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The effect of the carbonyl moiety on the spin density delocalization in the iminoxy radicals. Hybrid density functional studies
Chemical Physics Letters 367 (2003) 678–689 www.elsevier.com/locate/cplett
The effect of the carbonyl moiety on the spin density delocalization in the iminoxy radicals. Hybrid density functional studies Adrian R. Jaszewski, Katarzyna Tabaka, Julia Jezierska *, Jadwiga Kedzierska Faculty of Chemistry, University of Wroclaw, 14 Joliot-Curie Street, Wroclaw 50-383, Poland Received 5 June 2002; in final form 22 October 2002
Abstract UB1LYP method was used to study the influence of carbonyl group in 1,2-diphenyl-1-oxoethan-2-iminoxyl and 1-oxoethan-2-iminoxyl, on radical structure and unpaired electron spin density distribution. A significant change of radical properties was found due to the second carbonyl group in 2,4-dioxopenthan-3-iminoxyl and 1,3-dioxopropan-2iminoxyl. A specific impact of the carbonyl groups on the EPR hyperfine couplings with 1 H and 13 C nuclei (calculated at UB1LYP/EPR-III level in comparison with the experimental data) was determined for the stable isomeric forms of the radicals. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Iminoxyls are the persistent r-type radicals observed in the lichen thalli (Lasallia pustulata and Umbilicaria polyphylla) treated with NO2 under laboratory experiment as well as grown naturally in the polluted environment [1]. They are also formed by reaction between NO and tyrosyl radical occurring in photosystem II [2], prostaglandin H synthase-2 enzyme [3] and in non-protein systems [4].
*
Corresponding author. Fax: +48-71-328-23-48. E-mail address: [email protected] (J. Jezierska).
Generally the presence of C@O moiety increases the life time of the iminoxyls as can be easily seen for the radicals formed in reaction of NO2 with metal acetylacetonates [5], ketones having a free active methylene group [6] and b-diketones [7]. The radicals were studied in the rare gas matrices by infrared spectroscopy [8] and in the liquid and frozen solution by electron paramagnetic resonance (EPR) spectroscopy [5,9]. Stability of the iminoxyls containing C@O group provides a possibility to observe not only hyperfine splitting of the EPR spectra for nitrogen but also carbon; the assignment of the hyperfine constants (hfccs) to the particular carbon nuclei is not straightforward [10,11]. This work is a continuation of our studies on the effect of substituents in vinyl (RHC@CH) [12]
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 7 4 3 - 8
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679
Scheme 1.
and iminoxy (R1 R2 C@NO) radicals on hyperfine couplings with 1 H and 13 C nuclei [13]. The calculations of hfccs at HFD level showed that their
values depends mainly on isomeric form (E and Z) adopted by the radicals. Theoretical spin densities also showed the asymmetric (preferred in syn po-
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sition) delocalisation of unpaired electron in R1 ¼ Ph, R2 ¼ PðOÞðOCH3 Þ2 [14] consistent with the change of experimental values of hyperfine coupling constants. The main goal of this work is to study the effect of the substituents R1 , with one C@O group attached directly to iminoxyl carbon of the radicals on values of EPR hyperfine couplings and delocalization of unpaired electron spin density, calculated using UB1LYP method and the basis sets dependent on the radicals. The influence of the second C@O provided by substituent R2 on the properties of the radical isomers was also examined. The results are compared with those obtained for the radical without C@O group and with OH group instead of C@O. The available EPR data for the radicals are used to verify the results of the computations. The calculations also appeared to be a valuable criterion of assignment of the experimental hyperfine couplings.
nection methods, namely B1LYP [19] hybrid density functional, was applied. For geometry optimization and determination of thermodynamic properties it was used together with 6-31+G(d,p) basis set [20,21] for 1,2-diphenyl-1oxoethan-2-iminoxyl (I), 6-311+G(d,p) basis set [21,22] for 2,4-dioxopenthan-3-iminoxyl (II), and 6-311++G(3df,3pd) basis set [21,22] for ethan-1iminoxyl (III), 1-hydroxyethan-2-iminoxyl (IV), 1-oxoethan-2-iminoxyl (V) and 1,3-dioxopropan2-iminoxyl (VI). Hyperfine couplings (hfccs) were calculated using EPR-III basis set of Barone [23,24], developed for the calculations of the EPR parameters. The optimizations were performed without any constraint (except Cs symmetry if necessary). The existence of energy minima was proved by vibrational analyses. Harmonic frequencies and normal modes were computed at the same level as geometry optimizations. These frequencies were used, without scaling, to calculate analytically zero-point vibrational energy (ZPVE) contributions. No attempt was made to determine the effect of zero-point nuclear vibrations on the hfccs. Graphical representations of spin densities were generated using SURFER [25] program. Natural atomic orbitals (NAOs) occupancies were achived
2. Methods All the calculations have been performed by the GA U S S I A N 98 program system [15]. The Becke (B) exchange functional [16] combined with the Lee– Yang–Parr (LYP) correlation functional [17] using a single-parameter [18] version of adiabatic con-
Table 1 and deg), total (Etot in hartrees) and ZPVEs (in kJ=mol) and vibrational corrected energy differences (DE in Geometry parameters (A kJ=mol) for the isomers of 1,2-diphenyl-1-oxoethan-2-iminoxyl (I) at UB1LYP/6-31+G(d,p) level, for the most stable isomer of 2, 4-dioxopenthan-3-iminoxyl (II) at UB1LYP/6-311+G(d,p) level and for the isomers of ethan-1-iminoxyl (III) at UB1LYP/ 6-311++G(3df,3pd) level ½H3 CCð@OÞ2 C@NO
H3 CCð@NOÞH
I-2
Parameters
II
Parameters
III-1
III-2
C1
Symmetry r(NO) r(CN) a(CNO) a(NC3 C4 ) a(NC3 C7 ) /(ONC3 C4 ) /(NC3 C4 O5 ) /(NC3 C7 O8 )
Cs
Symmetry r(NO) r(CN) a(CNO) a(NC3 C4 ) a(NC3 H5 ) /(ONC3 C4 ) /(NC3 C4 H6 )
Cs
Cs
1.224 1.273 134.8 121.5 117.9 180.0 0.0
1.225 1.274 134.4 124.0 115.2 0.0 0.0
Etot ZPVE
)474.41328 282.2
Etot ZPVE DE
)208.48367 158.4 2.2
)208.48472 158.9 0.0
PhCð@OÞCð@NOÞPh Parameters
I-1
Symmetry C1 r(NO) 1.227 r(CN) 1.299 a(CNO) 135.0 a(NC3 C4 ) 113.7 a(NC3 C12 ) 122.9 /(ONC3 C4 ) 175.1 14.6 /(NC3 C12 C13 ) /(NC3 C4 O5 ) 149.0 /(C3 C4 C6 C7 ) 30.8 Etot )744.30690 ZPVE 540.7 DE 0.0
1.231 1.293 134.9 118.9 119.1 2.6 9.0 127.3 19.2 )744.30420 539.8 7.1
1.204 1.305 131.9 116.7 116.5 0.0 0.0 180.0
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from UB1LYP/EPR-III calculations according to Carpenter and Weinhold [26].
3. Results and discussion All the isomers obtained from the UB1LYP geometry optimizations for the radicals studied are presented on Scheme 1 with the numbering scheme used. Two stable isomers are determined for I (Table 1). E (I-1) form is thermodynamically more stable than Z (I-2). UB1LYP/EPR-III computations lead to Aiso ð14 NÞ of 33.2 G (E) and 29.8 G (Z) and
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Aiso ð13 CÞ for iminoxy C3 carbon of 21.4 G (E) and 14.8 G (Z) that is in an excellent agreement with experimental EPR data 32.65 and 29.95 G for Aiso ð14 NÞ and 22.0 and 15.0 G for Aiso ð13 C3 Þ [10]. Hfccs with carbons connected to C3 atom are dependent on their position (syn or anti) relating to NO bond of CNO moiety. The interaction with syn carbon (10.6 G for C12 in I-1 and 9.5 G for C4 in I-2) is stronger than with anti carbon (3.5 G for C4 in I-1 and 3.6 G for C12 in I-2). The couplings with two phenyl protons (after averaging the values for H23 and H27 ) of 1.8 G (I-1) and of 0.9 G (I-2) are also larger than for benzoyl ortho-protons 0.5 G (I-1) and )0.1 G (I-2) (after averaging the
Fig. 1. Contour maps of UB1LYP/EPRIII unpaired spin density (e/au3) in the plane containing C@NO iminoxy moiety for the isomers of ethan-1-iminoxy radical together with gray scale used; dashed line denotes the surface of zero spin density.
