selected ion infrared spectroscopy of carbocations
TRANSCRIPT
Selected Ion Infrared Spectroscopy of Carbocations
Michael A. Duncan
Department of Chemistry, University of Georgia, Athens, GA 30602 [email protected]
http://www.arches.uga.edu/~maduncan
NSF
Gary Douberly Allen Ricks Tim Cheng Biswajit Bandyopadhyyay
Collaborator: Prof. Paul v. R. Schleyer
H2+
H3+
CH+
CH2+
CH3+
CH5+
CH4
C2H3+
C2H2
C3H+
C3H3+
C4H2+
C4H3+
C6H5+
C6H7+ C6H6
H2
H2
H2
H2
H2
C
e
C+
e
C+
C
H
C2H2
H2 e
OH+ H2O+
H3O+ H2O
OH e
O H2
H2
HCO+ CO
HCN CH3NH2
CH3CN
C2H5CN
CH CH2CO
CH3OH
CH3OCH3
CH3+
C2H5+ e
C2H4
e C3H2
e C3H
e C2H
Carbocations in Astrochemistry
H2+
H3+
CH+
CH2+
CH3+
CH5+
CH4
C2H3+
C2H2
C3H+
C3H3+
C4H2+
C4H3+
C6H5+
C6H7+ C6H6
H2
H2
H2
H2
H2
C
e
C+
e
C+
C
H
C2H2
H2 e
OH+ H2O+
H3O+ H2O
OH e
O H2
H2
HCO+ CO
HCN CH3NH2
CH3CN
C2H5CN
CH CH2CO
CH3OH
CH3OCH3
CH3+
C2H5+ e
C2H4
e C3H2
e C3H
e C2H
H2+
H3+
CH+
CH2+
CH3+
CH5+
CH4
C2H3+
C2H2
C3H+
C3H3+
C4H2+
C4H3+
C6H5+
C6H7+ C6H6
H2
H2
H2
H2
H2
C
e
C+
e
C+
C
H
C2H2
H2 e
OH+ H2O+
H3O+ H2O
OH e
O H2
H2
HCO+ CO
HCN CH3NH2
CH3CN
C2H5CN
CH CH2CO
CH3OH
CH3OCH3
CH3+
C2H5+ e
C2H4
e C3H2
e C3H
e C2H
Carbocations Expected in Space
Ion chemistry important for the synthesis of complex organics. Like other astrophysical molecules, most ions are detected with microwave spectroscopy. H3
+ is an exception – detected with IR. IR signatures are useful for other ions, especially those with no dipole moment, e.g., cyclic-C3H3
+. Low resolution spectra refine potentials for theory and guide high resolution experiments.
From Ben McCall, Illinois
First seen by Mary Lea Heger in 1922 Now hundreds of lines throughout visible and ultraviolet region One of oldest mysteries in astronomy. Believed to come from absorption of starlight by small molecules.
Visible Interstellar Spectra: The Diffuse Interstellar Bands (DIBs)
Radicals and cations likely to have visible spectra.
Believed to come from polycyclic aromatic molecules or their ions.
Infrared Interstellar Spectra: The Unassigned Infrared Bands (UIRs)
0 25 50 75 100 125
9771
6545
20
7751
123131
105
103
63
39
79
53
n = 1 2 3 4 5
m/z
H+(C2H2)n27
43
Ion Spectroscopy: Ions are produced in “hot” conditions (discharges, plasmas, etc.). Need cooling. Mixture of species produced; neutral precursors have similar spectra. Density of any one species is low. Density per quantum state even lower. Success stories on small ions via IR absorption: H3
+, H3O+, N2H+, HCO+, etc. Oka, Saykally, Linnartz, Nesbitt, etc. Need separation/purification, i.e., mass-selection. Need bright light source, i.e., IR laser. Need sensitive detection scheme – density too low for absorption. We use IR photodissociation spectroscopy as done by Y. T. Lee, Oka, Dopfer, Johnson, etc.
Ions are familiar in mass spectrometry, but their spectroscopy is quite difficult.
Ion Production: Dissociation/electron impact: C3H3Cl + e−(fast) → C3H3• C3H3• + e−(fast) → C3H3
+ Proton transfer (aka “chemical ionization”): H2 + e−(fast) → H3
+
H3
+ + C2H2 → C2H3+ + H2
Scavenge electrons to limit cation-electron recombination (very fast at low temperature): e− + H2O → OH −
Gas pulse
discharge
Pulsed discharge occurs in center of gas pulse
Production of cold cations with pulsed discharge in a supersonic expansion. Mass selection of cations by high throughput time-of-flight MS. Photodissociation spectroscopy with an infrared OPO laser. Fragment yield vs wavelength = IR spectrum.
full mass spectrum
activate mass gate; select one cluster mass
excite at turning point
parent ion depletion
photofragments
Laser Photodissociation in a Reflectron ToF-MS
~106 ions
OPO OPA
1 crystal angle tuned
4 crystals angle tuned
signal (not used)
idler
532 nm
1064 nm
KTP oscillator
KTA diff. gen.
