Experimental and computational study of the HXeIHY complexes (Y = Br and I) Masashi Tsuge, Slavomir Berski, Markku Räsänen, Zdzislaw Latajka, and Leonid Khriachtchev Citation: J. Chem. Phys. 138, 104314 (2013); doi: 10.1063/1.4794309 View online: http://dx.doi.org/10.1063/1.4794309 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v138/i10 Published by the American Institute of Physics.
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THE JOURNAL OF CHEMICAL PHYSICS 138, 104314 (2013)
Experimental and computational study of the HXeI· · ·HY complexes (Y = Br and I) Masashi Tsuge,1,a) Slavomir Berski,2 Markku Räsänen,1 Zdzislaw Latajka,2 and Leonid Khriachtchev1,a) 1 2
Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014, Finland Faculty of Chemistry, University of Wroclaw, 14, F. Jollot-Curie Str., 50-383 Wroclaw, Poland
(Received 16 January 2013; accepted 20 February 2013; published online 13 March 2013) The complexes of HXeI with hydrogen halides HY (Y = Br and I) are studied computationally and experimentally in a xenon matrix. The calculations at the CCSD(T)/def2-TZVPPD level of theory predict several energy minima for the HXeI· · ·HY complexes with interaction energies from −4.69 to −0.23 kcal mol−1 . We have identified three bands of the HXeI· · ·HI complexes in the H−Xe stretching region with the monomer-to-complex blue shifts from +37 to +96 cm−1 , and three bands of the HXeI· · ·HBr complexes with blue shifts from +88 to +157 cm−1 . The structural assignments are done on the basis of the strong H−Xe and HY stretching bands and the decomposition rates upon broadband IR irradiation. The experimental bands with larger shifts are assigned to the most stable structures of the HXeI· · ·HY complexes with the Y−H· · ·I hydrogen bond. © 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794309] I. INTRODUCTION
Noble-gas hydrides with the general formula of HNgY (Ng = a noble-gas atom and Y = an electronegative fragment) are a part of modern noble-gas chemistry showing fascinating properties.1–3 A large number of HNgY molecules have been prepared experimentally including argon compound HArF.4, 5 The standard procedure to synthesize HNgY molecules is photodissociation of HY precursors and subsequent thermal mobilization of H atoms in noble-gas matrices, which leads to the reaction H + Ng + Y → HNgY. The H + Ng + Y triad is higher in energy than HNgY in the experimentally prepared molecules.6 The HNgY molecules are metastable with respect to Ng + HY and their decomposition to the global energy minimum is protected by a bending barrier.1, 3 The HNgY molecules are characterized by the strong (HNg)+ Y− ion-pair character. The high intensity (∼1000 km mol−1 and higher) of the H−Ng stretching vibration enables one to easily detect these molecules by IR absorption spectroscopy. Non-covalent interactions determine many physical properties of matter and play an important role in chemical reactions. Vibrational frequency shifts caused by intermolecular interactions are useful for characterizing the complexes. Interaction of HNgY with other molecules has attracted both theoretical and experimental interest. The complexes of HArF,7 HKrF,7 HKrCl,7–9 HXeBr,10–12 HXeCl,10, 11 HXeCCH,13, 14 and HXeOH (Ref. 15) have been observed experimentally and the number of calculated complexes is much bigger. Interaction with other molecules and environment has a strong effect on vibrational properties of HNgY molecules due to their weak bonding and large dipole moment. All the experimentally prepared HNgY complexes exhibit blue shifts of the H−Ng stretching mode while there are theoretical prea) Authors to whom correspondence should be addressed. Electronic
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dictions for the HArF· · ·P2 and HXeBr· · ·Xe complexes with red shifts.16, 17 The blue shift in HNgY complexes seems to be the “normal” effect because the complexation strengthens the (HNg)+ Y− ion-pair character.18 The HXeY· · ·HX complexes (X, Y = Cl and Br) show blue shifts of the H−Xe stretching mode up to ∼150 cm−1 .11 A blue shift of ∼300 cm−1 was observed for the HKrCl· · ·HCl complex, which is probably the largest blue shift reported for 1:1 complexes.9 The HKrCl· · ·N2 complex also shows a larger blue shift (up to 100 cm−1 ) as compared with the HXeBr· · ·N2 and HXeCl· · ·N2 complexes (∼10 cm−1 ).7, 8, 10 The stronger complexation effect on HKrCl can be connected with its weaker bonding compared to HXeCl and HXeBr. It is interesting to study complexes of other HNgY species in order to test the correlation between the strength of the H−Ng bond and the complexation-induced shift. The HXeI molecule has the H−Xe stretching frequency of 1193 cm−1 ,19 which is the second smallest H−Xe stretching frequency of the experimentally observed HXeY molecules (after HXeSH with absorption at 1119 cm−1 ).20 In other words, the HXeI molecule probably has one of the lowest stability among the identified HNgY molecules; thus, it may show exceptional spectral changes upon interaction with other molecules. In the present work, we report the experimental preparation of the HXeI· · ·HI and HXeI· · ·HBr complexes in a xenon matrix. These species are identified using IR absorption spectroscopy in a xenon matrix and assigned with the aid of quantum chemical calculation.
