1199
CRYSTAL STRUCTURE OF N-TRIFLUOROETHYL-N,2,4,6-TETRANITROANILINE
This result is in good agreement with the theoretical calculations of Barfield.26 The results of these calculations can be expressed very simply26as
J”
=
cos2
+ cos2
cp2
- 0.7
where JHHis the indirect contribution to the four-bond H H coupling and cp1 and cpz are the dihedral angles in the two CH-C-C fragments. This gives JtMeH and JgMeH values of 0.8 and 0.05 cps, respectively. Barfield states that there may be a direct (through space?) mechanism giving a negative contribution to the coupling when the two coupling protons are close together. Alternatively, the calculations would still be consistent with a slightly modified equation, such as JHH= 1.2(c0s2 cp1 cos2cp2 - l.O),
+
which gives the correct answers for JIMaH (0.6 cps) and JQMeH ( -0.3 CPS). Acknowledgments. This work was performed in part during the tenure of a Mellon Institute Visiting Fellowship. I am grateful to Professor A. A. Bothner-By for the sample of meso-2,3-dibromobutane, to both him and Professor J. A. Pople for some very helpful criticism, and to Mr. M. A. Cooper for the results for the 1,1,2trichloroethane. Professor T. Vladimiroff very kindly performed the numerical integration. ( 2 5 ) M. Barfleld, J . Chem. Phys., 41, 3825 (1964). ( 2 6 ) R. J. Abraham, H. Goltschalck, H. Paulsen, and W. A. Thomas, J. Chem. Soc., 6268 (1965).
The Crystal Structure of N- ( p, 0,p-Trifluoroethyl)-N,2,4,6-tetranitroaniline by James R. Holden and Charles Dickinson L’.S. Naval Ordnance Laboratory, White Oak, Silver Spring, Maryland
20910
(Received May 24, 1 9 6 8 )
The crystal structure of N- (/?,P,P-trifl~oroethyl)-N,2,4,6-tetranitroaniline ( C B H ~ F ~ N ~has O Bbeen ) determined by X-ray diffraction. The unit cell is orthorhombic (a = 11.98,b = 11.84,c = 9.29 A), space group PnaZl with four molecules per cell. The nitramine group is rotated 86’ out of the plane of the benzene ring and the 2-,4-,and 6-nitro groups are rotated 19,12,and 39O,respectively. A list of rotation angles for other nitroaromatic compounds is included for comparison. The shortest oxygen-fluorine separation in the structure (3.00A) is between the atoms with the longest C-F and N-0 bond distances.
Introduction The crystal structure of N- (p,p,p-trifluoroethy1)N,2,4,6-tetranitroanilinehas been determined as part of a study of the effect of fluorine on molecular structure. In this case, a, trifluoromethyl group has been added to the methyl group of N-methyl-N,2,4,6tetranitroaniline, the subject of a recent structure determination by Cady.’ Comparison of the lengths of equivalent bonds in the two compounds should provide an indication of the scope and magnitude of the influence of the trifluoromethyl group. Experimental Section A sample of N- (&&P-trifluoroethyl)-N,2,4,6-tetranitroaniline containing suitable crystals was obtained from the Denver Research Institute.2 Cell dimensions and reflection intensities were determined from the same 0.5 X 0.3 X 0.1 mm crystal rotated about its longest dimension, the a axis. The following orthorhombic cell dimensions were calculated from measure-
ments of precession films obtained with Zr-filtered Mo K a radiation (X = 0.7107 A) : a = 11.98 f 0.03 A; b = 11.84 f 0.03 A; c = 9.29 f 0.03 A. These values give a calculated density of 1.79 g/cc with four molecules per cell. The density as determined by flotation in an aqueous lead perchlorate solution is 1.77 g/cca3 The errors in the cell dimensions were estimated from variations among individual film measurements. Reflection intensities were determined from doubly integrated Weissenberg films obtained with a Nonius camera and Ni-filtered Cu K a radiation (X = 1.5418A), Four-film packs were collected for levels OkZ through 9kl and measured with a semiautomatic microdensitometer designed and built at this laboratory. The densitometer scanned in the 9 direction and the optical (1) H. H. Cady, Acta Crystallogr., 23, 601 (1967). (2) Compound synthesized
by the Denver Research Institute under Contract AF 08 (635)-2109, Project 2858, “Research on Fluorochemicals.” (3) H. T. Simmons, private communication. Volume 73, Number 6 M a y 1969
JAMES R. HOLDENAND CHARLESDICKINSON
1200 density difference between the peak and the interpolated background was correlated with exposure by means of a known gray scale. Of the 1331 reflections in the region of reciprocal space measured, 871 had measurable intensity. The two sets of periodic absences observed ( h = 2n - 1 in h01 and IC 1 = 2n - 1 in Olcl) indicate that the space group is Pna21 or Pnam. However, Pnam would require impossible molecular symmetry; therefore, the space group is Pna21.
