Theoretical and Experimental Study of Valence Photoelectron

Article pubs.acs.org/JPCA

Theoretical and Experimental Study of Valence Photoelectron Spectrum of D,L-Alanine Amino Acid H. Farrokhpour,†,* F. Fathi,† and A. Naves De Brito‡,§ †

Chemistry Department, Isfahan University of Technology, Isfahan 84156-83111, Iran Laboratório Nacional de Luz Síncrotron (LNLS), Box 6192-CEP, 13084-971 Campinas-Sp, Brazil § Instituto de Física “Gleb Wataghin”, Universidade Estadual de Campinas, 13083-859 Campinas-SP, Brazil ‡

S Supporting Information *

ABSTRACT: In this work, the He−I (21.218 eV) photoelectron spectrum of D,L-alanine in the gas phase is revisited experimentally and theoretically. To support the experiment, the high level ab initio calculations were used to calculate and assign the photoelectron spectra of the four most stable conformers of gaseous alanine, carefully. The symmetry adapted cluster/ configuration interaction (SAC-CI) method based on single and double excitation operators (SD-R) and its more accurate version, termed generalR, was used to separately calculate the energies and intensities of the ionization bands of the L- and D-alanine conformers. The intensities of ionization bands were calculated based on the monopole approximation. Also, natural bonding orbital (NBO) calculations were employed for better spectral band assignment. The relative electronic energy, Gibbs free energy, and Boltzmann population ratio of the conformers were calculated at the experimental temperature (403 K) using several theoretical methods. The theoretical photoelectron spectrum of alanine was calculated by summing over the spectra of individual D and L conformers weighted by different population ratios. Finally, the population ratio of the four most stable conformers of alanine was estimated from the experimental photoelectron spectrum using theoretical calculations for the first time.

1. INTRODUCTION Alanine is the simplest chiral natural α-amino acid that the crystal structure of its L-form has been determined by X-ray diffraction.1,2 It has also been studied in the gas phase by rotational spectroscopy.3−5 Godfrey et al.5 reported the rotational spectra of two conformers of alanine together with molecular orbital calculations at the Hartree−Fock (HF)/631G** level of theory and identified two conformers for this amino acid as alanine I and IIA (see Figure 1) in the gas phase. Cao et al.6 studied the stability of 13 different stable conformers of the neutral alanine molecule in the gas phase using the ab initio methods including the HF and second-order Moller− Plesset perturbation theory (MP2) methods using 6-31G* and 6-311G** basis sets. They found that the HF computational study with 6-31G** optimized geometry of conformer I is in agreement with one of the two experimental sets of rotational constants and dipole moment components of Godfrey et al.,5 while, at the MP2 level of theory, the corresponding parameters of the less stable conformer (IIA) agree with the same set of rotational constants and dipole moment components. In an alternative study, Godfrey et al.7 explained the loss of some conformers of alanine in the seeded supersonic jets through relaxation using ab inito calculations of the molecular potential energy hypersurface at the MP2/6-31G** level of theory. They found that the population of the conformers obtained from the rotational spectrum cannot be explained by only considering © 2012 American Chemical Society

the equilibrium structures on the potential energy surface of alanine and population ratio using the Boltzmann equation. In a completely theoretical work, Csaszar8 determined the accurate geometries, rotational and quartic centrifugal distortion constants, dipole moments, harmonic vibrational frequencies, and infrared intensities of 13 conformers of alanine using different computational methods and basis sets. They found that the theoretical prediction of the relative energies, even after zero-point vibrational energy corrections, deviate significantly from the experimental lower limits, reported by Godfrey et al.5 using their recorded rotational spectra. Their theoretical structural results supported the molecular constants measured for two conformers of lower relative energy. In an alternative study, Kaschner et al.9 investigated the alanine conformers using density functional theory (DFT) within a plane-wave pseudopotential scheme. They used local density approximation (LDA) along with three different gradient corrections for the systematic study of conformers. Blanco et al. 3 recorded the jet-cooled rotational spectrum of neutral alanine using laser-ablation molecular beam Fourier transform microwave spectroscopy (LA-MB-FTMW) and determined the structure of conformers I and IIA (Figure 1) Received: March 12, 2012 Revised: June 6, 2012 Published: June 21, 2012 7004

dx.doi.org/10.1021/jp3023716 | J. Phys. Chem. A 2012, 116, 7004−7015

The Journal of Physical Chemistry A

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measured the core level X-ray photoelectron and NEXAFS spectra of alanine in the gas phase at the carbon, nitrogen, and oxygen K edges and interpreted the results in the light of the theoretical calculations. They also determined the conformer abundance ratio (NI/NIIa ≈ 5) for alanine, which was in good agreement with the prediction of Blanco et al. and Balabin.3,4 In this work, the joint theoretical and experimental investigation of the valence photoelectron spectrum of alanine amino acid is reported, and the photoelectron spectrum of alanine is revisited. There are several motivations behind this effort: (1) To optimize the temperature so that there is sufficient concentration of the sample in the gas phase, while it is low enough for the sample not to decompose. Klasinc and Powis et al.11,24 performed their experiments at relatively high temperatures (286 and 268 °C, respectively) in order to reach a constant pressure of about 1 × 10−6 mbar in the main chamber. Most amino acids can be sublimated at temperatures below 230 °C;27 however, some of them are decomposed before going into the gas phase. This is the most challenging part of the photoelectron spectroscopy of amino acids, and one should avoid using a temperature at which the amino acid decomposition occurs. The species resulting from the decomposition of the sample adds new features to the spectrum leading to the changes in the relative intensity of the peaks in the main spectrum. (2) To the best of our knowledge, there is no reliable He−I photoelectron spectrum of alanine in the literature. The only reported He−I spectrum of alanine is that by Klasinc,11 which seemingly has a large amount of background on the high binding energy region. Therefore, the relative intensities of the features are not very accurate in the spectrum, and also, the peaks with lower intensities cannot be resolved in the background. By considering the He−I spectrum as a fingerprint of each compound and the importance of this spectral information in chemistry and biochemistry, it is desirable to revise the He−I spectrum of alanine. (3) The only existing ab inito calculations of the valence photoelectron spectrum of alanine are those reported by Powis et al. in which the outer-valence Green’s function (OVGF) method and cc-pVDZ basis set have been applied for theoretical calculations. They also have used CMS-Xα continuum multiple scattering in order to predict the intensity of vertical ionizations. It seems that no satisfactory agreement was observed between their theoretical spectrum and the spectrum recorded by Klasinc in terms of the intensity and position of the features, despite the fact that they pointed to such an agreement. Moreover, their calculated photoelectron spectrum resembled the theoretically calculated spectrum of conformer I. Therefore, a more advanced quantum calculation on the photoelectron spectrum of alanine seems to be necessary.

