CRYSTAL GROWTH & DESIGN
Molecular and Crystal Structure of Low Melting Nitrotoluene Isomers
2009 VOL. 9, NO. 1 57–65
Andrey A. Yakovenko,† Mikhail Yu. Antipin,†,‡ and Tatiana V. Timofeeva*,† Department of Natural Sciences, New Mexico Highlands UniVersity, Las Vegas, New Mexico 87701, and A. N. NesmeyanoV Institute of Organoelement Compounds, Russian Academy of Sciences, VaViloV St. 28, 119991 Moscow, Russia ReceiVed June 22, 2008
ABSTRACT: The molecular and crystal structures of low melting isomers of nitrotoluene (orto- and meta- derivatives) have been determined. The single crystals of o- and m-isomers have been grown using the miniature zone melting in situ crystallization technique. It is found that m-nitrotoluene undergoes on cooling an order-disorder phase transition from monoclinic to triclinic phase at about 200 K. X-ray diffraction analysis and quantum calculations on the MP2/6-31G** level show that all nitrotoluenes have similar structural parameters close to those found in toluene and nitrobenzene. In crystals of the isomers studied, molecules are linked in sheets by short N-O · · · H interactions. Introduction Structural studies of substituted nitrobenzenes have been carried out for more than 5 decades by different experimental (X-ray diffraction,1-3 electron diffraction,4-6 microwave spectroscopy,7 NMR8) and computational methods.4,5,9 Interest in these compounds is related in part to their properties as highenergetic materials.10 Mono- and disubstituted nitro aromatic molecules might also be used as models for evaluation of substitution effects on molecular geometry, reactivity, and other properties in crystallographic applications, in particular for formation of adducts or co-crystals. For nitrobenzene (1), a liquid at room temperature, numerous structural data are presented in the literature, including its first crystallographic characterization by Trotter1 and electron density distribution analysis by Boese et al.2 On the other hand, in the series of methyl substituted nitrobenzenes (Scheme 1), only the structure of para-nitrotoluene (2) was studied before,3 while the structures of ortho- (3) and meta- (4) nitrotoluenes, which are liquids at room temperature, were unknown. Molecules 1-4 have been found as solvates in clathrate adducts or, to use the term introduced by Etter and Panunto,11 co-crystals. We found in the CSD (November 2006) 162 records on nitrobenzene as a solvate molecule in adducts. By contrast, adducts (co-crystals) with 2-4 have been described only for a few examples, namely, three examples with para-,12,13 one with ortho-,14 and one with meta-nitrotoluene.15 A group of host-guest compounds, calyx[n]pyrroles, have been studied to find dimensions of cycles suitable for recognition of anions and neutral molecules. It was found that calyx[3]pyrroles with smaller cycles can bind only anions, while large cyclic ligands, such as calyx[6]pyrroles, can host different neutral guest molecules, including ortho- and meta-nitrotoluene.14 Due to the complicated structure of the co-crystals14 and disorder of guest molecules,12,13 information on the molecular geometry of 2 and 3 is limited. Nevertheless, it is possible to note that in these co-crystals nitrotoluene molecules participate in hydrogen bond formation between nitro groups of nitrotoluenes and amino groups of pyrrole moieties. The only example * To whom correspondence should be addressed. E-mail: tvtimofeeva@ nmhu.edu. † New Mexico Highlands University. ‡ Russian Academy of Sciences.
