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Phase transition in polar 2-nitroanilinium nitrate. Graph-set approach of hydrogen bonding patterns and analysis of vibrational spectra
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 207 (2019) 313–320
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Phase transition in polar 2-nitroanilinium nitrate. Graph-set approach of hydrogen bonding patterns and analysis of vibrational spectra Piotr Rejnhardt, Jan Baran, Marek Daszkiewicz ⁎ Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna str. 2, P.O. Box 1410, 50-950 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 20 June 2018 Received in revised form 19 September 2018 Accepted 22 September 2018 Available online 24 September 2018 Keywords: 2-Nitroaniline Crystal structure Phase transition Elementary graph-set descriptor Vibrational spectroscopy Second harmonic generation
a b s t r a c t A phase transition in new compound 2-nitroanilinium nitrate, (H2NA)NO3, was found. A symmetry lowering from orthorhombic, Pmn21, to monoclinic one, P21, at 249 K is observed. During the phase transition, the H2NA+ and nitrate ions displace from the mirror plane in high-temperature phase. As a result, hydrogen bonding network constructed by the ammonio group and NO3− anion is changed. Especially, a bifurcated hydrogen bond disappears. Mathematical operations using both elementary and hydrogen bond graph-set descriptors were used for the first time to describe a mechanism of phase transition. Changes in hydrogen bonding network were also studied by means of vibrational spectroscopy. Band shift associated with stretching and bending vibrations of the ammonio group indicates that the energy of intermolecular interactions rises along with temperature decrease. SHG response for the studied compound is higher than KDP, I(H2NA)NO3 = 1.1·IKDP, although a decay of the signal was observed due to instability of the sample. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Among three isomers 2-, 3- and 4-nitroaniline, only the 3nitroaniline crystallizes without a center of symmetry and therefore second harmonic generation is expected for this compound [1–14]. However, since nitroanilines are Brønsted bases, their salts of various properties can be easily obtained. Therefore, a search of new functional materials can be expanded for large group of organic ionic crystals. Recently, among salts of 2-methy-4-nitroaniline, the first non-linear organic compound, two polymorphs of chloride were reported as noncentrosymmetric, P-421c and P212121 [15]. In the case of 2nitroaniline, it possesses the smallest dipole moment among nitroanilines. Therefore, it can be expected the biggest chance for obtaining non-centrosymmetric salt for that isomer, because there is a general tendency of molecules with large dipole moment to crystallize with a center of symmetry. More recently, hybrid organic-inorganic compounds of 2-methoxyaniline with SnCl3− and SnCl62− were obtained [16,17], where the former compound crystallizes without a center of symmetry but the latter one is centrosymmetric. Such a result may be associated with a molecular structure of the octahedral SnCl62− anion which possesses inversion center and therefore facilitates organization of the crystal structure as centrosymmetric [18–24]. Although it is worth noting that a use of a pyramidal SnCl3− anion in synthesis is not a sufficient condition for non-centrosymmetry [25].
⁎ Corresponding author. E-mail address: [email protected] (M. Daszkiewicz).
https://doi.org/10.1016/j.saa.2018.09.041 1386-1425/© 2018 Elsevier B.V. All rights reserved.
Besides, 2-nitroaniline is interesting molecule that it forms N\\H⋯O intramolecular hydrogen bond, which additionally stabilizes position of terminal groups (NH2 and NO2). If the amino group is protonated, a formation of intramolecular hydrogen bond is also energetically privileged for isolated H2NA+ ion [26]. However, a rotation barrier for the NH3+ group is relatively small, 2.87 kcal/mol, and it depends on the position of the nitro group. Nevertheless, the position of the ammonio group can be easily changed by intermolecular hydrogen bonds as was observed for 2-nitroanilinium chloride and bromide [26,27]. In this work, we present crystal structure of room- and lowtemperature phases of 2-nitroanilinium nitrate, which are noncentrosymmetric. An analysis of phase transition and especially change of hydrogen bonding patterns during this process is presented. Mathematic approach using operations on graph-set descriptors as a tool for description of molecular mechanism of phase transition is used for the first time. Additionally, non-linear properties for the studied compound were evidenced by means of SHG experiment. 2. Materials and Methods 2.1. Synthesis The starting compounds, 2NA [Aldrich, purum, ≥98% (NT)] and nitric acid (Sigma-Aldrich, 70%), were used as supplied. The 2NA (1 mmol, 0.1381 g) was dissolved in a mixture of 5 ml of methanol and 5 ml of isopropanol, and then 0.5 ml HNO3 was added. Colorless crystals of 2nitroanilinium nitrate were obtained after six days by slow evaporation of the mixture.
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2.2. Single Crystal X-ray Diffraction Studies X-ray diffraction data were collected on an Oxford Diffraction fourcircle single crystal diffractometer equipped with a CCD detector using graphite-monochromatized MoKα radiation (λ = 0.71073 Å). The raw data were treated with the CrysAlis Data Reduction Program (version 1.171.38.34a). The intensities of the reflections were corrected for Lorentz and polarization effects. The crystal structures were solved by direct methods [28] and refined by full-matrix least-squares method using SHELXL-2014 program [28]. Non-hydrogen atoms were refined using anisotropic displacement parameters. H-atoms connected with C atoms from the aromatic rings were visible on the Fourier difference maps, but placed by geometry and allowed to refine “riding on” the parent atom with Uiso = 1.2 Ueq(C). Coordinates of hydrogen atoms of the ammonio group were refined, but Uiso = 1.5 Ueq(N). Visualizations of the structures were made using Diamond 3.2 k [29]. Low temperature X-ray diffraction experiments were carried out using OxfordCryosystems device in the ranges 300 K–100 K with ΔT = 10 K and 250 K – 240 K with ΔT = 1 K. High pressure experiment was carried out using Merrill-Bassett diamond anvil cell. The pressure was measured collecting a set of approximately hundred reflections for CaF2 which was a standard, and referring a unit cell volume to known characteristics vs. pressure. Both standard and sample crystals were placed between diamonds in a hole of stainless steel gasket. A mineral oil was used to fill the chamber. 2.3. Spectroscopic Measurements Room temperature FT-IR spectra in the 4000–400 cm−1 range were measured on the Bruker IFS-88 spectrometer with 2 cm−1 resolution. Nujol and fluorolube mull techniques have been used in the measurements. Closed cycle helium cryostat system of APD Cryogenics INC. (DE-202 Expander, Compressor Model HC-2, Microprocessor-based Temperature Indicator/Controller Model 5500-1-25) was used for low temperature measurements (300−12 K). The temperature of the sample was maintained with an accuracy of ±0.1 K. Powder FTRaman spectrum at room temperature was recorded with FRA-106 attachment to the Bruker IFS-88 spectrometer over the frequency range 3600–50 cm−1 with resolution of 2 cm−1; applying Nd:YAG laser (λ = 1064 nm, power ca. 150 mW).
were constrained, but the other geometry parameters were optimized similarly to the procedure written in Ref. [26]. 3. Results and Discussion 3.1. Crystal Structure of Ortho-Nitroanilinium Nitrate Title compound crystallizes at room temperature in orthorhombic crystal system, non-centrosymmetric space group Pmn21 (Table S1). X-ray experiment at low temperature revealed change of the crystal symmetry to monoclinic one, P1121 space group. Non-standard setting of the unit cell was used for low-temperature phase to inherit a setting from high-temperature phase. The crystal twinned into two domains after phase transition, which was associated with a change of lattice gamma angle (Fig. 1). Detailed diffraction experiments showed that this angle started to change at 249 K (Fig. 1b). (Changes of the other lattice parameters upon cooling were deposited as Supplementary Information.) Since the crystal symmetry was lowered during phase transition, both the mirror and n glide planes disappeared. Therefore, the violation of extinction rule, h + l = 2n, for n glide plane was observed (Fig. 2). High pressure experiment was carried out at 0.3(1) GPa. Due to low quality of the diffractions pattern and small number of reflections only the lattice parameters could be determined: a = 6,44(10) Å, b = 8013 (12) Å, c = 7760(4) Å, γ = 91,6 (4) deg and V = 401(3) Å3. The values can be referred to those obtained at low temperature, 100 K. In orthorhombic phase, (H2NA)+ and NO3− ions lie on the mirror plane, however two hydrogen atoms of ammonio group and two oxygen atoms of the nitrate lay in general positions (Fig. 3). As a result of phase transition, the mirror plane disappears and all the ions lay in
2.4. Second Harmonic Generation Measurements SHG measurements for powdered samples of the noncentrosymmetric (H2NA)NO3 and that of KH2PO4 (KDP), which was used as a reference material, were mounted between separate microscope glass slides and excited by tunable femtosecond laser pulses from the laser system consisting of a Quantronix Integra-C regenerative amplifier operating as an 800 nm pump and a Quantronix-Palitra-FS BIBO crystal-based optical parametric amplifier. This system delivers wavelength-tunable pulses of ~130 fs length and was operated at the repetition rate of 1 kHz. A detailed description of the experiment can be found elsewhere [30]. Caution! Work with the high-power laser brings danger to the eyes, especially in spectral range in which the beam is invisible. Adequate eye protection should be used during measurements. 2.5. Computational Details All the computations were performed using density-functional theory (DFT) and hybrid Becke's three-parameter the Lee-Yang-Parr correlation functionals (B3LYP) [31–35] and the 6-31G(d,p) basis set was used. The calculations were carried out for the singlet electronic ground state and no symmetry restrictions were applied. All the atomic positions were taken from crystallographic data. Due to keeping preserved the H2NA+ molecular structure existed the crystal structure of (H2NA)NO3, dihedral angles of both the ammonio and nitro groups
Fig. 1. (a) A section of reciprocal space with (hk0) plane for a measurement at 100 K. Large group of spots are split due to crystal twinning. (b) Changes of the γ angle with temperature indicate a phase transition at 249 K.
