Electronic spectroscopy of CHBr and CDBr

Chong Tao, Calvin Mukarakate, Mihaela Deselnicu and Scott A. Reid

Department of Chemistry, Marquette UniversityWisconsin 53233

Simple carbenes

• Carbenes have a divalent carbon

which leads to energetically similar

singlet and triplet states

Typical molecule

S0

T1

S1

Carbene

S0

T1

S1

C C

S0 T1 S1

C

Or reversed

Motivations

• The two lowest singlet states are components of a Renner-Teller pair, and thus simple carbenes are model systems for study of the Renner-Teller effect

• Mapping the singlet-singlet spectra is important for subsequent studies of the triplet state properties

Ene

rgy

X1A

Ã1A

a3A

180o

Methodology

Laser

G

HV

CHBr was produced from pulsed

discharge of CHBr3 precursors

seeded in He, with a backing P=40psi

CDBr was produced from CDBr3

precursor

Spectrum was measured under jet-

cooled condition

Spectra show polyad structure due to similar frequencies of 2 and 3.

Example survey spectra: CHBr

14900 15000 15100 15200 15300

25

1

21

033

0

23

031

0

22

032

0

Inte

nsi

ty

wavenumber in cm-1

24

0

15800 15900 16000 16100

23

033

1

24

032

1

23

032

0

22

033

0

25

0

Inte

nsity

wavenumber in cm-1

24

031

0

At high energies, we also observe levels in the C-H stretching progression.

Example survey spectra: CHBr

18800 19000 19200 19400 19600 198000.00

0.05

0.10

0.15

0.20

0.25

11

024

031

0

11

025

0

11

025

031

0

11

026

0

28

031

0

29

0

Inte

nsi

ty

wavenumber in cm -1

28

0

Br2 dissociation

limit

Example high resolution spectra: CHBr

16940 16950 16960 169700.0

0.2

0.4

0.617830 17840 17850 17860

0.1

0.2

0.3

0.418730 18740 18750 18760

0.0

0.1

Inte

nsi

ty

uncalibrated wavenumber in cm-1

26

0

Inte

nsity 27

0

Inte

nsi

ty 28

0

Dixon plots for CHBr

• Dashed lines are polynomial

fits, solid lines are calculations

of Yu, et al…

• The derived barrier heights

are:

2n 1680 cm-1

2n31 2470 cm-1

2n32 2760 cm-1

15000 16000 17000 18000800

810

820

830

840

850

12000 14000 16000 18000 20000800

820

840

860

880

900

920

14000 16000 18000800

820

840

860

880

900

(G+1 + G/2 in cm -1

2n

032

0

2n

031

0

2n

0

G+

1 - G

(in

cm

-1)

Isotope shifts: CHBr

The isotope shifts with increasing quanta of C-Br stretch (upper panel) are similar for all bands and consistent with a simple harmonic prediction (solid line)

A small dependence of isotope shift on bendingquantum number is observed (lower panel)

0 1 2 3

0

2

4

6

0 2 4 6 81

2

3

4

Isot

ope

shift

[T

(CH

81B

r)-T

(CH

79B

r)]

in c

m-1

m

n = 1n = 2n = 3n = 4n = 5n = 6n = 7n = 8

n

2n

032

0

2n

031

0

2n3m

79Br, 81Br

Excited state C-H stretching frequency

We observe bands in theCH stretching progression(112n) for the first time

A linear extrapolation to n = 0 of the term energy difference between these bands and the 2n bands returns:

E(112n)- E(2n) = 1o +n x12

0

1´ = 3100 cm-1

x120 = -41 cm-1

4 6 82800

2820

2840

2860

2880

2900

Ter

m e

ner

gy

dif

fere

nce

(11 02

n 0-2

n 0)

bending quantum number

Picture of bands observed for CDBr

0 2 4 6 8 10 1213000

14000

15000

16000

17000

18000

19000

20000 2n

2n3

1

2n3

2

112

n

112

n3

1

Waven

um

bers

(cm-1

)

Bending quanta

• Dashed lines are polynomial

fits (third order)

• The derived barrier heights

are:

2n 2110 cm-1

2n31 2890 cm-1

2n32 3501 cm-1

Dixon plots for CDBr

14000 16000 18000

590

600

610

620

630

640

650

660

15000 16000 17000 18000 19000

590

600

610

620

630

640

650

12000 14000 16000 18000

590

600

610

620

630

640

650

660

2n

031

0

[G(v

+1)

-G(v

)] in

cm

-1

[G(v) + G(v+1)]/2 in cm-1

2n

032

0

[G(v

+1)

-G(v

)] in

cm

-1

[G(v

+1)

-G(v

)] in

cm

-1

2n

0

CDBr isotope splitting

1 2 3 4 5 6 7 8 9 10 11 120.40.60.81.01.21.41.61.82.02.22.42.62.83.03.23.4

2n

2n3

1

2n3

2

Iso

top

e s

plit

tin

g in

cm-1

Bending quanta

Excited state C-D stretching frequency

6 8 10

2100

2120

2140

2160

2180

E(1

1 2n )-E

(2n )

Bending quanta

Extrapolated excited stateC-D stretching frequency

1´ = 2351 cm-1

x120 = -25 cm-1

LIF spectra of CHBr/CDBr in the 450-750nm region were recorded and rotationally analyzed.

CH/CD stretching excitation spectra were observed for the first time. Their frequencies were determined to be 3100, and 2351 cm-1, respectively.

The minima in Dixon plots indicate the barrier heights to linearity for CHBr and CDBr are 1680 cm-1

and 2110 cm-1, respectively, above vibrationless level.

Summary

• Members from Dr. Scott Reid’s group

• Funding from NSF and ACS/PRF

Acknowledgements