Is tetramethyleneethane a ground state triplet?

Chakrabarti, Aparna; Albert, I. D. L.; Ramasesha, S.; Lalitha, S.; Chandrasekhar, Jayaraman

doi:10.1007/bf02841350

Cited by 6 publications

(11 citation statements)

References 33 publications

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“…Ab initio calculations on species 1 − 7 were performed at various levels, namely, UHF, R(O)HF, UB3LYP, R(O)B3LYP, UCCSD, and UCCSD(T), by employing STO-3G, split-valence as well as a few polarized basis sets. In every case, the singlet−triplet energy gap varies with the rigor of calculation as well as the basis set, and this observation is in agreement with the trends noticed earlier in refs , a, c, and 14h. The UHF methodology generally yields the spin-contaminated geometry, and fails to give the correct singlet−triplet energy gap.…”

Section: Energy Differencessupporting

confidence: 91%

Ab Initio Quantum Chemical Investigation of the Spin States of Some Fused Ring Systems

2004

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The ground-state spins of seven diradicals belonging to the fused ring system have been investigated by ab initio restricted and unrestricted formalisms. The systems under study are (1) 4-oxy-2-naphthalenyl methyl, (2) 1,8-naphthalenediylbis(methyl), (3) 8-imino-1-naphthalenyl methyl, (4) 1,8-naphthalenediylbis(amidogen), (5) 8-methyl-1-naphthyl carbene, (6) 8-methyl-1-naphthalenyl imidogen, and (7) 8-methyl-1-naphthyl diazomethane. Out of the seven molecules, only 1 was theoretically investigated earlier. To our knowledge, for 2-7, this work represents the first ab initio investigation. A variety of basis sets have been employed in these calculations. For each spin state, the molecular geometry has been fully optimized at the unrestricted Hartree-Fock (UHF) level using the STO-3G, 4-31G, 6-311G(d), and 6-311G(d,p) basis sets. The UHF optimized geometries have been used for Møller-Plesset (MP) and coupled cluster (CC) calculations as well as the density functional (UB3LYP) treatment. Results in the unrestricted formalism have been given only at UHF and UB3LYP levels for the 6-311G(d) basis. The UHF calculations yield an unrealistically large singlettriplet (S-T) splitting. Splittings calculated with different bases disagree seriously. The S-T gap is smaller in the split-valence bases. The basis set truncation error can be considerably overcome by calculations involving electron correlation. For these diradicals, any meaningful result would require larger bases with polarization functions. Apart form this difficulty, the optimized geometry turned out to be highly spin-contaminated. The spin-contamination can be significantly reduced by the density functional UB3LYP treatment. Nevertheless, for most of the diradicals, the UB3LYP method did not yield a systematic trend. To avoid spin contamination completely, we have repeated computations in the restricted (open-shell) Hartree-Fock framework. Geometry optimizations were carried out using p) bases at the R(O)HF level and 6-311G(d,p) basis at the R(O)B3LYP level for each spin state. The R(O)B3LYP/6-311G(d,p) optimized geometry yields the best total energy for each spin state and hence the most reliable S-T energy difference. Molecules 1-6 are found as ground-state triplets. The calculated results are in agreement with the available experimental findings. Molecules 3 and 7 have widely different geometries in the singlet and triplet states. The calculations using 6-311G(d) and 6-311G(d,p) basis sets show that in molecule 3 the substituents of naphthalene are -NH 2 and -CH in singlet but -NH and -CH 2 in triplet. The two optimized geometries are tautomeric forms. Molecule 7 is expected to be either a ground-state triplet with a very little S-T gap or a ground-state singlet. This prediction is borne out by the computed results. The R(O)B3LYP/6-311G(d,p) calculation yields a S-T splitting of -21.9 kcal mol -1 . The singlet state becomes stabilized by forming an additional condensed ring. The UHF spin density plots obtained from the 4-31G optimized geometries manifest the ...

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Section: Energy Differencessupporting

confidence: 91%

Ab Initio Quantum Chemical Investigation of the Spin States of Some Fused Ring Systems

2004

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“…The idea of spin alternation is quite native in valence bond theory. In fact, several researchers have prepared their valence bond methodologies by considering alternative up and down spins on successive atoms and then averaging the results with a balanced wave function for the singlet state. 25c,,,, Although in most cases the valence bond procedure would show with alacrity spin alternation in the triplet state when the triplet is the ground state, the situation for a singlet ground state remains justifiably unclear. For a long time, the molecular orbital treatment remained largely silent on this issue.…”

