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The Journal of Physical Chemistry, Vol. 83, No. 22, 1979
cussed above, for the former compounds the C-Se scission must be accompanied by a reorientation of the S e B r bond and by a breakdown of the dimer structure. The 35ClTII direction, for PhzSeCl trapped in Ph3Se+C1-,is not perpendicular to any combination of two C-Se crystallographic directions and this could indicate that, in this matrix, the radical is not trapped in its most stable conformation.
Acknowledgment. We are grateful to the Swiss National Fund for its financial support.
References and Notes (1) (2) (3) (4)
On leave from University of Paris-Sud, Orsay, France. Hal is the abbrevlatlon for halogen. M. Geoffroy, J. Chem. Phys., 70, 1487 (1979). L. D. Kispert, R. Reeves, and T. C. S. Chen, J. Chem. Soc., Faraday Trans. 2 , 74, 871 (1977). (5) A. Hasegawa and F. Williams, Chem. Phys. Leff., 46, 66 (1977). (6) G. W. Neilson and M. C. R. Symons, Mol. Phys., 27, 1613 (1974). (7) G. W. Neilson and M. C. R. Symons, J. Chem. SOC.,Faraday Trans. 2, 68, 1582 (1972). (8) F. Krafft and W. Forster, Berichte, 26, 2818 (1893).
Tancredi et al. (9) H. M. Leicester and F. W. Bergstrom, J. Am. Chern. Soc., 51, 3587 (1929). (10) J. D.McCullough and 0. Hamburger, J. Am. Chem. SOC.,64, 508 (1942). (11) J. D.McCullough and G. Hamburger, J . Am. Chem. Soc., 63, 803 (1941). (12) T. Berclaz, J. Diolot, M. Geoffroy, and L. Ginet, J . Phys. E , 10, 871 (1977). (13) F. James and M. Roos, C.E.R.N. Program Llbrary, subroutine MINUIT. (14) E.A. C. Lucken and C. Mazellne, J. Chem. Phys., 48, 1942 (1968). (15) R. E. Watson and A. J. Freeman, Phys. Rev., 123, 521 (1961); 124, 1117 (1961). (16) F. Herman and S. Skillman “Atomic Structure Calculations”, Prentice-Hail, Englewood Cllffs, N.J., 1963. (17) R. G. Barnes and W. V. Smith, Phys. Rev., 93, 95 (1974). (18) J. R. Morton and K. F. Preston, J . Magn. Reson., 30, 577 (1978). (19) J. H. Mackey and D. E. Wood, J . Chem. Phys., 52, 4914 (1970). (20) J. G. King and V. Jaccarino, Phys. Rev., 94, 1610 (1954); 83, 471 (1951). (21) M. Geoffroy, L. Ginet, and E. A. C. Lucken, J. Chem. Phys., 65, 729 (1976). (22) K. Nlshikida and F. Williams, Chem. Phys. Lett., 34, 302 (1975). (23) Jung-Si Lee and D.D.Titus, J . Cryst. Mol. Struct., 6, 279 (1976). (24) L. Bonazzoia, N. Leray, and J. Roncln, Can. J . Chem., 55, 1617 (1977). (25) T. Berclaz and M. Geoffroy, Mol. Phys., 30, 549 (1975). (26) T. Berclaz and M. Geoffroy, Helv. Chlm. Acta, 61, 684 (1978). i
Collision Complexes. 3. A ‘H Nuclear Magnetic Resonance Study of the Complexes Caffeine-Mesitylene and Caffeine-Diphenylmethane Teodorico Tancredl, L.M.I.6. of C.N.R., Arco Felice, Italy
Francesco Leij, Lucian0 Ferrara, Salvatore Andini, and Piero A. Temussi” Istituto Chimico, Universiti di Napoli, 80 134, Naples, Italy (Received March 17, 1979)
Mesitylene and diphenylmethane were used as donor molecules in a NMR study of collision complexes formed by caffeine. The chemical shift data can only be interpreted on the basis of 1:l and 1:2 complexes. The peculiar features and shapes of the two donors were very useful to discriminate among the different geometrical arrangements in the complexes.
