Mass analyzed threshold ionization spectroscopy of N-methylaniline, N-ethylaniline, and N,N-dimethylaniline cations: Influence of N-alkyl substitution on the ionization energy and molecular vibration

Abstract

We have applied two-color resonant two-photon mass analyzed threshold ionization spectroscopy to investigate some ionic properties of N-alkylanilines. The respective adiabatic ionization energies of N-methylaniline (NMA), N-ethylaniline (NEA), and N,N-dimethylaniline (DMA) are determined to be 59 822, 59 204, and 58 018 cm−1 with an uncertainty of about 5 cm−1. This indicates that the longer alkyl chain gives rise to a larger redshift in the IE due to a stronger interaction between the alkyl group and the nitrogen atom in the ionic state. Because the alkyl group gives rise to an increase in the electron density around the nitrogen atom of the neutral species, the IE of DMA is lower than that of NMA. In addition, the N-alkyl substitution also influences the frequency of the internal motion of the cations. However, the frequency variation is dependent upon the vibrational pattern and the extent of the coupling between the N-alkyl group and ring vibrations.

Vibrational Spectra and Theoretical Calculations of cis- and trans-3-Fluoro-N-methylaniline in the Neutral (S0) and Cationic (D0) Ground States

“…In the S 0 state, the NH­(CH 3 ) group displays a pyramid-like nonplanar structure, with the amino H atom and CH 3 group being out of ring plane, while upon S 1 ← S 0 excitation and ionization, the increased electron delocalization from the nitrogen to the ring causes a planar 3FNMA with C s symmetry. Such molecular skeleton changes are similar to those reported for aniline and NMA. , Since the S 1 ← S 0 transition is mainly subject to the π* ← π excitation of the ring, the increased population of antibonding orbitals leads to ring expansion, and the ring geometry distorts further from that of a perfect hexagon, with four long and two short bonds. Upon D 0 ← S 1 transition, although only few changes take place on the ring structure, the differences of the six ring C–C bond lengths become smaller than those in the S 1 state, which may account for the frequency variations between the S 1 and the D 0 states.…”

“…The frequencies of observed MATI bands, along with the calculated values and their possible assignments, are summarized in Tables and . We tentatively assigned the vibrational modes mainly on the basis of comparison with the available data of halogen-substituted aniline , and NMA, , as well as the predicted values from the density functional theory (DFT) calculations.…”

“…To study the substitution effect brought by the fluorine atom and the NHCH 3 group, we also compared the vibrational features of 3-fluoroaniline (3FA) and NMA with those of 3FNMA. It is found that whether the vibrational frequency of 3FNMA is close to that of 3FA or NMA mainly depends upon the degree of involvement of the fluorine atom and the NHCH 3 group in the vibration motions.…”

Ionization Energies and Aqueous Redox Potentials of Organic Molecules: Comparison of DFT, Correlated ab Initio Theory and Pair Natural Orbital Approaches

“…Relevant experimental data with respect to ionization energies (IE), redox potentials ( E 0 ), and Henry’s constants ( H C ), which are used to derive solvation energies, are listed in Table . − We note that the experimental uncertainties for the redox potentials are reported in the range 0.01–0.05 eV. For the errors in the experimentally derived Henry’s constants there are large differences depending on the methods employed; therefore, the choices were made according to the recommendations in ref : Henry’s constant is first converted to the dimensionless constant sometimes called “water-air partitioning coefficient” where R is the ideal gas constant, and T is temperature.…”

REMPI spectroscopy and theoretical calculations of cis and trans 3-fluoro-N-methylaniline

“…Besides, as a typical secondary aromatic amine, NMA is often used as the substrate in studying some catalytic reactions [11,12]. Therefore, the structural and electronic properties of NMA in the neutral ground (S 0 ), electronic excited (S 1 ), and cationic ground (D 0 ) states have been extensively investigated using various experimental methods, including vapor phase infrared [13], microwave [14], laser-induced fluorescence (LIF) [15], resonance-enhanced multi-photon ionization (REMPI) [16,17], and mass-analyzed threshold ionization (MATI) [16,17] spectroscopy. As for halogen-substituted NMA, however, the related studies are limited in the literature up to now.…”