Scheme 1
found for meta-nitrotoluene15 shows an ordered structure with staking interaction between aromatic fragments of the host and guest molecules. A limited number of examples demonstrates that nitrotoluenes in some cases form adducts with different host molecules due to staking interactions and/or hydrogen bonding N-O · · · H. Thus, host-guest chemistry of nitrotoluenes has not been studied on a large number of examples, and new data in this area will help to present a more detailed picture of their supramolecular organization useful for their recognition and extraction. In the present paper, we describe results of the X-ray diffraction study of compounds 3 and 4 and reinvestigation of the crystal structure of 2, which was studied prviously.3 To reveal the influence of the crystal environment and substitution on molecular geometry in the series of nitrotoluenes, we compared molecular structures of 1-4 with results of quantum calculations of isolated molecules. We also used compounds 1-4 for co-crystallization with the Lewis acid 1,2-bis(chloromercurio)tetrafluorobenzene, and we will describe results in the next publication in this issue.16 Experimental Section All nitrotoluenes were purchased from Aldrich Chemicals and used without further purification. o-Nitrotoluene (3) (mp ) 269 K) and m-nitrotoluene (4) (mp ) 287 K) are liquids at room temperature, while p-nitrotoluene (2) is a low melting solid (mp ) 327 K). Crystals of 3 and 4 were grown using the procedure described below, and crystals of 2 were obtained from an ethanol solution. o-Nitrotoluene and m-nitrotoluene each were drawn into a 0.3 mm diameter Lindemann capillary (∼20 mm length). Capillaries were sealed at both ends with a standard butane burner and attached to a goniometer head, which was mounted in the diffractometer, equipped with a CRYOFLEX low-temperature device and an optical heating crystallization
10.1021/cg800659f CCC: $40.75 2009 American Chemical Society Published on Web 12/04/2008
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Figure 1. (a) Molecular structure of 2 at 100 K drawn at 50% ellipsoidal probability level; (b) centrosymmetric dimers in a crystal of 2, formed by stacking interactions of the benzene ring and the NO2 group and (c) projection of a pair of π-π interacting molecules on a plane of molecule 2; and (d) structure of the sheet (half of a layer) in a crystal of 2. device (OHCD) for laser-assisted crystallization17 (CO2 laser, λ ) 10.57-10.63 µm). The polycrystalline masses were obtained by freezing at 260 and 230 K, for m- and o-nitrotoluene, respectively. The temperature of the polycrystalline solids was then elevated up to 270 and 260 K, that is, close to the melting points of m- and o-nitrotoluenes, respectively. After that, using laser heating (2.0 W laser power), we established solid-liquid equilibrium in a narrow zone of the capillary, and crystals were grown by applying the same laser power along approximately 1 cm of the capillary over a duration of 30 min. The laser power was subsequently reduced to 0 W during the next 20 min, at the end of the crystallization cycle. In the case of p-nitrotoluene and o-nitrotoluene, crystals were cooled starting from room temperature (crystal 2) and from 260 K (crystal 3), respectively, down to 100 K, with determination of the unit cell parameters for both samples every 50 K. No significant changes were observed. The monoclinic crystal of 4, on cooling below 200 K, undergoes a phase transition to the triclinic phase. This transition is accompanied
by crystal caking; however, we found in the capillary a 0.6 mm long single crystalline fragment and carried out diffraction experiments at 270 K for the monoclinic phase and at 100 K for the triclinic one. Crystal data, details of data collection, and structure refinement parameters for compounds 2-4 are presented in Table 1. The intensities of reflections were measured with a Bruker SMART APEX II diffractometer with a CCD area detector (graphite monochromated Mo KR radiation, l ) 0.71073 Å, ω-scans with a 0.5° step in ω). The semiempirical method SADABS18 was applied for absorption corrections. The structures were solved by direct methods and refined by the full-matrix least-squares technique against F2, with anisotropic temperature parameters for all non-hydrogen atoms. All hydrogen atoms were placed geometrically and refined with the riding model. Data reduction and further calculations were performed using the Bruker SAINT+19 and SHELXTL NT20 program packages. The structure of the o-nitrotoluene 3 contains two symmetrically independent molecules A and B. Atomic displacement parameters in molecule B are found to be twice those in molecule A. We found also
