Presentation on theme: "Calculated Band Widths of Water Dimer Transitions Henrik G. Kjaergaard Department of Chemistry, University of Copenhagen, Copenhagen, Denmark. CAVIAR,"— Presentation transcript:
Calculated Band Widths of Water Dimer Transitions Henrik G. Kjaergaard Department of Chemistry, University of Copenhagen, Copenhagen, Denmark. CAVIAR, The Cosener’s House, December 15, 2009
Calculated clear sky direct solar flux at Earth’s surface from known absorbers in the atmosphere. VisibleNIRIR O2O2 O3O3 H 2 O, v=2 Solar flux at Earth’s surface H2OH2O H 2 O, v=3 H2OH2O Electronic transitions Vibrational transitions
Contribution depends on position, intensity and shape of spectroscopic transitions as well as atmospheric abundance. O 2O 2 and O 2N 2 complexes have been shown to absorb about 1 W/m 2 of incoming solar radiation. Hydrated complexes, H 2 OX, are likely to contribute. H 2 OH 2 OH 2 OO 2 H 2 ON 2 Complexes in the Atmosphere Nature, 1969. Vaida, Daniel, Kjaergaard, Goss, Tuck, QJRMS 2001. Solomon, Portmann, Sanders, Daniel, JGR 1998.
0.960Å 0.965Å 0.958Å H donor H acceptor CCSD(T)/aug-cc-pV5Z optimized geometry Water monomer, R OH = 0.959Å Water dimer, H 2 OH 2 O OH bond involved in hydrogen bonding is significantly longer frequency red shift CCSD(T)/CBS R OO = 2.9125Å +anharm corr R OO = 2.97Å Expt. (Dyke) R OO = 2.976Å Lane, Kjaergaard, JCP 2009.
We use the Harmonically Coupled Anharmonic Oscillators (HCAO) local mode model for each of the H 2 O units. Determine local mode parameters and dipole moment functions from ab initio calculations. Water dimer, H 2 OH 2 O Simple vibrational model for water dimer Each H 2 O unit is modeled by two OH-stretching and one HOH-bending local mode oscillator. Low, Kjaergaard, JCP 1999.Schofield, Kjaergaard, PCCP 2003.
Dipole moment function HCAO model for donor unit
Stretch fundamental region Long path length 351K with water monomer abs subtracted. Paynter, Ptashnik, Shine, Smith, GRL 2007Schofield, Kjaergaard, PCCP 2003
Water dimer, far-IR Matrix isolation experiment versus anharmonic calculation Mode Band frequencies (cm -1 ) Ne-matrixp-H 2 -matrixVPT2 (calc) 7522.4485495 8309.1299.1304 9173145.9144 10151121.2122 11122.2121 1275.785 Kjaergaard et al, JPCA 2008Ceponkus et al, JPCA 2008
Equilibrium constants Comparison of calculated (HCAO and VPT2) and observed vapor phase intensities in different regions 1200-7500 cm -1 lead to K eq in the range 0.011 to 0.054 atm -1 at 298K Region (cm -1 ) K eq (atm -1 ) VPT2HCAO 1200-2000 4.77 × 10 -2 5.41 × 10 -2 3000-4000 1.61 × 10 -2 1.10 × 10 -2 5000-5600 3.19 × 10 -2 3.26 × 10 -2 6800-75004.38 × 10 -2 4.72 × 10 -2
11500 14750 H 2 OH 2 O (calc) H 2 O (observed) wavenumber (cm -1 ) 13100 Water dimer in the atmosphere v OH = 3½ v OH = 4 Water dimer conc. depends on water conc. squared! Schofield, Kjaergaard, PCCP 2003
Different conformers present at room temperature and seen in the overtone spectra. Ethylene Glycol (~58%)(~26%)(~10%) Small molecule, so high level ab initio calculations are possible: CCSD(T)/aug’-cc-pVTZ. Howard, Jørgensen, Kjaergaard, JACS 05. Hydrogen bonding
Ethylene Glycol 1f 1b 2b2f Higher overtones better but also more difficult!
Larger diols QCISD/6-311++G(2d,2p) calculations show stronger hydrogen bonding from EG - PD - BD. Larger frequency red shifts AO local mode calculation indicate similar intensities of bonded and free OH modes. Propanediol, and Butanediol have similar structures to Ethylene Glycol. Howard and Kjaergaard, JPC A 06. EG PD BD
V=3V=4 OH-stretching in diols What happens to the hydrogen bonded OH-stretching vibrations?
Vibrational band profile important for detection and effect. Kjaergaard, Robinson, Howard, Daniel, Headrick, Vaida, JPCA 2003 Hydrated complexes
Water dimer, band profile Vibrational band profile important for detection and effect. Rotational profiles depend on direction of TDM Garden, Halonen, Kjaergaard, JPCA 2008 HbHb HfHf
Water dimer, band profile Effect of coupling to low frequency modes? Adiabatic separation of methyl torsion and CH- stretching has explained CH-stretching overtone spectra in toluenes and xylenes. Rong, Kjaergaard, JPCA 2002 108001100011200 Third CH-stretch overtone, p-xylene. We can separate adiabatically, the fast OH-stretching motion from the slow intra-molecular motion.
OO-stretch coupling Use variation in OH-stretch mode with OO displacement to construct effective OO-stretch potential Garden, Halonen, Kjaergaard, JPCA 2008
OO-stretch coupling Garden, Halonen, Kjaergaard, JPCA 2008 Shift in position of minimum. Both in s and E. Little change for OH f.
OH b -stretching transition is wide, OH f -stretch is not. Direction of TDM changed. Garden, Halonen, Kjaergaard, JPCA 2008 HbHb HfHf OO-stretch coupling
Accepter wag coupling Separate adiabatically, the fast OH-stretching motion from the slow acceptor wag motion. Garden unpublished
Accepter wag coupling Similar spreading of intensity. Closer to 1D transition. Double well changes order. Garden unpublished
Water dimer, band profile Garden, unpublished Combining OH-stretch + OO-stretch + Acceptor wag Four more intermolecular modes!
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 OH b stretching transitions is very wide - making observation elusive - but increases impact on solar radiative transfer.
Acknowledgements Bryan R. Henry, Guelph Veronica Vaida, Boulder Poul Jørgensen, Aarhus Lauri Halonen, Helsinki John Stanton, Austin Benny Gerber, Irvine Keith Shine, Reading Igor Ptasnik, Reading MARSDEN FUND John S. Daniel, NOAA Geoffrey R. Low Timothy W. Robinson Daniel P. Schofield Joseph R. Lane Anna L. Garden Daryl Howard Ben Miller
Copenhagen by night
Optical depth of self continuum (H 2 O only) compared to that calculated for water dimer (K = 0.01 to 0.12 atm -1 ). Continuum Daniel, Solomon, Kjaergaard, Schofield, GRL 2004