Electronic structures of acyl nitrites and nitrates

Spectrochimica Acta Part A 64 (2006) 949–955

Electronic structures of acyl nitrites and nitrates Xiaoqing Zeng a,b , Li Yao a,b , Weigang Wang a,b , Fengyi Liu a , Qiao Sun a , Maofa Ge a,∗ , Zheng Sun a , Jianping Zhang a , Dianxun Wang a a

State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun, Haidian, Beijing 100080,China b Graduate School of Chinese Academy of Sciences, Beijing 100039, China Received 31 May 2005; accepted 2 September 2005

Abstract The gas phase electronic structures of CM3 C(O)ONO and CM3 C(O)ONO2 (M = H, Cl, F) are studied by photoelectron spectroscopy (PES) combined with the outer valence Green’s function (OVGF) calculations at 6-311 + G(d, p) basis sets. The highest occupied molecular orbital (HOMO) for each compound is the carbonyl oxygen lone pair (nO ), the ionizations of these orbitals are associated with the vibrational frequency about 1750 and 1820 cm−1 reflected on the first band, respectively, for acyl nitrites and nitrate. Comparing with the calculated energies, it can be concluded that the syn conformers with Cs overall symmetry, a planar C C(O) ONO skeleton in nitrites, and a planar C C(O) O N skeleton in nitrates, respectively, are the most stable in the gas phase. © 2005 Elsevier B.V. All rights reserved. Keywords: Acyl nitrites; Nitrates; Electronic structures

1. Introduction Acyl nitrites, acyl nitrates and their peroxy counterpart compounds are of considerable interest. On one hand, they play important roles for the transportation of nitric oxides NOx in the atmosphere, for example, the resources for the oxy, peroxy radicals CX3 C(O)Oy (X = H, Cl, F; y = 1, 2) are widely used in many industrial processes (refrigeration, foam blowing, propellant and cleaning applications, etc.) [1–3]; besides, the use of 1,1,1-trichloroethane (methylchloroform) has been confirmed to be involved in the ClOx catalytic cycle for stratospheric ozone depletion, which is the primary precursor for CCl3 C(O) radical [4]; the formation and dissociation of FC(O)ONO have been detailed discussed during the photooxidation process of chlorofluorocarbon by Francisco [5]. On the other hand, nitration reactions have acquired great importance in the chemistry of organic compounds, as a number of nitro aromatics find applications as solvents, dyestuffs, pharmaceuticals, perfumery chemicals, agrochemicals, reagents and explosives, acetyl nitrates serve as useful intermediates in nitrating reactions with regioselectivity catalyzed by zeolites [6,7].

∗

Corresponding author. Tel.: +86 10 62554518; fax: +86 10 62559373. E-mail address: [email protected] (M. Ge).

1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.09.003

Only a limited number of nitrates MONO2 are known which are liquids or gases at room temperature. These compounds possess predominantly covalent character with some polarity of the O M bond, which increases with decreasing electronegativity of the substituent M. In covalent nitrates, the NO3 moiety adopts Cs symmetry in comparison to the D3h symmetry for radical NO3 [8]. Gas phase structure studies of the parent compound, nitric acid HONO2 , of methyl nitrate, trifluoromethyl nitrate and of the halogen nitrate MONO2 (M = F, Cl, Br, I) using microwave spectroscopy (MW), gas electron diffraction (GED) and photoelectron spectroscopy (PES) have recently been reported in the Ref. [9–13]. As for covalent nitrites, the number of stable nitrites is much smaller; the study of gaseous structure of these compounds is hampered by their chemical instability. The parent compound nitrous acid HONO, decomposes in the gas phase and can be only investigated in the presence of its decomposition products NO, NO2 and H2 O. Organic nitrites RONO with R = methyl, ethyl, isopropyl are thermally stable and their structures have also been studied by MW spectroscopy as well as electronic structures with the aid of photoelectron spectroscopy [9,14]. The study of electronic structures of carboxylic acids and their derivatives reveals the interactions between carbonyl oxygen lone pair (nO ) [15] and the approximately non-bonding anti-symmetric ␲2 orbital [16]. The latter orbital arises from

