PAUL CHERINAND MICHAELBURACK
1470
The N-IsopropylcarbazolePicryl Chloride System
by Paul Cherin and Michael Burack Fundamental Research Laboratory, Xeroz Corporation, Webater, New Y O T ~(Received October 18, 1966)
N-Isopropylcarbazole (NIPC) combines with picryl chloride (PC) to form a 1: 1 chargetransfer complex. Although NIPC crystallizes in an orthorhombic form, space group Ic2a, and the complex NIPC-PC crystallizes in a monoclinic form, space group P2Jc, the crystal structures are believed to be very similar. The probable stacking arrangement of molecules in each case is described.
N-Isopropylcarbazole (NIPC) and picryl chloride (PC) combine to form a charge-transfer complex having a molecular ratio of 1:l. I n combining, the NIPC becomes the electron donor, and the PC, the electron acceptor. The NIPC-PC combination occurs in both the solid state and in solution; this has been confirmed by Sharp1 using absorption techniques and by Hoeg12 who utilized differential thermal analysis. This article is devoted to the solid state aspects of the NIPC-PC system only. Single crystals of the parent compounds (NIPC and PC), as well as the complex, have been prepared and studied by X-ray diffraction methods. The NIPC and PC molecules are of similar size; a molecule of PC could easily replace NIPC without disrupting the NIPC structure to any great extent, which facilitates their molecular combination in the solid state. Our studies of the charge-transfer complex (NIPC-PC) revealed a remarkable similarity between its lattice parameters and that of the parent compound NIPC.
Experimental Procedure Single crystals of both NIPC and N I P G P C were grown from solution. Although different solvents were tried in preparing the solution, dichloromethane proved to be the most effective. The solution was allowed to evaporate and yielded needle-like crystals of NIPC and NIPC-PC in the respective experiments. Subsequent examination with a polarizing microscope suggested a crystallographic axis parallel to the needle direction. For this reason, the crystals were mounted on fibers which were parallel to the needle direction. X-Ray photographs of the mounted crystals were taken with Weissenberg and precession cameras. The resulting photographs were used subsequently to deterThe Journal: of Phyaieal Chemistry
mine the space group and the approximate lattice parameters of the crystals. Polycrystalline samples were studied with a diff ractometer, and the resulting data permitted a precise determination of the unit-cell dimensions. The density of NIPC and NIPC-PC was determined by suspending the respective crystals in a mixture of solvents. The densities were then measured with a pycnometer.
Discussion and Findings The systematic absences for pure NIPC were observed to be of the type, h k 1 = 2n 1 for 1 for Okl, and reflections in general, with l ( k ) = 2n h(1) = 2n 1 for hkO reflections, respectively. This suggests that the NIPC space group is either Ic2a or Icma. Apiezoelectric measurement was made and found to be positive, and that fact led to the conclusion that the noncentrosymmetric space group Ic2a was correct. The systematic absences for N I P G P C (1:1 ratio) 1 for OkO reflecwere found to be of the type k = 2n tions and 1 = 2n 1 for h01 reflections. This indicated that the space group of the NIPC-PC unit cell was P21/c. Conventional nomenclature was used in designating the axes of the monoclinic cell containing NIPC-PC; the axes of the orthorhombic cell were designated to illustrate the similarity to those of the monoclinic cell. Table I lists the refined unit-cell dimensions. Picryl chloride (PC) unit-cell dimensions were taken from the work reported by Golder, Zdanov, and Umanskij .*
+ +
+
+
+
+
+
(1) J. H. Sharp, J . Phys. Chem., 70, 584 (1966). (2) H. Hoegl, Battelle Memorial Institute, Geneva, private communication. (3)G. A. Golder, G. 9. Zdanov, and M. M. Umanskij, Dokl. A M . Nauk SSSR, 92, 311 (1953).
N-ISOPROPYLCARBAZOLE-PICRYL CHLORIDE SYSTEM
1471
Table I : Unit-Cell Dimensions Material
NIPC PC NIPC-PC
Space
a,
group
A
b, A
18.01 & 0.02 11.10 18.12 f 0.07
7.963 f 0.006 6.83 6.962 f 0.013
Ic2a P2,/c P21/~
An independent set of PC unit-cell dimensions was derived as a check against the referent data and, although a slight difference between the referent data and our data exists, it was concluded that the differences were not significant enough for separate discussion. The d spacings for NIPC are tabulated in Table 11, and those for IVIPC-PC are listed in Table 111.
