, sole H. Steinberg and R. J . Brotherton, “Organoboron Chemistry,” Vol. II, Wiley, New York, N.Y., 1966, pp. 174-434; 0 . T. Beachley, Jr.; J . Amer. Chem. Soc., 93 5066 (1971), and references thersin. (12) For examples, see 6.T.O ’ K o ~ ~ l c“Determination i, of Organic Structures by Physical Methods,” F. 6 . Nachod mid W. D. Phillips, Ed., Academic Press, New York, N. H.,1962, Chapter 11. (13) The values of the nuclear quadrupole moments used in this paper arc (in esu cmz) e Q ( 1 W ) = 1.56 4: 10~‘””;‘4” e&(%) = 8.04 X 10-26;14b eQ(I1B) 3,863 X 10-2e.Y4b (14) (a) C . T. O’Konski and T. Ha, J’. (>hem. P h y ~ .49, , 5354 (1968) ; Bee also C. Lin, P h p . Rev., 119, 1027 (1960); (b) R. Bchaefer, R. Klemm, and F. .Barr& ibid., €76,49 (1968).
e.
2873 Xn cc"quence, in the absence of association between borasole and othcr components of the solution, the tliff usional motion of boron and nitrogen nuclei with respect to the external magnetic field should be identical. Thus, the rtatio T B / T ~ ,a t the simplest level of ~ ~ ~ e r p78 ould ~ e be~ expected a ~ , to ~ be ~ unity. ~ ~ ~ ~ The technique used to determine relaxation times in this work yields s ~ m ~ ~ t a n e measurements ous on nitrogen and boron, tvvo nuclei having magnetic dipole ~ ~ ~ a at~ diflcreni, ~ ~ ~ frequencies ~ i o n [ Y O~( W) = 4.33 19.25 3IHz a t 14 kG1, and charac~ f ~ chemical e ~ ~shift ~ anisotropies ~ ~ , and ~ ~ ~ o ~ e - interactions d i ~ o ~ e witjh other spins in the sohtion. Since t:omparii;on of the ratio of the relaxation times of these t ~ nuclei o should serve to reduec the importance of several possihle types of experimental ~~~~~~~~Q~~ to relaxation rates from I the fairly restricted set covered by I might be dckknble in borazole that would be cult to detact in other compounds. Thus, in particular, Buctua ti s in the magnitude of the parameter (eq 3b) on ging solution composition, viscosity, or tempera hould be attributable either to a failure of eq 1, or to the influence of specific Intermolecular association between solvent and thc boron or nitrogen nuclei, since q 1 clcarly irnplies that, j sbould be iwarianL to t h e nat~ireof the medium in the absence of ~ ~ t e r m o ~ e association cu~a~ effects. The influence of quadrupole relaxation on the 'H nmr spectrum of azole and its analogs has been disciiabed previoudy .1.5
A
R
7-
Figure 1. 1H nmr spectra a t 60 MHz of (A) borazole in DMSO-& (68Y0 v/v) a t -62", (B) borazole in toluene-& (3370 v/v) at -80", (C) borazole (neat) aft -72", and (D) borazole The sharp line a t low field in each spectrum is (neat) at -8'. i,he benzene used as an internal line width standard.