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the p orbitals on C, O and X (mainly O and X) in the R COX molecules, where X is OH, OR or NR2 . If the conjugation between X and the carbonyl group were removed, the ␲2 orbital would correspond to the lone pair p orbital on the X group. In this work, we present the photoelectron spectra of acyl nitrates and nitrites, describing general characteristics of their geometric and electronic structures in combination with high-level quantum calculations. 2. Experimental and theoretical section Acyl nitrites and acyl nitrates were prepared in situ by the heterogeneous reactions of ClNO and ClNO2 with corresponding acyl silver salts: CH3 C(O)OAg, CF3 C(O)OAg and CCl3 C(O)OAg (Eqs. (1) and (2)). Nitryl chloride and nitrosyl chloride were prepared according to Ref. [17]; the PE spectra of both compounds were the same with Ref. [18,19]. The silver salts were purchased from ARCOS Company and had been dried for 3 h in vacuum (10−4 Torr) (1 Torr = 0.1333 kPa) at 60–70 ◦ C before experiment. In a typical method for the preparation of acetyl nitrite CH3 C(O)ONO, 1.0 g CH3 C(O)OAg were placed in the reaction tube designed specially for PE spectroscopy apparatus, according to Ref. [20], the PE spectrum of pure acetyl nitrite CH3 C(O)ONO could be obtained in situ easily by passing the gaseous ClNO through the solid silver salt. The He I PE spectrum was recorded on a double-chamber machine [21], which was built specifically to detect transient species at a resolution of about 30 meV as indicated by the Ar+ (2 P3/2 ) photoelectron band. Experimental vertical ionization energies (Iv in eV) are calibrated by simultaneous addition of a small amount of argon to the sample. CM3 C(O)OAg(s) + NOCl(g)

Chart 1.

cal calculation standard methods (HF, MP2) [9], the geometrical calculations were carried out at the B3LYP level of theory with 6311+G(d, p) basis set. The B3LYP is a hybrid functional method based on the Becke’s three-parameter non-local exchange functional [22], with the non-local correlation due to Lee et al. [23]. The total energies for all conformers were calculated with ab initio (HF/6-31+G(d, p) and MP2/6-31G(d, p)) and DFT methods (B3PW91/6-311+G(d), B3LYP/6-31+G(d)). To assign the PES bands, the OVGF calculations with 6311+G(d, p) basis set based on the optimized most stable structures have been performed for all acyl nitrites and nitrates. The vertical ionization energies (Ev ) were calculated at the ab initio level according to Cederbaum’s outer valence Green’s function (OVGF) method [24], which includes the effect of electron correlation and reorganization beyond the Hartree–Fock approximation. The self-energy part was expanded up to third-order, and contributions of higher-orders were estimated by means of a renormalization procedure. All above calculation were performed using Gaussian 98 Program [25]. 3. Results and discussion 3.1. Molecular structures

→ CM3 C(O)ONO(g) + AgCl(s)

(1)

CM3 C(O)OAg(s) + NO2 Cl(g) → CM3 C(O)ONO2 (g) + AgCl(s)

(2)

The geometries of syn and anti-conformers for all acyl nitrites and acyl nitrates were fully optimized with density functional theory methods. Because the B3LYP method was found to reproduce all bond lengths well for covalent nitrites and nitrates from the point of experiment, in comparison to other quantum chemi-

Similar to CH3 C(O)OONO2 (PAN) and CF3 C(O)OONO2 (FPAN) [2], all acyl nitrites and acyl nitrates can exist in two conformeric forms, with the acetyl C O bond syn or anti with respect to the O N bond (Chart 1). Spectroscopic investigations of PAN demonstrated the presence of essentially a single conformer, with a contribution of about 1% of a second form, according to NMR data, however, for FPAN there was no indication of a second conformer [26,27]. With quantum calculations, two similar conformers with syn and anti-symmetries are located for all acyl nitrites and nitrates. The calculated energy differences between syn and anti-conformers are listed in Table 1,

Table 1 Calculated energy difference:
E = E(anti) − E(syn) (kcal mol−1 ) of the conformers of the investigated acyl nitrites and nitrates Method

