ROTATIONAL SPECTRA OF THE N2OH+ AND CH2CHCNH+ MOLECULAR IONS

Oscar Martinez Jr.Valerio LattanziMichael C. McCarthy

Harvard-Smithsonian Center for AstrophysicsSchool of Engineering and Applied Science, Harvard University

Sven ThorwirthMax-Planck-Institut für Radioastronomie

I. Physikalisches Institut, Universität zu Köln

Fourier-Transform Microwave Spectrometer

• Operating range: 5 to 42 GHz • Pulsed nozzle (6Hz) supersonic

molecular beam (~Mach 2)– 2.5 kTorr stagnation pressure

behind nozzle, – Total flow 20 sccm– Results in Trot ~ 1 – 3 K– DC discharge used to create

radicals and ions• MW-MW double resonance

capability effectively extends range to 60+ GHz

McCarthy et al.,ApJ Suppl. Ser.(2000)

Protonated Molecules• Protonation plausibly occurs in our gas-phase hydrogen discharge via

H2 + H2+ → H3

+ + H

H3+ + A → AH+ + H2

• A number of protonated species studied by our group, including HNC2N+, HC3NH+, CH3CNH+, HSCO+, HSCS+, HSO2

+, HNCOH+, and H2NCO+

• Interstellar weeds– Internal rotors

• e.g., methanol (CH3OH), methyl formate (HCOOCH3), and dimethyl ether (CH3OCH3)

– and heavy species with no internal rotation• e.g., propionitrile (CH3CH2CN) and acrylonitrile (CH2CHCN)

– High density of spectral lines– Complicated spectra

• Can lead to misidentifications (e.g., glycine – NH2CH2COOH)

Protonated Nitrous Oxide(N2OH+ and HN2O+ isomers)

• N2O detected in ISM (Sgr B2) Ziurys et al., ApJ (1994)

• Theoretical work has examined structureRice et al., Chem. Phys. Lett. (1986)

Koichi and Mookuma, Chem. Phys. Lett. (1986)Martin and Lee , J. Chem Phys. (1993)

• Prior experimental work detected ground-state isomer (O-protonated)

IR by Amano et al., Chem. Phys. Lett. (1986)and Jacox and Thompson, J. Chem. Phys. (2005)

mm and sub-mm (130-406 GHz) Bogey et al., Astron. Astrophys. (1986)

and J. Chem. Phys. (1988)

• No prior detection of nitrogen hfs for O-protonated isomer• No report of N-protonated isomer

HN2O+ Experiment

• Computation:– Structure calculations at CCSD(T)/cc-pwCVQZ level

of theory– Structure corrected for zero-point vibrational effects

at CCSD(T)/cc-pVTZ level of theory

• Optimization on 10,1 → 00,0 transition of NNOH+

McCarthy and Thaddeus J. Mol. Spec. (2010)– DC discharge (~1 kV) – 0.3% N2O heavily diluted in mixture of H2 (10%) and

He (90%)

H

ON N

N N O

H

4.35 kcal/mol (Martin and Lee, J. Chem Phys. 1993)

E

4.05 kcal/mol (this work)

HN2O+

• 6 hyperfine-split lines detected in Ka=0 ladder for N2OH+ isomer

• 7 such lines detected for HN2O+ isomer

NNOH+ HNNO+

Constant Experimental Previous* Experimental CalculatedBeff 11 192.9194(4) 11 192.9214(12) 11 796.3517(4) 11 802.6Χaa(Nouter) -3.330(4) … 2.737(0) 2.760 Χaa(Ninner) 0.949(6) … -0.423(8) -0.466

*Bogey et al. Phys. Rev. Lett. (1987)

HN2O+

N2OH+

Protonated Vinyl Cyanide(CH2CHCNH+)

• Vinyl cyanide detected in ISM– Towards Sgr B2, in TMC-1 and in IRC+10216

1st det by Gardner and Winnewisser, ApJ (1975)

• Dubbed an ‘interstellar weed’• Relatively high proton affinity of vc

(784.7 kJ/mol, ~30% higher than CO)• PVC essentially unreactive with many major interstellar

constituents (e.g. H2, CO, N2, O2 CH4 and C2H2)Petrie et al., MNRAS (1992)

• Relevant to planetary atmospheres• PVC μa =2.1 D

• Other protonated nitriles detected (HC3NH+ and CH3CNH+)Turner and Feldman, ApJ (1990)

PVC Experiment• Search conditions first optimized on low-J line

(20,2 → 10,1 transition at 22.5 GHz) of HSCO+

McCarthy and Thaddeus, J. Chem. Phys. (2007)

• Search facilitated by computational support– Theoretical structure calculations at CCSD(T)/cc-pwCVQZ and

corrected for zero-point vibrational effects– Theoretical rotational constants scaled by ratio of measured to

calculated rate constants for vinyl acetylene (typically accurate to w/in 0.1%)

• Confirming lines found under optimized conditions for candidate (20,2 → 10,1 ) PVC line

– 1.1 kV discharge– 40:1 flow ratio H2:1% vinyl cyanide in H2

PVC

1 kV DC1% VC

0.7 kV DC1% VC

0.7 kV DC0.1% VC

PVC• MW-MW double resonance used to extend

measurements to 46 GHzLattanzi et al., J. Chem. Phys. (2010)

– Enabled determination of line frequency for 50,5 → 40,4 transition

• 15 a-type rotational transitions– 8 from Ka = 0 ladder

– 7 from Ka = 1 ladderCalculated

Constant Experimental Equilibrium Vib. Contrib. Ground Vib. StateA 46 187.0(69) 46 303.2 69.1 46 234.1B 4 791.136 7(11) 4 801.9 17.1 4 784.8C 4 334.793 51(74) 4 350.7 20.8 4329.9μa 2.08μb 0.47Χaa 0.197 8(69) 0.286Χbb 0.325 0.325

Conclusion• Rotational detection of HN2O+ and measurement of hfs for

both HN2O+ and N2OH+

• Detection of protonated vinyl cyanide

• New cations are excellent candidates for radioastronomical detection

• Future targets:– Extend protonated VC data set into the mm-wave – Detection of other astronomically-relevant protonated species

• high proton affinity• low reactivity with abundant neutrals• high column density of neutral counterparts• large dipole moments

Acknowledgements

• Harvard-Smithsonian Center for Astrophysics• Funding

Cambridge:NSF: CHE-0701204NASA: NNX08AE05G

Cologne:Deutsche Forschungsgemeinschaft (TH1301/3-1)