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

Published

November 1, 2000

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

Pub Type

Journals

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 NH₂D and ND₂H, Journal of Physical Chemistry A (Accessed July 26, 2024)