Vibrational Spectrum of HXeSH revisited : Combined computational and experimental study

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY-NC-ND 4.0 https://creativecommons.org/licenses/by-nc-nd/4.0/ Vibrational Spectrum of HXeSH revisited : Combined computational and experimental study © 2020 Elsevier B.V. All rights reserved. Accepted version (Final draft) Cukras, Janusz; Ahokas, Jussi M.E.; Lundell, Jan Cukras, J., Ahokas, J. M., & Lundell, J. (2020). Vibrational Spectrum of HXeSH revisited : Combined computational and experimental study. Chemical Physics Letters, 741, Article 137083. https://doi.org/10.1016/j.cplett.2019.137083 2020 Journal Pre-proofs Research paper Vibrational Spectrum of HXeSH revisited: Combined computational and experimental study Janusz Cukras, Jussi M.E. Ahokas, Jan Lundell PII: S0009-2614(19)31064-4 DOI: https://doi.org/10.1016/j.cplett.2019.137083 Reference: CPLETT 137083 To appear in: Chemical Physics Letters Received Date: 4 November 2019 Revised Date: 30 December 2019 Accepted Date: 31 December 2019 Please cite this article as: J. Cukras, J.M.E. Ahokas, J. Lundell, Vibrational Spectrum of HXeSH revisited: Combined computational and experimental study, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/ j.cplett.2019.137083 This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V. Vibrational Spectrum of HXeSH revisited: Combined computational and experimental study Janusz Cukras1*, Jussi M.E. Ahokas2 and Jan Lundell3 1 Department of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warsaw, Poland, [email protected] 2 NanoScience Center and Department of Chemistry, University of Jyväskylä, P.O.Box 35, 40014 University of Jyväskylä, Finland, [email protected] 3 Department of Chemistry, University of Jyväskylä, P.O.Box 35, 40014 University of Jyväskylä, Finland, [email protected] * Corresponding author Abstract Vibrational spectrum of HXeSH embedded in low-temperature matrix is experimentally studied. To support the spectrum interpretation, anharmonic vibrational analysis is performed using different models and basis sets and the data is compared with previous experimental and theoretical analyses. Computations of overtones and combination modes allowed for new band assignments. The HXeSH molecule exhibits high anharmonicity similarly as other molecules from the noble-gas hydride family. Comparison of the employed computational methods shows once again that the modelling of the noble-gas compounds faces theoretical challenges to yield quantitatively reliable results. Keywords: Xenon, noble gas, hydride, computational chemistry, anharmonicity, infrared spectrum, vibrational spectroscopy, matrix isolation Introduction The noble-gas chemistry has experienced a steady interest since its renaissance in the 90’s when a group of then-novel compounds was synthesized in low-temperature matrices [1–3]. These compounds included HNgY-type (where Ng=noble gas atom, Y= electronegative atom or group) molecules like HXeBr, HKrCl [1], HXeCN [3], HXeOH [4], HXeCCH [5], HArF [6], to name a few. They have been investigated both experimentally and computationally [7,8] and extensively reviewed [9–11]. They are intriguing to chemists not only because they are formed by atoms with full valence shell and they exhibit blueshifting complexes with other molecules [12–16], but also because they may shed light on many intriguing properties exhibited by noble gases like neuroprotective properties [17,18], anaesthetic properties [19,20] and the puzzling problem of missing xenon [21–23]. Among the fascinating group of noble-gas hydride compounds is the HXeSH molecule first synthesized in 1998 [24]. Being the first example of the Xe–S bond it shows similar features as other molecules in the family. It is formed in a photodissociation and subsequent process of H2S in xenon matrix. The Xe–S bond exhibits strong ionic character and the SH group bears a significant negative charge, yielding a system that can be approximated as (HXe)+(SH)–. Its most intense infrared (IR) band is the νXe–H stretch ca. 1118 cm–1 and its first overtone was tentatively assigned at ca. 2089 cm-1 giving the anharmonicity (ωexe) of – 74.5 cm–1 [25]. In general, the anharmonic effects in