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Ionization and Fragmentation of Formamide Induced by Synchrotron Radiation in the Valence Region via Photoelectron Photoion Coincidence Measurements and Density Functional Theory Calculations Manuela Souza Arruda, Aline Medina, Josenilton Nascimento Sousa, Luiz Antonio Vieira Mendes, Ricardo R. T. Marinho, and Frederico Vasconcellos Prudente J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b07464 • Publication Date (Web): 25 Sep 2015 Downloaded from http://pubs.acs.org on September 27, 2015

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The Journal of Physical Chemistry

Ionization and Fragmentation of Formamide Induced by Synchrotron Radiation in the Valence Region via Photoelectron Photoion Coincidence Measurements and Density Functional Theory Calculations Manuela S. Arruda,∗,†,‡ Aline Medina,† Josenilton N. Sousa,† Luiz A. V. Mendes,† Ricardo R. T. Marinho,† and Frederico V. Prudente∗,† †Instituto de F´ısica, Universidade Federal da Bahia, 40170-115, Salvador, BA, Brazil ‡Centro de Ciências Exatas e Tecnol´ ogicas, Universidade Federal do Recˆ oncavo da Bahia, 44380-000, Cruz das Almas, BA, Brazil E-mail: [email protected]; [email protected]

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Abstract We have performed a theoretical and experimental study of the formamide (HCONH2 ) photofragmentation and photoionization processes in the gas phase. The experiment was perfomed by using a time of flight mass spectrometer using the photoelectron photoion coincidence (PEPICO) technique in the valence region, from photons with energy between 10 eV and 20 eV. We have obtained both mass and partial ion yield spectra, identified by the mass to charge ratio as a function of the photon energy. With this set up we could ascertain the threshold energy for the production of formamide cation and its cationic fragments. The theoretical analysis of the formamide photofragmentation channels are fulfilled by the density functional theory (DFT) and the time dependent density functional theory (TD-DFT). The theoretical analysis allowed us to estimate, for example, which atoms are lost during the photofragmentation. We have also developed a theoretical-experimental analysis of the main fragments produced in + + the dissociation: m/q = 45 (HCONH+ 2 ), m/q = 44 (CONH2 ), m/q = 29 (HCO ), + m/q = 17 (NH+ 3 ) and m/q = 16 (NH2 ).

Introduction The study of the interaction of radiation with prebiotic molecules includes the aminoacids formation from its precursors, such as molecules with carboxylic, amine and amide groups, as well as the nucleotides generation, that are constituents of the RNA and DNA. The photophysics and photochemistry properties of these prebiotic molecules, in condensed and gas phases, and in the UV and VUV regions, have many applications. For example, they are of direct interest for the knowledge of photostability, they can contribute to radioastronomy research, comets science, and for exobiology study. A hypothesis to explain the origin of life suggests that the biomolecules are formed in space and carried to Earth by collisions of comets and meteorits. 1 Furthermore the bombardment of prebiotic molecules, in condensed, liquid and gas phases, by energetic particles 2 ACS Paragon Plus Environment

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Figure 1: Optimized geometry of the neutral HCONH2 molecule in the ground state. or photons can lead to fragmentation of these molecules, that can be decomposed in low molecular weight radicals or ions. 2 Formation of new molecules can also occur from recombination these products. Thus several theoretical and experimental studies simulate the stellar environment, involving the formamide in the gas, liquid, and condensed phases. 2–5 Formamide is an organic molecule that has already been detected in space in condensed and gas phases, in different environments like interstellar ice, the comet C/1995 O1 HaleBopp, and the protostellar source NGC 7538 IRS9. 6–8 This molecule has 45 u, 24 electrons and 6 atoms, in the fundamental state, organized as shown in Figure 1. It is liquid at room temperature and pressure and has a peptide bond, which is the kind of bond that connects the amine group of an aminoacid to the carboxyl group of another aminoacid, forming the aminoacid chains that originate the proteins. Besides the peptide bond, the formamide is composed by C, O, N, and H atoms, the constituent elements of the nucleobasese the most common elements of the Universe. 2 Experiments have already demonstrated that it is possible to create complex prebiotic molecules from simple molecules in environments similar to the interstellar medium. It has also been observed that the purine nucleobase is formed when the formamide is subjected to



