Vacuum Ultraviolet Photofragmentation of Sarcosine - ACS Publications

Feb 23, 2010 - and 60 (CH2COOH2. +. ) were discussed in detail with the help of calculations at the G3B3 and B3LYP/6-31++G(d,p) levels. 1. Introductio...
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J. Phys. Chem. A 2010, 114, 3411–3417

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Vacuum Ultraviolet Photofragmentation of Sarcosine: Photoionization Mass Spectrometric and Theoretical Insights Huijun Guo, Lidong Zhang, Liulin Deng, Liangyuan Jia, Yang Pan, and Fei Qi* National Synchrotron Radiation Laboratory, UniVersity of Science and Technology of China, Hefei, Anhui 230029, People’s Republic of China ReceiVed: NoVember 17, 2009; ReVised Manuscript ReceiVed: February 5, 2010

Vacuum ultraviolet (VUV) photon-induced ionization and fragmentation of N-methyl glycine (sarcosine) were investigated with infrared laser desorption/tunable synchrotron VUV photoionization mass spectrometry (IR LD/VUV PIMS) and theoretical calculations. Fragment-controllable mass spectra of sarcosine were measured at various photon energies. By tuning the photon energy, the fragments at m/z 44, 45, 43, 42, 30, and 60 were gradually detected. The ionization energy of the precursor was obtained by measuring the photoionization efficiency spectrum. Possible formation pathways of the fragment ions at m/z 44 (CH3NHCH2+), 45 (CH3NH2CH2+), 43 (CH2NHCH2+), 42 (CH2NCH2+), 30 (CH2NH2+), and 60 (CH2COOH2+) were discussed in detail with the help of calculations at the G3B3 and B3LYP/6-31++G(d,p) levels. 1. Introduction Amino acid, a type of important bioorganic small molecules, plays a crucial role in the chemoautotrophic origins of life on the earth.1-3 It was postulated that prebiotic amino acids are produced from primitive atmosphere discharge,4-6 volcanic precipitates autocatalysis,1,2,7 extraterrestrial meteorite, and prebiotic thermal energy synthesis.8-11 Sarcosine belonging to the family of secondary amino acid, which is yielded by organisms and acts as osmoprotectants and osmolytes to balance cellar osmotic stress and protect organisms from damages.12 Under monomeric sarcosine oxidase, it can be oxidized to yield glycine and formaldehyde by demethylation.13 Conformers of neutral sarcosine in the gas phase were investigated with experimental and theoretical methods.14-16 Headley et al. studied the conformers of sarcosine with high level ab initio calculations and proposed eight stable conformers of sarcosine.16 The conformer with N-H · · · OdC intramolecular hydrogen bond (HB) and cis carboxylic group configuration is the most stable one, which is analogous to the most stable conformer of glycine.17 The second most stable conformer displays a strong O-H · · · N intramolecular HB and a trans carboxylic group.14,15 Though the conformers of neutral sarcosine have been investigated, the fragmentation pathways from different conformers have not been reported yet. In this work, we investigated the photoionization and dissociative photoionization behaviors of sarcosine with IR LD/ vacuum ultraviolet (VUV) PIMS and theoretical calculations. The photoionization mass spectra of sarcosine were obtained at different photon energies. The ionization energy (IE) of sarcosine was obtained by measuring the photoionization efficiency spectrum (PIE) spectrum. Furthermore, on the basis of calculations at the G3B3 and B3LYP/6-31++G(d,p) levels, the major dissociation pathways to form fragments at m/z 44 (CH3NHCH2+), m/z 45 (CH3NH2CH2+), m/z 43 (CH2NHCH2+), m/z 42 (CH2NCH2+), m/z 30 (CH2NH2+), and m/z 60 (CH2COOH2+) were presented. * To whom correspondence should be addressed. E-mail: [email protected]. Tel: +86-551-3602125. Fax: +86-551-5141078.