1064 - idler
AgGaSe2 diff. gen.
Tunable 4.5-17 µm
1 crystal angle tuned
Tunable 2.3-5.0 µm
LaserVision Tunable Infrared Laser System Optical Parametric Oscillator (OPO)
Pumped by pulsed, seeded YAG e.g., Spectra Physics PRO-230.
2000-4500 cm-1
600-2200 cm-1
Spectroscopy made possible by new IR-OPO laser:
Combined tuning range: 600-4500 cm-1 Linewidth: ~1.0 cm-1
+ hν (tunable IR)
IR Spectroscopy of Ions: Photodissociation via Rare Gas Tagging
The density of mass-selected ions is too low for absorption spectroscopy. Use photodissociation, with mass spectrometer detection. Typical bonds are too strong (D0>20 kcal/mol) to be broken with IR light (e.g., C-H stretch of 3100 cm-1 = 9 kcal/mol). Laser power is too low for multiphoton dissociation. Use “spectator atom/molecule method” (Y.T. Lee and coworkers) aka “tagging”: Attach weakly bound (1-2 kcal) “tag” atom to enhance fragmentation efficiency. Tag elimination when light is absorbed provides indirect evidence of absorption. Detect fragment ion on zero background vs frequency. Now used throughout ion spectroscopy (Mikami, Johnson, Maier, Dopfer, Meijer, Bieske, Lisy, Brechignac, Kappes, Chang, Nishi, etc., etc.)
mass selected ion fragment ion detected vs IR frequency
+ +
Ar
Select mass 67 Detect mass 27
e.g., protonated acetylene
C=C-H H H
+ H-C≡C-H
H+
C2H3+
m/z=27
“classical” vinyl cation
“non-classical” protonated acetylene
Rotationally resolved spectrum [Oka, JPC 99, 15611 (1995)] in C-H stretching region was consistent with non-classical structure, but coulomb explosion experiments questioned this. Proton stretch not seen before!
∆E ~ 2-3 kcal/mol
2200 2400 2600 2800 3000 3200 3400 3600
-40
-30
-20
-10
0
10
20
30
(C2H2)H+Ar (classical)
(C2H2)H+ (classical)
(C2H2)H+ (non-classical)
νproton stretchνasym
Signal (a.u.)
Wavenumber (cm-1)
(C2H2)H+Ar (non-classical) x 10
νsym
Protonated acetylene in “non-classical” form!
J. Phys. Chem. A 112, 1897 (2008).
Argon on proton causes large shift
Trot=100K
C3H3+
m/z=39
propargyl cation
+28 kcal/mol cyclopropenyl cation
0.0 kcal/mol (CCSD(T) cc-pvTZ)
The smallest aromatic ring.
Mass 39 is common fragment in mass spectrometry from many organic molecules. Theory shows that there are two stable isomers. In space,
C3H2 + H3+ ↔ c-C3H3
+ + H2 Seen with radiofrequency spectroscopy
C3H3+
cyclopropenyl vs propargyl
J. Chem. Phys. 132, 051101 (2010).
Linear precursors produce more propargyl Higher energy discharge makes more cyclic
How can we have two isomers? Consider potential for C3H3
+ rearrangement
• Large barrier to isomer interconversion due to C-C bond breaking
• Linear C3H3+ dominates at higher
temperature while cyclic C3H3+ seems to
dominate at lower temperature1 • There must be another route to make the
cyclic isomer • Need cyclic precursor.
singlet
3000 3100 3200 3300
cm-1
3004 30743107
31323182
3235
C3H3+Ar
Assignment of the C-H stretching regions is difficult. Six bands are detected. If argon binds on C-H, then l- and c-C3H3
+ each have three bands. Argon shifts bands significantly when it attaches on C-H. However, recent high level theory (Botschwina; explicitly correlated coupled cluster) suggests that argon binds above ring and on side of chain. If this is true, then l-C3H3
+ has three C-H stretches, but c-C3H3
+ has only one (and then there are too many bands present). Full anharmonic calculations (Botschwina, Lee, Bartlett) can detect combination bands, but cannot include the argon.
Determining the argon binding site is not trivial, and this is critical to the assignment of the spectrum. High levels of theory can determine this, but so far cannot solve for vibrational spectra including the argon! Argon isomers possible??
