The HKrCCH⋯CO2 complex: an ab initio and matrix-isolation study

Sergey V. Ryazantsev ab, Daniil A. Tyurin b, Kirill B. Nuzhdin b, Vladimir I. Feldman *b and Leonid Khriachtchev a
aDepartment of Chemistry, University of Helsinki, P. O. Box 55, FI-00014 Helsinki, Finland
bDepartment of Chemistry, Lomonosov Moscow State University, 119991 Moscow, Russia. E-mail: feldman@rad.chem.msu.ru

Received 9th July 2018 , Accepted 1st November 2018

First published on 5th November 2018


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.


1. Introduction

The first noble-gas compound, xenon hexafluoroplatinate, was reported by Neil Bartlett in 1962.1 To date, many noble-gas compounds have been synthesized.2–5 Matrix isolation infrared spectroscopy was extensively used for characterization of such species from the beginning of these studies.6,7 In particular, noble-gas hydrides with the general formula HNgY (Ng is a noble-gas atom and Y is an electronegative atom or fragment) were discovered by Pettersson et al. in 1995.8 These molecules have a charge-transfer character (HNg)+Y and a strong H–Ng stretching absorption.4,9 About thirty noble-gas hydrides have been prepared, including an argon molecule HArF10,11 and the most recent species HKrCCCl, HXeCCCl, and C6H5CCXeH.12,13 In particular, an organic krypton molecule HKrCCH was prepared by photochemical insertion of a krypton atom into acetylene.14 Synthesis of noble-gas hydrides usually includes photolysis8–14 or radiolysis15,16 of the HY precursor in low-temperature solid matrices and subsequent annealing to mobilize the photogenerated hydrogen atoms and induce the H + Ng + Y reaction. In some cases, noble-gas hydrides are formed directly during UV photolysis of the HY/Ng matrices.10,17,18

Complexes involving noble-gas hydrides is an interesting case of noncovalent interactions, and a number of such species have been prepared in noble-gas matrices.19,20 The computational studies on the HNgY complexes were even more extensive (see, for example, ref. 21–37). The agreement between theory and experiment for the spectroscopic characteristics of these complexes is qualitatively reasonable, but far from being perfect, and there is a motivation for further work combining experiment and high-level calculations. Furthermore, characterization of the intermolecular complexes may be considered as an important step to preparation of noble-gas hydrides in molecular environment, which is a one of the major challenges in the noble gas hydride chemistry.25,26,35,36,38

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.

2. Experimental details and results

Experimental details

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.

Experimental results

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).
image file: c8cp04327b-f1.tif
Fig. 1 FTIR spectra showing the absorption bands of C2H⋯CO2 complexes, which were produced by two-step 193/275 nm photolysis of propiolic acid in krypton and argon matrices. The band marked with an asterisk (1839.7 cm−1) is assigned to a less stable configuration of the C2H⋯CO2 complex observed only in an argon matrix.47 The bands marked with dots originate from HCO (monomeric or complexed).48

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).


image file: c8cp04327b-f2.tif
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).

3. Computational details and results

Computational details

The molecular geometries were optimized (tolerance on gradient 2 × 10−7 a.u.) at the scalar-relativistic56,57 valence-correlated MP2 and scalar-relativistic56,57 valence-correlated CCSD(T)58,59 levels of theory. The augmented scalar-relativistic correlation consistent basis set of type L2a_3 (which is a further-developed version of L2a basis set) was used.60 This basis set is analogous to the aug-cc-pVTZ basis set,61–63 but it includes a larger number of primitive Gaussian functions (the comparison of the basis set contraction schemes for the L2a_3 and aug-cc-pVTZ basis sets is presented in Table S1, ESI). This basis set L2a_3 (and similar basis set L2a with shorter primitive expansion) was successfully used in our previous studies of intermolecular complexes (see, for example, ref. 53, 64 and 65). The minimum points on the potential energy surface (PES) were verified by vibrational analysis. For the optimized geometries, the harmonic vibrational frequencies, zero-point vibrational energies (ZPVE), and IR intensities were calculated at the same level of theory. The H–Kr stretching anharmonic frequencies (for HKrCCH molecule and its complex with CO2) were extracted from the anharmonic vibrational levels calculated using numerical potential curve obtained by energy scan along the H–Kr stretching mode at the scalar-relativistic CCSD(T)/L2a_3 level. The charges on atoms were computed by the zero-bond-dipole intrinsic minimal atomic basis scheme66 with the effective Hamiltonian extracted67 from the CCSD(T) or MP2 density matrix. The GAPT atomic charges68 were also computed. The interaction energy was found as a difference between energies of the complex and the monomers taking into account the basis set superposition error (BSSE)69 and ZPVE corrections. All calculations were performed with the PRIRODA computer code.70

Computational results

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.
image file: c8cp04327b-f3.tif
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.

Table 1 Calculated harmonic frequencies (cm−1) and IR intensities (km mol−1, in parentheses) of the HKrCCH⋯CO2 complex and the corresponding values of HKrCCH and CO2 monomers computed at the CCSD(T)/L2a_3 level of theory
Complex Assignment Monomers
3429.0 (36.8) C–H str. 3434.0 (36.2)
2386.0 (517.1) CO2 antisymm. str. 2387.1 (648.6)
1992.5 (1.2) C[triple bond, length as m-dash]C str. 1997.7 (0.5)
1424.4 (2144.0) H–Kr str. 1364.0 (2218.8)
1345.9 (0.2) CO2 sym. str. 1345.3 (0.0)
672.0 (27.9) Out-of-plane bend. (CO2 + HCC + HKrC) 670.9 (27.1)
668.8 (0.0) Out-of-plane bend. HKrC 664.2 (4.5)
667.5 (1.9) In-plane bend. HKrC 664.2 (4.5)
652.9 (47.8) In-plane bend. CO2 670.9 (27.1)
633.1 (35.8) Out-of-plane bend. HCC 626.9 (38.1)
632.5 (46.4) In-plane bend. HCC 626.9 (38.1)
308.8 (118.5) C–Kr str. 311.0 (110.3)
151.0 (18.5) In-plane bend. KrCC 117.1 (14.8)
120.1 (15.1) Out-of-plane bend. KrCC 117.1 (14.8)
98.9 (1.748) Intermol. vibr.
86.6 (0.2) Intermol. vibr.
50.5 (1.7) Intermol. vibr.
33.2 (0.4) Intermol. vibr.


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.

4. Concluding remarks

In this work, we have identified the HKrCCH⋯CO2 complex, which is a new complex of a noble-gas hydride HKrCCH and the first example of a krypton hydride with carbon dioxide. This species is produced in the annealing-induced H + Kr + C2H⋯CO2 reaction in a krypton matrix. The C2H⋯CO2 complexes are generated by a two-step 193/275 nm photolysis of propiolic acid in a krypton matrix (Fig. 1). The H–Kr stretching band of the HKrCCH⋯CO2 complex is observed at 1316 cm−1 and it is blue-shifted by 74 cm−1 from the strongest feature of HKrCCH monomer (Fig. 2). This blue shift shows the stabilization of the H–Kr bond upon complexation, which is the normal effect for complexes of noble-gas hydrides.19

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.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

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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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cp04327b
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