Ab Initio Quantum Chemical Investigation of the Spin States of Some Chain and Monocyclic Diradicals

Datta, Sambhu N.; Mukherjee, Prasun; Jha, Praket P.

doi:10.1021/jp0300344

Cited by 22 publications

(16 citation statements)

References 77 publications

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“…The 2/3 of an unpaired electron on each terminal site cannot be paired to one another under the alternancy-pairing condition (0) of pairing solely between sites of the same type (starred or unstarred), so that there are 2 fully unpaired spins and the ground state is a spin triplet. The result here also agrees largely with numerous ab initio quantum-chemical computations 39,40,41,42,43,44,45 , as well as experiment 46,47,48 .…”

Section: Application To Small Moleculessupporting

confidence: 89%

Mean-field resonance-theoretic view of benzenoid networks

Klein¹,

Ivanciuc²

2005

Arkivoc

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A simple classically based "mean field" resonance-theoretic approach is described to anticipate the effects of various modes of electron pairing (as in π-bond formation) for general "alternant" benzenoid conjugated π-networks. This simplified approach avoids generation and manipulation of individual resonance structures, whence application is facilitated, even for very large systems -so large that there might be hundreds or millions or moles or even substantially greater numbers of resonance structures. Some of the predictions so facilitated for several general circumstances are conveniently manifested as explicit algebraic formulas for numbers of unpaired electrons. The simplicity (and apparent reliability) of these algebraic formulas is emphasized; they involve no more than counts of different "kinds" of π-center carbons, e.g., the numbers of π-centers of different functionalities (primary, secondary, or tertiary) which are "starred" (in an "alternant" system) as well as those which are "unstarred". The argumentation also provides some information about rough locations for the (unpaired) spin densities, and ultimately also about the presence of low-lying excited states, magnetic moments, reactivities, and more. In particular these easily used ideas, which are extensions of those already familiar in "classical" organic chemistry should then aid in nano-technological developments.

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Section: Application To Small Moleculessupporting

confidence: 89%

Mean-field resonance-theoretic view of benzenoid networks

Klein¹,

Ivanciuc²

2005

Arkivoc

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“…Instead of considering bonded atoms as connected by springs here we assume that Table 1. Exact values of the thermal Green's function for the nonequivalent atoms in octatetraene as well those calculated by using formula (14). The labeling of the atoms is given in Figure 1.…”

Section: Discussionmentioning

confidence: 99%

“…In addition, according to the spin alternation rule [14][15][16], which states that the singlet spin pairing is preferred solely between sites in different subsets, the free valences on the starred and unstarred sites might be identified with "up" and "down" spin.…”

Section: Theoretical Modelmentioning

confidence: 99%

Atomic displacements due to spin–spin repulsion in conjugated alternant hydrocarbons

Estrada

Benzi

2013

Chemical Physics Letters

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We develop a theoretical model to account for the spin-induced atomic displacements in conjugated alternant hydrocarbons. It appears to be responsible for an enlargement of the distance between pairs of atoms separated by two atoms and located at the end of linear polyenes. It also correlates very well with the bond dissociation enthalpies for the cleavage of the C-H bond as well as to the spin density at carbon atoms in both open and closed shell at graphene nanoflakes (GNFs). Finally, we have modified the Schrödinger equation to study the propagation of the spin-induced perturbations through the atoms of GNFs.2

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“…23 As the species are very large, the UCCSD͑T͒ calculations could not be performed on the n-mers with reliable accuracy. Therefore, the J values could not be accurately determined.…”

Section: B the N-mersmentioning

confidence: 99%

On the variation of magnetic susceptibility of a molecular crystal with temperature: The 2,4,6-triphenylverdazyl system

Datta

Navada

2004

The Journal of Chemical Physics

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Magnetic susceptibilities of spin-1 2 systems of orthorhombic and higher crystal symmetries have been numerically investigated while taking possible anisotropy in the coupling constants along different crystal axes into account. The work relies on the magnon-based theory of ferromagnetic ͑FM͒ and antiferromagnetic ͑AFM͒ crystal systems of types FFF, AFF, AAF, and AAA ͓J. Chem. Phys. 111, 9009 ͑1999͔͒. The AAF crystal, in particular, shows interesting changes in the temperature dependence of magnetic susceptibility when the ferromagnetic exchange coupling constant is varied. We especially show that the susceptibility anomalies of molecular crystals fit naturally within the framework of the extended magnon-theoretical formalism, and do not necessarily imply a FM→AFM or a reverse phase transition. A real system, molecular crystal of 2,4,6-triphenylverdazyl ͑2,4,6-TPV͒, has been investigated here. It was previously interpreted as an AAF system from observed susceptibility data ͓Tomiyoshi et al., Phys. Rev. B 49, 16031 ͑1994͔͒. The trend of the temperature dependence of magnetic susceptibility studied in the present work also indicates that the crystal belongs to the AAF category with a less prominent FM exchange coupling constant. To reinforce our conclusions, we have adopted a two-pronged strategy. First, the geometry of the 2,4,6-TPV monomer has been optimized here by ab initio unrestricted Hartree-Fock ͑UHF͒ calculations using the STO-3G basis set. The optimized geometry is almost planar. A subsequent calculation has been carried out with the phenyl rings twisted out of the plane of the nitrogen atoms. The STO-3G optimized geometry, and the same geometry except for the twisted phenyl rings, have been used to perform ab initio coupled-cluster ͑UCCSD-T͒ calculations with the same basis, and UHF as well as density-functional ͑UB3LYP͒ calculations using the 6-31G basis set. The calculated data can easily rationalize the twists while the species remains in crystal. The magnetic category of the crystal has been unambiguously confirmed as AFA from ab initio UHF and UB3LYP calculations of the total energy in different spin states of dimers and trimers along the crystal axes. The computed energy values, however, fail to yield accurate estimates of the exchange coupling constants J a , J b , and J c , because the latter are on the order of 1k B K corresponding to energy differences on the order of 10 Ϫ6 hartree between different spin states. In the second approach, the observed features of the susceptibility minimum and maximum have been used to determine the best values of the exchange coupling constants from the theoretical formulas for an anisotropic AFA crystal. The AFM (J a and J c ) and FM (J b ) exchange coupling constants and the Néel temperature (T N ) found from this analysis correspond to J a ϩJ c ϭϪ1.05 k B K, J b ϭ1.35 k B K, and T N ϭ1.75 K. The calculated J values significantly differ from those estimated from a linear Heisenberg chain model, but generate a susceptibility versus temperature graph that mimics...

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Ab Initio Quantum Chemical Investigation of the Spin States of Some Chain and Monocyclic Diradicals

Cited by 22 publications

References 77 publications

Mean-field resonance-theoretic view of benzenoid networks

Mean-field resonance-theoretic view of benzenoid networks

Atomic displacements due to spin–spin repulsion in conjugated alternant hydrocarbons

On the variation of magnetic susceptibility of a molecular crystal with temperature: The 2,4,6-triphenylverdazyl system

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