ts
The % nmr spectrum of a sample of neat borazole containing a small amount of benzene as an internal resolution standard a t - 8" consists of a triplet arising from prorons bonded to I 4 n T [J('H, I4N) = 54 f 2 Hz J centered at 8 5.47, superimposed on a quartet due to I%) = 135 f 3 Hz, 6 4.431; the protons on llBIJ(lHH, expected septet due to protons on loB (-19% natural abundance) is obscured by the more intense "B-N resonances (Figure 1). Lowering the temperature of the sample to -72" results in partial collapse of the NH and 32H signals to broad superimposed singlets. Although complete collapse cannot be observed in samples C Q K Q O S C ~ mainly of borazole because these samples freeze at --,%O, samples in dimethyl-& ~ ~ 68%~ v/v)~ show S two ~ relatively ~ ~ sulfoxide ( sharp peaks a t -- 62 " . Samples of borazole in tolueneds could be cooled to -80" before freezing; spectra a t these l o r ilernperatures resemble those observed for 68To boraaole in MSO-de. The line widths characteristie of all of these spectra were never sufficiently narrow Io permit observation of 1H-N -B-'H, 'W--Bspin-spin coupling, although partial decoupljng: indicated that two bond heteronuclear COup h g mere ~ not important : thus, the *Hnmr spectrum
-
of neat borazole a t 30" showed that no significant change in the shape of the lines comprising the HJ4N triplet accompanied collapse of the H-lIB quartet on irradiation a t the boron resonance frequency. The difference in the appearance of the borazole spectra in neat samples and in toluene and DMSO solution are primarily a reflection of the differences in viscosities of these samples, rather than of sample temperatures.*6 The Gierer and Wirtz modification of the Stokes equation provides a fairly satisfactory empirical relation between the temperature and buPk viscosity of the sample, dimension of solvent and solute molecules, and the nmr rotational correlation times o f the solute o~s (eq 4).17 This equation contains the a s s u ~ p t ~ that
~
,
(15) H. Watanabi, T, Totani, and M.Ohtsuru, Mol. Phys., 14, 367 (1968). A recent paper by E. K. Mellon, B. M. Coker, and P. B. Dillon, Inorg. Chem., 11, 852 (1972), describss an investigation of the nmr spectrum of borazole closely related to that described in this article. (16) See 6.M .Whitesides and W. L. Mitchell, S. Amer. Chem. SOC., 91, 2245 (1969), for a discussion of this polnt. (17) A. Gierer and K. Wirtz, Z . Naturfoorsch. A, 8 , 532 (1953); D. Herbison-Evans and R. E. Richards, Mol. Phvs., 7, 515 (1964). See R. A. AssLnk, J. DeZwaan, and J. Jonas, J . @hem. Phz!s., 56, 4975 (1972), for a recent experimental investigation of certain of the assumptions underlying this treatment. Th'e Journal of Physical Chemistry, Vol. 76, N o . 90,1972
Here rsolutean-%rsolveneare hard-sphere radii for solute and solvent rn,lecr.tles (cm), 79 i s the Stokes correlation time jsec), V ,,lute ID the hard-sphere "volLime" of the solute, and 7 XB the macroscopic solution viscosity (poise). On { h e kask of this equation, T X should depend on both ?; and ?": however, in practice, the change in sample viscosity q on changing temperature is noranaUg .much la? ger p~-~q-,~ri,i~nately than the change in the E C G ~ ~ I ' O Ltemperature; ~I hence, the spectra of Figure 1 Hllusl rate i ~ r ~ ~ athe ri~ influence y of solution viscosity on tbe spin- le k.tmrelaxation sates of "B and '4W of boraz307c elaxatJim rates for boron and nitrogen were extracted from the experimcntal spectra using a procedure outlined pri:~iorasly~~ by treating the relaxation processes responsible for the collapse of tlie I4N'H, IIBH, and I0BFF rnuHiplets separately as equivalcnt to c~hemrcal exchangch between equally populated sites separated in i'requercy by mrJx,H, corresponding to the (21 -k 1) s~h81;atcsof of a nucleus having spin 1. 'Fire appropriate transition rates between the mI subElleees are g1vetl by t y 5--7.ja ii.m r a j l
km+l m
(7) \Kth these equations,the calculation of the amplitude of the proton signal at frequency w for any values of T1,!41\, ti,^^^^ and /II,IOp, is reduced to the solution of eq 8. i(w>
0:
+
Rei ((l,'Ji, [m14N
+
x i ~ l -i)l ((l/dOA1l. [iollB+ Rilg + l . ~ l - l ~ lt~ >((1/,)0.191. [iwloB4-RloR+ Here, e.g., w 4 N is a diagonal matrix whose elements are the vadues (mvJK,ri - woN - a); RlnNis a diagonal matrix ahose dements describe the widths of the NH lines in the a b m m of quadrupole relaxation; and the matrix (l/!l"l,lN ) 14N contains the transition rate constants obtained from eq 6-7, describing the transfer of
(18) Numerical values for these rat,e euiist,ants are given in e q 4.43 (for I = 1) and 4.44 (for 1 = 3 / ~ ) of reE '7, and 1 = 3 i n J , Ba.con, 11. d. Gilleopie, J. 3. Hartman, and U. 1%.11. Rao, J 4 d Fhys., 15, 561 (197aj. They d l nod be repi-oduced here. For discussions of the liii~itationsof chis computat,iorial prot:eclure, see N , C. Pjiper, ib.ld., 21, 8'7'T (19711, and references t1ieroi.n. (19) For reviews OF techniques, see C;. 8.Johnson, Jr",iidvan. Mag%. Resono.nce. 1, 33 (1966). carried out using a versioii of the program er, Ph.D. Thesis, Mssmcliusetts Institute ol , Mass., 1971. Matrix inversions were carbmed on 'the roiltilie ALLMAT (cf. C . Uinsch, 1.304 (1969); R.E:.Scbirmer, J, Ep. Noggle, 91, 6340 (1969); E,. G . Gordc8ai and R . 1'. J . , 49, 2455 (IBRR)).
