ARTICLE pubs.acs.org/JPCA
Electronic Structure and Conformation Properties of Halogen -Substituted Acetyl Acrylic Anhydrides, CX3C(O)OC(O)CHdCH2 (X = H, F, or Cl) Wang XiaoPeng, Tong ShengRui,* Wang WeiGang, Ge MaoFa,* and Wang DianXun Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, People's Republic of China
bS Supporting Information ABSTRACT: Acetyl acrylic anhydride (CH3C(O)OC(O)CHCH2) and its halogen-substituted derivatives (CF3C(O)OC(O)CHCH2 and CCl3C(O)OC(O)CHCH2) were prepared by the heterogeneous reaction of gaseous CH2dCHC(O)Cl with CX3C(O)OAg (X = H, F, or Cl). The molecular conformations and electronic structure of these three compounds were investigated by HeI photoelectron spectroscopy, photoionization mass spectroscopy, FT-IR, and theoretical calculations. They were theoretically predicted to prefer the [ss-c] conformation, with each CdO bond syn with respect to the opposite OC bond and the CdC bond in cis orientation to the adjacent CdO bond. The experimental first vertical ionization potential for CH3C(O)OC(O)CHCH2, CF3C(O)OC(O)CHCH2, and CCl3C(O)OC(O)CHCH2 was determined to be 10.91, 11.42, and 11.07 eV, respectively. In this study, the rule of the conformation properties of anhydride XC(O)OC(O)Y was improved by analyzing the different conformations of anhydrides with various substitutes.
1. INTRODUCTION Anhydrides of carboxylic acids are commonly used as acylating agents. They are preferred for the following reasons: (1) the cost of manufacturing anhydrides is lower than that of producing other acylating agents, such as acyl chloride; (2) the alkanoylation using anhydrides is less dangerous and easier to control because they are less reactive; (3) the byproduct of the alkanoylation using anhydrides is water; and (4) anhydrides are less susceptible to water. Anhydrides of carboxylic acids are used in biological signaling and pharmaceutical and textile industries.1 For example, acetic anhydride is used to produce o-acetoxybenzoic acid (aspirin), which can relieve pain and suppress the symptom of mild fever. The reaction of alkanoylation of carboxylic acid is rapid and not reversible. It does not require catalyst and produces a very good yield. Therefore, preparing esters from alkanoylation of carboxylic acid is better than alcoholysis of carboxylic acid. Although anhydrides of carboxylic acids are widely used, there has been little research on their molecular and electronic structure.2,3 Anhydrides of the type XC(O)OC(O)Y can adopt different conformations, depending on the orientations of the two carbonyl groups. Using the IUPAC nomenclature,4 the orientation of each CdO bond relative to the opposite OC bond can be synperiplanar (sp, δCOCdO = 0 ( 30°), synclinal (sc, δCOCdO = 60 ( 30°), anticlinal (ac, δCOCdO = 120 ( 30°), or antiperiplanar (ap, δCOCdO = 180 ( 30°). Therefore, four kinds of conformers can be concluded, [s, s], [s, a], [a, a], and [a, s], where s can be sp or sc and a can be ap or ac. The planar r 2011 American Chemical Society
forms are shown in Figure 1. Different conformation properties have been reported in the literature depending on the substituents X and Y, such as formic anhydride (HC(O)OC(O)H),5 formic acetic anhydride (CH3C(O)OC(O)H),6 acetic anhydride (CH3C(O)OC(O)CH3),2,710 bis(fluoroformyl) ether (FC(O)OC(O)F),11 and halogen-substituted acetic anhydrides CH3C(O)OC(O)CCl3 and CH3C(O)OC(O)CF3.12 The research methods mainly included gas electron diffraction (GED), spectral analysis, and quantum chemical calculations. These investigations indicate that HC(O)OC(O)H and CH3C(O)OC(O)H favor a single [s, a] conformer, while FC(O)OC(O)F, CH3C(O)OC(O)CCl3, and CH3C(O)OC(O)CF3 prefer a single [s, s] conformer at room temperature in the gas phase. It follows the rule that only formic anhydride and mixed anhydrides containing the formic moiety can occur in the energetically favorable [s, a] conformer, and all other acyclic anhydrides must stay in another form.5 However, distinct conformation properties of CH3C(O)OC(O)CH3 had been reported in several studies. The earlier electron diffraction 7 together with vibrational spectroscopy 8,9 and ab initio calculations10 maintained the preference of a single [s, s] conformer with C2 symmetry in the gas phase. The more recent GED study2 resulted in an experimental “conformer” ratio of [s, s]/[s, a] = 37((15)%/63((15)%, and it was in close agreement with Received: August 17, 2011 Revised: December 11, 2011 Published: December 12, 2011 560
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Figure 1. Schematic representation of the possible conformers of XC(O)OC(O)Y.
