Formation of the HHF complex in the reaction of thermal fluorine atoms with hydrogen molecules in solid Ar

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21 March 1997

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CHEMICAL PHYSICS LETTERS

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ELSEVIER

Chemical Physics Letters 267 (1997) 288-293

Formation of the H-HF complex in the reaction of thermal fluorine atoms with hydrogen molecules in solid Ar A.U. Goldschleger, E.Ya. Misochko, A.V. Akimov, I.U. Goldschleger, V.A. Benderskii Institute of Chemical Physics in Cherru~golovka of the Russian Academy of Sciences, Chernogolovka 142432, Moscow Region, Russian Federation Received 20 November 1996

Abstract The reaction of F atoms with H 2 in an Ar matrix was studied with EPR spectroscopy. F2 photolysis produces two well-known doublets of the H atom trapped in different crystal sites. Heating gives rise to an EPR spectrum assigned to the H2F complex (A H = 50.8, A F = 1.85, A n ~<0.15 mT), the formation of which is attributed to the reaction of diffusing thermal Ft atoms with H 2. The shape and width of the spectrum of the complex changes drastically with temperature due to the transitions between two stable configurations: A (a F = 3.2 mT, a H ~<0.1 mT) and B (a v = 0.5 mT, a H = 0.4 mT) with an Arrhenius-like dependence of rate (E a = 80 cal/mol, w o ~ 3 × 101° s- l).

1. Introduction

( v = 3826 cm -~ ) formed from a F 2 - H 2 pair trapped in the matrix upon deposition:

The reaction of an F atom with an H 2 molecule,

F+H 2 ~

HF(v')+H,

A H 0 = - 32.07 k c a l / m o l

(1) is one o f the most extensively studied chemical reactions [1-5]. Recent theoretical work has predicted [6,7] that the minimum energy path o f reaction (1) involves the weakly bonded state H2F, which was not observed in molecular beam experiments because the products fly apart. FTIR spectroscopy studies by Andrews et al. [8,9] demonstrated that the photolysis of F 2 - H z mixtures in an A r matrix mainly yields the (HF) 2 dimer

hv

[F2...H2]

~

[HF...HF]

(2)

They also detected a weak band at 3900 c m - J and tentatively assigned it to the H F stretch in the H - H F complex. Heating to 17 K results in the appearance of IR bands assigned to the H F clusters, while the band of the complex disappears. Being widely used in matrix isolation studies, FTIR spectroscopy reliably identifies molecular products formed due to reactions in pairs and larger clusters, whereas the IR bands o f radical products possess low intensities which make their assignment difficult. At the same time, EPR spectroscopy allows the detection and accurate assignment of the radical products of a t o m - m o l e c u l e reactions, i.e. the mod-

0009-2614/97/$17.00 Copyright © 1997 Published by Elsevier Science B.V. All rights reserved. PH S 0 0 0 9 - 2 6 1 4 ( 9 7 ) 0 0 1 1 5 - 2

A.U. Goldschlegeret al./ ChemicalPhysicsLetters267 (1997)288-293 elling of true gas phase reactions of type (1) at zero excitation energy in crystalline matrices. This opportunity can be realised if an atom produced by photolysis migrates from the parent site to a distant partner-molecule. Apkarian et al. [10] have demonstrated that F atoms in an Ar matrix are appropriate species to achieve this goal: translationally excited atoms migrate through several lattice spacings, while above 20 K thermal atoms diffuse over the distance of ~ 10 nm on a timescale of ~ 103 s. In recent papers we have used F atoms to obtain and study intermediate species of the F + RH reaction in ternary mixtures A r / F 2 / R H with EPR technique [11-14]. Photolysis of F 2 below 15 K yields isolated radicals and stabilised F atoms. Upon heating above 20 K diffusing F atoms react with impurity molecules. Because the relative kinetic energy of the products is low they cannot fly apart and thus radical complexes are frozen in the matrix. This situation provides a unique opportunity to explore the bonded states of gas phase reaction products. In particular, we have detected the hydrogen-bonded CH 3-HF complex in the UV irradiated A r / C H 4//F2 system [13,14]. In this Letter we report the identification and properties of the radical HzF complex produced in the reaction of thermal F atoms with H 2 molecules isolated in an Ar matrix.

