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

Previously Studied Pyridine & Acetylene Complexes 1 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.

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…

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

Pyridine - Acetylene MP2/aug-cc-pvdz Rotational Constants (MHz) 2.149 Å 3.474 Å MP2/aug-cc-pvdz Rotational Constants (MHz) 5824.79 829.80 726.32 Binding energy (counterpoise corrected): 2.4 kcal/mol 163° μa = 3.1021D μb = 0.8092 D These rotational constants were in good agreement with what Tony had fit

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

Dual Chirp/Cavity FTMW

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

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

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

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 413 - 312 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

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

Spin Statistics for Internal Motion Hb 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

Spin Statistics and Relative Intensities 0+ state was identified as lower frequency set of transitions 0+ 5:3 ratio 0- 413 - 312 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+ 404 - 303

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) 826.60449(64) 825.2931(10) 823.6950(13) 819.2894(10) C (MHz) 724.00895(60) 721.03296(98) 719.7948(12) 718.2010(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) -0.6057(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

Parent Single State Fits K=0,1 K=0,1,2,3 Constant 0+ State 0- State A (MHz) 5866.4(14) 5866.57(64) 5826.3(58) 5884.(10) B (MHz) 826.6814(14) 826.54924(61) 826.762(23) 826.610(35) C (MHz) 722.4006(12) 726.14271(54) 722.425(22) 726.154(32) ID (amu Å2) 2.100 -1.600 1.543 -1.311 14N χaa (MHz) -3.8807(73) -3.9168(32) -3.8789(61) -3.89(18) 14N χbb-χcc (MHz) -2.670(19) -2.6796(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) -0.9047(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”

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

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 10-100 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

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.

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 5.1078 Å

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

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?

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

Rotational Constants- Single State PYR-HCCH 0+ State 0- State PYR-DCCH PYR-HCCD A (MHz) 5866.4(14) 5866.57(64) 5842.9(14) 5841.5(14) 5853.6(22) 5853.7(11) B (MHz) 826.6814(14) 826.54924(62) 820.1535(11) 820.0154(10) 789.1357(15) 789.03307(78) C (MHz) 722.4006(12) 726.14271(54) 717.0639(10) 720.88199(88) 693.6132(13) 696.37269(68) ID (amu Å2) 2.101 -1.600 2.095 -1.763 1.861 -1.109 14N Xaa (MHz) -3.8807(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) -0.9047(43) 1.1190(80) -0.8809(94) 0.789(14) -0.6134(85) RMS (kHz) 10 4 5 6 3 N 48 54 30 26 28 25 K Levels 0,1

Parent Combined Fit Constant 0+ State 0- State A (MHz) 5850.57(35) 5851.67(34) B (MHz) 863.06(11) 862.93(11) C (MHz) 724.2739(19) 724.2307(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) -3.9332(23) 14N Xbb-Xcc (MHz) -2.6436(68) Fab (MHz) 426.84(66) ΔE01 (MHz) -263.67(66) RMS (kHz) 5 N 167

Preliminary Structure Analysis for Pyridine - Acetylene Isotope Rcm, Å Θ2° N-H, Å <(NHC)° PYR-HCCH 5.10883 -8.2 2.2070 0.06 PYR-DCCD 5.10813 -27.25 2.3600 32.28 PYR-DCCH 5.04860 -9.910 2.2103 2.67 PYR-HCCD 5.16852 -13.94 2.2201 10.36 Avg. 5.10852 2.2494 Max-min 0.120 0.15