2875 I
Figure 3. Comparison of the observed spectrum of borazole in DMSO-de (68% v/v) at -44' with spect,ra calculated assuming the following values of j' (eq 7): f = 1.0 (-); f = 1.1 (----); and f = 0.9 ( . . . .). The single resonance downfield from the NH line is benzene used as an internal line width standard.
-----------0 0001 *.E
--0.001 sac
0,OIO 0.031
A
0.006
0.000
0
l,WO
2.000
3.000
WOO
4.000
8.oao
50.0 *IC
Figure 2. Calculated IN nmr line shapes for borazole as a Function of a uniform relaxation time T1Q. The individual daxation times are related to T1° by the following expressions: 7 ' 1 , ~= ~
TI'//6.73; T l , i i ~= Tt0/I1.06; T1,iog = T1'/7.20.
pears to give better agreement between calculated and experimental line shapes than f = 1.1 or f = 0.90. The results of similar experiments in several other media, are s ~ ~ ~ i ~in ~Table a , I. r ~ ~ e ~ le 1: Values off (Eq 8 ) for Borazole in Several Solvents
Meat DLlSO-dc (6870) CDCl, (44%) Toluene-d8 (33yo) Cyclohexane-& (42%) CCla (60%) with added di-lert-but yinitroxyla a
1.03tO.I 1.0 i:0.1 1 . 0 =k 0 . 1 1.0 f 0 . 1 1.0 f0.1 0.17 It 0.08
-37
-44 25 - 11 25 25
The conrentration of nitroxyl radical was -5%.
The significant coiicIlusion to be drawn from these data is that j = 1 in a variety of "typical" solvents witbin the limits of the experimental technique (- 2:10%) Thus, these data provide support for the contention that cq 1 does provide a useful functional relationship between the molecular motions charae~
Figure 4. Plot of q / T as a function of l/!Pa,in~. for borazole in DMSO-& (68% v/v) (0); borazole in toluene-d8 and borazole (neat) (0). Relaxation times for 11 obtained from this plot using the relation l . / T ' ~ v ~=i ~ 1.64(~ / T I , I ~ N ) .
terized by TX and the spin-lattice relaxation time of a quadrupolar nucleus and that other factors not included in eq 1 that might in principal contribute to 171 apparently are not experimentally significant. The one medium in which there is a significant deviation off from unity is one composed of cu. 60% borazole in CC1, containing the stable free radical di-tert-butylnitroxyl, in which the 14N is selectively relaxed.21 Whether the selective relaxation of I4Nis due to a ucomplex formation between a boron atom of borazole and nitroxyl radical (1) or hydrogen bonding between a proton bound to nitrogen and nitroxyl. (2), both of which could selectively place the IGN nuclei in close proximity to an unpaired electron, or to some other more complex mechanism, is not clear. Wowever, it is pertinent to considerations of the litility of eq 1 that only a drastic change in medium (.;.e,, addition of a paramagnetic species to the solution containing borazole) is capable of inducing an observable deviation off from its expected value.22s23 (21) Increasing the concentration of di-tert-butylnitroxyl broadens and collapses the N H resonances more rapidly than the BH resonances. Since the solvent viscosity is not appreciably changed by addition of nitroxyl, the deviation of f from unity thus appears to be due to a decrease in TI,Nrather than an increase in TI,B.
