Infrared spectra of molecular ions derived from the hydrogen and methyl halides trapped in solid neon

Journal of Molecular Structure 449 (1998) 111–118

Infrared spectra of molecular ions derived from the hydrogen and methyl halides trapped in solid neon Catherine L. Lugez1, Daniel Forney1, Marilyn E. Jacox1,*, Karl K. Irikura2, Warren E. Thompson1,2 National Institute of Standards and Technology, Gaithersburg, MD 20899, USA 3 Received 21 January 1997; accepted 7 April 1997

Abstract A number of molecular ions have been stabilized in solid neon in sufficient concentration for detection of their infrared spectra. The neon matrices were prepared by codepositing the ion precursor, diluted in an excess of neon, at approximately 5 K with a beam of excited neon atoms. The results of these experiments are surveyed, and the studies of the hydrogen halides (X = Cl, Br, I) and methyl halides (X = F, Cl, Br) are described in greater detail. The position of the n+2 absorption of uncharged (HX) 2 is exceptionally sensitive to the matrix material. Rotation of simple hydrides, including H 2O, HX, and CH 3, in solid neon is inhibited by the electric field associated with the presence of ions. The fundamental absorptions of HX + in the neon matrix lie near the gas-phase band centers. Absorptions of (HX)+2 are also identified, and the XHX − anion contributes to the infrared spectrum. In studies of the methyl halides, infrared absorptions of both the conventional (CH 3X +) and the ylidion (H 2CXH +) isomers are identified and assigned with the aid of ab initio calculations. The conventional structure is significantly distorted from threefold symmetry by Jahn–Teller interaction. q 1998 Elsevier Science B.V. Keywords: CH 3X +; HX +; H 2CXH +; (HX) 2; (HX)+2

1. Infrared spectra of molecular ions Although the infrared spectra of molecular ions in liquid solution and in crystalline materials are familiar, spectral data for these species in the gas phase or in non-interactive media are sparse. Because many small molecular ions are so highly reactive that the probability of their reaction is greater than 10% on a single collision with an atom or molecule, their gasphase spectroscopic study is extraordinarily difficult. * Corresponding author 1 Optical Technology Division. 2 Physical and Chemical Properties Division. 3 Technology Administration, U.S. Department of Commerce.

Recently, we have used the photoionization and Penning ionization of small molecules by neon atoms excited to between 16.6 and 16.8 eV in a microwave discharge to produce molecular cations, which are then trapped at approximately 5 K in an excess of solid neon. In an earlier generation of experiments, a similar sampling configuration was used with argon atoms excited in a microwave discharge and an argon matrix [1]. However, most of the argon atoms were excited only to between 11.5 and 11.8 eV, an energy range somewhat below that necessary to photoionize many small molecules. Knight and co-workers [2,3] demonstrated that when neon is substituted for argon, in order to take advantage of the higher energy of the

0022-2860/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 02 2- 2 86 0 (9 8 )0 0 45 5 -4

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first excited states of neon atoms, sufficient concentrations of molecular cations are stabilized in the neon matrix for studies of their electron spin resonance spectra. In order to test whether this configuration could produce a concentration of ions sufficient for infrared studies, Jacox and Thompson [4] conducted experiments on Ne:CO 2 samples. The n 3 fundamental of CO+2 was readily detected only 1.4 cm −1 from the gas-phase band center. Electrons which result from the ionization process can be captured by molecules both in the gas phase and in the solid deposit, through which they can diffuse. Consequently, molecular anions are also stabilized. In the Ne:CO 2 experiments, a prominent absorption contributed by n 3 of CO¯2 was also present. Subsequent experiments in this laboratory have yielded a considerable body of information regarding the properties of this sampling configuration and the infrared spectra of molecular cations and anions in a neon matrix. It was soon discovered that the technique could also be used to study the infrared spectra of dimer ions, such as O+4 and O−4 [5,6]. These interesting species are considerably more strongly bound than are van der Waals dimers, and very little spectroscopic data have heretofore been available for them. Other dimer ions for which infrared spectra have been obtained using this type of sampling include N+4 [7] and (CO)+2 and (CO)−2 [8]. The close correspondence of two bands in the high-resolution zero kinetic energypulsed field ionization photoelectron spectrum of gasphase (NO) 2 [9] to the neon-matrix positions of infrared absorptions which had initially been assigned to trans-ONNO − [10,11] dictated the reassignment of these bands to trans-ONNO +. Products of several other ion–molecule reactions have also been detected in the ion stabilization experiments. Three absorptions of the CO 2 moiety in CO 2…O−2 were identified [12], as was the n 3 absorption of NO−3, produced from Ne:NO:O 2 samples [13]. Moreover, photoionization of N 2O and subsequent dissociative electron capture by N 2O provided a source of O − suitable for obtaining infrared spectra of NNO−2 [14] and O−3 [15], formed by the reaction of O − with N 2O and O 2, respectively, in the neon matrix. The infrared spectra of the products indicate that, under the sampling conditions used for these experiments, there is very little backstreaming of the parent molecule into the discharge region. The yield of cations resulting from dissociative ionization