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Fig. 2. A comparison of diffraction pattern on the (h0l) plane obtained (a) at 300 K and (b) at 100 K. Additional h + l odd reflections indicate a disappearance of the n glide plane.
general positions. In low-temperature phase the ions displace from their positions in comparison with the respective positions at room temperature. Change of conformation of ammonio and nitro groups is also observed, because they rotate around C\\N bonds. In the studied crystal, the most important hydrogen bonds are formed between ammonio group and nitrate anion arranged in a layer parallel to ac plane (Table 1). Theoretically, interacting functional group can be described by elementary graph-set descriptors using Etter's notation [36,37], GNH3 = {3E01(1); 3E02(3)}, GNO3 = {3E10(1); 3E20(3)}, which were used in our earlier studies on relations between descriptors in hydrogen bonding systems [15,26,38]. Summation each elementary graph-set descriptor GNH3 with GNO3 gives one of the following descriptors related to hydrogen bonding pattern: GHB = {D11(2); C11(2); G12(4); G21(4); G22(6)}, G = {R, C}. In the studied crystal at room temperature, the C22(6) chain parallel to a axis exists (Fig. 4). Additionally, two chains C12(4) run along [101] and [−101] directions. It is worth noting here that the other two chains can be distinguished, because one hydrogen atom is
engaged in bifurcated hydrogen bond, C 2 2(6) and C 34 (10). The chains cross to each other resulting in ring patterns, R 4 6 (14) and R21(4). The chain and ring descriptors can be expressed by following summations: E0 2 ð3ÞHNH þ E2 0 ð3ÞONO ¼ C2 2 ð6Þ; E0 2 ð3ÞHNH þ E1 0 ð1ÞO ¼ C1 2 ð4Þ; E0 2 ð3ÞHNH þ E2 0 ð3ÞONO þ E0 2 ð3ÞHNH þ E1 0 ð1ÞO ¼ C3 4 ð10Þ; h i 2 E0 2 ð3ÞHNH þ E1 0 ð1ÞO þ E0 2 ð3ÞHNH þ E2 0 ð3ÞONO ¼ R4 6 ð14Þ; E0 1 ð1ÞH þ E2 0 ð3ÞONO ¼ R2 1 ð4Þ: Overall, hydrogen bonding network changes during phase transition. Two chains C22(6) and C34(10) from high temperature phase are preserved in the second phase but C12(4) chain disappears. Besides, one new ring R56(16) can be found in low temperature phase
Table 1 Geometry parameters for selected hydrogen bonds found in high- and low-temperature phases of (H2NA)NO3. D\ \H⋯A 300 K N1\ \H1A⋯O4i N1\ \H1A⋯O4ii N1\ \H1B⋯O4 C3\ \H3⋯O3iii C6\ \H6⋯O2iv 100 K N1\ \H1A⋯O5v N1\ \H1B⋯O5vi N1\ \H1C⋯O4 C3\ \H3⋯O3vii C6\ \H6⋯O2iv Fig. 3. Molecular structure and a labeling scheme for room-temperature phase of (H2NA) NO3.
D\ \H (Å)
H⋯A (Å)
D⋯A (Å)
D\ \H⋯A (°)
0.91 (5) 0.91 (5) 1.00 (3) 0.93 0.93
2.21 (5) 2.21 (5) 1.86 (4) 2.63 2.60
3.035 (4) 3.035 (4) 2.845 (3) 3.562 (5) 3.205 (5)
151.2 (7) 151.2 (7) 168 (3) 178.9 122.9
0.97 (3) 0.91 (3) 0.93 (3) 0.93 0.93
1.89 (3) 1.90 (3) 1.91 (3) 2.58 2.59
2.851 (3) 2.801 (3) 2.813 (3) 3.509 (3) 3.174 (3)
171 (2) 168 (3) 164 (3) 176.8 121.1
Symmetry code(s): (i) –x + 1/2, −y + 2, z-1/2; (ii) x-1/2, −y + 2, z-1/2; (iii) –x + 1/2, −y + 1, z + 1/2; (iv) x, y, z-1; (v) –x + 1, −y + 1, z-1/2; (vi) x-1, y, z; (vii) –x + 1, −y + 2, z + 1/2.
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set descriptor of the H⋯O pathway is subtracted. It is worth noting that the hydrogen and oxygen atoms are common in both rings, R21(4) and R46(14). In the crystal structure of studied compound, several weak intermolecular HB interactions occur. Hirshfeld surface analysis reveals short contacts between aromatic ring and oxygen atom of the nitro group, i.e. C5\\H5⋯O2 and C6\\H6⋯O2 [39,40]. Both interactions connect the ions along crystallographic c direction, so they additionally stabilize N\\H⋯O hydrogen bonding network parallel to the ac plane. However, it is worth noting that none of N\\H⋯O hydrogen bonds is present in orthogonal direction i.e. along b axis. The only important interaction found on the Hirshfeld surface is short contact between C3\\H3 group and nitrate anion of neighboring layer (Fig. 5a). Therefore, it appears that the C3\\H3⋯O3 interaction is important for molecular self–assembly because it connects molecules together, which exist in two adjacent layers of N\\H⋯O hydrogen bonds. In the studied crystal structure, non-covalent interaction with NO2 also exists, which importance in molecular self-assembly was recently reported for many structures [38,41–49]. In the crystal structure of (H2NA)NO3, the nitro group forms π⋯π interaction along with adjacent aromatic ring and thereby a skeleton of the H2NA+ ion is stabilized. Fig. 5b shows that NO2 group is slightly shifted outside of the ring's center and a comparison of this interaction for two phases indicates that smaller region on the Hirshfeld surface is covered by the C⋯N and C⋯O contacts for high-temperature phase (14.5% at 300 K and 15.6% at 100 K). So, the π⋯π interaction becomes a little bit stronger upon cooling. Besides, interestingly this interaction stabilizes adjacent aromatic rings along the a direction, hence parallel to the plane of hydrogen bonding layer. On the other hand, it was observed that obtained crystals have the smallest dimension along a axis, but it is reasonable to expect
Fig. 4. A comparison of hydrogen bonding patterns constructed by the ammonio groups and nitrate anions (a) for high-temperature phase and (b) for low-temperature phase.
and arises from the following summation of elementary graph-set descriptors: h i E0 2 ð3ÞHNH þ E1 0 ð1ÞO þ 2 E0 2 ð3ÞHNH þ E2 0 ð3ÞONO ¼ R5 6 ð16Þ: Differences between hydrogen bonding networks are associated with the change of the position of H2NA+ ion and particularly ammonio group. The organic cation displaces from the mirror plane during phase transition and therefore the changes of molecular packing force a lowering of symmetry to monoclinic one. As a result of such molecular rearrangement, one hydrogen bond engaged in formation of R21(4) ring is broken and smaller ring R21(4) becomes a part of the bigger one, R46 (14), during phase transition. Result of those changes is formation of R56(16) pattern. This process can be described by the following operation among graph-set descriptors: R2 1 ð4Þ þ R4 6 ð14Þ–E1 1 ð2ÞH⋯O ¼ R5 6 ð16Þ: The elementary graph-set descriptor E11(2)H⋯O in this equation describes atomic pathway in broken hydrogen bond, so elementary graph-
Fig. 5. Hirshfeld surface around the H2NA+ ion in the high-temperature phase indicates (a) weak HB C\ \H⋯O interactions and (b) π⋯π interaction. Both C⋯N and C⋯O contacts on the Hirshfeld surface are presented.