Table 2 and deg), total (Etot in hartrees) and ZPVEs (in kJ=mol) and vibrational corrected energy differences (DE in Geometry parameters (A kJ=mol) for the isomers of 1-hydroxyethan-2-iminoxyl (IV) at UB1LYP/6-311++G(3df,3pd) level Parameters
Symmetry r(NO) r(CN) a(CNO) a(NC3 C4 ) a(NC3 C6 ) /(ONC3 C4 ) /(NC3 C4 O5 ) /(C3 C4 O5 H9 ) Etot ZPVE DE
H2 CðOHÞCð@NOÞH IV-1
IV-2
IV-3
IV-4
IV-5
IV-6
IV-7
C1
C1
Cs
C1
Cs
C1
C1
1.219 1.274 135.0 120.4 119.2 178.8 120.1 169.2 )283.70254 172.0 3.6
1.219 1.276 135.2 120.4 119.0 179.4 117.0 62.6 )283.70426 173.0 0.1
1.221 1.270 134.8 122.1 119.2 180.0 0.0 180.0 )283.69926 170.6 10.8
1.224 1.271 134.8 121.1 119.3 179.9 5.0 48.3 )283.70170 171.9 5.6
1.227 1.272 132.6 122.1 117.8 0.0 180.0 180.0 )283.70296 171.9 2.3
1.222 1.275 134.7 123.4 116.5 0.7 131.2 66.4 )283.70449 173.6 0.0
1.226 1.273 132.9 121.2 116.6 1.0 48.7 61.2 )283. 70387 173.7 1.8
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values for H18 and H22 ). Thus experimental value [10] for the phenyl protons of I-1 (1.4 G) is quite well reproduced. UB1LYP/6-311+G(d,p) calculations predict only one stable conformer of II (Table 1). Other structures that can be obtained by the change of dihedral angle N2 C3 C4 O5 from 0° to 180° and/or N2 C3 C7 O8 from 180° to 0° (with or without additional rotation of the methyl groups) correspond
to first-order saddle points according to the vibrational analysis. This is in contrast to previous Extended-H€ uckel calculations [11] showing that conformer with N2 C3 C4 O5 and N2 C3 C7 O8 angles of 180° has the lowest electronic energy. The assignment of the experimental EPR hyperfine couplings presented was unambiguous in spite of this shortcoming [11]. Our calculations give Aiso ð14 NÞ of 26.4 G very close to the experimental value of
Table 3 UB1LYP/EPR-III spin populations (e) on the natural atomic orbitals of some atoms of ethan-1-iminoxyl (III) and 1-hydroxyethan-2iminoxyl (IV) Totala
Isomer
Atom
Natural atomic orbitals s-type
px -type
py -type
pz -type
III-1
C3 C4
)0.001 0.002
0.010 0.002
)0.006 0.000
)0.071 0.001
)0.067 0.005
III-2
C3 C4
0.000 0.004
0.009 0.012
)0.005 0.027
)0.071 0.002
)0.066 0.045
IV-1
C3 C4 O5 H6
)0.001 0.002 )0.001 0.040
0.012 0.002 0.000 0.000
)0.006 0.001 0.000 0.000
)0.071 0.001 )0.002 0.000
)0.066 0.005 )0.002 0.040
IV-2
C3 C4 O5 H6
)0.001 0.002 0.000 0.040
0.012 0.002 )0.001 0.000
)0.006 0.001 )0.001 0.000
)0.071 0.001 )0.001 0.000
)0.065 0.005 )0.004 0.040
IV-3
C3 C4 O5 H6
)0.002 0.002 0.000 0.041
0.011 0.002 0.001 0.000
)0.007 0.001 0.001 0.000
)0.072 0.001 0.000 0.000
)0.069 0.006 0.002 0.041
IV-4
C3 C4 O5 H6
)0.002 0.002 0.000 0.038
0.013 0.002 0.003 0.000
)0.006 0.001 0.000 0.000
)0.069 0.001 0.000 0.000
)0.064 0.005 0.003 0.038
IV-5
C3 C4 O5 H6
0.000 0.005 0.001 0.005
0.010 0.007 )0.001 0.000
)0.005 0.026 0.001 0.000
)0.073 0.002 0.000 0.000
)0.068 0.041 0.001 0.005
IV-6
C3 C4 O5 H6
)0.002 0.004 0.000 0.007
0.011 0.006 0.000 0.000
)0.005 0.031 0.009 0.000
)0.071 0.002 0.000 0.000
)0.067 0.043 0.008 0.007
IV-7
C3 C4 O5 H6
)0.003 0.005 0.000 0.006
0.011 0.016 0.010 0.000
)0.005 0.019 0.003 0.000
)0.074 0.002 )0.001 0.000
)0.069 0.042 0.012 0.006
a
A summary of spin population on s-, px -, py -, and pz -type NAOs.
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683
Fig. 2. Contour maps of UB1LYP/EPRIII unpaired spin density (e/au3) in the plane containing C@NO iminoxy moiety for the isomers of 1-hydroxyethan-2-iminoxy radical together with gray scale used; dashed line denotes the surface of zero spin density.
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28.0 G. Additionally, DFT results show dramatic change of the carbon hyperfine interactions upon binding of the second carbonyl group to iminoxy CN@O moiety (in comparison with I). Aiso ð13 C3 Þ is strongly reduced to 7.3 G, which correlates with the experimental value of 8.12 G [11]. A larger hfcc predicted for methyl C6 (9.0 G) should be assigned to the experimental constant of 10.0 G [11]. Aiso ð13 CÞ is larger for C4 (4.0 G) of syn C@O (in respect to CN@O) than for C7 (3.0 G) of anti C@O; the values are close to 4.90 G from EPR spectrum [11]. The averaged hyperfine interaction with H13 , H14 , H15 protons of syn CH3 group (relatively N@C) but attached to anti C@O is larger (0.7 G) than that for H10 , H11 , H12 of CH3 attached to syn C@O ()0.3 G). As hfccs for methyl protons calculated using DFT are usually larger than EPR spectral values [14,27] we assign the experimental constants of 0.39 G to H13 , H14 and H15 but 0.12 G to H10 , H11 and H12 . This is in contrast with WoldÕs and Lagercrantz suggestion, but one should remember that the methyl group connected to the syn carbonyl moiety is in the anti position in relation to the N@C double bond in our calculations whereas it is in the syn position in the structure examined by these authors. 3.1. Spin density distribution Since the presence of C@O group strongly influences the value of iminoxy Aiso ð13 CÞ we decided
to study spin density delocalization in the series of model iminoxy radicals: from III that contains no other-than-iminoxy oxygen and IV that contains OH group to V with one and VI with two carbonyl moieties (Scheme 1). 3.1.1. Ethan-1-iminoxyl DFT calculations reveal two stable isomers of III corresponding to E (III-1) and Z (III-2) forms (Table 1) with Aiso ð14 NÞ of 29.0 and 31.5 G and Aiso ð13 C3 Þ of 31.3 and 29.3 G, respectively. Significantly smaller Aiso ð1 H5 Þ for III-2 (8.5 G) than for III-1 (26.8 G) is typical for iminoxyl radicals with R1 ¼ H, but Aiso ð13 C4 Þ is slightly larger for III-2 (7.5 G) than for III-1 (4.1 G). The absolute value of the averaged hfcc for CH3 protons (H6 , H7 , H8 ) is also slightly larger for Z ()2.6 G) than for E ()2.4 G) isomer. Unsymmetrical spin distribution on atoms syn to iminoxy group (NO bond) with larger spin densities is evidenced from the spin density maps (Fig. 1) and from the results of the natural atomic orbital analysis (Table 3). The data in Table 3 indicate that spin population is larger on r-type (s, px and py ) NAOs of C4 atom for Z form of III. It is remarkable that unsymmetrical spin density delocalization generally does not affect the population of pz -type orbitals on C3 and C4 atoms (p-type with the axis system used, iminoxyls are treated as r-type radicals; an unpaired electron is localized on p-type orbital derived mainly from p orbital of iminoxy oxygen and sp2 orbital of iminoxy nitrogen lying in the
Fig. 3. Contour maps of UB1LYP/EPRIII unpaired spin density (e/au3) in the plane containing O5 atom and parallel to the plane containing C@NO iminoxy moiety for the IV-6 (left) and IV-7 (right) isomers of 1-hydroxyethan-2-iminoxy radical together with gray scale used; dashed line denotes the surface of zero spin density.