α-dicarbonyl compounds.1820 CX3C(O)OC(O)CHCH2 is composed of interesting α,β-unsaturated carbonyl and anhydride structure. In this work, acetyl acrylic anhydride and its halogen-substituted derivatives, CX3C(O)OC(O)CHCH2 (X = H, F, or Cl) were prepared and characterized by PES, photoionization mass spectrometry (PIMS), and FT-IR. For comparison with the values of experimental ionization potentials, a completely theoretical study involving structure calculations of stable conformers at different levels, MO analysis, and ionization potential calculations were also performed.
the quantum chemical results. Thus, the rule of the conformational properties of anhydrides XC(O)OC(O)Y is not systematic. It is very necessary to investigate the conformational properties of anhydrides XC(O)OC(O)Y with different substituents X and Y. The electron structure of many carbonyl compounds was investigated by HeI photoelectron spectroscopy (PES) to identify the mechanism that carbonyl groups interact among themselves and with other functional groups, either through-space or through-bond.13,14 Many theoretical works also concerned the nature of the carbonylcarbonyl interaction and attempted to rationalize the extent of through-space and through-bond interaction.15 There are two types of linear combinations of the two carbonyl oxygen nonbonding atomic orbitals, the inphase combination (n+) and the out-of-phase combination (n). The magnitude of the interaction between the two carbonyl groups can be measured by the n+/n splitting (denoted by Δn, Δn = In+ In), which is experimentally determined by PES. The average value of Δn for a given set of dicarbonyls (i.e., the α-, β-, γ-, and δ- sets) was found to be a distinguishing character in that Δn ≈ Δn for most of the members.13 The value of Δn for α-, β-, and γ-dicarbonyls is 1.9 ( 0.15, 0.65 ( 0.17, and 0.3 ( 0.13 eV, respectively. Molecules for which Δn 6¼ Δn do exist, but they are always exceptional in some particularly obvious ways. Formic anhydride, (HCO)2O, is the simplest β-dicarbonyl anhydride. There were two different n+/n splittings, 1.1516 and 1.2817 eV, reported up to now. The former study suggested that the extent of the splitting was a result of a large through-bond coupling mechanism, owing to a coplanar geometry. Carbonyl groups favored interaction with the σ levels, which were close in energy to the carbonyl levels, thereby giving a maximum perturbation. The latter held that the geometry of formic anhydride gave rise to a significant “through-space” interaction, which was absent in other β-dicarbonyl compounds, caused the exceptionally large splitting in formic anhydride. When the hydrogen atoms in formic anhydride are replaced by other groups, the n+/n splitting may have great changes due to the different conformations. The splitting of n+/n in acetic anhydride (CH3C(O)OC(O)CH3) is 0.55 eV, which is of the same order with most of the β-dicarbonyls compounds.17 The splitting of the perfluorine-substituted derivative (CF3CO)2O is 0.93 eV. The increased separation derives mainly from the through-bond interaction that is caused by a greater interaction between the lone pairs and the appropriate lower-lying σ levels. Based on the above analysis, it can be concluded that the conformation properties and electronic structure of anhydride XC(O)OC(O)Y mainly depend on the substituents X and Y. Many researchers focused their interests on the compounds that the substituents were methyl- or halogen-substituted alkyl. However, none has been studied for the conformations and electronic structure of vinyl-substituted β-dicarbonyl compounds (CX3C(O)OC(O)CHCH2). There were just few studies on the assignments of the ionization potentials of the vinyl-substituted
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The anhydrides, CX3C(O)OC(O)CHCH2 (X = H, F, or Cl), were prepared by the heterogeneous reactions between acryloyl chloride and corresponding acyl silver salts at room temperature. The reaction route is illustrated in eq 1.
