O

O

O

O O

(H2O)n

Electronic and vibronic spectroscopy of crown ether water complexes: benzo-15-crown-5 (B15C)

and 4’-aminobenzo-15-crown (ABC)

V. Alvin Shubert and Timothy S. Zwier

Purdue University, Department of Chemistry, West Lafayette, IN 47907

O

O

O

O O

H2N

(H2O)n

ABC-(H2O)n B15C-(H2O)n

• Crown ethers long noted for ability to selectively bind substrates, especially cations

• Much work has focused on structure and binding energy of crown-cation complex in solution

• However, oxygen-rich pocket is ideally suited to binding other types of substrates, including water

Motivations

• We can study the binding of water to crown ethers in the absence of ions using jet-cooled gas phase spectroscopy

• As a first step, we present the IR and UV spectra of water clusters of 4’-aminobenzo-15-crown-5 and benzo-15-crown-5 ethers

O

O

O

O O

Na+

Resonant 2 photon ionization (R2PI): Records spectra in mass selective fashion

Experimental Methods

R2PI: Electronic spectrum

Biomolecule*

(S1)

Biomolecule+

+ e-

Biomolecule (S0)

UV-UV Hole-burning: Conformation specific electronic spectrum

Hole

-bu

rn

Pro

be

Conformer A Conformer B

Hole

-bu

rn

Pro

be

UV source (20 Hz) tuned UV hole-burn (10 Hz)

Laser Timin

g200 ns

Resonant ion dip infrared spectroscopy (RIDIRS)

UV Source fixed IR Source tuned

Laser Timin

g200 ns

Also used analogous laser induced fluorescence (LIF) methods.

A

C

A

ABC C

AAB BBB B

B B

Boltzmann distribution of conformers in the pre-expansion

Collisional cooling to zero-point vibrational levels

Laser(s)

B*

B*B*C

AB*

C

C

B A

AC

Computational Methods

• Build water clusters from optimized monomers (see monomer presentation, FD07)

• Place water in position such that it can form two H-bonds to crown oxygens

• Optimize with DFT B3LYP/6-31+G(d), ultrafine grid and tight convergence options using the GAUSSIAN03 suite of programs

• For uniquely optimized structures, perform frequency calculations

Results: ABC-(H2O)n=1,2 R2PI and UV-UV HB

• ABC-(H2O)1 – 1 conformation

• ABC-(H2O)2 – 1 conformation

32500

3

2

1

0

-1

-2

ion

in

ten

sity (

arb

itra

ry u

nits)

325003240032300322003210032000wavelength (nm, Scanmate)

*

32000 32100 32200 32300 32400Frequency (cm-1)

Ion

in

ten

sity

/dep

leti

on

(arb

. u

nit

s)

R2PI taken in ABC-(H2O)1 mass channel

ABC-(H2O)1

ABC-(H2O)2

O

O

O

O O

H2N

(H2O)n

• Vertical dashed lines show monomer origins, 4th of which is ~1200 cm-1 blue of these (see FD07)

Results: ABC-(H2O)n=1,2 RIDIRS in NH and OH stretch regions

• ABC-(H2O)1 – NH2 stretches most similar to ABC-C monomer (see FD07)

• Both OH groups in H-bonds to crown oxygens and observe 3 AS OH stretches!

Ion

dep

letio

n(ar

bitr

ary

units

)

36603640362036003580356035403520350034803460344034203400Frequency (cm

-1)

OH2 asymmetic stretchOH2 ASIo

n d

ep

leti

on

(arb

. u

nit

s)

3400 3460 3520 3580 3640Frequency (cm-1)

ABC-(H2O)1

ABC-(H2O)2

O

O

O

O O

H2N

(H2O)n

-NH2 symmetric stretch

(3401.9)

-NH2 anti-symmetric stretch

(3484.3)H2O

symmetric stretch(3568.7)

H2O asymmetric stretch

(3633.2, 3634.7, 3636.6)

Results: ABC-(H2O)1 RIDIRS: OH stretches

• No obvious shoulders on SS stretch band

• Could multiple conformations with overlapping origins and SS OH stretch frequencies be contributing to triplet?

