Flow of Vibrational Energy in Polyatomic Molecules: Using Acetylenic Anharmonic Couplings to Follow Vibrational Dynamics Steven T. Shipman and Brooks H. Pate Department of Chemistry University of Virginia

Bright State First Tier Bath States Second Tier Tier Model of IVR

Solvent Accommodates Energy Difference at Each Step of the Relaxation “Cascade” IR Pump C-H Stretch Solute Vibrational Levels Vibrational Energy Relaxation (VER) of Polyatomic Molecules in Solution

Watching Localized Energy Transport Many IVR studies follow the departure of energy from an initially prepared state rather than the arrival of energy elsewhere in the molecule. Exceptions: Time-Resolved IR-Raman (Dlott) IR pump - UV probe (Crim, Abel) A few difficulties: most low-frequency modes are delocalized across the entire molecule and are hard to directly monitor. Terminal acetylenes have several nice features: C  C-H bend is at low frequency (625 cm -1 ) but localized Stretch-bend coupling larger than linewidths in RT sol’n phase C  C-H stretch is very intense

Using Anharmonic Couplings to Follow Dynamics Following late-stage relaxation dynamics is hindered because directly probing low-frequency modes is difficult. So let’s indirectly probe them… E(v 1,v 2,v 3,…) = ∑  i (v i + 1/2) + ∑ ∑ x ij (v i + 1/2) (v j + 1/2) + … i ij ≥ i Anharmonic interaction between acetylenic C–H stretch (3330 cm -1 ) and acetylenic C–H bend (600 cm -1 ) is -20 cm -1. The transient absorption signal at 3330, 3310, and 3290 cm -1 allows us to extract the dynamics of the low-frequency bending mode.

Two Color Transient Absorption Spectroscopy OPA 2 PUMP OPA 1 PROBE Variable Delay Stage CaF 2 lens Sample λ/2 plate Chopper Probe Reference Pump Reference InSb Monochromator 1 kHz repetition rate Independently tunable OPAs Tuning range ~1–6 µm, ~5–10 µJ/pulse Gas and liquid samples Transmitted Intensity Polarizer

Vibrational Transitions of Terminal Acetylenes Ground State Pump v = 1 –C–H v = 1 =C–H 3100 cm cm -1 Probe v = 1 ≡C–H v = 0 bend v = 1 bend v = 2 bend 3310 cm cm cm -1

What States are Being Prepared? Ultrafast pulses are broad! High-resolution data is necessary to know what states are prepared. mbey ( MHz) 1.5 ps pulse (20 cm -1 FWHM)

Molecular Beam Data – Butyne GSD-DP Measurements 2 02 – 1 01 ( MHz) 10.8 cm -1 / hr, 20 averages

Butyne – C–H Stretch Identification Scaled Harm Anharm Cubic Rediag asym –CH 3 asym –CH 2 sym –CH 3 sym –CH 2 Mode DescriptionIntensity

2827 cm ps 3290 cm / 29.7 ps Butyne in CCl 4 – The 2941 cm -1 Band v = 0 v = 1 v = 2 v = 0 v = 1

Molecular Beam Data – Methylbutenyne GSD-DP Measurements 2 02 – 1 01 ( MHz) 10.8 cm -1 / hr, 20 averages

Methylbutenyne in CCl 4 – The 3101 cm -1 Band v bend = 1 v bend = cm cm / 31.0 ps 15.3 / 32.1 ps

v bend = 1 v bend = 2 Methylbutenyne in CCl 4 – The 2925 cm -1 Band Delay Time (ps) Transient Absorption (mOD) 3310 cm cm cm -1 v bend = / 34.6 ps 16.1 / 32.0 ps 9.9 / 21.0 ps

Pump 2925 cm -1 Pump 3101 cm -1 Pump Probe Bend Fast 10.7 (1.1) 16.1 (1.3) 9.9 (2.2) 18.4 (3.4) 15.3 (5.9) Slow 34.6 (5.4) 32.0 (1.9) 21.0 (5.1) 31.0 (4.6) 32.1 (8.2) (0.5)0N/A Methylbutenyne Results

Butyne and Methylbutenyne – Relaxation Tiers Bright State First Tier Bath States Second Tier t = 0 ps t = 5 ps t = 15 ps t = 30 ps –C–H stretches –C–H bends ≡C–H bends Solvent modes

Summary The strong stretch-bend coupling of the acetylenic C–H stretch can be exploited to probe late stages of the vibrational relaxation process. In methylbutenyne, a comparison of the dynamics of states with v bend = 1 and v bend = 2 indicates that the methylic and ethylenic stretches explore qualitatively different relaxation pathways. Gas phase measurements need to be made for comparison, but the solvent contribution to the relaxation is no faster than 30 ps.

Acknowledgements NSF - Chemistry Current and former Pate Lab members

GSD-DP Measurements 2 02 – 1 01 ( MHz) 10.8 cm -1 / hr, 20 averages Weak Butyne Bands Near 2981

(x –1.05) Methylbutenyne

pump 2941 probe (0.4) ps recovery Butyne one-color measurement One-color measurements hindered by stimulated emission.

Vibrational Dynamics in Solution Acetylenic C-H Stretch Fundamental Measurements: k TOT = k IVR + k VER E k IVR k VER v=1 v=0 Yoo, H.S. et al. J. Phys. Chem A (8) Yoo, H.S. et al. J. Phys. Chem A (8) Yoo, H.S. et al. J. Phys. Chem A (8)

Frequency (cm) CH stretch and (CC stretch + 2CH bend ) v=2 CH bend CC stretch CH bend H C C R Acetylenic CH stretch is a local mode oscillator with motion dominated by the terminal H-atom. This atom extends approximately 3A from the R group. -CH 2 CH 3 -CHFCH 3 -C(CH 3 )=CH 2 -CH(CH 3 ) 2 -CH 2 Br -CH 2 Cl -CH 2 F 3 -C(CH 3 ) 3 -Si(CH 3 ) 3 R group structure Properties of Acetylenic Compounds

Bright State W  C-H Dark States Molecular Eigenstates Bright States vs. Eigenstates

Pulsed-IR FTMW Detector OPO/OPA 0.02 cm -1 bandwidth Pulsed IR Nd:YAG Laser 10 Hz repetition rate Tuning range ~1 – 4 µm, ~5 – 10 mJ/pulse Molecular Beam Samples – No Hot Bands!

The Molecules Under Study 1-butyne methylbutenyne tert-butyl acetylene trimethyl silylacetylene

2973 / 2973 cm ps 2964 / 2964 cm ps Preliminary Data – TBA and TMSA in CCl 4 Long lifetime of bright state implies that couplings are not large.

2982 cm ps Butyne – Coupling to Other C–H Bends 5 cm -1 resolution makes identification of probed band difficult… … But the bleach indicates a frequency shift of at least that amount. From X ij, most likely couplings are to bending modes near 1470 cm -1. (Couplings on the order of 20 cm -1 to all –CH 3 -based stretches.)

Pump 2941 Probe 3290 Bend 1 Fast 16.9 (2.2) Slow 29.7 (2.9) (0.7) 13.4 (0.7) 1.4 (0.4) N/A Butyne Results 3290 time constants are in agreement with methylbutenyne results. Same general relaxation pathways? Need to check at 3270… Difficult to say if 2982 and 3290 time constants are in fact identical.