Analysis of the Rotational Structure in the High-Resolution Infrared Spectrum of trans-Hexatriene-1,1-d2 and -cis-1-d1

Norman C. Craig, Hannah A. Fuson, and Hengfeng Tian Department of Chemistry and Biochemistry, Oberlin College, Oberlin, OH

44074, USA Norm.Craig@oberlin.edu

Thomas A. BlakeEnvironmental Molecular Sciences Laboratory, Pacific Northwest National

Laboratory, Richland, WA 99352

Goal

Do the isomers of hexatriene (HTE) show greater structural effects of -electron delocalization than does butadiene?

Preliminary data for the C6 backbone of cis-HTE say “Yes,” as do B3LYP calculations with a cc-pVTZ basis set.

New work: analysis of C-type bands in high-resolution IR spectra of trans-HTE-1,1-d2 and -cis-1-d1.

Method: Semi-Experimental Equilibrium Structure

• Obtain ground state rotational constants from the rotational structure in high-resolution IR bands. (Nonpolar trans-HTE is MW silent.)

• Do so for a full series of isotopologues.• Use quantum chemistry (triple zeta level) to compute

vibration-rotation constants (alphas). • Find equilibrium rotational constants (k = a,b,c)

• Obtain an re structure from a global fit of all the equilibrium rotational constants.

Be, k B0, k 1/ 2 k,i1

n

Synthesis

IR spectrum (0.1 cm-1 resol.) of trans-HTE-1,1-d2 and -cis-1-d1

d2

d2

d2

d1

C-Type Bands Analyzed

tHTE-1,1-d2

29 (902 cm-1) CH2 out-of-plane flapping

31 (722 cm-1) CD2 out-of-plane flapping

tHTE-cis-1-d1

31 (803 cm-1) CD out-of-plane flapping

C-type Band (0.0015 cm-1 resol.) of trans-HTE-1,1-d2

Detail of C-type Band at 902 cm-1 of trans-HTE-1,1-d2

Finer Detail of C-type Band at 902 cm-1

C-type Band (0.0015 cm-1 resol.) of trans-HTE-1,1-d2

Rotational Constants for trans-Hexatriene-1,1-d2 Ground state 29(a") C-type

902 cm-1 A/cm-1 0.801882(1) 0.795541(4) B/cm-1 0.041850(2) 0.041881(2) C/cm-1 0.039804(1) 0.039767(2)

J 1010/cm-1 0.9218a 0.9218a

K 108/cm-1 0.9723a 0.9723a

K 106/cm-1 1.837(2) 1.837b

JK 108/cm-1 -2.65(5) -2.65b

J 109/cm-1 1.62(4) 1.671(4) -0.9946 -0.9944 0/cm-1 902.05

std. dev./cm-1 0.00033 0.00042 c/amu Å2 -0.3223 -0.2064

No. trans. 980d 292 Ka' 5-22e, 8-19f 5-7 Jmax 95 85

a Calculated with the B3LYP/cc-pVTZ model. b Ground state values. c Inertial defect, = Ic - Ia

- Ib. d 599 GSCDs from the 902 cm-1 band; 381 from the 722 cm-1 band. e 902 cm-1 band ( 29). f 722 cm-1 band ( 31).

C-type Band (0.0015 cm-1 resol.) of trans-HTE-cis-1-d1

Loomis-Wood Display for C-type Band of trans-Hexatriene-cis-1-d1

• RR13 yellow RR16 blue

• RR14 magenta RR17 brown

• RR15 red

Rotational Constants for trans-Hexatriene-cis-1-d1

Ground state 31(a") C-type

803 cm-1 A/cm-1 0.809388(1) 0.8053839(9) B/cm-1 0.043532(2) 0.0435516(8) C/cm-1 0.041319(1) 0.0412748(9)

J 1010/cm-1 1.0870a 1.0870a

K 108/cm-1 1.0844a 1.0844a

K 106/cm-1 2.150(2) 1.901(5)

JK 108/cm-1 -3.03(4) -2.61(2)

J 109/cm-1 1.67(4) 1.666(3) -0.99424 -0.99404 0/cm-1 803.01823(3)

std. dev./cm-1 0.00035 0.00038 b/amu Å2 -0.093759 0.420146

No. trans. 893 1409 Ka' 3-21 3-14 Jmax 94 98

a Calculated with the B3LYP/cc-pVTZ model. b Inertial defect, = Ic - Ia

- Ib.

ConclusionsAnalysis of the rotational structure in the high-resolution

(0.0015 cm-1) infrared spectrum of two C-type bands of trans-hexatriene-1,1-d2 and of a C-type band of trans-hexatriene-cis-1-d1 has yielded ground state rotational constants for the two species.

Prior work gave ground state rotational constants for the normal species.1

Good progress has been made toward the rotational constants needed for a semi-experimental structure of trans-hexatriene. See the following talk.

1. N. C. Craig, M. C. Leyden,M. C. Moore, Amie K. Patchen, T. van den Heuvel, T. A. Blake, T.

Masiello, R. L. Sams J. Mol. Spectrosc. 2010, 262, 49-60.

Support

Dreyfus Foundation

Environmental and Molecular Sciences Laboratory, PNNL

National Science Foundation

Oberlin College