CH2 dCHCðOÞClðgÞ þ CX3 CðOÞOAgðsÞ f CH2 dCHCðOÞOCðOÞCX3 ðgÞ þ AgClðsÞ X ¼ H, F, or ClÞ ð1Þ The precursor acryloyl chloride (CH2dCHC(O)Cl), CH3C(O)OAg, and CF3C(O)OAg were purchased from Alfa Aesar Company and their purity was better than 96%. Before the reaction, CH3C(O)OAg and CF3C(O)OAg were dried in vacuum (1 104 Torr) for 2 h at 50 °C. Silver trichloroacetate, CCl3C(O)OAg, was prepared according to the previous report,21 rapidly mixing equimolar amounts of silver nitrate and sodium trichloroacetate in saturated aqueous solution. The precipitate was washed successively with water, acetone, and ether. CCl3C(O)OAg was dried in vacuum and stored at 80 °C in the dark before it is used. 2.2. IR Spectroscopy. IR spectra of the studied samples were recorded in the spectral range from 4000 to 700 cm1 at a resolution of 4 cm1 with 32 scans, using a Thermo Nicolet FTIR spectrometer 6700 equipped with a liquid-nitrogen-cooled narrow band mercurycadmium-telluride (MCT) detector. All compounds had sufficient vapor and were pumped continuously through the cell using a rotary pump. The samples were prepared in a reaction vessel that was wrapped with foil to minimize the amount of stray light available to the reaction sample during the reaction. After evacuation of the vessel, 1 mmol acryloyl chloride was condensed on the silver salt at 196 °C. Then the temperature of the vessel was warmed up to room temperature (20 °C) and held for 24 h. Subsequently, the volatile products were slowly pumped and separated by trapto-trap condensation to remove a minor impurity of acryloyl chloride. 2.3. Photoelectron Spectroscopy. The compounds CX3C(O)OC(O)CHCH2 (X = H, F, or Cl) were heterogeneously generated at room temperature by passing acryloyl chloride 561
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Figure 2. Conformational energy profile for CH3C(O)OC(O)CHCH2 obtained using the relax scan of potential energy surface at the B3LYP/6-31+G* level.
Figure 3. Conformational energy profile for CF3C(O)OC(O)CHCH2 obtained using the relax scan of potential energy surface at the B3LYP/6-31+G* level.
vapor over the finely powdered acyl silver salts, and in situ PES and PIMS were recorded. The experimental apparatus used in this work has been described previously.22 Briefly, the photoelectron and photoionization mass spectrometer consists of two parts: one part is the double-chamber UPS-II machine, and the other is a time-of-flight mass spectrometer. The PES was recorded with the doublechamber UPS-II machine, which was built specifically to detect transient species at a resolution of about 35 meV as indicated by the Ar+ (2P3/2) photoelectron band. Experimental vertical ionization potentials were calibrated by the simultaneous addition of a small amount of argon and methyl iodide to the sample. Mass analysis of ions was achieved with the time-of-flight mass analyzer mounted directly to the photoionization point. The relatively soft ionization is provided by single-wavelength HeI radiation. The PES and PIMS can be recorded within seconds of each other under identical conditions. 2.4. Quantum Chemical Calculations. All quantum chemical calculations were performed using the Gaussian 03 program package.23 Relaxed scans of the potential energy surface were performed to search for the possible conformers of CX3C(O)OC(O)CHCH2 (X = H, F, or Cl) at the B3LYP/6-31+G* level. The geometries were optimized with HF, B3LYP and MP2 methods at the 6-311++G** basis set. To assign the PES, the outvalence Green’s function (OVGF/6-311++G**) calculations, which include sophisticated correlation effects of the self-energy, were applied to the most stable conformer of each compound to give accurate results for the vertical ionization potentials. Threedimensional MO plots were obtained with the Gauss View program by using the 0.05 isodensity.
Figure 4. Conformational energy profile for CCl3C(O)OC(O)CHCH2 obtained using the relax scan of potential energy surface at the B3LYP/6-31+G* level.
to the CdO double bond. The potential functions for internal rotation around the C2O2, O2C3, and C3C4 bonds (the numbers of the atoms were shown in Figure 5) were derived by structure optimizations at fixed dihedral angles between 180° to 180° in steps of 10° using the B3LYP method with the 6-31 +G* basis set (Figures 24). As shown in Figures 2 and 3, minima occur at the syn or anti orientation of O1dC2O2C3 with δO1dC2O2C3 around 30 or 150° for CH3C(O)OC(O)CHCH2 and CF3C(O)OC(O)CHCH2. However, there is a single minimum (δO1dC2O2C3 = 30°) in the calculated potential curve of CCl3C(O)OC(O)CHCH2 (Figure 4), indicating that the conformer with anti orientation of the O1dC2 double bond relative to the O2C3 single bond is unstable. This is mainly because of the space steric hindrance between Cl and O3 atom in anti orientation of O1dC2O2C3, which is resulted by the larger atomic radius of Cl (1.62 Å) than H or F.
3. RESULTS AND DISCUSSION 3.1. Molecular Structure. To ascertain the relative magnitude of the through-space and through-bond interactions, the conformations of the studied anhydrides must be done first. Various conformers of CX3C(O)OC(O)CHCH2 (X = H, F, or Cl) are feasible, depending on the configuration of the anhydrides moiety and the orientation of the CdC double bond relative 562
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Figure 5. Schematic representation of the five conformers of CX3C(O)OC(O)CHCH2 (X = H, F, or Cl): (a) as-c; (b) ss-t; (c) as-t; (d) sa-c; (e) ss-c.
