ZEKE Photoelectron Spectroscopy of thecis andtrans Isomers of Formanilide

Abstract

Cationic states are involved in long-range charge transfer in polypeptides, [1] an area of intense chemical interest. [2] While a number of theoretical and time-resolved studies have contributed to our mechanistic understanding of this problem, [1] there is currently little spectroscopic information available for cationic amides. [3,4] Basic questions regard the preference for cis or trans conformations and barrier heights for interconversion. Here we present initial results that indicate that zero electron kinetic energy (ZEKE) spectroscopy, [5,6] a highresolution variant of photoelectron spectroscopy, may represent a powerful technique for obtaining vibrationally resolved spectra of cationic amides and model peptides. Spectra are presented for both the cis and trans isomers of formanilide, an aromatic molecule with an amide side chain. The aromatic group provides a convenient chromophore, while significant charge delocalization occurs from the aromatic ring to the functional group in the cation, so that formanilide is a useful model system for studying the properties of cationic amides.Local-minimum geometric structures of the neutral (S 0 ) and cationic (D 0 ) isomers of formanilide obtained from MP2(fc)/ 6-31G* ab initio calculations are presented in Figure 1. [7] For trans-formanilide, the calculations predict that the molecule Figure 1. Optimized structures of the neutral (S 0 ) and cationic (D 0 ) isomers of formanilide at the MP2(fc)/6-31G* level of theory illustrating the ionization-induced geometry changes. Total energies are given relative to the energy of the S 0 state of trans-formanilide (À 399.693829 Hartree).[9] More recently, aerobic intramolecular oxidative amination has been achieved with a [Pd(OAc) 2 ]/O 2 /DMSO catalyst system that lacks a cocatalyst: a) R.

Rotamers of o- and m-Dimethoxybenzenes Studied by Mass-Analyzed Threshold Ionization Spectroscopy and Theoretical Calculations

“…Similar results are also reported for the position isomers of dihydroxybenzene and methoxyphenol. 4,10,11,15,27,[40][41][42] Vibrations 6b, 6a, and 1 are found to be active for the position isomers of dimethoxybenzene. The cation vibrational frequencies of modes 6b, 6a, and 1 appear at 458, 571, and 755 cm -1 for ODMB; 488, 541, and 692 cm -1 for MDMB-a; and 540, 395, and 816 cm -1 for trans-PDMB, 15 respectively.…”

“…Table 3 lists the frequencies of the observed bands in Figure 7 and their possible assignments. The spectral assignment to these MATI features of the ODMB cation in the D 0 state was made by comparing with (1) the experimental data of this molecule in the S 0 state, 26 (2) the experimental data of the ODMB, PDMB, m-methoxyphenol, 4 and resorcinol, 11 and (3) the calculated values. Most of the bands with frequency higher than 300 cm -1 arise from the active ring vibrations.…”

“…Table 5 lists the electronic excitation and ionization energies of benzene, phenol, anisole, and their derivatives on the basis of the LIF, R2PI, ZEKE, or MATI experiments. 4,10,11,15,27,[31][32][33][34][35][36][37][38][39][40][41][42] Analyses on these data indicate that both OH and OCH 3 substituents cause red shifts in the electronic excitation and IE. The red shifts of the S 1 r S 0 transition energy are 2437, 2174, and 2336 cm -1 for catechol, o-methoxyphenol, and o-dimethoxybenzene, respectively, whereas the IEs are 8679, 10 562, and 12 940 cm -1 , respectively.…”

“…One of its important applications is to identify molecular conformers. 4 With a similar experimental setup, mass-analyzed threshold ionization (MATI) spectroscopy 5 involves detection of ZEKE ions rather than electrons. When the pulsed field sequence is carefully designed, MATI spectroscopy can provide unambiguous mass information with a comparable spectral resolution to that of ZEKE spectroscopy.…”

Entropy-driven rearrangement of the water network at the hydrated amide group of the trans-formanilide–water cluster in the gas phase

“…The geometries and vibrational structures of FA and hydrated FA were investigated by resonant two-photon ionization (R2PI) spectroscopy, IR-dip spectroscopy, and ZEKE (Zero kinetic energy) photoelectron spectroscopy. [15][16][17][18][19][20] However, little is known about the the dynamics of these molecules. We report here for the first time the observation of an entropy-driven rearrangement of the water network at the amide group of [FA-(H 2 O) 4 ] + .…”