1. IR spectra of Methanol Clusters (CH3OH)n Studied by IR Depletion and VUV Ionization Technique with TOF Mass Spectrometer
Department of Applied Chemistry
National Chiao Tung University
Taiwan
Hui-Ling Han and Yuan-Pern Lee
Good afternoon everyone, my name is Hui-Ling Han from national chiao tung university, Taiwan.
Today, I’m going to give a presentation on IR spectra of methanol clusters studied by IR depletion and VUV ionization technique with TOF mass spectrometer.

2. Outline 1. Introduction Importance Previous work on methanol clusters
2. Experimental setup
VUV ionization and IR depletion technique
3. Results and discussion TOF spectra and action spectra IR spectra of methanol monomer and clusters
4. Summary and future work
First, I will give a brief introduction on methanol clusters. Then, I will describe the experimental setup for vuv ionization and IR depletion technique.
Following, I will show some TOF spectra and the action spectra of methanol clusters when IR laser is employed. Finally, I will give a summary about this study

3. Significance of Methanol Cluster
(H2O)n
two OH bonds
(CH3OH)n
one OH bond
n=2
n=3
3. Hydrogen-bonded clusters has been the subject of intense studies in the past decade. Water and methanol molecules received the greatest amount of attention.
In contrast to water which has two OH bonds and each water molecule in its cluster can form up to four hydrogen bonds, methanol has only one OH bond and each methanol molecule in its cluster can only form up to two hydrogen bonds. The bulky methyl group and the dipole it produces result in a more complicated and asymmetrical clusters compared with water.
n=4
n=5
F. N. Keutsch and R. J. Saykally, PANS, (2001)
U. Buck, J. G.Siebers, R. J. Wheatley, J. Chem. Phys. 108, 20 (1998)

4. momentum transfer method in a scattering experiment
Previous Work on Methanol Clusters ─ OH region
IR spectra shows severe overlap for the size larger than trimer
IR predissociation spectra
FTIR
Häber et al., Phys. Chem. Chem. Phys. 1, 5573 (1999)
jet‐cooled
methanol (0.1 % in He)
Provencal et al., J. Phys. Chem. 110, 4258 (1999)
IR-CRLAS
Huisken’s and Buck’s groups
momentum transfer method in a scattering experiment
4. Numerous experiments on vibrational spectra of methanol clusters in the gaseous phase were performed to derive information about the structures and dynamics of intermolecular interactions.
Most spectra investigation on methanol clusters was done in the OH stretching region. Two nice reviews maybe found in the chem.review and adv chem phys.
Several groups studied jet-cooled (CH3OH)n by direct IR absorption method, such as FTIR, cavity-ring down. However, clusters are typically generated with a distribution of sizes, spectra of larger clusters typically suffer from severe overlap among various clusters as can be seen here.
Huisken’s and Buck’s groups they employed the momentum transfer method in a scattering experiment to select a neutral cluster of specific size. In combination with IR predissociation they obtained the IR spectra of size selected cluster in OH stretching region.
J. Chem. Phys. 108, 33 (1998)
J. Chem. Phys. 95, 3924 (1991)
Reviews : M. A. Suhm, Chem. Rev. 100, 3863 (2000)
M. A. Suhm, Adv. Chem. Phys. 142, 1(2009)

5. Previous Work on Methanol Clusters ─ CH stretching region
Infrared plus vacuum ultraviolet spectroscopy of neutral and ionic methanol monomer and clusters
monitor
They reported the variation in intensity of (CH3OH)nH+ generated by VUV laser ionization upon IR irradiation at various wavenumber.
(CH3OH)+
(CH3OH)H+
(CH3OH)2H+
5. In contrast to the OH region, the CH stretching region is little studied partly because of severe overlap.
Bernstein group used IR laser predissociation plus vuv ionization to study methanol clusters. They mentioned measured the variation in intensities of (CH3OH)nH+ generated by VUV laser ionization of methanol clusters and reported these as the IR spectra of clusters. (point out the spectra) For trimer, the spectrum shows positive and negative bands, and for clusters larger than tetramer, the spectra look similar. Our laboratory is trying to setting up an IR-VUV-TOF system to have the mass selectivity so that the IR spectrum may be free from interferences from precursors or other products. We have chosen methanol clusters as our first target for investigation.
(CH3OH)3H+
(CH3OH)4H+
(CH3OH)5H+
(CH3OH)6H+
(CH3OH)7H+
Hu et al., J. Chem. Phys. 125, (2006).

