34
P. T.
NARASIMHAN AND
pass through the origin. I n view of the simplicity of the model the small deviation of the best straight line (11.3 for the observed slope and 0.3 for the intercept) from these values of slope and intercept seems quite reasonable. The large half-intensity width for the C1-C1 stretching vibration (Fig. 6) is extremely interesting, as has been pointed out by Collin and D’Or.8” For all the complexes studied until ~ O W , ~the J weaker complexes had small values for Av’/Z and this parameter increased as the strength of the interaction increased. We have interpreted3 this fact as an argument against the formation of “contact” charge-transfer complexesz0 in these solutions. However, we have perhaps some indication here in the half-intensity width that this extremely weak benzene-Clz complex may actually be an example of a “contact charge-transfer complex.” In conclusion it should be noted that this resonance model provides a basis for the correlation recently found by Bellamy, et aLZ1 They found that when the X-H stretching vibrations of a number of compounds were examined in a large number of solvents, the frequency shifts observed could be correlated in the following way. If A v / v for a particular acceptor molecule, such as HzO, in a given solvent was plotted against A v / v for a standard acceptor molecule, such as pyrrole, in the same solvent, then a series of points was obtained, as different solvents were used, which all fell on a straight line. Different acceptor molecules gave a series of lines with different slopes. Now, if we use the diatomic rnolecule formula for v, such as was ( 2 0 ) L. E. Orgel and R. S. Mulliken, THIS JOURNAL, 79, 4830 (1957). (21) L. J , Belldmy, H. E. Hallam and R . L. Williams, Tyans. F a r a d a y SOC.,64, 1120 (1958).
[CONTRIBUTION FROM THE
h h x T. ROGERS
VOl. 82
done in refs. 2 and 3, then
Taking the derivative of both sides, and dividing the left-hand by v2 and the right hand side by k / 4 T C2p,then 2(dv/v) = dk/k
(1)
Thus as long as Vgas - vsoin is small enough that second-order effects in equation 1 can be ignored, then the parameter used by BHW21 and the “relative change in force constant,’’ A k / k , used here differ only by the factor of two. According to the argument advanced in ref. 3 the relative change in force constant A k / k is directly proportional to the weight of the resonance structure b. This, in turn, will be determined by the donor strength of D and the acceptor strength of the X-Y bond. I n the comparison of BHLV, the donors are the solvents, and the acceptors are the X-Y bonds. For a given solvent and acceptor, A v / v (or A k / k ) is determined by the coefficient of #b, which will differ in general for a different acceptor bond in this solvent because of the difference of the acceptor strength of the two X-H bonds. The difference is constant, however, so that a plot of A v / v for one acceptor and for a series of donors against this quantity for the other acceptor gives a straight line. This conclusion was of course already reached by BHW. However, it is further indication of the correlations provided by the simple resonance model proposed above. Acknowledgment.-Financial assistance from the Office of Ordnance Research, U. S. rlrniy, is gratefully acknowledged. IOWA CITY,IOWA
KEDZIECHEMICAL LABORATORY, MICHIGAT STATE
vNIVERSITE.1
High Resolution Proton Magnetic Resonance Spectrum of Mercury Diethyl : Analysis of an A,B, System BY P. T. NARASIMHAN AKD MAXT. KOGERS RECEIVED APRIL11, 1969 The high resolution proton magnetic resonance spectrum of mercury diethyl has been studied a t both 40 and 60 mc. ’ I l c spectrum of this compound can be satisfactorily interpreted as a n A3Bz system. The ratio of the spin coupling constant J t o the internal chemical shift 6 between CHa ( A ) and CH2 ( B ) groups is about 0.73 a t 40 mc and about 0.49 at 60 mc. T h e presence in the sample of the fairly abundant Hglo8isotope (with nuclear spin I = I / % ) makes possible a direct determination of both the J and 6 values for this compound despite the complexity of the spectrum. Theoretical calculations of the line frequencies and relative intensities have been made and the agreement with experiment is satisfactory. Details of the theoretical method for calculating line frequencies and relative intensities of the n.1n.r. (nuclear magnetic resonance) spectrum of an A3Bt system (with nuclei of nuclear spin I = l / 2 ) are also given.
