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Timestamp: 2019-04-22 12:02:37+00:00

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We report an experimental and theoretical study on new noble-gas hydride complex HKrCCH⋯CO2, which is the first known complex of a krypton hydride with carbon dioxide. This species was prepared by the annealing-induced H + Kr + CCH⋯CO2 reaction in a krypton matrix, the CCH⋯CO2 complexes being produced by UV photolysis of propiolic acid (HCCCOOH). The H–Kr stretching mode of the HKrCCH⋯CO2 complex at 1316 cm−1 is blue-shifted by 74 cm−1 from the most intense H–Kr stretching band of HKrCCH monomer. The observed blue shift indicates the stabilization of the H–Kr bond upon complexation, which is characteristic of complexes of noble-gas hydrides. This spectral shift is slightly larger than that of the HKrCCH⋯C2H2 complex (+60 cm−1) and significantly larger than that of the HXeCCH⋯CO2 complex (+32 and +6 cm−1). On the basis of comparison with ab initio computations at the MP2 and CCSD(T) levels of theory, the experimentally observed absorption is assigned to the quasi-parallel configuration of the HKrCCH⋯CO2 complex. The calculated complexation-induced spectral shift of the H–Kr stretching band (60.4 or 72.7 cm−1 from the harmonic calculations at the MP2 and CCSD(T) levels, respectively) agrees well with the experimental value.
Due to the relatively weak bonding and large dipole moments, the HNgY molecules can be strongly affected by the interaction with other species. As a rule, these complexes show blue shifts of the H–Ng stretching mode, which means complexation-induced stabilization of the H–Ng chemical bond. The largest shift (ca. +300 cm−1) has been reported for the H–Kr stretching mode of the HKrCl⋯HCl complex in a krypton matrix.39 It is significantly larger than the shift of the H–Xe stretching mode of the HXeCl⋯HCl complex studied in a xenon matrix (up to +116 cm−1).40 The shifts of the H–Ng stretching modes in comparable structures of the HKrCCH⋯C2H2 and HXeCCH⋯C2H2 complexes (+60 cm−1 and about +25 cm−1)41,42 as well as of the HKrCl⋯N2 and HXeCl⋯N2 complexes (+32 cm−1 and about +10 cm−1)43,44 are also remarkably different. These differences are presumably explained by weaker bonding of the krypton compounds rather than by differences in interaction energies that are similar for these pairs of complexes. For example, the interaction energies of the HKrCl⋯N2 and HXeCl⋯N2 complexes are 1.42 and 1.34 kcal mol−1, respectively (at the MP2(full)/6-311++G(2d,2p) level of theory, after the basis set superposition error corrections).43,44 From this point of view it is interesting to identify the HKrCCH⋯CO2 complex, keeping in mind that the HXeCCH⋯CO2 complex has been reported in a xenon matrix (shifts of the H–Xe stretching mode of +32 and +6 cm−1 from that of HXeCCH monomer).45 It is worth noting that, among the complexes of noble-gas hydrides, the complexes with carbon dioxide are of particular interest in view of possible preparation of HNgY in this simple molecular medium. Indeed, the stabilization of HXeBr was reported in a CO2 matrix.38 However, to the best of our knowledge, the complexes of any krypton hydrides with CO2 are still unknown.
The current work presents an ab initio and matrix isolation study of the HKrCCH⋯CO2 complex. The preparation of this complex is not an easy task. Indeed, it has been shown that 193 nm photolysis of propiolic acid (HC3OOH, PA) in noble-gas matrices produces the C2H2⋯CO2 complex with a high yield.46 However, the C2H⋯CO2 complexes do not appear in prolonged 193 nm photolysis of the C2H2⋯CO2 complexes in a krypton matrix, which is due to photodecomposition of former species.47 On the other hand, the C2H⋯CO2 complexes can be prepared in a krypton matrix by two-step 193/275 nm photolysis.47 At the second step of photolysis (275 nm), the C2H⋯CO2 complexes most probably originate from the HC2O⋯CO complexes (not from the C2H2⋯CO2 complexes). The C2H⋯CO2 complexes in a krypton matrix can be then reacted with thermally mobilized hydrogen atoms to produce the HKrCCH⋯CO2 complexes. The experiments are supported by extensive quantum chemical calculations at the MP2 and CCSD(T) levels of theory.