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ultraviolet radiation while being heated. 9,10 In experiments 11 with the heating, irradiation and laser-induced dielectric breakdown dissociation of formamide in solid and liquid phases under various conditions (temperature, UV exposure, and the presence of catalysts), it was possible to detected all five basic nucleobases that occur in contemporary genetic materials (uracil, thymine, adenine, cytosine, and guanine), as well as the base purine and the amino acid glycine. Fourier transform-infrared spectroscopy (FTIR), time-resolved emission spectroscopy, and GC-MS were used to monitor the dissociation of this molecule. 11 Electronic structure calculations, density functional theory (DFT) and coupled-cluster theory (CCSD(T)), were used to propose a reaction pathway for the formation of both pyrimidine and purine nucleobases involving •CN radical chemistry. 11 In the condensed phase FTIR spectroscopy and temperature programmed desorption have been used to examine the thermal processing of formamide ice and its isotopes adsorbed on a SiO2 interstellar grain analogue under ultrahigh vacuum (UHV) conditions. 12 These experimental techniques also were used to study H2O:HCONH2 ice mixtures and pure HCONH2 on high-surface-area SiO2 nanoparticles bombarded by photon and electron-beam. 13 Several computational methods are employed in formamide theoretical studies. The synthesis of pyrimidine and purine heterocycles from formamide were investigated using quantum chemical calculations at DFT level. 14 The chemical transformations of formamide were investigated using methods of electronic structure computations CCSD(T) and RiceRampserger-Kassel-Marcus theory.

Quantum chemical calculations using the CCSD(T)

whose energies are extrapolated to the complete basis set (CBS) limit were carried out to construct the formamide potential energy surface. 15 Reaction channels leading to decompositions of the neutral and protonated formamide were investigated in the ground and excited states. Different decomposition channels were explored using CCSD(T)/CBS method and RASPT2(18,15) computations. 16 Concerning the context of chemical evolution of the universe, we have been performing theoretical and experimental studies of the photofragmentation processes of molecules of


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carboxylic acids and the amine and amide groups in gas phase. In the experimental point of view, we have focused the synchrotron radiation to fragment the sample and utilized a time of flight mass spectrometer to acquire partial ion yield data. The synchrotron radiation, among other characteristics, is a source of high brightness and wide broadband. 17 Its high brightness is related to large flow of photons, allowing that samples be examined more quickly and the resulted spectra have a good resolution. Its wide broadband, allied with the use of a monochromator, allows selecting photons of different energies in a simple way. Thus, it can make a scan on energy. Through this scan, we can, for example, determine the opening energy of fragmentation pathways. To determine these fragmentation pathways we have calculated the electronic structure using the DFT and the TD-DFT. The simplest carboxylic acid molecule, the formic acid, was previously studied by our group. 18 In the present work, we have developed a similar analysis for the simplest molecule of the amide group, the formamide. This paper is organized as follows. Section 2 describes technical details for experimental and theoretical procedures employed. We present the mass spectrometer and beamline characteristics that are important for the experiment. Then we present the calculation computacional and corrections employed in the ionization and fragmentation pathways energies. In section 3, we present experimental spectra, theoretical results and we compared theoretical and experimental results. The conclusions are in section 4.

Technical Details Experimental The experiment was performed at the Brazilian Synchrotron Laboratory, at the TGM Vacuum Ultraviolet Beamline. 19 This beamline has a monochromator with three toroidal grating that produces photons with energies between 7.3 eV and 310.0 eV and resolution of E/∆E = 500 for 100 µm slits. An important feature of the TGM is the krypton-neon 5 ACS Paragon Plus Environment


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and neon gas filters 20 that eliminate photons with energies higher than 14.00 eV and 21.54 eV, respectively. These filters ensure that there is no molecular fragmentation by higher harmonic orders while work with low energy photons. The experimental station has a time of flight mass spectrometer 21 based on the photoelectron photoion coincidence (PEPICO) technique. The spectrometer relates the mass to charge ratio of the ionic fragments, produced after the interaction of the synchrotron radiation with the gaseous molecular jet, with the time that the ions spend to flight until reach the detector. The lighter the ions, smaller are their time of flight. There is no information about the neutral fragments. The formamide is liquid at room temperature and was purchased from Sigma-Aldrich with purity better than 99%. All of our measurements were achieved with the sample holder at room temperature and we kept the chamber pressure between 1.1×10−6 and 1.3×10−6 mbar. The sample holder was directly connected to the main chamber by a needle. The inlet system is similar to reference , 22 but besides the low vapour pressure of the formamide, it was not necessary to heat it. Glass wool was used to prevent the liquid drops and the sample was evaporated by the pressure difference.

