1. Pyridine – Acetylene: Microwave Spectrum, Structure, and Internal Dynamics
Becca Mackenzie
Chris Dewberry, Ken Leopold
Department of Chemistry, University of Minnesota
Emma Jaret, A.C. Legon
Department of Chemistry, University of Bristol
69th International Symposium on Molecular Spectroscopy

2. Previously Studied Pyridine & Acetylene Complexes1 2 4 3
Give full references
Split into two slides, one with pyridine and one with acetylene
MOSCOWITZ!!!!!!
[1] Cooke et al., J. Chem. Soc., Faraday Trans., 1998
[2] Fraser et al., J. Chem. Phys., 1984
[3] Jaman et al., Chemical Physics, 1991
[4] Doran et al., (2012). JMS. 1019, 191.

3. Results from Bristol Spectra of HCCH-pyr and DCCD-pyr
Small inertial defect
14N nuclear hyperfine structure indicates that the C2 axis of pyridine does not coincide with the acetylene axis
Not the straight on geometry of HCCH – NH3 or H5C5N – HCl
One more thing…

4. Results from Bristol Two states observed
HCCH-Pyridine spectrum from Tony Legon
For example, in acetylene dimer
DCCD also studied
Frequency, MHz

5. Pyridine - Acetylene MP2/aug-cc-pvdz Rotational Constants (MHz)
2.149 Å
3.474 Å
MP2/aug-cc-pvdz
Rotational Constants (MHz)
829.80
726.32
Binding energy (counterpoise corrected):
2.4 kcal/mol
163°
μa = D
μb = D
These rotational constants were in good agreement with what Tony had fit

6. Experimental Introduction of Sample to Cavity
Ar over
H5C5N bubbler
1 % HCCH in Ar
Over H5C5N bubbler
Introduction of Sample to Cavity
HCCH through pulsed-nozzle
Pyridine through pulsed-nozzle and continuous flowline
Pyridine on both lines increased the signal intensity x80, critical for seeing 13C-pyridine in natural abundance
For comparing relative intensities we used our new chirp at UMN
Cavity

7. Dual Chirp/Cavity FTMW

8. Chirp and Cavity Spectra
Chirp spectrum
Cavity spectrum
Chirp spectrum of pyridine-acetylene
7000
8000
(MHz)

9. Switching Between Cavity and Chirp
Switch between cavity and chirp in under 5 min
Dr. Chris Dewberry
We’ll be talking more about our chirp spectrometer more next year, but in case this might help people right away, here’s a neat thing we added to allow us to switch between cavity and chirp experiments under 5 minutes… obviously without breaking vacuum
Add Chris’ picture
Absorber arm down
Absorber arm up

10. Isotopic Substitutions
DCCH –
same splitting as HCCH
HCCD-
same splitting as DCCD
Does not involve an interchange of the inner and outer hydrogens, that was really the only other option that we were entertaining, as other acetylene complexes, particularly acetylene dimer, have displayed such internal motion
Split into two slides, first slide shows that we’re not interchanging the acetylene hydrogens
Second slide should have bent configuration with C13 substitutions mention the ab initio calculations and that they support the minimum energy structure is bent
Other motion that maintains planarity
Based on the ab initio calculations, we were thinking it was a wagging motion but we wanted to check to make sure it wasn’t a flipping motion that interchaned the inner and outer acetylene hydrogens
Not an exchange of the two acetylene hydrogens

11. Isotopic Substitutions
13C substitutions breaks symmetry in pyridine,
Observed in natural abundance
Quenches tunneling!
Found four sets of rotational spectra corresponding to four distinct
non-interconverting structures O2 M2
Frequency, MHz
10 MHz
22 MHz
6 MHz M1 M2
Parent - B State
Parent - A State O1 O2 O1 M1
Add animation for ortho and meta highlights
Does not involve an interchange of the inner and outer hydrogens, that was really the only other option that we were entertaining, as other acetylene complexes, particularly acetylene dimer, have displayed such internal motion
Split into two slides, first slide shows that we’re not interchanging the acetylene hydrogens
Second slide should have bent configuration with C13 substitutions mention the ab initio calculations and that they support the minimum energy structure is bent
Other motion that maintains planarity

