Frank Lee Emmert III, Stephanie Thompson, and Lyudmila V. Slipchenko Purdue University, West Lafayette, IN 47907

ELECTRONIC STRUCTURE OF ETHYNYL SUBSTITUTED CYCLOBUTADIENES

Frank Lee Emmert III, Stephanie Thompson, and Lyudmila V. Slipchenko

Purdue University, West Lafayette, IN 47907

Fullerene formationBergman

CyclizationRing

Closure

Retro [2+2]

Coalescence and

Annealing

Hunter, J. M.; Fye, J. L.; Roskamp, E. J.; Jarrold, M. F. J. Phys. Chem. 1994, 98, 1810−1818.

Proposed mechanism for the formation of fullerenes.

H H

HH

1,2,3,4-tetraethynylcyclobuta-1,3-diene

Fullerene formation

HH

H

H

1,2,3-triethynylcyclobuta-1,3-diene

H

H

H

H

1,2-diethynylcyclobuta-1,3-diene

H

H H

H


H

H

H H

1-ethynylcyclobuta-1,3-diene

HH

H H

cyclobuta-1,3-diene

H

H

H

H


We cut the alkyne tails and looked at ethynyl substituted cyclobutadienes.

H H

HH

1,2,3,4-tetraethynylcyclobuta-1,3-diene

Fullerene formation

We calculated:equilibrium geometries adiabatic S-T gap energiesvertical S-T gap energiesstabilization energy of the ethynyl

substituentsspin densities (not discussed)natural charges (not discussed)

H

H

H H

1-ethynylcyclobuta-1,3-diene

HH

H H

cyclobuta-1,3-diene

H

H

H

H


Cyclobutadiene orbitalsSinglet cyclobutadiene undergoes Jahn-Teller distortion and becomes

rectangular.

1.34211.3676

1.56861.5679

1.43911.43961.4516

MP2EOM-SF-CCSD

ROMP2UMP2EOM-SF-CCSD

Triplet

Singlet

Singlet cyclobutadiene undergoes Jahn-Teller distortions to make a rectangular structure. Spin Flip variant of the Equation of Motion Coupled Cluster with single and double excitations was

emplolyed. Accuracy of Møller-Plesset 2nd order perturbation theory was tested employing both a restricted open shell

and unrestricted reference

SingletTriplet

L. V. Slipchenko and A. I. Krylov J. Chem. Phys. 2002, 117, 4694

Optimized singlet geometries

1 2

34

56

EOM-SF-CCSD/cc-pVDZ

Singlet Bond Lengths (Å)

Molecule 1 2 3 40-c 1.567 1.367 1.567 1.3671-c 1.557 1.353 1.557 1.371

2-c short 1.551 1.368 1.551 1.3702-c long 1.563 1.374 1.554 1.3742-c trans 1.564 1.377 1.564 1.377

3-c 1.542 1.385 1.546 1.3794-c 1.558 1.388 1.558 1.388

Ethynyl Substituents (Å)

bond 5 6average 1.42 1.22

Main pattern of alternating bond lengths does not change with substituent addition.

The singlet geometries become more square with increased diradical character.

Substituent bond lengths remain nearly constant.

Optimized triplet geometries

1 2

34

56

EOM-SF-CCSD/cc-pVDZ

Triplet Bond Lengths (Å)

Molecule 1 2 3 40-c 1.451 1.451 1.451 1.4511-c 1.466 1.466 1.438 1.4382-c 1.455 1.483 1.455 1.423

2-c trans 1.453 1.453 1.453 1.4533-c 1.456 1.442 1.442 1.4564-c 1.456 1.456 1.456 1.456



Square structure is maintained when substituents are added symmetrically.

The triplet geometries have increasing bond lengths; decreasing the aromaticity.


Adiabatic energies

0-c 1-c 2-c short 2-c trans 2-c long 3-c 4-c0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

EOM-SF-CCSD UMP2

ROMP2 ROCCSD(T)

Ene

rgy

(eV

)

The singlet triplet gap decreases with substituent addition.UMP2 does not follow the trend of decreasing singlet-triplet gap energy.Main source of error is spin contamination of the triplet state.

