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Vibrational Relaxation of CH2ClI in Cold ArgonAmber JainSibert Group1

Introduction2Interpreting spectra of relaxing molecules using theorySlow vibrational relaxation in picosecond iodine recombination in liquids

Time scale of 100 ps observed spectroscopicallyTheory reveals mechanism of energy transfer:Diffusive relative motionVibrational-translation relaxation followed by vibrational-vibrational relaxationNesbitt, Hynes, J. Chem. Phys. 77, 2130Experiments excite molecule electronically and probe subsequent relaxation.Theory helps interpret experiment and provide mechanistic detailsOne good example is iodine.2Introduction3Interpreting spectra of relaxing molecules using theoryNesbitt, Hynes, J. Chem. Phys. 77, 2130Slow vibrational relaxation in picosecond iodine recombination in liquidsA molecular dynamics simulation of NBr trapping in an argon matrixFraenkel, Haas, Chem. Phys. Lett. 220, 77A femtosecond study of the infrared-driven cis-trans isomerization of nitrous acidSchanz, Botan, Hamm, JCP 122, 44509The dynamical role of solvent on the ICN photodissociation reaction: connecting experimental observables directly with molecular dynamics simulationsRivera et al. Phys. Chem. Chem. Phys. 13, 8269Many more examples present in literature3Chloroiodomethane and Its IsomerFeatures halogen halogen bond.Low vibrational frequencies leads to better coupling with bath.Large amplitude motions leads to rearrangement of bath molecules.

Parent MoleculeIsomer Molecule44The Experiment* T. J. Preston et al., J. Chem. Phys. 135, 114503 (2011)

267 nmProbe at 435 nm and 485 nmDecay rates : 0.45 ps1 0.07 ps1 5

* Molecule excited electronically in argon. Fragments having nowhere else to go, they can sometimes recombine as isomer. Isomer so produced is hot vibrationally.* 485 nm having lower energy, it probes excited isomer molecule selectively. That gives decay rates of

5ModelExcited mode being C-I stretch (577 cm-1), only lowest 2 modes included : C-Cl-I bend () (159 cm-1) Cl-I stretch (r) (235 cm-1)CH2 treated as a point mass.C-Cl bond frozen CH2ClIr6Potential Energy Surface

Adiabatic SurfaceParentIsomerCH2ClIr* Chang; Miller, J. Phys. Chem. 94, 5884 (1990)Basis : H,C,Cl : 6-31++G** I : MidiX Theory : MP27Isomer is 21000 cm-1 higher in energy7

TrajectoriesReactive trajectory Statistics : 334/1500CH2ClIrColor bar shows time in fsColor bar shows # of trajectories8Trajectories do not follow minimum energy path.Trajectories in phase8

TrajectoriesNon reactive trajectoryStatistics: 1155/1500Color bar shows # of trajectoriesCH2ClIrColor bar shows time in fs9TrajectoriesReactive trajectory Statistics : 334/1500Non reactive trajectoryStatistics: 1155/1500CH2ClIr

10Trajectories that lose more energy with collisions with argon are trapped in isomer.10Energy Decay

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Energy Decay

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Energy DecayE=1527e-0.28t + 2085e-1.42t + 5kbT13Temperature of Bath14

Shell 1 Shell 2Shell 3

Temperature of Bath15

Shell 1 Shell 2Shell 316CH2ClIArCAST

17CH2ClIArCAST

Hot argons are red. Focus on these 2 argon atoms. Also look at energy.17Effect of Initial ConditionsTemperature of bathInitial energy deposited in the moleculeTemperature quench18Effect of Initial ConditionsTemperature of bathInitial energy deposited in the moleculeTemperature quenchT(K)Slow Decay Rate (ps-1)Fast Decay Rate (ps-1)40.267 0.0051.31 0.03200.28 0.031.4 0.1400.34 .021.7 0.2600.28 0.031.5 0.319Initial Energy (cm -1) Slow Decay Rate (ps-1)Fast Decay Rate (ps-1)% of Isomerizing Trajectories375000.28 0.031.4 0.122.3%425000.33 .021.81 0.0225.6%475000.39 0.031.9 0.141.2%525000.40 0.012.19 0.154.9%T:300 K 20 KDecay rates : 0.18 (0.01) ps1, and 1.5 (0.3) ps 119Effect of Initial ConditionsTemperature of bathInitial energy deposited in the moleculeTemperature quenchT(K)Slow Decay Rate (ps-1)Fast Decay Rate (ps-1)40.267 0.0051.31 0.03200.28 0.031.4 0.1400.34 .021.7 0.2600.28 0.031.5 0.320Initial Energy (cm -1) Slow Decay Rate (ps-1)Fast Decay Rate (ps-1)% of Isomerizing Trajectories375000.28 0.031.4 0.122.3%425000.33 .021.81 0.0225.6%475000.39 0.031.9 0.141.2%525000.40 0.012.19 0.154.9%T:300 K 20 KDecay rates : 0.18 (0.01) ps1, and 1.5 (0.3) ps 1Rates are relatively constantComparisionTheoretical decay rate (20K, 37500 cm-1) 0.28 ps-1 & 1.4 ps-1

Experimental decay rate 0.07 ps-1 & 0.45 ps-1

21Good comparison. 21Future Directions

Preliminary Surface

Decay rates : 0.22 (0.02) ps1 & 1.47 (0.04) ps 1

Surface which dissociates at large Cl-I distance requiredExcited surface for early time dynamics(Earlier : 0.28 (0.03) ps1 & 1.4 (0.1) ps 1 )E = 1941e-0.26t + 2085e-1.52t + 5kbT22Fine details of pes does not matter!22SummaryCalculated potential energy surface with respect to Cl-I distance and C-Cl-I angleObserved multi-step energy decay:Binary collision with argon atoms, losing 12000 cm-1 in first 1 psVibrational energy relaxation with decay rates of ~ 0.28 ps-1 and 1.42 ps-1

23AcknowledgmentsNed SibertSibert GroupFleming Crim and T.J. PrestonNSF for fundingThank You24