+
Determination of Structure All calculations for the determination of this structure were performed on an IBM 7090 computer using the crystallographic computing system X-ray 63.4 The intensity measurements within each film pack were correlated giving greater weight to those from the middle of the useful optical density range. These correlated intensities were converted to structure factors and to quasi-normalized structure factors (Karle-Hauptman E’s) 6 using form factors for neutral atomsa6 No absorption correction was applied. The angular orientation of the benzene ring of the asymmetric unit was obtained from an interpretation of the sharpened, origin-removed Patterson function calculated from the normalized structure factors (€E2- 1). This was facilitated by calculating the function on hemispheres 2.4 and 2.8 k . from the origin and searching for the planar array of interatomic vectors produced by the part of the molecule of known approximate geometry, i.e., the benzene ring and the nitrogen atoms attached to it. The search was carried out by means of a computer code which formed “synthetic” Patterson shells from a molecular model and then compared all possible orientations of these shells with the observed Patterson values. This procedure indicated three possible orientations for the benzene ring. Determination of the x and y parameters of the center of the ring (the z parameter is arbitrary) was accomplished with another computer code which translated the partial molecular model through the cell and determined all possible positions, ie., those with no impossibly short intermolecular distances (distances smaller than the sum of the van der Waals radii minus 0.3 A), For this step, the oxygen atoms of the ring nitro groups were added to the model which was still assumed t o be planar. This was done for all three angular orientations but only one produced a likely looking location. A structure factor calculation using this location of the partial molecular model gave an agreement index ( R ) of 0.49 and an electron density calculation using these indicated phases contained peaks at reasonable positions for all the nonhydrogen atoms of the molecule. With all the atoms placed as indicated, the R value fell to 0.28. Full-matrix least-squares refinement using individual isotropic temperature factors lowered the R value to The Journal of Physical Chemistry
0.14, and the introduction of anisotropic temperature factors for all of the nonhydrogen atoms except the benzene ring, N(2), N(4), and N(6) lowered R to 0.10, At this point, the eight reflections whose measurement involved the largest optical densities were omitted from the refinement. The calculated structure factors for these reflections were much larger than their observed values and it was assumed that the corresponding exposures had exceeded the usable range of film response. The R value without these reflections was 0.095. The four hydrogen atoms of the molecule were placed as indicated by the adjacent heavy-atom positions; i e . , C(3) and C(5) assumed trigonal and C(7) tetrahedral (see Figures 1 and 2). All C-H bond lengths were set equal to 1.075A and the isotropic temperature factors of the hydrogen atoms were set at 3.70, the value found for the overall temperature factor by the E scaling program.? The hydrogen parameters were not entered into the least-squares refinement but were adjusted between cycles to maintain normal bond distances and angles.
0121 10.341
1.48
12”
A N141 [-O.OSl
7
u Figure 1.
[-0.33]
lO.09l
(-0.021 Projection of molecule onto pIane
of benzene ring.
