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TAUTOMERISM: a special form of isomerism, inter molecular proton transfer. Prototype case:. Challenge for theory: completely different electronic structures!. Motivation: an embarrassing DISCREPANCY between theory and experiment about CYTOSINE.
a special form of isomerism,
intermolecular proton transfer
Challenge for theory: completely different electronic structures!
an embarrassing DISCREPANCY
between theory and experiment
Theoretical results, e.g.:
Watch out, black-box users!:
DFT gives a qualitatively different picture!
1 2b 3a
1.51 0. 1.49
-0.54 0. 1.27
-0.28 0. 1.74
From molecular beam MW and noble-gas matrix IR,
no reliable quantitative results, but the picture is:
Abundancies (for isolated specii): 2b > 1 >> 3a
While theory from above:2b > 1 3a
Serious discrepancy about the ‘rare’IMINO form
between theory and experiment!!
Check the reliability of several methods
on small molecules.
Test calculations on three systems:
RHF, B3LYP, MP2, CCSD(T)
from 6-31G(d,p) to 6-311++G(3df, 3pd)
and cc-PVTZ to aug-cc-PV5Z
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.
C Geometry selected as standard: MP2/aug-PVTZ
Acetaldimine - Vinylamine pair, incl. Trans. State
a Energy relative to vinylamine
aEnergy relative to vinylamine. bAt MP2/aug-PVTZ geometry.
Formamide – Formamidic acid
Even the best results are uncertain to ~ 1 kcal/mol
a water molecule added explicitly
The special case of formaldehyde-hydrate: b3lyp/631G(d,p) works reasonably,
as compared to “best”: 10.1 vs. 9.2 and 19.4 vs. 22.8
1: b3lyp/31Gdp, 2: b3lyp/311G++2d2p, 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
Try to ‘see’ the process of water-mediated tautomerization
Specifically: is there an intermediate state ?
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)
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).
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.
1 fs time steps, just 200 steps per trajectory.
But several hundred trajectories.
The QC method: DFT(B3LYP)/6-31G**
Call PQS to show dynamics
Two different cases:
1391c and 1397c