Mendeleev Communications Electronic Version, Issue 4, 1999 (pp. 129–170) EPR detection of the radical–molecule complex NH2–HF stabilised in solid argon Ilya U. Goldschleger, Alexander V. Akimov and Eugenii Ya. Misochko* Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russian Federation. E-mail: misochko@icp.ac.ru The radical–molecule complex NH2–HF is stabilised in a solid argon matrix as an intermediate species in the chemical reaction of F atoms with NH3 molecules.The initial interest to study the atom–molecule chemical reaction in a gas phase: was motivated by attempts to use it as a source for chemical lasers.1 However, these attempts to obtain an inverse HF excitation in reaction (1) were unsuccessful, despite the high exothermicity. 2,3 It was assumed that the reason was the formation of long-lived FNH3 intermediate species during the reaction. The existence of this intermediate complex can cause randomisation of the excess energy among its internal modes. The ab initio calculations performed by Goddard et al.4 predicted the formation of the NH2–HF complex in the exit channel of reaction (1) with the binding energy ~12 kcal mol–1.We have observed stabilised NH2–HF complexes formed in reaction (1) and measured their hyperfine (hf) constants for the first time. A comparison of experimental hf constants with the calculated data shows that the structure of the complex is close to that predicted by Goddard et al.4 Recently we have proposed5 an experimental technique to study reaction (1) in a solid argon matrix at cryogenic temperatures.This approach is based on the widely used matrix isolation technique and high mobility of fluorine atoms in solid argon.6 It was shown that translationally excited fluorine atoms generated by UV photolysis of F2 molecules migrate through several lattice periods, whereas thermal F atoms diffuse in Ar at T > 20 K.Upon heating above 20 K, diffusing F atoms react with impurity molecules. The crystalline environment, which prevents reaction products from flying apart and promotes fast relaxation of excess energy, results in stabilization of reaction intermediates. EPR spectroscopy allowed us to detect and to identify the radical products of atom–molecule reaction (1).7–9 Two facts drastically diminish inhomogeneous broadening of EPR spectra: zero nuclear spin of Ar and homogeneous distribution of radicals, [R] £ 1016 cm–3, in the sample.Hyperfine interaction of an unpaired electron with magnetic nuclei in the radical–molecule complex, which is an intermediate of reaction (1), is very sensitive to the distance between the radical R and the molecule HF and to their mutual orientation.Therefore, the geometry of the complex can be determined from a comparison of the measured hf constants with the results of quantumchemical calculations. The experimental technique has been described elsewhere.8 Solid argon films with the reactant molecules F2 and NH3 were formed by vapour deposition of the gases through two separate gas inlets onto a cold substrate at 14 K.Typically, we used the dilution ratio Ar:F2:NH3 = 1000:1:1. Fluorine atoms were generated by UV photolysis of F2 molecules at l = 337 nm (N2 pulsed laser, average power of 20 mW). The EPR spectra of freshly prepared samples exhibit no lines due to paramagnetic species. Photolysis of the samples with Ar:F2:NH3 = 1000:1:1 at 7.7 K leads to the appearance of a complex anisotropic spectrum.Temperature changes in the spectra of photolysed samples are reversible in the region 7.7–18 K and make it possible to distinguish two paramagnetic species generated during photolysis. The EPR spectrum of one of these species consists of nine narrow lines: two triplet groups with hf splitting of 1.05 and 2.40 mT, and g = 2.0058. Both the hf constants and the g-factor are in good agreement with published data10 for the radical NH2 in solid Ar.It allows us to conclude that one of the photolysis products is the free radical NH2, which is formed in reaction (1) between a translationally excited F atom with an NH3 molecule. The EPR lines of the other species are anisotropic and exhibit strong reversible temperaturedependent broadening.They become practically invisible at 7.7 K. To initiate reactions of thermal F atoms, we annealed the photolysed samples at T > 20 K. A comparison of the spectra before and after annealing shows that the concentration of NH2 radicals remained unchanged, whereas the intensity of lines of the other radical increased by a factor of four becuase of the reaction of diffusing F atoms.Upon heating above 30 K, the lines of this radical become narrow and isotropic [Figure 1(a)]. The EPR spectrum consists of 14 lines with a width of 0.10 mT and corresponds to three hf splittings: the triplet 1:1:1 with aN = 1.20 mT, the triplet 1:2:1 with aH = 2.40mT, and the doublet with aF = 0.70 mT. Because aN ª 2×aH, four lines of the spectrum are compound lines. Thus, only 14 lines are resolved in the spectrum, instead of 18 lines corresponding to this assignment.To ascribe the hf coupling constants to magnetic nuclei, a series of similar experiments with isotopically substituted NH3 was carried out. The EPR spectra obtained after annealing of the photolysed Ar/F2 /15NH3 and Ar/F2/14ND3 samples are shown in Figures 1(b) and 1(c). The isotopic substitution for nitrogen atom, 14N (S = 1) 15N (S = 1/2), F + NH3 HF + NH2; DH0 0 ª –31 kcal mol–1 (1) (a) (b) (c) aF a14N aH aF a15N aH aF aD a14N 311 314 317 320 H/mT Figure 1 EPR spectra of the samples after photolysis at 15 K and subsequent annealing at 25 K: (a) Ar/14NH3/F2; (b) Ar/15NH3/F2; (c) Ar/14ND3/F2. All spectra were recorded at 35 K.