MP2 vertical energies

Vertical S-T gaps should be larger at singlet geometries and smaller at triplet geometries.

MP2 cannot properly describe the diradical singlet state at the triplet geometries where the two π-orbitals of the ring are degenerate.

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

0-c 1-c 2-c short 2-c long 2-c trans 3-c 4-c

Ene

rgy

(eV

)

RMP2//UMP2 UMP2//RMP2

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

0-c 1-c 2-c short 2-c long 2-c trans 3-c 4-c

Ene

rgy

(eV

)

RMP2//ROMP2 ROMP2//RMP2

Adiabatic S-T Gap

Vertical S-T gap at triplet geometry

Vertical S-T gap at singlet geometry

S

T

EOM vertical energies

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0-c 1-c 2-c trans 2-c long 2-c short 3-c 4-c

Ene

rgy

(eV

)

EOM-SF-CCSD Singlet

EOM-SF-CCSD TripletAdiabatic S-T Gap

EOM-SF-CCSD shows correct vertical behavior for all substituents.Vertical S-T gap energies decrease with substituent addition at the singlet

geometries while remaining almost constant at the triplet geometries.

MP2 had a lot of trouble and CCSD(T) would have a lot of trouble because it is mostly a problem of the HF reference.

S

T

EOM vertical energies

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0-c 1-c 2-c trans 2-c long 2-c short 3-c 4-c

Ene

rgy

(eV

)

EOM-SF-CCSD Singlet

EOM-SF-CCSD TripletAdiabatic S-T Gap

Adiabatic S-T Gap

The triplet surface is becoming flatter or the singlet geometry is becoming more like the triplet geometry.

S

T

ST

Isodesmic Reactions

CH3 H C4H(4-n)(CCH)n + nCH41. C4H4 + n

H C4H(4-n)(CCH)n + n2. C4H4 + n

H C4H(4-n)(CCH)n + n3. C4H4 + n

H C4H(4-n)(CCH)n + n4. C4H4 + n

H C4H(4-n)(CCH)n + n5. C4H4 + n


H C4H(4-n)(CCH)n + n2. C4H4 + n

H C4H(4-n)(CCH)n + n3. C4H4 + n

Isodesmic

Homodesmotic

Isodesmic reactions preserve the number and type of bonds (single, double, triple).

Homodesmotic reactions preserve the hybridization, the number and types of bonds of the carbon atoms, and the number of hydrogen atoms bonded to individual carbon atoms.

Wheeler, S.E., et al., J. Am. Chem. Soc., 2009. 131(7): p. 2547.

Stabilization energiesProduct Eqn. 1 Eqn. 2 Eqn. 3. Eqn. 4 Eqn. 5

Singlet RMP2

1-c -11.74 -3.73 -4.26 -2.40 -1.712-cs -24.73 -8.69 -9.77 -6.05 -4.662-cl -24.17 -8.14 -9.22 -5.49 -4.102-ct -23.08 -7.04 -8.12 -4.40 -3.013-c -36.75 -12.70 -14.32 -8.73 -6.654-c -50.01 -17.95 -20.11 -12.66 -9.88

Triplet ROMP2

1-c -12.07 -4.05 -4.59 -2.73 -2.042-cs -26.38 -10.35 -11.42 -7.70 -6.312-cl -26.38 -10.35 -11.42 -7.70 -6.312-ct -23.48 -7.45 -8.53 -4.80 -3.413-c -37.92 -13.87 -15.49 -9.91 -7.824-c -51.70 -19.63 -21.79 -14.34 -11.56

Both the triplet and the singlet are stabilized with substituent addition.The triplet is more stabilized then the singlet.Each reaction gives the same pattern and S-T differences.

ConclusionsS-T gaps are decreased with ethynyl substituent addition

but the singlets are always lower in energy.

Results are effected by spin contamination of the triplet states; UMP2 fails to properly describe the system.

Based on isodesmic reactions, triplet states becomes more stabilized then the singlet states as substituents are added.