Table 1. Calculated Relative Energiesa (kcal/mol) of Different Conformers for the Three Anhydrides
The resulting potential functions for internal rotation around the O2C3 bond (Figures 24) for all these anhydrides possess minima for the syn (δC2O2C3dO3≈30°) and anti (δC2O2C3dO3 ≈ 130°) positions. In Figures 24, two structures correspond to minima in all the potential energy curves at δO3dC3C4dC5d0° or 180° with the cis(c) or trans(t) orientation of the CdC double bond relative to the CdO double bond. From the above analysis, there are eight possible conformers for CH3C(O)OC(O)CHCH2 and CF3C(O)OC(O)CHCH2, and four for CCl3C(O)OC(O)CHCH2. Full geometry optimizations and frequency calculations for feasible conformers were performed using density functional theory (DFT) and ab initio methods at the 6-311++G** basis set. According to the results of the theoretical calculations, five of the eight conformers for CF3C(O)OC(O)CHCH2 and CH3C(O)OC(O)CHCH2 are found to be stable structure by vibrational analysis. A scheme of the five conformers [ss-c], [ss-t], [as-c], [as-t] and [sa-c] is presented in Figure 5. However, only three stable conformers [ss-c], [ss-t], and [sa-c] are achieved for CCl3C(O)OC(O)CHCH2. Relative energies (ΔE) of different conformers are listed in Table 1. All the computational methods predict that [ss-c] conformers of these three studied compounds are the most stable conformers. The second stable forms are [ss-t] conformers, higher in energy by 1.66, 1.26, and 0.92 kcal mol1 for CH3C(O)OC(O)CHCH2, CF3C(O)OC(O)CHCH2, and CCl3C(O)OC(O)CHCH2, respectively, based on the MP2 method at the 6-311++G** basis level. With differences in ΔE < 2 kcal mol1, the two conformers should be present together in detectable quantities in the gas phase at ambient temperature. It is important to mention that either the [as-c] or the [as-t] conformer in CH3C(O)OC(O)CHCH2 is more stable than the [sa-c] conformer because of the intermolecular O 3 3 3 H interaction. The rule of the conformation properties of anhydride XC(O)OC(O)Y can be improved based on the reported results of CX3C(O)OC(O)CHCH2 (XdH, F, or Cl), HC(O)OC(O)H, CH3C(O)OC(O)H, FC(O)OC(O)F, CH3C(O)OC(O)CCl3, CH3C(O)OC(O)CH3, and CH3C(O)OC(O)CF3. Formic anhydride and mixed anhydrides containing the formic moiety adopt the [s, a] conformation as a consequence of the intermolecular O 3 3 3 H bond. Other acyclic anhydrides prefer the [s, s] conformation in the gas phase. However, the [s, a] conformer may also exist in the gas phase for other acyclic anhydrides. The existence of the [s, a] conformer could be inferred from the energy difference between the [s, a] and [s, s] conformers according to the theory calculation. The [s, a] conformer might exist in the gas phase at room temperature if ΔE e 2 kcal mol1. For CH3C(O)OC(O)CH3, the ΔE is 0.3878 kcal mol1 at the B3LYP/6-311+G* level.12 The [s, a] conformer could be
a
ss-c
ss-t
sa-c
as-c
as-t
CH3C(O)OC(O)CHCH2
0.0
1.66
11.6
5.63
6.62
CF3C(O)OC(O)CHCH2
0.0
1.26
21.5
24.6
28.2
CCl3C(O)OC(O)CHCH2
0.0
0.92
20.6
MP2/6-311++G** results.
