Vibronic structure of the cyclopentadienyl radical and its nonrigid van der Waals cluster with nitrogen

Abstract

Fluorescence excitation and two color mass resolved excitation spectroscopy are employed to study the D 1 (2 A 2 Љ)←D 0 (2 E 1 Љ) vibronic transitions of the cyclopentadienyl radical ͑cpd͒ and its van der Waals cluster with nitrogen. The radical is created by photolysis of the cyclopentadiene dimer and cooled by expansion from a supersonic nozzle. The cpd͑N 2 ͒ 1 cluster is generated in this cooling process. Mass resolved excitation spectra of cpd are obtained for the first 1200 cm Ϫ1 of the D 1 ←D 0 transition. The excitation spectrum of cpd͑N 2 ͒ 1 shows a complicated structure for the origin transition. With the application of hole burning spectroscopy, we are able to assign all the cluster transitions to a single isomer. The features are assigned to a 55 cm Ϫ1 out-of-plane van der Waals mode stretch and contortional ͑rotational͒ motions of the N 2 molecule with respect to the cpd radical. Empirical potential energy calculations are used to predict the properties of this cluster and yield the following results: ͑1͒ the N 2 molecular axis is perpendicular to the cpd fivefold axis and parallel to the plane of the cpd ring with the two molecular centers of mass lying on the fivefold ring axis; ͑2͒ the binding energy of cpd͑N 2 ͒ 1 is 434 cm Ϫ1 ; and ͑3͒ the rotational motion of the N 2 molecule is essentially unhindered about the cpd fivefold axis. The molecular symmetry group D 5h ͑MS͒ is applied to the nonrigid cluster, and optical selection rules exclude even↔odd transitions ͑⌬nϭ0, Ϯ2, Ϯ4,... allowed͒ between the different contortional levels. Tentative assignments are given to the observed contortional features based on these considerations. The barrier to internal rotation is also small in the excited state. The results for the cpd͑N 2 ͒ 1 van der Waals cluster are compared to those for the benzene ͑N 2 ͒ 1 and benzyl radical ͑N 2 ͒ 1 clusters.

STRUCTURAL DYNAMICS OF FREE MOLECULES AND CONDENSED STATE OF MATTER. Part II. TRANSIENT STRUCTURES IN CHEMICAL REACTIONS

“…As a test of the method for calculating energy differences, [124] have also optimized the D 5h minimum structure (labeled M(D 2 )) on D 2 ( 2 A' 2 ). The value for the 0-0 electronic transition was calculated as 86.6 kcal mol -1 , in good agreement with the experimental value of 84.6 kcal mol -1 (29,573 cm -1 ) [146,147]. Table 1 of ref.…”

Effect of Noncovalent Interactions on Vibronic Transitions: An Experimental and Theoretical Study of the C2H⋅⋅⋅CO2 Complex

“…[68] Experimental studies of electronic transitions in complexes involving radicals are even scarcer.S un and Bernstein have compared the interaction of benzene and benzyl and cyclopentadienyl radicals with an itrogen molecule. [69] They found that the origin of the first electronic transition of thesem olecules in the complexes could be either blue-or red-shifted compared with the monomers. Thec loseste xample to our system seems to be the complexes of NCO radical with Ar,N 2 , CH 4 ,a nd CF 4 .…”

On the à and B̃ electronic states of NCO and its clusters with nonpolar solvents

“…As is described in previous work, 31,32 the experiment is carried out in a stainless steel vacuum chamber at a pressure of ca. 10 Ϫ4 Torr.…”

“…Cluster geometry and binding energy are obtained employing an atom-atom potential energy calculation. 31,32 All molecules are considered as rigid entities in this calculation. The interaction energy is determined as a sum of various terms: short range repulsion, induction/dispersion, Coulomb, and hydrogen bonding.…”