12. Pyridine – Acetylene PES
David Tew, University of Bristol
fc-CCSD(T)(F12*)/cc-pVDZ=F12 μb μb
Include David Tew
44 cm-1 μa μa

13. Spin Statistics for Internal MotionHb Hc Hd He Hf Hg Hf Hg Ha Ha Hd He Hb Hc
Two pairs of protons are exchanged
When K is odd 0+:0- :: 5:3
When K is even 0+:0- :: 3:5

14. Spin Statistics and Relative Intensities
0+ state was identified as lower frequency set of transitions 0+
5:3 ratio 0-
0+ state was above the 0- state in energy… spin statistic identifies the symmetry of the state which tells you if it’s 0+ or 0- BUT doesn’t necessarily mean that the 0+ is lower in energy than 0-… would happen from major perturbations
3:5 ratio 0- 0+

15. Rotational Constants- 13C Substitutions
ORTHO
META
Constants
1 (INNER)
2 (OUTER)
A (MHz)
5830.4(11)
5731.1(17)
5691.9(21)
5804.6(19)
B (MHz)
(64)
(10)
(13)
(10)
C (MHz)
(60)
(98)
(12)
(10)
ID (amu Å2)
-0.043
0.365
-0.225
-0.242
14N χaa (MHz)
-3.905(21)
-3.915(33)
-3.917(41)
-3.889(35)
14N χbb-χcc (MHz)
-2.692(68)
-2.72(11)
-2.76(14)
-2.64(11)
ΔJ (kHz)
1.3220(90)
-0.516(14)
-0.548(18)
1.040(15)
ΔJK (kHz)
-49.22(43)
-39.60(69)
-53.91(87)
-50.89(73)
δJ (kHz)
(66)
0.856(11)
0.655(13)
-0.242(11)
RMS (kHz) 2 3 4 N 24 22
K Levels Included
0,1
C13 fits were relatively well-behaved
The rotational constants and the angle from 14N hyperfine structure is consistent with the angle from ab initio structure
Consider adding these numbers in if you have them

16. Parent Single State Fits
K=0,1
K=0,1,2,3
Constant
0+ State
0- State
A (MHz)
5866.4(14)
(64)
5826.3(58)
5884.(10)
B (MHz)
(14)
(61)
(23)
(35)
C (MHz)
(12)
(54)
(22)
(32)
ID (amu Å2)
2.100
-1.600
1.543
-1.311
14N χaa (MHz)
(73)
(32)
(61)
-3.89(18)
14N χbb-χcc (MHz)
-2.670(19)
(84)
-2.655(18)
-2.56(52)
ΔJ (kHz)
-0.924(14)
1.8673(59)
-0.94(10)
2.06(16)
ΔJK (kHz)
-49.12(90)
-46.49(37)
19.34(90)
10.2(31)
δJ (kHz)
1.099(10)
(42)
1.38(17)
-0.67(26)
RMS (kHz) 10 4
184
292 N 48 54 87 80
MAKE MORE READER FRIENDLY
Such a scenario may be consistent with large amplitude motion. Considering that we have two states and poor single state fits, we attempted to do a combined fir with coriolis
“If there’s an in plane motion, there’s a coriolis interaction with these two steates, one state goes down, one state goes up… that may impact the signs of the ID by shifting the rotational constants – this is relevant for when you haven’t taken the perturbation out”

17. Pyridine – Acetylene Tunneling Frequency
The doubled spectra provides definitive evidence that the complex is tunneling between equivalent configurations but it doesn’t provide direct measurement of the tunneling frequency.
Can we do a combined fit of the states to indirectly estimate the tunneling frequency?
The real bench mark for computation is the tunneling frequency, which we may be able to get from the doubled spectra, and would provide a touchstone for calculating experimentally accurate tunneling splittings and subsequently the potential energy surfaces

18. Single State Fits Combined State Fits Pros ID is small
Fits K=0,1,2,3 for both states with RMS of 5 kHz
Treats Coriolis interaction, fits ΔE01
Distortion constant signs are correct
Cons
ID is large and positive
kHz standard errors in rotational constants
ΔE01 < 0 ??
Pros
ID is small
Fits K=0,1 with RMS < 10 kHz
Cons
Can’t fit K=2,3 RMS ~100s kHz
Doesn’t treat obvious perturbations, can’t fit ΔE01
Some distortion constants are negative
Not totally clear what to make of it

19. Schematic of Structure Determination
Using experimental moments of inertia for monomers
Rcm
Fit from C rotational constants
Rcm θ1
From 14N nuclear hyperfine Θ1 Θ2 θ2
Using Rcm and θ1 adjusted θ2 to reproduce A or B
Leopold, K. R. (2012). JMS, 278, 27–30.

