Photoelectron spectra of diazabasketene, diazadeltacyclene, and related polycyclic cis-azoalkanes

Abstract

The He(1) photoelectron spectra of diazabasketene (Z), the cyclobutanated [2.2.2] system 3, and diazadeltacyclene (5) are reported. For all three substances level assignments lead to the MO sequence: n-> P > n+. In order to assist with the assignment of diazabasketene, the PE spectrum of basketene has been measured. Unlike the situation for simple cis-azo bicyclics, high-lying CT orbitals associated with the cyclopropane and cyclobutane moieties fall between n-and ?r(NN). The orbital interactions responsible are simultaneously the cause for a n-/n+ splitting of about 3.8 eV for diazabasketene and 4.0 eV for diazadeltacyclene. These values are the largest so far observed for cis-azoalkanes. The importance of ground-state homoconjugation for multicyclic azo compounds containing three-and four-membered rings is discussed in terms of molecular conformation and nitrogen extrusion rates. Caution is urged in the application of PES determined MO sequences to the relative energy of the corresponding transition states for retrocycloaddition reactions involving nitrogen. The partial determination of structural parameters for the title compounds and complete determination for some related bicyclics by MO calculations are discussed. And finally a problem associated with the interpretation of the PE spectra of large molecules is described.The study of the electronic structure of organic molecules and the rationalization of the pathways of chemical reactions have been given new impetus recently by the work of F~k u i ,~~ H~f f m a n n ,~~ P e a r~o n ,~~ and others4 on orbital interactions. These interactions are either intermolecular or intramolecular in nature. The former provide the basis for various set^^.^ of symmetry rules for chemical reactions which have been applied with considerable success to a large variety of systems. Intramolecular interactions are especially interesting in molecules which have two or more localized, or semilocalized, orbitals of the A bonding and lone pair types. These localized orbitals, or groups of localized orbitals, may interact either directly or indirectly with each other. The relative importance of these two modes of interaction, known as through-space and through-bond ,interaction, respectively, can only be evaluated by means of molecular orbital (MO) calculations whereas the magnitude of the splitting associated with the interaction of two localized orbitals can be investigated experimentally by photoelectron spectroscopy (PES).In this paper we are concerned with the interaction of lone pair orbitals on adjacent nitrogen atoms and their further interaction with various three-and four-membered ring carbon units in a multicyclic molecular frame. PES has been used to determine the splitting between the symmetric combination of lone pair orbitals n+ and the antisymmetric combination n-in diimide,6 trans-azomethane,' diazirine,x several aliphatic cyclic and multicyclic cis-and trans-azo m o l e c~l e s ,~-~ and As a sequel to our earlier study l o of the 2,3-diazabicyclo[2.2.n]alk-2-enes (n ...

Molecular dynamics studies of the thermal decomposition of 2,3-diazabicyclo(2.2.1)hept-2-ene

“…The first theoretical determination of the DBH structure was reported by Boyd et al 25 using complete neglect of differential overlap ͑CNDO͒ calculations. Despite only a fairto-moderate agreement for bond distances, the calculated bond angles accurately reproduced the microwave results.…”

Correlation of Oxidation and Ionization Potentials for Azoalkanes

“…f This work, see also ref . g Reference 37. h Mean value from refs and . i Mean value of E p obtained in ref (1.42 V), in ref (1.44 V), and this work (1.49 V).…”

“…The data for IP v are also subject to various experimental uncertainties, which may include errors in reproducibility for the same instrument (typically ±0.04 eV for our data) and errors due to different instruments, recording techniques, or calibration. For example, values in the range 8.83−8.96 eV have been reported in four independent measurements for 2,3-diazabicyclo[2.2.1]hept-2-ene ( 1 ), ,, which indicates a typical error of ±0.07 eV. There may also be an error in locating the band maximum, which becomes particularly important when bands with vibrational fine structure are compared with unresolved bands.…”

“…In such problematic cases, the strongest vibrational peak, which is often given as IP v for fine-structured bands, does not necessarily coincide with the maximum of an unresolved band (Franck−Condon envelope). To illustrate, the vibrational fine structure for the lowest ionization of 2,3-diazabicyclo[2.2.2]oct-2-ene ( 5 ) has been resolved in one case (IP v = 8.13 eV), but in another study an unresolved band was obtained with the maximum at 8.32 eV taken as IP v , the maximum of the Franck−Condon envelope was used in Table even for cases with vibrational fine structure. ,, …”

“…Azoalkanes constitute a typical class of substrates for which such a dilemma between the solution oxidation and gas phase ionization potentials obtains. Through intensive PES investigations in the 1970s, which persist to date, more than 100 ionization potentials for azoalkanes have been reported, ,− but since the one-electron oxidation of azoalkanes is mostly irreversible, , only a few peak oxidation potentials have been reported. − However, since in recent years mechanistic ,− and spectroscopic ,− studies on azoalkane radical cations have gained increasing attention, this unsatisfactory situation must be remedied. Hence, in the present work we introduce a convenient and useful correlation between the experimentally available oxidation and ionization potentials of azoalkanes, which allows us to estimate one if the other is known.…”