expected in the gas phase at room temperature, and it was confirmed by the GED study.2 However, theory calculation could only give the potential existence of different isomers, and it needs further confirmation by the experimental study. Tables S13 list the selected geometric parameters for the most stable conformers and the radical-cationic forms of these three anhydrides. Similar structural parameters were obtained with HF, B3LYP and MP2 methods at the 6-311++G** basis set. The results of the experimental and computational geometric parameters of the similar anhydrides, CH3C(O)OC(O)CH37,12 and CF3C(O)OC(O)CF324 show that the B3LYP method has good performance at the calculation of the structure geometry of anhydrides. Therefore, the following discussion is only referred to the results of the B3LYP method. It can be clearly seen that the calculated structure parameters are very similar due to the main framework of the CdOOCdOCdC moiety in these three molecules. The difference of the bond length mainly comes from the C2O2 bond, which is 1.400, 1.357, and 1.361 Å, accordingly. It is obviously that the bond length of C2O2 bond decreases gradually with the increase of the negativity of the withdrawing groups on the anhydride terminal. Besides the C2O2 bond length, another important structure parameter for anhydrides is the torsional angle, which greatly influences the whole structure of the molecule. The δO1C2O2C3 of the three molecules is 30.89, 24.26, and 28.85°, respectively, and δC2O2C3O3 is 27.76, 26.35, and 23.83°, accordingly. The twist orientation of the CdOOCdOCdC moiety in CCl3C(O)OC(O)CHCH2 is reversed to that of CH3C(O)OC(O)CHCH2 and CF3C(O)OC(O)CHCH2. All calculated results show that the radical-cationic forms of these three anhydrides are the planar conformations with Cs symmetry after the vibrational analysis. Furthermore, apparent changes can be found in bond length and bond angle between the neutral molecule and the radical-cationic form. 3.2. Vibrational Analysis. There has been no experimental infrared spectrum reported for CX3C(O)OC(O)CHCH2 (X = H, F or Cl) so far. In this work, the gas-phase IR spectra of these 563
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Figure 7. PIMS spectrum of CH3C(O)OC(O)CHCH2.
by comparison with the spectrum of the related compound CH3C(O)OC(O)H, in which the ν(CdO) stretching modes are observed at 1808 and 1792 cm1.6 The bands at 1133 and 1004 cm1 are associated with the COC- stretching vibration mode.2 The stretching mode of the CdC double bond is observed at 1641 cm1, and the deformation of the CH2 group is at 1409 cm1 according to the assignment of the spectrum of CH3C(O)CHdCH2.25 The absorption at 906 cm1 is mainly from the stretching vibration of the C(CH3)C(O) group.2 The CH3 symmetric deformation vibration mode is observed at 1375 cm1.6 CF3C(O)OC(O)CHCH2. Replacing the hydrogen atoms in CH3 moiety by fluorine atoms moves the IR absorptions of the ν(CdO) stretching vibration modes to higher frequency (1855, 1789 cm1), and the same shift happens when the hydrogen atoms in acetyl trifluoroacetate are substituted with fluorine atoms. The carbonyl stretching vibration absorptions of acetyl trifluoroacetate are at 1780 and 1850 cml and that of trifluoroacetic anhydride are at 1800 and 1860 cml.26 The IR absorptions at 1088 and 1053 cm1 are readily assigned to the COC stretching modes.26 The bands at 1635, 1406, and 990 cm1 are associated with the CdC stretching mode, the deformation mode of the CH2 group, and the wagging vibration mode of CH2 group, respectively.25 The relatively strong bands at 1244 and 1193 cm1 are arising from the antisymmetric and symmetric stretching vibrations of CF3 group.27 CCl3C(O)OC(O)CHCH2. The assignments are easily after the discussion of the above two compounds. The IR absorptions at 1814 and 1765 cm1 are assigned to the ν(CdO) stretching modes. The COC stretching vibration modes are observed at 1241 and 1063 cm1.28 The bands at 1636, 1434, 1130, and 998 cm1 are associated with the CdC stretching mode, the deformation mode of the CH2 group, the rocking vibration mode of the CH2 group, and the wagging vibration mode of the CH2 group, respectively.25 The symmetric and antisymmetric stretching vibrations of the CCl3 group appear in the 870670 cm1 region. The bands usually split into many bands owing to the rotation of the CCl3 group. The bands below 950 cm1 are mainly arising from the stretching, deformation, and torsional vibration modes of CCl3 group. 3.3. Photoelectron Mass Spectroscopy. Figures 79 present the first PIMS of CX3C(O)OC(O)CHCH2 (X = H, F, or Cl).
Figure 6. Infrared spectra (gas phase) of CX3C(O)OC(O)CHCH2 (X = H, F, or Cl).
three anhydrides were reported (Figure 6). Experimental frequencies and the assignments are given in Tables S46. The CH stretching vibrations may occur in the 3500 2800 cm1 region, but the bands are very weak.5 Thus, we mainly discussed the wavenumbers of vibrations below 2800 cm1. There are some general features in the vibration spectra of anhydrides. The CdO stretching vibrations give rise to two prominent IR absorptions in the 18801785 cm1 range. The high-frequency must be assigned to the in-phase stretching vibration of the two CdO bonds, and the low-frequency to the out-of-phase stretching vibration. The COC- stretching mode appears in the 11301000 cm1 region and has one or two infrared absorptions. The details of the tentative assignments of the bands in the IR absorption spectra of CX3C(O)OC(O)CHCH2 (X = H, F, or Cl) are as follows. CH3C(O)OC(O)CHCH2. The IR absorptions at 1834 and 1777 cm1 are readily assigned to the ν(CdO) stretching modes 564
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Figure 8. PIMS spectrum of CF3C(O)OC(O)CHCH2.
