MICROWAVE SPECTROSCOPY OF 2-PENTANONE - PowerPoint Presentation

Slide1

MICROWAVE SPECTROSCOPY OF 2-PENTANONE

M. ANDRESEN, H. V. L. NGUYEN, W. STAHLInstitut fur Physikalische Chemie, Raum Aachen, GermanyI. KLEINERLaboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), CNRS, Universités Paris Est et Paris Diderot, Créteil, FranceSlide2

Objectives

Study relatively large organic molecules (esters and ketones, amines) to obtain very precise molecular structures and compare with ab initio calculations

odorant

molecules

, phytohormones,

alarm

pheromone

:

what

are

their

molecular

structure ?

A

complex

issue …Slide3

Internal rotation and structure of alkyl acetates

Collaboration

with

Institute of Physical Chemistry, RWTH Aachen (Germany):

W. Stahl, L. Nguyen, H.

Mouhib

, T. Attig, Y. Zhao, D. Jelisavac, L. Sutikdja, R. Kannengisser

The barrier to internal rotation of the acetyl methyl group in acetates is always around 100 cm−1

JMS 2013

JMS 2013

N-hexyl acetate

Fruity

JMS 2014

N-butyl acetate

pear

N-pentyl acetate

rose

Isoamyl acetate Banana oil

JMS 2012

due to an almost C

2v

symmetric charge distribution over the COO group Slide4

Internal

rotation and structure of

ketones

No

systematic

study

of ketones by MW spectroscopy- Too many atoms to determine

their structure by

isotopic substitution- Large internal rotation splittings, serve as test of our models

Diethyl ketone

Nguyen et al,JCP 2014

3

Tulimat et al JMS 2015

Nguen and Stahl,

ChemPhysChem 2011

Methyl vinyl

ketone

Wilcox et al

JMS 2011

Methyl ethyl

ketone

Allyl acetone

barrier heights to internal rotation vary in a wide range.

accurate predictions of the barrier height of an acetyl methyl group are still difficult to make and no conclusive trends could be determined.

V

3

=

434 cm-1 ap

377 cm-1 sp

V

3

=

233 cm

−1

V

3

=

183

cm-1

V

3

=

772

cm-1Slide5

Methyl propyl ketone (

MPK

) or 2-Pentanone

additive in the food industry

pheromone in bee secretions

Slide6

Methyl Butyl ketone and

Methyl propyl ketone (MPK)Slide7

Ab initio calculations

MP2

,

B3LYP and B3PW91 method in combination with 26 different basis sets each

Plan C1-C5-C7 and plan C5-C7-C10:

-60, +60, 180°

 3

2 = 9 conformers but 4 are stableSlide8

Energy of the lowest conformers of 2-pentanoneSlide9

Rotational constants, dipole moments and relative energies of the most stable conformers of 2-pentanone (MPK)

C

1

C

s

MP2-6-311++G(d,p), Slide10

Internal rotation in 2-pentanone: 2 terminal methyl groups

acetyl methyl group

V

3

= 170 cm

−1

– 250 cm

−1,

alkyl methyl group V

3 1000 cm−1 Slide11

RED experimental value

Potential energy curves of the acetyl methyl group of

conformer 1

of methyl propyl ketone (acetyl methyl group)

BLUE: MP2/6-311++G(

d,p

)

GREEN: MP2/cc-

pVDZORANGE: B3LYP/6-311G(3df,3pd)PURPEL: B3PW91/6-311G(3df,3pd)Slide12

Potential energy curves of the acetyl methyl group of

conformer 2

of methyl propyl ketone

BLUE: MP2/6-311++G(d,p)

GREEN: B3LYP/6-311++G(df,pd)

ORANGE: B3PW91/6-31G(3df,3pd)

double minimum for calculations at the

MP2/6-311++G(d,p)

and the B3LYP/6-311++G(df,pd)  Near Cs structureSlide13

Experimental

technique

at

Aachen

Molecular beam Fourier transform microwave (MB-FTMW) spectrometers

Frequency ranges: 3 to 26.5 GHz

a gas mixture of approximately 1%

of 2-pentanone in helium

expanded through a pulsed

nozzle into the cavity at a total

pressure of about

200 kPa Slide14

Analysis of Conformer 2 : C

s

Calc

Obs

A species ____

E species ____

____ assigned to Conformer 1

No c-type transitions found

calculations at the MP2/6-311++G(d,p) allow a good Initial prediction

Arrows

: First A

transitions

assignedSlide15

FT-MB-MW spectrometer

Aachen University:

High resolution mode

Conformer 2: C

s.