(4) J. M. Stewart, et al., Technical Report TR 64-6, NsG-398 Computer Science Center of the University of Maryland, 1964. (5) H. Hauptman and J. Karle. “Solution of the Phase Problem. I. The Centrosymmetric Crystal,” ACA Monograph No. 3, Polycrystal Book Service. Pittsburgh, Pa., 1953. (6) (a) Carbon, nitrogen, and oxygen: J. Berghuis, I. M. Haanappel. M. Potters, B. 0 . Loopstra, C. H. MacGillavry, and A. L. Veenendaal. Acta Crystallogr., 8, 478 (1955); (b) hydrogen: R. McWeeny, i b i d . , 4, 513 (1951); (c) fluorine: A. J. Freeman, ibid.. 12, 261 (1969). (7) 0. Dickinson, J. M. Stewart, and J. R. Holden, ibid., 21, 663 (1966).
1201
CRYSTAL STRUCTURE O F N-TRIFLUOROETHYL-N,2,4,6-TETRANITROANILINE
Table I: Btomic Paramet,ers 5
0.0612(09) 0.0919(08) 0,1654(09) 0.2128(08) 0.1911 (09) 0.1143(08) 0.0214(11) 0.0480(11) -0.0111 (07) 0.041 1(09) 0.2873(08) 0.0897(08) -0.1250(09) -0.0446(08) 0.0897(09) 0.3189(12) 0.3122(09) 0.1697(08) 0.0058 (08) -0.1482(07) -0.1920 (07) -0.0421 (08) 0.0787(09) 0.1264(07) 0.0939 -0.0461 0.1853 0.2302
-
Y
z
0.1908(07) 0.2642(07) 0.2379(07) 0.1312(06) 0.0518(07) 0.0850(06) 0.2962(06) 0.2381 (09) 0.2179(06) 0.3783(07) 0.0959(07) -0.0012(06) 0.2040(08) 0.3994(06) 0.4471 (06) 0.1682(08) -0.0014(06) -0.0556(06) -0.0106(06) 0.1576(08) 0.2457(08) 0.1819(06) 0.3099(07) 0.1588(06) 0.3418 0.3545 0.2967 -0.0299
0.4815 0.3683(13) 0.2605( 13) 0.2663 (12) 0.3722(13) 0.4798(12) 0.7031 (14) 0.8416(14) 0.5925(12) 0.3604 (13) 0.1481 (14) 0.5905(12) 0.5595( 15) 0.4255(14) 0.2842(16) 0.0674(17) 0.1390(15) 0.6302( 14) 0.6328(13) 0.4508(13) 0.6489(14) 0.8947 (12) 0.9388(12) 0.8264(12) 0.6676 0.7211 0.1763 0.3726
B or B I I
2.96( 14) 3.01 (14) 3.49(15) 3.12(14) 3.29(15) 2.78( 13) 6.09(73) 5.82(74) 1.49(49) 4.26( 16) 4.65( 17) 3.65( 14) 2.90(65) 5.27(48) 8.56(67) 10.64(64) 7.92(65) 7.14(57) 5.57(49) 2.95(41) 3.55(52) 8.97(58) 10.89(66) 6 84(44) 3.70 3.70 3.70 3.70 I
Six final cycles of least-squares refinement using mixed isotropic and anisotropic temperature factors as mentioned above lowered the R value to 0.084. The average shift: error ratio for all parameters in the last cycle was 0.02; the maximum was 0.07. Including
b13
Bes
Baz
Bas
B12
2.14(28) 4.10(50) 3.30(28)
3.90(38) 3.75(41) 4.17(32)
-0.28(32) -0.01(44) 0.28(23)
-0.02(38) -0.04(44) -0.14(27)
-0.52 (35)
5.25(38) 4.09(30) 3.76(29) 5.45(37) 4.68(33) 5.17(34) 3.39 (24) 7.93(46) 7.05(41) 6.12(33) 7.00(39) 6,91(33)
6.19(50) 7.96(50) 10.55(74) 9.55(70) 6.66(45) 5.36(36) 5,66(34) 5.38(40) 7.37(47) 5.44(33) 4.80(34) 5.11(29)
0.76(36) 1,75(29) 1.60(34) -0.05(43) 2.39(35) 1.73(34) 1.04(23) -1.00(34) 1.56(34) -0.36(32) -0.82(39) 2 28(36)
0.91(42) 1.33(42) 3.01(57) 6.18(64) 2.49(49) -0.52 (39) 1.55(38) -0.66(31) 1,18(45) 3.12(37) 1.61(39) -1.22(31)
-0.26(39) 1.83(33) 3.64(39) 0.82(43) -0.14(35) 1.26(33) 0.36(27) -1.30(56) -0.29(43) 1.02(27) -2.13(31) -0.07(29)
-
I
-
-0.09(28)
-0.12(23)