HF/6-31+G(d, p)

MP2/6-31G(d, p)

B3PW91/6-311+G(d)

B3LYP/6-31+G(d)

B3LYP/6-311+G(d, p)

Nitrites CH3 C(O)ONO CF3 C(O)ONO CCl3 C(O)ONO

2.71661 5.98983 9.41747

2.86759 2.96147 5.29053

1.94352 3.27698 4.63529

1.71793 3.00132 4.60033

2.17125 3.03062 4.49002

Nitrates CH3 C(O)ONO2 CF3 C(O)ONO2 CCl3 C(O)ONO2

3.97628 6.46667 16.63038

5.74422 6.35259 11.45255

4.89338 6.85535 10.5399

4.61652 6.25263 10.44508

4.98531 6.59663 10.28909

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Table 2 ˚ bond angles (◦ ) and torsion angles (◦ ) for acyl nitrites and acyl nitrates at the B3LYP/6-311+G(d, p) level Calculated bond lengths (A), Parameters

CM3 C(O)ONO X=H

C2 C2 C2 C1 C1 O2 O3

M1 M C1 O1 O2 N N

M M M M1 C2 C2 C1 O2

C2 M  C2 M 1 C2 C 1 C2 C 1 C1 O1 C1 O2 O2 N N O3

∅(M ∅(M1 ∅(C2 ∅(C1

C2 C2 C1 O2

1.088 1.092 1.508 1.199 1.378 1.560 1.144

C1 O2 ) C1 O 2 ) O2 N) N O3 )

CM3 C(O)ONO2 X=F 1.333 1.346 1.563 1.197 1.330 1.675 1.124

X = Cl 1.776 1.799 1.571 1.196 1.333 1.660 1.126

X=H 1.088 1.092 1.504 1.195 1.384 1.525 1.187

X=F 1.330 1.342 1.561 1.193 1.339 1.604 1.177

X = Cl 1.776 1.796 1.563 1.193 1.344 1.586 1.179

107.5 110.1 109.9 109.2 109.5 126.3 113.8 108.4

108.1 108.4 110.5 110.6 122.2 109.3 113.4 107.5

110.0 109.8 108.6 109.9 123.0 109.7 113.1 107.6

107.9 110.1 110.0 108.8 127.9 109.1 111.6 113.3

108.5 108.7 110.5 109.9 123.8 108.8 110.6 111.8

110.3 110.0 108.6 109.3 124.6 109.4 110.6 112.1

59.1 180.0 180.0 180.0

59.9 180.0 180.0 180.0

59.8 180.0 180.0 180.0

59.3 180.0 180.0 91.6

60.0 180.0 180.0 91.6

60.0 180.0 180.0 91.7

as it can be seen, the energy gap increases with the enlarge of the CM3 substituent group, it means that the spatial interactions between the CM3 groups and the ONO or ONO2 moieties decreases the stabilities of all anti-conformers. These calculated energy gaps are all larger than that of PAN and FPAN, 3.5 and 4.1 kcal mol−1 , respectively, at B3PW91/6-311+G(d) level of theory [2]. The calculations also predict the structures with Cs overall symmetry, a planar C C(O) ONO skeleton in nitrites and a planar C C(O) O N skeleton in nitrates, respectively, which are similar to the structures of CH3 OC(O)SCl [28], (CF3 )2 NONO2 [29] and FC(O)ONO [5]. The calculated (B3LYP/6-311+G(d, p)) geometric parameters for all thermally favourable syn conformers are showed in Table 2 (for atom numbering, see Fig. 1). In both the nitrites and nitrates, the N O bond length increases ˚ (1.525 A) ˚ with increasing electronegativity of M from 1.560 A ˚ (1.604 A) ˚ in in CH3 C(O)ONO (CH3 C(O)ONO2 ) to 1.675 A CF3 C(O)ONO (CF3 C(O)ONO2 ), this trend can also be sum˚ CH3 ONO marized in other series of nitrites HONO (1.379 A), ˚ ClONO (1.48 A) ˚ and (CF3 )2 NONO (1.572 A); ˚ (1.398 A), ˚ ˚ ˚ HONO2 (1.410 A), CH3 ONO2 (1.402 A), CF3 ONO2 (1.493 A) ˚ and CF3 C(O)OONO2 (1.526 A) for nitrates [9]. As explained

