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Comment on `A computational study of the novel metastable compound HKrSH' [S.A.C. McDowell, Chem. Phys. Lett. 372 (2003) 553–556]
Chemical Physics Letters 388 (2004) 228–229 www.elsevier.com/locate/cplett
Comment
Comment on ‘A computational study of the novel metastable compound HKrSH’ [S.A.C. McDowell, Chem. Phys. Lett. 372 (2003) 553–556] J. Lundell *, L. Khriachtchev, M. Pettersson, M. R€ as€ anen Department of Chemistry, University of Helsinki, P.O. Box 55 (A.I.Virtasen aukio 1), FIN-00014 Helsinki, Finland Received 23 May 2003; in final form 23 May 2003 Published online: 20 March 2004
Abstract A recent computational study published in this journal suggests that a novel metastable molecule HKrSH exists and calculations at the MP2/6-311++G(2d,2p) level indicate a barrier high enough to trap this molecule in its local stable configuration. However, the report fails to present all factors influencing its stability and these inadequacies greatly affect the conclusions on the existence and experimental characterisation of such species. Ó 2004 Elsevier B.V. All rights reserved.
In a recent Letter published in this journal, McDowell proposed the existence of a novel metastable krypton-containing molecule, HKrSH, based on quantum chemical calculations at the MP2/6-311++G(2d,2p) level [1]. The new species was found to be a local minimum energy structure as verified by frequency analysis. It was also shown that the molecule is stabilised by a barrier of about 17 kJ mol1 from its decomposition to the global energy minimum configuration corresponding to a krypton atom and a H2 S molecule. However, there are, we believe, major inadequacies in the characterisation of the HKrSH molecule, which greatly affect the conclusions on its existence and experimental detection. The chemical nature of rare gas hydrides (HRgY; where Rg ¼ rare gas, Y ¼ electronegative fragment) is well established and the major factors affecting their generation and formation are generally understood [2,3]. All these molecules are high energy local minima configurations on their potential energy surfaces. Thus, the compounds are metastable but given a suitable preparation strategy such species can be made if the protecting barriers are sufficiently high to prevent their decay. The rare gas hydrides are stabilised by two barriers. The *
Corresponding author. Fax: +358919150466. E-mail address: jan.lundell@helsinki.fi (J. Lundell).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.03.006
bending barrier separates HKrSH from the global energy minimum being Kr + H2 S. According to McDowell, this barrier is about 17 kJ mol1 when zero-point energy is taken into account, whereas the global minimum was computed to be )477 kJ mol1 below the molecule at the MP2 level. The computed energy barrier indicates only that the molecule is stable with respect to its dissociation via the bending coordinate leaving the question on dissociation via the H–Kr–SH stretching coordinate open. In fact, all the experimentally prepared rare gas hydrides have been shown to favour dissociation into neutral fragments (H + Rg + Y) [2–5]. This demonstrates the crucial weakness of the computational study on HKrSH [1]. Moreover, up to now, all experimentally observed rare gas hydrides have been found computationally to lie below the neutral atom H + Rg + Y limit [2,3]. This is the case, for example, with the Xe-analog of HKrSH, for which the H + Xe + SH dissociation limit is +19.4 kJ mol1 above the molecular (HXeSH [6]) energy at the MP2/LJ18,6-311++G(2d,2p) level of theory [2,3]. In comparison with the Xe-analog of the insertion compound with H2 S, the HKrSH molecule appears much more unstable. The MP2//MP2 and CCSD(T)// MP2 computed energetics of this molecule is shown in Fig. 1. At the MP2 level, both the bending and
J. Lundell et al. / Chemical Physics Letters 388 (2004) 228–229
+ 30.8 - 95.9
+ 7.6 - 36.9 0.0
- 93.6 - 66.8
HKrSH H + Kr + SH
- 474.2 - 450.8 Kr + H2S Fig. 1. Computed energetics of the HKrSH molecule at the MP2//MP2 and CCSD(T)//MP2 (in italics) levels of theory using the 6311++G(2d,2p) basis set for all atoms. All energies are given in kJ mol1 .