Acknowledgements:Thank you:

Professor McMahon – University of WisconsinLevi Haupert

Visualization Software:MacMolPltChemBioDraw12

Packages:Q-ChemGAMESSCFOUR

Funding provided by:ACS-PRFPurdue University

Thank you

H H

HH

Graphene and fullerene formation

Bergman Cyclization

Ring Closure

Retro [2+2]

Coalescence and

Annealing

Hunter, J. M.; Fye, J. L.; Roskamp, E. J.; Jarrold, M. F. J. Phys. Chem. 1994, 98, 1810−1818.

Cyclobutadiene orbitals

Cyclobutadiene undergoes Jahn-Teller distortions.

SingletTriplet

Optimized Geometries1.43911.43961.4516

1.43501.46131.4385

1.44731.42171.4666

1.44731.42171.4666

1.43501.46131.4385

1.45511.40661.4830

1.44081.44011.4553

1.43681.49331.4239

1.44081.44011.4551

1.44451.44411.4536

1.44471.47291.4563

1.44471.47291.4564

1.44991.42441.4427

1.44981.42441.4426

1.45221.45091.4562

Singlet Triplet

1.35341.3724

1.58161.5578

1.55101.5576

1.34211.3717

1.34211.3676

1.56861.5679

1.35471.3749

1.59991.5631

1.35471.3749

1.53021.5546

1.56211.5510

1.34241.3707

1.36871.3859

1.56441.5559

1.35281.3779

1.54271.5453

1.36881.3855

1.57991.5467

1.35281.3798

1.36891.3884

1.55871.5478

Adiabatic energies

0-c 1-c 2-c short 2-c trans 2-c long 3-c 4-c0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

UEOM ROEOM UMP2

ROMP2 UCCSD(T) ROCCSD(T)

Ene

rgy

(eV

)

The singlet triplet gap decreases with substituent addition. UMP2 does not follow the trend of decreasing singlet-triplet gap energy. Main source of error is spin contamination.

Natural Charges Singlets-0.072

-0.187

-0.097

-0.146

-0.209

-0.076

-0.067

-0.173

-0.147

-0.040

-0.175

-0.078

-0.176

-0.075

-0.161

-0.079

-0.011

-0.040

-0.011

-0.081

-0.205

-0.181

( -0.837) ( -0.629) ( -0.428)

( -0.430)

( -0.426)

( -0.234)

( -0.045)

-0.169

-0.094

-0.119

-0.080

-0.157

-0.118

-0.066

-0.077

-0.165

-0.151

Natural Charges Triplets

-0.210

-0.184

-0.085

-0.082

-0.170

-0.085

-0.168

-0.047-0.165

-0.127

-0.079

-0.050

-0.021

-0.161

-0.012

-0.205

( -0.841) ( -0.627) ( -0.421)

( -0.430) ( -0.235) ( -0.048)

-0.173

-0.076

-0.089

-0.079

-0.159

-0.115

-0.148

-0.080

Spin Densities

0.4990.530

0.1220.831

0.5270.663

0.003-0.652

0.5000.596

0.003-0.665

0.1250.836

0.5370.651

0.3330.239

0.4050.425

0.001-0.571

0.4060.468

0.4290.614

0.0920.691

0.4020.414

0.3450.088

(1.999, 2.123)

(1.873, 1.944)

(1.741, 1.783)

(1.623, 1.703)

(1.714, 1.738)

(1.609, 1.659)

0.0920.662

0.002-0.545

0.4060.426

0.001-0.649

0.0940.807

0.4720.186

0.0950.705

0.001-0.578

0.0050.010

0.0410.267

0.018-0.035

-0.065-0.294

0.0150.107

-0.067-0.316

0.0440.310

0.0280.047

0.003-0.014

0.0070.298

-0.051-0.247

0.014-0.154

0.0190.252

0.0360.202

0.0190.017

-0.003-0.104

(0.020, 0.040)

(0.048, 0.075)

(0.062, 0.066)

(0.052, 0.178)

(0.066, 0.100)

(0.076, 0.068)