scale factors, the structure is based on 180 parameters; therefore, the overdetermination ratio is 4.8 (863/180). The value of the z parameter of C(l) was fixed (not refined) in order to establish the position of the molecule as a whole. The data were grouped by Weissenberg level (all reflections with the same Miller index h ) and a scale factor assigned to each level. These level scale factors were determined during the early cycles of least-squares refinement using isotropic temperature factors. They were not included in the final refinement because of the high correlation which would exist between the scale factors and the B11 component of the anisotropic temperature factors.8 This procedure could introduce or mask an overall anisotropic trend in the temperature factors; therefore, any such trend may have no physical reality. In all least-squares refinements, the quantity minimized was w(F, - FJ2, where w is the weight assigned to the reflection. For unobserved reflections, (F, - F m i n ) ' was included in the sum when the calculated structure factor, F,, was greater than Fmi,, the structure factor calculated from the minimum observable intensity. No contribution was included for unobserved reflections when F, was smaller than Fmin. The weight was 1.0 for reflections with F,1 less
2
Figure 2. Projection of molecule onto plane of C(1), C(7), and N(7).
(8) E. C . Lingafelter (1966).
and J. Donohue, Acta CryslaElogr.,
2 0 , 321
Volume 75, Number 6 May 1060
1202
JAMESR. HOLDEN AND CHARLES DICKINSON
+
than 10.0 and (10.0/(5.0 0.5F,,1))~for reflections with Frelgreater than 10.0. The maximum F,,1, 100.1, had a weight of 0.182 with this scheme. The final atomic parameters are given in Table I. The numbers in parentheses are the errors in the last two digits as estimated from the inverse matrix from the last least-squares cycle. The anisotropic temperature factors are of the form expr -$ (h2a*2Bll
+ le2b*2Bzz+ 12c*2Bas+ 2hlcu*b*B12 -/- 2hh*C*Bla + 2klb*c*&a)]
These Bij values are on the same scale as the isotropic B values listed for some of the atoms. A table of observed and calculated structure factors has been deposited with the ASIS National Auxiliary Publication Service.B
Discussion Figure 1 is a diagram of the molecule projected onto the least-squares plane10 of the six carbon atoms of the benzene ring. The equation of this plane is 8.99s
+ 4 . 3 1 ~+ 5.122 = 3.85
Table 11: Bond Angles (Degrees) C(2)-C(l)-C(6) C(l)-C(2)-C(3) C(2)-C( 3)-C(4) C(3)-C(4)-C(5) C(4)-C(5)-C(6) C(5)-C(6)-C( 1)
115.0 124.8 116.3 124.7 115.8 123.4
C(6)-C (1)-N( 1) C(2)-C (1)-N( 1) C(l)-C(2)-N(2) C(3)-C(2)-N(2) C(3)-C(4)-N(4) C(5)-C(4)-N(4) C(5)-C(6)-N(6) C(l)-C(B)-N(B)
119.9 125.1 119.4 115.8 118.5 116.8 115.4 121.2
C(2)-N(2)-0( 1) C(2)-N (2)-0 (2) C(4)-N(4)-0(3) C(4)-N(4)-0(4) C(B)-N(6)-0(5) C(6)-N(6)-0(6) N(l)-N(7)-0(7) N(l)-N(7)-0(8)
120.5 116.1 117.1 118.6 114.9 118.4 118.0 115.4
0(1)-N(2)-0(2) 0(3)-N(4)-0(4) 0 (5)-N (6)-0 (6) 0(7)-N(7)-0(8)
123.4 124 5 126.7 126.6
C(l)-N( 1)-N(7) C(l)-N(l)-C(7) C(7)-N(l)-N(7)
114.6 120.9 119.5
N(l)-C(7)-C(8)
112.1
C(7)-C(8)-F( 1) C(7)-C(8)-F(2) C(7)-C(8)-F(3)
111.6 111.1 112.5
F(l)-C(8)-F(2) F(l)-C(8)-F(3) F(2)-C(8)-F(3)
107.1 104.6 109.7
I
