ISSN 00360244, Russian Journal of Physical Chemistry A, 2014, Vol. 88, No. 4, pp. 625–628. © Pleiades Publishing, Ltd., 2014. Original Russian Text © I.V. Vorotyntsev, I.I. Grinvald, I.Yu. Kagalaev, A.N. Petukhov, E.A. Sutyagina, A.V. Vorotyntsev, E.V. Derbisher, N.A. Petukhova, V.M. Vorotyntsev, 2014, published in Zhurnal Fizicheskoi Khimii, 2014, Vol. 88, No. 4, pp. 611–614.
STRUCTURE OF MATTER AND QUANTUM CHEMISTRY
IR Spectroscopic Study of the Complex Formation between Ammonia and Water Molecules in a KBr Matrix I. V. Vorotyntseva, I. I. Grinvalda, I. Yu. Kagalaeva, A. N. Petukhova, E. A. Sutyaginaa, A. V. Vorotyntseva, E. V. Derbisherb, N. A. Petukhovaa, and V. M. Vorotyntseva a
Nizhni Novgorod State Technical University, Nizhni Novgorod, 603600 Russia b Volgograd State Technical University, Volgograd, 400131 Russia email:
[email protected] Received April 5, 2013
Abstract—The formation of complexes of ammonia and water molecules in a potassium bromide matrix is studied by means of IR spectroscopy. Ammonia and water complexes of variable composition are stabilized in a solid matrix using different approaches to saturating KBr powder with the initial components. Proton transfer can occur, leading to the formation of ammonium salts. Keywords: complex formation, ammonia, water, potassium bromide, IRspectroscopy. DOI: 10.1134/S003602441404030X
INTRODUCTION Today’s considerable interest in investigating the interaction in ammonia–water systems is due to a number of problems, e.g., studying the structure of a planet’s surface [1–4] or manufacturing high purity gases for optical or micro and nanoelectronics [5]. Ammonia is an important chemical component of the Solar System, being a reservoir of nitrogen in celestial bodies. Ammonia has been found in the atmospheres of Jupiter [1], Saturn [1], Uranus [2], and Neptune [3], and in some comets. The ice on the surfaces of these bodies contains ammonia at a level of 1% [4]. Solid ammonia and its hydrates inside icy bodies can play an important role in lowering their melting points relative to pure ice and thus ensuring heat loss by con vection [6–8]. Ammonia can therefore accelerate geo logical changes in icy bodies in the middle and outer parts of the Solar System [9]. Ammonia–water systems are usually studied by means of lowtemperature IR spectroscopy in a vari ety of matrices. The spectra of the 2NH3···H2O and NH3···H2O systems were studied in [10–12]. The IR spectra of crystalline hydrates of ammonia, obtained experimentally for water–ammonia systems with the maximum ammonia concentration (25%) in С3Н8 and СF3Cl, were studied in [13, 14]. An ammonia– water system with different contents of ammonia (from 0 to 99%) was studied over a wide range of tem peratures (10–130 K) [15]. It was found in [16] that ammonia acts as a complexing agent, due to its physi cochemical properties. Quantumchemical calcula tions show that ammonia forms stable ammonia hemi and monohydrates. A mixture of ammonia with water (~1000 ppm) is used as the initial raw material in
deep ammonia refining, and we therefore used it to study the formation of ammonia complexes with water. To study the formation of possible intermolecu lar compounds, we used a procedure we devised for the clustering of ammonia and water molecules on nano particles of a KBr matrix’s surface. EXPERIMENTAL Two methods of preparing samples according to the procedure proposed in [17] were used to study the IR spectra of ammonia and water complexes in a KBr matrix. In the first case (referred to as variant 1), KBr powder was saturated with ammonia filtered through a layer of purified water. In the second (variant 2), ammonia and water vapors were mixed in the gas phase and KBr powder was saturated with this mix ture. Bidistilled water and ammonia with purities of 99.9999% (KHORST, Russia) and ammonia of 99.98% purity were used to record standard IR spectra of water and ammonia in the KBr matrix. The KBr powder was then pressed into pellets for 30 min under a pressure of 30 MPa. Each pellet was immersed in a Dewar cell containing liquid nitrogen for 15 min at 89 K and a distance of 0.5 cm from the surface, after which the cell was placed in a spectral apparatus. IR spectra were measured on an FSM 1202 FTIR spec trometer (OOO Monitoring, Russia). The IR spectra (Fig. 1) were recorded at 500–4000 cm–1 with 30 scans and a resolution of 1 cm–1. CALCULATION PROCEDURE The Gaussian03 quantumchemical software package was used to calculate our IR spectra [18]. The
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T, %
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Fig. 2. IR spectrum of an ammonia–water mixture in the KBr matrix (variant 1).