A.R. Jaszewski et al. / Chemical Physics Letters 367 (2003) 678–689 Table 4 and deg), total (Etot in hartrees) and Geometry parameters (A ZPVEs (in kJ=mol) and vibrational corrected energy differences (DE in kJ=mol) for the isomers of 1-oxoethan-2-iminoxyl (V) at UB1LYP/6-311++G(3df,3pd) level Parameters
HCð@OÞCð@NOÞH V-1
V-2
V-3
Symmetry r(NO) r(CN) a(CNO) a(NC3 C4 ) a(NC3 H6 ) /(ONC3 C4 ) /(NC3 C4 O5 ) Etot ZPVE DE
Cs
Cs
Cs
V-4
Cs 1.204 1.200 1.208 1.203 1.287 1.288 1.287 1.292 135.1 135.0 134.5 132.5 118.0 119.7 121.0 120.8 120.4 119.0 117.8 117.2 180.0 180.0 0.0 0.0 180.0 0.0 180.0 0.0 )282.5044 )282.4992 )282.5040 )282.5027 110.4 110.3 111.1 111.3 0.0 13.7 1.7 5.3
nodal plane of a molecular p-type C@N double bond). 3.1.2. 1-Hydroxyethan-2-iminoxyl We have localized seven stable conformers of IV according to the vibrational analysis (Table 2): from IV-1 to IV-4 (E isomers) and from IV-5 to
685
IV-7 (Z isomers). IV-6 (Z) has the lowest total energy. IV-2 (with total energy of 0.1 kJ/mol larger) represents thermodynamically most stable E form. Generally, the calculations indicate slightly larger interaction with iminoxy nitrogen for Z than for E forms of the radical (29.1, 28.8, 31.0, 28.0 G for IV-1, IV-2, IV-3, IV-4 and 31.3, 30.8, 31.6 G for IV-5, IV-6, IV-7, respectively), except of 0.2 G larger value for IV-3 than for IV-6. IV-3 has the highest total energy, 10.8 kJ/mol larger than IV-6. Unsymmetrical syn-preferential spin distribution is manifested by Aiso ð1 HÞ for H6 proton (26.8, 27.2, 26.7, 25.3 G for IV-1, IV-2, IV-3, IV-4 and 6.8, 8.8, 7.6 G for IV-5, IV-6, IV-7, respectively), by Aiso ð13 CÞ for C4 (8.6, 7.9, 7.5 G for IV-5, IV-6, IV-7 and 4.0, 3.9, 3.0, 3.3 G for IV-1, IV-2, IV-3, IV-4, respectively) and by the spin populations on s- and r-type orbitals (Table 3). Aiso ð13 C3 Þ of IV-1 (29.6 G) and IV-2 (30.2 G) are slightly smaller than for IV-3 (27.7 G) and IV-4 (26.6 G) as a result of spin density flow to the p-type orbital of O5 oxygen in C@NO plane (Fig. 2, Table 3). Aiso ð13 C3 Þ for IV-5 is equal to 27.9 G and spin density is delocalized on sp3 orbital of O5 (in the radical plane). Interaction with C3 decreases from
Table 5 UB1LYP/EPR-III spin populations (e) on the natural atomic orbitals of some atoms of 1-oxoethan-2-iminoxyl (V) Isomer
Atom
Totala
Natural atomic orbitals s-type
px -type
py -type
pz -type
V-1
C3 C4 O5 H6
)0.002 0.003 )0.001 0.037
0.016 0.002 0.000 0.000
)0.005 0.001 0.000 0.000
)0.070 0.007 )0.028 0.000
)0.061 0.013 )0.028 0.037
V-2
C3 C4 O5 H6
)0.002 0.002 0.000 0.039
0.015 0.001 0.002 0.000
)0.006 0.002 0.000 0.000
)0.076 0.007 )0.029 0.000
)0.069 0.013 )0.028 0.039
V-3
C3 C4 O5 H6
)0.004 0.005 0.000 0.007
0.015 0.002 0.000 0.000
)0.003 0.026 0.034 0.000
)0.070 0.006 )0.025 0.000
)0.060 0.039 0.010 0.007
V-4
C3 C4 O5 H6
)0.003 0.003 0.001 0.005
0.018 0.007 0.027 0.000
)0.002 0.009 0.005 0.000
)0.067 0.003 )0.024 0.000
)0.054 0.024 0.008 0.005
a
A summary of spin population on s-, px -, py -, and pz -type NAOs.
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IV-6 (21.8 G) to IV-7 (19.7 G) due to delocalization of spin density on p-type orbitals of O5 in the plane parallel to C@NO (Fig. 3, Table 3). 3.1.3. 1-Oxoethan-2-iminoxyl Four stable forms of Cs symmetry are predicted for V (Table 4). V-1 E isomer has the lowest total energy, about 1.7 kJ/mol smaller than for V-3 Z. UB1LYP/EPR-III calculations give Aiso ð14 NÞ values similar to those observed for other radicals (29.2, 31.8, 30.0 and 28.1 G for V-1, V-2, V-3 and V-4, respectively). Unsymmetrical syn-preferential spin distribution is reflected by Aiso ð1 H6 Þ, greater for E (27.6 G (V-1) and 28.0 G (V-2)) than for Z isomers (8.6 G (V-3) and 6.3 G (V-4)) as well as by Aiso ð13 C4 Þ, greater for Z (7.7 G (V-3) and 8.4 G (V-4)) than for E isomers (5.0 G (V-1) and 3.6 G
(V-2)). The presence of C@O group leads to new effects as compared to the iminoxyls III and IV. Aiso ð13 C3 Þ is strongly reduced to 24.9 G (V-1) and 24.1 G (V-2) for E and to 13.2 G (V-3) and 12.7 G (V-4) for Z isomers. This effect, caused by effective delocalization of an unpaired electron onto the molecule p system, is distinctly indicated by spin populations on pz -type orbitals of O5 and C4 (Table 5). The results support ChiuÕs et al. [28] suggestion that hyperfine parameters of iminoxyls can partially originate from spin density on pp -like orbitals. Further reduction of Aiso ð13 C3 Þ for Z isomers can be associated with the spin density flow to r-type orbitals (in the plane containing CN@O) of oxygen, mainly of p-type (Fig. 4 and Table 5). Additionally, carbonyl moiety syn to C@N bond enhances strongly Aiso ð1 H7 Þ from 5.7 G (V-3) to 19.2 G (V-4). This correlates with the
Fig. 4. Contour maps of UB1LYP/EPRIII unpaired spin density (e/au3) in the plane containing C@NO iminoxy moiety for the isomers of 1-oxoethan-2-iminoxy radical together with gray scale used; dashed line denotes the surface of zero spin density.