II. COMPUTATIONAL DETAILS AND RESULTS
The structural optimizations and calculations of energies are performed by the CCSD(T) and MP2(full) methods.21, 22 The def2-TZVPPD basis sets for H, Br, I, and Xe atoms,23 which are triple-zeta-valence basis sets augmented with two
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sets of polarization and diffuse basis functions, are taken from the EMSL Basis Set Library using the Basis Set Exchange software.24, 25 For I and Xe atoms, 28 electrons are replaced by an effective core potential. The minima on the potential energy surface are verified by harmonic vibrational analysis, showing no imaginary frequencies. The interaction energy (Eint ) is defined as a difference between the total energies of the complex and of the HXeI and HY (Y = Br and I) monomers (with the structures in the complex). The Eint values are corrected (Eint CP ) for the basis set superposition error (BSSE) using the counterpoise procedure (CP).26 The optimization of the structures and harmonic vibrational analysis at the CCSD(T) level of theory is performed using the MOLPRO program (Ref. 27) and at the MP2 level using the GAUSSIAN 09 program.28 In order to estimate Eint and Eint CP values, the single point energies and BSSE at the MP2 and CCSD(T) levels are calculated using the GAUSSIAN 09 program. The natural population analysis is performed using the GAUSSIAN 09 program at the CCSD/def2-TZVPPD//CCSD(T)/def2-TZVPPD level.29 The structural optimization of the HXeI· · ·HY complexes (Y = Br and I) at the CCSD(T)/def2-TZVPPD level of theory yields four minima on the potential energy surface (Figure 1). Similar structures are obtained using the MP2 method. Structures I and III are stabilized by the Y−H· · ·I and Xe−H· · ·Y hydrogen bonds, respectively. Structure II comprises the interaction between the I and Y atoms (halogen bonding) and structure IV is stabilized by the dihydrogen Xe−H· · ·H−Y bond. The large dipole moments of the interacting molecules (μ(HI) = 0.44 D, μ(HBr) = 0.85 D, and μ(HXeI) = 6.10 D at the MP2(full)/def2-TZVPPD level) suggest that the intermolecular interactions are essentially due to by electrostatic forces but probably with a substantial contribution of dispersion interactions due to large polarizability of Br, I, and Xe atoms. The partial atomic charges are shown in Table I. The increase of positive charges on the (HXe) part is seen for structures I, II, and III of both complexes and this increase is somewhat greater for structure I. For structure IV of HXeI· · ·HI, the (HXe) charge decreases.
TABLE I. Partial atomic charges (in elementary charges) for the HXeI· · ·HI and HXeI· · ·HBr complexes calculated at the CCSD/def2TZVPPD//CCSD(T)/def2-TZVPPD level of theory. Monomers Structure I Structure II Structure III Structure IV
q(H)HXeI q(Xe)HXeI q(I)HXeI q(H)HI q(I)HI
−0.04 +0.58 −0.54 +0.07 −0.07
HXeI· · ·HI −0.01 −0.03 +0.61 +0.60 −0.55 −0.56 +0.08 +0.05 −0.13 −0.07
−0.02 +0.59 −0.59 +0.08 −0.06
−0.07 +0.58 −0.50 −0.05 +0.03
q(H)HXeI q(Xe)HXeI q(I)HXeI q(H)HBr q(Br)HBr
−0.04 +0.58 −0.54 +0.18 −0.18
HXeI· · ·HBr −0.01 −0.03 +0.61 +0.59 −0.56 −0.55 +0.19 +0.17 −0.24 −0.17
−0.02 +0.59 −0.58 +0.18 −0.16
−0.08 +0.58 −0.49 +0.15 −0.16
FIG. 1. Structures of the HXeI· · ·HY complexes (Y = Br and I). The structural parameters at the MP2(full)/def2-TZVPPD and CCSD(T)/def2TZVPPD levels of theory are given in Table III.
The interaction energies (Eint CP ) of the HXeI· · ·HY complexes (Y = Br and I) are collected in Table II. For the HXeI· · ·HI complexes, all the structures are characterized by negative interaction energies; thus, their formation decreases the total energy. For the HXeI· · ·HBr complexes, only three structures (I–III) are energetically favorable. Structure IV of this complex has a positive Eint CP value; therefore, its formation does not decrease the total energy, and we exclude
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TABLE II. BSSE corrected interaction energies Eint CP (in kcal mol−1 ) of the HXeI· · ·HI and HXeI· · ·HBr complexes calculated at the CCSD(T)/def2TZVPPD and MP2(full)/def2-TZVPPD levels of theory. Structure I
Structure II
Structure III
Structure IV
CCSD(T) MP2
− 3.88 − 5.68
HXeI· · ·HI − 2.69 − 3.83
− 1.23 − 1.99
− 0.23 − 0.35
CCSD(T) MP2
− 4.69 − 6.21
HXeI· · ·HBr − 1.74 − 2.53
− 1.00 − 1.49
+0.10 +0.35
it from the consideration. Similar results are obtained at the MP2(full)/def2-TZVPPD level of theory. The optimized geometrical parameters are presented in Table III. The formation of the complexes results in lengthening of the H−Y bond for all structures. For the HXeI· · ·HBr complex, this change is the largest in structure I (+0.018 Å at CCSD(T)) and substantially decreases for structures II and III with weaker interaction between HXeI and HBr. Similar results are obtained for the H−I bond length in the HXeI· · ·HI complexes (structures I–III). The lengthening of the H−I bond in structure IV (+0.006 Å at CCSD(T)) is larger than that in structure III (+0.001 Å at CCSD(T)) despite the stronger interaction in structure III. The Xe−I bond, playing the role of proton acceptor in structure I and involved in the I· · ·Y interaction in structure II, becomes longer in both HXeI· · ·HY complexes. The elongation is larger for the strongest structure I than for structure II. The
elongation of the Xe−I bond is also seen in structure III while this bond shortens in structure IV. The H−Xe bond shortens in structures I–III and lengthens in structure IV. This bond participates in the Xe−H· · ·Y hydrogen bond in structure III and in the dihydrogen Xe−H· · ·H−I bond in structure IV. The calculated vibrational frequencies for the H−Xe and HY stretching modes are presented in Table IV. For structure I, the complexation-induced blue shifts of H−Xe stretching mode are the largest (>100 cm−1 at the CCSD(T) level). Structures II and III have smaller blue shifts (from +40 to +100 cm−1 ). The blue shifts of the H−Xe stretching mode are attributed to the enhancement of the (HXe)+ Y− ionpair character upon complexation.18 This charge redistribution should correlate with the shortening of the H−Xe bond, the blue shifts of its vibrational frequency, and the decrease of its absorption intensity.11 This trend takes place for structures I and II, but structure III shows the largest decrease of the absorption intensity while the blue shift of the H−Xe stretching mode is not the largest. This might be due to the hydrogen bonding in structure III, which perturbs the H−Xe stretching mode, i.e., the normal mode is no longer the pure H−Xe stretching mode. Structure IV of HXeI· · ·HI shows a red shift of the H−Xe stretching mode, which is reasonable because the positive charge on the (HXe) part decreases upon complexation (Table I) and the H−Xe bond elongates. All the structures show red shifts of the HY stretching mode. The spectral red shift is accompanied by an increase of the HY absorption intensity. The HY red shift and the increase of intensity is the largest for structure I.
