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Conformational Transformations in Aromatic Nitroso Oxides Alfia R. Yusupova, Rustam L. Safiullin, and Sergey L. Khursan* Ufa Institute of Chemistry of the Russian Academy of Sciences, 71 pr. Oktyabrya, Ufa 450054, Russian Federation S Supporting Information *

ABSTRACT: A systematic theoretical study on conformational transformations of monosubstituted (ortho- and para-) aromatic nitroso oxides R-C6H4NOO was performed. The existence of two rotation axes enables two types of conformational transitions in substituted arylnitroso oxides: trans/cis (rotation around the N−O bond) and syn/anti (rotation around the C−N bond, which is important in ortho isomers). The complete set of conformers was localized for RC6H4NOO using four selected density functional (M06-L, mPWPW91, OLYP, and HCTH) and augmented polarization basis set of triple splitting. It was found that the activation enthalpy of the trans-cis conformational transition is nearly insensitive to the nature of R and ranges within 58−60 kJ/mol for para isomers. The ortho substituent has an insignificant effect on ΔH≠trans→cis: it increases this value by ∼5 kJ/mol in syn isomers and decreases it by ∼3 kJ/mol in anti isomers. On the contrary, the syn-anti conformational barrier is considerably affected by the substituent R; an increase in the electron-withdrawing properties of R decreases ΔH≠syn→anti. The activation enthalpies grow with increasing polarity of the solvent, as it was found using IEFPCM calculation. The values of relaxation time for all conformational equilibria were calculated and compared with known lifetimes of aromatic nitroso oxides. Our results suggest that syn/anti transitions occur fast enough in the scale of the experimental lifetime. However, trans/cis transformations proceed more slowly. And under certain conditions discussed in the paper, the rate of this conformational transition limits that of irreversible decay of nitroso oxide.

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INTRODUCTION Nitroso oxides RNOO are 1,3-dipolar compounds containing the −N−O−O monovalent functional group. The simplest representative of these compounds HNOO (X = HN, Scheme 1) is an isoelectronic analogue of ozone (X = O) and

It is known1 that nitroso oxides exist as two planar isomeric forms, cis and trans, which differ in the position of the terminal oxygen atom with respect to the R moiety:

Scheme 1

The thermodynamic preference of a certain conformer depends on the electronic properties of substituent R. The high polarity of nitroso oxide results in a strong effect of the solvent on the properties of RNOO, which also affects the population of cis/trans conformers. The structure of nitroso oxides was studied using various quantum-chemical methods, but calculations that include the major fraction of both static and dynamic electron correlation have only been performed for the simplest nitroso oxide representative, specifically, HNOO.7,22 The chemical properties of nitroso oxides have been mostly studied for aromatic nitroso oxides that have moderate reactivity due to the molecule stabilization through conjugation of the 4π-electron system of −NOO with the aromatic moiety. Theoretical calculations of the properties of aromatic nitroso oxides present challenge due to the complicated electronic structure of ArNOO. In fact, electronic spectra of a series of aryl nitroso oxides were calculated in ref 9, and a qualitative agreement between the

peroxymethylene H2COO (X = H2C).1−3 Nitroso oxides are recognized to be highly reactive intermediates in a number of oxidative processes, such as photooxidation of inorganic and organic azides4 and deoxygenation of aromatic nitroso compounds with phosphorus(III) compounds.5 Furthermore, HNOO is generated in ammonia flames.6 In all cases, RNOO compounds are formed in reactions of triplet nitrenes RN: with molecular oxygen.1,2 Many theoretical publications and experimental studies addressed the properties of nitroso oxides.1,7−20 A specific feature of the electronic structure of RNOO is that it has a three-centered 4π-electron orbital system. The electronic structure of nitroso oxides can be represented by a superposition of several resonance structures.21 The most important ones are shown in Scheme 1. © XXXX American Chemical Society

A

DOI: 10.1021/acs.jpca.6b04282 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A calculated and experimental results was found. Using the MCQDPT2 formalism12 and the density functional theory (DFT),12,13 a reaction of nitroso oxides with olefins was studied. It is important to note that only trans-nitroso oxides are involved in this reaction. It has been shown using a number of model systems that cycloaddition is a typical reaction of nitroso oxides with alkenes, though the reaction with tetracyanoethylene occurs via the epoxidation mechanism.14,23 Yet another example of the cis/trans chemoselectivity of aryl nitroso oxides is that only trans-ArNOO reacts with trivalent phosphorus compounds: the kinetics parameters of the reactions studied18,24 are in favor of the mechanism of (3 + 1) cycloaddition of the nitroso oxide moiety to the phosphorus atom, and the results of theoretical calculations25 confirm this mechanism. In ref 11, unusual intramolecular cyclization of an aromatic nitroso oxide to give a conjugated nitrile oxide was described (Scheme 2).

Scheme 3

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SELECTION OF THE APPROPRIATE CALCULATION METHOD The wave function of nitroso oxides has multi-configuration nature. This fact limits the range of methods that adequately describe the system of interest. In fact, multi-configuration methods account for both dynamic and static electron correlation, which is principally important for molecular systems such as nitroso oxides. However, calculations by these methods would consume unreasonable computational and time resources. The use of the methods based on the DFT provides a good alternative. The theoretical calculations in the papers cited above were performed using the B3LYP functional. On the one hand, it was shown30 that hybrid functionals containing a constant fraction of the Hartree−Fock exchange poorly describe the non-Lewis systems. On the other hand, mGGA gradient-corrected methods that account for electron density nonuniformity by inclusion of both density gradient and its Laplacian into the functional are characterized by the smallest deviation of the computational results from the experimental data or data obtained by the high-precision multi-configuration methods.30 Selecting the calculation methods, we used the agreement between theoretical and experimental data as the main criterion. IR and UV spectroscopies are the main tools for experimental studies of nitroso oxides, so comparison of spectral characteristics is of interest. We used trans-HNOO as a model compound, since its experimental IR spectrum is known.7 In the paper cited, the structure of peroxynitrene and the IR spectrum of trans-HNOO were obtained by the CCSDT-3(Qf)/cc-pVTZ method that shows good agreement with experimental data. It allows us using it as the reference method for description of the structural features of nitroso oxides. Furthermore, because of the multi-configuration character of the HNOO wave function, we used for comparison the following approximations: MR-AQCC(8;7)/cc-pVTZ, CCSD(T)/6-311+G(d,p), as well as CASSCF (18;13), which includes all valence electrons.22 Correct reproduction of bond lengths is highly important in simulations of IR spectra. Therefore, when choosing the best DFT functional, we paid special attention to the conformity of N−O and O−O interatomic distances in the nitroso oxide group to the reference methods. Separate description of the movement of electrons with opposite spins is more flexible for building the wave function. Therefore, taking into account the partially biradical nature of nitroso oxides, the unrestricted Kohn−Sham method was used in all cases. The wave functions were

