O Hydrogen Bonded Methacrolein Dimer upon Non-Resonant Multi

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Barrierless Proton Transfer in the Weak C-H···O Hydrogen Bonded Methacrolein Dimer upon NonResonant Multi-Photon Ionization in the Gas Phase Piyali Chatterjee, Arup K. Ghosh, Monoj Samanta, and Tapas Chakraborty J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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The Journal of Physical Chemistry

Barrierless Proton Transfer in the Weak C-H···O Hydrogen Bonded Methacrolein Dimer upon NonResonant Multi-Photon Ionization in the Gas Phase Piyali Chatterjee1, Arup K. Ghosh2, Monoj Samanta1, and Tapas Chakraborty1*

1

Department of Physical Chemistry, Indian Association for the Cultivation of Science, 2A Raja S C Mullick Road, Jadavpur, Kolkata 700032, India 2

Department of Chemistry, Dharmsinh Desai University, Nadiad-387001, Gujarat, India

*Corresponding Author: Email: [email protected] Tel: +91 33 2473 4971 (ext 1470) Fax: +91 33 2473 2805

1 Environment ACS Paragon Plus

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Intermolecular proton transfer (IMPT) in a C-H···O hydrogen bonded dimer of an α,βunsaturated aldehyde, methacrolein (MC), upon non-resonant multi-photon ionization by 532 nm laser pulses (10 ns), has been investigated using time-of-flight (TOF) mass spectrometry under supersonic cooling condition. The mass peaks corresponding to both the protonated molecular ion [(MC)H+] and intact dimer cation [(MC)2].+ show up in the mass spectra, and the peak intensity of the former increases proportionately with the latter with betterment of the jet cooling conditions. The observations indicate that [(MC)2].+ is the likely precursor of (MC)H+ and, based on electronic structure calculations, IMPT in the dimer cation has been shown to be the key reaction for formation of the latter. Laser power dependences of ion yields indicate that at this wavelength the dimer is photoionized by means of 4-photon absorption process, and the total 4photon energy is nearly the same as the predicted vertical ionization energy of the dimer. Electronic structure calculations reveal that, the optimized structures of [(MC)2].+ correspond to a proton transferred configuration wherein the aldehydic hydrogen is completely shifted to the carbonyl oxygen of the neighboring moiety. Potential energy scans along the C-H···O coordinate also show that the IMPT in [(MC)2].+ is a barrierless process.

I. INTRODUCTION Proton transfer (PT) reaction is ubiquitous in chemistry, and spans from the very basic acidbase laboratory reactions to complex chemistry that controls biological processes as well as the reactions occurring in the earth's atmosphere.1-11 In life processes PT plays crucial role in


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different enzymatic reactions, photoactive proteins, ion transport and also in giving rise to photostability of nucleic acids.5-7,

12-15

In atmospheric reactions occurring in aerosols, on

water/ice surfaces and in nitrous acid formation from hydrated nitrosonium ion, PT plays key role.8,11 A common feature of all such PT reaction is that, it occurs in networks of strong hydrogen bonds. PT reactions induced by stimuli like electronic excitation or ionization of any of the donor-acceptor partners have also been investigated over the past several decades.16-24 Studies in microsolvated clusters have provided information regarding the minimum cluster size essential for occurrence of PT.21-29 For example, studies with 1-napthol-ammonia clusters have shown that at least 8 ammonia molecules are required for PT to occur in the ground electronic state (S0), whereas in electronically excited S1 state, PT takes place in clusters containing only 5 ammonia molecules.22-25 Likewise, a large number of studies have also been devoted to investigate PT induced by ionization in hydrogen bonded clusters of protic molecules, e.g., water, ammonia, alcohols, phenols, amines, aliphatic acids, etc., in the gas phase 26-28, 30-45 Recent studies, on the other hand, have demonstrated that intermolecular proton transfer (IMPT) in small molecular complexes can occur involving donors without having an active X-H group (where X = O, N etc.), viz., aldehydes and ketones, upon photoionization in the gas phase.18, 46-50 However, to explain the PT process in such cases, a tautomeric conversion from the keto to enol form was considered, and this assumption was corroborated by the fact that upon ionization of the concerned ketones the tautomeric preferences are altered, and PT eventually takes place in the network of strongly bound conventional hydrogen bonds. For example, Fujii and co-workers probed IMPT in a vacuum ultraviolet (VUV) photoionized dimeric complex of acetone, and by means of infrared spectroscopy the authors unambiguously demonstrated that the process occurs in an O-H···O hydrogen bonded linkage of the dimer cation.47 In one of our



previous studies we have shown occurrence of IMPT in the photoionized jet-cooled dimeric complex of a small cyclic ketone, cyclopentanone.50 In that case, the enol form was found to be more stable in the cationic ground state, and the energy barrier for keto to enol conversion was shown to reduce drastically in the hydrogen bonded dimeric adduct. Recently, an almost barrierless IMPT was claimed to be occurring in the dimeric complex of acetaldehyde, upon 1photon VUV ionization, and the same was corroborated also by means of electronic structure theory methods.49 However, PT occurring directly in a C-H···O hydrogen bonded network is very rare, and the only available study that has been reported recently by Signorell and co-workers is within a dimeric complex of dimethyl ether, and IMPT in the complex was shown to be occurring upon 1-photon VUV ionization.51 We report in the present paper, a new molecular prototype and an alternative excitation scheme for the demonstration of barrierless IMPT in a C-H···O hydrogen bonded dimeric complex. The molecule is an α,β-unsaturated aldehyde (Figure 1), methacrolein (MC), and the ionization scheme is simple non-resonant multi-photon ionization.