Theoretical We have employed the computational method of the density functional theory (DFT) 23 to obtain the adiabatic energy of the neutral molecules and its neutral and cationic fragments in the fundamental state. For the electronic excited states, we have used the time dependent density functional theory (TDDFT), 24,25 that can provide us the vertical energy of the excited states of the ionized molecule. In both cases we have employed the atomic basis function set 6−311++G(3df, 3pd) and the functional B3LYP, 26,27 with the GAMESS-US 28,29 parametrization. The unrestricted Hartree-Fock method 30 was used to obtain part of the exchange energy of the functional B3LYP. The energy of the dissociation (fragmentation) channel appearances are obtained con6 ACS Paragon Plus Environment

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sidering that the neutral and ionized formamide molecules and their neutral and cationic fragments are in the fundamental state. Therefore, we considered that the molecular energy is the adiabatic electronic energy in its equilibrium geometry plus the vibrational energy in the fundamental state, within the harmonic approximation, known as zero point energy (ZPE) correction.

Results There is a discussion in the literature regarding the planarity or non-planarity of the formamide. 31,32 In our work, we have performed calculations of geometry optimization for the neutral and ionized formamide molecule in the ground state with Cs and C1 symmetry. Once we found the same energy values, we have used the planar molecule with Cs symmetry in our study. However, other possible isomers are found in the literature. For example, nine possible conformational, tautomeric and structural isomers were studied in the work of Robb and Csizmadia. 33 The theoretical calculations were initiated with the optimized geometry of the neutral and cationic molecules in their ground electronic states; Figure 1 shows the equilibrium geometry of neutral formamide. In the Table 1 we present the geometrical parameters of the HCONH2 and HCONH+ 2 . In the second column we present our results for neutral molecule, and compare with a theoretical geometry acquired by the CCSD(T) method (third column) 34,35 and also with an experimental geometry found by microwave spectroscopy (fourth column). 35 The fifth column shows the equilibrium geometry of the ionized molecule in the ground state, while in the last one the difference between the present (DFT) geometric parameters of the ionized and neutral molecules. Our geometric parameters calculated at B3LYP/6311++G(3df, 3pd) level are essentially the same obtained by ter Steege et al. 36 using 6311+G* basis and another B3LYP parametrization. Our theoretical results for the equilibrium geometry of the neutral molecule in the ground



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Table 1: Equilibrium geometry of the HCONH2 and HCONH+ 2 molecules in the electronic ground state. ∆ represents the difference between present (DFT) geometric parameters of the ionized and neutral molecules. The bond length is given in ˚ A and angles in degree. Geometric Parameters r(C=O) r(C-H) r(C-N) r(N-H2 ) r(N-H1 ) HCO OCN CNH1 H2 NH1

DFT (this work) 1.210 1.104 1.358 1.005 1.007 122.7o 124.9o 119.5o 119.2o

HCONH2 (11 A′ ) CCSD(T)/PVTZ 34,35 1.211 1.107 1.356 1.000 1.097 122.8o 125.0o 119.3o 121.1o

Microwave 35 1.193 1.102 1.376 1.014 1.002 122.8o 123.8o 120.6o 118.9o

2 ′ HCONH+ 2 (1 A ) DFT (this work) 1.262 1.103 1.295 1.014 1.016 112.7o 126.3o 121.9o 118.2o