Scheme 2

The latter may be stable enough to be isolated or undergoes further intramolecular rearrangement. The reaction gives carboand heterocyclic compounds with various structures, depending on the electronic nature of substituent and its position in the aromatic ring of the nitroso oxide.26−29 Note that only the cis form of phenyl nitroso oxide undergoes this reaction. Theoretical simulation of the properties of aromatic nitroso oxides was performed most commonly using the B3LYP density functional. In some cases, the G3MP2B3 composite method was used where the calculation scheme also included optimization and calculation of frequencies at the B3LYP/631G(d) level of theory. However, it was noted11 that the use of the B3LYP functional for ArNOO, which is characterized by a noticeable contribution of biradical resonance III to the wave function (Scheme 1), results in a considerable overestimation of the stability of singlet states. Also note that conformational transformations of ArNOO that precede the above-mentioned irreversible chemical transformations of arylnitroso oxides have almost not been studied. Taking into account the selectivity of chemical action of ArNOO isomers noted above (intramolecular transformation of cis forms and reactions of transArNOO with a suitable substrate, that is, an olefin, phosphite, or phosphine, conformational transitions in arylnitroso oxides can affect the experimentally determined kinetics features of these reactions. The availability of two rotation axes enables two types of conformational transformations in substituted arylnitroso oxides (see Scheme 3). In view of the above, this paper reports on the results of a systematic theoretical study on monomolecular transformations of aromatic nitroso oxides, starting from a description of cistrans (rotation around the N−O bond) and syn-anti (rotation around the C−N bond) isomerization of monosubstituted phenylnitroso oxides, where R1 or R2 = NMe2, MeO, Me, Br, H, NO2. Much attention in this study is given to the choice of a calculation method that would adequately describe both structural and electronic properties of aromatic nitroso oxides. B


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The Journal of Physical Chemistry A Table 1. Geometrical Parameters and Frequencies of Stretching Vibrations of Arylnitroso Oxidesa rN−O

rO−O

method CCSD(T) M06-L mPWPW91 OLYP HCTH

rN−O − rO−O

rC−N

νN−O

1.317 1.299 1.313 1.308 1.294

1.312 1.299 1.315 1.305 1.291

1.405 1.387 1.393 1.394 1.387

trans-C6H5NOO 0.005 0.001 −0.002 0.003 0.003 cis- C6H5NOO

1117 1076 1088 1103

(161) (71) (71) (127)

exp36 CCSD(T) M06-L mPWPW91 OLYP HCTH exp37 O2 37 M06-L 18 O2 mPWPW91 18 O2 HCTH 18 O2 OLYP 18 O2

νO−O

νN−O − νO−O

cm−1

Å

(163) (130) (158) (155)

−50 −42 −51 −53

1031 (172) 966 (179) 995 (176) 1009 (183)

114 118 102 113

1167 1118 1139 1156 1000 990

1.305 1.289 1.303 1.298 1.286

1.322 1.320 1.340 1.328 1.314

1.400 1.380 1.386 1.387 1.380

−0.017 −0.032 −0.037 −0.030 −0.028 trans-4-NO2C6H4NOO

1145 1084 1097 1122

(15) (16) (14) (12)

958 (107) 945 (86) 1021 (147) 997 (110) 1076 (182) 1047 (191) 1044 (155) 1015 (158)

1131 1069 1161 1096 1132 1076 1168 1109 1153 1092

(242) (227) (154) (136) (128) (87) (160) (131)

−203 −151 −111 −79 −92 −62 −109 −77

1139 1115 1078 1053 1117 1094 1091 1066

1017 968 1055 (271) 998 (206) 977 (161) 929 (172) 1028 (245) 973 (196) 1012 (234) 958 (195)

84 117 101 124 89 121 79 108

1093.5 1098 (120) 1024 (56) 1044 (74) 1076 (88)

1166.5 1131 (300) 1085 (316) 1106 (324) 1121 (302)

−73 −33 −61 −62 −45

1160.5 1131 (18) 1071 (25) 1084 (22) 1112 (14)

971 1009 (153) 974 (238) 945 (228) 986 (243)

189.5 122 97 139 126

18

1.307

1.291

1.385

0.016

1.316

1.306

1.392

0.010

1.298

1.283

1.385

0.015

1.311

1.297

1.393

0.014 cis-4-NO2C6H4NOO

exp37 18 O2 37 M06-L 18 O2 mPWPW91 18 O2 HCTH 18 O2 OLYP 18 O2

1.290

1.312

1.382

−0.022

1.304

1.332

1.387

−0.028

1.287

1.306

1.382

−0.019

1.299

1.320

1.389

−0.021

(11) (8) (8) (7) (11) (14) (3) (11)

trans-4-NH2C6H4NOO

a

exp8 M06-L mPWPW91 OLYP HCTH

1.305 1.319 1.313 1.300

1.310 1.325 1.315 1.301

1.374 1.381 1.382 1.374

exp8 M06-L mPWPW91 OLYP HCTH

1.294 1.308 1.303 1.290

1.332 1.352 1.339 1.325

1.368 1.373 1.375 1.368

−0.005 −0.006 −0.002 −0.001 cis-4-NH2C6H4NOO −0.038 −0.044 −0.036 −0.035

Basis set: 6-311+G(d,p). The numbers in parentheses indicate the integral intensities of bands (km·mol−1).

A wide range of density functionals was studied,30,35 both nonhybrid (GGA, mGGA) and hybrid ones (GH-GGA, GHmGGA, RSH-GGA). The computational results for the key geometrical and spectral parameters of trans-HNOO are provided in the section S1 of the Supporting Information. Among the DFT methods studied, the M06-L, mPWPW91, OLYP, and HCTH functionals describe the structure and IR spectrum of trans-HNOO most reliably. The use of these

constructed using the Dunning or Pople polarization basis sets of triple valent splitting−cc-pVTZ or 6-311+G(d,p). Quantumchemical calculations were performed on a cluster supercomputer of Ufa Institute of Chemistry of the RAS using Gaussian 09,31 Firefly V.7.1,32 and CFOUR V1.233 software packages. Visualization was performed in ChemCraft program.34 C


Article

The Journal of Physical Chemistry A methods for calculation of the properties of cis-HNOO is also characterized by satisfactory agreement with the data of the reference methods. Since the stated purpose of this work is to study the conformational potential of aromatic nitroso oxides, we tested chosen methods to adequately reproduce the energies of nitroso oxide conformers and transition states between them. The computational results for the reaction enthalpies ΔH° and activation energy ΔH‡ for the trans-cis conformational transformation of peroxynitrene are presented in the section S2 of the Supporting Information. Overall, good agreement is observed between the results of the DFT methods chosen and the reference method MR-CISD(18;13)+Q. All the approximations used correctly predict the higher stability of the cis isomer of peroxynitrene. The Cartesian coordinates of the peroxynitrene structure obtained by selected methods are presented in the section S3 of the Supporting Information. Using these methods, full optimization of the structures of all isomeric forms of the unsubstituted and monosubstituted aromatic nitroso oxides was performed. The structures of transition states of conformation transitions were optimized. Correspondence of the structures found to the minima on the potential energy surface was ascertained by the absence of negative elements in the diagonalized Hessian matrix, while correspondence to the transition states was identified by the only negative element. Reaction and activation enthalpies were calculated as the difference between the absolute enthalpies of the final (or transition) and initial states of a transformation of interest. The absolute enthalpies were calculated as the sum of the total energy, zero point vibration energy, and thermal correction for enthalpy change from zero to 298 K.