stretching barriers for the HKrSH molecule indicate a stable local minimum on the potential energy surface. On the contrary, single-point calculations at the CCSD(T) level using the MP2-computed equilibrium structures indicate that these barriers disappear when more correlated calculations are employed. This was verified by CCSD(T) optimisation starting from the MP2 geometry, which led to the three-body dissociation limit. Also, both computational levels predict that the HKrSH molecule is higher in energy than its three-body dissociation limit H + Kr + SH. It is rather evident from these calculations that HKrSH is most probably not a stable configuration. The case of HKrSH is similar to HNeF [7] and HKrOH [8,9] for which the MP2 calcu-
229
lations yield stable but shallow minima. Added electron correlation (CCSD(T)), however, removes the local energy minima from the potential energy surface. In conclusion, the original Letter on HKrSH [1] fails to present conclusively the energetic aspects of the molecule and therefore the conclusions drawn from the computational results have no strong basis. Based on systematic evaluation of all energetic aspects controlling the formation of HKrSH it is very probable that such a Kr-containing species does not exist. Finally, it should be mentioned here that the photolysis of H2 S and subsequent thermal mobilisation of hydrogen atoms in Ar, Kr or Xe matrices revealed only formation of HXeSH in a Xe-environment [10].
References [1] S.A.C. McDowell, Chem. Phys. Lett. 372 (2003) 553. [2] M. Pettersson, J. Lundell, M. R€as€anen, Eur. J. Inorg. Chem. (1999) 729. [3] J. Lundell, L. Khriachtchev, M. Pettersson, M. R€as€anen, Low Temp. Phys. 26 (2000) 680. [4] M. Pettersson, J. Nieminen, L. Khriachtchev, M. R€as€anen, J. Chem. Phys. 107 (1997) 8423. [5] L. Khriachtchev, M. Pettersson, N. Runeberg, J. Lundell, M. R€as€anen, Nature 406 (2000) 874. [6] M. Pettersson, J. Lundell, L. Khriachtchev, E. Isoniemi, M. R€as€anen, J. Am. Chem. Soc. 120 (1998) 7979. [7] J. Lundell, G.M. Chaban, R.B. Gerber, Chem. Phys. Lett. 331 (2000) 308. [8] S.A.C. McDowell, Phys. Chem. Chem. Phys. 5 (2003) 1530. [9] J. Lundell, L. Khriachtchev, M. Pettersson, M. R€as€anen, Phys. Chem. Chem. Phys. 5 (2003) 3334. [10] E. Isoniemi, M. Pettersson, L. Khriachtchev, J. Lundell, M. R€as€anen, J. Phys. Chem. A 103 (1999) 679.
Comment
Comment on ‘A computational study of the novel metastable compound HKrSH’ [S.A.C. McDowell, Chem. Phys. Lett. 372 (2003) 553–556] J. Lundell *, L. Khriachtchev, M. Pettersson, M. R€ as€ anen Department of Chemistry, University of Helsinki, P.O. Box 55 (A.I.Virtasen aukio 1), FIN-00014 Helsinki, Finland Received 23 May 2003; in final form 23 May 2003 Published online: 20 March 2004
Abstract A recent computational study published in this journal suggests that a novel metastable molecule HKrSH exists and calculations at the MP2/6-311++G(2d,2p) level indicate a barrier high enough to trap this molecule in its local stable configuration. However, the report fails to present all factors influencing its stability and these inadequacies greatly affect the conclusions on the existence and experimental characterisation of such species. Ó 2004 Elsevier B.V. All rights reserved.