Figure 1. Optimized structures of the homodimer of methacrolein (MC) in neutral (S0) and cationic (D0) ground states obtained by performing calculations at M06-2X/6-311++G(3df,3pd)


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level. Bond lengths are given in Å units. The optimized structures indicate IMPT on ionization occurs without encountering any energy barrier. MC is an atmospherically important volatile organic compound, whose primary atmospheric source is tropospheric oxidation of isoprene, the most abundant nonmethane hydrocarbon released into the troposphere as biogenic emission.52 Isoprene is oxidized to MC by the atmospheric oxidants like OH radical, NO3 radical and O3.53-55 MC is oxidized further by these oxidants, and in turn plays a very significant role in tropospheric ozone production by getting involved into the atmospheric nitrogen oxides (NOx) reaction cycle. Many studies, therefore, have been devoted in the recent years to the investigation of gas phase oxidation of MC.56-61 Recently our group has reported the fragmentation behavior of this molecule upon collisions with low-energy electrons.62 However, to the best of our knowledge, no photoionization study of this molecule is available. In this paper, the focus is on multi-photon ionization of the C-H···O hydrogen bonded dimeric complex of the molecule, and low-energy pathways for fragmentation of the dimer cation.

II. EXPERIMENTAL AND THEORETICAL METHODS A. Experiment MC (95%) was purchased from Sigma-Aldrich, and purified further by vacuum distillation before use. The dimer and higher complexes of MC were generated by means of supersonic jet expansion method. Following laser ionization, the mass spectra of the fragment ions were recorded using a homebuilt time-of-flight (TOF) mass spectrometer, and details of the apparatus has been described earlier.50, 63



Briefly, MC was taken in a small perforated glass capsule behind the nozzle valve and heated to a temperature of 80 oC to increase its vapor pressure, and the carrier gas argon of purity 99.999% was passed through this heated capsule for uniform mixing of MC vapor into argon. The gas mixture was expanded through the pulsed nozzle valve (General Valve Corporation series-9) of orifice diameter 250 µm into the expansion chamber of the TOF mass spectrometer. The background pressure of this chamber was 2x10-6 mbar. The size distribution of the clusters formed as a result of supersonic expansion was controlled by varying the total backing pressure of the nozzle, from 100 mbar to 1 bar, and also by varying the sample concentrations in the expansion gas mixture by changing the temperature of the heated capsule. The central portion of the gas pulse of 10 Hz repetition rate and 200 µs pulse width was skimmed by a circular skimmer of 2 mm diameter placed at the entrance of ionization chamber. The skimmed jet was photoionized by second (532 nm) and third (355 nm) harmonic wavelengths of a pulsed (10 ns, 10 Hz) Q-switched Nd:YAG laser (Spectra Physics, Quanta Ray Lab-150 series). A master trigger generator was used to trigger the nozzle driver and also to fire the laser, and the latter was delayed from the former by ~600 µs using a digital delay generator. The laser beam was focused at the center of the skimmed jet by a quartz lens of focal length 25 cm at a distance of 15 cm from the nozzle orifice, in between the backing and extractor plates. The ions generated by photoionization were accelerated into the flight tube by the electric fields applied to the three parallel plates of the mass spectrometer of Wiley–McLaren design, backing, extractor and accelerating plates, to which the applied voltages were 3300, 2100 and 0 V/cm, respectively. To maintain an ultraclean environment in the ionization region of the mass spectrometer, the ionization chamber was externally cooled by liquid nitrogen. A gate valve was used to separate the ionization chamber from detector, and working pressure maintained in this


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region was less than 2x10-6 mbar. The ions were spatially separated according to their m/z values in the 1.05 m long field free flight tube, and detected by a microchannel plate (MCP) ion detector (Advance Performance Detector 2 APTOF, Photonis). The collection diameter of the plate was 25 mm and the detector gain was of the order of 107. The electric current from the detector was routed to a 600 MHz digital storage oscilloscope (Lecroy, Wavesurfer 62Xs) which displays the ion signal on the screen of its inbuilt computer after averaging over 1000 laser shots. The mass resolution of this TOF mass spectrometer was 300 at 100 Da. The UV absorption spectrum of the sample vapor was recorded using a homebuilt static gas cell of 1 m path length. Light from a Xe arc lamp (Newport, Oriel Instruments, 66901) was dispersed by a small monochromator (Optometrics, SDMC101), and the selected wavelengths were passed through the gas absorption cell using a telescopic collimating arrangement. Light intensity after traversing through the gas cell was measured with (I) and without (I0) sample vapor by a photomultiplier tube (IP-28). The output signal of the photomultiplier tube was amplified and acquired into a personal computer using a homemade data acquisition system. The spectrum was obtained by plotting the values of log(I0/I) against wavelengths (nm) of light.