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Fig. 1. IR spectra of (a) water and (b) ammonia in the KBr matrix.
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energy parameters of water, ammonia, and ammonia crystalline hydrates were determined earlier using the density functional theory (DFT) with full geometry optimization in the spinrestricted model using the Becke–Lee–Yang–Parr potential (B3LYP) in the 631+G(d) basis set [16]. Our calculations yielded the IR spectra of ammonia, water, and three types of ammonia crystalline hydrates. The most useful data are given in Table 1. Using the data presented in Table 1, we can distin guish vibrations corresponding to formed hydrogen bonds. Analysis of the vibrations and their intensities shows that strong hydrogen bonds of the O–H···N and O–H···O types are formed. For NH3···H2O, the vibra tional frequency is 3479 cm–1, corresponding to the O–H···N hydrogen bond. For NH3···2H2O, the vibra tional frequency (3507 cm–1) corresponds to the O– H···O hydrogen bond, while a frequency of 3286 cm–1 corresponds to the O–H···N hydrogen bond. For hemihydrate 2NH3···H2O, the frequencies at 3364 and 3325 cm–1 correspond to O–H···N and O–H···O hydrogen bonds. The vibrational frequency of these
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Fig. 3. IR spectrum of an ammonia–water mixture in the KBr matrix (variant 2).
hydrogen bonds for ammonia mono and dihydrates lies between those of N–H ammonia bonds, impeding their further identification. Based on the energy and vibrational quantumchemical calculations in [16], we may conclude that the formation of O–H···Ntype bonds is energetically more advantageous than the for mation of O–H···O bonds, and O–H···N bonds are more stable. The positions of the peaks for νas(H–O), νas(N–H), and δs(N–H) bonds change in all three ammonia crystalline hydrates as a consequence of hydrogen bonding.
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IR SPECTROSCOPIC STUDY OF THE COMPLEX FORMATION BETWEEN AMMONIA Table 1. Vibrational parameters of ammonia crystalline hy drates, calculated by DFT B3LYP/631+G(d) ν, cm–1 3815.8 3595.5 3593.1 3479.2 3462.4 1719.2 1713.0 1700.4 1144.9 3825.3 3816.3 3595.5 3557.6 3507.5 3427.5 3286.8 1731.9 1730.6 1695.5 1692.7 1186.7 3824.7 3593.1 3585.7 3554.2 3533.8 3428.7 3364.6 3325.1 1747.6 1736.1 1735.3 1701.7 1680.6 1196.5 1162.5
Types of vibrations NH3…H2O νas (H(5)–O(6)–H(7)) νas (H(3)–N(1)–H(4)) νas (H(3,4)–N(1)–H(2)) H(2,3,4)–N(1)…(H(5)–O(6)–H(7)) νs N(1)–H(2,3,4) δs (H(5)–O(6)–H(7)) + δas N(1)–H(2,3,4) δs N(1)–H(2,3,4) NH3…2H2O νas (H(5)–O(6)–H(7)) νas (H(10)–O(9)–H(8)) νas (H(3)–N(1)–H(2)) νas (H(3,4)–N(1)–H(2)) (H(7)–O(6)–H(5))…O(9)–H(8) νs N(1)–H(2,3,4) O(9)–H(8)…N(1)–H(2,3,4) δas N(1)–H(2,3,4) + δs H(10)–O(9)–H(8) + δas N(1)–H(2,3,4) + δs (H(5)–O(6)–H(7)) δs N(1)–H(2,3,4) 2NH3…H2O νas (H(5)–O(6)–H(7)) νas N(1)–H(3,4) νas N(9)–H(10,11) νas N(9)–H(8,10,11) νas N(1)–H(2,3,4) νs N(9)–H(8,10,11) N(1)–H(2)…N(9)–H(8,10,11) + O(6)–H(5)…N(1)–H(2,3,4) δas N(1)–H(2,3,4) + δas N(9)–H(8,10,11) + δs (H(5)–O(6)–H(7))
δs N(1)–H(2,3,4) + δs N(9)–H(8,10,11)
Note: νas are asymmetrical stretching vibrations, νs are symmetrical stretching vibrations, δas are asymmetrical bending vibra tions, and δs are symmetrical bending vibrations.
