I' Modeling the Reaction between Vinylamine and Singlet Oxygen

1
I. Modeling the Reaction between Vinylamine and
Singlet Oxygen

• A Semi-Empirical Molecular Orbital Computational
  Study

2
Singlet Oxygen

• Singlet oxygen (1O2) is an electronically excited
  form of molecular oxygen.
• It is short-lived (10-5 s), but very reactive.
• Usually generated by dye-sensitization
  
  1Dye
  ____h?____gt 1Dye
  1Dye ____(isc)____gt 3Dye
  3Dye 3O2 __________gt 1O2
  1Dye

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Electron Configuration of Singlet Oxygen (HOMO)
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Typical Modes of Reaction of Singlet Oxygen
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Reaction of 1O2 with Enamines

• Based on product studies in more complex enamines
  (vinylamine is unstable and very difficult to
  prepare it is modeled because it is small, and
  amenable to rapid calculation)

6
Modeling Singlet Oxygen

• Calculated ?Hf in kcal/mol
• Structure MNDO AM1 MINDO/3
• singlet O2 12.2 0.7 22.8
• (Experimental value is 22.5 kcal/mol)

7
Intermediates Proposed for Reaction of Singlet
Oxygen with Enamines
8
Background

• We had strong kinetic (substituent effect)
  evidence that favored rate-limiting formation of
  a charge-transfer complex in the
  photo-oxygenation (reaction with singlet oxygen)
  of 1-benzyl-3,4-dihydroisoquinolines, which are
  in equilibrium with an enamine tautomer.
• We decided to model a similar mechanism for
  vinylamine, the simplest enamine.

9
Modeling an Interaction

• An AMPAC input file was created by merging two
  independently optimized structures vinylamine
  and singlet oxygen
• Various orientations were tried
• When one geometry showed
  attractive interaction, it was
  examined more
  closely.

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Optimizing the Geometry

• The N-O bond length was varied
• The N-O-O bond angle was varied
• The C-N-O-O dihedral angle was varied
• After optimizing one variable,
  it was fixed to
  optimize
  another then two were fixed
  to optimize the
  third.
• Finally the structure was allowed to optimize
  starting with the best values of the three
  variables.

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Attraction between Singlet Oxygen and Vinylamine
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Proposed Orbital Interaction
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Optimized Geometry of the Charge-Transfer Complex

• An energy minimum was found at an N-O bond
  length of 1.55 Ao
• The lowest energy complex was found to have an
  N-O-O bond angle of 111o
• The lowest energy complex was found to have a
  C-N-O-O dihedral angle of 139o

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Reaction Pathway CalculationInput File to Vary
Bond Length

• t5000 MINDO3
• vinylamine
• 7 0.00000 0 0.0000 0 0.0000 0 0 0 0
• 6 1.38210 1 0.0000 0 0.0000 0 1 0 0
• 1 1.09000 1 113.9000 1 0.0000 1 2 1
  0
• 6 1.34000 1 124.8000 1
  179.9000 1 2 1 3
• 1 0.99900 1 122.4000 0
  179.9000 1 1 2 3
• 1 0.99900 1 120.8000 1 0.0000 1 1 2
  3
• 1 1.08000 1 124.7000 1
  0.0000 1 4 2 1
• 1 1.08000 1 121.3000 1
  179.9000 1 4 2 1
• 8 1.55000 -1 111.4000 1
  137.1000 1 1 2 4
• 8 1.13000 1 111.1000 1
  138.9000 1 9 1 2
• 0 0.00000 0 0.0000 0 0.0000 0 0 0 0
• 1.6, 1.65, 1.7, 1.75, 1.8, 1.85, 1.9, 2, 2.2,
  2.4, 2.6, 3, 4, 6, 8, 10

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Locating the Transition Structure

• The keywords SADDLE, POWELL, and SIGMA were
  employed sequentially with the geometry of the
  reactant, a guess at the T.S. geometry, and the
  product geometry.
• A FORCE calculation was done on the T.S. geometry
  provided by this method.
• This yielded only 1 imaginary (negative)
  frequency, confirming the saddle point.