The PA/Ng (Ng = Ar and Kr) mixtures were made in a glass bulb by using standard manometric procedures. PA (≥98%, Alfa Aesar) was degassed by several freeze–pump–thaw cycles. Argon (99.9999%, Linde) and krypton (99.999%, Linde) were used as purchased. Since PA is efficiently adsorbed on glass surfaces, the bulb was passivated with these vapors by several fill-keep-evacuate cycles prior to the mixture preparation. The PA/Ng (1/1000) matrices were deposited onto a CsI window cooled by a closed-cycle helium cryostat (RDK-408D2, Sumitomo Heavy Industries, Ltd). The matrices were deposited at 15 and 20 K for argon and krypton, respectively. The PA/Ng matrices were photolyzed with an excimer laser at 193 nm (MSX-250, MPB, 1 Hz, ∼4 mJ cm−2) and with an optical parametric oscillator at 275 nm (OPO Sunlite, Continuum, 10 Hz, ∼5 mJ cm−2). The IR absorption spectra in the 4000–500 cm−1 range were measured at 4.3 K using an FTIR spectrometer (Vertex 80, Bruker) equipped with an MCT-B detector using 1 cm−1 resolution and 500 scans.
The C2H⋯CO2 complexes were produced in krypton and argon matrices by following the approach described in our earlier work.47 First, ∼95% of matrix-isolated PA had been photodecomposed by ∼1000 laser pulses at 193 nm, that results in formation of the C2H2⋯CO2 complex (parallel configuration) as the major primary product and some amounts of other photoproducts, one of each had been identified as ketenyl radical (HC2O), presumably complexed with a CO molecule.47 Then, the matrices were irradiated at 275 nm and the characteristic absorptions of the C2H⋯CO2 complex (1852.3 cm−1 and 1856.8 cm−1 for krypton and argon matrix, respectively)47 were observed in FTIR spectra (Fig. 1). It should be pointed out that 275 nm radiation bleaches particularly the bands attributed to HC2O⋯CO,47 and decay of this species correlates well with production of the C2H⋯CO2 complexes. The latter complexes in krypton matrices are efficiently bleached by 193 nm light, that is why a two-step 193/275 nm photolysis was used for its preparation. It should be mentioned that the C2H⋯CO2 complex is also characterized by vibronic absorption bands of the C2H subunit in the near-infrared region as well as absorption bands of the CO2 subunit (see ref. 47 for the detailed spectroscopic information). In this work, we have found that shoulder at 1862.6 cm−1 observed in an argon matrix (marked by dot in Fig. 1) had been previously47 misassigned to the C2H⋯CO2 complexes. This conclusion is based on monitoring of the annealing-induced changes in the spectra (see below). Indeed, the intensity of the absorption at 1862.6 cm−1 increased upon annealing, whereas all the other absorptions of the C2H⋯CO2 complexes did not grow. In fact, this feature (as well as its counterpart, the shoulder at 1859.6 cm−1 observed in a krypton matrix) should be assigned to the HCO radicals48 (most probably complexed with some undefined molecule).
Annealing at about 20 and 30 K is known to mobilize H atoms in argon and krypton matrices, respectively.49–52 Upon annealing of photolyzed matrices at these temperatures, a number of bands rise. The presence of H atoms in the matrices is evidenced by the formation of C2H3 radicals (complexed with CO2,53 spectra a and c in Fig. 2) and HCO radicals. Most of the HCO radicals in a krypton matrix are complexed with some undefined molecule based on the C–H stretching frequency of 2488 cm−1 (blue-shifted from this mode of HCO monomer by 21 cm−1).48 In addition, the annealing of a photolyzed PA/Kr matrix produces small amounts of HKrCCH monomers with the most intense infrared band at 1242 cm−1 and a broad satellite with maximum at 1254 cm−1 (spectrum a in Fig. 2). The principal finding of this work is the discovery of a new HKrCCH related species, which manifests itself in the appearance of annealing-induced band at 1316 cm−1 shifted by +74 cm−1 from the strongest band (and by +62 cm−1 from the second strongest band) of HKrCCH monomer. A similar band is not observed after photolysis and annealing of C2H2/Kr matrices14 and of PA/Ar matrices (spectrum c in Fig. 2). This band at 1316 cm−1 is assigned here to the H–Kr stretching mode of the HKrCCH⋯CO2 complex. Other modes of this complex are too weak to be observed in our experiments. It should be mentioned that the 1316 cm−1 band does not appear after annealing of the matrix photolyzed at 193 nm only (when the C2H⋯CO2 complexes are not observed).