HXeSH, and in these HNgY molecules in general, are shown to be important and non-negligible [26,27]. Here, we revisit the infrared spectrum of the HXeSH molecule and provide new assignments of overtones and combination bands. Understanding the IR spectra for noble-gas hydrides is important because it is the main investigation method used to study them and the anharmonicity itself is a subject of interest. For our investigation, we combine experimental measurements in low-temperature matrices with computational predictions. This improves both the understanding of the IR spectrum of HXeSH and the computational methodology used to predict the properties of the noble-gas hydrides. Figure 1. The structure of the HXeSH molecule. The colours white, yellow and turquoise designate hydrogen, sulfur and xenon atoms, respectively. Experimental details The matrices were deposited onto a MgF2 substrate placed in a closed-cycle helium refrigerator (Displex DE-202A) at 45 K. The total amount of deposited gas was ∼3 mmol. As a matrix film of sufficient thickness was grown, the sample was annealed at 50–55 K in order to minimize changes in the optical characteristics during the actual experiments. All spectral measurements were carried out at ca. 10 K. The temperature of the cold substrate was measured with a silicon diode, and controlled with a LakeShore 330 controller unit. The dilution ratio between the precursor and Xe was typically 1:1000 to 1:500. H2S of 99.8% purity and Xe of 99.997% purity were obtained from Messer Griesheim and AGA, respectively, and were used without further purification. The IR absorption spectra were measured with a Nicolet Magna IR 760 spectrometer equipped with a KBr beam splitter and a HgCdTe detector. The MgF2 substrate, used for subsequent UV measurements reported elsewhere [28], limited the detection window in the IR to the range >1000 cm-1. The samples were irradiated using a 193 nm excimer laser (Lambda Physik, Optex) and a 308 nm excimer laser (Estonian Academy of Sciences, ELI94). The full details of experimental setup and processes have been published earlier in Refs [28] and [29]. Computational details To support the analysis of the experimental data, anharmonic vibrational frequencies were computed by the vibrational self-consistent field (VSCF) and its extension by corrections via second-order perturbation theory (CC-VSCF) [30–33]. The correlation-consistent VSCF algorithm is used to calculate the vibrational wave functions and energies. Only interactions between pairs of normal modes are included in the calculations, since interactions of triples and higher were taken negligible [30,31]. Each pair of normal modes were pictured with a 16 × 16 PES grid and the mode-mode couplings were then evaluated by ab initio calculations over this grid. A more detailed description of the MP2/CC-VSCF method is given in Refs [31] and [32]. All CC-VSCF calculations in this work were performed in the framework of GAMESS electronic structure program [34]. Electron correlation was considered via MøllerPlesset perturbation theory [35,36] to second order (MP2) and aug-cc-pVDZ-PP basis set was employed which includes the scalar relativistic effect by means of the Effective Core Potentials (ECP) with 8 valence electron shell for xenon [37]. The CC-VSCF calculations were utilized to gain insight of the anharmonicity and overtone spectrum of HXeSH on a computational level used previously for other noble gas hydrides [26,27]. Additional computations on the structure and frequencies of HXeSH were performed using the Gaussian program [38] in order to see the effect of the method and the quality of basis sets used on the harmonic and anharmonic spectrum. Very tight convergence threshold was used throughout the geometry optimization calculations. For H and S atoms, the aug-cc-pVxZ (x=D, T or Q) basis sets were used. For Xe atom, appropriate relativistic variants, i.e. the aug-cc-pVxZ-PP (x=D, T or Q) basis sets. The basis sets are shortly called ‘dz’, ‘tz’ and ‘qz’ in the text. The structures were optimized and vibrational analysis was done using the second order of the Møller-Plesset Perturbation Theory (MP2 variant of MPPT), the fourth order of MPPT including singles, doubles