Figure 1. Geometrical structures of the five most stable neutral conformers of the L-alanine molecule in the gas phase.

of alanine for the first time. They also determined the relative abundance of conformers I and IIA (NI/NIIa = 3.75) in the supersonic expansion. The results were quite different from the hypothetical equilibrium relative populations of the lowestenergy conformers at 298 K calculated from the Gibbs free energies (NI/NIIa/NIIb/NIIIa = 68:10:11:11). 10 On the basis of their findings, this discrepancy could be attributed to the collisional relaxation during expansion leading to the conversion of the high energy conformers to the low energy conformers. Balabin et al.4 recorded the jet-cooled spontaneous Raman spectrum of alanine and observed four conformers for alanine, including two new conformers that are not reported by Blanco et al.3 and Godfrey et al.5 The evaluated conformer abundance ratio (NI/NIIa = 5) reported by Balabin et al. is close to the value obtained by Blanco. Moreover, they reported the experimental conditions under which the high energy conformers depopulated to the lower energy conformers in the jet. In addition to the rotational spectroscopy, core and valence photoelectron spectroscopy, and X-ray absorption spectroscopy, near edge X-ray absorption fine structure (NEXAFS) has been applied to most of the amino acids and nucleobases in the gas phase using a helium lamp and synchrotron as radiation sources.10−25 The elegant work by Klasinc and Debies11,13 as pioneers in the UV photoelectron spectroscopy of amino acids in the gas phase using He−I (21.218 eV) light is a distinguished example. Nowadays, both core and valence photoelectron spectroscopy have been significantly studied, employing computational modeling in order to determine the population ratios of different conformers and tautomers of amino acids and nucleotides in the gas phase.20−26 Powis et al.25 recorded the valence and C1s photoelectron spectrum of L-alanine using synchrotron radiation along with the theoretical calculations of its valence photoelectron spectrum. They found that good agreement between theoretical and experimental spectra may be achieved by considering just the contribution of a single molecular conformation (conformer I). In addition, Feyer et al.

2. THEORETICAL AND EXPERIMENTAL METHODS 2.1. Computations. Alanine has 13 different stable conformers in the gas phase,6 and only five lowest lying conformers were considered for the theoretical calculations in this work (Figure 1). The structures of the conformers were separately optimized at two different levels of theory including 7005



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the DFT-B3LYP28 and MP229 methods. The basis sets including 6-311++G**, 6-311G**, and cc-pVTZ were used for the geometry optimization. The optimized structures of alanine conformers at the MP2/6-311++G** level of theory were used for the calculation of the electronic energy by more advanced and expensive computational methods such as coupled cluster (CC) including all single and double excitations (CCSD), a perturbative correction for the contributions from the connected triple excitation (CCSD(T)), and SAC-CI methods (Table 1). In addition, the extrapolated MP2/cc-

correlation effects. The SAC-CI SD-R wave function is based on a cluster expansion of a CI calculation carried out over selected configurations. In the applied SAC/SAC-CI SD-R calculations, all of the one-electron operators and important two-electron operators were selected through the linked operators by perturbative selection procedure.39,40 The triple and quadruple excitation operators were generated as the product of the previous oneand two-electron excitation operators and were treated as unlinked operators. The highest recommended level (level three λg = 1.0 × 10−6; λe = 1.0 × 10−7) was used for the selection of the most important two electron excitation operators. The 6-311++G(d,p) basis set was used for the SAC/SAC-CI calculations. In addition to the SAC-CI SD-R calculations, the SAC-CI General-R method, with the same basis set, was used for calculating the photoelectron spectra of alanine conformers. The main objective was to consider the electron correlation more accurately and to obtain more accurate information about the energy position of ionization bands of conformers, especially in the range of 18 eV to 20 eV. Furthermore, NBO calculations using Gaussian NBO, version 3.1,41 at the HF/6-311++G** level of theory was used to calculate the molecular orbitals at the ground electronic state for better spectral band assignment. All calculations were carried out using the Gaussian09 Quantum Chemistry package.

Table 1. Theoretical Relative Electronic Energies (cal·mol−1) of L-Alanine Conformers (See Figure 1) Calculated in This Work method

I

IIB

IIA

IIIA

IIIB

6-311++G** MP2(Full) 6-311++G** SAC-CI//6-311+ +G**MP2(Full) 6-311++G** CCSD//6-311+ +G**MP2(Full) 6-311++G** CCSD(T)//6-311+ +G**MP2(Full) 6-311++G** B3LYP 6-311G** B3LYP cc-pVTZ MP2 6-311+G ** CCSD(T) // ccpVTZ MP2 6-311+G ** MP2//MP2 ccpVTZ 6-311+G**CCSD(T)//6-311+ +G** MP2(Full) 6-311+G**MP2//6-311++G** MP2(Full) E*limitb E**limitc

0 0

476 3022

146 6140

957 1606

1289a

0

1027

845

1033

0

603

427

994

0 0 0 0

13 407 123 608

28 −13 −224 467

1067 1370 1103 1000

0

480

163

951

0

588

425

991

0

456

142

955

0 0

252 608

76 451

1154 992

1123a

42

2.2. Experiments. The commercial sample of D,L-alanine was used as the crystalline powder without further purification and with the minimum purity of 99%. The D,L-alanine was sublimed using a heated oven in the ultrahigh vacuum chamber as described elsewhere.43 Before recording the photoelectron spectrum, the thermal degradation and stability of the alanine were checked at different temperatures, in a separate experiment, by mass spectrometry technique using a quadruple mass spectrometer (PrismaPlus-Pfeiffer Germany). Figure 2 shows the pressure normalized mass spectra of D,L-alanine recorded at different temperatures. The recorded mass spectra at different temperatures were compared with the standard mass spectrum

Note that the potential energy surface is extremely flat around the stationary point. bE*limit = EMP2/cc‑pVTZ + (ECCSD(T)/6‑311+G** − EMP2/6‑311+G**). cE**limit = EMP2(full)/6‑311++G**+ (ECCSD(T)/6‑311+G** − EMP2/6‑311+G**). a

pVTZ and MP2/6-311++G** energies of conformers were calculated. The potential energy surface was extremely flat around the stationary point for conformer IIIB; therefore, only four conformers (I, IIA, IIB, and IIIA) of alanine were considered. The optimized structures of alanine conformers at the MP2/ 6-311++G** level of theory were used to separately calculate the energies of the vertical outer valence photoelectron spectra using SAC/SAC-CI single and double SD-R and General-R methods.30−35 This computational method was also used in the previous work for calculating the He−I valence photoelectron spectra of hypoxanthine, xanthine, and caffeine.36 In fact, SAC/ SAC-CI SD-R is a recommended level of theory for studying the ionization of valence and Rydberg electrons.37 This method is able to describe the electron correlation effects in valence and Rydberg electronic states in an accurate and balanced way and therefore to provide a standard methodology for studying various kinds of fine spectroscopy. The ionization crosssections were calculated using the monopole approximation,38 which allows for the correct estimation of the relative intensity of the peaks. In calculating the monopole intensity, the correlated SAC wave function and SAC-CI SD-R wave function were used for the ground state and ionized states, respectively. This approach allows for the inclusion of both initial and final

Figure 2. Mass spectra of D,L-alanine at different temperatures and the pressure of the gas phase sample. 7006