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687
Table 6 and deg), total (Etot in hartrees) and ZPVEs (in kJ=mol) and vibrational corrected energy differences (DE in Geometry parameters (A kJ=mol) for the isomers of 1,3-dioxopropan-2-iminoxyl (VI) at UB1LYP/6-311++G(3df,3pd) level Parameters
½HCð@OÞ2 C@NO VI-1
VI-2
VI-3
VI-4
Symmetry r(NO) r(CN) a(CNO) a(NC3 C4 ) a(NC3 C7 ) /(ONC3 C4 ) /(NC3 C4 O5 ) /(NC3 C7 O8 ) Etot ZPVE DE
Cs
Cs
Cs
Cs
1.191 1.306 132.0 118.9 118.9 0.0 0.0 0.0 )395.8215 135.6 14.7
1.196 1.302 131.8 119.4 117.6 0.0 0.0 180.0 )395.8271 136.1 0.0
1.197 1.298 134.6 118.9 118.7 0.0 180.0 0.0 )395.8221 135.6 13.2
1.201 1.299 134.2 117.4 115.6 0.0 180.0 180.0 )395.8200 134.9 18.6
Fig. 5. Contour maps of UB1LYP/EPRIII unpaired spin density (e/au3) in the plane containing C@NO iminoxy moiety for the isomers of 1,3-dioxopropan-2-iminoxy radical together with gray scale used; dashed line denotes the surface of zero spin density.
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largest Aiso ð13 CÞ for C6 of II (in the same position in respect to CN@O, Scheme 1). 3.1.4. 1,3-Dioxopropan-2-iminoxyl DFT calculations give four stable forms of VI with Cs symmetry. The lowest total energy has VI2 (Table 6) which structurally corresponds to the only stable form for II. Introduction of the second carbonyl group implies that E and Z forms cannot be defined. For clarity in all structures C4 =C7 atom is in syn/anti position to NAO bond. The only difference between the conformers is C@O location relating to C@N bond (Scheme 1), where C4 @O5 and C7 @O8 moieties are hereafter denoted as first and second, respectively.
As can be seen on Fig. 5 unsymmetrical synpreferential spin density delocalization is also observed, except of the relative values of hfccs for carbons bound to iminoxy C3 and lying syn or anti to NAO. Introduction of the second carbonyl group is reflected by further reduction of Aiso ð13 C3 Þ for all conformers of VI (as compared to Aiso ð13 C3 Þ for V). Interestingly, syn-to-anti change of the second C@O group modifies the hyperfine parameters of the radical; Aiso ð14 NÞ, Aiso ð13 C3 Þ and Aiso ð13 C4 Þ decrease from 30.0, 10.2 and 6.6 G for VI-1 to 26.6, 9.3, 4.3 G for VI-2 and from 31.8, 11.0 and 6.5 G for VI-3 to 28.4, 8.3 and 4.3 G for VI-4, respectively. Aiso ð13 C7 Þ increases in the same order, from 2.2 G (VI-1) to
Table 7 UB1LYP/EPR-III spin populations (e) on the natural atomic orbitals of some atoms of 1,3-dioxopropan-2-iminoxyl (VI) Isomer
Atom
Totala
Natural atomic orbitals s-type
px -type
py -type
pz -type
VI-1
C3 C4 C7 O5 O8 H6 H9
)0.003 0.002 0.001 0.001 0.000 0.023 )0.001
0.025 0.006 0.001 0.030 0.000 0.000 0.000
)0.003 0.009 0.002 0.005 0.000 0.000 0.000
)0.057 0.003 0.006 )0.021 )0.024 0.000 0.000
)0.038 0.020 0.011 0.015 )0.024 0.023 )0.001
VI-2
C3 C4 C7 O5 O8 H6 H9
)0.003 0.001 0.002 0.001 0.000 0.023 0.002
0.025 0.007 0.002 0.036 )0.001 0.000 0.000
)0.002 0.009 0.000 0.007 )0.001 0.000 0.000
)0.056 0.002 0.006 )0.015 )0.023 0.000 0.000
)0.035 0.019 0.010 0.028 )0.024 0.023 0.002
VI-3
C3 C4 C7 O5 O8 H6 H9
)0.004 0.004 0.002 0.000 0.000 )0.002 )0.001
0.021 0.001 0.002 0.000 0.000 0.000 0.000
)0.004 0.025 0.003 0.037 0.000 0.000 0.000
)0.064 0.006 0.007 )0.023 )0.022 0.000 0.000
)0.051 0.036 0.014 0.014 )0.022 )0.002 )0.001
VI-4
C3 C4 C7 O5 O8 H6 H9
)0.004 0.003 0.003 0.001 0.000 )0.002 0.003
0.023 0.000 0.002 0.000 0.000 0.000 0.000
)0.002 0.026 0.001 0.048 )0.001 0.000 0.000
)0.060 0.004 0.007 )0.017 )0.021 0.000 0.000
)0.042 0.033 0.013 0.032 )0.022 )0.002 0.003
a
A summary of spin population on s-, px -, py -, and pz -type NAOs.
A.R. Jaszewski et al. / Chemical Physics Letters 367 (2003) 678–689
4.4 G (VI-2) and from 3.0 G (VI-3) to 6.1 G (VI4). On the other hand NAO analysis shows that syn-to-anti change of the second carbonyl group reduces b spin density on pz -type orbital of O5 atom and enhances of the a spin density on rtype orbitals (mainly p-type) of the same atom (Table 7). DFT calculations indicate also much stronger isotropic interaction with H6 protons for E (VI-1 (21.7 G) and VI-2 (23.0 G)) than for Z isomers (VI-3 (4.8 G) and VI-4 (6.6 G)). This effect corresponds to that observed for the hyperfine interactions with nuclei located similarly; the greatest Aiso ð13 CÞ was stated for C6 of only stable isomer of II and Aiso ð1 HÞ for H7 is greater for V-4 than for V-3.
4. Conclusions The values of hyperfine coupling constants and natural atomic occupations calculated using hybrid UB1LYP/EPR-III of R1 R2 C@NO radicals reveal unsymmetrical spin distribution with the preference of atoms in the syn position related to iminoxy group. This effect is manifested by a larger hyperfine interactions with the proton and carbon nuclei syn in respect to NO bond than with the nuclei at anti position. The presence of C@O group provided by R1 causes strong reduction of hyperfine coupling with iminoxy carbon due to an unpaired electron delocalization on molecule p-system (mainly C@O p-bond) and a spin density flow to the carbonyl oxygen orbitals of r-type. Introduction of the second C@O group by R2 further decreases the interaction with iminoxy carbon and with other nuclei. Furthermore, syn-to-anti change of the second C@O moiety (relatively to C@N bond) decreases Aiso for iminoxy carbon and nitrogen.
Acknowledgements The computers of Wroclaw Center of Networking and Supercomputing (Grant No. 02/98) were used for DFT calculations.
689
References [1] A. Jezierski, E. Byli nska, M.R.D. Seaward, Atmos. Environ. 33 (1999) 4629. [2] Y. Sanakis, C. Goussias, R.P. Mason, V. Petrouleas, Biochemistry 36 (1997) 1411. [3] M.R. Gunther, L.C. Hsi, J.F. Curtis, J.K. Gierse, L.J. Marnett, T.E. Eling, R.P. Mason, J. Biol. Chem. 272 (1997) 17086. [4] B.E. Sturgeon, R.E. Glover, Y.-R. Chen, L.T. Burka, R.P. Mason, J. Biol. Chem. 276 (2001) 45516. [5] W.H. Wolodarsky, J. Faniran, J.K.S. Wan, Can. J. Chem. 51 (1973) 4072. [6] H. Caldararu, A. Caragheorgheopol, M. Moraru, V.E. Sahini, J. Phys. Chem. 79 (1975) 646. [7] W.M. Fox, J.A. McRae, M.C.R. Symons, J. Chem. Soc. A (1967) 1773. [8] M. Nakata, H. Frei, J. Am. Chem. Soc. 114 (1992) 1363. [9] W.M. Fox, M.C.R. Symons, J. Chem. Soc. A (1966) 1503. [10] B.C. Gilbert, R.O.C. Norman, J. Chem. Soc. B (1966) 86. [11] S. Wold, G. Lagercrantz, Acta. Chem. Scand. 23 (1969) 1878. [12] A.R. Jaszewski, J. Jezierska, Chem. Phys. Lett. 340 (2001) 581. [13] A.R. Jaszewski, Chem. Phys. Lett. 342 (2001) 239. [14] A.R. Jaszewski, Z. Siatecki, J. Jezierska, Chem. Phys. Lett. 331 (2000) 403. [15] M.J. Frisch et al., GA U S S I A N 98, Revision A.6, Gaussian, Inc, Pittsburgh, PA, 1998. [16] A.D. Becke, Phys. Rev. A 38 (1988) 3098. [17] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [18] A.D. Becke, J. Chem. Phys. 104 (1996) 1040. [19] C. Adamo, V. Barone, Chem. Phys. Lett. 274 (1997) 242. [20] P.C. Hariharan, J.A. Pople, Theoret. Chim. Acta 28 (1973) 213. [21] T. Clark, J. Chandrasekhar, P.v.R. Schleyer, J. Comp. Chem. 4 (1983) 294. [22] R. Krishnam, J.S. Binkley, R. Seeger, J.A. Pople, J. Chem. Phys. 72 (1980) 650. [23] V. Barone, in: D.P. Chong (Ed.), Recent Advances in Density Functional Methods, World Scientific Publishing, Singapore, 1995. [24] N. Rega, M. Cossi, V. Barone, J. Chem. Phys. 105 (1996) 11060. [25] SURFER 5.0 (1994), Golden Software, 1993–1994. [26] J.E. Carpenter, F. Weinhold, J. Mol. Struct. (Theochem.) 169 (1988) 41. [27] A.R. Jaszewski, J. Jezierska, A. Jezierski, Chem. Phys. Lett. 319 (2000) 611. [28] M.F. Chiu, B.C. Gilbert, B.T. Sutcliffe, J. Phys. Chem. 76 (1972) 553.