and quadruples (MP4(SDQ)), Density Functional Theory (DFT) with B3LYP potential and Coupled Cluster Singles and Doubles method (CCSD). MP2 and B3LYP calculations were carried out using the anharmonic analytical algorithm whereas MP4(SDQ) and CCSD calculation were done using the harmonic and numerical approach. Because of the cost of the calculations the latter were not done with the quadruple zeta basis set, i.e. qz. Results and discussion Experimental results The IR absorption spectra of our H2S/Xe matrix were similar to what is previously reported for H2S doped Xe matrices [39]. The symmetric (ν1) and asymmetric (ν3) stretching modes of H2S are found at 2596 and 2620 cm–1 exhibiting a multiple band structure due to the hindered rotation the molecule undergoes in the matrix [39]. The H2S bending mode appears as a very weak band at ca. 1180 cm–1. According to Isoniemi et al. [39], the main dissociation channel of H2S in noble gas matrices employing 193 nm photolysis yields H atom and SH radical with a secondary channel to H2 molecule and S atom also available. Moreover, SH radicals photodissociate further to S and H atoms [40,41]. Here, upon 193 nm photolysis of the matrix, more than 90% of the total amount of H2S, including monomers and multimers, was dissociated as evidenced by Figure 2. Parallel to the decrease of the H2S lines, formation of a photolysis product was indicated by a new line at 2550.5 cm–1. This absorption has previously been assigned as the SH…H2S complex [39], indicating the formation of SH radicals from the H2S dimer. The absorption associated with isolated SH radical could not be observed in these experiments. Annealing of the photolyzed sample at 48 K induced new absorption bands at 1112, 1119, 1136, 1166, and 1181 cm–1 due to formation of HXeSH (first three) and HXeH (last two) [2,24]. An additional photolysis at a longer wavelength, 308 nm, bleached the HXeSH absorption bands completely, in accordance with selective photolysis of the noble gas molecules reported previously [42]. This photolysis wavelength coincides with the previously observed electronic transition of HXeSH between 260–325 nm with a maximum at 290 nm [28]. All these processes are found in Fig. 2 including a subsequent photolysis with 193 nm pulses bleaching the formed photoproducts and reproduction of the noble gas molecules upon annealing. These processes take place without notable losses of the forming noble gas molecules HXeH and HXeSH. Employing the reversible processes of forming and decomposing the noble gas molecules are helping in locating and identifying other vibrational absorptions belonging to the noble gas molecules than the fundamental ones. This is demonstrated in Fig. 2 on the left panel, where bands exhibiting the same appearance and bleaching patterns to the fundamental modes are found. Clearly, vibrational absorptions belonging to studied noble gas molecules are found at 2004, 2070 and 2087 cm–1, as indicated by arrows in Fig. 2. The vibrational absorption at 2004 cm–1 is previously known and it has been assigned as the ν1+ν3 combination mode between the symmetric and asymmetric Xe-H stretching modes of HXeH [27]. The two absorptions at 2070 and 2087 cm–1 can be associated with HXeSH and are discussed later. There is also a bump in the recorded spectrum around 2100–2110 cm–1 that behaves similarly to the other HXeSH absorptions mentioned. This spectral feature is most probable due to complexes involving HXeSH molecule, i.e. either a dimer (HXeSH)2 or HXeSH-H2S [16]. Both of these are plausible products following photodecomposition of one or both subunits in (H2S)2, as demonstrated earlier by Isoniemi et al. [39] being able to produce H2S-SH complex from H2S dimer. This could also be the origin of the weak broad spectral feature at 1125– 1150 cm–1, following the photochemical and thermal behaviour of the HXeSH molecule. Moreover, another vibrational absorption associated with HXeSH appears at 1325 cm–1, which is outside the spectral regions shown in Fig. 2. Figure 2. IR absorption spectra of a H2S/Xe matrix: (a) after approximately 90% of H2S was photolyzed with a 193 nm laser, (b) after annealing the photolyzed matrix at 48 K, (c) second photolysis at 308 nm with 