p-Phenylbisphosphinidene and Its Carbene and Nitrene Analogues:  An ab Initio Study

Abstract

Density functional and complete active space (CAS) SCF and CASPT2 calculations have been performed to investigate structural and electronic features of p-phenylbisphosphinidene and the analogous p-phenylbisnitrene, p-phenylbismethylene, p-phenylbis(fluoromethylene), and p-phenylbis(chloromethylene) molecules. Similar to the other species investigated, with the exception of p-phenylbis(fluoromethylene), p-phenylbisphosphinidene exhibits an open-shell singlet diradical ground state with a quinone-like structure. The singlet−triplet energy gap, computed with CASPT2 on CASSCF(10,10)/6-31+G* optimized structures, amounts to 1.78 kcal/mol. At the DFT (B3LYP/6-31+G*) level a value of 5.14 kcal/mol is obtained. Due to the highly multiconfigurational character of the p-phenylbisphosphinidene singlet and triplet wave functions, the DFT treatment becomes less accurate. Going to p-phenylbisnitrene, p-phenylbis(chloromethylene), p-phenylbis(fluoromethylene), and p-phenylbismethylene energies are better described by DFT calculations as the triplet wave functions have only one and the singlet wave functions exactly two leading configurations. The calculated singlet−triplet energy gaps are found to lie in a range of 1.3−2.2 kcal/mol at both DFT and CASPT2 levels of theory. p-Phenylbis(fluoromethylene) is a special case in the series as it has a closed-shell singlet ground state. The multiconfigurational treatment shows, however, that the singlet diradical state is very close in energy to the closed-shell singlet, even though the stability of the diradical is somewhat overemphasized by this method. The relative energies and the respective IR spectra are also discussed.

Singlet stabilization of oxazole- and isoxazolenitrene-n-oxides by radical delocalization

“…First, most substituents do not have a large effect on radical stabilities. Whereas they might have multifold effects on relative rates of radical reactions, it translates into small (0-3 kcal/mol) [17,[20][21][22][23][24][25][26], in energy lowering. Even ''super radical stabilizers'' [25], such as 4-pyridinyl-n-oxide or 2-furyl have modest effects (ca.…”

4,4-p-Biphenyl bis-phosphinidene: generation of a bis-W(CO)5 complex and ab initio calculation of its electronic structure

“…20 (iv) In the case of p-phenyl bis-phosphinidene (P-C 6 H 4 -P), 12 where a coupling of both terminal P-electrons through the phenyl ring becomes possible, a quinone-like molecular structure having an open-shell singlet diradical state has effectively been identified as its ground electronic state. 12 In spite of such a large geometric change, the resulting triplet-singlet separation gap is very small, being only 0.077 eV (7 kJ mol Ϫ1 ).…”

“…In this work, we have adopted a theoretical approach similar to that previously employed for treating phenyl phosphinidene (C 6 H 5 -P) 20 and p-phenyl bisphosphinidene (P-C 6 H 4 -P). 12 In view of the existence of several (1) (2)…”

“…In constructing adequately such wave functions, the complete active space selfconsistent field method (CASSCF) 17 was used in conjunction with the 6-31G(d) basis set including d-polarization functions on C and P atoms. The active spaces have been selected as follows: in a BPBP structure, the presence of two phenyl rings generates twelve (12) π-orbitals whereas the two terminal phosphorus atoms bring about four (4) π-orbitals (3p x and 3p y ). Thus, a full active space enclosing the whole π-system consists of 16 electrons in 16 orbitals.…”

Novel quintet and triplet (nitrenoethynyl)halomethylenes at theoretical levels

“…Thus, chemists have long been interested in high-spin molecules containing more than one reactive center. The chemistry of biradicals [1], biscarbenes [2-6], bisnitrenes [3,[6][7][8], and carbenonitrenes [3,5,7,9] is considerably richer and more complicated than that of the corresponding one reactive center species. For example, considering a carbene as a diradical species (i.e., as a two electron in two orbitals) gives rise to six electronic configurations [10], which can be arranged in four different electronic states (one triplet and three singlets).…”