AA-EA: splittings due to V3 = 186 cm-1

AA-AE: splittings due to V3=1000 cm-1

EE and EE*: coupling of the two tops

200 kPaRes: 2 kHz isolated lines

9.0 - 25.0 GHz Slide16

Molecular parameters of

conformer 2

of methyl propyl ketone obtained from the

XIAM Fit

AA/AE

in comparison to the BELGI-Cs Fit Slide17

Two-tops fit using XIAM for Conformer 2

use BELGi-Cs-2Tops to improve rms

V

31

= 188.755)(74) cm

-1

V

32

= 1036(16) cm

-1

=218 transitionsSlide18

Analysis of Conformer 1 : C

1

Calc

Obs

305 lines assigned to Conformer 1, 218 lines assigned to Conformer 2

27 unassigned lines (with squares) : conformer 3 or 13C ?

A species

___

E species

___Slide19

XIAM 1top

XIAM 2-tops

N(AA, AE)

Molecular parameters for the conformer 1 (C1 symetry)

with XIAM and BELGI-C1 transformed in the PAM system

BELGI-C

1

1-top

_________

6.49826 (22)

2.153824(71)

1.929372(64)

163.9

fixed

238.360(80)

26.87

71.66

71.115.3105/91

Fit with another 2-top

code (C1 symetry)Slide20

MP2/6-311++G(d,p)

and

B3LYP/6-311++G(df,pd

) level of theory :



Conformer 2 :“near-Cs” structure, alkyl chain is tilted out of the Cs plane by ca. 10°

B3PW91/6-31G(3df,3pd) level and the experimental results -“true-Cs” structure same phenomenon has been observed forconformer 4 of Methyl butyl ketone (Andresen et al unpublished) methyl ethyl ketone (2-butanone) Kroll et al 2014, Nguyen et al 2014methyl neopentyl ketone (Zhao et al JMS 2012)Slide21

methyl neopentyl ketone (Zhao et al JMS 2012)

methyl ethyl

ketone

(2-butanone)

Nguyen et al 2014Slide22

the barrier heights of two different conformers of MPK differ by ca. 50 cm−1 (238.59 cm−1 vs. 188.76 cm−1).

Contrarly to acetate : the influence of the conformational structure on the barrier height is negligible. For ketone: more sensitive to their environnement. The CH3 “smells” the structure more!

ConclusionsSlide23

“out-of-plane” structure, C1

symmetry and acetyl methyl barrier heights of about

240

± 10 cm

−1

conformer 1 of MPK (238.59 cm−1) allyl acetone (233.145 cm−1)methyl isobutyl ketone (250 cm−1)

2 categories of conformers in ketones

B) “in-plane” conformers with Cs symmetry or near-Cs and barrier heights of about 180 cm−1

- conformer 2 of MPK

(188.76 cm−1) - ethyl ketone (183.170 cm−1)- methyl neopentyl ketone (174.1 cm−1)Slide24

Thank you !Slide25
Slide26
Slide27

Potential energy curves of the propyl methyl group of

conformer 1

of methyl propyl ketone (alkyl methyl group)

BLUE: MP2/6-311++G(d,p)

GREEN: MP2/cc-pVDZ

ORANGE: B3LYP/6-311G(3df,3pd

PURPEL: B3PW91/6-311G(3df,3pd)

RED experimental valueSlide28

Rotational constants for the 4 lowest conformers of

2-pentanone

A

B

C

Experimental