Pair Association in Matrix Isolated Pyridine

Absorption spectra of pyridine‐H5 in frozen nitrogen exhibit concentration dependent sidebands due to pairwise interactions in the Λ10 region at 606 and 607.5 cm−1, in the Λ26 region at 706 and 709 cm−1, and in the Λ14 region at 1442 and 1443 cm−1. For pyridine‐D5 pair absorption bands were observed at 584.5, 586(Λ10), 533, 535(Λ26). Calculations of exciton splittings and shifts for various pair configurations using nonbonded atom–atom potentials including short‐range repulsive, dispersion, and electrostatic terms were performed. The results of the calculations predict that pairs of pyridine molecules face each other with the nitrogen atom opposite to the hydrogen atom of the partner, the distance separating them being 3.4 A.