Spin density and spin correlation in triplet states

McLachlan, Anton

doi:10.1080/00268976200100061

Cited by 67 publications

(5 citation statements)

References 35 publications

(39 reference statements)

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“…It should be noted that this approximation is based strictly on distance and positive spin densities; spin-orbit coupling and negative spin densities have not been taken into account. The neglect of negative spin densities, 10 which are significant in both MPH and TMM, might account for the large deviation observed in the calculated D values. Although this method is strictly empirical and limited in its scope for predicting reasonable D values to non-disjoint biradicals (without an empirical factor) and disjoint biradicals (with an empirical factor), it does represent an improvement for calculating D values for the biradicals that do fall into these categories.…”

mentioning

confidence: 87%

“…Figure 1 is a plot of the calculated D values using Eqn (1) (D nonav ) (where, again, i and j refer to the non-disjoint SOMOs), and Eqn (2) (D av ) versus the experimental D values for biradicals 1-8. We included less stable carbon-based biradicals and other biradicals that are not bisphenoxy-or bisnitroxy-based (hereafter referred to as non-bisoxy biradicals) (9)(10)(11)(12)(13)(14) in this investigation to see how applicable the approximation method was for other delocalized non-disjoint biradicals. Figure 2 is a plot of all of the compounds given in Scheme 1.…”

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confidence: 99%

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Improved method for estimating the ZFS parameterD for delocalized biradicals

Sandberg

Shultz

1998

J. Phys. Org. Chem.

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Calculating zero-field splitting (ZFS) parameters, D and E, as a function of molecular geometry is one of the few methods available for elucidating the conformations of organic molecules having more than one unpaired electron.1,2 The ZFS parameter D gives an indication of the dipolar interaction of unpaired electrons in molecules in the absence of spin-orbit coupling and therefore is related to interelectronic distance.Rigorous theoretical determinations of D using highquality ab initio wavefunctions have been performed for the excited-triplet states of formaldehyde, 3 benzene and naphthalene 4 and ground-triplets states of methylene 5 and trimethylenemethane. 6 Although it is theoretically possible to determine D with ab initio basis sets and electron correlation, it may not be computationally practical-a fact which has led to the derivation of approximate methods to simplify the task of evaluating the large number of integrals involved for even moderatesized molecules.7 These various approximations used for the theoretical determination of D have been presented in the literature over the last several decades. 1,4,[8][9][10][11][12][13][14][15][16][17][18][19][20][21] An approximation which is frequently used by Mukai and co-workers 1,19,20 iswhere r ij is the distance between atoms i and j, m ij is the distance vector along the axis which gives rise to the largest dipole-dipole interaction and r i and r j are the spin densities on atoms i and j. This equation has been used to give reasonable D value approximations for biradicals with localized electron distributions that can be separated into monoradical halves with atoms i and j belonging to different halves (hereafter referred to as localized biradicals.) The spin densities used are those of each monoradical half. It has been noted that this equation provides only crude approximations for delocalized biradicals, 21 which initiated an investigation in our laboratory into finding a simple approximate method that would provide more reasonable D values for delocalized biradicals owing to our interest in moderate-sized nondisjoint organic molecules with S b 1/2. 22 The problem we encountered in trying to use Eqn (1) for delocalized radical systems was that the delocalized system could not be separated into monoradical halves due to shared spincontaining atoms (hereafter referred to as delocalized biradicals). Since the dipole-dipole interaction, and thus the D value, strongly depend on the average distance between the two unpaired electrons (varying as 1/r where atom i is a spin-containing atom (spin density !0.01) in one SOMO and atom j is a spin-containing atom (also with spin density !0.01) in the other SOMO, and r ij refers to the distance between spin-containing

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confidence: 87%

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confidence: 99%

Improved method for estimating the ZFS parameterD for delocalized biradicals

Sandberg

Shultz

1998

J. Phys. Org. Chem.

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“…If we retain the Du nuclear geometry, but require the individual orbitals to transform according to the lower symmetry point group C2", we now have three electron configurations. la/ 2a,2 lb22 3a,2 4a,2 5a,22b22 6a,2 7a,2 3b/ 4b/ 8a,2 5b/ lb,2 la2 2b, « 'B, (5) (2)-V<la/ 2a/ lb/ 3a/ 4a/ 5a/ 2b/ 6a/ 7a/ 3b/ 4b/ 8a/ 5b/ lb/ la/) ± (2)-V<la/ 2a/ lb/ 3a/ 4a/ 5a/ 2b/ 6a/ 7a/ 3b/ 4b/ 8a/ 5b/ lb/ 2b/) 'A, (6) The correspondence between Dih and C2" electronic…”

Section: Theoretical Aspectsmentioning

confidence: 99%

Triplet electronic ground state of trimethylenemethane

Yarkony¹,

Schaefer²

1974

J. Am. Chem. Soc.

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“…This resuh has been independently confirmed by Van der Waals [12] and underlines the fact that a wave function which gives a significant improvement in the calculated energy may give poor values for the expectation values of other operators, especially two electron operators [12]. The occurrence of negative spin densities in triplet states [4,19] also shows that configuration is necessary even with open shell type wave functions. Nevertheless, it is perhaps surprising that this type of wave function gives the poorest agreement with experiment.…”

Section: Discussionmentioning

confidence: 65%

Zero-field splittings in aromatic triplet states

Thomson

1966

Molecular Physics

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The zero-field splitting parameters of the triplet state, D, E and D*, have been calculated in the two configuration approximation for the hydrocarbons pyrene and 1,12-benzperylene, and the results compared with the experiment. For calculations on molecules of this size it is a reasonable approximation to neglect two centre dipolar interaction integrals between 7r-electron centres more than 12 Bohr radii apart. Results in a one configuration approximation for the naphthalene molecule using open sheU-SCF-MO's are also compared with other calculations and experiment.

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Spin density and spin correlation in triplet states

Cited by 67 publications

References 35 publications

Improved method for estimating the ZFS parameterD for delocalized biradicals

Improved method for estimating the ZFS parameterD for delocalized biradicals

Triplet electronic ground state of trimethylenemethane

Zero-field splittings in aromatic triplet states

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