Effects of Reactant Rotational excitation on Cl + CH4 / CHD3 Reactions

Speaker: Huilin PanSupervisor: Kopin Liu

69th ISMS, June 16-20, 2014

Reactant rotation: a key to stereodynamics

• Disentangle the stereodynamical properties experimentally

• Rationalize the reaction stereodynamics mechanism

Cl + CHD3(v1=1, JK) HCl + CD3

Cl + CH4(v3=1, JNl) HCl + CH3

Energy levels involved in optical transition of methane

CH4 (ν3 = 1 , JNl)

Herzberg, 1945; Chem. Phys., 2009,356,131; Phys. Chem. Chem. Phys., 2014, 16, 444

Research background on Cl + methane reaction

J – total angular momentumN – rotational angular momentuml – vibrational angular momentumJ = N + lSelection rule: ΔN = 0

|JNl>

P(1) |01-1>Q(1) |110>R(0) |101>R(1) |211>R(2) |321>

Energy levels involved in optical transition of methane

CHD3 (ν1 = 1, JK)

Herzberg, 1945; Chem. Phys., 2009,356,131; Phys. Chem. Chem. Phys., 2014, 16, 444

Research background on Cl + methane reaction

J – total angular momentumK – projection of J on C-H bond

|JK>

P(1) |0,0>Q(1) |1,±1>R(0) |1,0>R(1) |2,0>+|2,±1>

Experimental setup

Rev.Sci.Instrum.2003,74,2495; Rev.Sci.Instrum.2008,79,033105

MCP + Phosphor

CD3 / CH3 probe ~333 nm

Cl beam

ICCD camera

UVscan

CHD3 / CH4 beam

CHD3 v1 / CH4 v3

IR

ion packet

Ion optics

Results of Cl + CH4(ν3=1, JNl) → HCl(ν) + CH3(ν=0)

Raw images of R(0) branch

5.0 kcal/mol IR off

IR on

2.0 kcal/mol

At same collision energies, similar images were obtained from reactants with different rotational states.

CH4CH4

0∘0∘

3.2 kcal/mol

CH4

0 1000 2000 3000

0.00

0.02

0.04

0.00

0.03

0.06

0 90 180

0.000

0.015

0.030

HCl =1 HCl =0

T

ransl

atio

nal d

istr

ibutio

n

u (m/s)

P1 Q1 R0 R1 R2

Angu

lar

dis

trib

utio

n

HCl =1

HCl =0

(degree)

Translational and angular distributions of different branches

5.0 kcal/mol

Similar translational and angular distributions.

Results of Cl + CH4(ν3=1, JNl) → HCl(ν) + CH3(ν=0)

Integral cross section(ICS) of different branches

At High Ec, there is little diversityfrom rotationally excited reactants.

Ec~2kcal/mol, reactivity differs.

2 3 4 50

1

2

3

EC (kcal/mol)

|110>

|01-1>

|211>

|101>

|J Nl>=|321>

J /

J=0

P(1) Q(1) R(0) R(1) R(2)

Reactivity diversity rises from the entrance valley. The exit valley is the same.

Results of Cl + CH4(ν3=1, JNl) → HCl(ν) + CH3(ν=0)

Integral cross section of different branches

The reactivity increases with the increasing total angular momentum.

The ICS of R(0) and Q(1) cross at around 2kcal/mol: above 2kcal/mol, reactivity of R(0) is in general bigger than Q(1); under 2kcal/mol, the situation is opposite.

1 2 3 4 5 60

1

2|2,0>+|2,+1>

|1,0>|1,+1>

J /

J=0

EC (kcal/mol)

P(1) Q(1) R(0) R(1)

|0,0>

Results of Cl + CHD3(ν1=1, JK) → HCl(ν) + CD3(ν=0)

Reaction mechanisms on Cl + CHD3(ν1=1, JK) → HCl(ν) + CD3(ν=0)

Origin of the reactivity diversity

Cl

H

Cl

D

D

H C D ?

Energy ?More energy enhances

reactivity effectively

σJ /σJ=0 ~ 2

Long range forces ?

Nature Chem. 4, 636 (2012); J. Chem. Phys. 103, 7313 (1995); J. Phys. Chem. 91, 1400 (1987).

CH4: R(2), J=3, ~ 63 cm-1

CHD3: R(1), J=2, ~ 20 cm-1

Ec: 2 kcal/mol ~ 700 cm-1

Angle 180

Energ

y

Origin of the reactivity diversity: Short range forces

Position of barrier Bending ⊥ reaction coordinateLate barrier: Saddle point, more product-like

r↑, BC’s moment of inertia↑.The initial rotation helps to overcome the repulsive torque. Enhance reactivity

Early barrier: Saddle point, more reactant-like

r unchanged. For J>0, the interactions tend to constrict the access to transition state. Reduce reactivity

Softer bend allows larger range of initial γRotational excitation enlarge γ, enhancing the reactivity

The factor dominates in rotational reactivity diversity is not sure.

Reaction mechanisms on Cl+CHD3(ν1=1, JK) HCl(ν) +CD3(ν=0)

In both reactions Cl + CH4(ν3=1, JNl) → HCl(ν) + CH3(ν=0)

Cl + CHD3(ν1=1, JK) → HCl(ν) + CD3(ν=0)

Further investigations are needed to gain deeper insights

Conclusion

The translational and angular distributions keep the same at different reactant rotation.

