J. Phys. Chem. 1982,86,332-335
332
Bimanes. 13. Crystal Structure of 9, IO-Dioxa-syn -(hydro,chloro)bimane (3,7-Dlchloro- 1,5-dlazabicyclo[ 3.3.0]octa-3,6-diene-2,8-dione) and I t s Relation to Solld-State Ultraviolet-Visible Spectroscopic Shifts Israel Goldberg“’
and Edward M. Kosower”
Department of Chemistry, Tel-Aviv University, Ramet-Aviv, TekAviv, Israel, and Department of Chemistry, State University of New York, Stony Brook, New York 11794 (Received: July 14, 1981: I n Final Form: October 7, 1981)
The ultraviolet and visible absorption maximum for 9,10-dioxa-syn-(hydro,chloro) bimane (3,7-dichloro-1,5diazabicyclo[3.3.0]octa-3,6-diene-2,8-dione, syn-(H,Cl)B) appears at much longer wavelengths in the crystal (458 nm) than in solution or the glassy state (389 nm). The shift occurs in stages (glassy (I) I1 (416 nm) crystal (111)).An X-ray crystal structure determination shows that the molecules of syn-(H,Cl)Bare tightly packed (d = 1.863 g ~ m - in ~ space ) group Pi,associating via dipolar and C-H. -0hydrogen bonds. The latter gives rise to a particularly strong C-H IR absorption at stage 111. Electrostatic stabilization of an excited-state relative to the ground state is assessed by using the observed intermolecular distances and estimates for the charge distributions in ground and excited states. It is shown to account for most of the spectroscopic shift between the glassy (I) and crystalline (111) stages.
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The longest wavelength maximum of 9,lO-dioxa-syn(hydro,chloro)bimane (1) shifts from 389 nm in CH3CN
H
H
syn-(H,Cl)B
solution or the glassy state (stage I) to 458 nm in the crystalline form with an intermediate stage (stage 11) having a maximum at 416 nm.2 Shorter wavelength maxima and IR spectra of 1 also show striking changes on annealing thin which prompted us to undertake an X-ray diffraction study of the crystalline solid. The chemical and photophysical properties of 9,lO-dioxabimanes have been discussed at length in other articles.= We herein present the structure of 1, indicate its relationship to other bimane structure^,^+'^ and suggest how the crystal packing can account for the spectroscopic observations made with the thin films. Structure. Single crystals of syn-(H,C1)B14were obtained by slow evaporation of a CH3CN solution. The crystal data are a = 6.630 (2), b = 7.192 (l), c = 9.264 (1) A, a = 75.54 (l), 0 = 110.92 (2), y = 117.47 (2)O, space group Pi,Z = 2, D,= 1.863 g ~ m - ~ Intensity . data were collected by the 0-26 scan method (26,, = 60’) on an Enraf-Nonius CAD4 diffractometer using monochromatized Mo Kcu radiation. The structure was solved by direct methods and refined to an R value of 0.031 for 1499 intensities above the threshold of 3a. A list of final atomic coordinates and thermal parameters is given in Table I. The molecular packing in the solid is unusually tight for an organic species, as illustrated in Figure 1. The crystal structure consists of layers of planar molecules approximately perpendicular to the b axis, adjacent layers being related to each other by inversion. Within each layer, the molecules associate mainly through C-H. .O=C bonds (along a direction parallel to a ) as well as through CCl-..C-Cl dipolar interactions (along directions parallel
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* Address correspondence to the following address: Department of Chemistry, Tel-Aviv University, Ramat-Aviv, Tel-Aviv, Israel. 0022-3654/82/2086-0332$01.25IO