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the fastest crystal growth along direction of well-established intermolecular interactions, e.g. hydrogen bonds. So in this case, it appears that the π⋯π interaction significantly affects the crystal growth of being a delay factor. 3.2. Conformation of Ortho-Nitroanilinium Ion +
In the HoNA ion, both ammonio and nitro groups can rotate around C\\N bonds. Thus, two torsional angles define relative position of these groups i.e. H1C-N1-C1-C2 (NH3) and O1-N2-C2-C1 (NO2). Crystal data for 300 K reveal that all the atoms of NO2 group and N atom of NH3 group are co-planar with the aromatic ring and they lay at special position. So, aforementioned torsional angles are equal to 60 deg. and 0 deg for NH3 and NO2 group, respectively. According to the Ref. [26], the energy of a such conformation refers to the saddle point and is higher by only 2.87 kcal/mol than the energy of global minimum. After phase transition, torsional angles change reaching the value of −67.22 deg. (NH3) and 4.43 deg. (NO2) at 100 K. The energy conformation equals 2.84 kcal/mol (in relation to energy of global minimum), so it is lower than that before phase transition. 3.3. Formal Classification of the Fundamental Modes for the (H2NA)NO3 Crystal at 300 K The sites for the space group Pmn21 = C2v7 are following C1(4), Cs(2). The nitrate anions (NO3−) are placed on the symmetry planes yz (perpendicular to the a axis). In a such way, the nitrogen atom (N3) and one oxygen atom, O(3), lay in this plane; whereas two other oxygen atoms O(4) and O(4′) are equivalent to each other and lay outside of that mirror plane. Thus, the internal modes of the NO3− transform as 4A′ + 2A″. In the case of H2NA+ cation, only two protons H1B and H1B′ do not lay in the yz plane (perpendicular to the a axis). The internal modes of this cation transform as follows: 30A′ + 15A″. Due to the Davydov type coupling, each A′ type mode should split into following two unit cell modes A1 + B2, whereas each A″ type internal mode should split into following two unit cell modes A2 + B1. The result of formal classification of the fundamental modes for the title crystal are given in the Table 2 for T = 300 K (orthorhombic system, space group Pmn21 = C2v7, Z = 2) and in the Table 3 for T = 100 K (monoclinic system, space group P21 = C22,Z = 2; the sites for that space group are following C1(2)). As follows from the Table 2 the bands observed in the measured spectra should arise from the internal modes of the NO3− anions and from the internal modes of the H2NA+ cations. The assignment of the bands arising from the internal vibrations ν1(A1′) = 1050 cm−1, ν2 (A2″) = 831 cm−1, ν3(E′) = 1390 cm−1 and ν4(E′) = 720 cm−1 of the nitrate anions seems to be a quite simple taking into account the well-known literature data [50] and the selection rules (Table 2a). The internal modes of the H2NA+ cation may be distinguished as those of the ammonio group (NH3+), nitro group (\\NO2) and internal
Table 2 Formal classification of the fundamental type modes (k = 0) for the orthorhombic (H2NA) NO3 crystal at 300 K. U.C.G (FG)a Lattice modes C2v
Internal modes
Ac T L
Selection rules
NO− 3
H2NA+
IR Raman
Total ν1 ν2 ν3 ν4 Total NH+ NO2 3 A1 A2 B1 B2
1 1 1
3 2 1 3
2 4 4 2
4 2 2 4
1 0 0 1
1 0 0 1
1 1 1 1
1 1 1 1
30 15 15 30
7 5 5 7
6 3 3 6
Z X Y
xx, yy, zz xy zx yz
Ac – Acoustic modes, T – Translational type lattice modes, L – Librational type lattice modes. a U.C.G (FG) - unit cell group which is equivalent to the factor group.
317
Table 3 Formal classification of the fundamental type modes (k = 0) for the monoclinic (H2NA) NO3 crystal at 100 K. U.C.G (FG)a
Lattice modes
Internal modes
Selection rules
C2
Ac
T
L
NO− 3
H2NA+
IR
Raman
A B
1 2
5 4
6 6
6 6
45 45
Z X,Y
xx, yy, zz, xy yz, zx
Ac – Acoustic modes, T – Translational type lattice modes, L – Librational type lattice mode. a U.C.G (FG) – unit cell group which is equivalent to the factor group.
modes of the benzene ring. The assignment of the bands arising from the internal vibrations of the H2NA+ may be done taking into account the literature data published for the 2-nitroanilinium chloride [26], 2nitroanilinium bromide [51], on 2-nitroaniline and its cations [52] and also on 4-nitrophenol [53]. A list of experimental frequencies and assignment of the bands is deposited as Supplementary Information (Table S2). 3.4. The Internal Vibrations of the Ammonio Groups All hydrogen atoms of the ammonio groups participate in the hydrogen bonds. The H1A atom which is positioned on the symmetry plane, participate in two symmetry equivalent hydrogen bonds with two symmetry equivalent O4 atoms (O4a and O4b) of the same nitrate ion (Table 1). These hydrogen bonds give rise to the medium strong bands observed at ca. 2900 and at ca. 2650 cm−1 with additional shoulders (2912, 2862, 2790 sh and 2667, 2634 and 2609 cm−1, respectively). The higher frequency band is much broader and its integrated intensity is larger than that of the lower frequency one. Their Raman analogues are extremely weak; especially for the higher frequency band (ca 2900 cm−1) an extremely weak intensity is noticed in Raman spectrum. For the lower analogue two very weak bands are noticed at ca. 2678 and 2639 cm−1 in the powder FT-Raman spectrum. Taking into account the paper [54] one should expect the stretching νNH bands at ca. 3030 cm−1 and at ca. 3260 cm−1 for the N1\\H1B⋯O4 and N1\\H1A⋯O4 hydrogen bonds, respectively. These literature data are not in agreement with our spectra discussed above. This may be due to participation of each proton in two intermolecular interactions (bifurcated hydrogen bonds). Due to this, bands of the stretching N\\H vibrations are downshifted much more that it appears in the case of the paper cited above [54]. Thus, the H1B atoms participate in one hydrogen bond, dN1⋯O4 = 2.845(3) Å, but it is worth noting the presence of a contact of the ammonio group to the second oxygen atom of the nitrate anion. The geometry parameters of this contact are dN1⋯O3 = 3.2561(3) Å) and bN\\H⋯O = 130(2) degrees. A quite small value (130(2) deg) of the N1\\H1B⋯O3 angle shows that this interaction may be very weak with respect to the well defined HB, whose N1\\H1B⋯O4 angle equals to 168(2) degrees. Nevertheless, it considerably lowers (till ca. 2900 cm−1) the frequency of the stretching vibration of the vN1\\H1B bond with respect to the expected one (3030 cm−1) for only N1\\H1B⋯O4 hydrogen bond. The N1\\H1A bond participates in the two symmetry equivalent hydrogen bonding interactions with two O(4) oxygen atoms being symmetry equivalent of the same nitrate ion. Therefore, its stretching frequency may be shifted to ca. 2600 cm−1. Such assignment seems to be supported by the integrated intensities of two bands observed in IR spectra measured for fluorolube mulls. The observed integrated intensity is lager for the higher frequency band than that of the lower frequency band. It follows from the fact that to the higher frequency bands (ca. 2900 cm−1) contributions arise from two symmetry dependent N1\\H1B bonds participating in identical hydrogen bonds, whereas the lower frequency band (ca. 2650 cm−1) arising from the one N1\\H1A bond participating in two symmetry equivalent hydrogen bonds. Moreover, the high frequency band exhibits clear splitting into two components (2912 and 2862 cm−1), which may be due to the out-of phase (2912 cm−1, A″)
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and in-phase coupling (2862 cm−1; A′) between the vibrations of the stretching vibrations of two N1\\H1B⋯O4 hydrogen bonds related to each other by symmetry plane operation perpendicular to the a axis. We hope to support this statement by measurement of the polarized spectra (IR and Raman) for single crystal samples. At present we try to grow a single crystal suitable for such experiment. The greatest changes of the IR spectra on lowering of temperature are observed in the ca. 1700–1250 cm−1 range (Fig. 6). The well shaped strong band at 1578 cm−1 observed in IR spectrum measured at T = 300 K arises from the deformation vibrations δaNH3. Such assignment may be supported by the powder Raman spectrum, where its analogue appears at 1581 cm−1 with a weak intensity. In the IR spectrum measured at low temperature (13K) it splits into few components (1564wsh, 1577ssh, 1580s, 1588s, 1606m). This splitting is already observed in the IR spectrum measured at 100 K (Fig. 7). 3.5. The Vibrations of the –NO2 (Nitro Group) and Nitrate (NO3−) Ions −1
The very strong band at 1535 cm observed in the IR spectrum measured at 300 K arises from the νasNO2. In the IR spectrum measured at 13 K, a shoulder is weakly noticed on top of this band at ca. 1535 cm−1 and the maximum of this band shifts to 1539 cm−1. In the Raman spectrum measured at T = 300 K, there is a weak band at 1538 cm−1, and very weak bands at 1512, 1496, 1490sh, 1470, 1456 and at 1412 cm−1. In IR spectrum measured for fluorolube mulls at T = 300 K, beside a very strong band at 1535 cm−1, one observes three other sharp bands at 1487w, 1455w and at 1395s cm−1. Their analogues are also observed in the IR spectrum measured for Nujol sample, although they are slightly masked by the Nujol bands. In the IR spectrum measured at T = 13 K, one clearly sees a very strong band at 1397 cm−1, the strong bands at 1410 and 1457 cm−1 and a medium sharp band at 1488 cm−1. The bands at 1397 and 1410 cm−1 arise from the ν3NO3 mode (ν3(E′) = 1390 cm−1 for isolated ion) which splits at low temperature. This splitting is not observed in the IR spectrum measured at room temperature. In the Raman spectrum a very weak band at ca.1412 cm−1 is observed. The other double degenerated mode ν4(E′)
Fig. 7. Changes of vibrational frequency of selected bands in IR spectra upon cooling the sample of (H2NA)NO3.