Molecular and Crystal Structures of Nitrotoluenes
Crystal Growth & Design, Vol. 9, No. 1, 2009 59
Table 1. Crystal Data Collection and Structure Refinement Details formula cryst syst space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 T, K Tmelt,°C/K Z F(000) Dcalcd, g/cm3 µ, mm-1 cryst size, mm3 no. of rfls collected (RC) no. of indep RC (IRC) final R indices [I > 2σ(I)] R for IRC goodness of fit largest diff peak and hole, eÅ-3 a
2
3
4 (triclinic)
4 (monoclinic)
para-C7H7NO2 orthorhombic Pbca 13.6516 (19) 6.3323 (9) 15.573 (2) 90 90 90 1346.3 (3) 100 54/327 8 576 1.353 0.101 0.50 × 0.30 × 0.10 17417 1768 [R(int) ) 0.0490] R1 ) 0.0438, wR2 ) 0.0936 R1 ) 0.0584, wR2 ) 0.1010 1.009 0.260 and -0.253
ortho-C7H7NO2 monoclinic P21/n 8.4053 (4) 10.9028 (6) 15.1685 (8) 90 105.9170 (10) 90 1336.77 (12) 100 -4/269 8 576 1.363 0.101 0.3 mm diameter 7678 1996 [R(int) ) 0.0116] R1 ) 0.0629, wR2 ) 0.1303 R1 ) 0.0663, wR2 ) 0.1324 1.028 0.463 and -0.414
meta-C7H7NO2 triclinic P1j 7.370 (4) 7.795 (4) 11.442 (6) 91.034(11) 93.199 (10) 90.452 (12) 656.2 (6) 100 ≈-73/≈200a 4 288 1.388 0.103 0.3 mm diameter 1633 1633 [R(int) ) 0.000] R1 ) 0.0671, wR2 ) 0.1485 R1 ) 0.0792, wR2 ) 0.1581 1.062 0.189 and -0.192
meta-C7H7NO2 monoclinic P21/c 7.888 (13) 11.611 (18) 7.537 (11) 90 90.15 (3) 90 690.4 (19) 270 14/287 4 288 1.319 0.098 0.3 mm diameter 1803 953 [R(int) ) 0.0202] R1 ) 0.0439, wR2 ) 0.0907 R1 ) 0.0602, wR2 ) 0.0994 1.041 0.108 and -0.108
Phase transition temperature (estimated). Table 2. Molecular Geometrical Parametrs of Compounds 1-4 Determined by X-ray Diffractiona 1 (ref 2)
2
3
4 monoclinic
4 triclinic
Mol.A/Mol.B/Mol.B′
postion1/position2
Mol. A/Mol.B
O(1)-N(1) O(2)-N(1) N(1)-C(1) C(1)-C(2) C(1)-C(6) C(2)-C(3) C(3)-C(4) C(4)-C(5) C(5)-C(6) C(n)-C(7)
1.225 1.228 1.467 1.387 1.387 1.385 1.388 1.389 1.386 N/A
(1) (1) (1) (1) (1) (1) (1) (1) (1)
1.231 1.227 1.469 1.387 1.389 1.390 1.399 1.403 1.387 1.507
(2) (2) (2) (2) (2) (2) (2) (2) (2) (2)
bond length, Å 1.227 (3)/1.215 (5)/1.230 (1) 1.222 (4)/1.242 (5)/1.230 (1) 1.472 (4)/1.476 (6)/1.466 (2) 1.386 (5)/1.392 (6)/ 1.395 (1) 1.394 (4)/1.397 (7)/ 1.379 (1) 1.403 (5)/1.387 (7)/1.396 (1) 1.390 (5)/1.363 (7)/1.379 (1) 1.376 (6)/1.396 (6)/1.388 (1) 1.378 (5)/1.367 (6)/1.388 (1) 1.513 (5)/1.506 (7)/1.523 (1)
1.230 1.230 1.466 1.371 1.379 1.374 1.397 1.377 1.358 1.515
(2)/1.230 (2) (2)/1.2300 (2) (2) (3) (4) (4) (4) (4) (5) (4)
1.218 1.212 1.475 1.383 1.385 1.381 1.397 1.402 1.369 1.510
(4)/1.214 (4)/1.233 (5)/1.466 (5)/1.385 (5)/1.379 (5)/1.381 (5)/1.389 (5)/1.379 (6)/1.388 (5)/1.522
(4) (4) (5) (5) (5) (6) (5) (6) (7) (5)
O(1)-N(1)-O(2) O(1)-N(1)-C(1) O(2)-N(1)-C(1) C(2)-C(1)-C(6) C(2)-C(1)-N(1) C(6)-C(1)-N(1) C(1)-C(2)-C(3) C(2)-C(3)-C(4) C(5)-C(4)-C(3) C(6)-C(5)-C(4) C(5)-C(6)-C(1) C(m)-C(n)-C(7) C(l)-C(n)-C(7)
123.2 118.3 118.5 122.7 118.5 118.8 118.2 120.2 120.5 120.3 118.1 N/A N/A
(1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1)
123.2 118.5 118.3 122.5 118.8 118.7 118.2 121.3 118.7 121.0 118.4 120.5 120.7
(1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1)
bond angles, deg 123.3 (3)/122.6 (4)/123.0 (2) 118.5 (3)/120.5 (4)/118.5 (2) 118.2 (3)/117.0 (4)/118.5 (2) 123.5 (3)/122.4 (4)/120.6 (1) 121.0 (3)/121.5 (4)/119.9 (6) 115.5 (3)/116.1 (4)/119.5 (6) 116.0 (3)/115.9 (4)/118.69 (9) 121.4 (4)/122.5 (4)/120.6 (1) 120.5 (3)/120.7 (5)/120.4 (1) 120.1 (3)/118.7 (5)/119.4 (1) 118.6 (4)/119.7 (4)/120.4 (1) 125.4 (3)/124.9 (4)/120.7 (1) 118.6 (3)/119.2 (4)/120.6 (1)
123.0 118.5 118.5 121.2 119.6 119.2 120.7 118.0 120.4 121.2 118.5 121.5 120.4
(2)/123.0 (2) (1)/118.5 (1) (1)/118.5 (1) (2) (2) (2) (2) (2) (3) (3) (3) (3) (3)
122.9 118.7 118.4 122.3 119.0 118.7 119.3 119.5 119.8 120.8 118.3 120.6 119.9
(4)/122.7 (3)/119.0 (3)/118.3 (4)/122.3 (3)/118.8 (3)/118.9 (3)/119.5 (3)/118.7 (4)/121.3 (3)/120.3 (3)/117.9 (3)/121.6 (4)/119.7
(3) (3) (3) (4) (3) (3) (3) (3) (4) (3) (3) (3) (4)
φ(Ph/NO2)
2.12
dihedrals angles, deg 31.8 (4)/17.3 (5)/19 (2)
8.4 (3)/15.4 (3)
a
1.3 (1)
2.9 (6)/5.6 (6)
For compound 2: n ) 4, m ) 3, and l ) 5; for compound 3: n ) 2, m ) 1, and l ) 3; for compound 4: n ) 3, m ) 2, and l ) 4.