B. Computation Geometry optimizations and single point energy calculations of the isomers of MC monomer and dimer in the neutral (S0) and ionic (D0) ground states were performed primarily using the density functional theory (DFT) method with the Becke three parameter hybrid functionals and correlation functional of Lee, Yang, and Parr (B3LYP functional) along with 6-311++G(d,p) basis set. However, for weakly bound molecular dimers, where dispersion interactions contribute significantly to binding energies, the DFT/B3LYP method is inadequate. Therefore, calculations



using M06-2X and ωB97X-D functional in conjunction with 6-311++G(3df,3pd) basis set were performed both for S0 and D0 states. The restricted and unrestricted options were employed for all structures with closed-shell (neutral) and open-shell (radical cation) electronic configurations, respectively. The basis set superposition errors (BSSE) in binding energies of the dimers were corrected using the counter-poise method of Boys and Bernardi.64 The vertical ionization energies (VIEs) were calculated by computing the single point energies for the ionized openshell doublet electron configurations of the neutral molecular geometries, while the adiabatic ionization energies (AIEs) were computed by performing further geometry optimizations. Potential energy scans (PESs) in the D0 state of the relatively more stable dimer isomers, starting from their Franck-Condon geometries, were performed at DFT/B3LYP/6-311++G(d,p) level. Natural bond orbital (NBO) analysis was performed at each step of PES to calculate partial atomic charges from which the charge distribution on each monomer moiety of the dimeric complexes was evaluated. Gaussian G09 suite of program was employed to perform all the calculations presented here.65

III. RESULTS AND DISCUSSION 1. Multi-photon ionization mass spectra of jet-cooled MC: The TOF mass spectrum of MC vapor upon non-resonant multi-photon ionization by the 2nd harmonic (532 nm) pulses (10 ns) of a Q-switched Nd:YAG laser, in a supersonic jet expansion, is shown in Figure 2.


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λMPI = 532 nm

379 nm (S0 → S1 )

Absorption [log (I 0 /I)]

0.20

0.15

0.10

0.05

0.00 300

0

10

20

30

40

350

400

50

60

70

450

500

550

600

Wa velength (nm)

[(MC)2]•+

[(MC)-H]+ (MC)•+ (MC)H +

C3H3+ + CH 3CO + C3H 5

CH 3+

C2H3+ HCO +

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

90

100

110

120

130

140

150

m/z (amu)

Figure 2. TOF mass spectrum of MC upon non-resonant multi-photon ionization by the pulsed (10 ns) 532 nm laser of energy 7 mJ/pulse. The ionization was performed by cooling MC in a seeded supersonic jet expansion of argon. Total backing pressure of jet expansion was ~200 mbar, which was enough to detect MC dimer signal at an acceptable signal to noise ratio. The inset shows the UV absorption spectrum of MC vapor to depict that ionization by 532 nm wavelength is indeed a non-resonant process. To avoid formation of higher clusters of MC in the supersonic jet expansion, the backing pressure of argon used was relatively low, ~200 mbar. Nevertheless, the cooling achieved under this condition is enough to see small but distinct mass peak for the dimer cation [(MC)2].+ in the photoionization mass spectrum. The laser pulse energy of the photoionization laser (532 nm) was also kept low, only 7 mJ/pulse, which is just enough to record the mass spectrum with a satisfactory signal to noise ratio. The electronic absorption spectrum of MC vapor is presented in



the inset of Figure 2, and it looks identical to the one reported earlier by Ravishankara and coworkers.57 The spectrum indicates clearly that the electronic absorption occurs only in the UV region. Furthermore, the compound is a colorless liquid, which also indicates that it does not have absorption in the visible range of the electromagnetic spectrum. Therefore, the mass spectrum depicted here for use of 532 nm wavelength must be the result of non-resonant multiphoton ionization of MC and its clusters in the supersonic jet expansion. The most notable feature of the mass spectrum is appearance of an intense triplet type peak structure around the molecular ion mass, with m/z values 69, 70 and 71, and these can be corresponded to H-atom loss molecular ion [(MC)-H]+, intact molecular ion (MC).+ and protonated molecular ion (MC)H+, respectively. The mass peak corresponding to the cation of the homodimer appears weakly at m/z = 140. In the following sections we offer experimental evidences and calculated data to suggest that (MC)H+ could be produced only via an IMPT reaction within the dimeric cation [(MC)2].+. We offer discussion about the mechanism for production of low-energy [(MC)2].+ ion by means of non-resonant multi-photon ionization with 532 nm laser.

