We present new experimental data demonstrating specific, photoactivated positive charge migration in isolated peptide radical cations. The effect exhibits a threshold behavior, which we can directly correlate with energetics of local electronic states. A new very efficient mechanism for charge transfer in cations is proposed that involves an extended coulomb state (EC) of shakeup character. Our investigations are performed on laserdesorbed, cooled, neutral peptides in the gas phase. Charge localization in the peptide is achieved by resonant UV two-photon ionization at an aromatic chromophore. Charge flow in the cations can be activated by absorption of a first visible (VIS) photon. Presence of charge in the aromatic chromophore is probed by resonant absorption of a second VIS photon and monitored by dissociation. While this charge detection is found to work in isolated, positively charged chromophores or amino acids, it is efficiently quenched in some peptides. We explain this by photoactivated charge transfer and charge storage in nonaromatic groups of the peptides. At threshold this process is found to be strongly dependent on amino acid substitution even far away from the site of photoactivation. For analysis we first set up a local molecular orbital model for peptide cations and subsequently obtain a landscape of local electronic cation states formed by local hole and low-lying extended coulomb states. Charge transfer is found to be a through-bond mechanism involving energetically accessible electronic states along the path of charge flow. Charge transfer between hole states is mediated with very high efficiency through saturated carbon bridges by extended coulomb states. This new mechanism seems to be generally applicable to large extended molecular radical cations. Only barriers of the size of a full length of a certain defined amino acid are found to block charge transfer. The model qualitatively accounts for the order of the rates of the processes involved.

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The 2-phenylethyl-N,N-dimethylamine (PENNA) radical cation offers two functional groups for a positive charge to reside, the benzyl ring and the substituted amine group. Previously published HeI photoelectron spectra (PES) and our B3LYP data of the cation ground state agree that the amine site has the lowest ionization energy (IE). In this work we present evidence that by resonant laser multiphoton ionization the electron is removed from the phenyl site. B3LYP calculations of the neutral molecule predict that by far the most stable structure is the nonsymmetric unfolded anti conformer and that no other conformer should be significantly populated. This is confirmed by the S0−S1 resonant two-photon ionization (R2PI) spectrum in which one conformer is found to be predominant. The presence of the vibrational fingerprint of the phenyl chromophore and the absolute energetic position of the R2PI spectrum clearly show the local character of the first photoexcitation. Surprisingly, the R2PI mass spectra taken via S1 resonances show strong fragmentation. The parent-to-fragment-ion ratio is about 1:10 and mostly independent of laser intensity. The metastable character of the decay excludes a fragmentation caused by cation photoabsorption. The strong dissociation directly after ionization is explained by (i) a local ionization at the phenyl chromophore, (ii) a fast charge transfer (CT) to the lower-energetic amine site, and (iii) a subsequent metastable dissociation. A first detailed analysis of the ionization process indicates that intersite R2PI ionization between local states is a one-photon two-electron process which is expected to be improbable and that a failure of local ionization only happens in cases of mixed intermediate S1 states or mixed cation states. PENNA with two possible charge sites spaced by a −CH2−CH2− bridge is an ideal model system to study the dynamics of a downhill charge transfer after local ionization to the first excited cation state as presented in a forthcoming paper.

In the past decade halogen bond (XB) catalysis has gained considerable attention. Halo-triazoles are known XB donors, yet few examples detail their use as catalysts. As a continuation of our previous work the catalytic properties of substituted enantiomerically pure halo-triazolium salts were explored in the reaction between an imine and Danishefsky’s diene leading to the formation of dihydropyridinone. The catalytic activity of the XB donors was highly dependent on the choice of the halogen atom and on the counterion. Also, it was found that impurities in the diene affected the rate of the reaction.

A complex derived from the enantiomeric bipiperidine and copper(II) acetate hydrate is an efficient catalyst for the enantioselective Henry reaction. The easy availability of both catalyst components, mild reaction conditions, high yield, and good to excellent enantioselectivity make the catalyst useful for everyday practice.