TABLE III. Geometrical parameters for the HXeI· · ·HY (Y = Br and I) complexes optimized at the MP2(full)/def2-TZVPPD and CCSD(T)/def2-TZVPPD levels of theory.a r(IHXeI · · ·HHY /Y)
r(HHXeI –Xe)
r(Xe–I)
MP2
1.704
2.971
CCSD(T)
1.768
3.024
Structure I/MP2 Structure I/CCSD(T) Structure II/MP2 Structure II/CCSD(T) Structure III/MP2 Structure III/CCSD(T) Structure IV/MP2 Structure IV/CCSD(T)
1.679 1.740 1.692 1.751 1.702 1.752 1.738 1.786
3.008 3.044 2.985 3.034 3.010 3.040 2.949 3.016
2.466 2.749 3.756 3.959 -
Structure I/MP2 Structure I/CCSD(T) Structure II/MP2 Structure II/CCSD(T) Structure III/MP2 Structure III/CCSD(T)
1.677 1.734 1.697 1.757 1.693 1.752
3.011 3.051 2.978 3.029 3.001 3.036
2.466 2.662 3.778 3.980 ... ...
a b
The bond lengths are in Ångströms and angles in degrees. See Figure 1.
r(HHXeI · · ·HHY /Y)
Angle 1b
Angle 2b
Angle 3b
HXeI· · ·HI 1.622 1.628 1.595 1.617 1.588 1.613 1.603 1.618
... ... ... ... 2.664 3.075 1.562 1.955
72.1 74.5 72.8 74.0 ... ... ... ...
... ... ... ... 180.0 179.4 180.0 180.0
169.2 164.6 171.0 170.6 84.7 84.5 180.0 180.0
HXeI· · ·HBr 1.439 1.438 1.410 1.422 1.408 1.421
... ... ... ... 2.642 3.025
71.8 73.9 66.4 69.9 ... ...
... ... ... ... 178.2 178.3
166.9 164.7 170.3 169.7 94.8 95.1
r(HHY –Y)
Monomers (HXeI, HI, and HBr) 1.586 (HI) 1.406 (HBr) 1.612 (HI) 1.420 (HBr)
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TABLE IV. Computational monomer-to-complex frequency shifts (cm−1 ) of the H−Xe and HY stretching modes of the HXeI· · ·HI and HXeI· · ·HBr complexes calculated at the CCSD(T)/def2-TZVPPD and MP2(full)/def2-TZVPPD levels of theory.a Monomer
Structure I
CCSD(T) MP2
1327.2 1663.2 (2790)
+120.4 +111.0 (1980)
CCSD(T) MP2
2325.1 2447.5 (0.4)
−160.8 −387.4 (1228)
CCSD(T) MP2
1327.2 1663.2 (2790)
+148.7 +124.8 (2029)
CCSD(T) MP2
2662.3 2743.4 (16)
−221.2 −408.6 (1162)
Structure II
Structure III
Structure IV
+71.8 +51.5 (2355)
+98.3 −26.8 (455)
−56.1 −263.5 (5508)
−27.2 −49.3 (30)
−3.2 −11.5 (8)
−69.4 −221.8 (284)
+42.8 +28.8 (2521)
+101.7 +50.0 (946)
... ...
−17.6 −33.5 (2)
−6.6 −15.7 (33)
... ...
HXeI· · ·HI
HXeI· · ·HI
HXeI· · ·HBr
HXeI· · ·HBr
a
The IR intensities calculated in the MP2 method are shown in parentheses (km mol−1 ).