2. Experimental and Theoretical Methods 2.1. Experimental Method. The experiment was completed at the National Synchrotron Radiation Laboratory, Hefei, China. Sarcosine with purity of 98% was obtained commercially and used without further purification. The IR LD/VUV PIMS setup was described in detail in previous publications.18-20 Briefly, a beam of 1064 nm output of a pulsed Nd:YAG laser (Surelite I-20, Continuum, U.S.A.) with a duration of 7 ns and repetition rate of 10 Hz was utilized for desorption of the analyte on the surface of a stainless steel substrate without any matrix. To generate the plume of intact neutral molecules, the laser power for desorption was controlled at about 9 mJ · pulse-1. The desorbed neutral molecules in the gas phase near the stainless steel substrate surface were ionized by the crossed synchrotron VUV light, and the generated ions were detected by a homemade reflection time-of-flight (RTOF) mass spectrometer. The ion signals were amplified by a preamplifier (VT120C, EG & G, ORTEC, U.S.A.) and recorded by a P7888 multiscaler (FAST Comtec, Germany). Time delay between the laser and the pulse of repeller field of RTOF is 150 µs, which was controlled by a homemade pulse/delay generator. Synchrotron VUV radiation from an undulator beamline of 800 MeV electron storage ring was monochromatized by a 1 m Seya-Namioka monochromator with a laminar grating (1500 grooves · mm-1, Horiba Jobin Yvon, France). The grating covered the photon energy range of 7.8-24 eV with the energy resolution (E/∆E) of about 1000. The average photon flux was measured to be 1 × 1013 photons · s-1 at the ionization region. A silicon photodiode (SXUV-100, International Radiation Detectors Inc., U.S.A.) was used to monitor the photon flux for normalizing the ion signals. The higher order harmonic radiation was eliminated by a gas filter filled with argon or neon. 2.1. Theoretical Method. The dissociation pathways of sarcosine cation were calculated using the G3B3 and DFT methods with the Gaussian 03 program package.21 The geometries were optimized at the B3LYP/6-31++G(d,p) level.22,23 The harmonic frequencies were computed at the same level to verify the minima and transition state (TS). The zero-point energies (ZPE) were also obtained from the frequency calculations. Furthermore, the photoionization and dissociation were

10.1021/jp9109095  2010 American Chemical Society Published on Web 02/23/2010

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TABLE 1: Calculated Energies at the G3B3 and B3LYP/6-31++G(d,p) Levels (All Energies with ZPE Correction; Unit: Hartree) species b

I I+ TS1 TS2 TS3 TS7 TS11 TS12 TS13 TS16 TS17 TS20 INT1 ITN3 INT5 INT6 INT7 INT8 CH3NHCH2+ COOH-1 COOH-1+ CH3NHCH2 CH2NHCH2+ HOCOH CH2NCH2+ HCO CH2NH2+ CH2COOH-1 CH3COOH+ CH2NH

B3LYPa –323.65656 –323.35096 –323.33114 –323.33015 –323.32618 –323.32578 –323.32972 –323.2962 –246.83340 –323.27903 –323.31249 –323.21808 –323.33875 –323.32680 –323.32574 –246.88657 –246.86355 –323.33754 –134.22938 –189.08587 –188.77932 –134.44755 –133.58572 –189.6747 –132.99917 –113.84736 –94.93039 –228.39225 –228.65979 –94.60087

G3B3 –323.51796 –323.20116 –323.18190 –323.17923 –323.18081 –323.17222 –323.17989 –323.14869 –246.71294 –323.13453 –323.16162 –323.05717 –323.18898 –323.17545 –323.20005 –246.77213 –246.73743 –323.19548 –134.16547 –189.00211 –188.70833 –134.37793 –133.51463 –189.59330 –132.93306 –113.79410 –94.88630 –228.28873 –228.55318 –94.55726

species

B3LYP

G3B3

II II+ TS4 TS5 TS6 TS8 TS9 TS10 TS14 TS15 TS18 TS19 TS21 TS22 INT2 INT4 INT9 INT10 INT11 COOH-2 COOH-2+ CH3NH2CH2+ CO2 CH2NHCH2CO+ H 2O CO CH2NHCH+ H2 CH2COOH-2 CH2COOH2+

