Closed-shell Singlet Ground State Research Articles

In Search of Singlet Phosphinidenes.

We have examined singlet-triplet energy separations in different phosphinidenes (RP) substituted by first- and second-row elements, making use of ab initio molecular orbital theory. Our main purpose is to find out the substituents that particularly favor the singlet electronic state. The QCISD(T)/6-311++G(3df,2p) + ZPE level has been applied to small molecules and the CISD(Q) and QCISD(T) with the 6-311G(d,p) basis set for all species considered. We have identified few factors that come into play rendering the singlet phosphinidene more stable than the triplet. The parent phosphinidene, PH, has a triplet ground state lying 28 kcal/mol below the closed-shell singlet excited state. The triplet ground state is mainly favored when negative hyperconjugation is involved. In the boryl-, alkyl-, and silyl-substituted phosphinidenes, the triplet state remains by far the ground state. When the substituents have pi-type lone pair electrons (i.e., -NX(2), -PX(2), -OX, -SX), the singlet state becomes stabilized by such an amount that both states have similar energies or even a change in ground state occurs. The most stabilized singlet ground states are attributed to PSF and PSCl. P and S have similar p-orbital sizes, making pi-delocalization easier. Implantation of alkyl and/or amino groups in the beta-position of amino- and phosphinophosphinidenes also contributes to a singlet stabilization. Bulky beta-groups also destabilize the triplet state by a steric effect. From a practical viewpoint, amino (P-NR(2)) and phosphino (P-PR(2)) derivatives bearing large alkyl groups (R) are the most plausible and feasible targets for preparing phosphinidenes possessing a closed-shell singlet ground state.

How Do Different Fluorine Substitution Patterns Affect the Electronic State Energies of Phenylnitrene?

Substituted phenylnitrenes, reacting out of their closed-shellsinglet (CSS) states, can serve as photoaffinity labeling reagents if they engage in bond insertion reactions.1-3 Such reactivity is similar to that exhibited by singlet phenylcarbene.4 However, the reactivity of unsubstituted CSS phenylnitrene (PhN) is dominated by unimolecular ring expansion to form didehydroazepine.4,5 Substituents on the aromatic ring are required for bimolecular insertion reactions to compete with ring expansion, e.g., polyfluorinated phenylnitrenes, which were first examined by Banks and Prakash.6,7 Platz has demonstrated that the barrier to ring expansion in 2,6-difluorophenylnitrene ((o,oF2)PhN) is increased by 3-5 kcal/mol compared to unsubstituted PhN.4,8 Platz has also shown that proper location of the fluorines on the aromatic ring is critical to this effect; 3,5difluorophenylnitrene ((m,m-F2)PhN), like PhN, undergoes ring expansion faster than it participates in insertion reactions.2-4 Elucidating the energies for different electronic states of PhN has been the goal of several experimental9,10 and theoretical11-13 studies. Theory and experiment are in good agreement on the relative ordering of the lowest energy states. The ground state is a triplet (A2), the first excited state is an open-shell singlet (A2) lying about 18 kcal/mol higher in energy, and higher still is the first CSS (1A1). No experimental number is available for the relative energy of the latter state, but reasonable quality singleand multireference configuration interaction (CISD+Q) calculations place it 31-39 kcal/mol above the triplet. The electronic configuration of this state is dominated by double occupation of the nitrogen p orbital that lies in the plane of the aromatic ring. The next higher lying CSS (2A1) is derived from double excitation of that pair of electrons out of the inplane p orbital and into the aromatic π system. Kim et al.12 have estimated the energy of this state to be about 52 kcal/mol above the triplet, but the errors associated with this calculation may be substantial (vide infra). The energy of this state bears on the ring expansion reactivity of PhN insofar as it correlates with the CSS ground state of didehydroazepine, i.e., the ring expansion is a Wagner-Meerwein shift of a C-C bond to the electron-deficient nitrogen using the empty in-plane p orbital. On the basis of this analysis, Platz has speculated that different fluorine substitution patterns may preferentially stabilize the 1A1 state and hence lead to higher activation barriers for ring expansion.4 Of course, a more complete explanation includes the recognition that there must be an avoided crossing in the state-symmetry correlation diagrams, but the crux of the argument remains a differential effect on the relative energies of the two CSS states (Figure 1). In order to address this question, we have employed density functional theory14 (DFT) to calculate15,16 the relative energies of the triplet and CSS states of PhN, (o,o-F2)PhN, (m,m-F2)PhN, and 4-fluorophenylnitrene (p-FPhN) and their corresponding didehydroazepines. To assess the quality of this level of theory, we have compared the DFT results for PhN to multireference second-order perturbation theory (CASPT2N)17,18 employing an eight-electron/eight-orbital active space and also to single-reference coupled cluster19 calculations including all single, double, and perturbative triple20 excitations (CCSD(T)). These calculations are summarized in Table 1. The gap between the A2 and 1A1 states for PhN appears to be well predicted by DFT. On the basis of the results for methylene, Roos has suggested that the CASPT2N level overstabilizes triplets relative to singlets by 3-5 kcal/mol.21 Our own calculations for various carbenes and nitrenium ions have shown similar overstabilizations of triplet states by up to 8 kcal/ mol.22 The CCSD(T) calculations also support the DFT results. Because of the size of PhN, calculations at this level were not performed using a triple-u basis. The larger stabilization of the triplet at this level of theory compared to DFT is consistent with lesser flexibility in the basis set and the observed tendency of CCSD(T) to overstabilize triplets relative to CSSs by 1-2 kcal/mol in carbenes and nitrenium ions.22 Similar levels of DFT have previously been shown to accurately predict singlet-

