Formation of Vibrationally Excited Methyl Radicals Following State-Specific Excitation of Methylamine

Abstract

The photochemistry of methylamine has been investigated following state-specific excitation of the S1 state. 2+1 resonance-enhanced multiphoton ionization was used to detect nascent methyl radical products via the 3p 2 A2″-X̃2A2″ electronic transition. Methyl radicals were formed at all photolysis wavelengths used over the range 222 -240 nm. The nascent products showed significant rotational excitation and several quanta of vibrational excitation in ν3, the degenerate C-H stretch. The partially deuterated methyl-d3-amine isotopologue yielded methyl-d3 fragments with vibrational distributions entirely consistent with those measured for the fully protiated species; no mixed isotopologues were detected.Energetic constraints require that the vibrationally excited methyl radicals be produced in conjunction with electronic ground state NH2 X̃2B1 radicals on the S0 surface, negating the previous interpretation that dissociation occurs on the upper adiabat. New ab initio calculations characterizing the C-N bond cleavage coordinate confirm the presence of a barrier to dissociation on S1 that is insurmountable at the photolysis wavelengths used in this work. We propose a "semi-direct" mechanism in which frustrated aminyl H-atom loss on the upper adiabatic potential energy surface leads to internal conversion at the exitchannel conical intersection at extended N-H distance on its return. It is proposed that C-N bond cleavage then occurs promptly and non-statistically on the S0 surface.

A Quasi-Diabatic Representation of the 1,21A States of Methylamine

“…The ab initio [H d ] predicted dissociation energy for the CH 2 NH + H and CH 3 + NH 2 channels are 33 549 [33 639] cm −1 and 28 395 [28 543] cm −1 , respectively, which are in good agreement with experimental values, 34 250 cm −1 (refs 2 and 4) and 29 300 cm −1 . 5 The ab initio (H d ) determined T 0 (NH 2 (A ̃1A)) is 11 124 [10 850] cm −1 , which is in good agreement with the experimentally determined value, 11 122 cm −1 given in ref 12. This is much lower than the energy of the excited state of CH 3 so that the 2 1 A asymptotically constrained optimization, 42 753 [42 730] cm −1 gives ground state CH 3 and excited state NH 2 .…”

Evidence for quantum effects in the predissociation of methylamine isotopologues

“…A very appealing system that received considerable interest over recent decades is the prototypical model of methylamine (CH 3 NH 2 ) and its partially, or fully deuterated isotopologues, and particularly their photolysis following excitation to the first UV absorption band. This is due to their similarity with ammonia and relative simplicity, which made them liable to numerous experimental [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] and theoretical [28][29][30][31][32][33][34] studies. In particular, five dissociation channels have been discussed and identified, 9,10,[27][28][29][30] including N-H, C-H and C-N bond cleavage that leads to CH 3 + NH 2 , or CH 4 + NH, as well as H 2 elimination from both the amino and methyl groups.…”

“…This is due to their similarity with ammonia and relative simplicity, which made them liable to numerous experimental [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] and theoretical [28][29][30][31][32][33][34] studies. In particular, five dissociation channels have been discussed and identified, 9,10,[27][28][29][30] including N-H, C-H and C-N bond cleavage that leads to CH 3 + NH 2 , or CH 4 + NH, as well as H 2 elimination from both the amino and methyl groups. Nevertheless, most studies focused on the H(D) photofragment detection 11,12,15,[18][19][20][21][22][23][24][25] resulting from N-H(D) and C-H(D) bond cleavage, where the former was found to be the dominant one.…”

H/D substitution makes difference in photochemical studies: the case of dimethylamine

“…As the simplest primary alkyl amine, methylamine (MA) has been widely studied for many years. [1][2][3][4][5][6][7][8][9][10][11][12] Recent many theoretical and experimental studies show that the MA photochemistry is very rich in terms of its interesting diverse relaxation pathways and detailed mechanisms involved. Photochemistry of the secondary alkyl amine, however, has not been much studied to date.…”

“…As the predissociation of N-H or N-D occurs via tunneling through a barrier, the NH/ND isotope substitution influence is expected to be quite substantial. [4][5][6][7][8][9][10][11][12] Indeed, the much more resolved R2PI spectrum could be obtained for DMA-d 1 , Figure 2(a). And yet, the spectrum is overall quite broad and individual peaks are Lorentzian shaped, indicating that the excited state, in spite of NH/ND substitution, decays very fast.…”