∆ DFT (this work) 0.052 -0.001 -0.063 0.009 0.009 -10.0o 1.4o 2.4o -1.0o

state are in agreement with the experimental and theoretical values found in the literature. Concerning the equilibrium geometry of the ionized molecule, the lenght of the C=O bound and both N−H bounds increased, while the length of the C−H and C−N bounds decreased. The major variation of the geometrical parameter of the ionized molecule compared to the neutral one was for the angle between the HCO atoms, that decreased 10.04o . This variation can be explained by the charge distribution in atoms of these molecules. From the Mulliken atomic population analysis, we realized that the H and O atoms have a negative charge in the neutral molecule, while in the ionized molecule the H atom stays with a slightly positive charge and the charge of the O atom practically disappears. The C=O bound behavior was also observed for the formic acid, for which the double bound characteristic is lost. 18 This phenomenon of characteristic loss of the double bond C= O, together with the loss of single bound characteristic of the C−N, may be explained from the difference of the electronic configuration of neutral and ionized molecule in their ground states. From our DFT calculations, the electronic configuration of the nine occupied valence orbitals of the molecule HCONH2 is (4a′ )2 (5a′ )2 (6a′ )2 (7a′ )2 (8a′ )2 (9a′ )2 (1a′′ )2 (2a′′ )2 (10a′ )2 , while for the cationic molecule there is an exchange between the two occupied orbitals of higher energy, getting the electronic configuration given by (4a′ )2 (5a′ )2 (6a′ )2 (7a′ )2 (8a′ )2 (9a′ )2 (1a′′ )2 (10a′ )2 (2a′′ )1 . That is, we found that, when formamide is ionized, the symmetry of the Highest Occupied Molecular Orbital (HOMO) undergoes a change of symmetry of σ type 8 ACS Paragon Plus Environment

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Figure 2: Highest Occupied Molecular Orbitals (HOMO) of HCONH2 (panel a) and HCONH+ 2 (panel b). to π type, explaining the observed changes in its equilibrium geometry. This change can be seen in Figure 2, where the contours of HOMO of HCONH2 , orbital 10a′ , and of HCONH+ 2, orbital 2a′′ , are shown respectively in panels (a) and (b). A similar effect is observed for the acid formic molecule. 18 Ionization energies (IE) of formamide are showed in Table 2. Our results are estimated by using DFT and TDDFT methods. The adiabatic ionization energy (AIE) is determined assuming that the ionized molecule relaxes, changing to a new equilibrium geometry. This value represents the lowest photon energy value necessary to be able occur the ionization of the molecule, which is associated with the appearance energy of the formamide cation. On the other hand, in the calculation of the vertical IEs, we have considered that an ionized molecule does not relax; in other words, the cation keeps neutral molecule equilibrium geometry. For the DFT calculations of the vertical ionization energy (VIE) of an excited state, we infer that the vertical ionization energy of the ground state is equal to the value obtained by TDDFT calculations. Then the VIE of the excited state is obtained by adding the vertical ionization energy of the ground state and the difference between the HOMO energy and the corresponding energy of the molecular orbital which the electron was excited. The TDDFT calculations for the ionization potentials were performed optimizing ten electronic states. From these, we show in Table 2 the states in which the largest contribution consists



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Table 2: Ionization Potential Energies of Formamide. Values in eV. Molecular

Adiabatic

Verticala

Verticala

Orbital

+ZPEa

DFT

TDDFT

10a′

10.03

10.25

2a′′

-

1a′′ 9a′ 8a′ 7a′ 6a′ 5a′ 4a′ C1s N1s O1s a

Present results (see the text for details).

b

EXPb

THEOc

10.25

10.35

10.31

10.76

10.68

10.71

10.75

14.23

14.43

14.13

14.38

14.47

14.80

14.80

14.79

16.26

16.42

16.36

16.57

18.31

18.83

18.79

18.69

20.20

20.39

20.47

20.66

27.94

-

28.6

28.73

32.8

32.28

294.40

294.22

406.38

406.68

537.77

537.70

31.38 282.59 393.40 522.66

Best experimental estimate from Ref. 32 using B3LYP geometry.