that several additional high residual peaks of the electron density are located in the plane of the molecule B. These peaks are attributed to statistical disorder of molecule B, which can be presented as two positions (B and B′) located in the same plane. In the process of structure refinement of the monoclinic form of m-nitrotoluene 4 at 270 K, it is found that the oxygen atoms O(1) and O(2) of the nitro group have enlarged displacement ellipsoids oriented perpendicular to the plane of the benzene ring. This is attributed to rotational disorder of the nitro group around the C(1)-N(1) bond. The low temperature structure of m-nitrotoluene (triclinic form) does not show any disorder. The description of the disorder refinement in the crystal structures of compounds 3 and 4 (monoclinic phase) is presented in the Supporting Information. The calculation of molecular geometries of compounds 1-4 was performed with the Gaussian 03 program package.21 The ab initio MP2 method, in conjunction with the 6-31G** basis set, was applied for
the optimization to the stationary points of molecules 1-4. We also optimized geometry using density functional theory (DFT) calculations with the functionals B3LYP and PBE in conjunction with the 6-31G*, 6-311G**, and 6-311++G** basis sets (see the Supporting Information and Table 5) for molecule 3. Structures were visualized using the program packages SHELXTL NT20 and MERCURY.22
Results and Discussion All studied compounds have been examined with respect to the propensity of generating molecular packing motifs particularly involving N-O · · · H and π-π staking interactions. In fact, no similarities between packing modes of these compounds were found. For compound 4, a solid state phase transition between very similar structures has been observed. A detailed description
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Table 3. Important Geometry Parameters of Compounds 1-4 Determined by Quantum Calculations on the MP2/6-31G** Levela 1
2
3
4
1.243 1.244 1.466 1.402 1.394 1.402 1.395 1.395 1.392 1.506
1.243 1.243 1.471 1.392 1.392 1.398 1.402 1.396 1.394 1.505
124.8 117.6 117.7 123.4 120.5 116.6 115.7 122.3 120.0 119.6 119.0 124.5 119.9
124.7 117.7 117.6 122.9 118.6 118.5 119.1 118.8 121.0 120.6 117.6 120.4 120.8
34.3
0.8
bond length, Å O(1)-N(1) O(2)-N(1) N(1)-C(1) C(1)-C(2) C(1)-C(6) C(2)-C(3) C(3)-C(4) C(4)-C(5) C(5)-C(6) C(n)-C(7)
1.242 1.242 1.472 1.392 1.392 1.394 1.397 1.397 1.394 N/A
O(1)-N(1)-O(2) O(1)-N(1)-C(1) O(2)-N(1)-C(1) C(2)-C(1)-C(6) C(2)-C(1)-N(1) C(6)-C(1)-N(1) C(1)-C(2)-C(3) C(2)-C(3)-C(4) C(5)-C(4)-C(3) C(6)-C(5)-C(4) C(5)-C(6)-C(1) C(m)-C(n)-C(7) C(l)-C(n)-C(7)
124.7 117.6 117.6 122.8 118.6 118.6 118.1 120.5 120.0 120.5 118.1 N/A N/A
1.242 1.242 1.469 1.392 1.392 1.393 1.401 1.401 1.393 1.505
bond angles, deg 124.7 117.7 117.7 122.3 118.8 118.8 118.2 121.5 118.4 121.5 118.2 120.8 120.8
dihedral angles, deg φ(Ph/NO2)
0.0
0.0
For compound 2: n ) 4, m ) 3, and l ) 5; for compound 3: n ) 2, m ) 1, and l ) 3; for compound 4: n ) 3, m ) 2, and l ) 4. a