is limited to that characteristic of processes which occur below 16.8 eV; fragment ions which have appearance potentials higher than that energy are not observed. The limited extent of fragmentation has been very useful for identifying the cations formed from a number of simple fluorides, including CF 4 [16], SiF 4 [17], BF 3 [18], PF 5 [19], and SF 6 [20], which have relatively high ionization potentials. Where both gas-phase and neon-matrix data are available, the neon-matrix absorption of the ion usually lies within about 1% of the gas-phase band center. The comparison shown in Fig. 1 includes data obtained since the publication of a review of matrix shifts [21] in 1994. A recent gas-phase study of the N+4 dimer cation [22] demonstrated that the gas-phase band center for n 3 of that species lies only 3.1 cm −1 below the absorption in a neon matrix [7]. This observation, together with the good correspondence already noted for n 5 of trans-ONNO +, suggests that the above generalization is also true for some vibrations of dimer ions. Larger shifts are sometimes observed in argon-matrix experiments. While the neon-matrix shifts for n 5 of trans-ONNO + [9] and for n 3 of HCCH + [23] are less than 2 cm −1, their argon-matrix shifts are 25 cm −1 [11] and 30 cm −1 [23], respectively. Exposure of the ionic species in the solid deposit to various ranges of visible and ultraviolet radiation is helpful not only as an aid in product identification but

Fig. 1. Deviation (%) of the positions of infrared absorptions of molecular ions trapped in solid neon from the corresponding gasphase band centers.

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also as a source of information regarding the photostability of ionic species. Photodissociation of BF+3 into BF+2 + F [18] and of BCl+3 into BCl+2 + Cl [24] have been observed. Furthermore, reversible photoisomerization between HCN + and HNC + has been studied [25]. Electron detachment from anions, with subsequent capture of the electrons by cations or by other species, is observed in most systems. Because NO−2 has a higher energy photodetachment threshold than do many other anions, it is sometimes possible to selectively photodetach a less strongly bound anion. Capture of the released electrons by NO 2 in the sample results in growth in the intensity of the NO−2 absorptions. Anion identifications can sometimes be supported by doping a sample with a small amount of NO 2 and then observing the response of the infrared absorptions to increasingly energetic irradiation. In a study of the ions produced from NF 3 [26], not only did the absorptions assigned to NF−2 decrease, with concomitant growth of the NO−2 absorptions, but when the sample was subsequently exposed for a brief time to radiation of energy sufficiently great to photodetach the NO−2 some of the electrons were recaptured by NF 2, resulting in the reappearance of a small amount of NF−2. After a longer period of irradiation, electron capture by NF+2 and NF+3 in the system predominated, and the NF−2 absorptions disappeared irreversibly.

