Singlet-triplet Energy Separation Research Articles

Donor–acceptor interactions with electrophilic terminal phosphinidene complexes

Donor–acceptor interactions of Lewis bases, such as amine, phosphine, cyanic acid and phosphaalkine with various phosphinidene complexes carrying a W(CO) 5 fragment are studied by means of density functional calculations. Accordingly, the donor interactions can be divided into two categories. In terms of energy an amine binds stronger than a phosphine to a phosphinidene, the resulting DoP bond is longer (NP) or equal (PP) with respect to a corresponding single bond. This tendency is also revealed in the corresponding transition metal complexes. In all cases the singlet-triplet energy separation values of the resulting donor-adducts are fairly small. A donor with a π-system, such as HCN yields a shorter NP bond, but the resulting donor-adduct is even less stable with respect to decomposition into a singlet PH complex and cyanic acid. Similar considerations with HCP reveal only a weak donor addition of this fragment to the parent PH complex but a facile rearrangement of the adduct to the diphosphirene transition metal complex. The diaminophosphino–PH possesses a singlet ground state, its corresponding transition metal complex reveals a fairly small singlet–triplet energy separation. It can be considered as a nucleophilic PH complex in contrast to the other homologues which reveal an electrophilic behaviour.

Computational study on the stability of imidazol-2-ylidenes and imidazolin-2-ylidenes

The dimerization reactions of 12 cyclic diaminocarbenes have been studied using density functional theory (DFT). The activation energies ( E a) and reaction energies ( ΔE) for the dimerizations of imidazol-2-ylidenes are larger than those of imidazolin-2-ylidenes. It was observed that E a of dimerization is approximately proportional to the singlet–triplet energy separation ( ΔE S–T), aromatic stabilization energy (ASE), and ΔE of the carbene. Excellent linear correlation is seen between E a and ASE. Contrary to previous suggestions, we found that 4,5-dichloro substitutions decrease the stability of imidazol(in)-2-ylidenes. Steric effects on E a occur noticeably as isopropyl ( i-Pr) substitutions are introduced to the carbenes.

Density Functional Study on the Reactivity of Carbenes Toward 1,2‐H Shifts

AbstractThe 1,2‐H shift reactions of simple carbenes (CH2Y‐C‐X) have been studied using density functional theory (DFT). The influence of the substituent X and Y groups on the activation energy (Ea) of 1,2‐H shifts were examined. The ‘by stander’ Y substituents lower Ea in the order of Me, F > Cl, Br > H. Our analysis shows that the X effect is more significant than the by stander Y effect. X substitutions increase Ea of carbenes in the order of F > Cl > Br > Me > H. The influence of X on Ea is governed by the singlet‐triplet energy separation (ΔES‐T) of the carbene, i.e., Ea of a carbene is larger as its ΔES‐T in creases due to an X substitution. The X effect was also found to be related to the magnitude of the exothermicity: Ea of reaction is smaller when the reaction is more exothermic. Origin of the Y effect is attributed to the inter play between two factors: ‘lateness’ of transition state on the potential energy surface, and the exothermicity of the reaction.

The interplay of theory and experiment in the study of phenylnitrene.

The intra- and intermolecular chemistry of phenylnitrene (PhN), its singlet-triplet energy separation, and its electronic spectra are interpreted with the aid of ab initio molecular orbital theory. The key to understanding singlet PhN is the recognition that this species has an open-shell electronic structure, in contrast to the related species, phenylcarbene, which has a closed-shell electronic structure. The thermodynamics of nitrenes, benzazirines, dehydroazepines, aminyl radicals, and their hydrocarbon analogues are also discussed.

Magneto–structural studies of monohydroxo-bridged dicopper(II) complexes M[Cu 2L 2(OH)]·2H 2O (M=Na + ( 1) and K + ( 2); H 2L=2,6-bis[ N-(phenyl)carbamoyl]pyridine). Effect of CuOHCu bridge angle on antiferromagnetic coupling

Using a tridentate bis-amide ligand 2,6-bis[ N-(phenyl)carbamoyl]pyridine (H 2L), in its deprotonated form, two new monohydroxo-bridged dicopper(II) complexes M[Cu 2L 2(OH)]·2H 2O (M=Na + ( 1) and K + ( 2)) have been prepared and characterised by a number of methods, including X-ray crystallography. Each copper(II) ion is terminally coordinated by one pyridyl and two amide nitrogen donors. The two copper(II) centres are bridged by a hydroxo group, with each copper(II) centre assuming a distorted square planar geometry. The observation of short CuN py and long CuN amide bonds is caused by the steric requirement of the ligand. Interestingly, each cation Na +/K + is coordinated to four different [Cu 2L 2(OH)] − units through the amide O-donors, in an uncommon distorted tetrahedral coordination environment. Temperature-dependent magnetic susceptibility measurements revealed that the compounds have S=0 ground state with singlet–triplet energy separation, 2 J=−334 and −296 cm −1 for 1 and 2, respectively. The larger CuOHCu bridge angle in 1 (131.1(6)°) causes better antiferromagnetic exchange coupling than that in 2 (125.7(6)°).