Introduction The high resolution proton magnetic resonance spectra of several metal alkyls have been studied in recent especially with a view to correlating the electronegativity of the metal atom with the proton chemical shifts. Particular men(1) ‘C, S. Piper and G.Wilkinson, J . Iizorg. hruclear Chem., 3, 104 (1956). ( 2 ) A. L. Allred and E. G. Kochow, ibid., I , 269 (1957). (3) E. B. Baker, J . Chem. P h y s . , 2 6 , 960 (1957).
tion niay be made of the interesting proton resonance spectrum of lead tetraethyl recorded by Baker3 a t 30 mc., arid of his ingenious interpretation of the observations. The spectrum consists of a rather broad, intense central line with satellite “ethyl group”-like triplet and quartet lines on either side. These satellite lines were shown by Baker to be the result of unequal spin coupling of the PbZ07isotope (I = abundance 21.11%) with the CH2 and CHa groups, the dif-
Jan. 5, 1960
PROTON I~AGNETIC RESONANCE SPECTRUM OF L~ERCURY DIETHYL
35
ference in these coupling constants acting very much like a chemical shift between CH2 and CHP to produce the characteristic "ethyl group" appearance; the spin coupling constant JCH,-CH, (called J1hereafter) could be obtained from the line separation within either multiplet. Since this pattern appears on either side of the main peak, corresponding to the two possible values of the magnetic spin quantum number of PbZo7,the values of the spin coupling constants JcH~-P~, (42 c.P.s.) and J C H (124 ~ Pc.P.s.) ~ could be obtained from the separation between the quartets and triplets, respectively. Since the chemical shift ~ C H ~ C is H ,less than 1 C.P.S. a t 30 mc. for lead tetraethyl, the satellite lines appear to be placed almost symmetrically about the main peak. The spectrum of mercury diethyl should show some similarity to that of lead tetraethyl since a I1 I mercury isotope, Hg199, of nuclear spin I = ' 1 2 and abundance 16.86% exists. We have therefore made an intensive study of the spectrum of this substance in order to obtain the various spin coupling constants J and the chemical shift 6. To confirm our interpretation of the spectrum a Fig. 1.-Experimental and theoretical proton resonance calculation of the frequencies and relative intenspectra of mercury diethyl (60 mc.). sities of the lines in an AaBz type spectrum (for nuclear spin quantum numbers I = l/J has been lines well separated from one another, those apmade. Details of this calculation are presented pearing on the higher field side seem to overlap. along with a comparison of the spectra calculated Also, the main part of the resonance spectrum conand observed a t 40 and 60 mc. The only previous sists of a complex multiplet pattern with some indireport of an exact solution of the A3Bz spin system cation that the methyl group proton resonance falls is that by Anderson4n5who obtained a solution for a t a lower field than the methylene group resonance. the ratio J/6 = 0.136 only; he gives no details of Since the two ethyl groups in diethyl mercury are the method so his report was not of use for the known to be chemically equivalent, the spectrum present problem. Perturbation theory methods4v6 may be interpreted as an A3B2 system (in the notaeven to second and third order, are not satisfactory tion of Bernstein, Pople and Schneider*) if one for large values of J!6, particularly in the intensity neglects all interactions between the two ethyl calculations. groups. In this event the complex central part of Experimental the resonance spectrum could be caused by the comMercury diethyl was synthesized' from the ethyl Gri- parable magnitudes of J1and 6, the spin coupling gnard reagent and mercuric bromide, and was purified by constant and the chemical shift, respectively, distillation. A liquid sample contained in a 5 mm. 0.d. Pyrex tube was degassed and the tube was sealed off. The between CHBand CH2 group protons. However, spectrum was observed with a Varian V4300B spectrometer the complexity of this part of the spectrum did not and ( a t 60 mc.) V-4311 R.F. unit. The usual spinning seem to offer much hope for the direct determinatechnique and sideband modulation were employed. tion of the J1 and 6 values for the CH3-CHz group Results protons in this compound. Fortunately the lack of The proton resonance spectrum obtained a t 60 symmetry of the satellite spectrum mentioned mcs. is shown in Fig. 1. The frequencies and in- earlier gave us a clue to the determination of tensities of the 40 rncs. peaks are listed in Table V. both the J1 and 6 values, and as we shall see presThe spectra show certain similarities to that of ently the values thus obtained can be justified by lead tetraethyl in that satellite "ethyl group"- the agreement between the experimental spectrum like lines appear on either side of a central peak. and the theoretically calculated spectrum a t both The triplet and quartet lines presumably result 40 and 60 mc. from the unequal coupling of the Hglg9 nucleus Determination of the Coupling Constants and with the CH2 and CHI protons and, as for lead Chemical Shifts from the Satellite Spectrum.tetraethyl, the coupling constant J C H I - " g appears The effect of the nuclear spin coupling of the Hglg9 to be larger than JcH,-"~. isotope on the proton resonance spectrum of the However, a closer inspection of the satellite lines ethyl group of mercury diethyl can be adequately (see Fig. I) shows a lack of symmetry with respect treated by means of first-order perturbation theory.9 to the center of the spectrum and in particular it Thus denoting the resonance frequency of the CH3 will be noted that while the satellite lines appearing group protons by VA and that of CHZ group by on the lower field side show the triplet and quartet V B we may expect the following four lines in the proton resonance spectrum of the Hg199(C2H5)2 ( 4 ) W. A. Anderson, Phys. Rev., 102, 151 (1956).