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of D,L-alanine reported in the NIST Chemistry WebBook44 for any spurious peak related to the decomposition of alanine. In order to obtain a temperature limit for the thermal stability and degradation of D,L-alanine, the relative peak intensities of the ionic fragments to the intensity of a reference peak (44 m/z) were investigated by the gradual temperature changes of the oven applying 5 °C temperature intervals. To ensure the thermal equilibrium, 30 min of incubation time was performed at each step, and then the mass spectrum was recorded. Since, at 130 °C, there was sufficient concentration of alanine in the gas phase and the pressure was in the range of 10−6 torr, the temperature was not increased any more. After determining the suitable temperature, the photoelectron spectrum of D,L-alanine was recorded in an alternative experiment using the photons from a commercial noble gas resonance lamp (VSW Scientific Instrument) with helium (He−I, hν = 21.21 eV) at the Laboratório Nacional de Luz ́ Sincrotron (LNLS) in Brazil. A modern hemispherical electron analyzer (Scienta R-4000) mounted in a chamber at the right angle perpendicular to the light direction was used to record the photoelectron spectrum of alanine. The He−I light of the lamp was not polarized so that the measurements were insensitive to the photoelectron asymmetry parameter, β; hence, the electron analyzer was not placed at the magic angle (54.7°). The pass energy of the electron analyzer was selected as 100 eV. Before starting the experiment and after finishing it, the valence photoelectron spectrum of the noble gas (krypton) was recorded to obtain the total resolution (photon + analyzer) of the experiment, which is about 100 meV. The recorded valence photoelectron spectrum of Kr was also used for the kinetic energy calibration of the recorded alanine photoelectron spectrum. To avoid the deposition of alanine vapor on the entrance of the electron analyzer, its lenses, slits, and the internal walls of the experimental chamber, the temperature of the chamber and electron analyzer was kept in the range of 80− 90 °C. Before recording the photoelectron spectrum of the sample, the mass spectrum of alanine was recorded. Twenty successive scans of photoelectron spectra were then recorded and summed to increase the signal-to-noise. The mass spectrum of D,L-alanine was recorded soon after and compared with the ones recorded at the beginning of the experiment so as to follow the concentration of the sample in the chamber. It should be mentioned that there was no changes in the intensity of the peak fragments and this showed that the pressure of the sample was nearly constant during the recording of the first set of the photoelectron spectra. The sum of the first set of photoelectron spectra was normalized to the pressure of alanine at the beginning of the experiment. During the recording of photoelectron spectra, four sets of photoelectron spectra, each set containing 20 spectra, were recorded; then, the sets were summed together to have the photoelectron spectrum of D,Lalanine. It should be mentioned that the recorded photoelectron spectrum was corrected for the transmission of the electron analyzer.

Figure 3. Comparison of the He−I (21.218 eV) valence photoelectron spectrum of D,L-alanine, recorded in this work (red trace) with (a) He−I valence photoelectron spectrum recorded by Klasinc11 and (b) valence photoelectron spectrum of L-alanine, recorded by Powis et al.,24 using synchrotron radiation at 92 eV photon energy. (The present spectrum was scaled so that to be comparable with the Powis spectrum. The vertical dashed lines are for comparing the position of corresponding features in the experimental spectra.)

lines) of the spectrum presented in Figure 3 (present work) with the one recorded by Klasinc, especially below 16 eV. However, there is an unwanted background in the Klasinc spectrum, which increases with the binding energy so that the feature appearing around 20 eV is completely absent in our spectrum. Therefore, it can be concluded that the relative intensity of the features appearing above 12 eV in Klasinc spectrum are not reliable. For example, the intensities of two features, marked with asterisks in Figure 3a are equal in the Klasinc spectrum, while their intensities are not equal in the spectrum obtained in the present work. Figure 3b also demonstrates that the He−I recorded spectrum in the present work can well describe the features in the Powis et al. spectrum. It can be seen that the relative intensity of the features in the Powis et al. spectrum is completely different from that of our spectrum because of the change in the ionization cross-section resulting from the change in the photon energy from 21 to 92 eV. For example, the intensity of the first three features in the Powis et al. spectrum is much higher than those observed in the present spectrum. Interestingly, the feature appearing at nearly 19 eV in the spectrum of Powis et al. is clearly seen in the present spectrum (inset of Figure 3b) but cannot be distinguished in the Klasinc spectrum due to its unwanted background.

3. RESULTS AND DISCUSSION 3.1. Experimental Valence Photoelectron Spectra. The recorded He−I (21.218 eV) photoelectron spectrum of D,Lalanine is displayed in Figure 3. The spectra recorded by Klasinc 11 (Figure 3a) and the one recorded by Powis et al.24 using the synchrotron radiation at 92 eV of photon energy (Figure 3b) are also included. It is clear that there is a good correspondence between the peak positions (vertical dashed 7007



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3.2. Thermochemistry of Alanine. In order to confirm the experimental result, it is necessary to calculate the photoelectron spectrum of alanine. However, before calculating the photoelectron spectrum, we should determine the population ratio of the four stable conformers of alanine at the temperature of the experiment (403 K). For this purpose, the structures of the four conformers of L-alanine (Figure 1) were optimized by applying different theoretical methods including MP2 level using 6-311++G** and cc-pVTZ basis sets and B3LYP using 6-311++G** basis set (see Table 1). The optimized structures at the MP2 and B3LYP/6-311++G** levels of theory were also used to calculate the relative electronic energies of conformers using more expensive and correlated methods such as CCSD, CCSD(T), and SAC-CI and the same basis set (Table 1). The latter calculations are not used in the Csaszar’s article.8 It is evident that the order of the relative stability of conformers, based on their electronic energy, is dependent on the applied theoretical method and basis set, resulting from the presence of a delicate balance of covalent and nonocovalent interactions (especially hydrogen bonding) in the alanine conformers. The correct description of this balance strongly depends on the employed theoretical level and basis set. For example, the order of the stability of the three conformers (IIA, IIB, and IIIA) using SAC-CI/6-311++G** is IIIA > IIB > IIA, while this order changes to IIA > IIB > IIIA (Table 1) for CCSD(T)/6-311++G**. It is also evident (Table 1) that all the applied theoretical methods predicted that the conformer I is the most stable conformer except for MP2/ccpVTZ and B3LYP/6-311G** levels of theory, which predicted IIA as the most stable conformer. This shows that consideration of the diffuse function in the applied basis set is necessary for studying the stability of alanine conformers because of the intramolecular hydrogen bonding interactions. In addition, similar to the work reported in ref 45, the extrapolated relative MP2 energy of the alanine conformers, which was calculated in this work, is reported in Table 1. In this calculation, the energy of the optimized structures at MP2/ccpVTZ and MP2/6-311++G** were used separately as a starting point for the calculation of the extrapolated relative electronic energy (Elim). Because of higher electron correlation effects (estimated as the differences of MP2 and CCSD(T) energies for the 6-311+G** basis set), corrections were added to these values. Two different values of Elim for each conformer are reported in Table 1, which demonstrates that the predicted order of the stability of the conformers is similar to the prediction of CCSD(T)/6-311++G**. Meanwhile, the relative electronic energies of the four conformers of D-alanine were calculated at the MP2 level of theory, and it was concluded that the relative stabilities of different conformers for L- and Dalanine are almost the same. Table 2 reports the values of Gibbs free energy and the Boltzmann population ratio of L-alanine conformers at 403 K obtained from MP2/6-311++G**, B3LYP/6-311++G**, and B3LYP/6-311G** calculations and their extrapolated values (G*L, G**L) based on the values of Elim, reported in Table 1. To obtain Glim, the Gibbs free energies (evaluated at the B3LYP/6-311G** level of theory), zero point vibrational energy, temperature-dependent enthalpy, and entropy were added to Elim. For example, the predicted hypothetical equilibrium relative population using G**L, is NI/NIIa/NIIb/ NIIIa = 57:13:6:24. Considering that the conformers IIIA and IIB in the supersonic jet expansion are converted to conformers I and IIA,3,4 respectively, a postexpansion relative population

