TY - JOUR

T1 - The electron-pair origin of antiaromaticity

T2 - Spectroscopic manifestations

AU - Zilberg, Shmuel

AU - Haas, Yehuda

PY - 1999

Y1 - 1999

N2 - It is shown that the antiaromatic character of certain conjugated cyclic hydrocarbons is due to the presence of an even number of distinct electron pairs in the system (such as, but not necessarily π electrons). In these systems, the ground state is constructed from an out-of-phase combination of two valence bond (VB) structures, and its equilibrium geometry is necessarily distorted along the coordinate that interchanges these structures. If a new symmetry element appears during the transition between the two structures, the ground electronic state at the symmetric point transforms as one of the nontotally symmetric irreducible representations of the point group. The conjugate excited state, formed from the in-phase combination of the same two structures, transforms as the totally symmetric representation of the group and is strongly bound. Its structure is similar to that of the ground state at the symmetric point, and the energy separation between the two states is small compared to that of conjugated cyclic hydrocarbons having an odd number of distinct electron pairs. Motion along the "Kekulé-type" vibrational mode on the excited-state potential surface is very similar to motion along the reaction coordinate connecting the two distorted structures on the ground-state surface. It is characterized by a significantly higher vibrational frequency compared to frequencies of similar modes in ground-state molecules. These qualitative predictions are supported by quantum chemical calculations on cyclobutadiene, cyclooctatetraene, and pentalene.

AB - It is shown that the antiaromatic character of certain conjugated cyclic hydrocarbons is due to the presence of an even number of distinct electron pairs in the system (such as, but not necessarily π electrons). In these systems, the ground state is constructed from an out-of-phase combination of two valence bond (VB) structures, and its equilibrium geometry is necessarily distorted along the coordinate that interchanges these structures. If a new symmetry element appears during the transition between the two structures, the ground electronic state at the symmetric point transforms as one of the nontotally symmetric irreducible representations of the point group. The conjugate excited state, formed from the in-phase combination of the same two structures, transforms as the totally symmetric representation of the group and is strongly bound. Its structure is similar to that of the ground state at the symmetric point, and the energy separation between the two states is small compared to that of conjugated cyclic hydrocarbons having an odd number of distinct electron pairs. Motion along the "Kekulé-type" vibrational mode on the excited-state potential surface is very similar to motion along the reaction coordinate connecting the two distorted structures on the ground-state surface. It is characterized by a significantly higher vibrational frequency compared to frequencies of similar modes in ground-state molecules. These qualitative predictions are supported by quantum chemical calculations on cyclobutadiene, cyclooctatetraene, and pentalene.

UR - http://www.scopus.com/inward/record.url?scp=0001022775&partnerID=8YFLogxK

U2 - 10.1002/(SICI)1097-461X(1999)71:2<133::AID-QUA2>3.0.CO;2-P

DO - 10.1002/(SICI)1097-461X(1999)71:2<133::AID-QUA2>3.0.CO;2-P

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AN - SCOPUS:0001022775

SN - 0020-7608

VL - 71

SP - 133

EP - 145

JO - International Journal of Quantum Chemistry

JF - International Journal of Quantum Chemistry

IS - 2

ER -