–323.65615 –323.33133 –323.32035 –323.30936 –323.31658 –323.32833 –323.29166 –246.88263 –134.03499 –134.08783 –323.26558 –323.30240 –323.26558 –323.31605 –323.31971 –323.35088 –323.32495 –323.32495 –323.31813 –189.08849 –188.77932 –134.77331 –188.57883 –246.88657 –76.41285 –113.31231 –132.88231 –1.16885 –228.38283 –228.68296

–323.51603 –323.17842 –323.17103 –323.15885 –323.17114 –323.17877 –323.14852 –246.76496 –133.96822 –134.02300 –323.12227 –323.15283 –323.12227 –323.17357 –323.17352 –323.20005 –323.18385 –323.18385 –323.17514 –189.00486 –188.70833 –134.70612 –188.50435 –246.77214 –76.38373 –113.26997 –132.81758 –1.16748 –228.28080 –228.57994

c

B3LYP stands for the B3LYP/6-31++G(d,p) level. b In the dissociation pathways of I+ discussed in this work, the energy of I is defined as zero. c In the dissociation routes of II+, the energy of II is defined as zero. a

studied at the G3B3 level,24,25 which is an approximation for the QCISD(T)/GTlarge energy. Further single-point energies were calculated at the QCISD(T,E4T)/6-31G(d), MP4/631+G(d), MP4/6-31G(2df,p), and MP2(full)/GTlarge levels with the structures optimized at the B3LYP/6-31G(d) level. A semiempirical correction was also applied to account for the high level correlation effect. All the theoretical energies used in this work are electronic energies with ZPE correction, as listed in Table 1. The energies of the neutral conformers are defined as zero. The relative energies of all transition states, intermediates, and corresponding dissociation products discussed in this work were obtained with the G3B3 method unless otherwise indicated. The optimized two stable conformers of sarcosine and the corresponding cations at the B3LYP/6-31++G(d,p) level are shown in Figure 1. The geometries of two stable conformers of neutral sarcosine in this work are the same as that reported previously.15,16 The conformer I including C3-H · · · O1 and N-H · · · O1 intramolecular HBs is more stable than the conformer II with O2-H · · · N intramolecular HB. For IfI+, the intramolecular HB of N-H · · · O1 breaks and the HB length of C3-H · · · O1 decreases from 2.754 to 2.254 Å. Simultaneously, the bond angel of H-N-C2 increases from 108.3 to 117.7°. For IIfII+, the intramolecular HB of O2-H · · · N is elongated from 1.927 to 2.344 Å. In the ionization process of the neutral sarcosine, the C1-C2 bond length is elongated (1.528 Å in I to 1.585 Å in I+, 1.537 Å in II to 1.658 Å in II+), accompanied by C3-N and C2-N bonds shortening. The elongation of the C1-C2 bond length indicates that the C1-C2 bond cleaves easily and the formation of corresponding fragment ions are more favorable than that of the product ions derived from other bond cleavage.

Figure 1. Geometries of the two stable conformers of neutral sarcosine and corresponding cations at B3LYP/6-31++G(d, p) level. (All distances are in Å, the bond angle is in degrees.)

Meanwhile, the conformer cation I+ can be transformed to the conformer cation II+ via two transition states TS1 and TS2 with the energy barriers of 0.52 and 0.27 eV, respectively, as shown in Scheme 1. The geometries of transition states and intermediates in the interconversion between I+ and II+ are shown in Scheme 1 as well. 3. Results and Discussion 3.1. Photoionization Mass Spectra. The photoionization and dissociative photoionization mass spectra of sarcosine at dif-

VUV Photofragmentation of Sarcosine

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SCHEME 1: Conformational Conversion between I+ and II+ at the G3B3 Levela

Figure 3. PIE spectrum of molecular ion.

a

The values in brackets were calculated at the B3LYP/6-31++ G(d, p) level. All the geometries were optimized at the B3LYP/631++G(d, p) level. (Bond length is in Å, and bond angle is in degrees).

SCHEME 2: Formation Pathways of CH3NHCH2+ (m/z 44) Calculated at the G3B3 Levela

a

The values in brackets are obtained at the B3LYP/6-31++G(d,p) level. Figure 2. Photoionization mass spectra of sarcosine at different photon energies with the partial ion signals amplified by different factors labeled in the figure.

ferent photon energies are shown in Figure 2. Partial ion signals are amplified by different factors, as labeled in the figure. At low photon energy (9.0 eV), only the molecular ion at m/z 89 was detected by near-threshold single-photon ionization (SPI), which is a soft ionization technique.26-28 With photon energy increasing to 11.0 and 12.0 eV, the fragments at m/z 30, 42, 43, 44, 45, and 60 were obtained. Among the mass spectra of sarcosine, the fragment ion m/z 44 has the strongest intensity, indicating that the formation pathway of m/z 44 is more favorable compared to the formations of other fragment ions. Similar results have been reported in the electron-impact ionization (EI) mass spectrum of sarcosine at 70 eV29 and in the photoionization processes of R-alanine.30 Consecutively altering VUV photon energy, PIE spectrum of the molecular ion can be obtained. The neutral plume of sarcosine was generated during IR laser desorption process.

Thus, the hot-band effect will result in a thermal tail in PIE of the molecular ion, which makes it difficult to ascertain the accurate threshold. In addition, poor Franck-Condon factor near ionization threshold causes a not-obvious onset. Some methods have been employed to determine the ionization threshold.31-33 In this work, it is assumed that the thermal tail near ionization threshold is dominantly affected by thermal energy from laser heating. IE of sarcosine is determined to be 8.42 ( 0.10 eV (see Figure 3) based on the first discernible onset, which is comparably to the previous value of 8.40 eV with photoelectron spectroscopy.34 The calculated adiabatic IE value of sarcosine is determined to be 8.62 eV, which is a little higher than the experimental value. 3.2. Fragmentation Pathways. Detailed formation pathways of dominant fragment ions were established with the help of calculations at the G3B3 and B3LYP/6-31++G(d,p) levels. The calculated energies at the two levels are listed in Table 1. The proposed fragmentation channels for the main product ions are shown in Schemes 2-7, which display the involved geometries of transitions, intermediates and some products in the dissociation processes of sarcosine cation.

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SCHEME 3: Formation Pathway of CH3NH2CH2+ (m/z 45) from II+ Calculated at the G3B3 Levela

HB of C3-H · · · O1 are elongated to 3.068 and 2.910 Å, respectively. Then the C1-C2 bond cleaves to lose the COOH-1 radical to generate CH3NHCH2+. The dissociation channel from II+ is similar to that from I+. From II+, the fragment CH3NHCH2+ is formed via transition state TS4 with an energy barrier of 0.20 eV, as shown in Scheme 2. Along the pathway, the C1-C2 bond length and the O2-H · · · N HB length are stretched to 2.383 and 3.333 Å in TS4. With the loss of COOH-2 radical, the final product of CH3NHCH2+ is yielded. (2) Formation Pathway of CH3NH2CH2+ (m/z 45). The analogous pathways for the generation of the fragment ion at m/z 45 have been reported previously in the photoionization investigations of R-alanine and β-alanine,30,36,37 which are the isomers of sarcosine. In this work, similar process of sarcosine was investigated. The formation of CH3NH2CH2+ (m/z 45) with CO2 releasing from the conformer cation II+ is briefly described as reaction c

a The values in brackets were obtained at the B3LYP/6-31++G(d,p) level.

CH3NHCH2COOH+(II+) f CH3NH2CH2+ + CO2

(1) Formation Pathways of CH3NHCH2+ (m/z 44). The fragment ion at m/z 44 assigned to CH3NHCH2+ is derived from the C1-C2 bond fission. As reported previously, the loss of COOH radical is the most favorable dissociation pathway of amino acids,35 which is supported by this work. As seen in Figure 2, the fragment at m/z 44 has the highest intensity. The dissociation pathways to form CH3NHCH2+ from I+ and II+ are described briefly in reactions a and b

I+ f CH3NHCH2+ + COOH-1

(a)

II+ f CH3NHCH2+ + COOH-2

(b)

Detailed formation pathways of CH3NHCH2+ with potential energy surface are depicted in Scheme 2. From I+, the CH3NHCH2+ ion can be formed via transition state TS3 with an energy barrier of 0.55 eV. In TS3, the C1-C2 bond and the