Table 2. Theoretical Relative Gibbs Free Energies (cal·mol−1) and Boltzmann Population Ratios of Alanine Conformers (See Figure 1) Calculated at 403 K method

I

IIB

IIA

IIIA

6-311++G** MP2(full) 6-311++G** B3LYP 6-311G** B3LYP G*La G**Lb

0 (0.58)

1151 (0.14)

1196 (0.13)

1079 (0.15)

0 0 0 0

849 (0.19) 721 (0.20) 1379 (0.09) 1736 (0.06)

795 (0.17) 759 (0.19) 847 (0.19) 1223 (0.13)

1000 (0.14) 1065 (0.13) 848 (0.19) 687 (0.24)

(0.50) (0.48) (0.53) (0.57)

G*L = E*limit + G B3LYP/6‑311G** − E B3LYP /6‑311G**. bG**L = E**limit + G B3LYP/6‑311G** − E B3LYP /6‑311G**.

a

NI/NIIa = 4.26 is obtained based on the G**L. This ratio is in good agreement with the one obtained from the experiment,3,4 although the population ratio of conformer IIIA was greater than that of IIA and IIB. 3.3. Hartree−Fock Molecular Orbitals. Table 3 reports the energies of the HF main valence molecular orbitals for conformers I and IIA calculated at the HF/6-311++G(d,p) level of theory. The HF energies of the main valence molecular orbitals of conformer IIB are reported in Tables S1 in the Supporting Information. In addition, the contribution of the dominant natural bonding orbitals (NBOs) obtained from NBO calculations 41 at the same level of theory is listed in Tables 3 and S1, Supporting Information. Here, only the molecular orbitals of conformers I and IIA are explained, and the molecular orbital shapes of the conformers I and IIA are shown in Figure 4. The shapes of the molecular orbital of conformers IIB and IIIA are also given in Figure S1 in the Supporting Information. On the basis of the point group of molecules obtained during geometry optimization (C1), there is no symmetry element and symmetry-based orbital classification scheme to offer convenient labeling and characterization of the orbitals. Therefore, various molecular orbitals were identified with a sequence number by increasing the eigenvalue. As reported in Table 3, the highest occupied molecular orbital (HOMO) of the conformer I is due to the lone pair of the nitrogen atom (N1 in Figure 1), while there is a contribution from the lone pair of oxygen of carbonyl (O10) for the conformer IIA in addition to the lone pair of N1. Therefore, the HOMO of conformers I and IIA were considered as σ orbital in the present work. The molecular orbital No. 23 in the conformer I corresponds mainly to the lone pair of oxygen of the carbonyl group (O10) and could be assumed as σ. In addition, there is a considerable contribution (37%) from the π molecular orbital of the CO bond and the lone pair of N1 and O12 of NH2 and OH groups in the conformer IIA. The third molecular orbital (No. 22) in conformers I and IIA has the character of π originating from the CO bond and the lone pair of O12. Molecular orbital No. 22 can be considered as an OCO π* molecular orbital in conformers I and IIA. The prediction of NBO calculation about the π* character of molecular orbital No. 22 is in agreement with the assignments for the first three photoelectron spectra bands suggested by Cannington and Ham. 46 The remaining molecular orbitals of conformers I and IIA of alanine can be regarded as σ (except No. 16) and do not need further discussion. The molecular orbital No. 16 had the π character and could be assumed as the OCO π orbital. 3.4. Theoretical Valence Photoelectron Spectra. As mentioned before, the SAC-CI SD-R method was used to 7008



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Table 3. Calculated HF Energies E (eV) and the Dominant Natural Bonding Orbital Contributions (σ, π, and Lone Pair) in the Molecular Orbitals of L-Alanine (Conformers I and IIA). conformer I

conformer IIA

main characters > 14%

E (eV)

type σ

MO

E (eV)

25 (LUMO) 24 (HOMO)

1.11 −11.15

σ

70% LP N1

0.76 −11.47

23

−12.60

σ

60% LP O10

−12.57

π*

22

−13.33

π*

58% π (C6− O10)

−12.81

σ

21

−13.95

σ

−14.43

σ

20

−14.58

σ

−14.72

σ

19

−14.97

σ

−15.21

σ

18

−16.32

σ

−16.04

σ

17

−16.61

σ

27% σ (C5− H7) 19% σ (N1− H11) 25% LP O12

−17.02

π

16

−17.33

π

−17.13

σ

15

−18.36

σ

40% LP O12 29% π (C6− O10) 38% σ (N1− H2)

−18.40

σ

14

−19.27

σ

28% LP O12

−19.31

σ

13

−19.50

σ

35% LP O10

−19.78

σ

12

−21.79

σ

−21.94

σ

11

−24.75

σ

23% σ (O12− H13) 21% σ (C6− O12) 26% σ (C3− C6) 15% σ (O12− H13)

−24.6

σ

type

27% LP O12 28% σ (C3− H4) 26% σ (C5− H7) 20% σ (C3− C5) 43% σ (C5− H8) 17% σ (N1− C3) 54% σ (C5− H9)

main characters > 14% 39% LP O10 29% LP N 1 37% π (C6− O10) 17% LP N 1 16% LP O12 31% LP N 1 22% π (C6− O10) 17% LP O12 22% σ (C3− H4) 21% σ (C5− H7) 18% σ (C3− C5) 24% σ (C3− C5) 17% σ (C5− H9) 50% σ (C5− H8) 22% σ (C5− H7) 24% σ (C5− H9) 18% σ LP O12 20% π (C6− O10) 19% LP O12 29% σ LP O10

Figure 4. (a) Significant HF molecular orbitals of conformer I and (b) conformer IIA of alanine (the HF/6-311++g(d,p) energy of each molecular orbital is also shown).

