On the electronic structure aspects of spin-forbidden processes in N2O

The crossing surfaces corresponding to the intersection of the lowest singlet surface of N2O and the three triplet surfaces correlating with N2+O(3P) are studied using multireference configuration interaction wave functions comprised principally of 300 000–500 000 terms, but with some calculations using expansions as large as 1 700 000 terms. These (1 1A′,1 3A′), (1 1A′,1 3A″), and (1 1A′,2 3A″) crossing surfaces are characterized in the vicinity of their minimum energy points. The minimum energy crossing structures are each linear and correspond to (X 1Σ+,3Π) and (X 1Σ+,3Σ−) intersections. The minimum energy point on (X 1Σ+,3Π) crossing surface, was found to be ∼58 kcal/mol above the N2O(X 1Σ+) minimum. The minimum energy point on (X 1Σ+,3Σ−) crossing surface was found to be 71 kcal/mol above the N2O(X 1Σ+) minimum. The N–N bond distance is similar at the (X 1Σ+,3Π) and (X 1Σ+,3Σ−) minimum energy crossing structures, being 1.116 and 1.113 Å, respectively, and approximately equal to that in isolated N2(X 1Σg+). The N–O bond is 1.72 and 1.96 Å for the (X 1Σ+,3Π) and (X 1Σ+,3Σ−) minimum energy crossing structures, respectively, and significantly stretched when compared with its value, 1.18 Å, at the equilibrium geometry of N2O(X 1Σ+). The spin–orbit couplings were also evaluated on each of the (1 1A′,1 3A′), (1 1A′,1 3A″), and (1 1A′,2 3A″) crossing surfaces. The (X 1Σ+,3Π) and (X 1Σ+,3Σ−) spin–orbit interactions were found to be ∼90 and ∼9 cm−1 at their respective minimum energy crossing structures. The crossing surfaces persist in the vicinity of the minimum energy crossing points. However for increased deviations from collinearity avoided crossings are found.