The singlet–triplet separation in dichlorocarbene: A surprising difference between theory and experiment

The recently reported experimental singlet–triplet energy separation in CCl2, namely 3±3 kcal/mol, lies far below extant theoretical predictions. This problem is investigated here at much higher levels of theory than previously applied. The present theoretical prediction is 19.5±2 kcal/mol.

On the Electronic Properties of Substituted Phosphanylcarbenes

A phosphanyl group exerts much less π-conjugation properties than an amino group. On this basis, corresponding carbene structures exhibit much smaller singlet–triplet energy separations. Of the various structures investigated quantum-chemically the largest singlet–triplet energy separations are predicted for cyclic diphosphanylcarbenes, in which the two functional groups are incorporated into a ring system and the phosphorus atoms are substituted by phosphoraniminato groups. Then the singlet–triplet energy separations become essentially larger than for the Bertrand-type (push-pull) carbenes.

On the Electronic Properties of Substituted Phosphanylcarbenes

A phosphanyl group exerts much less pi-conjugation properties than an amino group. On this basis, corresponding carbene structures exhibit much smaller singlet-triplet energy separations. Of the various structures investigated quantum-chemically the largest singlet-triplet energy separations are predicted for cyclic diphosphanylcarbenes, in which the two functional groups are incorporated into a ring system and the phosphorus atoms are substituted by phosphoraniminato groups. Then the singlet-triplet energy separations become essentially larger than for the Bertrand type (push-pull) carbenes.

Performance of the hybrid density functionals in the determination of the geometric structure, vibrational frequency and singlet–triplet energy separation of CH 2, CHF, CF 2, CCl 2 and CBr 2

The geometric structure, vibrational frequency, dipole moment and singlet–triplet energy separation of CH 2, CHF, CF 2, CCl 2 and CBr 2 has been investigated using B3LYP, B3P86 and B3PW91 hybrid density functionals. For CH 2, the density functionals predict the equilibrium C–H bond length, H–C–H bond angle and vibrational frequency to an accuracy comparable to high level ab initio methods. However, only B3LYP functional produces reasonable singlet–triplet energy separation. Also estimates of the singlet–triplet energy separation of CHF and CF 2 indicate B3LYP produces a better agreement with experiment than B3P86 or B3PW91.

A comparative study of the singlet–triplet energy separation of carbenes using density functional theory and coupled-cluster methods

The singlet–triplet energy separation (Δ E S–T) of carbenes has been studied using density functional theory (DFT) and the single, double, plus perturbative triple excitations coupled-cluster {CCSD(T)} approaches. Δ E S–T predictions from DFT were compared with those of CCSD(T). We found that BLYP and G96LYP predictions are closest to the CCSD(T) results, with the average absolute deviations within 1 kcal/mol. Many of the tested functionals overestimate Δ E S–T. It was observed that best comparisons are displayed by the choice of correlation (LYP) functionals, while the effect of choosing among exchange functionals is less significant. The hybrid functionals provide poorer, overestimated predictions for Δ E S–T.

Different bases for different correlation effects: multireference Møller–Plesset perturbation theory in the extended basis function space

A method to improve virtual orbitals of CASSCF theory has been proposed by expanding the virtual orbital space through the addition of basis functions without disturbing already established CASSCF function space. It is a natural extension of work used in Hartree–Fock calculations. The improved virtual orbitals are then used to estimate the dynamical correlation. That is, a moderate basis set is used in CASSCF to include nondynamical correlation and a sufficiently large basis set is used in multireference Møller–Plesset perturbation theory to estimate the slowly convergent dynamical correlation. The present formalism is tested on the barrier height of the H 2CO → H 2+CO reaction, singlet–triplet energy separation in CH 2 and valence π–π * excitation energies in butadiene. Numerical results show that good accuracy can be obtained.

