“An In-Depth AB Initio Study of Thermodynamics and Stabilization Energies of Mono- and Di-substituted

Methyl Halides”

David JamesChem 6304March 31st, 2010

OutlineIntroduction to compounds of interest

Stabilization energiesAnomeric Effect (bond separation reactions)

Geometry optimizationSubstitution Reactions

Trends in Mulliken Charges

Problems at the MP4 basis set

Conclusions

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Molecules of Interest

Methane : CH4

Methyl Halides: CH3X

Methylene Halides: CH2X2

Di-substitued Methyl Halides: CH2YX

C

H

HH

H

C

H

HH

F C

H

HH

Cl C

H

HH

Br

C

F

HH

F C

Cl

HH

ClC

Br

HH

Br

C

Cl

HH

F C

Br

HH

FC

Br

HH

Cl

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Computational Details

Levels of Theory: Basis Sets: HF STO-3GMP2 6-31GB3LYP 6-31+G(MP4)** 6-31++G(f2d,p)

Optimizations and Frequencies calculated at:

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The Anomeric Effect

CY

Xπ - donor

σ - acceptor

-Most stable systems will consist of a good π – donor and a good σ – acceptor substituents

-As shown, C is a good π – acceptor and good σ – donor

-If both X and Y have only accepting or donating properties, the compound will be destabilized

-Acceptor/acceptor pull too much electron density away from C center-Donor/donor will give too much electron density to C center

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X-CH2-Y + CH4 CH3-X + CH3-Y

Bond Separation Reactions

F: Very good σ acceptor and fairly weak π donor.Cl: good σ acceptor and very weak π donor.Br: good σ acceptor and very weak π donor.

- Isodesmic Reaction: Should be able to calculate accurately at low levels of theory and basis sets

X, Y

CY

Xπ - donor

σ - acceptor

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Predicting Stabilization using Geometry

VS.

Compound C-X (Å) Difference Compound Bond C-X (Å)

CH 3 F 1.3623 C-F 1.3369

CH 2 F 2 1.3334 C-Cl 1.7706

CH 3 Cl 1.7855 C-F 1.3378

CH 2 Cl 2 1.7676 C-Br 1.9142

CH 3 Br 1.9244 C-Cl 1.7675

CH 2 Br 2 1.9088 C-Br 1.9099

2.12%

1.01%

0.81%

CH 2 FCl

CH 2 FBr

CH 2 ClBr

Can give insight into π – donating ability of substituents

C

H

HH

F C

F

HH

F

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Calculated at the HF/6-31++G(f2d,p) level.

Anomeric Stabilization Energies

F: Very good σ acceptor and fairly weak π donor.Cl: good σ acceptor and very weak π donor.Br: good σ acceptor and very weak π donor.

Theory Basis setAnomeric

Stabilization Energy (KJ/mol)

Theory Basis setAnomeric

Stabilization Energy (KJ/mol)

Theory Basis setAnomeric

Stabilization Energy (KJ/mol)

HF STO-3G 20 HF STO-3G -6.2 HF STO-3G 17.4HF 6-31G -1.8 HF 6-31G -23 HF 6-31G -8.2HF 6-31++G(f2d,p) 13 HF 6-31++G(f2d,p) -12 HF 6-31++G(f2d,p) 6.5MP2 STO-3G 27.6 MP2 STO-3G 2.3 MP2 STO-3G 22.3MP2 6-31G 9.5 MP2 6-31G -8.2 MP2 6-31G 4.4MP2 6-31++G(f2d,p) 22 MP2 6-31++G(f2d,p) 4.5 MP2 6-31++G(f2d,p) 15.9B3LYP STO-3G 40.4 B3LYP STO-3G 3.3 B3LYP STO-3G 32.1B3LYP 6-31G 13.4 B3LYP 6-31G -9.7 B3LYP 6-31G 8.4B3LYP 6-31++G(f2d,p) 19.7 B3LYP 6-31++G(f2d,p) -1.3 B3LYP 6-31++G(f2d,p) 15.4

