23 MarieAline MartinDrumel 45 Sandra Eibenberger 6 David Patterson 6 John Stanton 7 Barney Ellison 3 and Michael McCarthy 4 1 JILA amp Department of Physics University of Colorado Boulder ID: 629428
Download Presentation The PPT/PDF document "Bryan Changala 1 , Joshua Baraban" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Slide1
Bryan Changala1, Joshua Baraban2,3, Marie-Aline Martin-Drumel4,5, Sandra Eibenberger6, David Patterson6, John Stanton7, Barney Ellison3, and Michael McCarthy4
1JILA & Department of Physics, University of Colorado Boulder 2Department of Chemistry, Ben-Gurion University 3Department of Chemistry and Biochemistry, University of Colorado Boulder 4Harvard-Smithsonian Center for Astrophysics, Harvard University 5Institut des Sciences Moléculaires d’Orsay, CNRS, Univ. Paris-Sud, Université Paris-Saclay 6Department of Physics, Harvard University 7Department of Chemistry, University of Florida
Structure and tunneling dynamics of
gauche
-1,3-butadieneSlide2
Diels-Alder cycloaddition
trans
isomer more stable than “
cis”“cis” gauche
Butadiene: a prototypical conjugated π
-systemSlide3
FTMW spectrometers & gauche productionMcCarthy and Patterson labs
10K He buffer gas cell
inlet
Cavity enhanced FTMW w/ supersonic expansion
1 kV electric discharge
5 kHz resolution
Chirped-pulse FTMW in cryogenic buffer gas cell
Heated inlet (230°C)
80 kHz resolution
1
2Slide4
Rotational spectroscopy of gauche-butadiene
Goals: 1. Equilibrium gauche structure: Beq = B0 - Δ
B02. Tunneling splitting and its effects on the ground state rotational structureSlide5
Ab initio ro-vibrational corrections and tunneling splitting Accurate ΔB0 and tunneling predictions require full dimensional, anharmonic rovibrational calculations.Problem 1: C4H6 is relatively large. Numerically exact variational nuclear motion calculations would be prohibitively expensive.
Problem 2: Standard approximate methods like VPT2 are efficient, but do not treat tunneling systems.Our approach: a VMP2-based rovibrational method for non-rigid moleculesSlide6
Rotational VMP2 (see talk TD02)Energy
Target states not isolated
Treat subset of states non-
perturbatively
Account for remaining (weak) interactions
perturbatively
(via contact/Van
Vleck
transformation)
1. Find optimal
Hartree
product vibrational wavefunction in terms of 3N-6
curvilinear
internal vibrational coordinates q
i
2
. Treat correlation between vibrational coordinates and rotation-vibration interactions up to 2
nd
order in perturbation theory
The internal vibrational coordinate system is critical user input!
What is the “correct” choice?
PBC & J.H. Baraban,
J. Chem. Phys.
145
, 174106 (2016)
0
+
0
-Slide7
0
o
34
o-34o
Rotational VMP2 with an isomerization path coordinate systemSlide8
0
o
34o
-34o
“Steepest descent” reaction path
Rotational VMP2 with an isomerization path coordinate systemSlide9
gauche rotational constantsParameterExpt.VMP2VMP2 + Beq correctionsv = 0+
A0 / MHz212232112821219B05671
56145669C0
457745414573
v = 0-A021232
2113521226B05667
56105665
C04581
4544
4576
Average Error/MHz
63 (0.8%)
4
(0.05%)
Non-planar
B
eq
= B
0
-
Δ
B
0
Slide10
gauche structural featuresC=C—C=C conjugation (favors planarity) vs.steric interactions of terminal H’s (non-planarity)
1
Craig,
Groner, and McKean,
J. Phys. Chem. A 110, 7461 (2006)2This work, partial structureSemi-empirical equilibrium structure (r
se)Slide11
gauche-gauche tunneling splitting
v=0
-
v=0
+
is a vibrationally forbidden transition!
How can we measure splitting?
Parameter
Expt.
VMP2
Tunneling splitting / cm
-1
0.55
0.58
J
a
J
c
+J
c
J
a
interaction / MHz
23.4
23.3
For the d
6
isotopologue
:
v=0
+
v=0
-
30
p
s
tunneling time
Via perturbations!Slide12
Conclusionsgauche-butadiene has a significantly non-planar structure and readily tunnels between gauche enantiomersCurvilinear rotational VMP2 successfully treats the large amplitude nuclear motion and rotation-vibration interactionsA complete structure fit requires additional isotope substitution measurements … in progress!AcknowledgementsExpt. assistance: E.S. Palmer, P. Antonucci, G. T. BuckinghamFunding: NSF, Welch Foundation, Austrian Science Fund
Thanks for you attention!Slide13
Summary of measured constantsSlide14
1D torsional levelsSlide15
Experimental: Fourier transform microwave (FTMW) spectrometers; production of gauche-butadieneSpectroscopy of tunneling gauche-butadieneStructure and dynamics: theory & expt.Semi-empirical gauche structureDetermination of tunneling frequencyOutline1
23