(The 14ND3 used in experiments contained ~10% 14NH3.Therefore, weak outer lines in the spectra correspond to the complex 14NH2–HF).Mendeleev Communications Electronic Version, Issue 4, 1999 (pp. 129–170) leads to the replacement of the triplet aN (14N) = 1.20 mT with the doublet aN (15N) = 1.55 mT. In Ar/F2/14ND3 samples, only the triplet aH = 2.40 mT is replaced by the quintet aD = 0.37 mT. These findings allows us to assign unambigously the hf constants aN and aH to the NH2 group.At the same time, the doublet splitting 0.70 mT (which remained unchanged by the above isotopic substitution) should be attributed to the 19F atom, because it is the only atom having the magnetic nucleus S = 1/2 in the system. Since NH2F is a closed-shell molecule, we can attribute the considered EPR spectrum to the radical–molecule complex NH2–HF, assuming that the hf constant at the H atom of the HF molecule aH is less than 0.05 mT.We carried out quantum-chemical computations in order to clarify the structure of the NH2–HF complex. All of the calculations were performed using the GAUSSIAN-94 program11 at the N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences (grant no. 98-07-90290 from the Russian Foundation for Basic Research). The density functional method B3LYP with the EPR-3 basis set12 was used. The calculated steady-state configuration of the complex NH2–HF corresponds to the collinear C2v geometry, which is similar to that calculated earlier by Goddard et al.4 The binding energy of the complex is equal to 12.3 kcal mol–1.The geometry and hf constants of the complex are given below: The calculated hf constants aN, aH and aF are in good agreement with the experimental data. The hf constant aH' at the proton of the HF molecule is less than 0.05 mT, and this splitting cannot be resolved under given experimental conditions. Therefore, the radical–molecule complex NH2–HF is the main product in the reactions of thermal F atoms with NH3 molecules in a solid argon matrix: We optimised the arrangement of the complex in an argon lattice starting from the calculated structure.Each isolated NH3 molecule occupies a singly substitutional argon lattice site; therefore, we anticipated that the NH2 group of the complex is also located at the same type of sites. The minimisation of the total energy of a doped argon cluster containing 365 atoms was done using molecular dynamics simulation as described in ref. 8. The resulting configuration of the complex is shown in Figure 2. The position of the nitrogen atom of the NH2 group is close to the substituted site (0, 0, 0). The fluorine atom of the HF molecule occupies the nearest octahedral interstitial site Oh (–a/2, 0, 0), where a = 0.54 nm is the fcc lattice parameter. This is possible only due to the fact that the distance between F and N atoms in the complex is close to the one-half lattice period a/2.This commensurability leads to a minor deformation of the complex in the crystal lattice: the distance between the NH2 radical and the H atom of HF is 0.005 nm shorter than that in the gas-phase complex, and the out-of-plane deformation of the complex does not exceed 4°.These distortions do not result in changes of the hf constants with respect to those obtained for an equilibrium geometry of the complex. In summary, the radical–molecule complex NH2–HF was observed for the first time as an intermediate product in reactions of mobile F atoms with NH3 molecules in solid argon.The EPR spectrum of the complex is characterised by three hyperfine constants (aN = 1.20 mT, aH = 2.40 mT, and aF = = 0.70 mT). The hf constant for the H atom of the HF molecule is less than 0.05 mT. The quantum-chemical calculation revealed that the NH2–HF complex has a planar C2v structure and a binding energy of 12.3 kcal mol–1. The calculated hf parameters of the complex and the experimental data are in good agreement.The complex in somewhat distorted in an argon lattice with respect to its equilibrium geometry in the gas phase. This work was supported by the Russian Foundation for Basic Research (grant no. 98-03-33175). References 1 W. H. Duewar and D. W. Setser, J. Chem. Phys., 1973, 58, 2310. 2 D. J. Donaldson, J. J. Sloan and J. D. Goddard, J. Chem.Phys., 1985, 82, 4524. 3 S. Wategaonkar and D. W. Setser, J. Chem. Phys., 1987, 86, 4477. 4 D. Goddard, D. J. Donaldson and J. J. Sloan, Chem. Phys., 1987, 114, 321. 5 V. A. Benderskii, A. U. Goldschleger, A. V. Akimov, E. Ya. Misochko and C. A. Wight, Mendeleev Commun., 1995, 245. 6 J. Feld, H. Kunti and V. A. Apkarian, J. Chem. Phys., 1990, 93, 1009. 7 E. Ya. Misochko, V. A. Benderskii, A.U. Goldschleger and A. V. Akimov, J. Am. Chem. Soc., 1995, 117, 11997. 8 E. Ya. Misochko, V. A. Benderskii, A. U. Goldschleger, A. V. Akimov, A. V. Benderskii and C. A. Wight, J. Chem. Phys., 1997, 106, 3146. 9 A. U. Goldschleger, E. Ya. Misochko, A. V. Akimov, I. U. Goldschleger and V. A. Benderskii, Chem. Phys. Lett., 1997, 267, 288. 10 S. N. Foner, E. L. Cochran, V.A. Bowers and C. K. Jen, Phys. Rev. Lett., 1958, 1, 91. 11 M. J. Frisch, G.W. Trucks, H. B. Schlegel, P. M.W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez and J. A. Pople, Gaussian-94, Revision D.1, Gaussian, Inc., Pittsburgh PA, 1995. 12 V. Barone, in Recent Advances in Density Functional Methods, ed. D. P. Chong, World Scientific, Singapore, 1995, part 1, p. 287. x y z Ar F H N H H Figure 2 Arrangement of the complex NH2–HF in an argon lattice. Twelve nearest neighbouring Ar atoms are shown. H F H' N H 0.937 Å 1.783 Å 1.022 Å 105.14° a(calc)/mT –0.70 0.02 1.15 –2.30 a(exp)/mT –0.70 < 0.05 1.20 –2.40 (2) F + NH3 NH2–HF (3) Received: 16th March 1999; Com. 99/1463