Ion

de

ple

tion

(arb

itra

ry u

nits

)

3650364036303620361036003590358035703560Frequency (cm

-1)

OH2 asymmetic stretchOH2 AS

3560 3590 3620 3650Frequency (cm-1)

Ion

dep

leti

on

(arb

. u

nit

s)

O

O

O

O O

H2N

(H2O)n

3633.2

3634.7

3636.63568.7

• dimethoxy benzene – only a single H-bonded OH and only an AS OH stretch singlets

1.0

0.8

0.6

0.4

0.2

Ion

In

ten

sity

(a

rbitr

ary

un

its)

374037203700368036603640362036003580Frequency (cm

-1)

3585.9

3724.8

OCH3H3CO

• IR-UV HB attributes triplet to a single conformation

• Could triplet be due to water motion (e.g. rotation) or a tunneling splitting?

Results: ABC-(H2O)1 IR-UV HB on AS OH stretch bands

ion

inte

nsity

32.10x103

32.0832.0632.0432.0232.00undoubled wavelength (nm)

32000 32020 32040 32060 32080 32100Frequency (cm-1)

Ion

in

ten

sity

/dep

leti

on

(arb

. u

nit

s)

R2PI in ABC-(H2O)1 mass channel

IR-UV HB (3633.2)

IR-UV HB (3634.7)

IR-UV HB (3636.6)

Io

n d

ep

letio

n(a

rb

itra

ry u

nits)

36443640363636323628Frequency (cm

-1)

OH2 asymmetic stretchOH2 AS

Io

n d

ep

letio

n(a

rb

itra

ry u

nits)

36443640363636323628Frequency (cm

-1)

OH2 asymmetic stretchOH2 AS

Io

n d

ep

letio

n(a

rb

itra

ry u

nits)

36443640363636323628Frequency (cm

-1)

OH2 asymmetic stretchOH2 AS

Results: ABC-(HDO)1 RIDIRS in OH and OD stretch regions

-0.6

-0.4

-0.2

0.0

Ion

In

ten

sity (

arb

itra

ry u

nits)

36403620360035803560Frequency (cm

-1)

3583.1 3619.7

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Ion

In

ten

sity (

arb

itra

ry u

nits)

27002680266026402620Frequency (cm

-1)

2633.8 2660.7

2620 2640 2660 2680 2700 3560 3580 3600 3620 3640Frequency (cm-1)

Ion

dep

leti

on

(arb

. u

nit

s)

2633.8

2660.7

3583.1

3619.7

* *

• 2 OH and 2 OD stretches

• No triplet!

• Asterisks label ABC-(H2O)1 bands -1.0

-0.8

-0.6

-0.4

-0.2

0.0

Ion

In

ten

sity

(a

rbitr

ary

un

its)

32.10x103

32.0932.0832.0732.0632.0532.0432.0332.0232.0132.0031.99Frequency (cm

-1)

• IR-UV HB on 3583.1 (green) and 3619.7 (blue) demonstrated the UV-spectra are too closely overlapped to obtain clean conformer specific RIDIR spectra

Laser Timing

3.5 μs 200 ns

UV Source fixed (10 Hz) Provides selectivity IR2 tuned (10 Hz) IR1 fixed, hole-burn laser (5 Hz)

Conformer A Conformer B

B (S0)

B* (S1)

A (S0)

A* (S1)

In ABC-HDO, two OH and two OD stretches were seen. IR-UV hole-burning indicated that this was due to two different ABC-HDO species, IR-RIDIRS confirmed this and showed which pairs belonged together.

IR-IR-UV Hole-burning: Conformation specific IR spectrum when electronic spectra overlap

Results: ABC-(HDO)1: IR-IR-UV HB, 2 conformations

2620 2640 2660 2680 27002600Frequency (cm-1)

1.5

1.0

0.5

0.0

-0.5

Ion

In

ten

sity (

arb

itra

ry u

nits)

270026802660264026202600Frequency (cm

-1)

Ion

dep

leti

on

(arb

. u

nit

s)

2633.8

2660.71.5

1.0

0.5

0.0

-0.5Io

n I

nte

nsity (

arb

itra

ry u

nits)

36403620360035803560Frequency (cm

-1)