Figure 9. PIMS spectrum of CCl3C(O)OC(O)CHCH2.
There is no molecular ion peak in the spectrum of each studied anhydride. This indicates that the molecular ion peak is unstable. To validate the level of electronic structure theory used in the energetics calculation of dissociation pathways, we computed the adiabatic ionization energies of these three anhydrides from the energies of the identified fragments. The calculated results are shown in Table S7 along with the experimentally measured values. Since the calculated and experimental IPs of some fragments are in very good agreement, our calculated IPs for these fragments could estimate the actual values. These IPs are necessary to construct the energy diagrams of the dissociation pathway shown in Figure 10. The quite low energies for the O2C3 bonds dissociation (1.12 eV for CH3C(O)OC(O)CHCH2, 0.48 eV for CF3C(O)OC(O)CHCH2, and 0.66 eV for CCl3C(O)OC(O)CHCH2) in the parent ions suggest that, once the parent ions are formed, dissociation into CH2CHCO+ is very easy. This fact explains the absence of the molecular ion peaks in PIMS of CX3C(O)OC(O)CHCH2 (X = H, F, or Cl). The generation of CX3C(O)OC(O)CHCH2 (X = H, F, or Cl) under our experimental conditions can be confirmed by the following discussion, although there is no molecular ion peak (M+). First, fragments that are originally from the silver salts are observed in the PIMS of these three anhydrides. They are from the dissociation of the molecular ion of the product CX3C(O)OC(O)CHCH2 (X = H, F, or Cl) because there is no signal when only the silver salt is left in the PES apparatus. Second, there is no molecular ion peak of acryloyl chloride in the PIMS of CX3C(O)OC(O)CHCH2 (X = H, F, or Cl). It is concluded that the peaks CH2CH+ and CH2CHCO+ are also fragments from the molecular ion of the product, because the molecular ion peak of acryloyl chloride is distinct in the PIMS in our previous study.22 In addition, the generation of CX3C(O)OC(O)CHCH2 (X = H, F, or Cl) is also confirmed by the IR absorption spectra. The spectrum of CH3C(O)OC(O)CHCH2 is relatively simple. The main peaks are ions (m/z): CH3+ (15), CH2CH+ (27), CH3CO+ (43), and CH2CHCO+ (55). As shown in Figure 10, the energy of the direct dissociation of the C2O2 bond in the parent ion to generate CH3CO+ is lower than other pathways, with the exception of the dissociation of the O2C3 bond. Therefore, the peak of CH3CO+ is also the main peak in the PIMS. The energy of the subsequent dissociation of CH2CHCO+ to CH2CH+/CO is 2.89 eV and that of CH3CO+
to CH3+/CO is 3.52 eV. It means that CH2CHCO+ is easier to subsequently dissociate than CH3CO+. Therefore, the intensity of CH2CHCO+ is relatively small compared with CH3CO+. Six peaks (m/z): CH2CH+ (27), CO+ (28), CH2CHCO+ (55), CF3+ (69), CF3CO+ (97), and CF3C(O)OCO+ (141) are shown in the spectrum of CF3C(O)OC(O)CHCH2, with the dominant feature being the CH2CHCO+ peak. There are two pathways to generate CH2CH+, one is the direct dissociation of the C3C4 bond in CF3C(O)OC(O)CHCH2+, and the other is the subsequent dissociation of CH2CHCO+. Therefore, the intensity of the peak CH2CH+ is relatively high. The peak CF3+ is comparable to the peak CH2CH+, which can be accounted for the quite low energy for the dissociation of CF3CO+ to CF3+/ CO (0.74 eV). The peaks in the spectrum of CCl3C(O)OC(O)CHCH2 are CH2CH+ (27), Cl+ (35,37), CCl+ (47, 49), CH2CHCO+ (55), CH2CHCOO+ (71), and CCl2+ (82, 84, 86). The assignments of the peaks are mainly based on the mass number of the fragments and the isotopic abundance of chlorine. The dominant feature is the peak CCl2+ in the PIMS, and it is formed by the dissociation of CCl3+. CCl3+ can be easily formed by the direct dissociation of the C1C2 bond in CCl3C(O)OC(O)CHCH2+ (1.09 eV) as well as the subsequent dissociation of CCl3CO+. The energy of the direct dissociation of the C2O2 in CCl3C(O)OC(O)CHCH2+ to generate CCl3CO+ is only 1.75 eV. Besides, as shown in Figure 10, CCl3CO+ is unstable and dissociates to CCl3+ once it is formed. Therefore, there is not the peak CCl3CO+ in the PIMS of CCl3C(O)OC(O)CHCH2. 