2. [(MC)2].+, the precursor of (MC)H+: To verify that the dimer cation is the most likely precursor of the protonated MC, the TOF mass spectra were recorded by changing the expansion condition of the supersonic jet while keeping the same laser ionization condition. In Figure 3, three photoionization mass spectra, measured for three different backing pressures of the argon carrier gas are displayed. The temperature of the nozzle and sample compartment behind the nozzle was kept the same, 80 oC, and the pulse energy of the photoionization laser (532 nm), 7 mJ/pulse, was the same in all the three cases.


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[(MC)2 ]• +

Stagnation Pressure = 300 mbar

[(MC)2 ]• +


[(MC)-H]+ (MC)H+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[(MC)-H]+ (MC)• + (MC)H+

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60

70

80

90

100

110

120

m/z (amu)

130

140

150

Figure 3. 532 nm multi-photon ionization mass spectra of jet-cooled MC with increase of the stagnation pressure from 100 to 500 mbar. The laser fluences were kept the same (7 mJ/pulse) for each backing pressure. The relative increase in (MC)H+ and [(MC)2].+ ions indicate that the latter is the precursor of the former. It is seen that at the minimum backing pressure (100 mbar), intensity of the (MC)H+ ion peak is the smallest and the dimer cation peak at m/z 140 does not appear in the mass spectrum. With increase in backing pressure, the (MC)H+ ion peak intensity is sharply increased with a



simultaneous increase of that for [(MC)2].+ ion. This observation led us to infer that [(MC)2].+ is the major precursor of (MC)H+ ion, and the theoretically calculated data in support of this proposal is presented in sections 5 and 6. The possibility that (MC)H+ could be produced also from larger cluster ions, [(MC)n].+ (n ˃ 2), cannot be completely ruled out, but in our recorded mass spectra the ion signals corresponding to higher cluster ions are almost invisible. In the lower two mass spectral traces, a small mass peak at m/z = 141 can be corresponded to the protonated dimer [(MC)2H+] ion, and its appearance indicates that the higher clusters are formed under the supersonic jet expansion condition used. The absence of higher cluster ion peaks in the mass spectra implies that the abundances of such clusters could be small, and the cluster cations could be more prone to fragmentation even under the same multi-photon ionization condition for having different extents of excess internal energies. However, the apparent possibility that (MC)H+ ion could be generated via direct protonmolecule collision in the ionization region of the mass spectrometer can be eliminated considering the fact that under the experimental condition, the mass spectrometer chamber was practically collision-free (as the working pressure in this chamber was in the lower range of ~106

mbar). It is worth mentioning that in one of our previous studies concerning low-energy

electron ionization of MC in an effusive beam, a condition of very soft ionization, the (MC).+ ion peak appeared almost exclusively in the mass spectrum achieved by tuning the electron kinetic energy down to the level of the vertical ionization threshold of the molecule (9.92 eV).62 However, in that study, no (MC)H+ ion was detected under such condition, and that happened because MC dimer was absent under the effusive beam condition. Therefore, it is inferred that under the experimental condition used for the present study the (MC)H+ ion has to be formed from larger species via a dissociation process.


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3. Multi-photon ionization mechanism of (MC)2: A pertinent issue here is the mechanism by which the MC dimer is ionized in non-resonant multi-photon ionization and number of photons involved in the process. The known value of VIE of MC is 9.92 eV,66 but in the case of (MC)2 the experimentally measured value of VIE is not available. Although dimerization via hydrogen bonding in general have very pronounced effect in lowering of molecular ionization energies, but in the case of (MC)2, the hydrogen bond being a weaker variant, C-H···O type, the lowering in VIE is not expected to be large from that of the monomer. The electronic structure calculation data presented in Table S1 shows that the lowering of VIE due to dimerization of MC is only ~0.5 eV. Therefore, it could be likely that the dimer is ionized via 4-photon absorption process as the total 4-photon energy of 532 nm, 4x2.33 = 9.32 eV, is comparable with the calculated VIE. For experimental verification of the photon number involved in the ionization process, we have measured ion yields as a function of laser power, and the results presented below show that the observations are consistent with the prediction. In a non-resonant multi-photon ionization process, the ionization yield (W) could be expressed as, W = σIn, where, σ is the ionization cross-section, n is the number of photons involved and I is the laser intensity. The log-log plot of the ionization yield vs. laser intensity should be a straight line with a slope value equals to the number of photons involved in the ionization process. Such plots for the protonated molecular ion (MC)H+ and dimer cation [(MC)2].+, obtained by varying the laser intensity and keeping the other parameters same, are shown Figure 4.