3560 3580 3600 3620 3640

3583.1

3619.7

• IR-IR-UV HB proves OD and OH stretches are due to two conformations

• H-bonds of unequal strength – A: OD stronger, OH weaker

B: OH stronger, OD weaker

• Raises possibility that triplet in AS OH stretch could be do to multiple conformations – need to do IR-IR-UV HB to check

ABC-(HDO)-B

ABC-(HDO)-B

ABC-(HDO)-A ABC-(HDO)-

A

Results: B15C-(H2O)1: UV-UV HB, 2 conformations

• Unlike ABC-H2O, UV-UV HB resolves 2 B15C-(H2O)1 conformations

• For more on B15C monomer, see FD07

-3

-2

-1

0

1

36.035.935.835.7x10

3

35800 35900 3600035700Frequency (cm-1)

B15C-(H2O)1-A

B15C-(H2O)1-B

B15C-A

B15C-B

Flu

ore

scen

ce d

ep

leti

on

(arb

. u

nit

s)

O

O

O

O

O

(H2O)n

Results: B15C-(H2O)1: RIDIRS in OH stretch region

• Observe a doublet in B15C-(H2O)1-A AS OH stretch

• Singlet in B15C-(H2O)1-B AS OH stretch

• Why doublet? Same possibilities as for ABC-(H2O)1 AS OH stretch triplet

fra

ctio

na

l de

ple

tion

3700365036003550photon energy (cm

-1) power scan

3600 3650 37003550Frequency (cm-1)

B15C-(H2O)1-A

B15C-(H2O)1-B

Flu

ore

scen

ce d

ep

leti

on

(arb

. u

nit

s)

3568.53635.0

3638.1

3583.9

3639.7

Results: RIDIRS in alkyl CH stretch region

• B15C-(H2O)1-A and ABC-(H2O)1 have almost identical alkyl CH stretch spectra

• B15C-(H2O)1-B is also very similar

• Data is evidence that all three share the same crown conformation but differing in water-binding site

fluor

esce

nce/

ion

dip

2950290028502800frequency (cm

-1)

2850 2900 29502800Frequency (cm-1)

B15C-(H2O)1-A

B15C-(H2O)1-B

Flu

ore

scen

ce/i

on

dep

leti

on

(a

rb. u

nit

s)

ABC-(H2O)1

fluor

esce

nce/

ion

dip

2950290028502800frequency (cm

-1)

Results: RIDIRS in alkyl CH stretch region• Furthermore, the crown conformation for the water clusters may be different from those seen in monomer (see FD07).

• Assumes water does not significantly perturb CH stretch frequencies.

• However, in tryptamine with H2O bound to -NH2 group, alkyl CH stretches were perturbed.

• Would offer insight into water binding site

• If crown conformation of water clusters is different from monomer, demonstrates crown is flexible and adjusts to accommodate the water

2850 2900 29502800Frequency (cm-1)

B15C-(H2O)1-A

B15C-(H2O)1-B

Flu

ore

scen

ce/i

on

dep

leti

on

(a

rb. u

nit

s)

ABC-(H2O)1

B15C-C

B15C-B

B15C-A

Conclusions

• ABC-(H2O)1 : 1 conformer; B15C-(H2O)1: 2 conformers

• Crown conformation same in all three water clusters (alkyl CH stretch)

• Triplet/doublet observed in AS OH stretch of ABC-(H2O)1/B15C-(H2O)1

• In ABC-(H2O)1, disappears upon substitution with HDO

• Two conformers associated with two different strength H-bonds

• Tunneling splitting? – observed intensity ratio is reverse of that predicted by spin statistics

• Water motion, rotation?

• Multiple overlapped conformations?Future Work• Perform IR-IR-UV HB on AS OH stretch peaks (for both ABC and B15C-A)

• Measure water binding energies

• Study higher order water clusters

Acknowledgements

• Zwier group:Prof. Timothy S. Zwier

Jasper R. ClarksonEsteban E. BaqueroTracy LegreveNathan PillsburyJosh NewbyWilliam H. James IIIChirantha RodrigoChing Ping LiuChristian MüllerJosh Sebree

• Funding:

National Science Foundation

• Computing Resources

Information and Technology at Purdue (ITaP), Rosenbaum Computing Center (RCC)