1000 1500 2000 2500 3000 3500
Wavenumber (cm-1)
MP2 / 6-311+G(2d,2p) 0.95 scale factor
allyl
2-propenyl
3110
30042933
2798
18771584
14181277
1227
C3H5+: allyl vs 2-propenyl isomers
Bowers et al. suggested two isomeric forms of C3H5
+
J. Am. Chem. Soc. 102, 4830 (1980); Our IR spectra here (from ethylene discharge) indicate about 1:1 ratio.
J. Chem. Phys. 128, 021102 (2008).
0.0 kcal/mol
+8.0 kcal/mol Barrier = 18 kcal/mol
Change the precursor! IR spectra from discharge with cyclopropyl bromide produce more allyl and less 2-propenyl. N2 tagging works just like Ar tagging.
1000 1500 2000 2500 3000 3500
C3H5+-N2
loss of N2
Wavenumbers (cm-1)
C3H5+-Ar
loss of Ar
allyl
2-propenyl
ethylene discharge
cyclopropyl bromide discharge
Protonated benzene, C6H7+
Structure was characterized with 1H and 13C NMR in superacid solutions
- Rapidly equilibrating with ~8 kcal/mol barrier to H+ migration - 7 equivalent H’s and 6 equivalent C’s above ~150 K. - 13C-NMR data consistent with allylic π electron density
JACS, 1978, 100, 6299.
800 1000 1200 1400 1600 2600280030003200cm-1
1456
16071238
2821
963828
3107
w/out Ar
with Ar
B3LYP/6-311+g (d,p)
IRPD with Ar tagging; argon binding energy is ~200 cm-1
Protonated Benzene, C6H7+
J. Phys. Chem. A 112, 4869 (2008)
Sigma protonation, as for benzene Alpha position favored Argon has negligible effect
Protonated Naphthalene: C10H9+
B3LYP/6-311+G(d,p)
Astrophys. J. 702, 301 (2009).
(790 cm-1)
(1600 cm-1)
(1300 cm-1)
(1160 cm-1)
(890 cm-1)
6.2 micron band has been especially difficult to explain.
Emission seen from carbon-rich regions. Usually associated with PAH’s. (Léger, Tielens, Ehrenfreund, Hudgins, Peeters, Joblin, etc.)
Laboratory experiments on neutral, cation and anion PAH’s in gas phase and in matrices failed to find suitable match to band pattern. (Allamandola, Salama, Vala, Brechignac, Meijer, Oomens,Bakker, Maitre, Dopfer, Pino,etc.) Theory on protonated and nitrogen substituted species found poor match (Bauschlicher). Reaction studies of H+(PAH)s show that they are inert (Snow, Bierbaum, etc.)
Unidentified Infrared Bands (UIR’s)
Tielens et al. Astron. & Astrophys. (2002), 390, 1089
4 6 8 10 12µm
7.7
8.6
6.2
3.56.6
6.9
7.4
3.3 10.4
11.26.2 7.7
8.6
3.5 µm band is the sp3 C-H stretch 6.2 µm is allylic C-C-C stretch 7.7 µm is H-C-H scissors bend 8.6 µm band is CH2/CH wag Theory suggests many H+(PAH) species have these same vibrations! 11.2 µm band not seen; low laser power and dissociation yield. 6.6 & 6.9 µm bands seen here but not in UIR’s. Theory suggests that larger H+(PAH) species do not have these. This protonated naphthalene study suggests that protonated PAH’s account for main features of UIR’s. Need spectra for larger systems.
Astrophys. J. 702, 301 (2009).
UIRs
See also Dopfer et al., Astrophys. J. 706, L66-L70 (2009).
Electron impact ionization of methanol or ethanol produces fragment at m/z=31.
Small Oxygen –Containing Ions
31
Protonated Formaldehyde versus Methoxy Cation
Same spectrum obtained from methanol and ethanol, but branching is different. Branching also varies with discharge conditions. m/z =32 is both protonated formaldehyde and methoxy!
Amano: 3422.8 cm-1
Neon tag: 3400 cm-1
J. Phys. Chem. A 116, 9287 (2012).
m/z=30 ion is apparently both formaldehyde cation and hydroxy methylene (more work to be done).
also under study: CH3OH+ vs CH2OH2
+
H5+ has a shared proton structure
But H7+ and H9
+ have solvated H3
+ structures.
H5+ has shared proton structure
and complex vibrations. Theory collaboration with Bowman and coworkers.
IR-MPD at FELIX free electron laser
J. Phys. Chem. Lett. 3, 3160 (2012).
J. Phys. Chem. Lett. 1, 758 (2010).
Conclusions Carbocation spectra in full range of IR provide structures and identifies isomers. Protonated naphthalene and other proto-PAH’s explain UIR bands. Low resolution IR spectra with mass selection provides “finder scope” for future high resolution ion spectra.
Paul Schleyer Allen Ricks and Gary Douberly