Chemistry Reference
In-Depth Information
2.2.8 Choice of Computational Method for Modeling Nitrenes
From a practical standpoint, open-shell singlet nitrenes represent a particular
challenge to theory, because these species generally require multideterminant (or
multiconfigurational) computational methods to accurately describe their electronic
structure. Multiconfigurational self-consistent field methods (MCSCFs)—most often
implemented using the complete active space self-consistent field (CASSCF)
procedure—have been the
de facto
computational methods of choice for describing
these open-shell singlet species. For more accurate energies, the CASSCF energies
can be augmented with second-order perturbation theory (CASPT2) to capture
dynamical correlation outside of the active space. CASPT2 energy corrections are
particularly important for obtaining accurate relative energies of nitrene excited
states (such as when computing the UV-Vis spectra of nitrenes).
Unfortunately, these expensive MCSCF calculations limit the size of the species
that can be studied. However, there are some less-expensive, alternative
approaches for modeling larger open-shell singlet nitrenes. While restricted
methods, which assume that orbitals are doubly occupied, cannot be used in
these cases, unrestricted singlet single-reference computations, which allow the
a
and
spins to break spatial symmetry, have in some cases been used to describe
open-shell nitrenes, especially in combination with density functional theory
[unrestricted density functional theory (UDFT)].
38
Unfortunately these broken-
symmetry computations suffer from spin contamination and tend to be less
desirable. However, spin purification procedures can help mitigate this problem
of spin contamination and can lead to useful results in some cases.
39,40
For larger
open-shell singlet nitrenes that are intractable by CASSCF, restricted active space
CASSCF computations (RASSCF
41
), or the spin-flip methods of Krylov (such as
SF-DFT and SF-CC
42,43
), may prove to be a cheaper alternative to complete
CASSCF computations that still includes a description of the multireference nature
of the open-shell electronic state. However, such calculations have seen less use in
computing the properties of nitrenes thus far and remain largely with benchmark-
ing of these reactive intermediates.
In contrast to open-shell singlet nitrenes, the single reference nature of triplet
nitrenes and closed-shell singlet nitrenes expands the number of theoretical methods
that are capable of modeling these species. While singlet nitrenes with significant
open-shell character generally require multireference computational methods (e.g.,
MCSCF) to accurately describe their electronic structure, triplet nitrenes and singlet
nitrenes that are essentially closed-shell can often be reasonably described using
single-reference methods (e.g., coupled cluster, DFT). DFT performs admirably for
these species, although singlet energies tend to be underestimated relative to the
triplet energies by several kilocalories per mole. Compound methods such as G2,
44
and complete basis set methods (CBS),
25,45,46
which are based on MP2, quadratic
configuration interaction and coupled cluster calculations extrapolated to the
“complete” basis set limit, have also been shown to be quite robust for simple
nitrenes that have closed-shell singlet states, as well as related species such as closed-
shell singlet carbenes.
47
b