c

Theoretical results from Ref. 32 by

in the transition from a specific internal orbital to the HOMO orbital, thus indicating the ionization energy of an inner orbital. Moreover, in Table 2 are also showed a theoretical and the best experimental estimative of vertical ionization energies stated by Chong. 32 The presented theoretical VIE’s of Chong were the ones determined using the proposal of Stener et al. 37 to the B3LYP geometry. From this Table, we can note a good agreement between our theoretical estimatives with the ones obtained by Chong., 32 mainly for the valence orbitals. In Figure 3 we show the formamide mass spectra for different photon energies. The dependence of the ionic fragments with the energy can be observed. The ions with mass to + + + + charge ratio 45 (HCONH+ 2 ), 44 (HCONH and/or CONH2 ), 29 (HCNH2 , HCO and/or + + + COH+ ), 17 (NH+ 3 and/or OH ) and 16 (NH2 and/or O ) correspond to approximately

97% of the total ions produced. Hence, we have concentrated our theoretical study of the formamide fragmentation paths in these more abundant ions. Nevertheless, it is possible to observe peaks with lower intensity, that correspond to ions with m/q = 46, 43, 30, 28 and 18, that will be experimentally analyzed. We present the Partial Ion Yield (PIY) spectra, which provides the quantity of collected


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+

+

HCONH2

HCO

+

NH3

+

CONH2

+

NH2

Intensity (counts)

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20.0 eV

16.5 eV

12.5 eV 10.5 eV 10

12

14

16

18

20

22

24

26

28

30

32

34

36

38

40

42

44

46

48

mass/charge (a.m.u.)

Figure 3: The formamide mass spectra for different photon energies. ions with particular m/q as a function of photon energy, in two Figures, according to the yield intensity of ions. These spectra are obtained for the photon energy region from 10.00 to 20.00 eV, in steps of 0.02 eV. The four most intense ions (m/q = 45, 44, 29 and 17) are shown in Figure 4, while the other five ones (m/q = 46, 43, 28, 18 and 16) are shown in Figure 5. Additionally, the spectrum in black in Figure 4, called ”Full PExPICO”, represents the sum of intensities for all ions mapped in that Figure. This curve is obtained by adding point to point (for each energy value) intensities of all ions. Note that the opening of these nine ionization and fragmentation channels are explicitly observed in the PIY spectra. For the theoretical determination of the energy of fragmentation and ionization channel appearances (opening energies), we have calculated the adiabatic electronic energy with ZPE correction in the harmonic approximation. We consider ZPE correction, that represents the energy of the molecular vibrational ground state, to obtain better accuracy for calculated energies. The channel opening energy is obtained by adding the energies (adiabatic + ZPE) of cationic and neutral fragments produced and subtracting the energy (adiabatic + ZPE) of neutral formamide molecule. These results are shown in Table 3 for the main ions produced, together with experimental values previously published by other research groups. 3,38



18

10x10

8

Intensity (counts)

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6

4

2

0 10

11

12

13

14

15

16

17

18

19

20

Energy (eV)

Figure 4: The formamide partial ion yields for mass to charge ratio 45 (—), 44 (- · -), 29 (- -) and 17 (– - –) and the sum of them (- ·· -) as a function of the photon energy. The parent ion production (m/q = 45) is observed since 10.14 ± 0.05 eV and correspond to the electron loss of a neutral molecule, HCONH+ 2 . We obtained two theoretical ionization energies: the vertical, 10.25 eV, and the adiabatic with ZPE correction, 10.03 eV. The difference between these theoretical values and the experimental results achieved in this work are, respectively, 0.11 eV above and below the experimental value, which it can be considered as an adiabatic one. In the work of Leach et al., 3 with photoionization mass spectrometry Table 3: Energy Values of Fragmentation Channel Appearances of Formamide. m/q

45 44 44 29 29 17 17 17 16 16

Channels

− HCONH+ 2 + e CONH+ + H + e− 2 HCONH+ + H + e−

HCO+ + NH2 + e− − HCNH+ 2 + O + e − NH+ + CO + e 3 OH+ + HCNH + e− + OH + CNH2 + e− − NH+ 2 + HCO + e O+ + HCNH2 + e−

Theoretical Adiab.+ZPE Vert. 10.03 10.25 11.28 15.52 12.44 17.81 10.53 21.22 22.01 15.90 24.84

Energy (eV) Experimental PEPICO PIMS 3 10.14 10.220 (10.42) 11.24 11.29 13.12

13.11

11.36

11.37

15.86

15.57


EI 38 12.0 13.70

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15

200x10

150

Intensity (counts)