appears as a strong band at 736 cm−1 in the IR spectrum measured at room temperature. In the powder FT-Raman spectrum three very weak bands at 743, 729 and 715 cm−1 are observed therein. In the IR spectrum at T = 13 K one observes an additional sharp band at 722 cm−1. It may be due to Nujol absorption. At T = 300 K it is also observed, however, as a shoulder only. According to the selection rules for the isolated NO3− ion of D3h type symmetry, the ν2(A2″) mode (831 cm−1) is allowed only in the IR spectrum. Surprisingly, this selection rules is fulfilled/observed in the case of title crystal. Thus, in the powder IR spectra (both at T = 300 and 13 K) there is a sharp band of medium intensity at 818 cm−1, which analogue is not present in the powder Raman spectrum. According to the X-ray data, the site symmetry of the NO3 is Cs(v), which predicts the activity of the ν2(A′; X,Y, xx,yy,zz,xy) in both type (IR, R) spectra. The ν1 (A1′; 1050 cm−1) for the isolated (D3h) NO3− ion is allowed only in the Raman spectrum (xx + yy, zz). Unfortunately, in this (ca.
Fig. 6. A juxtaposition of FT-IR and FT-Raman spectra for (H2NA)NO3.
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1040–1050 cm−1) region there are also other modes expected therein (ωNH3, νCC ring breathing [26] or γCH [51]). Thus, in the case of ortho-nitroanilinium chloride crystal the bands at 1035 and 1051 cm−1 are observed in the IR spectrum and at 1046 and1034 cm−1 in the Raman spectrum [26]. In the case of 2nitroanilinium bromide [51], the bands at 1030 and 1086 cm−1 in the IR spectrum and the bands at 1038 and 1053 cm−1 in the Raman spectrum appear. In the IR spectra of the title crystal there are two bands at 1048vw and at 1038w cm−1 in T = 300 K and at 1050vw and at 1040m cm−1 in T = 13 K. A medium band at 1041 cm−1 with a shoulder at 1046 cm−1 appears in the powder FT-Raman spectrum of the title crystal. One can weakly suggest that the band at ca. 1040 cm−1 may be arising from the ν1NO3 vibration, although it is possible that it may arise from other modes, as many bands appear in that region of the vibrational spectra of other (non-nitrate) crystal, as it is shown above. The other bands in this region may arise from skeleton vibrations of benzene ring and in-plane bending vibrations of C\\H bonds. It seems important to mention now, that the stretching νCH bands appear as shoulders at 3153, 3111, 3083 and at 3047 cm−1 in the IR spectrum and as weak bands in the Raman spectrum (ca. 3150vvw, 3101, 3081 and 3056 cm−1) at T = 300 K. In the IR spectrum measured at T = 13 K one sees three quite sharp bands, although of weak intensity, at 3115, 3089 and at 3050 cm−1 and two shoulders at 3150 and 3019 cm−1. The group of overlapping bands (1367ssh, 1353ssh, 1342vs, 1317vs, 1302vssh, 1220 wsh) at T = 300 K, splits into well defined bands at 1356 m, 1344vs, 1329vs, 1319s, 1313vs, 1308vs, 1225vw and as a shoulders at 1291msh, 1249wsh, 1243wsh and at 1211vwsh cm−1 in the IR spectrum measured at T = 13 K. The strongest IR band at 1344 and a medium band at 1356 cm−1 arise from the νsNO2, to which corresponds the strongest Raman bands appearing at 1344 cm−1 with a shoulder at 1354 cm−1. The other bands appearing in this region may arise from various deformation type modes of the benzene ring. The broad strong band at ca. 1317 cm−1 in the IR spectrum may arise from the δsNH3 mode, however corresponding Raman band is not observed therein. A very large temperature changes are observed for the relatively weak bands appearing in the region 1200–1060 cm−1. Thus, the intensity of the bands 1155 cm−1 increases very much, whereas the band at 1170 cm−1 splits into three components (1175, 1170 and 1164sh) at 13 K. A weak and broad band at ca. 1113 cm−1 with shoulder at ca. 1120 cm−1 changes into sharp of medium intensity band at 1121 cm−1 with a very weak shoulder at ca. 1112 cm−1. These bands arise from the deformation vibrations of the benzene ring and NH3+ group.
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3.6. SHG Studies X-ray diffraction experiments revealed that the 2-nitroanilinium nitrate crystallizes without a center of symmetry and therefore SHG experiments were carried out. Fig. 8 shows that intensity of SHG response is significant in relation to KDP, I(H2NA)NO3 = 1.1·IKDP. However, a decay of the signal is observed due to instability of the sample which decomposes to the substrates. This effect is probably associated with the heating of the sample by the laser beam and instability of 2nitroanilinium nitrate itself at ambient conditions. Besides, the SHG signal is falling down because in situ obtained 2-nitroaniline is centrosymmetric, P21/n space group [1–4]. 4. Conclusions Crystal structure of the new compound, 2-nitroanilinium nitrate, has been determined by the X-ray diffraction at 300 K and 100 K temperatures. A series of X-ray diffraction experiments reveal a phase transition at 249 K and changes symmetry of the crystal structure from orthorhombic (Pmn21) to monoclinic one (P1121) upon cooling. A mechanism of the phase transition is connected to the displacement of the nitrate anion and ammonio group of the 2-nitroanilinium ion from the mirror plane. As a consequence, monitored changes of the lattice parameters show significant increase beyond 90 degrees of the γ angle. Therefore, a twinning of the measured crystal is observed. Additionally, both m and n type planes disappear, where the lack of the latter symmetry element is seen in diffraction pattern since h + l odd reflections in (h0l) plane is observed. As a result of the molecular movement, the hydrogen bonding network is changed. Especially, the bifurcated hydrogen bond existed in high-temperature phase disappears during phase transition because one hydrogen bond described by the elementary graph-set descriptor E11(2)H⋯O was broken. In order to describe a mechanism of phase transition in hydrogen bonding patterns, mathematical operation of the graph-set descriptors was applied for the first time. Overall, basing on the vibrational spectra, it is noticed that network of N\\H⋯O hydrogen bonds becomes stronger upon cooling the crystal. Besides, the analysis of C⋯N and C⋯O contacts on the Hirshfeld surface suggest the same tendency for π⋯π interaction alike the energy of hydrogen bonds. Since the studied compound crystallizes in non-centrosymmetric space group, SHG activity was checked. Intensive signal was reported, although its gradual decrease associated with decomposition of the sample was observed. Therefore, prior potential application of the compound, a future studies on improvement of its stability is required. Acknowledgement We would like to thank Prof. M. Samoć and J. K. Zaręba for the SHG measurements. We also acknowledge ILT&SR PAS for financial support by statutory activity subsidy, grant no. K15. Calculations have been carried out in Wroclaw Centre for Networking and Supercomputing (http://www.wcss.wroc.pl). Appendix A. Supplementary data Dependencies of lattice parameters and unit cell volume on temperature, IR spectra and a table with assignment of the bands were deposited as Supplementary Material. CCDC 1567755-1567756 contain the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures. Supplementary data to this article can be found online at doi:https://doi.org/10.1016/j.saa.2018.09.041. References
Fig. 8. Dependence of SHG response of (H2NA)NO3 on exposure time.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Phase transition in polar 2-nitroanilinium nitrate. Graph-set approach of hydrogen bonding patterns and analysis of vibrational spectra Piotr Rejnhardt, Jan Baran, Marek Daszkiewicz ⁎ Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna str. 2, P.O. Box 1410, 50-950 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 20 June 2018 Received in revised form 19 September 2018 Accepted 22 September 2018 Available online 24 September 2018 Keywords: 2-Nitroaniline Crystal structure Phase transition Elementary graph-set descriptor Vibrational spectroscopy Second harmonic generation
