Abstract
Using ab initio cubic force constants, estimates are made for four higher order interaction matrix elements recently determined recently from the infrared spectrum. The predicted (observed) values for the matrix elements are as follows: 〈ν2‖Heff‖ν3 + 2ν±24Δe〉 = 6.4(5.4) × 10−5 cm−1 ×√J(J+1)[J(J+1)−2], 〈2ν3‖Heff‖3ν4 + ν5(∑+)〉 = −0.14(0.24) cm−1, 〈3ν4 + ν5(Δε)‖Heff‖2ν3〉 = 3.1(3.2) × 10−5 cm−1 × J(J + 1), 〈2ν2‖Heff‖ν1 + 2ν04〉 = 4.0(3.8) cm−1. The level of agreement between predicted and observed interaction matrix elements is excellent. This results supports the hope that even slow IVR rates may be predictable in terms of the low order anharmonic force constants which can be estimated by ab initio calculations.

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