where 2, y, and x are fractional coordinates in the examples of this arrangement are found in 1,3,5-triorthorhombic cell. The standtrd deviation of these amino 2,4,6 trinitrobenzene,161,3 - diamino-2,4,6 - triatoms from the plane is 0.005A. The distances (in nitrobenzene,lg1,3-dichloro-2,4,6-trinotroben~ene,~~ and Angstroms) of the individual atoms of the molecule trinitrobenzene in complexes with skatole,ls indo1e,l8 from the plane are given in parentheses in Figure I. azulene,19 tricarbonylchromium anisole,14 acepleiadylFigure 2 is a projection of the molecule onto the plane ene,20anthracene,21and 2,4,6-tri (dimethylamino) -1,3,5of C (1), C (7), and N ( 7 ) . The numbers in parentheses triazine.22 in this figure are the distances of the atoms from this Because the stabilization energy due to resonance plane. Note that the plane of N(1), C (7), and C (8) is interaction is greatest for a coplanar configuration, approximately perpendicular t o that of C (1), C ( 7 ) , unhindered aromatic nitro groups lie approximately in and N(7) and that the trifluoromethyl group is turned the plane of the ring to which they are attached. so as to maximize the distances between the fluorine Spectroscopic evidence for such resonance has been atoms and N (1). The estimated standard deviations in the bond (9) The structure factor table has been deposited as Document No. NAPS-00215 with the ASIB National Auxiliary Publication distances given in Figures 1 and 2 range from 0.012 to Service, c / o CCM Information Sciences, Inc., 22 West 34th St., 0.018A. Those in the bond angles listed in Table I1 New York, N. Y. 10001. A copy may be secured by citing the document number and by remitting $1.00 for microflche or $3.00 range from 0.7 t o 1.3". The rotation angles given in for photocopies. Advance payment is requfred. Make checks or Figure 1 are the dihedral angles between the ring plane money orders payable to: ASIS-NAPS. and the planes specified by the three atoms attached to (10) V. Schomaker, J. Waser, R. E. Marsh, and G. Bergman, Acta Crystallogr., 12, 600 (1959). each of the four substituent nitrogen atoms. (11) P. M. Harris, "Structures of Trinitro-aromatic Crystals and Within the accuracy of the determination, all of the Related Substances," Ohio State University Research Foundation Report AFOSR-TR-59, 1959, p 165. nitro groups are planar. However, the amine group is (12) H. H. Cady, A. 0. Larson, and D . T . Cromer, Acta Crystallogr., not planar; N ( l ) lies 0.18A from the plane of C ( l ) , 16, 617 (1963). N (71, and C (7) (see Figure 2 ) . Such nonplanarity is (13) A. 9. Bailey and 0. K. Prout, J . Chem. Soc., 4867 (1965). also observed in N-methyl-N,2,4,6-tetranitroaniline1 (14) 0. L. Carter, A. T . McPhail. and G . A. Sim, i b i d . , A, 822 (19661. (15) H. H. Cady and A. C. Larson, Acta Crystallogr., 18, 485 (1965). and in the other nitramine structures cyclotrimethylene(16) J. R . Holden, ( b i d . , 22, 545 (1967). trinitramine" and the a and p polymorphs of cyclo(17) J. R. Holden and C. Dickinson, J . Phys. Chem., 71, 1129 tetramethylenetetranitrarnine.l2 (1967). The bond distances and angles in the trinitrophenyl (18) A. W. Hanson, Acta Crystallogr., 17, 559 (1964). (19) A. W. Hanson, {bid., 19, 19 (1965). group are normal when compared to other structures. (20) A. W. Hanson, i b i d . , 21, 97 (1966). The internal angles of the ring are greater than 120" at (21) D . 8. Brown, S. 0. Wallwork, and A. Wilson, ( b i d . , 17, 168 the carbon atoms carrying nitro groups and less than (1964). 120" at the three remaining carbon a t o r n ~ . ' ~ JOther ~ (22) R. M . Williams and S. 0. Wallwork, i b i d . , 21, 406 (1966).