Fig. 2 Difference FTIR spectra showing results of (a) annealing at 35 K of a PA/Kr matrix after 193/275 nm photolysis, (b) irradiation at 254 nm of the previous matrix, (c) annealing at 25 K of a PA/Ar matrix after 193/275 nm photolysis, (d) irradiation at 254 nm of the previous matrix. The labeled bands are from the HKrCCH⋯CO2 complex (C), HKrCCH monomer (M), vinyl radical in the C2H3⋯CO2 complex (V), and acetylene in the C2H2⋯CO2 complex (Ac).
Noble-gas hydrides can be easily destroyed by UV light, which helps one to find relatively weak bands of their complexes.39–41,54,55 In particular, the bands of the HKrCCH⋯C2H2, HXeCC⋯C2H2, and HXeCCXeH⋯C2H2 complexes were observed only in the decomposition spectra.41,55 The 254 nm light from a low-pressure mercury lamp is found to bleach the bands of the HKrCCH⋯CO2 complex and HKrCCH monomer in a krypton matrix with similar efficiency (spectrum b in Fig. 2). In a photolyzed and annealed PA/Ar matrix, irradiation at 254 nm does not lead to notable changes in this spectral region (spectrum d in Fig. 2).
The properties of HKrCCH monomer (structural parameters, charge distribution, and harmonic vibrational spectra) are consistent with those reported previously.14 Geometries of HKrCCH and CO2 monomers obtained in present work at MP2/L2a_3 and CCSD(T)/L2a_3 levels of theory are presented in Fig. S1 (ESI†). A quasi-parallel configuration of the HKrCCH⋯CO2 complex (Fig. 3) was found to be the only true energy minimum on the potential-energy surface by both MP2 and CCSD(T) calculations. Cartesian atomic coordinates, dipole moments, and total energies for the HKrCCH⋯CO2 complex and both HKrCCH and CO2 monomers are given in Table S2 (ESI†). Upon complexation, the H–Kr bond slightly shortens (by 0.014 Å or 0.018 Å, according to MP2 or CCSD(T) computations, respectively) and the Kr–C bond elongates (by 0.012 Å – MP2, 0.011 Å – CCSD(T)), which is characteristic for the complexes of noble-gas hydrides. In the complex, the CO2 and HKrCCH units are somewhat nonlinear, in contrast to the monomers. The CO2 molecule in the complex is tilted to the Kr atom, and this may be due to the additional interaction of a lone pair of the O2 atom (see Fig. 3 for atom labeling) of the carbon dioxide with the krypton atom of HKrCCH. According to the CCSD(T)/L2a_3 computations, the interaction energy in the complex is −3.16 kcal mol−1 taking into account both BSSE and ZPVE corrections (the corresponding value from the MP2/L2a_3 computations is −3.98 kcal mol−1; the effect of corrections is shown in Table S3, ESI†). The charge separation in HKrCCH increases upon complexation, which is also typical for complexes of noble-gas hydrides. In particular, the positive charge on the HKr group increases by 0.026e (GAPT charge increase by 0.041e) as predicted by CCSD(T). The negative charges on the carbon atoms increase accordingly (see Table S4, ESI† for more details). It should be mentioned that two other stationary points (linear configurations) were found for the HKrCCH⋯CO2 system (both in MP2/L2a_3 and CCSD(T)/L2a_3 computations); however, they appear to be saddle points at the present computational levels and hence are not considered. We can only notice that these two linear configurations are essentially (by ca. 3 kcal mol−1) higher in energy than the quasi-parallel configuration.
Fig. 3 Structure of the HKrCCH⋯CO2 complex obtained at the CCSD(T)/L2a_3 level of theory (the structural parameters obtained at the MP2/L2a_3 level of theory are shown in parentheses). The distances are in Å.
The vibrational spectroscopic characteristics of the HKrCCH⋯CO2 complex and HKrCCH and CO2 monomers obtained from the CCSD(T)/L2a_3 computations are presented in Table 1 (the corresponding results of MP2/L2a_3 computations are given in Table S5 (ESI†)). The strongest infrared absorption (calculated infrared intensity >2000 km mol−1) in the HKrCCH⋯CO2 complex is provided by the H–Kr stretching mode, and the corresponding absorption band was detected in the experiment. This mode experiences a blue shift of 60.4 cm−1 according to the CCSD(T)/L2a_3 results (72.7 cm−1 – MP2/L2a_3), which correlates with the shortening of the H–Kr bond. It should be mentioned that computations within harmonic approximation give some overestimation for the H–Kr stretching frequency: for HKrCCH (monomer), the theoretical value is 1364 cm−1 (CCSD(T)/L2a_3, harmonic), whereas the experimental one is 1242 cm−1. The theoretically predicted (at the CCSD(T)/L2a_3 level) anharmonic frequencies of the H–Kr stretching mode are 1221.2 cm−1 and 1273.8 cm−1 for the HKrCCH monomer and the HKrCCH⋯CO2 complex, respectively, that is closer to experimental values. However, we may note that the value of complexation-induced shift (+52.6 cm−1) calculated with anharmonic corrections is not significantly different from that obtained in the harmonic approximation.