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+
to c and a c). Every molecule is thus in close contact with six neighbors. The carbons bearing the hydrogens involved in the hydrogen bonding are probably somewhat positive, according to the hydrogen chemical shift in the NMR spectrum (7.95 ppm)14and the appearance of a strong C-H stretch in the stage I11 IR spectrum.2 The geometry of the C-H...O interaction (Figure 2) resembles that of a normal hydrogen bond between more electronegative atoms (e.g., N or 0) and a nonbonded electron pair on oxygen. The interlayer stacking arrangement is largely stabilized by electrostatic interactions of antiparallel C=O dipoles and antiparallel C-C1 dipoles. The overlap between individual molecules located in adjacent layers is small. In fact, only molecules displaced along the [lll]direction partially overlap, the C1 atom of one molecule being located above the nearest five-membered ring of its neighbor. The one-dimensional stacks thus formed contain alternately oriented parallel molecules and are characterized by an intrastack distance of 3.4 A. The crystal structure undoubtedly reflects the strength of the closest nonbonding interactions, which include the (1) (a) Tel-Aviv University; (b) State University of New York, Stony Brook. (2) Bimanes. 11. Kosower, E. M.;Hermolin, J.; Ben-Shoshan, M.; Faust, D. J. Org. Chem. 1981,46,4578-80. (3) Bimanes. 5. Kosower, E. M.;Pazhenchevsky, B. J. Am. Chem. SOC.1980, 102,4983-93. (4) Bimanes. 6. Kosower, E. M.;Pazhenchevsky, B.; Dodiuk, H.; Kanety, H.; Faust, D. J. Org. Chem. 1981, 46, 1666-73. (5)Bimanes. 7. Kosower, E. M.; Pazhenchevsky, B.; Dodiuk, H.; Ben-Shoshan, M.; Kanety, H. J. Org. Chem. 1981,46, 1673-9. (6) Bimanes. 8. Kosower, E. M.;Kanety, H.; Dodiuk, H. J. Org. Chem., submitted for publication. (7) Bimanes. 9. Kosower, E. M.;Kanety, H.; Dodiuk, H.; Hermolin, J. J. Phys. Chem. In press. (8) Bimanes. 10. Kanety, H.; Dodiuk, H.; Kosower, E. M. J . Org. Chem. In press. (9) Bimanes. 12. Huppert, D.; Kanety, H.; Pines, E.; Kosower, E. M. J. Phys. Chem. 1981,85,-3387-91. (10) Kosower, E.M.;Bernstein, J.; Goldberg, I.; Pazhenchevsky, B.; Goldstein, E.J. Am. Chem. SOC.1979, 101, 1620-1. (11) Bernstein, J.;Goldstein, E.; Goldberg, I. Cryst. Struct. Commun. 1980,9, 295-9. (12) Bernstein, J.; Goldstein, E.; Goldberg, I. Cryst. Struct. Commun. 1980, 9,301-5. (13) Goldberg, I. Cryst. Struct. Commun. 1980, 9, 329-33. (14) Bimanes. 14. Kosower, E. M.:Faust, D.; Ben-Shoshan, M.; Goldberg, I. J. Org. Chem. In press.
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 3, 1982 333
Crystal Structure of sun-(H,Ci)B
TABLE I: Final Atomic Coordinates and Thermal Parameters of syn-(H,C1)Ba X
0.6558(3) 0.5126 ( 3 ) 0.6585 (3) 0.8626 ( 3 ) 0.8639(3) 1.0018 ( 3 ) 0.8861 (3) 0.6577 ( 3 ) 0.3179 ( 2 ) O(10) 0.5036 (3) Cl(11) 0.5712(1) Cl(12) 0.9792(1) H(4) 0.992 (5) H(6) 1.157 ( 5 )
N(1) C(2) C(3) C(4) N(5) C(6) C(7) C(8) 0(9)
Y
2
U' I
UZ2
u3
U'=
u2
U'
0.7496(3) 0.7586 (3) 0.7673 (3) 0.7594 ( 3 ) 0.7497 (4) 0.7395 (3) 0.7321 (3) 0.7407 ( 3 ) 0.7575 (3) 0.7421 (3) 0.7845(1) 0.7155 (1) 0.752 (4) 0.734 (4)
0.7500(2) 0.5955 (3) 0.5050 ( 3 ) 0.6008 (3) 0.7500 (2) 0.8989 ( 3 ) 0.9947 ( 3 ) 0.9040 ( 3 ) 0.5567 ( 2 ) 0.9433 (2) 0.3075(1) 1.1925 (1) 0.581 (3) 0.920 (3)
0.0288(8) 0.0292 (8) 0.0358 (9) 0.0339 (9) 0.0304 (8) 0.0283 (9) 0.0310 (9) 0.0299 (8) 0.0305 (7) 0.0385 (7) 0.0480(3) 0.0493 (3) 0.050 (8) 0.049(8)