Integral cross sections change with reactant rotational states.

Short-range interactions of reactants.

Thanks for your attention!!

Prof. Kopin Liu

Dr. Yuan Cheng

Dr. Ondrej Tkac

Queiya Chang

Dr. Fengyan Wang

Juisan Lin

Acknowledgement

All the people working in our lab

Methane’s angular momentumCH4(ν3=1) J’’ N’’ J’ N’ l’

P(1) 1 1 0 1 -1

Q(1) 1 1 1 1 0

R(0) 0 0 1 0 1

R(1) 1 1 2 1 1

R(2) 2 2 3 2 1

CHD3(ν1=1) J’’ J’ K’

P(1) 1 0 0

Q(1) 1 1 1

R(0) 0 1 0

R(1) 1 2 1

Experimental setup and data analysis

Experimental setup

Reaction mechanisms on Cl+CHD3(ν1=1, JK)HCl(ν) +CD3(ν=0)Origin of the reactivity diversity: Long range forces

Chem. Phys. 104, 213 (1986); Chem. Phys. 112, 85 (1987);J=1 --- ; J=3 -·-·; J=5 ···

J=0

J>0

J=0

J>0

Reaction mechanisms of Cl + CH4 (ν3=1, JNl) → HCl(ν)+ CH3(ν=0)

Reaction mechanisms of two channels of HCl

HCl (v=1)HCl (v=0)

Cl H C

Cl H

Cl

H CCl H

J. Chem. Phys., 133, 124304 (2010); Proc. Natl. Acad. Sci. USA 105, 12667 (2008).

HH

H

HH

H

With the CH4 reactant excited to different J at ν3=1, the translational and angular distributions are the same.

Direct reboundBack/sideways

Short-lived complexforward

PC(event counting)

MCP + Phosphor

CD3 / CH3 probe ~333 nm

Cl beam

ICCD camera

UVscan

CHD3 / CH4 beam

CHD3 v1 / CH4 v3

IR

ion packet

Ion optics

Rev.Sci.Instrum.2003,74,2495; Rev.Sci.Instrum.2008,79,033105

Experimental setup

Experimental setup and data analysis

Product pair correlation measurement

IR laser multipass reflector

3 3 3 33 3( , ) ( , ' ) ( , ' )CHD CHD HCl HCl CD CDCl CHD J HCl J CD J

Well-defined

3c r CHD availE H E E

3 3 3

2 21 1

2 2HCl HCl HCl CD CD CDm v E m v E

3 3HCl HCl CD CDm v m v

imaging REMPI

Energy conservation

Momentum conservation

Experimental setup and data analysis

Experimental setup

Rev.Sci.Instrum.2003,74,2495; Rev.Sci.Instrum.2008,79,033105

IR off

IR on

Data Analysis

Reactant CH4/CHD3 Product CH3/CD3

1 gS = Soff

= Son

D

1 D

+

(1 ) gD S

sD S+

Ss = [Son – (1 – D)Soff] / D

Results of Cl + CH4(ν3=1, JNl) → HCl(ν) + CH3(ν=0)

Obtaining the relative cross sections

Depletion Test: F+CH4(v3=1, JNl) HF+CH3(ν=0) F+CHD3(v1=1, JK) DF+CHD2(ν=0)

Ss = [Son – (1 – D)Soff] / D

ss σu[Cl][CH4]

Relative σ

σ: integral cross section u: relative velocity [Cl],[CH4]: molecular beam density

D P(1) Q(1) R(0) R(1) R(2)

CH4 0.135 0.271 0.234 0.3385 0.085

CHD3 0.045 0.168 0.105 0.245

TranslationalAngular

Results of Cl + CH4(ν3=1, JNl) → HCl(ν) + CH3(ν=0)

Science 316, 1723 (2007); Proc. Natl. Acad. Sci. USA 105, 12667 (2008).

0

6

12 vibrationally nonadiabatic transition region

(0,00)S

(1,00)S

EC+IR

Energ

y(kc

al/m

ol)

Reaction coordinate

ΔΗ0= 1.7 kcal/mol

Barrier height ~4

kcal/mol

Late barrier reaction

:)0,0( 0 g

Research background on Cl + methane reaction

Schematics of the potential energy levels of Cl+CHD3

Results of Cl + CHD3(ν1=1, JK) → HCl(ν) + CD3(ν=0)

Raw images of R(1) branch at low collision energies

1.5 kcal/mol 1.1 kcal/mol

Similar images were obtained from reactants with different rotational excitation of reactants at the same Ec.

IR onCHD3

0∘ 0∘

0 1000 2000

0.00

0.04

0.08

0.00

0.02

0.04

0 90 180

0.00

0.02

Tra

nsl

atio

nal d

istr

ibutio

n

u (m/s)

P1 Q1 R0 R1

Angula

r dis

trib

utio

n

HCl =1

HCl =0

(degree)

Similar translational and angular distributions in these branches.

At low collision energy 1.5kcal/mol, the angular distribution of product pair (1,00)s spreads to the whole range.

Translational and angular distributions of different branches

1.5 kcal/mol

Results of Cl + CHD3(ν1=1, JK) → HCl(ν) + CD3(ν=0)