by Oberhammer, the N O single bond length in gaseous nitrates XO–NO2 depends strongly on the substituent X, the variation of N O bond lengths can be attributed to the electronic effects, qualitatively, the variation in the electronic effects can be divided into two contributions: (1) decreasing polarity of the bond with increasing electronegativity of X and (2) change in the covalent bond strength. 3.2. Spectra assignment The He I PE spectra of CH3 C(O)ONO, CCl3 C(O)ONO and CF3 C(O)ONO are depicted in Fig. 2 (the expanded spectra in the low ionization region (10.0–13.0 eV)) of the three acyl nitrites are also presented on the right side). On the whole, there exist two bands in the low ionization region for all three compounds, and the second band is much broader than the first band; meanwhile, clear vibrational progressions can be seen on the first band from their expanded spectra. Assignments were made with reference to the results of the OVGF/6-311+G(d, p) calculations. The experimental and calculated vertical ionization potentials are listed in Table 3. Similar to the PE spectra of acetic acid and its derivatives,

Fig. 1. Molecular models of CM3 C(O)ONO and CM3 C(O)ONO2 (M = H, Cl, F).

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Fig. 2. PES of CM3 C(O)ONO (M = H, Cl, F) and expanded PE spectra in the low ionization region (10.0–13.0 eV).

Table 3 Experimental vertical ionization energies (Iv in eV), computed ionization energies (Ev in eV) by OVGF calculation with 6-311+G(d, p) basis sets and molecular orbital-ionized characters for syn isomers of acyl nitrites Ev (eV)

MO

Iv (eV)

Character

CH3 C(O)ONO 10.58 11.30 11.99 13.86 14.68 15.27

19a 18a 4a 3a 17a 16a

10.62 11.51 13.40 14.97 15.60

nO(C O) ␴ONO ␲2 a ␲CH3 , ␲OCO ␲CH3 ␴C O,CH3

CF3 C(O)ONO 11.25 11.99 12.35 15.48 15.51 15.60

26a 25a 9a 24a 8a 23a

11.01 12.05 12.49 15.19 15.65 16.02

nO(C O) ␴ONO ␲2 a nF nF , ␲OCO nF , ␴ C O

CCl3 C(O)ONO 10.66 11.06 11.19 11.50 12.00 12.07 12.16 12.45 12.56 14.92

33a 14a 32a 13a 31a 12a 30a 11a 29a 10a

10.72 11.23

nO(C O) , nCl nCl nCl nCl nCl , ␴ONO nCl , ␲2 a nCl nCl , ␲C O nCl ␲OCO

a

Ref. [16].

11.59 12.09

12.64 15.16

such as CH3 C(O)OH (10.87 eV), CH3 C(O)OCH3 (10.48 eV) and CF3 C(O)OH (11.77 eV) [16], the first ionization process happens on the carbonyl oxygen lone pair (nO ), the experimental first vertical ionization potentials for CH3 C(O)ONO, CCl3 C(O)ONO and CF3 C(O)ONO are 10.62, 10.72 and 11.01 eV, respectively, which are close to the calculated values 10.58, 10.66 and 11.25 eV, the electronic withdrawing effects have been clearly reflected in this series of compounds. By analyzing the calculated characteristics of molecular orbitals (Table 3), that the highest occupied molecule orbital (HOMO) for each compounds comes from the oxygen lone pair on the C O group, the vibration frequencies at about 1750 ± 60 cm−1 in the expanded PE spectra (Fig. 2) coincides well with the calculated (B3LYP/6-311+G(d, p)) 1815 cm−1 (CH3 C(O)ONO), 1811 cm−1 (CCl3 C(O)ONO) and 1820 cm−1 (CF3 C(O)ONO) within the spectroscopic resolution. Owing to the participation of the chlorine lone pair in the HOMO for the second nitrite, the vibration seems to be blurred. The second band for CH3 C(O)ONO (11.51 eV) and CF3 C(O)ONO (12.05 eV) exhibits some similarity, the approximate intensity is two times of the first band for each compounds, it means that there are two ionization processes happen in this region, a shoulder can be seen in these two broad bands from the expanded spectra. With the aid of MO analysis (Table 4), we can easily assign these two broad bands as follows: as for CH3 C(O)ONO, it can be deemed as the overlap of two different orbitals 18a and 4a , which are the ionization of ␴ONO [14] and ␲2 (␲C O , nO ) orbitals [16], with the calculated ionization energies 11.30 and 11.99 eV; as for the latter compound, the

