1. Characterization of Intermolecular Interactions in the
2,2,2-trifluoroethanol Trimer Using Cavity & Chirped-Pulse
Microwave Spectroscopy
Nathan Seifert, Javix Thomas, Wolfgang Jäger, Yunjie Xu
University of Alberta

2. The missing trans conformer
≤ 10 cm-1
-----
ZPE leads to a ground-state trans- structure
above the gt barrier
No experimental gas-phase observation
of trans- conformer
Neutron diffraction studies of liquid TFE
shows a 60:40 g:t ratio1
B2PLYP-D3/aug-cc-pVTZ
trans
No-go in dimer either --- trans still not fully stabilized2: φ
ΔE (kJ mol-1)
Add gauche structures to PES; be more clear about the axial chirality of the gauche conformers
gauche
gauche
φ(C-C-O-H)
1. I. Bako, T. Radnai, M. C. B. Funel, J. Chem. Phys., 121, (2004).
2. Figure adapted from: T. Scharge, C. Cezard, P. Zielke, A. Schutz, C. Emmeluth, M. A. Suhm, Phys. Chem. Chem. Phys. 9, 4472 (2007).

3. Chirality Synchronization
Trifluoroethanol dimer exhibits two primary isomers:
a-het
i-hom g+ g- g+ g+
However…
ΔE0BSSE < 0.1 kJ MP2/ g(2d,p)
Jet cooled FTIR : only i-hom is detected1
Raman : weak evidence of hethom conversion during jet expansion2
FTMW : i-hom is preferred experimentally 10:1 over i-het3
Chirality synchronization plays a significant role: tunneling process
effectively cools nearly all transiently heterochiral dimers into homochiral species
Does this synchronization continue to play a role in larger TFE clusters?
T. Scharge, T. Haeber, M. A. Suhm, Phys. Chem. Chem. Phys. 8, 4664 (2006).
T. Scharge, C. Cezard, P. Zielke, A. Schutz, C. Emmeluth, M. A. Suhm, Phys. Chem. Chem. Phys. 9, 4472 (2007).
J. Thomas, Y. Xu, J. Phys. Chem. Lett. 5, 1850 (2014).

4. Trimer Search g+g+t (0.0) g+g-g- (2.2) g+g+g+ (2.2) g+g-g- II (4.7)
< 5 kJ mol-1
Trimers strongly prefer compactification due to
large variety of non-covalent linkages
Trimers can only stabilize one trans- subunit;
additional t- subunits relax into g+/g- upon optimization
g+g+g+ III (9.0)
g+g+g- II (6.1)
g+g+t II (6.4)
g+g-t (7.4)
g+g+g+ II (5.7)
< 10 kJ mol-1
Show 10kj classes and then fade them when done
g+g+g- III (11.2)
g-g-g- (21.0)
g+g-t III (15.0)
g+g+g- IV (11.8)
> 10 kJ mol-1
MP2/ g(2d,p), counterpoise corrected

5. Experimental Details Trimer 1 measured initially using cavity FTMW
CP-FTMW data taken using
new 2-6 GHz CP-FTMW spectrometer2
(TH01)
0.15% TFE in 3 atm backing pressure
Two bands used:
2-3 GHz, 700k averages
3-4.5 GHz, 1.46 million averages
Change a-c-het I to a-het and i-c-hom to i-hom to be consistnet
(takes advantage of 𝐼∝ ∆𝑣 −0.5
scaling in weak pulse limit)
Trimers 2/3 identified using AUTOFIT3
C. Perez, S. Lobsiger, et al. Chem. Phys. Lett. 571, 1 (2013).
N. A. Seifert, I. A. Finneran, et al. J. Mol. Spectrosc. 312, 13 (2015).

6. Assignments g+g+t g+g-g- g+g+g+ 392 417 463 292 250 230 210 191 181
Trimer 1
B3LYP-D3
Trimer 2
Trimer 3
A / MHz
(89)
392
(60)
417
(27)
463 B
(56)
292
(34)
250
(29)
230 C
(53)
210
(35)
191
(35)
181
DJ / kHz
(16)
0.0566(21)
0.0388(15)
DJK
(91)
(92)
[0] DK
0.0829(12) d1
(47)
(89)
(71) d2
(86)
N / σ (kHz)
174 / 3.1
47 / 5.5
53 / 6.5
g+g+t
g+g-g-
g+g+g+
ΔE0BSSE = 0.0 kJ mol-1
ΔE0BSSE = 2.2 kJ mol-1
ΔE0BSSE = 2.2 kJ mol-1

7. Chirality synchronization in (TFE)3?
A small thought experiment:
All three trimers follow pattern of
homochiral dimer + monomer subunit
Trimer 1: g+g+t
i-hom (g+g+)
Replace spectral figure with structure figure + animations to show dimer + monomer idea
t-TFE

8. Chirality synchronization in (TFE)3?
i-hom (g-g-)
A small thought experiment:
“Complex” Algebra:
g+g+t ≈ (i-hom+ + t)
All three trimers follow pattern of
homochiral dimer + monomer subunit
Trimer 2: g+g-g-
Replace spectral figure with structure figure + animations to show dimer + monomer idea
g+-TFE

9. Chirality synchronization in (TFE)3?
A small thought experiment:
“Complex” Algebra:
g+g+t ≈ (i-hom+ + t)
g+g-g- ≈ (i-hom- + g+)
All three trimers follow pattern of
homochiral dimer + monomer subunit
i-hom (g+g+)
Trimer 3: g+g+g+
g+-TFE
Replace spectral figure with structure figure + animations to show dimer + monomer idea

