ROTATIONAL SPECTRA OF THE
N2OH+ AND CH2CHCNH+
MOLECULAR IONS
Oscar Martinez Jr.
Valerio Lattanzi
Michael C. McCarthy
Harvard-Smithsonian Center for Astrophysics
School of Engineering and Applied Science, Harvard University
Sven Thorwirth
Max-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 structure
Rice 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:
H
– Structure calculations at CCSD(T)/cc-pwCVQZ
level of
theory
4.05 kcal/mol (this work)
N
N
– Structure corrected for zero-point vibrational effects
at CCSD(T)/cc-pVTZ level of theory
O
H
4.35 kcal/mol
+
N
•O Optimization
on
1
→
0
transition
of
NNOH
N
0,1
0,0
(Martin and Lee, J. Chem Phys. 1993)
McCarthy andEThaddeus J. Mol. Spec. (2010)
– DC discharge (~1 kV)
– 0.3% N2O heavily diluted in mixture of H2 (10%) and
He (90%)
HN2O+
• 6 hyperfine-split lines detected in Ka=0 ladder
for N2OH+ isomer
• 7 such lines detected for HN2O+ isomer
N2OH+
HN2O+
Constant
Beff
Χaa(Nouter)
Χaa(Ninner)
NNOH+
Experimental
Previous*
11 192.9194(4)
11 192.9214(12)
-3.330(4)
…
0.949(6)
…
HNNO+
Experimental
Calculated
11 796.3517(4)
11 802.6
2.737(0)
2.760
-0.423(8)
-0.466
*Bogey et al. Phys. Rev. Lett. (1987)
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 DC
1% VC
0.7 kV DC
1% VC
0.7 kV DC
0.1% VC
PVC
• MW-MW double resonance used to extend
measurements to 46 GHz
Lattanzi 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 ladder
Constant
A
B
C
μa
μb
Χaa
Χbb
Experimental
46 187.0(69)
4 791.136 7(11)
4 334.793 51(74)
0.197 8(69)
0.325
Equilibrium
46 303.2
4 801.9
4 350.7
2.08
0.47
0.286
0.325
Calculated
Vib. Contrib.
69.1
17.1
20.8
Ground Vib. State
46 234.1
4 784.8
4329.9
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-0701204
NASA: NNX08AE05G
Cologne:
Deutsche Forschungsgemeinschaft
(TH1301/3-1)