2. Ions produced from HX The infrared spectra of Ne:HX samples deposited without a discharge are of intrinsic interest. The Table 1 Positions (cm −1) of the n+2 absorption of (HX) 2 in various environments Species 35

(H Cl) 2 (HBr) 2 (HI) 2

Gas phase 2854.06

b

6 0.2 cm −1. Ref. [28]. c Ref. [32]. d Refs. [29,30]. e Ref. [35]. f Tentative assignment. g Refs. [30,31].

a

b

Ne matrix a c

2842.2 2518.7 e 2202.9 ef

Ar matrix 2818 d 2496.4 d 2187 g

rotation of HCl in rare-gas matrices has been studied by many workers. The spectra recorded in our studies for simple Ne:HCl deposits agree well with the data previously reported by Andrews and co-workers [27]. The neon-matrix absorption arising from the R(0) transition of H 35Cl appears only about 7 cm −1 below the corresponding gas-phase absorption. As shown in Table 1, the prominent n+2 absorption of (HCl) 2 is quite strongly dependent on the medium. In a neon matrix, this absorption appears about 12 cm −1 below the gasphase term value [28], but in an argon matrix [29,30] it appears some 36 cm −1 below the gas-phase value. Infrared spectra had previously been reported for HBr, HI, and their multimers trapped in an argon matrix [29–31], but not for these species in a neon matrix. The positions of the HX-stretching absorptions of (HBr) 2 and (HI) 2 trapped in an argon matrix lie considerably below those observed in this work for these dimers trapped in a neon matrix. Although the positions of the gas-phase absorptions of (HBr) 2 and (HI) 2 have not been reported, analogy with the observations for (HCl) 2 suggests that the neon-matrix absorptions lie closer to the gas-phase band centers than do the argon-matrix absorptions. The positions of the HXstretching absorptions of the trimer and of HX…H 2O are also strongly matrix dependent. The positions of the absorptions which have been assigned to the HX + and DX + cations, produced in the present series of experiments on Ne:HX samples, are summarized in Table 2. In the discharge sampling experiments on Ne:HCl and Ne:DCl mixtures, the Table 2 Ground-state fundamentals (cm −1) of HX + and DX + Species

Gas phase

Ne matrix a

H 35Cl + H 37Cl + D 35Cl + D 37Cl + H 81Br + D 81Br + HI + DI +

2568.62 b 2566.75 b 1864.03 b 1861.33 b 2346.72 d 1690.00 d 2118.96 f

2543.8 c 2542.4 c 1846.7 c 1844.1 c 2355.7 e 1694.9 e 2120.9 e 1521.7 e

6 0.2 cm −1. Refs. [33,34]. c Ref. [32]. d Ref. [36]. e Ref. [35]. f Ref. [37].

a

b

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infrared absorption of the corresponding cation [32] appears approximately 1% below its gas-phase band center [33,34]. The absorptions of HBr + and DBr + trapped in a neon matrix [35] each appear a few cm −1 above the corresponding gas-phase band center [36]. The agreement is even closer for HI +, for which the high-resolution gas-phase determination of the band center was reported [37] while these experiments were in progress. The first identification of the infrared absorption of DI + has resulted from observations on Ne:HI:DCl samples, for which isotopic exchange occurs in the gas mixture before the sample is deposited. As was also found for H 2O rotation in our study of the ionization of H 2O by excited neon atoms [38], the electric field of the ions inhibits rotation of the hydrogen halide in the neon matrix and activates the Qbranch absorption of the nonrotating molecule. As summarized in Table 3, in the neon matrix an HX-stretching absorption of the charge-delocalized (HX)+2 complex also appears at a frequency intermediate between that of the vibrational fundamentals of HX and of HX + [32,35]. The (HCl)+2 complex may photodecompose to form H 2Cl +. The OO- and HCl-stretching absorptions of the O 2…HCl + complex are also present in many of the experiments on the Ne:HCl system [32]. The pattern of the OO-stretching absorptions observed for oxygen-18 enriched samples indicates that the two O atoms are equivalent or nearly equivalent in the complex. The experiments suggest that a relatively low Table 3 Absorptions (cm −1) of isotopically substituted (HX)+2, X = Cl, Br, I Species (HCl)+2 [(HCl)(DCl)] +