0.0340.170

-0.051-0.227

0.019-0.209

-0.056-0.340

0.0370.395

0.0190.156

0.0380.279

-0.053-0.277

HF Triplet

ROEOM Triplet

Spin Densities-0.067-0.314

0.0570.330

0.019 0.083

0.009-0.035

0.0050.009

-0.071-0.320

0.0230.039

0.0580.323

-0.002-0.016

0.0220.074

-0.005-0.065

-0.060-0.294

0.0470.298

-0.054-0.274

0.0530.347

-0.063-0.325

0.021-0.054

0.0200.120

0.0140.012

-0.053-0.272

-0.001-0.032

0.001 0.012

(0.021, 0.038) (0.029, 0.028) (0.041, 0.047)(0.033, 0.017)

(0.033, 0.003) (0.045, 0.013) (0.054, 0.047)

0.0510.312

0.016 0.101

0.000-0.100

-0.066-0.304

0.0540.288

-0.010-0.221

0.0140.169

-0.045-0.272

0.0380.312

0.0430.278

Optimized singlet geometries

1 2

34

56

EOM-SF-CCSD/cc-pVDZ

Singlet Bond Lengths (Å)

Molecule 1 2 3 40-c 1.5679 1.3676 1.5679 1.36761-c 1.5578 1.3534 1.5576 1.3717

2-c short 1.5510 1.3687 1.5510 1.37072-c long 1.5631 1.3749 1.5546 1.37492-c trans 1.5644 1.3779 1.5644 1.3779

3-c 1.5427 1.3855 1.5467 1.37984-c 1.5587 1.3884 1.5587 1.3884



Main pattern of alternating bond lengths does not change with substituent addition.

The singlet geometries have decreasing single bond lengths and increased double bond lengths; becoming more square.


Optimized triplet geometries

1 2

34

56

EOM-SF-CCSD/cc-pVDZ

Triplet Bond Lengths (Å)

Molecule 1 2 3 40-c 1.4516 1.4516 1.4516 1.45161-c 1.4666 1.4666 1.4385 1.43852-c 1.4553 1.4830 1.4551 1.4239

2-c trans 1.4536 1.4536 1.4536 1.45363-c 1.4563 1.4426 1.4427 1.45644-c 1.4562 1.4562 1.4562 1.4562



Square structure is maintained when substituents are added symmetrically.

The singlet geometries have decreasing single bond lengths and increased double bond lengths, becoming more square.

Stabilization energiesProduct Eqn. 1 Eqn. 2 Eqn. 3. Eqn. 4 Eqn. 5

Singlet RMP2

1-c -11.74 -3.73 -4.26 -2.40 -1.712-cs -24.73 -8.69 -9.77 -6.05 -4.662-cl -24.17 -8.14 -9.22 -5.49 -4.102-ct -23.08 -7.04 -8.12 -4.40 -3.013-c -36.75 -12.70 -14.32 -8.73 -6.654-c -50.01 -17.95 -20.11 -12.66 -9.88

Triplet ROMP2

1-c -12.07 -4.05 -4.59 -2.73 -2.042-cs -26.38 -10.35 -11.42 -7.70 -6.312-cl -26.38 -10.35 -11.42 -7.70 -6.312-ct -23.48 -7.45 -8.53 -4.80 -3.413-c -37.92 -13.87 -15.49 -9.91 -7.824-c -51.70 -19.63 -21.79 -14.34 -11.56

Product Eqn. 1 Eqn. 2

Singlet RCCSD(T)

1-c -10.71 -3.422-cs -22.05 -7.482-cl -21.67 -7.102-ct -21.16 -6.583-c -32.74 -10.884-c -44.21 -15.06

Triplet ROCCSD(T)

1-c -12.34 -5.052-cs -26.85 -12.272-cl -26.85 -12.272-ct -22.65 -8.073-c -37.03 -15.164-c -49.12 -19.97


H C4H(4-n)(CCH)n + n2. C4H4 + n

H C4H(4-n)(CCH)n + n3. C4H4 + n

H C4H(4-n)(CCH)n + n4. C4H4 + n

H C4H(4-n)(CCH)n + n5. C4H4 + n

Documents

Frank Lee Emmert III, Stephanie Thompson, and Lyudmila V. Slipchenko Purdue University, West Lafayette, IN 47907