of the molecular and crystal structures of compounds 2-4 and a comparison with data from quantum computations and other structural methods are presented below. para-Nitrotoluene (2). Compound 2 is a low melting material, but it is solid at room temperature. The molecular structure of p-nitrotoluene is presented in Figure 1a. Molecules of 2 have approximate C2V symmetry. Selected geometry parameters of 2 in comparison with the geometry of molecules 1, 3, and 4 are presented in Table 2. A comparison of the geometrical parameters of 2 with analogous values from other structural investigations and quantum computations1-9 is presented in the Supporting Information. The ipso-angle C(2)-C(1)-C(6) at the nitro group in 2 is larger than 120° (122.5(1)) and is close to the similar value in nitrobenzene (122.7(1)°).2 The ipso-angle at the methyl group (118.7(1)°) is larger than in toluene (117.8°).23 Molecule 2 in the crystalline form is almost planar; the dihedral angle between the nitro group and the benzene ring
(φ(Ph/NO2)) is 1.3 (1)°; which corresponds to the result of quantum computations (Table 3). A similar situation was found in nitrobenzene2 where the same angle is 2.12°. We expect conjugation between the aromatic system of the ring and the nitro group; however, the bond length N(1)-C(1) is not shortened in comparison with the other nitrotoluenes where the value of φ(Ph/NO2) is much higher. Quantum calculations at the MP2/6-31G** level (Table 3) show results similar to X-ray data. Molecules 1 and 2 have C2V symmetry. The C-C bonds and the bond angles in the aromatic rings in both molecules have similar values, except for the angle C(5)-C(4)-C(3) which is smaller in p-nitrotoluene (118.4°) than in nitrobenzene (120.0°). This can be explained by the presence of the methyl group in 2. In crystal 2, molecules are linked into centrosymmetric dimers by π-π staking interactions (Figure 1b). The distance between planes of the two parallel benzene rings is 3.36 Å; the distance between centroids of these rings CR(1) · · · CR(1A) is 3.72 Å, so the benzene rings of two molecules are shifted by 1.60 Å (Figure 1c). We can say that stacking includes an interaction between the π-system of the aromatic rings and the conjugated π-system of the nitro group; this observation is supported also by shortened intermolecular distances C(1) · · · CR(1A) and N(1) · · · C(4A) (3.39 and 3.41 Å, respectively). Molecular dimers are linked into layers parallel to the crystallographic plane bc by N-O · · · H interactions (Table 4). The layer contains two antiparallel sheets, which are linked by π-π staking and N(1)-O(1) · · · H(6)2-x,1-y,1-z interactions. ortho-Nitrotoluene (3). A single crystal of 3 was grown by the in situ crystallization technique17 at 260 K and then was cooled down to 100 K. o-Nitrotoluene crystallizes in a monoclinic space group P21/n with two crystallographically nonequivalent molecules A and B (Figure 2a). Molecule B in the crystal of 3 is disordered at two positions (B and B′) as shown in Figure 2b with the relative occupancies 84 and 16%, for B and B′, respectively. We focused our discussion on molecule B; the geometrical parameters of both molecules are given in Table 2. The methyl and nitro substituents in 3 adjust to neighboring atoms of the ring; therefore, steric interactions between those groups are expected. As a result of this interaction, the molecular structure of o-nitrotoluene should have some characteristics which cannot be found in other nitrotoluenes and nitrobenzene. One such effect is rotation of the nitro group around the C-N bond: the dihedral angles φ(Ph/NO2) are equal to 31.8 (4) and 17.3 (5)° for molecules A and B, respectively. This is the largest value of this angle in the series of compounds 1-4 (Table 2). Quantum computation gives for this angle a value of 34.3°
Table 4. List of Short Intermolecular N-O · · · H Contacts in Crystals 2-4 comp 2