2.8 +

m/z 71 (MC)H

2.4

Log [Ion Signal]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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.+

m/z 140 [(MC)2]

2.0 1.6 1.2

W =σ In

0.8

log W = log σ + n log I

0.4 -1.4

-1.3

-1.2

-1.1 -1.0 -0.9 Log [Laser Fluence]

-0.8

-0.7

Figure 4. Logarithmic plots for the variation of yields of (MC)H+ and [(MC)2].+ ions with increasing laser pulse energies of 532 nm excitation wavelength. It is seen that the plots are satisfactorily linear in the low pulse energy region, but deviations from linearity occurs at higher pulse energies due to saturation effects. In the linear regions, estimated values of the slopes for both the ions are nearly the same, ~4. Therefore, it appears that a 4-photon process is primarily responsible for ionization of the dimer. It is already shown in the previous section that the dimer cation is the precursor of (MC)H+, and the mechanism suggested for the latter to be produced from the former is an IMPT followed by dissociation. According to the discussion presented above, 4-photon absorption can excite the dimer only up to its ionization threshold, and the excess energy available for vibrational excitation is likely to be very small. We show below that the stability of the dimer cation is extremely sensitive to excess internal energy with which it is produced upon multi-photon ionization. A mass spectral trace recorded upon multi-photon ionization by 355 nm laser pulses of the jetcooled molecules, keeping the expansion condition the same as before, is shown in Figure 5.


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2.0 m/z 69 [M-H] + m/z 41 C3H5

1.6

C3 H3 +

Log [Ion Signal]

m/z 39 C3H 3

λMPI = 355 nm

1.2

+

+

m/z 29 HCO

+

0.8 0.4 0.0 -0.4 -0.8 -1.56

-1.51

-1.46

-1.41

-1.36

-1.31

-1.26

-1.21

0

10

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30

40

[(MC)-H]+ (MC) •+

CH3 +

C3 H5 +

Log [Laser Fluence] C2 H3 + HCO+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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m/z (amu)

Figure 5. Multi-photon ionization mass spectrum of MC recorded using 355 nm laser pulses under the same supersonic jet expansion condition as that for Figure 2. Laser pulse energy was only 120 µJ. The inset shows logarithmic plots for some of the fragment ions indicating that those are produced by a 3-photon process. It is seen that the mass spectral features of Figure 5 are drastically different compared to that of Figure 2, here the ion signal corresponding to (MC)H+ ion is absent along with the signal for the intact dimer cation. The laser pulse energy was kept as low as only 120 µJ/pulse, which is barely enough to record mass spectrum, but in spite of that fragmentation becomes very extensive. The logarithmic plots (shown in the inset of Figure 5) for ion currents vs. laser pulse energies (in the lower energy region) with respect to some of the larger fragments show that a 3-photon process is responsible for formation of these ions. Total 3-photon energy at this wavelength (355 nm) is ~10.48 eV (3x3.49 eV), which is ~0.5 eV higher than the ionization energy of MC monomer, and



according to the prediction of electronic structure theories it could be about 1 eV larger compared to IE of the dimer. With this large excess of internal energy the survival possibility of the intact dimer cation is much less compared to the case of 532 nm ionization. It is also worth mentioning that the molecular absorption at 355 nm is quite strong (shown in the inset of Figure 2), and therefore monomer will be strongly ionized which is reflected in the experimental observation of extensive fragmentation (depicted in Figure 5) even for the use of low laser fluence. Therefore, it is likely that the signals for small amount of intact dimer cation and its primary fragment (MC)H+ ion (m/z 140 and 71) survived under the experimental condition became undetectable over the large background of the mass spectrum. To verify this possibility, the spectrum has been recorded using a more sensitive vertical scale of the recorder (here oscilloscope), which causes saturation of the intense peaks. A spectral trace recorded by such means is presented in the supporting information (Figure S1), and the mass peaks for both (MC)H+ and [(MC)2].+ are now become visible, although the spectrum is quite noisy. This also corroborates our earlier inference that in the case of 4-photon ionization by 532 nm laser wavelength, [(MC)2].+ is produced with only a little internal energy and thus the mass peak prominently shows up in the TOF spectrum. In the following sections we discuss about the predictions of electronic structure theory concerning production of (MC)H+ ions via IMPT within [(MC)2].+ followed by dissociation. IMPT in the dimer cation is shown to be a barrierless process and subsequent dissociation of the proton transferred species is also predicted to be a quite low energy demanding process (Sec 5 and 6).