The effect of the carbonyl moiety on the spin density delocalization in the iminoxy radicals. Hybrid density functional studies Adrian R. Jaszewski, Katarzyna Tabaka, Julia Jezierska *, Jadwiga Kedzierska Faculty of Chemistry, University of Wroclaw, 14 Joliot-Curie Street, Wroclaw 50-383, Poland Received 5 June 2002; in final form 22 October 2002
Abstract UB1LYP method was used to study the influence of carbonyl group in 1,2-diphenyl-1-oxoethan-2-iminoxyl and 1-oxoethan-2-iminoxyl, on radical structure and unpaired electron spin density distribution. A significant change of radical properties was found due to the second carbonyl group in 2,4-dioxopenthan-3-iminoxyl and 1,3-dioxopropan-2iminoxyl. A specific impact of the carbonyl groups on the EPR hyperfine couplings with 1 H and 13 C nuclei (calculated at UB1LYP/EPR-III level in comparison with the experimental data) was determined for the stable isomeric forms of the radicals. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Iminoxyls are the persistent r-type radicals observed in the lichen thalli (Lasallia pustulata and Umbilicaria polyphylla) treated with NO2 under laboratory experiment as well as grown naturally in the polluted environment [1]. They are also formed by reaction between NO and tyrosyl radical occurring in photosystem II [2], prostaglandin H synthase-2 enzyme [3] and in non-protein systems [4].
*
Corresponding author. Fax: +48-71-328-23-48. E-mail address: [email protected] (J. Jezierska).
Generally the presence of C@O moiety increases the life time of the iminoxyls as can be easily seen for the radicals formed in reaction of NO2 with metal acetylacetonates [5], ketones having a free active methylene group [6] and b-diketones [7]. The radicals were studied in the rare gas matrices by infrared spectroscopy [8] and in the liquid and frozen solution by electron paramagnetic resonance (EPR) spectroscopy [5,9]. Stability of the iminoxyls containing C@O group provides a possibility to observe not only hyperfine splitting of the EPR spectra for nitrogen but also carbon; the assignment of the hyperfine constants (hfccs) to the particular carbon nuclei is not straightforward [10,11]. This work is a continuation of our studies on the effect of substituents in vinyl (RHC@CH) [12]
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 7 4 3 - 8
A.R. Jaszewski et al. / Chemical Physics Letters 367 (2003) 678–689
679
Scheme 1.
and iminoxy (R1 R2 C@NO) radicals on hyperfine couplings with 1 H and 13 C nuclei [13]. The calculations of hfccs at HFD level showed that their
values depends mainly on isomeric form (E and Z) adopted by the radicals. Theoretical spin densities also showed the asymmetric (preferred in syn po-
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sition) delocalisation of unpaired electron in R1 ¼ Ph, R2 ¼ PðOÞðOCH3 Þ2 [14] consistent with the change of experimental values of hyperfine coupling constants. The main goal of this work is to study the effect of the substituents R1 , with one C@O group attached directly to iminoxyl carbon of the radicals on values of EPR hyperfine couplings and delocalization of unpaired electron spin density, calculated using UB1LYP method and the basis sets dependent on the radicals. The influence of the second C@O provided by substituent R2 on the properties of the radical isomers was also examined. The results are compared with those obtained for the radical without C@O group and with OH group instead of C@O. The available EPR data for the radicals are used to verify the results of the computations. The calculations also appeared to be a valuable criterion of assignment of the experimental hyperfine couplings.
nection methods, namely B1LYP [19] hybrid density functional, was applied. For geometry optimization and determination of thermodynamic properties it was used together with 6-31+G(d,p) basis set [20,21] for 1,2-diphenyl-1oxoethan-2-iminoxyl (I), 6-311+G(d,p) basis set [21,22] for 2,4-dioxopenthan-3-iminoxyl (II), and 6-311++G(3df,3pd) basis set [21,22] for ethan-1iminoxyl (III), 1-hydroxyethan-2-iminoxyl (IV), 1-oxoethan-2-iminoxyl (V) and 1,3-dioxopropan2-iminoxyl (VI). Hyperfine couplings (hfccs) were calculated using EPR-III basis set of Barone [23,24], developed for the calculations of the EPR parameters. The optimizations were performed without any constraint (except Cs symmetry if necessary). The existence of energy minima was proved by vibrational analyses. Harmonic frequencies and normal modes were computed at the same level as geometry optimizations. These frequencies were used, without scaling, to calculate analytically zero-point vibrational energy (ZPVE) contributions. No attempt was made to determine the effect of zero-point nuclear vibrations on the hfccs. Graphical representations of spin densities were generated using SURFER [25] program. Natural atomic orbitals (NAOs) occupancies were achived
2. Methods All the calculations have been performed by the GA U S S I A N 98 program system [15]. The Becke (B) exchange functional [16] combined with the Lee– Yang–Parr (LYP) correlation functional [17] using a single-parameter [18] version of adiabatic con-
Table 1 and deg), total (Etot in hartrees) and ZPVEs (in kJ=mol) and vibrational corrected energy differences (DE in Geometry parameters (A kJ=mol) for the isomers of 1,2-diphenyl-1-oxoethan-2-iminoxyl (I) at UB1LYP/6-31+G(d,p) level, for the most stable isomer of 2, 4-dioxopenthan-3-iminoxyl (II) at UB1LYP/6-311+G(d,p) level and for the isomers of ethan-1-iminoxyl (III) at UB1LYP/ 6-311++G(3df,3pd) level ½H3 CCð@OÞ2 C@NO
H3 CCð@NOÞH
I-2
Parameters
II
Parameters
III-1
III-2
C1
Symmetry r(NO) r(CN) a(CNO) a(NC3 C4 ) a(NC3 C7 ) /(ONC3 C4 ) /(NC3 C4 O5 ) /(NC3 C7 O8 )
Cs
Symmetry r(NO) r(CN) a(CNO) a(NC3 C4 ) a(NC3 H5 ) /(ONC3 C4 ) /(NC3 C4 H6 )
Cs
Cs
1.224 1.273 134.8 121.5 117.9 180.0 0.0
1.225 1.274 134.4 124.0 115.2 0.0 0.0
Etot ZPVE
)474.41328 282.2
Etot ZPVE DE
)208.48367 158.4 2.2
)208.48472 158.9 0.0
PhCð@OÞCð@NOÞPh Parameters
I-1
Symmetry C1 r(NO) 1.227 r(CN) 1.299 a(CNO) 135.0 a(NC3 C4 ) 113.7 a(NC3 C12 ) 122.9 /(ONC3 C4 ) 175.1 14.6 /(NC3 C12 C13 ) /(NC3 C4 O5 ) 149.0 /(C3 C4 C6 C7 ) 30.8 Etot )744.30690 ZPVE 540.7 DE 0.0
1.231 1.293 134.9 118.9 119.1 2.6 9.0 127.3 19.2 )744.30420 539.8 7.1
1.204 1.305 131.9 116.7 116.5 0.0 0.0 180.0
A.R. Jaszewski et al. / Chemical Physics Letters 367 (2003) 678–689
from UB1LYP/EPR-III calculations according to Carpenter and Weinhold [26].