III. EXPERIMENTAL DETAILS AND RESULTS
Hydrogen iodide was synthesized from 1,2,3,4tetrahydronaphthalene (tetralin) and iodine.30 Tetralin (100 g) was heated to 200 ◦ C in a 500 ml flask and stirred in argon atmosphere. A solution of I2 (30 g) in tetralin (100 g) was added to the flask through a dropping funnel. The gas generated in the reaction was collected by passing through two traps cooled to −30 ◦ C and −80 ◦ C. Hydrogen iodide collected in the −80 ◦ C trap was then purified by low-temperature distillation and stored in a blackened glass bulb. The gas mixtures of HI, HBr (≥99%, Aldrich), and Xe (≥99.995%, AGA) were made by standard manometric procedures with concentration ratios of HI/Xe = 1/(600–3000) and HI/HBr/Xe = 1/1/600. These mixtures were deposited onto a cold CsI substrate at temperatures from 30 to 50 K in a closed-cycle helium cryostat (DE 202A, APD or RDK408D2, SHI). The FTIR spectra in the 4000–400 cm−1 range with 1 cm−1 resolution were measured with a Vertex 80 V spectrometer (Bruker) co-adding 200 scans. The spectra were measured at 10 K with the DE 202A cryostat and at 3 K with the RDK-408D2 cryostat. The matrices were photolyzed by an excimer laser (MSX-250, MPB) operating at 193 nm (∼10 mJ cm−2 ). After deposition of HI/Xe matrices, the known absorptions of the HI monomer, HI multimers, and HI· · ·H2 O complex are observed in the FTIR spectra (Figure 2).31 The HI monomer spectrum consists of several bands separated by 2–3 cm−1 , which possibly originate from hindered rotation of HI in a xenon matrix. In addition, HI dimer and trimer are observed at 2166.2 and 2153.6 cm−1 , respectively. The band at 2145.9 cm−1 previously assigned to the HI· · ·H2 O complex properly correlates with the amount of water impurity. The relative amount of HI multimers decreases for smaller HI amounts and lower deposition temperatures. Irradiation at 193 nm decomposes the HI species. The HI dimer and trimer are photolyzed faster than the HI monomer. After 100 pulses of 193 nm radiation, up to 80% of the dimers and practically all trimers are decomposed while the decomposition of the HI monomer is less than 50%. Upon photoly-
sis, new absorption appears at 2164.2 cm−1 . This absorption can be assigned to the I· · ·HI complex, which is the precursor of the HXeI· · ·HI complex, but no bands to be assigned to the I· · ·(HI)2 complex are found. The formation of (XeHXe)+ and HI2 − is also seen after photolysis.32, 33 It should be noted that 193 nm photolysis of HY molecules in a xenon matrix is “selflimited” which limits the amount of photolyzed species.34 Annealing of photolyzed matrices mobilizes hydrogen atoms produced by photolysis. Hydrogen atoms are known to diffuse in solid xenon upon annealing above 35 K.35, 36 As a result of annealing, the formation of HXeI (1193.2 cm−1 )19 and HXeH (1181.2 and 1166.2 cm−1 )37 is observed (Figure 3). The HXeI main band is accompanied by a broad side band on the blue side (1213 cm−1 , marked as L) and a sharp side band at 1186.7 cm−1 . The L band has a characteristic behavior for librational bands, i.e., the relative intensity of the blue side band decreases at elevated temperatures and another broad band rises on the red side (1176 cm−1 ).17, 38 Several other bands can be seen in the same spectral region. A band at 1322.0 cm−1 originates from the (ν H−Xe + ν Xe−I ) combination mode of the HXeI monomer.39 A band at 1332.1 cm−1 is most probably due to the HXeI· · ·H2 O
FIG. 2. FTIR spectra of HI/Xe (1/600 and 1/3000) matrices measured after deposition. The upper spectrum was measured at 3 K and the lower spectrum at 10 K.
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FIG. 3. FTIR spectra of HI/Xe (1/600 and 1/3000) matrices showing the results of photolysis at 193 nm followed by annealing at 60 K. The absorbances of HXeI monomer are scaled to 1.0. The upper spectrum was measured at 3 K and the lower spectrum at 10 K.
complex since the intensity of this band correlates with the amount of the HI· · ·H2 O complex observed after deposition. (The results on the HXeY· · ·H2 O complexes will be reported separately.) At least three bands (1230.4, 1268.0, and 1289.2 cm−1 ) correlate with the amount of HI dimer after deposition. A band with a similar concentration dependence is seen in the HI region at 2048.0 cm−1 . These bands at 1230.4, 1268.0, 1289.2, and 2048.0 cm−1 are efficiently bleached by UV light as known for HXeY molecules40, 41 and assigned to the HXeI· · ·HI complexes. As shown below, weak bands at 1281, 1303, and 1350 cm−1 (marked as C) originate from the HXeI· · ·HBr complexes explained by a small amount of HBr present in these HI/Xe matrices (from previous experiments with HBr). No bands of the possible HXeI· · ·(HI)2 complex are found. After deposition of an HI/HBr/Xe = 1/1/600 matrix at 50 K, monomeric HBr bands are observed at 2565.8 cm−1 (coupled with translational mode), 2531.1 cm−1 (strong absorption, R branch), and 2519.6 cm−1 (Q branch) (Figure 4).11 In addition to the HBr dimer (2493.0 cm−1 ) and trimer (2475.1 cm−1 ) bands,11 a strong band is observed at 2468.5 cm−1 , which is induced by the presence of HI. This
FIG. 4. FTIR spectra of HI/HBr/Xe (1/1/600, 1/0/600, and 1/0/3000) matrices measured after deposition. The mixed trimer (tentative assignment), (HBr2 )HI or HBr(HI)2 , band is marked as T. The top and middle spectra were measured at 3 K and the bottom spectrum at 10 K.
FIG. 5. FTIR spectra of HI/HBr/Xe (1/1/600 and 1/0/3000) matrices showing the results of photolysis at 193 nm followed by annealing at 60 K (top and middle spectra) and subsequent UV photolysis by a low-pressure mercury lamp (bottom spectrum). The top spectrum was measured at 10 K and the middle and bottom spectra at 3 K.