6. Experiments Cluster formation Cluster detection
time
TOF spectrum
signal
time
TOF spectrum
signal
Cluster formation
① Methanol clusters were formed through supersonic expansion ⑤
Ion detector
Cluster detection
② Clusters were ionized by VUV at 118 nm without fragmentation.
③ Cluster ions were detected by TOF MS.
TOF MS ③
6. Now, let me explain the experimental technique. Methanol clusters were formed through supersonic expansion. The size distribution may be controlled by stagnation pressure, nozzle size and concentration. After passing through a skimmer, the jet-cooled beam was introduced into the ionization region of the TOF mass spectrometer.
Then, neutral clusters are softly ionized by the 118 nm vuv light. The ions are extracted to the flight tube and their arrival time depends on their mass. Ions were detected with a MCP detector. As shown in this TOF spectra, different size of methanol clusters were detected at different arrival time.
For IR dissociation exp, we introduce the tunable IR light several hundred ns prior to the vuv light pulse. If the cluster absorbs this IR light, it might undergoes dissociation, so as to change the size distribution of clusters, hence changing the relative intensities of the TOF peaks. (By scanning the IR frequency, and monitoring the changes of ion intensity at observed mass channel simultaneously, we can obtain action spectra. )
Cluster ions
IR dissociation of clusters
④ Variation of intensity of each m/z peak ①
skimmer
0 V ④
1500 V
Action spectra of clusters
⑤ Scan IR frequency while monitoring all m/z peaks simultaneously
Pulsed valve
1700 V ②
(CH3OH)n IR
VUV

7. Results (CH3OH)n+1 + 10.5 eV → (CH3OH)nH+ + CH3O +e‐ (n≥2)
(CH3OH) eV → (CH3OH)H+ + CH3O +e‐
→ (CH3OH)2+
CH3OH+IR eV → CH3OH+ (IP=10.8 eV)
Results
(CH3OH)n eV → (CH3OH)nH+ + CH3O +e‐ (n≥2)
Time‐of‐Flight Mass Spectra
CH3OH eV → CH3OH+
7. Here we show a typical TOF mass spectrum of methanol clusters induced by 118 nm VUV ionization. The X axis is the flight time; and the y axis is the ion intensity. Here shows mass peaks of protonated methanol clusters with n = 1-5, which are from ionized clusters. The ionized molecule further dissociate one methoxy group through rapid proton-transfer reaction to generate a protonated cluster ion with one less carbon. For example, ionization of the tetramer produces protonated trimer ion. Here we also observed a peak of dimeric ion because the direct ionization of dimer also takes place. Methanol monomer ion is not observed here, because the ionization energy of methanol monomer 10.8 eV is higher than our ionization energy of 10.5 eV.
In our experiments upon ionization at 118 nm, we found no evidence for further fragmentation of protonated cluster ions; so the detected distribution of protonated cluster ions represents the distribution of neutral clusters.

8. Results Time‐of‐Flight Mass Spectra CH excitation
(CH3OH)n eV → (CH3OH)nH+ + CH3O +e‐ (n≥2)
Time‐of‐Flight Mass Spectra
CH3OH+
IR+VUV ionization
8. The middle trace shows the TOF spectrum observed upon IR irradiation at 2950 cm-1.All clusters are expected to absorb infrared light at this frequency, so we observed decreased intensities of all mass peaks, whereas signals due to (CH3OH)+ appear because of the (1+1) IR+VUV ionization of methanol monomer.
The bottom trace shows the TOF spectrum observed with IR irradiation at 3150 cm-1. Only clusters larger than tetramer absorb this infrared light, therefore, only peaks of protonated trimer to pentamer decreases in intensity significantly.
CH excitation
OH excitation for n ≥ 4

9. Mechanism of Ionization and Dissociation
(CH3OH)+
(CH3OH)H+
(CH3OH)2+
(CH3OH)3H+
(CH3OH)2H+
IR+ VUV
VUV
VUV
VUV
VUV
9. To summarize this part, we use this diagram to explain the mechanism of ionization and dissociation.
The bottom part shows the clusters of different sizes that are present in the supersonic jet, all clusters are ionized with 118 nm VUV light to form protonated cluster ions, except monomer which needs an extra IR photon to be ionized .
When we introduce the IR irradiation, the dissociation of clusters changes the distribution of cluster, therefore changes the intensity distribution of protonated clusters.
And the change in intensity of each protonated cluster depends on the decomposition and production (point at two arrows) processes of each neutral cluster.
(CH3OH)4
(CH3OH)3
(CH3OH)2
(CH3OH) IR IR IR

10. TOF and Action Spectra
Monitoring percentage variation in intensity of all mass channel while scan the IR frequency to obtain the action spectra
TOF spectrum
Action spectra
These are the action spectra recorded for each mass peak by monitoring percentage variation in intensity while scanning the IR frequency. For protonated tetramer and trimer, only loss was obtained, whereas for protonated dimer and monomer, gain or loss was observed depending on the wavenumber.