3. Results and discussion All the isomers obtained from the UB1LYP geometry optimizations for the radicals studied are presented on Scheme 1 with the numbering scheme used. Two stable isomers are determined for I (Table 1). E (I-1) form is thermodynamically more stable than Z (I-2). UB1LYP/EPR-III computations lead to Aiso ð14 NÞ of 33.2 G (E) and 29.8 G (Z) and
681
Aiso ð13 CÞ for iminoxy C3 carbon of 21.4 G (E) and 14.8 G (Z) that is in an excellent agreement with experimental EPR data 32.65 and 29.95 G for Aiso ð14 NÞ and 22.0 and 15.0 G for Aiso ð13 C3 Þ [10]. Hfccs with carbons connected to C3 atom are dependent on their position (syn or anti) relating to NO bond of CNO moiety. The interaction with syn carbon (10.6 G for C12 in I-1 and 9.5 G for C4 in I-2) is stronger than with anti carbon (3.5 G for C4 in I-1 and 3.6 G for C12 in I-2). The couplings with two phenyl protons (after averaging the values for H23 and H27 ) of 1.8 G (I-1) and of 0.9 G (I-2) are also larger than for benzoyl ortho-protons 0.5 G (I-1) and )0.1 G (I-2) (after averaging the
Fig. 1. Contour maps of UB1LYP/EPRIII unpaired spin density (e/au3) in the plane containing C@NO iminoxy moiety for the isomers of ethan-1-iminoxy radical together with gray scale used; dashed line denotes the surface of zero spin density.
Table 2 and deg), total (Etot in hartrees) and ZPVEs (in kJ=mol) and vibrational corrected energy differences (DE in Geometry parameters (A kJ=mol) for the isomers of 1-hydroxyethan-2-iminoxyl (IV) at UB1LYP/6-311++G(3df,3pd) level Parameters
Symmetry r(NO) r(CN) a(CNO) a(NC3 C4 ) a(NC3 C6 ) /(ONC3 C4 ) /(NC3 C4 O5 ) /(C3 C4 O5 H9 ) Etot ZPVE DE
H2 CðOHÞCð@NOÞH IV-1
IV-2
IV-3
IV-4
IV-5
IV-6
IV-7
C1
C1
Cs
C1
Cs
C1
C1
1.219 1.274 135.0 120.4 119.2 178.8 120.1 169.2 )283.70254 172.0 3.6
1.219 1.276 135.2 120.4 119.0 179.4 117.0 62.6 )283.70426 173.0 0.1
1.221 1.270 134.8 122.1 119.2 180.0 0.0 180.0 )283.69926 170.6 10.8
1.224 1.271 134.8 121.1 119.3 179.9 5.0 48.3 )283.70170 171.9 5.6
1.227 1.272 132.6 122.1 117.8 0.0 180.0 180.0 )283.70296 171.9 2.3
1.222 1.275 134.7 123.4 116.5 0.7 131.2 66.4 )283.70449 173.6 0.0
1.226 1.273 132.9 121.2 116.6 1.0 48.7 61.2 )283. 70387 173.7 1.8
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values for H18 and H22 ). Thus experimental value [10] for the phenyl protons of I-1 (1.4 G) is quite well reproduced. UB1LYP/6-311+G(d,p) calculations predict only one stable conformer of II (Table 1). Other structures that can be obtained by the change of dihedral angle N2 C3 C4 O5 from 0° to 180° and/or N2 C3 C7 O8 from 180° to 0° (with or without additional rotation of the methyl groups) correspond
to first-order saddle points according to the vibrational analysis. This is in contrast to previous Extended-H€ uckel calculations [11] showing that conformer with N2 C3 C4 O5 and N2 C3 C7 O8 angles of 180° has the lowest electronic energy. The assignment of the experimental EPR hyperfine couplings presented was unambiguous in spite of this shortcoming [11]. Our calculations give Aiso ð14 NÞ of 26.4 G very close to the experimental value of
Table 3 UB1LYP/EPR-III spin populations (e) on the natural atomic orbitals of some atoms of ethan-1-iminoxyl (III) and 1-hydroxyethan-2iminoxyl (IV) Totala
Isomer
Atom
Natural atomic orbitals s-type
px -type
py -type
pz -type
III-1
C3 C4
)0.001 0.002
0.010 0.002
)0.006 0.000
)0.071 0.001
)0.067 0.005
III-2
C3 C4
0.000 0.004
0.009 0.012
)0.005 0.027
)0.071 0.002
)0.066 0.045
IV-1
C3 C4 O5 H6
)0.001 0.002 )0.001 0.040
0.012 0.002 0.000 0.000
)0.006 0.001 0.000 0.000
)0.071 0.001 )0.002 0.000
)0.066 0.005 )0.002 0.040
IV-2
C3 C4 O5 H6
)0.001 0.002 0.000 0.040
0.012 0.002 )0.001 0.000
)0.006 0.001 )0.001 0.000
)0.071 0.001 )0.001 0.000
)0.065 0.005 )0.004 0.040
IV-3
C3 C4 O5 H6
)0.002 0.002 0.000 0.041
0.011 0.002 0.001 0.000
)0.007 0.001 0.001 0.000
)0.072 0.001 0.000 0.000
)0.069 0.006 0.002 0.041
IV-4
C3 C4 O5 H6
)0.002 0.002 0.000 0.038
0.013 0.002 0.003 0.000
)0.006 0.001 0.000 0.000
)0.069 0.001 0.000 0.000
)0.064 0.005 0.003 0.038
IV-5
C3 C4 O5 H6
0.000 0.005 0.001 0.005
0.010 0.007 )0.001 0.000
)0.005 0.026 0.001 0.000
)0.073 0.002 0.000 0.000
)0.068 0.041 0.001 0.005
IV-6
C3 C4 O5 H6
)0.002 0.004 0.000 0.007
0.011 0.006 0.000 0.000
)0.005 0.031 0.009 0.000
)0.071 0.002 0.000 0.000
)0.067 0.043 0.008 0.007
IV-7
C3 C4 O5 H6
)0.003 0.005 0.000 0.006
0.011 0.016 0.010 0.000
)0.005 0.019 0.003 0.000
)0.074 0.002 )0.001 0.000
)0.069 0.042 0.012 0.006
a
A summary of spin population on s-, px -, py -, and pz -type NAOs.
A.R. Jaszewski et al. / Chemical Physics Letters 367 (2003) 678–689
683
Fig. 2. Contour maps of UB1LYP/EPRIII unpaired spin density (e/au3) in the plane containing C@NO iminoxy moiety for the isomers of 1-hydroxyethan-2-iminoxy radical together with gray scale used; dashed line denotes the surface of zero spin density.