3.4. Photoelectron Spectra. The contribution of the [ss-t] form could be expected in the PES, because the computational energy difference was small ( CCl3C(O)OC(O)CHCH2 > CH3C(O)OC(O)CHCH2. The same shift was also observed by our group in the PES of acetic anhydride and halogen substituted acetic anhydrides, CH3C(O)OC(O)CH3, CCl3C(O)OC(O)CH3, and CF3C(O)OC(O)CF3. Their first vertical ionization potentials are 10.73, 11.06, and 11.53 eV, respectively.12 Calculations (B3LYP/6-311++G**) were also performed to analyze the nature of the cation formed in the first ionization process. The Mulliken population analysis of the charges for both neutral and radical-cationic forms is shown in Tables S810. The results demonstrate that the atomic charges are delocalized all over the molecule, with an appreciable fraction localized on oxygen atoms in the two carbonyl groups. It can be deduced that the first ionization happens primarily on the electrons of the oxygen atoms in the two carbonyl groups, that is, the oxygen 2p lone pair electrons. The second band in the PES of CH3C(O)OC(O)CHCH2 at 11.44 eV was assigned primarily to the ionization of the πCdC orbital. A shift (0.66 eV) comparing to the second band of CH3C(O)C(O)CHCH2 was due to the stronger inductive effect of oxygen lone pairs in carbonyl group which was produced by 568
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The Journal of Physical Chemistry A CF3C(O)OC(O)CHCH2, and CCl3C(O)OC(O)CHCH2 are 2.892, 2.885, and 2.869 Å, respectively. A separation of 2.53.0 Å between two interaction atoms makes direct overlap negligible.15 Thus, the direct through-space interaction between the two lone pairs of carbonyl oxygen atoms is impossible. The larger value of Δn of either CH3C(O)OC(O)CHCH2 or CF3C(O)OC(O)CHCH2 might be the result of the indirect through-bond coupling between the two lone pairs of carbonyl oxygen atoms and the lower lying σ levels. The standard restriction is that only orbitals of like symmetry could interact with each other. Therefore, the larger value of Δn of either CH3C(O)OC(O)CHCH2 or CF3C(O)OC(O)CHCH2 than that of CCl3C(O)OC(O)CHCH2 was probably due to the different conformation properties. The ionization of fluorine lone pairs occurs at relatively high ionization potential region. The ionization of fluorine lone pairs of the CF3 group in CF3C(O)OC(O)CHCH2 is harder than that of the chlorine lone pairs in CCl3C(O)OC(O)CHCH2 owing to the higher electronegativity of fluorine. The sharp bands at 15.55 and 15.92 eV were assigned to the ionization of the fluorine lone pairs. The ionization of the chlorine lone pairs occurred at 11.36 eV. The other bands at 11.83, 12.15, 12.64, and 12.89 eV, which were distinctive and less overlapped in the PES of CCl3C(O)OC(O)CHCH2, were also attributed to the ionization of chlorine lone pairs of the CCl3 group. In the high ionization potential region (13.520 eV), bands were related mainly to the σ-type orbital, the fluorine lone pairs of the CF3 group, and the π orbital of CC, CO, and CH bond. They seriously overlap and are much broader than the bands in the low ionization potential region, making it difficult to make further assignments. The precise correspondence was not as clear as that for the outer valence orbitals, but a suggested distribution was given by the calculations (Tables 24).
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culated and experimental ionization potentials of CX3C(O)OC(O)CHCH2 (X = H, F, or Cl) and their fragments. Tables S810 list the atomic charges for molecular and radical-cationic form of CX3C(O)OC(O)CHCH2 (X = H, F, or Cl). This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: (+86)10-62558682 (S.T.); (+86)10-62554518 (M.G.). Fax: (+86) 10-62559373 (S.T. and M.G.). E-mail: tongsr@ iccas.ac.cn (S.T.);
[email protected] (M.G.).
