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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.

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Presentation Transcript
slide2

TAUTOMERISM:

a special form of isomerism,

intermolecular proton transfer

Prototype case:

Challenge for theory: completely different electronic structures!

slide3

Motivation:

an embarrassing DISCREPANCY

between theory and experiment

about CYTOSINE

slide5

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/6-311++G(2d,2p): B3PW91/6-311++G(2d,2p):

-0.54 0. 1.27

-0.28 0. 1.74

slide6

Compare with experiment:

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!!

slide7

Section 1. Test calculations

Check the reliability of several methods

on small molecules.

Test calculations on three systems:

slide8

Vinylalcohol

Acetaldehyde

slide9

Acetaldimine

Vinylamine

slide10

Formamide

Formamidic acid

slide11

Methods:

Electronic theory:

RHF, B3LYP, MP2, CCSD(T)

Basis sets:

from 6-31G(d,p) to 6-311++G(3df, 3pd)

and cc-PVTZ to aug-cc-PV5Z

slide12

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.

slide13

Acetaldehyde – Vinylalcohol Tautomer Pair Contnd.

C Geometry selected as standard: MP2/aug-PVTZ

slide14

Test 2.

Acetaldimine - Vinylamine pair, incl. Trans. State

_________________________________________

a Energy relative to vinylamine

slide15

Acetaldimine - Vinylamine contnd.

______________________________________________________

aEnergy relative to vinylamine. bAt MP2/aug-PVTZ geometry.

slide16

Test 3.

Formamide – Formamidic acid

slide18

CONCLUSIONS ON THE TESTS:

_________________________________________________________________

Even the best results are uncertain to ~ 1 kcal/mol

slide19

Section 2. The effect of water

Model: supermolecule,

a water molecule added explicitly

slide22

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

slide23

Section 3: Dynamics:

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)

slide24

About ab initio dynamics

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).

slide25

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.

slide26

Dynamics details:

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