0.0731(13) 0.0385 (11) 0.0421 (11) 0.0454 (12) 0.0802(14) 0.0445 (12) 0.0407 (11) 0.0392 (11) 0.0655 (11) 0.0670 (11) 0.0703(4) 0.0698 ( 4 )
0.0505(10) 0.0507 (12) 0.0505 (12) 0.0586 (13) 0.0568 (11) 0.0597 (13) 0.0511 (12) 0.0508 ( 1 2 ) 0.0569 (9) 0.0567 (9) 0.0490(3) 0.0492 (3)
0.0289(8) 0.0151 (8) 0.0172(8) 0.0185 (9) 0.0317 (9) 0.0200 (8) 0.0185 (8) 0.0169 (8) 0.0261 (7) 0.0303 (8) 0.0257(3) 0.0340 (3)
-0.0116(9) -0.0044 (9) -0.0031 (9) -0.0030 (10) -0.0120 (10) -0.0039 (10) -0.0020 (9) -0.0050 (9) -0.0051 (8) -0.0040 (8) -0.0033(3) -0.0035 (3)
0.0113(7) 0.0145 (8) 0.0196 (9) 0.0233 (9) 0.0133 (8) 0.0120 (9) 0.0113 (8) 0.0138 (8) 0.0122 (6) 0.0196 ( 7 ) 0.0190(2) 0.0094 (2)
' The anisotropic temperature factor is in the form exp[- 2n2(hiai)(hjd)@j],where hi and ai are reflection indices and reciprocal unit-cell edges, respectively.
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Figure 1. Stereoscopic packing diagram for the crystal structure of syn-(H,CI)B viewed approximately down the b axis. Dipoles within the chosen interactlon dlstance (6.00 A) from the molecule undergoing excitation (shaded) are emphasized with an X .
'200
300
400
500
I- - 7 7 ' K , S T A G E
600
700
I
BOG
WAVELENGTH (nm)
Flgure 3. Spectroscopic data for thin films of syn-(H,CI)B in random form, after some annealing (stage 11) and after more thorough annealing (stage HI). Spectra for solution and for the crystals dispersed in a KBr matrix are also shown. (Reprlnted with permission from ref 2.)
Figure 2. Two molecules of syn-(H,Ci)B displaced by a , showing the C-H.. .O bonds and 40% probability elllpsoi&. The bond distances and angles are included; a v w a w standard ~ v ~of at bond ~ s and a bond angle are 0.003 A and 0 . 2 O , respectively. The c-H bond lengths are not corrected for polarization.
following: (a) C...O 3.19 A, H...O 2.2 A for the C-H...O bonds, (b) Cl-.-C13.50 A within the layers, and (c) C.*-O 3.20 A and C1. SO3.32 A for electrostatic interactions between layers. The mean-square vibration amplitudes along a (roughly parallel to the C-H. -0 direction) are
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consistently smaller than those along other directions. Certain details of the molecular structure (bond distances and bond angles), shown in Figure 2, are very similar to those observed previously in a number of other 9,lOdi~xabimanes.'~Moreover, in spite of the dense packing, the molecules of syn-(H,Cl)B exhibit considerable flexibility in the solid, as shown by the anisotropic shapes of the thermal ellipsoids on nitrogen. Relationship of Structure to Spectra. The UV-vis spectroscopic results described elsewhere are illustrated in Figure 3. We should explain the shifts of the maxima from the glassy stage I through the partially ordered stage I1 to the crystalline stage 111. The possible contribution of Davydov (exciton) splitting to the spectrum must also be considered. The interaction energy of a molecule in a crystal lattice is the sum of electrostatic, polarization, dispersion, and contributions.Light absorption by atom-atom a given molecule in the crystal changes the charge distribution within the molecule but does not instantaneously change its environment. Since the charge distributions in the ground and excited states are considerably different, electrostatic interactions should have a pronounced effect on the absorption spectrum. (15) Bimanes. 15. Goldberg, I.; Bernstein, J.; Kosower, E. M.; Goldstein,E.; Pazhenchevsky, B., to be submitted for publication.