’ ACKNOWLEDGMENT This project was supported by Knowledge Innovation Program (Grant Nos. KJCX2-EW-H01, KJCX2-YW-N24, and KZCX2-YW-Q02-03) of the Chinese Academy of Sciences, the National Basic Research Program of China (973 Program, Nos. 2011CB933701 and 2011CB403401) of the Ministry of Science and Technology of China, and the National Natural Science Foundation of China (Contract Nos. 40925016, 40830101, 21077109, and 41005070). ’ REFERENCES (1) Towler, D. A.; Gordon, J. I.; Adams, S. P.; Glaser, L. Annu. Rev. Biochem. 1988, 51, 69–97. (2) Wu, G.; Van Alsenoy, C.; Geise, H. J.; Sluyts, E.; Van der Veken, B. J.; Shishkov, I. F.; Khristenko J. Phys. Chem. A 2000, 104, 1576–1587. (3) Andreassen, A. L.; Zebelman, D.; Bauer, S. H. J. Am. Chem. Soc. 1971, 93, 1148–1152. (4) IUPAC (1974). Rules for the Nomenclature of Organic Chemistry; Pergamon Press: Oxford, 1974. (5) Wu, G.; Shlykov, S.; Vanalsenoy, C.; Geise, H. J.; Sluyts, E.; Vanderveken, B. J. J. Phys. Chem. 1995, 99, 8589–8598. (6) Wu, G.; Shlykov, S.; VanAlsenoy, C.; Geise, H. J.; Sluyts, E.; VanderVeken, B. J. J. Phys. Chem. 1996, 100, 11620–11629. (7) Vledder, H. J.; Mijlhoff, F. C.; Leyte, J. C.; Romers, C. J. Mol. Struct. 1971, 7, 421–429. (8) Mirone, P.; Fortunato, B.; Canziani, P. J. Mol. Struct. 1970, 5, 283–295. (9) Vledder, H. J.; Van Kleef, F. S. M.; Mijlhoff, F. C.; Leyte, J. C. J. Mol. Struct. 1971, 10, 189–202. (10) Colthup, N. B. Appl. Spectrosc. 1985, 39, 1030–1032. (11) Mayer, F.; Oberhammer, H.; Berkei, M.; Pernice, H.; Willner, H.; Bierbrauer, K.; Paci, M. B.; Arg€uello, G. A. Inorg. Chem. 2004, 43, 8162–8168. (12) Ma, C. P.; Zeng, X. Q.; Ge, M. F. J. Mol. Struct. 2008, 875, 143–151. (13) Dougherty, D.; Brint, P.; McClynn, S. P. J. Am. Chem. Soc. 1978, 100, 5597–5603. (14) Amett, J. F.; Newkome, G.; Mattice, W. L.; McGlynn, S. P. J. Am. Chem. Soc. 1974, 96, 4385–4392. (15) Hoffmann, R. Acc. Chem. Res. 1971, 4, 1–9. (16) Colbourne, D.; Frost, D. C.; McDowell, C. A.; Westwood, N. P. C J. Electron Spectrosc. Relat. Phenom. 1978, 14, 391–403. (17) de Leeuw, D. M.; Mooyman, R.; de Lange, C. A. Chem. Phys. Lett. 1979, 63, 57–60. (18) Kramme, R.; Martin, H. D.; Mayer, B.; Weimann, R. Angew. Chem. 1986, 98, 1134–1136. (19) Bigotto, A.; Galasso, V.; Distefano, G.; Modelli, A. J. Chem. Soc., Perkin Trans. 2 1979, 11, 1502–1506.
4. CONCLUSIONS Acetyl acrylic anhydride and its halogen substituted derivatives, CX3C(O)OC(O)CHCH2 (X = H, F, or Cl), were synthesized by the reaction between acryloyl chloride and the corresponding silver salts. They were investigated by theoretical calculations and various techniques. The results show that the most stable conformers for the three anhydrides are the [ss-c] forms, with each CdO bond syn with respect to the opposite OC bond and the CdC bond in cis orientation to the adjacent CdO bond. The first vertical ionization potential is 10.91, 11.42, and 11.07 eV, respectively, which is attributed mainly to the ionization of the out-of-phase combination of the two lone pairs of carbonyl oxygen atoms. The n+/n splittings are 1.18, 1.15, and 0.63 eV, respectively. The large values of Δn for CH3C(O)OC(O)CHCH2 and CF3C(O)OC(O)CHCH2 mainly derive from the through-bond interaction between the two lone pairs of carbonyl oxygen atoms and the lower lying σ levels. Also, the IR spectra and PIMS of these anhydrides were reported for the first time in this article. ’ ASSOCIATED CONTENT
bS
Supporting Information. Tables S13 list the calculated geometric parameters for the [ss-c] conformer of CX3C(O)OC(O)CHCH2 (X = H, F, or Cl) and for their radicalcationic forms. Tables S46 list experimental vibrational data (cm1) and assignments of the vibrational modes for CX3C(O)OC(O)CHCH2 (X = H, F, or Cl). Table S7 lists cal569
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The Journal of Physical Chemistry A
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