band is assigned to the HBr stretching mode of the HI· · ·HBr complex, in which HBr acts as a hydrogen donor. In the HI stretching region, two HBr induced bands are observed at 2177.3 and 2150.9 cm−1 . The band at 2177.3 cm−1 is assigned to the HBr· · ·HI complex, in which HI is a proton donor, and the band at 2150.9 cm−1 is tentatively assigned to a mixed trimer ((HBr2 )HI or HBr(HI)2 ). In the spectrum of the photolyzed and annealed HI/HBr/Xe (1/1/600) matrix (Figure 5), the 1281.0, 1302.7, and 1349.9 cm−1 bands are strong (Figure 3) and these are assigned to the HXeI· · ·HBr complex. The HBr induced bands at 2254.2 and 2141.5 cm−1 are also observed. These bands are assigned to the HBr stretching of the HXeI· · ·HBr complex. Two additional bands at 1422.7 and 1434.8 cm−1 are formed upon annealing, and we tentatively assign these HBr induced bands to the mixed trimer HXeI· · ·(HBr)2 or HXeI· · ·HBr· · ·HI. In the H−Xe stretching region of HXeBr, we do not observe bands which can be attributed to the HXeBr· · ·HI complexes whereas the HXeBr· · ·HBr bands are observed.11 Ultraviolet irradiation by a low-pressure mercury lamp bleaches the HXeI monomer and its complexes with a small recovery of HI monomer (the bottom spectrum in Figure 5). The HXeI monomer is known to be decomposed by IR radiation in the 2950–3800 cm−1 region.42 The HXeI monomer band at 1193.2 cm−1 is efficiently bleached by broad-band IR light but its sharp side band at 1186.7 cm−1 does not change much. The HXeI· · ·H2 O complex (1332.1 cm−1 ) is also quite photostable. The IRdecomposed species are quantitatively recovered by annealing at 60 K. The photodecomposition kinetics for the HXeI monomer and HXeI· · ·HI complexes were measured for
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FIG. 7. Lifetimes of the HXeI· · ·HY complexes under broadband IR irradiation as a function of the blue shifts of the H−Xe stretching mode.
FIG. 6. Time dependences of the integrated absorptions under broadband IR radiation: (a) the HXeI and HXeI· · ·HI bands and (b) the HXeI· · ·HBr bands. The lines show single exponential fits.
HI/Xe matrices (Figure 6(a)). The integrated intensities were derived from fitting of the bands by Lorentzian functions and the kinetic data were fitted by a single exponent. The HXeI main band at 1193.2 cm−1 decomposes most quickly (lifetime of 1.0 h). The 1230.4 cm−1 band has a similar lifetime with the HXeI main band. The lifetimes of the 1268.0 and 2048.0 cm−1 bands are longer (4.3 and 4.7 h, respectively). The 1289.2 cm−1 band is the most photostable HXeI· · ·HI band having a lifetime of 5.5 h. The photodecomposition kinetics for the HXeI· · ·HBr bands are measured for HI/HBr/Xe matrices (Figure 6(b)). The lifetimes of the HXeI· · ·HBr bands are 7.3 h (1281.0 cm−1 ), 21 h (1302.7 and 2254.2 cm−1 ), and 100 h (1349.9 cm−1 ). The data for the 2141.5 cm−1 band of the HXeI· · ·HBr complex are fitted by an exponent with a lifetime of 108 h with a minor contribution (16%) of the second exponent with a lifetime of 2.6 h. For the bands at 1422.7 and 1434.8 cm−1 , no detectable decomposition is observed after 12 h of IR irradiation. The lifetimes determined for the HXeI· · ·HY complexes are shown in Figure 7 as a function of the complexation-induced blue shift of the H−Xe stretching mode. IV. DISCUSSION A. HXeI· · ·HI complex
The bands observed at 1230.4, 1268.0, 1289.2, and 2048.0 cm−1 are assigned to the HXeI· · ·HI complex in a xenon matrix (Table V). The relative intensities of these bands correlate with each other and with the amount of HI dimer.
Upon photolysis, one of the HI molecules in the HI dimer dissociates to H and I atoms. The H atom can exit the cage and the I· · ·HI complex remains in the cage. The I· · ·HI complex participates in the annealing induced reaction H + Xe + (I· · ·HI) to yield the HXeI· · ·HI complex as discussed previously for other HNgY· · ·HX complexes (X, Y = Cl and Br).9, 11 The amount of the HXeI· · ·HI complex should have a maximum for maximum [I· · ·HI][H]. In the initial stage of annealing, the bands of the HXeI· · ·HI complex increase synchronously with the HXeI monomer bands. The amount of the HXeI· · ·HI complex increases somewhat upon prolonged annealing at 60 K, which probably indicates that HI molecules can move at this temperature, similarly to HCCH in the case of the HXeCCH· · ·HCCH complex.13 In agreement, prolonged annealing at 60 K decreases the absorption of HXeI monomer possibly due to reactions with thermally mobilized molecules or atoms.1, 3, 12, 43 IR-induced decomposition is a characteristic feature of HXeI.42 As shown in Figure 6(a), one of the HXeI· · ·HI complex band at 1230.4 cm−1 decreases upon IR irradiation as fast as the HXeI monomer main band, while the other bands decompose more slowly. These results suggest that there are at least two structures of the HXeI· · ·HI complexes in a xenon matrix. The decomposition rates of the bands at 1268.0 and 2048.0 cm−1 are very close, suggesting that these two bands originate from the H−Xe and HI stretching modes of the same structure. The structural assignments are based on the computational results (Figure 1 and Tables II and IV). The experimental band at 2048.0 cm−1 is assigned to the HI vibration of structure I of the HXeI· · ·HI complex because the experimental shift of −166.9 cm−1 agrees with the computational shift of −160.8 cm−1 at the CCSD(T) level of theory whereas the calculated shifts for the other structures are much smaller (from −3.2 to −69.4 cm−1 ). The calculation at the MP2 level predicts a high intensity of the HI stretching transition only for structure I (1228 km mol−1 , Table IV). Accordingly, the 1268.0 cm−1 band is assigned to structure I based on the same IR decomposition rates of these two bands (Figure 6(a)). The calculated shift for the H−Xe stretching mode of structure I is +120.4 cm−1 , which is somewhat larger than the experimental shift of +74.8 cm−1 , and this difference is discussed below. The band observed at 1289.2 cm−1 most likely also belongs to structure I in a different matrix site because it has a large
