Selected Ion Infrared Spectroscopy of Carbocations

Michael A. Duncan

Department of Chemistry, University of Georgia, Athens, GA 30602 maduncan@uga.edu

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