3
4 (monoclinic) 4 (triclinic)
N-O · · · H-C
r(O · · · H)/Å
(O · · · H-C)/deg
r(O · · · C)/Å
symmetry operation
N(1)-O(1) · · · H(7C)-C(7) N(1)-O(1) · · · H(6)-C(6) N(1)-O(2) · · · H(5)-C(5) N(1)-O(2) · · · H(7A)-C(7) N(1A)-O(1A) · · · H(5AA)-C(5A) N(1B)-O(2B) · · · H(7BC)-C(7B) N(1A)-O(1A) · · · H(5BA)-C(5B) N(1B)-O(2B) · · · H(5AA)-C(5A) N(1B)-O(1B) · · · H(7AB)-C(7A) N(1)-O(2′) · · · H(5A)-C(5) N(1)-O(1) · · · H(7C)-C(7) N(1)-O(1′) · · · H(7C)-C(7) N(1A)-O(1A) · · · H(2AA)-C(2A) N(1A)-O(2A) · · · H(7BC)-C(7B) N(1B)-O(1B) · · · H(5AA)-C(5A) N(1B)-O(2B) · · · H(7AB)-C(7A)
2.71 2.50 2.68 2.70 2.55 2.47 2.58 2.66 2.70 2.61 2.67 2.61 2.70 2.55 2.70 2.60
169.5 144.7 148.8 117.0 142.2 171.7 153.1 122.5 142.8 142.2 170.5 161.1 143.1 156.6 144.4 161.0
3.678 (4) 3.311 (3) 3.250 (4) 3.259 (4) 3.346 (6) 3.447 (6) 3.451 (6) 3.267 (6) 3.527 (6) 3.396 (6) 3.622 (6) 3.531 (6) 3.508 (4) 3.472 (5) 3.516 (5) 3.544 (4)
x, 1/2 - y, -1/2 - z 2 - x, 1 - y, 1 - z x, 1/2 - y, -1/2 - z x, -1/2 - y, -1/2 - z 1 /2 + x, 1/2 - y, 1/2 + z -x, -y, -z 1 - x, 1 - y, -z -1/2 + x, 1/2 - y, 1/2 + z -1 + x, y, z 1 - x, 1/2 + y, 1/2 - z -1 + x, 1/2 - y, -1/2 + z -1 + x, 1/2 - y, -1/2 + z - x, 1 - y, 2 - z -1 + x, 1 + y, z 1 - x, 1 - y, 1 - z x, 1 + y, z
Molecular and Crystal Structures of Nitrotoluenes
Crystal Growth & Design, Vol. 9, No. 1, 2009 61
Figure 2. (a) Asymmetric unit of 3 at 100 K drawn at 50% ellipsoidal probability level; (b) Disordered positions of molecule B in the crystal structure of 3 presented as two positions, B (solid line) and B′ (open line), with occupancies 84 and 16%, respectively; (c) structure of a molecular layer in crystal 3 (red, molecules A; blue, molecules B); (d) view of the layer in crystallographic plane ac; and (e) view of the same layer along the crystallographic direction [1 0 -1].
(Table 3), that is very close to the rotation of the nitro group in molecule A. The smaller value in molecule B can be attributed to disorder and/or the crystal environment. The same effect was found for other ortho-substituted nitrocompounds. For example, in 2,3-dichloronitrobenzene and in 2,5-dichloronitrobenzene, the same dihedral angle is equal to 54.09 and 43.41°, respectively.24 The second effect is deviation from ideal values of the exocyclic angles in the benzene ring at atoms C(1) and C(2). Table 2 shows that bond angles C(6)-C(1)-N(1) and C(3)C(2)-C(7) are smaller than the ideal values [C(6)-C(1)-N(1) is 115.5 (3) and 116.1 (4)°; C(3)-C(2)-C(7) is 118.6 (3) and 119.2 (4)° for molecules A and B, respectively], while angles C(2)-C(1)-N(1) and C(1)-C(2)-C(7) are larger than 120° [C(2)-C(1)-N(1) is 121.0 (3) and 121.5 (4)°; C(1)-C(2)-C(7)
is 125.4 (3) and 124.9 (4)° for molecules A and B, respectively]. It is noteworthy that the values of similar angles in molecules 1 and 2 are close to 120° (Table 3). We should also mention the change in the endocyclic bond anglesofthearomaticbenzenering.ThebondangleC(6)-C(1)-C(2) at the ipso-carbon of the nitro groups is equal to 123.5 (3) and 122.4 (4)° for molecules A and B, respectively, and these values are very similar to those in nitrobenzene (122.7 (1)°).2 The ipsoangles at the methyl group are equal to 116.0 (3) and 115.9 (4)° for molecules A and B, respectively, which are even smaller than that in toluene (117.8°).23 Therefore, the geometry of the benzene ring in compound 3 is defined by a combination of electronic effects from both nitro and methyl groups.
