Condensed phase vs. Isolated gas phase spectra

Solution phase

AA

A

A A

AW

W

W

W

W

W W

WWW

WW

W

W

WW

W

W: waterA: sample 12

10

8

6

4

2

0

x10

3

30000295002900028500

Wavenumbers / cm-1

MonomerOrigin

* ** **

****

252

418 252+418

(350.88 nm) (333.33 nm)

Isolated gas phase (Supersonic jet)

AA A

A

AA A

S0

S1

252418

Supersonic expansion technique

Pump

Sample seeded in Helium (~ 2 atm)

V m/s

f (v

) s

/ m

P ~ 1x10-6 torr

Trot ~ 5 KTvib < 50 K

Supersonic expansion

S0

S1

V=0

V=0

V=0

V=0

V=0

V=0

V=0

V=0

Dihedral angles

(i) Conformational isomersSchematic potential energy diagram

(ii) Non-covalently bondedcomplexes

LIF (Laser Induced Fluorescence)

S0,v=0

l1

S1

monitor total fluorescence

Fluorescence excitation spectroscopy

Can we perform spectroscopy in a mass spectrometer?

Yes, we can perform high resolution electronic and vibrational spectroscopy in a mass spectrometer.

How?

Laser Desorption and REMPI (ResonantlyEnhanced MultiphotonIonization)

Time of FlightMass Spectrometry

Supersonic jetexpansion

Ionization source Mass analyzerFormation of non-covalently bonded clusters and high resolution spectroscopy

+ +

Jet-cooled Laser Desorption Time of Flight Mass spectrometer

Home-built Jet-cooled Laser Desorption REMPI (Resonantly Enhanced Multiphoton Ionization)Time OF Flight Mass Spectrometer

Biomolecules (peptides)

Low vaporpressure

Decompose uponheating

Vapor?

Reduce the time-scale for heating

dT/dt = 1011 K/sec

1000 K temp jump in a 10 ns laser pulseDesorption without fragmentation

To bring the molecules in vapor phase

(i) Thermal heating(ii) Laser desorption

Computation: M05-2X, M06-2X, DFT-D level of calculation using Gaussian 09, Gamess programs

Experimental Set up and computational methods

Spectroscopic techniques

S0 v = 0

S1

D0

h1 (UV)tuned

h1 (UV)

1C-R2PI(one color Resonant twophoton ionization)

Mass selectedElectronic spectrum

S0 v = 0

S1

D0

h2 (UV)fixed

h2 (UV)

h1 (IR)tuned

100 ns

RIDIRS(Resonant ion dipInfrared spectroscopy)

IR spectrum

S0 v = 0

S1

D0

h2 (UV)tuned

h2 (UV)

h1 (IR)fixed

100 ns

IR-UV hole-burning spectroscopy

Discriminate different conformers

S0 v = 0

S1

D0

h2 (UV)fixed

h2 (UV)

h1 (UV)tuned

100 ns

UV-UV hole-burning spectroscopy

48040032024016080

Mass (a.m.u.)

Ind

+(I

nd

...H

2O

)+

[In

d...

(H2O

) 2 ]+

(In

d...

Py)

+

(In

d) 2

+ [(In

d) 2

...P

y]+

[(In

d) 2

...P

y...H

2O

]+

(In

d) 3

+

Time of Flight mass spectrum of complexes of indole and pyridine

27.525.022.520.017.515.0Time of Flight (s)

Ind

+

(In

d...

H2O

)+

[In

d...

(H2O

) 2 ]+

(In

d...

Py)

+

(In

d) 2

+ [(

Ind

) 2...

Py]

+

[(In

d) 2

...P

y...H

2O

]+

(In

d) 3

+

Electronic spectra of complexes of indole and pyridine

S0

S1

35240 cm-1

S0

S1

Indole Pyridine

34776 cm-1

S0

S1

34969 cm-1

S0

S1

Indole…pyridinedimer

(Indole)2…pyridinetrimer

35164 cm-1

(35240)

D0D0

D0

D0

h

(349

69)

(351

64)

One-color R2PI spectra

IR spectra of species A and B

36003500340033003200

3526

3411

3309

3269

3281

(a) Exc. at 0-0 band of indole

(b) Exc. at A00

+ 27 cm-1

(c) Exc. at B00

+ 30 cm-1

Wavenumber (cm-1

)

S0 v = 0

S1

D0

h2 (UV)fixed

h2 (UV)

h1 (IR)tuned

100 ns

RIDIRS

Single conformer or multiple conformers of dimer and trimer?

Are all these peaks due totransitions of one or multiple conformers of the trimer?

Are all these peaks due to transitions of one or multiple conformers of the dimer?

IR-UV holeburning spectra of A and B

UV spectrum (IR off)

UV spectrum(IR fixed at 3411 cm-1 band of the trimer)

UV spectrum(IR fixed at 3281 cm-1 band of the trimer)

UV spectrumIR fixed at 3269 cm-1 band of the dimer

S0 v = 0

S1

D0

h2 (UV)tuned

h2 (UV)

h1 (IR)fixed

100 ns

IR-UV holeburningspectroscopy

36003500340033003200

3526

3411

3309

3269

3281

(a) Exc. at 0-0 band of indole

(b) Exc. at A00

+ 27 cm-1

(c) Exc. at B00

+ 30 cm-1

Wavenumber (cm-1

)

Conclusion: One conformer of the dimer and one conformer of the trimer are observed.

Determination of the structures of the observed dimer and trimer

36003500340033003200

(c)

(b)

(a)

(N-H...)(N-H...N)

(Ind)2.Py - 1

(Ind)2.Py - 2

(Ind)2.Py - 3

3269

3309

3411 3281

(N-H...N)

3526

HB3

Wavenumber (cm-1

)

Comparison of experimental and theoretical IR spectra

1.95 Å

(ind)2.py-1

Indole

S0 v = 0

S1

D0

h2 (UV)fixed

h2 (UV)

h1 (IR)tuned

100 ns