J . Phys. Chem. 1988, 92, 5094-5096
5094
direction has already been reported.’O Accurate VB calculations of avoided crossing diagrams and QMRE’s should be very useful to this end, and are currently in progress. Finally, the model (Figure 2, eq 3) opens some new avenues to think about organic reagents. For example, why should radicals be “free”? Is it not possible to design organic radicals which will (70) Shaik, S. S. J . Org. Chem. 1987, 52, 1563
cluster in analogy with, e.g., Li,? Good prospects may lie beyond first-row elements. Other examples are the new “hyper-iodo” reagents, (Rf),I- (Rf = fluoroalkyl),’l which are simple analogues of I+Iiiaki X. Irazabalbeitia,I and Alberto Gonzalez Guerrerol Kimika- Fisikoa Departamentoa and Kimika Organikoko Departamentoa, Euskal Herriko Unibertsitatea, Posta kutxa 1072, 20080 Donostia, Spain (Received: August 1 1 , 1987)
A computational analysis of the structures and electronic charge density distributions of the transoid cis- and trans- 1.2dinitrosoethylene has been carried out by means of an ab initio self-consistent-field molecular orbital method. Complete geometry optimization was carried out within the 4-31G and 4-31G* basis sets, and the 6-31G**//4-31G* single-point calculations were carried out to improve energy calculations, charge distributions, and other properties. The calculations demonstrate that the electronic structures of both cis- and trans- 1,2-dinitrosoethyIene isomers are mainly determined by the cooperative enhanced charge withdrawal of the two nitroso groups. The overall result is a strengthening of the two C-N bonds and a weakening of the C=C bond (relative to nitrosoethylene). The 4-31G* optimized geometry of furoxan has been found to compare quite well with available experimental data on related molecules. We observe that the furoxan extraannular oxygen induces a strong charge withdrawal effect and should be regarded as a very reactive site toward electrophiles.
Introduction The interest in the chemistry of conjugated nitroso compounds has been renewed in recent years. Thus, Gilchrist’ has reviewed the methods of generation and characteristic physical and chemical properties of the vinyl-nitroso compounds. Politzer et al.2 have analyzed the effect of the nitroso and nitro groups upon aromatic reactive properties both with and without an additional N H , substituent and upon the C=C bond.5 They found that the nitroso group deactivates the aromatic ring toward nucleophilic attack, this being the overall result even in the presence of the resonance-donor N H 2 substituent; however, it was found that the degree of deactivation was less than for the nitro gr0up~3~ and that the presence of the nitro group, in itself, strengthens the C=C bond; NO substitution, on the other hand, weakens it.5 Vinyl-dinitroso compounds have long ago been identified as transient species6,’ in furoxan ring isomerizations. Recently, Willer and Moore8 have suggested that ring isomerization in furoxano[3,4-b]piperazines undergoes through a vinyl-dinitroso intermediate. Also, the synthetic interest in the nucleophilic opening of annelated furoxans to dinitroso products has been pointed This paper reports the results of a study using a b initio molecular orbital theory of l ,2-dinitrosoethylene and furoxan (I). Only the transoid structures of both the cis (11) and the trans (111) isomers of 1,2-dinitrosoethylene were found to be stable. Kimika-Fisikoa Departamentoa. Kimika Organikoko Departamentoa.