11. Analysis of the Action Spectra
Assumptions:
The cluster size only decreases by 1 during IR dissociation.
No multi-photon dissociation
small IR cross section (~10-19 cm2/molecule)
3. The PIE for formation of (CH3OH)nH+ (n=2-5) at 10.5 eV, set to be 1. Set correction factors for formation of (CH3OH)H+ and (CH3OH)2+ due to smaller PIE, 0.4 and 0.055, respectively.
Now we want to process the action spectra to yield actual absorption spectra. In our model, we are assuming that
The cluster size only decreases by 1 during IR dissociation. This is because dissociation of one unit of CH3OH takes about XX kJmol-1, and the IR photon in this region is about XXkJmol-1.
There is no multiphoton dissociation, because the IR cross section is small and we are using a laser energy of 5mJ, with a fluence of 1mJ/mm2.
The PIE for formation of protonated cluster ions at 10.5 eV is set to be 1, except for formation of protonated monomeric and dimeric ion, which are 0.4 and according to literature values.
For example, the negative action spectra of the highest clusters ion, protonated tetramer ion, gives the absorption spectrum of pentamer, because there is only loss mechanism for this cluster. The action spectra of protonated trimer increase is due to absorption of pentamer to produce neutral tetramer, and loss is due to absorption of tetramer leading to predissociation.
A : absorption spectrum  : action spectrum (% change)
-(Δ[(CH3OH)4H+]) = A[(CH3OH)5]
-(Δ[(CH3OH)3H+]) = A[(CH3OH)5]-A[(CH3OH)4]
Kostko et al., J. Phys. Chem. A, Vol. 112, No. 39, 2008

12. IR Spectra of (CH3OH)n n =1−5 cooperative effect ↑ C −H C −H O −H O −HPD PD PA PA
3576
3675
12. After going through this process, we obtained IR spectra of methanol clusters. In separate experiments with different cluster distribution, we also obtained similar IR absorption spectra, supporting the feasibility of our model. The spectra in the O-H-stretching region agree with previous experimental reports. (Perhaps it is worthwhile to prepare a slide for comparison as a spare slide) For monomer, OH stretching is observed at 3685 cm-1. The dimer shows two bands near 3675 and 3576 cm1, corresponding to proton acceptor (PA) and proton donor (PD), respectively, indicating a chain structure. From trimer to pentamer, the absence of absorption of the free O-H-stretching near 3670 cm1 is consistent with previous experimental and computational results that indicate the most stable structures to be cyclic. Furthermore, the increased red shifts of O-H-stretching fundamentals as n increases indicate that the cooperative effect is enhanced as the size of the cluster increases.
Then, I would like to discuss more on CH stretching region.
3685

13. IR Spectra of (CH3OH)n in CH stretching region
13. In the C−H region, each absorption peak of CH3OH splits into two components in the dimer spectrum, indicating a chain structure with PD and PA. However, for (CH3OH)3, the splitting diminishes and the band become narrower, indicating a preferred cyclic structure, as predicted by theory. For cluster larger than tetramer, the spectral pattern remains similar to (CH3OH)3 but the band widths becomes greater, supporting that the most stable structure is cyclic.
Although the CH stretching mode are not directly involved in the hydrogen bonding of the cluster, the fact that absorption in the CH region also results to dissociation of clusters indicates that energy redistribution is rapid.

14. Experiment Calculation By H. Witek’s group
ANO1-DFT-B3LYP-VPT2
Fundamental: ν2, ν9, ν3
Combination: ν4+ν10, ν4+ν5, ν5+ν10,
Oventone: 2ν4, 2ν10,
a quartic force field calculation is performed in order to compute the fundamental frequencies with second order vibrational perturbation theory (VPT2)
Our collaborator help us calculate harmonic and anharmonic frequencies of methanol monomer, dimer and trimer. As can be seen in the right panel, fundamental frequencies with relative intensities are shown as tall dark lines, and combinational modes without intensity information are shown as short, light lines. the splitting due to PA and PD of dimer and the quenching of the splitting are clearly seen, Our experimental results are consistent with this general picture.

15. Summary
1. The IR-VUV-TOF system has been successfully implemented.
2. IR Spectra of (CH3OH)n (n≤5) have been determined.
3. Spectra support that (CH3OH)n (n≥3) are cyclic and (CH3OH)2 has a chain structure.
4. The C-H excitation also leads to photodissociation, indicating rapid energy redistribution.
Tunable VUV laser light is going to be implemented to study the IR spectra of free radicals. Ex: CH3S, IP= 9.26 eV
Future work
15. In summary, the IR-VUV-TOF system has been successfully implemented, the mass selectivity enable us to observe spectra of methanol clusters of various size that overlap with each other. And, IR Spectra of (CH3OH)n (n≤5) have been determined. Observed Spectra support that (CH3OH)n (n≥3) are cyclic and (CH3OH)2 has a chain structure. The C-H excitation also leads to photodissociation, indicating rapid energy redistribution in these clusters.

16. Thank C. Y. Ng’s group in UC Davis Thanks for your attention.
Acknowledgment
Taiwan
Thank C. Y. Ng’s group in UC Davis
Thanks for your attention.

17. PIE curves for protonated methanol monomer and methanol clusters ((CH3OH)nH+) for size n=2-6
M = methanol (CH3OH)
Kostko et al., J. Phys. Chem. A, Vol. 112, No. 39, 2008

18. Vibrations of methanol dimer ν1 ν2 ν9 ν3PA PD