a b s t r a c t A phase transition in new compound 2-nitroanilinium nitrate, (H2NA)NO3, was found. A symmetry lowering from orthorhombic, Pmn21, to monoclinic one, P21, at 249 K is observed. During the phase transition, the H2NA+ and nitrate ions displace from the mirror plane in high-temperature phase. As a result, hydrogen bonding network constructed by the ammonio group and NO3− anion is changed. Especially, a bifurcated hydrogen bond disappears. Mathematical operations using both elementary and hydrogen bond graph-set descriptors were used for the first time to describe a mechanism of phase transition. Changes in hydrogen bonding network were also studied by means of vibrational spectroscopy. Band shift associated with stretching and bending vibrations of the ammonio group indicates that the energy of intermolecular interactions rises along with temperature decrease. SHG response for the studied compound is higher than KDP, I(H2NA)NO3 = 1.1·IKDP, although a decay of the signal was observed due to instability of the sample. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Among three isomers 2-, 3- and 4-nitroaniline, only the 3nitroaniline crystallizes without a center of symmetry and therefore second harmonic generation is expected for this compound [1–14]. However, since nitroanilines are Brønsted bases, their salts of various properties can be easily obtained. Therefore, a search of new functional materials can be expanded for large group of organic ionic crystals. Recently, among salts of 2-methy-4-nitroaniline, the first non-linear organic compound, two polymorphs of chloride were reported as noncentrosymmetric, P-421c and P212121 [15]. In the case of 2nitroaniline, it possesses the smallest dipole moment among nitroanilines. Therefore, it can be expected the biggest chance for obtaining non-centrosymmetric salt for that isomer, because there is a general tendency of molecules with large dipole moment to crystallize with a center of symmetry. More recently, hybrid organic-inorganic compounds of 2-methoxyaniline with SnCl3− and SnCl62− were obtained [16,17], where the former compound crystallizes without a center of symmetry but the latter one is centrosymmetric. Such a result may be associated with a molecular structure of the octahedral SnCl62− anion which possesses inversion center and therefore facilitates organization of the crystal structure as centrosymmetric [18–24]. Although it is worth noting that a use of a pyramidal SnCl3− anion in synthesis is not a sufficient condition for non-centrosymmetry [25].
⁎ Corresponding author. E-mail address: [email protected] (M. Daszkiewicz).
https://doi.org/10.1016/j.saa.2018.09.041 1386-1425/© 2018 Elsevier B.V. All rights reserved.
Besides, 2-nitroaniline is interesting molecule that it forms N\\H⋯O intramolecular hydrogen bond, which additionally stabilizes position of terminal groups (NH2 and NO2). If the amino group is protonated, a formation of intramolecular hydrogen bond is also energetically privileged for isolated H2NA+ ion [26]. However, a rotation barrier for the NH3+ group is relatively small, 2.87 kcal/mol, and it depends on the position of the nitro group. Nevertheless, the position of the ammonio group can be easily changed by intermolecular hydrogen bonds as was observed for 2-nitroanilinium chloride and bromide [26,27]. In this work, we present crystal structure of room- and lowtemperature phases of 2-nitroanilinium nitrate, which are noncentrosymmetric. An analysis of phase transition and especially change of hydrogen bonding patterns during this process is presented. Mathematic approach using operations on graph-set descriptors as a tool for description of molecular mechanism of phase transition is used for the first time. Additionally, non-linear properties for the studied compound were evidenced by means of SHG experiment. 2. Materials and Methods 2.1. Synthesis The starting compounds, 2NA [Aldrich, purum, ≥98% (NT)] and nitric acid (Sigma-Aldrich, 70%), were used as supplied. The 2NA (1 mmol, 0.1381 g) was dissolved in a mixture of 5 ml of methanol and 5 ml of isopropanol, and then 0.5 ml HNO3 was added. Colorless crystals of 2nitroanilinium nitrate were obtained after six days by slow evaporation of the mixture.
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2.2. Single Crystal X-ray Diffraction Studies X-ray diffraction data were collected on an Oxford Diffraction fourcircle single crystal diffractometer equipped with a CCD detector using graphite-monochromatized MoKα radiation (λ = 0.71073 Å). The raw data were treated with the CrysAlis Data Reduction Program (version 1.171.38.34a). The intensities of the reflections were corrected for Lorentz and polarization effects. The crystal structures were solved by direct methods [28] and refined by full-matrix least-squares method using SHELXL-2014 program [28]. Non-hydrogen atoms were refined using anisotropic displacement parameters. H-atoms connected with C atoms from the aromatic rings were visible on the Fourier difference maps, but placed by geometry and allowed to refine “riding on” the parent atom with Uiso = 1.2 Ueq(C). Coordinates of hydrogen atoms of the ammonio group were refined, but Uiso = 1.5 Ueq(N). Visualizations of the structures were made using Diamond 3.2 k [29]. Low temperature X-ray diffraction experiments were carried out using OxfordCryosystems device in the ranges 300 K–100 K with ΔT = 10 K and 250 K – 240 K with ΔT = 1 K. High pressure experiment was carried out using Merrill-Bassett diamond anvil cell. The pressure was measured collecting a set of approximately hundred reflections for CaF2 which was a standard, and referring a unit cell volume to known characteristics vs. pressure. Both standard and sample crystals were placed between diamonds in a hole of stainless steel gasket. A mineral oil was used to fill the chamber. 2.3. Spectroscopic Measurements Room temperature FT-IR spectra in the 4000–400 cm−1 range were measured on the Bruker IFS-88 spectrometer with 2 cm−1 resolution. Nujol and fluorolube mull techniques have been used in the measurements. Closed cycle helium cryostat system of APD Cryogenics INC. (DE-202 Expander, Compressor Model HC-2, Microprocessor-based Temperature Indicator/Controller Model 5500-1-25) was used for low temperature measurements (300−12 K). The temperature of the sample was maintained with an accuracy of ±0.1 K. Powder FTRaman spectrum at room temperature was recorded with FRA-106 attachment to the Bruker IFS-88 spectrometer over the frequency range 3600–50 cm−1 with resolution of 2 cm−1; applying Nd:YAG laser (λ = 1064 nm, power ca. 150 mW).
were constrained, but the other geometry parameters were optimized similarly to the procedure written in Ref. [26]. 3. Results and Discussion 3.1. Crystal Structure of Ortho-Nitroanilinium Nitrate Title compound crystallizes at room temperature in orthorhombic crystal system, non-centrosymmetric space group Pmn21 (Table S1). X-ray experiment at low temperature revealed change of the crystal symmetry to monoclinic one, P1121 space group. Non-standard setting of the unit cell was used for low-temperature phase to inherit a setting from high-temperature phase. The crystal twinned into two domains after phase transition, which was associated with a change of lattice gamma angle (Fig. 1). Detailed diffraction experiments showed that this angle started to change at 249 K (Fig. 1b). (Changes of the other lattice parameters upon cooling were deposited as Supplementary Information.) Since the crystal symmetry was lowered during phase transition, both the mirror and n glide planes disappeared. Therefore, the violation of extinction rule, h + l = 2n, for n glide plane was observed (Fig. 2). High pressure experiment was carried out at 0.3(1) GPa. Due to low quality of the diffractions pattern and small number of reflections only the lattice parameters could be determined: a = 6,44(10) Å, b = 8013 (12) Å, c = 7760(4) Å, γ = 91,6 (4) deg and V = 401(3) Å3. The values can be referred to those obtained at low temperature, 100 K. In orthorhombic phase, (H2NA)+ and NO3− ions lie on the mirror plane, however two hydrogen atoms of ammonio group and two oxygen atoms of the nitrate lay in general positions (Fig. 3). As a result of phase transition, the mirror plane disappears and all the ions lay in
2.4. Second Harmonic Generation Measurements SHG measurements for powdered samples of the noncentrosymmetric (H2NA)NO3 and that of KH2PO4 (KDP), which was used as a reference material, were mounted between separate microscope glass slides and excited by tunable femtosecond laser pulses from the laser system consisting of a Quantronix Integra-C regenerative amplifier operating as an 800 nm pump and a Quantronix-Palitra-FS BIBO crystal-based optical parametric amplifier. This system delivers wavelength-tunable pulses of ~130 fs length and was operated at the repetition rate of 1 kHz. A detailed description of the experiment can be found elsewhere [30]. Caution! Work with the high-power laser brings danger to the eyes, especially in spectral range in which the beam is invisible. Adequate eye protection should be used during measurements. 2.5. Computational Details All the computations were performed using density-functional theory (DFT) and hybrid Becke's three-parameter the Lee-Yang-Parr correlation functionals (B3LYP) [31–35] and the 6-31G(d,p) basis set was used. The calculations were carried out for the singlet electronic ground state and no symmetry restrictions were applied. All the atomic positions were taken from crystallographic data. Due to keeping preserved the H2NA+ molecular structure existed the crystal structure of (H2NA)NO3, dihedral angles of both the ammonio and nitro groups
Fig. 1. (a) A section of reciprocal space with (hk0) plane for a measurement at 100 K. Large group of spots are split due to crystal twinning. (b) Changes of the γ angle with temperature indicate a phase transition at 249 K.