0022-3654/88/2092-5094$01.50/0
I
I1
I11
Of some relevance to the present study is the work of Bhujle et a1.,I0who determined by a C N D O calculation that the transoid structure of the nitrosoethylene is of slightly lower energy than the cisoid, and the work of Calleri et al.,” who carried out HF-SCF a b initio (IBMOL program) calculations using a minimal Gaussian basis set and CNDO/2 calculations for furoxan and several di( I ) Gilchrist, T. L. Chem. SOC.Reu. 1983, 12, 53. (2) Politzer, P.; Bar-Adon, R. J . Phys. Chem. 1987, 91, 2069. (3) Politzer, P.; Abrahmsen, L.; Syoberg, P. J. Am. Chem. SOC.1984, 106,
855. (4) Politzer, P.; Laurence, P. R.; Abrahmsen, L.; Zilles, B. A,; Syoberg, P. Chem. Phys. Lett. 1984, 111, 75. (5) Politzer, P.; Bar-Adon, R. J. Am. Chem. SOC.1987, 109, 3529. (6) Mallory, F. B.; Manatt, S . L.; Wood, C. S . J . Am. Chem. SOC.1965, 87, 5433. Mallory, F. B.; Cammarata, A. J . Am. Chem. SOC.1966, 88, 61. (7) Katritzky, A. R.; Lagonski, J. M. Chemistry of Heterocyclic N-Oxides; Academic: New York, 1971; pp 336-338. (8) Willer, R. L.; Moore, D. W. J . Org. Chem. 1985, 50, 5123. (9) Tennant, G.; Wallace, G. M. Chem. Commun. 1982, 267. ( I O ) Bhujle, V . ; Wild. U. P.; Baurnann. H.; Wagniere, G. Tetrahedron 1976. 32, 467. ( I I ) Calleri, M.; Ranghino, G.; Ugliengo, P.; Viterbo, D. Acta Crystallogr., Sect 5 1986, 542, 84.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 18, 1988 5095
1,2-Dinitrosoethylene and Furoxan TABLE I: 4-31G and 4-31G' Optimized Geometries distances, 8, species 4-31G 4-31G* trans-1,2-di- C-C 1.425 1.433 C-C-N C-N 1.264 1.262 C-N-0 nitrosoethylene N-0 1.248 1.206 C-C-H C-H 1.087 1.090 cis-1,2-dinitrosoethylene
C-C C-N N-0
C-H
1.483 1.280 1.223 1.063
1.502 C-C-N 1.279 C-N-0 1.190 C-C-H 1.069
95.8 143.5 136.3
93.7 144.5 138.0
107.2 108.2 107.7 105.8 110.9 133.2 121.9 128.1
107.0 108.1 107.2 105.1 110.7 134.7 122.1 128.0
H3-C-C H4-C-C H5-C-C N-C-C 0-N-C
120.5 121.6 125.5 118.4 112.6
120.2 121.9 125.7 118.2 112.3
NON ONC CCN CCH
110.4 105.8 109.0 130.2
1.423 1.285 1.411 1.392 1.293 1.258 1.060 1.063 transoid
nitrosoethylene
CI-C2
CI-C3 CI-H4 C2-H5 C2-N N-0
1,2,5-oxadia-C-C zole (C", C-N
N-0 C-H
1.313 1.072 1.070 1.071 1.441 1.224 1.421 1.300 1.380 1.760
1.316 1.077 1.076 1.078 1.440 1.183
angles, deg 4-31G 4-31G* 116.3 115.9 137.6 137.2 118.6 118.0
'Taken from ref 20.
substituted furoxans. They found the furoxan ring to be a planar electron-overcrowded molecule which tends to release this excess charge toward the substituents.
Results and Discussion All results presented in this paper are based upon a b initio self-consistent-field molecular orbital calculations using the GAUSSIAN 82 program.I2 Complete geometry optimizations were carried out at the 4-31G and 4-31G* levels, which have been Our calculated structures shown to be effective for this for the furoxan and the cis and trans isomers of 1,2-dinitrosoethylene are shown in Table I. To improve energy calculations, charge distributions, and other properties, 6-3 1G** single-point calculations were carried out on the 4-3 1G* optimized structures. the stabilization energy gained by improving the basis set is given in the fifth column of Table 11. We begin the discussion looking at the open structures, Le., the transoid cis and trans isomers of 1,2-dinitrosoethylene, and then afterward at the furoxan ring. From hereafter the x-y plane will be the molecular plane. ( a ) 1,2-Dinitrosoethylene. Both cis and trans isomers were found to be planar. Since the nitroso group is known as a strong electron-attracting s~bstituent,'~ it can cause an effective inductive charge withdrawal from the carbon-carbon double bond and so bring about a strong interaction between the N O groups and the C-C double bond. This effect can be placed in perspective by examining the optimized C-C and C-N bond lengths (Table I) for both isomers and comparing them to their average values. The C-C bond lengths, Le., 1.42 and 1.43 %, for the trans isomer at the 4-3 1G and 4-3 l G * levels, respectively, and 1.48 and 1SO %, for the cis isomer at the 4-31G and 4-31G* levels, respectively, compare remarkably well with the average length of 1.48 8,
-
(12) Binkley, J. S.; Frisch, M.; Raghavachari, K.; DeFrees, D.; Schlegel, H. B.; Whiteside, R.; Fluder, E.; Seeger, R.; Pople, J. A. GAUSSIANBZ, Release A, Carnegie-Mellon University, Pittsburgh, PA. (13) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J . Chem. Phys. 1971, 54,724.(b) For an extensive revision see: Hehre, W. J.; Radon, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley-Interscience: New York, 1986. (14) March, J. Advanced Organic Chemistry, 3rd ed.; Wiley-Interscience: New York, 1985; p 1103.
of a single s p 2 s p 2C-C bond.I5 The C-N optimized bond lengths of 1.264 and 1.262 %, for the trans isomer at the 4-3 1G and 4-3 1G* levels, respectively, and 1.28 and 1.279 8, for the cis isomer at the 4-31G and 4-31G* levels, respectively, resemble the average length of 1.28 8, of the carbon-nitrogen bond characteristic of compounds like oximines and imines.16 However, although the main trends of the 4-31G and 4-31G* optimized structures of both cis and trans isomers of 1,2-dinitrosoethylene appear to be nearly the same, we notice some remarkable discrepancies. The splitvalence basis sets are known to overestimate the N=O bond length; therefore, for molecules which contain the N=O bond it is absolutely necessary to optimize the molecular geometry with basis sets including d-type functions, in order to obtain a reliable structure. From Table I we notice that the 4-31G* N=O bond length has been reduced by 0.042 8, in the trans isomer and by 0.033 8,in the cis isomer with respect to the corresponding 4-31G optimized bond lengths. Although calculations at the split-valence level have previously been found to be very reliable for bond length prediction^,'^^ their poor performance here can be traced out by comparing the 4-31G and 4-31G* wave functions and associated electron populations. We find that the d-functions on nitrogen contribute to the bonding by enabling T-electron donation from oxygen 2p lone pairs into the empty nitrogen d-orbitals. This back-donation may be responsible for the bond length shortening and for the lower theoretical dipole maments for the cis isomer at the 4-31G* level compared with the 4-31G level. For the cis isomer the dipole moments are 4.76 D (4-31G*) and 6.52 D (4-3 lG*), respectively. It is interesting to contrast the structures of the cis- and trans-1,2-dinitrosoethyleneisomers with that of the nitrosoethylene, whose 4-31G* optimized geometry is shown in Table I. It is observed that the substitution of a proton by a nitroso group on carbon i3 lengthens the C-C bond and shrinks the C-N bond. This effect emphasizes the nitroso group's effective inductive charge withdrawal from the carbon-carbon double bond and is reflected in the 6-31G**//4-31G* net atomic charges, as shown in Table 111. A similar behavior has been recently reported by Politzer et aL5 for mono- and dinitrosoacetylene. Inspection of Table I1 shows that the relative stability of the species under study is the same for the 4-31G, 4-31G*, and 631G** basis sets. The trans isomer is more unstable than the cis, the energy difference between the two being 56.28 kcal/mol. On the other hand, the stabilization of the total energy gained by replacing the 4-31G* basis set with the 6-31G** basis set (shown in the fifth column of Table 11) is larger for the trans than for the cis isomer. ( b ) Furoxan. Geometry optimizations using both the 4-31G and 4-31G* basis sets were found to predict a planar structure, in close agreement with available experimental data on related The major differences between the 4-3 1G and 4-3 l G * calculated structures were found for the extraannular N+-O- bond parameters. From Table I one may observe that the extraannular Nf-O- distance is reduced by 0.055 8, as a consequence of including d-type functions on heavy atoms. The different performance of the 4-31G* basis set with respect to the 4-31G can be placed in perspective once again by examining their corresponding wave functions and associated electron population. We like to emphasize that although the presence of diffuse functions has often been recognized to have a disastrous effect on the Mulliken population analysis" due to the very large coefficients entering the S C F MO's when such diffuse orbitals are included in the basis set, we have found no orbital populations
-
(15) Somayayulu, P. J . Chem. Phys. 1959, 31, 919. (16) Levine, R. J . Chem. Phys. 1963, 38, 2366. (17) Baker, J. Theor. Chim. Acta 1985, 68, 221. (18) (a) Irazabalbeitia, I. X.Ph.D. Dissertation, Basque Country University. (b) Irazabalbeitia, I. X.; Gonzalez Guerrero, A,, to be published. (19) Seagebarth, E.; Cox, A. P. J . Chem. Phys. 1965, 43, 166. (20)Harmony, M. D.; Laurie, V. W.; Duczkowski, R. L.; Schwendeman, R. H.; Ramsay, D. A,; Lovas, F. J.; Lafferty, W. L.; Maki, A. G. J . Phys. Chem. Ref. Data 1979, 8 , 619. (21) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley-Interscience: New York, 1986.
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The Journal of Physical Chemistry, Vol. 92, No. 18, 1988
Sedano et al.
TABLE 11 total energy, au 4-3 l G *
species
4-31G
trans- 1,2-dinitrosoethyIene cis- 1,2-dinitrosoethylene furoxan
-334.486 57 -334.666 34 -334.781 62
-334.51924 -334.674 55 -334.79094
6-3 1G**//4-3 1G*
difference, au
re1 energy, kcal/mol
-335.1 11 86 -335.201 55 -335.319 98
0.592 61 0.587 10 0.53278
130.53 14.25
0.00
TABLE III: 6-31G**//4-31C* Net Atomic Charges species
atom
net charge, e
trans-l,2-dinitrosoethylene
C N 0 H
0.12 -0.38 0.18
cis- 1,2-dinitrosoethyIene
C N 0 H
0.04 0.27 -0.53 0.23
furoxan
0, N2
-0.32 0.41 0.01 0.09 -0.09 -0.54 0.21 0.23
c3 c4
NS 0 6
H7
Ha
0.08
greater than two or less than zero which are known to be the two major deficiencies of the Mulliken population analysis.22 Therefore, one is allowed to use this information to find out what role the d-functions in the 4-31G* basis set play in bringing about this bond length reduction. However, due to the strong basis set dependence, conclusions derived from the Mulliken population analysis should be. treated with caution. A more rigorous method, such as electrostatic potential map,23 is necessary to arrive at conclusive and meaningful results. The 4-3 l G * gross orbital populations show that the 2p, orbital on furoxan's O6has donated 0.365 electron, and the d,,, orbital on Nzaccepts electrons with a calculated population of 0.041. In addition, there is a significant overlap population between the dyzorbital on N2and the 2p, orbital totaling 0.044 electron. Therefore, the resultant bonding on 06, can be described as a-acceptance-o-donation by nitrogen (see Figure 1). The lower dipole moment at the 4-31G* level (4.1 D) compared with the 4-31G level (5.8 D) corroborates this effect. A similar NO bond has been described by Radom et al. for ammonia oxide, N H 3 0 . 2 4 Comparison of the furoxan C-C and C-N bond lengths and angles with those of 1,2,5-oxadiazole (see Table I) reveals the similarity between the two rings and leads (22) Huzniaga, S.; Narita, S. Isr. J . Chem. 1980, 19, 242. (23) Politzer, P.; Truhlar, D. G., Eds. Chemical Applications of Atomic and Molecular Electrostatic Potentials; Plenum: New York, 1981. (24) Radom, L.; Binkley, J. S.; Pople, J. A. Ausr. J . Chem. 1977, 30,699.
,5-c\ H7
H8
Figure 1. The extraannular NO bond in furoxan.
to the conclusion that the furoxan carbon-carbon bond can be described as a single sp2-sp2 bond and the carbon-nitrogen as a C(sp2)-N bond. The differences between the C-C bond length in furoxan with respect to the C-C bond length in 1,2-dinitrosoethylene are due to ring strain.25 The furoxan molecule appears to be the most stable of the three species that we have considered (see Table 11) at the 4-31G, 4-31G*, and 6-31G** levels. Finally, analysis of the data of Table I11 reveals a noticeable inductive charge withdrawal by the extraannular oxygen. This produces the largest net negative and positive charges that are associated with the extraannular oxygen and its neighbor nitrogen and should be regarded as very reactive sites toward electrophiles and nucleophiles, respectively. Another important feature of the furoxan molecule is that the sum of the net atomic charges of the six atoms of the furoxan moiety is -0.44 au. This indicates that the furoxan is an electron-overcrowded moiety. As suggested by Calleri et al.," the reduced probability of delocalizing this excess charge could be responsible for the fact that this molecule has not been synthesized so far. Acknowledgment. This work has been supported by the Basque Country University (Euskal Herriko Unibertsitatea), Grant UPV 203.512-49/87. The authors gratefully acknowledge the Computing Center of the Basque Country University (Euskal Herriko Unibertsitatearen Donostiako Kalkulu-Zentroa) for providing us with the necessary computer facilities and the Basque Gipuzkoako Foru Aldundia for a grant to I.X.I. Registry No. I, 497-27-8; 11, 115482-75-2; 111, 115482-76-3. (25) For a discussion of how C(sp2)-C(sp2) distances vary with the structure of the molecule, see: Kuchitsu, T.; Fukuyama, F.; Morino, H. J . Mol. Srruct. 1968, 1 , 463.