Ab initiostudies of X-3and X3(X=F, Cl)

The electronic structures, optimized geometries and harmonic vibrational frequencies of F- 3, Cl- 3, F3 and Cl3 have been studied by ab initio calculations, using various basis sets and different correlation methods, including MP2, CISD and CASSCF. For the anions, reasonably consistent results were obtained for the closed-shell singlet ground states. However, for the open-shell doublet states of the neutral trihalogens and the triplet states of the anions the optimized geometries and hence the computed harmonic vibrational frequencies were found to be rather basis set and method dependent. Calculations based on a single configuration are inadequate for the neutral trihalogens, and the electronic structures of the various open-shell states of both the neutral trihalogens and their anions are rather complex. The different effects due to basis set size and dynamic and non-dynamic electron correlation on the various electronic structures, optimized geometries and computed harmonic vibrational frequencies obta...

The electronic structure and conformation of dimethylaminobenzonitrile

Ab initio molecular orbital (MO) computations on the closed shell singlet ground state ofN, N-dimethylaminobenzonitrile (DMABN) are reported. Fully optimized structures of several conformers of DMABN were calculated at the HF/6-31G level of theory. Our results indicate that for each of these conformations the minimum energy structure has a trigonal (sp2 hybridized] amino nitrogen. The most stable DMABN conformer was found to be planar with its methyl groups eclipsed. The Koopmans ionization potentials and dipole moments of the various ground state conformers are compared. The implications for dynamical models of twisted-intramolecular charge transfer (TICT) are discussed. Moreover, the use of qualitative MO theory arguments provides an interpretation of the computational results in a simple orbital interaction framework.

Orbital coupling in double-vacancy and two-electron systems

In double-core-hole states where two electrons are removed from different core orbitals, the SCF approach produces artificially low singlet-state energies, leading to energy separations between corresponding singlet and triplet states which can be wrong by orders of magnitude. We present a theoretical analysis of this singlet-triplet separation problem, revealing that the SCF breakdown depends exclusively on the coupling between the core orbitals involved and can be viewed as a particular form of “spin-constrained” variational collapse in the singlet state. The SCF procedure on this state tries to mimic the energetically lower closed-shell singlet ground state even though it has to work with an open-shell configuration. We further prove analytically that it is still possible to obtain reliable estimates of the singlet-triplet splitting in a one-particle picture and show how to construct suitable orbitals. The theoretical analysis is complemented and supported by results on SiX 2+ 4 (X=H, F) obtained by a propagator method, as well as a full treatment of the related two-electron system Si 12+.

UPS of 2–30-atom carbon clusters: Chains and rings

Ultraviolet photoelectron spectra (UPS) of negative carbon clusters are reported in the size range from 2 through 29 atoms. The clusters were prepared in a supersonic beam by laser vaporization, and a F 2 excimer laser (7.9 eV) was used for photodetachment. The resultant UPS data indicate that carbon clusters in the 2–9-atom size range take the form of linear chains: the even-numbered chains having open shell electronic structures with high electron affinity, the odd chains having closed shell singlet ground states (for the neutral) and substantially lower electron affinity. Clusters in the 10–29-atom range give UPS patterns consistent with a monocyclic ring structure.