ROTATIONAL SPECTROSCOPY AND QUANTUM CHEMICAL CALCULATIONS OF A FRUIT ESTER: THE MICROWAVE SPECTRUM OF n-BUTYL ACETATE T. ATTIG, L.W. SUTIKDJA, R. KANNENGIEßER,

Präsentation zum Thema: "ROTATIONAL SPECTROSCOPY AND QUANTUM CHEMICAL CALCULATIONS OF A FRUIT ESTER: THE MICROWAVE SPECTRUM OF n-BUTYL ACETATE T. ATTIG, L.W. SUTIKDJA, R. KANNENGIEßER,"—  Präsentation transkript:

1 ROTATIONAL SPECTROSCOPY AND QUANTUM CHEMICAL CALCULATIONS OF A FRUIT ESTER: THE MICROWAVE SPECTRUM OF n-BUTYL ACETATE T. ATTIG, L.W. SUTIKDJA, R. KANNENGIEßER, W. STAHL Institut fur Physikalische Chemie, Raum Aachen, Germany I. KLEINER Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), CNRS, Universités Paris Est et Paris Diderot, Créteil, France

2 Objectives Study relatively large organic molecules (esters and ketones, thiophenone) to obtain very precise molecular structures and compare with ab initio calculations Fruit esters and odorant molecules Relationship odor-molecular structure ? A complex issue …

3 Internal rotation Last year in Columbus Nguen and Stahl
Jelisavac et al JMS.(2009). C1 : isoamyl acetate Nguyen et al Mol. Phys.2010 Nguen, Stahl JMS 2010 Last year in Columbus Cs : methyl propionate Cs: methyl acetate Tudorie et al JMS 2011 Nguen and Stahl ChemPhysChem 2011

4 This year: Lam Nguyen talk
Linalool Methyl acetate

5 Increasing the length of the chain …
100 cm-1 1000 cm-1 JMS 2011 JMS 2009 Methyl acetate Ethyl acetate Increasing the length of the chain …

6 Under course Present talk Mol Phys 2012 Submitted JMS N-pentyl-acetate Under course

7 Experimental technique at Aachen
Molecular beam Fourier transform microwave (MB-FTMW) spectrometers Frequency ranges: 4 to 26.5 GHz (big cavity) 26.5 to 40 GHz (small cavity)

8 Strategy: ab initio / two internal rotation codes / experimental data
XIAM BELGI 1) ab initio calculation (MP2) or quantum chemical calculation (DFT)  A, B, C for all conformers 2) use the scan spectrum to assign transitions for the A species 3) guess values for V3, Ig and use XIAM to predict the E species 4) use the high resolution spectrum to assign E species 5) fit the A-E data with XIAM and BELGI 6) compare with ab initio values the structural parameters (A,B,C, angles of the methyl group)

9 the conformers & nomenclature
the geometries of n-butyl acetate Cis Struktur von Isoamyl Acetate, Vorkommen in Banana, Wie wichtig eine systematische Nomenklatur zu definieren ist, zahlreiche Konformere aufgrund mehrere Drehmöglichkeit um die C-C-Bindung. Einführung in Nomenklatur für sp3-Hybridisierungssysteme, welche 3 günstige Energie-minima besitzt. Ausgehend von CIP-Regel für Helix-Systeme und Alkylhalogenide, wird eine Nomenklatur für allgemeine Kohlenwasserstoffe Verbindungen definiert. Newmann-Projektion. Benennung des am Beispiel angezeigten Bildes. Wieviele mögliche Konformere könnten existieren für isoamyl acetate. 3 x 3 x 3 = 27 cis-conformers 1 Cs – Symmetry + 26 C1-Symmetry trans conformers: high torsional barrier of about 40 kJ/mol around θii lilian w. sutikdja, 9 9

10 Relative energies of the observed conformers
Aaa Cs PPa C1 aPa C1

11 quantum chemical calculation
on MP2/ G(d,p) level of theory Experimental Value VS. Quantum Chemical Calculation Rotational Constants A, B, C Aus 27 Konformere, gab es nur 25 erfolgreiche Rechnungen. Ein Enantiomerpaar nicht optimiert werden konnte aufgrund der sterischen Hinderung. Die theoretischen Daten wurden mit den experimentellen Daten verglichen. Verglichen wurden die Rotationskonstante des Moleküls. Übereinstimmung findet man für Enantiomerpaar aM(M,a)/aP(a,P). Durch Vergleichen der Winkel zwischen der Achse des internen Rotors und den Hauptträgheitachsen a, b, und c wird ebenfalls Übereinstimmung gefunden. lilian w. sutikdja, 23rd February 2012 11 11

12 conformer aPa : Jmax = 16, |Kmax | = 6 aaa: Jmax = 13, |Kmax| = 4 ;
Molecular parameters for the aPa, aaa and PPa conformers using BELGI and XIAM N °Param. N° raies A/E s conformer aPa : Jmax = 16, |Kmax | = 6 aaa: Jmax = 13, |Kmax| = 4 ; PPa : Jmax = 9, Kmax = 6

13 Broadband spectrum recorded in Aachen for n-butyl acetate
Obs Calc (3 conf.) calc

14 Conclusions Combining ab initio, microwave spectroscopy in a MB and effective hamiltonian methods to study rather large esters led to rather consistent results, apart sometimes for the moment of inertia of the top (non-rigidity effects from the rest of the molecule). - How can we study the excited torsional states now ? - How the molecular structures for the lowest conformers change from n-butyl acetate to n-pentyl acetate?

