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Computational note on the dimerisation of lithium alkoxides
Journal of Molecular Structure: THEOCHEM 866 (2008) 81
Contents lists available at ScienceDirect
Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem
Computational note
Computational note on the dimerisation of lithium alkoxides Sten O. Nilsson Lill Department of Chemistry, University of Gothenburg, SE-412 96, Göteborg, Sweden
Lithium alkoxides (R–OLi) are useful intermediates and reagents in both organic and inorganic chemistry [1]. It is known that the mainly ionic [2] lithium alkoxides can form aggregates (e.g. dimers, Scheme 1) [1,3]. However, no study has elucidated what computational methods is needed to predict aggregation energies and aggregate structures accurately. These will be useful results for studies on large aggregates or on more substituted alkoxides. To investigate this, dimerisation of lithium methoxide [4] (R = CH3) was studied with DFT and ab initio methods in Gaussian98 [5a]. It is found that HF, MP2, and B3LYP equally well predicts the O–Li, Li–Li, and O–O distances in the dimer (Table S1). For the monomer it is found that HF or B3LYP gives a slightly too short O–Li bond while MP2 and QCISD are in better agreement with the benchmark CCSD(T) distances using the same basis set (BSI). However, using a smaller basis set (BSII) with HF or DFT gives better agreement with the benchmark distances. The dimerisation energies at the HF/BSIII, MP2/BSIII, and B3LYP/BSIII level of theory are 59.1, 59.4 and 56.1 kcal mol 1, respectively. Dimerisation energies using BSI or BSIII combined with HF; MP2 or B3LYP are within 1 kcal mol 1. BSI is therefore preferable due to reduction of computational time. Using the electron correlation method CCSD(T)/BSI//CCSD(T)/BSII or QCISD/BSI// QCISD/ BSI gives more negative dimerisation energies: 60.7 and 60.5 kcal mol 1, respectively. Thus, B3LYP gives an error in the dimerisation energy by as much as 4–5 kcal mol 1. The calculated dimerisation energies using the extrapolation methods G2, G2(MP2), CBS4, CBSQ, and CBSQB3 are 57.2, 56.4, 52.9, 57.1, and 56.2 kcal mol 1, respectively. These are also too positive compared with the benchmark values. Using Gaussian03,[5b] the G3 method was used resulting in a more negative dimerisation
E-mail address: [email protected] 0166-1280/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2008.07.005
Li
2 R-O-Li
R
O
O
R
Li Scheme 1.
energy of 58.9 kcal mol 1, thus closer to the benchmark. These studies indicate that the dimerisation energy of lithium methoxide is sensitive to the level of electron correlation and basis set used. However, HF and MP2 converges closer to the benchmark value than what B3LYP does. In conclusion, to treat large aggregates of lithium alkoxides, it is recommended to use HF/BSI or MP2/BSI calculated aggregation energies on B3LYP/BSII or HF/BSII geometry optimised structures. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.theochem.2008.07.005. References [1] D.C. Bradley, R.C. Mehrotra, I.P. Rothwell, A. Singh, Alkoxo and Aryloxo Derivatives of Metals, Academic Press, London, 2001. [2] A.E. Reed, R.B. Weinstock, F. Weinhold, J. Chem. Phys. 83 (1985) 735. [3] S. Matsuta, T. Asada, K. Kitaura, J. Electrochem. Soc. 147 (2000) 1695. [4] P.I. Wheatley, Nature 185 (1960) 681. [5] (a) Gaussian 98, Rev. A3, Gaussian Inc., Pittsburgh, PA, 1998.; (b) Gaussian 03, Rev. D. 01, Gaussian, Inc., Wallingford CT, 2004.
Contents lists available at ScienceDirect
Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem
Computational note
Computational note on the dimerisation of lithium alkoxides Sten O. Nilsson Lill Department of Chemistry, University of Gothenburg, SE-412 96, Göteborg, Sweden
Lithium alkoxides (R–OLi) are useful intermediates and reagents in both organic and inorganic chemistry [1]. It is known that the mainly ionic [2] lithium alkoxides can form aggregates (e.g. dimers, Scheme 1) [1,3]. However, no study has elucidated what computational methods is needed to predict aggregation energies and aggregate structures accurately. These will be useful results for studies on large aggregates or on more substituted alkoxides. To investigate this, dimerisation of lithium methoxide [4] (R = CH3) was studied with DFT and ab initio methods in Gaussian98 [5a]. It is found that HF, MP2, and B3LYP equally well predicts the O–Li, Li–Li, and O–O distances in the dimer (Table S1). For the monomer it is found that HF or B3LYP gives a slightly too short O–Li bond while MP2 and QCISD are in better agreement with the benchmark CCSD(T) distances using the same basis set (BSI). However, using a smaller basis set (BSII) with HF or DFT gives better agreement with the benchmark distances. The dimerisation energies at the HF/BSIII, MP2/BSIII, and B3LYP/BSIII level of theory are 59.1, 59.4 and 56.1 kcal mol 1, respectively. Dimerisation energies using BSI or BSIII combined with HF; MP2 or B3LYP are within 1 kcal mol 1. BSI is therefore preferable due to reduction of computational time. Using the electron correlation method CCSD(T)/BSI//CCSD(T)/BSII or QCISD/BSI// QCISD/ BSI gives more negative dimerisation energies: 60.7 and 60.5 kcal mol 1, respectively. Thus, B3LYP gives an error in the dimerisation energy by as much as 4–5 kcal mol 1. The calculated dimerisation energies using the extrapolation methods G2, G2(MP2), CBS4, CBSQ, and CBSQB3 are 57.2, 56.4, 52.9, 57.1, and 56.2 kcal mol 1, respectively. These are also too positive compared with the benchmark values. Using Gaussian03,[5b] the G3 method was used resulting in a more negative dimerisation
E-mail address: [email protected] 0166-1280/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2008.07.005
Li
2 R-O-Li
R
O
O
R
Li Scheme 1.
energy of 58.9 kcal mol 1, thus closer to the benchmark. These studies indicate that the dimerisation energy of lithium methoxide is sensitive to the level of electron correlation and basis set used. However, HF and MP2 converges closer to the benchmark value than what B3LYP does. In conclusion, to treat large aggregates of lithium alkoxides, it is recommended to use HF/BSI or MP2/BSI calculated aggregation energies on B3LYP/BSII or HF/BSII geometry optimised structures. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.theochem.2008.07.005. References [1] D.C. Bradley, R.C. Mehrotra, I.P. Rothwell, A. Singh, Alkoxo and Aryloxo Derivatives of Metals, Academic Press, London, 2001. [2] A.E. Reed, R.B. Weinstock, F. Weinhold, J. Chem. Phys. 83 (1985) 735. [3] S. Matsuta, T. Asada, K. Kitaura, J. Electrochem. Soc. 147 (2000) 1695. [4] P.I. Wheatley, Nature 185 (1960) 681. [5] (a) Gaussian 98, Rev. A3, Gaussian Inc., Pittsburgh, PA, 1998.; (b) Gaussian 03, Rev. D. 01, Gaussian, Inc., Wallingford CT, 2004.