david james chem 6304 march 31 st , 2010
DESCRIPTION
“ An In-Depth AB Initio Study of Thermodynamics and Stabilization Energies of Mono- and Di-substituted Methyl Halides”. David James Chem 6304 March 31 st , 2010. Outline. Introduction to compounds of interest Stabilization energies Anomeric Effect (bond separation reactions) - PowerPoint PPT PresentationTRANSCRIPT
“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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