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TABLE V. IR absorptions and assignments of HXeI monomer and its complexes in a xenon matrix. Frequencya 1176 1186.7 1193.2 1213 1230.4 (+37.2) 1268.0 (+74.8) 1281.0 (+87.8) 1289.2 (+96.0) 1302.7 (+109.5) 1322.0 1332.1 1349.9 (+156.7) 1422.7 (+229.5) 1434.8 (+241.6) 2048.0 (−166.9) 2141.5 (−378.5) 2190 2210 broad 2254.2 (−265.8)
Species
Assignments
HXeI HXeI HXeI HXeI HXeI· · ·HI HXeI· · ·HI HXeI· · ·HBr HXeI· · ·HI HXeI· · ·HBr HXeI HXeI· · ·H2 O HXeI· · ·HBr HXeI· · ·(HBr)2 or HXeI· · ·HBr· · ·HI HXeI· · ·(HBr)2 or HXeI· · ·HBr· · ·HI HXeI· · ·HI HXeI· · ·HBr HXeI HXeI HXeI· · ·HBr
ν H–Xe libration ν H–Xe , side bandb,c ν H–Xe , main bandb,c ν H–Xe librationb ν H–Xe , structure II ν H–Xe , structure I ν H–Xe , structure III ν H–Xe , structure I ν H–Xe , structure I ν H–Xe + ν Xe–H d ν H–Xe e ν H–Xe , structure I ν H–Xe e ν H–Xe e ν HI , structure I ν HBr , structure I 2ν H–Xe c,d 2ν H–Xe libration or phononc,d ν HBr , structure I
Frequencies are in cm−1 and monomer-to-complex frequency shifts are in the parentheses. Reference 42 (The 1213 cm−1 band was tentatively assigned to the phonon-vibration or phonon-libration interactions in this reference). c Reference 19. d Reference 39. e Tentative assignment. a
b
spectral shift (+96.0 cm−1 ) and similar photostability to the 1268.0 cm−1 band. The calculated shifts of structures II and III are +71.8 and +98.3 cm−1 , which are smaller than that of structure I, and therefore, both structures II and III are candidates for the 1230.4 cm−1 band with the experimental shift of +37.2 cm−1 . Because the interaction energy of structure II (−2.69 kcal mol−1 ) is twice bigger than that of structure III (−1.23 kcal mol−1 ), structure II is more probable for the 1230.4 cm−1 band. In general, the blue shift of the H−Ng stretching frequency upon the complex formation is known to be the normal effect because complexation enhances the (HNg)+ Y− ion-pair character.18 The H−Xe stretching frequency shift correlates with the changes in the (HXe) positive charge (Table I). In structure I, the charge separation between the (HXe) part and iodine atom might be enhanced due to the (HXe)+ (IHI)− structure. The difference between the computational (+120.4 cm−1 ) and experimental (+74.8 cm−1 ) frequency shifts of the H−Xe stretching mode of structure I may be connected by the fact that the experiments are performed in a xenon matrix whereas the calculations refer to the complex in vacuum, i.e., this is connected to the solvation by a xenon matrix. The solvation effect on the H−Xe stretching frequency is presumably due to an increase of (HNg)+ Y− charge separation in a polarizable medium.44 The experience accumulated in matrix-isolation research suggests that the frequency in a neon matrix is close to the value in vacuum. The H−Xe stretching shifts of HXeBr and HXeCl from neon to xenon matrices are +51 and +36 cm−1 , respectively.45 This effect may be even stronger for HXeI than for HXeBr and
HXeCl because of the weaker H−Xe bond of HXeI. When the HXeI· · ·HI complex forms, one of the surrounding xenon atoms is presumably substituted by one HI molecule, i.e., solvation of the HXeI molecule changes. In the first approximation, the frequency shift in a xenon matrix represents a difference between the specific HXeI· · ·Xe and HXeI· · ·HI interactions; thus, it is probably smaller than the computational shift between HXeI and HXeI· · ·HI in vacuum. In addition, the discrepancy between the experimental and calculated shifts can be contributed by an inaccuracy in theoretical description of these complicated species. The calculations are also complicated by strong anharmonicity of the H−Xe stretching mode.39, 46 The IR decomposition rate strongly depends on the matrix site: the HXeI monomer main band at 1193.2 cm−1 is much less stable than the side band at 1186.7 cm−1 . It has been suggested that, after IR decomposition, the nascent H atom stays in a nearest interstitial site.42 For photostable HXeI, one can speculate that the cage exit of H atom is prevented by a high barrier and/or by the absence of a local energy minima in the vicinity. The recovery of the HXeI· · ·HI complexes upon subsequent annealing occurs similarly to that of the HXeI monomer, suggesting that the HXeI· · ·HI complexes decompose upon IR irradiation to the H + Xe + (I· · ·HI) system. The 1268.0 and 1289.2 cm−1 bands are assigned to structure I of HXeI· · ·HI in different matrix sites, and the different photostability (see Figure 6(a)) may also be explained by the matrix site effect. The stabilization of the H−Xe bond may have some effects on photostability but it cannot explain the huge difference between the
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photostabilities of HXeI monomer main band and the sharp side band, whose frequencies are very close to each other.
