Pure Rotational and Ultraviolet- Microwave Double Resonance Spectroscopy of Two Water Complexes of para-methoxyphenylethylamine (pMPEA) Justin L. Neill , Matt T. Muckle and Brooks H. Pate, Department of Chemistry, University of Virginia Ryan G. Bird, David W. Pratt, Department of Chemistry, University of Pittsburgh
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Pure Rotational and Ultraviolet-Microwave Double Resonance Spectroscopy of Two Water Complexes of para-methoxyphenylethylamine (pMPEA) Justin L. Neill,
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Pure Rotational and Ultraviolet-Microwave Double Resonance Spectroscopy of
Two Water Complexes of para-methoxyphenylethylamine (pMPEA)
Justin L. Neill, Matt T. Muckle and Brooks H. Pate,Department of Chemistry, University of Virginia
Ryan G. Bird, David W. Pratt,Department of Chemistry, University of Pittsburgh
Spectroscopy of pMPEA and pMPEA-waterpMPEASeven conformers reported: Martinez et al, J. Mol. Spectrosc. 158 (1993) 82-92.Complete (correct) structural assignment: Robertson, Simons, and Mons, J. Phys. Chem. A 105 (2001) 9990. Yi, Robertson, and Pratt, Phys. Chem. Chem. Phys. 4 (2002) 5244-5248. (rotationally resolved LIF)Douglass et al., MF02, ISMS (2006) (CP-FTMW and UV-MW)Cortijo, Alonso, and López, Chem. Phys. Lett. 466 (2008) 214-218. (MW)
pMPEA-waterTwo clusters found, binding energies measured,assigned to structures: Unamuno et al, Chem. Phys. 271 (2001) 55-69.
New CP-FTMW MeasurementsSample acquired from Aldrich (98%); placed in reservoir within nozzle, heatedto approximately 100°C, seeded in He/Nesupersonic expansion. (No water added)
CP-FTMW sample reduction techniques:3 nozzles10 FIDs per valve pulse
Collected 995,000 FIDs, using298,500 valve pulses, equivalentsensitivity to 8.955 million pulses witha single nozzle!
3 nozzles, 995,000 FIDs (298,500 pulses) versus 1 nozzle, 80,000 FIDs (80,000 pulses)Scaled to match signal heights on strongest transitions
New CP-FTMW Measurements
2008 spectrum much richer than 2006…(used same bottle)
3 nozzles, 995,000 FIDs (298,500 pulses) versus 1 nozzle, 80,000 FIDs (80,000 pulses)Scaled to match signal heights on strongest transitions
New Transitions
Several unassigned Q-branches observed. For near-prolate top, b/c-type Q-branches are locatedat approximately (A-(B+C)/2)*(2K-1), so the ratio between two Q-branches gives you their K assignmentsand A-(B+C)/2.
Pattern of the Q-branches gives (B-C), then (A+B+C) can be varied until the strong b/c-type R-branchesare fit. Two new spectra were assigned this way.
rms error/kHz 12.7µa/D 0.47µb/D c only 0.49µc/D 2.32
Observed structures are analogous to those of other similar structures: tryptamine (Felker, J. Phys. Chem. 96 (1992) 7844);2-phenylethylamine (Melandri, et al, RC13)
Ab initio: b3pw91/6-311+g(df,pd), using effective Q and recommended basis set of W.C. Bailey (http://homepage.mac.com/wcbailey/nqcc/)
All fits performed using SPFIT (Pickett), with standard errors determined by PIFORM (Kisiel). (Quartic distortion parameters not listed)
Coherence-Converted Population Transfer UV-FTMW Spectroscopy
T.J. Balle and W.H. Flygare, Rev. Sci. Instrum. 52, 33 (1981)M. Nakajima, Y. Sumiyoshi, and Y.Endo, Rev. Sci. Instrum. 73, 165 (2002)
The assignments of Unamuno et al are correct—35670 cm-1 feature is due towater with conformer 5; 35681 cm-1 feature is due to water withconformer 8.
Their assignments were based on structural stability—conformer 8+water goesto strongest peak, conformer 5+water to second-strongest—andlow-frequency vibrational mode calculations.
Residual Spectrum
No residual transitions with resolved quadrupole hyperfine splitting—not pMPEA, or simply a functionof cluster size? (large number of hyperfine-resolved transitions for assigned pMPEA-H2O clusters)
Possibilities:Other conformers with water; water molecule on the methoxy group? (Unlikely due to energetics)Two waters or more? (more likely—ab initio calculations needed)Remeasure CP-FTMW spectrum with water added! MW-MW double resonance spectroscopy needed
Strongest pMPEA transition intensity 120 µV
Acknowledgements
Funding:NSF CRIF:ID (CHE-0618755)Jefferson Scholars Foundation (J.Neill)
Observed structures are analogous to those of other similar structures: tryptamine (Felker, J. Phys. Chem. 96 (1992) 7844);2-phenylethylamine (Melandri, et al, RC13)