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100

50

0 10

11

12

13

14

15

16

17

18

19

20

Energy (eV)

Figure 5: The formamide partial ion yields for mass to charge ratio 46 (—), 43 (- · -), 28 (- -), 18 (– - –) and 16 (- ·· -) as a function of the photon energy. (PIMS), the adiabatic and vertical ionization energies obtained were, respectively, 10.220 ± 0.005 eV and 10.42 ± 0.01 eV. Compared to these values, our results of vertical and adiabatic theoretical energy are 0.17 eV and 0.19 eV below the respective experimental data. However, our experimental and theoretical results for the first ionization energy are in a better agreement with the experimental values obtained by Brundle et al. 39 with the He I PES technique. This group found 10.32 eV for the vertical energy and 10.13 eV for the adiabatic energy. Our vertical and adiabatic energy values are 0.07 eV and 0.10 meV below the respective experimental data obtained by Brundle et al., while our experimental energy result diverges 10 meV from the adiabatic energy found by the same author. Comparing the difference between the vertical and adiabatic theoretical energies, our result is 0.22 meV. Using PIMS and He I PES techniques, this difference is, respectively, 0.20 meV and 0.19 meV. This shows that the theoretical vertical and adiabatic energy difference is close to the experimental value for both techniques. The continuous curve in Figure 4 represent the parent ion production for the energy



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range between 10.00 eV and 20.00 eV. We can observe, between 10.00 eV and 11.22 eV, inclination variations of the PIY curve. This can be associated to the parent ion production in excited states. The first change in the inclination occurs at 10.22 eV, that coincides with the adiabatic ionization energy obtained by Leach et al. 3 The second inclination variance is at 10.36 eV, near the adiabatic energy of 10.42 eV reported by the same authors. This value (10.36 ± 0.05 eV) can be considered as our estimative for the first vertical ionization potential. Another inclination change occur at the energies 10.52 eV and 10.70 eV. The second energy value can be related to the ion production from the ionization of the 2a′′ molecular orbital (corresponding to 12 A′′ electronic state), whose theoretical value that we obtained was 10.68 eV for TDDFT calculation and 10.76 eV for the estimative from the molecular orbital energy. There is a minor decrease in this ion production at 11.22 eV, that we relate to the opening channel for the formation of the cation with m/q = 44. Another decrement in the production is observed at 12.66 eV and it is associated to the threshold for the production of the cation with m/q = 29. The first molecular fragmentation, m/q = 44 , occurs around 11.24 ± 0.05 eV caused by the hydrogen loss of the neutral molecule. In accordance with our theoretical result, presented in Table 3, the lost hydrogen is initially bonded to the carbon. However, our theoretical calculation indicates the opening of the fragmentation channel in which lost hydrogen is initially attached to the nitrogen, constituting the HCONH+ ion, at 15.52 eV. From this energy, the ion with m/q = 44 is the result of a fragmentation of these two channels. The ion with mass to charge ratio 29 is experimentally observed from 13.12 ± 0.05 eV. Our obtained theoretical energy value for the opening channel indicates that at 12.44 eV the neutral molecule fragments in HCO+ + NH2 . At 17.81 eV the neutral molecule should loose the oxygen forming the HCNH+ 2 ion. We point out the excellent agreement of our result with the one obtained by Leach et al. using the PIMS technique, 3 while it is smaller than the Electron Impact (EI) results approximately 0.60 eV. According to our experimental results, at 11.36 ± 0.05 eV the production of the ion with


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+ mass to charge ratio 17, corresponding to the formation of the NH+ ion, starts. 3 or OH