HX stretch a 2704.1 2703.1 2711.2

(DCl)+2 (HBr)+2 [(HBr)(DBr)] + (DBr)+2 (HI)+2 [(HI)(DI)] + (DI)+2

2440.3 2444.3 2143.0 2148.6

DX stretch a

Table 4 Absorptions (cm −1) of XHX − ClHCl −

1967.7 1965.0 1962.2 1960.6 1759.8 1756.4 1542.8 1537.0

Ar Kr Xe a

722.90 728.9 b 737.9 b

BrHBr − n1 + n3

Medium n 3 Gas Ne

Ne matrix, 6 0.2 cm −1. For X = Cl, Ref. [32]; for X = Br, I, Ref. [35]. a

barrier to charge exchange, forming O+2 and HCl, may be surmounted by photoexcitation. Several experimental and theoretical studies [39– 43] conclude that gas-phase HX experiences dissociative electron attachment or that HX − is only very weakly bound. The resulting X − can diffuse through the neon matrix, leading to the stabilization of XHX −, which is also detected. The positions observed for the XHX − absorptions in the present neon-matrix experiments [32,35] and in earlier experiments using heavier rare-gas matrices [44–49] are summarized in Table 4, together with the position of the n 3 band center of gasphase ClHCl − [50]. The unusually large shifts in the positions of the XHX − absorptions when the rare-gas matrix material is changed has been attributed [49] to charge delocalization in the matrix, which becomes increasingly important as the mass of the rare-gas matrix material is increased. When relatively concentrated Ar:HCl samples were deposited with concurrent irradiation at 10.2 eV, well below the ionization potential of HCl (12.75 eV), ClHCl − was formed [44]. However, ClHCl − did not appear when neon was used as the matrix material [32], suggesting that charge delocalization in the matrix, with consequent formation and stabilization of ions at relatively low excitation energy, is considerably less important for solid neon than for solid argon. X −, X−2, XHX −, and possibly weakly bonded HX − contribute to the overall charge neutrality of the deposit. The photodetachment of electrons from

n1 + n3

n3

n1 + n3

903.8 c 908.5 c 918.0 c 892 f 845.1 e 797.5 e

644.7 c 653.3 c 671.8 c 682 g 647.4 e 580.8 e

768.5 c 786.6 c 803 g 764.1 e 689.8 e

a

695.6 de 662.8 e 644.1 e

992.6 b 741.2 c 1001.0 b 745.4 c 752.9 c 954.9 de 728 f 915.6 e 686.6 e 892.9 e 645.6 e

Ref. [50]. 6 0.2 cm −1. Ref. [32]. c 6 0.2 cm −1. Ref. [35]. d Ref. [44]. e Ref. [49]. f Refs. [45,46]. g Refs. [47,48]. b

n3

IHI −

C. L. Lugez et al./Journal of Molecular Structure 449 (1998) 111–118

these species and subsequent electron capture processes are expected to lead to major changes in the absorption spectrum on exposure of the deposit to filtered visible and near ultraviolet radiation. The matrix cage may somewhat stabilize HX −, for which ab initio calculations have suggested the possibility of a shallow potential minimum at a large H…X − separation [51–54]. The resulting HX − species would be expected to undergo photodetachment at low energies—probably in the near infrared or visible spectral region. It is likely that some X − is trapped in the neon matrix. Although atoms can diffuse to some extent through the rare-gas lattices, the mobility of X − is probably lower than that of X atoms because of its polarization interaction with atoms of the matrix material. The electron affinities of Cl, Br, and I are 3.617(3) eV, 3.365(3) eV, and 3.0591(4) eV, respectively [55]. Since the neon matrix somewhat inhibits the photodetachment of electrons, the X − photodetachment thresholds in the matrix experiments are expected to be approximately 5 eV. The 254 nm output of a medium-pressure mercury arc corresponds to 4.88 eV. Thus, the release of electrons from X − trapped in a neon matrix is likely to be slow even when the sample is exposed to unfiltered mercuryarc radiation. Ayala, Wentworth, and Chen [56] have summarized the electron attachment properties of Cl 2, Br 2, and I 2. Each of the diatomic halogen molecular anions has a dissociative state which correlates with X + X − below the threshold for electron detachment. The photodissociation thresholds range from 1.26 eV for Cl−2 to 0.86 eV for I−2. In the neon matrix, the onset of X−2 photodissociation is expected to occur in the visible spectral region. The reaction of these photodissociation products is, therefore, likely to be a major contributor to the observed changes in the product distribution on filtered tungsten-lamp or mercury-arc irradiation of the deposit at energies below the photodetachment threshold of X − and X−2. The absorptions assigned to XHX − grow as the sample is exposed to successively shorter wavelengths of radiation until the thresholds for photodestruction of BrHBr − and IHI − are reached near 260 nm. Presumably, the XHX − anions result from the migration of X − formed with excess kinetic energy and its reaction with HX in the neon lattice. Recent