is favorable for enhancing the contribution of resonance form I. According to the valence pictures shown in Scheme 1, a growth of the contributions of resonance forms I and III is accompanied by the corresponding decrease or increase in the rN−O − rO−O difference, in full agreement with the data in Table 1. The density functionals that we chose correctly reproduce the vibrational spectra of arylnitroso oxides. The experimental and calculated results were compared for para-nitro- and paraaminophenylnitroso oxides (Table 1), for which stretching vibration bands of O−O and N−O bonds were recorded in the IR spectra. Arylnitroso oxides were obtained by photolysis of the corresponding azides at 30−50 K in inert gas matrix doped with oxygen.8,37 The description quality of the spectral characteristics of ArNOO is demonstrated by the following facts: • satisfactory agreement is observed between the frequencies of stretching vibrations of O−O and N−O bonds with unscaled calculated νO−O and νN−O values (Table 1). Taking into consideration the complex nature of the calculated vibrations, the modes with the maximal contribution of the corresponding stretching vibrations were selected for assignment to a particular type (νO−O or νN−O). • the intensity of the O−O vibrations is higher than that of νN−O; this regularity is reproduced both in calculations and in experiments.8 It is interesting to note that the band of the O−O bond vibrations in peroxide compounds usually has low intensity and, therefore, is low informative.38 Arylnitroso oxides give an opposite example, which is certainly explained by considerable polarization of the nitroso oxide moiety. • isotopic substitution with 18O shifts the vibration frequency of the O−O bond downfield, by 62 cm−1 for the trans isomer and by 49 cm−1 for the cis isomer of 4NO2C6H4NOO.37 The isotopic shift is quantitatively reproduced in the calculations using the selected density functionals: ΔνO−O = 56−65 (trans isomer) and 48−57 cm−1 (cis isomer). • the splitting of the characteristic bands of N−O and O− O vibrations in 4-NH2C6H4NOO reported previously8 is satisfactorily described by our calculations (Table 1): νN−O − νO−O = −738 and from −33 to −62 cm−1 (calculation, trans form), 189.58 and from 97 to 139 cm−1 (calculation, cis form). Thus, the four density functionals that showed adequate results in describing the properties of the simplest nitroso oxide, HNOO, also demonstrate the capability to correctly describe the structure and spectral properties of aromatic nitroso oxides. Conformational Transitions in Para-Substituted Phenylnitroso Oxides. The conformational potential of ArNOO is determined by the availability of two rotation axes in the nitroso oxide group. Apart from cis and trans isomerization, aromatic nitroso oxides can undergo conformational transitions between syn and anti isomers of ortho- and meta-substituted ArNOO due to internal rotation of the molecule around the C−N nitroso oxide bond. The possible conformational transitions of arylnitroso oxides are shown in Scheme 3. In this paper, we consider monosubstituted ArNOO containing the R moiety at para or ortho position relative to the nitroso oxide group. According to Scheme 3, description of the

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RESULTS AND DISCUSSION Structure and Infrared Spectra of ArNOO. The DFT approximations that we selected were used to study the structural and spectral properties of aromatic nitroso oxides. The Cartesian coordinates of all the structures studied are presented in the sections S4−S6 of the Supporting Information. Note that characteristic changes in the structure of the NOO group are observed in arylnitroso oxides in comparison with the simplest nitroso oxide. In particular, it has been found that the N−O bond in cis-PhNOO is ca. 0.01 Å shorter than in the trans isomer, whereas elongation of the O−O bond is more pronounced (by 0.020−0.025 Å). Furthermore, sign inversion of the rN−O − rO−O difference is observed (Table 1). The interatomic distances in the nitroso oxide group of transPhNOO are nearly equal. This geometric picture agrees with the concept that zwitterionic resonance structure (I) predominates in cis-phenylnitroso oxide, whereas the contribution of biradical resonance structure (III) is more pronounced in the trans form (see Scheme 1). The character of dependence of electronic properties on the substituent in the aromatic ring of nitroso oxide fully agrees with these conclusions. In fact, an electron acceptor (4NO2C6H4NOO, Table 1) increases, whereas a donor (4NH2C6H4NOO, Table 1) decreases the rN−O − rO−O difference both in trans and cis isomers of arylnitroso oxides in comparison with C6H5NOO. A natural explanation of this regularity is based on the assumption that a powerful electronwithdrawing group (−NO2) pulls the electron density from the nitroso oxide moiety and thus increases the contribution of the less polarized biradical resonance structure III into the wave function of the molecule. And vice versa, doping of electron density from the −NH2 substituent to the nitroso oxide moiety D


Article

The Journal of Physical Chemistry A conformational behavior of ortho-ArNOO requires calculations for four stable isomers and four transition states that connect the isomers on the conformational potential energy surface (PES). If the para-ArNOO substituent does not violate the local C2v symmetry of the aromatic moiety, the number of unique stationary points on the PES decreases. To study the effect of substituent nature on the corresponding conformational transformations (Scheme 3), a series of atoms and groups with various electron-donating and electron-withdrawing capabilities were used as R1 or R2, namely: -NMe2, -MeO, -Me, -Br, -H, and -NO2. Table 2 shows the results of calculation of the relative energies of stationary points on the conformational PES for a

azide ArN3 gives cis and trans isomers of the corresponding ArNOO with similar initial concentrations.8,14,39 Note that the M06-L and mPWPW91 density functionals slightly overestimate ΔH°, whereas the OLYP and HCTH functionals underestimate it in comparison with the results of optimizing calculation based on CCSD(T)/6-311+G(d,p) for PhNOO (ΔH° = −1.9 kJ/mol). But the range of ΔH° variation does not exceed 7 kJ/mol and is the same for all the para-ArNOO compounds studied. The absolute values and character of variation of conformational transition activation enthalpies deserve attention. The reproducibility of results for the four approximations used becomes somewhat worse, namely, to 9 kJ/mol for syn-anti transitions and to 15 kJ/mol for cis-trans transitions. However, all the functionals reproduce the same qualitative pattern of ΔH≠ variation on varying the para substituent. A characteristic feature is the weak, almost unnoticeable increase in the activation enthalpy of cis-trans transitions in the series from para-Me2N- to para-NO2-substituted phenylnitroso oxides and a strong decrease in ΔH≠ of both syn-anti transitions (Figure 1). Therefore, while the ΔH≠trans→cis and ΔH≠syn→anti values are

Table 2. Reaction Enthalpies (ΔH°) and Activation Enthalpies (ΔH≠) for trans-cis Isomerization of Peroxynitrene (kJ/mol) para-Phenylnitroso Oxides (Gas Phase)a method

ΔH°

M06-L mPWPW91 OLYP HCTH

−12.4 −11.3 −4.7 −5.7


−11.4 −10.6 −4.1 −5.0


−7.5 −6.3 −0.2 −1.0


−7.5 −6.5 −0.4 −1.1


−6.2 −4.9 1.1 0.4


−2.6 −2.5 3.1 2.5

ΔH‡trans→cis R2 = NMe2 57.8 65.3 68.2 69.5 R2 = MeO 58.3 66.9 69.9 71.3 R2 = Me 58.7 69.0 72.0 73.2 R2 = Br 57.9 68.3 71.4 72.5 R2 = Hb 59.7 70.3 73.4 74.5 R2 = NO2 60.0 69.9 72.8 74.0