photoelectron spectra of the conformers were calculated. The photoelectron spectrum of each conformer was produced by convoluting the discrete photoelectron bands with Lorentzian distribution function. The same Lorentzian width (600 meV; full width at the half-maximum) was considered for all the photoelectron bands. The bandwidth is due to the instrumental resolution, natural ionic state lifetimes, and unresolved vibrational structures. Figure 5 shows the calculated photoelectron spectra of four L-alanine conformers up to 21 eV. The calculated photoelectron spectra of D-alanine conformers were quite similar to the L-type conformers (Figure S2, Supporting Information). Tables 4−6 report the results of SAC-CI SD-R calculations for conformers I, IIA, IIB, and IIIA and compare them with the latest theoretical calculations at the OVGF/ccpVTZ//B3LYP/6-31G** level of theory.47 As indicated in Tables 4−6, the ionization bands of three conformers up to 21 eV are primarily due to one-electron ionization processes, while the effects of shakeup (1 hole and 1 photoelectron) and satellite (2 holes and 1 photoelectron) processes on the spectra are small. The first ionization band of conformers I, IIA, IIB, and IIIA was related to the ionization from their HOMOs. On the basis of the NBO calculations, their HOMOs are mainly due to the lone pairs of N1 and O10 (Figure 1). The second ionization band of conformer I is related to ionization from orbital No. 23, which has σ character due to the lone pair of O10. However,

28% σ (N1− H2) 23% σ LP O10 17% σ (N1− C3) 50% σ (N1− H11) 29% σ (C3− H4) 17% σ (C3− C6) 16% σ LP O12 42% σ (O12− H13) 31% σ (C6− O12) 30%σ (C3− C6)

calculate and assign the ionized states of L-alanine conformers up to 20 eV. Using the calculated positions and intensities of ionization bands based on the monopole approximation, the 7009



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Table 5. Calculated Ionization Potential (IP), Monopole Intensities, and the Main Configurations of the Ionized States of Conformer IIA by the SAC-CI SD-R Level of Theory and Using 6-311++G(d,p) Basis Set (The Ionization Energies of Conformer IIA Calculated by the OVGF Method24 Are Included As Well) ROVGF/ccpVDZ ionization energy (eV)

SAC-CI/6-311++G(d,p) states 2 A 1 2 3 4 5 6 7 8

Figure 5. Photoelectron spectra of L-alanine conformers calculated by SAC-CI/6-311++G** level of theory. (The red vertical arrows show the positions and the cross-sections of the ionization bands calculated in this work. The spectra shown with the dashed lines are the theoretical spectra of L-alanine calculated at OVGF/cc-PVDZ// B3LYP/6-31G** taken from ref 24. The blue vertical arrows show only the position of the ionization bands taken from ref 24).

9 10 11 12 13 a

Table 4. Calculated Ionization Potential (IP), Monopole Intensities, and the Main Configurations of the Ionized States of Conformer I by the SAC-CI SD-R Level of Theory and Using 6-311++G(d,p) Basis Set (Ionization Energies of Conformer I Calculated by OVGF Method24 Are Also Included)

states 2 A

main configuration (|C| > 0.3)

1 2 3 4 5 6 7 8 9 10 11 12

0.97(24) 0.96(23) 0.95(22) 0.93(21) −0.84(20) + 0.39(19) −0.84(19) − 0.44(20) 0.82(17) − 0.45(18) 0.84(18) + 0.48(17) −0.95(16) −0.91(15) − 0.30(13) 0.87(14) − 0.38(13) 0.83(13) + 0.39(14) − 0.30 (15) −0.61 (22, 24−35) − 0.53 (22, 24−34) + 0.33 (22, 24−39)

13 a

intensity

Ia

Ib

9.581 10.55 11.797 12.606 13.186 13.538 14.549 14.949 15.533 16.64 16.908 17.472

0.95135 0.94052 0.9444 0.95671 0.95587 0.95564 0.94711 0.95461 0.94021 0.94517 0.93491 0.94155

9.58 10.77 11.88 12.58 13.22 13.59 14.82 14.84 15.37 16.60 17.00 17.30

9.51 10.79 11.86 12.62 13.27 13.57 14.90 14.78 15.45 16.67 16.96 17.37

20.538

0.00441

19.64

19.62

B3LYP/6-31G** optimized geometries. +G** optimized geometries.

b

intensity

IIAa

IIAb

9.668 10.778

0.94533 0.9492

9.72 11.05

9.77 11.05

11.144

0.94714

11.08

11.07

12.735

0.9459

12.93

13.00

13.053

0.95253

12.96

13.00

13.928 14.344 14.95

0.95752 0.95124 0.9442

13.93 14.36 15.10

13.95 14.31 15.07

15.359

0.94609

15.14

15.24

16.541 17.779

0.94581 0.94928

16.42 17.63

16.47 17.63

17.82

0.9481

17.59

17.64

0.93367

19.54

19.56

19.725

b

Alternative MP2/6311+

the second ionization band of conformer IIA is due to a mix of two one electron ionization determinants of molecular orbital Nos. 23 and 22 with π* and σ characters, respectively. The second ionization band of conformer IIB is primarily due to ionization from molecular orbital No. 22, which is a π* orbital. It is interesting to note that, in contrast to the Koopmans' theorem,48 the order of molecular orbitals 22 and 23 in conformer IIB changed. The third photoelectron band in conformer I is mainly due to the ionization from OCO π* molecular orbital and its ninth ionization band is due to the ionization from OCO π molecular orbital. For the conformers IIA and IIB, the one electron ionization determinants of π molecular orbital is mixed with the determinant of molecular orbital Nos. 17 and 15. By using the NBO tables and the information in Tables 4−6, it is possible to distinguish the ionized molecular orbitals of each ionization band from each other. The comparison of NBO tables with Tables 4−6 indicates that the ordering of the photoelectron bands predicted by the Koopmans' theorem is different from the ordering obtained from SAC-CI SD-R calculations. This energy rearrangement is related to the electron correlations in the conformers. In addition, it is evident that electron correlations decrease the energy of photoelectron bands in comparison with Koopmans' theorem. The number of mixed one-electron ionization determinants for conformers I and IIA is larger than that of the conformer IIB, which reflects the higher electronic correlations in conformers I and IIA.

ionization energy (eV)


0.95(24) 0.80(23) + 0.51(22) −0.81(22) + 0.52(23) 0.69(20) − 0.59(21) 0.72(21) + 0.62(20) 0.93(19) −0.92(18) −0.76(17) + 0.51(16) 0.77(16) + 0.55(17) 0.93(15) 0.75(14) − 0.61(13) 0.75(13) + 0.60(14) 0.96(12)



ROVGF/ccpVDZ SAC-CI/6-311++G(d,p)

main configuration (|C| > 0.3)


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intensity by the OVGF method is much higher than that of the SAC-CI SD-R. The SAC-CI SD-R method predicted one ionization band above 18 eV, which has a very low intensity compared with the OVGF method. A similar discussion can be considered for the conformers IIB and IIA about the comparison of SAC-CI and OVGF methods as evident from Figure 5. It should be mentioned that the difference in the energy positions as well as the intensity of ionization bands, calculated by the SAC-CI SD-R and OVGF methods for conformer IIB is more than that for the other conformers. These differences are related to the accuracy of the applied theoretical method, the selected basis set, the method used for calculating the intensities, and the employed theoretical level used for the geometry optimization. Figure 6 compares the He−I spectrum, recorded in this work, with the calculated spectrum of each conformer at the