(c) As shown in Scheme 3, the hydroxyl hydrogen atom migration toward the N atom leads to the formation of intermediate INT2 via the transition state TS5 with an energy barrier of 0.53 eV. The O2H · · · N bond and O2-H bond lengths are 1.336 and 1.204 Å in TS5, respectively. While the corresponding bond lengths are 2.344 and 0.971 Å in II+. Then the H-O2 bond cleaves and the intramolecular hydrogen transfer (IHT) from hydroxyl oxygen to nitrogen atom undergoes to form the intermediate INT2. With the C1-C2 bond length increasing to 1.805 Å in transition state TS6, the C1-C2 bond cleaves to yield the product ion CH3NH2CH2+ accompanied by CO2 releasing via TS6 with an energy barrier of only 0.06 eV. Additionally in Scheme 2, if the positive charge is transferred to carboxyl group in transition states TS3 and TS4, the fragment ions of COOH-1+ and COOH-2+ (also m/z 45) are yielded from I+ and II+, respectively. Because of the energy barriers for the formation of COOH-1+ and COOH-2+ are higher than that of CH3NH2CH2+, the charge transfer pathways to form COOH-

SCHEME 4: Formation Pathways of CH2NHCH2+ (m/z 43) from I+ and II+ Calculated at the G3B3 Levela

a

The values in brackets were obtained at the B3LYP/6-31++G(d, p) level.

VUV Photofragmentation of Sarcosine

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SCHEME 5: Formation Pathways of CH2NCH2+ (m/z 42) and CH2NHCH+ (m/z 42) Calculated at the G3B3 Levela

a

The values in brackets were obtained at the B3LYP/6-31++G(d, p) level.

SCHEME 6: Formation Pathways of CH2NH2+ (m/z 30) Calculated at the G3B3 Levela

a

The values in brackets were obtained at the B3LYP/6-31++ G(d, p) level.

1+ and COOH-2+ are less favorable than the pathway to form CH3NH2CH2+ from II+. (3) Formation Pathways of CH2NHCH2+ (m/z 43). The fragment ion of CH2NHCH2+ (m/z 43) can be produced via two proposed formation pathways, which are displayed in Scheme 4. For the formation pathway of CH2NHCH2+ from I+, the IHT to carbonyl oxygen (O1 atom) from C3 atom occurs to form intermediate INT3 via transition state TS7 with an energy barrier of 0.79 eV. Then the fragment ion of CH2NHCH2+ is produced via C1-C2 bond fission. For the formation pathway of CH2NHCH2+ from II+, the HB of N · · · H-O2 cleaves and one hydrogen atom at C3 atom gets close to O2 atom to form intermediate INT4 via transition state TS8. Then the hydrogen atom from C3 atom migrates to O2 atom via transition state TS9 with an energy barrier of 1.40 eV. Meanwhile, the C1-O2 bond length increases to 1.718 Å

in TS9, which is 1.335 Å in INT4. Subsequently the H2O moiety in TS9 is removed to form CH2NHCH2CO+ (m/z 71), which is not found in Figure 2. The probable reason is that CH2NHCH2CO+ is unstable and tends to dissociate. Then the C1-C2 bond length increases to 1.937 Å in transition state TS10 with an energy barrier of 0.20 eV. And the product ion CH2NHCH2+ (m/z 43) is formed with the removal of carbon monoxide. Comparing the two formation pathways of CH2NHCH2+ (m/z 43), the energy barrier of CH2NHCH2+ yielded from II+ is lower than that from I+, thus the latter one from II+ is more favorable. (4) Formation Pathways of CH2NCH2+ (m/z 42). As shown in Figure 2, the ion signal of fragment ion at m/z 42 appears with the increase of photon energy. Two formation pathways of the m/z 42 ion are postulated, one pathway undergoing dehydration and aldehyde-removing from I+ and the other one involving dehydrogenation of CH3NHCH2+, which are depicted briefly in reactions d and e, respectively

CH3NHCH2COOH+(I+) f CH2NH2CH2+ + H2O + CO2 (d) CH3NHCH2+ f CH2HCH2+ + H2

(e)