ΔH≠cis→cis

ΔH≠trans→trans

61.7 58.8 52.9 53.6

59.7 54.0 51.1 51.7

49.9 47.5 42.3 43.1

49.4 43.8 41.5 42.1

39.2 37.2 32.3 33.2

42.9 37.8 35.5 36.3

38.5 36.5 31.6 32.4

42.8 37.5 35.1 35.8

33.8 32.4 27.7 28.4

38.7 34.5 32.3 32.9

27.2 26.4 22.2 22.5

37.4 32.2 30.0 30.5

Figure 1. Effect of substituent on the relative energies of conformational states of para-substituted phenylnitroso oxides. Calculation by the M06-L/6-311+G(d,p) approximation.

comparable for para-Me2NC6H4NOO, the activation enthalpies differ drastically for the last member of the series studied, specifically, para-NO2C6H4NOO (Table 2). The conformational barrier of the syn-anti transitions in arylnitroso oxides appears due to the conjugation of the nitroso oxide group with the aromatic system. For example, in nitrosomethane, where this kind of conjugation is absent, the rotation barrier of the NO group is ca. 4 kJ/mol. The significant increase in ΔH≠syn→anti in ArNOO containing an electron-donating substituent can be explained by an increase in the π-nature of the C-NOO bond due to occurrence of a zwitterionic resonance structure (Scheme 4) that is shown for

Basis set: 6-311+G(d,p). ΔH° = H°cis − H°trans. bΔH°(trans−cis) = −1.9 kJ/mol, CCSD(T) optimization. a

series of para-substituted phenylnitroso oxides. Symmetry conditions predetermine the existence of two stable states, namely, cis and trans forms of ArNOO, the relative stability of which was characterized by the heat effect of conformational transition ΔH°. The data presented in Table 2 indicate that the energies of both isomers are similar, though the cis isomers of ArNOO with electron-donating substituents are somewhat more stable. An increase in the acceptor capability of the parasubstituent reduces ΔH° almost to zero. This result agrees with the experimental observation that flash photolysis of aromatic

Scheme 4

E


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The Journal of Physical Chemistry A

Figure 2. Comparison of geometrical parameters of cis isomers of para-Me2NC6H4NOO and para-NO2C6H4NOO. Calculation by the M06-L/6311+G(d,p) approximation.

the cis isomer of para-dimethylaminophenylnitroso oxide as an example. This assumption is confirmed by comparison of C−C bond lengths in the benzene ring of this nitroso oxide and its “antipode”para-NO2C6H4NOO (Figure 2). In the former case, considerable alternation of C−C interatomic contacts is observed, just like it is expected from valence Scheme 4. A similar behavior is observed for trans isomers (Figure S1). The Solvent Effect. It has been reliably established15 that the solvent significantly affects the ArNOO reactivity. The rate constants of ArNOO monomolecular decay and the activation parameters obtained in nonpolar (n-hexane), aromatic (benzene), and polar solvents (acetonitrile) have been collected and analyzed in a recent review.1 It can be assumed that the conformational features of arylnitroso oxides that affect their absolute reactivity also depend on the properties of the medium where the conformational transformation occurs. To identify the effect of the solvent on the energies of the key conformational states of para-substituted phenylnitroso oxides, we performed the DFT calculations using the IEFPCM polarized continuum model. The results of ArNOO structure optimization and the relative energies obtained in M06-L/6311+G(d,p) + IEFPCM approximation for the three solvents (n-hexane, benzene, and acetonitrile) are presented in Table 3. Note that the other density functionals give a similar picture of the solvent effect (Tables S3−S5). It has been found that an increase in the solvent dielectric constant favors a more efficient polarization of ArNOO and hence an increase in the contribution of zwitterionic resonance structures (Schemes 1 and 4) to the wave function of the molecule. The solvent effect is accompanied by elongation of O−O bonds and shortening of C−N and N−O bonds for both ArNOO isomers, as well as by convergence of enthalpies of the cis and trans forms. The latter effect is apparently due to the more efficient stabilization of the trans isomer by the solvent in comparison with cis-ArNOO. One should also note the systematic growth of enthalpies of conformational transitions that increase with the solvent polarity. The most obvious explanation of this regularity is that the transition states of conformational transitions have an essentially biradical nature, they are less polar, and are stabilized by the solvent to a smaller

extent than the ArNOO isomers. In fact, the greatest solvent effect is observed for para-Me2NC6H4NOO, the structure of which implies the greatest contribution of zwitterionic resonance states in the nitroso oxide series studied. In this compound, the O−O and C−N bond lengths and ΔH≠syn→anti values change most strongly (Table 3). However, in total, the trends of variation of enthalpies of isomerization and activation enthalpies identified in the arylnitroso oxide series studied in the gas phase (see Figure 1 and the discussion above) are preserved in all the solvents studied and for all the density functionals used. Kinetics Analysis and Activation Barrier of trans-cis Conformational Transformation. To what extent do the results of our calculations match the experimental data, and how do they agree with each other? To answer this question, we will perform a formal kinetics analysis of a scheme describing irreversible consumption of arylnitroso oxides under real experimental conditions. Studies of ArNOO reactivity by flash photolysis method14,39 revealed the following regularities: • cis and trans isomers of ArNOO differ in optical absorption spectra in the visible region, which enables separate monitoring of the decay kinetics of both forms. • for all the arylnitroso oxides studied, except for 4Me2NC6H4NOO, the cis form is consumed more quickly than the trans form; the ratio of the decay rate constants of the isomers depends on the nature of the substituent at the aromatic ring of the nitroso oxide and on the solvent, and varies within 2−30 at room temperature. • in most cases, the kinetics reaction order with respect to both isomeric forms equals 1. These facts and a number of other regularities (effect of an active additive on the decay kinetics, ArNOO transformation products study) made it possible to establish that both isomers undergo the reversible transformation into each other, but only cis-ArNOO is irreversibly consumed. Hence, the kinetics scheme of arylnitroso oxide decay is as follows: where T and C are the trans and cis forms of ArNOO, respectively; P is the product; and kT, k−T, and kC are the kinetic rate constants of the corresponding transformations. F


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Table 3. Effect of Solvent on the Structure Δr (Å), Reaction Enthalpies ΔH°, and Activation Enthalpies ΔH≠ (kJ/mol) for the Conformational Transitions of para-Phenylnitroso Oxidesa R -NMe2

solvent hexane benzene acetonitrile

-OMe

hexane benzene acetonitrile

-Me


-Br


-H


-NO2


ΔH≠trans→cis

ΔrN−O

ΔrO−O

ΔrC−N

Δ(rN−O − rO−O)

ΔH°

0.001 0.000 0.001 −0.001 0.004 0.001 0.000 −0.002 −0.001 −0.002 0.000 −0.003 0.000 −0.002 −0.001 −0.003 −0.002 −0.005 −0.001 −0.002 −0.001 −0.003 −0.002 −0.006 0.000 −0.003 −0.001 −0.003 −0.002 −0.006 −0.001 −0.008 −0.002 −0.008 −0.003 −0.012

0.009 0.011 0.012 0.014 0.030 0.037 0.007 0.009 0.010 0.011 0.022 0.027 0.006 0.007 0.007 0.009 0.018 0.023 0.005 0.007 0.006 0.008 0.015 0.021 0.005 0.007 0.006 0.008 0.015 0.020 0.003 0.004 0.005 0.005 0.010 0.013

−0.004 −0.005 −0.005 −0.006 −0.013 −0.015 −0.003 −0.003 −0.003 -0.004 −0.009 −0.011 −0.002 −0.003 −0.003 −0.003 −0.007 −0.008 −0.002 −0.002 −0.002 −0.002 −0.006 −0.006 −0.002 −0.002 −0.002 −0.002 −0.006 −0.006 −0.001 0.001 −0.001 0.001 −0.003 −0.001

−0.008 −0.011 −0.011 −0.015 −0.026 −0.035 −0.007 −0.010 −0.011 −0.013 −0.022 −0.030 −0.006 −0.009 −0.009 −0.013 −0.020 −0.028 −0.006 −0.009 −0.007 −0.011 −0.018 −0.027 −0.005 −0.010 −0.007 −0.011 −0.018 −0.026 −0.004 −0.011 −0.007 −0.013 −0.014 −0.025

−9.9

61.9 (62 ± 0.3)

−9.2

62.7

−3.8

67.6 (13.7 ± 0.3)

−8.9

61.6(66.9 ± 0.7)

−8.3

62.6 (67.2 ± 0.9)

−3.3

70.0 (70.4 ± 0.6)

−5.2

61.5

−4.7

62.2

−0.6

68.0

−5.2

60.5

−4.6

61.2

−0.6

66.3 (54 ± 1)

−4.1

62.1

−3.6

62.7

0.1

67.6 (62 ± 2)

−1.5

60.8

−0.9

61.4

3.0

65.3

ΔH≠syn→anti 67.8 67.3 69.5 69.5 80.7 85.1 52.7 53.5 53.4 54.7 58.3 63.0 41.0 45.7 41.5 46.5 44.4 52.2 39.3 44.8 39.5 45.3 40.8 49.0 34.2 40.6 34.5 41.1 36.5 45.0 26.6 36.9 26.5 38.3 25.8 38.3

a

Calculation by the M06-L/6-311+G(d,p) + IEFPCM approximation. The numbers in parentheses indicate the experimental activation energies of trans isomer decay. The results for the trans isomers are given in italic font; Δr = r(solvent) − r(gas); ΔH° = H°cis − H°trans.

To describe the kinetics of this process, a five-parameter biexponential equation was adopted:39 A = A∞ + A 0I exp( −kIt ) + A 0II exp( −kIIt )

member A∞ = (C0 + T0)εPl (C0 and T0 are the initial concentrations of C and T; εP is the extinction coefficient of the reaction product, and l is the optical path length) agrees with the conclusions made in ref 39, and the effective rate constants kI and kII are determined by the expressions

(1)

where AI0, AII0 , kI, and kII have the physical sense of initial optical densities and rate constants of consumption of trans and cis isomers, respectively; A∞ is the residual optical density resulting from the absorption of reaction products. The analytical solution of the system of equations describing Scheme 5 in terms of optical density was obtained using Maple 12.0 software.40 In fact, the form of the analytical solution matches the form of Equation 1; the physical sense of the free

kI = 0.5(σ − δ)

kII = 0.5(σ + δ) σ = k T + k −T + k C δ = [(k C + k −T − k T)2 − 4k Tk −T]1/2

The expressions for AI0 and AII0 are rather cumbersome and provided in the section S7 of the Supporting Information. Our calculated data indicate that in most cases, cis-ArNOO are thermodynamically preferable isomers, or at least that both isomers have comparable stability (Table 3 and Table S7). Thus, we obtain, on the one hand, an obvious relationship: kT ≥ k−T. On the other hand, since the decay rate of cis-ArNOO is noticeably higher than that of the trans form due to

Scheme 5

G


Article


Table 4. Effect of Solvent on the Reaction Enthalpies ΔH° and Activation Enthalpies ΔH≠ (kJ/mol) for the Conformational Transitions of ortho-Phenylnitroso Oxides syn

a

R

solvent

-NMe2


-OMe


-Me


-Br


-NO2


syn → anti

anti

ΔH°

ΔH≠trans→cis

ΔH°

ΔH≠trans→cis

ΔH°cisa

ΔH°transb

ΔH≠cis

ΔH≠trans

−15.7 −12.9 −12.2 −6.6 1.6 5.3 6.2 12.7 2.7 6.1 6.9 12.5 −0.5 2.2 2.8 7.8 −12.4 −11.0 −10.6 −8.0

62.3 66.1 67.2 75.2 66.3 69.2 70.1 76.8 66.6 70.0 70.9 77.1 66.7 69.1 72.2 74.5 64.7 66.1 66.5 69.7

−5.8 −3.8 −3.3 0.7 −6.5 −4.3 −3.7 0.6 −4.3 −2.3 −1.7 1.8 −4.2 −2.0 −1.5 2.7 −4.5 −2.3 −1.7 2.6

54.1 58.3 59.6 68.5 55.1 58.0 58.8 65.2 56.9 59.4 60.2 65.3 56.6 58.9 59.6 64.4 58.0 59.7 60.2 63.7

1.0 0.5 0.3 −1.3 −8.3 −9.7 −10.1 −12.7 −12.2 −12.7 −12.8 −13.7 −14.0 −14.4 −14.5 −15.4 5.4 5.5 5.5 6.0

−8.9 −8.6 −8.6 −8.6 −0.1 −0.1 −0.1 −0.6 −5.3 −4.4 −4.2 −3.0 −10.3 −10.2 −10.2 −10.3 −2.5 −3.2 −3.4 −4.6

44.4 48.1 49.2 55.5 25.1 26.2 26.4 29.0 17.8 18.5 18.8 20.2 11.4 12.1 12.3 13.6 10.1 10.7 10.8 12.7

36.5 41.9 43.4 54.7 39.0 43.0 44.2 52.2 33.3 36.3 37.0 42.7 23.6 27.2 30.3 43.1 16.1 17.8 18.0 19.0

Calculation by the M06-L/6-311+G(d,p) + IEFPCM approximation. ΔH°cis = H°cis‑anti − H°cis‑syn. bΔH°trans = H°trans‑anti − H°trans‑syn.

experimental results (the results obtained by the other methods are presented in Tables S3−S5 of the Supporting Information). Me2NC6H4NOO is the only but very indicative exception. In nhexane, the cis isomer is more stable by 9.9 kJ/mol than the trans form, while the activation enthalpy of trans−cis isomerization nearly coincides with the activation energy of trans-Me2NC6H4NOO consumption (Table 3), that is, the mechanism of nitroso oxide decay is in good agreement with Scheme 5. ArNOO decay occurs in quite a different way in polar acetonitrile at room temperature. This follows from the considerable growth of effective rate constants, the strong decrease in the activation energies of consumption of both isomers,14,39 and the change in the ArNOO transformation products studied previously.29 It was found in the study referred to that Me2NC6H4NOO was converted to the corresponding nitro and nitroso compounds under the conditions specified, and almost no products of intramolecular cyclization by the mechanism shown in Scheme 2 were observed. The conclusion was made that nitroso oxide decay followed a bimolecular mechanism, which explained the observed low activation energy. The effective first order with respect to the concentrations of Me2NC6H4NOO isomers noted previously14,39 undoubtedly deserves an explanation. Presumably, it results from the parallel consumption of nitroso oxide through first- and second-order reactions, the ratio of which, apart from the factors noted above (temperature, solvent), is also affected by the concentration of the starting azide and the intensity of reaction mixture irradiation.29 Conformational Transitions in ortho-Substituted Phenylnitroso Oxides. The chemical potential of aromatic nitroso oxides is so high that they can undergo intramolecular transformation (Scheme 2) accompanied by destruction of the stable aromatic system. In this case, the terminal oxygen atom of the cis isomer attacks the adjacent (ortho) carbon atom in the benzene ring. It can be assumed a priori that an ortho substituent would sterically hinder this transformation.

intermolecular orthocyclization (Scheme 2), which is possible for cis form only, kC > kT, it can be assumed that kC ≫ k−T. It formally means that competition of reversible and irreversible decay of the cis isomer is in favor of the latter transformation. Under this assumption, analytical solution for Scheme 5 becomes much simpler, while the biexponential form of (1) is preserved:

kI = k T

kII = k C A 0I =

T0l(k TεT − k CεT − k TεC + k CεP) k T − kC

A 0II =

C0l(εC − εP)(k T − k C) + T0l(εC − εP)k T k T − kC

In the above equations, εT and εC are the extinction coefficients of the isomeric forms of ArNOO. It follows from the form of the equations for AI0 and AII0 that on increasing the kC/kT ratio, these parameters tend to lim kC/ k T→∞ A 0I = T0εTl = A 0T lim kC/ k T→∞ A 0II = C0εCl = A 0C

which fully agrees with the interpretation of the physical sense of the parameters in Equation 1 used in ref 39. Thus, kinetics analysis of Scheme 5 indicates that, under reasonable assumptions, the decay rate of trans-ArNOO is determined by the rate of the trans−cis conformational transition. Hence, the experimentally determined activation energy for the kI rate constant corresponds to the activation barrier of this transition. The activation energies are presented in Table 3 in parentheses. One can see that in general, the ΔH≠trans→cis values obtained by the M06-L/6-311+G(d,p) method are in good agreement with H


Article


Figure 3. Violation of planarity in stable conformers of ortho-NO2C6H4NOO as an illustration of steric repulsion of nitroso oxide and nitro groups. Dihedral angles are given relative to the benzene ring plane. Calculation by the M06-L/6-311+G(d,p) approximation.

However, reality is much more complex and interesting. First, the results of theoretical study11 demonstrate the opposite effect: the presence of an ortho substituent favors the transformation in question. Moreover, the first experimental example of transformation of arylnitroso oxides according to Scheme 2 has been found for ArNOO with a methoxy substituent at the ortho position.26 In view of this, studies on the conformational potential of ortho-substituted phenylnitroso oxides is of obvious scientific interest. ArNOO compounds of this type possess a full set of conformational transitions (Scheme 3) and require the calculation of four stable states and four saddle points of conformational transformations. All the specified stationary points of the conformational PES were calculated for the set of substituents used in this work, both without consideration for the solvent and in three media with various solvating power (n-hexane, benzene, and acetonitrile).

The density theory functional only qualitatively affects the conformational picture, therefore Table 4 presents the main results obtained in the M06-L/6-311+G(d,p) approximation. The other data can be found in the Table S6 of the Supporting Information. Analysis of the data presented in Table 4 indicates the absence of a simple relationship of ΔH° = H°cis − H°trans variation in the series of nitroso oxides under study. Let us note the main characteristic features of the data obtained. An interesting behavior is observed in ArNOO with syn-oriented substituents: formation of the cis conformer is clearly preferable in nitroso oxides with the strongest electron-donating (NMe2) or electron-withdrawing (NO2) substituents. For the other ArNOO, the trans isomer is somewhat more stable. The cis and trans isomers in the anti forms of nitroso oxides have approximately equal stability. Finally, the solvent effect both I


Article

The Journal of Physical Chemistry A in the syn and anti forms is the same as that in para-ArNOO: the polar solvent stabilizes the trans isomer more efficiently. The complex character of ΔH° variation is explained by a superposition of three factors: electronic and steric effects of the substituent and the solvent effect. The steric effect manifests itself most clearly if the enthalpies of cis isomers with syn and anti structures are compared. Owing to spatial proximity of the nitroso oxide group and the ortho substituent in 2-R-C6H4NOO with R = OMe, Me, and Br, the anti form is much more stable that the syn form, ΔH°cis = H°cis‑anti − H°cis‑syn < 0 (Table 4). It is logical that this effect is less pronounced for the syn/anti pair in trans-ArNOO. Quite an opposite behavior is observed in nitroso oxides containing NMe2 and NO2: the syn-cis conformer is somewhat more stable than the anti-cis conformer, while the opposite relation is observed for the trans syn/anti pair. The reason for this effect demonstrated in Figure 3 is that bulky substituents that preserve the molecule planarity in para-substituted ArNOO must turn around the C−N bond in ortho-substituted arylnitroso oxides, thus reducing steric repulsion but losing conjugation with the aromatic ring. The steric effect is equally significant both in cis and trans isomers. An additional stabilization factor of the syn-cis isomer results from interaction of the terminal oxygen atom of the nitroso oxide group and the nitrogen atom of the dimethylamine and nitro substituents (Figure 3). In particular, AIM41 analysis of electron density distribution indicates that a bond critical point (3, −1) exists between the mentioned atoms. The parameters of the critical point in the syn-cis form of ortho-NO2C6H4NOO are ρ = 0.0184, ∇2ρ = +0.0867, |λ1/λ2| < 1, which indicates that this bonding is mostly ionic (“closed-shell interaction”). The presence of a substituent at an ortho-position of the aromatic ring of a nitroso oxide oppositely affects the energy barrier of the trans−cis conformational transition in syn and anti forms of ortho-ArNOO in comparison with the para isomers. As expected, the ΔH≠trans→cis value increases due to steric interaction in nitroso oxides with the syn-oriented substituents (by 4.0−9.0 kJ/mol) and slightly decreases in the anti form. The electronic effect of the substituent is insignificant for this conformational transition, similarly to para-ArNOO (Figure 1). The activation barrier of syn-anti transitions for both isomers having an ortho substituent is much lower than that for 4-R-C6H4NOO. Presumably, the observed effect is related to the fact that the nitroso oxide group in the syn form of cis and trans isomers of ortho-ArNOO deviates from the benzene ring plane, thus making the geometrical structure closer to the transition state of syn-anti conformational transition, which in turn results in a decrease in the reaction activation barrier. Moreover, in accordance with Hammond’s postulate,42 an early transition state is realized in ortho-ArNOO, with a lower activation energy in comparison with the para isomer. A strong electronic effect of the substituent on the activation energy of syn-anti conformational transition is observed (Table 4), which was also noted above for paraArNOO (Table 3, Figure 1). Analysis of the overall picture of the conformational potential of ortho-substituted arylnitroso oxides leads us to an important practical conclusion. Because of the essentially different steric and electronic effects of substituents on the activation barriers of trans-cis and syn-anti conformational transitions, the ΔH≠trans→cis ≫ ΔH≠syn→anti relation is valid for the majority of arylnitroso oxides (except those containing strong electron donors). It means that a syn-anti equilibrium should be

established on the time scale of nitroso oxide decay reaction. Hence, analysis of ArNOO kinetics and transformation products should be performed with consideration for competition of intramolecular cyclization reactions (Scheme 2) of cis-ArNOO at both ortho directions to give different products.