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A.R. Jaszewski et al. / Chemical Physics Letters 367 (2003) 678–689
28.0 G. Additionally, DFT results show dramatic change of the carbon hyperfine interactions upon binding of the second carbonyl group to iminoxy CN@O moiety (in comparison with I). Aiso ð13 C3 Þ is strongly reduced to 7.3 G, which correlates with the experimental value of 8.12 G [11]. A larger hfcc predicted for methyl C6 (9.0 G) should be assigned to the experimental constant of 10.0 G [11]. Aiso ð13 CÞ is larger for C4 (4.0 G) of syn C@O (in respect to CN@O) than for C7 (3.0 G) of anti C@O; the values are close to 4.90 G from EPR spectrum [11]. The averaged hyperfine interaction with H13 , H14 , H15 protons of syn CH3 group (relatively N@C) but attached to anti C@O is larger (0.7 G) than that for H10 , H11 , H12 of CH3 attached to syn C@O ()0.3 G). As hfccs for methyl protons calculated using DFT are usually larger than EPR spectral values [14,27] we assign the experimental constants of 0.39 G to H13 , H14 and H15 but 0.12 G to H10 , H11 and H12 . This is in contrast with WoldÕs and Lagercrantz suggestion, but one should remember that the methyl group connected to the syn carbonyl moiety is in the anti position in relation to the N@C double bond in our calculations whereas it is in the syn position in the structure examined by these authors. 3.1. Spin density distribution Since the presence of C@O group strongly influences the value of iminoxy Aiso ð13 CÞ we decided
to study spin density delocalization in the series of model iminoxy radicals: from III that contains no other-than-iminoxy oxygen and IV that contains OH group to V with one and VI with two carbonyl moieties (Scheme 1). 3.1.1. Ethan-1-iminoxyl DFT calculations reveal two stable isomers of III corresponding to E (III-1) and Z (III-2) forms (Table 1) with Aiso ð14 NÞ of 29.0 and 31.5 G and Aiso ð13 C3 Þ of 31.3 and 29.3 G, respectively. Significantly smaller Aiso ð1 H5 Þ for III-2 (8.5 G) than for III-1 (26.8 G) is typical for iminoxyl radicals with R1 ¼ H, but Aiso ð13 C4 Þ is slightly larger for III-2 (7.5 G) than for III-1 (4.1 G). The absolute value of the averaged hfcc for CH3 protons (H6 , H7 , H8 ) is also slightly larger for Z ()2.6 G) than for E ()2.4 G) isomer. Unsymmetrical spin distribution on atoms syn to iminoxy group (NO bond) with larger spin densities is evidenced from the spin density maps (Fig. 1) and from the results of the natural atomic orbital analysis (Table 3). The data in Table 3 indicate that spin population is larger on r-type (s, px and py ) NAOs of C4 atom for Z form of III. It is remarkable that unsymmetrical spin density delocalization generally does not affect the population of pz -type orbitals on C3 and C4 atoms (p-type with the axis system used, iminoxyls are treated as r-type radicals; an unpaired electron is localized on p-type orbital derived mainly from p orbital of iminoxy oxygen and sp2 orbital of iminoxy nitrogen lying in the
Fig. 3. Contour maps of UB1LYP/EPRIII unpaired spin density (e/au3) in the plane containing O5 atom and parallel to the plane containing C@NO iminoxy moiety for the IV-6 (left) and IV-7 (right) isomers of 1-hydroxyethan-2-iminoxy radical together with gray scale used; dashed line denotes the surface of zero spin density.
A.R. Jaszewski et al. / Chemical Physics Letters 367 (2003) 678–689 Table 4 and deg), total (Etot in hartrees) and Geometry parameters (A ZPVEs (in kJ=mol) and vibrational corrected energy differences (DE in kJ=mol) for the isomers of 1-oxoethan-2-iminoxyl (V) at UB1LYP/6-311++G(3df,3pd) level Parameters
HCð@OÞCð@NOÞH V-1
V-2
V-3
Symmetry r(NO) r(CN) a(CNO) a(NC3 C4 ) a(NC3 H6 ) /(ONC3 C4 ) /(NC3 C4 O5 ) Etot ZPVE DE
Cs
Cs
Cs
V-4
Cs 1.204 1.200 1.208 1.203 1.287 1.288 1.287 1.292 135.1 135.0 134.5 132.5 118.0 119.7 121.0 120.8 120.4 119.0 117.8 117.2 180.0 180.0 0.0 0.0 180.0 0.0 180.0 0.0 )282.5044 )282.4992 )282.5040 )282.5027 110.4 110.3 111.1 111.3 0.0 13.7 1.7 5.3
nodal plane of a molecular p-type C@N double bond). 3.1.2. 1-Hydroxyethan-2-iminoxyl We have localized seven stable conformers of IV according to the vibrational analysis (Table 2): from IV-1 to IV-4 (E isomers) and from IV-5 to
685
IV-7 (Z isomers). IV-6 (Z) has the lowest total energy. IV-2 (with total energy of 0.1 kJ/mol larger) represents thermodynamically most stable E form. Generally, the calculations indicate slightly larger interaction with iminoxy nitrogen for Z than for E forms of the radical (29.1, 28.8, 31.0, 28.0 G for IV-1, IV-2, IV-3, IV-4 and 31.3, 30.8, 31.6 G for IV-5, IV-6, IV-7, respectively), except of 0.2 G larger value for IV-3 than for IV-6. IV-3 has the highest total energy, 10.8 kJ/mol larger than IV-6. Unsymmetrical syn-preferential spin distribution is manifested by Aiso ð1 HÞ for H6 proton (26.8, 27.2, 26.7, 25.3 G for IV-1, IV-2, IV-3, IV-4 and 6.8, 8.8, 7.6 G for IV-5, IV-6, IV-7, respectively), by Aiso ð13 CÞ for C4 (8.6, 7.9, 7.5 G for IV-5, IV-6, IV-7 and 4.0, 3.9, 3.0, 3.3 G for IV-1, IV-2, IV-3, IV-4, respectively) and by the spin populations on s- and r-type orbitals (Table 3). Aiso ð13 C3 Þ of IV-1 (29.6 G) and IV-2 (30.2 G) are slightly smaller than for IV-3 (27.7 G) and IV-4 (26.6 G) as a result of spin density flow to the p-type orbital of O5 oxygen in C@NO plane (Fig. 2, Table 3). Aiso ð13 C3 Þ for IV-5 is equal to 27.9 G and spin density is delocalized on sp3 orbital of O5 (in the radical plane). Interaction with C3 decreases from
Table 5 UB1LYP/EPR-III spin populations (e) on the natural atomic orbitals of some atoms of 1-oxoethan-2-iminoxyl (V) Isomer
Atom
Totala
Natural atomic orbitals s-type
px -type
py -type
pz -type
V-1
C3 C4 O5 H6
)0.002 0.003 )0.001 0.037
0.016 0.002 0.000 0.000
)0.005 0.001 0.000 0.000
)0.070 0.007 )0.028 0.000
)0.061 0.013 )0.028 0.037
V-2
C3 C4 O5 H6
)0.002 0.002 0.000 0.039
0.015 0.001 0.002 0.000
)0.006 0.002 0.000 0.000
)0.076 0.007 )0.029 0.000
)0.069 0.013 )0.028 0.039
V-3
C3 C4 O5 H6
)0.004 0.005 0.000 0.007
0.015 0.002 0.000 0.000
)0.003 0.026 0.034 0.000
)0.070 0.006 )0.025 0.000
)0.060 0.039 0.010 0.007
V-4
C3 C4 O5 H6
)0.003 0.003 0.001 0.005
0.018 0.007 0.027 0.000
)0.002 0.009 0.005 0.000
)0.067 0.003 )0.024 0.000
)0.054 0.024 0.008 0.005
a
A summary of spin population on s-, px -, py -, and pz -type NAOs.