The detailed formation pathway of CH2NCH2+ from I+ along reaction d is shown in Scheme 5a. Because of the carboxyl group rotation about the C1-C2 bond, one methyl hydrogen atom approaches to the hydroxyl oxygen atom (O2 atom), and the distance between the two atoms is 3.505 Å in intermediate INT5. Then the hydrogen atom migrates to the O2 atom and the H2O moiety is removed to produce intermediate INT6 with the C1-O2 bond elongated to 1.892 Å in transition state TS12, which has an energy barrier of 1.40 eV. In INT6, one hydrogen atom on the N atom is transferred to the C1 atom to form the intermediate INT7 with an aldehyde group via transition state TS13 with an energy barrier of 1.61 eV, as shown in Scheme

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SCHEME 7: Formation Pathways of CH3COOH+ (m/z 60) and CH2COOH2+ (m/z 60) Calculated at the G3B3 Levela

a

The values in brackets were obtained at the B3LYP/6-31++G(d, p) level.

5a. Also the C1-C2 bond is elongated to 1.654 Å in INT7 from 1.520 Å in TS13 and meantime the C2-N bond is shortened. Then C1-C2 bond cleaves to yield CH2NCH2+ (m/z 42) accompanied by loss of HCO radical with barrier of 0.28 eV. The further dissociation of CH3NHCH2+ (m/z 44) produced from II+ to form CH2NHCH+ (m/z 42) and H2 via two competitive formation pathways is shown in detail in Scheme 5b. The dissociation of CH3NHCH2+ (m/z 44) yielded from I+ is similar to the process shown in Scheme 5b, which is not discussed in this work. For CH2NCH2+ formation along reaction e, two hydrogen atoms are removed from N and C3 atoms via transition state TS14 with an energy barrier of 5.36 eV. However for CH2NHCH+ formation, two hydrogen atoms are taken out from C2 and C3 atoms by transition state TS15 with an energy barrier of 3.87 eV. Because of that the relative energy of CH2NCH2+ is lower than CH2NHCH+ by 3.15 eV, and the formation pathway to form stable CH2NCH2+ is more favorable than that to form CH2NHCH+, though the energy barrier of TS14 is higher than that of TS15 by 1.49 eV. Furthermore, comparing the two formation pathways of CH2NCH2+, shown in Scheme 5, the appearance energy of CH2NCH2+ along reactions d and e is calculated to be 11.46 and 14.77 eV, respectively. Thus the formation pathway of CH2NCH2+ along reaction d is more favorable. (5) Formation Pathways of CH2NH2+ (m/z 30). Two proposed formation pathways of CH2NH2+ (m/z 30) derived from C2-N bond fission from I+ and II+ are presented in Scheme 6. The backbone cleaving to yield the same product CH2NH2+ (m/z 30) has been reported in the photodissociation of β-alanine.37 From I+, the IHT from C3 atom to N atom occurs to form intermediate INT8 via transition state TS16 with an energy barrier of 1.81 eV. Then the C2-N bond is elongated to 2.090 Å in TS17 from 1.527 Å in INT8. The transition state TS17 has an energy barrier of 0.92 eV. The product ion CH2NH2+ is yielded with CH2COOH-1 radical releasing. From II+, the formation pathway of CH2NH2+ is similar to that from I+. The hydrogen atom from C3 atom migrates to N atom to form intermediate INT9 via transition state TS18 with an energy barrier of 1.52 eV. Then the C2-N bond length