2. Experimental details The experimental technique of matrix isolation EPR spectroscopy applied in this work is described elsewhere [11,14]. The samples were prepared by vacuum co-deposition of gas mixtures Ar: H 2 ( A r / D 2) and Ar: F 2 from two separate jets onto a sapphire rod (5 × 2 × 50 mm 3) held at 13 K. Reactant concentrations in a typical sample were A r : H 2 :F 2 = 1000:1 : 1. The sample width was ~ 100 ixm. No radicals or H atoms were detected prior to UV irradiation. The samples were irradiated through the window in the microwave cavity. Photolysis of F 2 at h = 337 nm (pulsed N2-1aser, repetition frequency 1000 Hz, average power 30 mW) produces translationally excited F atoms with kinetic energy ~ 1 eV per atom.

289

3. Results and discussions 3.1. Photolysis and subsequent annealing of Ar / H 2 / F2 samples UV irradiation at 14 K leads to the accumulation of stabilised hydrogen atoms, the EPR spectrum of which is demonstrated in Fig. 1. The spectrum consists of two doublets, assigned to H atoms trapped in different crystal sites. The parameters of the spinHamiltonian of atoms I and II are similar to those reported for H atoms in an Ar matrix generated by the photolysis of HBr or HI: I a H = 50.46 mT, II a H = 50.76 mT, g = 2.0026 [15,16]. The concentrations of I and II ([I]/[II] = 1.5) are proportional to time in the beginning of photolysis and then reach the limiting value of ~ 1016 cm -3. The concentration of the atoms remains constant for several hours at 14K. After photolysis the samples are heated at a rate of ~ 0.2 K / m i n up to 25 K. From 18 to 19 K EPR spectrum of a new radical I I I appears. This spectrum consists of two lines on the outside of lines I and II (see Fig. 1). The accumulation of radical III is not accompanied by any changes in the concentrations of I and II. The concentration of III increases up to a limiting value of 3 X 1016 cm -3 at 25 K. The shape of its spectrum depends drastically on temperature as demonstrated in Fig. 2. Let us emphasise that these changes are completely reversible in the temperature range 7-25 K. The spectrum III irreversibly disappears above 27 K in several minutes, but this does not affect the intensities and shapes of lines I and II, which disappear at 41 and 32 K, respectively. II

'J~--

-

-

------J~,~w------"~ III

, . ,

~,~- ~__

~ •

14K 17K 19.5 K 22 5 K

~ - - - 28.5 K , , , . , . , 338,5 339,5 340.5 341.5 H, mT

Fig. 1. EPR spectrum of the H atom at different temperatures (only mH= ½lines are shown). The curve at 26.5 K demonstrates the decompositionof the complex.

290

A.U. Goldschleger et al./ Chemical Physics Letters 267 (1997) 288-293 IV/I IV

76K

(a)

(b)

//

(b)

10.2 K

t"

~

7.6 K IV

'd

K

~ ~

)

~

~t~V,j ~ 338

19.7 K

340

24.7K

232 K

25.2 K

266 K 342

344

338,0

H, mT

340,0

3~2

342,0

H, mT

Fig. 2. Temperature dependence of the shape of the H-HF spectrum obtained in several wanning-cooling cycles: (a) differential spectra; (b) integrated spectra; intensities of the spectra are corrected in accordance with Curie's law.

3.2. Formation o f H e F (D 2 F) complexes The formation of H atoms upon F 2 photolysis in solid argon includes the photodissociation of F2: F2

14,6K

K

19.8 K

fl

336

~.5.8

337 nm ~ F* + F ~

and the reaction of " h o t " molecules: F* + H 2 ~ H + H F *

(3) atoms [10] with H 2 (4)

Since the concentration of H 2 molecules is low, a majority of F atoms are stabilised in the Ar lattice. The H atoms of types I and I I are formed exclusively in reaction (4), whereas radicals I I I appear in the temperature region where thermal atoms F t are able to diffuse [10,14]. This allows us to attribute I I I to the product of the reaction of F1 with isolated H 2. To assign spectrum I I I additional experiments with A r / D 2 / F 2 were carried out. Photolysis of F 2 at 14 K yields the well-known triplet of D atoms [15]. As for A r / H 2 / F 2 samples, heating above 19 K gives rise to a new EPR spectrum demonstrated in Fig. 3. The new spectrum disappears above 27 K similarly to spectrum I I I in A r / H 2. Each component of the 1 : 1 : 1 triplet of the D atom ( S = 1, A l = 7.67 mT) is split into a doublet (S = ½, Aj/2 = 1.6 roT). Thus EPR spectrum I I I in Ar/H 2 at 24 K is interpreted as an H atom doublet ( A n -= 50.8 mT) additionally split on two non-equivalent nuclei with spin 1 ~-. The corresponding hyperfine (hf) constants are

~3

32.

