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STEPHEN KUKOLICH, Chemistry Dept., University of Arizona, MICHAEL PALMER School of Chemistry, University of Edinburgh, PETER GRONER, Chemistry, University of Missouri-Kansas City, and CHAKREE TANJAROON, Chemistry, University of Alberta,

Experimental Rotational Spectra for MnRe(CO)10

and o-C6H4† Required Accurate theoretical

Calculations for Successful Analysis

C1C2

C3

C4 C5

C6

H1

H2

a

b

MnRe

† Measured at Harvard with Pat Thaddeus and Mike McCarthy

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Observing and Measuring the Rotational Transition Frequencies was Difficult

B = 200.3687 MHz (187Re ) eQq(187Re) = 370.42(38) MHz eQq(55Mn) = -16.52(5) MHz

MnRe

C1C2

C3

C4 C5

C6

H1

H2

a

b

Normal Isotopomer transitions were readily observed 13C Isotopomer lines often obscured by other products of Discharge (Shotgun effect)

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MnRe(CO)10

+ >Symmetric top with C4v symmetry- >Combination of small B-values with two

quadrupoles resulted in congested and difficult-to-assign spectra (J=11→ 12 and 12→ 13)

+ > Michael Palmer and Martyn Guest (Edinburgh) calculated the eQq values sufficiently accurately to allow unambiguous assignments of the spectra

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Example Spectra for 8, and 10 MHz “pieces” of the spectrum

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Pair of measured transitions

2000 Shots

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C4v symmetry Only K=4n transitions

observed Staggered or

Eclipsed? † † F. A. Cotton, Austin 2004 (E) (S)

MnRe

MnRe

(S)

(E)

xx

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Experimental and theoretical molecular parameters for Mn187Re(CO)10 .

Calculated Values (B3LYP) from M. H. Palmer, et al. (J. Chem. Phys. 121, 7187

(2004)). B and eQq values in MHz.

Parameter Mn187Re(CO)10

MICROWAVE

Mn187Re(CO)10

Calculated

(STAGGERED)

Mn187Re(CO)10

Calculated

(ECLIPSED)

MnRe(CO)10

X-ray1

eQq(55Mn) -16.52(5) 5.87 0.68 -

eQq(187Re) 370.42(38) 310.11 327.6 -

B 200.36871(8) 188.77 178.40 204.2304

rMnRe (Å) 2.99 3.086 Å 3.224 2.909(1)

De(kJ/mol) 212(1) 199(1)

Calculated from the geometry 1 A. L. Rheingold, W. K. Meckstroth, and D.P. Ridge, Inorg. Chem. 25, 3706-3707 (1986)

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The structure of o – benzyne and vibrational averaging effects.

• Spectra Measured on HARVARD FTMWS1

• Discharge 0.5% BENZENE in NEON• Normal Isotopomer - 27 b-dipole transitions

• 13C6 - 12 b-dipole transitions

• D1 - 26 b-dipole transitions• D2 - 23 b-dipole transitions

• 13C1(1) - 10 b-dipole transitions

• 13C1(3) - 9 b-dipole transitions

• 13C1(5) - 10 b-dipole transitions1. S. G. Kukolich, M. C. McCarthy and P. Thaddeus, J. Phys. Chem.

108, 2645-2651, (2004)

DIFFICULTTo MEASURE

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Search for 13C lines… in a sea of lines from other

radicals and molecules produced in the DISCHARGE.

JUNK

WRONG

MOLECULEGOOD ONE

C1C2

C3

C4 C5

C6

H1

H2

a

b

kHz

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Least-squares structure fit The inertial defect for the normal isotopomer of o-benzyne is = ICC –

IAA – IBB = 0.06935(1) amu Å2, consistent with a PLANAR STRUCTURE.

This value is sufficiently large that when trying to fit the measured A, B, and C rotational constants with a planar structure, some of the deviations will be as large as 1 MHz.

STANDARD DEVIATION FOR FIT = 1.05 MHz

Experimental errors are < 2 kHz!

EXPT CALC. DEV.

NORMAL A 6989.729 6989.915 -0.186

B 5706.806 5706.891 -0.085

C 3140.371 3141.789 -1.418

13C1 A 6859.730 6859.886 -0.155

B 5679.516 5679.369 0.147

C 3105.740 3107.029 -1.289

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Least-squares fit to determine the structure

Most of the same vibrational averaging effects which contribute to the differences between the r0 and re coordinates, will also contribute to this inertial defect.