B. HXeI· · ·HBr complex
The precursor of HXeI· · ·HBr is the HI· · ·HBr complex. In this complex HI can act as a proton donor and acceptor, HBr· · ·HI and HI· · ·HBr, respectively. We assign the 2468.5 cm−1 band to the HBr vibration in the HI· · ·HBr complex (Figure 4) based on the following reasons. The intensity of this band correlates with the amount of HI in the matrices, and the large red shift (−51.1 cm−1 from the Q branch of HBr) indicates that HBr acts as a proton donor. In the HI region, there are two bands, at 2177.3 and 2150.9 cm−1 , which appear only when both HI and HBr are present in a matrix. The frequency shifts of these two bands from the strongest band of HI monomer at 2214.9 cm−1 are −37.6 and −51.1 cm−1 while that of HI dimer is −48.7 cm−1 . These bands cannot originate from the HI· · ·HBr complex because the red shift of this complex is expected to be very small and it should be located near the structured absorption of the HI monomer. According to the MP2 calculations by Latajka and Scheiner,47 the frequency shift of HI in the HBr· · ·HI complex is −22 cm−1 and the shift in the HI dimer is −36 cm−1 . Based on this, we assign the 2177.3 cm−1 band to the HBr· · ·HI complex. The 2150.9 cm−1 band probably originates from some of the mixed trimers like (HBr)2 HI and HBr(HI)2 . Upon 193 nm photolysis of HI/HBr/Xe matrices, the formation of the I· · ·HBr complex is indicated by a band at 2465.1 cm−1 . Because HBr is more photostable than HI under 193 nm radiation, the formation of the I· · ·HBr complex is more probable compared to the Br· · ·HI complex. This case is similar to the HCl/HBr/Xe system where the Br· · ·HCl formation is the dominating channel in the photolysis of the HBr· · ·HCl complex.11 The I· · ·HBr complex participates in the annealing induced reaction H + Xe + (I· · ·HBr) to form the HXeI· · ·HBr complex, which absorbs at 1281.0, 1302.7, 1349.9, 2141.5, and 2254.2 cm−1 . The assignment is supported by efficient bleaching of these bands by UV light, similarly to the HXeI monomer bands (Figure 5). The negligible formation of the Br· · ·HI complex explains the absence of the HXeBr· · ·HI complex in the photolyzed and annealed HI/HBr/Xe matrices. The structural assignments of the HXeI· · ·HBr complexes are done by using the computational results similarly to the HXeI· · ·HI complexes (Table V). The bands at 2141.5 and 2254.2 cm−1 are assigned to the HBr stretching mode of structure I because it has high calculated intensity (1162 km mol−1 , Table IV) and a large red shift from the HBr monomer (experimental shifts −265.8 and −378.5 cm−1 ; calculated shift −221.2 cm−1 ). Based on the agreement of photodecomposition rates, the 1302.7 (+109.5) and 1349.9 (+156.7) cm−1 bands are assigned to the H−Xe stretching mode of the same structure and the agreement with the calculated shift is reasonable (+148.7 cm−1 at the CCSD(T)/def2-TZVPPD level). The band at 1281.0 (+87.8) cm−1 probably originates from structure III (calculated shift of +101.7 cm−1 ) since the calculated shift for structure II is much smaller (+42.8 cm−1 ).
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The formation of structure III requires precursor HBr and HI molecules in the neighboring cages, and similar structures were observed for the HXeBr· · ·HBr, HXeCl· · ·HCl, and HKrCl· · ·HCl complexes.9, 11 The calculated shift of the HBr stretching mode in structure I of the HXeI· · ·HBr complex (−221.2 cm−1 at the CCSD(T)/def2-TZVPPD level) is somewhat smaller than the experimental shifts of −265.8 and −378.5 cm−1 , i.e., the calculations underestimate the complexation effects on the HBr moiety. A similar deviation (underestimate) has been noted for the HI· · ·HCl complex, in which the experimental shift of the HCl vibration in an argon matrix (−94 cm−1 ) is bigger than the MP2 value (−55 cm−1 ).47–50 Furthermore, it has been shown that the calculated shift of the HY vibration in the HXeY· · ·HX complexes (X, Y = Cl and Br) varies widely depending on the basis sets, and this has been attributed to the extent of a basis set borrowing which depends on the size of the basis set.11 C. Comparison
The experimental frequency shifts of the H−Xe stretching mode are from +37.2 to +96.0 cm−1 and from +109.5 to +156.7 cm−1 for the HXeI· · ·HI and HXeI· · ·HBr complexes, respectively, in qualitative agreement with the calculations (Tables IV and V). For both HXeI· · ·HY complexes (Y = Br and I), structure I, which is stabilized mainly by the Y−H· · ·I hydrogen bond, is most stable. The interaction energy of these complexes (see Table II) is typical for medium strength hydrogen bonds, e.g., in the H2 O· · ·HOH (−4.5 kcal mol−1 ), H3 N· · ·H2 O (−5.8 kcal mol−1 ), and CH2 O· · ·HF (−5.5 kcal mol−1 ) complexes calculated at the MP2/6-311++G(d,p) level of theory.51 It is worth noticing that the Eint CP values of the HXeI· · ·HBr complex are bigger by 0.81 (CCSD(T)) and 0.53 (MP2) kcal mol−1 than those of the HXeI· · ·HI complex. This difference may be explained by the contribution of electrostatic forces taking into account the larger dipole moment of HBr (0.82 D) than that of HI (0.44 D).52 For the other structures (II–IV), the interaction is, in contrast, stronger for the HXeI· · ·HI complex, which may be attributed to larger polarizability of iodine (∼5 Å3 ). In a similar way, the lack of stability for the Br−H· · ·H−Xe−I complex (structure IV) may be explained by larger dipole-dipole electrostatic repulsion and smaller contribution of stabilizing dispersion energy due to smaller polarizability of bromine (3.1 Å3 ). Next, we compare the HXeY· · ·HY complexes (Y = Cl, Br, and I). The interaction energies of the most stable structures are −8.1 kcal mol−1 (HXeCl· · ·HCl), −6.8 kcal mol−1 (HXeBr· · ·HBr) at the MP2(full)/cc-pVTZ-PP[Xe, Br] level,11 and −5.7 kcal mol−1 (HXeI· · ·HI) at the MP2(full)/def2-TZVPPD level. The smaller interaction energy of the HXeI· · ·HI complex may be attributed to the weaker electrostatic (dipole-dipole) interaction. This may be explained by the fact that the dipole moments of HI (0.44 D) and HXeI (6.10 D) are smaller than the values for Y = Br (0.82 and 6.45 D) and for Y = Cl (1.08 and 6.45 D), where the dipole moments of HY is from Ref. 52 and those of HXeY are calculated at the MP2(full)/def2-TZVPPD level. Other components of the