H, m'r

325

328

__/ 322

323

~

L_J: L25.2_K 324

H, mT

325

326

Fig. 3. Temperature dependence of the shape of the D-DF spectrum: (a) differential spectra; (b) integrated spectra. Only the mD = 1 component is shown. 1.8 and ~ 0.2 mT (the latter splitting is less than the linewidth in A r / D 2 since '~H//'~D ~ 6.5). A comparison of spectra I I I in A r / H 2 / F 2 and A r / D 2 / F 2 samples allows us to make the following conclusions on the structure of the radical: (a) it contains two atoms H (D) and one F atom; (b) almost all the spin wavefunction is localised on a proton (this condition is required to obtain such a large hf splitting which is a distinguishing feature o f the free H atom); (c) the larger additional splitting belongs to 19F in both systems A r / H 2 and A r / D 2, whereas A'~/2 = 0.2 mT belongs to the proton of HF. In the A r / D 2 / F 2 system it is less than the linewidth, ~< 0.03 mT. The above reasoning implies that I I I belongs to H atoms bonded with an H F molecule, i.e. an H 2 F ( D z F ) complex. Hence the radical complex is the only product of the reaction of Ft with H 2 (D2) in Ar: F 1 + H 2 ( D 2 ) --* H 2 F ( D 2 F )

(5)

3.3. Slow internal motion in the H e F complex Let us consider the temperature dependence of the shape and linewidth of the DzF spectrum (see Fig. 3) which is more simple than the spectrum o f H2F. At T = 25 K the widths of perfectly isotropic lines I I I are ~ 0.15 roT. Upon cooling they crucially broaden (up to ~ 1.2 mT at 10 K) and shift towards lines I and II. Below 20 K in addition to I I I we observe a broad background spectrum, which narrows upon further cooling and becomes resolved below 15 K (in

291

A.U. Goldschleger et a l . / Chemical Physics Letters 267 (1997) 288-293

Fig. 3a the spectrum is marked with IV). Spectrum IV contains a doublet of 1 : 1 : 1 triplets ( a D, = 0.07 mT, a F = 0.5 mT). In the A r / H 2 system spectrum IV is observed below 9 K (see Fig. 2a). It retains the greater splitting a F = 0.5 roT; the splitting on the proton is about 0.45 mT, being in accordance with the ratio TH/TD- The inner lines of quartet IV are masked by the intense spectra I and II. The contribution of spectrum IV to the total spectrum of complexes ( I I I + IV) is ~ 20% in both systems. The listed changes of the spectrum are usually ascribed to the effect of slow motion [17,18]. According to calculations [6,7], the equilibrium distance between the H atom and the HF molecule in the complex is ~ 3 ,~. Since broadening of the lines of the complex (up to ~ 1.0-1.5 mT) far exceeds the expected anisotropy of the hf interaction with distant HF nuclei (~< 0.1 mT), the origin of the observed temperature changes is thermally activated transitions among different stable conformers of the complex. Each of them is characterised by different hf constants a F and az,. Exchange among the conformers results in a modulation of the isotropic hf interaction with the HF nuclei. The most simple model considers conformers to be energetically equivalent. However, it fails to explain the temperature behaviour below 20 K. Within the model the centre of the spectrum retains its position, whereas line I I I shifts towards lines I and II (see Fig. 3). This testifies the increasing population of configurations with smaller hf splittings. Below l0 K (i.e. in the slow exchange region) instead of the expected spectrum of both stable configurations (two narrow lines for each component (mF,mH,)) we observe only spectrum IV, corresponding to one of the configurations. Because of this we try to simulate the experimental spectra within the model of exchange between two energetically non-equivalent configurations A and B. The equations of motion of the density matrix elements, pq, of configuration A read (see e.g. Refs. [17,19]):

