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Competition Between N-H and N-D Bond Cleavage in the Photodissociation of NH2D and ND2H

Published

Author(s)

J P. Reid, R A. Loomis, S R. Leone

Abstract

The adiabatic dissociation dynamics of NH2D( )and ND2H( ) have been probed by time-resolved Fourier transform infrared emission spectroscopy. A product-state pattern recognition technique is employed to separate out the emission features arising from the different photofragmentation channels following the simultaneous excitation of mixtures of the four parent molecules NH3, NH2H and ND2H and ND3 at 193.3 nm. The rotational energy partitioning about the primary α-axis of the fragments NH2( ,υ2' = 0) from NH2D( ) and ND2H( ), respectively, is bimodal. We suggest that the origin of this excitation reflects the competition between two distinct dissociation mechanisms that sample two different geometries during the bond-cleavage. A larger quantum yield for producing ND2( , υ2' = O) from the photodissociation of ND2 H than ND3 is attributed to the lower dissociation energy of the N-H as compared with the N-D bond and to the enhanced tunneling efficiency of H-atoms over D atoms through the barrier to dissociation. Similarly, the quantum yield for producing the NH2( , υ2' = 0) fragment is lower when an N-D bond must be cleaved in comparison to an N-D bond. Photodissociation of ND2H by cleavage of an N-H bond leads to an ND2 ( ) fragment with a much larger degree of vibrational excitation (υ2' = 1.2), accompanied by substantial rotation about the minor b/c axes, than when a N-D bond is cleaved in the photodissociation of ND3. The quantum yield for producing NHD( ) is larger for cleavage of an N-H bond from NH2D than by cleavage of an N-D from ND2H.spectroscopy, A product-statc spectral pattern recognition technique is employed to separate out the emission features arising from the dil'l'ereipilotofragiticntation channels following the simultaneous excitation ol'inixtures of the four parent molecules N3, NH,D, ND2Ll and ND3193.3 ni-n. The rotational energy partitioning about the primary a-axis of the fragments NH2(AIUI'=O) and ND,(A,U,'=O) I-i-oin Nfi,D(A) aiND,H(A), respectively, is bimodal. We suggest that the origin of this excitation reflects the competition between two distinct dissociatemechanisms that sample two different geometries during the boiid-cleavage. A larger quantum yield lor producing ND,(,k,i),'=O) from tlphotodissociation of ND,H than ND3 is attributed to the lower dissociation energy ol' the N-H as compared with the N-D bond iiid to tlenhanced tunneling efficiency of H-atoi-ns over D-atoins through the barrier to dissociation. Similarly, the quantum yield l'oi- producing tilNH,(A,u,'=O) fragment is lower when an N-D bond must be cleaved in comparison to ail N-1-1 bond. I)Iiotodissociatioii ot'ND,ll by cleiivaof an N-H bond leads to an ND,(,k) fragment with amuch larger degree of vibrational excitation (i),'=I,2), accompanied by substantial i-olatiabout the minor b/c axes, than when aN-D bond is cleaved in the pilotodissociation of ND,. The quantum yield for producing NIID(A) is largfor cleavage of an N-H bond from NH2D than by cleavage ot'an N-D from ND,H.
Citation
Journal of Physical Chemistry A

Keywords

chemical dynamics, photodissociation, spectroscopy

Citation

Reid, J. , Loomis, R. and Leone, S. (2000), Competition Between N-H and N-D Bond Cleavage in the Photodissociation of NH<sub>2</sub>D and ND<sub>2</sub>H, Journal of Physical Chemistry A (Accessed April 14, 2024)
Created October 31, 2000, Updated October 12, 2021