Section: Spin Alternation In Uhfmentioning

confidence: 99%

“…Theoretical investigations carried out on molecules 1a − c have been fairly extensive. Several authors have investigated TMM, ,,− TME, ,,− and m -xylylene. ,,− General theoretical 38-47,48a and experimental 48a,b work has been reported on a number of other diradicals containing π electrons. The systematic investigation of radical species by Klein and co-workers 44a-k employing different methodologies is of general interest.…”

Section: Introductionmentioning

confidence: 99%

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

2003

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Ab initio calculations have been performed on nine organic diradicals to find the spin multiplicities in their electronic ground states. Three diradicals, namely, trimethylene methane (TMM), tetramethylene ethane (TME), and m-xylylene, were previously investigated in detail by various authors. These have been used as test cases so as to establish the reliability of the calculated results. The basis sets used in this work are mainly STO-3G and 4-31G. For the smaller molecules, the 6-31G basis set has also been tried. In every case, the molecular geometry has been optimized at the unrestricted Hartree−Fock (UHF) level for each basis set and for each spin state. Calculations have also been performed at the post-Hartree−Fock Møller−Plesset (MP) and coupled-cluster (CCSD) levels. Results have been quoted only for the UHF, UCCSD, and UCCSD(T) levels because the MP-level calculations do not generate reliable singlet−triplet energy gaps. The UHF calculation generally yields an unrealistically large splitting, but the UCCSD and especially the UCCSD(T) methods reduce the gap to a significant extent. From a comparison with the best results already reported on the test cases, we find that the UCCSD(T) singlet−triplet energy gaps are of the correct order. In fact, the singlet−triplet energy gaps calculated by the UCCSD(T) method using the 4-31G basis sets are more or less in agreement with the results of previous detailed investigations on TME and m-xylylene. Six more diradicals, two of them of the linear chain type (3-methylene pentane-2,4-diyl and tris(methylimino)methane) and four of the monocyclic variety (2-isopropylidene cyclopentane-1,3-diyl, 2,3-bis(methylene) cyclohexane-1,4-diyl, 3-methylene phenoxyl, and tetramethyl m-xylylene), have been investigated here. To our knowledge, this work is the first report on an ab initio post-Hartree−Fock calculation of the spin states of these six species. Out of the six diradicals, one has a singlet ground state, and the rest are ground-state triplets. These findings agree with the experimental observations without fail. The UCCSD(T)/4-31G results on tris(methylimino) methane, for which the hyperfine splitting spectrum is available, can explain the number of lines as well as the average hyperfine coupling constant. The molecule 2,3-bis(methylene) cyclohexane-1,4-diyl has been found to have almost degenerate singlet and triplet states, as observed experimentally. The UCCSD(T)/4-31G singlet−triplet gap is −0.84 kcal mol-1 for this species. The UHF spin-density plots show that the ground states of all nine diradicals can be successfully predicted by the rule of spin alternation in the UHF treatment.

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“…The calculations also support the notion of a singlet ground state for TME. 22 It is suggested22 that perhaps the observed ESR spectrum represents a trapped excited state of TME following photochemical excitation. With the observation of TME at 15 K and above being on the order of hours, this is not a tenable position.…”

Section: Synthesismentioning

confidence: 99%

“…The infrared spectrum (neat) showed bands at 3423 (m), 2538 (m), 2233 (m), 2125 (m), 1225 (w), 1362, 1120, and 1043 cm-1. The mass spectrum (70 eV) showed peaks at m/e (rel intens): 112 (3, M+ -CD3), 93 (22), 74 (14), 65 (100), 46 (69). Anal.…”

Section: Experimental Partmentioning

confidence: 99%

Tetramethyleneethane-d8

Dowd¹,

Chang²,

Partian³

et al. 1993

J. Phys. Chem.

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Tetramethyleneethane (TME) is a triplet non-KekulC molecule that is currently the source of a difference between theory and experiment. The best theory places the energy of the triplet about 1 kcal/mol below the singlet at an angle of twist of 50° about the central carbon4arbon bond. However, the triplet ESR spectrum shows no splitting between the x-and y-lines, indicating that the zero field parameter, E, is 0 or very small. If it is the former, that would be consistent with a D u configured TME. In order to explore this point by removing proton hyperfine coupling, TME-ds was prepared. Again, no splitting was observed between the xand y-lines for TME-dg. This sets an upper limit for E of approximately 0.001 cm-l.

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Is tetramethyleneethane a ground state triplet?

Cited by 6 publications

References 33 publications

Ab Initio Quantum Chemical Investigation of the Spin States of Some Fused Ring Systems

Ab Initio Quantum Chemical Investigation of the Spin States of Some Fused Ring Systems

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

Tetramethyleneethane-d8

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