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Dipole-bound anions - from crude estimates to accurate results

Dipole-bound anions - from crude estimates to accurate results. Piotr Skurski Department of Chemistry, University of Gdańsk, Poland Department of Chemistry, University of Utah, SLC. Dipole-bound anions.

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Dipole-bound anions - from crude estimates to accurate results

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  1. Dipole-bound anions - from crude estimates to accurate results Piotr SkurskiDepartment of Chemistry, University of Gdańsk, PolandDepartment of Chemistry, University of Utah, SLC

  2. Dipole-bound anions neutral molecule + e dipole-bound anion(closed-shell, μ>2.5 D) • - usually weakly bound • excess electron localized • outside the molecule • - fully symmetric, diffuse MO (CH3CN)–

  3. Electron correlation effects are NOT important (?) Salt Lake City, UT / Pittsburgh, PA Jack Simons&Ken Jordan1987 KT / SCF approach considered satisfactory

  4. Electron correlation effects are NOT important (?) Ken Jordan 1987 Jack Simons

  5. Correlation effects are CRUCIAL! Gdańsk, Poland Maciek Gutowski (~1984) …electron binding energies calculated for dipole-bound anions are underestimated …something is missing…

  6. Correlation effects are CRUCIAL! Why not all of the KT/SCF results were underestimated? Gdańsk, Poland Pople’s basis sets of double-zeta quality lead to overestimated dipole moments of the neutral molecules … which sometimes compensates the lack of stabilizing electron correlation effects molecules with artificially larger dipole moments bind the excess electrons stronger

  7. Correlation effects are CRUCIAL! when the proper basis sets are employed the lack of stabilizing correlation contributions can be clearly seen CH3CN- 56 cm-1 108 cm-1(HF)2- 179 cm-1 387 cm-1urea- 37 cm-1 122 cm-1glycine- 323 cm-1 668 cm-1 total electron binding energies SCF-calculated binding energies

  8. Demonstrating the importance of the electron correlation effects 1996 dispersion correlation contributions

  9. Satisfactory accuracy achieved!(1996-1998) (H2O…HCl)- 475 cm-1 436 cm-1 (H2O…NH3)- 111 cm-1 123-129 cm-1 (C3H2)- 173 cm-1 171 ± 50 cm-1 (CH3CN)- 108 cm-1 93-145 cm-1 (still… there were/are species for which the agreement is not so good…) How to get the correct VDEs? 1. the proper choice of the basis set2. the inclusion of the correlation effects (CCSD(T) level is recommended)

  10. Binding TWO electrons (Salt Lake City, 1999) Ab initio (full CI) calculations for two charges +q and –q separated by a distance R e/2e µ = q×R +q -q R Basis set: uncontracted aug-cc-pVQZ for hydrogen supplemented with the 6s6p4d For charges |q|>1e – basis set was scaled according to Huzinaga (scaling factor q2 ). In these cases the basis set was supplemented until the satisfactory small exponents were achieved (up to 12s12p set).

  11. What is the smallest positive charge that is capable of binding of two electrons? A single proton binds two electrons (see: the existence and stability of the H– , EA=0.7 eV) what is the critical positive charge for binding two electrons?in other words:how can we decrease the positive charge in the H atom to keep the H– anion electronically stable? +q q=0.91161e Conclusion: to construct a model dipole we need to use a charge q greater than 0.91 a.u. (q>0.91161 e)

  12. Calculating the critical value of a dipole moment µCRITneeded to bind two electrons for given values of q, from q=0.91161 e to q=10 000 e extrapolation to the point dipole limit,q→∞and R→0 Goal:

  13. At this point it is clear that the critical binding conditions are different for one and for two electrons binding. critical conditions dependon the charge q forming a dipole (q>0.91161e) one electron two electrons critical conditions do not dependon the charge q forming a dipole 2e e +q -q +q -q

  14. Results: the critical value of µ strongly depends on q values of µ are small for large values of q (such q’s are not possible at the molecular level, when we think of them as of atomic partial charges). values of µ are very large for small values of q

  15. Extrapolating the results to the point dipole limitq→∞ , R→0 The critical µ goes to some horizontal asymptote as the charge q increases and R decreases this horizontal asymptote should be found in the µ<2 D region

  16. Extrapolating the results to the point dipole limitq→∞ , R→0 Critical value for binding of two electrons µ2eCRIT < 2 D Critical value for binding of one electron µ1eCRIT = 1.625 D Is it possible that µ2eCRIT equals to µ1eCRIT ?

