TUNNELING-SYMMETRY-RESOLVED VIBRATIONAL SPECTROSCOPY AND DYNAMICS OF THE C2H4-H2S COMPLEX MEASURED USING COHERENCE-DETECTED FOURIER TRANSFORM MICROWAVE(FTMW)-- INFRARED SPECTROSCOPY. MATT T. MUCKLE, JUSTIN L. NEILL AND BROOKS H. PATE University of Virginia AND MAUSAMI GOSWAMI AND E. ARUNAN, Department of Inorganic and Physical Chemistry, Indian Institute of Science What can FTMW-IR do? Outline Method Tunneling splittings can be Coherence Detected FTMW-IR assigned with pure Pulse sequences Instrumentation microwave Applications to weekly bound Can IR be used as a tool for clusters investigating tunneling and Advantages over other methods looking at dynamics at the H2S-C2H4 same time Analysis of IR Data Band shifts Predissociation Infrared-Microwave Spectroscopy Balle-Flaygare 8-18GHz MW Nd-YAG pumped OPO/OPA tunable IR laser IR-Multipass mirror cavity to increase effective path length. Laser freq. scanned across various MW transitions Pulse valve sample introduction Supersonic expansion cooling to ~2K Experimental Setup Coherence Detected FTMW-IR MW-IR-MW Pulse sequence 1st MW “π/2” polarizes sample and eliminates population difference IR Pulse transfers population out of monitored state 2nd MW “-π/2” pulse cancels remaining coherent light Population is converted into a coherence Induced population difference causes the second pulse to be greater or less than an exact -π/2 pulse No Laser Resonance Laser Resonant with Lower State Laser Resonant with Upper State Applications Of IR To Weak Clusters Previous Hydride Stretch Work Current Bolometer Detection Cavity FTMW (pulsed jet) (Miller, Fraser, Scoles) High sensitivity of weak clusters Direct Absorption (Nesbitt) Lower Resolution High Resolution .02cm-1 (600Mhz) .001-.0001cm-1 (3-30MHz) Limits Lifetime Determination Limited to small clusters IR-MW double resonance Simplifies spectrum (only two base J states) Applications of FTMW-IR to C2H4–H2S Band Origin Shifts Lifetime Complexation effects Mode-Specific Predissociation dynamics Tunneling dynamics Excited Geometry Competition of IVR and Predissociation H2S-C2H4 Dimer Loosely Bound (0.3196kcal/mol) 4 Tunneling Components A 26GHz B 1972.90MHz C 1866.69MHz Dj 14.3kHz Djk 1.061MHz º6-311++G** with ZPE correction M. Goswami, PK Mandal,DJ Ramdass, E Arunan Chem Phys Lett 2004 Tunneling Components Larger splitting(10MHz) from H2S Proton Exchange Smaller splitting(1MHz) C2H2 Rotation around C=C axis S-H Stretching Modes 2 IR active stretching modes in free H2S Band Origins 2614.4080cm-1 symmetric 2628.4551cm-1 asymmetric Expectations Complexation should make a strong red shifted “bound” and a weakly shifted “free” S-H stretch (shift)2 should be proportional to lifetime* *R.E. Miller, Science, 240 (1988) p.447. *Le Roy, J Phys Chem, 95 (1991) p. 2167. S-H Stretches S-H Stretch K1 Band Origins Monomer Complex Shift Type LL (1L) 2614.335 2607.88 6.455 A LU (1U) 2628.431 2622.18 6.251 A UL (2L) 2614.335 2608.30 5.035 A UU (2U) 2628.431 2625.72 2.711 A Symmetric stretch predicted to be redshifted farther than the asymmetric Both bands have a nearly identical shift Predissociation Broader Linewidth in Lower band ~ 0.05cm-1 linewidth in both K0 and K1 100ps lifetime Does NOT agree with Miller’s rule (shift)2 lifetime Experimental vs Predicted Lifetime R.E. Miller, Science, 240 (1998) p.447. 100ps lifetime would be a 300cm-1 shift in band origins 6.53 and 6.28cm-1 shifts from Monomer Large deviation from Miller’s Rule Deviation in upperstate rotational constant may speed up reaction rate (Le Roy++ ) ++ Le Roy, J Phys Chem, 95 (1991) p. 2167 Combination Band ~50cm-1 shift from S-H stretch Possible coupling to intermolecular mode Same lifetime broadening in lower band C-H Stretches Expectations No large shift to Band Origins H2S not directly interacting with C-H mode Tunneling No shift for large splitting Different shifts for each mode on small splitting C-H Stretch Band Origins C2H4 monomer Band Origins 2988.643cm-1 ** 3104.887cm-1 ** Very little deviation from complex Indicative of small binding interaction Identical B type Origins ( Lower Band) Thee C type bands (Upper band Lower doublet) LL LH HL HH Type 2987.12 2987.12 2987.12 2987.12 B 3103.31 3103.35 3103.42 3103.51 C 3103.56 3103.56 - - C 3104.49 3104.40 - - C **ref: J. Mol. Spectrosc., 185, 31-47 (1997) Lower Band C-H stretch Coupling Perturbations Not coupled to S-H modes No perturbations on S-H bands Not coupled to C2H2 No perturbations in monomer Most likely coupled to intermolecular mode No lifetime broadening!
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