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Table 4 Characters of the three highest occupied molecular orbitals (HOMO) for CM3 C(O)ONO (M = H, F, Cl) Species

CH3 C(O)ONO

CF3 C(O)ONO

CCl3 C(O)ONO

HOMO

SHOMO

THOMO

ionizations on the similar orbitals 25a and 9a with almost same orbital components. However, it is quite different for CCl3 C(O)ONO. The second band ranges from 11.23 to 12.64 eV, obviously, it is the overlapping results of ionizations from several orbitals, in comparison to the PE spectra of CHCl3 and CCl3 C(O)Cl [30], the lone pair electrons of the three chlorine

atoms account for this broad band, the calculated values for the these ionizations are listed in Table 3, the characteristics of these orbitals are depicted in Table 4, and the analysis of these orbitals further supports the assignments of this band. Of course, other ionization occasions also take part in this broad region.

Fig. 3. PES of CM3 C(O)ONO2 (M = H, Cl, F) and expanded pe spectra in the low ionization region (10.0–14.0 eV).

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Table 5 Experimental vertical ionization energies (Iv in eV), computed ionization energies (Ev in eV) by OVGF calculation with 6-311+G(d, p) basis sets and molecular orbital-ionized characters for syn isomers of acyl nitrites Ev (eV)

MO

Iv (eV)

Character

CH3 C(O)ONO2 11.57 12.36 12.56 12.63 13.28 14.59

19a 8a 7a 18a 6a 5a

11.46 12.00 12.20 12.75 13.18 14.32

nO(C O) ␴ONO2 nO(NO2 ) ␲2 a , nO(NO2 ) ␲ONO2 , ␲C O ␲CH3 , ␲OCO

CF3 C(O)ONO2 12.28 13.20 13.20 13.62 14.26 16.20

26a 13a 12a 25a 11a 24a

12.40 13.25

nO(C O) nO(NO2 ) ␲2 a ␴ONO2 ␲ONO2 nF

CCl3 C(O)ONO2 11.42 11.50 11.59 11.84 12.52 12.59 13.03 13.05 13.09 13.84 15.66

18a 33a 32a 17a 31a 16a 15a 14a 30a 29a 13a

11.34 11.68

a

13.75 14.25 15.69

12.06 12.37 12.68

13.94 15.63

Ref. [16].

nCl , nO(C nCl nCl nCl nCl nCl , ␲2 a ␴ONO2 nCl nO(NO2 ) nCl ␲ONO2

O)

There remains one broad from 13.40 to 17.0 eV for CH3 C(O)ONO, it can be reasonably assigned to the ionization of ␲CH3 , comparing to the PE spectra of other organic molecules like CH3 C(O)CH3 and CH3 NO2 [30], and other inner molecular orbitals in the high ionization region (>14.0 eV). In the case of CCl3 C(O)ONO, the band centered at 15.16 eV with low intensity might be assigned to the ionization of ␴C Cl , it can be expected in the PE spectra of similar organic compounds with CCl3 moiety [30]. Similarly, the ionization of fluorine lone pair of the CF3 moiety always causes the appearance of the characteristic bands from 15.19 to 17.0 eV. The PE spectra of the three acyl nitrates (CM3 C(O)ONO2 , M = H, Cl, F) are depicted in Fig. 3, the expanded spectra in the low ionization region are also presented on the right side. In comparison to the above discussed spectra of the nitrite counterparts, some general aspects could be concluded, in the low ionization region (10.0–14.0 eV), two broad bands exist as well, and the profiles are similar to those of the acyl nitrites; the first vertical ionization potentials for these three compounds shift to the higher energy side by almost 1.0 eV, it can be supposed to be the electron–acceptor nature of the NO2 group; the first band for each compound shows the characteristic vibrational frequencies of C O about 1820 ± 60 cm−1 , it means that the first ionizations occur primary on the carbonyl oxygen lone pair (mainly localized on the oxygen atom); finally, the bands in the high ionization region are almost the same with those nitrite compounds; here, we only discuss those bands in the low ionization area. The experimental first vertical ionization potentials for CH3 C(O)ONO2 , CCl3 C(O)ONO2 and CF3 C(O)ONO2 are calibrated to be 11.46, 11.34 and 12.40 eV, respectively, coincides well with the calculated values 11.57, 11.42 and 12.28 eV. However, according to the electronegativity of H and Cl, the former two energies should be reversed, by the analysis of the HOMO

