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Volumn 127, Issue 15, 2005, Pages 5552-5562

Role of conformation and electronic structure in the chemistry of ground and excited state o-pyrazolylphenylnitrenes

Author keywords

[No Author keywords available]

Indexed keywords

CONFORMATIONS; ELECTRONIC STRUCTURE; MOLECULAR DYNAMICS; PHOTOCHEMICAL REACTIONS; REACTION KINETICS;

EID: 17744362499     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja043988w     Document Type: Article
Times cited : (33)

References (61)
  • 11
    • 0000430416 scopus 로고
    • Abramovitch, R. A., Ed.; Plenum Press: New York
    • Wentrup, C. In Reactive Intermediates; Abramovitch, R. A., Ed.; Plenum Press: New York, 1980; Vol. 1; p 263.
    • (1980) Reactive Intermediates , vol.1 , pp. 263
    • Wentrup, C.1
  • 13
    • 0004260899 scopus 로고
    • Wiley-Interscience: New York, Chapter 4
    • Wentrup, C. In Reactive Molecules; Wiley-Interscience: New York, 1984; Chapter 4.
    • (1984) Reactive Molecules
    • Wentrup, C.1
  • 17
    • 2542495781 scopus 로고    scopus 로고
    • Moss, R. A., Platz, M. S., Jones, M., Eds.; Wiley: New York
    • Platz, M. S. In Reactive Intermediate Chemistry; Moss, R. A., Platz, M. S., Jones, M., Eds.; Wiley: New York, 2003.
    • (2003) Reactive Intermediate Chemistry
    • Platz, M.S.1
  • 38
    • 17744375743 scopus 로고    scopus 로고
    • note
    • The B3LYP/6-31G*-calculated Arrhenius activation energies for the rotation of the pyrazolyl groups are 2.76 kcal/mol (1a(NH) → 1a(NN)) and 0.52 kcal/mol (1a′(NH) → 1a′(NN)) at 298 K, and those for rotation of the azide group are 4.31 kcal/mol (1a(NN) → 1a′(NN)) and 3.68 kcal/mol (1a(NH) → 1a′(NH); full details are given in the Supporting Information). Thus, the four conformations of azide 1a equilibrate rapidly at room temperature.
  • 39
    • 17744371106 scopus 로고    scopus 로고
    • note
    • 32a.
  • 40
    • 17744364457 scopus 로고    scopus 로고
    • note
    • s symmetry, so only this conformation is considered here.
  • 41
    • 17744371305 scopus 로고    scopus 로고
    • note
    • Note that the CASSCF dipole moment of 2a(NN) is much higher than that of 2a(NH) (4.3 vs 1.7 D). Thus, the energy difference between the two conformers may be strongly attenuated in polar solvents, e.g. ethanol.
  • 42
    • 17744383543 scopus 로고    scopus 로고
    • note
    • The extent of this effect depends quite strongly on the size and the nature of the active space in the CASSCF calculations. Removing the σ lone pair on the nitrene N atom from the active space does not affect the energy difference between the two conformers in the triplet state but reduces that difference in the singlet state from 6.4 to 4.5 kcal/mol (and increases the S/T gap in the H-bonded conformer to 20.4 kcal/mol, only 1 kcal/mol less than in the other conformer).
  • 43
    • 17744394245 scopus 로고    scopus 로고
    • note
    • The transition states were found to be stationary points in a saddle point search. Unfortunately, we were unable to perform frequency calculations at the CASSCF(14,12) level to ascertain the nature of these stationary points and to obtain vibrational data for thermal corrections. However, the transition state found for the triplet nitrene looks very similar to that found (and characterized) by B3LYP, so we have reason to be confident that we have actually located true saddle points also by CASSCF.
  • 44
    • 17744398393 scopus 로고    scopus 로고
    • note
    • To convince ourselves that we had not missed a secondary minimum, we carried out relaxed scans of the dihedral angle, the results of which are summarized in Figure S1 of the Supporting Information.
  • 45
    • 17744376749 scopus 로고    scopus 로고
    • note
    • 12b equilibrate rapidly at room temperature.
  • 47
    • 17744362251 scopus 로고    scopus 로고
    • note
    • a, corresponds to an enthalpy difference (augmented by RT for a unimolecular reaction).
  • 48
    • 17744375336 scopus 로고    scopus 로고
    • note
    • One may argue that these differences between the parent system and the dimethyl derivative come about by an error in the calculation of the singlet/ triplet gap. However, to explain a 5 kcal/mol difference in activation energies would require the relative S/T gap of the parent system and the dimethyl derivative to be in errror by a similar amount. In the absence of H-bonds, the S/T gaps of the two compounds are calculated to be about the same (21 ± 0.2 kcal/mol), which is reasonable. Thus we believe that the differences in exothermicities and activation barriers for cyclization to azirines are not artifactual.
  • 50
    • 0001015446 scopus 로고
    • Photoinduced imermolecular H-abstractions of triplet nitrenes are known, cf.: Levya, E.; Munoz, D.; Platz, M. S. J. Org. Chem. 1989, 54, 5938.
    • (1989) J. Org. Chem. , vol.54 , pp. 5938
    • Levya, E.1    Munoz, D.2    Platz, M.S.3
  • 51
    • 17744371616 scopus 로고    scopus 로고
    • note
    • The primary product of hydrogen abstraction from the methyl group is biradical 18. This turns out to be an interesting species, because it has an open-shell singlet ground state that is almost degenerate with the corresponding triplet state. The corresponding closed-shell singlet state (which is what is obtained in a DFT calculation) corresponds to a polyeneimine and lies vertically about 22 kcal/mol above the open-shell singlet ground state of 17. However, on geometry optimization that state ends up only ca. 5 kcal/mol above that state (all CASSCF/CASPT2 results). We did not succeed in finding a transition state for the cyclization of the open-shell singlet state to dihydroquinoxaline 8 (all attempts to shorten the distance betweeen the two radical centers resulted in collapse to 8), so we assume that this process is essentially barrierless.
  • 52
    • 3142763621 scopus 로고    scopus 로고
    • Moss, R. A., Platz, M. S., Jones, M., Eds.; Wiley: New York
    • Bally, T. In Reactive Intemediate Chemistry; Moss, R. A., Platz, M. S., Jones, M., Eds.; Wiley: New York, 2003.
    • (2003) Reactive Intemediate Chemistry
    • Bally, T.1


* 이 정보는 Elsevier사의 SCOPUS DB에서 KISTI가 분석하여 추출한 것입니다.