Spectroscopy and dynamics of methylamine. I. Rotational and vibrational structures of CH3NH2 and CH3ND2 in à states

Abstract

Accurate and precise ionization energies of methylamines (CH 3 NH 2 and CH 3 ND 2 ) are determined to be 9.0422Ϯ0.0012 and 9.0532Ϯ0.0012 eV, respectively, by ͑1ϩ1Ј͒ two-photon mass-analyzed threshold ionization ͑MATI͒ spectroscopy. From selective ionizations from specified intermediate quantum states, fundamental frequencies of amino-wagging and CH 3 -rocking modes of CH 3 NH 2 ϩ (CH 3 ND 2 ϩ ) in D 0 states are determined to be 738 ͑573͒ and 1013 ͑1024͒ cm Ϫ1 , respectively. The frequency of the amino wag is largely blueshifted from that of the neutral S 1 state, while the CH 3 -rocking frequency is little shifted from that of S 1 . Internal rotational constants associated with the nearly free internal rotation of the top ͑amino group͒ with respect to the frame ͑methyl group͒ about the C-N axis are accurately determined, from which the geometries of methylamine ions are revealed. Barrier heights for torsional motion in CH 3 ND 2 ϩ are determined to be 25Ϯ5 and 34Ϯ5 cm Ϫ1 at the origin and first ND 2 -wagging bands, respectively. Mode-resolved spectroscopy using the MATI signal for resolving overlapped spectral features in the intermediate state is employed for clarifying the vibrational assignment of the intermediate state. An ab initio calculation at the QCISD level is carried out, giving good agreement with the experiment.

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

“…[32][33][34] The most detailed spectroscopic analysis comes from Kim and co-workers, who have recorded 1+1 REMPI spectra of methylamine (as well as partially and fully deuterated isotopologues) and modeled the vibronic features using a hindered internal rotor Hamiltonian. [35][36][37] Spectra of the protiated methylamine show homogeneous broadening consistent with a lifetime of 0.38 ps at the origin and decreasing as the excitation energy increases. Analogous spectra of the partially deuterated methylamine-d2 species display sharper lines, and a longer excited state lifetime of around 8.8 ps.…”

“…The effect of vibrational excitation in ν9 on passage through the conical intersection region was also noted by Waschewsky et al, who suggested that the anticipated vibrational excitation would lead to trajectories sampling regions of phase space in which the upper and lower adiabats are well separated. However, based on the more recent spectroscopic assignments of Kim and co-workers, [35][36][37] excitation at 222 nm likely prepares the 2ν7+2ν9 mode in S1, which as an even mode might be expected to favor formation of ground state products. In any event, this point is moot as it seems unlikely that the predicted barrier to direct dissociation on S1 can be overcome at any of the excitation wavelengths used.…”

“…53,54 In all geometries, the magnitudes of SO are small, in agreement with Xiao et al 24 The calculated values of ISC are reported in Table 3, where they are compared with predissociation rates derived from previously measured homogeneous line-broadening in 1+1 REMPI spectra of CH3NH2. 36 The golden rule calculations suggest energy dependent ISC rates that are ~14,000-400,000 slower than predissociation at the geometry of S1 (or T1). We conclude that, although the CN cleavage pathway is minor, the rate of ISC is so slow relative to predissociation that the photodissociation mechanism does not involve ISC to the triplet surface in the FranckCondon region.…”

Nuclear motion captured by the slow electron velocity imaging technique in the tunnelling predissociation of the S1 methylamine

“…[16][17][18][19][20][21][22][23][24][25] It displays intriguing dynamical features such as a huge H/D isotope effect, indicating that the N-H(D) bond dissociation occurs via tunnelling at the zero-point level of S 1 . [16][17][18][19][20][21] Predissociation dynamics is state dependent and the conical intersection seems to play an important role in the final product properties. 22 Even though product analyses reflect the exit dynamics, it is extremely hard to follow the trajectory back from the product to reactant for the irreversible chemical processes.…”

A new method for determining absorption cross sections out of initially excited vibrational states

“…Because of its importance, methylamine and its deuterated isotopologues have been a target for numerous experimental and theoretical studies exploring their dissociation, [9][10][11][12][13][14][15][16][17] and structural and spectroscopic [18][19][20][21][22][23][24][25][26] properties. Most of the previous dissociation studies have been performed without characterizing the initial quantum state of the methylamine on the ground, X , or the first electronic state, Ã .…”