Problem 1. We are trying to fit a PLANAR, (re ) structure to EXPERIMENTAL A, B, and C which have non-zero inertial defect, (characteristic of the r0 coordinates )

Problem 2. The r0 coordinates are different for each of the measured isotopomers

SOLUTION: Find the k, vibration-rotation constants, so we fit the re structure, same for all isotopomers

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VIBRATIONAL AVERAGING EFFECTS – RELATED TO

VIBRATION-ROTATION INTERACTION CONSTANTS k A r o t a t i o n a l c o n s t a n t f o r t h e G R O U N D V I B R A T I O N A L S T A T E , B o , i s r e l a t e d t o t h e E Q U I L I B R I U M r o t a t i o n a l c o n s t a n t , B e b y : ( s u m m e d o v e r k v i b r a t i o n a l s t a t e s )

k

k

ααe

ααo αBB

2

1

T h e c o r r e s p o n d i n g r e l a t i o n f o r M O M E N T S O F I N E R T I A i s :

ααk

k

ααe

ααo II

2

1

a n d t h e I N E R T I A L D E F E C T i s :

bbo

aao

ccoo IIIΔ

C1C2

C3

C4 C5

C6

H1

H2

a

b

Values for these k were calculated by Peter Groner1, University of Missouri

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Final Structure of o-benzyne

C C

C C

C C

12

3

4 5

61

2

a

b

1.255

1.383

1.403

1.405

1.080

1.082

127

111

119

o

o

o

H

H

127o

122

o

r(C1-C2) for HCCH =1.203 Å

r(C1-C2) for H2CCH2 =1.332 Å

Now the fit is MUCH improved ( < 30 kHz)

r(C2-C3) for benzene =1.3914 Å

P. Groner and S. G. Kukolich, J. Mol. Struct. 780-781, 178 (2006)

r(C2-C3), NMR value → 1.24(2) ÅGrant, Michl, et al.

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The re structure of o-benzyne (Distances r in Å)

Structural parameter

reMP2/6-31G(d)

rs /

Kraitch-man

B3LYP/631G(d,p)

BPW91/ cc-pVDZ

Ref. This work This work [a] [b] [c]

r(C1-C2) 1.255(3) 1.268 1.255(8) 1.251 1.266

r(C2-C3) 1.383(2) 1.389 1.40(2) 1.385 1.391

r(C3-C4) 1.403(2) 1.405 1.39(2) 1.412 1.423

r(C4-C5) 1.405(3) 1.410 1.404(14) 1.407 1.412

r(C3-H1) 1.080(1) 1.086 1.08(2) 1.085 1.098

r(C4-H2) 1.082(1) 1.088 1.084(9) 1.087 1.100

Benzene          

r(C-C) b 1.3914(1) 1.395      

r(C-H) b 1.0825(3) 1.087       [a] S. G. Kukolich, M. C. McCarthy, P. Thaddeus, J. Phys. Chem A 108 (2004) 2645-2651. [b] S. G. Kukolich, C. Tanjaroon, M. C. McCarthy, P. Thaddeus, J. Chem. Phys. 119 (2003) 4353-4359. [c] C. J. Cramer, Nash, J. J. and R. R. Squires, Chem. Phys. Lett. 277 (1997) 311-320.

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Acknowledgements

• N$F - This material is based upon work supported by the National Science Foundation under Grant No. CHE-0304969. This support from the National Science Foundation is gratefully acknowledged

• Willis Flygare and Terry Balle

•Harvard: Pat Thaddeus, Mike McCarthy

•Arizona: Kristen Keck

•Edingburgh: Martyn Guest, Phillip Camp

•Department of Chemistry, University of Arizona.

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Other isomers of benzyne

The structural isomers of didehydrobenzene – ortho-benzyne, meta-benzyne and para-benzyne.

 

Calculations show Hf(o-benzyne) >Hf(m-benzyne)>Hf(p-benzyne)

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O - BENZYNE Brown, Godfrey, Rodler, Robertson (1st

microwave, no structure)

Pyrolyzed: pthalic anhydride, or ninhydrin, or benzocyclobutene-R (1986, 2003)

Lineberger, Squires, et al. (1998)- electron afinities, singlet triplet splittings & vibrational frequencies

• PRESENT WORK (2002…) Discharge 0.5% BENZENE in NEON HARVARD SPECTROMETER (Sabbatical)