908 J. Phys. Chem. A, Vol. 102, No. 6, 1998
Ng et al.
Finally, 3,5-ethynylpyridine can be considered an intermediate
case with a single n-π′cc interaction. However, band overlap
precludes a detailed analysis.
Conclusion
The order of ionic states in pyridine (n < π < π) is different
from ethynylpyridines, where HOMO ionization corresponds
to π-orbital ionization. Since Koopmans’ (and even non-
Koopmans’) approaches failed for pyridine, we have based our
analysis on He I/He II band intensities rather than MO
calculations. The latter were used only for a qualitative
description of electron distributions in ionic states.
We have provided experimental evidence for intramolecular
n-π′cc interactions. The interactions are reflected in orbital
mixing in 2- and 2,6-derivatives, as shown in Figure 5. Our
results are also useful in predicting gas-phase (Lewis) basicities.
The basicity among monoethynylpyridines should follow the
sequence 4 > 3 > 2. The order for diethynylpyridines should
be 2,5 > 3,5 > 2,6. In fact, in the absence of UPS data, the
basicity can be used as an indicator of such interactions.
The UPS study of 2-halopyridines3a could provide an answer
to the possible occurrence of similar interactions with a halogen
substituent. Unfortunately, band overlap and inferior resolution
in the He II spectra make the answer merely suggestive.
It appears that the intensity of the n band is not enhanced
significantly on going from He I to He II. This could be due
to an admixture of halogen np character, because it is well-
established that Cl, Br, and I np-1 ionization cross sections
decrease very significantly upon increasing the photon energy.
Acknowledgment. This research was supported by Grant
RP900624 from the National University of Singapore. X.Y.
thanks the University for a postgraduate scholarship.
References and Notes
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Figure 5. Some MOs in 2- and 2,6-ethynylpyridine which show n-π′cc
mixing (interaction).
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but not in 3- and 4-ethynylpyridine. The n-π′cc interaction
introduces some N 2p character into π′cc and the intensity
variations of C˜ and D˜ will be reversed on going from the 2- to
the 3- and 4-ethynyl derivatives. Deductions about n-π′cc
interactions in the spectra of 2,6-ethynylpyridine and 3,5-
ethynylpyridine can be made simultaneously. In the 2,6-
derivative only minor differences occur between the intensities
of the π′′cc and π′cc ionizations, while in the 3,5-derivative
differences of He I/He II intensities can be easily observed.
A tentative rationalization assumes that n-π′cc interactions
are only possible in the 2,6- and not in the 3,5-derivative,
implying an increase of N 2p character of the π′cc orbitals in
2,6-ethynylpyridine and a reduction of differences between the
orbital characters of π′cc and π′′cc. In the 3,5-derivative there
are no through-space interactions and the π′cc and π′′cc orbitals
retain their distinct compositions, a fact reflected in the band
intensity changes.
and Asymmetry Parameters; Gordon and Breach: Langhorne, 1993.