In a recent Letter published in this journal, McDowell proposed the existence of a novel metastable krypton-containing molecule, HKrSH, based on quantum chemical calculations at the MP2/6-311++G(2d,2p) level [1]. The new species was found to be a local minimum energy structure as verified by frequency analysis. It was also shown that the molecule is stabilised by a barrier of about 17 kJ mol1 from its decomposition to the global energy minimum configuration corresponding to a krypton atom and a H2 S molecule. However, there are, we believe, major inadequacies in the characterisation of the HKrSH molecule, which greatly affect the conclusions on its existence and experimental detection. The chemical nature of rare gas hydrides (HRgY; where Rg ¼ rare gas, Y ¼ electronegative fragment) is well established and the major factors affecting their generation and formation are generally understood [2,3]. All these molecules are high energy local minima configurations on their potential energy surfaces. Thus, the compounds are metastable but given a suitable preparation strategy such species can be made if the protecting barriers are sufficiently high to prevent their decay. The rare gas hydrides are stabilised by two barriers. The *
Corresponding author. Fax: +358919150466. E-mail address: jan.lundell@helsinki.fi (J. Lundell).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.03.006
bending barrier separates HKrSH from the global energy minimum being Kr + H2 S. According to McDowell, this barrier is about 17 kJ mol1 when zero-point energy is taken into account, whereas the global minimum was computed to be )477 kJ mol1 below the molecule at the MP2 level. The computed energy barrier indicates only that the molecule is stable with respect to its dissociation via the bending coordinate leaving the question on dissociation via the H–Kr–SH stretching coordinate open. In fact, all the experimentally prepared rare gas hydrides have been shown to favour dissociation into neutral fragments (H + Rg + Y) [2–5]. This demonstrates the crucial weakness of the computational study on HKrSH [1]. Moreover, up to now, all experimentally observed rare gas hydrides have been found computationally to lie below the neutral atom H + Rg + Y limit [2,3]. This is the case, for example, with the Xe-analog of HKrSH, for which the H + Xe + SH dissociation limit is +19.4 kJ mol1 above the molecular (HXeSH [6]) energy at the MP2/LJ18,6-311++G(2d,2p) level of theory [2,3]. In comparison with the Xe-analog of the insertion compound with H2 S, the HKrSH molecule appears much more unstable. The MP2//MP2 and CCSD(T)// MP2 computed energetics of this molecule is shown in Fig. 1. At the MP2 level, both the bending and
J. Lundell et al. / Chemical Physics Letters 388 (2004) 228–229
+ 30.8 - 95.9
+ 7.6 - 36.9 0.0
- 93.6 - 66.8
HKrSH H + Kr + SH
- 474.2 - 450.8 Kr + H2S Fig. 1. Computed energetics of the HKrSH molecule at the MP2//MP2 and CCSD(T)//MP2 (in italics) levels of theory using the 6311++G(2d,2p) basis set for all atoms. All energies are given in kJ mol1 .
stretching barriers for the HKrSH molecule indicate a stable local minimum on the potential energy surface. On the contrary, single-point calculations at the CCSD(T) level using the MP2-computed equilibrium structures indicate that these barriers disappear when more correlated calculations are employed. This was verified by CCSD(T) optimisation starting from the MP2 geometry, which led to the three-body dissociation limit. Also, both computational levels predict that the HKrSH molecule is higher in energy than its three-body dissociation limit H + Kr + SH. It is rather evident from these calculations that HKrSH is most probably not a stable configuration. The case of HKrSH is similar to HNeF [7] and HKrOH [8,9] for which the MP2 calcu-
229
lations yield stable but shallow minima. Added electron correlation (CCSD(T)), however, removes the local energy minima from the potential energy surface. In conclusion, the original Letter on HKrSH [1] fails to present conclusively the energetic aspects of the molecule and therefore the conclusions drawn from the computational results have no strong basis. Based on systematic evaluation of all energetic aspects controlling the formation of HKrSH it is very probable that such a Kr-containing species does not exist. Finally, it should be mentioned here that the photolysis of H2 S and subsequent thermal mobilisation of hydrogen atoms in Ar, Kr or Xe matrices revealed only formation of HXeSH in a Xe-environment [10].
References [1] S.A.C. McDowell, Chem. Phys. Lett. 372 (2003) 553. [2] M. Pettersson, J. Lundell, M. R€as€anen, Eur. J. Inorg. Chem. (1999) 729. [3] J. Lundell, L. Khriachtchev, M. Pettersson, M. R€as€anen, Low Temp. Phys. 26 (2000) 680. [4] M. Pettersson, J. Nieminen, L. Khriachtchev, M. R€as€anen, J. Chem. Phys. 107 (1997) 8423. [5] L. Khriachtchev, M. Pettersson, N. Runeberg, J. Lundell, M. R€as€anen, Nature 406 (2000) 874. [6] M. Pettersson, J. Lundell, L. Khriachtchev, E. Isoniemi, M. R€as€anen, J. Am. Chem. Soc. 120 (1998) 7979. [7] J. Lundell, G.M. Chaban, R.B. Gerber, Chem. Phys. Lett. 331 (2000) 308. [8] S.A.C. McDowell, Phys. Chem. Chem. Phys. 5 (2003) 1530. [9] J. Lundell, L. Khriachtchev, M. Pettersson, M. R€as€anen, Phys. Chem. Chem. Phys. 5 (2003) 3334. [10] E. Isoniemi, M. Pettersson, L. Khriachtchev, J. Lundell, M. R€as€anen, J. Phys. Chem. A 103 (1999) 679.