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Figure 3. (a) Molecular structure of 4 at 270 K (monoclinic form) drawn at 30% ellipsoidal probability level; (b) disorder of nitro group in 4 at 270 K (monoclinic form) around the C(1)-N(1) bond with two equally populated positions of oxygen atoms O(1), O(2) and O(1′), O(2′); (c) sheet structure of 4 at 270 K, and (d) view of the same sheet along the crystallographic direction [-1 0 2].
Because of steric interaction between methyl and nitro substitutes in 3, we should expect an increase of the C(1)-C(2) bond length in comparison with other distances in the aromatic ring. But we do not observe this effect in the X-ray diffraction experiment. However, this effect was detected in the molecular structure of 3 determined by MP2/6-31G** quantum calculations (Table 3): the values of C(1)-C(2) and C(2)-C(3) bonds (1.402 Å for both distances) are larger than other C-C bonds in the benzene ring. For molecule 3, several optimizations of geometry on different levels of theory were done (Table 5). The bond lengths and bond angles are similar in all calculations; however, the value of the dihedral angle φ(Ph/NO2) depends on the level of calculation (Table 5). This angle increases not only with an elevation of the level of theory but also with an increase in the size of the basis set. The DFT methods (B3LYP and PBE), even with very large basis sets, do not describe correctly the relative orientation of phenyl and nitro groups in 3; therefore, we used the MP2 approximation with the basis set 6-31G**. On this level of calculation, the value of angle φ(Ph/NO2) is 34.3°,
which is very close to our experimental data for molecule A (31.8(4)°) and to results of electron diffraction analysis (38(1)°).4 Therefore, the MP2/6-31G** level of calculations probably gives the best description of steric interaction between nitro and methyl groups in molecule 3. In crystal 3, molecules A and B are linked in goffered layers (Figure 2c), formed via weak N-O · · · H interactions (Table 4). The mean plane of these layers is (1 0 -1), shown in Figure 2d. The “ridges” of this layer are formed by chains of molecule A (Figure 2d, e). Molecules of A are linked in such chains by contacts N(1A)-O(1A) · · · H(5AA)1/2+x, 1/2-y,1/2+z, (Table 4). Molecules of B are located in between the chains and are liked into dimers by contacts N(1B)-O(2B) · · · H(7BC)-x,-y,-z. The link between the chains and dimers is formed by three intermolecular N-O · · · H contacts (Table 4), with an average distance between oxygen and hydrogen atoms of 2.65 Å. This is greater than the N-O · · · H distances in dimers of molecule B and chains of molecule A. It is interesting that in 3 we do not find any π-π stacking interactions. The shortest distance between carbons of the
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Figure 4. (a) Asymmetric unit of 4 at 100 K (triclinic form) drawn at 50% ellipsoidal probability level; (b) sheet structure of 4 at 100 K separated by symmetry equivalence (red, molecules A; blue, molecules B); and (c) view of the same sheet along the crystallographic direction [4 2 2]. Table 5. Comparison of Dihedral Angle φ(Ph/NO2) in Molecule 3 with Results from Quantum Optimization on Different Levels of Theory level of calculation
φ(Ph/NO2)
HF/6-31G* (ref 9) B3LYP/6-31G* (ref 4) B3LYP/6-311G** B3LYP/6-311++G** PBE/6-311G** PBE/6-311++G** MP2/6-31G* (ref 4) MP2/6-31G**
0 12.9 20.9 26.5 19.3 26.3 34.7 34.3
aromatic ring is 3.45 Å (C(4A) · · · C(3B)), which is larger than double the van der Waals radius of a carbon atom (3.40 Å).25 One reason for this may be the nonplanar structure of molecule 3. meta-Nitrotoluene (4). By using the same in situ crystallization technique,17 a single crystal of compound 4 was grown at 270 K. At this temperature, 4 has a monoclinic unit cell with the parameters presented in Table 1. While we cooled crystal 4 down, it cracked when the temperature dropped below 200 K. The crystal system changed from monoclinic to triclinic with the unit cell parameters being close to monoclinic (Table 1).