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Fig. 2. A comparison of diffraction pattern on the (h0l) plane obtained (a) at 300 K and (b) at 100 K. Additional h + l odd reflections indicate a disappearance of the n glide plane.
general positions. In low-temperature phase the ions displace from their positions in comparison with the respective positions at room temperature. Change of conformation of ammonio and nitro groups is also observed, because they rotate around C\\N bonds. In the studied crystal, the most important hydrogen bonds are formed between ammonio group and nitrate anion arranged in a layer parallel to ac plane (Table 1). Theoretically, interacting functional group can be described by elementary graph-set descriptors using Etter's notation [36,37], GNH3 = {3E01(1); 3E02(3)}, GNO3 = {3E10(1); 3E20(3)}, which were used in our earlier studies on relations between descriptors in hydrogen bonding systems [15,26,38]. Summation each elementary graph-set descriptor GNH3 with GNO3 gives one of the following descriptors related to hydrogen bonding pattern: GHB = {D11(2); C11(2); G12(4); G21(4); G22(6)}, G = {R, C}. In the studied crystal at room temperature, the C22(6) chain parallel to a axis exists (Fig. 4). Additionally, two chains C12(4) run along [101] and [−101] directions. It is worth noting here that the other two chains can be distinguished, because one hydrogen atom is
engaged in bifurcated hydrogen bond, C 2 2(6) and C 34 (10). The chains cross to each other resulting in ring patterns, R 4 6 (14) and R21(4). The chain and ring descriptors can be expressed by following summations: E0 2 ð3ÞHNH þ E2 0 ð3ÞONO ¼ C2 2 ð6Þ; E0 2 ð3ÞHNH þ E1 0 ð1ÞO ¼ C1 2 ð4Þ; E0 2 ð3ÞHNH þ E2 0 ð3ÞONO þ E0 2 ð3ÞHNH þ E1 0 ð1ÞO ¼ C3 4 ð10Þ; h i 2 E0 2 ð3ÞHNH þ E1 0 ð1ÞO þ E0 2 ð3ÞHNH þ E2 0 ð3ÞONO ¼ R4 6 ð14Þ; E0 1 ð1ÞH þ E2 0 ð3ÞONO ¼ R2 1 ð4Þ: Overall, hydrogen bonding network changes during phase transition. Two chains C22(6) and C34(10) from high temperature phase are preserved in the second phase but C12(4) chain disappears. Besides, one new ring R56(16) can be found in low temperature phase
Table 1 Geometry parameters for selected hydrogen bonds found in high- and low-temperature phases of (H2NA)NO3. D\ \H⋯A 300 K N1\ \H1A⋯O4i N1\ \H1A⋯O4ii N1\ \H1B⋯O4 C3\ \H3⋯O3iii C6\ \H6⋯O2iv 100 K N1\ \H1A⋯O5v N1\ \H1B⋯O5vi N1\ \H1C⋯O4 C3\ \H3⋯O3vii C6\ \H6⋯O2iv Fig. 3. Molecular structure and a labeling scheme for room-temperature phase of (H2NA) NO3.
D\ \H (Å)
H⋯A (Å)
D⋯A (Å)
D\ \H⋯A (°)
0.91 (5) 0.91 (5) 1.00 (3) 0.93 0.93
2.21 (5) 2.21 (5) 1.86 (4) 2.63 2.60
3.035 (4) 3.035 (4) 2.845 (3) 3.562 (5) 3.205 (5)
151.2 (7) 151.2 (7) 168 (3) 178.9 122.9
0.97 (3) 0.91 (3) 0.93 (3) 0.93 0.93
1.89 (3) 1.90 (3) 1.91 (3) 2.58 2.59
2.851 (3) 2.801 (3) 2.813 (3) 3.509 (3) 3.174 (3)
171 (2) 168 (3) 164 (3) 176.8 121.1
Symmetry code(s): (i) –x + 1/2, −y + 2, z-1/2; (ii) x-1/2, −y + 2, z-1/2; (iii) –x + 1/2, −y + 1, z + 1/2; (iv) x, y, z-1; (v) –x + 1, −y + 1, z-1/2; (vi) x-1, y, z; (vii) –x + 1, −y + 2, z + 1/2.
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set descriptor of the H⋯O pathway is subtracted. It is worth noting that the hydrogen and oxygen atoms are common in both rings, R21(4) and R46(14). In the crystal structure of studied compound, several weak intermolecular HB interactions occur. Hirshfeld surface analysis reveals short contacts between aromatic ring and oxygen atom of the nitro group, i.e. C5\\H5⋯O2 and C6\\H6⋯O2 [39,40]. Both interactions connect the ions along crystallographic c direction, so they additionally stabilize N\\H⋯O hydrogen bonding network parallel to the ac plane. However, it is worth noting that none of N\\H⋯O hydrogen bonds is present in orthogonal direction i.e. along b axis. The only important interaction found on the Hirshfeld surface is short contact between C3\\H3 group and nitrate anion of neighboring layer (Fig. 5a). Therefore, it appears that the C3\\H3⋯O3 interaction is important for molecular self–assembly because it connects molecules together, which exist in two adjacent layers of N\\H⋯O hydrogen bonds. In the studied crystal structure, non-covalent interaction with NO2 also exists, which importance in molecular self-assembly was recently reported for many structures [38,41–49]. In the crystal structure of (H2NA)NO3, the nitro group forms π⋯π interaction along with adjacent aromatic ring and thereby a skeleton of the H2NA+ ion is stabilized. Fig. 5b shows that NO2 group is slightly shifted outside of the ring's center and a comparison of this interaction for two phases indicates that smaller region on the Hirshfeld surface is covered by the C⋯N and C⋯O contacts for high-temperature phase (14.5% at 300 K and 15.6% at 100 K). So, the π⋯π interaction becomes a little bit stronger upon cooling. Besides, interestingly this interaction stabilizes adjacent aromatic rings along the a direction, hence parallel to the plane of hydrogen bonding layer. On the other hand, it was observed that obtained crystals have the smallest dimension along a axis, but it is reasonable to expect
Fig. 4. A comparison of hydrogen bonding patterns constructed by the ammonio groups and nitrate anions (a) for high-temperature phase and (b) for low-temperature phase.
and arises from the following summation of elementary graph-set descriptors: h i E0 2 ð3ÞHNH þ E1 0 ð1ÞO þ 2 E0 2 ð3ÞHNH þ E2 0 ð3ÞONO ¼ R5 6 ð16Þ: Differences between hydrogen bonding networks are associated with the change of the position of H2NA+ ion and particularly ammonio group. The organic cation displaces from the mirror plane during phase transition and therefore the changes of molecular packing force a lowering of symmetry to monoclinic one. As a result of such molecular rearrangement, one hydrogen bond engaged in formation of R21(4) ring is broken and smaller ring R21(4) becomes a part of the bigger one, R46 (14), during phase transition. Result of those changes is formation of R56(16) pattern. This process can be described by the following operation among graph-set descriptors: R2 1 ð4Þ þ R4 6 ð14Þ–E1 1 ð2ÞH⋯O ¼ R5 6 ð16Þ: The elementary graph-set descriptor E11(2)H⋯O in this equation describes atomic pathway in broken hydrogen bond, so elementary graph-
Fig. 5. Hirshfeld surface around the H2NA+ ion in the high-temperature phase indicates (a) weak HB C\ \H⋯O interactions and (b) π⋯π interaction. Both C⋯N and C⋯O contacts on the Hirshfeld surface are presented.