■

CONCLUSION A systematic theoretical study on the conformational transformations of aromatic nitroso oxides preceding irreversible chemical transformations of arylnitroso oxides has been performed. The multi-configuration nature of the wave function of the compounds under study requires a thorough selection of methods that allow, within a single approximation, accurate description of the geometrical structure and reproduce the energies of nitroso oxides and transition states of reactions involving these compounds. Upon testing various methods of density functional theory, we succeeded in selecting the M06-L, mPWPW91, OLYP, and HCTH functionals, which most reliably describe the structure, spectral, and energy properties of both the simplest nitroso oxide, peroxynitrene, and arylnitroso oxides ArNOO. We used the DFT methods that we selected to study the conformational potential of para- and ortho-substituted phenylnitroso oxides that result from the availability of two free rotation axes in these compounds. The conformational rotation around the Ar-NOO bond determines the existence of syn-anti isometry in ortho-substituted phenylnitroso oxides. Rotation around the ArN-OO bond gives rise to formation of cis- and trans-ArNOO isomers. All stable states in the ortho- and paraR-C6H4NOO (R = NMe2, MeO, Me, Br, H, NO2) have been localized, as well as the saddle points (transition states) of all conformational transformations. Both interatomic contacts (C−N and N−O) determining the conformational potential of arylnitroso oxides contain a noticeable contribution of πbonding due to the presence of a three-centered 4π-electron system in nitroso oxides (-NOO) and its conjugation with the π-aromatic system. Because of this π-bonding, rotation around the C−N and N−O bonds is retarded by considerable conformational barriers. In particular, it has been found that the activation enthalpy of the trans-cis conformational transition is nearly insensitive to the nature of the substituent at the benzene ring and ranges within 58−60 kJ/mol for paraArNOO (gas phase; here and below, the results of calculation in the M06-L/6-311+G(d,p) approximation are given, Table 2). The ortho substituent has an insignificant effect on ΔH≠trans→cis: it increases this value by ∼5 kJ/mol in syn isomers and decreases it by ∼3 kJ/mol in anti-isomers of ortho-ArNOO (Table 4). Calculations within the IEFPCM model with consideration for the solvent effect show an increase in ΔH≠trans→cis by 5−15 kJ/mol (Tables 5 and 6) on transition from the gas phase to solvents with various polarities (n-hexane, benzene, acetonitrile). The maximum values of the activation barrier for the trans-cis conformational transition in the most polar solvent (acetonitrile) are explained by more efficient solvation of the starting conformer, in which electron density distribution shows a partial zwitterionic character in comparison with stabilization of the less polar biradical transition state by the solvent. In contrast to the trans-cis transition, the syn-anti conformational barrier is considerably affected by the substituent in the aromatic ring; moreover, an increase in the electron-withdrawing properties of the substituent decreases ΔH≠syn→anti. J


Article

The Journal of Physical Chemistry A Table 5. Comparison of Relaxation Times τa of Conformational Transitions and Lifetimes t of trans and cis Isomers of para-R-C6H4NOO in n-Hexane Solvent

arylnitrenes would result in comparable amounts of cis and trans isomers, in agreement with experimental observations. Obviously, this distribution of conformers (two for parasubstituted and four for ortho-substituted ArNOO) does not match the equilibrium distribution; hence, subsequent conformational transitions to more stable isomers are possible. Conformational transitions change the starting population of ArNOO isomers and result in quite a diverse picture of the equilibrium distribution of isomers, depending on the electronic nature of the substituent (Figure 4). However, we need to understand whether there is enough time for establishment of conformational equilibrium on the ArNOO lifetime scale determined by irreversible decay of the nitroso oxide. To answer this question, Table 5 compares the experimental lifetimes t of a series of para-substituted ArNOO with the relaxation times of conformational equilibria τ derived from the results of our calculations. The lifetimes t = 1/k were calculated from the data provided in ref 14. The relaxation time τ for first-order reversible reactions is τ = (k+ + k−)−1, where k+ and k− are the rate constants of the direct and reverse reactions, respectively. The rate constants were calculated by the Eyring equation:

R τ or t, s

− NMe2

τtrans−cis

0.03

0.02

τcis−cis

0.29

τtrans−trans

0.25

ttrans tcis

0.1 0.5

4.5 × 10−4 7.5 × 10−4 1.0 0.2

−OMe

−Me 5.0 × 10−3 6.3 × 10−6 2.7 × 10−5 3.8b

−Br

−H

0.01

0.016

2.9 × 10−6 2.6 × 10−5 3.6b

3.5 × 10−7 5.1 × 10−6 3.3b

−NO2 5.8 × 10−3 2.5 × 10−8 1.1 × 10−6

a

Calculation by the M06-L/6-311+G(d,p) approximation. bThe cis and trans isomers are spectrally and kinetically indistinguishable.

This effect ranges from 62 and 44 kJ/mol (gas phase, R = NMe2, para and ortho isomers, respectively) to 27 and 10 kJ/ mol (R = NO2). The syn-anti barrier grows with the solvent polarity. The effect of the solvent on ΔH≠syn→anti is similar to the trans-cis barrier, both in effect magnitude and mechanism. The results of conformational analysis of arylnitroso oxides allow calculation of the thermodynamic probability of the formation of each stable conformer. However, it should be taken into account that under real experimental conditions, ArNOO is formed in the reaction ArN: + O2 that occurs with a rate constant of ∼1 × 106 L/mol·s at room temperature. It may be assumed that the cis and trans isomers are formed at similar rates: this situation is definitely realized for cis/trans HNOO isomers;22 hence, it can be expected that oxidation of

k=κ

⎛ −ΔG≠ ⎞ kBT exp⎜ ⎟ h ⎝ RT ⎠

where κ is the transmission coefficient that was taken to be 1, and ΔG‡ is the Gibbs activation energy (Tables S7, S8). Comparison of t and τ values (Table 5) leads to the obvious conclusion that cis−cis and trans−trans transitions in all paraR-C6H4NOO, except for that with R = Me2N, occur much

Figure 4. Equilibrium population of conformational forms of para (a) and ortho (b) phenylnitroso oxides. K



■

faster than the nitroso oxide is consumed. Since these transitions occur between structurally identical states, this conformational transformation does not affect para-ArNOO decay kinetics and products. The relaxation time of cis-trans equilibrium τtrans−cis is smaller by 1−2 orders than the ArNOO lifetime (Table 5). If the real transmission coefficient is κ < 1, then the t and τ values will be commensurable, and hence irreversible decay of an arylnitroso oxide will violate the conformational cis-trans equilibrium, while the composition and ratio of the reaction products will be determined by ArNOO generation conditions. A similar comparison of t and τ for ortho-substituted ArNOO is impossible due to lack of experimental data. However, if the same relations of t and τ for para-ArNOO are also true for ortho isomers, then analysis of the transformation kinetics and products of such nitroso oxides should be performed with consideration for the existence of all four stable states of ortho-substituted ArNOO. Experimental studies in this direction are currently in progress in our laboratory.

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REFERENCES

(1) Chainikova, E. M.; Khursan, S. L.; Safiullin, R. L. The Chemistry of Nitroso Oxides. In the Chemistry of Peroxides; Liebman, J. F., Greer, A., Eds.; John Wiley & Sons Ltd: Chichester, U.K, 2014; Vol. 3, pp 357− 421. (2) Gritsan, N. P. Study of Photochemical Transformations of Organic Azides by Matrix Isolation Techniques and Quantum Chemistry. Russ. Chem. Rev. 2007, 76, 1139−1160. (3) Sawwan, N.; Greer, A. Rather Exotic Types of Cyclic Peroxides: Heteroatom Dioxiranes. Chem. Rev. 2007, 107, 3247−3285. (4) Laursen, S. L.; Grace, J. E.; DeKock, R. L.; Spronk, S. A. Reaction of NH (X) with Oxygen in a Solid Xenon Matrix: Formation and Infrared Spectrum of Imine Peroxide, HNOO. J. Am. Chem. Soc. 1998, 120, 12583−12594. (5) Cadogan, J. I. G. Reduction of Nitro- and Nitroso-Compounds by Tervalent Phosphorus Reagents. Q. Rev., Chem. Soc. 1968, 22, 222− 251. (6) Dean, A. M.; Bozzelli, J. W. Combustion Chemistry of Nitrogen. In Gas-Phase Combustion Chemistry; Gardiner, W. C., Jr; SpringerVerlag: New York, 2000; pp 125−341. (7) DeKock, R. L.; McGuire, M. J.; Piecuch, P.; Allen, W. D.; Schaefer, H. F.; Kowalski, K.; Kucharski, S. A.; Musiał, M.; Bonner, A. R.; Spronk, S. A.; et al. The Electronic Structure and Vibrational Spectrum of trans-HNOO. J. Phys. Chem. A 2004, 108, 2893−2903. (8) Pritchina, E. A.; Gritsan, N. P.; Bally, T. Matrix Isolation and Computational Study of the Photochemistry of p-Azidoaniline. Phys. Chem. Chem. Phys. 2006, 8, 719−727. (9) Talipov, M. R.; Ryzhkov, A. B.; Hursan, S. L.; Safiullin, R. L. Theoretical Investigation of the Electronic Spectra of Aromatic Nitrosooxides with Allowance for Solvent Effects. J. Struct. Chem. 2006, 47, 1051−1058. (10) Talipov, M. R.; Khursan, S. L.; Safiullin, R. L. Quantum Chemical Study of Isomerization of the Phenyl Nitroso Oxide. Bashk. Khim. Zh. 2006, 13, 105−107. (11) Talipov, M. R.; Khursan, S. L.; Safiullin, R. L. A New Intramolecular Transformation of Aromatic Nitroso Oxides. Russ. J. Phys. Chem. A 2012, 86, 235−243. (12) Talipov, M. R.; Khursan, S. L.; Safiullin, R. L. The Reaction of Nitroso Oxides with Olefins: Concerted or Nonconcerted Addition? Russ. J. Phys. Chem. A 2011, 85, 364−376. (13) Talipov, M. R.; Khursan, S. L.; Safiullin, R. L. The QuantumChemical Modeling of the Reaction of Nitroso Oxides with Olefins. Russ. J. Phys. Chem. B 2009, 3, 529−536. (14) Chainikova, E. M.; Khursan, S. L.; Safiullin, R. L. Isomeric Forms of Arylnitroso Oxides: Electronic Spectra and Reactivity. Kinet. Catal. 2006, 47, 549−554. (15) Chainikova, E. M.; Khursan, S. L.; Safiullin, R. L. Strong Solvent Effect on the Reactivity of Polar Intermediates X−O−O: Isomerization of 4-(N,N-Dimethylamino)Phenylnitroso Oxide. Dokl. Phys. Chem. 2004, 396, 138−140. (16) Chainikova, E. M.; Safiullin, R. L. Kinetics of Reactions between Arylnitroso Oxides and Methyl Vinyl Ketone. Kinet. Catal. 2009, 50, 97−102. (17) Chainikova, E. M.; Khursan, S. L.; Safiullin, R. L. Phenylnitroso Oxide: Formation and Decay Kinetics. Dokl. Phys. Chem. 2003, 390, 163−165. (18) Chainikova, E. M.; Safiullin, R. L. Reactivity of Arylnitroso Oxides to Triphenylphosphine. Kinet. Catal. 2009, 50, 527−529. (19) Chainikova, E. M.; Safiullin, R. L.; Spirikhin, L. V. Redox Isomerization of Aromatic Nitroso Oxides. Dokl. Chem. 2012, 442, 12−14. (20) Chainikova, E. M.; Safiullin, R. L.; Spirikhin, L. V.; Abdullin, M. F. A Revised Mechanism of Thermal Decay of Arylnitroso Oxides. J. Phys. Chem. A 2012, 116, 8142−8147. (21) Kahn, S. D.; Hehre, W. J.; Pople, J. A. Hartree-Fock Descriptions of 1,3-Dipoles. Zwitterions, 1,3-Diradicals, or Hypervalent Species? J. Am. Chem. Soc. 1987, 109, 1871−1873.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b04282. Geometrical parameters and frequencies of stretching vibrations of peroxynitrene; reaction enthalpies ΔH° and activation enthalpies ΔH‡ for trans−cis isomerization of peroxynitrene; Cartesian coordinates of the stationary points of HNOO, PhNOO, 4-RPhNOO, and 2RPhNOO in gas phase; cis and trans isomers structures of para-Me2NC6H4NOO and para-NO2C6H4NOO in the M06-L/6-311+G(d,p) approximation; analytic solution of differential equations; effect of a solvent on structure Δr, reaction enthalpies ΔH°, and activation enthalpies ΔH‡ for the conformational transitions in para-phenylnitroso oxides calculated by different methods; effect of a solvent on reaction enthalpies ΔH° and activation enthalpies ΔH‡ for the conformational transitions in ortho-phenylnitroso oxides (mPWPW91/ 6-311+G(d,p) + IEFPCM); Gibbs free energy ΔG° of reaction and Gibbs free energy activation ΔG‡ for the conformational transitions in para- and ortho-phenylnitroso oxides calculated in the M06-L/6-311+G(d,p) + IEFPCM approximation; comparison of relaxation times of conformational transitions and lifetimes of trans and cis isomers of para-R-C6H4NOO in benzene and acetonitrile solutions. (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Cand. chem. Sci. M. Yu. Ovchinnikov for his assistance in the kinetics analysis of the nitroso oxide decay scheme. All theoretical calculations were performed using the equipment installed in the Center for collective use “Khimiya” (Chemistry) at Ufa Institute of chemistry of the Russian Academy of Sciences. L


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