Table 2. Main maxima of absorption peaks (cm–1) for the samples at 89 K Type of vibrations νas (H–N) N…H–O νs (H–N) δas (H–N) + δs (H–O) δs (H–O) + δ ( NH 4 ) δs (H–N) δ (H3O+)
Variant 1 Variant 2 3402 3248 3120 1635 1458 1401 1154 1104 750
3402 3237 3158 1640 1461 1401 1120 1063 750
[10, 13, 14] ~3440 ~2950–3500 ~3330 ~1640 ~1500 ~1400 [19] ~950–1100 ~700–800 [19]
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RESULTS AND DISCUSSION The spectra of samples obtained by saturating KBr powder with ammonia filtered through the water layer (variant 1) in the region of C–H and N–H bond stretching vibrations (3600–3000 cm–1) contain only one new band at 3248 cm–1 (marked with an asterisk in Fig. 2), which corresponds to the formation of N···H–O hydrogen bonds. This confirms the formation of only one type of ammonia crystalline hydrate, i.e., mono hydrate. No such absorption is observed in the stan dard spectra of pellets prepared from KBr powder sat urated with water or ammonia vapors (Fig. 1). The wave numbers of the spectra recorded for complexes obtained with variants 1 and 2 are shown in Table 2 for identification. A new band characteristic of ammonia salts [19] appears in the mediumfrequency range at 1401 cm–1 at the same time as a band that can be attributed to the bending vibrations of water and ammonia (1640 cm–1). This suggests that an intermediate between water and ammonia molecules, n[(H2O) · (H3O+)]···m(NH3), is formed in the potassium bromide matrix. The pres ence of hydroxonium cations is indicated by the band at ~750 cm–1 [20]. In this system, hydrogen can be transferred to ammonia molecules according to the scheme +
NH3···H2O · (H3O+) → NH 4 OH– · (H2O). (1) The spectra of the pellets prepared from potassium bromide powder saturated with ammonia and water according to variant 2 in the range of 3600–3000 cm–1 display the band of the n[(H2O) · (H3O+)] · m(NH3) intermediate at ~3237 cm–1 (marked with an asterisk in Fig. 3), shifted somewhat relative to the one observed in the spectra of the pellets prepared accord ing to variant 1. The intensity of this band relative to the absorption of ammonia and water is much greater (Fig. 3). At the same time, the intensity of the band corresponding to the ammonium fragment in the mediumfrequency spectral range declines. We may therefore conclude that unlike variant 1, in which both intermediate and ammonium cations are formed, ammonia–water complex is the main product of the interaction between the components when a pellet is prepared according to variant 2. CONCLUSIONS Our experimental and calculated data show that ammonia and water complexes close in composition to ammonia monohydrate (NH3 · H2O) can form in a potassium bromide matrix. The structure of the com plex can include hydroxonium cations whose mobile protons are transferred to the ammonia molecules. ACKNOWLEDGMENTS This work was supported by the RF Ministry of Education and Science, contract no. 14.V37.21.2056 Vol. 88
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Translated by E. Yablonskaya
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