100 pulses, and (d) third photolysis at 193 nm with 630 pulses, and e) after reannealing the matrix at 48 K. The arrows indicate the positions of the observed overtone and combination bands of the noble gas compounds HXeH and HXeSH. Computational results The geometry of the HXeSH molecule is presented in Fig. 1 and numerical values of the bond lenghts and the H–Xe–S angle for different methods employed are given in Table 1. The results correspond well to what is established in the literature [24,43] with rXe–H being ca. 1.7 Å and the αHXeS being close to a right angle. All bond distances and the angle tend to decrease with the size of the basis set within a particular calculation method. There is no particular trend in this respect when the level of electron correlation increases (e.g. MP2<CCSD<MP4(SDQ)). The B3LYP potential tends to give slightly larger values. Table 1: The calculated bond distances (in Å) and angles (in degrees) of the HXeSH molecule using different basis sets and methods. Basis set H–Xe Xe–S S–H ∠(HXeS) MP2/CC-VSCF dz 1.763 2.747 1.353 91.0 MP2 dz 1.765 2.751 1.355 91.1 tz 1.749 2.694 1.340 90.4 qz 1.743 2.678 1.338 90.4 B3LYP dz 1.797 2.756 1.359 92.7 tz 1.789 2.735 1.347 92.5 qz 1.788 2.732 1.345 92.4 CCSD dz 1.784 2.768 1.357 91.8 tz 1.756 2.709 1.343 91.5 MP4(SDQ) dz 1.777 2.765 1.356 91.8 tz 1.753 2.707 1.342 91.6 The values of all the calculated vibrational frequencies, overtones and combinations modes for all the methods are presented in Supplementary Information Tables 1–7, and the selected values which are relevant for the experiment together with the experimental values are presented in Table 2. Table 2: The chosen experimental and calculated vibrational frequencies in cm–1 and the anharmonicities ωexe for of the ν2 mode for the HXeSH molecule. ν2 Xe–H ν2+ν6 Xe–H + Xe–S ν2+ν3 Xe–H + bend 2ν2 Xe–H ν6 Xe–S ν2→2ν2 anharmonicity (ωexe) experimental 1119 1325 2070 2087 206a –75.5 CCSD(T)b 1148b 218b cc-VSCF 1373 1615 1983 2666 244 –40.2 dz 1372 1616 1978 2644 248 –50.1 tz 1448 1708 2069 2807 263 –44.2 MP2 qz 1445 1708 2069 2818 266 –36.0 dz 1285 1517 1982 2253 234 –158.7 tz 1245 1479 1935 2129 236 –180.5 anharmonic + analytical B3LYP qz 1232 1466 1919 2096 236 –184.1 dz 1366 244 CCSD tz 1495 258 dz 1406 246 harmonic + numerical MP4(SDQ) tz 1514 259 a The number was calculated from the experimental values, see Conclusions. b The harmonic CCSD(T) results are from Ref. [24]. Let us discuss the MP2 computational results. The most intense band, 1445 cm−1 on harmonic MP2/qz level is still over three hundred wave numbers off from the experimental result. The experimental H-Xe stretching vibrational mode has been reported to be at 1119 cm−1 [24], and this low value has been connected with weak molecular stability [8]. The low energy barrier preventing the molecule to dissociate into a three-body dissociation channel H+Xe+SH also indicates large anharmonicity on the potential energy surface as was demonstrated especially for HArF by Runeberg et al. [7]. The molecules, thus, are much more stable in theory than in experiment. The computed frequencies tend to increase with the size of the basis set, both harmonic and anharmonic ones. For instance, the Xe–H bond oscillation increases from 1372 to 1445 cm−1 when going from dz to qz at anharmonic level. This means the bond is predicted to be shorter and stronger with larger basis sets which is evidenced in Table 1 as well. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. 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Ga’ Highlights  Vibrational spectrum of HXeSH has been studied experimentally and computationally  New vibrational mode assignments were done for HXeSH  HXeSH exhibits large anharmonic effects Janusz Cukras: Conceptualization, Methodology, Software, Formal analysis, Investigation, Resources, Writing – Original Draft, Writing – Review & Editing, Visualization, Supervision, Project Administration, Funding acquisition Jussi Ahokas: Conceptualization, Methodology, Investigation, Visualization Jan Lundell: Conceptualization, Methodology, Software, Formal analysis, Investigation, Resources, Writing – Original Draft, Writing – Review & Editing, Visualization, Supervision, Funding acquisition