10. : : Chirality synchronization in (TFE)3? 15 1 1 16 1 1
A small thought experiment:
“Complex” Algebra:
g+g+t ≈ (i-hom+ + t)
g+g-g- ≈ (i-hom- + g+)
g+g+g+ ≈ (i-hom+ + g+)
All three trimers follow pattern of
homochiral dimer + monomer subunit
Since trans- not stabilized in vacuo, g+g+t ≈ (i-hom+ + t)
probably not plausable
g+g-g- / g+g+g+ have similar energies
but mismatched axial chirality in addition scheme
 Chirality synchronization in the jet?
Replace spectral figure with structure figure + animations to show dimer + monomer idea : : 15 1 1
Est. 100 K: 16 1 1
Experimental Abundances :

11. The g+g+ Subunit Basin of Stability
Relaxed potential energy scan
g+g+g+ III
ΔE (kJ mol-1)
g+g+g-
Shift PES so trans minimum is shown clearly φ
g+g+t
trans subunit
efficient g-  t cooling
pathway?
B2PLYP-D3/aug-cc-pVTZ
φ(C-C-O-H), trans subunit

12. Non-covalent contact type Contact interaction energy
Non-covalent interactions: g+g+t
Principle of Least Action:
Quantum Theory of Atoms and Molecules (QTAIM)
Relaxation of g+ subunit
of i-hom to t upon
addition of new g+
monomer?
g+g+t
i-hom+
overlapping g+ subunit
Focus on H-F contacts and compare and contrast with others. Get rid of top addition schema and animate the last one. Show explicitly the addition of dimer + overlay + addition of third subunit. The key is to focus on that the trans gets stabilized by these F-H contacts first off, then talk about the formation mechanism (“principle of least action”)
Bifurcated interactions
“lock” trans conformer into place
Non-covalent contact type
# of contacts
Contact interaction energy
(kJ mol-1)
O-H ∙∙∙ O-H 3
89.0
C-H ∙∙∙ F-C 4
18.6
C-F ∙∙∙ F-C
21.4
Strong
interaction

13. Non-covalent interactions: g+g-g-
g+g+t
g+g-g-
Focus on H-F contacts and compare and contrast with others. Get rid of top addition schema and animate the last one. Show explicitly the addition of dimer + overlay + addition of third subunit. The key is to focus on that the trans gets stabilized by these F-H contacts first off, then talk about the formation mechanism (“principle of least action”)
Non-covalent contact type
# of contacts
Contact interaction energy
(kJ mol-1)
O-H ∙∙∙ O-H 3
89.0
C-H ∙∙∙ F-C 4
18.6
C-F ∙∙∙ F-C
21.4
Non-covalent contact type
# of contacts
Contact interaction energy
(kJ mol-1)
O-H ∙∙∙ O-H 3
88.4
H-C∙∙∙F-C 1
3.2
C-H ∙∙∙ F-C 2
11.0
C-F ∙∙∙ F-C
19.2

14. Non-covalent contact type Contact interaction energy
Non-covalent interactions: g+g-g-
g+g-g-
Formation pathway requires significant structural
relaxation beyond basic (i-hom- + g+)
van der Waals-like association
g+g-g-
i-hom-
overlapping g+ subunit
Focus on H-F contacts and compare and contrast with others. Get rid of top addition schema and animate the last one. Show explicitly the addition of dimer + overlay + addition of third subunit. The key is to focus on that the trans gets stabilized by these F-H contacts first off, then talk about the formation mechanism (“principle of least action”)
Non-covalent contact type
# of contacts
Contact interaction energy
(kJ mol-1)
O-H ∙∙∙ O-H 3
88.4
H-C∙∙∙F-C 1
3.2
C-H ∙∙∙ F-C 2
11.0
C-F ∙∙∙ F-C
19.2

15. Non-covalent interactions: g+g+g+
Strong
C-F∙∙∙H-O
contact
g+g+g+
g+g-g-
Less structural relaxation
required for (i-hom+ + g+) addition
than in (i-hom- + g+)  g+g-g- case
g+g+g+
i-hom+
overlapping g+ subunit
Focus on H-F contacts and compare and contrast with others. Get rid of top addition schema and animate the last one. Show explicitly the addition of dimer + overlay + addition of third subunit. The key is to focus on that the trans gets stabilized by these F-H contacts first off, then talk about the formation mechanism (“principle of least action”)
Non-covalent contact type
# of contacts
Contact interaction energy
(kJ mol-1)
O-H ∙∙∙ O-H 3
88.4
H-C∙∙∙F-C 1
3.2
C-H ∙∙∙ F-C 2
11.0
C-F ∙∙∙ F-C
19.2
Non-covalent contact type
# of contacts
Contact interaction energy
(kJ mol-1)
O-H ∙∙∙ O-H 2
79.2
C-F ∙∙∙ O-H 1
7.0
C-H ∙∙∙ F-C
9.8
C-F ∙∙∙ F-C 3
15.5

16. Conclusions
(TFE)3 cluster conformations driven strongly by non-covalent topology & energetics
“Chirality synchronization” plays less of a role in determining trimer populations in the jet
The mysterious trans conformer shows its face!
Stability driven by local hydrogen bonding environment
Experimental molecular beam abundance t:g ≈ 3:7, close to 4:6 seen in liquid!
Tetramer: Still prefers only one trans conformer?
Initial conformational search suggests g+g+g+t is global minimum and energetically preferred
g+ insertion
overlapping t subunit
g+g+t trimer
g+g+g+t

17. Thanks for your attention!!
Research funded by:
Thanks for your attention!!

18. g+g+t g+g+g+ NCI NCI Reduced density gradient Reduced density gradient
Weakly
interacting
Weakly
interacting
Strong
interaction
Repulsive
𝑠𝑖𝑔𝑛( 𝜆 2 )𝜌
𝑠𝑖𝑔𝑛( 𝜆 2 )𝜌