Introduction The study of collision complexes by means of NMR spectroscopy has many interesting implications both from general phy~icochemical~-~ and spectroscopic5@points of view. We have recently studied the system caffeine-C6D6 in CC14 solutions7in some detail and a few surprising results have stimulated further study of systems involving caffeine and aromatic donors. The main features of the NMR study performed on the CC14 solutions of caffeine as a function of increasing benzene molalities may be summarized as follows: (i) one of the chemical shifts induced by the ring current of the benzene changes sign at high benzene molalities (i.e., it goes to lower field with respect to the position in absence of benzene); (ii) all chemical shifts have a marked nonlinear dependence on benzene molality; (iii) the apparent formation constants, measured on the basis of one 1:l complex, are different if measured from each of the four groups of the molecule. These observations can be reconciled on the basis of a model involving at least a 1:2 caffeine-benzene complex beside the 1:l complex (further complexations, although very likely, were not taken into consideration in order to 0022-3654/79/2083-2902$01 .OO/O
keep the model as simple as possible). The presence of multiple complexations bears also on the critical issue of the “geometry” of collision complexes. Strictly speaking it is doubtful whether it is even possible to refer to a geometry of a loose complex in solution. In fact the term cannot possibly refer to a static structure since it is implicit in the definition of a collision complex that we are dealing with an average situation and NMR data faithfully reflect it. On the other hand, any interpretation of experimental data based on a given structural description has great advantages, at least for comparison with other systems containing collision complexes. A structural interpretation retains much of its impact even if it only describes a weighted average and not the geometry of a single static complex. This is true, for instance, for a qualitative evaluation of the forces responsible for the complexation. Particular situations may favor a “geometric” description more accurate than usual; such, we feel, is the case for the systems described in this paper and, in general, for most systems containing mixtures of 1:l and 1:2 complexes. Thus, the importance of a second (or, rather, multiple complexation) may reflect, among other causes, either the 0 1979 American Chemical Society
The Journal of Physical Chemistty, Vol. 83, No. 22, 1979 2903
’H NMR Study of Collision Complexes
“stacking” (i.e., self-association) of two complexed benzene molecules or a “sandwich complexation with two benzene molecules placed on opposite sides of the caffeine. Here we present a NMR study of two model systems containing aromatic donors especially chosen to clarify these issues. Ideal model systems in this case would be of course one in which only the sandwich complexation is possible and another in which only the stacking of donor molecules is permitted. It is evident, however, that preventing complexation on opposite sides of caffeine is virtually impossible; thus we preferred to choose two donors for which self-association is hindered to a different extent. Mesitylene is known to be able to act as donor with respect to amidic N-CH3groups even more effectively than benzeneFQhowever, its methyl groups ought to hinder self-association to some extent. Accordingly, we may expect that, if the major cause of nonlinear behavior in the system caffeine-benzene was the stacking of benzene molecules, the system caffeine-mesitylene should behave in a linear fashion, Le., its NMR data ought to be consistent with a simple 1:l complex model. Stacking of donor molecules can be excluded a fortiori for the bulky molecules of diphenylmethane (DPM). A marked nonlinear behavior in the system caffeine-DPM would thus be consistent only with a sandwich model. It must be noted that DPM was chosen also because a single donor molecule can place its two phenyl rings on opposite sides of a given caffeine molecule. The study was performed at a very high field (6.34 T), a circumstancethat makes it feasible to observe even small perturbations that may have thus far escaped detection. In fact most studies on collision complexes have been made at very low fields’ (of the order of 1.41 T).
Experimental Section Materials. Caffeine was purchased from Merck (Milano, Italy). Mesitylene, diphenylmethane, and spectrograde carbon tetrachloride were purchased from EGA (Steinheim/Albuch, West Germany). Procedure. The molar concentration of caffeine was kept constant in all solutions (0.03 M), while the molality of the donor varied from 0.2 to 5.8 M for mesitylene and from 0.4 to 14.9 M for diphenylmethane. The molar ratio of donor:caffeine is always high enough to assure the applicability of the Benesi-Hildebrand equation.’ ‘H NMR spectra were recorded on a WH-270 spectrometer at probe temperature (29 “C). Chemical shift measurements are relative to internal cyclohexane (less than 0.5% in all cases) and are accurate to better than 0.5 Hz. Equilibrium Quotients. It has been shown by many authors’ that the shifts induced by the ring current of aromatic hydrocarbons can be used to estimate both the equilibrium quotient and the shift of the pure complex in systems in which a 1:l complex is formed. The NMR data can be analyzed by the Benesi-Hildebrand equation’ 1 1 1 _1 -- -(1) A QAcm A, or by its equivalent forms A / m = -QA + &Ac
+-
_m -- - m1 + A
Ac
1 QAc
where A is the observed induced shift at molality m, A,, is the corresponding shift for the pure complex and Q is the equilibrium quotient for association of the complex.
1
/
0
o
t
0 00
-
7
1.60
1
I
3 20
’
I
4 80
I
I
6 40
m Figure 1. Plot of experimental m/A ratios of the caffeine methyl groups vs. mesitylene molality. The squares, octagons, and triangles refer to methyls 1, 3, and 7 of caffeine, respectively, in this and all subsequent figures.
The introduction in the model of a 1:2 complex leads to the equation’J0
mQiAio+ mQzAzO (2) 1 + mQi + n2QiQz where Q1 is the equilibrium quotient for 1:l complexation between an acceptor molecule (A) and a donor (D): A+D=AD A: is the corresponding limiting shift, Q2 is the equilibrium quotient for the complex AD2: AD + D AD2 and A; is the corresponding limiting shift. The analysis of NMR data on the basis of eq 2 is best performed with an automatic procedure which we have briefly described in a recent paper.’ A=
Results Figure 1shows one of the conventional chemical shift plots (based on eq 1)for the system caffeine-mesitylene. Although the dependence on donor molality looks approximately linear for all shifts, the equilibrium quotients calculated from the NMR data show that the model based on a simple 1:l complexation is not satisfactory. That is, the equilibrium quotients one would extract from the shifts of different groups of caffeine would be different from one another, a situation that has not gained any physical significance from its repeated observation in many donor-acceptor s y ~ t e m s .A~good ~ ~ interpretation of the experimental shifts, however, can be obtained rather easily if we treat our system as a mixture of 1:l and 1:2 complexes. Figure 2 shows the best fitting of the data given by the automatic minimization procedure we have recently developed’ which uses the powerful algorithm proposed by Fletcher.” Table I reports a comparison of experimental shifts with those calculated by using two equilibrium quotients (Q1 and Qz) and also with data calculated by imposing the minimization of a single equilibrium quotient (Q1’) in the automatic procedure. The better fit of the two-constant model is self-evident. Previous workQon the system caffeine-mesitylene interpreted the shifts of methyl 7 of caffeine on the basis of a single Q, but no attempt was
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The Journal of Physical Chemistry, Vol. 83, No. 22, 7979
Tancredi et al.
TABLE I : Comparison of Experimental and Calculated Shifts for the System Caffeine-Mesitylene best fit with one Q (RMSE = 1.48,) calcd shift, exptl - calcd,
best fit with two Q (RMSE = 0.57,) donor concn, mol kg-*
exptl chemical shift, Hz
calcd shift, Hz
exptl - calcd, Hz
Hz
Hz
1
0.2334 0.2900 0.3957 0.777 1.1571 2.0397 3.2251 4.7888 5.8574
6.47 6.47 1.94 13.09 16.39 21.18 24.12 21.80 21.80
5.869 6.901 8.586 13.027 16.153 20.629 24.280 27.115 28.510
0.601 - 0.431 - 0.646 0.063 0.331 0,551 -0.160 0.625 -0.710
4.466 5.400 7.016 11.754 15.268 20.759 25.111 28.461 29.966
2.004 1.070 0.924 1.336 4.122 0.421 -0,991 - 0.661 - 2.166
3
0.2334 0.2900 0.3957 0.1770 1.1571 2.0397 3.2257 4.1888 5.8574
8.92 8.92 12.60 18.48 23.63 31.72 39.81 46.43 48.63
1.688 9.169 11.676 18.823 24.121 32.679 39.884 45.713 48.532
1.232 - 0.249 0.924 -0.343 - 0.491 -0.959 - 0.074 0.657 0.098
7.151 8.654 11.245 18.838 24.411 33.270 40.245 45.614 48.026
1.763 0.266 1.355 - 0.358 -0.841 - 1.550 -0.435 0.816 0.604
7
0.2334 0.2900 0.3957 0.777 1.1571 2.0391 3.2257 4.7888 5.8574
26.21 29.88 39.44 62.98 81.36 110.04 134.98 154.90 162.99
25.76 30.734 39.161 63.228 81.095 109,998 134.355 154.212 163.609
0.450 - 0.854 0.279 - 0.248 0.265 0.042 0.625 0.628 - 0.619
24.238 29.301 38.081 63.793 82.867 112.664 136.286 154.466 162.634
1.972 0.573 1.359 - 0.813 - 1.501 - 2.624 1.694 0.434 0.356
Me group
0
--
4
N
m/a
L11
0 0-
21
93 i
3 0
N-
1
81 3-
-
EA
gt-I 0.00
I
1.60
I
I
3.20
I
I
I
I
6.40
4.80
m Flgure 2. Best fitting of the methyl chemical shifts of caffeine induced by mesitylene ring current.
made to fit the smaller shifts of methyls 1 and 3. Thus, even in the absence of spectacular nonlinear behavior the recourse to a model based on multiple complexation is made absolutely necessary to avoid the paradox of different formation constants in different parts of the acceptor molecule. According to the hypotheses put forward in the Introduction, the more linear behavior of the system containing the bulkier donor would seem an indication against a stacking model for this system but in favor of self-association of benzene in the system caffeine-benzene. However, it is more difficult to decide whether this finding can be regarded as a clear answer to our original question on the model of the 1:2 complex in caffeine-benzene. At any rate the issue is clarified by the results obtained with the second model donor. Here the stacking model is very unlikely a priori and thus any departure from the
000
400
E00
1200
m Flgure 3. Plot of experimental m / A ratios of the caffeine methyl grwp vs. diphenylmethanemolality. The continuous lines represents a best fitting with two formation quotients.
linear behavior typical of a single 1:l complexation may be regarded as an indication in favor of the sandwich model. The nonlinear dependence of the ring current shifts induced by increasing molalities of diphenylmethane is quite marked, indeed, the graphs of Figures 3 and 4 are reminiscent of those for caffeine-benzene. The continuous lines represent the best fitting that can be obtained with two formation constants by means of the automatic procedure. Table I1 shows the comparison of experimental and calculated shifts. Table I11 summarizes all relevant parameters for the systems caffeine-benzene,’ caffeinemesitylene, and caffeine-DPM. The whole body of NMR data for these three systems seems in favor of a model for the 1:2 complex in which the two donor molecules
The Journal of Physical Chemistry, Vol. 83, No. 22, 1979 2905
'H NMR Study of Collision Complexes
TABLE I1 : Comparison of Experimental and Calculated Shifts for the System Caffeine-Diphenylmethane best fit with one Q best fit with two Q (RMSE = 1.486) (RMSE = 0.385) Me group
donor concn, mol kg-'
exptl chemical shift, Hz
calcd shift, Hz
exptl - calcd, Hz
calcd shift, Hz
exptl - calcd, Hz
1 -
0.4215 0.49480 0.67160 0.98940 1.3407 2.02960 4.94780 5.20580 8.05720 11.75440 14.91700
7.940 8.680 9.410 11.220 12.360 13.830 12.360 12.360 9.410 7.940 6.470
7.325 8.126 9.697 11.590 12.797 13.738 12.236 12.022 9.852 7.770 6.493
0.615 0.554 - 0.287 - 0.370 - 0.437 0.092 0.124 0.338 - 0.442 0.170 - 0.023
3.607 4.056 4.998 6.316 7.396 8.842 11.401 11.516 12.361 12.904 13.172
4.333 4.624 4.412 4.904 4.964 4.988 0.959 0.844 - 2.951 - 4.964 - 6.702
3
0.4215 0.49480 0.67160 0.98940 1.34070 2.02960 4.9478 5.20580 8.05720 11.75440 14.917
14.070 15.540 18.480 22.890 26.570 31.720 39.070 39.810 40.540 42.010 42.010
13.524 15.173 18.597 23.285 27.007 31.745 38.953 39.221 40.970 41.834 42.163
0.546 0.367 -0.117 -0.395 - 0.437 - 0.025 0.117 0.589 - 0.430 0.176 -0.153
12.216 13.737 16.927 21.392 25.051 29.949 38.614 39.003 41.865 43.707 44.614
1.854 1.803 1.553 1.498 1.519 1.771 0.456 0.807 - 1.325 - 1.697 - 2.604
7
0.4215 0.4 9 4 8 0 0.67160 0.9894 1.3407 2.02960 4.94780 5.20580 8.05720 11.75440
65.920 73.270 90.180 114.100 134.310 160.780 208.580 210.050 225.490 236.520
65.147 73.305 90.449 114.524 134.315 160.882 208.060 210.181 225.787 235.824
0.773
65.783 73.972 91.148 115.193 134.898 161.271 207.932 210.026 225.439 235.357
- 0.035 - 0.269 - 0.424 - 0.005
-0.102 0.520 -0.131 - 0.297 0.696
0.137 - 0.702 - 0.968
-1.093 - 0.589 - 0.491
0.648 0.024 0.051 1.163
TABLE I11 : Relevant Equilibrium Parameters for the Systems Caffeine-Benzene, Caffeine-Mesitylene, and Caffeine-Diphen ylmethanea
benzene 3.7 f 1.3 0.291 t 0.021 10.1 i 0.4 -4.2 t .5 11.1 f 0.8 36.5 f 0.5 30 i 3 243.5 mesityl- 2.4 i 1.4 0.31 + 0.08 1 4 .L 6 37.4 f 1.1 17 + 9 67.5 i 2.1 57 ? 30 228.0 lene DPM 0.99 i 0.03 0.16 + 0.02 25.5 i 0.5 -1.2 t 0.8 44.0- 1.2 42.6 i 0.7 208 + 6 261.0
i i
0.6 49 6.0
t
4 243.0
f
0.5
I0.9
a All A ' s are exaressed in Hz. The benzene A ' s were taken at 2.35 T and have been normalized t o 6.34 T for the sake of comparison with i h e shifts for the other systems.
sandwich the acceptor caffeine molecule on opposite sides of the main molecular plane. A more careful examination of the data of Table I11 gives useful indications also on the geometry of the 1:l complex for the system caffeine-DPM. In fact both the Qs and the A's for mesitylene are similar to those for benzene but the limiting shifts for DPM are quite different. In the case of DPM this behavior may depend on the peculiar geometry of the 1:l complex. In fact the shape of DPM is such that 1:2 complexes involving stacking (i.e., self-associating) donor molecules ought to be inhibited. Besides, the shifts induced on a group facing one of the phenyl rings are diminished (with respect to benzene) by the presence of the second phenyl ring. For instance, an elementary calculation performed on the basis of the model of Waugh and Fessenden12shows that the average shift induced at 3.4 A at the center of one phenyl ring (while the other ring rotates over 360O) is 1.1 ppm vs. 1.3 ppm induced by benzene. Figure 5 illustrates the two situations now discussed. Indeed the first limiting shifts (Ala' s) are so high, with respect to those found for benzene and mesitylene (and also with respect to the same A? s induced by DPM itself),
/A
I
80 1- , 000
/ I
,
080
r
1
v
160
-
2 40
'/m Figure 4. Plot of experimental A-' values of the caffeine methyl groups vs. m-' values of diphenylmethane. The continuous lines are best fiing lines for a model with two 0's.