Synthesis, magnetism, 1H NMR and redox activity of dicopper(II) complexes having a discrete {Cu2(μ-phenoxide)2}2+ unit supported by a non-macrocyclic ligand environment. Crystal structure of [Cu2(L)2(OClO3)2] [HL = 4-methyl-2,6-bis(pyrazol-1-ylmethyl)phenol] †

Reaction between 4-methyl-2,6-bis(pyrazol-1-ylmethyl)phenol (HL) or its 3,5-dimethylpyrazole derivative (HL′) and Cu(ClO4)2·6H2O afforded [CuII2(L/L′)2(OClO3)2] 1 and 2. Complex 1 has been structurally characterized showing that each copper(II) centre is square pyramidal with two bridging phenoxide oxygens and two terminal pyrazole nitrogens in the equatorial plane and a perchlorate oxygen atom axially co-ordinated. Variable-temperature magnetic susceptibility measurements revealed that the dicopper(II) centres are strongly antiferromagnetically coupled [singlet–triplet energy separation, 2J (in cm–1): –1204 for 1 and –798 for 2]. The complexes exhibit 1H NMR spectra within δ 0–10 due to their S = 0 ground state. In MeCN solution they exhibit ligand field transitions in the range 14 300–16 600 cm–1 and phenolate-to-copper(II) charge-transfer transition at ≈22 700 cm–1. In MeCN solution each complex displays three consecutive irreversible responses (scan rate of 50 mV s–1) with Epc values (V vs. SCE) at –0.02, –0.54 and –0.86 (1) and 0.00, –0.42 and –0.80 (2). The first two responses are due to CuII–CuI and the most cathodic response to CuI–Cu0 redox processes, respectively.

A density functional study of the internal rotation in the quadruply bonded Mo 2Cl 4(PH 3) 4 complex

Internal rotation around the Mo–Mo bond in Mo 2Cl 4(PH 3) 4 is studied by DFT calculations with the B3LYP functional. The lowest singlet ( 1δ 2) and triplet ( 3δδ *) states are studied, full geometry optimization being performed for each value of the rotational angle ( χ). Compared to experimental data and to previous CASSCF calculations, the singlet–triplet energy separation is satisfactorily reproduced provided approximate spin-projected broken-symmetry calculations are used for the singlet state. The geometrical changes due to the internal rotation or the nature of the electronic state are discussed with respect to experimental data. Structural correlations are derived from the geometry optimizations for both singlet and triplet states.

Isoelectronic Arduengo-Type Carbene Analogues with the Group IIIa Elements Boron, Aluminum, Gallium, and Indium

While Arduengo-type carbenes are now well established for the Group IVa elements carbon, silicon and germanium, they are experimentally unknown for the isoelectronic anions with the Group IIIa elements boron, aluminum, gallium, and indium. In the present quantum chemical investigations, the bonding features of this class of compounds are explored. As a result, they are predicted to be stable species with sizeable singlet-triplet energy separations and electron affinities. Hence, they may be considered as valid targets for experimental investigations. An analysis of the electron density distribution in the case of boron and aluminum reveals a strongly polar B-N bond for the former, and a half Al-N bond with positive charge at the aluminum, emphasizing the donor-acceptor formulation for the latter.

On the electronic structure of dihalogenophosphenium cations

The bonding properties of the dihalogenophosphenium cations, PX 2 (+), X = F, Cl, Br, I, are evaluated by relativistic effective core potential calculations with a DZP basis set. Our investigations include an analysis of the singlet-triplet energy separation in the cations and the determination of the relative cation and anion stabilities in the gas phase, via corresponding group transfer reactions. A comparison is made with other stabilized, previously reported low-coordinated π-bonded phosphorus cations.

Configuration interaction and pairing in superconducting cuprates

We propose a quantum-mechanical effect specific of the symmetry of the Cu-O plane of cuprate superconductors. The mechanism considers hole pairs in single-particle degenerate states which, because of the configuration interaction, form singlet states with zero on-site repulsion (W = 0 pairs). In the many-body problem the effective interaction due to the spin exchange with the holes ofb1 symmetry is attractive. The exact diagonalization of the extended Hubbard Hamiltonian with 4 holes using well-established parameters for several clusters shows that the ground state has1B2(xy) symmetry and that binding energies are of the order of 10 meV. Unlike other electronic mechanisms proposed so far, this effect is due to the symmetry of the system and does not rely on off-site interactions; when the symmetry is broken the repulsion wins. Besides the correct symmetry of the pair, we obtain good agreement with experiment for the binding energies and the singlet-triplet energy separation of the quasiparticles.