CH 2 FCl + CH 4 --> CH 3 F + CH 3 Cl CH 2 ClBr + CH 4 --> CH 3 Cl + CH 3 Br CH 2 FBr + CH 4 --> CH 3 F + CH 3 Br

Expectations: • CH2FCl CH2FBr CH2ClBr

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• All levels of theory and basis sets will give similar values (isodesmic reactions)

> >

Anomeric Stabilization Energies

Theory Basis setAnomeric

Stabilization Energy (KJ/mol)

Theory Basis setAnomeric

Stabilization Energy (KJ/mol)

Theory Basis setAnomeric

Stabilization Energy (KJ/mol)

HF STO-3G 44.6 HF STO-3G -14.5 HF STO-3G -4.4HF 6-31++G(f2d,p) 51.4 HF 6-31++G(f2d,p) -9.1 HF 6-31++G(f2d,p) -11.5B3LYP STO-3G 57.5 B3LYP STO-3G -4.8 B3LYP STO-3G 3.3B3LYP 6-31++G(f2d,p) 50.3 B3LYP 6-31++G(f2d,p) 0.2 B3LYP 6-31++G(f2d,p) -0.2

CH 2 F 2 + CH 4 --> 2CH 3 F CH 2 Cl 2 + CH 4 --> 2CH 3 Cl CH 2 Br 2 + CH 4 --> 2CH 3 Br

CH2F2 > CH2Cl2 ~ CH2Br2

**AB Initio Molecular Orbital Theory: CH2F2 = 60 kJ/mol (HF/3-21G)** CH2Cl2 = -17 kJ/mol (HF/3-21G(*))

10 kJ/mol difference

Shows that second row elements are very sensitive to polarization function in these ISODESMIC reactions!

My calculations of CH2Cl2 with HF/3-21G = -27 kJ/mol

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Substitution Reactions

2X-CH2 + Y2 2CH3-Y + X2

Level of Theory Basis Set Energy (KJ/mol) Level of Theory Basis Set Energy (KJ/mol) Level of Theory Basis Set Energy (KJ/mol)HF 6-31G 264.4 HF 6-31G 62.6 HF 6-31G 376.9HF 6-31+G 291.0 HF 6-31+G 59.5 HF 6-31+G 373.9HF STO3G 64.4 HF STO3G 82.8 HF STO3G 147.2HF 631++G(2df,p) 358.6 HF 631++G(2df,p) 56.0 HF 631++G(2df,p) 414.6MP2 6-31G 219.6 MP2 6-31G 48.0 MP2 6-31G 219.2MP2 6-31+G 259.5 MP2 6-31+G 45.6 MP2 6-31+G 305.0MP2 STO3G 38.0 MP2 STO3G 65.4 MP2 STO3G 103.3MP2 631++G(2df,p) 320.2 MP2 631++G(2df,p) 48.9 MP2 631++G(2df,p) 369.2B3LYP 6-31G 227.6 B3LYP 6-31G 52.2 B3LYP 6-31G 279.8B3LYP 6-31+G 259.3 B3LYP 6-31+G 52.0 B3LYP 6-31+G 218.7B3LYP STO3G 67.0 B3LYP STO3G 62.9 B3LYP STO3G 129.9B3LYP 631++G(2df,p) 319.0 B3LYP 631++G(2df,p) 52.5 B3LYP 631++G(2df,p) 371.6

2CH 3 F + Br 2 --> 2CH 3 Br + F 22CH 3 Cl + Br 2 --> 2CH 3 Br + Cl 22CH 3 F + Cl 2 --> 2CH 3 Cl + F 2

Polarization functions must be used to fully describe the process when 2nd and 3rd row elements are present!

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Trends in Mulliken Charges

Compound Theory C H X Y CompoundTheory C H X YCH4 HF -0.26262 0.065655 CH4 HF -0.62182 0.155425

MP2 -0.24509 0.061217 MP2 -0.62074 0.155124B3LYP -0.31133 0.077794 B3LYP -0.57811 0.144469

CH3F HF -0.04648 0.064171 -0.14603 CH3F HF -0.02473 0.162787 -0.46363MP2 -0.03959 0.0608 -0.14281 MP2 -0.03692 0.167417 -0.46533B3LYP -0.11403 0.071525 -0.10054 B3LYP -0.08848 0.147933 -0.35532

CH3Cl HF -0.13535 0.102377 -0.17178 CH3Cl HF -0.56122 0.219302 -0.09668MP2 -0.11885 0.098971 -0.17807 MP2 -0.55328 0.218987 -0.10368B3LYP -0.18585 0.112295 -0.15104 B3LYP -0.53617 0.201579 -0.06857

CH3Br HF -0.22799 0.086379 -0.03115 CH3Br HF -0.63735 0.225803 -0.04006MP2 -0.20913 0.082193 -0.03745 MP2 -0.6324 0.224385 -0.04076B3LYP -0.27306 0.095835 -0.01444 B3LYP -0.59878 0.206139 -0.01963

CH2F2 HF 0.166368 0.06458 -0.14783 CH2F2 HF 0.289639 0.20188 -0.34668MP2 0.162349 0.062816 -0.14399 MP2 0.496317 0.187207 -0.43532B3LYP 0.064879 0.066366 -0.09881 B3LYP 0.323713 -0.31831 -0.31822

CH2Cl2 HF -0.02795 0.131824 -0.11778 CH2Cl2 HF -0.55395 0.250328 0.026115MP2 -0.07708 0.140036 -0.10156 MP2 -0.55395 0.250328 0.027182B3LYP -0.07708 0.140036 -0.10143 B3LYP -0.55395 0.250328 0.027182

CH2Br2 HF -0.20229 0.101588 -0.00045 CH2Br2 HF -0.7268 0.28339 0.080184MP2 -0.18169 0.097839 -0.00696 MP2 -0.55457 0.288357 -0.01084B3LYP -0.24158 0.109583 0.011189 B3LYP -0.66771 0.257548 0.07669

CH2FCl HF 0.084004 0.10332 -0.11152 -0.17912 CH2FCl HF -0.02263 0.227146 -0.40811 -0.02356MP2 0.091496 0.101511 -0.10737 -0.18715 MP2 -0.01931 0.232021 -0.41277 -0.03197B3LYP 0.013247 0.109338 -0.04771 -0.18422 B3LYP -0.09468 0.20556 -0.29598 -0.02046

CH2FBr HF -0.01111 0.085847 -0.12715 -0.03343 CH2FBr HF -0.01111 0.085847 -0.12715 -0.03343MP2 -0.0026 0.08311 -0.12338 -0.04024 MP2 -0.08721 0.235926 -0.41361 0.028967B3LYP -0.07375 0.091705 -0.06932 -0.04035 B3LYP -0.14532 0.209564 -0.29632 0.022509

CH2ClBr HF -0.11667 0.116342 -0.15138 0.035368 CH2ClBr HF -0.65607 0.278168 0.019511 0.080221MP2 -0.09843 0.113267 -0.1586 0.030498 MP2 -0.63018 0.276976 0.003282 0.072941B3LYP -0.16202 0.124357 -0.13973 0.053038 B3LYP -0.61088 0.253708 0.025854 0.077615

STO-3G 6-31G• STO-3G shows Cl to carry a larger negative charge then F• 6-31G properly assigns charge in order of electronegativity

• Charge on C increases with increasing electronegativity of substituent.

• Charge of H decreases with increasing electronegativity of substituent.

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Problems with MP4

-At the MP4 level of theory, the initial geometry is used for all calculations throughout, meaning that redundant internal coordinates (RIC) are not used.

-Probably do to cost of computing.

-Symmetry of compound is lost.

-Optimized structures must be used.

-Even CH4 did not use RIC!-ie: Predicts CH4 to be CV3 symmetry

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Conclusions

Geometry Optimizations comparing bond lengths can give insight into π-donating abilities of substituents.

Not all isodesmic reactions can be fully described by low levels of theory and small basis sets.

d polorizable functions are necessary to fully describe second row elements.

Mulliken charges can be very random. Trends in basis sets can be observed, but the magnitude of the charges must be investigated using additional information for complete understanding.

MP4 level of theory does not use RIC; therefore, giving incorrect values when optimized structures are not initially used.

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