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TAUTOMERISM a special form of isomerism,
intermolecular proton transfer
Prototype case
Challenge for theory completely different
electronic structures!
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Motivation Theoretical studies on CYTOSINE
The role of the four bases in DNA ..
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1 the canonical oxo form 2b enol 3a imino
Theoretical results, e.g. CCSD(T)f.c./cc-pVTZ/
/ rfg
Watch out, black-box users! DFT gives a
qualitatively different picture!
1 2b 3a 1.51 0. 1.49
B3LYP/6-311G(2d,2p) B3PW91/6-311G(2d,2p)
-0.54 0. 1.27 -0.28 0. 1.74
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What about experiment? From molecular beam MW
and noble-gas matrix IR, no reliable
quantitative results, but the picture
is Abundancies (for isolated specii) 2b gt 1
gtgt 3a Thus, serious discrepancy between
theory and experiment!!
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Section 1. Test calculations
Check the reliability of several methods on small
molecules. Test calculations on three systems
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Vinylalcohol
Acetaldehyde
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Acetaldimine
Vinylamine
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Formamide
Formamidic acid
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Methods Electronic theory RHF, B3LYP, MP2,
CCSD(T) Basis sets from 6-31G(d,p) to
6-311G(3df, 3pd) and cc-PVTZ to
aug-cc-PV5Z
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Test 1.
Acetaldehyde Vinylalcohol Tautomer Pair
a In calculations with Pople-type basis sets
(6-31) geometries and energies calculated at the
same level. b E in atomic units.
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Table 1. Contnd. Acetaldehyde Vinylalcohol
Tautomer Pair
C Geometry selected as standard MP2/aug-PVTZ
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Test 2.
Acetaldimine - Vinylamine pair, incl. Trans. State
_________________________________________ a
Energy relative to vinylamine
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Acetaldimine - Vinylamine contnd.
__________________________________________________
____ aEnergy relative to vinylamine. bAt
MP2/aug-PVTZ geometry.
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Test 3.
Formamide Formamidic acid
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Formamide Formamidic acid, contnd.
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Section 2. The effect of water
Model supermolecule a water molecule added
explicitly
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Formamide plus water water may mediate proton
transfer
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(Formamide Formamidic acid) Monohydrate
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CONCLUSIONS ON THE TESTS
__________________________________________________
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Even the best results are uncertain to 1
kcal/mol
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The special case of formaldehyde-hydrate
b3lyp/631G(d,p) works reasonably, as compared to
best 10.1 vs. 9.2, 19.4 vs. 22.8
1 b3lyp/31Gdp, 2 b3lyp/311G2d2p, 3
MP2/31Gdp, 4 MP2/aug-PVTZ, 5 MP2/PVQZ, 6
CCSD/aug-PVTZ, 7 CCSD(T)/aug-PVTZ, 8
CCSD/PVQZ, 9 CCSD(T)/PVQZ
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Section 3 Dynamics Try to see the process of
tautomerization
Method ab initio dynamics Contrary to
Car-Parrinello, the wave function is truly
recalculated at each point . Pulay, P.,
Fogarasi, G. Fock matrix dynamics, CPL 386,
272-278 (2004)
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The notion of reaction mechanisms is based on the
Born-Oppenheimer (B-O) approximation atoms move
on a potential energy surface (PES) defined by
the electronic energy as a function of nuclear
positions. In the simplest models reactions
follow the minimum energy pathway (MEP), going
through a transition state (TS). The MEP
expressed in mass-weighted Cartesians is referred
to as the internal reaction coordinate, IRC.
Recent computations have shown that reactions may
follow a route totally different from the
IRC. (W.L. Hase, Science 2002 M. Dupuis, Science
2003).
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True dynamics calculations require knowledge of
the complete PES, and recent methods generate it
"on the fly". The well-known Car-Parrinello
method is most efficient computationally because
the electronic wave function is "propagated", and
not optimized, at the trajectory points. As a
consequence, the system is moving close to, but
not exactly on the B-O surface. In B-O dynamics,
the wave function of a QC method is fully
optimized in each step along the trajectory.
Energy and first derivatives are determined from
ab initio wf, and atomic movements calculated
from them classically. This is the approach
adopted here using Verlet's algorithm.22 The QC
method was DFT(B3LYP)/6-31G and
/6-311(2d,2p).
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Call PQS to show dynamics Two different
cases 601c and 604c
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The End