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molecular beam photoelectron spectroscopic studies have determined electron detachment thresholds of 4.896(5) eV for ClHCl − [57,58], 4.47 eV for BrHBr − [58], and 3.93 eV for IHI − [59,60]. Photodestruction of HX + proceeds primarily by electron capture. As HX + is destroyed, the initially prominent absorptions assigned to the Q(0) transition of uncharged HX and to the n 2 absorption of nonrotating H 2O decrease in intensity, and the R(0) absorption of HX, as well as the n 2(1 1,1 –0 0,0) absorption of H 2O [38], sharpens and grows in intensity. This observation supports the hypothesis that molecular rotation is inhibited by the electric field of ions trapped in the solid and suggests that the contour of the n 2 absorption of H 2O should provide a measure of the extent of ion stabilization in the matrix.

3. Ions produced from CH 3X In similar experiments on Ne:CH 3X samples (X = F, Cl, Br), the infrared spectrum of the solid deposit includes several absorptions which can be assigned to cation products with both the conventional (CH 3X +) and the ylidion (H 2CXH +) structures [61]. The spectrum obtained in studies on CH 3F also includes prominent absorptions which have previously been assigned to H 2CF, HCF, and CF. However, in the studies of CH 3Cl and CH 3Br the corresponding free radical absorptions make only a minor contribution to the spectrum. The identification of the new absorptions is aided by studies of the isotopically substituted methyl halides, by ab initio calculations at the MP2 and QCISD levels for both the conventional and the ylidion structures, and by earlier mass spectrometric studies [62], which demonstrated that the H 2CFH + ylidion is more stable than CH 3F +. Although for both the chloride and the bromide the conventional structure is more stable, the difference in stability of the two isomers is small. The key to the ylidion identification is the appearance in each CH 3X system of a prominent absorption intermediate in frequency between the absorptions of HX and HX +. This absorption is unshifted on carbon13 substitution in the methyl halide, consistent with its assignment to the XH-stretching fundamental of the ylidion. Ab initio calculations were conducted for all three H 2CXH + species [61] using the Gaussian 94

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package at the QCISD level [63], the frozen core approximation, and the 6-311 + G** basis set for all of the atoms except Br, for which the Hay–Wadt effective core potential [64] was used. These calculations yield a similar relative position for the XHstretching absorption of the ylidion species, intermediate between the absorptions of HX and HX +, and indicate that for each of these ylidions this absorption should be much more prominent than any other. The ab initio and experimental values of the per cent separation of the new absorption from that of HX + relative to the (HX − HX +) separation are plotted in Fig. 2. (For the ab initio comparison, supplementary calculations at the QCISD level were performed for HX and HX +, using the same basis sets as for H 2CXH +.) All of the per cent separations of the observed absorptions from those of HX + are smaller than the calculated values. However, both the calculations and the observations suggest that the separation of the new absorption from the corresponding HX + absorption is smallest for X = Cl. The observed position of the new XH-stretching absorption is always somewhat closer to the position of the HX + absorption than to that of HX, suggesting that a significant fraction of the positive charge resides in the XH moiety of the ylidion. Other absorptions of the H 2CXH + ylidions have also been observed in the neon-matrix experiments

Fig. 2. Separation of the XH-stretching frequency of H 2CXH + from that of HX +, compared with the (HX–HX +) separation.

