0 ipc kiel The rotational spectrum of the pyrrole-ammonia complex Heinrich Mäder, Christian Rensing and Friedrich Temps Institut für Physikalische Chemie der Universität Kiel

1 ipc kiel outline motivation ab initio calculations experimental setup double resonance technique results summary outlook

2 ipc kiel motivation pyrrole is a component of larger aromatic rings e.g. porphyrins (heme), chlorines (chlorophyll) and corrin (vitamin B12) interest in microsolvation effects of pyrrole with strong H-acceptors like ammonia in view of mode specific dissociation of the pyrrole N-H bond structure of the pyrrole-ammonia dimer nuclear quadrupole hyperfine structure

3 ipc kiel problems no structure published in literature → ab initio predictions assignment of spetra → double resonance complex hyperfine structure expected due to quadrupole coupling by two 14 N nuclei →initial analysis of the complex pyrrole- 15 N-ammonia with simplified hyperfine structure (only one quadrupole nucleus)

4 ipc kiel ab initio calculations input : geometry optimization on MP2 level with the basis sets aug-cc-pVDZ and G**, considering of the basis set superposition error (BSSE) and using the counterpoise method. output : rotational constants, dipole moment components → prediction of a spectrum (a-type)

5 ipc kiel scheme of the MBFTMW-spectrometer GHz ≈ 160 MHz signal MW source pump MW source MW ab +160 MHz ≈ ab pump MW

6 ipc kiel scheme of the MBFTMW-spectrometer GHz ≈ 160 MHz signal MW source pump MW source MW ab +160 MHz ≈ ab pump MW

7 ipc kiel pulse sequence Molecular pulse MW-signal pulse Detection period MW-pump pulse (optional) ~1µs~1µs ~ 1ms ~ 20 µ s ~ µ s ~ µ s

8 ipc kiel single resonance broadband scan search for rotational transitions of pyrrole -ammonia

9 ipc kiel interpretation of the spectrum problems when searching for rotational transitions of a specific species: formation of dimers in the molecular beam is not selective ! →composition of species in the molecular beam is unknown →observed spectrum is a superposition of spectra from various species

10 ipc kiel interpretation of the spectrum pyrrole & misc. 13 C substituted species pyrrole dimer & misc. 13 C substituted species pyrrole-neon dimer, trimer… ammonia dimer solution: double resonance

11 ipc kiel double resonance bc pump transition signal transition ab EcEc EbEb EaEa requirements for double resonance: - 3 level system - 2 transitions sharing one energy level → line splittings ("Javan doublets") for resonant and homogeneous pump radiation

12 ipc kiel Effect of pump field inhomogeneity : dephasing (loss of coherence) → quenching of the signal in the time domain time end of MW pulse data acquisition double resonance no pumpwith pump

13 ipc kiel double resonance: broadband scans bc E c pump transition E a E b signal transition ab pump MW time I signal pump frequency signal MW ab bc

14 ipc kiel double resonance: broadband scans fixed signal transition 3 12 ↔ 2 11 (approx MHz) 4 13 ↔ 3 12 (approx MHz) Single resonance broadband scan

15 ipc kiel rotational transitions investigated double resonances

16 ipc kiel pyrrole - 14 NH 3 : nqhfs frequency / MHz intensity rotational transition 2 11 – 1 10 with hfs components IF - I'F' (I=I 1 +I 2, F=I+J)

17 ipc kiel results : spectroscopic constants pyrrole - 15 NH 3 pyrrole - 14 NH 3 A / MHz (49) (29) B / MHz (11) (72) C / MHz (11) (88)  J / kHz 0.528(16)0.551(12)  JK / kHz 38.86(13)40.765(93)  J / kHz 0.128(20)0.165(18)  + pyrrole ( 14 N) / MHz (38) (35)  - pyrrole ( 14 N) / MHz (84)3.6480(68)  + ammonia ( 14 N) / MHz (25)  - ammonia ( 14 N) / MHz (68)

18 ipc kiel structure and dynamics a r s -structure analysis confirms the ab initio prediction that the N-atom is located on the a-principal axis of the complex the inertial defect Δ = 0.5 uÅ 2 differs from a rigid complex (Δ  -2.7 uÅ 2 ) this difference indicates a nearly free internal rotation of NH 3 around its C 3 -symmetry axis (Δ  0) as well as a contribution from a large in plane bending vibration of ammonia in the plane of pyrrole (Δ > 0)

19 ipc kiel summary high resolution rotational spectra of pyrrole - 15 NH 3 and pyrrole - 14 NH 3 complexes were recorded, assigned and confirmed 14 rotational transitions containing 65 hyperfine components (pyrrole - 15 NH 3 ) 12 rotational transitions containing 154 hyperfine components (pyrrole - 14 NH 3 )

20 ipc kiel summary ab initio calculations: useful tool for initial line search double resonance technique: very helpful for the assignment and confirmation of the spectra

21 ipc kiel outlook extension of structure determination by investigation of further isotopomeres analysis of internal dynamics (internal rotation and large amplitude vibration of the NH 3 moiety

22 ipc kiel Thank you for your attention!