Table 6. Calculated Ionization Potential (IP), Monopole Intensities, and the Main Configurations of the Ionized States of Conformer IIB by the SAC-CI SD-R Level of Theory and Using 6-311++G(d,p) Basis Set (Ionization Energies of Conformer IIB Calculated by the OVGF Method24 Are Also Included) ROVGF/ccpVDZ ionization energy (eV)

SAC-CI/6-311++G(d,p) states 2 A 1 2 3 4 5 6 7 8 9 10 11 12 13 a

main configuration (|C| > 0.3) 0.96(24) 0.95(22) 0.96(23) 0.80(20) − 0.47(21) 0.81(21) + 0.47(20) 0.92(19) −0.91(18) 0.86(17) − 0.31(16) 0.83(16) + 0.36(17) 0.91(15) − 0.32(16) 0.94(14) 0.95(13) −0.96(12)


intensity

IIBa

IIBb

9.593 10.927 11.168 12.653

0.94568 0.9472 0.94681 0.94539

9.74 10.96 11.16 12.86

9.68 11.19 10.99 12.99

12.994

0.95481

13.01

12.89

14.056 14.485 14.927

0.95483 0.94961 0.94686

14.06 14.51 15.04

14.07 14.48 15.09

15.373

0.94658

15.40

15.35

16.203

0.9483

15.99

16.08

17.059 18.453 19.7

0.94471 0.94759 0.93234

16.87 18.26 19.56

17.01 18.23 19.53


b


3.5. Comparison of Theory and Experiment. In this part, the theoretical photoelectron spectra of L-alanine conformers, calculated by the SAC-CI SD-R method using 6311++G(d,p) basis set, were used to obtain the photoelectron spectrum of D,L-alanine. For this purpose, the weighted photoelectron spectra of alanine conformers including I, IIA, IIB, and IIIA were summed. Before comparing the calculated photoelectron spectra with the recorded experimental spectrum, a comparison was made between the calculated spectra of conformers obtained at the SAC-CI/6-311++G(d,p)//MP2/6311++G(d,p) level of theory and those obtained using the OVGF/cc-pVDZ//B3LYP/6-31G** level of theory by Powis et al. presented in Figure 5. As can be noted, there are differences between the energy position and intensity of ionization bands predicted by SAC-CI SD-R and OVGF levels of theory, especially for binding energies higher than 12 eV. The difference in the predicted intensity of ionization bands between the SAC-CI and OVGF levels of theory is more obvious than that of the energy positions. The positions of ionization band of conformer I, in the range of 8 to 14 eV is in good agreement with those predicted by the OVGF method. As is evident, the features in the SAC-CI SD-R calculated spectrum in the range of 11 to 14 eV are more resolved than the one in the OVGF method for conformer I. The SAC-CI SD-R and the OVGF methods predicted three ionization bands for conformer I in the range of 14 to 16 eV, but two of these bands are nearly in the same position in the OVGF calculation. Also, there is a considerable difference between the predicted intensities by OVGF and SAC-CI SD-R methods. Both methods predicted three ionization bands in the range of 16 to 18 eV for conformer I, while the predicted

Figure 6. Comparison of the experimental spectra, He−I valence photoelectron spectrum recorded in this work (red trace) and the valence photoelectron spectrum recorded at 92 eV (green trace) with the photoelectron spectrum of each conformer calculated at the SACCI SD-R/6-311++G**//MP2/6-311++G** level (solid black trace). (Also, the photoelectron spectra of the conformers calculated using the SAC-CI General-R method are included in the figure (dashed traces).) The black vertical arrows show only the position of the ionization bands of conformers calculated in this work.

SAC-CI SD-R level of theory. The spectrum recorded at photon energy 92 eV by Powis et al. is also included in Figure 6. Conformers I, IIA, and IIB (especially conformer I) can relatively well produce the first and second features in the experimental spectra, while conformer IIIA cannot describe them. It is also evident that the relative intensity of the first and second features in the experimental spectrum is more consistent with the relative intensity of the two features in the calculated spectrum of conformer I. On the basis of this comparison from the energy and relative intensity point of view, it seems that the contribution of conformer I in producing the experimental spectrum is much more than that of other conformers involved. This conclusion is also in good agreement 7011



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It should be mentioned that the shape of our recorded experimental spectrum in the range of 12 to 14 eV is more identical to the calculated spectrum of conformer I compared with other conformers. None of the conformers IIA and IIB could predict the correct position for the experimentally very distinct third peak in the range of 12 to 14 eV in the experimental spectra. As mentioned before (Figure 5), both SAC-CI SD-R and OVGF methods predicted three ionization bands in the 14 to 16 eV, while the two first ionization bands, calculated by the OVGF method, are very close to each other. Therefore, the feature located around 14.3 eV in the experimental spectra (which is more visible in the Powis et al. spectrum) could not be predicted and assigned based on the OVGF calculations, while it can be well attributed to the first ionization band of conformer I in this region based on the SAC-CI calculations (see Figure 6). Again, it can be observed that the shape of the experimental spectrum in the range of 14 to 16 eV is more identical to the SAC-CI calculated spectrum of conformer I. To compare the experimental spectra with the calculated spectrum of each conformer for binding energies above 16 eV, Figure 6 was expanded in the range of 16 to 20 eV (Figure 7) to see the fine features in the recorded He−I spectrum more clearly. In addition, in order to see the small features, the scale of the peak around 16.5 eV was changed to that of the corresponding feature in the Powis et al. spectrum. Figure 7 shows that the features in 16−18 eV region in the He−I experimental spectrum is again well described by conformers I and IIB; but it can be observed that the shape of the experimental spectrum is more identical to that of the spectrum of conformer I, confirming the existence of a higher population ratio of conformer I compared to others. There are two features with low intensity in the range of 17.6 to 18.7 eV in the recorded He−I spectrum, the first of which is also observed in the Powis et al. spectrum, while the second one is not. As is evident in Figure 7, the conformers I and IIIA have no ionization bands in this region and these two features are due to the conformers IIA and IIB (vertical dashed lines in Figure 7). In the range of 19 to 20 eV, there is a good correspondence between the He−I recorded spectrum and Powis et al. spectrum, and two features can be distinguished in this region. The SAC-CI SD-R calculations did not predict any ionization band for conformers I and IIIA in this region. On the basis of the SAC-CI SD-R calculations, the features appearing in this region could be attributed to conformers IIA and IIB, though their ionization bands are located in higher binding energy than those of the experiment. The very low intensity of these features in the He−I spectrum confirmed that the population ratio of conformers IIA and IIB should be lower than that of conformer I in the present experiment. To support this conclusion, it is necessary to check the validity of the wave function of the thirteenth ionization state of the conformers I, IIA, IIB, and IIIA. As seen in Table 4 (conformer I), the most important configurations of this state are originating from the shakeup configurations, and the one-hole configurations, related to the ionization from molecular orbital Nos. 12 and 13, have a very small contribution. To clear this ambiguity, more accurate calculation is necessary. It is well established that the amount of the electronic correlations increases with the binding energy while going to the correlated inner valence region. Therefore, to obtain more accurate information about the energy position of the ionization band of the conformers, especially in the range of

with the predictions of the thermochemistry calculations, reported in Table 2. It can also be concluded that the total experimental population ratio of conformers IIB, IIA, and IIIA should be lower than the corresponding values reported in Table 2. The reason behind this conclusion will be explained in relation to Figure 7. Figure 6 shows that the third feature in the