Table I1 : d Spacings and X-Ray Data for N-Isopropylcarbazole (NIPC) h kl
dobad
dcalcd
1/11
101 200 002 202 211 112 103 400 004 312 303 402 204
12.32 9.02 8.44 6.15 5.62 5.53 5.36 4.51 4.21 4.16 4.10 3.97 3.81
12.30 9.01 8.42 6.15 5.62 5.51 5.36 4.51 4.21 4.17 4.10 3.97 3.81
5 15 100 6 55 10 2 3 18 50 5 13 7
220 501 222 321 413
3.64 3.52 3.34 3.26 3.21
3.64 3.52 3.34 3.26 3.21
4 2 1 6
3
The unit-cell dimensions of the charge-transfer complex are very similar to the parent compound, NIPC; the major difference between them being in the lengths of their b axes. This condition suggests that only minor structural differences exist between the two. The major cleavage planes of the NIPC cell are parallel to the ab planes. This suggests the possibility that the essentially planar NIPC molecule is parallel to the ab planes. The intensity distribution of the 001 reflections observed on Weissenberg photographs
16.82 f 0.01 12.62 16.70 f 0.08
90 102.5 100.1 f 0 . 1
8 4 8
1.14
1.15
...
...
1.47
1.46
Table 111: d Spacings and X-Ray Data for Charge-Transfer Complex NIPC-PC
100 200 002 102 102 202 110 1i i 300 202 210 21i 012 211 212 310
17.88 8.96 8.24 8.06 7.03 6.68 6.50 6.17 5.94 5.57 5.48 5.40 5.30 5.04 4.81 4.52
17.80 8.91 8.20 8.02 7.01 6.65 6.47 6.16 5.94 5.57 5.48 5.38 5.32 5.04 4.81 4.52
68 39 100 42 31 8 2 6 5 4 12 4 8 3 3 3
'02} 400
4.46
i:;:}
13
113
4.32
4.32
2
312 103 203 104 4ii
4.19 4.16 4.02 3.85 3.78
312 402 500
3.66 3.56
3.75 3,75) 3.66 3.57
1 1
3.50
3.50
2
3.40 3.36 3.32
3.40 3.36 3.32
23 6 4
3.27
i:::)
2
3.22
3.22
io
204 02 1 12i 121
22i
3.75
4.20 4.16 4.01 3.86 3.78
15 6 3 2 6
seem to confirm this implication. The intensity of the 001 reflections decrease rather uniformly as a function of (sin @/A. It is interesting to note that this observation is an exception to the findings of Boeyens and Herbstein4 in which they indicated that, in general, the short dimension (usually about 6.7 to 7.4 A) was the stacking direction for r-molecular compounds. Our findings are suggestive of molecular stacking in the moderately long c direction for both NIPC and NIPCPC. Further indirect evidence that the stacking direction is not parallel to the short axis was yielded by thermal diffuse scattering. Boeyens and Herbstein reported that oscillation photographs taken about the short direction, the needle axis of r-molecular crystals, showed strong thermal diffuse scattering from planes located on the second-layer line with spacings of about 3.2 to 3.5 A. In contrast, NIPC and NIPC-PC show (4) J. C. A. Boeyens and
F. H. Herbstein, J. Phy8. Chem., 69,2153
(1965).
Vo~unzeYO, Number 6
May 1966
PAUL CHERINAND MICHAELBURACK
1472
strong thermal diffuse scattering in planes distributed throughout reciprocal space in a more complex manner. Analysis based on the data accumulated and presented in this note leads us to suspect that the planar molecules of NIPC are almost parallel to the ab planes; but, because the van der Waals width of the molecule across the benzene rings is 9.3 A, and because the length of the b dimension is only 8 A, it is necessary that the KIPC molecules be slightly tilted out of the ab plane. Taking the van der Waals radius of the hydrogen atcm to t e 1.2 A, the tilt was calculated to be approximately 15". The structure of the NIPC-PC complex is believed to be very similar to the parent NIPC structure. This belief is supported by the similarity in the lattice parameters, and by the relative intensities of the hOO, OkO, and 001 reflections observed on the Weissenberg photographs. It is further believed that the PC molecules replace alternate NIPC molecules in the
The J O U T ~ Cof~ Physical ~ Chemistry
structure to maintain the ?r-bond interaction that exists in solution. In addition because the short axis is only 7 A in the 1 : 1 complex, the molecules must be tilted out of the ab plane to a greater extent than in the parent NIPC crystal, assuming our basic model is correct.
Summary The crystal structure of the charge-transfer complex, NIPC-PC, appears to be similar to its parent compound, NIPC, and the molecules lie in stacks as in the case of other similar compounds. It would seem, however, that the short dimension is not in the stacking direction.
Acknowledgments. The authors wish to thank J. Weigl and J. Sharp, Xerox Corporation, for their encouragement and many valuable discussions, and H. Hoegl, Battelle Memorial Institute, for his help in obtaining samples.