The Journal of Physical Chemistrv, Vol. 76, N o . 90,1978
2878
2b
"irtx ieqnation (eq 4) predicts a linear B l,'Tls14N(and T/T,,l,B) and v / T . This relalion is only approximately obeyed by the relaxation time data obtained in this study (Figure 4): %L ploc of the recipeoed relaxation times os. r/T shows a significant cur-iaburrb. This curvature has the sense that would be expceted if reorientation of borazole around its Ga axis were faster than that around the CI prexently available data are not R between a failure of the approximations untiedying i h e Gierer-Wirts equation'l and errors in the line shape analysis a t the fast- or slowe:rchange Piartitb, due 4 o neglect of spin-spin couplings, cross-relaxat ion e :ts, or other factors influencing "ition that the relaxation data for b o r a d e in llolucw~e,DMSO, and borazol e solutions fall ~ ~ ~ alongr a common ~ xcurve ~provides ~ support &or t h e assertion that specific interactions bt$rveen boracol e a x d typical diamagnetic solvent molecules do not strongly influence the relaxation bchavior of the fori". Keeping in iaind Ilie failure of the relaxation data obtained m tilit sfirdy to obey eq 4 in detail, these data can waonehhele~:.,k~ , ! a d in conjunction with eq 4 to ima Ces of the quadrupole coupling opnlaa and boron in hora.de. TO
(28) The vamliieof the coupling constant, Cor llBis of interest in a qualit,ative cstiniat,ion of the n-bond order of the N--B bond of borazole. Xing and KoskiZBhave mggested that the *-bond order of B-N multiple bond. ca.n be determined hy t h e expression II'U--N = E .- [ @ ~ ~ ~ / ~ : l i ~ Our ~ / ~estimate . ~ ! , ~ . of the quadrupole coupling constant lea,ds t o n~-.rq = 0.33 f 0.13, in rough agreement with previous estimates of .this pa.rameter ( - 4 G j . 3 * , 3 1 The eslimate of this ~ ~ a ~by~ Meilon, r ? ~et eaZ.,Is ~ based on similar quadrupole coupling conatarih asppearsto be too high by R factor of 2 . (29) M. Ring and goski, J. Chem. Phgs., 35, 381 (1961). For other recr?nt, discussions pertinent to the electronic structure of ~ ~.see D. R,. ~ Armstrong ~ ande D, 'I". ~ ~ Commun., borazole, Clark, Chsm. 99 (197'0); 13. Bock arid W. Fuss, Agnew. Chetn., Int. E d . Engl., IDv 182 (1971); G. J, Uullen and N. H. Clark, J', Chem. Soc. A, 992 (1970); 8.D. Peyerirnhoff and R. a. Buenker, J. Chem. f'hhys., 49, Biick, J. W. Dawson, and IC. Wioderizzs, Inorg. 0); 8 . U. FBeyerimhdT and 1%.dl. Baenker, 7 ' k ~ ~ . Ch.im. A d a , 19, 4. (lS70). (2 W. Bchaefler, and J R H z l t , J Chem rhus., (30) R ntanabe, IC. Ito, and M Piubo, J Amcr Chem. oi representative "N quadnupole coupling con(31) For iic compounds, see E A. 6, Lucken in "Physical stants in oaychc Chernistry," Vol. HV, A It. Katiitalcy, ~ d i l ~, o a L i t ~ h"rc,&,, i ~ r ( ~ NOW Yolk, N.Y . , iwi, Gianptei 2
micro viscometer modified for operation under a nitrogen a ~ m o ~ The ~ ~ viscometer ~ ~ e " was calibrated a ~ ~ literature i n ~ values ~ €or toluene32 (temperature, vi;soosity ili UP): 2 4 O , 63.560; tio, 0.720; -12", bileascared vdues for the solutions of borazole U~W: 24', 0410; Go, 0.532; -12", 0.759. Values for solutions of borazole in DMSO were 24", 1.19;6", 1.61; --12", 2.47. ~~~~~~2~ was ~~~~~~r~~ using a literature procedure33 and wa8 ~~~~e~ hy three successive bulb-to-bulb ~ ~ s ~ i ~ ~~ ~ ~~ t i ~o before ~ ~~ s use. ~ Samples ~ a for~ s ~ e ~ ~ex& ~ ~~~~~0~ ~ s ~were o ~prepared ~ c on a vacuum