The second strongest infrared absorption of the complex corresponds to the antisymmetric stretching mode of CO2 (calculated infrared intensity is ca. 500 km mol−1); however, it experiences a very small shift, which obviously complicates its unambiguous experimental detection. The bending modes of HKrCCH and CO2 are non-degenerate in the complex due to symmetry reasons, but the corresponding absorptions are not intense enough to be detected experimentally.
The spectral effect of CO2 on HKrCCH is slightly larger than that of C2H2 (+60 cm−1).41 On the other hand, the present complexation-induced shift is much larger than that in the HXeCCH⋯CO2 complex (+32 and +6 cm−1).45 The latter observation agrees with the trend for other known pairs of complexes of noble-gas hydrides: HNgCCH⋯C2H2, HNgCl⋯HCl, and HNgCl⋯N2 (Ng = Kr and Xe).39–44 In all four cases, the experimental shift is two–three times bigger for the krypton species than for the xenon ones. The strict theoretical interpretation of this trend is outside the scope of this work. We suppose that it is connected with the weaker bonding of the krypton hydrides as compared to the xenon analogues. The weaker H–Kr bonding leads to the larger anharmonicity of the H–Kr stretching mode compared to the H–Xe one, which is experimentally indicated by the smaller H/D frequency ratios. These ratios are 1.335 and 1.376 for HKrCl and HXeCl,8 respectively, and 1.350 and 1.379 for HKrCCH and HXeCCH,14,16,71 respectively.
On the basis of the calculations, the structural assignment of the experimental complex is straightforward. Both scalar-relativistic MP2/L2a_3 and scalar-relativistic CCSD(T)/L2a_3 computations provide consistent results – only one (quasi-parallel) configuration (Fig. 3) is predicted to be a true minimum on the potential energy surface. The absolute values of the H–Kr stretching harmonic frequency of the HKrCCH monomer and the HKrCCH⋯CO2 complex are somewhat overestimated, while the H–Kr stretching anharmonic frequency values are more consistent with experimental results. Meanwhile, the calculated complexation-induced shift of the H–Kr stretching mode is in good agreement with the experimental value and it does not depend much on the anharmonicity effect.
Finally, it should be noted that three computationally stable configurations (one parallel and two linear) were obtained for the HXeCCH⋯CO2 complex using the MP2/6-311++G(2d,2p) and MP2/aug-cc-pVDZ calculations.45 It is in contrast to the present scalar-relativistic MP2/L2a_3 and scalar-relativistic CCSD(T)/L2a_3 calculations for the HKrCCH⋯CO2 complex showing only one (quasi-parallel) stable configuration. This difference is probably not due to the change of the noble gas (krypton vs. xenon) but rather due to the computational level (higher in the present work). In fact, our preliminary calculations of the HXeCCH⋯CO2 complex at the scalar-relativistic CCSD(T)/L2a_3 level of theory features only one true minimum (quasi-parallel configuration) with the complexation-induced shift of +28.5 cm−1 for the H–Xe stretching mode (which is very close to the experimental value of +32 cm−1 reported in ref. 45). Thus, we may note that using the scalar-relativistic MP2 and CCSD(T) theories allow us to reliably predict the complexation-induced spectral shifts for the complexes of noble gas hydrides. In the present case, the agreement is nearly quantitative. Meanwhile, the detailed comparison of different theoretical approaches for accurate description of the properties of noble-gas hydrides and their complexes is still a challenge for future work, beyond the scope of this manuscript.
This work was supported by the Academy of Finland (Projects No. 1277993 and 1288889) and the Division of Chemistry and Material Sciences of the Russian Academy of Science (sub-program “Nature of chemical bond and mechanisms of the most important chemical reactions and processes”). The Joint Supercomputer Center of the Russian Academy of Sciences (Moscow) is gratefully acknowledged for granting computation resources. The authors are grateful to D. N. Laikov for implementation of the method for anharmonic frequencies calculation.
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‡ Deceased 16 March 2018.

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