15 n-butyl acetate n-pentyl acetate hydrogen atoms are
marked in white, the carbonate atoms in grey, and the oxygen atoms in dark grey aPaa: lower energy

16

17 From microwave spectroscopy to perfum analysis ?
H. Mouhib, W. Stahl, M. Lüthy, M. Büchel, P. Kraft, Angew. Chem. Int. Ed. 2011, 50, 5576 (2S,5S)-Cassyrane (2S,5R)-Cassyrane important blackcurrant odorants for perfumery, the two cassyrane stereoisomers were studied by high resolution microwave spectroscopy The interesting point about this study is that the cassis odor strength of these molecules is critically dependent upon the stereochemistry. To determine the relative stereochemistry and gas phase structure of the Cassyrane, microwave spectroscopy in combination with quantum chemistry was perfect in this case since it gave directly access to the gas phase structure, which was then used in superposition analyses. For x-ray crystallography, crystalline derivatives would have to be synthesized that would considerably affect the structure. The two stereoisomers (2S,5S) and (2S,5R)-Cassyrane (and 2 of its dihydro- derivatives), were synthesized and studied by rotational spectroscopy. The gas phase structures deduced were then used directly in molecular modeling calculations. The most pronounced fruity character of the (2S,5S)-cassyrane stereoisomer versus the (2S,5R) stereoisomer could be explained by superposition analysis on the Oxane molecule25. (H. Mouhib, W. Stahl, M. Lüthy, M. Büchel, P. Kraft, Angew. Chem. Int. Ed. 2011, 50, (2S,5S)- Most fruity (2S,5R)-

18 Cassyrane – Superposition Analysis –
H. Mouhib, W. Stahl, M. Lüthy, M. Büchel, P. Kraft, Angew. Chem. Int. Ed. 2011, 50, 5576 Cassyrane – Superposition Analysis – For structural correlations: gas-phase structure of the most fruity (2S,5S)-Cassyrane superposed with the (+)‑(2S,4R)-Oxane (“Cassis base 345B”) For the superposition: Cassyrane fixed (black) (+)‑(2S,4R)-Oxane superimposed on the structure by rotation (in silver) No deformation of bond lengths and angles Methyl groups which have a direct effect on the olfactory properties of Cassyrane Oxane only overlay well if C-5 of Cassyrane is (S)-configured and the C-4 of Oxane is (R)-configured. 18

19 Theoretical Model: the global approach for one top
RAM = Rho Axis Method (axis system) for a Cs (plane) frame : get rid of Jxpa HRAM = Hrot + Htor + Hint + Hc.d. Torsional operators and potential function V(a) Constants 1 1-cos3a p2a Japa 1-cos6a p4a Jap3a V3/2 F r V6/2 k4 k3 J2 (B+C)/2* Fv Gv Lv Nv Mv k3J Ja2 A-(B+C)/2* k5 k2 k1 K2 K1 k3K Jb2 - Jc2 (B-C)/2* c2 c1 c4 c11 c3 c12 JaJb+JbJa Dab or Eab dab Dab dab6 DDab ddab Rotational Operators Rotational constants give the structure. Using the diagonalization of the inertial tensor, one can get the orientation angles of the CH3 group relative to the PAS = angle of torsion, r = couples internal rotation and global rotation, ratio of the moment of inertia of the top and the moment of inertia of the whole molecule Kirtman et al 1962 Lees and Baker, 1968 Herbst et al 1986 Hougen, Kleiner, Godefroid JMS 1994

20 the conformers & nomenclature
the geometries of isoamyl acetate Struktur von Isoamyl Acetate, Vorkommen in Banana, Wie wichtig eine systematische Nomenklatur zu definieren ist, zahlreiche Konformere aufgrund mehrere Drehmöglichkeit um die C-C-Bindung. Einführung in Nomenklatur für sp3-Hybridisierungssysteme, welche 3 günstige Energie-minima besitzt. Ausgehend von CIP-Regel für Helix-Systeme und Alkylhalogenide, wird eine Nomenklatur für allgemeine Kohlenwasserstoffe Verbindungen definiert. Newmann-Projektion. Benennung des am Beispiel angezeigten Bildes. Wieviele mögliche Konformere könnten existieren für isoamyl acetate. qiii≈ 180° qiv≈ 180° qv≈ 60°, -60° 11a1-14a3-(16P8, 20M8) 11a1-14a3-(16P8, 20M8) aa(P, M) 3 x 3 x 3 = 27 cis-conformers lilian w. sutikdja, 23rd February 2012 20 20

21 Experimental technique
Two different modes of the spectrometers High resolution mode Scan mode Methyl propionate

22 Molecular parameters for the aPa conformer using BELGi-C1 and XIAM

23 Comparison of methyl rotor angles