Figure 7. Expanded range of 16 to 20 eV of Figure 6; in order to see the small features, the scale of the peak around 16.5 eV is changed to that of the corresponding feature in the Powis et al. spectrum.

experimental spectra is exclusively due to the conformers I and IIIA because the SAC-CI SD-R calculations did not predict any ionization line around 12 eV for conformers IIB and IIA. The relative intensity of the first three features in the experimental spectrum is very similar to the ones in the calculated spectrum of conformer I, and this again confirms that the conformer I is dominant in the experimental spectrum. The energy position of the next three ionization bands of conformers I and IIIA located in the range of 12 to 14 eV are in relatively good agreement with the appeared features in the experimental spectra (especially for conformer I). The features are more resolved in the Powis et al. spectrum than in the present recorded spectrum, and it can be seen that the energy position of the features in this region are more consistent with the one in the theoretical spectrum of conformer I than conformer IIIA. Moreover, the last feature below 14 eV, which is more visible in the Powis et al. spectrum (marked with asterisk), can be attributed to the sixth ionization band of conformers I and IIIA. 7012



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18 to 20 eV, it was decided to calculate the photoelectron spectrum of each conformer using a more expensive and accurate version of SAC-CI, known as the general-R method with the same basis set. The theoretical spectrum of alanine conformers calculated by the general-R method using 6-311+ +G** is included in Figure 6 (and the expanded region in Figure 7). In the employed general-R method, higher excitation operators up to quadruple excitation operators were included to investigate the accuracy of the wave functions of the ionic states of the conformers, especially their 13th ionic state of conformer I. It is evident that the calculated spectra of conformers I, IIA, and IIB using the general-R method are very similar to the one calculated by the SD-R method, except that the general-R method predicts an ionization band around 19.75 eV for conformer I. This shows that the results obtained for the ionization energies and the relative intensity of the features using the SD-R method are as accurate as the general-R method for the conformers below 19 eV. The energy position of the last ionization band of conformer I shifted from 20.538 to 19.754 eV, and the main configurations contributing to the wave function of this state changed from the shake up to one hole configuration, corresponding to the molecular orbital Nos. 12, 13, and 14 with the coefficients 0.96, 0.04, and 0.07, respectively. It should be mentioned that, based on our general-R calculation, the contribution of the shakeup configurations in the wave function of the ionic states increase suddenly for the energies above 20 eV in the case of alanine. Considering the general-R calculated spectrum of conformer I and its higher population ratio, it seems that the feature located around 19 eV in the experimental spectra can also be attributed to the last calculated ionization band of conformer I (19.75 eV), although there is some difference between theory and experiment. It seems that, to obtain reasonable results for the alanine ionization states above 19 eV, one should use the larger and more flexible basis sets along with higher excitation operators in the general-R calculation. Figure 8 compares the recorded He−I photoelectron spectrum of alanine obtained in this work with the calculated photoelectron spectra obtained from summing different weighted photoelectron spectra of alanine conformers I, IIB, IIA, and IIIB. The photoelectron spectrum of Powis et al.,24 calculated at the OVGF/cc-pVTZ//B3LYP/6-311++G** level of theory (spectrum a), is also included. The theoretical d spectrum was calculated using the Boltzmann populations ratio of conformers reported in Table 2, which was obtained from the calculated Gibbs free energy G*L and was shifted by 120 meV to align the first theoretical peak with its corresponding peak in the experimental spectrum. It should be noted that, if one uses the remaining population ratios reported in Table 2, the relative intensity and position of the features in the resulting spectrum does not change significantly. The calculated spectra using the population ratios reported by Balabin4 (NI/NIIa = 4, spectrum e) is also included in Figure 8 along with the theoretical spectrum, considering 25% population ratio for each conformer (spectrum c). Trace b shows the spectrum that was calculated considering 33% population ratio for conformers I, IIA, and IIB in order to compare with the photoelectron spectrum calculated by Powis et al.24 (trace a in Figure 8). Comparing the theoretical spectrum calculated by Powis et al. (trace a) and the spectrum b (with the same population ratio (0.33) of conformers I, IIA, and IIB) with the experimental spectrum indicates that the spectrum b is more informative and can describe the experimental spectrum much better than the

Figure 8. Sum over weighted photoelectron spectra of alanine conformers including I, IIA, IIB, and IIIA (b to e traces). (Trace a is the OVGF theoretical spectrum of L-alanine of Powis et al. obtained from the sum over the equal weighted photoelectron spectra of conformers I, IIA, and IIB taken from ref 24. The lowest trace is the experimental He−I photoelectron spectrum of D,L-alanine recorded in this work.)

spectrum a. For example, spectrum b shows the third feature in the experimental spectrum, which is absent in the spectrum a. It can also describe the features in the experimental spectrum for the binding energies larger than 12 eV much better than spectrum b. This shows that the predicted relative ionization energies and their intensities corresponding to the alanine conformers, calculated at the SAC-CI level of theory using 6311++G**, are more accurate than those predicted by the OVGF/cc-pVTZ level of theory. Therefore, it is concluded that the electronic wave functions of the neutral ground electronic state and their ionic states predicted by the SAC-CI SD-R level of theory is more accurate than the ones obtained by the OVGF method; moreover, the monopole approximation used for calculating the ionization cross-section of ionization bands is more accurate than the continuum multiple scattering treatment, which utilizes an Xα local-exchange potential, (CMS-Xα) applied by Powis et al. 3.6. Estimating the Population Ratio of the Conformers. In this part, the population ratio of alanine conformers is estimated from the recorded spectrum by a fitting procedure. The first two features of the present experimental spectrum are selected to be fitted in the expression composed of the sum over the intensity of the two ionization bands (Ii) of four individual conformers that were calculated by the SAC-CI SD-R method and multiplied by their population ratio (Ai) as adjustable parameters. 7013


The Journal of Physical Chemistry A I = A Ia IIa + A IIa IIIa + A IIbIIIb + A IIIa IIIIa