[61]. Both of the CH-stretching fundamentals of H 2CClH + and of H 2CBrH + are quite prominent. The CF-stretching absorption of H 2CFH + is identified near 715 cm −1, in reasonable agreement with the 596 cm −1 value predicted by the QCISD calculation and substantially below the position (ca. 1040 cm −1) of the CF-stretching absorption of CH 3F. The calculations suggest that the CX-stretching absorption of each of the other two ylidion species should lie somewhat below the CX-stretching absorption of the uncharged methyl halide, but the fractional change is much less than for H 2CFH +. Still other absorptions can be assigned to CH 3Cl + and CH 3Br +. Since the ground state of each of these cations possesses 2E electronic symmetry, Jahn– Teller distortion can occur. Ab initio calculations were also performed for the CH 3X + species [61]. Their results indicate that this distortion is substantial, resulting in C s symmetry for the cation, which has nine distinct infrared-active vibrational absorptions. Three of these absorptions have been identified for CH 3Cl +, four each for CD 3Cl + and CD 3Br +, and all nine for CH 3Br +. The behavior of the product absorptions on exposure of the deposit to filtered visible and ultraviolet radiation is consistent with the proposed assignments. There is some evidence for photoisomerization between the ylidion and the conventional cation structures, which behave quite differently as the sample is exposed to successively shorter wavelengths (higher energies) of radiation. The transfer of 16.6 eV to 16.8 eV excitation energy from the neon atom to any of the methyl halide species may also lead to dissociative ionization, with CH+3 + X + e the principal products. No infrared absorption of CH+3 was identified in any of these experiments. The contour of the n 2 absorption of uncharged CH 3, also present in the product spectra, changes during subsequent mercury-arc irradiation of the deposit. CH 3, like H 2O and the hydrogen halides, can rotate in rare-gas matrices [65,66]. This rotation, like that of the other simple hydrides, is severely quenched in the initial deposit but grows in importance as photodestruction of ionic species proceeds [61]. In the gas phase, CH 3X, like HX, undergoes dissociative electron attachment at near-zero electron energies [67–69]. X − formed in this process may

C. L. Lugez et al./Journal of Molecular Structure 449 (1998) 111–118

undergo subsequent reactions leading to the stabilization of a small amount of XHX − in the initial deposit. As for the Ne:HX system, reaction of X with X − in the solid and photodestruction of X−2 are important processes. (The electron affinities of the F atom [55] and of F 2 [56] are 3.401 eV and 3.08 eV, respectively, and the threshold for the photodissociation of F−2 into F + F − is 1.34 eV [56].) As the energy of the radiation is increased, photodissociation of X−2 grows in importance, and reaction of the resulting X − results in the growth of prominent, broad absorptions of XHX − (or XDX −) in the CH 3Cl and CH 3Br experiments. Because the major anion species, X −, X−2, and XHX −, all have relatively high thresholds for electron detachment, cations persist until the sample is subjected to prolonged irradiation by the full light of a medium-pressure mercury arc. Relatively little change in the absorption pattern of n 2 of H 2O occurs until the sample is exposed to the 254 nm output of the unfiltered mercury arc, when the absorption of the rotating species intensifies and sharpens and the absorption of nonrotating H 2O diminishes in intensity.

4. Conclusions

infrared absorptions of the ylidion structures, H 2CXH +, have been identified. For these ylidions, a prominent HX-stretching absorption intermediate between the positions of the vibrational fundamentals of the diatomic HX and HX + species is characteristic. The ground states of the CH 3X + species, of 2E symmetry, are predicted and observed to experience Jahn–Teller distortion from threefold symmetry. Several absorptions have been assigned to CH 3Cl +, and all nine vibrational fundamentals of CH 3Br + have been identified.

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

[10]

n+2

The positions of the hydrogen-stretching absorptions of the uncharged dimers of the hydrogen halides are unusually sensitive to the matrix material. For (HCl) 2, the neon-matrix values correspond most closely to the gas-phase values, suggesting a similar correspondence for (HBr) 2 and (HI) 2. When ions are present in the deposit, the resulting electric field inhibits the rotation of the HX molecule, as well as that of H 2O desorbed from the walls of the vacuum system and trapped in the neon matrix. The vibrational fundamentals observed for HCl +, HBr +, and HI + isolated in a neon matrix lie within approximately 1% of their gas-phase values. Infrared absorptions in the HX-stretching region can also be assigned to the (HX)+2 dimer cations. Other absorptions can be assigned to the bihalide anions, XHX −, which contribute to the maintenance of overall charge neutrality in the deposit. Consideration of the matrix shifts for XHX − in the various rare-gas matrices suggests that charge delocalization into the matrix is minimal for XHX − isolated in solid neon. In similar experiments on the methyl halides (X = F, Cl, Br),

117

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