increases to 2.055 Å in transition state TS19, which has an energy barrier of 0.84 eV. The product ion CH2NH2+ is yielded with loss of CH2COOH-2 radical. (6) Formation Pathways of CH2COOH2+ (m/z 60). The fragment ions at m/z 60 assigned to CH3COOH+ or CH2COOH2+ are proposed to be formed with the loss of CH2NH along two different formation pathways from I+ and II+, respectively, as shown in Scheme 7. In the formation pathway of CH3COOH+ from I+, the hydrogen atom from N atom gets close to C2 atom via transition state TS20 with an energy barrier of 3.92 eV, in which the C2-N bond length increases to 2.248 Å. Subsequently, the hydrogen atom transfers to C2 atom accompanied by the migration of the other hydrogen atom on the C3 atom to N atom simultaneously. And the product ion CH3COOH+ is yielded via C2-N cleaving to lose CH2NH moiety. In the formation pathway of CH2COOH2+ from II+, one hydrogen atom is transferred from C3 atom to N atom to produce the intermediate INT10 with the C2-N bond length elongation to 1.527 Å via transition state TS21 with an energy barrier of 1.52 eV. In INT10, one hydrogen atom on the N atom approaches to the carbonyl oxygen atom (O1 atom) to form an intramolecular HB (NH · · · O1 bond) with the bond length of 1.815 Å. Then the hydrogen atom is transferred to the O1 atom to form intermediate INT11 via transition state TS22 with an energy barrier of 0.28 eV. The C2-N bond is elongated to 1.457 Å in INT11 from 1.430 Å in TS22. Then the product ion CH2COOH2+ is yielded with the loss of CH2NH. The energy barrier of TS21 is lower than that of TS20 by 2.40 eV, thus the formation pathway of CH2COOH2+ is more favorable than that of CH3COOH+. 4. Conclusion The photoionization and dissociative photoionization of sarcosine have been investigated with IR LD/VUV PIMS and theoretical calculations at the G3B3 and B3LYP/6-31++G(d,p) levels. The dominant fragments at m/z 44, 45, 43, 42, 30, and 60 are assigned to CH3NHCH2+, CH3NH2CH2+, CH2NHCH2+,

VUV Photofragmentation of Sarcosine CH2NCH2+, CH2NH2+, and CH2COOH2+, respectively. The IE of sarcosine was obtained by measuring the PIE spectrum. The detailed fragmentation pathways of the dominant fragment ions from the conformers I+ and II+ are discussed in detail with the help of theoretical calculations at the G3B3 level. In summary, CH3NHCH2+ (m/z 44) is formed via the C1-C2 bond cleaving to lose the COOH-1 and COOH-2 radicals from I+ and II+, respectively. CH3NH2CH2+ (m/z 45) derived from C1-C2 bond fission is produced with the release of CO2 from II+. CH2NHCH2+ (m/z 43) is formed from II+ with the losses of H2O and CO and from I+ with the loss of HOCOH. CH2NCH2+ (m/z 42) is produced from I+ via dehydration and aldehyde-removing. The C2-N bond cleaves to produce CH2NH2+ (m/z 30) with the losses of the CH2COOH-1 and CH2COOH-2 radicals, and CH2COOH2+ (m/z 60) is formed via the loss of CH2NH derived from C2-N bond fission too. Acknowledgment. This work has been supported by grants from Chinese Academy of Sciences (YZ200764), the Natural Science Foundation of China (10705026 and 10805047), and Special Grade of the Financial Support from China Postdoctoral Science Foundation (200801223). References and Notes (1) Huber, C.; Wachtershauser, G. Science 2006, 314, 630. (2) Huber, C.; Wachtershauser, G. Science 1998, 281, 670. (3) Miller, S. L.; Orgel, L. E. The Origins of Life on the Earth; PrenticeHall. Inc.: Englewood Cliffs, NJ, 1974. (4) Miller, S. L. Science 1953, 117, 528. (5) Miller, S. L. Biochim. Biophys. Acta 1957, 23, 480. (6) Miller, S. L.; Urey, H. C. Science 1959, 130, 245. (7) Wachtershauser, G. Science 2000, 289, 1307. (8) Cronin, J. R.; Pizzarello, S. Science 1997, 275, 951. (9) Oro´, J.; Gibert, J.; Lichtenstein, H.; Wikstrom, S.; Flory, D. A. Nature 1971, 230, 105. (10) Lawless, J. G.; Boynton, C. D. Nature 1973, 243, 405. (11) Lawless, J. G.; Kvenvolden, K. A.; Peterson, E.; Ponnamperuma, C.; Moore, C. Science 1971, 173, 626. (12) Mendum, M. L.; Smith, L. T. Appl. EnViron. Microbiol. 2002, 68, 813. (13) Wagner, M. A.; Jorns, M. S. Biochemistry 2000, 39, 8825.

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