4. Structure of the MC dimer cation:


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For an α,β-unsaturated aldehyde, two distinct rotational isomers (s-cis and s-trans) are possible depending on the relative orientation of the C=O and C=C bonds across the C-C single bond. In the present case of MC, the conformer s-trans is predicted to be more stable by 3.4 kcal mol-1 in the neutral (S0) ground state, but for the molecular cation (D0) the order of stability is reversed and the s-cis isomer is predicted to be more stable by 1.7 kcal mol-1 according to calculation at DFT/B3LYP/6-311++G(d,p) level (Figure S2). For geometry optimization of various isomers of MC dimer, we have used as reference the similar structures of the neutral dimer isomers of the simplest α,β-unsaturated aldehyde, acrolein, reported by Desiraju and co-workers.67 Depending upon the initial monomer conformation, four isomers for the dimer are possible, s-cis/s-cis, scis/s-trans, s-trans/s-cis and s-trans/s-trans. Considering all these structures and different initial geometries, nearly 30 optimized geometries have been initially generated. However, in the neutral ground state, the isomer wherein both moieties are s-trans type and they are bound by two aldehydic C-H···O hydrogen bonds in a planar anti-parallel symmetric geometry, is predicted to be the most stable species (Figure S3). This prediction is consistent with the structural preferences of acrolein dimer reported in Ref. 67. As the hydrogen bonds in the dimer are of weaker variant, the structures have been further optimized by methods like M06-2X/6311++G(3df,3pd) and ωB97XD/6-311++G(3df,3pd) that take into account of dispersion interactions. It has been noted that there are a number of isomeric geometries which have similar binding energies, and the relative stability slightly depends on the level of calculation used. Presented in Table 1 are the BSSE corrected binding energies and relative energies of the five most stable isomers of MC dimer, and it is seen that those are within ~1 kcal mol-1. Therefore, under the experimental condition of supersonic jet expansion, more than one isomeric species are likely to coexist.



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ωB97X-D/6311++G(3df,3pd)

M06-2X/6311++G(3df,3pd)

DFT/B3LYP/6311++G(d,p)

Binding Energy

Relative Energy

Binding Energy

Relative Energy

Binding Energy

Relative Energy

Isomer A

3.68

0.27

3.51

0.12

2.35

0

Isomer B

3.20

0.75

2.63

1.01

1.52

0.84

Isomer C

3.95

0

3.64

0

2.24

0.12

Isomer D

3.10

0.85

2.77

0.87

1.86

0.49

Isomer E

3.15

0.80

2.64

0.99

1.75

0.60

Table 1. BSSE corrected binding energies (in kcal mol-1) and relative energies (in kcal mol-1) of the five most stable structural isomers of MC dimer in the neutral (S0) ground state, calculated at ωB97XD/6-311++G(3df,3pd), M06-2X/6-311++G(3df,3pd) and DFT/B3LYP/6-311++G(d,p) levels. The geometries of the aforementioned five isomers of the dimer in S0 and D0 states optimized by M06-2X/6-311++G(3df,3pd) method are shown in Figure 6 along with their BSSE corrected relative energies. It is seen that the structures can be categorized into two groups based on the nature of C-H···O hydrogen bond(s) present. In neutral S0 state, the isomers with aldehydic C-H as hydrogen bond donors, viz., the isomers A, C and D, are predicted to be more stable. In order to optimize the geometries of the corresponding dimer cations in the D0 state, the optimized S0 geometries were given as input. A scrutiny of the optimized geometric parameters reveals that in cations of the aforesaid three isomers, the aldehydic hydrogen involved in C-H···O hydrogen bonding is transferred completely to the carbonyl oxygen of the other moiety.


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Figure 6. Geometric structures, BSSE corrected relative energies (in kcal mol-1) of five structural isomers of MC dimer in neutral (S0) and cationic (D0) ground states, optimized at M062X/6-311++G(3df,3pd) level. Bond lengths are given in Å units.



Thus, in isomer A, the aldehydic C-H (donor) bond length is increased from ~1.1 Å in neutral precursor to ~1.8 Å in the cation, and accordingly the H···O hydrogen bond is turned into a covalent bond with a reduced bond length of ~1.01 Å. Likewise for isomers C and D, at the energy minima configurations of the cation, the donor C-H distances are 1.9 and 1.8 Å, respectively. The three dimeric isomers having aldehydic C-H···O hydrogen bond(s) are of particular interest as they attain proton transferred geometries in the cationic ground state. However, it is worth mentioning that in case of isomer E, wherein an H-atom from the =CH2 group takes part in the C-H···O interaction, a cationic minimum energy geometry without Htransfer is found for each of the three levels of calculations used. On the other hand, for the isomer B methyl groups act as the hydrogen bond donor in the neutral ground state but different types of minimum energy geometries in the D0 state are predicted by different levels of calculations which are separately shown in the supporting information (Figure S4). Nonetheless, for both the two isomers, B and E, the proton transfer process is predicted to be higher energy demanding as compared to the other isomers which undergo barrierless PT as shown in the following section.

5. Barrierless IMPT across C-H···O hydrogen bond: IMPT within ionic clusters resulting in protonated molecular ion formation has been demonstrated earlier in many systems, but in most of such cases the concerned molecules in the clusters are bound either by conventional hydrogen bonds or upon ionization one of the moieties undergoes tautomeric conversion to enolic form and eventually PT occurs across the conventionally hydrogen bonded bridge.30-45, 47, 49-50 In the dimer of the present α,β-unsaturated