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the fastest crystal growth along direction of well-established intermolecular interactions, e.g. hydrogen bonds. So in this case, it appears that the π⋯π interaction significantly affects the crystal growth of being a delay factor. 3.2. Conformation of Ortho-Nitroanilinium Ion +
In the HoNA ion, both ammonio and nitro groups can rotate around C\\N bonds. Thus, two torsional angles define relative position of these groups i.e. H1C-N1-C1-C2 (NH3) and O1-N2-C2-C1 (NO2). Crystal data for 300 K reveal that all the atoms of NO2 group and N atom of NH3 group are co-planar with the aromatic ring and they lay at special position. So, aforementioned torsional angles are equal to 60 deg. and 0 deg for NH3 and NO2 group, respectively. According to the Ref. [26], the energy of a such conformation refers to the saddle point and is higher by only 2.87 kcal/mol than the energy of global minimum. After phase transition, torsional angles change reaching the value of −67.22 deg. (NH3) and 4.43 deg. (NO2) at 100 K. The energy conformation equals 2.84 kcal/mol (in relation to energy of global minimum), so it is lower than that before phase transition. 3.3. Formal Classification of the Fundamental Modes for the (H2NA)NO3 Crystal at 300 K The sites for the space group Pmn21 = C2v7 are following C1(4), Cs(2). The nitrate anions (NO3−) are placed on the symmetry planes yz (perpendicular to the a axis). In a such way, the nitrogen atom (N3) and one oxygen atom, O(3), lay in this plane; whereas two other oxygen atoms O(4) and O(4′) are equivalent to each other and lay outside of that mirror plane. Thus, the internal modes of the NO3− transform as 4A′ + 2A″. In the case of H2NA+ cation, only two protons H1B and H1B′ do not lay in the yz plane (perpendicular to the a axis). The internal modes of this cation transform as follows: 30A′ + 15A″. Due to the Davydov type coupling, each A′ type mode should split into following two unit cell modes A1 + B2, whereas each A″ type internal mode should split into following two unit cell modes A2 + B1. The result of formal classification of the fundamental modes for the title crystal are given in the Table 2 for T = 300 K (orthorhombic system, space group Pmn21 = C2v7, Z = 2) and in the Table 3 for T = 100 K (monoclinic system, space group P21 = C22,Z = 2; the sites for that space group are following C1(2)). As follows from the Table 2 the bands observed in the measured spectra should arise from the internal modes of the NO3− anions and from the internal modes of the H2NA+ cations. The assignment of the bands arising from the internal vibrations ν1(A1′) = 1050 cm−1, ν2 (A2″) = 831 cm−1, ν3(E′) = 1390 cm−1 and ν4(E′) = 720 cm−1 of the nitrate anions seems to be a quite simple taking into account the well-known literature data [50] and the selection rules (Table 2a). The internal modes of the H2NA+ cation may be distinguished as those of the ammonio group (NH3+), nitro group (\\NO2) and internal
Table 2 Formal classification of the fundamental type modes (k = 0) for the orthorhombic (H2NA) NO3 crystal at 300 K. U.C.G (FG)a Lattice modes C2v
Internal modes
Ac T L
Selection rules
NO− 3
H2NA+
IR Raman
Total ν1 ν2 ν3 ν4 Total NH+ NO2 3 A1 A2 B1 B2
1 1 1
3 2 1 3
2 4 4 2
4 2 2 4
1 0 0 1
1 0 0 1
1 1 1 1
1 1 1 1
30 15 15 30
7 5 5 7
6 3 3 6
Z X Y
xx, yy, zz xy zx yz
Ac – Acoustic modes, T – Translational type lattice modes, L – Librational type lattice modes. a U.C.G (FG) - unit cell group which is equivalent to the factor group.
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Table 3 Formal classification of the fundamental type modes (k = 0) for the monoclinic (H2NA) NO3 crystal at 100 K. U.C.G (FG)a
Lattice modes
Internal modes
Selection rules
C2
Ac
T
L
NO− 3
H2NA+
IR
Raman
A B
1 2
5 4
6 6
6 6
45 45
Z X,Y
xx, yy, zz, xy yz, zx
Ac – Acoustic modes, T – Translational type lattice modes, L – Librational type lattice mode. a U.C.G (FG) – unit cell group which is equivalent to the factor group.
modes of the benzene ring. The assignment of the bands arising from the internal vibrations of the H2NA+ may be done taking into account the literature data published for the 2-nitroanilinium chloride [26], 2nitroanilinium bromide [51], on 2-nitroaniline and its cations [52] and also on 4-nitrophenol [53]. A list of experimental frequencies and assignment of the bands is deposited as Supplementary Information (Table S2). 3.4. The Internal Vibrations of the Ammonio Groups All hydrogen atoms of the ammonio groups participate in the hydrogen bonds. The H1A atom which is positioned on the symmetry plane, participate in two symmetry equivalent hydrogen bonds with two symmetry equivalent O4 atoms (O4a and O4b) of the same nitrate ion (Table 1). These hydrogen bonds give rise to the medium strong bands observed at ca. 2900 and at ca. 2650 cm−1 with additional shoulders (2912, 2862, 2790 sh and 2667, 2634 and 2609 cm−1, respectively). The higher frequency band is much broader and its integrated intensity is larger than that of the lower frequency one. Their Raman analogues are extremely weak; especially for the higher frequency band (ca 2900 cm−1) an extremely weak intensity is noticed in Raman spectrum. For the lower analogue two very weak bands are noticed at ca. 2678 and 2639 cm−1 in the powder FT-Raman spectrum. Taking into account the paper [54] one should expect the stretching νNH bands at ca. 3030 cm−1 and at ca. 3260 cm−1 for the N1\\H1B⋯O4 and N1\\H1A⋯O4 hydrogen bonds, respectively. These literature data are not in agreement with our spectra discussed above. This may be due to participation of each proton in two intermolecular interactions (bifurcated hydrogen bonds). Due to this, bands of the stretching N\\H vibrations are downshifted much more that it appears in the case of the paper cited above [54]. Thus, the H1B atoms participate in one hydrogen bond, dN1⋯O4 = 2.845(3) Å, but it is worth noting the presence of a contact of the ammonio group to the second oxygen atom of the nitrate anion. The geometry parameters of this contact are dN1⋯O3 = 3.2561(3) Å) and bN\\H⋯O = 130(2) degrees. A quite small value (130(2) deg) of the N1\\H1B⋯O3 angle shows that this interaction may be very weak with respect to the well defined HB, whose N1\\H1B⋯O4 angle equals to 168(2) degrees. Nevertheless, it considerably lowers (till ca. 2900 cm−1) the frequency of the stretching vibration of the vN1\\H1B bond with respect to the expected one (3030 cm−1) for only N1\\H1B⋯O4 hydrogen bond. The N1\\H1A bond participates in the two symmetry equivalent hydrogen bonding interactions with two O(4) oxygen atoms being symmetry equivalent of the same nitrate ion. Therefore, its stretching frequency may be shifted to ca. 2600 cm−1. Such assignment seems to be supported by the integrated intensities of two bands observed in IR spectra measured for fluorolube mulls. The observed integrated intensity is lager for the higher frequency band than that of the lower frequency band. It follows from the fact that to the higher frequency bands (ca. 2900 cm−1) contributions arise from two symmetry dependent N1\\H1B bonds participating in identical hydrogen bonds, whereas the lower frequency band (ca. 2650 cm−1) arising from the one N1\\H1A bond participating in two symmetry equivalent hydrogen bonds. Moreover, the high frequency band exhibits clear splitting into two components (2912 and 2862 cm−1), which may be due to the out-of phase (2912 cm−1, A″)
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and in-phase coupling (2862 cm−1; A′) between the vibrations of the stretching vibrations of two N1\\H1B⋯O4 hydrogen bonds related to each other by symmetry plane operation perpendicular to the a axis. We hope to support this statement by measurement of the polarized spectra (IR and Raman) for single crystal samples. At present we try to grow a single crystal suitable for such experiment. The greatest changes of the IR spectra on lowering of temperature are observed in the ca. 1700–1250 cm−1 range (Fig. 6). The well shaped strong band at 1578 cm−1 observed in IR spectrum measured at T = 300 K arises from the deformation vibrations δaNH3. Such assignment may be supported by the powder Raman spectrum, where its analogue appears at 1581 cm−1 with a weak intensity. In the IR spectrum measured at low temperature (13K) it splits into few components (1564wsh, 1577ssh, 1580s, 1588s, 1606m). This splitting is already observed in the IR spectrum measured at 100 K (Fig. 7). 3.5. The Vibrations of the –NO2 (Nitro Group) and Nitrate (NO3−) Ions −1
The very strong band at 1535 cm observed in the IR spectrum measured at 300 K arises from the νasNO2. In the IR spectrum measured at 13 K, a shoulder is weakly noticed on top of this band at ca. 1535 cm−1 and the maximum of this band shifts to 1539 cm−1. In the Raman spectrum measured at T = 300 K, there is a weak band at 1538 cm−1, and very weak bands at 1512, 1496, 1490sh, 1470, 1456 and at 1412 cm−1. In IR spectrum measured for fluorolube mulls at T = 300 K, beside a very strong band at 1535 cm−1, one observes three other sharp bands at 1487w, 1455w and at 1395s cm−1. Their analogues are also observed in the IR spectrum measured for Nujol sample, although they are slightly masked by the Nujol bands. In the IR spectrum measured at T = 13 K, one clearly sees a very strong band at 1397 cm−1, the strong bands at 1410 and 1457 cm−1 and a medium sharp band at 1488 cm−1. The bands at 1397 and 1410 cm−1 arise from the ν3NO3 mode (ν3(E′) = 1390 cm−1 for isolated ion) which splits at low temperature. This splitting is not observed in the IR spectrum measured at room temperature. In the Raman spectrum a very weak band at ca.1412 cm−1 is observed. The other double degenerated mode ν4(E′)
Fig. 7. Changes of vibrational frequency of selected bands in IR spectra upon cooling the sample of (H2NA)NO3.