The fluorine effect on the stability of phosphaalkenes, phosphasilenes, oxophosphane, thioxophosphane and their rearranged isomers

Ab initio molecular orbital calculations at the QCISD(T)/6-311G(d,p)//(U)MP2/6-31G(d,p) + ZPE level have been applied to determine the relative energies between the species featuring double bond to phosphorus, X 2C PX, X 2Si PX, XP O and XP S, and their rearranged products XC PX 2, X 3C P, XSi PX 2, X 3Si P, POX and PSX. The energy ordering between isomers is strongly dependent on the halogen substituents. When P is bonded to a more electronegative element, such as C, O and S, fluorination at P enormously stabilizes the corresponding species. The molecular stability is overwhelmingly dominated by the strength of the P F bond. When P is bonded to a more electropositive element, such as Si, a reversal of role occurs. The relative stability is now mainly determined by the Si F bond strength. Perfluorination amplifies the effect in such a way that in the SiPX 3 system, the P SiF 3 phosphinidene turns out to be the most stable isomer, making it an attractive target for a laboratory preparation. While the negative hyperconjugation effect in some cases plays a noticeable effect, the ‘cis-effect’ is not operative. The effect of chlorine has also been examined in both XPO and XPS cases. The singlet—triplet energy separations were also estimated for the carbenes, silylenes and phosphinidenes involved.

Density functional investigation of the molecular geometries, harmonic vibrational frequencies, singlet-triplet energy separations, adiabatic ionization potentials, and electron affinities of XY2 (X = Si, Ge, Sn; Y = F, Cl) systems

The geometrical and spectroscopic parameters of SiF2, SiCl2, GeF2, GeCl2, SnF2, and SnCl2 were determined using the linear combination of Gaussian-type orbitals-local spin density (LCGTO-LSD) method and employing both the local (LSD) and nonlocal functionals (NLSD) for the exchange-correlation energy. A general good agreement with available experimental information and with previous high-level correlated computations was found. Data on cations and anions are reported for the first time and can be used, together with those on neutral systems, to stimulate future and desirable experimental work on this significant class of molecules. © 1997 John Wiley & Sons, Inc.

Singlet-triplet energy separation and barrier for ring closure for trimethylenemethane and its complexes

The energy separations between the 3A′2 ground state and the excited electronic 1B1 and 1A1 states of trimethylenemethane (TMM) have been studied using quantum mechanical methods. The 1B1 minimum TMM is predicted to lie 15.7 kcal/mol higher in energy than the 3A′2 ground state at the TZ2P + f CISD + Q level. This is by far the highest level of theory employed to date to the vexing TMM singlet-triplet separation. The 1A1 minimum of TMM is 1.5 kcal/mol higher in energy than the 1B1 minimum at the DZP CISD + Q level. The transition state for ring closure of TMM to methylenecyclopropane has been located, with a barrier of 1.4 kcal/mol on the 1A1 surface at the DZP CISD + Q level. The above energies relative to the 3A′2 ground state for isolated TMM are much higher than Dowd's experimental maximum (7.8 kcal/mol) for the singlet-triplet separation. The discrepancy is discussed in the light of possible solvation effects. Direct evidence for the magnitude of the solvation effect has been obtained from a study of the TMM · methylenecyclopropane (TMM · MCP), TMM · H2CO, TMM · CO, and TMM · Ne complexes. There is essentially no change of the triplet-singlet energetics upon forming the TMM · Ne complex. However, the 3A′2-1A1 energy difference is 0.24 kcal/mol lower in the TMM · H2CO, 0.20 kcal/mol lower for the TMM · MCP complex, and 0.10 kcal/mol lower in the TMM · CO complex than for isolated TMM, respectively, at the DZP CISD + Q level. There is no barrier for ring closure for the TMM · H2CO complex on the 1A1 surface at the DZP TC-CISD level, even though the barrier is 0.2 kcal/mol at the DZP TCSCF level. It has been found that the electronic states of TMM interact with surrounding molecules in the order: 1B1 < 3A′2 < 1A1 state.

Methyl Substitution in Carbenes. A Theoretical Prediction of the Singlet-Triplet Energy Separation of Dimethylcarbene

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMethyl Substitution in Carbenes. A Theoretical Prediction of the Singlet-Triplet Energy Separation of DimethylcarbeneS. Matzinger and M. P. FuelscherCite this: J. Phys. Chem. 1995, 99, 27, 10747–10751Publication Date (Print):July 1, 1995Publication History Published online1 May 2002Published inissue 1 July 1995https://doi.org/10.1021/j100027a012RIGHTS & PERMISSIONSArticle Views173Altmetric-Citations53LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts