The first magic angle spinning NMR spectrum of a captive intermediate: direct observation of a singlet ground state biradical, 3,4-dimethylenefuran

Zilm, Kurt W.; Merrill, Ron A.; Greenberg, Marc M.; Berson, Jerome A.

doi:10.1021/ja00239a046

Cited by 36 publications

(6 citation statements)

References 3 publications

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“…This is in agreement with the experimental observation that the singlet-triplet splitting is negative for 6. [66] The data in Table 2 together with the information provided by Figure 6 confirm what was discussed in the case of the benzyne biradicals. The hybrid functionals always lead to a stronger explicit account of multi-reference effects, which is clearly due to the fact that the self-interaction error is reduced by the admixture of exact exchange and, by this, also the long-range correlation effects simulated by the exchange functional.…”

Section: Is the Difference [R(c2−c3) − R(c2−c1)] S − [R(c2−c3) − R(c2supporting

confidence: 79%

Implicit and Explicit Coverage of Multi-reference Effects by Density Functional Theory

Cremer

Filatov²,

Polo³

et al. 2002

IJMS

112

135

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Multi-reference effects can be covered by density functional theory (DFT) either implicitly via the exchange-correlation functional or explicitly via the form of the Kohn-Sham wave function. With the help of the exchange hole it is shown that the self-interaction error of the exchange functional will mimic long-range electron correlation effects if restricted Kohn-Sham theory is used. Functionals based on Slater or Becke exchange have a relatively large self-interaction error and, therefore, lead to a relatively large implicit coverage of long-range correlation, which, because of the possibility of doublecounting of electron correlation, has to be considered when using these functionals in connection with two-or multi-configurational descriptions based on ensemble DFT methods such as REKS (spinRestricted Ensemble-referenced KS-DFT). Arguments are given that a REKS description of a multireference problem avoids a double-counting of long-range correlation effects, in particular as in this situation the self-interaction error of the exchange functional simulates more short-rather than longrange correlation effects. There is, however, no guarantee that the short-range effects are not doublecounted, namely once via the exchange and once via the correlation functional. Therefore, one should use hybrid functionals such as B3LYP in connection with multi-reference DFT methods because for hybrid functionals the self-interaction error and by this the implicit coverage of long(short)-range correlation effects is reduced due to the admixture of exact exchange. This rule applies also to broken-symmetry UDFT, which performs better with hybrid rather than GGA functionals. A way of avoiding the implicit coverage of multi-reference effects is given by the combination of wave function theory and DFT methods. The advantages and disadvantages of CAS-DFT are discussed and it is shown that an effective reduction of a double-counting of correlation effects is possible within this method.

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Section: Is the Difference [R(c2−c3) − R(c2−c1)] S − [R(c2−c3) − R(c2supporting

confidence: 79%

Implicit and Explicit Coverage of Multi-reference Effects by Density Functional Theory

Cremer

Filatov²,

Polo³

et al. 2002

IJMS

112

135

View full text Add to dashboard Cite

show abstract

“…NMR spectroscopy of matrix isolated species was first reported in 1978 [24,25], including the determination of a spectrum consistent for (HCl)2 in solid 40 Ar with a linear hydrogen bond and a Cl ...Cl separation of 3.7 Å, with the second unit 122° to the first HCl molecule [25]. Subsequent developments include the study of o-H2 in solid Ne, Ar, Kr and p-H2 [26], 13 C CPMAS studies of photochemically generated molecules in frozen organic glasses [27,28], the design and application of an NMR spectrometer capable of working at 26 K [29] and the study of N2/Ar mixtures [30][31][32][33]. Although it is possible to use 13 C cross-polarisation spectra from species containing a pair of adjacent 13 C nuclei to reflect the 13 C- 13 C dipolar coupling of matrix isolated organic species to obtain interatomic distances, this has not found widespread use.…”

Section: Matrix Isolationmentioning

confidence: 99%

The application of synchrotron radiation and in particular X-ray absorption spectroscopy to matrix isolated species

Young

2014

Coordination Chemistry Reviews

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This review provides comprehensive coverage of the application of X-ray absorption spectroscopy (XAS, XAFS, EXAFS, XANES) to matrix isolated species. As X-ray absorption spectroscopy provides structural data without the need for long range order it has been applied to a large number of systems to yield unique structural data about both the matrix isolated species, and their interactions with the matrix host. To put the work into perspective there is a tutorial introduction to the theoretical background of X-ray absorption spectroscopy, data content, processing and analysis. In addition there is brief coverage of the use of other synchrotron radiation techniques for the study of matrix isolated species, and a consideration of future perspectives.

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“…Ab initio calculations for this species predict a twist angle of zero degrees (i.e., a planar diradical). 74, 76 Additionally, the ground state of a furano analog of TME had been demonstrated to be the singlet by Zilm et al 77 To explain these trends, Nash, Dowd, and Jordan 78 performed small MRCI calculations for these, TME, and four other analogous systems (Fig. 6), and showed that (i) the employed level of theory successfully predicted the correct ground state for all of the known systems (TME itself was restricted to planarity and predicted to have a singlet ground state at that geometry, a situation not really addressed by experiment but included for consistency with the other planar diradicals) and (ii) the calculated splittings correlated reasonably well (quadratically) with the restricted open-shell Hartree-Fock (ROHF) orbital energy splittings between the two SOMOs in the triplets (thereby providing in principle a simple method to predict S-T splittings in new TME derivatives).…”

Section: Unsolved Mysteriesmentioning

confidence: 99%

Paul Dowd and diradicals

Cramer¹

1998

J. Chem. Soc., Perkin Trans. 2

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The first magic angle spinning NMR spectrum of a captive intermediate: direct observation of a singlet ground state biradical, 3,4-dimethylenefuran

Cited by 36 publications

References 3 publications

Implicit and Explicit Coverage of Multi-reference Effects by Density Functional Theory

Implicit and Explicit Coverage of Multi-reference Effects by Density Functional Theory

The application of synchrotron radiation and in particular X-ray absorption spectroscopy to matrix isolated species

Paul Dowd and diradicals

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