-
The Journal of Physical Chemiatry
-
1203
CRYSTAL STRUCTURE OF N-TR1FLU0R0ETHYL-N12,~,6-TETRAN1TR0AN1LINE
Table 111: Nitro Group Rotation Angles in Nitrobenzenes (Degrees) No ortho
One ortho
T w o ortho
0 9 13,13
NitrobenzeneB7 p-Dinitrobenzene28 wDinitrobenzene20 1,3,&Trinitrobenzene-complex with Skatole1* Azulenele Tricarbonylchromium anisole14 AcepleiadyleneZ0 Anthracenez1
0,4, 14 (3, 8, 3) (4,4, 3) (21 2, 1) 0,18
2,4,6-Tri(dimethylamino)-l,3,5-triazin~z
010
55 66
Hexanitrobenzene*o Nitromesit ylene" o-Nitrobenzaldehydesz p-Nitrobenzoic acid88 o-Nitrobenzoic acid" o-Nitroperoxybenzoic acid85 Potassium hydrogen di(p-nitrobenzoate)m 2-Chloro-5-nitrobenzoic acidn Potassium p-nitrophenyldicyanomethide" 4-NitroanilineaD ZChloro-4-nitroaniline40
27 14
55 28
65
7 (11 2 4
19 (sh, 5h)
2,3,4,6-Tetranitroaniline7 1,3-Diamino-2,4,6-trinitrobenzene1~ 1,3,5-Triamino-2,4,6-trinitrobenzene1~ N-Methyl-N,2,4,6-tetranitroaniline1 N,3-Dirnethyl-4-brom0-2,6-dinitroaniline~~
3h,
45,64
(39 (3h, 35 lh)
24
N-Picryl-p-iodoaniline IQ N-Picryl-p-iodoaniline I P N,N-Dimethyl-p-nitroaniline~~ p-Nitrophenyl azide44 Picryl azide-bis (8-hydroxyquinolinato) copper(II)l* a-p-Nitrophenol46 p-p-Nitropheno14~
2,4,6-Trinitr0phenetole'~
0 0 3 (4)
7 2 7 4
o-Nitrobenzenesulfenic acid@ Propargyl-2-bromo-3-nitrobenzene4e
26,44 6h 28,45 28,55
73
14,58
32,61 0'
60
2,4-Dinitrochlorobenzene60 2,4-Dinitrobromobenzenem 2,4,6-Trinitrochloroben~ene11 l13-Dichloro-2,4,6-trinitrobenzene'7
15 15 4
4,4'-DinitrodiphenyP1
0,ll
39 40 40,71 37
75
40
63
-
Average
6
reported by Nagakura, Kojima, and Maruyama.a* rotated by more than 20'. Intermolecular hydrogen bonding is proposed as the cause for this displacement.' However, the stabilization energy is small (8.4 kcal/mol for nitrobenzenez3)and the C-N bond is not shortenedSz4 Trotterz6 and Dashevskii, Struchkov, and Akopyanze (23) S.Nagakura, M. Kojima, and Y. Maruyama, J. Mol. Spectrosc., 13, 174 (1964). proposed similar values and stated that the energy (24) J. Trotter, Tetrahedron, 8 , 13 (1960). diminishes as the cosine or square of the cosine of the (25) J. Trotter, Can. J. Chem., 37, 905 (1959). angle between aromatic and nitro group planes. That (26) V. G. Dashevskii, Yu. T. Struchkov, and Z. A. Akopyan, Zh. Struk. Khdm., 7 , 594 (1966). is, an 18" rotation would reduce the energy by only 5 (27) J. Trotter, Acta Crystallogr., 12, 884 (1959). or 10%. Therefore, it is not surprising that nitro (28) 8. C . Abrahams, dbdd., 3, 194 (1950). groups are often rotated to allow closer intermolecular (29) J. Trotter, ( b i d . , 14, 244 (1961). packing. However, in the absence of ortho substit(30) Z. A. Akopyan, Yu. T. Struchkov, and V. 0.Dashevskii, Zh. Strukt. K h i m . , 7 , 408 (1956). uents, such displacements are usually small; of the 41 (31) J. Trotter, Acta Crystallogr., 1 2 , 605 (1959). crystallographically independent nitro groups in the (32) P. Coppens and G . M. J. Schmidt, ibdd., 17, 222 (1954). substituted benzenes listed in Table IIIl2'-61 only the (33) T. D. Sakore and L. M. Pant, Inddan J . Pure A p p l . Phys., 3, 4-nitro group of N-methyl-N,2,4,6-tetranitroanilineis 143 (1965). Volume Y8, Nurnbdr 6 May 1960
