The equilibrium molecular structure of 3-methyl-4-nitro- and 4-methyl-3-nitrofuroxans by gas-phase electron diffraction and coupled cluster calculations

Highlights

• Isomeric 3-/4-nitrofuroxans were characterized by gas-phase electron diffraction.

• Structural parameters were refined by high-level quantum-chemical calculations.

• The use of orbital descriptor revealed distinctions between 4- and 3-nitroisomers.

Abstract

The equilibrium molecular structures of the isomeric 3-methyl-4-nitro- and 4-methyl-3-nitrofuroxans have been determined for the first time by gas-phase electron diffraction (GED) supported by coupled cluster calculations up to CCSD(T)/cc-pVTZ level of theory, in frame of dynamic model with relaxation of all structural parameters. The best fit of the experimental scattering intensities was obtained for a model of C_s symmetry. The small differences between similar geometric parameters were constrained at the theoretical values. To compare structural parameters of the titled compounds, the geometries of parent furazan and furoxan molecules were calculated at the CCSD(T)-AE/aug-cc-pVQZ and CCSD(T)-AE/cc-pVQZ levels of theory, respectively. Mean amplitudes and vibrational corrections necessary for GED analysis were computed at the B3LYP/SNDS levels of theory using quadratic and cubic force fields. Peculiar features of electronic and geometric structures of the studied isomeric nitrofuroxans were analyzed with the use of different topological, magnetic and orbital descriptors and the reasons of the significant distinctions between 4- and 3-nitroisomers structures were revealed. Due to hyperconjugation a lone electron pair of exocyclic oxygen atom in both isomers was found to delocalize into antibonding orbital of adjacent ON endocyclic bond, n_π(O)→σ∗(ON). The peculiarity of the 3-nitroisomer structure was provided with the additional interaction of exocyclic and nitro group oxygen atoms.

Introduction

The chemistry of heterocyclic compounds is an important field of modern organic chemistry and materials science. Heterocyclic compounds play a special role in medicinal chemistry: they form the basis of most pharmaceuticals, including natural products. Heterocycles are utilized as precursors or intermediates in the synthesis of a broad range of functional derivatives and acyclic products. Moreover, the majority of modern energetic materials are comprised of nitrogen heterocycles, first of all, of the five-membered azoles [1,2].

Among the variety of azoles, 1,2,5-oxadiazole (furazan, 1) and 1,2,5-oxadiazole 2-oxide (furoxan, 2) attract particular attention because of their potential applicability in the so-called dual-use technologies: as pharmacologically active compounds and as components of energetic formulations. On the one hand, furoxans are effective nitric oxide (NO) donors, which determine the broad spectrum of their pharmacological activities as vasodilating, antiplatelet, antiparasitic and antitumor agents [[3], [4], [5], [6], [7], [8]]. On the other hand, furoxans contain two active oxygen atoms and have high positive enthalpies of formation, which in combination with high density make these compounds suitable for the synthesis of highly energetic materials for various purposes, particularly for national defense and civilian technologies [[9], [10], [11], [12], [13]].

Nitrofuroxans are of special interest both as NO-donors [14,15] and as potential components of energetic formulations [[16], [17], [18]], 4-nitroisomers being more thermodynamically preferable, than 3-nitroisomers [19]. In this series, alkylnitrofuroxans are either low-melting compounds or stable distillable liquids. They incorporate four active oxygen atoms in the molecule and are promising ingredients (e.g., plasticizers) for energetic formulations [20]. In particular, 3-methyl-4-nitrofuroxan (3) has low melting point (68–69 °C), rather high density (1.64 g cm⁻³) and positive formation enthalpy (169.9 kJ mol⁻¹). Isomeric 4-methyl-3-nitrofuroxan (4) was synthesized quite recently [21]. It has lower melting point (41–42 °C), but nearly the same formation enthalpy and therefore can be more attractive as potential plasticizer. Moreover, energetic furoxan derivatives have higher combustion rate than known energetic compounds. This property of furoxan derivatives connects with a peculiarity of the furoxan ring structure in which a length of endocyclic O(1)N(2) bond is almost equal to the length of a single bond (1.44–1.50 Å).

Structural study of nitrofuroxans is limited only to several X-ray diffraction examples: four examples of 3-nitro derivatives and four examples of 4-nitro derivatives [17,[22], [23], [24], [25], [26], [27], [28]]. It should be emphasized, that furoxans can exist as two regioisomers A and Aʹ, which undergo interconversion through dinitrosoethylene intermediate DNE at heating (Scheme 1). Recent calculation studies have shown that dinitrosoethylene intermediate possess presumably a diradical character DNE’ [24]. Isomerization conditions are controlled by various electronic and structural factors of substituents at the furoxan ring. This feature may restrict unambiguous structural determination of furoxan derivatives. Therefore, structural investigations of isomeric nitrofuroxans by gas-phase electron diffraction (GED) remain highly urgent.

Herein, we report the semi-experimental equilibrium molecular structures of free 3-methyl-4-nitro- (3) and 4-methyl-3-nitro-furoxan (4) molecules which were determined by gas phase electron diffraction method using dynamic model with relaxation of all structural parameters. Small differences in similar geometrical parameters were fixed on the basis of CCSD(T) calculations using quadruple-ζ basis sets. Cubic force constants were used to calculate anharmonic vibrational correction between thermal average and equilibrium structural parameters. For comparison, geometry optimizations of parent furazan 1 and furoxan 2 molecules were performed at CCSD(T)-AE/aug-cc-pQTZ level of theory. Additional consideration of aromaticity, NBO, NCI, AIM descriptors, heats of formation and bond dissociation energies of the weakest bonds in the furoxan ring was also performed.