Table 6 Characters of the three highest occupied molecular orbitals (HOMO) for CM3 C(O)ONO2 (M = H, F, Cl) Species

HOMO

SHOMO

THOMO

CH3 C(O)ONO2

CF3 C(O)ONO2

CCl3 C(O)ONO2

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of both two molecules depicted in Table 6, we can find that the ionization of the HOMO for CCl3 C(O)ONO2 includes not only the carbonyl oxygen lone pair electrons, but also the lone pair electrons on the chlorine atom, in other chlorine contained compounds like Cl2 and ClSO2 N3 , same ionization occasions occur at 11.59 eV [30] and 11.43 eV [31]. The second band for CH3 C(O)ONO2 displays a shoulder at about 12.0 eV, it is ascribed to the ionization of 8a (␴ONO2 ) orbital, the calculated energy for this orbital is 12.36 eV. The next peak band at 12.20 eV corresponds to the lone pair electrons wholly or dominantly localized on the two oxygen atoms of the NO2 group. The following two bands overlap, and the vertical ionization energies are 12.75 and 13.18 eV, which are in agreement with the calculated values 12.63 and 13.28 eV, the former is the result of the ionization of the approximately non-bonding anti-symmetric ␲2 orbital, but it arises from the p orbitals on the two oxygen atoms of the C O group and the C O moiety, the oxygen lone pair electrons of NO2 group should be also included; the latter involves the contribution of both ␲ONO2 and ␲C O . The remained band centered at 14.32 eV is the typical ionization of ␲CH3 , besides the participation of ␲OCO . The PE spectra of CCl3 C(O)ONO2 and CF3 C(O)ONO2 are quite similar to those of CCl3 C(O)ONO and CF3 C(O)ONO, so the assignment of the bands can refer to the analysis of their nitrite counterparts, the experimental and calculated spectroscopic data, molecular orbitals and MO characteristics are summarized in Table 5, the characteristics of the first three highest occupied molecular orbitals are also given in Table 6. As it can be seen, the main differences can be attributed to the participation of oxygen lone pair electrons on the NO2 group. 4. Conclusion The electronic structures of CM3 C(O)ONO and CM3 C(O)ONO2 (M = H, Cl, F) were discussed. The assignments of their PE spectra were accomplished in combination with quantum calculations, the highest occupied molecular orbital for each compound is the carbonyl oxygen lone pair (nO ), which is associated with the vibrational frequency about 1800 cm−1 reflected on the first band. Meanwhile, the geometric structures for these two series of compounds are optimized at B3LYP/6-311+G(d, p) level of theory. Comparing with the calculated energies, it can be concluded that the syn conformers with Cs overall symmetry, a planar C C(O) ONO skeleton in nitrites, and a planar C C(O) O N skeleton in nitrates, respectively, are the most stable in the gas phase. Acknowledgements This project was supported by Chinese Academy of Sciences (contract no. KJCX2-SW-H8 and hundred talents fund) and the National Natural Science Foundation of China (contract nos. 20477047, 20473094, 50372071). Xiaoqing Zeng, Li Yao, Weigang Wang and Fengyi Liu thank the Chinese Academy of Sciences for scholarship during the period of this work.

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