According to our theoretical result, the opening of the channel corresponding to the NH+ 3 ion is at 10.53 eV. The difference between the experimental and theoretical opening energies for this channel is of 0.83 eV. This discrepancy can be caused by the existence of a transition + state for the fragmentation HCONH+ 2 −→ NH3 + CO with a maximum energy that was

not considered in our calculations. Opening channel of the cation with m/q = 16 is found to be 15.86 ± 0.05 eV, forming the ionic fragment NH+ 2 that, according to the theoretical calculation, is formed at 15.90 eV. Our energy value of ion with m/q = 16 appearance is about 0.3 eV higher than that observed with PIMS technique. 3 The other fragmentations are observed around 10.12 eV, 15.98 eV, 14.18 eV and 11.94 eV, that correspond to the ions with mass to charge ratio 46, 43, 28 and 18, respectively. A hypothesis for the ion with m/q = 46, (HCONH2 )·H+ , is that it may have been produced from the fragmentation of formamide dimers that were formed in the experimental chamber (see in Refs., 40–42 and references therein, the discussion of the formamide dimers formation). In our study of the photofragmentation and photoionization of the formic acid, 18 it was observed in the mass spectrum an ion with mass to charge ratio one unity larger than the parent ion. In similar studies, other authors also verified the presence of this ion, as for the acetic acid. 43 The explanation usually found in the literature for the formation of these ions is the presence of the

13

C isotope. However, this explanation has not been verified in recent

study on photoionization and photofragmentation of organic molecules. 44 The ion with mass to charge ratio 18, H2 O+ , can be resultant of the formamide fragmentation or of the water ionization, which theoretical energy is 12.53 eV. 18 In Figure 5 it can be observed that around 12.50 eV exists a significative increase in the PIY curve (in yellow), corresponding to the production of this ion, what indicates a contamination of the sample or the experimental chamber by water. The ion with m/q = 43 can be formed by the loss of two hydrogens, that can be the two hydrogens bounded to the nitrogen or one of these with another bounded to the carbon. The ion with mass to charge ratio 28 can be resultant



1.0 0.9 0.8

Relative Intensity

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0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 11

12

13

14

15

16

17

18

19

20

Energy(eV)

Figure 6: Relative production of ions with mass to charge ratio 45 (—), 44 (- · -), 29 (- - -), 17 (– - –) and 16 (- ·· -) as a function of the photon energy. of the loss of the NH3 , forming the CO+ ion, or resultant of the loss of the OH, forming the HCNH+ or CNH+ 2 ions. In Figure 6 we present the relative ion yield spectrum (RIY), also called the branching ratio, of ions obtained from the formamide photoionization and photofragmentation for the energy region from 10.12 eV to 20.00 eV, in intervals of 0.02 eV. The initial parent ion yield corresponds to 98% of the total production until 11.36 eV, when this branching ratio decreases because of the NH+ 3 production. At 18.73 eV the parent ion is no longer the most abundant, it is the HCO+ ion. And at 20.00 eV the parent ion formation corresponds only to 29% of the total production. The NH+ 3 (m/q = 17) reaches the maximum of its yields at 13.34 eV, corresponding approximately to 34% of the total production and remains as the second most produced ion until its production decrease to 24% of the total production, at 17.24 eV. In this energy, the HCO+ ion, which production always increase, becomes the second most produced ion. The channel that the formamide loose an hydrogen starts with a yield larger than the HCO+ . At 14.74 eV, the production of both channels is equated. Among the less produced ions, the mass to charge ratio 16 is more important, with a maximum branching 16 ACS Paragon Plus Environment

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ratio around 8%, followed by the ions with mass to charge ratio 43 and 28, with maximum production of 5% and 4%, respectively. The ions with mass to charge ratio 46 and 18 do not go beyond 1% of the total production in the energy range studied. In Figure 6, among the less produced ions we present only the ion with mass to charge ratio 16. We finish this study with the production percentage of the main ionization and fragmentation paths of formamide. The fragmentation was perfomed in the energy region from 10.18 to 20.00 eV. As shown in Table 4, the parent ion contributes more than half of the total production, 51.47%, followed by the production of NH+ 3 , with 20.12% of all produced ions. The third most produced ion is HCO+ with 13.13% of the total production and, among the main ions, the less produced is the NH+ 2 with 1.82% of production. In HCONH2 photofragmentation, as in formic acid photofragmentation, 18 the ion with one unit bigger towards his father ion (m/q = 46), is also detected and represents 0.72% of total production. Table 4: Main Formamide Photofragmentation Pathways by Photons with Energies between 10.18 and 20.00 eV. m/q

