Flow of Vibrational Energy in Polyatomic Molecules:Using Acetylenic Anharmonic Couplings to Follow

Vibrational Dynamics

Steven T. Shipman and Brooks H. Pate

Department of ChemistryUniversity of Virginia

 

BrightState

FirstTier

Bath StatesSecondTier

Tier Model of IVR

Solvent Accommodates Energy Difference at Each Step of the Relaxation “Cascade”

IR Pump

C-H StretchSolute 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:

CC-H bend is at low frequency (625 cm-1) but localizedStretch-bend coupling larger than linewidths in RT sol’n phaseCC-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(v1,v2,v3,…) = ∑ i (vi + 1/2) + ∑ ∑ xij (vi + 1/2) (vj + 1/2) + …i i j ≥ 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

CaF2 lens

Sample

λ/2 plate

Chopper

ProbeReference

PumpReference

InSb

InSb InSb

Monochrom

ator

1 kHz repetition rateIndependently tunable OPAsTuning range ~1–6 µm, ~5–10 µJ/pulseGas and liquid samples

Transmitted Intensity

Polarizer

Vibrational Transitions of Terminal Acetylenes

Ground StatePump

v = 1–C–H

v = 1=C–H

3100

cm

-1

2950

cm

-1

Probe

v = 1≡C–H

v = 0bend

v = 1bend

v = 2bend

3310

cm

-1

3290

cm

-1

3270

cm

-1

What States are Being Prepared?

Ultrafast pulses are broad!High-resolution data is necessary to know what states are prepared.

mbey 202-101 (13567.966 MHz)1.5 ps pulse (20 cm-1 FWHM)

Molecular Beam Data – Butyne

GSD-DP Measurements202 – 101 (17259.905 MHz)10.8 cm-1 / hr, 20 averages

Butyne – C–H Stretch Identification

3007.13000.32943.32929.82914.5

Scaled Harm2982.92979.32911.22969.12920.0

Anharm2984.22983.12912.62986.62939.5

Cubic Rediagasym –CH3

asym –CH3 asym –CH2

sym –CH3

sym –CH2

Mode Description Intensity26.830.96.7

32.120.0

2827 cm-1

4.9 ps

3290 cm-1

16.9 / 29.7 ps

Butyne in CCl4 – The 2941 cm-1 Band

v = 0

v = 1

v = 2

v = 0

v = 1v = 1

Molecular Beam Data – Methylbutenyne

GSD-DP Measurements202 – 101 (13567.966 MHz)10.8 cm-1 / hr, 20 averages

Methylbutenyne in CCl4 – The 3101 cm-1 Band

vbend = 1

vbend = 2

3290 cm-1

3270 cm-1

18.4 / 31.0 ps

15.3 / 32.1 ps

vbend = 1

vbend = 2

Methylbutenyne in CCl4 – The 2925 cm-1 Band420

-2

0 50 100 150 200-50Delay Time (ps)

420

-2420

-2

Tra

nsie

nt A

bsor

ptio

n (m

OD

) 3310 cm-1

3290 cm-1

3270 cm-1

vbend = 0

10.7 / 34.6 ps

16.1 / 32.0 ps

9.9 / 21.0 ps

Pump 2925 cm-1 Pump 3101 cm-1

Pump

2925

3101

Probe

331032903270

32903270

Bend

012

12

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)

2925 2925 3.7 (0.5)0 N/A

Methylbutenyne Results

Butyne and Methylbutenyne – Relaxation Tiers

BrightState

FirstTier

Bath StatesSecondTier

t = 0 ps

t = 5 ps t = 15 pst = 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 vbend = 1 and vbend = 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 Measurements202 – 101 (17259.905 MHz)10.8 cm-1 / hr, 20 averages

Weak Butyne Bands Near 2981

32903310 (x –1.05)

Methylbutenyne

pump 2941probe 2941

1.4 (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:kTOT = kIVR + kVER

E kIVR

kVER

v=1

v=0

Yoo, H.S. et al. J. Phys. Chem A 2004 108(8) 1348-1364Yoo, H.S. et al. J. Phys. Chem A 2004 108(8) 1365-1379Yoo, H.S. et al. J. Phys. Chem A 2004 108(8) 1380-1387

Fre

quen

cy (

cm)

-1

500

1000

1500

2000

2500

3000

3500

CHstretch and (CCstretch + 2CHbend)

v=2 CHbend

CCstretch

CHbend

H

C

C

RAcetylenic CH stretch is a local mode oscillator with motion dominated by the terminal H-atom.

This atom extends approximately 3A from the R group.-CH2CH3

-CHFCH3

-C(CH3)=CH2

-CH(CH3)2

-CH2Br

-CH2Cl

-CH2F

-CH3

-C(CH3)3

-Si(CH3)3

R group structure

Properties of Acetylenic Compounds

Bright State

W

C-H

Dark StatesMolecular

Eigenstates

Bright States vs. Eigenstates

Pulsed-IR FTMW Detector

OPO/OPA0.02 cm-1 bandwidth

Pulsed IRNd:YAG

Laser

10 Hz repetition rateTuning range ~1 – 4 µm, ~5 – 10 mJ/pulseMolecular Beam Samples – No Hot Bands!

The Molecules Under Study

1-butyne methylbutenyne

tert-butyl acetylene trimethyl silylacetylene

2973 / 2973 cm-1

15.4 ps2964 / 2964 cm-1

8.4 ps

Preliminary Data – TBA and TMSA in CCl4

Long lifetime of bright state implies that couplings are not large.

2982 cm-1

13.4 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 Xij, most likely couplings are to bending modes near 1470 cm-1.(Couplings on the order of 20 cm-1 to all –CH3-based stretches.)

Pump

2941

Probe

3290

Bend

1

Fast

16.9 (2.2)

Slow

29.7 (2.9)

294128272982

29414.9 (0.7)

13.4 (0.7)

1.4 (0.4)00

0N/AN/A

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.