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IV-6 (21.8 G) to IV-7 (19.7 G) due to delocalization of spin density on p-type orbitals of O5 in the plane parallel to C@NO (Fig. 3, Table 3). 3.1.3. 1-Oxoethan-2-iminoxyl Four stable forms of Cs symmetry are predicted for V (Table 4). V-1 E isomer has the lowest total energy, about 1.7 kJ/mol smaller than for V-3 Z. UB1LYP/EPR-III calculations give Aiso ð14 NÞ values similar to those observed for other radicals (29.2, 31.8, 30.0 and 28.1 G for V-1, V-2, V-3 and V-4, respectively). Unsymmetrical syn-preferential spin distribution is reflected by Aiso ð1 H6 Þ, greater for E (27.6 G (V-1) and 28.0 G (V-2)) than for Z isomers (8.6 G (V-3) and 6.3 G (V-4)) as well as by Aiso ð13 C4 Þ, greater for Z (7.7 G (V-3) and 8.4 G (V-4)) than for E isomers (5.0 G (V-1) and 3.6 G
(V-2)). The presence of C@O group leads to new effects as compared to the iminoxyls III and IV. Aiso ð13 C3 Þ is strongly reduced to 24.9 G (V-1) and 24.1 G (V-2) for E and to 13.2 G (V-3) and 12.7 G (V-4) for Z isomers. This effect, caused by effective delocalization of an unpaired electron onto the molecule p system, is distinctly indicated by spin populations on pz -type orbitals of O5 and C4 (Table 5). The results support ChiuÕs et al. [28] suggestion that hyperfine parameters of iminoxyls can partially originate from spin density on pp -like orbitals. Further reduction of Aiso ð13 C3 Þ for Z isomers can be associated with the spin density flow to r-type orbitals (in the plane containing CN@O) of oxygen, mainly of p-type (Fig. 4 and Table 5). Additionally, carbonyl moiety syn to C@N bond enhances strongly Aiso ð1 H7 Þ from 5.7 G (V-3) to 19.2 G (V-4). This correlates with the
Fig. 4. Contour maps of UB1LYP/EPRIII unpaired spin density (e/au3) in the plane containing C@NO iminoxy moiety for the isomers of 1-oxoethan-2-iminoxy radical together with gray scale used; dashed line denotes the surface of zero spin density.
A.R. Jaszewski et al. / Chemical Physics Letters 367 (2003) 678–689
687
Table 6 and deg), total (Etot in hartrees) and ZPVEs (in kJ=mol) and vibrational corrected energy differences (DE in Geometry parameters (A kJ=mol) for the isomers of 1,3-dioxopropan-2-iminoxyl (VI) at UB1LYP/6-311++G(3df,3pd) level Parameters
½HCð@OÞ2 C@NO VI-1
VI-2
VI-3
VI-4
Symmetry r(NO) r(CN) a(CNO) a(NC3 C4 ) a(NC3 C7 ) /(ONC3 C4 ) /(NC3 C4 O5 ) /(NC3 C7 O8 ) Etot ZPVE DE
Cs
Cs
Cs
Cs
1.191 1.306 132.0 118.9 118.9 0.0 0.0 0.0 )395.8215 135.6 14.7
1.196 1.302 131.8 119.4 117.6 0.0 0.0 180.0 )395.8271 136.1 0.0
1.197 1.298 134.6 118.9 118.7 0.0 180.0 0.0 )395.8221 135.6 13.2
1.201 1.299 134.2 117.4 115.6 0.0 180.0 180.0 )395.8200 134.9 18.6
Fig. 5. Contour maps of UB1LYP/EPRIII unpaired spin density (e/au3) in the plane containing C@NO iminoxy moiety for the isomers of 1,3-dioxopropan-2-iminoxy radical together with gray scale used; dashed line denotes the surface of zero spin density.
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largest Aiso ð13 CÞ for C6 of II (in the same position in respect to CN@O, Scheme 1). 3.1.4. 1,3-Dioxopropan-2-iminoxyl DFT calculations give four stable forms of VI with Cs symmetry. The lowest total energy has VI2 (Table 6) which structurally corresponds to the only stable form for II. Introduction of the second carbonyl group implies that E and Z forms cannot be defined. For clarity in all structures C4 =C7 atom is in syn/anti position to NAO bond. The only difference between the conformers is C@O location relating to C@N bond (Scheme 1), where C4 @O5 and C7 @O8 moieties are hereafter denoted as first and second, respectively.
As can be seen on Fig. 5 unsymmetrical synpreferential spin density delocalization is also observed, except of the relative values of hfccs for carbons bound to iminoxy C3 and lying syn or anti to NAO. Introduction of the second carbonyl group is reflected by further reduction of Aiso ð13 C3 Þ for all conformers of VI (as compared to Aiso ð13 C3 Þ for V). Interestingly, syn-to-anti change of the second C@O group modifies the hyperfine parameters of the radical; Aiso ð14 NÞ, Aiso ð13 C3 Þ and Aiso ð13 C4 Þ decrease from 30.0, 10.2 and 6.6 G for VI-1 to 26.6, 9.3, 4.3 G for VI-2 and from 31.8, 11.0 and 6.5 G for VI-3 to 28.4, 8.3 and 4.3 G for VI-4, respectively. Aiso ð13 C7 Þ increases in the same order, from 2.2 G (VI-1) to
Table 7 UB1LYP/EPR-III spin populations (e) on the natural atomic orbitals of some atoms of 1,3-dioxopropan-2-iminoxyl (VI) Isomer
Atom
Totala
Natural atomic orbitals s-type
px -type
py -type
pz -type
VI-1
C3 C4 C7 O5 O8 H6 H9
)0.003 0.002 0.001 0.001 0.000 0.023 )0.001
0.025 0.006 0.001 0.030 0.000 0.000 0.000
)0.003 0.009 0.002 0.005 0.000 0.000 0.000
)0.057 0.003 0.006 )0.021 )0.024 0.000 0.000
)0.038 0.020 0.011 0.015 )0.024 0.023 )0.001
VI-2
C3 C4 C7 O5 O8 H6 H9
)0.003 0.001 0.002 0.001 0.000 0.023 0.002
0.025 0.007 0.002 0.036 )0.001 0.000 0.000
)0.002 0.009 0.000 0.007 )0.001 0.000 0.000
)0.056 0.002 0.006 )0.015 )0.023 0.000 0.000
)0.035 0.019 0.010 0.028 )0.024 0.023 0.002
VI-3
C3 C4 C7 O5 O8 H6 H9
)0.004 0.004 0.002 0.000 0.000 )0.002 )0.001
0.021 0.001 0.002 0.000 0.000 0.000 0.000
)0.004 0.025 0.003 0.037 0.000 0.000 0.000
)0.064 0.006 0.007 )0.023 )0.022 0.000 0.000
)0.051 0.036 0.014 0.014 )0.022 )0.002 )0.001
VI-4
C3 C4 C7 O5 O8 H6 H9
)0.004 0.003 0.003 0.001 0.000 )0.002 0.003
0.023 0.000 0.002 0.000 0.000 0.000 0.000
)0.002 0.026 0.001 0.048 )0.001 0.000 0.000
)0.060 0.004 0.007 )0.017 )0.021 0.000 0.000
)0.042 0.033 0.013 0.032 )0.022 )0.002 0.003
a
A summary of spin population on s-, px -, py -, and pz -type NAOs.
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4.4 G (VI-2) and from 3.0 G (VI-3) to 6.1 G (VI4). On the other hand NAO analysis shows that syn-to-anti change of the second carbonyl group reduces b spin density on pz -type orbital of O5 atom and enhances of the a spin density on rtype orbitals (mainly p-type) of the same atom (Table 7). DFT calculations indicate also much stronger isotropic interaction with H6 protons for E (VI-1 (21.7 G) and VI-2 (23.0 G)) than for Z isomers (VI-3 (4.8 G) and VI-4 (6.6 G)). This effect corresponds to that observed for the hyperfine interactions with nuclei located similarly; the greatest Aiso ð13 CÞ was stated for C6 of only stable isomer of II and Aiso ð1 HÞ for H7 is greater for V-4 than for V-3.
4. Conclusions The values of hyperfine coupling constants and natural atomic occupations calculated using hybrid UB1LYP/EPR-III of R1 R2 C@NO radicals reveal unsymmetrical spin distribution with the preference of atoms in the syn position related to iminoxy group. This effect is manifested by a larger hyperfine interactions with the proton and carbon nuclei syn in respect to NO bond than with the nuclei at anti position. The presence of C@O group provided by R1 causes strong reduction of hyperfine coupling with iminoxy carbon due to an unpaired electron delocalization on molecule p-system (mainly C@O p-bond) and a spin density flow to the carbonyl oxygen orbitals of r-type. Introduction of the second C@O group by R2 further decreases the interaction with iminoxy carbon and with other nuclei. Furthermore, syn-to-anti change of the second C@O moiety (relatively to C@N bond) decreases Aiso for iminoxy carbon and nitrogen.
Acknowledgements The computers of Wroclaw Center of Networking and Supercomputing (Grant No. 02/98) were used for DFT calculations.
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