Fragmentation Paths

Production (%) 52.47

17

− HCONH+ 2 + e − NH+ 3 + CO + e

29

HCO+ + NH2 + e−

13.13

44

CONH+ 2 + H NH+ + HCO 2

45

16 46 18

(HCONH2 H2

O+

20.12

e−

9.20

+ e−

1.82

+

)·H+

+

+ CN +

e−

e−

0.72 0.69

Conclusions Attempting to contribute to a better knowledge of the formation process of biomolecules, we performed a theoretical and experimental study of the photoionization and photofragmentation of the formamide, an important prebiotic molecule. We have analyzed partial ion yield spectra, obtained after the interaction between the molecular gas jet and photons in the energy range from 10 eV to 20 eV. From these spectra we could ascertain the fragmenta17 ACS Paragon Plus Environment


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tion opening channels energy, and experimental values were compared with theoretical ones, obtained from DFT electronic structure calculations. The theoretical calculations of the different fragmentation channels allowed us to identify which ions were being formed, whereas experimentally we identify only the mass to charge ratio. Our theoretical opening channels + energy values for the main produced cations, i.e., the HCONH+ 2 (m/q = 45), CONH2 (m/q + = 44), HCO+ (m/q = 29), NH+ 3 (m/q = 17) and NH2 (m/q = 16), are in agreement with

the experimental results. For these ions, the difference between theory and experiment, in absolute values, are, respectively, 0.11 eV, 0.04 eV, 0.58 eV, 0.83 eV and 0.04 eV. The major discrepancy is for the NH+ 3 ion, 0.83 eV, that we speculated to be due to the existence of a barrier caused by a transition state in this fragmentation path. Further investigations need to be done about it. We have also verified the experimental energy for the less intense ions, with mass to charge ratio 46, 43, 28 and 18. We highlight the presence of the ion with mass to charge ratio 46, the (HCONH2 )·H+ . This origin can be assigned to the formamide dimers fragmentation, least because this ion has an appearance energy lower than the parent ion (m/q = 45), as noted in the Results section. We have fulfilled a detailed theoretical study of the parent ion. Besides the first adiabatic ionization energy, we have also calculated the vertical ionization energies of electronic ground and excited states. We compared this result with other experimental and theoretical values for the first vertical and adiabatic ionization energy of the formamide, obtained with different techniques, and we found discrepancies up to 300 meV. Our vertical (10.25 eV) and adiabatic (10.03 eV) theoretical results are in better agreement with Brundle et al. 39 Another data presented in this work is the relative intensity spectrum of the partial ion yield. We conclude our study of the formamide photofragmentation with a comparative analysis of the ions production ratio. We have verified that the main produced ion in the studied region is the HCONH+ 2 cation. Similar result was observed in our previous study about formic acid molecule. 18 Despite the analysis performed in the present work has as focus the production of


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ions during the formamide photoionization and photofragmentation processes, the created neutral and radical fragments also play an important role in photochemical of prebiotic molecules. 45,46 Here, we have estimated which neutral species are produced from the analysis of fragmentation pathways, but without detailed information can be obtained. Moreover, in this energy range the absorbed photons do not necessarily generate ions and can produce highly excited neutral molecules (ionization quantum efficiency is lesser than 1). Thus, it is necessary to use other experimental techniques, 47,48 in conjunction with the TOF-MS, for conducting detailed studies on the produced neutral and radicals fragments. Future studies may be done in this direction. A specific interest of this work for astrochemistry is in determining, with a good accuracy, the partial and relative ion yields (PIY and RIY) spectra to formamide. These data, together with the knowledge of the absolute total photoionization cross section, allow estimating the absolute value of the partial photoionization cross sections (or production of each specific ion), an important measure for understanding the photostability of molecules in space. 49 The realization of such measures for formamide and other molecules are being planned by our group using a double-ion chamber spectrometer. 50,51

Acknowledgement This work has been supported by Coordena¸caõ de Aperfei¸coamento de Pessoal de N´ıvel Superior (CAPES/MEC, Brazil), Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnológico (CNPq/MCT, Brazil), Funda¸cão de Amparo à Pesquisa do Estado da Bahia (FAPESB, Brazil) and Laboratório Nacional de Luz S´ıncrotron (LNLS, Brazil).

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