Sequence of Highest Occupied Molecular Orbitals in the Phosphorin System

Abstract

Residual triethylamine is removed by dissolution in ethyl acetate and extraction with cold 0.1 N HCl. The organic phase is evaporated down, 1 N NaOH (300ml) is added followed by enough methanol, THF, or dioxane to give a homogeneous solution. The mixture is stirred for the prescribed length of time, evaporated down to half its original volume in uacuu, and extracted three times with ethyl acetate [after prior acidification in the case of ( 5 ) !].If necessary the extract is decolorized with animal charcoal, dried over MgSO,, and the solvent is evaporated off. The resulting oils soon crystallize on standing. The aromatic phosphorins, which are analogs of pyridine, contain a trivalent phosphorus atom of coordination number 2 whose bonding has hitherto hardly been studied. A quantum-mechanical studyt3' has shown that the highest occupied molecular orbital (HOMO) is a A-MO, whereas the corresponding orbital in pyridine describes the lone pair (n-MO) on the nitrogen atom. The fundamental importance of the HOMO for properties and reactivity of a compound prompted us to seek experimental confirmation of the theoretical predictions by studying the photoelectron spectrum of 2,4,6-tri-tert-butylphosphorin (I). The band maxima in the spectrum of ( I ) lie at 8.98.6, and 10.0 eV, those in the spectrum of (2) at 8.6 and 9.3 eV. As expected, the phosphorins are more readily ionized than corresponding pyridines. The band maxima in unsubstituted pyridine are found at 9.6,9.7, and 10.5 eVC6]. Hence the three tert-butyl groups in (2) lower the ionization potential (ZP) by 1.0 to 1.2 eV.The size of molecules (1) and (2) precludes direct CNDO/2 calculations[71; instead calculations were performed on 2,4,6-trimethylphosphorin (3) and 2,4,6-trimethylpyridine (4). A series of model calculations on the mono-, di-, and tri-methyl and the mono-and di-tert-butyl substituted compounds showed that replacement of a methyl by a tert-butyl group shifts the orbital energy by an additive component but that the orbital sequence is retained. For compound (4) it is n, A, A, o and for (3) A, n, Q, K (see Fig. 2).These results strongly suggest that the first photoelectron band of (2) arises from overlap of an n-and a A ionization, and the second band from a K ionization. The intensity ratio of the first to the second band (2 : 1) may be regarded as supporting evidence. The same orbital sequence has recently also been suggested for pyridine itself by photoelectron spectroscopy [']. Figure 2 the first and third band ofphosphorin (1) should be assigned to a A ionization, while the second band should result from an n-and a o ionization. Experience shows, however, that semiempirical valenceelectron calculations on unsaturated systems afford (3 orbitals of too high energy"], thus making assignment of A, n, A, and Q ionization to the three photoelectron bands more likely. According toIt is to be expected from Figure 2 that the ZP of the lone pairs of ( I ) and (2) are almost equal. The ZP of the lone pairs of NH, and PH, also differ by only 0.25 eVtlol. On...

Aromatic heterocycles XII. Semiempirical PM3 study of Diels-Alder cycloaddition reaction of substituted phosphabenzenes

“…The absolute values are close to the values of ¼ ionization energies of the HOMO orbital given by photoelectron spectra of phosphabenzene of 9.12 eV [10] and 8.6eV for 2,4,6-tritertbutyl-¸3{phosphabenzene [15].…”

“…The converse situation is that of pyridine where the nitrogen atom is negatively charged. The lone pair orbital is the HOMO in pyridine where in¸3-phosphabenzene the lone pair is the third occupied level while the LUMO of phosphabenzene is lower in energy conferring very interesting coordinating properties [10], [15].…”

Applications of photoelectron spectroscopy. 41. Photoelectron spectra of phosphabenzene, arsabenzene, and stibabenzene

“…Recent studies have indicated that gas-phase ion chemistry can be used to help in understanding solution thermodynamic properties. [13][14][15][16] It is now apparent that in some cases insight can be gained into transition state properties as well. Work is currently underway to elucidate the nature of the activation barrier.…”

Aromatic λ3 heterocycles XIV. Fosfinine-ether macrocyclesheterocycles XIV. Fosfinine-ether macrocycles

“…Even if crown ethers and phosphine-ether macrocycles display interesting coordinative behavior towards metals or neutral molecules, due to optical isomerism of phosphorus atoms they posses many practical problems [5][6][7][8][9][10][11][12][13]. The aromatic character of phosphinine was demonstrated earlier by structural and magnetic criteria and also by spectral investigation [14][15][16][17][18]. The exploration of coordinative abilities of phosphinine showed that η 1 coordination is preferred with low oxidation state metals, whereas π-complexes may be obtained with different metallic species [19][20][21][22][23][24][25][26][27].…”