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Reaction Energy Profiles Zwitterionic Peroxide
(a) and CT Complex followed by Zwitterionic
Peroxide (b)
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Conclusion

• A charge-transfer complex may be competitive with
  direct zwitterionic peroxide formation in the
  reaction of singlet oxygen with enamines,
  particularly in those cases where more complex
  structures may stabilize the intermediate complex
  or facilitate its rearrangement.

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II. Modeling the Quenching of Singlet Oxygen by
Amines

• A Semi-Empirical Molecular Orbital Computational
  Study

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Singlet Oxygen

• Singlet oxygen (1O2) is an electronically excited
  form of molecular oxygen.
• It is short-lived (10-6 s), but very reactive.
• Usually generated by dye-sensitization
  
  1Dye
  ____h?____gt 1Dye
  1Dye ____(isc)____gt 3Dye
  3Dye 3O2 __________gt 1O2
  1Dye

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Singlet Oxygen...

• It is quenched efficiently by tertiary amines,
  especially DABCO (diazabicyclooctane)
• DABCO is sterically unhindered, unlike most
  tertiary amines, and has a low IP...therefore it
  should be a good electron donor.

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Quenching of Singlet Oxygen by Amines
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Quenching

• No chemical reaction involved...amines are
  unchanged
• Physical quenchingrate related inversely to
  ionization potential of the amine
• Very sensitive to steric effects in the vicinity
  of nitrogen
• This data suggests a charge-transfer mechanism.

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Proposed Orbital Interaction (same as for C.T.
complex formation)
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Our Approach

• Model various amines and singlet oxygen
  (optimized separately) in proximity to one
  another and look for an interaction (attraction)
  AMPAC/MINDO3 was used.
• When an attraction is observed, examine the
  complex more closely to determine the optimum
  geometry.
• Use knowledge gained from geometry of
  vinylamine-singlet oxygen complex.

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Determining the Optimum N-O Bond
Distance
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Determining the Optimum N-O-O Bond Angle
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Determining the Optimum N-O-O-O Dihedral
Angle
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Typical Optimum Geometry of Amine-1O2 Complexes

• N-O bond length of 1.55 Ao
• N-O-O bond angle of 119o
• C-N-O-O dihedral angle of 180o
• Transfer of 0.3 esu of charge from N to distal O
  is observed (more than in vinylamine).

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Modeling Charge-Transfer Complexation of Amines
with Singlet Oxygen

• N-O bond distance 1.55 Ao
• ?qN 0.35esu ?qOdistal -0.33 esu

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Unexpected Results!
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An Alternative Hypothesis

• Perhaps rate-limiting step is NOT formation of
  the charge-transfer complex, but instead is
  intersystem crossing (ISC) to the triplet
  complex
• 1O2 . . . 1NR3 ___ISC___gt 3O2 . .
  . 3NR3
• This process can not be modeled, but can be
  estimated by single point calculations.

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Considering Triplet Complex

• Triplet with
• Singlet geometry
• Singlet Triplet

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Consistent with Slow ISC !
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Acknowledgments Financial and Technical Support

• UNCW Office of Information Technology for use of
  the VAX computer
• M.J.S. Dewar (now deceased) and Eamon F. Healy
  for providing a free copy of AMPAC
• NSF for an ILI grant (1996) to Integrate
  Molecular Modeling into the Chemistry Curriculum
  (computers and software)

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Acknowledgments The Workers

• Reaction of Singlet Oxygen with Vinylamine David
  B. Allen and Kelly N. Taylor
• Quenching of Singlet Oxygen by Amines Charles K.
  Marschke, Jr., Chris A. Cottle and Noah W. Allen,
  III.