A= 2iV[ p,2e - "0' _ ( PI2 )* e~O,,] -- tOe J + ~Oe ~ -- T ? ' ( A -- 3 ( ° ) ) [)12 = i V A e i ° ' t

+

+ t5e t5~2

[ i t o A - ( T 2 ' + toe)]

P,2 (6)

where A = p l I --/922; V = TeHl; H l is the microwave magnetic field of frequency to = 7e" H; tog is the Zeeman frequency; toe = tOo exp[-Ea/T] and &e = tOo e x p [ - ( E a - AE)/T] are the frequencies of transitions A ~ B and B ~ A, respectively; AE is the difference in energies of A and B; Tt and T2 are longitudinal and transversal spin-relaxation times. The variables (/~, Pl2) characterise the evolution of configuration B with resonance frequency toB and the same relaxation times. The solution of Eqs. (6) within the linear response approximation gives the following shape of each line (m F, raN): G(to) = {(t% - ~oA)2(pA% + PB ~ ) + 2T~

x { [r~ '(toe + '~e + T; ' ) - (~, - to~)(~o - ~o.)l ~ + [ to(~oe + ,~e)- ('~e toA + ~'o~o~)]"} -'

(7)

where PA,a = [1 + exp( + A E/T)]- I. It follows from Eq. (7) that at fast exchange (toe + ~e > ItoB -- toAD as in the case of h E = 0 the spectrum consists of one Lorentzian line with width: TeA H I / 2 = (tOA

-

-

rOB) 2

2toz& 2 X

~

+T2 ~

4

(toe+toe) [P tOe

(8) In the region of slow exchange the lines are separated. Their widths and intensities are different and depend on A E:

"yeAn[#)2=tOe+r;',

G(tOA) -----pA

%A - - "r4(r3) 1 / 2 = ~e + T~- l ,

G(tos)

(9)

~PB

Spectra I I I and IV of D2F are fitted within the suggested model. Simulated and observed spectra are compared in Fig. 4. The best-fit parameters are: III A E < 3 K , E a = 6 0 K , t O 0 = 3 X l 0 m rad/s. T h e h f constants of the conformers equal: A a F = 3.2 mT, a D, < 0.1 mT; B a F =- 0.5 mT, a D, - 0.07 mT. The exchange frequency remains high, ~ 2 × l0 s s -~ even at 10 K. The parameters for the spectrum IV are: AE = 25 K, E, =- 120 K, too = 3 × 10 m rad/s. Its hf constants are the same as for III. Below 10 K, spectrum IV corresponds to the pure configuration B since configuration A is not populated ( A E > T).

A.U. Goldschleger et al. / Chemical Physics Letters 267 (1997) 288-293

292

~::~ .f

-~:.'

~'.

~

--

experiment

.........

III

-. . . . . .

IV ....

11.9K

...:/.!~

14.6K

......~

19.7K

~

_ ,...

L

324

~ ....

, ....

325 326 H, mT

25.2K , 327

Fig. 4. Comparison of simulated and observed spectra of the D - D F complex. Line ! of free D atoms is subtracted, only one

componentof the D' triplet (mD= 1, m D, = 1, mF = ½) is demonstrated for spectrum IV. The H2F spectrum is described with the same values of a F, the hf splittings on the proton are: A a n < O.1 mT; B ar~ - 0.4 mT. The temperature dependence of toe is characterized by the same prefactor, too = 3 × 10 ~° rad/s, whereas the height of the barrier separating the configurations is lower than that for DaF: III E a = 40 K ( A E = 0, the experimental spectra reveal no difference in energies of A and B even at 7 K); IV: E ~ = 8 0 K (AE=- 20 K). The widths of lines of H2F in the region of fast exchange are different because: IAFAI> IABF[, Ia~l < l a ~ l

(10)

so that:

mF _t/, -V_. mH -~½ +V2

"" J "

I

I

+V2 -V;~

+% +~

i ",

i

(11)

Y f

'lll //

1 -- I

l

tion (1). Because of inequality (10), the H - F distance in conformer B is larger than that in A, whereas the H - H ' distance in B is less than that in A. Thus we suggest that the stable conformer B corresponds to H-HF. Configuration A can be tentatively assigned to the hydrogen-bonded H - F H complex stabilised in the matrix. The energy of the isolated H - F H complex is ~ 100 K [7] higher than that of H - H F (without zero-point energy correction). It is worth mentioning that spectra I I I and IV belong to different types of complexes. The differences in E a and A E arise from different crystalline environments rather than differences in the geometries of the conformers of both types because their hf constants are similar. As we believe, the hf constants found for the complexes open an opportunity for further elucidating the structure of the weakly-bonded states of the products of reaction (1).