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The Journal of Physical Chemistry, Vol. 86, No. 3, 1982
TABLE 11: Calculated Energies (kcal/mol) for the Interaction of a syn-(H,Cl)Bimane Molecule in the Ground and Excited Statesa with the Dipoles of Surrounding syn-(H,Cl)Bimane Molecules in the Crystal Latticeb Atomic Charges Q l ( C ( C 0 ) )= 0.45, Qa(O(C0))= 0.45, Q3(C(CC1))= 0.21, Q4(CI(CCI))= 0.21, Qfi(O*(CO)) = 0.65, Q6(N*) = 0.4 Ground-State Interaction Energies total E(C0,CO) = - 7.475 total E(CC1,CCl) = -1.721 total E(C0,CCI) = 0.238 total ground-state interaction = - 8.958 Excited-State Interaction Energies total *E(CO,CO) = -8.111 total *E(CCI,CCI)= - 1 . 7 2 1 total *E(CO,CCI) = 4.48 total *E(N+,CCl,CO)= - 14.939 total excited-state interaction = - 20.291 Total Difference excited - ground = - 11.333 a * indicates an excited-state charge or interaction All dipoles within 6.00 A were considered. energy.
These interactions were estimated by using the Coulomb equation for the dipolar bonds (C=O, C-C1) located within 6.00 A from a particular dipole (C=O, C-C1) or charge (N+)in the absorbing molecule. Only through-space interactions (dielectric constant = 1.0) with the dipoles in the first "coordination shell" were included. Atomic charges were estimated from typical dipole moments and bond distances. In the ground state, the charges on the C and 0 ( ( 2 4 ) are f0.45, on the C and C1 (C-C1) are f0.21, and on the 0-N is 0.l6 Table 11illustrates the way in which the different dipolar interactions contribute to the overall result. The major difference between the excited-state and ground-state stabilizations is due to a shift in the center of positive charge to the nitrogen of the excited molecule. Changes in the geometry of the molecule on conversion to the excited state would have a negligible effect on the calculated interaction energies since the distances between the dipoles would change little in the crystal. The greater the distance between the central molecule and the molecules bearing the dipoles, the more would the increased dielectric constant of "organic material" (>2) diminish the interaction energies. At the present time, we prefer a simple, straightforward model as a basis for calculations, pending the investigation of additional molecules. Although some parallels between our approach and that of crystal field theory exist, we are not aware of any other attempts to calculate the effects of dipoles on the electronic spectra of the solid state of organic molecules in the literature. We (16) Dipole moments: CBH6Cl,1.73 D; CH3COCH3,2.88 D. Hill, N.; Vaughan, W. E.; Price, A. H.; Davies, M. "Dielectric Properties and Molecular Behavior"; Van Noatrand New York, 1969, p 580. Bond distances: C-0, 1.21 A; C-C1, 1.70 A (present work). Carbonyl stretching frequency in syn-bimanes is similar to that in cyclopentanone (1745 cm-') (see ref 3). Solvent sensitivities of absorption and emission maxima for a number of syn-bimanes are reported in ref 6. These correspond to 0.20-0.25 of that for half of the 2-value solvent polarity parameter, based on the charge-transfer transition of a pyridinium iodide (Kosower, E. M. J. Am. Chem. SOC.1968,80,3253). The basic idea is that the charge-transfer transition defines the solvent sensitivity to be expected for a change of 1 in charge distribution. The solvent is organized around the ground state of an a,p-unsaturated carbonyl system; the increaae in charge separation is given by the change in emission (or absorption) energy divided by the change in 0.5A.Z for the same solvent pair. (The '2-value model" for a charge-transferelectronic transition is discussed in the reference cited.) The Coulomb interactions were obtained in kcal/mol by the equation E = 330q1q2/Dr, in which q, = fractional charge on an atom, D = dielectric constant (taken as 1.0) and r = distance in A.
Goldberg and Kosower
are fortunate in that the spectroscopic shifts observed for syn-(H,Cl)B are large and the crystal structure of this bimane is uncomplicated. For these reasons, a simple model works well. The change in charge distribution in the excited state is estimated from the solvent sensitivity of absorption and emission in comparison to that for a charge-transfer transition of a pyridinium iodide.16 The charges thus derived for the excited state are as follows: C, +0.45; 0, -0.65 (C=O); 0-N, +0.4; with no change for the C-C1 bond. The excited-state stabilization is greater than the ground-state stabilization by 11.3 kcal/mol, to be compared with the 11.8 kcal/mol for the spectroscopic shift from the glassy state (I) to the crystalliie stage (111). Increasing the charge to f0.5 (on C=O) in the ground state and to +0.5 (C) and -0.7 (0)in the excited state leads to an excitedstate stabilization of 12.5 kcal/mo1.16 X-ray crystal structure and spectroscopic analyses for 4,6-substituted and 4,g-bridged bimanes show that the more bent (smaller ring-ring dihedral angle) the bimanes are in the crystal, the more the absorption maxima for solutions are shifted toward shorter wavelengths." Assuming that syn-(H,Cl)B is somewhat bent in solution, the spectroscopic shift from stage I (random) to stage I1 may be due in part to increased planarity of the molecules. The appearance of a strong C-H IR absorption at stage 111: with C-H.. -0 bonding as the most likely origin, shows that the partially ordered arrangement of stage I1 does not contain such hydrogen bonds and may thus consist of one-dimensional stacks. The electrostatic contributions were estimated as 3.0 kcal/mol for a molecule within a one-dimensionalstack (a series of molecules along the [ l l l ] direction, perpendicular to the average C=O direction). Adding another row of molecules to the stack leads to an electrostatic stabilization too large for a stage I to stage I1 shift, assuming that the intermolecular distances are the same in stages I1 and 111. Perhaps the remaining portion (1.8 kcal/mol) of the total shift (4.8 kcal/mol) might be assigned to increased planarity of the bimane molecules resulting from the stage I stage I1 transformation. An estimate of the Davydov (exciton) splitting to be expected in the syn-(H,Cl)B crystal for the longest wavelength band gave a value of 126 cm-', using an oscillator strength of 0.1 (the absorption coefficient is around lOooO), a wavelength of 460 nm, and an intermolecular distance of 3.4 The shoulders on the main absorption bands might then arise from exciton splitting, but the primary shifts must come from electrostatic interactions. The two absorption bands observed at stage 111probably arise from molecules in two different sites, those "outside" (small shift, at 430 nm), at the surfaces and ends of stacks, and those "inside" (large shift, at 458 nm), within the bulk of the crystal. The thickness of the crystalline thin layer is only about 1000 A, corresponding to less than 300 molecules. It is unlikely that the surface is perfectly flat, and additional distortion must also be expected for molecules close to the sapphire window on which the material is deposited. Assuming equal absorption coefficients at the two sites, the ratio of the two types of molecules is about 2:1, with the bulk crystalline type (largest shift) dominant. We may thus infer that the domains within the stage I11 crystalline material have a limited size, given the
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(17) Kosower, E. M.; Goldberg, I., work in progress. (18) Atkins, P. W. 'Physical Chemistry";Oxford University Press: Oxford, 1978; pp 586-7. (19) Birks, J. B. "Photophysics of Aromatic Molecules", Wiley-Interscience: New York, 1970; pp 523-8.
J. Phys. Chem. 1082, 86, 335-340
relatively low ratio of inside to outside molecules.
Conclusions The absorption spectroscopic measurements on annealed
thin films of bimanes yield well-defined optical absorption spectra similar to those for crystals dispersed in a KBr matrix.2 The present article suggests that increased electrostatic stabilization of the excited state can account for a major part of the spectroscopic shift to longer wavelengths resulting from the change from a random arrangement of the molecules of syn-(H,Cl)B to the crystalline form. Transformation of the bent into a planar molecule may be a second, but less important, factor contributing to the shift. A simple model is useful for estimating the electrostatic stabilization of the ground and
335
excited states using the distances from an accurate crystal structure. Further work should involve the preparation of synbimanes in which the electronegative chlorine has been replaced with a first-row element (e.g., F); crystals of such bimanea could have smaller intermolecular separations and thus augmented intermolecular electrostatic interactions. In addition, the electrostatic stabilization calculations should be reconsidered when more accurate functions of molecular charge distributions in the ground and excited states become available from theoretical calculations. Supplementary Material Available: Structure factor tables (6 pages). Ordering information is given on any current masthead page.