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REFERENCES

(1) Simpson, H. J., Jr.; Marsh, R. E. Acta Crystallogr. 1966, 20, 550− 555. (2) Dunitz, J. D.; Ryan, R. R. Acta Crystallogr. 1966, 21, 617−618. (3) Blanco, S.; Lesarri, A.; Lopez, J. C.; Alonso, J. L. J. Am. Chem. Soc. 2004, 126, 11675−11683. (4) Balabin, R. M. Phys. Chem. Chem. Phys. 2010, 12, 5980−5982. (5) Godfrey, P. D.; Firth, S.; Hatherley, L. D.; Brown, R. D.; Pierlot, A. P. J. Am. Chem. Soc. 1993, 115, 9687−9691. (6) Cao, M.; Newton, S. Q.; Pranata, J.; Sch afer, L. J. Mol. Struct. 1995, 332, 251−267. (7) Godfrey, P. D.; Firth, S.; Rodgers, F. M. J. Mol. Struct. 1996, 376, 65−81. (8) Csaszar, A. G. J. Phys. Chem. 1996, 100, 3541−3551. (9) Kaschner, R.; Hohl, D. J. Phys. Chem. A 1998, 102, 5111−5116. (10) Feyer., V.; Plekan, O.; Richter, R.; Coreno, M.; Prince, K. C.; Carravetta, V. J. Phys. Chem. A 2008, 112, 7806−7815. (11) Klasinc, L. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 161−164. (12) Cannington, P. H.; Ham, N. S. J. Electron Spectrosc. Relat. Phenom. 1983, 32, 139−151. (13) Debies, T. P.; Rabalais, J. W. J. Electon. Spectrosc. Relat. Phenom 1974, 3, 315−322. (14) Cannington, P. H.; Ham, N. S. J. Electron Spectrosc. Relat. Phenom. 1979, 15, 79−82. (15) Hush, N. S.; Cheung, A. S. Chem. Phys. Lett. 1975, 34, 11−13. (16) Dougherty, D.; McGlynn, S. P. J. Chem. Phys. 1977, 67, 1289− 1290. (17) Lin, J.; Yu, C.; Peng, S.; Akiyama, I.; Li, K.; Lee, L. K.; Lebreton, P. R. J. Phys. Chem. 1980, 84, 1006−1012. (18) Ajo, D.; Fragala, I.; Granozzi, G.; Tondello, E. Spectrochim. Acta 1978, 34, 1235−1238. (19) Dougherty, D.; Younathan, E. S.; Voll, R.; Abdulnur, S.; McGlynn, S. P. J. Electron. Spectrosc. Relat. Phenom. 1978, 13, 379−393. (20) Plekan, O.; Feyer, V.; Richter, R.; Coreno, M.; De Simone, M.; Prince, K. C.; Trofimov, A. B.; Gromov, E. V.; Zaytseva, I. L.; Schirmer, J. Chem. Phys. 2008, 347, 360−375. (21) Plekan, O.; Feyer, V.; Richter, R.; Coreno, M.; Vall-llosera, G.; Prince, K. C.; Monica de Simone; Trofimov, A. B.; Zaytseva, I. L.; Moskovskaya, T. E.; et al. J. Phys. Chem. A 2009, 113, 9376−9385. (22) Zaytseva, I. L.; Trofimov, A. B.; Schirmer, J.; Plekan, O.; Feyer, V.; Richter, R.; Coreno, M.; Prince, K. C. J. Phys. Chem. A 2009, 113, 15142−15149. (23) Feyer, V.; Plekan, O.; Richter, R.; Coreno, M.; Prince, K. C.; Carravetta, V. J. Phys. Chem. A 2009, 113, 10726−10733. (24) Powis, I.; Rennie, E. E.; Hergenhahn, U.; Kugeler, O.; BussySocrate, R. J. Phys. Chem. A 2003, 107, 25−34. (25) Powis, I. J. Phys. Chem. A 2000, 104, 878−882. (26) Feyer, V.; Plekan, O.; Richter, R.; Coreno, M.; Vall-llosera, G.; Prince, K. C.; Trofimov, A. B.; Zaytseva, I. L.; Moskovskaya, T. E.; Gromov, E. V.; et al. J. Phys. Chem. A 2009, 113, 5736−5742. (27) Gross, D.; Grodsk, G. J. Am. Chem. Soc. 1955, 77 (6), 1678− 1680. (28) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785−789. (29) Moller, C.; Plesset, M. S. Phys. Rev. 1934, 46, 618−622. (30) Nakatsuji, H.; Hirao, K. J. Chem. Phys. 1978, 68, 2053−2065. (31) Nakatsuji, H. Chem. Phys. Lett. 1978, 59, 362−364.

4. CONCLUSIONS In the present work, the valence photoelectron spectrum of D,Lalanine is investigated and revisited both experimentally and theoretically. The correct and reliable He−I photoelectron spectrum of alanine is reported for the first time. Parallel to the experiment, high level and expensive ab initio calculations at the SAC-CI level of theory is used to interpret and assign the features in the experimental spectrum. The comparison of the simulated spectrum with other theoretical spectra in the literature shows the superiority of the SAC-CI level of theory to the OVGF theory. By fitting the experimental photoelectron spectrum, the population ratio of alanine conformers is successfully reported at 403 K for the first time. ASSOCIATED CONTENT

S Supporting Information *

Calculated HF energies and dominant natural bonding orbital contributions in the molecular orbitals of L-alanine; calculated ionization potential, monopole intensities, and the main configurations of the ionized states of conformer IIIA; significant HF molecular orbitals of conformers IIB and IIA of alanine; comparision of calculated photoelectron spectra of D and L forms of conformers I and IIA. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

We would like to thank Isfahan University of Technology for its financial support. This work has been also supported by CNPq ́ and Laboratório Nacional de Luz Sincrotron (LNLS) in Brazil. F.F. wishes to thank the CNPq for PCI financial support. H.F. and F.F. wish to thank Professor M. Amirnasr and Professor M. Tabrizchi of IUT for their insightful comments. We would also like to thank the technical staffs of LNLS for their technical supports.

The reason for selecting the initial parts of the experimental spectrum for fitting is the accuracy of the SAC-CI theory in the case of energy positions and intensity. The first two features of the experimental spectrum were selected to be fitted because they were clearly resolved and had the minimum overlap with other features in the spectrum. The accuracy of the SAC-CI SD-R method, monopole approximation with the employed basis set (6-311++G**), in predicting the energy position and intensity of outer valence ionization bands is also higher than that of other ionization bands located at higher binding energy. The population ratio of the conformers obtained from the fitting are AIa = 70%, AIIa = 14%, AIIb = 10%, and AIIIa = 6% at 403 K. If the transformation of the conformer IIIA to I and IIB to IIA is considered, the ratio of NI/NIIa would be 3.2, which is very close to the value reported by Blanco et al.3 The comparison of these population ratios with the ones obtained from thermochemistry calculations reported in Table 2 indicated that the ab initio calculations are not able to predict the correct Boltzmman population distribution over the conformers at the considered temperature. A similar method was used by Vitally et al. for estimating the population ratio of alanine conformers by fitting the N1s photoemission spectrum of alanine. It should be mentioned that they could report only the sum of the population ratio of conformers I + IIIA and IIA + IIB from their spectrum through the fitting. Because they had only the N1s binding energy of conformers from ab initio calculation without any information about the intensities; the N1s binding energies of conformers I and IIIA and IIB and IIA were also very close to each other. However, in the present work, the population ratio of each conformer could be estimated from the fitting of the spectrum, using the position and intensities of ionization energies from ab initio calculations.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. 7014



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Theoretical and Experimental Study of Valence Photoelectron

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