  17. Extrapolating the results to the point dipole limitq→∞ , R→0 is µ2eCRIT equal to µ1eCRIT ? Hmm, it took me a while… Indeed, in the point dipole limit µ1eCRIT= µ2eCRIT

  18. Extrapolating the results to the point dipole limitq→∞ , R→0 One needs to remember that µ2eCRIT = µ1eCRIT is true only in the point dipole limit and for „critical binding conditions” which means that the electron is only slightly bound This means that for any larger electron binding energy, like a few cm-1, for example, the value of µ2e (needed for two electron binding) is different than µ1e (for one electron binding)

  19. Interpretation How to explain this result? ( µ2eCRIT = µ1eCRIT ) 1. point dipole limit2. true for infinitesimally small electron binding energies3. are two extra electrons extremely diffused?4. perfect correlation?

  20. ACTC meeting, 1999, Boulder, Colorado I will believe when I see the molecule. Show me the molecule!

  21. Searching for a molecule, the molecule (Utah, 1999/2000) dipole-bound dianion – stability vs. instability the stability region (for the hypothetical dianion) the instability region(of the hypothetical dianion)

  22. Searching for a molecule for „chemically accesible” q’s (q<1.5e) required µ’s are large (greater than 20 D) it is expected that for real molecules the values of µ should be even larger than those derived from our table (due to destabilizing valence repulsion effects caused by the core-electrons)

  23. Tested species I would try calcium… molecule that meets the required conditions ( µ=55.5D)

  24. ”Jordanium” Let’s call it ”Jordanium” atomic partial charge qCa=+1.73 a.u.dipole moment µ=55.5D Equilibrium structure –C2v, symmetry – two 5-member carbon rings,– 3 highly electronegative superhalogen groups –PF5– electropositive Ca atom

  25. ”Jordanium” The energy of the dianion is smaller than that of the monoanion so the system is electronically stable This is a stable dianion but is it of a dipole-bound nature?

  26. ”Jordanium” 1. ground electronic state of the parent neutral molecule is a closed shell singlet state (a1) 2. Its monoanion daughter is a dipole-bound anion 3. The excess electron charge distribution in dianion is very similar to that in the monoanion

  27. ”Jordanium” singly occupied MO in the monoanion (a1 symmetry) the highest doubly occupied MO in the dianion (a1 symmetry)

  28. Jordanium ”Jordanium” is the very first dipole-bound dianion reported

  29. Unusual molecular anions Design new molecules! what ya got? mixed valence/dipole dianions anions with extremely large correlation contributions

  30. Mixed valence/dipole-bound dianion dianion having: - one excess electron dipole-bound and localized on the positive side of the molecular dipole - second excess electron bound by valence interaction to the functional group at a distant part of a molecule The stability of such a dianion is possible when the Coulomb repulsion 1/r is overcome by the dipole-binding (dipole binding potential is expected to produce weak binding) the system should be long enough to both: reduce 1/r repulsion and increase the dipole moment

  31. Mixed valence/dipole-bound dianion EKT(mono) – KT electron binding energy of the dipole-bound monoanionEKT(di) – KT electron binding energy of the dianionr – spacer length this formula helped to design a proper system and allowed to predict the resulting stability of the dianion

  32. Mixed valence/dipole-bound dianion we ended up with : LiCN···LiCC-PF5 positive side

  33. Mixed valence/dipole-bound dianion predicting the stability of the LiCN…LiCC-PF5 dianion LiCN···LiCC-PF5 1. replacing P with S to construct the closed-shell LiCN…LiCC-SF5 system to estimate the dipole binding strength of LiCN…LiCC-PF5 neutral radical2. The KT binding of LiCN…LiCC-SF5 is 1.35 eV3. The distance between terminal Li and P is r=10 Å which produces 1/r Coulomb repulsion of 1.4 eV Conclusion: the electronic stability of such a dianion is likely

  34. Mixed valence/dipole-bound dianion ground state of (LiCN…LiCC-PF5)- is a closed-shell singlet state the second electron binding energy (calculated at the KT level) is 0.045 eV (so the dianion is electronically stable) including orbital relaxation effects (SCF level) leads to a decent estimate of the total electron binding energy (calculated as 0.120 eV at the CCSD/aug-cc-pVDZ+4s4p level)

  35. Mixed valence/dipole-bound dianion The equilibrium structure and MOs for the dianion valence-bound electron dipole-bound electron

  36. Dipole-bound anions with extremely large correlation contributions HCN  HNC tautomerization Both HCN and HNC form weakly stable dipole-bound anions Will the excess electron stay with the system during the (HCN)- (HNC)- tautomerization? HCN binds an extra electron by only 10 cm-1HNC binds an extra electron by 43 cm-1

  37. Dipole-bound anions with extremely large correlation contributions Will the excess electron stay with the system during the (HCN)- (HNC)- tautomerization? (HCN)-  (HNC)- isomerization should result in autodetachment of the excess electron(because bent structures cannot support stable dipole-bound anions)

  38. Dipole-bound anions with extremely large correlation contributions What is unusual about HNC- anion? the correlation contribution to the total electron binding energy is responsible for 92% of this value!

  39. Thank you for your attention AcknowledgmentsJack SimonsMaciek GutowskiAlex BoldyrevKen Jordan

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