2906
The Journal of Physical Chemistry, Vol. 83, No. 22, 1979
e
+
C
:
Q
-L H H
1 pam
t13ppm
Flgure 5. Schematic illustration of the effect of the second phenyl group of DPM on a typical ring current shift.
Tyrreli
forming a 1:l complex but more or less like a single benzene when it forms a 1:2 complex. The details of the geometries of the average 1:l and 1:2 complexes are too difficult to determine in all three systems, but the whole picture seems to point consistently to the relevance of 1:2 complexes in which the two molecules of donor are situated on different sides with respect to the main plane of the acceptor molecule. It is not inconceivable that even higher order complexations play a role in very concentrated donor solutions. In any case it seems in order to collect more data at high enough fields before it may be possible to go back to detailed geometrical pictures, because most of the data previously collected at 1:41 T are probably of little significance.
11
Acknowledgment. We thank the “Centro Interfacolti di Metodologie Chimico-Fisiche” of the University of Naples for the use of their WH-270 spectrometer. The skillful technical help of Miss Lucia Pastore, Universiti della Calabria, and of Mr. Raffaele Turco of L.M.I.B., is also gratefully acknowledged.
\\
References and Notes 1 2
Flgure 6. Complexation scheme proposed for the system caffeinediphenyimethane.
that it seems possible to propose a unique geometrical arrangement for the 1:l complex. Figure 6 shows this model &e., the one in which a molecule of DPM “chelates” that of caffeine by placing the two phenyl rings on opposite sides of the acceptor) together with the scheme of the facile interconversion between different 1:l complexes by internal rotation. This model explains immediately the trend of the limiting shifts; each DPM behaves as two benzenes when
(1) R. Foster, “Organic Charge Transfer Complexes”, Academic Press, New York, 1969. (2) R. Foster in “Molecular Complexes”, Voi. 2, R. Foster, Ed., Eiek Science, London, 1974. (3) J. V. Hatton and R. E. Richards, Mol. Phys., 5, 139 (1962). (4) J. Ronayne and D. H. Williams, J . Chem. SOC. B , 540 (1967). (5) R. E. Kllnck and J. 6 . Stothers, Can. J. Chem., 44, 37 (1966). (6) M. 1. Foreman, R. Foster, and D.R. Twiseiton, Chem. Commun., 1316 (1969). (7) S. Andini, L. Ferrara, P.A. Temussi, F. Lelj, and T. Tancredi, J. Phys. Chem., 83, 1766 (1979). (6) A. A. Sandoval and M. W. Hanna, J. Phys. Chem., 70, 1203 (1966). (9) M. W. Hanna and A. Sandoval, Biochim. Siophys. Acta, 155, 433 (1968). (10) B. Dodson, R. Foster, and A. A. S.Bright, J , Chetn. SOC.B, 1283 (1971). (11) R. Fletcher, Hatwell Technical Report No. R. 6799 AERE, U.K. (1971). (12) J. S. Waugh and R. W. Fessenden, J. Am. Chem. SOC.,79, 846 (1957).
Electronic Structure and One-Electron Properties of the Isoelectronic Molecules HCN, HNC, HBO, HOB, HBF’, and HFB’ James Tyrrell Department of Chemistry and 6iocbemistry, Southern Illinois University at Carbondale, Carbondale, Illlnois 6290 1 (ReC8lVed April 6, 1979)
A double quality basis set was used to determine the equilibrium geometries, energies, and one-electron properties of a number of isoelectronic molecules. The properties determined included multipole moments, electric field gradients, and diamagnetic susceptibilities. The geometries and properties were compared with experimental results and previous calculations where available.