line in flame-dried glassware using standard procedure~.~~
Acknowledgment. We are indebted Mro Calvin owell and Dr. Jeanne K. Krieger for extensive assistance with the computer programs used in this work. (32) E. G . Belinskaya, Uch. Zap., Mosk. Oblasl. Pedagoy. Inst., 33, 221 (1955); Chem. Abstr., 52, 8216 (1958). (33) $1. Watanabe and M. Kubo, J . Amer. C h e m . Soc., 82, 2428 (1960). e ~ y (34) Boraaole is inert t o oxygen, but sensitive to moisture: A, Stock and E. Pohland, Chem. Ber., 59, 2215 (1926).
Steric Interactions in tert-Butylethylenes Observed through Proton
13C
Satellite Spectra
.Nicholas,* C. J. Carman, B. 8'. Goodrich Research Center, Brecksville, Ohio 4141
A. R. Tarpley, Jr., and J. H. Goldstein Department of Chemistry, Emory University, Atlanta, Georgia SO3dd (Received March 6,1978) ~ ~ ~ b l ~costs c a assisted ~ ~ o nby the B . F . Goodrich Research Center
Steric interactions in a series of alkyl-substituted tert-butylethylenes have been studied through their proton satellite spectra. The parent compound, mono-tert-butylethylene, has an unusually low V I S C - H value for the ethylenic C-H bonded to tert-butyl (150.07 Ha). Further methyl or tert-butyl substitution, regardless of stereochemistry, causes little change in this value. Assuming that the inductive effect on VIIC-H for alkyl substituents is relatively small, the usual interpretation of lJlac-H leads to the conclusion that these low values reflect sterically induced changes in s character, but particularly interesting is the fact that most of this change occurs with the introduction of a single tert-butyl group on ethylene. tert-Butyl substituent effects These accuhave also been determined from the three ethylenic 1J13c-R values in mono-tert-butylethylene. rately predict 'Jl3C-H for trans-di-tert-butylethylene. The observed deviation from additivity of substituent effecls for the cis isomer reflects the steric interaction of the cis-tert-butyl groups in this compound" I3C cbemical shifts and proton nmr parameters are also reported for this series.
Introduction There ha,s been considerable interest in cis-di-tertt-Bu '\
(1)
c-c
/
H'
/
Rz
(2) \
compound indicates that an undistorted structure with normal bond angles and bond lengths would result in nearly rigid Conformations of the tert-butyl groups. Strain in this molecule is reflected in the large differences in heats of combustion and hydrogenation be-
Ri 1
~ ~ t ~ l e t (Ib) ~ y ~because e n ~ of the apparently large steric eff eets of the tert-butyl substit~ents.l-~Examination of the Fisher-Himchfelder model for this
(1) W. Puterbaugh and M. S. Newman, J. Amer. Chem. SOC.,81,
1611 (1959). (2) R. E. Turner, D. E. Nettleton, and M.PerePman, ibjd., 80, 1430 (1958). (3) F. H. A. Rummens, Reel. Trav. Chem. Pays-Bas, 84, 5 (1965). (4) R. C. Fahey, J. Amer. Chem. SOC., 88, 4681 (1966). (5) R. W. Murray, R. D. Youssefyeh, and P. R. Story, ibid., 89, 2429 (1967).
The Journal of Physical Chemistry, Vol. 76, No. 80,19%