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aldehyde, the structural constraints do not allow tautomerization of the cationic moiety into an enol form. Nevertheless, IMPT has been predicted and it occurs by direct dissociation of the C-H bond. To elucidate the nature of PT pathways, the potential energies along the C-H···O hydrogen bonds of the three isomeric dimer cations (A, C and D) have been calculated at DFT/B3LYP/6311++G(d,p) level, and the results are depicted in Figure 7. It is seen that as the H-atom of the donor C-H bond is moved stepwise towards the carbonyl oxygen of the acceptor moiety and rest of the geometric parameters are allowed to relax, the overall energy of the system decreases, i.e., the IMPT in the dimer cations, starting from the Franck-Condon (FC) structure (which is the structure of the vertically ionized dimer), is a barrierless process. It is worth mentioning that among these three isomers the potential energy scan for the isomer C initially goes downhill from the vertically ionized FC point and reaches to the first minimum at the C-H distance of ~1.2 Å (structure is shown in Figure 7). As the RC-H is increased further, a very small energy barrier of ~0.006 eV (0.15 kcal mol-1), which is much smaller compared to the zero-point vibrational energy, is encountered. Moreover, calculation predicts that PT vibrational frequency of the dimer cation at the first minimum is ~143 cm-1, and thus the proton could effectively tunnel through the barrier and the system will ultimately assume the global energy minimum proton transferred configuration, for which RC-H is ~1.8 Å.



Figure 7. Constrained geometry optimization of the vertically ionized MC dimer (isomer A, C and D) at DFT/B3LYP/6-311++G(d,p) level, relative energies (∆E) in eV are plotted as a


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function of C-H bond length, RC-H, of the proton donating monomer. Minimum energy geometries are also shown. Bond lengths are given in Å units. Furthermore, as mentioned in the previous section, the PT process for the other two isomers is not at all barrierless. The potential energy scan along the donor C-H stretching for isomer E (shown in Figure S5) demonstrates that the PT is an uphill process from the FC point and it needs to cross a barrier of ~0.6 eV. On the other hand, the energy barrier of PT in the isomer B is predicted to be still larger, ~2.7 eV, at the same level of calculation and it is shown in Figure S6. To distinguish whether the process actually involves a proton or an H-atom transfer, the partial charges (Q) and spin density distributions (S) of the donor and acceptor moieties were also calculated and shown in Figure 8 for all the three dimer isomers (A, C and D). With increase in C-H distance (RC-H), the value of Q on the donor (QD) decreases and that on the acceptor (QA) increases, whereas the changes with respect to the parameter S are opposite. Furthermore, most of the unpaired electron spin density parameter (> 80%) resides on the donor moiety in the energy minimum structures, which indicates occurrence of a PT rather than an H-atom transfer.



0.9

Charge or Spin Density

0.8 0.7 0.6

Isomer A

0.5 0.4

QD

0.3

QA

0.2

SDA SDD

0.1 0.0 1.0

1.2

1.4

1.6

° RC-H (A)

1.8

2.2

2.0

0.9


0.8

SDA

0.7

SDD QA

Isomer C

0.6

QD 0.5 0.4 0.3 0.2 0.1 0.0 1.0

1.2

1.4

1.6

1.8

2.0

2.2

° RC-H (A)

0.9 0.8


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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SDA

0.7

SDD

Isomer D

0.6

QD QA

0.5 0.4 0.3 0.2 0.1 0.0 1.0

1.2

1.4

1.6

1.8

° RC-H (A)


2.0

2.2

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Figure 8. At each C-H bond length of Figure 7, partial charge distribution and spin densities of the donor (QD and SDD respectively) and acceptor (QA and SDA respectively) calculated at DFT/B3LYP/6-311++G(d,p) level, excluding the H-atom being transferred, are plotted.

6. Energetics for [(MC)2].+ fragmentation: The low-energy major fragmentation channels of [(MC)2].+ ion as revealed in the mass spectrum presented in Figure 2 have been summarized below. As IMPT in [(MC)2].+ ion has been shown as a barrierless process, the dimer cation exists as an adduct of protonated molecular ion and proton loss radical in the lowest energy configuration (Figure 1). The energetic data of different reaction channels of the dimer cation shown below are with respect to this energy minimum PT configuration. [(MC)H+…C4H5O.]PT

(MC)H+ + C4H5O. ................(1) ∆E = 0.8 eV

[(MC)H+…C4H5O.]PT

(MC).+ + MC .........................(2) ∆E = 1.4 eV

[(MC)H+…C4H5O.]PT

[(MC)-H]+ + C4H6O + H. ......(3) ∆E = 2.4 eV

The lowest energy reaction is found to be the direct fragmentation of the adduct (reaction 1), and according to calculation at DFT/B3LYP/6-311++G(d,p) level, the required energy is ~0.8 eV. According to the calculated data presented in Table S1, AIEs for all the dimer isomers considered here are more than 0.5 eV smaller compared to their respective VIEs, which is consistent with the fact that substantial changes in geometry occur with IMPT in dimer cations. Therefore, as the [(MC)2].+ is vertically ionized, the FC configuration relaxes to the minimum energy structure, i.e., the proton transferred geometry, and thus it gains more than 0.5 eV of