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intermolecular interaction, such as polarization, dispersion, and charge transfer interactions,53, 54 can also be different for these species, but this analysis exceeds the scope of the present work. For structure I, the calculated shifts of the H−Xe stretching mode are similar for the HXeY· · ·HY complexes: +117 cm−1 (Y = Cl), +119 cm−1 (Y = Br),11 and +111 cm−1 (Y = I) at the MP2 level. However, to consider the complexation effect, we should compare the shifts normalized by the frequency of the HXeY monomer. The harmonic frequencies of the HXeCl, HXeBr, and HXeI monomers are 1920.1, 1818.3, and 1663.2 cm−1 ; thus, the normalized shifts are 6.1%, 6.5%, and 6.7% for the HXeCl· · ·HCl, HXeBr· · ·HBr, and HXeI· · ·HI complexes, respectively. HXeI has a weaker H−Xe bond than HXeCl and HXeBr, so that one could expect the strongest effect for the case of HXeI. However, the calculated shift in HXeI is not exceptionally large, which might be due to a weaker interaction between HXeI and HI. The experimental H−Xe stretching frequency shifts for the HXeCl· · ·HCl, HXeBr· · ·HBr, and HXeI· · ·HI complexes are up to +116, +145,11 and +96 cm−1 , respectively, which corresponds to the normalized shifts of 7.0%, 9.6%, and 8.0%, in agreement with the calculations. The value for the HXeI· · ·HI complex is probably the most affected (decreased) by solvation in a xenon matrix. A remarkably large shift is observed for the HXeI· · ·HBr complex. The largest H−Xe stretching shift in this complex is +156.7 cm−1 , corresponding to a normalized shift of 13.2%, which is the biggest value among all HXeY· · ·HX (Y, X = I, Br, and Cl) complexes. This shift shows that the H−Xe bond in the HXeI· · ·HBr complex is stronger by 28%, in terms of the force constant, than in the HXeI monomer. The IR decomposition rates of the HXeI· · ·HY complexes correlate with their H−Xe stretching shifts (Figure 7). In other words, the complexes with larger blue shifts are more stable under IR irradiation. The blue shift of the H−Xe stretching mode is a measure for the strength of the H−Xe bond. In general, the photostability should be connected with the strength of the H−Xe bond because the HXeI· · ·HY complexes are presumably decomposed to H + Xe + (I· · ·HY). Indeed, other factors exist such as the position of the excited states and the matrix effect. However, the observed correlation between the shift and photostability is remarkable. The study of thermal stability of these (and other) complexes is underway in our laboratory. V. CONCLUSIONS
The HXeI· · ·HY complexes (Y = Br and I) have been prepared in a xenon matrix and identified by IR spectroscopy and quantum chemical calculations. The HXeI· · ·HI complexes were synthesized by photolyzing the HI dimer to the I· · ·HI complex and subsequent thermal mobilization of hydrogen atoms. The H−Xe stretching mode of the HXeI· · ·HI complex was observed at 1230.4, 1268.0, and 1289.2 cm−1 with the monomer-to-complex shift of +37.2, +74.8, +96.0 cm−1 , respectively. The HI stretching of the HI moiety was observed at 2048.0 cm−1 , which is shifted from HI monomer by −166.9 cm−1 . The quantum chemical
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calculation at the MP2(full) and CCSD(T) levels of theory with the def2-TZVPPD basis set predicted four minima for the HXeI· · ·HI complex with the counterpoise-corrected interaction energies of −3.88, −2.69, −1.23, and −0.23 kcal mol−1 (Figure 1). The observed HI stretching band is assigned to the most stable structure I based on the large IR intensity calculated for this structure at the MP2(full) level (1228 km mol−1 ) and the frequency shift calculated at the CCSD(T) level (−160.8 cm−1 ). The 1268.0 cm−1 band (shift +74.8 cm−1 ) is also assigned to structure I based on the photodecomposition experiments. The 1230.4 cm−1 band (shift +37.2 cm−1 ) is assigned to the second most stable structure. The difference between the experimentally observed and computational H−Xe stretching frequency shifts is presumably connected to the solvation of HXeI in a xenon matrix and to inaccuracy of the theoretical description (harmonic approximation, etc.). The HXeI· · ·HBr complexes in a xenon matrix were prepared from the HI· · ·HBr complex and observed at 2254.2, 2141.5, 1349.9 1302.7, and 1281.0 cm−1 with the monomerto-complex shift of −265.8, −378.5, +156.7, +109.5, +87.8 cm−1 , respectively. The calculations at the CCSD(T) level predicted three stable and one meta-stable structures. The 2254.2 and 2141.5 cm−1 bands are assigned to the HBr stretching of the most stable structure I of the HXeI· · ·HBr complex. The 1349.9 and 1302.7 cm−1 bands are assigned to the H−Xe stretching mode of the same structure. The band at 1281.0 cm−1 most probably originates from structure III. The normalized shift of the H−Xe stretching mode in the HXeI· · ·HBr complex is up to 13% which is the largest value among the experimentally prepared HXeY· · ·HX complexes (X, Y = I, Br, and Cl). ACKNOWLEDGMENTS
This work was supported by the Academy of Finland (Grant No. 139105). S.B. and Z.L. acknowledge the National Center for Research and Development of Poland for the support of this research (Grant ERA-CHEMISTRY2009/01/2010). The authors thank the Wroclaw Centre for Networking and Supercomputing and the CSC—IT Center for Scientific Computing Ltd. (Espoo, Finland) for generous allocation of computer time. 1 L.
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