Molecular Motion of Micellar Solutes: A I3C NMR Relaxation Study R. E. Stark,' M. L. Karakevlch, and J. W. Grangert Department of Chemlstty, Amherst College, Amherst, Massachusetts 0 1002 (Recelved:July 29, 198 1)
A series of simple NMR relaxation experiments have been performed on nitrobenzene and aniline dissolved in the ionic detergents SDS and CTAB. Using 13C relaxation rates at various molecular sites, and comparing data obtained in organic media with those for micellar solutions, we have estimated the viscosity at the solubilizationsite and derived a detailed picture of motional restrictions imposed by the micellar environment. Viscosities of 8-17 CPindicate a rather fluid environment for solubilized nitrobenzene; both additives exhibit altered motional preferences in CTAB solutions only. As an aid in interpretation of the NMR data, quasi-elastic light scattering and other physical techniques have been used to evaluate the influence of organic solutes on micellar size and shape. The NMR methods are examined critically in terms of their general usefulness for studies of solubilization in detergent micelles.
Introduction Micelles are aggregates formed by amphipathic molecules in aqueous (or organic) solutions, and they have attracted particular interest as model membranes and moderately effective catalysts.' Often at the center of their physiological function is the ability to solubilize organic compounds which dissolve only slightly in water, yet many structural and dynamic details of the solubilization process remain unclear. We have been particularly interested in the motional behavior of small organic molecules in the micellar environment, and our approach has been to focus on the physical characteristics of additives which may be introduced at concentrations low enough to avoid disruption of the micellar aggregate. In the present study, we demonstrate the usefulness of simple NMR relaxation measurements to obtain dynamic information on monosubstituted benzenes in sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium bromide (CTAB) micella. We critically compare micellar viscosities and solute reorientation rates derived from 13Cand from combined 13C/14Nstudies, in terms of their reliability, informational content, and general applicability for the study of solubilization. Theoretical Section The connection between nuclear magnetic relaxation and molecular reorientation has long been recognized by both theoreticians and experimentalists in the field.2J NSF-URP participant, 1980. 0022-3654/82/2086-0335$01.25/0
Briefly, 13C nuclear dipoles undergo transitions between spin states in order to return to equilibrium after a perturbation; these transitions often result from fluctuating magnetic fields induced by motion of 13C-lH dipoles in the molecule, which is more or less efficient in reestablishing equilibrium depending on the overall molecular tumbling time Teff. It is hardly surprising, then, that the characteristic NMR relaxation rate depends on such factors as temperature and viscosity. Alternatively, nuclei with an electric quadrupole moment (14N,%) interact with electric field gradients (efg's) of the surrounding molecular environment; again transitions may occur to the extent that the efg's fluctuate at a rate matching the NMR resonance frequency. As shown previously in studies of 50% (v/v) nitrobenzene in organic s0lvents,4*~ it is useful to employ the formalism developed by Huntress,G Woessner,' and Hubbard8 to express measured I3C and 14Nrelaxation rates in (1)(a) J. H. Fendler and E. J. Fendler, "Catalysis in Micellar and Macromolecular Systems", Academic Press, New York, 1975; (b) B. Lindman and H. Wennerstr6m, Top. Curr. Chem., 87,1 (1980). (2)A. Abragam, 'Principles of Nuclear Magnetism", Oxford University Press, London, 1961. (3) T. C. Farrar and E. D. Becker, "Pulse and Fourier Transform NMR", Academic Press, New York, 1971. (4)D. R. Bauer, J. I. Brauman, and R. Pecora, J.Am. Chem. SOC.,96, 6840 (1974),and references therein. ( 5 ) R. E. Stark, R. L. Vold, and R. R. Vold, Chem. Phys., 20, 337 (1977). ~ - -.,..
(6) W. T. Huntress, Jr., Adu. Magn. Reson., 4, 1 (1970);J. Chem. Phys., 48,3524 (1968). (7)D.E.Woessner, J. Chem. Phys., 37,647 (1962).
0 1982 American Chemical Society