energy. This energy is utilized for its fragmentation into (MC)H+ by reaction channel 1. According to the observed mass spectrum, the other two very basic and accompanying reaction channels could be dissociation of the dimer cation via channel 2 resulting in formation of molecular cation (MC).+, and channel 3 leading to H-atom loss molecular ion [(MC)-H]+ formation. The required energies for these two reaction channels, are ~1.4 and 2.4 eV, respectively, according to calculations at the same level. The reaction in channel 2 can compete with reaction 1, but for that to occur the [(MC)2]PT.+ needs to absorb one more photon of 532 nm. Production of [(MC)-H]+ ion from the dimer cation by channel 3 is predicted to be even more energy demanding. However, the same fragment ion could be produced by hydrogen loss from the monomer cation. The ion current vs. laser fluences plot for this fragment ion also shows linear variation, with the slope value close to 5 (shown in Figure S7). This indicates that in the mass spectrum shown in Figure 2, a direct 5-photon ionization of MC could be the major source of this ion along with (MC).+. This possibility is more vividly revealed in Figure 3, which shows that unlike (MC)H+ ion the increase in intensities of (MC).+ and [(MC)-H]+ ions do not occur in proportion with [(MC)2].+ ion with increasing backing pressure. We would like to further mention here that all these energetics data presented above were calculated for the most stable isomer, isomer A, at this level of calculation. The dissociation energies corresponding to the proton transferred structures of the other isomers, including the proton transferred structure for isomer B, have been listed in Table S2.

IV. CONCLUSION


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This paper presents a TOF mass spectrometric study for IMPT in a C-H···O hydrogen bonded homodimer cation of MC. The dimer is prepared in a supersonic jet expansion and photoionized non-resonantly using the 2nd harmonic wavelength (532 nm) of a Q-switched Nd:YAG laser. The occurrence of PT reaction is inferred by observing an intense mass peak directly at the m/z value corresponding to the protonated molecular ion, whose intensity in the mass spectra are found to be correlated with the intensity of the dimer cation mass peak. Thus, signal intensities for both the protonated molecular ion and dimer cation mass peaks increase simultaneously as the cooling condition of supersonic jet expansion is made better by increasing backing pressure of argon carrier gas. A barrierless IMPT in the dimer cation followed by its fragmentation are proposed to be the key steps for formation of the protonated molecular ion from the homodimer cation. It has been revealed further by recording the mass spectra upon varying the laser pulse intensities (532 nm) that the dimer is photoionized via a 4-photon process, and the total 4-photon excitation energy (9.32 eV) is very similar to the VIE of the dimer predicted by electronic structure theory methods. The stability of the produced dimer cation and protonated molecular ion from it have been shown to be critically dependent on the excess internal energies with which it can be produced by means of multi-photon ionizations. Thus, both the mass peaks corresponding to the [(MC)2].+ and (MC)H+ ions are found to be very less in intensity compared to other fragment ions for ionization with 355 nm wavelength. For this wavelength, 3-photon absorption is required for ionization of the dimer. However, in contrast to the abovementioned 4-photon process at 532 nm, the excess internal energy with which the dimer cations could be produced in 3-photon ionization by 355 nm, is nearly 1 eV. This energy is significantly large to cause prompt fragmentation of the dimer



Page 28 of 40

cation, resulting in very small ion intensity of the intact dimer cation and its primary fragment protonated molecular ion. According to the predictions of electronic structure theory, the IMPT reaction in the MC dimer cation is a barrierless process. While the aldehydic C-H is found to form C-H···O hydrogen bonded geometry in the neutral ground state, in the optimized structure of the cationic ground state one of the aldehydic hydrogen is found to be completely transferred to the carbonyl oxygen of other monomer moiety. The barrierless nature of the PT process is more vividly revealed from the potential energy scan along C-H···O hydrogen bond coordinate. The calculations also reveal that among different fragmentation pathways of the dimer cation, the one resulting in formation of (MC)H+ ion has the least energy barrier, only ~0.8 eV, which can be compensated by the energy released upon ionization of the dimer due to significantly lower value of the AIE compared to the VIE.

Supporting Information Geometric structures and relative energies of different isomers of MC monomer and dimer in neutral (S0) and cationic (D0) ground states, optimized at the DFT/B3LYP/6-311++G(d,p) level of theory, VIEs and AIEs of both the monomer and dimer isomers calculated at DFT/B3LYP/6311++G(d,p),

M06-2X/6-311++G(3df,3pd)

and

ωB97X-D/6-311++G(3df,3pd)

level,

logarithmic plot for the ion yield variation of (MC-H)+ with increasing laser pulse energies at 532 nm, TOF mass spectrum for 355 nm photoionization, potential energy scans for PT in isomers B and E, dissociation energies of the proton transferred forms corresponding to different isomer of the MC dimer.


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ACKNOWLEDGEMNTS: PC thanks Department of Science and Technology, Government of India for the INSPIRE Fellowship. AKG thanks CSIR, Government of India for the fellowship of RA-I. MS thanks UGC, Government of India for research fellowship.

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O Hydrogen Bonded Methacrolein Dimer upon Non-Resonant Multi

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