1204
JAMES R. HOLDENAND CHARLEB DICKINSON
ortho substituents other than hydrogen effectively prevent aromatic nitro groups from approaching coplanarity. A notable exception is the amine group which apparently forms an intramolecular hydrogen bond with the adjacent nitro group oxygen atom and thereby allows the nitro group to lie in the aromatic plane. The rotation angles for nitro groups involved in such bonds are marked with an “h” in Table 111. Another exception occurs in o-nitrobenzenesulfenic acid where the coplanarity of the nitro group has been attributed to orbital interaction between sulfur and oxygen (marked “i” in Table 111). These values have been omitted from the calculation of average rotation angles for nitro groups with one and two nonhydrogen ortho substituents. Of the nitro groups with noninteracting ortho substituents, only the 4-nitro group of 2,3,4,6-tetranitroaniline’ and the 6-nitro group of picryl azidela are rotated by less than 25” from the ring plane. The short nonbonded distances are 2.61 A to the nitrogen atom of the adjacent nitro group and 2.52 A to the central azide nitrogen atom. Rotation angles in parentheses in Table I11 are calculated values not obtained from the structure reference. The N- (p,&3-trifluoroethyl)-N,2,4,6-tetranitroaniline molecule contains two nitro groups adjacent to the trifluoroethylnitramine group, The shortest nonbonded distance affecting the position of the 2-nitro group is 2.68 A from O(1) to N(1) which is approximately the same as the corresponding distance in N-methyl-N,2,4,6-tetranitroaniline.*This group is rotated only 19” from the plane of the ring. As mentioned above, the two other similarly situated nitro groups with rotations this small also involve close approach to nitrogen atoms. The 39” rotation of the 6-nitro group appears to be controlled by F(3) and by F(1’) of the neighboring x ) (see Figure 1). molecule located at (-2, -y, -3 0 ( 5 ) and O(6) are 3.17 and 3.12A from F(3) and both are 3.06 A from F ( l’), The 4-nitro group is displaced from the aromatic plane so as to increase its distance from the methylene group of the molecule located at - 2, y - 4, z - +) (see Figure 1). O(4) is 3.17A from C(7”) and only 2.19 A from the assumed position of H(1”). This probably indicates that the position of H ( l ) is somewhat in error. The 12” rotation found for the 4-nitro group is within the range of displacement reported for unhindered nitro groups in the compounds listed in Table 111. Since N- (p,P,P-trifluoroethyl) -N,2,4,6-tetranitroaniline differs from N-methyl-E,2,4,6-tetranitroaniline only by the addition of a trifluoromethyl group to the methylnitramine side chain, it is interesting to compare equivalent bonds in the two compounds. N(1)C(1) (1.38 A) and N(l)-C(7) (1.44 A) appear to be
+
(3
The Joumal of Physical Chemistry