4. Conclusion The present study revealed two reaction channels in UV irradiated A r / F 2 / H 2 samples. The reaction of translationally excited F atoms with H 2 gives isolated H atoms stabilised in the Ar lattice. In contrast, the reaction of thermal F atoms yields uniquely H2F complexes. The complex is stable over a wide temperature range, 7 ~< T ~< 27 K, and possesses two energetically inequivalent configurations. The most stable configuration is tentatively assigned to H-HF. Thus we have observed the theoretically predicted [6,7], bonded state in the exit channel of reaction (1). The data obtained in the present study and our recent papers [13,14], demonstrate the possibility of observing and investigating weakly-bonded states of products which decompose on a femtosecond timescale if the reaction is carried out in the gas phase.

1

A(___ ~,_+ 3) < A ( _ + ~ , + ~ ) ( A ( _ + ~ , + ~ ) = 1.1 1 I roT, A(_+ ~,T-~-) = 1.6 roT), and according to formula (8) the width of inner lines is greater than that of the outer lines. Recent calculations [6,7], have predicted a collinear H - H F complex in the exit channel of reac-

Acknowledgements The authors wish to thank the Russian Foundation for Basic Research (Grant No. 95-03-08509) for financial support of this work.

A.U. Goldschleger et al./ Chemical Physics Letters 267 (1997) 288-293

References [1] T.S. Schafer, P.E. Siska, J.M. Parson, J.M. Parson, F.P. Tully, Y.C. Wong and Y.T. Lee, J. Chem. Phys. 53 (1970) 3385. [2] M.J. Berry, J. Chem. Phys. 59 (1973) 6229. [3] D.S. Perry and Polanyi, Chem. Phys. 12 (1974) 419. [4] M.J. Berry, J. Chem. Phys. 59 (1973) 6229. [5] D.M. Neumark, A.M. Wodtke, G.N. Robinson, C.C. Hayden, R. Shobatake, R.K. Sparks, T.P. Schafer and Y.T. Lee, J. Chem. Phys. 82 (1985) 3067. [6] S.I. Mielke, G.C. Lynch, D.G. Truhlar and D.W. Schwenke, Chem. Phys. Lett. 213 (1993) 10. [7] K. Stark, H.-J. Werner, J. Chem. Phys. 104 (1996) 6516. [8] L. Andrews and G.L. Johnson, Chem. Phys. Lett. 96 (1983) 133. [9] R.D. Hunt and L. Andrews, J. Chem. Phys. 82 (1985) 4442. [10] J. Feld, H. Kuntu and V.A. Apkarian, J. Chem. Phys. 93 (1990) 1009.

293

[11] E.Ya. Misochko, V.A. Benderskii, A.U. Goldschleger and A.V. Akimov, Mend. Commun. 5 (1995) 198. [12] V.A. Benderskii, A.U. Goldschleger, A.V. Akomov, E.Ya. Misochko and C.A. Wight, Mend. Commun. 6 (1995) 203. [13] E.Ya. Misochko, V.A. Benderskii, A.U. Goldschleger, A.V. Akimov, A.V. Benderskii and C.A. Wight, J. Chem. Phys., in press. [14] E.Ya. Misochko, V.A. Benderskii, A.U. Goldschleger, A.V. Akimov, A.V. Benderskii and C.A. Wight, J. Chem. Phys., in press. [15] S.N. Foner, E.L. Cochran, V.A. Bowers and C.K. Jen, J. Chem. Phys. 32 (1960) 963. [16] F. Adrian, J. Chem. Phys. 32 (1960) 972. [17] A. Abragam, The Principles of Nuclear Magnetism (Clarendon Press, Oxford, 1961). [18] C.P. Slichter, Principles of Magnetic Resonance (SprmgerVerlag, Berlin, 1980). [19] B.M. Fain and Ya.l. Khanin, Quantum Radiophysics ("Soy. Radio", Moscow, 1965; in Russian).