Mechanism of Carbene Formation from the Excited States of Diazirine and Diazomethane: An MC-SCF Study

Abstract

Three radiationless decay pathways for the photochemical decomposition of diazirine and diazomethane have been characterized using the MC-SCF method with a 6-31G* basis. From diazirine, two almost barrierless paths exist on SI. One leads, via a diradicaloid IDur conical intersection at a bent, in-plane, diazomethane-like structure, to ground-state diazomethane; the other leads, via another I D . . conical intersection at a ring-opened diazirine diradicaloid geometry, directly to lCH2 + Nz. The triplet pathway starts at a )u-u* diazirine minimum, passing over a 9 kcal mol-' barrier to a %-7r* ' D, bent diazomethane-like minimum from which the barrier to Nz extrustion is 7 kea1 mol-'. In the absence of sensitizers, this triplet path can be entered from the singlet manifold via intersystem crossing at a point that has been characterized by finding the lowest energy point on the singlet-triplet crossing surface. This crossing point occurs at a geometry that is very similar to the transition state that occurs on the singlet path between diazirine and ground-state diazomethane. However, the efficiency of intersystem crossing (spin-orbit coupling) is predicted to be low. These data rationalize the temperature dependence of the fluorescence, the fact that diazomethanes and diazirines are observed as products of photolysis of diazirines and diazomethanes, respectively, the fact that there is CHz + N2 formation from both diazirines and diazomethanes, and the fact that no triplet states seem to be involved in the reaction.

Accurate Excited-State Geometries: A CASPT2 and Coupled-Cluster Reference Database for Small Molecules

“…82 The structure of the lowest ES structure of diazomethane, constrained in the C 2 v symmetry, was previously determined at both CASPT2/6-31G(d) 26 and CC2/TZVP 25 levels of theory, whereas a CASSCF/6-31G(d) investigation of the ES pathways relating diazirine to diazomethane has also been performed. 83 …”

Ground and excited state CASPT2 geometry optimizations of small organic molecules

“…24 Lastly, the investigation of the diazo chromophore -NAN (a n 3 * chromophore) was performed, taking diazomethane as a model. 25 The S 0 /n 3 * conical intersection, leading to diazomethane decomposition, was also located.…”

In This Molecule There Must Be a Conical Intersection

“…Examples occur in the conical intersections involved in a lone pair photoexcitation, [89][90][91][92][93][94][95] in the conical intersections involved where an isolated negative charge occurs as the result of a charge transfer, 65,[96][97][98][99][100][101][102][103][104][105][106][107] or in the conical intersections involved as a result of the charge transfer and a proton transfer. Examples occur in the conical intersections involved in a lone pair photoexcitation, [89][90][91][92][93][94][95] in the conical intersections involved where an isolated negative charge occurs as the result of a charge transfer, 65,[96][97][98][99][100][101][102][103][104][105][106][107] or in the conical intersections involved as a result of the charge transfer and a proton transfer.…”