"I
( 5 ) Narasimhan and Rogers, unpublished results. (6) J. T. Arnold, P h y s . Rev., 102, 130 (1956). (7) We are indebted t o Mr. Terry Haas for carrying out this synthesis.
(8) H. J. Bernstein, J. A. Pople and W. G. Schneider, Canad. J . Chem., SS, 65 (1957). (9) H . S. Gutowsky, D. McCall and C. P. Slichter, J. Chcm. Phys., 21, 279 (1953).
P. T. NARASIMHAN AND MAXT. ROGERS
Vol. 82
system due to the HgIg9-H spin interactions (Table I ) . J2 and J3 are the Hglg9-H spin-spin coupling constants for the CH3 and CHr group protons, respectively. -1s before 6 = v.4 - Y B , the internal chemical shift between the CH:] and CH, group protons. The H(CH3)-H(CH?) proto11 spin iiiter-
equal to J s ; thus J 2 and J3 can be determined directly from the experimental spectrum. J1 is obtained easily from the separation within either of the multiplet groups of the satellite spectrum. The quantity marked A in Fig. 2 is the separation between the centers of the triplet and quartet groups on the low field side. It will be seen readily that TABLE I ~ C H ~ - C I= I ~ ] / 2 J 2 - (A l/:!J3). Thus the satellite Frequency Relative intensit> spectrum also yields directly the values of J1 and 6 CWt group v.4' = u , 4- 1 .'J, :3 0 for the ethyl group protons. The consistency of 7,'' ,;, -- V 2 J I , 3 (1 these values can be tested by noting the constancy CH? group b U ' = v.4 - F -+ ' 2.0 of the J1 value a t both 40 and 60 mc. and also from 3.1) the ratio of the 6 values obtained a t these two freaction is represented by the quantity J1. This quencies. Table I11 gives a summary of the spininteraction would serve to split the CHI grpup spin coupling constants and chemical shifts derived lines into triplets and those of the CH2 group into from an analysis of the satellite lines in the observed quartets if the magnitude of IJ3-J31is largelo spectra of mercury diethyl. compared to J1. Such a first-order perturbation TABLE I11 treatment for the CH3-CH2 interaction in the SPIS-SPINCOUPLIKGCOYSTANTS A N D PROTON ISTERXAL Hglg9(C2Hb)2case seems to be adequate as seen CHEMICAL SHIFT I>rZTA FOR XIERCURY n I E T I I Y L from the experimental spectrum. Thus, including Frequency the CHs-CHz proton spin interaction, the HgIg910 mc. i i 0 mc. (CzHj)? spectrum may be expected theoretically to jJ1l H(CH3) - H(CII?I 7.0c.p s. 7 . 0 c.p.s consist of the lines listed in Table 11. ~ J ?Hg'" i - H(CH3) 115.2 llA.2
+
lJa(Hgig9- H ( C H n )
TABLE I1 Frequency
CH3 group
VI
=
VA
-
v.4
+
+
'/?J?
A
Relative intensity
6 =
UA
-
VI3
X7.R 23.4
-R.B
87.6 28.2 -14.4
3.0
J1
+ '/2J? + ' / ? J ?- JI - ] / ? J ?+ J I
Theoretical Calculation of the Nuclear Magnetic Resonance Spectrum of an A3B2 System (Nuclei of Spin 1 = ','a).-The equivalence of the two ethyl ~4 = Y A groups in diethyl mercury and the assumption of ~j = u.4 - ' 1 : J : the lack of interactions between these two groups V G = v.< - '/qJ! - Ji simplifies the problem of the theoretical calculation CH? group v7 = V A - 6 l / ? J 3- 3 / 2 J 1 of the proton n.m.r. spectrum of this compound. vs = VA - 6 'I'TJ: ','?Ji Using these simplifications the problem reduces VQ = VA - 6 '/?Ja to the computation of the nuclear magnetic energy V I [ ) = Vd - L + ','2J1 levels of an d& system and finding the allowed vI1 = v.1 - 6 - V s J aT 3/2J1 transition frequencies and transition probabilities. vi: = U A - 6 - '/2J3 '/ Ji \Then the ratio of the spin coupling constant to the v i 3 = v.4 - 6 - ' / ? J 1 - '/oJi chemical shift, that is, J 6, is quite small simple V I - 6 - ','Ja - 3,'2J1 perturbation theory can be employed to obtain The satellite spectrum should then consist of the theoretical spectrum. However, when the fourteen lines, seven on either side of the central ratio is fairly large (as is the case in mercury multiplet group which is due to Hgzo0(C2H6)2 diethyl) the use of higher order perturbation and Hg201(C2Hj)z.But the experimental spectrum theory4,G is not feasible and one has to find the energy levels and transition probabilities by direct recorded at 40 mc. shows eleven satellite linesseven lines on the low field side, as expected, but solution of the secular equations. We wish to deal only four lines on the high field side. It was with this general problem here and arrive a t the noticed, however, that although the four lines on the various expressions that would enable one to calhigh field side have the frequency separation culate the spectrum of an S a B e system. We therecharacteristic of a CH:! group, they do not exhibit fore proceed by writing the Hamiltonian4--"-'1-14 of the simple 1:3:3:1 intensity ratio. Actually, the the system in a magnetic field as ratio of the line intensities experimentally measured H = Hi + HQ Ha (1) from the 40 rnc. spectrum is 1:6:9:4 owing to the superposition of the CH3 multiplets on the CH2 spectrum. I n order to confirm this fact, we obtained the proton resonance spectrum a t 60 mc. which clearly shows the separated triplet and quartet lines, although it will be noticed that they do fall in the same region. The separation (experimentally measured in cycles per second) between (11) E. L. Hahn and D . E , >faxwell, P h y s Rev., 84, 124G (1951). 88, lilies 2 and ,5 (see Fig. 2) is equal to Jz,while that (1932). between the centers of the €3 and B' quartets is 1070 (12) P;. F. Ramsey and E. >I,Purcell, i b i d . , 8 6 , 143 (1952). ~ . i
1'3
= u.4
+ + +
6.0 3 .0 3.0 6.0 3 0 1.0 3.0 3.0 1.0 1.0 3.0 3.0 1.0
+
+
+
+
-
+
(10) More correctly the quantity ' 6 '/zJz l / t J a / or 16 - 1 / z J % should he >>.Ti. J I and Ja may be shown t o have opposite signs from :L simple c o n s ~ d e r n t i r mnf these conditions. 1/2JaI
(13) h ' . F. Ramsey, i b i d . , 91, 303 (1953). (14) H. M. McConnell, A . D. M r L e a n a n d C . A . Rcilly, J . C h e m P h y s . , 23, 11-52 (193R)
PROTON L
Jan. 5, 1960
M RESONANCE ~ SPECTRUM ~ ~OF MERCURY ~ DIETHYL ~ ~
ci'
and I , is the z-component (in the direction of the external field) of the spin angular momentum
cut
4
vector I , y is the magnetogyric ratio of the nuclei, and H A and HB are the magnetic fields a t the nuclei A and B, respectively. The factor l / 2 r has been introduced in order to express the energy levels in units of cycles per second. HI and H2 correspond to the interaction of the A and B group nuclei with the externally applied magnetic field while H3 corresponds to the spin-spin interaction between the two groups of nuclei. I n the case of mercury diethyl the A and B groups would correspond to the CH3 and CH2 group protons, respectively. As zero-order eigenfunctions we may start by using the simple product spin functions of the five nuclei which will be 26 = 32 in number. These spin eigenfunctions will be the correct stationary state functions in the absence of the H3 term in the Hamiltonian of equation 1. The presence of this spin-spin coupling term however causes some mixing among the product functions and in order to get the stationary state functions we have to use linear combinations of the zero-order spin functions. We may note here that since the total z-component spin angular momentum, that is, I,(A) I,(B) = ZI,, is a good quantum number there is no mixing between states with different values of XI,. The secular equation
+
IH,, - E,6,1
= 0
where, ,6
= 1 if m =
n =Oifm=n
1
(2)
can thus be factorized according to the various allowed values of XIs. In our present case these would be + 5 ' 2 , +3/2, +1/2, -1/2, -312 and -512. The evaluation of the various matrix elements and the solution of the secular equations would then yield the energy levels of the system. The observed resonance frequencies correspond to the transitions among these energy levels subject to well-known selection rules.14 For the calculation of the transition probabilities one has to first get the correct stationary state functions expressed as linear combinations of the zero-order functions. The coefficients in these expressions may be obtained from the secular equations by substituting the roots and normalizing. The transition probability P m n for a transition between two energy levels due to a perturbing radiofrequency (r.f.) field is given by pmn
=
27r(Mx)mn2P(V)
(3)
where p ( v ) is the energy density of the r.f. field as a function of frequency and ( p x ) m n is the matrix element of the x-component of the nuclear magnetic moment . (pcx)mn = ($rn
1 1 qn) ELI
(4)
where fim and fin are the stationary state functions corresponding to states m and n. Assuming the energy density function p ( v ) to be independent of frequency we may obtain the relative intensity of a line as ( p x ) m n 2 for the transition. The method of evaluation of the relevant matrix elements has been discussed by several authors.8,4a11J4-17 Although the simple product (15) M. K. Banerjee, T. P. D a s and A K. Saha, Proc. Roy. SOC. (London), 8226, 490 (1954). (16) D. R Whitman, Ph.D. Thesis, Yale University, 1957. (17) E. B.Wilson, J . Chem. Phys.. 27, 60 (1957).
~ 37
I
+-I
J3------4
e
A
b-4 SCALC
IOCPS
Fig. 2.-The effect of HglB9-Hand H-H spin coupling on the proton magnetic resonance spectrum of Hglgs ( C2H5)2 (60 mc.). Heights of the lines correspond to relative intensities. The internal chemical shift 6 between CHz and CH2 group protons is equal to '/zJZ - ( A '/z J 3 ) .
+
spin functions are straightforward to handle, they are often unwieldy when one encounters larger systems. Further, the evaluation of the offdiagonal matrix elements is often tedious when one uses the simple product functions. McConnell, et ~ l . , ' and ~ Wilson" have shown that in the cases of equivalent (interchangeable) nuclei, or nuclei arranged in a symmetric framework, it is easier to use spin functions that belong to the irreducible representations of the symmetry group common to these nuclei. Thus the three equivalent protons in the methyl group (AS) and the two equivalent protons in the methylene group (B2) may be shown to have the symmetry spin functions given in Table IV. These functions have been derived TABLE IV
Zero-order symmetry functions
Designation according to hlcConnell,
et
~1.14
Designation according to Whitman16 Q8
QI
a-
1
Q-
3
Di D- I Di' D- 'I TI To
T-I
sa
earlier by McConnell, et ~ 1 . ) ' following ~ group theoretical methods and we give them here not only for the sake of completeness but also to follow a notation different from these authors. Our present notation is based on the multiplicity character of the spin functions and is the same as that used by Whitman.16 As usual CY and p denote spin functions
P. T. NARASIMHAN AND MAXT. ROGERS MATRICES FOR SCALAR PRODUCTS O F
ANGULARM O M E N T A Q(A)*T(B)
FOR T W O
Vol. SI! GROUPS As AND Bz
G 0
246
2d6 2 -6 2 4 0 2dC 0 442 0 442 2 2
4d2
2vG 4v5 0 0 246 -6 0 246 246 0
6
+
with the z-components of 1/2 and In Table IV the last column gives the notations for the symmetry functions with Q, T , D and So representing quartet, triplet, doublet and singlet states, respectively. It will be noticed that the eight functions of the A3 group and the four functions of the Bz group yield the 32 product spin functions for the A3BZ system. These can be simply written, for example, as follows: Q3T1, D-ISo, etc. Owing to their symmetry properties the Q and D states do not mix nor do the T and SOstates. Thus the secular equations can be set up separately for QT, Q s , DT, DSo, D’T and D’So functions. The simplification resulting from this procedure is obvious. We shall now direct our attention to some details concerning the evaluation of the various matrix elements required in this problem. We note that matrix elements involving HI and H2 may appear only in the diagonal while terms involving H3 may appear both in the diagonal and off-diagonal elements. It can be shown16that (
~
+ i
~ z ) n n=
-
jqmhvi
+
(5)
Thus, for example
+
( Q ~ T L I H I Hz’lQaTi) =
- 3 / 2 ~ ~
- YE = -6/2VA
$- 6
(6)
These matrix elements thus can be very easily written by inspection. For the evaluation of the matrix elements in $13 we shall follow the method of generalized angular momentum matrices as outlined by Whitrnan.l6 According to this method one proceeds to find the matrix elements of H3 by constructing the relevant matrices for the scalar products of angular momenta of the various “spin-particles.” Since, as mentioned before, the Q and D functions do not mix, a group of three interchangeable nuclei each with spin of l/z can be thought of as a single “spin-particle” with spin 31/2 or l / Z . The total z-component of the angular momentum of such a group can be shown to be a good quantum number and hence the concept of a “spin-particle” is useful. Now we have
(fan =
[7;*, + 0.6,
39
18.05
0 2.761 3.29
shielded than those of the CH2 g ~ o u p . The internal chemical shift may be used to calculate the electronegativity of the Hg atom in this compound. The Dailey-Shoolery e q ~ a t i o n ’gives ~ a value of 1.54 for the electronegativity of mercury. The probable value of the electronegativity of the divalent mercuryatom is 1.9 (Pritchard and Skinner) ,20 To extend the Dailey-Shoolery equation to atoms of low electronegativity Shooleryzl has suggested that the value of the constant in the original equation be changed from 1.71 to 2.10. Use of this modified equation leads to a value of 1.93 for the electronegativity of mercury. The better agreement with the modified Dailey-Shoolery equation seems to illustrate a characteristic feature of the (10) B. P. Dailey and J. N. Shoolery, Tn18 JOURNAL, 77, 3977 (1957). ( 2 0 ) H. 0. Pritchard and H. A. Skinner, Chem. Reus., 55, 745 (1955). (21) J. N. Shoolery, Lecture Notes, Variau Associates Conference, (Nov., 1957).
H IR OSHI TsUB OMURA
40
Vol. is2
latter equation when one is concerned with atoms or groups with low electronegativity values.21-22 Acknowledgment.--We wish to thank the staff of the MISTIC computer laboratory of hrichigan
State University for their valuable cooperation. This work was supported by a grant from the Research Corporation to whom we are particularly indebted.
(22) P T Xarasinihan, N Laine aud 31 T Rogers, J C i i m n Phys , 28, 1257 (1938), 23, 1181 '19.58)
EASTT,ASSING,
[CONTRIBUTION FROM THE DEPARTMENT OF PI-IYSICS,
MICHIC.A\
LABORATORY O F ?rfOLECULAR
S?.RUCTURE A N D SPECTRA, ~ S 1 V E R S I T . Y
O F CHICAGO]
Molecular Complexes and Their Spectra. X. Molecular Complexes between Iodine and N ,N-Dimethylaniline Derivatives BY HIROSHITSUBONURA' RECEIVED MAY14, 1959 %-Heptane solutions of the iodine complexes with S,S-dimethylaniline or its methyl derivatives have been found to be stable when the iodine concentrations in the solutions are quite low (of the order of 3 x 10-5 df). All these solutions show strong absorption bands in the near ultraviolet, which are interpreted as the charge transfer absorption bands due to the complexes. The equilibrium constants and the heats of formation of the 1: 1 complexes have been determined from the intensity measurements of these bands. While the heats of formation obtained for the iodine complexes with S,N-dimethylaniline and N,N-dimethyl-@toluidine are very large ( -8.2 and -8.3 kcal.!mole, respectively), those for the iodine coinplexes with N,N-dimethyl-o-toluidine and N,N-dimethyl-2,6-xylidine ( -2.3 and -1.7 kcal./mole, respectively) are much smaller than the former two. From the inspection of the energies and shapes of the molecular orbitals of K,N-dimethylaniline, it has been concluded that the charge transfer in the complex between N,N-dimethylaniline and iodine occurs mainly from the lone pair of electrons of the nitrogen atom. I n the ortho-methylated N,S-dimethylanilines, the ortho methyl group approaches the 1one.pair of electrons of nitrogen, owing to the twisting of the dimethylamino group. T h e decrease in donor strengths of these ortho-methylated l\;,l\;-dimethylanilines is therefore attributed to the steric effect of the methyl group on the iodine molecule approaching the lone pair of electrons.
Introduction The donor molecules which form donor-acceptor complexes with iodine may be divided into two groups : hydrocarbon donors such as olefins and aromatic hydrocarbons (T-donors) and compounds which contain heteroatoms such as oxygen or nitrogen atoms with lone-pair electrons (n-donors). Generally speaking, the latter group has stronger donating properties than the former, and especially aliphatic amines and aza-aromatic compounds like pyridine2 interact very strongly with iodine. Although the anilines are expected to have considerable donor strengths toward iodine, no study has ever been made on the molecular compounds between the anilines and iodine. As donor molecules, the anilines cannot be classified rigorously as either n- or a-donors, because the amino group and the benzene ring form nT mixed donor orbitals, as well as a pure a-donor orbital, as a consequence of the resonance between them. (For details, see below.) It is a problem, therefore, to decide whether the anilines behave mainly as n-donors or as Tdonors in complex formation with iodine; in other words, whether the orbital of the electron which is donated to the iodine molecule is mainly concentrated on the nitrogen lone-pair electron orbital or spreads over the molecule. When the hydrogen atom a t the ortho position of N,N-dimethylaniline is substituted by a methyl group, the dimethylamino group is rotated around the nitrogen-carbon bond, due to steric hindrance between the ring and amino methyl groups and, consequently, resonance of the lone-pair on the (1) On leave of absence from the Institute for Solid S t a t e Physics, University of Tokyo, Meguroku, Tokyo, Japan. (2) C. Reid and R . S. Mulliken, THISJOURNAL, 76, 3869 (1954); S. P i n g n k u m , ibid., 80, 520 (1958).
nitrogen atom with the benzene a-electrons will be prevented to a large extent. The twisted dimethylamino group plus the ring methyl group will also produce a steric effect toward the iodine molecule approaching the nitrogen atom. It is interesting to see what change will occur in the donor strength of this compound with iodine. The donating properties of these hindered anilines toward aromatic acceptors such as trinitrobenzene3 and chlorani14have been studied already. I t was found that the complexing strength of N,Ndimethylaniline is greatly diminished by the substitution of a methyl or a larger group a t the ortho position. Somewhat similar studies were made for the donor strengths of hindered biphenyls toward s-trinitrobenzene.b It was again concluded that the complexing power is large only when the molecule retains the coplanar structures. These results are quite reasonable from the theoretical viewpoint that the benzene rings of the donor and acceptor in these cases should lie parallel and as close as possible t o each other to obtain a large binding energy. The steric requirements for the iodine-aniline complexes may be different from those of the above mentioned a-Tcomplexes. There is evidence that, in many complexes formed between halogens and n-donors, the halogen lies in a straight line passing through the n-donor, like 0-X-X Even in the case of benzene-haloor N-X-X.6 gen complexes, there are some experimental results which support the structure with the halogen mole(3) J. Landauer and H. McConnell, ibid.,74, 1221 (1952) ; R . Foster and D. L. Hammick, J. Chem. SOL,2685 (l95d); S. D. Ross and M. M. Labes, THISSOURNAL, 79, 76 (1957). (4) Pi. H. Smith, Thesis, Univ. Chicago, 1955. ( 5 ) C. E. Castro, L. J. A n d r e w s a n d R . hl. Keefer, TITISJOURNAL, 80, 2322 (1958). ( 0 ) 0. Hassel, .%id.Phyr., 1, 211 (193R), a n d earlier papers.