We suggest that, around 200 K, compound 4 undergoes a phase transition from the monoclinic to triclinic form. The volume and unit cell parameters of both phases are similar (Table 1). The molecular structure of 4 is shown in Figure 3a. The most important geometry molecular parameters are presented in Table 2. The monoclinic form of 4 contains one independent molecule per asymmetric unit, where the nitro group is disordered due to rotation around the C(1)-N(1) bond with two evenly populated positions characterized by angles φ(Ph/NO2) equal to 8.4 (3) and 15.4 (3)° (Figure 3b). In the monoclinic phase, molecules 4 are linked into corrugated sheets which are parallel to the crystallographic plane (-1 0 2) (Figure 3c, d). Molecules in the sheets are linked by three N-O · · · H intermolecular contacts (Table 4), with distances in the range 2.61-2.67 Å. No π-π stacking interactions are found in this crystal. At 100 K, 4 has a space group P1j, with two independent molecules (A and B) in the asymmetric unit (Figure 4a). Both molecules in the triclinic form of 4 have slightly different dihedral angles φ(Ph/NO2) equal to 2.9 (6)° in A and 5.6 (6)° in B (Table 2). The order in the triclinic phase (single positions of the nitro group) versus disorder in the monoclinic phase
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understanding of characteristics of the molecular geometry of simple nitroaromatic compounds. Presented examples of nitrotoluenes might be used, as has been done for nitrochlorobenzenes,24 as models of crystal structure prediction for polymorphs and computational characterization of order-disorder phase transitions. Also, the nitrotoluenes might be used for crystal engineering via a co-crystallization procedure. We present examples of organometallic co-crystals of nitrobenzenes with Lewis acids in another publication.16 Acknowledgment. The authors are grateful for NSF support via the NM EPSCoR program and DMR/NSF Grant No. 0420863 and to Professor Roland Boese for invaluable directions.
Figure 5. Superposition of molecular positions for monoclinic (red) and triclinic (blue) phases of compound 4.
(“dynamic” positions of the nitro group) may indicate the “order-disorder” nature of the transition.26 Figure 5, with superposition of monoclinic and triclinic phases, clearly shows that there is no significant difference in molecular packing in these two phases. Quantum computations show the same characteristics for molecular structure 4 in terms of bond lengths and angles as in molecules 2 and 3 (Table 3 and the Supporting Information). According to calculations, molecule 4 is almost planar with a dihedral angle φ(Ph/NO2) equal to 0.8°. In the triclinic crystal, molecules are linked into sheets (Figure 4b, c) by N-H · · · O interactions (Table 4) in which the average plane has Miller indices (4 2 2). As shown in Figure 4b, molecules A in these sheets are linked into dimers by two N(1A)-O(1A) · · · H(2AA)-x, 1-y, 2-z. intermolecular interactions. Dimers are linked to each other via molecules B, by three N-O · · · H interactions with distances between oxygen and hydrogen atoms in the range 2.55-2.70 Å (av ) 2.61 Å). Conclusion Crystal growths of two low melting isomers of nitrotoluene and molecular and crystal structures of all three isomers (2-4) have been presented. All molecules have analogous geometrical parameters for the benzene ring that are similar to those in toluene and nitrobenzene (1). In the experimentally determined structures, we did not find significant changes of C-C bonds in the benzene ring in comparison to nitrobenzene and toluene. However, it is shown that, in molecular structures determined by quantum calculations on the MP2/6-31G** level, C-C bonds attached to the ipso-carbon in the methyl group are slightly elongated (Table 3). The ipso-angle at the nitro group is increased by 1-3°, if compared to 120°, while the ipso-angle at the methyl group is decreased by 2-3° (Tables 2 and 3) in all isomers, according to experimental and calculational data. The dihedral angle between planes of the benzene ring and the nitro group φ(Ph/NO2) is dependent on relative positions of nitro and methyl groups in the molecule. This value is increased in a series of isomers, para-, meta-, and orthonitrotoluene (Table 2). The largest value of this angle is found in molecule 3 (31.8 (4)°), while the smallest one is found in planar molecule 2 (1.3 (1)°). Similar regularity is found in quantum calculations (Table 3). Finally, we need to articulate that structures of all isomers of nitrotoluene have been described, that increase the basic
Supporting Information Available: X-ray crystallographic information files for structures of compounds 2-4 (CIF); description of the disorder refinement in crystal structures of compounds 3 and 4 (monoclinic phase) (PDF); Cartesian coordinates and absolute energies for the MP2/6-31G** optimized structures of nitrotoluenes (2-4) and nitrobenzene (1) (PDF); Cartesian coordinates and absolute energies for the B3LYP/6-311G**, B3LYP/6-311++G**, PBE/6-311G**, and PBE/6-311++G** optimized structures of 3 (PDF); comparison of nitrobenzene (1) and nitrotoluenes (2-4) structural parameters determined by different methods (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
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