Calculated Band Widths of Water Dimer Transitions

1. Calculated Band Widths of Water Dimer Transitions CAVIAR, The Cosener’s House, December 15, 2009 Henrik G. Kjaergaard Department of Chemistry, University of Copenhagen, Copenhagen, Denmark.

3. Visible NIR IR Solar flux at Earth’s surface Electronictransitions O3 O2 H2O, v=3 Vibrationaltransitions H2O H2O, v=2 H2O Calculated clear sky direct solar flux at Earth’s surface from known absorbers in theatmosphere.

4. H2O•H2O H2O•N2 H2O•O2 Complexes in the Atmosphere O2•O2 and O2•N2 complexes have been shown to absorb about 1 W/m2 of incoming solar radiation. Solomon, Portmann, Sanders, Daniel, JGR 1998. Hydrated complexes, H2O•X, are likely to contribute. Contribution depends on position, intensity and shape of spectroscopic transitions as well as atmospheric abundance. Nature, 1969. Vaida, Daniel, Kjaergaard, Goss, Tuck, QJRMS 2001.

5. Water dimer,H2O•H2O CCSD(T)/aug-cc-pV5Z optimized geometry CCSD(T)/CBS ROO = 2.9125Å 0.965Å 0.958Å +anharm corr ROO = 2.97Å 0.960Å Water monomer, ROH = 0.959Å Expt. (Dyke) ROO = 2.976Å H donor H acceptor OH bond involved in hydrogen bonding is significantly longer  frequency red shift Lane, Kjaergaard, JCP 2009.

6. Water dimer,H2O•H2O Simple vibrational model for water dimer Each H2O unit is modeled by two OH-stretching and one HOH-bending local mode oscillator. We use the Harmonically Coupled Anharmonic Oscillators (HCAO) local mode model for each of the H2O units. Determine local mode parameters and dipole moment functions from ab initio calculations. Low, Kjaergaard, JCP 1999. Schofield, Kjaergaard, PCCP 2003.

7. HCAO model for donor unit Dipole moment function

8. Stretch fundamental region Long path length 351K with water monomer abs subtracted. Paynter, Ptashnik, Shine, Smith, GRL 2007 Schofield, Kjaergaard, PCCP 2003

9. Water dimer, far-IR Matrix isolation experiment versus anharmonic calculation Ceponkus et al, JPCA 2008 Kjaergaard et al, JPCA 2008

10. Equilibrium constants Comparison of calculated (HCAO and VPT2) and observed vapor phase intensities in different regions 1200-7500 cm-1 lead to Keq in the range 0.011 to 0.054 atm-1 at 298K

11. Water dimer in the atmosphere DvOH = 3½ DvOH = 4 H2O•H2O (calc) H2O (observed) Water dimer conc. depends on water conc. squared! 11500 13100 14750 wavenumber (cm-1) Schofield, Kjaergaard, PCCP 2003

13. Ethylene Glycol Different conformers present at room temperature and seen in the overtone spectra. Hydrogen bonding (~58%) (~26%) (~10%) Small molecule, so high level ab initio calculations are possible: CCSD(T)/aug’-cc-pVTZ. Howard, Jørgensen, Kjaergaard, JACS 05.

14. Ethylene Glycol 1f 1b 2f 2b Higher overtones better but also more difficult!

15. Larger diols Propanediol, and Butanediol have similar structures to Ethylene Glycol. EG QCISD/6-311++G(2d,2p) calculations show stronger hydrogen bonding from EG - PD - BD. Larger frequency red shifts PD BD AO local mode calculation indicate similar intensities of bonded and free OH modes. Howard and Kjaergaard, JPC A 06.

16. OH-stretching in diols V=3 V=4 What happens to the hydrogen bonded OH-stretching vibrations?

17. Hydrated complexes Vibrational band profile important for detection and effect. Kjaergaard, Robinson, Howard, Daniel, Headrick, Vaida, JPCA 2003

18. Water dimer, band profile Vibrational band profile important for detection and effect. Rotational profiles depend on direction of TDM Hf Hb Garden, Halonen, Kjaergaard, JPCA 2008

19. 10800 11000 11200 Water dimer, band profile Effect of coupling to low frequency modes? Third CH-stretch overtone, p-xylene. Adiabatic separation of methyl torsion and CH-stretching has explained CH-stretching overtone spectra in toluenes and xylenes. We can separate adiabatically, the fast OH-stretching motion from the slow intra-molecular motion. Rong, Kjaergaard, JPCA 2002

20. OO-stretch coupling Use variation in OH-stretch mode with OO displacement to construct effective OO-stretch potential Garden, Halonen, Kjaergaard, JPCA 2008

21. OO-stretch coupling Shift in position of minimum. Both in s and E. Little change for OHf. Garden, Halonen, Kjaergaard, JPCA 2008

22. OO-stretch coupling <0|0> <1|0> Garden, Halonen, Kjaergaard, JPCA 2008

23. OO-stretch coupling Hf Direction of TDM changed. Hb OHb-stretching transition is wide, OHf-stretch is not. Garden, Halonen, Kjaergaard, JPCA 2008

24. Accepter wag coupling Separate adiabatically, the fast OH-stretching motion from the slow acceptor wag motion. Garden unpublished

25. Accepter wag coupling Similar spreading of intensity. Closer to 1D transition. Double well changes order. <0|0> <1|0> Garden unpublished

26. Water dimer, band profile Combining OH-stretch + OO-stretch + Acceptor wag Four more intermolecular modes! Garden, unpublished

27. Conclusion • The local mode model gives a good description of the dominant OH-stretching overtone transitions. • We can calculate quite accurate absolute overtone intensities ab initio for species that have not been observed. • Guide experimental efforts to observe these species. • Provide input for atmospheric impact studies. • Water dimer band profile/width of OHb stretching transitions is very wide - making observation elusive - but increases impact on solar radiative transfer.

29. Acknowledgements Bryan R. Henry, Guelph Veronica Vaida, Boulder Poul Jørgensen, Aarhus LauriHalonen, Helsinki John Stanton, Austin Benny Gerber, Irvine Keith Shine, Reading Igor Ptasnik, Reading Geoffrey R. Low Timothy W. Robinson Daniel P. Schofield Joseph R. Lane Anna L. Garden Daryl Howard Ben Miller John S. Daniel, NOAA MARSDEN FUND

31. Copenhagen by night

32. Continuum Optical depth of self continuum (H2O only) compared to that calculated for water dimer (K = 0.01 to 0.12 atm-1). Daniel, Solomon, Kjaergaard, Schofield, GRL 2004