slightly shorter than the equivalent bonds in the methyl compound (1.419 and 1.466 A),l but N(l)-N(7) (1.41 A) is much longer than its equivalent (1.347 A). Also, N(7)-O(7) (1.18 is different from N(7)-O(8) (1.26A) whereas the equivalent bonds are the same length (1.232, 1.230 in the methyl compound.’ The average length of the three C-F bonds, 1.33 A, is the same as the average value, 1.333 A, given by the tables of ref 52 for this bond in poly-substituted compounds. The variation among the individual bonds may be due to the influence of neighboring atoms. The shortest bond, 1.29A, is to F(2) which is not close to any other atom. The intermediate bond, 1.34 A, is to F(3) which is close to N ( l ) (2.82 A) and N(6) (2.93 A). The longest bond, 1.36 A, is to F(1) which is close to N ( l ) (2.86A) and O(8) (3.00A). Note that O(8) is the oxygen atom with the longest N-0 bond, 1.26 Although this is the shortest P-0 distance in the structure, it is longer than the estimated van der Waals contact distance using the radii given by Bondi.63 Other closer approaches to oxygen atoms produce no change in the N-0 bond distance; therefore, the long C(8)-F(l) and N(7)-0(8) bonds may indicate an interaction between partial negative charges carried by the fluorine and oxygen atoms.
A) A)
A,
Acknowledgment. The authors gratefully acknowledge financial support for this work by the U,S. Air Force Systems Command, Eglin Air Force Base, Fla., Project PG-3-19, and by the Foundational Research Fund, Task FR-44 of the U. S. Naval Ordnance Laboratory, White Oak, Silver Spring, Md. (34) T. D. Sakore, S, 9. Tavale, and L. M. P a n t , Acta Crystallogr., 22, 720 (1967). (35) M. Sax, P. Beurskens, and S. Chu, ibid., 18, 252 (1965). (36) H . N. Shrivastava and J. C. Speakman, J. Chem. Soc., 1151 (1961). (37) G. Ferguson and G. A. Sim, i b i d . , 1767 (1962). (38) R. Sass and C. Bugg, Acta Crystallogr., 2 3 , 282 (1967). (39) K. N. Trueblood, E. Goldish, and J. Donohue, i b i d . , 14, 1009 (1961). (40) A. T . McPhail and G. A . Sim, J. Ghem. Soc., 227 (1965). (41) 9. Abrahamsson, M.Innes, and B. Lamm, Acta Chem. Scand., 21, 224 (1967). (42) E. Grison. Acta Crystallogr., 2 , 410 (1949). (43) T. C. W.M a k and J. Trotter, ( b i d . , 18, 68 (1965). (44) A. Mugnoli and C. Mariani, Gam. Ghim. Ital., 94, 666 (1964). (45) P. Coppens and G. M. J. Schmidt, Acta Crystallogr.. 18, 62 (1965). (46) P. Coppens and G. M. J. Schmidt, ibid., 18, 654 (1965). (47) 0. M. Gramaccioli, R . Destro, and M. Simonetta, i b i d . , B24, 129 (1968). (48) W. C. Hamilton and S. J. LaPlaca, J. Amer. Chem. Soc., 86, 2289 (1964). (49) J. 0. Calabrese, A. T. McPhail, and G. A. Sim, J . Chem. Soc., B , 1235 (1966). (50) K.J. Watson, Nature, 188, 1102 (1960). (51) E. G. Boonstra, Acta Crystallogr., 16, 816 (1963). (52) L. E. Sutton, “Tables of Interatomic Distances and Configuration in Molecules and Ions,” Supplement 1956-1959, Special Publication No. 18, The Chemical Society, London, 1964. (53) A. Bondi, J . Phys. Chem., 68,441 (1964).