appears as a strong band at 736 cm−1 in the IR spectrum measured at room temperature. In the powder FT-Raman spectrum three very weak bands at 743, 729 and 715 cm−1 are observed therein. In the IR spectrum at T = 13 K one observes an additional sharp band at 722 cm−1. It may be due to Nujol absorption. At T = 300 K it is also observed, however, as a shoulder only. According to the selection rules for the isolated NO3− ion of D3h type symmetry, the ν2(A2″) mode (831 cm−1) is allowed only in the IR spectrum. Surprisingly, this selection rules is fulfilled/observed in the case of title crystal. Thus, in the powder IR spectra (both at T = 300 and 13 K) there is a sharp band of medium intensity at 818 cm−1, which analogue is not present in the powder Raman spectrum. According to the X-ray data, the site symmetry of the NO3 is Cs(v), which predicts the activity of the ν2(A′; X,Y, xx,yy,zz,xy) in both type (IR, R) spectra. The ν1 (A1′; 1050 cm−1) for the isolated (D3h) NO3− ion is allowed only in the Raman spectrum (xx + yy, zz). Unfortunately, in this (ca.
Fig. 6. A juxtaposition of FT-IR and FT-Raman spectra for (H2NA)NO3.
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1040–1050 cm−1) region there are also other modes expected therein (ωNH3, νCC ring breathing [26] or γCH [51]). Thus, in the case of ortho-nitroanilinium chloride crystal the bands at 1035 and 1051 cm−1 are observed in the IR spectrum and at 1046 and1034 cm−1 in the Raman spectrum [26]. In the case of 2nitroanilinium bromide [51], the bands at 1030 and 1086 cm−1 in the IR spectrum and the bands at 1038 and 1053 cm−1 in the Raman spectrum appear. In the IR spectra of the title crystal there are two bands at 1048vw and at 1038w cm−1 in T = 300 K and at 1050vw and at 1040m cm−1 in T = 13 K. A medium band at 1041 cm−1 with a shoulder at 1046 cm−1 appears in the powder FT-Raman spectrum of the title crystal. One can weakly suggest that the band at ca. 1040 cm−1 may be arising from the ν1NO3 vibration, although it is possible that it may arise from other modes, as many bands appear in that region of the vibrational spectra of other (non-nitrate) crystal, as it is shown above. The other bands in this region may arise from skeleton vibrations of benzene ring and in-plane bending vibrations of C\\H bonds. It seems important to mention now, that the stretching νCH bands appear as shoulders at 3153, 3111, 3083 and at 3047 cm−1 in the IR spectrum and as weak bands in the Raman spectrum (ca. 3150vvw, 3101, 3081 and 3056 cm−1) at T = 300 K. In the IR spectrum measured at T = 13 K one sees three quite sharp bands, although of weak intensity, at 3115, 3089 and at 3050 cm−1 and two shoulders at 3150 and 3019 cm−1. The group of overlapping bands (1367ssh, 1353ssh, 1342vs, 1317vs, 1302vssh, 1220 wsh) at T = 300 K, splits into well defined bands at 1356 m, 1344vs, 1329vs, 1319s, 1313vs, 1308vs, 1225vw and as a shoulders at 1291msh, 1249wsh, 1243wsh and at 1211vwsh cm−1 in the IR spectrum measured at T = 13 K. The strongest IR band at 1344 and a medium band at 1356 cm−1 arise from the νsNO2, to which corresponds the strongest Raman bands appearing at 1344 cm−1 with a shoulder at 1354 cm−1. The other bands appearing in this region may arise from various deformation type modes of the benzene ring. The broad strong band at ca. 1317 cm−1 in the IR spectrum may arise from the δsNH3 mode, however corresponding Raman band is not observed therein. A very large temperature changes are observed for the relatively weak bands appearing in the region 1200–1060 cm−1. Thus, the intensity of the bands 1155 cm−1 increases very much, whereas the band at 1170 cm−1 splits into three components (1175, 1170 and 1164sh) at 13 K. A weak and broad band at ca. 1113 cm−1 with shoulder at ca. 1120 cm−1 changes into sharp of medium intensity band at 1121 cm−1 with a very weak shoulder at ca. 1112 cm−1. These bands arise from the deformation vibrations of the benzene ring and NH3+ group.
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3.6. SHG Studies X-ray diffraction experiments revealed that the 2-nitroanilinium nitrate crystallizes without a center of symmetry and therefore SHG experiments were carried out. Fig. 8 shows that intensity of SHG response is significant in relation to KDP, I(H2NA)NO3 = 1.1·IKDP. However, a decay of the signal is observed due to instability of the sample which decomposes to the substrates. This effect is probably associated with the heating of the sample by the laser beam and instability of 2nitroanilinium nitrate itself at ambient conditions. Besides, the SHG signal is falling down because in situ obtained 2-nitroaniline is centrosymmetric, P21/n space group [1–4]. 4. Conclusions Crystal structure of the new compound, 2-nitroanilinium nitrate, has been determined by the X-ray diffraction at 300 K and 100 K temperatures. A series of X-ray diffraction experiments reveal a phase transition at 249 K and changes symmetry of the crystal structure from orthorhombic (Pmn21) to monoclinic one (P1121) upon cooling. A mechanism of the phase transition is connected to the displacement of the nitrate anion and ammonio group of the 2-nitroanilinium ion from the mirror plane. As a consequence, monitored changes of the lattice parameters show significant increase beyond 90 degrees of the γ angle. Therefore, a twinning of the measured crystal is observed. Additionally, both m and n type planes disappear, where the lack of the latter symmetry element is seen in diffraction pattern since h + l odd reflections in (h0l) plane is observed. As a result of the molecular movement, the hydrogen bonding network is changed. Especially, the bifurcated hydrogen bond existed in high-temperature phase disappears during phase transition because one hydrogen bond described by the elementary graph-set descriptor E11(2)H⋯O was broken. In order to describe a mechanism of phase transition in hydrogen bonding patterns, mathematical operation of the graph-set descriptors was applied for the first time. Overall, basing on the vibrational spectra, it is noticed that network of N\\H⋯O hydrogen bonds becomes stronger upon cooling the crystal. Besides, the analysis of C⋯N and C⋯O contacts on the Hirshfeld surface suggest the same tendency for π⋯π interaction alike the energy of hydrogen bonds. Since the studied compound crystallizes in non-centrosymmetric space group, SHG activity was checked. Intensive signal was reported, although its gradual decrease associated with decomposition of the sample was observed. Therefore, prior potential application of the compound, a future studies on improvement of its stability is required. Acknowledgement We would like to thank Prof. M. Samoć and J. K. Zaręba for the SHG measurements. We also acknowledge ILT&SR PAS for financial support by statutory activity subsidy, grant no. K15. Calculations have been carried out in Wroclaw Centre for Networking and Supercomputing (http://www.wcss.wroc.pl). Appendix A. Supplementary data Dependencies of lattice parameters and unit cell volume on temperature, IR spectra and a table with assignment of the bands were deposited as Supplementary Material. CCDC 1567755-1567756 contain the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures. Supplementary data to this article can be found online at doi:https://doi.org/10.1016/j.saa.2018.09.041. References
Fig. 8. Dependence of SHG response of (H2NA)NO3 on exposure time.
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