20. Preliminary Structure Analysis for Pyridine - Acetylene
0.15 Å longer than ab initio prediction
NH Distance ~2.25(15) Å
8.2-27°
23°
Why is there a second minimum out there?
Ortho interactions
Check on the distance change from the center of the CC bond to the othro hydrogen between the minimum energy state and the transition state
Structural analysis is in progress, variation between isotopologues is larger than typically expected, which may be indicative of the large amplitude of motion. A more precise way to define the complex is to use the center of mass of pyridine to the center of the CC bond, which is isotopically invariant to within 4 significant figures and is in fact, a better way of characterizing the structure of the complex Å

21. Conclusions Unexpected bent equilibrium structure
Doubling of the rotational spectrum was assigned to a rocking motion that does not exchange acetylene hydrogens
Difficulty fitting higher K levels and isotopic variability in the intermolecular distance are both consistent with a system undergoing large amplitude motion

22. Future Work Look for potential b-type lines in the chirp spectrum
Direct measurement of tunneling frequency
Assess validity of combined fit
Wrap up the structure analysis
Finish structure incorporating 13C pyridine isotopologues
Try with the rotational constants from the combined fit???
Pyridine – HCN: will the dipole-dipole interaction trump the ortho hydrogen – π interaction?

23. Funding & Acknowledgements
Leopold Group
Dr. Chris Dewberry and Dr. Brooke Timp
University of Bristol
A.C. Legon
David Tew
Lester C. and Joan M. Krogh Fellowship

24. Rotational Constants- Single State
PYR-HCCH
0+ State
0- State
PYR-DCCH
PYR-HCCD
A (MHz)
5866.4(14)
(64)
5842.9(14)
5841.5(14)
5853.6(22)
5853.7(11)
B (MHz)
(14)
(62)
(11)
(10)
(15)
(78)
C (MHz)
(12)
(54)
(10)
(88)
(13)
(68)
ID (amu Å2)
2.101
-1.600
2.095
-1.763
1.861
-1.109
14N Xaa (MHz)
(73)
-3.92(32)
-3.905(54)
-3.908(39)
-3.877(61)
-3.903(27)
14N Xbb-Xcc (MHz)
-2.67(19)
-2.679(84)
-2.73(16)
-2.69(12)
-2.71(18)
-2.73(85)
ΔJ (kHz)
-0.924(14)
1.8673(59)
-0.973(11)
1.798(12)
-0.550(19)
1.409(11)
ΔJK (kHz)
-49.12(90)
-46.49(38)
-54.48(78)
-51.13(62)
-20.53(91)
-18.82(45)
δJ (kHz)
1.099(10)
(43)
1.1190(80)
(94)
0.789(14)
(85)
RMS (kHz) 10 4 5 6 3 N 48 54 30 26 28 25
K Levels
0,1

25. Parent Combined Fit Constant 0+ State 0- State A (MHz) 5850.57(35)
(34)
B (MHz)
863.06(11)
862.93(11)
C (MHz)
(19)
(20)
ID (amu Å2)
25.8
ΔJ (kHz)
0.4444(80)
0.4948(75)
ΔJK (kHz)
18.96(36)
17.52(35)
δJ (kHz)
9.04(60)
8.63(42)
14N Xaa (MHz)
(23)
14N Xbb-Xcc (MHz)
(68)
Fab (MHz)
426.84(66)
ΔE01 (MHz)
(66)
RMS (kHz) 5 N
167

26. Preliminary Structure Analysis for Pyridine - Acetylene
Isotope
Rcm, Å
Θ2°
N-H, Å
<(NHC)°
PYR-HCCH
-8.2
2.2070
0.06
PYR-DCCD
-27.25
2.3600
32.28
PYR-DCCH
-9.910
2.2103
2.67
PYR-HCCD
-13.94
2.2201
10.36
Avg.
2.2494
Max-min
0.120
0.15